Synthesis and characterization of a new acid molten salt and the study of its thermal behavior and catalytic activity in Fischer esterification

Abstract

A new acid molten salt was prepared and its structure elucidation was conducted by FTIR, 1D NMR, 2D NMR, and mass spectrometry. Further support to elucidate the chemical structure of the 1H,4H-piperazine-N,N′-diium ring of the new acid molten salt was achieved by ¹H and ¹³C NMR, and COSY analyses of 1H,4H-piperazine-N,N′-diium dibromide, which is synthesized and characterized for the first time in the current work. The analysis of FTIR and NMR spectra as well as pH and titrimetric analysis excluded the formation of [SO₄]²⁻ and the presence of an excess of H₂SO₄. Moreover, no distinguishing peak was detected for the acid proton of [HSO₄]⁻ in DMSO-d₆. The thermal phase transition and thermal stability of the acid molten salt were also recorded, which approved the strong interaction between a dication and hydrogen sulfate anions. According to the acidity of the new molten salt, we encourage the study of its catalytic activity for the acetylation of n-pentanol using glacial acetic acid. Pentyl acetate was obtained in 89.0% conversion and 78.0% isolated yield. The ¹H NMR spectrum of the residue showed an excess of HOAc together with molten salt, whereas the ¹H NMR spectrum of the upper phase exhibited pure pentyl acetate. After separation of the upper phase, the residue was concentrated and used in the next run without further purification. No significant changes in the chemical structure and catalytic activity of the new molten salt were observed even after the 5th run. Two chiral alcohols, including (−)-menthol and (+)-borneol, as well as α-tocopherol (α-TCP) were also acetylated with acetic acid in the presence of the new acid molten salt under optimized reaction conditions, which afforded the desired acetates in high yields.