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Keywords:

  • diphenyl carbonate;
  • diphenyl oxalate;
  • methyl phenyl oxalate;
  • transesterification;
  • TiO2/SiO2;
  • monolayer;
  • acid sites;
  • disproportionation

Abstract

Physicochemical properties of silica-supported titanium oxide catalysts as well as their performances for transesterification of dimethyl oxalate (DMO) with phenol to methyl phenyl oxalate (MPO) and diphenyl oxalate (DPO) have been investigated systematically. Various wt % of TiO2 were loaded on SiO2 by a two-step wet impregnation method. The surface properties of TiO2/SiO2 catalysts were explored by various characterization techniques (BET, SEM, ICP, XPS, XRD, FTIR of pyridine adsorption, and NH3-TPD). Catalytic performances of TiO2/SiO2 catalysts were found to be strongly dependent on TiO2 dispersion and surface acidity. Monolayer dispersion capacity of TiO2 on silica was estimated to be about 4.0 TiO2 molecules per nm2 (SiO2) and no crystalline TiO2 was detected at TiO2 loading less than 12 wt %. FTIR and TPD analysis suggested that weak Lewis acid sites on the surface of TiO2/SiO2 were responsible for their unique selectivity to the target products, MPO and DPO. An optimization of reaction conditions for the transesterification of DMO with phenol was performed over 12 wt % TiO2/SiO2 calcined at 550°C. In addition, we studied the disproportionation reaction from MPO to DPO via a catalytic distillation process, which is highly efficient to promote formation of the desired DPO. © 2008 American Institute of Chemical Engineers AIChE J, 2008