Characterization and reactivity of zirconia-doped phosphate ion catalyst prepared by sol–gel route and mechanistic study of acetic acid esterification by ethanol

Abstract

Phosphated zirconia prepared by sol–gel method has been used as catalyst in esterification reaction of acetic acid with ethanol. Optimization of different preparation parameters on the catalyst was studied, such as the effect of molar ratio n P/n Zr, surfactant assisted synthesis, calcination temperature, and the effect of the drying mode. Catalysts were characterized by N2 physisorption at −196 °C, X-ray diffraction, FTIR spectroscopy, and 31P MAS NMR spectroscopy. The obtained results show that an increase in phosphate content partially inhibits the development of tetragonal t-ZrP phase and leads to a rise of both specific area and pore size of the catalyst. Besides, the introduction of the surfactant in the preparation step develops this phase and enhances the size of pores, but decreases specific area. However, calcination of the catalyst allows the development of tetragonal ZrO2 phase and causes the disappearance of ZrP phase. The evacuation of the solvent at its supercritical conditions promotes the development of both tetragonal phase of zirconia and pore size but slows that of the phases related to the ZrP species. Catalytic properties of acid esterification by ethanol were correlated to catalyst characteristic data, suggesting that t-ZrP phase and doping agent-support interaction stabilize active sites of the catalyst. Kinetic and mechanism study shows that catalytic reaction occurs with a first order and takes place through Eley–Rideal mechanism in which adsorbed acetic acid species react with ethanol in fluid phase to form corresponding ester. Application of Eyring theory shows that the adsorption step is characterized by an endothermic character and a rapid associative mechanism occurs between adsorbed species and the second reactant. Eley–Rideal mechanism of acetic acid esterification by ethanol over xerogel catalyst XZrP0.05