Gases In Porous Materials Research Articles

Isosteric heat and selectivity in adsorption of gases in rough pores: In Henry’s law region

Henry constant and isosteric heat of adsorption are important parameters for characterizing the gas-solid interaction in an adsorption process. In order to better understand the adsorption behavior of gas in rough pores, we construct rough slit pores by using two rectangular corrugated substrates, and then calculate the potential profile in it. By utilizing classical statistical mechanics, the longitudinal Henry constant and isosteric heat of <inline-formula><tex-math id="M3">\begin{document}$ \text{H}_{2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20212348_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20212348_M3.png"/></alternatives></inline-formula> are further calculated in the Henry region. The results suggest that both geometric morphology and pore width can significantly influence and modulate the longitudinal Henry constant and isosteric heat of the gas in the pore. Furthermore, the selectivity of adsorption in the binary <inline-formula><tex-math id="M4">\begin{document}$ \text{CO}_{2}/ \text{H}_{2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20212348_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20212348_M4.png"/></alternatives></inline-formula> mixture is also calculated and investigated in the Henry region. In addition, the effects of corrugated substrates with different geometries on adsorption properties are also calculated and studied. The result can provide reliable theoretical basis for understanding the adsorption, separation and purification of the gas in porous materials, and it is also expected to provide helpful idea for designing and exploring new nano functional materials.

Toward comprehensive exploration of the physisorption space in porous pseudomaterials using an iterative mutation search algorithm.

We describe an updated algorithm for efficiently exploring structure-property spaces relating to physisorption of gases in porous materials. This algorithm uses previously described "pseudomaterials," which are crystals of randomly arranged and parameterized Lennard-Jones spheres, and combines it with a new iterative mutation exploration method. This algorithm is significantly more efficient at sampling the structure-property space than previously reported methods. For the sake of benchmarking to prior work, we apply this method to exploring methane adsorption at 35 bars (298K) and void fraction as the main structure-property combination. We demonstrate the effect and importance of the changes that were required to increase efficiency over prior methods. The most important changes were (1) using "discrete" mutations less often, (2) decreasing degrees of freedom, and (3) removing biasing from mutations on bounded parameters.

Simultaneous quantitative recognition of all purines including N6-methyladenine via the host-guest interactions on a Mn-MOF

Simultaneous quantitative recognition of all purines including N6-methyladenine via the host-guest interactions on a Mn-MOF

High-capacity methane storage in flexible alkane-linked porous aromatic network polymers

Adsorbed natural gas (ANG) technology is a viable alternative to conventional liquefied or compressed natural-gas storage. Many different porous materials have been considered for adsorptive, reversible methane storage, but fall short of the US Department of Energy targets (0.5 g g−1, 263 l l−1). Here, we prepare a flexible porous polymer, made from benzene and 1,2-dichloroethane in kilogram batches, that has a high methane working capacity of 0.625 g g−1 and 294 l l−1 when cycled between 5 and 100 bar pressure. We suggest that the flexibility provides rapid desorption and thermal management, while the hydrophobicity and the nature of the covalently bonded framework allow the material to tolerate harsh conditions. The polymer also shows an adsorbate memory effect, where a less adsorptive gas (N2) follows the isotherm profile of a high-capacity adsorbate (CO2), which is attributed to the thermal expansion caused by the adsorption enthalpy. The high methane capacity and memory effect make flexible porous polymers promising candidates for ANG technology. Adsorbing natural gas in porous materials is a potential storage alternative to conventional approaches based on liquefaction or compression, but higher capacities are required for commercial viability. Here, the authors employ porous covalent organic polymers that are flexible but robust, leading to high storage capacities and cyclability.

General Theory of Absorption in Porous Materials: Restricted Multilayer Theory.

In this article, we present an approach for the generalization of adsorption of light gases in porous materials. This new theory goes beyond Langmuir and Brunauer-Emmett-Teller theories, which are the standard approaches that have a limited application to crystalline porous materials by their unphysical assumptions on the amount of possible adsorption layers. The derivation of a more general equation for any crystalline porous framework is presented, restricted multilayer theory. Our approach allows the determination of gas uptake considering only geometrical constraints of the porous framework and the interaction energy of the guest molecule with the framework. On the basis of this theory, we calculated optimal values for the adsorption enthalpy at different temperatures and pressures. We also present the use of this theory to determine the optimal linker length for a topologically equivalent framework series. We validate this theoretical approach by applying it to metal-organic frameworks (MOFs) and show that it reproduces the experimental results for seven different reported materials. We obtained the universal equation for the optimal linker length, given the topology of a porous framework. This work applied the general equation to MOFs and H2 to create energy-storage materials; however, this theory can be applied to other crystalline porous materials and light gases, which opens the possibility of designing the next generations of energy-storage materials by first considering only the geometrical constraints of the porous materials.

Macroscopic description of capillary transport of liquid and gas in unsaturated porous materials

A new macroscopic description of capillary transport of liquid and gas in porous materials is presented within the framework of multiphase continuum mechanics. It is assumed that unsaturated porous material form a macroscopic continuum composed of three constituents: gas, mobile liquid and capillary liquid, while the skeleton is rigid with an isotropic pore space structure. The capillary liquid forms a thin layer on the internal contact surface with the skeleton, is immoveable and contains the whole capillary energy. The remaining part of the liquid, surrounded by the layer of the capillary liquid and menisci, forms the constituent called mobile liquid. Both liquids exchange mass, momentum and energy in the vicinity of menisci surfaces during their motion, which is described by an additional macroscopic velocity field. A macroscopic scalar quantity is introduced, parametrizing changes of saturations, which for quasi static and stationary processes is equal to the capillary pressure. This quantity extends the dimensionality of the description of mechanical processes taking place in unsaturated porous materials. For the three fluid constituents of the medium, balance equations of mass and linear momentum and evolution equations for saturations are formulated. The constitutive relations for quantities describing mechanical processes in such a medium are proposed based on the dissipation inequality of mechanical energy for the whole system. A new approach is proposed, similar to that used in rational thermodynamics, based on entropy inequality analysis and the Lagrange multipliers method.

Locating Gases in Porous Materials: Cryogenic Loading of Fuel-Related Gases Into a Sc-based Metal-Organic Framework under Extreme Pressures.

An alternative approach to loading metal organic frameworks with gas molecules at high (kbar) pressures is reported. The technique, which uses liquefied gases as pressure transmitting media within a diamond anvil cell along with a single-crystal of a porous metal-organic framework, is demonstrated to have considerable advantages over other gas-loading methods when investigating host-guest interactions. Specifically, loading the metal-organic framework Sc2BDC3 with liquefied CO2 at 2 kbar reveals the presence of three adsorption sites, one previously unreported, and resolves previous inconsistencies between structural data and adsorption isotherms. A further study with supercritical CH4 at 3-25 kbar demonstrates hyperfilling of the Sc2 BDC3 and two high-pressure displacive and reversible phase transitions are induced as the filled MOF adapts to reduce the volume of the system.

Locating Gases in Porous Materials: Cryogenic Loading of Fuel‐Related Gases Into a Sc‐based Metal–Organic Framework under Extreme Pressures

AbstractAn alternative approach to loading metal organic frameworks with gas molecules at high (kbar) pressures is reported. The technique, which uses liquefied gases as pressure transmitting media within a diamond anvil cell along with a single‐crystal of a porous metal–organic framework, is demonstrated to have considerable advantages over other gas‐loading methods when investigating host–guest interactions. Specifically, loading the metal–organic framework Sc2BDC3 with liquefied CO2 at 2 kbar reveals the presence of three adsorption sites, one previously unreported, and resolves previous inconsistencies between structural data and adsorption isotherms. A further study with supercritical CH4 at 3–25 kbar demonstrates hyperfilling of the Sc2BDC3 and two high‐pressure displacive and reversible phase transitions are induced as the filled MOF adapts to reduce the volume of the system.

Diffusion of H 2, CO, N 2, O 2 and CH 4 Through Nanoporous Carbon Membranes

Diffusion of pure H 2, CO, N 2, O 2 and CH 4 gases through nanoporous carbon membrane is investigated by carrying out non-equilibrium molecular dynamics (NEMD) simulations. The flux, transport diffusivity and activation energy for the pure gases diffusing through carbon membranes with various pore widths were investigated. The simulation results reveal that transport diffusivity increases with temperature and pore width, and its values have a magnitude of 10 −7 m 2·s −1 for pore widths of about 0.80 to 1.21 nm at 273 to 300 K. The activation energies for the gases diffusion through the membrane with various pore widths are about 1–5 kJ·mol −1. The results of transport diffusivities are comparable with that of Rao and Sircar (J. Membr. Sci., 1996), indicating the NEMD simulation method is a good tool for predicting the transport diffusivities for gases in porous materials, which is always difficult to be accurately measured by experiments.

How molecules huddle in holes

The process of adsorption and subsequent desorption of gases in porous materials often shows hysteretic behaviour. A combination of diffusion measurements and numerical modelling could now explain why.

Modelling sorption and diffusion in activated carbon: a novel low pressure pulse-response technique

A novel low pressure pulse-response technique, referred to as Multitrack, was applied for the simultaneous determination of the adsorption and diffusion parameters of gases in porous materials in a packed bed. Measurements were performed on two different carbons, Kureha and Sorbonorit B3, in the temperature range between 473 and 700 K. The modelling procedure of the pulse responses consisted of two parts. The transport parameters through the packed bed were determined from the pulse responses of non-adsorbing argon. Adsorption and diffusion parameters were determined for the adsorbing gases n-butane and hexafluoropropylene. As a general rule it is concluded that microporous materials must be described with a basic three-parameter model, while macroporous materials must be described with a simplified two-parameter model. The heats of adsorption, determined from the adsorption equilibrium constant, agree well with the values determined independently from uptake experiments. With Multitrack a single value for the diffusivity is obtained at a given temperature, valid for the low pressure limit, whereas with uptake a range of diffusivities is found, covering a wide concentration range. The values were of the same order of magnitude. Multitrack is a promising method for the simultaneous determination of adsorption and diffusion parameters.

Visualization of gas flow and diffusion in porous media.

The transport of gases in porous materials is a crucial component of many important processes in science and technology. In the present work, we demonstrate how magnetic resonance microscopy with continuous flow laser-polarized noble gases makes it possible to "light up" and thereby visualize, with unprecedented sensitivity and resolution, the dynamics of gases in samples of silica aerogels and zeolite molecular sieve particles. The "polarization-weighted" images of gas transport in aerogel fragments are correlated to the diffusion coefficient of xenon obtained from NMR pulsed-field gradient experiments. The technique provides a unique means of studying the combined effects of flow and diffusion in systems with macroscopic dimensions and microscopic internal pore structure.

Steady-State and Dynamic Mass Transfer of Gases in Porous Materials

The diffusion of binary gas mixtures through a porous asymmetric tubular membrane has been studied experimentally. A modified Wicke-Kallenbach diffusion cell consisting of two gas compartments separated by the membrane was used. Steady-state experiments with pure gases and binary mixtures were carried out in order to determine the transport parameters of the membrane with respect to the dusty gas model. To verify these parameters, the dynamic transport of binary mixtures was examined applying the dynamic diffusion cell technique proposed by Novak et al. The measured transients were in relative good agreement with the model predictions.

EFFECTIVE DIFFUSION COEFFICIENT OF GAS IN POROUS MATERIALS AND GRANULAR BEDS

To predict the effective diffusion coefficient of a porous material, a theoretical equation was derived by using a pore model. However, it cannot be applied to a granular bed of single pore structure. From this point of view, the tortuosity factor is introduced into the expression of the effective diffusion coefficient and calculated from penetration experiments of gas in porous materials. As a result, for a granular bed, in the region of large void fraction, the effective diffusion coefficient may be estimated by the presented equation. It may be applied to the diffusion of gas in porous materials.

Experimental Investigation of Factors Affecting Laboratory Relative Permeability Measurements

Abstract Laboratory studies of several factors affecting measurements of relativepermeability were made using the three-section plastic-covered core technique.Results show that the core assembly, properly constructed, will perform as asingle unit, and that the testing technique will, under suitable conditions ofpressure gradient, gas expansion, and migration of partial water saturation, permit measurement of flow characteristics not affected by technique.Wettability equilibrium is readily established in cores exhibiting strongwetting preference to water or oil when initially saturated with water.Laboratory tests must be conducted so that saturation changes represent thosethat occur in the reservoir. Immediate implications of saturation history arethat the possibility exists of increasing the displacement efficiency ofsolution gas drive reservoirs over the natural process, andresidual gassaturations following water flooding in gas or gas condensate reservoirs willbe 15 to 50 per cent pore space rather than 1 to 11 per cent as generallybelieved. Introduction Solutions of petroleum reservoir problems pertaining to productionperformance require the use of true relative permeability characteristics. Thisrelationship of fluid conductivity and saturation has been obtained byreservoir engineers in four ways, namely:From past gross reservoir performance and the extrapolation of this databased on experience,By using published fluid flow relationships obtained in laboratory studieson general type porous materials,By attempting a mathematical derivation of flow behavior, using someexperimentally obtained characteristics of reservoir rocks, andBy laboratory flow tests using representative rock samples of areservoir. The first three methods listed above have shortcomings which make their uselimited or questionable. Production characteristics of only certain processesare obtained from field data and these are not available at the beginning of areservoir's producing life, at which time they are desirable. It is fortuitousif general fluid flow characteristics obtained experimentally have accurateapplication to specific field problems. Also, it is felt that at this timethere is not sufficient knowledge of the flow behavior of oil, water, and gasin porous materials to enable applicable analytical description of this to bemade based on other measured rock characteristics. Measurement of relative permeability in the laboratory offers the only directmethod subject to adequate checking for determination of flow characteristicsapplicable to field problems. Primarily, this paper deals with laboratoryexperiments to establish the effects of several factors on the measurement ofrelative permeability and the practical significance of this knowledge. T.P. 3053