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

Abstract

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.