Thermochemical Mapping of Levulinic Acid Conversion to Pentane in Supercritical Water within the Framework of Density Functional Theory

Abstract

Undoubtedly, fossil fuels being the primary source of energy have led us to milestone achievements to date. However, its alarming rate of depletion has raised concerns on a global level, which has channelized significant scientific research on biomass-to-biofuel conversion. Nonetheless, the economics of biomass conversion still poses a challenge due to low yield and conversion. In this regard, experimental studies have suggested supercritical water as a medium, which can facilitate better heat and mass transfer amidst the products and reactants. Thus, in this study, the authors applied the density functional theory (DFT) to simulate upgrading of a bio-oil model compound, levulinic acid, under supercritical water conditions. The supercritical water was defined at densities 0.089, 0.109, 0.190, and 0.360 g/cc, and the corresponding parameters (such as dielectric constant, refractive index, hydrogen-bond acidity, basicity, etc.) were manually set in the SMD implicit solvation model. The present method of model description for supercritical water within the framework of DFT is first of its kind and is the most reliable approach owing to validation with experimental results. Further, in this study, numerous pathways elaborating the conversion of levulinic acid to pentane via intermediates like pentanol and acetopropanol were simulated. The kinetic mapping of the pathways was then done by evaluating Gibbs free-energy change and enthalpy change. The supercritical water showed an advantage in deoxygenating compounds with a higher number of oxy groups. However, in some reactions like conversion of 5,5-dihydroxypentan-2-one to 5-hydroxypentan-2-one (γ-Acetopropanol), the effects of temperature and pressure were seen to offset the solvent effect. Of the four supercritical conditions, ρ = 0.109 and 0.360 g/cc were found to be the most favorable supercritical water densities for the conversion of levulinic acid. Overall, the production of pentane from levulinic acid is found to be most advantageous in the supercritical water density of ρ = 0.109 g/cc till pentane-1,4-diol and further conversion to pentane under supercritical conditions of ρ = 0.360 g/cc is the best pathway. Furthermore, the gas phase was found to be the least favorable medium in almost all of the reactions. In contrast, the presence of supercritical water showed an advantage in nearly all of the reactions suggesting supercritical water to be a suitable solvent for the production of biomass-derived chemicals. Thus, this line of investigation warrants further study, especially by experimental groups, to corroborate the findings of this study and scale-up potential.

Kinetic Modeling of Conversion of Levulinic Acid to Valeric Acid in Supercritical Water Using the Density Functional Theory Framework

“…However, for the supercritical phase, all the descriptors were manually defined in the model. Agrawal et al have elaborated upon the definition and parameterization of these descriptors in detail. Since the water does not contain any halogen atom or an aromatic ring, the halogenicity (ψ) and aromaticity (ϕ) were set to null.…”

Energetics of acid catalyzed biomass reactions: how and why does the solvent model matter?

“…For the reactions mentioned earlier, some of their steps reveal (i) differences in the energy profiles of the reaction in the gas phase and implicit solvent and (ii) the changes in energetics between the implicitly modeled solvents are negligible despite having different dielectric constants. 55,[74][75][76] Fig. 4b and c show the most stable product conformers for the gas phase and implicit solvent environment respectively.…”