Acetylated heteroxylan from Agave sisalana and its behavior in alkaline pulping and TCF/ECF bleaching

The heteroxylan from sisal (Agave sisalana), an O-acetyl-(4-O-methylglucurono)xylan with a molecular weight (Mw) of 18 kDa, was isolated by extraction of peracetic holocellulose with Me2SO and thoroughly characterized by wet chemistry, and NMR spectroscopy. The heteroxylan backbone is composed of (1 → 4)-linked β-d-xylopyranosyl units (Xylp) partially branched with terminal (1 → 2)-linked 4-O-methyl-α-d-glucuronosyl (MeGlcpA, 9 mol%) and a small proportion of α-d-glucuronosyl (GlcpA, <1 mol%) residues. Roughly 61 mol% of Xylp residues are acetylated (DS = 0.70). During soda/AQ pulping of sisal fibers, MeGlcpA and GlcpA are mostly removed or converted to 4-deoxy-β-l-threo-hex-4-enopyranosyluronic acid (HexA), although about 15% of the initially present MeGlcpA was maintained intact upon cooking. The major part of acetyl groups (95%) was hydrolyzed during pulping. It was proposed that during bleaching, a low molecular weight xylan fraction associated to residual lignin was removed from pulp and small proportion of MeGlcpA was additionally converted to HexA. The profiles of uronosyl residues in xylans from TCF and ECF bleached sisal pulps were rather different.