Acetic acid hydrogenation over supported platinum catalysts

Acetic acid was chosen to probe the kinetic behavior of carboxylic acid hydrogenation over platinum supported on TiO2, SiO2, eta-Al2O3, and Fe2O3. The reaction was studied in the vapor phase under conditions of 423-573 K, 100-700 Torr hydrogen, and 7-50 Torr acetic acid in a differential, fixed-bed reactor. Product selectivity was strongly dependent on the oxide supports. Carbon-containing products during hydrogenation at low conversions consisted of about: 50% CO and 50% CH4 over Pt/SiO2; 8% ethanol, 4% ethyl acetate, 10% ethane, 40% CH4, 33% CO, and 5% CO2 over Pt/eta-Al2O3; 50% ethanol, 30% ethyl acetate, and 20% ethane over Pt/TiO2 reduced at either 473 or 773 K; and about 80% acetaldehyde and 20% ethanol over Pt/Fe2O3. The TiO2-supported Pt catalysts were the most active, and both their activities and their turnover frequencies were up to two orders of magnitude larger than those for Pt dispersed on SiO2 eta-Al2O3, or Fe2O3. The activity dependence on the partial pressures of hydrogen, P-H2, and acetic acid, P-A, was determined for Pt/TiO2 catalysts at three operating temperatures-422, 445, and 465 K-after reduction at either 473 or 773 K. The apparent reaction order with respect to Hz was found to vary between 0.4 and 0.6, while that with respect to acetic acid was between 0.2 and 0.4. One reaction model that correlated these data well involves a Langmuir-Hinshelwood-type catalytic sequence that incorporates dissociative hydrogen and acetic acid adsorption on one type of site existing on the Pt surface, but only molecular acetic acid adsorption at another type of site on the oxide surface. Only the latter species on the titania surface was considered catalytically significant in the formation of desired products, i.e., acetaldehyde, ethanol, and ethane. The resulting rate expression in terms of acetic acid disappearance has the form r(HOAc) = k(1)P(A)P(H2)(1/2)/[(K2PH21/2 + K3PA/P-H2(1/2)) (1 + K4PA)]. Values of enthalpy and entropy of adsorption obtained from the optimized rate for hydrogen on platinum and acetic acid on titania were reasonable and thermodynamically consistent. (C) 2000 Academic Press.