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, H₂SO_4, 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.

Graphical Abstract

Keywords:

• HNbMoO₆
• superacid
• catalysis
• heterogeneous catalysis
• biodiesel

Acknowledgments

The authors would like to thank the support of the National Council for Scientific and Technological Development (CNPq), the National Council for the Improvement of Higher Education (CAPES), the Federal University of Itajubá (UNIFEI), the structural characterization laboratory of the Federal University of Itajubá (LCE), the Federal University of Paraná (UFPR), Laboratory of Advanced Materials (LAQMA – UFPR), Brazilian Metallurgy and Mining Company (CBMM) and the Chemical Mining Network of Minas Gerais for the research support (RQ-MG).

Disclosure statement

No potential conflict of interest was reported by the author(s).