期刊
ACS SUSTAINABLE CHEMISTRY & ENGINEERING
卷 3, 期 7, 页码 1339-1347出版社
AMER CHEMICAL SOC
DOI: 10.1021/acssuschemeng.5b00070
关键词
Density functional theory; Microkinetic modeling; alpha-O-4; Ether bond
资金
- National Science Foundation [CHE-1314063]
- National Advanced Biofuels Consortium - DOE BioEnergy Technologies Office through American Recovery and Reinvestment Act Funds
- Office of Science of the U.S. Department of Energy [DE-AC02-05CH11231]
- Department of Energy [DE-EE0005006]
- Virent, Inc.
Lignocellulosic biomass offers a vast, renewable resource for the sustainable production of fuels and chemicals. To date, a commonly employed approach to depolymerize the polysaccharides in plant cell walls employs mineral acids, and upgrading strategies for the resulting sugars are under intense development. Although the behavior of cellulose and hemicellulose is reasonably well characterized, a more thorough understanding of lignin depolymerization mechanisms in acid environments is necessary to predict the fate of lignin under such conditions and ultimately to potentially make lignin a viable feedstock. To this end, dilute acid hydrolysis experiments were performed on two lignin model compounds containing the alpha-O-4 ether linkage at two temperatures concomitant with dilute acid pretreatment. Both primary and secondary products were tracked over time, giving insight into the reaction kinetics. The only difference between the two model compounds was the presence or absence of a methyl group on the alpha-carbon, with the former being typical of native lignin. It was found that methylation of the alpha-carbon increases the rate of reaction by an order of magnitude. Density functional theory calculations were performed on a proposed mechanism initiated by a nucleophilic attack on the alpha-carbon by water with a commensurate protonation of the ether oxygen. The values for the thermodynamics and kinetics derived from these calculations were used as the basis for a microkinetic model of the reaction. Results from this model are in good agreement with the experimental kinetic data for both lignin model compounds and provide useful insight into the primary pathways of alpha-O-4 scission reactions in acid-catalyzed lignin depolymerization. The distribution of primary and secondary products is interpreted as a function of two barriers of formation exhibiting opposite trends upon methylation of the alpha-carbon (one barrier is lowered while the other is increased). Such insights will be needed to construct a comprehensive model of how lignin behaves in a common deconstruction approach.
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