Pyrolysis of Lignin Model CompoundsMay 30, 2013
Lignin is an underused but major component of biomass. In order to make biomass a viable alternative to fossil fuels it is of great importance to develop effective processing techniques of all components of biomass, in particularly lignin, not only for energy but also chemicals and materials production. Due to the complexity of lignin itself and its degradation pathways, the study of model compounds is common practice. Model compounds of lignin represent typical linkages found in the three-dimensional polyphenolic network. The most abundant linkage is the Î²-O-4 ether linakge for which phenethyl phenyl ether (PPE) is the simplest model compound. Our efforts have been focused on the thermal degradation of PPE complementing experimental research conducted in the Chemical Sciences Division at ORNL. Experimental conditions foster a free radical chain mechanism for the pyrolysis of PPE for which we investigate key mechanistic steps using computational methods. We address questions that are difficult to answer with experimental techniques: how do substituents influence individual parts of the reaction mechanism, what is their effect on reaction energies and barriers, and how does the electronic structure change with substituent pattern. Since experiment measures overall product distributions, not all reaction steps are visible to experiment; e.g., when the reactions yield identical products, but are investigated by us with computational methods. Kinetic information is obtained by locating transition states and calculating rate constants for the corresponding reactions. The rate constants are used in simplified kinetic models based on rate determining reaction steps only and kinetic simulations of the complete mechanism. Our approach gives, on one hand, valuable insight into the details of lignin model degradation that is not attainable by experiment while, on the other hand, returning product distributions that are directly comparable to experimental values and validate our methodology.