Upgrading of glycerol acetals by thermal catalyst-free transesterification of dialkyl carbonates under continuous-flow conditions

At 250-300 degrees C and 30-50 bar, a continuous-flow (CF) transesterification of different dialkyl and alkylene carbonates (dimethyl-, diethyl-, dibenzyl-, and propylene carbonate, respectively) with two glycerol derived acetals (glycerol formal and solketal) was investigated without any catalyst. An unprecedented result was obtained; not only the desired process occurred, but also the formation of the corresponding mono-transesterification products took place with an excellent selectivity (up to 98%) in all cases. Under isothermal conditions, a study on the effect of pressure allowed us to optimize the conversion of acetals (up to 95%) for the reactions of dimethyl- and diethyl-carbonate (DMC and DEC, respectively). This proved that an abrupt progress of the reaction occurred for very small increments of pressure. For example, at 250 degrees C, the thermal transesterification of DMC with glycerol formal showed a sharp increase of the conversion from 1-2% at 30 bar to similar to 85% at 37 bar. The lower the temperature, the lower the pressure interval at which the onset of the reaction is achieved. The absence of catalysts allowed us to run CF-reactions virtually indefinitely and with a very high productivity (up to 68 mg min(-1)) compared to the capacity (1 mL) of the used CF-reactor. Products of the transesterification of DMC and DEC were isolated in good-to-almost quantitative yields. In the case of heavier carbonates, steric reasons were responsible for the considerably lower reactivity of propylene carbonate (PC) with respect to DMC and DEC, while the transesterification of dibenzyl carbonate (DBnC, solid at room temperature) with glycerol formal required the presence of acetone as an additional solvent/carrier. Although the reactions of both PC and DBnC were not optimized, results offered a proof-of-concept on the extension of thermal transesterification processes to higher homologues of linear and alkylene carbonates.