Robin Babu

Dual-porous metal organic framework for room temperature CO2 fixation via cyclic carbonate synthesis

A novel approach of employing a dual-porous metal organic framework (MOF) in CO2 fixation at room temperature was demonstrated using a micro–mesoporous MOF, UMCM-1-NH2, in the synthesis of various five-membered cyclic carbonates under solventless conditions. Mesopores allow easy guest diffusion and molecular accessibility, while micropores are predominantly helpful in regulating the catalytic interactions in active centres; thus outperforming the properties of pure microporous or pure mesoporous MOFs in cycloaddition. Structural features, acid–base characteristics and physical properties were studied in detail for carrying out a systematic investigation on the cooperative influences of porosity, functionalization and synergism with quaternary ammonium salts in the cycloaddition reaction of CO2 with propylene oxide, so as to arrive at the underlying mechanism. The catalyst was totally recyclable up to five times without compromising the activity and the extent of heterogeneity was also studied. The effects of various reaction parameters like catalyst–cocatalyst ratio, reaction time and reaction temperature have been investigated.

Advancements in the Conversion of Carbon Dioxide to Cyclic Carbonates Using Metal Organic Frameworks as Catalysts

Global warming has begun to show its impact on the environment, and it is time to take steps to manage CO2 emissions, so as to regain the balance of carbon cycle. In addition to various capture and sequestration techniques, conversion of CO2 to value added products is high relevant. However, the inertness of CO2 makes catalysts an indispensable part of the process. CO2 undergoes cycloaddition with epoxides to produce cyclic carbonates, which have utility in various applications. Considering the necessity for heterogeneity and activity under ambient conditions, metal organic framework (MOF) catalysts have recently emerged as prospective candidates for cyclic carbonate synthesis. These porous hybrid inorganic–organic crystals are also excellent materials for gas storage and separation, including CO2 gas. Thus, MOFs could efficiently capture CO2 and catalytically convert them to cyclic carbonates. In this review, we discuss the recent advancements in the design of MOF catalysts for cyclic carbonate synthesis.

Ionic liquid tethered post functionalized ZIF-90 framework for the cycloaddition of propylene oxide and CO2

A novel heterogeneous one-component catalyst was developed by the covalent post functionalization of zeolitic imidazolate framework-90 (ZIF-90) with a pyridinium based ionic liquid (IL) to generate IL supported ZIF-90 (IL-ZIF-90). The synthesized catalyst has been characterized using a range of analytical, spectroscopic, and electron microscopy techniques followed by successful employment for the solventless cycloaddition of epoxides and CO2 to yield cyclic carbonates under mild reaction conditions. The effects of reaction parameters like catalyst amount, reaction time, reaction temperature, and CO2 pressure have been investigated. A reaction mechanism for the IL-ZIF-90 catalyzed PO–CO2 cycloaddition has been proposed on the basis of density functional theory (DFT) calculations. The catalyst system was separable by centrifugation and reused four consecutive times successfully. In general, it can be suggested that IL supported porous metal organic frameworks (MOFs) may introduce a new class of highly porous catalyst species showing excellent CO2 transforming capability.

An lcy-topology amino acid MOF as eco-friendly catalyst for cyclic carbonate synthesis from CO2: Structure-DFT corroborated study

The concept of bio-metal-organic framework (bio-MOF) catalysts for CO2 transformation was devised using L-glutamic acid as the natural surrogate for synthetic ligands, and demonstrated their catalytic efficacy for the first time, in the cycloaddition of CO2 with epoxides, supplemented with the structure–DFT correlation. The water stable amino acid bio-MOF, zinc-glutamate-MOF (ZnGlu), with a rare 3D topology (33·59·63)-lcy was synthesized as single crystals and bulk, through an ecofriendly protocol in aqueous medium, from zinc and the proteinogenic amino acid, L-glutamic acid. Amino acid MOFs (AA-MOFs), owing to their economic and environmental factors, are expected to be the future of MOF chemistry at industrial levels. The ZnGlu catalyst with open metal sites was successfully demonstrated as the first bio-MOF catalyst for cyclic carbonate synthesis from CO2 and epoxides, and its efficiency was compared with those of prominent synthetic MOFs reported so far in the process. The as-synthesized catalyst operated even under moist conditions, was thermally and chemically stable; heterogeneous, easily separable (due to its high selectivity, absence of synthesis solvents, and easy catalyst recovery by filtration) and was recycled up to four times. Mechanistic aspects, possible intermediates, transition states and pathways were portrayed using a combination of the experimental inferences, previous reports and ab initio quantum mechanical calculations (DFT techniques) by its correlation with the single crystal XRD structure and topology.

A solid solution zeolitic imidazolate framework as a room temperature efficient catalyst for the chemical fixation of CO2

An energy efficient and economically viable bimetallic heterogeneous catalyst system composed of Co and Zn as active centers and 2-methylimidazole as a linker has been synthesized in water at room temperature. The synthesized material (CZ-ZIF) possesses a sodalite topology, similar to the parent materials, ZIF-8 and ZIF-67, with a high surface area of >1400 m2 g−1. The Zn and Co metal ions were shown to occupy equivalent sites throughout the framework in similar proportions, as confirmed by inductively coupled plasma atomic emission spectroscopy and energy dispersive X-ray spectroscopy techniques. CZ-ZIF rendered a high catalytic conversion of epoxides to five-membered cyclic carbonates using CO2 as the C1 source under solvent- and co-catalyst-free conditions with excellent selectivity and manifested better catalytic abilities than ZIF-67 and enhanced framework stability compared to ZIF-8. Furthermore, CZ-ZIF exhibited catalytic activity even at room temperature in the presence of a co-catalyst, and was reusable over a minimum of five cycles with no noticeable decrease in activity. A plausible mechanism for CZ-ZIF catalyzed solvent- and co-catalyst-free epoxide–CO2 cycloaddition has been proposed.

A computational study of the mechanistic insights into base catalysed synthesis of cyclic carbonates from CO2: bicarbonate anion as an active species

The standalone catalytic potential of common organic bases such as imidazole, pyridine and dimethylaminopyridine (DMAP) for the solvent-free cycloaddition of CO2 with epoxides yielding five-membered cyclic carbonates is reported here. Appreciable conversion of various epoxides with excellent selectivity towards the desired products was materialized in this metal/halide/hydrogen bond donors/solvent-free reaction. The presence of catalytic amounts of water was found significantly advantageous in this base catalyzed chemical fixation of CO2 and the conversion almost got doubled or tripled under the same reaction conditions. A definitive mechanism for the activation of base catalysis was also proposed with the aid of ab initio calculations performed at the B3LYP/6-31G(d,p) level. Besides, a bicarbonate anion mediated catalytic cycle was also proposed utilizing computational calculations. The possible intermediates and transition states as well as the related energy constraints of the base alone and base–water catalyzed reactions were deduced and the activation energy obtained was found higher for the former (∼30 kcal mol−1) than for the latter (∼12 kcal mol−1), which rationalizes the experimental observation of the higher activity of the latter.

Inverse relationship of dimensionality and catalytic activity in CO2 transformation: a systematic investigation by comparing multidimensional metal–organic frameworks

The correlation between dimensionality and active sites on deciding the catalytic performance of an MOF catalyst in CO2–epoxide cycloaddition reactions has been studied. Seven In(III) based MOFs built from carboxylic and N-donor ligands possessing different dimensionalities and distinct coordination environments were chosen as solid acid catalysts for this study. The origin of the catalytic activity of an In3+/TBAB bifunctional system in a CO2–PO reaction was studied in detail by performing density functional theory (DFT) calculations at the M06/LACVP**++ level. The energy barrier of the propylene oxide ring opening in the presence of In3+/Br− is 11.5 kcal mol−1, which is significantly lower than those of un-catalyzed (55–63 kcal mol−1) and Br−-catalyzed (19.5 kcal mol−1) reactions, which confirms the importance of the In3+/Br− binary catalytic system in the CO2–epoxide cycloaddition reactions. The one-dimensional (1D) MOF with unsaturated metal centers exhibited higher catalytic activity (PO conversion: 91%, temperature: 50 °C, and time: 12 h) than the two- and three-dimensional MOFs. The roles of dimensionality and unsaturated metal centers in cycloaddition reactions were explained on the basis of the results of activity testing and structural investigations. In addition, a plausible reaction mechanism for the catalytic activity of the 1D MOF was proposed with reference to our structure-density functional theory correlations.

Microwave-induced synthesis of a bimetallic charge-transfer metal organic framework: a promising host for the chemical fixation of CO2

The aqueous synthesis of a bimetallic metal organic framework (MOF) with Ni and Co as the active metal centers and benzene-1,4-dicarboxylic acid as the linker has been achieved rapidly in high yield using microwave irradiation. The synthesized MOF is investigated for its catalytic efficacy in the synthesis of cyclic carbonates from epoxides and CO2. The Ni–Co-MOF provides high conversion rates of epoxides to cyclic carbonates with >99% selectivity under solvent-free conditions. The bimetallic framework (Ni–Co-MOF) exhibits superior catalytic activity to those of the corresponding single metal MOF catalysts (Ni-MOF and Co-MOF) and their mechanical combination (Ni-MOF + Co-MOF), which indicates the existence of a synergistic catalytic effect based on charge transfer between the Ni and Co metal centers and is highly advantageous for the catalytic chemical fixation of CO2. The catalytic potential of the Ni–Co-MOF is also applied to terminal and cyclic epoxides, and a recyclability study over a minimum of six cycles is also conducted. Finally, a plausible reaction mechanism for Ni–Co-MOF-catalyzed epoxide-CO2 cycloaddition reactions is also proposed.

Zirconium-based isoreticular metal-organic frameworks for CO 2 fixation via cyclic carbonate synthesis

Two highly stable isoreticular metal-organic frameworks comprising chains of zirconium coordinated with linkers of 1,4-H2BDC (1,4-benzenedicarboxylic acid) and 4,4′-H2BPDC (4,4′-biphenyldicarboxylic acid), denoted as MIL-140A and MIL-140C, were synthesized. The catalytic activity of these frameworks was studied for the coupling reaction of CO2 and epoxides to produce cyclic carbonates under solvent-free conditions. Excellent activity was observed for both catalysts: they yielded high epoxide conversion with >99% selectivity toward the cyclic carbonate, and were fully reusable even after four cycles without any considerable loss of initial activity. The enhancement in the catalytic activity was explained based on acidity/basicity studies. The influence of various reaction parameters such as catalyst amount, reaction time, reaction temperature, and CO2 pressure was also investigated. Reaction mechanism was proposed on the basis of experimental evidence and our previous DFT (density functional theory) studies.

Bifunctional Pyridinium-Based Ionic-Liquid-Immobilized Diindium Tris(diphenic acid) Bis(1,10-phenanthroline) for CO2 Fixation

A pyridinium-based ionic-liquid-decorated 1 D metal-organic framework (MOF; IL-[In2 (dpa)3 (1,10-phen)2 ]; IL=ionic liquid; dpa=diphenic acid; 1,10-phen=1,10-phenanthroline) was developed as a bifunctional heterogeneous catalyst system for CO2 -oxirane coupling reactions. An aqueous-microwave route was employed to perform the hydrothermal reaction for the synthesis of the [In2 (dpa)3 (1,10-phen)2 ] MOF, and the IL-[In2 (dpa)3 (1,10-phen)2 ] catalyst was synthesized by covalent postfunctionalization. As a result of the synergetic effect of the dual-functional sites, which include Lewis acid sites (coordinatively unsaturated In sites) and the I- ion in the IL functional sites, IL-[In2 (dpa)3 (1,10-phen)2 ] displayed a high catalytic activity for CO2 -epoxide cycloaddition reactions under mild and solvent-free conditions. Microwave pulses were employed for the first time in MOF-catalyzed CO2 -epoxide cycloaddition reactions to result in a high turnover frequency of 2000-3100 h-1 . The catalyst had an excellent reusability and maintained a continuous high selectivity. Furthermore, only a small amount of leaching was observed from the spent catalyst. A plausible reaction mechanism based on the synergistic effect of the dual-functional sites that catalyze the CO2 -epoxide cycloaddition reaction effectively is proposed.