Aqueous-phase reforming of methanol and ethylene glycol over alumina-supported platinum catalysts

The rates of aqueous-phase reforming of methanol and ethylene glycol to form H2 and CO2 were measured under kinetically controlled reaction conditions at temperatures of 483 and 498 K over alumina-supported platinum catalysts. Results show that the rates of formation of H2 from aqueous solutions of methanol (from 1 to 10 wt%) are similar to the rates of conversion of ethylene glycol, suggesting that CC bond cleavage is not rate limiting for ethylene glycol reforming. Aqueous-phase reforming of both oxygenated hydrocarbons over Pt/Al2O3 leads to nearly 100% selectivity for the formation of H2 (compared to the formation of alkanes), suggesting that methanation or Fischer–Tropsch reactions involving CO/CO2 and H2 do not appear to be important over platinum-based catalysts under the conditions of the present study. The rate of production of hydrogen is higher order in methanol (0.8) compared to ethylene glycol (0.3–0.5), and the reaction is weakly inhibited by hydrogen (−0.5 order) for both feedstocks. The rates of aqueous-phase reforming of methanol and ethylene glycol show apparent activation barriers of 140 and 100 kJ/mol, respectively, from 483 K and 22.4 bar total pressure to 498 K and 29.3 bar total pressure. Low levels of CO (<300 ppm) are detected in the gaseous effluents from aqueous-phase reforming of methanol and ethylene glycol over alumina-supported Pt catalysts, suggesting that water–gas shift processes are operative under the aqueous-phase reforming conditions of this study. The observed reaction kinetics for ethylene glycol of this study can be explained by a reaction scheme involving quasi-equilibrated adsorption of ethylene glycol, water, H2, and CO2, combined with irreversible steps involving dehydrogenation of adsorbed ethylene glycol to form adsorbed C2O2 species, cleavage of the CC bond to form adsorbed CO species, further dehydrogenation leading to adsorbed CO∗, and removal of adsorbed CO∗ by water-gas shift. Aqueous-phase reforming of methanol may take place by a similar reaction scheme, without the step involving cleavage of the CC bond. The nearly first-order reaction kinetics with respect to methanol can be explained by weaker adsorption of methanol compared to molecular adsorption of ethylene glycol.