(399g) Selective Hydrogenations in Proton Exchange Membrane Reactor

Conventional selective hydrogenation reactions are carried out thermocatalytically at high temperatures and pressures in tubular plug-flow reactor that often yield low selectivities to the desired products. An alternative involves the use of PEM reactors that operate low temperatures and pressures, and can utilize renewable electricity. Additionally, these reactors allow for control over protons, enabling higher hydrogen coverages on the catalyst surface.

As is the case for other types of catalytic reactions, the selectivity for electrocatalytic hydrogenations is determined primarily by the cathode materials. Often noble metals such as Pt, Rh, and Pd are often employed for its high activity and selectivity for desired hydrogenated products; however, due to its high cost of these precious metals, they have been loaded onto high surface area supports to minimize amount used and retain its high performance. For example, our group has investigated the use of early transition metal carbide based catalysts as a support. Significant improvements in product yields and selectivities were demonstrated for the electrocatalytic hydrogenation of triglycerides using Pd/W₂C materials. These improved rates have been linked to hydrogen spillover and synergistic effects between Pd and the W₂C support.

The aim of research described in this paper was to investigate the impact of hydrogen spill-over in the electrocatalytic hydrogenation of furfural, a model biomass compound, in a PEM reactor by varying the cathode catalyst. The performance of the traditional electrocatalyst (Pd black) was compared to the performance of cathode catalyst mixture consisting of the traditional electrocatalyst with a metal active for hydrogenation (Pd and Pt) supported on traditional thermocatalyst supports (SiO₂ and Al₂O₃). The hydrogenation rates and selectivities were linked to the structural and compositional properties of the catalyst mixtures in which the cathode containing the thermocatalyst had improved selectivity towards completely hydrogenated products. The determine the synergistic effect of the Pd black and thermocatalyst in the cathode mixture, the performance of that mixture was compared to a cathode mixture utilizing carbon as its electrocatalyst. The selectivity towards completely hydrogenated products was attributed to the thermocatalyst while the improved hydrogenation rate was attributed to hydrogen spill-over from Pd black.