Oxidative Carbonylation Of Phenol Over CeO₂-nanotube Supported Pd Catalyst

In recent years, the synthesis of diphenyl carbonate（DPC） has been paid more attention because it can be used as raw material in manufacturing of polycarbonate, which is one of the important engineering plastics and has extensive application in many fields. Oxidative carbonylation of phenol is a "green" route for DPC synthesis with advantages of low toxicity and cheap raw materials, which shows much development potential and achieves simple and non-polluting industrial process.Oxidative carbonylation of phenol is a multi-step electron transfer reaction and is normally catalyzed by Pd compounds with various co-catalysts. Ce compounds are effective cocatalysts. Ce O2 is one of the most significant rare earth oxides in industrial catalysis. The redox ion pair（Ce3+/Ce4+） of CeO₂ can achieve rapid reversible redox circulations,which has been studied and applied extensively in heterogeneous catalysis.In this paper, one-dimensional structure CeO₂ nanotubes（CeO₂-NT） were prepared through liquid phase deposition-hydrothermal method with hard templates of multi-walled carbon nanotubes, ethanol as dispersing agent, Ce（NO3）3·6H2O as precursor for Ce. Variety of characterization techniques were used to characterize samples: transmission electron microscopy（TEM）, selected area electron diffraction（SAED）, X-ray diffraction（XRD） and N2 adsorption-desorption techniques, temperature programmed reduction by hydrogen（H2-TPR）, temperature programmed desorption of carbon monoxide（CO-TPD）, Raman spectra and X-ray photoelectron spectroscopy（XPS）. The wall of CeO₂-NT was composed of small interconnected nanocrystallites ranging from 4 to 9 nm in sizes. The specific surface area of CeO₂-NT was 108.8 m2/g with an outer diameter of 25 nm and length > 300 nm. Compared with zero-dimensional CeO₂ particles（CeO₂-P）, which were synthesized through water-in-oil microemulsion method, CeO₂-NT had more surface oxygen vacancies, activated the molecular oxygen easily and produced more adsorbed reductive oxygen species.As Pd-supported catalysts, Pd-O/CeO₂-NT and Pd-O/CeO₂-P were prepared respectively using CeO₂-NT and CeO₂-P as support by in-situ method. Pd-O/CeO₂-NT was used as catalyst for oxidative carbonylation of phenol and it displayed better activity and DPC selectivity than Pd-O/CeO₂-P. According to characterization results, Pd-O/CeO₂-NT catalyst showed high phenol conversion because of increased surface oxygen species. Meanwhile, the highly dispersed Pd and CeO₂ were associated through a strong interaction. This is in favor of electron transferring from Pd（0） to Ce（IV）; therefore the active sites could be regenerated to Pd（II）, which can promote the synthesis of DPC and increase its selectivity. Reaction conditions were optimized for oxidative carbonylation of phenol over Pd-O/CeO₂-NT catalyst. When CO pressure was 6.6 MPa and catalyst loading was Pd/Phenol = 1/425（molar ratio）, phenol conversion was 67.7% with 93.3% DPC selectivity at 110 °C for 7 h. The reusability of Pd-O/CeO₂-NT catalyst is poor. The re-evaluation results of oxidative carbonylation of phenol to DPC showed that only 29.3% phenol was converted with 62.3% DPC selectivity. The deactivation resulted from the destruction of the tubular structure of Pd-O/CeO₂-NT during the reaction and the subsequent decrease of surface oxygen species. Moreover, Pd leaking was obvious and could also led to activity decreasing.