Hydrogenation/dehydrogenation reactions: isopropanol dehydrogenation over copper catalysts

For accurate catalyst comparisons and kinetic modeling, data free of mass and heat transfer effects must be acquired over a range of temperature and reactant concentrations using well-characterized heterogeneous catalysts. For semibatch reactors operated at constant pressure, selectivity vs conversion profiles at several temperatures, preferably obtained with catalysts reduced in situ, are required to allow complicated reaction networks to be defined. Vapor-phase isopropyl alcohol (IPA) dehydrogenation over carbon-supported Cu catalysts in a differential fixed-bed reactor was studied and compared to UHP Cu powder and a Cu chromite catalyst. Low dispersions of Cu (ca. 0.02-0.17) were obtained in all these catalysts. Cu dispersed on an activated carbon heat-treated at 1223 K had the highest turnover frequency (0.052 s(-1) at 448 K) of all the Cu/C catalysts, more than double that on an industrial Cu chromite catalyst; thus the rate with this 5.0% Cu/C catalyst (6.0 mumol s(-1) g(-1)) was close to that of the chromite catalyst containing 41% Cu (10.6 mumol s(-1) g(-1)). The steady-state selectivity to acetone was 100% for all catalysts except those containing the nitric-acid-treated carbon. In the absence of Cu, the nitric-acid-treated carbon produced propylene under steady-state reaction conditions, and reduction at 573 K decreased activity compared to reduction at 423 K. This dehydration reaction is associated with the presence of oxygen-containing acidic groups on the C surface. The apparent activation energy for acetone formation was typically near 20 kcal mol(-1) for all the Cu/C catalysts and the Cu powder, but it was near 12 kcal mol(-1) for the Cu chromite catalyst. A DRIFT spectrum under reaction conditions indicated the presence of an isopropoxide species and molecularly adsorbed IPA on the surface. A Langmuir-Hinshelwood mechanism, which assumed removal of the first hydrogen atom as the rate-determining step and incorporated adsorbed IPA, hydrogen, acetone, and a surface isopropoxide species into the site balance, fit the kinetic data well and gave physically meaningful values for the enthalpies and entropies of adsorption. (C) 2003 Elsevier Science (USA). All rights reserved.