Synergistic effects of bimetallic Co-SnOx species in selective hydrogenation of methyl oleate: The crucial role of reduction characteristics

Abstract

This study investigates the catalytic performance of bimetallic Co-Sn catalysts supported on alumina (Al2O3) for the selective hydrogenation of methyl oleate to oleyl alcohol. A strategic sequential reduction methodology, employing sodium borohydride followed by thermal hydrogen treatment (NaBH4-H2), was utilized to precisely tune the catalytic activity and chemoselectivity. Catalysts reduced solely with hydrogen at 300 °C exhibited limited efficiency, primarily favoring the formation of oleic acid via hydrogenolysis, which is attributed to the prevalence of cobalt oxide species. In contrast, the CoSnBH/Al2O3 catalyst synthesized via a strategic sequential reduction methodology demonstrated a markedly enhanced selectivity toward oleyl alcohol, achieving a maximum selectivity of 54.1% at 270 °C and 8 MPa. Comprehensive characterization and kinetic analyses revealed that the superior performance stems from the synergistic coexistence of metallic cobalt (Co0) and low-valent tin oxides (SnOx), which form a bimetallic interface that preferentially polarizes the C=O bond. The catalytic efficiency was found to be highly sensitive to the nature of the support, the Co/Sn atomic ratio, and the total metal loading. Notably, turnover frequency (TOF) analysis indicated that while increased loading improves conversion, it may lead to site agglomeration and a reduction in surface-active concentration. Mechanistically, we propose that the CoSnBH/Al2O3 system facilitates a direct hydrogenation pathway to oleyl alcohol, effectively bypassing the formation of heavy ester intermediates typically reported in literature.