Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Zinc Porphyrin/Imidazolium Integrated Multivariate Zirconium Metal−Organic Frameworks for Transformation of CO2 into Cyclic Carbonates Jun Liang,†,‡ Ya-Qiang Xie,† Qiao Wu,‡ Xiu-Yun Wang,§ Tao-Tao Liu,‡ Hong-Fang Li,‡ Yuan-Biao Huang,*,‡ and Rong Cao*,†,‡,∥ †
Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China § National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002, China ‡
S Supporting Information *
ABSTRACT: The design and synthesis of metal−organic frameworks (MOFs) enclosed with multiple catalytic active sites is favorable for cooperative catalysis, but is is still challenging. Herein, we developed a sequential postsynthetic ionization and metalation strategy to prepare bifunctional multivariate Zr-MOFs incorporating zinc porphyrin and imidazolium functionalities. Using this facile strategy, tetratopic [5,10,15,20-tetrakis(4-carboxyphenyl)porphyrinato]zinc(II) (ZnTCPP) ligands were successfully installed into the cationic Zr-MOF to obtain ZnTCPP⊂(Br−)EtimUiO-66. These MTV-MOFs, including TCPP⊂Im-UiO-66, TCPP⊂(Br−)Etim-UiO-66, and ZnTCPP⊂(Br−)Etim-UiO-66, were well characterized and used in CO2 capture and conversion into cyclic carbonate from allyl glycidyl ether and CO2 under cocatalyst-free and 1 bar CO2 pressure conditions. It was found that the structural features and CO2 affinity properties of these MTV-MOFs can be tuned by introducing imidazolium groups or doping zinc sites. Additionally, ZnTCPP⊂(Br−)Etim-UiO-66 exhibited enhanced catalytic activities compared to other MTVMOFs herein for obtaining the 3-allyloxy-1,2-proplyene carbonate product, which was attributed to the cooperative effect of Zn2+ sites and Br− ions in this microporous ionic MTV-MOF. ZnTCPP⊂(Br−)Etim-UiO-66 can be recycled easily and used at least three times.
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INTRODUCTION
Recent homogeneous catalysis studies showed that metalloporphyrins with strong Lewis acid centers could function synergistically with nucleophilic halogen anions in the double activation of epoxides and CO2 to produce cyclic carbonates.22 Besides, ionic liquid moieties have been promising to not only enhance the CO2 affinity through dipole−quadruple interactions23,24 but also facilitate the catalytic CO2 fixation process by providing nucleophilic ions.25−27 Thus, it is expected that MOFs incorporated with metalloporphyrin and ionic liquids could endow them with the ability for both CO2 capture and conversion (Figure 1).28 However, to our best knowledge, very few metalloporphyrin-based MOFs have been reported for CO2-involved chemical conversions.29−32 The challenge lies in incorporating these multifunctionalities into the porous multivariate frameworks.33−35
CO2-involved chemical synthesis of valuable products has attracted much attention.1−3 Particularly, the catalytic fixation of CO2 into epoxides to produce cyclic carbonates is attractive because the products have been widely used as electrolytes, aprotic solvent, and organic intermediates.4−6 To date, various kinds of porous heterogeneous catalysts have been investigated in this reaction.7−13 Metal−organic frameworks (MOFs) are promising porous solid materials for this reaction because of their high surface areas and tunable components and pore sizes.14−20 In general, MOFs containing active Lewis acidic sites can promote this CO2 fixation process with or without the use of a nucleophilic reagent such as tetrabutylammonium bromide (TBAB).7,21 Although many MOF catalysts have been reported for this reaction, it is challenging to obtain task-specific MOFs with both Lewis acidic and nucleophilic sites for CO2 chemical fixation. © XXXX American Chemical Society
Received: November 22, 2017
A
DOI: 10.1021/acs.inorgchem.7b02983 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
the Zr6 cluster.47,48 Recently, using the admirable one-pot synthetic strategy initiated by Zhou’s group, tetratopic TCPP ligands have been successfully integrated into UiO-66 framework while maintaining the crystal structure of UiO-66.42,43 We envision that the integration of zinc porphyrin carboxylate ligand into imidazolium functionalized ionic UiO-66 might give a recyclable porous catalyst for the synthesis of cyclic carbonates from epoxides and CO2 (Scheme 1). To our best knowledge, there is no report thus far on the cationic MOFs containing porphyrins. It was previously reported by our group that imidazoliumbased ionic liquid decorated UiO-66, denoted (I−)Meim-UiO66, could be easily obtained via postsynthetic ionization (PSI) method;27 the incorporation of TCPP into Im-UiO-66 was thus the key step, as shown in Scheme 1a. Herein, we report on the preparation, characterization, and CO2 adsorption and conversion properties of the imidazolium functionalized ZrMOFs integrated with zinc porphyrins.
Figure 1. Dual activation mode of epoxides over the ionic MTV-MOF containing metalloporphyrins.
Multivariate metal−organic frameworks (MTV-MOFs)36,37 have attracted growing attention as promising materials for enhanced gas adsorption and catalysis because multifunctionalities can be incorporated into one single phase by a mixedligand strategy, either through one-pot synthesis38,39 or by postsynthetic modification (PSM).40,41 However, reports on the multifunctional MTV-MOFs with cooperative active sites are still quite limited, which can be realized by judicious choice of compatible ligands and metals.42−46 As one class of the most stable MOFs, Zr-MOFs have been widely applied in many fields including gas adsorption and catalysis because of their high stability and versatile connectivity (e.g., 6, 8, and 12) of
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EXPERIMENTAL SECTION
Materials and Instrumentation. 2-(Imidazol-1-yl)terephthalic acid (Im-H2BDC·HCl·H2O) and [(Br−)(Etim-H2BDC)+] were prepared according to previous literature.27 Other reagents were
Scheme 1. Schematic Illustration of the Synthesis of ZnTCPP⊂(Br−)Etim-UiO-66 Incorporating Imidazolium, Zincporphyrin, and Typical Zr6 Clusters via (a) Sequential Mixed-Ligand of Different Geometry, Postsynthetic Ionization, and Postsynthetic Metalation Strategy and (b, c) de Novo One-Pot Syntheses Based on Ionic [(Br−)(Etim-H2BDC)+] Liganda
a
Im-Zr6, imidazole functionalized Zr6 cluster; (Br−)Etim-Zr6, imidazolium functionalized Zr6 cluster. Yellow sphere represents the pore cavity. B
DOI: 10.1021/acs.inorgchem.7b02983 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 2. (a) Simulated PXRD patterns of UiO-66 and that of as-synthesized samples of TCPP⊂Im-UiO-66, TCPP⊂(Br−)Etim-UiO-66, and ZnTCPP⊂(Br−)Etim-UiO-66. (b) TEM image of ZnTCPP⊂(Br−)Etim-UiO-66. N 1s XPS spectra of (c) TCPP⊂Im-UiO-66 and (d) TCPP⊂(Br−)Etim-UiO-66. (e) Solid-state 13C NMR spectra of TCPP⊂(Br−)Etim-UiO-66 and ZnTCPP⊂(Br−)Etim-UiO-66. (f) UV/vis solid-state absorption spectra of (Br−)Etim-UiO-66, TCPP⊂(Br−)Etim-UiO-66, and ZnTCPP⊂(Br−)Etim-UiO-66. purchased from commercial sources and used as received. N2 and CO2 sorption isotherms for three MTV-MOFs were measured by using a Micrometrics ASAP 2020 instrument at 77 K and 273 and 298 K, respectively. Before the measurement, the samples were evacuated and activated at 393 K in vacuum for 12 h. Powder X-ray diffraction patterns (PXRD) were recorded on a Rigaku Dmax 2500 diffractometer equipped with a Cu Kα radiation source (λ= 1.54056 Å) over the 2θ range of 4−40° with a scan speed of 3° min−1 at room temperature. Infrared (IR) spectra were recorded using KBr pellets on a Bruker VERTEX70 in the range of 400−4000 cm−1. Elemental analyses of C, H, and N were carried out on an Elementar Vario EL III analyzer. The 1H NMR measurements were performed on an AVANCE III Bruker Biospin spectrometer, operating at 400 MHz. About 12 mg of MOF samples was dissolved in 500 μL of DMSO-d6, and then 30 μL of HF was added. The solid-state 13C NMR (SS 13C NMR) data were measured on an AVANCE III HD Bruker-BioSpin. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis Ultra DLD system with a base pressure of 10−9 Torr. Diffuse reflectance ultraviolet−visible (UV−vis) spectra
were recorded using a PerkinElmer instrument (Lambda 35). The morphologies of MOFs were studied using a FEIT 20 transmission electron microscopy (TEM) instrument working at 200 kV and a scanning electron microscopy (SEM) instrument working at 10 kV. The gas chromatography (GC) measurements were performed on a G7890A-GC equipped with a HP-5 column, FID detector, and autosampler. The gas chromatography−mass spectrometry (GC-MS) measurements were performed on a Varian 450-GC/240-MS. Preparation of [(PF6−)(Etim-H2BDC)+]. [(Br−)(Etim-H2BDC)+] (150 mg) was dissolved in 1 mL of distilled water before adding NH4PF4 to obtain a white precipitate product. The product was further washed by a small amount of water, centrifugated, and dried at 70 °C under vacuum overnight. 1H NMR (400 MHz, DMSO-d6, 25 °C): δ (ppm) = 1.49 (t, 3H, CH3), 4.30 (q, 2H, −CH2−), 8.00 (s, 1H, N−CH−CH−N), 8.06 (s, 1H, N−CH−CH−N), 8.21−8.27 (br, 3H, Ar−H), 9.60 (s, H, N−CH-N), 13.91 (br, 2H, COOH). IR characteristics (KBr, υmax, cm−1): 3426 (w), 3153 (s), 3102 (s), 2991 (s), 2944 (w), 2864 (w), 1721 (s), 1574 (w), 1552 (s), 1500 (s), 1444 (s), 1409 (s), 1274 (s), 1250 (m), 1104 (w), 1059 (s), 760 (s). C
DOI: 10.1021/acs.inorgchem.7b02983 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. (a) Typical STEM image of ZnTCPP⊂(Br−)Etim-UiO-66 (left top) and the corresponding EDX elemental mapping images (Zr, C, N, Br, and Zn). (b) N2 sorption isotherms at 77 K and (c) CO2 sorption isotherms at 298 K and (d) the Qst values for CO2 adsorption at low coverage for TCPP⊂Im-UiO-66, TCPP⊂(Br−)Etim-UiO-66, and ZnTCPP⊂(Br−)Etim-UiO-66. Solid symbols denote adsorption; open symbols denote desorption. Synthesis of ZnTCPP⊂(Br−)Etim-UiO-66 via a Sequential Postsynthetic Ionization and Metalation Strategy. ZrCl4 (25 mg, 0.11 mmol), Im-H2BDC·HCl·H2O (30 mg, 0.11 mmol), TCPP (4 mg, 0.0048 mmol), and acetic acid (0.57 mL) in 3 mL of DMF were ultrasonically dissolved in a 30 mL Pyrex vial. The mixture was heated in an oven at 120 °C for 12 h. After the mixtrue cooled to room temperature, brown precipitates were collected by centrifugation. The solids were washed with DMF three times to remove unreacted precursors and then solvent exchanged with acetone three times. The resulting brown TCPP⊂Im-UiO-66 powder was obtained by centrifugation and dried in an oven at 70 °C. Then, a CH3CN (15 mL) suspension of TCPP⊂Im-UiO-66 (0.25 g) and bromoethane (0.8 mL) was heated at 80 °C for 48 h under N2 atmosphere. The product was collected by centrifugation, washed with EtOH (10 mL × 2), acetone (10 mL × 2), and ether (10 mL × 2) to generate TCPP⊂(Br−)Etim-UiO-66 as brown powder solids after drying at 70 °C under vacuum for 12 h. Zinc bromide (200 mg) was dissolved into DMF (100 mL). TCPP⊂(Br−)Etim-UiO-66 (500 mg) was added to this solution, and the solution was heated at 100 °C for 24 h. The microcrystalline powder were collected by filtration and washed with DMF (3 × 100 mL). The DMF was then replaced with acetone (3 × 200 mL) over a three-day period. Finally, the volatile acetone was removed by heating at 120 °C under vacuum for 12 h to give ZnTCPP⊂(Br−)Etim-UiO-66. Elemental analysis for TCPP⊂Im-UiO66 (found): C, 33.65; N, 6.10; H, 4.75; Zr, 18.92 (%). TCPP⊂(Br−)Etim-UiO-66 (found): C, 30.93; N, 5.43; H, 3.69; Zr, 18.03 (%). ZnTCPP⊂(Br−)Etim-UiO-66 (found): C, 32.68; N, 5.63; H, 3.55; Zr, 25.16; Zn, 2.63 (%). The empirical formula for each compound could not be determined because of their multiple components. The deviation on the C, H, and N results of three compounds might be a result of coordination flexibility of ligands, ionization, Zn doping, and residual DMF during the tandem PSM process. IR characteristics (KBr, υmax, cm−1): 2959 (w), 2922 (w), 1716 (s), 1597 (s), 1502 (s), 1415 (s), 1392 (s), 1377 (s), 1205 (s), 1114 (s), 993 (s), 771 (s), 653 (s), 484 (s). Synthesis of (Br−)Etim-UiO-66 via de Novo Synthesis. ZrCl4 (28.8 mg, 0.12 mmol), (Br−)Etim-H2BDC (41 mg, 0.12 mmol), and
acetic acid (0.34 mL) in 3 mL of DMF were ultrasonically dissolved in a 30 mL Pyrex vial. The mixture was heated in an oven at 120 °C for 24 h. After the mixture cooled to room temperature, white precipitates were collected by centrifugation. The solids were washed with DMF three times to remove unreacted precursors and then solvent exchanged with acetone three times. The resulting powder was obtained by centrifugation and dried in an oven at 70 °C. Elemental analysis (%) calcd for C13H11.6Zr1N2O5.3Br1 (desolvated): C, 34.56; N, 6.2; H, 2.59; Zr, 20.19; Br, 17.69. Found: C, 31.07; N, 4.88; H, 3.90; Zr, 22.16. IR characteristics (KBr, υmax, cm−1): 2966 (w), 1711 (s), 1598 (w), 1411 (m), 1205 (m), 1115 (s), 772 (s), 651 (s), 484 (m). Trying to Synthesize TCPP⊂(Br−)Etim-UiO-66 via de Novo Synthesis. ZrCl4 (28.8 mg, 0.12 mmol), (Br−)Etim-H2BDC (36 mg, 0.10 mmol), TCPP (2 mg or 4 mg or 6 mg), and acetic acid (0.2 or 0.34 mL) in 3 mL of DMF were ultrasonically dissolved in a 30 mL Pyrex vial. The mixture was heated at 120 °C for 12 h in an oven. After the mixture cooled to room temperature, purple and pale white precipitates were collected by centrifugation. The solids were washed with DMF three times and acetone three times. The resulting powder was obtained by centrifugation and dried in an oven at 70 °C. The PXRD suggested the coexistence of PCN-224 and (Br−)Etim-UiO-66. Trying to Synthesize TBPP⊂Im-UiO-66 via de Novo Synthesis. A process similar to that of TCPP⊂Im-UiO-66 was conducted, except that tetrakis(4-bromophenyl)porphyrin (TBPP) (4 mg or 8 mg) was used instead of TCPP. The resulting powder was obtained by centrifugation and dried in an oven at 70 °C. The PXRD and 1H NMR results indicated that pure Im-UiO-66 was obtained. IR characteristics (KBr, υmax, cm−1): 1656 (s), 1594 (s), 1500 (s), 1424 (s), 1388 (s), 1057 (s), 772 (s), 656 (s), 488 (s). Typical Procedures for Cyclic Carbonate from Allyl Glycidyl Ether and CO2. In a typical procedure, ZnTCPP⊂(Br−)Etim-UiO-66 (43 mg, equal to 0.0475 mmol ionic liquid (Br−)Etim-H2BDC) and 5 mmol allyl glycidyl ether (AGE) were placed in a 15 mL thick-walled reaction tube equipped with a magnetic stirrer. After being sealed, the tube was purged thrice with CO2. The reaction was carried out at 140 °C for 14 h and constant 1 bar CO2. The qualitative analysis of product was determined by GC-MS and quantitative analysis by GC. D
DOI: 10.1021/acs.inorgchem.7b02983 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry When homogeneous catalyst, such as (Br−)Etim-H2BDC, ZnTCPP/ (Br−)Etim-H2BDC, or (PF6−)Etim-H2BDC, was used, extraction process were conducted with distilled water/ethyl acetate, and a portion of the ethyl acetate mixture was diluted with ethyl acetate before GC analysis. Recyclability Test for ZnTCPP⊂(Br−)Etim-UiO-66. The recyclability test was conducted as follows: after reaction, the catalyst was separated by centrifugation and washed by ethyl acetate twice and by ether once. After being dried at 70 °C for 10 h in vacuum, the catalyst was directly used in the next catalytic reaction.
Table 2. Synthesis of Cyclic Carbonates from Epoxides and CO2 Catalyzed by ZnTCPP⊂(Br−)Etim-UiO-66a
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RESULTS AND DISCUSSION Synthesis and Characterization of MTV-MOFs. Following the synthetic route outlined in Scheme 1a, initially, Table 1. Catalytic Cycloaddition Reaction of Allyl Glycidyl Ether with CO2 by Different Catalystsa
entry
catalyst
conversion (%)b
selectivity (%)b
c
blank UiO-66 TCPP⊂Im-UiO-66 TCPP⊂(Br−)Etim-UiO-66 ZnTCPP⊂(Br−)Etim-UiO-66 ZnTCPP (Br−)Etim-H2BDC ZnTCPP/(Br−)Etim-H2BDC (PF6−)Etim-H2BDC
trace trace 64(5) 78(2) 77(4) trace 94(1) 98(1) 11(2)
− − 90 65d 92 − 100 100 100
1 2 3 4 5 6e 7f 8g 9h
a
Reaction conditions: solvent free, catalyst (0.95 mol % based on imidazolium), CO2 (constant 1 bar), 140 °C, 14 h. bDetermined by GC-MS.
a
Reaction conditions: solvent free, MOF catalyst (43 mg), CO2 (constant 1 bar), 140 °C, 6 h. bDetermined by GC-MS, an average value of three runs (error in parentheses). cNo catalyst was used. dAllyl derivatives as byproducts. eZnTCPP (0.0018 mmol). f(Br−)EtimH2BDC (0.0475 mmol). gZnTCPP (0.0018 mmol), (Br−)Etim-UiO66 (0.0475 mmol). h(PF6−)Etim-H2BDC (0.0475 mmol).
nitrogen in the protonated and unprotonated imidazole rings.50 After ionization, the N 1s XPS spectrum of TCPP⊂(Br−)EtimUiO-66 (Figure 2d) showed one main peak at 401.5 and two peaks at 400.0 and 398.6 eV, indicating the dominant role of imidazolium and residual imidazole groups near the surfaces. After metalation, signals in the N 1s XPS and Zn 2p XPS spectra of ZnTCPP⊂(Br−)Etim-UiO-66 also indicated the introduction of ZnTCPP into the ionic framework (Figure S3). In addition, compared with the 13C NMR spectrum of Im-UiO66, X⊂(Br−)Etim-UiO-66 (X= TCPP, ZnTCPP) samples showed new peaks at around 44 and 11 ppm, respectively, which were ascribed to the methylene and methyl carbon of the ethyl groups in these catalysts (Figure 2e). UV/vis spectra of X⊂(Br−)Etim-UiO-66 (X= TCPP, ZnTCPP) are shown in Figure 2f. The strong absorption bands at around 420 nm (Soret band) and 520−650 nm (four Q bands) clearly indicated the existence of the porphyrin chromophore in TCPP⊂(Br−)Etim-UiO-66. An apparent red shift to 431 nm in the Soret band (compared to 420 nm for TCPP⊂(Br−)ImUiO-66) further confirmed the insertion of Zn2+ into TCPP in ZnTCPP⊂(Br−)Etim-UiO-66. Furthermore, two instead of four Q bands were predominantly present because of the higher symmetry of the zinc porphyrin.32 These results indicated the successful synthesis of targeted ionic MOFs. To quantify the amount of functional Br− ions, Zn2+ in the corresponding catalysts, 1H NMR and CHN elemental analyses and ICP analysis were performed (Figures S4−S6). The 1H NMR analysis results revealed that cal. 58.8% of the imidazole groups for X⊂(Br−)Etim-UiO-66 (X= TCPP, ZnTCPP) were
solvothermal reactions of ZrCl4, Im-H2BDC·HCl·H2O, and 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP) in DMF at 120 °C for 12 h yielded brown powders (Figure S1). After washing with DMF and acetone, the product denoted as TCPP⊂Im-UiO-66 remained brown, indicating the successful incorporation of TCPP into the framework of ImUiO-66. The powder X-ray diffraction (PXRD) patterns of TCPP⊂Im-UiO-66 were consistent with those of phase-pure Im-UiO-66 27 and UiO-66 49 (Figure 2a), implying the formation of MTV-MOF composed of Im-H2BDC and TCPP. Subsequently, functional imidazolium moieties and zinc porphyrins were sequentially introduced into TCPP⊂ImUiO-66 via a postsynthetic ionization and metalation (PSIM) method, respectively, to obtain TCPP⊂(Br−)Etim-UiO-66 and ZnTCPP⊂(Br−)Etim-UiO-66 as depicted in Scheme 1. In addition to the consistent PXRD patterns of these MTV-MOFs (Figure 2a), the TEM and SEM images showed that the powders of TCPP⊂Im-UiO-66 remained uniform submicrospheres (100−300 nm) during the postsynthetic modification processes (Figures 2b and S2), which further confirmed the phase purity of each product. To verify the successful syntheses of TCPP⊂(Br−)Etim-UiO66 and ZnTCPP⊂(Br−)Etim-UiO-66, XPS, solid 13C NMR and UV/vis spectra analyses were conducted. The N 1s XPS spectrum of TCPP⊂Im-UiO-66 (Figure 2c) exhibited three peaks at 401.2, 400.0, and 398.5 eV, indicating the existence of E
DOI: 10.1021/acs.inorgchem.7b02983 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Scheme 2. Plausible Mechanism of ZnTCPP⊂(Br−)Etim-UiO-66 Catalyzed Cycloaddition Reaction of Epoxides and CO2a
a
The hashed bonds represent electrostatic interactions.
adsorption (Qst) of these MTV-MOFs from the corresponding adsorption data obtained at 273 and 298 K (22.8 kJ mol−1 for TCPP⊂Im-UiO-66, 29.3 kJ mol−1 for TCPP⊂(Br−)Etim-UiO66, and 30.9 kJ mol−1 for ZnTCPP⊂(Br−)Etim-UiO-66) (Figure 3d). The results indicated the high binding ability with CO2 of these ionic MTV-MOFs, which was attributed to the microporous structures and ionic surfaces.24,34 Discussion of the Synthesis of Ionic MTV-MOFs. To validate the advantages of the postsynthetic modification approach for constructing ionic MTV-MOF catalysts, the one-pot synthetic approaches have also been conducted (Scheme 1b,c). However, unfortunately, X⊂(Br−)Etim-UiO66 (X= TCPP, ZnTCPP) could not be obtained by direct synthesis based on (Br−)Etim-H2BDC ligand. Although the pure phase of (Br−)Etim-UiO-66 could be obtained (Figure S10), the addition of TCPP into the reaction system of (Br−)Etim-BDC− and Zr4+ led to the formation of mixed phases of PCN-224 and (Br−)Etim-UiO-66 (Figures S11 and S12). This was perhaps due to the bulky imidazolium groups on (Br−)Etim-BDC linkers and the high thermodynamic stability of PCN-224, which inhibited the incorporation of TCPP into (Br−)Etim-UiO-66 and facilitated the formation of PCN-224.29a On the other hand, all attempts to prepare ZnTCPP containing MTV-MOFs via direct synthesis failed because of the instability of ZnTCPP linkers under current conditions where acetic acid was used.29b To further confirm that TCPP incorporated into Im-UiO-66 via coordination to the Zr6 clusters, we used tetrakis(4bromophenyl)porphyrin (TBPP), with similar size to TCPP but without carboxylic acid groups, to replace ZnTCPP and TCPP to conduct contrast experiments. However, only white powders of pure Im-UiO-66 were obtained as characterized by PXRD (Figure S13). 1H NMR results for the digested sample of the product further revealed that TBPP was not incorporated (Figure S14). The difficulty in incorporating TBPP suggested
converted to imidazolium, based on the molar ratio of the (Br−)Etim-BDC2− and Im-BDC2− linkers (for details, see Figures S4−S6). Additionally, the Br loading was calculated as 1.1 mmol g−1 based on the N content measurement, and the Zn loading was approximately 0.4 mmol g−1 from the ICP results. Thus, the Br/Zn ratio was 2.75 and the ZnTCPP loading was 0.042 mmol g−1 in ZnTCPP⊂(Br−)Etim-UiO-66. Moreover, the energy dispersive X-ray (EDX) spectroscopy mapping images of the sample demonstrated a homogeneous distribution of the elements including N, Br, and Zn, implying the integration of ZnTCPP into the framework (Figure 3a). N2 and CO2 Sorptions. The textural properties of these MTV-MOFs were investigated by analyzing the nitrogen sorption isotherms obtained at 77 K. These MTV-MOFs showed rapid N2 uptake at low relative pressures (P/P0 < 0.1), which is typical Type I behavior for microporous materials. Accordingly, TCPP⊂Im-UiO-66 showed a Brunauer−Emmett−Teller (BET) surface area of 721 m2 g−1 and a pore volume of 0.44 cm3 g−1 and good CO2 adsorption uptakes of 5.92 wt % at room temperature (Figure 3b,c and Table S1). After the incorporation of ethyl groups and Br−, X⊂(Br−)EtimUiO-66 (X= TCPP, ZnTCPP) still showed moderate BET surface area and a total pore volume of 519 m2 g−1 and 0.33 cm3 g−1, 242 m2 g−1 and 0.26 cm3 g−1, respectively, and acceptable CO2 adsorption uptakes of 5.08 and 3.26 wt % at room temperature, which would be favorable for the conversion of CO2 (Figure 3b,c and Table S1). It can be understandable that the abundant ethyl groups, bromide anions, and zinc centers in the microporous structure might lead to the decreased surface area for the resultant ionic MTV-MOFs. Additionally, nonlocal density functional theory (NLDFT) revealed the pore size distribution (PSD) width ( TCPP⊂(Br−)Etim-UiO-66 > ZnTCPP⊂(Br−)Etim-UiO-66 (Figures S7−S9). Moreover, we calculated the isosteric heat of F
DOI: 10.1021/acs.inorgchem.7b02983 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
TCPP⊂(Br−)Etim-UiO-66 was due to its larger amount of Br− ions instead of Cl− (not detected) (Figure S3), while the reason for the lower selectivity remained unknown.52 To our delight, the ZnTCPP⊂(Br−)Etim-UiO-66 containing both Zn2+ sites and bromide ions led to an acceptable conversion (77%) and higher selectivity (92%) than that of TCPP⊂(Br−)Etim-UiO-66 (entries 4 and 5), although the former had lower surface area (Figure 3b). The enhanced catalytic performance for 3-allyloxy1,2-proplyene carbonate was tentatively attributed to the cooperative effect of Zn2+ and Br− ions in this microporous ionic MTV-MOF. In the presence of Lewis acidic site, the ringopening of the epoxide by Br− was perhaps more effectively accelerated by the metal coordination.22,53 On the other hand, we conducted homogeneous control experiments. While Lewis acidic ZnTCPP molecules alone showed very poor activity for this reaction (entry 6), 3-allyloxy-1,2-proplyene carbonate was obtained with 94% yield in the presence of (Br−)Etim-H2BDC (entry 7). Furthermore, the combination of ZnTCPP and (Br−)Etim-H2BDC led to excellent yield (98%) (entry 8), which led us to believe that the ZnTCPP/(Br−)Etim-H2BDC system was different from the simple (Br−)Etim-H2BDC system for this reaction.53a In addition, we found that the corresponding cyclic carbonate could be obtained with only 11% yield in the presence of (PF6−)Etim-H2BDC (entry 9), which indicated the crucial role of nucleophilic Br− ions for this reaction.54 Thus, ZnTCPP⊂(Br−)Etim-UiO-66 was chosen for further investigations. As a solid catalyst, ZnTCPP⊂(Br−)Etim-UiO-66 can be easily separated by centrifugation. Thus, to evaluate the catalyst life and stability of this ionic MTV-MOF, three recycled runs were carried out in the cycloaddition reaction of AGE with CO2. As shown in Figure S19, the catalyst still showed good activity (84% yield) after four runs. Then, the PXRD patterns (the first run) and UV−vis spectra of the recovered catalyst were in agreement with the synthesized one (Figures 2a and S20), indicating that ZnTCPP⊂(Br−)Etim-UiO-66 was chemically stable. However, it was physically unstable and decomposed gradually during the reaction process, as indicated by the PXRD patterns (the fourth run) and UV−vis spectra of the filtrate (Figures S21−S23). The scope of this catalytic CO2 cyclic addition by ionic MTV-MOF can be well extended to other substrates, and the results are shown in Table 2. Several typical epoxides could be efficiently converted into corresponding cyclic carbonates to give good yields (19.8−91.1%) by ZnTCPP⊂(Br−)Etim-UiO66 under optimized conditions. These results indicated that the catalyst ZnTCPP⊂(Br−)Etim-UiO-66 has a wide substrate scope. Based on the structural features of ZnTCPP⊂(Br−)EtimUiO-66 and reported work,33,34,54 a plausible cooperative activation mechanism was proposed, as shown in Scheme 2. Initially, an epoxide ring was adsorbed and stabilized by Lewis acid electrostatic interactions, such as zinc sites. Then the ringopening step occurred because of the activation effect of the Lewis acid sites and the nucleophilic attack by the Br− ion on the less hindered side of the epoxide. After that, an acyclic carbonate can be formed by the nucleophilic attack from the alcoholate toward CO2, which was converted to a carbonate by intramolecular cyclization with the liberation of the original catalyst. The catalytic mechanism for TCPP⊂(Br−)Etim-UiO66 was similar except that no zinc site was involved. These results indicated that the introduction of extra zinc sites
that TCPP containing carboxylates can readily participate in the coordination to the Zr6 clusters by partially substituting ImBDC or acetic acid during the Im-UiO-66 formation. It is generally recognized that UiO-66 derivatives can form defective frameworks under certain conditions (Figure S15).51 In our case, further analyses on the molecular size (about 1.8 nm) of TCPP and possible defective space (2.0 nm) generated by missing cluster in Im-UiO-66 indicated that the smaller TCPP can be geometrically incorporated into defective ImUiO-66 (Figure S16 and Table S2). Once TCPP is incorporated, the small pore windows of Im-UiO-66 will in turn prevent its leaking. Moreover, the formation mechanism of TCPP Im-UiO-66 was similar to that reported by Zhou et al.42 Although TCPP could competitively coordinate with the Zr6 cluster during the nucleation process of Im-UiO-66 and therefore participate in its growing process, the nucleation of Im-UiO-66 is still dominant because of the much higher concentration of Im-BDC than TCPP. As a result, Im-UiO-66 preserved its three-dimensional structures when the Zr6 cluster is partially occupied by TCPP, leaving some defects in the framework (Figure S17). Therefore, pure phase Im-UiO-66 could be obtained with TCPP participating in the coordination to Zr6 clusters. CO2 Cycloaddition Catalyzed by MTV-MOFs. Considering the coexistence of Lewis acidic Zn2+ sites and imidazolium bromide moieties in ZnTCPP⊂(Br−)Etim-UiO-66, we set out to investigate the catalytic CO2 cycloaddition reactions based on this ionic MTV-MOF. As zinc porphyrins and imidazolium bromide moieties were integrated together, there was no need to add any cocatalyst. In our initial tests on the chemical fixation of CO2 with epoxides, allyl glycidyl ether (AGE) was selected as a model substrate to explore the optimized reaction conditions (Table S3). First, when 0.3 mol % ZnTCPP⊂(Br−)Etim-UiO-66 was used in the reaction under 1 bar CO2 and solvent-free conditions, a yield of 5.4% for 3-allyloxy-1,2proplyene carbonate was obtained at 100 °C. Then, it was found that increasing the temperature would be beneficial for enhancing the yield without sacrificing selectivity (90%), but the yield was not more than 25% unless loading more catalyst (entries 2 and 3). As shown in entries 4 and 5, the yield can reach 57.5% and 71% when 0.64 and 0.95 mol % catalyst was used over 6 h, respectively. Furthermore, the time course of the reaction by using 0.95 mol % catalyst is shown in Figure S18. The reaction proceeded smoothly, and the yield reached 85.6% in 14 h under atmospheric pressure of CO2. Thus, taking into account the conversion and selectivity, the optimized reaction conditions should be 0.95 mol % catalyst ZnTCPP⊂(Br−)EtimUiO-66 at 140 °C. The CO2 cycloaddition with epoxides was performed under 1 atm pressure because ZnTCPP⊂(Br−)EtimUiO-66 has a good affinity toward CO2. We then compared the catalytic performances of these ionic MTV-MOFs in the cycloaddition reaction of AGE and CO2 at 140 °C for 6 h under solvent-free and 1 bar CO2. The results are shown in Table 1. First, it was noticed that the reaction could not occur without any catalyst (entry 1), and only a trace of AGE was converted with the existence of UiO-66 (entry 2). In contrast, 64% of AGE was converted when TCPP⊂Im-UiO66 was used as a catalyst with in situ protonated and unprotonated imidazole groups and Cl− ions (entry 3).52 As expected, a higher conversion of 78% was obtained when TCPP⊂(Br−)Etim-UiO-66 with bromide ions was used (entry 4). However, the selectivity decreased from 90% to 65% (entries 3 and 4). We deemed that the higher activity of G
DOI: 10.1021/acs.inorgchem.7b02983 Inorg. Chem. XXXX, XXX, XXX−XXX
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(6) Martín, C.; Fiorani, G.; Kleij, A. W. Recent Advances in the Catalytic Preparation of Cyclic Organic Carbonates. ACS Catal. 2015, 5, 1353. (7) Liang, J.; Huang, Y.-B.; Cao, R. Metal-organic frameworks and porous organic polymers for sustainable fixation of carbon dioxide into cyclic carbonates. Coord. Chem. Rev. 2017, DOI: 10.1016/ j.ccr.2017.11.013. (8) Xie, Y.; Wang, T. T.; Liu, X. H.; Zou, K.; Deng, W. Q. Capture and conversion of CO2 at ambient conditions by a conjugated microporous polymer. Nat. Commun. 2013, 4, 1960−1966. (9) Ji, G.; Yang, Z.; Zhang, H.; Zhao, Y.; Yu, B.; Ma, Z.; Liu, Z. Hierarchically Mesoporouso-Hydroxyazobenzene Polymers: Synthesis and Their Applications in CO2 Capture and Conversion. Angew. Chem., Int. Ed. 2016, 55, 9685−9689. (10) Lin, S.; Diercks, C. S.; Zhang, Y.-B.; Kornienko, N.; Nichols, E. M.; Zhao, Y.; Paris, A. R.; Kim, D.; Yang, P.; Yaghi, O. M.; Chang, C. J. Covalent organic frameworks comprising cobalt porphyrins for catalytic CO2 reduction in water. Science 2015, 349, 1208−1213. (11) Han, Y.-H.; Zhou, Z.-Y.; Tian, C.-B.; Du, S.-W. A dual-walled cage MOF as an efficient heterogeneous catalyst for the conversion of CO2 under mild and co-catalyst free conditions. Green Chem. 2016, 18, 4086−4091. (12) Li, P. Z.; Wang, X. J.; Liu, J.; Lim, J. S.; Zou, R.; Zhao, Y. A Triazole-Containing Metal-Organic Framework as a Highly Effective and Substrate Size-Dependent Catalyst for CO2 Conversion. J. Am. Chem. Soc. 2016, 138, 2142−2145. (13) Liu, X.-H.; Ma, J.-G.; Niu, Z.; Yang, G.-M.; Cheng, P. An Efficient Nanoscale Heterogeneous Catalyst for the Capture and Conversion of Carbon Dioxide at Ambient Pressure. Angew. Chem., Int. Ed. 2015, 54, 988−991. (14) He, H.; Perman, J. A.; Zhu, G.; Ma, S. Metal-Organic Frameworks for CO2 Chemical Transformations. Small 2016, 12 (46), 6309−6324. (15) Corma, A.; García, H.; Llabrés i Xamena, F. X. Engineering Metal Organic Frameworks for Heterogeneous Catalysis. Chem. Rev. 2010, 110, 4606−4655. (16) Liu, J.; Chen, L.; Cui, H.; Zhang, J.; Zhang, L.; Su, C.-Y. Applications of metal-organic frameworks in heterogeneous supramolecular catalysis. Chem. Soc. Rev. 2014, 43, 6011−6061. (17) Nath, I.; Chakraborty, J.; Verpoort, F. Metal organic frameworks mimicking natural enzymes: a structural and functional analogy. Chem. Soc. Rev. 2016, 45, 4127−4170. (18) Howarth, A. J.; Liu, Y.; Li, P.; Li, Z.; Wang, T. C.; Hupp, J. T.; Farha, O. K. Chemical, thermal and mechanical stabilities of metal− organic frameworks. Nat. Rev. Mats. 2016, 1, 15018. (19) Chughtai, A. H.; Ahmad, N.; Younus, H. A.; Laypkov, A.; Verpoort, F. Metal-organic frameworks: versatile heterogeneous catalysts for efficient catalytic organic transformations. Chem. Soc. Rev. 2015, 44, 6804−6849. (20) Gao, W.-Y.; Wu, H.; Leng, K.; Sun, Y.; Ma, S. Inserting CO2 into Aryl C-H Bonds of Metal−Organic Frameworks: CO2 Utilization for Direct Heterogeneous C-H Activation. Angew. Chem., Int. Ed. 2016, 55, 5472−5476. (21) Trickett, C. A.; Helal, A.; Al-Maythalony, B. A.; Yamani, Z. H.; Cordova, K. E.; Yaghi, O. M. The chemistry of metal-organic frameworks for CO2 capture, regeneration and conversion. Nat. Rev. Mats. 2017, 2, 17045−17060. (22) (a) Ema, T.; Miyazaki, Y.; Shimonishi, J.; Maeda, C.; Hasegawa, J.-Y. Bifunctional Porphyrin Catalysts for the Synthesis of Cyclic Carbonates from Epoxides and CO2: Structural Optimization and Mechanistic Study. J. Am. Chem. Soc. 2014, 136, 15270−15279. (b) Maeda, C.; Taniguchi, T.; Ogawa, K.; Ema, T. Bifunctional Catalysts Based onm-Phenylene-Bridged Porphyrin Dimer and Trimer Platforms: Synthesis of Cyclic Carbonates from Carbon Dioxide and Epoxides. Angew. Chem., Int. Ed. 2015, 54, 134−138. (c) Maeda, C.; Shimonishi, J.; Miyazaki, R.; Hasegawa, J.; Ema, T. Highly Active and Robust Metalloporphyrin Catalysts for the Synthesis of Cyclic Carbonates from a Broad Range of Epoxides and Carbon Dioxide. Chem. - Eur. J. 2016, 22, 6556−6563.
possibly enhanced the catalytic selectivity with a dual activation mode.
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CONCLUSION In summary, we have successfully prepared a family of ionic multivariate metal−organic frameworks incorporating porphyrin/metalloporphyrin and imidazolium moieties via a sequential postsynthetic ionization and metalation strategy for the first time. It was found that the structural features and CO2 affinity property of these MTV-MOFs can be tuned by introducing imidazolium groups or zinc ion doping. As a representative of ionic MTV-MOF catalysts, ZnTCPP⊂(Br−)Etim-UiO-66 exhibited enhanced catalytic performance for the cyclic carbonate from AGE and CO2 under solvent-free and 1 atm conditions. The enhanced catalytic performance was attributed to the cooperative effect of Lewis acidic Zn2+ sites and Br− ions in this microporous ionic MTV-MOF. Various expoxides can be catalyzed by ZnTCPP⊂(Br−)Etim-UiO-66. The porphyrin containing ionic MTV-MOFs studied herein may also find other applications, such as photocatalysis.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02983. Experimental data and photographs (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Yuan-Biao Huang: 0000-0003-4680-2976 Rong Cao: 0000-0003-2384-791X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support from the 973 Program (2014CB845605); Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000); NSFC (21671188, 21521061, and 21331006); Key Research Program of Frontier Science, CAS (QYZDJ-SSW-SLH045); and Youth Innovation Promotion Association, CAS (2014265).
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DOI: 10.1021/acs.inorgchem.7b02983 Inorg. Chem. XXXX, XXX, XXX−XXX
