Synthesis and characterization of methyl xylan

Abstract

The synthesis of methyl xylan using methyl chloride and methyl iodide as etherifying agent under varying reaction conditions was studied. The reaction of xylan with an excess of methyl chloride under pressure in the presence of 40% aqueous NaOH led to a methyl xylan with a degree of substitution (DS) of 0.94. The conversion of xylan with methyl iodide yielded DS values of about 0.5 independent of the molar ratio of methyl iodide to anhydroxylose unit homogeneously in 25% aqueous NaOH or after addition of acetone leading to a heterogeneous slurry. The DS and the distribution of the substituents were determined by means of one and two-dimensional NMR spectroscopy.

Introduction

The hemicellulose xylan is most abundant in hardwoods and other plants such as grasses, cereals, and herbs. The biopolymer backbone comprises mainly of β-1 → 4-linked xylose units (Ebringerova & Heinze, 2000). Recently, xylans as one of the most important renewable resources gain increasing importance as basis for new biopolymeric materials and functional polymers accessible by chemical modification reactions (Heinze et al., 2004, Lindblad and Albertsson, 2004). Due to the functional properties of xylans various application fields are considered to be of interest. Important possible applications of xylan and its derivatives are drug carrier, wound dressing materials, or additives in papermaking (Ebringerova, 2006). A problem that limits the use of xylans is their usually bad solubility in water or aqueous systems at neutral pH value. To get soluble polysaccharide derivatives, etherification in particular methylation, hydroxyalkylation, and carboxymethylation of the hydroxyl groups is a suitable path (Petzold, Schwikal, Günther and Heinze, 2006).

Etherification of hemicelluloses was performed under heterogeneous and homogeneous conditions as reviewed by Lindblad and Albertsson (2004). In particular methylation seems to be an appropriate functionalization to get water-soluble products. Up to now, the methylation of xylan was used both to analyze the non-reducing end groups and to get information about the branching of hemicelluloses already in the 1960s (Lindblad and Albertsson, 2004, Zinbo and Timell, 1965). For methylation of xylan, the methylsulfinyl anion (Na dimsyl) in dimethyl sulfoxide (DMSO) as reaction medium, introduced for linkage analysis by Hakomori (1964), was widely used (Aspinall, 1982, Fang et al., 2001, Han and Swan, 1968, Shimizu, 1976). This method substituted the dimethyl sulfate procedure, which led to partially methylated products (Kislitsyn et al., 1970, deBelder et al., 1962, Croon and Timell, 1960, Heuser and Ruppel, 1922). An efficient methylation method for carbohydrates was introduced about 20 years ago using methyl iodide, DMSO, and powdered NaOH (Ciucanu & Kerek, 1984).

The present paper deals with investigations of the methylation of xylan using methyl halides as etherification agent in the presence of NaOH in order to get water-soluble products. The main focus of the studies was the detailed structure characterization regarding the degree of substitution (DS) and the substitution pattern by means of one and two-dimensional NMR spectroscopic techniques.