The Research On One-pot Synthesis Of DMC

Abstract: In past decads, much attention has been paid to the synthesis and application of dimethyl carbonate (DMC) because of its low toxicity and versatile chemical properties. It can be used as an environment-friendly intermediate and starting material for organic synthesis substituting the poisonous phosgene and dimethyl sulfate in carbonylation and methylation reaction. In addition, DMC is considered to be a promising gasoline octane enhancer to replace methyl tert-butyl ether (MTBE) due to its higher oxygen content and excellent blending properties.Some studies on the two-step synthesis of DMC from ethylene oxide (EO) or propylene oxide (PO), CO₂ and methanol have already been carried out with various catalysts, and the transesterification of ethylene or propylene carbonate with methanol have been commercialized. The major disadvantages of the two-step process are its high energy consumption, high investment and production costs. To eliminate the difficulties in the two-step process, the one-pot reaction is a better way to produce DMC. It couples the cycloaddition of CO₂ with EO/PO and the transesterification of methanol and the resulting cyclic carbonate. Meanwhile, the alcoholysis of EO/PO happens in this reaction simultaneously. Therefore, the key to carry out the one-pot synthesis is that the catalyst can facilitate the target reaction, while it should not be active to the alcoholysis of epoxides.In the present work, the synthesis of dimethyl carbonate and propylene glycol from methanol, propylene oxide, and carbon dioxide over various catalysts was reported. The catalysts were characterized using a combination of techniques of X-ray photoelectron energy spectrum (XPS), X-ray diffraction (XRD), Temperature-Programmed Desorption of CO₂ (CO₂-TPD) and N₂ physical adsorption. The influence of active components, support prosperities, and various reaction conditions e.g. temperature, CO₂ pressure, reaction time, molar ratio of CH₃OH/PO on the reaction was investigated. Among the used catalysts, NaOH supported on activated carbon support with a 15% (wt) loading, heat treated at 600℃in Ar atmosphere exhibits the best catalytic performance in terms of the conversion of methanol and PO, the selectivity to DMC and PG. Under the reaction conditions of 423K, 4.0MPa initial pressure of CO₂, the molar ratio of CH₃OH to PO of 8:1 and reaction time of 3h, 28.1%, 99.9% conversion of methanol and PO, 39.8%, 40.5% selectivity of DMC and PG were achieved over the NaOH/C catalyst. After six recycles the activity and selectivity of the catalyst decrease slightly, which is mainly due to loss of the catalyst during handling and transferring the reaction mixture since both XPS and XRD confirm that no changes of the surface and phase structures was observed on the used catalyst in comparison with the fresh one. In addition, the characterization results confirm that catalytic activity has close relations with strong base sites on the catalysts.