Direct Observation of Surface Intermediates Formed by Selective Oxidation of Alcohols on Silica-Supported Molybdenum Oxide

The behavior of surface species, formed from the adsorption of methanol and isobutyl alcohol (2-methyl-1-propanol) on MoO 3 /SiO 2, was studied in an oxygen-containing atmosphere by in situ Fourier transform infrared spectroscopy (FTIR). At 250 °C, methanol adsorption resulted in the formation of surface methoxide species (CH 3 O−), whereas isobutyl alcohol adsorption resulted in surface isobutoxide species ((CH 3 ) 2 CHCH 2 O−). Kinetic studies indicated that the rate-determining step for the alcohol oxidation was the cleavage of the α-C−H bond in the alkoxide intermediates. Although the α-C is a primary carbon in the case of methanol ( C H 3 OH) and a secondary carbon in the case of isobutyl alcohol ((CH 3 ) 2 CH C H 2 OH), the activation energies for the rate-determining step were found to be similar. Theoretical molecular orbital calculations confirmed these experimental results by also predicting similar activation energies for both alkoxide species. During oxidation of the alkoxide species in the absence of alcohol in the gas phase, new bands were observed, which were assigned to the C O bands of adsorbed acyl species. In the case of these species, further reaction with oxygen produced CO 2 . The acyl species are therefore likely not to be intermediates that lead to aldehyde products in alcohol oxidation.