Adsorption of Small Molecules in LTA Zeolites. 1. NH3, CO2, and H2O in Zeolite 4A

Tailoring ammonia capture in MOFs and COFs: A multi-scale and machine learning comprehensive investigation of functional group modification

Our study aims to examine the impact of ligand functionalization on the ammonia adsorption properties of MOFs and COFs, by combining multi-scale calculations with machine learning techniques. Density Functional Theory calculations were performed to investigate the interactions between ammonia (NH3) and a comprehensive set of 48 strategically chosen functional groups. In all of the cases, it is observed that functionalized rings exhibit a stronger interaction with ammonia molecule compared to unfunctionalized benzene, while -O2Mg demonstrates the highest interaction energy with ammonia (15 times stronger than the bare benzene). The trend obtained from the thorough DFT screening is verified via Grand Canonical Monte-Carlo calculations by employing interatomic potentials derived from quantum chemical calculations. Isosteric heat of adsorption plots provide a comprehensive elucidation of the adsorption process, and important insights can be taken for studies in fine-tuning materials for ammonia adsorption. Furthermore, a proof of concept machine learning (ML) analysis is conducted, which demonstrates that ML can accurately predict NH3 binding energies despite the limited amount of data. The findings derived from our multi-scale methodology indicate that the functionalization strategy can be utilized to guide synthesis towards MOFs, COFs, or other porous materials for enhanced NH3 adsorption capacity.

Sorption of Polar Sorbates NH3, H2O, SO2 and CO2 on Selected Inorganic Materials

In this paper, the sorption of NH₃, H₂O, SO₂ and CO₂ was tested for several selected inorganic materials. The tests were performed on samples belonging to two topologies of materials, faujasite (FAU) and framework-type MFI, the structures of which differ in pore size and connectivity. All sorbates are important in terms of reducing their emissions to the environment. They have different chemical nature: basic, alkaline, and acidic. They are all polar in structure and composition and two of them (ammonia and water vapor) can form hydrogen bonds. These differences result in different interactions with the surface of the adsorbents. This paper presents experimental data and proposes a mathematical description of the sorption process. The best fit of the experimental data was obtained for the Toth and GAB models. The studies showed that among the selected samples, faujasite has the best sorption capacity for ammonia and water vapor, while the best sorbent for sulfur dioxide is the MFI framework type. These materials behave like molecular sieves and can be used for quite selective adsorption of relevant gases. In addition, modification of the faujasite with organic silane resulted in a drastic reduction in the surface area of the sorbent, resulting in significantly lower sorption capacities for gases.

Ammonia Capture in Rhodium(II)-Based Metal-Organic Polyhedra via Synergistic Coordinative and H-Bonding Interactions

Ammonia (NH₃) is among the world's most widely produced bulk chemicals, given its extensive use in diverse sectors such as agriculture; however, it poses environmental and health risks at low concentrations. Therefore, there is a need for developing new technologies and materials to capture and store ammonia safely. Herein, we report for the first time the use of metal-organic polyhedra (MOPs) as ammonia adsorbents. We evaluated three different rhodium-based MOPs: [Rh₂(bdc)₂]₁₂ (where bdc is 1,3-benzene dicarboxylate); one functionalized with hydroxyl groups at its outer surface [Rh₂(OH-bdc)₂]₁₂ (where OH-bdc is 5-hydroxy-1,3-benzene dicarboxylate); and one decorated with aliphatic alkoxide chains at its outer surface [Rh₂(C₁₂O-bdc)₂]₁₂ (where C₁₂O-bdc is 5-dodecoxybenzene-1,3-benzene dicarboxylate). Ammonia-adsorption experiments revealed that all three Rh-MOPs strongly interact with ammonia, with uptake capacities exceeding 10 mmol/g_MOP. Furthermore, computational and experimental data showed that the mechanism of the interaction between Rh-MOPs and ammonia proceeds through a first step of coordination of NH₃ to the axial site of the Rh(II) paddlewheel cluster, which triggers the adsorption of additional NH₃ molecules through H-bonding interaction. This unique mechanism creates H-bonded clusters of NH₃ on each Rh(II) axial site, which accounts for the high NH₃ uptake capacity of Rh-MOPs. Rh-MOPs can be regenerated through their immersion in acidic water, and upon activation, their ammonia uptake can be recovered for at least three cycles. Our findings demonstrate that MOPs can be used as porous hosts to capture corrosive molecules like ammonia, and that their surface functionalization can enhance the ammonia uptake performance.

Zeolites in Adsorption Processes: State of the Art and Future Prospects

Zeolites have been widely used as catalysts, ion exchangers, and adsorbents since their industrial breakthrough in the 1950s and continue to be state-of the-art adsorbents in many separation processes. Furthermore, their properties make them materials of choice for developing and emerging separation applications. The aim of this review is to put into context the relevance of zeolites and their use and prospects in adsorption technology. It has been divided into three different sections, i.e., zeolites, adsorption on nanoporous materials, and chemical separations by zeolites. In the first section, zeolites are explained in terms of their structure, composition, preparation, and properties, and a brief review of their applications is given. In the second section, the fundamentals of adsorption science are presented, with special attention to its industrial application and our case of interest, which is adsorption on zeolites. Finally, the state-of-the-art relevant separations related to chemical and energy production, in which zeolites have a practical or potential applicability, are presented. The replacement of some of the current separation methods by optimized adsorption processes using zeolites could mean an improvement in terms of sustainability and energy savings. Different separation mechanisms and the underlying adsorption properties that make zeolites interesting for these applications are discussed.

Study of Water Adsorption on EDTA-Modified LTA Zeolites

The present work deals with the study of water adsorption on acid-modified zeolites A. Commercial zeolites 4A (Na form) and 5A (Ca form) were subjected to EDTA dealumination, and their structure, textural properties and stability were checked by XRD, EDX, NMR and N₂ physisorption analyses. The water adsorption isotherms of the parent zeolites and their modified forms were measured at a temperature of 25 °C and up to a relative pressure of 0.9. The results show that the treatment with EDTA drastically changes the structural properties of the zeolites and increases the water adsorption capacity by up to 10%. The changes depend on the type of extra-framework cations (Na⁺ and Ca²⁺) and the EDTA concentration.

Adsorption of water in Na-LTA zeolites: an ab initio molecular dynamics investigation

The very wide range of applications of LTA zeolites, including the storage of tritiated water, implies that a detailed and accurate atomic-scale description of the adsorption processes taking place in their structure is crucial. To unravel with an unprecedented accuracy the mechanisms behind the water filling in NaA, we have conducted a systematic ab initio molecular dynamics investigation. Two LTA structural models, the conventional Z4A and the reduced one ZK4, have been used for static and dynamic ab initio calculations, respectively. After assessing this reduced model with comparative static DFT calculations, we start the filling of the α and β cages by water, molecule by molecule. This allowed us to thoroughly study the interaction of water molecules with the zeolite structure and between water molecules, progressively forming H-bond chains and ring patterns as the cage is being filled. The adsorption energies could then be calculated with an unprecedented accuracy, which showed that the interaction of the molecules with the zeolite weakens as their number increases. By these methods, we have been able to highlight the primary role of Na⁺ cations in the interaction of water with zeolite, and inversely, the role of water in the displacement of cations when it is sufficiently solvated, allowing the passage between the α and β cages. This phenomenon is possible thanks to the inhomogeneous distribution of water molecules on the cationic sites, as shown by our AIMD simulations, which allows the formation of water clusters. These results are important because they help in understanding how the coverage of cationic sites by water will affect the adsorption of other molecules inside the Na-LTA zeolite.

Zeolite (In)Stability under Aqueous or Steaming Conditions

Zeolites are among the most environmentally friendly materials produced industrially at the Megaton scale. They find numerous commercial applications, particularly in catalysis, adsorption, and separation. Under ambient conditions aluminosilicate zeolites are stable when exposed to water or water vapor. However, at extreme conditions as high temperature, high water vapor pressure or increased acidity/basicity, their crystalline framework can be destroyed. The stability of the zeolite framework under aqueous conditions also depends on the concentration and character of heteroatoms (other than Al) and the topology of the zeolite. The factors critical for zeolite (in)stability in the presence of water under various conditions are reviewed from the experimental as well as computational sides. Nonreactive and reactive interactions of water with zeolites are addressed. The goal of this review is to provide a comparative overview of all-silica zeolites, aluminosilicates and zeolites with other heteroatoms (Ti, Sn, and Ge) when contacted with water. Due attention is also devoted to the situation when partial zeolite hydrolysis is used beneficially, such as the formation of hierarchical zeolites, synthesis of new zeolites or fine-tuning catalytic or adsorption characteristics of zeolites.

Selective Adsorption of CO2 on Zeolites NaK-ZK-4 with Si/Al of 1.8-2.8

Zeolites with appropriately narrow pore apertures can kinetically enhance the selective adsorption of CO₂ over N₂. Here, we showed that the exchangeable cations (e.g., Na⁺ or K⁺) on zeolite ZK-4 play an important role in the CO₂ selectivity. Zeolites NaK ZK-4 with Si/Al = 1.8-2.8 had very high CO₂ selectivity when an intermediate number of the exchangeable cations were K⁺ (the rest being Na⁺). Zeolites NaK ZK-4 with Si/Al = 1.8 had high CO₂ uptake capacity and very high CO₂-over-N₂ selectivity (1190). Zeolite NaK ZK-4 with Si/Al = 2.3 and 2.8 also had enhanced CO₂ selectivity with an intermediate number of K⁺ cations. The high CO₂ selectivity was related to the K⁺ cation in the 8-rings of the α-cage, together with Na⁺ cations in the 6-ring, obstructing the diffusion of N₂ throughout the zeolite. The positions of the K⁺ cation in the 8-ring moved slightly (max 0.2 Å) toward the center of the α-cage upon the adsorption of CO₂, as revealed by in situ X-ray diffraction. The CO₂-over-N₂ selectivity was somewhat reduced when the number of K⁺ cations approached 100%. This was possibly due to the shift in the K⁺ cation positions in the 8-ring when the number of Na⁺ was going toward 0%, allowing N₂ diffusion through the 8-ring. According to in situ infrared spectroscopy, the amount of chemisorbed CO₂ was reduced on zeolite ZK-4s with increasing Si/Al ratio. In the context of potential applications, a kinetically enhanced selection of CO₂ could be relevant for applications in carbon capture and bio- and natural gas upgrading.

Thermodynamic evidence of flexibility in H2O and CO2 absorption of transition metal ion exchanged zeolite LTA

Absorption thermodynamics on the framework flexibility of TMI-exchanged zeolite LTA driven by water/CO₂ molecules.

Separation based adsorption of ethanol-water mixture in azeotropic solution by single-walled carbon, boron-nitride and silicon-carbide nanotubes

The separation of the azeotropic ethanol-water mixture (95.57wt% ethanol) over a wide range of pressures (100-100000kPa) was studied on armchair SWCNTs, SWSiCNTs and SWBNNTs with different diameters at 351.30K using GCMC simulations. The GCMC results demonstrated that ethanol and water molecules form a monolayer single-file, chain together in the center of (6,6) SWCNT, while a spiral ring of ethanol and water is formed in the center of (8,8), (10,10) and (12,12) SWCNTs. It was found that in SWCNTs, the adsorption of ethanol reduces the function of pressure, while water adsorption increases its function. Water selectivity rises as a function of pressure. Also, in SWBNNTs, the adsorption of water increases as a function of pressure, while ethanol adsorption is almost constant. However, in the case of SWSiCNTs, ethanol and water adsorptions are very similar to those of SWBNNTs, whereas the adsorptivities of SWSiCNTs are more than those of SWBNNTs. Our findings regarding adsorption and slope of adsorption indicate that higher pressures are favorable for separating water and ethanol by SWCNTs, while SWBNNTs and SWSiCNTs are demonstrate higher ethanol adsorptivities in lower pressures. Also, MD simulations have been performed to study the microscopic structure and diffusion of binary mixtures of water and ethanol within SWCNTs, SWSiCNTs and SWBNNTs. The MD simulations imply that the oxygen atoms are highly well-organized around themselves. Also, the MD results illustrate a similar tendency for oxygen of water (OW) and oxygen of ethanol (OE) to the wall of the nanotubes in all the pressures. In addition, from the MD results, self-diffusion of water and ethanol in all nanotubes were calculated and discussed.

A Simulation Study of Alkanes in Linde Type A Zeolites

Monte Carlo simulations were performed to study the adsorption and diffusion of small hydrocarbons in Linde Type A zeolites as a function of their calcium/sodium ratio. The diffusion studies were focused on methane whereas the adsorption simulations were performed from methane up to pentane. The results obtained showed that an increase in the number of cations in the structure (exchange of univalent sodium ions by divalent calcium ions) led to an increase in the adsorption of linear alkanes at low and medium pressure, but caused a decrease in adsorption at the highest pressures. An increase in the amount of cations favours molecular attraction and hence results in lower mobility. At higher cation loading the ions block the windows interconnecting the LTA cages, leading to a further decrease in diffusion. Methane self-diffusion coefficients obtained from our simulations were twice as high for the Linde Type 5A zeolite as for the Linde Type 4A zeolite. These results are consistent with previous experimental studies and provide a molecular picture of the influence of the zeolite type, the amount of cations contained and their location in the structure.

Potential and actual uses of zeolites in crop protection

In this review, it is demonstrated that zeolites have a potential to be used as crop protection agents. Similarly to kaolin, zeolites can be applied as particle films against pests and diseases. Their honeycomb framework, together with their carbon dioxide sorption capacity and their heat stress reduction capacity, makes them suitable as a leaf coating product. Furthermore, their water sorption capacity and their smaller particle sizes make them effective against fungal diseases and insect pests. Finally, these properties also ensure that zeolites can act as carriers of different active substances, which makes it possible to use zeolites for slow-release applications. Based on the literature, a general overview is provided of the different basic properties of zeolites as promising products in crop protection.

Transferable force fields for adsorption of small gases in zeolites

We provide transferable force fields for oxygen, nitrogen, and carbon monoxide that are able to reproduce experimental adsorption in both pure silica and alumino-substituted zeolites at cryogenic and high temperatures.

Effect of hydrophilic defects on water transport in MFI zeolites

The subnanometer pore structure of zeolites and other microporous materials has been proposed to act as a molecular sieve for various water separation technologies. However, due to the increased interaction between the solid and water in these nanoconfined spaces, it is unclear which type of interface, be it hydrophilic or hydrophobic, offers an advantageous medium for enhancing transport properties. In this work, we probe the role of hydrophilic defects on the transport of water inside the microporous hydrophobic MFI zeolite pore structure via combined sorption and high-pressure infiltration experiments. While the inclusion of defects was observed to increase the amount of water within the zeolite pore network by up to 7 times at the saturation pressure, the diffusivity of this infiltrated water was lowered by up to 2 orders of magnitude in comparison to that of water within the nearly defect-free hydrophobic MFI zeolite. Subsequently, the permeability of water within the more defective MFI zeolite was an order of magnitude lower than that of the nearly defect-free zeolite. The results from these experiments suggest that the intrinsic hydrophobic pore structure of MFI zeolites can facilitate faster water transport due to the decreased attraction between the water and the defect-free surface. While the strong attraction of water to the defects allows for water to infiltrate the porous network at lower pressures, the results suggest that this strong attraction decreases the mobility of the infiltrated water. The insights gained from this study can be utilized to improve the design of future membranes for water desalination and other separation techniques.

Zeolites and related sorbents with narrow pores for CO2 separation from flue gas

Adsorbents with small pores are especially relevant for capturing carbon dioxide at large emission sources.

Sorbents for CO(2) capture from flue gas--aspects from materials and theoretical chemistry

Predictions of future climate change have triggered a search for ways to reduce the release of greenhouse gases into the atmosphere. Carbon capture and storage (CCS) assists this goal by reducing carbon dioxide emissions, and CO(2) adsorbents in particular can reduce the costs of CO(2) capture. Here, we review the nanoscale sorbent materials that have been developed and the theoretical basis for their function in CO(2) separation, particularly from N(2)-rich flue gases.

Molecular simulation of water/alcohol mixtures' adsorption and diffusion in zeolite 4A membranes

The COMPASS (condensed-phase optimized molecular potentials for atomistic simulation studies) force field with two sets of partial atomic charges of water was used to simulate adsorption and diffusion behavior of water/methanol and water/ethanol mixtures in zeolite 4A at 298 K. The adsorption of alcohol first increased and then decreased with increasing pressure, whereas the adsorption of water increased progressively until an adsorption equilibrium was reached. Both the adsorbed molecules and the zeolite framework were treated as a fully flexible model in MD simulations. The simulation results show that the effects of the size and steric hindrance of the diffusing molecules on diffusivity are significant. The diffusivity of water, methanol, and ethanol molecules decreases by 1 order of magnitude in the order of water > methanol > ethanol. The diffusivity of water molecules depends on the mass fraction and the partial charges of water in zeolite 4A. The ethanol and methanol molecules have restricted motion through the alpha-cages, whereas the water molecules can easily pass through the alpha-cages window at low feed alcohol concentrations. And the extent of hydrogen bonding increased with increasing water concentration.

Advances in principal factors influencing carbon dioxide adsorption on zeolites

We report the advances in the principal structural and experimental factors that might influence the carbon dioxide (CO₂) adsorption on natural and synthetic zeolites. The CO₂ adsorption is principally govern by the inclusion of exchangeable cations (countercations) within the cavities of zeolites, which induce basicity and an electric field, two key parameters for CO₂ adsorption. More specifically, these two parameters vary with diverse factors including the nature, distribution and number of exchangeable cations. The structure of framework also determines CO₂ adsorption on zeolites by influencing the basicity and electric field in their cavities. In fact, the basicity and electric field usually vary inversely with the Si/Al ratio. Furthermore, the CO₂ adsorption might be limited by the size of pores within zeolites and by the carbonates formation during the CO₂ chemisorption. The polarity of molecules adsorbed on zeolites represents a very important factor that influences their interaction with the electric field. The adsorbates that have the most great quadrupole moment such as the CO₂, might interact strongly with the electric field of zeolites and this favors their adsorption. The pressure, temperature and presence of water seem to be the most important experimental conditions that influence the adsorption of CO₂. The CO₂ adsorption increases with the gas phase pressure and decreases with the rise of temperature. The presence of water significantly decreases adsorption capacity of cationic zeolites by decreasing strength and heterogeneity of the electric field and by favoring the formation of bicarbonates. The optimization of the zeolites structural characteristics and the experimental conditions might enhance substantially their CO₂ adsorption capacity and thereby might give rise to the excellent adsorbents that may be used to capturing the industrial emissions of CO₂.

Influence of Cation Na/Ca Ratio on Adsorption in LTA 5A: A Systematic Molecular Simulation Study of Alkane Chain Length

Recent adsorption isotherms of n-alkanes on Ca,Na-LTA-type zeolite afford development of a force field describing the interactions between calcium and n-alkanes in configurational-bias Monte Carlo simulations. The force field of Calero et al. (J. Am. Chem. Soc. 2004, 126, 11377-11386) is able to accurately describe the adsorption properties of linear alkanes in the sodium form of FAU-type zeolites. Here, we extend upon this type of force field by including calcium-type ions. The force field was fitted to reproduce the calcium and sodium cations positions on LTA 5A and the experimental adsorption properties of n-alkanes over all range of temperatures and pressures. This opens up a vast amount of experimental data on LTA 5A, both on adsorption and diffusion. Furthermore, evaluation of half a century of reported n-alkane adsorption data on LTA-type zeolites indicates that there are many inconsistencies between the various data sets, possibly as a result of (i) undisclosed calcium and sodium contents, (ii) less than perfect drying of the hygroscopic zeolite, and (iii) coadsorption of contaminants such as vacuum grease. Having obtained our force field, and confirmed its reliability on predictions outside the calibration set, we apply the force field on two "open" problems: (a) the heats of adsorption and Henry coefficient as a function of chain length and (b) the effect of cations in LTA-type zeolites. The molecular simulations shed new light on previous experimental findings, and we provide rationalizations on the molecular level that can be generalized to the class of cage/window-type nanoporous materials.

Gibbs ensemble Monte Carlo simulation of supercritical CO2 adsorption on NaA and NaX zeolites

Adsorption of supercritical carbon dioxide on two kinds of zeolites with identical chemical composition but different pore structure (NaA and NaX) was studied using the Gibbs ensemble Monte Carlo simulation. The model frameworks for the two zeolites with SiAl ratio being unity have been chosen as the solid structures in the simulation. The adsorption behaviors of supercritical CO2 on the NaA and NaX zeolites, based on the adsorption isotherms and isosteric heats of adsorption, were discussed in detail and were compared with the available experimental results. A good agreement between the simulated and experimental results is obtained for both the adsorbed amount and the bulk phase density. The intermediate configurational snapshots and the radial distribution functions between zeolite and adsorbed CO2 molecules were collected in order to investigate the preferable adsorption locations and the confined structure behavior of CO2. The structure behaviors of the adsorbed CO2 molecules show various performances, as compared with the bulk phase, due to the confined effect in the zeolite pores.

Incidence and properties of nanoscale defects in silicalite

Crystal intergrowths are predicted to form more readily than other extended defects in a model zeolite membrane and cause a reduction in molecular flux.