NMR Study of the Hydrolysis and Dehydration of Inulin in Water: Comparison of the Catalytic Effect of Lewis Acid SnCl₄ and Brønsted Acid HCl

Abstract

Various NMR techniques were employed to study the catalytic performance of the Lewis acid SnCl₄ and the Brønsted acid HCl in the conversion of inulin to value-added compounds by hydrolysis and subsequent dehydration. The hydrolysis of inulin was examined to reveal the catalytic abilities of SnCl₄ besides its intrinsic acidity by in situ ¹H and ¹³C NMR at 25 °C. The dehydration reaction of inulin with SnCl₄ as catalyst was followed by high temperature in situ ¹H NMR at 80 °C. The fructose moieties were dehydrated to 5-(hydroxy­methly)­furfural (5-HMF), but the glucose fragment of inulin was inactive for dehydration reaction under this condition. The formation of 5-HMF and its transformation into formic acid and levulinic acid through a rehydration reaction could be monitored by in situ NMR spectroscopy. Moreover, diffusion ordered spectroscopy NMR revealed that the Lewis acid ion, Sn⁴⁺ interacts with the inulin model compounds, i.e., sucrose and fructose. The synergistic effects of complexation and acidity from the hydrolysis of SnCl₄ results in a higher catalytic ability of this Lewis acid catalyst compared with a Brønsted acid