TiO2: As a versatile catalyst for the ortho-selective methylation of phenol

Abstract

The investigation describes synthesis of active pure and mixed phase TiO2 catalysts by a TiCl3-urea-oxalic acid precursor method. Pure and mixed phase TiO2 catalysts could be obtained by varying the concentration of oxalic acid during synthesis. A minimum of 25 mol percent of oxalic acid during synthesis was found to bring about complete phase transition from rutile to anatase. The catalysts were characterized by XRD, BET surface area measurements, pore-size analysis, TPD studies with NH3 and CO2 as probe molecules for acidity and basicity. In situ FTIR studies using pyridine as a probe molecule for determining the type of acid sites.The various TiO2 catalysts were evaluated for the methylation of phenol reaction. While all the samples were near 100 percent ortho-selective, they showed a unique activity profile as the concentration of oxalic acid was increased during the synthesis. The observed yield and product selectivity is correlated with the surface properties of the catalysts. The overall catalytic activity did not show particular phase dependence. Strong Lewis acid-weak Lewis base pairs were identified as the active centers for ortho-selectivity. The catalytic activity was primarily governed by the concentration of effective strong Lewis acid-weak Lewis base pairs. The selectivity between the ortho-products o-cresol and 2,6-xylenol is greatly influenced by the pore-size distribution.