Structures and Energetics of Unimolecular Thermal Degradation of Isopropyl Butanoate as a Model Biofuel: Density Functional Theory and Ab Initio Studies Ahmed M. El-Nahas,* Lobna A. Heikal, Ahmed H. Mangood, and El-Sayed E. El-Shereefy Chemistry Department, Faculty of Science, El-Menoufia UniVersity, Shebin El-Kom, Egypt ReceiVed: April 15, 2010; ReVised Manuscript ReceiVed: June 21, 2010 Density functional theory (DFT)/BMK and CBS-QB3 ab initio calculations have been carried out to study the structures and energetics of unimolecular decomposition reactions of isopropyl butanoate (IPB, C 3 H 7 C(O)OCH(CH 3 ) 2 ) as a model biofuel. The results show a good performance of the BMK method. Among seven different dissociation channels of IPB, formation of butanoic acid and propene via a six-membered ring transition state is the most favorable reaction. On the other hand, formation of lower esters is hindered by high-energy barriers and unlikely occurs except at elevated temperatures. Simple bond scission costs less energy than lower ester formation. A comparison with methyl and ethyl esters indicates faster decomposition of IPB. The changes in bond lengths along minimum energy paths are discussed. 1. Introduction Increased demand for energy, climate change, air pollution, the higher cost for crude oil, and limited reserves of fossil fuel together push the search for alternative renewable fuels. Biofuels represent very promising sources of energy for the transportation sector, which will lessen the dependence on petroleum. They produce fewer greenhouse gases than fossil fuels and have the potential to reduce soot emissions. 1 Biofuel can be produced from different biomasses. One important class of biofuels consists of large methyl and ethyl esters derived from vegetable and animal oils and fats. 2-6 This is the biodiesel which is a renewable fuel that is expected to relieve demand for imported fossil fuel. Biodiesel is suitable for use within the current transport sector infrastructure, as it has physical properties similar to those of conventional diesel fuel. 7 Some experimental and few theoretical studies have been reported for real biofuel. 8-24 Direct studies of typical biodiesels are difficult because experiments would have to be carried out on complex, largely involatile mixtures and also because of the computational cost needed for such large molecules. However, better understanding of the combustion mechanism can be accomplished using a model biofuel. For practical reasons it is more convenient to use simple molecules for combustion modeling. Experimental 25-44 and theoretical 43-47 work has been done on model esters. These small esters include the ester moiety and part of the carbon chain in the acid side and, therefore, can be used to provide insights into the combustion chemistry of the real biodiesel. One of the problems of biodiesel is its performance in cold weather. The crystallization properties of methyl and ethyl biodiesel esters can be improved by introduction of branching. 48,49 Methyl and ethyl esters of fatty acid have been intensively studied. Introduction of branching into a linear, long-chain ester is expected to disrupt intermolecular associations at low temperatures, which reduces the crystallization temperature. The crystallization point of isopropyl soyate (IPS) was found be lower than that of methyl soyate (MS) (-9 vs -2 °C, respectively). 50 IPS has emission behavior similar to that of MS and superior low-temperature performance. 50 Moreover, the cetane numbers of fatty acid esters are not altered significantly by branching in the alcohol moiety. 19 Branching decreases the viscosity of fatty acid esters. 51 Therefore, studying isopropyl ester as a model biodiesel will shed some light onto the combustion behavior of a branched ester as a biodiesel. There are some experimental studies on isopropyl acetate and isopropyl propanoate esters concerned with determination of the rate coefficients of their unimolecular decompositions. 52-61 Only one theoretical study has been reported on the mechanistic details of pyrolysis of isopropyl acetate to propene and acetic acid. 62 Better simulation of fatty acid esters can be achieved by increasing the alkyl chain length at the acid side. Therefore, we expect esters of butanoic acid to be better than either acetic or propanoic acid analogues. The six-centered decomposition channel is the dominant reaction for alkyl esters. 25,29,30,45 Branching at the alcohol side of the ester increases the rate of decomposition compared to that of methyl, ethyl, and propyl esters. 34 Branching has been shown to reduce the activation energy for ester six-centered decomposition reactions by 12.5-23 kJ/mol. 63 Isopropyl butanoate (IPB) has some advantages as a biofuel. Its carbon content fits with the number of carbons in gasoline, and therefore, it can be considered as a real biogasoline (high flash point compared to that of biobutanol). With increasing branching, the octane number is expected to increase. IPB is highly flammable and not corrosive compared to bioethanol. It does not need any additives as its freezing point is high. Therefore, it does not freeze in winter. To the best of our knowledge, there are a few experimental studies and no theoretical studies on IPB regarding its unimolecular decom- position. Most of the studies on IPB concentrated on one dissociation reaction, namely, the production of butanoic acid and propene. An earlier study declared that the enol form of the corresponding acetate ester (alkoxyvinyl alcohol, CH 2 dC(OH)OR) is one of the observed channels in the decomposition of esters having a γ-hydrogen. 64 In this paper we describe the initial results from our ongoing work on thermochemistry and kinetics of pathways of decom- position of some branched esters. Low-temperature combustion usually dominates oxidative decomposition and thermal decom- * To whom correspondence should be addressed. E-mail: amelnahas@ hotmail.com. J. Phys. Chem. A 2010, 114, 7996–8002 7996 10.1021/jp103397f  2010 American Chemical Society Published on Web 07/12/2010