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Binary metal-organic frameworks: Catalysts for the efficient solvent-free CO2 fixation reaction via cyclic carbonates synthesis
Applied Catalysis A, General 571 (2019) 1–11
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Binary metal-organic frameworks: Catalysts for the efficient solvent-free CO2 fixation reaction via cyclic carbonates synthesis Jintu Francis Kurisingal, Yadagiri Rachuri, Yunjang Gu, Ga-Hyeong Kim, Dae-Won Park
T
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Division of Chemical and Biomolecular Engineering, Pusan National University, Busan 46241, Republic of Korea
ARTICLE INFO
ABSTRACT
Keywords: CO2 UiO-66/Cu-BTC Binary MOF Epoxide Cyclic carbonate
Two types of binary metal-organic frameworks (MOFs) using Cu (from Cu-BTC) and Zr (from UiO-66) as the metal centers have been synthesized by the solvothermal method and characterized using different physicochemical techniques. The catalytic potential of binary MOFs was clearly demonstrated for the cycloaddition reaction of carbon dioxide (CO2) with epoxides under solvent-free conditions. The effects of various parameters such as catalyst amount, temperature, reaction time, and CO2 pressure were studied, and a moderate set of reaction conditions (0.16 mol% of the catalyst, 60 °C, 8 h and 1.2 MPa CO2 pressure) was selected for detailed analysis. The synthesized Cu/Zr MOFs were used for the CO2-epoxide cycloaddition reaction with a tetrabutylammonium bromide (TBAB) co-catalyst. The UiO-66/Cu-BTC displayed excellent conversion of epichlorohydrin (ECH) with > 99% selectivity. The appreciable conversion of ECH with the UiO-66/Cu-BTC/TBAB system was influenced by the synergistic effect of the Cu and Zr metals and the Br ion from TBAB. The scope of extending this catalysis to various epoxides was established, and a recyclability study was also conducted. Finally, based on our previous DFT (density functional theory) studies and experimental inferences, a plausible reaction mechanism for the binary MOF-catalyzed epoxide-CO2 cycloaddition reaction was proposed.
1. Introduction Anthropogenic emission of waste CO2 gas has reached frightening levels in the biosphere, and is suspected to be causing ocean acidification and global warming [1]. Despite this fact, high-availability, non-flammability, renewability, and non-toxic nature of CO2 have attracted much attention, and therefore, it has been used for several organic transformations [2–5]. Several strategies have been proposed for CO2 reduction reactions, including carbon capture and storage, and carbon control and sequestration (CCS) techniques such as absorption (ionic liquids, zeolites, metal-organic frameworks (MOFs)), adsorption (MOF physisorption or chemisorption), membrane gas separation, gas storage (saline formations), solid storage (carbonate formations), and ocean storage [6–11]. Nonetheless, CSS technologies present major concerns, such as CO2 leakage, ocean acidification (carbolic acid formation), and low CO2 selectivity. An effective and attractive way to mitigate CO2 levels might be found with carbon capture and utilization technologies, in which captured CO2 is effectively converted into a variety of valuable products such as urethanes, formic acid, dimethyl carbonates, and cyclic carbonates [12–25]. Among these products, cyclic carbonates from CO2 and epoxides have gained much attention from the scientific community because of their 100% atom economy ⁎
(Scheme 1). The produced five-membered cyclic carbonates are industrially attractive due to their wide range of uses, such as for the electrolytic elements of lithium-ion batteries, in agriculture, as aprotic solvents for cosmetics, as intermediates in the synthesis of acyclic carbonates, in fine chemicals, pharmaceuticals, and resins, and as precursors for polymer synthesis [26–32]. For this reason, various efficient catalysts, including organometallic compounds, metal oxides, siliconbased main chain polyimidazolium salts, metal complexes, zeolites, tetraarylphosphonium salts, polymer-supported catalysts, ion-exchanged resins, N-heterocyclic carbenes, and ionic liquids (ILs) have been reported [33–45]. However, most of them required intense reaction conditions and have inherent limitations such as difficulties in separation and recycling. MOFs are a class of crystalline coordination polymer materials built from metals and organic linker compounds with a wide range of applications. MOFs are powerful platforms for gas storage, separation, drug delivery, sensors, and catalysis owing to their high surface area, chemical and thermal stability, and different functional groups [46–53]. In particular, the catalytic application of MOFs is gaining momentum in the field of carbon dioxide capture, storage, and utilization. The synthesis of Cu-BTC or HKUST-1, first addressed by Stephen Chui and co-workers in 1999 [54], forms face-centered cubic crystals
Corresponding author. E-mail address: [email protected] (D.-W. Park).
https://doi.org/10.1016/j.apcata.2018.11.035 Received 8 September 2018; Received in revised form 31 October 2018; Accepted 16 November 2018 Available online 07 December 2018 0926-860X/ © 2018 Published by Elsevier B.V.
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Scheme 1. Schematic presentation of the synthesis of cyclic carbonate from epoxides and CO2.
Scheme 2. Packing diagram of (a) Cu-BTC and (b) UiO-66.
Fig. 1. PXRD spectra of synthesized samples.
and a framework consisting of dimeric cupric tetracarboxylate units in which the three Cu2+ ions of the formula unit are coordinated with the 12 carboxylate oxygens from the two benzene-1,3,5-tricarboxylate (BTC) ligands. Later, Macias et al. [55] reported on Cu-BTC catalyzed CO2-epichlorohydrin cycloaddition reactions with moderate epoxide conversions and moderate selectivity for epichlorohydrin (ECH) carbonate at 100 °C. Furthermore, UiO-66(Zr) is a benzene-1,4-dicarboxylic acid (BDC)-based MOF with a Zr6O4(OH)4 octahedral zirconium vertex having both octahedral and tetrahedral cavities. Kim et al. [56] investigated the cycloaddition of CO2 to styrene oxide over Zr-based UiO-66 MOFs under relatively mild reaction conditions (2.0 MPa, 100 °C) in chlorobenzene solvent. In the present study, we investigated the preparation of a binary MOF with Cu (from Cu-BTC) and Zr (from UiO-66) as the metal centers for assessing its catalytic potential for cyclic carbonate synthesis from CO2 and epoxide under solvent-free conditions. Herein, we adopted simple solvothermal synthesis method to prepare the binary organic framework from
inexpensive metal salts and linker units. This work compared the catalytic potentials of the binary MOFs with the mono metallic MOFs towards the cycloaddition of CO2 with epoxide. To date, there is no binary MOF catalyst systems have been reported in the field of carbon capture and utilization (CCU) and found very few bimetallic MOFs [18,28] and other bimetallic catalyst [57,58] systems for the cycloaddition reactions of CO2. 2. Experimental section 2.1. Materials Copper nitrate trihydrate (Sigma-Aldrich); trimesic acid (SigmaAldrich); ZrCl4 (Sigma-Aldrich); terephthalic acid (Sigma-Aldrich); dimethylformamide (DMF), (Alfa-Aesar); ethanol (Sigma Aldrich); DI water; epoxides (Sigma-Aldrich); dichloromethane (Sigma-Aldrich); toluene (Sigma-Aldrich); all co-catalysts (Sigma-Aldrich) 2
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Fig. 2. FE-SEM images of the synthesized samples: (a) Cu-BTC (b) UiO-66 (c) UiO-66/Cu-BTC and (d) Cu-BTC/UiO-66.
Fig. 3. EDS elemental mapping of UiO-66/Cu-BTC catalyst.
2.2. Synthesis of Cu-BTC
2.3. Synthesis of UiO-66(Zr)
For the preparation of Cu-BTC, 1.1 g of copper nitrate trihydrate and 0.5 g of trimesic acid were dissolved in separate beakers containing 15 mL of ethanol. The above solutions were mixed and stirred thoroughly for 10 min. After stirring, the entire solution was transferred into a 100 mL autoclave and heated for 24 h at 125 °C. After the reaction was finished, the mixture was centrifuged, washed with ethanol, and dried.
For the preparation of UiO-66, 1.1 g of ZrCl4 and 0.5 g of terephthalic acid were dissolved in separate beakers containing 15 mL of DMF. The above solutions were then mixed and stirred thoroughly for 10 min. After stirring, the entire solution was transferred into a 100 mL autoclave and heated for 24 h at 125 °C. After the reaction was finished, the mixture was centrifuged, washed with ethanol, and dried.
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Fig. 4. FT-IR spectra of synthesized catalysts.
Fig. 5. Thermal gravimetric analysis (TGA) of synthesized catalysts.
2.4. Synthesis of UiO-66/Cu-BTC
ZrCl4 was dissolved in 1 mL of DMF, and 0.02 g of terephthalic acid was dissolved in another 1 mL of DMF and stirred for 4 h at 60 °C. After stirring, the entire solution was transferred into a 100 mL autoclave and heated for 24 h at 125 °C. After the reaction was finished, the mixture was centrifuged, washed with ethanol, and dried at 100 °C.
For the preparation of UiO-66/Cu-BTC (Zr:Cu = 27%:73%) [59], 1.1 g of copper nitrate trihydrate and 0.5 g of trimesic acid were dissolved in separate beakers containing 15 mL of ethanol. The solutions were then mixed together and stirred for 4 h at 60 °C. Then, 0.05 g of 4
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Table 1 Elemental analysis (EA) and inductively coupled plasma optical emission spectrometry (ICP-OES) of synthesized catalysts. Catalyst
C
H
N
O
Cu
Zr
Cu-BTC UiO-66 Cu-BTC/UiO-66 UiO-66/Cu-BTC
33.86 29.91 29.52 28.30
2.02 4.33 3.32 3.23
0.70 0.17 2.53 1.52
18.12 35.53 31.69 30.63
32.90 – 8.56 32.77
– 29.45 28.89 12.45
Table 2 Concentration of acidic and basic sites in the catalysts.
2.5. Synthesis of Cu-BTC/UiO-66
Entry
Catalyst
Acidic sites NH3-TPD (mmol/g)
Basic sites CO2-TPD (mmol/g)
1 2 3 4
Cu-BTC UiO-66 Cu-BTC/UiO-66 UiO-66/Cu-BTC
0.74 0.55 3.21 5.44
0.25 0.12 0.89 1.18
The FT-IR analysis revealed characteristic peaks for UiO-66, CuBTC, and their composite MOFs (Fig. 4). The broad band in the range of 3400-3450 cm−1 was attributed to the OeH stretching frequency of the coordinated water molecule in Cu-BTC, and the μeOH functional group in UiO-66. The medium intensity peak at 745 cm−1 corresponded to the Zr-O stretching frequency in UiO-66 and its composite MOF. The band at 728 cm−1 was attributed to the Cu-O stretching frequency in UiO66/Cu-BTC and its parent MOF, Cu-BTC. Moreover, the bands in the range of 1550-1630 cm−1 and 1450-1580 cm−1 were attributed to the symmetric and asymmetric stretching frequencies of C]O and C]C in the aromatic ring of all parent and composite materials, respectively. Thermal stabilities of pristine and binary materials were studied by thermogravimetric analysis (TGA) (Fig. 5). The pristine and binary MOFs of Cu-BTC showed lesser thermal stabilities than Zr-based UiO-66 pristine and binary MOFs. The TGA profiles of Cu-BTC indicated that weight loss in the range of 100–120 °C was attributable to the expulsion of lattice and coordinated water molecules and was followed by framework degradation at 325 °C. Additionally, the composite material of Cu-BTC, UiO-66/Cu-BTC, showed similar thermal stability. However, in the case of UiO-66, the pristine material showed high thermal stability up to 525 °C, whereas the binary MOF, Cu-BTC/UiO-66, showed less stability than the parent MOF due to the inclusion of less stable Cu-BTC in the pores of UiO-66. The TGA patterns of all materials clearly confirmed that Zr4+-based pristine and binary MOFs showed greater thermal stabilities (up to 525 °C), while Cu2+ based MOFs, Cu-BTC, and its binary MOF showed lesser thermal stabilities (up to 325 °C). The compositions of Cu-BTC, UiO-66, Cu-BTC/UiO-66, and UiO-66/ Cu-BTC were analyzed using elemental analysis (EA), and inductivelycoupled plasma optical emission spectrometry (ICP-OES) (Table 1). ICPOES analysis confirmed that the Cu and Zr content of UiO-66/Cu-BTC were 32.77 and 12.45 wt%, and for Cu-BTC/UiO-66, were 8.56 and 28.89 wt%, respectively. The acid-base characteristics of all the catalysts were examined by performing temperature programmed desorption (TPD) experimental analyses. NH3-TPD analysis illustrated the acidic sites, and CO2-TPD determined the basic sites present in the
For the preparation of Cu-BTC/UiO-66 (Cu:Zr = 23%:77%) [59], 1.1 g of ZrCl4 and 0.5 g of terephthalic acid were dissolved in separate beakers containing 15 mL of DMF. The solutions were mixed and stirred for 4 h at 60 °C. Then, 0.05 g of copper nitrate trihy drate was dissolved in 1 mL of ethanol, and 0.02 g of trimesic acid was dissolved in another 1 mL of ethanol and stirred for 4 h at 60 °C. After stirring, all solutions were mixed together and transferred to a 100 mL autoclave and heated for 24 h at 125 °C. After the reaction was finished, the mixture was centrifuged, washed with ethanol, and dried at 100 °C. 3. Results and discussion 3.1. Characterization of catalysts Packing diagram of pristine MOFs such as Cu-BTC and UiO-66 are shown in Scheme 2. Both the single (UiO-66 and Cu-BTC) and binary MOFs (Cu-BTC/UIO-66 and UIO-66/Cu-BTC) were synthesized by the solvothermal method and subjected to different analytical techniques, such as FE-SEM, Powder XRD, FT-IR, TGA, and XPS, for characterization and phase purity confirmation. The phase purity and crystallinity of single and binary MOFs was studied by Powder XRD analysis (Fig. 1). The Powder XRD patterns of the composite materials revealed that they were in good agreement with the parent MOFs (Cu-BTC and UiO-66), and also revealed a similar degree of crystallinity in all materials. The bulk sample morphology was investigated by FE-SEM analysis (Fig. 2). Distinct octahedral-shaped microcrystals were observed for Cu-BTC and its composite material (UiO-66/Cu-BTC). UiO-66 and its binary MOF (Cu-BTC/UIO-66) had analogous cubic-shaped crystals with a particle size in the range of 100–200 nm. To investigate the distribution of metal ions in binary MOF catalysts EDS elemental mapping analyses were carried out (Fig. 3 and Fig. S1). EDS mapping of each element in binary catalyst shows that all metals are spread out within the area of the crystalline particles.
Fig. 6. NH3-TPD (left) and CO2-TPD (right) results of synthesized catalysts.
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Fig. 7. XPS spectra of synthesized catalysts.
Fig. 8. N2 adsorption-desorption analysis of prepared catalysts: UiO-66 (red), Cu-BTC (black), Cu-BTC/UiO-66 (green), UiO-66/Cu-BTC (blue): closed symbols – adsorption, open symbols – desorption (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Table 3 BET surface area and the amount of CO2 adsorption for the synthesized MOFs. Catalysts
BET surface area (m2/g)
CO2 adsorption (cm3/g)
UiO-66 Cu-BTC Cu-BTC/UiO-66 UiO-66/Cu-BTC
1272 1116 255 693
19.07 23.93 29.62 31.81
Table 4 Catalytic activity comparison of catalysts for the cycloaddition reaction of ECH and CO2.
catalysts. As shown in Fig. 6 and Table 2, UiO-66/Cu-BTC showed a higher number of acidic and basic sites than did other catalysts. XPS analyses of all the catalysts were employed to identify the chemical composition and states of the metals in the prepared catalyst (Fig. 7 and Fig S2). The definitive binding energy peak located at approximately 934.5 eV and 182.2 eV corresponded to 2p3/2 of Cu2+ and 3d5/2 of Zr4+ in the binary metal organic catalysts. The XPS survey spectrum verified the presence of C, O, N, and metals in the synthesized catalysts. Fig. 8 shows the N2 adsorption-desorption isotherms obtained for all of the catalysts at 77 K. UiO-66/Cu-BTC catalyst showed more N2 uptake than Cu-BTC/UiO-66 catalyst systems and exhibited a BET surface area of 693 m2 g−1 at p/p° = 0.99. The pristine MOF catalysts showed the higher BET surface area (Table 3). Cu-BTC/UiO-66 showed different hysteresis loop in the N2 physisorption graph which may be attributed to the mesoporous nature (Fig. S3). The CO2 adsorption-desorption isotherms obtained for all the prepared catalysts at 298 K are shown in Fig. 9. Eventhough, the binary MOF catalysts have lower BET surface area than the pristine MOFs, they showed highest CO2 uptake capacities for CO2-epoxide cycloaddition reactions, 29.62 cm3 g−1 for UiO-66/CuBTC and 31.81 cm3 g−1 for Cu-BTC/UiO-66 (Table 3). Formation of small voids and higher number of basic sites in binary MOFs is expected to be responsible for CO2 adsorption.
Entry
Catalyst
Conversionf (%)
1 2 3 4 5a 6a 7b 8b 9c 10d 11e
None TBAB ZrCl4/H2BDC Cu(NO3)2.3H2O/H3BTC UiO-66 Cu-BTC Cu-BTC/UiO-66 UiO-66/Cu-BTC UiO-66/Cu-BTC UiO-66/Cu-BTC UiO-66/Cu-BTC
– 38 21 33 67 78 84 91 69 41 98
Selectivityf (%) – > 99 95 98 > 99 > 99 > 99 > 99 > 99 > 99 > 99
Reaction Conditions: a Epichlorohydrin (ECH) = 25 mmol, Catalyst = 0.16 mol % with respect to metal, TBAB = 0.5 mol%, Pressure = 1.2 MPa CO2, Temperature = 60 °C, Time = 8 h, semi batch reaction. b catalyst = 0.16 mol% with respect to metals (0.06 g), c TBAC = 0.5 mol%. d TBAI = 0.5 mol%. e Catalyst = 0.26 mol% with respect to metals, TBAB = 0.5 mol%, Pressure = 1.2 MPa CO2, Temperature = 100 °C, Time = 8 h. f Determined by GC.
ECH carbonate (Table 4, entry 1). When the reaction was carried out in the presence of co-catalyst, TBAB yielded 38% ECH conversion (Table 4, entry 2). A combination of metal precursors and ligands were used for the cycloaddition reaction, as shown in Table 4, entries 3 and 4. The synthesized heterogeneous catalysts Cu-BTC, UiO-66, Cu-BTC/ UiO-66, and UiO-66/Cu-BTC provided excellent ECH conversion. The monometallic Cu-BTC and UiO-66 showed ECH conversions of 78% and 67%, respectively, in the presence of a co-catalyst (Table 4, entries 5 and 6). Meanwhile, the binary MOFs showed much better catalytic conversion of ECH than the monometallic MOFs under the optimized reaction conditions (Table 4, entries 7 and 8). The excellent catalytic performance of the UiO-66/Cu-BTC binary MOF was influenced by the synergistic effect of the Cu and Zr metals. In order to investigate the effect of anions in the tetraalkylammonium salt co-catalysts, cycloaddition reactions over the UiO-66/Cu-BTC were carried out with TBAB, tetrabutylammonium chloride (TBAC), or tetrabutylammonium iodide (TBAI). In spite of the order of nucleophilicity (I− > Br− > Cl−),
3.2. Catalytic performance The catalytic potential of synthesized catalysts was evaluated with ECH in the cycloaddition reaction, and the reactions were performed at 60 °C and 1.2 MPa CO2 pressure for 8 h using 0.16 mol% of catalyst (mol% based on metals, 0.06 g) in the presence of 0.5 mol% co-catalyst TBAB (Table 4). In the absence of catalyst, ECH was not converted to
Fig. 9. CO2 adsorption-desorption analysis of prepared catalysts: UiO-66 (red), Cu-BTC (black), Cu-BTC/UiO-66 (green), UiO-66/Cu-BTC (blue): closed symbols – adsorption, open symbols – desorption (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Fig. 10. Effect of reaction parameters for UiO-66/Cu-BTC on the cycloaddition of ECH and CO2: (a) reaction temperature (0.06 g (0.16 mol%), 8 h, 1.2 MPa PCO2), b) reaction time (0.06 g (0.16 mol%), 60 °C, 1.2 MPa PCO2), (c) catalyst amount (60 °C, 8 h, 1.2 MPa PCO2), and (d) CO2 pressure (0.06 g (0.16 mol%), 60 °C, 8 h).
Scheme 3. Synthesis of cyclic carbonates from various epoxides (Reaction conditions: epoxide = 25 mmol, UiO-66/Cu-BTC = 0.06 g (0.16 mol% based on metals), TBAB = 0.5 mol%, semi batch reaction, pressure = 1.2 MPa CO2, temperature = 60 °C, and time = 8 h).
Cl− and I− ions. Theoretically, the catalytic performance of TBAI should provide the highest conversion of epoxides, in compliance with the order of nucleophilicity (I− > Br− > Cl−). However, the low
bromide anions from TBAB exhibited higher ECH conversion than chloride (from TBAC) and iodide ions (TBAI) (Table 4, entries 9 and 10) [18]. The use of Br− resulted in the highest conversion followed by the 8
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The effect of varying catalyst concentration on UiO-66/Cu-BTCcatalyzed ECH–CO2 cycloaddition was also determined at 1.2 MPa CO2 pressure and a reaction temperature of 60 °C for 8 h. The conversion of ECH increased with the amount of UiO-66/Cu-BTC, ranging from 0.02 g to 0.08 g with 25 mmol of ECH. As shown in Fig. 10c, a significant increase in the ECH conversion was observed when the amount of UiO66/Cu-BTC was increased from 0.02 g to 0.06 g. The maximum ECH conversion of 91% was observed for UiO-66/Cu-BTC at 0.06 g (0.16 mol % based on metals) of the catalyst under the same reaction conditions. Similarly, the role of CO2 pressure was studied in the presence of 0.16 mol% of UiO-66/Cu-BTC at 60 °C for 8 h (Fig. 10d). A CO2 pressure of 1.2 MPa resulted in the highest ECH conversion of 91%, with > 99% selectivity toward the product. However, a high CO2 pressure of 1.6 MPa slightly reduced the ECH conversion due to the dilution effect of epoxide and the breakage of metal-epoxide bonds at high pressure [13,18]. After optimizing the reaction conditions, the cycloaddition reactions of CO2 with different epoxides catalyzed by UiO-66/Cu-BTC were studied at 60 °C and 1.2 MPa CO2 pressure for 8 h (Scheme 3). Terminal epoxides (propylene oxide, (3a); ECH, (3b); and allyl glycidyl ether, (3c)) showed excellent catalytic activity toward the corresponding cyclic carbonates with high selectivity. The aromatic epoxide styrene oxide exhibited a maximal conversion of almost 26% over UiO-66/CuBTC, with nearly 99% selectivity under the optimized condition (3d), while the internal epoxide, cyclohexene oxide, showed the least catalytic activity (selectivity remained appreciable) due to steric and electronic effects (3e) [18,60].
Fig. 11. Catalyst reusability test (60 °C, 8 h, 1.2 MPa CO2 pressure, 0.06 g (0.16 mol%) UiO-66/Cu-BTC, 0.5 mol% co-catalyst, 25 mmol of ECH, > 99% selectivity towards the ECH carbonate).
reactivity observed for TBAI can be explained in terms of its steric factor; despite its high nucleophilicity, the I− ion is too bulky to access the pores of MOF. The reaction parameters (temperature, time, catalyst amount, and CO2 pressure) had a prominent effect on the catalytic activity of UiO66/Cu-BTC in the synthesis of ECH carbonate in a batch process. The influence of reaction temperature in the cycloaddition reaction of ECH and CO2 was studied in the presence of a co-catalyst at 1.2 MPa CO2 pressure for 8 h (Fig. 10a). The higher catalytic activity of UiO-66/CuBTC was observed while increasing the reaction temperature from 40 °C to 80 °C. The effect of reaction time on the UiO-66/Cu-BTC catalyzed ECH–CO2 cycloaddition is shown in Fig. 10b. ECH conversion increased with increase in reaction duration and reached the highest conversion of 91% at 8 h, which established the optimum reaction time as 8 h.
3.3. Catalyst recycling In order to check the heterogeneity of the UiO-66/Cu-BTC catalysts, recyclability studies were carried out using ECH as the substrate under the optimal reaction conditions (60 °C, 8 h, 0.16 mol% of the catalyst, and 1.2 MPa CO2 pressure) after the recovery of catalyst from the reaction mixture. The UiO-66/Cu-BTC catalysts were easily separated by simple centrifugation, and the catalysts were washed and dried at 60 °C
Scheme 4. Proposed mechanism for the cycloaddition of ECH and CO2 catalyzed by UiO-66/Cu-BTC/TBAB. 9
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Table 5 Comparison of the catalytic potential of the UiO-66/Cu-BTC catalyst with previously reported MOFs. No
MOF
T (˚C)
P CO2 (MPa)
t (h)
Catalyst amount (mol%)
Yield (%)
Ref.
1 2 3 4 5 6 7 8 9 10
ZIF-95 MOF-53 Ni-Salphen NH2-MIL-125 MIL-101-P(n-Bu)3 {Co(μ3-L)(H2O).0.5H2O)}n ZnTCPP⸦(Br−)Etim-UiO-66a MIL-101-N(n-Bu)3 ZnMOF-1-NH2 UiO-66/Cu-BTCb
80 100 80 100 80 50 140 80 80 60
1.2 1.6 2 2 2 0.1 0.1 2 0.8 1.2
2 2 4 6 8 36 14 8 8 8
0.4 0.1 3 1.6 0.9 0.19 0.95 0.9 0.21 0.16
76 78 84 85 85 87 87 88 89 91
55 56 57 58 59 60 61 59 35 This work
a
mol% based on imidazolium,
b
mol% based on metals.
time of 36 h (Table 5, entry 6). Even though ZnTCPP⸦(Br−)Etim-UiO66 provided catalytic activity at atmospheric pressure without the use of a co-catalyst, it required a high reaction temperature of 140 °C and 14 h of reaction time (Table 5, entry 7). However, the UiO-66/Cu-BTC provided 91% ECH carbonate yield with a short reaction time. MIL-101N(n-Bu)3 [66] and amine-functionalized Zn(ii) MOF (ZnMOF-1-NH2) [42] showed ECH carbonate yields of 88 and 89%, respectively, under the reaction condition of 80 °C reaction temperature for 8 h (Table 5, entries 8 and 9). Compared to the above catalyst systems, the UiO-66/ Cu-BTC/TBAB system operated at a lower reaction temperature and CO2 pressure (except for entries 6, 7, and 9). We believed that the UiO66/Cu-BTC results reported here stand alongside of or a step ahead of the MOFs reported earlier in the cycloaddition reaction of epoxide with CO2.
for 10 h. The reusability results over six consecutive cycles with the recovered UiO-66/Cu-BTC showed only a slight decrease in the ECH carbonate yield under the optimal reaction conditions (Fig. 11). The PXRD and FT-IR patterns of reused UiO-66/Cu-BTC were similar to those of synthesized catalysts (Figs. S4 and S5). This result indicated the chemical stability without noticeable structural change after recycling test. 3.4. Reaction mechanism Based on our previous DFT calculation [60,61] studies and experimental results, we proposed a plausible reaction mechanism behind the cycloaddition reaction catalyzed by UiO-66/Cu-BTC in combination with TBAB as co-catalyst to synthesize cyclic carbonates (Scheme 4). The catalytic activity of UiO-66/Cu-BTC originated from the co-existence of Lewis acidic metal centers (Cu or Zr) and linker units. Initially, the unsaturated metal atom commences an electrophilic attack on the epoxide oxygen atom. Subsequently, the Br ion from TBAB attacks the least-hindered carbon atom, and thereby helps to open the epoxide ring. Then, CO2 molecules become polarized by the negatively charged oxygen of the epoxide to form a carbonate complex. Finally, ring closure takes place upon the removal of the Br ion to produce cyclic carbonate.
4. Conclusion In this work, MOFs with Cu-BTC and UiO-66 were synthesized using the solvothermal method and employed as novel catalysts for cycloaddition reaction under solvent‐free conditions. The ability of synthesized binary MOF catalysts to engage in synergistic catalysis with a TBAB co-catalyst was studied for the first time in CO2-epoxide cycloaddition reactions. Higher catalytic activities were obtained with UiO-66/Cu-BTC and Cu-BTC/UiO-66 than with Cu-BTC and UiO-66 in the cycloaddition of ECH and CO2 in the presence of TBAB as a cocatalyst, with > 99% selectivity for ECH carbonate. The appreciable ECH conversion with the binary MOF/TBAB system was influenced by the synergistic effect of the Cu and Zr metals with the Br ion of TBAB. Various reaction parameters for the cycloaddition reaction, such as the amount of catalyst, temperature, reaction time, and pressure, were investigated with 0.16 mol% of the catalyst, at a temperature of 60 °C, with a reaction time of 8 h, and under a CO2 pressure of 1.2 MPa. The binary catalyst UiO-66/Cu-BTC was subjected to reuse over six repetitions, and no obvious loss in catalytic activity was found. Finally, based on the literature and experimental inferences, a plausible reaction mechanism for the binary MOF catalyzed epoxide-CO2 cycloaddition reaction was also proposed.
3.5. Comparison study with other MOFs A comparison of catalytic potentials of UiO-66/Cu-BTC catalysts with other reported MOFs in catalyzing the ECH-CO2 cycloaddition reaction is given in Table 5. For the comparison study, reactions were performed with 0.16 mol% of the catalyst and 0.5 mol% of TBAB at 60 °C under a CO2 pressure of 1.2 MPa for 8 h. ZIF-95 achieved an ECH carbonate yield of only 76% under the reaction conditions of 80 °C, 1.2 MPa pressure, and 2 h duration (Table 5, entry 1) [62]. The MOF-53 catalyst reported by Demir et al. [63] displayed almost the same ECH carbonate yield (78%) at 100 °C and 1.6 MPa of CO2 pressure for 2 h (Table 5, entry 2). Ni-salphen [64] afforded a slightly higher ECH carbonate yield than ZIF-95 or MOF-53 under the reaction conditions of 80 °C for 4 h with the use of high CO2 pressure of 2 MPa. NH2-MIL-125 [65] and the bifunctional heterogeneous catalyst MIL-101-P(n-Bu)3 [66] gave the same ECH carbonate yield (85%) at a high CO2 pressure of 2 MPa and a reaction temperature of 100 and 80 °C, respectively. Notably, UiO-66/Cu-BTC exhibited a much higher ECH conversion (91%) and ECH carbonate selectivity (> 99%) compared to NH2-MIL125 and MIL-101-P(n-Bu)3, even at 60 °C and under 1.2 MPa CO2 pressure over 8 h. Both {Co(μ3-L)(H2O).0.5H2O)}n [67] and ZnTCPP ⸦(Br−)Etim-UiO-66 [68] catalysts afforded 87% ECH carbonate production. {Co(μ3-L)(H2O).0.5H2O)}n achieved a ECH carbonate yield under mild reaction conditions, such as 50 °C reaction temperature and an atmospheric pressure of 0.1 MPa, but required a prolonged reaction
Conflict of interest The authors declare no competing financial interest. Acknowledgments This study was supported by the National Research Foundation of Korea through Basic Research Program (2016- 03931325). 10
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Appendix A. Supplementary data
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Binary metal-organic frameworks: Catalysts for the efficient solvent-free CO2 fixation reaction via cyclic carbonates synthesis Jintu Francis Kurisingal, Yadagiri Rachuri, Yunjang Gu, Ga-Hyeong Kim, Dae-Won Park
T
⁎
Division of Chemical and Biomolecular Engineering, Pusan National University, Busan 46241, Republic of Korea
ARTICLE INFO
ABSTRACT
Keywords: CO2 UiO-66/Cu-BTC Binary MOF Epoxide Cyclic carbonate
Two types of binary metal-organic frameworks (MOFs) using Cu (from Cu-BTC) and Zr (from UiO-66) as the metal centers have been synthesized by the solvothermal method and characterized using different physicochemical techniques. The catalytic potential of binary MOFs was clearly demonstrated for the cycloaddition reaction of carbon dioxide (CO2) with epoxides under solvent-free conditions. The effects of various parameters such as catalyst amount, temperature, reaction time, and CO2 pressure were studied, and a moderate set of reaction conditions (0.16 mol% of the catalyst, 60 °C, 8 h and 1.2 MPa CO2 pressure) was selected for detailed analysis. The synthesized Cu/Zr MOFs were used for the CO2-epoxide cycloaddition reaction with a tetrabutylammonium bromide (TBAB) co-catalyst. The UiO-66/Cu-BTC displayed excellent conversion of epichlorohydrin (ECH) with > 99% selectivity. The appreciable conversion of ECH with the UiO-66/Cu-BTC/TBAB system was influenced by the synergistic effect of the Cu and Zr metals and the Br ion from TBAB. The scope of extending this catalysis to various epoxides was established, and a recyclability study was also conducted. Finally, based on our previous DFT (density functional theory) studies and experimental inferences, a plausible reaction mechanism for the binary MOF-catalyzed epoxide-CO2 cycloaddition reaction was proposed.
1. Introduction Anthropogenic emission of waste CO2 gas has reached frightening levels in the biosphere, and is suspected to be causing ocean acidification and global warming [1]. Despite this fact, high-availability, non-flammability, renewability, and non-toxic nature of CO2 have attracted much attention, and therefore, it has been used for several organic transformations [2–5]. Several strategies have been proposed for CO2 reduction reactions, including carbon capture and storage, and carbon control and sequestration (CCS) techniques such as absorption (ionic liquids, zeolites, metal-organic frameworks (MOFs)), adsorption (MOF physisorption or chemisorption), membrane gas separation, gas storage (saline formations), solid storage (carbonate formations), and ocean storage [6–11]. Nonetheless, CSS technologies present major concerns, such as CO2 leakage, ocean acidification (carbolic acid formation), and low CO2 selectivity. An effective and attractive way to mitigate CO2 levels might be found with carbon capture and utilization technologies, in which captured CO2 is effectively converted into a variety of valuable products such as urethanes, formic acid, dimethyl carbonates, and cyclic carbonates [12–25]. Among these products, cyclic carbonates from CO2 and epoxides have gained much attention from the scientific community because of their 100% atom economy ⁎
(Scheme 1). The produced five-membered cyclic carbonates are industrially attractive due to their wide range of uses, such as for the electrolytic elements of lithium-ion batteries, in agriculture, as aprotic solvents for cosmetics, as intermediates in the synthesis of acyclic carbonates, in fine chemicals, pharmaceuticals, and resins, and as precursors for polymer synthesis [26–32]. For this reason, various efficient catalysts, including organometallic compounds, metal oxides, siliconbased main chain polyimidazolium salts, metal complexes, zeolites, tetraarylphosphonium salts, polymer-supported catalysts, ion-exchanged resins, N-heterocyclic carbenes, and ionic liquids (ILs) have been reported [33–45]. However, most of them required intense reaction conditions and have inherent limitations such as difficulties in separation and recycling. MOFs are a class of crystalline coordination polymer materials built from metals and organic linker compounds with a wide range of applications. MOFs are powerful platforms for gas storage, separation, drug delivery, sensors, and catalysis owing to their high surface area, chemical and thermal stability, and different functional groups [46–53]. In particular, the catalytic application of MOFs is gaining momentum in the field of carbon dioxide capture, storage, and utilization. The synthesis of Cu-BTC or HKUST-1, first addressed by Stephen Chui and co-workers in 1999 [54], forms face-centered cubic crystals
Corresponding author. E-mail address: [email protected] (D.-W. Park).
https://doi.org/10.1016/j.apcata.2018.11.035 Received 8 September 2018; Received in revised form 31 October 2018; Accepted 16 November 2018 Available online 07 December 2018 0926-860X/ © 2018 Published by Elsevier B.V.
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Scheme 1. Schematic presentation of the synthesis of cyclic carbonate from epoxides and CO2.
Scheme 2. Packing diagram of (a) Cu-BTC and (b) UiO-66.
Fig. 1. PXRD spectra of synthesized samples.
and a framework consisting of dimeric cupric tetracarboxylate units in which the three Cu2+ ions of the formula unit are coordinated with the 12 carboxylate oxygens from the two benzene-1,3,5-tricarboxylate (BTC) ligands. Later, Macias et al. [55] reported on Cu-BTC catalyzed CO2-epichlorohydrin cycloaddition reactions with moderate epoxide conversions and moderate selectivity for epichlorohydrin (ECH) carbonate at 100 °C. Furthermore, UiO-66(Zr) is a benzene-1,4-dicarboxylic acid (BDC)-based MOF with a Zr6O4(OH)4 octahedral zirconium vertex having both octahedral and tetrahedral cavities. Kim et al. [56] investigated the cycloaddition of CO2 to styrene oxide over Zr-based UiO-66 MOFs under relatively mild reaction conditions (2.0 MPa, 100 °C) in chlorobenzene solvent. In the present study, we investigated the preparation of a binary MOF with Cu (from Cu-BTC) and Zr (from UiO-66) as the metal centers for assessing its catalytic potential for cyclic carbonate synthesis from CO2 and epoxide under solvent-free conditions. Herein, we adopted simple solvothermal synthesis method to prepare the binary organic framework from
inexpensive metal salts and linker units. This work compared the catalytic potentials of the binary MOFs with the mono metallic MOFs towards the cycloaddition of CO2 with epoxide. To date, there is no binary MOF catalyst systems have been reported in the field of carbon capture and utilization (CCU) and found very few bimetallic MOFs [18,28] and other bimetallic catalyst [57,58] systems for the cycloaddition reactions of CO2. 2. Experimental section 2.1. Materials Copper nitrate trihydrate (Sigma-Aldrich); trimesic acid (SigmaAldrich); ZrCl4 (Sigma-Aldrich); terephthalic acid (Sigma-Aldrich); dimethylformamide (DMF), (Alfa-Aesar); ethanol (Sigma Aldrich); DI water; epoxides (Sigma-Aldrich); dichloromethane (Sigma-Aldrich); toluene (Sigma-Aldrich); all co-catalysts (Sigma-Aldrich) 2
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Fig. 2. FE-SEM images of the synthesized samples: (a) Cu-BTC (b) UiO-66 (c) UiO-66/Cu-BTC and (d) Cu-BTC/UiO-66.
Fig. 3. EDS elemental mapping of UiO-66/Cu-BTC catalyst.
2.2. Synthesis of Cu-BTC
2.3. Synthesis of UiO-66(Zr)
For the preparation of Cu-BTC, 1.1 g of copper nitrate trihydrate and 0.5 g of trimesic acid were dissolved in separate beakers containing 15 mL of ethanol. The above solutions were mixed and stirred thoroughly for 10 min. After stirring, the entire solution was transferred into a 100 mL autoclave and heated for 24 h at 125 °C. After the reaction was finished, the mixture was centrifuged, washed with ethanol, and dried.
For the preparation of UiO-66, 1.1 g of ZrCl4 and 0.5 g of terephthalic acid were dissolved in separate beakers containing 15 mL of DMF. The above solutions were then mixed and stirred thoroughly for 10 min. After stirring, the entire solution was transferred into a 100 mL autoclave and heated for 24 h at 125 °C. After the reaction was finished, the mixture was centrifuged, washed with ethanol, and dried.
3
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Fig. 4. FT-IR spectra of synthesized catalysts.
Fig. 5. Thermal gravimetric analysis (TGA) of synthesized catalysts.
2.4. Synthesis of UiO-66/Cu-BTC
ZrCl4 was dissolved in 1 mL of DMF, and 0.02 g of terephthalic acid was dissolved in another 1 mL of DMF and stirred for 4 h at 60 °C. After stirring, the entire solution was transferred into a 100 mL autoclave and heated for 24 h at 125 °C. After the reaction was finished, the mixture was centrifuged, washed with ethanol, and dried at 100 °C.
For the preparation of UiO-66/Cu-BTC (Zr:Cu = 27%:73%) [59], 1.1 g of copper nitrate trihydrate and 0.5 g of trimesic acid were dissolved in separate beakers containing 15 mL of ethanol. The solutions were then mixed together and stirred for 4 h at 60 °C. Then, 0.05 g of 4
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Table 1 Elemental analysis (EA) and inductively coupled plasma optical emission spectrometry (ICP-OES) of synthesized catalysts. Catalyst
C
H
N
O
Cu
Zr
Cu-BTC UiO-66 Cu-BTC/UiO-66 UiO-66/Cu-BTC
33.86 29.91 29.52 28.30
2.02 4.33 3.32 3.23
0.70 0.17 2.53 1.52
18.12 35.53 31.69 30.63
32.90 – 8.56 32.77
– 29.45 28.89 12.45
Table 2 Concentration of acidic and basic sites in the catalysts.
2.5. Synthesis of Cu-BTC/UiO-66
Entry
Catalyst
Acidic sites NH3-TPD (mmol/g)
Basic sites CO2-TPD (mmol/g)
1 2 3 4
Cu-BTC UiO-66 Cu-BTC/UiO-66 UiO-66/Cu-BTC
0.74 0.55 3.21 5.44
0.25 0.12 0.89 1.18
The FT-IR analysis revealed characteristic peaks for UiO-66, CuBTC, and their composite MOFs (Fig. 4). The broad band in the range of 3400-3450 cm−1 was attributed to the OeH stretching frequency of the coordinated water molecule in Cu-BTC, and the μeOH functional group in UiO-66. The medium intensity peak at 745 cm−1 corresponded to the Zr-O stretching frequency in UiO-66 and its composite MOF. The band at 728 cm−1 was attributed to the Cu-O stretching frequency in UiO66/Cu-BTC and its parent MOF, Cu-BTC. Moreover, the bands in the range of 1550-1630 cm−1 and 1450-1580 cm−1 were attributed to the symmetric and asymmetric stretching frequencies of C]O and C]C in the aromatic ring of all parent and composite materials, respectively. Thermal stabilities of pristine and binary materials were studied by thermogravimetric analysis (TGA) (Fig. 5). The pristine and binary MOFs of Cu-BTC showed lesser thermal stabilities than Zr-based UiO-66 pristine and binary MOFs. The TGA profiles of Cu-BTC indicated that weight loss in the range of 100–120 °C was attributable to the expulsion of lattice and coordinated water molecules and was followed by framework degradation at 325 °C. Additionally, the composite material of Cu-BTC, UiO-66/Cu-BTC, showed similar thermal stability. However, in the case of UiO-66, the pristine material showed high thermal stability up to 525 °C, whereas the binary MOF, Cu-BTC/UiO-66, showed less stability than the parent MOF due to the inclusion of less stable Cu-BTC in the pores of UiO-66. The TGA patterns of all materials clearly confirmed that Zr4+-based pristine and binary MOFs showed greater thermal stabilities (up to 525 °C), while Cu2+ based MOFs, Cu-BTC, and its binary MOF showed lesser thermal stabilities (up to 325 °C). The compositions of Cu-BTC, UiO-66, Cu-BTC/UiO-66, and UiO-66/ Cu-BTC were analyzed using elemental analysis (EA), and inductivelycoupled plasma optical emission spectrometry (ICP-OES) (Table 1). ICPOES analysis confirmed that the Cu and Zr content of UiO-66/Cu-BTC were 32.77 and 12.45 wt%, and for Cu-BTC/UiO-66, were 8.56 and 28.89 wt%, respectively. The acid-base characteristics of all the catalysts were examined by performing temperature programmed desorption (TPD) experimental analyses. NH3-TPD analysis illustrated the acidic sites, and CO2-TPD determined the basic sites present in the
For the preparation of Cu-BTC/UiO-66 (Cu:Zr = 23%:77%) [59], 1.1 g of ZrCl4 and 0.5 g of terephthalic acid were dissolved in separate beakers containing 15 mL of DMF. The solutions were mixed and stirred for 4 h at 60 °C. Then, 0.05 g of copper nitrate trihy drate was dissolved in 1 mL of ethanol, and 0.02 g of trimesic acid was dissolved in another 1 mL of ethanol and stirred for 4 h at 60 °C. After stirring, all solutions were mixed together and transferred to a 100 mL autoclave and heated for 24 h at 125 °C. After the reaction was finished, the mixture was centrifuged, washed with ethanol, and dried at 100 °C. 3. Results and discussion 3.1. Characterization of catalysts Packing diagram of pristine MOFs such as Cu-BTC and UiO-66 are shown in Scheme 2. Both the single (UiO-66 and Cu-BTC) and binary MOFs (Cu-BTC/UIO-66 and UIO-66/Cu-BTC) were synthesized by the solvothermal method and subjected to different analytical techniques, such as FE-SEM, Powder XRD, FT-IR, TGA, and XPS, for characterization and phase purity confirmation. The phase purity and crystallinity of single and binary MOFs was studied by Powder XRD analysis (Fig. 1). The Powder XRD patterns of the composite materials revealed that they were in good agreement with the parent MOFs (Cu-BTC and UiO-66), and also revealed a similar degree of crystallinity in all materials. The bulk sample morphology was investigated by FE-SEM analysis (Fig. 2). Distinct octahedral-shaped microcrystals were observed for Cu-BTC and its composite material (UiO-66/Cu-BTC). UiO-66 and its binary MOF (Cu-BTC/UIO-66) had analogous cubic-shaped crystals with a particle size in the range of 100–200 nm. To investigate the distribution of metal ions in binary MOF catalysts EDS elemental mapping analyses were carried out (Fig. 3 and Fig. S1). EDS mapping of each element in binary catalyst shows that all metals are spread out within the area of the crystalline particles.
Fig. 6. NH3-TPD (left) and CO2-TPD (right) results of synthesized catalysts.
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Fig. 7. XPS spectra of synthesized catalysts.
Fig. 8. N2 adsorption-desorption analysis of prepared catalysts: UiO-66 (red), Cu-BTC (black), Cu-BTC/UiO-66 (green), UiO-66/Cu-BTC (blue): closed symbols – adsorption, open symbols – desorption (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Table 3 BET surface area and the amount of CO2 adsorption for the synthesized MOFs. Catalysts
BET surface area (m2/g)
CO2 adsorption (cm3/g)
UiO-66 Cu-BTC Cu-BTC/UiO-66 UiO-66/Cu-BTC
1272 1116 255 693
19.07 23.93 29.62 31.81
Table 4 Catalytic activity comparison of catalysts for the cycloaddition reaction of ECH and CO2.
catalysts. As shown in Fig. 6 and Table 2, UiO-66/Cu-BTC showed a higher number of acidic and basic sites than did other catalysts. XPS analyses of all the catalysts were employed to identify the chemical composition and states of the metals in the prepared catalyst (Fig. 7 and Fig S2). The definitive binding energy peak located at approximately 934.5 eV and 182.2 eV corresponded to 2p3/2 of Cu2+ and 3d5/2 of Zr4+ in the binary metal organic catalysts. The XPS survey spectrum verified the presence of C, O, N, and metals in the synthesized catalysts. Fig. 8 shows the N2 adsorption-desorption isotherms obtained for all of the catalysts at 77 K. UiO-66/Cu-BTC catalyst showed more N2 uptake than Cu-BTC/UiO-66 catalyst systems and exhibited a BET surface area of 693 m2 g−1 at p/p° = 0.99. The pristine MOF catalysts showed the higher BET surface area (Table 3). Cu-BTC/UiO-66 showed different hysteresis loop in the N2 physisorption graph which may be attributed to the mesoporous nature (Fig. S3). The CO2 adsorption-desorption isotherms obtained for all the prepared catalysts at 298 K are shown in Fig. 9. Eventhough, the binary MOF catalysts have lower BET surface area than the pristine MOFs, they showed highest CO2 uptake capacities for CO2-epoxide cycloaddition reactions, 29.62 cm3 g−1 for UiO-66/CuBTC and 31.81 cm3 g−1 for Cu-BTC/UiO-66 (Table 3). Formation of small voids and higher number of basic sites in binary MOFs is expected to be responsible for CO2 adsorption.
Entry
Catalyst
Conversionf (%)
1 2 3 4 5a 6a 7b 8b 9c 10d 11e
None TBAB ZrCl4/H2BDC Cu(NO3)2.3H2O/H3BTC UiO-66 Cu-BTC Cu-BTC/UiO-66 UiO-66/Cu-BTC UiO-66/Cu-BTC UiO-66/Cu-BTC UiO-66/Cu-BTC
– 38 21 33 67 78 84 91 69 41 98
Selectivityf (%) – > 99 95 98 > 99 > 99 > 99 > 99 > 99 > 99 > 99
Reaction Conditions: a Epichlorohydrin (ECH) = 25 mmol, Catalyst = 0.16 mol % with respect to metal, TBAB = 0.5 mol%, Pressure = 1.2 MPa CO2, Temperature = 60 °C, Time = 8 h, semi batch reaction. b catalyst = 0.16 mol% with respect to metals (0.06 g), c TBAC = 0.5 mol%. d TBAI = 0.5 mol%. e Catalyst = 0.26 mol% with respect to metals, TBAB = 0.5 mol%, Pressure = 1.2 MPa CO2, Temperature = 100 °C, Time = 8 h. f Determined by GC.
ECH carbonate (Table 4, entry 1). When the reaction was carried out in the presence of co-catalyst, TBAB yielded 38% ECH conversion (Table 4, entry 2). A combination of metal precursors and ligands were used for the cycloaddition reaction, as shown in Table 4, entries 3 and 4. The synthesized heterogeneous catalysts Cu-BTC, UiO-66, Cu-BTC/ UiO-66, and UiO-66/Cu-BTC provided excellent ECH conversion. The monometallic Cu-BTC and UiO-66 showed ECH conversions of 78% and 67%, respectively, in the presence of a co-catalyst (Table 4, entries 5 and 6). Meanwhile, the binary MOFs showed much better catalytic conversion of ECH than the monometallic MOFs under the optimized reaction conditions (Table 4, entries 7 and 8). The excellent catalytic performance of the UiO-66/Cu-BTC binary MOF was influenced by the synergistic effect of the Cu and Zr metals. In order to investigate the effect of anions in the tetraalkylammonium salt co-catalysts, cycloaddition reactions over the UiO-66/Cu-BTC were carried out with TBAB, tetrabutylammonium chloride (TBAC), or tetrabutylammonium iodide (TBAI). In spite of the order of nucleophilicity (I− > Br− > Cl−),
3.2. Catalytic performance The catalytic potential of synthesized catalysts was evaluated with ECH in the cycloaddition reaction, and the reactions were performed at 60 °C and 1.2 MPa CO2 pressure for 8 h using 0.16 mol% of catalyst (mol% based on metals, 0.06 g) in the presence of 0.5 mol% co-catalyst TBAB (Table 4). In the absence of catalyst, ECH was not converted to
Fig. 9. CO2 adsorption-desorption analysis of prepared catalysts: UiO-66 (red), Cu-BTC (black), Cu-BTC/UiO-66 (green), UiO-66/Cu-BTC (blue): closed symbols – adsorption, open symbols – desorption (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Fig. 10. Effect of reaction parameters for UiO-66/Cu-BTC on the cycloaddition of ECH and CO2: (a) reaction temperature (0.06 g (0.16 mol%), 8 h, 1.2 MPa PCO2), b) reaction time (0.06 g (0.16 mol%), 60 °C, 1.2 MPa PCO2), (c) catalyst amount (60 °C, 8 h, 1.2 MPa PCO2), and (d) CO2 pressure (0.06 g (0.16 mol%), 60 °C, 8 h).
Scheme 3. Synthesis of cyclic carbonates from various epoxides (Reaction conditions: epoxide = 25 mmol, UiO-66/Cu-BTC = 0.06 g (0.16 mol% based on metals), TBAB = 0.5 mol%, semi batch reaction, pressure = 1.2 MPa CO2, temperature = 60 °C, and time = 8 h).
Cl− and I− ions. Theoretically, the catalytic performance of TBAI should provide the highest conversion of epoxides, in compliance with the order of nucleophilicity (I− > Br− > Cl−). However, the low
bromide anions from TBAB exhibited higher ECH conversion than chloride (from TBAC) and iodide ions (TBAI) (Table 4, entries 9 and 10) [18]. The use of Br− resulted in the highest conversion followed by the 8
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The effect of varying catalyst concentration on UiO-66/Cu-BTCcatalyzed ECH–CO2 cycloaddition was also determined at 1.2 MPa CO2 pressure and a reaction temperature of 60 °C for 8 h. The conversion of ECH increased with the amount of UiO-66/Cu-BTC, ranging from 0.02 g to 0.08 g with 25 mmol of ECH. As shown in Fig. 10c, a significant increase in the ECH conversion was observed when the amount of UiO66/Cu-BTC was increased from 0.02 g to 0.06 g. The maximum ECH conversion of 91% was observed for UiO-66/Cu-BTC at 0.06 g (0.16 mol % based on metals) of the catalyst under the same reaction conditions. Similarly, the role of CO2 pressure was studied in the presence of 0.16 mol% of UiO-66/Cu-BTC at 60 °C for 8 h (Fig. 10d). A CO2 pressure of 1.2 MPa resulted in the highest ECH conversion of 91%, with > 99% selectivity toward the product. However, a high CO2 pressure of 1.6 MPa slightly reduced the ECH conversion due to the dilution effect of epoxide and the breakage of metal-epoxide bonds at high pressure [13,18]. After optimizing the reaction conditions, the cycloaddition reactions of CO2 with different epoxides catalyzed by UiO-66/Cu-BTC were studied at 60 °C and 1.2 MPa CO2 pressure for 8 h (Scheme 3). Terminal epoxides (propylene oxide, (3a); ECH, (3b); and allyl glycidyl ether, (3c)) showed excellent catalytic activity toward the corresponding cyclic carbonates with high selectivity. The aromatic epoxide styrene oxide exhibited a maximal conversion of almost 26% over UiO-66/CuBTC, with nearly 99% selectivity under the optimized condition (3d), while the internal epoxide, cyclohexene oxide, showed the least catalytic activity (selectivity remained appreciable) due to steric and electronic effects (3e) [18,60].
Fig. 11. Catalyst reusability test (60 °C, 8 h, 1.2 MPa CO2 pressure, 0.06 g (0.16 mol%) UiO-66/Cu-BTC, 0.5 mol% co-catalyst, 25 mmol of ECH, > 99% selectivity towards the ECH carbonate).
reactivity observed for TBAI can be explained in terms of its steric factor; despite its high nucleophilicity, the I− ion is too bulky to access the pores of MOF. The reaction parameters (temperature, time, catalyst amount, and CO2 pressure) had a prominent effect on the catalytic activity of UiO66/Cu-BTC in the synthesis of ECH carbonate in a batch process. The influence of reaction temperature in the cycloaddition reaction of ECH and CO2 was studied in the presence of a co-catalyst at 1.2 MPa CO2 pressure for 8 h (Fig. 10a). The higher catalytic activity of UiO-66/CuBTC was observed while increasing the reaction temperature from 40 °C to 80 °C. The effect of reaction time on the UiO-66/Cu-BTC catalyzed ECH–CO2 cycloaddition is shown in Fig. 10b. ECH conversion increased with increase in reaction duration and reached the highest conversion of 91% at 8 h, which established the optimum reaction time as 8 h.
3.3. Catalyst recycling In order to check the heterogeneity of the UiO-66/Cu-BTC catalysts, recyclability studies were carried out using ECH as the substrate under the optimal reaction conditions (60 °C, 8 h, 0.16 mol% of the catalyst, and 1.2 MPa CO2 pressure) after the recovery of catalyst from the reaction mixture. The UiO-66/Cu-BTC catalysts were easily separated by simple centrifugation, and the catalysts were washed and dried at 60 °C
Scheme 4. Proposed mechanism for the cycloaddition of ECH and CO2 catalyzed by UiO-66/Cu-BTC/TBAB. 9
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Table 5 Comparison of the catalytic potential of the UiO-66/Cu-BTC catalyst with previously reported MOFs. No
MOF
T (˚C)
P CO2 (MPa)
t (h)
Catalyst amount (mol%)
Yield (%)
Ref.
1 2 3 4 5 6 7 8 9 10
ZIF-95 MOF-53 Ni-Salphen NH2-MIL-125 MIL-101-P(n-Bu)3 {Co(μ3-L)(H2O).0.5H2O)}n ZnTCPP⸦(Br−)Etim-UiO-66a MIL-101-N(n-Bu)3 ZnMOF-1-NH2 UiO-66/Cu-BTCb
80 100 80 100 80 50 140 80 80 60
1.2 1.6 2 2 2 0.1 0.1 2 0.8 1.2
2 2 4 6 8 36 14 8 8 8
0.4 0.1 3 1.6 0.9 0.19 0.95 0.9 0.21 0.16
76 78 84 85 85 87 87 88 89 91
55 56 57 58 59 60 61 59 35 This work
a
mol% based on imidazolium,
b
mol% based on metals.
time of 36 h (Table 5, entry 6). Even though ZnTCPP⸦(Br−)Etim-UiO66 provided catalytic activity at atmospheric pressure without the use of a co-catalyst, it required a high reaction temperature of 140 °C and 14 h of reaction time (Table 5, entry 7). However, the UiO-66/Cu-BTC provided 91% ECH carbonate yield with a short reaction time. MIL-101N(n-Bu)3 [66] and amine-functionalized Zn(ii) MOF (ZnMOF-1-NH2) [42] showed ECH carbonate yields of 88 and 89%, respectively, under the reaction condition of 80 °C reaction temperature for 8 h (Table 5, entries 8 and 9). Compared to the above catalyst systems, the UiO-66/ Cu-BTC/TBAB system operated at a lower reaction temperature and CO2 pressure (except for entries 6, 7, and 9). We believed that the UiO66/Cu-BTC results reported here stand alongside of or a step ahead of the MOFs reported earlier in the cycloaddition reaction of epoxide with CO2.
for 10 h. The reusability results over six consecutive cycles with the recovered UiO-66/Cu-BTC showed only a slight decrease in the ECH carbonate yield under the optimal reaction conditions (Fig. 11). The PXRD and FT-IR patterns of reused UiO-66/Cu-BTC were similar to those of synthesized catalysts (Figs. S4 and S5). This result indicated the chemical stability without noticeable structural change after recycling test. 3.4. Reaction mechanism Based on our previous DFT calculation [60,61] studies and experimental results, we proposed a plausible reaction mechanism behind the cycloaddition reaction catalyzed by UiO-66/Cu-BTC in combination with TBAB as co-catalyst to synthesize cyclic carbonates (Scheme 4). The catalytic activity of UiO-66/Cu-BTC originated from the co-existence of Lewis acidic metal centers (Cu or Zr) and linker units. Initially, the unsaturated metal atom commences an electrophilic attack on the epoxide oxygen atom. Subsequently, the Br ion from TBAB attacks the least-hindered carbon atom, and thereby helps to open the epoxide ring. Then, CO2 molecules become polarized by the negatively charged oxygen of the epoxide to form a carbonate complex. Finally, ring closure takes place upon the removal of the Br ion to produce cyclic carbonate.
4. Conclusion In this work, MOFs with Cu-BTC and UiO-66 were synthesized using the solvothermal method and employed as novel catalysts for cycloaddition reaction under solvent‐free conditions. The ability of synthesized binary MOF catalysts to engage in synergistic catalysis with a TBAB co-catalyst was studied for the first time in CO2-epoxide cycloaddition reactions. Higher catalytic activities were obtained with UiO-66/Cu-BTC and Cu-BTC/UiO-66 than with Cu-BTC and UiO-66 in the cycloaddition of ECH and CO2 in the presence of TBAB as a cocatalyst, with > 99% selectivity for ECH carbonate. The appreciable ECH conversion with the binary MOF/TBAB system was influenced by the synergistic effect of the Cu and Zr metals with the Br ion of TBAB. Various reaction parameters for the cycloaddition reaction, such as the amount of catalyst, temperature, reaction time, and pressure, were investigated with 0.16 mol% of the catalyst, at a temperature of 60 °C, with a reaction time of 8 h, and under a CO2 pressure of 1.2 MPa. The binary catalyst UiO-66/Cu-BTC was subjected to reuse over six repetitions, and no obvious loss in catalytic activity was found. Finally, based on the literature and experimental inferences, a plausible reaction mechanism for the binary MOF catalyzed epoxide-CO2 cycloaddition reaction was also proposed.
3.5. Comparison study with other MOFs A comparison of catalytic potentials of UiO-66/Cu-BTC catalysts with other reported MOFs in catalyzing the ECH-CO2 cycloaddition reaction is given in Table 5. For the comparison study, reactions were performed with 0.16 mol% of the catalyst and 0.5 mol% of TBAB at 60 °C under a CO2 pressure of 1.2 MPa for 8 h. ZIF-95 achieved an ECH carbonate yield of only 76% under the reaction conditions of 80 °C, 1.2 MPa pressure, and 2 h duration (Table 5, entry 1) [62]. The MOF-53 catalyst reported by Demir et al. [63] displayed almost the same ECH carbonate yield (78%) at 100 °C and 1.6 MPa of CO2 pressure for 2 h (Table 5, entry 2). Ni-salphen [64] afforded a slightly higher ECH carbonate yield than ZIF-95 or MOF-53 under the reaction conditions of 80 °C for 4 h with the use of high CO2 pressure of 2 MPa. NH2-MIL-125 [65] and the bifunctional heterogeneous catalyst MIL-101-P(n-Bu)3 [66] gave the same ECH carbonate yield (85%) at a high CO2 pressure of 2 MPa and a reaction temperature of 100 and 80 °C, respectively. Notably, UiO-66/Cu-BTC exhibited a much higher ECH conversion (91%) and ECH carbonate selectivity (> 99%) compared to NH2-MIL125 and MIL-101-P(n-Bu)3, even at 60 °C and under 1.2 MPa CO2 pressure over 8 h. Both {Co(μ3-L)(H2O).0.5H2O)}n [67] and ZnTCPP ⸦(Br−)Etim-UiO-66 [68] catalysts afforded 87% ECH carbonate production. {Co(μ3-L)(H2O).0.5H2O)}n achieved a ECH carbonate yield under mild reaction conditions, such as 50 °C reaction temperature and an atmospheric pressure of 0.1 MPa, but required a prolonged reaction
Conflict of interest The authors declare no competing financial interest. Acknowledgments This study was supported by the National Research Foundation of Korea through Basic Research Program (2016- 03931325). 10
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Appendix A. Supplementary data
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