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

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

DOSY NMR: A Versatile Analytical Chromatographic Tool for Lignocellulosic Biomass Conversion

The diffusion ordered NMR spectroscopy (DOSY) protocol for the analysis of reaction mixture of lignocellulosic biomass conversion has been developed and investigated systematically. Model reaction mixtures from cellulose, hemicellulose and lignin conversion, real reaction mixtures of sucrose and glucose dehydration, were facilely separated and assigned in the diffusion dimension without any prior separation or isolation. The shift reagent, EuFOD, was successfully utilized to increase the difference in diffusion and thereby resolution in lignin degradation model. DOSY NMR offers an easy and robust method for the structure identification and reaction mixture separation in biomass conversion