MECHANISMS OF PYROLYSIS OF POLYSACCHARIDES .4. THERMAL SYNTHESIS AND PYROLYSIS OF A XYLAN

A xylan has been synthesized in yields up to 64% by thermal polymerization of methyl beta-D-xylopyranoside, catalyzed by phosphoric acid. The polymer is highly branched, with d.p. approximately 20. The xylose units in the polymer are predominantly pyranose, with some furanose, and 14% of the nonreducing end groups are in the furanose form. The ''reducing'' end groups are at least partly in the form of methyl xylosides. Vacuum pyrolysis of the pure synthetic xylan gave a high char yield (48%, compare, for example 5% from pure cellulose), 1,4-anhydro-alpha-D-xylopyranose (5.7%), and 1,5-anhydro-4-deoxypent-1-en-3-ulose (2.4%). The structure of the latter compound was verified by degradation experiments and it was shown to be the compound previously isolated from pyrolysis of natural xylans and incorrectly assigned as 3-hydroxy-2-penteno- 1,5-lactone. This glyculose has considerable importance as a marker compound for xylans in pyrolysis of biomass materials.