The facts and hypotheses relating to the phenomenological model of cellulose pyrolysis: Interdependence of the steps

The quantum-mechanical computations of Nimlos et al. [M.R. Nimlos, S.J. Blanksby, G.B. Ellison, R.J. Evans, J. Anal. Appl. Pyrol. 66 (2003) 3–27] predict that peak temperatures of dehydration of non-protonated forms of alcohols at the heating rate of about 0.033 K/s (2 °C/min) exceed 600 °C. Peak temperatures for completely protonated alcohols lie at about 100 °C, while the experimental peak temperature of cellulose dehydration is equal approximately to 300 °C. The latter value is very close to the peak of the rate of overall mass loss, ≈300 °C. Hence, one may conclude that the dehydration is a fast secondary reaction with respect to cellulose depolymerization. Neither considerable dehydration nor other reactions of the β-elimination seems to occur inside the solid matrix of polymer cellulose. The elimination needs an acid catalyst for protonation of oxygen at the α-position. But this catalyst is absent in the matrix. High-boiling liquid tar arising as a result of transglycosylation launches the ionic mechanisms, filling up the cavities in cellulose and playing the role of an electrolyte. Volatile acids dissolved in the tar are the strong catalysts accelerating various heterolytic (ionic) reactions, including depolymerization by the acid-catalyzed β-elimination. A two-level kinetic model summarizes such conclusions. The transglycosylation leads to the appearance of non-reducing ends. In the regime of their fast destruction and a quasi-stationary removal of the corresponding volatile acids from the pyrolysis zone the apparent activation energy of the formation of light gases, Egas, is the sum of activation energy of transglycosylation, Etar, and the true activation energy of the β-elimination, Eβ, namely: Egas = Etar + Eβ. One can evaluate Eβ = Egas − Etar ≈ 50–60 kJ/mol.