Isotopic transient analysis of the ethanol coupling reaction over magnesia

Isotopic transient analysis of ethanol coupling to butanol over MgO in a fixed-bed reactor at 673 K revealed a surface coverage of adsorbed ethanol equivalent to about 50% of the exposed Mg-O atomic pairs. DRIFTS of ethanol reaction at 673 K confirmed that the surface was populated primarily with adsorbed ethoxide and hydroxide, presumably from the dissociative adsorption of ethanol. The coverage of reactive intermediates leading to butanol was an order of magnitude lower than that of adsorbed ethanol, and about half the surface base sites counted by adsorption of CO2. The intrinsic turnover frequency for the coupling reaction at 673 K determined by isotopic transient analysis was 0.04 s(-1), which is independent of any assumptions about the nature of the active sites. Although the ethanol coupling reaction appears to involve aldol condensation of an aldehyde intermediate, the high coverage of ethanol under steady-state conditions apparently inhibits unproductive C-C coupling reactions that deactivate the catalyst at high temperature. (C) 2012 Elsevier Inc. All rights reserved.