Methyl ester synthesis catalyzed by nanoporous gold: from 10-9 Torr to 1 atm

The oxidative coupling reaction of aldehydes with methanol occurs in the vapor phase over a support-free nanoporous gold (npAu) catalyst over a wide pressure range-from 10(-9) Torr to 1 atm. The dependence of the aldehyde-to-ester reaction rate on the oxygen, methanol and aldehyde partial pressures suggests that the rate-limiting step for coupling is the reaction of the aldehyde with surface sites saturated with adsorbed methoxy. Stable catalyst activity is achieved for aldehyde-methanol coupling in flowing reactant mixtures at 70 degrees C. While the conditioned npAu catalyst exhibits high selectivity for methanol-aldehyde coupling, its activity for the self-coupling reaction of methanol to methyl formate is reduced by the exposure to the alcohol-aldehyde mixture in a manner that is consistent with the buildup of spectator species. The activity for methanol self-coupling can be regenerated by extended exposure to flowing methanol, CO and O-2 at 70 degrees C. Overall, the observed catalytic esterification is consistent with model studies of both the npAu catalyst and single crystal gold in ultrahigh vacuum.