INORGANIC CHEMISTRY FRONTIERS RESEARCH ARTICLE Cite this: Inorg. Chem. Front., 2023, 10, 6329 Received 19th July 2023, Accepted 25th August 2023 DOI: 10.1039/d3qi01386c rsc.li/frontiers-inorganic Diaspore as an efficient halide-free catalyst for the conversion of CO 2 into cyclic carbonates† Antarip Mitra, a Khushboo S. Paliwal, a Sourav Ghosh, a Saikat Bag, a Avishek Roy, a Aditi Chandrasekar* b and Venkataramanan Mahalingam * a Efficient fixation of carbon dioxide (CO 2 ) into epoxides under atmospheric pressure generally necessitates the use of halide ion-containing co-catalysts. However, the use of halide ion-containing materials as cat- alysts is less encouraged, particularly from an industrial point of view. This demands the development of a suitable halide-free catalyst for the successful fixation of CO 2 into epoxides to prepare cyclic carbonates under atmospheric pressure. In this work, we report diaspore [α-AlO(OH)] as an efficient halide-free cata- lyst for CO 2 fixation. Diaspore in the presence of a small amount of dimethyl formamide is able to convert a range of epoxides into their corresponding cyclic carbonates. Hardly any loss in the catalytic activity or change in the functional/chemical characteristics of diaspore was observed after five cycles. DFT calcu- lations reveal the spontaneity of the diaspore-catalyzed cycloaddition reaction compared to that of the diaspore-free reaction. The stabilization of the substrates and intermediates on diaspore resulted in an overall negative change in Gibb’s free energy of the reaction. Introduction Carbon dioxide (CO 2 ) is the major contributor to greenhouse gases. There has been a huge surge in the CO 2 concentration in the atmosphere post industrial revolution. This results in many adverse impacts which include global warming, climate change and more. 1 CO 2 capture and their subsequent utiliz- ation as C 1 feedstock for the synthesis of various value-added chemicals is one of the sustainable ways to mitigate these excess CO 2 related challenges. 2 Among various reactions invol- ving CO 2 , the reaction of epoxides with CO 2 to produce cyclic carbonates has gained much attention. 3 This is mainly ascribed to the advantageous atom-economical and non-reduc- tive features of the pathway. 4 In addition, cyclic carbonates are commercially important as they find significant applications like as electrolytes in lithium-ion batteries, in the preparation of vicinal diols, as high-boiling polar aprotic solvents, and more. 5–7 However, the high activation energy for the cyclo- addition reaction of CO 2 and epoxides necessitates the use of efficient catalysts in addition to the requirement of high temp- erature and pressure. 8–11 The rate-determining step of the reaction of epoxides with CO 2 to produce cyclic carbonates involves a nucleophilic attack to open the epoxide ring. 12,13 Primarily, halide ions such as chlorides, bromides, or iodides are used to facilitate this ring- opening step. 14–16 Moreover, hydrogen bond donors (e.g., OH, –NH, and –COOH) or Lewis acidic centers (e.g., metal ions) can assist this ring opening by increasing the electrophilicity of the epoxides. 17–23 However, the corrosive nature of the halide ions limits their scope, particularly for scaling up synthesis from an industrial point of view. In this regard, halide-free heterogeneous catalysts have gained attention in recent years for the synthesis of cyclic carbonates. 24,25 For instance, North and co- workers used salophens as catalysts to prepare cyclic carbonates using CO 2 under 10 bar pressure at 120 °C. 26 Similarly, Zhang et al. developed a homogeneous halide-free organocatalyst which catalyzed the cycloaddition reaction under 2 MPa pressure and at 120 °C. 19 Recently, Ma et al. developed a halogen-free solid solu- tion of CeO 2 –ZrO 2 and explored CO 2 fixation into epoxides. The catalysis was performed at 150 °C under a CO 2 pressure of 6 MPa. 27 In another recent report, Bragato et al. developed ionic liquids to prepare cyclic carbonates at 120 °C under 2 MPa pressure of CO 2 . 28 The above-mentioned works reported the halide-free synthesis of cyclic carbonates under high pressure. Nevertheless, to our knowledge, there are relatively few reports on halide-free heterogeneous catalysts to produce cyclic carbonates with satisfactory yields under atmospheric pressure. For instance, pyridyl salicylimines, ionic porous polymers and zinc MOFs have been used as catalysts for CO 2 fixation. 17,24,29 Our group has recently developed a pyridine dicarboxylic acid coordinated to alu- † Electronic supplementary information (ESI) available. See DOI: https://doi.org/ 10.1039/d3qi01386c a Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur, West Bengal 741246, India. E-mail: mvenkataramanan@yahoo.com b School of Arts and Sciences, Azim Premji University, Bangalore 562125, India. E-mail: aditi.chandrasekar@apu.edu.in This journal is © the Partner Organisations 2023 Inorg. Chem. Front. , 2023, 10, 6329–6338 | 6329 Published on 30 August 2023. Downloaded by Indian Institute of Science Education & Research Kolkata on 4/29/2024 12:07:00 PM. View Article Online View Journal | View Issue