The significant effect of K+ counterion in the enhancement of physicochemical and structural properties of amorphous aluminosilicates xerogels used as catalysts in the transesterification of waste frying oils

Abstract

With the aim of better directing the polycondensation of oligomers during the synthesis of amorphous aluminosilicate xerogels by the sol-gel process using Na₂SiO_3, we reinforced the sols with K⁺ counterion and studied its effect on the physicochemical and structural properties of the xerogels using XRD, FTIR, N₂ physisorption, SEM, EDX, and TGA techniques as well as quantitative analysis of the acid sites by N-butylamine adsorption followed by potentiometric titration. Based on the results, the introduction of K⁺ generated xerogels with good porosity and conferred resistance to matrix cracking. The amorphous character of the xerogels did not change after adding K⁺. FTIR spectra showed that the composition of the xerogels was improved after the addition of K⁺. Cation exchange and sulfur removal were much more efficient in the presence of K⁺ which improved the acidity of the xerogels. The best percentages of Si and Al were observed at the Na⁺/K⁺ ratios of 4 and 1.The enhanced xerogel prepared with Na⁺/K⁺ ratio of 1 gave a biodiesel yield of 80.44% at the optimum parameters and was used for three reaction cycles without significant loss of biodiesel yield.