(276c) Selective Hydrogenation of Furfural to Furfuryl Alcohol over Supported Pd-Re Catalysts

PdRe/Al₂O₃ catalysts are demonstrated to be highly active and selective for hydrogenation of furfural to furfuryl alcohol (FAL). Bimetallic PdRe catalysts (1:1 atomic ratio) were prepared by incipient wetness co-impregnation of γ‑Al₂O₃ and by two sequential impregnation methods: (1) impregnation of Re/Al₂O₃ with Pd²⁺ with subsequent calcination and (2) impregnation of reduced and passivated Pd/Al₂O₃ with perrhenic acid, [HReO₄], without subsequent calcination.  Catalyst structure after in situ reduction at 400°C depends strongly on the Pd precursor and the sequence of metal precursor addition. Extended x-ray absorption fine structure (EXAFS) spectroscopy and temperature-programmed hydride decomposition (TPHD), showing essentially complete suppression of β-PdH_x formation, evidence alloy formation in catalysts prepared by co-impregnation. Due to strong electrostatic adsorption of [Pd(NH₃)₄]²⁺ at pH 10-11, catalysts prepared from [Pd(NH₃)₄(NO₃)₂] contain small (<5 nm) Pd particles, whereas [Pd(NO₃)₂]-derived catalysts contain larger (>10 nm) particles. The latter observation correlates with formation of PdO crystallites during calcination, as detected by Raman spectroscopy. Preparation of PdRe/Al₂O₃ catalysts by sequential impregnation facilitates low-temperature Re reduction and increases furfural hydrogenation activity and FAL selectivity. Varying degrees of Pd-Re interaction in each catalyst are indicated by EXAFS spectroscopy, scanning transmission electron microscopy with energy dispersive x-ray analysis (STEM-EDX), and CO diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Furfural turnover frequency and FAL selectivity correlate strongly with suppression of H₂ chemisorption; a H/CO chemisorption ratio of ~0.2 appears to be optimum for selective hydrogenation of furfural to FAL.