Pyrolysis of carbohydrate-derived abiotic macromolecules: limitations in monitoring carbohydrate signature of geopolymers

Abstract

Two series of black-colored macromolecular substances were respectively prepared by a) acid-catalyzed dehydration of pure glucose and b) dry-state heating of crystalline cellulose. For molecular characterization, these substances were analyzed by standard chemical and spectroscopic techniques, revealing a more or less smoothed carbohydrate signature. The pyrolysis results showed striking differences between the two series of carbohydrate-derived substances. The carbohydrate origin of the charred celluloses (even after 48% wt loss) were evident by the characteristic cyclic ketones, alkylfuranes and the major peak of levoglucosan. The alkylbenzenes and aikylphenols were in comparatively low amounts. On the contrary, the acid-catalyzed dehydration substances, showed pyrolysis patterns where aromatic compounds (phenols, benzofurans, indenes, naphthalenes) dominate vs the infrequent furan peaks. The residual, acid-soluble, polyvinylpyrrolidone-retained fraction yielded furanes in addition to acetic and levulinic acids. The results obtained suggest that carbohydrate dehydration in media of intense ionic strength may lead to macromolecular material showing pyrolysis patterns compatible with the "carbohydrate-free" sedimentary macromolecules. Two non-excludent hypotheses are suggested from the above results: a) analytical pyrolysis is largely limited to yield diagnostic compounds from diagenetically altered (eg, OH-depleted) carbohydrate structures, and/or b) carbohydrate do really turn into highly unsaturated threedimensional networks yielding, upon laboratory degradation, unspecific fragments consisting of phenolic, heterocyclic and short-chain alkyl structures.