LAYERED DOUBLE HYDROXIDES (LDHs) – CARBON NANOSTRUCTURE (CNS) COMPOSITES: SYNTHESIS AND APPLICATION IN BASE- CATALYSED CONDENSATION REACTIONS

NICOLAS MORALES VEGA

Department of Chemical Engineering Imperial College London

Submitted as fulfilment of the thesis requirement for the degree of

Doctor of Philosophy July 2017

Declaration of Originality

I hereby declare that this thesis is the result of my own work, and that any ideas or quotations from the work of other people are appropriately referenced.

Nicolas Morales Vega Imperial College London London, United Kingdom 2017

The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work.

Abstract

The activity and selectivity produced by novel Layered Double Hydroxides (LDH) supported in both multi-walled carbon nanotubes (MWNT) and graphene oxide (GO) was investigated for a set of base-catalysed aldol condensation reactions. The MWNT/LDH composites were studied in the self-condensation of acetone and retroaldolization of diacetone alcohol (DAA), while the GO/LDH materials were used in the latter reaction, as well as the benzaldehyde-acetone condensation, where the material was activated in-situ. The effect of carbon content was investigated with the aim to compare the performance of the solids during reaction.

Adding the LDH into either carbon support increases dispersion and generally decreases LDH particle size, which produces an increment in both surface area and the availability of basic sites in the hybrids. As a result, activity in all three condensation processes is enhanced by 3 to 4 times, in comparison to the unsupported LDH. Similarly, stability during reaction is also increased.

However, problems regarding the use of LDH and the composites were identified.

- Activated unsupported meixnerite-type LDH (OH interlayer anion) has high initial activity, but tends to deactivate faster. This effect can be reduced by using the nanocarbon supports. The use of MWNT as support allows increased rates to be achieved compared to GO, but high contents of this less volume efficient support are necessary. The pre-treatment stage of MWNT is also extremely lengthy, while care should be taken during MWNT purification before composite synthesis, as carbon debris could hinder activity greatly, especially at higher loadings. Conversely, very low amounts of GO material lead to the best activity in this study, probably related to the coherence between the natural charge and geometry between the LDH and GO layers. Nevertheless, both types of composites tend to adsorb reaction products at long reaction times, which possibly reduces the availability of basic sites of the catalysts.

The self-condensation of acetone is an equilibrium limited process where the highest conversion is achieved at 273 K. This hinders the ability to compare the basic properties of the materials appropriately. The problem is avoided by studying the catalytic effect of the composites in the DAA retroaldolization, which requires low amounts of catalyst and rates can be compared at mild conditions. The in-situ activation study of benzaldehyde-acetone condensation showed that higher Mg content and the use of the carbon supports increases both activity and selectivity towards the desired product, benzalacetone.

I

Acknowledgements

I would like to extend a special and sincere thanks to Prof. David Chadwick for his continuous support throughout the development of my PhD. His advice and knowledge in a range of practical topics, and ability to relate them to every part of my research, allowed me to develop my skills extensively.

I am also grateful to Prof. Milo Shaffer for allowing me to be part of his research group from time to time, and obtain some of the expertise necessary to produce this work. I also would like to acknowledge Dr. Martina de Marco for her important and careful advice.

I would like to address my sponsors: Consejo Nacional de Ciencia y Tecnologia (CONACyT), Secretaria de Educacion Publica (SEP), Bio Nano Consulting. Also, my sincere thanks to Imperial College London for opening its doors to continue my professional and personal development.

Many thanks to Kai, Saeed and Xiaowen and all the UG and MSc students which I had the opportunity to supervise and work with. I definitely learned something from each one of you.

Special thanks to my friends Raul, Omar, Pablo, Katherina, Motaz and specially Diana and Ines, who always had time to share a good moment, and for always having a word of advice on difficult and stressful moments.

I am deeply grateful to Raquel for all her love and support every single day. For her time and her company during difficult times, and a smile whenever it was necessary. Thank you for always being there with me.

Finally, I would like to thank my family. My brothers and sister, Roberto, Victor and Cynthia and my parents, Nicolas and Maria de los Angeles, for always pushing me to keep going forward with unconditional love and care. The confidence you always had in me allowed me to keep going forward every single day.

This thesis is dedicated to all of you. Nicolas Morales Vega London, England July 2017

II Contents

Abstract ...... I

Acknowledgements ...... II

List of Figures………………………………………………………………………………...... VII

List of Tables………………………………………………………………………………...... X

Nomenclature.………………………………………………………………………………...... XI

Chapter 1 Introduction ...... 1

1.1 The state of biorefining applications ...... 1

1.2 Research objectives ...... 5

1.3 Structure of the thesis ...... 6

References ...... 7

Chapter 2 Literature Review ...... 8

2.1 Aldol condensation reactions on biorefining applications ...... 8

2.2 Solid catalysts used on aldol condensation and upgrading processes...... 11

2.3 Layered Double Hydroxides ...... 14

2.3.1 Clay materials, properties and classification ...... 14

2.3.2.-Layered Double Hydroxides: Solid materials ...... 14

2.3.3.-Structure and composition ...... 16

2.3.4.-Anionic species on the interlamellar space of LDH ...... 17

2.3.5- LDH materials synthesis methods ...... 19

2.4.-Layered Double Hydroxides: applications ...... 21

2.4.1.-LDH applications on base catalysed aldol condensation reactions ...... 23

2.4.2.- Diacetone alcohol retroaldolization: an of basic activity on the gas phase ...... 28

2.4.3.- Benzaldehyde-acetone condensation for the analysis of in-situ rehydration ...... 29

References ...... 32 Contents

Chapter 3 Experimental Methodology ...... 40

3.1 Synthesis of catalytic materials ...... 40

3.1.1 Unsupported Mg-Al LDH ...... 40

3.1.2 Synthesis of LDH supported on MWNT materials ...... 41

3.1.3 Synthesis of LDH supported on GO materials ...... 43

3.1.4 Activation of synthesised LDH and hybrids ...... 43

3.2 Characterisation of catalytic materials ...... 44

3.2.1 X-Ray Powder Diffraction (XRD) ...... 44

3.2.2 Nitrogen Physisorption ...... 44

3.2.3 Transmission-Electron Microscopy/Energy Dispersive Spectroscopy (TEM/EDS) and Scanning-Electron Microscopy (SEM) ...... 45

3.2.4 Inductively Coupled Plasma (ICP) ...... 45

3.2.5 Thermogravimetric Analysis (TGA) ...... 45

3.2.6 CO2-Temperature-Programmed Desorption (CO2-TPD) ...... 46

3.3 Catalytic Tests ...... 47

3.3.1 LDH/MWNT catalytic activity on the liquid phase self-condensation of acetone...... 47

3.3.2 LDH/MWNT and LDH/GO catalytic activity study on retroaldolization of diacetone alcohol in the gas phase ...... 50

3.3.3 In-situ activated LDH/GO activity tests on the condensation of benzaldehyde and acetone ...... 53

3.4 Conversion and Selectivity ...... 55

References ...... 56

Chapter 4 Acetone self-condensation: analysis of basic strength on LDH/MWNT composites ...... 58

4.1 Introduction ...... 58

4.2 Initial testing with activated LDH samples: Looking for optimal reaction conditions ....61

4.2.1 Test with activated composite, pure and debris LDH samples in reactor 1 ...... 61

IV Contents

4.2.2 Mass transfer limitation research: Comparison between mixing in reactors 1 and 2 ...... 64

4.2.3 Analysis of lower rate and production of DAA: Activity in reactors 1, 2 and 3 ...... 67

4.3 MWNT/LDH composites: Synthesis of a range of solids and characterisation studies ...... 70

4.3.1 Thermogravimetric analysis of composites: Carbon quantification of synthesised MWNT/LDH catalysts ...... 71

4.3.2 Phase composition analysis: X-ray diffraction of MWNT/LDH composites ...... 74

4.3.3 Analysis of morphology in the solids MWNT/LDH catalysts: Microscopy studies ..77

4.3.4 Dispersion in the LDH/MWNT: An analysis of surface area by N2 adsorption ...... 78

4.3.5 CO2-TPD analysis of solid catalysts: Dependence of the availability of basic sites with carbon amount ...... 80

4.4 Catalytic testing of synthesise MWNT/LDH hybrids ...... 83

4.5 Concluding Remarks ...... 88

References ...... 90

Chapter 5 DAA retroaldolization: Relative basic strength comparison of LDH/CNS composites in the gas phase ...... 93

5.1 Introduction ...... 93

5.2 Testing with activated LDH: Defining an ideal set of conditions and materials ...... 95

5.2.1 Analysing the effect LDH catalyst activation and reactor temperature on the retroaldolization process ...... 95

5.2.2 Effect of space velocity: adjusting system parameters to reduce conversion and avoid mass transfer limitations ...... 97

5.3 CNS composites: Synthesis of solid catalysts and characterisation ...... 100

5.3.1 Synthesis methodologies and composition on carbon hybrids ...... 101

5.3.2 Thermogravimetric analysis of synthesised composites: General quantification of carbon on CNS/LDH hybrids ...... 103

5.3.3 Phase composition of solid materials: XRD of CNS/LDH composites ...... 105

5.3.4 Morphology on the solids CNS/LDH catalysts: analysis via microscopy ...... 110

5.3.5 Surface area quantification of CNS/LDH materials by N2 adsorption ...... 113

V Contents

5.3.6 CO2 adsorption of solids CNS hybrids. General relation and availability of basic sites ...... 116

5.4 Catalytic testing of synthesise MWNT/LDH hybrids ...... 118

5.4.1 MWNT catalysts ...... 122

5.4.2 GO containing LDH solid catalysts ...... 123

5.5 Deactivation measurements on selected rehydrated samples ...... 127

5.6 Concluding Remarks ...... 129

References ...... 131

Chapter 6 Benzaldehyde-acetone aldol condensation: An analysis of the effect of in- situ rehydration ...... 134

6.1 Introduction ...... 134

6.2 Benzaldehyde-acetone: sample selection and characterisation...... 137

6.3 Activity on the benzaldehyde-acetone condensation: Conditions and comparison of selected LDH and LDH hybrid materials...... 138

6.4 Concluding Remarks ...... 142

References ...... 144

Chapter 7 Conclusions ...... 145

References ...... 148

Appendix A ...... 149

Appendix B ...... 153

Appendix C ...... 155

VI Contents

List of Figures

Figure 1.1 First and second generation biorefining processes 3

Figure 2.1 A brucite-like layer 16

Figure 2.2 A layered double hydroxide (LDH) 17

Figure 2.3 Classification of LDH synthesis methods 19

Figure 2.4 Applications of LDH compounds 22

Figure 2.5 Scheme of the self condensation of acetone for production of MIBK 24

Figure 2.6 Scanning Electron Microscopy (SEM) of LDH samples[54] 26

Figure 2.7 Condensation reaction between benzaldehyde and acetone 30

Figure 3.1 Schematic of a) reactor 1 b) reactor 2 48

Figure 3.2 General distribution of reaction system used for the DAA retroaldolization process 51

Figure 3.3 Carrousel system used on benzaldehyde-acetone condensation studies 53

Figure 4.1 DAA molar concentration as a function of time. Treac= 273 K, stirring speed = 1000 rpm, Magnetic stirring, NAo= 0.25 mol acetone, mcat = 0.05 g, treac = 50 h. Insert included to show Equilibrium concentration 63

Figure 4.2 DAA molar concentration as a function of time. Rehydrated LDH catalyst. Treac=

273 K, stirring speed = 1000 rpm, Mechanical stirring, NAo= 0.80 mol acetone, mcat=0.05 g, treac = 50 h 66

Figure 4.3 DAA molar concentration as a function of time. Rehydrated LDH catalyst. Treac= -1 273 K, Stirring speed = 1000 rpm, Ccat =0.83 gcat L , treac 50 h. 68

Figure 4.4 TGA decomposition pattern for synthesised samples. (a) MWN/LDHhl-20, (b)

MWN/LDHhl-33, (c) MWN/LDHhl-50, (d) MWN/LDHhl-67. Data obtained by Dr. Almudena Celaya-Sanfiz 72

Figure 4.5 X-ray diffractogram of synthesised solid catalysts. (a) MWN/LDHhl-20, (b)

MWN/LDHhl-33, (c) MWN/LDHhl-50, (d) MWN/LDHhl-67. Data obtained by Dr. Almudena Celaya-Sanfiz 74

Figure 4.6 Dependence of the LDH crystallite size in relation with carbon content. Data obtained by Dr. Almudena Celaya-Sanfiz 76

Figure 4.7 SEM of synthesised solid catalysts. (a) MWN/LDHhl-20, (b) MWN/LDHhl-33, (c)

MWN/LDHhl-50, (d) MWN/LDHhl-67. Square areas represent MWNT. Circle areas represent LDH. Data obtained by Dr. Almudena Celaya-Sanfiz 78

VII Contents

Figure 4.8 BET surface area dependence with the amount of MWNT loaded on the catalysts. Data obtained by Dr, Almudena Celaya-Sanfiz 79

Figure 4.9 CO2 TPD profiles for pure materials and synthesised compounds. Data obtained by Dr. Diana Iruretagoyena Ferrer 82

Figure 4.10 DAA molar concentration as a function of time. Treac= 273 K, stirring speed =

1000 rpm, Magnetic stirring, NAo= 0.8 mol acetone, mcat = 0.05 g, treac = 50 h. Selected data obtained by Dr. Almudena Celaya-Sanfiz 84

Figure 5.1 DAA conversion as a function of time for different activated LDH materials. Treac =

313 K, WHSV = 2.21 h-1, mcat = 0.5 g, treac = 5 h 96

Figure 5.2 DAA conversion as a function of time at different WHSV, with modification of flow.

Treac = 303 K, mcat = 0.025 g, treac = 5 h 97

Figure 5.3 DAA conversion as a function of time at different WHSV with modification of catalyst mass. Treac = 303 K, treac = 5 h 98

Figure 5.4 DAA conversion as a function of time at different WHSV of LDH samples activated by different methods. Treac = 303 K, treac = 5 h 99

Figure 5.5 Thermogravimetric analyses of LDH samples and composites. a) Precipitated MWNT b) Impregnated MWNT c) Precipitated GO 105

Figure 5.6 XRD patterns of CNS/LDH hybrid catalysts. a) Precipitated MWNT b) Impregnated MWNT c) Precipitated GO d) Selected calcined samples 107

Figure 5.7 Representative SEM micrographs of hybrid catalysts. a) SynthLDH b) 15MWNT/LDH c)29MWN/LDH d) Impregnated 80MWNT/LDH e) Pure GO 111

Figure 5.8 Representative TEM images of hybrid catalysts. a) 1.5GO/LDH b)15MWNT/LDH c)15MWNT/LDH (different zone) d)11GO/LDHc 112

Figure 5.9 Hybrid surface area vs carbon amount on thermally activated samples. a) MWNT solids b) GO solids 114

Figure 5.10 Conversion vs time for the optimal samples of synthesised CNS/LDH hybrid -1 catalysts. Treac = 303 K, WHSV = 19.80 h ,mcat = 0.015 g, treac = 5 h 120

Figure 5.11 Relationship of Initial rate of reaction vs amount of CO2 adsorbed. a) MWNT/LDH solids b) GO/LDH hybrids 121

Figure 5.12 TGA of pure thermally activated LDH catalyst before and after reaction 125

Figure 5.13 DAA conversion vs time of Rehydrated LDH and 1.50GO/LDH catalysts. Insert -1 includes conversion until 5h of reaction. Treac = 303 K, WHSV = 19.80 h , mcat = 0.015 g, treac = 45 h 128

Figure 5.14 TGA of as LDH catalyst activated by rehydration before and after 45 h of reaction 129

VIII Contents

Figure 6.1 General scheme of condensation reaction between benzaldehyde and acetone. Isomers of benzalacetone and dibenzalacetone are possible by-products 136

Figure A.1 Correlation between DAA amount and area measured by FID detector. Study of acetone-self condensation 149

Figure A.2 Correlation between DAA amount and area measured by FID detector. Study of retroaldolization of DAA 150

Figure A.3 Correlation between benzaldehyde amount and area measured by FID detector 151

Figure A.4 Correlation between benzalacetone amount and area measured by FID detector 151

Figure A.5 GC-MS Chromatogram of selected sample. The insert indicates the magnetic sector and identification of the compound 152

Figure B.1 TGA analysis of synthesised LDH used on section 4.2 153

Figure B.2 XRD analysis of synthesised LDH used on section 4.2 154

IX Contents

List of Tables

Table 2.1 Reported anionic structures intercalated into LDH sheets 18

Table 4.1 Identifiers and initial rates of reaction obtained in the self-condensation of acetone for preselected LDH samples 62

Table 4.2 Initial rates of reaction for pure activated LDH catalysts for different types of mixing and reactors 65

Table 4.3 Initial rates of reaction for pure activated LDH catalysts at different stirring speed 67

Table 4.4 Initial rates of reaction for pure activated LDH catalysts for different types of mixing and reactors 70

Table 4.5 MWNT/LDH samples synthesised to catalyse the self-condensation of acetone. Selected data obtained by Dr. Almudena Celaya-Sanfiz 71

Table 4.6 MWNT/LDH samples synthesised to catalyse the self-condensation of acetone. Data obtained by Dr. Diana Iruretagoyena Ferrer 81

Table 4.7 Initial rate of reaction of MWNT/LDH samples synthesised to catalyse the self- condensation of acetone. Correlation with amount of CO2 adsorbed 84

Table 5.1 Identifiers, synthesis method and composition of the carbon supported catalysts before thermal activation 102

Table 5.2 Crystallite size of uncalcined CNS/LDH hybrid materials 109

Table 5.3 Normalised CO2 adsorption measurements of synthesised LDH/CNS 117

Table 5.4 Rate of reaction and deactivation of hybrid CNS/LDH catalysts 119

Table 6.1 Nomenclature used for the compounds used on the chapter 136

Table 6.2 Identifiers of selected LDH catalysts: Parameters for selection and characteristics 138

Table 6.3 Conversion of B and selectivity towards BA: Effect of reaction temperature. In-situ rehydrated commercial LDH. B/A/W molar ratio 1:5:10. Water volume = 1.8 mL, mcat = 0.05 g 140

Table 6.4 Conversion of B and selectivity towards BA: Comparison on unsupported and supported LDH materials. Treac = 353 K, B/A/W molar ratio 1:5:10. Water volume = 1.8 mL 141

Table B.1 Characterisation data of sample unsupported LDH analysed in section 4.2 153

Table C.1 BET surface area of CNS/LDH composite materials activated via heat-treatment 155

X Nomenclature

-1 Ccat Catalyst concentration (g L ) mcat Mass of catalyst (g)

NAo Moles of acetone at the start of reaction (mol)

S Selectivity treac Reaction time (h)

Treac Reaction temperature (K)

WHSV Weight hourly Space velocity (h-1)

X Conversion

Acronyms

A Acetone

APR Aqueous Phase Reforming

B Benzaldehyde

BA Benzalacetone

BET Brunauer–Emmett–Teller

CeZrOx Cerium Zirconium Oxide

CNF Carbon nanofibers

CNS Carbon nanostructure

ComLDH Commercial LDH

CVD Chemical Vapour Deposition

DAA Diacetone alcohol

DBA Dibenzalacetone

EDS Energy Dispersive Spectroscopy Contents

FTIR Fourier Transformed Infrared

GO Graphene Oxide

HMF Hydroxymethyl furfural

HT Hydrotalcite

HT-c Hydrotalcite in calcined form

HT-rg Hydrotalcite reconstructed in the gas phase

HT-rl Hydrotalcite reconstructed in the liquid phase

LDH Layered Double Hydroxide

LDHhl Layered Double Hydroxide reconstructed in the liquid phase

MgAlOx Magnesium - Aluminium mixed oxides

MIBK Methyl isobutyl ketone

MO Mesityl oxide

MWNT Multi-walled carbon nanotubes

MWNT/LDHhl-XX MWNT-LDH composite reconstructed on the liquid phase, with XX

wt% of carbon

P Product

SEM Scanning Electron Microscopy

SS Steady State

SWNT Single-walled carbon nanotubes

SynthLDH Synthesised LDH

SynthLDH-X Synthesised LDH wit Mg/Al ratio of X

TEM Transmission Electron Microscopy

TGA Thermogravimetric analysis, analyser

TPD Temperature programmed desorption

W Water

XRD X-ray diffraction

XXGO/LDH GO-LDH composite with XX wt% of GO

XII Contents

XXGO/LDH-Y GO-LDH composite with XX wt% of GO and Y Mg/Al metal ratio

XXMWNT/LDH MWNT-LDH composite with XX wt% of MWNT

XIII

Chapter 1

Introduction

1.1 The state of biorefining applications

Nowadays, as the supply of available fossil sources keeps diminishing, research interests focus on the search and development on renewable energy sources, ranging from solar and wind energy among others, towards the production of cleaner energies.

Nevertheless, the dependence on the production of fuels and other important chemicals necessary for everyday activities still depends on the use of petroleum at its core. Consequentially, environmental concerns are also more prevalent as the production of harmful compounds such as nitrous oxides and carbon dioxide continues as well[1]. For these reasons among others, the efficient transformation and use of biomass has become one of the more prevalent technical challenges on the chemical industry.

Biorefining applications focus on the transformation of renewable feedstock into fuel and useful chemicals in order to reduce the use of petroleum-derived materials to an extent. Interestingly, biofuels represent an important research opportunity as their similarities to the petroleum-derived fuel counterparts makes their transportation and use at the application level do not require extensive changes[2]. Currently, starch and oil derived materials are the most common feedstock to produce biofuels, with ethanol and biodiesel being the most common, as their transformation is relatively simple and well known from the current processes on the chemical industry. However, problems still exist in the actual industrial integration of these transformations, as their relative novelty makes them far from being easily applied and developed on site[3], while the fundamental differences of the starting materials and the transformation routes to the desired compounds make their application a great challenge. As such, the development of new biobased processes also continues to be a focus of research.

Newer routes of bio conversion typically focus on select individual processes (selective oxidation, pyrolysis, condensation, etc) for the transformations towards the synthesis and use Introduction

of key building blocks, whereas the approach on petrochemicals has been mostly based on selected target compounds like ethanol, butanol and diesel[4]. The process-based development is slightly riskier since the engineering and monetary analysis are less approachable and secure in comparison, as there is a lack of actual data to confirm the effectiveness on the production of certain building blocks along with continuous advancements in technology, which makes these developments complex [5]. As such, the identification of good building blocks, feedstock, along with the optimal processes for such conversions becomes one of the most important bases to analyse while working on these topics.

The selection of the feedstock itself is important as different processes are commonly obtained due to the different structures of each starting biosource. Sugar derived materials are typically used in the production of ethanol via glucose derived compounds, while triglyceride feedstocks mainly work with the process of glycerol and fatty acids to produce diesel via catalytic processes[6]. Nevertheless, the sugars and oils typically used as the main production feedstock of such biofuels are derived from corn and sugar cane, typical sunflower or soybean oils, among others, which are mainly obtained from edible supplies. The application of bio-based processes that rely solely on these materials makes difficult to justify the use of biomass as a possible pathway to partially replace the use of petroleum as the main source for energy consumption requirements.

One of the proposed ways to take advantage of bio sustainable processes without relying on these feedstocks is the use of non-edible materials and instead focus on the use of lignocellulosic sources, which are abundant in forms of natural biological waste, algae, etc.[2]. However, current process technology for the pre-treatment and conversion of biomass has an increased cost over the use of the so called first generation processes with corn, etc. As a more in depth pre-processing involving physical separations and chemical treatment, like milling or hydrolysis on either acidic or base medium respectively[7] is necessary before the actual production of fuels starts, the existing drawbacks prevent still a wider use of this cellulose derived materials. A general description of these processes is shown in Figure 1.1.

Studies show that it is possible to leverage the costs and make the use of biomass and these transformation processes and similar technologies feasible by not only focusing in obtaining the desired fuels, but also approaching the processing with the aim to obtain bioderived higher-value products along with them[8]. Different compounds have been studied and some are well defined by either their production on the oil processing industry, or as important useful intermediates in other production processes.

2 Introduction

Edible Non-edible Building Transport Limited Available Blocks Fuels

Cellulose HMF, LA Ethanol Starch Corn,Corn, Lignocellulose Hemicellulose Furfural Liquid alkanes Sunflower,Sunflower etc Liquid alkanes Biodiesel Oil

Lignin Bio-oil

Figure 1.1 First and second generation biorefining processes. Adapted from [9]

Cellulose-type materials are currently used to obtain a range of building blocks (HMF, furfural and bio-oils, among others), which are commonly used in the production of alkanes and transport fuels[6]. Once pre-treatment of biomass is completed, the aim is to remove oxygen from the remaining structures while creating carbon bonds to control molecular weight of the end products and requiring an amount of hydrogen from external sources as minimal as possible. There are different approaches to fulfilling these requirements, the following being the most common:

• A gasification process is used in order to obtain synthesis gas, a CO-H2 mixture, to then proceed with a Fischer-Tropsch synthesis, obtaining hydrocarbons as end producctts, which can be used to produces diesel-like fuels.

• Anaerobic pyrolysis in the presence of H2 to obtain liquid oils that, after processing, resulting in compounds with added carbon bonds and a lower amount of oxygen containing molecules. • Aqueous phase reforming to produce sugars, which are then broken down via reaction into a different range from alcohols, ketones and organic acids, used to produce hydrocarbon fuels via catalytic carbon coupling processes like alkylation over acid zeolites, aldol condensations of alcohols and ketones over basic or bifunctional metal catalysts, or ketonization over basic oxides.

3 Introduction

• Formation of aqueous solutions of sugars, which are catalytically dehydrated to produce furan compounds like furfural or hydroxymetylfurfural (HFM). The compounds are later used in aldol condensations over basic catalysts to produce hydrocarbons useable on diesel applications.

Each process presents certain advantages and disadvantages, like possibility of using a range or feedstocks instead of a single one on the gasification, or the different range of bio- oils produced that need additional treatment to be used as fuel on the production by the pyrolysis process [6]. As a result, research continues developing new technologies, while further developing the ones currently used as well.

Specifically, the catalytic routes for bio-conversion are important on the general application scheme of bioderivaties, as the use, study and selection of a catalytic material still represent a challenge which could lead to improved rates, selectivity or reduction of the general energy requirements on some processes.

One of the most important areas in catalytic research is the development of solid bases. Perhaps the main reason which makes the development of solid bases a challenging prospect lies in the actual number of chemical processes which proceed via a basic mechanism, which are not many in comparison to acid catalytic processes[10]. Along with this, in some cases the use of liquid base still presents an easier and more cost-effective alternative in comparison to the use of solid materials.

As such, the development of easy and economical solid materials with basic characteristics, which could present enhance properties in comparison to commonly used liquid or solid bases represents a topic of interest from both a chemical and engineering point of view.

4 Introduction

1.2 Research objectives

The present work is focused on the study of the application of novel and tuneable solid base catalysts to be used on aldol condensation processes, similar to the ones used for fuel derivative production as well as the functionalisation of compounds like furfural or HMF where, in some cases, strong liquid bases or basic solid oxides like NaOH and MgO respectively are still used. Specifically, layered double hydroxides (LDH) materials present useful characteristics as solid base catalysts, and the study by supporting them on both multi-walled carbon nanotubes (MWNT) and graphene oxide (GO) is investigated. Some of the materials and research developed throughout the thesis was developed in collaboration with the Materials Chemistry group in the Department of Chemistry at Imperial College London. The application and feasibility of use of the composites in the self-condensation of acetone, the retroaldolization of diacetone alcohol (DAA), and the condensation between benzaldehyde and acetone, as well as different activation procedures is studied. Characterisation studies allowed differences between the materials to be elucidated, in order to identify the effect of loading the LDH on the carbon support. Specific objectives are as follows:

• Synthesise composite materials using supports that are compatible in both electronic charge and geometry, either by pre-treatment or naturally. It is expected to obtain materials with increased surface area and accessibility. • Produce a detailed characterisation study of the composites over a range of compositions, in order to identify the effects in the availability and number of basic sites that intervene on the selected condensation processes. • Research the effect of the loading of support under different reaction conditions, in terms of rate, conversion and stability during reaction.

5 Introduction

1.3 Structure of the thesis

The thesis is divided into seven chapters, including the introduction. In Chapter 2, a specification of the characteristics of LDH materials, along with studies about their general applications, as well as information related to their use with different carbon supports is included.

In Chapter 3, the procedures followed to synthesise, characterise and test the composite materials used during the thesis are described. Specific methodologies followed for the synthesis of the materials, standard operating procedures, and description and information about the equipment used are included as well.

Chapter 4 focuses on the analysis of selected MWNT/LDH composite syntheses and their application on the self-condensation of acetone on the liquid phase. Optimization of reaction variables to produce a detailed comparison between the composites, along with characterisation and description of the results of reaction is given. A comparison of different carbon loading helps to compare the effectiveness of using the MWNT as a support for the LDH in catalytic processes.

Chapter 5 includes the optimization and analysis of the activity of both MWNT/LDH and GO/LDH composites, in the retroaldolization of DAA via the gas phase process. In a similar way to Chapter 4, initial studies as well as comparison between activation methods of the hybrid materials, reaction parameters, characterisation and reaction results are presented, A study of the deactivation of the material at long reaction time is added as well.

Chapter 6 aims to show possible enhancements on the condensation of benzaldehyde and acetone by activating GO/LDH composites in-situ during reaction. The optimization of parameters of reaction, LDH metallic ratio, as well as the relation between reactants, water amount and catalyst mass is described.

Finally, Chapter 7 includes the overall conclusions of the research.

6 Introduction

REFERENCES

[1] G.W. Huber, S. Iborra, A. Corma, Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering, Chem. Rev. 106 (2006) 4044–4098. doi:10.1021/cr068360d. [2] G.W. Huber, A. Corma, Synergies between bio- and oil refineries for the production of fuels from biomass, Angew. Chemie - Int. Ed. 46 (2007) 7184–7201. doi:10.1002/anie.200604504. [3] K. Weissermel, H.-J. Arpe, Industrial Organic Chemisty, 4th ed., WILEY-VCH Verlag, 2003. doi:10.1002/9783527619191. [4] D.J. Hayes, An examination of biorefining processes, catalysts and challenges, Catal. Today. 145 (2009) 138–151. doi:10.1016/j.cattod.2008.04.017. [5] J.J. Bozell, G.R. Petersen, Technology development for the production of biobased products from biorefinery carbohydrates—the US Department of Energy’s “Top 10” revisited, Green Chem. 12 (2010) 539. doi:10.1039/b922014c. [6] D.M. Alonso, J.Q. Bond, J.A. Dumesic, Catalytic conversion of biomass to biofuels, (2010) 1493–1513. doi:10.1039/c004654j. [7] N. Mosier, C. Wyman, B. Dale, R. Elander, Y.Y. Lee, M. Holtzapple, M. Ladisch, Features of promising technologies for pretreatment of lignocellulosic biomass, Bioresour. Technol. 96 (2005) 673–686. doi:10.1016/j.biortech.2004.06.025. [8] J.J. Bozell, Feedstocks for the future - Biorefinery production of chemicals from renewable carbon, Clean - Soil, Air, Water. 36 (2008) 641–647. doi:10.1002/clen.200800100. [9] D.M. Alonso, J.Q. Bond, J. a Dumesic, Catalytic conversion of biomass to biofuels, Green Chem. 12 (2010) 1493–1513. doi:10.1039/c004654j. [10] Y. Ono, Solid base catalysts for the synthesis of fine chemicals, J. Catal. 216 (2003) 406–415. doi:10.1016/S0021-9517(02)00120-3.

7

Chapter 2

Literature Review

The work developed throughout the project focuses on the study of layered double hydroxides (LDH) supported on carbon nanostructures (CNS) for enhanced catalytic activity on a diverse range of base-catalysed aldol condensation reactions, a process typically used in biorefining applications. In this chapter, relevant aspects to the materials and the reactions are addressed, such as common aldol condensation reactions used in the production and transformation of biomass intermediates and different types of base catalysts used in aldol condensation along with details on the background and characteristics of the base solids used on each reaction system studied.

2.1 Aldol condensation reactions on biorefining applications

Biorefining focuses on the process of renewable feedstock for cleaner synthesis and transformations towards fuels and chemicals typically obtained from petroleum derived processes. Specifically, the production of fuels and chemical building blocks focuses on the linkage of compounds via carbon bonding. To do so, different reactions for synthesis are used ranging from oligomerization of alkenes or condensation of ketones[1].

Generally, biomass pre-processing focuses on the transformation towards compounds such as bio-oils and polyols, after which the materials can be converted to desirable compounds such as fuels via different reaction processes, where aqueous-phase reforming (APR) is one of the most commonly used. In this process, cellulose based biomass is used to produce both hydrogen and an aqueous solution of carboxygenated derived materials[2], which can then be broken down to simpler organic compounds with functional groups such as hydroxyls, carbonyls or carboxyls. APR was initially developed as a hydrogen production technology and it has been demonstrated that oxygenated feedstocks produced by biomass, such as glucose, sorbitol, glycerol, ethylene glycol, and methanol can be processed to produce hydrogen easily[3]. As a result, further processing of the biomass in reactions such as Literature Review hydrogenation is less dependent on hydrogen obtained from petroleum or other external sources.

Depending on the process followed during APR, the functional intermediates obtained can then be used to produce hydrocarbon fuels by routes such as dehydration, alkylation over zeolites[4], aldol-condensation of alcohols and ketones over bifunctional metal/basic catalysts[5], and ketonization of carboxylic acids over basic oxides[6]. These processes are typically used to control the carbon coupling to the desired length of the compound while removing the necessary amount of oxygen for the materials to be useful on different applications.

One example of such conversion is presented while working with APR on conditions that enhance the cleavage of carbon-oxygen bonding rather than carbon-carbon, such as high concentration of oxygenates on the feed and/or high pressure. In this case, Pt catalysts are able to transform the sugar solutions to produce monofunctional intermediates, which range from 2-ketones, secondary alcohols, and carboxylic acids[7]. These compounds can then be used to produce short alkanes, like hexane, or syngas. Aldol condensation is a useful process for upgrading the intermediates, as C–C coupling and oxygen removal is part of the process, and these characteristics are desirable on the processing of the biomass. By using condensation reactions, larger hydrocarbons ranging from C5 to C6 can be obtained from biomass using basic catalysts such as solid oxides, metals supported on combinations of oxides [8,9] or hydrotalcites [10–12],

Aldol condensation is a C-C bond-forming reaction which requires at least a carbonyl compound with an α-hydrogen atom, and the reaction is generally carried out in the presence of a base catalyst. Initially, the base catalyst abstracts the α-hydrogen from the carbonyl compound to form an intermediate carbanion species, which can then interact with the carbon atom of a separate carbonyl group on another molecule. The aldol adduct typically forgoes a dehydration to form an unsaturated aldehyde or ketone, but this depends on the desired end- product. Factors such as reaction temperature, solvent, reactant molar ratio, structure of reactant molecules, and the nature of the catalyst determine the selectivity of the process.

The process itself is important in the production of alkanes from biomass, as some intermediate compounds containing C-O groups can be obtained directly from biomass pre- processing and then used to obtain longer chain materials, Specifically, glucose and xylose cannot be aldol-condensed because their carbonyl groups tend to form cyclic structures. However, by following a dehydration process, 5-hydroxymethylfurfural (HMF) and furfural can be produced instead. These compounds then can be processed via aldol condensation with other molecules like acetone, dihydroxyacetone, or glyceraldehyde to produce light alkane

9 Literature Review compounds[13]. Another method developed to produce fuel derivatives is the conversion of the sugars and oils by using bifunctional metal basic catalysts. In addition to the aldol condensation, the metallic components allow the increase of the chain length of secondary alcohols when hydrogen is present on stream via reaction with 2-ketones[5,14].

Gasoline can also be obtained from biomass intermediates via full hydrogenation of ketones and carboxylic acids to alcohols using a H-ZSM-5 zeolite[4]. A similar procedure is used to dehydrate alcohols over solid catalysts of acidic nature to obtain olefins[7,15]. In order to obtain less branching on the linear alkenes, materials such as Cu-Mg-Al hydrotalcites have been used to produce C8–C12 ketones from the starting compound mixture of pre-processed biomass[15]. One disadvantage while using these Cu-Mg-Al solid catalysts is that they tend to deactivate as the reactions advance due to an effect of poisoning of their basic sites. This is caused by the amount of organic acids on the feed, produced by the original biomass transformation. When there is a low concentration of acid it is possible to neutralise the contents with a low amount of a strong base, but some types of biomass processed materials, like glucose, tend to produce higher concentrations of acid due to the higher amount of oxygen on the compound. This makes the neutralisation process not ideal in some cases. Instead, other processing techniques use catalysts like CeZrOx, which allow the condensation process to occur before the production of acids takes place. This allows to obtain intermediates that can be converted to the desired fuel compounds with little amount of acid on stream. The effluents of the process are then processed on a separate stage of reaction system by using a bifunctional Pd/CeZrOx catalyst, a solid resistant to organic acids, to obtain the desired end- products[6,16].

Another common biomass processing route apart from the direct production of fuels is the transformation of the sugar-containing solutions towards useful building blocks, via catalytic dehydration. Typical compounds obtained as building blocks include furan derivatives like furfural and HMF, which can then be used as feedstock for aldol-condensation reactions over basic catalysts to produce hydrocarbons. These hydrocarbons have been used in different applications to obtain materials such as light alkanes[17], syngas[18] and monofunctional compound derivatives[15]. The processes of conversion of these furanic compounds involve compound coupling to obtain higher chained materials, while reducing the production of branching groups on each alkane chain.

The use of furans as precursors of hydrocarbon fuels is an important pathway to obtain linear alkanes of molecular weights usable to produce diesel. Initially, biomass sugars can be hydrolysed to produce monomers which, after dehydration via a process with an acid catalyst, result in a range of furanic compounds. The products obtained are HMF, 5-methylfurfural, or

10 Literature Review

2-furaldehyde (furfural). As an added benefit, these compounds can be produced from the pre-processed intermediates obtained from both cellulose and hemicellulose, which combined account for more than 80% of lignocellulosic biomass, the most abundant type of biomass available.

Furfural and HMF can be produced in high quantities from xylose and fructose respectively [8,19] while yields are lower for glucose unless a solvent is added into the processed solution, resulting in higher selectivity towards HMF[20]. Reducing the water concentration on this solution is critical to the selective preparation of HMF, as the compound is easily hydrated in water to form levulinic acid and formic acid[21]. To reduce the decomposition of the HMF, furanic compounds can also be extracted from the sugar derived aqueous layer using organic solvents. However, the use of solvents requires an extra separation step after the synthesis, which is not ideal as energy costs tend to increase[22].

HMF can be transformed by hydrogenolysis to 2,5-dimethylfuran (DMF) which can be used as blender in transportation fuels[23]. HMF and other furfural derivatives can also be upgraded by aldol condensation with ketones such as acetone over basic catalysts to form larger hydrocarbons at 298 K [24]. For example, single condensation of HMF and acetone produces a C9 intermediate, which can then react with more HMF to produce a C15 intermediate. These condensation products can then undergo a hydrogenation and dehydration process with a metal/acid bifunctional catalyst to produce linear C9 or C15 alkanes, hydrophobic compounds easy to separate from the aqueous phase usable as substitutes of fuels[9,12].

Aldol condensation is a useful coupling reaction which, in combination with other processes like hydrogenation or hydrogenolysis, can lead to high yields of useful compounds, including fuels, fuel derivatives and other important chemicals. The research of different solid catalysts used in synthesis process of biofuels, such as solid oxides, zeolites, and hydrotalcites continues in order to increase activity, selectivity, reduce deactivation or look for separate pathways to continue processing biomass on a more efficient way in either the chemical, environmental and economical fronts.

2.2 Solid catalysts used on aldol condensation and upgrading processes

As previously introduced, aldol condensation is a process that is currently used in the transformation of biomass intermediates to obtain higher value products. Liquid bases along with a wide range of solid catalysts have been studied in this process, while the use of solid materials represents a good opportunity to reduce standard liquid waste production and separation from the reactant streams. Nevertheless, research on solid materials requires

11 Literature Review defined characteristics like good turnover numbers, small deactivation, etc. which bring the challenge to take advantage of the properties of each material, while reducing the drawbacks while in use.

Combinations of solid oxides with metal supported materials are by far the most common used in these kind of processes, as the range of properties that the metals supported on the solids or the nature of the solids themselves provides an ease of usability and stability on many applications. For example, platinum supported on alumina (Pt/Al2O3) is used for the reforming towards monofunctional intermediates on APR. These catalysts present high activity and selectivity towards the production of C7-C9 alkane compounds without the use of alcohols as solvents on the reactor[12]. However, problems arise while the reforming processes occur, because the alumina tends to be converted to boehmite (AlO(OH)), while the platinum metal particles suffer an undesirable process of sintering, which lowers the effective activity of the catalyst as the reactions take place[18,25]. In this example, the following condensation process take place with a mixed oxide Mg-Al catalyst produced by thermal treatment of hydrotalcite-like-compound, a common solid base used for other reaction processes.

Other materials such as palladium supported on a mixed magnesia/zirconia (Pd/MgZrO) are used to increase carbon compound chains by binding oxygenated intermediates like HMF. The process involves a single stage process where the compounds are initially linked via a condensation reaction and then hydrogenated afterwards on the same reactor. The enhancement effect in comparison to other catalysts is directly related to the increased surface area on the mixed solid, which leads to a better dispersion of the metallic component and an increased interaction of the oxides with the compounds on stream. Similar tests with alumina and other types of hydrotalcites produce comparable results, but recyclability is enhanced with the zirconia, as the hydrotalcites tend to forgo structural changes in the presence of water [8,9]. Bimetal catalysts (Pt-Pd, Pd-Ni, Ni-Sn, etc) are also used in order to increase the activity in comparison with single metal catalysts. In particular, authors have shown that Ni and Ru containing catalysts generally show higher activity and selectivity than the single supported materials, as they favour the cleavage of the carbon oxygen bonds from the produced intermediates during the process[26].

Using solid oxides is an efficient and relatively easy-to-use alternative while working with aldol condensation, whether they are used for biorefining applications or other processes. However, being technically stable materials, further modifications to their structure to enhance activity is not viable, which could hinder their use on certain applications that require a material with specific characteristics. Other routes to increase activity have been investigated, like using combinations of oxides or modification of the metals supported on the solids as well as

12 Literature Review their loadings. By following these procedures, it is possible to obtain materials with higher activity or better stability[8,9,12,26].

Studies with more structurally complex materials have shown that it is possible to enhance properties like selectivity as well, while also presenting some disadvantages during application. In this regard, hydrotalcites and the mixed oxides produced by their calcination, along with zeolites and carbon materials have had the most use due to their common application in other areas on chemical research and industrial applications. Mg-Al mixed oxides are commonly used to test new base catalysed processing pathways to obtain larger chained materials via aldol condensation, as the solids are easy to obtain via synthesis and their properties are well-understood. Although studies show that they are effective when converting compounds like HMF and furfural to higher molecular weight materials, the yield of the alkanes is not as good as it would be required for industrial application. Another inherent issue resides in the deactivation of the mixed oxide, which the material forgoes when in contact with water or acidic particles, as structural changes or poisoning occurs.

The H-ZSM-5 zeolite has also been used in a wide range of processes of industrial importance, and its application in the upgrading of alcohols obtained from pre-processing of biomass has shown good results in terms of production of olefins. However, while working on the necessary reaction conditions defined for the production process, the final olefins obtained tend to get branched carbon groups. To reduce this effect, using materials like Cu-Mg-Al mixed oxides has shown that the transformation of alcohols is possible with a better conversion towards higher carbon chain, C8-C12 and less branching. Nonetheless, as previously mentioned, deactivation occurs as the reaction progresses due to traces of acidic material on stream produced on the processing of the biomass[15]. At this point, using slightly less active catalysts is the optimal choice.

There is no specific study that shows a specific material to have a marginally better performance without general deficiencies while in use. The objective of this research study is to search for possible enhancements to activity and stability on the current materials used for biorefining applications, as well as the development of new catalytic materials. Specifically, the study aims to reduce deactivation via modification of the structure and enhance activity via increments in surface area and defects on hydrotalcite compounds, for application on aldol condensation reactions.

13 Literature Review

2.3 Layered Double Hydroxides

Development of any kind of material requires basic understanding of its properties, synthesis methods as well as current applications in order to locate possible areas of use and optimization. As such, this section focuses on Layered Double hydroxide materials, showing detailed descriptions of LDH synthesis and their use as the catalysts independently or supported on other materials.

2.3.1 Clay materials, properties and classification

Clay materials are solids normally composed of particles of a size up to 2휇푚 [27] which are typically used for a range of different industrial and non-industrial processes, such as ceramics and in pharmaceutical synthesis, and as adsorbents, catalysts or catalyst supports, ion exchangers among many others[28]. Generally, one of the main properties of clay materials is their ability to undergo ionic exchange. Also, depending on the type of ion types they can exchange with, clays can be classified as[29]:

-Cationic Clays: Inherently lacking positive charge on their structure as a result of the isomorphic substitution of metallic ions by others with lower oxidation state, the overall charge is compensated by the introduction of cations into the structure. These materials are relatively common and therefore easily found as part of minerals deposits, but synthesis according to the requirements for different applications is the optimal pathway to follow. Since the 1970s, studies to understand their properties and possible applications have been heavily developed. One of the most well known materials in this group are zeolite materials, which can be found and synthesised on different shapes and sizes, like the commonly known H-ZSM5, which use ranges from commercial hydrocarbon cracking to fine chemistry applications[28].

-Anionic Clays: The nature of their structure creates an excess of positive charge on the materials. As a result, anions are naturally introduced into the structure or by synthetic techniques in order to obtain a neutral material [30]. Generally, the nature of the materials makes them good solid bases of medium strength and, just like the cationic clays, acidity can be a part of the structure depending on the components. The number of natural minerals which can be classified on this group is smaller than the cationic clays, so synthesis is the most common method to obtain them and their application is less developed than the cationic clays.

2.3.2.-Layered Double Hydroxides: Solid materials

Amongst anionic clays, layered double hydroxides (LDH), or hydrotalcite like compounds (HLC or HT) are the most typical and well-known materials included on this group. LDH take their name from the stacked sheets that form the base of the material, which have

14 Literature Review an overall positive charge. This excess charge is balanced by the introduction of anions in the interlamellar space, where water molecules are also introduced. The name “hydrotalcite” is derived from the mineral of the same name, which is the most abundant of all the compounds known with this structure in nature.

The condensed structural formula is as follows [31]:

퐼퐼 퐼퐼퐼 푥+ 푛− [( ) ( ) ( ) ] 푥 푀 1−푥 푀 푥 푂퐻 2 (퐴 ⁄푛) 푚 퐻2푂

Where each of the components represents:

푀퐼퐼 and 푀퐼퐼퐼 Mono, di and trivalent metals respectively

푛− 푥 - Anion with n charge (퐴 ⁄푛)

푥 Metallic molar ratio

 M 3  x   2 3  in the 0.20-0.33 interval M  M 

푚 Number of water molecules

15 Literature Review

2.3.3.-Structure and composition

The main building block of an Mg-Al LDH layer, is derived from the octahedral

2+ organisation of brucite (Mg(OH)2). These octahedral cells have a Mg cation at its centre, while a hydroxide group (OH-) is located at each of its vertices. Each cell shares a hydroxyl group with the next cell [30] (Figure 2.1). The arrangement is repeated and each set of “sheets” is stacked over another, forming a compact crystalline structure.

Figure 2.1 A brucite-like layer

When the inner Mg2+ ion is isomorphically substituted by a trivalent ion, Al3+, the layer loses its neutral charge, which results in a separation between layers. To compensate the charge imbalance, anionic species on the medium of the material enter the space between the sheets, and water molecules enter also the space (Figure 2.2).

16 Literature Review

Brucite-like layer

Interlamellar

region

Anion

Water

Figure 2.2 A layered double hydroxide (LDH)

2.3.4.-Anionic species on the interlamellar space of LDH

Studies have tested the capacity of different anions to be introduced and interchanged into the layers of LDH materials, mainly depending on the strength of the bonding to the layers. Some of the classifications on interaction are as follows [32,33]

2− 2− − 2− − − 퐶푂3 > 푆푂4 > 퐶푙 > 퐶푙푂4 > 푁푂3 > 퐶퐻3퐶푂푂

− − − − − − 푂퐻 > 퐹 > 퐶푙 > 퐵푟 > 푁푂3 > 퐼

2− 2− 2− 3− 2− 퐶푂3 > 퐶푟푂4 > 푆푂3 > 푃푂4 > 푊푂4

Generally, most of the divalent anions are more stable when bonded to the sheets, as it is the case with the carbonate ion which, given the existence of large amounts of CO2 in the atmosphere, is practically impossible to remove fully from the structure via substitution. When modification of the anion is a requirement on a specific process, it is recommended to

- - synthesize LDH with monovalent anions, such as nitrate (NO3 ) or acetate (CH3COO ) to reduce the interaction and facilitate the interchange, or use other synthesis methods to remove the previous anion of the structure, like calcination.

The main challenges to obtain LDH with different anions is the compatibility of the synthesis method as well as the availability of the precursors necessary to produce the LDH. Different reported anions which have been intercalated into the LDH structure are presented in Table 2.1[28,32,34,35]

17 Literature Review

Table 2.1 Reported anionic structures intercalated into LDH sheets

Inorganic Organic Others

- - - - - 3- F Cl Br I (NO3) Acetate, Oxalate, (PMo12O40)

- - - 3 (ClO3) (ClO4) (IO3) Valerate, Succinate, (PW12O40)

- 2- 2- (OH) (CO3) (SO4) Malonate

2- 2- (S2O3) (WO4) Protoporphyrin 2- 3- (CrO4) [Fe(CN)6] Phthalocyanine

The LDH ability to interchange anions as well as the multiple combinations of metals that can form the layers gives the material a very broad range of application, as it is possible to use it in different fields, such as adsorption or catalytic applications [36,37]. The ability of the material to interact via the layers allows it to also be used in conjunction with other types of materials, like oxides or carbon, which increases the general properties the individual materials have on different applications[38]. The diversity of synthesised LDH materials along with the properties that differentiate them from oxides or other types of material (basicity, porosity, surface area, etc) make them good candidates to be used as solid bases for catalytic processes.

Different synthesis methods have also been developed, which enhance or limit the properties of the end materials. A summary of some of these methods of synthesis is presented in the following section.

18 Literature Review

2.3.5- LDH materials synthesis methods

Depending on the requirements of the application, synthesising LDH by different methodologies is a suitable way to obtain or enhance different properties, like a better defined crystalline structure or an increment in the available surface area of the material. Certain changes, such as the application of microwave or ultrasonic irradiation to these methods, have also been reported to increase activity in some reactions, or adsorption of different molecules, etc.[39,40]. The synthesis of LDH materials can be classified in two broad groups, as presented in Figure 2.3:

Coprecipitation Direct Sol-Gel LDH synthesis methods Anionic Interchange Indirect Structure reconstruction

Figure 2.3 Classification of LDH synthesis methods

Constraints to the introduction of different anionic species other than carbonate is also an important factor to consider, as the LDH materials are not modifiable by simple anionic exchange once this anion is introduced. In all cases, synthesis in an inert atmosphere to avoid this issue must be considered.

2.3.5.1.-Direct synthesis

Standard coprecipitation

The most well-known and used method for LDH synthesis is based on the simultaneous precipitation of aqueous solutions of salts, which have different valence cations, along with a precipitating agent. This procedure can be done at either constant or variable pH.

In this method, both salt precursors are dissolved in a single solution, while the precipitating agent is kept separate. The solutions are heated independently up to a range

19 Literature Review between 60-80 °C along with stirring. In the constant pH method, coprecipitation proceeds at a constant speed to maintain the pH of the synthesis in the range of 10-11 while at variable pH the salt solution is deposited into the precipitating agent. The LDH synthesized with the latter procedure tends to yield materials with a less ordered crystalline structure than those produced by the constant pH method. Factors such as stirring are also important, as properties such as crystallite size and surface area depend on good mixing. The mixture obtained at the end of the precipitation is left aging at a controlled temperature and pressure anywhere between 24 and 48h. Once the process is finished, the final mixture is filtered and washed thoroughly to remove any undesired impurities and obtain the end final LDH material.[32]

Sol-gel method

This synthesis involves the formation of atom networks by the dispersion of metallic alkoxides on a solution, typically known as sol. The sol is thermally treated to produce crystalline compounds that have high contact surface area compared to the materials obtained by the coprecipitation method. The sol is a colloidal dispersion of solid particles while the gel is a polymeric system produced by a chemical polycondensation. Further thermal treatment of the gel produces materials with even higher surface areas[39].

2.3.5.2.-Indirect synthesis

The following methods are generally used to synthesize LDH materials with different anionic species, although modifications of the surface properties of the materials are a common result of the processes as well.

Anionic interchange

A simple way to describe this synthesis method is by analysing the following interchange:

[푀퐼퐼 − 푀퐼퐼퐼 − 푋−] + 푌− → [푀퐼퐼 − 푀퐼퐼퐼 − 푌−] + 푋−

The foundation of the method relies on the intrinsic properties of the LDH materials allowing to introduce Banions during the original synthesis of the LDH, as some anionic species are preferred by the layered structure because of the electrostatic interaction between the positive layers and the new anionic species, which makes the material more thermodynamically stable[41]. The process occurs at any point in which anionic species are readily available on either liquid or gas phase and they are more coherent to the LDH materials in relation to factors such as the interlayer distance on the LDH or valence of the substituting anions. Examples of this method include both carbonate salt solutions surrounding nitrate

20 Literature Review

containing LDHs or carbonate produced by the available CO2 in the air surrounding of the synthesis area of the materials.

Issues while working with this synthesis method could range from the lack of control of the physical properties of the final LDH, like the surface area or the particle size, to the time required for the interchange process to be completed. Nevertheless, efforts to minimise have been done to control these variables with the application of either use of ultrasonic or microwave irradiation for example[39,42]

Structure reconstruction

The regeneration of collapsed LDH structure by thermal treatment has been studied to synthesize different LDH compounds. This regeneration is produced by a retro- topotactical transformation, more commonly known as the “memory effect”[33,43], inherent to the LDH as well as other compounds like magnetite. This property allows the compounds to recover their original crystalline structure.

In-situ thermal analysis of LDHs formed by Al-Mg show that when heated up to temperatures between 100 and 190 °C, the interstitial water (surface water) moves out of the structure in the gas phase, as it is neatly attached to the hydroxyl groups of the layers. In the range of 190 - 280 °C, the first stage of the collapse takes place as the hydroxyl groups interacting with Al begin to separate from the structure, while at the range from 280 to 405 °C, the interacting hydroxyls with the Mg species exit the structure as well. The process converts these hydroxyls into water and leaves the remaining solid as a slightly amorphous material. From this point forward and until 580 °C the elimination of the carbonate species occurs while the remaining solid completes a change towards a solid solution of MgO and

Al2O3[44].

This mixture is used in different applications as it tends to have a high surface area and stability towards thermal and some chemical events, etc. Along with these, the material also has the ability to reorganise its components when in contact with anions even if the initial LDH structure had different components[32]. Different parameters like the pH of the dispersion, stirring speed, etc. can be modified to control the textural properties of the reconstructed material. Other methods, such as irradiation have been applied to have a better control these properties during a reconstruction synthesis[39].

2.4.-Layered Double Hydroxides: applications

One of the main challenges on the chemical research is the synthesis and development of solid bases. As most processes in industry are based on heterogeneous acid

21 Literature Review catalysis transformations, base catalysts are still mostly liquid bases like NaOH or organometallic complexes like Grignard reagents [45]. Problems with these catalysts are mainly related to issues related to environmental topics as well as extra separation processes required[46]. As such, the current development of basic materials mainly focuses on either the modification of other well-known used materials, such as intercalation or modification of zeolites, oxides, etc. to obtain weakly base characteristics on acid or acid-base solids, or the development of pure solid oxide materials for general used applications.

The first application of as synthesised coprecipitated LDH as catalysts was reported on 1970, when a BASF patent described a Ni-Al LDH used as part of a hydrogenation process. It was suggested that the layered structure was the main parameter to obtain the final activity on the system[32,47]. A wide range of research has focus on possible applications for LDH materials, from catalysis as well as medical or other industrial applications. Figure 2.4 shows some applications of LDH [32,35,48,49].

Layered Double Hydroxide fields of application

Industrial Catalysis Others

Hydrogenation Waste water Ion exchangers Reactions treatment

Metal particle Molecular Sieves CO Adsorbents Supports 2

Photodynamic Flame retardants Steam reforming therapy

Figure 2.4 Applications of LDH compounds

In this regard, applications are typically divided between the use of LDH materials as layered, unmodified species and as the mixed oxides produced by thermal treatment. The oxides produced by the high temperature treatment are reported to present better properties, like larger surface area than the uncalcined materials, which are useful for specific processes[28].

22 Literature Review

Modification of the metals in the brucite-like layer from Mg-Al to transition metals like Co, Ni, La and Rh helps to reduce the temperature for the thermal decomposition of nitrous oxide. Results show that with a Co/Al LDH doped with 0.7 wt% of Rh, the temperature necessary to decompose the N2O lowers about 400 K in comparison to the standard Mg/Al LDH[50].

The metal ratio of an Mg/Al LDH was changed in values from 2 to 4 for the hydrogen transfer of 2-propanol to cyclohexanone. In this case reducing the ratio increases the rate for the hydrogen transfer as well as the production of cyclohexanol, which indicates an improvement when a slightly less basic material is used[51].

In this study, the research focuses on the usage of LDH materials on base catalysed aldol condensation processes related to biorefining applications, seeking to increase properties such as stability, surface area, porosity, etc.

2.4.1.-LDH applications on base catalysed aldol condensation reactions

Many applications exist in the chemical industry directly related to the use of aldol condensation reactions[52]. Among these, one the most important fields of use is the synthesis and transformation of fine chemicals. For example, LDH has been used as a catalyst for the condensation of arylsulfones with substituted and non-substituted benzyl aldehydes[53]. Both are higher-value compounds, while the arylsulfone is used in the activation of chemical groups to make alkylation and addition reactions. In this case, the mixed oxides produced by the thermal treatment of the LDH increased basicity and consequently higher activity than either

MgO or Al2O3. Other uses also include the condensation of citral and acetone to produce pseudoionone, an important intermediate used in the production of vitamin A and E along with applications in the fragrance industry[54].

On a more general industrial scale, the most important and well-known processes related to aldol condensation are the production of methyl isobutyl ketone (MIBK) via the self- condensation of acetone, as well as the production of pseudoionone via condensation between citral and acetone (see Figure 2.5 and Figure 2.7). However, in this synthesis as well as other condensation reactions, liquid solutions of NaOH and other bases are used in most cases; as such, extra undesired steps such as purification, recovery and waste treatment are still common[55]. Studies have examined the possibility of using a diverse range of solid catalysts to replace the use of these bases on a single stage process, which not only requires a good activity but also the presence of acid sites on the same catalyst to finish the dehydration process. Materials such as KOH supported on alumina and combinations of MgO and SiO2

23 Literature Review among others have been good candidates[55] along with combinations of Pd with zeolites, Ni supported on MgO[56] or Al2O3[57] or hydrotalcites for the conversion process [58].

Not only is acetone self-condensation an important industrial process, but also the general activity on the reaction allows for the characterisation and comparison of basic strength between solids[43]. To test the basic properties of the LDH materials and composites produced on this study, the reaction was used following a similar procedure previously reported [59]. Along with the self-condensation, other model reactions have also been used to compare basic activity. Specifically, the retro aldolization of diacetone alcohol (DAA) towards acetone is also a easy to use process to compare basic strength, while having a reduced effect due to equilibrium limitations present on the system, among other advantages[60].

In the following section, the self-condensation process is introduced as an example to show the usage of an LDH composite materials where the interlayer space is occupied by a different anionic species.

2.4.1.1.-Self condensation of acetone: analysis of basic strength of solids

The aldolization of two molecules of acetone (Ac) produces the compound 4-methyl- 4-hydroxy-2-pentanone, also known as diacetone alcohol (DAA). Once the process of dehydration is completed on this molecule the end product is mesityl oxide, an important precursor of other chemicals like hexylenglycol and MIBK[61] (see Figure 2.5). Adding up to the common use of DAA, it has also been suggested that in specific cases DAA can be a useful replacement of acetone as a solvent, as the compound is less volatile.

Figure 2.5 Scheme of the self condensation of acetone for production of MIBK

The aldol reaction is an equilibrium limited process which favours acetone, with a maximum conversion at nearly 24%, reachable at 273 K[60]. The use of heterogeneous catalysis like alumina (Al2O3) or zeolites has brought advantages like an easier separation, less corrosion and, the possibility of catalyst regeneration[62–64].

24 Literature Review

Meixnerite type LDH materials (hydroxyl substituted LDH, like

푀푔6퐴푙2(푂퐻)16(푂퐻)2 4퐻2푂) achieve equilibrium conversion to DAA on relatively short periods under specific reaction conditions, as the materials are highly active due to increased basicity and changes in morphology. Some examples include the use of water vapour-reconstructed LDH, which achieved 24% conversion of acetone at around 30min[65]. In comparison, under

2- - the same experimental conditions, LDHs with CO3 and Cl as compensating anions took around 24 h to get a conversion between 6-16%. Reports of 20% conversion for meixnerite type LDH with different modifications to the metals and ratios used in the structure have also been reported, specifically Mg/Ga and Ni/Al LDH. The Ga LDH achieves 20% conversion after 300 min of reaction time whereas the Ni containing LDH attains 14% conversion at most, which is around the same amount for the Mg/Al LDH[66].

The process of introducing hydroxyls into the LDH structure has also been studied and compared against the carbonated structure of as-synthesised LDH. By rehydrating the mixed oxides obtained from thermal treatment of the original sample (HT/c) with water on both the liquid and gas-phase (HT-rl and HT-rg, respectively), the structure recovers its original layered state to some capacity. However, while the gas-phase process results in a better and almost complete reconstruction, the liquid-phase treated materials present more activity on reactions like acetone condensation[54]. According to the experimental measurements, the gas phase rehydration reduces the available surface area of the reconstructed material, making the number of active sites available for the reaction lower than the liquid phase rehydrated material (Figure 2.7). The increase in the surface area is related to the lower degree of reconstruction of the layered structure and the defects produced by the process.

25 Literature Review

Figure 2.6 Scanning Electron Microscopy (SEM) of LDH samples. Reproduced from [54]

2.4.1.2.-Defect inducing strategies on LDH materials

While studying the self-condensation of acetone reaction with meixnerite type LDH it was found that only a fraction of the total active sites available on the catalyst interact with the acetone to produce DAA, and these active sites are mainly located on the edges of the LDH platelets[67]. This behaviour is generally similar for other condensation reactions. In relation to these results, studies have focused on techniques to increase the availability of edge sites on the materials.

Research with both magnetic and mechanical stirring as well as the application of ultrasonic and microwave irradiation during the reconstruction of the mixed oxides towards the meixnerite structure have been used to identify possible enhancements. The application of mechanical stirring leads to better mixing and a slightly more crystalline structure, while the use of both irradiation methods has shown that there is an increase on the surface area and, as a result, an increase in the amount of basic sites of the end materials, mainly related to a higher amount of the defects on the edge of the layers of the LDH[68].

Supporting the LDH onto other materials has also been a relatively common technique used to this end. Studies on the synthesis of LDH composites with carbon nanomaterials

26 Literature Review materials, such as carbon nanofibers (CNF), have achieved 4 times the activity of pure activated LDH [59,69]. CO2-TPD results indicate that the number of basic sites increased significantly in the supported material in comparison to the as-synthesized and unsupported LDH and hence, the specific activity per unit of solid base is higher than the unsupported one. A similar supporting technique has been used as well in other applications. For example, in the synthesis of glycerol carbonate from glycerol, increased activity is obtained when working with a composite CNF-LDH[70].

2.4.1.3 Carbon Nanomaterials: properties and application as support

Carbon has been considered a good material for different applications, such as an adsorbent for metal contaminants or as a support for different materials in catalysis applications[71]. This is mainly derived from the large surface area and porosity resulting from the chemical structure of the material, along with chemical resistance to different reaction media. Different forms of carbon are currently studied and specifically, nanomaterial developed structures have gathered great importance over the last decade, as they exhibit enhanced properties in comparison with graphitic carbon, like better diffusion due to coherent particle spacing, allowing better use of their qualities[72].

Depending on the way carbon sheets are oriented, the resulting structures end up with different enhancements. The basic unit structure of some of these nanostructures is graphene, formed by carbon atoms arranged in a non-volumetric lattice. Specifically, the nature of the carbon bonds forming the graphene layer gives the material great mechanical strength, high heat resistance and electrical conductivity[73]. When graphene layers are stacked, the three- dimensional structure of graphite is obtained while the cylindrical single-walled carbon nanotube structure is the result of a sheet bending to form a hollow cylinder (SWNTs). When multiple layers of graphene (starting at two) become part of a concentric cylindric arrangement, the multi-walled carbon nanotubes (MWNTs) structure is obtained [74].

The most common synthesis method of carbon nanostructures is the carbon vapour deposition (CVD) with metallic particle catalysts. The method involves the nucleation and growth via CO or hydrocarbons at high temperature[75]. A relatively simpler method to obtain the graphene structure is by a thermal exfoliation reduction of graphene oxide. Graphene oxide is typically obtained by an oxidation procedure of graphite in organic solvents and wave irradiation. An important property of graphene oxide in comparison to other carbon materials is its ability to solubilise in both water and organic solvents, which generally increases its range of application.

27 Literature Review

Apart from catalysis, the use of carbon LDH composites as CO2 capture devices has shown interest results. Studies have shown that MWNTs as well as GO aid to increase the adsorption capacity by of 60% and 30% with respect of that of the unsupported material, with a similar mass of active adsorbent[36,38]. Results show that this rise is related to the modification of morphology, which not only enhances the dispersion of the LDH on the composite, but also nucleates the structure to produce a more active material for the adsorption. Adding to these results, the process of doping metals into the composite materials increased adsorption further, showing an enhancement effect with the use of carbon structures[76]. However, there are some disadvantages from the use of these carbon derivatives, with the most common being the production of molecular debris, formed by carboxylated carbonaceous fragments, when the carbon material is functionalized to increase its reactivity and solubility[77,78]. This debris carries most of the functionalization of the carbon[79] and its effect on catalytic activity has not been determined completely. Modifications to the characteristics of the LDH/CNS composites due to removal of this debris in catalysis applications has not been studied in detail as well.

In summary, to increase the efficiency of LDH materials as catalysts for base activated processes, different methods have been used to alter inherent properties of the solids. Modifications during the synthesis procedure, doping with alkali metals, etc. and supporting on other higher surface area materials, among other techniques have been tested with good results, generally increasing activity.

2.4.2.- Diacetone alcohol retroaldolization: an of basic activity on the gas phase

Acetone self-condensation is relatively simple and easy-to-use model reaction employed to compare the basic strength of solid materials. Along with it, other processes have also been used to compare activity, with retroaldolization of DAA being one of the most promising. The retro aldol process presents a set of advantages in comparison to the forward self-condensation of acetone, but most importantly is that the reaction is only catalysed by basic sites and does not suffer from the effects of equilibrium limitations as severely as the forward reaction[80]. This results in shorter reaction times, less adsorption, etc. allowing a better kinetic comparison between basic catalysts. Another advantage is the ability to study the reaction at higher temperature, which allows the analysis of the behaviour of catalysts on the gas phase, in order to understand the effect on activity on gas-phase continuous flow systems. Nevertheless, study on the reaction has not been as broad and complete as that of the self-condensation process, possibly due to the simplicity of that process.

Initial reports on retroaldolization date back to the 70s, when the basic strength of solid oxides was compared with the reaction on the liquid phase, concluding that the general trend

28 Literature Review

of basicity between oxides was Al2O3

More recently, studies on the retroaldol process were undertaken to characterise basicity of Mg-Al LDH mixed oxides solids on the gas phase at atmospheric pressure. To do so, a saturation system was used to vaporise the DAA and the catalysts used were calcined at different temperatures, gas atmospheres and heating rates. The Al/(Al+Mg) molar ratio in the samples was 0.2. The results indicate that there is no direct correlation of the temperature ramp or the calcination temperature towards the conversion of DAA into acetone. However, the use of a dry air atmosphere increased activity slightly[85].

Solid oxides have also been studied with the reaction on the gas phase. Magnesia, alumina, zirconia and a mixture of magnesia zirconia was tested at atmospheric pressure. According to the results, the solids can be classified in two groups depending on their basic strength. The low basic strength solids were alumina and zirconia, while the higher strength were magnesia, Mg-Al LDH mixed oxides and zinc-doped magnesia. The authors demonstrate that the criteria of basic strength matches with the electronegativities of the metals present in the bulk oxides, while also indicating that partial substitution of Mg by Zn in brucite decreases the general basic strength of the mixed oxide formed[80].

Overall, the application of retroaldolization of DAA as a model reaction to compare the strength of basic strength of solid materials has not been studied with a lot of detail in comparison to the self-condensation of acetone. However, research has shown that it is possible to obtain an accurate comparison between solids in both the gas phase and the liquid phase. For this reason, the process is ideal to analyse the application of newly developed modified, supported, doped, etc. LDH materials.

2.4.3.- Benzaldehyde-acetone condensation for the analysis of in-situ rehydration

Another commonly known condensation process is the reaction between acetone and benzaldehyde. This is typically used to produce benzalacetone and dibenzalacetone. Specifically, dibenzalacetone is commonly used as an additive in the production of photoprotectors as well as a ligand compound on processes involving organometallic chemistry. Figure 2.7 shows a schematic of the process of condensation.

29 Literature Review

Figure 2.7 Condensation reaction between benzaldehyde and acetone

Initial studies with calcined LDH materials showed that starting with a carbonate hydrotalcite precursor increases activity in comparison with a chloride substituted one. According to the authors, this is partially related to the small percentage of chloride left in the sample before the interchange of anions to carbonates takes place, as the heat of adsorption difference between each species is as high as 150 kJ/mol. The calcination temperature had a small effect as well, showing an optimum for the work at 823 K. As with other base catalysed reactions, a higher Mg/Al molar ratio increases the activity of the catalysts [86].

Research on the reaction at 273 K shows that the mixed oxides produced by LDH thermal treatment managed to obtain a conversion of benzaldehyde of 94% in 3h of reaction time, while the selectivity of the aldol compound reached 86%. The LDH used in the study had Mg/Al ratios between 2 and 3, and the best activity was achieved with the higher one, in comparison with the self-condensation of acetone. Interestingly, comparing the activity and the effect of mass transfer on the catalyst particles, the authors suggest that there is a competing adsorption for the active sites of the solid between benzaldehyde and acetone which, at the temperature the experiments were conducted, results in benzaldehyde being heavily adsorbed in the catalyst[87]. Modifying the reaction temperature to 298 K increases the activity of the catalyst 3 times, while a change in selectivity occurs, reducing the amount of aldol from 86 to 71% while the benzalacetone is increased from 6.3 to 22%. A rehydration procedure was also applied to the materials, increasing activity of the reaction while working at 273 K almost five times in comparison with the pure calcined LDH material. When there were substitution groups on the benzaldehyde aromatic ring, the activity increased as a function of the nucleophilic strength of the substituents (p-NO2>m-Cl>p-Cl>Me>p-OMe).

Other authors have worked with meixnerite type LDH with a Mg/Al ratio of 3, obtained by rehydration in the gas phase. The study was performed at a reaction temperature of 273 K as well and the results achieved a selectivity higher than 95% for the aldol compound, with conversion between 84% at 73% for the benzaldehyde[88].

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A more recent study focused on the analysis of activity and different parameters of the same type of LDH, but rehydrated in-situ instead of the gas phase[89]. An analysis on the molar ratio of benzaldehyde, acetone and water, B, A and W respectively, revealed that to achieve the higher activity in the process, an optimum exists at around 1:5:5 of B:A:W. At higher quantities of water and a fixed amount of acetone and benzaldehyde the selectivity towards benzalacetone (BA) tends to be lowered, possibly due to an excess of the required amount of water for the reconstruction of the LDH material to take place. Conversely, when the ratio of benzaldehyde and water was set to a constant value, maximum activity was obtained as the amount of acetone was fixed to the same value of the water in the system. Higher or lower quantities decreased the yield of the reaction sharply. No trend was found when analysing the cooperative effect of acetone and water in the system, specifically while setting the ratio of both compounds equally in successive experiments (B:A:W as in 1:1:1, 1:3:3, 1:5:5, etc.).

Analysis by XRD comparing the as-synthesised LDH material, the mixed oxides obtained after calcination and catalysts after reaction show that partial reconstruction of the LDH structure occurred during the reaction process to the mixed oxides. When the fraction of W/A is less than 1, the reconstructed material tends to have a crystalline structure more similar to that of the mixed oxides. However, even at lower W/A ratios, when there is enough water into the system, like with a relationship of 1:15:5, it is possible to reconstruct the structure of the LDH material, as the XRD analysis shows that at these conditions a semi-crystalline LDH is obtained.

Experiments with multiple solids, including as-synthesised LDH, alumina, magnesia, a physical mixture of the last two materials, as well as no catalyst in the system showed that no activity was present in the reactor. The use of mixed oxides when no water was present on the system only converted a small amount of benzaldehyde, around 7%. With a liquid phase ex-situ rehydrated material, the conversion increased to 25-32% and a yield of 11% towards benzalacetone was obtained. 98% conversion and 78% selectivity was achieved when working on the in-situ rehydrated material at the optimum ratio of reactants found on the initial analysis, showing the LDH material is a good, active catalyst for this aldol condensation reaction. Another advantage of the in-situ process is the general lack of deactivation of the rehydrated material before reaction, as the active meixnerite type LDH is never in contact with a CO2 containing medium, like atmospheric air surrounding the system. Currently, attempts to reuse the in-situ rehydrated LDH catalyst have not been successful, mainly due to high adsorption of the reactants and products on the active surface of the material.

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39

Chapter 3

Experimental Methodology

The experimental methodologies followed during the development of this study are described in this chapter. In order to have the best understanding of the procedures followed, the characteristics, elements and design from all equipment and techniques used are described in detail. It is important to note that, for the sake of clarity, any differences in preparation or application techniques which differ from the ones presented here will be described and discussed as a part of the corresponding section on each chapter.

3.1 Synthesis of catalytic materials

3.1.1 Unsupported Mg-Al LDH

The co-precipitation method at supersaturation was followed to synthesise different amounts of unsupported LDH. Unless otherwise noted, the Mg-Al atomic ratio of the LDH materials used was 2, as this has been reported to give good results on other catalytic experiments, as well as other applications such as CO2 adsorption [1–3]. To obtain the material, an aqueous solution (50mL) with 0.1mol of Mg(NO3)2·6H2O and 0.05mol of Al(NO3)3·9H2O was added dropwise to a second aqueous solution (75mL) containing 0.35mol of NaOH and

0.09mol of Na2CO3, the latter under stirring (300 rpm). While the salt solution was added, the resulting white suspension was continuously heated at 333K. After the precipitation was finished, the end material was kept at the synthesis temperature and stirring speed for a period of 12h. All the compounds used were acquired from Sigma-Aldrich. The obtained precipitate was filtered under vacuum using 0.4μm polycarbonate membranes (Millipore, HTTP Isopore membrane) and washed with deionised water at 303K until a pH of 7 was reached. The white precipitate was dried in an oven for 12h at 393K. The procedure followed for the synthesis of the materials is similar to others used in literature as well [2,4,5]. Experimental Methods

Hybrid catalysts were prepared by supporting the synthesised LDHs on carbon. Two different types of carbons were used: CVD-grown MWNT and GO. The general synthesis procedures used are as follows:

3.1.2 Synthesis of LDH supported on MWNT materials

A commercial batch of MWNT (CVD-ARKEMA Graphistrength® product, average diameter 10-15 nm) was used to synthesise the composites. However, MWNT typically contain remnants of amorphous carbon, graphite particles and metallic residue used to catalyse their synthesis via the CVD method[6,7]. For this reason, in order to purify the materials and allow better solubilisation in water, the MWNT are first oxidised with a mixture of strong acids and then washed with a dilute base treatment to remove the debris loose on the surface of the material[8]. This method introduces oxygen-containing acidic groups, which are converted into their conjugate salts when the base is added. The carboxylate anions then allow the aqueous dispersion of the MWNTs to solubilise under alkaline conditions, and this favours the deposition of LDH nanocrystals during the coprecipitation of Mg2+ and Al3+ ions.

Specifically, work in this research has shown that depending on the application, a further treatment of the materials with base might be needed to increase the removal of the debris on the MWNT[3], which makes the synthesis procedure very time consuming.

A typical batch of MWNTs was first oxidized by adding 7mL of concentrated solution of H2SO4 and HNO3 with a volumetric ratio of 3 to 1, to every set of 200mg of MWNTs. This mixture was kept under reflux for 30 min at 393K while stirring, after which the solids were recovered by filtration under vacuum using 0.4μm polycarbonate membranes and washed with 1L of 0.01M NaOH[2]. The resulting material was then washed with deionised water until the filtrate reached a neutral pH. It is important to note that this purification procedure is inherently slow, which makes the synthesis procedure very time consuming. The oxidised MWNTs were further purified before each composite synthesis in order to remove any carbonaceous debris that could hinder their activity[3]. This treatment was done with the stoichiometric amount of NaOH required to synthesise the quantity of LDH that would be introduced, following the procedure on section 3.1.1. For example, for a 20 wt% MWNT/LDH catalyst, the carbon nanotubes were treated in 2.06mL of 4.8M NaOH under stirring at 333K for 12h, reproducing the LDH synthesis conditions. The solubilized carbonaceous residues in

41 Experimental Methods the base solution were then separated from the remaining nanotubes through a 0.4μm polycarbonate filter*.

Depending on the application of each catalyst, two methods of synthesis were used to obtain the LDH/MWNT composites: precipitation or impregnation.

For the precipitated hybrids, the debris-free MWNT were dispersed in an aqueous solution (2.06 mL) containing 9.9 mmol of NaOH and 2.5mmol of Na2CO3, and then titrated with 1.39 mL of a salt solution of 2.8 mmol Mg(NO3)2·6H2O and 1.4 mmol Al(NO3)3·9H2O. The amounts of both the salt and base solutions were revised on each batch synthesis to obtain the desired amount of carbon in each catalyst. The resulting black powder suspensions were aged at 333 K and stirred at 300 rpm for 12 h and afterwards filtered under vacuum using 0.4 μm polycarbonate membranes, washed with deionised water at 333 K until neutral pH was measured, and then dried in an oven for 12 h at 393 K*.

The impregnation method followed a similar procedure reported in studies with synthesised carbon nanofibers[9]. The debris-free MWNT were first filtered until a black solid was recovered and then dried in static air. Afterwards, the obtained material was impregnated with 1.39 mL of a salt solution of 2.8mmol Mg(NO3)2·6H2O and 1.4 mmol Al(NO3)3·9H2O. The solid was kept at 303 K for 1h in air, and then again dried at 393 K for 1 h. Afterwards, an aqueous solution (2.06mL) containing 9.9 mmol of NaOH and 2.5 mmol of Na2CO3 was added drop by drop to the previously impregnated MWNT, after which the sample was introduced in

- a horizontal furnace and kept for 18 h at 333 K under a water-saturated N2 flow (200 mL min 1). The quantities of base and salt solutions were adjusted according to the desired amount of LDH. Each solid was thoroughly washed with deionised water and dried at 393 K for 18 h.

* Some of the MWNT/LDH catalysts used were prepared in the Department of Chemistry by Dr. Almudena Celaya-Sanfiz, part the research group of Prof. Milo Shaffer, following the same procedures shown here. Work done by the previously mentioned authors will be indicated as such in the corresponding chapters.

42 Experimental Methods

3.1.3 Synthesis of LDH supported on GO materials

In the case of GO/LDH materials used, small batches of pure commercial GO (ACS chemicals) were used. As the GO inherently has the acidic functional groups necessary to solubilise it in water, no prior oxidation treatment was required.

The GO catalysts were synthesised via the precipitation method, in a similar way to the MWNT. In this case, the pure commercial GO were dispersed in an aqueous solution (2.06 mL) containing 9.9 mmol of NaOH and 2.5 mmol of Na2CO3, and then titrated with 1.39 mL of a salt solution of 2.8 mmol Mg(NO3)2·6H2O and 1.4 mmol Al(NO3)3·9H2O. The amounts of both the salt and base solution were modified according to the amount of carbon in each catalyst. The resulting black powder suspensions were aged at 333K, and stirred at 300 rpm for 12 h. The obtained samples were afterwards filtered under vacuum using 0.4 μm polycarbonate membranes, washed with deionised water at 333K until neutral pH was measured, and then dried in an oven for 12h at 393K.

3.1.4 Activation of synthesised LDH and hybrids

Typically, crystalline Mg-Al LDHs with carbonate as interlamellar anions show low basic activity; as such, the materials require an activation procedure to increase their intrinsic efficiency. Two methods of activation were used depending on the characteristics and required conditions needed for the analysis of the activity on each reaction system.

A) Via calcination A thermal treatment was done to the catalysts to form pseudo-amorphous mixed oxides. The unsupported and supported as synthesised materials were calcined ex situ at 673

-1 K for 4 hours flowing 100 mL min of N2 using a tubular quartz reactor (ID = 5 cm) placed in a horizontal concentric furnace. The furnace temperature was selected according to previous studies on the adsorption of CO2[2,10], which indicated that the LDH derivatives produced at 673 K show an optimum balance between surface area and basic sites. This relationship is a desirable characteristic for the application of base catalysts for reaction.

B) Via anion modification The modification of the interlamellar anions in the structure of the LDH allows the enhancement of properties in the materials like textural area and basic strength. In the present study, the substitution of the original carbonate anions with hydroxyl anions increased the basic activity. Initially, the catalysts were calcined following the procedure of activation on section 3.1.2.A. The calcined materials were then introduced into a glass reactor with a height- to-diameter aspect ratio of 5 and reconstructed via liquid phase hydration with decarbonated

43 Experimental Methods water, heated to 303 K and stirred at 300 rpm for 1h[11]. Subsequently, the temperature was increased to 373K and the process continued for another 2h, without stirring. The complete rehydration procedure took place in inert atmosphere, with argon flowing at a rate of 100 mL min-1 continuously purging the system. The remaining water in the system was filtered via cannula filtration and the resulting solids were dried at 303K overnight, also under inert atmosphere.

3.2 Characterisation of catalytic materials

The unsupported and supported LDHs in the as synthesised and activated forms were characterised using a range of physicochemical techniques. In particular, the samples were analysed by X-ray diffraction (XRD), N2 physisorption, transmission electron microscopy/ energy dispersive spectrometry (TEM/EDS), scanning electron microscopy (SEM), inductively couple plasma (ICP), thermogravimetric analysis (TGA), and temperature-programmed desorption of CO2 (CO2-TPD).

3.2.1 X-Ray Powder Diffraction (XRD)

The diffraction patterns of the powder catalyst materials presented in this work were obtained using two different pieces of equipment. The first one, a PANalytical X’Pert Pro multipurpose apparatus with the Cu-Kα radiation selected, was operated in the reflection mode at 40 kV and 30 mA. The second one was a BRUKER Pro Multi-Purpose Diffractometer, also using the Cu-Kα radiation, but working at 30 kV and 10mA. The samples were placed on a silicon holder and scanned at room temperature varying the 2θ diffraction angle between 5° and 80°. The coherence length of the materials in the c-direction (stacking direction of the LDH layers, reflection at 2θ = 11.7°) were estimated using the Scherrer equation from the full width at half maximum (FWHM). After thermal treatment, the particle size measurement for some of the calcined materials was calculated with the same procedure, with the diffraction signal at 2θ = 43.2°.

3.2.2 Nitrogen Physisorption

Physical adsorption of N2 was carried out at 77 K on either a Micrometrics Tristar 3000 or a Micromeritics Tri-flex 3000 instrument, depending on the type of sample and conditions required for analysis. Initially, the samples were dried at 393 K under N2 for periods of 12 hours. The specific surface areas were obtained using the Brunauer–Emmett–Teller (BET) method.

44 Experimental Methods

3.2.3 Transmission-Electron Microscopy/Energy Dispersive Spectroscopy (TEM/EDS) and Scanning-Electron Microscopy (SEM)

A JEOL-2010 microscope operating at 200 kV was used to obtain high resolution transmission electron microscopy (HR-TEM) images. Before analysis, the materials were dispersed in isopropanol with a concentration of 10mg material per 1 mL of solvent. A drop of this suspension was dropped onto holey carbon copper grid (300 mesh, Agar Scientific) and left to dry before starting analysis. Elemental analysis was performed using an Oxford Instruments INCA energy dispersive X-ray spectrometer. The morphology of the catalysts was analysed via scanning-electron microscopy using a Zeiss Auriga FIB/FEG-SEM microscope. Before imaging, the samples were adhered to carbon tape and then coated with a fine layer of chromium in order to increase conductivity.

3.2.4 Inductively Coupled Plasma (ICP)

The composition of the solids was determined by digesting the materials on a mixture of strong acids. Approximately, 10 mg of sample was crushed into fine powder and then treated with 5 mL of aqua regia (acid solution of concentrated HCl and HNO3 on a ratio 3:1 by volume). To help the digestion process, 5 mL of water were added were added dropwise at a very slow rate to avoid a harsh reaction with the acid. The samples were left for digestion for 12h, and then they were prepared for analysis. Typically, the samples were diluted to a total volume of 50 mL with deionised water. The measurements were carried out in a Perkin Elmer Optima 2000DV apparatus, which was calibrated using standard solutions (TraceCERT, Fluka).

3.2.5 Thermogravimetric Analysis (TGA)

The decomposition patterns of the LDH, the composites as well as the MWNT and GO were measured by thermal analysis on a TA Instruments TAQ500 analyser. In a typical run, approximately 5 mg of the sample were first dried at 393 K under nitrogen for 20 minutes and then heated up to 1073 K at a rate of 10 K min-1, flowing 20 mL min-1 of air. Following a similar procedure, the actual carbon loadings of the composites were measured. For the pure LDH and for the carbon materials, the ratios between the mass of the dry sample and the remnant at the end of the run were reproducible. Thus, it was possible to determine the actual loading of carbon by considering each component additive and equal to the residue of the hybrid.

45 Experimental Methods

3.2.6 CO2-Temperature-Programmed Desorption (CO2-TPD)

A thermogravimetric analyser was also used in order to analyse the CO2 adsorption capacity of the materials, in order to make a qualitative comparison of their basic strength. Argon (BOC, CP grade) was used on the sample pre-treatment and during desorption, while a premixed certified gas mixture of 2% CO2 in Ar (BOC, CP grade) was used for the adsorption. The argon was further purified through the use of a drier and an oxygen trap. Similarly, to the standard thermogravimetric analysis, approximately 5 mg of sample was loaded into the TGA crucible. The samples were pre-treated to similar conditions to their actual catalytic usage. So, the oven was heated to a temperature of 673 K at 10 K min-1 under argon flow (20 mL min-1) and kept at the set temperature for 4h. Afterwards, the samples were cooled down to 353 K and kept for 10 min for stabilization. At this point, the gas was switched to the CO2/Ar mixture and kept flowing for 1h. Subsequently, the reactor was purged with argon and the temperature increased to 373 K and kept at these conditions for 45 min to remove weakly adsorbed CO2. The adsorption capacity of the materials was obtained from the difference between the difference on the value at this point and before the adsorption process started.

A separate analysis was undertaken in some samples, where a continuous flow atmospheric pressure microreactor connected to a mass spectrometer via a heated quartz capillary was used†. On each run, the catalyst (20mg) was heated at 393K(10 K min−1) while flowing argon for 1h in order to remove the physically adsorbed water. The sample was then cooled to 303 K and exposed to CO2 (20% CO2 in argon) for 1h. After this, the system was purged in flowing argon for 2h to remove any physisorbed CO2 and then the temperature was

−1 increased, from 303 K to 1273 K at 50 K min . The concentration of the CO2 desorbed into the argon carrier gas was monitored with the mass spectrometer. The amount of CO2 was quantified by comparing the area under the TPD profile to the peak areas of the CO2 calibration pulses injected before each TPD experiment.

† The CO2-TPD studies at high temperature to identify the types of CO2 adsorbed were developed by Dr Diana Iruretagoyena Ferrer, from the Chemical Engineering Department at Imperial College London

46 Experimental Methods

3.3 Catalytic Tests

In order to understand the effect of the loading of the LDH on the carbon supports, different reaction systems were preselected in order to test the synthesised composites. From the systems used throughout the duration of the research, the most promising ones were chosen and studied in detail. The experimental set-ups along with the standard procedures followed are described in the following sections.

3.3.1 LDH/MWNT catalytic activity on the liquid phase self-condensation of acetone

3.3.1.1 Experimental Setup The self-condensation of acetone is a well-known reaction procedure typically used to analyse the basic strength of solid catalysts on batch reaction systems. As such, systems were selected and used based on previous studies for LDH materials as well as other nanocarbon supported LDHs[11,12].

Three different reactors were used to test the catalytic composites. Reactor 1 was a reaction miniclave consisting of a 25 mL Borosilicate glass reactor (Büchiglasuster, maximum pressure: 10 bar) and a stainless-steel head and a protective mesh. The head was connected to a pressure gauge, a relief valve and a t-piece connected to a valve for filling and removal of gases, and finally a Swagelok compression fitting nut with a septum for sampling. Reactor 2 was a 100 mL jacketed reactor along with a three-neck flat flange lid (Lenz Laborglass) coupled with an overhead stirrer (IKA, stirring speed 0-1000 rpm). Both reactors were equipped with O-rings seals to avoid elimination of the acetone during reaction. Reactor 3 was a tubular simplified borosilicate glass reaction system with 15.0 and 2.2 cm of height and diameter respectively, which was sealed with a rubber septum. The cooling bath used with reactors 1 and 3 was located over a magnetic plate, which was used to stir the reaction mixture during the process. A schematic of reactor 1 and 2 is presented in Figure 3.1.

47 Experimental Methods

Figure 3.1 Schematic of a) reactor 1 b) reactor 2 In order to avoid the dehydration of DAA into mesityl oxide and further products, the temperature of the reaction is set to 273 K, which is reportedly the optimal temperature to maximise selectivity of the process [3,9,13]. To do so, a circulation system (Huber MPC-K6, operation temperature -30 °C- 250 °C) was adapted to work with the three available reaction systems. For reactors 1 and 3, a 6 cm six-spire copper coil was constructed (tube I.D. 6.35 mm) and connected to the circulation system, in order to lower the temperature of a cooling bath to which the reactors were placed in. The jacket on reactor 2 was connected directly to the circulation system before use.

In order to ensure the temperature was accurate, a handheld temperature measuring system was used to read the actual reaction temperature. In some cases, in order to avoid reducing the loses of acetone to a minimum, the thermocouple was not connected to the system as the reaction advanced. However, the reproducibility of the temperature was corroborated before the reaction at least three times, to ensure accuracy during the reaction process.

48 Experimental Methods

3.3.1.2 Standard experimental procedure The experimental procedure followed for the condensation of acetone was similar for the three reaction systems, but specific differences make the description of each process important. For this reason, the procedure followed for each one will be will be described independently.

Initially, reactor 1 was introduced into the bath and the cooling system was set to the desired temperature, 273 K, in order to start the reaction process at the selected conditions. Once the temperature stabilised, the reactor was opened and the activated catalyst was introduced as fast as possible to avoid small deactivation from contact of the solid with the atmosphere, along with a suitable magnetic stirrer. As the reactor was closed, a filling- emptying process with argon was done in order to purge the system from air, increasing the pressure inside the reactor to 4 bar and then venting the gas. The process was followed three times in order to dilute the amount of air into argon much as possible. Once the set up process was finished, a syringe was used to introduce the desired amount of acetone via the sampling point to start the reaction, while the stirring speed was set to 1000 rpm.

For reactor 2, the cooling solution was flowed through the jacket and the temperature was set to 273 K. As the temperature was reached and stable, the reactor was opened and the selected catalyst was introduced quickly to avoid deactivation. After the reactor was closed, argon was flowed through the reactor at a rate of 20 mL min-1 to purge the air from the system. As with reactor 1, the acetone was introduced into the reaction system with a syringe through one of the lid necks to start the reaction just as the stirring speed was set to 1000 rpm on the overhead stirrer.

Reactor 3 was similarly located into the bath previously cooled to 273 K. As the temperature stabilised, the sealing septum was removed, and the activated catalyst was introduced along with a magnetic stirrer. The sealing septum was put back in position and argon was flown through the system to remove air. The acetone was then introduced to start the reaction and the stirring speed was set to 1000 rpm.

3.3.1.3 Product analysis Aliquots of 0.1 mL were taken from each of the reactors via their respective sampling points at defined intervals for periods ranging from 3 to 100 h of reaction time. In order to analyse the reaction effect, the samples were prepared for analysis via offline GC. Decane was added to each sample as internal standard and then diluted to a total volume of 1 mL with ethyl acetate, used as solvent. Each sample was then analysed on a Shimadzu Gas Chromatograph (GC-2014) with a flame ionization detector (FID) and a BP-1 column (Length: 30 m, I.D. 0.325 μm). In order to obtain accurate measurements of the amount of compound

49 Experimental Methods analysed, the GC was calibrated in advance using prepared solutions of known concentration of acetone, diacetone alcohol and the internal standard. The concentration of acetone and diacetone alcohol in the calibration solutions was varied in the range of the amounts expected from reaction, whereas the concentration of the internal standard was constant in all samples. The calibration for this analysis is included in Appendix A. It should be noted that the initial rates of reaction obtained from the self-condensation of acetone process and reported on Chapter 4 were calculated via the derivative of a 3rd order polynomial obtained from the experimental data up to 3h of reaction time. This period was selected in order to observe the effect of the catalyst on activity without deactivation being a significant part of the calculation.

3.3.2 LDH/MWNT and LDH/GO catalytic activity study on retroaldolization of diacetone alcohol in the gas phase

3.3.2.1 Experimental Setup The retroaldolization of DAA on the gas phase is a simple process which has been used to test qualitatively the basic strength of solid catalysts [18]. A continuous tubular jacketed reactor system was used to measure the activity of the composite materials. The tubular reactor itself consisted on a set of concentric stainless steel tubes, fixed together via a pair of 12.70 mm diameter T-piece Swagelok compression fittings. The reactor itself had the measurements: 푑푡 = 6.35 mm and length = 300 mm while the outer tubing, used as jacket, measured 푑푡 = 12.70 mm and length = 150 mm. The position of the arrangement was selected so that the middle length of both the reactor and the jacket would coincide to ensure the heat transfer between the heating liquid and the catalyst bed, positioned on the middle of the reactor, was optimal. The temperature of the jacket was controlled by coupling a circulation system (Huber MPC-K6, operation temperature -30 °C- 250 °C) and insulation was used over the jacket to avoid heat loss. All the lines on the system were stainless steel tubing with dt = 3.175 mm and were connected via Swagelok compression fittings, trace heated and insulated to prevent condensation. The temperature on the evaporator, condenser, reactor and the lines was measured via K-type thermocouples connected to each element via a central control unit on the reaction enclosure.

The reactant was fed using a mass flow controller (Bronkhorst, EL-FLOW) calibrated for argon, which was used as a carrier and was saturated with the diacetone alcohol. The DAA was supplied by a saturator system connected in series, consisting of a borosilicate glass evaporator, set at the flash temperature of the DAA at atmospheric pressure, and a glass condenser at the desired temperature for each experiment, in order to obtain the selected mole fraction of the DAA on-stream. A valve was used to purge the saturated argon from the

50 Experimental Methods system while the reactor reached the selected temperature for the reaction run. A separate line from the saturation system containing pure argon was also connected to the entrance of the reactor to purge the system from air and stabilise it at the chosen conditions before starting the reaction procedure. The selection of stream flowing through the reactor was done via a two-way valve just before the entrance of the reactor. A diagram of the reaction system used appears in Figure 3.2.

Figure 3.2 General schematic of the reaction system used for the DAA retroaldolization process

The catalyst particles were diluted with inert silicon carbide (100 μm) to a total volume of 0.1 mL in all cases to maintain isothermal operation and increase the available surface area for the interaction between the catalyst and the reactant. A second bed of silicon carbide of 0.1 mL was used below the catalyst bed to minimise catalyst loss while the gas was flowing

51 Experimental Methods through the reactor. The pressure in the system was checked for each run to ensure no significant changes occurred due to the introduction of the catalyst bed. In most cases, the changes were kept to a minimum and they were taken into consideration to ensure accurate measurement of activity.

3.3.2.2 Standard experimental procedure and product analysis A set of synthesised catalysts were tested at constant temperature for DAA retroaldolization. Initially, argon was flowed through the saturation system while the evaporator was set to 333 K and the condenser to 293 K. The argon was continuously vented to atmosphere to ensure the amount of DAA on stream became constant. This process typically lasted for 2h. The selected flow and catalyst bed mass were chosen to ensure the reactor would operate in differential mode, reducing axial and angular effects on the reaction process. In the meantime, a defined amount of catalyst was loaded into the reactor for each run, and then the reactor was connected to the reaction system. Pure argon was flowed immediately through the reactor at a rate of 100 mL min-1 to reduce possible deactivation of the catalysts from contact with air. The system was tested for leaks and then the reaction temperature was set and the circulation system started. Once all temperatures were stable, the reaction process started.

To do so, the flow of pure argon was stopped and the vent line of saturated argon was closed, diverting its flow towards the reactor itself. The initial dead volume of gas inside the reactor was vented and then the reaction products were analysed online with an automatic sampling valve, located after the reactor, Typically, sampling was undertaken every 0.25h for 5h. For specific catalysts, measurements continued after 5h, approximately every 1h for up to 48h, to check stability.

The analysis was perrformed with a Shimadzu gas chromatograph system (GC-2014) equipped with a BP-1 column (Length: 30 m, I.D. 0.325 μm) where the end compounds were separated and then analysed with a flame ionisation detector (FID). The GC was calibrated by manual injection of solutions of known concentration of diacetone alcohol and acetone. The concentration in each solution was modified on the range of the amounts expected from reaction. For the quantification of the products, it was assumed that the volume of each injection was constant. As such, it is possible to obtain proportionality factors between the fraction of the products and the area of the peaks given by the FID and calculate the moles converted of DAA in the reactor. The concentration of DAA was tested every few experiments with no catalyst loaded into the system, in order to corroborate the values obtained from the calibration, as well as the baseline calculated for each measurement. For details of the calibration, see Appendix A.

52 Experimental Methods

3.3.3 In-situ activated LDH/GO activity tests on the condensation of benzaldehyde and acetone

3.3.3.1 Experimental Setup Recent studies on in-situ activated LDH have shown that there are further modifications on the surface and structure of materials depending on the type of rehydration. These changes generally increase activity or modify the selectivity to the end products in some reaction systems[16,17]. As such, investigation of these modifications and possible enhancements due to different types of activation for the carbon composites becomes a topic of interest.

In order to increase the efficiency of each reaction run, a 12-position carrousel system (Radleys, Carrousel 6 Plus) was used to test the catalysts. The system consisted of a specially designed metallic base, placed on a magnetic plate, which allowed heat to be evenly transferred along each of the reactors in the system. Each reactor was constructed from borosilicate glass for a total reaction mixture volume of up to 5 mL, and the heads were sealed individually with a manufacturer designed accessory, which allowed sampling via a septum port along its centre. The actual head of the reaction system allowed to control the temperature of the top of each reactor via liquid circulation. In order to reduce the loses of the volatile components, a circulation system (Huber MPC-K6, operation temperature -30 °C- 250 °C) was connected to the head to reduce condensation to a minimum. A schematic of the carrousel is presented in Figure 3.3.

Figure 3.3 Carrousel system used for benzaldehyde-acetone condensation studies

53 Experimental Methods

3.3.3.2 Standard experimental procedure The experimental procedure followed was similar to other condensation processes in batch reactors, the main difference being the activation procedure of the LDH catalysts.

Initially, the head of the reactor was cooled down to 278 K to reflux the reaction mixture and minimise possible loses of benzaldehyde and acetone. At the same time, the temperature on the plate was set to the selected value for each individual experiment. Once both temperatures were reached and were stable, each reactor used was removed from the system and the desired amount of calcined activated catalysts was weighed and then introduced into the reactor, along with a suitable stirrer. Once all the catalysts were ready, argon as flown through to remove any traces of air inside via the sampling septum port.

In order to increase reproducibility and accuracy between measurements, a mother solution of benzaldehyde, acetone and water was used. The desired amounts of each component were weighed according to the chosen molar ratio for each run and once ready, the solution was heated to the selected reaction temperature on a separate hotplate, making sure any compound losses were kept to a minimum. As the mixture reached the same temperature as the reactors, the individual chosen amount of reaction mixture was weighed and then introduced into its corresponding reactor with a syringe. The reaction process started and then proceeded for a period between 3-5h.

At the end of the reaction runs, each reaction mixture was transferred to a container and centrifuged for 5 min. The separated liquids were moved into other flasks and then aliquots weighting 0.1 g were taken from each one. In most of the runs, however; heavy adsorption of the reaction products was evidentt on the catalysts at the end of each run. As a result, an extraction process was followed before sample analysis, in order to separate the reaction products from the catalysts.

After the first centrifugation, the catalysts were separated from the reaction mixture and 5 mL of ethyl acetate were added to the container with the solids. The newly added mixture was stirred for a period of 5 minutes, before another centrifugation was performed to separate the solid a second time. The end solution after the centrifugation was weighted and added to the original reaction mixture for analysis and aliquots of these mixtures were taken for analysis. All solution weights were considered to calculate the conversion and selectivities of the reactions.

3.3.3.3 Product analysis Analysis mixtures were prepared with each aliquot, along with 0.1 g of decane, used as internal standard, and ethyl acetate as solvent. The amount of ethyl acetate added to each

54 Experimental Methods sample was the difference of the total weight at the end of the preparation, 10 g in all cases, and the amount originally recovered from the reactor. Each sample was then analysed on a Shimadzu Gas Chromatograph (GC-2014) with a flame ionization detector (FID) and a BP-1 column (Length: 30 m, I.D. 0.325 μm). The GC was calibrated using prepared solutions of known concentration of acetone, benzaldehyde, benzalacetone, and the internal standard. The concentration of each reactant and product was varied in the range from zero to 100% conversion on benzaldehyde, whereas the concentration of the internal standard was constant on all samples. For specific values in the calibration, see Appendix A.

3.4 Conversion and Selectivity

Depending on the reaction system, different definitions of conversion and selectivity are used. In the case of the batch reactors and considering a constant reactor volume, conversion X would be defined as:

0 푡 퐶퐴 − 퐶퐴 푋 = 0 퐶퐴

-1 where CA0 and CAt are the concentration in mol L of the limiting reactant at the start of the reaction and at time t, respectively. Similarly, selectivity S can be defined as:

푡 0 퐶푖 − 퐶푖 푆푖 = 푡 ∑ 퐶푖

-1 where Cit is the concentration in mol L at reaction time t.

In the case of the continuous flow reactor, similar definitions are used considering the change of the molar flow rates of the different species at different positions depending on the catalyst bed. As such:

0 퐹퐴 − 퐹퐴 푋 = 0 퐹퐴

퐹푖 푆푖 = ∑ 퐹푖 where FA0, FA and Fi are the inlet and outlet molar flowrates of the limiting reactant, and the flowrate of the product i at the end of the catalyst bed. All the flowrates are considered at reaction time t.

55 Experimental Methods

REFERENCES

[1] J. Yang, J. Kim, Hydrotalcites for adsorption of CO at high temperature, 23 (2006) 77– 80. [2] A. Garcia-Gallastegui, D. Iruretagoyena, M. Mokhtar, A.M. Asiri, S.N. Basahel, S. a. Al- Thabaiti, A.O. Alyoubi, D. Chadwick, M.S.P. Shaffer, Layered double hydroxides supported on multi-walled carbon nanotubes: preparation and CO2 adsorption characteristics, J. Mater. Chem. 22 (2012) 13932. doi:10.1039/c2jm00059h. [3] A. Celaya-Sanfiz, N. Morales-Vega, M. De Marco, D. Iruretagoyena, M. Mokhtar, S.M. Bawaked, S.N. Basahel, S. a. Al-Thabaiti, A.O. Alyoubi, M.S.P. Shaffer, Self- condensation of acetone over Mg–Al layered double hydroxide supported on multi- walled carbon nanotube catalysts, J. Mol. Catal. A Chem. 398 (2015) 50–57. doi:10.1016/j.molcata.2014.11.002. [4] M.J. Climent, A. Corma, S. Iborra, K. Epping, A. Velty, Increasing the basicity and catalytic activity of hydrotalcites by different synthesis procedures, 225 (2004) 316–326. doi:10.1016/j.jcat.2004.04.027. [5] A. Garcia-Gallastegui, D. Iruretagoyena, V. Gouvea, M. Mokhtar, A.M. Asiri, S.N. Basahel, S. a. Al-Thabaiti, A.O. Alyoubi, D. Chadwick, M.S.P. Shaffer, Graphene Oxide as Support for Layered Double Hydroxides: Enhancing the CO 2 Adsorption Capacity, Chem. Mater. 24 (2012) 4531–4539. doi:10.1021/cm3018264. [6] H.-S.P. Wong, D. Akinwande, Carbon Nanotube and Graphene Device Physics:, Cambridge University Press, Cambridge, 2010. doi:10.1017/CBO9780511778124. [7] A. Krueger, Carbon Nanotubes, in: Carbon Mater. Nanotechnol., Wiley-VCH Verlag GmbH & Co. KGaA, 2010: pp. 123–281. doi:10.1002/9783527629602.ch3. [8] S. Fogden, R. Verdejo, B. Cottam, M. Shaffer, Purification of single walled carbon nanotubes: The problem with oxidation debris, Chem. Phys. Lett. 460 (2008) 162–167. doi:10.1016/j.cplett.2008.05.069. [9] F. Winter, V. Koot, a Vandillen, J. Geus, K. Dejong, Hydrotalcites supported on carbon nanofibers as solid base catalysts for the synthesis of MIBK, J. Catal. 236 (2005) 91– 100. doi:10.1016/j.jcat.2005.09.020. [10] D. Iruretagoyena, X. Huang, M.S.P. Shaffer, D. Chadwick, Influence of Alkali Metals (Na, K, and Cs) on CO2 Adsorption by Layered Double Oxides Supported on Graphene Oxide, Ind. Eng. Chem. Res. 54 (2015) 11610–11618. doi:10.1021/acs.iecr.5b02762. [11] S. Abelló, F. Medina, Aldol condensations over reconstructed Mg–Al hydrotalcites: structure–activity relationships related to the rehydration method, … -A Eur. J. 11 (2005) 728–739. doi:10.1002/chem.200400409. [12] F. Winter, a J. van Dillen, K.P. de Jong, Supported hydrotalcites as highly active solid

56 Experimental Methods

base catalysts., Chem. Commun. (Camb). (2005) 3977–9. doi:10.1039/b506173c. [13] E.C. Craven, The alkaline condensation of acetone, J. Biochem. Toxicol. 13 (1963) 71– 77. doi:10.1002/jbt.2570130205. [14] J.C. a. . Roelofs, a. . van Dillen, K.. de Jong, Base-catalyzed condensation of citral and acetone at low temperature using modified hydrotalcite catalysts, Catal. Today. 60 (2000) 297–303. doi:10.1016/S0920-5861(00)00346-1. [15] S. Abelló, S. Dhir, G. Colet, J. Pérez-Ramírez, Accelerated study of the citral–acetone condensation kinetics over activated Mg–Al hydrotalcite, Appl. Catal. A Gen. 325 (2007) 121–129. doi:10.1016/j.apcata.2007.03.022. [16] C. Xu, Y. Gao, X. Liu, R. Xin, Z. Wang, Hydrotalcite reconstructed by in situ rehydration as a highly active solid base catalyst and its application in aldol condensations, RSC Adv. 3 (2013) 793. doi:10.1039/c2ra21762g. [17] J.A. Van Bokhoven, J.C.A.A. Roelofs, K.P. De Jong, Unique Structural Properties of the Mg ± Al Hydrotalcite Solid Base Catalyst : An In Situ Study Using Mg and Al K-Edge XAFS during Calcination and, (2001) 1258–1265. doi:10.1002/1521-3765(20010316)7. [18] W.M. Antunes, C.D.O. Veloso, C.A. Henriques, Transesterification of soybean oil with methanol catalyzed by basic solids, Catal. Today. 133–135 (2008) 548–554. doi:10.1016/j.cattod.2007.12.055.

57

Chapter 4

Acetone Self-Condensation: Analysis of basic strength on LDH/MWNT composites

4.1 Introduction

The development of solid base catalysts is a continuous challenge in the chemical research, as their use in current standardised industrial processes has not been heavily required, in comparison to solid acidic materials[1]. As a result, their synthesis and property optimisation has not been as developed.

As previously introduced, some chemical transformations in biorefining processing; whether related to the production of fuels or synthesis of higher value compounds via building blocks, are based on base catalysed reactions. Along with the continuous search for newer transformations like biorefining, further development and optimisation of currently available base catalysed processes which are still based in homogeneous liquid base catalysts requires attention[2]. As such, the development of base catalysts with better properties like higher surface area, increased basic site availability and lower deactivation during reaction would facilitate better activity and selectivities along with less after-process separations, etc. in a wide range of different process.

Specifically, aldol condensation reactions are used in biorefining and other applications to increase the length of carbon compound chains. Research in this area has shown that the use of different solid bases like magnesia, magnesium doped zeolites, hydrotalcites and mixed oxides results in good activity with a large range of condensation processes, both related and unrelated to biorefining[3]. Nevertheless, problems still exist after these solid bases are in use, such as generally lower activity than homogeneous bases, leeching, structural modification and poisoning of active sites.

Acetone self-condensation has been a prime example of both an application of LDH materials as base catalysts, as well as a good model process to characterise the base site Acetone Self-Condensation: LDH/MWNT composites activity strength of solids. Studies with modified LDH materials have shown that it is possible to achieve increments of activity by modifying the inherent properties of the LDH itself, such as surface area, degree of crystallinity and morphology as well as structural composition[4].

A more recent approach which has increased activity further in comparison to the pure material is the use of high surface carbons as supports for the LDH, specifically CNFs. Studies examine the effect of the loading of the LDH over nanofibers, approximately 11 and 89 wt% respectively, and attribute the enhancement of activity of up to 4 times per unit of activated Mg-Al LDH to a higher accessible amount of hydroxyl ions at the edges of the layers of the LDH. This effect is related to a better dispersion of the LDH in the carbon, as little to no activity was obtained with the pure carbon and activated LDH[5,6].

While the study shows that CNFs can be used as a good support to increase activity in condensation processes, these materials also present issues when applied as supports. Most notably, CNFs tend to suffer fracture as force is applied to their structure, which makes their application in some processes where mixing or other mechanical effort is involved less than optimal. Furthermore, common synthetic procedures to obtain CNFs leave sites prone to chemical attack; it is reported that under certain conditions the edge of the fibres suffer oxidation or other chemical modifications[7].

The current research focuses on the use of composite materials formed by Mg-Al LDH and MWNTs as support in the self-condensation of acetone. The selection of MWNT arises from specific properties which are desirable in comparison to the CNFs. For example, the morphology of carbon nanotubes is more structurally defined and generally has smaller diameter in comparison to that of the CNFs. This generally results in a higher accessible surface area to support the LDH particles. Furthermore, the structure of the MWNTs is sturdier, which allows it to maintain its properties during use[8]. Studies in other applications, like CO2 capture and storage, have shown that the use of MWNT as well as other types of carbons as composited with LDH materials can enhance the availability of basic sites in the final solids, in a similar way than the carbon nanofibers[9]. Generally, increments in the amount of CO2 adsorbed are a result of a better interaction of the LDH with the MWNTs.

A wider range of carbon loadings are used in comparison to other research reported in literature [5,6,10,11] and to the authors knowledge, this study was the first related to LDH/MWNT composites materials for the aldol condensation of acetone in the liquid phase at the time of publication.

The chapter comprises the experimental testing used to define the optimal conditions to study the carbon hybrids, including analysis of the process with multiple reactors, stirring

59 Acetone Self-Condensation: LDH/MWNT composites activity speed and stirring type, catalyst concentration as well as an analysis of the carboxylated carbonaceous debris component removed from the MWNT during preparation for composite synthesis (section 4.2). At the same time, analysis of the structural and physical properties of the catalysts studied by different characterisation techniques, such as TGA and XRD, is included (section 4.3). The influence of the MWNT loading in the catalytic performance and possible causes for the deactivation and activity reduction are included in section 4.4. Finally, a summary of the results and final remarks, along with possible suggestions for a better performance are included in section 4.5.

The research outlined in this chapter was undertaken in collaboration with the Materials Chemistry group of Professor Milo Shaffer from the Department of Chemistry at Imperial College London. The procedures to synthesise the catalysts used were developed by the Chemistry group. Some the reaction rates obtained from selected composites reported were measured by Dr Almudena Celaya Sanfiz, who was member of that group. Additionally, the

CO2 adsorption-desorption studies included were measured by Dr Diana Iruretagoyena Ferrer, member from the group of Prof David Chadwick from the Chemical Engineering Department at Imperial College London.

60 Acetone Self-Condensation: LDH/MWNT composites activity

4.2 Initial testing with activated LDH samples: Looking for optimal reaction conditions

As mentioned in section 3.1.2, the synthesis of the MWNT/LDH composite materials is a time-consuming process, as the pre-treatment required to solubilise and activate the MWNT is slow. Hence, in order to maximise the amount of synthesised composite produced for the research, it was decided to define the optimal conditions to study the hybrid materials. To do so, small batches of unsupported and supported samples were selected and used to test multiple reaction variables like catalyst concentration and mixing inside the reactors. Additionally, this allowed to compare the feasibility of use of the different reaction systems available for the research.

To summarise, three different reactors were used throughout the development of this chapter. Reactor 1 was a reaction miniclave consisting of a 25 mL borosilicate glass reactor and a stainless-steel head and a protective mesh. Reactor 2 was a 100 mL jacketed reactor along with a three-neck flat flange lid coupled with an overhead stirrer. Reactor 3 was a tubular simplified borosilicate glass reaction system with 15.0 and 2.2 cm of height and diameter respectively, which was sealed with a rubber septum.

4.2.1 Test with activated composite, pure and debris LDH samples in reactor 1

A pure, synthesised Mg-Al LDH was obtained by following the procedure in section 3.1.1. Separately, a hybrid MWNT/LDH material was synthesised for testing. This composite was synthesised by coprecipitation following the method in section 3.1.2 with a selected amount of MWNT of 20 wt%. This amount was preselected to analyse if there were any major increments in activity with a relatively low quantity of nanotubes into the LDH composite material.

In order to determine if the activity in the condensation was affected by this carboxylated debris, the recovered solution from the purification of the nanotubes was used for a separate synthesis of a Debris-LDH hybrid. The material was synthesised by the same coprecipitation method described in section 3.1.1, but adding the solution of debris during the addition of the salt and base solutions. The molar ratio of LDH/Debris was approximately 10.

All catalysts were activated following the rehydration procedure in section 3.1.4B in order to obtain higher activity in comparison to LDH mixed oxides or carbonated LDH[12]. In brief, the materials were calcined at 673 K in a N2 atmosphere. Once the process finished, the solids were then placed into a sealed glass vessel where decarbonated water was introduced. Activation proceeded in a two-way process starting at 303 K for 1h and then at 373 K for 2h

61 Acetone Self-Condensation: LDH/MWNT composites activity

while flowing N2 to the system. After activation, excess water was removed from the container and the catalysts were dried with inert atmosphere overnight at 303 K with the same gas flow.

The samples used along with their identifiers and the initial rates calculated for each experiment appear in Table 4.1, while Figure 4.1 shows the concentration of DAA produced by each of the catalysts used as a function of time.

Table 4.1 Identifiers and initial rates of reaction obtained in the self-condensation of acetone for preselected LDH samples*

Initial rate Initial rate Sample name Material -1 -1 -1 -1 (mmolDAA gcat h ) (mmolDAA gLDH h ) Synthesised Mg-Al SynthLDH 6.9 6.9 LDH

Pure MWNT/LDH MWNT-LDH hybrid 27.4 34.2

Debris from MWNT- Debris-LDH 3.1 3.4 LDH hybrid

* Details on the characterisation of sample SynthLDH appear in Appendix B.

62 Acetone Self-Condensation: LDH/MWNT composites activity

Figure 4.1 DAA molar concentration as a function of time. Treac= 273 K, stirring speed

= 1000 rpm, magnetic stirring, NAo= 0.25 mol acetone, mcat = 0.05 g, treac = 50 h. Insert included to show equilibrium concentration

The reactions were undertaken at 273 K to maximise selectivity towards DAA [13,14] and analysis continued for 50h of reaction time. The amounts of acetone and catalyst were preselected following similar conditions as reported in [5] and the experiments were performed in reactor 1. The standard operating procedure followed is included in section 3.3.1.2 while the product analysis appears in section 3.3.1.3. Specifically, the purification process necessary to remove the carboxylated carbonaceous debris mentioned in section 3.1.2 was followed for the MWNT/LDH sample. Hence the name “pure”.

Firstly, Figure 4.1 shows that the hybrid MWNT/LDH material is more active in comparison to the unsupported LDH, as the molar concentration of DAA produced by it is always higher. The increment is continuous and gets slightly larger as the reaction proceeds. Also, an analysis of the production of DAA from the Debris-LDH sample demonstrates a severe decrease in activity in comparison to the other 2 catalysts. Considering that the amount of active LDH material in the Debris sample is around 90% of the pure activated LDH material at the same catalyst mass, it is clear that the carbon component in this sample is hindering the conversion of acetone. Most likely, the effect is related to the carboxylated carbonaceous fragments included into the Debris-LDH, as analyses by FTIR in the literature have shown that these carbon leftovers are mostly formed by carboxylic and ketone acidic groups[15,16], which

63 Acetone Self-Condensation: LDH/MWNT composites activity hinders the ability of basic sites to interact with other compounds. This effect has been demonstrated in other process, like adsorption[17].

Considering the values obtained for the rate in comparison with the activated Mg-Al LDH and the MWNT hybrid, the increment between both rates is nearly 4 times, which shows there is an actual effect while loading LDH particles to the MWNT. However, the values of rate for both the pure LDH and the MWNT/LDH in this work are much lower than that reported in the literature, at around 1/10 of the values of studies with CNF composites[5,10]. It could be stated that the use of the MWNT to increase the activity of the LDH is detrimental. Nevertheless, as the rate for the pure LDH material is lower as well, it is feasible to think that other factors rather than the sole interaction of the LDH with the MWNT is the cause of such decreased activity. A note to add is that multiple experiments with pure activated LDH were done to corroborate the results obtained, and in all of them results of the calculations of rate were similar.

The first factor to consider while making the comparison is that the study with CNFs approaches the equilibrium of the reaction[14] in a short period of time, which could make any decline of activity less clear with the values of initial rate. Another aspect could be the amount of active material in both composites, 11 wt% for the CNF study in the literature against 80 wt% LDH in this research. However, as experiments with pure MWNT and literature with CNFs can confirm both types of carbon present no activity in the self-condensation of acetone independently, which would indicate that it is possible that the interaction with the tubes is decremental at higher loadings of MWNT. As it is, a comparison with greater amounts of MWNT would be useful, as the enhancing effect might not be similar considering the morphological differences between the MWNT, the CNFs and the LDH materials.

4.2.2 Mass transfer limitation research: Comparison between mixing in reactors 1 and 2

As the focus of the study is to compare the general activity between catalysts, it is also important to verify that the results are not affected by mass transfer limitations. While working with solid-fluid heterogeneous reaction systems, external particle mass transfer effects are caused by the boundary layer surrounding the catalyst particles. For the reaction to be under kinetic conditions, the particles in the surrounding fluid should move and adsorb into the pores of the catalyst at a higher rate than the rate of reaction, hence not affecting the rate of conversion of reactants to products.

An efficient method to ensure the transport rate is higher than the reaction rate is to increase the movement around the particles. To do so, in a batch reaction system this is

64 Acetone Self-Condensation: LDH/MWNT composites activity

typically done by modifying the mixing behaviour of the fluid either with different stirring speeds, stirring types, the introduction of baffles into the system, etc. In order to analyse whether this could be affecting the results obtained with reactor 1, an analysis with pure activated LDH was developed with reactor 2, while testing the effect of mechanical stirring in the system with a different, and typically better, type of mixing. The use of a more standardised laboratory reactor allowed to perform multiple tests on shorter periods of time. Pure LDH material and the same amount of catalyst used in the experiments with reactor 1 were selected to analyse and make the comparisons between the reactors and other variables of the process. Also, this was decided as it is generally difficult to obtain good amounts of pure MWNT to synthesise the LDH/MWNT composite materials in reasonable periods of time.

The reaction conditions were the same applied to reactor 1 as well, Treac = 273 K and stirring speed = 1000 rpm. The procedure followed is described in section 3.1.1.2. The reaction was left to run for 50 h and the initial rates of reaction obtained in each experiment appear in Table 4.2. The production of DAA for 1000 rpm appears in Figure 4.2

Table 4.2 Initial rates of reaction for pure activated LDH catalysts for different types of mixing and reactors

Reactor Mixing Mass of Acetone Catalyst Initial rate -1 -1 -1 number type catalyst (g) amount (mol) concentration (gcat L ) (mmolDAA gcat h )

1 Magnetic 0.25 2.70 6.9 0.05 2 Mechanical 0.80 0.83 66.9

65 Acetone Self-Condensation: LDH/MWNT composites activity

Figure 4.2 DAA molar concentration as a function of time. Rehydrated LDH catalyst.

Treac= 273 K, stirring speed = 1000 rpm, mechanical stirring, NAo= 0.80 mol acetone,

mcat=0.05 g, treac = 50 h

The amount of acetone in the study with reactor 2 was selected after trial runs showed that lower quantities could cause a high amount of separation from the solid catalyst to the walls of the reactor at the start of the reaction process. As such, the catalyst concentration in the reaction is lower than the studies with reactor 1 and the literature. Nonetheless, the study at these conditions is useful to solely analyse the effect of the solid catalyst in the rate, as the reaction process would approach equilibrium more slowly, while also keeping a good balance between low and high activity to make a better comparison between the materials.

Table 4.2 shows that even at this lower catalyst concentration there is increase of activity nearing ten times the initial rate obtained with reactor 1. The result is interesting as the value of the rate of the reaction could be expected to be lower when working at the same temperature and stirring speed. Analysing the process during the complete reaction period; however, shows a continuous drop in the amount of DAA produced as the reaction continues at further times than 10 h, as seen in Figure 4.2.

This deactivation pattern could be attributed to a different number of factors. Regarding the reaction system, an amount of solid was slowly removed from the reaction mixture to the reactor wall once the reaction crossed the 10 h mark, probably inducing the loss in activity at longer reaction periods. This effect was similar to the initial pretesting with reactor 2 but the behaviour only occurred at longer reaction times under the currently selected reaction

66 Acetone Self-Condensation: LDH/MWNT composites activity conditions. Several tests were done to corroborate this effect, yielding similar results. Another factor would be that the recovered catalysts show small traces of adsorption, as small yellow colouring appears in the surface of the material, similar to other reports in the literature, which could indicate deactivation of the solids as the active sites in the surface remain with molecules adsorbed[18].

As an increment in the rate was noted with this setup, it is important to define if mass transfer limitations could be intervening in the reaction process with this reactor as well. As such, the condensation was tested at multiple stirring speeds with the mechanical stirrer in reactor 2. Conditions were similar to the previous experiments. The initial rates of reaction obtained are shown in Table 4.3.

The results indicate that no significant modification of the values of rate are obtained when modifying the stirring speed. Consequently, no mass transfer limitations exist under the current conditions, while it is also assumed that further increases in mixing velocity would not modify the rate. As such, studies were continued applying a stirring speed of 1000 rpm.

Table 4.3 Initial rates of reaction for pure activated LDH catalysts at different stirring speed

Initial rate Stirring speed (rpm) -1 -1 (mmolDAA gcat h )

600 64.6

800 68.0

1000 66.9

4.2.3 Analysis of lower rate and production of DAA: Activity in reactors 1, 2 and 3

While aiming to elucidate whether the low rate of reaction in reactor 1 and the reduction of DAA production observed in reactor 2 are related to the design of the reactors or adsorption on the catalyst, reactor 3 was preselected to prevent the catalyst loss at longer reaction times, as its high length-to-diameter aspect ratio avoids this behaviour.

Selecting this reactor also allowed the study of possible differences between magnetic and mechanical stirring as the method for mixing, as the design only allows the use of magnetic mixers. Also, as the volume of the reactor is smaller than that of reactor 2, the

67 Acetone Self-Condensation: LDH/MWNT composites activity amount of catalyst and acetone were modified to match the catalyst concentration used in reactor 2 as well. This was done to corroborate whether a higher rate could be achieved with a lower catalyst concentration, similar to the change obtained from using reactor 1 to reactor

-1 2. As such, the reaction conditions were set at Treac = 273 K, Ccat = 0.83 gcat L and stirring speed = 1000 rpm. The reaction was left to run for 50 h. A comparison between the production of DAA between reactors 2 and 3 appears in Figure 4.3, while the insert includes the production DAA per amount of catalyst used to calculate the rate in the 3 reactors used.

Figure 4.3 DAA molar concentration as a function of time. Rehydrated LDH catalyst.

-1 Treac= 273 K, stirring speed = 1000 rpm, Ccat =0.83 gcat L , treac 50 h.

Figure 4.3 shows that both reactor 2 and 3 achieved comparable initial rates in the production of DAA. However, as the reactions proceed, the amount of DAA starts to decrease while using reactor 2. A visual analysis of the recovered catalyst from reactor 3 suggests that there is a small amount of adsorption occurring, analogous to the catalysts recovered from reactor 2. As this adsorption is present with both systems, it would be feasible to assume that the deactivation of the samples with reactor 2 could be related to the splashing effect inside the reactor to a higher degree. Multiple tests confirmed the behaviour of catalyst with each reactor. Also, an important factor to consider is that even with the higher rate and lower

68 Acetone Self-Condensation: LDH/MWNT composites activity deactivation in reactor 3, both tests do not approach equilibrium even at reaction times of 50 h, indicating that the process of conversion itself is very slow. Characteristics like the low temperature to achieve good selectivity partly influence this behaviour.

Analysing the insert in Figure 4.3, a comparison between the normalised moles of DAA produced shows that the study in each subsequent reactorr increases the rate. When comparing the initial reaction rate in both reactors 2 and 3, a higher value can be achieved when using the latter, but the increment is not as large if experimental error is considered. However, the profile of mixing via mechanical stirring in reactor 2 shows there could be issues inherent to the use of the reactor at longer times. This effect could be reduced using different stirrer types or stirrer material, as well as different positioning of the propeller to increase efficiency. For reactor 1; however, the rate was extremely low in comparison to 2 and 3.

Considering the catalyst concentration used in the set of experiments with reactor 1 was a bit over 3 times of the ones used in both reactors 2 and 3, with the same activated LDH catalyst and reaction conditions, both mechanical and magnetic stirring as well as different stirring speeds, an issue with the use of reactor 1 is probable. Adding to these results, a large set of experiments were done in reactor 1 initially to corroborate the values of rate, obtaining similar low results in most cases.

An analysis of the behaviour of the reactor led to the most probable conclusion that the design was not optimal to work at the conditions required for the self-condensation process. As the reactor was developed to study high pressure processes like hydrogenation or oxidation, the safety mechanisms include a thick glass vessel to uphold the pressure, a stainless steel protective mesh, and a bursting disc on the head, along with a pressure gauge. In order to increase the selectivity of the process, the reactor had to be introduced during each test in a cold bath held at 273 K. This increase condensation along the head and possibly modified the mixing profile inside of the vessel. Adding to these characteristics, acetone was typically difficult to work with the reactor, as the compound is volatile even at low temperatures, which could have made the interaction with the solid catalyst decrease.

Following these results, it was important to corroborate whether the Debris-LDH composite material presented low activity, as previously measured in reactor 1. The reaction process was repeated in reactor 3 under the same conditions as the test with reactor 1, Treac=

273 K, Stirring speed = 1000 rpm, magnetic stirring, NAo= 0.25 mol acetone, treac = 50 h. The results of the analysis appear in Table 4.4.

69 Acetone Self-Condensation: LDH/MWNT composites activity

Table 4.4 Initial rates of reaction for pure activated LDH catalysts for different types of mixing and reactors

Mass of Acetone Catalyst Initial rate Reactor Mixing -1 Catalyst catalyst amount Concentration (mmolDAA gcat number type -1 (g) (mol) (gcat L ) h-1)

1 Debris-LDH Magnetic 0.050 0.25 2.70 3.4

Debris-LDH 16.9 3 Magnetic 0.015 0.25 0.83 SynthLDH 80.6

The rate obtained for the Debris-LDH samples was around 5 times higher than the one obtained in reactor 1, even at a lower catalyst concentration. However, considering that the Debris-LDH sample has a molar ratio of LDH to carbon debris of 10:1, the results from reactor 3 indicate that the activity is reduced almost an order of magnitude in comparison to the pure LDH compound obtained as well. This result would indicate that even with any issues that were present in the initial studies with reactor 1, under the methodology used for testing in reactor 3 the activity of the LDH materials is heavily reduced due to the presence of the debris material. In conclusion, it is important to ensure that debris material is removed from the MWNT as much as possible before starting the synthesis process with LDH materials to produce basic catalysts.

4.3 MWNT/LDH composites: Synthesis of a range of solids and characterisation studies

The research in Chapter 4 was partially funded by CONACyT as well as BioNano Consulting, two independent sponsors. As such, the work within this Chapter was developed by multiple members of the research team. The carbon samples obtained in the following sections as well as the characterisation work, apart from the ones introduced in section 4.2, were synthesised by Dr Almudena Celaya-Sanfiz, from the materials group lead by Prof. Milo Shaffer of the Chemistry Department at Imperial College London. The information is included to aid in the discussion of the results.

A selection of MWNT/LDH materials with multiple carbon loadings were selected for testing in the self-condensation of acetone, following the procedures described in Chapter 3. The synthesised samples were the following:

70 Acetone Self-Condensation: LDH/MWNT composites activity

Table 4.5 MWNT/LDH samples synthesised to catalyse the self-condensation of acetone. Selected data obtained by Dr. Almudena Celaya-Sanfiz

Nominal Carbon Actual Carbon Sample name amount (wt%) amount (wt%)

LDHhl 0 0

MWNT/LDHhl-20 20 17

MWNT/LDHhl-33 33 30

MWNT/LDHhl-50 50 40

MWNT/LDHhl-67 67 67

The range of MWNT carbon loading was selected to observe the behaviour of the activity of the catalysts with different amounts of support, as literature indicates that a similar degree of activity to this could be achieved at high carbon loadings nearing 90 wt%. The subscript -hl added to each identifier noted that the sample was activated via rehydration, following the procedure outlined in Chapter 3. Hence, any samples included in the results without the subscript indicate that the solid was tested or analysed as-synthesised. The real amount of carbon was estimated via a thermogravimetric analysis, described in the following section.

4.3.1 Thermogravimetric analysis of composites: Carbon quantification of synthesised MWNT/LDH catalysts

Studying the catalysts via thermogravimetry allows to analyse the decomposition profile of the synthesised catalysts, in order to understand the differences of composition, as well as calculate the actual amount of carbon of each sample. Each profile measured appears in Figure 4.4.

71 Acetone Self-Condensation: LDH/MWNT composites activity

Figure 4.4 TGA decomposition pattern for synthesised samples. (a) MWN/LDHhl-20, (b)

MWN/LDHhl-33, (c) MWN/LDHhl-50, (d) MWN/LDHhl-67. Data obtained by Dr. Almudena Celaya-Sanfiz

Figure 4.4 shows two declines in weight for the LDHhl. The first one, appearing in the range between 373 and 473 K is associated with the elimination of interlayer water on the LDH layers. The second is typically found in the range of 473 to 773 K, is due to the loss of hydroxyls from the LDH layers. No loss of carbonate species is present in this sample as it has been activated via rehydration[19]. Analysing the sample of LDHhl after reaction shows a third weight loss in the range 673 to 873 K, indicating the presence of additional material being part of the sample after the reaction process was completed. This was useful to corroborate the that the resulting colouring obtained during the pretesting was due to the presence of adsorbed compounds into the basic sites of the solid catalysts.

As expected, the initial loss events for the MWNT/LDH composites is similar to the pure LDH material, although the degree of the decrease depends on the amount of water incorporated after rehydration in each sample. Similarly, for the second loss, the elimination of the hydroxyls in the solids is dependent on the amount of LDH loaded into each of the catalysts. A third weight loss can be identified for the composite materials in the range of 773 to 973 K. An analysis of the pure MWNT shows that, similar to other reports in the literature, the weight decrease is attributed to the oxidation of the MWNT present in the solids[20]. The temperature at which this loss is located could also elucidate the nature of the adsorbed components of the catalysts after reaction, as the range where it appears is similar for both

72 Acetone Self-Condensation: LDH/MWNT composites activity

the MWNT and the LDHhl sample tested after the process. This could indicate that a process similar to coking could be the effect occurring to the solids at long reaction times, although further analysis would have to be done to corroborate the actual compounds incorporated into the materials. The pure MWNT sample also shows that leftovers are present after analysis, typically associated with the presence of graphitic carbon.

The amount of residue after each TGA analysis was used to estimate the actual amount of LDH on each solid while assuming that these remnants were additive according to the individual analysis for the pure LDH and the MWNT. The calculated values appear in Table 4.5. The differences found in comparison to the nominal values could be attributed to the loss of MWNT during the liquid base removal step, as the filtration process tends to last extended periods of time, allowing small quantities of MWNT to be removed and filtered through the equipment. Similar reports regarding this issue can be found in other literature work[9]. Increasing the carbon content of the samples results on a higher amount of remnants in each sample at the end of the analysis, as the concentration of carbon increases.

73 Acetone Self-Condensation: LDH/MWNT composites activity

4.3.2 Phase composition analysis: X-ray diffraction of MWNT/LDH composites

The phase composition of each synthesised solid was obtained via XRD. The diffractogram of each sample appears in Figure 4.5.

Figure 4.5 X-ray diffractogram of synthesised solid catalysts. (a) MWN/LDHhl-20, (b)

MWN/LDHhl-33, (c) MWN/LDHhl-50, (d) MWN/LDHhl-67. Data obtained by Dr. Almudena Celaya-Sanfiz

The pattern obtained for both the as-synthesised and the rehydrated LDH confirms that a standard LDH structure material was produced according to the LDH power data (JPDS 14–191).

The lattice parameters were calculated in these samples, obtaining values of 2.98 and 22.39 Å for the a and c parameters in the as-synthesised LDH and 2.98 and 22.48 Å for the hydrated LDH catalyst. The values obtained are slightly different for the commonly known Mg- Al 3:1 LDH; however, these depend on the composition of the layers as well as the interlayer components[21]. In this case, the materials synthesised have a ratio 2:1 for the metallic composition, as it is reported that they have better activity than the standard 3:1 ratio[5], but this modification does not seem to be modifying parameter a, related to the in-plane distance of the lattice in the layer. In the case of the c parameter, which indicates the out-of-plane distance between each layer, a modification is present possibly due to the anions inside each sample. Once the LDH was heat treated, the structure is modified into a periclase like

74 Acetone Self-Condensation: LDH/MWNT composites activity structure, a mixture of alumina and magnesia (MgAlOx), according to the (JCPDS 87-0653). As a result, the original carbonates within the structure are removed from the material. Once the rehydration procedure is applied, the material suffers a reconstruction due to the inherent “memory effect” of the structure. This allows the solid to partially recover the layered orientation it had before the thermal treatment. In this case, the sample (LDHhl) includes -OH groups in the interlayer, as they are available in the “reconstruction solution”. This OH groups allow the material to create Brønsted base sites within the reformed layered structure [22].

Loading the LDH material into the MWNT does not change the general phases originally obtained for the single material, but the width of the peaks and position of the (003) signal of the LDH is slightly modified. The shift is about 2° lower for the diffraction (003), which typically indicates an increase of the interlamellar distance of the solid. Generally, the signals tend to get broader with increasing carbon content, indicating further dilution of the LDH into the carbon. The extra signal around 26° in all catalysts is related to graphitic material from the MWNT, as it can also be identified in the pure MWNT. The broadness and intensity is directly related to the amount of both components in the solids. The calculated values for the directions of the crystallites were obtained via the Scherrer equation and are shown in Figure 4.6 in relation to the MWNT loading in each solid. However, as peak broadening by defects such as stacking faults and other effects are more common and inherent to the MWNT structure, the actual size might be modified in comparison to the actual or real measurement of the crystallites. As such, the lateral and thickness dimensions calculated from the (110) and (003) reflections respectively, should be more appropriately considered an ‘apparent’ crystallite size[22].

75 Acetone Self-Condensation: LDH/MWNT composites activity

Figure 4.6 Dependence of the LDH crystallite size in relation with carbon content. Data obtained by Dr. Almudena Celaya-Sanfiz

After activation, apparent LDH crystallites sizes are either constant or increase in the lateral dimension, whereas in the c direction they are constant or decrease. This contrasting change suggests there is a modification of the actual shape of the crystallite size. The apparent crystallite size in c direction is in reasonable agreement with the information obtained from microscopy. However, the calculated size in the lateral dimension is smaller than the observed in the images, and likely to be generated by the defects of the structure of the tubes. Upon addition of MWNT, the sizes in both directions are systematically reduced compared to the as-synthesized samples, probably as a result of heterogeneous nucleation effects during the initial synthesis. After activation, the same trend is evident, although on a smaller scale. Overall, the hydrated hybrids contain relatively thinner and smaller LDH platelets compared to the pure hydrated LDH.

76 Acetone Self-Condensation: LDH/MWNT composites activity

4.3.3 Analysis of morphology in the solids MWNT/LDH catalysts: Microscopy studies

In order to identify sites of interaction between the LDH and the MWNT in the catalysts, each solid was analysed via SEM to check the morphology of the material. An initial analysis of the pure LDH shows that the material is formed by hexagonal platelets, as with other reports in literature[11].

Once the LDH is loaded into the MWNT, no significant change in the shape is identified. Particularly, a uniform distribution of LDH platelet sets is obtained in all samples but

MWNT/LDHhl-67, where a segregation between the LDH and MWNT phases can be identified in some areas. A set of micrographs is included for reference in Figure 4.7.

The LDH particles appear to be nucleated the open framework of the MWNT, which seems to be acting as a support of the material, while measurements of some of these LDH layers shows a similar reduction in comparison with the pure LDH material, as was calculated via XRD analysis. However, as the LDH is classified as a layered structure and the tubes are mainly a framework with a high aspect ratio along their axis, the interaction between both would probably be small in comparison to other combinations of solids with a more coherent structure (both 2D, rather than 1D vs 2D). This could lead to a less effective catalyst-support interaction and rather to a better dispersion and accessibility for compounds into the LDH materials. As a result, characteristics such as surface area are improved in the supported solids.

77 Acetone Self-Condensation: LDH/MWNT composites activity

Figure 4.7 SEM of synthesised solid catalysts. (a) MWN/LDHhl-20, (b) MWN/LDHhl-33,

(c) MWN/LDHhl-50, (d) MWN/LDHhl-67. Square areas represent MWNT. Circle areas represent LDH. Data obtained by Dr. Almudena Celaya-Sanfiz

For the hydrated samples, the average lateral size was comparable to all the hybrid

MWNT/LDHhl samples, at approximately 62 nm, although these measurements do not agree to those calculated via the Scherrer equation. The pure LDHhl showed a slightly bigger average lateral size, 69 nm. On average, larger LDH platelets were observed after hydration than for the as-synthesized MWNT/LDH, with a similar trend being observed for reported CNF/LDH hybrids [28]. EDS analysis of the samples results in location of C, O, Mg and Al as the main components of the catalysts, locating the ratio of the Mg and the Al metals in the range of 1.9 for all solids, with an estimated error of up to 2.5%.

4.3.4 Dispersion in the LDH/MWNT: An analysis of surface area by N2 adsorption

A N2 physisorption analysis was used to determine possible differences in the available surface area of the catalysts due to the addition of the LDH particles on the surface of the MWNT. The calculated results of BET area dependence with the amount of carbon on each sample is included in Figure 4.8.

78 Acetone Self-Condensation: LDH/MWNT composites activity

Figure 4.8 BET surface area dependence with the amount of MWNT loaded on the catalysts. Data obtained by Dr, Almudena Celaya-Sanfiz

Figure 4.8 shows a decrease of the surface areas of all samples once they were hydrated to obtain activity, while increasing the carbon content of each one increases the overall available surface area, similarly to other reported results[9]. When working with physical mixtures of materials, the general trend of surface area obtained is a linear correlation. However, for the synthesised hybrids, the trend follows instead a curvature peaking at around 70 wt% MWNT. While analysing the materials before and after activation, the trend is maintained in all samples but the one hydrated with a MWNT content of 67wt%. This could indicate that at higher contents of MWNT, the reconstruction process of the LDH material might become more difficult due to the natural framework of the MWNT, reducing accessibility and dispersion along the surface of the solid[23]. Segregation at high values of carbon tubes could intervene with the measurements as well, as the micrographs of this sample show slight separation between both materials more consistently.

In combination with the phase and morphology analysis, the more cohesive interaction between the LDH and the MWNT could be the result of the modification of the platelet distance when both materials are interacting between one another. However, a variation of the packing of each component of the solid may be modifying the area values determined for the hybrids, as MWNT packing is affected by degree of oxidation procedure as well as capillarity effects[23]

79 Acetone Self-Condensation: LDH/MWNT composites activity

4.3.5 CO2-TPD analysis of solid catalysts: Dependence of the availability of basic sites with carbon amount

A general analysis of the strength and concentration of the basic centres in the

† MWNT/LDH catalysts was measured via a CO2-TPD analysis . The data obtained for the desorption is shown in Figure 4.9.

The strength of sites available in base materials is typically measured by analysing the desorption signal at the highest temperature of the analysis, as it indicates the CO2 adsorbed to the material more strongly. First, the as-synthesised LDH material shows a single signal at around 673 K, mainly produced by the decomposition of the carbonates in the structure of the solid. Once the material is hydrated however, there is an increase in temperature at which the signal of desorption appears, nearing 750 K. This would be an expected result as the material is intrinsically more basic once the OH- groups are introduced into the solid. Information in literature indicates that the sites at that desorb at this temperature are the ones related to the activity in the self-condensation of acetone process[12,24,25]. This signal also appears in all four solids synthesised in combination with the MWNT, which would indicate that any activity generated by each would be related to a similar type of site and no modification appears regarding the adsorption. This would be expected when related to the other characterisation work, as evidence exists that the LDH materials used the MWNT as a support to disperse, rather than synergistically interact to increase activity.

When analysing the pure MWNT material, a small desorption is identified in a range of lower temperature, possibly related to the elimination of surface groups containing oxygen from the pre-treatment, such as carboxylic acids, along with traces of physisorbed material. However, this desorption is not as significant as the one measured in the composites. A quantitative measurement of the CO2 desorbed was done and is included in Table 4.6. In general, the amount of CO2 adsorbed is similar for all catalysts, normalised by the amount of LDH, but no particular trend can be identified, within the error of the measurements.

† The CO2-TPD analysis was developed by Dr Diana Iruretagoyena Ferrer from the Chemical Engineering Department at Imperial College London, to complement the research on the project done by the workgroup.

80 Acetone Self-Condensation: LDH/MWNT composites activity

Table 4.6 MWNT/LDH samples synthesised to catalyse the self-condensation of acetone. Data obtained by Dr. Diana Iruretagoyena Ferrer

mmol g -1 mmol g -1 Sample name CO2 cat CO2 LDH adsorption adsorption

LDHhl 1.4 1.4

MWNT/LDHhl-20 1.5 1.8

MWNT/LDHhl-33 1.1 1.6

MWNT/LDHhl-50 1.4 2.4

MWNT/LDHhl-67 0.6 1.9

81 Acetone Self-Condensation: LDH/MWNT composites activity

Figure 4.9 CO2 TPD profiles for pure materials and synthesised compounds. Data obtained by Dr. Diana Iruretagoyena Ferrer

82 Acetone Self-Condensation: LDH/MWNT composites activity

4.4 Catalytic testing of synthesised MWNT/LDH hybrids

In a similar way to the set of results in the reactor testing phase, the conditions and equipment used were selected to study the synthesised MWNT/LDH, considering the following:

• All catalysts were rehydrated to obtain higher activity in comparison to the mixed oxides obtained by calcination. • The temperature for the study was set at 273 K to maximise selectivity towards DAA. • Magnetic stirring is used to keep the mixing profile while aiming to avoid loses of catalyst to the reactor walls as much as possible. • The stirrer speed was set to 1000 rpm to ensure that external mass transfer limitations were avoided.

-1 • The catalyst concentration in the reactor was set to 0.83 gcat L , which allowed to obtain initial rates of reaction similar to the literature. • All reactions were tested in a period of 50 h of reaction time. • A reactor with the same specifications as reactor 2 was used for testing, as rates obtained were similar between reactors 2 and 3‡.

The self-condensation of acetone was carried out over the pure LDH, the MWNT and the synthesised composite materials with varying amounts of carbon, as described in Table 4.5. As expected, selectivity to DAA was >99% for all the catalytic tests except for the pure activated MWNT, where no activity was measured during the reaction period, as found in the reactor testing procedures. The production of DAA against time for the samples is included in Figure 4.10, while initial rates of production of diacetone alcohol (DAA) were calculated in order to compare the catalysts performance. The initial rates obtained along with a relationship with the total amount of CO2 adsorbed are included in Table 4.7. The error of the measurement was estimated by a polynomial fitting of the results.

‡ The reactor along with magnetic stirring was also used by Dr. Almudena Celaya Sanfiz due to availability of equipment in the Chemistry Department.

83 Acetone Self-Condensation: LDH/MWNT composites activity

Figure 4.10 DAA molar concentration as a function of time. Treac= 273 K, stirring speed

= 1000 rpm, Magnetic stirring, NAo= 0.8 mol acetone, mcat = 0.05 g, treac = 50 h. Selected data obtained by Dr. Almudena Celaya-Sanfiz

Table 4.7 Initial rate of reaction of MWNT/LDH samples synthesised to catalyse the self- condensation of acetone. Correlation with amount of CO2 adsorbed

-1 -1 Initial rate Initial rate mmolac mmolCO2ads h Sample name -1 -1 -1 -1 (mmolDAA gcat h ) (mmolDAA gLDH h )

LDHhl 71.1±11 71.1±11 101.6

MWNT/LDHhl-20 162.7±20 196.2±20 218.0

MWNT/LDHhl-33 77.0±3 110.0±3 139.2

MWNT/LDHhl-50 73.8±20 123.2±20 102.2

MWNT/LDHhl-67 3.2±3 9.8±1 10.4

As the initial pre-study research and other works suggested, an increase of initial rate is found when loading the LDH into the nanotubes. With the set of catalysts synthesised for the study in this chapter, the increase nears 3 times of the one obtained for the pure activated LDH material with the catalyst including 20 wt% of MWNT. Particularly, all other samples but the one containing 67 wt% of MWNT increase both initial rate and DAA production as well. As

84 Acetone Self-Condensation: LDH/MWNT composites activity shown in Table 4.7, the initial rates for the supported samples, normalized by the amount of LDH, have a maximum in activity at a loading of 20 wt% of MWNT, to then decrease as higher amounts of nanotubes are introduced. A similar behaviour is found when the rates are normalised by the amount of catalyst, although the differences in this case are not significant.

The result of the tests shows that the amount of DAA produced by the catalysts is in agreement to the initial rate obtained for each one. However, similarly to the studies in reactor 2, a small deactivation effect can be seen at long reaction times, possibly related to small loses of catalyst from the reaction mixture process. Along with it, decreases of rate could be related to adsorption of compounds due to the duration of the reaction and the reaction being inherently slow as well, both to a lower degree.

Interesting to note is the fact that the increment in the rate in the literature was achieved with a more diluted catalyst than the current MWNT/LDH system, 11 vs 80 wt% of LDH respectively, and the current initial rates are lowered at any further contents of MWNT material, suggesting an inhibiting effect at higher loadings. A possible explanation is that carboxylated debris could have remained in the surface of the nanotubes and, although thoroughly washed with dilute base, a more concentrated base solution like the one used during the synthesis of the composites and the reactor pretesting (0.1 vs 4.8 M) could have caused the release of acidic debris impurities[26].

As tested in the initial research, the debris material tends to reduce the activity of the LDH even at low loadings of debris. Higher amounts of carbon would in this case produce further decreases, like the drop of activity in the catalyst sample with 67 wt% of MWNT. It is a reasonable hypothesis that the carboxylated fragments may bind to the basic sites in the LDH, but not be completely removed during thermal activation or hydration. It is also possible that some of the of oxygen-containing groups in the debris could participate in the condensation reaction or other side processes, although no compounds apart from acetone and DAA were found in the reaction mixture after reaction.

It is important to consider; however, that in order to use a high mass to volume ratio material in practical applications, the amount of solid to be used is of great importance, as large quantities make new design or readjustment of current technology a difficult process. The MWNT/LDH-20 catalyst synthesised offers an improvement of absolute activity compared

−1 −1 to both the pure material and the CNF system, 162 vs. 61 mmolDAA gcat h , while introducing a lower quantity of voluminous material.

The relationship of the support of LDH particles along with modifications to the morphological structure have been heavily discussed in literature research. Most studies are

85 Acetone Self-Condensation: LDH/MWNT composites activity in agreement that the Brønsted-base OH sites, associated with the hydration of the LDH interlayer, are the active sites that allow the acetone to self-condensation and produce DAA [10,19]. However, discussions have also suggested that activity is not solely dependent on the availability of these sites and the surface area of the materials but other factors, such as the size and dispersion of the LDH platelets, also modify activity. Adding to this, other research indicates that the majority of the activity is related to the sites located at the edge of this platelets themselves, rather than the whole surface of the materials[12,18,27,28].

Different techniques have been used to modify the number of active sites in the edge of the platelets, the most common being the reduction of the size of the platelets themselves. [5,6,12,28–30]. One method used with particular effectiveness has been the support of the LDH into other materials, like the aforementioned CNFs[6,10], which achieve a reduction of the size of these from the most common 60 nm to a value of 20 nm. Improved catalytic activity of these solids was partially attributed to the reduction of the LDH platelet size, while increased accessibility to the active sites also played a part in the enhancement due to the heavily porous structure of the CNF.

In this study, slightly smaller LDH crystallites sizes were observed when supporting the LDH in MWNT, compared with pure LDH activated material, as Figure 4.6 and 4.7 show. Decrease of these crystallite sizes could be related to the increment of activity in the MWNT/LDH catalysts, as a reduction of this parameter typically results into an increase in the total available surface area of a solid. Amongst the MWNT/LDHhl hybrids, the LDH crystallite thicknesses decrease very slightly when increasing the weight percentage of MWNT.

Nevertheless, the catalytic activity does not follow the same behaviour, as the highest initial rate obtained is achieved with the catalyst with the lowest loading of MWNT, at 20 wt%. This would indicate that other factors might be affecting activity as well. While the presence of MWNTs helps to increase the activity of the pure LDH by nucleating the particles an improving accessibility, the associated debris leftover studied previously, could also be reducing the enhancements at higher MWNT loadings.

A ratio between the initial rate and the number of total basic sites assigned to the desorption peak at nearly 750 K in the CO2-TPD analysis, both normalised by the amount of LDH, was calculated, showing that this relationship also has a maximum at the optimal loading of carbon, 20 wt% MWNT (Table 4.7). It is important to note; however, that the sites considered in this calculation are not only related to the particle size of the LDH, and in turn to the surface area accessible to acetone, but also to the number of defects in the LDH structure, which might also act as basic centres, as suggested in other studies[18,28]. Additionally, the LDH

86 Acetone Self-Condensation: LDH/MWNT composites activity particles tend to disperse more homogeneously in the porous MWNT network, as shown in the electron microscopy, which should improve access to sites as well.

However, each of the measurements of rate do not correlate directly to the to the sites independently measured by CO2-TPD, which might imply either that not all active centres are accessible to the acetone during reaction[5], that the sites measured in the adsorption- desorption cycling are not equally active, or the number of sites quantified is larger than those intervening on the reaction. This correlation of rate and adsorption shows that the catalytic sites that adsorb CO2 are more active for the majority of the MWNT supported samples, but with a systematic decrease with increasing MWNT content. The result is consistent with a direct enhancement of the intrinsic activity of the LDH due to the presence of MWNT as a support, but as previously mentioned, it is also probably affected by the presence of increasing debris content with each sample.

87 Acetone Self-Condensation: LDH/MWNT composites activity

4.5 Concluding Remarks

A set of composite materials with different amounts of commercially-available MWNT and LDH were synthesised via a previously reported methodology to be tested as base catalysts in the self-condensation of acetone. A wide range of parameters were tested to optimise the reaction process via the selection of a specific reactor design to reduce catalyst loss, stirrer type and speed to avoid mass transfer limitations, etc.

These results also provided initial insight of possible increments in activity shown in the initial rate of reaction using MWNT as support of the LDH catalyst material, while also inferring a possible reduction of activity caused by the debris material in the MWNT surface. At this stage, problems related to deactivation due to catalyst contamination via adsorption in the solids, and a general low activity even at long reaction times also became apparent, mainly due to the low temperature required to achieve good selectivity.

An increment of activity was also identified when using a different series of synthesised catalysts with varying degrees of carbon content. The increments in activity were related to both an increased dispersion of the LDH into the porous MWNT surface, along with a reduction of the size of the LDH platelets when loaded in the carbon support, which led to increases in the number of available basic sites for the acetone to interact with the catalyst, according to the XRD, SEM and N2 physisorption analyses.

However, the number of basic sites quantified by CO2-TPD did not correlate with the activity of each catalysts, showing that there is a maximum of activity obtained with a low concentration of carbon in the LDH/MWNT system synthesised. This would imply that the loading of the LDH into the MWNT is detrimental at higher carbon loadings, hindering the reaction by contaminating the active sites during synthesis, activation, and the reaction.

Specifically, this poisoning effect in the LDH material is apparently produced during the composite synthesis by carboxylated debris released from the MWNT. This debris is produced during the oxidation reaction used to solubilise and activate the carbon material before use. The washing treatment applied to the oxidized nanocarbon before catalyst deposition should therefore be designed to reproduce the subsequent synthesis conditions to ensure that all mobile contaminants are removed. Considering this effect, there is an optimum concentration of MWNT that balances the benefits of using the low mass-volume ratio support with the activity of the composite system. In the set of materials tested, the optimum MWNT content was found at 20 wt%, which increases activity 3 times over the pure LDH activated catalyst, as well as an improvement in activity per gram of supported catalyst compared to studies of CNF.

88 Acetone Self-Condensation: LDH/MWNT composites activity

The results obtain show that the MWNT/LDH solid base catalysts may also be useful to use in base-catalysed processes. However, care should be taken when synthesising the composite materials, as further enhancements in activity might be inhibited if the debris material is not eliminated. Also, the selection of different base reaction processes with less requirements to achieve good activity and selectivity is desirable, as the and the materials synthesised MWNT/LDH are not very active under the required conditions to research the self- condensation of acetone process.

89 Acetone Self-Condensation: LDH/MWNT composites activity

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92

Chapter 5

DAA retroaldolization: Relative basic strength comparison of LDH/CNS composites in the gas phase

5.1 Introduction

A series of studies has shown that LDH materials are a useful set of solid bases, which allow to obtain good activity for the self-condensation of acetone in the liquid phase. As introduced on Chapter 4, further increments on activity with these solids can be achieved when loading the LDH particles onto other materials used as supports. Particularly, the combination of LDH and carbon materials, like MWNT, present increased activity as a result of a better dispersion and a modified crystallite size of the LDH particles, allowing the solids to achieve higher rates and slightly better production of DAA.

However, the study of the reaction also showed problems when analysing the relative basic strength of the synthesised solids. Firstly, on the self-condensation process, only the production of the aldol compound is typically considered, as it is the only step that occurs thorough a purely basic mechanism. Consequently, the reaction is normally studied at 273 K to maximise both the equilibrium conversion of the process[1] while also achieving good selectivity towards DAA, as well as reducing the possibility of further condensations from taking place.

Generally, the reduction of temperature hinders the activity of reaction processes, and research has shown that acetone condensation reactions are affected in a similar way[2]. Results in Chapter 4 show that this uncommon but necessary specification results in long and less than desirable reaction times at the conditions used in the process. In addition, traces of adsorption were identified on the surface of the catalysts, leading to deactivation of the material. DAA retroaldolization: LDH/CNS composites activity

In a similar way to reaction activity, adsorption is a process directly related to the temperature of the system. As such, this hindering effect produced while working on the forward condensation of acetone towards DAA, should be directly related to both the temperature selected for the reaction, as well as the duration of the process to a higher degree in comparison to studies at higher temperature.

Other reaction processes, like the self-condensation of acetone, have been used as model reactions to compare the activity of solid base catalysts, which has allowed the identification of differences between a range of solid materials on different applications. Specifically, when focusing on condensation, the study on the interaction of citral, benzaldehyde or furfural with acetone is particularly relevant in the context of biorefining applications[3–5], as each of the aforementioned compounds can produce a range of useful chemicals which could be used in industrial level bio-applications[6].

However, while keeping focus on the acetone-DAA system, an alternative approach used to study solid catalysts which helps to overcome the issues present on the forward reaction has been to work on the reverse condensation, or retroaldolization, of DAA towards acetone. The use of this process has some inherent advantages in comparison, such as allowing to study the effect of activity at a higher temperature and in both the liquid and the gas phase. The retro aldol process tends to be more selective towards the production of mesityl oxide (MO) and isophorone at the typical reaction temperature for the study with LDH, 333 K and above, which favours the elimination of water from the DAA molecule[7,8]. As such, the selection of an optimal reaction temperature is key to avoid further DAA condensation towards the mentioned compounds.

Studies with other solid materials, like oxides such as zirconia, alumina or magnesia in the gas phase have shown that the activity analysis can be carried out at temperature as low as 293 K [9,10], while the nature of the continuous process helps to avoid further condensation of either the acetone or the DAA into higher molecular weight compounds[11]. Further condensation is also hindered by the mechanism of the reverse process, which only occurs via basic sites in comparison to the forward reaction[12].

As such, the retroaldolization reaction is a viable process to classify the basicity of solid materials, as it allows analysing the nature of the active sites of the catalysts under conditions which are not necessarily as specific as the self-condensation of acetone.

This chapter includes initial testing experiments to define the optimal materials along with real time conditions to study the carbon composites in section 5.2. After defining specific conditions for analysis, structural and physical properties of the catalysts were determined by

94 DAA retroaldolization: LDH/CNS composites activity characterisation techniques. The results are included in section 5.3. The influence of the use of carbon nanostructures activated via thermal treatment, comparison between their activity and the degree of deactivation appears are part of section 5.4. A brief analysis of the deactivation of rehydrated samples in the reaction system is included on section 5.5, while some concluding remarks on the study are part of section 5.6.

5.2 Testing with activated LDH: Defining an ideal set of conditions and materials

The retroaldolization of DAA in the gas phase is a simple process which has been used to compare the basic strength of solid catalysts qualitatively[12]. The reaction system shown in Figure 3.2 was used to obtain all the measurements of activity in this study. The system consisted of a continuous tubular jacketed reactor where the temperature was controlled by circulation of fluid. An evaporator-condenser system was used to control the fraction of DAA in the inlet stream by modifying the temperature of the condenser.

A basis of this study is to control the behaviour of the reactor to work in differential mode in order to reduce the dependence of activity through the catalyst bed. Hence, modifications to the weight hourly space velocity (WHSV) were applied via changes to the amount of catalyst loaded, along with the flow of gas of the system. The procedure used on the reaction process appears on section 3.3.2.2.

In a similar way to the studies in Chapter 4, selected pure and supported LDH synthesised catalysts were used to define a good set of operating conditions of the system before studying the synthesised hybrid materials, as their production was extremely time consuming.

5.2.1 Analysing the effect LDH catalyst activation and reactor temperature on the retroaldolization process

The initial conditions used were preselected based on other studies reported in literature[11,12]. Initially, as-synthesised along with both rehydrated and thermal treated activated LDH materials were used to analyse the activity of the reaction system. The procedures followed for activation are described on Chapter 2 and were similar to those used on the catalysts studied on Chapter 3. The results obtained from this reaction process appear in Figure 5.1.

95 DAA retroaldolization: LDH/CNS composites activity

Figure 5.1 DAA conversion as a function of time for different activated LDH materials.

-1 Treac = 313 K, WHSV = 2.21 h , mcat = 0.5 g, treac = 5 h

As expected from the as synthesised LDH material, little to no activity was identified under the reaction conditions selected. In comparison, the activated samples fully convert the DAA flowing through the reactor at the start of the reaction, indicating that both materials have a high concentration of basic sites, allowing to complete the process in a short contact time. At these reaction conditions, it is not possible to discern the difference in activity between either of the activated catalysts.

While aiming to find a proper set of conditions to study the materials, as well as to reduce the time between reactions, the next set of analyses were studied only with the LDH material activated by thermal treatment. A comparison with other methods of activation is included later.

With the aim to decrease the conversion of DAA of the solids, the temperature of the reactor was reduced to a minimum of 303 K, in steps of 5 K at a time. Each test was done independently and further reductions were not considered as cooling of the system would have to be applied, which is not desirable from a practical standpoint, and not the aim of the study.

However, no reduction of the conversion of DAA could be achieved even at 303 K. As the aim of the initial studies was to lower the conversion of the active materials, all further measurements were obtained at 303 K.

96 DAA retroaldolization: LDH/CNS composites activity

5.2.2 Effect of space velocity: adjusting system parameters to reduce conversion and avoid mass transfer limitations

As there was a low impact on activity with the lowering of the temperature of the reaction, changes in space velocity were considered next. The space velocity is a parameter that modifies the general interaction of the reactants with the solid catalyst bed in the continuous flow reactor. As such, not only the conversion of a catalyst is modified, but also the effects of external mass transfer are highly dependent on it.

First, the amount of solid on the catalyst bed was reduced initially to 0.025 g, but the change did not affect the activity of the system. As such, the gas flow of DAA was modified to obtain different values of WHSV. The measurement at 2.21h-1 was repeated by recalculating the required flow to match the space velocity and allow comparison. The tests presented in Figure 5.2 were obtained by modifying the gas flow on the system and reducing the amount of catalyst loaded into the reactor.

Figure 5.2 DAA conversion as a function of time at different WHSV, with modification

of flow. Treac = 303 K, mcat = 0.025 g, treac = 5 h

Figure 5.2 shows a clear decrease on the conversion of DAA to a range nearing 35% once the amount of DAA flowing through the system is increased. However, it is known that in order to analyse the behaviour of the activity of the reactor in differential mode, the conversion of the system should be as low as possible, typically on the range of 5-10 %, to minimise the effect of concentration and temperature changes to the rate. Specifically, the flow

97 DAA retroaldolization: LDH/CNS composites activity used on the measurement at 11.88 h-1 was obtained with the maximum flow capacity available of the MFC installed on the system. As a result, further decrements on activity would have to be related to the amount of catalyst in the reactor.

A test with calcined LDH material was undertaken with a lower amount of catalyst, approximately 15 mg, to further decrease the conversion. Other conditions of the system were maintained at the same level as the measurements done to analyse the effect of flow changes on the system. The activity obtained in these tests is presented on Figure 5.3.

Figure 5.3 DAA conversion as a function of time at different WHSV with modification

of catalyst mass. Treac = 303 K, treac = 5 h

Figure 5.3 clearly presents a further reduction of activity with a conversion of DAA in the range of 7%. Even though the value of conversion was in the desired range for the analysis of activity, further decreases in the catalyst loaded in the reactor were implemented. The results obtained increased the error of the measurements substantially. This was probably due to loss of accuracy on the amount of solid loaded onto the system before reaction, as it was difficult to introduce lower amounts into the reactor.

These tests give a general idea on a good set of conditions to analyse the activity of calcined LDH catalysts. Consequently, a set of pure rehydrated LDH catalysts was synthesised, activated via reconstruction, and analysed at the defined reaction conditions to discern whether the activity could be lowered similarly. The results of the analysis are included in Figure 5.4. The calcined samples are included for comparison purposes.

98 DAA retroaldolization: LDH/CNS composites activity

Figure 5.4 DAA conversion as a function of time at different WHSV of LDH samples

activated by different methods. Treac = 303 K, treac = 5 h

According to Figure 5.4, it is possible to discern between the activity of both catalysts at higher values of space velocity. When comparing both methods of activation, a similar effect as the forward condensation process on the liquid phase is obtained, indicating that once the material is rehydrated, a larger amount of more active sites is generated increasing the activity greatly in comparison to the catalysts solely activated by heat treatment.

Conversely, a clear difference between the conversion profiles of rehydrated and heat- treated samples is present. While the heat-treated samples have a stable and almost constant conversion pattern during the whole reaction process, the rehydrated samples show a deactivation behaviour starting almost at the beginning of the reaction procedure. This is probably related to an adsorption effect on the active sites of the material, as the solid catalysts after reaction present a slight colouring, not unrelated to the results obtained for the catalysts studied on Chapter 3.

The result shows also a problem related to the conversion of DAA obtained with the rehydrated materials, since at the conditions used, the amount converted is increased significantly in comparison with the calcined samples tested previously. As such, the analysis in differential mode would not be possible with rehydrated LDH catalysts.

Attempts to increase the amount of DAA on stream to lower the conversion further were made by modifying the temperature of the condenser, part of the saturation system, while

99 DAA retroaldolization: LDH/CNS composites activity also measuring and taking into consideration possible modifications to the flow and pressure on the system lines. However, no apparent decrease on the values obtained previously could be achieved.

As the original aim of the research in this chapter is to compare the effect of the loading of LDH on carbon supports, rather than the sole quantification of possible enhancements on the catalysts, it was decided to continue the studies with materials activated only by heat- treatment to allow the reactor to approach operation in differential mode as much as possible. It has been shown that the rehydrated materials increase activity more significantly than the calcination procedure; however, under the conditions for the analysis this characteristic is not necessarily desired.

5.3 CNS composites: Synthesis of solid catalysts and characterisation

Once a set of conditions was defined to study the activity of the DAA-acetone system, a range of carbon compositions was selected to study differences between loadings. In the acetone condensation work, the optimal loading was obtained with the sample containing 20 wt% of MWNT, after which further loadings let to a reduction of activity, ranging from 80 – 90% of that of the 20wt% MWNT/LDH sample. The result obtained contrasted with reports in the literature of the same reaction, which reported similar increases on activity with 89 wt% of CNFs[13].

One of the key differences between both studies, apart from the type of carbon material and loading, was the synthesis methodology. While the study presented on Chapter 3 used a modified coprecipitation method to obtain pure LDH materials, the research reported in the literature employed a modified impregnation technique to load the LDH particles onto the CNF support.

As such, in order to identify possible modifications of activity on the LDH-MWNT solid materials, both synthesis methodologies were used to synthesise the catalysts. Small modifications in relation to the referenced work were performed to be able to impregnate the LDH particles into the carbon. The procedure is described in section 3.1.2.

Along with the use of MWNTs for the supporting of LDH particles for catalytic studies, research on the materials has been reported for other applications. Specifically, the use of

MWNT/LDH materials for CO2 adsorption has provided information on the efficiency and availability to use the basic sites. In that research area, carbon species like CNF have been used with varying results[14].

100 DAA retroaldolization: LDH/CNS composites activity

GO hybrids have showed similar adsorption capacity as the MWNT composite materials. This leads to a hypothesis that increased activity due to a similar, higher availability of basic sites would be possible on the GO hybrids in condensation reactions. Moreover, the natural 2D layered structure of GO could present a more compatible interaction between the carbon support and the LDH[15].

Particularly, GO/LDH hybrids have been tested as catalysts in the self-condensation of acetone process in the liquid phase. Results on reaction show that an increased capacity of CO2 and the production of DAA is increased on a similar way to MWNT/LDH materials. This is mainly related to an increased surface area in the solids after calcination, and a low loading of GO had a better effect in these parameters[16]. It is important to note that the study referenced was published while this thesis was in preparation.

5.3.1 Synthesis methodologies and composition on carbon hybrids

The samples synthesised and studied in the gas phase continuous flow retroaldolization of DAA are presented in Table 5.1, along with the Mg/Al ratio calculated for each. All catalysts used in reaction and properties related to activity were measured, unless otherwise noted, on the materials activated by heat treatment only. The activation was done following the procedure on section 3.1.4.A. The character ‘c’ has been added at the end of the name of each sample where required to identify the difference between thermally activated and unsupported materials on the following sections.

101 DAA retroaldolization: LDH/CNS composites activity

Table 5.1 Identifiers, synthesis method and composition of the carbon supported catalysts before thermal activation

Actual Synthesis Mg/Al Carbon Catalyst method Ratio Weight (%)

SynthLDH Precipitation 1.82 -

12.5MWNT/LDH 1.99 12.5

15MWNT/LDH 1.97 15.5 Precipitation 24MWNT/LDH 1.98 24.0

29MWNT/LDH 2.01 29.1

73MWNT/LDH 2.14 73.0 Impregnation 80MWNT/LDH 2.15 80.0

0.5GO/LDH 1.96 0.4

1.5GO/LDH 2.04 1.4 Precipitation 6GO/LDH 2.01 5.8

11GO/LDH 2.05 10.6

In the case of nanotubes, catalysts with 12.5 to 29.1 wt% of carbon were obtained via precipitation, while the impregnation method was used to obtain composites with 73 and 80 wt% respectively. For the GO, the precipitation method was used to obtain materials with 0.4 to 10.6 wt% of GO. The selection of a specific range of carbon amount and the specific synthesis methodology was due to the inherent requirements of each method to successfully produce a composite material. In the case of the GO, reports indicate that higher loadings produce restacking of the GO sheets at the conditions of the synthesis[17]. For both synthesis methodologies, highly concentrated solutions of the metal salts and the precipitating agent are used during synthesis. This limits the applicability of each method to a defined range of carbon material, to some extent.

102 DAA retroaldolization: LDH/CNS composites activity

With the impregnation method, only high loadings of carbon on the catalyst can be used, as the capillary effect which allows the metals into the carbon is available and enhanced only with low volumes of solution. In the case of the precipitation, it is only possible to work with reduced loadings due to the low mass density of the carbons, as at medium and high loadings the heterogeneity of the end products is greatly increased, leading to mixtures of carbon with small traces of LDH. These differences can clearly be seen on a macro level.

Quantification of the metallic species in the catalysts was done via ICP. In general, both the precipitated and the impregnated catalysts have values similar to the aimed metallic composition, which would indicate the loading of the particles onto each carbon structure does not affect the formation of the LDH layers. The amount of remaining Na, used during synthesis as the precipitating agent as NaOH, was measured as less than 0.2 wt% for all the materials.

5.3.2 Thermogravimetric analysis of synthesised composites: General quantification of carbon on CNS/LDH hybrids

The actual carbon weight percentage on each sample was determined by TGA analysis, along with the general pattern of decomposition for a standard as-synthesised hydrotalcite. The mass measurements during the thermal treatment are presented in on Figure 5.5.

The TGA decomposition profile for pure LDH shows two weight loss regions. The first one, appearing in the range between 393 to 473 K, is typically related to the elimination of water in both the interlayer of the LDH as well as the surface of the material. A second loss is located in the range between 473 and 773 K, which appears as the LDH layers of the solids are both dehydroxylated and decarbonated[18]. In comparison with standard Mg-Al LDHs, the carbon containing hybrids show an additional, third weight loss in the range of 673 to 873 K. Authors agree that this loss is related to the oxidation of the CNS[14].

The actual carbon amount on the catalysts was obtained from the mass loss at this final step, while taking into consideration the remnants of both the pure LDH and each CNS. In general, the real amount of carbon present in each solid, calculated from this region, was lower than the nominal values. However, material loss typically occurs during the purification step in the synthesis procedure, with further drops as the carbon loading increases in the solid[14,19]. The actual carbon contents obtained from the TGA for each catalyst appear in Table 5.1.

103 DAA retroaldolization: LDH/CNS composites activity

104 DAA retroaldolization: LDH/CNS composites activity

Figure 5.5 Thermogravimetric analyses of LDH samples and composites. a) Precipitated MWNT b) Impregnated MWNT c) Precipitated GO

5.3.3 Phase composition of solid materials: XRD of CNS/LDH composites

The XRD phase composition patterns of the as-synthesised LDH and hybrid materials as well as the general pattern for each type of hybrid material thermal activation are shown in Figure 5.6.

The precipitated catalysts samples show the phase structure of a LDH with a metallic molar ratio of 2, in accordance to the JPDS 01-070-2151 profile for Mg-Al LDH material. As such, the profile obtained is indexed accordingly. A characteristic peak related to the (002) plane corresponding to pure GO typically appears at 11.3° [20]. However, this is not observed for the present GO hybrid catalysts, due to the low amount of carbon in each sample. For the MWNT containing hybrids, the signal for the (002) plane is located at 26.3°[14] (Figure 5.6b), which appears at carbon contents of 24 wt% and upwards. This can be corroborated by analysing the carbon samples in Figure 5.6d, as the elements related to the LDH material are removed due to the heat treatment process, making the signals of carbon more pronounced. Standard crystal signals for periclase can also be identified in the calcined samples.

105 DAA retroaldolization: LDH/CNS composites activity

106 DAA retroaldolization: LDH/CNS composites activity

Figure 5.6 XRD patterns of CNS/LDH hybrid catalysts. a) Precipitated MWNT b) Impregnated MWNT c) Precipitated GO d) Selected calcined samples

The reflection of the LDH corresponding to (003) appears at 11.7°. It can be noted this signal appears in all but the calcined samples, and the peak width and height is modified in accordance to the amount of LDH on each solid sample. The width of the peak is also directly correlated with the normal length to the plane of a layer in the c-direction, or layer-stacking direction, or to a modification of structural disorder, specifically with the appearance of stacking

107 DAA retroaldolization: LDH/CNS composites activity incoherence between the layers[21]. The values of the length normal to the plane for each catalyst, estimated by the Scherrer equation, are given in Table 5.2

Table 5.2 shows that no significant modification in crystallite size could be identified by adding either species of carbon in comparison to the pure synthesised LDH solid and instead, a minimum size seems to be achieved with each support at specific carbon loadings.

In the case of the MWNT hybrids, where loadings are higher in comparison to the GO composites, the increment of carbon nearing 30 wt% shows that no apparent change occurs with higher amounts of carbon on the solid catalysts, apart from the sample with 24 wt% of nanotubes. The drop for this sample is most likely related to experimental errors in the measurement.

The trend on the results contrasts with that obtained for the samples in Chapter 3, where a general reduction of the c direction length is identified. Values for the size on the impregnated materials were obtained, but the nature of the signal makes the calculation subject to a high error. As such, these values were not included on the table

However, the nature of the solids analysed on Chapter 3 could correlate with possible differences on the catalysts, as the existence of leftover debris from the initial synthesis procedure on the samples could have led to morphology modifications to a lower apparent crystallite size on amounts of carbon on the range studied. While this might not be an inherent factor for the actual modification of crystallite length, the microscopy studies presented in the following section show slight differences between both sets of catalysts. The samples in this chapter were obtained with further purified carbon in comparison to the ones included in Chapter 3. As such, a more heterogenous morphology overall is achieved.

Another possible explanation for the difference between sample sets could be the presence of an actual distribution of particle sizes of varying degrees in the samples of this chapter. Analysis done by XRD would only identify the measurement of the particles of the specific samples during each study, rather than the overall value for the solid. In a similar way to the previous point, microscopy indicates that, at the scale of the analysis, both the MWNT and the GO hybrids present a slightly heterogenous distribution on their surface, as the particles are dispersed unevenly along the carbon solids.

In the case of the GO samples, considering that the amount of carbon present is relatively low in comparison to the MWNT, it could be argued that no significant modifications of particle size would be apparent, or that the variation would be small. However, a high reduction is identified on the sample with 1.5 wt% of carbon, after which the size of the platelets

108 DAA retroaldolization: LDH/CNS composites activity increases. The drop in the size of the crystallites could be related to a higher interaction between the LDH and the voluminous GO support materials once a certain threshold is crossed. This effect seems to be produced only at very low loadings of carbon, where dispersion is increased on the GO surface, after which the supporting effect is not as significant. A similar effect is reported on other works[15,22]

Table 5.2 Crystallite size of uncalcined CNS/LDH hybrid materials

Crystallite Size Catalyst Uncalcined materials (003) (nm)

SynthLDH 26.08

12.5MWNT/LDH 27.81

15MWNT/LDH 23.18

24MWNT/LDH 7.95

29MWNT/LDH 26.08

73MWNT/LDH -

80MWNT/LDH -

0.5GO/LDH 20.86

1.5GO/LDH 11.28

6GO/LDH 26.08

11GO/LDH 29.80

The behaviour itself differs from reports for different applications of GO/LDH composites, where a constant trend of reduction in crystallite size can be identified with increased amounts of carbon support[16,20]. As with the MWNT/LDH materials, a similar heterogeneity is identified on the surface on the GO/LDH solids in the microscopy analysis, raising the possibility of having a wide distribution of particle size instead of a single, individual value for each solid. The position of the (003) plane for LDH also shows that for low values of

109 DAA retroaldolization: LDH/CNS composites activity

GO, the (002) plane for carbon possibly overlaps with the main signal for LDH. It is possible as well that further reduction on the calculated size could be found in the range between 1.5 and 6 wt% for the GO. Further studies would be necessary to identify if this is the case.

5.3.4 Morphology of the solid CNS/LDH catalysts: analysis via microscopy

Analysis by SEM and TEM was undertaken on the synthesised catalysts to analyse possible morphological differences between carbon supports and different loadings. Selected micrographs are presented in Figure 5.7 and 5.8.

Firstly, the micrographs corroborate the formation of the LDH platelet structure in the composite catalysts. The methodology followed during synthesis along with the conditions selected typically produce LDH materials with clusters of crystallite LDH rather than the more coherent layered structure, as generally seen with other synthesis methods[23]. This can be clearly seen for both the synthesised LDH pure and the 15MWNT/LDH samples, shown in Figures 5.7a and 5.7b.

Interestingly, the sections where the MWNT and the LDH particles are located do not appear to follow a continuous or repeated pattern in between them, but they are rather dispersed unevenly throughout the surface seen on the micrographs. In a similar way to the pure material, the particles are mostly incoherent to the whole structure and with small clumps appearing on sections of the carbon nanotubes. This would indicate that, for the most part, the hybrids are not necessarily homogenous throughout the whole surface available on each solid. Figure 5.7c is a good example of this, as it becomes clear that the tubes interact with the LDH on certain sections of the material only. A similar effect is identified for the MWNT/LDH catalysts synthesised via impregnation, where the carbon is loaded with different sections of LDH material dispersed along the surface of the tubes.

For the GO composite materials, as well as pure GO, the sheets cannot be seen in the micrographs, as the method used is not able to discern materials with very small thickness. In this case, the manufacturer specification for the GO used indicates that the sheets are very thin as well as large, being ca. 1 nm and ca. 1-5 µm respectively. For reference for pure GO, see Figure 5.7e. However, elemental analysis on the GO samples via EDS verified the composition of different areas of the materials to distinguish the zones with LDH platelets, oxidised carbon and places where both materials were located. A similar result was obtained for the MWNT/LDH solids.

110 DAA retroaldolization: LDH/CNS composites activity

Figure 5.7 Representative SEM micrographs of hybrid catalysts. a) SynthLDH b) 15MWNT/LDH c)29MWN/LDH d) Impregnated 80MWNT/LDH e) Pure GO

111 DAA retroaldolization: LDH/CNS composites activity

Figure 5.8 Representative TEM images of hybrid catalysts. a) 1.5GO/LDH b)15MWNT/LDH c)15MWNT/LDH (different zone) d)11GO/LDHc

The particle size of the LDH layers ranges from 38 to 44 nm for all synthesised catalysts, according to the measurements from the micrographs obtained. Also, the images show that no apparent nucleation or sintering effect is present on the LDH particles when they are introduced into the carbon, even at increasing carbon loadings. This would indicate that little to no change in the overall size of the platelets occurs due to modification in the structure of the catalyst on the sections analysed, making at least some of the increment in rate or measurements of active basic sites due to increase dispersion of particles, rather than the formation of smaller ones.

For the samples synthesised via the impregnation method, sizes measured from the images are on average 40 nm, which would indicate a similar value to those obtained in the materials with lower contents of carbon.

Generally, these measurements are slightly larger than the values calculated via the Scherrer equation from the XRD measurements. This could be in part to related to the positioning of the layers, as the values measured in the micrographs appear to be related to the lamellar a-direction, rather than the c-direction, or interlayer spacing, in the structure of the LDH[14].

112 DAA retroaldolization: LDH/CNS composites activity

In their calcined state, as in Figure 5.8d, the materials present no apparent increase in the size of their particles, either via nucleation or other effects after the heat treatment, apart from the morphological modification to the Mg-Al mixed oxide species.

5.3.5 Surface area quantification of CNS/LDH materials by N2 adsorption

The textural properties were measured on the solid hybrids after calcination, as the solids were used to catalyse the retroaldolization in their calcined form. The determination of the surface area was done via N2 physisorption and the values were calculated via the via the BET method. The values of area related to the amount of carbon on the samples are included in Figure 5.9, while specific values appear in Appendix C. All the CNS/LDH samples studied show a hysteresis loop assigned to mesoporous materials (type IV isotherm, IUPAC classification).

The unsupported synthesised LDH has a surface area of 137 m2 g-1, which is on par with Mg-Al mixed oxides obtained with similar thermal activation conditions in other studies[24]. This increment in area in comparison to an as-synthesised materials is mainly related to the loss of both the initial anions on the structure, typically carbonates, and the interstitial water residing in between the layers and the octahedra of the crystallites as hydroxyls[25]. For reference, the area of a general unsupported LDH material was measured at 86 m2 g-1.

For the MWNT/LDH solids, the synthesis method used appears to have an impact on the dispersion of the particles on the solid, as the surface area increases with carbon content. This is also identified in some of the micrographs in section 5.3.4. This behaviour is similar to other studies with MWNT, where increments are identified with similar materials[14,19].

113 DAA retroaldolization: LDH/CNS composites activity

Figure 5.9 Hybrid surface area vs carbon amount for thermally activated samples. a) MWNT solids b) GO solids

Generally, the MWNT solids show a linear correlation between the values obtained for both the pure MWNT and LDH materials, but this trend does not continue to the samples obtained via the impregnation method, as seen in Figure 5.9a. It could be argued that this effect is due to the better segregation of the LDH particles on the MWNT in the impregnated materials, as seen on the SEM analysis. This is likely caused by the nature of the method of

114 DAA retroaldolization: LDH/CNS composites activity synthesis, a capillary effect producing the impregnation, combined with the relatively weak size effects observed in XRD.

In comparison to the MWNT materials, the GO samples show a decrease in area with a very low amount of carbon added when comparing to the pure thermally activated LDH. After this, however; increments of area were measured in all proceeding samples, showing a synergistic effect on the composites. The values near 240 m2 g-1 on average with small amounts of GO ranging from 1 to 5 wt % of GO.

It could be assumed that the increments measured are partly due to a better dispersion of the particles of LDH in the surface of GO, as no direct change from particle or crystallite size was found from the XRD and microscopy characterisation on the samples and areas selected.

However, as previously mentioned, a possible explanation on the apparent lack of size change could be the existence of zones with particles of varying size throughout the solid catalysts due to their general heterogeneity, rather than no modification occurring. This would produce increases in area measured on the zones where smaller particles are located, whereas the bulk of higher size particles are mainly identified via XRD and microscopy.

After reaching the maximum near 240 m2 g-1 with approximately 1.5 wt% of GO, a drop is seen at further loadings. The effect in this case should be related to the reduction of synergy between the LDH and the carbon and the rather approximation towards a mechanical mixture between the components related to the nature of the mixing on the coprecipitation methodology, as higher loadings make the values of area approach that of pure GO, measured as 32 m2 g-1.

A possible reason for which the sample with 6 wt% GO has a slightly larger surface area along with a larger particle size might be the state of the LDH solid measured on each analysis. The particle size identified, which helps to define the intrinsic properties of the layered LDH, was measured on the as-synthesised samples, while the surface area values obtained in the analyses were from the thermally activated materials which were used in the reaction.

115 DAA retroaldolization: LDH/CNS composites activity

5.3.6 CO2 adsorption for solid CNS hybrids. General relation and availability of basic sites

In order to correlate the performance of the synthesised catalysts with the general number of basic sites available on each one, CO2 was adsorbed on the solids and the total amount was measured. The method used differs from the one in Chapter 3, as the whole chemically adsorbed-desorbed amount was considered via a gravimetric measurement with a

TGA, instead of following the desorption of CO2 via a continuous analysis of the mass number, which is also a standard methodology also used on similar studies for basic site measurement[26]. This was done for the relatively easiness to analyse the samples as well as the short time required to obtain the measurements, along with the low amounts of sample required for analysis. The procedure used is described in section 3.2.6.

As with surface area analysis, the values were obtained with the materials thermally activated, as the performance on reaction for all solids was compared in that chemical state. The amount was also obtained for the pure materials, LDHc, MWNT and GO for comparison purposes. The measurements obtained are included on Table 5.3

Firstly, both the MWNT and GO achieve little to no adsorption when analysed independently, which indicates that no basic sites with enough strength to adsorb the CO2 are present on both types of carbon supports. This could also be related to the chemical nature of the carbons themselves, as both possess acidic groups on their surface due to their original synthesis or pre-treatment.

A measurement on an uncalcined, pure LDH also confirms the inability of the carbonate-substituted material to adsorb CO2. However, once the LDH is calcined basic sites are generated on the surface of the solid, allowing for the adsorption of the acidic CO2 molecule.

The calcined hybrid materials show a further increase in adsorption in comparison to the pure calcined LDH. The increment on the number of basic sites is likely related to the dispersion of the particles of LDH on the surface of the carbons, which was improved with the addition of the carbon itself, along with a slight heterogeneity in-between them.

116 DAA retroaldolization: LDH/CNS composites activity

Table 5.3 Normalised CO2 adsorption measurements of synthesised LDH/CNS hybrids

Adsorbed CO2 Adsorbed CO2 Catalyst

(umolCO2/gcat) (umolCO2/gLDHc)

SynthLDH 296 296

12.5MWNT/LDH 296 312

15MWNT/LDH 362 428

24MWNT/LDH 332 437

29MWNT/LDH 247 348

73MWNT/LDH 80 296

80MWNT/LDH 43 215

0.5GO/LDH 322 323

1.5GO/LDH 378 388

6GO/LDH 314 323

11GO/LDH 124 178

Pure GO 0 0

Pure MWNT 0 0

This enhancement peaks at around 1.5 wt% for the GO and at 24 wt% for the MWNT, after which both the surface area and the adsorption capacity decrease with further additions,

-1 both towards the values of the pure materials (296 and 0 umolCO2 gLDHc and 136.83 and 32.72

2 -1 m g ). An interesting fact about the results is that the error calculated on the amount of CO2 adsorbed at higher loadings of carbon tends to increase with the carbon content itself, furthering the idea that the materials become more heterogeneous with higher amounts of MWNT or GO. Another possible explanation would be the increased number of acidic particles on the surface of the carbon, which would make the measurement of basic sites deviate from the correct value.

117 DAA retroaldolization: LDH/CNS composites activity

5.4 Catalytic testing of synthesise MWNT/LDH hybrids

As mentioned in the introduction, to analyse and compare the performance of the solid materials as base catalysts, the liquid phase self-condensation of acetone is a common and easy to use methodology. However, only the transformation of acetone into DAA is typically considered[27]. Consequently, the reaction is studied at 273 K to maximise the equilibrium conversion [1], as well as to reduce the possibility of further condensations from taking place. At the same time, these conditions also result in the process taking more time to achieve good conversion and the rates of reaction being typically low. An alternative approach has been to work in the gas phase reverse aldolization of DAA, which does not require similar conditions to produce activity.

The catalytic performance of the LDH/carbon hybrids and the pure materials, all in their calcined state, were evaluated on the retroaldolization of DAA in the gas phase. The reaction conditions were selected for the reactor to operate in differential mode (low conversion, small catalyst bed thickness, etc), as defined by the initial experiment work in section 5.2. The initial rates obtained for the catalysts, as well as the rates obtained once steady state was achieved, along with their degree of deactivation are given in Table 5.4. For analysis and comparison purposes, the conversion of DAA obtained with selected samples appears in Figure 5.10

Firstly, both the pure GO and MWNT materials were inactive when tested at the conditions used, confirming that the mechanism of reaction occurs solely due to the presence of the basic sites, similar to the self-condensation of acetone process. The basic site strength of the hybrids is shown to be an effect of the introduction of the LDH particles, in calcined form, loaded onto each sample in agreement with other reports[19,28]. This result is also consistent with CO2 adsorption measurements, which showed no basicity for either of the pure carbon solids.

118 DAA retroaldolization: LDH/CNS composites activity

Table 5.4 Rate of reaction and deactivation of hybrid CNS/LDH catalysts

Initial [email protected] SS Rate@5h Deactivation at Catalyst -1 -1 * -1 -1 steady state (%) (mmolDAAr gLDH h ) (mmolDAAr gLDH h )°

Synth LDH 9.09 4.76 48

12.5MNT/LDH 26.04 17.85 31

15MWNT/LDH 24.58 18.00 27

24MWNT/LDH 29.73 21.96 26

29MWNT/LDH 24.68 20.47 17

73MWNT/LDH 36.06 22.82 38

80MWNT/LDH 37.01 19.11 49

0.5GO/LDH 16.67 12.53 25

1.5GO/LDH 37.03 30.75 17

6GO/LDH 21.80 14.42 34

11GO/LDH 8.86 6.38 28

Pure GO 0 0 -

Pure MWNT 0 0 -

*Rate calculated at 15 minutes of reaction from a linear polynomial from time of reaction 0-1h °Rate calculated at 5h of reaction from a linear polynomial from time of reaction 4-5h

119 DAA retroaldolization: LDH/CNS composites activity

Figure 5.10 Conversion vs time for the optimal samples of synthesised CNS/LDH

-1 hybrid catalysts. Treac = 303 K, WHSV = 19.80 h ,mcat = 0.015 g, treac = 5 h

On a general basis, one of the main characteristics of solid oxides is their thermal and chemical stability under different reaction conditions. In the case of the mixed Mg-Al oxides obtained via the thermal treatment, the rate of conversion of DAA is reduced almost 50% as steady state behaviour is achieved with the sample. Although a considerable decrease, the initial rate of this sample is very small from the start of the reaction in comparison to all other samples. This loss in activity is also in a similar range to the rehydrated LDH materials studied in section 5.1, but the rate values for the rehydrated samples were higher.

The introduction of the LDH into both the MWNT and the GO results in improvements on the rate of reaction of DAA towards the production of acetone for all the loadings of carbon tested. The effect in both systems is related to an increased availability of basic sites in comparison to the pure activated LDH material, which allows the DAA to be adsorbed and converted, as corroborated via the CO2 adsorption measurements done to the samples.

120 DAA retroaldolization: LDH/CNS composites activity

Figure 5.11 Relationship of Initial rate of reaction vs amount of CO2 adsorbed. a) MWNT/LDH solids b) GO/LDH hybrids

As previously mentioned, this increase on available sites is mostly related to accessibility as a result of obtaining higher surface areas by loading the LDH onto the carbons. In addition, the heterogeneity identified on the materials on the microscopy analysis could imply that an actual reduction of the size of the particles on the solids also could have occurred; however, it is possible that no change could be measured on the characterisation studies due

121 DAA retroaldolization: LDH/CNS composites activity to the specific sections analysed in each solid. The increment in rate is also identified disregarding the synthesis methodology used, however, the trend for each carbon support is not completely similar.

5.4.1 MWNT catalysts

For the MWNT/LDH hybrids obtained via precipitation, the increment in rate reaches a maximum which, on average, is very similar within all the carbon loadings studied. These values are approximately an increase of 3 times in the rate in comparison to the pure thermally activated LDH. The influence of an increased surface area for a better dispersion of particles, as well as a possible crystallite size reduction allows to have a generally better availability of more basic sites for DAA interaction with the catalysts.

However, it is apparent that the actual amount of carbon in the solid is not as significant in comparison to the presence of the carbon itself, as little modification on the rate of reaction, around 10%, can be appreciated between the all the loadings of MWNT used. Figure 5.11a) shows that for the MWNT/LDH solids, an approximate correlation between available basic sites and rate is obtained, rather than a 1:1 relation between both quantities. This could be due to a combination of factors, such as heterogeneity on the solids, lack of coherence between the MWNT and the LDH particles and the presence of different particles size of varying degrees, not directly correlated with the amount of carbon present in each solid.

Interestingly, by extrapolating the values towards the CO2 adsorption axis, the results would indicate that basic sites are present in the solids, but that no reaction would occur. This behaviour, which differs from other studies in literature[28], indicates that a difference is present in the way sites are measured on the methodology used in each study. The gravimetric technique in this work would then either measure the presence of basic sites of different strength which might not intervene on the DAA retroaldol process, or it could also be possible that the treatment before the start of the chemically adsorbed measurement does not remove the physically adsorbed CO2 molecules completely.

Nevertheless, the values of rate follow the trend obtained in the surface area available for each amount of carbon loaded onto the materials, with both the normalised amount of area and rate at their peak for the sample 24MWNT/LDH. This would indicate that the enhancements obtained by loading the LDH particles on the MWNT are at least partially due to an increased dispersion of the basic component on the carbon.

In the case of the impregnated catalysts, the increment in rate is more significant as

-1 -1 the value reaches a high 37 mmolDAA gLDH h , which is on par with the activity of the optimum

122 DAA retroaldolization: LDH/CNS composites activity catalyst synthesised with GO, as shown in the following section. This result is also important, as this activity can be achieved with only a quarter of the amount of active material in the reaction procedure, 98.5 wt% LDH on a 1.5 wt% GO catalyst against 27 and 20 wt% LDH on the 73 and 80 wt% MWNT/LDHc catalysts. Nevertheless, care must be taken when analysing the total activity of the catalysts, since factors like the total volume of the catalyst become much more important when considering the design of a reaction system on a larger scale.

Similar to other literature studies with impregnated materials[13], a better rate of reaction in this case is due to a much higher and better dispersion of the active LDH particles throughout the surface of the nanotubes. The basic principle of the dry impregnation methodology, usage of capillarity to enhance the introduction and better localisation of the solutions to produce the LDH during synthesis, is key to increase interaction and obtain highly active materials in comparison to the coprecipated and pure solid samples. Micrographs from the impregnated catalysts evidence that a further dispersion of the LDH crystallite sections on the surface of the nanotubes is indeed present.

The reduction of activity for the coprecipitated MWNT/LDH solids also tends to be less significant than the pure LDH material after 5h of reaction. The results seem to indicate that this is caused mainly by the LDH component on the hybrids as well, because the composites reduce their activity to a lower degree with increasing carbon content. On other studies, the addition of carbon to composite materials not only increases activity or adsorption capacity, but also stability at long analysis time[13,14]. Studies to analyse the drop of rate were performed on used catalysts and are discussed in the following sections.

5.4.2 GO containing LDH solid catalysts

For the GO solid samples, the reaction rate achieves an increased value of 37 mmolDAA

-1 -1 gLDH h with a loading of approximately 1.5 wt% of GO. In this case, the rate represents a 4- time increase in comparison with the unsupported synthesised LDH, which is the highest for all the synthesised catalysts, apart from the impregnated MWNT sample with 80 wt% of carbon.

Once the loading of GO crosses the 5 wt% mark, the rate of reaction as well as the characteristics which define the activity of the catalysts, surface area, base site concentration, by CO2 adsorption capacity, tend to decrease towards the quantities measured on the

-1 -1 individual components (9 and then 0 mmolDAA gLDH h in the case of the rate for pure LDH and GO). As such, the enhancements occur only with very low carbon loadings.

123 DAA retroaldolization: LDH/CNS composites activity

Figure 5.11b) shows that a practically linear correlation between the rate of reaction and the normalised number of basic sites is obtained, apart from the sample 11GO/LDH, showing that the improvements present with the synthesised catalysts are mostly related to the higher number of sites available on the material to interact with the DAA. The highest relation between rate and sites is the catalyst with 1.5 wt% of GO, after which a subsequent drop is identified showing that possible enhancements are reduced significantly at further loadings, being 6 and 11%. The effect of increased availability at low carbon loadings is similar to other LDH-GO studies, which used the materials solely as adsorbents in research with CO2 capture [15,20].

In a similar way to the MWNT/LDH samples, the pattern of desorption would indicate that no rate would be achieved with sites still present on the material. It could be argued that the measurements in this case could have detected sites of different strength as well, or molecules that were not completely desorbed during pre-treatment before the CO2 adsorption measurements. Other catalytic studies with GO/LDH hybrids show a trend which is similar to this study, but the difference is not as large in comparison[16].

The practical reduction of surface area available for reaction, which follows a similar trend of a drop, then increase and peaking in-between 1.5 and 6 wt% GO to then drop significantly at higher carbon loadings would suggest more clearly that the material suffers from reduction of dispersion on the LDH particles with increasing carbon content. In this case, although not as clearly visible, the microscopy analysis slightly shows a more agglomerated structure of the LDH along the surface of the carbon.

A possible explanation of both the behaviour of the rate and the surface area could also be that acidic groups are located on the surface of the initial structure of the GO, inherent to it before the catalysts are synthesised. This is a basic contrast in comparison to the MWNT, where the groups are introduced during pre-treatment. As such, increasing the GO content should naturally increase the concentration of these groups, possibly hindering the enhancement on rate at higher loadings.

Similarly, the presence of these groups could decrease reactant accessibility at higher loadings of carbon. As results show that the materials become more heterogeneous with increased carbon loadings while employing the precipitation synthesis method, it is also feasible that the interaction of the solids with DAA could be affected.

124 DAA retroaldolization: LDH/CNS composites activity

Figure 5.12 TGA of pure thermally activated LDH catalyst before and after reaction

For the GO/LDH catalysts, a varying degree of deactivation is found in comparison to the MWNT solids. In this case, the most active solid, 1.5 wt% of GO/LDH, is the one that presents the least reduction once steady state is achieved. The drop on rate on this catalyst is also lower than the pure LDH material, and on a similar range to the MWNT solids. This shows that not only an increase activity can be achieved with the use of the carbon as support, but also a certain degree of stability is obtained.

To unuderstand whether part of the deactivation on the catalysts was caused by a reduction of the amount of basic sites available by irreversible adsorption, a pure calcined LDH material was analysed by TGA after reaction to understand if changes of mass were present on the solids. Figure 5.12 presents the results of the analysis before and after reaction.

The measurement shows a continuous mass loss from the start of the analysis, continuing all through the complete range of temperature increase used. Apart from water elimination under 400 K, there is a continuous loss of mass on the range 673–873K.

From literature work as well as the initial analysis on the carbon catalysts, it is reasonable to think that the continuous elimination step in the decomposition pattern indicates the presence of organic carbon compounds on the surface of the catalysts once the reaction finishes. This would in turn produce deactivation at further reaction times. In particular, the catalyst show showed an unusual yellow colouring, which is typically related to adsorption of DAA in the catalyst surface[29].

125 DAA retroaldolization: LDH/CNS composites activity

In the case of the MWNT materials, an increased rate is obtained by using a low amount of LDH material with the impregnation synthesis method, similar to other reports in literature with other condensation reactions. Nevertheless, as mentioned in Chapter 3, the effective use of hybrids in scalable applications should put into consideration the feasibility of using low density materials, as the final volume of the composites to obtain the required rates and activity could be extremely large, making their use not possible from a design or practical standpoint in current equipment in use. Apart from this, with the current synthesis protocol used the time required to obtain the purified MWNT to synthesised the hybrids makes the use of high carbon loadings not ideal as well.

A final topic to consider is the cost of both carbon supports, which are currently high in comparison to the LDH components and other oxides of similar nature, which would make the use of a lower carbon loading better at this time.

In the case of the GO hybrids, the material combination can achieve a similar rate to the best one obtained with the MWNT materials. Furthermore, the ability of the GO to be used without an initial pre-treatment in comparison to the carbon tubes makes the procedure of synthesis of the actual composites more approachable and less time consuming.

As the amount of GO required to obtain good activity was much lower in comparison to the nanotubes, at 1.5 and 80 wt% respectively, the analysis for possible applications of the materials and possible concerns related to the volume of the catalysts required to achieve good rates becomes less important as well. Also, the GO support is a much more volume efficient support in comparison to the MWNT.

126 DAA retroaldolization: LDH/CNS composites activity

5.5 Deactivation measurements on selected rehydrated samples

Once the analysis of the CNS/LDH composites activated by thermal treatment was completed, attempts to examine the degree of deactivation for the rehydrated catalysts were made. To do this, the catalyst sample 1.5GO/LDH along with the pure LDH material were both activated via rehydration following the procedure detailed in section 3.1.4.B and they were used to catalyse the DAA retroaldolization, under the same conditions used on section 5.3. The analysis lasted for a period of 45h and the conversion patterns obtained are included in Figure 5.13

Figure 5.13 shows that, similar to the pre-analyses on section 5.2, the rehydrated LDH sample presents a high initial activity, but it also deactivates at long reaction times. Interestingly, the 1.5GO/LDH sample does not convert as much DAA as the pure material initially, but after crossing the 1 hour mark, the conversion of DAA becomes more stable as time proceeds.

The pure unsupported LDH sample stops converting DAA from a practical standpoint once the reaction time nears 10h. Although the DAA conversion by the GO/LDH hybrid is reduced continuously as the process occurs as well, the decrease of activity is much less significant. The degree of deactivation of this sample matches the one of the unsupported material almost at the end of the reaction testing period, at 45 h. By extrapolating the behaviour of the conversion of this catalyst, it would be feasible that the sample completely stops converting DAA afterwards.

The result at long reaction times, although negative in comparison to the more stable thermally activated hybrids, shows in a clear way that the addition of carbon not only enhances dispersion and possibly increases particle size of the LDH particles, but also increases the stability of the solids when the LDH is deposited on the carbon. A visual analysis on the spent catalysts also showed a yellow colouring after the reaction period was completed, but to a much higher degree in comparison to the catalyst used on section 5.4. This gave a possible insight into the reason for the deactivation of this set of samples.

127 DAA retroaldolization: LDH/CNS composites activity

Figure 5.13 DAA conversion vs time of Rehydrated LDH and 1.50GO/LDH catalysts.

-1 Insert includes conversion until 5h of reaction. Treac = 303 K, WHSV = 19.80 h , mcat =

0.015 g, treac = 45 h

The sample was then tested on a similar way to that of the thermally activated catalysts, in order to corroborate if the behaviour of deactivation was similar. The TGA testing showed a similar mass loss on the range of 673–873K, corresponding to carbon elimination from the solid. Pore blocking as well as irreversible adsorption would be the most likely factor producing the drop on activity. The TGA of this sample appears in Figure 5.14.

128 DAA retroaldolization: LDH/CNS composites activity

Figure 5.14 TGA of as LDH catalyst activated by rehydration before and after 45 h of reaction

5.6 Concluding Remarks

The reverse aldolization of DAA was used to compare the basic characteristics of solid hybrid materials with Mg-Al LDH and either MWNT and GO nanostructures. Initial testing allowed to select optimal conditions and corroborate that the active component in the hybrids is the LDH particles loaded on carbon. Both a thermal treatment as well as a rehydration method were used to activate the solids. However, in order to operate the reaction system in differential mode, the calcined materials were selected to continue the study.

Supporting LDH into each carbon species enhances the properties on the solids, such as available surface area, where the best MWNT/LDH sample presents an increment of over 100% in comparison to the unsupported LDH. For the GO/LDH hybrids, the major increment is of 78%. Characterisation by microscopy indicates that the enhancement is partially produced by a better dispersion of the crystallites of LDH onto the carbon supports, which results in an improved availability of basic sites, as shown by the CO2 adsorption measurements. The nature of the geometry and charge of the GO is probable reason a lower amount of support is needed to maximise activity with the solids.

It could also be argued that, even though not identified by XRD, a particle size distribution with a lower size than the unsupported material might be present on the hybrids. This could be caused by the general heterogeneity identified on the material and, as only

129 DAA retroaldolization: LDH/CNS composites activity specific sections and traces of solid are studied, the localization of these particles might not be completely straightforward. This could also be the reason for the increments in area available with no apparent modification of particle size with changing carbon content on the XRD analysis.

As a result of loading the LDH on low amounts of carbon, a higher rate of conversion of the DAA is achieved, up to a maximum of 4 times over the unsupported material, with either carbon support. The increment appears to be mostly related to the additional surface available on all hybrids. However, further additions of carbon over 5-10 wt% hinders the increase of the rate. The main reason is possibly related to a further development of heterogeneity on the materials, as the particles of LDH tend to get more localised into specific sections of the carbon, reducing availability of basic sites. This effect can be reduced by using the impregnation method where an increase of activity is identified with a very low loading of active phase.

The use of the carbon also increases the stability of the solids, as the materials take a longer time to deactivate in comparison to pure unsupported LDH. Analysis of unsupported LDH after long reaction times showed that carbon adsorption occurs, indicating that a combination of pore blocking and irreversible desorption of the active sites is the possibly reason for the reduction of activity. In specific catalysts, a lower nominal loading of the active LDH material tends to reduce this effect.

130 DAA retroaldolization: LDH/CNS composites activity

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133

Chapter 6

Benzaldehyde-acetone aldol condensation: An analysis of the effect of in-situ rehydration

6.1 Introduction

Current research has shown that using LDH materials as solid bases increases activity forr condensation reactions. In addition, the use of supports such as carbon nanostructures increases the initial activity further. In most cases, this effect is related to a better dispersion of the particles on the surface of the carbons, with a general reduction of their.

These results have been confirmed for two different reaction systems, acetone self- condensation as well as the reverse process of that reaction, retroaldolization of DAA. Similarly, the studies on each reaction have taken place in both the liquid and the gas phase, respectively. For each type of reaction system used, the effect of the catalytic materials has been analysed along with modification of variables such as stirring speed and space velocity, which are directly related to activity of either system. Both processes are useful model reactions when aiming to compare the differences between solid bases, whether related to the type or the amount of carbon nanostructure loaded to obtain activity. Also, self- condensation is an important process used on the production of higher-value chemicals[1].

In order to use LDH materials for any base-catalysed process, an important step to follow before the reaction is an activation procedure, which generates basic sites on the surface of the solids. Generally, the most common methods to do so are either thermal activation, which collapses the layered structure on the solid to obtain a mixture of solid oxides with enhanced basic properties; or rehydration, which allows the LDH material to be partially reconstructed into a layered structure, with hydroxyls as interlamellar anions.

Studies on the rehydration procedure have shown that the materials obtained with this method generally present higher activity than the solids activated by thermal treatment. This is mainly related to the nature and strength of basic sites available on the rehydrated material in comparison to the mixed metal oxides, and just as importantly, to the general incoherence in the layers of the LDH, shown by the modifications to particle size[2,3]. Depending on the Benzaldehyde-Acetone condensation: In-situ activated composite activity nature of the process, the increased activity of rehydrated samples might or might not be beneficial for different aspects, like a better selectivity towards specific compounds, etc.[4]

Even though the rehydration procedure is relatively easy to apply, topics like a generally rapid material deactivation due to carbonate formation during manipulation and transport in the surroundings, the time needed to complete the activation procedure itself, as well as the necessity of using independent vessels for the activation and reaction process, could represent problems for application or other instances.

A relatively easy method to eliminate or reduce these issues is the use of a one-pot method, or in-situ rehydration. In this, both the process of activation and reaction occur at the same time in the reactor vessel. Recently, reports on this kind of process have shown that it is possible to obtain both good activity and selectivity on the production of benzalacetone (BA) with LDH materials, via the aldol condensation between benzaldehyde and acetone (B and A, respectively)[5]. The general schematic of the reaction along with possible by-products is presented in Figure 6.1.

The main parameter studied was the variation of the molar ratio of the components introduced to the reactor: benzaldehyde, acetone and water(W). In this case, results indicated that a good ratio between the reactants, as well as a minimum amount of water is critical to obtain good activity, because these parameters could impact the reconstruction of the LDH from the starting mixed oxides, and end up lowering activity.

Also, the modification of the Mg/Al ratio was studied to analyse modifications of activity of the system, with a ratio of metals of 3 producing an optimum output of conversion of benzaldehyde and production of benzalacetone. The activity for in-situ rehydration increased the conversion of benzaldehyde more than twice compared to a similar activation process done ex-situ methodology, while selectivity towards benzalacetone was increased almost three times.

135 Benzaldehyde-Acetone condensation: In-situ activated composite activity

Table 6.1 Nomenclature used for the compounds in this chapter

Compound Name

Benzaldehyde B

Acetone A

Water W

Benzalacetone BA

2-buten-1-one,1-phenyl P

Figure 6.1 General scheme of the condensation reaction between benzaldehyde and acetone. Isomers of benzalacetone and dibenzalacetone are possible by-products

The published set of results obtained with this methodology show that possible enhancements applied to the activation could increase the activity of the synthesised CNS/LDH hybrids further. The aim of this chapter is then to establish whether a further increment of activity could be achieved by supporting the LDH particles on carbon supports. Specifically, the GO/LDH hybrids are of interest, as they present good activity in the DAA retroaldolization system studied in Chapter 5, while not requiring as long a synthesis time and processing as their MWNT/LDH counterparts. As such, the materials are tested for the benzaldehyde-acetone condensation reaction, following a similar methodology as used in [5].

136 Benzaldehyde-Acetone condensation: In-situ activated composite activity

The chapter includes a comparison between selected synthesised LDH materials tested for reaction, hybrid materials composed by GO and LDH, and a set of characterisation results. Samples used throughout the chapter as well as initial testing results are included in section 6.1. Properties measured for the materials appear in section 6.2, while the results of reaction and discussion on changes pertinent to it are included in section 6.3. A summary of the results is included as part of section 6.4.

A part of the studies developed iin this chapter was developed by the Yan Guo, a student who was part of the research group of Prof. David Chadwick, during the development of her MSc research studies. As such, her contribution will be referenced in the contents of this chapter.

6.2 Benzaldehyde-acetone: sample selection and characterisation

The selection of the aldol condensation between B and A was derived from literature studies about in-situ rehydrated unsupported LDH solids[5], along with the materials used for the research on both the self-condensation and retroaldolization processes, studied in both Chapter 4 and 5. As such, unsupported LDH materials with different Mg/Al molar ratios were selected to be analysed on the reaction. Similarly, the selection of the carbon support was based on the results obtained on the studies of retroaldolization of DAA. The samples tested for the benzaldehyde-acetone reaction are listed in Table 6.2

The reactor system used allowed multiple tests of activity measurement to be undertaken concurrently. Commercial LDH solid (Sigma-Aldrich) was used during initial testing. This material is useful as it shares the same basic characteristics of the synthesised counterparts, while also being easy to obtain in large quantities from the supplier.

Interestingly, elemental analysis of the commercial material revealed that the actual Mg/Al molar ratio on the solid was 2.16, instead of 3 for the standard hydrotalcite material. As such, it is important to consider possible differences when comparing the results obtained with the synthesised catalysts.

137 Benzaldehyde-Acetone condensation: In-situ activated composite activity

Table 6.2 Identifiers of selected LDH catalysts: Parameters for selection and characteristics

Catalyst Material Nominal Mg/Al Actual Mg/Al

ComLDH Commercial LDH 3 2.16

SynthLDH-3 Synthesised LDH 3 2.81

1.5GO/LDH-2 GO/LDH composite 2 2.04

1.5GO/LDH-3 GO/LDH composite 3 -

A set of pure LDH was also synthesised with a Mg/Al ratio of 3, as the report of the study of the condensation between benzaldehyde and acetone indicates this ratio is optimal to obtain a high activity in comparison to materials with other ratios. In this case, the actual ratio on the sample was measured as 2.81. In general, all synthesised materials present a slightly lower value than the nominal amount, as some leaching takes place during the washing step of the synthesis process[6].

In the case of the supported LDH/GO materials, two solids were selected to be used in the reaction. The first one, 1.5GO/LDH-2, was obtained from the catalysts synthesised for the studies on the DAA retroaldolization. Consequently, the full characterisation of the material is included on section 5.2. A solid with the same carbon amount was synthesised, but with a Mg/Al metallic ratio of 3, in order to compare the activity for the reaction. The sample was defined as 1.5GO/LDH-3.

6.3 Activity on the benzaldehyde-acetone condensation: Conditions and comparison of selected LDH and LDH hybrid materials

Initial studies of the benzaldehyde-acetone condensation reaction were done using commercial LDH. As the samples were going to be activated during the reaction process, a thermal treatment, as described on section 3.1.4, was initially done in order to obtain mixed oxides from the solid. A similar procedure was followed for all other LDH samples used in this chapter, whether supported or unsupported.

The reaction system used was a 12-position carrousel system (Radleys, Carrousel 6 Plus, see section 3.3.4.1 for details), which allows to test multiple samples under similar conditions, such as reactor temperature, stirring speed, etc. Each of the 12 reactors on the

138 Benzaldehyde-Acetone condensation: In-situ activated composite activity system allowed up to 5 mL of reaction mixture to be analysed at a time. As such, reactant amounts were modified from the referenced work to be used in reactors of this volume. The proportion between the catalyst and reactants was 1:10. The reduction of the amount of catalyst used was helpful, considering that the synthesis process of the composite materials is time consuming. Depending on the catalyst, between 1 and 4 reactors were used for comparison.

Yan Guo provided the optimal relationship between the amounts of reactants and water to be used during the process. As part of her research, the B:A:W molar ratio was modified to 1:5:1, 1:5:5, 1:5:10, 1:5:15 and 1:5:20, and tested in the reaction process at 348 K with 0.4245 g of commercially available LDH, among other materials. The results indicate that the optimal relation between conversion of B and selectivity towards BA is obtained with the ratio B:A:W of 1:5:10, while also showing that the amount of water necessary in order to reconstruct the layered LDH material and obtain activity is highly important. The values reported for conversion of B and selectivity towards BA are 42 and 32 % respectively[7]. As such, the ratio 1:5:10 was selected to continue with the studies on this chapter. It is important to note that the optimal molar ratio referenced was obtained on a different reactor and with reactant and catalysts amounts different from the ones presented in the current study, but conserving the same reactant ratio.

A visual comparison of the spent unsupported LDH catalysts used during pretesting, against the materials studied in Chapters 4 and 5 showed that a higher degree of adsorption is present due to the benzaldehyde-acetone condensation. Specifically, the LDH materials changed from their common white colouring to a very tinted yellow, which is typical in the production of BA and dibenzalacetone. As such, a process of extraction with solvent was applied to the solid catalysts immediately after reaction, in order to recover adsorbed compounds. All the results presented in this chapter were obtained following this methodology, ensuring that correct mass balances were obtained from the recovered reaction mixtures.

Initial testing was done to define the optimal reaction temperature, as very low values of conversion of B and selectivity towards BA were obtained when working at the conditions presented in the literature work[5]. Also, no conversion was achieved with as-synthesised LDH. Reportedly, 100% conversion of B and a selectivity of BA of 70% could be achieved at 338K. A range of temperatures nearing this value were used to test the in-situ rehydrated commercial LDH catalyst. while other variables were set as: stirring speed = 1000 rpm, mcat =

0.05 g, treac = 3 h. The results of the condensation process are given in Table 6.3.

139 Benzaldehyde-Acetone condensation: In-situ activated composite activity

Table 6.3 Conversion of B and selectivity towards BA: Effect of reaction temperature. In-situ rehydrated commercial LDH. B/A/W molar ratio 1:5:10. Water volume = 1.8 mL, mcat = 0.05 g

Temperature (K) Conversion B (%) BA selectivity (%) P selectivity (%)

338 56 46 53

348 57 51 48

353 52 63 36

First, the results included on Table 6.3 show that it is possible to catalyse the conversion of B and obtain the desired BA product when using thermally activated LDH which reform their layered structure during reaction. Additional analysis via GC-MS allowed to identify the second product of the reaction as 2-buten-1-one,1-phenyl. Traces completed the mass balance. See Appendix A for details.

Analysis of the activity of this catalyst shows that a very small modification occurs to the conversion of benzaldehyde by varying the temperature on the system. At 348K, there is a small increase in conversion and selectivity in comparison to the values at 338 K, but the percentage of each one is 20% higher to those obtained by [7], with nearly half of the catalysts concentration in this study. The effect is more significant when comparing the values on the selectivity of BA, where an increment in the temperature to 353 K enhances the selectivity nearly 30% in comparison to the value obtained at 338 K. It is reasonable to consider that the change of temperature affects the rate of desorption of the product from the sites on the solids, allowing to obtain a similar conversion without producing higher molecular weight compounds by further condensation reactions, or a rearrangement of the structure of the products obtained. With this set of results, 353K was selected to compare the activity of the supported GO/LDH solids.

As mentioned before, activity is reportedly increased with a Mg/Al ratio of 3. As such, when comparing the activity of the composites, it was desired to confirm whether the higher ratio would affect activity for either the supported and unsupported solids. The reaction was tested with each of the solids given in Table 6.2, at a stirring speed = 1000 rpm, mcat = 0.05 g, treac = 3 h and Treac = 353 K. The results from the process, along with a test with a lower amount of the catalyst 1.5GO/LDH-2 are included on Table 6.4.

140 Benzaldehyde-Acetone condensation: In-situ activated composite activity

Table 6.4 Conversion of B and selectivity towards BA: Comparison of unsupported and

supported LDH materials. Treac = 353 K, B/A/W molar ratio 1:5:10. Water volume = 1.8 mL

Conversion B Material Catalyst mass (g) BA selectivity (%) P selectivity (%) (%)

ComLDH 52 63 36

SynthLDH-3 97 71 28 0.050 1.5GO/LDH-3 99 64 35

1.5GO/LDH-2 88 68 31

1.5GO/LDH-2 0.025 29 61 38

Initially, the analysis between the commercial and the synthesised LDH samples shows that a very large increase of activity is identified when using the latter material. Considering that the number of basic sites is proportional to the amount of Mg in the solids, obtaining practically 100% conversion shows that the basicity of the synthesised solid is greatly increased due to the higher Mg/Al ratio. While selectivity towards the production of BA is also enhanced on a similar way, the effect is less significant.

Similar to the study with the unsupported samples, the GO/LDH composite with a ratio of 2 presents both lower conversion and activity in comparison to the material with ratio of 3, further suggesting a higher density of basic sites on the solids with the higher Mg content. However, the difference between each of the materials is not as large as their unsupported counterparts, showing that the introduction of the GO produces an enhancement in the composite solid.

When comparing the conversion obtained with the commercial LDH and the sample 1.5GO/LDH-2, both with a similar Mg/Al ratio, it can be shown that an enhancement is present once the LDH is supported. As shown in previous studies with materials with similar characteristics, particularly as the carbon composite used was the same in Chapter 5, this enhancement is probably related to an increased dispersion of the LDH on the carbon.

Interestingly, the same pattern is not repeated when comparing the samples with Mg/Al ratio nearing 3, SynthLDH-3 and 1.5GO/LDH-3. It is difficult to discern if there could be an actual enhancement on conversion, as both values are practically 100%, but even these small

141 Benzaldehyde-Acetone condensation: In-situ activated composite activity differences could have a heavy effect in the rate of reaction. Further studies with a lower amount of the 1.5GO/LDH-3 catalyst should be developed to understand whether this is the case. Selectivity seems to be affected in some way as well, as the amount produced of BA in comparison to the total amount of B reacted is reduced by 7%. This could be related to a possible limit on the enhancement that the GO provides as the basic strength of the solid loaded on the carbon structure increases, or to experimental error on the measurements.

A comparison of the amount of catalyst mass on reaction with the composite 1.5GO/LDH-2 is included as well. The result indicates that the amount of solid, and as a result the availability of basic sites, directly relates to the transformation of B and production of BA, as conversion (and activity to a much higher degree) is reduced to almost a third of the 88% conversion obtained with 0.05g with the same catalyst.

6.4 Concluding Remarks

The aldol condensation between benzaldehyde and acetone was studied with LDH materials, in both supported and unsupported state. The LDH material included on the composite was activated via rehydration in-situ during the reaction process, which allowed to obtain activity in the reaction, on a similar way to a standard activation done ex-situ.

For the unsupported LDH samples studied, a higher Mg/Al metallic ratio increased the conversion of benzaldehyde, indicating that a higher density of basic sites is present in the material with the increased Mg loading. An enhancement on selectivity towards BA is also identified, but the effect is lower in comparison to the actual conversion.

Activity is also increased by supporting the LDH materials into the carbon support, GO, but the effect is not similar for each ratio of Mg/Al content. For the composite with ratio 2, the conversion of B is increased by 36% in comparison to the unsupported sample. This indicates an improvement in the availability of the basic sites on this composite, which increases the rate of reaction and as a result, the end conversion with the solid supported materials. According to the characterisation obtained for these samples in Chapter 5, the effect should be mainly related to increased dispersion and a possible reduction of particle size on the solids.

However, in the samples with a ratio of 3, the selectivity drops slightly when the composite is used, possibly showing a limit on the enhancement that the carbon could offer when used as support. A possible explanation on the behaviour would be that, as the basicity of the LDH increases due to an increased Mg content, the LDH particles could interact strongly with the carbon support. Considering the natural geometry of the GO and the acidities located

142 Benzaldehyde-Acetone condensation: In-situ activated composite activity on its surface, it would be feasible that the amount of available sites for reaction is decreased. The effect occurring with this metallic ratio would need to be studied further for comparison purposes with different catalysts amounts, similar to the reaction with the materials with Mg/Al ratio of 2.

143 Benzaldehyde-Acetone condensation: In-situ activated composite activity

REFERENCES

[1] A.A. Nikolopoulos, B.W.-L. Jang, J.J. Spivey, Acetone condensation and selective hydrogenation to MIBK on Pd and Pt hydrotalcite-derived MgAl mixed oxide catalysts, Appl. Catal. A Gen. 296 (2005) 128–136. doi:http://dx.doi.org/10.1016/j.apcata.2005.08.022.

[2] J.C. a. a. Roelofs, D.J. Lensveld, A.J. van Dillen, K.P. de Jong, On the Structure of Activated Hydrotalcites as Solid Base Catalysts for Liquid-Phase Aldol Condensation, J. Catal. 203 (2001) 184–191. doi:10.1006/jcat.2001.3295.

[3] R. Chimentao, S. Abello, F. Medina, J. Llorca, J. Sueiras, Y. Cesteros, P. Salagre, Defect-induced strategies for the creation of highly active hydrotalcites in base- catalyzed reactions, J. Catal. 252 (2007) 249–257. doi:10.1016/j.jcat.2007.09.015.

[4] M.J. Climent, A. Corma, S. Iborra, K. Epping, A. Velty, Increasing the basicity and catalytic activity of hydrotalcites by different synthesis procedures, 225 (2004) 316–326. doi:10.1016/j.jcat.2004.04.027.

[5] C. Xu, Y. Gao, X. Liu, R. Xin, Z. Wang, Hydrotalcite reconstructed by in situ rehydration as a highly active solid base catalyst and its application in aldol condensations, RSC Adv. 3 (2013) 793. doi:10.1039/c2ra21762g.

[6] A. Garcia-Gallastegui, D. Iruretagoyena, V. Gouvea, M. Mokhtar, A.M. Asiri, S.N. Basahel, S. a. Al-Thabaiti, A.O. Alyoubi, D. Chadwick, M.S.P. Shaffer, Graphene Oxide as Support for Layered Double Hydroxides: Enhancing the CO 2 Adsorption Capacity, Chem. Mater. 24 (2012) 4531–4539. doi:10.1021/cm3018264.

[7] Y. Guo, Nanoengineered layered double hydroxides on graphene oxide supports and networks reconstructed by in situ rehydration as a highly active solid base catalyst and its application in aldol condensations, Imperial College London, 2015.

144

Chapter 7

Conclusions

Biorefining applications are key promising alternatives towards the production of both fuels and building block compounds. In particular, base catalysed aldol condensation processes are one of the most important reaction mechanisms used in these transformations[1]. In a similar way, the synthesis and optimisation of solid bases still present a great overall challenge. Layered Double Hydroxides (LDHs) are basic solids which have been used with a varying degree of success for diverse transformations of bio-blocks, since they have large surface area in calcined form and are easily regenerable. However, their application is hindered by slightly lower activity in comparison to materials like doped zeolites, etc[2].

Attempts to enhance LDH activity in different processes has ranged from adjustments of the synthesis procedure, modification and reconstruction of the layered structure of the LDH via calcination, rehydration and doping via alkali metal particles, among others. The use of supports, such as multi-walled carbon nanotubes (MWNT) and graphene oxide (GO), has shown it is also possible to obtain LDH with improved properties as composite materials. The enhancements are mostly based on the decrease of particle size and better dispersion, which results in increased surface areas and general stability. The aim of these studies; however, has been mostly on adsorption processes aimed at CO2 capture applications.

In this work, analysis of the activity of novel supported composites, LDH supported in different types of nanostructured carbon, has been made for model condensation reactions, aiming to compare possible differences between loadings. This work represents the first time activity measurements have been investigated on these composites, with only one report being published while this thesis was completed[3]. To do so, physical and chemical properties of the catalysts have been researched via a range of characterisation techniques, with the aim to quantify the modification of the basic properties and relate the changes to the activity measurements on each system studied. The effect of an in-situ activation was studied as well.

The synthesis of MWNT/LDH and GO/LDH composites via coprecipitation of Mg and Al nitrate salts produced materials with good interaction between the positively charged LDH Conclusions layered particles and the negative surface of each carbon species, throughout the complete range of loadings studied. However, microscopy analysis showed that, even though the LDH particles were attached to the surface of each carbon solid, they also tend to agglomerate in different areas of the surface of the carbon. This led to a good dispersion of the active phase of the material overall, but also increased the general heterogeneity throughout the whole surface of the composites, with the effect being more pronounced at increased carbon loadings.

On the self-condensation of acetone in the liquid phase, the oxidised pure MWNTs were inactive, showing that the conversion on the reaction is solely associated with the LDH component in the hybrids. As low loadings of MWNT are introduced and the samples are activated by rehydration, the composites presented enhancements in surface area, related to the dispersion in the more open structure of the carbon, and a partial decrease in particle size, measured by N2 physisorption and XRD respectively. This caused the initial rate to be increased 4 times in comparison with the unsupported solid. However, with loadings over 30 wt%, the enhancements were reduced significantly. One of the factors that restricts the analysis of the hybrids in the liquid phase process is the necessary conditions of study of the reaction. The condensation of acetone is an equilibrium limited transformation, which has the highest production of DAA at 273 K. This leads to a relatively slow process. Another factor to consider is the presence of carboxylated carbonaceous debris in some of the composites. These acidic impurities reduce the activity of the materials, possibly due to lower interaction of the basic sites with the acetone during reaction. At higher carbon loadings, the presence of the debris become more prominent, which hinders activity further.

The diacetone alcohol (DAA) retroaldolization in the gas phase allows higher reaction temperatures to be used. No activity was detected with either carbon species independently, while an increased rate was identified with composites of varying MWNT and GO loading. The composites had to be studied in calcined form to reduce activity. A similar trend of maximum activity and then a reduction occurred, but remarkably, the increase with the GO composites reaches higher activity with only 1.5 wt% of carbon added, in comparison to the 24wt% required for the tubes to reach a slightly lower activity. As characterisation indicates a general similarity between surface area and amount of basic sites available, it is feasible that the difference between the amount of carbon needed to achieve similar activity is related to the compatibility of charge and geometry between the GO and LDH. The use of impregnation in the synthesis of MWNT/LDH composites reduces the issues related to the loading amount and the rate is increased to values comparable to the best GO/LDH composite; however, the main problems in the use of MWNTs, low volume efficiency as well as extremely long pre- treatment process, necessary to functionalise material, are heavily increased. Also, the

146 Conclusions presence of adsorbed compounds was confirmed in spent catalysts via TGA analysis, with increased quantities identified at long reaction times.

Analysis with a rehydrated GO/LDH composite indicated that it is possible to obtain enhanced activity in comparison to the unsupported LDH in the benzaldehyde-acetone condensation process, while activating the LDH in-situ. Studies with different Mg/Al loadings with unsupported LDH showed that a higher content of Mg increases both conversion of benzaldehyde and the selectivity towards benzalacetone. However, the difference is not as siggnificant when comparing the ratio with the GO/LDH composites material, as selectivity is decreased with higher Mg content. Further studies with a lower catalyst amount would give insight of this behaviour.

Overall, the present thesis confirms the enhancement effect that the use of MWNT and GO produces in catalytic activity in comparison to the unsupported LDH, in a range of 3 to 4 times, depending on the reaction process. Similarly, increments in stability during reaction can be achieved by using the carbon nanostructures as supports. Nevertheless, care should be taken while considering the synthesis and application of these composite materials, especially in the case of MWNT, as high amounts are required to obtain the highest activity, while activity could also be hindered severely if debris is not removed completely during pre-treatment. In comparison, GO brings enhancements with lower amounts of material, possibly related to the natural coherence between the materials.

As current environmental requirements continue to intensify, the development and research in bioprocessing and solid bases will also keep increasing in importance. As such, key points will have to be addressed before the LDH/CNS composites could be considered candidates for application. In the case of the MWNT/LDH materials, the optimization of the oxidation procedure necessary for the composite synthesis is the main challenge to overcome, as the time required to complete the process is extremely lengthy, and the acidities present, even with the removal of surface debris, could reduce activity depending on the process.

The commercial manufacture of GO is a relatively new and developing process, where costs are still high in comparison with the established production of carbon nanotubes. As such, even when GO appears to be the optimal support when aiming to achieve increased rates and selectivities in the processes studied, its actual application could become more complex in the near future. Conversely, the current results show that only small quantities of the support are necessary to achieve higher activity. Further research aiming to elucidate the optimal amount in processes directly related to biorefining applications would be critical to continue the composites development towards actual application.

147 Conclusions

REFERENCES

[1] S.N. Naik, V. V. Goud, P.K. Rout, A.K. Dalai, Production of first and second generation biofuels: A comprehensive review, Renew. Sustain. Energy Rev. 14 (2010) 578–597. doi:10.1016/j.rser.2009.10.003.

[2] R. Karinen, K. Vilonen, M. Niemelä, Biorefining : Heterogeneously Catalyzed Reactions of Carbohydrates for the Production of Furfural and Hydroxymethylfurfural, (2011) 1002–1016. doi:10.1002/cssc.201000375.

[3] M.G. Álvarez, D. Tichit, F. Medina, J. Llorca, Role of the synthesis route on the properties of hybrid LDH-graphene as basic catalysts, Appl. Surf. Sci. 396 (2017) 821– 831. doi:10.1016/j.apsusc.2016.11.037.

148 Appendix A

The calibration curves used for the quantification of reactants and products throughout the thesis are included in this appendix.

A.1 Calibration for the study of the self-condensation of acetone in the liquid phase

For the self-condensation of acetone study in Chapter 4, the FID in the GC was calibrated with solutions of known concentration of diacetone alcohol (DAA), as high error was detected when using acetone as the calibration compound, mainly related to volatility. The correlation between the area and mass appears in Figure A.1.

Figure A.1 Correlation between DAA amount and area measured by FID detector. Study of acetone-self condensation

A.2 Calibration for the study of diacetone alcohol retroaldolization in the gas phase

In the case of the retroaldolization of DAA in Chapter 5, a correlation between area and mass of DAA was obtained for the online GC connected to the reaction system, by manually injecting solutions of known concentration of DAA directly into the injection port. This correlation is shown in Figure A.2.

Appendices

Figure A.2 Correlation between DAA amount and area measured by FID detector. Study of retroaldolization of DAA

The reaction system used to measure the activity of the CNS/LDH composites on the retroaldolization was equipped with an automatic two-position injection valve (VICI-VALCO). To quantify the number of moles of DAA reacting in the system, a 0.5 mL injection loop was installed in the valve. By knowing the volume of injection to the GC and the information of Figure A.2, it was possible to determine the number of moles of DAA leaving the reactor at any time.

150 Appendices

A.3 Calibration for the study of in-situ rehydrated composites on the benzaldehyde- acetone condensation

Figure A.3 Correlation between benzaldehyde amount and area measured by FID detector

Figure A.4 Correlation between benzalacetone amount and area measured by FID detector

During the study of the benzaldehyde-acetone condensation in Chapter, 6, two products were mainly identified, benzalacetone (3-buten-2-one,1-phenyl) and 2-buten-1- one,1-phenyl, with traces of a third compound in a couple cases. The calibration for the

151 Appendices benzaldehyde is included in Figure A.3, while the information for benzalacetone is included in Figure A.4.

Similarly, to identify the product 2-buten-1-one,1-phenyl, a qualitative GC-MS analysis was done to some samples. Selected results appear on Figure A.5

Figure A.5 GC-MS Chromatogram of selected sample. The insert indicates the magnetic sector and identification of the compound

152 Appendices Appendix B

The characterisation of the pure synthesised LDH used throughout the initial studies in Chapter 4, section 4.2, is included in this appendix.

Table B.1 Characterisation data of sample unsupported LDH analysed in section 4.2

Crystallite size Surface area (m2 g-1) Property Mg/Al ratio (003) (nm)

Measurement 2.21 17.38 86.02

Figure B.1 TGA analysis of synthesised LDH used on section 4.2

153 Appendices

Figure B.2 XRD analysis of synthesised LDH used on section 4.2

Figure B.1 shows the pattern of decomposition of the LDH as temperature increases. Two principal weight loss events are identified. The first one, appearing in the range between 393 to 473 K, is related to the elimination of water in both the interlayer as well as the surface of the material. A second loss is located in the range between 473 and 773 K, which appears as the LDH layers of the solids are both dehydroxylated and decarbonated[1].

The XRD appearing on Figure B.2 shows the phase composition of the LDH, which is in accordance to the JPDS 01-070-2151 profile for Mg-Al LDH material. As such, the profile obtained is indexed accordingly. The reflection of the LDH corresponding to (003) appears at 11.7°. The estimated value of the length normal to the plane of the LDH, estimated by the Scherrer equation, is given in Table B.1.

154 Appendices Appendix C

Appendix C include information regarding the composites studied on Chapter 5.

Table C.1 BET surface area of CNS/LDH composite materials activated via heat- treatment

Surface area of Catalyst calcined 2 materials(m /gcalc)

SynthLDH 136.83

12.5MWNT/LDH 161.96

15MWNT/LDH 139.21

24MWNT/LDH 168.13

29MWNT/LDH 200.07

73MWNT/LDH 283.09

80MWNT/LDH 243.39

0.5GO/LDH 81.65

1.5GO/LDH 234.45

6GO/LDH 243.19

11GO/LDH 156.38

Pure MWNT 262.83

Pure GO 32.78

155 Appendices

REFERENCES

[1] S. Abelló, F. Medina, D. Tichit, J. Pérez-Ramírez, X. Rodríguez, J.E. Sueiras, P. Salagre, Y. Cesteros, Study of alkaline-doping agents on the performance of reconstructed Mg–Al hydrotalcites in aldol condensations, Appl. Catal. A Gen. 281 (2005) 191–198. doi:10.1016/j.apcata.2004.11.037.

156