(190e) Spectroscopic Insights into Cellulose Hydrolysis Utilizing Molten Salt Hydrates As Reaction Media. Understanding Structural Changes at the Molecular Level

The diminishing availability of fossil resources as well as the need of production fuels and chemicals via alternative routes have led to the development of new, sustainable and environmental friendly technologies that utilize biomass as raw material. In this respect, non-edible lignocellulosic biomass is a promising renewable feedstock since it is abundant, does not directly compete with the food chain, and could lead to nearly carbon-free processes with concomitant reduction in CO₂ emissions. Development of methods for cellulose hydrolysis under mild conditions has been a topic of great interest, since they can be considered suitable for large-scale applications for the production of valuable sugar hexoses.

The utilization of molten salt hydrates as cellulose solvents/ modifiers is known to promote cellulose hydrolysis under mild conditions (low acid concentration / temperature). In this respect, the present study focuses on the investigation of the effect of molten salts (LiBr, LiCl, ZnBr₂ and ZnCl₂) on cellulose structure as well as on the reactivity for the hydrolysis reaction. Our catalytic results revealed that complete cellulose hydrolysis could efficiently take place at temperatures as low as 85°C and low acid concentration (0.05M H₂SO₄) with glucose yields ~85% by using LiBr molten hydrate as reaction media.

We pay special attention to the role of the nature as well as the concentration of each salt on the structure of cellulose by means of vibrational spectroscopies (ex-situ and in-situ ATR/FTIR, and Raman). Careful analysis of pertinent wavenumber regions allowed us to elucidate the effect of salts on the inter/intra-molecular hydrogen bond network, on the strength as well as the conformation of the glycosidic bond. It became clear that the treatment of cellulose, even at room temperature, with molten salt hydrates is effective in breaking the inter- and intra-molecular hydrogen bond network within cellulose chains explaining the enhanced catalytic activity during cellulose hydrolysis. The vibrational spectra also indicate an interaction of the salt hydrates with cellulose around C₆ of each anhydroglucose units. Changes in specific vibrational motions with time under realistic reaction conditions, monitored by in-situ ATR-FTIR spectroscopy, revealed the possibility of two different catalytic steps taking place for cellulose hydrolysis.