Framework Oxygen Atoms Research Articles

Water in the micropores of CPO-27 metal-organic frameworks: A comprehensive study

The metal-organic framework CPO-27 exhibits free coordination sites (open metal sites) and can be prepared with a wide range of metals that influence its properties. It is therefore an intriguing structure to study sorption phenomena. We analyze the water resistance and sorption behavior of these frameworks, with particular attention to the sorption mechanism in detail and the structure of the confined water molecules. For this purpose, we use manometric water vapor sorption analysis and FTIR spectroscopy. The respective metal center orchestrates both the adsorption behavior and the arrangement of the water molecules in the micropores of the framework. The extent to which water molecules form hydrogen bonds (with each other and with framework oxygen atoms) plays a crucial role in the stability of the framework towards water. Water adsorption is governed by the coordination of water molecules to the open metal sites (except for CPO-27-Cu) and subsequent H-bonding. A stepwise adsorption of water is observed, with significant differences depending on the choice of metal.

Water-mediated super-correlated proton-assisted transport mode for solid-state K−O2 batteries

A comprehensive understanding of the behavior nature of fast ions at the atomic level is essential for the development of advanced solid-state ionic conductors. The inadequate inter-ion correlation effects of current ion transport models lead to a conductivity bottleneck in designing conductors. Herein, based on water-mediated proton-assisted ion transport, a novel transport mode with simultaneous anion-cation and inter-cation coupling is designed, enabling the K-ions of the modeled solid-state ionic conductor K2Fe4O7 crystals to achieve an ultra-high conductivity of 7.6 × 10–2 S cm–1, an enhancement of two orders of magnitude. The principle of the accelerated K-ion diffusion through the rotation and vibration of water molecules around the framework oxygen atom is elaborated, and the coupling correlation between proton and K-ion transport is confirmed using in-situ impedance spectroscopy under labeled isotopes. The application of this mechanism enabled the fabricated K−O2 solid-state battery exhibit an ultra-low overpotential (0.1 V) and excellent rate performance. Further, the mechanism is also applicable for Li and Na-ion conductors, providing significant theoretical guidance for breaking existing universal design rules and for the development for faster ionic conductors.

Competitive Adsorptive Mechanism of H2/N2 in LTA/FAU Zeolites by Molecular Simulations and Experiments.

For industrial tail gas to be converted into high-purity hydrogen, the H2-N2 mixture needs to be separated efficiently. This work examined the adsorption characteristics and competitive mechanisms of H2 and N2 on LTA- and FAU-type zeolites, at 77 K, 298 K, and 0.1-10 bar by thoroughly analyzing results of adsorption capacity experiments and molecular simulations. In the Grand Canonical Monte Carlo (GCMC) simulations, the force field causing a molecular dipole of H2 and the polarization force field of N2 are first applied. The accuracy of the force field was experimentally verified. The findings indicate that N2 and H2 loading on Ca-FAU (Ca-LTA) are higher than Na-FAU (Na-LTA). On NaX at 77 K, the highest adsorption selectivity (N2/H2) is observed; on NaA at 298 K, it is the opposite. The GCMC data findings demonstrate that H2 and N2 have remarkably similar adsorption sites, with framework oxygen atoms and non-framework cations serving as the main adsorption sites for adsorbate molecules. Furthermore, the rate at which H2 diffuses is higher than that of N2. The study of redistribution charge before and after adsorption demonstrated that N2 has a greater affinity for the framework oxygen atoms than H2. This study provides a molecular theoretical foundation for the adsorption behavior of H2-N2 mixture in zeolites.

Thermal stability of templated ZSM-5 zeolites: An in-situ synchrotron X-ray powder diffraction study

The topological symmetry of the as-synthesized ZSM-5 is orthorhombic Pnma, with 12 crystallographically independent tetrahedral sites in the unit cell. Highly crystallized ZSM-5 zeolite can be synthesized using organic species as structure directing agents (SDA), including tetra-n-propylammonium (TPA) and Na cations. This work investigates the thermal stability and high temperature structural evolution by in situ synchrotron (X-Ray) powder diffraction of TPA-ZSM-5 with different Si/Al ratios (15, 25, 50, and 100) and their structural modifications induced by template calcination. Rietveld refinements of the room temperature data revealed that in all samples tetrapropylammonium ions are located at the intersection of the sinusoidal and the straight channels. The refined TPA+ content increases with the increasing of the Si/Al ratio of the zeolite, reaching the theoretical value of 4 when the Si/Al ratio = 84, in very good agreement with both chemical and thermogravimetric analyses. The bond distances evidenced the lack of interactions between the TPA+ and the framework oxygen atoms. The high temperature refinements confirmed the high thermal stability of all four ZSM-5 samples, which remain crystalline until the end of the thermal treatment (800 °C). Burning of the template does not cause any relevant variations in the framework geometry and channel shape, confirming the high flexibility of the MFI framework.In addition, thermal analysis proved insights about the effect of aluminum content on the interaction between SDA and zeolite structure.

Lead, zinc, nickel and chromium ions removal from polluted waters using zeolite formed from bauxite, obsidian and their combination with red mud: Behaviour and mechanisms

Synthetic zeolites obtained combining natural sources (bauxite and obsidian), pure alumina/silica reagents, and waste material (red mud) were tested for heavy metals (i.e., Pb2+, Zn2+, Ni2+, Cr3+) removal. The adsorption capabilities of the formed sodalite, zeolite A and zeolite X (LTA and FAU topology, respectively), were compared through thermodynamic and kinetic experiments. Although all the newly-formed zeolites were able to remove the pollutant elements within 24 h, Zeolite X and sodalite synthesized combining obsidian (natural material as silica source) and red mud (waste material as alumina source) proved to be a better sorbent phase (qmax 20–25 mg g−1) compared to Zeolite A formed from treated bauxite (qmax 4–18 mg g−1). Their removal efficiency was also evaluated in polluted waters (wastewater treatment plant [WWTP] effluent, heavy metals mixture, native pH). Depending on synthesized zeolite type, the adsorption mechanism was accredited to ion exchange and precipitation mechanisms. The location of metal-ions inside the zeolite channels was defined by X-ray Powder Diffraction (XRPD) Rietveld analysis. Host–guest interactions among the framework oxygen atoms, co-adsorbed water molecules, and metal-ions were highlighted by the refined bond distances. Finally, magnetic characterization allowed the recognize of different magnetic properties as a function of raw materials used for zeolite synthesis.

Three-dimensional inhomogeneity of zeolite structure and composition revealed by electron ptychography.

Structural and compositional inhomogeneity is common in zeolites and considerably affects their properties. Thickness-limited lateral resolution, lack of depth resolution, and electron dose-constrained focusing limit local structural studies of zeolites in conventional transmission electron microscopy (TEM). We demonstrate that a multislice ptychography method based on four-dimensional scanning TEM (4D-STEM) data can overcome these limitations. Images obtained from a ~40-nanometer-thick MFI zeolite exhibited a lateral resolution of ~0.85 angstrom that enabled the identification of individual framework oxygen (O) atoms and the precise determination of the orientations of adsorbed molecules. Furthermore, a depth resolution of ~6.6 nanometers allowed probing of the three-dimensional distribution of O vacancies, as well as the phase boundaries in intergrown MFI and MEL zeolites. The 4D-STEM ptychography can be generally applied to other materials with similar high electron-beam sensitivity.

Regulating Extra-Framework Cations in Faujasite Zeolites for Capture of Trace Carbon Dioxide.

The development of cost‐effective sorbents for direct capture of trace CO2 (<1 %) from the atmosphere is an important and challenging task. Natural or commercial zeolites are promising sorbents, but their performance in adsorption of trace CO2 has been poorly explored to date. A systematic study on capture of trace CO2 by commercial faujasite zeolites reveals that the extra‐framework cations play a key role on their performance. Under dry conditions, Ba−X displays high dynamic uptake of 1.79 and 0.69 mmol g−1 at CO2 concentrations of 10000 and 1000 ppm, respectively, and shows excellent recyclability in the temperature‐swing adsorption processes. K−X exhibits perfect moisture resistance, and >95 % dry CO2 uptake can be preserved under relative humidity of 74 %. In situ solid‐state NMR spectroscopy, synchrotron X‐ray diffraction and neutron diffraction reveal two binding sites for CO2 in these zeolites, namely the basic framework oxygen atoms and the divalent alkaline earth metal ions. This study unlocks the potential of low‐cost natural zeolites for applications in direct air capture.

PFAS as emerging pollutants in the environment: A challenge with FAU type and silver-FAU exchanged zeolites for their removal from water

The adsorption efficiency of Zeolites Y (FAU-type topology) with different silica/alumina ratio towards perfluorooctanoic acid (PFOA, C8HF15O2) and perfluorooctane sulfonate (PFOS, C8F17SO3H), was investigated by combining batch adsorption and synchrotron X-ray powder diffraction (XRPD) analyses. Adsorption isotherms show that the saturation capacity of Y390 as-synthesized and Ag-exchanged sample, was for PFOA was ~43% and ~62%, and for PFOS was ~17% and ~32% in weight respectively. XRPD Rietveld refinements provided the exact location of PFOA and PFOS in the zeolites network before and after functionalization with Ag. PFAS are located at the center of the supercage, H- bonded to both co-adsorbed water molecules and framework oxygen atoms. Y-PFOA/PFOS and AgY-PFOA/PFOS oligomers strongly interact with oxygen atoms of the zeolite framework, thus immobilizing the pollutants in the zeolite microporosity. These findings indicated that these materials can be successfully used for the decontamination of water from perfluoroalkyl chemicals. Additionally, introduction of Ag onto zeolites could confer combined antifouling and adsorption properties thus exhibiting improved performances compared to zeolite as single compounds thus working synergistically in removal of PFAS from water.

Insights into the role of titanium sites in cyclohexanone ammoximation over titanium silicalite-1

The discovery of titanium silicalite-1(TS-1) is a milestone in the development of heterogeneous catalysts for the eco-benign and mild catalytic oxidation process. However, the interpretation of specific Ti location and its role in oxidation is severely hindered due to the limited crystal size of as-synthesized TS-1. Here, a high quality TS-1 (ca. 10 μm) was successfully fabricated by the pre-hydrolysis silica source strategy. As evidenced by powder X-ray diffraction crystallography, the preferred Ti locations were mainly found at T2, T4, and T11 in Ti-containing MFI framework. Combining multiple spectroscopy methods (i.e., FT-IR, UV–Vis, and UV Raman) and the reaction kinetics, the active center in TS-1 was demonstrated as an exposed site of hexa-coordinated titainum species. Furthermore, the powder X-ray crystallography also revealed that the ketone and oxime adsorbed in the zeolite host with intense host-guest interaction of framework oxygen atoms, and exhibiting an overwhelming spatial intimacy to the Ti species at the molecular scale. This unique phenomenon affords a plausible interpretation for the oxime reaction.

Elaboration and characterization of a new phosphate Mg1.74Cu1.26(PO4)2·H2O with morphotropic structure

The single crystal of Mg1.74Cu1.26(PO4)2·H2O was synthesized hydrothermally and its crystal structure was characterized through X-ray diffraction data. This new phosphate crystallizes in monoclinic system with P21/n space group. Its structure is discussed as a new member of the morphotropic series: MII3(PO4)2·nH2O Three types of cationic sites and two (PO4)3− anions make up its crystal structure. The first and second cationic sites are filled by Cu2+ and Mg2+ respectively, while the last one is occupied by a mixture of the two cations Mg2+ and Cu2+.The connecting of the basic building results in a net framework with two kinds of layers extending parallel to (−101). The first is built up of [Cu2O8] dimers joined by edge-sharing to PO4 tetrahedra. Common edges connecting [(Cu/Mg)2O8(H2O)2] dimers and [MgO6] octahedra form the second layer, which is more corrugated. Adjacent layers are linked by PO4 groups, revealing channels running parallel to the [101] direction and contain H atoms from coordinated H2O. Across these channels, water molecules connect with framework oxygen atoms through OH---O hydrogen bonding interactions.

On the nature of Pb species in Pb-exchanged zeolite stellerite (STI): A combined experimental and theoretical study

Structural properties of Pb-exchanged zeolites are of interest because of their applications in environmental remediation and in industrial processes. In this study, we report on a Pb-exchanged stellerite (Pb-STI), with particular focus on the cationic species, which form inside the zeolitic pores as a result of the exchange experiments. The produced zeolite had chemical composition Pb 13.4 (OH) 10 Al 17.4 Si 54.6 O 144 ∙38H 2 O, indicating that Pb was uptaken in form of [Pb(OH) x ] (2-x) species. The STI framework maintained the Fmmm space group characteristic of the type material. However, the extraframework occupants, Pb 2+ , H 2 O and OH − , were characterized by a strong positional-disorder. The latter was resolved and interpreted combining Extended X-ray Absorption Fine Structure (EXAFS) analysis with Molecular Dynamics (MD) simulations. On average, Pb 2+ ions are coordinated by 2 OH − and 1H 2 O at distances <2.5 Å, whereas bonds to framework oxygen-atoms were found only at longer distances (>2.8 Å). Pb 2+ adopts mainly a sided distorted coordination, indicating a stereochemical activity of the lone pair electrons. The obtained results were compared with those of other mono-cationic forms of STI zeolites. Based on the analysis of the framework distortion experienced after the incorporation of different metal ions, considerations are drawn on the potential effect of Pb 2+ on the thermal stability of STI framework type zeolites. • A STI zeolite was Pb-exchanged in 0.5 M Pb(CH 3 COO) 2 solution at pH = 5-6. • No framework distortion from the Fmmm symmetry was observed. • The EF species (Pb 2+ , OH − , and H 2 O) were highly disordered in the cavities. • Pb 2+ is sided coordinated by OH − and H 2 O. • No significant interactions occur between Pb and framework oxygen.

Molecular dynamics simulations of oxide ion migration in La2Ga3O7.5 with completely ordered interstitial oxide ions

Molecular dynamics simulations were performed on metastable La2Ga3O7.5 melilite ceramics, which were synthesized previously by direct crystallization of an under-cooled melt. La2Ga3O7.5 possesses the theoretically maximum content of interstitial oxide ions featuring complete ordering but ~3 orders of magnitude lower oxide ion conductivities compared with disordered La1·54Sr0.46Ga3O7.27 within 573–773 ​K. One dimensional diffusion paths of interstitial oxide ion was identified in La2Ga3O7.5 based on the molecular dynamics simulations, which shows that the interstitial oxide ions can migrate from the pentagonal ring containing GaO5 pyramid unit to its neighboring pentagonal ring free of interstitial oxide ion mainly along the a axis through a synergic mechanism involving continuous breaking and reformation of Ga2O8 units assisted by rotation and deformation of GaOn polyhedra and knock-on process between interstitial and framework oxygen atoms. Such constrained dimensionality on the diffusion paths arising from the confining effect for a large number of oxygen interstitials in the highly relaxed and ordered melilite superstructure accounts for the lower oxide ion conductivity in La2Ga3O7.5.

Side-chain Alkylation of Toluene with Methanol, Modification and Deactivation of Zeolite Catalysts of the Reaction

The review deals with main aspects of the toluene methylation reaction on basic catalysts. The side reactions of decomposition of methanol to CO and H2 on strong basic sites and ring alkylation of toluene on Lewis acid sites (cations of high polarizing ability) hinder obtaining high yields of the target products – styrene and ethylbenzene. Both types of sites are necessary for the course of the target reaction. So optimizing their strength and quantity is an important prerequisite for the selectivity of the side-chain alkylation catalysts. The advantage of fojasite-based systems for this reaction was confirmed by the works of many researchers. However, the possibilities of use of zeolites of other structural types and representatives of a new generation of molecular sieves are being studied, as well as ways of modifying such materials to increase their catalytic efficiency. The main direction of modification is to regulate the balance of acidity and basicity. Effective charge of framework oxygen atoms, which determines basicity of zeolite framework, increases due to the introduction of guest compounds into the catalyst, and this effect is more significant than influence on basicity of ion exchange for cations of elements of low electronegativity. However, the role of this method of modifying in increasing the selectivity remains crucial due to potentiality to decrease the Lewis acidity of cations. Compounds of other elements and transition metals also are used for modification, as well as promotion with metallic copper and silver. Techniques are applied, but not widely, to deprive the external surface of crystallites of active sites. This method of modification is effective for slowing down their deactivation by coke. Acid sites, in particular BAS, are most often distinguished among the sites responsible for coke formation. The mechanism of coke formation in the absence of such centers is also proposed. On the whole, this issue not fully disclosed and requires a deeper study.

Cd2+ incorporation in small-pore LEV/ERI intergrown zeolites: A multi-methodological study

Small-pore zeolites are successfully employed as catalysts, sorbents and molecular sieves. Their physiochemical properties can be tuned by modifying their extraframework cation (EF) composition via ion exchange. In this study, we investigate the crystal structure of a Cd-exchanged levyne ( LEV ) intergrown with erionite ( ERI ) by combining Single Crystal X-ray Diffraction (SCXRD), Molecular Dynamic simulations (MD) and Extended X-ray Absorption Fine-Structure spectroscopy (EXAFS). Data obtained from the different techniques consistently indicate that Cd 2+ in LEV is arranged in a nearly ordered fashion. In contrast, strong disorder of the EF species (Cd 2+ and H 2 O) is observed in the ERI cavities. Here, Cd 2+ forms aqueous complexes that are more mobile in comparison to LEV , where Cd 2+ bonds to both H 2 O and framework-oxygen atoms. The formation of Cd-clusters is excluded based on EXAFS analysis. Finally, to discriminate between thermal and static disorder, we propose a new approach based on combined MD and geometry optimization. • The crystal structure of Cd-LEV and Cd-ERI was determined by SCXRD. • Cd 2+ has a fairly regular octahedral coordination in Cd-LEV. • Cd 2+ is strongly disordered in the eri cavity. • Cd aqueous complexes form in Cd-ERI. • The disorder of the EF species was resolved by combining MD simulations and XAFS.

Thermal activation of NH4 precursor of acidic omega zeolite: A neutron and in-situ synchrotron powder diffraction combined study

Abstract For the first time, the real-time thermal activation of the NH4-omega zeolite (NH4-omega) was investigated through in situ synchrotron X-ray powder diffraction combined with neutron diffraction. The experimental approach allowed to study the precursor thermal behaviour upon heating, continuously monitoring the evolution of its structural features determined by NH4 ions and TMA calcination and dehydration processes. The activation of the precursor (H-omega sample) is fully reached at ⁓600 °C, when all the NH4 content has been released, dehydration mostly occurred and TMA degradation completed. From a structural point of view, Rietveld structural refinements highlighted relevant structural changes. In particular variations in intertetrahedral angles allowed to make hypothesis about the possible distribution of Bronsted acid sites on the framework oxygen atoms. The hypothesis was corroborated through the neutron Rietveld refinement of the deuterated and calcined omega (D-omega sample) that revealed the presence of two acid sites: D1 and D2 located on O5 and O2 framework oxygen atoms, respectively. Besides, neutron data confirmed the strong relation among framework and Bronsted acid sites geometry and short- and/or long-range interactions. In summary, non-conventional sources were successfully used to probe both activation and formation of acidic sites in omega precursor as well as determine the number and location of Bronsted sites in the deuterated omega.

Effect of zeolite topology on NH3-SCR activity and stability of Cu-exchanged zeolites

The catalytic activity and stability of Cu-exchanged zeolites Cu-ZSM-5 (MFI topology), Cu-TNU-9 (TUN), Cu-FER (FER) and steamed Cu-Y (FAU) were evaluated in the selective catalytic NO reduction by ammonia (NH3-SCR). The NH3-SCR activity of the investigated catalysts strongly depended on the feed composition, i.e., the presence of H2O (5.0 Vol.-% of H2O). The catalysts revealed high stability during 24 h of NH3-SCR reaction, also in the presence of water vapour. DR UV–vis and FT-IR with probe molecules (CO or NO) showed the presence of isolated Cu+/Cu2+ and aggregated copper oxide species. Although Cu-ZSM-5 is less active than Cu-TNU-9 in catalysing NO oxidation to NO2, both catalysts revealed similar activity in the NH3-SCR reaction. Applying Rapid-Scan FT-IR spectroscopy and 2D COS analysis the reaction mechanism of NH3-SCR on Cu-exchanged zeolites was investigated. It is proposed that NH3-SCR over the studied Cu-exchanged catalysts proceeds via [Cu(OH)(NH3)2(NO)]+ intermediates followed by the reduction of Cu(II) to Cu(I). The differentiated speciation of copper sites is reflected in their various susceptibility for the reduction and finally affects the catalytic activity and stability of the zeolitic catalysts. The formation of the [Cu(OH)(NH3)2(NO)]+ mixed ligand species is governed by the competition between the O2− and NH3 ligands. Also the stability of the forms initiating NH3-SCR, i.e., [Cu(NH3)4]2+, [Cu(OH)(NH3)3]+ and [Cu(NH3)2]+, is ruled by the confine interaction of ammonia molecules, which can be both adsorbed on a protonic site and ligated to copper sites with the framework oxygen atoms.

Comparative study of direct methylation of benzene with methane on cobalt-exchanged ZSM-5 and ZSM-11 zeolites

Methane exists in massive quantity on the Earth, and considerable efforts have been made to develop effective methods to convert chemically inert methane into value-added products. Recently, cobalt-exchanged ZSM-5 zeolite has been found to be a competent catalyst for non-oxidative methylation of benzene using methane as a methylation agent. Herein, cobalt-exchanged ZSM-11 zeolite, with close topological structure to that of ZSM-5 zeolite, was discovered to be capable of catalyzing non-oxidative methylation of benzene with methane for the first time. ZSM-5 and ZSM-11 zeolites both with Si/Al molar ratio of ca. 36 and 18 were synthesized. However, higher cobalt loadings were obtained on ZSM-11 zeolites, and cobalt-exchanged ZSM-11 catalysts displayed superior toluene production rates and turnover frequencies. In addition, low silica zeolite catalysts possessed higher methylation selectivity. Diffuse reflectance visible–near infrared spectra revealed that divalent cobalt cations on the ion exchange sites existed mainly in two different coordination states, and the cobalt cation on the α site, coordinating weakly to the framework oxygen atoms, was supposed to be the active site because of the positive correlation of its concentration and the methane activation performance. Since more α-type Co2+ could be created in ZSM-11 zeolites than in ZSM-5 zeolites with the same Si/Al molar ratio, it was concluded that cobalt-exchanged ZSM-11 catalyst was a promising candidate for the non-oxidative methylation of benzene with methane.

Hydrogen Bond Formation of Brønsted Acid Sites in Zeolites

Bronsted acid sites (BAS) in dry aluminosilicate zeolites, also known as bridging OH–H groups, may form hydrogen bonds with other framework oxygen atoms, O, in addition to BAS that are free of such...

Zeolite‐Encaged Single‐Atom Rhodium Catalysts: Highly‐Efficient Hydrogen Generation and Shape‐Selective Tandem Hydrogenation of Nitroarenes

AbstractSingle‐atom catalysts are emerging as a new frontier in heterogeneous catalysis because of their maximum atom utilization efficiency, but they usually suffer from inferior stability. Herein, we synthesized single‐atom Rh catalysts embedded in MFI‐type zeolites under hydrothermal conditions and subsequent ligand‐protected direct H2 reduction. Cs‐corrected scanning transmission electron microscopy and extended X‐ray absorption analyses revealed that single Rh atoms were encapsulated within 5‐membered rings and stabilized by zeolite framework oxygen atoms. The resultant catalysts exhibited excellent H2 generation rates from ammonia borane (AB) hydrolysis, up to 699 min−1 at 298 K, representing the top level among heterogeneous catalysts for AB hydrolysis. The catalysts also showed superior catalytic performance in shape‐selective tandem hydrogenation of various nitroarenes by coupling with AB hydrolysis, giving &gt;99 % yield of corresponding amine products.

Zeolite-Encaged Single-Atom Rhodium Catalysts: Highly-Efficient Hydrogen Generation and Shape-Selective Tandem Hydrogenation of Nitroarenes.

Single-atom catalysts are emerging as a new frontier in heterogeneous catalysis because of their maximum atom utilization efficiency, but they usually suffer from inferior stability. Herein, we synthesized single-atom Rh catalysts embedded in MFI-type zeolites under hydrothermal conditions and subsequent ligand-protected direct H2 reduction. Cs -corrected scanning transmission electron microscopy and extended X-ray absorption analyses revealed that single Rh atoms were encapsulated within 5-membered rings and stabilized by zeolite framework oxygen atoms. The resultant catalysts exhibited excellent H2 generation rates from ammonia borane (AB) hydrolysis, up to 699 min-1 at 298 K, representing the top level among heterogeneous catalysts for AB hydrolysis. The catalysts also showed superior catalytic performance in shape-selective tandem hydrogenation of various nitroarenes by coupling with AB hydrolysis, giving >99 % yield of corresponding amine products.