An Investigation of the Mechanism for Methanol Oxidation to Formaldehyde on V-MCM-48
Jason L. Bronkema and Alexis T. Bell. Chemical Engineering, University of California, Berkeley, 201 Gilman Hall, Berkeley, CA 94720
The investigation of reaction mechanism and catalyst structure-function relationships is greatly facilitated by the use of single-site catalysts which have the advantage that the composition and structure of all active sites are essentially the same. We report here a study of the mechanism of methanol oxidation to formaldehyde catalyzed by isolated vanadate species supported on the surface of MCM-48, a high surface area, mesoporous silica. Isolated vanadate species were deposited onto MCM-48 by gas phase sublimation, using vanadyl acetylacetonate as the precursor. Samples were prepared with vanadium surface densities of 0.1-0.75 V/nm2, well below the theoretical monolayer value on silica of 2 V/nm2. Raman spectra taken after dehydration revealed that vanadium was present exclusively as isolated vanadate species. By XANES, the vanadium was determined to have a +5 oxidation state with a distorted tetrahedral geometry. EXAFS analysis confirmed the absence of any V-V scattering, with the best fit structure consisting of one V=O and three V-O bonds. In-situ Raman and infrared spectroscopy experiments were performed to identify the reaction intermediates involved in the oxidation of methanol to formaldehyde. Upon contact with methanol vapor at 323 K, the silanol groups on the silica surface reacted completely to form Si-OCH3 species and methanol reacted with the V-O-Si bonds of the supported vanadate species to produce both V-OCH3 and Si-OCH3 species. A new peak at 1080 cm-1 was observed, which was assigned to V=O stretching vibrations of V atoms that are bonded as well to a methoxy group. During temperature-programmed oxidation of adsorbed methanol, Raman spectroscopy revealed a decrease in the intensity of the Raman band for methoxy groups bonded to V and the band for V=O stretching vibrations at the same time that formaldehyde formation was observed by mass spectrometry. Similar studies were carried out for methanol adsorbed on silica in the absence of vandia. In-situ Raman and isotopic labeling experiments carried out with 18O2 show that the distribution of 18O in the reoxidized vanadium is less than that in the gas phase, suggesting that oxygen from the silica support is used to reoxidize the reduced vanadate species. Complimentary experiments in which adsorbed methanol undergoes programmed heating in the absence of oxygen provided additional information about the elementary process that Si-OCH3 and V-OCH3 species undergo at high temperatures. The accumulated evidence is used to propose a reactions network for the oxidation of methanol to formaldehyde.