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2010-01-01 Oxidation Of Dibenzothiophene To Dibenzothiophene Using Metal Nanoparticles Supported On Silica Karina Castillo University of Texas at El Paso, [email protected]

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Recommended Citation Castillo, Karina, "Oxidation Of Dibenzothiophene To Dibenzothiophene Using Metal Nanoparticles Supported On Silica" (2010). Open Access Theses & Dissertations. 2656. https://digitalcommons.utep.edu/open_etd/2656

This is brought to you for free and open access by DigitalCommons@UTEP. It has been accepted for inclusion in Open Access Theses & Dissertations by an authorized administrator of DigitalCommons@UTEP. For more information, please contact [email protected]. OXIDATION OF DIBENZOTHIOPHENE TO DIBENZOTHIOPHENE

SULFONE USING METAL NANOPARTICLES

SUPPORTED ON SILICA

KARINA CASTILLO Department of Chemistry

APPROVED:

Russell R. Chianelli, Ph.D., Chair

Keith Pannell, Ph.D.

Wen-Yee Lee, Ph.D.

Christian Botez, Ph.D.

Patricia D. Witherspoon, Ph.D. Dean of the Graduate School

Copyright by Karina Castillo 2010

Dedication

I want to dedicate my dissertation to my husband Jason Parsons who has always be there to help me, support me and guide me.

OXIDATION OF DIBENZOTHIOPHENE TO DIBENZOTHIOPHENE

SULFONE USING METAL NANOPARTICLES

SUPPORTED ON SILICA

by

KARINA CASTILLO, MS

DISSERTATION

Presented to the Faculty of the Graduate School of The University of Texas at El Paso in Partial Fulfillment of the Requirements for the Degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry THE UNIVERSITY OF TEXAS AT EL PASO May 2010 Acknowledgements

I want to thank my advisor Dr. Chianelli for all his support and help him has given me throughout these years. I want to thank my committee members: Dr. Lee, Dr. Botez and Dr. Pannell for their time and help. Especially I want to thank Dr. Lee for her help in my dissertation and Dr. Botez for his help in x- ray diffraction. I also want to acknowledge Dr. Arteaga for all the guidance she provided me when I started my studies in chemistry. Dr. Alvarez she gave me the opportunity to be part of the RISE program and I thank her for that. The RISE program gave the necessary foundations to learn research skills.

Additional programs such as MARC and Bridges to the Future provided me also with skills that I used during my master’s and doctorial degrees. I am really thankful to the coordinators of these programs. I also want to thank Dr. Mahmoud for giving me the opportunity to teach at El Paso Community College while pursuing my doctorial degree. This teaching experience helped me academically and financially during my doctorial degree. I want also thank the chemistry secretaries Lucema and Grace for helping me with my paperwork. I want to acknowledge Kristine Gonzalez for proofreading my thesis and distract me by making me adopt cats. I want to acknowledge Renzo Arias and David Chavez for helping with some of the experiments. I also want to thank Brenda Torres for helping me relax by making me go to the yoga and in general for being my friend. I am thankful to Alejandro Fernandez for checking on me throughout these years and making sure I am ok. Lastly I want to thank my mom, sisters, and my little brother for all their emotional support. I want to thank my husband Jason for all the things he has taught me. Without him getting this degree would have been harder and it would have taken a lot longer. Special thanks for financial support to the Welch Foundation, UNEEC Program (NASA) and Stanford Synchrotron

Radiation Lab (SSRL) and Department of Energy (DOE).

v Abstract

Silica and nanoparticles of Pt, Au, and Ag supported on silica were tested for the ability to oxidize dibenzothiophene (DBT) to sulfone. High performance liquid chromatography was used to study the removal of DBT from solution. In addition, X- ray diffraction, infrared spectroscopy and Raman

Spectroscopy were used to characterize the product of the oxidation reaction. Further studies involved the use X-ray absorption spectroscopy to characterize the nanoparticle catalysts before and after the oxidation reaction.

To better understand the reaction, silica was synthesized at different pHs using three different acids. The acids used to synthesize the silica were HCl, HNO3, and H2SO4. The effect of SiO2 calcination temperature on the reaction was studied by drying silica at different temperatures. In addition, the oxidation reactions were performed at different temperatures and in different solvents. The reaction temperature used to test silica ranged between 115oC and 160oC when using decahydronaphthalene

(decalin) as a solvent. Further studies were performed using high boiling point solvents tetrahydronaphthalene (tetralin) and n-dodecane. The percentage removal of DBT was similar when any of the three solvents was used in the reaction. However, when lower boiling point solvents were tested, such as hexane and heptane, no oxidation was observed. Furthermore, oxidation was not observed when the reactions were performed at the boiling point of each tested solvent. Silica synthesized at pH 0 using

HCl, removed up to 70% of dibenzothiophene. However, a 90% removal was observed when commercially available silica was tested in this reaction.

Studies were performed to determine activation energy of the reactions using silica and then using the metal nanoparticles supported on silica. The activation energy was determined by testing each catalyst at three different temperatures. Each reaction was sampled every 30 minutes for up to 2 h. The

Pt/SiO2 catalyst had an activation energy of 20.26 kJ/mol, the Au/SiO2 catalyst an activation energy of

47.04 kJ/mol, and the activation energy of the Ag/SiO2 catalyst was 77.31 kJ/mol. In addition, the vi activation energy of SiO2 catalyst was determined to be 115.56 kJ/mol. The Pt/SiO2 catalyst had the

lowest activation energy of all the studied catalysts. Recycling studies showed the Au/SiO2 catalyst maintained the highest catalytic activity when tested four consecutive times in the reaction.

Infrared spectroscopy, X-ray diffraction and Raman spectroscopy confirmed that dibenzothiophene was oxidized to dibenzothiophene sulfone in the reaction. Furthermore, extended X-ray absorbed fine structure (EXAFS) showed the metal nanoparticles grew after the

reaction. In addition, EXAFS studies also showed the silver catalyst converted to Ag2S after use in the oxidation reaction.

vii Table of Contents

Acknowledgements…………………… ...... ………………………………………………...V

Abstract ...... VI

Table of Contents…………………… ...... ………………………………………………...VIII

List of Tables…………………… ...... ………………………………………………...XI

List of Figures ...... XII

Chapter 1: Background and Significance ...... 1

1.1 Petroleum and its components...... 1

1.1.1 Sulfur compounds in petroleum...... 4

1.1.2 Petroleum Separation...... 5

1.2 Classification parameters of petroleum...... 7

1.3 Hydrodesulurization Process...... 9

Chapter 2: Introduction ...... 11

2.1 Oxidative Desulfurization an alternative method for HDS...... 11

2.2 Metal nanoparticle support ...... 12

2.3 Reducing agents used to reduce metal nanoparticles ...... 13

2.4 General methods to synthesize metal nanoparticles ...... 15

2.5 Synthesis of metal nanoparticles on porous supports ...... 15

Chapter 3: Material and Methods ...... 19

3.1 Dibenzothiophene model reaction ...... 19

3.2 Synthesis of the support (SiO2) ...... 20

3.3 Synthesis of metal nanoparticles on silica ...... 23

viii 3.4 Bimetallic Metal nanoparticles ...... 23

3.5 Catalyst Recycling ...... 23

3.6 X-ray diffraction analyses (XRD) ...... 24

3.7 XAS Studies ...... 24

3.8 XAS Data Analysis ...... 25

3.9 Raman Studies ...... 26

3.10 FTIR Studies ...... 26

3.11 High Performance Liquid Chromatography (HPLC) ...... 26

3.12 Kinetics and Thermodynamics Studies ...... 28

Chapter 4: Results and Discussion ...... 32

4.1 Silica Synthesized at different pH results ...... 32

4.2 Oxidation of DBT using metal nanoparticles supported on silica ...... 38

4.2.1 Silica Activation Energy Results ...... 39

4.2.2 Silica gold nanoparticle activation energy determination ...... 41

4.2.3 Platinum silica activation energy determination ...... 45

4.2.4 Silver silica activation energy determination ...... 48

4.2.5 Palladium silica activation energy determination ...... 50

4.3 X- Ray diffraction results...... 59

4.4 Infrared spectroscopy results ...... 62

4.5 XAS results ...... 64

4.5 Raman spectroscopy results ...... 75

4.6 TEMPO experiment ...... 78

Conclusions ...... 81

ix References ...... 85

Curriculum Vita………...…………………………………………………………………..……93

x List of Tables

Table1: Solubility of oxygen in decalin at different temperatures ...... 36

Table 2: Summary of the different reactions performed at different temperatures, different solvents and different gases atmospheres ...... 38

Table 3: Different palladium gold silica catalysts synthesized and the % removal of DBT...... 52

Table 4: Calculated activation energies of SiO2, Ag/SiO2, Au/SiO2 and Pt/SiO2 ...... 57

Table 5: Metal nanoparticles and coordination number ...... 67

Table 6: RAMAN vibrations found in the DBT, and the Gold-DBT reaction samples as a precipitated crystal and the spent catalysts, the Platinum- DBT reaction samples as a precipitated crystal and the spent catalysts ...... 77

xi List of Figures

Figure 1: Possible structures of the molecules present in asphalthenes ...... 3

Figure 2: Common sulfur containing molecules present in crude oil ...... 5

Figure 3: Oil Refinery distillation tower ...... 7

Figure 4: The structures of the different solvents used in the oxidation reaction are shown A)Decalin, B) Tetralin, C) Hexane, D) Heptane and E) n-dodecane ...... 20

Figure 5: Different silica samples synthesized at different pH and calcinated at different temperatures ... 22

Figure 6: HPLC of continuous injections of DBT solutions at different concentrations ...... 27

Figure 7: Calibration curve obtained using the DBT standard solutions ...... 28

Figure 8: Catalyzed Reaction and uncatalyzed reaction (from left to right) ...... 29

Figure 9: The plot of a first order kinetics reaction and the linear plot obtained by taking the ln of the concentration (from left to right) ...... 30

Figure 10: Plot of the rate constant (k) vs temperature and the linear plot obtained by taking the ln of the k to determine activation energy (from left to right)...... 31

Figure 11: Types of acids used in the synthesis of silica vs the % removal of DBT. Hydrochloric Acid, Nitric Acid, Sulfuric Acid, and commercial silica (from left to right)...... 32

Figure 12: Removal of DBT in ppm using synthesized silica calcinated at different temperatures ...... 33

Figure 13: Removal of DBT in ppm using commercial silica and linear plot obtained when taking the ln of the DBT concentration ...... 34

Figure 14: Ln concentration of DBT vs 1/T when using commercial silica at different temperatures ...... 35

Figure 15: % DBT removal using commercial silica and metal nanoparticles in decalin for 2 h ...... 39

Figure 16: k values at three different temperatures are given by the slopes of each line...... 40

Figure 17: Activation energy of silica is giving by the slope of the line ...... 41

Figure 18: DBT removal using gold supported on silica at three different temperatures ...... 42

Figure 19: k values of each reaction at three different temperatures correspond to the slopes of each line42

xii Figure 20: Activation energy of silica gold is giving by the slope of the line ...... 43

Figure 21: DBT removal when using silica platinum at three different temperatures ...... 45

Figure 22: k values at each temperature correspond to the slope of each line...... 46

Figure 23: Activation energy of silica platinum is giving by the slope of the line...... 46

Figure 24: DBT removal using silver supported on silica follows first order kinetics ...... 48

Figure 25: k values of silica silver at each temperature correspond to the slopes of the lines ...... 49

Figure 26: Linear plot of lnk vs 1/TR to determine the activation energy of silver (activation energy is given by the slope)...... 49

Figure 27: DBT concentration when using palladium supported on silica at two different temperatures 120oC and 160oC ...... 51

Figure 28: Radial distribution of s,p, and d electrons ...... 54

Figure 29: Recyclability studies using the different catalysts ...... 59

Figure 30: X-ray diffraction of DBT-sulfone standard and DBT after reacted with the catalyst (from top to bottom) ...... 60

Figure 31: DBT-sulfone obtained experimentally and DBT sulfone simulated. The assignments of the planes hkl are shown ...... 61

Figure 32: Far IR of DBT after reaction with silica gel grade22 at 140oC (A). DBT after reaction lab synthesized silica at pH 1 (B). DBT sulfone standard (C). DBT standard (D)...... 62

Figure 33: Mid-IR of DBT after reaction with silica gel grade22 at 140oC (A). DBT after reaction lab synthesized silica at pH 1 (B). DBT sulfone standard (C). DBT standard (D)...... 62

Figure 34: Au LIII XANES of gold foil, gold nanoparticles before the reaction with DBT, and gold nanoparticles after the reaction with the DBT take from 11.90 to 11.99 keV. The insert shows the expanded view of the edge features from 11.921 to 11.90 keV...... 65

Figure 35: Au LIII EXAFS of gold foil, gold nanoparticles before the reaction with DBT, and gold nanoparticles after the reaction with the DBT. The insert shows the expanded view of the Fourier transformed EXAFS...... 66

Figure 36: Pt LIII XANES spectra taken from 11.525-11.565 keV of the platinum nanoparticles supported on silica before the reaction with DBT, nanoparticles supported on silica after the reaction with DBT, and a Pt(0) metal foil for reference. The insert shows an expanded version of the XANES region...... 69

xiii Figure 37: Pt LIII EXAFS spectra taken of the platinum nanoparticles supported on silica before the reaction with DBT, nanoparticles supported on silica after the reaction with DBT, and a Pt(0) metal foil for reference. The insert shows an expanded version of the first shell Fourier transformed EXAFS...... 70

Figure 38: XANES spectra of silver nanoparticles before and after use in the oxidation of DBT to DBT Sulfone and the XANES spectra of the bulk silver standard...... 73

Figure 39: A. Fourier transformed EXAFS and the Fourier Transform of the fitting of the EXAFS from 2.0 to 12.2 Å-1 of the silver nanoparticles before use in the reaction. B. Fourier transformed EXAFS and the Fourier Transform of the fitting of the EXAFS from 2.0 to 12.2 Å-1 of the silver nanoparticles after use in the reaction. C. Fourier transformed EXAFS and the Fourier Transform of the fitting of the EXAFS from 2.0 to 12.2 Å-1 of bulk silver standard...... 74

Figure 40: A. Raman Spectra of DBT Sulfone and DBT taken from 160 cm-1 to 1200 cm-1. B. Top is the Raman spectra of the precipitated crystals from the reaction of gold nanoparticles with DBT and bottom is the Raman Spectra of the spent catalysts. C. Top is the Raman spectra of the precipitated crystals from the reaction of platinum nanoparticles with DBT and bottom is the Raman Spectra of the spent catalysts. ... D. Top is the Raman spectra of the precipitated crystals from the reaction of silver nanoparticles with DBT and bottom is the Raman Spectra of the spent catalysts...... 75

Figure 41: TEMPO (2,2,6,6, tetramethylpiperidine-1-oxyl) molecule structure...... 79

Figure 42: Oxidation of DBT to DBT sulfone ...... 80

xiv Chapter 1: Background and Significance

1.1 PETROLEUM AND ITS COMPONENTS

According to the most widely accepted theory, oil is composed of compressed

hydrocarbons. It was formed millions of years ago in a process that began when aquatic plant

and animal remains were covered by layers of sediment1. Different mixtures of plant and animal remains as well as pressure, heat, and time, have caused hydrocarbons to appear today in a variety of forms such as crude oil, natural gas, and coal.

The largest challenge for the future is to meet energy consumption demands. The U.S. alone consumes 130 billion gallons (almost 500 billion liters) of gasoline per year2. There is a

need to increase the amount of refined hydrocarbons or to develop new technologies. Petroleum

encompasses liquid hydrocarbons referred to as crude oil, natural gas and solid hydrocarbons

such as tars3. The composition of petroleum can be distilled out to obtain different fractions for

various applications. The United States has approximately three hundred refineries that produce

from 40,000 to 365,000 barrels of oil a day4. The main elements present in crude oils are:

™ Carbon - 84% ™ Hydrogen - 14% ™ Sulfur - 1 to 3% (hydrogen sulfide, sulfides, disulfides, elemental sulfur) ™ Nitrogen - less than 1% (basic compounds with amine groups) ™ Oxygen - less than 1% (found in organic compounds such as carbon dioxide, phenols, ketones, carboxylic acids) ™ Metals - less than 1% (nickel, iron, vanadium, copper, arsenic) ™ Salts - less than 1% (sodium chloride, magnesium chloride, calcium chloride). Crude oils contain light fractions such as hydrocarbons with short alkane chains. Light

fractions include hydrocarbons with a carbon chain length up to seven carbon atoms. Crude oils

also contain complex structures such as asphalthenes. Asphalthenes are formed by molecules

1 with a polyaromatic structure containing alkyl chains as well as oxygen, nitrogen, and sulfur5,6.

They can be separated by the addition of low molecular weight alkanes. Figure 1 shows possible asphalthene molecule structures.

2 Figure 1: Possible structures of the molecules present in asphalthenes

3 1.1.1 Sulfur compounds in petroleum

Crude oils contain different amounts of sulfur depending upon the region they are

produced. For example, some crudes from Mesopotamia have from 7% to 8% sulfur7. In the oils from the Americas the richest in sulfur are those from the oil fields of California, Ohio,

Canada and Mexico; where the oil may contain as much as 3% to 4% sulfur7.

Elemental sulfur and a few types of combined sulfur have been identified in petroleum

and its products8. The compounds found include hydrogen sulfide, carbon disulfide, mercaptans,

thioalcohols, thioethers, thiophens, organic disulfides and polysulfides, and organic sulfates. In

oil refining, sulfur compounds may be removed or converted into other sulfur compounds which

are considered less harmful. During refining of fuel oils, such as gasoline, the sulfur is unwanted

because of its corrosiveness to metals9. In addition, all of the sulfur, whether elemental or combined, is converted to sulfur dioxide during the combustion process.

Through time, research has been performed to determine the maximum amount of sulfur permissible in gasoline without causing serious corrosion. The tendency of the sulfur dioxide to

collect in the lubricant, where it combines with traces of moisture to form sulfurous acid, attacks

the bearing surfaces. It was found that in motors manufactured during 1942 gasoline containing

0.46% sulfur produced very corrosive conditions, whereas gasoline with .15% a slight but

appreciable corrosion was observed; however, gasoline with .04% sulfur was harmless9. From

the research that was conducted in 1942, the maximum limit of sulfur content in gasoline was

about .10% which was required by governmental specifications as well as a safe allowance at the

time. Figure 2 shows the most common sulfur containing molecules present in crude oil.

4 H H H H

H H

S H H3C

H H H H

H H

S H H

Figure 2: Common sulfur containing molecules present in crude oil

However, recent studies show that sulfur in crude oils is a source of pollution starting at

10 the crude oil production with emissions of H2S to its end use where it is converted into SO2 .

SO2 emissions react with moisture of the environment causing acid rain. It has also been found that sulfur compounds poison catalytic converters in automobiles11.

1.1.2 Petroleum separation

Once the crude oil is obtained from the ground, it is separated into different fractions.

The most common way to separate crude oil into its components is by using fractional distillation12 (Figure. 3). It consists in heating petroleum to change it into a gas. The gases are

passed through a distillation column which becomes cooler as the height increases. The volatile

fraction, which is about ¾ of the whole, is fractionally distilled giving as a result the following

5 products: hydrocarbons (such as methane, which is the one with the lowest boiling point),

followed by propane and then butane. Then higher boiling petroleum fractions produced include

light gasoline, heavy gasoline, kerosene, light gas oil, coke, fuel oil, and lubricant oil13.

Although all fractions of petroleum are usable, the greatest demand is for gasoline. One barrel of crude petroleum contains about 30-40% gasoline14. Transportation demands require that over

50% of the crude oil be converted into gasoline14. To meet this demand some petroleum fractions

which are heavy oil (crude oil) must be converted to gasoline. Fluid catalytic cracking (FCC), or

"cat cracking," is the basic gasoline-making process. Using intense heat (about 1,000 degrees

Fahrenheit), low pressure and a powdered catalyst, the cat cracker can convert most relatively

heavy fractions into smaller gasoline molecules15. However, the atmospheric residues or the

vacuum distillates, which constitute FCC feedstocks, contain significant amounts of sulfur,

which is generally .5-1.5wt%.. Consequently FCC gasoline, which represents 30-40% of the

total gasoline pool is by far the most important sulfur contributor in gasoline up to 85-95%15.

This energy intensive process is carried out to meet current energy consumption demands.

6

Figure 3: Oil Refinery distillation tower

1.2 CLASSIFICATION PARAMETERS OF PETROLEUM

Crude oil is classified according to three main parameters: the geographical region where

it is produced, the American Petroleum Institute (API), and sulfur content. Brent Crude is

obtained from the North Sea region. The name “Brent” comes from the naming policy of Shell

UK Exploration and Production operating on behalf of Exxon and Shell which originally named all of its fields after birds such as the Brent Goose16. It is used to price two thirds of the world’s

internationally traded crude oil supplies. Dubai Crude is a light sour crude oil extracted from the

Persian Gulf region17. West Texas Intermediate (WTI), also known as Texas Light Sweet Crude,

is a type of crude oil extracted from Texas region.

7 The second parameter to classify a crude oil is based on the American Petroleum Institute gravity, or API gravity18. If the value of the API gravity is greater than 10, the crude floats on water. If the value of the API gravity is less than 10, it sinks in water. API gravity is measured using a hydrometer instrument.

Crude oil is classified as light, medium or heavy, according to its measured API gravity:

• Light crude oil is defined as having an API gravity higher than 31.1 °API

• Medium oil is defined as having an API gravity between 22.3 °API and 31.1 °API

• Heavy oil is defined as having an API gravity below 22.3 °API.

The third parameter to classify crude oil is sulfur content. According to the sulfur content crudes are sweet or sour crude7. Sweet oil has less than 0.5% sulfur content by weight. Brent crude and West Texas Intermediate crudes are considered sweet crudes due to the low percentage of sulfur present. The other type of crude is sour crude. Sour crude has more than 0.5% sulfur content. Sour crude is commonly found in Venezuela, Canada and some regions in the US.

The use of sour crude oils is becoming more popular than in the past, largely because fields with higher quality varieties were the first to be developed. Refiners’ preference for quality crudes has led to the depletion of those reserves over the past 100 years and reduced the amount of the sweet crude that remains. According to the Energy Information Administration (EIA), the

US consumes 20,680,000 barrels of oil per day. The available amount of sweet oil is not enough to meet these consumption demands. In order to meet these energy consumption demands, it is necessary to be able to process sour crude. One of the major problems in processing sour crude is the presence of sulfur.

8 1.3 HYDRODESULFURIZATION (HDS) PROCESS TO REMOVE SULFUR

Sulfur is unwanted in oil for three main reasons. First, when fuel is combusted, sulfur is

released as SO2. Then it combines with the oxygen and moisture of the atmosphere leading to

the formation of acid rain. Acid rain causes environmental and health problems. Second, sulfur

poisons catalytic converters in automobiles. Third, sulfur regulations are getting stricter each

year. The Environmental Protection Agency (EPA) has revised the allowable amount of sulfur

in gasoline from 500 ppm to 50 ppm. Oil refineries must now meet this requirement when

processing crude oil. By 2014, the allowable sulfur content in diesel fuel will be below 15 ppm19. To meet these specifications at the pump, refiners must produce diesel containing one-

half that amount of sulfur before it enters the distribution system, because the low-sulfur product is expected to pick up trace amounts of sulfur as it moves through pipelines and other distribution channels. The widely used process to remove sulfur from crude oil is

hydrodesulfurization (HDS). This process requires a transition metal sulfide as the catalyst, high temperature (350oC) and high hydrogen pressure (3MPa)20,21.

From an economic and environmental point of view, it would be more suitable if low-

temperature and low-pressure systems could be developed to remove sulfur from fuels. Higher

temperature and pressure processes decrease the lifetime of the catalyst and it involves higher H2

consumption which results in higher costs and the byproduct of HDS, H2S is an environmental contaminant. Another important reason to find an alternative method to remove sulfur is that substituted sulfur compounds such as methyl, ethyl, 4,6 dimethyl dibenzothiophene and other sulfur compounds are non-reactive in the hydrodesulfurization process20,22 in conventional

hydrotreating, sulfur removal proceeds through direct desulfurization for low boiling sulfur

compounds. With increased complexity of sulfur compounds, the hydrodesulfurization follows

9 hydrogenolysis of substituted benzothiophenes. The DMDBT (dimethyl dibenzothiophene) is a

typical complex molecule, with its sulfur atom having limited access to active sites on catalyst

due to steric hindrance by the methyl substituents. When alkyl groups are added on the 4th and 6th positions of the sulfur compound, the angle of approach to an active site is restricted.

Hydrogenolysis leads to increased hydrogen consumption and makes the process more energy intensive. Severe operating conditions result in higher levels of desulfurization but also lead to undesirable side reactions. High operating temperatures favor increased coke formation and accelerated catalyst deactivation. There is a general interest in alternative technologies such as adsorptive desulfurization, bio-desulfurization, and oxidative desulfurization. In the oxidative desulfurization (ODS) process, the divalent sulfur of dibenzothiophenes (DBTs) is oxidized by the electrophilic addition of oxygen from a suitable oxidant in the presence of a catalyst to the hexavalent sulfur of sulfones. Subsequently, these sulfones are removed either by adsorption or extraction23.

10 Chapter 2: Introduction

2.1 OXIDATIVE DESULFURIZATION AND ALTERNATIVE PROCESS FOR HDS

An alternative method to HDS is oxidative desulfurization. Oxidative desulfurization utilizes a catalyst to oxidize sulfur containing molecules to their respective sulfones. Sulfones are insoluble in non-polar solvents facilitating their removal by simple filtration.

Mo/Al2O3 catalyst has been studied in conjunction with hydrogen peroxide as the

23 oxidizing agent . Another example is the Ti3(PW12O40)4 catalyst which also requires the use

24 H2O2 as an oxidizing agent . In addition, studies using a decalin solution of sulfur compounds and tert-buthyl hydroperoxide (TBHP) have showed oxidation of the sulfur compounds in the presence of a molybdenum oxide catalyst25. Potassium superoxide has been shown to remove sulfur at greater than 90% and as high as 99% with samples of benzothiophene, dibenzothiophene, and a number of selected diesel oil samples26.

The advantage of using these catalysts is that sulfur can be removed at relatively low temperatures and atmospheric pressure. The major drawback to ODS is that current catalytic systems require the use of an oxidizing agent such as H2O2. The constant use of strong oxidizing agents such as hydrogen peroxide can become expensive and it makes these catalysts cost prohibitive to be used on a large scale. In addition, peroxide is a dangerous material to be used on a large scale.

Attempts have already been made to directly generate H2O2 in situ from H2+O2, to replace the current process since it is the most atom-efficient method to synthesize H2O2. Metals such as platinum, palladium, and gold, supported on titanosilicate or other porous materials have

27 been explored as catalysts for this reaction . The strategy is to synthesize H2O2 on the noble-

11 metal in situ and use it in oxidative reaction on titanosilicate. The main problem in this process

28 is how to synthesize H2O2 efficiently, rendering the process economically viable .

Metal nanoparticles are being investigated in oxidation reactions. It is believed in-situ production of H2O2 is possible by using metal nanoparticles and hydrogen and oxygen as the

reactants. For example, the most common oxidation reaction in which metal nanoparticles are

used is in the oxidation reaction of CO to CO2. Silver nanoparticles supported on mesostructured

silica are able to oxidize CO to CO2 at room temperature and with a higher percentage than when

either Au or Pt nanoparticles are used29. Ru/Pt core/shell nanoparticles have also been studied in

the oxidation of CO. It has been suggested that the enhanced catalytic activity for the core-shell

Ru/Pt nanoparticle can be obtained from the combination of an increased availability of CO-free

Pt surface sites on the Ru/Pt nanoparticles29. It also has been reported that gold nanoparticles

catalyze the oxidation of 4-(dimethylamino) benzaldehyde to 4-(dimethylamino) benzoic acid30.

In addition, it has also been reported that palladium nanoparticles have been used in the oxidation of formic acid31. Studies have shown that the catalytic activity of these metal

nanoparticles depends on the amount or percentage of the metal used, the size, the type of the

support and the reducing agent used. Ultimately, the method of synthesis is what determines

each of these characteristics.

2.2 METAL NANOPARTICEL SUPPORT

The support used for the metal nanoparticles plays a crucial role in the catalytic activity.

There are many different supports; however, the most common supports used are acidic oxides

such as are alumina, silica, ceria, and titania. The support for the catalyst is chosen according to

the reaction that is being catalyzed. Metal nanoparticles have been supported on various oxides

such as TiO2, Al2O3, Fe2O3, CoO4, NiO, SiO2, ZrO2, or CeO2. The support stabilizes and

12 disperses the metal particles, but also ensures an adequate metal-support interaction. The

chemical composition and physical state can strongly influence the activity of the resulting

catalyst. For example, in the oxidation of CO two catalysts have been tested Au/Mg(OH)2 and

Au/MgO32. These catalysts exhibit high activity for low temperature CO oxidation even at -89

°C. In this study where gold is supported on MgO and Mg(OH)2, the high activity of the support

to supply reactive oxygen is suggested as the origin of this high activity, and the depletion of

these oxygen species with increasing temperature causes the negative apparent activation energy

at intermediate temperatures. However, the exact nature of the oxygen species remains still

elusive, and further experiments are in progress32.

2.3 REDUCING AGENTS USED TO PRODUCE METAL NANOPARTICELS

There is some controversy regarding the catalytic capabilities of metal nanoparticles such

as gold, which was thought to be inert. Some hypotheses relate the catalytic activity of gold to

the reduced metal ions or the metal on the zero valence33. The reduced metal ions provide a

surface for a reaction to occur. Other hypothesis relates the catalytic activity of gold to the

oxidized gold ions on the surface. The metal ions are reduced; however, the outermost layer on

the nanoparticle may be oxidized due to the exposure to air34. Also it has been suggested the

gold catalytic activity is due to the ratio of the reduced to the unreduced gold ions on the surface

of the nanoparticle35. Different studies on gold nanoparticles have shown the reduced gold is a

key factor in its catalytic capabilities33. Different reducing agents have been used such as

sodium citrate, sodium borohydride and gases such as hydrogen. When gold nanoparticles are

synthesized in silica matrices, the gold is reduced by using hydrogen. It has been shown the

reducing agent affects the size and morphology of the nanoparticles.

13 Stabilizing agents are required to prevent agglomeration or further growth of the particles, and some reducing agents (e.g., sodium citrate, block copolymers) also act as stabilizers. Because of its simple preparation procedure, the classical citrate reduction of a gold precursor with Na3C6H5O7 in aqueous solution near the boiling point remains one of the most reliable pathways of monodispersed gold colloids36. A multistep mechanism of particle formation has been proposed. First the complete amount of Au (III) is quickly reduced to Au

(II). Then it slowly reduces to Au (0) forming clusters. Followed by their assembly into larger polycrystalline particles and completed by further aggregation37.

− TEM and UV−vis-based study suggested that the initial fast reduction of AuCl4 is followed by formation of clusters up to a mean radius of 2.5 nm38. Then they assemble into chains and networks of approximately 4 nm crystalline particles interconnected by amorphous gold. Those chains increase in diameter to finally collapse and cleave while at the same time individual particles grow and reach a final size of about 7.5 nm radius. Sodium citrate is used to reduce the metal ions39. It also serves as a stabilizer allowing the dispersion of the nanoparticles onto the surface of the support. This method yields sizes between 5 and 7 nm.

Sodium borohydride is another common reducing agent used in the synthesis of nanoparticles. Sodium borohydride is sometimes used in addition to sodium citrate. In this type of synthesis the role of the sodium citrate is to stabilize the particles by capping them and the role of the sodium borohydride is to reduce the metal ions. Using capping agents yields nanoparticles with a diameter range from 5 to 40 nm. It was shown the advantage of capping the nanoparticles is that secondary nucleation is avoid40.

14 2.4 GENERAL METHODS TO SYNTHESIZE METAL NANOPARTICLES

There are different ways to synthesize metal nanoparticles. Methods such as deposition, co-precipitation and impregnation are the most common ones to synthesize metal nanoparticles.

The catalytic activities of nanoparticles are strongly dependent on the size of the metal nanoparticles. Many catalytic reactions require the homogeneous distribution of small metal nanoparticles with diameters between 2 and 5 nm. In order to achieve the controlled synthesis of dispersed gold nanoparticles in the narrow size range, several methods have been developed.

These methodologies include co precipitation from an aqueous solution of hydrogen

tetrachloroaurate (III) or any other salt precursor can be used depending on the metal

nanoparticle of interest41. In the co precipitation method the salt precursor of the support and the salt precursor of the metal are mixed together and reacted. Then the solution is centrifuged and the precipitate is washed and calcinated. Another method of synthesis of gold nanoparticles is the deposition–precipitation method41. In the deposition-precipitation method, the salt precursor

of the support is dissolved in water usually a nitrate salt is used as the precursor of the support.

Then the oxide of the support is precipitated by adding a base usually NaOH or NH3. Once the precipitate forms, it is washed and calcinated. Then the support is resuspended in water and the salt precursor of the gold or the salt of any metal, is added. The precipitate is subsequently centrifuged and calcinated. Common methods of synthesis of metal nanoparticles are the adsorption of the metal colloids on metal oxides42, and chemical vapor deposition of metal

nanoparticles on porous oxide matrixes42.

2.5 SYNTHESIS OF METAL NANOPARTICLES ON POROUS SUPPORTS

The key drawback associated with synthesis methods described on the previous section,

is the difficulty to control both location and size of the metal nanoparticles in oxide matrixes.

15 Metal nanoparticles are normally either formed on external surfaces of the oxide particles

(support) or embedded in microporous oxide matrixes. When metal nanoparticles are present on

external surfaces are susceptible to aggregation because of the decreased melting point of

nanoparticles and the lack of space confinements. When the metal nanoparticles are embedded in the microporous matrixes of the support, they are inaccessible to reactants for effective catalytic reactions.

It has been suggested that a solution for these two problems would be to trap the metal nanoparticles inside the pores of nanoporous materials. The formation of metal nanoparticles

with size 2–4 nm has been already successfully achieved in porous materials such as MCM-41

matrixes43. MCM-41 is a new family of silicon oxides discovered by scientists at Mobil Oil

Research and Development44. These materials exhibit large internal surface areas and narrow

pore size distributions similar to those found in microporous crystalline zeolites. Zeolites can

have a maximum pore size of 20 Å; however, MCM-41 oxides can have a pore diameter up to

100 Å44.

It has been reported that the formation of gold nanoparticles with size less than 5nm

inside the mesoporous materials has been achieved by this co-synthesis sol–gel process. In this

method, the tetraorthosilicate (TEOS) silicon precursor solution, is combined with the salt

precursor of the metal nanoparticle which in the case of gold is hydrogen tetrachloroaurate (III)

trihydrate combined with a chelating ligand. The resulting solution is then added to a surfactant.

Typical surfactants used in the synthesis of nanoparticles are dodecylamine and Triton X-100.

The precipitate is centrifuged and calcinated to eliminate the surfactant45. Then the gold ions are

reduced using a reducing agent gas such as hydrogen gas. Mesoporous materials containing

metal nanoparticles such as gold nanoparticles have been prepared by using several procedures.

16 A successful method of incorporating gold nanoparticles inside periodic mesoporous

organosilica materials is through the wet impregnation methodology46.

A chemical vapor deposition was used for the deposition of a gold precursor (dimethyl

gold acetylacetonate) on MCM-41. Subsequent treatments gave rise to gold nanoparticles with

the size between 3 and 4 nm47. Gold nanoparticles (20 nm) have been synthesized inside a monolithic mesoporous material functionalized with 3-aminopropyltrimethoxysilane48. Gold nanoparticles (3.5 nm) have also been synthesized through autoreduction of gold ions on MCM-

41 functionalized with the primary amine group49. The different approaches described to

synthesize gold nanoparticles have the same goal to synthesize small particles on an exposed

surface. These methods use a surfactant to create a confiment to stop the nanoparticles from growing. Also, these types of synthesis use supports with large pore size. The purpose of large pores on the support is for the reactants to have access to the metal nanoparticles (catalyst).

In the current study, the oxidative properties of the metal nanoparticles of gold, silver, platinum and palladium were investigated. The oxidative capability of the metal nanoparticles at low temperature and atmospheric pressure was investigated using dibenzothiophene. The synthesis of metal nanoparticles on acidic silica gel is being reported. Lab synthesized silicas were tested in the oxidation reaction. A commercially available silica gel was also tested in the oxidation reaction of DBT. Metal nanoparticles were supported on commercial silica. Silica was dispersed in water and the metal ion solution was added to it. Metal ions were reduced using sodium borohydride. After the addition of sodium borohydride the nanoparticles were washed several times with water, and subsequently oven dried at 100oC.

Four different metal nanoparticles were used which included gold, platinum, palladium

and silver since all of these have been shown to exhibit oxidative properties in different

17 reactions. The catalysts were characterized before and after reaction using x-ray absorption spectroscopy. This technique provides information on the size of the nanoparticles before and after reaction and the oxidation states of the metals present. The original size of the nanoparticles was compared to the size after reaction to investigate any growth or change in the particles size or oxidation state. EXAFS also provided information on the formation of a new interaction between the metal and the sulfur containing molecules. Infrared spectroscopy,

Raman Spectroscopy and X-ray diffraction were used to characterize the product of the oxidation of DBT. High performance liquid chromatography was used to determine the amount of DBT removed from solution.

The reactions were carried out in different solvents. to develop an understanding of the reaction. The solvents used included tetrahydronaphthalene (tetralin), decahydronaphthalene

(decalin), heptanes and n-dodecane. The kinetics and thermodynamics information was obtained of each metal from the oxidation reaction. The activation energy was calculated for each catalyst. The activation energy was determined by using each catalyst at three different temperatures and sampling each reaction every 30 minutes up to 2h.

In addition, the catalysts were examined for their recyclability in three consecutively reactions. The oxidative capability was measured using HPLC to determine if the oxidative properties changed after the different cycles.

18 Chapter 3: Materials and Methods

3.1 DIBENZOTHIOPHEN MODEL REACTION

The dibenzothiophene model reaction has been successful in predicting how catalysts

behave in the real crude feed oil. This reaction is used to test catalysts and by monitoring the

dibenzothiophene removal, the efficiency of the catalyst is measured. Each catalyst was tested

using this reaction with slightly modifications. The model reaction uses decahydronaphtalene as

a solvent, dibenzothiophene as the sulfur containing molecule, 5kPA of hydrogen pressure and a

temperature of about 400oC50. Two modifications were made to the model reaction. One

modification was the temperature range at which the reaction was tested. The temperature range was between 100oC and 210oC. The second modification was to run the reactions under

atmospheric pressure conditions. In addition, a reflux set up was used instead of a pressurized reactor. Decahydronaphthalene was used as a solvent; however, other solvents were tested which included tetrahydronaphthalene, hexane, heptanes and n-dodecane (Figure 4). The differences in the solvents are their boiling point and their ability to donate hydrogen radicals.

The starting concentration of dibenzothiophene was 10,000 ppm which corresponded to 2000 ppm concentration of sulfur.

19

H

H H H H H HH H H H

H H H H H HH HH H H H H H H

A) Decalin B.P. 187oC B) Tetralin B.P. 206oC

C) n-Hexane B.P. 69oC D) n-Heptane B.P.98.42oC

E) n-Dodecane B.P.217oC

Figure 4: The structures of the different solvents used in the oxidation reaction are shown

A)Decalin, B) Tetralin, C) Hexane, D) Heptane and E) n-dodecane

3.2 SYNTEHSIS OF THE SUPPORT (SIO2)

There are several factors that can vary when synthesizing silica gel. The pH during the

synthesis can be varied, the drying temperature of the gel can be varied, the source of the Si, and

20 the acid or base used in the titration. In this study, the pH, the acid used, and the drying

temperature of the gel were varied.

Silica was synthesized at different pHs. Silica gel was prepared with slight modifications

51 as previously reported . Sodium metasilicate (Na2SiO3) and hydrochloric acid were obtained

from Alfa Aesar. Na2SiO3 was dissolved in water which produced a basic solution (pH 14). HCl

was added to the solution under vigorously stirring to bring the pH down to 10. The gel was then

washed several times with water to remove any NaCl. The gel was then dried at different

temperatures 100oC (24h), 140oC (24h), 180oC (24h), 220oC(24h), 260oC(24h), 300oC(24h),

o o 340 C(24h), and 380 C(24h). The same procedure was followed to synthesize SiO2 at pH 9, 8

and 7 (Figure 5).

To synthesize SiO2 gel at pH 6, 5, and 4 two different methods were used. One method

consisted of performing a fast titration with HCl to avoid gel formation at pH 7. The second method consisted in bringing the pH down to approximately 2, subsequently the pH was adjusted to either pH 4, 5, 6 using sodium hydroxide. Finally, SiO2 was synthesized under acidic pH

conditions using three different acids including HCl, HNO3 and H2SO4.

HCl was added to the solution of Na2SiO3 in water under vigorously stirring and very fast to reach a pH of 1. Then the solution was heated to remove H2O and obtain a gel. The gel was

then washed with water and dried at previously mentioned temperatures. Similarly, HNO3 was added to the solution of Na2SiO3 in water under vigorously stirring very quickly to reach a pH of

1. Then the solution was heated to obtain a gel. The gel was then washed with water and dried

at the aforementioned different temperatures. H2SO4 was added to the solution of Na2SiO3 in water under vigorously stirring and very fast to reach a pH of 1. Then the solution was heated to

21 obtain a gel. The gel was then washed with water and dried at the varying different temperatures.

The oxidative properties of SiO2 synthesized at different pH were tested using the DBT model reaction. DBT (0.5 g) was dissolved in decalin (50mL), 0.5g of SiO2 was added and the reaction was refluxed for 4 h under stirring. The reactions were tested at 6 different temperatures: 50oC, 100oC, 120oC, 140oC, and 160oC. In addition, to the synthesized silica, commercial silica was also tested in the reaction (silica grade 22). Different temperatures were tested to determine the optimum temperature for the reaction to occur.

In addition, different solvents were tested which included decalin, tetralin, hexane,

heptane and n-dodecane. Different solvents were used to determine if the solvent or the

temperature were important factors in controlling the oxidation reaction of DBT to DBT-sulfone.

Figure 5: Different silica samples synthesized at different pH and calcinated at different temperatures

22 3.3 SYNTHESIS OF METAL NANOPARTICELS ON SILICA

Metal nanoparticles were synthesized as previously reported with slightly

52 modifications . Silica was dispersed in water and salts of the metals of interest (KAuCl4,

KPtCl4, KPdCl4 and AgNO3) were added to the silica/water suspension. Then the metal ions

were reduced to the zero valent using NaBH4. The nanoparticles were then washed several times

- - with distilled water and centrifuged to wash out the Cl and NO3 ions from the salt precursors.

The metal ions reduction reactions were carried out at room temperature and the samples were

oven dried at a temperature of 100oC overnight.

3.4 BIMETALLIC METAL NANOPARTICELS

A bimetallic system of gold-palladium nanoparticles was synthesized and tested. The

bimetallic system was synthesized using three different methods to obtain different core-shell

structures. The first method was to disperse the support in the solvent and then a gold solution

was added onto the support. The gold ions were reduced, then a palladium solution was added to

the reduced gold ion solution. Palladium ions were reduced on the surface of the reduced gold

ions. The second method was by dispersing the support in the solvent then the palladium

solution is added. The palladium ions were reduced onto the support. Then the gold solution was

added followed by the subsequent reduction of the gold ions on top of the reduced palladium

ions. The third method was to disperse the support in the solvent and adding both solutions gold

and palladium at the same time. Then the metals ions are reduced simultaneously. The three

different synthesis methods were used to synthesize the bimetallic Au/Pd nanoparticles.

3.5 CATALYST RECYCLING

Each catalyst SiO2, AgSiO2, AuSiO2, and PtSiO2 were three times consecutively. Each

reaction lasted 2 hours. The solvent used in the recycling experiment was decahydronaphthalene

23 and the temperature was 140oC. After each reaction the amount of dibenzothiophene was

measured using high performance liquid chromatography. The recycling reactions consisted in

filtering out the catalyst and making a new solution of dibenzothiophene and

decahydronaphthalene. The same catalyst was used three times. In total each catalyst was tested

for 8 hours.

3.6 X-RAY DIFFRACTION ANALYSIS (XRD)

The x-ray diffraction data was collected using a Scintag XDS 2000. X-ray

diffractometer with Cu-Kα radiation which its wavelength corresponded to 1.541 nm. The

electron flux in the equipment was 10kev. The slits used were .1 and 1.0. X-ray analyses were

performed on the silica before the reaction with DBT and after the reaction to investigate any

changes that have may occur in the silica structure.

3.7 XAS STUDIES

X-ray absorption studies were performed at Stanford Synchrotron Radiation Laboratory

(SSRL) on beam-line 7-3. The samples were prepared by taking the gold and platinum nanoparticles before and after the reaction and packing the samples into 1 mm aluminum sample holders with Kapton® tape windows. The operating conditions of the beam-line were as follows: a current ranging between 80 and 100 mA, a beam energy of 3.0 GeV, a Si double crystal monochromator (Si 220 φ 90º), and an Au(0) foil was collected simultaneously with each sample, by placing the arsenic foil between the I1 and I2 ion chambers for calibration purposes.

In addition, a 1.0 mm by 10 mm upstream slit was used to reduce signal to noise ratios, which

resulted in an energy resolution of 1-2 eV. The bulk metal foils were collected in the same

manner as the nanoparticles spectrum, only a 1 µm film of the metal was deposited on top of a

piece of Mylar.

24 3.8 XAS DATA ANALYSIS

Spectra of metal foils and the nanoparticle samples were analyzed using the WinXAS

software and standard data reduction techniques53,54. The general procedure for the data

extraction and analysis is outlined below. Sample spectra were first energy calibrated using the internal Au(0) foil LIII edge (11.918 keV) or Pt(0) Foil LIII edge (11.564 keV). The energy

calibration was performed by taking a second degree derivative of the spectra of the metal foil,

and shifting the sample spectra according to the difference between the real metal foil energy and

the energy determined from the spectra. Once the energy calibration was performed, all of the spectra collected were background corrected. The background correction was performed by fitting a 1 degree polynomial to the pre-edge region and 4 degree polynomial to the post edge region. Spectra were then normalized to give a 1 absorption unit change across the absorption edge. Next, the XANES spectra were extracted from the whole spectrum by sectioning each spectrum from 11.90 to 11.99 keV for the gold samples and 11.525 to 11.565 keV for the platinum samples.

The EXAFS regions were extracted from the energy calibrated; background corrected,

and normalized XAS spectra. The first step in the EXAFS extraction was converting the spectra

into wave number or k space (Å-1). The conversion into k space was performed by first

determining the E0 for each of the samples. This was performed by taking a second derivative of

the absorption edge of sample spectra. Then, the XAS spectra were fitted from 2 to 12.2 Å-1 in a

Hanning window. The Fourier transformed EXAFS spectra were then back transformed to include the first two coordination shells and subsequently fitted using linear least squared fitting techniques and calculated outputs from the FEFF V8.00 software55. The parameters determined

for the fittings were: interatomic distances [R(Å)], coordination numbers, energy shifts, and

25 2 Debye-Waller factors. The S0 for the spectra were determined using the model compound

fittings which ranged between 0.85-0.95. The inputs for the FEFF V8.00 software were created

using the ATOMS 3.0 beta software, and crystallographic data from the model compounds Au

56 metal and Pt metal .

3.9 RAMAN STUDIES

The Raman spectra were collected using a Bruker SENTERRA Dispersive Raman

Microscope equipped with a 754 nm excitation laser. The samples were mounted on microscope

slides and sections of the samples were investigated. In addition, a collection time of 20 seconds

and a co-addition of 5 spectra were used to obtain the Raman data.

3.10 FT-IR DATA COLLECTION

Infrared studies were carried out with a Bruker Thermo Nicolet Nexus 420. The samples for IR measurements were prepared as pellets by embedding the sample in a polycrystalline KBr

matrix. A Craver pellet presser was used to make the pellets. The pressure applied to make the pellets was 5MPa. A background collection was obtained before each sample.

3.11 HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)

High performance liquid chromatography was used to determine the removal of DBT

from solution. High performance liquid chromatography (HPLC) was performed with a ODS (5

micro) silica based column, a Shamatzu LC-600 gradient pump, UV2000 detector, and Winner

for Windows Software. The mobile phase was acetonitrile. The flow rate was set to 1 mL/min.

and the detector was set to a 254 nm wavelength, all at room temperature. A 5 µl aliquot of

analyte was used. A calibration curve was obtained by using four different concentrations of

DBT.

26 The lowest concentration of DBT used was the blank with no DBT and the highest

concentration of DBT used was 500 ppm. Even though the samples measured had a maximum of 2000 ppm of sulfur, a dilution factor was taken into consideration when obtaining the DBT concentrations of the samples. Figure 6 shows the chromatogram of the DBT solutions at different concentrations to obtain the calibration curve.

1.0

0.8

0.6

0.4

Normalized intensity Normalized 0.2

0.0

-200 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Time minutes

Figure 6: HPLC of continuous injections of DBT solutions at different concentrations

Each peak was fitted using Origin 6.0 software using a Gaussian fitting function. The

Gaussian fitting provided the area under the curve. The area under the curve was related to the

concentration of DBT. When using higher concentrations of DBT the detector was saturated and

it did not provide a readable signal. Therefore, the calibration curve was obtained using lower concentrations than the DBT samples tested. Figure 7 shows one of the calibration curves obtained where the R factor is 0.99 and the DBT concentration ranged from 0 ppm to 500 ppm.

27 Calibration y = 0.0107x + 0.1065 R² = 0.9967 7 6 5 4

3 2Area 1 0 0 200 400 600 DBT Concentration

Figure 7: Calibration curve obtained using the DBT standard solutions

The R factor of the calibration curve was 0.99 or better. Then using the line equation y=mx+b, y corresponded area under the curve, m to the slope of the curve and x was calculated to determine the concentration of DBT remaining in the solution. To determine y from the experimental data, the peak obtained from the chromatogram was fitted using a Gaussian curve in Origin 6.0 software. The software calculated the area under the curve for each sample. The area under the curve corresponded to the value of y.

3.12 KINETICS AND THERMODYNAMICS

A kinetic and thermodynamic study was conducted to determine the activation energy of each

of catalysts used. The activation energy is one of the most important characteristics of a catalyst.

A catalyst lowers the activation energy of a reaction thus allows the reaction to occur faster.

Figure 8 shows the difference between a catalyzed and uncatalyzed reaction. Figure 8 is

showing a comparison of the activation energy barriers of an uncatalyzed reaction and the same

reaction with a catalyst. The catalyst lowers the energy barrier but does not affect the actual

energies of the reactants or products.

28

Figure 8: Catalyzed Reaction and uncatalyzed reaction (from left to right)

The kinetics of the reactions were determined by monitoring the removal of DBT as function of time. Samples were taken out every 30 minutes. The results were plotted and from the obtained graph the constant rate (k) was determined. The reactions could have been zero-order reactions, first or second order reactions. Zero order reactions have a constant rate. This rate is independent of the concentration of the reactants. The rate law is: rate = k, with k having the units of M/sec. First order reaction has a rate proportional to the concentration of one of the reactants. The rate law is: rate = k[A] (or B instead of A), with k having the units of sec-1.

Second order reactions have a rate proportional to the concentration of the square of a single reactant or the product of the concentration of two reactants: rate = k[A]2 (or substitute B for A or k multiplied by the concentration of A times the concentration of B), with the units of the rate constant M-1sec-1.

The determination of the reaction order can be obtained graphically. For example,

Figure 9 shows the plot of the concentration vs time. That plot corresponds to a first order

kinetics. To determine the rate constant (k) the ln of the concentration is applied to obtained a

29 straight line and the rate constant (k) corresponds to the negative slope (Figure 9). Second order

reactions will not be discussed since all the catalysts tested exhibited first order behavior.

Figure 9: The plot of a first order kinetics reaction and the linear plot obtained by taking the ln of the concentration (from left to right)

By plotting the results of the DBT concentration vs time the order of the reaction was

determined. The reactions were run at three different temperatures 120oC, 130oC and 140oC.

The k constant was obtained for each reaction run at the different temperature. Triplicates of each reaction at each temperature was performed, the temperature was controlled to within a couple degrees up of the intended temperature. The average temperature for each run is the one reported. Using the reaction constant (k) calculated at different temperatures, the activation energy to start the reaction (Ea) was determined. Figure 10 shows on the left the graph of rate

constant (k) vs different temperatures. Then by taking the ln of the rate constants and the inverse

function of the temperature a straight line can be obtained.

30

Figure 10: Plot of the rate constant (k) vs temperature and the linear plot obtained by taking the ln of the k to determine activation energy (from left to right)

The activation energy of the reaction can be determined by plotting ln k vs 1/TR in

Kelvin and determining the slope of the resulting line.

E Slope = − a RT

The activation energy was determined from the slope of the plot of ln k vs 1/TR and the value of R was 8.314 J/K mol. The Ea of the reaction determines which metal is the most suitable or most catalytic for the oxidation reaction by having the lowest activation energy.

31 Chapter 4: Results and Discussion

4.1 SILICA SYNTHESIZED AT DIFFERENT PH RESULTS

It was observed that (DBT) did not oxidize to dibenzothiophene sulfone when SiO2 was synthesized at a basic pH. The DBT was also not oxidized when SiO2 was synthesized in the pH

range of 3-6. However, when SiO2 was synthesized in the pH range of 1-2, the oxidation of

DBT to DBT sulfone was observed. Additionally, the oxidation of the DBT to DBT sulfone was

observed to be the highest when HCl was used as the acid in the synthesis of the silica. Only

minimal oxidation was observed when HNO3 was used in the synthesis of the SiO2 and no oxidation of DBT was observed when H2SO4 was used for the synthesis (Figure 11). HNO3 and

H2SO4 acids are strong oxidizing agents; HCl is not an oxidizing agent. It is possible their

oxidizing properties influenced their behavior in the reaction.

Figure 11: Types of acids used in the synthesis of silica vs the % removal of DBT. Hydrochloric Acid, Nitric Acid, Sulfuric Acid, and commercial silica (from left to right)

32

10000 9000

8000

7000 6000 5000

4000 DBT Concentration (ppm) 3000 30 60 120 180 240

Time (Min) 100 C 180 C 260 C 340 C 140 C 220 C 300 C 380 C

Figure 12: Removal of DBT in ppm using synthesized silica calcinated at different temperatures

A kinetic study of synthesized silica using HCl and calcinated at different temperatures was performed. The percentage of DBT removal at different intervals in time when using silica dried from 100oC to 380oC is shown in Figure 12. As it is shown in Figure 12, the removal of

DBT using synthesized silica exhibited first order kinetics at the conditions tested. Figure 12

shows the largest percentage removal occurred when silica was calcinated in the temperature

range of 100oC to 300oC. The percentage removal of DBT decreases when silica is calcinated at

higher temperatures. It is possible at higher calcinations temperatures silica losses alkoxy

groups. Alkoxy groups may be important in radical formation during the oxidation reaction.

Commercial silica obtained from Alfa Aesar was also tested in the DBT reaction and it also

exhibited first order kinetics as shown in Figure 13. Commercial silica had a higher DBT

33 percentage removal. This is may be due to particle size. Lab synthesized silica was not sieved; therefore, different particle sizes were present. Another possible reason is aging. Commercial silica is aged. The lab synthesized in this study was not aged. Aging may provide Lewis acid sites in the silica which interact with sulfur (Lewis base).

Figure 13: Removal of DBT in ppm using commercial silica and linear plot obtained when taking the ln of the DBT concentration

34 Figure 14: Ln concentration of DBT vs 1/T when using commercial silica at different temperatures

The oxidation reaction was tested at different temperatures to determine the optimum temperature of the reaction. Decalin was used as the solvent and commercial silica was used as the catalyst in this reaction. From the temperatures tested, the reaction run at 140oC was the

reaction temperature that showed the largest removal of DBT in decalin as shown in Figure 14.

It is expected each solvent would have an optimum temperature range. Tetralin had a similar behavior to decalin at lower temperatures the percentage removal of DBT was low. A 90% DBT removal occurred when the reaction was run at 180oC. When the reaction was run in n-

dodecane, oxidation of DBT to DBT-sulfone was observed. The percentage DBT removed

corresponded to 93%. However, this percentage removal was obtained only when the reaction was run at 190oC in n-dodecane. When the reaction was run at 160oC in n-dodecane only a small

percentage removal of DBT was observed. When the reaction was run either in hexane or

heptanes oxidation of DBT was not observed any temperature tested. The reactions were also

tested at the boiling point of each solvent and the oxidation reaction was not observed. It is

suggested that both the temperature and availability of oxygen are the determining factors in this

oxidation reaction. If the temperature is too high the solvent boils and oxidation does not take

place. A possible explanation for this observation is the degassing of the solvents at high

temperatures.

Tetralin is considered a good hydrogen radical donor. Decalin is considered a poor hydrogen

radical donor. n-Dodecane is considered a non hydrogen radical donor. The oxidation reaction

occurs when using either of these solvents regardless of their ability to donate hydrogen radicals.

35 It is suggested that oxidation does not occur when using heptanes or hexane solvents because the

boiling points are too low (110oC-70oC) to initiate the reaction.

In addition to the temperature, oxygen availability is also an important factor in this reaction.

As previously mentioned, the availability of the oxygen is dependent on temperature. According

to Henry’s law (eq.1), when the temperature of a solvent increases the solubility of the gas

decreases:

Where: c is concentration (mol/Latm), k is Henry’s constant (M/atm) and P is pressure (atm)

The solubility of oxygen in decalin can be calculated at different temperatures using Henry’s

Law using the following formula:

ln 4100 1 1

Table 1: Solubility of oxygen in decalin at different concentrations

Temperature Oxygen solubility in decalin o -4 140 C 1.58X10 mol O2/L atm o -4 160 C 1.00X10 mol O2/L atm o -5 180 C 5.4X10 mol O2/L atm

As was shown in Table 1, the solubility of oxygen in decalin decreases when the temperature

increases in the solvent. The availability of oxygen is important because it is a reactant in the reaction and without it there would not be any reaction. In addition, to confirm the importance

of the oxygen in the reaction, the reactions were run under different gases atmospheres. The reactions were performed under nitrogen and carbon dioxide atmospheres. When the reactions

were run under either nitrogen or carbon dioxide atmospheres the oxidation of DBT to DBT

36 sulfone was not observed. Table 2 shows a summary of solvent, temperature, and gas conditions

and the subsequent effect on the removal of DBT. Lab synthesized silica oxidized DBT to DBT sulfone when synthesized at acidic pH. The highest removal of DBT occurred when the reaction temperature was 140oC in decalin as the solvent. Oxidation of DBT occurred when the reaction

temperature was 180oC and the solvent used was tetralin. The highest DBT removal occurred

when the reaction temperature was 190oC when using n-dodecane as the solvent and commercial

silica as the catalyst. When the reaction was carried out in hexane or heptanes oxidation did not take place. The reaction was carried out in nitrogen and carbon dioxide atmosphere and no oxidation occurred.

37 Table 2: Summary of the different reactions performed at different temperatures, different solvents and different gases atmospheres

Catalyst Reaction Temp Solvent %DBT removal Atmosphere o SiO2 grade 22 160 C Decalin 80% Open reflux (air) o SiO2 grade 22 160 C Decalin 0% N2 o SiO2 grade 22 160 C Decalin 0% CO2 o SiO2 grade 22 160 C Tetralin 80% Open reflux (air) o SiO2 grade 22 100 C Heptane 0% Open reflux (air) o SiO2 grade 22 140 C Decalin 97% Open reflux (air) o SiO2 grade 22 160 C Dodecane 0% Open reflux (air) o SiO2 grade 22 190 C Dodecane 93% Open reflux (air) o SiO2 neutral pH 160 C Decalin 0% Open reflux (air) o SiO2 basic pH 160 C Decalin 0% Open reflux (air) o SiO2 acidic pH 160 C Decalin 80% Open reflux (air) SiO neutral pH 160oC Tetralin 0% Open reflux (air) 2 o SiO2 basic pH 160 C Tetralin 0% Open reflux (air) o SiO2 acidic pH 160 C Tetralin 80% Open reflux (air)

4.2 OXIDATION OF DBT USING METAL NANOPARTICELS SUPPORTED ON

SILICA

Metal nanoparticles were supported on commercial silica and tested in the reaction of

DBT to DBT sulfone. The metals used included gold, silver, platinum and palladium. The results are shown in Figure 15. As it is shown in Figure 15, after the reaction run for two hours the removal of DBT is approximately 95% when using the metals supported on silica. When using silica by itself the removal is about 70% at two hours. The oxidation reaction needs to be run for four hours when using silica by itself to reach a DBT removal of about 95%. When silica is used by itself there is no DBT removal in the first 30 min.

38

Ag/SiO2 Au/SiO2 Pt/SiO2 100 SiO2

80

60

40 % DBT removal

20

0 30 60 90 120 Time

Figure 15: % DBT removal using commercial silica and metal nanoparticles in decalin for 2 h

4.2.1 Silica activation energy results

As discussed earlier, the rate constant (k) is giving by the negative of the negative slope

of the line obtained by plotting concentration vs time. The k or rate constant is a constant of

proportionality between the reaction rate and the concentration of the reactant. Silica exhibited

first order kinetics. When the temperature increases, an increase removal of DBT is observed;

however, as shown in Figure 14 there is range where the reaction occurs if the temperature goes

too high oxidation does not take place. For these studies, the three temperatures tested included

120oC, 130oC and 140oC. It was shown previously that higher temperatures approaching or at

the boiling point of the solvent there is no DBT removal; however, if the temperature is too low the necessary energy for the appropriate number of collisions for the molecules to react is not

applied. To determine the activation energy of silica, the ln of three k values obtained from the

reactions run at three different temperatures were plotted (Figure 16). The activation energy was

39 determined by plotting the inverse of the temperature vs the ln k to obtain the plot shown in

Figure 17. The slope of this plot corresponds to the activation energy of silica by itself, which was determined for the oxidation of DBT to be 115.30 kJ/mol.

Figure 16: k values at three different temperatures are given by the slopes of each line.

40

-3 y = 23.117 - 11530x R2= 0.99971

-3.5

-4

-4.5

Ln K -5

-5.5

-6

-6.5 0.0023 0.00235 0.0024 0.00245 0.0025 0.00255 1/T (K-1)

Figure 17: Activation energy of silica is giving by the slope of the line

4.2.2 Silica gold nanoparticle activation energy determination

Silica supported gold nanoparticles were tested in the model reaction at three different temperatures to determine the rate constant (k). The temperatures were 125oC, 132oC and

144oC and the reaction was followed for 2 hours at these temperatures. Samples were analyzed every 30 minutes for up to 2 hours. Each reaction showed a first order reaction as shown in

Figure 18. .

41 AuSiO2 144C AuSiO2 132C AuSiO2 125C 14000

12000

10000

8000

DBT ppm DBT 6000

4000

2000

0 -20 0 20 40 60 80 100 120 140 time minutes

Figure 18: DBT removal using gold supported on silica at three different temperatures

AuSiO2 144C AuSiO2 132C AuSiO2 125C 10 y = 9.7276 - 0.034173x R= 0.96897 y = 9.5984 - 0.025102x R= 0.97988 y = 9.5672 - 0.017561x R= 0.97629 9

8

7 Ln DBT ppm Ln DBT

6

5 -20 0 20 40 60 80 100 120 140 Time minutes

Figure 19: k values of each reaction at three different temperatures correspond to the slopes of each line

42

Au/SiO2 Ea y = 10.217 - 47043x R= 0.98526 -3.3 -3.4

-3.5 -3.6 Ln k Ln -3.7

-3.8

-3.9

-4

-4.1 0.000288 0.000292 0.000296 0.0003 0.000304 1/RT

Figure 20: Activation energy of silica gold is giving by the slope of the line

In order to get a straight line the ln of the DBT concentration was plotted against time at the three different temperatures in Figure 19. The slopes of the plots corresponded to the k or reaction constants. The ln k and the inverse of the temperature values were calculated and plotted. The activation energy of silica gold for DBT oxidation was determined from the linear plot (Figure 20). The activation energy of silica gold was determined to be 47.04 kJ/mol for the oxidation of DBT to DBT sulfone. The value of the activation energy of gold catalysts reported in the literature for the oxidation of CO is similar to the value obtained for the oxidation of DBT to DBT sulfone. The activation energy of Au/TiO2 catalysts has been determined for the CO oxidation and the apparent activation energy is 32 kJmol−1 at 80 oC57. The resulting value of the apparent activation energy agrees closely with the value obtained from a similar evaluation of the

43 final rates in the temperature-dependent measurements. The result of approximately 30 kJ/ mol

agrees closely with the results compared in previous studies, which reported apparent activation

58,59 energies between 27 and 37 kJ/ mol for the oxidation of CO to CO2 .

It has also been reported molybdenum and tungsten based catalysts are efficient in the

oxidation of DBT to DBT sulfone. The catalysts used were Cp*2M2O5 (M= Mo or W) in the

60 presence of H2O2 in MeCN . The study consisted of calculating the kinetics for the oxidation of

different sulfur containing molecules. As mentioned in the introduction, refractory sulfur

compounds such as 4-methyl dibenzothiophene and 4,6 dimethyl dibenzothiophene are very

resistant to HDS. The calculated activation energy for the first oxidation process (thiophene to

sulfoxide) is almost 4 kcal/mol higher for BT than for DBT, and the second oxidation (sulfoxide

to sulfone) has similar activation energies for both substrates, are similar to the first activation

energy60. The activation energy reported in the literature in the first step of the oxidation is 7.76 kcal/mol and the second activation energy obtained when the sulfoxide is completely oxidized to sulfone is 10.39 kcal/mol. These numbers translate into 32.46 kJ/mol and 43.47 kJ/mol. As can be seen, the activation energy is higher when completely oxidizing the sulfoxide to sulfone. In our case, the intermediate was not detected60. The activation energy for the oxidation of DBT to

DBT sulfone when we used silica is about 115 kJ/mol. This is a really large value when

compared to the activation energy of silica gold which is 47.04 kJ/mol. It is important to

emphasize the values of the activation energy reported in the literature for the oxidation of DBT,

were obtained from reactions where hydrogen peroxide was used. Similar values are obtained in the current study without the use of peroxide which is a very aggressive oxidation agent. In addition, in the current reaction DBT sulfoxide was not detected when using infrared spectroscopy.

44 4.2.3 Platinum silica activation energy determination

The platinum nanoparticles supported on silica catalyst were tested in the model reaction

of DBT in decalin for 2 hours as with the gold catalyst the reactions with the platinum catalysts

also exhibited first order reaction with respect to the removal of DBT. Reactions were

performed at three different temperatures. The reactions exhibited first order kinetics with

respect to DBT. The change in concentration of DBT was followed by analyzing samples every

30 minutes for 2 hours. The plot showing the change in concentration of DBT vs time is shown

in Figure 20.

Pt SiO2 138C Pt SiO2 146C Pt SiO2 117C 14000

12000

10000

8000

DBT ppm DBT 6000

4000

2000

0 0 20 40 60 80 100 120 140 time minutes

Figure 21: DBT removal when using silica platinum at three different temperatures

The lnk and the inverse values of the three temperatures at which the reactions were run

were calculated and plotted in Figure 22.

45 Pt SiO2 138C y = 9.4599 - 0.029589x R= 0.99015 Pt SiO2 146C y = 9.4496 - 0.033408x R= 0.99695 Pt SiO2 117C y = 9.7362 - 0.021802x R= 0.96521 10

9

8

Ln ppm DBT 7

6

5 0 20 40 60 80 100 120 140 time minutes

Figure 22: k values at each temperature correspond to the slope of each line.

-3.3 y = 2.4159 - 20269x R= 0.99981 -3.4

-3.5

Ln k -3.6

-3.7

-3.8

-3.9 0.000285 0.00029 0.000295 0.0003 0.000305 0.00031 1/RT

Figure 23: Activation energy of silica platinum is giving by the slope of the line.

Figure 23 shows the plot of Ln k vs 1/TR to calculate the activation energy. The

activation energy of silica platinum is 20.27 kJ/mol. Silica platinum exhibits the lowest

activation energy from the metals tested making it the most active catalyst for the oxidation of

46 DBT to DBT sulfone. This low value for the activation energy using silica platinum is possible

for the reaction of DBT to DBT sulfone. There have been experiments reported where

quaternary ammonium bromide and phosphotungstic acid were used as the catalysts at different

temperatures60. Using this catalyst the activation energy obtained from the plot of ln(k) vs

1/(RT), corresponded to 28.70 kJ/mol61. This value is similar to the value obtained for the silica

platinum catalyst in the current study. However, this value is quite low, compared to other

values reported in the literature. For example, the apparent activation energy for DBT oxidation has been determined to be 52.83 kJ/mol in the H2O2/acetic acid system using polyoxometalates

as catalysts62.

It was also reported the activation energy of DBT oxidation in the H2O2-formic acid

system is 60 kJ/mol. The literature also reports oxidative desulfurization of sulfur compounds in

hexadecane in the presence of H2O2 at different temperatures, using a vanadium oxide-based

catalyst. The apparent activation energies of the sulfur compounds ranged between 48.4 and

35.3kJ/mol. The oxidative reaction of each sulfur compound listed followed a first-order kinetics

reaction63.

In addition, it has been reported that the activation energy value of two commercially

available Ni-Mo catalysts with alumina support64. Tert-butyl hydroperoxide (TBHP) was used

as an oil soluble oxidant. The peroxy group (–O–O–), a chain-like structure containing two

oxygen atoms present in TBHP, is a powerful oxidizing agent. TBHP in the presence of a Mo-

based catalyst enhances the oxidation reactions of refractory sulfur compounds present in

hydrocarbon samples including diesel. During oxidation, TBHP is reduced to its corresponding

alcohol and therefore does not form any persistent residual toxic compound. To calculate the

activation energy64, the oxidation reactions were carried out at 80oC, 100oC, 120oC, and 140oC.

47 The reactions exhibited first-order kinetics corroborating the bulkier compounds are more easily oxidized, which is opposite from conventional hydrodesulfurization, where the rate constants are lower for the bulkier compounds. The activation energies of the oxidative reactions were obtained from the slopes of the Arrhenius plots, and their ranges were found to be between 27 and 30 KJ/mol which agree with the values obtained in this study without using a harsh oxidizing agent. The silica platinum catalyst synthesized in this study exhibited an activation energy of

20.27kJ/mol which is lower activation energy than most catalysts reported in the literature for the oxidation of DBT to DBT sulfone.

4.2.4 Silver-silica activation energy calculations

144C AgSiO2 131C AgSiO2 134C AgSiO2 14000

12000

10000

8000 DBT ppm DBT 6000

4000

2000

0 0 20 40 60 80 100 120 140 Time minutes

Figure 24: DBT removal using silver supported on silica follows first order kinetics

48 AgSiO2 144C AgSiO2 131C y = 9.0702 - 0.032881x R= 0.97056 AgSiO2 134C y = 9.6135 - 0.015746x R= 0.90481 10 y = 9.2148 - 0.019924x R= 0.9427

9

8

Lnppm DBT 7

6

5 0 20406080100120140 Time minutes

Figure 25: k values of silica silver at each temperature correspond to the slopes of the lines

-3.4 y = 18.895 - 77314x R= 0.99512 -3.5

-3.6

-3.7 Lnk -3.8

-3.9

-4

-4.1

-4.2 0.000288 0.00029 0.000292 0.000294 0.000296 0.000298 1/RT

Figure 26: Linear plot of lnk vs 1/TR to determine the activation energy of silver (activation energy is given by the slope).

The silver nanoparticles supported on silica catalyst were tested in the model reaction of

DBT in decalin for 2 hours as previously mentioned. Reactions were run at three different temperatures 131oC, 134oC and 144oC. The reactions exhibited first order kinetics under the

49 conditions tested. The change in concentration or removal of DBT was followed for 2 hours.

The plot showing the change in the concentration of DBT vs time is shown in Figure 24. The k

values of the reactions at different temperatures are shown in Figure 25.

The activation energy of silica silver was calculated and was determined to be 77.33

kJ/mol (Figure 26). For comparison purposes, the value of the activation energy calculated for

silica gold was 47.04kJ/mol. The activation energy of silver is almost double that of gold. This

data indicates that silver is a much weaker oxidization catalyst than either gold or platinum.

However, the addition of the silver nanoparticles does make it a better catalyst than SiO2 alone.

4.2.5 Palladium Silica activation energy calculations

The palladium nanoparticles supported on silica catalyst was tested in the model reaction of DBT in decalin for 4 hours. Reactions were run at three different temperatures 120oC, 130oC

and 140oC and there was no observable removal of DBT. The reactions were also run at 120oC

and 160oC. The change in concentration or removal of DBT was followed for 4 h.

50

Figure 27: DBT concentration when using palladium supported on silica at two different temperatures 120oC and 160oC

Figure 27 shows the DBT concentration when using Palladium silica catalyst. The reactions were tested at two different temperatures 120oC and 160oC. The reactions using the palladium catalyst did not show any catalytic activity as the concentration of DBT remained relatively constant throughout the 4 hours of reaction. The removal of DBT using palladium silica catalyst was null.

Some catalysts can be promoted or activated by combining them with different elements.

For example, it is well known in the hydrodesulfurization process (HDS) one of the catalysts that

65 has the highest catalytic activity is RuS2 . However, Ru is an expensive metal, so CoMoS2 and

NiMoS2 are used. MoS2 by itself has a relatively low catalytic activity compared to RuS2, but when is combined with cobalt or nickel MoS2 catalytic activity is increased almost as high as that

of RuS2. The synthesis of a bimetallic catalyst of Au/Pd was performed in three different ways

51 to produce different core/shell structures. A core shell of Au/Pd catalyst was synthesized, a core shell of Pd/Au catalyst was synthesized, and both metals ions were reduced at the same time.

The results of these experiments are shown in Table 3.

Table 3: Different palladium gold silica catalysts synthesized and the % DBT removal

Catalyst % of DBT removed

AuPdSiO2 67.5 %

PdAuSiO2 57.16%

Together SiO2 29.41%

AuSiO2 88.6%

PdSiO2 0%

Table 3 shows the percent removal of DBT when using AuPdSiO2, PdAuSiO2, and

together AuPd reduced on top of the silica and PdSiO2. It can be seen that the best result was obtained with the gold catalyst. When the gold is on the surface of the palladium the percent removal of DBT is 67.5%; however if the palladium is on the surface of the gold the percent removal decreases to 57%. This data suggests the gold is catalytic since improves the oxidative capabilities of palladium from 0% to almost 70%. The catalytic activity of gold is still better when using it by itself than when combined with palladium in this reaction. If the palladium and the gold are reduced at the same time on top of the silica, there is minimal oxidation approximately 30% of the DBT to DBT sulfone. This experiment shows palladium does not catalyze the oxidation of DBT to DBT sulfone whereas gold is catalytically active in this reaction.

52 Palladium was the only metal that did not oxidize DBT to DBT sulfone under our

reaction conditions. The range values of the activation energies obtained from the different

catalysts tested are between 115 kJ/mol and 20 kJ/mol. Silica had the highest activation energy

for the oxidation of DBT to DBT-sulfone; however, when coated with different metal

nanoparticles the activation energy decreases which shows the catalytic activity of the

nanoparticles. The catalyst that exhibited the lowest activation energy was silica platinum.

The activation energy of silver quadruples that of platinum and doubles that of gold.

The differences in activation energy values obtained for both metals may be due to their

differences in electronic configuration and position in the periodic table. The main difference

between the Ag and both Pt and Au metals is the presence of f electrons in gold. Gold and silver

are in the same group sometimes called the coinage group in the periodic table. This group has been characterized by their chemical unreactivity when the elements are in the bulk state.

In the case of gold and platinum, f electrons have an effect in its characteristics. The d and f orbitals do not penetrate the core and, with the contraction of the s and p orbitals, they are more strongly shielded from the nucleus. However, gold was thought to be inert since it has a full d shell. In addition it has a full f shell. As it was previously mentioned, the f electrons don’t penetrate to the core they are shielded by the s and p electrons. But s electrons can penetrate to the core and calculations of the radial distribution of these electrons have shown they spent more time away from the nucleus (Figure 28). Since the f electrons have poor shielding properties and the s electron experiences a strong nuclear effective charge making this electron to have an ionization energy of 890KJ/mol. The general trend regarding ionization energy is a decrease of ionization energy going down a group in the periodic table. Gold exhibits the highest ionization energy in its group due to the presence of the f electrons which accounts for its non-reactive

53 nature in bulk. Silver does not have f electrons and similarly to gold has a high ionization energy of 731kJ/mol. Electrons in the d orbitals, as f electrons, also have poor shielding properties and that account for the high ionization energy and unreactivity in bulk.

Figure 28: Radial distribution of s,p, and d electrons

The electron configuration of gold is [Xe] 6s14f145d10 and the electronic configuration of silver is [Kr] 5s14d10. The common oxidation states of gold are +1 and +3, but +3 is thermodynamically preferred giving gold an electronic configuration of 5d8. In the case of silver the common oxidation state is +1 giving silver a 4d10 configuration. Both gold and silver are not easily oxidized as it can be noticed from their standard reduction potentials:

3 1.50

1 .85

Typical metals such as lithium have low reduction potentials; for example the reduction potential of lithium is -3.05 V. In the nanophase though, the increase of surface area contributes to the catalytic capabilities of silver and gold. In addition, as it will be shown in the EXAFS section, there is an oxidized layer on the nanoparticles which may account for their catalytic properties. Since it is not energetically easy to oxidize these metals, only a small surface layer

54 gets oxidized and the core remains reduced. In the nanophase the increase of surface area leads

to more ions available for reaction. The different behavior of these two metals may be due to

presence of f electrons in gold and the fact that silver acquires a full electronic configuration

after losing the s electron. From the economical point of view, if the this reaction will be

performed at large scale, silver will be the most suitable metal to use since it is the least

expensive of the three metals tested.

Palladium did not oxidize DBT to DBT sulfone. Platinum had the lowest activation

energy from all the metals tested. Palladium and Platinum are in the same group in the periodic

table. The electronic configuration of Palladium is [Kr] 4d10 and the electronic configuration of

Platinum is [Xe] 6s14f145d9. The palladium-catalyzed Wacker process is one of the most

successful aerobic oxidation reactions in the chemical industry66. This process and numerous

related reactions require co catalysts such as copper salts to mediate dioxygen-coupled oxidation

of reduced palladium during catalysis. The Wacker oxidation of ethylene to acetaldehyde is a

historically important industrial process, involving the use of palladium. In this process, Pd(II)

is reduced to Pd(0), which is re-oxidized to Pd(II), via reduction of Cu(II) to Cu(I). To complete

66 the catalytic cycle, Cu(I) is classically re-oxidized to Cu(II) by O2 . New oxidation reactions

employ a palladium catalyst with pyridine or bidentate nitrogen ligands suggesting that such

ligands promote the reaction between reduced palladium and dioxygen67. The Wacker process uses palladium catalyst and it uses an oxidizing agent which is copper(II) chloride. In the case of the oxidation of DBT to DBT sulfone, no oxidizing agent is being used. Palladium did not catalyze the oxidation reaction of DBT to DBT sulfone may be because this reaction is based on radical chemistry mechanism rather than reductive elimination mechanism as it is the case of the

Wacker reaction.

55 It has been reported Platinum catalyzes the oxidation of CO to CO2 due to the presence of

oxidized ions68. The partially oxidized catalyst had a lower oxygen coordination number and

shorter Pt–O bond length as expected for bulk PtO2. The catalyst structure showed a dynamic

behavior68. The catalyst at low activity is metallic covered with carbon monoxide, which

hampers oxygen from reacting. At high activity, there is a disordered surface oxide which it is

rich in defects. Oxygen interacts with the surface, and enhances the catalytic activity. The

thickness of the disordered oxide layer strongly depends on particle size and the support and

could be partially related to the temperature at which it formed68. This data reported regarding

CO oxidation, where the oxidized surface plays a key role in the oxidation process, concur with

the EXAFS data obtained from the reaction of DBT to DBT sulfone which will be discussed

later.

Another factor that may influence the oxidation catalytic activity of the different metals

tested is the values of the enthalpy of formation. As is already discussed, metal nanoparticles are

69 known to oxidize CO to CO2. The enthalpy of formation of CO2 is -393.5kJ/mol . The enthalpy of formation of CO2 is an exothermic where the total energy of the products is less than the energy of the reactants. On the other hand, the value of the enthalpy of formation of DBT is

120kJ/mol70. No thermochemical data was found for DBT-sulfone but it is expected the

enthalpy of formation will be similar to that of DBT. The value of the enthalpy of formation of

DBT is positive meaning it is an endothermic process where the difference between the energy

of the products and the energy of the reactants is equal to the heat supplied to the system by the

surroundings. More energy is required to oxidize DBT to DBT-sulfone than to oxidize CO to

CO2.

56 According to the data obtained, in the oxidation reaction of DBT to DBT sulfone using the different metal nanoparticles, each metal is behaving slightly differently in this reaction.

Silver has a value for the activation energy that is doubled the value of gold. As will be shown in the section of the EXAFS data, the changes in the catalysts after the reaction suggest they behave differently in this reaction. Platinum and Palladium are in the same group; however, platinum had the lowest activation energy and palladium did not catalyze the oxidation of DBT to DBT sulfone.

Following the decrease of DBT with high performance liquid chromatography, the activation energy of silica, silica-gold, silica silver, silica platinum and silica palladium was calculated. A summary is provided in Table 4. The metal that exhibited the lowest activation energy was silica platinum. Silica gold and Silica silver had different activation energy values.

Palladium is not an oxidative catalyst in the reaction of DBT to DBT sulfone. The amount of

DBT removed when using silica gold, silica silver and silica platinum was about the same.

Table 4: Calculated activation energies of SiO2, Ag/SiO2, Au/SiO2 and Pt/SiO2

Activation Energy Sample Equation Correlation Factor (kJ/mol)

SiO2 y=91.6204-366560.7x .99984 366.56kJ/mol

Ag/SiO2 y=18.895-77314x .99512 77.31kJ/mol Au/SiO2 y=10.217-47043x .98528 47.04kJ/mol Pt/SiO2 y=2.4159-20269x .99981 20.27kJ/mol

Recycling experiments were carried out to determine which catalyst could be used consecutively for three times maintaining its catalytic activity. Silica, silica gold, silica silver and silica platinum were run once in the oxidation reaction. Then the catalysts were recovered through filtration and tested them again. Each catalyst was tested three times in addition to the 57 first cycle. The results are shown in Figure 29. The data in Figure 29 shows at cycle zero silica removes 80% of DBT, silica gold and silica platinum remove 75% and silica silver removes 70% of DBT. In the first cycle, silica removed 55% of DBT, silica platinum removed 30% of DBT, silica gold removed 65% of DBT and silica silver removes 52 % of DBT. During the second cycle, silica removed 40% of DBT, silica platinum removed 30% of DBT, silica gold removed

55% and silica silver removed 55% of DBT. During the third cycle, silica removed 30% of

DBT, silica platinum removed 20% of DBT, silica gold removed 50% of DBT and silica silver removed 35% of DBT. The catalysts were tested for a total of 8 h.

According to the data obtained from kinetics and thermodynamics, silica platinum has the lowest activation energy. However, during the recycling experiment, silica platinum catalyst died in the second cycle. In the zero cycle removed 80% of DBT, by the first cycle removed only 30% of DBT. Even though silica platinum had the lowest activation energy, the catalytic activity decreases fast. Silica which had the highest activation energy, exhibited a decrease in the catalytic activity which each cycle; however, the decrease was not as high in the second cycle. There is a decrease in the catalytic activity of silica silver after cycle zero. Then the catalytic activity stayed relatively constant between the first and second cycle. During the third, recycling the amount of DBT removed decreased.

The removal of DBT by silica gold stayed constant during cycle 0, cycle 1, cycle 2, cycle

3. The removal of DBT started at 75% at cycle 0 and during the last cycle the removal of DBT was 50%. Silica gold catalyst gave the best removal of DBT after the three cycles, followed by silica silver catalyst, silica, and silica platinum catalyst.

58 90 80 70 60 SiO2 50 Pt 40 Au 30 Ag 20 10 0 Cycle 0Cycle 1Cycle 2Cycle 3

Figure 29: Recyclability studies using the different catalysts

4.3 X-RAY DIFFRACTION

The X-ray diffraction patterns of DBT-sulfone and of DBT-sulfone standard are shown in

Figure 30. Cerius software was used to assign the h k l planes. DBT-sulfone was modeled using the cell parameters and atomic positions as previously reported71. The unit cell parameters used

were a = 10.09 Å, b = 13.89 Å, c = 7.22 Å, and b = 93.5 Å. The space group used was C2/c. The

X-ray diffraction patterns showed a difference in the intensities (Figure 31). Reflections at 13.30

and 22.14o 2θ correspond to the 020, and 220 planes and are in the same intensity in DBT- sulfone standard and SiO2-reacted DBT. The treated sample may have adopted preferred

orientation in the 110, 111, 021, 040, 221, 041, and 113 planes since they are the predominant

peaks in the treated sample. It is possible that the height peaks are different because the treated

sample was more crystalline than the purchased sulfone. The way the treated sample was

extracted from the solvent was by slow evaporation which led to the formation of large DBT-

sulfone crystals.

When the reaction was run at higher temperatures at 140oC-145oC DBT-sulfone crystals

could be extracted and observed in the X-ray diffraction pattern. When the reaction was run at a 59 lower temperature between 100oC and 130oC DBT-sulfone could not be completely isolated and the X-ray diffraction pattern showed silica (a broad diffraction peak at about 20 in 2theta) and

DBT-sulfone diffraction peaks. As previously discussed, the optimum temperature where the oxidation of DBT to DBT sulfone in decalin occurs is between 140o-150oC. The fact that when the reaction was run at 130oC the DBT sulfone could not be extracted, means there is a strong interaction between the surface of the catalyst and the DBT. According to the data, at 140oC the interaction between the surface of the catalyst and DBT is not as a strong, since the temperature of 140oC is moving away from the optimum temperature range of this reaction.

Figure 30: X-ray diffraction of DBT-sulfone standard and DBT after reacted with the catalyst (from top to bottom)

60

Figure 31: DBT-sulfone obtained experimentally and DBT sulfone simulated. The assignments of the planes hkl are shown

61 4.4 INFRARED SPECTROSCOPY RESULTS

Figure 32: Far IR of DBT after reaction with silica gel grade22 at 140oC (A). DBT after reaction lab synthesized silica at pH 1 (B). DBT sulfone standard (C). DBT standard (D).

Figure 33: Mid-IR of DBT after reaction with silica gel grade22 at 140oC (A). DBT after reaction lab synthesized silica at pH 1 (B). DBT sulfone standard (C). DBT standard (D).

62 In Figure 32, the infrared spectrum is shown only from 800 to 400 cm-1 wavenumbers where the C-S and S-O ring vibrations occur. It is important to emphasize the difference in the infrared spectrum after the oxidation reaction with silica (commercial and synthesized) at 140oC

(Figure 32A and 32B). The shift in the peak at 744 cm-1 occurs in the treated samples and the

absorption bands corresponding to S-O vibrations appear in the 600-500cm-1 range (Figure 32A and 32 B). Comparing the treated sample to DBT standard (Figure 32D) there is only a fraction of DBT left in the sample treated with the synthesized silica. Comparing the treated sample to

DBT sulfone standard (Figure 32C), all the vibrations of the DBT sulfone standard are present in the treated samples. Figure 33 shows the Mid IR of the product of the oxidation reaction. Figure

33A and 33B are the products of the reaction when using commercial silica and synthesized respectively. At around 1100 cm-1, the vibrations corresponding to the S-O appear in both

treated samples.

The main differences that were observed when the reaction is run at different

temperatures are in the range of 1000 to 1500cm-1. The absorption bands corresponding to DBT-

sulfone start appearing when the reaction is ran at 100oC (data not shown). However, DBT-

sulfone vibrations between 1000 cm-1 and 1500cm-1 are masked by the presence of the silica,

which absorbs in the range of 1000 and 1500cm-1 and thus not visible in the FT-IR spectra. As the reaction temperature increased, the absorption bands of DBT-sulfone standard in the range of

-1 1000 to 1500cm , begin to become visible in the samples treated with the SiO2. Based on this

data, it is suggested that a lower temperatures DBT is absorbed onto the silica first and

subsequently oxidized. As the temperature of the reaction is increased (140oC), the silica

releases the oxidized DBT (DBT-sulfone) back to the solution. The oxidation reaction was

63 studied at the boiling temperature of the decalin (190oC). At boiling point of the decalin no

oxidation was observed not even partial oxidation was observed (data not shown).

4.5 XAS RESULTS

Figure 34 shows the Au LIII XANES spectra of the gold nanoparticles before the reaction,

the gold nanoparticles after the reaction, and a gold metal reference. The insert shows an

expanded view of the overlaid edges of the gold nanoparticles and gold metal. The XANES

spectra for the most part look identical; however, the gold nanoparticles from the nanoparticle

sample taken before the reaction show a small white-line feature in the absorption edge. The

small white-line feature in nanoparticles indicates the presence of oxidized gold in the sample.

The white line feature is induced in the gold samples through the electronic hole in the 5d band

of the metal72; reduced gold generally does not contain this feature because of the full d10 configuration of gold metal and thus no electron can be transferred into the d shell. However, in oxidized gold samples the electron transition is allowed because of the vacancy in the d shell for example the 5d8 electronic configuration for the gold(III) ions allows the movement of electrons

into and out of the 5d shell.

64

1.0

0.8

0.6

3.0 0.4 Normalized Absorption 11.94 11.96 11.98 2.5 Energy [keV]

2.0 Au Nanoparticles After Reaction

Au Nanoparticles before Reaction 1.5

Au(0) Metal Foil 1.0

Normalized Absorption 0.5

11.92 11.94 11.96 11.98 Energy [keV]

Figure 34: Au LIII XANES of gold foil, gold nanoparticles before the reaction with DBT, and gold nanoparticles after the reaction with the DBT take from 11.90 to 11.99 keV. The insert shows the expanded view of the edge features from 11.921 to 11.90 keV.

65

10

9.5

9 10 8.5

8 8

Magnitude Transform Fourier 7.5

6 7 2.62.833.23.4 R(A) 4

2 Fourier Transform MagnitudeFourier

0 012345678

R(A) AuAu MetalMetal AuAu NanoparticelsNanoparticles BeforeBefore ReactionReaction AuAu NanoparticelsNanoparticles AfterAfter ReactionReaction

Figure 35: Au LIII EXAFS of gold foil, gold nanoparticles before the reaction with DBT, and gold nanoparticles after the reaction with the DBT. The insert shows the expanded view of the Fourier transformed EXAFS.

Figure 35 and Table 5 show the Fourier transformed EXAFS and the fittings using calculations

generated from FEFF V8.00 of the back transformed First shell EXAFS of the gold foil, gold

nanoparticles before and the gold nanoparticles after the reaction, respectively. As can be seen in

the Fourier transformed EXAFS the nanoparticle sample before the reaction has an incomplete

first shell coordination, which is indicative of a nanoparticle as has been shown in the

literature73,74. However, the first shell coordination environment of the gold nanoparticles in the

66 sample after reaction has increased and looks the similar to the bulk metal. From the fitting data

presented in Table 5, it can be seen that the gold nanoparticles before the reaction have a first

shell coordination environment consisting of 9.4 gold atoms at an interatomic distance of 2.86 Å.

Whereas, the gold nanoparticles after the reaction have a first shell coordination environment

that consists of 11.0 neighboring atoms at an interatomic distance of 2.85 Å. The bulk gold

sample shows a complete first shell coordination environment for a FCC metal of 12.0 with an

interatomic distance of 2.85 Å, which is the interatomic distance for gold-gold neighbors74. It

has been shown in the literature that the average grain size of nanoparticles of metals which take

on a FCC packing can be calculated based on the first shell coordination environment75. From the data collected in this study, the nanoparticles before the reaction with DBT have an average diameter of 21 nm and after the reaction the nanoparticles have an average diameter of approximately 51 nm. Comparatively, to the literature the average size of gold nanoparticles that are usually shown to be catalytic are in the range of 1-10 nm76. We are observing the reaction to occur on rather large nanoparticles, with a lowered activation energy than using the SiO2 alone a decrease in the activation energy.

Table 5: Results of linear least squared fittings of calculations generated using FEFF V8.00 for the gold and platinum nanoparticles supported on acidic silica before and after the oxidation of DBT to DBT –sulfone.

2 2 2 Sample Interaction CN R(Å) σ (Å ) So Bulk Gold Au-Au 12.0 2.85(1) 0.0079 0.95 Gold nanoparticles before reaction Au-Au 9.4 2.86(1) 0.0086 Gold nanoparticles after reaction Au-Au 11.0 2.85(6) 0.0087 Bulk Platinum Pt-Pt 12.0 2.76(4) 0.0046 0.95 Platinum nanoparticles before reaction Pt-Pt 9.0 2.76(1) 0.0045 Platinum nanoparticles after reaction Pt-Pt 10.5 2.75(8) 0.0047 Bulk Silver Ag-Ag 12.0 2.85(1) 0.0087 0.95 Silver nanoparticles before reaction Ag-Ag 3.8 2.88(1) 0.0098 Silver nanoparticles after reaction Ag-S 1.0 2.54(2) 0.0088 Ag-Ag 3.0 3.01(7) 0.0079

67 Figure 36 shows the LIII XANES spectra taken from the nanoparticles before and after the

reaction with DBT and a bulk platinum metal sample. The insert shows that the edges of the

platinum nanoparticles are very similar to the XANES of the bulk metal. However, the major difference is the white line feature at 11.5666 keV, which is slightly more pronounced in the

XANES of both the nanoparticle samples. The more pronounced white line feature indicates that there is some oxidized platinum in the sample, which has been shown in the literature with gold

nanoparticles77. The presence of oxidized platinum in the sample is no surprise in nanoparticle

samples as many pure metal nanoparticles are now being shown to consist of a core shell type

system with a reduced core and an oxidized shell as shown in Figure 35. However, the XANES

features at 11.5807 and 11.5948 keV are in the same location in both the nanoparticle samples

and the bulk metal sample showing the samples do indeed consist of the platinum metal before

and after the reaction. Figure 36 and Table 5 show the Fourier transformed EXAFS of the metal

foil, and the nanoparticles before and after the reaction with DBT, and the fittings generated using calculated fittings from the FEFF V8.00 software, respectively.

68

1.25 1.0

0.75

0.5

3.0

Normalized Absorption 11.58 11.6 Energy [keV] 2.5

2.0 Platinum Metal

Platinum Nanoparticles after Reaction 1.5 Platinum Nanoparticles before Reaction 1.0 Normalized Absorption 0.5

11.55 11.575 11.6 11.625 Energy [keV]

Figure 36: Pt LIII XANES spectra taken from 11.525-11.565 keV of the platinum nanoparticles supported on silica before the reaction with DBT, nanoparticles supported on silica after the reaction with DBT, and a Pt(0) metal foil for reference. The insert shows an expanded version of the XANES region.

69 18.4

17.6

16.8

16 15.2

14.4

13.6

15 Magnitude Transform Fourier 12.8

12 22.533.54 R(A) 10

5

Fourier TransformMagnitude

0 012345678 R(A)

PtPt MetalMetal PtPt NanoparticlesNanoparticles BeforeBefore reactionreaction PtPt NanoparticlesNanoparticles AfterAfter reactionReaction

Figure 37: Pt LIII EXAFS spectra taken of the platinum nanoparticles supported on silica before the reaction with DBT, nanoparticles supported on silica after the reaction with DBT, and a Pt(0) metal foil for reference. The insert shows an expanded version of the first shell Fourier transformed EXAFS.

As can be seen in Figure 36 and Table 5, both the before and after nanoparticles samples have incomplete first shell coordination, which is indicative of nanoparticles74. However, the samples are also indicating that there is some growth in the average grain size of the nanoparticles after the reaction with the DBT as the amplitude of the EXAFS increases and so too does the coordination number. The coordination number increases from 9.0 platinum atoms at an interatomic distance of 2.76Å to 10.5 neighboring platinum atoms at 2.75 Å. This change in the coordination number corresponds to an approximate doubling of the size of the platinum

70 nanoparticles from approximately 16 nm to approximately 33 nm. This is based on a calculation first present by Borowoski on copper nanoparticles as published elsewhere78. However, the XAS data does not show the coordination of sulfur to the platinum, and it does not show any changes other than the increase in the amplitude of first shell EXAFS which has been linked to the growth of nanoparticles74,75,78. As with the gold nanoparticles, the platinum nanoparticles show a lowering of the activation energy of the reaction, which shows the catalytic activity of the platinum nanoparticles is higher than the gold nanoparticles.

A growth in the average grain size for both the gold and platinum nanoparticles was observed after the reaction, this may be due to the fact that the nanoparticles were not stabilized before the reaction. The nanoparticles were only adsorbed to the silica and then allowed to react with the DBT in the decalin solution. During the reaction at a temperature 140 ºC, the nanoparticles may have aggregated, which would increase the average diameter of the nanoparticles.

From the EXAFS data on the determination of the average size of the nanoparticles before and after the reaction showed that the gold nanoparticles grew to a much larger size than the platinum nanoparticles under the same conditions. In addition, the XANES data also showed that the platinum nanoparticles changed less than the gold nanoparticles, the presence of the oxidized platinum before and after the reaction stayed at approximately the same amount.

Whereas in the gold nanoparticles samples, the sample at the end appears to have the same spectrum as the bulk gold there appears to be no oxidized gold present in the sample. This change in the average grain size of nanoparticles may mean that the nanoparticles in their current state that is non-stabilized which may only be a one-time use catalyst, however, more study is needed to determine the recyclability of the platinum and gold nanoparticles as oxidation

71 catalysts for the oxidation of DBT to DBT-sulfone. The association of the DBT and the DBT sulfone at the end of the reaction with the gold and platinum nanoparticles may be indicating the gold and platinum are involved in activating the DBT for the oxidation. The conditions of the reaction do strongly suggest that the reaction occurs through the synthesis of OH˙ and studies have shown that the formation of radical species occurs on both gold and platinum nanoparticles79,80,81.

Figure 38 shows the K edge spectra for the silver nanoparticles on the silica before the reaction, after the reaction and the silver metal standard. The spectrum of the metal foil and the silver nanoparticles supported on the silica are very similar with features appearing at 25.531,

25.555, and 25.581 keV. The similarity of the XANES features between the silver nanoparticles and the bulk silver indicates that the nanoparticles are indeed silver(0). In addition, the inflection point of the edge which was determined to be 25.514 keV which is the energy for elemental silver. The spectrum for the spent catalysts shows some similarity to the spectrum for the unused silver catalysts and the bulk silver metal. For example, the feature at 25.531 keV is still present and the inflection point of the spectrum is still at 25.514 keV. However, the other features in the

XANES spectrum change after the reaction only a broad peak exists at 25.567keV. This change in the XANES spectrum indicates a change in the local structure of the silica but the inflection point of the edge remaining at the same energy as the bulk metal indicates the addition of an electron donating material such as sulfur to the structure.

72 1.6

1.4

1.2 1 0.8

0.6

0.4

Normalized Absorption 0.2 0 25450 25550

Energy (eV)

Ag metal Ag Nanoparticles Before RXN Ag Nanoparticles After RXN

Figure 38: XANES spectra of silver nanoparticles before and after use in the oxidation of DBT to DBT Sulfone and the XANES spectra of the bulk silver standard.

Figure 39 shows the Fourier Transformed EXAFS of the silver metal standard, the silver on silica catalyst before use and the silica catalysts after use. As can be observed spectra for the unused catalysts in comparison to the bulk metal has much lower amplitude, which as mentioned earlier indicates that the silver is present as a nanomaterial. The observed coordination number in the bulk silver is 12 Ag nearest neighboring atoms with an interatomic distance of 2.86 Å and the unused catalyst has approximately 4 with an interatomic distance of 2.86 Å. The unused silver catalyst has a calculated average size of 14.9 nm. However, after the silver catalyst is used in a reaction the coordination environment changes a sulfur ligand is added to the coordination environment at 2.54 Å, and the silver interaction has been shifted to 3.04 Å. This shift in the coordination of the silver becomes similar to that of silver sulfide82. The data indicates that the

silver nanoparticles have changed their structure after the reaction to form silver sulfide

nanoparticles.

73 3.5

3 A

2.5

2

1.5

1

Fourier Transform Magnitude Magnitude Transform Fourier 0.5

0 2.5 B 2

1.5

1

0.5 Fourier Transform Magnitude

0 14

12 C 10

8

6

4

Fourier Transform Magnitude Magnitude Transform Fourier 2

0 0123456

R(A)

Data Fitting

Figure 39: A. Fourier transformed EXAFS and the Fourier Transform of the fitting of the EXAFS from 2.0 to 12.2 Å-1 of the silver nanoparticles before use in the reaction. B. Fourier transformed EXAFS and the Fourier Transform of the fitting of the EXAFS from 2.0 to 12.2 Å-1 of the silver nanoparticles after use in the reaction. C. Fourier transformed EXAFS and the Fourier Transform of the fitting of the EXAFS from 2.0 to 12.2 Å-1 of bulk silver standard.

74 4.5 RAMAN SPECTROSCOPY RESULTS

1.0

A B

1.0 Normalized Intensity

0.0 2.0 C D 1.5

1.0

0.5 Normalized Intensity

0.0 250 500 750 1000 250 500 750 1000 Raman Shift [cm-1] Raman Shift [cm-1]

Figure 40: A. Raman Spectra of DBT Sulfone and DBT taken from 160 cm-1 to 1200 cm-1. B. Top is the Raman spectra of the precipitated crystals from the reaction of gold nanoparticles with DBT and bottom is the Raman Spectra of the spent catalysts. C. Top is the Raman spectra of the precipitated crystals from the reaction of platinum nanoparticles with DBT and bottom is the Raman Spectra of the spent catalysts. D. Top is the Raman spectra of the precipitated crystals from the reaction of silver nanoparticles with DBT and bottom is the Raman Spectra of the spent catalysts.

Figure 40 shows the Raman spectra of the DBT compound DBT sulfone compound at the start of the reaction , the product precipitated after the reaction for both the gold and the platinum

nanoparticles, as well as the spent platinum and gold catalysts from 160 cm-1 to 1200 cm-1.

75 Table 5 shows the peak assignments83,84,85 for the different visible Raman vibrations for the samples presented in Figure 40. Figure 40 A shows the Raman spectra of the DBT sulfone (top spectrum) DBT compound (bottom spectrum) which shows the presence of the C-S-C peaks at

234, 497, 703, and 768 cm-1 and shows none of the peaks associated with the oxidized form of

DBT the sulfone. Figure 40 B shows the Raman spectra of the gold nanoparticles after the

reaction with DBT. The spectrum shown in Figure 40 B are as follows the top Raman spectrum

is from the crystals that precipitate after the reaction and the bottom spectrum is a spectrum of

the spent catalyst after the reaction. The top Raman spectra shows a clear conversion of the DBT

to the DBT sulfone, as indicated by the peaks at 565 and 595 cm-1 , which show the presence of

the S=O. In addition, the disappearances of the C-S-C at vibration of the ring leaving only the C-

H stretches of the rings at 702 cm-1 indicates a change in the structure of the DBT. The C-S-C stretches in the DBT are moved to a higher wave number after the addition of the two oxygen atoms to the sulfur to be located at 744 and 746 cm-1. In addition, these changes are also shown

in the platinum samples as shown in Raman Spectra in Figure 40 C. There are two different sets

of samples present in Figure 40 B and Figure 40 C for the gold and platinum samples, respectively. Each of these samples is the precipitated crystal from the decalin sample and the spent catalysts. The precipitated crystal show the presence of the DBT Sulfone; and the spent catalysts show also the vibrations corresponding to the DBT sulfone. These results showing the presence of DBT sulfone associated with the silica supported nanoparticles is indicating that surface of the particle is extremely important in the activation of the DBT for reaction. In addition, it has also been shown in the literature that platinum and gold nanoparticles are extremely efficient in dissociating H2 and O2 gases to form radicals such as the hydroxyl radical

(OH˙) which are extremely reactive species20.

76 Table 6: RAMAN vibrations found in the DBT, and the Gold-DBT reaction samples as a precipitated crystal and the spent catalysts, the Platinum- DBT reaction samples as a precipitated crystal and the spent catalysts

DBT DBT-Au DBT-Pt DBT DBT Ag Assignment DBT DBT-Au DBT-Pt Sulfone Spent Spent Ag Spent C=C-C 219 202 202 201 201 202 203 203 C-S 234 SO2 twist 253 252 253 254 254 254 254 285 C=C-C 354 355 C-C torsion 367 367 368 369 367 367 367 C-C=C 376 404 405 407 407 404 405 405 410 C-C2 wag C-C-C 421 422 421 421 421 C-C ring 458 457 456 458 458 458 458 C-S-C ring 497 478 489 492 492 478 477 477 def 505 O-S-O 551 552 552 566 566 O-S-O 567 565 566 566 567 O-S-O 580 599 595 594 596 596 C-C=C 614 615 615 617 617 γCH/C-S-C 703 698 700 698 699 641 C-S 734 744 742 741 698 698 698 C-S 756 751 751 732 753 753 C-S 768 760 767 761 761 738 763 763 C-H 779 786 793 792 761 793 793 C-S 846 847 847 779 850 850 Γch 879 875 874 876 876 γCH/ γring 929 C-H bend 980 980 C-H bend 989 990 δC-H 1001 1010 1010 C-C/C-H 1023 1024 1025 1022 1022 1022 1022 1022 C-C 1040 1041 1040 1040 CH,C=C C-H bending 1051 1055 1054 1054 1050 1052 1052 C-H 1067 1070 1071 C-H, and 1087 C=C δC-H 1116 1116 1117 1115 1115 1115 1117 1117 δCH 1128 C-C 1135 1133 1140 1140 O-S-O 1154 1155 1153 1153 1155 1155 1155 δC-H 1168 1163 1163 1160 1160 1163 1162 1162

77 4.6 TEMPO EXPERIMENT The series of experiments performed showed the importance of oxygen solubility in decalin in the reaction of oxidation of DBT to DBT sulfone. The mechanisms for the oxidation of DBT to DBT sulfone suggested in the literature involve the in situ production of hydrogen peroxide by dissociating H2 and O2 into radicals, the hemolytic or heterolytic dissociation of the

peroxide used in the reaction as the oxidizing agent and by the use of oxometals25. In the current

study DBT is oxidized to DBT sulfone using just silica without metal nanoparticles; therefore,

the oxidation reaction is not occurring only because the use of metal nanoparticles. In addition,

the in situ production of hydrogen peroxide seems unlikely since there is no source of H2.

Protons are present since silica is acidic but seems rather energetically unfavorable to produce H2 and then produce H2O2 followed by the dissociation of H2O2.

However, the data suggests the oxidation of DBT to DBT sulfone may be due to the

presence of radicals. To test this hypothesis 2,2,6,6, tetramethylpiperidine-1-oxyl (TEMPO)

which is a tetraalkylnitroxyl radical (Figure 41), was tested in the reaction of DBT to DBT

sulfone86. The reaction was tested by studying the standard reaction radical, DBT, silica, decalin

and tetraalkylnitroxyl. The reaction was refluxed for 4 hours and the removal of DBT was

measured using HPLC. The observed removal of DBT was 0%. The results suggest that the

reaction is radical driven since in the presence of Tetraalkylnitroxyl radical the oxidation did not

occur. It is suggested tetraalkylnitroxyl radical reacted with the radicals produced in the

reaction; therefore no oxidation took place. The TEMPO acts like a radical scavenger to remove

the OH radicals that may be generated in solution

78

Figure 41: TEMPO (2,2,6,6, tetramethylpiperidine-1-oxyl) molecule structure.

Acid-catalyzed silica gels contain higher concentrations of adsorbed water, silanol

groups, and unreacted alkoxy groups than the base-catalyzed precipitates80. Oxidation occurs only in the presence of acidic silica maybe due to the presence of the SiOH+. The oxidation of

DBT occurs only when silica gel is synthesized at an acidic pH, suggests, H+ protons are necessary to drive the oxidation reaction. It has been reported that sulfides can be oxidized to sulfoxides using HNO3 supported on silica gel and polyvinylpyrrolidone (PVP) in the presence of a catalytic amount. of KBr or NaBr81. Further studies are necessary to understand the mechanism of this reaction.

79 S

SiO2 or Au/SiO2, Ag/SiO2, Pt/SiO2

140 °C n-dodecane or decalin

S

O O

Figure 42: Oxidation of DBT to DBT sulfone

80 Conclusions

The oxidation of DBT to DBT sulfone was influenced by various factors. The pH at

which silica gel was synthesized influenced the oxidation reaction. Silica gel synthesized in the

pH range of 3-9- did not oxidize DBT. Oxidation of DBT only occurred when silica was

synthesized at pH in the range of 1-2. The structure of the silica is depending upon what pH the

silica gel was synthesized. A silica gel synthesized at acidic pHs will have H+ embedded in the

matrix, whereas a silica gel synthesized under basic pH will have OH- embedded in the matrix.

Depending upon the pH, silica adopts different structures. Silica synthesized at an acidic pH

forms linear structures, whereas silica synthesized at basic pH forms branched structures80. In

order for silica to catalyze the reaction, reactants should be able to access the surface. In a

branched structure the access to the surface may be difficult making basic silica un-reactive in

this reaction. The drying temperature of the synthesized silica gel did have an impact in the oxidation reaction. The oxidation reaction where silica gel dried at 300oC was used as the catalyst had the best percentage conversion of DBT to DBT sulfone. Drying temperatures usually may induce structural changes, for example, high temperature drying of SiO2 usually

yields a crystalline quartz structure. The temperatures used in this study to dry the synthesized

silica were not high enough to yield a crystalline structure; however, at higher temperatures silica loses protons and OH. That may account for the low catalytic activity of silica when dried at higher temperatures. Many catalysts are active due to their disordered structure21. It is possible

this is the case when using silica as a catalyst. Quartz a crystalline phase of silica was tested in

the oxidation reaction and no conversion of DBT to DBT sulfone was observed.

The reactions were carried in heptanes and n-dodecane at different temperatures.

Oxidation of DBT did not occur in heptanes or hexanes but did it occur in n-dodecane. When

81 the reaction was carried out in n-dodecane at 190oC (very close to the boiling point of dodecane)

DBT oxidized to DBT sulfone. When the reaction was carried out in dodecane at 160oC the oxidation reaction did not occur. Oxidation did take place when either decalin or tetralin was used as the solvent. However, it was shown decalin has a working temperature range which yields a bell shape curve. Each solvent yields different conversion percentages at different temperatures. Further studies are necessary to determine the working temperature range of dodecane although according to the data it is suggested the temperature range for the reaction to occur is between 175oC-190oC. The ability to donate hydrogen radicals was the difference in the

tested solvents. Tetralin is considered a good hydrogen donor molecule due to the stabilization of

the pi system in the ring. Decalin is considered a poor hydrogen donor and n-dodecane is

considered a non-hydrogen donor molecule. Regardless of their ability to donate hydrogen,

when the reaction was performed in these solvents there was up to 90% removal of DBT.

However, the 90% DBT removal occurred at different temperatures. The reaction is both

temperature and solvent dependent. If the reaction would have been only temperature

dependent, then it would have worked at 140oC independently from the solvent used. Even

though n-dodecane is not a hydrogen donor, in the presence of a catalyst, it forms hydrogen

radicals87. It is possible the reaction is radical driven, and hydrogen radicals are provided by the

solvents. Since n-dodecane is considered a non-hydrogen donor solvent that may be the reason

why the reaction does not occur at 140oC but it does occur at 190oC. Higher temperatures are required to activate the hydrogen atoms in n-dodecane. Oxidation of DBT using H2O2 is

accomplished by the formation of OH radicals88. Acidic silica at the right temperature and with

the right solvent catalyzes the oxidation reaction of DBT. Protons and OH groups may activate

the hydrogen atoms in the solvent leading to the formation of OH radicals.

82 Metal nanoparticles were supported on commercial silica and the activation energy was

calculated. Pt/SiO2 activation energy was 20.26 kJ/mol, the value of the activation energy of

Au/SiO2 was 47.04 kJ/mol, the activation energy of Ag/SiO2 was 77.31 kJ/mol and the activation energy of SiO2 was 115 kJ/mol. Pt/SiO2 had the lowest activation energy for this reaction. Metal

nanoparticles enhanced the oxidative capabilities of silica since the activation energy is lower

than in silica by itself. Metal nanoparticles are known to serve as the surface of dissociation for

oxygen89. This may account for the lower activation energy exhibited by the metal

nanoparticles. It is possible silica activates the solvent and the metal nanoparticles break O2.

Metal nanoparticles had lower activation energy and the reaction occurred faster. The oxidation reaction was completed in 2 h, whereas when using silica by itself the reaction was completed after 4 h. In the industry setting, metal nanoparticles may be a better option to use as the catalysts. By cutting the reaction time in half the time, companies save money in different ways such as less consumption of electricity. In addition, the percentage of metal is about 1 % which makes it cost accessible. The recycling experiments showed Au/SiO2 kept its catalytic activity

constant through the four reaction cycles. The platinum catalyst dies as a side effect of its

reactivity. Platinum is the catalyst that removed the most DBT in the first cycle and then it died

probable due to poising of the catalyst. EXAFS showed a new phase appearing in the silver

catalyst. The new phase was Ag2S. Silver catalyst dies may be due to the presence of Ag2S on the surface. This means DBT is stucked onto surface of the catalyst blocking the access of the reactants to the surface. EXAFS also showed the nanoparticles grew after the reaction which agrees with studies in the literature90. The metal nanoparticles resembled the bulk metal they

were synthesized from. Silica may activate the solvent and the metals activate the oxygen

leading to a low activation energy of the catalysts. To confirm the reaction is radical driven

83 TEMPO a radical scavenger was used. In the presence of TEMPO no oxidation was observed.

This leads to the conclusion this reaction is radical driven. Silica activates the solvent and the

metal nano particles activate the oxygen to form OH radicals. X-ray diffraction, Infrared

Spectroscopy and Raman Spectroscopy techniques successfully confirmed DBT was oxidized to

DBT sulfone. Further studies are necessary to determine the mechanism of the oxidation of DBT using silica and metal nanoparticles supported on silica.

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92 Curriculum Vita

Karina Castillo was born in El Paso TX on April 26, 1981. She graduated from a high

school in Juarez, Chih. She moved to El Paso TX and got enrolled at El Paso Community

College in English classes. After two years she started my bachelor’s degree in chemistry at

UTEP. While doing my bachelor’s degree, she was part of different programs such as Bridges to the Future, RISE and MARC programs. These programs provided me with the necessary research skills that she needed during my master’s and doctorial degrees. As an undergraduate researcher, she studied the salt bush Atriplex canescens and its potential to desalinate water. As an undergraduate she was awarded the poster presentation in the division of inorganic chemistry at the SACNAS meeting in 2005. She also did an internship at the University of Houston. She graduated from by bachelor’s degree in 2005 as a cum laude student and she got the Outstanding

Undergraduate Research Student award.

In 2006, she started my master’s degree in chemistry and graduated in May 2007.

During her master’s degree she did research in hydrogen storage materials. She graduated as the

Graduate Student Marshall for the College of Science. In the summer of 2007 Karina started her

PhD degree and as May 2010 Karina is a doctorial candidate. As a doctorial student she got a fellowship from the UNEEC program through NASA. My dissertation deals with a novel way to oxidize sulfur compounds to their respective sulfones. So far, she has 3 publications and 2 patents. Karina has been teaching general chemistry for three years. Two years she taught at El

Paso Community College and Karina has been teaching almost for a year at South Texas

College.

Permanente Address: 508 Martha Lousie

Edinburg, TX 78539

93 This dissertation was typed by Karina Castillo

94