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

Abstract

The faster depleting natural reserves of fossil fuel and growing global climate change crisis have shifted the focus of researchers toward the extraction of bio-fuel and value-added chemicals from biomass. In this quest, supercritical (SC) water as a medium has been experimentally explored to derive bio-oil from biomass and deoxygenate the oxygenated compounds of it. Levulinic acid (LA) and pentanoic acid or valeric acid (VA) are two standard value-added products obtained from the biomass treatment. Thus, in this study, the authors report the kinetics of the conversion of levulinic acid to valeric acid at four different supercritical conditions using an implicit solvation model available within the framework of density functional theory (DFT) and compare them with their gas and aqueous phase counterparts. Prior to obtaining the new results, the present approach is first benchmarked with the existing experimental and theoretical literature under the supercritical water conditions. The conversion of levulinic acid is studied in two competing pathways. For each of the reaction pathways, the enthalpy and Gibbs free energy changes have been discussed. It is found that the production of valeric acid is equally likely to proceed by the protonation of the fourth carbon of the acid or by the protonation of the oxo-group at the fourth carbon atom. The solvent effects are found to be favorable, especially under two supercritical conditions, SC1 (ρ = 0.089 g/cc, T = 773 K, P = 250 bar) and SC2 (ρ = 0.109 g/cc, T = 723 K, P = 250 bar) compared to SC3 (ρ = 0.190 g/cc, T = 700 K, P = 304 bar) and SC4 (ρ = 0.360 g/cc, T = 723 K, P = 463 bar) conditions.

Supercritical Carbon Dioxide in Presence of Water for the Valorization of Spent Coffee Grounds: Optimization by Response Surface Methodology and Investigation of Caffeine Extraction Mechanism

“…The modelling of this particular system using Density Functional Theory (DFT) could enlighten knowledge about interactions of this trilateral agreement between SC-CO 2 /H 2 O/CAF SCG . The DFT already proved its worth to get a better comprehension of binding energies of molecules during biomass extraction, biomass transformation, or in a supercritical system like supercritical H 2 O [ 89 , 90 , 91 ]. Hence, it could be useful to have a better understanding of the SC-CO 2 /H 2 O system to apply it to other biomasses.…”

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

“…The energy barrier for the protonation of 5-HMF C 4 is close to a similar reaction, such as the protonation of a ketone carbon from levunilic acid (LA) to form pentanoic acid of ∼84 kJ mol −1 using an implicit water. 74 Also, various biomass reactions reported in the literature, such as acid-catalyzed hydrolysis of pyranose, acid catalyzed conversion of LA to valeric acid or pentane, and the cleavage of cellulose glycosidic bonds, have been studied using DFT-implicit solvent environments. 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.…”

“…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.…”