Investigation of the pyrolysis characteristics of guaiacol lignin using combined Py-GC × GC/TOF-MS and in-situ FTIR

To investigate the pyrolysis behavior of guaiacol lignin (G-lignin) at 400–750 °C and its major decomposition mechanisms, ginkgo MWL as a typical G-lignin was decomposed in a micro-pyrolyzer, coupling two-dimensional gas chromatography with time-of-flight mass spectrometric detection (Py-GC × GC/TOF-MS) and an in-situ infrared pyrolysis reactor (in-situ FTIR). The pyrolysis products showed that a large amounts of guaiacol phenols were produced at low temperature, which indicated that guaiacol phenols were an important intermediate product, and other lignin-derived aromatic chemicals formed from guaiacol phenols by de-methyl, de-methoxyl reactions, and broken off branch structures. Relatively high contents of 2-methoxyphenol and l-(2-hydroxy-5-methylphenyl) ethanone were obtained at 450 °C, up to 19.46% and 16.35%, respectively. However, when temperature increased, 2-methoxyphenol and l-(2-hydroxy-5-methylphenyl) ethanone should be transformed to catechol and p-cresol by the removal of the carbonyl group from the branch and methyl from the methoxyl group, respectively. The methyl breakage of 4-ethyl-2-methoxyphenol and de-methoxyl of mequinol caused the formation of creosol and phenol, respectively, when temperatures increased. Two-dimensional perturbation correlation infrared spectroscopy (2D-PCIS) was introduced to analyze the in-situ FTIR spectra, and it was found that the cleavage of alcoholic or phenolic C-O bond was the major reaction in G-lignin decomposition, and the methyl in methoxyl groups could be easily removed from methoxyl to form CH4. Based on 2D-PCIS, reaction pathways involving the change sequence of several functional groups from G-lignin were proposed. Three reaction paths were provided from a model lignin with β-O-4, α-O-4 and 5-5′ connection to 1-(2-hydroxy-5-methylphenyl) ethanone, 2-methoxyphenol, catechol, trans-isoeugenol, p-cresol, phenol and benzene.