Thermal degradation of softwood lignin and hardwood lignin by TG-FTIR and Py-GC/MS

The structural characteristics of softwood (Chinese fir) lignin and hardwood (Maple) lignin prepared by Klason method were identified by elemental analysis and Fourier transform infrared (FTIR) spectrometry, and the pyrolytic behaviors of lignin were examined by means of thermogravimetric-Fourier transform infrared spectrometry (TG-FTIR) and Pyrolylisis-gas chromatography/mass spectrometry (Py-GC/MS). It was found that maple (hardwood) lignin contained more methoxyl groups than Chinese fir (softwood) lignin due to the enrichment of syringol units, presenting the chemical formula as C4.64H4.017O2.482 against C4.939H5.255O2.219 for Chinese fir lignin. The amounts of phenolics, methanol and CH4 evolved from pyrolysis of maple lignin were all remarkably larger than that of Chinese fir lignin through TG-FTIR analysis. For both two lignins, aromatic compounds (such as benzene, toluene and xylene) were predominantly released between 650 °C and 800 °C, due to the intensive cleavage of aryl-O-R linkages and dehydroxylation reaction on benzene-ring. The distribution of produced volatiles during lignin fast pyrolysis against furnace temperature was intensively discussed, finding that the cleavage of typical inter-unit linkages under relatively low temperature produced the guaiacol-type and syringol-type compounds, whereas the elevated temperature facilitated the cracking of methoxyl group, giving rise to the notable increase of phenol-type, catechol-type compounds and aromatic hydrocarbons.