Aromatic Tar and Coke Precursor Formation in the Early Stages

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes, Geochemistry, and Astrochemistry

PAH/Aromatic Tar and Coke Precursor Formation in the Early Stages of Triglyceride (Triolein) Pyrolysis Ibrahim Alhroub, Evguenii Kozliak, Alena Kubatova, and Mark Sulkes J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11340 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

PAH/Aromatic Tar and Coke Precursor Formation in the Early Stages of Triglyceride (Triolein) Pyrolysis Ibrahim Alhroub1, Evguenii Kozliak2*, Alena Kubátová2, Mark Sulkes1* 1 Chemistry Department, Tulane University, New Orleans, Louisiana 2 Department of Chemistry, University of North Dakota, Grand Forks, North Dakota 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 non-trivial relative product fractions. With an increase of temperature to ~350 ºC, 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 ºC. 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 and facilitated large-MW PAH formation as a result of TG cracking.

*corresponding authors Evguenii Kozliak [email protected] phone 701-777-2145 Mark Sulkes [email protected] phone 504-862-3587

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction In recent years there have been a growing number of real time molecular beam (MB) based studies of lignocellulosic biomass pyrolysis products, using soft photoionization followed by time-of-flight mass spectrometry (TOFMS).1-12 In contrast, molecular beam studies applied to pyrolysis chemistry of triglycerides (TGs) have been limited.13 More generally as well, the full range of studies on woody biomass far exceeds those on TGs--all understandable from the fact that potential fuel production from pyrolysis of woody biomass is much larger.14 On the other hand, pyrolysis of TGs can produce high yields of transportation fuel components (C6-C17 alkanes and aromatic hydrocarbons) with little further refining, whereas the bio oil resulting from biomass pyrolysis is still far from being suitable transportation fuel.14 In addition, some algae have TG production rates 45-220 times higher than terrestrial biomass.14 If algae production costs are lowered, TGs as renewable fuel sources may assume greater future importance. The chemical mechanisms of TG pyrolysis, either thermal or catalytic, are far from elucidated.15-19 In particular, little is known about the chemistry of thermal repolymerization of reactive intermediates, resulting in the formation of solids and extremely viscous liquids, coke and tar, respectively. These byproducts, whose formation appears to be inherent to the process, result not only in a fuel yield reduction; they also significantly reduce solid catalyst surface area. Furthermore, tars are partially soluble in the liquid product fraction, which results in fuel contamination with high molecular weight (MW) components, polycyclic aromatic hydrocarbons (PAHs). The formation mechanisms of potentially carcinogenic PAHs with multiple (>5) condensed rings are obscure because they, as well as their lower-MW precursors, have an extremely low volatility; as a result, they are not amenable to GC, the “workhorse” of the modern analysis providing accurate identification and quantification of individual chemical species.16 Nonetheless, understanding PAH formation mechanisms is essential: They are precursors of soot, ultimately leading to graphite. The other, TG-specific, motivation to study their thermal processing is the similarity of this process to those occurring during cooking with vegetable oil. The types and abundances of PAHs formed obviously contribute to the product toxicity to humans. Mass spectrometry seems to be the method of choice for PAH detection. A study using matrix assisted laser desorption ionization (MALDI) TOFMS to investigate PAHs in coal tar pitch showed the occurrence of PAHs even beyond 550 m/z.20 MB-based methods that incorporate soft photoionization followed by TOFMS are also effective for neutral PAH studies, particularly when employing REMPI (resonance enhanced multiphoton ionization).21-23 PAHs with masses approaching 103 amu have been observed in flames using these methods.24 VUV single photon ionization allows for the detection of nearly all of the neutral species present, whether aromatic or not. Jia et al.2 studied lignocellulosic biomass pyrolysis products using tunable VUV single photon ionization, with photon energies ranging from 9 to 12 eV. For detection of all neutral products, it turns out to be a fortuitous fact that 10.5 eV (118 nm) photons, our VUV wavelength, are near-optimal, enabling efficient photoionization of virtually all species of interest except small molecules (e.g., CO) with minimal fragmentation of the ions


Page 2 of 28

Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


produced. Within an order of magnitude, 118 nm single photon ionization cross sections reflect the populations of the species present.25-27 It has been reported since the 1990s that excitation within the absorbance wavelength range of aromatics (~240-270 nm) is effective in inducing two photon REMPI, or near REMPI, involving intermediate S1 or possibly T1 levels, leading to their selective ionization.23,28 Of particular note for our study, PAH species can be selectively soft photoionized with minimal fragmentation,23 then mass detected with great sensitivity, by employing REMPI. The use of the Nd:YAG 4th harmonic, 266 nm, is a convenient possibility.23,28 Selective photioinization enhancement can extend to heteroatom aromatics as well.23 However, in the subsequent discussion, our analysis using size/stoichiometry considerations is consistent with the major species under consideration being PAHs with no heteroatoms. The mechanisms of soot and soot precursor formation have been studied in many other systems, ranging from flames and internal combustion engines to interstellar clouds,21-23,29-32 but not for TG pyrolysis. The new experiments proceeded by carrying out laser photoionization TOFMS on the ongoing products of TG thermal pyrolysis at gradually incremented temperatures, aiming for the elucidation of the initial steps of PAH formation. Our attention here is focused on the detection and likely identification of individual chemical species, whereas our initial report13 made only limited and tentative product characterizations. For a clearer overview of ongoing PAH chemistry and likely product identifications, we have proceeded by employing a stoichiometric analysis approach. Methods Experimental Details of the MB-based TOFMS analysis have been described in our preliminary publication.13 Heated vapors of TGs were entrained in He carrier gas flows. The model TG under study was triolein (>99%, Sigma-Aldrich), consisting of a glycerol backbone with three ester linked oleic acid (ω-9 C18:1) moieties. Oleic acid is prevalent in crop oils such as canola. In our reflectron TOF apparatus, the mass m of a singly ionized cation can be related to its flight time t with good precision according to a fitting relation m = c1 + c2·t2, where c1 and c2 are fitting constants. These constants were established in prior experiments by compiling experimental fit (t,m) values for a variety of calibration compounds. In subsequent experiments, several factors could cause the calibrated masses to be slightly off. A photoionization laser beam alignment that is different from the standard one is probably the most important factor (cations created in slightly different locations in the ion extraction region). Gas expansion/skimmer conditions can also have small effects. Initially, the standard fitting calibration was always used to establish preliminary mass values. In some cases where slightly shifted masses were observed, mass recalibrations were done, using certain prominent peaks (e.g., m/z 352) as secondary standards. In general, mass assignments above ~200 m/z have an uncertainty of ~1 m/z. Accordingly, mass values are reported as integers. All experiments employed a two stage sample holder as schematized in Figure 1. Both stages had separate temperature controlled heaters. Deactivated glass wool was used as the 3 ACS Paragon Plus Environment


medium in both stages. In each experiment, one drop of triolein (~16 mg) was deposited in the glass wool of the first stage. According to our observations, the chemical speciation at a given temperature can depend sensitively on details and dimensions of the stages, internal medium, and deposition method. Typical He carrier gas pressures were ~0.5 atm absolute. As the two stages were heated to higher temperatures, the gas exiting the second stage pinhole began to contain volatilized pyrolysis products. After exiting the ~100 µm pinhole, the ensuing gas mixtures underwent cooling expansions; the center stream was skimmed, after which laser pulses were employed for soft ionization of neutral species present. The detected ions then underwent TOF mass analysis. We could obtain good quality results with a continuous flow pinhole arrangement, but not with 350 °C, may occur with a shift of the mechanism from paths II and III to IV. Methods similar to MB were applied before to study the soot precursors formed in other processes such as petroleum hydrocarbon combustion.22,53,54 A comparison to those studies shows that TG decomposition produces large PAHs at significantly lower temperatures. Presumably this is due to the facilitated formation of PAH building blocks within the particulate phase, as discussed above in this section. The C21-22 and C27-28 peaks (276 and 352 amu, respectively) are characteristic for intermediate temperatures while the C36 peak appears at higher temperatures and becomes predominant. The increased growth of high mass PAHs with temperature is therefore consistent with increasing association of C7 - C11 fragments. Conclusions 13 ACS Paragon Plus Environment


This study of TG pyrolysis chemistry by MB TOFMS shows the abundant formation of PAHs; their size and chemical speciation are significantly affected by temperature. The exact structures of PAHs observed cannot be predicted based on the available information (although some predictions for lower-MW PAHs can be made). However, predictions of their sizes based on the stoichiometry analysis are feasible. The large PAHs selectively formed as a result of TG decomposition appear to be coke precursors; they were not observed earlier as soot precursors in other PAH-generating processes, possibly due to longer analysis time and low volatility. At lower temperatures, below 300-350°C, the aromatic products are near C14- C20 in size, therefore likely to have been formed by the condensation (intermolecular reaction) of two to three C7 or similar size fragments. The most prominent PAHs observed at high temperatures (at 276, 352 and 444 amu) appear to be C21-22, C27-28 and C34-36 in size. Based on size, they are likely to be formed by the condensation (intermolecular reaction) of 3-5 fragments, C7 or C9-10, consistent with the earlier proposed TG cracking patterns. In summary, increasing temperature leads to stepwise addition of PAH building blocks of a certain size. This pattern explains the observed selectivity of PAH formation. One striking feature of observed formation of multi-ring PAHs is their trace appearance (m/z 276, 352, 444) at remarkably low temperatures, ~260-280 ºC, or even somewhat lower, whereas in combustion or simulated interstellar processes (laser ablation of graphite)31 the threshold temperatures are considerably higher (~103 °C). The reason for this facility may be that TGs have a “head start” for producing PAHs: In the evolving long-chain hydrocarbon products, there is the possibility that an end chain radical can fold back for ring formation, followed by ring aromatization due to dehydrogenation.16 On the other hand, combustion and laser ablation based PAH formation depend on initial association of rather small carbon/hydrocarbon fragments. The MB pyrolysis results are particularly striking in their detection of high mass PAHs-indeed, in nontrivial relative concentrations--whereas previous TG pyrolysis product analyses did not report any such PAHs. In these experiments, three factors may be significant for PAH production or detection: (1) The “chromatographic flow” preparative methods may have facilitated associative chemical reactions, leading to PAH production. (2) PAH products exit the sample holder pinhole and are subjected to PI-TOFMS before there is any opportunity for condensation or subsequent chemistry. (3) Soft photoionization of the PAH molecules in the molecular beam leads to detection without any complications. In particular, use of near-REMPI leads to sensitive and selective real time detection. For all the foregoing reasons, these methods are especially suited to studies of PAH chemistry in TG pyrolysis. Acknowledgements MS thanks the Tulane Committee on Research for support of this wo rk. The authors are grateful to Dr. I. Smoliakova (UND) for valuable suggestions on organic chemistry, helpful insights and discussion.


Page 14 of 28

Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References 1. Dufour, A.; Weng, J.; Jia, L.; Tang, X.; Sirjean, B.; Fournet, R.; Le Gall, H.; Brosse, N.; Billaud, F.; Mauviel, G.; Qi, F. Revealing the Chemistry of Biomass Pyrolysis by Means of Tunable Synchrotron Photoionisation-Mass Spectrometry. RSC Adv. 2013, 3 (14), 4786-4792. 2. Jia, L.; Le Brech, Y.; Mauviel, G.; Qi, F.; Bente-von Frowein, M.; Ehlert, S.; Zimmermann, R.; Dufour, A. Online Analysis of Biomass Pyrolysis Tar by Photoionization Mass Spectrometry. Energy Fuels 2016, 30 (3), 1555-1563. 3. Jia, L.; Le-Brech, Y.; Shrestha, B.; Frowein, M. B.; Ehlert, S.; Mauviel, G.; Zimmermann, R.; Dufour, A. Fast Pyrolysis in a Microfluidized Bed Reactor: Effect of Biomass Properties and Operating Conditions on Volatiles Composition as Analyzed by Online Single Photoionization Mass Spectrometry. Energy Fuels 2015, 29 (11), 7364-7374. 4. Le Brech, Y.; Jia, L.; Cissé; Mauviel, G.; Brosse, N.; Dufour, A. Mechanisms of Biomass Pyrolysis Studied by Combining a Fixed Bed Reactor With Advanced Gas Analysis. J. Anal. Appl. Pyrolysis 2016, 117, 334-346. 5. Li, J.; Cai, J.; Yuan, T.; Guo, H.; Qi, F. A Thermal Decomposition Study of Polymers by Tunable Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry. Rapid Commun.Mass Spectrom. 2009, 23 (9), 1269-1274. 6. Weng, J.; Jia, L.; Wang, Y.; Sun, S.; Tang, X.; Zhou, Z.; Kohse-Hoinghaus, K.; Qi, F. Pyrolysis Study of Poplar Biomass by Tunable Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry. Proc.Combust.Inst. 2013, 34 (2), 2347-2354. 7. Weng, J.; Jia, L.; Sun, S.; Wang, Y.; Tang, X.; Zhou, Z.; Qi, F. On-line Product Analysis of Pine Wood Pyrolysis Using Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry. Anal.Bioanal.Chem. 2013, 405 (22), 7097-7105. 8. Ge, S.; Xu, Y.; Tian, Z.; She, S.; Huang, L.; Zhang, Z.; Hu, Y.; Weng, J.; Cao, M.; Sheng, L. Pyrolysis Study of Pectin by Tunable Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry. J.Therm.Anal.Calorim. 2015, 120 (2), 13991405. 9. Fendt, A.; Geissler, R.; Streibel, T.; Sklorz, M.; Zimmermann, R. Hyphenation of Two Simultaneously Employed Soft Photo Ionization Mass Spectrometers with Thermal Analysis of Biomass and Biochar. Thermochim.Acta 2013, 551, 155-163. 10. Fendt, A.; Streibel, T.; Sklorz, M.; Richter, D.; Dahmen, N.; Zimmermann, R. On-Line Process Analysis of Biomass Flash Pyrolysis Gases Enabled by Soft Photoionization Mass Spectrometry. Energy Fuels 2012, 26 (1), 701-711.



11. He, T.; Zhang, Y.; Zhu, Y.; Wen, W.; Pan, Y.; Wu, J.; Wu, J. Pyrolysis Mechanism Study of Lignin Model Compounds by Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry. Energy Fuels 2016, 30 (3), 2204-2208. 12. Streibel, T.; Fendt, A.; Geißler; Kaisersberger, E.; Denner, T.; Zimmermann, R. Thermal Analysis/Mass Spectrometry using Soft Photo-ionisation for the Investigation of Biomass and Mineral Oils. J. Therm. Anal. Calorim.2009, 97 (2), 615-619. 13. Alhroub, I.; Shen, W.; Yan, J.; Sulkes, M. Thermal Cracking of Triacylglycerols: Molecular Beam Studies. J. Anal. Appl. Pyrolysis 2015, 115, 24-36. 14. Huber, G. W.; Corma, A. Synergies Between Bio- and Oil Refineries for the Production of Fuels from Biomass. Angew.Chem., Int.Ed. 2007, 46 (38), 7184-7201. 15. Kubatova, A.; Geetla, A.; Casey, J.; Linnen, M. J.; Seames, W. S.; Smoliakova, I. P.; Kozliak, E. I. Cleavage of Carboxylic Acid Moieties in Triacylglycerides During Non-Catalytic Pyrolysis. J. Am. Oil Chem. Soc.2015, 92 (5), 755-767. 16. Kubatova, A.; St'avova, J.; Seames, W. S.; Luo, Y.; Sadrameli, S. M.; Linnen, M. J.; Baglayeva, G. V.; Smoliakova, I. P.; Kozliak, E. I. Triacylglyceride Thermal Cracking: Pathways to Cyclic Hydrocarbons. Energy Fuels 2012, 26 (1), 672-685. 17. Kubatova, A.; Luo, Y.; Stavova, J.; Sadrameli, S. M.; Aulich, T.; Kozliak, E.; Seames, W. New Path in the Thermal Cracking of Triacylglycerols (Canola and Soybean Oil). Fuel 2011, 90 (8), 2598-2608. 18. Jenab, E.; Mussone, P.; Nam, G.; Bressler, D. Production of Renewable Hydrocarbons from Thermal Conversion of Abietic Acid and Tall Oil Fatty Acids. Energy Fuels 2014, 28 (11), 6988-6994. 19. Maher, K. D.; Bressler, D. C. Pyrolysis of Triglyceride Materials for the Production of Renewable Fuels and Chemicals. Bioresour.Technol. 2007, 98 (12), 2351-2368. 20. Fan, X.; Fei, Y.; Chen, L.; Li, W. Distribution and Structural Analysis of Polycyclic Aromatic Hydrocarbons Abundant in Coal Tar Pitch. Energy Fuels 2017, 31 (5), 4694-4704. 21. Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H.; Kettrup, A.; Boesl, U. On-line Emission Analysis of Polycyclic Aromatic Hydrocarbons Down to pptv Concentration Levels in the Flue Gas of an Incineration Pilot Plant with a Mobile Resonance-enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometer. Anal. Chem. 1999, 71 (1), 46-57. 22. Mueller, L.; Jakobi, G.; Orasche, J.; Karg, E.; Sklorz, M.; Abbaszade, G. Ã.; Weggler, B.; Jing, L.; Schnelle-Kreis, J.; Zimmermann, R. Online Determination of


Page 16 of 28

Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Polycyclic Aromatic Hydrocarbon Formation from a Flame Soot Generator. Anal. Bioanal. Chem. 2015, 407 (20), 5911-5922. 23. Ahrens, J.; Keller, A.; Kovacs, R.; Homann, K. H. Large Molecules, Radicals, Ions, and Small Soot Particles in Fuel Rich Hydrocarbon Flames: Part III: REMPI Mass Spectrometry of Large Flame PAHs and Fullerenes and their Quantitative Calibration through Sublimation. Ber. Bunsenges. Phys. Chem. 1998, 102 (12), 1823-1839. 24. Keller, A.; Kovacs, R.; Homann, K. H. Large Molecules, Ions, Radicals and Small Soot Particles in Fuel-Rich Hydrocarbon Flames. Part IV. Large Polycyclic Aromatic Hydrocarbons and their Radicals in a Fuel-Rich Benzene-Oxygen Flame. Phys. Chem. Chem. Phys. 2000, 2 (8), 1667-1675. 25. Yang, B.; Wang, J.; Cool, T. A.; Hansen, N.; Skeen, S.; Osborn, D. L. Absolute Photoionization Cross-Sections of Some Combustion Intermediates. Int. J. Mass Spectrom. 2012, 309, 118-128. 26. Xie, M.; Zhou, Z.; Wang, Z.; Chen, D.; Qi, F. Determination of Absolute Photoionization Cross-Sections of Oxygenated Hydrocarbons. Int. J. Mass Spectrom. 2010, 293 (1), 28-33. 27. Zhou, Z.; Zhang, L.; Xie, M.; Wang, Z.; Chen, D.; Qi, F. Determination of Absolute Photoionization Cross Sections of Alkanes and Cyclo-Alkanes. Rapid Comm. Mass Spectrom. 2010, 24 (9), 1335-1342. 28. Zimmermann, R.; Boesl, U.; Heger, H.-J.; Rohwer, E. R.; Orthner, E. K.; Schlag, E. W.; Kettrup, A. Hyphenation of Gas Chromatography and Resonance-Enhanced Laser Mass Spectrometry (REMPI-TOFMS): A Multidimensional Analytical Technique. J. Sep. Sci. 1997, 20 (9), 461-470. 29. Eschner, M. S.; Zimmermann, R. Determination of Photoionization Cross-Sections of Different Organic Molecules using Gas Chromatography Coupled to SinglePhoton Ionization (SPI) Time-of-Flight Mass Spectrometry (TOF-MS) with an Electron-Beam-Pumped Rare Gas Excimer Light Source (EBEL): Influence of Molecular Structure and Analytical Implications. Appl. Spectrosc. 2011, 65 (7), 806-816. 30. Radischat, C.; Sippula, O.; Stengel, B.; Klingbeil, S.; Sklorz, M.; Rabe, R.; Streibel, T.; Harndorf, H.; Zimmermann, R. Real-Time Analysis of Organic Compounds in Ship Engine Aerosol Emissions using Resonance-Enhanced Multiphoton Ionisation and Proton Transfer Mass Spectrometry. Anal. Bioanal. Chem. 2015, 407 (20), 5939-5951.



31. Jager, C.; Huisken, F.; Mutschke, H.; Jansa, I. L.; Henning, T. Formation of Polycyclic Aromatic Hydrocarbons and Carbonaceous Solids in Gas-Phase Condensation Experiments. Astrophys. J. 2009, 696 (1), 706. 32. Joblin, C.; Tielens, A. G. G. M.; Jager, C.; Mutschke, H.; Henning, T.; Huisken, F. From PAHs to Solid Carbon. Eur. Astronom. Soc. Publs. Series 2011, 46, 293-304. 33. Eveleigh, A.; Ladommatos, N.; Hellier, P.; Jourdan, A. L. An Investigation into the Conversion of Specific Carbon Atoms in Oleic Acid and Methyl Oleate to Particulate Matter in a Diesel Engine and Tube Reactor. Fuel 2015, 153, 604-611. 34. Fialkov, A. B.; Dennebaum, J.; Homann, K. H. Large Molecules, Ions, Radicals, and Small Soot Particles in Fuel-Rich Hydrocarbon Flames Part V: Positive Ions of Polycyclic Aromatic Hydrocarbons (PAH) in Low-Pressure Premixed Flames of Benzene and Oxygen. Combust. Flame 2001, 125 (1-2), 763-777. 35. Zhirong, Z. A New Method for Analysis of Coke on Catalysts. Analyst 2001, 126 (10), 1669-1673. 36. Lappi, H.; Alen, R. Production of Vegetable Oil-Based Biofuels: Thermochemical Behavior of Fatty Acid Sodium Salts during Pyrolysis. J. Anal. Appl. Pyrolysis 2009, 86 (2), 274-280. 37. Lappi, H.; Alen, R. Pyrolysis of Vegetable Oil Soaps-Palm, Olive, Rapeseed and Castor Oils. J. Anal. Appl. Pyrolysis 2011, 91 (1), 154-158. 38. Asomaning, J.; Mussone, P.; Bressler, D. C. Thermal Deoxygenation and Pyrolysis of Oleic Acid. J.Anal.Appl.Pyrolysis 2014, 105, 1-7. 39. Wang, K.; Wang, S.; Guo, X.; Luo, Z.; Fransson, T. Comparison of Pyrolysis Characteristics of Degreased and Synthesized Mongolian Pine. AIP Conf.Proc. 1251[2nd Internl. Symp. Aqua Sci., Water Resource and Low Carbon Energy, 2009] 2010, 221-224. 40. Raj, A.; Prada, I. D. C.; Amer, A. A.; Chung, S. H. A Reaction Mechanism for Gasoline Surrogate Fuels for Large Polycyclic Aromatic Hydrocarbons. Combust. Flame 2012, 159 (2), 500-515. 41. Wang, Y.; Raj, A.; Chung, S. H. A PAH Growth Mechanism and Synergistic Effect on PAH Formation in Counterflow Diffusion Flames. Combust. Flame 2013, 160 (9), 1667-1676. 42. Raj, A.; Al Rashidi, M. J.; Chung, S. H.; Sarathy, S. M. PAH Growth Initiated by Propargyl Addition: Mechanism Development and Computational Kinetics. J. Phys. Chem. A 2014, 118 (16), 2865-2885.


Page 18 of 28

Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


43. Bian, H.; Wang, Z.; Zhang, F.; Wang, Z.; Zhu, J. Unimolecular Reaction Properties for the Long-Chain Alkenyl Radicals. Int. J. Chem. Kinetics 2015, 47 (11), 685-694. 44. Wang, K.; Villano, S. M.; Dean, A. M. Reactivity-Structure-Based Rate Estimation Rules for Alkyl Radical H Atom Shift and Alkenyl Radical Cycloaddition Reactions. J. Phys. Chem. A 2015, 119 (28), 7205-7221. 45. Myshkin, V. E.; Shostenko, A. G.; Zagorets, P. A. Rate Constants of Alkyl Radical Addition to Olefins. Kinet. Katal. 1979, 20 (2), 298-301. 46. Degirmenci, I.; Coote, M. L. Comparison of Thiyl, Alkoxyl, and Alkyl Radical Addition to Double Bonds: The Unusual Contrasting Behavior of Sulfur and Oxygen Radical Chemistry. J. Phys. Chem. A 2016, 120 (10), 1750-1755. 47. Sinha, S.; Raj, A. Polycyclic Aromatic Hydrocarbon (PAH) Formation from Benzyl Radicals: A Reaction Kinetics Study. Phys. Chem. Chem. Phys. 2016, 18 (11), 8120-8131. 48. Hawari, J. A.; Engel, P. S.; Griller, D. Rate Constants for the Reactions of Alkyl Radicals with 1, 4-Cyclohexadiene. Int. J. Chem. Kin. 1985, 17 (11), 1215-1219. 49. Wang, H.; Yang, H.; Chuang, W.; Ran, X.; Shi, Q.; Wen, Z. Pyrolysis Mechanism of Carbon Matrix Precursor Cyclohexane - The Formation of Condensed-Ring Aromatics and the Growing Process of Molecules. J. Mol. Graph. Model. 2007, 25 (6), 824-830. 50. Indarto, A.; Giordana, A.; Ghigo, G.; Maranzana, A.; Tonachini, G. Polycyclic Aromatic Hydrocarbon Formation Mechanism in the "Particle Phase". A Theoretical Study. Phys. Chem. Chem. Phys. 2010, 12 (32), 9429-9440. 51. Sinha, S.; Rahman, R. K.; Raj, A. On the Role of Resonantly Stabilized Radicals in Polycyclic Aromatic Hydrocarbon (PAH) Formation: Pyrene and Fluoranthene Formation from Benzyl-Indenyl Addition. Phys. Chem. Chem. Phys. 2017, 19 (29), 19262-19278. 52. McEnally, C. S.; Pfefferle, L. D.; Atakan, B.; Kohse-Höinghaus, K. Studies of Aromatic Hydrocarbon Formation Mechanisms in Flames: Progress Towards Closing the Fuel Gap. Prog. Energy Comb. Sci. 2006, 32 (3), 247-294. 53. Oektem, B.; Tolocka, M. P.; Zhao, B.; Wang, H.; Johnston, M. V. Chemical Species Associated with the Early Stage of Soot Growth in a Laminar Premixed EthyleneOxygen-Argon flame. Combust. Flame 2005, 142 (4), 364-373. 54. Dilger, M.; Orasche, J.; Zimmermann, R.; Paur, H. R.; Diabate, S.; Weiss, C. Toxicity of Wood Smoke Particles in Human A549 Lung Epithelial Cells: The Role of PAHs, Soot and Zinc. Arch. Toxicol. 2016, 90 (12), 3029-3044. 19 ACS Paragon Plus Environment


Figure 1. Schematic of the apparatus used. The flight trajectories of the ions are reflected (reflectron) to afford better mass resolution.


Page 20 of 28

Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


276

266 nm PI

444

352

263 276 281

118 nm PI

444

352 360

0

100

200

300

499

400

500

m/z

Figure 2. Comparative triolein TOFMS at ~380 °C with 266 nm PI and 118 nm PI. The 266 nm PI TOFMS contains aromatic product peaks; all these strong peaks are also evident in the 118 nm PI TOFMS. Other peaks in the 118 nm TOFMS are due to non-aromatic products.


The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 22 of 28

444 276 360/375 °C, 1440 V

352 Figure 3. TOFMS of triolein using 266 nm PI as a function of temperature and detector voltage from a single experimental run. The detector response produces a 10x sensitivity increase for an increase of ~200-250 V. An apparent homology series is marked with the corresponding MW values starting with 204 amu.

340/350 °C, 1500 V

320/330 °C, 1530 V

285/300 °C, 1550 V

250/260 °C, 1630 V

200/220 °C, 1730 V

260 232 184 218 246 274 288 204

50/60 °C, 1820 V

0

100

200

300

400

m/z 22 ACS Paragon Plus Environment

500

Page 23 of 28


184 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

2 2 2

92 116

80

100

120

144

130

158

140

160

172

180

200

m/z Figure 4. Magnified portion of the 250/260 ºC TOFMS from Figure 3. Note numerous 2 m/z product spacings, a few indicated. An apparent homology series is also indicated.



ratio to m/z 276 peak 2.5 2 1.5 260 amu 1

352 amu

0.5

444 amu

0 250 270 290 310 330 350 370

temperature, °C Figure 5. Product peak trends versus temperature in pyrolysis of triolein using measured peak heights from 266 nm PI TOFMS. Growth of the m/z 352 and 444 peaks relative to the m/z 276 peak as a function of temperature, based on peak height measurements. The m/z 276, 352, and 444 products all display persistence or growth at higher temperatures. In contrast, the m/z 260 product, significant relatively at lower temperatures, displays decreasing relative importance at higher temperatures.


Page 24 of 28

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Names, structures and stoichiometry of low-MW PAHs.



Scheme 2. Potential reactions leading to PAH formation


Page 26 of 28

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Tentative Arhenius activation energies of reactions leading to PAH formation

Reaction

Ea range, kJ/mol

Reference

Radical recombination

0

40-42, 47

1,5- and 1,6- H atom

7-15

43

migration in alkenyl radicals

6-13

44

Intramolecular endo-cyclization

6-7

44

Facilitated radical addition to alkenes

͂7

45

Radical addition to alkenes

29-36

46

Radical addition to arenes

10-15

42

Facilitated intermolecular

5-7

48

up to 37

44

Dehydrogenation/aromatization

͂350

49

Facilitated dehydrogenation, e.g.,

͂13

47

of alkenyl radicals

H-atom abstraction Non-facilitated intermolecular H-atom abstraction

phenantrene from stilbene



82x44mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 28 of 28

Aromatic Tar and Coke Precursor Formation in the Early Stages

Recommend Documents