Morphological Synthesis And Catalytic Performance Of The Cerium-manganese Oxides In Catalytic Synthesis Of Diphenyl Carbonate

Catalytic oxidative carbonylation of phenol to diphenyl carbonate?DPC?is very appealing because it is an environmentally friend techonology with phenol,CO and O2 as raw materials.Many attentions have been focused by both domestic and international scholars since 1980s.However,due to harsh operation condition,requirements of high temperature and pressure,and an enormous cost in noble metals catalysts,the industrialization for oxidative carbonylation process can not realize up to now.The yield of DPC in the reaction of oxidative carbonylation is low compared to the phosgene processes and the ester exchange method.Therefore,how to prepare and to select a catalyst with high activity,selectivity,and long service life is the key point for oxidative carbonylation of phenol,with which the yield of DPC can be promoted.At present,the catalysts for DPC synthsis by oxidative carbonylation of phenol include homogeneous and supported heterogeneous catalysts.Homogeneous catalysts can improve the yield of DPC in oxidative carbonylation reaction to some extent,but they are difficult to separate from the products.Oxidative carbonylation of phenol to diphenyl carbonate with the palladium supported catalysts make excellent prospects for the disadvantages of easy-separation.During the preparation of the palladium supported catalysts,the active Pd species are easy to be sintered and difficult to be reoxidated if it is reduced.So how to prepare and to select the carrier for supported palladium catalyst with high activity becomes a hot topic.The preparation methods and the morphology of the carrier have a great effect on the activity of catalysts.The oxygen species and the interaction between metal and support in different crystallographic plane of the carrier were studied,aiming to solve the problem of Pd²⁺easily to be reduced to form the Pd⁰.The sintering of active Pd species should be solved via the modification of palladium species with different dispersants.All of these are to improve the catalytic activity and to offer an effective way to inhibit the catalyst deactivation for oxidative carbonylation of phenol.The following works has been done:1.Nanorods and nanocubes of Pd/MnO_x-CeO₂ were synthesized by hydrothermal method.The morphology of crystals,the oxygen species and the effect of Mn on Pd/MnO_x-CeO₂ were evaluated and characterized by the X-ray diffraction?XRD?,transmission electron microscope?TEM?,Raman,X-ray photoelectron spectroscopy?XPS?techniques.And the catalytic performance of the catalysts with different contents of Mn was also investigated in the oxidative carbonylation of phenol to DPC.The catalysts with different morphology mainly exposed different crystals,separately.The results showed that the oxygen vacancy was easier to produce in the CeO2 nanocubes which mainly exposed the?100?crystal than that in the CeO2 nanorods.Mn elements were doped and could enter into the lattice of CeO₂ by replacing the cerium disorderly,and Mn elements doped in the CeO₂ were conducive to the formation of oxygen vacancies.Moreover,the interaction between MnO_x and CeO₂was helpful for the releasing of active oxygen.The interaction between MnO_x and CeO₂ was much stronger for the nanocubes catatlysts.We concluded that the oxygen vacancy on the surface could enhance the adsorption of Pd species which prefers to be loaded on the O-bridge site?O2-site?.Therefore,the amount of two valence Pd²⁺in the Pd/Mn₃Ce₁O-cube could be increased to 85.19%.The yield of DPC in the oxidative carbonylation reaction reached 6.31%for the Pd/Mn₃Ce₁O-cube catalyst.2.The theoretical simulation was used to elucidate the effect of Mn dopants on the atom charge of MnO_x-CeO₂ nanoparticles.The net charge and the state density of the Ce,Mn,O element in CeO₂,?100?and?111?crystal plane of MnO_x-CeO₂,were optimized by CASTEP calculation,respectively.The DFT calculations obviously demonstrated that the interaction between MnO_x and CeO₂ in the?100?crystal plane was stronger than that in the?111?crystal plane.With Mn doping,the charge of lattice oxygen increased which was conducive to the migration of lattice oxygen.The lattice oxygen was easier to migrate to form the oxygen vacancy in the?100?crystal plane.3.A series of supported palladium catalysts have been prepared through the precipitation method and the reduction method,using polyvinyl alcohol?PVA?and polyvinylpyrrolidone?PVP?as dispersants.The effects of the dispersants on the catalyst structure were evaluated and the catalytic performance of the new materials was investigated in the oxidative carbonylation of phenol to diphenyl carbonate?DPC?.The catalysts were also characterized by the XRD,TEM,XPS,Brunner-Emmet-Teller?BET?measurements and techniques.The results showed that addition of the dispersants had no effect on the crystal phase of the catalysts.However,the dispersion of Pd particles improved when the dispersants are used.Moreover,the particle sizes of Pd nanoparticles modified by PVA were smaller than that modified by PVP.The highest yield of DPC was 17.9%with the PVA-Red catalyst.Analysis of the used catalysts showed that the Pd species in the PVA-Red catalyst remained mostly divalent.The Pd species remained mostly divalent in the used catalysts means that the catalysts prepared by the reduction method and modified by PVA could efficiently prevent the deactivation for oxidative carbonylation of phenol.