Extra-framework Cations Research Articles

Extra-framework cations promoted zeolite-encaged cobalt catalysts towards efficient ethylbenzene dehydrogenation

The transformation of ethylbenzene to styrene through direct dehydrogenation is essential process in chemical industry, indispensable for the synthesis of polymers and plastics. Herein, faujasite zeolite encaged cobalt catalyst with potassium ion as the extraframework cations, namely K-Co@Y, is reported to be a robust catalyst for ethylbenzene direct dehydrogenation, showing ethylbenzene conversion of ∼45 % and styrene selectivity of ∼95 % simultaneously under employed conditions. The superior performance of K-Co@Y in comparison with Na-Co@Y is attributed to the effective modulation of zeolite-encaged Co2+ species by extraframework K+ ions, which increases the electron density of cobalt sites and facilitates CH bond activation. Although coke deposits cause gradual deactivation, the structural integrity of K-Co@Y can be well preserved during the dehydrogenation reaction and the performance can be full restored via a simple calcination regeneration step, making K-Co@Y an eligible catalyst for long-term running in the continuous regeneration mode. This study highlights the vital role of extraframework cations in zeolite-encaged single-site catalysts for target chemical transformations and opens up a new way to modulate the performance of zeolite catalysts.

Effect of temperature and extraframework cation type on CHA framework flexibility

The sorption properties of zeolites are controlled by several factors, i.e. Si/Al ratio of the aluminosilicate framework [AlSiO4]-, the type and position of the extraframework (EF) cations, and the applied temperature. Here we investigate the flexibility of CHA framework as a function of EF cation-content and temperature (20–350 °C). Two CHA forms (Na- and Cu-CHA) with Si/Al = 2 were analysed. The main objectives were: (i) to shed light on the HT behaviour of Na-CHA, for which contrasting results exist in literature; (ii) define the role of temperature and EF cation-type in the response to the heating stimuli. We show that at 75 °C, Na-CHA undergoes a severe contraction of the unit-cell volume (-12%) accompanied by a symmetry lowering (R-3m to I2/m). The transformation is reversible, if the dehydrated Na-CHA is exposed to ambient conditions. In contrast, Cu-CHA experiences a significant different dehydration path, which involves minor changes of the CHA framework, and a net positive thermal-expansion after dehydration. The implications of the observed transformations for gas separation processes are finally discussed.

A highly active and stable palladium zeolite catalyst for wet methane combustion

Natural gas engines are the most viable alternative to diesel and gasoline ones. However, the current state-of-art catalyst, palladium (Pd) on alumina, for eliminating unburnt methane from engine exhaust suffers from high light-off temperature (> 400 °C) and poor water tolerance. Here we systematically investigated the effects of zeolite structure and Si/Al ratio, catalyst calcination temperature and extra-framework cation type on catalytic performance of zeolite-supported Pd catalysts for wet methane combustion to overcome these limitations. We found that a 3.0 wt% Pd catalyst supported on Na+-post-exchanged 500 °C-calcined IWV zeolite with a Si/Al ratio of 45 has a light-off temperature as low as 290 °C for methane combustion in the presence of 10 % water vapor while maintaining ca. 85 % methane conversion at 330 °C for 100 h. This study provides a new direction for bringing supported Pd catalysts close to real-world applications.

Characterization and potential toxicity of asbestiform erionite from Gawler Downs, New Zealand

Abstract Erionite is the name for a zeolite mineral series originating from diagenesis or hydrothermal alteration of volcanic rocks. The particular erionite “species” is based on the dominant extra-framework cation, erionite-Ca, erionite-K, or erionite Na. Irrespective of the species, erionite can display a fibrous/asbestiform morphology and has been linked with cases of malignant mesothelioma, a disease typically associated with asbestos exposure. Characterization of new discoveries of erionite is therefore important to assess any potential exposure hazards. This study describes a new asbestiform erionite from vesicles within the Upper Cretaceous Mt. Somers Volcanics Group (MSVG), Canterbury, New Zealand. The erionite is within the Hinds River Dacite, the youngest unit within the MSVG at Gawler Downs, ~100 km west of Christchurch, in the foothills of the Southern Alps. A multi-analytical approach was taken to analyze the sample which included micro-Raman spectroscopy, thermogravimetric analysis, electron microscopy, electron microprobe analysis, and X-ray powder diffraction with the Rietveld method. Results confirmed the mineral as fibrous erionite-K. The chemical composition of the mineral is unique due to the presence of higher levels of Mg. While Fe was also identified, this was due to smectite flakes occurring on the surface of the erionite fibers. According to the World Health Organization (WHO) respirable mineral fiber definition, where length ≥5 μm, width ≤3 μm, and aspect ratio (L/w) ≥3:1, the Gawler Downs erionite fibers are respirable, while the fibers themselves exceed respirable thickness. In addition to morphology, a value for the potential toxicity model was computed to be 2.28 for the Gawler Downs erionite. This is similar to those of other carcinogenic erionites from Karain, Turkey (2.33), and Nevada, U.S.A. (2.28). Taken together, results indicate Gawler Downs erionite represents an environmental hazard. Nevertheless, further investigation is required to determine potential environmental exposure pathways by which erionite may become airborne and assess the actual environmental risk in the Gawler Downs area.

The Rôle of Iron in Zeolite Beta for deNOx Catalysis

Selective catalytic reduction with ammonia (NH3-SCR) is a widely used deNOx process, employing zeolitic catalysts. Here, we examine framework substituted and extra-framework exchanged transition metal zeolite catalysts, in the extensively studied Fe – zeolite Beta focusing on the influence of the transition metal cation on the NH3-SCR reaction, and aiming to unravel the underlying mechanisms and their impact on catalytic performance. We use hybrid multiscale density functional theory (DFT)/molecular mechanics to investigate the NH3-SCR at various Fe-BEA active sites including systems with both framework and extra-framework Fe cations. The catalytically active sites under investigation include Fe-FeF-BEA, Fe-AlF-BEA, and Cu-FeF-BEA, (where the subscript F indicates framework species) on which the formation and consumption of key intermediates of both oxidation and reduction half cycles species are analysed. We show the distinctive ability of framework Fe sites to promote the formation of key nitrate and nitrosamine intermediates. Furthermore, we find a strong binding affinity of NH3 with framework Fe systems including Cu(II)-FeF-BEA, Fe(III)-FeF-BEA, and H-FeF-BEA compared to framework Al systems (Fe(III)-AlF-BEA). We explore the reduction potential of the framework Fe3+/Fe2+ in BEA zeolite and establish its feasibility in the presence of a Brønsted acid site. Our calculations clearly predict the bimetallic Cu-FeF system to be the best catalyst which supports a recent experimental report by Yue et al. (Chem. Eng. J. 2020, 398, 125515) for a related FeCu-SSZ-13 (CHA) zeolite. We provide a new understanding of the reactivity of Fe-BEA zeolites, suggesting the advantages of framework Fe over Al sites in NH3-SCR reactions, and establishing a foundation for interpretation of the rôle of framework cations in metal-exchanged zeolites.

The evolution of yugawaralite structure at high pressure: A single-crystal X-ray diffraction study

The structural evolution of yugawaralite |Ca1.93Na0.08(H2O)8.1|[Al3.94Si12.06O32], space group Pc, a = 6.72309(15), b = 13.9990(2), c = 10.0477(2) Å, β = 111.164(2)°, V = 881.87(3) Å3, compressed in penetrating (ethanol:water 4:1) and non-penetrating (paraffin oil) media, was studied up to 3.2 GPa. Under the compression in a penetrating medium, yugawaralite experiences pressure-induced hydration. Additional H2O molecules occupy two positions which do not enter the coordination of the extra-framework cations: one of them is partially filled under ambient conditions, and the second is initially vacant. At around 2 GPa, yugawaralite experiences a phase transition with doubling of the a- and b-parameters of the unit cell. At high pressure the diffraction frames contain sharply broadened low-intensity diffuse reflections corresponding to a multiple increase of the unit cell metric. The structure of the high-pressure phase solved in the sub-cell, corresponding to the initial metric, is characterized by the disordering of the framework sub-chains in which the SiO4-tetrahedra become alternatively oriented. The pressure-induced structural changes are completely reversible. The compression of yugawaralite in a non-penetrating medium does not lead to radical structural changes. The structure symmetry is preserved; there are no signs of diffuse scattering or the splitting of atomic positions.

Mechanistic Insights into Cs-Ion Exchange in the Zeolite Chabazite from In Situ Powder X-Ray Diffraction.

Zeolites contain extraframework cations that are exchangeable under favorable aqueous conditions; this is the fundamental feature for their application in water purification and necessary to produce cation forms for other applications such as catalysis. Optimization of the process is common, but there is little fundamental understanding based on real-time experiments of the mechanism of exchange for most zeolites. The sodium and potassium forms of zeolite chabazite selectively uptake Cs+ by ion exchange, leading to its application in removing radioactive 137Cs+ from industrial nuclear waste streams, as well as from contaminated environments in the aftermath of the Fukushima and Three Mile Island accidents. In this study, in situ synchrotron powder X-ray diffraction patterns have been collected on chabazite as it undergoes Cs-ion exchange. Applying Rietveld refinement to these patterns has revealed the time-resolved structural changes that occur in the zeolite as exchange progresses, charting the changes in the spatial distribution of the extraframework cations and water molecules in the structure during the reaction. Ultimately, a detailed mechanistic understanding of how this dynamic ion-exchange reaction occurs has been obtained.

Investigation of Anionic Metal–Organic Frameworks with Extra-Framework Cations for Room Temperature Hydrogen Storage

Investigation of Anionic Metal–Organic Frameworks with Extra-Framework Cations for Room Temperature Hydrogen Storage

Molecular sieve 13X activated zeolite for CO2 filter in air purifier

Indoor air quality is crucial for health. Room with high CO2 concentration can cause sick building syndrome such as dizziness, nausea, fatigue, and breathing difficulties. In addition to the air circulation, air filtering can reduce CO2 and improve the indoor air quality. The integration of activated zeolite into conventional air cleaning filters, which lack specific CO2 gas selectivity, presents a promising solution for CO2 reduction. Possessing cage-like porous structures and extra framework cations, activated zeolites have been known as gas adsorbers via physisorption and chemisorption mechanism. Activated zeolites have been reported to have high selectivity to CO2 gas via electrostatic interaction and trapping in small pores. In this study, we integrated the molecular sieve13X activated zeolite (MSAZ) with typical pore size around 1 nm to the conventional filters in air purifier which does not have specific selectivity to CO2 gas. By varying the MSAZ masses, we demonstrated that the MSAZ addition effectively reduce around 147 up to 316 ppm CO2 depending on the MSAZ mass and the air purifier fan speed. This value is extremely higher compared to 4 ppm CO2 reduction by commercial carbon filter without MSAZ. Larger mass and faster fan speed resulted in higher CO2 reduction. This study proves the prospective application of the MSAZ for CO2 reduction, to improve the indoor air quality and to avoid adverse effects of CO2 exposure.

Solid-state NMR study of the stability of MOR framework aluminum

MOR zeolite has been effectively utilized for dimethyl ether (DME) carbonylation reaction due to its unique pore structure and acidity. During industrial production, the transformation of ammonium type MOR zeolite (NH4-MOR) into proton type MOR zeolite (H-MOR) causes inevitable dealumination. Therefore, understanding the influencing factors and dynamic evolution mechanism of zeolite dealumination is crucial. In this work, the stability of framework aluminum was studied by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, 29Si, 27Al, 1H magic angle spinning nuclear magnetic resonance (MAS NMR), and DME carbonylation performance evaluation. These results indicate that extra-framework cation Na+ and NH4+ could better preserve the aluminum structure of the MOR zeolite framework compared to H+, primarily due to the different ‘attraction’ of the framework to water. Furthermore, the impact of water on the zeolite framework aluminum at high temperature was studied by manipulating the humidity of the calcination atmosphere, revealing the formation of extra-framework six-coordinated aluminum (Al(VI)-EF) and the mechanism of water influence on the zeolite framework aluminum.

Incorporating solvent effects in DFT: insights from cation exchange in faujasites.

Zeolites are versatile materials renowned for their extra-framework cation exchange capabilities, with applications spanning diverse fields, including nuclear waste treatment. While detailed experimental characterization offers valuable insight, density functional theory (DFT) proves particularly adept at investigating ion exchange in zeolites, owing to its atomic and electronic resolution. However, the prevalent occurrence of zeolitic ion exchange in aqueous environments poses a challenge to conventional DFT modeling, traditionally conducted in a vacuum. This study seeks to enhance zeolite modeling by systematically evaluating predictive differences across varying degrees of aqueous solvent inclusion. Specifically focusing on monovalent cation exchange in Na-X zeolites, we explore diverse modeling approaches. These range from simple dehydrated systems (representing bare reference states in vacuum) to more sophisticated models that incorporate aqueous solvent effects through explicit water molecules and/or a dielectric medium. Through comparative analysis of DFT and semi-empirical DFT approaches, along with their validation against experimental results, our findings underscore the necessity to concurrently consider explicit and implicit solvent effects for accurate prediction of zeolitic ionic exchange.

The surface characteristics of natural heulandites/clinoptilolites with different extra-framework cations

Abstract Natural tuff samples in western Anatolia (Türkiye) originating from Miocene rhyolitic–pyroclastic rocks with >80 wt.% heulandite/clinoptilolite zeolites were investigated for their surface characteristics determined according to nitrogen adsorption after degassing at 150°C (specific surface area, pore volume and pore diameter). Additionally, these surface characteristics were correlated with the cationic compositions of the heulandite/clinoptilolite group minerals. The examined samples were characterized by two main pore diameters that were not related to specific surface area and pore volume but were partially related to the types and occupancy of extra-framework cations. One set of samples has a pore diameter of ~24 Å, total cation content (Na + K + Ca + Mg) ranging from 3.46 to 4.40 and a (Na + K)/(Ca + Mg) ratio ranging from 0.34 to 0.92. The total cation contents and (Na + K)/(Ca + Mg) ratios of the remaining samples with a pore diameter of ~37 Å are 4.30–5.08 and 1.48–2.85, respectively. After degassing at 300°C, there is a slight difference in the pore diameters of these two sets of samples (~37 and 38 Å). The pore sizes of the samples with a (Na + K)/(Ca + Mg) ratio < 1 (heulandite composition) increased from 24 to 36–38 Å with increasing degassing temperature, whereas the pore sizes of the samples with a (Na + K)/(Ca + Mg) ratio > 1 (clinoptilolite composition) increased from 37 to only 38–39 Å. However, there is no correlation between the Si/Al ratios and the cation-exchange capacities of the samples and their surface characteristics obtained by degassing at the two temperatures.

Order-disorder transition and thermal behavior of paracelsian-type MGa2Ge2O8 (M = Sr, Ba) compounds

Two Ga–Ge disordered feldspar-related compounds MGa2Ge2O8 (M = Sr, Ba) with paracelsian topology were synthesized for the first time using the melt crystallization methods (1300–1400 °C) and rapid cooling. Their stability was studied under high-temperature conditions using in situ single-crystal (up to 1000 °C) and powder (up to 1050 °C) X-ray diffraction. Both compounds are stable in the studied temperature range. The refinement of the crystal structure of BaGa2Ge2O8 at different temperatures does not demonstrate any ordering processes. The thermal expansion of both compounds has anisotropic character (αmax/αmin = 3.7 and 3.9 for SrGa2Ge2O8 and BaGa2Ge2O8, respectively), wherein the maximum and minimum expansion is observed within the bc plane. The anisotropy degree of MGa2Ge2O8 increases as the size of extraframework cation increasing. Simultaneously this process leads to the decreasing of MGa2Ge2O8 stability range. The influence of the crystallization procedure on order-disorder process of MGa2Ge2O8 is discussed.

Unusual large pore copper silicate for CO2 adsorption

The variable ratio of tetrahedrally coordinated atoms with different valences in the zeolite framework determines the amount and distribution of extraframework cations, thereby regulating properties such as adsorption, separation, and catalysis. However, similar control is not possible for stoichiometric microporous mixed coordination polyhedra silicates. Here we identify synthesis conditions that reduce or completely eliminate the alkali cations in the 12-ring channels of a copper silicate while preserving the original framework stoichiometry that enables CO2 adsorption. Furthermore, despite the absence of cation positions, the emptied channels can undergo ion exchange and accommodate substantial amounts of Cs+ and Sr2+ ions. These results introduce a pioneering example of cation-free large pores in transition metal silicates and demonstrate how the same framework heteropolyhedral silicates can exhibit significantly distinct pore composition and adsorption properties controlled by the synthesis.

Modification of zeolite Na-X, sodalite and cancrinite in salt melts

The possibility of occlusion in melts of useful components of some of the most economically advantageous materials, obtained during zeolitization of fly ash – Na-X, sodalite and cancrinite, was investigated. A very high degree of occlusion of ZnCl2 in zeolite Na-X was achieved along with complete exchange of Na extra framework cations with Zn2+. By elution, it was proved that, in the interaction of zeolite Na-X with ZnCl2 melt, occlusion takes place without destroying the zeolite aluminum-silicate framework. In a KNO3 melt, in the narrowest pore structure – the sodalite structure, occlusion does not proceed, only ion exchange occurs. Through computer modeling, the ion exchange of both the charge-balancing cations and the synthetically incorporated cation-anion pairs has been proven. The interaction of cancrinite-SO4 with a KNO3 melt results in the formation of a material with an altered cationic composition and occluded KNO3, which gradually comes out of the structure upon elution. The results obtained in the work can be a basis for the development of future applications of the modified minerals as materials with antibacterial properties (occluded ZnCl2 in the structure of zeolite Na-X) and as slow-acting fertilizers or micro-fertilizers (occluded with KNО3 cancrinite and sodalite forms).

Effect of mono- and divalent extra-framework cations on the structure and the internal surface area of chabazite zeolites

Effect of mono- and divalent extra-framework cations on the structure and the internal surface area of chabazite zeolites

Illuminating the Black Box: A Perspective on Zeolite Crystallization in Inorganic Media.

ConspectusSince the discovery of synthetic zeolites in the 1940s and their implementation in major industrial processes involving adsorption, catalytic conversion, and ion exchange, material scientists have targeted the rational design of zeolites: controlling synthesis to crystallize zeolites with predetermined properties. Decades later, the fundamentals of zeolite synthesis remain largely obscured in a black box, rendering rational design elusive. A major prerequisite to rational zeolite design is to fully understand, and control, the elementary processes governing zeolite nucleation, growth, and stability. The molecular-level investigation of these processes has been severely hindered by the complex multiphasic media in which aluminosilicate zeolites are typically synthesized. This Account documents our recent progress in crystallizing zeolites from synthesis media based on hydrated silicate ionic liquids (HSIL), a synthesis approach facilitating the evaluation of the individual impacts of synthesis parameters such as cation type, water content, and alkalinity on zeolite nucleation, growth, and phase selection. HSIL-based synthesis allows straightforward elucidation of the relationship between the characteristics of the synthesis medium and the properties and structure of the crystalline product. This assists in deriving new insights in zeolite crystallization in an inorganic aluminosilicate system, thus improving the conceptual understanding of nucleation and growth in the context of inorganic zeolite synthesis in general. This Account describes in detail what hydrated silicate ionic liquids are, how they form, and how they assist in improving our understanding of zeolite genesis on a molecular level. It describes the development of ternary phase diagrams for inorganic aluminosilicate zeolites via a systematic screening of synthesis compositions. By evaluating obtained crystal structure properties such as framework composition, topology, and extraframework cation distributions, critical questions are dealt with: Which synthesis variables govern the aluminum content of crystallizing zeolites? How does the aluminum content in the framework determine the expression of different topologies? The crucial role of the alkali cation, taking center stage in all aspects of crystallization, phase selection, and, by extension, transformation is also discussed. New criteria and models for phase selection are proposed, assisting in overcoming the need for excessive trial and error in the development of future synthesis protocols.Recent progress in the development of a toolbox enabling liquid-state characterization of these precursor media has been outlined, setting the stage for the routine monitoring of zeolite crystallization in real time. Current endeavors on and future needs for the in situ investigation of zeolite crystallization are highlighted. Finally, experimentally accessible parameters providing opportunities for modeling zeolite nucleation and growth are identified. Overall, this work provides a perspective toward future developments, identifying research areas ripe for investigation and discovery.

A multi-functionalized MOF with unique molecular-sized pockets for excellent lead(II) removal and selective separation of C3H8/CH4

Design and synthesis of metal-organic frameworks (MOFs) with functional pore environments are of critical significance for the development of the porous adsorbents to solve energy and environmental crises. Herein, we utilized the structural and coordinated characteristics of adenine (HAD) ligands to develop a multi-functional MOF material (JLU-MOF68) with unique molecular-sized pockets for gas separation and Pb2+ removal. Benefiting from the internal surface environment and the size of 4.6–5.9 Å of the molecular pockets, JLU-MOF68 exhibited ultrahigh C3H8/CH4 selectivity coupled with the relatively high adsorption capacity of C3H8. Both N sites and extraframework organic cations were conducive to the adsorption of Pb2+ through weak chemical coordination and ion exchange, respectively. The activated JLU-MOF68 showed not only an ultrahigh Pb2+ adsorption capacity of 805 mg g−1, but also exceptionally high Pb2+ removal efficiency under the condition of different pH and coexisting ions. Moreover, the adsorbent can be reused at least five times, and the removal efficiency remains almost constant after five cycles. Therefore, JLU-MOF68 had a wide potential application for the C3H8/CH4 selective separation and capture of Pb2+.

Insights into adsorption of various gases on extra-framework cations of zeolite: A dispersion corrected DFT study on zeolite cluster models with Li[formula omitted], Na[formula omitted] and K[formula omitted] charge compensating ions

Design of an economical and sustainable gas separation material is relevant in several industrial processes. Zeolites with tunable pore sizes are ideal molecular sieves of many gases. The adsorption centers of these molecular sieves are extra-framework Lewis acid centers. In this study, we attempt to delineate the electronic properties of such centers (Li+, Na+ and K+) and their sorption properties towards N2, O2, CO, CO2 and H2. Negative framework of zeolites are modeled using different cluster models that present distinct electronic environment and role of this environment on the Lewis acidity of the cation. The sorption property towards different gases is evaluated using dispersion corrected DFT studies. The results obtained are benchmarked for one of the studied model using CCSD calculations. The results indicate that while the local environment modulates the adsorption properties, the relative adsorption properties between different ions follow the same order irrespective of the type of negative framework modeled. This reveals that intrinsic atomic properties of the charge compensating cations drive the sorption properties of the zeolites. Adsorption energies compounded with the analysis of IR stretching frequencies of the adsorbed gases reveals that Li centers shows molecular adsorption (charge donation to the cationic centers) towards N2 as compared to O2 and towards CO2/CO as compared to H2, demonstrating the applicability of Li-Zeolites as ideal membranes for oxygen concentrators and syngas separation. These adsorption studies are ratified by the BOMD simulations at 300 K, where H2 and O2 desorbs while N2, CO and CO2 remains adsorbed to the cationic site.

Interplay between alkali-metal cations and silanol sites in nanosized CHA zeolite and implications for CO2 adsorption

Silanols are key players in the application performance of zeolites, yet, their localization and hydrogen bonding strength need more studies. The effects of post-synthetic ion exchange on nanosized chabazite (CHA), focusing on the formation of silanols, were studied. The significant alteration of the silanols of the chabazite nanozeolite upon ion exchange and their effect on the CO2 adsorption capacity was revealed by solid-state nuclear magnetic resonance (NMR), Fourier-transform infrared (FTIR) spectroscopy, and periodic density functional theory (DFT) calculations. Both theoretical and experimental results revealed changing the ratio of extra-framework cations in CHA zeolites changes the population of silanols; decreasing the Cs+/K+ ratio creates more silanols. Upon adsorption of CO2, the distribution and strength of the silanols also changed with increased hydrogen bonding, thus revealing an interaction of silanols with CO2 molecules. To the best of our knowledge, this is the first evidence of the interplay between alkali-metal cations and silanols in nanosized CHA.