Impact of gas-solid direct contact on gas-liquid-solid reaction performance in a flow reactor

Although gas-liquid-solid reactions, such as catalytic hydrogenation, have a long history, a fundamental understanding of the flow behavior and its effect on the reaction is lacking for flow chemistry applications using powder catalysts. This study revealed the distinctive effect of gas-solid direct contact on the surface of a powder catalyst. Direct gas–solid contact accelerates the reaction beyond the theoretical maximum of the batch reaction system, where gaseous species are supplied to the catalyst surface after dissolution into the liquid. The benefit of direct contact is further pronounced in systems with low-solubility gaseous species. Liquid holdup analysis revealed that the micro-concavities of the catalyst support is crucial for sustaining the liquid using capillary forces and supplying the liquid substrate to the catalyst surface even under high gas flow rate conditions. The gas-to-liquid flow rate ratio (G/L) is a decisive factor for direct gas–solid contact, whereas the flow direction, whether upflow or downflow, has no impact on powder catalysts with a size of a few hundred microns. Utilization of a powder catalyst in flow reactor enables direct gas-solid contact. Accelerated reactions through gas-solid direct contact surpass the maximum rate in the reaction-controlled regime. Micro-concavities of particles retain liquid in reactor and achieve stable reaction rate.