Direct mass-spectrometric studies of the pyrolysis of carbonaceous fuels: III. Primary pyrolysis of lignin

Abstract The primary pyrolysis of lignins derived from different types of biomass and by the major separation methods has been investigated by molecular-beam sampling mass spectrometry. The lignins separated by the steam-explosion and kraft processes have altered pyrolysis characteristics while ball-milled lignins yield nearly the same product slate as observed in native biomass samples. The predominant and first-formed products, as determined by mass spectrometry, appear to be the precursor monomers, coniferyl and sinapyl alcohol. There is a distinct lack of higher-molecular-weight oligomers as is commonly observed in the pyrolysis of other types of natural and synthetic polymers. A group of peaks, with the most predominant being m/z 272, appear at masses greater than the monomer masses, but are significantly below the dimer mass range. A third group of predominant peaks are present at masses lighter than the monomers and consist of methoxyphenols that have double bonds and carbonyl groups in the alkyl side chains that are in conjugation with the aromatic ring, enhancing thermal stability. The results indicate that the thermolysis of the alkyl-aryl ether linkage, which is the major bonding unit in lignin, and the limited availability of transferable hydrogen, are the major factors that lead to the predominance of these products. The observed product distributions are indicative of specific, sterically favored rearrangement reactions, which allow devolatilization from the hydrogen deficient, solid matrix and favor double bond formation in the alkyl side chain of the products. A discussion of possible mechanisms of formation is given based on these results and the results of other workers in lignin pyrolysis.