Selective one-step synthesis of dimethoxymethane via methanol or dimethyl ether oxidation on H3+nVnMO12-nPO40 Keggin structures

The one-step selective synthesis of dimethoxymethane (DMM; CH3OCH2OCH3) was achieved by oxidation of dimethyl ether (DME) or methanol (CH3OH) with O-2 at low temperatures (453-513 K) on unsupported and SiO2-Supported heteropolyacids with Keggin structures [H3+nPVnMo12-nO40 (n = 0-4)]. These materials provide redox and Bronsted acid sites required for bifunctional DMM synthesis pathways. Supported structures at submonolayer coverages (0.1-0.28 Keggin units per nm(2)) are much more accessible than bulk structures and remove diffusional constraints. Their higher dispersions lead to marked improvements in DMM synthesis rates and selectivities and to lower CO2 yields using either CH3OH or DME reactants. The presence of H2O during DME oxidation increases DMM synthesis rates because of a consequent increase in the rate of DME hydrolysis reactions, which form CH3OH molecules required as intermediates in the DMM synthesis reaction sequence. Pure CH3OH reactants form DMM at much higher rates than DME reactants. The replacement of some Mo atoms in H3PMo12O40 structures with V increases DMM synthesis rates and selectivities while inhibiting the formation of COx. In fact, COx was not detected on H3+nPVnMo12-nO40 (n = 2, 4; similar to0.1 KU/nm(2)) even at high CH3OH conversions (similar to50%). CH3OH converts to DMM via primary CH3OH reactions to form formaldehyde (HCHO) and subsequent secondary reactions of HCHO with CH3OH in steps requiring both redox and acid sites; CH3OH also reacts to form DME on acid sites. These pathways are consistent with the effects of changes in residence time and of the partial removal of acidic OH groups from Keggin structures on reaction selectivities. High CH3OH pressures and conversions favor HCHO-CH3OH acetalization reactions and DMM synthesis rates and selectivities. Thermal treatments that cause dehydroxylation and loss of Bronsted acid sites without destroying the primary Keggin structures decrease DME formation rates without significant changes in DMM synthesis rates. These findings suggest that acid sites are not involved in the rate-limiting step for DMM synthesis and that much higher DMM selectivities can be achieved by further increases in the ratio of the rates of redox and acid catalysis. This study represents the first report of high DMM selectivity and yields on stable molecular oxide clusters and provides an effective approach to the rational design of oxide materials for the one-step synthesis of dimethoxymethane from either dimethyl ether or methanol.