Bimetallic Ru-Ni Catalyzed Aqueous-Phase Guaiacol Hydrogenolysis at Low H₂ Pressures

Aqueous-phase hydrogenolysis of renewable biomass at low H₂ pressures is an attractive route to selectively produce renewable fuels and valuable chemicals. Here, we show that Ru and Ni nanoparticles (NPs) dispersed on HZSM-5 with an optimum H^• radical transfer catalyzed a rapid rate (152 mmol g^-1 h^-1) of hydrogenolysis of C-O bonds in lignin-derived guaiacol at 240 °C and 2 bar H₂ pressure in water. The coimpregnated individual Ru and Ni nanoparticles (NPs) on HZSM-5 were highly dispersed and did not present an alloy structure, but the individual Ru and Ni NPs were in close proximity. The guaiacol hydrogenolysis rates were proportional to the amounts of the adjacent RuO₂ and NiO NPs on the calcined samples, suggesting that the closely contacted Ru and Ni NPs on HZSM-5 are the active sites. In the water phase at low H₂ pressures, Ru dissociated the hydrogen molecules to H^• radicals (H^•), and then such radicals were transferred to adjacent Ni atoms to activate the capability of inert Ni centers. The adjustment of the H^• transfer length between Ru and Ni NPs led to shorter H^• transfer lengths, which resulted in activities as high as 118 mmol g^-1 h^-1. The transferring and anchoring of H^• radicals was considered to be achieved by the Si-OH groups and their defects on HZSM-5, as demonstrated by a temperature-programmed desorption of hydrogen coupled with mass spectroscopy (TPD/H₂-MS) experiment. To further shorten the H^• transfer length over uniformly formed Ru and Ni nanoparticles, the isolated Ni islands were removed through the incorporation of a Ru precursor that initially occupied the Brønsted acid sites on HZSM-5. By fully activating the two metals in the aqueous phase via an H^• transfer mechanism at low H₂ pressures, the rational design of bimetallic Ru-Ni catalysts provides a promising approach for achieving substantially high rates in selective hydrogenolysis steps.