Properties of Copper Chromite Catalysts in Hydrogenation Reactions

A pretreatment involving a 4-h reduction under H2at 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 H2and 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+1sites, which reached a maximum intensity after a 573 K pretreatment, while XRD patterns also provided evidence for a CuCrO2phase and showed a continuous increase in Cu0as temperature increased. Selectivity to furfuryl alcohol ranged from 35–80% and reaction orders on furfural and H2were 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 H2dependence 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 H2and crotonaldehyde.

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