Investigating the activity of zirconia as a catalyst and as a support for noble metals in organic oxidation reactions

Abstract

Alcohols and cyclic alkanes oxidation, in an environment friendly protocol, was carried out in a typical batch reactor. These reactions were carried out in solvent free conditions and/or in eco-friendly solvents using molecular oxygen as the only oxidant and ZrO2 and/or ZrO2 supported noble metals (Pt, Pd) as catalysts. The influence of different reaction parameters (speed of agitation, reaction time, and temperature), catalyst parameters (calcination temperature and loading), and oxygen partial pressure on the catalyst performance was studied. Different modern techniques such as (FT-IR, XRD, SEM, EDX, surface and pores size analyzer and particle size analyzer) were used for the characterization of catalyst. ZrO2 calcined at 1223 K was found to be more active as a single catalyst than the one calcined at 723 K for alcohol oxidation to the corresponding carbonyl products under solvent free conditions and in ecofriendly solvent as well. Platinum supported on zirconia was highly active and selective for oxidation of benzyl alcohol to benzaldehyde in n- heptane, and toluene to benzoic acid in both solvent free conditions and in aqueous medium. Similarly, zirconia supported Pt or Pd catalysts were tested for cyclohexane oxidation in solvent free conditions and for phenol oxidation in aqueous medium. Both catalysts have shown magnificent catalytic activity. Bismuth was added as a promoter to these catalysts. Bismuth promoted Pt/ZrO2 has shown outstanding catalytic performance. These catalysts are insoluble in the reaction mixture and can be easily separated by simple filtration and reused. Typical batch reactor’s kinetic data were obtained and fitted to the classical Langmuir–Hinshelwood, Mars–van Krevelen and as well as to the Eley-Rideal model of heterogeneously catalyzed reactions. In alcohol oxidation reactions the Langmuir-Hinshelwood model was found to give a better fit. The rate-determining step was proposed to involve direct interaction of an adsorbed oxidizing species with the adsorbed reactant or an intermediate product of the reactant. While in toluene oxidation the Eley-Rideal model was found to give a better fit. Eley-Rideal mechanism envisages reaction between adsorbed oxygen with hydrocarbon molecules from the fluid phase. The calculated apparent activation energy and agitation effect have shown the absence of mass transfer effect.