The production of aldehydes that are microbial inhibitors may occur when hexoses and pentoses
in an aqueous solution are exposed to temperatures above 150 °C under acidic conditions common
to acid-catalyzed lignocellulose biomass pretreatment. Concentrations greater than 0.1% of the
degradation product, furfural, strongly inhibit fermentation, as was confirmed for hydrolysate
that contained 0.5% (w/o) furfural. Methods of furfural removal that have been reported include
sulfite or alkali addition to achieve chemical reduction, ion exchange, hydrophobic adsorption,
and irreversible adsorption on activated carbon. This paper reports the removal of furfural from
biomass hydrolysate by a polymeric adsorbent, XAD-4, and desorption of the furfural to
regenerate the adsorbent using ethanol. Liquid chromatographic analysis showed that furfural
concentrations were less than 0.01 g/L compared to the initial concentrations that were in the
range of 1−5 g/L. Fermentation of the resulting biomass hydrolysate with recombinant
Escherichia coli ethanologenic strain K011 confirmed that the concentration of furfural in the
hydrolysate caused negligible inhibition. Fermentation of XAD-4-treated hydrolysate with E.
coli K011 was nearly as rapid as the control medium that was formulated with reagent-grade
sugars of the same concentration. Ethanol yields for both fermentations were 90% of theoretical.
Modeling of the adsorptive properties of this styrene-based adsorbent indicates that it is suitable
for on−off chromatography and could be useful in a continuous processing system for removing
small amounts of aldehydes that might otherwise inhibit fermentation.
The pretreatment of cellulose in corn fiber by liquid hot water at 160 degrees C and a pH above 4.0 dissolved 50% of the fiber in 20 min. The pretreatment also enabled the subsequent complete enzymatic hydrolysis of the remaining polysaccharides to monosaccharides. The carbohydrates dissolved by the pretreatment were 80% soluble oligosaccharides and 20% monosaccharides with <1% of the carbohydrates lost to degradation products. Only a minimal amount of protein was dissolved, thus enriching the protein content of the undissolved material. Replication of laboratory results in an industrial trial at 43 gallons per minute (163 L/min) of fiber slurry with a residence time of 20 min illustrates the utility and practicality of this approach for pretreating corn fiber. The added costs owing to pretreatment, fiber, and hydrolysis are equivalent to less than 0.84 dollars/gal of ethanol produced from the fiber. Minimizing monosaccharide formation during pretreatment minimized the formation of degradation products; hence, the resulting sugars were readily fermentable to ethanol by the recombinant hexose and by pentose-fermenting Saccharomyces cerevisiae 424A(LNH-ST) and ethanologenic Escherichia coli at yields >90% of theoretical based on the starting fiber. This cooperative effort and first successful trial opens the door for examining the robustness of the pretreatment system under extended run conditions as well as pretreatment of other cellulose-containing materials using water at controlled pH.
Pressure cooking of corn fiber in liquid water at 160 °C and a pH maintained at 4-7 produces an aqueous stream of dissolved glucans, xylans, proteins, phenolics, and minerals. We report hydrolysis of these oligosaccharides to glucose and xylose in a fixed-bed reactor packed with a macroreticular strong cation exchanger. The aqueous stream is first contacted with the cation exchanger at room temperature where proteins, phenolics, minerals, and other catalyst fouling components are removed. The material is then passed over a packed-bed of the same catalyst at 130 °C to give 88% hydrolysis for a space time of 105 min. Comparison of cation exchanger in a plug-flow versus a batch reactor for hydrolysis of oligosaccharides as well as for hydrolysis of the disaccharide cellobiose shows that yields at 110-160 °C are greatest for a plug-flow reactor. Maximum glucose yield increases as hydrolysis temperature increases and reaches 90% at 160 °C, which was the highest temperature tested in this study. A model of reactor performance based on first-order kinetics with diffusion resistance fit the data for cellobiose with an observed hydrolysis yield of 90% at a residence time of 3.5 min at 160 °C. A preliminary economic analysis shows 1 lb of catalyst that generates 1000 lb of glucose will give incremental costs of between $0.01 and $0.18/gal of ethanol, depending on catalyst cost. Further improvements in catalyst life and selectivity could result in an alternative or complimentary approach to enzyme hydrolysis for biomass pretreatment processes that generate water-soluble glucans and xylans from corn fiber and other cellulosic residues. Ultimately a sequential, continuous pretreatment and hydrolysis system is envisioned that has the added benefit of minimizing reactor volumes in large-scale cellulose to ethanol plants.
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