Carbonic Anhydrase-Mimicking Keplerate Cluster Encapsulated Iron Trimesate for Base-Free CO2 Hydrogenation

Producing formic acid (FA) via base-free CO2 hydrogenation suffers from low conversion efficiency because of the difficulty of activating CO2 and the low solubility of CO2 in neutral water. In this paper, we demonstrate a dual catalyst system, incorporating CO2 hydration and Pd/C catalysts to increase the FA yield from base-free CO2 hydrogenation for the first time. As the CO2 hydration catalyst, a carbonic anhydrase-mimicking Keplerate-type Mo-132 cluster was used, and its heterogenized catalyst (Mo-132@MIL-100(Fe)) was prepared by encapsulating in a mesoporous iron trimesate (MIL-100(Fe)) structure. Notably, the addition of a Mo-132 cluster greatly increased the FA yields and turnover numbers (TONs) of Pd/C by similar to 3.8x through a ligand exchange-derived CO2 hydration process, as confirmed by a NMR study. Mo-132@MIL-100(Fe) was not only separable and recycled along with Pd/C for consecutive reactions without leaching and notable loss of activity, but also showed 2.8x higher TON (1257) compared to the Mo-132 cluster (TON = 452) under the same reaction conditions. The present strategy is applicable to various catalytic reactions, especially in aqueous-phase CO2 conversion systems.