The behavior and derivatizations of carbohydrates in hydrogen fluoride

Abstract

SynopsisLow molecular weight /j, 1 + 4-glucans (cellodextrins) are favorably prepared by cleavage of cellulose in liquid hydrogen fluoride at temperatures between -15 and -30° C. This is due to the suppression of the reversion reaction in that low temperature range. Under conditions favorable for reversion the presence of water leads to a competing hydrolysis and lowers the average degree of polycondensation. For the preparation of defined 0-glycosides from glucose and alcohols, hydrogen fluoride is not'suitable as a reaction medium. Under reversion conditions, monohydroxy compounds are inferior in their reactivity to the competing carbohydrate molecules, and polyols like sorbitol furnish mixtures of isomeric glycosides. Gaseous hydrogen fluoride represents a highly suitable agent for the degradation of carbohydrate and lignin containing biomass, such as waste wood, for the purpose of providing fermentation raw material. As a model, lignocellulose was studied and the heat of reaction of the hydrogen fluoride sorption and desorption processes were examined. The practically important desorption value was found to be approximately 870 kJ/kg HF:

Gravimetric analysis of hydrogen fluoride vapor absorption by bigtooth aspen wood

“…As would be expected, the heat of vaporization gets closer to the heat of vaporization for pure HF as the HF loading increases. The AHv's calculated here tend to be 2-3 times larger than those reported by Franz et al (1987) using the same type of analysis of isotherm data for prehydrolyzed wood. Some, but probably not all, of the differences in AHV between the two studies could be because the lignocellulose in each was slightly different.…”

“…Recently, HF desorption isotherms for prehydrolyzed (hemicellulose removed) wood appeared in the literature (Franz et al, 1987). For these experiments, the weight of a reactor filled with lignocellulose was measured.…”

“…From the absorption isotherms, it is possible to calculate the heat of vaporization of HF from aspen wood at various levels of HF loading using the Clausius-Clapeyron equation. This approach was originally suggested by Franz et al (1987). These heats of vaporization are expected to be useful in process design studies for calculating heat loads and also for studying the effects of heat transfer on the kinetics of the HF saccharification process.…”