Synthesis, characterization and application of supported nickel catalysts for the hydrogenation of octanal.
Three nickel based catalysts were prepared by the impregnation method (Ni/Al2O3 and Ni/SiO2) and co-precipitation method (Ni/ZnO). The catalysts were characterized by XRD, ICP-OES, BET-surface area and pore volume, SEM, TEM, TPR, NH3-TPD and in-situ XRD reduction. The catalytic activity of the catalysts in the liquid phase hydrogenation of octanal was studied at 110 °C and 50 bar. The effect of water as a co-feed on the catalytic activity of the catalysts was also investigated. Generally, all the catalysts were crystalline materials. The Ni/Al2O3 and Ni/ZnO catalysts contained NiO species that were “hard” to reduce, whereas the Ni/SiO2 catalyst was the easiest to reduce, according to the TPR and in-situ XRD reduction studies. The total acidity (μmol NH3/gcatal.) of the catalysts decreased in the following sequence: Ni/Al2O3 > Ni/ZnO > Ni/SiO2. The Ni/SiO2 and Ni/ZnO catalysts had intermediate and strong acidic sites, respectively, while the Ni/Al2O3 catalyst had weak-intermediate and strong acidic sites. The BET-surface area and pore volume of the catalysts decreased in the following order: Ni/Al2O3 > Ni/SiO2 > Ni/ZnO. The conversion of octanal for all the catalysts was ca. 90 %. The Ni/SiO2 and Ni/ZnO catalysts had octanol selectivities of over 99 % and the Ni/Al2O3 catalyst had 95 % octanol selectivity. The alumina support was observed to catalyze the formation of heavy products (C24 acetal, dioctyl ether and 2-hexyl-1-decanol). The water present in the feed poisoned the alumina sites that were responsible for the formation of heavy products thereby, making the catalyst more selective (> 99 %) to octanol. For the Ni/SiO2 catalyst the presence of water in the feed caused the octanal conversion to decrease with time-on-stream. The deactivation of the Ni/SiO2 catalyst, when water was used as a co-feed, was caused by the mechanical failure of the catalyst and also by the leaching of nickel metal during the reaction.