A STRUCTURE-ACTIVITY RELATIONSHIP FOR THE OXIDATION OF METHANOL BY HETEROPOLYANION CATALYSTS (VANADIUM OXIDE, LFER, FORMALDEHYDE, DIMETHYL-ETHER)

Heteropolyanions having the Keggin structure, PW(,12)O(,40) ('3-), substituted Keggin structure, PW(,12-n)V(,n)O(,40) ('(3+n)-), and bi-capped Keggin structure, PV(,14)O(,42) ('9-), are discrete metal oxide clusters. They were used here to determine the relationship between their structure and activity for the oxidative dehydrogenation of methanol to formaldehyde. Since these clusters have well defined stoichiometries and surfaces, they may serve as suitable models of the surfaces of bulk tungsten and vanadium oxides commonly used as catalysts in oxidation reactions.;When the total conversion of methanol was less than 10%, the formaldehyde reaction rate depended linearly on the vanadium ion content in the samples even when the number of adjacent vanadium ions in the starting clusters were varied. Additional experiments with supported vanadium oxide samples showed that the formaldehyde reaction rate did not depend on the method of sample preparation. Temperature programmed reaction and electron spin resonance spectroscopy results suggest that a single vanadium ion and its oxide ligands are responsible for the catalysis, and that structures beyond the first coordination sphere of the vanadium do not participate in the reaction mechanism.;In contrast, the activity per cluster for the production of dimethyl ether decreased exponentially as the charge on the starting clusters increased. This trend is described quantitatively by allowing the activation energy for the etherification reaction to increase with an assumed Coulombic attraction between the anionic cluster and a cationic intermediate.;Sodium salts of the heteropolyanions were supported on silica in order to obtain highly exposed samples. Characterization of the samples by infrared spectroscopy, transmission electron microscopy, and x-ray diffractometry showed that the Keggin structures remained intact on the silica surface, and had aggregated into crystallites about 3 nm across. Samples were tested as methanol conversion catalysts in a fixed bed reactor. With excess di-oxygen in the feed, the principal reaction products were formaldehyde and dimethyl ether.