Size Effect of a Ni Nanocatalyst on Supercritical Water Gasification of Lignin by Reactive Molecular Dynamics Simulations

Decomposition Mechanism of Isoprenoid Hydrocarbon p-Menthane in the Presence of Pt@FGS Nanoparticles: A ReaxFF-MD Study

Pt@FGS nanoparticles have shown effective enhancement in the decomposition of hydrocarbon fuels. To further explore the potential enhancing mechanisms of Pt@FGS nanoparticles, the catalytic decomposition of p-menthane, a bioderived isoprenoid "drop-in" fuel with great promise, is investigated here using the reactive force-field molecular dynamics (ReaxFF-MD) simulations. The results show that the Pt@FGS nanoparticles exhibit good catalytic reactivity with a reduction of the activation energy by nearly 62%. Possible initial reactions of enhanced p-menthane (PMT) decomposition are discussed, which suggests that the supported Pt-cluster plays a key role in the dehydrogenation of PMT, as does the oxygen-containing functional group of the functionalized graphene sheets (FGS). It is also interesting to note that the presence of Pt@FGS causes the initial reactions, which are dominated by H-abstraction, favorable in both kinetics and thermodynamics.

Revealing of Supercritical Water Gasification Process of Lignin by Reactive Force Field Molecular Dynamics Simulations

Gasification with supercritical water is an efficient process that can be used for the valorization of biomass. Lignin is the second most abundant biopolymer in biomass and its conversion is fundamental for future energy and value-added chemicals. In this paper, the supercritical water gasification process of lignin by employing reactive force field molecular dynamics simulations (ReaxFF MD) was investigated. Guaiacyl glycerol-β-guaiacyl ether (GGE) was considered as a lignin model to evaluate the reaction mechanism and identify the components at different temperatures from 1000 K to 5000 K. The obtained results revealed that the reactions and breaking of the lignin model started at 2000 K. At the primary stage of the reaction at 2000 K the β-O-4 bond tends to break into several compounds, forming mainly guaiacol and 1,3-benzodioxole. In particular, 1,3-benzodioxole undergoes dissociation and forms cyclopentene-based ketones. Afterward, dealkylation reaction occurred through hydroxyl radicals of water to form methanol, formaldehyde and methane. Above 2500 K, H2, CO and CO2 are predominantly formed in which water molecules contributed hydrogen and oxygen for their formation. Understanding the detailed reactive mechanism of lignin’s gasification is important for efficient energy conversion of biomass.

Study of Conversion of Bio-oil Model Compounds in Supercritical Water Using Density Functional Theory

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.