SYNTHESIS OF METAL-ORGANIC FRAMEWORKS AS CATALYST FOR THE CONVERSION OF CARBON DIOXIDE TO CYCLIC CARBONATES by Mandeep Kaur B.Sc. in Chemistry Honors School, Guru Nanak Dev University, Amritsar, 2019 THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN CHEMISTRY UNIVERSITY OF NORTHERN BRITISH COLUMBIA August 2022 © Mandeep Kaur, 2022 ii Abstract As more attention is focused on the emission of CO2 into the environment, CO2 produced by industrial sources such as fossil-fuel power plants can be used as a potential source for the manufacturing of useful chemicals. Carbonate synthesis is a general approach for the conversion of CO2 using epoxides. The reaction of CO2 with epoxides to produce cyclic carbonates requires a catalyst with a high surface area, Lewis active sites, and an affinity for CO2. In this study, copperbased MOF catalysts (MOF-199, Cu-BDC, NH2-Cu-BDC) were synthesized by solvothermal method under mild reaction conditions. These catalysts were characterized by powdered X-ray diffraction, Fourier-Transform infrared spectroscopy, thermal gravimetric analysis, scanning electron microscope, and Brunauer-Emmett-Teller analysis. MOF-199 was subsequently used as a heterogeneous catalyst to catalyze the reaction of CO2 with epoxides to produce value-added cyclic carbonates. More than 65% conversion of epoxides (E1-E4) to the corresponding cyclic carbonates was observed using MOF-199 catalysts (C1, C2-S6, C2-S4, and C2-S6) and TBAB under mild reaction conditions. The conversion to cyclic carbonates was analyzed using 1H-NMR Spectroscopy. Keywords: Metal-organic framework, Catalyst, Carbon dioxide conversion, Cyclic carbonate iii Table of Contents Abstract .......................................................................................................................................... iii List of Figures ................................................................................................................................ vi List of Tables ............................................................................................................................... viii List of Abbreviations and Symbols................................................................................................ ix Acknowledgements ......................................................................................................................... x 1. Introduction ............................................................................................................................. 1 1.1 Importance of CO2 gas conversion........................................................................................ 1 1.1.1 General routes to convert CO2 gas to chemicals .................................................................... 2 1.1.2 Methods to produce dialkyl carbonates and cyclic carbonates from CO2 gas ............... 4 1.2 Catalysis .......................................................................................................................... 6 1.2.1 Why Catalysts for CO2 gas conversion? ................................................................................... 6 1.2.2 Metal-Organic Frameworks as a novel catalyst for CO2 gas conversion ........................ 8 1.2.3 Background study on MOF-199, Cu-BDC MOF, and NH2-Cu-BDC MOF ................ 14 2. 1.3 Research Aim and Objectives ....................................................................................... 15 1.4 Novelty of the study ...................................................................................................... 16 Materials and methods .......................................................................................................... 17 2.1 Materials ............................................................................................................................. 17 2.2 Synthesis of Cu (OH)2 precursor ........................................................................................ 17 2.3 Synthesis of Catalyst C1 ..................................................................................................... 18 2.4 Synthesis of catalyst C2 ...................................................................................................... 18 2.5 Synthesis of catalyst C3 ...................................................................................................... 19 2.6 Synthesis of catalyst C4 ...................................................................................................... 19 2.7 Synthesis of catalyst C5 ...................................................................................................... 19 2.8 Activation of MOF-199 (C1, C2, C3) Catalysts ................................................................. 20 2.9 Optimization of the synthesis of catalysts C1, C2, C3, C4, and C5 ................................... 20 2.9.1 Effect of solvent ratio....................................................................................................... 21 2.9.2 Effect of temperature and time ........................................................................................ 22 2.10 Catalysts characterization ................................................................................................. 23 2.11 Catalytic conversion of CO2 and epoxides to cyclic carbonates (value-added product) .. 24 iv 2.11.1 Catalytic reaction of epoxides (E1-E4) with CO2 to synthesize corresponding cyclic carbonates ...................................................................................................................................................... 24 3. Results and discussion .............................................................................................................. 26 3.1 Optimizing reaction conditions ..................................................................................... 26 3.1.1 Effect of solvent ratio ...................................................................................................................... 26 3.1.2 Effect of temperature and time...................................................................................................... 27 3.2 Comparison of copper precursor, reaction parameters and yields for the solvothermal synthesis of MOF-199 catalysts ................................................................................................ 30 3.3 Catalysts characterization (XRD, FTIR, SEM, TGA, and BET) .................................. 31 3.4 Catalytic Activity .......................................................................................................... 40 1 3.5 H-NMR results for the catalytic performance of C1, C2-S6, C3-S5, and C3-S6........ 41 1 3.5.1 H-NMR results for the catalytic conversion of propylene oxide and CO2 to propylene carbonate ........................................................................................................................................................ 42 3.5.2 1H-NMR results for the catalytic conversion of styrene oxide and CO2 to styrene carbonate ........................................................................................................................................................ 44 3.5.3 1H-NMR results for the catalytic conversion of 1,2-epoxybutane and CO2 to 1,2butylene carbonate....................................................................................................................................... 46 3.5.4 1H-NMR results for the catalytic conversion of epichlorohydrin and CO2 to chloropropene carbonate ........................................................................................................................... 48 4. Conclusion and Future Recommendations ........................................................................... 51 References .................................................................................................................................... 53 Appendix ....................................................................................................................................... 61 v List of Figures Figure 1: Conversion of CO2 gas into useful chemicals [17] ......................................................... 3 Figure 2: Typical transformation Mechanisms of carbon dioxide gas [16] .................................... 4 Figure 3: 3D Metal-organic frameworks formed by the coordination of metal ions with organic ligands [54] ..................................................................................................................................... 9 Figure 4: Diagrammatic representation for the solvothermal synthesis of MOF [75].................. 11 Figure 5: Reaction mechanism of CO2 with epoxide by using MOF as a catalyst and TBAB as a cocatalyst [81] ............................................................................................................................... 13 Figure 6: Proposed reaction mechanism for CO2 with epoxide by using a catalyst with acid-base pairs (A, acid; B, base) [90] .......................................................................................................... 14 Figure 7: Paddle-wheel structure of MOF-199 formed by bonding two Cu2+ ions with four benzene 1,3,5-tricarboxylate (BTC) linker molecules [92] .......................................................... 14 Figure 8: (A) Reproduction of a theoretical structure of Cu-BDC, (B) Schematic synthesis of NH2-Cu-BDC [97] ........................................................................................................................ 15 Figure 9: (A) 350 mL stainless-steel reactor; (B) Catalysis reaction setup .................................. 25 Figure 10: Variation of the yield of catalyst C2 with a solvent ratio of DMF: H2O .................... 27 Figure 11: Effect of temperatures and times on the yield of C2 catalysts (C2-S1 to C2-S9) at a solvent ratio of 7:1 (DMF: H2O) ................................................................................................... 28 Figure 12: Effect of temperatures and times on the yield of C3 catalysts (C3-S1 to C3-S9) at a solvent ratio of 7:1 (DMF: H2O) ................................................................................................... 29 Figure 13: Powder X-ray Diffraction pattern of catalyst C1 in comparison with the simulated XRD pattern of MOF-199 ............................................................................................................. 32 Figure 14: Powder X-ray Diffraction Patterns for MOF-199 catalysts (A) C1, (B) C2-S6, (C) C3S4, (D) C3-S5, and (E) C3-S6 ...................................................................................................... 33 Figure 15: Powder X-ray Diffraction patterns of (A) catalyst C4 (Cu-BDC), (B) catalyst C5 (NH2-Cu-BDC) ............................................................................................................................. 34 Figure 16: SEM images of catalyst C1 at different scales (A) 10 µm, (B) 200 µm ..................... 34 Figure 17: FT-IR Spectra of MOF-199 catalysts (A) catalyst C1, (B) catalyst C2-S6, (C) catalyst C3-S4, (D) catalyst C3-S5, and (E) catalyst C3-S6 ...................................................................... 35 vi Figure 18: FTIR Spectra of (A) NH2-BDC ligand, (B) catalyst C4 (Cu-BDC), and (C) catalyst C5 (NH2-Cu-BDC) ............................................................................................................................. 36 Figure 19: TGA profiles of (A) catalyst C1, (B) catalyst C2, (C) catalyst C3-S5, and (D) catalyst C3-S6. TGA performed on TGA Instruments Discovery TGA, from 25°C to 700°C, at 10°C/min under a nitrogen flow rate of 25mL/min ....................................................................................... 38 Figure 20: TGA profiles of (A) catalyst C4 (Cu-BDC), (B) catalyst C5 (NH2-Cu-BDC) ........... 39 Figure 21: 1H-NMR spectrum of the reaction mixture of propylene oxide (E1) and CO2 catalyzed by catalyst C1................................................................................................................................ 43 Figure 22: 1H-NMR spectrum of the reaction mixture of styrene oxide (E2) and CO2 catalyzed by catalyst C1................................................................................................................................ 46 Figure 23: 1H-NMR spectrum of the reaction mixture of 1,2-epoxybutane (E3) and CO2 catalyzed by catalyst C1................................................................................................................ 48 Figure 24: 1H-NMR spectrum of the reaction mixture of epichlorohydrin (E4) and CO2 catalyzed by catalyst C1 ................................................................................................................................ 50 vii List of Tables Table 1: Different solvent ratios (DMF: H2O) for catalyst C2 preparation .................................. 21 Table 2: Various set of temperatures and times for catalyst C2 and C3 preparation.................... 22 Table 3: Concentration of epoxides (E1-E4), catalyst C1 and TBAB for the synthesis of cyclic carbonates ..................................................................................................................................... 25 Table 4: Comparison of copper precursor, reaction parameters and yields for the solvothermal synthesis of MOF-199 catalysts .................................................................................................... 31 Table 5: BET surface area, micropore volume and pore radius of MOF-199 catalysts ............... 39 Table 6: BET surface area, micropore volume and pore radius of catalyst C4 (Cu-BDC) and C5 (NH2-Cu-BDC) ............................................................................................................................. 40 Table 7: Comparison of catalytic performance of MOFs for the conversion of propylene oxide and CO2 to propylene carbonate.................................................................................................... 43 Table 8: 1H-NMR chemical shift assignment of propylene oxide and propylene carbonate protons ....................................................................................................................................................... 44 Table 9: Comparison of catalytic performance of MOFs for the conversion of styrene oxide and CO2 to styrene carbonate .............................................................................................................. 45 Table 10: 1H-NMR chemical shift assignment of styrene oxide and styrene carbonate protons . 46 Table 11: Comparison of catalytic performance of MOFs for the conversion of 1,2-epoxybutane and CO2 to produce 1,2-butylene carbonate ................................................................................. 47 Table 12: 1H-NMR chemical shift assignment of 1,2-epoxybutane oxide and 1,2-butylene carbonate protons .......................................................................................................................... 48 Table 13: Comparison of catalytic performance of MOFs for the conversion of epichlorohydrin and CO2 to produce chloropropene carbonate............................................................................... 50 Table 14: 1H-NMR chemical shift assignment of epichlorohydrin and chloropropene carbonate protons........................................................................................................................................... 51 viii List of Abbreviations and Symbols CO2 Carbon dioxide MOF Metal-organic framework COF Covalent organic framework MOF-199 Metal-organic framework-199 HKUST-1 Hong Kong University of Science and Technology-1 BTC Benzene-1,3,5-tricarboxylic acid BDC Benzene-1,4-dicarboxylic acid Cu-BDC Copper (1,4-benzene dicarboxylate) NH2-Cu-BDC Amino copper (1,4-benzene dicarboxylate) Cu (OH)2 Copper hydroxide NaOH Sodium hydroxide SBU Secondary building unit TBAB Tetrabutylammonium bromide 3D Three dimensional KPa Kilopascal XRD X-ray diffraction FT-IR Fourier Transform infrared spectroscopy TGA Thermogravimetric analysis SEM Scanning electron microscope BET Brunauer-Emmett-Teller NMR Nuclear magnetic resonance CDCl3 Deuterated chloroform Ppm Parts per million DMF N, N-dimethylformamide Psi Pounds per square inch Cps Counts per second ix Acknowledgements Firstly, I would like to thank Almighty God for giving me energy and patience during these years to accomplish my goal of the successful completion of graduate studies. Then, I would like to express my great gratitude to my supervisor Dr. Hossein Kazemian and my co-supervisor Dr. Kerry Reimer for accepting me into their group and helping me pursue my passion for chemistry research. They have given me self-determination in exploring the unknown in research while providing adequate guidance to keep me from getting lost. I greatly appreciate their specialized training in doing research and academic writing. I feel so fortunate and blessed to have supervisors who are so energetic, extremely supportive, and caring at the same time. I was always motivated by their dedication and passion for research. In addition, I would like to express my great gratitude to my dissertation committee member, Dr. Samuel Hanson for offering his time and support to help me achieve my research completion. Also, I am very thankful to the analytic specialists Erwin Rehl and Charles Bradshaw at the Northern Analytic Laboratory Services (NALS), for their help in the training of characterization and corporation of data analysis. In addition, I am thankful to Mya Schouwenburg, Ann Duong, and Dominic Reiffarth for their assistance during my research work. I acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) for covering the research expenses of this project through Discovery Grant [RGPIN2019-06304]. I also want to special thanks to Navjot Kaur, a graduate student at UNBC, for her great help and motivation in my research. I am also very thankful to all my friends at UNBC, especially Sarah Haghjoo, Hossein Zeinalzadeh, Dorna Sobhani, Hoory Jahanbani, Naghdi Shaghayegh, Lon Kerr and to those who completed their graduate studies last year, Simisola Idim, Farzana Nargis, Sahar x Ebadzadsahraei, for being with me whenever I needed them and motivating me to continue with my research. Finally, my heartful gratitude goes to my family for their love, support and kind cooperation and inspiration, which helps me in completing my thesis. xi 1. Introduction 1.1 Importance of CO2 gas conversion Humankind is highly dependent on fossil fuel as a primary source of energy. Though industrialization and civilization have brought convenience, but it has also led to increased pollution through vehicles, factories, and chemical plants [1]. The industrialized world derives energy from burning fossil fuels leading to high emissions of carbon dioxide and other gases into the environment [2]. A continuous increase in atmospheric CO2 concentration has been observed from approximately 280 ppm in the early 1800s to over 400 ppm in 2018 [3] [4]. High emissions of CO2 have led to serious environmental problems such as ocean acidification, global warming, and species extinction [5]. Over the past decades, though industries focused on controlling emissions of other toxic pollutants such as nitrogen and sulfur oxides, CO2 emissions didn’t get as much attention. Though in recent years, a link between CO2 gas and global warming caught the great attention of science, as well as the public, through the well-known name “greenhouse effect” [6]. Due to an increasing number of recent studies and global awareness, now the focus has shifted to the reduction of CO2 emissions in the atmosphere [7]. The most straightforward strategy to reduce anthropogenic CO2 emissions lies in removing CO2 from point sources, such as the flue gas from fossil-fuel-burning power plants. CO2 conversion already happens in nature by photosynthesis, by which plants convert CO2 to some useful products such as sugars (simple and complex carbon chains) and oxygen [8]. The stress on the environment can be reduced by using some green alternative approaches by converting CO2 to some useful chemicals using renewable energy sources in chemical industries [9]. 1 1.1.1 General routes to convert CO2 gas to chemicals Large number of chemical products can be synthesized using CO2 if the right technology is utilized [10]. For instance, various industrial chemical production processes use CO2 as a raw material to produce urea, a chemical used in fertilizers [11]. The process to convert CO2 to chemical products not only reduces CO2 emissions but is also a cost-effective alternative to chemical production. Carbon dioxide is renewable, abundant, economical, and non-toxic substitute in the chemical production when compared to harsh conventional raw material, such as phosgene [12]. To convert CO2 into value-added chemicals, CO2 first needs to be captured from the air or industrial emissions and then purified and concentrated [13]. Because CO2 is the most oxidized state of carbon, the major difficulty for establishing industrial processes based on CO2 as a raw material is its low energy level. In other words, a large energy input is needed to transform CO2. [14] [15]. There are four main methodologies to convert CO2 into a useful chemical [16]: (1) Reaction of CO2 with high energy materials such as small membered ring compounds (epoxides), hydrogen, unsaturated compounds (acetylene, diyne, diene, allene), organometallics and some other high energy starting materials. (2) Production of low energy oxygen-containing products such as carbonate, carbamate, carboxylic acid, ester, and lactone. (3) Shift the equilibrium of the reaction toward the product side. (4) Provide physical energy to the reaction mixture such as light or electricity for the completion of the reaction. 2 Figure 1: Conversion of CO2 gas into useful chemicals [17] The conversion of CO2 to five-membered ring compounds consists of the formation of a carbonyl group using nucleophile and oxidative cycloaddition for the generation of a five-membered ring [18]. The CO2 reacts with a nucleophile to produce organic carbonates and carbamates. The CO2 has a strong affinity toward nucleophiles and electron-donating groups such as amine, water, alkoxides and organometallic reagents [19]. On the other hand, the low valent metal complexes such as nickel (0) and palladium (0) form five-membered ring compounds by reacting with CO2 and unsaturated compounds [16]. 3 Figure 2: Typical transformation Mechanisms of carbon dioxide gas [16] 1.1.2 Methods to produce dialkyl carbonates and cyclic carbonates from CO2 gas Cyclic carbonates are raw materials that can be used in various chemical reactions for conversion to polycarbonates [20]. Cyclic carbonates and polycarbonates have a wide range of industrial applications: they are used as aprotic polar solvents, production of pharmaceuticals, electrolytes in lithium batteries, used for cleaning, stripping, and degreasing [21]. Other than CO2, various chemicals such as phosgene have also been used as a raw material to produce cyclic carbonate [22]. Phosgene is a poisonous gas at room temperature and pressure; and was used in world wars as a chemical weapon. Also, this reaction produces hydrogen chloride as a byproduct, which is corrosive to the eyes and skin [23]. Research has been done to find the safest method for the synthesis of carbonates that does not include phosgene or isocyanate as reactants. The replacement of phosgene with CO2 is the most common and effective route to synthesizing carbonates [24]. CO2 is a renewable, nontoxic, and easily available reactant that displays 100% atom efficiency as all the atoms of the reactant are incorporated into the product. Also, the reaction using CO2 can be carried out without using a solvent. In addition, the by-product in this reaction is just water; far safer than HCl [24]. 4 Scheme 1: Synthesis of dialkyl carbonate with phosgene [23] Scheme 2: Synthesis of dialkyl carbonate with carbon dioxide [24] The synthesis of five-membered cyclic carbonates has been industrialized by reacting CO2 with epoxides [25]. Some epoxides such as ethylene oxides are toxic and volatile (gas at room temperature) [26]. Different strategies have been investigated to reduce the toxicity of ethylene oxide for the safe production of fully renewable cyclic carbonates. This can be done by the synthesis of cyclic carbonates using alkenes in two sequential steps: epoxidation of an alkene and cycloaddition of the formed epoxide. This approach helps to avoid the purification and handling of toxic epoxides and thus increases the sustainability of the process [27]. However, the development of a catalyst to promote this two-step reaction is challenging. Research has been performed on the conversion of alkenes to cyclic carbonates using Nb-catalyzed oxidative carboxylation of olefins [28]. The other green method for the synthesis of cyclic carbonate is the carboxylation of diols with CO2 using CeO2-ZrO2 or Bu2SnO catalysts [29]. An iron or coppercatalyzed reaction of CO2 with ketals have also been proposed for the synthesis of cyclic carbonates [30]. 5 Scheme 3: Synthesis of cyclic carbonate from epoxide [25] Scheme 4: Synthesis of cyclic carbonate via oxidative carboxylation of alkene [27] Scheme 5: Reaction of a diol with CO2 gas under optimized conditions [29] Scheme 6: Reaction of ketal with supercritical CO2 gas to synthesize cyclic carbonate [30] 1.2 Catalysis 1.2.1 Why Catalysts for CO2 gas conversion? A catalyst increases the rate of a chemical reaction without itself being consumed, by providing another reaction pathway of lower energy [31]. The reaction of epoxides with CO2 gas to produce cyclic carbonates requires a catalyst to activate CO2 at lower temperatures and pressure [32]. A catalyst allows the efficient production of many useful chemicals such as polymers, 6 pharmaceutical and edible oils, and refinery operations [33]. There are two types of catalysts: homogeneous catalysts and heterogeneous catalysts. Homogeneous catalysts are those that exist in the same phase as the reactants, whereas heterogeneous catalysts are in a different phase from the reactants [34]. In heterogeneous catalysis, the catalytic reaction occurs at a solid-liquid/gas interface which results in a large surface area with an inner porous structure of the catalyst [35]. The inner porous structure promotes the reaction between the specific molecules. The porous structures not only add to the surface area of a catalyst but also increase the selectivity by having specific pore sizes [36]. The other important feature of a catalyst is recyclability and reusability several times in different reactions. This feature increases the economic feasibility of expensive catalysts. Moreover, the operation time of recycled catalyst can be decreased if the catalyst is replaced less often, hence easing the operation when scaling to industrial production [37]. CO2based cyclic carbonates are synthesized using quaternary ammonium salts by Chimei-Asahi Corporation [38]. The quaternary ammonium salt is homogeneous and is less expensive as compared to alternate catalysts. However, this reaction needs high temperature and pressure for the completion of the reaction [39]. Research has been performed to identify a catalyst that can activate CO2 at lower temperatures and pressure [40]. Some homogeneous catalysts such as metal halides, metal complexes [20], and ionic liquids have been used for the reaction of CO2 with epoxides [41]. Metal complexes as a catalyst are toxic and sensitive to water and air, so extra care needs to be taken while handling metal complexes [42]. Some catalysts such as CH3SnBr3, nBu3SnI and Ph4SbBr effectively synthesized the product by allowing the reaction to proceed at a lower temperature. In contrast, these catalysts need to be in higher concentration which creates problems in the separation and purification of product [43]. Research has been performed using Schiff bases, porphyrins, and phthalocyanines as catalysts that reduce the reaction temperature and 7 pressure [44]. Additionally, these catalysts also require a co-catalyst such as a tetraalkylammonium halide or N, N’-dimethyl aminopyridine to obtain a high yield of product [44]. Heterogeneous catalysts are widely used for carbonate synthesis reactions. One benefit of the use of heterogeneous catalysts compared to homogeneous catalysts is that they are easy to separate from the product using filtration [45]. Catalyst deactivation, the loss of catalyst reactivity and selectivity over a period, is a problem for the industrial catalytic process [46]. Poisoning, aging, and fouling/coking are three main reasons for the catalyst deactivation [47]. Catalyst poisoning refers to the partial or complete deactivation of a catalyst by a chemical compound such as carbon monoxide, phosphate, cyanide, and others [48]. Aging occurs in almost all catalysts over a large period which results in changing crystal structure into a structure that no longer supports the desired reaction [49]. Finally, the closing of pores of a catalyst by carbonaceous species of any material represents fouling/coking which results in the deactivation of a catalyst [50]. 1.2.2 Metal-Organic Frameworks as a novel catalyst for CO2 gas conversion Metal-organic frameworks (MOFs) are crystalline porous materials consisting of metal ions coordinated to organic ligands to form one-, two-, or three-dimensional structures. The chemical bond connecting metal ions and organic linkers is a coordination bond which is weaker than a covalent bond [51]. MOFs differ in their geometries such as octahedron, trigonal, trigonal prism, and square planar depending on their coordination number, coordination geometry of metal ions and nature of functional groups [52]. Apart from the organic linkers, the oxidation state of metal ions has also led to structural diversity in MOFs. Divalent metal ions usually form four or sixcoordinated metal centers, whereas the trivalent metal ions can form six, eight, or even sevencoordinated metal centers in MOFs [53] 8 Figure 3: 3D Metal-organic frameworks formed by the coordination of metal ions with organic ligands [54] MOFs have numerous applications such as gas storage, separation of gaseous molecules, catalysis, sensing, optics, microelectronic, water adsorption, bioreactors, and drug delivery. MOFs can adsorb a large amount of gas because of their large surface area. Due to their useful applications, they are advantageous to many industrial processes [55]. As is known; many efforts have been devoted to designing bulk MOFs with new structural compositions of frameworks [56]. Various adsorbents such as zeolites, porous carbons, porous organic polymers, covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) have been developed to capture and store CO2. Of these adsorbents, MOFs are a new class of crystalline porous materials that serve as promising adsorbents and catalysts because of their unique advantages such as large surface area, ultrahigh porosity, and tunable structures [57]. Several heterogeneous catalysts, including metal oxides, zeolites, silica-supported salts, and microporous polymers have been used for CO2 conversion reactions. But due to the increased number of recycling steps in the chemical reactions, these types of catalysts require elevated temperature, pressure, separation, and purification steps which require more energy, time, and cost. Zeolites are also porous, just like MOFs, but the surface area of zeolites is smaller than MOFs. Additionally, the product yield using zeolites as a catalyst in CO2 conversion reactions is low. On the contrary, crystalline solids (MOFs) can easily be 9 separated from the reaction stream and be recycled [58].Therefore, CO2 gas can be effectively converted to value-added products using MOF as a catalyst [59] [60]. There are adsorbents such as zeolites that can adsorb a higher quantity of CO2 gas at low operating pressure, as compared to activated carbon (<20 KPa). Despite, the higher adsorption capacity of zeolites at low pressure, the adsorption capacity reduces in the case of CO2/H2O mixture. These adsorbents require significantly higher temperature for regeneration because the presence of water significantly decreases their adsorption capacity. The decrease in adsorption capacity, leads to a decrease in strength and heterogeneity of the electric field and thus favoring the formation of bicarbonates [61]. MOF's utility as reticular adsorbents were discovered 20 years ago. Over time, many MOFs have been developed by scientists to maximize selective CO2 uptake for gas mixtures. MOF-based materials can serve as highly effective catalysts for numerous value-added CO2 conversion reactions, especially photocatalyst and electrocatalyst CO2 reductions [62]. CO2 reaction with epoxides is one of the most intensively MOF-catalyzed conversion reactions [63]. In the past few years, various synthetic strategies for MOF catalysts have been reported, such as ultrasonic synthesis [64] [65] [66], microwave-assisted synthesis [67], mechanochemical synthesis [68] and conventional hydrothermal/solvothermal synthesis [69]. Conventional methods normally involve the use of high temperature and pressure to achieve the formation of MOF catalyst in a shorter period [70]. However, the synthesis of MOF catalyst requires a long reaction time (from days to weeks) to achieve synthesis under milder reaction conditions [71]. Other methods involve synthesis of MOF catalysts using specific instruments, such as a high-pressure autoclave, which is associated with safety issues and high costs [72] [73] [74]. This research has been focused on developing a solvothermal cost-effective synthetic method for MOF-199 catalyst, Cu-BDC MOF 10 catalyst and NH2-Cu-BDC MOF catalyst using copper hydroxide [Cu(OH)2] precursor under mild reaction conditions (at atmospheric pressure and lower temperature) in a shorter period. The solvothermal synthesis of MOFs involves a mixture of inorganic and organic compounds in high boiling polar solvents such as N, N-dimethylformamide (DMF). The reaction mixture is heated in an oven under vibration-free conditions. Metal ions and organic components are selfassembled to yield a framework that is obtained as a crystalline powder. High-quality crystals depend on reaction conditions such as temperature and duration of reaction, the stoichiometry of reactants, concentration, and solvent composition. The temperature of reaction has a strong influence on product formation. Different MOFs can be produced from the same reaction mixture, depending on reaction temperature and reaction time [75]. Figure 4: Diagrammatic representation for the solvothermal synthesis of MOF [75] Most of the copper-based MOFs have been synthesized using soluble metal salts such as copper nitrate trihydrate as an inorganic precursor [70] [76] [77]. The nitrate is an oxidizer and may be a safety hazard in industrial use [78]. Other commonly used soluble precursors such as acetates are cost-wise unreasonable. The desirable substitute, from a safety and financial standpoint, is copper 11 (II) hydroxide. A simple acid-base reaction of copper hydroxide [Cu (OH)2] and benzene-1,3,5tricarboxylic acid (H3BTC) is waste-free and produces H2O by-product [79]. Scheme 7: Acid-base reaction of Cu(OH)2 with H3BTC to produce Cu3(BTC)2 The other precursors such as copper nitrate trihydrate, and copper acetate produce nitrate and acetate by-products. Due to the exothermic nature of MOF synthesis, large scale synthesis using nitrate trihydrate requires special precaution. Large scale synthesis using copper acetate produces high concentrations of acetate that is harmful to plants, animals, and aquatic life. In addition, salt removal in wastewater is costly. Considering these factors, Cu (OH)2 precursor is very useful in laboratory practice. Despite its insoluble character, its structure is relatively flexible [80]. The metal ions in MOFs (also known as secondary building unit; SBU) activate epoxides by binding with the oxygen atoms of epoxides. This interaction results in weakening the strength of the CO bond due to the transfer of electrons from the oxygen atom to the metal center. Then the nucleophilic bromine ion from tetrabutylammonium bromide (TBAB) (co-catalyst) attacks at the less hindered carbon atom of coordinated epoxide, cleaving the CO bond of the epoxide ring. An alkyl carbonate forms by the nucleophilic attack of the former epoxide oxygen atom on the electrophilic carbon atom of CO2. Finally, intramolecular ring closure results in the formation of cyclic carbonate [81]. 12 Figure 5: Reaction mechanism of CO2 with epoxide by using MOF as a catalyst and TBAB as a cocatalyst [81] The open metal sites in the MOFs act as a Lewis acid that fulfills the role of a catalyst [82]. There is also the need for a co-catalyst (Lewis’s base) such as tetrabutylammonium bromide (TBAB) to promote the reaction under mild conditions. However, the organic linker can be functionalized with a nucleophilic group such an amine that can fulfil the role of a cocatalyst in the CO2 conversion reaction [82]. In this case, the MOF will have a Lewis acidic and Lewis basic site that can act as a catalyst and cocatalyst, and one need not add an external cocatalyst [83]. To date, several MOFs with functional linkers [84] [85] [86] [87] [88] have been studied for the catalytic conversion of CO2 to cyclic carbonates. Among them, amine (NH2) is one of the most studied functional groups which act as a nucleophilic group and takes the place of an externally added nucleophilic co-catalyst [89]. The action of Lewis acidic and Lewis basic sites in MOF is effective for the reaction of CO2 with epoxide. The CO2 activates on the basic site of MOF by nucleophilic attack. Activated CO2 reacts with the epoxide that adsorbs on the acidic site of MOF. This results in the formation of cyclic carbonate without an external co-catalyst in the reaction [90]. 13 Figure 6: Proposed reaction mechanism for CO2 with epoxide by using a catalyst with acid-base pairs (A, acid; B, base) [90] 1.2.3 Background study on MOF-199, Cu-BDC MOF and NH2-Cu-BDC MOF MOF-199, also known as HKUST-1 is a copper-based MOF which was first reported by Williams and co-workers in 1999 [87]. The chemical formula of MOF-199 is Cu3(BTC)2(H2O)3. MOFs are crystalline materials, where metals are linked with organic ligands to form a three-dimensional structure. In MOF-199, a pair of Cu2+ ions (metallic group) coordinates with four carboxylate bridges to make a paddle-wheel moiety, each carboxylate group is a part of benzene 1,3,5tricarboxylate (BTC), organic ligand [91]. One H2O molecule is coordinated to the fifth binding site (out of plane) on each Cu2+ ion. The water molecules can easily be removed by thermal treatment of MOF-199 in a vacuum oven at about 120˚C [92]. Figure 7: Paddle-wheel structure of MOF-199 formed by bonding two Cu2+ ions with four benzene 1,3,5-tricarboxylate (BTC) linker molecules [92] 14 MOF-199 has thermal stability up to 300˚C under N2 atmosphere. Above this temperature, the catalyst (MOF-199) starts to lose weight due to the degradation of the organic linker [93]. MOF199 has a crystalline porous structure with a large surface area [94]. MOF-199 has been used in research for various applications such as carbonation reactions, hydrogen adsorption, gas separation and storage and drug delivery [95]. Cu-BDC and NH2-Cu-BDC MOFs are copper-based MOFs. Copper, due to its low cost, non-toxicity, and high complexation strength, is one of the most commonly used metals in the synthesis of MOFs [96]. In copper-based MOFs, there is a strong interaction between the copper atom and oxygen atoms of the organic ligands. This strong interaction leads to the formation of stable metal-organic framework. In Cu-BDC MOF, the organic ligand is terephthalic acid which has very low toxicity and is easily available. The COOH group in terephthalic acid strongly interacts with Cu2+ cations via coordination interaction [97]. The organic ligand for the synthesis of NH2-Cu-BDC is amino terephthalic acid [98]. Figure 8: (A) Reproduction of a theoretical structure of Cu-BDC, (B) Schematic synthesis of NH2-Cu-BDC [97] 1.3 Research Aim and Objectives This study aimed to use MOF-199 as a heterogeneous catalyst for the reaction of CO2 with epoxides to manufacture cyclic carbonates as value-added products. 15 To fulfill the research objective the following activities will be executed during this research project: A. Synthesis of MOF-199 catalysts following three methods: i. Catalyst C1: Solvothermal synthesis of MOF-199 catalyst using copper nitrate trihydrate precursor at elevated pressure. ii. Catalyst C2: Solvothermal synthesis of MOF-199 catalyst using copper hydroxide precursor under atmospheric pressure at lower temperature and shorter crystallization time. iii. Catalyst C3: Solvothermal synthesis of MOF-199 catalyst using copper nitrate trihydrate precursor under atmospheric pressure at lower temperature and shorter crystalline time. B. In addition, solvothermal synthesis of catalysts C4 (Cu-BDC) and C5 (NH2Cu-BDC) using copper hydroxide precursor under milder reaction conditions (atmospheric pressure, lower temperature, and time). C. Study of different process parameters such as solvent ratio, temperature, and time to analyze the effect on the product yield of MOF-199 catalysts. D. Determine the crystallinity, surface area, size and morphology, thermal stability and functional groups of catalysts using XRD, BET, SEM, TGA and FT-IR. E. Investigate the performance of catalysts C1, C2 and C3 towards converting CO2 to valueadded cyclic carbonates. F. Comparison of catalytic activity of C1, C2 and C3 with the literature data. 1.4 Novelty of the study To the best of the author’s knowledge, the following points are considered novel in this study: 16 • Solvothermal synthesis of catalyst C2, C4 and C5 using green metal salt [Cu (OH)2] at atmospheric pressure and milder reaction conditions. • The catalytic performance of C2 and C3 for the conversion of CO2 and epoxides (E1, E2, E3 and E4) to cyclic carbonates have not been studied yet. • Copper-based MOF-199 (C1, C2 and C3) has never been used for the reaction of CO2 and 1,2-epoxybutane to 1,2-butylene carbonate. 2. Materials and methods 2.1 Materials All chemicals, including 1,3,5-benzene tricarboxylic acid (H3BTC, 98%, Alfa Aesar, China), 1,4benzene dicarboxylic acid (H2BDC, 98%, Sigma Aldrich, USA), 2-aminobenzene-1,4dicarboxylic acid (2-NH2-H2BDC, 99%, Alfa Aesar, Canada), copper nitrate trihydrate (Cu (NO3)2·3H2O, 99.0-104.0%, Sigma-Aldrich, USA), sodium hydroxide (NaOH, Fisher Biotech, USA), ethanol (C2H5OH, Greenfield Specialty alcohols, Canada), N,N-dimethylformamide (C3H7NO, 99.8%, TCI Chemicals, Japan), acetone (C3H6O, 99.5%, Fisher Chemical), deionized water, chloroform-D1 (CDCl3-D1, 99.8% Sigma-Aldrich, Switzerland), tetrabutylammonium bromide (TBAB, 98.0%, Sigma-Aldrich), propylene oxide (99+%, Alfa Aesar, USA), styrene oxide (98+%, Alfa Aesar, USA), epichlorohydrin (99%, Alfa Aesar, South Korea) and 1,2epoxybutane (Sigma-Aldrich, France) were used as purchased without any further purifications. 2.2 Synthesis of Cu (OH)2 precursor Cu (OH)2 precursor was synthesized by using the previously reported procedure by Wang et al [99]. In a typical synthesis, Cu (NO3)2.3H2O (0.96 g, 4.0 mmol) dissolved in H2O (160 mL). To that solution, NaOH (0.32 g, 8.0 mmol in 160 mL H2O) was added dropwise at room temperature under continuous stirring on a magnetic stirrer. The obtained blue precipitate was collected by 17 using vacuum filtration and the sample was washed with H2O (3×10 mL). The blue-coloured Cu (OH)2 sample was air-dried at room temperature for 24 h. 2.3 Synthesis of Catalyst C1 Catalyst C1 was synthesized by modification of the method reported by Cui et al [100]. In a typical synthesis, Cu (NO3)2 ·3H2O (0.65g, 2.72 mmol) and H3BTC (0.35g, 1.68 mmol) were dissolved in ultrapure water (12 mL) and ethanol (36 mL). The solution was stirred for 30 minutes in a 100 mL Teflon reactor. Then, the Teflon reactor was placed into the stainless-steel autoclave which then heated at 120 °C for 8 h. After synthesis, the stainless-steel autoclave was cooled to room temperature. Blue-coloured solid particles were separated from the solution by centrifugation at 3500 rpm. The sample was washed sequentially with water (3×10 mL) and ethanol (3×10 mL). The resultant solid material was dried at 120 °C for 3 h which gave catalyst C1 as a blue powder (0.48g, 0.840 mmol, 96% yield). 2.4 Synthesis of catalyst C2 To develop an improved synthesis of MOF-199 catalyst at atmospheric pressure, catalyst C2 was prepared by modification in the synthetic method used for the preparation of C1 catalyst. The reaction parameters were optimized as mentioned in subsection 2.9. In a typical synthesis, Cu (OH)2 (0.46g, 4.80 mmol) and H3BTC (0.67g, 3.20 mmol) were dissolved in DMF (35 mL) and H2O (5 mL) in a 100 mL beaker. The mixture was stirred for 1 h on the magnetic stirrer. The beaker was then placed in an oven at 70°C for 8 h. The solid product was separated from the solution using vacuum filtration. The obtained sample was then washed with DMF (2×5 mL) and ethanol (2×5 mL). After washing, the sample was redispersed in ethanol for 12 h followed by washing with ethanol (3×10 mL). The resultant solid material was dried at 80°C for 12 h which gave catalyst C2 as a blue powder (0.93 g, 1.50 mmol, 97%). 18 2.5 Synthesis of catalyst C3 Catalyst C3 was synthesized under the same conditions as those for catalyst C2, except rather than using Cu (OH)2 as the metal precursor, Cu (NO3)2·3H2O was used. Also, except for 1h of stirring time, the reaction was stirred for 5 min. Cu (NO3)2·3H2O (1.30 g, 5.40 mmol) and H3BTC (0.71 g, 3.40 mmol) were dissolved in DMF (35 mL) and H2O (5 mL) in a 100 mL beaker. This mixture in the beaker was stirred for 5 min on the magnetic stirrer. After 5 min of stirring, the solution becomes clear with light blue colour. The beaker containing a clear solution was then placed in an oven at 70°C for 8 h. The solid sample was separated from the solution using vacuum filtration. The obtained solid material was then washed sequentially with DMF (2×5 mL) and H2O (3×5 mL). After washing, the sample was redispersed in ethanol for 12 h followed by washing with ethanol (3×10 mL). The resultant solid material was dried at 80°C for 12 h which gave catalyst C3 as a blue powder (1.02 g, 1.60 mmol, 94%). 2.6 Synthesis of catalyst C4 Catalyst C4 was prepared under the same conditions as those for catalyst C2, except that the organic ligand was H2BDC. Cu (OH)2 (0.40 g, 4.19 mmol) and H2BDC (0.69 g, 4.20 mmol) were dissolved in DMF (35 mL) and H2O (5 mL) in a 100 mL beaker. The solution in the beaker was stirred for 1 h on the magnetic stirrer. The beaker was then placed in an oven at 70°C for 8 h. After the reaction, the solid sample was filtered out and washed with DMF (2×5 mL) and ethanol (2×5 mL) using vacuum filtration. After washing, the sample was redispersed in ethanol for 12 h followed by washing with ethanol (2×5 mL). The resultant solid material was dried at 80°C for 12 h which gave catalyst C4 as a blue powder (0.88 g, 3.80 mmol, 93%). 2.7 Synthesis of catalyst C5 19 Catalyst C5 was synthesized under similar conditions as those for catalyst C4, except that the organic ligand was NH2BDC. Cu (OH)2 (0.40 g, 4.19 mmol) and NH2BDC (0.76 g, 4.20 mmol) were dissolved in DMF (35 mL) and H2O (5 mL) in a 100 mL beaker. The solution in the beaker was stirred for 1 h on the magnetic stirrer. The beaker was placed in an oven at 70°C for 8 h. After the reaction, the solid sample was filtered out and washed with DMF (2×5 mL) and ethanol (2×5 mL) using vacuum filtration. After washing, the sample was redispersed in ethanol for 12 h followed by washing with ethanol (2×5 mL). The resultant solid material was dried at 80°C for 12 h which gave catalyst C5 as a green powder (0.86 g, 3.50 mmol, 87%). 2.8 Activation of MOF-199 (C1, C2, C3) Catalysts The activation of catalysts C1, C2 and C3 was carried out by modification of the previously reported procedure by Gupta et al [101]. For activation of C1, the solvent exchange was accomplished by immersing the as-prepared catalyst C1 in acetone for 12 h. The sample was then collected by vacuum filtration and washed with acetone (2×5 mL). Finally, the C1 sample was dried at 120°C for 3 h to achieve a completely solvent-free catalyst. Activation of the catalyst was recognized by its colour change from light blue to dark blue. Activation of catalysts C2 and C3 was carried out in a similar fashion. 2.9 Optimization of the synthesis of catalysts C1, C2, C3, C4, and C5 In this study, the improved synthesis of catalysts C2, C3, C4 and C5, under mild reaction conditions, has been reported. To optimize the yield of catalyst C2, the solvent ratio, reaction temperature, and reaction time were varied. The solvent ratio, that gave the highest yield of C2 was applied to C3, C4 and C5 preparation. The effect of temperature and time was tested for C2 and C3 catalysts. 20 Over the past years, attempts have been made by varying reaction temperatures and times to improve the quality and yield of MOF-199 catalyst using the solvothermal method (as summarized in Table 3). In general, low reaction temperatures lead to the formation of MOF-199 catalyst with high purity. However, long reaction times are required to achieve high yields [102]. Most of the previous studies compared the subsequent products based on BET surface area and pore volume. However, only a few studies have examined the effect of the alteration of the reaction conditions on the yield of MOF-199 catalyst [76] [102] [103]. To attain a fundamental understanding of reaction conditions on the yield of MOF-199 catalyst, a systematic study was carried out. 2.9.1 Effect of solvent ratio The effect of solvent ratio on product yield was studied for catalyst C2. The solvent ratio that gave the highest yield of catalyst C2 was applied to the synthesis of catalysts C3, C4 and C5. To evaluate the effect of solvent ratio on the yield of catalyst C2, a set of experiments were conducted by varying the DMF and H2O ratios (Table 1). Table 1: Different solvent ratios (DMF: H2O) for catalyst C2 preparation Entry Solvent ratio Volume of DMF Volume of H2O (DMF: H2O) (mL) (mL) 1 1:0 40 mL ---- 2 7:1 35 mL 5 mL 3 3:1 30 mL 10 mL 4 1:1 20 mL 20 mL 5 1:3 10 mL 30 mL 6 1:7 5 mL 35 mL 7 0:1 ---- 40 mL 21 To run these experiments, Cu (OH)2 (0.46 g, 4.80 mmol) and H3BTC (0.67 g, 3.20 mmol) were dissolved in a solution (40 mL) of DMF and H2O in a 100 mL beaker (solvent ratios are given in Table 1). The solution was stirred for 1 h on a magnetic stirrer. The beakers were then placed in an oven at 50˚C for 8 h. The obtained samples were washed with DMF (2×5 mL) and ethanol (2×5 mL). After washing, the samples were redispersed in ethanol for 12 h followed by washing with ethanol (3×10 mL). The resultant samples were dried in an oven at 80°C for 12 h. 2.9.2 Effect of temperature and time Based on the solvent ratio that gave the highest yield of catalyst C2, the effect of temperature and time was studied for catalysts C2 and C3. To evaluate these reaction parameters on catalysts C2 and C3, set of experiments with varying temperatures and times were performed using a solvent ratio of 7:1 (DMF: H2O) (Tables 2). The temperature and time that showed the highest yield of catalyst C2 were applied for the synthesis of catalysts C4 and C5. Table 2: Various set of temperatures and times for catalyst C2 and C3 preparation Catalysts C2 Catalysts C3 Temperature Time Solvent ratio (˚C) (h) (DMF: H2O) C2-S1 C3-S1 30˚C 6h 7:1 C2-S2 C3-S2 50˚C 6h 7:1 C2-S3 C3-S3 70˚C 6h 7:1 C2-S4 C3-S4 30˚C 8h 7:1 C2-S5 C3-S5 50˚C 8h 7:1 C2-S6 C3-S6 70˚C 8h 7:1 C2-S7 C3-S7 30˚C 10 h 7:1 C2-S8 C3-S8 50˚C 10 h 7:1 C2-S9 C3-S9 70˚C 10 h 7:1 22 2.10 Catalysts characterization The catalysts that resulted in higher yield (C1, C2-S6, C3-S5, C3-S6, C4 and C5) were characterized using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA), Brunauer-Emmett-Teller (BET) and Fourier Transform Infrared Spectroscopy (FTIR). In addition, catalyst C3-S5 was also analyzed using XRD to determine the effect of temperature of reaction on crystallinity of the product. Phase identification of catalysts was carried out using X-ray diffraction analysis on a Miniflex 600 6G (Rigaku, Japan) diffractometer (CuKα1, λ=1.5406 Ȧ, 40 kV, 15 mA,) in the range from 5° to 60° of 2θ. The phase identification of catalyst C1 was assisted by the search and match algorithm of Smart Lab Studio II software (version 4.1.0.219) based on ICDD Powder Diffraction File-2 (PDF-2) 2019 database. Scanning Electron Microscope (SEM) XL30 Series SEM (Philips, Netherlands) was performed to determine the particle size and morphology of synthesized catalysts. The thermal stability of obtained catalysts was obtained by thermo-gravimetric analysis (TGA, TA Instruments Discovery, USA) using two heating programs under N2 atmosphere (5.0 purity, Canada) at a flow rate of 25 mL/min. The sample was heated from 25°C to 700°C at a heating rate of 10°C /min to obtain the first TGA curve. It was then heated from 25°C to the targeted temperature at a heating rate of 10°C /min to obtain a second TGA curve and held at 200 °C and 240 °C for 2 h. Pore size and the surface area of the catalysts were determined using Brunauer-Emmett-Teller (Quantachrome Instruments, USA). The sample was outgassed at 105°C for 21 h and measured for surface area and adsorption/desorption on a NOVA 2000e surface area analyzer. The sample cell was 6 mm, small bulb, using the non-elutriation plug. The functional groups were detected using Fourier Transform Infrared Spectroscopy (Bruker, ALPHA II). 23 2.11 Catalytic conversion of CO2 and epoxides to cyclic carbonates (value-added product) Scheme 8: Various epoxides used for CO2 conversion to cyclic carbonates For the catalytic conversion of CO2 and epoxides, catalysts C1, C2-S6, C3-S5 and C3-S6 were selected based on their high production yield and BET surface area. 2.11.1 Catalytic reaction of epoxides (E1-E4) with CO2 to synthesize corresponding cyclic carbonates Initially, the catalytic reaction of epoxide with CO2 to produce cyclic carbonate was carried out using propylene oxide (E1) as a model substrate. The reaction of E1 and CO2 for their conversion to propylene carbonate was performed using catalyst C1. In this reaction, E1 (5mL, 71.4 mmol), catalysts C1 (2.59 g, 4.20 mmol, 6 mol %) and TBAB (0.46 g, 1.40 mol, 2 mol%) were added in 350 mL stainless steel reactor. The reactants were mixed with a glass rod and the reactor was sealed properly. It was then pressurized with CO2 at 70 psi. The pressurized reactor was placed in an oil bath at 100˚C for 26 h. After the reaction, the reactor was cooled to room temperature and unreacted CO2 gas was vented. The reaction mixture was collected in a glass vial for further testing to check the performance of catalyst C1. The synthesis of styrene carbonate, 1,2-butylene carbonate and chloropropene carbonate was carried out in a similar fashion. The activity of catalysts C2-S6, C3-S5 and C3-S6 for the reaction of epoxides (E1-E4) and CO2 was tested with the similar parameters. Table 3 shows the concentration of epoxides, catalyst C1 and TBAB for the reaction of different epoxides (E1-E4) with CO2 to synthesize their corresponding cyclic carbonates. 24 Table 3: Concentration of epoxides (E1-E4), catalyst C1 and TBAB for the synthesis of cyclic carbonates Epoxides Amount of epoxide Amount of catalyst C1 Amount of TBAB Propylene oxide (E1) 71.4 mmol 4.20 mmol 1.40 mmol Styrene oxide (E2) 43.7 mmol 2.60 mmol 0.80 mmol 1,2-epoxybutane (E3) 57.5 mmol 3.40 mmol 1.10 mmol Epichlorohydrin (E4) 62.7 mmol 3.80 mmol 1.20 mmol Figure 9: (A) 350 mL stainless-steel reactor; (B) Catalytic reaction setup For the catalytic reaction setup (Figure 9), the oil bath was heated at 100°C, the pressurized reactor was placed in oil bath once temperature of oil bath rises to 100°C. The drop in temperature was observed after placing the reactor in oil bath. The temperature becomes constant after 1 h of placing the reactor in the oil bath. The measurement of reaction time was started once the temperature of 25 oil bath becomes constant. Therefore, temperature inside the reactor was assumed similar to the reaction in oil bath. 3. Results and discussion 3.1 Optimizing reaction conditions 3.1.1 Effect of solvent ratio The effect of solvent ratio was studied based on production yield of catalyst C2. A set of seven experiments were performed where DMF to H2O ratios were varied. After experimentation, it was found that the 7:1 of DMF: H2O gives the highest yield of C2 (86%) compared to other solvent ratios (Figure 10). The yield of catalyst was calculated using Eq. 1, the theoretical yield based on limiting reactant. Similarly, catalyst C3 preparation has been done using the best solvent ratio (7:1 of DMF: H2O) observed for the C2 catalyst. (%) = ℎ ℎ 26 × 100 ( . 1) 100% 90% 80% Yield (%) 70% 60% 50% 40% 30% 20% 10% 0% 1:0 7:1 3:1 1:1 1:3 1:7 0:1 DMF: Water Percentage Yield (Average) Standard deviation Figure 10: Variation of the yield of catalyst C2 with a solvent ratio of DMF: H2O 3.1.2 Effect of temperature and time The effect of temperature and time on the production yield of catalysts C2 and C3 were analyzed. To study temperature and time on catalysts C2 and C3, a total of eighteen experiments were performed (Table 2). After studying the solvent ratio, 7:1 of DMF: H2O was proved best for the highest yield of catalyst C2. The same solvent ratio was implemented for catalyst C3. Different temperatures and times were studied for catalysts C2 and C3 at a static solvent ratio of DMF: H2O (7:1). 9 combinations were made for each catalyst, at three temperatures (30˚C, 50˚C, 70˚C) and three different times (6 h, 8 h, 10 h). Each combination for temperature and time gave different results for the yield of C2 and C3 catalysts. 70˚C with 8 h time of synthesis gave the highest yield of C2 and C3 catalysts (Figure 11, 12). The temperature was found to be the most effective parameter on the yield of C2 and C3 catalysts. A sharp increment was observed in the yield of catalysts with an increase in temperature from 30°C to 50°C. Further, there was noticeable but not as steep increment in the yield of catalyst at 50°C and 70°C. Also, the yield of catalysts C2 and C3 27 increased with an increase in reaction time from 6 h to 8 h and stagnated after further increase in temperature. Time = 6 hr Temp = 30° C 80 80 60 60 Yield (%) 100 Yield (%) 100 40 40 20 20 0 0 30 35 40 45 50 55 60 65 70 6 75 7 11 9 10 11 9 10 11 100 80 80 60 Yield (%) Yield (%) 10 Temp = 50° C Time = 8 hr 40 60 40 20 20 0 0 30 35 40 45 50 55 60 65 70 6 75 7 8 Time (h) Temperature (°C) Time = 10 hr Temp = 70° C 100 80 80 Yield (%) 100 Yield (%) 9 Time (h) Temperature (°C) 100 8 60 40 60 40 20 20 0 0 30 35 40 45 50 55 60 65 70 6 75 7 8 Time (h) Temperature (°C) Figure 11: Effect of temperatures and times on the yield of C2 catalysts (C2-S1 to C2-S9) at a solvent ratio of 7:1 (DMF: H2O) 28 Temp = 30° C Time = 6 hr 100 100 80 60 40 20 Yield (%) 80 60 40 20 0 30 35 40 45 50 55 60 65 70 0 75 6 8 9 Time (h) Time = 8 hr Temp = 50° C 100 10 11 10 11 10 11 100 80 80 60 Yield (%) Yield (%) 7 Temperature (°C) 40 60 40 20 20 0 0 6 30 35 40 45 50 55 60 65 70 75 7 8 9 Time (h) Temperature (°C) Time = 10 hr Temp = 70° C 100 80 80 60 Yield (%) Yield (%) 100 40 60 40 20 20 0 0 30 35 40 45 50 55 60 65 70 6 75 7 8 9 Time (h) Temperature (°C) Figure 12: Effect of temperatures and times on the yield of C3 catalysts (C3-S1 to C3-S9) at a solvent ratio of 7:1 (DMF: H2O) 29 3.2 Comparison of copper precursor, reaction parameters and yields for the solvothermal synthesis of MOF-199 catalysts The synthesis of MOF-199 catalyst has already been carried out at high pressure (in autoclave), atmospheric pressure (in beaker) and at low pressure (in capped glass vial) and is described in Table 4. This study showed the successful solvothermal synthesis of MOF-199 catalysts under elevated pressure and atmospheric pressure at lower reaction time and temperature. However, in most of the existing studies, MOF-199 catalyst synthesis was performed at high pressure using copper nitrate trihydrate precursor. The reaction at elevated pressure requires a high temperature and a long reaction time to achieve a high yield [70] [71]. As per Table 4 (entry 4), a maximum of 89.4% yield of MOF-199 catalyst was obtained at elevated pressure. The previously studied synthesis of MOF-199 at atmospheric pressure resulted in a 92% yield using copper nitrate trihydrate (Table 4, entry 6). In this study, MOF-199 catalysts (C2-S6) with improved yield was obtained using Cu (OH)2 precursor at atmospheric pressure and mild reaction conditions. In addition to catalyst yield, this study focused on the safety and cost of the synthesis method. Synthesis at low temperature and time saves cost in heating and the use of copper hydroxide precursor results in green synthesis. Moreover, the synthesis of the C3 catalyst has been performed using copper nitrate trihydrate at the same reaction conditions that were studied for catalyst C2 synthesis. The results were compared to the yield as shown in Table 4. Catalysts with the highest yield were tested for the catalytic performance towards the conversion of epoxides and CO2 to cyclic carbonates. 30 Table 4: Comparison of copper precursor, reaction parameters and yields for the solvothermal synthesis of MOF-199 catalysts Entry Catalysts Precursors Pressure Temperature Time (°C) (h) Solvents Yield Reference (%) 1 MOF-199 Cu (NO3)2.2.5H2O/ H3BTC Elevated pressure in an autoclave 110 18 EtOH/ H2O 74 [104] 2 MOF-199 Cu (NO3)2.3H2O/ H3BTC Elevated pressure in an autoclave 120 12 EtOH/ H2O 60 [70] 3 MOF-199 Cu (NO3)2.2.5H2O/ H3BTC Elevated pressure in an autoclave 75 320 EtOH/ H2O 80 [102] 4 MOF-199 Cu (NO3)2.3H2O/ H3BTC Elevated pressure in an autoclave 100 24 EtOH/ H2O 89.4 [76] 5 MOF-199 Cu (NO3)2.2.5H2O/ H3BTC Elevated pressure in an autoclave 140 8 H2O N/A [105] 6 MOF-199 Cu (NO3)2.3H2O/ H3BTC Atmospheric pressure 80 12 DMF 92 [77] 7 MOF-199 Cu (OAc)2.H2O/ H3BTC Atmospheric pressure 25 23 DMF/EtOH/ H2O 44 [106] 8 MOF-199 Cu (NO3)2.2.5H2O/ H3BTC Low pressure 60 5 EtOH 41.6 [107] 9 MOF-199 Cu (NO3)2.2.5H2O/ H3BTC Low pressure 79 5 DMF 55.0 [107] 10 C1 Cu (NO3)2.3H2O/ H3BTC Elevated pressure in an autoclave 120 8 EtOH/ H2O 96 This work 11 C2-S5 Cu (OH)2/ H3BTC Atmospheric pressure 50 8 DMF/ H2O 86 This work 12 C2-S6 Cu (OH)2/ H3BTC Atmospheric pressure 70 8 DMF/ H2O 97 This work 13 C3-S5 Cu (NO3)2.3H2O/ H3BTC Atmospheric pressure 50 8 DMF/ H2O 73 This work 14 C3-S6 Cu (NO3)2.3H2O/ H3BTC Atmospheric pressure 70 8 DMF/ H2O 94 This work 15 C4 Cu (OH)2/ H2BTC Atmospheric pressure 70 8 DMF/ H2O 93 This work 16 C5 Cu (OH)2/ NH2-H2BTC Atmospheric pressure 70 8 DMF/ H2O 87 This work 3.3 Catalysts characterization (XRD, FTIR, SEM, TGA, and BET) The X-ray diffraction (XRD) pattern of catalysts C1, C2-S6, C3-S4, C3-S5 and C3-S6 agreed with the reference pattern of MOF-199 from the PDF-2 powder diffraction database (DB card number: 00-062-1183) (Figure 13, 14). The clear and sharp peaks showed the good crystallinity of these catalysts. The diffraction peaks of catalyst C1 matched the standard pattern (Figure 13). The XRD pattern analysis for catalysts C3-S4, C3-S5 and C3-S6 was performed to study the effect of varying reaction temperature on crystallinity. These three XRD patterns matched with the XRD patterns of catalysts C2-S6 and C1. The XRD peaks are well-matched with the standard spectra, confirming 31 the successful synthesis of C1, C2-S6, C3-S4, C3-S5 and C3-S6. Among these catalysts, catalyst C2-S6 showed intense and sharp XRD peaks. Sharpness in peaks is due to the high electron cloud, indicating the higher crystal size of the catalyst [108] [109]. Therefore, catalyst C2-S6 has high crystallinity compared to catalysts C1, C3-S4, C3-S5 and C3-S6. Figure 13: Powder X-ray Diffraction pattern of catalyst C1 in comparison with the simulated XRD pattern of MOF-199 32 Figure 14: Powder X-ray Diffraction Patterns for MOF-199 catalysts (A) C1, (B) C2-S6, (C) C3S4, (D) C3-S5, and (E) C3-S6 The crystallinity of catalysts C4 (Cu-BDC) and C5 (NH2Cu-BDC) was also studied using X-ray diffraction (Figure 15), the main characteristic peaks of catalysts C4 and C5 appeared at 2Ɵ = 10.2˚, 12.0˚, 16.3˚, 17.5˚ and 25.6˚ which matched to the simulated pattern to the previously reported XRD spectra [99] [110] [111]. There was no difference in XRD peaks observed in catalysts C4 and C5. The perfect match of the XRD pattern for catalysts C4 and C5 with the standard pattern confirming the successful synthesis of these MOF catalysts. 33 Figure 15: Powder X-ray Diffraction patterns of (A) catalyst C4 (Cu-BDC), (B) catalyst C5 (NH2-Cu-BDC) Catalysts C1 was also analyzed using scanning electron microscopy (SEM). Scanning electron microscopy helps to determine the surface morphology and crystal size of metal-organic framework catalysts. The SEM image of catalysts C1 was compared to previously reported SEM images of MOF-199 [105] [112]. As indicated (Figure 16), catalyst C1 showed a clear pyramidal shape with distinct edges, well-matched with the SEM image of MOF-199 previously reported [105] [112]. Catalyst C1 has a clear pyramidal shape with distinct edges. Figure 16: SEM images of catalyst C1 at different scales (A) 10 µm, (B) 200 µm 34 The FTIR spectra of catalysts C1, C2-S6, C3-S4, C3-S5 and C3-S6 are illustrated in Figure 17. The FTIR spectra of these catalysts were in good agreement with the published FT-IR data on MOF-199 [95]. FTIR analysis was performed to study the functional groups present in MOF-199 catalysts. FTIR shows different bands at 1700-450 cm-1 due to the vibrations of the main MOF functional groups. The peak at 2973 cm-1 is attributed to the hydroxyl groups. The band at 1648 cm-1 is due to H-O-H vibration, which shows the presence of water and OH group in the structure of MOF-199 catalysts. The bands at 1369 cm-1 and 1447 cm-1 are due to the vibrations of carboxylate groups in 1,3,5-benzene tri carboxylate which corresponds to the bidentate behaviour of COO moiety. The band at 1041 cm-1 is attributed to the C-O-Cu stretching of MOF-199 catalysts. The bands at 728 cm-1 and 760 cm-1 are attributed to metal Cu substitution on benzene groups. The band at 490 cm-1 is attributed to the metal-oxygen bond. The FTIR spectra of catalysts C1, C2-S6, C3-S4, C3-S5 and C4-S6 observed a similar pattern. Figure 17: FT-IR Spectra of MOF-199 catalysts (A) catalyst C1, (B) catalyst C2-S6, (C) catalyst C3-S4, (D) catalyst C3-S5, and (E) catalyst C3-S6 35 The FTIR spectra of C4 and C5 catalysts were in good agreement with the reported FTIR of CuBDC and NH2Cu-BDC [113] [114]. The sharp peaks with high intensity at 1569 and 1396 cm-1 are attributed to the asymmetric and symmetric stretching of coordinated carboxylate in terephthalic acid respectively (Figure18). The δ (C-H) and γ (C-H) vibrations of aromatic rings resulted in weak and narrow bands at 1125 and 735 cm-1 respectively. Therefore, the existence of an aromatic ring shows the presence of an organic linker in the final product. The band at 1492 and 775 cm-1 are due to the vibrations of the phenyl ring. The weak band at 2926 cm-1 is due to the aliphatic (C-H) asymmetric stretching vibration of dimethylformamide (DMF). Whereas the broad peaks in the region of 3400-3600 cm-1 in catalyst C5 are due to the presence of water in the final product or the acidic OH of the carboxylic group. Moreover, the presence of the amine group was clearly shown in the FTIR spectra of catalyst C5 in the range of 3200 to 3600 cm-1, which was matched with the spectra of NH2-BDC ligand. Figure 18: FTIR Spectra of (A) NH2-BDC ligand, (B) catalyst C4 (Cu-BDC), and (C) catalyst C5 (NH2-Cu-BDC) 36 The thermal stabilities of catalysts C1, C2-S6, C3-S5, C3-S6, C4 and C5 were tested using a thermogravimetric analyzer. TGA was performed from room temperature to 700°C at a constant heating rate of 10˚C/min under a nitrogen atmosphere. The TGA curves of catalysts (C1, C2-S6, C3-S5 and C3-S6) (Figure 19) are comparable with those reported by Cui et al. [100]. The TGA curve presents that the decomposition of catalyst C1 occurs in three stages. An initial weight loss of 6.10% is seen from 49°C to 89 °C, possibly due to the evaporation of water in the sample. In the second stage, the slow loss of coordinated water with copper ion or crystal water of catalyst C1 was observed from 217°C to 240°C. The last intense mass loss of about 37.8% from 331°C to 356 °C is attributed to the degradation of the organic linker and therefore decomposition of catalyst C1 structure. For catalysts C2-S6, C3-S5 and C3-S6; the TGA behaviours are like those of catalyst C1. The catalyst C3-S6 showed an increased weight loss of about 10.40% from 47°C to 75°C because of the evaporation of water in the sample. Additionally, 49.70% weight loss was observed in the range of 334°C to 357°C due to the degradation of catalyst structure. Whereas catalysts C2S6 and C3-S5 result in 5.87% and 6.30% weight loss in the range of 46°C to 93°C and 48°C to 82°C respectively, due to the water evaporation. In the last stage, catalyst C2-S6 resulted in 45.60% weight loss in the range of 338°C to 358°C. Whereas the catalyst C3-S5 showed 35.22% weight loss from 343°C to 356°C due to the degradation of the catalyst. The TGA curves prove that the MOF-199 catalysts are thermally stable up to 300°C under N2 atmosphere. 37 Figure 19: TGA profiles of (A) catalyst C1, (B) catalyst C2, (C) catalyst C3-S5, and (D) catalyst C3-S6. TGA performed on TGA Instruments Discovery TGA, from 25°C to 700°C, at 10°C/min under a nitrogen flow rate of 25mL/min The thermal stability of catalysts C4 and C5 was also studied by Thermo Gravimetric Analysis (TGA). The results shown in Figure 20, are in good agreement with the TGA reported by Alamgholiloo et al [115]and Zhang et al [116]. TGA was performed from room temperature to 700°C at a constant heating rate of 10˚C/min under a nitrogen atmosphere. For catalyst C4, there is a 23.04% weight loss observed in the range of 213°C to 241°C due to the loss of moisture. A sharp decrease in weight of about 41.0% from 398°C to 444°C was observed, which is be attributed to the decomposition of organic BDC linker and collapsing of catalyst structure. Whereas for catalyst C5, the huge and intense weight loss of about 64.4% observed in the range of 252°C to 38 296°C indicated the least stability of catalyst C5 compared to C4. Figure 20: TGA profiles of (A) catalyst C4 (Cu-BDC), (B) catalyst C5 (NH2-Cu-BDC) The specific surface area and porosity of catalysts C1, C2, C3-S5, C3-S6, C4 and C5 were studied using Brunauer-Emmett-Teller (Table 5, 6). The surface area of catalyst C1 and C2-S6 is comparable to surface area of MOF-199 reported by Mahmoodi et al [105]. Whereas, catalyst C3S5 and C3-S6 report to have significantly larger surface area. Table 5: BET surface area, micropore volume and pore radius of MOF-199 catalysts Catalysts Surface area Micropore volume Pore radius C1 319.5 m2/g 0.2432 cm3/g 9.234 Å C2-S6 279.6 m2/g 0.2234 cm3/g 9.234 Å C3-S5 781.9 m2/g 0.4784 cm3/g 7.374 Å C3-S6 793.2 m2/g 0.5179 cm3/g 7.713 Å 39 Moreover, the surface area and porosity of catalysts C4 and C5 has been also studied (Table 6). The BET surface area and micropore volume of catalyst C4 are in good agreement with previously reported by Singh et al [117]. Whereas the surface area of catalyst C5 is comparable to those previously reported by Rezki et al [118]. The low surface areas of C4 and C5 catalysts can be due to the very small pore size of these catalysts. The small pore size might be a reason for the inability of nitrogen molecules to enter the pores resulting in less surface area. The nitrogen uptake by this type of materials is strongly dependent on the activation process [117]. Table 6: BET surface area, micropore volume and pore radius of catalyst C4 (Cu-BDC) and C5 (NH2-Cu-BDC) Catalysts Surface area Micropore volume Pore radius C4 29.23 m2/g 0.0333 cm3/g 9.234 Å C5 31.70 m2/g 0.0417 cm3/g 9.234 Å 3.4 Catalytic Activity Scheme 9: General catalytic reaction of epoxide with CO2 to produce cyclic carbonate The catalytic reaction of epoxides with CO2 was performed using MOF-199 catalysts (C1, C2-S6, C3-S5 and C3-S6) and TBAB. Catalysts with satisfactory production yield that was obtained for the catalyst synthesis (Table 4) were chosen for the conversion of epoxides to their corresponding carbonates using CO2. A series of experiments were performed to test the catalytic activity of C1, 40 C2-S6, C3-S5and C3-S6 in converting CO2 and epoxides (E1-E4) to cyclic carbonates. Initially, the reaction of E1 with CO2 was tested using catalyst C1. The reaction of CO2 (70 psi) and E1(71.4 mmol) in the presence of C1(6 mol%) and TBAB (2 mol%) at a maximum temperature of 100°C was performed for 26 h. The reactions of E1-E4 with CO2 using catalysts C1, C2-S6, C3-S5 and C3-S6 were carried out in a similar fashion. 3.5 1H-NMR results for the catalytic performance of C1, C2-S6, C3-S5, and C3-S6 Upon completion of catalytic reactions, the solid material was removed by filtration and a portion of filtrate was analysed using 1H-NMR Spectroscopy (Bruker, 300 MHz).For the 1H-NMR spectroscopy, CDCl3 solvent has been used to prepare a sample for 1H-NMR analysis. The chemical shift of CDCl3 was observed at 7.27 ppm in all 1H-NMR spectra. The catalytic product conversion was evaluated using 1H-NMR spectroscopy through the integration of Proton A (clearly resolved signal) from the epoxide, and Proton A from the carbonate, according to the given Eq. 2 [101]. (%) = ( ( ) )+ ( ) × 100 ( . 2) Where I HA(carbonate) and I HA(epoxides) are the integration values of proton A form the carbonate and epoxide respectively. The reaction of E1 with CO2 using catalyst C1, showed 96% product conversion (Table 7, entry 7), which was calculated using 1H-NMR. Eq 2 was used for the calculation of conversion (%) using the integration of PC(A) and PO(A) as shown in Figure 21. To study the importance of catalyst and co-catalyst for epoxide and CO2 conversion, the reaction of E1 and CO2 was also performed without using catalyst and co-catalyst. The catalyst and co-catalyst free transformation 41 reaction showed 27.2% product conversion of E1 and CO2 (Figure A15). Similarly, the catalytic activity of C2-S6, S3-S5, and C3-S6 was investigated with different epoxides (E1-E4). The product conversion of epoxides with catalysts and without catalysts was analyzed using 1H-NMR (Figure A11-A30). 3.5.1 1H-NMR results for the catalytic conversion of propylene oxide and CO2 to propylene carbonate Scheme 10: Catalytic reaction of propylene oxide (E1) with CO2 to produce propylene carbonate The reactions of propylene oxide (E1) with CO2 have already been studied using different catalysts as mentioned in Table 7. As per Table 7 (entry 4), MOF-Zn-1 showed a prominent conversion of E1 to propylene carbonate using CO2. However, this catalytic reaction had also been studied using Cu-MOF-199 and Mn-MOF-199 (Table 7, entries 1, 2) under specific reaction conditions, which showed 49.2% and 94.5% conversions respectively. To improve the conversion percentage with Cu-MOF-199, the reaction of E1 with CO2 was catalyzed by using copper-based catalysts C1, C2S6, C3-S5 and C2-S6 with improved conditions of temperature, pressure, time, and concentration of reactants (Table 7, entries 7, 8, 9, 10). The reaction without catalyst and co-catalyst produced a conversion of propylene oxide of 27.2% to propylene carbonate. Among all these catalysts, the C1 catalyst showed the highest conversion (96%) of propylene oxide and CO2 to propylene carbonate. 42 Table 7: Comparison of catalytic performance of MOFs for the conversion of propylene oxide and CO2 to propylene carbonate Entry MOF Catalysts Amount Substrate Temperature Time Pressure Conversion (˚C) (h) (psi) (%) (MOF/TBAB) References 1 Cu-MOF-199 0.031 mmol/ 1.80 mmol E1 25˚C 48 h 14.5 psi 49.2 % [119] 2 Mn-MOF-199 0.496 mmol/ ---- E1 105˚C 9h 435 psi 94.5% [120] 3 MOF-505 0.031 mmol/ 1.80 mmol E1 25˚C 48 h 14.5 psi 48% [119] 4 MOF-Zn-1 0.031 mmol/ 1.80 mmol E1 80˚C 3h 145 psi 99% [121] 5 MOF-Zn-1 0.031 mmol/ 0.77 mmol E1 80˚C 24 h 145 psi 98% [121] 6 MOF-Zn-1 0.031 mmol/ 0.77 mmol E1 25˚C 24 h 14.5 psi 19% [121] 7 Catalyst C1 4.20 mmol/ 1.40 mmol E1 100˚C 26 h 70 psi 96% This work 8 Catalyst C2-S6 4.20 mmol/ 1.40 mmol E1 100˚C 26 h 70 psi 97% This work 9 Catalyst C3-S5 4.20 mmol/ 1.40 mmol E1 100˚C 26 h 70 psi 97.4% This work 10 Catalyst C3-S6 4.20 mmol/ 1.40 mmol E1 100˚C 26 h 70 psi 97% This work 11 Without catalyst ----- E1 100˚C 26 h 70 psi 27.2% This work Figure 21: 1H-NMR spectrum of the reaction mixture of propylene oxide (E1) and CO2 catalyzed by catalyst C1 43 Table 8: 1H-NMR chemical shift assignment of propylene oxide and propylene carbonate protons Assigned 1 H-NMR shift/ppm 1 Protons of Propylene oxide Propylene carbonate A 1.14 ppm 1.48 ppm B 2.74 ppm 4.56 ppm C 2.42 ppm 4.02 ppm D 2.98 ppm 4.86 ppm H-NMR shift/ppm of 3.5.2 1H-NMR results for the catalytic conversion of styrene oxide and CO2 to styrene carbonate Scheme 11: Catalytic reaction of styrene oxide (E2) with CO2 to produce styrene carbonate The reactions of styrene oxide (E2) with CO2 have previously been examined with different catalysts as mentioned in Table 9. Among these reactions, Co-MOF-74 and UiO-66-NH2 catalysts showed 96% conversion of styrene oxide (E2) to styrene carbonate using CO2 (Table 9, entries 6 and 7). This catalytic reaction has also been studied using Cu-MOF-199 (Table 9, entry 1) at specific reaction conditions, which showed 48% conversion. To enhance this conversion percentage with Cu-MOF-199, catalysts C1, C2, C3-S5 and C2-S6 with improved conditions of temperature, pressure, time, and concentration of reactants were used (Table 9, entry 8, 9, 10, 11). Among these catalysts, C3-S6 showed the highest conversion (95%) of E2 and CO2 to styrene 44 carbonate (Figure A19). No conversion was observed without catalyst and co-catalyst (Table 9, entry 12). Table 9: Comparison of catalytic performance of MOFs for the conversion of styrene oxide and CO2 to styrene carbonate Entry MOF Catalysts Amount Substrate (MOF/TBAB) Temperature Time Pressure Conversion (˚C) (h) (psi) (%) Reference 1 Cu-MOF-199 0.333 mmol/ -- E2 100˚C 4h 290.0 psi 48% [122] 2 Cu-MOF 5 wt%/ 1 mmol E2 70˚C 10 h 14.69 psi 92% [123] 3 Cu-MOF 0.01 mmol/ 0.50 mmol E2 100˚C 4h 14.50 psi 69% [124] 4 MOF-5 0.16 mmol/ 0.50 mmol E2 50˚C 15 h 870.2 psi 92% [125] 5 MOF-Zn-1 0.031 mmol/ 1.80 mmol E2 80˚C 3h 145 psi 54% [121] 6 Co-MOF-74 0.042 mmol/ --- E2 100˚C 4h 290 psi 96% [126] 7 UiO-66-NH2 0.003 mmol/ --- E2 100˚C 4h 290 psi 96% [122] 8 Catalyst C1 2.60 mmol/ 0.80 mmol E2 100˚C 26 h 70 psi 87% This work 9 Catalyst C2-S6 2.60 mmol/ 0.80 mmol E2 100˚C 26 h 70 psi 83% This work 10 Catalyst C3-S5 2.60 mmol/ 0.80 mmol E2 100˚C 26 h 70 psi 85% This work 11 Catalyst C3-S6 2.60 mmol/ 0.80 mmol E2 100˚C 26 h 70 psi 95% This work 12 None ------ E2 100˚C 26 h 70 psi 0% This work 45 Figure 22: 1H-NMR spectrum of the reaction mixture of styrene oxide (E2) and CO2 catalyzed by catalyst C1 Table 10: 1H-NMR chemical shift assignment of styrene oxide and styrene carbonate protons H-NMR shift/ppm 1 Protons of Styrene oxide of Styrene carbonate A’ 2.81ppm 4.35 ppm A 3.16 ppm 4.81 ppm B 3.87 ppm 5.69 ppm C 7.27 ppm 7.2-7.5 ppm Assigned 1 H-NMR shift/ ppm 3.5.3 1H-NMR results for the catalytic conversion of 1,2-epoxybutane and CO2 to 1,2butylene carbonate Scheme 12: Catalytic reaction of 1,2-epoxybutane (E3) with CO2 to produce butylene carbonate 46 The reactions of 1,2-epoxybutane (E3) with CO2 were formerly conducted using different catalysts as mentioned in Table 11. According to Table 11 (entries 1, 2), Cu-MOF and Mn-MOF-199 showed comparable conversion of 1,2-epoxybutane to 1,2-butylene carbonate using CO2. CuMOF-199 had not been investigated for its catalytic activity for 1,2-epoxybutane. Therefore, reactions in Table 11 (entries 4, 5, 6, 7) have been conducted using catalysts C1, C2, C3-S5 and C3-S6 under mild reaction conditions. Of these catalysts, C3-S6 results in maximum conversion (88%) of 1,2-epoxybutane to its carbonate using CO2. However, there was little to no conversion observed without using a catalyst and co-catalyst (Table 11, entry 8). Table 11: Comparison of catalytic performance of MOFs for the conversion of 1,2-epoxybutane and CO2 to produce 1,2-butylene carbonate Entry MOF Catalysts Amount (MOF/TBAB) Substrate Temperature Time Pressure Conversion (˚C) (h) (psi) (%) References 1 Cu-MOF 5 wt %/ 1.00 mmol E3 30˚C 10 h 14.6 psi 99% [123] 2 Mn-MOF-199 0.496 mmol/ ---- E3 105˚C 9h 435 psi 97.5% [120] 3 MOF-Zn-1 0.031 mmol/ 1.80 mmol E3 80 ˚C 3h 145 psi 90% [121] 4 Catalyst C1 3.40 mmol/ 1.10 mmol E3 100˚C 26 h 70 psi 70% This work 5 Catalyst C2-S6 3.40 mmol/ 1.10 mmol E3 100˚C 26 h 70 psi 75% This work 6 Catalyst C3-S5 3.40 mmol/ 1.10 mmol E3 100˚C 26 h 70 psi 87% This work 7 Catalyst C3-S6 3.40 mmol/ 1.10 mmol E3 100˚C 26 h 70 psi 88% This work 8 None ------ E3 100˚C 26 h 70 psi 1.4% This work 47 Figure 23: 1H-NMR spectrum of the reaction mixture of 1,2-epoxybutane (E3) and CO2 catalyzed by catalyst C1 Table 12: 1H-NMR chemical shift assignment of 1,2-epoxybutane oxide and 1,2-butylene carbonate protons Assigned 1 H-NMR shift/ppm 1 Protons of 1,2-epoxybutane 1,2-butylene carbonate A 2.83 ppm 4.61 ppm B 2.65 ppm 4.47 ppm C 2.36 ppm 4.01 ppm D 1.56 ppm 1.69 ppm E 0.85 ppm 0.92 ppm H-NMR shift/ppm of 3.5.4 1H-NMR results for the catalytic conversion of epichlorohydrin and CO2 to chloropropene carbonate 48 Scheme 13: Catalytic reaction of epichlorohydrin (E4) with CO2 to produce chloropropene carbonate The catalytic reactions of epichlorohydrin (E4) with CO2 have already been performed with different catalysts (Table 13). As per Table 13 (entries 5, 6), MOF-5 and MOF-Zn-1 showed conversion of 93% and 63% respectively. The catalytic reaction of E4 and CO2 was also tested with Mn-MOF-199 and Co-MOF-199. Of these catalysts, Mn-MOF-199 showed the highest conversion of 98%. Moreover, Cu-MOF-199 (Table 13, entry 1) at specific reaction conditions, showed 34% conversion of E4 to its corresponding carbonate. To improve the conversion (%) with Cu-MOF-199, the reaction of E4 with CO2 was catalyzed using copper-based catalysts C1, C2-S6, C3-S5 and C2-S6 with mild conditions of temperature, pressure, time, and concentration of reactants (Table 13, entry 7, 8, 9, 10). Out of these catalysts, C3-S6 showed the highest conversion (93%) of E4 to its carbonate using CO2. However, there was little to no conversion observed without using a catalyst and co-catalyst (Table 13, entry 11). 49 Table 13: Comparison of catalytic performance of MOFs for the conversion of epichlorohydrin and CO2 to produce chloropropene carbonate Entry MOF Amount (MOF/TBAB) Substrate Temperature Time Pressure Conversion (˚C) (h) (psi) (%) References 1 Cu-MOF-199 0.165 mmol/ --- E4 100˚C 4h 101.52 psi 34% [127] 2 Mn-MOF-199 0.496 mmol/ ---- E4 105˚C 9h 435 psi 98% [120] 3 Co-MOF-199 0.496 mmol/ ---- E4 120˚C 9h 435 psi 97.9% [120] 4 Ni-MOF-199 0.496 mmol/ ---- E4 120˚C 16 h 435 psi 91.9% [120] 5 MOF-5 0.16 mmol/ 0.50 mmol E4 50˚C 12 h 870.2 psi 93% [125] 6 MOF-Zn-1 0.031 mmol/ 1.80 mmol E4 80˚C 3h 145 psi 63% [121] 7 Catalyst C1 3.80 mmol/ 1.20 mmol E4 100˚C 26 h 70 psi 74% This work 8 Catalyst C2-S6 3.80 mmol/ 1.20 mmol E4 100˚C 26 h 70 psi 89% This work 9 Catalyst C3-S5 3.80 mmol/ 1.20 mmol E4 100˚C 26 h 70 psi 92% This work 10 Catalyst C3-S6 3.80 mmol/ 1.20 mmol E4 100˚C 26 h 70 psi 93% This work 11 None ------ E4 100˚C 26 h 70 psi 0.5% This work Figure 24: 1H-NMR spectrum of the reaction mixture of epichlorohydrin (E4) and CO2 catalyzed by catalyst C1 50 Table 14: 1H-NMR chemical shift assignment of epichlorohydrin and chloropropene carbonate protons Assigned 1 H-NMR shift/ppm of 1 H-NMR shift/ppm of Protons Epichlorohydrin Chloropropene carbonate A 3.61 ppm 3.77 ppm B 3.20 ppm 4.99 ppm C 2.91 ppm 4.43 ppm D 2.67 ppm 4.61 ppm 4. Conclusion and Future Recommendations In summary, the first half of this study reports the facile synthesis of catalysts C2-S6, C3-S5, and C3-S6, C4, and C5 under atmospheric pressure at lower temperature and shorter crystallization time. The yields of C2-S6, C3-S5 and C3-S6, C4, and C5 were comparable to C1, synthesized at elevated pressure. Out of these catalysts, C2-S6 prepared by using green metal precursor [Cu (OH)2] showed the highest production yield (97%). Furthermore, catalysts C1, C2-S6, C3-S5, C3S6, C4, and C5 were successfully analyzed using XRD, SEM, FTIR, TGA, and BET. Among these catalysts, catalyst C3-S6 had highest BET surface area. In the second half of the research, catalysts C1, C2-S6, C3-S5, and C3-S6 were used to study the feasibility of the conversion of CO2 and epoxides to value-added cyclic carbonates. These catalysts showed excellent catalytic performance at milder conditions of CO2 pressure and temperature. Satisfactory results were obtained for the conversion of epoxides and CO2 using copper-based MOF-199 catalysts (C1, C2-S6, C3-S5, and C3-S6). Among these catalyst, catalyst C3-S6 had larger surface area (almost twice the surface area of catalyst C2-S6) and therefore resulted better conversion of epoxides and CO2 to corresponding cyclic carbonates. 51 For future investigation, catalysts C1, C2-S6, C2-S5, and C2-S6 can be functionalized by amine functional group using NH2-1,3,5-benzene tricarboxylic acid as an organic linker. Amine functionalized catalysts will have a Lewis acidic and Lewis basic site that can act as a catalyst and cocatalyst and one need not add cocatalyst externally in the reaction. Catalysts C4 and C5 can be utilized for the catalytic conversion of epoxides and CO2 to cyclic carbonates. 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Appendix 61 Figure A1: Pictorial representation for the synthesis of Catalyst C1 by solvothermal method using stainless steel autoclave reactor [105] Figure A2: Stainless Steel Autoclave Reactor with 100 mL Teflon Vessel 62 Figure A3: Pictures of MOF-199 catalysts after activation; (A) Catalyst C1, (B) Catalyst C2-S6, (C) Catalyst C3-S5, (D) Catalyst C3-S6 Figure A4: Pictures of Cu-BDC and NH2-Cu-BDC catalysts after synthesis; (A) Catalyst C4, (B) Catalyst C5 63 Figure A5: Adsorption-desorption isotherm of Catalyst C1 Figure A6: Adsorption-desorption isotherm of Catalyst C2-S6 64 Figure A7: Adsorption-desorption isotherm of Catalyst C3-S5 Figure A8: Adsorption-desorption isotherm of Catalyst C3-S6 65 Figure A9: Adsorption-desorption isotherm of Catalyst C4 Figure A10: Adsorption-desorption isotherm of Catalyst C5 66 Figure A11: 1H-NMR spectrum of propylene oxide (E1) in CDCl3 solvent Figure A12: 1H-NMR spectrum of the reaction mixture of propylene oxide (E1) and CO2 catalyzed by C2-S6 67 Figure A13: 1H-NMR spectrum of the reaction mixture of propylene oxide (E1) and CO2 catalyzed by C3-S5 Figure A14: 1H-NMR spectrum of the reaction mixture of propylene oxide (E1) and CO2 catalyzed by C3-S6 68 Figure A15: 1H-NMR of reaction mixture of propylene oxide (E1) and CO2 without catalyst and co-catalyst Figure A16: 1H-NMR spectrum of styrene oxide (E2) in CDCl3 solvent 69 Figure A17: 1H-NMR spectrum of the reaction mixture of styrene oxide (E2) and CO2 catalyzed by C2-S6 Figure A18: 1H-NMR spectrum of the reaction mixture of styrene oxide (E2) and CO2 catalyzed by C3-S5 70 Figure A19: 1H-NMR spectrum of the reaction mixture of styrene oxide (E2) and CO2 catalyzed by C3-S6 Figure A20: 1H-NMR spectrum of the reaction mixture of styrene oxide (E2) and CO2 without using catalyst and co-catalyst 71 Figure A21: 1H-NMR spectrum of 1,2-epoxybutane (E3) in CDCl3 solvent Figure A22: 1H-NMR spectrum of the reaction mixture of 1,2-epoxybutane (E3) and CO2 catalyzed by C2-S6 72 Figure A23: 1H-NMR spectrum of the reaction mixture of 1,2-epoxybutane (E3) and CO2 catalyzed by C3-S5 Figure A24: 1H-NMR spectrum of the reaction mixture of 1,2-epoxybutane (E3) and CO2 catalyzed by C3-S6 73 Figure A25: 1H-NMR spectrum of the reaction mixture of 1,2-epoxybutane (E3) and CO2 without catalyst and co-catalyst Figure A26: 1H-NMR spectrum of epichlorohydrin in CDCl3 solvent 74 Figure A27: 1H-NMR spectrum of the reaction mixture of epichlorohydrin (E4) and CO2 catalyzed by C2-S6 Figure A28: 1H-NMR spectrum of the reaction mixture of epichlorohydrin (E4) and CO2 catalyzed by C3-S5 75 Figure A29: 1H-NMR spectrum of the reaction mixture of epichlorohydrin (E4) and CO2 catalyzed by C3-S6 Figure A30: 1H-NMR spectrum of the reaction mixture of epichlorohydrin (E4) and CO2 without a catalyst and co-catalyst 76