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Nanoporous materials are important in industrial separation, but their application is subject to strong interfacial barriers to the entry and transport of fluids. At certain conditions the fluid inside and outside the nanoporous material can be viewed as a two-phase system, with an interface between them, which poses an excess resistance to matter flow. We show that there exist two kinds of phenomena which influence the interfacial resistance: hydrodynamic effects and thermodynamic effects, which are independent of each other. Here, we investigate the role of the thermodynamic effects in carbon nanotubes (CNTs) and slit pores and compare the associated thermodynmic resistance with that due to hydrodynamic effects traditionally modeled by the established Sampson expression. Using CH4 and CO2 as model fluids, we show that the thermodynamic resistance is especially important for moderate to high pressures, at which the fluid within the CNT or slit pore is in the condensed state. Further, we show that at such pressures the thermodynamic resistance becomes comparable with the internal resistance to fluid transport at length scales typical of membranes used in fuel cells, and of importance in membrane-based separation, and nanofluidics in general.

We investigate permeation in non-ideal mixed-matrix membranes (MMMs) having interfacial defects based on a multiscale simulation approach, involving sequential nanoscale and macroscale simulations. Our approach combines insight from equilibrium molecular dynamics (EMD) simulation of the transport in the individual phases, and in the interfacial region, with macroscopic simulation of the transport through solution of coupled 3-D partial differential equations (PDEs) via a finite-element method (FEM). Thus, unlike existing adaptations of classical effective medium theory (EMT), our approach avoids the empirical fitting of filler-polymer interfacial properties, such as interfacial thickness and permeability, against experimental permeation data. While the proposed multiscale simulation approach is general and applicable to any MMM system, it is illustrated in the context of single CO 2 and CH 4 permeation in two different polyimide-based MMMs; with one using MFI-type zeolite as filler phase and the other using carbon molecular sieves (CMS). For both systems, our nanoscale simulations reveal an interfacial region with rigidification of the polymer and reduced permeability at the polymer-adsorbent interface. Comparison of permeability predictions of classical EMT-based models considering interfacial rigidification, with our macroscale simulations, demonstrates the former to be best at low particle loadings. Further, our macroscale simulations reveal the existence of an optimum filler particle size, never discussed before, which maximizes the permeability in non-ideal MMMs; this arises from the competing effects of decrease in permeability and decrease of the relative interfacial resistance with increase of filler particle size. Moreover, the CO 2 / CH 4 perm-selectivity is found strongly sensitive to the interfacial resistance for both finite uniform and non-uniform particle size distributions, an effect not captured by existing EMT models and overlooked in prior fundamental approaches.

Cobalt oxide silica membranes were prepared and tested to separate small molecular gases, such as He (dk = 2.6 Å) and H2 (dk = 2.89 Å), from other gases with larger kinetic diameters, such as CO2 (dk = 3.47 Å) and Ar (dk = 3.41 Å). In view of the amorphous nature of silica membranes, pore sizes are generally distributed in the ultra-microporous range. However, it is difficult to determine the pore size of silica derived membranes by conventional characterization methods, such as N2 physisorption-desorption or high-resolution electron microscopy. Therefore, this work endeavors to determine the pore size of the membranes based on transport phenomena and computer modelling. This was carried out by using the oscillator model and correlating with experimental results, such as gas permeance (i.e., normalized pressure flux), apparent activation energy for gas permeation. Based on the oscillator model, He and H2 can diffuse through constrictions narrower than their gas kinetic diameters at high temperatures, and this was possibly due to the high kinetic energy promoted by the increase in external temperature. It was interesting to observe changes in transport phenomena for the cobalt oxide doped membranes exposed to H2 at high temperatures up to 500 °C. This was attributed to the reduction of cobalt oxide, and this redox effect gave different apparent activation energy. The reduced membrane showed lower apparent activation energy and higher gas permeance than the oxidized membrane, due to the enlargement of pores. These results together with effective medium theory (EMT) suggest that the pore size distribution is changed and the peak of the distribution is slightly shifted to a larger value. Hence, this work showed for the first time that the oscillator model with EMT is a potential tool to determine the pore size of silica derived membranes from experimental gas permeation data.

We have used nonequilibrium molecular dynamics simulations to investigate the transport diffusion of methane, at 300 K and pressures of up to 15 bar, in 30 nm-long (10, 10) carbon nanotubes (CNTs) held between two flanges mounted at the ends to represent the surface layers of an embedding matrix material. Strong interfacial resistance to the entry and exit of molecules is found in the 30 nm-long CNTs, which reduces their permeability by more than 2 orders of magnitude. Increasing the adsorption affinity and surface area of the flange reduces the interfacial resistance and consequently enhances the methane diffusivity in CNT membranes. Curved streamlines near the flange surface make a significant contribution to the permeability, even when the adsorption on the matrix surface is negligible. We propose a model to calculate the separate components of the interfacial resistance, the flange resistance, which increases with increase in the membrane porosity, and the entrance-exit resistance, which is independent of the membrane porosity. While the flange resistance accounts for the reduction of interfacial resistance with decrease in the membrane porosity, the entrance-exit resistance is responsible for the reduction of interfacial resistance with increase in the flange adsorption affinity. The flange resistivity demonstrates a complex dependency on the flange adsorption affinity, which is attributed to the competition between the enhanced adsorption and the enhanced migration time of the molecules on the flange. It is concluded that the embedding matrix adsorption affinity and membrane porosity separately play critical roles in determining the interfacial resistance and permeability in CNT membranes. Our simulation results can help reduce the interfacial resistance and improve the permeance in CNT membranes by appropriate choice of intertube spacing and flange material and are readily applied to all nanoporous membranes with a passive matrix.

Over the past three decades, mixed-matrix membranes (MMMs), comprising an inorganic filler phase embedded in a polymer matrix, have emerged as a promising alternative to overcome limitations of conventional polymer and inorganic membranes. However, while much effort has been devoted to MMMs in practice, their modeling is largely based on early theories for transport in composites. These theories consider uniform transport properties and driving force, and thus models for the permeability in MMMs often perform unsatisfactorily when compared to experimental permeation data. In this work, we review existing theories for permeation in MMMs and discuss their fundamental assumptions and limitations with the aim of providing future directions permitting new models to consider realistic MMM operating conditions. Furthermore, we compare predictions of popular permeation models against available experimental and simulation-based permeation data, and discuss the suitability of these models for predicting MMM permeability under typical operating conditions.

Three-dimensional ordered mesoporous polymer (OMP) single-crystals were synthesized for the first time via a simple soft-template strategy. Ordered mesoporous nitrogen-doped carbon (OMNC) was obtained through carbonization of OMP and post ammonia activation. The evolution of specific surface area, pore volume, and nitrogen content, and species of OMNC were systematically investigated by adjusting the activation temperature and time.

A series of Li4SiO4 was synthesized using LiNO3 and six different silicon precursors. The precipitated-silica-derived Li4SiO4 presented the highest CO2 capacity in a 10 h sorption test, and ZSM-5-derived Li4SiO4 demonstrated the most rapid CO2 sorption. The CO2 sorption kinetics predominantly followed the nucleation mode and could be accurately described by the Avrami-Erofeev model. The Avrami-Erofeev model provided an in-depth analysis of correlation between sorption performance and material properties. Both the nucleation speed and nucleation dimensionality affected the overall sorption kinetics. The kinetics and pore-size distribution suggest that the sorption kinetics was dependent on the quantity of ∼4 nm-pores which favors nucleation dimensionality. For the cyclic tests, the precipitated-silica-derived sample presented the poorest performance with the capacity decreasing from 31.33 wt% at the 1st cycle to only 11.52 wt% at the 30th cycle. However, the sample made from fumed silica displayed an opposite trend with the capacity increasing from 19.90 wt% at the 1st cycle to 34.23 wt% at the 30th cycle. The radically distinct behaviour of samples during cycles was on account of the alternation of sorption kinetics. The decrease in ∼4 nm-pores after cycles was responsible for the decrease of nucleation dimensionality for the precipitated-silica-derived sample. The rearrangement during cycles could enrich the pores of ∼4 nm for the fumed silica-derived sample, which improved the nucleation growth, thus enhancing the kinetics with cycles.

We illustrate here the interplay of decoordination and accessible pore volume in nanosized supercapacitors, using constant voltage Gibbs ensemble based grand canonical Monte Carlo simulations for three different microporous carbon electrodes of known atomistic structure and 1-ethyl-3-methylimidazolium boron tetrafluoride (EMI-BF4) as electrolyte. We demonstrate that the counterion coordination number decreases with pore size, and this trend is similar for the electrodes considered, despite their different structures, suggesting that the pore shape is less important to this relation, at least for the carbons examined here. It is seen that ions with low coordination and/or completely decoordinated ions induce maximum charge, while those with higher coordination induce less, in accordance with recent MD simulation results which demonstrate that ions in high degree of confinement (DOC) induce more charge than those in low DOC. Our results indicate that electrodes with different pore volumes can exhibit simila...

This work investigated the permeation of binary gas mixtures in non-reducing (He/CO2) and reducing (H2/Ar) conditions at temperatures ranging from 200 to 500 °C. A common performance aspect under both non-reducing and reducing conditions was that the He and H2 purity in the permeate stream was independent of temperature for the tested binary gas mixtures, except at very high He/CO2 or H2/Ar concentrations (⩾90/10) in the retentate stream. Under non-reducing conditions, the transport of gases was consistent with molecular sieving properties of silica derived membranes, and He permeance was constant irrespective of the He/CO2 binary concentration tested. An anomalous H2 transport was observed under reduced conditions, as unexpectedly the H2 permeance was higher for gas mixtures instead of single gas. Further tests showed that H2 permeance increased 170% as the gas mixture changed from single H2 gas to H2/Ar gas mixtures. This was attributed to the experimental procedure, as the membranes were partially reduced each day and tested for gas permeation from pure H2 to lower H2 concentration in gas mixtures. Under these partial reducing conditions, H2 slowly reacts with the surface of the dense Co3O4 particle, thus forming a porous CoO region. The increase in H2 permeance was therefore attributed to improved pore connectivity between the silica structure and the porous CoO region.

In seeking environmentally clean technologies, this research has assessed a new alternative to synthesize activated carbon of Colombian mineral coal by using gases from partial combustion of propane (hereafter called flue gases), which are generally exhausted to the atmosphere. The activation was carried out by two methods of synthesis: in the first, pyrolysis was performed prior to activation, while in the second, pyrolysis and activation were carried out simultaneously. This alternative of synthesizing activated carbon was compared with conventional activation of coal using CO2. The activated carbon with flue gases and CO2 were characterized by adsorption of argon at −186 °C, and parameters such as specific surface area, pore size distribution, total pore volume, and micropore volume were determined. The activation with flue gases reduced combustion gas components such as SO2, NOx, and CO2 during the coal activation process. Activation with flue gases resulted in a surface area of 804 m2/g and a total p...

We investigate the separation of helium isotopes by quantum tunneling through graphene nanopores, recently proposed as an alternative to conventional methods for 3He production. We propose here a novel defective nanopore created by removing two pentagon rings of a Stone–Thrower–Wales (STW) defect, which significantly decreases the helium tunneling barrier by 50–75%. The barrier height is fine-tuned by adjusting the effective pore size, which is achieved by pore rim passivation using an appropriate functionalizing atom. This fine-tuning leads to positive deviation in the tunneling probability of 3He compared to that of 4He in the low-energy region, and thereby to high selectivity and transmission of the former isotope. It is found that fluorine-passivated nanopores restrict helium atom penetration because of their highly reduced pore size. Defective nanopores in nitrogen- and oxygen-passivated structures exhibit relatively high transmission values of 10–3 for the oxygen variant and improved selectivity val...

We perform constant voltage Gibbs ensemble based grand canonical Monte Carlo simulations for nanosized supercapacitors comprising graphene slit electrodes in symmetric and asymmetric electrolytes. Our simulations demonstrate that external electrolyte at the electrode surface can be exploited to positively influence the structure and packing of that inside the slit, when the system is engineered to allow these to interact. Oscillatory dependence of capacitance on slit-pore size, seen in recent results from molecular dynamics simulation and density functional theory, is observed also in our Monte Carlo simulations. A detailed analysis suggests that maximum in capacitance occurs in subnanometre pores because of the interference between internal double layers (largely the Helmholtz parts) on the opposite sides of the slit, expelling the co-ions; and that the oscillatory character of capacitance with pore width is due to relative changes in counterion and co-ion populations with pore width, also dictated by th...

We investigate the adsorption equilibrium and uptake kinetics of CO2 in nanoporous silicon carbide-derived carbon (SiC-DC) experimentally, as well as by simulation through grand canonical Monte Carlo (GCMC) and equilibrium molecular dynamics (EMD) simulations using a hybrid reverse Monte Carlo (HRMC) simulation-based atomistic structural model of the carbon. The kinetics is best explained when a grain surface barrier resistance mechanism is incorporated in a bidisperse pore structure model, considering particle scale diffusion in large micropores and a local grain scale diffusion in ultra-micropores. Good agreement is found between simulated and experimental isotherms; however, experimental particle scale diffusivities are almost two orders of magnitude smaller than those obtained from EMD, suggesting the presence of long range barriers for CO2 not captured by the HRMC model structure. These barriers also lead to about two orders of magnitude reduction in the particle-scale diffusivity of CO2 compared to CH4. On the other hand activation barriers for grain scale diffusion of CO2 are comparable to those for CH4 in the SiC-DC, and in carbon molecular sieves, consistent with the ultra-microporous nature of the grains. The activation barrier for interfacial mass transfer coefficient at the grain surface is also consistent with values for carbon molecular sieves.

We explore the effect of fluorine doping on hydrophobicity of nanoporous silicon carbide-derived carbon (SiCDC), and investigate the underlying barriers for adsorption and diffusion of water vapor and CO2 in the fluorinated and nonfluorinated structures. We develop atomistic models of fluorine-doped SiCDC at three different levels of fluorination, based on a hybrid reverse Monte Carlo constructed model of SiCDC, and develop a novel first-principles force field for the simulation of adsorption and transport of water and CO2 in the fluorine-doped carbon materials. We demonstrate an apparent dual effect of fluorination, showing that while fluorination generates more hydrophilic carbon surfaces, they actually act as more hydrophobic structures due to enhanced energy barriers in the disordered network of microporous carbon. While an increase in adsorption energy and in water uptake is seen for fluorine-doped carbon, large internal free energy barriers as well as the results of MD simulations demonstrate that the increased adsorption is kinetically limited and not experimentally observable on practical time scales. We show that an increase in apparent hydrophobicity due to fluorination is mediated by larger free energy barriers arising from stronger binding of fluid molecules inside the pore network, as opposed to repulsion or steric hindrance to the diffusion of molecules through narrow pore entries. For carbon dioxide, adsorption enthalpies and activation energy barriers are both decreased on fluorination, indicating weakened solid-fluid binding energies in the fluorinated systems.

We explore different multi-stage and multi-constraint modeling strategies using the Hybrid Reverse Monte Carlo (HRMC) technique to develop realistic models for the amorphous structure of silicon carbide derived-carbon, and investigate the effect of modeling parameters on the development of nano-structural features of the constructed models. It is shown that application of long simulations with slow thermal quench rate is essential for modeling of amorphous structures. Nevertheless, very slow quenching rates are shown to lead to the formation of configurations with large fraction of sp2 carbon, lacking the level of disorder required to match structure-related experimental data. The predicted gas adsorption isotherms are very sensitive to the pore size distribution (PSD), thus the final structure must reasonably reproduce the total pore volume and pore size distribution of the experimental sample. The frequently-observed strong first peak of the DFT-based PSD obtained from argon adsorption is shown to be an artifact of argon inaccessibility. Pore accessibility analysis of the constructed models, as well as MD simulations of gas transport demonstrate that the HRMC constructed structures contain short-range structural anisotropy, however the models are successful in capturing the long range internal energy barriers of amorphous carbon for methane.

We report a comparison of the adsorption of CH4 and CO2/CH4 mixtures of different composition in three different types of nanoporous carbons including carbon nanotubes, and activated carbon fibre (ACF-15) and silicon carbide derived carbon (SiC-DC) having distinctly different disordered structures, using Monte Carlo simulation. CO2 is represented as a linear molecule, and both the united-atom and full-atom models are investigated for CH4. It is found that the united-atom model of CH4 overestimates the adsorption of CH4 in all these adsorbents compared to the 5-site model, as a consequence of the enhanced 1-site CH4-adsorbent potential energy. Moreover, the selectivities of the nanoporous carbons for CO2 relative to CH4 calculated using the 1-site CH4 model are underestimated compared to those from the 5-site model, at pressures up to 3.0 MPa. However, differences in the structural disorder of porous carbon models have little impact on CO2 selectivity. Our simulations reveal that the selectivity of an adsorbent for a particular species is strongly dependant on adsorbate-adsorbate interaction effects, comprising the adsorbate-adsorbate potential interactions and an adsorbate sieving effect. As a balance between the confinement and adsorbate-adsorbate effects, it is found that increasing the concentration of CO2 in the gas phase increases the selectivity of (10, 10) CNT dramatically, while having negligible impact on the selectivities in amorphous carbons. Further, it is shown that increasing the temperature reduces the performance of all the carbons in separating CO2, and that an isolated (7,7) CNT has the best performance for CO2/CH4 separation in comparison to the disordered nanoporous carbons investigated. (C) 2014 Elsevier Ltd. All rights reserved.

We investigate the transport diffusion of methane at 300 K and pressures of up to 15 bar in dry and wetted carbon nanotubes (CNTs) having diameters ranging from 0.95 to 2.034 nm using nonequilibrium molecular dynamics (NEMD) simulation. Because of their strong hydrogen bonding, preadsorbed water molecules transport in the form of clusters and block the diffusion of methane, reducing the Onsager coefficient of methane dramatically compared to that in dry CNTs. The reduction in the methane Onsager coefficient is greater in narrower CNTs or at higher water densities. Because the diameter of the water clusters is almost invariant with water density, the Onsager coefficient of water in the (10, 10) CNT increases linearly with water density. It is further found that whereas decreasing the CNT diameter from 2.034 to 0.95 nm enhances the Onsager coefficient of pure methane by about 1 order of magnitude, the Onsager coefficient of water is almost independent of the CNT diameter at a water density of 0.05 g/cm3. We...

The adsorption of pure H2O, CO2/CH4, and H2O/CO2/CH4 mixtures in carbon nanotubes (CNTs) ranging from (6,6) to (15,15), and in a reconstructed model of a silicon carbide derived carbon (SiC-DC), was studied using Monte Carlo simulations, at 300 K. We also investigated the effect of preadsorbed water and the choice of water model including SPC, SPC/E, TIP3P, TIP4P, and TIP5P, on the adsorption of CO2/CH4 and H2O/CO2/CH4 mixtures in the CNTs and SiC-DC. Using the SPC model, our simulations reveal that, below saturation pressure, the adsorption of water is negligible in CNTs with diameters ranging from 0.81 to 2.03 nm, and in the SiC-DC. Water fills the CNTs and the SiC-DC model suddenly when the pressure reaches a critical value that is above the saturation pressure, and exhibits nonmonotonic variation with CNT diameter. It was also found that the small amount of water adsorbed from the saturated H2O/CO2/CH4 mixture has little impact on the adsorption of the components CO2 and CH4. When water is preadsorbed...

We investigate the effect of pore confinement and molecular geometry on the adsorption and self-diffusion of H2O, CO2, Ar, CH4, C3H6, SF6 and C5H12, in a realistic model of nanoporous silicon carbide derived carbon (SiC-DC), constructed using hybrid reverse Monte Carlo simulation. Adsorption isotherms, adsorbate–adsorbate and adsorbate–adsorbent contributions to the isosteric heat of adsorption are determined to study the effect of pore confinement, microporosity and molecular geometry on adsorption of these gases. We describe the cooperative effect of pore confinement and hydrogen bonding on the formation of water clusters and anomalous adsorption behaviour of water compared with non-polar gases. We find that, in contrast to literature results based on the slit-pore model, pore-filling does not occur below the saturation pressure in hydrophobic amorphous carbon materials such as SiC-DC and activated carbon fibre. We also compare self-diffusivities and activation energy barriers of water and non-polar gas...

We investigate macroscopic uptake kinetics of CH4 in silicon carbide-derived carbon (SiC-DC). Ultra-microprosity in SiC-DC is found based on CO2 adsorption at 273 K, but which has poor accessibility to Ar at 87 K. The adsorption kinetics of CH4 is found to follow a bidisperse pore structure model, considering relatively rapid particle scale diffusion in large micropores, and a much slower local grain (or microparticle) scale diffusion in ultra-micropores. The grain scale activation energies are comparable with values for carbon molecular sieves, and consistent with values expected for the size range of the ultra-micropores, while the activation energies for transport in the larger particle scale micropores are comparable to those for conventional activated carbons. The particle scale diffusivities compare well with the results of equilibrium molecular dynamics simulations using a hybrid reverse Monte Carlo simulation constructed model of SiC-DC, with similar activation energy. On the other hand microscopic quasi-elastic neutron scattering measurements are found to probe only short-range barriers with lower activation energy. It is anticipated that ultra-micropores will not make a significant contribution to the transport in any membrane or adsorption-based process based on SiC-DC, due to the extremely slow transport in these ultra-micropores and their small pore volume.

Ordered structures such as zeolites play an important role in a myriad of technological applications, from catalysis to adsorptive separations. Moreover, in recent years, a new class of zeolites with a “hierarchical” structure has been the subject of intense research, due to its potential to dramatically improve yield in those processes where the critical dimensions of the pore and the fluid are similar. In these hierarchical structures, a mesoporous network is purposefully embedded into the microporous framework; the presence of mesopores in the interior of microporous particles may significantly improve their transport properties. We analyze here our recent experiments on mass transfer in hierarchical CHA-like zeolite, microporous SAPO-34, having embedded mesopores as transport pathways providing access to intracrystalline micropores. We use effective medium theory (EMT), where no topological configuration of the phases is assumed other than they mix in an isotropic manner. Our results suggests that, wh...

We investigate gas adsorption and transport in a disordered microporous silica membrane having mean pore diameter 1.5 nm, coated on a porous tubular asymmetric support. The adsorption isotherms are found to be Langmuirian, with equilibrium constants that are accurately predicted for nonpolar gases, considering Lennard–Jones (LJ) interactions with a single layer of oxygen atoms on the pore surface. For the polar gas, CO2, the hydroxyls groups on the pore walls strongly increase the affinity with the pore walls, and a superposition of the LJ potential and an empirically represented electrostatic interaction is found to be adequate in correlating the Langmuirian equilibrium constant. The gas transport in the microporous silica layer is investigated using effective medium theory, with single pore transport represented by combination of pore mouth and internal pore diffusion resistances. Good agreement is observed for all the gases using different coordination numbers, indicating that the essential features of the transport in the silica micropores are captured in the approach. It is found that the overall transport resistance is dominated by the pore mouth barrier; however, the internal diffusion resistance in the relatively smaller pores is significant, especially for weakly adsorbed gases at higher temperature. In addition, the dependence of the pore mouth barrier coefficient on temperature and diffusing species are in good agreement with predictions of transition-state theory, with larger more strongly adsorbed molecules having higher activation energy. The proposed methodology is validated against experiment by comparison of the predicted flux for different gases in the supported membrane at various feed pressures in the low pressure range of 200–400 kPa, using the parameters obtained at 200 kPa.

This work presents a computational fluid dynamic (CFD) model to investigate the effect of binary gas (H/Ar) composition and fluid dynamics at high temperatures (from 200 up to 500°C) for a reasonably sized membrane module containing two cobalt oxide silica membrane tubes in series for H separation. The model provided the local information of velocity, pressure and H fraction for the driving force analysis. The H molar fraction was found to be the most influential factor affecting the driving force, though the total pressure varied slightly along the axial length. In feed domain the H molar fraction showed a clear decline in the axial direction from feed inlet to retentate outlet by 45.34% (for the case of feed fraction 50% H and feed flow rate 100Nmlmin). In permeate domain, H fraction showed the same trend but the decline slope was much less than feed domain being 2.22%. Concentration-polarizations in both feed and permeate domains were very weak with the concentration polarization degree less than 0.4% and can be ignored in cobalt-oxide-silica membrane module. High temperature promoted the performance of pure gas permeation, but had little impact on mixed gas separation as the driving force reduction at higher temperature is more significant.

We investigate the influence of structural heterogeneity on the transport properties of simple gases in a Hybrid Reverse Monte Carlo (HRMC) constructed model of silicon carbide-derived carbon (SiC-DC). The energy landscape of the system is determined based on free energy analysis of the atomistic model. The overall energy barriers of the system for different gases are computed along with important properties, such as Henry constant and differential enthalpy of adsorption at infinite dilution, and indicate hydrophobicity of the SiC-DC structure and its affinity for CO2 and CH4 adsorption. We also study the effect of molecular geometry, pore structure and energy heterogeneity considering different hopping scenarios for diffusion of CO2 and CH4 through ultramicropores using the Nudged Elastic Band (NEB) method. It is shown that the energy barrier of a hopping molecule is very sensitive to the shape of the pore entry. We provide evidence for the influence of structural heterogeneity on self-diffusivity of methane and carbon dioxide using molecular dynamics simulation, based on a maximum in the variation of self-diffusivity with loading. A comparison of the MD simulation results with self-diffusivities from quasi-elastic neutron scattering (QENS) measurements and, with macroscopic uptake-based low-density transport coefficients, reveals the existence of internal barriers not captured in MD simulation and QENS experiments. Nevertheless, the simulation and macroscopic uptake-based diffusion coefficients agree within a factor of 2-3, indicating that our HRMC model structure captures most of the important energy barriers affecting the transport of CH4 in the nanostructure of SiC-DC.

The interpretation of experimental data on transport in porous materials is often based on the use of a single representative pore size, overlooking effects of the pore size distribution (PSD) and pore network connectivity, and fitting a tortuosity into which all such uncertainties are consigned. Using literature data on the diffusion of N2, Xe and i-C4H10 in mesoporous Shell silica spheres, we demonstrate that the tortuosity depends on the choice of the representative pore radius as well as gas species. Both the Knudsen model and the Oscillator model considering dispersive fluid–solid interactions, developed in this laboratory, are found to adequately interpret the data in conjunction with effective medium theory (EMT) by fitting a network coordination number instead of tortuosity. This insensitivity to model is due to the large mean mesopore radius of 7.4 nm for this silica; however, the Oscillator model is found to yield a value of the coordination number closer to the range of values expected for this material. Using the EMT we demonstrate that the tortuosity is dependent on temperature and diffusing species, because of differences in temperature dependence between the conductance at the representative pore radius and the true conductance which depends on the network connectivity and PSD. In the slip flow regime, which is obtained at large pore size, we show that the superposition of Knudsen and viscous mechanisms leads to temperature and species dependence of tortuosity, because of the different pore size dependence of the two contributions. This leads to different limiting tortuosities and PSD dependence in the Knudsen and viscous flow regimes. These critical aspects are largely unappreciated in the literature, and even systematic variations of tortuosity with temperature or diffusing species usually overlooked, often leading to misrepresentation of the underlying mechanism.

We investigate the accessibility of Ti3SiC2-derived carbons (Ti3SiC2-DCs) synthesized non-isothermally using a temperature ramp. The microstructure of the Ti3SiC2-DCs is characterized using TEM, XRD, Raman spectroscopy and gas adsorption. For the characterization by gas adsorption, we adopt our Finite Wall Thickness (FWT) model to invert Ar adsorption isotherms at 87 K to obtain pore size and pore wall thickness distributions of the Ti3SiC2-DCs. Accordingly, we identify a pore accessibility problem in the Ti 3SiC2-DCs, as reported for Ti3SiC 2-DCs prepared at 1073 K in our previous work. A striking feature is that Ti3SiC2-DC prepared at the slowest ramping rate (2 K/min) has a very narrow pore size distribution, while the Ti 3SiC2-DCs synthesized at higher ramping rates (5 and 15 K/min) have much broader pore size distributions centered around 5.2 A. A significant amount of previously unreported ultra-microporosity is observed based on low pressure CO2 adsorption at 273 K. Our results indicate that slow ramping rate could potentially be utilized for fine control of the ultra-microporosity of carbide-derived carbons. Finally, we have found that fast ramping rate above 5 K/min leads to subtle changes in microstructure, with long and periodic graphitic multilayers having some large pores formed in between.

Strong interfacial interactions between the filler(s) and chosen polymer in mixed matrix membranes (MMMs) is a crucial factor in obtaining high gas separation efficiency. However, it is challenging to obtain excellent filler/polymer contact simply by direct incorporation of micron-sized metal-organic frameworks (MOFs) into a polymer matrix without modification. In this study micron-sized ZIF-67 particles were coated with a thin layer of 3 different ionic liquids (ILs) and characterized for the change in adsorption properties. The coated ZIF-67 particles were then incorporated into 6FDA-durene polymer at different loadings to fabricate MMMs for gas separation. Playing the role of interfacial binder, all ILs effectively enhanced the polymer/ZIF adhesion, minimizing the formation of non-selective interfacial defects, which was evidenced by the scanning electron microscopy (SEM) as well as by the focus ion beam scanning electron microscopy (FIB-SEM), leading to an increment in CO2/N2, CO2/CH4 and C3H6/C3H8 selectivity. The successful combination of ZIF-67/IL is proposed as a potential method to overcome the interfacial issues in MMMs, particularly in the application of larger micron-sized fillers.

Understanding the transport behavior of fluid molecules in confined spaces is central to the design of innovative processes involving porous materials and is indispensable to the correlation of process behavior with the material structure and properties typically used for structural characterizations such as pore dimension, surface texture, and tortuosity. [...]

The widespread developments in the use of nanomaterials in catalysis, adsorption, and nanofluidics present significant new challenges in achieving optimal adsorbed fluid flow characteristics. Here we demonstrate, using molecular dynamics simulations of nanoconfined fluids, that at nanoscales, fluid-solid friction is not restricted to a sharp interface as is commonly assumed; instead it is distributed over the whole adsorbed fluid phase, and is strongest in an interfacial region that is not negligible in comparison to the system size. Our simulations yield position-dependent dynamical fluid-solid friction coefficients, and lead to a modification of conventional hydrodynamics, incorporating distributed momentum loss in the fluid due to fluid-solid interaction. The results demonstrate that the usual concepts of slip length or interfacial friction coefficient are meaningful only for uniform fluids, and lose their significance for adsorbates in nanospaces, which are intrinsically inhomogeneous. We show that static friction coefficients, based on equilibrium density distributions, follow the same spatial dependence as the dynamical coefficients. These results open up possibilities for tailoring nanomaterials and surfaces to engineer low friction pathways for adsorbed fluid flow by tuning the potential energy landscape.

High surface area silicon carbide-derived carbons (Si-CDCs) synthesized by chlorination of beta silicon carbide (βSiC) with two different particle sizes (6 μm and 50 nm) show different porosities with graphitic structure. Transmission electron microscopy, Raman spectroscopy and argon (Ar) and carbon dioxide (CO2) sorption analyses are used to examine the textural properties of the Si-CDCs. The results show that the particle size of the precursor affects the surface area and porosity of carbons. Furthermore, an additional heat treatment of the Si-CDC with 50-nm particle size for 24 h at 1,000 °C results in a collapse of the pore structure and reduces the surface area. The capacitive behaviours are investigated in H2SO4 and in tetraethyl ammonium tetrafluoroborate (TEABF4)/acetonitrile (AN). The electrochemical performance of the Si-CDCs is influenced by the particle size, surface area, pore volume and pore size distribution. The Si-CDCs exhibit capacitances in 1 M H2SO4 of up to 179 F g−1 and very stable charge–discharge performance over 5,000 cycles. This study shows the crucial importance of ultramicropores less than 1 nm combined with nanosized particles for achieving high capacitance in aqueous electrolyte. Moreover, the graphitic degree at the surface of the Si-CDCs enhances considerably the rate capability and stability in both electrolytes.

This work shows the application of a validated mathematical model for gas permeation at high temperatures focusing on demonstrated scale-up design for H2 processing. The model considered the driving force variation with spatial coordinates and the mass transfer across the molecular sieve cobalt oxide silica membrane to predict the separation performance. The model was used to study the process of H2 separation at 500 °C in single and multi-tube membrane modules. Parameters of interest included the H2 purity in the permeate stream, H2 recovery and H2 yield as a function of the membrane length, number of tubes in a membrane module, space velocity and H2 feed molar fraction. For a single tubular membrane, increasing the length of a membrane tube led to higher H2 yield and H2 recovery, owing to the increase of the membrane area. However, the H2 purity decreased as H2 fraction was depleted, thus reducing the driving force for H2 permeation. By keeping the membrane length constant in a multi-tube arrangement, the H2 yield and H2 recovery increase was attributed to the higher membrane area, but the H2 purity was again compromised. Increasing the space velocity avoided the reduction of H2 purity and still delivered higher H2 yield and H2 recovery than in a single membrane arrangement. Essentially, if the membrane surface is too large, the driving force becomes lower at the expense of H2 purity. In this case, the membrane module is over designed. Hence, maintaining a driving force is of utmost importance to deliver the functionality of process separation.

An atomistic model of the nanoparticle size Silicon Carbide Derived Carbon (SiC-CDC) is constructed using the Hybrid Reverse Monte Carlo (HRMC) simulation technique through a two-step modeling procedure. Pore volume and three-membered ring constraints are utilized in addition to the commonly used structure factor and energy constraints in the HRMC modeling to overcome the challenges arising from uncertainties involved in determining the structure. The final model is characterized for its important structural features including pore volume, surface area, pore size distribution, physical pore accessibility, and structural defects. It is shown that the microporous structure of SiC-CDC 800 possesses a high pore volume and surface area, making it potentially a good candidate for gas adsorption applications. The HRMC model reveals the SiC-CDC 800 structure to be highly amorphous, largely comprising twisted graphene sheets. It is found that these distorted graphene-like carbon sheets comprising the carbon struct...

We investigate the low pressure transport of several gases in a disordered mesoporous silica membrane of mean pore diameter 3.70 nm, deposited on a porous tubular asymmetric support. The transport data for the supported membrane is examined using three different diffusion models, the classical Knudsen model, a version corrected for finite molecular size, as well as the Oscillator Model developed in this laboratory, using a representative pore size in each layer. We show that the correlation by the Knudsen approach, or its modification for finite molecular size, overestimates the diffusion coefficient in the membrane layer, and yields unrealistically high tortuosities, which vary with temperature and gas species. The apparent tortuosity is significantly reduced to a more reasonable range based on the Oscillator Model, which accounts for dispersive fluid–solid interaction and the effect of adsorption on the transport. In order to overcome the lack of transferability associated with fitting of a variable tortuosity, effective medium theory is used to model the transport in the different layers, while considering the entire pore size distribution for each layer, using only fundamental structural parameters. It is found that the classical and corrected Knudsen models yield significant deviations even with an unrealistically high thickness for the membrane layer, due to the overestimation of the diffusivity. The most satisfactory results are obtained with the Oscillator Model, in which the fitting error is significantly reduced with acceptable membrane thickness. The results indicate that the Knudsen model fails to represent the transport for the mesopores of mean size of 3.70 nm in silica, and that the Oscillator Model provides a more accurate apparent diffusivity which accounts for the effects of adsorption. We also show that use of the effective medium theory provides a satisfactory option to model the transport, using only fundamental structural parameters that are transferable.

The presence of mesopores in the interior of microporous particles may significantly improve their transport properties. Complementing previous macroscopic transient sorption experiments and pulsed field gradient NMR self-diffusion studies with such materials, the present study is dedicated to an in-depth study of molecular uptake and release on the individual particles of mesoporous zeolitic specimens, notably with samples of the narrow-pore structure types, CHA and LTA. The investigations are focused on determining the time constants and functional dependences of uptake and release. They include a systematic variation of the architecture of the mesopores and of the guest molecules under study as well as a comparison of transient uptake with blocked and un-blocked mesopores. In addition to accelerating intracrystalline mass transfer, transport enhancement by mesopores is found to be, possibly, also caused by a reduction of transport resistances on the particle surfaces.

The adsorption of carbon dioxide in the presence of water in single-walled carbon nanotubes is studied using Monte Carlo simulation, at 300, 325, and 350 K. We also investigate the influence of the diameter and chirality of the nanotubes on the adsorption isotherms of CO2. It is observed that increasing the nanotube diameter from 1.36 nm (10, 10) to 2.03 nm (15, 15) leads to enhanced CO2 capacity, while change in chirality has little effect on the adsorption capacity of carbon nanotubes. Our results show that the influence of preadsorbed water on CO2 adsorption is dependent on both the effects of excluded volume and H2O–CO2 interactions. The maximum adsorbed amount of CO2 decreases linearly with the loading of water, and drops more rapidly in narrower nanotubes. The structure of water in hydrophobic nanopores is in the form of hydrogen-bonded clusters, and its adsorption does not affect the arrangement and orientation of CO2 molecules (i.e., it does not affect the mechanism of CO2 adsorption). The average...

Molecular transport in confined spaces plays a fundamental role in well-established and emerging technologies such as catalysis, gas separation, electrochemical energy storage, and nanofluidics. In all these applications, fluids penetrate the voids of porous materials often having a complex morphology. This demands a deep understanding of how structural variables such as pore size distribution and pore connectivity affect transport in order to design or optimize processes and devices. In previous work from this laboratory, the effect of these structural variables had been studied at infinite dilution. Here, we consider nonvanishing densities and investigate the influence of adsorbate density on the effective diffusivity and tortuosity. At finite densities the transport and self-diffusivity are not equivalent at the single pore level, leading to differing effective diffusion coefficients and tortuosities. The tortuosity has previously been shown to be governed not only by the pore network topology, but als...

An experimentally validated theoretical model was developed to investigate the influence of operating conditions on the performance of a multi-tube membrane module containing cobalt oxide silica (COxS) membranes with molecular sieving properties. The model investigated the separation process for a binary gas mixture consisting of H 2 and Ar at 400 °C. Engineering parameters such as feed flow rate, feed pressure, module size and flow configuration were systematically varied in order to optimise the separation performance promoting three main goals: H 2 yield, H 2 purity and H 2 recovery. Changing these parameters led to different flows and H 2 fractions in the feed domain, thus altering the driving forces for the preferential permeation of H 2 . The simulated results suggest that gas separation was greatly improved by reducing the module radius which meets all of the three aforementioned optimisation criteria. Interestingly, increasing the feed flow rate and feed pressure were found to be beneficial but the former led to lower H 2 recovery whilst the latter did not deliver the same purity when compared to lower feed pressure. In addition, two flow configurations, counter-current and co-current, were compared. It was observed that the results of counter-current were effectively the same as the co-current. This was attributed to the high gas-through-gas diffusion for high-temperature membrane operation. Finally, neglecting diffusion effects, or considering advection only, leads to over prediction of H 2 permeate molar fraction.

We investigate the low pressure diffusion of several gases in a disordered mesoporous γ-alumina membrane of pore diameter 10.4. nm, coated on a macroporous α-alumina tubular support. Transport data for the uncoated support is also separately obtained and interpreted using the conventional slip flow correlation with a single representative pore size, and with effective medium theory while considering the entire pore size distribution. It is shown that the conventional correlation based on a single pore size yields significant anomalies in the tortuosity and pressure profile for all gases, which are eliminated on using the effective medium theory, demonstrating the importance of considering the pore size distribution (PSD). These anomalies lead to failure of the correlation for the membrane layer. The effective medium theory is also extended to the disordered membrane layer, using the classical slip flow model and a version corrected for finite molecular size, as well as the Oscillator Model developed in this laboratory. All the diffusion models fitted the experimental data accurately, without the artifacts observed with the commonly used single pore size model. The results make evident the importance of correctly taking the pressure profile into account, in any investigation for a multi-layered supported membrane (i.e. with a macroporous support and a mesoporous membrane layer). It is seen that the slip flow model leads to significantly higher apparent diffusivities than the Oscillator flow model at small pore sizes. Nevertheless, the performance of all three diffusion models is comparable, and it is not possible to distinguish between them for the large pore diameter of 10.4. nm of the mesoporous γ-alumina membrane layer. The results unequivocally show that it is critical to consider the full PSD for each layer rather than using a single representative pore size in modeling membrane transport, and in interpreting experimental permeability data.

We explore the temperature dependence of the self-, corrected-, and transport-diffusivities of CO2, CH4, and N2 in a polyethylene (PE) polymer membrane through equilibrium molecular dynamics simulations. We also investigate the morphology of the polymer membrane based on the intermolecular radial distribution function, free volume, and pore size distribution analysis. The results indicate the existence of 1.5–3 A diameter pores in the PE membrane, and with the increase in the temperature, the polymer swells linearly with changing slope at 450 K in the absence of gas and exponentially in the presence of gas. The gas adsorption isotherms extracted via a two-step methodology, considering the dynamics and structural transitions in the polymer matrix upon gas adsorption, were fitted using a “two-mode sorption” model. Our results suggest that CO2 adsorbs strongly, whereas N2 shows weak adsorption in PE. The results demonstrate that CO2 is more soluble, whereas N2 is least soluble. Further, it is found that an i...

The reliable characterization of nanoporous carbons is critical to the design and optimization of their numerous applications; however, the vast majority of carbons in industrial use are highly disordered, with complex structures whose understanding has long challenged researchers. The idealized slit pore model represents the most commonly used approximation to a carbon nanopore; nevertheless, it has been only partially successful in predicting adsorption isotherms and fails significantly in predicting transport properties because of its inability to capture structural disorder and its effect on fluid accessibility. Atomistic modeling of the structure has much potential for overcoming this limitation, and among such approaches, hybrid reverse Monte Carlo simulation has emerged as the most attractive. This method reconstructs the structure of a carbon based on the fitting of its experimentally measured pair distribution function and appropriate properties such as porosity while minimizing the energy. The method is shown to be best implemented using a multistage strategy, with the first stage used to attain a deep minimum of the energy and subsequent stages to refine the structure based on the fitting of specific properties. Methods to determine the accessibility of gases based on the atomistic structure are outlined, and it is shown that energy barriers are very sensitive to small differences in the sizes of constrictions and pore entries. The ability to accurately predict macroscopic transport coefficients of adsorbates in nanoporous carbons appears to be the greatest limitation of such models. Overcoming this will require the fitting of properties more sensitive to long-range disorder than the currently used pair distribution and the use of a suitable multiscaling strategy, which is suggested as a future direction for advancing atomistic models. The inclusion of heteroatoms in the structure is also an important area requiring further attention, particularly in the development of computationally efficient force fields incorporating their interactions.

International audience; We investigate the effect of fluorine-functionalisation of nanoporous silicon carbide-derived carbon (SiCDC), on its structural as well as adsorption properties, with the aim of assessing its suitability for CO2 capture. The morphology and structure of samples fluorinated to three different F/C ratios are characterized by several analysis techniques, as well as gas adsorption. Fluorination is seen to have a stabilising effect on SiCDC, and stronger C–F bonds are formed at high levels of fluorination. Further, increasing fluorination level leads to a decrease of specific surface area and total pore volume, consistent with recent simulation results from this laboratory. Fluorination has little effect on the ultra-microporosity at low levels of fluorination, but leads to significant decrease at high levels of fluorination. Sub-atmospheric pressure CO2 adsorption kinetics is interpreted using a bidisperse pore structure model, considering particle scale diffusion in large micropores and local grain scale diffusion with an interfacial barrier in ultra-micropores. The comparison of the CO2 uptake-time curves for the fluorinated and non-fluorinated samples shows slightly slower uptake with increasing fluorination level, largely due to decrease in pore volume and surface area. The isosteric heat of adsorption and activation energy barriers both decrease mildly with increased level of fluorination.

We examine the contributions to the shear stress from fluid–fluid and fluid–solid interaction in a confined fluid, using methane in a cylindrical silica nanopore as a model system. The shear stress...

The infiltration of fluids through the voids of porous materials is common place to several well established and emerging technologies, and optimizing their performance relies heavily on developing a deeper understanding of how the fluid flow is affected by the topology of the pore space. This is particularly challenging when the medium is unconsolidated, since it is not only the pore size distribution and pore network connectivity that must be considered, but also the pore aspect ratio distribution will often assume importance. In this work, we introduce a modified effective medium theory (EMT) where the effect of non-vanishing aspect ratios is considered in order to describe the transport mechanism of several light gases through unconsolidated macroporous α-alumina tubular supports. It is confirmed that the tortuosity is not just a property of the medium, but also depends (weakly) on the operating conditions due to the combined effect of viscous and Knudsen contributions. The coordination number and average pore length are the only adjustable parameters in the theory, which allowed adequate prediction of the variation of the apparent tortuosity with temperature for the investigated gases.

We determine the pore size distribution for five activated carbons (comprising carbide derived as well as commercial activated carbon samples) by the interpretation of experimental small angle neutron scattering (SANS) intensity profiles, based on the primary assumption of an infinitely dilute solution of hollow spherical particles. The interpretation yields the pore size distribution of the carbon samples that have predominantly micropore populations (size 20 A, which are not present in the PSD results based on adsorption isotherms of either Ar at 87 K or CO2 273 K. This inaccessible porosity becomes accessible to CO2 and Ar on heat treatment, leading to increase in the adsorption based pore volume. However, the surface area does not commensurately increase, indicating the inaccessible microporosity to predominantly comprise surface defects and roughness that are removed on heat treatment or activation. This finding sheds the light onto the evolution of porosity of activated carbons during gasification or post synthesis-treatment.

This work reviews the development of computational fluid dynamics (CFD) modeling for hydrogen separation, with a focus on high temperature membranes to address industrial requirements in terms of membrane systems as contactors, or in membrane reactor arrangements. CFD modeling of membranes attracts interesting challenges as the membrane provides a discontinuity of flow, and therefore cannot be solved by the Navier-Stokes equations. To address this problem, the concept of source has been introduced to understand gas flows on both sides or domains (feed and permeate) of the membrane. This is an important solution, as the gas flow and concentrations in the permeate domain are intrinsically affected by the gas flow and concentrations in the feed domain and vice-versa. In turn, the source term will depend on the membrane used, as different membrane materials comply with different transport mechanisms, in addition to varying gas selectivity and fluxes. This work also addresses concentration polarization, a common effect in membrane systems, though its significance is dependent upon the performance of the membrane coupled with the operating conditions. Finally, CFD modeling is shifting from simplified single gas simulation to industrial gas mixtures, when the mathematical treatment becomes more complex.

The unusual dynamics of water under confinement is important to a number of conventional and emerging industrial processes and to life in general; nevertheless, our understanding of this critical a...

We report a comparison of the adsorption and transport characteristics of one-site and five-site molecular models of methane in silica nanopores, using grand canonical Monte Carlo and equilibrium molecular dynamics simulations. It is found that while the two models show similar effective molecular sizes, based on similar high-pressure densities in the bulk and nanopore fluids, the conventional parameters of the one-site model yield somewhat stronger intermolecular and pore wall interaction. This leads to higher densities in the bulk and adsorbed fluids at intermediate pressures, for the one-site model. However, the self- and collective-diffusion coefficients are similar for the two models for most nanopores, except at low densities in large mesopores. In this case, the five-site model shows slightly larger low-density diffusivity, due to its weaker interaction with the pore surface. On the basis of comparison with molecular dynamics simulations for the five-site model fluid, the predictive ability of our recent frictional theory of transport in nanopores is confirmed over a wide range of densities and pore diameters, using only the low-density diffusivity from a single simulation. Exceptions are found in the region of the critical point where the correlation length of the fluid diverges and when intermolecular interactions become significant in narrow nanopores where the fluid is nearly one-dimensional. In such cases, the local average density model used to estimate local transport properties becomes inaccurate.

There has been increasing interest in carbon dioxide sequestration in mineral form, because there are large deposits of silicate minerals in peridotite and basalt that have the potential for this option. We examined the dehydroxylation of natural serpentine ore, considered to be a candidate for mineralization of CO2. In this study, serpentine and dehydroxylated serpentine have been characterized using different techniques and the dehydroxylation kinetics of serpentine has been investigated by non-isothermal thermogravimetry. The results indicated that the thermal decomposition of magnesium serpentine [Mg3Si2O5(OH)(4)] proceeds via the removal of physisorbed water and, subsequently, the hydroxyl group. Kinetic modeling of the dehydroxylation shows that the reaction follows a three-dimensional diffusion-controlled mechanism in the particles. The effect of the heating rate and particle size on the dehydroxylation reaction has been investigated, and the results are found to be consistent with diffusion-controlled kinetics. The gas solid heat-transfer resistance is shown to influence the results at a high heating rate and large particle size.

The entry of water into hydrophobic nanospaces is critical to a variety of biological processes and in nanotechnologies for desalination and separation by nanofluidic devices. Idealized models of hydrophobic carbons have hitherto been used in simulations to investigate the anomalous adsorption of water, but the answer to the difficult question of how water enters such spaces has remained elusive. Here we show that while water entry does not occur in idealized independent carbon slit pores it is observed in realistic models of carbons having connected pore spaces. Good agreement with experimental water adsorption data is obtained for a realistic atomistic model of a disordered hydrophobic activated carbon fiber. Upon analyzing the adsorption in this atomistic model, and in another comprising adjacent slit pores connected by a small window in the separating wall, we find that the critical factor governing the behavior is the formation of sufficiently large and stable water clusters at windows or nanospaces connecting small and large pores. When this window size is large enough for the formation of a stable water cluster, condensation in a small pore induces the filling of empty large connected pores. This unique feature is not observed for nonpolar or weak polar gases (e.g., Ar or N-2) at subcritical conditions and explains why the Kelvin equation fails to estimate the condensation pressure for water.

Experiments of H2O and N2 adsorption in silicon carbide-derived carbon (SiC-CDC) are reported, showing only weak adsorption of N2, with H2O adsorption being highly kinetically restricted due to the strong hydrophobicity of this material. Such results suggest that SiC-CDC is an attractive option for adsorptive CO2, as our prior experiments have shown much more significant adsorption of this gas. This is confirmed by grand canonical Monte Carlo simulations, which show that in a 2-stage adsorption system the CO2 mole fraction can be increased from 15% (typical of flue gas) to 80–99.6% for the pore sizes that comprise the SiC-CDC.