Optimization of fuel ratio in solution combustion method for fabrication of nickel aluminate spinel used in the esterification reaction

In this study, the solution combustion method as a simple, fast, and cost-effective method was utilized for the fabrication of spinel nickel aluminate as a stable material to use in the esterification reaction. The effect of fuel amount (urea) as an important parameter of the solution combustion method on the structure, properties, and performance of the sample was evaluated. The results of characterization analyses revealed the highest crystallinity with the desired diffusion of nickel cations in alumina lattice was obtained for the sample prepared at a fuel ratio of 1.5. Moreover, a large pore size without any agglomerated particle was observed because of releasing a huge amount of gases and high reaction temperature formed during the combustion reaction. The sulfate groups were impregnated on the NiAl2O4 surface to increase the sample activity in the esterification reaction. The chelating bidentate structure can confirm suitable bonding of sulfate groups with the surface of NiAl2O4. Evaluating the nanocatalyst activity in the esterification reaction of oleic acid confirmed the high activity of SO4 2-/ NiAl2O4 nanocatalyst (94.2%) at the optimum condition of 120℃, 6 molar ratio of methanol/oleic acid, 3 wt.% of catalyst, and 3 h reaction time. In addition, stability assessment of nanocatalyst with and without post-treatment after each run exhibited that the porosity blocking and poisoning of the surface functional group were the major reasons for reducing the activity of the nanocatalyst. This activity was increased more than two times when the nanocatalyst was treated by washing and calcination (five cycles) after each use.