Thermal decomposition of lignin structural modification in termite digested softwood (II)

In light of increasing demand for renewable fuels and chemicals derived from nonfood biomass (lignocellulose), deciphering the mechanism of wood-cellulose utilization by wood-feeding termites became more imperative. To further explore lignin unlocking process of termite demonstrated in our previous investigations, study on thermal processing of termite digested softwood was continued using thermogravimetric analysis (TGA) at different heating rates from 2 to 50 °C min−1, with the aid of the Friedman method. The differential thermogravimetry (DTG) curve was also deconvoluted using the Gauss and Lorentz model, into three fractions (lumps), each of which was represented by a single first-order reaction. The results showed that termite feces exhibited alterations in both the required activation energies at each thermal decomposition stage and the conversion rates of the three components of cellulose, hemicelluloses, and lignin, respectively. The activation energy for lignin decomposition decreased at its earlier decomposition stage, but increased when conversion reached 80%. The impact of structural modifications upon changes in kinetic parameters in different decomposition stages was then discussed. The fractional cellulose decomposition was decelerated but required higher temperature for the same thermal conversion extent as speculated to be a result of decrease in amorphous cellulose in termite feces. Moreover, termite digestion resulted in a more thermo-reactive hemicellulose, which possibly resulted from the bond cleavage within hemicellulose fraction and between hemicelluloses and lignin. It was proposed based on the active thermal behavior of the lignin fraction that preferential cleavage of bonds with higher dissociation energy as lignin structure unlocking process occurred within termite guts.