Title: Cr(III) Incorporated Melamine‐Terephthalaldehyde Porous Organic Framework Nanosheet Catalyst for Carbon Dioxide Fixation Reaction

Abstract Cr(III) centers have been incorporated into melamine‐terephthalaldehyde porous organic framework (MT‐POF) through N‐donor site coordination. Morphological studies by SEM give evidence of the transformation of spherulitic texture of non‐metalated MT‐POF to aggregated platelike structure in chromium metalated POF, MT‐Cr(III) POF (bulk). The flattening in the surface morphology of bulk MT‐Cr(III) POF might be a consequence of conformational change in POF caused by the template effect of the Cr(III) coordination. On sonication, bulk MT‐Cr(III) POF transformed into a distinguishable stacked nanosheet structure in MT‐Cr(III) POFN as revealed by FE‐SEM and TEM images. MT‐Cr(III) POFN could catalyse the carbon dioxide cycloaddition reaction to epoxides to yield cyclic carbonates in presence of tetrabutylammonium bromide (TBAB) co‐catalyst. It was found that bulkier epoxide substrates showed lesser conversions to their corresponding cyclic carbonates (styrene oxide, 41 %; allyl glycidyl ether, 70 %; butyl glycidyl ether, 63 % conv.), whereas smaller substrates showed much higher conversions (epichlorohydrin, 96 %; epibromohydrin, 95 % conv.) under the reaction conditions [MT‐Cr(III) POFN, 10 mg; TBAB, 2 mol%; 5 bar CO 2 ; 60 °C] giving a clear evidence of the substrate discrimination. The reason for the lesser conversions of the bigger substrates can be attributed to the steric clash rendered by the POF network.