Pyrolysis of bioethanol and biobutanol: A thermodynamic and kinetic study

Context

Bioethanol and biobutanol are renewable oxygenated fuels derived from biomass, commonly blended with gasoline for use in gasoline engines. These alcohol-based fuels have high oxygen content, promoting more complete combustion and reducing carbon dioxide emissions compared to petroleum fuels. However, during combustion, oxygenated radicals can interact and lead to the formation of formaldehyde, a highly toxic compound. This study delves into the thermodynamic and kinetic study of biofuel pyrolysis using quantum chemical methods. Our results identify C–C bond as the weakest in the initiation step, with bond dissociation enthalpy around 86 kcal/mol. Notably, ethanol exhibits higher bond dissociation energies than butanol. While the initiation step predominantly involves C–C bond breaking, the propagation step reveals a competition between H abstraction and C–C bond cleavage. Analyzing the computed rate constants and Gibbs free energies for radical reactions in the propagation steps indicates the likelihood formation of acetaldehyde, formaldehydes, methane, and ethylene. These products indeed present significant risks to both human health and the environment. This emphasizes the importance of carefully controlling macroscopic thermodynamic variables, such as temperature and pressure, during the pyrolysis of alcohol. Proper regulation of these factors is crucial in minimizing the formation of harmful aldehydes and ensuring a safer and more sustainable process.

Methods

The reaction mechanisms of thermal decomposition are analyzed using UωB97XD/6–311 + G(3 df,2p), G4MP2, and G4 computational methods. The latter offers highly accurate enthalpies of formation, with a deviation from experiment values approximately 1 kcal/mol, though it is computationally expensive compared to DFT. To evaluate the diradical character of certain open-shell intermediate species, CASSCF and MP2-CASSCF methods, which effectively account for static correlation effects, are employed with the cc-pVDZ basis set. Thermodynamic and kinetic analyses are carried out using both ab initio and semi-empirical approaches through Gaussian 09 and OpenSMOKE + + 0.21.0 programs.