Zeolite Cluster Research Articles

Theoretical Study of the Molecular Complexes between Pyridyne and Acid Sites of Zeolites

The main interaction between pyridine and zeolites leads to form a hydrogen bond between the N atom of pyridine and OH groups of zeolites. The present work reports a theoretical study about the structural, vibrational and topological properties of the charge distribution of the molecular complexes between pyridine and a series of acids sites of zeolites. The calculated structural parameters are the highest occupied molecular orbital energy (EHOMO), lowest unoccupied molecular orbital energy (ELUMO), energy gap (ΔE), hardness (η), softness (S), the absolute electronegativity (χ), the electrophilicity index (ω) and the fractions of electrons transferred (ΔN) from zeolites molecules to pyridine. We show N atom of pyridine attacks to the H atom of the OH bridged group of zeolite clusters.

Confinement of Highly Luminescent Lead Clusters in Zeolite A

Metal clusters confined inside zeolite frameworks display unique electronic, catalytic, and optical properties. However, so far only confined silver clusters have shown peculiar luminescent properties, displaying high photoluminescent quantum efficiencies reaching almost unity. In this study, we demonstrate the self-assembly and confinement of highly luminescent lead (Pb) clusters into the molecular-sized cavities of Linde Type A (LTA) zeolites. These Pb-LTA samples display an intense deep-blue emission with external quantum efficiencies up to 69% in their partially dehydrated state. A tetrahedral lead cluster (Pb4) with unusually short Pb–Pb distances and hydroxyl ligands was identified as responsible for the luminescence as determined by X-ray absorption fine structure (XAFS) analysis. The in-depth characterization of the Pb-zeolites, reported here, sets the stage for elucidating the structure-to-luminescent relationship of other zeolite-embedded clusters.

Impact of long-range electrostatic and dispersive interactions on theoretical predictions of adsorption and catalysis in zeolites

In this paper, we review the importance of long-range zeolite framework interactions in theoretical predictions for a variety of zeolite-catalyzed processes, and we show why such interactions must be determined accurately in order to reproduce experimentally measured adsorption and activation energies. We begin with an overview of the different strategies that have been used to account for long-range coulombic and dispersive interactions of zeolite framework atoms with species adsorbed at an active site. These methods include full periodic-DFT calculations and multi-layer hybrid techniques. Electrostatic interactions are observed to have a more significant impact than dispersive interactions on the geometries of ion-pair transition states and adsorbed species. Stabilization of the TS relative to reactant complexes is also dictated by electrostatic interactions. Dispersion effects are found to significantly stabilize both transition and reactant states for adsorbed species, especially those which have dimensions that provide good fits within the zeolite pore or cavity. We also show that the relevance of particular active site configurations can be missed, if the effects of long-range interactions are neglected. As a case in point, we demonstrate that a site previously considered inactive for ethane dehydrogenation, [GaH2]+ may in fact be more active than previously thought, when the impact of long-range interactions on the predicted activation energy is taken into account. Finally, the use of hybrid quantum mechanics/molecular mechanics approaches on extended, finite zeolite clusters has emerged as an accurate, highly cost-effective, and versatile alternative towards overcoming some of the present-day limitations of periodic calculations.

Non-resonant enhancement mechanism in SERS effect due to copper oxide quantum dots stabilized in synthetic zeolite F9-NaX

This study shows the presence of SERS effect caused by the interaction of synthetic zeolite F9-NaX with stabilized CuO quantum dots. The quantum dots synthesized by the ion-exchange properties of the zeolite show a good size distribution, and a spheroidal shape of 4 nm approximately. The simulation of the structural unit of the zeolite (d4r-8T) and small clusters of (CuO)n performed with density functional theory (DFT), allowed to determine the non-resonant effects from the interaction between the zeolite and CuO as the main contribution to SERS effect by the chemical bonding effect (CBE). Quantum dots CuO show good antibacterial activity against Gran positive and Gran negative bacteria.

Computational study of ethanethiol conversion reactions catalyzed by acidic zeolites

The reaction mechanism, of the ethanethiol transformation over acidic zeolite surface: ethanethiol-Hp-zeolite → Hp-zeolite + ethene + H2S, has been investigated computationally with the use of ab initio, density functional theory (DFT) and MM methods. The investigation has been carried out by employing two model zeolite clusters, each including one hydroxyl Brønsted acidic site (BAS). The first model, denoted 20T, has been studied with the help of the two-layered ONIOM2 (M06-2X/6-31 + G(d,p):UFF) methodology and the second one denoted 38T, has been investigated using the three-layered ONIOM3 (M06-2X/6-31 + G(d,p):HF:UFF) approach. The calculations using both models show that the reaction mechanism involves two competing channels, a direct t2 type dehydrosulfidation (DHS) channel, and a stepwise Sw2 type pathway through the intermediate formation of an alkoxy species. The detailed examination of the two pathways, i.e., the direct mechanism and the stepwise mechanism, indicate that the latter is strongly favored over the direct t2 type mechanism. Finally, the consistency of the results from both methodologies suggests that the ONIOM approach yields an accurate and practical method to study the degradation mechanism of sulfur containing compounds on acidic zeolite surface.

Many-body expansion of the Fock matrix in the fragment molecular orbital method.

A many-body expansion of the Fock matrix in the fragment molecular orbital method is derived up to three-body terms for restricted Hartree-Fock and density functional theory in the atomic orbital basis and compared to the expansion in the basis of fragment molecular orbitals (MOs). The physical nature of many-body corrections is revealed in terms of charge transfer terms. An improvement of the fragment MO expansion is proposed by adding exchange to the embedding. The accuracy of all developed methods is demonstrated in comparison to unfragmented results for polyalanines, a water cluster, Trp-cage (PDB: 1L2Y) and crambin (PDB: 1CRN) proteins, a zeolite cluster, a Si nano-wire, and a boron nitride ribbon. The physical nature of metallicity is discussed, and it is shown what kinds of metallic systems can be treated by fragment-based methods. The density of states is calculated for a fully closed and a partially open nano-ring of boron nitride with a diameter of 105 nm.

Silver Clusters in Zeolites: From Self-Assembly to Ground-Breaking Luminescent Properties.

Interest for functional silver clusters (Ag-CLs) has rapidly grown over years due to large advances in the field of nanoscale fabrication and materials science. The continuous development of strategies to fabricate small-scale silver clusters, together with their interesting physicochemical properties (molecule-like discrete energy levels, for example), make them very attractive for a wide variety of applied research fields, from biotechnology and the environmental sciences to fundamental chemistry and physics. Apart from useful catalytic properties, silver clusters (Agn, n < 10) were recently shown to also exhibit exceptional optical properties. The optical properties and performance of Ag-CLs offer strong potential for their integration into appealing micro(nano)-optoelectronic devices. To date, however, the rational design and directed synthesis of Ag-CLs with specific functionalities has remained elusive. The inability for rational design stems mainly from a lack of understanding of their novel atomic-scale phenomena. This is because accurately studying silver cluster systems at such a scale is hindered by the perturbations introduced during exposure to various experimental probes. For instance, silver possesses a strong tendency to cluster and form ever-larger Ag aggregates while probed with high-energy electron beams and X-ray irradiation. As well, there exists a need to provide a stabilizing environment for which Agnδ+ clusters can persist, setting up a complex interacting guest-host system, as isolated silver clusters are confined within a suitable hosting medium. Fundamental research into Agnδ+ formation mechanisms and their important optical properties is paramount to establishing truly informed synthesis protocols. Over recent years, we have developed several protocols for the ship-in-a-bottle synthesis of highly luminescent Ag-CLs within the microporous interiors of zeolite frameworks. This approach has yielded materials displaying a wide variety of optical properties, offering a spectrum of possible applications, from nano(micro)photonic devices to smart luminescent labels and sensors. The versatility of the Ag-zeolite multicomponent system is directly related to the intrinsic and complex tunability of the system as a whole. There are several key zeolite parameters that confer properties to the clusters, namely, the framework Si/Al ratio, choice of counterbalancing ions, silver loading, and zeolite topology, and cannot be overlooked. This Account is intended to shed light on the current state-of-the-art of luminescent Ag-CLs confined in zeolitic matrices, emphasizing the use of combinatorial approaches to overcome problems associated with the correct characterization and correlation of their structural, electronic, and photoluminescence properties, all to establish the important design principles for developing functional silver-zeolite-based materials. Additionally, examples of emerging applications and future perspectives for functional luminescent Ag-zeolite materials are addressed in this Account.

Subnanometric Hybrid Pd-M(OH)2, M = Ni, Co, Clusters in Zeolites as Highly Efficient Nanocatalysts for Hydrogen Generation

Summary Hybrid multi-metallic nanocatalysts have attracted increasing attention because of the synergistic effect of metal species and considerably improved catalytic performance, but they often suffer from severe sintering and poor stability. Here, we show a facile strategy for preparing subnanometric hybrid bimetallic clusters Pd-M(OH) 2 (M = Ni, Co) within silicalite-1 (S-1) zeolite via a hydrothermal synthesis method. The hybrid bimetallic nanocatalysts exhibit excellent shape-selective catalytic performance and superior thermal stability. The incorporation of secondary Ni(OH) 2 species in S-1 can considerably increase the catalytic activity of the Pd nanoclusters for the dehydrogenation of formic acid (FA) as a result of the electron-enriched Pd surface and bimetallic interfacial effect. Notably, the 0.8Pd0.2Ni(OH) 2 @S-1 catalyst affords the highest initial turnover frequency value, up to 5,803 hr −1 toward complete FA decomposition without any additives at 60°C. The superior catalytic properties and excellent stability of the subnanometric hybrid bimetallic clusters confined in zeolites create new prospects for their practical high-performance catalytic application.

A density functional theory study of hydrogen adsorption on Be-, Mg-, and Ca-exchanged LTL zeolite clusters.

Hydrogen molecule adsorption on frameworks consisting of alkaline earth metal atoms (Be, Mg, or Ca) in LTL zeolite was investigated via density functional theory. A 24T zeolite cluster model was used in this study. HOMO and LUMO energy, chemical potential, chemical hardness, electronegativity, adsorption energy, and adsorption enthalpy values were calculated. The Mg-LTL and Ca-LTL clusters were found to have much lower chemical potentials and adsorption energies than those of the Be-LTL cluster. Additionally, the calculations indicated that the Mg-LTL and Ca-LTL clusters are softer (considering their lower chemical hardness values) and more chemically reactive than the Be-LTL cluster. The calculated hydrogen adsorption enthalpies were -14.7 and -9.4kJ/mol for the Mg-LTL and Ca-LTL clusters, respectively, which are significantly larger than the enthalpy of liquefaction for the hydrogen molecule. These results imply that the Mg-LTL and Ca-LTL zeolite structures are promising cryoadsorbents for hydrogen storage. Graphical abstract Hydrogen adsorption was theoretically investigated on Be-, Ca- and Mg-LTL clusters. Ca- and Mg-LTL zeolites are potential cryoadsorbent materials for hydrogen storage.

Structural Design of Zeolitic Cluster Organic Frameworks from Hexamethylentetramine and Copper-Halide Clusters

Zeolitic cluster organic frameworks constructed with two different tetrahedral units, hexamethylenetetramine (hmt) and hmt-type [Cu4Cl6]2– clusters, are realized in this work for the first time. By using a tetrahedral hmt ligand to assemble in situ hmt-type Cu4Cl6 clusters, a three-dimensional cluster organic framework with lonsdaleite (lon) topology is successfully built to host the two-dimensional honeycomb-like metal–organic complex. Furthermore, a series of cluster organic frameworks based on hmt and other well-defined copper-halide clusters, such as Cu2Cl3, Cu3Cl2, Cu4Cl4, Cu5Cl6, and Cu6Cl6, are also presented, which exhibit rich topological nets. These compounds are further investigated by infrared, powder X-ray diffraction, thermogravimetric analysis, infrared and photoluminescent spectroscopy. Photoluminescent studies reveal that these compounds display yellow green emissions based on halide to metal charge transfer. This work reveals a versatile synthetic method to generate a large family of copper-halide cluster organic frameworks with hmt ligands.

A DFT study of hydrogen adsorption on Be, Mg and Ca frameworks in erionite zeolite

The molecular hydrogen adsorption was investigated on additional frameworks with earth alkaline metal atoms (Be, Mg and Ca) in 24T ERI zeolite cluster model by means of Density Functional Theory study. HOMO and LUMO energy values, chemical potential, chemical hardness, electronegativity, adsorption energy and adsorption enthalpy values have been calculated in this study. Mg-ERI and Ca-ERI clusters have much lower chemical potentials with much lower adsorption energy values when compared to the value of Be-ERI cluster. Additionally, they are softer than Be-ERI cluster with respect to their lower chemical hardness values. Hydrogen adsorption enthalpy values were computed as −3.6 and −3.9kJ/mol on Mg-ERI and Ca-ERI clusters, respectively. These adsorption enthalpy values are significantly larger than the enthalpy value of liquefaction for hydrogen molecule. This consequently specifies that Mg-ERI and Ca-ERI zeolite structures which have higher chemical reactivity appear to be a promising candidate cryoadsorbent for hydrogen storage.

In Situ Confinement of Ultrasmall Pd Clusters within Nanosized Silicalite-1 Zeolite for Highly Efficient Catalysis of Hydrogen Generation.

Well-dispersed and ultrasmall Pd clusters in nanosized silicalite-1 (MFI) zeolite have been prepared under direct hydrothermal conditions using [Pd(NH2CH2CH2NH2)2]Cl2 as precursor. High-resolution scanning transmission electron microscopy studies indicate that the Pd clusters are encapsulated within the intersectional channels of MFI, and the Pd clusters in adjacent channels visually aggregate, forming nanoparticles (NPs) of ∼1.8 nm. The resultant catalysts show an excellent activity and highly efficient H2 generation toward the complete decomposition of formic acid (FA) under mild conditions. Notably, thanks to the further reduced Pd NP size (∼1.5 nm) and the additionally introduced basic sites, the Pd/S-1-in-K catalyst affords turnover frequency values up to 856 h(-1) at 25 °C and 3027 h(-1) at 50 °C. The easy in situ confinement synthesis of metal clusters in zeolites endows the catalysts with superior catalytic activities, excellent recyclability, and high thermal stability, thus opening new perspectives for the practical application of FA as a viable and effective H2 storage material for use in fuel cells.

Stability and Structures of Silver Subnanometer Clusters in EMT Zeolite with Maximum Aluminum Content

The stability of small silver clusters, Agn with n = 3, 4, 6, and 8, supported in a sodalite of EMT zeolite with a maximum possible aluminum content (Si/Al = 1) was studied with the density-functional-theory-based cluster approach. The results showed that various silver clusters, partially or completely reduced, can be stabilized in this zeolite with the maximum negative defects. Computed formation energies for two reduction reactions reveal that completely reduced Ag clusters can be stabilized through hydrogenation, giving some preference to the formation of hydrogenated Ag6 species. The hydrogenated neutral AgnHm/EMT clusters remain stable at T = 300 K, as obtained from the analysis of the Born–Oppenheimer molecular dynamics trajectories. Ion exchange of Na+ by Ag+ cations is obtained to be exothermic with energy gain of 0.5 eV per Ag cation. The formation of Ag–O ionic bonds, known to stabilize metal clusters in zeolitic frameworks, is found to be preponderant in the structures with fewer than six silv...

Adsorption of hydrogen sulfide as initial step of H2S removal: A DFT study on metal exchanged ZSM-12 clusters

The molecular adsorption of hydrogen sulfide has been theoretically studied via DFT on additional framework with cations of metals (Fe, Co, Ni, Cu and Zn) in 14T-channel ZSM-12 zeolite cluster model. The method employed in this study is B3LYP functional with LanL2DZ and 6–31G(d,p) basis sets. Cu–ZSM-12 cluster has the lowest chemical potential with minimum adsorption energy value with negative ΔG, highest electronegativity and lowest energy gap between HOMO and LUMO with respect to other clusters. It is softer than other clusters because of its lower chemical hardness value. Accordingly, based on these data it can be mentioned that Cu–ZSM-12 is a promising candidate catalyst as for removal of hydrogen sulfide via activation of S–H bond than other metal ZSM-12 zeolites.

Adsorption and transformations of ethene on hydrogenated rhodium clusters in faujasite-type zeolite. A computational study

Phase diagrams from DFT modeling suggest that zeolite-supported Rh4 clusters may be appropriate for the catalytic hydrogenation of ethene to ethane, whereas Rh3 clusters favor the formation of the stable adsorbed ethylidyne species, preventing further hydrogenation.

Protonated thiophene-based oligomers as formed within zeolites: understanding their electron delocalization and aromaticity.

In an earlier work, protonated thiophene-based oligomers were identified inside ZSM-5 zeolites. The novel compounds exhibited π-π* absorption wavelengths deep within the visible region, earmarking them for possible use as chromophores in a variety of applications. In this computational study, we determine the factors that cause such low-energy transitions, and describe the electronic structure of these remarkable compounds. DFT calculations of conjugated thiophene-based oligomers with up to five monomer units reveal that the main absorption band of each protonated oligomer is strongly red-shifted compared to the unprotonated form. This effect is counterintuitive, since protonation is expected to diminish aromaticity, and thereby increase the HOMO-LUMO gap. We find that upon protonation the π-electrons remain delocalized over the entire π-conjugated molecule, but the positive charge is localized predominantly on the protonated side of the molecule. A possible explanation for this ground-state charge localization is the participation of the C-H bond in the π-system of the protonated ring, locally providing aromatic stabilization for the positive charge. The addition of the proton stabilizes all electronic orbitals, but due to the ground state π-electron distribution away from the added nucleus, the HOMO is stabilized less than the LUMO. The main absorption peak upon protonation corresponds to the charge transfer excitation involving the frontier orbitals, and the small band gap explains the observed red shift. Analogue calculations on thiophene within a ZSM-5 zeolite cluster model confirm the same trends upon protonation as observed in the non-interacting compounds. Understanding the electronic structure of these compounds is very relevant to correlate UV-Vis bands with acidic strength and possibly environment in zeolites and to improve their performance in catalytic and energy related applications.

Vision of the Cross-Border Zeolitic Tuff Cluster

The natural consequence of observed changes and trends in the macroeconomic scale include formation of the clusters. Nowadays, clustering is an increasingly more popular form of business organization, and it is becoming more significant from year to year. In Poland, clusters are relatively new forms of self-organization business, which are supported by the administrative authorities, and is based on mutual cooperation between entrepreneurs or producers and development research units on a specific area. The aim of the project carried out by University of Management and Administration in Zamość entitled “Description of an innovative model of cross-border application of zeolitic tuffs” was establishment of a functioning institutionalized cooperation networks in the form of a cross-border zeolitic tuff cluster on a Polish-Ukrainian cross-border area. The objective of this article is to present mission and vision of the Cross-border Zeolitic Tuff Cluster. Mainly, the authors focus on a various possibilities of application of zeolitic tuffs and main areas of functioning of the cluster. On this basis, authors described strengths, weaknesses, opportunities and threats of functioning of the Cross-border Zeolitic Tuff Cluster and proposed the development trends of cluster.

Genesis of Delaminated-Zeolite Morphology: 3-D Characterization of Changes by STEM Tomography.

Zeolite delamination increases the external surface area available for catalyzing the conversion of bulky molecules, but a fundamental understanding of the delamination process remains unknown. Here we report morphological changes accompanying delamination on the length scale of individual zeolite clusters determined by 3-D imaging in scanning transmission electron microscopy. The results are tomograms that demonstrate delamination as it proceeds on the nanoscale through two distinct key steps: a chemical treatment that leads to a swelled material and a subsequent calcination that leads to curling and peeling off of delaminated zeolite sheets over hundreds of nanometers. These results characterize the direct, local, 3-D morphological changes accompanying delaminated materials synthesis and, with corroboration by mercury porosimetry, provide unique insight into the morphology of these materials, which is difficult to obtain with any other technique.

On the Rational Design of Zeolite Clusters

We have applied density functional theory calculations to systematically investigate zeolite cluster-size convergence for two acid-zeolite-catalyzed processes related to the conversion of biomass: (1) the keto–enol tautomerization of acetone in HZSM-5 and HY and (2) the protonation and ring opening of furan in HZSM-5. We have used these reactions as platforms to study two different approaches for constructing successively larger cluster models of zeolites, with the aim of determining a protocol that converges the energy differences with minimal system size. One approach for cluster design involves counting framework bonds from the Brønsted acid-site aluminum atom. Another approach involves applying multicentered spherical cutoffs based on geometries of the zeolite active site, the adsorbed reactant, and the adsorbed product. We have investigated the convergence of reaction energies using single-point calculations on clusters containing as many as 166 tetrahedral (T) atoms and geometry optimizations on clusters with as many as 78 T atoms. For all reactions studied, convergence rates of single-point reaction energies agree well with those from geometry-optimized clusters. In addition, converged and optimized reaction energies agree well with previously published values for all reactions. Our central finding is that clusters generated with multicentered spherical cutoffs yield converged reaction energies with smaller system sizes than clusters generated by counting framework bonds. This method, employing a single length scale (5 Å), converges reaction energies with respect to system size to within chemical accuracy (±1 kcal/mol), and it includes between 15 and 34 T atoms in the cluster depending on the process and zeolite framework under investigation. We suggest a general protocol for generating such clusters for subsequent use in computational studies of zeolites and other heterogeneous catalysts.

C2-Hydrocarbon Adsorption in Nano-porous Faujasite: A DFT Study

The shape, interaction and stability of adsorbed C2-hydrocarbon, ethane and ethylene, molecule in a nano-porous cages of zeolite molecular sieves, e.g. Faujsite, were studied by cluster based Density Functional Theory (DFT). The interactions of ethylene and ethane on nano-porous Faujasite with varying size of extra-framework cations, e.g. Li < Na < K < Rb < Cs, have been investigated using generalized gradient approximation (GGA) with PW91 correlation functional. A cluster model was taken from the single crystal data of Faujasite to investigate the interaction between nano-porous zeolite and C2-hydrcarbon. Additionally, ethane and ethylene with naked monovalent cations were optimized using the same PW91 level theory. DFT were employed to get the geometries, Mulliken partial charges and adsorption binding energies for zeolite cluster with varying size of cations. These geometric and energetic data were analyzed with respect to the size of cation. DFT results suggest that if smaller the size of the extra-framework cations stronger will be the hydrocarbon interaction in nano-porous Faujasite even if there would specific interaction.