Highly efficient CO2 conversion on a robust metal-organic framework Cu(I)-MFU-4l: Prediction and mechanistic understanding from DFT calculations

As a bioinspired metal-organic framework (MOF) with a unique trigonal pyramidal Cu(I) site in a bowl-shaped pocket, Cu(I)-MFU-4 l has emerged as an advanced single-site heterogeneous catalyst for several important reactions. Herein, we computationally investigate Cu(I)-MFU-4 l combining with tetrabutylammonium bromide (TBAB) as a high-performance catalyst for CO 2 conversion with propylene oxide (PO) into propylene carbonate (PC), a reaction of great significance in achieving carbon neutrality and sustainable development. Four paths are proposed depending on the initial activation of either PO or CO 2 by Cu(I), as well as the ring-opening of PO at C α -O bond or C β -O bond. Density functional theory calculations over a large cluster model reveal that Cu(I) is prone to interact with PO first and the most favorable path (Path 3) involves the sequential ring-opening of PO at C β -O bond, CO 2 insertion and ring closure with Gibbs energy barriers of 8.0, 2.8 and 10.3 kcal/mol, respectively. In view of such low energy barriers, Cu(I)-MFU-4 l /TBAB is predicted to be highly active for CO 2 cycloaddition with PO at ambient conditions. Significant confinement effect is demonstrated in Cu(I)-MFU-4 l by comparing Path 3 over both large and small cluster models. The observed confinement not only stabilizes the intermediates and transition states thermodynamically, but also promotes the ring-opening of PO kinetically, which is mainly attributed to the van der Waals (vdW) interactions between reactive fragments and surrounding frameworks through non-covalent interaction (NCI) analysis. A mechanism of electronic effect competing with steric effect is proposed to rationalize the regioselectivity of ring-opening of PO under different scenarios via natural bond orbital (NBO) charge and NCI analysis. Our results also suggest that Cu(I)-MFU-4 l /TBAB outperforms Cu(II) paddle-wheel MOFs/TBAB reported previously for CO 2 cycloaddition with PO. The structural robustness, the soft electron-rich Cu(I) site and the inherent confinement promote Cu(I)-MFU-4 l to be an appealing candidate catalyst for a wide range of reactions. • Cu(I)-MFU-4 l /TBAB is computationally predicted to be highly active for CO 2 cycloaddition under ambient conditions. • Significant confinement is demonstrated and the importance of the environment beyond active site is highlighted. • Electronic effect competes with steric effect to dominate the regioselectivity of ring-opening of epoxides. • Cu(I)-MFU-4 l /TBAB outperforms Cu(II) paddle-wheel MOFs/TBAB for CO 2 cycloaddition with propylene oxide.