Systematic study on the properties of nickel aluminate (NiAl2O4) as a catalytic precursor for aqueous phase hydrogenolysis of glycerol

Abstract

A systematic study for determining the maximum amount of nickel incorporated into the γ-Al2O3 structure to form NiAl2O4 was performed, and its behavior as a catalytic precursor for obtaining Ni/γ-Al2O3 catalysts for glycerol hydrogenolysis and cyclohexane conversion as a model reaction was investigated. The catalysts were prepared with 2, 10, 18, and 33 wt%Ni and characterized by N2 physisorption, TPR, XRD-Rietveld refinement, XPS, CO chemisorption, and SEM-EDS. NiAl2O4 was detected by XRD for all catalytic precursors, and NiO was only detected for the solid containing 33 wt%Ni. TPR analysis of catalytic precursors and catalysts showed that the insertion of Ni into the γ-Al2O3 structure is a reversible process. TPR and XRD results indicated that the maximum amount of Ni that can be incorporated into the NiAl2O4 structure is probably 20 wt%Ni. A tetragonal model with I41/amd space group symmetry adjusted to the description of NiAl2O4 structure in Rietveld refinement. XRD, CO chemisorption, and SEM images showed that exsolution of Ni from NiAl2O4 led to the formation of uniformly sized Ni0 particles for catalysts containing 2 and 10 wt%Ni. For 18 and 33 wt%Ni, SEM-EDS results showed larger Ni0 particles and the formation of gridshell type structures, with the alumina covering the Ni particles. XPS analysis showed that the nature of the Ni0 sites depended on if they are obtained entirely from Ni exsolution from NiAl2O4 or from exsolution combined with the reduction of excess NiO, which affected the catalytic activity for cyclohexane conversion. For glycerol hydrogenolysis, the activity decreased with the increase of Ni content. In this case, the formation of the gridshell could be the determining factor affecting the catalytic activity, especially in liquid-phase reactions.