Separation and Catalysis of Carboxylates: Byproduct Reduction during the Alkylation with Dimethyl Carbonate

Quaternary ammonium carboxylates (both ionic liquids and resins) can be used for formation of methyl carboxylate esters in an alkylation reaction with dimethyl carbonate (DMC). This reaction is pursued in the context of a low-waste process for recovery and upgrading of fermentative carboxylates. Byproducts, such as methanol, are produced during the alkylation reaction with DMC. A molar yield of methyl acetate on acetate of 0.84 and modest selectivity (0.54-0.95 mol of methyl acetate/mol of methanol) are obtained with an anion exchanger fully in the acetate form. Water and DMC concentrations have a strong effect on the yield and selectivity of the reaction. Model calculations indicate that an excess of 50 mol of DMC_in/mol of acetate_in in DMC is needed to achieve yields above 0.98 mol of methyl acetate/mol of acetate_in, and water concentrations in the resin below 0.04 g of water/g of resin to achieve a selectivity above 1.1 mol of methyl acetate/mol of methanol. The explanation for the modest selectivity is the hydrolysis and absence of methylating activity of the methylcarbonate anion. This indicates that 1 mol of DMC is consumed for the alkylation of 1 mol of a monocarboxylate salt. The utilization of a low amount of DMC is critical, especially for low value products such as methyl acetate.