Two-step Synthesis of Dialkyl Carbonates through Transcarbonation and Disproportionation Reactions Catalyzed by Calcined Hydrotalcites

A two-step methodology was implemented to prepare dialkyl carbonates from both primary and secondary alcohols, including glycerol acetals, tetrahydrofurfuryl alcohol and cyclohexanol. Accordingly, alcohols were initially subjected to a batch transcarbonation (carbonate interchange) reaction with the non-toxic dimethyl carbonate (DMC), providing the corresponding asymmetrical methyl alkyl carbonates (ROCO2Me). These compounds were then used as reactants for a continuous-flow (CF) disproportionation reaction, producing the corresponding symmetrical dialkyl carbonates (ROCO2R). Although transcarbonation and dismutation took place at different operating temperatures (90 and 180-275 °C, respectively), both reactions were catalyzed by the same heterogeneous Mg/Al mixed oxides catalyst, obtained upon calcination of commercially available hydrotalcites (HTs). Yields and selectivities for methyl alkyl carbonates were excellent, in the range of 93-96% and 95-98%, respectively. CF-disproportionation reactions were strongly affected by the nature/structure of reactants, nevertheless, they provided the corresponding dialkyl carbonates with selectivities and productivities up to 92% and 164 mgprod∙(gcat∙min)-1, respectively. Overall, the reported methodology displays attractive sustainability features, including a straight-forward upgrade of biomass derivatives, development of a continuous-flow intensified process, improved catalyst recycle and products’ purification, while disclosing stimulating perspectives for further investigations on the reaction mechanism and on the role of HTs as catalyst precursors for the synthesis of organic carbonates.