Ruichang Xiong

Structure–Property Relationships in Hydroxide‐Exchange Membranes with Cation Strings and High Ion‐Exchange Capacity

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.

Solvent-tuned hydrophobicity for faujasite-catalyzed cycloaddition of biomass-derived dimethylfuran for renewable p-xylene

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.

Molecular screening of alcohol and polyol adsorption onto MFI-type zeolites.

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, Henrys 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. Henrys 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.

Adsorption of HMF from Water/DMSO Solutions onto Hydrophobic Zeolites: Experiment and Simulation

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.

Kinetic regimes in the tandem reactions of H-BEA catalyzed formation of p-xylene from dimethylfuran

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.

Isosteric heats of gas and liquid adsorption.

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.

Quantum Chemically Estimated Abraham Solute Parameters Using Multiple Solvent–Water Partition Coefficients and Molecular Polarizability

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 chemicals 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.