Mechanisms of Formation of H, HO, and Water and of Water Desorption in the Early Stages of Cellulose Pyrolysis

Abstract

Here, we report the results from a combined first-principles and experimental investigation of the initial stages of decomposition of cellulose during heating in pyrolysis. Density functional theory calculations with periodic boundary conditions were performed to investigate the formation of H and HO radicals and of the molecular products H2O, H2, and H2O2 originating from their recombination. The stabilization that alcohol groups impart to adjacent C-radicals and the allylic recombination of unpaired electrons of neighboring C-radicals play decisive roles in the decomposition mechanism. This makes the simultaneous formation of H• from C2 and HO• from C3 the most favorable process. The recombination of these radicals to form water leads to an additional stabilization of the reaction. The computed temperature-dependent reactions Gibbs’ free energies reveal that desorption of H2O from intact cellulose occurs at T = 147 °C and that gas-phase water forms spontaneously from the decomposition of cellulose at T = 282 °C. These results are in excellent agreement with our experimental study of the pyrolysis done with pyrolysis–gas chromatography/mass spectrometry at different temperatures. The experiments show that upon heating, a small amount of water is released from cellulose at 210 °C, and a considerably larger amount starts to be released at 280 °C.