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Reaction kinetics and pathways of p-xylene formation from 2,5-dimethylfuran (DMF) and ethylene via cascade reactions of Diels–Alder cycloaddition and subsequent dehydration over H-BEA zeolite (Si/Al = 12.5) were characterized. Two distinct kinetic regimes were discovered corresponding to the rate limiting reaction, namely Diels–Alder cycloaddition and cycloadduct dehydration, as the concentration of Bronsted acid sites decreases. At catalyst loadings with effective acid site concentrations exceeding a critical value (~2.0 mM), the rate of formation of Diels–Alder products becomes constant. Under these conditions, the measured activation energy of 17.7 ± 1.4 kcal mol−1 and reaction orders correspond to the [4 + 2] Diels–Alder cycloaddition reaction of DMF and ethylene. Conversely, at catalyst loadings below the critical value, the formation rate of p-xylene becomes first order in catalyst loading, and the measured activation energy of 11.3 ± 3.5 kcal mol−1 is consistent with dehydration of the Diels–Alder cycloadduct to p-xylene. Experimental comparison between H-BEA and H-Y zeolite catalysts at identical conditions indicates that the micropore structure controls side reactions such as furan dimerization and hydrolysis; the latter is supported via molecular simulation revealing a substantially higher loading of DMF within H-Y than within H-BEA zeolites at reaction conditions.

Liquid-liquid extraction of fructose, 5-hydroxymethylfurfural (HMF), and HMF rehydration products, levulinic acid (LA) and formic acid (FA), from aqueous solutions to C4 to C7 linear and branched alcohols was studied. The alcohol-water partition coefficients for these compounds were measured at 30 °C and 1 atm. The COSMO-SAC method was used to predict these partition coefficients and the water/alcohol and HMF/water/alcohol liquid-liquid equilibria. The predictions are in good agreement with experiments, and both show that the partition coefficients decrease with increasing the number of carbon atoms in the alcohols. Furthermore, with the same alcohol, the partition coefficient of HMF is higher than that of fructose and the HMF rehydration products. The agreement between the results of experiments and calculations indicates that the COSMO-SAC method is reasonable for the prediction of such thermodynamic data, and can be used to provide insights into the separations of these systems that is generally lacking in biomass processing. This is an important step towards screening organic solvents for liquid-liquid extraction that may preferentially extract HMF from an aqueous phase into an organic phase, thereby preventing HMF rehydration.

Polyparameter Linear Free Energy Relationships (pp-LFERs), also called Linear Solvation Energy Relationships (LSERs), are used to predict many environmentally significant properties of chemicals. A method is presented for computing the necessary chemical parameters, the Abraham parameters (AP), used by many pp-LFERs. It employs quantum chemical calculations and uses only the chemical's molecular structure. The method computes the Abraham E parameter using density functional theory computed molecular polarizability and the Clausius-Mossotti equation relating the index refraction to the molecular polarizability, estimates the Abraham V as the COSMO calculated molecular volume, and computes the remaining AP S, A, and B jointly with a multiple linear regression using sixty-five solvent-water partition coefficients computed using the quantum mechanical COSMO-SAC solvation model. These solute parameters, referred to as Quantum Chemically estimated Abraham Parameters (QCAP), are further adjusted by fitting to experimentally based APs using QCAP parameters as the independent variables so that they are compatible with existing Abraham pp-LFERs. QCAP and adjusted QCAP for 1827 neutral chemicals are included. For 24 solvent-water systems including octanol-water, predicted log solvent-water partition coefficients using adjusted QCAP have the smallest root-mean-square errors (RMSEs, 0.314-0.602) compared to predictions made using APs estimated using the molecular fragment based method ABSOLV (0.45-0.716). For munition and munition-like compounds, adjusted QCAP has much lower RMSE (0.860) than does ABSOLV (4.45) which essentially fails for these compounds.

A modified COSMO-SAC model is presented to calculate thermodynamic properties of pure fluids and mixtures using statistical thermodynamics and the surface charge density of each compound obtained from a quantum mechanics (QM) calculation. The main differences from the previous models are that the new model includes a dispersion contribution in the mixture interaction, and is reparametrized using measured pure and mixture thermodynamic data simultaneously. With a single set of universal parameters, the new model provides higher accuracy than our previous models for predicting mixture thermodynamic properties while maintaining the same accuracy for pure compound thermodynamic properties. The overall root-mean-square deviation (RMSD) in the logarithms of partition coefficients for 992 octanol–water partitioning systems and 829 other solvent–water partitioning systems with this new model is reduced by about 10% compared to the results from previous models. Also, the agreement between the predicted and measure...

Low-defect siliceous zeolites of framework types BEA ∗ , CHA, FER, MFI, and STT have been prepared in a fluoride medium. The adsorption isotherms of CO 2 and N 2 on these materials were measured at various temperatures up to a pressure of 1 atm. Heats of adsorption of CO 2 and N 2 at the limit zero coverage are in a range of 18.6–27.2 kJ/mol and N 2 (11.2–16.6 kJ/mol), respectively, because CO 2 has larger polarizability. Grand Canonical Monte Carlo (GCMC) simulations were used to calculate the adsorption of nitrogen and carbon dioxide separately in purely siliceous models of these zeolites using various force fields proposed in the literature. Predictions for the sorption thermodynamics are in excellent agreement with the experiment, and both simulation and experimental data show a decrease in heat of adsorption with an increase of pore/window size for a given adsorbate.

The adsorption of 5-hydroxymethylfurfural (HMF), DMSO, and water from binary and ternary mixtures in hydrophobic silicalite-1 and dealuminated Y (DAY) zeolites at ambient conditions was studied by experiments and molecular modeling. HMF and DMSO adsorption isotherms were measured and compared to those calculated using a combination of grand canonical Monte Carlo and expanded ensemble (GCMC-EE) simulations. A method based on GCMC-EE simulations for dilute solutions combined with the Redlich-Kister (RK) expansion (GCMC-EE-RK) is introduced to calculate the isotherms over a wide range of concentrations. The simulations, using literature force fields, are in reasonable agreement with experimental data. In HMF/water binary mixtures, large-pore hydrophobic zeolites are much more effective for HMF adsorption but less selective because large pores allow water adsorption because of H2 O-HMF attraction. In ternary HMF/DMSO/water mixtures, HMF loading decreases with increasing DMSO fraction, rendering the separation of HMF from water/DMSO mixtures by adsorption difficult. The ratio of the energetic interaction in the zeolite to the solvation free energy is a key factor in controlling separation from liquid mixtures. Overall, our findings could have an impact on the separation and catalytic conversion of HMF and the rational design of nanoporous adsorbents for liquid-phase separations in biomass processing.

The heat of adsorption is an indicator of the strength of the interaction between an adsorbate and a solid adsorbent. This parameter can be determined from the heat released in calorimetric experiments or from the analysis of adsorption isotherms at different temperatures. The latter, called isosteric heats of adsorption, are commonly used in the characterization of materials for gas- and liquid-phase adsorption. Although the equations for the determination of isosteric heats of adsorption from the gas phase are well-known, approximate equations are frequently used for liquid-phase adsorption. We present here the rigorous equations for determining the isosteric heats of gas- and liquid-phase adsorption and their relation to the commonly used approximate equations. These equations are used to compute the isosteric heats of liquid adsorption based on the adsorption isotherms obtained from simulations for two well-defined systems, one ideal and the other nonideal. The results of using the rigorous equations are compared with those from the approximate equations. The main conclusion is that the commonly used approximate equations provide reasonable, but not perfect, estimates of the isosteric heats of liquid adsorption using only the experimental adsorption isotherms. The more accurate rigorous equations require additional information, including the heat of vaporization and, for nonideal mixtures, the heat of mixing.

A methodology based on the combination of grand canonical Monte Carlo (GCMC) and expanded ensemble (EE) simulations has been used to study the adsorption of alcohols from dilute aqueous solution onto the silicalite zeolite. The chemical potential of the guest alcohol molecules in bulk aqueous solutions is calculated by the EE method, and the adsorption isotherms of the alcohols are calculated using GCMC simulations. This approach results in adsorption isotherms that relate the loadings to external concentrations without using an equation of state, experimental data, or performing computationally expensive simulations such as two-phase Gibbs ensemble Monte Carlo (GEMC). Also, the method is “force-field-consistent”, in that the same force field is used for the bulk fluid and the adsorption calculations, and the resulting isotherms can be directly compared with experimental data. We have established the validity of the method used by comparing the calculated adsorption isotherms with experimental data for me...

In this work, the adsorptive and diffusive behaviours of molecular hydrogen in 10 different isoreticular metal–organic frameworks (IRMOFs) are studied using molecular-level simulation. Hydrogen adsorption isotherms and heats of adsorption at 77 and 300 K were generated for 10 MOFs at low-pressure conditions (up to 10 bar) using Path Integral Grand Canonical Monte Carlo simulations. Self-diffusivities and activation energies for diffusion were generated using molecular dynamics simulation. Density distributions showing the location and the shape of the adsorption sites are also provided. Statistical correlations for all of the properties as a function of surface area (SA), accessible volume (AV) and binding energy are provided. Based on this work, we observe that at pressures up to 10 bar at 300 K, the adsorption process is virtually completely governed by entropic considerations, resulting in a strong correlation between the amount of hydrogen adsorbed and the AV of the adsorbent. At 77 K, we observe more...

Classical molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) simulations were used to generate self-diffusivities, adsorption isotherms and density distributions for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in five isoreticular metal-organic frameworks (IRMOFs), which varied in the cage size and in the presence and location of amine groups. These simulations were performed at room temperature (300 K) and low pressures (up to 1 ppm RDX). The atomic charges required for MD and GCMC simulations were calculated from quantum mechanical (QM) calculations using two different charge generation methods—Lowdin Population Analysis and Natural Bond Orbital Analysis. Both charge sets show that the presence of amine groups increases the amount of RDX adsorbed. The cage size and the location of amine groups also affect the loading of RDX. The amount of RDX adsorbed is correlated with the energy of adsorption. The activation energy for diffusion of RDX is not positively correlated with the energy of adsorption. The density distributions identify the location of the adsorption sites of RDX-exclusively in the big cage around the metal complex vertices and between benzene rings. In the absence of amine groups on the framework, one of nitro groups on RDX interacts closely with the metal complex. In the IRMOFs functionalized with amine groups, a second nitro group of the RDX interacts with an amine group, enhancing adsorption. With regard to the application as a smart nanoporous preconcentrator, these IRMOFs are found to concentrate RDX up to 3000 times compared to the gas phase, on a volumetric basis. From a simple Langmuir estimation, the selectivity of RDX over butane is up to 5000. The diffusion of RDX is sufficiently high for real time sensor applications. These results indicate IRMOFs can be tailored with functional groups to be highly selective nanoporous preconcentrators.

In order to test the feasibility of using metal-organic frameworks (MOFs) to pre-concentrate explosive molecules for detection, molecular simulations of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) within IRMOF-1 were performed. Grand canonical Monte Carlo (GCMC) simulations were used to generate adsorption isotherms for pure RDX, RDX in dry air, and RDX in wet air. In addition to the isotherms, the GCMC simulations provide adsorption energies and density distributions of the adsorbates within the MOF. Molecular dynamics simulations calculate diffusivities and provide a detailed understanding of the change in conformation of the RDX molecule upon adsorption. The presence of dry air has little influence on the amount of RDX that adsorbs. The presence of wet air increases the amount of RDX that adsorbs due to favourable interactions between RDX and water. We found a Henry's law constant of 21.2 mol/kg/bar for both pure RDX and RDX in dry air. The RDX adsorption sites are located (i) in big cages, (ii) near...

Molecular dynamics (MD) simulations of a Lennard-Jones fluid in an inhomogeneous external field generate steady-state profiles of density and pressure with nanoscopic heterogeneities. The continuum level of mass, momentum, and energy transport balances is capable of reproducing the MD profiles only when the equation of state for pressure as a function of density is extracted directly from the molecular level of description. We show that the density profile resulting from simulation is consistent with both a molecular-level theoretical prediction from statistical mechanics as well as the solution of the continuum-level set of differential equations describing the conservation of mass and momentum. 1. Introduction. The principles of conservation of mass, momentum, and en- ergy in classical systems are equally valid at both the continuum and the molecular levels. At the continuum level, the density and velocity distributions are given by solutions of partial differential equations (PDEs) describing the mass and momentum balances. It is sufficient for the purposes of this work to limit the investigation to single-component fluids in an isothermal system. Therefore, the relevant continuum equations are a mass balance (1), ∂ρ ∂t = −∇ · (ρv), (1a)

A series of poly(2,4-dimethyl-1,4-phenylene oxide) hydroxide-exchange membranes (HEMs) with cation strings containing a well-defined number of cations (CS-n) and similar, high ion-exchange capacities are synthesized to investigate the effect of cation distribution on key HEM properties. As the number of cations on each string grows, the size of the ionic clusters increases from 10 to 55 nm. Well-connected ion pathways and a hydrophobic framework are observed for n≥4. The enhanced phase segregation increases the hydroxide conductivity from CS-1 to CS-6 (30 to 65 mS cm(-1) ) and suppresses the water uptake (from 143 % to 62 %). Moreover, molar hydroxide conductivities for CS-n membranes show two distinctive stages as n increases: ∼23 S cm(2) mol(-1) for n≤3; and ∼34 cm(2) mol(-1) for n≥4.

Configurational-bias grand canonical Monte Carlo (CB-GCMC) simulations and expanded ensemble (EE)-CB-GCMC simulations were performed to obtain adsorption isotherms of alcohols and polyols onto MFI-type zeolites from the gas phase and aqueous solution. In adsorption from both phases, Henry's constants and heats of adsorption at infinite dilution for straight-chain alcohols, diols, and triols in silicalite-1 are found to increase, and the saturation loadings decrease with increasing carbon number. Adsorption of straight-chain alcohols is more favorable than that of branched-chain alcohols. Henry's constants increase with increasing number of hydroxyl groups for gas-phase adsorption but decrease for adsorption from aqueous solution due to the strong hydrophilic solvent effect of water. The location of the hydroxyls does not affect significantly the adsorption from aqueous solution but does so in gas-phase adsorption. The saturation pressures for gas-phase adsorption decrease by orders of magnitude from the alcohols to the triols. Nonframework cations increase the adsorption of the small alcohols by an order magnitude at low concentrations (1 mg/mL), but result in only a factor of 2 increase for larger alcohols like butanol at low concentrations (0.03 mg/mL), and then decrease the adsorption at higher concentrations. Overall, the simulated results are in reasonable agreement with available experimental data.

A general random walk theory for diffusion in the presence of nanoscale confinement is developed and applied. The random-walk theory contains two parameters describing confinement: a cage size and a cage-to-cage hopping probability. The theory captures the correct nonlinear dependence of the mean square displacement (MSD) on observation time for intermediate times. Because of its simplicity, the theory also requires modest computational requirements and is thus able to simulate systems with very low diffusivities for sufficiently long time to reach the infinite-time-limit regime where the Einstein relation can be used to extract the self-diffusivity. The theory is applied to three practical cases in which the degree of order in confinement varies. The three systems include diffusion of (i) polyatomic molecules in metal organic frameworks, (ii) water in proton exchange membranes, and (iii) liquid and glassy iron. For all three cases, the comparison between theory and the results of molecular dynamics (MD) simulations indicates that the theory can describe the observed diffusion behavior with a small fraction of the computational expense. The confined-random-walk theory fit to the MSDs of very short MD simulations is capable of accurately reproducing the MSDs of much longer MD simulations. Furthermore, the values of the parameter for cage size correspond to the physical dimensions of the systems and the cage-to-cage hopping probability corresponds to the activation barrier for diffusion, indicating that the two parameters in the theory are not simply fitted values but correspond to real properties of the physical system.

Quantum mechanical (QM) calculations, classical grand canonical Monte Carlo (GCMC) simulations, and classical molecular dynamics (MD) simulations are performed to test the effect of charge distribution on hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) adsorption and diffusion in IRMOF-10. Several different methods for mapping QM electron distributions onto atomic point charges are explored, including the electrostatic potential (ESP) method, Mulliken population analysis, Lowdin population analysis, and natural bond orbital analysis. Classical GCMC and MD simulations of RDX in IRMOF-10 are performed using 15 combinations of charge sources of RDX and IRMOF-10. As the charge distributions vary, interaction potential energies, the adsorption loading, and the self-diffusivities are significantly different. None of the 15 combinations are able to quantitatively capture the dependence of the energy of adsorption on local configuration of RDX as observed in the QM calculations. We observe changes in the charge distributions of RDX and IRMOF-10 with the introduction of an RDX molecule into the cage. We also observe a large dispersion contribution to the interaction energy from QM calculations that is not reproduced in the classical simulations, indicating that the source of discrepancy may not lie exclusively with the assignment of charges.

To avoid drilling hole and stealing oil in pipeline, a method of monitoring man made damage in advance is offered based on low frequency vibration signal. The signals come from the complex activities such as welding, hammering and damaging metallic pipe. Spectral analysis and wavelet packet decomposition are used to extract feature information from gathered signal. The test distance between sensor and damage site is extended to 2.4 km, 7.4 km and even 13 km much longer than before. The results show that the method is effective when sensor spacing is 2.4 to 3.0 km, and instrumentation system can guarantee the security of oil pipeline and alarm in time.

Phase equilibria of the high temperature/high pressure heterogeneous catalytic reaction of 2,5-dimethylfuran (DMF), a biomass-derived furan, with ethylene to produce p-xylene was studied as a function of DMF conversion in a constant-pressure reactor. Adsorption of reactants and products onto a zeolite catalyst was computed using a configurational-bias Grand Canonical Monte Carlo (CB-GCMC) simulation, and the vapor–liquid behavior was computed using ASPEN Plus with an appropriate equation of state. It was found that the amount of water adsorbed in H–Y (Si/Al = 2.6) increases significantly as DMF is consumed, but is small in the presence of an aliphatic solvent, here n-heptane, even at high DMF conversion. The presence of the solvent reduces side reactions with water, such as hydrolysis followed by oxyalkylation, and increases p-xylene selectivity, in agreement with experiment. The decrease in the amount of water adsorbed is due to the increased hydrophobic environment in the zeolite as a result of the addition of n-heptane. The increase of ethylene inside the cage by the addition of n-heptane results in a slight increase of p-xylene alkylation. This work presents the first use of molecular simulation to understand mechanistic effects of solvents on the catalytic production of p-xylene in an H–Y zeolite.

Path integral grand canonical Monte Carlo (PI-GCMC) simulations using standard force fields are carried out to calculate the adsorption of H2 in five metal-porphyrin frameworks (MPFs), a new class of metal organic framework (MOF)-type materials. These simulations are performed at 77 K and room temperature (300 K). The adsorption isotherms of H2 in IRMOF-1 and IRMOF-10 are also calculated as a comparison. All calculations indicate that all MPFs adsorbed a higher weight fraction of H2 than both IRMOF-1 and IRMOF-10, with one exception (MPF-2). The gravimetric hydrogen capacities are still well short of practical goals. The MPFs provide additional adsorption sites due to the porphyrin. A statistical mechanical lattice model predicts the adsorption well at room temperature. The prediction by this model showed that a weight fraction of hydrogen of 6 wt. % adsorbed in pores of the size found in IRMOF-1 at ambient temperature and modest pressures required a binding energy of about 17 kJ/mole, which is consistent with other findings.