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Metal–organic frameworks for the chemical fixation of CO2 into cyclic carbonates
Coordination Chemistry Reviews 408 (2020) 213173
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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr
Review
Metal–organic frameworks for the chemical fixation of CO2 into cyclic carbonates Tapan K. Pal a,⇑, Dinesh De b,⇑, Parimal K. Bharadwaj c,⇑ a
Department of Science, Pandit Deendayal Petroleum University, Gandhinagar 382007, Gujarat, India Department of Basic Science, Vishwavidyalaya Engineering College, Lakhanpur, Sarguja University, Chhattisgarh 497116, India c Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, Uttar Pradesh, India b
a r t i c l e
i n f o
Article history: Received 1 July 2019 Received in revised form 26 November 2019 Accepted 23 December 2019
Dedicated to Professor G.K. Lahiri celebrating his 60th birthday. Keywords: Metal–organic frameworks Chemical fixation of CO2 Cyclic carbonate Energy conversion Heterogeneous catalysis
a b s t r a c t Presently, we find a rapid pace of CO2 emission into the atmosphere causing major problems facing our planet. If no action is taken, it would have harmful consequences to humanity and the biosphere. More CO2 in the atmosphere will cause global warming. This will lead to climate upheavals disturbing the ecosystems, modification of the conditions and cycles of plant reproduction and numerous associated problems. Therefore, present CO2 content in the atmosphere should be drastically reduced to a much lower level on an urgent basis. Alternatively, CO2 represents an abundant C1 feedstock and its chemical utilization has caught the imagination of chemists in recent years. Thus, fixation of CO2 with epoxide to form cyclic carbonate (hereafter, CC) via cycloaddition reaction is significantly important and vigorously pursued in different laboratories around the world. This is because removal of CO2 takes place from the atmosphere and simultaneously it can be converted into value-added products. Metal organic frameworks (MOFs) have attracted enormous attention in recent years as potential systems for gas storage, separation, heterogeneous catalysis and so on, owing to their unique features such as designable architecture, controllable pore size, high surface area, permanent porosity, etc. In the present review, we discuss the recent progress made on catalytic conversion of CO2 to CCs by specially designed MOFs. It should be emphasized here that in the present review the literature survey is not exhaustive and we apologize for missing any important result in this review. Ó 2020 Elsevier B.V. All rights reserved.
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Synthesis of cyclic carbonates (CCs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Catalysis due to the presence of open coordination sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1.1. Presence of ionic part (ionic liquid/ions) in MOF channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2. Lewis basic linker-promoted catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.2.1. Post synthetic modification of the functional site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.3. Cooperative bifunctional catalysis due to open coordination sites and Lewis basic functional linker. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.4. Catalysis due to the presence of defect in the crystal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.5. Catalysis due to the presence of accessible acidic functional sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.6. Catalysis due to the accessible Lewis and Brønsted acid bifunctional sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Conclusion and future perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
⇑ Corresponding authors. E-mail addresses: [email protected] (T.K. Pal), [email protected] (D. De), [email protected] (P.K. Bharadwaj). https://doi.org/10.1016/j.ccr.2019.213173 0010-8545/Ó 2020 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Global green house gas emission, (b) coutrywise CO2 emission, adapted from Inter-governmental Panel on Climate Change (IPCC), 2014 [2].
1. Introduction According to United Nations 2014 report, by 2050 the World urbanization will increase by 66% and it will lead to population explosion in the urban area [1]. Such a quick growth will definitely enhance the overlap between residential and industrial areas. CO2 is a major contributor to the global warming and with increasing human activities, the emission of CO2 from the power plants, biomass or fuel combustion, automobile exhaust and industrial
processes will also increase (Fig. 1) [2]. It is well known that CO2 assists to warm the earth [3] and the release of it by combustion or other activities is balanced through photosynthesis that helps to maintain the carbon cycle. However, the concentration of CO2 increasing to an alarming proportion leading [4] to several natural disasters and other calamity including rising sea level, ecosystem disturbance, crop failure and extermination of living species. On the other hand, fossil fuels supply us huge amount of energy in our daily lives [5]. So, we are living in a dilemma and there is an urgent need to find a solution that can optimise these two situations. An extremely sagacious approach would be very low emission and on the spot capture and fixation of anthropogenic CO2 emission. Till now, several technologies have been developed like CO2 capture and geological sequestration [6], various chemical
Fig. 2. Several potential applications of MOFs.
Fig. 3. Schematic representation for cycloaddition reaction between CO2 and epoxide to cyclic by MOF.
Scheme 1. General schematic representation for cycloaddition reaction mechanism between epoxides and CO2 catalyzed by Lewis acidic open coordination sites of a MOF in presence of TBAB.
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Fig. 4. A perspective view of the structure of Co-MOF-74 (The cobalt, carbon and oxygen atoms are marked as yellow, grey and red colour respectively). Reproduced with permission from [25] Elsevier, Copyright 2012.
strategies to convert CO2 to value added chemicals [7], and so on. Particularly, the possibility of converting CO2 to value-added products have been the subject of enormous attention worldwide in recent years. In this regard, several important properties of CO2 like non-toxic, non-flammable, inexpensive, highly abundant, easy to handle, etc., are added advantages. Sometimes, however, CO2 suffers from low reactivity due to the highest oxidation state of carbon and very high C@O bond enthalpy (+805 kJ mol1) [8]. Although, this problem can be surmounted by reducing high energy barrier with the help of highly reactive substrates and use of catalysts. Thus, CO2 capture and its conversion into valuable chemicals (CCs, formic acid, substituted urea, dimethyl carbonate, etc.) have attracted attention from a large number of researchers [9,10]. The CCs are useful in a number of industries like pharmaceuticals, dye, electrolyte in the lithium ion batteries, intermediates for the synthesis of ethylene glycol, polymeric materials, precursor of
Fig. 5. Perspective views of the Cu(II)-MOF having two types of pores (blue and red) and is decorated with unsaturated cupper metal sites and acylamide groups. Reproduced with permission from [27] American Chemical Society, Copyright 2017.
Table 1 Conversion of different epoxides to their corresponding CCs upon reaction with CO2. Entry
Epoxides
Small substrates 1
Product
O
O
Yields (%)
O
96
O 2
H3C 3
Cl 4
Br Larger substrates 5
O
O
O H3C
O
O O
Cl
O Br
O
85
O
88
O
O
O
90
O
O
O
10
O
O
6
O
O
7
O 7
O
O
O
O
O
O
6
4
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
polycarbonates, etc [11]. A number of homogeneous catalysts such as quaternary ammonium salts, metal complexes, metal halides, etc. have been employed to synthesize CC in an industrial scale under mild conditions [12]. But, homogeneous catalysts are usually associated with disadvantages such as separation of the products, their purification, recycling of the catalyst, and so on. Ionic liquids (ILs) possess active centres which enhance the rate of cycloaddition reactions between epoxides and CO2. Besides, some of the features like non-flammability, non-volatile, low vapour pressure and high melting points of ILs, make them suitable catalysts for CO2 cycloaddition reactions [13]. But the recyclability of IL is very
expensive and the catalyst is ineffective at room temperature. To overcome these issues, ILs have been immobilized in traditional polymeric materials [14]. Still, leaching of reacting species in to the reaction system and inadequacy at milder conditions are of major concern [15]. On the other hand, MOFs can be designed to be useful as potential heterogeneous catalysts. These compounds represent a new class of porous materials that can be synthesized easily from metal ions/metal ion clusters and rigid multidentate organic ligands. Depending upon the nature of the node and the structure of the linkers, a myriad number of structures can be easily constructed under mild conditions. These materials have
Table 2 Syntheses of various carbonates from styrene oxide derivatives catalysed by WMMOFs.a
a b c
Entry
R-
Temp. [K]
Time [h]
Con.b [%]
Sel.b [%]
1 2 3 4
PhMeCl-
333 r.t.c 333 333
30 24 8 16
99 96 99 96
100 94 93 100
O
Reaction conditions: 20 mg catalyst (0.34 mol % Zr), 5% TBAB, no solvent, 17.5 mmol epoxide, CO2 balloon. Determined by GC–MS. Room temperature.
Fig. 6. (a) Conversion of styrene epoxide to corresponding carbonate by WM-MOF, (b) Kinetics of CO2 conversion by different catalysts, (c) Recyclability experiment of WMMOF, (d) PXRD patterns before reaction and after reaction of WM-MOF (TEM image (scale bar 50 nm) after reaction of WM-MOF). Reproduced with permission from [28] American Chemical Society, copyright 2017.
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attracted enormous attention in recent years due to versatility in the structure, crystalline nature, ultra high surface area with voids of different shapes and sizes that make them useful in many practical applications (Fig. 2) [16]. Highly porous MOFs with one or more solvent molecules attached to each metal centre can be made via design of nodes as well as linkers. In a robust MOF, removal of both metal coordinated and lattice solvent molecules can be achieved through heating without disturbing the overall architecture, leading to a highly porous MOF. Reactants can easily enter these pores and can also access the empty metal centres. Leaching of metal ions into the solution during or after catalytic reactions is rare. Consequently, MOFs incorporate the merits of ‘‘metal complex (homogeneous catalyst) in terms of reactivity and accessibility of active sites” and ‘‘recyclability of a heterogeneous catalyst”. Catalytic application by a MOF has been demonstrated earlier [17] and now it has been shown to be an excellent heterogeneous catalyst in various organic transformations [18]. Recently, CC synthesis by MOFs, eminent as a excellent catalysts (Fig. 3) and a number of reports have been published based on the carbon dioxide capture and sequestration (CCS) [19]. Adsorption studies with CO2 showed that the capacity can be enhanced through (a) proper ligand designing (amino functionalised MOFs, those containing N and F in the linker), (b) hybrid composition (MOF-CNF and MOF-templated carbon) and in some cases (c) generation of open metal sites [20]. In 2009, Han and
co-workers first showed CC synthesis by using a binary catalytic system, MOF-5/n-Bu4NBr at 323 K. The catalyst was stable up to three cycles and reaction was completed within 6 h with high selectivity [21]. Since then, a massive number of MOFs and their derivatives have been used by different workers to form CCs by reacting CO2 with different epoxides. In the present review, we discuss the use of MOFs in the formation of CCs utilizing CO2 and epoxide which is documented as a 100% atom economic reaction. At the outset, it should be emphasized here that the discussions are not exhaustive and we apologize in case we have missed few important references. 2. Synthesis of cyclic carbonates (CCs) The conversion of CO2 into chemical feedstock has attracted attention in recent years as it is an abundant C1 resource [22] and the conversion of CO2 into value-added fine chemicals is of environmental and practical importance [23]. The synthesis of CC via cycloaddition reaction between an epoxide and CO2 is a profitable reaction with wide applications in pharmaceutical and fine chemical industries [24]. In general, catalytic activity (here, cycloaddition of CO2 with epoxides) of MOFs arises from different structural characteristics: (i) presence of open metal coordination sites (generation of its takes place upon elimination of loosely bound solvent molecules) acting as Lewis acidic sites that can be used with a Lewis basic co-catalyst like tetrabutyl ammonium
(a)
(b)
(c)
Fig. 7. (a) The Zn(II) coordination environment, (b, c) The hexagonal double-walled channels viewed down the crystallographic c-axis. Reproduced with permission from [29]. Copyright 2017, American Chemical Society.
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Table 3 Cycloaddition of CO2 with different epoxides catalysed by the Zn(II)-MOF in the presence of TBAB.a Entry
Epoxide
1
Cl 2
H3C
Product
O
O
O Cl
O
Time (h)
Yieldb %
343
12
>99
303
12
>99
343
12
>99
343
12
>99
343
12
88
343
12
68
343
22
68
343
32
4c
343
32
9d
O O
O H3C
O
3
Temperature (K)
O
O
O
O O
4
H3C
O
O H3C
O
O
5
O
O
O O
6
O 7
O
O
O
O
O
O
O
O 8
Cl 9
Cl a b c d
O
O
O
O Cl
O
O O
Cl
O
O O
Reaction conditions: catalyst (10 wt%), epoxide (20 mmol) and TBAB (1 mmol). Calculated from 1H NMR analysis. Catalysed by MOF only. Catalysed by the co-catalyst TBAB alone.
HOOC
COOH
HOOC
COOH
(c)
(b)
(a)
(d)
(e)
Fig. 8. (a) Structure of H4TCPE ligand; (b) Ni2 bimetallic unit present in Ni–TCPE1; (c) 1D nanotube (pink column) representingthe channel; (d) view from the top of the nanotube; (e) view along the b axis showing the packing pattern. The Ni, C, N and O atoms are read as cyan, grey, blue and red color code respectively. Reproduced with permission from [30]. Copyright 2015, American Chemical Society.
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bromide (TBAB); (ii) Lewis basic functionalized linkers can be functionalized during or after synthesis) that can act as the site of CO2 fixation; (iii) presence of Lewis acidic or basic defect sites inside/surface of the MOF, (iv) presence of accessible Brønsted acidic COOH groups, and (v) presence of accessible Lewis and Brønsted acid bifunctional sites.
2.1. Catalysis due to the presence of open coordination sites Presence of empty coordination sites on the metal in a MOF is important as the catalysis cycle begin with the epoxide ring binding at the open metal site. The nucleophile Br ion (from cocatalyst TBAB) attacks on the sterically less crowding side of the
Fig. 9. (a) The Ni3 unit present in Ni–TCPE2, (b) coordination environments of TCPE; (c) 3D structure of Ni–TCPE2, (d) topological figure showing PtS network. The Ni, C, N and O atoms are read as cyan, grey, blue and red color code respectively. Reproduced with permission from [30]. Copyright 2015, American Chemical Society.
Table 4 Coupling of epoxides with CO2. Entry
Substrate
1a
Ni-TCPE1
O
2a
O
O
3a
O
O
Ni-TCPE2
Yield (%)
TON
Yield (%)
TON
>99
2000
86.2
1720
>99
2000
97.7
1950
95.7
1910
94.2
1890
93.7
1880
92.6
1850
O 4b
O
O
O
O
a Conditions of the reaction: MOF catalyst (10 lmol, on the basis of Ni2+), epoxide (20 mmol), , and TBAB (0.3 mmol), 1 MPa of CO2 pressure for 12 h at 373 K. % yields were calculated from 1H NMR study. b Epoxide (10 mmol), other conditions are same.
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metal-bound epoxide with ring opening leading to a metal-bound alkoxide in an SN2-type reaction. Next, cycloaddition of CO2 to the alkoxide occurs forming metal bound carbonate that subsequently form CC with the regeneration of the catalyst (Scheme 1). Emphasis had been given to created open coordination sites with large pores to accomadate epoxides. Besides, open metal sites positively influence adsorption of CO2 by the MOF. Thus, CC synthesized was experienced with Co-MOF-74 in 2012 [25]. Thereafter, a growing number of reports utilizing open metal catalytic sites have been reported [26]. The crystal structure of the compound shown in Fig. 4 consisted of 1D honeycomb channels of dimension 1.1–1.2 nm and helical chains of edge-condensed metal–oxygen coordination octahedra located at the intersections of the honeycomb. The metal coordination was square–pyramidal with one site occupied by a water molecule. Upon heating in vacuum, the framework created an open metal site that was used to catalyze the reactionbetween CO2 and styrene oxide in chlorobenzene solvent without using any co-catalyst at 20 bar pressure and 373 K temperature. Interestingly, it afforded a maximum of 96% conversion. The conversion decreased with decreasing temperature, attributed to lower solubility of CO2 in chlorobenzene. Its catalytic activity was intact for at least three cycles. Zhao and co-workers solvothermally synthesized a Cu(II)-MOF using an acylamide incorporated tetracarboxylate ligand and Cu (II) ions [27]. Presence of accessible nitrogen-rich groups and unsaturated metal coordination sites made the MOF (Fig. 5) a highly promising candidate for selective adsorption of CO2. Conversion of the adsorbed CO2 to CC was carried out in a Schlenk tube under 1 bar pressure of CO2 at room temperature for 48 h using 20 mmol of epoxide and 0.5 g of the co-catalyst, TBAB and the activated MOF catalyst without using any solvent. Smaller epoxides afforded good yields of the product while larger epoxides gave substantially low yields as diffusion of these substrates inside the coordination space was difficult (Table 1). A series of zirconium–metallo porphyrinic MOFs decorated with mesoporous channels [28] led to catalytic sites (Lewis acid sites) exposed within the framework. This displayed excellent catalytic efficacy for the CCs formation with TBAB (co-catalyst). A 99% conversion to CC was reported with 100% product selectivity in case of styrene oxide as the substrate (Table 2). The recycling test was carried out simply by separating the catalyst by centrifugation and could be used again without significant loss in catalytic activity at least up to five cycles (Fig. 6).
(b)
(a)
(c)
(d)
(e)
Fig. 10. Structures of (a) [CuL(ClO4)2], (b) [NiL(ClO4)2]H2O, (c) [CuLCl2],(d) [NiLCl2] 2DMF, (e) packing diagram viewed down [111] and depicting the cyclam linkers connecting the Zr6-clusters along the cell body diagonals. Reproduced with permission from [31]. Copyright 2018, American Chemical Society.
Table 5 Chemical fixation of CO2 with epoxides catalyzed by VPI-100 MOFs. Entry
Epoxide
Product
Catalyst
Conversion (%)a d
1 2 3 4 5 6
Cl
H3C
O
O Cl
b c d
TOF (h1)c
TONb d
R1
R2
R3
R1
R2
R3
R1
R2
R3
VPI-100 (Cu), TBAB VPI-100 (Ni), TBAB
94 96
95 98
94 98
431 443
439 452
431 453
72 74
73 75
72 76
VPI-100 (Cu), TBAB VPI-100 (Ni), TBAB
60 50
52 50
40 43
273 229
236 229
185 199
46 38
39 38
31 33
VPI-100 (Cu), TBAB VPI-100 (Ni), TBAB
13 18
16 20
18 17
60 85
76 92
81 78
10 14
13 15
14 13
O
O
O H3C
O O
O
O
O a
O
d
1
O
Conversion were calculated from the H NMR spectra. Turnover number [product (mmol)/active site (mmol)], where ‘‘active site” is the total no of unsaturated Zr and Cu/Ni metal centers. Turnover frequency [TON/time (h)]. R1, R2, and R3 stands for 1st, 2nd and 3rd cycles, using the same VPI-100 MOF as catalyst.
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An anionic porous framework, {[(CH3)2NH2][Zn2(L)(H2O)PO4] 2DMF}n with double-walled hexagonal channels had been constructed [29] using the dicarboxylate ligand H2L (Fig. 7a). As expected, the isophthalate group in the linker formed a paddle-wheel dimeric SBU where each metal was bonded to a water molecule. The de-solvated framework obtained upon heating, showed microporous nature with good CO2 uptake capacity (111.7 cc g1 or 22 wt% at 273 K, 1 bar pressure) due to the presence of imidazole units as well as accessible open metal sites inside the channels. The framework showed efficient catalytic activity in the conversion of adsorbed CO2 to CCs under mild conditions in the presence of TBAB as the co-catalyst
9
(Table 3). Due to its highly robust nature, the MOF has been recycled for a number of catalytic cycles with nominal change in the product yield. An aromatic tetracarboxylate linker was obtained by substituting all the four hydrogen atoms of ethylene. This linker upon reacting solvothermally with hexa aqua nickel nitrate salt in presence of L-proline (L-Pro) in mixed DMF and H2O for 3d at 373 K [30] afforded a single walled discrete metal–organic nanotube (Ni– TCPE1) and a 3D extended network (Ni–TCPE2). The L-Pro was not present in the crystal structure of Ni–TCPE1 and it served only as atemplate. The framework is having large channel, internal diameter (2.1 nm) and the external diameter is 3.6 nm (Fig. 8).
Fig. 11. Representation for the assembly of (FJI-C10) form BDCs, (Br) Etim-BDCs and Cr3O clusters (green). The circle region in the cluster indicates the water molecule. Reproduced with permission from [32]. Copyright 2018, The Royal Society of Chemistry.
Scheme 2. The probable mechanism for the cycloaddition reaction by FJI-C10. The weak interaction between the reagents indicated by hashed bond. Reproduced with permission from [32]. Copyright 2018, The Royal Society of Chemistry.
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The 3D network (Ni–TCPE2) was obtained when the amount of LPro was increased keeping every other chemical in the same amounts as in the previous case. It crystallized in chiral space group C2 with one of the Ni ions was coordinated one L-Pro molecule (Fig. 9a). Compound Ni–TCPE1 having a large pore, exhibited excellent catalytic activity in presence of TBAB in the cycloaddition reactions of CO2 with various epoxides, with a highest TON of 17,500 and an initial TOF 1000 (Table 4). Interestingly, it showed excellent enantioselectivity (ee~92%) when enantiopure styrene oxide (R or S) was used. Compound Ni–TCPE2 however, showed significantly lower catalytic activity than that of Ni–TCPE1 possibly due to pore blockage in the former. Two highly stable three dimentional Zr-MOFs, VPI-100 (Ni) and VPI-100 (Cu) reported by Morris and co-worker and the frameworks were assembled from eight-connected Zr6 clusters and [Cu(cyclam)]2+ or [Ni(cyclam)]2+ (cyclam = 1,4,8,11-tetraaza cyclotetradecane) linkers (Fig. 10) [31]. The firmly placed metal ion in the cavity of cyclam was readily accessible by epoxides
while empty coordination sites on the Zr4+ ion showed good CO2 uptake capacity. Both these factors helped in the reaction of CO2 with epoxides, including sterically larger epoxides (Table 5) in presence of TBAB as the co-catalyst. 2.1.1. Presence of ionic part (ionic liquid/ions) in MOF channels It is well-established that in the cycloaddition of CO2 to epoxide, the nucleophile Br- ion attacks the less-hindered side of the metalbound epoxide and cleave the ring to afford a metal-bound alkoxide in an SN2-type reaction. In the previous section, all examples had the nucleophile Br- ion coming from an external source like Bu4NBr (TBAB). It might be a good strategy to have the Br ion present in the coordination space of the MOF near the Lewis acidic reaction centres. Encapsulation of flexible cationic polymer having bromide counter anion into a MOF could function synergistically leading to lower activation energy pathway and fixes the carbon dioxide into CCs with various epoxides. Following this strategy, Cao and co-workers constructed a mesoporous cationic Cr-MOF,
Scheme 3. Synthesis of ZnTCPP (Br)Etim-UiO-66 through: (a) step by step post synthetic ionization and then metalation strategy; (b, c) direct syntheses from the ionic ligand [(Br)(Etim-H2BDC)+], Im-Zr6, imidazolium functionalized Zr6SBU; (Br)Etim-Zr6, imidazolium functionalized Zr6 SBU. Pore cavity is designated by yellow sphere. Reproduced with permission from [33]. Copyright 2018, American Chemical Society.
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termed FJI-C10, [32] using the linker 2-(3-ethyl-imidazol-1-yl)-ter ephthalic acid [(Etim-H2BDC)+Br] along with the H2BDC ligand. The structure was similar to MIL-101(Cr) having Lewis acidic vacant Cr3+ sites and free halogens in the channels (Fig. 11). FJI-C10 exhibited excellent CO2 uptake capacity (20.2 wt% at 1 bar and 273 K) as the ionic surface could boost the interactions between CO2 and the surface through dipole–quadrupole interactions. Owing to the co-existence of Cr3+ (Lewis acidic sites) and adjacent halogen ions in the framework, the FJI-C10 exhibited catalytic efficiency for the CO2 conversion into CCs under atmospheric pressure at room temperature in 24 h. Unfortunately, this compound gave low conversion (27.3%) and product selectivity (60.7%). However, upon increasing the temperature to 333 K, the reaction practically completed (99.7%) within 24 h with improved selectivity (88.2%). On the hand, under the same conditions, the neutral MIL-101 gave inferior yield of 12% and a selectivity of 29% for the same reaction. Furthermore, the catalyst retained its activity at least six times in the cycloaddition reaction of CO2 with epichlorohydrin. The proposed mechanism for this reaction could be found in Scheme 2. Initially, an epoxide was activated by the influence of Lewis acidic Cr3+centre. The ring opening of the epoxide was driven by the combined influences of the Cr3+open sites and a Br ion on the less-crowded carbon atom of the epoxide. After that, a CC was formed by the nucleophilic attack from the alcoholate to CO2, which converted to a CC by intermolecular cyclization, giving the original catalyst for the next cycle. The ionic MOF, ZnTCPP (Br)Etim-UiO-66, (ZnTCCP = [5,10,1 5,20-tetrakis(4-carboxyphenyl)-porphyrinato]zinc(II)) had been synthesized [33] by a sequential post-synthetic metalation strategy as well as de novo one-pot syntheses (Scheme 3). The MOF exhibited excellent catalytic performance in solvent-free CO2 conversion into CC with allyl glycidyl ether under cocatalyst free condition at 1 bar CO2 pressure at 413 K for 6 h. As the Zn2+ sites in the porphyrin ring and imidazolium bromide were integrated within the MOF, no external TBAB cocatalyst was needed. The results are summeried in Table 6. The superior catalytic activity was ascribed to the combined effects of available Zn2+ sites and Br ion in the micropore channel of the MOF. ZnTCPP (Br)Etim-UiO-66 under the optimized conditions could efficiently convert several epoxides into respective CCs (Table 7) with the yield ranging between 19.8 and 91.1%. A probable mechanism had been proposed (Scheme 4). Firstly, the epoxide was coordinated to Lewis acidic Zn(II) sites. The cleavage of the activated epoxide ring occurred by the regioselective attack of Br nucleophile. Then, an acyclic carbonate was formed by the reaction of ring opened epoxide and CO2. Acyclic carbonate was then transformed to a CC by the removal of Br ion with the regeneration of catalyst. Further utilization of this concept was made by Ma and coworkers [34] who synthesized a synergistic catalytic system by encapsulating an ionic polymer (IP) into the well known MOF, MIL-101(Cr). The Br counter ions (function as co-catalyst) in this catalytic system (MIL-101-IP) present close to the open Cr(III) reactive sites (Fig. 12). The imidazolium polymer could be encapsulated within the pores of the MOF by stirring MIL-101 with the monomer (3-ethyl-1-vinyl-1H-imidazol-3-ium bromide) in DMF overnight followed by the addition of free-radical initiator, azobis-isobutyronitrile (AIBN) to induce polymerization resulting in MIL-101-IP (Fig. 12). In spite of the encapsulation of the polymer, the MIL-101-IP still possessed sufficient open space as supported by the nitrogen and carbon dioxide adsorption investigation. The surface area of the daughter MOF was slightly reduced to 2232 m2g1 compared to the parent MOF, MIL-101 with a BET surface area of 2610 m2 g1. The cycloaddition of CO2 with epichlorohydrin was complete
Table 6 Fixation of CO2 with AGE.a
O O
CO2
O
Catalyst
O O
1 atm
O
Allyl Glycidyl Ether (AGE)
Entry c
1 2 3 4 5e 6 7f 8g 9h
Catalyst
Con. (%) [sel. %]b
Blank UiO-66 TCPP (Br)Etim-UiO-66 TCPP Im-UiO-66 ZnTCPP ZnTCPP (Br)Etim-UiO-66 ZnTCPP/(Br)Etim-H2BDC (Br)Etim-H2BDC (PF 6 )Etim-H2BDC
Trace Trace 78(2) 64(5) Trace 77(4) 98(1) 94(1) 11(2)
[–] [–] [65d] [90] [–] [92] [100] [100] [100]
a Reaction conditions: no solvent, catalyst (43 mg), CO2 pressure: 1 bar, temperature: 413 K, time: 6 h. b Determined by GC–MS, values are obtained by averaging three runs, error written in parentheses. c Absence of catalyst. d By products are allyl derivatives. e ZnTCPP (0.0018 mmol). f ZnTCPP (0.0018 mmol) and (Br)Etim-UiO-66 (0.0475 mmol). g (Br)Etim- H2BDC (0.0475 mmol). h (PF6)Etim-H2BDC (0.0475 mmol).
Table 7 ZnTCPP (Br)Etim-UiO-66 catalyzed chemical fixation of CO2 with epoxides for the synthesis of CCs.a Entry
Epoxide
1
Cl
Yield (%)b
Product
O
O
O Cl
2
86.9
O
O
O
O
O 3
O
O
O O
O
4
90.0
O
O
O
O
91.1
O O
52.8
O 5
H3C
O 3
O H3C
O
38.2
O 3
6
H3C
O 5
O H3C
O
19.8
O 5
a Reaction conditions: no solvent, catalyst (0.95 mol % on the basis of imidazolium), CO2(1 bar), 413 K for 14 h. b calculated by using GC–MS.
(99%) at 323 K and 1 atm CO2 in 68 h. The catalytic activity of MIL-101-IP was better than free MIL-101 (32%), IP (3%) and the physical mixture of both under the same reaction conditions (Table 8).
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Scheme 4. Probable Mechanism for chemical fixation of CO2 with epoxides catalyzed by ZnTCPP (Br)Etim-UiO-66. Reproduced with permission from [33]. Copyright 2018, American Chemical Society.
Table 8 Cycloaddition of CO2 with epichlorohydrin.a Entry
Catalyst
Yield (%)
1 2 3 4
Ionic Polymer (IP)b MIL-101-IPc MIL-101 + IPd MIL-101e
3 99 80 32
a Reaction conditions: epichlorohydrin (1 g, 10.5 mmol), 1 atm CO2 for 68 h, at 323 K. b 6.3 mg of IP (0.0313 mmol Br). c 50 mg of MIL-101-IP (0.0313 mmol Br). d Physical mixture of 43.7 mg MIL-101 and 6.3 mg IP (0.0313 mmol Br). e 50 mg of MIL-101.
Table 9 Cycloaddition rection of CO2 into various epoxides.a Entry
Epoxides
1
H3C Fig. 12. The polymer, MIL-101-IP entrapped within the channels. Reproduced with permission from [34]. Copyright 2018, Wiley-VCH.
2
Products
O
O H3C
O
O
95 [48]
O
O
99 [48]
O
As shown in the Table 9, the composite acted as an effective catalyst for the synthesis of various CC from their corresponding component with high yield. The catalyst maintained its activity at least up to four cycles with insignificant change in the performance. From kinetic studies, the activation energy for MIL-101-IP was found to be approximately 50% lower than that of the physical mixture of MIL-101 + IP (63.6 vs. 91.6 kJmol1). Later Dong group utilized the same concept for the CCs synthesis [35].
O
3
O
O O
O 4
O
O
5
O
O O
2.2. Lewis basic linker-promoted catalysis
a
O
84 [96]
O
O O
O
Unlike the Lewis acid catalyzed cycloaddition reactions, a modified reaction mechanism was believed to be operational [36] with
Yields [%] [T (h)]
O
82 [96]
O O
33 [72]
O
Reaction conditions: MIL-101-IP (50 mg), epoxide (1 g) at 1 atm CO2 and 298 K.
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COOH
COOH
x
4 Zn(NO3)2
(3 - x) COOH
NH2 COOH
Fig. 13. Synthesis mixed-linker MOF-5 from 2-amino-BDC and BDC, where BDC = benzene-1,4-dicarboxylate in different ratio. Reproduced with permission from [37] Copyright 2009, Wiley-VCH.
Fig. 14. (a) Large channels (LC) and tight channels (TC) in MIL-68(In)-NH2, (b) In-O-In chain in MIL-68(In)-NH2. Reproduced with permission from [39] Copyright 2012, Wiley-VCH.
linkers incorporating Lewis basic sites like side chain amino groups. Here, the first step involved a nucleophilic attack on the C atom of CO2 by the amino group to activate CO2 through formation of a carbamate intermediate that could react with the epoxide. To test this mechanistic pathway, some of the BDC linkers of MOF5 had been replaced [37] by 2-amino-BDC that led to mixedlinkered MOF-5, Zn4O(BDC)x(BDC-NH2)3x] (Fig. 13) where 40% of 2-amino-BDC linker was incorporated. Together with the cocatalyst TBAB, this framework behaved as a suitable catalyst for the reaction of CO2 with propylene oxide to afford propylene carbonate (63% yield). Using amino group based MIL series of MOFs such as MIL-125NH2 and (MIL-68(In)–NH2)) by Ahn et al. reported [38] synthesis of CCs. Following similar methodology, Farrusseng and co-workers
Scheme 5. The conversion of styrene epoxide (SO) to CC.
modified MIL-68 showed the reactivity of MOF towards synthesis of CC increases by introducing amine functionality into the ligand system [36,39]. The frameworks, MIL-68(In)-NH2 and MIL-68(In) were synthesized through precipitation method by reaction between aminoterephthalic acid/terephthalic acid with indium nitrate in dimethylformamide as a solvent. Both the frameworks were charecterized by a number of spectroscopic techniques. Calculations using the density functional theory predicted the structure of MIL-68(In)-NH2 to be one-dimentional rod shaped framework having two types of channels: hexagonal large channels (LC) and triangular tight channels (TC) (Fig. 14a) forming an
Table 10 SO conversion table by MOF MIL-68(In) and MIL-68(In)-NH2.
X
Entry
Catalyst
SO conversion %
1 2 3 4 5 6 7
Blank TBAB MIL-68(In)X MIL-68(In)X MIL-68(In)-NH2X MIL-68(In)-NH2X Recyle-MIL-68(In)-NH2
8 87 36 42 68 74 53
Different run under same experimental condition.
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infinite chain structure (Fig. 14b) via bridging hydroxyl groups. Interestingly, the framework MIL-68(In)-NH2 was able to performed cycloaddition reaction between CO2 and epoxide in DMF solvent (Scheme 5) with much greater efficiency compared to MIL-68(In) (Table 10). Theoretical and experimental study concluded that by introducing –NH2 in MIL-68(In)-NH2 resulted in higher conversion rate of epoxide to CC with CO2. However, the catalyst was recycled with lower conversion of epoxide rate attributed to occupancy of the channels by reactant molecules which was also confirmed by lower BET surface area (1100 m2/g for pristine MOF and 720 m2/g for recycled MOF). The CO2 fixation by chiral MOFs are rarely available in the literature. An interesting example reported by Qi and co-workers where they had performed tandem assymetric catalysis reaction from alkene to CC using a polyoxametalate based homochiral MOF [40]. This MOF was synthesized solvothermally by reacting Zn(NO3)26H2O, TBA4W10O32, L-N-tert-butoxy-carbonyl-2-(imida zole)-1-pyrrolidine (L-BCIP) and NH2-BPY in aqueous acetonitrile and the framework obtained was assigned as ZnW-PYI1. Similarly, when D-BCIP was used instead of L-BCIP, the corresponding MOF
was ZnW-PYI2. As expected, each framework was a mirror image of the other (Fig. 15a). The framework, ZnW-PYI1 had a labile anion, W10O4 32 that reacted with Zinc metal ions to form a kegging type anions, ZnW12O6 40 and each kegging anions binds to four zinc metal ions with their terminal oxygen atoms. In the framework each zinc metal ions form a 2D square grid sheets which is then linked by NH2-BPY ligands to form 3D architecture (Fig. 15b) having solvent accessible void space 289.1 Å. Fig. 16 indicates that the framework consisting four basic units and the framework was used for the tandem reaction for the synthesis of CC from olefins. The framework, ZnW-PYI1 showed epoxidation reaction of styrene with 92% yield and 79% enatiomeric excess after 5 days at 323 K in presence of t-butylhydroperoxide (in 70% decane). In different experiment the cycloaddition reaction between racemic styrene epoxide with CO2 by MOF, ZnW-PYI1 (0.1% mol) produce CC (negligible ee) with yield greater than 99% under solvent free condition at 0.5 MPa pressure and 323 K temperature for 48 h in presence of 1 mol% of the cocatalyst TBAB (Table 11). Importantly, when chiral (either R or S) epoxide was used for cyclo addition reaction, the chirality was preserved in the carbonate product (99%ee, yield
Fig. 15. (a) Synthetic procedure of ZnW-PYI1 (left) and ZnW-PYI2 (right) and both are mirror iamge to each other. The yellow one is the rearrangement of precursor [W10O32]4 in both the framework, (b) 3D infinite structure of ZnW-PYI1 along crystallographic b direction [color code, Zn = cyan, C = gray, nitrogen = blue, PYI = orange]. Reproduced with permission from [40] Copyright 2015, Nature.
Fig. 16. (a) Representation of MOF component and tandem catalytic reaction for the conversion of olefin to CC, (b) probable mechanism for the tandem reaction. Reproduced with permission from [40] Copyright 2015, Nature group of publications.
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173 Table 11 Cycloaddition by the catalyst, Zn(II)-PYIs between CO2 and different epoxides.a Entry
Reactant
Product
1
2
O
O
O
O
Temp. (K)
Time(h)
Yieldb%
ee%c
323
120
92(94)
79(76)
323
48
>99
trace
323
48
>99
90
323
48
>99
96
323
96
92(90)
80(77)
323
96
83(85)
70(73)
323
96
72(70)
55(59)
O 3
O
O
O
O 4
O
O
O
O O
5
O
O O
6
O
O 7
O
O O
a Reaction conditions: Entry 1: ZnW PYIs (0.01 mmol), olefin (10 mmol), TBHP (20 mmol); Entry 2–4: MOF: 0.01 mmol, styrene oxide: 10 mol, TBABr 0.1 mmol; Entry 5–7: MOF: 0.01 mmol, olefin: 10 mmol, TBHP: 20 mmol. b,c The number in the parentheses indicates the yield (1H NMR analysis) of the product and ee (chiral OD-H column HPLC analysis) by the catalyst Zn-WPYI2.
Scheme 6. Post synthetic modification of aldehyde in ZIF-90 to quaternary ammonium cation to produce F-ZIF-90.
>99%) (Table 11). Moreover, one pot assymetric CO2 coupling was also softly achieved. In a typical reaction, heating the mixture of catalyst ZnW-PYI1, CO2, t-butylhydroperoxide, for 96 h, under 5 bar and 323 K, the olefins converted to the CC. The desired carbonate (R)-phenyl(ethylencarbonate) was accomplished in 92% yield with 80% enantiomeric excess. The good selectivity in the assymetric reaction was attributed to the –NH2 moiety inside the network architecture that increased the concentration of CO2 around the active centre and facilitated CC formation (Fig. 16). 2.2.1. Post synthetic modification of the functional site Post synthetic modification (PSM) technique in MOF chemistry had a great impact in broadening the scopes of these materials for
practical applications [41]. In the synthesis of CCs from CO2 and epoxides, the PSM technique could be of impotance through generation of MOFs with new architecture [42]. In 2015, Park and co-wrokers synthesized ZIF-90 from the Zn (NO3)24H2O and imidazolate-2-carboxyaldehyde and via PSM technique, converted [43] ZIF-90 to F-ZIF-90 (Scheme 6). In spite of having smaller pore volume in F-ZIF-90 compared to ZIF-90, it exhibited significantly greater efficiency of CC formation (yield 96.6%) than in case of ZIF-90 (yield 43%) under similar experimental condition (Table 12). The catalyst was reusable and the XRD patterns of the catalyst before and after the reaction, confirmed maintenance of the structural integrity. However, the yield of the product (96% for fresh and
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Table 12 Cycloaddition rection of CO2with allyl glycidyl ether (AGE) by catalyst, ZIF-90 and F-ZIF-90.a Catalyst a
Epoxides
Products
O
ZIF-90
O
O O
O a
O
F-ZIF-90
O
O b
O
ZIF-90
O
O
b
O
F-ZIF-90
O
O
6
393 K
40(61.8)
6
393 K
72.7(94.6)
6
393 K
43.4(50.4)
6
393 K
96.6(97.9)
O O
O
Yields [%] (sel. %)
O O
O
Temp. (K)
O O
O
T [h]
O
a Reaction conditions: Catalyst (20 mg i.e. 0.177 mol%), AGE (18.1 mmol). Batch operationa CO2 pressure initially of 1.17 MPa, Semi batchb having pressure of CO2 is constant, 1.17 MPa.
Table 13 The various cyclo addition reactions between CO2 with different epoxide by catalyst, F-ZIF-90.a Entry
Epoxides
1
Products
O
O
O
T [h]
Temp. (K)
Yields [%] (sel. %)
6
393 K
62.4(99.3)
6
393 K
94.3(99.1)
6
393 K
87.8(98.6)
6
393 K
96.3(98.7)
6
393 K
2.4(98.0)
O O
2
Cl 3
O Cl
O
O O
O
O O
4
O
O
O O
5
O
O O
a
O O
O
Reaction conditions: F-ZIF-90 (20 mg, 0.177 mol%), epoxide (18.1 mmol), pressure of CO2 is 1.17 MPa semi batch.
Scheme 7. Synthesis of Im-UiO-66 (isoreticular chemistry) and (I) Meim-UiO-66 (PSM method). Zr6 clusters (Green polyhedral) andmicropores (yellow ball). For clarity only linkerparts are shown. Adapted with permission from [45]. Copyright 2017, The Royal Society of Chemistry.
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
58.3% for reused) and BET surface area (763 m2/g for fresh and 548 m2/g for reused) of the reused catalyst decreased from the original one probably due to blockage of the active sites in the catalyst by carbonaceous products and low stability of the catalyst. The framework is also acts as a versatile catalyst in different CC formation (Table 13). The mechanism for CC formation was quite similar as in the case of Lewis acid based catalysis. The same group performed
17
another PSM between ZIF-90 and ionic liquid (IL) (1-aminopyridinium iodide) to from IL-ZIF-90 [44]. The newly formed catalyst, IL-ZIF-90 could execute cylcoaddition reactions between CO2 with a number of epoxide affording cexcellent yields at 393 K for 3 h reaction and unlike the previous case, this catalyst was reusable upto five cycles without losing activity. A zirconium based imidazolium bifunctional functionalized ZrMOF, (I)Meim-UiO-66 had been synthesized [45] from the parent
Scheme 8. The probable mechanism for the cycloaddition reaction by (I) Meim-UiO-66. The hashed bonds indicate p p, cationic p or hydrogen bonding interactions. Reproduced with permission from [45]. Copyright 2017, The Royal Society of Chemistry.
Scheme 9. (a) Synthesis of MOF, UiO-66-IL-Br and its anion exchanged product (UiO-66-IL-X, where X = anions, SO3CF 3 , PF6 ), (b) schematic representation for the synthesis of cross linked MOF membrane of UiO-66-IL-ClO4. Reproduced with permission from [46]. Copyright 2017, American Chemical Society.
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Fig. 17. (a) Loading of MOFs in the membrane UiO-66-IL-ClO4, (b) IR spectra of the membranes and oligomer. Reproduced with permission from [46]. Copyright 2017, American Chemical Society.
Table 14 Cycloaddition reaction results with membrane bound UiO-66-IL-ClO4 (membrane MOF). Entry
Epoxides
1
Cl 2
Cl 3
Br a
Products
O
O Cl
O Cl
Membrane MOF (0.7 mol% MOF)
29
O
TBAB (1 mol%)/membrane MOF (0.7 mol% MOF)
99
TBAB (1 mol%)/membrane MOF (0.7 mol% MOF)
99
O
O Br
Yields [%]a
O O
O
O
Catalyst
O O
1
The yield of different CCs were determined by H NMR spectroscopy.
Scheme 10. The post synthetic modification of UTSA-16 by alkali metal cations to M-UTSA-16 followed by cycloaddition reaction. Reproduced with permission from [47]. Copyright 2017, Elsevier.
Table 15 The BET surface area of MOFs (M-UTSA-16). MOF
BET surface area (m2/g)
Cs-UTSA-16 Rb-UTSA-16 K-UTSA-16 Na-UTSA-16 Li-UTSA-16
291.3 461.5 847.0 544.1 519.4
Im-UiO-66 adopting a post-synthetic modification technique (Scheme 7). The bifunctional framework, (I) Meim-UiO-66, consisting both iodide ions and Brønsted acid sites, exhibited superior catalytic activity in absence of any co-catalyst. In this particular MOF, the I ions present in the channel near the catalytic centre of Zr6 clusters, acted as a co-catalyst. The cycloaddition reactivity between CO2 and epichlorohydrin was found to be 88.5% conversion in 24 h under 1 bar pressure
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Fig. 18. (a) CO2 uptake by M-UTSA-16 at 273 K, (b) conversion of cycloaddition reaction by M-UTSA-16. Reproduced with permission from [47]. Copyright 2017, Elsevier.
Table 16 Cyclo addition reaction between CO2 with different epoxide epoxides by Li-UTSA-16.a Entry
Epoxides
1
H3C 2
Products
O
O
O H3C
T [h]
Temp. (K)
Conversions (%) (sel.)[%]
6
393 K
25.3 (>99)
6
393 K
25.9 (>99)
6
393 K
99 (>99)
O
O
O
O
O 3
Cl a
O
O Cl
O O
Reaction conditions: Catalyst (60 mg), epoxide (3.5 mL), pressure of CO2 1.5 MPa.
Scheme 11. The synthetic scheme for the preparation of polyILs@MIL-101. Reproduced with permission from [48]. Copyright 2018, The American Chemical Society.
at 373 K. The lower conversion rate may be due to the lower BET surface area (328 m2 g1) offered vy MOF, (I) Meim-UiO-66. Interestingly, when the temperature was raised to 393 K, a quantitative conversion was obtained. This catalyst could be recycled six
times in the cycloaddition reaction of epichlorohydrin with CO2, without a noticeable loss in activity. An acid/base synergistic catalysis mechanism has been proposed where the ring-opening of the epoxide occurs due to the synergistic activation effect by
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nucleophilic Lewis base, iodide anion and Brønsted-acid (Zr–OH/ Zr–OH2) (Scheme 8). A membrane-based MOF reported by Dong et al. that exhibited a cycloaddition reaction between CO2 and epoxide [46]. An ionic liquid (imidazolium ion) based dicarboxy ligand reacted with ZrCl4 solvothermally led to the formation of the MOF, UiO-66-IL-Br (Scheme 9). The Br anion could be exchanged to form UiO-66 IL-X (X = anion, SO3CF 3 , PF6 , ClO4 , Br ). Upon reacting with a polyurethane oligomer it formed a cross-linked membrane bound MOF (Scheme 9). Amongst all the anion exchanged ionic liquid based MOFs, UiO-66-IL-ClO4 and cross-linked membrane bound
UiO-66-IL-ClO4 (Fig. 17) showed very good selectivity of CO2 adsorption and excellent yields for the cycloaddition reactions in presence of TBAB (Table 14). The mechanism of cycloaddition reaction was proposed to be similar to that of acid catalysed reaction. In another report, the well known MOF, UTSA-16 had been post synthetically converted [47] to the series of alkali metal based MOF, M-UTSA-16 where the pore of the framework contained alkaline metal cations (M = Cs, Rb, K, Na, Li) (Scheme 10) with different BET surface area (Table 15). The CO2 uptake capacity at 273 K and 298 K increased in the order Cs+ < Rb+ < Li+ < Na+ < K+ commensurate with the BET surface area (Fig. 18).
Fig. 19. (a) The adsorption isotherm of N2 and CO2 by polyILs@MIL-101 at different temperatures, (b) Qst for CO2 adsorption by polyILs@MIL-101 and MIL-101. Adapted with permission from [49]. Copyright 2018, The American Chemical Society.
Table 17 Cycloaddition by polyILs@MIL-101 between CO2 and different epoxides.a Entry
Epoxide
Product
O
1
O
O
Time (h)
Temperature (K)
Sel.(%)b
Con.(%)b
2
318
100
94
1
343
100
89
1
343
100
85
2
343
100
81
1
343
100
95
3
383
100
68
1
343
100
>99
O
O
2
O
O
O 3
Br
O
O Br
O
4
O O
O
O O
5
O
O
O O
6c
O
O
O 7
HO a b c
O
O HO
O O
O O O
Reaction conditions: catalyst polyILs@MIL-101 (100 mg), epoxide (1 mmol), PCO2 = 1 bar, solvent acetonitrile (2 mL). GC was used to determine % of con. and sel. DMF 2 mL was used.
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All the cation exchanged frameworks were able to perform the cycloaddition reaction between propylene oxide (PO) and CO2 (Fig. 18b, Table 16) and among them the activity was showing by Li-UTSA-16 (72%) was much greater compare to the other frameworks (M-UTSA-16). The possible reason is due to the highest charge density (since smallest radii) of Li+ ion compare to the other alkali metal cations can diffuse the epoxide and CO2 effectively. The catalyst was reusable upto five cycles without losing its conversion efficiency. Recently, Jian and co-workers post-synthetically modified MIL101 (Scheme 11) with a poly ionic liquid (poly IL) to have,
HOOC
polyILs@MIL-101 which could efficiently perform cycloaddition reaction between CO2 and epoxide [48]. Although polyILs@MIL-101 had lower BET surface area and pore volume of (2462 m2/g and 1.26 cm3/g) compared to the pristine MOF, MIL-101 (3603 m2/g and 1.83 cm3/g), it exhibited significant CO2 adsorption capacity with greater heat of adsorption (Fig. 19) due to increased aromatic p-surface and the presence of confined pore size [49] compared to the MIL-101. The MOF, polyILs@MIL101 was able to performed cycloaddition reactions with a variety of epoxides (Table 17) and can be re-used without loss of catalytic activity. The reaction mechanism was similar to the acid catalysed one.
COOH
NH2
SBU HOOC
NH2 group decorated cage
COOH
(b)
(a)
Fig. 20. (a) Tetracarboxylate linker, (b)–NH2 decorated pore within MOF network. Reproduced with permission from [51]. Copyright 2015, Wiley VCH.
Table 18 The styrene oxide (SO)# cycloaddition reactions with CO2 and catalysed by Cu(II)-MOF. Entry
SO:MOF
Time (h)
Temperature (K)
Pressure (bar)
Conversion (%)*
TON
1 2 3
667 667 667
6 12 12
393 393 393
20 20 50
43 50 13
286 333 86.7
# The activated Cu(II)-MOF immediately transferred inside glove box at room temperature. Styrene oxide and TBAB were added within the autoclave already placed inside the glove box. * 1 H NMR helps to analyze the conversion of CC by integration of SO peak with CC peak.
NH2 HO2C
CO2H
CO2H
(a)
CO2H
(b)
Fig. 21. (a) Schematic diagram of the ligand, (b) the pore along a-axis. Reproduced with permission from [52]. Copyright 2017, The Royal Society of Chemistry.
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in presence of TBAB as the co-catalyst. However, a maximum of 50% conversion was found in this reaction. When the ligand was replaced with a bent tetracarboxylate (Fig. 21) incorporating an outward –NH2 group in the middle benzene ring, a paddle-wheel Cu2(COO)4 SBU was formed with metal bound solvent molecules [52]. Upon heating, a porous MOF embellished with unsaturated metal centres and free –NH2 groups was formed showing very high adsorption of CO2 gas (60% by weight) at 298 K and 32 bar pressure. The activated MOF catalyzed the adsorbed CO2 to CCs with epoxides at room temperature in presence of the co-catalyst TBAB. Table 19 indicates the MOF is able to convert different epoxide to their corresponding carbonate by reacting with CO2. Most significantly, the CO2 present in the atmosphere was successfully converted to the CC at room temperature when the air was passed through the suspended solution of MOF in presence of TBAB (Table 20). The catalyst was reused upto four cycles with out change in its reactivity. The good yields of the CCs under ambient condition was due to the easy accessibility of the vacant Cu(II) sites for activation of the
2.3. Cooperative bifunctional catalysis due to open coordination sites and Lewis basic functional linker Although the cycloaddition of CO2 to epoxides was mainly catalyzed by Lewis acidic open metal sites, still literature report divulged that the cooperative effect contribute from both Lewis acidic and basic component parts in the MOF can execute the cyclo addition reaction between CO2 and epoxide under mild condition [50]. For example, a hanging amine moiety in the linear tetracarboxylic acid, that formed a nbo type Cu(II)-MOF (Fig. 20) had been reported [51]. The high thermal stability of this MOF allowed the removal of loosely bound solvents from metal as well as from cavity on heating to get a porous framework. The presence of coordinatively unsaturated metal sites, accessible Lewis basic NH2 group and large voids made the compound an excellent heterogeneous catalyst for the cycloaddition of CO2 to epoxides under relatively mild reaction conditions (20 bar, 393 K). As seen from Table 18, the compound could serve as a good recoverable catalyst for the cycloaddition of CO2 to styrene oxide under solvent-free condition
Table 19 Cycloaddition of various epoxides with CO2 catalyzed by activated MOF using CO2 gas at 1 bar and at room temperature. Entry
Epoxide
Product
O
O Cl
Time (h)
Yields (%)
8
95a 19b 10c
12
92
8
89
12
88
8
85
Time (h)
Yields (%)
24
62
24
45
24
57
8
30
O O
Cl 2
O
O
O O
3
O
O O O
4
O
O O
O O
O
5
O
O O
a b c
O
In presence of both MOF and TBAB. Absence of co-catalyst TBAB. Absence of MOF.
Table 20 Cycloaddition of various epoxides with CO2 through purging of atmospheric air at 298 K. Entry
Epoxide
1
Product
O
O O
Cl
O
Cl 2
O
O O O
3
O
O O
O O
4
O
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Scheme 12. The probable mechanism for the cycloaddition reaction catalyzed by Cu(II)-MOF in presence of TBAB. Reproduced with permission from [52]. Copyright 2017, The Royal Society of Chemistry.
epoxide ring and also free-NH2 groups for non-covalent interaction with CO2. The proposed mechanism [53] could be found in Scheme 12. The open Cu(II) sites of the activated MOF accelerated the opening of the epoxide ring by the use of TBAB to afford a
Cu-bound alkoxide. Next, cycloaddition of CO2 to the metal bound epoxide occurred in presence of TBAB. The metal bound carbonate subsequently formed CC with the regeneration of the catalyst. Recently, Neogi and co-workers by using 2-amino terephthalic acid constructed popular framework, UiO-66-NH2 (known as ZrOF) [54]. They incorporated metal nanoparticles (NPs) inside the pores via post-synthetic modification (Scheme 13) and characterized the NP embedded framework (Ni@ZrOF) by a number of spectroscopic techniques that suggested that the ultra-small Ni NPs well dispersed inside the pores. Although BET surface area was reduced compared to the original MOF, the CO2 adsorption capacity improved by 35% with 10 kJ/mol rise in the heat of adsorption value. This compound in presence of TBAB as the co-catalyst, catalysed solvent-free CO2 cycloaddition with styrene oxide (SO) in a heterogeneous manner in 98% yield with 99% selectivity (Table 21, entry 10) at 1.0 MPa CO2 pressure and 343 K for 6 h. The catalyst could be recycled at least five times without any significant loss in activity and was quite versatile for other substrates as well (Fig. 22). A tentative mechanism had been provided as shown in Scheme 14. The efficient catalytic activity of the Ni@ZrOF system was ascribed to its empty network architecture with Zr4+ acidic sites, introduction of Ni nanoparticles, and pendent basic amino groups that cooperatively augmented its catalytic abilities. Evidently, the presence of both Nio and Zr4+ metals synergistically affect the catalytic performance of Ni@ZrOF, by activating the epoxide oxygen atom (Scheme 14). Li et al. reported [55] a 3D Ba-MOF, [Ba2(BDPO)(H2O)] (H4BDPO = N,N-bis(isophthalic acid)-oxalamide) with oxophilic alkaline-earth Ba2+ ion (Fig. 23). It had 1D honeycomb hexagonal channels with a window diameter of ~8.2 Å. The activated framework exhibited efficient CO2 chemical fixation owing to the
Scheme 13. Synthesis of Ni@ZrOF using the Solution Impregnation Method. Reproduced with permission from [54] Copyright 2019, American Chemical Society.
Table 21 Cycloaddition of styrene oxide (SO) and CO2 using various catalysts.a
a
Entry
Catalyst
Temperature [K]
Selectivity [%]
Yield [%]
1 2 3 4 5 6 7 8 9 10 11
none ZrCl4 NH2–BDC TBAB NH2–BDC + ZrCl4 + TBAB ZrOF Ni@ZrOF ZrOF/TBAB Ni2 + @ZrOF/TBAB Ni@ZrOF/TBAB Ni@ZrOF/TBAB
343 343 343 343 343 343 343 343 343 343 393
0 – – 98 98 99 99 97 98 99 99
0 – – 25 30 41 45 52 62 98 99
Reaction conditions: catalyst, 0.35 mol %, SO, 30.6 mmol; co-catalyst, 0.11 mol %; 1.0 MPa of CO2 pressure for 6 h at 343 K.
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
presence of the open metal site (Lewis acidic) and the oxalamide groups (Lewis basic) inside the tubular channel (Table 22, entry 1). The pleasing catalytic activity were indorsed due to the rod like metal–organic chains, suitable for epoxides activation (Scheme 15) aided by the oxalamide groups that effectively interacted with CO2 molecules. Lower conversion for the larger epoxides indicated that the cycloaddition reactions took place within the cavity. The MOF showed no loss of activity after five runs with epichlorohydrin and CO2. Solvothermal synthesis of two frameworks, Zn-NTTA and CuNTTA with the tripodal ligand (H6NTTA) (Fig. 24) had been reported [56]. In Zn-NTTA, two kinds of dinuclear paddle-wheel SBUs {Zn2(OOC)3} and {Zn2(OOC)4} were present leading to three cages of diameters 16.0, 15.6, and 19.0 Å. The framework showed high and selective uptake of CO2 (115.6 cm3g1) at 273 K. Presence of highly dense vacant coordination sites along with acylamide functional groups made these MOFs highly catalytically active with TON values up to 110,000 per paddle-wheel unit and 30 rounds of recyclability under mild conditions. Within 8 h of reaction, about 0.25 mol % of Zn-NTTA (based on a paddle-wheel unit) provided nearly complete conversion (yield ~98%) (Table 23) when the reactions were conducted under solvent-free condition at 373 K in the presence of TBAB co-catalyst with CO2 purged at 1.0 MPa. Under similar conditions Cu-NTTA also showed very good catalytic activity. The cycloaddition reactions using mixed gas as CO2 source under the same reaction conditions afforded a significant yield of 87.3% after 8 h. This showed no adverse effect of moisture (Fig. 25) and the MOFs could be used for the selective adsorption and chemical conversion of CO2 releasing from a power plant. The thermally robust anionic Zn(II)-MOF, {[(CH3)2NH+2]2[Zn3 ((l3-O))(L)2(H2O)]4DMF2H2O}n, synthesized solvothermally [57], was highly porous having free amino groups. The linker L used was a tricarboxylate ligand shown in Fig. 26. This anionic 3D framework showed a two-fold interpenetrated structure. Loss of metal bound and lattice water molecules by heating afforded a porous structure with 45.1% void volume. In presence of TBAB as
O
O Cl
O
H3C
Scheme 14. The probable mechanism for CO2 conversion with substituted styrene epoxide by Synergic Ni@ZrOF/TBAB Catalyst. Reproduced with permission from [54]. Copyright 2019, American Chemical Society.
co-catalyst, this activated MOF exhibited outstanding catalytic ability in transforming CO2 to CCs with various epoxides under atmospheric pressure and solvent free conditions. The maximum amount of conversion was 99% (Table 24, entry 1), in case of styrene oxide as substrate at 20 bar pressure and 373 K for a reaction time of 6 h. It could be reused at least three times without much loss of catalytic activity. Another amine-functionalised framework built on MIL-101, NH2-MIL-101(Al) by solvothermal (1S) and microwave (1M) methods, was reported by Neogi and co-workers [58]. Using styrene oxide (SO) as the substrate, 96% styrene carbonate was obtained with 99% selectivity and 23.5 h1 turnover frequency (TOF) when
O
O
O
O
O
O
H2C
8
24
O
CH3
Fig. 22. The bar diagram indicates the yield (red) and selectivity (green) for cycloaddition reaction catalysed by Ni@ZrOF/Bu4NBr. Reproduced with permission from [54] Copyright 2019, American Chemical Society.
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Fig. 23. (a) Ligand, H4BDPO, (b) left-handed 21 helical rod SBUs, (c) yellow rod indicates tubular structure, (d) representation of the porous surface, (e) the pore along caxis. Reproduced with permission from [55]. Copyright 2018, American Chemical Society.
Table 22 Results of the cycloaddition reactions of CO2 with different epoxides catalyzed by Ba-MOF. Entry
Epoxides
1a
Products
O
O
O
Conversion [%]
TONe
98.1
196.2
99.2
198.4
95.7
191.4
94.5
189.0
90.2
180.4
94.1
188.2
97.5
195.0
19.8
39.6
52.3
104.6
O 2b
O
O
O O
3a
H3C 4a
Br 5a
Cl 6c
Cl 7d
Cl 8a
O
O
O H3C
O Br
O
O
O
O O
Cl
O Cl
O O
O Cl
O
O O
O
O
O
O O
O
O O
9a
O
O
O O
a b c d e
O O
Reaction conditions: 0.5%mmol catalyst, 20 mmol epoxides, 2% mmolTBAB,CO2 (1 atm), room temperature, 48 h. The same conditions but CO2 pressure increased to 10 atm and the time reduced to 6 h. The same conditions with temperature enhanced to 323 K. Under same circumstances but temperature increased to 343 K. TON (product (mmol)/catalyst (mmol)).
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activated 1S/TBAB system was used at 393 K, 18 bar CO2 pressure for 6 h of reaction (Table 25, entry 9). Under the same reaction condition, the activated 1M/TBAB system showed almost equal conversion (93.6%) with 99% selectivity (Table 25, entry 10). The catalyst could be reused five times. A variety of aliphatic and aromatic epoxides showed excellent conversion. A reaction mechanism was also proposed (Scheme 16). Solvothermal reaction of 2,5-di(1H-1,2,4-triazol-1-yl)tereph thalic acid, H2BTTA (Fig. 27a) with Cu(NO3)2 in H2O at 393 K for 3 d afforded the framework FJI-H14 [59]. The structure contained 1D hexagonal channels along the crystallographic caxis (Fig. 27c) and had high density of open metal sites (OMS) and also Lewis basic sites (LBS) within the channel. This framework
captured CO2 effectively and selectively and displayed remarkable stability in presence of acid/ base and high volumetric uptake (171 cm3 cm3) of CO2 under ambient conditions (298 K, 1 atm). The CO2 present in the flue gas could be efficiently transformed into CCs by the FJI-H14 catalyst and TBAB as co-catalyst. Such high CO2 uptake capacity and reasonable catalytic performance may result from the cooperative effect of the various active sites. Compound FJI-H14 displayed a far higher catalytic performance for the reaction of styrene oxide with the CO2 present in simulated flue gas compared to other catalysts, as shown in Table 26. The larger size epoxide, 1,2-epoxyoctane gave less yield (27%), signifying that the reactions occurred within the channels of FIJ-H14.
Scheme 15. The CO2 conversion mechanism. Reproduced with permission from [55] Copyright 2018, American Chemical Society.
Fig. 24. Acylamide-bridging ligand, H6NTTA and three types of cages present in Zn-NTTA. Spheres represent the void spaces. H atoms and coordinated water molecules are not shown for clarity. Reproduced with permission from [56] Copyright 2016, American Chemical Society.
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2.4. Catalysis due to the presence of defect in the crystal In one of the earlier reports in 2009, MOF-5 in presence of TBAB was utilized for the fixation of CO2 with epoxides exclusively based on defect in crystal structural [21]. The MOF-5/TBAB was a very efficient catalyst system and the reaction could be completed within 6 h at 323 K and 6 MPa CO2 pressure with very high selectivity and could be recycled. Other epoxides could also be transformed effectively under mild conditions (Table 27). In a similar way, MOF-205/TBAB system showed excellent catalytic activity in the room temperature CO2 fixation [60]. Given that the metal nodes were fully saturated, defect sites in the structure were proposed [61] to be responsible for its catalytic activity. Taking a cue from this result, Carreon and his group demonstrated that zeolitic imidazole framework-8 (ZIF-8) showed effective catalytic activity in the synthesis of chloropropene carbonate as the main product by the cycloaddition of CO2 to epichlorohydrin [62]. It was proposed that as a consequence of surface defects, ZIF8 was highly active in the conversion of epichlorohydrin under solvent free condition (98.2% conversion at 373 K) without the aid of any co-catalyst. Unfortunately, the product selectivity was low (33.4%). To increase the selectivity and yield, ZIF-8 was postsynthetically modified by functionalization on the surface with ethylene diamine (labelled as ZIF-8-f). This post-synthetically modified sample offered both improved product selectivity (chloropropene carbonate, 73.1%) and conversion (100%) at temperature of 353 K demonstrating the importance of combining defect sites at the surface with post-synthetic modification as a helpful approach for enhancing the catalytic performance of MOFs. Jiang group synthesized a stable, Al-containing MOFknown as
Fig. 25. Fixation of CO2 present in wet gases with epoxides. Reaction conditions: 20 mmolMOF, 100 lLH2O, 5 lmolcatalyst (based on a paddle wheel unit), and TBAB (0.3 mmol) under 1.0 MPa CO2 or mixed gas pressure, temperature: 373 K, time: 8 h. Reproduced with permission from [56] Copyright 2016, American Chemical Society.
USTC-253, isoreticular to MOF-253, by substituting bipyridine-5,50 -dicarboxylate ligands with sulfone-functionalized 4,40 -dibenzoic acid-2,20 -sulfone ligand (Fig. 28) [63]. When some amount of modulator like trifluoroacetic acid (TFA) was used during the synthesis, USTC-253-TFA was formed with structural defects within the crystals (Fig. 29).
Table 23 Coupling of epoxides with CO2.
Entry
Substrate
Zn-NTTA
Cu-NTTA
Yield (%)
TON
Yield (%)
TON
1a
98.2
3920
95.9
3830
2a
64.1
2560
56.3
2250
3a
97.2
3880
97.4
3895
4b
82.5
3300
89.4
3570
a Reaction conditions: 20 mmol epoxide, 5 lmolcatalyst (based on a paddlewheel unit), and TBAB (0.3 mmol), 1 MPa CO2pressure, 373 K for 8 h. The yields were calculated from 1H NMR spectra. b All experimental conditions are same except 10 mmol epoxide was used.
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Fig. 26. (a) Structure of ligand H3L, (b) trinuclear SBU of in the framework showing metal bound water molecule, (c) coordination modes of ligand showing Lewis basic –NH2 group, (d) before and (e) after two-fold interpenetration in the framework. Reproduced with permission from [57]. Copyright 2017, American Chemical Society.
Table 24 Cycloaddition reactions of CO2 with various epoxides.a
Time (h)
Temp. (K)
P (bar)
% Yieldb
1
6
373
20
99
2
6
373
10
95
3
12
373
1
96
4
12
323
1
92
5
12
323
1
94
6
12
323
1
87
Entry
Epoxide
Product
29
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173 Table 24 (continued) Entry
Epoxide
Product
7
a b
Time (h)
Temp. (K)
P (bar)
% Yieldb
6
323
10
98
Reaction conditions: epoxides (25 mmol, 1 equiv), Activated MOF (10 wt%), 40 mg TBAB, (0.005 equiv). solvent free. Conversion was estimated from the 1H NMR spectra.
Upon activation by heating, these defects led to the generation of Lewis acidic open metal sites showing greater CO2 uptake property (168–182%) with respect to the defect-free pristine USTC-253, isoreticular MOF-253, and MIL-53. Additionally, the presence of large no of easily accessible Lewis and Brønsted acid sites, USTC-253-TFA exhibited a reasonable catalytic activity to give cyclic carbonates at ambient temperature and 1 bar CO2 pressure. The catalytic activity of USTC-253-TFA was superior to that of parent USTC-253 and some other MOFs (Figs. 30 and 31). It was believed that large no of easily accessible Lewis and Brønsted acidic sites in USTC-253-TFA could increase the cooperative effect with TBAB to help the CO2 cycloaddition reaction (Fig. 32).
MOF-892 possessed large hexagonal one-dimensional channels of pore aperture of ~24 27 Å2 along the crystallographic c-axis while MOF-893 had triangular 1D channels along the crystallographic a-axis with much smaller pore aperture of ~8.6 9.9 Å2. MOF-894 was an overall 2D anionic framework having two crystal-
2.5. Catalysis due to the presence of accessible acidic functional sites With a linker having multi-carboxylate groups, one or more acid groups might remain uncoordinated in the MOF. Following this approach, a new 3D framework, [Ni(btzip)(H2btzip)] 2DMF2H2O (1) had been reported (H2btzip = 4,6-bis(triazol-1-yl) isophthalic acid) [64]. The framework architecture consisted of 2D interconnected channels and the pore of the channels exposed with free acidic functional groups and the nitrogen atom of the triazole unit (Fig. 33). The activated framework (1a) efficiently catalyzed the transformation of CO2 to CCs with various epoxides in presence of co-catalyst TBAB. The maximum yield (98.9%) was found when CO2 was reacted with propylene oxide under 2.0 MPa pressure of CO2 and 353 K for 4 h (Table 28, entry 4). In this case, activation of the epoxide was possible through the free –COOH groups in the channels which was then attacked by the Br ion of TBAB on the less hindered C atom of epoxide. In the final step, the alkyl carbonate anion was converted into the CC, and simultaneously TBAB was regenerated. A hexatopic linker, 10 ,20 ,30 ,40 ,50 ,60 -hexakis(4-carboxyphenyl)ben zene had been used to construct three MOFs, viz., Zirconium based (MOF-892 and MOF-893) and Indium based MOF-894 respectively, (Fig. 34) [65]. All the frameworks were highly porous and stable. The overall structure of MOF-892 and MOF-893 contained Zr6 cluster and two out of the six carboxylic acid groups remained free.
Scheme 16. The general (above, A = acidic site; B = basic site) and probable mechanism (below) for CO2 conversation with epoxides using 1S/TBAB system. Reproduced with permission from [58] Copyright 2018, The Royal Society of Chemistry.
Table 25 Cycloaddition reaction of CO2 with styrene oxide. Entry
Catalyst
Temp. [K]
Conversion [%]
Selectivity [%]
Yield [%]
1 2 3 4 5 6 7 8 9 10
None AlCl36H2O NH2-BDC TBAB AlCl36H2O + NH2-BDC + TBAB 1S 1M 1S/TBAB 1S/TBAB 1M/TBAB
393 393 393 393 393 393 393 RT 393 393
0 Trace Trace 47 48 26 22 Trace 99 93.6
0 – – 99 98 78 72 – 99 99
0 – – – – 14 11 – 96 95
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Fig. 27. Structural diagram of FJI-H14. (a) The linker H2BTTA used to construct FJI-H14. (b) Four-connected metal and ligand binding mode. (c) 1-D channels along the caxis. (d) USF topological figure (Cu atoms are in cyan, C atoms are ingray, O atoms are in red, N atoms are in blue and H atoms are in white colour). Reproduced with permission from [59] Copyright 2017, Nature Communication.
Table 26 Cycloaddition of epoxides and CO2.a
Catalysts
Time (h)
Yield (%)b
1
FJI-H14
24
86
2
–
24
52
3
Cu(OAc)2
24
45c
4
Cu(NO3)2 + H2BTTA
24
70
5
HKUST-1
24
67
Entry
Substrates
31
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173 Table 26 (continued) Catalysts
Time (h)
Yield (%)b
6
FJI-H14
24
95
7
FJI-H14
24
27
Entry
Substrates
a Reaction conditions: 20.0 mmol styrene oxide, 0.48 mol % catalyst (per Cu(II) units), 2.5 mol %TBAB, the reactions were conducted in a Schleck tube connected with a condenser, simulated post-combustion flue (CO2 = 0.15 atm, N2 = 0.85 atm) was purged at 1 atm and 353 K for 24 h. b Determined by 1H NMR. c Some by-products were formed in case of Cu(OAc)2 catalyst.
lographically distinct monoatomic indium ions. All the MOFs are higly effective and selective for transformation of CO2 to CCs under solvent free condition of CO2 pressure 1 atm at 353 K (Table 29). A plausible mechanism had been proposed for these transformations as shown in (Scheme 17) for MOF-892. The epoxide was
effectively activated due to the coexistence of accessible Zr units and free carboxylic acid moieties. Hydrothermal synthesis of a 2D porous MOF, {[Zn2(TBIB)2 (HTCPB)2]9DMF19H2O}n using a mixture of ligands 1,3,5-tri(1Hbenzo[d]imidazol-1-yl)benzene (TBIB) and 1,3,5-tris(40 -carboxy
Table 27 Synthesis of cyclic carbonates catalyzed by MOF-5 in the presence of TBAB.a Entry
a b
Time [h]
Yield [%]
1
Epoxides
Products
3
56
2b
4
97
3
12
93
4
15
92
Reaction conditions: epoxide 20 mmol with 2.5 mol% TBAB, 0.1 g MOF-5, CO2 pressure 0.1 MPa, reaction temperature 323 K. CO2 pressure 6 MPa.
Fig. 28. Framework structure of USTC-253 (left) and different ligands for the constraction of isostructural MOFs (right). Reproduced with permission from [63]. Copyright 2015, Wiley VCH.
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(Table 30). The catalyst was reused up to five times. The reaction mechanism was similar to other cases having free –COOH groups (Scheme 17). A non-interpenetrated suphonate based MOF, TMOF-3 had been synthesized hydrothermally [67] by reacting 1,2,4,5-benzenetetra methane sulfonate, 4,40 -bipyridine, and Cu(NO3)23H2O at 448 K for 72 h (Scheme 18). Crystallographic investigation revealed two sulphonate groups were deprotonated and were distributed in the pore of the channels. The framework showed highest void space of 42.5% amongst the MOFs having organosulfonate ligand. It exhibited high CO2 adsorption capacity. When TMOF-3 was placed in 10 mL aqueous solution of silver nitrate (10 mmol/L) for one hour it afforded TMOF-3-Ag (Scheme 18). This TMOF-3Ag framework adsorbed significant amount of acetylene (103 cm3/g and 75 cm3/g at 273 and 298 K (Fig. 36a). Also, TMOF-3-Ag catalyzed the reaction between CO2 and propargyl alcohol in presence of the base DBU at room temperature to form carbonate (Table 31). The probable mechsnism of this catalysis reaction could be as in Fig. 36b.
2.6. Catalysis due to the accessible Lewis and Brønsted acid bifunctional sites
Fig. 29. Synthesis of (a) USTC-253 and (b) defect-engineered USTC-253-TFA. Reproduced with permission from [63]. Copyright 2015, Wiley VCH.
Fig. 30. Fixation of CO2 with propylene oxide usingvarious catalysts. Reaction conditions: 28.6 mmol propylene oxide, 0.289 mmol catalyst, 1.86 mmol TBAB, under 1 bar CO2 pressure and 298 K for 72 h. Reproduced with permission from [63]. Copyright 2015, Wiley VCH.
phenyl-)benzene (H3TCPB) had been reported [66]. The 2D layer structure formed an overall 3D structure with 1D channels via supramolecular interactions (Fig. 35). Free –COOH groups and uncoordinated–N atoms of the TBIB ligand decorating the channels acted as an excellent catalyst for the formation of CCs from various epoxides at room temperature
Ma and coworker utilized the linker 2,4-bis(3,5-dicarboxyphe nylamino)-6-oltriazine (H4BDPO) for the synthesis of buffer CuMOF, (JUC-1000) [68] where the phenolic component showed weak acidic behavior and triazine and amino groups displayed weak basic activity (Fig. 37). The MOF maintained its structural integrity at variour pH range in aqueous media owing to the combine effect put forward by basic and acidic functional groups as buffer guards (Scheme 19). The buffer ligand H4BDPO reacted with copper nitrate to form the microporous MOF, JUC-1000 having copper paddle-wheel SBUs. The high CO2 uptake capacity by JUC-1000 exerted by the presence of open metal center and the basic part allowed to performed CO2 fixation reaction with epoxide under solvent free condition and TBAB as co catalyst at 1 atm and room temperature (Table 32). It behaved as a superior catalyst compared to the other homologous MOFs like HKUST-1 (62% yield, entry 3) and MOF-505 (61%, entry 2) under similar conditions. The lower yiled was observed when the size of the epoxide increases (entries 4, 5, and 6). It is quite interesting to note that the epoxide ring has been activated synergistically: The oxygen atom of the epoxide can interact with the open Cu(II) metal site and also the hydrogen bonding interaction with the phenolic –OH and the amino group, already placed in the ligand (Fig. 38). The ring opening was facilitated by Br ion from TBAB and the resultant oxide intermediate then rapidly reacted with the activated CO2 molecules (amino and triazine moiety activated the CO2) to produce alkyl carbonate. The final CC was achieved through the cyclization step. The buffer mediated platform in the MOF encouraged for such an efficient CO2 cycloaddition reaction with epoxide. An tetrazolyl-carboxylate ligand, 5-(4-(tetrazol-5-yl)phenyl)iso phthalic acid (H3tzpa) has been used to form a 3D Co(II)-based MOF [69] in which l3–OH groups and free carboxylate oxygen atoms are adorned inside 1D tubular channels (Fig. 39). The framework was chemically stable in aqueous acidic and basic solutions. This activated MOF, 1a showed high capacity, selective CO2 adsorption and facile catalytic activity in the cycloaddition of CO2 to epoxides to form CCs at 298 K and 100 kPa (Scheme 20) in presence of TBAB as co-catalyst (Table 33). Maximum of 93.8% yield of propylene carbonate was reported at a molar ratio of 100/10/1for epoxide/TBAB/1a for 48 h (Table 33).
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
33
Fig. 31. Chemical fixation of CO2: (a) propylene oxideas substrate using various catalysts, (b) various substituted epoxides as substrate using USTC-253-TFA as catalyst. Reaction conditions: 28.6 mmol epoxide, 0.289 mmol catalyst, 1.86 mmol TBAB, under 1 bar CO2 pressure and 298 K for 72 h. Reproduced with permission from [63]. Copyright 2015, Wiley VCH.
Fig. 32. Proposed mechanistic diagram for the fixation of CO2 with epoxideusing USTC-253-TFA catalyst. Reproduced with permission from [63]. Copyright 2015, Wiley VCH.
Fig. 33. (a) The H2btzip ligand, (b, and c) the pore along a and b axes in 3D framework showing free –COOH groups and 2D channels respectively. Adapted with permission from [64]. Copyright 2018, Wiley-VCH.
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Table 28 Cycloaddition reactions of various epoxides catalyzed by 1a in presence of TBAB. Entry
Catalysts
Epoxides
Products
Yields [%]
1
1a
5.2
2
TBAB
10.8
3
1a/TBAB
9.0
4
1a/TBAB
>98.9
5
1a/TBAB
98.3
6
1a/TBAB
83.1
7
1a/TBAB
90.9
8
1a/TBAB
40.0
Epoxide (20 mmol), 1a (0.2 mmol), TBAB (2 mmol). Reaction conditions: entry 3: 0.1 MPa, 298 K, 48 h; entries 1–2 and 4–8: 2.0 MPa, 353 K, 4 h.
Fig. 34. Hexatopic linker and structure of MOF-892, MOF-893, and MOF-894. Reproduced with permission from [65].Copyright 2018, American Chemical Society.
35
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173 Table 29 Reactions between CO2 and styrene epoxide by MOF-894, MOF-893, and MOF-892.a
a b c
Entry
Catalyst
Conversion [%]b
Selectivity [%]b
T [h]
Yield [%]b
1 2 3 4 5 6 7 8
MOF-894 MOF-894 MOF-893 MOF-893 MOF-892 H3CBPc H3CBP None
86 67 98 66 96 23 79 55
91 99 90 96 86 86 24 88
20 16 23 16 16 16 16 16
78 66 88 63 82 20 19 48
Reaction conditions: MOF/catalyst (0.32 mol %), styrene oxide (6.87 mmol), nBu4NBr (1 mol %), 1 atm CO2 (balloon pressure) and 353 K. The selectivity (sel), catalytic conversion (and yield were measured by GC–FID with the help internal standard(biphenyl). 0.16 mol % catalyst.
Scheme 17. The cyclo addition reaction of SO with CO2 and catalysed by MOF-92. Reproduced with permission from [65]. Copyright 2018, American Chemical Society.
COOH
N N
N
N
Zn(II)
N
HOOC
COOH
H3TCPB
N
TBIB
Fig. 35. Structure of ligands H3TCPB, TBIP and MOF. Reproduced with permission from [66].Copyright 2019, American Chemical Society.
The mechanism has been proposed based on the Lewis (vacant Co2+ ions) and Brønsted acidic (l3–OH groups) bifunctional catalytic sites (Scheme 21). First, the epoxy ring was activated by open Co2+ ions (formed strong Co O interactions with epoxide) and O–H O hydrogen bonding with l3-OH groups (Fig. 40). Then, the Brions from TBAB attacked the less hindered C atom of the epoxide followed by insertion of CO2 formed the CCs through the ring-closing step.
3. Conclusion and future perspective CO2 is one of the naturally abundant C1 feedstock. Its catalytic conversion to fine chemicals (CC) has been a hot research topic now-a-days. This is due to the fact that CCs are useful chemicals for a number of industries such as pharmaceuticals, dye, electrolyte in the lithium ion batteries, intermediates for the synthesis of ethylene glycol, polymeric materials, precursor
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Table 30 Coupling of various epoxides with CO2 at atmospheric pressure.a Entry
Epoxide
CC
Yieldb (%)
Catalyst
Co-catalyst
Time (h)
1
MOF
TBAB
24
>99
2
MOF
TBAB
24
>99
3
MOF
TBAB
12
>99
4
MOF
TBAB
24
82
5
MOF
None
24
7
6
None
TBAB
24
12
7
H3TCBP
TBAB
24
20
a At room temperature CO2 gas was bubbled through the two-neck round-bottom flask (10 mL) and no solvent was used: MOF (0.0025 equiv), Epoxide (20 mmol, 1 equiv), TBAB (0.05 equiv). b 1 H NMR helped to analyze the yield by integrating the epoxide peak with the cycliccarbonate peaks.
Scheme 18. (a) Synthetic scheme for TMOF-3 and dangling sulphonate group within the framework, (b) schematic representation of the transmetalation of silver(I) cation within the framework to form TMOF-3-Ag. Reproduced with permission from [67]. Copyright 2019, American Chemical Society.
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Fig. 36. (a) Acetylene and ethylene uptake by TMOF-3-Ag, (b) possible mechanism for CC formation. Reproduced with permission from [67]. Copyright 2019, American Chemical Society.
Table 31 Cycloaddition of CO2 to different propargylic alcohola catalyzed by TMOF-3-Ag.
Temperature (K)
Time (h)
Yieldb %
1
RT
6
>99
2
RT
6
>99
3
RT
6
>87
4
RT
6
>86
5
RT
6
77
6
RT
6
90
7
RT
6
25
Entry
a b
Epoxide
Product
Reaction conditions: TMOF-Ag (10 mol %), propargylic alcohol (0.2 mmol), DMF (2 mL). DBU (0.1 equiv.). Calculated from 1H NMR analysis.
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Fig. 37. Illustration of the buffer strategy for the construction of JUC-1000. Reproduced with permission from [68]. Copyright 2018, Wiley-VCH.
Scheme 19. Illustration of the buffer strategy in a MOF. Reproduced with permission from [68]. Copyright 2018, Wiley-VCH.
Table 32 Synthesis of CCs from epoxides and CO2 using different catalysts. Entry
Catalyst
Epoxide
Product
Yield (%)d
a
1
JUC-1000
96
2a
MOF-505
61
3a
HKUST-1
62
4a
JUC-1000
58
5a
JUC-1000
81
6a
JUC-1000
29
39
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173 Table 32 (continued) Entry b
a b c d
Catalyst
Epoxide
Product
Yield (%)d
7
JUC-1000
96
8c
–
3
Reaction conditions: 0.25 mol % catalyst (per exposed copper site), 20.0 mmol epoxide, 0.65 g TBAB, under 1 atm CO2 for 48 h at room temperature. Yields after five cycles. Absence of catalyst. Yields were calculated by 1HNMR spectroscopy.
Fig. 38. The different electronic environment within JUC-1000 help to facile CO2 converstion to CC. Reproduced with permission from [68]. Copyright 2018, Wiley-VCH.
of polycarbonates, and so on. Besides, it also would help reduce the amount of environmental CO2. Therefore, the development of catalysts for the conversion of CO2 into CCs is of immense importance. Especially, development of MOF based heterogeneous catalysts should be vigorously pursued. From the above discussions it is clear that the presence of open metal site (Lewis acid) and the high positive charge on the metal ion (Cr3+, Zr4+, Al3+) will be very useful in fabricating catalysts for the cycloaddition of CO2 to epoxides. Moreover, the presence of Lewis base can do the same and importantly the cooperative effect of both (Lewis acids and Lewis base) greatly enhance the
reaction rate by optimizing other reaction conditions (temperature, pressure and reaction time etc). A suitably designed system can do this job without the use of the expensive cocatalyst, TBAB. The versatile structural features (crystalline, porous, flexible, tunable composition etc) of MOFs offer an important podium for further investigation on interaction between reactants and the MOF and dynamics of catalytic reaction. In the present review, we have discussed the conversion of CO2 into CC with a number of examples. It should be emphasized here that various other reactions that chemically fix CO2 are available where MOFs play important roles. Overall, there is
40
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Fig. 39. Structure of H3tzpa ligand and 3D framework of 1showing 1D tubular channels along the c-axis. Reproduced with permission from [69]. Copyright 2017, American Chemical Society.
Scheme 20. Chemical Fixation of CO2 with Epoxides.
Table 33 Chemical fixation of CO2 with epoxides.a Entry
Catalysts
Amount of catalysts (mmol)
1
Co(NO3)2/TBAB
0.2/2
Epoxides
Products
36.5
Yield (%)
2
1a
0.2
0
3
TBAB
2
16.8
4
1a/TBAB
0.1/2
57.1
5
1a/TBAB
0.2/2
93.8
6
1a/TBAB
0.2/2
53.6
41
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173 Table 33 (continued)
a
Entry
Catalysts
Amount of catalysts (mmol)
7
1a/TBAB
0.2/2
Epoxides
Products
Yield (%) 48.3
Reaction conditions: epoxide (20 mmol); 100 kPa CO2 pressure; 98 K; 48 h.
Scheme 21. Proposed mechanism for the fixation of CO2 with epoxides. Reproduced with permission from [69]. Copyright 2017, American Chemical Society.
Fig. 40. Two different modes of interaction (a and b) of ethylene oxide molecules in 1a. Reproduced with permission from [69]. Copyright 2017, American Chemical Society.
an urgent need to develop MOF based technology to accomplish a spot fixation of CO2. Acknowledgements P.K.B. gratefully acknowledges the financial support received from the DST and the MNRE, New Delhi, India. PKB wishes to thank all his students who made significant contributions in the area of MOF based CO2 adsorption and its fixation over the years. TKP acknowledge to PDPU/ORSP/R&D/SRP/2019-20/1361/1 and DD acknowledge to the ‘‘TEQIP Collaborative Research Scheme (CRS)”.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ccr.2019.213173.
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Contents lists available at ScienceDirect
Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr
Review
Metal–organic frameworks for the chemical fixation of CO2 into cyclic carbonates Tapan K. Pal a,⇑, Dinesh De b,⇑, Parimal K. Bharadwaj c,⇑ a
Department of Science, Pandit Deendayal Petroleum University, Gandhinagar 382007, Gujarat, India Department of Basic Science, Vishwavidyalaya Engineering College, Lakhanpur, Sarguja University, Chhattisgarh 497116, India c Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, Uttar Pradesh, India b
a r t i c l e
i n f o
Article history: Received 1 July 2019 Received in revised form 26 November 2019 Accepted 23 December 2019
Dedicated to Professor G.K. Lahiri celebrating his 60th birthday. Keywords: Metal–organic frameworks Chemical fixation of CO2 Cyclic carbonate Energy conversion Heterogeneous catalysis
a b s t r a c t Presently, we find a rapid pace of CO2 emission into the atmosphere causing major problems facing our planet. If no action is taken, it would have harmful consequences to humanity and the biosphere. More CO2 in the atmosphere will cause global warming. This will lead to climate upheavals disturbing the ecosystems, modification of the conditions and cycles of plant reproduction and numerous associated problems. Therefore, present CO2 content in the atmosphere should be drastically reduced to a much lower level on an urgent basis. Alternatively, CO2 represents an abundant C1 feedstock and its chemical utilization has caught the imagination of chemists in recent years. Thus, fixation of CO2 with epoxide to form cyclic carbonate (hereafter, CC) via cycloaddition reaction is significantly important and vigorously pursued in different laboratories around the world. This is because removal of CO2 takes place from the atmosphere and simultaneously it can be converted into value-added products. Metal organic frameworks (MOFs) have attracted enormous attention in recent years as potential systems for gas storage, separation, heterogeneous catalysis and so on, owing to their unique features such as designable architecture, controllable pore size, high surface area, permanent porosity, etc. In the present review, we discuss the recent progress made on catalytic conversion of CO2 to CCs by specially designed MOFs. It should be emphasized here that in the present review the literature survey is not exhaustive and we apologize for missing any important result in this review. Ó 2020 Elsevier B.V. All rights reserved.
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Synthesis of cyclic carbonates (CCs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Catalysis due to the presence of open coordination sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1.1. Presence of ionic part (ionic liquid/ions) in MOF channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2. Lewis basic linker-promoted catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.2.1. Post synthetic modification of the functional site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.3. Cooperative bifunctional catalysis due to open coordination sites and Lewis basic functional linker. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.4. Catalysis due to the presence of defect in the crystal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.5. Catalysis due to the presence of accessible acidic functional sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.6. Catalysis due to the accessible Lewis and Brønsted acid bifunctional sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Conclusion and future perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
⇑ Corresponding authors. E-mail addresses: [email protected] (T.K. Pal), [email protected] (D. De), [email protected] (P.K. Bharadwaj). https://doi.org/10.1016/j.ccr.2019.213173 0010-8545/Ó 2020 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Global green house gas emission, (b) coutrywise CO2 emission, adapted from Inter-governmental Panel on Climate Change (IPCC), 2014 [2].
1. Introduction According to United Nations 2014 report, by 2050 the World urbanization will increase by 66% and it will lead to population explosion in the urban area [1]. Such a quick growth will definitely enhance the overlap between residential and industrial areas. CO2 is a major contributor to the global warming and with increasing human activities, the emission of CO2 from the power plants, biomass or fuel combustion, automobile exhaust and industrial
processes will also increase (Fig. 1) [2]. It is well known that CO2 assists to warm the earth [3] and the release of it by combustion or other activities is balanced through photosynthesis that helps to maintain the carbon cycle. However, the concentration of CO2 increasing to an alarming proportion leading [4] to several natural disasters and other calamity including rising sea level, ecosystem disturbance, crop failure and extermination of living species. On the other hand, fossil fuels supply us huge amount of energy in our daily lives [5]. So, we are living in a dilemma and there is an urgent need to find a solution that can optimise these two situations. An extremely sagacious approach would be very low emission and on the spot capture and fixation of anthropogenic CO2 emission. Till now, several technologies have been developed like CO2 capture and geological sequestration [6], various chemical
Fig. 2. Several potential applications of MOFs.
Fig. 3. Schematic representation for cycloaddition reaction between CO2 and epoxide to cyclic by MOF.
Scheme 1. General schematic representation for cycloaddition reaction mechanism between epoxides and CO2 catalyzed by Lewis acidic open coordination sites of a MOF in presence of TBAB.
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Fig. 4. A perspective view of the structure of Co-MOF-74 (The cobalt, carbon and oxygen atoms are marked as yellow, grey and red colour respectively). Reproduced with permission from [25] Elsevier, Copyright 2012.
strategies to convert CO2 to value added chemicals [7], and so on. Particularly, the possibility of converting CO2 to value-added products have been the subject of enormous attention worldwide in recent years. In this regard, several important properties of CO2 like non-toxic, non-flammable, inexpensive, highly abundant, easy to handle, etc., are added advantages. Sometimes, however, CO2 suffers from low reactivity due to the highest oxidation state of carbon and very high C@O bond enthalpy (+805 kJ mol1) [8]. Although, this problem can be surmounted by reducing high energy barrier with the help of highly reactive substrates and use of catalysts. Thus, CO2 capture and its conversion into valuable chemicals (CCs, formic acid, substituted urea, dimethyl carbonate, etc.) have attracted attention from a large number of researchers [9,10]. The CCs are useful in a number of industries like pharmaceuticals, dye, electrolyte in the lithium ion batteries, intermediates for the synthesis of ethylene glycol, polymeric materials, precursor of
Fig. 5. Perspective views of the Cu(II)-MOF having two types of pores (blue and red) and is decorated with unsaturated cupper metal sites and acylamide groups. Reproduced with permission from [27] American Chemical Society, Copyright 2017.
Table 1 Conversion of different epoxides to their corresponding CCs upon reaction with CO2. Entry
Epoxides
Small substrates 1
Product
O
O
Yields (%)
O
96
O 2
H3C 3
Cl 4
Br Larger substrates 5
O
O
O H3C
O
O O
Cl
O Br
O
85
O
88
O
O
O
90
O
O
O
10
O
O
6
O
O
7
O 7
O
O
O
O
O
O
6
4
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
polycarbonates, etc [11]. A number of homogeneous catalysts such as quaternary ammonium salts, metal complexes, metal halides, etc. have been employed to synthesize CC in an industrial scale under mild conditions [12]. But, homogeneous catalysts are usually associated with disadvantages such as separation of the products, their purification, recycling of the catalyst, and so on. Ionic liquids (ILs) possess active centres which enhance the rate of cycloaddition reactions between epoxides and CO2. Besides, some of the features like non-flammability, non-volatile, low vapour pressure and high melting points of ILs, make them suitable catalysts for CO2 cycloaddition reactions [13]. But the recyclability of IL is very
expensive and the catalyst is ineffective at room temperature. To overcome these issues, ILs have been immobilized in traditional polymeric materials [14]. Still, leaching of reacting species in to the reaction system and inadequacy at milder conditions are of major concern [15]. On the other hand, MOFs can be designed to be useful as potential heterogeneous catalysts. These compounds represent a new class of porous materials that can be synthesized easily from metal ions/metal ion clusters and rigid multidentate organic ligands. Depending upon the nature of the node and the structure of the linkers, a myriad number of structures can be easily constructed under mild conditions. These materials have
Table 2 Syntheses of various carbonates from styrene oxide derivatives catalysed by WMMOFs.a
a b c
Entry
R-
Temp. [K]
Time [h]
Con.b [%]
Sel.b [%]
1 2 3 4
PhMeCl-
333 r.t.c 333 333
30 24 8 16
99 96 99 96
100 94 93 100
O
Reaction conditions: 20 mg catalyst (0.34 mol % Zr), 5% TBAB, no solvent, 17.5 mmol epoxide, CO2 balloon. Determined by GC–MS. Room temperature.
Fig. 6. (a) Conversion of styrene epoxide to corresponding carbonate by WM-MOF, (b) Kinetics of CO2 conversion by different catalysts, (c) Recyclability experiment of WMMOF, (d) PXRD patterns before reaction and after reaction of WM-MOF (TEM image (scale bar 50 nm) after reaction of WM-MOF). Reproduced with permission from [28] American Chemical Society, copyright 2017.
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attracted enormous attention in recent years due to versatility in the structure, crystalline nature, ultra high surface area with voids of different shapes and sizes that make them useful in many practical applications (Fig. 2) [16]. Highly porous MOFs with one or more solvent molecules attached to each metal centre can be made via design of nodes as well as linkers. In a robust MOF, removal of both metal coordinated and lattice solvent molecules can be achieved through heating without disturbing the overall architecture, leading to a highly porous MOF. Reactants can easily enter these pores and can also access the empty metal centres. Leaching of metal ions into the solution during or after catalytic reactions is rare. Consequently, MOFs incorporate the merits of ‘‘metal complex (homogeneous catalyst) in terms of reactivity and accessibility of active sites” and ‘‘recyclability of a heterogeneous catalyst”. Catalytic application by a MOF has been demonstrated earlier [17] and now it has been shown to be an excellent heterogeneous catalyst in various organic transformations [18]. Recently, CC synthesis by MOFs, eminent as a excellent catalysts (Fig. 3) and a number of reports have been published based on the carbon dioxide capture and sequestration (CCS) [19]. Adsorption studies with CO2 showed that the capacity can be enhanced through (a) proper ligand designing (amino functionalised MOFs, those containing N and F in the linker), (b) hybrid composition (MOF-CNF and MOF-templated carbon) and in some cases (c) generation of open metal sites [20]. In 2009, Han and
co-workers first showed CC synthesis by using a binary catalytic system, MOF-5/n-Bu4NBr at 323 K. The catalyst was stable up to three cycles and reaction was completed within 6 h with high selectivity [21]. Since then, a massive number of MOFs and their derivatives have been used by different workers to form CCs by reacting CO2 with different epoxides. In the present review, we discuss the use of MOFs in the formation of CCs utilizing CO2 and epoxide which is documented as a 100% atom economic reaction. At the outset, it should be emphasized here that the discussions are not exhaustive and we apologize in case we have missed few important references. 2. Synthesis of cyclic carbonates (CCs) The conversion of CO2 into chemical feedstock has attracted attention in recent years as it is an abundant C1 resource [22] and the conversion of CO2 into value-added fine chemicals is of environmental and practical importance [23]. The synthesis of CC via cycloaddition reaction between an epoxide and CO2 is a profitable reaction with wide applications in pharmaceutical and fine chemical industries [24]. In general, catalytic activity (here, cycloaddition of CO2 with epoxides) of MOFs arises from different structural characteristics: (i) presence of open metal coordination sites (generation of its takes place upon elimination of loosely bound solvent molecules) acting as Lewis acidic sites that can be used with a Lewis basic co-catalyst like tetrabutyl ammonium
(a)
(b)
(c)
Fig. 7. (a) The Zn(II) coordination environment, (b, c) The hexagonal double-walled channels viewed down the crystallographic c-axis. Reproduced with permission from [29]. Copyright 2017, American Chemical Society.
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Table 3 Cycloaddition of CO2 with different epoxides catalysed by the Zn(II)-MOF in the presence of TBAB.a Entry
Epoxide
1
Cl 2
H3C
Product
O
O
O Cl
O
Time (h)
Yieldb %
343
12
>99
303
12
>99
343
12
>99
343
12
>99
343
12
88
343
12
68
343
22
68
343
32
4c
343
32
9d
O O
O H3C
O
3
Temperature (K)
O
O
O
O O
4
H3C
O
O H3C
O
O
5
O
O
O O
6
O 7
O
O
O
O
O
O
O
O 8
Cl 9
Cl a b c d
O
O
O
O Cl
O
O O
Cl
O
O O
Reaction conditions: catalyst (10 wt%), epoxide (20 mmol) and TBAB (1 mmol). Calculated from 1H NMR analysis. Catalysed by MOF only. Catalysed by the co-catalyst TBAB alone.
HOOC
COOH
HOOC
COOH
(c)
(b)
(a)
(d)
(e)
Fig. 8. (a) Structure of H4TCPE ligand; (b) Ni2 bimetallic unit present in Ni–TCPE1; (c) 1D nanotube (pink column) representingthe channel; (d) view from the top of the nanotube; (e) view along the b axis showing the packing pattern. The Ni, C, N and O atoms are read as cyan, grey, blue and red color code respectively. Reproduced with permission from [30]. Copyright 2015, American Chemical Society.
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bromide (TBAB); (ii) Lewis basic functionalized linkers can be functionalized during or after synthesis) that can act as the site of CO2 fixation; (iii) presence of Lewis acidic or basic defect sites inside/surface of the MOF, (iv) presence of accessible Brønsted acidic COOH groups, and (v) presence of accessible Lewis and Brønsted acid bifunctional sites.
2.1. Catalysis due to the presence of open coordination sites Presence of empty coordination sites on the metal in a MOF is important as the catalysis cycle begin with the epoxide ring binding at the open metal site. The nucleophile Br ion (from cocatalyst TBAB) attacks on the sterically less crowding side of the
Fig. 9. (a) The Ni3 unit present in Ni–TCPE2, (b) coordination environments of TCPE; (c) 3D structure of Ni–TCPE2, (d) topological figure showing PtS network. The Ni, C, N and O atoms are read as cyan, grey, blue and red color code respectively. Reproduced with permission from [30]. Copyright 2015, American Chemical Society.
Table 4 Coupling of epoxides with CO2. Entry
Substrate
1a
Ni-TCPE1
O
2a
O
O
3a
O
O
Ni-TCPE2
Yield (%)
TON
Yield (%)
TON
>99
2000
86.2
1720
>99
2000
97.7
1950
95.7
1910
94.2
1890
93.7
1880
92.6
1850
O 4b
O
O
O
O
a Conditions of the reaction: MOF catalyst (10 lmol, on the basis of Ni2+), epoxide (20 mmol), , and TBAB (0.3 mmol), 1 MPa of CO2 pressure for 12 h at 373 K. % yields were calculated from 1H NMR study. b Epoxide (10 mmol), other conditions are same.
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metal-bound epoxide with ring opening leading to a metal-bound alkoxide in an SN2-type reaction. Next, cycloaddition of CO2 to the alkoxide occurs forming metal bound carbonate that subsequently form CC with the regeneration of the catalyst (Scheme 1). Emphasis had been given to created open coordination sites with large pores to accomadate epoxides. Besides, open metal sites positively influence adsorption of CO2 by the MOF. Thus, CC synthesized was experienced with Co-MOF-74 in 2012 [25]. Thereafter, a growing number of reports utilizing open metal catalytic sites have been reported [26]. The crystal structure of the compound shown in Fig. 4 consisted of 1D honeycomb channels of dimension 1.1–1.2 nm and helical chains of edge-condensed metal–oxygen coordination octahedra located at the intersections of the honeycomb. The metal coordination was square–pyramidal with one site occupied by a water molecule. Upon heating in vacuum, the framework created an open metal site that was used to catalyze the reactionbetween CO2 and styrene oxide in chlorobenzene solvent without using any co-catalyst at 20 bar pressure and 373 K temperature. Interestingly, it afforded a maximum of 96% conversion. The conversion decreased with decreasing temperature, attributed to lower solubility of CO2 in chlorobenzene. Its catalytic activity was intact for at least three cycles. Zhao and co-workers solvothermally synthesized a Cu(II)-MOF using an acylamide incorporated tetracarboxylate ligand and Cu (II) ions [27]. Presence of accessible nitrogen-rich groups and unsaturated metal coordination sites made the MOF (Fig. 5) a highly promising candidate for selective adsorption of CO2. Conversion of the adsorbed CO2 to CC was carried out in a Schlenk tube under 1 bar pressure of CO2 at room temperature for 48 h using 20 mmol of epoxide and 0.5 g of the co-catalyst, TBAB and the activated MOF catalyst without using any solvent. Smaller epoxides afforded good yields of the product while larger epoxides gave substantially low yields as diffusion of these substrates inside the coordination space was difficult (Table 1). A series of zirconium–metallo porphyrinic MOFs decorated with mesoporous channels [28] led to catalytic sites (Lewis acid sites) exposed within the framework. This displayed excellent catalytic efficacy for the CCs formation with TBAB (co-catalyst). A 99% conversion to CC was reported with 100% product selectivity in case of styrene oxide as the substrate (Table 2). The recycling test was carried out simply by separating the catalyst by centrifugation and could be used again without significant loss in catalytic activity at least up to five cycles (Fig. 6).
(b)
(a)
(c)
(d)
(e)
Fig. 10. Structures of (a) [CuL(ClO4)2], (b) [NiL(ClO4)2]H2O, (c) [CuLCl2],(d) [NiLCl2] 2DMF, (e) packing diagram viewed down [111] and depicting the cyclam linkers connecting the Zr6-clusters along the cell body diagonals. Reproduced with permission from [31]. Copyright 2018, American Chemical Society.
Table 5 Chemical fixation of CO2 with epoxides catalyzed by VPI-100 MOFs. Entry
Epoxide
Product
Catalyst
Conversion (%)a d
1 2 3 4 5 6
Cl
H3C
O
O Cl
b c d
TOF (h1)c
TONb d
R1
R2
R3
R1
R2
R3
R1
R2
R3
VPI-100 (Cu), TBAB VPI-100 (Ni), TBAB
94 96
95 98
94 98
431 443
439 452
431 453
72 74
73 75
72 76
VPI-100 (Cu), TBAB VPI-100 (Ni), TBAB
60 50
52 50
40 43
273 229
236 229
185 199
46 38
39 38
31 33
VPI-100 (Cu), TBAB VPI-100 (Ni), TBAB
13 18
16 20
18 17
60 85
76 92
81 78
10 14
13 15
14 13
O
O
O H3C
O O
O
O
O a
O
d
1
O
Conversion were calculated from the H NMR spectra. Turnover number [product (mmol)/active site (mmol)], where ‘‘active site” is the total no of unsaturated Zr and Cu/Ni metal centers. Turnover frequency [TON/time (h)]. R1, R2, and R3 stands for 1st, 2nd and 3rd cycles, using the same VPI-100 MOF as catalyst.
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
An anionic porous framework, {[(CH3)2NH2][Zn2(L)(H2O)PO4] 2DMF}n with double-walled hexagonal channels had been constructed [29] using the dicarboxylate ligand H2L (Fig. 7a). As expected, the isophthalate group in the linker formed a paddle-wheel dimeric SBU where each metal was bonded to a water molecule. The de-solvated framework obtained upon heating, showed microporous nature with good CO2 uptake capacity (111.7 cc g1 or 22 wt% at 273 K, 1 bar pressure) due to the presence of imidazole units as well as accessible open metal sites inside the channels. The framework showed efficient catalytic activity in the conversion of adsorbed CO2 to CCs under mild conditions in the presence of TBAB as the co-catalyst
9
(Table 3). Due to its highly robust nature, the MOF has been recycled for a number of catalytic cycles with nominal change in the product yield. An aromatic tetracarboxylate linker was obtained by substituting all the four hydrogen atoms of ethylene. This linker upon reacting solvothermally with hexa aqua nickel nitrate salt in presence of L-proline (L-Pro) in mixed DMF and H2O for 3d at 373 K [30] afforded a single walled discrete metal–organic nanotube (Ni– TCPE1) and a 3D extended network (Ni–TCPE2). The L-Pro was not present in the crystal structure of Ni–TCPE1 and it served only as atemplate. The framework is having large channel, internal diameter (2.1 nm) and the external diameter is 3.6 nm (Fig. 8).
Fig. 11. Representation for the assembly of (FJI-C10) form BDCs, (Br) Etim-BDCs and Cr3O clusters (green). The circle region in the cluster indicates the water molecule. Reproduced with permission from [32]. Copyright 2018, The Royal Society of Chemistry.
Scheme 2. The probable mechanism for the cycloaddition reaction by FJI-C10. The weak interaction between the reagents indicated by hashed bond. Reproduced with permission from [32]. Copyright 2018, The Royal Society of Chemistry.
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The 3D network (Ni–TCPE2) was obtained when the amount of LPro was increased keeping every other chemical in the same amounts as in the previous case. It crystallized in chiral space group C2 with one of the Ni ions was coordinated one L-Pro molecule (Fig. 9a). Compound Ni–TCPE1 having a large pore, exhibited excellent catalytic activity in presence of TBAB in the cycloaddition reactions of CO2 with various epoxides, with a highest TON of 17,500 and an initial TOF 1000 (Table 4). Interestingly, it showed excellent enantioselectivity (ee~92%) when enantiopure styrene oxide (R or S) was used. Compound Ni–TCPE2 however, showed significantly lower catalytic activity than that of Ni–TCPE1 possibly due to pore blockage in the former. Two highly stable three dimentional Zr-MOFs, VPI-100 (Ni) and VPI-100 (Cu) reported by Morris and co-worker and the frameworks were assembled from eight-connected Zr6 clusters and [Cu(cyclam)]2+ or [Ni(cyclam)]2+ (cyclam = 1,4,8,11-tetraaza cyclotetradecane) linkers (Fig. 10) [31]. The firmly placed metal ion in the cavity of cyclam was readily accessible by epoxides
while empty coordination sites on the Zr4+ ion showed good CO2 uptake capacity. Both these factors helped in the reaction of CO2 with epoxides, including sterically larger epoxides (Table 5) in presence of TBAB as the co-catalyst. 2.1.1. Presence of ionic part (ionic liquid/ions) in MOF channels It is well-established that in the cycloaddition of CO2 to epoxide, the nucleophile Br- ion attacks the less-hindered side of the metalbound epoxide and cleave the ring to afford a metal-bound alkoxide in an SN2-type reaction. In the previous section, all examples had the nucleophile Br- ion coming from an external source like Bu4NBr (TBAB). It might be a good strategy to have the Br ion present in the coordination space of the MOF near the Lewis acidic reaction centres. Encapsulation of flexible cationic polymer having bromide counter anion into a MOF could function synergistically leading to lower activation energy pathway and fixes the carbon dioxide into CCs with various epoxides. Following this strategy, Cao and co-workers constructed a mesoporous cationic Cr-MOF,
Scheme 3. Synthesis of ZnTCPP (Br)Etim-UiO-66 through: (a) step by step post synthetic ionization and then metalation strategy; (b, c) direct syntheses from the ionic ligand [(Br)(Etim-H2BDC)+], Im-Zr6, imidazolium functionalized Zr6SBU; (Br)Etim-Zr6, imidazolium functionalized Zr6 SBU. Pore cavity is designated by yellow sphere. Reproduced with permission from [33]. Copyright 2018, American Chemical Society.
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termed FJI-C10, [32] using the linker 2-(3-ethyl-imidazol-1-yl)-ter ephthalic acid [(Etim-H2BDC)+Br] along with the H2BDC ligand. The structure was similar to MIL-101(Cr) having Lewis acidic vacant Cr3+ sites and free halogens in the channels (Fig. 11). FJI-C10 exhibited excellent CO2 uptake capacity (20.2 wt% at 1 bar and 273 K) as the ionic surface could boost the interactions between CO2 and the surface through dipole–quadrupole interactions. Owing to the co-existence of Cr3+ (Lewis acidic sites) and adjacent halogen ions in the framework, the FJI-C10 exhibited catalytic efficiency for the CO2 conversion into CCs under atmospheric pressure at room temperature in 24 h. Unfortunately, this compound gave low conversion (27.3%) and product selectivity (60.7%). However, upon increasing the temperature to 333 K, the reaction practically completed (99.7%) within 24 h with improved selectivity (88.2%). On the hand, under the same conditions, the neutral MIL-101 gave inferior yield of 12% and a selectivity of 29% for the same reaction. Furthermore, the catalyst retained its activity at least six times in the cycloaddition reaction of CO2 with epichlorohydrin. The proposed mechanism for this reaction could be found in Scheme 2. Initially, an epoxide was activated by the influence of Lewis acidic Cr3+centre. The ring opening of the epoxide was driven by the combined influences of the Cr3+open sites and a Br ion on the less-crowded carbon atom of the epoxide. After that, a CC was formed by the nucleophilic attack from the alcoholate to CO2, which converted to a CC by intermolecular cyclization, giving the original catalyst for the next cycle. The ionic MOF, ZnTCPP (Br)Etim-UiO-66, (ZnTCCP = [5,10,1 5,20-tetrakis(4-carboxyphenyl)-porphyrinato]zinc(II)) had been synthesized [33] by a sequential post-synthetic metalation strategy as well as de novo one-pot syntheses (Scheme 3). The MOF exhibited excellent catalytic performance in solvent-free CO2 conversion into CC with allyl glycidyl ether under cocatalyst free condition at 1 bar CO2 pressure at 413 K for 6 h. As the Zn2+ sites in the porphyrin ring and imidazolium bromide were integrated within the MOF, no external TBAB cocatalyst was needed. The results are summeried in Table 6. The superior catalytic activity was ascribed to the combined effects of available Zn2+ sites and Br ion in the micropore channel of the MOF. ZnTCPP (Br)Etim-UiO-66 under the optimized conditions could efficiently convert several epoxides into respective CCs (Table 7) with the yield ranging between 19.8 and 91.1%. A probable mechanism had been proposed (Scheme 4). Firstly, the epoxide was coordinated to Lewis acidic Zn(II) sites. The cleavage of the activated epoxide ring occurred by the regioselective attack of Br nucleophile. Then, an acyclic carbonate was formed by the reaction of ring opened epoxide and CO2. Acyclic carbonate was then transformed to a CC by the removal of Br ion with the regeneration of catalyst. Further utilization of this concept was made by Ma and coworkers [34] who synthesized a synergistic catalytic system by encapsulating an ionic polymer (IP) into the well known MOF, MIL-101(Cr). The Br counter ions (function as co-catalyst) in this catalytic system (MIL-101-IP) present close to the open Cr(III) reactive sites (Fig. 12). The imidazolium polymer could be encapsulated within the pores of the MOF by stirring MIL-101 with the monomer (3-ethyl-1-vinyl-1H-imidazol-3-ium bromide) in DMF overnight followed by the addition of free-radical initiator, azobis-isobutyronitrile (AIBN) to induce polymerization resulting in MIL-101-IP (Fig. 12). In spite of the encapsulation of the polymer, the MIL-101-IP still possessed sufficient open space as supported by the nitrogen and carbon dioxide adsorption investigation. The surface area of the daughter MOF was slightly reduced to 2232 m2g1 compared to the parent MOF, MIL-101 with a BET surface area of 2610 m2 g1. The cycloaddition of CO2 with epichlorohydrin was complete
Table 6 Fixation of CO2 with AGE.a
O O
CO2
O
Catalyst
O O
1 atm
O
Allyl Glycidyl Ether (AGE)
Entry c
1 2 3 4 5e 6 7f 8g 9h
Catalyst
Con. (%) [sel. %]b
Blank UiO-66 TCPP (Br)Etim-UiO-66 TCPP Im-UiO-66 ZnTCPP ZnTCPP (Br)Etim-UiO-66 ZnTCPP/(Br)Etim-H2BDC (Br)Etim-H2BDC (PF 6 )Etim-H2BDC
Trace Trace 78(2) 64(5) Trace 77(4) 98(1) 94(1) 11(2)
[–] [–] [65d] [90] [–] [92] [100] [100] [100]
a Reaction conditions: no solvent, catalyst (43 mg), CO2 pressure: 1 bar, temperature: 413 K, time: 6 h. b Determined by GC–MS, values are obtained by averaging three runs, error written in parentheses. c Absence of catalyst. d By products are allyl derivatives. e ZnTCPP (0.0018 mmol). f ZnTCPP (0.0018 mmol) and (Br)Etim-UiO-66 (0.0475 mmol). g (Br)Etim- H2BDC (0.0475 mmol). h (PF6)Etim-H2BDC (0.0475 mmol).
Table 7 ZnTCPP (Br)Etim-UiO-66 catalyzed chemical fixation of CO2 with epoxides for the synthesis of CCs.a Entry
Epoxide
1
Cl
Yield (%)b
Product
O
O
O Cl
2
86.9
O
O
O
O
O 3
O
O
O O
O
4
90.0
O
O
O
O
91.1
O O
52.8
O 5
H3C
O 3
O H3C
O
38.2
O 3
6
H3C
O 5
O H3C
O
19.8
O 5
a Reaction conditions: no solvent, catalyst (0.95 mol % on the basis of imidazolium), CO2(1 bar), 413 K for 14 h. b calculated by using GC–MS.
(99%) at 323 K and 1 atm CO2 in 68 h. The catalytic activity of MIL-101-IP was better than free MIL-101 (32%), IP (3%) and the physical mixture of both under the same reaction conditions (Table 8).
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Scheme 4. Probable Mechanism for chemical fixation of CO2 with epoxides catalyzed by ZnTCPP (Br)Etim-UiO-66. Reproduced with permission from [33]. Copyright 2018, American Chemical Society.
Table 8 Cycloaddition of CO2 with epichlorohydrin.a Entry
Catalyst
Yield (%)
1 2 3 4
Ionic Polymer (IP)b MIL-101-IPc MIL-101 + IPd MIL-101e
3 99 80 32
a Reaction conditions: epichlorohydrin (1 g, 10.5 mmol), 1 atm CO2 for 68 h, at 323 K. b 6.3 mg of IP (0.0313 mmol Br). c 50 mg of MIL-101-IP (0.0313 mmol Br). d Physical mixture of 43.7 mg MIL-101 and 6.3 mg IP (0.0313 mmol Br). e 50 mg of MIL-101.
Table 9 Cycloaddition rection of CO2 into various epoxides.a Entry
Epoxides
1
H3C Fig. 12. The polymer, MIL-101-IP entrapped within the channels. Reproduced with permission from [34]. Copyright 2018, Wiley-VCH.
2
Products
O
O H3C
O
O
95 [48]
O
O
99 [48]
O
As shown in the Table 9, the composite acted as an effective catalyst for the synthesis of various CC from their corresponding component with high yield. The catalyst maintained its activity at least up to four cycles with insignificant change in the performance. From kinetic studies, the activation energy for MIL-101-IP was found to be approximately 50% lower than that of the physical mixture of MIL-101 + IP (63.6 vs. 91.6 kJmol1). Later Dong group utilized the same concept for the CCs synthesis [35].
O
3
O
O O
O 4
O
O
5
O
O O
2.2. Lewis basic linker-promoted catalysis
a
O
84 [96]
O
O O
O
Unlike the Lewis acid catalyzed cycloaddition reactions, a modified reaction mechanism was believed to be operational [36] with
Yields [%] [T (h)]
O
82 [96]
O O
33 [72]
O
Reaction conditions: MIL-101-IP (50 mg), epoxide (1 g) at 1 atm CO2 and 298 K.
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COOH
COOH
x
4 Zn(NO3)2
(3 - x) COOH
NH2 COOH
Fig. 13. Synthesis mixed-linker MOF-5 from 2-amino-BDC and BDC, where BDC = benzene-1,4-dicarboxylate in different ratio. Reproduced with permission from [37] Copyright 2009, Wiley-VCH.
Fig. 14. (a) Large channels (LC) and tight channels (TC) in MIL-68(In)-NH2, (b) In-O-In chain in MIL-68(In)-NH2. Reproduced with permission from [39] Copyright 2012, Wiley-VCH.
linkers incorporating Lewis basic sites like side chain amino groups. Here, the first step involved a nucleophilic attack on the C atom of CO2 by the amino group to activate CO2 through formation of a carbamate intermediate that could react with the epoxide. To test this mechanistic pathway, some of the BDC linkers of MOF5 had been replaced [37] by 2-amino-BDC that led to mixedlinkered MOF-5, Zn4O(BDC)x(BDC-NH2)3x] (Fig. 13) where 40% of 2-amino-BDC linker was incorporated. Together with the cocatalyst TBAB, this framework behaved as a suitable catalyst for the reaction of CO2 with propylene oxide to afford propylene carbonate (63% yield). Using amino group based MIL series of MOFs such as MIL-125NH2 and (MIL-68(In)–NH2)) by Ahn et al. reported [38] synthesis of CCs. Following similar methodology, Farrusseng and co-workers
Scheme 5. The conversion of styrene epoxide (SO) to CC.
modified MIL-68 showed the reactivity of MOF towards synthesis of CC increases by introducing amine functionality into the ligand system [36,39]. The frameworks, MIL-68(In)-NH2 and MIL-68(In) were synthesized through precipitation method by reaction between aminoterephthalic acid/terephthalic acid with indium nitrate in dimethylformamide as a solvent. Both the frameworks were charecterized by a number of spectroscopic techniques. Calculations using the density functional theory predicted the structure of MIL-68(In)-NH2 to be one-dimentional rod shaped framework having two types of channels: hexagonal large channels (LC) and triangular tight channels (TC) (Fig. 14a) forming an
Table 10 SO conversion table by MOF MIL-68(In) and MIL-68(In)-NH2.
X
Entry
Catalyst
SO conversion %
1 2 3 4 5 6 7
Blank TBAB MIL-68(In)X MIL-68(In)X MIL-68(In)-NH2X MIL-68(In)-NH2X Recyle-MIL-68(In)-NH2
8 87 36 42 68 74 53
Different run under same experimental condition.
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
infinite chain structure (Fig. 14b) via bridging hydroxyl groups. Interestingly, the framework MIL-68(In)-NH2 was able to performed cycloaddition reaction between CO2 and epoxide in DMF solvent (Scheme 5) with much greater efficiency compared to MIL-68(In) (Table 10). Theoretical and experimental study concluded that by introducing –NH2 in MIL-68(In)-NH2 resulted in higher conversion rate of epoxide to CC with CO2. However, the catalyst was recycled with lower conversion of epoxide rate attributed to occupancy of the channels by reactant molecules which was also confirmed by lower BET surface area (1100 m2/g for pristine MOF and 720 m2/g for recycled MOF). The CO2 fixation by chiral MOFs are rarely available in the literature. An interesting example reported by Qi and co-workers where they had performed tandem assymetric catalysis reaction from alkene to CC using a polyoxametalate based homochiral MOF [40]. This MOF was synthesized solvothermally by reacting Zn(NO3)26H2O, TBA4W10O32, L-N-tert-butoxy-carbonyl-2-(imida zole)-1-pyrrolidine (L-BCIP) and NH2-BPY in aqueous acetonitrile and the framework obtained was assigned as ZnW-PYI1. Similarly, when D-BCIP was used instead of L-BCIP, the corresponding MOF
was ZnW-PYI2. As expected, each framework was a mirror image of the other (Fig. 15a). The framework, ZnW-PYI1 had a labile anion, W10O4 32 that reacted with Zinc metal ions to form a kegging type anions, ZnW12O6 40 and each kegging anions binds to four zinc metal ions with their terminal oxygen atoms. In the framework each zinc metal ions form a 2D square grid sheets which is then linked by NH2-BPY ligands to form 3D architecture (Fig. 15b) having solvent accessible void space 289.1 Å. Fig. 16 indicates that the framework consisting four basic units and the framework was used for the tandem reaction for the synthesis of CC from olefins. The framework, ZnW-PYI1 showed epoxidation reaction of styrene with 92% yield and 79% enatiomeric excess after 5 days at 323 K in presence of t-butylhydroperoxide (in 70% decane). In different experiment the cycloaddition reaction between racemic styrene epoxide with CO2 by MOF, ZnW-PYI1 (0.1% mol) produce CC (negligible ee) with yield greater than 99% under solvent free condition at 0.5 MPa pressure and 323 K temperature for 48 h in presence of 1 mol% of the cocatalyst TBAB (Table 11). Importantly, when chiral (either R or S) epoxide was used for cyclo addition reaction, the chirality was preserved in the carbonate product (99%ee, yield
Fig. 15. (a) Synthetic procedure of ZnW-PYI1 (left) and ZnW-PYI2 (right) and both are mirror iamge to each other. The yellow one is the rearrangement of precursor [W10O32]4 in both the framework, (b) 3D infinite structure of ZnW-PYI1 along crystallographic b direction [color code, Zn = cyan, C = gray, nitrogen = blue, PYI = orange]. Reproduced with permission from [40] Copyright 2015, Nature.
Fig. 16. (a) Representation of MOF component and tandem catalytic reaction for the conversion of olefin to CC, (b) probable mechanism for the tandem reaction. Reproduced with permission from [40] Copyright 2015, Nature group of publications.
15
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173 Table 11 Cycloaddition by the catalyst, Zn(II)-PYIs between CO2 and different epoxides.a Entry
Reactant
Product
1
2
O
O
O
O
Temp. (K)
Time(h)
Yieldb%
ee%c
323
120
92(94)
79(76)
323
48
>99
trace
323
48
>99
90
323
48
>99
96
323
96
92(90)
80(77)
323
96
83(85)
70(73)
323
96
72(70)
55(59)
O 3
O
O
O
O 4
O
O
O
O O
5
O
O O
6
O
O 7
O
O O
a Reaction conditions: Entry 1: ZnW PYIs (0.01 mmol), olefin (10 mmol), TBHP (20 mmol); Entry 2–4: MOF: 0.01 mmol, styrene oxide: 10 mol, TBABr 0.1 mmol; Entry 5–7: MOF: 0.01 mmol, olefin: 10 mmol, TBHP: 20 mmol. b,c The number in the parentheses indicates the yield (1H NMR analysis) of the product and ee (chiral OD-H column HPLC analysis) by the catalyst Zn-WPYI2.
Scheme 6. Post synthetic modification of aldehyde in ZIF-90 to quaternary ammonium cation to produce F-ZIF-90.
>99%) (Table 11). Moreover, one pot assymetric CO2 coupling was also softly achieved. In a typical reaction, heating the mixture of catalyst ZnW-PYI1, CO2, t-butylhydroperoxide, for 96 h, under 5 bar and 323 K, the olefins converted to the CC. The desired carbonate (R)-phenyl(ethylencarbonate) was accomplished in 92% yield with 80% enantiomeric excess. The good selectivity in the assymetric reaction was attributed to the –NH2 moiety inside the network architecture that increased the concentration of CO2 around the active centre and facilitated CC formation (Fig. 16). 2.2.1. Post synthetic modification of the functional site Post synthetic modification (PSM) technique in MOF chemistry had a great impact in broadening the scopes of these materials for
practical applications [41]. In the synthesis of CCs from CO2 and epoxides, the PSM technique could be of impotance through generation of MOFs with new architecture [42]. In 2015, Park and co-wrokers synthesized ZIF-90 from the Zn (NO3)24H2O and imidazolate-2-carboxyaldehyde and via PSM technique, converted [43] ZIF-90 to F-ZIF-90 (Scheme 6). In spite of having smaller pore volume in F-ZIF-90 compared to ZIF-90, it exhibited significantly greater efficiency of CC formation (yield 96.6%) than in case of ZIF-90 (yield 43%) under similar experimental condition (Table 12). The catalyst was reusable and the XRD patterns of the catalyst before and after the reaction, confirmed maintenance of the structural integrity. However, the yield of the product (96% for fresh and
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Table 12 Cycloaddition rection of CO2with allyl glycidyl ether (AGE) by catalyst, ZIF-90 and F-ZIF-90.a Catalyst a
Epoxides
Products
O
ZIF-90
O
O O
O a
O
F-ZIF-90
O
O b
O
ZIF-90
O
O
b
O
F-ZIF-90
O
O
6
393 K
40(61.8)
6
393 K
72.7(94.6)
6
393 K
43.4(50.4)
6
393 K
96.6(97.9)
O O
O
Yields [%] (sel. %)
O O
O
Temp. (K)
O O
O
T [h]
O
a Reaction conditions: Catalyst (20 mg i.e. 0.177 mol%), AGE (18.1 mmol). Batch operationa CO2 pressure initially of 1.17 MPa, Semi batchb having pressure of CO2 is constant, 1.17 MPa.
Table 13 The various cyclo addition reactions between CO2 with different epoxide by catalyst, F-ZIF-90.a Entry
Epoxides
1
Products
O
O
O
T [h]
Temp. (K)
Yields [%] (sel. %)
6
393 K
62.4(99.3)
6
393 K
94.3(99.1)
6
393 K
87.8(98.6)
6
393 K
96.3(98.7)
6
393 K
2.4(98.0)
O O
2
Cl 3
O Cl
O
O O
O
O O
4
O
O
O O
5
O
O O
a
O O
O
Reaction conditions: F-ZIF-90 (20 mg, 0.177 mol%), epoxide (18.1 mmol), pressure of CO2 is 1.17 MPa semi batch.
Scheme 7. Synthesis of Im-UiO-66 (isoreticular chemistry) and (I) Meim-UiO-66 (PSM method). Zr6 clusters (Green polyhedral) andmicropores (yellow ball). For clarity only linkerparts are shown. Adapted with permission from [45]. Copyright 2017, The Royal Society of Chemistry.
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
58.3% for reused) and BET surface area (763 m2/g for fresh and 548 m2/g for reused) of the reused catalyst decreased from the original one probably due to blockage of the active sites in the catalyst by carbonaceous products and low stability of the catalyst. The framework is also acts as a versatile catalyst in different CC formation (Table 13). The mechanism for CC formation was quite similar as in the case of Lewis acid based catalysis. The same group performed
17
another PSM between ZIF-90 and ionic liquid (IL) (1-aminopyridinium iodide) to from IL-ZIF-90 [44]. The newly formed catalyst, IL-ZIF-90 could execute cylcoaddition reactions between CO2 with a number of epoxide affording cexcellent yields at 393 K for 3 h reaction and unlike the previous case, this catalyst was reusable upto five cycles without losing activity. A zirconium based imidazolium bifunctional functionalized ZrMOF, (I)Meim-UiO-66 had been synthesized [45] from the parent
Scheme 8. The probable mechanism for the cycloaddition reaction by (I) Meim-UiO-66. The hashed bonds indicate p p, cationic p or hydrogen bonding interactions. Reproduced with permission from [45]. Copyright 2017, The Royal Society of Chemistry.
Scheme 9. (a) Synthesis of MOF, UiO-66-IL-Br and its anion exchanged product (UiO-66-IL-X, where X = anions, SO3CF 3 , PF6 ), (b) schematic representation for the synthesis of cross linked MOF membrane of UiO-66-IL-ClO4. Reproduced with permission from [46]. Copyright 2017, American Chemical Society.
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Fig. 17. (a) Loading of MOFs in the membrane UiO-66-IL-ClO4, (b) IR spectra of the membranes and oligomer. Reproduced with permission from [46]. Copyright 2017, American Chemical Society.
Table 14 Cycloaddition reaction results with membrane bound UiO-66-IL-ClO4 (membrane MOF). Entry
Epoxides
1
Cl 2
Cl 3
Br a
Products
O
O Cl
O Cl
Membrane MOF (0.7 mol% MOF)
29
O
TBAB (1 mol%)/membrane MOF (0.7 mol% MOF)
99
TBAB (1 mol%)/membrane MOF (0.7 mol% MOF)
99
O
O Br
Yields [%]a
O O
O
O
Catalyst
O O
1
The yield of different CCs were determined by H NMR spectroscopy.
Scheme 10. The post synthetic modification of UTSA-16 by alkali metal cations to M-UTSA-16 followed by cycloaddition reaction. Reproduced with permission from [47]. Copyright 2017, Elsevier.
Table 15 The BET surface area of MOFs (M-UTSA-16). MOF
BET surface area (m2/g)
Cs-UTSA-16 Rb-UTSA-16 K-UTSA-16 Na-UTSA-16 Li-UTSA-16
291.3 461.5 847.0 544.1 519.4
Im-UiO-66 adopting a post-synthetic modification technique (Scheme 7). The bifunctional framework, (I) Meim-UiO-66, consisting both iodide ions and Brønsted acid sites, exhibited superior catalytic activity in absence of any co-catalyst. In this particular MOF, the I ions present in the channel near the catalytic centre of Zr6 clusters, acted as a co-catalyst. The cycloaddition reactivity between CO2 and epichlorohydrin was found to be 88.5% conversion in 24 h under 1 bar pressure
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Fig. 18. (a) CO2 uptake by M-UTSA-16 at 273 K, (b) conversion of cycloaddition reaction by M-UTSA-16. Reproduced with permission from [47]. Copyright 2017, Elsevier.
Table 16 Cyclo addition reaction between CO2 with different epoxide epoxides by Li-UTSA-16.a Entry
Epoxides
1
H3C 2
Products
O
O
O H3C
T [h]
Temp. (K)
Conversions (%) (sel.)[%]
6
393 K
25.3 (>99)
6
393 K
25.9 (>99)
6
393 K
99 (>99)
O
O
O
O
O 3
Cl a
O
O Cl
O O
Reaction conditions: Catalyst (60 mg), epoxide (3.5 mL), pressure of CO2 1.5 MPa.
Scheme 11. The synthetic scheme for the preparation of polyILs@MIL-101. Reproduced with permission from [48]. Copyright 2018, The American Chemical Society.
at 373 K. The lower conversion rate may be due to the lower BET surface area (328 m2 g1) offered vy MOF, (I) Meim-UiO-66. Interestingly, when the temperature was raised to 393 K, a quantitative conversion was obtained. This catalyst could be recycled six
times in the cycloaddition reaction of epichlorohydrin with CO2, without a noticeable loss in activity. An acid/base synergistic catalysis mechanism has been proposed where the ring-opening of the epoxide occurs due to the synergistic activation effect by
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nucleophilic Lewis base, iodide anion and Brønsted-acid (Zr–OH/ Zr–OH2) (Scheme 8). A membrane-based MOF reported by Dong et al. that exhibited a cycloaddition reaction between CO2 and epoxide [46]. An ionic liquid (imidazolium ion) based dicarboxy ligand reacted with ZrCl4 solvothermally led to the formation of the MOF, UiO-66-IL-Br (Scheme 9). The Br anion could be exchanged to form UiO-66 IL-X (X = anion, SO3CF 3 , PF6 , ClO4 , Br ). Upon reacting with a polyurethane oligomer it formed a cross-linked membrane bound MOF (Scheme 9). Amongst all the anion exchanged ionic liquid based MOFs, UiO-66-IL-ClO4 and cross-linked membrane bound
UiO-66-IL-ClO4 (Fig. 17) showed very good selectivity of CO2 adsorption and excellent yields for the cycloaddition reactions in presence of TBAB (Table 14). The mechanism of cycloaddition reaction was proposed to be similar to that of acid catalysed reaction. In another report, the well known MOF, UTSA-16 had been post synthetically converted [47] to the series of alkali metal based MOF, M-UTSA-16 where the pore of the framework contained alkaline metal cations (M = Cs, Rb, K, Na, Li) (Scheme 10) with different BET surface area (Table 15). The CO2 uptake capacity at 273 K and 298 K increased in the order Cs+ < Rb+ < Li+ < Na+ < K+ commensurate with the BET surface area (Fig. 18).
Fig. 19. (a) The adsorption isotherm of N2 and CO2 by polyILs@MIL-101 at different temperatures, (b) Qst for CO2 adsorption by polyILs@MIL-101 and MIL-101. Adapted with permission from [49]. Copyright 2018, The American Chemical Society.
Table 17 Cycloaddition by polyILs@MIL-101 between CO2 and different epoxides.a Entry
Epoxide
Product
O
1
O
O
Time (h)
Temperature (K)
Sel.(%)b
Con.(%)b
2
318
100
94
1
343
100
89
1
343
100
85
2
343
100
81
1
343
100
95
3
383
100
68
1
343
100
>99
O
O
2
O
O
O 3
Br
O
O Br
O
4
O O
O
O O
5
O
O
O O
6c
O
O
O 7
HO a b c
O
O HO
O O
O O O
Reaction conditions: catalyst polyILs@MIL-101 (100 mg), epoxide (1 mmol), PCO2 = 1 bar, solvent acetonitrile (2 mL). GC was used to determine % of con. and sel. DMF 2 mL was used.
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All the cation exchanged frameworks were able to perform the cycloaddition reaction between propylene oxide (PO) and CO2 (Fig. 18b, Table 16) and among them the activity was showing by Li-UTSA-16 (72%) was much greater compare to the other frameworks (M-UTSA-16). The possible reason is due to the highest charge density (since smallest radii) of Li+ ion compare to the other alkali metal cations can diffuse the epoxide and CO2 effectively. The catalyst was reusable upto five cycles without losing its conversion efficiency. Recently, Jian and co-workers post-synthetically modified MIL101 (Scheme 11) with a poly ionic liquid (poly IL) to have,
HOOC
polyILs@MIL-101 which could efficiently perform cycloaddition reaction between CO2 and epoxide [48]. Although polyILs@MIL-101 had lower BET surface area and pore volume of (2462 m2/g and 1.26 cm3/g) compared to the pristine MOF, MIL-101 (3603 m2/g and 1.83 cm3/g), it exhibited significant CO2 adsorption capacity with greater heat of adsorption (Fig. 19) due to increased aromatic p-surface and the presence of confined pore size [49] compared to the MIL-101. The MOF, polyILs@MIL101 was able to performed cycloaddition reactions with a variety of epoxides (Table 17) and can be re-used without loss of catalytic activity. The reaction mechanism was similar to the acid catalysed one.
COOH
NH2
SBU HOOC
NH2 group decorated cage
COOH
(b)
(a)
Fig. 20. (a) Tetracarboxylate linker, (b)–NH2 decorated pore within MOF network. Reproduced with permission from [51]. Copyright 2015, Wiley VCH.
Table 18 The styrene oxide (SO)# cycloaddition reactions with CO2 and catalysed by Cu(II)-MOF. Entry
SO:MOF
Time (h)
Temperature (K)
Pressure (bar)
Conversion (%)*
TON
1 2 3
667 667 667
6 12 12
393 393 393
20 20 50
43 50 13
286 333 86.7
# The activated Cu(II)-MOF immediately transferred inside glove box at room temperature. Styrene oxide and TBAB were added within the autoclave already placed inside the glove box. * 1 H NMR helps to analyze the conversion of CC by integration of SO peak with CC peak.
NH2 HO2C
CO2H
CO2H
(a)
CO2H
(b)
Fig. 21. (a) Schematic diagram of the ligand, (b) the pore along a-axis. Reproduced with permission from [52]. Copyright 2017, The Royal Society of Chemistry.
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in presence of TBAB as the co-catalyst. However, a maximum of 50% conversion was found in this reaction. When the ligand was replaced with a bent tetracarboxylate (Fig. 21) incorporating an outward –NH2 group in the middle benzene ring, a paddle-wheel Cu2(COO)4 SBU was formed with metal bound solvent molecules [52]. Upon heating, a porous MOF embellished with unsaturated metal centres and free –NH2 groups was formed showing very high adsorption of CO2 gas (60% by weight) at 298 K and 32 bar pressure. The activated MOF catalyzed the adsorbed CO2 to CCs with epoxides at room temperature in presence of the co-catalyst TBAB. Table 19 indicates the MOF is able to convert different epoxide to their corresponding carbonate by reacting with CO2. Most significantly, the CO2 present in the atmosphere was successfully converted to the CC at room temperature when the air was passed through the suspended solution of MOF in presence of TBAB (Table 20). The catalyst was reused upto four cycles with out change in its reactivity. The good yields of the CCs under ambient condition was due to the easy accessibility of the vacant Cu(II) sites for activation of the
2.3. Cooperative bifunctional catalysis due to open coordination sites and Lewis basic functional linker Although the cycloaddition of CO2 to epoxides was mainly catalyzed by Lewis acidic open metal sites, still literature report divulged that the cooperative effect contribute from both Lewis acidic and basic component parts in the MOF can execute the cyclo addition reaction between CO2 and epoxide under mild condition [50]. For example, a hanging amine moiety in the linear tetracarboxylic acid, that formed a nbo type Cu(II)-MOF (Fig. 20) had been reported [51]. The high thermal stability of this MOF allowed the removal of loosely bound solvents from metal as well as from cavity on heating to get a porous framework. The presence of coordinatively unsaturated metal sites, accessible Lewis basic NH2 group and large voids made the compound an excellent heterogeneous catalyst for the cycloaddition of CO2 to epoxides under relatively mild reaction conditions (20 bar, 393 K). As seen from Table 18, the compound could serve as a good recoverable catalyst for the cycloaddition of CO2 to styrene oxide under solvent-free condition
Table 19 Cycloaddition of various epoxides with CO2 catalyzed by activated MOF using CO2 gas at 1 bar and at room temperature. Entry
Epoxide
Product
O
O Cl
Time (h)
Yields (%)
8
95a 19b 10c
12
92
8
89
12
88
8
85
Time (h)
Yields (%)
24
62
24
45
24
57
8
30
O O
Cl 2
O
O
O O
3
O
O O O
4
O
O O
O O
O
5
O
O O
a b c
O
In presence of both MOF and TBAB. Absence of co-catalyst TBAB. Absence of MOF.
Table 20 Cycloaddition of various epoxides with CO2 through purging of atmospheric air at 298 K. Entry
Epoxide
1
Product
O
O O
Cl
O
Cl 2
O
O O O
3
O
O O
O O
4
O
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Scheme 12. The probable mechanism for the cycloaddition reaction catalyzed by Cu(II)-MOF in presence of TBAB. Reproduced with permission from [52]. Copyright 2017, The Royal Society of Chemistry.
epoxide ring and also free-NH2 groups for non-covalent interaction with CO2. The proposed mechanism [53] could be found in Scheme 12. The open Cu(II) sites of the activated MOF accelerated the opening of the epoxide ring by the use of TBAB to afford a
Cu-bound alkoxide. Next, cycloaddition of CO2 to the metal bound epoxide occurred in presence of TBAB. The metal bound carbonate subsequently formed CC with the regeneration of the catalyst. Recently, Neogi and co-workers by using 2-amino terephthalic acid constructed popular framework, UiO-66-NH2 (known as ZrOF) [54]. They incorporated metal nanoparticles (NPs) inside the pores via post-synthetic modification (Scheme 13) and characterized the NP embedded framework (Ni@ZrOF) by a number of spectroscopic techniques that suggested that the ultra-small Ni NPs well dispersed inside the pores. Although BET surface area was reduced compared to the original MOF, the CO2 adsorption capacity improved by 35% with 10 kJ/mol rise in the heat of adsorption value. This compound in presence of TBAB as the co-catalyst, catalysed solvent-free CO2 cycloaddition with styrene oxide (SO) in a heterogeneous manner in 98% yield with 99% selectivity (Table 21, entry 10) at 1.0 MPa CO2 pressure and 343 K for 6 h. The catalyst could be recycled at least five times without any significant loss in activity and was quite versatile for other substrates as well (Fig. 22). A tentative mechanism had been provided as shown in Scheme 14. The efficient catalytic activity of the Ni@ZrOF system was ascribed to its empty network architecture with Zr4+ acidic sites, introduction of Ni nanoparticles, and pendent basic amino groups that cooperatively augmented its catalytic abilities. Evidently, the presence of both Nio and Zr4+ metals synergistically affect the catalytic performance of Ni@ZrOF, by activating the epoxide oxygen atom (Scheme 14). Li et al. reported [55] a 3D Ba-MOF, [Ba2(BDPO)(H2O)] (H4BDPO = N,N-bis(isophthalic acid)-oxalamide) with oxophilic alkaline-earth Ba2+ ion (Fig. 23). It had 1D honeycomb hexagonal channels with a window diameter of ~8.2 Å. The activated framework exhibited efficient CO2 chemical fixation owing to the
Scheme 13. Synthesis of Ni@ZrOF using the Solution Impregnation Method. Reproduced with permission from [54] Copyright 2019, American Chemical Society.
Table 21 Cycloaddition of styrene oxide (SO) and CO2 using various catalysts.a
a
Entry
Catalyst
Temperature [K]
Selectivity [%]
Yield [%]
1 2 3 4 5 6 7 8 9 10 11
none ZrCl4 NH2–BDC TBAB NH2–BDC + ZrCl4 + TBAB ZrOF Ni@ZrOF ZrOF/TBAB Ni2 + @ZrOF/TBAB Ni@ZrOF/TBAB Ni@ZrOF/TBAB
343 343 343 343 343 343 343 343 343 343 393
0 – – 98 98 99 99 97 98 99 99
0 – – 25 30 41 45 52 62 98 99
Reaction conditions: catalyst, 0.35 mol %, SO, 30.6 mmol; co-catalyst, 0.11 mol %; 1.0 MPa of CO2 pressure for 6 h at 343 K.
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
presence of the open metal site (Lewis acidic) and the oxalamide groups (Lewis basic) inside the tubular channel (Table 22, entry 1). The pleasing catalytic activity were indorsed due to the rod like metal–organic chains, suitable for epoxides activation (Scheme 15) aided by the oxalamide groups that effectively interacted with CO2 molecules. Lower conversion for the larger epoxides indicated that the cycloaddition reactions took place within the cavity. The MOF showed no loss of activity after five runs with epichlorohydrin and CO2. Solvothermal synthesis of two frameworks, Zn-NTTA and CuNTTA with the tripodal ligand (H6NTTA) (Fig. 24) had been reported [56]. In Zn-NTTA, two kinds of dinuclear paddle-wheel SBUs {Zn2(OOC)3} and {Zn2(OOC)4} were present leading to three cages of diameters 16.0, 15.6, and 19.0 Å. The framework showed high and selective uptake of CO2 (115.6 cm3g1) at 273 K. Presence of highly dense vacant coordination sites along with acylamide functional groups made these MOFs highly catalytically active with TON values up to 110,000 per paddle-wheel unit and 30 rounds of recyclability under mild conditions. Within 8 h of reaction, about 0.25 mol % of Zn-NTTA (based on a paddle-wheel unit) provided nearly complete conversion (yield ~98%) (Table 23) when the reactions were conducted under solvent-free condition at 373 K in the presence of TBAB co-catalyst with CO2 purged at 1.0 MPa. Under similar conditions Cu-NTTA also showed very good catalytic activity. The cycloaddition reactions using mixed gas as CO2 source under the same reaction conditions afforded a significant yield of 87.3% after 8 h. This showed no adverse effect of moisture (Fig. 25) and the MOFs could be used for the selective adsorption and chemical conversion of CO2 releasing from a power plant. The thermally robust anionic Zn(II)-MOF, {[(CH3)2NH+2]2[Zn3 ((l3-O))(L)2(H2O)]4DMF2H2O}n, synthesized solvothermally [57], was highly porous having free amino groups. The linker L used was a tricarboxylate ligand shown in Fig. 26. This anionic 3D framework showed a two-fold interpenetrated structure. Loss of metal bound and lattice water molecules by heating afforded a porous structure with 45.1% void volume. In presence of TBAB as
O
O Cl
O
H3C
Scheme 14. The probable mechanism for CO2 conversion with substituted styrene epoxide by Synergic Ni@ZrOF/TBAB Catalyst. Reproduced with permission from [54]. Copyright 2019, American Chemical Society.
co-catalyst, this activated MOF exhibited outstanding catalytic ability in transforming CO2 to CCs with various epoxides under atmospheric pressure and solvent free conditions. The maximum amount of conversion was 99% (Table 24, entry 1), in case of styrene oxide as substrate at 20 bar pressure and 373 K for a reaction time of 6 h. It could be reused at least three times without much loss of catalytic activity. Another amine-functionalised framework built on MIL-101, NH2-MIL-101(Al) by solvothermal (1S) and microwave (1M) methods, was reported by Neogi and co-workers [58]. Using styrene oxide (SO) as the substrate, 96% styrene carbonate was obtained with 99% selectivity and 23.5 h1 turnover frequency (TOF) when
O
O
O
O
O
O
H2C
8
24
O
CH3
Fig. 22. The bar diagram indicates the yield (red) and selectivity (green) for cycloaddition reaction catalysed by Ni@ZrOF/Bu4NBr. Reproduced with permission from [54] Copyright 2019, American Chemical Society.
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Fig. 23. (a) Ligand, H4BDPO, (b) left-handed 21 helical rod SBUs, (c) yellow rod indicates tubular structure, (d) representation of the porous surface, (e) the pore along caxis. Reproduced with permission from [55]. Copyright 2018, American Chemical Society.
Table 22 Results of the cycloaddition reactions of CO2 with different epoxides catalyzed by Ba-MOF. Entry
Epoxides
1a
Products
O
O
O
Conversion [%]
TONe
98.1
196.2
99.2
198.4
95.7
191.4
94.5
189.0
90.2
180.4
94.1
188.2
97.5
195.0
19.8
39.6
52.3
104.6
O 2b
O
O
O O
3a
H3C 4a
Br 5a
Cl 6c
Cl 7d
Cl 8a
O
O
O H3C
O Br
O
O
O
O O
Cl
O Cl
O O
O Cl
O
O O
O
O
O
O O
O
O O
9a
O
O
O O
a b c d e
O O
Reaction conditions: 0.5%mmol catalyst, 20 mmol epoxides, 2% mmolTBAB,CO2 (1 atm), room temperature, 48 h. The same conditions but CO2 pressure increased to 10 atm and the time reduced to 6 h. The same conditions with temperature enhanced to 323 K. Under same circumstances but temperature increased to 343 K. TON (product (mmol)/catalyst (mmol)).
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
activated 1S/TBAB system was used at 393 K, 18 bar CO2 pressure for 6 h of reaction (Table 25, entry 9). Under the same reaction condition, the activated 1M/TBAB system showed almost equal conversion (93.6%) with 99% selectivity (Table 25, entry 10). The catalyst could be reused five times. A variety of aliphatic and aromatic epoxides showed excellent conversion. A reaction mechanism was also proposed (Scheme 16). Solvothermal reaction of 2,5-di(1H-1,2,4-triazol-1-yl)tereph thalic acid, H2BTTA (Fig. 27a) with Cu(NO3)2 in H2O at 393 K for 3 d afforded the framework FJI-H14 [59]. The structure contained 1D hexagonal channels along the crystallographic caxis (Fig. 27c) and had high density of open metal sites (OMS) and also Lewis basic sites (LBS) within the channel. This framework
captured CO2 effectively and selectively and displayed remarkable stability in presence of acid/ base and high volumetric uptake (171 cm3 cm3) of CO2 under ambient conditions (298 K, 1 atm). The CO2 present in the flue gas could be efficiently transformed into CCs by the FJI-H14 catalyst and TBAB as co-catalyst. Such high CO2 uptake capacity and reasonable catalytic performance may result from the cooperative effect of the various active sites. Compound FJI-H14 displayed a far higher catalytic performance for the reaction of styrene oxide with the CO2 present in simulated flue gas compared to other catalysts, as shown in Table 26. The larger size epoxide, 1,2-epoxyoctane gave less yield (27%), signifying that the reactions occurred within the channels of FIJ-H14.
Scheme 15. The CO2 conversion mechanism. Reproduced with permission from [55] Copyright 2018, American Chemical Society.
Fig. 24. Acylamide-bridging ligand, H6NTTA and three types of cages present in Zn-NTTA. Spheres represent the void spaces. H atoms and coordinated water molecules are not shown for clarity. Reproduced with permission from [56] Copyright 2016, American Chemical Society.
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
2.4. Catalysis due to the presence of defect in the crystal In one of the earlier reports in 2009, MOF-5 in presence of TBAB was utilized for the fixation of CO2 with epoxides exclusively based on defect in crystal structural [21]. The MOF-5/TBAB was a very efficient catalyst system and the reaction could be completed within 6 h at 323 K and 6 MPa CO2 pressure with very high selectivity and could be recycled. Other epoxides could also be transformed effectively under mild conditions (Table 27). In a similar way, MOF-205/TBAB system showed excellent catalytic activity in the room temperature CO2 fixation [60]. Given that the metal nodes were fully saturated, defect sites in the structure were proposed [61] to be responsible for its catalytic activity. Taking a cue from this result, Carreon and his group demonstrated that zeolitic imidazole framework-8 (ZIF-8) showed effective catalytic activity in the synthesis of chloropropene carbonate as the main product by the cycloaddition of CO2 to epichlorohydrin [62]. It was proposed that as a consequence of surface defects, ZIF8 was highly active in the conversion of epichlorohydrin under solvent free condition (98.2% conversion at 373 K) without the aid of any co-catalyst. Unfortunately, the product selectivity was low (33.4%). To increase the selectivity and yield, ZIF-8 was postsynthetically modified by functionalization on the surface with ethylene diamine (labelled as ZIF-8-f). This post-synthetically modified sample offered both improved product selectivity (chloropropene carbonate, 73.1%) and conversion (100%) at temperature of 353 K demonstrating the importance of combining defect sites at the surface with post-synthetic modification as a helpful approach for enhancing the catalytic performance of MOFs. Jiang group synthesized a stable, Al-containing MOFknown as
Fig. 25. Fixation of CO2 present in wet gases with epoxides. Reaction conditions: 20 mmolMOF, 100 lLH2O, 5 lmolcatalyst (based on a paddle wheel unit), and TBAB (0.3 mmol) under 1.0 MPa CO2 or mixed gas pressure, temperature: 373 K, time: 8 h. Reproduced with permission from [56] Copyright 2016, American Chemical Society.
USTC-253, isoreticular to MOF-253, by substituting bipyridine-5,50 -dicarboxylate ligands with sulfone-functionalized 4,40 -dibenzoic acid-2,20 -sulfone ligand (Fig. 28) [63]. When some amount of modulator like trifluoroacetic acid (TFA) was used during the synthesis, USTC-253-TFA was formed with structural defects within the crystals (Fig. 29).
Table 23 Coupling of epoxides with CO2.
Entry
Substrate
Zn-NTTA
Cu-NTTA
Yield (%)
TON
Yield (%)
TON
1a
98.2
3920
95.9
3830
2a
64.1
2560
56.3
2250
3a
97.2
3880
97.4
3895
4b
82.5
3300
89.4
3570
a Reaction conditions: 20 mmol epoxide, 5 lmolcatalyst (based on a paddlewheel unit), and TBAB (0.3 mmol), 1 MPa CO2pressure, 373 K for 8 h. The yields were calculated from 1H NMR spectra. b All experimental conditions are same except 10 mmol epoxide was used.
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Fig. 26. (a) Structure of ligand H3L, (b) trinuclear SBU of in the framework showing metal bound water molecule, (c) coordination modes of ligand showing Lewis basic –NH2 group, (d) before and (e) after two-fold interpenetration in the framework. Reproduced with permission from [57]. Copyright 2017, American Chemical Society.
Table 24 Cycloaddition reactions of CO2 with various epoxides.a
Time (h)
Temp. (K)
P (bar)
% Yieldb
1
6
373
20
99
2
6
373
10
95
3
12
373
1
96
4
12
323
1
92
5
12
323
1
94
6
12
323
1
87
Entry
Epoxide
Product
29
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173 Table 24 (continued) Entry
Epoxide
Product
7
a b
Time (h)
Temp. (K)
P (bar)
% Yieldb
6
323
10
98
Reaction conditions: epoxides (25 mmol, 1 equiv), Activated MOF (10 wt%), 40 mg TBAB, (0.005 equiv). solvent free. Conversion was estimated from the 1H NMR spectra.
Upon activation by heating, these defects led to the generation of Lewis acidic open metal sites showing greater CO2 uptake property (168–182%) with respect to the defect-free pristine USTC-253, isoreticular MOF-253, and MIL-53. Additionally, the presence of large no of easily accessible Lewis and Brønsted acid sites, USTC-253-TFA exhibited a reasonable catalytic activity to give cyclic carbonates at ambient temperature and 1 bar CO2 pressure. The catalytic activity of USTC-253-TFA was superior to that of parent USTC-253 and some other MOFs (Figs. 30 and 31). It was believed that large no of easily accessible Lewis and Brønsted acidic sites in USTC-253-TFA could increase the cooperative effect with TBAB to help the CO2 cycloaddition reaction (Fig. 32).
MOF-892 possessed large hexagonal one-dimensional channels of pore aperture of ~24 27 Å2 along the crystallographic c-axis while MOF-893 had triangular 1D channels along the crystallographic a-axis with much smaller pore aperture of ~8.6 9.9 Å2. MOF-894 was an overall 2D anionic framework having two crystal-
2.5. Catalysis due to the presence of accessible acidic functional sites With a linker having multi-carboxylate groups, one or more acid groups might remain uncoordinated in the MOF. Following this approach, a new 3D framework, [Ni(btzip)(H2btzip)] 2DMF2H2O (1) had been reported (H2btzip = 4,6-bis(triazol-1-yl) isophthalic acid) [64]. The framework architecture consisted of 2D interconnected channels and the pore of the channels exposed with free acidic functional groups and the nitrogen atom of the triazole unit (Fig. 33). The activated framework (1a) efficiently catalyzed the transformation of CO2 to CCs with various epoxides in presence of co-catalyst TBAB. The maximum yield (98.9%) was found when CO2 was reacted with propylene oxide under 2.0 MPa pressure of CO2 and 353 K for 4 h (Table 28, entry 4). In this case, activation of the epoxide was possible through the free –COOH groups in the channels which was then attacked by the Br ion of TBAB on the less hindered C atom of epoxide. In the final step, the alkyl carbonate anion was converted into the CC, and simultaneously TBAB was regenerated. A hexatopic linker, 10 ,20 ,30 ,40 ,50 ,60 -hexakis(4-carboxyphenyl)ben zene had been used to construct three MOFs, viz., Zirconium based (MOF-892 and MOF-893) and Indium based MOF-894 respectively, (Fig. 34) [65]. All the frameworks were highly porous and stable. The overall structure of MOF-892 and MOF-893 contained Zr6 cluster and two out of the six carboxylic acid groups remained free.
Scheme 16. The general (above, A = acidic site; B = basic site) and probable mechanism (below) for CO2 conversation with epoxides using 1S/TBAB system. Reproduced with permission from [58] Copyright 2018, The Royal Society of Chemistry.
Table 25 Cycloaddition reaction of CO2 with styrene oxide. Entry
Catalyst
Temp. [K]
Conversion [%]
Selectivity [%]
Yield [%]
1 2 3 4 5 6 7 8 9 10
None AlCl36H2O NH2-BDC TBAB AlCl36H2O + NH2-BDC + TBAB 1S 1M 1S/TBAB 1S/TBAB 1M/TBAB
393 393 393 393 393 393 393 RT 393 393
0 Trace Trace 47 48 26 22 Trace 99 93.6
0 – – 99 98 78 72 – 99 99
0 – – – – 14 11 – 96 95
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Fig. 27. Structural diagram of FJI-H14. (a) The linker H2BTTA used to construct FJI-H14. (b) Four-connected metal and ligand binding mode. (c) 1-D channels along the caxis. (d) USF topological figure (Cu atoms are in cyan, C atoms are ingray, O atoms are in red, N atoms are in blue and H atoms are in white colour). Reproduced with permission from [59] Copyright 2017, Nature Communication.
Table 26 Cycloaddition of epoxides and CO2.a
Catalysts
Time (h)
Yield (%)b
1
FJI-H14
24
86
2
–
24
52
3
Cu(OAc)2
24
45c
4
Cu(NO3)2 + H2BTTA
24
70
5
HKUST-1
24
67
Entry
Substrates
31
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173 Table 26 (continued) Catalysts
Time (h)
Yield (%)b
6
FJI-H14
24
95
7
FJI-H14
24
27
Entry
Substrates
a Reaction conditions: 20.0 mmol styrene oxide, 0.48 mol % catalyst (per Cu(II) units), 2.5 mol %TBAB, the reactions were conducted in a Schleck tube connected with a condenser, simulated post-combustion flue (CO2 = 0.15 atm, N2 = 0.85 atm) was purged at 1 atm and 353 K for 24 h. b Determined by 1H NMR. c Some by-products were formed in case of Cu(OAc)2 catalyst.
lographically distinct monoatomic indium ions. All the MOFs are higly effective and selective for transformation of CO2 to CCs under solvent free condition of CO2 pressure 1 atm at 353 K (Table 29). A plausible mechanism had been proposed for these transformations as shown in (Scheme 17) for MOF-892. The epoxide was
effectively activated due to the coexistence of accessible Zr units and free carboxylic acid moieties. Hydrothermal synthesis of a 2D porous MOF, {[Zn2(TBIB)2 (HTCPB)2]9DMF19H2O}n using a mixture of ligands 1,3,5-tri(1Hbenzo[d]imidazol-1-yl)benzene (TBIB) and 1,3,5-tris(40 -carboxy
Table 27 Synthesis of cyclic carbonates catalyzed by MOF-5 in the presence of TBAB.a Entry
a b
Time [h]
Yield [%]
1
Epoxides
Products
3
56
2b
4
97
3
12
93
4
15
92
Reaction conditions: epoxide 20 mmol with 2.5 mol% TBAB, 0.1 g MOF-5, CO2 pressure 0.1 MPa, reaction temperature 323 K. CO2 pressure 6 MPa.
Fig. 28. Framework structure of USTC-253 (left) and different ligands for the constraction of isostructural MOFs (right). Reproduced with permission from [63]. Copyright 2015, Wiley VCH.
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(Table 30). The catalyst was reused up to five times. The reaction mechanism was similar to other cases having free –COOH groups (Scheme 17). A non-interpenetrated suphonate based MOF, TMOF-3 had been synthesized hydrothermally [67] by reacting 1,2,4,5-benzenetetra methane sulfonate, 4,40 -bipyridine, and Cu(NO3)23H2O at 448 K for 72 h (Scheme 18). Crystallographic investigation revealed two sulphonate groups were deprotonated and were distributed in the pore of the channels. The framework showed highest void space of 42.5% amongst the MOFs having organosulfonate ligand. It exhibited high CO2 adsorption capacity. When TMOF-3 was placed in 10 mL aqueous solution of silver nitrate (10 mmol/L) for one hour it afforded TMOF-3-Ag (Scheme 18). This TMOF-3Ag framework adsorbed significant amount of acetylene (103 cm3/g and 75 cm3/g at 273 and 298 K (Fig. 36a). Also, TMOF-3-Ag catalyzed the reaction between CO2 and propargyl alcohol in presence of the base DBU at room temperature to form carbonate (Table 31). The probable mechsnism of this catalysis reaction could be as in Fig. 36b.
2.6. Catalysis due to the accessible Lewis and Brønsted acid bifunctional sites
Fig. 29. Synthesis of (a) USTC-253 and (b) defect-engineered USTC-253-TFA. Reproduced with permission from [63]. Copyright 2015, Wiley VCH.
Fig. 30. Fixation of CO2 with propylene oxide usingvarious catalysts. Reaction conditions: 28.6 mmol propylene oxide, 0.289 mmol catalyst, 1.86 mmol TBAB, under 1 bar CO2 pressure and 298 K for 72 h. Reproduced with permission from [63]. Copyright 2015, Wiley VCH.
phenyl-)benzene (H3TCPB) had been reported [66]. The 2D layer structure formed an overall 3D structure with 1D channels via supramolecular interactions (Fig. 35). Free –COOH groups and uncoordinated–N atoms of the TBIB ligand decorating the channels acted as an excellent catalyst for the formation of CCs from various epoxides at room temperature
Ma and coworker utilized the linker 2,4-bis(3,5-dicarboxyphe nylamino)-6-oltriazine (H4BDPO) for the synthesis of buffer CuMOF, (JUC-1000) [68] where the phenolic component showed weak acidic behavior and triazine and amino groups displayed weak basic activity (Fig. 37). The MOF maintained its structural integrity at variour pH range in aqueous media owing to the combine effect put forward by basic and acidic functional groups as buffer guards (Scheme 19). The buffer ligand H4BDPO reacted with copper nitrate to form the microporous MOF, JUC-1000 having copper paddle-wheel SBUs. The high CO2 uptake capacity by JUC-1000 exerted by the presence of open metal center and the basic part allowed to performed CO2 fixation reaction with epoxide under solvent free condition and TBAB as co catalyst at 1 atm and room temperature (Table 32). It behaved as a superior catalyst compared to the other homologous MOFs like HKUST-1 (62% yield, entry 3) and MOF-505 (61%, entry 2) under similar conditions. The lower yiled was observed when the size of the epoxide increases (entries 4, 5, and 6). It is quite interesting to note that the epoxide ring has been activated synergistically: The oxygen atom of the epoxide can interact with the open Cu(II) metal site and also the hydrogen bonding interaction with the phenolic –OH and the amino group, already placed in the ligand (Fig. 38). The ring opening was facilitated by Br ion from TBAB and the resultant oxide intermediate then rapidly reacted with the activated CO2 molecules (amino and triazine moiety activated the CO2) to produce alkyl carbonate. The final CC was achieved through the cyclization step. The buffer mediated platform in the MOF encouraged for such an efficient CO2 cycloaddition reaction with epoxide. An tetrazolyl-carboxylate ligand, 5-(4-(tetrazol-5-yl)phenyl)iso phthalic acid (H3tzpa) has been used to form a 3D Co(II)-based MOF [69] in which l3–OH groups and free carboxylate oxygen atoms are adorned inside 1D tubular channels (Fig. 39). The framework was chemically stable in aqueous acidic and basic solutions. This activated MOF, 1a showed high capacity, selective CO2 adsorption and facile catalytic activity in the cycloaddition of CO2 to epoxides to form CCs at 298 K and 100 kPa (Scheme 20) in presence of TBAB as co-catalyst (Table 33). Maximum of 93.8% yield of propylene carbonate was reported at a molar ratio of 100/10/1for epoxide/TBAB/1a for 48 h (Table 33).
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
33
Fig. 31. Chemical fixation of CO2: (a) propylene oxideas substrate using various catalysts, (b) various substituted epoxides as substrate using USTC-253-TFA as catalyst. Reaction conditions: 28.6 mmol epoxide, 0.289 mmol catalyst, 1.86 mmol TBAB, under 1 bar CO2 pressure and 298 K for 72 h. Reproduced with permission from [63]. Copyright 2015, Wiley VCH.
Fig. 32. Proposed mechanistic diagram for the fixation of CO2 with epoxideusing USTC-253-TFA catalyst. Reproduced with permission from [63]. Copyright 2015, Wiley VCH.
Fig. 33. (a) The H2btzip ligand, (b, and c) the pore along a and b axes in 3D framework showing free –COOH groups and 2D channels respectively. Adapted with permission from [64]. Copyright 2018, Wiley-VCH.
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Table 28 Cycloaddition reactions of various epoxides catalyzed by 1a in presence of TBAB. Entry
Catalysts
Epoxides
Products
Yields [%]
1
1a
5.2
2
TBAB
10.8
3
1a/TBAB
9.0
4
1a/TBAB
>98.9
5
1a/TBAB
98.3
6
1a/TBAB
83.1
7
1a/TBAB
90.9
8
1a/TBAB
40.0
Epoxide (20 mmol), 1a (0.2 mmol), TBAB (2 mmol). Reaction conditions: entry 3: 0.1 MPa, 298 K, 48 h; entries 1–2 and 4–8: 2.0 MPa, 353 K, 4 h.
Fig. 34. Hexatopic linker and structure of MOF-892, MOF-893, and MOF-894. Reproduced with permission from [65].Copyright 2018, American Chemical Society.
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173 Table 29 Reactions between CO2 and styrene epoxide by MOF-894, MOF-893, and MOF-892.a
a b c
Entry
Catalyst
Conversion [%]b
Selectivity [%]b
T [h]
Yield [%]b
1 2 3 4 5 6 7 8
MOF-894 MOF-894 MOF-893 MOF-893 MOF-892 H3CBPc H3CBP None
86 67 98 66 96 23 79 55
91 99 90 96 86 86 24 88
20 16 23 16 16 16 16 16
78 66 88 63 82 20 19 48
Reaction conditions: MOF/catalyst (0.32 mol %), styrene oxide (6.87 mmol), nBu4NBr (1 mol %), 1 atm CO2 (balloon pressure) and 353 K. The selectivity (sel), catalytic conversion (and yield were measured by GC–FID with the help internal standard(biphenyl). 0.16 mol % catalyst.
Scheme 17. The cyclo addition reaction of SO with CO2 and catalysed by MOF-92. Reproduced with permission from [65]. Copyright 2018, American Chemical Society.
COOH
N N
N
N
Zn(II)
N
HOOC
COOH
H3TCPB
N
TBIB
Fig. 35. Structure of ligands H3TCPB, TBIP and MOF. Reproduced with permission from [66].Copyright 2019, American Chemical Society.
The mechanism has been proposed based on the Lewis (vacant Co2+ ions) and Brønsted acidic (l3–OH groups) bifunctional catalytic sites (Scheme 21). First, the epoxy ring was activated by open Co2+ ions (formed strong Co O interactions with epoxide) and O–H O hydrogen bonding with l3-OH groups (Fig. 40). Then, the Brions from TBAB attacked the less hindered C atom of the epoxide followed by insertion of CO2 formed the CCs through the ring-closing step.
3. Conclusion and future perspective CO2 is one of the naturally abundant C1 feedstock. Its catalytic conversion to fine chemicals (CC) has been a hot research topic now-a-days. This is due to the fact that CCs are useful chemicals for a number of industries such as pharmaceuticals, dye, electrolyte in the lithium ion batteries, intermediates for the synthesis of ethylene glycol, polymeric materials, precursor
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Table 30 Coupling of various epoxides with CO2 at atmospheric pressure.a Entry
Epoxide
CC
Yieldb (%)
Catalyst
Co-catalyst
Time (h)
1
MOF
TBAB
24
>99
2
MOF
TBAB
24
>99
3
MOF
TBAB
12
>99
4
MOF
TBAB
24
82
5
MOF
None
24
7
6
None
TBAB
24
12
7
H3TCBP
TBAB
24
20
a At room temperature CO2 gas was bubbled through the two-neck round-bottom flask (10 mL) and no solvent was used: MOF (0.0025 equiv), Epoxide (20 mmol, 1 equiv), TBAB (0.05 equiv). b 1 H NMR helped to analyze the yield by integrating the epoxide peak with the cycliccarbonate peaks.
Scheme 18. (a) Synthetic scheme for TMOF-3 and dangling sulphonate group within the framework, (b) schematic representation of the transmetalation of silver(I) cation within the framework to form TMOF-3-Ag. Reproduced with permission from [67]. Copyright 2019, American Chemical Society.
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Fig. 36. (a) Acetylene and ethylene uptake by TMOF-3-Ag, (b) possible mechanism for CC formation. Reproduced with permission from [67]. Copyright 2019, American Chemical Society.
Table 31 Cycloaddition of CO2 to different propargylic alcohola catalyzed by TMOF-3-Ag.
Temperature (K)
Time (h)
Yieldb %
1
RT
6
>99
2
RT
6
>99
3
RT
6
>87
4
RT
6
>86
5
RT
6
77
6
RT
6
90
7
RT
6
25
Entry
a b
Epoxide
Product
Reaction conditions: TMOF-Ag (10 mol %), propargylic alcohol (0.2 mmol), DMF (2 mL). DBU (0.1 equiv.). Calculated from 1H NMR analysis.
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Fig. 37. Illustration of the buffer strategy for the construction of JUC-1000. Reproduced with permission from [68]. Copyright 2018, Wiley-VCH.
Scheme 19. Illustration of the buffer strategy in a MOF. Reproduced with permission from [68]. Copyright 2018, Wiley-VCH.
Table 32 Synthesis of CCs from epoxides and CO2 using different catalysts. Entry
Catalyst
Epoxide
Product
Yield (%)d
a
1
JUC-1000
96
2a
MOF-505
61
3a
HKUST-1
62
4a
JUC-1000
58
5a
JUC-1000
81
6a
JUC-1000
29
39
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173 Table 32 (continued) Entry b
a b c d
Catalyst
Epoxide
Product
Yield (%)d
7
JUC-1000
96
8c
–
3
Reaction conditions: 0.25 mol % catalyst (per exposed copper site), 20.0 mmol epoxide, 0.65 g TBAB, under 1 atm CO2 for 48 h at room temperature. Yields after five cycles. Absence of catalyst. Yields were calculated by 1HNMR spectroscopy.
Fig. 38. The different electronic environment within JUC-1000 help to facile CO2 converstion to CC. Reproduced with permission from [68]. Copyright 2018, Wiley-VCH.
of polycarbonates, and so on. Besides, it also would help reduce the amount of environmental CO2. Therefore, the development of catalysts for the conversion of CO2 into CCs is of immense importance. Especially, development of MOF based heterogeneous catalysts should be vigorously pursued. From the above discussions it is clear that the presence of open metal site (Lewis acid) and the high positive charge on the metal ion (Cr3+, Zr4+, Al3+) will be very useful in fabricating catalysts for the cycloaddition of CO2 to epoxides. Moreover, the presence of Lewis base can do the same and importantly the cooperative effect of both (Lewis acids and Lewis base) greatly enhance the
reaction rate by optimizing other reaction conditions (temperature, pressure and reaction time etc). A suitably designed system can do this job without the use of the expensive cocatalyst, TBAB. The versatile structural features (crystalline, porous, flexible, tunable composition etc) of MOFs offer an important podium for further investigation on interaction between reactants and the MOF and dynamics of catalytic reaction. In the present review, we have discussed the conversion of CO2 into CC with a number of examples. It should be emphasized here that various other reactions that chemically fix CO2 are available where MOFs play important roles. Overall, there is
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T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173
Fig. 39. Structure of H3tzpa ligand and 3D framework of 1showing 1D tubular channels along the c-axis. Reproduced with permission from [69]. Copyright 2017, American Chemical Society.
Scheme 20. Chemical Fixation of CO2 with Epoxides.
Table 33 Chemical fixation of CO2 with epoxides.a Entry
Catalysts
Amount of catalysts (mmol)
1
Co(NO3)2/TBAB
0.2/2
Epoxides
Products
36.5
Yield (%)
2
1a
0.2
0
3
TBAB
2
16.8
4
1a/TBAB
0.1/2
57.1
5
1a/TBAB
0.2/2
93.8
6
1a/TBAB
0.2/2
53.6
41
T.K. Pal et al. / Coordination Chemistry Reviews 408 (2020) 213173 Table 33 (continued)
a
Entry
Catalysts
Amount of catalysts (mmol)
7
1a/TBAB
0.2/2
Epoxides
Products
Yield (%) 48.3
Reaction conditions: epoxide (20 mmol); 100 kPa CO2 pressure; 98 K; 48 h.
Scheme 21. Proposed mechanism for the fixation of CO2 with epoxides. Reproduced with permission from [69]. Copyright 2017, American Chemical Society.
Fig. 40. Two different modes of interaction (a and b) of ethylene oxide molecules in 1a. Reproduced with permission from [69]. Copyright 2017, American Chemical Society.
an urgent need to develop MOF based technology to accomplish a spot fixation of CO2. Acknowledgements P.K.B. gratefully acknowledges the financial support received from the DST and the MNRE, New Delhi, India. PKB wishes to thank all his students who made significant contributions in the area of MOF based CO2 adsorption and its fixation over the years. TKP acknowledge to PDPU/ORSP/R&D/SRP/2019-20/1361/1 and DD acknowledge to the ‘‘TEQIP Collaborative Research Scheme (CRS)”.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ccr.2019.213173.
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