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Porous metal-organic framework with Lewis acid−base bifunctional sites for high efficient CO2 adsorption and catalytic conversion to cyclic carbonates
Inorganic Chemistry Communications 106 (2019) 70–75
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Porous metal-organic framework with Lewis acid−base bifunctional sites for high efficient CO2 adsorption and catalytic conversion to cyclic carbonates Jing Lia,1, Wen-Jun Lib,1, Shi-Cheng Xuc, Bo Lic, Ying Tangd,e, Zhen-Fang Linf,
T
⁎
a
School of Chemical Engineering, Xi'an University, Xi'an 710065, China Prenatal Diagnosis Center, The Seventh People's Hospital of Chengdu, Chengdu, Sichuan, China c Department of Neurology, Bazhong Central Hospital, Bazhong, Sichuan, China d Shaanxi Province Key Laboratory of Environmental Pollution Control and Reservoir Protection Technology of Oilfields, Xi'an Shiyou University, Xi'an 710065, China e State Key Laboratory of Petroleum Pollution Control, CNPC Research Institute of Safety and Environmental Technology, Beijing 102206, China f Department of Neurology, Affiliated Sichuan Provincial Rehabilitation Hospital of Chengdu University of TCM, Sichuan Bayi Rehabilitation Center, Chengdu, Sichuan, China b
GRAPHICAL ABSTRACT
A highly porous MOF incorporating both exposed metal sites and nitrogen-rich melamine was successfully constructed via solvothermal reaction. The obtained Ni(II)MOF-1 features high CO2 loading capacity and excellent CO2 affinity due to the Lewis-base property together with abundant micropores. The heterogeneous catalytic activity of the activated Ni(II)-MOF-1 was confirmed by remarkably high efficiency on CO2 cycloaddition with small epoxides under ambient conditions. Moreover, the Ni(II)-MOF-1 exhibited satisfied stability and versatility, and it was easy to recycle with no obvious decrease of catalytic activity. Then the feasible synergistic mechanism of Ni(II)-MOF/Bu4NBr catalysts for CO2 conversion was proposed.
ARTICLE INFO
ABSTRACT
Keywords: Metal-organic framework Bifunctional sites CO2 sorption CO2 conversion
A highly porous three-dimensional (3D) metal−organic framework (MOF) [Ni3(BTC)2(MA)(H2O)](DMF)7 (1) incorporating both exposed metal sites and nitrogen-rich melamine was successfully constructed via solvothermal assembly of 1,3,5-benzenetricarboxylic acid (H3BTC), melamine (MA) and Ni(II) ions. The resulting activated Ni(II)-MOF-1a features high CO2 loading capacity and excellent CO2 affinity due to the Lewis-base property together with abundant micropores. The heterogeneous catalytic activity of the activated Ni(II)-MOF1a was confirmed by remarkably high efficiency on CO2 cycloaddition with small epoxides under ambient conditions. Moreover, the Ni(II)-MOF-1a exhibited satisfied stability and versatility, and it was easy to recycle with no obvious decrease of catalytic activity. Then the feasible synergistic mechanism of Ni(II)-MOF/Bu4NBr catalysts for CO2 conversion was proposed.
Corresponding author. E-mail address: [email protected] (Z.-F. Lin). 1 These authors contributed equally to this work. ⁎
https://doi.org/10.1016/j.inoche.2019.05.031 Received 24 April 2019; Received in revised form 14 May 2019; Accepted 27 May 2019 Available online 28 May 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 106 (2019) 70–75
J. Li, et al.
The rapid development of petrol and automobile industries has brought about massive accumulating of CO2 in atmosphere and the large-scale consumption of natural gas, which is the main case for the global warming [1,2]. The development of CO2 capture and sequestration (CCS) technologies is imperative to selectively capture CO2 from existing emission sources [3]. Besides the physical adsorption and permanent underground deposition of CO2, an alternative and more attractive strategy for addressing anthropogenic CO2 emission issues is catalytically chemical conversion of CO2 into value-added chemicals and materials, so that the emitted CO2 can be reused in the carbon recycling on the earth [4]. A promising strategy for CO2 fixation is coupling with epoxides to form five-membered cyclic carbonates, which are currently used as valuable chemicals in numerous applications including green nonprotonic solvent, precursors for polymer, intermediates of medicines and pesticides, electrolytes of lithium-ion battery and so on. Although some homogeneous catalysts have been used for the formation of cyclic carbonates in industry under mild conditions, catalyst separation and disposal present both environmental and economic drawbacks [5,6]. Metal-organic frameworks (MOFs), a special type of porous materials consisting of metal clusters and organic linkers, might be the most promising candidates to satisfy the dual challenge of CO2 capture and conversion [7–9]. They possess large surfaces and well-ordered porous structure and, most importantly, could be designed to selectively capture CO2 via selection of suitable building blocks. Many strategies are employed to reinforce the CO2 capture ability of MOFs. For example, the creation of unsaturated metal sites and decoration of the pores with basic molecules at the open metal sites and the basic pore surface could result in effective enhancement of the CO2 adsorption [10–13]. Recently, some MOFs with excellent CO2 adsorption capacity have been applied in the CO2 conversion. For instance, the ZIF-67 and ZIF-90 have been developed and presented excellent catalytic performance under
high temperatures [14,15]. MIL-101 and MOF-505 exhibited moderate catalytic performance under room temperature for over 48 h [9]. To meet the requirement of industrialization for conversion of CO2 into cyclic carbonates, more effective MOF catalysts need to be developed for proceeding the reaction under milder conditions, especially at low temperature to further reduce the energy consumption and production cost. Pioneering theoretical and experimental results have proved that the synergistic effects of Lewis acid − base sites and the assistance of nucleophile are crucial for CO2 cycloaddition to epoxide, wherein epoxide substrate and CO2 species are activated by Lewis acid and Lewis base, respectively, then nucleophile helps promote the ring-opening reaction of epoxide [16–18]. Moreover, the existence of accessible hydroxyl group and nitrogen-rich units in the structures of porous MOFs help to further improve the affinity of CO2 molecule to the catalysts. Taking these into account, in this study, a highly porous three-dimensional (3D) metal−organic framework (MOF) [Ni3(BTC)2(MA)(H2O)] (DMF)7 (1) incorporating both exposed metal sites and nitrogen-rich melamine was successfully constructed via solvothermal assembly of 1,3,5-benzenetricarboxylic acid, melamine and Ni(II) ions. The resulting activated Ni(II)-MOF-1a features high CO2 loading capacity and excellent CO2 affinity due to the Lewis-base property together with abundant micropores. The heterogeneous catalytic activity of the activated Ni(II)-MOF-1a was confirmed by remarkably high efficiency on CO2 cycloaddition with small epoxides under ambient conditions. Moreover, the Ni(II)-MOF-1a exhibited satisfied stability and versatility, and it was easy to recycle with no obvious decrease of catalytic activity. Then the feasible synergistic mechanism of Ni(II)-MOF/ Bu4NBr catalysts for CO2 conversion was proposed. Single crystal X-ray diffraction analysis reveals that the Ni(II)-MOF1 crystallizes in the tetragonal space group I4/m and displays a 3D framework with two types of channels with different window sizes running along the [0 0 1] directions. There is one and a half Ni(II)
Fig. 1. (a) The {Ni3(MA)(COO)6(H2O)2} cluster in 1; (b) The cuboctahedral cage and the compressed octahedral cage in 1; (c) View for the two types of the 1D channels in 1; (d) The solvent accessible volume of 1 at the probe radius of 1.8 Å. 71
Inorganic Chemistry Communications 106 (2019) 70–75
J. Li, et al.
cations, one BTC3− ligand, a half MA ligand and one coordinated water molecule in the asymmetric unit. The Ni1 atom is located in a slightly disordered octahedral coordination environment and connected with four carboxyl O atoms from four different BTC3− ligand, one N atom from the MA ligand and one coordinated water (Fig. 1a); The tetrahedral coordination environment of Ni2 atom is completed by three carboxyl O atoms from three different BTC3− ligands and one N atom from the MA ligand. The average distances of Ni−N and Ni−O are 2.03 and 2.07 Å, respectively, which agree with the results in previous reports. Two four-coordinated Ni2 atoms and one six-coordinated Ni1 atoms are held together via the MA ligand and six carboxyl groups from the BTC3− ligands to afford the {Ni3(MA)(COO)6(H2O)2} fragment with the Ni1-Ni2 distance of 3.718 Å. The BTC3− ligand adopts a μ3-η1: η1: η1: η1: η1: η0 bridging mode to coordinate with five Ni(II) centers in three {Ni3(MA)(COO)6(H2O)2} fragments in which two carboxyl groups are in the bidentate mode and one carboxyl group is in the monodentate mode. There are two types of polyhedral cages in the Ni(II)-MOF-1, i.e. one is the cuboctahedral cage composed of 12 {Ni3(MA)(COO)6(H2O)2} fragments as vertices and 32 BTC3− ligands as edges (Fig. 1b). The pore diameter amounts to approximately 9.6 Å. The other is the compressed octahedral cage that shaped by 6 {Ni3(MA)(COO)6(H2O)2} fragments as vertices and 6 BTC3− ligands as edges whose inner diameter is 7.2 Å. The packing of the two types of cages in the three-dimensional direction results in the formation of a 3-dimensional non-interpenetrated network. It is noteworthy that in the framework of the Ni-MOF-1 there is coexistence of two types of one-dimensional channels (Fig. 1c). The channel A with a pore aperture of 10.62 Å fills with water coordination unsaturated Ni(II) centers, and the channel B with a pore aperture of 6.45 Å fills with open O donors. For a clear description of the framework, the {Ni3(MA)(COO)6(H2O)2} cluster could be considered as a 6connected node and the BTC3− ligand could be judged as a 3-connected node, so the overall 3D network of 1 can be rationalized as a (3,6)connected topology with the point symbol of {4^2.6}2{4^4.6^2.8^9} (Fig. S1). Fig. S2 shows the PXRD patterns of prepared Ni-MOF-1, the results were well matched with the simulated single crystal pattern based on the diffraction data, which confirmed the structural integrity and phase purity of the synthesized Ni-MOF-1 sample. Furthermore, no obvious loss of crystallinity for Ni-MOF-1 was found after being soaked in water for 12 h. The high water stability of Ni-MOF-1 may benefit from the increased coordinative bond strength between the Ni2+ ions and the μ3chelating melamine ligand. In addition, the TGA analysis was conducted to detect the thermal stability and skeleton construction of NiMOF-1 (Fig. S3). The Ni-MOF-1 shows a continues weight loss of about 42.2% from room temperature to 386 °C without obvious plateau, which could be attributed to the loss of one coordinated water and
seven DMF molecules in the crystal lattice (calcd: 42.5%). To remove the high boiling point DMF molecules, 100 mg of as-prepared samples of Ni-MOF-1 were soaked in 15 mL of anhydrous methanol in a 20 mL of glass vial for three days, during which the solution was replaced with fresh MeOH every 12 h. The solvent-exchanged MOF showed a weight loss of ∼16% in the range of 50–200 °C due to loss of guest MeOH molecules and one coordinated water molecule. After a long plateau till 381 °C, a sharp weight loss was observed, indicating the collapse of the framework. Whereas activated Ni-MOF-1a prepared by heating the MeOH exchanged Ni-MOF-1 at 393 K for 12 h under dynamic vacuum does not show any weight loss in the temperature range of 55–351 °C, supporting the absence of guest solvents and the coordinated water molecules. The permanent porosity of activated Ni-MOF-1a was proven by N2 adsorption isotherms at 77 K, which exhibited type-I adsorption isotherm behavior with the maximum uptake capacity of 411 cm3/g, evidencing its microporous feature (Fig. 2a). The calculations based on the N2 sorption isotherm at 77 K with the nonlocal density functional theory were also carried out to reveal that its pore size distribution mainly centers at 6.2 and 10.1 Å, which are consistent with the observation from its crystal structure. Furthermore, CO2 adsorption isotherms of Ni-MOF-1a carried out at 273 K and 298 K follow a typical type-I profile with the volumetric uptake of 9.8 and 5.9 mmol/g at 1 bar, respectively (Fig. 2b). The observed CO2 uptake capacity of the Ni-MOF-1a is comparable to the analogues Cu-MOF reported by Gao and co-workers [19]. The corresponding isosteric heats of CO2 adsorption (Qst) at zero coverage were calculated to be 33 kJ/mol, by fitting the CO2 isotherms at 273 and 298 K with the virial method, which reflects a very strong interaction between the framework and the Ni-MOF-1a (Fig. S5) [20]. As shown in the Fig. 1c, the nano-sized channels of Ni-MOF-1 are decorated with Ni(II) sites with coordinated water molecules, which could afford the unsaturated, Lewis acidic Ni(II) sites by activation. Motivated by the high CO2 uptake capacity and high density of open metal sites as well as uncoordinated nitrogen atoms, we envisioned that the activated MOF can act as efficient heterogeneous catalysts for cycloaddition of CO2 to afford the cyclic carbonates. Therefore, the catalytic activity of Ni-MOF-1a was investigated for cycloaddition of CO2 and styrene oxide as a model substrate at the condition of 0.1 MPa CO2 and 60 °C in the presence of 1 mol% TBAB as cocatalyst, and the results are summarized in the Table 1. It can be seen that the yield for styrene carbonate was very low when the catalyst or TBAB was absent in the system under the employed reaction conditions. While the test on the combination of Ni-MOF-1a and TBAB as a binary catalyst it was found that it can serve as an excellent catalyst for the synthesis of styrene carbonate with a yield of 94% over 6 h. Thus, both Ni-MOF-1a and TBAB perform a critical role and are indispensable in the reaction. It is
Fig. 2. (a) The N2 sorption isotherm of the Ni(II)-MOF-1a at 77 K; (b) The CO2 sorption isotherm of the Ni(II)-MOF-1a around room temperature. 72
Inorganic Chemistry Communications 106 (2019) 70–75
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Table 1 Cycloaddition of CO2 and styrene using different conditions.
Catalyst
Cocatalyst
None Ni-MOF-1a None Ni-MOF-1a Ni-MOF-1a
none none Bu4NBr Bu4NBr Bu4NBr
Time (h)
Temp (°C)
6 6 6 6 24
60 60 60 60 25
Yield (%) <5 8 18 94 92
TON – 3.2 – 37.6 36.8
TOF – 0.53 – 6.3 1.5
Reaction conditions: 20 mmol styrene oxide, 0.05 mol% Ni-MOF-1a, and 1 mol% TBAB (nBu4NBr) under 0.1 MPa CO2 at the given temperature. The yields were determined by GC–MS. Table 2 Cycloaddition of CO2 and various epoxides under the given conditions.
Entry
Epoxide
Product
Yield
TON
TOF
1
99%
39.6
6.6
2
97%
38.8
6.5
3
88%
35.2
5.9
4
95%
38
6.3
Reaction conditions: epoxide (20 mmol), Ni-MOF-1a (0.05 mol%), and TBAB (1 mol%) under 0.1 MPa CO2, 60 °C (25 °C for propylene oxide), and 6 h. The percent yields were determined by GC − MS.
known that TBAB acts as a nucleophilic cocatalyst and facilitates the ring opening of the epoxides [9]. It should also be noted that the reaction yield catalyzed by Ni-MOF-1a from styrene carbonate is higher than that of the benchmark Cu(II)–carboxylate MOF, HKUST-1, which showed a moderate yield [21]. Given similar pore size and the same embedded Lewis acid metal sites in both Ni-MOF-1a and HKUST-1, the high catalytic activity of Ni-MOF-1a should be ascribed to the increase of the CO2 affinity via the introduction of the nitrogen-rich triazole groups into the framework. Then, we extended this work to various epoxides under similar conditions in order to check the generality for such CO2 cycloaddition reactions (Table 2). Owing to its nanoscale
channels, the Ni-MOF-1a also exhibited very high activities for the substrates of different sizes, to the large substrates, cyclohexene oxide and benzyl phenylglycidyl ether. Thus, Ni-MOF-1a is a suitable heterogeneous catalyst for carbon fixation. The stability and recyclability of the catalyst are very important for heterogeneous catalysis. The recycling experiments were carried out after the catalyst recovered from the cycloaddition reaction by thorough a washing and drying process. Compared to the freshly prepared Ni-MOF-1a, its PXRD pattern after catalysis showed no change in the peak intensity and position, which indicated the stability of Ni-MOF-1a. The catalytic activity of Ni-MOF1a still remained equal to the level of that of the freshly prepared during 73
Inorganic Chemistry Communications 106 (2019) 70–75
J. Li, et al.
Fig. 3. (a) Proposed mechanism for the cycloaddition of CO2 catalyzed by Ni-MOF-1a/TBAB.
four cycles of recyclability tests for the cycloaddition of CO2 with propylene oxide (Fig. S6). Based on the previously reported literature and the crystal structure of Ni-MOF-1, an assumptive mechanism for the catalytic process was proposed [22]. First, the Lewis acid open Ni sites activated the epoxide by forming Ni–O adduct. Then the Br− of the TBAB acted as nucleophile to attack the β‑carbon atom with less sterically hindered C of epoxide to facilitate ring-opening reaction. Simultaneously, CO2 molecule was activated by Lewis base of amine groups with the formation of a carbamate salt, then which was attacked by the ring-opened epoxide as a nucleophilic. Finally, the ring-closure proceeded with the formation of cyclic carbonate, and the catalyst was regenerated for the next cycle (Fig. 3). In conclusion, we have successfully prepared a new porous Ni(II)MOF with Lewis acid − base bifunctional sites via solvothermal assembly of 1,3,5-benzenetricarboxylic acid, melamine and Ni(II) ions. The resulting activated Ni(II)-MOF-1a possessed abundant micropore, high density of open metal sites along with nitrogen-rich units, and showed excellent CO2 adsorption property around room temperature. Furthermore, the Ni(II)-MOF-1 also presented excellent catalytic performance cooperating with Bu4NBr for the cycloaddition of CO2 to epoxides under mild conditions, even at ambient temperature. Additionally, the Ni(II)-MOF-1 catalyst also showed good versatility and recyclability, which indicate its potential use in the in the practical utilization of CO2 to valuable chemicals.
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223–336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:https://doi.org/10.1016/j.inoche.2019.05.031. References [1] W.-M. Liao, M.-J. Wei, J.-T. Mo, P.-Y. Fu, Y.-N. Fan, M. Pan, C.-Y. Su, Acidity and Cd2+ fluorescent sensing and selective CO2 adsorption by a water-stable Eu-MOF, Dalton. Trans. 48 (2019) 4489–4494. [2] W.-M. Liao, J.-H. Zhang, Z. Wang, Y.-L. Lu, S.-Y. Yin, H.-P. Wang, Y.-N. Fan, M. Pan, C.-Y. Su, Semiconductive amine-functionalized Co(II)-MOF for visible-light-driven hydrogen evolution and CO2 reduction, Inorg. Chem. 57 (2018) 11436–11442. [3] E.S. Rubin, C. Chen, A.B. Rao, Cost and performance of fossil fuel power plants with CO2 capture and storage, Energy Policy. 35 (2007) 4444–4454. [4] W.-M. Liao, J.-H. Zhang, Y.-J. Hou, H.-P. Wang, M. Pan, Visible-light-driven CO2 photo-catalytic reduction of Ru(II) and Ir(III) coordination complexes, Inorg. Chem. Commun. 73 (2016) 80–89. [5] W.J. Kruper, D.D. Dellar, Catalytic formation of cyclic carbonates from epoxides and CO2 with chromium metalloporphyrinates, J. Org. Chem. 60 (1995) 725–727. [6] H. Yasuda, L.-N. He, T. Sakakura, Cyclic carbonate synthesis from supercritical carbon dioxide and epoxide over lanthanide oxychloride, J. Catal. 209 (2002) 547–550. [7] J. Song, Z. Zhang, S. Hu, T. Wu, T. Jiang, B. Han, MOF-5/n-Bu4NBr: an efficient catalyst system for the synthesis of cyclic carbonates from epoxides and CO2 under mild conditions, Green Chem. 11 (2009) 1031. [8] J. Kim, S.-N. Kim, H.-G. Jang, G. Seo, W.-S. Ahn, CO2 cycloaddition of styrene oxide over MOF catalysts, Appl. Catal. A Gen. 453 (2013) 175–180. [9] W.-Y. Gao, Y. Chen, Y. Niu, K. Williams, L. Cash, P.J. Perez, L. Wojtas, J. Cai, Y.S. Chen, S. Ma, Crystal engineering of an nbo topology metal-organic framework for chemical fixation of CO under ambient conditions, Angew. Chemie Int. Ed. 53 (2014) 2615–2619. [10] X. Zhang, H.-L. Liu, D.-S. Zhang, L. Geng, A multifunctional anionic 3D Cd(II)-MOF derived from 2D layers catenation: Organic dyes adsorption, cycloaddition of CO2 with epoxides and luminescence, Inorg. Chem. Commun. 101 (2019) 184–187. [11] L. Wang, R. Zou, W. Guo, S. Gao, W. Meng, J. Yang, X. Chen, R. Zou, A new microporous metal-organic framework with a novel trinuclear nickel cluster for selective CO2 adsorption, Inorg. Chem. Commun. 104 (2019) 78–82. [12] D.-M. Chen, N.-N. Zhang, C.-S. Liu, M. Du, Template-directed synthesis of a luminescent Tb-MOF material for highly selective Fe3+ and Al3+ ion detection and VOC vapor sensing, J. Mater. Chem. C. 5 (2017) 2311–2317. [13] X. Mu, C. Liu, N. Zhao, J. Liu, F. Sun, Synthesis, structures and selective adsorption properties of two novel zinc(II) metal-organic frameworks based on a tetrazolate ligand, Inorg. Chem. Commun. 102 (2019) 256–261. [14] B. Mousavi, S. Chaemchuen, B. Moosavi, Z. Luo, N. Gholampour, F. Verpoort,
Acknowledgements This work was supported by the Shaanxi Province Department of Education (17JK1117). Appendix A. Supplementary material CCDC (1) contains the supplementary crystallographic data for the title compound. This data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge 74
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(II)-based metal-organic framework with open-metal sites and basic groups, Inorg. Chem. Commun. 102 (2019) 199–202. [20] D.-M. Chen, J.-Y. Tian, M. Chen, C.-S. Liu, M. Du, Moisture-stable Zn(II) metal–organic framework as a multifunctional platform for highly efficient CO2 capture and nitro pollutant vapor detection, ACS Appl. Mater. Interfaces. 8 (2016) 18043–18050. [21] E.E. Macias, P. Ratnasamy, M.A. Carreon, Catalytic activity of metal organic framework Cu3(BTC)2 in the cycloaddition of CO2 to epichlorohydrin reaction, Catal. Today. 198 (2012) 215–218. [22] R. Babu, A.C. Kathalikkattil, R. Roshan, J. Tharun, D.-W. Kim, D.-W. Park, Dualporous metal organic framework for room temperature CO2 fixation via cyclic carbonate synthesis, Green Chem. 18 (2016) 232–242.
75
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Porous metal-organic framework with Lewis acid−base bifunctional sites for high efficient CO2 adsorption and catalytic conversion to cyclic carbonates Jing Lia,1, Wen-Jun Lib,1, Shi-Cheng Xuc, Bo Lic, Ying Tangd,e, Zhen-Fang Linf,
T
⁎
a
School of Chemical Engineering, Xi'an University, Xi'an 710065, China Prenatal Diagnosis Center, The Seventh People's Hospital of Chengdu, Chengdu, Sichuan, China c Department of Neurology, Bazhong Central Hospital, Bazhong, Sichuan, China d Shaanxi Province Key Laboratory of Environmental Pollution Control and Reservoir Protection Technology of Oilfields, Xi'an Shiyou University, Xi'an 710065, China e State Key Laboratory of Petroleum Pollution Control, CNPC Research Institute of Safety and Environmental Technology, Beijing 102206, China f Department of Neurology, Affiliated Sichuan Provincial Rehabilitation Hospital of Chengdu University of TCM, Sichuan Bayi Rehabilitation Center, Chengdu, Sichuan, China b
GRAPHICAL ABSTRACT
A highly porous MOF incorporating both exposed metal sites and nitrogen-rich melamine was successfully constructed via solvothermal reaction. The obtained Ni(II)MOF-1 features high CO2 loading capacity and excellent CO2 affinity due to the Lewis-base property together with abundant micropores. The heterogeneous catalytic activity of the activated Ni(II)-MOF-1 was confirmed by remarkably high efficiency on CO2 cycloaddition with small epoxides under ambient conditions. Moreover, the Ni(II)-MOF-1 exhibited satisfied stability and versatility, and it was easy to recycle with no obvious decrease of catalytic activity. Then the feasible synergistic mechanism of Ni(II)-MOF/Bu4NBr catalysts for CO2 conversion was proposed.
ARTICLE INFO
ABSTRACT
Keywords: Metal-organic framework Bifunctional sites CO2 sorption CO2 conversion
A highly porous three-dimensional (3D) metal−organic framework (MOF) [Ni3(BTC)2(MA)(H2O)](DMF)7 (1) incorporating both exposed metal sites and nitrogen-rich melamine was successfully constructed via solvothermal assembly of 1,3,5-benzenetricarboxylic acid (H3BTC), melamine (MA) and Ni(II) ions. The resulting activated Ni(II)-MOF-1a features high CO2 loading capacity and excellent CO2 affinity due to the Lewis-base property together with abundant micropores. The heterogeneous catalytic activity of the activated Ni(II)-MOF1a was confirmed by remarkably high efficiency on CO2 cycloaddition with small epoxides under ambient conditions. Moreover, the Ni(II)-MOF-1a exhibited satisfied stability and versatility, and it was easy to recycle with no obvious decrease of catalytic activity. Then the feasible synergistic mechanism of Ni(II)-MOF/Bu4NBr catalysts for CO2 conversion was proposed.
Corresponding author. E-mail address: [email protected] (Z.-F. Lin). 1 These authors contributed equally to this work. ⁎
https://doi.org/10.1016/j.inoche.2019.05.031 Received 24 April 2019; Received in revised form 14 May 2019; Accepted 27 May 2019 Available online 28 May 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 106 (2019) 70–75
J. Li, et al.
The rapid development of petrol and automobile industries has brought about massive accumulating of CO2 in atmosphere and the large-scale consumption of natural gas, which is the main case for the global warming [1,2]. The development of CO2 capture and sequestration (CCS) technologies is imperative to selectively capture CO2 from existing emission sources [3]. Besides the physical adsorption and permanent underground deposition of CO2, an alternative and more attractive strategy for addressing anthropogenic CO2 emission issues is catalytically chemical conversion of CO2 into value-added chemicals and materials, so that the emitted CO2 can be reused in the carbon recycling on the earth [4]. A promising strategy for CO2 fixation is coupling with epoxides to form five-membered cyclic carbonates, which are currently used as valuable chemicals in numerous applications including green nonprotonic solvent, precursors for polymer, intermediates of medicines and pesticides, electrolytes of lithium-ion battery and so on. Although some homogeneous catalysts have been used for the formation of cyclic carbonates in industry under mild conditions, catalyst separation and disposal present both environmental and economic drawbacks [5,6]. Metal-organic frameworks (MOFs), a special type of porous materials consisting of metal clusters and organic linkers, might be the most promising candidates to satisfy the dual challenge of CO2 capture and conversion [7–9]. They possess large surfaces and well-ordered porous structure and, most importantly, could be designed to selectively capture CO2 via selection of suitable building blocks. Many strategies are employed to reinforce the CO2 capture ability of MOFs. For example, the creation of unsaturated metal sites and decoration of the pores with basic molecules at the open metal sites and the basic pore surface could result in effective enhancement of the CO2 adsorption [10–13]. Recently, some MOFs with excellent CO2 adsorption capacity have been applied in the CO2 conversion. For instance, the ZIF-67 and ZIF-90 have been developed and presented excellent catalytic performance under
high temperatures [14,15]. MIL-101 and MOF-505 exhibited moderate catalytic performance under room temperature for over 48 h [9]. To meet the requirement of industrialization for conversion of CO2 into cyclic carbonates, more effective MOF catalysts need to be developed for proceeding the reaction under milder conditions, especially at low temperature to further reduce the energy consumption and production cost. Pioneering theoretical and experimental results have proved that the synergistic effects of Lewis acid − base sites and the assistance of nucleophile are crucial for CO2 cycloaddition to epoxide, wherein epoxide substrate and CO2 species are activated by Lewis acid and Lewis base, respectively, then nucleophile helps promote the ring-opening reaction of epoxide [16–18]. Moreover, the existence of accessible hydroxyl group and nitrogen-rich units in the structures of porous MOFs help to further improve the affinity of CO2 molecule to the catalysts. Taking these into account, in this study, a highly porous three-dimensional (3D) metal−organic framework (MOF) [Ni3(BTC)2(MA)(H2O)] (DMF)7 (1) incorporating both exposed metal sites and nitrogen-rich melamine was successfully constructed via solvothermal assembly of 1,3,5-benzenetricarboxylic acid, melamine and Ni(II) ions. The resulting activated Ni(II)-MOF-1a features high CO2 loading capacity and excellent CO2 affinity due to the Lewis-base property together with abundant micropores. The heterogeneous catalytic activity of the activated Ni(II)-MOF-1a was confirmed by remarkably high efficiency on CO2 cycloaddition with small epoxides under ambient conditions. Moreover, the Ni(II)-MOF-1a exhibited satisfied stability and versatility, and it was easy to recycle with no obvious decrease of catalytic activity. Then the feasible synergistic mechanism of Ni(II)-MOF/ Bu4NBr catalysts for CO2 conversion was proposed. Single crystal X-ray diffraction analysis reveals that the Ni(II)-MOF1 crystallizes in the tetragonal space group I4/m and displays a 3D framework with two types of channels with different window sizes running along the [0 0 1] directions. There is one and a half Ni(II)
Fig. 1. (a) The {Ni3(MA)(COO)6(H2O)2} cluster in 1; (b) The cuboctahedral cage and the compressed octahedral cage in 1; (c) View for the two types of the 1D channels in 1; (d) The solvent accessible volume of 1 at the probe radius of 1.8 Å. 71
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cations, one BTC3− ligand, a half MA ligand and one coordinated water molecule in the asymmetric unit. The Ni1 atom is located in a slightly disordered octahedral coordination environment and connected with four carboxyl O atoms from four different BTC3− ligand, one N atom from the MA ligand and one coordinated water (Fig. 1a); The tetrahedral coordination environment of Ni2 atom is completed by three carboxyl O atoms from three different BTC3− ligands and one N atom from the MA ligand. The average distances of Ni−N and Ni−O are 2.03 and 2.07 Å, respectively, which agree with the results in previous reports. Two four-coordinated Ni2 atoms and one six-coordinated Ni1 atoms are held together via the MA ligand and six carboxyl groups from the BTC3− ligands to afford the {Ni3(MA)(COO)6(H2O)2} fragment with the Ni1-Ni2 distance of 3.718 Å. The BTC3− ligand adopts a μ3-η1: η1: η1: η1: η1: η0 bridging mode to coordinate with five Ni(II) centers in three {Ni3(MA)(COO)6(H2O)2} fragments in which two carboxyl groups are in the bidentate mode and one carboxyl group is in the monodentate mode. There are two types of polyhedral cages in the Ni(II)-MOF-1, i.e. one is the cuboctahedral cage composed of 12 {Ni3(MA)(COO)6(H2O)2} fragments as vertices and 32 BTC3− ligands as edges (Fig. 1b). The pore diameter amounts to approximately 9.6 Å. The other is the compressed octahedral cage that shaped by 6 {Ni3(MA)(COO)6(H2O)2} fragments as vertices and 6 BTC3− ligands as edges whose inner diameter is 7.2 Å. The packing of the two types of cages in the three-dimensional direction results in the formation of a 3-dimensional non-interpenetrated network. It is noteworthy that in the framework of the Ni-MOF-1 there is coexistence of two types of one-dimensional channels (Fig. 1c). The channel A with a pore aperture of 10.62 Å fills with water coordination unsaturated Ni(II) centers, and the channel B with a pore aperture of 6.45 Å fills with open O donors. For a clear description of the framework, the {Ni3(MA)(COO)6(H2O)2} cluster could be considered as a 6connected node and the BTC3− ligand could be judged as a 3-connected node, so the overall 3D network of 1 can be rationalized as a (3,6)connected topology with the point symbol of {4^2.6}2{4^4.6^2.8^9} (Fig. S1). Fig. S2 shows the PXRD patterns of prepared Ni-MOF-1, the results were well matched with the simulated single crystal pattern based on the diffraction data, which confirmed the structural integrity and phase purity of the synthesized Ni-MOF-1 sample. Furthermore, no obvious loss of crystallinity for Ni-MOF-1 was found after being soaked in water for 12 h. The high water stability of Ni-MOF-1 may benefit from the increased coordinative bond strength between the Ni2+ ions and the μ3chelating melamine ligand. In addition, the TGA analysis was conducted to detect the thermal stability and skeleton construction of NiMOF-1 (Fig. S3). The Ni-MOF-1 shows a continues weight loss of about 42.2% from room temperature to 386 °C without obvious plateau, which could be attributed to the loss of one coordinated water and
seven DMF molecules in the crystal lattice (calcd: 42.5%). To remove the high boiling point DMF molecules, 100 mg of as-prepared samples of Ni-MOF-1 were soaked in 15 mL of anhydrous methanol in a 20 mL of glass vial for three days, during which the solution was replaced with fresh MeOH every 12 h. The solvent-exchanged MOF showed a weight loss of ∼16% in the range of 50–200 °C due to loss of guest MeOH molecules and one coordinated water molecule. After a long plateau till 381 °C, a sharp weight loss was observed, indicating the collapse of the framework. Whereas activated Ni-MOF-1a prepared by heating the MeOH exchanged Ni-MOF-1 at 393 K for 12 h under dynamic vacuum does not show any weight loss in the temperature range of 55–351 °C, supporting the absence of guest solvents and the coordinated water molecules. The permanent porosity of activated Ni-MOF-1a was proven by N2 adsorption isotherms at 77 K, which exhibited type-I adsorption isotherm behavior with the maximum uptake capacity of 411 cm3/g, evidencing its microporous feature (Fig. 2a). The calculations based on the N2 sorption isotherm at 77 K with the nonlocal density functional theory were also carried out to reveal that its pore size distribution mainly centers at 6.2 and 10.1 Å, which are consistent with the observation from its crystal structure. Furthermore, CO2 adsorption isotherms of Ni-MOF-1a carried out at 273 K and 298 K follow a typical type-I profile with the volumetric uptake of 9.8 and 5.9 mmol/g at 1 bar, respectively (Fig. 2b). The observed CO2 uptake capacity of the Ni-MOF-1a is comparable to the analogues Cu-MOF reported by Gao and co-workers [19]. The corresponding isosteric heats of CO2 adsorption (Qst) at zero coverage were calculated to be 33 kJ/mol, by fitting the CO2 isotherms at 273 and 298 K with the virial method, which reflects a very strong interaction between the framework and the Ni-MOF-1a (Fig. S5) [20]. As shown in the Fig. 1c, the nano-sized channels of Ni-MOF-1 are decorated with Ni(II) sites with coordinated water molecules, which could afford the unsaturated, Lewis acidic Ni(II) sites by activation. Motivated by the high CO2 uptake capacity and high density of open metal sites as well as uncoordinated nitrogen atoms, we envisioned that the activated MOF can act as efficient heterogeneous catalysts for cycloaddition of CO2 to afford the cyclic carbonates. Therefore, the catalytic activity of Ni-MOF-1a was investigated for cycloaddition of CO2 and styrene oxide as a model substrate at the condition of 0.1 MPa CO2 and 60 °C in the presence of 1 mol% TBAB as cocatalyst, and the results are summarized in the Table 1. It can be seen that the yield for styrene carbonate was very low when the catalyst or TBAB was absent in the system under the employed reaction conditions. While the test on the combination of Ni-MOF-1a and TBAB as a binary catalyst it was found that it can serve as an excellent catalyst for the synthesis of styrene carbonate with a yield of 94% over 6 h. Thus, both Ni-MOF-1a and TBAB perform a critical role and are indispensable in the reaction. It is
Fig. 2. (a) The N2 sorption isotherm of the Ni(II)-MOF-1a at 77 K; (b) The CO2 sorption isotherm of the Ni(II)-MOF-1a around room temperature. 72
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Table 1 Cycloaddition of CO2 and styrene using different conditions.
Catalyst
Cocatalyst
None Ni-MOF-1a None Ni-MOF-1a Ni-MOF-1a
none none Bu4NBr Bu4NBr Bu4NBr
Time (h)
Temp (°C)
6 6 6 6 24
60 60 60 60 25
Yield (%) <5 8 18 94 92
TON – 3.2 – 37.6 36.8
TOF – 0.53 – 6.3 1.5
Reaction conditions: 20 mmol styrene oxide, 0.05 mol% Ni-MOF-1a, and 1 mol% TBAB (nBu4NBr) under 0.1 MPa CO2 at the given temperature. The yields were determined by GC–MS. Table 2 Cycloaddition of CO2 and various epoxides under the given conditions.
Entry
Epoxide
Product
Yield
TON
TOF
1
99%
39.6
6.6
2
97%
38.8
6.5
3
88%
35.2
5.9
4
95%
38
6.3
Reaction conditions: epoxide (20 mmol), Ni-MOF-1a (0.05 mol%), and TBAB (1 mol%) under 0.1 MPa CO2, 60 °C (25 °C for propylene oxide), and 6 h. The percent yields were determined by GC − MS.
known that TBAB acts as a nucleophilic cocatalyst and facilitates the ring opening of the epoxides [9]. It should also be noted that the reaction yield catalyzed by Ni-MOF-1a from styrene carbonate is higher than that of the benchmark Cu(II)–carboxylate MOF, HKUST-1, which showed a moderate yield [21]. Given similar pore size and the same embedded Lewis acid metal sites in both Ni-MOF-1a and HKUST-1, the high catalytic activity of Ni-MOF-1a should be ascribed to the increase of the CO2 affinity via the introduction of the nitrogen-rich triazole groups into the framework. Then, we extended this work to various epoxides under similar conditions in order to check the generality for such CO2 cycloaddition reactions (Table 2). Owing to its nanoscale
channels, the Ni-MOF-1a also exhibited very high activities for the substrates of different sizes, to the large substrates, cyclohexene oxide and benzyl phenylglycidyl ether. Thus, Ni-MOF-1a is a suitable heterogeneous catalyst for carbon fixation. The stability and recyclability of the catalyst are very important for heterogeneous catalysis. The recycling experiments were carried out after the catalyst recovered from the cycloaddition reaction by thorough a washing and drying process. Compared to the freshly prepared Ni-MOF-1a, its PXRD pattern after catalysis showed no change in the peak intensity and position, which indicated the stability of Ni-MOF-1a. The catalytic activity of Ni-MOF1a still remained equal to the level of that of the freshly prepared during 73
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Fig. 3. (a) Proposed mechanism for the cycloaddition of CO2 catalyzed by Ni-MOF-1a/TBAB.
four cycles of recyclability tests for the cycloaddition of CO2 with propylene oxide (Fig. S6). Based on the previously reported literature and the crystal structure of Ni-MOF-1, an assumptive mechanism for the catalytic process was proposed [22]. First, the Lewis acid open Ni sites activated the epoxide by forming Ni–O adduct. Then the Br− of the TBAB acted as nucleophile to attack the β‑carbon atom with less sterically hindered C of epoxide to facilitate ring-opening reaction. Simultaneously, CO2 molecule was activated by Lewis base of amine groups with the formation of a carbamate salt, then which was attacked by the ring-opened epoxide as a nucleophilic. Finally, the ring-closure proceeded with the formation of cyclic carbonate, and the catalyst was regenerated for the next cycle (Fig. 3). In conclusion, we have successfully prepared a new porous Ni(II)MOF with Lewis acid − base bifunctional sites via solvothermal assembly of 1,3,5-benzenetricarboxylic acid, melamine and Ni(II) ions. The resulting activated Ni(II)-MOF-1a possessed abundant micropore, high density of open metal sites along with nitrogen-rich units, and showed excellent CO2 adsorption property around room temperature. Furthermore, the Ni(II)-MOF-1 also presented excellent catalytic performance cooperating with Bu4NBr for the cycloaddition of CO2 to epoxides under mild conditions, even at ambient temperature. Additionally, the Ni(II)-MOF-1 catalyst also showed good versatility and recyclability, which indicate its potential use in the in the practical utilization of CO2 to valuable chemicals.
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Acknowledgements This work was supported by the Shaanxi Province Department of Education (17JK1117). Appendix A. Supplementary material CCDC (1) contains the supplementary crystallographic data for the title compound. This data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge 74
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