From Batch to Continuous Operation: Hydrogenation of Bicarbonate to Formate at Multiphase Boundaries in a Continuous Stirred-Tank Reactor.

The utilization of carbon dioxide (CO2) as a C1 building block has emerged as a promising strategy for sustainable chemical production. Among various CO2-derived products, formate and formic acid are particularly attractive due to their roles as hydrogen carriers, fuel cell feedstocks, and industrial intermediates. Recent advances in Ru-based homogeneous catalysis have enabled efficient hydrogenation of bicarbonate, which was prepared from CO2, in biphasic and triphasic systems. In this study, we employed a continuous stirred-tank reactor (CSTR) for the triphasic hydrogenation of KHCO3 under high-pressure conditions (50 bar of H2), and optimized the stirring conditions using a view cell to ensure efficient mixing. A kinetic model assuming a slow-reaction regime and incorporating the reverse reaction was developed, which accurately predicted the residence time-yield relationship and enabled high formate yields through residence-time optimization. Furthermore, the addition of tris-(2,4-di-tert-butylphenyl)-phosphite as an antioxidant effectively suppressed residual oxygen contamination, which is a known challenge in flow systems. Catalyst recycling and phase separation were successfully integrated into the flow setup, demonstrating the practicality and scalability of the process. These findings provide a rational framework for designing continuous triphasic hydrogenation systems and contribute to the development of resource-efficient chemical technologies based on CO2 utilization.