PYROLYSIS OF XYLAN 



The hemicellulose fraction of cottonwood is mainly 0-acetyl-4-0-methylglucuronoxylan 

 (i.e., xylan) . The isolation of the polysaccharide was described in a previous section. 

 The pyrolysis studies in the past have mainly dealt with cellulose and there is little 

 thermal degradation data on xylan. Beall 9 has reported a thermal analysis study (TGA 

 and DTA) of hemicelluloses from different species of wood, without inorganic chemical 

 analysis of samples. The early investigation of the products formed from xylan pyroly- 

 sis has been limited to identification of a few major compounds, especially furfural 

 (2-furaldehyde) (Heuser and Scherer 1923) . 



A recent report deals with the thermal analysis of xylan isolated from beechwood, 

 and includes a complete chemical, analysis of the substrate (Shimizu, Teratani, and 

 Miyazaki 1968). This study showed that pyrolysis of xylan takes place at about 215° C. 

 when the' polysaccharide structure disappears, as shown by IR spectroscopy. When xylan 

 was heated further to 290° C, fragmentation of the compound occurred. Shimizu and co- 

 workers also claimed that some polymerization, evidenced by the appearance of xylobiose 

 above 275° C, could take place. The fragmentation and polymerization gave a net 

 exothermic reaction which was not resolved. Maximum weight loss occurred between 215° 

 and 290° C. 



Further studies by McGinnis* have shown that the initial pyrolytic reaction 

 involves breaking of the 3 1^4 glycosidic bond through a carbonium ion mechanism. This 

 conclusion is mainly based on investigation of methyl, and substituted phenyl B-D- 

 xylosides, used as model compounds for xylan. The nature of the aglycone for these 

 xylosides provided a wide range of glycosidic bonds. By subjecting the models to DTA 

 and TGA, it was shown that the thermal stability of the glycosidic bonds was determined 

 by the electron withdrawing power of the aglycone. Analysis of the products showed 

 that the aglycone could be recovered quantitatively and therefore the l->4 glycosidic 

 bond must have been broken during the initial stage of the reaction. 



Two kinds of xylan were described previously: the DMSO-extracted xylan which 

 remains acetylated as in the native form, and the base-extracted xylan which has been 

 deacetylated by the alkali. In this part of the program, both types of treated and 

 untreated xylan were subjected to DTA and TGA as well as pyrolysis gas chromatography. 



Thermal analysis of both types of xylan showed very little difference between 

 them. The TGA (fig. 9) shows that pyrolysis began at 215° C. as indicated by weight 

 loss and proceeded rapidly up to 275° C. The maximum rate of pyrolysis was 0.11 mg./°C. 

 and a residue of 6 percent remained at 450° C. (table 5). 



In the DTA curve there was an endotherm at the onset of rapid weight loss. This 

 was followed by a sharp exotherm (fig. 10). Figures 9 and 10 show the effects of 5 

 percent ZnCl 2 on the TGA and DTA of base-extracted xylan. On TGA, weight loss began 

 at 175° C. as compared to 215° C. for the untreated. The maximum rate of weight loss 

 was 0.06 mg./°C. and a residue of 2 to 5 percent remained at 450° C. On DTA, the 

 endotherm corresponding with rapid weight loss occurred from 175° to 225° C. and was 

 followed by a broad exothermic region (fig. 10). 



Beall, op. cit. 

 McGinnis, op. cit. 



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