PAH/Aromatic Tar and Coke Precursor Formation in the Early Stages of Triglyceride (Triolein) Pyrolysis

Abstract

There has been a limited understanding of high MW polycyclic aromatic hydrocarbon (PAH) product chemistry in the pyrolysis of triglycerides (TGs), though the subject has important implications for both fuel production from TGs and food science. Previous TG pyrolysis studies have been able to identify only relatively low MW GC-elutable aromatics occurring in the bulk liquid phase; products occurring in the solid phase have remained inaccessible to chemical analysis. In contrast, cold gas expansion molecular beam methods, where pyrolysis products are analyzed in real time as they are entrained in gas expansions, remove product collection difficulties, thereby allowing for analysis of coke/tar PAH precursors. In this study, the model TG triolein was heated and the ensuing products in the molecular beam were soft photoionized, enabling time-of-flight mass detection. Use of 266 nm pulses enabled selective photoionization of aromatic products. Unlike previous work on analysis of the liquid phase TG cracking products, a different and distinct pattern of rather large PAHs, up to 444 amu, was observed, at nontrivial relative product fractions. With an increase of temperature to ∼350 °C, a small number of PAHs with MW ≥ 276 amu increasingly dominated the aromatic product distribution. Surprisingly, PAH product detection ensued at rather low temperatures, as low as ∼260 °C. For tentative PAH product identification and product chemistry rationalization, we observed the product homology pattern and applied a stoichiometric analysis. The latter, combined with the known homology profiles of TG cracking products, indicated specific patterns of intermediate fragment association that facilitated large-MW PAH formation as a result of TG cracking.

Pathways toward PAH Formation during Fatty Acid and Triglyceride Pyrolysis

“…We observed a connection between the growth of PAH product peaks and the concurrent appearance of certain TG cracking products, particularly of C 7 size. 2,3 After considering the abundant literature on molecular mechanisms of soot formation, 20−26 we concluded that stable C 7 sized products observed using 118 nm PI could be interpreted as "markers" for reactive C 7 species that were precursors of the high MW PAHs. In particular, important reactive C 7 species include benzyl radicals; we deemed them to be essential process intermediates, 2 as was suggested earlier for soot formation upon combustion.…”

“…2,3 After considering the abundant literature on molecular mechanisms of soot formation, 20−26 we concluded that stable C 7 sized products observed using 118 nm PI could be interpreted as "markers" for reactive C 7 species that were precursors of the high MW PAHs. In particular, important reactive C 7 species include benzyl radicals; we deemed them to be essential process intermediates, 2 as was suggested earlier for soot formation upon combustion. 27,28 Formation of benzyl radicals is followed by their condensation with other C 7 and C 8 fragments to produce larger selectively formed C 22 , C 28 , and C 36 sized PAHs.…”

“…3 Using 266 nm photoionization to analyze aromatic pyrolysis products, we observed at rather low temperatures (∼200 °C) the beginning appearance of aromatic product peaks attributable to homologous series of multiring PAHs. 2,3 Particularly for T > 300 °C, these clustered peaks were replaced by three selectively formed product peaks, m/z 276, 352, and 444, tentatively identified by their molecular weight and application of Based on the MB results, TG pyrolysis now can be viewed from a new perspective, as a novel and experimentally tractable platform for studying soot formation from PAH precursors. Compared to combustion, PAH formation begins at far lower temperatures.…”

Pitch‐Based Laminated Carbon Formed by Pressure Driving at Low Temperature as High‐Capacity Anodes for Lithium Energy Storage Systems

“…As is well known, coal tar pitch is agood choice of precursor to preparec arbon materials with high graphitization and betterc onductivity even under an on-graphitization temperature [14,15] because of its high degree of aromatic ring structure. [16,17] In previous literature reports, pitch or aromatic hydrocarbon substances are generally used as lithium anode materials after being directly carbonized or graphitized.W hen the precursor is only carbonized, bulk carbon is obtained by the treatment and its capacity is low,a sW ang's work [18] shows. When the precursor is graphitized like in Reference [19],a sheet-like structure can be obtained, which displays ab etter capacity, but requires an experimental temperature that is harder to reach.…”

“…As is well known, coal tar pitch is a good choice of precursor to prepare carbon materials with high graphitization and better conductivity even under a non‐graphitization temperature [14, 15] because of its high degree of aromatic ring structure [16, 17] . In previous literature reports, pitch or aromatic hydrocarbon substances are generally used as lithium anode materials after being directly carbonized or graphitized.…”