Theoretical Studies On The Reaction Kinetics Of Biodiesel Surrogates

The growing energy demand in industrialized societies and the environmental problems caused by the widespread use of fossil fuels,have greatly contributed to the development of clean and renewable energy alternatives.Biodiesel is considered to be one of the most promising alternative fuels because of its environmentally friendly renewable,properties close to diesel fuel.At present,biodiesel and conventional hydrocarbon-based diesel blends can be used directly in existing standard engines,and thus are widely available in the diesel retail market.Pure biodiesel can be used as a fuel without mixing with alkanes,but it is necessary to modify existing engines according to their special combustion behavior to avoid problems such as unsatisfactory performance loss.The chemical reaction mechanism of biodiesel combustion has important guiding significance for its actual combustion process.Therefore,the primary goal of this paper is to study the reaction kinetics mechanism of biodiesel surrogate molecules from theoretical perspectives.In order to develop a cleaner and more efficient combustion technology,the low-temperature oxidation technology,such as HCCI fire of homogeneous charge compression point,its kinetic mechanism of the target fuel is urgently needed.Methyl actate(MA)is the smallest methyl ester containing carbon chain,only one carbon atom attached to the methyl ester group.It is also an important reaction intermediate in the biodiesel pyrolysis process and a potential pollutant for atmospheric degradation.As an important surrogate molecule to explore the kinetics of methyl ester radical oxidation,the addition reaction of MA and O2 reaction contains the characteristic reaction of biodiesel and meets the high-level calculation requirements.Theoratical studies on the chemistry of MA and O2 was conducted to get further understanding of biodiesel combustion.Reactions of the first oxygen addition to MA radicals has been investigated by high level quantum chemical methods,and rate constants were computed by using microcanonical variational transition state theory coupled with Rice-Ramsberger-Kassel-Marcus/Master-Equation theory.The calculated rate constants agree reasonably well with both theoretical and experimental results of chain-like alkoxy radicals.We considered each step in the oxidation process as a class of reaction,including all the possible reactions taking place,only the formation and re-dissociation of initial adducts are critical for the low temperature combustion of MA.The current study is an extension of kinetic data for such chain propagation reactions for MA oxidation in a wider pressure and temperature range,which can be used for the modeling study of low temperature oxidation of methyl esters.The catalytic oxidation ofMA over the H-ZSM-5 zeolites,which plays a cr,itical role in energy supply,environmental protection and industrial applications,has been investigated systematically and thoroughly by using calculations at the M06-2X/6-311++G(d,p)level of theory.Feasible reactions of the protolytic cracking channels are of particular interest in the present study,and it is found that the formation reactions of the ketene in the concerted mechanism and the formation reaction of acetyloxy+CH3 are competitive during MA consumption.Furthermore,energy comparisons between the catalytic pyrolysis and conventional pyrolysis of MA are carried out to demonstrate the benefits due to the introduction of catalysts in MA combustion.It is demonstrated that the energy barriers of the dissociation reactions for MA over H-ZSM-5 zeolites decrease significantly with respect to those of the corresponding reactions in the conventional pyrolysis of MA,which lowers down the ignition temperature considerably.This work provides new insight into the mechanism of the catalytic pyrolysis of MA that will guide the improvements in the engine combustion efficiency and in the control of volatile organic compounds,and will also help to improve the selectivity of the conversion of methanol to hydrocarbon and olefin products.The isomerization and dissociation reactions of methyl decanoate(MD)radicals were theoretically investigated by using high-level theoretical calculations based on a two-layer ONIOM method,employing the QCISD(T)/CBS method for the high layer and the M06-2X/6-311++G(d,p)method for the low layer.Temperature-and pressure-dependent rate coefficients for involved reactions were computed by using the transition state theory and the Rice-Ramsperger-Kassel-Marcus/Master-Equation method.The structure-reactivity relationships were explored for the complicated multiple-well interconnected system of ten isomeric MD radicals.Comparative studies of methyl butanoate(MB)and MD were also performed systematically.Results show that the isomerization reactions are appreciably responsible for the population distribution of MD radicals at low and intermediate temperatures,while the p-scission reactions are dominant at higher temperatures.Although the rate constants of MB specific to methyl esters are close to those of MD in certain temperature ranges,MB is unable to simulate most of dissociation reactions due to its short aliphatic chain.Significant differences of rate constants for isomerization reactions were observed between the calculated results and the literature data,which were estimated by analogy to alkane systems,but the rate constants of ?-scissions show generally good agreement between theory and experiment.The current work extends kinetic data for isomerization and dissociation reactions of MD radicals,and it serves as a reference for the studies of detailed combustion chemistry of practical biodiesels.A two-layer ONIOM[QCISD(T)/CBS:DFT]method was proposed for the high-level single-point energy calculations of large biodiesel molecules and was validated for the hydrogen abstraction reactions of unsaturated methyl esters that are important components of real biodiesel.The reactions under investigation include all the reactions on the potential energy surface of CnH2n-1COOCH3(n=2-5,17)+ H·,including the hydrogen abstraction,the hydrogen addition,the isomerization(intramolecular hydrogen shift),and the ?-scission reactions.By virtue of the introduced concept of chemically active center(CAC),a unified specification of chemically active portion(CAP)for the ONIOM method was proposed to account for the additional influence of C=C double bond.The predicted energy barriers and heat of reactions by using the ONIOM method are in very good agreement with those obtained by using the widely accepted high-level QCISD(T)/CBS theory,as verified by that the computational deviations are less than 0.15 kcal/mol,for almost all the reaction pathways under investigation.The method provides a computationally accurate and affordable approach to combustion chemists for high-level theoretical chemical kinetics of large biodiesel molecules.High-level activation energy prediction is of great importance in determining accurate kinetic parameters of practical biodiesel combustion mechanism.However,its determination of practical biodiesels by electronic structure-based theory is challenging for huge computational load.Here,the Bell-Evans-Polanyi relations specific to biodiesels were introduced specifically to predict accurate activation energies.Thermal energetics of feasible hydrogen abstraction reactions for biodiesel surrogates by H and OH radicals for the Bell-Evans-Polanyi relations and their relationships between the reactivity and the structure have been investigated via high level orbital-based methods.Reaction classes within the biodiesel family were defined though distinctive electron interactions to verify the reaction patterns due to the complex characteristic of esters.It was observed that there was significantly linear Bell-Evans-Polanyi relationship between activation energies and corresponding reaction enthalpies within a given reaction class.Reliability of the fitted Evans-Polanyi relations of biodiesels was confirmed though comparisons with available high-level calculation results by deviations less than 0.90kcal/mol.Fitted Evans-Polanyi relations within different reaction classes provide effective solutions to the long-term kinetic issues of biodiesel combustion.