439 / 2018-09-01 17:02:05

Pericyclic Chemistry in the Anomerization and Pyrolysis of Mono- and Polysaccharides

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As key steps in making bio-oil thermally, pyrolysis and isomerization (anomerization) of non-solvated glucose and cellulose is dominated by pericyclic reactions, as has been shown in the past seven years; the reactions can be unimolecular or can be homogeneously catalyzed by hydroxyl groups in H2O vapor, molecular intermediates, or the saccharides themselves. The present work supports the hypothesis that this domination extends to hemicellulose and its constituent saccharides. Our approach is to extend experimental pyrolysis data (TGA/DSC and Pyroprobe/GCxGC-TOFMS) using calculations of monomer thermochemistry and kinetics using computational quantum chemistry at the CBS-QB3 level. Transition states and rates of anomerization by reversible ring-opening and -closing reactions were predicted for the monosaccharides D-glucose, D-mannose, D-galactose, D-xylose, L-arabinose, and D-glucuronic acid. Activation energies for the ring-opening were 162 to 217 kJ/mol for four-membered pericyclic separation of the lactol group. They were reduced by catalytic participation of an alcohol group within the monosaccharide or an external R-OH group represented by an explicit water molecule, reaching activation energies as low as 97 kJ/mol and 67 kJ/mol, respectively. Decomposition reactions of the hemicellulose monosaccharides and small oligomers should be very analogous to those of cellulose except for routes involving the -(C6)H2-OH group of glucose. Calculations indicate that this is true, and we are seeking experimental support of this hypothesis using identification and yields of captured intermediates and products.