Theoretical Investigations Into Combustion Kinetics Of Small Alcohol Systems

Energy crisis is one of the major issues human beings face nowadays. The rapid depletion of fossil fuels is compelling people to try to find new energy resources to replace the conventional fuels. Biofuels, including alcohols, ethers, and esters, are considered as the most promising alternative fuels, Ini particular, alcohols have already been widely used as fuel additives into the engine, and even directly used as a fuel in some regions. Consequently, it becomes very necessary to understand in depth the combustion behavior of alcohols under relevant conditions. This has led to the first goal of this dissertationexploring the combustion kinetics of representative alcohols from a theoretical perspective.Radical-initiated H abstraction and unimolecular decomposition are two sorts of significant reactions among the major consumption pathways, In this dissertation, we have chosen proper alcohol systems to study these two specific sorts of reactions. With respect to H abstraction reaction, we have chosen OH+methanol as the target system with the aim to explore in great detail the reaction kinetics of hydrogen abstraction from methanol by OH radical. H abstraction is amongst the intractable reactions which the kinetic theorists find it pretty tricky to deal with. From this point, it bares great significance in exploring the reaction kinetics of H abstraction and developping appropriate theoretical treatment. Here in this dissertation, we have taken the OH+methanol as a good case study to investigate and analyze the possible complexities that may be encountered in this sort of reactions. We hope that this work can shed some useful light into related studies in the future.When methanol reacts with OH radical, a pre-reaction complex is first formed via Van der Waals1force, and then the complex overcomes different transition states leading to various products. OH radial can abstract H atom from either the methyl site (Al) or the hydroxyl site (A2) in CH3OH, reaction pathways being as follows: OH+CH3OHâ†'[HO…CH3OH]â†'CH2OH+H2O(Al) OH+CH3OHâ†'[HO…CH3OH]â†'CH3O+H2O(A2)When making theoretical predictions for the mechanism above, there exist a variety of complexities that are also probably encountered in H abstraction of similar systems. We are giving a brief summary here.(1) Sensitivity to electronic structure method. Molecular geometries and rovibrational properties are rather sensitive to electronic structure method. A number of different levels need to be applied in the same task and compared to pick the appropriate one.(2) Reaction path variational effect. Path (A1) possesses quite a strong variational effect, and hence the variational transition state theory is used for rate predictions.(3) Quantum mechanical tunneling. Path (A2) bares a negligible variational effect, yet the quantum tunneling is pretty strong at low temperature. The magnitude of tunneling effect is determined greatly by the imaginary frequency of saddle point, which is much more sensitive to electronic structure method as opposed to normal vibrational frequencies.(4) Strongly coupled hindered rotors. A two-dimensional hindered rotor treatment is used to mimic the two coupled hindered rotors in transition states. Balancing computational efficiency and energy accuracy, B2PLYPD3/6-311++G(d,p) is a good compromise to scan the two-dimensional hinderance potential curves.(5) Existence of pre-reaction complex. At high temperature, the influence of this complex could be ignored, whereas quantum effect dominates the reaction at low temperature and thereby it becomes very important. The existence of the pre-reaction complex changes the competition between (A1) and (A2), and brings about some pressure dependence at low temperature.(6) Excited transition state. In this case, the excited state of transition state makes a very minor contribution, i.e., less than6%, to the overall rate.(7) Isotopic effect. We also study the deuterated reaction of OH+CH3OH and obtain corresponding rates for them:OH+CH3OD, OH+CD3OH.Beside H abstraction reaction, another important reaction type we investigate in this dissertation is the unimolecular decomposition of small alcohols. For this purpose we have chosen these target systems:ethylene glycol, n-propanol,1,2-and1,3-propanediol, and glycerol. Up to date, monohydroxy alcohols, ethanol and butanol, are the dominant biofuels, and propanol is another potential representative. However, polyols are considered to have a great potential to become the new energy platform. For these alcohol systems, on one hand, we have explored in depth the unimolecular decomposition mechanism. On the other hand, we have discussed the influence of OH group number on the decomposing kinetics especially on C-C bond fissions.Direct bond fission reactions and H2O elimination reactions are the most important pathways in the unimolecular decomposition of alcohol systems, either monohydroxy or polyhydroxy alcohols. Other reaction pathways, like H2elimination, make a very minor contribution to the overall decomposition mechanism. Specifically, H2O elimination reactions dominate the low temperature decomposition mechanism, while C-C bond fission becomes more important at high temperature. It is the combined effect of enthalpy and entropy that results in the varying competition with temperature between these two sorts of reactions. At low temperature region, the enthalpy effect contributes a larger part to the reaction rate coefficients, and therefore the H2O eliminations, barriers of those being lower than C-C bond fission reactions, play a dominant role. With temperature increasing, the entropy effect becomes more significant, finally making C-C bond fission the fastest process.For C3molecules studied in this dissertation, the number of OH groups tends to influence to some extent the magnitude of the C-C bond dissociation energy. When OH is bonded with one of the C atoms involved in C-C bond fission, the C-C bond dissociation energy is inclined to decrease. When OH groups are bonded with both two C atoms in the dissociating C-C bond, the dissociation energy seems to continue to decrease. However, the author is conservative about this conclusion being generalized to larger molecules. That will definitely require more efforts, which is out of the scope of present work.Propene is amongst the most important products of unimolecular decomposition of propanol. It is the most typical member after ethylene in the alkene family, is a key intermediate in the combustion of many higher hydrocarbons and alcohols. Its pyrolysis kinetics plays an integral part of Cl-C4hydrocarbon mechanism. In view of its great significance in combustion chemistry, we perform an exhaustive theoretical analysis of the thermal kinetics for the C3H6system. There are a number of important combustion relevant reactions that occur over the C3H6potential energy surface, for example, the propene unimolecular decomposition, the recombination of CH3+C2H3and H+CH2CHCH2, the isomerization between propene and cyclo-propane, and so on. The recombination of CH3+C2H3is very typical amongst the radical-radical reactions, and is the simplest reaction between alkyl and alkenyl radicals. At low temperature, the major product of this reaction is propene while as temperature increases it leads to more H+CH2CHCH2. The recombination of H and CH2CHCH2is suggested to play a central role in aromatic ring formation and provides a valuable prototype for related larger resonantly stabilized radicals. Moreover, the reaction of singlet methylene (’CH2) with ethylene (C2H4) gives rise to the resonantly stabilized allyl radical at temperature higher than1500K, the key temperature region for soot formation, which supports the viewpoint that allyl radical plays a role in the formation of aromatic rings. Generally speaking, the theoretical predictions for relevant channels are generally in quite satisfactory agreement with existing experimental data. It is worth mentioning that the greatest uncertainty in our predictions arises from that in the treatment of collisional energy transfer. This shortcoming still remains a problematic issue for combustion chemistry which certainly deserves more research efforts in future.Another important goal of this work is to show how reaction kinetics plays its role in fundamental combustion research. Actually, this has already been illustrated in detail when predicting the rate coefficients of a reaction. Only a brief description is given here. Application of reaction kinetics in combustion has developed greatly in past several decades, and already become the most efficient methodology in theoretical predictions. By solving the eigenvalues of the master equation, which is based on the RRKM theory, one can obtain the rate coefficients of elementary reactions. The RRKM-master equation method is now the most widely employed approach in theoretical investigations of combustion kinetics, and is also the method used in this work. General procedures of estimating reaction rate coefficients are as follows. Specifically, one first selects the appropriate quantum chemistry models to construct the potential energy surfaces of the reaction system, i.e., try to find all the thermochemically accessible reaction pathways. Once we get the energetics of the system along with rovibrational parameters and moments of inertia, etc., we could then calculate the high-pressure-limit rate coefficients for the target reaction. After that, collision energy transfer is taken into account and the pressure-dependent rates of the reaction are obtained via solving the master equation. We have always been using the method described above to perform the theoretical study in this dissertation, and we have illustrated every step in our specific calculations.