Zeolite Framework Structure Research Articles

Using electron microscopy to complement X-ray powder diffraction data to solve complex crystal structures

High-resolution X-ray powder diffraction and electron microscopy techniques yield remarkably complementary information for crystal structure determination. By combining the two types of data, structures of polycrystalline materials that resist solution by more conventional methods can be tackled. In particular, crystallographic phase information extracted either from high-resolution transmission electron microscopy (HRTEM) images or from precession electron diffraction (PED) data has proven to be extremely useful in such cases. PED data can also be used to identify weak reflections in the diffraction pattern, and this information, in turn, can be exploited to improve the partitioning of the intensities of reflections that overlap in the powder diffraction pattern. The dual-space (diffraction and real space) structure determination programs Focus and pCF have been shown to be especially well-suited for combining the two different types of data. The power of this approach is illustrated with the details of the solutions of the structures of TNU-9, IM-5 and SSZ-74, the three most complex zeolite framework structures known.

Spectroscopic Signatures of Nitrogen-Substituted Zeolites

Nanoporous acid catalysts such as zeolites form the backbone of catalytic technologies for refining petroleum. With the promise of a biomass economy, new catalyst systems will have to be discovered, making shape-selective base catalysts especially important because of the high oxygen content in biomass-derived feedstocks. Strongly basic zeolites are attractive candidates, but such materials are notoriously difficult to make due to the strong inherent acidity of aluminosilicates. Several research groups have endeavored to produce strongly basic zeolites by treating zeolites with amines, but to date there is no compelling evidence that nitrogen is incorporated into zeolite frameworks. In this communication, we detail synthesis, NMR spectroscopy, and quantum mechanical calculations showing that nitrogen adds onto both surface and interior sites while preserving the framework structure of zeolites. This finding is crucial for the rational design of new biomass-refinement catalysts, allowing 50 years of zeolite science to be brought to bear on the catalytic synthesis of biofuels.

Temperature-dependent structural study of microporous CsAlSi 5O 12

CsAlSi 5O 12 crystals were synthesized at high temperature by slow cooling of a vanadium oxide flux. Single-crystal X-ray diffraction structure analysis and electron microprobe analyses yielded the microporous CAS zeolite framework structure of Cs 0.85Al 0.85Si 5.15O 12 composition. High-temperature single-crystal and powder X-ray diffraction studies were utilized to analyze anisotropic thermal expansion. Rietveld refined cell constants from powder diffraction data, measured in steps of 25 °C up to 700 °C, show a significant decrease in expansion above 500 °C. At 500 °C, a displacive, static disorder–dynamic disorder-type phase transition from the acentric low-temperature space group Ama2 to centrosymmetric Amam ( Cmcm in standard setting) was found. Thermal expansion below the phase transition is governed by rigid-body TO 4 rotations accompanied by stretching of T–O–T angles. Above the phase transition at 500 °C all atoms, except one oxygen (O6), are fixed on mirror planes. Temperature-dependent polarized Raman single-crystal spectra between −270 and 300 °C and unpolarized spectra between room temperature and 1000 °C become increasingly less resolved with rising temperature confirming the disordered static–disordered dynamic type of the phase transition.

Photo-degradation of acid green dye over Co–ZSM-5 catalysts prepared by incipient wetness impregnation technique

Co–ZSM-5 catalysts with different Co-loadings (2–30 wt.%) were prepared by incipient wetness impregnation method. The prepared solid catalysts were characterized by X-ray diffraction, FTIR, in situ FTIR of pyridine adsorption and surface area measurements. The XRD data presented disintegration in the zeolitic crystalline structure accompanied by an increase in particle size of the prepared solids. New phases, Co 3O 4 and Co 2SiO 4, were detected with increasing the Co-loading, which indicate the strong interaction of cobalt ions with the ZSM-5 zeolite. FTIR study proved the presence of Co ions in stabilized sites inside the ZSM-5 framework. The in situ FTIR of adsorbed pyridine determined the type and relative strength of acidity on the surface of the prepared solids. The acidity switched from B-acid sites to L-acid sites with impregnation of cobalt ions in ZSM-5 zeolite. The acidity decreased with increasing Co-loading, which might be due to the destruction of zeolite framework and presence of new phases such as cobalt silicate and cobalt oxide on the surface. The surface texture characteristics changed with the promotion of ZSM-5 by cobalt ions, since a decrease of surface area, mean pore radius and pore volume was observed. The assessment of the catalytic activity was performed by the use of the photo-degradation of acid green (AG) dye as a probe reaction in presence of H 2O 2 as an oxidant. The pH value controlled the degradation rate since a gradual increase of AG degradation rate was observed with increasing pH value and the optimum H 2O 2 concentration was 61.6 mmol/l. It was found that, the AG degradation rate increased until an optimum value of Co-loading ( ca. 10 wt.%), beyond which a monotonic decrease of reaction rate was recognized. The experimental data pointed to the importance of both the cobalt moieties and the zeolite framework structure in the AG degradation reaction.

The flexibility window in zeolites

Today synthetic zeolites are the most important catalysts in petrochemical refineries because of their high internal surface areas and molecular-sieving properties. There have been considerable efforts to synthesize new zeolites with specific pore geometries, to add to the 167 available at present. Millions of hypothetical structures have been generated on the basis of energy minimization, and there is an ongoing search for criteria capable of predicting new zeolite structures. Here we show, by geometric simulation, that all realizable zeolite framework structures show a flexibility window over a range of densities. We conjecture that this flexibility window is a necessary structural feature that enables zeolite synthesis, and therefore provides a valuable selection criterion when evaluating hypothetical zeolite framework structures as potential synthetic targets. We show that it is a general feature that experimental densities of silica zeolites lie at the low-density edge of this window--as the pores are driven to their maximum volume by Coulomb inflation. This is in contrast to most solids, which have the highest density consistent with the local chemical and geometrical constraints.

Coke deposition and characterization on titanium silicalite-1 catalyst in cyclohexanone ammoximation

In this work, the changes of catalytic activity, as well as the main deactivation mode were investigated on a commercial titanium silicalite-1 (TS-1) catalyst under simulated industrial conditions in continuous tank slurry reactor. The conclusions about deactivation behavior have been achieved based on the changes of activity, conversion and selectivity. XRD and FTIR analyses indicate the MFI framework structure of zeolite cannot be changed evidently during the reaction. It is found by comparative experiments that the main reason for catalyst deactivation is coke formation on the surface of TS-1. Results from TGA-DTA, N 2 adsorption and pyridine TPD analyses of used catalysts demonstrate there are two types of coke deposited mainly on micropores. Soft coke is located near Ti sites and can be removed by oxidation under 350 °C, while refractory coke can be oxidized by air until 700 °C. Analysis results of GC–MS for soluble coke and NMR, FTIR spectra for insoluble coke indicate that N-containing polycyclic aromatic and/or polyalkene species are the main components of coke deposits.

A systematic topological search for the framework of ZSM-10

The application of a new technique for zeolite framework structure solution is described that exhaustively enumerates every possible topology consistent with known unit-cell dimensions and space-group symmetry. It is shown that computer-generated on-line databases of hypothetical crystal structures can radically augment structure building in the pre-refinement stage.

Methane dehydroaromatization over Mo/HZSM-5 catalysts: The reactivity of MoC x species formed from MoO x associated and non-associated with Brönsted acid sites

The catalytic performances of methane dehydroaromatization (MDA) under non-oxidative conditions over 6 wt.% Mo/HZSM-5 catalysts calcined for different durations of time at 773 K have been investigated in combination with ex situ 1H MAS NMR characterization. Prolongation of the calcination time at 773 K is in favor of the diffusion of the Mo species on the external surface and the migration of Mo species into the channels, resulting in a further decrease in the number of Brönsted acid sites, while causing only a slight change in the Mo contents of the bulk and in the framework structure of the HZSM-5 zeolite. The MoO x species associated and non-associated with the Brönsted acid sites can be estimated quantitatively based on the 1H MAS NMR measurements as well as on the assumption of a stoichiometry ratio of 1:1 between the Mo species and the Brönsted acid sites. Calcining the 6 wt.% Mo/HZSM-5 catalyst at 773 K for 18 h can cause the MoO x species to associate with the Brönsted acid sites, while a 6 wt.% Mo/SiO 2 sample can be taken as a catalyst in which all MoO x species are non-associated with the Brönsted acid sites. The TOF data at different times on stream on the 6 wt.% Mo/HZSM-5 catalyst calcined at 773 K for 18 h and on the 6 wt.% Mo/SiO 2 catalyst reveal that the MoC x species formed from MoO x associated with the Brönsted acid sites are more active and stable than those formed from MoO x non-associated with the Brönsted acid sites. An analysis of the TPO profiles recorded on the used 6 wt.% Mo/HZSM-5 catalysts calcined for different durations of time combined with the TGA measurements also reveals that the more of the MoC x species formed from MoO x species associated with the Brönsted acid sites, the lower the amount of coke that will be deposited on it. The decrease of the coke amount is mainly due to a decrease in the coke burnt-off at high temperature.

The Art of Zeolite Structure Analysis

AbstractFor Abstract see ChemInform Abstract in Full Text.

Ammonium removal from aqueous solution by natural zeolite, Transcarpathian mordenite, kinetics, equilibrium and column tests

The scope of this study was ammonium ions removal from synthetic aqueous solutions by raw and pretreated natural zeolite, Transcarpathian mordenite under static and dynamic conditions. The cation exchange capacity of the Transcarpathian mordenite regarding ammonium ions was evaluated as 1.64 meq/g at 1000 mg/l initial NH 4–N concentration. The dynamic exchange capacity exceeded one estimated in equilibrium study at the same initial concentration that may be conditioned by the constant removal of ion exchange products. Ammonium uptake rate was controlled by particle diffusion with diffusion coefficients determined in the range of 0.7–3.6 × 10 −12 m 2/s. Efficiency of ammonium sorption may be improved by slowing down of initial solution rate in the column test and non-significantly by NaCl and HCl pretreatment of the mordenite. Ammonium sorption by the mordenite increased from the coarser to the finer fraction but this dependence became weaker to low flow rates. It was established that hydrogen ions displaced exchangeable cations on the mordenite in distilled water and hydrochloric acid with destroy of the zeolite framework structure in the last case. NH 4 +-ions removal from aqueous solutions occurs mainly by ion exchange with Na +- and Ca 2+-ions at the practically equal parts of them because of the weakest affinity of the mordenite to these cations.

Skeletal isomerization of octadecane on bifunctional ZSM-23 zeolite catalyst

Octadecane was isomerized over three different Pt/H-ZSM-23 samples. The distributions of methylheptadecane and dimethylheptadecane skeletal isomers obtained on the Pt/ZSM-23 samples were very similar. Positional isomer distributions are fingerprints of the zeolite framework structure. The independence of skeletal isomer distributions from Al-gradients and particle size constitutes a strong argument in favor of pore-mouth catalysis.

Ammonium sorption from aqueous solutions by the natural zeolite Transcarpathian clinoptilolite studied under dynamic conditions

The scope of this study is ammonium-ion uptake from synthetic aqueous solutions onto raw and pretreated forms of the natural zeolite Transcarpathian clinoptilolite under dynamic conditions. Hydrogen ions displaced exchangeable cations on the clinoptilolite in distilled water (sodium ions) and hydrochloric acid (sodium, potassium, and calcium ions) and destroyed the zeolite framework structure in the last case. Ammonium uptake onto the zeolite occurs by exchange with Na +, Ca 2+, and K + ions. Although Na + ions were observed to be more easily exchanged for both hydrogen and ammonium ions, the role of Ca 2+ ions increased with zeolite saturation by NH + 4 ions. The maximum sorption capacity of the clinoptilolite toward NH + 4 ions, estimated under dynamic conditions, is significantly higher than that measured under static conditions; proximity of the values of a distribution coefficient and a retardation factor for different conditions (215–265 dm 3/kg and 979–1107, respectively) allows us to use these parameters to model ammonium uptake onto the clinoptilolite. Slowing down or interruption in filtration resulted in the improvement of ammonium sorption properties of the zeolite. The ammonium removal improves with use of the finer fractions of the clinoptilolite up to 0.35 mm. A recycling study results confirmed the importance of external diffusion for ammonium sorption by the clinoptilolite. Preliminary treatment of the sorbent confirmed the predominant importance of the ion-exchange mechanism. The advantage of prior NaCl treatment of the clinoptilolite in improvement of ammonium removal over the other techniques was shown.

Symmetry-Based 29Si Dipolar Recoupling Magic Angle Spinning NMR Spectroscopy: A New Method for Investigating Three-Dimensional Structures of Zeolite Frameworks

A new 29Si solid-state MAS NMR experiment is described for investigating the framework structures of pure silica zeolites. The symmetry-based homonuclear dipolar recoupling sequence SR26411 has been incorporated into a two-dimensional NMR experiment to probe the Si-O-Si bonding connectivities and long-range Si-Si distances in zeolite frameworks. This dipolar recoupling sequence is shown to have a number of advantages over the J-coupling-based INADEQUATE experiment. For the clathrasil Sigma-2, it is demonstrated that there is excellent agreement between experimental double-quantum build-up curves obtained from a series of two-dimensional double-quantum correlation spectra and simulated curves which consider all Si-Si distances out to 8 A. This result suggests that this experiment could be used to solve zeolite frameworks with unknown structures.

Improvements to a Peak Assignment Algorithm for Two-Dimensional NMR Correlation Spectra of Zeolites Using Graph Theory

An improved version of a peak assignment algorithm for two-dimensional NMR correlation spectra of zeolites is described. Both the zeolite framework structure and the two-dimensional NMR correlation spectrum can be expressed as graphs such that the task of peak assignment is to find labelings of the vertices of the graphs which show that the two graphs are homomorphic. In this paper, it is shown that an initial analysis of the structure and spectrum graphs provides information about the possible identities of the peaks which can lead to large gains in the efficiency of the peak assignment algorithm. The improvements in the algorithm are demonstrated for the peak assignment of solid state magic-angle spinning 29Si NMR spectra of the purely siliceous zeolite ZSM-5 from two-dimensional 29Si INADEQUATE NMR spectra.

Zeolite-encapsulated Co(II), Ni(II) and Cu(II) complexes as catalysts for partial oxidation of benzyl alcohol and ethylbenzene

Co(II), Ni(II) and Cu(II) complexes of dimethylglyoxime and N, N′-ethylenebis(7-methylsalicylideneamine) have been synthesized in situ in Y zeolite by the reaction of ion-exchanged metal ions with the flexible ligand molecules that had diffused into the cavities. The hybrid materials obtained have been characterized by elemental analysis, SEM, XRD, surface area, pore volume, magnetic moment, FTIR, UV-Vis and EPR techniques. Analysis of data indicates the formation of complexes in the pores without affecting the zeolite framework structure, the absence of any extraneous species and the geometry of encapsulated complexes. The catalytic activities for hydrogen peroxide decomposition and oxidation of benzyl alcohol and ethylbenzene of zeolite complexes are reported. Zeolite Cu(II) complexes were found to be more active than the corresponding Co(II) and Ni(II) complexes for oxidation reactions. The catalytic properties of the complexes are influenced by their geometry and by the steric environment of the active sites. Zeolite complexes are stable enough to be reused and are suitable to be utilized as partial oxidation catalysts.

An FT-Raman Spectroscopic Study of the Conformational Properties of Chlorocyclohexane in Zeolites

The conformational behavior of chlorocyclohexane (ClCH) adsorbed inside several representative zeolites such as completely siliceous ZSM-5, Na−Y, and siliceous Y (Si−Y) has been investigated by FT-Raman spectroscopy. The results have clearly shown that the conformational and dynamic properties of ClCH depend on the zeolitic framework structure, the presence of charge-balancing cations, as well as the Si/Al ratio. Compared to pure liquid, the population of equatorial conformation increases upon adsorption into ZSM-5. Apparently, the ZSM-5 framework stabilizes the equatorial conformation (with a larger molecular volume) via maximizing the van der Waals interactions between the host and guest. Within the ZSM-5 framework, the axial and equatorial conformers are still in dynamic equilibrium. It continues to shift toward the equatorial configuration upon lowering the temperature, and at 153 K the ClCH in ZSM-5 adopts the equatorial conformation exclusively. For the ClCH/Na−Y system, the situation is remarkably ...

An FT-Raman Spectroscopic Study of the Conformational Behavior of trans-1,4-Dichlorocyclohexane Adsorbed in Zeolites

We have examined the conformational properties of trans-1,4-dichlorocyclohexane (DCC) adsorbed in several zeolites, including completely siliceous ZSM-5, Na−Y and siliceous Y, by using FT-Raman spectroscopy. The results indicate that the conformational behavior of DCC is dependent on the zeolite framework structure, Si/Al ratio, and the charge-balancing cations. The conformational behavior of DCC in ZSM-5 differs from that in nonpolar solvents such as carbon tetrachloride in that the population of diequatorial (ee) conformer is significantly increased relative to the diaxial (aa) form upon adsorption. The ee and aa conformers are still in dynamic equilibrium as in solution, and lowering the temperature shifts the equilibrium further toward the ee configuration. The situation for DCC incorporated inside Na−Y is markedly different. Upon adsorption, the conformation of DCC is frozen by interactions with the extraframework sodium ions in the supercages of Na−Y. These interactions also lock the guest molecules...

Redox catalysis over metallo-zeolites: Contribution to environmental catalysis

The paper reviews development in analysis and understanding of the structure of active sites in metallo-zeolites and their function in SCR–NOx, NO–NO2 equilibration reaction, and benzene oxidation with N2O to phenol. The opportunities and limitations of various spectral techniques (FTIR, UV-Vis, ESR, EXAFS, Moessbauer, Raman spectroscopy) in connection with these studies are discussed. The attention is centered on Co and Fe species planted in pentasil ring zeolites of various topologies, specifically on Co-beta and Fe-oxo species in zeolites, exhibiting stable SCR–NOx activity under presence of high excess of water vapor existing in real exhaust gases from combustion processes. Also participation of the protonic, Al-Lewis sites and specific Fe-oxo sites is analyzed with respect to these redox reactions. It is shown that the zeolite matrix has a dramatic effect on the SCR–NOx activity of the hosted metal ion species. Main parameters controlling metallo-zeolite activity are the type and structure of metallo-species, distances between the cationic sites and concentration and distribution of Al in the framework. Specific Fe-oxo structures coordinated to zeolite framework exhibit extremely high activity in benzene oxidation with N2O to phenol in contrast to protonic and Al-Lewis sites which contribute to these reactions only by activation of hydrocarbons. Analogous (but probably not identical) Fe-oxo structures bear a potential to be applied in SCR–NOx with C3+ paraffins under high excess of water vapor. Stable SCR–NOx activity of Co-beta zeolite indicates either presence of Co-oxo species of extremely high activity or positive effect of high density of aluminium in specific local framework structure of Co-beta zeolite.

The effect of the framework structure on the chemical properties of the vanadium oxide species incorporated within zeolites

V-silicalite catalysts (VS-1 and VS-2) prepared by hydrothermal synthesis have been studied by ESR, XAFS (XANES and EXAFS) and photoluminescence spectroscopy. The in situ characterization of these V-silicalites shows that vanadium is present within the zeolitic framework as a highly dispersed tetrahedrally coordinated V-oxides, VO 4 unit, having a short VO bond length. Photoluminescence spectroscopy in static and dynamic mode, as well as XAFS studies allow to detect in the V-silicalites different V species than that present in V-HMS or V/SiO 2, in terms of VO bond length, vibrational energy, ▪ bond angle and lifetime of the excited triplet state. It is suggested that the combined contribution of the neighboring SiOH group attached to the VO 4 unit and the zeolitic rigid framework structure of V-silicalites cause a more significant and pronounced effect on the chemical properties of the VO 4 unit than the flexible structure of V-HMS or V/SiO 2. Moreover, the dynamic quenching of the phosphorescence by the addition of reactant molecules such as NO or propane indicates that the V species in the excited triplet state can be expected to be the active sites for the photocatalytic reactions.

Ti atom in MFI zeolite framework: a large cluster model study by ONIOM method

Ti atom siting in MFI zeolite framework was studied using one large cluster model containing about 200 atoms, which was made by trimming of MFI zeolite crystal, by means of ONIOM quantum mechanics/molecular mechanics (QM/MM and QM/QM hybrid) method. Si atoms at 12 T sites were replaced succeedingly with a Ti atom and resulted Ti(OSi) 4 moiety was locally relaxed by ONIOM (B3LYP/Lanl2dz:UFF) method. The other atoms at crystallographic structure of MFI zeolite framework were treated as an inorganic matrix and fixed at its original position. Single-point energies of T sites were calculated for the optimized structure by ONIOM (B3LYP/Lanl2dz:B3LYP/Lanl2mb) and the relative stability between T sites was compared.