Synthesis Of Methyl Propionate From N-propyl Alcohol Using Gold Catalysts

In the early years, gold was considered as an inert metal which is impossible to be used in the catalytic reactions. However, in the 1980s, Haruta discovered that gold nanoparticles could be active for low temperature aerobic oxidation of carbon monoxide. Since then, gold catalyst had been studied and used extensively in many environmental and chemical fields, such as the catalytic abatement of toxic gas (NOx, CO), the combustion of hydrocarbon, the selective addition of unsaturated hydrocarbon, and the decomposing of chloric and fluoric hydrocarbon. Nowadays, gold catalyst also exhibits favorable activity in the oxidation of alcohols.In this paper, gold nanoparticles supported on various supports were prepared using deposition-precipitation (DP) method with urea/NaOH as precipitate and were characterized by ICP, XRD, TEM, H₂-TPR, UV-vis, NH₃-TPD and pyridine adsorbed FT-IR spectroscopy techniques. The catalysts were used in the oxidation of n-propyl alcohol to synthesize methyl propionate. The effects of preparing methods, supports, calcinating temperature and gold loading on the performance of the catalysts were studied. The results showed that Au/TiO₂ catalyst prepared by DP-urea method exhibited high gold loading and small gold particles with a uniform distribution. The 5% Au/TiO₂ calcined at 673 K exhibited excellent catalytic performance in the synthesis of methyl propionate.The effect of base additive, catalyst weight, n-propyl alcohol/methanol molar ratio, oxygen pressure, reaction temperature and reaction time were studied. The addition of small amount of base dramatically improved the n-propyl alcohol conversion and slowed down the deactivation rate of AU/TiO₂. On optimum conditions, the maximum n-propyl alcohol conversion was 63% and the selectivity of methyl propionate could reach 99% with the only by-product propyl propionate. Au/TiO₂ could be reused for six cycles without significant deactivation in the presence of base additive.