Carbon dioxide utilisation in the synthesis of dimethyl carbonate (DMC) is attractive due to the extensive application of DMC in a variety of processes. Previously, high selectivity to DMC has been correlate with high pressure operating environments employing supercritical CO2. Applying high-operating pressure (8 Mpa~16.5 MPa) in chemical plants is, however, cost and energy intensive and introduces significant safety concerns. Optimisation of reaction conditions with the aim of minimizing the operating pressure while increasing reaction rate, selectivity and catalyst lifetime should be a key goal of process development. Herein, one-pot synthesis of DMC from propylene oxide, methanol and CO2 using alkali halide catalysts under low-operating pressure (2 MPa at room temperature) was studied. This study has evaluated the catalytic activity of alkali halide-based catalysts via the one-pot route of DMC synthesis and the results showed that a DMC selectivity of 15.9% can be obtained when K2CO3-NaBr-ZnO is used as the catalyst. However, selectivity to DMC is limited by the relatively high yield of PG through the hydrolysis of PO. Therefore, both a physical drying agent (3Å molecular sieve) and a chemical dehydrating agent (acetonitrile) are applied in DMC synthesis. The production of PC increases significantly (from 1.04 mmol/ml to 1.48 mmol/ml and 1.59 mmol/ml for 3Å molecular sieve and acetonitrile, respectively) owing to the inhabitation of the PO hydrolysis, however, the influence of adding dehydrating agents on improving the yield of DMC is not obvious (from 0.53 mmol/ml to 0.51 mmol/ml and 0.49 mmol/ml for 3Å molecular sieve and acetonitrile, respectively). The catalyst applied in the DMC synthesis reactions are characterised using TGA, XRD, SEM, SEM/EDS, AAS and ICP-OES.
Additionally, Zn powder shows great possibility of achievement to remove water through the reaction with steam water under reaction condition. Results indicate that the addition of Zn powder to the K2CO3-NaBr-ZnO catalyst system increases the DMC selectivity from 15.9% to 36.1% at 2 MPa and 160 °C for 5 h. Catalyst characterisation showed that Zn powder increases the stability of the catalyst by preventing the active ingredients on the catalyst surface from leaching. The results also show an increase in propylene oxide conversion to DMC attributed to the increase of Zn2+ ions in the reaction solution. Zn both promotes DMC formation through the new reaction route and transesterification of propylene carbonate and methanol.
