Molecular simulation of water/alcohol mixtures' adsorption and diffusion in zeolite 4A membranes

Methyl-Functionalized Al-Based MOF ZJU-620(Al): A Potential Physisorbent for Carbon Dioxide Capture

Developing Al-based metal-organic frameworks (MOFs) with moisture-resistance ability is a challenge for carbon dioxide (CO2) capture. Methyl-functionalized Al-based MOF ZJU-620(Al), with excellent chemical-thermal stability and specific surface area of 1347 m2/g, observed here, is a potential adsorbent for CO2 capture with good recyclability and large capacity up to 4.25 mmol/g at 298 K and 1 atm. CO2 molecules are largely trapped on two types of sites. One (site I) is near the AlO6 clusters, and another (site II) is between two parallel benzene rings with a distance of 6.64 Å. ZJU-620(Al) can be used for CO2/N2 (15/85) separation with the excellent selectivity up to 107.20 at 273 K and 31.93 at 298 K, and the separation factor of 13.68. It is also with excellent moisture-resistance ability due to 5% breakthrough time (outlet concentration reached the 5% of inlet concentration) without reduction at 80% relative humidity than under dry conditions. Water molecules occupy a small amount of CO2 adsorption site I, but they almost do not occupy the CO2 adsorption site II due to hydrophobic methyl-functional ligands. Moreover, CO2 can be adsorbed on the ZJU-620(Al) surface through C═O···H binding of water molecules with high affinity. Thus, ZJU-620(Al) is a candidate adsorbent for CO2 capture and separation especially under humidity conditions.

Computational simulation-driven discovery of novel zeolite-like carbon materials as seawater desalination membranes

Freshwater is a scarce and vulnerable resource that has never encountered such an extensive focus on a nearly worldwide scale as it does today. Recently, it has been found that desalination powered by two-dimensional (2D) carbon materials as separation membranes has significantly reduced the operational costs and complexity but presents heavy requirements for the structural stability and separation properties of the membrane materials. Here, we combined carbon materials with promising adsorption properties and zeolites characterized by a regular pore structure to obtain a zeolite-like structured carbon membrane Zeo-C and investigated the suitability of the Zeo-C membrane for seawater desalination based on the computational-simulation-driven approach. The results of molecular dynamics (MD) simulations and density functional theory (DFT) calculations revealed that the periodic pore distribution conferred favorable structural stability and mechanical strength to the Zeo-C desalination membrane. The rejection rate of Na⁺ and Cl^- is ensured at 100% under a pressure of 40-70 MPa, and that of Na⁺ could reach 97.85% even though the pressure increases to 80 MPa, exhibiting superior desalination properties. The porous nature of the zeolite-like structure and the low free energy potential barrier are conducive for reliable adsorption and homogeneous diffusion of salt ions, which facilitates the acquisition of desirable water molecule permeability and salt ion selectivity. In particular, the interlinked delocalized π-network imparts inherent metallicity to Zeo-C for self-cleaning in response to electrical stimulation, thereby extending the lifetime of the desalination membrane. These studies have greatly encouraged theoretical innovations and serve as a guiding reference for desalination materials.

Molecular Simulation for Guiding the Design and Optimization of Mixed Matrix Membranes (MMMs) in the Pervaporation Process

Molecular simulation has been used extensively in the study of pervaporation membranes as a new economical and environmentally friendly research method. In this paper, A-SiO₂/PDMS-PTFE mixed matrix membranes (MMMs) were prepared by molecular-simulation-guided experiments to achieve the separation of dimethyl carbonate/methanol (DMC/MeOH)) azeotropes. The interaction energy, X-ray diffraction pattern mean square displacement, and density field between PDMS and inorganic particles were analyzed by molecular dynamics simulations. The dissolution and diffusion processes of the DMC/MeOH azeotropes system in the MMM were simulated, and the surface-silylated silica (A-SiO₂) with relatively better performance was screened. Based on the simulation results, A-SiO₂/PDMS-PTFE MMMs were prepared by the coblending method, and the pervaporation separation performance of MMM membranes for DMC/MeOH azeotropes with different A-SiO₂ loadings was investigated. When the A-SiO₂ loading was 15 wt %, the separation factor of DMC/MeOH azeotropes at 50 °C was 4.74 and the flux was 1178 g m^-2 h^-1, which was consistent with the expected results of the simulation. The MMMs showed good stability in pervaporation over a period of up to 120 h. This study demonstrates that molecular simulations can provide a viable means for pretest screening and validation of experimental mechanisms, and to a certain extent, guide the design and optimization of pervaporation membranes.

Investigating the Interaction between Methanol and the Heulandite-type Zeolite using First Principle Molecular Dynamic

The interaction between methanol and the Heulandite-type zeolite has been unveiled to give an atomic scale detail regarding the catalytic activity of this zeolite for methanol conversion. The study was carried out by first principle molecular dynamics to get an insight into the structure and electronic behaviour of methanol inside the zeolite structure at different temperatures. The behaviour of methanol was studied when the location of the proton of Bronsted acid sites was varied to give both possible direct and less interaction with methanol. The results show that methanol interacts with the proton from zeolite to give a cationic species of [CH3OH2]+ both in 300K and 573K conditions. However, when the proton is located at different location far from possible interaction with methanol, the formation of a cationic species is hindered. This study provides an insight into the design of Heulandite type zeolite to give a catalytic activity toward methanol transformation.

Physical mixing of a catalyst and a hydrophobic polymer promotes CO hydrogenation through dehydration

In many reactions restricted by water, selective removal of water from the reaction system is critical and usually requires a membrane reactor. We found that a simple physical mixture of hydrophobic poly(divinylbenzene) with cobalt-manganese carbide could modulate a local environment of catalysts for rapidly shipping water product in syngas conversion. We were able to shift the water-sorption equilibrium on the catalyst surface, leading to a greater proportion of free surface that in turn raised the rate of syngas conversion by nearly a factor of 2. The carbon monoxide conversion reached 63.5%, and 71.4% of the hydrocarbon products were light olefins at 250°C, outperforming poly(divinylbenzene)-free catalyst under equivalent reaction conditions. The physically mixed CoMn carbide/poly(divinylbenzene) catalyst was durable in the continuous test for 120 hours.

Molecular dynamic simulation studies of adsorption and diffusion behaviors of methanol and ethanol through ZSM-5 zeolite

Due to the importance of synthesis gas's entire conversion to methanol, the separation of methanol from unconverted synthesis gas is an industrial challenge. In this work, the influence of temperature, guest molecules concentrations (methanol and ethanol), and acid site density (Si/Al) of zeolites on the diffusion of methanol and ethanol, pure and binary mixture (80% methanol and 20% ethanol) in silicalite-1 and HZSM-5 (Si/Al = 47 and 23) were studied by using of the COMPASS force-field molecular dynamics method. Also, the adsorption of pure methanol and ethanol and binary mixture through these zeolites has been studied by using the Grand Canonical Monte Carlo (GCMC) method. The calculated adsorption rate and isosteric heat of adsorption for ethanol are lower and higher than methanol, respectively. The results of the binary mixture show that HZSM-5 (Si/Al = 23) has the lowest adsorption selectivity and most diffusion selectivity. The calculated diffusion coefficients of methanol and ethanol guest molecules decreased with rising guest molecule concentration and Si/Al-ratios. The effect of both agents was investigated by analysis of mean square displacement (MSD) and RDF diagram.

Zeolite-Polymer Composite Materials as Water Scavenger

The influence of the charge compensating cation nature (Na+, Mg2+) on the water adsorption properties of LTA-type zeolites used as filler in composite materials (zeolite/polymers) was investigated. Large scale cation exchanges were performed on zeolite powder at 80 °C for 2 h using 1 M magnesium chloride (MgCl2) aqueous solutions. XRF, ICP, and EDX analyses indicate a successful cationic exchange process without the modification of the zeolite structure as shown by XRD and solid-state NMR analyses. Composite materials (granulates and molded parts) were manufactured using to extrusion and injection processes. In the case of MgA zeolite, nitrogen adsorption-desorption experiments allowed us to measure a microporous volume, unlike NaA zeolite, which is non-porous to nitrogen probe molecule. SEM and EDX analyses highlighted the homogeneous distribution of zeolite crystals into the polymer matrix. Water adsorption capacities confirmed that the trends observed in the zeolite powder samples are preserved after dragging zeolites into composite formulations. Granulates and molded parts composite samples containing the magnesium exchanged zeolite showed an increase of their water adsorption capacity up to +27% in comparison to composite samples containing the non-exchanged zeolite. The MgA composite is more promising for water decontamination applications due to its higher water adsorption properties than the NaA composite.

Steric and Electrostatic Effects on the Diffusion of CH4/CH3OH in Copper-Exchanged Zeolites: Insights from Enhanced Sampling Molecular Dynamics and Free Energy Calculations

Copper-exchanged zeolites have demonstrated high selectivity in methane-to-methanol conversion carried out on copper-oxo centers. Nevertheless, the reaction can only occur if the methane molecules reach the active site while the methanol molecules must leave the material without high energetic cost for the migration. In this context, we have used force field-based molecular dynamics simulations with the potential of mean force method to estimate the energy barrier in cage to cage diffusion of methane and methanol molecules in the chabazite framework type zeolite. The results show considerably higher energy barrier for methanol diffusion. The steric effect of the active site and the electrostatic environment favors the CH₃OH diffusion toward nonactive cages where it tends to accumulate due to the strong interactions with the zeolite. The same behavior is observed in the water molecules distribution, which emphasizes the control of the electrostatic potential over the polar molecules migration. For high concentration of polar molecules, the electrostatic effect is shielded and the driving force is reduced for CH₃OH diffusion. The results show that if the electrostatic environment can be controlled, the product migration may be facilitated, which can improve the catalytic process.

Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids

Nanoporous solids are ubiquitous in chemical, energy, and environmental processes, where controlled transport of molecules through the pores plays a crucial role. They are used as sorbents, chromatographic or membrane materials for separations, and as catalysts and catalyst supports. Defined as materials where confinement effects lead to substantial deviations from bulk diffusion, nanoporous materials include crystalline microporous zeotypes and metal–organic frameworks (MOFs), and a number of semi-crystalline and amorphous mesoporous solids, as well as hierarchically structured materials, containing both nanopores and wider meso- or macropores to facilitate transport over macroscopic distances. The ranges of pore sizes, shapes, and topologies spanned by these materials represent a considerable challenge for predicting molecular diffusivities, but fundamental understanding also provides an opportunity to guide the design of new nanoporous materials to increase the performance of transport limited processes. Remarkable progress in synthesis increasingly allows these designs to be put into practice. Molecular simulation techniques have been used in conjunction with experimental measurements to examine in detail the fundamental diffusion processes within nanoporous solids, to provide insight into the free energy landscape navigated by adsorbates, and to better understand nano-confinement effects. Pore network models, discrete particle models and synthesis-mimicking atomistic models allow to tackle diffusion in mesoporous and hierarchically structured porous materials, where multiscale approaches benefit from ever cheaper parallel computing and higher resolution imaging. Here, we discuss synergistic combinations of simulation and experiment to showcase theoretical progress and computational techniques that have been successful in predicting guest diffusion and providing insights. We also outline where new fundamental developments and experimental techniques are needed to enable more accurate predictions for complex systems.

Influence of the Compensating Cation Nature on the Water Adsorption Properties of Zeolites

The influence of the compensating cation (Na⁺, Li⁺, Mg²⁺) nature on the water adsorption properties of LTA and FAU-type zeolites was investigated. Cation exchanges were performed at 80 °C for 2 h using 1 M aqueous solutions of lithium chloride (LiCl) or magnesium chloride (MgCl₂). XRF and ICP-OES analyses indicate that the cation exchange yields reach values between 59 to 89% depending on the number of exchange cycles and the nature of the zeolite and cation, while both zeolites structures are preserved during the process, as shown by XRD and solid state NMR analyses. Nitrogen adsorption-desorption experiments indicate a higher available microporous volume when sodium cations are replaced by smaller monovalent lithium cations or by divalent magnesium cations because twice less cations are needed compared to monovalent cations. Up to 15% of gain in the available microporous volume is obtained for FAU-type zeolites exchanged with magnesium cation. This improvement facilitates the adsorption of water with an increase in the water uptake up to 30% for the LTA and FAU type zeolites exchanged with magnesium. These exchanged zeolites are promising for uses in water decontamination because a smaller amount is needed to trap the same amount of water compared to their sodium counterparts.

Multi-factor study of the effects of a trace amount of water vapor on low concentration CO2 capture by 5A zeolite particles

CO₂ adsorption amount is enhanced with below 0.1 ppm humidity, and water molecule partial charge is a dominant factor in adsorption.

Predicting Accurate Solvation Free Energy in n-Octanol Using 3D-RISM-KH Molecular Theory of Solvation: Making Right Choices

Molecular theory of solvation, a.k.a., three-dimensional reference interaction site model theory of solvation with Kovalenko-Hirata closure relation (3D-RISM-KH), is an accurate and fast theory predicting solvation free energy and structure. Here we report a benchmark study of n-octanol solvation free energy calculations using this theory. The choice of correct force field parameters is quintessential for the success of 3D-RISM theory, and we present a guideline to obtain them for n-octanol solvent. Our best prediction of the solvation free energy on a set of 205 small organic molecules supplemented with the so-called "universal correction" scheme yields relative mean unsigned error of 0.94 kcal/mol against the reported database. The best agreement is obtained with the united atom (UA) type force field parametrization of n-octanol with the van der Waals parameters of hydroxyl hydrogen reported by Kobryn et al. [ Kobryn , A. E. ; Kovalenko , A. J. Chem. Phys. 2008 , 129 , 134701 ].

Nanosized Na-EMT and Li-EMT zeolites: selective sorption of water and methanol studied by a combined IR and TG approach

Nanosized EMT-type zeolite crystals in sodium (Na-EMT) and ion-exchanged lithium (Li-EMT) forms were prepared. The sorption behavior of Li(Na)-EMT samples towards water, methanol and a mixture of both (50 : 50) was studied by combined thermogravimetric and infrared spectroscopic methods. The stability of the samples prior to and after the sorption measurements in two subsequent cycles was confirmed by X-ray diffraction, N₂ sorption and NMR spectroscopy. The high sorption capacity of the Li-EMT sample towards water was demonstrated. It was found that the methanol is replaced by water faster in the Li-EMT sample in comparison to the Na-EMT sample. At low temperature, the methanol shows weak adsorption on each cationic site and no side products during desorption for both samples were obtained.

The adsorption and fast transport of Xe in single walled carbon nanotubes

Combined GCMC and MD simulations have been used to investigate the adsorption and diffusion of Xe gases in carbon nanotubes (CNTs) at different conditions.

Correlation of the depletion layer with the Helmholtz layer in the anatase TiO2–H2O interface via molecular dynamics simulations

By correlating the Helmholtz layer with the depletion layer, the depletion layer widths of anatase TiO₂ surfaces (001), (100), and (101) are calculated.

Unraveling the adsorption mechanism of mono- and diaromatics in faujasite zeolite

Monte Carlo simulations are performed to study the adsorption of aromatic molecules (toluene, styrene, o-xylene, m-xylene, p-xylene, 1,3,5-trimethylbenzene, and naphthalene) in all-silica faujasite (FAU) zeolite. For monoaromatics, a two-stage "ideal adsorption" and "insertion adsorption" mechanism is found by careful inspection of locations and distributions of the adsorbed toluene molecules. The validity of this mechanism is confirmed for all monoaromatics considered in the current study. Remarkably, the number of C atoms per unit cell corresponding to the inflection point of adsorbate loading (CI-P) is defined as a valid and convenient characterizing factor in the packing efficiency of monoaromatics in the FAU zeolite. For the case of naphthalene, a type of diaromatic, the three-stage mechanism is proposed, which consists of the first two stages and a third stage of "overideal adsorption". The so-called overideal adsorption is labeled because the naphthalene molecules start to occupy the S site nonideally at loadings that approach saturation, leading to a more localized feature of the adsorbates. The explicit adsorption mechanism can be used to understand the loading dependence of isosteric adsorption heat for the aromatics concerned.

Pervaporation performance of PPO membranes in dehydration of highly hazardous mmh and udmh liquid propellants

Polyphenylene oxide (PPO) membranes synthesized from 2,6-dimethyl phenol monomer were subjected to pervaporation-based dehydration of the highly hazardous and hypergolic monomethyl hydrazine (MMH) and unsymmetrical dimethyl hydrazine (UDMH) liquid propellants. Membranes were characterized by TGA, DSC and SEM to study the effect of temperature besides morphologies of surface and cross-section of the films, respectively. Molecular dynamics (MD) simulation was used to study the diffusion behavior of solutions within the membrane. CFD method was employed to solve the governing mass transfer equations by considering the flux coupling. The modeling results were highlighted by the experimental data and were in good agreement. High separation factors (35-70) and reasonable water fluxes (0.1-0.2 kg/m(2)h) were observed for separation of the aqueous azeotropes of MMH (35 wt%) and UDMH (20 wt%) and their further enrichment to >90% purity. Effect of feed composition, membrane thickness and permeate pressure on separation performance of PPO membranes were investigated to determine optimum operating conditions.

Microwave synthesis of high-flux NaY zeolite membranes in fluoride media

High-flux NaY zeolite membranes were synthesized using low-cost mullite supports by microwave heating in fluoride media.

Hydrogen bonding effects in adsorption of water-alcohol mixtures in zeolites and the consequences for the characteristics of the Maxwell-Stefan diffusivities

This work highlights a variety of peculiar characteristics of adsorption and diffusion of polar molecules such as water, methanol and ethanol in zeolites. These peculiarities are investigated with the aid of configurational-bias Monte Carlo (CBMC) simulations of adsorption isotherms, and molecular dynamics (MD) simulations of diffusivities in FAU, MFI, DDR, and LTA zeolites. Because of strong hydrogen bonding, significant clustering of the guest molecules occurs in all investigated structures. Because of molecular clustering, the inverse thermodynamic factor 1/Gamma(i) identical with (d[ln c(i)])/(d[ln f(i)]) exceeds unity for a range molar concentrations c(i) within the micropores. The degree of clustering is lowered as the temperature is increased. For the concentration ranges for which 1/Gamma(i) > 1, the Fick diffusivity, D(i), for unary diffusion is often lower than both the Maxwell-Stefan, D(i), and the self-diffusivity, D(i,self). For water-alcohol mixtures, the hydrogen bonding between water and alcohol molecules is much more predominant than for water-water, and alcohol-alcohol molecule pairs. Consequently, the adsorption of water-alcohol mixtures shows significant deviations from the predictions of the ideal adsorbed solution theory (IAST). The water-alcohol bonding also leaves its imprint on the mixture diffusion characteristics. The Maxwell-Stefan diffusivity, D(i), of either component in water-alcohol mixtures is lower than the corresponding values of the pure components; this behavior is distinctly different from that for mixtures of nonpolar guest molecules. The binary exchange coefficient D(12) for water-alcohol mixtures is also significantly lower than either self-exchange coefficients D(11) and D(22) of the constituent species. This implies that correlation effects are significantly stronger in water-alcohol mixtures than for the constituent species. Correlation effects are found to be significant for water-alcohol mixture diffusion in DDR and LTA zeolites, even though such effects are negligible for the pure constituents. The major conclusion to emerge from this investigation is that, unlike mixtures of nonpolar molecules, it is not possible to estimate water-alcohol mixture adsorption and diffusion characteristics on the basis of pure component data.