Hydrogenation of aliphatic carboxylic acids to corresponding aldehydes over Cr 2O 3-based catalysts

Abstract

Direct hydrogenation of aliphatic carboxylic acids to the corresponding aldehydes catalyzed by Cr–ZrO 2 and supported Cr 2O 3 catalysts were investigated. A variety of Cr–ZrO 2 catalysts were prepared by doping of Cr 2O 3 into ZrO 2, which itself catalyzes the intermolecular ketonization of aliphatic carboxylic acids. This Cr–ZrO 2 catalyst exhibited higher catalytic performance toward the hydrogenation of caprylic acid than pure Cr 2O 3, having maxima both of the catalyst activity and of specific surface area of the catalyst at Cr:Zr = 15:100 in atomic ratio. It is estimated that high dispersion of chromia on the Cr–ZrO 2 catalyst surface will improve the catalyst performance. Furthermore, the α-Al 2O 3-supported Cr 2O 3 catalyst showed over 90% aldehyde selectivity in the hydrogenation of stearic acid. However, employment of other supports such as γ-Al 2O 3, SiO 2 and TiO 2 to Cr 2O 3 resulted in poor aldehyde selectivity and also encouraged undesirable ketonization to give predominantly C 35-ketone species. Pre-treatment of the γ-Al 2O 3 support at or above 1080 °C, enabled the Cr 2O 3 (10 wt.%)/γ-Al 2O 3 catalyst to catalyze the hydrogenation over 90% selectivity. The novel catalyst of 10 wt.% Cr 2O 3 supported by commercial α-Al 2O 3 was found to hydrogenate various aliphatic carboxylic acids as well as benzoic acid and methyl nicotinate in moderate to high conversion and in high aldehyde selectivity. The effects of catalyst supports and their thermal pre-treatment in promoting the hydrogenation are also discussed.