NMR Insights into the Unexpected Interaction of SnCl4withd-Glucosamine and Its Effect on 5-HMF Preparation in ZnCl2Molten Salt Hydrate Medium

Abstract

SnCl4, a versatile Lewis acid catalyst in the biomass conversion, plays a positive role for the cellulose biomass conversion but negative for chitin biomass conversion as realized in our previous research. In this report, the unexpected effects of SnCl4 on the conversion of d-glucosamine (GlcNH2) to 5-hydroxymethylfurfural (5-HMF) in ZnCl2 molten salt hydrate medium were investigated. The strong interaction between SnCl4 and GlcNH2 in ZnCl2 molten salt hydrate medium was examined by nuclear magnetic resonance (NMR) techniques. Additionally the d-glucose and N-acetyl-d-glucosamine were also utilized in NMR titration experiments to reveal the complexation of amino group with Sn4+ ions during the catalytic process. The observed drifts of chemical shifts and signal broadening of the C2 position of GlcNH2 in 13C NMR spectra indicates that the amino group in GlcNH2 is probably the Sn4+ ion complexation site. The correlations in the 1H-15N HSQC NMR spectra provide the direct evidence and further confirm that the amino group is the complexation site between SnCl4 and GlcNH2. This very specific complex between the amino group and the Sn4+ ion inhibit the conversion of GlcNH2 to 5-HMF in ZnCl2 molten salt hydrate medium.