All-silica Zeolite Research Articles

The effect of perfluoroalkyl chain length and the type of acid group on PFAS adsorption from water

Widespread use of per- and polyfluoroalkyl substances (PFAS) poses significant ecological and health risks due to their non-degradability in the environment and consequent bioaccumulation in humans. In this work, we carried out a systematic computational study to unravel the effect of perfluorinated carbon chain length and the type of acid head group on the adsorption mechanism of PFAS from water. We considered the adsorption of perfluoroalkyl carboxylic acids (PFCA) and perfluoroalkyl sulfonic acids (PFSA) with chain lengths of three to eight perfluorinated carbons in hydrophobic all-silica zeolite Beta. Molecular dynamics (MD) simulations coupled with enhanced sampling calculations showed that adsorption free energies of both PFCA and PFSA decreased with increasing number of perfluorinated carbons. Calculated free energies further showed that PFCA and PFSA, which are anions in water, must overcome substantially large interfacial energy barriers to be adsorbed in hydrophobic pores. Subsequent ab initio MD simulations revealed that PFCA and PFSA gain a proton at the water–zeolite interface and are eventually adsorbed in neutral form. Breakdown of guest–host interaction energies into hydrophobic and polar components showed that hydrophobic interactions dominate PFCA adsorption whereas polar interactions are dominant in PFSA adsorption. This is due to sulfonic acid giving rise to stronger electrostatic interactions and the rearrangement of the electron density distribution in the perfluorinated carbon chain, which render the hydrophobic interactions relatively weaker. Our study reveals that a combination of adsorbents that provide optimized hydrophobic and polar interactions should be used for the capture of diverse PFAS from water.

Monte Carlo Simulations of Water Pollutant Adsorption at Parts-per-Billion Concentration: A Study on 1,4-Dioxane.

1,4-dioxane, an emerging water pollutant with high production volumes, is a probable human carcinogen. The inadequacy of conventional treatment processes demonstrates the need for an effective remediation strategy. Crystalline nanoporous materials are cost-effective adsorbents due to their high capacity and selective separation in mixtures. This study explores the potential of all-silica zeolites for the separation of 1,4-dioxane from water. These zeolites are highly hydrophobic and can preferentially adsorb nonpolar molecules from mixtures. We investigated six zeolite frameworks (BEA, EUO, FER, IFR, MFI, and MOR) using Monte Carlo simulations in the Gibbs ensemble. The simulations indicate high selectivity by FER and EUO, especially at low pressures, which we attribute to pore sizes and shapes with a greater affinity to 1,4-dioxane. We also demonstrate a Monte Carlo simulation workflow using gauge cells to model the adsorption of an aqueous solution of 1,4-dioxane at a 0.35 ppb concentration. We quantify 1,4-dioxane and water coadsorption and observe selectivities ranging from 1.1 × 105 in MOR to 8.7 × 106 in FER. We also demonstrate that 1,4-dioxane is in the infinite dilution regime in the aqueous phase at this concentration. This simulation technique can be extended to model other emerging water contaminants such as perfluoroalkyl and polyfluoroalkyl substances (PFAS), chlorofluorocarbons, and others, which are also found in extremely low concentrations.

Computational Screening of Hydrophobic Zeolites for the Removal of Emerging Organic Contaminants from Water.

The pollution of water resources by pharmaceuticals and agents of personal care products (PPCPs) poses an increasingly pressing issue that has received considerable attention from scientists and government agencies alike. Hydrophobic zeolites can serve as selective adsorbents to remove these contaminants from aqueous solution. So far, the adsorption of PPCPs in zeolites has often been investigated in case studies focusing on a small number of contaminants and one or a few zeolites. We present a computational screening approach to investigate the interaction of 53 PPCPs with 14 all-silica zeolites, aiming at a more comprehensive understanding of factors that are beneficial for a strong host-guest interaction and thus an efficient adsorption. The systems are modelled on the classical force field level of theory, allowing for the efficient computational treatment of a large number of PPCP-zeolite combinations and evaluated in terms of the interaction energy between PPCP and zeolite framework. For selected PPCP-zeolite combinations additional Free Energy Perturbation simulations are employed to compute Free Energies of Transfer between the aqueous phase and the adsorbed state. These results can serve as a starting point for experimental studies of relevant PPCP-zeolite combination or more in-depth theoretical investigations.

Hydrophobic nanosheet silicalite-1 zeolite for iodine and methyl iodide capture

Effective capture of radioactive iodine from nuclear fuel reprocessing is of great importance for public safety as well as the secure utility of nuclear energy. In this work, a hydrophobic nanosheet silicalite-1 (NSL-1) zeolite with an adjustable size was developed for efficient iodine (I2) and methyl iodide (CH3I) adsorption. The optimized all-silica zeolite NSL-1 exhibits an excellent I2 uptake capacity of 553 mg/g within 45 min and a CH3I uptake capacity of 262 mg/g within 1 h. Benefiting from the reduced thickness and enhanced porosity, microporous NSL-1 possesses enhanced iodine adsorption capacity and fast adsorption kinetics, which is a considerable high value among inorganic materials. Unexpectedly, the remarkable characters of high hydrophobicity, acid-resistance and anti-oxidation endow it a higher iodine uptake capacity than traditional aluminosilicate zeolites. More importantly, the high uptake selectivity toward I2 possessed by NSL-1 owing to its hydrophobic skeleton under simulated dynamic conditions. The low cost, facile and scalable synthesis of NSL-1 further highlights great prospects for applications in the nuclear industry. This work provides useful insights for designing efficient adsorbents for iodine capture.

SSZ-13 membranes on novel silica carbide monoliths for efficient CO2 separation

The usage of normal alumina supports brings the uncertain and undesirable aluminum transformation to the high-silica zeolite membranes to reduce Si/Al ratio in framework. In this current study, a novel aluminum-free silica carbide (SiC) monolithic support is used for the synthesis of SSZ-13 membranes with enhanced Si/Al ratio for the first time. Moreover, this high-porosity and monolithic SiC support enables membrane to increase gas permeance and surface-to-volume ratio. High-quality SSZ-13 membranes were prepared after rational modification of synthesis parameters such as synthesis temperature and time and support material. The effects of test temperature, pressure drop, feed flow rate and humidity on separation performance of the monolithic membranes were studied. The formation mechanism of SSZ-13 membrane was also investigated. The Si/Al ratio and CO2 permeance of SSZ-13 membranes on SiC monolith were 100% and 60% higher than that of membrane on alumina monolith, respectively. This monolithic membrane had comparable separation performance but about 2.3 times higher surface-to-volume ratios to the reported tubular zeolite membranes. This solution could be suitable to prepare other high-silica and all-silica zeolite membranes.

Fundamental insights into the variety of factors that influence water/alcohol membrane permeation selectivity

The primary objective of this article is to develop a fundamental understanding of water/methanol and water/ethanol mixture permeation across microporous membrane constructs. Configurational-Bias Monte Carlo (CBMC) simulations were undertaken for water/methanol and water/ethanol mixture adsorption in all-silica zeolites (CHA, DDR, FAU, LTA) and ZIF-8. Additionally, Molecular Dynamics (MD) simulations were used to determine the intra-crystalline diffusivities in water/alcohol mixtures. The combination of CBMC and MD simulations allow the calculation of membrane permeation selectivities. This study provides insights into the influence of the structural properties of the microporous layer on the permeation selectivity. Another key result that emerges is that the water/alcohol permeation selectivity becomes increasingly in favor of water as the feed mixtures becomes richer in alcohol. The reason for the dependence can be traced to strong hydrogen bonding between water and alcohol molecular pairs resulting in cluster formation and enhanced water ingress; these results provide a rationalization of a number of membrane permeation studies in the literature. A corollary to the reported results is that the use of the Ideal Adsorbed Solution Theory (IAST) is unable to provide a quantitative description of mixture adsorption equilibrium and thermodynamic non-idealities need to be accounted for in modelling membrane permeation of water/alcohol mixtures.

Thermal Conductivity of a Fluid-Filled Nanoporous Material: Underlying Molecular Mechanisms and the Rattle Effect.

Nanoporous materials are central to the energy and environmental crisis, with key applications in adsorption, separation, and catalysis. While confinement and surface effects on fluids severely confined in their porosity are well documented, the thermal behavior of nanoporous solids subjected to fluid adsorption remains puzzling in many aspects. With striking phenomena such as the so-called rattle effect, through which fluid/solid collisions decrease the overall thermal conductivity, the solid thermal conductivity and, more generally, heat transfer and dispersion in these complex systems challenge classical approaches (e.g., mixing rules including effective medium approaches fail to capture such effects as shown here). In particular, a robust molecular framework to describe the crossover between the decrease in thermal conductivity through the rattle effect in very narrow pores and the increase in thermal conductivity when replacing vacuum with a fluid phase in larger pores is still missing. Here, using a prototypical model of fluid-filled nanoporous materials (a Lennard-Jones phase confined in an all-silica zeolite), we perform a molecular simulation study to shed light on the parameters that govern the rattle effect in nanoporous solids. First, by varying the fluid/fluid, fluid/solid, and solid/solid interaction strengths as well as the fluid number density and mass density, we unravel the ingredients that lead to the essential coupling between fluid adsorption and phonon transport. Second, despite this complex interplay, inspired by pioneering molecular approaches on the rattle effect, we show that all data obey a simple statistical physics model that relies on the change in the speed of sound due to the fluid adsorbed density and the decrease in phonon lifetime due to scattering by fluid molecules. This framework, which provides a simple formalism to rationalize the thermal behavior of this class of solid/fluid composites, points to a decrease in thermal conductivity upon fluid confinement (up to 30% in some cases). Such an effect paves the way for the design of novel applications involving fluids in interaction with nanoporous materials.

Studying the adsorption of emerging organic contaminants in zeolites with dispersion-corrected density functional theory calculations: From numbers to recommendations.

Adsorption energies obtained from dispersion-corrected density functional theory (DFT) calculations show a considerable dependence on the choice of exchange-correlation functional and dispersion correction. A number of investigations have employed different approaches to compute adsorption energies of small molecules in zeolites, using reference values from high-level calculations and/or experiments. Such comparative studies are lacking for larger functional organic molecules such as pharmaceuticals or personal care products, despite their potential relevance for applications, e. g., in contaminant removal or drug delivery. The present study aims to fill this gap by comparing adsorption energies and, for selected cases, equilibrium structures of emerging organic contaminants adsorbed in MOR- and FAU-type all-silica zeolites. A total of 13 dispersion-corrected DFT approaches are compared, including methods using a pairwise dispersion correction as well as non-local van der Waals density functionals. While absolute values of adsorption energies vary widely, qualitative trends across the set of zeolite-guest combinations are not strongly dependent on the choice of functional. For selected cluster models, DFT adsorption energies are compared to reference values from coupled cluster (DLPNO-CCSD(T)) calculations. Although all DFT approaches deliver systematically more negative adsorption energies than the coupled cluster reference, this tendency is least pronounced for the rev-vdW-DF2 functional.

Preparation of nano-MFI zeolites doped with Al/Ti and their performance in VOC sorption.

Highly crystalline nano-MFI-type zeolites containing different elements were synthesized, and the sorption effects of the elements on volatile organic compounds (VOCs) were investigated. The results showed that the optimal toluene and acetone breakthrough time of the synthesized zeolites was 2.1 and 1.9 times as long as that of the commercial zeolite, respectively. For a weakly polar toluene molecule, MFI zeolites (aluminum-free) showed better adsorption properties than aluminum-containing zeolites. For the highly polar acetone molecule, zeolites with a Si/Al ratio of 87 showed the highest adsorption capacity, which was 7% higher than that of the all-silica zeolite and 1.4 times that of the commercial zeolite. Furthermore, MFI zeolites with Ti replacing part of Al proved to have better performance for highly polar molecules. In the adsorption process of VOCs, in addition to internal diffusion, diffusion on the external surface of the zeolite also played a remarkable role, and the adsorption data of all samples fitted better with the pseudo-first-order model. This study may provide a reliable structure-performance relationship for the synthesis of nanosized zeolite-based adsorbents and their use in the industrial recovery/treatment of VOCs.

Adsorption of PFAS by All-Silica Zeolite β: Insights into the Effect of the Water Matrix, Regeneration of the Material, and Continuous PFAS Adsorption.

Per- and polyfluoroalkyl substances (PFAS) are man-made organic compounds ubiquitously present in the environment. Due to their persistency and bioaccumulative nature, and because of increasingly stringent regulations of PFAS, their removal from the environment is necessary. Our initial study identified all-silica zeolite β as an alternative adsorbent with a high selectivity, affinity, and capacity for perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) removal. Here, we study the influence of the PFAS chain length on the affinity and capacity of a novel material, all-silica zeolite β, showing that the *BEA zeolite is an ideal sorbent for the removal of PFAS with 8 carbons. The solution pH and the addition of cations or natural organic substances to the water matrix have minimal influence on PFOA/PFOS removal with the zeolite. Next, regeneration of a PFOS-loaded zeolite was assessed; besides thermal, solvent-driven regeneration of the zeolite is also possible, using well-selected combinations of non-noxious solvents. Lastly, continuous adsorption experiments show that zeolite can be used for larger-scale applications.

Bridging the Gap between Zeolites and Dense Silica Polymorphs: Formation of All-Silica Zeolite with High Framework Density from Natural Layered Silicate Magadiite.

A silica zeolite (RWZ-1) with a very high framework density (FD) was synthesized from highly crystalline natural layered silicate magadiite, bridging the gap between the two research areas of zeolites and dense silica polymorphs. Magadiite was topotactically converted into a 3D framework through two-step heat treatment. The resulting structure had a 1D micropore system of channel-like cavities with an FD of 22.1 Si atoms/1000 Å3 . This value is higher than those of all other silica zeolites reported so far, approaching those of silica polymorphs (tridymite (22.6) and α-quartz (26.5)). RWZ-1 is a slight negative thermal expansion material with thermal properties approaching those of dense silica polymorphs. It contributes to the creation of a new field on microporous high-density silica/silicates. Synergistic interactions are expected between the micropores with molecular sieving properties and the dense layer-like building units with different topologies which provide thermal and mechanical stabilities.

The role of surface thermodynamics and kinetics in the removal of PFOA from aqueous solutions

Perfluorooctanoic acid (PFOA) has been extensively used as surfactant in industrial applications. Human exposure to PFOA through contaminated water has been linked to serious adverse health effects. In this work, the removal of PFOA from water in all-silica zeolites, which are hydrophobic materials with diverse pore geometries and exceptional hydrothermal stability, is studied. Molecular scale structure, dynamics, kinetics, and free energy landscapes associated with PFOA adsorption are characterized. Interfacial adsorption constitutes the rate limiting step and the adsorption of PFOA is orientation competitive. The PFOA orientation where the hydrophobic perfluorinated methyl group is adsorbed first on the zeolite surface is thermodynamically favored; whereas the adsorption kinetics is faster when the hydrophilic carboxyl group is adsorbed first. Furthermore, the adsorption of PFOA in deprotonated state in hydrophobic pores is thermodynamically prohibitive. Based on computed permeabilities in the pores and kinetic rates associated with the adsorption of PFOA from water, three zeolites, MTW, VET and GON, are estimated to exhibit several orders of magnitude better PFOA removal performance compared to the benchmark material, zeolite Beta (BEA).

Structure-property relationships that influence platinum stability in all-silica or highly siliceous zeolites

In this study we examine the role of alkali cations and siloxy/silanol nests in stabilizing platinum clusters on all-silica zeolites. This work complements a recent study by the Corma group at Instituto de Tecnología Química (ITQ) that investigated platinum stability within Pt–K/MFI zeolites prepared by one-pot methods [1]. Here we demonstrate that stable platinum particles are likewise produced either by impregnating platinum onto an alkali-cation-containing all-silica MFI or by deboronating a borosilicate to create silanol nests and then co-impregnating with potassium and platinum sources. In this context, stability is assessed by resistance to sintering as measured by hydrogen chemisorption. Platinum impregnation alone onto a deboronated or borosilicate MFI does not produce stable platinum clusters after reduction. Furthermore, a one-pot synthesis of a Pt–K/B-MFI (borosilicate) or exchange of Pt onto a K-containing borosilicate does not lead to stable Pt particles. This further demonstrates the necessity of internal silanol/siloxy nests in stabilizing platinum clusters. In summary, silanols and alkali cations together create silanol/siloxy nests that stabilize Pt clusters. Hence, stable Pt clusters can be achieved by 1) a one-pot synthesis of the all-silica zeolite in the presence of Pt and alkali cations, 2) platinum impregnation on an all-silica sample that has occluded a high enough concentration of an alkali cation during the synthesis, or 3) by co-impregnating platinum and alkali metal cations together onto a calcined zeolite that has a high density of internal silanol nests. These nests can be generated by removal of boron from the framework of the zeolite. In the current study, we have examined these systems by 11B and 29Si NMR spectroscopy.

Revealing the structure–property relationship of all-silica zeolites for the carbon dioxide capture: a high throughput screening study

Abstract The structural parameters and performance evaluation metrics of 242 promising all-silica zeolites to rank and screen out the top candidates for CO2/N2 separation are obtained by using molecular simulations combined with high-throughput calculations. By setting the pore size range of the material, three top zeolites with the highest adsorption selectivity for CO2 molecules are screened to further investigate the adsorption and separation for CO2 and N2 molecules. The GCMC method was used to calculate the adsorption isotherm, heat of adsorption, adsorption selectivity and centroid distribution density under different feed ratios of CO2/N2 mixed components. The zeolite exhibits a high CO2 adsorption capacity with the maximum pore size of 4–8 Å, the surface area of 1400 m2/g and the pore volume of 0.2–0.4 cm3/g, and best adsorption selectivity for CO2 at the pore size of 4 Å and pore volume of 0.2 cm3/g in the CO2/N2 mixed systems. Zeolite has higher adsorption selectivity for CO2 with low amount of CO2, which is favorable for the removal of a small amount of CO2 molecules. The geometrical structure of the material plays an important role for the adsorption capacity and spatial distribution of the adsorbate molecules in all-silicon zeolites.

Removal of Low Trace ppb-Level Perfluorooctanesulfonic Acid (PFOS) with ZIF-8 Coatings Involving Adsorbent Degradation.

For the first time, low trace-level removal of perfluorooctanesulfonic acid (PFOS), i.e., 20-500 μg/L (ppb), from aqueous solutions using zeolitic imidazolate framework-8 (ZIF-8)-coated copper sheet (ZIF-8@Cu) composite is reported here. In comparison with different commercial activated carbon (AC) and all-silica zeolites, the composite showed the highest removal rate of 98%, which remained consistent over a wide range of concentrations. Additionally, no adsorbent leaching from the composite was noticed, which eradicated pre-analysis steps such as filtration and centrifugation, unless needed for other adsorbents studied here. The composite displayed fast uptake with saturation reaching within 4 h, irrespective of the initial concentration. However, the morphological and structural characterization revealed surface degradation of ZIF-8 crystals, along with a decline in the crystal size. The adsorption of PFOS on ZIF-8 crystals was linked to chemisorption, as the surface degradation surges with an increase in PFOS concentration or with cyclic exposure at low concentrations. Methanol seemingly removed surface debris (partially), thus providing access to ZIF-8 beneath the surface debris. Overall, the findings demonstrate that at low trace ppb-level PFOS concentrations ZIF-8 can be considered as a possible candidate for PFOS removal, even though it suffers slow surface degradation, it also removes efficiently PFOS molecules from aqueous solutions.

Adsorption of Carbamazepine in All-Silica Zeolites Studied with Density Functional Theory Calculations.

The anticonvulsant drug carbamazepine (-) is an emerging contaminant of considerable concern due to its hazard potential and environmental persistence. Previous experimental studies proposed hydrophobic zeolites as promising adsorbents for the removal of carbamazepine from water, but only a few framework types were considered in those investigations. In the present work, electronic structure calculations based on dispersion-corrected density functional theory (DFT) were used to study the adsorption of CBZ in eleven all-silica zeolites having different pore sizes and connectivities of the pore system (AFI, ATS, BEA, CFI, DON, FAU, IFR, ISV, MOR, SFH, SSF framework types). It was found that some zeolites with one-dimensional channels formed by twelve-membered rings (IFR, AFI) exhibit the highest affinity towards CBZ. A "good fit" of CBZ into the zeolite pores that maximizes dispersion interactions was identified as the dominant factor determining the interaction strength. Further calculations addressed the role of temperature (for selected systems) and of guest-guest interactions between coadsorbed CBZ molecules. In addition to predicting zeolite frameworks of particular interest as materials for selective CBZ removal, the calculations presented here also contribute to the atomic-level understanding of the interaction of functional organic molecules with all-silica zeolites.

Zirconia-supported all-silica zeolite CHA membrane with unprecedentedly high selectivity in humidified CO2/CH4 mixture

Continuous and thin all-silica zeolite CHA membranes were prepared successfully on the tubular zirconia/alumina supports by a novel fluoride-free synthesis approach using lithium hydroxide as mineralizer. Membrane synthesis using the uniform and low-viscosity gel displayed good reproductivity. The influences of pressure and temperature on separation data of the membrane in the dry and wet mixtures were comprehensively investigated. The typical membrane had CO2/CH4 and CO2/N2 selectivities of 170 and 33 and carbon dioxide permeances of 950 × 10−9 and 1200 × 10−9 mol/(m2 s Pa) [∼3600 GPU, Gas Permeation Unit: 1GPU = 3.348 × 10−10 mol/(m2 s Pa)] at 298 K in dry CO2/CH4 and CO2/N2 (50/50 in mol) mixtures, respectively. The unprecedentedly high CO2/CH4 selectivity of 430 was obtained for the zeolite membrane at 298 K in wet CO2/CH4 mixture with saturated water vapor. The long-term stability of this membrane was also investigated in humidified mixture with relative humidity of close to 100%. The possible water-gated molecular sieve mechanism in the presence of moisture was proposed.

Structure-property relationships that influence platinum stability in all-silica or highly siliceous zeolites

In this study we examine the role of alkali cations and siloxy/silanol nests in stabilizing platinum clusters on all-silica zeolites. This work complements a recent study by the Corma group at Instituto de Tecnología Química (ITQ) that investigated platinum stability within Pt–K/MFI zeolites prepared by one-pot methods [1]. Here we demonstrate that stable platinum particles are likewise produced either by impregnating platinum onto an alkali-cation-containing all-silica MFI or by deboronating a borosilicate to create silanol nests and then co-impregnating with potassium and platinum sources. In this context, stability is assessed by resistance to sintering as measured by hydrogen chemisorption. Platinum impregnation alone onto a deboronated or borosilicate MFI does not produce stable platinum clusters after reduction. Furthermore, a one-pot synthesis of a Pt–K/B-MFI (borosilicate) or exchange of Pt onto a K-containing borosilicate does not lead to stable Pt particles. This further demonstrates the necessity of internal silanol/siloxy nests in stabilizing platinum clusters. In summary, silanols and alkali cations together create silanol/siloxy nests that stabilize Pt clusters. Hence, stable Pt clusters can be achieved by 1) a one-pot synthesis of the all-silica zeolite in the presence of Pt and alkali cations, 2) platinum impregnation on an all-silica sample that has occluded a high enough concentration of an alkali cation during the synthesis, or 3) by co-impregnating platinum and alkali metal cations together onto a calcined zeolite that has a high density of internal silanol nests. These nests can be generated by removal of boron from the framework of the zeolite. In the current study, we have examined these systems by 11B and 29Si NMR spectroscopy.

In Silico Screening of Metal-Organic Frameworks and Zeolites for He/N2 Separation.

In silico screening of 10,143 metal-organic frameworks (MOFs) and 218 all-silica zeolites for adsorption-based and membrane-based He and N2 separation was performed. As a result of geometry-based prescreening, structures having zero accessible surface area (ASA) and pore limiting diameter (PLD) less than 3.75 Å were eliminated. So, both gases can be adsorbed and pass-through MOF and zeolite pores. The Grand canonical Monte Carlo (GCMC) and equilibrium molecular dynamics (EMD) methods were used to estimate the Henry's constants and self-diffusion coefficients at infinite dilution conditions, as well as the adsorption capacity of an equimolar mixture of helium and nitrogen at various pressures. Based on the obtained results, adsorption, diffusion and membrane selectivities as well as membrane permeabilities were calculated. The separation potential of zeolites and MOFs was evaluated in the vacuum and pressure swing adsorption processes. In the case of membrane-based separation, we focused on the screening of nitrogen-selective membranes. MOFs were demonstrated to be more efficient than zeolites for both adsorption-based and membrane-based separation. The analysis of structure-performance relationships for using these materials for adsorption-based and membrane-based separation of He and N2 made it possible to determine the ranges of structural parameters, such as pore-limiting diameter, largest cavity diameter, surface area, porosity, accessible surface area and pore volume corresponding to the most promising MOFs for each separation model discussed in this study. The top 10 most promising MOFs were determined for membrane-based, vacuum swing adsorption and pressure swing adsorption separation methods. The effect of the electrostatic interaction between the quadrupole moment of nitrogen molecules and MOF atoms on the main adsorption and diffusion characteristics was studied. The obtained results can be used as a guide for selection of frameworks for He/N2 separation.

The Synthesis of Different Series of Cobalt BEA Zeolite Catalysts by Post-Synthesis Methods and Their Characterization

Three series of zeolite catalysts Co all-silica and Co Al-containing zeolites beta were prepared for use in the selective oxidative dehydrogenation of propane to propylene. Two series of zeolite catalysts Co all-silica were prepared by a two-step postsynthesis method at pH = 2.5 and pH = 3.0–9.0, respectively, which allows the incorporation of cobalt into SiBEA zeolite in the form of isolated framework pseudo-tetrahedral Co(II) species. The incorporation of Co ions into vacant T-atom sites and their reaction with silanol groups were demonstrated by NMR and FTIR methods. The generation of Lewis acid sites without the formation of Brønsted sites was proved by FTIR using pyridine and CO as probe molecules. The state of cobalt in three series of prepared and calcined zeolite catalysts was characterized by DR UV-vis. This technique allowed to show that for low Co content (<2 wt.%) cobalt is present in the form of framework pseudo-tetrahedral Co(II) species. For higher Co content (>2 wt.%), both framework pseudo-tetrahedral and extra-framework octahedral Co(II) species are present. The Co Al-containing zeolite beta series prepared on non-dealuminated support shows the presence of extra-framework octahedral Co(II) only.