Hardwoods pretreatment

The effective pretreatments for enzymatic hydrolysis of cellulose in hardwoods and agricultural residues have been developed over the past 90 years and new or improved ones are still being developed. The initial impetus for pretreatment research was the expansion of a feed base for ruminants (i.e., cattle and sheep). The concentration of pretreatment research on animal feed preparation placed an additional burden on researchers, because the product must be not only fully convertible to animal live-weight, but also nontoxic and palatable. Early feeding trials of pretreated fine sawdust to cattle produced very dismal results (55). This may be the reason, besides the... 

The Purdue concepts have been applied to several different agricultural products, such as corn stalks, alfalfa, orchard grass, tall fescue, and sugarcane bagasse. No experiments have been reported on either hardwoods or softwoods. The processes have been explored in two major modes. In the first, the entire agricultural residue is treated with solvent in the second, a dilute acid pretreatment to remove hemicellulose precedes solvent treatment. The first process is especially desirable for making furfural or fermentation products from hemicellulose as a separate activity. Then, the hemicellulose-free raw material can be converted to substantially pure glucose. 

Vinzant, T. B., Ehrman, C. I., and Himmel, M. E., SSF of pretreated hardwoods Effect of native lignin content. Appl Biochem Biotechnol 1997, 62, 97-102. 

Schultz, T. R, Blermann, C. J., and McGinnis, G. D., Steam explosion of mixed hardwood chips as a biomass pretreatment. Industrial Engineering Chem Product Res Dev 1983, 22 (2), 344-348. 

Preparation of Hot-Melt and Self-Bonded Boards. Reaction conditions for benzylation are dependent on wood species and specimen size. For Akamatsu particles, pretreatments and reaction conditions are as follows wood is first oven-dried at 105°C for 24 h and then immersed in 40% sodium hydroxide solution for 1-2 h at room temperature. These particles are then squeezed to remove excess sodium hydroxide and reacted with benzyl chloride at 120°C for 1-2 h. Benzylated particles are then washed in water and any unreacted reagent is removed by squeezing. Particles are then washed further in a mixture of water-methanol (1 2 v/v) and air-dried for 48 h at 20 C. In general, benzylation of softwood species is more difficult than for hardwoods, and the reactivities of wood from fast-growing trees, i.e., willow or monocotyledons,... 

Another method of improving the dispersibility of cellulose fibers in a thermoplastic matrix is described in U.S. Pat. No. 4,414,267 [36], according to which cellulose fibers (hardwood kraft, from 1 to 40% by weight of the final composite) are pretreated by slurrying them in water, contacting them with an aqueous suspension of a mixture of a vinyl chloride polymer and a plasticizer, and drying the thus-treated fibers. 

Pew (26) found digestibility of hardwoods by cellulases improved markedly by pretreatment with aqueous NaOH the effect was considerably more pronounced with hardwoods than with softwoods. Stranks (35) reported that the in vitro rate of digestion of hardwoods by rumen-inhabiting bacteria, measured by succinic acid production, was markedly increased by pretreatment with 1-5% NaOH, whereas softwoods were unaffected by the treatment. The effect of NaOH in improving the digestibility of straw has long been known (5). 

Economically feasible processes for biomass conversion to ethanol requires the fermentation of the sugars generated in the pretreatment and hydrolysis steps. In agricultural residues and hardwoods, xylose constitutes about 45% of the total sugars, and therefore xylose conversion to ethanol is important for high yields. Dilute acid hydrolysis of cellulosic biomass generates inhibitory compounds such as furfural, hydroxymethyl furfural, and acetic acid [1], These inhibitors affect the ability of yeasts to ferment the hydrolyzates, and therefore a detoxification step is usually included in fermenting acid... 

Hardwood and switchgrass residues after a two-stage hydrolysis still contain significant amounts of cellulose (22-47%) and hemicellulose (9-11%). Hydrothermal treating fractions containing cellulose and/or hemicellulose resulted in a high yield of residue and gases. Our experiments reveal that a better pretreatment process needs to be developed. 

Of the freeze-drying methods tested, pretreatment with PEG 200 and 400 worked well with undeteriorated wood. Very highly deteriorated hardwoods responded best to pretreatment with a combination of higher molecular weights of PEG or PEG dissolved in solvents such as f-butyl alcohol... 

Fermentation of the lignoceliulosic hydrolyzates is more difficult than the well-established processes of ethanol production from e.g. molasses and starch. Hydrolyzates contain a broader range of inhibitory compounds, where the composition and the concentration of these compounds depend on the type of lignoceliulosic materials and the chemistry and nature of foe pretreatment and hydrolysis processes. Secondly, foe hydrolyzates of hemicelluloses contain not only hexoses but also pentoses, where xylose is foe dominant sugar in foe hydrolyzates from hardwood hemicelluloses. Therefore, foe fermenting microorganism should be able to produce efoanol from foe hydrolyzates with a... 

Grethlein HE, Allen DC, Converse AO. (2004). A comparative study of the enzymatic hydrolysis of acid-pretreated white pine and mixed hardwood. Biotechnol Bioeng, 26,1498-1505. 

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