Catalytic activity in methyl esterification reactions and characterization of the superacid HNbMoO6 treated with different inorganic acids

Abstract

The superacid HNbMoO6 was synthesized from the treatment of the precursor LiNbMoO6 with different acids (HNO3, H2SO4, or H3PO4) in the concentrations of 2, 4, and 6 mol L−1. The solids obtained were characterized to elucidate their composition, acidity, and properties. The materials presented similar diffraction patterns and spectra regardless of the acid type and concentration used. The microscopy analysis showed that at lower concentrations of acid, lower crystal agglomeration was observed. The results of the energy-dispersive X-ray spectroscopy indicated that HNbMoO6 synthesized with H2SO4 6 mol L−1 had the highest Nb5+/Mo6+ ratio (1.95), besides the highest surface area among the synthesized solids (8.53 m2 g−1), and highest Lewis acidity (1.5 mmol of n-butylamine). All solids were efficient in the methyl esterification of lauric acid and did not undergo structural changes after the reactions. The best result was obtained with HNbMoO6 treated with H2SO4 6 mol L−1, with a high conversion rate of 81.79% in methyl laurate, an expressive catalytic gain of 36.82 p.p in contrast to the thermal conversion.