Insights into the Influence of ZrO2 Crystal Structures on Methyl Laurate Hydrogenation over Co/ZrO2 Catalysts

Six kinds of ZrO2 supports (ZRX, X = 1–6) with different crystal structures (monoclinic ZrO2: m-ZrO2; tetragonal ZrO2: t-ZrO2; and mixed-phase of monoclinic and tetragonal ZrO2: mix-ZrO2) were synthesized, and their effects on the properties of Co/ZrO2 catalysts were investigated for methyl laurate hydrogenation. Among the prepared catalysts, Co/mix-ZrO2 (Co/ZR3), with the highest concentration of surface Co2+, oxygen vacancies (Ov), and complex types of exposed Co crystal planes, exhibited the highest activity and stability toward methyl laurate hydrogenation, with nearly 100% yield of liquid alkanes produced at 240 °C, or 90.5% of fatty alcohol (selectivity: 96%) at 180 °C. By exploring reaction kinetics and catalyst properties, oxygen vacancies in the support of Co/ZrO2 catalysts were found to play an important role in tuning the valence state of Co, while the crystal structure of ZrO2 influenced exposed lattice planes of cobalt. The high Ov content and active β-Co(102) crystal face of Co/ZR3 were efficient for reducing activation energies for C–C or C═O bond cleavage, causing high activity toward fatty ester hydrogenation. Less-active Co, such as α-Co(111) and α-Co(200), or oxygen vacancies made the fatty alcohol intermediate difficult to be deoxygenated to alkanes.