Pyrolysis Kinetics and Mechanisms of Fuel Model Compounds

This research probes the kinetics and mechanisms underlying the pyrolysis of organic species that are representative of corresponding structural units in complex organic materials such as biomass and coal, where there is a current lack of fundamental understanding. For example, lignin model compounds (Figure 1) are being investigated in the gas phase under flash vacuum pyrolysis (ca. 500°C, < 500ms residence time) to highlight the primary products and reaction channels.¹ The effects of substituents on the pyrolysis rates and product distribution are being systematically investigated both in the gas phase and in solution (lower temperature and longer residence times), where secondary reactions can be elucidated. This research involves multi-step synthesis of target lignin model compounds. The selectivity in product formation as a function of substituents reflects a subtle balance in the rates of bond homolysis, competitive hydrogen abstraction, 1,2-phenyl shift, and b-scission steps. Additional insights are being provided by DFT calculations.

We have also been examining the pyrolysis chemistry of aromatic carboxylic acids^2,3 whose decarboxylation chemistry is thought to be associated with the low efficiency in processing low-rank coals. Both model compounds and polymers (Figure 2) have been employed to unravel the fundamental ionic and free radical pathways involved. Anhydrides have been found to be important intermediates under conditions where water is removed from the system. Research on polymer models also involves the use of FTIR, solid-state 13C-NMR, TGA, and TG-MS (Figure 2) to probe the underlying chemistry. Current studies are examining the pyrolysis chemistry of corresponding aromatic carboxylic acid salts and esters.

Another research interest is the elucidation of pathways for polycyclic aromatic hydrocarbon (PAH) formation during pyrolysis of biomass. Recent studies have focused on the pyrolysis of plant steroids and sterol esters as a function of temperature (600-800°C), residence time, and concentration. These studies revealed that PAHs such as phenanthrene could be produced from the native steroid ring structure by dealkylation and dehydrogenation reactions rather than by pyrosynthesis from small precursor fragments.⁴ In addition, we have now found that PAH yields depend on the steroid structure and are most sensitive to the number of double bonds in the steroid B-ring (ergosterol>cholesterol>dihydrocholesterol).⁵ Current studies are investigating the formation of PAHs and nitrogen-containing PAHs from pyrolysis of amino acids and mixtures of amino acids with reducible sugars.

1. P. F. Britt, A. C. Buchanan, III, M. J. Cooney, D. R. Martineau, Flash Vacuum Pyrolysis of Methoxy-Substituted Lignin Model Compounds J. Org. Chem. 2000, 65, 1376-1389.
2. T. P. Eskay, P. F. Britt, and A. C. Buchanan, III, Pyrolysis of Aromatic Carboxylic Acids: Potential Involvement of Anhydrides in Retrograde Reactions in Low Rank Coals Energy & Fuels 1997, 11, 1278-1287.
3. P. F. Britt, W. S. Mungall, A. C. Buchanan, III Pyrolysis of a Polymeric Model of Aromatic Carboxylic Acids in Low-Rank Coal Energy & Fuels 1998, 12, 660-661.
4. P. F. Britt, A. C. Buchanan, III, M. M. Kidder, C. Owens, J. R. Ammann, J. T. Skeen, and L. Luo Mechanistic Investigation Into the Formation of Polycyclic Aromatic Hydrocarbons from the Pyrolysis of Plant Steroids Fuel 2001, 80, 1727-1746.
5. P. F. Britt, A. C. Buchanan, III, M. K. Kidder, and C. V. Owens, Jr. Influence of Steroid Structure on the Pyrolytic Formation of Polycyclic Aromatic Hydrocarbons J. Anal. Appl. Pyrolysis 2003, 66, 73-95