Ideal Adsorbed Solution Theory Method Research Articles

Efficient adsorption separation of methane from C2–C3 hydrocarbons in a Co(II)-nodes metal–organic framework

Methane (CH4) as a substitute for other mineral fuels plays a crucial role in reducing energy consumption and preventing environmental pollution. The present study employs a solvothermal method to fabricate a porous framework Co-metal–organic framework (Co-MOF) containing two distinct secondary building units (SBUs): an anionic [Co2(μ2-OH)(COO)4(H2O)] and a neutral [CoN2(COO)2]. Notably, within the anionic SBUs, the coordinated water molecules induce the generation of divergent unsaturated Co(II) centers in the unidirectional porous channels, thereby creating open metal sites. The adsorption performance of Co-MOF towards pure component gases was systematically investigated. The results demonstrated that Co-MOF exhibits superior adsorption capacity for C2–C3 hydrocarbons compared to CH4, which offering the potential for efficient adsorption and separation of CH4 from C2–C3 hydrocarbons. The gas selectivity separation ratios of Co-MOF for C2H6/CH4 and C3H8/CH4 were calculated using the ideal adsorbed solution theory method at 273/298 K and 0.1 MPa. The results revealed that Co-MOF achieved remarkable equilibrium separation selectivity for CH4 and C2–C3 hydrocarbon gases among non-modified MOFs, signifying the potential of the synthesized Co-MOF for efficient recovery and purification of CH4 from C2–C3 hydrocarbons. Breakthrough experiments further demonstrate the ability of Co-MOF to purify methane from C2–C3 hydrocarbons in practical gas separation scenarios. Additionally, molecular simulation calculations further substantiate the propensity of anionic SBUs to interact with C2–C3 hydrocarbon compounds. This study provides a novel paradigm for the development of porous MOF materials in the application of gas mixture separation.

Interchangeable effect of polyols-based zeolite on the separation of CO2, CH4, and N2 gases

The adsorptive separation process for CO2/CH4 and CH4/N2 gas separation has commercial potential, in biogas, Coalbed Methane process etc. although it remains to strive due to the absence of effective adsorbents. Zeolite NaY was used in this study due to its uniform pore structures, surface properties, and potential gas separation adsorbent. Zeolite NaY was modified by polyether polyols (PEPO) to get an adsorbent for selective separation of CO2/CH4 and CH4/N2 gas mixtures. The PXRD, FT-IR, GPC, 1H NMR, TEM, and surface area analysis were used to characterize the PEPO and PEPO-modified zeolite. The single component equilibrium adsorption desorption isotherms with PEPO modified NaY zeolites and NaY zeolite for CO2, CH4 and N2 were measured at 298, 313, and 328 K and up to 5 bar pressure. The isotherm data of CO2, CH4 and N2 were fitted in Dual Site Langmuir equations, and adsorption selectivity of CO2/CH4 and CH4/N2 gas mixtures was predicted using Ideal Adsorbed Solution Theory (IAST) method for 90:10, 50:50 (vol/vol) CH4/N2 and 80:20, 50:50 (vol/vol) CO2/CH4 feed gas mixtures representing coal bed methane gas composition and pressure conditions. The high surface area and hydroxyl groups in PEPO-modified NaY enhanced selectivity for CO2/CH4 and CH4/N2 gas separation. The selectivity predicted by the IAST method were 106.12 and 95.34 for 0.5 % PEPO-NaY (80:20 v/v and 50:50 v/v) for CO2/CH4 and 3.95 and 3.52 for 2 % PEPO-NaY (90:10 v/v and 50:50 v/v) for CH4/N2 at 298 K up to 5 bar respectively.

A robust ethane-selective metal-organic framework with nonpolar pore surface for efficient C2H6/C2H4 separation

A robust chiral chain-based microporous MOF with nonpolar pore surface and suitable cavity space was designed and synthesized for C 2 H 6 /C 2 H 4 separation. The MOF shows simultaneously high C 2 H 6 uptake and inverse C 2 H 6 /C 2 H 4 selectivity, affording the one-step purification of C 2 H 4 from C 2 H 6 -C 2 H 4 mixture under ambient condition. • Adopting the methyl-modified ligands afforded a Zn-MOF with nonpolar channels. • The MOF shows excellent stability and uncommon preferential adsorption for C 2 H 6 . • The methyl groups in MOF are crucial for the separation of C 2 H 6 over C 2 H 4 . • The MOF achieves effective one-step purification of C 2 H 4 from C 2 H 6 -C 2 H 4 mixture. Due to similar physical and chemical properties, the separation of ethane (C 2 H 6 ) and ethylene (C 2 H 4 ) is an indispensable and extremely challenging task in petrochemical process. Although traditional C 2 H 4 -selective porous materials realize the separation of C 2 H 6 -C 2 H 4 mixture, but subsequent multiple separation operations are required to produce high-purity C 2 H 4 . Herein, we designed a hydrophobic methyl group-modified metal-organic framework (MOF) for one-step efficient purification of C 2 H 4 from C 2 H 6 -C 2 H 4 mixture. The as-synthesized MOF constructed from inexpensive carboxylic acid and pyrazole mixed linkers shows high thermal stability and chemical stability (water resistance). Cooperative study on the pure-component sorption isotherm, Ideal Adsorbed Solution Theory method, adsorption heat calculation and grand canonical Monte Carlo (GCMC) simulation supports the unique C 2 H 6 -selective feature of the MOF. Breakthrough experiments revealed the one-step C 2 H 4 purification from C 2 H 6 -C 2 H 4 (1/1, 1/10 and 1/15) mixtures through the MOF around room temperature condition with nice recyclability.

Adsorption of CO2, N2 and CH4 on a Fe-based metal organic framework, MIL-101(Fe)-NH2

MIL-101(Fe)-NH2 was synthesized using solvothermal method to assess the selective adsorption of CO2 over N2 and CH4. The BET surface area of adsorbent sample was 915 m2.g-1. The adsorption capacities of CO2, N2 and CH4 on MIL-101(Fe)-NH2 at 4 MP and 298 K were achieved equal to 13, 3.8 and 5 mmol.g-1 respectively. Khan adsorption equilibrium model was applied to correlate the adsorption isotherms of pure gases. In addition to Khan model, the adsorption isotherm of CO2, which showed type II behavior, was also obtained by BET model as well as a newly presented hybrid model, which resulted in excellent agreement with the experimental data. Moreover, the selectivity of CO2 over N2 and CH4 was estimated by Ideal Adsorbed Solution Theory (IAST). Comparison of the achieved results from IAST method with experimental data verified the accuracy of this method in the lower pressure. The selectivity coefficient was about 2.2 for CO2 over N2 and CH4 at 16 bar. The achieved results from a classical micropore diffusion model showed that the diffusion coefficient of the pure gases was approximately constant with pressure. Moreover, the estimated values for diffusion time constants (DC/rC2 × 103) for CO2, N2 and CH4 were obtained equal to 1.178, 1.118, and 0.802 respectively. The adsorption kinetic data were also correlated with empirical models. Ultimately, the results showed that pseudo-nth order empirical model achieved the best agreement with the experimental data, and it can be successfully applied for kinetic modeling of gas adsorption by MIL-101(Fe)-NH2 MOF.

Computing the temperature dependence of adsorption selectivity in porous solids

We describe a method that computes gas adsorption equilibria in porous solids, at arrays of temperature and mixture compositions. The method uses an initial dataset of single component adsorption isotherms, obtained either from experiment or grand canonical Monte Carlo simulations. It utilizes the Clausius Clapeyron equation to expand the number of the available isotherms for the adsorbate components at a specified sequence of temperatures. It employs the ideal adsorbed solution theory (IAST) on the expanded data for the complete range of mole fractions in the mixture. We show case the method considering experimental isotherms of CO2 and CH4 adsorption at near ambient temperatures, for an activated carbon cloth, an ordered mesoporous carbon and on the alpha phase of magnesium formate. In the same context, we produce a set of simulated adsorption isotherms for a zeolitic imidazolate framework, namely ZIF69. The adsorbate mixture capacities of CO2 and CH4 on ZIF69, predicted by the IAST method are validated with explicit mixture adsorption simulations performed at the same conditions of interest. The method is appropriate for fast screening the selectivity performance of porous solids aiming to minimize the effort regarding experiments for multicomponent physisorption.

Role of Surface Phenolic-OH Groups in N-Rich Porous Organic Polymers for Enhancing the CO2 Uptake and CO2/N2 Selectivity: Experimental and Computational Studies.

Design and successful synthesis of phenolic-OH and amine-functionalized porous organic polymers as adsorbent for postcombustion CO2 uptake from flue gas mixtures along with high CO2/N2 selectivity is a very demanding research area in the context of developing a suitable adsorbent to mitigate greenhouse gases. Herein, we report three triazine-based porous organic polymers TrzPOP-1, -2, and -3 through the polycondensation of two triazine rings containing tetraamine and three dialdehydes. These porous organic polymers possess high Brunauer-Emmett-Teller (BET) surface areas of 995, 868, and 772 m2 g-1, respectively. Out of the three materials, TrzPOP-2 and TrzPOP-3 contain additional phenolic-OH groups along with triazine moiety and secondary amine linkages. At 273 K, TrzPOP-1, -2, and -3 displayed CO2 uptake capacities of 6.19, 7.51, and 8.54 mmol g-1, respectively, up to 1 bar pressure, which are considerably high among all porous polymers reported till date. Despite the lower BET surface area, TrzPOP-2 and TrzPOP-3 containing phenolic-OH groups showed higher CO2 uptakes. To understand the CO2 adsorption mechanism, we have further performed the quantum chemical studies to analyze noncovalent interactions between CO2 molecules and different polar functionalities present in these porous polymers. TrzPOP-1, -2, and -3 have the capability of selective CO2 uptake over that of N2 at 273 K with the selectivity of 61:1, 117:1, and 142:1 by using the initial slope comparing method, along with 108.4, 140.6, and 167.4 by using ideal adsorbed solution theory (IAST) method, respectively. On the other hand, at 298 K, the calculated CO2/N2 selectivities in the initial slope comparing method for TrzPOP-1, -2, and -3 are 27:1, 72:1, and 96:1, whereas those using IAST method are 42.1, 75.7, and 94.5, respectively. Cost effective and scalable synthesis of these porous polymeric materials reported herein for selective CO2 capture has a very promising future for environmental clean-up.

Effects of pore structure of prepared coal-based activated carbons on CH4 enrichment from low concentration gas by IAST method

In this study, low rank bituminous coals were used as raw material for the preparation of a series of coal-based activated carbons (CACs) with similar surface properties but different pore structures, and these CACs could be used to enrich methane from low concentration gas by pressure swing adsorption (PSA) method. The surface morphology of raw material changed from smooth to porous after activating reaction. In addition, the high-pressure adsorption isotherms of methane and nitrogen indicated that the adsorption performance of CACs for methane is stronger than nitrogen. The prediction results by Ideal Adsorbed Solution Theory (IAST) method also shows that prepared CACs have a high selectivity for methane and nitrogen mixture at low pressure. At higher-pressure conditions, the selectivity increases more rapidly with increasing molar fraction of methane in the gas phase. In addition, selectivity is mainly affected by micropores in the range of 0.55–0.85 nm, while mesopores and macropores have no effects on the selectivity. Present research results would provide some guidance for the preparation of excellent adsorbents and improve the efficiency of adsorption separation.

On the use of the IAST method for gas separation studies in porous materials with gate-opening behavior

Highly flexible nanoporous materials, exhibiting for instance gate opening or breathing behavior, are often presented as candidates for separation processes due to their supposed high adsorption selectivity. But this view, based on "classical" considerations of rigid materials and the use of the Ideal Adsorbed Solution Theory (IAST), does not necessarily hold in the presence of framework deformations. Here, we revisit some results from the published literature and show how proper inclusion of framework flexibility in the osmotic thermodynamic ensemble drastically changes the conclusions, in contrast to what intuition and standard IAST would yield. In all cases, the IAST method does not reproduce the gate-opening behavior in the adsorption of mixtures, and may overestimates the selectivity by up to two orders of magnitude.

Evidence of Amine-CO2 Interactions in Two Pillared-Layer MOFs Probed by X-ray Crystallography.

Two pillared-layer metal-organic frameworks (MOFs; PMOF-55 and NH2 -PMOF-55) based on 1,2,4-triazole and terephthalic acid (bdc)/NH2 -bdc ligands were assembled and display framework stabilities, to a certain degree, in both acid/alkaline solutions and toward water. They exhibit high CO2 uptakes and selective CO2 /N2 adsorption capacities, with CO2 /N2 selectivity in the range of 24-27, as calculated by the ideal adsorbed solution theory method. More remarkably, the site and interactions between the host network and the CO2 molecules were investigated by single-crystal X-ray diffraction, which showed that the main interaction between the CO2 molecules and PMOF-55 is due to multipoint supramolecular interactions of C-H⋅⋅⋅O, C⋅⋅⋅O, and O⋅⋅⋅O. Amino functional groups were shown to enhance the CO2 adsorption and identified as strong adsorption sites for CO2 by X-ray crystallography.

Adsorption Equilibria and Kinetics of Methane + Nitrogen Mixtures on the Activated Carbon Norit RB3

The separation of methane and nitrogen from binary mixtures using a commercial activated carbon, Norit RB3, was investigated. The adsorption of pure fluids and CH4 + N2 mixtures were measured at temperatures of 242, 273, and 303 K, at pressures ranging from 53 to 5000 kPa using a high pressure volumetric apparatus and at pressures from 104 to 902 kPa using a dynamic column breakthrough apparatus (DCB). The pure gas equilibrium adsorption capacities were regressed to Toth, Langmuir, Langmuir–Freundlich, and Sips isotherm models; the Toth model gave the best prediction of measured capacities at pressures from 800 to 5000 kPa. The uptake of components from gas mixtures calculated using the Ideal Adsorbed Solution Theory (IAST), Extended Langmuir and Multi-Sips models were all within the uncertainties of the measured adsorption capacities, suggesting that for this adsorbent there is no significant advantage in using the more computationally intensive IAST method. A linear driving force (LDF)-based model of ad...

Testing the Accuracy of Correlations for Multicomponent Mass Transport of Adsorbed Gases in Metal−Organic Frameworks: Diffusion of H2/CH4 Mixtures in CuBTC

Mass transport of chemical mixtures in nanoporous materials is important in applications such as membrane separations, but measuring diffusion of mixtures experimentally is challenging. Methods that can predict multicomponent diffusion coefficients from single-component data can be extremely useful if these methods are known to be accurate. We present the first test of a method of this kind for molecules adsorbed in a metal-organic framework (MOF). Specifically, we examine the method proposed by Skoulidas, Sholl, and Krishna (SSK) ( Langmuir, 2003, 19, 7977) by comparing predictions made with this method to molecular simulations of mixture transport of H 2/CH 4 mixtures in CuBTC. These calculations provide the first direct information on mixture transport of any species in a MOF. The predictions of the SSK approach are in good agreement with our direct simulations of binary diffusion, suggesting that this approach may be a powerful one for examining multicomponent diffusion in MOFs. We also use our molecular simulation data to test the ideal adsorbed solution theory method for predicting binary adsorption isotherms and a method for predicting mixture self-diffusion coefficients.

An evaluation of multicomponent absorption equilibria models

An investigation into the modelling of competitive adsorption equilibria data using the ideal adsorbed solution theory (IAST), the modified simplified competitive adsorption models (MSCAM), and the equivalent background compound (EBC) method is presented. The models were evaluated with the only competitive isotherms available which quantified both natural organic matter (NOM) and synthetic organic chemical (SOC) adsorption. The data used a mixture of 1,1,2-trichloroethane (TCEA) and NOM from river water measured as dissolved organic carbon (DOC). The IAST method was applied by representing the NOM in the river water as pseudocomponents. It was observed that the laborious and time-consuming regressions of only the TCEA isotherm data yield different sets of parameters, many of which do not describe the NOM data well. Simultaneous regression of TCEA and NOM isotherm data, like TCEA isotherm data regression, is highly dependent on the initial guesses of the parameters. MSCAM simulated the NOM adsorption very well but TCEA less so. TCEA simulation with the EBC method provided good results, but unrealistic EBC characteristics led to poor predictions. Model predictions of the experiment showed that the MSCAM model described the data better than the IAST method, but not perfectly. DOC-based MSCAM simulation does not offer any significant improvements over molar-based simulation.