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

Various NMR techniques were employed to study the catalytic performance of the Lewis acid SnCl4 and the Bronsted 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 H-1 and C-13 NMR at 25 degrees C. The dehydration reaction of inulin with SnCl4 as catalyst was followed by high temperature in situ H-1 NMR at 80 degrees 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 Bronsted acid.