Exposure of metallic copper surface on Cu-Al2O3-carbon catalysts

The bifunctional nature of Cu---Al2O3-on-carbon catalysts, used in the direct catalytic conversion of ethanol to ethyl acetate, prompted an examination of the dispersion of Cu on the composite catalyst. For this, the N2O-method of Osinga et al. for estimation of bare metallic copper surface on composite copper catalysts has been adapted for use on a gravimetric adsorption unit and then applied to several promoted and supported copper catalysts. For catalysts with Cu/Al atomic ratio 0.8 to 26, all reduced at 300 °C, the copper surface is only 1–8% of the total surface of 500–560 m2/g, but 50–430 m2/g Cu. The maximum Cu dispersion is obtained for the catalyst with Cu/Al = 6, which is also one of the best catalysts for the esterification reaction. The Cu surface rapidly increases from 1 to 10% of the total surface as the temperature of reduction of the catalyst is raised from 100 to 400 °C. On a low area (32 m2/g) asbestos carrier, a relatively higher Cu coverage of 55% can be obtained, but the activity of this catalyst is very poor. On use in the esterification reaction, the carbon-supported catalyst gets a more reddish brown copper color, however there is no increase in the exposed copper area. X-ray microscan studies of the catalyst prove that the copper sites on the surface (in an 1 μ-thick layer) are mostly in close association with the Al sites of the alumina promoter. Results from catalyst testing show that these Cu---Al sites or junctions are necessary to catalyze the condensation to ester of the acetaldehyde formed in the primary dehydrogenation of ethanol over copper. The microscan studies on the same spot on one and the same catalyst particle indicate that the distribution of Cu and Al in the 1 μ-thick layer on the catalyst surface does undergo some alterations when the catalyst is reduced at 300 °C and subsequently used in the esterification reaction at 275 °C. The exact nature of these alterations is still not clear.