Cellulose Hydrolysis in Acidified LiBr Molten Salt
Hydrate Media
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Abstract
We
screened nine acidified molten salt hydrates (solutions with
water to salt molar ratio equal or less than the coordination number
of the cation) as reaction media for cellulose hydrolysis, and we
found that cellulose can be efficiently hydrolyzed in LiBr acidified
MSH under mild conditions (>90% yield to water-soluble products
in
0.05 M H<sub>2</sub>SO<sub>4</sub> at 85 °C for 30 min). The
effect of various factors (temperature as well as acid and initial
cellulose concentrations) on the kinetics of hydrolysis reaction was
also investigated. At the lowest temperatures examined (70 and 85
°C) low amounts of degradation products have been observed, and
glucose appears to be in equilibrium with its dimers and possibly
other oligomers. Higher temperatures (100–115 °C) enhanced
the formation of degradation products (organic acids and humins).
Analysis of the kinetic data indicate that hydrolysis rates are first
order in cellulose and in H<sub>2</sub>SO<sub>4</sub> concentration,
and the initial hydrolysis rates have an apparent activation energy
∼123 kJ/mol. X-ray diffraction, SEM, and FTIR were also used
to study cellulose’s structural/morphological changes upon
treatment in the LiBr MSH media, in an attempt to understand the effects
of the cellulose–salt interaction. Analysis of the data indicates
that the enhancement of the hydrolysis rates can be attributed to
the enhancement of the acidity of reaction media through synergistic
effect of dilute acid and MSH, the breaking of crystalline structure
through swelling, and the interaction of the salt with cellulose chains
affecting the conformation and flexibility of the glycosidic bonds