Study On MoO₃ Catalysts In Ester Exchange Between Diethyl Oxalate And Phenol

Diphenyl carbonate (DPC) has become an important organic chemical that is used as a starting material for phosgene-free manufacturing polycarbonates (PCs) by transesterification. Diethyl oxalate (DEO) with phenol to produce diphenyl oxalate (DPO), followed by the decarbonylation of DPO to produce DPC,is a green process with a promising prospect in an industrial application. In this paper, the ester exchange between diethyl oxalate and phenol was disscused using the heterogeneous catalysts, including the supported MoO₃/SiO₂ catalysts and the complex MoO₃-SiO₂ catalysts. The relation between the catalytic performance and the structure and chemical composition of the catalyst in the transesterification was deeply studied.Ethylphenyl Oxalate (EPO) as the intermediate product,and DPO as the terminal product were qualitatively analysed through GC-MS. Therefore, the synthesis of DPO followed a 2-step reaction module consisting of the transesterification of DEO with phenol into EPO and the disproportionation of EPO into DPO.By means of XRD, Raman, IR, XPS,the catalytic properties of MoO₃/SiO₂ and MoO₃-SiO₂ catalysts were closely related with the state of MoO₃. The MoO₃ species in the form of amorphous state was responsible for the synthesis of EPO and DPO, especially, for the disproportionation of EPO to DPO. When the crystal was formed, the catalyst activity and selectivity to DPO was decreased.The activity of MoO₃-SiO₂ mixed oxides by sol-gel method was better than the supported MoO₃/SiO₂ catalysts. IR and XPS investigation showed that the interaction of the MoO₃-SiO₂ catalysts between MoO₃ and SiO₂ were stronger than MoO₃/SiO₂ catalysts. Most MoO₃ species were inserted into the silica framework, suggesting the formation of Mo-O-Si bond. Accordingly, more activity sites were produced in more dispersed state, resulting in the improvement of the catalytic activity and selectivity to DPO.The characterization of FT-IR of pyridine adsorption and Temperature-programmed desorption of NH3 (NH3-TPD) indicated the uniquely existed weak Lewis acid sites in the MoO₃/SiO₂ and MoO₃-SiO₂ catalyst resulted in high selectivity to EPO and DPO. Furthermore, there were much more acid sites on MoO₃/SiO₂ than MoO₃-SiO₂, which was in favour of improving the catalytic performance. The catalyst of MoO₃-SiO₂ with 12% MoO₃ performed best, giving 71.0% conversion of DEO, 32.0% selectivity of DPO, 22.7% yield of DPO, and 100% total selectivity to DPO and EPO.