Catalysis Today, Vol.327, 226-234, 2019
A kinetic study of lignin pyrolysis over base catalyst during steam exploded depolymerization
As an effective method for comprehensive utilization of lignin, steam explosion coupled with base catalysis (SEBC) pretreatment was applied into lignin depolymerization in this paper. The pyrolytic behavior characteristics of raw lignin (RL) and SEBC pretreated lignin (SL) were investigated by thermogravimetric analysis (TGA) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) analytical technology. Pyrolysis of RL and SL was performed on a TG analyzer at multiple heating rates of 20, 60, 80, and 100 degrees C/min up to 720 degrees C to obtain the thermal behavior of lignin substrates. TGA results showed that the pyrolysis process of the two lignin samples has similar characteristics, and the main pyrolysis temperature range of both substrates is from 200 degrees C to 500 degrees C. A total loss weight of 44% and 47% at the heating rate of 20 degrees C/min for RL and SL was observed respectively. Derivative thermogravimetric (DTG) also showed that two major decomposition reaction stages (drying stage and degradation stage) took place at a specific heating rate for RL, while three major pyrolytic reaction stages for SL (drying stage, fast degradation stage, and slow degradation stage). The heating rate mainly influences the primary pyrolysis stage of substrate, while the maximum weight loss rate and corresponding temperature would change with increasing of heating rate. At a temperature above 200 degrees C, the pyrolysis of substrate was clearly accelerated, and all DTG curves indicated that the temperature corresponding to maximum pyrolytic rate shifted to higher temperature range with higher heating rate. The effect of steam explosion coupled with base catalysis is obvious. Based on Py-GC/MS results, it promoted the degradation of compounds from high molecular weight to low molecular weight, even accelerated cracking reactions of oxygenated products, e.g., phenol for SL's prominent product, instead of 2-methoxy-4-ethylphenol for RL's. Assuming that the thermal decomposition obeys first-order reaction, kinetic parameters of RL and SL pyrolysis were determined with two methods proposed by Kissinger and Ozawa. Both methods gave analogous values of activation energy and frequency factor for RL's two pyrolytic stages and SL's three pyrolytic stages, respectively. This study gave further confirmation that the SEBC pretreatment process could transform lignin biomass into an intermediate feedstock with favorable properties for thermo-chemical applications.