Effects of support on bifunctional methanol oxidation pathways catalyzed by polyoxometallate Keggin clusters

Abstract

H 5PV 2Mo 10O 40 polyoxometallate Keggin clusters supported on ZrO 2, TiO 2, SiO 2, and Al 2O 3 are effective catalysts for CH 3OH oxidation reactions to form HCHO, methyl formate (MF), and dimethoxymethane (DMM). Rates and selectivities and the structure of supported clusters depend on the surface properties of the oxide supports. Raman spectroscopy showed that Keggin structures remained essentially intact on ZrO 2, TiO 2, and SiO 2 after treatment in air at 553 K, but decomposed to MoO x and VO x oligomers on Al 2O 3. Accessible protons per Keggin unit (KU) were measured during CH 3OH oxidation by titration with 2,6-di- tert-butyl pyridine. For similar KU surface densities (0.28–0.37 KU/nm 2), the number of accessible protons was larger on SiO 2 than on ZrO 2 and TiO 2 and much smaller on Al 2O 3 supports, even though residual dimethyl ether (DME) synthesis rates after titrant saturation indicated that the fractional dispersion of KU was similar on the first three supports. These effects of support on structure and on H + accessibility reflect varying extents of interaction between polyoxometallate clusters and supports. Rates of CH 3OH oxidative dehydrogenation per KU were higher on ZrO 2 and TiO 2 than on SiO 2 at similar KU surface densities (0.28–0.37 KU/nm 2) and dispersion, indicating that redox properties of Keggin clusters depend on the identity of the support used to disperse them. ZrO 2 and TiO 2 supports appear to enhance the reducibility of anchored polyoxometallate clusters. Rates were much lower on Al 2O 3, because structural degradation led to less reactive MoO x and VO x domains. CH 3OH reactions involve primary oxidation to form HCHO and subsequent secondary reactions to form DMM and MF. These reactions involve HCHO–CH 3OH acetalization steps leading to methoxymethanol (CH 3OCH 2OH) or hemiacetal intermediates, which condense with CH 3OH on acid sites to form DMM or dehydrogenate to form MF. CO x formation rates are much lower than those of other reactions, and DME forms in parallel pathways catalyzed by acid sites. Secondary reactions leading to DMM and MF are strongly influenced by the chemical properties of support surfaces. Acidic SiO 2 surfaces favored DMM formation, while amphoteric or dehydrogenating surfaces on ZrO 2 and TiO 2 led to MF formation, as a result of the varying role of each support in directing the reactions of HCHO and CH 3OH and of the CH 3OCH 2OH intermediates toward DMM or MF, which was confirmed using physical catalyst–pure support mixtures. These support effects reflect the bifunctional pathways of CH 3OH reactions. These pathways are consistent with the effects of residence time and of the partial removal of H + sites by titration using 2,6-di- tert-butyl pyridine.