Selective hydrogenation of amides using bimetallic Ru/Re and Rh/Re catalysts

摘要

Heterogeneous Ru/Re and Rh/Re catalysts, formed in situ from Ru _3(CO)_(12)/Re_2(CO)_(10) and Rh _6(CO)_(16)/Re_2(CO)_(10) respectively, are effective for the liquid phase hydrogenation of cyclohexanecarboxamide (CyCONH_2) to CyCH_2NH_2 in up to 95% selectivity without the requirement for ammonia to inhibit secondary and tertiary amine formation. Good amide conversions are noted within the reaction condition regimes 50-100 bar H_2 and ≥150 (Rh) - 160 °C (Ru). Variations in Ru:Re and Rh:Re composition result in only minor changes in product selectivity with no evidence of catalyst deactivation at higher levels of Re. In situ HP-FTIR spectroscopy has shown that catalyst genesis occurs via decomposition of the metal carbonyl precursors. Ex situ characterization, using XRD, XPS and EDX-STEM, has provided evidence for the active components of these catalysts containing bimetallic Ru/Re and Rh/Re nanoclusters, the surfaces of which become significantly oxidized after use in amide reduction. Potential mechanistic pathways for amide hydrogenation are discussed, including initial dehydration to nitrile, a pathway potentially specifically accessible to primary amides, and evidence for often postulated imine intermediates.