Molecular Simulation Of Adsorption Research Articles

Molecular simulation of adsorption and separation of N2 and CF4 on carbon and boron nitride nanostructures

Grand canonical ensemble Monte Carlo (GCMC) molecular simulations were performed to study CF4 adsorption on carbon nanotubes, boron nitride (BN) nanotubes, carbon nanoscrolls, and BN nanoscrolls. At 298 K and 2 MPa, (15,15) BN nanotubes exhibited 4.57 mmol/g and 98 v/v in excess weight and volume adsorption for CF4, respectively, which is a 50 % and 44 % increase over (15,15) carbon nanotubes at 3.04 mmol/g and 68 v/v. BN nanoscrolls with a 1.3 nm interlayer spacing showed even higher adsorption at 13.63 mmol/g and 145 v/v, a 33 % increase over carbon nanoscrolls with a 10.23 mmol/g and 108 v/v. Utilizing Ideal Adsorbed Solution Theory (IAST), we predicted that (14,14) BN nanotubes would have a CF4 adsorption capacity of 4.27 mmol/g and a selectivity of 6.4 at 298 K and 4 MPa for equimolar CF4/N2 mixtures, surpassing (14,14) carbon nanotubes at 3.01 mmol/g and 4.5. BN nanoscrolls with a 1.2 nm spacing showed a 13.35 mmol/g and 8.1 adsorption capacity and selectivity for CF4, respectively, which is superior to carbon nanoscrolls at 9.33 mmol/g and 5.1. These results outperform other porous materials, indicating that BN nanoscrolls are highly promising for CF4 capture.

Molecular Simulation of Adsorption and Diffusion of Methane and Ethane in Kaolinite Clay under Supercritical Conditions: Effects of Water and Temperature

Grand Canonical Monte Carlo (GCMC) simulation and Molecular Dynamics (MD) simulations were used to study the effects of temperature (310 K to 400 K), pressure (≤30 MPa) and water content (0 molecule/nm3 to 9 molecule/nm3) on the adsorption and diffusion behavior of CH4 and C2H6 in 3 nm kaolinite slit under supercritical conditions. The obtained adsorption capacity, isosteric adsorption heat, concentration distribution and diffusion coefficient were analyzed and compared. The simulation results show that the adsorption capacity of C2H6 is higher under low pressure conditions, and the adsorption capacity of CH4 is higher under high pressure conditions due to the small molecular radius and increased adsorption space. The addition of water molecules and the increase in temperature will reduce the adsorption capacity and isosteric adsorption heat of the two gases. We analyzed the changes in Langmuir volume and Langmuir pressure of the two gases under different temperature and water content conditions. The addition of water molecules and the increase in temperature will reduce the saturation adsorption capacity (which has a greater effect on C2H6) and the adsorption rate of the two gases in the kaolinite slit. The water molecules occupy the adsorption site of the gas molecules (limiting the diffusion of the gas molecules), which reduces the interaction between gas molecules and the wall surface, thus altering the distribution of the two gases in the slit. The increase in temperature will accelerate the oscillation of the gas molecules, increasing diffusion, and also leads to a reduction in the peak value of the adsorption peaks of the two gases.

Efficient removal of fluoride ion by the composite forward osmosis membrane with modified cellulose nanocrystal interlayer

In this paper, the performance of three-layer film composite (TFC) forward osmosis (FO) membrane for the retention of fluoride ions was investigated. A cellulose nanocrystal (CNC) interlayer was introduced on the porous substrate by coating method to enhance the hydrophilicity and retention capacity of the FO membrane. In order to improve the dispersibility of CNC, boric acid (BA) and (3-aminopropyl)triethoxysilane (APTES) were used for chemical modification, respectively. The characterization results demonstrated the successful introduction of CNC, BA/CNC and APTES/CNC–CNC interlayers. The introduction of the interlayer enhanced the hydrophilicity of the membrane and increased the negative surface charge, which facilitated the transport of water molecules and the retention of fluoride ions. The FO tests indicated that the water flux of TFC/CNC membrane was increased by 46.4% compared to the original TFC membrane, and the water flux of TFC/APTES-CNC and TFC/BA-CNC membranes were higher than that of TFC/CNC membranes by 5.9% and 13.0%, respectively. 97.0% and 97.6% of the fluoride ions of TFC/BA-CNC and TFC/APTES-CNC could be retained. Based on the molecular simulation of adsorption, it was demonstrated that BA-CNC interlayer was more favorable for the adsorption of water molecules than CNC and APTES-CNC interlayer. This study contributes to the development of high-performance TFC-FO membrane for the treatment of wastewater containing fluoride.

Molecular simulation of adsorption and diffusion behavior of CO2 in pyrophyllite

Understanding the microscopic process of adsorption and diffusion of CO2 in clay minerals is important for geological storage of CO2. In this study, Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) simulations are used to obtain the adsorption and diffusion behavior of CO2 in pyrophyllite slit pores. The influence of split pore size, temperature, pressure, and water content were analyzed in detail. The adsorption characteristics of CO2 in pyrophyllite were revealed by discussing the distribution of CO2 density profile, adsorption capacity, isosteric heat of adsorption, interaction energy, mean square displacement, self-diffusion coefficient, and molecular orientation distribution. It shows that the excess adsorption isotherm of CO2 in pyrophyllite slit pores demonstrate significant supercritical adsorption characteristics. Surface attractions originating from both pore walls overlap in the middle of pore, leading to an enhanced adsorption capacity of CO2 in pyrophyllite slit pores when pore aperture is lower than 3 nm. In pyrophyllite pores, the presence of water has an enhancing effect on CO2 adsorption when the water content is below 30 wt%. A maximum value of CO2 adsorption amount is reached at the water content of 10 wt%, while negative growth occurs at 35 wt%. At low water content, water molecules form a weak adsorption layer due to the weak attraction of pyrophyllite to water. As water content increases, the self-diffusion coefficient of the CO2 decreases, and the diffusion capacity decreases. The adsorption state of the CO2 molecule on the mineral surface is primarily a parallel configuration, while the water molecule has no clear orientation. Our calculated results can provide guidance for further study of clay-water-adsorbate interface reaction of hydrophobic minerals.

Molecular simulation of adsorption and diffusion of H2 /CO2 /CO /MeOH /EtOH mixture into the zeolitic imidazolate framework ZIF-8

The adsorption and diffusion behaviors of H2, CO2, CO, MeOH, and EtOH into ZIF-8 were studied in pure and mixtures (binary and total) states by grand canonical Monte Carlo and molecular dynamics simulation, respectively. In addition, the separation of MeOH over the other guest molecules was also investigated in the mentioned mixtures. The results of molecular dynamics simulations showed that the diffusion coefficients of pure guest molecules followed the trend of H2> CO2> CO > MeOH > EtOH. Analysis of adsorption indicated that all of the guest molecules (except H2) have S-shaped adsorption isotherms with an initial low-uptake region, which can be ascribed to the cage-filling phenomenon. However, the trend of the initial sorption plateau range is CO2> CO > MeOH > EtOH. In the mixture used guest molecules, the most separation is related to the separation of MeOH from H2 and EtOH, which is the optimal separation of MeOH from H2 at pressures less than 1584 kPa and the optimal separation of MeOH from EtOH at a pressure of 10,000 kPa.

Molecular Simulation of Adsorption in Deep Marine Shale Gas Reservoirs

Deep marine shale gas reservoirs are extremely rich in the Sichuan basin in China. However, due to the in situ conditions with high temperature and high pressure (HTHP), in particular reservoir pressure being usually much higher than the test pressure, it is difficult to accurately clarify the adsorption behavior, as seepage theory plays an important role in shale gas reserves evaluation. Therefore, three kinds of sorbent, including illite, quartz and kerogen, and two simulation methods, containing the grand canonical ensemble Monte Carlo method and molecular dynamics method, are synthetically used to determine the methane adsorption behavior under HTHP. The results show that both absolute adsorption and excess adsorption decrease with the increase of temperature. When the pressure increases, the absolute adsorption increases quickly and then slowly, and the excess adsorption first increases and then decreases. The superposition of wall potential energy is strongest in a circular hole, second in a square hole, and weakest in a narrow slit. The effect of pore size increases with the decrease of the pore diameter. Under HTHP, multi-layer adsorption can occur in shale, but the timing and number of layers are related to the sorbent type.

Methane and carbon dioxide in dual‐porosity organic matter: Molecular simulations of adsorption and diffusion

AbstractShale gas, which predominantly consists of methane, is an important unconventional energy resource that has had a potential game‐changing effect on natural gas supplies worldwide in recent years. Shale is comprised of two distinct components: organic material and clay minerals, the former providing storage for hydrocarbons and the latter minimizing hydrocarbon transport. The injection of carbon dioxide in the exchange of methane within shale formations improves the shale gas recovery, and simultaneously sequesters carbon dioxide to reduce greenhouse gas emissions. Understanding the properties of fluids such as methane and methane/carbon dioxide mixtures in narrow pores found within shale formations is critical for identifying ways to deploy shale gas technology with reduced environmental impact. In this work, we apply molecular‐level simulations to explore adsorption and diffusion behavior of methane, as a proxy of shale gas, and methane/carbon dioxide mixtures in realistic models of organic materials. We first use molecular dynamics simulations to generate the porous structures of mature and overmature type‐II organic matter with both micro‐ and mesoporosity, and systematically characterize the resulting dual‐porosity organic‐matter structures. We then employ the grand canonical Monte Carlo technique to study the adsorption of methane and the competing adsorption of methane/carbon dioxide mixtures in the organic‐matter porous structures. We complement the adsorption studies by simulating the diffusion of adsorbed methane, and adsorbed methane/carbon dioxide mixtures in the organic‐matter structures using molecular dynamics.

Molecular Simulations of Adsorption and Energy Storage of R1234yf, R1234ze(z), R134a, R32, and their Mixtures in M-MOF-74 (M = Mg, Ni) Nanoparticles

The refrigerant circulation heat can be enhanced through the mutual transformation between thermal energy and surface energy during the adsorption and separation process of fluid molecules in porous materials. In this paper, the adsorption and energy storage of R1234ze(z), R1234yf, R32 and R134a, as well as their mixed refrigerants in Mg-MOF-74 and Ni-MOF-74 nanoparticles were investigated by means of molecular dynamics simulations and grand canonical Monte Carlo simulations. The results suggested that, in the case of pure refrigerant adsorption, the adsorption quantities of R32 and R134a in MOFs were higher than those of R1234yf and R1234ze(z). However, in the case of saturation adsorption, the desorption heat of R32 was lower than that of R1234yf and R1234ze(z). The addition of MOF-74 nanoparticles (NPs) could enhance the energy storage capacity of the pure refrigerant; besides, R1234yf and R1234ze(z) nanofluids had superior enhancement effect to that of R32 nanofluid. In mixed refrigerant adsorption, the adsorption quantities of R1234ze(z) and R1234yf were lower than those of R32 and R134a; with the increase in temperature, the adsorption of R1234ze(z) and R1234yf showed a gradually increasing trend, while that of R32 was gradually decreased.

Effects of Intrinsic Flexibility on Adsorption Properties of Metal–Organic Frameworks at Dilute and Nondilute Loadings

Molecular simulation of adsorption in nanoporous materials has become a valuable complement to experimental studies of these materials. In almost all cases, these simulations treat the adsorbing material as rigid. We use molecular simulations to examine the validity of this approximation for the adsorption in metal-organic frameworks (MOFs) that have framework flexibility without change in their unit cells because of thermal vibrations. All nanoporous materials are subject to this kind of framework flexibility. We examine the adsorption of nine molecules (CO2, CH4, ethane, ethene, propane, propene, butane, Xe, and Kr) and four molecular mixtures (CO2/CH4, ethane/ethene, propane/propene/butane, and Xe/Kr) in 100 MOFs at dilute and nondilute adsorption conditions. Our results show that single-component adsorption uptakes at nondilute conditions are only weakly affected by framework flexibility, but adsorption selectivities at both dilute and nondilute conditions can be significantly affected by flexibility. The most dramatic impacts of framework flexibility occur for adsorption uptake in the limit of dilute adsorption. These results suggest that the importance of including framework flexibility when attempting to make quantitative predictions of adsorption selectivity in MOFs and similar materials may have been underestimated in the past.

Molecular Simulations of Adsorption and Thermal Energy Storage of Mixed R1234ze/UIO-66 Nanoparticle Nanofluid

In the process of adsorption and separation of fluid molecules on the solid surface of porous nanomaterials, the mutual transformation of thermal energy and surface energy can improve the heat absorption and energy utilization efficiency of circulating working medium. In this study, the adsorption, thermal energy storage, and mean square displacement of the minimum energy adsorption configuration of R1234ze in UIO-66 were studied by molecular simulations, including molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations. The results show that the thermal energy storage density of R1234ze/UIO-66 mixed working medium is significantly higher than that of pure working medium in the temperature range of 20 K-140 K. However, the increase rate of thermal energy storage density decreases significantly as temperature rises, and the mean square displacement and diffusion coefficient increase with increasing temperature.

Quasi-Equilibrated Thermodesorption Combined with Molecular Simulation for Adsorption and Separation of Hexane Isomers in Zeolites MFI and MEL

Adsorption of hexane isomers in high silica MFI and MEL zeolites was studied by means of quasi-equilibrated temperature-programmed desorption and adsorption and Monte Carlo simulations. Configurational bias and continuous fractional component Monte Carlo were applied for these systems, and their efficiency and effectiveness were compared. All branched hexane isomers, i.e., 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, and 2,2-dimethylbutane, were used. The agreement between experimental and calculated adsorption isobars confirmed that quasi-equilibrated temperature-programmed desorption and adsorption is an effective method for studying adsorption of branched alkanes in zeolites. Detailed literature review on adsorption of branched paraffins in MFI revealed diffusion limitations which make reaching adsorption equilibria of these molecules difficult with isothermal approach at low temperatures. Temperature-dependent measurements conducted in this work largely allowed avoiding such limitations. Adso...

Molecular simulations of adsorption and separation of acetylene and methane and their binary mixture on MOF-5, HKUST-1 and MOF-505 metal–organic frameworks

In this work, the adsorption of acetylene and its binary mixture with methane on MOF-5, HKUST-1 and MOF-505 was studied using Grand Canonical Monte Carlo molecular simulations. The preferred adsorption sites of acetylene and methane molecules into metal–organic frameworks (MOFs) were investigated. The simulated adsorption isotherms of acetylene on MOF-5 and MOF-505 agreed well with the experimental ones without any reparameterisation of the potential parameters but for HKUST-1 the interaction parameters of the acetylene and copper ion were reparameterised. Comparisons of the calculated adsorption isotherms of acetylene in the studied MOFs showed that the MOF-5 had the lowest adsorption capacity. Our results revealed that guest molecules were most adsorbed on the entrance windows of the octagon pore of HKUST-1, while the preferred adsorption sites were large pores and on the metal ion cluster of MOF-505 and MOF-5, respectively. Adsorption of binary mixtures of methane and acetylene on MOF-5, HKUST-1 and MOF-505 revealed that acetylene adsorption is higher than that of methane. Finally, the results showed that C2H2/CH4 selectivity values on HKUST-1 are significantly higher than on MOF-505 and MOF-5. The preferred adsorption sites of acetylene and methane in an equimolar binary mixture were calculated and discussed.

Molecular simulation of adsorption and separation of pure noble gases and noble gas mixtures on single wall carbon nanotubes

Grand Canonical Monte Carlo simulations were performed to systematically study the adsorption and separation of noble gases on single wall carbon nanotube (SWCNT) bundles. Pure noble gases, as well as binary and ternary mixtures, were simulated in carbon nanotube systems under various conditions. Adsorption data was collected at 100K and 300K over a wide range of pressures. Carbon nanotube bundles present distinct adsorption capacities towards different noble gases. In particular, larger and heavier noble gases are easier to be adsorbed at low pressure, while lighter atoms with smaller sizes can be better stored at high pressure. For noble gas mixtures with equal molar loadings, selective adsorption was observed and the selectivity inverted at different pressure ranges according to the choice of mixtures. Furthermore, the influence of loading proportion of the components on the adsorption behavior was investigated by varying the loading partial pressure in binary mixtures from 1% to 99%. The results suggest gas–CNT interactions dominate the adsorption selectivity at low loading conditions, whereas the entropic effect plays a more important role at high loadings.

Molecular simulations of physical and chemical adsorption under gas and liquid environments using force field- and quantum mechanics-based methods

Here we review our simulations of adsorption on metal–organic frameworks (MOFs) and platinum (Pt) catalysts, focusing on the modelling methods required to understand these two very different systems. MOFs are porous, crystalline materials with large surface areas, which are promising for a variety of adsorption applications. We review our simulations of gas uptake in PCN-53 (porous coordination network) as well as gas storage in MOFs functionalised with metal alkoxide sites. While fluid–solid interactions in both systems can be modelled quite well using algebraic force fields, the alkoxide sites in the functionalised MOFs require specialised versions, in order to describe the stronger adsorption energies. We discuss grand canonical Monte Carlo (GCMC) simulations of both systems. Pt is a common catalyst, and simulations have proven quite useful for providing molecular level details to understand its functionality. This involves understanding adsorption phenomena, which often requires quantum mechanical calculations. We describe our periodic boundary condition density functional theory (DFT) simulations of Pt-catalysed NO oxidation, focusing on adsorbate geometries and coverage effects. Finally, we describe one of the current ‘grand challenges’ in molecular simulations of adsorption, modelling catalytic activity in aqueous phase, which requires a combination of algebraic force fields, DFT and GCMC.

Molecular Simulation of Adsorption in Microporous Materials

The development of industrial software, the decreasing cost of computing time, and the availability of well-tested forcefields make molecular simulation increasingly attractive for chemical engineers. We present here several applications of Monte-Carlo simulation techniques, applied to the adsorption of fluids in microporous solids such as zeolites and model carbons (pores < 2 nm). Adsorption was computed in the Grand Canonical ensemble with the MedeA<sup>®<sup/>-GIBBS software, using energy grids to decrease computing time. MedeA<sup>®<sup/>-GIBBS has been used for simulations in the NVT or NPT ensembles to obtain the density and fugacities of fluid phases. Simulation results are compared with experimental pure component isotherms in zeolites (hydrocarbon gases, water, alkanes, aromatics, ethanethiol, etc.), and mixtures (methane-ethane, <i>n<i/>-hexane-benzene), over a large range of temperatures. Hexane/benzene selectivity inversions between silicalite and Na-faujasites are well predicted with published forcefields, providing an insight on the underlying mechanisms. Also, the adsorption isotherms in Na-faujasites for light gases or ethane-thiol are well described. Regarding organic adsorbents, models of mature kerogen or coal were built in agreement with known chemistry of these systems. Obtaining realistic kerogen densities with the simple relaxation approach considered here is encouraging for the investigation of other organic systems. Computing excess sorption curves in qualitative agreement with those recently measured on dry samples of gas shale is also favorable. Although still preliminary, such applications illustrate the strength of molecular modeling in understanding complex systems in conditions where experiments are difficult.

Molecular Simulation of Adsorption and Transport in Hierarchical Porous Materials

Adsorption and transport in hierarchical porous solids with micro- (~1 nm) and mesoporosities (>2 nm) are investigated by molecular simulation. Two models of hierarchical solids are considered: microporous materials in which mesopores are carved out (model A) and mesoporous materials in which microporous nanoparticles are inserted (model B). Adsorption isotherms for model A can be described as a linear combination of the adsorption isotherms for pure mesoporous and microporous solids. In contrast, adsorption in model B departs from adsorption in pure microporous and mesoporous solids; the inserted microporous particles act as defects, which help nucleate the liquid phase within the mesopore and shift capillary condensation toward lower pressures. As far as transport under a pressure gradient is concerned, the flux in hierarchical materials consisting of microporous solids in which mesopores are carved out obeys the Navier-Stokes equation so that Darcy's law is verified within the mesopore. Moreover, the flow in such materials is larger than in a single mesopore, due to the transfer between micropores and mesopores. This nonzero velocity at the mesopore surface implies that transport in such hierarchical materials involves slippage at the mesopore surface, although the adsorbate has a strong affinity for the surface. In contrast to model A, flux in model B is smaller than in a single mesopore, as the nanoparticles act as constrictions that hinder transport. By a subtle effect arising from fast transport in the mesopores, the presence of mesopores increases the number of molecules in the microporosity in hierarchical materials and, hence, decreases the flow in the micropores (due to mass conservation). As a result, we do not observe faster diffusion in the micropores of hierarchical materials upon flow but slower diffusion, which increases the contact time between the adsorbate and the surface of the microporosity.

Molecular simulations of adsorption and separation of natural gas on zeolitic imidazolate frameworks

Grand canonical Monte Carlo simulations were employed to investigate the adsorption and separation of C2H6, CO2 and CH4 on two zeolitic imidazolate frameworks (ZIF-2 and ZIF-71). The adsorption isotherm and isosteric heat of pure gas, the separation performance of C2H6-CH4, CO2-CH4 and C2H6-CO2 binary mixtures and C2H6-CO2-CH4 ternary mixtures on two ZIFs were simulated and discussed. For single component gas adsorption at a low pressure, the adsorption amount depended on isosteric heat; at a high pressure, due to the limited pore volume, ZIFs preferably adsorbed smaller size gas molecules. For gas mixture separation, energetic effect dominated at low pressure, therefore, ZIFs selectively adsorbed gas component with strong interactions; packing effect usually played an important role at high pressures, consequently, smaller size component would be more entropically favorable. Results demonstrated that both ZIF-2 and ZIF-71 were of good separation performance for these three binary mixtures. For the ternary mixture separation, it was found that ZIF-2 cowld effectively separate C2H6 and CO2 from CH4 at 3000-4000 kPa and room temperature.

Molecular Simulation for Adsorption and Separation of CH4/H2 in Zeolitic Imidazolate Frameworks

Molecular Simulation for Adsorption and Separation of CH₄/H₂ in Zeolitic Imidazolate Frameworks

Molecular simulations of adsorption and diffusion of RDX in IRMOF-1

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 a vertex, and (iii) between benzene rings. The energy of adsorption of RDX at infinite dilution was found to be − 9.2 kcal/mol. The distributions of bond lengths, bond angles and torsion angles in RDX are uniformly slightly broader in the gas phase than in the adsorbed phase, but not markedly so. The self-diffusivity of RDX in IRMOF-1 is a strong function of temperature, with an activation energy of 6.0 kcal/mol.

Molecular Simulations of Adsorption and Diffusion Behaviors of Benzene Molecules in NaY Zeolite

In the article the Grand Canonical Monte Carlo (GCMC), molecular dynamics (MD), and kinetic Monte Carlo (KMC) simulations with particular focus on ascertaining the loading dependence of benzene diffusion in the zeolite were performed. First, a realistic representation of the structure of the sorbate-sorbent system was obtained based on GCMC simulation. The simulation clearly shows the characteristics of the adsorption sites of the benzene-NaY system, from which two kinds of preferably adsorbing sites for benzene molecules, called S II and W sites, are identified. The structure thus obtained was then used as a basis for KMC and MD simulations. A comparative study by introducing and comparing two different mechanisms underlying jump diffusion in the zeolite of interest shows that the MS diffusivity values predicted by the KMC and MD methods are fairly close to each other, leading to the conclusion that for benzene diffusion in NaY, the S II→W→S II jumps of benzene molecules are dominated, while the W→W jumps do not exist in the process. These findings provide further support to our previous conclusion about the absence of the W→W jumps in the process of benzene diffusion in NaY. Finally, two relations for predicting the self- and MS diffusivities were derived and found to be in fair agreement with the KMC and MD simulations.