The Influence of Nano-Confinement and Hydrogen Bonding on the Transition State of Model Lignin Compounds
Bobby G. Sumpter, Phillip F. Britt, A.C. Buchanan III
Despite the extensive research into the pyrolysis of lignin, the underlying chemical reactions that lead to the product formation are poorly understood. Detailed mechanistic studies on the pyrolysis of biomass and lignin under conditions relevant to current process conditions could provide insight into utilizing this renewable resource for the production of chemicals and fuel. Lignin is the second-most abundant naturally occurring biopolymer and a by-product of the pulping process, however, despite the extensive attempts to extend the use of lignin, there has only be modest success. This is mainly due to the structural diversity of lignin and the dependence of the chemical structure of lignin on the method of isolation. Lignin is a complex, heterogeneous, three-dimensional polymer formed from the enzyme-initiated, dehydrogenative, free-radical polymerization of three p-hydroycinnamyl alcohol precursors that differ by the number of methoxy groups on the aromatic ring. As opposed to other biopolymers such as cellulose, lignin has many different types of linkages between monomer units. This arises from the distribution of the p-electron density throughout the phylpropene monomer unit and the thermodynamic principles governing radical addition reactions. The dominant inter-unit linkage in lignin is the arylglycerol-β-aryl and arylglycerol-α-aryl ether linkages. If the substituents are removed from the arylglycerol-β-aryl linkage, a simplified model compoud, pheneethyl phenyl ether (PPE) is obtained. By combining experimental flash pyrolysis studies of model lignin compounds (PPE compounds with a variety of different substituents) with computational quantum chemistry, we have been able to develop a new mechanistic description for the reaction pathways. Reaction rates can be varied by as much as a factor of 20 by using substituents containing OH. This rather large change is caused by the formation of a hydrogen-bonded stabilized transition state (on the order of 3 Kcal/mol lower). The same chemical reactions can also be drastically altered by confining the system to the nanoscale, such as in mesoporous materials or carbon nanotubes.
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Oak Ridge National Laboratory, LDRD
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