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 utilizedfor 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 thestructure, properties, and performance of the sample was evaluated. The results of characterizationanalyses revealed the highest crystallinity with the desired diffusion of nickel cations in alumina latticewas obtained for the sample prepared at a fuel ratio of 1.5. Moreover, a large pore size without anyagglomerated particle was observed because of releasing a huge amount of gases and high reactiontemperature formed during the combustion reaction. The sulfate groups were impregnated on theNiAl2O4 surface to increase the sample activity in the esterification reaction. The chelating bidentatestructure can confirm suitable bonding of sulfate groups with the surface of NiAl2O4. Evaluating thenanocatalyst activity in the esterification reaction of oleic acid confirmed the high activity of SO42-/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 andwithout post-treatment after each run exhibited that the porosity blocking and poisoning of the surfacefunctional group were the major reasons for reducing the activity of the nanocatalyst. This activity wasincreased more than two times when the nanocatalyst was treated by washing and calcination (fivecycles) after each use.