Selective One-Step Synthesis of Dimethoxymethane via Methanol or Dimethyl Ether Oxidation on H 3+ n V n Mo 12- n PO 40 Keggin Structures

The one-step selective synthesis of dimethoxymethane (DMM; CH 3 OCH 2 OCH 3 ) was achieved by oxidation of dimethyl ether (DME) or methanol (CH 3 OH) with O 2 at low temperatures (453−513 K) on unsupported and SiO 2 -supported heteropolyacids with Keggin structures [H 3+ n PV n Mo 12 - n O 40 ( n = 0−4)]. These materials provide redox and Brönsted 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 CO x yields using either CH 3 OH or DME reactants. The presence of H 2 O during DME oxidation increases DMM synthesis rates because of a consequent increase in the rate of DME hydrolysis reactions, which form CH 3 OH molecules required as intermediates in the DMM synthesis reaction sequence. Pure CH 3 OH reactants form DMM at much higher rates than DME reactants. The replacement of some Mo atoms in H 3 PMo 12 O 40 structures with V increases DMM synthesis rates and selectivities while inhibiting the formation of CO x . In fact, CO x was not detected on H 3+ n PV n Mo 12 - n O 40 ( n = 2, 4; ∼0.1 KU/nm 2 ) even at high CH 3 OH conversions (∼50%). CH 3 OH converts to DMM via primary CH 3 OH reactions to form formaldehyde (HCHO) and subsequent secondary reactions of HCHO with CH 3 OH in steps requiring both redox and acid sites; CH 3 OH 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 CH 3 OH pressures and conversions favor HCHO−CH 3 OH acetalization reactions and DMM synthesis rates and selectivities. Thermal treatments that cause dehydroxylation and loss of Brönsted 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.