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

A systematic study for determining the maximum amount of nickel incorporated into the gamma-Al2O3 structure to form NiAl2O4 was performed, and its behavior as a catalytic precursor for obtaining Ni/gamma-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 gamma-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.

Automated summary

A systematic study was conducted to determine the maximum amount of nickel incorporated into the gamma-Al2O3 structure to form NiAl2O4. The behavior of this catalyst precursor for glycerol hydrogenolysis and cyclohexane conversion was investigated. Various characterization techniques were used to study the prepared catalysts with different nickel content. The results showed that Ni could be reversibly inserted into the gamma-Al2O3 structure, and the maximum amount of Ni incorporated into NiAl2O4 was approximately 20 wt%Ni. Different forms of Ni0 particles were observed depending on the Ni content, and their nature affected the catalytic activity. The formation of gridshell structures was found to be a determining factor for the catalytic activity in liquid-phase reactions.