Hyper-Cross-Linked Polymer-Based Self-Supported Reusable Ru-NHC Catalyst for Amine-Assisted Hydrogenation of CO<sub>2</sub> to Methanol
Ravi Kumar, Tanmoy Mandal, Arindom Bhattacherya, Madhusudan K. Pandey, Jitendra K. Bera, Joyanta Choudhury
Abstract
Research on the capture and catalytic conversion of CO 2 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 CO 2 and the subsequent hydrogenation via a formamide intermediate. Notably, amines not only function as CO 2 -capturing agents but also assist in crucial H 2 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 CO 2 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 H 2 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 CO 2 and H 2 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 CO 2 -to-MeOH production reaction, a polyamine, namely, pentaethylenehexamine (PEHA), was used due to its high efficiency in CO 2 capture as well as ability to act as a suitable organic base for deprotonative H 2 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.