Properties of copper chromite catalysts in hydrogenation reactions

A pretreatment involving a 4-h reduction under H-2 at 573 K, compared to either 473 or 673 K, gave the highest specific activity for furfural and crotonaldehyde hydrogenation over an unpromoted copper chromite catalyst. Both H-2 and CO chemisorption declined monotonically with increasing reduction temperature, but turnover frequencies also showed a maximum after a pretreatment at 573 K. DRIFTS identified a band, associated with CO adsorbed on Cu+1 sites, which reached a maximum intensity after a 573 K pretreatment, while XRD patterns also provided evidence for a CuCrO2 phase and showed a continuous increase in Cu-0 as temperature increased, Selectivity to furfuryl alcohol ranged from 35-80% and reaction orders on furfural and H-2 were near unity. In contrast, crotonaldehyde hydrogenation gave primarily butyraldehyde with only around 3% crotyl alcohol and a near zero-order dependence on crotonaldehyde, although the H-2 dependence was still near first order. IR spectra under reaction conditions revealed adsorbed crotonaldehyde, provided evidence for an unsaturated alkoxide intermediate and detected no butyraldehyde on the surface. A simple Langmuir-Hinshelwood sequence nicely explained both reactions, it was consistent with the DRIFTS results obtained during crotonaldehyde hydrogenation, and the resultant rate expression gave meaningful entropies and enthalpies of adsorption for H-2 and crotonaldehyde. (C) 1997 Academic Press.