Thermal (Catalyst-Free) Transesterification of Diols and Glycerol with Dimethyl Carbonate: A Flexible Reaction for Batch and Continuous Flow Applications

An innovative thermal transesterification protocol for the synthesis of linear and alkylene carbonates was investigated under both batch and continuous-flow (CF) conditions. Accordingly, model 1,n-diols (n = 2-4) and glycerol were set to react with dimethyl carbonate (DMC) at T and p of 150-260 degrees C and 1-50 bar, respectively, in the absence of any catalyst. 1,2-diols afforded the corresponding five-membered ring carbonates as the main products with a quantitative conversion and a selectivity up to 94%, whereas 1,3-diols gave the six-membered ring products along with linear mono- and dicarbonate derivatives. A complete conversion was attained also for glycerol, but the products distribution depended on reaction conditions: the CF mode allowed the synthesis of glycerol carbonate, whereas batch reactions yielded either glycerol carbonate or its derivative from a further transesterification reaction, i.e., methyl (2-oxo-1,3-dioxolan-4-yl)methyl carbonate. The selectivity toward these two compounds was in the range of 83%-94%. An addition of gaseous CO2 (up to 20 bar) allowed to control further the selectivity of batch reactions.