(349e) Effect of Zeolite Structure and Composition On Vapor-Phase Carbonylation of Dimethoxymethane

We have shown that the carbonylation of dimethoxymethane (DMM, a dialkyl acetal of formaldehyde) provides a highly selective, low-pressure route to methyl methoxyacetate (MMAc), a precursor to ethylene glycol [1]. By contrast, liquid-phase carbonylation of formaldehyde requires pressures of tens to hundreds of atmospheres to achieve desirable yields and selectivities [2]. DMM disproportionation, which produces dimethyl ether (DME) and methyl formate (MF), is the only byproduct of DMM carbonylation. Acidic zeolites are effective catalysts for producing MMAc from DMM and CO. While FAU, MFI, MOR, and BEA exhibited similar rates of MMAc synthesis, FAU showed a much higher selectivity to MMAc because it disfavored the disproportionation of DMM. The low rate of DMM disproportionation for FAU is consequence of the large supercages in this zeolite, which limit the transfer of a hydride from DMM in a critical step of the reaction mechanism leading to DME and MF. The proximity of acid sites to one another in high Al content zeolites led to lower carbonylation rates, and MMAc formation rates were observed to increase with increasing Si/Al ratio. It is thought that interaction between proximal adsorbed species increases the activation barrier to carbonylation and, hence, reduces the reaction rate. High selectivities to MMAc could be achieved by careful selection of the reactant concentrations, reaction temperature, zeolite framework type, and silicon to aluminum ratio. At a carbon monoxide pressure of three atmospheres, a MMAc selectivity of 79% at a DMM conversion of 13% was obtained using H-FAU with a Si/Al ratio of 30. In situ IR studies indicated that upon exposure to DMM, most of the acid sites in the zeolite reacted to form the intermediate CH₃OCH₂OZ, which then underwent slow carbonylation to form CH₃OCH₂COOZ. The latter species then reacted with DMM to form MMAc and CH₃OCH₂OZ.

[1] Celik, F.E.; Kim, T.; Bell, A.T. Angew.Chem. Int. Ed. In press

[2] Loder, D.J. U.S. Patent 2,152,852, 1939; Hendriksen, D.E. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 1983, 28, 176-190; Lee, S.Y.; Kim, J.C.; Lee, J.S.; Kim, Y.G. Ind. Eng. Chem. Res. 1993, 32, 253-259.