425 / 2018-08-01 16:31:48

Structural elucidation and comparative pyrolysis behaviors of seven lignins

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Lignin, the second most abundant component in biomass, can be pyrolyzed into variable valuable products. Pyrolysis behavior of lignin is greatly impacted by the starting materials, which could be attributed to their different structure. To elucidate the impact, thermal performance together with structural analysis were studied on seven lignins prepared with representative methods from hardwood, softwood and grass, including acid extracted birch lignin (ABL), ethanol extracted birch lignin (EBL), ethanol extracted maple lignin (EML), black liquor lignin (BLL), acid extracted pine lignin (APL), acid extracted corn stalk lignin (ACL), and cellulolytic enzyme corn stalk lignin (CECL). The structural characteristics of the lignins were identified by elemental analysis, fourier transform infrared (FTIR) spectroscopy, and two-dimensional heteronuclear single-quantum coherence nuclear magnetic resonance (2D HSQC NMR). The comparative pyrolysis behaviors of the lignins were examined by TG over a heating rate range from 10 ℃/min to 40℃/min. Results show that the distribution of elements, functional groups, phenylpropane units and interunit linkage bonds varied greatly across the lignins. Lignin from hardwood and grass had more linkages, while lignin from softwood had less linkages. Isolation under mild conditions helped to retain more linkages, especially weak linkages like β-O-4, while preparation under harsh conditions would bring the substantial cleavage of the interunit linkages. The two birch lignins (ABL, EBL) had higher ratios of syringyl units (84-86%) than other lignins, and APL had only guaiacyl units. p-Coumarates (pCA) and ferulates (FA) structure were also observed in two grass lignins from corn stalk. Pyrolysis results show that the slow pyrolysis of most lignins started at 200℃ and ended at 600℃.DTG curves of the four hardwood lignins (ABL, EBL, EML and BLL) had only one main mass loss peak between 376℃ and 390℃. APL had two obvious mass loss peaks at 320℃ and 392℃. While grass lignin of ACL and CECL had one main weight loss peak (383-384℃) and one obvious shoulder peak (250-280℃), and the shoulder peak is mainly due to the breakdown of the abundant ferulic acid and coumaric acid building blocks in grass lignins as revealed in structural analysis. With the heating rate increasing, the mass loss peak moved to higher temperature. The average activation energy obtained with Starink model-free method revealed that the Ea values follow the order as grass lignin>hardwood lignin>softwood lignin. In stage 1, pyrolysis of ABL and APL followed the O3 model, BLL follwed G5 model, while the two grass lignin ACL and CECL followed the G4 model. In stage 2, O3 was the best suitable model for EBL, EML APL ACL and CECL, while G6 was the best for ABL and BLL.