Insights into CO2 adsorption and chemical fixation properties of VPI-100 metal–organic frameworks

Abstract

Metal–organic frameworks (MOFs) have shown great promise as efficient CO₂ adsorbents, as well as an emerging class of heterogeneous catalysts for conversion of CO₂ to other useful chemicals. In this work, we synthesized and characterized two isostructural hafnium-based MOFs, denoted as Hf-VPI-100 (Cu) and Hf-VPI-100 (Ni). Both frameworks demonstrated high catalytic efficiency for cycloaddition of CO₂ to epoxides under ambient pressure. In situ PXRD, QCM and DRIFTs have been used to probe the interaction between the guest molecules (CO₂/epoxide) and Hf-VPI-100. The crystal structures of these frameworks were preserved during the exposure to CO₂ atmosphere up to 20 bar. The epoxide uptake per unit cell of VPI-100 and diffusion coefficients have been calculated by QCM analysis. Comparison of catalytic efficiency between Hf-VPI-100 and the previously reported Zr-based VPI-100, aided by electronic structure calculations revealed that the open metal centers in the metallocyclam act as the primary Lewis acid sites to facilitate the catalytic conversion of CO₂.