Size effect of Ru particles on the self-reforming-driven hydrogenolysis of a lignin model compound

Abstract

Particle size always has a great influence on catalytic performance. In this study, we investigated the size effect of Ru colloids on the self-reforming-driven hydrogenolysis of a lignin model compound by taking 4-(3-hydroxypropyl)-2-methoxyphenol (PG-OH) as the substrate and NiAl₂O₄ spinel as the support. This reaction provides a strategy to produce alkylphenols from lignin without the consumption of exogenous hydrogen. Three catalysts with different Ru particle sizes (Ru^1.5, Ru^2.6, and Ru^3.3) were prepared, and the catalytic results showed that the reaction activity presented a volcanic curve with the increase in the Ru particle sizes, in which Ru^2.6/NiAl₂O₄ possessed the best performance. Reaction pathway studies showed two different reaction routes over small (Ru^1.5 and Ru^2.6) and large (Ru^3.3) Ru particles. The reforming-driven hydrogenolysis of the methoxy group dominated over small Ru particles, with 4-ethylphenol (4-EP) as the main product; while the hydrogenation of hydroxypropyl and hydrolysis of the methoxy group dominated over the larger particles, with 4-propylcatechin (4-PC) as the main product. Comprehensive investigations indicates the difference in catalytic performance came from the interaction between Ru colloids and the NiAl₂O₄ spinel support, which led to different Ru^δ+/Ru⁰ ratios and finally changed the reaction routes or reaction rate in these three catalysts. This study proves that the particle size really affects catalytic reactions, and indeed, the self-reforming-driven hydrogenolysis of lignin (the model compound) to alkylphenols is structure-sensitive.