Chapter 20
Chemical Functionalization of Xylan: A Short Review 1
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Thomas Heinze , Andreas Koschella , and Anna E b r i n g e r o v á
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Department of Chemistry (FB9), Bergische University of Wuppertal, Gauss Strasse 20, D-42097 Wuppertal, Germany Institute of Chemistry, Slovak Academy of Sciences, Dubravska cesta 9, Bratislava, SK-842 38, Slovakia
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Typical functionalization reactions of xylan are reviewed. Moreover, comments about the analytical characterization of the xylan derivatives are given and some structure-property relationships are discussed.
The hemicellulose xylan belongs to the most abundant biopolymers present in wood and other plants such as grasses, cereals, and herbs. The xylan structure is rather complex. Depending on the natural source of the xylan, varying contents of glucose, xylose, mannose, galactose, arabinose, fucose, glucuronic acid, and galacturonic acid can be found. The backbone consists mainly of P~ l - » 4 - l i n k e d xylose units (7). Recently, xylan gains increasing importance as basis for new biopolymeric materials and functional polymers by chemical modification reactions. Moreover, agricultural wastes like cobs, blades, leaves are always available and interesting raw-products. In the present article, important functionalization methods for the preparation of xylan derivatives are discussed. In addition, examples for the structure analysis and applications are given.
Xylan esters Carboxylic acid esters of xylan are prepared under typical conditions used for polysaccharide esterification, i.e., activated carboxylic acid derivatives are allowed to react with the polymer both heterogeneously and homogeneously (Figure 1). The heterogeneous esterification of oakwood sawdust and wheat bran
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© 2004 American Chemical Society
In Hemicelluloses: Science and Technology; Gatenholm, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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313 hemicelluloses (extracted with alkaline media) with an excess of octanoyl chloride under different conditions including the use of bases and co-solvents was described (2). The conversion was characterized by the weight increase of the products and the ester content by a saponification method. A solid esterified material and a liquid fraction were isolated after a solvent-free reaction. The liquid fraction was separated into an insoluble part (acetylated cellulose) and soluble hemicellulose and lignin. Moreover, the values of the degree of substitution (DS) of the products after fractionation steps showed that products of high DS are soluble in the reaction medium and, therefore, DS values of solubilized parts are higher compared to the non-dissolved material. The conversion of hemicellulose and wood is accompanied by polymer degradation induced by acidic hydrolysis due to the HC1. The amount of liquefied fraction is diminished in the presence of pyridine.
ONa
Xyl—OH
—
Xyl—OS0 Na 3
n
R= Alkyl group
Xy|= — " ^ V ^ — \ Sulfating agent= H>S0 HOA-^^\—0-.. SQj
4
OH
Figure
CISQH
1. Introduction of ester groups into xylan.
The homogeneous acylation of the hemicellulose is mostly carried out in the solvent A^A^-dimethylformamide (DMF) in combination with LiCl. Hemicellulose extracted from poplar wood chips was acylated with various carboxylic acid chlorides in D M F / L i C l applying triethylamine and 4-N,Ndimethylaminopyridine (DMAP) as base and catalyst (3, Table 1). Depending on
In Hemicelluloses: Science and Technology; Gatenholm, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
314 the molar ratio of acyl chloride and anhydroxylose unit, DS values in the range from 0.32 to 1.51 were obtained under moderate reaction conditions (temperature < 7 5 ° C , time up to 45 min).
Table 1: Conditions and degree of substitution (DS) of esterified hemicellulose.
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Molar ratio" Acetyl Butyryl Octanoyl Decanoyl Decanoyl Stearoyl Stearoyl Stearoyl Oleoyl Oleoyl
Product
Conditions
Carboxylic acid chloride
3:1 3:1 3:1 2:1 3:1 2:1 3:1 3:1 2:1 3:1
Temp. rc) 45 75 75 65 75 65 75 75 65 75
Time (min) 30 35 40 30 40 30 40 45 30 40
TEA"
DS
4
104.08 104.08
102.50 102.50
73.39 74.02
73.39 73.64
84.39 74.62
68.61 77.29
65.78 63.80
104.05 104.05 203.73
102.48 102.48
73.56 74.15
73.56 74.15
84.28 74.54
68.51 77.22
65.75 63.80
l->3
Xylan 3-O-substituted 4-O-substituted
Br oxidized derivative 2
3-O-substituted 4-O-substituted CO
In Hemicelluloses: Science and Technology; Gatenholm, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
324 per repeating unit suitable for chemical functionalization and supramolecular structure formation make the polymers available by functionalization reactions an interesting class of polymeric materials whose attractiveness is expected to rise in the future.
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