It is well known that supercritical water is a favourable medium for biomass conversion followed by its hydrodeoxygenation (HDO). Moreover, the actual kinetics and mechanism of reaction occurring in the supercritical water are not yet completely understood, either by experimental or computational approaches. Within the framework of DFT, the major challenge is non-availability of models to simulate supercritical phase. In this study, the authors manually define the descriptors of a solvation model to describe an implicit supercritical phase. In order to examine the suitability of supercritical water for thermal and hydrotreatment of bio-oil model compounds, nine different reactions involving conversion of furfural, tetrahydrofuran, xylose, phenol, guaiacol, ferulic acid, acetic acid, 2-hydroxybenzaldehyde and hydroxyacetone have been considered. Further these reactions are also studied in gas and liquid phase to compare results of different phases, including supercritical water. It was found that while HDO of aromatic compounds like phenol and 2-hydroxybenzaldehyde was favourable in the supercritical phase, smaller molecules like acetic acid and hydroxyacetone did not show much advantage in the supercritical phase over gas and liquid phase. It was also found that the thermochemical parameter - Gibbs free energy change (ΔG) was equally influenced by the solvation effect and the effect of temperature-pressure under supercritical conditions. In several instances, the two effects were found to offset each other in the supercritical phase.

The conversion of guaiacol to benzene, toluene and o‐cresol along with several important intermediates like phenol, catechol and others in aqueous phase has been theoretically studied under the framework of density functional theory (DFT). The bond dissociation energy (BDE) calculation has been performed on optimized structures of guaiacol, phenol and anisole; and accordingly several reaction pathways have been proposed. The thermochemical parameters like Gibb's free energy change and enthalpy change of the reactions have also been reported using M06‐2X functional. In BDE study of the three compounds, i. e., guaiacol, phenol and anisole, it is observed that the scission of H. at fifth carbon position of an aromatic ring is the highest energy demanding dissociation, whereas the cleavage of bond from the functional group attached to the aromatic ring has the least BDE. The formation of phenol from guaiacol is more likely to occur by simultaneous hydrogenation and demethoxylation of guaiacol amongst all proposed pathways in aqueous phase. Further, decomposition of phenol to benzene is likely to occur via direct dehydroxylation of phenol. The simultaneous hydrogenation and dehydroxylation of guaiacol in aqueous phase are most likely to produce anisole which can further be reduced to phenol by direct cleavage of methyl group followed by hydrogenation. Further, free energy change landscape shows the conversion of guaiacol to phenol to be kinetically most favourable conversion at low temperature and high pressure in the aqueous phase. Finally, the increase in temperature causes a decrease in Gibb's free energy change and enthalpy change of overall reactions, thereby increasing favourability of most of the reactions in aqueous phase. Furthermore, the comparison between gaseous and aqueous phase results have been made wherever applicable.

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The hydrodeoxygenation of guaiacol is modelled over a (100) β-Mo2C surface using density functional theory and microkinetic simulations. The thermochemistry of the process shows that the demethoxylation of the guaiacol,...

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The fast pyrolysis of lignocellulosic biomass produces raw biooil that comprises of several oxygenated organic compounds which are disadvantageous and lower the quality of bio-oil as a fuel. In this numerical study, 2-hydroxy-6-methylbenzaldehyde (HMB) component, one such oxygenated compound which represents aromatic aldehyde category of bio-oil, is considered as model compound for its decomposition within the framework of density functional theory. The bond dissociation analysis of HMB component suggests that the dehydrogenation of methyl group is the least energy demanding amongst all nine possible bond scissions. Further, eight reaction pathways are investigated for the conversion of HMB into toluene as end product along with the analyses of their corresponding potential energy surfaces. Briefly, results indicate that the optimum reaction progress for the production of toluene from HMB requires an activation energy of 12.26 kcal/mol. It is further observed that the production of toluene from HMB includes m-cresol as an intermediate instead of 2-formyltoulene; and, the production of 2-hydroxybenzaldehyde is not favourable. Furthermore, the thermochemistry analyses for the production of toluene using optimum reaction pathway and for the production of 2-hydroxybenzaldehyde using reaction pathway 9 are performed over a wide range of temperature, i. e., 473-873 K at an interval of 100 K. The thermochemistry also suggests higher favourability for the production of toluene compared to the production of 2-hydroxybenzaldehdye by decomposing HMB.

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.