LAUSR

Abstract Metal-organic frameworks (MOFs) have emerged as unique catalysts for CO2 conversion, but the catalytic mechanism remains elusive. In this study, a density functional theory (DFT) study is reported on CO2 cycloaddition with propylene oxide (PO) to produce propylene carbonate (PC), catalyzed by a Cu-MOF (NTU-180) in the presence of tetrabutylammonium bromide (TBAB). This reaction was experimentally observed to have a remarkably high efficiency. We compute the electronic and Gibbs energies of the reactants, intermediates, transition states and products for two alternative reaction pathways. On NTU-180/TBAB, the three-member ring opening of PO is revealed to possess a higher activation barrier than the five-member ring closure of PC, and thus it is the rate-determining step. Pathway I is kinetically more favorable as its ring opening has a lower barrier than in Pathway II. Moreover, the activation barriers of ring opening and ring closure steps on NTU-180/TBAB are substantially lower than those in the gas phase, and also lower than those on TBAB alone and on a Cu2 paddle-wheel/TBAB complex. At a molecular level, these results unambiguously highlight the important roles of the Cu sites and the confinement effect of NTU-180 in enhancing the stabilities of reaction intermediates and transition states, and promoting the catalytic cycloaddition of CO2 with PO. In addition, the sensitivities of the DFT calculation results to the choice of functional and basis set are critically analyzed. While the electronic and Gibbs energies vary substantially with functional, the activation barriers in the rate-determining step are affected only slightly by the functional and marginally by the basis set. This theoretical study provides microscopic and quantitative insights into the high catalytic activity observed experimentally for CO2 cycloaddition on NTU-180/TBAB, and might facilitate the development of new catalysts for efficient CO2 conversion.