Dehydration and Dehydrogenation Kinetics of OH Groups in Biomass Pyrolysis

Experiments in pulse-injected flow reactors show that gas-phase pyrolysis of alcohols makes a transition from dehydrogenation dominance for mono-alcohols to dehydration and fragmentation for tested diols and a triol. These dehydration and dehydrogenation reactions are proposed to occur by unimolecular and bimolecular pericyclic reactions, as has been used to explain glucose and cellulose pyrolysis in the absence of ions (Seshadri and Westmoreland, 2012, 2015). Vaporizable mono-alcohols (ethanol, propanol, butan-2-ol, t-butyl alcohol, and neopentyl alcohol), diols (ethan-1,2-diol, propan-1,2-diol and propan-1,3-diol), and a triol (propan-1,2,3-triol) were pulse-injected into a helium flow, pyrolyzed at 400°C in a tubular flow reactor, and swept directly into a two-dimensional gas chromatograph with time-of-flight mass-spectrometric detection (Pegasus 4D, LECO Corp.). Methanol and 2- propanol were pyrolyzed in a tubular quartz reactor with solely mass-spectrometric analysis. All the mono-alcohols dehydrogenated except t-butyl alcohol, which has no H on the alcohol carbon. Calculations at a CBS-QB3 level of theory showed that the energetically favored transition states for dehydrogenation were six-centered pericyclic reactions with a hydrogen-bonded ROH molecule. These transition-state enthalpies were about 55 kcal/mol, compared to about 85 kcal/mol for four-centered H2 elimination. Non-hydrogen-bonded six-centered transition states can occur, but their enthalpies were 109 kcal/mol and higher. The diols displayed dehydrogenation, dehydration, and cyclic Grob fragmentation, and the triol yielded only dehydration and fragmentation products. Details of the analyses are presented, and the findings are compared and contrasted to other findings in the literature.