Cations In Zeolites Research Articles

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.

Hydrogenation of phenol to cyclohexanone catalyzed by isolated Pd cations in the micropores of zeolite

Cyclohexanone is a key raw material for manufacturing nylon and other chemicals. The sustainable conversion of biomass-derived phenol to cyclohexanone meets the requirements of sustainable development goal but still be challenging. Herein, a catalyst containing isolated Pd cations in zeolite, namely Pd1/ZSM-5, is investigated for the partial hydrogenation of phenol to cyclohexanone. Under 140 °C and 1.5 MPa of initial hydrogen pressure, the conversion of phenol reaches 95.6% with the selectivity to cyclohexanone of 99.9%. Besides, the turnover rate (TOR) of partial hydrogenation of phenol reaches 382.7 h−1. The mild adsorption affinity of cyclohexanone on catalytic sites leads to the high selectivity. Moreover, as unveiled by surface reactivity of phenol monitored by in-situ infrared spectrum, the oxygen atom of phenol hydroxyl group is adsorbed on the Brönsted acid site of Pd1/ZSM-5 as the key reaction step. The reaction intermediate involves the partial hydrogenation of benzene ring of phenol.

Thermodynamics and kinetics of interconversion between platinum nanoparticles and cations in zeolites

In metal-containing zeolites, sintering and redispersion processes exercise control over the identity of metal site structures, but the thermodynamic and kinetic factors that influence these molecular-level processes are not completely understood. Here, we assess the ability of first principles informed free energy models (for supported and unsupported nanoparticles) and kinetic models integrating Ostwald ripening with atom trapping to describe the interconversion between Pt cations and nanoparticles encapsulated in chabazite (CHA) zeolites. Density functional theory-derived thermodynamic phase diagrams show that the interconversion between cations, favored in oxidizing environments, and particles, favored in reducing environments, is fully reversible within a wide range of their respective conditions (temperatures and pressures) for CHA and several other zeolite topologies. Kinetic Monte Carlo simulations of Pt redispersion are consistent with experimentally observed redispersion kinetics of encapsulated Pt nanoparticles in CHA zeolites, and model results suggest the zeolite host imparts additional stability for Pt nanoparticles. We envision our thermodynamic and kinetic models for Pt-CHA are also capable of describing nanoparticle and cation interconversions for other zeolite frameworks under similar conditions.

Understanding Alkene Interaction with Metal-Modified Zeolites: Thermodynamics and Mechanism of Bonding in the π-Complex.

Zeolites modified with metal cations are perspective catalysts for converting light alkenes to valuable chemicals. A crucial step of the transformation is an alkene interaction with zeolite to afford π-complex with metal cations. The mechanism of alkene bonding with cations is still unclear. To address this problem, propene adsorption on H+ (Bro̷nsted acid site), Na+, Ca2+, Zn2+, Co2+, Cu2+, Cu+, and Ag+ cationic sites in ZSM-5 zeolite has been studied by quantum chemical calculations in terms of adsorption enthalpy, νC═C frequency, and natural bond orbital (NBO) analysis together with natural energy decomposition analysis (NEDA). It is revealed that the conventional concept of σ- and π-bonding is only partially applicable to alkene interaction with metal cations in zeolites. The orbital interaction between an alkene molecule and a metal site is more complex. Several different bonding mechanisms have been identified depending on the nature and electron configuration of the metal cation. This finding explains the complex correlations observed for propene π-complex stability and νC═C frequency shift or charge transfer from the alkene molecule. The results provide the basis for further understanding the interactions between alkenes and inorganic solid Bro̷nsted and Lewis acids.

Maximizing the utilization of Calcium species in the supercages of CaNa-FAU zeolite for efficient CO2 capture

Ca-FAU zeolite sorbents have been recognized as one of promising CO2 capture materials, while it is subject to the change in the speciations and locations of Ca2+ species as adsorption sites in the FAU zeolite due to its susceptible migration. In this paper, a facile strategy is reported by tuning calcination temperatures and Ca loadings, to maximize the utilization of Ca2+ species in the supercages of CaNaX zeolites. Detail characterizations of TG-MS, in situ FTIR spectroscopy and XRD Rietveld refinement reveal the changeable regular in the chemical speciations, locations and number of Ca2+ species in the CaNaX zeolites. Synthetic 0.05CaNaX-250 zeolite (calcined at 250 °C and Ca loading of 1.58 wt%) guarantees at least 21 Ca(OH)+ species (depicted as hydroxylated Ca2+ species) in the supercages per FAU cell unit, close to theoretical value. Such sorbent apparently improves the CO2 uptakes and the separation from CO2/N2 and CO2/CH4, along with fast kinetics, low heat of adsorption, measured by dynamic and gravimetric adsorption methods, respectively. In addition, it keeps a superior recyclability after 10 times regeneration. CO2-dosing FTIR experiments further illustrate that excellent CO2 adsorption performance are benefited from the formation of Ca(OH)+∙∙∙(CO2)2 adducts with an appropriate interaction. This work is of great significance to better understand the role of cations in FAU zeolites, and also provides an important reference value for the directional design of effective adsorption sites in other excellent CO2 zeolite sorbents.

Computational Screening of Cationic Zeolites for n-Butane/Methane Separations Using Quantitatively Accurate First-Principles-Derived Force Fields

Computational Screening of Cationic Zeolites for <i>n</i>-Butane/Methane Separations Using Quantitatively Accurate First-Principles-Derived Force Fields

Phenol removal from aqueous media using BEA zeolite: A molecular simulation study on the effect of zeolite mobile cations

Phenol removal from wastewater is crucial due to its high toxicity at low concentrations. In this study, the effectiveness of BEA zeolites (All-Si-BEA, H-BEA, Ca-BEA, and Na-BEA) for phenol removal was investigated using GCMC and MD simulations. The surface area and pore volume of the BEA structures were analyzed. The results showed that All-Si-BEA and H-BEA zeolites had slightly larger pore volumes (less than 2 %) compared to cationic BEA zeolites, indicating minimal impact of cations on pore volume. Efficient phenol adsorption was observed on all zeolite structures, even at low concentrations. All-Si-BEA and H-BEA exhibited high selectivity (105-106) and phenol adsorption capacity (1.86 mmol/g) at low phenol concentrations. Water adsorption was lower on All-Si-BEA and H-BEA, while cationic zeolites had higher water adsorption. The simulations showed that water molecules gather around mobile cations in the pores, causing variation in water adsorption in All-Si-BEA, H-BEA compared to cationic BEAs. The energy examination confirmed this observation. The diffusion coefficients of phenol in BEA (All-Si-BEA, H-BEA, Na-BEA, and Ca-BEA) were 5.34 × 10−5, 3.49 × 10−5, 5.32 × 10−6, and 1.02 × 10−6 (cm2/s), respectively, indicating higher mobility in All-Si-BEA and H-BEA compared to cationic BEA zeolites. Increasing the number of water molecules in the zeolite structures resulted in a decrease in the diffusion coefficient of phenol due to the hindrance caused by water molecules. Overall, All-Si-BEA and H-BEA showed higher potential for phenol separation from water compared to cationic BEA zeolites.

Coordination chemistry of Ca2+ sites in CaX zeolite: FTIR evidence of three coordination vacancies per cation

Adsorption plays an important role in many processes in nature and industry. In some cases, more than one adsorbate molecule can be coordinated to a single cation site, which dramatically increases the adsorption capacity. In this study, we report on the coordination chemistry of Ca2+ sites in CaX zeolite as revealed by the IR spectra of probe molecules (CO and N2) adsorbed at low temperature. The accessible Ca2+ sites were found to be uniform and occupy SII positions. Due to the extremely high adsorption capacity of the material, the bands of adsorbed CO were too intense, which prevented the determination of their maxima. Only Ca2+−CO species were unambiguously detected by a band at 2194 cm-1. We therefore studied the adsorption of CO containing 0.5% 13C18O and tracked the 13C18O bands. A stepwise formation of mono- (2092 cm-1) di- (2080 cm-1) and tricarbonyls (2072 cm-1) of Ca2+ was established. The results were confirmed by analysis of the 2ν(12C16O) overtone region, where bands due to mono- di- and tricarbonyls were recorded at 4362, 4336 and 4319 cm-1, respectively. However, examination of the 13C16O spectra can lead to misleading conclusions due to the overlap with some low-intensity 12C16O bands. Similar results were obtained after N2 adsorption. Mono-, di, and triligand species were detected by bands at 2342, 2337 and 2333 cm-1, respectively. Thus, the results imply that the accessible Ca2+ sites occupying SII positions in CaX possess three coordinative vacancies each. Similarities and differences between the coordination chemistry of Ca2+ cations in CaX and CaY zeolites are discussed.

Direct microwave energy input on a single cation for outstanding selective catalysis.

Microwave (MW)-driven catalytic systems are attracting attention not only as an aggressive electrification strategy of the chemical industry but also as creating a unique catalytic reaction field that conventional equilibrium heating cannot achieve. This study unlocked direct and selective heating of single alkali metal cations in the pores of aluminosilicate zeolites under MW. Selectively heated Cs+ cations in FAU zeolite exhibited selective CH4 combustion performance, that is, COx generation at the heated Cs+ cations selectively occurred while side reactions in the low-temperature gas phase were suppressed. The Cs-O pair distribution function revealed by synchrotron-based in situ x-ray total scattering gave us direct evidence of peculiar displacement induced by MW, which was consistent with the results of molecular dynamics simulation mimicking MW heating. The concept of selective monoatomic heating by MW is expected to open a next stage in "microwave catalysis" science by providing physicochemical insights into "microwave effects."

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.

Role of Strontium Cations in ZSM-5 Zeolite in the Methanol-to-Hydrocarbons Reaction.

The selectivity of the methanol-to-hydrocarbons (MTH) reaction can be tuned by modifying zeolite catalysts with alkaline earth metals, which typically increase propylene selectivity and catalyst stability. Here we employed Sr2+ as its higher atomic number in comparison to the zeolite T atoms facilitates characterization by scanning transmission electron microscopy and operando X-ray absorption spectroscopy. Sr2+ dispersed in the ZSM-5 micropores coordinates water, methanol, and dimethyl ether during the MTH reaction. Complementary characterization with nuclear magnetic resonance spectroscopy, thermogravimetric analysis combined with mass spectrometry, operando infrared spectroscopy, and X-ray diffraction points to the retention of substantially more adsorbates during the MTH reaction in comparison to Sr-free zeolites. Our findings support the notion that alkaline earth metals modify the porous reaction environment such that the olefin cycle is favored over the aromatic cycle in the MTH, explaining the increased propylene yield and lower deactivation rate.

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.

A systematic DFT screening of cationic faujasite-type zeolites for the adsorption of NO, NO2 and H2O

Unlocking the selective adsorption of NO and NO2 in presence of H2O in cationic zeolites: a systematic DFT screening of key questions and properties.

Shape-Selectivity of Cyclopentenyl Cation Isomerization: Investigating Kinetic Control in Medium-Pore Zeolites

We performed periodic density functional theory (DFT) calculations to study the dynamics of alkyl substituents on cyclopentenyl cations in medium-pore acid zeolites, H-ZSM-5 and H-ZSM-22. Our study seeks to shed light on how zeolite shape selectivity can influence key reaction intermediates in the methanol-to-hydrocarbon process, which can lead to both value-added products and coke species. We considered the isomerization of 4-ethyl-4,5,5-trimethylcyclopentenyl cation (A), which has been shown by Hernandez et al. [ACS Catal.2021, 11, 12893–12914] to lead to IR spectra and alkyl substitution patterns that vary with zeolite pore structure, in contrast to DFT-predicted thermodynamics. Here, we investigate the role of kinetic control on zeolite shape-selectivity by computing exhaustive DFT dynamics of substituent rearrangement in medium-pore zeolites starting with cation A. We used the Rule Input Network Generator (RING) code to enumerate isomerization pathways from A to five product isomers that differ mainly in the presence or absence of an alkyl substituent on the central allylic carbon of the cyclopentenyl ring, yielding a reaction network with a total of 24 distinct species. We combined metadynamics and climbing-image nudged elastic band (CI-NEB) methods to compute the free-energy landscapes, including barriers for all species in H-ZSM-5 and H-ZSM-22 zeolites. Integrating kinetic equations for the reaction network in the two zeolites predicts that equilibrium product distributions are obtained after 103 s in H-ZSM-5, while 108–109 s is required for equilibration in H-ZSM-22, suggesting the clear possibility of kinetic control depending on zeolite structure.

Stable and Uniform Extraframework Cations in Faujasite Zeolites.

Extraframework transition metal ions (TMIs) in zeolites can serve as active sites for adsorption and catalysis. However, due to the complexity and mobility of extraframework cation sites, their applications are significantly limited and the structure-performance relationship is poorly understood. In this Perspective, stable and uniform TMIs in zeolites are exemplified and their characteristics are discussed. A series of TMIs can be introduced to specific cation sites of faujasite via a ligand-protected in situ synthesis route to construct uniform TMIs in the zeolite matrix, namely, TMI@FAU (TMI= Co, Ni, Cu, Rh, and Pt). Coordinatively unsaturated TMIs within faujasite are active for small-molecule adsorption and activation, and therefore, TMI@FAU zeolites show unique properties in adsorption and catalysis. TMI@FAU zeolites appear to be ideal model systems, and the well-defined structure of TMI@FAU greatly facilitates the mechanism studies by spectroscopic investigations and theoretical simulations.

Novel and emerging concepts related to cationic species in zeolites: Characterization, chemistry and catalysis

This review surveys our efforts to form and characterize cationic species located in the micropores of various zeolites, both for metal and non-metal cations, with specific emphasis on their novel, emerging properties, and reactivity. Herein, it is described how we realized synthesis of such species with a focus on both cation identity as well as zeolite architecture, and synthesis methods. Well-defined cationic species could be produced by careful choice of metal incorporation methods, Al distribution tuning and synthesis parameters. Previously unknown phenomena in zeolite and cation-zeolite chemistry were discovered by employing these well-defined systems and will be covered in this review article. Methodology to dramatically improve stability of cations in zeolite for adsorption and catalytic applications, and usefulness of these materials in elucidating hitherto unknown catalytic intermediates and mechanisms will be discussed.

Can two CO2 molecules be simultaneously bound to one Na+ site in NaY zeolite? A detailed FTIR investigation

Efficient CO2 capture is necessary to solve the current environmental problems of global warming. Porous materials are promising for this purpose and adsorption capacity is a critical issue. Large cations in zeolites can often attach two small molecules, but it is not yet well established whether this applies to CO2. Here we present a detailed FTIR spectroscopic study of CO2 adsorption on NaY zeolite. At ambient temperature and pressures up to 50 mbar, Na+⋯O–C–O species are formed and the CO2 molecules are tilted, which allows interaction of the C-atom with framework oxygen. At higher pressure or at lower temperature reorientation of the CO2 molecules occurs, leading to a significant decrease in the extinction of the ν1 and 2ν2 bands and to a gradual red shift of the ν3(13CO2), (ν1+ν3) and (2ν2+ν3) modes with 3 cm−1. At lower temperature, two additional conversions occur: (i) formation of (Na+⋯O–C–O)2 dimers, causing an additional 2-3 cm−1 red shift of the combination bands, and (ii) attachment of a second CO2 molecule to the same Na+ site, resulting in a discrete red shift of ν3(13CO2), (ν1+ν3) and (2ν2+ν3) with 10 cm−1. Addition of N2 or CO to pre-adsorbed CO2 leads to compression of the formal CO2 adsorption layer. The obtained results demonstrate the possibility of simultaneous binding of two CO2 molecules to one cationic site in zeolites which opens up new horizons in the design of high-capacity CO2 adsorbents.

An initial evaluation of the thermodynamic or kinetic separation performance of cation-exchanged LTA zeolites for mixtures of propane and propylene

Adsorptive separation of propylene (C3H6) and propane (C3H8) may represent an energy-efficient alternative to conventional cryogenic distillation. In this perspective, a set of zeolitic adsorbents of LTA type structure (pure-silica zeolite (Si-LTA) and cation- (Na+, 33% of Li+, 50% of Mg2+ and 50% of Ca2+) containing zeolites) were prepared, then characterized using various techniques such as PXRD, gas adsorption measurement, TGA, XRF, SEM and EDX mapping. Thanks to an in-house manometric dosing setup coupled with a Tian-Calvet type microcalorimeter, the pure gas adsorption isotherms and their corresponding differential enthalpies of adsorption were measured at 303 K and for pressures up to 5 bar. To fit those adsorption data, the dual-site Langmuir was selected as the best fit model, and by using the Ideal Adsorbed Solution Theory (IAST), thermodynamic selectivities were determined. The mass transfer constants were also estimated by fitting, separately, the Linear Driving Force (LDF) and isothermal micropore diffusion models to adsorption kinetic curves, thus allowing kinetic type selectivities to be calculated. The combination of those selectivities reveals that the thermodynamic separation of C3H6 from C3H8 is highly favorable on CaNa-LTA, followed by its MgNa-LTA counterpart with IAST selectivities around 15 and 5, respectively. On the other hand, the monovalent cationic zeolites (i.e., Na- and LiNa-LTA) show a predominance of steric effects. Also, except for Si-LTA which shows a moderate kinetic separation, the studied materials are potential candidates, in the following order CaNa- > MgNa- > LiNa- > Na-LTA, for the separation of C3H6 from C3H8 by adsorption-based technologies.

Structural evidence of sunscreen enhanced stability in UV filter-Zeolite hybrids

Organic UV filters are used in many applications (personal-care products, components in packaging, dyes, etc). However, many issues remain related to their safety; their release into the environment is toxic for many organisms and their photodegradation products are possibly dangerous. Encapsulation of these molecules in a zeolitic matrix can avoid their dispersion and promote the enhancement of their properties. In this study, we report the structural characterization of the new LTL/OMC ZEOfilter, produced by encapsulating the organic UV filter octinoxate (OMC, C18H26O3) into synthetic zeolite L. In fact, for the further exploitation and the realization of tailor-made materials, it is fundamental to understand the interactions between the filter and the matrix. X ray powder diffraction, Rietveld structural analysis and Monte Carlo simulations result in a detailed description of the molecules’ setting and configuration. To fit the pores of the zeolite, the conformation of OMC molecules assumes a bent tail, allowing suitable intermolecular distances and host-guest interactions. These results were also corroborated by FT-IR experiments. The host-guest interaction is the key point for the stability and efficacy of LTL/OMC ZEOfilter and may explain why, among different ZEOfilters, the cationic zeolites, and especially zeolite L, display the most promising features.

Review of Hydrogen Sulfide Removal from Various Industrial Gases by Zeolites

Hydrogen sulfide (H2S) removal from various industrial gases is crucial because it can cause huge damage to humans, the environment, and industrial production. Zeolite possesses huge specific surface area and well-developed pore structure, making it a promising adsorbent for H2S removal. This review attempts to comprehensively compile the current studies in the literature on H2S removal in gas purification processes using zeolites, including experimental and simulation studies, mechanism theory, and practical applications. Si/Al ratio, cations of zeolite, industrial gas composition and operating conditions, and H2S diffusion in zeolites affect desulfurization performance. However, further efforts are still needed to figure out the influence rules of the factors above and H2S removal mechanisms. Based on an extensive compilation of literature, we attempt to shed light on new perspectives for further research in the future.