Effect of Solvent on the Interaction of Lignin with a Zeolite Nanosheet in the Condensed Phase

Lignin is the essential building block of lignocellulosic biomass, an excellent renewable source of different aromatic monomers for the polymer and biofuel industry. The depolymerization of lignin into value-added chemicals and fuels through the catalytic process poses a significant challenge due to the complex structure of lignin. Understanding lignin’s conformational diversity and dynamics in the liquid phase is crucial to designing an effective depolymerization process. Here, we conducted all-atom molecular dynamics simulations to understand the conformation and dynamics of softwood lignin on the all-silica zeolite nanosheet based on the MFI topology in a binary mixture of water–methanol at three different molar compositions (0%, 50%, and 100% methanol). We observed that the methanol–surface interaction is stronger than the water–surface interaction, and methanol readily diffused into the MFI core. Lignin surface contacts decrease with increasing methanol composition due to higher solubility and dynamics. Lignin dynamics on the surface in neat water is an order of magnitude smaller than methanol. We also found that lignin adopts a slightly extended conformation when it stays on the surface than in the bulk solution phase for the pure water case, whereas for pure methanol and the binary solution structures are statistically similar

Transferable Potentials for Chloroethenes: Insights into Nonideal Solution Behavior of Environmental Contaminants

Predicting the nonideal phase behavior of binary and multicomponent systems remains a significant challenge for particle-based simulations. Here, we develop a transferable force field for chloroethenes, common environmental contaminants, that can accurately model the vapor liquid phase equilibria including azeotrope formation. The new all-atom force field reproduces saturated liquid densities, saturated vapor pressures, boiling points, and critical properties within 1, 10, 1, and 1% of the experiment data, respectively. Furthermore, the vapor liquid equilibria of trichloroethylene and 1-propanol binary mixture, which forms a minimum boiling point azeotrope, is predicted with a reasonable accuracy. The microstructure of neat and binary systems is explored using pair correlation functions and spatial distribution functions. As the new force field is consistent with transferable potentials for phase equilibria (TraPPE) force field, it expands the applicability of TraPPE force field to chloroethenes

Transferable Potentials for Phase Equilibria. 9. Explicit Hydrogen Description of Benzene and Five-Membered and Six-Membered Heterocyclic Aromatic Compounds

The explicit hydrogen version of the transferable potentials for phase equilibria (TraPPE-EH) force field is extended to benzene, pyridine, pyrimidine, pyrazine, pyridazine, thiophene, furan, pyrrole, thiazole, oxazole, isoxazole, imidazole, and pyrazole. While the Lennard-Jones parameters for carbon, hydrogen (two types), nitrogen (two types), oxygen, and sulfur are transferable for all 13 compounds, the partial charges are specific for each compound. The benzene dimer energies for sandwich, T-shape, and parallel-displaced configurations obtained for the TraPPE-EH force field compare favorably with high-level electronic structure calculations. Gibbs ensemble Monte Carlo simulations were carried out to compute the single-component vapor−liquid equilibria for benzene, pyridine, three diazenes, and eight five-membered heterocycles. The agreement with experimental data is excellent with the liquid densities and vapor pressures reproduced within 1 and 5%, respectively. The critical temperatures and normal boiling points are predicted with mean deviations of 0.8 and 1.6%, respectively

Transferable Potentials for Phase Equilibria. 10. Explicit-Hydrogen Description of Substituted Benzenes and Polycyclic Aromatic Compounds

The explicit-hydrogen version of the transferable potentials for phase equilibria (TraPPE-EH) force field is extended to various substituted benzenes through the parametrization of the exocyclic groups F, Cl, Br, CN, and OH and to polycyclic aromatic hydrocarbons through the parametrization of the aromatic linker carbon atom for multiple rings. The linker carbon together with the TraPPE-EH parameters for aromatic heterocycles constitutes a force field for fused-ring heterocycles. Configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to compute vapor–liquid coexistence curves for fluorobenzene; chlorobenzene; bromobenzene; di-, tri-, and hexachlorobenzene isomers; 2-chlorofuran; 2-chlorothiophene; benzonitrile; phenol; dihydroxybenzene isomers; 1,4-benzoquinone; naphthalene; naphthalene-2-carbonitrile; naphthalen-2-ol; quinoline; benzo­[b]­thiophene; benzo­[c]­thiophene; benzoxazole; benzisoxazole; benzimidazole; benzothiazole; indole; isoindole; indazole; purine; anthracene; and phenanthrene. The agreement with the limited experimental data is very satisfactory, with saturated liquid densities and vapor pressures reproduced to within 1.5% and 15%, respectively. The mean unsigned percentage errors in the normal boiling points, critical temperatures, and critical densities are 0.9%, 1.2%, and 1.4%, respectively. Additional simulations were carried out for binary systems of benzene/benzonitrile, benzene/phenol, and naphthalene/methanol to illustrate the transferability of the developed potentials to binary systems containing compounds of different polarity and hydrogen-bonding ability. A detailed analysis of the liquid-phase structures is provided for selected neat systems and binary mixtures

Transferable Potentials for Phase Equilibria. 10. Explicit-Hydrogen Description of Substituted Benzenes and Polycyclic Aromatic Compounds

The explicit-hydrogen version of the transferable potentials for phase equilibria (TraPPE-EH) force field is extended to various substituted benzenes through the parametrization of the exocyclic groups F, Cl, Br, CN, and OH and to polycyclic aromatic hydrocarbons through the parametrization of the aromatic linker carbon atom for multiple rings. The linker carbon together with the TraPPE-EH parameters for aromatic heterocycles constitutes a force field for fused-ring heterocycles. Configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to compute vapor–liquid coexistence curves for fluorobenzene; chlorobenzene; bromobenzene; di-, tri-, and hexachlorobenzene isomers; 2-chlorofuran; 2-chlorothiophene; benzonitrile; phenol; dihydroxybenzene isomers; 1,4-benzoquinone; naphthalene; naphthalene-2-carbonitrile; naphthalen-2-ol; quinoline; benzo­[<i>b</i>]­thiophene; benzo­[<i>c</i>]­thiophene; benzoxazole; benzisoxazole; benzimidazole; benzothiazole; indole; isoindole; indazole; purine; anthracene; and phenanthrene. The agreement with the limited experimental data is very satisfactory, with saturated liquid densities and vapor pressures reproduced to within 1.5% and 15%, respectively. The mean unsigned percentage errors in the normal boiling points, critical temperatures, and critical densities are 0.9%, 1.2%, and 1.4%, respectively. Additional simulations were carried out for binary systems of benzene/benzonitrile, benzene/phenol, and naphthalene/methanol to illustrate the transferability of the developed potentials to binary systems containing compounds of different polarity and hydrogen-bonding ability. A detailed analysis of the liquid-phase structures is provided for selected neat systems and binary mixtures

Probing Early-Stage Aggregation of Low Molecular Weight Gelator in an Organic Solvent

Molecular gels are formed by the supramolecular assembly of low molecular weight gelators (LMWGs) in organic solvents or water. Despite significant advances in the field, our understanding of how gelator molecules lead to complex self-assembled fibrillar network (SAFIN) is rather poor. Here, we present molecular dynamics simulations to gain insights into the early-stage aggregation of self-assembled fibrillar network (SAFIN) of 12-hydroxyoctadecanamide (12-HSAm) in octane. Our simulations reveal that the hydroxyl group located at the 12th carbon position plays an important role in the fiber formation. If the hydroxyl group is removed from the backbone, then we find that the aggregates adopt a bilayer morphology rather than cylindrical fibers. Analysis of fibers reveals different morphologies such as cylindrical, tape, and junction zones. A typical cylindrical fiber diameter is 2.4–3.4 nm, while the tape-like fibers are 4.4–8.6 nm in width and 2.4–4.2 nm in depth. In the fibers, we observe that the majority of the gelator molecules interact with neighboring molecules with only one interaction site, leading to growth of the fiber in one dimension. Our simulations help explain the role of functional groups in the self-assembly of small molecules leading to gel formation

Vapor–Liquid Coexistence and Critical Behavior of Ionic Liquids via Molecular Simulations

Vapor–liquid coexistence curves and critical points are of great practical and fundamental importance. Our understanding of these phenomena is well-developed for most fluids but is severely lacking for ionic liquids, a class of salts that are liquid near ambient temperatures. Thermal stability limitations virtually eliminate direct experimental determination of these properties. In this Letter, we report the first vapor–liquid phase diagrams and critical points for ionic liquids obtained in silico with an atomistic force field. We show that within a homologous series of imidazolium-based ionic liquids, the critical temperature, critical density, critical pressure, boiling point, and enthalpy of vaporization all decrease with increasing length of the cation alkyl chain, while the saturation pressure increases with chain length. These trends are opposite to what is observed for alkanes and other nonionic polar compounds such as alcohols. In the vapor phase, we find that ions are distributed across clusters of different sizes with neutral ion pairs being the predominant aggregation state

Transferable Potentials for Phase Equilibria. 10. Explicit-Hydrogen Description of Substituted Benzenes and Polycyclic Aromatic Compounds

The explicit-hydrogen version of the transferable potentials for phase equilibria (TraPPE-EH) force field is extended to various substituted benzenes through the parametrization of the exocyclic groups F, Cl, Br, CN, and OH and to polycyclic aromatic hydrocarbons through the parametrization of the aromatic linker carbon atom for multiple rings. The linker carbon together with the TraPPE-EH parameters for aromatic heterocycles constitutes a force field for fused-ring heterocycles. Configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to compute vapor–liquid coexistence curves for fluorobenzene; chlorobenzene; bromobenzene; di-, tri-, and hexachlorobenzene isomers; 2-chlorofuran; 2-chlorothiophene; benzonitrile; phenol; dihydroxybenzene isomers; 1,4-benzoquinone; naphthalene; naphthalene-2-carbonitrile; naphthalen-2-ol; quinoline; benzo­[<i>b</i>]­thiophene; benzo­[<i>c</i>]­thiophene; benzoxazole; benzisoxazole; benzimidazole; benzothiazole; indole; isoindole; indazole; purine; anthracene; and phenanthrene. The agreement with the limited experimental data is very satisfactory, with saturated liquid densities and vapor pressures reproduced to within 1.5% and 15%, respectively. The mean unsigned percentage errors in the normal boiling points, critical temperatures, and critical densities are 0.9%, 1.2%, and 1.4%, respectively. Additional simulations were carried out for binary systems of benzene/benzonitrile, benzene/phenol, and naphthalene/methanol to illustrate the transferability of the developed potentials to binary systems containing compounds of different polarity and hydrogen-bonding ability. A detailed analysis of the liquid-phase structures is provided for selected neat systems and binary mixtures

Role of Silanol Group in Sn-Beta Zeolite for Glucose Isomerization and Epimerization Reactions

Density functional calculations are used to elucidate the role of the silanol group adjacent to the active site Sn metal center of the Sn-BEA zeolite in the isomerization and epimerization of glucose. We find that the silanol group plays an important role in the isomerization reaction, wherein hydride transfer and subsequent proton transfer occur in a single step with a lower energy of activation. Epimerization, on the other hand, proceeds via a mechanism similar to the Bílik mechanism and has lower activation barrier when the silanol group does not participate directly in the transition state. Our calculations indicate that cooperative effects, often encountered in enzymatic catalysis, promote hydride transfer in the isomerization reaction but not for the Bílik mechanism for epimerization