CAPÍTULO V: RESULTADOS
5.1. Resultados de la investigación
5.1.3. Verificación de los objetivos o contrastación de las hipótesis
We acknowledge the American Chemical Society – Petroleum Research Fund Grant # 51170 UNI6, the University of San Francisco Faculty Development Fund for financial support, the usage of the chemistry computer cluster at the University of San Francisco supported by professors Claire Castro and William Karney, and the Advanced Light Source (ALS) division at the Lawrence Berkeley National Laboratory for beamtime allocation. We also thank Dr. John Savee from Sandia National labs for sharing his insight throughout the writing of this paper. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02- 05CH11231. The Sandia authors and the development of the experimental apparatus are supported by the Division of Chemical Sciences, Geosciences, and Biosciences, the Office of Basic Energy Sciences, of the U.S. Department of Energy. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the National Nuclear Security Administration under contract DE-AC04- 94-AL85000.
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4.8 References
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38. Deng, L.; Li, J.; Lai, D. M.; Fu, Y.; Guo, Q. X., Catalytic Conversion of Biomass-Derived Carbohydrates into Gamma-Valerolactone without Using an External H2 Supply. Angew. Chem. Int. Ed. Engl. 2009,48 (35), 6529-32.
39. Fegyverneki, D.; Orha, L.; Láng, G.; Horváth, I. T., Gamma-Valerolactone-Based Solvents. Tetrahedron 2010,66 (5), 1078-1081.
40. Osborn, D. L.; Zou, P.; Johnsen, H.; Hayden, C. C.; Taatjes, C. A.; Knyazev, V. D.; North, S. W.; Peterka, D. S.; Ahmed, M.; Leone, S. R., The Multiplexed Chemical Kinetic Photoionization Mass Spectrometer: A New Approach to Isomer-Resolved Chemical Kinetics. Rev. Sci. Instrum. 2008, 79, 104103.
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Chapter 5
Study of Low Temperature Chlorine Atom Initiated Oxidation of Methyl and Ethyl Butyrate Using Synchrotron Photoionization TOF-Mass Spectrometry
5.1 Abstract
The initial oxidation products of methyl butyrate (MB) and ethyl butyrate (EB) are studied using a time- and energy-resolved photoionization mass spectrometer. Reactions are initiated with Cl• radicals in an excess of oxygen at a temperature of 550 K and a pressure of 6 Torr. Ethyl crotonate is observed as the sole isomeric product of concerted HO2-elimination from initial alkylperoxy radicals formed in the
oxidation of EB. Analysis of the potential energy surface of each possible alkylperoxy radical shows that the RγO2 radical is the isomer that undergoes this concerted HO2-elimination. Two lower-mass products
(formaldehyde and acetaldehyde) are observed in both methyl and ethyl butyrate reactions. Quantum chemical (CBS-QB3) calculations show that beta-scission of the RmO2 alkylperoxy radicals of MB or EB
can produce formaldehyde or acetaldehyde, respectively, from the methyl or ethyl group. However, secondary reactions of alkylperoxy radicals with HO2 radicals can form weakly bound complexes that
subsequently decompose into the aforementioned products and smaller radicals. These pathways are more energetically favorable and are a more likely explanation for the formation of formaldehyde and acetaldehyde. The time traces for formaldehyde and acetaldehyde product formation from MB confirm that they are products of secondary reactions.
5.2 Introduction
In recent years, the production of new biofuels has started to grow due to increased environmental awareness, cost effectiveness, and a drive for energy independence. For example, ethanol is used today as a fuel additive to boost combustion and decrease pollutants, but has the disadvantage of corrosive properties,1-2 which limit the amount that can be added. Therefore, other alternatives such as esters, ethers,
and furans3-5 have been sought in the last few years.
Esters, such as methyl butyrate (MB) and ethyl butyrate (EB), shown in Scheme 5-1, have been proposed as additives and even replacements to conventional gasoline.6-7 Both compounds have the
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advantages of relatively low freezing points and high boiling points. Methyl butyrate has a melting point of -85.8 ± 0.1 oC and a boiling point of 101.9 ± 0.1 oC, while ethyl butyrate has a melting point of -97 ± 6 oC and a boiling point of 121.1 ± 0.4 oC.8 This allows the fuels to be handled under normal and extreme
conditions without special precautions. The vapor pressures are relatively low (40.0 Torr at 30 oC for MB
and 15.5 Torr at 25 oC for EB),8 which makes their use in a homogeneous-charge compression-ignition
(HCCI) engine difficult, but possible.9 MB and EB have very low toxicity and indeed are already employed
in the food industry as flavor agents.
Scheme 5-1. The compounds studied in this investigation showing the labeling scheme of the carbons used in the text (Rα, β, γ, δ, m, e denotes a radical site while Cα, β, γ, δ, m, e denotes a saturated carbon).
These esters have gained considerable interest because they can be produced through both first and second generation biofuel production methods.10 The “first-generation” method used is primarily
transesterification of vegetable oils and also some lipids from animal fats.11-13 Second-generation biofuels
must use biomass from a non-food crop. In this regard, there is an extensive amount of research on producing biodiesel from non-food sources such as rapeseed, jatropha, hemp, and microalgae.14-17
In order to accurately assess the overall impact of new fuels, their oxidation mechanisms must be considered. Oxidation reactions of ethyl butyrate and ethyl propanoate have been studied through ab initio18- 22 methods and methyl butyrate combustion has been the subject of many experiments that include exhaust
analysis,23 shock tubes,20, 24 rapid compression machine,24 opposed flow diffusion flame,25-26 and a jet stirred
reactor.25-26 There have also been detailed kinetic modeling studies of methyl butyrate with various
methods.24, 26-28 More recently, the reaction of MB + Cl• + O
2 has been studied using photoionization mass
spectrometry to analyze the HO2-elimination and cyclic ether formation pathways from the alkyl peroxy
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in the high temperature kinetic and modeling investigation by Hakka et al.30 and the ignition delay time
measurements of Dayma et al.31 In this work chlorine initiated reactions of MB and EB are studied at 550
K in the presence of oxygen at a pressure of 6 Torr. The HO2-elimination pathways for the reaction of EB-
derived radicals with O2 are analyzed and compared to similar pathways in MB oxidation. CBS-QB3
calculations are performed for the HO2-elimination pathways and for bimolecular reactions between
organic peroxy radicals (RO2) and HO2 radicals. The aim is to understand the role of the esteric alkyl group
in the low-temperature oxidation reactions of esters, chemistry that can occur during the compression heating in HCCI and other advanced engine designs.
5.2.1 Fundamental Low Temperature Oxidation Chemistry of Oxygenates and Bimolecular Reactions
At low temperature and in the presence of oxygen, the reaction between an organic radical (R) and O2 yields an RO2 radical.32 Several well-known reaction pathways are possible for the decomposition of
RO2 species. Among them, the most common at low temperatures is the concerted HO2-elimination that
forms an unsaturated coproduct. However, the RO2 radical could also undergo internal H-abstraction to
form a hydroperoxyalkyl radical (QOOH). These QOOH species could then decompose or undergo bimolecular reactions with a second O2 to initiate a chain branching reaction. More details about the
decomposition of RO2 radicals can be found elsewhere.32