Tuesday, November 10, 2015: 9:08 AM
250F (Salt Palace Convention Center)
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-PdHx 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.