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Title: Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules

Abstract

Oxygenated biofuels provide a renewable, domestic source of energy that can enable adoption of advanced, high-efficiency internal combustion engines, such as those based on homogeneously charged compression ignition (HCCI). Of key importance to such engines is the cetane number (CN) of the fuel, which is determined by the autoignition of the fuel under compression at relatively low temperatures (550-800 K). For the plethora of oxygenated biofuels possible, it is desirable to know the ignition delay times and the CN of these fuels to help guide conversion strategies so as to focus efforts on the most desirable fuels. For alkanes, the chemical pathways leading to radical chain-branching reactions giving rise to low-temperature autoignition are well-known and are highly coincident with the buildup of reactive radicals such as OH. Key in the mechanisms leading to chain branching are the addition of molecular oxygen to alkyl radicals and the rearrangement and dissociation of the resulting peroxy radials. Prediction of the temperature and pressure dependence of reactions that lead to the buildup of reactive radicals requires a detailed understanding of the potential energy surfaces (PESs) of these reactions. In this study, we used quantum mechanical modeling to systematically compare the effects of oxygen functionalitiesmore » on these PESs and associated kinetics so as to understand how they affect experimental trends in autoignition and CN. The molecules studied here include pentane, pentanol, pentanal, 2-heptanone, methylpentyl ether, methyl hexanoate, and pentyl acetate. All have a saturated five-carbon alkyl chain with an oxygen functional group attached to the terminal carbon atom. The results of our systematic comparison may be summarized as follows: (1) Oxygen functionalities activate C-H bonds by lowering the bond dissociation energy (BDE) relative to alkanes. (2) The R-OO bonds in peroxy radicals adjacent to carbonyl groups are weaker than corresponding alkyl systems, leading to dissociation of ROO radicals and reducing reactivity and hence CN. (3) Hydrogen atom transfer in peroxy radicals is important in autoignition, and low barriers for ethers and aldehydes lead to high CN. (4) Peroxy radicals formed from alcohols have low barriers to form aldehydes, which reduce the reactivity of the alkyl radical. In conclusion, these findings for the formation and reaction of alkyl radicals with molecular oxygen explain the trend in CN for these common biofuel functional groups.« less

Authors:
ORCiD logo [1];  [2]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1];  [1]
  1. National Renewable Energy Lab. (NREL), Golden, CO (United States)
  2. National Renewable Energy Lab. (NREL), Golden, CO (United States)s
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE)
OSTI Identifier:
1373677
Report Number(s):
NREL/JA-5100-68971
Journal ID: ISSN 1089-5639
Grant/Contract Number:
AC36-08GO28308
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory
Additional Journal Information:
Journal Volume: 121; Journal Issue: 29; Journal ID: ISSN 1089-5639
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
English
Subject:
09 BIOMASS FUELS; biofuels; autoignnition; oxygenation

Citation Formats

Ciesielski, Peter N., Robichaud, David J., Kim, Seonah, McCormick, Robert L., Foust, Thomas D., Nimlos, Mark R., and Bu, Lintao. Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules. United States: N. p., 2017. Web. doi:10.1021/acs.jpca.7b04000.
Ciesielski, Peter N., Robichaud, David J., Kim, Seonah, McCormick, Robert L., Foust, Thomas D., Nimlos, Mark R., & Bu, Lintao. Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules. United States. doi:10.1021/acs.jpca.7b04000.
Ciesielski, Peter N., Robichaud, David J., Kim, Seonah, McCormick, Robert L., Foust, Thomas D., Nimlos, Mark R., and Bu, Lintao. Wed . "Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules". United States. doi:10.1021/acs.jpca.7b04000.
@article{osti_1373677,
title = {Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules},
author = {Ciesielski, Peter N. and Robichaud, David J. and Kim, Seonah and McCormick, Robert L. and Foust, Thomas D. and Nimlos, Mark R. and Bu, Lintao},
abstractNote = {Oxygenated biofuels provide a renewable, domestic source of energy that can enable adoption of advanced, high-efficiency internal combustion engines, such as those based on homogeneously charged compression ignition (HCCI). Of key importance to such engines is the cetane number (CN) of the fuel, which is determined by the autoignition of the fuel under compression at relatively low temperatures (550-800 K). For the plethora of oxygenated biofuels possible, it is desirable to know the ignition delay times and the CN of these fuels to help guide conversion strategies so as to focus efforts on the most desirable fuels. For alkanes, the chemical pathways leading to radical chain-branching reactions giving rise to low-temperature autoignition are well-known and are highly coincident with the buildup of reactive radicals such as OH. Key in the mechanisms leading to chain branching are the addition of molecular oxygen to alkyl radicals and the rearrangement and dissociation of the resulting peroxy radials. Prediction of the temperature and pressure dependence of reactions that lead to the buildup of reactive radicals requires a detailed understanding of the potential energy surfaces (PESs) of these reactions. In this study, we used quantum mechanical modeling to systematically compare the effects of oxygen functionalities on these PESs and associated kinetics so as to understand how they affect experimental trends in autoignition and CN. The molecules studied here include pentane, pentanol, pentanal, 2-heptanone, methylpentyl ether, methyl hexanoate, and pentyl acetate. All have a saturated five-carbon alkyl chain with an oxygen functional group attached to the terminal carbon atom. The results of our systematic comparison may be summarized as follows: (1) Oxygen functionalities activate C-H bonds by lowering the bond dissociation energy (BDE) relative to alkanes. (2) The R-OO bonds in peroxy radicals adjacent to carbonyl groups are weaker than corresponding alkyl systems, leading to dissociation of ROO radicals and reducing reactivity and hence CN. (3) Hydrogen atom transfer in peroxy radicals is important in autoignition, and low barriers for ethers and aldehydes lead to high CN. (4) Peroxy radicals formed from alcohols have low barriers to form aldehydes, which reduce the reactivity of the alkyl radical. In conclusion, these findings for the formation and reaction of alkyl radicals with molecular oxygen explain the trend in CN for these common biofuel functional groups.},
doi = {10.1021/acs.jpca.7b04000},
journal = {Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory},
number = 29,
volume = 121,
place = {United States},
year = {Wed Jul 05 00:00:00 EDT 2017},
month = {Wed Jul 05 00:00:00 EDT 2017}
}

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  • The phases La{sub 15}Ge{sub 9}Z, Z = Mn, Fe, Co, No, Cu, Ru, C, O, P, have been obtained from reactions of either the elements or suitable binary compounds in sealed Ta containers at 1200-1350{degrees}C. All exhibit an ordered {radical}3a {times} {radical}3a {times} C superstructure of the Mn{sub 5}Si{sub 3} type (La{sub 5}Ge{sub 3}, La{sub 5}Ge{sub 3}Z, P6{sub 3}/mcm) in which two-thirds of the slightly distorted confacial chains of trigonal antiprisms (La{sub 6/2}Ge{sub 6/2}) have Z bound in alternate, contracted interstitial sites. The room-temperature structures for Z=Mn, Fe, Co, Ni, C, P were refined from single-crystal data (space group P6{submore » 3}mc, Z=2; for the protypical Fe, a = 15.4810(2) {Angstrom} and c=6.8768(3) {Angstrom}). The unit cell lengths and volumes and d(Z-Z) values show notably smaller variations with Z than in La{sub 5}Ge{sub 3}Z or related systems. The buffering effect of empty cavities in the commensurate chains and the weak La-La bonding appear to be responsible. Magnetic susceptibility data and XPS core shifts are reported for the Fe and, in part, the Co and Ni phases La{sub 15}Ge{sub 9}Z. All three are metallic with decreasing moments of {approximately}1.83, {approximately}0.3, and {approximately}0{mu}{sub B}, respectively. In contrast, the Fe-richer La{sub 5}Ge{sub 3}Fe is a very soft ferromagnet at room temperature. Members of the sequence Fe, La{sub 5}Ge{sub 3}Fe, La{sub 15}Ge{sub 9}Fe exhibit regularly decreasing moments as well. The Fe 2p{sub 3/2} core levels show a 1.7 eV decrease over the same series, i.e., increasing reduction, while an evidently regular oxidation of La from the element through La{sub 5}Ge{sub 3} and La{sub 15}Ge{sub 9}Fe to La{sub 5}Ge{sub 9}Fe to La{sub 5}Ge{sub 3}Fe is indicated by a regular overall increase of 2.7 eV in La 3d{sub 5/2} binding energies.« less
  • The aim of this work is to establish the mechanisms of the elementary acts of photophysical and photochemical processes in an homologous series of phenylmethane molecules Ph/sub n/C-H/sub m-n/, where m = 4, n = 1,...,4, and pH is a phenyl radical. The molecules that form the homologous series belong to the same orbital class (in this case sigma..pi..) and the same spectral-luminescence systematization group (SLG). In the studied series of molecules, as a result of the change of the number of the (Ph) and (C-H) systems, an evolution of the orbital nature of the states occurs (in the givenmore » orbital class) on insignificant change of their relative position. As a result of this a considerable redistribution of the relaxation channels of the electron-excitation energy and the rate constants of the radiation and radiationless processes takes place. To establish the character of these changes they performed experimental and theoretical studies of the mentioned series of molecules.« less
  • Vibrational to translational (VT) energy-transfer rate constants (k[sub VT]) for two series of fluoroalkanes with argon as deactivator were measured by using time-resolved optoacoustics. A pulsed CO[sub 2] laser was used to excite the fluorinated alkanes; the average excitation energy, (E), was in the range 15 000-40 000 cm[sup [minus]1], k[sub VT] was found to be independent of (E), indicating that the average energy transferred per collision, (([Delta]E)), is linear with (E). It is observed that k[sub VT] decreases (by <30%) as the number of vibrational modes increase by a factor approximately 2.7 in this homologous series. This is contrarymore » to that observed for small molecules at low excitation energy, where k[sub VT] increases. Both the observed per-collision relaxation efficiency, [beta][sub obs], and (([Delta]E))/(E) decrease with an increase in the number of carbon or fluorine atoms. These results are in qualitative agreement with a model in which the relaxation occurs via a single low-frequency doorway oscillator which is in statistical equilibrium with the remaining oscillators (bath) such that E[sub osc] = g(E). This oscillator has an intrinsic efficiency, [beta][sub int] = (([Delta]E))/(E)[sub osc], which is equal for all members of the series, so [beta][sub obs] = g[beta][sub int]. The decrease in [beta][sub cos] and (([Delta]E))is due to the resulting decrease in (E)[sub osc] as the number of effective vibrational modes increases for a given total energy. 48 refs., 4 figs., 1 tab.« less