Oxidative Carbonylation Of Phenol Over Pd Catalyst Supported On Single Crystal CeO₂-nanotube

Diphenyl carbonate?DPC?is an important chemical intermediate,which can be used to synthesize many organic compounds and polymer materials?such as polycarbonate,PC?and so on.In recent years,the synthesis of diphenyl carbonate has attracted widespread concern because the DPC demand increased significantly.Oxidation carbonylation of phenol to DPC is a simple and environmentally-friendly process,with water being the only by-product in theory and avoiding the use of highly toxic phosgene.Pd and its compounds are the main catalysts for the oxidative carbonylation of phenol.Meanwhile,various co-catalysts are added to the system to promote the reoxidation of Pd⁰.Ce compound is an effective additive.In the preliminary study,CeO₂ polycrystalline nanotubes supported Pd catalyst was prepared for the oxidative carbonylation of phenol,which showed good catalytic performance.However,the stability of the catalyst is poor,the tubular structure collapses after the reaction,and its catalytic performance is also greatly reduced.In order to solve the problem of collapse of CeO₂ nanotubes,single crystal CeO₂nanotubes?CeO2-NTs?with regular structure were prepared by hydrothermal method.And Pd-O/CeO₂-NTs catalyst was prepared using CeO₂-NTs as support to catalyze the oxidation carbonylation of phenol.Firstly,CeO₂ nanotubes were prepared by hydrothermal method with CeCl₃·7H₂O as cerium source,and P123 as template.When initial cerium concentration was 0.125 mol/L and P123 dosage was P123/Ce=1/5?molar ratio?,regular CeO₂ nanotubes were prepared at 160?for 72 h,which the tube length was 1³?m and the diameter was<100 nm.Secondly,single crystal CeO₂-nanotubes supported Pd catalyst Pd-O/CeO₂-NTs were prepared by in-situ hydrothermal method,with an outer diameter of 30⁶0 nm,an inner diameter of about 15 nm and a length of 1³?m.The characterization results found that the oxygen vacancy in CeO2 increases and the reduction temperature of surface oxygen decreases when Pd is supported on CeO₂ nanotubes.Thirdly,the Pd-O/CeO₂-NTs catalyst was used for the oxidative carbonylation of phenol.When reaction time was 8 h,reaction temperature was 100?,catalyst loading was Pd/Phenol=1/425?molar ratio?and CO pressure was 6.6 MPa,phenol conversion was 53.2%and the selectivity of DPC was 96.7%.Finally,the stability of catalyst was investigated.It was found that the activity and selectivity of catalyst decreased significantly,after being reclaimed and reused for phenol oxidative carbonylation.The results showed that the tubular structure in the catalyst still exists,but the content of active component Pd is obviously reduced and the loss of Pd is serious,which is the main reason for catalyst deactivation.In addition,the reduction of oxidation state Pd in Pd-O/CeO₂-NTs catalyst to Pd?0?during the reaction is another important reason for catalyst deactivation.