A Comparative Study of NiO/Al2O3 Catalysts Prepared by Different Combustion Techniques for Octanal Hydrogenation

Abstract

Four catalysts with the molar ratio of Ni:Al (1:2) were prepared by solution combustion synthesis (SCS) and sol–gel auto combustion synthesis (SGCS) techniques. Two fuels [urea and oxalyldihydrazide (ODH)] with distinct oxygen/carbon (O/C) ratios and chelating capability were chosen for investigating the effect of the fuel on the syntheses of these catalysts. All catalysts were fully characterized by PXRD, ICP-OES, TEM, H2-TPR, pyridine IR, H2-chemisorption and nitrogen physisorption. Pure NiO/Al2O3 (free of spinel) was achieved by application of SGCS and urea as a fuel. Thus, catalysts prepared by urea as a fuel showed high activity for hydrogenation of octanal even under moderate conditions (110 °C). Optimisation showed that the catalyst prepared via SCS using ODH as a fuel gave the best comparative aldehyde conversion and alcohol selectivity (98 and 97 %, respectively) albeit at a higher temperature of 150 °C. The catalysts with NiAl2O4 as the dominant phase presented the lowest acidity, likely due to the non-availability of acidic sites of alumina via formation of the spinel, thus making these catalysts highly selective and also offering a slow release of fresh Ni sites.