Ruthenium Nanoparticles Supported on Carbon: An Active Catalyst for the Hydrogenation of Lactic Acid to 1,2-Propanediol

The hydrogenation of lactic acid to form 1,2-propanediol has been investigated using Ru nanoparticles supported on carbon as a catalyst. Two series of catalysts which were prepared by wet impregnation and sol-immobilization were investigated. Their activity was contrasted with that of a standard commercial Ru/C catalyst (all catalysts comprise 5 wt % Ru). The catalyst prepared using sol-immobilization was found to be more active than the wet impregnation materials. In addition, the catalyst made by sol-immobilization was initially more active than the standard commercial catalyst. However, when reacted for an extended time or with successive reuse cycles, the sol-immobilized catalyst became less active, whereas the standard commercial catalyst became steadily more active. Furthermore, both catalysts exhibited an induction period during the first 1000 s of reaction. Detailed scanning transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray absorption fine structure analysis data, when correlated with the catalytic performance results, showed that the high activity can be ascribed to highly dispersed Ru nanoparticles. Although the sol-immobilization method achieved these optimal discrete Ru nanoparticles immediately, as can be expected from this preparation methodology, the materials were unstable upon reuse. In addition, surface lactide species were detected on these particles using X-ray photoelectron spectroscopy, which could contribute to their deactivation. The commercial Ru/C catalysts, on the other hand, required treatment under reaction conditions to change from raft-like morphologies to the desired small nanoparticle morphology, during which time the catalytic performance progressively improved.