Carbon-Supported Copper Catalysts: II. Crotonaldehyde Hydrogenation

Abstract

The adsorption and catalytic behavior of Cu supported on different forms of carbons, i.e., activated carbon, graphitized carbon fibers, and diamond powder, were examined during crotonaldehyde hydrogenation. Activities as well as turnover frequencies (TOFs) varied significantly as a function of the support, the pretreatment temperature, and the catalyst preparation technique; for example, activities at 423 K ranged from 0.23 to 18 μmol/s/g cat while TOFs ranged from 0.001 to 0.13 s−1 after reduction at 573 K. A maximum in TOF was exhibited by each catalyst as a function the fraction of Cu0 at the surface, i.e., Cu0/(Cu0+Cu+1), which is attributed to enhanced reactivity of crotonaldehyde adsorbed on Cu+1 sites. Whereas typical Cu catalysts, including Cu powder, exhibited similar TOF values after a given reduction step, significantly higher TOFs were obtained with Cu dispersed on a nitric acid-treated activated carbon; this is attributed to additional adsorption sites on the carbon surface for crotonaldehyde which allow it to react with hydrogen spilled over from Cu0 sites. In contrast, unusually low TOFs and a higher selectivity to crotyl alcohol were exhibited by Cu deposited on the graphitized fibers by an ion-exchange technique, which may be due to small Cu particles preferentially stabilized along the edges of the graphitic basal planes. Selectivity to crotyl alcohol was routinely near zero after 3 h on stream. Both diffuse reflectance FTIR spectroscopy (DRIFTS) and kinetic data obtained during crotyl alcohol and butyraldehyde hydrogenation indicated stronger adsorption and higher reactivity of crotyl alcohol on Cu, presumably leading to these low steady-state selectivities. DRIFT spectra taken under reaction conditions revealed adsorbed crotonaldehyde, provided evidence for a monohydrogenated reactive intermediate with detectable surface coverage, and detected no adsorbed butyraldehyde. A Langmuir–Hinshelwood model incorporating this reaction intermediate gave an excellent fit of the kinetic data and provided physically meaningful values for enthalpies and entropies of adsorption.