of Biomass Model Compounds
Woody biomass is a potentially renewable source of fuels and valuable chemicals. In particular, the abundant lignin fraction could be a source of aromatic chemicals such as phenols and aldehydes, but there is currently no process for deconstructing the complex polymeric structure to produce the desired products with high selectivity. Thermochemical approaches are most often investigated, and we have been interested in developing an improved understanding of the fundamental reaction pathways involved. Our approach is to examine organic molecules that are key building blocks in the lignin structure.
Since the pyrolysis of lignin is conducted under varied conditions that result in different product mixtures, we also explore the pyrolysis reactions of model compounds under both conventional (low temperature, long residence time) and flash (high temperature, low pressure, short residence time) pyrolysis conditions. Flash pyrolysis conditions highlight unimolecular transformations (bond scissions, intramolecular rearrangements) while conventional pyrolysis in the liquid phase allows slower bimolecular reactions and radical chain processes to occur. This research involves synthesis of the model compounds, determination of reaction kinetics and mechanisms, and quantitative product identification (GC and GC-MS). We also use deuterium labeling to provide additional mechanistic insights and explore the influence of substituents on reaction rates and product selectivities.
It is sometimes difficult to unravel the exact influence of a substituent on the pyrolysis rate and product selectivity, since it can perturb multiple reaction steps in the mechanism such as bond scissions, competitive radical hydrogen abstractions, rearrangements, and radical scissions. This is where modern computational chemistry can provide tremendous new insights. We conduct this computational research using density functional theory (DFT) in collaboration with Dr. Ariana Beste (http://www.csm.ornl.gov/ccsg/html/staff/beste.html) of the Computer Sciences and Mathematics Division at ORNL. This research takes advantage of the high performance computers available through the National Center for Computational Sciences (NCCS) at ORNL (http://www.nccs.gov/).
Physical Organic Chemistry R & D Projects