Design and synthesis of metal organic frameworks doctor of philosophy major chemical engineering

Dissertation for the Degree of Doctor of Philosophy Design and Synthesis of Metal-Organic Frameworks for CO, CO2, and C7H8 Adsorption Le Van Nhieu Department of Chemical Engineering Graduate School Kyung Hee University Seoul, Korea June, 2021 Design and Synthesis of Metal-Organic Frameworks for CO, CO2, and C7H8 Adsorption Le Van Nhieu Department of Chemical Engineering Graduate School Kyung Hee University Seoul, Korea June, 2021 Design and Synthesis of Metal-Organic Frameworks for CO, CO2, and C7H8 Adsorption by Le Van Nhieu Advised by Prof Jinsoo Kim Submitted to the Department of Chemical Engineering and the Faculty of the Gradual School of Kyung Hee University in partial fulfillment of the requirement for degree of Doctor of Philosophy Dissertation Committee Chairman Prof Eun Yeol Lee Prof Bum Jun Park Prof Chang Kyoo Yoo Prof Kye Sang Yoo Prof Jinsoo Kim ABSTRACT Design and Synthesis of Metal-Organic Frameworks for CO, CO2, and C7H8 Adsorption Le Van Nhieu Department of Chemical Engineering Graduate School of Kyung Hee University Seoul, Korea To date, gas adsorption has attracted attention in the context of more serious air pollution as a result of industrialization and the growing population By the way, the gaseous contaminants are effectively managed to contribute to the improvement of air quality, simultaneously, supply several important chemicals (CO, CO2) used as raw materials for the industrial manufacturing process And, the derived-adsorbents from metal organic frameworks (MOFs) have gradually become a key contributor to the amelioration of gas adsorption performance The separation of CO out of gas mixture, especially containing CO is an important mission in the industrial production sector but encounter huge challenges due to the higher polarizability of CO2 than that of CO Most of the investigation showed that after introducing Cu(I) into pore system of MOF-support, the resulting materials exhibited a higher adsorption capacity of CO than CO2 whereas a contrary result was observed on the original MOFs This is due to -complexation formed between Cu(I) and CO species Among the reported MOFs, MIL-100(Fe) possesses high BET surface area, thermal stability ( ̴ 320 oC), and tunability of the oxidation state of iron ions (Fe(II) and Fe(III)) under high temperature (150 ̴ 250 oC) So, a simple route is employed to introduce Cu(I) on MIL-100(Fe), in which Cu(II) is directly transferred to Cu(I) thanks to Fe(II), but no requirement support from reducing agents However, MIL-100(Fe) is typically synthesized in closed batch systems, which is i not favorable for large-scale production Herein, we report a scalable MOF synthesis route based on a continuous flow tubular reactor equipped with microwave volumetric heating The system enabled continuous crystallization of MIL-100(Fe) with a high space-time yield of ~771.6 kg m-3 day-1 under relatively mild conditions in a range of temperature (100 ̴ 110 o C) and resident time of 50 The product quality is evaluated via porous property and crystallinity in comparison to the traditional method Ultimately, the MIL-100(Fe) was used as a support to prepare Cu(I)-modified π complexation adsorbents The adsorbents exhibited preferred CO adsorption over CO2, and the adsorption performance was confronted to, or even higher than most of the Cu(I)-modified π complexation adsorbents in previous reports Until now, the CO-selective adsorbents are kept developing towards the improvement of CO uptake capacity and CO/CO2 selectivity, but Cu(I)-incorporated MOFs are instability in the air This is the main reason for reducing CO separation performance in the real gas environment being usually available a certain amount of oxygen and moisture Recently, some reports have revealed a strategy to improve the stability of Cu(I)-incorporated MOFs, however, their CO adsorption capacity is modest Therefore, The development of a COselective adsorbent with large CO adsorption capacity, high CO/CO selectivity, and good stability is still a huge challenge In this dissertation, a novel Cu(I)-incorporated MIL-100(Fe) adsorbent for CO/CO2 separation is prepared using a host–guest redox strategy by combining the co-addition of Zn(II) and Cu(II) inside the MIL-100(Fe)’s pore system The addition of Zn(II) resulted in a higher Cu(I) yield of the adsorbent due to the facilitated regeneration of Fe(II), which was utilized for the reduction of Cu(II) Therefore, both CO uptake amount and achieved CO/CO2 selectivity on Cu(I)Zn@MIL-100(Fe) with only 10 wt% of Zn loading were considerably higher than that of the benchmark Cu(I)-incorporated adsorbents In addition, the presence of the Zn(II) in Cu(I)Zn@MIL-100(Fe)-10 improved the oxygen resistance This study opens a new perspective for developing efficient COselective π-complexation adsorbents with high CO/CO2 selectivity and superior oxygen resistance Unlike CO adsorption, the MOF-adsorbent for capturing the target gas like CO2 or ii C7H8 is relatively diverse, in which the adsorbent perhaps possesses positive factors for gas adsorption like a superior surface area, a suitable pore structure, and a large amount of adsorption sites having an affinity toward adsorbates Zirconium-based MOFs (UiO-66, UiO-67) are potential adsorbents for gas adsorption due to a quite large surface area, easily tunable pore structure as well as chemical surface, high chemical/thermal stability, and facilely large scale production thanks to using a microwave-assisted continuous tubular reactor However, they exhibited a modest uptake capacity for both CO2 and C7H8 in comparison with the others So, gas adsorption capacity should be improved For CO2 adsorption, an amino-defective UiO-66 was prepared by a one-step synthesis method using the mixed linkers of terephthalic acid and a cheap defect linker as 4aminobenzoic acid The presence of the 4-aminobenzoic acid in the reaction system, induced enhanced porosity owing to the missing-linker defects, simultaneously, created the amino (NH2) groups in the framework Both two factors contribute to the improvement of the CO2 capture capacity on modified UiO-66 as a result of the synergy effect Only with 10% in mole of used 4-amino benzoic acid in the mixed ligand, at 25 oC and bar, the obtained CO2 uptake amount and ideal adsorbed solution theory-based CO2/N2 selectivity on the resulting material increased 47.8% and afforded 2.8 higher times in comparison with the original UiO66 sample, respectively These results exhibited an efficient approach (a cheap linker as 4aminobenzoic acid and one-step synthesis) to prepare a defective UiO-66 adsorbent with amine functional groups, which not only improve CO2 separation performance but also reduce production cost For C7H8 adsorption, some defective Zr-based biphenyl dicarboxylate (UiO-67) MOFs were prepared via fast modulated synthesis under microwave-assisted continuous tubular reactor by using formic, acetic, propionic, and benzoic acid as modulators A surfacemodified UiO-67(Zr) framework with high porosity and crystallinity could be rapidly produced in a few minutes due to the incomplete exchange between the bridging ligand and the modulator The defect concentration in the products was tuned by controlling both the modulator species and their concentrations The adsorption ability toward toluene of the iii prepared UiO-67(Zr) MOFs was found to be related to their structural defects The defective UiO-67(Zr) MOF synthesized with HCOOH as the modulator exhibited the highest toluene adsorption capacity (467 mg g –1), surpassing also most of the previously reported adsorbent materials, such as zeolites, activated carbon, UiO-66(Zr), H2N-UiO-66(Zr), ZIF-67, and CuBTC Moreover, the experimental dynamic adsorption data were mathematically modeled to predict the adsorption behaviors of defective UiO-67(Zr) MOFs Additionally, zirconium-based MOFs has still had limitations in fixed-bed adsorption system owing to its tiny sub-micron crystallite size leading to inconvenience in transportation, difficult recovery and serious pressure loss in fixed-bed adsorption system Thus, an approach to construct mm-scale granules using UiO-66(Zr) powder is required In this work, UiO-66(Zr) particles were prepared by the solvothermal method under microwave irradiation for only 20 min, then fabricated into spherical granules of UiO-66/PVA by freeze granulation technique PVA was added as a binder to connect UiO-66 particles together to spherical beads with high mechanical strength, not affecting the crystalline and micropore structures of UiO-66 The regular octahedron of the UiO-66 individual particles remained intact and the pore size did not change with increasing PVA concentration However, PVA bound the particles together to form compact and cohesive network clusters that reduced the BET surface area and total pore volume This consequently lowered the toluene adsorption efficacy slightly due to the premature breakthrough that limited the toluene molecules exposure into the micropores of the individual UiO-66 particles iv Table of Contents ABSTRACT i List of Tables ix List of Figures xi CHAPTER - Introduction 1.1 Background 1.2 Motivation 1.3 Research objectives 10 1.4 Dissertation overview 12 CHAPTER – Literature Review 15 2.1 Adsorption technique 15 2.1.1 Adsorption definition 15 2.1.2 Adsorption models 18 2.1.3 Isosteric heat of adsorption 19 2.1.4 Ideal adsorption solution theory (IAST) selectivity 20 2.2 Metal organic frameworks 22 2.2.1 Iron-based metal organic framework (MIL-100Fe) 26 2.2.2 Zirconium-based metal organic framework (UiO-66 & UiO-67) 28 2.3 Application of MOFs for gas adsorption 30 2.3.1 Mechanism of gas adsorption 30 2.3.2 Metal organic frameworks for CO adsorption 33 2.3.3 Metal organic framework for CO2 adsorption 36 2.3.4 Metal organic framework for toluene adsorption 37 CHAPTER - Microwave-assisted continuous flow synthesis of MIL-100 (Fe) and its application to Cu(I)-loaded adsorbent for CO/CO2 separation 40 v 3.1 Introduction 40 3.2 Materials and methods 41 3.2.1 Chemicals 41 3.2.2 Synthetic procedures 41 3.2.3 Characterization 42 3.3 Results and discussion 43 3.3.1 Synthesis of MIL-100(Fe) in a microwave-assisted flow reactor 43 3.4 Conclusions 61 CHAPTER - A novel approach to prepare Cu(I)Zn@MIL-100(Fe) adsorbent with high CO adsorption capacity, CO/CO2 selectivity and stability 62 4.1 Introduction 62 4.2 Experimental 64 4.2.1 Materials 64 4.2.2 Synthesis of MIL-100(Fe) 64 4.2.3 Synthesis of Cu(I)@MIL-100(Fe) and Cu(I)Zn@MIL-100(Fe) adsorbents 64 4.2.4 Characterizations 65 4.2.5 CO and CO2 adsorption test 65 4.3 Results and discussion 68 4.3.1 Characterizations of MIL-100(Fe) and Cu(I)Zn@MIL-100(Fe) 68 4.3.2 CO and CO2 adsorption on Cu(I)Zn@MIL-100(Fe) adsorbents 75 4.3.3 Regeneration and stability test 84 4.3.4 Reduction mechanism 86 4.4 Conclusions 88 CHAPTER - Facile one-step synthesis of amino-defective UiO-66 using 4-amino benzoic acid for enhanced CO2 adsorption performance 90 5.1 Introduction 90 vi 5.2 Experimental 93 5.2.1 Materials 93 5.2.2 Synthesis of MOF materials 93 5.2.3 Characterizations 93 5.2.4 CO2 and N2 adsorption 94 5.3 Results and Discussions 94 5.3.1 Material characterizations 94 5.3.2 CO2 and N2 adsorption 110 5.3.3 Isosteric heat of CO2 adsorption and the regeneration of the adsorbent 114 5.3.4 Adsorption selectivity of CO2/N2 on UiO-66 and UiO-66#10-NH2 118 5.4 Conclusions 119 CHAPTER - Defect engineering of UiO-67(Zr) under continuous-flow microwave synthesis condition and application for toluene adsorption 120 6.1 Introduction 120 6.2 Experimental 122 6.2.1 Synthesis procedure 122 6.2.2 Characterization 123 6.2.3 Toluene recovery test 125 6.3 Results and discussion 125 6.3.1 Microwave-assisted continuous-flow synthesis of UiO-67(Zr) 125 6.3.2 Adsorption and desorption of toluene 142 6.3.3 Dynamic adsorption of toluene 146 6.4 Conclusions 148 CHAPTER - Facile synthesis of UiO-66/PVA spherical granules and their application for toluene adsorption 149 7.1 Introduction 149 vii 2.2.1 Iron-based metal organic framework (MIL-100Fe) The mesoporous iron (III) trimesate MIL-100 structure (MIL = Materials of Institute Lavoisier) revealed by Gerard Ferey’s researcher group [113, 114] is constructed from four iron-oxo-centered trimers and four bridges (1,3,5-benzene tricarboxylate) to establish a hybrid supertetrahedra These hybrid supertetrahedras are arranged in an order to form a three-dimensional structure containing two kinds of cages with a dimension of 25 Å and 29 Å , corresponding to two micro-windows of the pentagon (5.5 Å ) and hexagon (8.5 Å ) (Fig 2.4) Therefore, the pore structure of MIL-100(Fe) has a large volume of 380 nm3 and BET surface area up to 2400 m2 g-1 It should be noted that each trimer of iron (III) consists of three octahedra incorporated together via a sharing vertex of μ 3-oxo anion (Fig 2.4), simultaneously is connected to six carboxylate groups, next to two water molecules and one of the anions (OH-, F- or Cl-) depending on the used recipe These species can be departed to generate unsaturated metal sites as Lewis acid sites including Fe(II) and Fe(III) under high temperature in vacuum pressure (Fig 2.5), which have strong interaction to polar, quadrupolar, or unsaturated molecules like CO2, NOx, C2H4, C3H8 … Moreover, the MIL100(Fe) possesses high thermal stability (> 300 °C) and chemical stability Besides, the MIL100(Fe) is originated from iron metal well-known as a cheap and available raw material, environmental safe Therefore, MIL-100(Fe) is employed to apply for adsorption, catalysis, drug carrier, … [92, 113, 115-118] 26 Figure 2.4 Formation of MIL-100(Fe) structure Reprinted with ratification from ref [115] Copyright 2012 John Wiley and Sons Figure 2.5 Effect of activation temperature on the oxidation state of iron (open metal sites) in MIL-100(Fe) structure Reprinted with ratification from ref [113] Copyright 2019 Royal Society of Chemistry (Great Britain) 27 2.2.2 Zirconium-based metal organic framework (UiO-66 & UiO-67) Zirconium-based organic frameworks are firstly revealed by Lillerud's group at the University of Olso in 2008, singed as UiO [119] The arrangement between inorganic bricks with linker brides like benzendicarboxylate (BDC2-), biphenyldicarboxylate (BPDC2-), or terphenyldicarboxylate (TPDC2-) is in an orderly manner to establish UiO-66, UiO-67, or UiO-68 network, respectively (Fig 2.6) Notably, each inorganic brick is built from a nucleus at the center consisting of six cations of Zr(IV) cations linked to four 3-O anions and four 3-OH anions to form an octahedron Zr 6O4(OH)4 (Fig 2.7a) and surrounded twelve carboxylates coming from organic linkers, resulting in the formation of Zr6O4(OH)4(CO2)12 (Fig 2.7b) During activating at high temperature, dehydroxylation happens in a range of 250 oC ̴ 300 oC temperature, in which two water molecules are released out of zirconium cluster and formation of a dehydroxylated cluster (Zr 6O6) [119-121] (Fig 2.7d) Zirconium-based MOFs (UiO-66 & UiO-67) are well-known as structures with high thermal stability and chemical stability thanks to a strong bond of Zr-O [120-122] Besides, functionalization of UiO MOFs is facilely implemented by using the organic linker bearing functional groups like amino (-NH2) [58, 72, 76, 83], nitro (-NO2) [76, 123], hydroxy (-OH) [76, 85], methyl (-CH3) [121] … in a similar synthesis procedure to original UiO MOFs, as well as easily defect their frameworks by using monocarboxylic as modulator [85, 93], defective linker [72, 77] or surfactant [81, 82] during the synthesis process By the way, both the pore structure and chemical surface of UiO’s MOFs could be facilely tuned to fit with a given application Therefore, they are usually found in applications concerning to surface of the material like adsorption, catalyst, membrane 28 Figure 2.6 Zirconium-based UiO metal organic framework: (a) UiO-66 with benzen dicarboxylate, (b) UiO-67 with biphenyl dicarboxylate, (c) UiO-68 with terphenyl dicarboxylate Reproduced with ratification from ref [119] Copyright 2008 American Chemical Society Figure 2.7 Nucleus structures of UiO’s cluster Zirconium – red, oxygen – blue, carbon – grey and hydrogen – white Reprinted with ratification from ref [119] Copyright 2008 American Chemical Society 29 2.3 Application of MOFs for gas adsorption 2.3.1 Mechanism of gas adsorption Gas adsorption process on the MOF-based adsorbents depends on the natural properties of adsorbed gas species and MOF-hosts as well as their mutual interactions Based on this aspect, the mechanism of gas adsorption is usually described as follows: (i) there is a discrepancy between pore diameter of a given MOF-based adsorbent with shape and/or size of adsorbate gases So, only several molecular gases are permitted to access the pore system, followed by gas adsorption whereas the others are not In this case, the MOF-based adsorbent plays a role as a molecular sieve; (ii) gas adsorption on the MOF-based adsorbents perhaps happens thanks to the interaction between the adsorbed gases and the MOF’s surface, assigned to effect of thermodynamic equilibrium adsorption In this case, the MOF-based adsorbent possesses a larger pore size than kinetic diameter of all adsorbed gases This means that all adsorbate gases can enter the pore system, and the efficacy adsorption is determined via specific affinity between the target gas with the surface, whereas the other gas is not This result is completely based on the natural characterizations of adsorbate (polarizability, permanent dipole moment, quadrupole moment, free electron density, free hydrogen atom) and pore surface (free electron density on the organic linker, unsaturated metal sites, functional groups: -OH, -NH2, -SO3H, …), resulting formation of H-bonding, van der Waals, -complexation, - complexation Consequently, there is different adsorption enthalpy between the target gas and given adsorbent in comparison with the gas other; (iii) the adsorption process may also depend on diffusing rates, which is related to kinetic adsorption The molecular gases diffuse in the pore system with different paths, depending on shape and size of adsorbed gases as well as pore diameter and pore shape (Fig 2.8), leading to a discrepancy in diffusion rate Consequently, the adsorbate gas with a faster diffusion rate is faster adsorption than the others [124-126] 30 Figure 2.8 The diffusion models and effect of pore size on diffusion coefficient Reprinted with ratification from ref [124] Copyright 2020 Elsevier 31 For CO/CO2 separation via gas adsorption, MIL-100(Fe) with pore diameter around 5.8 angstrom which is much larger than the kinetic diameter of both CO and CO2, corresponding to 3.69 angstrom and 3.30 angstrom, respectively So, it showed higher adsorption capacity of CO2 than that of CO Because the polarizability of CO (19.5×10-25 cm3) is smaller than that of CO2 (29.11×10-25 cm3) [125] After doping Cu(I) into the MIL100(Fe)’s pore system, however, the resulting adsorbent showed a higher uptake amount of CO than that of CO2, because of -complexation established between CO species and Cu(I) sites [36, 38] Herein, the mechanism of gas adsorption is based on the characteristic interaction between the adsorbate and the adsorbent surface Similarly, CO2/N2 separation is also based on different adsorption interaction between the CO2 and UiO-66 from that between N2 and UiO-66 Typically, N2 with a kinetic diameter around 3.64 ̴ 3.80 angstrom is larger than that of CO2 in 3.30 angstrom These are smaller than the main pore diameter of UiO-66 in 5.6 angstrom Consequently, both CO2 and N2 easily access the UiO-66’s pore system, but the CO2 adsorption capacity is much higher than in comparison with N2 uptake capacity on the UiO-66 This is due to CO2 polarizability (29.11×10-25 cm3) and quadrupole moment (4.3×10-26 esu cm2) are larger than those of N2, corresponding to (17.403×10-25 cm3) and (1.52×10-26 esu cm2), respectively [125] For more improvement of CO2/N2 separation, the amino (-NH2) group was introduced into the original UiO-66’s framework [72, 83] Consequently, the N2 adsorption capacity on the resulting material was decreased owing to declining BET surface, but its CO2 uptake capacity was significantly improved because of establishing strong affinity between CO2 playing as a Lewis acid and NH2 group playing as a Lewis base, alongside formed H-bonding between CO2 and –NH2 groups For toluene adsorption, the toluene molecule possesses a kinetic diameter of 5.25 angstrom, alongside polarizability of 118×10-25 ̴ 123×10-25 cm3, and dipole moment of 0.375×10-18 esu cm [125] The defected framework of UiO-66 and UiO-67 hold main pore sizes around 7.5 and 5.8 angstrom, respectively The mechanism of toluene adsorption occurred as resulting formation of - complexation between -system of the organic linker 32 in the framework and -system in the benzene ring of toluene molecules [35, 127], complexation between unsaturated metal sites (Zr(IV)) and aromatic ring of toluene molecules [54, 81], and van der Waal interaction between toluene and pore wall of the adsorbents [127] 2.3.2 Metal organic frameworks for CO adsorption CO-selective adsorbents have been fabricated by incorporating transition metal salts, mostly Cu(I), Ag(I), or Pd (II), which are active metal-binding sites that can capture CO molecules through the formation of π-complexation thanks to -back bonding as shown Fig 2.9 [36, 38, 128] Among various salts, Cu(I) species have been mostly used as they are cheap and readily available Until now, many Cu(I)-incorporated adsorbents have been investigated to find promising adsorbents for CO/CO2 separation Wang et al [129] prepared a novel CuAlCl4 complex and introduced it into MIL-101(Cr) framework to produce CuAlCl4incorporated MIL-101(Cr) adsorbent, which enhanced CO uptake amount and CO/N2 selectivity Another route to synthesize CO-selective adsorbent on MIL-100(Fe) support via impregnating equimolar mixture of Cu(HCOO)2 and CuCl2, sequent reduction them in Cu(I) at high temperature under vacuum, then applied for CO adsorption with a capacity of 2.78 mmol g-1 at 298 K and 100 kPa [70], 3.1 mmol g-1 at 298 K and bar [36] or 3.52 mmol g-1 at 303 K and 100 kPa [38] In another aspect, Li et al [130] fabricated Cu(I)@MIL101(Cr) by using a two-step double solvent strategy and Na 2SO3 as a reduction agent to significantly reduce the aggregation of Cu species The resultant adsorbent exhibited an excellent CO uptake amount of 2.42 mmol g−1 at 25 oC and at In another work, Li and coworkers [131] fabricated Cu(I)-incorporated MIL-100(Fe) using a double- solvent/host−guest redox approach to uniformly distribute the Cu(I) sites within the pores of MOFs The prepared Cu(I)@MIL-100(Fe) adsorbent had greater CO uptake amount (3.75 mmolg-1 at 298K and bar) and CO/N2 selectivity (31) than those of the traditional πcomplexation adsorbents In general, these approaches have attempted to construct the Cu(I) 33 sites inside the pores of MOFs with high reduction yield and high dispersion of Cu(I), which remarkably enhance CO adsorption However, for practical applications, Cu(I)-based πcomplexation adsorbent requires not only high CO adsorption performance but also good air stability due to the possibility of Cu(I) oxidation Very recently, Yin et al [71] synthesized CuV-incorporated MIL-101(Cr), which showed good CO selectivity and remarkable stability under exposure to atmospheric air due to the assistance of vanadium species Nevertheless, the prepared CuV@MIL-101(Cr) showed modest CO adsorption capacity (1.3 mmol g-1), which was much lower than those of other Cu(I)-incorporated MOF adsorbents Generally, to fabricate CO-selective adsorbents, the adsorption sites of Cu(I) should be generated into the pore system of MOFs support with uniform distribution Consequently, CO adsorption capacity is enhanced owing to formation -complexation with Cu(I) whereas the adsorption capacity of the others (N2, CO2, CH4) is decreased due to decreasing BET surface area Moreover, it should be noted that Cu(I) is easily oxidized in the air This prevents the application of CO-selective adsorbent on an industrial scale Therefore, an adsorbent with a large CO adsorption capacity, high CO selectivity, and excellent air stability is highly desired 34 Figure 2.9 Formation of π-complexation between CO and metal atom Reproduced with ratification from ref [128] Copyright 2010-2021 Atlanta Publishing House LLC 35 2.3.3 Metal organic framework for CO2 adsorption It is well-known that MOFs possess a large total pore volume, high specific surface area, relative thermal stability, and high metal density as well as tuned pore structure and physicochemical properties facilely So, MOFs are widely applied for CO adsorption Generally, to enhance gas adsorption performance, the adsorbent should be meliorated for a relatively high surface area, suitably tuned pore structure, and a large number of adsorption sites having an affinity toward objective gases [75-77] Several investigations exhibited strong interaction between unsaturated metal sites in the framework and CO [91, 132-134] Also, CO2 uptake capacity was enhanced thanks to strong affinity between functional groups such as amino (NH2), hydroxy (OH), carboxylic (COOH), nitro (NO2), methoxy (OCH3), sulfonate (HSO3), and methyl (CH3), during preparation introduced into the framework [83, 92, 135, 136] Ethiraj et al [83] prepared UiO-66 derivatives by mixing ligands, including terephthalic and 2-amino terephthalic acid, at different concentrations Compared to that of raw UiO-66, at bar, the CO2 adsorption capacity increased from 17% to 46% with the increase in the amount of 2-amino terephthalic acid in the starting materials from 53% to 100% Hu and co-workers [84] successfully prepared UiO-66-SO3H materials With 15% of SO3H in the starting materials, the UiO-66-SO3H-0.15 sample showed 24.6% higher CO2 adsorption capacity than that by the pristine UiO-66 material at 25 oC and at Alternatively, Barkhordarian and Kepert [137] used N-heterocyclic compounds (pyridine or pyrazine) as linkers to produce a Zr-based MOF that contained N-donors as Lewis bases, which exhibited strong interaction with CO2 adsorbate Thus, the CO2 adsorption capacity of the resulted materials was significantly enhanced Besides, defect engineering has attracted more attention because the chemical surface, surface area, and pore size distribution are easily controlled, originating from the missing linker and/or missing cluster in the framework [72, 79] Some reports showed that a defective structure was one of the factors that contributed to the enhancement of the CO adsorption performance on the attained material For instance, Liang and co-workers [73] exerted two kinds of modulators (HCl and HCOOH) at various concentrations to defect UiO-66’s 36 framework Compared to that on the perfect UiO-66 material, the CO2 uptake capacities on defective materials were improved, and they depended on the porosity and pore size distribution tuned by defect engineering A similar result was reported by Wu et al [80] in which the CO2 uptake capacity on the modified-UiO-66 material prepared with supporting acetic acid was approximately 10% higher than that on UiO-66 prepared without a modulator Recently, a combination of both the defective structure and functionalization on Zrbased UiO-66 was investigated on a series of UiO-66(OH)2 materials produced from a mixture containing zirconium chloride (ZrCl4), 2,5-dihydroxyterephthalic acid (H2BDC(OH)2), and different concentrations of acetic acid The obtained materials showed that the CO2 uptake amount and CO2/N2 selectivity depended on the defective degree Additionally, owing to the possession of a defective structure and dihydroxy groups, the obtained materials exhibited impressive results in comparison to benchmark materials [85] Erkartal et al [77] decorated boronic fragments within the UiO-66 structure using the defect linker of 4carboxyphenylboronic (HBMCB(OH)2) mixed with terephthalic acid (H2BDC) during synthesis Compared to those on the original UiO-66, the CO2/CH4 and CO2/N2 selectivities on the obtained materials were significantly improved thanks to the appearances of the boronic (–B(OH)2) groups and defective structure in its framework By a different approach, Koutsianos and co-workers [72] successfully introduced nitrogen moieties into the defective structure of UiO-66 through the post-synthetic defect exchange (PSDE) technique The resulting materials showed approximately 25%–50% higher CO2 adsorption capacity than that of the original material without functional groups, depending on the kinds of used Nmoieties, owing to a partial functionalization in the defective structure Generally, enhancing CO2 adsorption capacity and selectivity for MOFs, typically UiO-66 is necessary However, selecting suitable pathways as well as cheap linkers should be considered to reduce production cost but also ensure high gas separation performance 2.3.4 Metal organic framework for toluene adsorption So far, a lot of MOFs have been applied for adsorption of VOCs, mainly based on 37 properties of the material such as BET surface area, total pore volume, pore size, chemical surface involving functional groups or unsaturated metal sites, so on These factors have synergistic effects together, leading to enhance interaction between target VOCs and host material and result in improvement of adsorption capacity For instance, Bahri et al [138] prepared MIL-101(Cr), MIL-53(Fe) and CPM-5(In), subsequently applied for toluene adsorption at 23 oC under dried gas, indicating that MIL-101(Cr) was an exceptional adsorbent with adsorption capacity up to 2115.7 mg g-1 in comparison with 730.4 mg g-1 for MIL-53(Fe) and 388.5 mg g-1 for CPM-5(In) The interaction between the aromatic ring of organic linker and toluene, and between unsaturated metal sites (Cr3+, Fe3+ and In3+) with system of toluene are reasons for capturing toluene on the adsorbents However, the dominant uptake amount of MIL-101(Cr) is due to superior surface area (2728 m2 g-1) whereas that of MIL53(Fe) and CPM-5(In) are 951 m2 g-1 and 1140 m2 g-1, respectively In another work, although UiO-66 (1414 m2 g-1) possesses a higher BET surface area than that of MOF-199 (1237 m2 g-1), they show equivalent toluene adsorption capacity, 166 mg g-1 for UiO-66 and 159 mg g-1 for MOF-199 This is ascribed to the discrepancy in adsorptive interaction, which is crucial for van der Waals between the adsorbed VOCs and pore wall in UiO-66 meanwhile -interaction between unsaturated metal sites (Cu(II)) in MOF-199 and benzene ring of toluene is dominant [54] Also, it is worth noting that the adsorptive mechanism based on hydrogen bonding between toluene and MOF’s adsorbent is more influent than van der Waals interaction, thus adsorption capacity is improved significantly [54] This was apparently evidenced in the report of Vo et al [58], in which UiO-66 and UiO66(NH2) were successfully prepared in a microwave-assisted tubular reactor Consequently, UiO-66(NH2) adsorbent showed a toluene uptake amount of 180 mg g-1 which was higher than 130 mg g-1 of UiO-66 even if the UiO-66’s specific surface area was 1300 m2 g-1, higher 935 m2 g-1 of UiO-66(NH2) Another way to modify the chemical surface of as-prepared material, Qin and co-workers [15] successfully prepared modified-MOF adsorbent, by impregnating a desired amount of PdCl2 onto the surface of MIL-101(Cr) with aiming increase adsorption sites (Pd(II)) strongly interacting to toluene moieties via formation of complexation Compared to parent MIL-101(Cr), the toluene adsorption capacity of 38 adsorbent increased from 1110 mg g-1 to 1285 mg g-1 after doping 3wt% PdCl2 into MIL101(Cr) support, particularly increment of 450% at low pressure regime (P/P o = 0.06) owing to decreasing pore size of PdCl2-modified MIL-101(Cr) Recently, defect engineering is considered a promising approach to tune properties of UiO-66 or UiO-67, including pore structure and specific surface area via the formation of missing linker and/or clusters, leading to improvement in toluene adsorption capacity For example, the BET surface area was enhanced from 707 m2 g-1 to 1320 m2 g-1 by changing the content of HCl playing a role as a modulator in a microwave-assisted tubular reactor to tune the pore structure of UiO-66 As a result, toluene adsorption capacity on the optimal sample achieved 130 mg g-1 [93] Zhang et al [81] observed considerably higher toluene adsorption capacity in the defective UiO-66(Zr) prepared with cetyltrimethylammonium bromide (CTAB) surfactant in comparison with its defect-free counterpart owing to the missing-linker defect sites and stronger π–π complexation In another report, Zhang et al [82] demonstrated that surface-modified UiO-66(Zr) prepared with polyvinylpyrrolidone (PVP) had 1.7 times higher toluene uptake capacity than the pristine UiO-66 39 CHAPTER Microwave-assisted continuous flow synthesis of MIL-100 (Fe) and its application to Cu(I)-loaded adsorbent for CO/CO2 separation 3.1 Introduction Metal organic frameworks (MOFs) have been utilized in various applications [139141] because of extensive strutural diversity originated from theoretically unlimited metal and organic linker combinations, together with ease of post-synthetic modification MOFs are typically synthesized in batch reactors Batch reactors, though having simple design, display a number of disavantages, such as large volume, thermal gradient inside reactors and heat control issues Conversely, continuous flow tubular reactors have the potential to overcome these drawbacks due to small reactor dimensions, as well as continuous production [142, 143] In contrast to conventional heating, microwave heating is volumetric, which enables uniform reaction throughout entire reactor volume and results in enhanced reaction rate [144, 145] Although microwave heating has been employed for rapid MOF crystallization [63, 102, 146-149], pairing a continuous flow tubular reactor with microwave heating for MOF crystallization is uncommon [69, 112, 150] Carbon monoxide is one of essential feedstocks for C1 chemistry processes Because CO sources exist as mixtures, the separation and purification of CO is essential Adsorptionbased CO separation has been actively studied in both academic [151] and commercial sectors [18, 38] The most extensively studied approaches include supported Cu(I)-modified π complexation adsorbents [6, 8, 18, 36, 38, 70, 129] with performance strongly relying on the properties of porous supports accommodating Cu(I) species (e.g., pore volume, surface area, and surface chemistry) [7, 18] In comparison to conventional porous materials with relatively limited pore texture and surface properties (e.g., carbon, zeolite, and porous ceramics), MOFs, known to possess exceptionally large surface area and pore volume, and tunable surface chemistry [152, 153], can be promising candidates as support materials 40