Mass spectrometric studies of the thermal decomposition of carbohydrates using C-13-labeled cellulose and glucose

The mechanism of the thermal decomposition of carbohydrates is very important to the development of fuels, fibers, and paper products. To help gain more insight into the pyrolysis chemistry of cellulose, we have carried out experimental studies using Acetobacter xylinum cellulose grown on D-(1-C-13)-glucose medium with incorporation levels of 1-C-13 of 14%, as determined by C-13 NMR analysis. Samples of the labeled cellulose, as well as D-(1-C-13)- and D-(2-C-13)-glucose, were pyrolyzed under fast-heating conditions and the products analyzed by molecular beam mass spectrometry (MBMS). From the labeled cellulose samples, statistically significant levels of enrichment were observed for the pyrolysis products that occur at m/z 110, 114, 126, 144, and 191, but not at m/z 98, 60, or 31. The lack of enrichment in the latter fragments indicates that they do not incorporate C-1. Samples were treated with 0.1% aqueous KOH to favor the formation of glycolaldehyde, but even in this case this major product was not enriched. These results suggest that the m/z 60 ion is an EI fragment ion of levoglucosan, formed by loss of neutral species containing C-1. However, the m/z 191 ion was found to contain two C-1 carbons. Collision-induced dissociation results for this ion suggest that it consists of a formate group. The structure of this ion is proposed to be the protonated formate of levoglucosan at the C-4 position, derived from either a reverse aldol reaction or a +/--Diels-Alder reaction. The (1-C-13)-glucose pyrolysis product distribution is similar to that for (1-C-13)-labeled cellulose. The (2-C-13)-glucose pyrolysis product distribution shows significant contribution from C-2 at the M + 1 peaks of m/z 32, 43, 60, 73, 85, and 97, indicating major incorporations of C-2 in glycolaldehyde in contrast to the lack of incorporation of the C-1 position in this major product.