Preparation of Temperature-Controlled Heteropolyacid Ionic Liquids and Their Application for Synthesis of Diphenyl Carbonate

Abstract

First, an intermediate ionic liquid [Bmim]Cl and Keggin-type heteropolyacids with different metal vacancies were prepared, and then prepared six heteropolyacid ionic liquids by the ion exchange method, and these liquids were tested and analyzed for chemical structures, crystal structures, thermal stability, and apparent morphology. After that, the six heteropolyacid ionic liquids were used to catalyze the transesterification reaction between phenol and dimethyl carbonate (DMC) to synthesize diphenyl carbonate (DPC), to explore the effects of reaction temperature, reaction time, and catalyst dosage on the catalytic performance, and reveal the catalytic mechanism. The results showed that the six heteropolyacid ionic liquids prepared were the target products with good thermal stability. Compared with traditional catalysts Cp₂TiCl₂ and C₁₆H₃₆O₄Ti, they had better catalytic performance in the catalytic process. Among them, Bmim₄[Ti(H₂O)TiMo₁₁O₃₉] manifested the best catalytic performance. Under the conditions that the molar ratio of raw materials DMC to phenol was 2:1, reaction temperature 180 °C, catalyst consumption 1.5% of the total mass of the raw materials, and reaction time 8 h, the conversion rate of phenol reached 46.17%, and the overall selectivity of products monophenyl carbonate (MPC) and DPC was 98.89%. As a phase transfer catalyst characterized by a "homogeneous phase at high temperature and separation at low temperature," the catalyst can be self-assembled in the catalytic process with the temperature change. Therefore, they can be recycled and reused through simple washing and filtering at the end of the reaction. After five times of recycling, the catalytic performance of the catalyst reduces significantly, in which case the fresh catalyst should be added to maintain a good catalytic performance.

Graphical Abstract