PNNL

Abstract

Pyrolytic lignin is a fraction of the pyrolysis bio-oil that contains a wide variety of phenolic compounds that can be used as platform chemicals in the energy and fuels, pharmaceutical and cosmetic industries. However, the characteristics of the lignin polymer structure makes it difficult to establish a pyrolysis mechanism and to determine the characteristics of the pyrolytic lignin structure. This study proposes dimer, trimer, and tetramer structures based on their relative thermodynamic stability for a hardwood model lignin in the pyrolysis process. Different configurations of oligomers were evaluated by varying the positions of the guaiacyl (G) and syringyl (S) units in the hardwood model lignin through electronic structure calculations. The homolytic cleavage of ?O4 bonds is assumed to occur and generate two free radical fragments. These can stabilize taking hydrogen radicals that may be in solution during the formation of the intermediate liquid (pathway 1) before the thermal ejection. An alternative pathway (pathway 2) could happen when the radicals use intramolecular hydrogen, turning themselves into stable products. Subsequently, a demethylation reaction can take place through the homolysis of the O-CH3 bond by temperature increasing, thus generating a methane molecule and a range of structures of varying stability and characteristic spectral signatures. The most probable resulting structures were characterized through simulated FTIR and NMR spectra, and the thermophysical properties were calculated using group contribution methods. The results give insights about the lignin oligomers structures and the mechanism through which some of these molecules are formed. They also provide useful information for the design of pyrolysis bio-oil separation and upgrading equipment.