Ab Initio Thermochemistry of Biofules

Abstract

Combustion plays an outsized role in our daily lives, such that 85% of the energy powering the US economy, for example, is produced through combustion processes. Combustion of liquid transportation fuels in particular contributes significantly to this, and consequently represents a major source of greenhouse gas emissions. Renewable biofuels is one of the viable alternatives to petroleum transportation fuels. Biofuels are CO2 neutral, meaning while they release this greenhouse gas during combustion, CO2 produced. Biofuels are largely similar to conventional petroleum fuels in use, but they show important differences. One of the most prominent features of ethanol and biodiesel biofuel combustion is that they produce less soot than conventional fuels, because the biofuels are oxygenated, leading to more complete oxidation. Examination of the chemistry and physics of combustion are needed in order to maximize the efficient and clean use of both oxygenated and non-oxygenated fuels. Computational tools enable combustion studies and are often used along with experimental investigations to elucidate combustion details. Ab initio theoretical modeling is particularly suitable for probing elementary reactions. Global properties of combustion chemistry can then be studied using computational kinetics models, which are assemblies of elementary reactions and the associated rate and thermochemical parameters. Reliable models are only possible with accurate rate and thermochemical parameters. This thesis work focuses on the computation of bond dissociation energies (BDEs), a thermochemical parameter. I propose ab initio multireference singles and doubles configuration interaction-based schemes to perform BDE calculations. I show that a size-extensivity correction in the form of the multireference averaged coupled-pair functional (MRACPF2) is necessary to obtain is used up during photosynthesis through which biofuel feedstocks are accurate energies. The scheme is thoroughly validated for accurate BDE calculations in hydrocarbons, alcohols, aldehydes, carboxylic acids, and methyl esters. I use smaller surrogates to estimate BDEs of biodiesel esters and then show that BDEs of larger molecules can also be directly calculated with a reduced scaling MRACPF2 method. I calculate BDEs for hydrocarbons, aldehydes, carboxylic acids, and methyl esters and explain trends in BDEs within and between the molecules. My calculated BDEs are used to make inferences on combustion.