Significance of extra-framework monovalent and divalent cation motion upon CO2 and N2 sorption in zeolite X

Porous hyper-cross-linked aromatic polymers are one ofthe emergingclasses of porous organic polymers with the potential for industrialapplication. Four different porous polymeric materials have been preparedusing different precursors (indole, pyrene, carbazole, and naphthalene),and the composition and textural properties were analyzed. The materialswere characterized in detail using different physicochemical techniqueslike scanning electron microscopy, transmission electron microscopy,nitrogen adsorption at 77 K, Fourier transform infrared spectroscopy,X-ray diffraction, etc. The effect of textural properties and nitrogenspecies on carbon dioxide and nitrogen adsorption capacities and selectivitywas studied and discussed. The carbon dioxide and nitrogen adsorptioncapacities were measured using a volumetric gas adsorption system.The adsorption data were fitted into different adsorption models,and the ideal absorbed solution theory was used to calculate adsorptionselectivity. Among the studied samples, POP-4 shows the highest carbondioxide and nitrogen adsorption capacities. While POP-1 shows maximumCO2/N2 selectivity of 78.0 at 298 K and 1 barpressure. It is observed that ultra-micropores, which are presentin the prepared materials but not measured during conventional surfacearea measurement via nitrogen adsorption at 77 K, play a very importantrole in carbon dioxide adsorption capacity and determining the carbondioxide selectivity over nitrogen. Surface nitrogen also increasesthe CO2 selectivity in the dual mode by increasing carbondioxide adsorption via the acid–base interaction as well asby decreasing nitrogen adsorption due to N–N repulsion.

Selective adsorption of supercritical carbon dioxide and methane binary mixture in shale kerogen nanopores

The surface areas obtained by application of the B.E.T. theory to adsorption isotherms of nitrogen and carbon dioxide gases at 77°K and 195°K respectively on homoionic samples of illite and montmorillonite clays have been examined. The isotherms were obtained using a standard volumetric adsorption system and the results are compared with those obtained by Thomas and Bohor (1968) using a dynamic sorption system.Small amounts of residual water have been shown to have a marked influence on the accessibility of the internal surfaces of the montmorillonite clays to nitrogen and carbon dioxide adsorption, in this respect the standard outgassing procedure under high vacuum seems more efficient than that used in dynamic systems. The present data indicate that provided the sample has been satisfactorily outgassed there is little penetration of nitrogen or carbon dioxide gases into the quasi-crystalline regions of montmorillonite clays. With the exception of the caesium saturated montmorillonites the surfaces of the clays are more accessible to the smaller nitrogen molecules than to carbon dioxide assuming the values used for molecular area are correct.

High pressure sorption of various hydrocarbons and carbon dioxide in Kimmeridge Blackstone and isolated kerogen

Surface thermodynamics of hydrocarbon vapors and carbon dioxide adsorption on shales

Sequestration of carbon dioxide in coal seams is a potential method of reducing atmospheric emissions of carbon dioxide. If carbon dioxide can be sequestered in coal seams and it simultaneously results in enhanced coal bed methane (ECBM) production, some of the sequestration costs can be recovered in the value of the methane produced. This requires knowledge of both the carbon dioxide and methane sorption behavior of coal at high pressures. However, the relationship between methane and carbon dioxide sorption at high gas pressure is not well understood. To elucidate their relationship, we investigated the sorption of carbon dioxide, methane, ethane, nitrogen, argon, krypton, xenon, carbon tetrafluoride, and sulfur hexafluoride by dry coals at 55 °C at pressures up to 20 MPa; all of these gases have critical temperatures below 55 °C. Sorption isotherms for the different gases were very different but all could be fitted by a modified Dubinin−Radushkevich model to within about 1% of their calculated maximum adsorption capacity. We found that the maximum adsorption capacity, determined from the isotherms, of the coals investigated for a supercritical gas increases linearly with the critical temperature of the gas when the maximum adsorption capacity is expressed on a (van der Waals) volume basis, except for gases that could not penetrate the coal as effectively as the other gases: carbon tetrafluoride and sulfur hexafluoride. This behavior is consistent with the idea that, in sorption of supercritical gases, the surface phase is not condensed but acts as a compressed gas. This provides a simple explanation of why the molar maximum sorption capacity at temperatures near ambient decreases in the order carbon dioxide > methane > nitrogen: their critical temperature decreases in the same order. The heats of sorption of different gases on a given coal, calculated from the isotherm, were closely related to their van der Waals attraction constant and were similar to those reported for sorption of these gases onto graphite at low pressure. The calculated heats of sorption for xenon and ethane on coal were higher than that for carbon dioxide on the corresponding coal. For the three bituminous coals examined, carbon tetrafluoride and sulfur hexafluoride did not penetrate the coal as completely as the other gases. About 3−5% (by volume) of each coal was calculated to be inaccessible to these two gases that were accessible by the other gases, which we attribute to their greater molecular diameter. Except for these two gases, the volume of each coal accessible by each gas was found to be similar (to within 1.5% of the coal volume).

Five nitrogen sources (glycine, β‐alanine, urea, melamine and nicotinamide) and three heating methods (thermal, monomodal microwave and multimodal microwave) are used to prepare nitrogen‐doped Starbons® derived from starch. The materials are initially produced at 250–300 °C (SNx300y), then heated in vacuo to 800 °C to produce nitrogen‐doped SNx800y’s. Melamine gives the highest nitrogen incorporation without destroying the Starbon® pore structure and the microwave heating methods give higher nitrogen incorporations than thermal heating. The carbon dioxide adsorption capacities of the nitrogen‐doped Starbons® determined gravimetrically, in many cases exceed those of S300 and S800. The carbon dioxide, nitrogen and methane adsorption isotherms of the most promising materials are measured volumetrically. Most of the nitrogen‐doped materials show higher carbon dioxide adsorption capacities than S800, but lower methane and nitrogen adsorption capacities. As a result, the nitrogen‐doped Starbons® exhibit significantly enhanced carbon dioxide versus nitrogen and methane versus nitrogen selectivities compared to S800.

Adsorption of carbon dioxide on hydrotalcite-like compounds of different compositions

Carbon dioxide (CO2) and methane (CH4) adsorption in organic-rich mudrocks can significantly be affected by clay minerals and kerogen as dominant components of the rock. Previous publications have explored the adsorption of CH4 and CO2 on kerogen and clay structures separately and overlooked their competitive adsorption behavior on the molecular scale. The objectives of this paper are to (a) evaluate the CH4 and CO2 adsorption capacity of different kerogen types and thermal maturity levels under reservoir pressure and temperature, (b) evaluate the CH4 and CO2 adsorption capacity of illite and kaolinite with different pore structure, and finally (c) improve the conventional Langmuir adsorption models to account for the competitive adsorption of CH4 and CO2 within the different components of organic-rich mudrocks. We used realistic kerogen molecular models that were condensed and optimized to mimic the actual kerogen structures. Kerogen molecules of different types (e.g., type I, II, and III) and different thermal maturity levels were transformed into dense porous structures through an annealing process. Meanwhile, illite and kaolinite samples were modeled, honoring their chemical composition, surface charges, and pore size. We then performed Grand Canonical Monte Carlo (GCMC) simulations to evaluate the CH4 and CO2 adsorption isotherms for kerogen and clay structures. To investigate the effect of reservoir temperature on the adsorption capacity, adsorption isotherms were constructed for a pressure range of 1 to 20 MPa under temperatures of 300, 330, and 360 K. The change of gas adsorbate density along with atomic radial distribution function (RDF) was evaluated to quantify the interfacial interactions between the gases and the adsorbent surface. Moreover, the diffusion coefficients of CH4 and CO2 were calculated for the simulated kerogen and clay structures. Results showed that the available pore volume for kerogen type III was found to be twice as much as kerogen type I. This is a result of the increase in the kerogen aromaticity from 29% for kerogen type I to about 57% for kerogen type III. In addition, increasing thermal maturity from kerogen type IIA through type IID led to an increase in the available pore volume and kerogen aromaticity by 50% and 92.6%, respectively. These changes in the kerogen geochemistry and pore structure led to variations in the adsorption capacity for both CH4 and CO2. Meanwhile, The CH4 and CO2 adsorption capacity values of illite and kaolinite were much lower than that of kerogen molecules. Pore size and surface area in clay minerals were proven to be the main controlling factor in determining the clay adsorption capacity for the tested gases. However, the negatively charged illite samples showed more adsorption affinity to the polar CO2 molecules than the nonpolar CH4 molecules. The proposed methods improve the conventional Langmuir adsorption models of organic-rich mudrocks that used kerogen as the only adsorbent component. Moreover, the improved adsorption models can be used as guidelines for enhanced gas recovery or CO2 sequestration applications in organic-rich mudrock. This also can be extended to different formations where both kerogen and clay particles may coexist.

Molecular level investigation of methane and carbon dioxide adsorption on SiO2 surface

Adsorption of carbon dioxide in slit-shaped carbon micropores at 273 K has been studied by means of the grand canonical Monte Carlo (GCMC) simulations and the nonlocal density functional theory (NLDFT). Three molecular models of CO2 have been used. Long-run GCMC simulations were performed with the three-center model of Harris and Yung (J. Phys. Chem. 1995, 99, 12021). For NLDFT calculations, we developed an effective Lennard-Jones (LJ) model. GCMC simulations of the effective LJ model of CO2 have been performed for comparison. For each model used, parameters of intermolecular potentials have been determined and validated against two-phase bulk equilibrium data and experimental adsorption isotherms on graphite at 273 and 195 K. In the range of pore widths from 3 to 15 Å, the NLDFT isotherms of CO2 adsorption are overall in a satisfactory agreement with the GCMC isotherms generated using the three-center model. Some deviations have been observed between 6.5 and 8.5 Å, where the adsorbate undergoes a transition from a single-layer to a two-layer structure. The models developed are recommended for studying carbon dioxide adsorption in microporous adsorbents and also for calculating pore size distributions in carbonaceous materials and soil particles. The NLDFT model has the advantage of being much less computationally demanding, whereas the three-center GCMC model serves as a benchmark for quantitative estimates and can be used for studying CO2 sorption at ambient conditions close to the critical temperature.

Amorphous materials are usually characterized using nitrogen adsorption isotherms at 77 K taken at pressures up to 1 bar to obtain pore size distributions. Activated carbons are amorphous microporous graphitic materials containing pores which can range from nanometers to microns in width and which can, in principle, be tailored to adsorb specific molecules or classes of molecule by changing the method of preparation (the activation process). For the physical chemist, they pose the challenge of understanding how gases adsorb in graphitic nanopores, that is, in restricted geometries, and of using that understanding to improve their characterization. In this paper, we compare pore size distributions of an ultrahigh surface area activated carbon (AX21) determined from nitrogen adsorption measurements up to 0.6 bar at 77 K with those determined from carbon dioxide adsorption measurements up to 20 bar at 298 K. Our analysis employs grand canonical and Gibbs ensemble Monte Carlo simulations together with accurate site−site interaction models of the adsorbates. We find that the calculated pore size distributions for each adsorbate are quite different, and the adsorption of one gas can be estimated from the adsorption of the other gas to within an error of 25% at the highest pressures only. At lower pressures, we speculate that large errors are due to the behavior of nitrogen in carbon micropores in which diffusion is severely limited. To substantiate this speculation, we have calculated the self-diffusion coefficient for nitrogen at 77 K and carbon dioxide at 298 K in carbon slit pores using equilibrium molecular dynamics. The results suggest that nitrogen is diffusionally limited, and possibly frozen, in such pores whereas carbon dioxide remains mobile. We conclude that room-temperature carbon dioxide adsorption isotherms up to the saturation pressure could provide a more accurate characterization of carbon microstructure than nitrogen isotherms at 77 K up to 1 bar.

Selectivity of new siliceous zeolites for separation of methane and carbon dioxide by Monte Carlo simulation

New experimental results for the solubility of nitrogen and carbon dioxide in polystyrene are reported, accompanied by data on the change in volume of the polymer caused by the sorption process. The two phenomena were measured simultaneously with a combined technique, in which the quantity of penetrating fluid introduced into the system was evaluated by pressure‐decay measurements in a calibrated volume, whereas a vibrating‐wire force sensor was employed for weighing the polymer sample during sorption inside of the high‐pressure equilibrium cell. The use of the two techniques was necessary because the effects of swelling and solubility could not be decoupled in a single gravimetric or pressure‐decay measurement. The sorption of nitrogen in polystyrene was studied along three isotherms from 313 to 353 K at pressures up to 70 MPa. The sorption of carbon dioxide was measured along four isotherms from 338 to 402 K up to 45 MPa. The results are compared with values from the literature when possible, although our data extend significantly the pressure ranges of the latter. The uncertainties affecting our measurements with nitrogen are 1 mg of N2/g of polystyrene in solubility and 0.1% of the volume of the polymer. For carbon dioxide, the uncertainties are 5 mg of N2/g of polystyrene and 0.5% respectively, carbon dioxide being about 1 order of magnitude more soluble than nitrogen. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2063–2070, 2001

Carbon dioxide removal for methane upgrade by a VSA process using an improved 13X zeolite