Molecular simulation study of the competitive adsorption of H2O and CO2 in zeolite 13X

Adsorption-Induced Deformation of Zeolites 4A and 13X: Experimental and Molecular Simulation Study

Gas adsorption in zeolites leads to adsorption-induced deformation, which can significantly affect the adsorption and diffusive properties of the system. In this study, we conducted both experimental investigations and molecular simulations to understand the deformation of zeolites 13X and 4A during carbon dioxide adsorption at 273 K. To measure the sample's adsorption isotherm and strain simultaneously, we used a commercial sorption instrument with a custom-made sample holder equipped with a dilatometer. Our experimental data showed that while the zeolites 13X and 4A exhibited similar adsorption isotherms, their strain isotherms differed significantly. To gain more insight into the adsorption process and adsorption-induced deformation of these zeolites, we employed coupled Monte Carlo and molecular dynamics simulations with atomistically detailed models of the frameworks. Our modeling results were consistent with the experimental data and helped us identify the reasons behind the different deformation behaviors of the considered structures. Our study also revealed the sensitivity of the strain isotherm of zeolites to pore size and other structural and energetic features, suggesting that measuring adsorption-induced deformation could serve as a complementary method for material characterization and provide guidelines for related technical applications.

Cooperative Carbon Dioxide Capture in Diamine-Appended Magnesium-Olsalazine Frameworks

Diamine-appended Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) metal-organic frameworks have emerged as promising candidates for carbon capture owing to their exceptional CO2 selectivities, high separation capacities, and step-shaped adsorption profiles, which arise from a unique cooperative adsorption mechanism resulting in the formation of ammonium carbamate chains. Materials appended with primary,secondary-diamines featuring bulky substituents, in particular, exhibit excellent stabilities and CO2 adsorption properties. However, these frameworks display double-step adsorption behavior arising from steric repulsion between ammonium carbamates, which ultimately results in increased regeneration energies. Herein, we report frameworks of the type diamine-Mg2(olz) (olz4- = (E)-5,5'-(diazene-1,2-diyl)bis(2-oxidobenzoate)) that feature diverse diamines with bulky substituents and display desirable single-step CO2 adsorption across a wide range of pressures and temperatures. Analysis of CO2 adsorption data reveals that the basicity of the pore-dwelling amine─in addition to its steric bulk─is an important factor influencing adsorption step pressure; furthermore, the amine steric bulk is found to be inversely correlated with the degree of cooperativity in CO2 uptake. One material, ee-2-Mg2(olz) (ee-2 = N,N-diethylethylenediamine), adsorbs >90% of the CO2 from a simulated coal flue stream and exhibits exceptional thermal and oxidative stability over the course of extensive adsorption/desorption cycling, placing it among top-performing adsorbents to date for CO2 capture from a coal flue gas. Spectroscopic characterization and van der Waals-corrected density functional theory calculations indicate that diamine-Mg2(olz) materials capture CO2 via the formation of ammonium carbamate chains. These results point more broadly to the opportunity for fundamentally advancing materials in this class through judicious design.

Screening of Alkali Metal-Exchanged Zeolites for Nitrogen/Methane Separation

Methane (CH4) is the primary component of natural gas and must be purified to a certain level before it can be used as pipeline gas or liquified natural gas (LNG). In particular, nitrogen (N2), a common contaminant in natural gas needs to be rejected to increase the heating value of the gas and meet the LNG product specifications. The development of energy-efficient N2 removal technologies is hampered by N2's inertness and its resemblance to CH4 in terms of kinetic size and polarizability. N2-selective materials are so rare. Here, for the first time, we screened 1425 alkali metal cation exchange zeolites to identify the candidates with the best potential for the separation of N2 from CH4. We discovered a few extraordinary zeolite frameworks capable of achieving equilibrium selectivity toward N2. Particularly, Li+-RRO-3 zeolite with a specific two-dimensional structure demonstrated a selective N2 adsorption capacity of 2.94 mmol/g at 283 K and 1 bar, outperforming the capacity of all known zeolites. Through an ab initio density functional theory study, we found that the five-membered ring of the RRO framework is the most stable cationic site for Li+, and this Li+ can interact with multiple N2 molecules but only one CH4, revealing the mechanism for the high capacity and selectivity of N2. This work suggests promising adsorbents to enable N2 rejection from CH4 in the gas industry without going for energy-intensive cryogenic distillations.

Carbon dioxide capture with zeotype materials

This review describes the application of zeotype materials for the capture of CO2 in different scenarios, the critical parameters defining the adsorption performances, and the challenges of zeolitic adsorbents for CO2 capture.

In silico design of a new Zn-triazole based metal-organic framework for CO2 and H2O adsorption

In search for future good adsorbents for CO₂ capture, a nitrogen-rich triazole-type Metal-Organic Framework (MOF) is proposed based on the rational design and theoretical molecular simulations. The structure of the proposed MOF, named Zinc Triazolate based Framework (ZTF), is obtained by replacing the amine-organic linker of MAF-66 by a triazole, and its structural parameters are deduced. We used grand-canonical Monte Carlo (GCMC) simulations based on generic classical force fields to correctly predict the adsorption isotherms of CO₂ and H₂O. For water adsorption in MAF-66 and ZTF, simulations revealed that the strong hydrogen bonding interactions of water with the N atoms of triazole rings of the frameworks are the main driving forces for the high adsorption uptake of water. We also show that the proposed ZTF porous material exhibits exceptional high CO₂ uptake capacity at low pressure, better than MAF-66. Moreover, the nature of the interactions between CO₂ and the MAF-66 and ZTF surface cavities was examined at the microscopic level. Computations show that the interactions occur at two different sites, consisting of Lewis acid-Lewis base interactions and hydrogen bonding, together with obvious electrostatic interactions. In addition, we investigated the influence of the presence of H₂O molecules on the CO₂ adsorption on the ZTF MOF. GCMC simulations reveal that the addition of H₂O molecules leads to an enhancement of the CO₂ adsorption at very low pressures but a reduction of this CO₂ adsorption at higher pressures.

Adsorption-induced clustering of CO2 on graphene

Utilization of graphene-based materials for selective carbon dioxide capture has been demonstrated recently as a promising technological approach. In this study we report results from density functional theory calculations and molecular dynamics simulations on the adsorption of CO2, N2, and CH4 gases on a graphene sheet. We calculate adsorption isotherms of ternary and binary mixtures of these gases and reproduce the larger selectivity of CO2 to graphene relative to the other two gases. Furthermore it is shown that the confinement to two-dimensions, associated with adsorbing the CO2 gas molecules on the plane of graphene, increases their propensity to form clusters on the surface. Above a critical surface coverage (or partial pressure) of the gas, these CO2-CO2 interactions augment the effective adsorption energy to graphene, and, in part, contribute to the high selectivity of carbon dioxide with respect to nitrogen and methane. The origin of the attractive interaction between the CO2 molecules adsorbed on the surface is of electric quadrupole-quadrupole nature, in which the positively-charged carbon of one molecule interacts with the negatively-charged oxygen of another molecule. The energy of attraction of forming a CO2 dimer is predicted to be around 5-6 kJ mol-1, much higher than the corresponding values calculated for N2 and CH4. We also evaluated the adsorption energies of these gases to a graphene sheet and found that the attractions obtained using the classical force-fields might be over-exaggerated. Nevertheless, even when the magnitudes of these (classical force-field) graphene-gas interactions are scaled-down sufficiently, the tendency of CO2 molecules to cluster on the surface is still observed.

Preferential Adsorption of Propylene over Propane on a Ag-Exchanged X-Type Zeolite Membrane

We investigated the adsorption properties of propylene and propane on an olefin-selective Ag-X membrane and discussed the contribution of adsorption selectivity to propylene/propane separation performance through this membrane. The isotherms of propylene and propane on Ag-X membranes were measured in unary systems at 313 K. The amount of propylene adsorbed on the Ag-X membrane at a lower pressure increased remarkably compared with that on the Na-X membrane. Such a change of adsorption property could induce excellent separation property for the Ag-X membrane. We compared the adsorption properties in a binary system calculated based on the Markham-Benton approach with the results of a permeation test. The molar fractions of propylene in the adsorbed phase in the binary system provided good agreement with propylene purity on the permeation side of the Ag-X membrane. These results clearly show that permeation selectivity of the Ag-X membrane for the propylene/propane mixture is mainly governed by adsorption selectivity.

Optimization of CO2 Adsorption on Solid-Supported Amines and Thermal Regeneration Mode Comparison

For improving the CO₂ adsorption capacity of solid-supported amines, five commercial porous supports have been selected and impregnated with tetraethylenepentamine (TEPA), and their CO₂ adsorption performances have been evaluated using a fixed-bed reactor coupled with mass spectrometry. For solid-supported amines, CO₂ adsorption capacities coincide with the texture characterization of the adsorbent supports (mesoporous alumina, montmorillonite, silica gel, porous resin, MCM-41 molecular sieve), and the optimum TEPA loading amount is mainly affected by the pore volume. The mesoporous supports were found to be more conducive to uniform loading of organic amine, with more than 370 mg/g CO₂ adsorbed per unit TEPA. Other components in flue gas, especially H₂O, favor CO₂ adsorption on solid-supported amines. SO₂ inhibited the CO₂ adsorption, which was mainly attributable to the strong and irreversible binding of SO₂ on some amine sites. NO had little effect on CO₂ adsorption. Thermal stabilities of solid-supported amines have been tested based on thermogravimetry curves, and the main weight loss peak for TEPA appears at 513 K for solid-supported amines. Linear and step regeneration modes have been compared, revealing that the temperature for step regeneration is 37 K lower than that for the linear regeneration mode. Moreover, the desorption peak area for the step regeneration mode is 20% higher than that for the linear regeneration mode, indicating that the step regeneration mode can be used in practical applications, to reduce energy consumption during regeneration.

Capsules of Reactive Ionic Liquids for Selective Capture of Carbon Dioxide at Low Concentrations

The task-specific ionic liquid (IL), 1-ethyl-3-methylimidazolium 2-cyanopyrolide ([EMIM][2-CNpyr]), was encapsulated with polyurea (PU) and graphene oxide (GO) sheets via a one-pot Pickering emulsion, and these capsules were used to scrub CO₂ (0-5000 ppm) from moist air. Up to 60 wt % of IL was achieved in the synthesized capsules, and we demonstrated comparable gravimetric CO₂ capacities to zeolites and enhanced absorption rates compared to those of bulk IL due to the increased gas/liquid surface-to-volume area. The reactive IL capsules show recyclability upon mild temperature increase compared to zeolites that are the conventional absorber materials for CO₂ scrubbing. The measured breakthrough curves in a fixed bed under 100% relative humidity establish the utility of reactive IL capsules as moisture-stable scrubber materials to separate CO₂ from air, outperforming zeolites owing to their higher selectivity. It is shown that thermal stability, CO₂ absorption capacity, and rate of uptake by IL capsules can be further modulated by incorporating low-viscosity and nonreactive ILs to the capsule core. This study demonstrates an alternative and facile approach for CO₂ scrubbing, where separation from gas mixtures with extremely low partial pressures of CO₂ is required.

Multi-factor study of the effects of a trace amount of water vapor on low concentration CO2 capture by 5A zeolite particles

CO₂ adsorption amount is enhanced with below 0.1 ppm humidity, and water molecule partial charge is a dominant factor in adsorption.

Zeolite-Based Sorbent for CO2 Capture: Preparation and Performance Evaluation

In this work, zeolite-based sorbents were developed from gasified rice husk. CO₂ capture capacity of the sorbents was examined at various temperatures and pressures employing a fixed-bed flow reactor and simulated flue gas. Various physicochemical properties such as thermal stability, pore size distribution, morphology, chemical composition, etc. of the in-house-developed materials were characterized in detail and were also compared with two commercially available zeolites. Tetra-ethylenepentamine was impregnated in the in-house-developed zeolite supports to investigate its suitability to improve the CO₂ adsorption capacity. The effects of reactor pressure, temperature, Si/Al ratio, and amine loading on CO₂ uptake capacity were examined. A declining trend in CO₂ adsorption capacity was observed with the increase in adsorption temperature and amine loading. At 30 °C, zeolite-Y (designated as Z-Y-3, silica to alumina ratio of 2.25) sample exhibited maximum adsorption capacity, and the obtained values were around 114 and 190 mg CO₂/g sorbent under atmospheric and 5 bar pressure, respectively. It was also observed that the presence of alkali metal ions influenced the adsorption capacity of the zeolites. The study inferred that the adsorbent was efficient and promising for multiple adsorption-desorption cycles without much deterioration of the capture capacity.

Complexes of Zn(II)-Triazoles with CO2 and H2O: Structures, Energetics, and Applications

Using a first-principle methodology, we investigate the stable structures of the nonreactive and reactive clusters formed between Zn²⁺-triazoles ([Zn²⁺-Tz]) clusters and CO₂ and/or H₂O. In sum, we characterized two modes of bonding of [Zn²⁺-Tz] with CO₂/H₂O: the interaction is established through (i) a covalent bond between Zn²⁺ of [Zn²⁺-Tz] and oxygen atoms of CO₂ or H₂O and (ii) hydrogen bonds through N-H or C-H of [Zn²⁺-Tz] and oxygen atoms of H₂O or CO₂, N-H···O. We also identified intramolecular proton transfer processes induced by complexation. Indeed, water drastically changes the shape of the energy profiles of the tautomeric phenomena through strong lowering of the potential barriers to tautomerism. The comparison to [Zn²⁺-Im] subunits formed with Zn²⁺ and imidazole shows that the efficiency of Tz-based compounds for CO₂ capture and uptake is due to the incorporation of more accessible nitrogen donor sites in Tzs compared to imidazoles. Since [Zn²⁺-Tz] clusters are subunits of an organometallic nanoporous materials and Zn-proteins, our data are useful for deriving force fields for macromolecular simulations of these materials. Our work also suggests the consideration of traces of water to better model the CO₂ sequestration and reactivity on macromolecular entities such as pores or active sites.

Challenges and opportunities for adsorption-based CO2 capture from natural gas combined cycle emissions

In recent years, the power sector has shown a growing reliance on natural gas, a cleaner-burning fuel than coal that emits approximately half as much CO2 per kWh of energy produced. This rapid growth in the consumption of natural gas has led to increased CO2 emissions from gas-fired power plants. To limit the contribution of fossil fuel combustion to atmospheric CO2 levels, carbon capture and sequestration has been proposed as a potential emission mitigation strategy. However, despite extensive exploration of solid adsorbents for CO2 capture, few studies have examined the performance of adsorbents in post-combustion capture processes specific to natural gas flue emissions. In this perspective, we emphasize the importance of considering gas-fired power plants alongside coal-fired plants in future analyses of carbon capture materials. We address specific challenges and opportunities related to adsorptive carbon capture from the emissions of gas-fired plants and discuss several promising candidate materials. Finally, we suggest experiments to determine the viability of new CO2 capture materials for this separation. This broadening in the scope of current carbon capture research is urgently needed to accelerate the deployment of transformational carbon capture technologies.

First principles Monte Carlo simulations of unary and binary adsorption: CO2, N2, and H2O in Mg-MOF-74

Dative bonding of adsorbate molecules onto coordinatively-unsaturated metal sites in metal–organic frameworks (MOFs) can lead to unique adsorption selectivities.

A Diaminopropane-Appended Metal-Organic Framework Enabling Efficient CO2 Capture from Coal Flue Gas via a Mixed Adsorption Mechanism

A new diamine-functionalized metal-organic framework comprised of 2,2-dimethyl-1,3-diaminopropane (dmpn) appended to the Mg2+ sites lining the channels of Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) is characterized for the removal of CO2 from the flue gas emissions of coal-fired power plants. Unique to members of this promising class of adsorbents, dmpn-Mg2(dobpdc) displays facile step-shaped adsorption of CO2 from coal flue gas at 40 °C and near complete CO2 desorption upon heating to 100 °C, enabling a high CO2 working capacity (2.42 mmol/g, 9.1 wt %) with a modest 60 °C temperature swing. Evaluation of the thermodynamic parameters of adsorption for dmpn-Mg2(dobpdc) suggests that the narrow temperature swing of its CO2 adsorption steps is due to the high magnitude of its differential enthalpy of adsorption (Δhads = -73 ± 1 kJ/mol), with a larger than expected entropic penalty for CO2 adsorption (Δsads = -204 ± 4 J/mol·K) positioning the step in the optimal range for carbon capture from coal flue gas. In addition, thermogravimetric analysis and breakthrough experiments indicate that, in contrast to many adsorbents, dmpn-Mg2(dobpdc) captures CO2 effectively in the presence of water and can be subjected to 1000 humid adsorption/desorption cycles with minimal degradation. Solid-state 13C NMR spectra and single-crystal X-ray diffraction structures of the Zn analogue reveal that this material adsorbs CO2 via formation of both ammonium carbamates and carbamic acid pairs, the latter of which are crystallographically verified for the first time in a porous material. Taken together, these properties render dmpn-Mg2(dobpdc) one of the most promising adsorbents for carbon capture applications.

Enhanced Selectivity for CO2 Adsorption on Mesoporous Silica with Alkali Metal Halide Due to Electrostatic Field: A Molecular Simulation Approach

Since adsorption performances are dominantly determined by adsorbate-adsorbent interactions, accurate theoretical prediction of the thermodynamic characteristics of gas adsorption is critical for designing new sorbent materials as well as understanding the adsorption mechanisms. Here, through our molecular modeling approach using a newly developed quantum-mechanics-based force field, it is demonstrated that the CO₂ adsorption selectivity of SBA-15 can be enhanced by incorporating crystalline potassium chloride particles. It is noted that the induced intensive electrostatic fields around potassium chloride clusters create gas-trapping sites with high selectivity for CO₂ adsorption. The newly developed force field can provide a reliable theoretical tool for accurately evaluating the gas adsorption on given adsorbents, which can be utilized to identify good gas adsorbents.

Gas adsorption in Mg-porphyrin-based porous organic frameworks: A computational simulation by first-principles derived force field

A novel type of porous organic frameworks, based on Mg-porphyrin, with diamond-like topology, named POF-Mgs is computationally designed, and the gas uptakes of CO₂ , H₂ , N₂ , and H₂ O in POF-Mgs are investigated by Grand canonical Monte Carlo simulations based on first-principles derived force fields (FF). The FF, which describes the interactions between POF-Mgs and gases, are fitted by dispersion corrected double-hybrid density functional theory, B2PLYP-D3. The good agreement between the obtained FF and the first-principle energies data confirms the reliability of the FF. Furthermore our simulation shows the presence of a small amount of H₂ O (≤ 0.01 kPa) does not much affect the adsorption quantity of CO₂ , but the presence of higher partial pressure of H₂ O (≥ 0.1 kPa) results in the CO₂ adsorption decrease significantly. The good performance of POF-Mgs in the simulation inspires us to design novel porous materials experimentally for gas adsorption and purification. © 2017 Wiley Periodicals, Inc.

Advances in theory and their application within the field of zeolite chemistry

Zeolites are versatile and fascinating materials which are vital for a wide range of industries, due to their unique structural and chemical properties, which are the basis of applications in gas separation, ion exchange and catalysis. Given their economic impact, there is a powerful incentive for smart design of new materials with enhanced functionalities to obtain the best material for a given application. Over the last decades, theoretical modeling has matured to a level that model guided design has become within reach. Major hurdles have been overcome to reach this point and almost all contemporary methods in computational materials chemistry are actively used in the field of modeling zeolite chemistry and applications. Integration of complementary modeling approaches is necessary to obtain reliable predictions and rationalizations from theory. A close synergy between experimentalists and theoreticians has led to a deep understanding of the complexity of the system at hand, but also allowed the identification of shortcomings in current theoretical approaches. Inspired by the importance of zeolite characterization which can now be performed at the single atom and single molecule level from experiment, computational spectroscopy has grown in importance in the last decade. In this review most of the currently available modeling tools are introduced and illustrated on the most challenging problems in zeolite science. Directions for future model developments will be given.