Unraveling the Interactions of Homogenous Lewis Acid Catalyst (aqueous Tin(IV) Chloride) Species with Glucose By Means of Raman Spectroscopy

: Utilizing renewable resources, such as lignocellulosic biomass, for the production of fuels and chemicals has attracted significant interest in recent years. A characteristic example involves the conversion of cellulose to 5-Hydroxymethylfurfural (HMF) through a series of catalytic steps. A bottleneck in this reaction network is the conversion of glucose to fructose via isomerization reaction. Lewis acid catalysts, such as Sn-Beta, are essential for the selective isomerization of glucose to fructose. In this work, we utilize Raman spectroscopy to study the interactions between aqueous tin chloride species (homogeneous Lewis acid catalyst) with glucose in an effort to establish structure function-reactivity relationships for the glucose isomerization reaction. Specifically, spectroscopic characterization of [Sn(H₂O)₆]⁴⁺, [SnCl(H₂O)₅]³⁺, [SnCl₂(H₂O)₄]²⁺, [SnCl₃(H₂O)₃]⁺, [SnCl₄(H₂O)₂], & [Sn(OH)₆]^2- complexes in aqueous solutions was performed by deconvoluting pertinent spectral envelopes (50-250cm^-1, 290-350cm^-1 and 400-700cm^-1). The deconvolution of the binary Tin(IV) Chloride/water as well as the ternary Tin(IV) Chloride/glucose/water solutions has revealed significant changes in the vibrational modes of specific aqueous tin species. A sharp decrease in the 65 cm^-1 band of Tin(IV) Chloride with the addition of glucose in the aqueous solution, could possibly be attributed to the binding of asymmetric tin species with glucose molecules. To further characterize the complexation between glucose and aqueous tin species, isotopic exchange studies using deuterated water will be discussed. Additionally, to determine the exact symmetry behind these dissociated Tin(IV) Chloride species, identifying the respective depolarization ratio for each significant Raman band has also been achieved. We also couple Raman spectroscopic measurements with chemical equilibrium calculations using Visual MINTEQ in order to provide information about changes in the species equilibrium as well as potential interactions between tin species and glucose. For the first time, the full characterization of aqueous Tin(IV) Chloride from 50-700cm^-1 along with the specific vibrational modes for each Raman active band have been elucidated.