Hemicellulose from Biodelignified Wood: A Feedstock for Renewable

Chapter 16

Hemicellulose from Biodelignified Wood: A Feedstock for Renewable Materials and Chemicals

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Arthur J. Stipanovic , Thomas E. Amidon , Gary M. Scott , Vincent Barber , and Misty K. Blowers 2

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Faculties of Chemistry and Paper Science and Engineering, State University of New York-College of Environmental Science and Forestry (SUNY-ESF), Syracuse, NY 13210 Air Force Research Laboratory, Information Directorate, Rome, NY 13440

The hemicellulose fraction of woody biomass, typically 2035% of the dry weight of wood, is currently an underutilized renewable resource potentially useful for biobased fuels, chemicals and polymeric materials. In this study, a fungal biodelignification pretreatment stage was successfully employed to enhance the accessibility of fast growing willow wood chips to hemicellulose extraction using water at 140160°C. Films of hardwood xylan hemicellulose blended with commercially available cellulose esters were prepared from solution in an initial effort to generate new, biodegradable hemicellulose-based materials.

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© 2006 American Chemical Society

In Feedstocks for the Future; Bozell, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Introduction st

The 21 century is envisioned to become the "age of biology" as sustainable biomass resources replace non-renewable petroleum in energy and industrial product applications (1,2). Motivated by concerns over national energy security, global C 0 reduction, an evolving need for biodegradable products, and enhanced rural economic development, the engineering and construction of "biorefineries" is now a critical national priority. The vision of a "wood-based" biorefinery becoming a commercial reality will require the development of new core competencies: Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: January 12, 2006 | doi: 10.1021/bk-2006-0921.ch016

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• Development of fast-growing woody species that can be used year round as the biorefinery feedstock. These feedstocks should enjoy "life cycle" energy benefits compared to alternative biomass resources such as agricultural crops or residues. • Efficient pretreatment and separation scenarios for woody biomass that provide at least three major process streams for the biorefinery: cellulose, hemicellulose and lignin. • Creation of a value-added portfolio of fuels, chemicals and materials from each of the three biorefinery process streams that mimic the diverse product slate characteristic of today's petroleum refineries. Although cellulose fiber is profitably exploited by the paper industry, the hemicellulose and lignin components of wood are generally underutilized. In the present study, attention was focused on the ability of a fungal biodelignification pretreatment stage to enhance the accessibility of wood to hemicellulose extraction and the use of the resulting hemicellulose as a component in biodegradable polymer blends. More specifically, a fast growing species of shrub willow (Salix sp.) was selected as a feedstock based on its proven potential as a renewable energy crop grown in an agro-forestry environment (3) and the fact that willow is a hardwood relatively rich in hemicellulose, primarily glucuronoxylan (xylan; « 22 wt% on a dry wood basis).

Biodelignification Lignin-degrading fungi have been used for several applications within the paper industry. Biopulping is probably the most studied application in which the fungi are used as a pretreatment to the pulping of wood or as a pulping method itself. These fungi have also been used for the treatment of pulp for both pulping and bleaching applications with some success. The use of white-rot fungi for the biological delignification of wood was first seriously considered by Lawson and Still (4) at the West Virginia Pulp and Paper Company research laboratory (now



212 Westvaco Corporation). Subsequent work showed that paper strength properties increased with the extent of natural degradation of pine by white-rot fungi (5,6). Related work was done at a Swedish research laboratory (STFI) in Stockholm, and the first published report on biopulping demonstrated that fungal treatment could result in significant energy savings for mechanical pulping (7). Considerable efforts by the Swedish group were directed toward developing cellulase-less mutants of selected white-rot fiingi to improve the selectivity of lignin degradation and, thus, the specificity of biopulping (8-10). Samuelsson and co-workers (77) applied the white-rot fungus Phlebia radiata and its cellulase-less mutant to chips and pulp. Eriksson and Vallander (72) treated wood chips (with or without added glucose) with white-rot fungi, in most cases a cellulase-less mutant of Phanerochaete chrysosporium. Pearce and co-workers (75) screened 204 isolates of wild-type wood decay fungi for biomechanical pulping of eucalyptus chips. Some of these fungi saved 40 to 50% electrical energy in refining and resulted in greater brightness of unbleached pulp as compared with that of pulps from untreated control chips. Some of the strains were found to be effective on unsterilized wood chips. Other details on biopulping research have been described in review articles and the literature cited therein (14,15). Recent work has shown that the white-rot fungus Ceriporiopsis subvermispora is very effective for both mechanical (16) and chemical pulping (77). Scott (18) demonstrated the feasibility of fungally treating wood chips on a semi-commercial scale.

Hemicellulose Composition, Extraction and Utilization Significant differences exist in the hemicellulose composition of hardwoods and softwoods. Typically, softwoods contain galactoglucomannan (15-20% of dry wood) and arabinoglucuronoxylan (5-10%) while hardwoods contain very little glucomannan (< 4%) and a preponderance of glucuronoxylan (20-35 wt%; 19). Hemicelluloses found in other plants such as grasses and cereal grain stalks may approach 50 wt % in some tissues (20) but the hemicellulose fraction typically contains two or more polymers. A n idealized backbone molecular structure for hardwood xylan is shown in Figure 1. In nature, this polymer contains one 4-O-methyl-glucuronic acid side chain per 4-16 backbone xylose residues (21,22) while acetyl groups are found on 3.5-7.0 repeat units per 10 xylose residues (22,23). For hardwoods, the terms xylan and glucuronoxylan are used interchangeably, while "xylans" from other sources may contain arabinofuranose-, glucurono(arabino)-, and rhamno- sugars as side chains.


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OH

HO' "^0

OH L

HO

OH OH J η


Figure 1: Idealized backbone structure of xylan (pyranose ring hydrogen atoms omitted for clarity). The acetyl groups on xylan (not shown in Figure 1) are critically important in maintaining the water solubility of the native polymer since they block potential sites for hydrogen bonding. As a result, crystallization is inhibited. If they are removed by base-catalyzed hydrolysis (or acid hydrolysis) xylan becomes insoluble in water and can only be re-dissolved in alkali or other more polar organic solvents. Despite the huge potential volume of xylan and other hemicelluloses available for commercial development, very few products currently exist which exploit this renewable resource. At least two reasons for this under-utilization are commonly cited: (1) most biological sources of hemicellulose and xylan produce a number of different heteropolysaccharides and the concentration of any single polymer is usually quite low [

Hemicellulose from Biodelignified Wood: A Feedstock for Renewable

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