Applications of Ionic Liquids in Membrane Separation

University of Arkansas, Fayetteville ScholarWorks@UARK

Theses and Dissertations

12-2019

Mohanad Kamaz University of Arkansas, Fayetteville

Follow this and additional works at: https://scholarworks.uark.edu/etd

Part of the Complex Fluids Commons, Membrane Science Commons, and the Transport Phenomena Commons

Citation Kamaz, M. (2019). Applications of Ionic Liquids in Membrane Separation. Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/3491

This Dissertation is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of ScholarWorks@UARK. For more information, please contact [email protected]. Applications of Ionic Liquids in Membrane Separation

A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Engineering

Mohanad Kamaz University of Basrah Bachelor of Science in Chemical Engineering, 2011 University of Arkansas Master of Science in Chemical Engineering, 2017

December 2019 University of Arkansas

This dissertation is approved for recommendation to the Graduate Council.

______Ranil Wickramasinghe, Ph.D. Dissertation Director

______David Ford, Ph.D. Ed Clauson, Ph.D. Committee Member Committee Member

______Xianghong Qian, Ph.D. Wen Zhang, Ph.D. Committee Member Committee Member

Abstract

Ionic liquids represent an emerging attractive material in membrane technology. The central theme of this doctoral dissertation is to develope novel membranes using ionic liquids. Two different approaches were used to prepare ionic liquid membranes including the immobilization of ionic liquid within the membrane pores or the use of pressure assembly method to deposit a selective ionic liquid layer on the top membrane surface.

In chapter 2, imidazolium ionic liquids with three different alkyl halides were successfully synthesized and used to prepare supported ionic liquid membranes (SILMs). SILMs preraper were tested for aqueous and nonaqueous applications. For nonaqueous applications, the trend of separation was found to be divinylbenzene> styrene> toluene, which was attributed to the extended conjugation of π electron cloud. For nonaqueous system, preferential separation of different dyes was achieved.

In chapter 3, SILMs have been tested for the fractionation of organic compounds.

Fractionation of a feed stream consisting of benzene, naphthalene, and phenanthrene in tetradecane and cis- and trans-stilbene in hexane were investigated. Differences in the π electron cloud density of the aromatic solutes influence their interactions with the imidazolium cation which can affect the rates of transport. In the case of the three aromatic solutes the degree of conjugation and molecular weight increase in the order of benzene, naphthalene, and phenanthrene. However, for cis-and trans-stilbene, the mass transfer coefficient was larger for all three ionic liquids. Aqueous

SILM was used as the liquid membrane phase to fractionate two nucleobases: thymine and cytosine in water. The mass transfer coefficient for thymine was 5.5 times greater than cytosine indicating the possibility of exploiting multimodal interactions fractionate compounds.

Chapter 4 constitutes the preparation of novel polyionic liquid augmented membranes that were fabricated using pressure assisted self-assembly and employed for π- electron cloud mediated separation of aromatics from their aliphatic homologs. The prepared membranes can be employed for the fractionation of aromatics such as benzene, toluene, p-xylene, nitrobenzene. Further, these augmented membranes were demonstrated to have preferential separation of one stereoisomer over others.

Finally, chapter 5 includes conclusions drawn out of this research and future directions and recommendations.

Dedication

To my beloved greatest mother, Azhar Almosowy.

To my father, whose soul is always present to support me throughout my journeys, Ali Kamaz.

Acknowledgment

First and foremost, I would like to express my sincere gratitude to my supervisor Dr.

Ranil Wickramasinghe for giving me the opportunity to this journey and fund me throughout this

Ph.D. study as well as introducing me to the membrane research. His significant scientific contribution and wise guidance made this Ph.D. research not impossible. Also, I would show my appreciation and gratitude to Dr. Arijit Sengupta for all the innovative input he had put in the research and the long pertaining discussion and explanations. His guidance and sharp notifications made this research recognizable.

My thanks also go to all the group members who helped me in my research and kept standing me on my feet during the hard times. Financial support from the Arkansas Research

Alliance and the University of Arkansas are gratefully acknowledged.

I would also like to express my great appreciation for my family in particular my mother.

I am so thankful for your unconditional love and support that motivates me to do the best.

Thanks for all the prayers you said to keep me blessed.

Besides my advisor, I would like to thank my dissertation committee: Prof. David Ford,

Prof. Ed Clauson, Prof. Xianghong Qian and Associate Prof. Wen Zhang. Thanks for being part of this dissertation and giving time to attend my defense. Also, thanks for your constructive comments and suggestions which widen and strengthen my research from many perspectives.

Last but not the least, I would like to thank my host family the Petersons here in the U.S. for their tremendous unconditional love and support, especially taking care of me during the downtimes and keeping me moving forward.

Mohanad Kamaz Ralph E Martin Department of Chemical Engineering University of Arkansas

Table of Contents

Chapter 1 Introduction ...... 1

1.1 Membrane Based Technology ...... 1

1.2 Membrane processes ...... 1

1.3 Advanced membrane separation ...... 4

1.4 Novel mechanisms in membrane separation ...... 5

1.4.1 Charge based separation mechanism ...... 6

1.4.2 Hydrophobic interactions ...... 8

1.4.3 Facilitated transport ...... 9

1.4.4 Pi electron cloud interaction ...... 11

1.5 Ionic liquids and poly(ionic liquids) ...... 14

1.5.1 Synthesis of ionic liquids ...... 18

1.6 Preparation of IL based membranes ...... 20

1.6.1 Supported ionic liquid membranes ...... 21

1.6.2 Physical coating using pressure assisted self-assembly ...... 22

1.7 Application of IL membranes ...... 24

1.8 References...... 30

Chapter 2 Pi electron cloud mediated separation of aromatics using supported ionic liquid membranes (SILMs) having antibacterial activity ...... 38

2.1 Introduction ...... 39

2.2 Experimental ...... 41

2.2.1 Materials ...... 41

2.2.2 Synthesis of ionic liquid ...... 41

2.2.3 Preparation of supported liquid membranes ...... 43

2.2.4 Characterization ...... 43

2.2.5 Transport studies ...... 44

2.2.6 Stability of the SIL membranes ...... 45

2.3 Results and discussion ...... 45

2.3.1 Characterization ...... 45

2.3.2 Pi- electron mediated separation of aromatics and dyes ...... 47

2.3.3 Stability of the SIL membranes ...... 52

2.4 Conclusions ...... 56

2.5 Acknowledgements ...... 56

2.6 References...... 57

Chapter 3 π Electron induced separation of organic compounds using supported ionic liquid membranes ...... 61

3.1 Introduction ...... 63

3.2 Materials and methods ...... 66

3.2.1 Materials ...... 66

3.2.2 Preparation of ionic liquids ...... 67

3.2.3 Preparation of SILMs ...... 68

3.2.4 Membrane characterization ...... 68

3.2.5 Membrane stability ...... 68

3.2.6 Membrane performance ...... 69

3.2.7 Solute detection ...... 74

3.3 Result and Discussion ...... 75

3.3.1 Membrane characterization ...... 75

3.3.2 SILM performance ...... 76

3.4 Conclusions ...... 90

3.5 Acknowledgements ...... 90

3.6 References...... 91

Chapter 4 Poly(ionic liquid) augmented membranes for π electron induced separation/fractionation of aromatics ...... 96

4.1 Introduction ...... 98

4.2 Experimental ...... 101

4.2.1 Materials ...... 101

4.2.2 Instrumentation...... 101

4.2.3. Synthesis of polyimide support ...... 102

3.2.4 Synthesis of PILs ...... 103

3.2.5 PIL augmentation by pressure assisted self-assembly ...... 104

3.2.6 Performance ...... 105

4.3 Result and discussion ...... 106

4.3.1. Characterization...... 106

4.3.1.1 NMR spectroscopy ...... 106

4.3.1.2 Water contact angle ...... 108

4.3.1.3 Surface morphology ...... 109

4.3.1.4 FTIR ...... 111

4.3.2 Applications ...... 113

4.3.2.1 Separation of aromatic from aliphatic homologue ...... 113

4.3.2.2 Nature of substituents in benzene ring on the fractionation...... 116

4.3.2.3 Influence of the extended conjugation on the fractionation ...... 117

4.3.2.4 Separation of stereoisomers ...... 119

4.3.2.5 Separation of regioisomers...... 120

4.3.3 Stability of the PIL augmented membranes ...... 121

4.4 Conclusions ...... 122

4.5 Acknowledgments ...... 123

4.6 References...... 124

Chapter 5 Conclusions and Future Direction ...... 128

5.1 Conclusions ...... 128

5.2 Future Direction and Suggestion...... 132

5.2.1 Membrane modification ...... 132

5.2.2 Membrane separation techniques ...... 132

Appendices ...... 134

Appendix I ...... 134

Appendix II ...... 138

HPLC protocol ...... 140

2.1 Running the HPLC system ...... 140

2.2 Removing the column ...... 141

2.3 Backflushing the C18 column ...... 141

Appendix III ...... 142

List of all papers representing UARK ...... 143

List of Figures

Figure 1.1: Salient features of Ionic Liquid...... 15

Figure 1.2: Cations and anions and their chemical structure that are commonly used in ionic liquids synthesis [63]...... 17

Figure 1.3: Schematic of the preparation of imidazolium halides...... 19

Figure 1.4: Schematic illustration of the supported liquid membrane setup...... 22

Figure 1.5: Schematic representation of membrane modification via pressure assisted self- assembly technique...... 23

Figure 2.1: The synthesis scheme for water soluble/insoluble ionic liquids...... 43

Figure 2.2: FTIR spectra for the base PTFE membrane and supported ionic liquid membranes. 47

Figure 2.3: SEM images for the supported ionic liquid membranes (a) 1 vinyl-3 allyl imidazolium bromide, (b) 1 vinyl-3 hexyl imidazolium bromide and (c) 1 vinyl-3 butyl imidazolium chloride...... 48

Figure 2.4: Separation of toluene, styrene and divinyl benzene by supported ionic liquid membranes (all the data are reported based on triplicate measurements with an error less than 5 %)...... 50

Figure 2.5: Transport of dyes through SILMs with water insoluble ionic liquids (all the data are reported based on triplicate measurement with an error less than 5 %)...... 52

Figure 2.6: The stability of supported liquid membranes in different media; (a) 1 hexyl 3 vinyl imidazolium bromide; (b) 1 allyl 3 vinyl imidazolium NTf2; (c) 1 allyl 3 vinyl imidazolium bromide; (d) 1 hexyl 3 vinyl imidazolium NTf2; (e) 1 butyl 3 vinyl imidazolium chloride; (f) 1 butyl 3 vinyl imidazolium NTf2...... 55

Figure 2.7: The % transport for toluene, styrene and DVB using SILMs with water soluble ionic liquid: a. allyl, original; a1. allyl, equilibration with hexane; b. chloro, original; b1. chloro, equilibration with hexane; c. bromo, original; c1. bromo, equilibration with hexane...... 56

Figure 3.1: Chemical structure of solutes...... 74

Figure 3.2: Schematic diagram of the lab scale supported ionic liquid apparatus...... 74

Figure 3.3: FTIR spectra for: (A) base polyrpopylene membrane and membrane pores filled with: (B) [C3vim+][Br−];(C) [C6vim+][Br−]; (D) [C6vim+][NTf2-];(E) [C8vim+][Br−]…………….77 Figure 3.4: Variation of benzene, naphthalene and phenanthrene concentration in the receiving + - + - solution as a function of time for membrane pores containing (a) [C3vim ][Br ], (b) [C6vim ][Br + - ], (c) [C8vim ][Br ] respectively ...... 78

Figure 3.5: Variation of benzene, naphthalene and phenanthrene flux with time for membrane + - + - + - pores containing (a) [C3vim ][Br ], (b) [C6vim ][Br ], (c) [C8vim ][Br ] respectively ...... 79

Figure 3.6: Variation of cis-stilbene and trans-stilbene concentration in the receiving solution as a + - + - function of time for membrane pores containing (a)[C3vim ][Br ], (b)[C6vim ][Br ], (c) + - [C8vim ][Br ] respectively...... 81

Figure 3.7: Variation of cis-stilbene and trans-stilbene flux with time for membrane pores + - + - + - containing (a) [C3vim ][Br ], (b) [C6vim ][Br ], (c) [C8vim ][Br ] respectively...... 82

Figure 3.8: Variation of cytosine and thymine concentration in the receiving solution as a function + - of time for membrane pores containing [C3vim ][NTf2 ]...... 83

Figure 3.9: Variation of cytosine and thymine flux with time for membrane pores + - containing[C3vim ][NTf2 ]...... 83

Figure 3.10: Variation of conductivity with time for hexane, tetradecane and water containing + − + − + − membranes with pores filled with (a)[C3vim ][Br ], (b)[C6vim ][Br ], (c)[C8vim ][Br ] ...... 89

Figure 3.11: Variation of conductivity with time for water containing a membrane with pores filled + - with [C6vim ][NTf2 ]...... 90

Figure 4.1: The schematic presentation of the crosslinking between P84 and HMD for fabrication of the support material...... 103

Figure 4.2: The schematic presentation for pressure assisted self-assembly to form PIL augmented membranes...... 105

Figure 4.3: The NMR spectra along with the synthetic scheme for the PIL obtained from 2 and 4 isomers of vinyl pyridine...... 108

Figure 4.4: Dynamic water contact angle measurement for PIL augmented membranes...... 109

Figure 4.5: Surface morphology of PIL augmented membranes by SEM (surface and cross sectional view), EDX spectroscopy and AFM (2D and 3D view) for 2 vinyl pyridine and 4 vinyl pyridine membranes...... 111

Figure 4.6: The FTIR spectra for PIL augmented membranes and polyimide support...... 113

Figure 4.7: Separation of aromatics from aliphatic homologues using PIL augmented membranes...... 115

Figure 4.8: The effect of substituents in benzene ring on the fractionation of aromatics by PIL augmented membranes...... 117

Figure 4.9: The effect of extended conjugation on the fractionation of aromatics by PIL augmented membranes...... 119

Figure 4.10: Separation of stereoisomer of stilbene using PIL augmented membranes...... 120

Figure 4.11: Separation of two regioisomers of bipyridine using PIL augmented membranes. . 121

Figure 4.12: The stability of PIL augmented membranes in different organic solvents of interest...... 123

Figure A.1: Chemical structure of dyes and aromatic compounds……………………………..127

Figure A.2: Schematic diagram and the photographic picture of the diffusion cell used in this study………………………………………………………………………………………….....128

Figure A.3: The linear relationship between concentration and absorption for the dyes using UV- vis at optimized wavelengths…………………………………………………………….……..129

Figure A.4: The linear relationship between concentration and absorption of the aromatic compounds using UV-vis at optimized wavelengths……………………………….…..………130

Figure A2.1: Standard curves for benzene, naphthalene, and phenanthrene using HPLC equipped with C18 column……………………………………………………………………………..…132

List of Tables

Table 1.1: Using of ILs in membrane separation applications...... 27

Table 3.1: Summary of diffusion experiments ...... 71

Table 3.2: HPLC detection wavelengths and retention time for solutes……………………..…..76

Table 3.3: Calculated mass transfer coefficients...... 85

Table 3.4: The separation factor (ℬ) using different SILMs.….………………………….....…..86

Table 4.1: Relative compositions for different solvent systems in feed and permeate...... 116

Table A3.1: Chemical structure of compounds used in this chapter and their distinct absorption wavelength with UV-Vis……………………………………………………….…….…………135

List of published papers generated from this dissertation

Ø Chapter 2: Mahmood Jebur, Arijit Sengupta, Mohanad Kamaz, Yunshan Chiao, Xianghong Qian, Ranil Wickramasinghe. (2018). Pi electron cloud mediated separation of aromatics using supported ionic liquid membranes (SILMs) having antibacterial activity. Journal of Membrane Science. 556, 1-11. [86]. (Published)

Ø Chapter 3: Mohanad Kamaz, Ronald Vogler, Mahmood Jebur, Arijit Sengupta, Ranil Wickramasinghe (2019). π Electron induced separation of organic compounds using supported ionic liquid membranes. Separation and Purification Technology, 116237. [87]. (Published)

Ø Chapter 4: Mohanad Kamaz, Arijit Sengupta, Sandrina DePaz, Yu-Hsuan Chiao, Ranil Wickramasinghe. (2019). Poly(ionic liquid) augmented membranes for π electron induced separation/fractionation of aromatics. Journal of Membrane Science. 579, 102-110. [88]. (Published)

Chapter 1 Introduction

1.1 Membrane Based Technology

A membrane is basically defined as a barrier that selectively permeates targeted species while limits the transport of other ones in a mixture. The membrane definition is so broad since the membrane could be; porous and nonporous, homogenous or heterogeneous, symmetric or asymmetric in structure, a charge or neutral surface, and made of organic or inorganic materials.

Ultimately a membrane has the ability to discriminate between molecules. Permeation of individual components through a membrane is achieved via a driving force. The driving forces could be established by concentration, pressure or temperature gradient or even by an electric field

[1]. Membrane based separation provides a clean, economically viable, easy to operate, efficient separation procedure with the ease of sizeable industrial scale adaptation. Hence, the pace of developing membrane separation processes has been accelerating for the last couple of decades

[2].

1.2 Membrane processes

Commercially implemented membranes are typically composed of a homogeneous organic or inorganic material which allows the components to be separated from a mixture and permeated through at different rates. Due to its favorable technological characteristics, membrane-based technology was extensively employed in the field of water treatment including wastewater and bio-pharmaceutical separation [3]. Nonaqueous based separation using membranes has also become popular in recent days. Though membrane based technology is widely used, constant R&D efforts have been conducted to improve the membrane performance either in terms of flux, selectivity or antifouling properties of membranes [4].

Improvement during membrane fabrication or via modification of the surface morphology of the membrane are the two approaches followed to achieve desired membrane performance.

Among the membrane processes, pressure driven membranes (microfiltration, ultrafiltration, nanofiltration and reverse osmosis), electrodialysis (ED), as well as membrane distillation (MD) have been extensively applied for water treatment including portable drinking water, a variety of industrial wastewater, brackish water, agricultural wastewater and even in municipal wastewater streams [5]. The membrane based separation is highly advantageous over other conventional techniques especially in food and pharmaceutical industries due to the mild experimental conditions, which assure the quality of such products in terms of taste, aroma, color, biological activity, etc. remain unaltered [4–7].

The separation mechanism in membrane processes is based on a difference in: size of molecules (micro and ultrafiltration membranes), vapor pressure (membrane distillation), affinity

(adsorption or absorption), solubility and diffusivity of materials in the membrane (membranes for gas separation), the electrical charge of the species (electrodialysis), or the facilitated transport

(liquid membranes). These classifications can be narrowed down to three main categories: porous membranes, dense membranes, and electrically charged membranes.

In spite of these wide range of membrane technology applications, some crucial challenges need to be overcome in order to have long term commercialization. Some of the significant challenges are as follows;

I. Membrane fouling is one of the critical problems faced by membrane technologists.

The interaction of feed components with the membrane materials leads to membrane

fouling resulting in deterioration of the overall membrane performance in terms of

reduction in flux, loss of selectivity, changes in rejection etc. [8]. Since surface

properties play a significant role in membrane performance, modification of

membrane surface has been demonstrated to be successful. However, the nature of

modification solely depends on the membrane materials, nature of feed constituents

and the operation conditions. Controlled modification of the membrane surface to

achieve desired performance without compromising other factors is challenging.

II. Size exclusion is the most common basis for the membrane-based separation.

Therefore, mutual separation of two components of the same molecular weight

remains one of the biggest challenges in membrane technology especially in

nonaqueous separation. Therefore, the development of membrane-based separation

guided by different physical properties is essential.

III. Concentration polarization leading to a deterioration of membrane performance is

another drawback that has to be addressed. Several approaches have been reported in

the literature either by feed conditioning or by incorporating reversible responsiveness

near the boundary layer of the membrane. The incorporation of different stimuli into

the membrane to achieve reversible responsiveness is essential.

IV. Biofouling due to the interaction of membrane materials with microorganisms is

another major challenge while processing wastewater [9]. The antimicrobial growth is

of concern during the preservation of the membrane materials. Therefore, the

membrane with inherent anti-microbial activity is desired to sustain membrane

performance without biofouling.

1.3 Advanced membrane separation

Due to the increasing concern about environmental pollution and the establishment of more stringent regulations, researchers and industrial corporations have intensively started seeking novel environmentally friendly and potentially more economical technologies [10]. In several industries, for example petroleum refineries, solvent mixtures accumulate due to recycling deficiency and difficulty of their separation. Breaking down of mixtures into their pure components is a necessity for their reuse. This has led to an accelerated focus on the development of membrane technology for the advanced separation because of favorable features that overcome the drawbacks associated with classical separation techniques. Features such as low energy consumption, moderate pressure, and perhaps the most preferred property is their selective permeation, are vital for their vast applications [11]. Given these advantages over the conventional processes, membrane technology could be found in industries like petroleum, electrochemical, and pharmaceutical for the treatment of waste stream and purification of downstream products.

In the petrochemical industry, membrane processes could be implemented for a selective permeation of aromatic over aliphatic compounds using various processes like pervaporation [12] adsorptive membranes [13] and supported liquid membranes [14] which otherwise require high cost and larger footprints via conventional methods. Thus, membranes with higher transport selectivities and permeabilities are necessary to achieve optimal performance. Chemical features of the membrane, and of the molecule to be selectively transported can be exploited to enhance transport selectivity, these include charge, chemical interactions, and molecular size. Traditionally, microporous membranes have been used for size based exclusion with high permeability for species smaller than the membrane molecular weight cutoff (MWCO) while large components are retained in the feed tank. Membrane MWCO is an essential tool to predict solute rejection by

4 membrane filtration because size exclusion is such a vital mechanism. Membrane suppliers provide MWCO information regarding solute rejection tests using macromolecules such as proteins and dextrans that are uncharged [15]. However, the need for developing advanced techniques with low membrane fouling, high selectivity and high permeate flux is necessary to meet industrial demands. The scope of recent research and development is to focus on radical enhancement in selectivity while maintaining the membrane high permeability and low fouling characteristics. In particular one can make use of surface interactions to obtain separations among species of similar molecular weight.

1.4 Novel mechanisms in membrane separation

To expand membrane applications in separation science efficiently, researchers have started looking at nonconventional separation mechanisms. Though membranes are generally characterized by their pore size and molecular weight cut off for solutes rejection, solutes with molecular size or collision diameter bigger than the pores diameter are retained in the feed side of the membrane process. Lately, novel approaches have been explored by scientists to make the most use of membranes in separation areas where conventional mechanisms have not met the demand.

1.4.1 Charge based separation mechanism

Microfiltration membranes (MF) are used to separate particles in the size range of 0.1–10.0

µm while ultrafiltration (UF) membranes with 1–100 nm pore size are suitable for the separation of proteins and other macromolecules [16]. Hence, UF membranes have been extensively commercialized in the pharmaceutical industry for polishing downstream products and concentrating protein feed. The separation mechanism of porous membranes is generally through sieving while novel charged membranes were recently developed for a more efficient separation mechanism involving both size and charge based exclusion for separation/fractionation of protein molecules. Molecules with a similar charge to the membrane surface could be retained by the electric repulsive force. Thus, if a membrane is negatively charged, then solutes with low cationic charge density and high anionic charge density are effectively repelled while neutral charge species are not included in this mechanism and as a result, their rejection is highly governed by the membrane pore size. In consequence, charged species much smaller than the membrane pore size could be rejected by a membrane of similar charge while the same size species that do not carry electric charge are transmitted through the membrane readily [17,18].

Membranes from different materials with positive or negative charges were generally used for selective protein separation due to interactions between transporting species and membrane surface. Not only is selective transport achieved via charged membranes but also low fouling prone membranes because of electrostatic repulsion with similar charge foulants. Several parameters including pH and ionic strength of the feed solution could significantly affect the overall membrane performance. Early studies by Zydney and his collaborators have shown that electrostatic interactions between charged proteins and the membranes surface, as well as the pH values and ionic strengths, have severe effects on protein separation [19–21].

For example, for the filtration of bovine serum albumin (BSA) and immunoglobulin G

(IgG) in high salt concentration, the selectivity was drastically modified from a value of only two, at pH 7 and high salt concentrations, to more than 30 simply by adjusting the pH to 4.7 and lowering the solution ionic strength [22]. This was attributed to the electrostatic repulsion of IgG with a positive charge at pH 4.7, while the uncharged BSA is highly permeable through the membrane. This has paved the way for more exciting new opportunities using electrostatic interactions in the optimization of membrane systems for protein purification. Another research reported preliminary results of protein filtration through both neutral and positively charged polyacrylonitrile membranes, with the latter chemically modified with a quaternary amine [23].

Considerably higher permeation of BSA through the positively charged membrane was observed in comparison with the base membrane at pH above the protein isoelectric point (negatively charged BSA). Miyama et al. [23] related this observation to the electrostatic interactions between the negatively charged BSA and the positively charged modified membrane.

Another application of charge based membranes is the purification of wastewater containing natural organic matter (NOM) based on molecular electrostatic interaction with a charged membrane surface [24]. Economic factors are directly linked to the membrane performance, such as selectivity toward the targeted components, flux deterioration, and perhaps more importantly fouling of the membranes. The ideal feature of a membrane employed for organic rich water purification is a negatively charged surface to repel natural organic matter that is negatively charged as well. Hence, this electrostatic repulsion will mitigate the possibility of membrane fouling due to NOM attachment and will maintain stable flux over the operation course

[25]. Further, this charge aversion is utilized to reject molecules based on their charge, regardless of their molecular size. NF membranes are extensively known for their negatively charged surface.

However, recently positively charged membranes have attracted a lot of attention. Some studies have shown that positively charged NF membrane tends to have a higher rejection rate for divalent cations and cationic dyes than that of a negatively charged NF membrane [26].

1.4.2 Hydrophobic interactions

The simplest definition of hydrophobic interaction is the aggregation of nonpolar species in a polar solvent such as water due to the hydrophobic effect. Thus, the association of these noncharged molecules in polar media is termed as hydrophobic interactions. Nonpolar molecules are unable to participate in hydrogen bonding so their presence in polar media is not favorable.

When a drop of oil is introduced to a polar medium, such as water, the water molecules tend to form cages around the oil molecules. Hydrophobic interaction is a thermodynamically governed phenomenon that determines the spontaneity of the mixing of hydrophobic molecules and water

[27].

For example, the presence of oil molecules in water induces the disassociation of hydrogen bonds among water molecules to have a space for the nonpolar molecules. This hydrogen bonding disassociation is endothermic as heat is added to the system to break the bonds. However, the effects of enthalpy are insignificant since it could be positive, negative, or even zero, depending on whether the hydrogen bonds disassociation is complete or partial. Water molecules disrupted by the presence of the nonpolar molecules will make new hydrogen bonds around the nonpolar species. This orientation makes the system more uniform and decreases the total entropy of the system. Thus, the entropy change is considerably large and dictates the mixing of hydrophobe and water [28].

This phenomenon is wildly implemented as a separation mechanism in membrane technology, especially for protein purification, particularly in the pharmaceutical industry.

Hydrophobic interaction chromatography (HIC) membrane is microporous support and the chromatographic ligands are attached to the surface of the membrane pores [29,30]. Membrane chromatographic is easy and cheap to produce massively. In the separation of biomolecules, such as proteins, the interaction between the immobilized hydrophobic ligands on and in the HIC membrane and nonpolar regions of molecules during the convective flow through the membrane pore is a thermodynamically favorable process. Phenyl-based HIC membrane adsorbers were used for purification of α1-antitrypsin (AAT) from human plasma fraction. The results demonstrated that recovery of 91.4% and a purity of 1.22 AAT/protein (g/g) could be achieved that were higher than the recovery reported by using hydrophobic interaction chromatography column [31]. Further, the same researchers later reported HIC membrane functionalized by dodecyl mercaptan ligands to fractionate IgG from human serum albumin (HSA) and showed the purity of IgG in the elution pool was above 94% [32].

1.4.3 Facilitated transport

In membrane technology facilitated transport means the involvement of a chemical agent to selectively carry from the feed composition and ultimately increase the rate of transport. A chemical agent can react reversibly with a specific solute and results in high selectivity and permeability. This makes facilitated transport a very selective separation technique. The carrier often is immobilized in the membrane pores so it conveys a selective solute from the feed side through the membrane and releases it in the permeate side due to the difference of the chemical conditions such as pH and electric charge and then diffuses back [33,34]. Thus, carrier selection is crucial to optimize the transport, and a carrier with high association and dissociation rate constant

9 is desirable [18]. Membrane selectivity indicates the permeation of desired solutes and retention of the others on the feed side. Since the transport involves a chemical reaction between the solute and the carrier, facilitated transport is not described by Fisk’s law. Hence, a solute could be selectively carried against its concentration and not affected by equilibrium.

Facilitated transport is applicable to transfer a broad range of molecules and ions based on several specific target permeants such as amino acids, organic acids, O2, CO2, olefin, and sugar, etc. by using suitable carrier agents in liquid or solid composite membranes. A conventional supported liquid membrane composed of microporous hydrophobic support filled by an organic phase (carrier) that separates two aqueous or nonaqueous phases [35]. An important feature of the carrier is its immiscibility with the feed and strip solutions and stability inside the membrane structure due to surface tensions and capillary forces. Hence, liquid membranes (LMs) act like liquid-liquid extraction systems in which the amount of deployed extractant is minimized. Due to their exciting feature of carrying a solute against its concentration gradient, they can be efficiently used for wastewater treatment, metals extraction and chemical/biomedical engineering systems

[36,37]. Furthermore, these systems could be operated with high permeability in comparison with solid membranes because of the higher diffusion coefficients in liquids, low operating costs, and easy feasibility [38,39]. In wastewater treatment, LMs offer the advantage of having an intensified process consisting of sorption and stripping units for an efficient technology to remove hazardous contaminants from wastewater streams and a selective cleanup.

1.4.4 Pi electron cloud interaction

Another vital interaction that could be implemented in a selective separation technology is the use of aromatic interactions. Aromatic compounds are unsaturated cyclic and planar molecules that contain electrons located above and below the plane of the aromatic ring, which gives rise to a π-electron cloud over the ring. The aromatic or π-cloud interaction is the interaction between aromatic π-electrons of compounds and those of the solute species. Because of this cloud of π electrons on top and below the flat face of an aromatic ring, the ring is negatively charged. Recently research has studied the aromatic interactions happening in nature to provide an insightful understanding of these interactions in terms of their usage as a driving force for selective noncovalent interactions [40]. Two π-system interactions are involved in aromatic moieties;

- π-π cloud interaction (electron donating/withdrawing interaction)

- Cation-π interaction

1.4.4.1 π-π cloud interaction (electron donating/withdrawing interaction)

Among these highlighted interactions of π systems, the non-covalent attraction between neutral closed to each other aromatic rings is often categorized using the term π-π interaction or π- stacking [41–43]. Distinguishable types of geometries, namely characterize this interaction;

- Edge-to-face or T-shaped

- Face-to-face

- Parallel displaced or stacked interactions

The T-shaped and stacking geometries are interactions between regions of positive and negative charge while face to face geometry has partial electrostatic repulsion causing overlapping of negatively charged regions. An extensive study by Hunter and Sanders demonstrated that the interaction strength between two rings in parallel stacking configuration is changed based on the

11 introduction of an electron-withdrawing/donating substituent. The introduction of a moiety to aromatic benzene ring will reduce or increase the repulsion of two systems and eventually change the charge distribution among the aromatic molecule. Hence, the overall π-π interaction is significantly modified by the strength of donor and acceptor substituent polarizability.

This interesting interaction was exploited in several areas such as analytical chemistry, bioseparation, and even absorption. For example, Peng et al. [44] studied the effects of π–π interactions on the adsorption of polyphenol from pomegranate peel extract on a graphene oxide– cotton fiber composite adsorbent. Their results showed that the concentration of the aromatic ellagic acid in the purified supernatant was tripled while the polyphenols concentration raised from

48% to 76%. In analytical chromatography, it was revealed that π–π interactions are directly linked to the retention behavior of high and low aromaticity on different chromatographic media [45,46].

Moreover, π systems were proposed to be responsible for the recalcitrance of natural organic matters in the soil, which is their resistance to being decomposed in the soil [47].

1.4.4.2 Cation-π interaction

The cation-π interaction is a noncovalent interaction between a cationic moiety and the planar surface of an aromatic π-system with an engagement distance of 6.0 Å [48]. It also could be explained electrostatically as the interaction between a positively charged ion and the negative surface of the ring represented by a cloud of electrons. Thus, electrostatic potential generally plays a major role in the qualitative prediction of the strength of the cation-π interaction as a function of ring substitution [49] where increasing electron-donating substituents owns an increase in the negative potential above and below the ring resulting in a stronger interaction with a cation.

Binding energies of benzene with monovalent cations (e.g., Li+, Na+, K+, and Rb+) were estimated experimentally and theoretically via computational studies to be as high as 167 kJ mol-

1 in the gas phase where it was found that bonding strength increases with the size of the cyclic aromatic π-system and the presence of electron inductive groups [50,51].

Here this research work seeks the use of this π-system interaction through easy tunable ionic liquids and membrane technology for continuous separation of aqueous and non-aqueous aromatics with low energy consumption, small footprint equipment, smooth facilitation operation, slick scale-up, and enhanced membrane properties.

1.5 Ionic liquids and poly(ionic liquids)

Ionic liquids (ILs) are organic or inorganic salts with a melting point lower than 100 °C or even lower than room temperature also referred to as room temperature ionic liquids (RTILs). This definition is widely used to distinguish ionic liquids salts from the traditional salts where their melting temperatures are much higher. Delocalization of their asymmetrical ions and the weak intermolecular forces prevent the formation of the stable crystal lattice and cause considerable low melting temperatures. As a consequence, they are known for their unique chemical properties such as high chemical and thermal stability, negligible volatility and inflammability, low vapor pressure and high ionic conductivity [52–54]. In view of the environmental concern in using volatile organic compounds (VOCs), ionic liquids have gained a worldwide acceptance as a potential alternative

‘green solvent’ to VOCs due to some favorable properties like low vapor pressure, high degree of chemical, radiolytic and thermal stability, a large potential window and a high degree of solubilizing power for different kinds of materials including polymers [55]. The most exciting properties of the ionic liquid are its easy tunability. A small change in either the cationic or the anionic part of the ionic liquid may lead to a large change in their physicochemical properties.

Therefore, the proper understanding of the structure-activity relationship can be utilized to fine tune the property depending on the requirements. Figure 1.1 illustrates the general salient features of ionic liquid.

Low Vapour Pressure Wide Thermal Electrochemical Decomposition Window

Ionic Solvating Poor conductors Property liquids

Resistance to High Viscosity radiation damage

Designer Solvent

Figure 0.1: Salient features of Ionic Liquid.

Recently, ILs have been utilized as building blocks of polymeric structures, generating a new class of polyelectrolyte compounds known as poly(ionic liquid)s (PILs) or also called polymerized ionic liquids. PILs have considerably grabbed the attention of researchers’ interest because of their inherent combined features of ILs, including thermal stability, high ionic conductivity, and adjustable solubility, with specific properties of polymers and processability.

Further, PILs appeared to play a significant role in the amplification of the IL functions, resulting in performance that could not readily be afforded by ILs. As an example, the preparation of membranes requires polymeric materials with excellent mechanical properties and film formation as well as high efficiency for selective transport.

The unique properties of ionic liquids have led to significant improvement in the separation area of various industries such as electrochemistry [56], organic synthesis [57], nuclear industry

[58], biomass processing [59], analytical chemistry [60] and fuel cell applications [61]. The large potential window of ionic liquids provides an opportunity for unconventional signature oxidation state of metal ions, which was not possible by the conventional medium. The extraordinarily high separation efficiency in the hydrometallurgical application of ionic liquid was mainly attributed to the ion exchange mechanism during the separation. It was also reported that structural modification in ionic liquid may promote the extraction mechanism to be ‘solvation’ instead of ion exchange

[62].

Unlike regular polyelectrolytes, PILs can be soluble not only in water but also in polar organic solvents, depending on the anion/cation combination. Their solubility in organic regimes is attributed to the hydrophobic character of their counter-anions and reduced columbic interactions. Indeed, the solubility of ionic liquids is mainly dictated by the nature of the counter anion which could be adjustable during the synthesis reaction or via efficient ion exchange. For example, hydrophilic halides like (Cl-, I-, Br-) have the tendency to get dissolved in water while

- the introduction of hydrophobic counter anion that includes tetrafluoroborate [BF4] ,

- - - hexafluorophosphate [PF6] , nitrate [NO3] and bis(trifluoromethanesulfonyl) imide [Tf2N] results in these ILs immiscible with water [63]. In figure 1.2 the common cations and ions used in the synthesis of ionic liquids are depicted.

Figure 0.2: Cations and anions and their chemical structure that are commonly used in ionic liquids synthesis [63].

Though ionic liquids provide several attractive properties, their industrial scale application suffers from their limitations. The ionic liquid is costly and highly viscous [64]. The high viscosity results in slower kinetics in the hydrometallurgical applications. Though the ion exchange mechanism improves the separation efficiency, it also leads to loss of either the cationic or anionic moiety of the ionic liquid [65,66]. Though ILs have been advantageously implemented in chemical synthesis and analytical chemistry, their use as novel functional materials for the various number of applications e.g. in membrane separation processes, has also been playing a vital role in their vast attraction.

In view of this, there is a need for a separation technique where the ionic liquid inventory is much less, there is no loss of ionic liquid, taking advantage of its higher viscosity and at the same time incorporating the tunable and unique properties of the ionic liquids.

1.5.1 Synthesis of ionic liquids

There are various ways to prepare ionic liquid salts which will be briefly described in this section since the main focus of this dissertation is the implementations and applications of ILs in membrane processes for a favorable performance in comparison with conventional membrane materials. In this section, the methods of preparing ILs with emphasis on imidazolium based ionic liquids as widely studied in this thesis and literature in general will be explained briefly. Ionic liquids could be made via two paths which are alkylation or protonation.

1.5.1.1 Preparing ILs by alkylation reaction

The alkylation of alkyl imidazole with a haloalkane is a widely used method for ionic liquid formation, which involves a quaternization reaction to form a halide salt. This reaction has inherent advantages which are the accessibility of relatively inexpensive alkyl halides and low temperature requirements. Several alkylammonium halides are commercially available and they can be used directly in metathesis reactions, among which are pyridinium and imidazolium halides [67]. Figure

1.3 is a schematic representation for imidazolium based ionic liquid synthesis via alkylation reaction. Nonconventional methods like microwave and ultrasound techniques are being applied in the quaternization reaction for higher purity ionic liquids [68]. Though this reaction is favorable for the preparation of ILs due to its simplicity and mild control requirements, small traces of contaminations of halide ions may react with solute materials and cause impurity for the final product.

Heat CH CH2 2 R-X + N N N N R - X

Figure 1.3: Schematic of the preparation of imidazolium halides.

1.5.1.2 Protonating for ILs synthesis

Another path that avoids contamination of the final product and is used for synthesizing ionic liquids is through acid base neutralization method. This reaction is described by the reaction of acid and base to form salt and water which is produced by the combination of H+ ions and

OH- ions. Salts are formed by neutralizing a mixture of equimolar of acid and base. The amount of acid gives one mole of protons (H+) while one mole of (OH-) is represented by the amount of base needed. There are straight forward reactions where an equal amount of the reactants are mixed together and heated up to a certain temperature to obtain ionic liquid salts without the formation of byproducts [69]. This method is extensively used to prepare high purity ionic liquids by the reaction of tertiary amines with halide acids or some other organic acids.

1.6 Preparation of IL based membranes

In spite of their attractive tunable properties, the high viscosities and considerably high production costs are the main challenges for implementations of ILs in large scale processes.

Hence, the integration of ILs with other existing processes is perhaps a more efficient approach.

Until recently, most of the literature pertaining to separation technology has reported the use of

ILs in liquid-liquid extraction, extractive distillation, or mixing with other chemicals to improve the separation performance. In this work, we tested the integration of ILs/PILs with membrane processes as novel materials to overcome the limitations associated with classical technologies.

Different routes were explored to integrate membranes with a minimal amount of IL, among which are using supported liquid membranes and the use of pressure assisted self-assembly to form a thin selective ionic liquid layer on an inert support. These routes will be explained in detail below.

1.6.1 Supported ionic liquid membranes

A supported liquid membrane (SILM) is formed by a thin layer of often organic phase that separates two aqueous phases of different compositions. This thin layer of organic phase can be immobilized onto hydrophobic microporous support, which when interposed in between two aqueous solutions, is termed as a supported ionic liquid membrane (SILM). In this multiphase extraction technique, the analytes are extracted from a continuously flowing aqueous sample through an organic liquid phase into another usually temporally stagnant, aqueous phase [70].

Supported liquid membrane processes have been used for the extraction, separation, and removal of valuable metal ions from various resources [7]. It is one of the promising technologies possessing attractive features such as high selectivity and it combines extraction and stripping into one single stage. It also acts on nonequilibrium mass transfer characteristics where the conditions of equilibrium do not limit the separation. These limitations, like aqueous organic phase ratio, emulsification, flooding and loading limits, phase disengagement, large solvent inventory, and so forth, can be avoided [71]. SILMs have applications for separation, preconcentration, and treatment of wastewater [70,71]. Thus, SILM technology has been considered as an attractive alternative over conventional unit operations for separation and concentration of metal ions in the hydrometallurgical process [72]. Figure 1.4 displays a schematic diagram of the SILM.

Figure 1.4: Schematic illustration of the supported liquid membrane setup.

1.6.2 Physical coating using pressure assisted self-assembly

Membrane modification is a method of altering the membrane surface properties by incorporating physical or chemical characteristics different from the membrane matrix. For a task specific implementation, appropriate membrane modifications could be used to obtain the optimal performance. Physical and chemical modifications are major modification techniques. The difference between the two is whether a chemical reaction takes place during the modification or not. Unlike chemical modification, physical modification alters the surface properties of the membrane surface via depositing of a modifier which is not chemically bonded to the membrane matrix. Physical modification could be classified into two categories; blending and coating. In this research, coating was used to deposit a selective PIL layer on an inert support via pressure assisted self-assembly. Pressure assisted self-assembly is basically a coating technique in which a material is coated on the membrane to form a selective layer that adheres to the surface. This modification

22 could be conducted using a dead-end membrane filtration setup where pressure is applied to force the attachment of modifying agents on the membrane’s active side. Figure 1.5 illustrates the membrane modification via a pressure assisted self-assembly technique apparatus.

Dead end filtration cell

Polymeric solution Membrane

cylinder cylinder

2 N

Collecting beaker Figure 1.5: Schematic representation of membrane modification via pressure assisted self- assembly technique.

1.7 Application of IL membranes

Due to their unique characteristics, e.g. low vapor pressure, high conductivity, thermal stability, and more importantly, their tunability, ILs have been recently incorporated in membrane technology for specific task applications, namely in supported liquid membranes and gas separation membranes. For example, in gas separation, the interaction between the quadrupole moment of CO2 and the electrical charge of ionic liquid provides an enhancement in the solubility of CO2 over other gases. SILMs have proved to be very promising for the selective separation of

CO2 from gas mixtures, such as N2, CH4, H2 and He [73]. The increase in IL content in the SILM was found to create a transition from diffusion-controlled transport to solubility-controlled transport. Hanioka and collaborators [74] revealed that the SILMs prepared from amino- functionalized imidazolium cation [C3NH2MIM] and [NTf2] anion have higher CO2 permeability in comparison with CH4 permeability, which was mainly attributed to the complex interaction between amines and CO2.

Further, SILMs have been applied to the separation of metal ions from aqueous, and showed significant performance improvement compared to conventional liquid membranes, specifically in terms of membrane lifetime and robustness [75]. As an example, the transport of chromium (III) from alkaline aqueous solutions using pseudo-emulsion based hollow fiber strip dispersion with [N8881][Cl] ionic liquid as the mobile carrier has been studied [75]. In addition, the extraction of chromium (VI) from aqueous acidic solutions was carried out using Cyphos 101

IL [76]. The results showed that the extraction of chromium up to 95% was obtained, and the transport of chromium increased with an increase in carrier concentration.

For alcohol separation, binary systems (1,3-proanediol-water and 1-butanol-water), ternary systems (such as 1- butanol-acetone-water and 1-butanol-isopropyl-water) and even quaternary systems (such as 1-butanol-acetone-ethanolwater) have been investigated in the literature using different ILMs. For the separation of the binary system (1,3- proanediol-water), Izaak et al. [77] studied the extraction of the 1,3-proanediol from an aqueous solution using [N3333][B(CN)4] ionic liquid impregnated in the NF ceramic membrane support by pervaporation. The separation factor of 1,3-propanediol was found to achieve a value of 177, which may be attributed to the fact that

1,3-propanediol preferentially passed through the membrane when the hydrophobic

[N3333][B(CN)4] is immobilized inside the pores of the ceramic membrane. Furthermore, for the binary (1-butanol-water) system, Kohoutova et al. [78] investigated the separation of 1-butanol from the aqueous mixture using the quasi-solidified [BBIM][BF4]- PDMS membranes. This kind of membrane showed high stability even if it contained [BBIM][BF4] and PDMS phases. The separation factor of 1-butanol raised to 37 when 30 wt.% of [BBIM][BF4] was accommodated in

PDMS. For the ternary systems, Izaak and coworkers [79] studied the separation of the 1-butanol and acetone from water using the supported [N3333][B(CN)4]-PDMS membrane with an ultrafiltration membrane as support material. In comparison with a PDMS membrane, the enrichment factors of 1-butanol and acetone increased from 2.2 to 10.9 and 2.3 to 7.9, respectively, when the supported [N3333] [B(CN)4]-PDMS membrane was used.

The potential applications of SILMs for the separation of aromatics from aliphatic hydrocarbons were also investigated by several researchers [7,14,80]. Selective transport of benzene, toluene, and xylene in (polyvinylidene fluoride) PVDF membranes containing different

ILs has been examined by Matsumoto et al. [14]. Although the permeation rates through SILMs followed the order of the increasing hydrophobicity of the ILs and were lower than those through

25 water-based SLMs because of the high viscosities of the ILs, the selectivity of these aromatic hydrocarbons was greatly improved by the SILMs. SILMs prepared by impregnating imidazolium- based ILs inside the pores of hydrophobic PVDF membrane have been applied to the vapor permeation process for the separation of benzene and cyclohexane [80]. A SILM with

[BMIM][PF6] and a hydrophobic PVDF membrane was utilized for the separation of toluene- cyclohexane mixtures [7]. The separation factor of the SILM reached 15-25 at 40 ⁰C after a 550 h, and the ILs were stable inside the PVDF membrane even under high vacuum conditions. This shows that the affinity between [BMIM][PF6] and PVDF membrane resulted in a robust capillary force that holds ILs firmly inside the membrane structure. Not only ILMs are applied in organic systems, but also SILMs comprised of a PVDF membrane and hydrophobic ILs based on phosphonium, imidazolium, ammonium, or pyridinium were used to extract bisphenol A from an aqueous solution [81].

SILM with [BMIM][PF6] as a liquid carrier and PVDF support exhibited a relatively high selective transport of secondary amines over tertiary amines (up to a 55:1 ratio) having similar boiling points, because of the stronger hydrogen bonds of the secondary amine to 2-H of the imidazolium cation. This result indicated that this methodology was feasible for the continuous separation of compounds from complex mixtures under simple conditions [82]. Table 1.1 summarizes the applications of SILMs in various applications.

Table 0.1: Using of ILs in membrane separation applications.

Membrane Process Ionic liquid Application Reference material configuration

Separation of CO2 [BMIM][NTf2] PVDF SILM [83] from N2

[EMIM][C2SO4] Separation of CO2 [EMIM][SCN] PTFE SILM [84] from CH4 [EMIM][DCA]

Separation of H2S [BMIM][BF4] PVDF SILM [85] from CH4

1-butanol from Dead end [BBIM][BF4] PDMS water in aqueous [78] filtration solution

Separation of

[BMIM][PF6] PVDF toluene and SILM [7]

cyclohexane

[BMIM][PF6] Separation of

[OMIM][PF6] PVDF benzene, toluene, SILM [14]

[HMIM][PF6] and p-xylene

[OMIM][PF6] Extraction of

[THTDP][Br] PVDF bisphenol A from SILM [81]

[THTDP][Cl] aqueous solution

Here this Ph.D. dissertation seeks the use of ionic liquids and polymerized ionic liquids in membrane technology for continuous separation processes of aqueous and non-aqueous systems that have low energy consumption, small footprint equipment, easy facilitation operation, slick scale-up, and enhanced membrane properties. Moreover, as part of this ongoing research, various membrane processes are explored with the incorporation of ILs as novel polymeric materials for supported liquid membranes and direct nanofiltration. The fractionation of aromatic compounds, as well as their separation from their monologue aliphatic molecules, is investigated via ionic liquid membrane contactors and dead-end filtration. The effects of different anion associated with

ILs and the length of the side chain attached are also investigated. Cl, Br, and LiNTf2 were used to study the effect of anions on the separation process. Furthermore, the lifespan of membrane processes is crucial, so the membrane stability for long term operation, challenged with polar and nonpolar solvents, is also explored. This dissertation is divieded into three chapters as follow;

Ø Chapter 2: Mahmood Jebur, Arijit Sengupta, Mohanad Kamaz, Yunshan Chiao, Xianghong Qian, Ranil Wickramasinghe. (2018). Pi electron cloud mediated separation of aromatics using supported ionic liquid (SIL) membrane having antibacterial activity. Journal of Membrane Science. 556, 1-11. [86]

Contribution: Mohanad kamaz made substantial contributions to conception and design, and acquisition of data, and analysis and interpretation of data.

1.8 References

[1] E. Drioli and M. Romano, “Progress and new perspectives on integrated membrane operations for sustainable industrial growth,” Ind. Eng. Chem. Res., vol. 40, no. 5, pp. 1277– 1300, 2001.

[2] S. Andrzej I. and M. Jacob A., “Process Intensification : Transfroming Chemcial Engineering,” Chem. Eng. Prog., pp. 22–34, 2000.

[3] F. Macedonio, E. Drioli, A. A. Gusev, A. Bardow, R. Semiat, and M. Kurihara, “Efficient technologies for worldwide clean water supply,” Chem. Eng. Process. Process Intensif., vol. 51, pp. 2–17, 2012.

[4] D. J. Miller, D. R. Dreyer, C. W. Bielawski, D. R. Paul, and B. D. Freeman, “Surface Modification of Water Purification Membranes,” Angew. Chemie - Int. Ed., vol. 56, no. 17, pp. 4662–4711, 2017.

[5] A. J. M. Von Gottberg, J. M. Persechino, and A. Yessodi, “Integrated Membrane Systems for Water Reuse,” pp. 1–6, 2012.

[6] M. Takht Ravanchi, T. Kaghazchi, and A. Kargari, “Application of membrane separation processes in petrochemical industry: a review,” Desalination, vol. 235, no. 1–3, pp. 199– 244, 2009.

[7] B. Wang, J. Lin, F. Wu, and Y. Peng, “Stability and Selectivity of Supported Liquid Membranes with Ionic Liquids for the Separation of Organic Liquids by Vapor Permeation Stability and Selectivity of Supported Liquid Membranes with Ionic Liquids for the Separation of Organic Liquids by Vapor ,” Society, pp. 8355–8360, 2008.

[8] M. David, J. Swaminathan, E. Guillen-, H. A. Arafat, J. H. L. V Scaling, and C. Link, “Scaling and fouling in membrane distillation for desalination applications : A review Citation Accessed Scaling and fouling In membrane distillation for desalination applications : A review,” vol. 356, pp. 294–313, 2016.

[9] P. Le-clech, V. Chen, and T. A. G. Fane, “Fouling in membrane bioreactors used in wastewater treatment,” vol. 284, pp. 17–53, 2006.

[10] M. E. Porter and C. van der Linde, “Toward a New Conception of the Environment- Competitiveness Relationship,” J. Econ. Perspect., vol. 9, no. 4, pp. 97–118, 1995.

[11] F. Zhang, H. Feng, W. Sun, W. Zhang, J. Liu, and Z. Ren, “Selective Separation of Toluene/n-Heptane by Supported Ionic Liquid Membranes with [Bmim][BF4],” Chem. Eng. Technol., vol. 38, no. 2, pp. 355–361, 2015.

[12] B. Wang, J. Lin, F. Wu, and Y. Peng, “Stability and selectivity of supported liquid membranes with ionic liquids for the separation of organic liquids by vapor permeation,”

Ind. Eng. Chem. Res., vol. 47, no. 21, pp. 8355–8360, 2008.

[13] G. W. Meindersma and A. B. De Haan, “Economical feasibility of zeolite membranes for industrial scale separations of aromatic hydrocarbons,” Desalination, vol. 149, no. 1–3, pp. 29–34, 2002.

[14] M. Matsumoto, Y. Inomoto, and K. Kondo, “Selective separation of aromatic hydrocarbons through supported liquid membranes based on ionic liquids,” J. Memb. Sci., vol. 246, no. 1, pp. 77–81, 2005.

[15] M. N. Sarbolouki, “A General Diagram for Estimating Pore Size of Ultrafiltration and Reverse Osmosis Membranes,” Sep. Sci. Technol., vol. 17(2), no. 1982, pp. 381–386, 1982.

[16] A. Saxena, B. P. Tripathi, M. Kumar, and V. K. Shahi, “Membrane-based techniques for the separation and purification of proteins: An overview,” Adv. Colloid Interface Sci., vol. 145, no. 1–2, pp. 1–22, 2009.

[17] L. Voswinkel and U. Kulozik, “Fractionation of whey proteins by means of membrane adsorption chromatography,” Procedia Food Sci., vol. 1, pp. 900–907, 2011.

[18] L. Millesime, C. Amiel, and B. Chaufer, “Ultrafiltration of lysozyme and bovine serum albumin with polysulfone membranes modified with quaternized polyvinylimidazole,” J. Memb. Sci., vol. 89, no. 3, pp. 223–234, 1994.

[19] R. Van Reis, J. M. Brake, J. Charkoudian, D. B. Burns, and A. L. Zydney, “High- performance tangential flow filtration using charged membranes,” J. Memb. Sci., vol. 159, no. 1–2, pp. 133–142, 1999.

[20] N. S. Pujar and A. L. Zydney, “for a Spherical Particle in a Cylindrical Pore,” vol. 349, no. 192, pp. 338–349, 1997.

[21] N. S. Pujar and A. L. Zydney, “Electrostatic effects on protein partitioning in size-exclusion chromatography and membrane ultrafiltration,” J. Chromatogr. A, vol. 796, no. 2, pp. 229– 238, 1998.

[22] S. Saksena and A. L. Zydney, “Effect of solution pH and ionic strength on the separation of albumin from immunoglobulins (IgG) by selective filtration,” Biotechnol. Bioeng., vol. 43, no. 10, pp. 960–968, 1994.

[23] H. Miyama, K. Tanaka, Y. Nosaka, N. Fujii, H. Tanzawa, and S. Nagaoka, “Charged ultrafiltration membrane for permeation of proteins,” J. Appl. Polym. Sci., vol. 36, no. 4, pp. 925–933, 1988.

[24] J. Cho, G. Amy, and J. Pellegrino, “Membrane filtration of natural organic matter: Factors and mechanisms affecting rejection and flux decline with charged ultrafiltration (UF) membrane,” J. Memb. Sci., vol. 164, no. 1–2, pp. 89–110, 2000.

[25] A. E. Childress and M. Elimelech, “Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes,” J. Memb. Sci., vol. 119, no. 2, pp. 253–268, 1996.

[26] P. S. Zhong, N. Widjojo, T. S. Chung, M. Weber, and C. Maletzko, “Positively charged nanofiltration (NF) membranes via UV grafting on sulfonated polyphenylenesulfone (sPPSU) for effective removal of textile dyes from wastewater,” J. Memb. Sci., vol. 417– 418, pp. 52–60, 2012.

[27] J. Chen, R. Peng, and X. Chen, “Hydrophobic interaction membrane chromatography for bioseparation and responsive polymer ligands involved,” Front. Mater. Sci., vol. 11, no. 3, pp. 197–214, 2017.

[28] M. E. Lienqueo, A. Mahn, J. C. Salgado, and J. A. Asenjo, “Current insights on protein behaviour in hydrophobic interaction chromatography,” J. Chromatogr. B Anal. Technol. Biomed. Life Sci., vol. 849, no. 1–2, pp. 53–68, 2007.

[29] E. Klein, “Affinity membranes: A 10-year review,” J. Memb. Sci., vol. 179, no. 1–2, pp. 1– 27, 2000.

[30] R. Ghosh, “Protein separation using membrane chromatography: opportunities and challenges,” J. Chromatogr. A, vol. 952, pp. 13–27, 2002.

[31] J. Fan, J. Luo, W. Song, X. Chen, and Y. Wan, “Directing membrane chromatography to manufacture α1-antitrypsin from human plasma fraction IV,” J. Chromatogr. A, vol. 1423, pp. 63–70, 2015.

[32] A. Vu, X. Qian, and S. R. Wickramasinghe, “Membrane-based hydrophobic interaction chromatography,” Sep. Sci. Technol., vol. 52, no. 2, pp. 287–298, 2017.

[33] E. S. Matulevicius and N. N. Li, “Facilitated transport through liquid membranes,” Sep. Purif. Rev., vol. 4, no. 1, pp. 73–96, 1975.

[34] Y. Li, S. Wang, G. He, H. Wu, F. Pan, and Z. Jiang, “Facilitated transport of small molecules and ions for energy-efficient membranes,” Chem. Soc. Rev., vol. 44, no. 1, pp. 103–118, 2015.

[35] T. Marino and A. Figol, “Arsenic removal by liquid membranes,” Membranes (Basel)., vol. 5, no. 2, pp. 150–167, 2015.

[36] V. Eyupoglu and R. A. Kumbasar, “Extraction of Ni(II) from spent Cr-Ni electroplating bath solutions using LIX 63 and 2BDA as carriers by emulsion liquid membrane technique,” J. Ind. Eng. Chem., vol. 21, pp. 303–310, 2015.

[37] F. J. Hernández-Fernández et al., “New application of supported ionic liquids membranes as proton exchange membranes in microbial fuel cell for waste water treatment,” Chem.

Eng. J., vol. 279, pp. 115–119, 2015.

[38] G. Zarca, I. Ortiz, and A. Urtiaga, “Facilitated-transport supported ionic liquid membranes for the simultaneous recovery of hydrogen and carbon monoxide from nitrogen-enriched gas mixtures,” Chem. Eng. Res. Des., vol. 92, no. 4, pp. 764–768, 2014.

[39] R. Molinari, P. Argurio, and T. Poerio, “Studies of various solid membrane supports to prepare stable sandwich liquid membranes and testing copper(II) removal from aqueous media,” Sep. Purif. Technol., vol. 70, no. 2, pp. 166–172, 2009.

[40] R. Anjana et al., “Aromatic-aromatic interactions in structures of proteins and protein-DNA complexes: a study based on orientation and distance,” Bioinformation, vol. 8, no. 24, pp. 1220–1224, 2012.

[41] S. B. Haderlein and R. P. Schwarzenbach, “Adsorption of Substituted Nitrobenzenes and Nitrophenols to Mineral Surfaces,” Environ. Sci. Technol., vol. 27, no. 2, pp. 316–326, 1993.

[42] S. B. Haderlein, K. W. Weissmahr, and R. P. Schwarzenbach, “Specific adsorption of nitroaromatic explosives and pesticides to clay minerals,” Environ. Sci. Technol., vol. 30, no. 2, pp. 612–622, 1996.

[43] K. W. Weissmahr, S. B. Haderlein, and R. P. Schwarzenbach, “Complex Formation of Soil Minerals with Nitroaromatic Explosives and other ir-Acceptors,” Soil Sci., vol. 62, no. 2, pp. 369–378, 1998.

[44] R. Peng, X. Chen, and R. Ghosh, “Preparation of graphene oxide-cotton fiber composite adsorbent and its application for the purification of polyphenols from pomegranate peel extract,” Sep. Purif. Technol., vol. 174, pp. 561–569, 2017.

[45] J. Léon, E. Reubsaet, and K. Jinno, “Characterisation of important interactions controlling retention behaviour of analytes in reversed-phase high-performance liquid chromatography,” TrAC - Trends Anal. Chem., vol. 17, no. 3, pp. 157–166, 1998.

[46] E. Reubsaet, R. Vieskar, and J. Leon, “Figure 10 shows large regional faults (blue lines), the aerial extent of possible Ellenburger Karsting and the A-A’ line of cross section for Figure 9.,” vol. 841, no. 1999, p. 10, 2000.

[47] M. Keiluweit and M. Kleber, “Molecular-level interactions in soils and sediments: The role of aromatic π-systems,” Environ. Sci. Technol., vol. 43, no. 10, pp. 3421–3429, 2009.

[48] A. J. Neel, M. J. Hilton, M. S. Sigman, and F. D. Toste, “Exploiting non-covalent π interactions for catalyst design,” Nature, vol. 543, no. 7647, pp. 637–646, 2017.

[49] K. D. Daze and F. Hof, “The Cation À π Interaction at Protein À Protein Interaction Interfaces : Developing and Learning,” Acc. Chem. Res., vol. 46, no. 4, pp. 937–945, 2013.

[50] R. A. Kumpf and D. A. Dougherty, “Potassium Channels:,” vol. 261, no. September, pp. 1– 3, 1993.

[51] K. J. Thomas, R. B. Sunoj, J. Chandrasekhar, and V. Ramamurthy, “Cation-π-interaction promoted aggregation of aromatic molecules and energy transfer within Y zeolites,” Langmuir, vol. 16, no. 11, pp. 4912–4921, 2000.

[52] Luís C. Branco, João G. Crespo, and Carlos A. M. Afonso, “Highly Selective Transport of Organic Compounds by Using Supported Liquid Membranes Based on Ionic Liquids13,” Angew. Chemie Int. Ed., vol. 41, no. 15, pp. 2771–2773, 2002.

[53] Z. Dai, R. D. Noble, D. L. Gin, X. Zhang, and L. Deng, “Combination of ionic liquids with membrane technology: A new approach for CO2 separation,” J. Memb. Sci., vol. 497, pp. 1–20, 2016.

[54] Q. Gan, D. Rooney, M. Xue, G. Thompson, and Y. Zou, “An experimental study of gas transport and separation properties of ionic liquids supported on nanofiltration membranes,” J. Memb. Sci., vol. 280, no. 1–2, pp. 948–956, 2006.

[55] R. D. Rogers and K. R. Seddon, “Perspective article: Ionic Liquids Solvents of the Future?,” Science (80-. )., vol. 302, p. 792, 2003.

[56] M. Armand, F. Endres, D. R. MacFarlane, H. Ohno, and B. Scrosati, “Ionic-liquid materials for the electrochemical challenges of the future,” Nat. Mater., vol. 8, no. 8, pp. 621–629, 2009.

[57] P. J. Dyson and T. J. Geldbach, “Applications of Ionic Liquids in Synthesis and Catalysis,” Electrochem. Soc. Interface, pp. 50–53, 2007.

[58] J. F. Brennecke and E. J. Maginn, “Ionic liquids: Innovative fluids for chemical processing,” AIChE J., vol. 47, no. 11, pp. 2384–2389, 2001.

[59] A. G. Fadeev and M. M. Meagher, “Opportunities for ionic liquids in recovery of biofuels,” Chem. Commun., no. 3, pp. 295–296, 2001.

[60] S. Pandey, “Analytical applications of room-temperature ionic liquids: A review of recent efforts,” Anal. Chim. Acta, vol. 556, no. 1, pp. 38–45, 2006.

[61] P. Wang, S. M. Zakeeruddin, J.-E. Moser, and M. Grätzel, “A New Ionic Liquid Electrolyte Enhances the Conversion Efficiency of Dye-Sensitized Solar Cells,” J. Phys. Chem. B, vol. 107, no. 48, pp. 13280–13285, 2003.

[62] M. L. Dietz, J. a. Dzielawa, I. Laszak, B. a. Young, and M. P. Jensen, “Influence of solvent structural variations on the mechanism of facilitated ion transfer into room-temperature ionic liquidsElectronic supplementary information (ESI) available: lengths of the average scattering paths and the Debye?Waller factors for the S,” Green Chem., vol. 5, no. 6, p. 682,

2003.

[63] E. Rynkowska, K. Fatyeyeva, and W. Kujawski, “Application of polymer-based membranes containing ionic liquids in membrane separation processes: A critical review,” Rev. Chem. Eng., vol. 34, no. 3, pp. 341–363, 2018.

[64] A. Sengupta and P. K. Mohapatra, “Extraction of radiostrontium from nuclear waste solution using crown ethers in room temperature ionic liquids,” Supramolecular Chemistry, vol. 24, no. 11. 2012. [65] M. Singh, A. Sengupta, M. S. Murali, S. K. Thulasidas, and R. M. Kadam, “Selective separation of uranium from nuclear waste solution by bis(2,4,4-trimethylpentyl)phosphinic acid in ionic liquid and molecular diluents: a comparative study,” J. Radioanal. Nucl. Chem., vol. 309, no. 3, pp. 1199–1208, 2016.

[66] P. K. Mohapatra, A. Sengupta, M. Iqbal, J. Huskens, and W. Verboom, “Diglycolamide- functionalized calix[4]arenes showing unusual complexation of actinide ions in room temperature ionic liquids: Role of ligand structure, radiolytic stability, emission spectroscopy, and thermodynamic studies,” Inorg. Chem., vol. 52, no. 5, pp. 2533–2541, 2013.

[67] A. J. S. McIntosh, J. Griffith, and J. Gräsvik, Methods of Synthesis and Purification of Ionic Liquids. Elsevier B.V., 2016.

[68] R. F. Cristina, “Ionic Liquids Synthesis – Methodologies,” Org. Chem. Curr. Res., vol. 04, no. 01, pp. 1–2, 2014.

[69] H. Ohno and M. Yoshizawa, “Ion conductive characteristics of ionic liquids prepared by neutralization of alkylimidazoles,” Solid State Ionics, vol. 154–155, pp. 303–309, 2002.

[70] P. K. Parhi, “Supported Liquid Membrane Principle and Its Practices: A Short Review, Supported Liquid Membrane Principle and Its Practices: A Short Review,” J. Chem. J. Chem., vol. 2013, 2013, p. e618236, 2012.

[71] J. Grünauer, V. Filiz, S. Shishatskiy, C. Abetz, and V. Abetz, “Scalable application of thin film coating techniques for supported liquid membranes for gas separation made from ionic liquids,” J. Memb. Sci., vol. 518, pp. 178–191, 2016.

[72] N. M. Kocherginsky, Q. Yang, and L. Seelam, “Recent advances in supported liquid membrane technology,” Sep. Purif. Technol., vol. 53, no. 2, pp. 171–177, 2007.

[73] J. E. Bara et al., “Guide to CO Separations in Imidazolium-Based Room-Temperature Ionic Liquids Guide to CO 2 Separations in Imidazolium-Based Room-Temperature Ionic Liquids,” Ind., pp. 2739–2751, 2009.

[74] S. Hanioka et al., “CO2separation facilitated by task-specific ionic liquids using a supported liquid membrane,” J. Memb. Sci., vol. 314, no. 1–2, pp. 1–4, 2008.

[75] F. J. Alguacil, M. Alonso, F. A. Lopez, and A. Lopez-Delgado, “Application of pseudo- emulsion based hollow fiber strip dispersion (PEHFSD) for recovery of Cr(III) from alkaline solutions,” Sep. Purif. Technol., vol. 66, no. 3, pp. 586–590, 2009.

[76] F. J. Alguacil, M. Alonso, F. A. Lopez, and A. Lopez-Delgado, “Pseudo-emulsion membrane strip dispersion (PEMSD) pertraction of chromium(VI) using CYPHOS IL101 ionic liquid as carrier,” Environ. Sci. Technol., vol. 44, no. 19, pp. 7504–7508, 2010.

[77] P. Izák, M. Köckerling, and U. Kragl, “Stability and selectivity of a multiphase membrane, consisting of dimethylpolysiloxane on an ionic liquid, used in the separation of solutes from aqueous mixtures by pervaporation,” Green Chem., vol. 8, no. 11, pp. 947–948, 2006.

[78] M. Kohoutová et al., “Influence of ionic liquid content on properties of dense polymer membranes,” Eur. Polym. J., vol. 45, no. 3, pp. 813–819, 2009.

[79] P. Izák, W. Ruth, Z. Fei, P. J. Dyson, and U. Kragl, “Selective removal of acetone and butan-1-ol from water with supported ionic liquid-polydimethylsiloxane membrane by pervaporation,” Chem. Eng. J., vol. 139, no. 2, pp. 318–321, 2008.

[80] M. Matsumoto, K. Ueba, and K. Kondo, “Vapor permeation of hydrocarbons through supported liquid membranes based on ionic liquids,” Desalination, vol. 241, no. 1–3, pp. 365–371, 2009.

[81] A. Panigrahi, S. R. Pilli, and K. Mohanty, “Selective separation of Bisphenol A from aqueous solution using supported ionic liquid membrane,” Sep. Purif. Technol., vol. 107, pp. 70–78, 2013.

[82] L. C. Branco, J. G. Crespo, and C. a M. Afonso, “Studies on the selective transport of organic compounds by using ionic liquids as novel supported liquid membranes,” Chem. Eur. J., vol. 8, no. 17, pp. 3865–3871, 2002.

[83] P. Jindaratsamee, A. Ito, S. Komuro, and Y. Shimoyama, “Separation of CO2from the CO2/N2mixed gas through ionic liquid membranes at the high feed concentration,” J. Memb. Sci., vol. 423–424, pp. 27–32, 2012.

[84] L. C. Tomé, C. Florindo, C. S. R. Freire, L. P. N. Rebelo, and I. M. Marrucho, “Playing with ionic liquid mixtures to design engineered CO 2 separation membranes,” Phys. Chem. Chem. Phys., vol. 16, no. 32, p. 17172, 2014.

[85] Y. I. Park, B. S. Kim, Y. H. Byun, S. H. Lee, E. W. Lee, and J. M. Lee, “Preparation of supported ionic liquid membranes (SILMs) for the removal of acidic gases from crude natural gas,” Desalination, vol. 236, no. 1–3, pp. 342–348, 2009.

[86] M. Jebur, A. Sengupta, M. Kamaz, Y. Chiao, X. Qian, R. Wickramasinghe, “Pi electron cloud mediated separation of aromatics using supported ionic liquid (SIL) membrane having antibacterial activity” J. Memb. Sc., vol. 556, p. 1-11, 2018.

[87] M. Kamaz, R. Vogler, M. Jebur, A. Sengupta, R. Wickramasinghe, “π Electron induced separation of organic compounds using supported ionic liquid membranes” Sep. Purif. Technol., 116237. 2019

[88] M. Kamaz, A. Sengupta, S. DePaz, Y. Chiao, R. Wickramasinghe, “Poly(ionic liquid) augmented membranes for π electron induced separation/fractionation of aromatics” J. Memb. Sc., vol. 579, 102-110. 2019.

Chapter 2 Pi electron cloud mediated separation of aromatics using supported ionic liquid

membranes (SILMs) having antibacterial activity

This chapter is adapted from a published paper by Mahmood Jebur, Arijit Sengupta, Yu-Hsuan

Chiao, Mohanad Kamaz, Xianhong Qian, Ranil Wickramasinghe “Pi electron cloud mediated separation of aromatics using supported ionic liquid membranes (SILMs) having antibacterial activity” Journal of Membrane Science, 2018.

(Reproduced with permission from reference [86], copyright (2018) Journal of Membrane

Science.)

Abstract

Supported ionic liquid membranes (SILMs) using imidazolium based ionic liquids and hydrophobic polytetrafluoroethylene (PTFE) base membrane were used for π electron mediated separation of aromatic compounds including dyes in aqueous and non-aqueous medium. The supported ionic liquid membrane was prepared by immobilizing three imidazolium based ionic liquids in the PTFE membrane using the pressure method. The Fourier-transform infrared spectroscopy spectra of SILMs showed the signature of the functionalities for both base membrane and the imidazolium moiety. The SEM image confirmed the pore filling of the base membrane with ionic liquids. The stability of the SILMs was studied using hexane, water, and DMF as solvents. Tuning the anionic moieties, ionic liquids were utilized for aqueous and non-aqueous applications through SILMs. For non-aqueous applications, the trend in separation efficiency was found to be divinylbenzene> styrene> toluene, which was attributed to the more π electron cloud density on extended conjugation leading to favorable interaction with ionic liquid influencing its transfer through SILMs. Preferential separation of Congo red (CR) and Ramazol Brilliant Blue R

(RBBR) was achieved in water based application of the SILMs.

2.1 Introduction

In view of the environmental concern in using volatile organic compounds (VOCs), ionic liquid gas gained a worldwide acceptance as a potential alternative ‘green solvent’ to VOCs due to some favorable properties like low vapor pressure, high degree of chemical, radiolytic and thermal stability, a large potential window, high degree of solubilizing power for different kinds of materials including polymers [1–3]. The most promising property of ionic liquid is its easy tunability. A small change in either cationic or anionic part of the ionic liquid may lead to a large variation in their physicochemical properties. Therefore, the proper understanding of the structure- activity relationship can be utilized to fine tune the property depending on the requirements [4,5].

The unique properties of ionic liquid lead to significant improvement in separation science, electrochemistry, organic synthesis, nuclear industry, biomass processing, and fuel cell applications [1,6–11].

The large potential window of ionic liquids provides an opportunity to a signature unusual oxidation state of metal ions, which was not possible by conventional medium [12]. The extraordinarily high separation efficiency in the hydrometallurgical application of ionic liquid was mainly attributed to the ion exchange mechanism during the separation [13]. It was also reported that structural modification in ionic liquid may promote the extraction mechanism to be ‘solvation’ instead of ion exchange [14]. Though ionic liquid provides several attractive properties, its industrial scale application suffers from the limitations. The ionic liquid is very expensive and highly viscous [15]. The high viscosity results in slower kinetics in the hydrometallurgical applications. Though the ion exchange mechanism, improves the separation efficiency, it also leads to loss of either cationic and anionic moiety of the ionic liquid [16,17]. In view of this, there is a need for separation procedure, where the ionic liquid inventory is very less, no loss of ionic

39 liquid, taking advantage of its higher viscosity and at the same time incorporate the tunable and unique properties of the ionic liquid.

Membrane based technology provides clean, energy efficient, environmentally benign technology for the separation and used modern age industrial application in ultrafiltration, microfiltration, nanofiltration, osmotic and membrane distillation [18]. Mainly, size exclusion is the basis of membrane based separation. Different surface modification was utilized either to selectively separate the targeted analytes or improve the performance of the membrane by reducing membrane fouling [19–21]. Membrane fouling and bacterial growth on the membrane is one of the serious problems faced by the researcher. Responsive membranes were utilized to address the issue [22–24]. Though several works of literature were seen in gas phase separation using supported ionic liquid (SILMs) [25–31], the applications of SILMs in liquid phase separation is minimal [32–34].

In the present investigation, three ionic liquids from the imidazolium family with structural modification have been synthesized and used for ionic liquid supported membranes using hydrophobic PTFE as the base membrane. The situation is similar to the ionic channel in the hydrophobic membrane. The surface characterization of SILMs was carried out using FTIR, SEM instruments. The affinity of ionic liquid towards the π electron cloud was exploited here for the separation of aromatics using water soluble ionic liquids and dyes using water insoluble ionic liquids. The stability of these SILMs was investigated in different diluents. SILMs are advantageous due to the small ionic liquid inventory, and high viscosity which imposed stability blended with unique properties of ionic liquid in separation.

2.2 Experimental

2.2.1 Materials

All reagents purchased were ACS reagent grade or higher unless specified otherwise. 1- vinyl imidazole, 1-bromohexane, 1-chlorobutane, allyl bromide, Congo Red (CR), Ramazol

Brilliant Blue R (RBBR), Erichrome Black T (EBT), Hexane, toluene, p-divinylbenzene, styrene and Bis(trifluoromethane)sulfonimide lithium salt (LiNTf2) were procured from Sigma Aldrich,

St. Louis, MO. Deionized water was produced by Thermo Fisher 18M Ω Barnstead Smart2Pure system (Schwerte, Germany). Commercially available hydrophobic PTFE membrane with pore size 0.45 µm was procured from Pall Corporation, San Diego, CA.

2.2.2 Synthesis of ionic liquid

For the preparation of hydrophilic ionic liquid equimolar amount of vinyl imidazole and three different alkyl halides (chlorobutane, bromohexane and allyl bromide) were taken in a glass container. Then the mixtures were heated at 60 °C for three hours with constant stirring of 150 rpm. A straw yellow color liquid was found to obtain, and it was found to form different phase

[11]. The newly generated phase was found to be soluble in water, while the unreacted reactants were insoluble. The newly generated phase is the hydrophilic ionic liquid having halide anion. The ionic liquid phase was washed with an equal volume of CCl4 three times for the removal of residual unreacted precursor molecules. To obtain a hydrophobic ionic liquid, the anion exchange reaction was carried out using hydrophilic ionic liquid and LiNTf2. 100mM of both these precursors were dissolved separately in double distilled water. The LiNTf2 solution was added to the aqueous solution of hydrophilic ionic liquid dropwise in a stirring condition at ambient temperature. The highly viscous phase was found to be separated due to the formation of the hydrophobic ionic

41 liquid. After complete phase separation, the ionic liquid phase was washed three times with DI water for the removal of water soluble impurities, side products or unreacted precursors. These hydrophilic and hydrophobic ionic liquids were used in the present investigation without further purification. The simplified scheme for the synthesis was shown in Figure 2.1.

Figure 0.1: The synthesis scheme for water soluble/insoluble ionic liquids.

2.2.3 Preparation of supported liquid membranes

Immobilization of the ionic liquids on the PTFE membrane of pore size 0.45 µm, was carried out using a 10 mL Amicon TH ultrafiltration unit and adding 3 mL of ionic liquid in accordance with a procedure reported in the literature [34,35]. Nitrogen pressure at 2×105 Pa (2 bar) was applied, and the ionic liquid was forced to pass through the pores of the membrane. The pressure was released once a thin layer of ionic liquid was left on the upper surface of the membrane. This procedure was repeated three times to ensure that all membrane pores were filled with liquid. Then, the membrane was left to drip overnight to remove the excess of ionic liquid from the membrane surface.

2.2.4 Characterization

FTIR spectroscopy provides qualitative information about functional groups at the top, approximately 2000 nm of the membrane. FTIR spectra were averaged over 100 scans covering a range of 1150–3650 cm−1 using the spectrometer procured from Shimadzu, IR Affinity-1, Kyoto,

Japan. Prior to analysis, membranes were dried overnight in a vacuum oven at 40 °C. Scanning electron microscopy (SEM) was used to image the pore filling with suitable ionic liquids. The images were analyzed using FEI Nova Nanolab 200 Duo-Beam Workstation (Hilsboro, OR). Prior to analysis, the samples were spotted with a 10 nm layer of gold and then scanned using a 15 kV electron beam.

2.2.5 Transport studies

The transport of toluene, styrene, divinylbenzene, and three dyes: congo red (CR), erichrome black T (EBT) and Ramazol Brilliant blue r (RBBR) were studied through the ionic liquid SLMs. The structures of these compounds are shown in figure A.1. The transport studies were performed at 30 °C using a glass diffusion cell with two independent compartments, with 30 mL each, separated by the membrane. O-rings were inserted on each side of the SILM. The entire assembly was held together by a threaded connector. The arrangement of the cell was shown in

Supplementary figure A.2. The effective diameter of the membrane used for the transport study is

2 cm. For the separation of toluene, styrene, and divinylbenzene, SILMs having water soluble ionic liquids (with halide ions) were employed. The initial concentration for toluene, styrene, and divinylbenzene were kept as 90% by volume in hexane (hexane was chosen as the solvent based on the stability study of the SILMs). Hexane was used as a receiving solution in all cases.

The transport experiment was begun by adding 30 mL of each solution into their respective compartments. Both compartments were mechanically stirred to avoid concentration polarization conditions at the membrane interfaces [34]. The solute concentrations were monitored by sampling

100 µL of each compartment at a definite time interval. UV–Vis absorption spectroscopy was employed for the quantification using the previously established calibration curves for these compounds at the characteristic absorption maxima (All the calibration curves are shown in

Supplementary figure A.3 and A.4). Similar experiments were also carried out for the dyes. But in this case, DI water was chosen as the medium, while SILM with water insoluble ionic liquids

(having NTf2- anion) was chosen as separating media.

2.2.6 Stability of the SIL membranes

The stability of the SILMs was investigated by immersing 25” piece of each membrane in a glass container containing 25 mL of DI water, hexane, and DMF (dimethylformamide) at a constant stirring condition (stirring speed ~ 100 rpm). The conductivity of the supernatant was checked at a regular time interval for a period of 2 months. If the ionic liquid is coming out of the membrane pores, the conductivity of the medium is expected to get modified due to the ions.

2.3 Results and discussion

2.3.1 Characterization

Figure 2.2 represents the FTIR spectra for the virgin membrane and supported ionic liquid membranes (filled with hydrophilic ionic liquid). The peaks around 1157 cm-1 to 1226 cm-1 were attributed to the characteristic peaks for the base PTFE membranes [36,37]. In SILMs, additionally, peaks were observed in the range 1435 cm-1 to 1643 cm-1, which was attributed to the stretching frequency for the imidazolium ring [38]. This existence of imidazolium peaks in SILMs is an indication of pore mouth exposure of the ionic liquid filled in the pores.

Figure 2.2: FTIR spectra for the base PTFE membrane and supported ionic liquid membranes.

Figure 2.3 is the SEM images after filling the pore of PTFE membranes with 1 vinyl-3 butyl imidazolium chloride, 1 vinyl-3 hexyl imidazolium bromide, and 1 vinyl-3 allyl imidazolium bromide. The images clearly indicate the filling of the membrane pores with the ionic liquid and also corroborate the fact that any modification on the surface property of the membrane is not due to the formation of the ionic liquid layer on the membrane surface but due to the exposure of the pore mouth. A similar observation was also reported in the literature [39]. Grünauer et al. reported the surface and cross sectional view of supported ionic liquid membranes to investigate the surface morphology, the pore structure. Pore size, pore size distribution [40].

Figure 0.3: SEM images for the supported ionic liquid membranes (a) 1 vinyl-3 allyl imidazolium bromide, (b) 1 vinyl-3 hexyl imidazolium bromide and (c) 1 vinyl-3 butyl imidazolium chloride.

2.3.2 Pi- electron mediated separation of aromatics and dyes

SILMs with water soluble ionic liquids were investigated for the separation of toluene, styrene, and divinylbenzene. The base PTFE membrane showed no selectivity for these organic molecules. The % transport for all these compounds was found to increase gradually with time up to 50 min, followed by a plateau or slow increase. SILMs with allyl homolog of the ionic liquid were found to be the least efficient for transferring these aromatics, while for bromo hexyl and chlorobutane analogues, the efficiency was almost found to be similar.

At 1 h, the maximum transport of toluene was found to be 12%, whereas, for styrene and divinylbenzene, it was 30% and 59%, respectively. For SILMs with a particular ionic liquid, the trend in % transport always followed: toluene < styrene < divinylbenzene. This fact can be attributed to the favorable interaction through the π electron cloud of the aromatics [41-43]. The ionic liquids were having an affinity for the π electron cloud. This π electron cloud density for toluene is lower than styrene followed by divinylbenzene, due to the presence of additional double bonds in conjugations with the aromatic π electron cloud. Due to this extended conjugation,

47 divinylbenzene interacted strongly with the ionic liquid sitting inside the pores. This might be responsible for bringing the selectivity in the separation for the SILMs.

The lower transport in the case of allyl ionic liquid might be attributed to the presence of electron withdrawing sp2 hybridized carbon of the ally group resulting reduction in the electron density over the imidazolium ring. On the other hand, for chloro and bromo ionic liquids, butyl and hexyl groups were present in the side chain of the imidazolium ring. The (+) inductive effect of the alkyl group containing sp3 hybridized carbon atom pushed the electron density towards the imidazolium ring. More electron cloud on the imidazolium ring, it will be more favorable for the

π electron interaction resulting enhancement in the separation efficiency. Figure 2.4 is the % transport of toluene, styrene, and divinylbenzene through the SILMs.

Figure 2.4: Transport of toluene, styrene, and divinylbenzene by supported ionic liquid membranes (all the data are reported based on triplicate measurements with an error less than 5 %).

A similar investigation was also carried out to understand the selectivity in the separation of different dyes using SILMs with water insoluble ionic liquids. In this case, three different dyes

CR, EBT, and RBBR were used. The SILMs were found to be more selective for CR. In the case of hexyl imidazolium ionic liquid, almost 60% CR was transferred to the receiver side within one and half hours, while for allyl imidazolium ionic liquid % transport of CR reached around 50% after two hours. For the other dyes RBBR and EBT, the % transport through both the SILMs were found to be less than 5%. Though all these dyes have aromatic moieties, their complicated structures with different functionalities and stereochemical arrangements might lead to various overall electron cloud density on them, resulting in different selectivity through SILMs. Figure 2.5 is the separation behavior of the dyes in the presence of these SILMs.

Figure 2.5: Transport of dyes through SILMs with water insoluble ionic liquids (all the data are reported based on triplicate measurement with an error of less than 5 %).

2.3.3 Stability of the SIL membranes

The stability of SILMs is of significant concern. Therefore, it is interesting to investigate the stability of the SILMs. Because of this, SILMs were allowed to be equilibrated in the presence of water, DMF and hexane and at different time intervals, the conductivity of the medium was measured as demonstrated in figure 2.6. For SILMs with water soluble ionic liquid, the conductivity of the medium was found to be modified in the presence of water and DMF.

In the presence of water, the water soluble ionic liquids came out of the membrane pore resulting in changes in water conductivity. DMF being polar in nature certainly interacts with the ionic liquid moieties in the membrane due to electrostatic interaction. As a result, ionic liquid might get dissolved in DMF. This also led to changes in conductivity in DMF, while in the case of hexane, up to 60 days, there is no change in the conductivity of the medium indicating the stability of the SILMs. Using these equilibrated membranes (30 days’ equilibration), the separation of toluene, styrene and divinyl benzenes were carried out. The modification in separation efficiency was found to be within 10–15 %, figure 12. These investigations revealed the high stability of SILMs with water soluble ionic liquid in the hexane medium.

Figure 2.7 compares the % transport after 1 hour for toluene, styrene, and divinylbenzene initially and after equilibration with hexane for 30 days. This 30 days’ equilibration is considered to be equivalent to the continuous operation of the membranes for 30 days. The initial % transport values for these aromatic compounds were found to be 10 – 15 % more than that obtained after the equilibration. This indicates a 10–15 % degradation in membrane performance after 30 days’ continuous use of these SILMs. Similar experiments were carried out using SILMs with water insoluble ionic liquid. The conductivity was found to be modified for hexane as well as DMF,

52 while for water the conductivity remained constant. This revealed that ionic liquid is getting dissolved for hexane and DMF but not for water.

This investigation revealed that SILMs with water soluble ionic liquid is suitable for separation in non-aqueous, non-polar medium, while that with water insoluble ionic liquid is a better choice for water treatment application. The high degree of stability of these supported ionic liquid membranes might be due to the higher viscosity coefficient of the ionic liquid.

a b

c d

e f

Figure 2.6: The stability of supported liquid membranes in different media; (a) 1 hexyl 3 vinyl imidazolium bromide; (b) 1 allyl 3 vinyl imidazolium NTf2; (c) 1 allyl 3 vinyl imidazolium

54 bromide; (d) 1 hexyl 3 vinyl imidazolium NTf2; (e) 1 butyl 3 vinyl imidazolium chloride; (f) 1 butyl 3 vinyl imidazolium NTf2.

Figure 0.7: The % transport for toluene, styrene and DVB using SILMs with water soluble ionic liquid: a. allyl, original; a1. allyl, equilibration with hexane; b. chloro, original; b1. chloro, equilibration with hexane; c. bromo, original; c1. bromo, equilibration with hexane.

2.4 Conclusions

Imidazolium based 3 SIL membranes were utilized for the π electron induced separation: separation of toluene, styrene, and divinylbenzene in hexane medium and separation of Congo red,

Eichrome black T and Ramazol brilliant blue R from aqueous medium. The presence of imidazolium ring stretching frequency along with the characteristic of PTFE membranes in SIL membranes was attributed to the exposure of pore mouth on the surface. The SEM images confirm the pore filling of the SIL membranes with the ionic liquids. The considerable modification of contact angle and the zeta potential for the SIL membranes indicates the pore filling with ionic liquid. The affinity of ionic liquid towards the π electron cloud was found to be the driving force for the separation resulting in the separation efficiency as divinylbenzene > styrene > toluene, while in case of dyes the complicated electronic configuration results in the high selectivity of

Congo red. The stability of the SIL was evaluated up to 60 days revealing SIL membranes with water soluble ionic liquid is highly stable in hexane while that for water insoluble ionic liquid in water.

2.5 Acknowledgements

Funding for this work was received from the Arkansas Research Alliance and the University of Arkansas.

Supplementary material

Supplementary data associated with this chapter can be found in Appendix I.

2.6 References

[1] T. Welton, Room-temperature ionic liquids. solvents for synthesis and catalysis, Chem. Rev. 99 (1999) 2071–2084.

[2] S.G. Cull, J.D. Holbrey, V. Vargas-Mora, K.R. Seddon, G.J. Lye, Room-temperature ionic liquids as replacements for organic solvents in multiphase bioprocess operations, Biotechnol. Bioeng. 69 (2000) 227–233.

[3] G.T. Wei, Z. Yang, C.J. Chen, Room temperature ionic liquid as a novel medium for liquid/liquid extraction of metal ions, Anal. Chim. Acta. 488 (2003) 183–192.

[4] K. Binnemans, Lanthanides and Actinides in Ionic Liquids, Chem. Rev. 107 (2007) 2592- 2614.

[5] K. Binnemans, Ionic liquid crystals, Chem. Rev. 105 (2005) 4148–4204.

[6] J.D. Holbrey, M.B. Turner, R.D. Rogers, Selection of ionic liquids for green chemical applications, 2003, 2–12.

[7] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Ionic-liquid materials for the electrochemical challenges of the future, Nat. Mater. 8 (2009) 621–629.

[8] R.F. De Souza, J.C. Padilha, R.S. Gonçalves, J. Dupont, Room temperature dialkylimidazolium ionic liquid-based fuel cells, Electrochem. Commun. 5 (2003) 728–731.

[9] P.K. Mohapatra, A. Sengupta, M. Iqbal, J. Huskens, W. Verboom, Highly efficient diglycolamide-based task-specific ionic liquids: synthesis unusual extraction behavior irradiation and fluorescence studies, Chem. – Eur. J. 19 (2013) 3230–3238.

[10] S. Priya, A. Sengupta, S. Jayabun, V.C. Adya, Piperidinium based ionic liquid in combination with sulphoxides: highly efficient solvent systems for the extraction of thorium, Hydrometallurgy 164 (2016) 111–117.

[11] R. Marcilla, J.A. Blazquez, R. Fernandez, H. Grande, J.A. Pomposo, D. Mecerreyes, Synthesis of novel polycations using the chemistry of ionic liquids, Macromol. Chem. Phys. 206 (2005) 299–304.

[12] A. Sengupta, M.S. Murali, P.K. Mohapatra, Role of alkyl substituent in room temperature ionic liquid on the electrochemical behavior of uranium ion and its local environment, J. Radioanal. Nucl. Chem. 298 (2013) 209–217.

[13] A. Sengupta, P.K. Mohapatra, M. Iqbal, J. Huskens, W. Verboom, A highly efficient solvent system containing functionalized diglycolamides and an ionic liquid for americium recovery from radioactive wastes, Dalton Trans. 41 (2012) 6970-6979.

[14] M.L. Dietz, J.A. Dzielawa, I. Laszak, B.A. Young, M.P. Jensen, Influence of solvent structural variations on the mechanism of facilitated ion transfer into room-temperature ionic liquids, Green Chem. 5 (2003) 682-685.

[15] A. Sengupta, P.K. Mohapatra, Extraction of radio strontium from nuclear waste solution using crown ethers in room temperature ionic liquids, Supramol. Chem. 24 (2012) 771-778.

[16] M. Singh, A. Sengupta, M.S. Murali, S.K. Thulasidas, R.M. Kadam, Selective separation of uranium from nuclear waste solution by bis(2,4,4-trimethylpentyl) phosphinic acid in ionic liquid and molecular diluents: a comparative study, J. Radioanal. Nucl. Chem. 309 (2016) 1199–1208.

[17] P.K. Mohapatra, A. Sengupta, M. Iqbal, J. Huskens, W. Verboom, Diglycolamide functionalized calix [4] arenes showing unusual complexation of actinide ions in room temperature ionic liquids: role of ligand structure, radiolytic stability, emission spectroscopy, and thermodynamic studies, Inorg. Chem. 52 (2013) 2533–2541.

[18] S.P. Nunes, K.V. Peinemann, Membrane Technology: in Chemical Industry (2001). Book.

[19] Y. Kwak, A.J.D. Magenau, K. Matyjaszewski, ARGET ATRP of methyl acrylate with inexpensive ligands and ppm concentrations of catalyst, Macromolecules 44 (2011) 811–819.

[20] C.H. Worthley, K.T. Constantopoulos, M. Ginic-Markovic, R.J. Pillar, J.G. Matisons, S. Clarke, Surface modification of commercial cellulose acetate membranes using surface-initiated polymerization of 2-hydroxyethyl methacrylate to improve membrane surface biofouling resistance, J. Membr. Sci. 385–386 (2011) 30–39.

[21] J.Q. Meng, C.L. Chen, L.P. Huang, Q.Y. Du, Y.F. Zhang, Surface modification of PVDF membrane via AGET ATRP directly from the membrane surface, Appl. Surf. Sci. 257 (2011) 6282–6290.

[22] D. Wandera, S.R. Wickramasinghe, S.M. Husson, Stimuli-responsive membranes, J. Membr. Sci. 357 (2010) 6–35.

[23] Q. Yang, H.H. Himstedt, M. Ulbricht, X. Qian, S. Ranil Wickramasinghe, Designing magnetic field responsive nanofiltration membranes, J. Membr. Sci. 430 (2013) 70–78.

[24] M. Marroquin, A. Vu, T. Bruce, S. Ranil Wickramasinghe, L. Zhao, S.M. Husson, Evaluation of fouling mechanisms in asymmetric microfiltration membranes using advanced imaging, J. Membr. Sci. 465 (2014) 1–13.

[25] R.D. Noble, D.L. Gin, Perspective on ionic liquids and ionic liquid membranes, J. Membr. Sci. 369 (2011) 1–4.

[26] J. Wang, J. Luo, S. Feng, H. Li, Y. Wan, X. Zhang, Recent development of ionic liquid membranes, Green Energy Environ. 1 (2016) 43–61.

[27] L.J. Lozano, C. Godínez, A.P. de los Ríos, F.J. Hernández-Fernández, S. Sánchez- Segado, F.J. Alguacil, Recent advances in supported ionic liquid membrane technology, J. Membr. Sci. 376 (2011) 1–14.

[28] P. Cserjési, N. Nemestóthy, K. Bélafi-Bakó, Gas separation properties of supported liquid membranes prepared with unconventional ionic liquids, J. Membr. Sci. 349 (2010) 6–11.

[29] M.A. Malik, M.A. Hashim, F. Nabi, Ionic liquids in supported liquid membrane technology, Chem. Eng. J. 171 (2011) 242–254.

[30] S. Hanioka, T. Maruyama, T. Sotani, M. Teramoto, H. Matsuyama, K. Nakashima, M. Hanaki, F. Kubota, M. Goto, CO2 separation facilitated by task-specific ionic liquids using a supported liquid membrane, J. Memb. Sci. 314 (2008) 1–4.

[31] J. Ilconich, C. Myers, H. Pennline, D. Luebke, Experimental investigation of the permeability and selectivity of supported ionic liquid membranes for CO2/He separation at temperatures up to 125 °C, J. Membr. Sci. 298 (2007) 41–47.

[32] J.F. Peng, J.F. Liu, X.L. Hu, G. Bin Jiang, Direct determination of chlorophenols in environmental water samples by hollow fiber supported ionic liquid membrane extraction coupled with high-performance liquid chromatography, J. Chromatogr. A 1139 (2007) 165–170.

[33] L.C. Branco, J.G. Crespo, C.A.M. Afonso, Studies on the selective transport of organic compounds by using ionic liquids as novel supported liquid membranes, Chem. Eur. J. 8 (2002) 3865–3871.

[34] B.H. Yao, J. Zhao, J.F. Niu, S. Wang, C. Wang, A novel application of supported liquid membrane based on hydrophobic ionic liquids to the selective transport of phenol and p- nitrophenol, Adva. Mater. Res. 233–235 (2011) 1349–1353.

[35] F.J. Hernández-Fernández, A.P. de los Ríos, M. Rubio, F. Tomás-Alonso, D. Gómez, G. Víllora, A novel application of supported liquid membranes based on ionic liquids to the selective simultaneous separation of the substrates and products of a transesterification reaction, J. Membr. Sci. 293 (2007) 73–80.

[36] F. Wang, H. Zhu, H. Zhang, H. Tang, J. Chen, Y. Guo, Effect of surface hydrophilic modification on the wettability, surface charge property and separation performance of PTFE membrane, J. Water Process Eng. 8 (2015) 11–18.

[37] W.-C. Chao, Y.-H. Huang, W.-S. Hung, Q. An, C.-C. Hu, K.-R. Lee, J.-Y. Lai, Effect of the surface property of poly(tetrafluoroethylene) support on the mechanism of polyamide active layer formation by interfacial polymerization, Soft Matter. 8 (2012) 8998-9004.

[38] N. Pekel, Z.M.O. Rzaev, O. Güven, Synthesis and characterization of poly(N-inylimidazole- co-acrylonitrile) and determination of monomer reactivity ratios, Macromol. Chem. Phys. 205 (2004) 1088–1095.

[39] A.P. de los Ríos, F.J. Hernández-Fernández, F. Tomás-Alonso, J.M. Palacios, D. Gómez, M. Rubio, G. Víllora, A SEM–EDX study of highly stable supported liquid membranes based on ionic liquids, J. Membr. Sci. 300 (2007) 88–94.

[40] J. Grünauer, V. Filiz, S. Shishatskiy, C. Abetz, V. Abetz, Scalable application of thin film coating techniques for supported liquid membranes for gas separation made from ionic liquids, J. Membr. Sci. 518 (2016) 178–191.

[41] C.G. Hanke, A. Johansson, J.B. Harper, R.M. Lynden-Bell, Why are aromatic compounds more soluble than aliphatic compounds in dimethylimidazolium ionic liquids? A simulation study, Chem. Phys. Lett. 374 (2003) 85–90.

[42] J.D. Holbrey, W.M. Reichert, M. Nieuwenhuyzen, O. Sheppard, C. Hardacre, R.D. Rogers, Liquid clathrate formation in ionic liquid-aromatic mixtures, Chem. Commun. (2003) 476–477.

[43] J.B. Harper, R.M. Lynden-Bell, Macroscopic and microscopic properties of solutions of aromatic compounds in an ionic liquid, Mol. Phys. 102 (2004) 85–94.

Chapter 3 π Electron induced separation of organic compounds using supported ionic liquid

membranes

This chapter is adapted from a published paper by Mohanad Kamaz, Ronald J Vogler, Mahmood

Jebur, Arijit Sengupta, Ranil Wickramasinghe “π Electron induced separation of organic compounds using supported ionic liquid membranes” Separation and Purification Technology,

2019.

(Reproduced with permission from reference [87], copyright (2019) Separation and Purification

Technology.)

Abstract

Supported ionic liquid membranes have been tested for the fractionation of organic compounds. Imidazolium based ionic liquids; 1-allyl-3-vinylimidazolium bromide, 1-hexyl-3- vinylimidazolium bromide and 1-octyl-3-vinylimidazolium bromide, were trapped in the pores of a polypropylene membrane. Fractionation of a feed stream consisting of benzene, naphthalene, and phenanthrene in tetradecane and cis- and trans-stilbene in hexane were investigated. The receiving phase was hexane. Differences in the π electron cloud density of the aromatic solutes influence their interactions with the imidazolium cation which can affect the rates of transport. In the case of the three aromatic solutes the degree of conjugation and molecular weight increase in the order of benzene, naphthalene, and phenanthrene. These opposing effects result in similar mass transfer coefficients for the three solutes for the three ionic liquids tested. However, for cis-and trans-stilbene, the mass transfer coefficient was larger for all three ionic liquids. It is likely that enhanced aromatic stacking of the trans- isomer led to a higher mass transfer coefficient. In addition, 1-hexyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide was used as the liquid

61 membrane phase to fractionate two nucleobases: thymine and cytosine in water. The receiving phase was also water. The mass transfer coefficient for thymine was 5.5 times greater than cytosine indicating the possibility of exploiting multimodal interactions with the imidazolium group to fractionate compounds.

3.1 Introduction

Supported liquid membranes where a liquid phase is trapped within the pores of a microporous membrane represents one of the early membrane contactors [1,2]. Supported liquid membranes are attractive as a solute can be selectively extracted into the liquid phase that is trapped within the membrane pores and then back extracted into the strip solution. Numerous module configurations have been described. Other potential advantages include low capital cost, low operating cost and energy consumption. Further, the process is modular and easy to scale up.

The commercialization of supported liquid membranes has been slow as the stability of the liquid membrane is limited. The liquid phase within the membrane pores can be lost to the bulk feed and strip phases on either side of the membrane as well as by evaporation. Even if the solubility of the phase within the membrane pores in the two bulk phases that contact the membrane is minimal, given the large difference in the volume of the liquid membrane phase and the bulk phases, loss of the liquid membrane occurs. Various attempts such as strip dispersion have been proposed to increase membrane stability [3–5].

Room temperature ionic liquids are salts in which the formation of a crystal lattice is suppressed at room temperature. They have been referred to as designer solvents as the properties can be tuned by using different ion pair combinations. Ionic liquids have negligible vapor pressure thus there will be no evaporation losses. By choosing appropriate ion pairs, hydrophobic ionic liquids with very low water solubility can be prepared. Further, due to their high surface tension and viscosity, which can be modified by choice of the ion pairs used, very stable supported ionic liquid membranes (SILMs) may be prepared.

Abejón et al. [6] indicate that interest in SILMs has increased rapidly since 2005. In general, four main applications for SILMs have been considered: carbon dioxide separation,

63 separation of other gases and pervaporation represent gas phase feed streams. Here we focus on liquid phase feed streams where numerous potential applications have been described.

Chakraborty et al. reported a highly efficient and selective separation of toluene using an ionic liquid membrane with Ag+ carrier [7]. Branco et al. reported a mutual separation of several organic compounds (1,4-dioxane, propan-1-ol, butan-1-ol, cyclohexanol, cyclohexanone, morpholine, and methyl morpholine) using imidazolium based ionic liquid membranes [8].

Removal of phenol, amino acids, ketones, and acetic acid have also been reported using bulk ionic liquid membranes [9–12]. The transport and experimental design were optimized for achieving higher efficiency without any compromise in ion selectivity [13]. Ionic liquid based membranes have also been used in microbial fuel cell applications [14,15]. One of the advantages was the specific energy gained from substrates i.e. glucose and acetate was found to be more in ionic liquid based membranes compared to the nafion based membrane [16].

Nevertheless, most interest has focused on hydrocarbon separations, recovery of products from fermentation broths, desulfurization of crude oil and fuels [17] and metal ion recovery [18–

20]. Here we focus on the separation of organic compounds.

Perhaps the first use of a SILM for separation of organic compounds was described by

Branco et al. [21]. They indicated a selective separation of secondary amines over tertiary amines using ionic liquids based on 1-n-alkyl-3methylimidazolium and hexafluorophosphate and tetrafluoroborate as anions. The high selectivity for secondary amines is a result of preferential interactions with the imidazolium ring. This is due to the formation of hydrogen bonds with the protons at the C-2 position [22]. In an early publication, Matsumoto et al. [23] indicated that SILMs may be used for selective transport of aromatic hydrocarbons. Hanke et al. [24] show that electrostatic interaction between the solute and ionic species in the solvent, in this case, the ionic

64 liquid cation and p electrons in the aromatic solute are important. For benzene and other aromatic compounds, with electron density on both sides of the plane of the aromatic ring, quadrupole and other higher order electrostatic moments arise. These quadrupole moments interact with the cations present in the solvent.

Chakraborty et al. [25] extended the work by Matsumoto et al. [23] by investigating the separation of various aromatic hydrocarbons using different imidazolium based ionic liquids as the liquid membrane phase. They also showed that ionic liquids may be used to separate aromatic hydrocarbons from aromatic aliphatic mixtures. Further studies by Zhang et al. [26] and Zhang et al. [27] have explored the feasibility of toluene/cyclohexane and toluene/n-heptane separations.

Again, the enhanced solubility of aromatic hydrocarbons led to the recovery of toluene in the strip solution. Wang et al. [28] have determined solubility parameters and selectivity for a range of organic solutes in ionic liquids. They demonstrate that ionic liquids show great promise for the separation of alkanes from aromatics. Further, by selecting the cation and anion of the ionic liquid the properties of the liquid membrane phase can be tailored for a specific separation. Lozano et al.

[29], Han and Row [30] and Martínez-Palou et al. [31] provided reviews of the use of SILMs for liquid phase separations.

Marták et al. [32] investigated the extraction of lactic acid using a SILM. These authors provide a detailed mass transfer analysis of the extraction process. They indicate that the protonated form of lactic acid may be extracted via hydrogen bonding with the ionic liquid [33].

On the other hand, the anionic form of lactic acid could also be extracted by ion exchange with the lactate anion.

Here we further explore the ability of imidazolium based ionic liquids to fractionate aromatic compounds. We consider separation of a three-component mixture consisting of benzene,

65 naphthalene, and phenanthrene in tetradecane. The three imidazolium based ionic liquids we investigate in the membrane phase are: 1-allyl-3-vinlyinidazoliumbromide, 1-hexyl-3- vinlyinidazolium bromide, and 1-octyl-3-vinlyinidazolium bromide. The strip solution consisted of hexane. The difference between the three solutes is their degree of conjugation. We also investigated the separation of two stereoisomers, cis- and trans-stilbene using the same three imidazolium based ionic liquids as the membrane phase. The feed and strip solutions consisted of hexane. Finally, we have investigated the separation of two nucleobases cytosine and thymine.

The feed and strip solutions consisted of water. In order to render the ionic liquid water insoluble, the bromide anion in 1-hexyl-3-vinylimidazolium bromide was exchanged with bis(trifluoromethylsulfonyl)imide. The membrane phase consisted of 1-hexyl-3-vinlyimidazolium bis(trifluoromethylsulfonyl) imide. Our results highlight the importance of interactions between the p electrons above and below the aromatic ring and the cation present in the ionic liquid.

3.2 Materials and methods

3.2.1 Materials

1-vinylimidazole (99%), lithium bis(trifluoromethylsulfonyl)imide (98+%), cis-stilbene, trans- stilbene, n-hexane (ACS grade), n-octane, and n-tetradecane (99%) were purchased from Alfa-

Aesar (Ward Hill, MA). Allylbromide (99%), bromooctane (99%), toluene (ACS grade), naphthalene, and phenanthrene were bought from Sigma-Aldrich (Munich, Germany). Thymine

(>98.0%) and cytosine (>98.0%) were obtained from Tokyo Chemical Industry Co., Ltd (Chuo- ku, Tokyo, Japan). Acetonitrile (HPLC grade) and ethyl acetate (ACS grade) were sourced by

Macron Fine Chemicals TM (Radnor, PA). 1-bromohexane (≥98%) was obtained from Merck

KGaA (Kenilworth, NJ). Deionized water used in all the experiments was produced using a

Thermo Fisher 18MΩ Barnstead Smart2Pure system (Schwerte, Germany). Polypropylene (PP) membranes provided by 3M Corporation (Maplewood, MN) and had a thickness of 135 µm, a pore size of 0.45 µm, and a porosity of 0.76.

3.2.2 Preparation of ionic liquids

Imidazolium based ionic liquids were prepared from an equimolar mixture (0.05 M) vinyl imidazole and one of the following alkyl halides (allyl bromide, bromo hexane and bromo octane).

The resulting mixture was heated to 60°C and stirred at 150 rpm for 3 hours. The mixture phase separated during reaction forming an ionic liquid phase: 1-allyl-3-vinylimidazolium bromide,

+ − + − [C3vim ][Br ], 1-hexyl-3-vinylimidazolium bromide, [C6vim ][Br ], 1-octyl-3-vinylimidazolium

+ − bromide, [C8vim ][Br ], and a phase containing the residual reactants [34]. To remove residual reactants and impurities, the mixture was rinsed three times with an equal volume of ethyl acetate.

After each rinse, the mixture was shaken, and the residual precursors were removed after phase separation. After rinsing, the ionic liquid was placed in a vacuum oven set to 40°C for at least 5 hours to remove any remaining impurities.

To obtain a more hydrophobic ionic liquid (1-hexyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide), an ion exchange procedure was carried out using 1-hexyl-3- vinylimidazolium bromide and lithium bis(trifluoromethylsulfonyl)imide (LiNTf2) as described earlier [35]. Prior to anion exchange, separate aqueous solutions of 1-hexyl-3-vinylimidazolium bromide and LiNTf2 were prepared using equimolar amounts (100mM) of each reactant. To initiate the ion exchange, the LiNTf2 solution was added dropwise to the aqueous solution of 1-hexyl-3- vinylimidazolium bromide. After stirring and phase separation, a hydrophobic ionic-liquid phase

(clear) and an aqueous phase were obtained, and the latter was removed. The remaining ionic liquid

+ - [C6vim ][NTf2 ] was rinsed with deionized (DI) water multiple times. To ensure that no water remained the ionic liquid was placed in a vacuum oven at a temperature of 40°C for about 3 hours.

3.2.3 Preparation of SILMs

The pores of PP microfiltration membranes were filled with one of the ionic liquids as described earlier [35]. A PP membrane was placed in a 5-mL Amicon® ultrafiltration cell EMD

Millipore (Bedford, MA, USA) and then 3 mL of each ionic liquid was loaded. Pressurized nitrogen (1.0-1.4 bar) was used to push the ionic liquid through the membrane. The process was stopped after 2 mL of ionic liquid were collected as permeate. This process was repeated three times to ensure all the membrane pores were filled with the ionic liquid. The membranes were removed from the cell and their surfaces were wiped to remove as much excess ionic liquid as possible. Thus, we assume there is minimal excess ionic liquid on the membrane surface. This method was shown in earlier studies to minimize excess ionic liquid on the membrane surface

[35].

3.2.4 Membrane characterization

Fourier Transform Infrared (FTIR) spectroscopic analysis of the surface of the PP membranes was conducted to verify the incorporation of ionic liquids in the membrane pores. An

IRAffinity-1 and a ZnSe crystal (Shimadzu) was used for these measurements. Prior to analysis, the surface of the membrane was wiped to remove residual ionic liquid. For each measurement,

300 scans were done in the range of 600 cm-1 to 4000 cm-1 at a resolution of 8 cm-1.

3.2.5 Membrane stability

+ − + For SILMs containing [C3vim ][Br ], [C6vim ][Br−] and [C8vim+][Br−], the stability in hexane, tetradecane and water was determined. 50 mL of each of these liquids was added to a container and the initial conductivity was recorded using a Cond 3310 Conductivity Meter, Cole-

Parmer (Vernon Hills, IL). Next, the SILM was added and the liquid gently shaken. The conductivity was measured over a 165 hour period. For the more hydrophobic ionic liquid

+ - [C6vim ][NTf2 ] the stability in water was determined using the same method.

3.2.6 Membrane performance

Table 3.1 summarizes the experiments that were conducted. Figure 3.1 gives the structure of the various solutes investigated. In the case of the three aromatic compounds, different feed concentrations were used. The experiments were run for 2 hours. As can be seen, the degree of conjugation of these three solutes is very different. For the two stereoisomers of stilbene the concentration of both stereoisomers was the same and the experiments were run for 2.6 hours.

Figure 3.1 gives the structure of these two isomers. Finally, the concentration of the two nucleobases in the feed was the same. These experiments were run for 48 hours. Each data point is reported based on the average of triplicate measurements. All readings were within 10% of the mean value.

Table 3.1: Summary of diffusion experiments.

Ionic liquid phase Aromatic solutes in Stereoisomers of Nucleobases:

Tetradecane stilbene in hexane cytosine,

thymine

1-allyl-3- Solute concentration: Solute concentration: vinylimidazolium benzene 0.819 M, cis-stilbene and trans- bromide naphthalene0.650 M, stilbene 0.0556 M,

phenanthrene 0.468M, experiment was run for

experiment run for 2 hr, 2.6 hr, strip solution

strip solution was hexane was hexane

1-hexyl-3- Solute concentration: Solute concentration: vinylimidazolium benzene 0.819 M, cis-stilbene and trans- bromide naphthalene 0.650 M, stilbene 0.0556 M,

phenanthrene 0.468 M, experiment was run for

experiment run for 2 hr, 2.6 hr, strip solution

strip solution was hexane was hexane

Table 3.1: Summary of diffusion experiments “Cont.”.

Ionic liquid phase Aromatic solutes Stereoisomers of Nucleobases:

in Tetradecane stilbene in cytosine,

hexane thymine

1-hexyl-3-vinylimidazolium Solute bis(trifluoromethylsulfonyl)imide concentration:

cytosine,

thymine, 0.0218

M, experiment

was run for 48

hr, strip solution

was water

1-octyl-3-vinylimidazolium Solute Solute bromide concentration: concentration:

benzene 0.819 M, cis-stilbene and

naphthalene trans-stilbene

0.650 M, 0.0556 M,

phenanthrene experiment was

0.468 M, run for 2.6 hr,

experiment run strip solution was

for 2 hr, strip hexane

solution was

hexane

Diffusion experiments were conducted in a glass diffusion cell consisting of two separable cylindrical compartments with a volume of 30 mL each (Figure 3.2). The SILM was placed between the two compartments and O-rings were inserted in between. The two compartments were put together using a threaded connector. The active area of the membrane was 1.27 cm². Due to the low solubility of aliphatic compounds in the ionic liquid [25] tetradecane and hexane were used as the feed and receiving solvents. The solution in both compartments was stirred at 500 rpm.

All experiments were conducted at room temperatures. The variation of solute concentration in the receiving cell (strip solution) was determined by collecting 100 µL samples at various times for analysis.

A mass balance over the receiving reservoir for the solute species is given by

�� − � = � (1) where A is the membrane surface area, k the solute mass transfer coefficient, V the volume of the receiving solution t the time Cf the feed concentration and C the receiving solution solute concentration. The feed concentration is given by C0 – C where C0 is the initial feed concentration.

Equation (1) may be integrated to give

�� = (2)

The overall mass transfer coefficient is given by

= + + (3) where 1/kF, 1/kM, 1/kS are the individual mass transfer coefficients for solute transfer through the feed side concentration boundary layer, through the membrane pores, and through the receiving

(strip) side concentration boundary layer. For stirred cell diffusion experiments the membrane phase mass transfer coefficient dominates [2]. The membrane mass transfer coefficient is given by

� (4) where e, d and t are the void fraction, thickness and tortuosity of the membrane and H is the partition coefficient that relates the concentration between phases.

The separation factor is also calculated to express the relative separation under the specified conditions, which is defined as follows.

( ) ℬ = (5) ( )

Figure 0.1: Chemical structure of solutes.

Figure 3.2: Schematic diagram of the lab-scale supported ionic liquid apparatus.

3.2.7 Solute detection

High Performance Liquid Chromatography (HPLC) equipped with Luna C18 column (5

µm, size 250*4.6 mm, from (Phoenix, USA) was used to detect all the solutes. Acetonitrile was used as the mobile phase at a flow rate of 0.5 mLmin-1 and the column temperature was kept at 29

°C. The injection sample volume was 20 µL. A diode array detector was used to detect the selected compounds. UV scan was conducted for each compound and the maximum wavelength identified.

A distinctive retention time for each compound was obtained. The results are summarized in Table

3.2.

Table 3.2: HPLC detection wavelengths and retention time for solutes.

Compound Wavelength (nm) Retention time (min)

Benzene 220 7.7

Naphthalene 250 8.5

Phenanthrene 300 10.0

Trans-stilbene 310 7.0

Cis-stilbene 310 9.3

Thymine 220 9.5

Cytosine 220 6.8

3.3 Result and Discussion

3.3.1 Membrane characterization

The presence of ionic liquid within the membrane pores was verified by FTIR spectroscopy analysis as shown in Figure 3.3. Spectra of the synthesized ionic liquids are given in the supplementary data. The spectrum for the base PP membrane showed peaks corresponding to C-

H bond stretching. For membranes with ionic liquid filled prores, extra peaks were observed in the range of 1500 cm−1 to 1700 cm−1, which were attributed to the stretching frequency of the imidazolium ring [36].

Figure 03: FTIR spectra for: (A) base polyrpopylene membrane and membrane pores filled with: (B) [C3vim+][Br−];(C) [C6vim+][Br−]; (D) [C6vim+][NTf2-];(E) [C8vim+][Br−].

3.3.2 SILM performance

Figures 3.4 (a), (b), and (c) give the change in concentration in the receiving solution as a function of time for benzene, naphthalene and phenanthrene for membranes filled with

+ - + - + - [C3vim ][Br ], [C6vim ][Br ], and [C8vim ][Br ] respectively. Since Table 3.1 indicates that the initial feed concentrations increase in order phenanthrene, naphthalene and benzene, it is not surprising that the increase in concentration in the receiving solution follows the same order. The concentration increases approximately linearly with time.

1 (a) 0.8 Benzene Naphthalene 0.6 Phenanthrene

0.4

0.2 Concentration (mmol/L) 0 0 20 40 60 80 100 120 Time (min) 1

0.8 Benzene (b) Naphthalene 0.6 Phenanthrene

0.4

0.2

Concentration (mmol/L) 0 0 20 40 60 80 100 120 Time (min)

0.8 Benzene (c) Naphthalene 0.6 Phenanthrene

0.4

0.2 Concentration (mmol/L) 0 0 20 40 60 80 100 120 Time (min)

Figure 3.4: Variation of benzene, naphthalene and phenanthrene concentration in the receiving + - solution as a function of time for membrane pores containing (a) [C3vim ][Br ], + - + - (b) [C6vim ][Br ], (c) [C8vim ][Br ] respectively.

Figures 3.5 (a), (b) and (c) give the benzene, naphthalene and phenanthrene flux defined as the concentration in the receiving solution divided by the membrane area and time for membrane

+ - + - + - pores filled with [C3vim ][Br ], [C6vim ][Br ], and [C8vim ][Br ] respectively. As can be seen,

77 there is an initial rapid increase in flux followed by a slow decrease. The slow decrease in flux occurs as the driving force for diffusion decreases with increasing solute concentration in the receiving solution and decreasing concentration in the feed solution.

1000 Benzene (a) 800 Naphthalene Phenanthrene 600

400

200

Flux (mol/cm2.min)*10^9 0 0 20 40 60 80 100 120 Time (min)

1000 Benzene (b) 800 Naphthalene Phenanthrene 600

400

200

Flux (mol/cm2.min)*10^9 0 0 20 40 60 80 100 120 Time (min)

1000 Benzene (c) 800 Naphthalene Phenanthrene 600

400

200

Flux (mol/cm2.min)*10^9 0 0 20 40 60 80 100 120 Time (min)

Figure 3.5: Variation of benzene, naphthalene and phenanthrene flux with time for membrane + - + - + - pores containing (a) [C3vim ][Br ], (b) [C6vim ][Br ], (c) [C8vim ][Br ] respectively.

Results for the stereoisomers of stilbene are given in Figures 3.6 and 3.7. Figures 3.6 (a),

(b) and (c) give the change in concentration in the receiving solution as a function of time for the

+ - + - two stereoisomers of stilbene for membranes pores filled with [C3vim ][Br ], [C6vim ][Br ],

+ - [C8vim ][Br ] respectively. Figures 3.7 (a), (b) and (c) give the corresponding flux values. The results are analogous to Figures 3.4 and 3.5. The concentration of both isomers increases with time. Again, the slow decrease in flux after about 60 min is due to the decrease in driving forces for diffusion with increasing solute concentration in the receiving solution and decreasing concentration in the feed solution. Though the initial feed concentration of the two stereoisomers is the same, Figure 3.6 indicates that for all three SILMs transport of trans-stilbene is faster than cis-stilbene.

0.03 Cis-stilbene 0.025 Trans-stilbene 0.02

0.015

0.01 (a) Concentration (mmol/L) 0.005

0 0 20 40 60 80 100 120 140 160 Time (min)

0.03 Cis-stilbene 0.025 Trans-stilbene 0.02

0.015

0.01 (b) 0.005 Concentration (mmol/L) 0 0 20 40 60 80 100 120 140 160 Time (min)

0.03 Cis-stilbene 0.025 Trans-stilbene 0.02

0.015

0.01 (c) 0.005 Concentration (mmol/L) 0 0 20 40 60 80 100 120 140 160 Time (min)

Figure 3.6: Variation of cis-stilbene and trans-stilbene concentration in the receiving solution as + - + - a function of time for membrane pores containing (a)[C3vim ][Br ], (b)[C6vim ][Br ], (c) + - [C8vim ][Br ] respectively.

50 (a) Cis-stilbene 40 Trans-stilbene

10 Flux(mol/cm2.min)*10^9 0 0 50 100 150

Time (min)

50 (b) Cis-stilbene 40 Trans-stilbene

10 Flux(mol/cm2.min)*10^9 0 0 50 100 150 Time (min)

50 Cis-stilbene (c) 40 Trans-stilbene

10 Flux(mol/cm2.min)*10^9 0 0 50 100 150 Time (min)

Figure 3.7: Variation of cis-stilbene and trans-stilbene flux with time for membrane pores + - + - + - containing (a) [C3vim ][Br ], (b) [C6vim ][Br ], (c) [C8vim ][Br ] respectively.

Finally Figures 3.8 and 3.9 give analogous results for the two nucleobases. The rate of solute transfer as well as the initial solute concertation is lower. Consequently, no decrease in flux

81 is observed at longer run times. Though the initial nucleobase concentration in the feed is the same, the rate of transport of thymine is much faster than cytosine.

1 0.9 Cytosine 0.8 Thymine 0.7 0.6 0.5 0.4

Concentration (mmol/L) 0.3 0.2 0.1 0 0 500 1000 1500 2000 2500 3000 Time (min)

Figure 3.8: Variation of cytosine and thymine concentration in the receiving solution as a + - function of time for membrane pores containing [C3vim ][NTf2 ].

50 45 Cytosine 40 Thymine 35 30 25 20

15 10 Flux (mol/cm2.min)*10^9 5 0 0 500 1000 1500 2000 2500 3000 Time (min)

Figure 3.9: Variation of cytosine and thymine flux with time for membrane pores + - containing[C3vim ][NTf2 ].

The results in Figure 3.4-3.7 indicate that as the length of the carbon chain present on the imidazolium ring increases the rate of transport of all solutes increases. The rate of diffusion through the liquid membrane will depend on the viscosity and density of the ionic liquid as well as interactions between the solute and the ionic liquid. Recent studies indicate that the groups on the imidazolium ring can alter viscosity in unpredictable ways [37]. In the case of increasing hydrocarbon chain length, it is likely the viscosity increases while the density decreases [38].

The anion of the ionic liquid is generally weakly coordinating in nature. Therefore, it will have a much weaker interaction with the ionic liquid cation or the solutes of interest. However, the anion can be used to tune the properties of the ionic liquid such as solubility and viscosity as we have done in this work. It is the interplay between these effects that results in the observed rate of mass transport. This observation highlights the tremendous potential for tailoring the properties of the liquid membrane phase.

Using equation (2) mass transfer coefficient were calculated and are given in Table 3.3.

For stilbene and the nucleobases, the feed and receiving solutions consisted of the same liquid phase. However, for benzene, naphthalene, and phenanthrene the feed liquid was tetradecane while the receiving solute was hexane. Though very similar, the solubility of the three solutes will be slightly different in the two distinct liquid phases. We have ignored these effects (assumed the partition coefficient is 1) in determining the overall mass transfer coefficient.

Table 3.3: Calculated mass transfer coefficients.

Solute SILM Mass transfer coefficient,

K x 10^9 m s-1

+ - Benzene [C3vim ][Br ] 2.78

+ - Naphthalene [C3vim ][Br ] 2.78

+ - Phenanthrene [C3vim ][Br ] 3.06

+ - Benzene [C6vim ][Br ] 3.06

+ - Naphthalene [C6vim ][Br ] 3.33

+ - Phenanthrene [C6vim ][Br ] 3.33

+ - Benzene [C8vim ][Br ] 3.89

+ - Naphthalene [C8vim ][Br ] 3.61

+ - Phenanthrene [C8vim ][Br ] 3.89

+ - Cis-stilbene [C3vim ][Br ] 1.67

+ - Trans-stilbene [C3vim ][Br ] 2.50

+ - Cis-stilbene [C6vim ][Br ] 1.94

+ - Trans-stilbene [C6vim ][Br ] 2.50

+ - Cis-stilbene [C8vim ][Br ] 2.22

+ - Trans-stilbene [C8vim ][Br ] 2.78

+ - Thymine [C6vim ][NTf2 ] 18.89

+ - Cytosine [C6vim ][NTf2 ] 3.33

The mass transfer coefficient for benzene naphthalene and phenanthrene are the same for a given ionic liquid though they increase slightly for all solutes with the increasing length of the carbon chain attached to the imidazolium ring. In the case of the two stilbene stereoisomers, the mass transfer coefficient for trans-stilbene is always larger than of cis-stilbene. Finally, the mass transfer coefficient is larger for thymine compared to the cytosine.

The separation factor was calculated using equation 4. It can be seen that the separation factors of phenanthrene to benzene and naphthalene to benzene using different SILMs are almost the same. However, the concentration of these solutes in the feed is very different. The separation factor for trans-stilbene to cis-stilbene is greater than for the same feed concentration indicating preferential transport of trans stilbene. A large separation factor is obtained in the case of thymine to cytosine.

Table 3.4: The separation factor (ℬ) using different SILMs.

Parameter SILM ℬ

+ - Thymine to Cytosine [C6vim ][NTf2 ] 5.12

Trans stilbene to Cis stilbene [C3vim+][Br−] 1.49

Trans stilbene to Cis stilbene [C6vim+][Br−] 1.47

Trans stilbene to Cis stilbene [C8vim+][Br−] 1.34

Phenanthrene to Benzene [C3vim+][Br−] 1.07

Phenanthrene to Benzene [C6vim+][Br−] 1.10

Phenanthrene to Benzene [C8vim+][Br−] 0.98

Naphthalene to Benzene [C3vim+][Br−] 0.96

Naphthalene to Benzene [C6vim+][Br−] 0.99

Naphthalene to Benzene [C8vim+][Br−] 0.91

The results indicate that the degree of conjugation and hence, electron cloud density by itself does not imply faster transport. Though the degree of conjugation increases in order benzene, naphthalene, phenanthrene, the molecular weight of these compounds also increases as follows benzene, 78.11 g mol-1; naphthalene, 128.17 g mol-1, phenanthrene 178.23 g mol-1. Thus, the mass transfer coefficient for all three compounds, which in the stirred diffusion cell used here is dominated by diffusion through the SILM, is the same.

The situation is different for the two stereoisomers of stilbene which have the same molecular weight. The presence of two phenyl rings on the same side of the carbon double bond in the cis- isomer (see Figure 3.1) may lead to deviations from planarity between the two groups and result in deterioration in aromatic stacking with the cation present on the ionic liquid [39–41].

As a result, the effects of extended conjugation of the π electron cloud is reduced. On the other hand, the trans- isomer does not have such stereo-chemical constrains and does not lose the effects of extended conjugation. This leads to preferential transport of trans-stilbene.

Thymine and cytosine have similar molecular weights; 126.11 and 111.1 g mol-1 respectively. However, thymine is preferentially transported through the SILM. The mass transfer coefficient is more than 5 times large. It is known that imidazole groups play an important role in nature [42]. They can form hydrogen bonds with both thymine and cytosine. It is likely that differences in the hydrogen bonding interactions between the cation in the ionic liquid and thymine which contains two nitrogen and two oxygen atoms compared to cytosine which contains three nitrogen and one oxygen atoms results in the enhanced transport of thymine.

The results indicate that cation π interactions may be used to enhance the selectivity of the

SILM for the given solute. However, simply increasing the degree of conjugation of the solute species does not imply enhanced rates of transport through the SILM. In the case of fractionation

86 of stereoisomers, our results suggest that a SILM could be used to enhance the concentration of one of the isomers. Our results for the two nucleobases indicate that exploiting multimodal interactions (hydrogen bonding, cation -p interactions, etc.) could result in very high selectivities for a given solute. A molecular level simulation study could identify the importance of these different interactions. A major advantage of SILMs over conventional supported liquid membranes is the ability to tune the properties of the cation and anion of the ionic liquid membrane.

The stability of the SLIMs like all supported liquid membranes is a major concern. The

+ − + − stability of the SILMs containing water soluble ionic liquids [C3vim ][Br ], [C6vim ][Br ] and

+ − [C8vim ][Br ], was determined by incubating them in hexane, tetradecane and water for up to 165 hours and measuring the conductivity of the solution. Figure 3.10 gives the results for (a)

+ − + − + − [C3vim ][Br ], (b) [C6vim ][Br ] and (c) [C8vim ][Br ]. As can be seen over 165 hours the conductivity of tetradecane and hexane did not increase due to loss of the liquid membrane.

However, as expected, the conductivity of the water continually increased for the SILMs containing water soluble ionic liquids. Thus, the SILMs containing the water-soluble ionic liquids were stable in the organic solutions tested here.

As discussed by Koók et al, for aqueous phase application, hydrophobic ionic liquids are relatively stable [43]. Further, by increasing the length of the alkyl side chain attached to the imidazolium cation, the hydrophobicity is increased and hence aqueous miscibility decreases. The nature of anion also plays a vital role in the stability of the SILM [44]. Similarly, the reverse is true for the present case, where an organic solvent is used. Previous studies indicate minimal loss of the ionic liquid from the pores under mild stirring conditions. However, some loss is observed due to excess ionic liquid on the membrane surface [45,46]. Our results are in keeping with these observations.

+ - The stability of water insoluble [C6vim ][NTf2 ] was determined in water. The result is shown in Figure 3.11. As can be seen, this SILM is less stable as over a 165 hour period the conductivity of the water gradually increased. However, our mass transfer results (see Figure 3.8 and 3.9) indicate that over 165 hours, loss of the ionic liquid did not lead to failure of the SILM.

80 70 Hexane Tetradecane 60 (a) Water 50 40 30 20

Conductivity ( µ S/cm) 10 0 0 50 100 150 Time (hour)

80 70 60 50 Hexane 40 (b) Tetradecane 30 Water 20

Conductivity ( µ S/cm) 10 0 0 50 100 150 Time (hour)

80 70 60 50 Hexane 40 (c) Tetradecane 30 20 Water

Conductivity ( µ S/cm) 10 0 0 50 100 150 Time (hour) Figure 3.10: Variation of conductivity with time for hexane, tetradecane and water containing + − + − + − membranes with pores filled with (a)[C3vim ][Br ], (b)[C6vim ][Br ], (c)[C8vim ][Br ].

Taken together these results indicate that the ability to tune the properties of ionic liquids makes them very attractive for highly specific separations. The development of a SILM is attractive as it minimizes the volume of expensive ionic liquid that is required. By combining extraction and back extraction, significant process intensification is possible. Nevertheless, significant engineering challenges remain regarding module design and the stability of the SILM will be critical. For practical processes, highly stable SILMS will be required.

If we lose traces of ionic liquid during the separation it will contaminate the component(s) to be purified and hence the removal of the trace amounts of ionic liquid may be necessary. Though not part of the current investigation an additional unit operation may be required. For example, as ionic liquids are known for their very low vapor pressure, while most of the organic compounds are highly volatile distillation could be feasible. Importantly inclusion of a SILM based separation in a manufacturing process will depend on the cost of any additional purification steps that may be required.

200 180 160 140 120 100 80 Water 60 Conductivity ( µ S/cm) 40 20 0 0 50 100 150 Time (hour)

Figure 3.11: Variation of conductivity with time for water containing a membrane with pores + - filled with [C6vim ][NTf2 ].

3.4 Conclusions

The results of the investigation conducted here indicate that SILMs may be developed for very specific separations. Here we have focused on the interaction between p electrons in the solute and the ions present in the ionic liquid that forms the liquid membrane phase. Given the ability to tailor the properties of the ionic liquid through the choice of the cation and anion as well as groups attached to these ions, tremendous potential exists to develop SILMs for highly specific separations. Our results indicate however that the rate of mass transfer will depend on the interplay of a number of factors. Simply increasing the degree of conjugation of the solute species does not guarantee faster rates of mass transfer.

Though supported liquid membranes are attractive for energy efficient separations that can lead to significant process intensification, the stability of the liquid membrane remains a major challenge. Ionic liquids are attractive as their properties can be tailored to minimize loss of the liquid membrane. Our results indicate that it will be essential to optimize the properties of the ionic liquid in order to maximize the stability in practical applications of SILMs for liquid phase separations.

3.5 Acknowledgements

Funding for this work was provided by the National Science Foundation (1659653), the

Arkansas Research Alliance, and the University of Arkansas through the Ross E Martin Chair in

Emerging Technologies.

Supplementary material

Supplementary data associated with this chapter can be found in Appendix II.

3.6 References

[1] M.S. Brennan, A.G. Fane, C.J.D. Fell, Natural gas separation using supported liquid membranes, AIChE J. 32 (1986) 1558–1560. doi:10.1002/aic.690320917.

[2] S. Majumdar, K. Sirkar, Hollow fiber contained liquid membrane, in: W.S. Ho, K. Sirkar (Eds.), Membr. Handb., Van Nostrand Reinhold, New York, 1992: pp. 746–808.

[3] A.K. Pabby, A.M. Sastre, State-of-the-art review on hollow fibre contactor technology and membrane-based extraction processes, J. Memb. Sci. 430 (2013) 263–303. doi:10.1016/j.memsci.2012.11.060.

[4] Z. Hao, M.E. Vilt, Z. Wang, W. Zhang, W.S. Winston Ho, Supported liquid membranes with feed dispersion for recovery of Cephalexin, J. Memb. Sci. 468 (2014) 423–431. doi:10.1016/j.memsci.2014.06.009.

[5] M.E. Vilt, W.S.W. Ho, In situ removal of Cephalexin by supported liquid membrane with strip dispersion, J. Memb. Sci. 367 (2011) 71–77. doi:10.1016/j.memsci.2010.10.044.

[6] R. Abejón, H. Pérez-Acebo, A. Garea, A bibliometric analysis of research on supported ionic liquid membranes during the 1995-2015 period: Study of the main applications and trending topics, Membranes (Basel). 7 (2017) 1–28. doi:10.3390/membranes7040063.

[7] M. Chakraborty, H.J. Bart, Highly selective and efficient transport of toluene in bulk ionic liquid membranes containing Ag+ as carrier, Fuel Process. Technol. 88 (2007) 43–49. doi:10.1016/j.fuproc.2006.08.004.

[8] L.C. Branco, J.G. Crespo, C.A.M. Afonso, Studies on the selective transport of organic compounds by using ionic liquids as novel supported liquid membranes, Chem. - A Eur. J. 8 (2002) 3865–3871. doi:10.1002/1521-3765(20020902)8:17<3865::AID- CHEM3865>3.0.CO;2-L.

[9] L.C. Branco, J.G. Crespo, C.A.M. Afonso, Ionic liquids as an efficient bulk membrane for the selective transport of organic compounds, J. Phys. Org. Chem. 21 (2008) 718–723. doi:10.1002/poc.1381.

[10] A.B. Lakshmi, S. Sindhu, S. Venkatesan, Performance of ionic liquid as bulk liquid membrane for chlorophenol removal, Int. J. ChemTech Res. 5 (n.d.) 1129–1137.

[11] S.A.M. Mohammed, M.S. Hameed, Extraction of 4-Nitrophenol from Aqueous Solutions using Bulk ionic Liquid Membranes, Int. J. Curr. Eng. Technol. 6 (2016) 542–550. http://inpressco.com/category/ijcet.

[12] R. Fortunato, M.J. González-Muñoz, M. Kubasiewicz, S. Luque, J.R. Alvarez, C.A.M. Afonso, I.M. Coelhoso, J.G. Crespo, Liquid membranes using ionic liquids: The influence of water on solute transport, J. Memb. Sci. 249 (2005) 153–162.

doi:10.1016/j.memsci.2004.10.007.

[13] N. Baylan, S. Çehreli, Removal of acetic acid from aqueous solutions using bulk ionic liquid membranes: A transport and experimental design study, Sep. Purif. Technol. 224 (2019) 51–61. doi:10.1016/j.seppur.2019.05.001.

[14] F.J. Hernández-Fernández, A. Pérez de los Ríos, F. Mateo-Ramírez, C. Godínez, L.J. Lozano-Blanco, J.I. Moreno, F. Tomás-Alonso, New application of supported ionic liquids membranes as proton exchange membranes in microbial fuel cell for waste water treatment, Chem. Eng. J. 279 (2015) 115–119. doi:10.1016/j.cej.2015.04.036.

[15] L. Koók, N. Nemestóthy, P. Bakonyi, G. Zhen, G. Kumar, X. Lu, L. Su, G.D. Saratale, S.H. Kim, L. Gubicza, Performance evaluation of microbial electrochemical systems operated with Nafion and supported ionic liquid membranes, Chemosphere. 175 (2017) 350–355. doi:10.1016/j.chemosphere.2017.02.055.

[16] L. Koók, N. Nemestóthy, P. Bakonyi, A. Göllei, T. Rózsenberszki, P. Takács, A. Salekovics, G. Kumar, K. Bélafi-Bakó, On the efficiency of dual-chamber biocatalytic electrochemical cells applying membrane separators prepared with imidazolium-type ionic liquids containing [NTf2]− and [PF6]− anions, Chem. Eng. J. 324 (2017) 296–302. doi:10.1016/j.cej.2017.05.022.

[17] G.O. Yahaya, F. Hamad, A. Bahamdan, V.V.R. Tammana, E.Z. Hamad, Supported ionic liquid membrane and liquid-liquid extraction using membrane for removal of sulfur compounds from diesel/crude oil, Fuel Process. Technol. 113 (2013) 123–129. doi:10.1016/j.fuproc.2013.03.028.

[18] F.J. Alguacil, M. Alonso, F.A. Lopez, A. Lopez-Delgado, Application of pseudo-emulsion based hollow fiber strip dispersion (PEHFSD) for recovery of Cr(III) from alkaline solutions, Sep. Purif. Technol. 66 (2009) 586–590. doi:10.1016/j.seppur.2009.01.012.

[19] F.J. Alguacil, M. Alonso, F.A. Lopez, A. Lopez-Delgado, I. Padilla, H. Tayibi, Pseudo- emulsion based hollow fiber with strip dispersion pertraction of iron(III) using (PJMTH+)2(SO42-) ionic liquid as carrier, Chem. Eng. J. 157 (2010) 366–372. doi:10.1016/j.cej.2009.11.016.

[20] F.J. Alguacil, M. Alonso, F.A. Lopez, A. Lopez-Delgado, Pseudo-emulsion membrane strip dispersion (PEMSD) pertraction of chromium(VI) using CYPHOS IL101 ionic liquid as carrier, Environ. Sci. Technol. 44 (2010) 7504–7508. doi:10.1021/es101302b.

[21] L.J. Branco, J.G. Crespo, C.A.M. Afonoso, High selectivity transport organic compounds by using supported liquid membranes, based on ionic liquids, Chem. Int. Ed. 41 (2002) 2771–2773.

[22] C.F. Poole, S.K. Poole, Extraction of organic compounds with room temperature ionic liquids, J. Chromatogr. A. 1217 (2010) 2268–2286. doi:10.1016/j.chroma.2009.09.011.

[23] M. Matsumoto, Y. Inomoto, K. Kondo, Selective separation of aromatic hydrocarbons through supported liquid membranes based on ionic liquids, J. Memb. Sci. 246 (2005) 77– 81. doi:10.1016/j.memsci.2004.08.013.

[24] C.G. Hanke, A. Johansson, J.B. Harper, R.M. Lynden-Bell, Why are aromatic compounds more soluble than aliphatic compounds in dimethylimidazolium ionic liquids? A simulation study, Chem. Phys. Lett. 374 (2003) 85–90. doi:10.1016/S0009-2614(03)00703-6.

[25] M. Chakraborty, D. Dobaria, P.A. Parikh, The separation of aromatic hydrocarbons through a supported ionic liquid membrane, Pet. Sci. Technol. 30 (2012) 2504–2516. doi:10.1080/10916466.2010.516618.

[26] F. Zhang, W. Sun, J. Liu, W. Zhang, Z. Ren, Extraction separation of toluene/cyclohexane with hollow fiber supported ionic liquid membrane, Korean J. Chem. Eng. 31 (2014) 1049– 1056. doi:10.1007/s11814-014-0021-7.

[27] F. Zhang, H. Feng, W. Sun, W. Zhang, J. Liu, Z. Ren, Selective Separation of Toluene/n- Heptane by Supported Ionic Liquid Membranes with [Bmim][BF 4 ], Chem. Eng. Technol. 38 (2015) 355–361. doi:10.1002/ceat.201400160.

[28] L.-S. Wang, X.-X. Wang, Y. Li, K. Jiang, X.-Z. Shao, C.-J. Du, Ionic Liquids: Solubility Parameters and Selectivities for Organic Solutes, Am. Inst. Chem. Eng. 59 (2013) 3034– 3041. doi:10.1002/aic.

[29] L.J. Lozano, C. Godínez, A.P. De Los Ríos, F.J. Hernández-Fernandez, S. Sanchez-Segado, F.J. Alguacil, Recent advances in supported ionic liquid membrane technology, J. Memb. Sci. 376 (2011) 1–14. doi:10.1016/j.memsci.2011.03.036.

[30] D. Han, K.H. Row, Recent applications of ionic liquids in separation technology, Molecules. 15 (2010) 2405–2426. doi:10.3390/molecules15042405.

[31] R. Martínez-Palou, N. V. Likhanova, O. Olivares-Xometl, Supported ionic liquid membranes for separations of gases and liquids: An overview, Pet. Chem. 54 (2014) 595– 607. doi:10.1134/S0965544114080106.

[32] J. Marták, Š. Schlosser, S. Vlčková, Pertraction of lactic acid through supported liquid membranes containing phosphonium ionic liquid, J. Memb. Sci. 318 (2008) 298–310. doi:10.1016/j.memsci.2008.02.064.

[33] J. Marták, S.̌ Schlosser, Phosphonium ionic liquids as new, reactive extractants of lactic acid, Chem. Pap. 60 (2006) 395–398. doi:10.2478/s11696-006-0072-2.

[34] S.K. Ethirajan, A. Sengupta, M. Jebur, M. Kamaz, X. Qian, R. Wickramasinghe, Single- Step Synthesis of Novel Polyionic Liquids Having Antibacterial Activity and Showing π- Electron Mediated Selectivity in Separation of Aromatics, ChemistrySelect. 3 (2018) 4959– 4968. doi:10.1002/slct.201800101.

[35] M. Jebur, A. Sengupta, Y.H. Chiao, M. Kamaz, X. Qian, R. Wickramasinghe, Pi electron cloud mediated separation of aromatics using supported ionic liquid (SIL) membrane having antibacterial activity, J. Memb. Sci. 556 (2018) 1–11. doi:10.1016/j.memsci.2018.03.064.

[36] H. El-Hamshary, M.M.G. Fouda, M. Moydeen, M.H. El-Newehy, S.S. Al-Deyab, A. Abdel- Megeed, Synthesis and antibacterial of carboxymethyl starch-grafted poly(vinyl imidazole) against some plant pathogens, Int. J. Biol. Macromol. 72 (2015) 1466–1472. doi:10.1016/j.ijbiomac.2014.10.051.

[37] L. Sun, O. Morales-Collazo, H. Xia, J.F. Brennecke, Effect of Structure on Transport Properties (Viscosity, Ionic Conductivity, and Self-Diffusion Coefficient) of Aprotic Heterocyclic Anion (AHA) Room-Temperature Ionic Liquids. 1. Variation of Anionic Species, J. Phys. Chem. B. 119 (2015) 15030–15039. doi:10.1021/acs.jpcb.6b03934.

[38] L. Sun, O. Morales-Collazo, H. Xia, J.F. Brennecke, Effect of structure on transport properties (viscosity, ionic conductivity, and self-diffusion coefficient) of aprotic heterocyclic anion (AHA) room temperature ionic liquids. 2. Variation of alkyl chain length in the phosphonium cation, J. Phys. Chem. B. 120 (2016) 5767–5776. doi:10.1021/acs.jpcb.6b03934.

[39] A. Warshel, Calculation of vibronic structure of the π→π* transition of trans- and cis- stilbene, J. Chem. Phys. 62 (1975) 214–221. doi:10.1063/1.430265.

[40] J.L. Oudar, Optical nonlinearities of conjugated molecules. Stilbene derivatives and highly polar aromatic compounds, J. Chem. Phys. 67 (1977) 446–457. doi:10.1063/1.434888.

[41] M. Kamaz, A. Sengupta, S.S. DePaz, Y.H. Chiao, S. Ranil Wickramasinghe, Poly(ionic liquid) augmented membranes for Π electron induced separation/fractionation of aromatics, J. Memb. Sci. 579 (2019) 102–110. doi:10.1016/j.memsci.2019.01.044.

[42] E.B. Anderson, T.E. Long, Imidazole- and imidazolium-containing polymers for biology and material science applications, Polymer (Guildf). 51 (2010) 2447–2454. doi:10.1016/j.polymer.2010.02.006.

[43] L. Koók, B. Kaufer, P. Bakonyi, T. Rózsenberszki, I. Rivera, G. Buitrón, K. Bélafi-Bakó, N. Nemestóthy, Supported ionic liquid membrane based on [bmim][PF6] can be a promising separator to replace Nafion in microbial fuel cells and improve energy recovery: A comparative process evaluation, J. Memb. Sci. 570–571 (2019) 215–225. doi:10.1016/j.memsci.2018.10.063.

[44] R. Fortunato, C.A.M. Afonso, J. Benavente, E. Rodriguez-Castellón, J.G. Crespo, Stability of supported ionic liquid membranes as studied by X-ray photoelectron spectroscopy, J. Memb. Sci. 256 (2005) 216–223. doi:10.1016/j.memsci.2005.02.023.

[45] P. Wasserscheid, T. Welton, eds., Ionic Liquids in Synthesis, John Wiley & Sons, 2008.

[46] R. Fortunato, C.A.M. Afonso, M.A.M. Reis, J.G. Crespo, Supported liquid membranes using ionic liquids: Study of stability and transport mechanisms, J. Memb. Sci. 242 (2004) 197–209. doi:10.1016/j.memsci.2003.07.028.

Chapter 4 Poly(ionic liquid) augmented membranes for π electron induced

separation/fractionation of aromatics

This chapter is adapted from a published paper by Mohanad Kamaz, Arijit Sengupta, Sandrina

Svetlana DePaz, Yu-Hsuan Chiao, and S. Ranil Wickramasinghe “Poly(ionic liquid) augmented membranes for π electron induced separation/fractionation of aromatics” Journal of Membrane

Science, 2019.

(Reproduced with permission from reference [88], copyright (2019) Journal of Membrane

Science.)

Abstract

Novel polyionic liquid (PIL) augmented membranes were fabricated using pressure assisted self-assembly and employed for π- electron cloud mediated separation of aromatics from their aliphatic homologues, which are otherwise difficult to achieve by conventional size exclusion based membrane filtration. The electron withdrawing/electron donating substituents on the aromatic ring, which can lead to an overall change in the π cloud density on the aromatic ring, can be employed in fractionating such aromatics such as benzene, toluene, p-xylene, nitrobenzene.

High π electron could density due to extended conjugation over more than one aromatic ring was also utilized in the fractionation of benzene, naphthalene, anthracene, and phenanthrene. The most interestingly, these membranes were demonstrated to have preferential separation of one stereoisomer over others. The preferential separation of trans-stilbene over cis-stilbene was attributed to stereochemical arrangement assisted deviating from planarity of aromatic moieties of the later resulting deterioration of overall extended conjugation and π electron cloud delocalization. These PIL membranes also exhibited the preferential separation of regioisomers of bipyridine. These PIL augmented membranes were found to stable in the presence of mainly non-

96 polar organic diluents due to the π- stacking interaction between the aromatic phenylic moieties of the polyimide support and aromatic pyridinium moieties of the poly (ionic liquid) polymers.

4.1 Introduction

Membrane technology has played a major role in developing more effective separation processes to produce downstream products with reducing consumption of raw materials and energy and the minimization of wastewater and solid waste [1–3]. The organic compounds are widely used as solvents in industries. A large amount of organic compounds wastewater is discharged into the aqueous environment from manufacturing, transportation, and purposeful disposal sources [4]. Because most of these chemicals are classified as flammable, toxic and carcinogenic substances, their present in aqueous surroundings in large amounts may have an adverse impact on water quality and thus serious risk on public and welfare health. Membrane processes have been introduced to water and wastewater treatment as a promising breakthrough technology to resolve water scarcity for the removal of microcontaminants. Recycling of water for the potential reuse and recovery of byproducts has been linked with membrane processes in industrial operations scale and various membrane processes are employed in the treatment of potable water, domestic and industrial wastewater [5,6].

In the petrochemical industry, separation and fractionation of the aromatics with different molecular weight is of major concern. Nowadays, the production of many aliphatic and cyclo chemicals has proceeded via the catalytic hydrogenation of aromatics at high temperatures such as the hydration of benzene to synthesis cyclohexane. Thus, the unreacted precursors should be separated from the product to produce high purity cyclohexane. Cost deficient technologies have been currently deployed for such purpose including azeotropic distillation and extractive distillation for the separation of benzene/ cyclohexane mixtures due to their close boiling points

[7].

Many researchers lately have explored membrane separation processes, especially the pervaporation and vapor permeation techniques as alternatives to the conventional processes because of the high operation costs [8]. Moreover, most of the membrane based separation is either based on size exclusion or diffusion. With the rapid worldwide industrial development involving different types of organic solvents, there is an urgent need for the development of the novel membranes to separate the solvents with the same number of carbon atoms.

Poly(ionic liquid) is the polymer of ionic liquid that has repetitive units [9–11]. They have the blended unique properties of ionic liquid including a high degree of tenability and advantageous properties of polymers including a high degree of chemical and mechanical stability.

The delocalized ionic charge on the organic cation in ionic liquid leads to the lowering of the lattice energy resulting in the liquid nature of these compounds even though they are ionic in nature. The charge moieties of ionic liquid induce electrostatic interaction, while the uncharged organic tail at the end of ionic liquid induces the lipophilic character. Moreover, small structural modification in ionic liquid can be intelligently utilized in order to have an ionic liquid of required physicochemical properties. The ionic liquid provides some unique chemistry, which otherwise is unconventional and hence finds a wide range of applications in the field of chemical synthesis, hydrometallurgy, catalysis, separation, electrochemistry and energy applications [11–15]. The antimicrobial activity of the ionic liquids and PIL provides an additional advantage in their biomedical applications and even for wastewater treatment [16,17].

In the present investigation, such technologically important materials, PIL have been augmented to provide a selective layer on polyimide porous support to achieve novel unconventional separation based on π cloud interaction of the PILs and the aromatics in the feed.

Our previous investigation involving the synthesis of ionic liquids and using of supported ionic liquid membranes demonstrated that the ionic liquid has an affinity towards the π electron cloud

[18–20]. Separation of aromatics from its aliphatic homologues (for example heptane-toluene separation) is challenging. Several membranes modification involving crosslinking, surface modification, and incorporation of the metal-organic framework into the membrane have been investigated [21–23].

However, though some of the results are moderately good, the constant needs of such separation are still required. As proposed, the π electron cloud density on the feed is the driving force of these PIL based membranes and hence more extensively conjugate systems would have more affinity towards the PIL active layer and therefore possible to achieve fractionation between benzene, naphthalene, anthracene, and phenanthrene. To the best of our knowledge, no other literature reports such unconventional separation using PIL membranes. However, recently, an increasing number of authors have suggested stronger adsorption mechanisms between π electron cloud density of organic compounds and sorption sites at mineral surfaces [24]. Specifically, electron donor-acceptor (EDA) interactions of aromatic compounds with mineral surfaces have been suggested that can comprise via either cation-π interactions [25] or n-π EDA interactions

[26].

100

Further, the interaction of natural organic matters with aromatic π cloud systems has been suggested to be involved in π-π aromatic cloud interactions, and polar-π interactions [27].

Therefore, π cloud interaction is proposed to be the driving force for the separation scheme of PIL membranes, any substituents on the aromatic ring that has electron donating or withdrawing capability should also be fractionated and the present investigation demonstrated this fact.

Additionally, these PIL membranes are confirmed to be suitable for the preferential separation of stereochemical isomers: like cis/trans stilbene. The tampering in planarity due to the stereochemical arrangement of one of the isomers leads to deterioration in planarity and hence overall conjugation. This is the basis of the stereochemical separation achieved in the present case and it is among the first ever reports involving PIL augmented membranes.

4.2 Experimental

4.2.1 Materials

2-vinyl pyridine, 4 vinyl pyridine, allyl bromide, toluene, naphthalene, anthracene, phenanthrene, cis-stilbene, trans-stilbene, n-hexane, n-heptane, n-decane, n-tetradecane, nitrobenzene have been procured from Sigma-Aldrich (Munich, Germany). All the reagents used in the present case are of analytical grade.

4.2.2 Instrumentation

A sessile drop contact angle goniometer (Model 100) procured from Rame-Hart Instrument

Company, Netcong, NJ was used for the measurement of water contact angle. A Nova Nanolab

200 Duo-Beam Workstation procured from FEI, Hillsboro, Oregon was used for scanning electron microscopic (SEM) imaging and energy dispersive X-ray spectroscopic investigations. IR Affinity with a horizontal ZnSe accessary; procured from Shimadzu, Columbia, MD, USA was used for

Fourier transform infrared (FTIR) spectroscopic analysis. The 1H NMR analysis of the synthesized

101

PILs was done using 300 MHz spectrometer (Model No: AVANCE 300) procured from Bruker,

Billerica, MA using BBO probe. UV–Vis spectrometer, GENESIS 10 S UV-V procured from

Thermo Scientific Waltham, MA was used for the determination of different aromatic compounds used in the present case. Atomic force microscopic (AFM) imaging was done using a Bruker instrument (Santa Barbara, CA). Thermo Fisher 18 MΩ Barnstead Smart2Pure system procured from Schwerte, Germany was used for the production of deionized water used throughout the investigation. The conductivity was estimated using a conductivity-meter procured from VWR,

Radnor, PA.

4.2.3. Synthesis of polyimide support

A 24 wt% solution of P84, provided by HP Polymer GmbH, Austria, stirred and degassed for 24 h. Then it was cast on nonwoven PET using a knife (200 µm), provided by Membrane

Science Inc., Taiwan. The membrane thus formed was used for crosslinking using 1–6- hexamethylenediamine (HMD) in isopropyl alcohol [28]. Then the resulting membrane was rinsed with isopropyl alcohol in order to remove the excess crosslinking agent. Then the membrane was kept inside the solution mixture of PEG400-isopropyl alcohol with a composition of 3:2. Finally, overnight air drying for the removal of isopropyl alcohol resulted in the polyimide support used for the present investigation. Figure 4.1 schematically presents the crosslinking between P84 and

HMD.

102

Figure 0.1: The schematic presentation of the crosslinking between P84 and HMD for the fabrication of the support material.

3.2.4 Synthesis of PILs

A mixture of 2-vinyl pyridine (2.5 mol) and allyl bromide (2.5 mol) was used for one pot synthesis of the PIL as reported earlier [18]. The mixture was stirred on a temperature controlled shaker at 333 K with a stirring speed of 100 rpm for 30 min. A yellow polymeric solid was obtained, which got separated from the medium. The solid was washed thrice with ethyl acetate thereafter for the removal of the trapped unreacted precursors. Lastly, it was dried and stored in a sealed glass container for the preparation of a PIL solution. A similar procedure was followed for the synthesis of the PIL from 4-vinyl pyridine. The light pink color solid polymer obtained by this method was dried overnight prior to further use.

103

3.2.5 PIL augmentation by pressure assisted self-assembly

The aqueous 1 mg/mL of PIL solution was pressurized to pass through the polyimide support into a pressurized Buchner funnel at 5 bar (72.52 psi) to create a selective layer on top

[29]. The filtrate was collected and conductivity was measure in order to ensure that no PIL is coming out of the system. Figure 4.2 schematically presents the pressure assisted self-assembly set up.

Figure 0.2: The schematic presentation for pressure assisted self-assembly to form PIL augmented membranes.

104

3.2.6 Performance

The membrane performance was tested by a series of filtration experiments carried out at

100 psi using a large pressurized Buchner funnel filtration system. The filtrate was collected in a glass beaker and covered with foil and parafilm to counteract volatility. The effective area of the membrane was 17.35 cm2, with the diameter being 4.7 cm. Flux data were recorded at 30 s intervals using the Balinx Flux Program and a computer connected weight balance. Permeate samples of 2 mL were collected in a small centrifuge tube during each filtration for UV–VIS

Absorption testing of the aromatics. The feed solutions were prepared by mixing the corresponding compound in a sonicator for 30 min to have homogeneous solutions. Deadend filtration mode was employed throughout the experiments. UV–vis spectroscopic technique was used for calculating the separation/fractionation factors of the separation schemes. Since all these compounds are aromatic and have extended conjugation, they have a very distinct absorption wavelength as shown in table A 3.1. Calibration curves were established for each compound with regression coefficients of more than 0.999. The aromatics in the filtrates were tested by UV–VIS absorption spectroscopy at respective wavelengths and the calibration curves were utilized for the concentration calculation.

All the data reported in the present investigation is the average of triplicate measurements with associated errors (unless otherwise specified).

105

4.3 Result and discussion

4.3.1. Characterization

4.3.1.1 NMR spectroscopy

The pyridinium based PILs have been synthesized in a single step combining the quaternization of vinyl pyridine isomers followed by self-polymerization using our previous literature. The synthesis of the PILs from 2 vinyl pyridine and 4 vinyl pyridine has been evidenced by their 1H-NMR spectra shown in Figure 4.3. The magnetic non-equivalence of the 1Hs was quantified in terms of different δ values. The δ value at 8.968 ppm corresponds to ortho H to pyridinium N for PIL from 2 vinyl pyridine. For PIL from 4 vinyl pyridine with 2 such ortho Hs showed a peak at δ 8.896 ppm (termed as 1). For 2 vinyl pyridine analogue, peaks at δ 8.018 ppm and 8.508 ppm, were attributed to the magnetically non-equivalent meta Hs, termed as 2 and 4, respectively. For 4 vinyl analogue, these 2 meta Hs were found to be magnetically equivalent and exhibit a single peak at δ 8.021 ppm. For 2 vinyl pyridine homologue the para H showed a peak at

8.813 ppm, while no such para proton was present for the 2 vinyl pyridine homologue (termed as

3). The H in between pyridinium N and allylic C (marked as 5) was found to show very high δ values as 5.117 and 5.215 for 2 and 4 vinyl pyridinium PILs, respectively. This peak corresponds to two protons. The single H attached to sp2 C having alkyl substitution showed a peak at 6.125 ppm for 2 vinyl pyridine PIL, whereas that for 4 vinyl homologue showed at 6.239 ppm. The two protons attached to the terminal sp2 C from allylic groups for both the PIL isomers showed a peak at δ 5.303 and 5.423 ppm, respectively. The peaks in the range δ 0.9 – 2.4 or broad peak in this region can be attributed to methylic and ethylic Hs individually or such Hs with very small magnetic nonequivalence leading to unresolvable broad structure.

106

Figure 0.3: The NMR spectra along with the synthetic scheme for the PIL obtained from 2 and 4 isomers of vinyl pyridine.

107

4.3.1.2 Water contact angle

Pressure assisted self-assembly of such PIL isomers on the polyimide support led to alteration of the surface properties of the resulting membrane. Though the polyimide support material is hydrophilic in nature, the augmentation of PILs on the polyimide surface was found to further enhanced the hydrophilicity of the membrane surface as shown in figure 4.4. The water contact angle for the polyimide surface was measured as 82º, which dropped down to 36º and 32º for 2 vinyl and 4 vinyl pyridine PIL membranes, respectively. The induction in hydrophilicity can easily be explained based on repetitive units of ionic liquids having charge separation inducing a high degree of electrostatic interaction with the water molecules. The water contact angle was measured in dynamic mode revealed almost no or negligible change in water contact angle and hence the hydrophilicity of the PIL augmented membrane surface over a period of 20 s.

Figure 4.4: Dynamic water contact angle measurement for PIL augmented membranes.

108

4.3.1.3 Surface morphology

The surface morphology of the PIL augmented membranes have been investigated through scanning electron microscopy and atomic force microscopy as shown in figure 4.5. The augmentation of PIL on membrane surface has also been evidenced by energy dispersive X-ray photoelectron spectroscopy. The cross sectional SEM images revealed the presence of dense PIL on the porous polyimide surface with a thickness of ~ 200 nm for both the membranes. The EDX spectra revealed that on augmentation of PILs lead to enhancement of N peak attributed to the repeated pyridinium unit. The appearance of Br peak can also be attributed to the existence of polymers of vinyl pyridinium bromide isomers. The self-assembled pressure assisted PILs deposition also led to a significant reduction in O peaks. The AFM images revealed a comparatively rough surface, which might be due to the repetitive presence of aromatic pyridinium moieties. The mean roughness (Ra); root mean square of Z values (Rr) and maximum vertical distance between the highest and lowest data points (Rmax) for these two membranes were evaluated as 28.5 nm, 42 nm and 313 nm respectively for 2 vinyl pyridinium bromide PIL augmented membrane, while that for 4 vinyl isomer was found to be 29.7 nm, 40 nm, and 378 nm, respectively.

109

Figure 0.5: Surface morphology of PIL augmented membranes by SEM (surface and cross sectional view), EDX spectroscopy and AFM (2D and 3D view) for 2 vinyl pyridine and 4 vinyl pyridine membranes.

110

4.3.1.4 FTIR

The FTIR spectra were used in order to evidence the modification in the surface functionalities after augmentation of PILs as displayed in figure 4.6. The FTIR spectra of the polyimide support exhibited two distinct peaks ~ 1534 cm-1 and 1648 cm-1, corresponding to the amide moieties of the polymer. The overall spectrum resembles with the FTIR spectra reported for polyimide support with crosslinking [31]. On incorporation of PILs, primarily, the intensity of these characteristic peaks of polyimide moieties were reduced, while some additional sets of peaks appeared in range of 880 – 1468 cm−1 correspond to the PIL obtained from 2 vinyl pyridine, [32] while for the PIL of 4 vinyl pyridine augmented membrane, additional peaks (in the range of 1214–

1468 cm−1 and 1588–1858 cm−1) characteristics of PIL of 4 vinyl pyridine were observed [33].

The peaks with 3000–3700 cm−1 can be attributed to the presence of pyridine moieties [33].

111

Transmittance %

-1 Wavelength (cm )

Figure 0.6: The FTIR spectra for PIL augmented membranes and polyimide support.

112

4.3.2 Applications

4.3.2.1 Separation of aromatic from aliphatic homologue

The membrane based separation mainly based on the size exclusion technique. However, very few works of literature demonstrating different driving forces to achieve unconventional separation. In this section, separation of aromatic from the aliphatic homologues with the same number of carbon atoms has been attempted, which otherwise very difficult by conventional size exclusion based membrane technology. The feed was containing aromatic: aliphatic (1: 99) v% was used in the present investigation. The separation factor for benzene from hexane was found to be ~ 60 and 64 for 2 vinyl and 4 vinyl pyridinium PIL augmented membranes, respectively. The separation efficiency for toluene from the toluene-heptane mixture was found to be more than that of benzene. The separation factor was found to follow the trend: anthracene from anthracene- tetradecane> naphthalene from naphthalene-decane mixture> p-xylene from p-xylene-octane mixture> toluene from toluene-heptane mixture> benzene from benzene hexane mixture. The selectivity of these PIL augmented membrane towards aromatic over aliphatic compound was attributed to the suitable π electronic interaction between aromatic pyridinium moieties in membrane surface and the aromatic in the feed solution [34–37]. Figure 4.7 shows the separation of aromatic compounds from their aliphatic homologues using PIL augmented membranes.

113

Figure 0.7: Separation of aromatics from aliphatic homologues using PIL augmented membranes.

If our argument is correct, then the increase/decrease in π electron density on the compound present in the feed solution will influence the separation factor. For toluene, the +inductive effect of methyl group enhanced the electron density on the benzene ring and hence enhancing the separation factor, while for p-xylene, two methyl groups at para position further enhances the π cloud density on benzene ring also evidence from the separation factor. Naphthalene exhibits the delocalization of π electron cloud over ten carbon atoms. This enhancement in electron cloud further enhances the separation factor, while for anthracene, the π electron cloud is further delocalized over three rings having fourteen carbon atoms, showing maximum separation factor.

Moreover, the increase in the number of carbon atoms in aliphatic homologues leads to enhancement in their overall size resulting in more improvement in the separation factor.

114

The separation factor using 4 vinyl pyridine PIL membrane was found to be more for all the aromatics compared to the 2 vinyl isomers. In case of 2 vinyl pyridinium PIL, the vinylic substituent at position 2 in pyridinium ring and allyl substituent on pyridinium N resides in very close proximity might result in a little deterioration from planarity and hence marginal reduction in π cloud density on ionic liquid compared to 4 vinyl pyridinium PIL, wherein both the substituents are far apart (in para position). Table 4.1 summarizes the relative compositions of the permeate and permeate flux through these novel PIL modified membranes. The initial flux values for different membranes vary based on the feed composition, as reported in table 1 summarizes the relative compositions of the permeate and permeate flux through these novel PIL modified membranes. Although 2 vinyl pyridine membrane has higher selectivity toward the studied aromatics, the flux was found to be lower than 4 vinyl pyridine membrane. For both 2 and 4 vinyl pyridine membranes, the flux data was found to follow the trend of benzene: hexane > toluene: heptane > xylene: octane > naphthalene: decane > anthracene: tetradecane. This could be explained by the ease of diffusion for the small molecules in comparison with relatively larger ones through the membrane.

Table 0.1: Relative compositions for different solvent systems in feed and permeate.

System Feed 2 Vinyl pyridine Permeate 4 Vinyl pyridine Permeate Initial flux Initial flux composition (LMH) composition (LMH) benzene: hexane 1:99 60:40 59 ± 2 64:36 41± 3 toluene: heptane 72:28 51± 2 76:24 42 ± 3 xylene: octane 79:21 24 ± 1 84:16 16 ± 2 naphthalene: decane 89:11 30 ± 3 93:7 16 ± 1 anthracene: tetradecane 95:5 3.3 ± 0.2 97:3 3.3 ± 0.3

115

4.3.2.2 Nature of substituents in benzene ring on the fractionation

Figure 4.8 reveals the effect of the substituent in the benzene ring on their fractionation when an equimolar of a mixture of benzene, nitrobenzene, toluene, and p-xylene were employed as feed solution. Though all the constituents in the feed have π electron density due to their aromaticity, the extent of the π electron density is highly influenced by the nature of substituents.

The electron withdrawing nitro group in nitrobenzene reduces the π cloud density on the benzene ring, while the methyl group with + inductive effect enhanced π cloud density for toluene. This further enhances by substituting two methyl groups in para position to each other. As a matter of fact, the fractionation factor was found to follow the trend of p-xylene > toluene > nitrobenzene > benzene.

Figure 0.8: The effect of substituents in benzene ring on the fractionation of aromatics by PIL augmented membranes.

116

4.3.2.3 Influence of the extended conjugation on the fractionation

The influence of extended delocalization over more than one benzene ring has been investigated using an equimolar amount of benzene, naphthalene, anthracene, and phenanthrene.

Benzene with minimum π electron cloud delocalization over one six membered aromatic rings showed the least fractionation factor in the permeate. Naphthalene with two aromatic rings in conjugation exhibits more π electron density resulting in improved fractionation factor. The electron density further increases for anthracene and phenanthrene, where three aromatic rings are involved [40–42]. Another interesting thing required to be mentioned is that, though anthracene and phenanthrene both have three aromatic rings for extended delocalization, their fractionation factor was yet different. The stereochemical arrangement of these two isomers might lead to an influence on the planarity or the π electron density on the aromatic system. This might be responsible for the difference in their fractionation factors. To understand the actual reason, further investigation is necessary, yet this result provides us another opportunity to explore the separation of stereoisomers associated with π systems. Figure 4.9 shows the fractionation results for benzene, naphthalene, anthracene, and phenanthrene using these PIL augmented membranes.

117

Figure 0.9: The effect of extended conjugation on the fractionation of aromatics by PIL augmented membranes.

118

4.3.2.4 Separation of stereoisomers

The π electron induced stereoisomer separation has also been attempted using cis and trans stilbene. A feed containing cis:trans stilbene in 70:30 ratio has been subjected to filtration. The separation factor for trans isomer was found to be 1.70 and 1.96 for the 2 and vinyl pyridine PIL augmented membranes, respectively. In cis isomer, the presence of two phenyl rings on the same side of the alkene groups may lead to deterioration from the planarity between these two phenyl groups and alkene moiety [43,44]. Consequently, the extended conjugation involving two phenyl rings bridged through the alkene moiety is lost for cis isomer. On the other hand, trans isomer does not have such stereo-chemical constrain to lose the extended conjugation. Therefore, trans stilbene isomer has been preferentially separated from the cis-trans mixture. Figure 4.10 shows the separation factors for trans isomer from a cis-trans mixture using these PIL augmented membranes.

Figure 4.10: Separation of stereoisomer of stilbene using PIL augmented membranes.

119

4.3.2.5 Separation of regioisomers

An attempt was also made using these PIL augmented membranes to separate regioisomers of bipyridine. A feed containing an equal concentration of 2,2ʹ bipyridine and 4,4ʹ bipyridine in hexane was allowed to filter through these PIL augmented membranes in dead end mode. The separation factors for both the membranes were found to be similar and a preferential separation of 4,4ʹ bipyridine over 2,2ʹ bipyridine regioisomer (PIL from 2 vinyl pyridine augmented membrane: 1.70 and that of 4 vinyl pyridine: 1.76). The close proximity of two nitrogen in 2,2ʹ bipyridine conformation might be responsible for a slight deterioration of the planarity between two pyridine rings, which is expected to be retained for 4,4ʹ bipyridine. Consequently, the extended resonance between two pyridine rings in 4,4ʹ isomer provides more π electron density for interaction with the active layer of membrane and hence got preferential separation. Figure 4.11 presents the separation factors for these two regioisomers for the PIL augmented membranes.

Figure 0.11: Separation of two regioisomers of bipyridine using PIL augmented membranes.

120

4.3.3 Stability of the PIL augmented membranes

Since the PIL augmentation is done by pressure assisted self-assembly method, the stability of the PIL augmented membranes can be a major concern even though we are getting very exciting separation results. The PIL augmented membranes were subjected to equilibration with the feed solutions encountered for separation and the conductivity of the supernatant solutions was monitored. Any dissolution of PIL would lead to enhancement in conductivity. All the organic solvents used in the present case did not show any change in conductivity over a time of 350 hours, while in the presence of water the conductivity was found to increase very rapidly with time. This investigation revealed that these PIL augmented membranes are highly stable in organic non-polar solvent media, while not stable in aqueous media or maybe any polar or protic solvents. The high degree of stability of these PIL augmented membranes might be due to high π stacking interaction between the aromatic moieties from polyimide support and the PILs [18,37]. This also is clearly demonstrated that a small amount of water impurity in the solvent can be problematic for the stability of these PIL augmented membranes. The stability test for these PIL augmented membranes have been depicted in figure 4.12.

121

Figure 0.12: The stability of PIL augmented membranes in different organic solvents of interest.

4.4 Conclusions

One-pot synthesis of novel pyridinium based PILs were augmented on polyimide porous support by pressure assisted self-assembly and employed for unconventional π electron mediated separation of aromatics from a mixture of their aliphatic homologues: benzene-hexane, toluene- heptane, xylene-octane, naphthalene- decane, anthracene-tetradecane. The aromatic compounds with more electron cloud density were found to get separated preferentially. The fractionation trend was observed to follow nitrobenzene < benzene < toluene < p-xylene, and was attributed to the electron withdrawing effect of the nitro group and electron cloud enhancing + inductive effect of methyl group/s. The fractionation of the aromatic moieties showed an order: phenanthrene > anthracene > naphthalene > benzene that was caused by extended conjugation in terms of the number of benzene rings. These PIL augmented membranes were found to have preferential

122 separation factors for trans stilbene compared to its stereoisomer due to the stereochemical arrangement of cis isomer. The proximity of the two benzene rings might results in loss of planarity in the most stable conformation, causing deviation from overall conjugation and hence reduced the extended π electron density. These PIL augmented membranes were also utilized for the separation of regioisomers of bipyridine (2,2ʹ and 4,4ʹ). A preferential separation of 4,4ʹ isomer was reported probably due to the proximity of two nitrogen in 2,2ʹ isomer conformation. These membranes were also found to be stable in the presence of non-polar, non-aqueous, common organic solvents used in the present study.

4.5 Acknowledgments

Funding for this work was received from the Arkansas Research Alliance and the University of

Arkansas.

Supplementary material

Supplementary data associated with this chapter can be found in Appendix III.

123

4.6 References

[1] N. Jacob, R. Radhakrishnan, V. Sunil, G. Kamalanathan, A. Sengupta, R. Wickramasinghe, Separation and Puri fi cation Technology Performance evaluation of novel nanostructured modi fi ed mesoporous silica / polyetherimide composite membranes for the treatment of oil/water emulsion, Sep. Purif. Technol. 205 (2018) 32–47. doi:10.1016/j.seppur.2018.05.007.

[2] M. Malmali, J. Askegaard, K. Sardari, S. Eswaranandam, A. Sengupta, S.R. Wickramasinghe, Evaluation of ultrafiltration membranes for treating poultry processing wastewater, J. Water Process Eng. 22 (2018) 218–226. doi:10.1016/j.jwpe.2018.02.010.

[3] E. Drioli, V. Calabro, Y. Wu, Microporous membranes in membrane distillation, Pure Appl. Chem. 58 (1986) 1657–1662. doi:10.1351/pac198658121657.

[4] K. Sardari, J. Askegaard, Y.H. Chiao, S. Darvishmanesh, M. Kamaz, S.R. Wickramasinghe, Electrocoagulation followed by ultrafiltration for treating poultry processing wastewater, J. Environ. Chem. Eng. 6 (2018) 4937–4944. doi:10.1016/j.jece.2018.07.022.

[5] T.Y. Cath, D. Adams, A.E. Childress, Membrane contactor processes for wastewater reclamation in space: II. Combined direct osmosis, osmotic distillation, and membrane distillation for treatment of metabolic wastewater, J. Memb. Sci. 257 (2005) 111–119. doi:10.1016/j.memsci.2004.07.039.

[6] E. Metcalf, H. Eddy, Wastewater engineering: treatment and reuse, 2003. doi:10.1016/0309-1708(80)90067-6.

[7] J. Neel, Q.T. Nguyen, R. Clement, L. Le Blanc, Fractionation of a binary liquid mixture by continuous pervaporation, J. Memb. Sci. 15 (1983) 43–62. doi:10.1016/S0376- 7388(00)81361-7.

[8] B. Smitha, D. Suhanya, S. Sridhar, M. Ramakrishna, Separation of organic-organic mixtures by pervaporation - A review, J. Memb. Sci. 241 (2004) 1–21. doi:10.1016/j.memsci.2004.03.042.

[9] N. Pekel, Z.M.O. Rzaev, O. Güven, Synthesis and characterization of poly(N- vinylimidazole-co-acrylonitrile) and determination of monomer reactivity ratios, Macromol. Chem. Phys. 205 (2004) 1088–1095. doi:10.1002/macp.200300130.

[10] Z. Zheng, Q. Xu, J. Guo, J. Qin, H. Mao, B. Wang, F. Yan, Structure-Antibacterial Activity Relationships of Imidazolium-Type Ionic Liquid Monomers, Poly(ionic liquids) and Poly(ionic liquid) Membranes: Effect of Alkyl Chain Length and Cations, ACS Appl. Mater. Interfaces. 8 (2016) 12684–12692. doi:10.1021/acsami.6b03391.

[11] H.L. Ricks-Laskoski, A.W. Snow, Synthesis and electric field actuation of an ionic liquid polymer, J. Am. Chem. Soc. 128 (2006) 12402–12403. doi:10.1021/ja064264i.

124

[12] J. Yuan, H. Schlaad, C. Giordano, M. Antonietti, Double hydrophilic diblock copolymers containing a poly(ionic liquid) segment: Controlled synthesis, solution property, and application as carbon precursor, Eur. Polym. J. 47 (2011) 772–781. doi:10.1016/j.eurpolymj.2010.09.030.

[13] A. Sengupta, P.K. Mohapatra, Multiple Diglycolamide Functionalized Ligands in Room Temperature Ionic Liquids : ‘ Green ’ Solvents for Actinide Partitioning, (2014) 47–51.

[14] A. Sengupta, M.S. Murali, P.K. Mohapatra, M. Iqbal, J. Huskens, W. Verboom, An insight into the complexation of UO22+with diglycolamide-functionalized task specific ionic liquid: Kinetic, cyclic voltammetric, extraction and spectroscopic investigations, Polyhedron. 102 (2015) 549–555. doi:10.1016/j.poly.2015.09.070.

[15] N.K. Gupta, A. Sengupta, S. Biswas, Quaternary ammonium based task specific ionic liquid for the efficient and selective extraction of neptunium, Radiochim. Acta. 105 (2017) 689– 697. doi:10.1515/ract-2016-2694.

[16] C.W. Cho, J.S. Park, S. Stolte, Y.S. Yun, Modelling for antimicrobial activities of ionic liquids towards Escherichia coli, Staphylococcus aureus and Candida albicans using linear free energy relationship descriptors, J. Hazard. Mater. 311 (2016) 168–175. doi:10.1016/j.jhazmat.2016.03.006.

[17] J. Guo, Q. Xu, Z. Zheng, S. Zhou, H. Mao, B. Wang, F. Yan, Intrinsically Antibacterial Poly(ionic liquid) Membranes: The Synergistic Effect of Anions, ACS Macro Lett. 4 (2015) 1094–1098. doi:10.1021/acsmacrolett.5b00609.

[18] S.K. Ethirajan, A. Sengupta, M. Jebur, M. Kamaz, X. Qian, R. Wickramasinghe, Single- Step Synthesis of Novel Polyionic Liquids Having Antibacterial Activity and Showing π- Electron Mediated Selectivity in Separation of Aromatics, ChemistrySelect. 3 (2018) 4959– 4968. doi:10.1002/slct.201800101.

[19] M. Jebur, A. Sengupta, Y.H. Chiao, M. Kamaz, X. Qian, R. Wickramasinghe, Pi electron cloud mediated separation of aromatics using supported ionic liquid (SIL) membrane having antibacterial activity, J. Memb. Sci. 556 (2018) 1–11. doi:10.1016/j.memsci.2018.03.064.

[20] A. Sengupta, S. Kumar Ethirajan, M. Kamaz, M. Jebur, R. Wickramasinghe, Synthesis and characterization of antibacterial poly ionic liquid membranes with tunable performance, Sep. Purif. Technol. 212 (2019) 307–315. doi:10.1016/j.seppur.2018.11.027.

[21] S. Basu, M. Maes, A. Cano-Odena, L. Alaerts, D.E. De Vos, I.F.J. Vankelecom, Solvent resistant nanofiltration (SRNF) membranes based on metal-organic frameworks, J. Memb. Sci. 344 (2009) 190–198. doi:10.1016/j.memsci.2009.07.051.

[22] N. Wang, S. Ji, J. Li, R. Zhang, G. Zhang, Poly(vinyl alcohol)-graphene oxide nanohybrid “pore-filling” membrane for pervaporation of toluene/n-heptane mixtures, J. Memb. Sci. 455 (2014) 113–120. doi:10.1016/j.memsci.2013.12.023.

125

[23] S. Nair, Z. Lai, V. Nikolakis, G. Xomeritakis, G. Bonilla, M. Tsapatsis, Separation of close- boiling hydrocarbon mixtures by MFI and FAU membranes made by secondary growth, Microporous Mesoporous Mater. 48 (2001) 219–228. doi:10.1016/S1387-1811(01)00356- 0. [24] M. Keluwett, M. Kleber, Molecular-Level Interactions in Soils and Sediments: The Role of Aromatic π-Systems, 43 (2009) 3421–3429.

[25] D. Zhu, E. Herbert Bruce, A. Schlautman Mark, R. Carraway Elizabeth, Characterization of cation-pi interactions in aqueous solution using deuterium nuclear magnetic resonance spectroscopy, J Env. Qual. 33 (2004) 276–284.

[26] S.B. Haderlein, R.P. Schwarzenbach, Adsorption of Substituted Nitrobenzenes and Nitrophenols to Mineral Surfaces, Environ. Sci. Technol. 27 (1993) 316–326. doi:10.1021/es00039a012.

[27] X. Qu, P. Liu, D. Zhu, Enhanced sorption of polycyclic aromatic hydrocarbons to tetra- alkyl ammonium modified smectites via cation-π interactions, Environ. Sci. Technol. 42 (2008) 1109–1116. doi:10.1021/es071613f.

[28] I. Soroko, Y. Bhole, A.G. Livingston, Environmentally friendly route for the preparation of solvent resistant polyimide nanofiltration membranes, Green Chem. 13 (2011) 162–168. doi:10.1039/c0gc00155d.

[29] W.S. Hung, Q.F. An, M. De Guzman, H.Y. Lin, S.H. Huang, W.R. Liu, C.C. Hu, K.R. Lee, J.Y. Lai, Pressure-assisted self-assembly technique for fabricating composite membranes consisting of highly ordered selective laminate layers of amphiphilic graphene oxide, Carbon N. Y. 68 (2014) 670–677. doi:10.1016/j.carbon.2013.11.048.

[30] M. Kamaz, P. Rocha, A. Sengupta, X. Qian, R.S. Wickramasinghe, Efficient removal of chemically toxic dyes using microorganism from activated sludge: Understanding sorption mechanism, kinetics, and associated thermodynamics, Sep. Sci. Technol. 00 (2018) 1–17. doi:10.1080/01496395.2018.1440305.

[31] S. Yang, H. Zhen, B. Su, Polyimide thin film composite (TFC) membranes: Via interfacial polymerization on hydrolyzed polyacrylonitrile support for solvent resistant nanofiltration, RSC Adv. 7 (2017) 42800–42810. doi:10.1039/c7ra08133b.

[32] L.M. Madikizela, P.S. Mdluli, L. Chimuka, Experimental and theoretical study of molecular interactions between 2-vinyl pyridine and acidic pharmaceuticals used as multi-template molecules in molecularly imprinted polymer, React. Funct. Polym. 103 (2016) 33–43. doi:10.1016/j.reactfunctpolym.2016.03.017.

[33] N.A. Rahim, F. Audouin, B. Twamley, J.G. Vos, A. Heise, Synthesis of poly(4-vinyl pyridine-b-methyl methacrylate) by MAMA-SG1 initiated sequential polymerization and formation of metal loaded block copolymer inverse micelles, Eur. Polym. J. 48 (2012) 990– 996. doi:10.1016/j.eurpolymj.2012.02.011.

126

[34] C.A. Hunter, J.K.M. Sanders, The Nature of π-π Interactions, J. Am. Chem. Soc. 112 (1990) 5525–5534. doi:10.1021/ja00170a016.

[35] C. Janiak, A critical account on n-n stacking in metal complexes with aromatic nitrogen- containing ligands, J. Chem. Soc. Dalt. Trans. (2000) 3885–3896. doi:10.1039/b003010o.

[36] P. Kölle, R. Dronskowski, Hydrogen Bonding in the Crystal Structures of the Ionic Liquid Compounds Butyldimethylimidazolium Hydrogen Sulfate, Chloride, and Chloroferrate(II,III), Inorg. Chem. 43 (2004) 2803–2809. doi:10.1021/ic035237l.

[37] R.P. Matthews, T. Welton, P.A. Hunt, Competitive pi interactions and hydrogen bonding within imidazolium ionic liquids, Phys. Chem. Chem. Phys. 16 (2014) 3238–3253. doi:10.1039/c3cp54672a.

[38] D. Lazdins, M. Karplus, The Inductive Effect in the Toluene Anion Radical, J. Am. Chem. Soc. 87 (1965) 920–921. doi:10.1021/ja01082a044.

[39] L.R. Radovic, I.F. Silva, J.I. Ume, J.A. Menéndez, C.A. Leon Y Leon, A.W. Scaroni, An experimental and theoretical study of the adsorption of aromatics possessing electron- withdrawing and electron-donating functional groups by chemically modified activated carbons, Carbon N. Y. 35 (1997) 1339–1348. doi:10.1016/S0008-6223(97)00072-9.

[40] J.M.L. Martin, J. El-Yazal, J.-P. François, Structure and Vibrational Spectrum of Some Polycyclic Aromatic Compounds Studied by Density Functional Theory. 1. Naphthalene, Azulene, Phenanthrene, and Anthracene †, J. Phys. Chem. 100 (1996) 15358–15367. doi:10.1021/jp960598q.

[41] C.F. Matta, J. Hernández-Trujillo, Bonding in Polycyclic Aromatic Hydrocarbons in Terms of the Electron Density and of Electron Delocalization, J. Phys. Chem. A. 107 (2003) 7496– 7504. doi:10.1021/jp034952d.

[42] J.R. Platt, The box model and electron densities in conjugated systems, J. Chem. Phys. 1448 (1954) 1448–1455. doi:10.1063/1.1740414.

[43] A. Warshel, Calculation of vibronic structure of the π→π* transition of trans- and cis- stilbene, J. Chem. Phys. 62 (1975) 214–221. doi:10.1063/1.430265.

[44] J.L. Oudar, Optical nonlinearities of conjugated molecules. Stilbene derivatives and highly polar aromatic compounds, J. Chem. Phys. 67 (1977) 446–457. doi:10.1063/1.434888.

127

Chapter 5 Conclusions and Future Direction

5.1 Conclusions

A membrane is basically illustrated as a barrier that selectively permeates targeted species while preventing the transport of other species. Membrane based separation provides clean, economically viable, easy to operate, efficient separation procedure with the ease of large industrial scale adaptation. Hence, developing membrane separation processes have continued to accelerate for the last couple of decades resulting in new technology [1]. To expand membrane applications in separation science, researchers have started looking at nonconventional separation mechanisms. Membranes are generally characterized by their pore size and solute sieving coefficient and hence transport through the pores requires the size of the pores to be smaller than the diameter of the unwanted components. Lately, novel approaches have been explored by scientists to make the most use of membranes in separation areas where conventional mechanisms have not met the demand.

This Ph.D. dissertation explored the use of ionic liquids and polymerized ionic liquids in membrane technology for continuous separation processes of aqueous and non-aqueous systems that have low energy consumption, small footprint equipment, and enhanced membrane properties.

The central theme of this research was to exploit the extradentary properties of ionic liquids in supported ionic liquid membranes and deadend filtration. The fractionation of aromatic compounds, as well as their separation from their monologue aliphatic molecules, was investigated by means of the supported ionic liquid membrane and deadend nanofiltration. The effects of different anion associated with ILs and the length of the side chain attached were also investigated.

Cl, Br, and LiNTf2 were used to study the effect of anions on the separation process. Furthermore,

128 the lifespan of membrane processes is crucial so the membrane stability for long term operation challenged with polar and nonpolar solvents was also explored.

Imidazolium based 3 SILMs were successfully prepared and utilized for the π electron induced separation: separation of toluene, styrene, and divinylbenzene in hexane medium and separation of congo red, eichrome black T and ramazol brilliant blue R from aqueous medium.

The presence of imidazolium ring stretching frequency attributed to the exposure of pore mouth on the surface along with the characteristic of the PTFE membrane was signified with FTIR and confirmed by SEM. The affinity of ionic liquid towards the π electron cloud was found to be the driving force for the separation resulting the separation efficiency as divinylbenzene > styrene > toluene, while in case of dyes the complicated electronic configuration combining the π electron cloud along with cation π electron cloud interaction resulted in the preferred selectivity of congo red. However, no clear conclusion was drawn for the favorable transport of congo red over other dyes in aqueous solution and further research is indeed required to understand the transport mechanism and their interaction with ionic liquids.

Different membrane material was also utilized to prepare SILMs with imidazolium based

ILs and employed for unconventional π electron mediated fractionation of aromatics. Here the influence of extended conjugation over more than one benzene ring was investigated using an equimolar amount of benzene, naphthalene, and phenanthrene. The fractionation of the aromatic molecules followed the order phenanthrene > naphthalene > benzene was attributed to extended conjugation of the number of benzene rings. Aromatic moieties with higher electron cloud density had favorable interaction with ionic liquids moieties resulted in higher % transport. Further, ionic liquids with different alkyl side chain length were impregnated in the same substrate. The results showed that ionic liquids with shorter side chains and higher viscosity in the case of [C3vim+][Br−]

129 membrane had lower % transport while % transport increased gradually for [C6vim+][Br−], and

[C8vim+][Br−] membranes, respectively. The larger the contacting surface area at the interface between the feed and impregnated ionic liquid, the higher % transport of species through the carrier to the receiver phase.

The π electron induced stereoisomer separation was attempted using cis and trans stilbene.

[C3vim+][Br−] membrane was found to have approximately similar % transport rate for cis and trans stilbene compared with [C6vim+][Br−], and [C8vim+][Br−] membranes. However, the separation factor for trans stilbene is increased as the ionic liquid alkyl length decreases with the highest value of 1.48 for [C3vim+][Br−] membrane which is attributed to the viscosity of the ionic liquid. SILMs were found to have a selective separation for trans stilbene over its stereoisomer.

This is explained by the fact that the stereo-chemical arrangement of cis isomer the loss of the planarity in the case of cis stilbene results in weaker interaction with the ionic liquid carrier through extended pi cloud interaction. One of the inherited drawbacks associated with supported liquid membranes in general is their stability over long term operation. Due to a small transmembrane pressure or mutual solubility with the feed/receiver phase, the carrier tends to leach out from the membrane matrix causing the loss of its selectivity. Thus, during this research, membrane stability was carefully tested. The stability of the SILM was evaluated for up to 60 days revealing SILMs with water soluble ionic liquid is highly stable in hexane while the water insoluble ionic liquid in water. Generally, water soluble SILMs were also found to be stable in the presence of nonpolar and nonaqueous organic solvents while the stability of SILM degraded in the presence the water.

Also, ionic liquids with higher viscosity showed better stability due to the stronger capillary forces with the membrane substrate.

130

Another approach for incorporating PILs in membranes was through direct deposition on the membrane surface. In this part of the dissertation, one pot synthesis of novel pyridinium based

PILs were augmented on polyimide porous support by pressure assisted self-assembly and later employed for unconventional π electron mediated separation of aromatics from a mixture their aliphatic homologues as following: benzene-hexane, toluene-heptane, xylene-octane, naphthalene- decane, anthracene-tetradecane. It was shown that aromatic moieties with a high electron cloud density were found to get separated preferentially over their aliphatic homologues. The fractionation of the aromatic moieties was possible and found to follow the order: phenanthrene> anthracene> naphthalene> benzene. This was attributed to extended conjugation in terms of the number of benzene rings. Separation of stereoisomers is challenging in chemical processes due to their very close physicochemical properties. These PIL augmented membranes were found to have preferential separation factor for trans stilbene compared to its stereoisomer, which is due to the fact that the stereo-chemical arrangement of cis isomer leads to the proximity of the two benzene rings might results in loss of planarity in the most stable conformation, resulting deviation from overall conjugation and hence extended π electron density. These PIL augmented membranes were also utilized for the separation of regioisomers of bipyridine (2,2 and 4,4′). A preferential separation of 4,4′ isomer was reported probably due to the proximity of two nitrogens in 2,2′ isomer conformation. These membranes were also found to be stable in the presence of non-polar, non- aqueous, common organic solvents used in the present study.

131

5.2 Future Direction and Suggestion

For future consideration regarding the use of ionic liquids in membrane technology, the necessity to develop more robust ionic liquid membranes could happen flowing two routes;

5.2.1 Membrane modification

Across the work of this Ph.D., two different routes were followed to incorporate the ionic liquids in different membrane substrates, namely; immobilizing ionic liquids inside membrane pores by means of pressure, and physical coating on top the membrane surface. However, these two techniques are associated with inherited drawbacks such as the wide window solubility of ionic liquids in polar solvents and their high viscosity. For future work, the chemical attachment of ionic liquid monomers on the membrane is highly recommended to be explored for several reasons. First, membrane stability is a serious technical requirement for a robust membrane process and chemical modification allows monomer brushes to be part of the membrane matrix and here nothing except ion exchange could alter surface membrane properties during operation. Second, although deposition of ionic liquids will improve membrane performance, limited operating conditions are required since the added layer could be washed out at certain conditions such as a high flow rate and the use of mutual miscible media. Chemical modification approaches using atom transfer radical polymerization, UV light, plasma, and interfacial polymerization could be deployed for the successful attachment of ionic liquid monomers owing to their bulk properties.

However, prior to ionic liquid attachment, surface preparation such as functionalizing membrane surface might be needed.

5.2.2 Membrane separation techniques

The membrane separation techniques such as microfiltration, ultrafiltration, reverse osmosis, electrodialysis, and supported liquid membranes, are being used in the industry for the

132 separation of different components from solutions. In all the membrane separation processes, the membrane is used as a barrier for allowing certain components to transport while rejects others.

Though supported liquid membrane has a wide range of advantages over other processes, namely, the intensified process of extraction and stripping in one unit, low solvent use, and high interfacial area per unit volume, the process is associated with some disadvantages. Membrane stability is the major concern for implementing SILM in industrial scale. Further, the loss of the carrier from the membrane pores by evaporation or mutual dissolution or dispersion into feed or receiver phases is of major concern. The loss of membrane carrier results in losing membrane selectivity. Transport of solutes in SILM takes place by means of diffusion which is relatively slow to compare with convection transport in other size exclusion membrane processes. Thus, other membrane processes are proposed here for better performance. Vapor permeation seems to be the most suitable process to adopt. During the vapor permeation process, the feed is heated up to generate a vapor phase for the targeted solute to transfer through a semipermeable membrane which could be easily modified with ionic liquids using a similar protocol to the one used in SILM preparation. Here membrane stability is not a concern since ionic liquids are known for their zero or negligible vapor pressure.

Also, the stripping phase occurs faster than SILM because of the use of vacuum.

133

Appendices

Appendix I

Pi electron cloud mediated separation of aromatics using supported ionic liquid (SIL) membrane having antibacterial activity

*This chapter is adapted from a published paper by Mahmood Jebur, Arijit Sengupta, Yu-Hsuan Chiao, Mohanad Kamaz, Xianhong Qian, Ranil Wickramasinghe “Pi electron cloud mediated separation of aromatics using supported ionic liquid (SIL) membrane having antibacterial activity” Journal of Membrane Science, 2018.

The structure of the tested dyes and aromatic compounds are illustrated in figure A.I.

Figure A.1: Chemical structure of dyes and aromatic compounds.

134

Figure A.2: Schematic diagram and photographic picture of the diffusion cell used in this study. UV-vis is used to determine the concentration by running a calibration curve prior to the experiments with a regression coefficient value close to unity for all compounds. The process started by preparing an aqueous dye solution at 50 mg/L and dilute it 10 times while evaluating

UV absorption at an identical wavelength for each chemical. In the case of aromatic solvents, each one was diluted hexane by means of volume and quantified by UV until reaching the lowest detectable concentration of the aromatic diluents. Figure A.II and Figure AIII show the calibration curves of dyes and aromatic compounds investigated in this study.

135

Figure A.3: The linear relationship between concentration and absorption for the dyes using UV-vis at optimized wavelengths.

136

Figure A.4: The linear relationship between concentration and absorption of the aromatic compounds using UV-vis at optimized wavelengths.

137

Appendix II

π Electron induced separation of organic compounds using supported ionic liquid membranes Detection of aromatic compounds

High Performance Liquid Chromatography (HPLC) equipped with Luna C18 column (5

µm, size 250*4.6 mm, from Phoenix, USA) was used to detect aromatic compounds and subsequently determine their fractionation factor. Acetonitrile was used as the mobile phase at a flow rate of 0.5 mL.min-1 and the column temperature was kept at 29 °C. The injection sample volume was set to be 20 µL. A diode array detector (DAD) was used to detect the selected compounds. UV scan spectra was firstly obtained for each compound to specify the maximum wavelength absorbance. The separation was efficiently obtained and distinctive retention time for each compound was achieved with regression coefficient value close to one as shown in figure A

2.1.

138

Figure A2.1: Standard curves for benzene, naphthalene, and phenanthrene using HPLC equipped with C18 column.

139

HPLC protocol

2.1 Running the HPLC system

1. Load method “EssMartinStartup.N”: DAD 29 oC or set flow rate to 0.1 mL/min.

2. Connect guard column inlet slowly with the flow on (0.1 ml/min) to remove any air bubble

trapped in the column or tubings.

3. Connect small tubing on the column outflow side to remove the sodium azide (column

storage solution) used as eluent to prevent microbial growth.

4. Secure the column with the two metal plates and check if there is any leaking from the

fittings or other connections.

5. Start to collect diluted sodium azide in a beaker and after collecting 3 mL connect the

column to DAD detector.

6. Increase the flow rate gradually from 0.1-0.5 mL/min over 30 min which is the operating

flow rate.

7. Now load method “Mohanadaromatics.N” and check the column pressure is within the set

range. Then name the samples in the sequence table save it under the known name it a

designated file and start the run.

140

2.2 Removing the column

1. Once the runs are over the shutdown method will automatically be activated and the flow

rate will go down to 0.1 mL/min with a (50:50 v) mixture of acetonitrile: water eluent.

2. Disconnect column outlet from the system.

3. Connect small tubing on the column outflow side to collect the sodium azide in a separate

beaker.

4. Disconnect the column and cap both ends and now can be stored at room temperature.

2.3 Backflushing the C18 column

1. To backflush the column in case of increasing the pressure across the column, connect the

column reversely to the pump.

2. Connect the other end of the column to a short tube where it ends in a collecting beaker.

3. Strat flushing the column with 100% acetonitrile at 0.2 mL/min for at least 3 h or once the

eluent volume collected is 6 times the column volume.

4. Then it’s followed by a sequence of flushing with isopropyl alcohol, methanol, and

chloroform if needed for an overnight.

5. Stop the flow rate and disconnect the column and connect it back to the HPLC in the right

direction and run the mobile phase at 0.2 mL/min.

6. Leave the column running for at least 3 hours and check the pressure. If it is needed repeat

the same flushing protocol with a more nonpolar solvent such as hexane to remove

hydrophobic compounds trapped inside the column media.

141

Appendix III

Poly(ionic liquid) augmented membranes for π electron induced separation/fractionation of aromatics

Table A3.1: Chemical structure of compounds used in this chapter and their distinct absorption wavelength with UV-Vis.

142

List of all papers representing UARK

First author papers

Mohanad Kamaz, Arijit Sengupta, Sandrina DePaz, Yu-Hsuan Chiao, Ranil Wickramasinghe. (2019). Poly(ionic liquid) augmented membranes for π electron induced separation/fractionation of aromatics. Journal of Membrane Science. 579, 102-110. (Published)

Mohanad Kamaz, Arijit Sengupta, Ashley Gutierrez, Yu-Husan Chiao, Ranil Wickramasinghe. (2019). Surface modification of PVDF Membranes for the Treating Produced Waters by Direct Contact Membrane Distillation. International Journal of Environmental Research and Public Health. 16 (5), 685. (Published)

Mohanad Kamaz, S. Ranil Wickramasinghe, Satchithanandam Eswaranandam, Wen Zhang, Steven M. Jones, Michael J. Watts, and Xianghong Qian. (2019). Investigation into Micropollutant Removal from Wastewaters by a Membrane Bioreactor. International journal environmental research and public health. International Journal of Environmental Research and Public Health. 16, 1363. (Published)

Mohanad Kamaz, Perla Rocha, Arijit Sengupta, Xianghong Qian, Ranil Wickramasinghe. (2018). Efficient removal of chemically toxic dyes using microorganisms from activated sludge: Understanding sorption mechanism, kinetics, and associated thermodynamics. Separation Science and Technology, 1-17. (Published)

Mohanad Kamaz, Ronald Vogler, Mahmood Jebur, Arijit Sengupta, Ranil Wickramasinghe (2019). π Electron induced separation of organic compounds using supported ionic liquid membranes. Separation and Purification Technology, 116237. (Published)

Co-author papers

Arijit Sengupta, Sudeesh Ethirajan, Mohanad Kamaz, Mahmood Jebur, Ranil Wickramasinghe. (2019). Synthesis and characterization of antibacterial polyionic liquid membranes with tunable performance. Separation and Purification Technology 212, 307-315. (Published)

Mahmood Jebur, Arijit Sengupta, Mohanad Kamaz, Yunshan Chiao, Xianghong Qian, Ranil Wickramasinghe. (2018). Pi electron cloud mediated separation of aromatics using supported ionic liquid (SIL) membrane having antibacterial activity. Journal of Membrane Science. 556, 1- 11. (Published)

Sudeesh Ethirajan, Arijit Sengupta, Mahmood Jebur, Mohanad Kamaz, Xianghong Qian, Ranil Wickramasinghe. (2018). Single-Step Synthesis of Novel Polyionic Liquids Having Antibacterial Activity and Showing π-Electron Mediated Selectivity in Separation of Aromatics. Chemistry Select 3 (17), 4959-4968. (Published)

143

Kamyar Sardari, John Askegaard, Yunshan Chiao, Sevesh Darvishmanesh, Mohanad Kamaz, and Ranil Wichramasinghe. (2018). Electrocoagulation followed by ultrafiltration for treating poultry processing wastewater. Journal of Environmental Chemical Engineering 6 (4), 4937- 4944. (Published)

Submitted papers

Mohanad Kamaz, Steven Jones, Xianghong Qian, Michael Watts, Wen Zhang (2019). Atrazine Removal from Municipal Wastewater using a Membrane Bioreactor. Journal of Water Process Engineering. (Under Review)

Tejas Tripathi, Mohanad Kamaz, S. Ranil Wickramasinghe, Arijit Sengupta. Designing electrically responsive ultrafiltration membranes. Journal name: International Journal of Environmental Research and Public Health. (Submitted)

144