Hyper-Cross-Linked Polymer-Based Self-Supported Reusable Ru-NHC Catalyst for Amine-Assisted Hydrogenation of CO₂ to Methanol

Research on the capture and catalytic conversion of CO2 to valuable chemicals and alternative fuel candidates has multifaceted relevance from the perspective of sustainable development goals (SDGs). Methanol, advocated as an alternative fuel, can be produced via amine-assisted integrated capture of CO2 and the subsequent hydrogenation via a formamide intermediate. Notably, amines not only function as CO2-capturing agents but also assist in crucial H2 activation by a metal catalyst during a series of hydrogenation steps to form methanol. Toward implementation of this cascade strategy of amine-assisted hydrogenation of CO2 to MeOH, herein, we developed an NHC-based pincer ligand-bound homogeneous molecular Ru catalyst (Ru-1), which was then translated into a porous hyper-cross-linked polymer (HCP)-based self-supported single-site heterogeneous catalyst, termed as HCP-Ru-1. Initially, a control mechanistic investigation was performed in order to get insights into the crucial H2 activation step at the Ru center of the catalyst assisted by the amine used in the reaction. Next, the formation of the formamide intermediate by both catalyst candidates Ru-1 and HCP-Ru-1 was probed and confirmed, employing several amines in the presence of pressurized CO2 and H2 at a relatively mild temperature of 120 °C, where the catalysts exhibited turnover numbers (TONs) up to 3520 and 15,750, respectively, indicating their high catalytic efficiency. Finally, for the catalytic CO2-to-MeOH production reaction, a polyamine, namely, pentaethylenehexamine (PEHA), was used due to its high efficiency in CO2 capture as well as ability to act as a suitable organic base for deprotonative H2 activation, and the Ru-1 and HCP-Ru-1 systems displayed single-run TONs of 400 and 1150, respectively. As the most significant advancement in this process, the heterogeneous HCP-Ru-1 catalyst turned out to be highly reusable, and in a 10-cycle reuse demonstration experiment, it achieved a cumulative TON of 10,370 with merely 1.13% loss in activity per cycle.