Structure of Dehydrated Zeolite Li−LSX by Neutron Diffraction: Evidence for a Low-Temperature Orthorhombic Faujasite

Monitoring by in situ neutron diffraction of simultaneous dehydration and Ni2+ mobility in partially exchanged NaY zeolites

The incoherent signal in neutron diffraction patterns allows correlating in real time hydration–dehydration levels and structural changes in zeolites.

Radiation Effects on Zeolite Nanomaterials - Some Potential Implications for Cleaning Liquid Nuclear Waste and for Enhanced Radioactive Decontamination

A series of group 1 and group 2 element cation-exchanged samples of zeolite X were prepared and exposed to a beam of spin-polarised, positive muons (of energy 28MeV/c) from a particle accelerator. Longitudinal field repolarisation measurements were made which identified, in each case, both diamagnetic and paramagnetic muon fractions. One of the paramagnetic components revealed a hyperfine field compatible with values reported previously for free muonium atoms (1585 G), while a secondary component was detected showing roughly half the hyperfine coupling measured for atomic muonium, leading to the conclusion that the muonium had become chemically-bound within the zeolite structure, probably via a single-electron bond to a coordinatively unsaturated aluminium atom. A significant “lost fraction” was also indicated along with superhyperfine and anisotropic components that repolarised in weaker magnetic fields (0-100G). As a general trend, the muonium atom yield was found to increase in order of the increasing radius/charge (riz) ratio, though it is constant within the experimental errors, while the chemically bound muonium was formed in similar proportion in all samples (14.1 + 1.9% of the total muonium polarisation), with the exception of HX and the parent NaX (from which the other zeolite samples were prepared by cation-exchange), in which it was absent. Along with data from zero-field experiments which were also undertaken, this is interpreted in terms of an entrapment or effective “scavenging” of radiolytic electrons by the cations (in the order Na+>K+; Mg2+>Ca2+>Sr2+>Ba2+) whose combination with positive muons is therefore impeded, thus reducing the yield of muonium atoms. Results for LiX are anomalous since the muonium yield is greater than expected, possibly due to a greater degree of covalency in the Li-0 bonding than pertains for the other cations. These results may have some implications for zeolites that suffer longterm radiation exposure/damage when they are used to clean radioactive cations (e.g. Cs +, Sr2+) from the waters of nuclear power stations and as materials whose cation-exchange capacity has been deliberately enhanced by prior exposure to ionising radiation.

Muonium--the second radioisotope of hydrogen: a remarkable and unique radiotracer in the chemical, materials, biological and environmental sciences

Muonium (Mu), may be regarded as a radioactive hydrogen atom with a positive muon as its nucleus, and is formed in a range of media which are irradiated with positive muons. This exotic atom can be considered as a second radioisotope of hydrogen, along with tritium. Addition of this light atom (with a mass 1/9th that of a normal hydrogen, protium, atom) to unsaturated organic molecules forms free radicals, in which the muon serves as a radioactive and magnetic probe of their kinetic and structural properties. Suitable examples are chosen to illustrate the very large functionality of organic radicals which have been measured using muons and various methods of muSR, where mu stands for muon, S for spin and R may refer to rotation, resonance or relaxation. The principal techniques illustrated are transverse-field muon spin rotation (TF-muSR), avoided level crossing muon spin resonance (ALC-muSR) and longitudinal-field muon spin relaxation (LF-muSRx). Structural studies of radicals, the determination of mechanisms for radical formation, the measurement of radical stabilisation energies, the determination of the kinetics of reactions of free muonium atoms and of free radicals have all been accomplished using TF-muSR methods. It is further shown that TF-muSR is most useful in measuring radical reaction rates in non-aqueous media, to provide information of relevance to cell membrane damage and repair Muonium may further be used as a mechanistic probe since it determines a true pattern of H-atom reactivity in molecules, against which results from similar radiolysed materials may be compared. [In many solid materials that are exposed to ionising radiation, apparent H-atom adduct radicals are detected but which originate from charge-neutralisation of positive holes (radical cations) and ejected electrons, without free H-atoms being formed. DNA is the superlative example of this. Free H-atoms normally feature in the province of radiolysed aqueous media]. The applications of ALC-muSR and LF-muSRx in studying the reorientation of reactive radicals on reactive surfaces forms the substantive proportion of the review: considered specifically are radicals sorbed in zeolites, in clays and in porous silica, in porous carbons and on ice-surfaces, in connection with their role as intermediates in catalytic systems, particularly hydrocarbon cracking and oxidation processes, and in atmospheric aerosol chemistry. The formation of muonium and other muon species in cation-exchanged zeolite-X samples are also considered, according to the evidence of longitudinal field repolarisation measurements. Finally, mention is given of the use of muSR techniques for studying radicals in the gas-phase.

Interaction of molecular nitrogen and oxygen with extraframework cations in zeolites with double six-membered rings of oxygen-bridged silicon and aluminum atoms: a DFT study

The interaction of N(2) and O(2) with extraframework cations of zeolite frameworks was studied by DFT, using the B3LYP method. The extraframework cation sites located in the vicinity of the double six-member rings (D6R) of FAU zeolites (SI, SI', SIII') were considered and clusters with composition (M(n)(+))(2/)(n)()H(12)Si(10)Al(2)O(18), M = Li(+), Na(+), K(+), Ca(2+), were selected to represent the adsorption centers. The cation sites SII in the center of single six-membered rings (S6R) were modeled by [M(I)H(12)Si(4)Al(2)O(6)](-) and M(II)H(12)Si(4)Al(2)O(6) clusters. The adsorption energy of N(2) and O(2) is the highest for Li(+) cations at the SIII' cation sites, while for the SI' and SII sites the adsorption energies decrease in the order Ca(2+) > Na(+) > Li(+). The calculated small N(2) adsorption energy for Li(+) cations at SII sites suggests that these sites do not take part in the sorption process in agreement with results of NMR studies and Monte Carlo simulations. The N(2) adsorption complexes with the extraframework cations are linear, while those of O(2) are bent regardless of the extraframework cation location. The SIII' cation sites are the most favorable ones with respect to N(2) adsorption capacity and N(2)/O(2) selectivity; the SII sites are less selective and the SI sites are not accessible.

Influence of extra-framework cations on the adsorption properties of X-faujasite systems: microcalorimetry and molecular simulations

Isotherms and differential enthalpies of adsorption are obtained for nitrogen at ambient temperature on monovalent (Li(+), Na(+), K(+)) and divalent (Ca(2+), Ba(2+), Sr(2+), Mn(2+)) substituted X-faujasite systems by microcalorimetry measurements. These experimental data are compared with those obtained by combining grand canonical Monte Carlo simulations and newly derived force fields for describing the interactions between the extra-framework cations and the adsorbates obtained from a simple model based only on the intrinsic properties of the cations. It is the first time that such good qualitative agreement is reported between experiment and simulation for a series of both monovalent and divalent cations.

Theoretical Study of CuIY Zeolite: Structure and Electronic Properties

The structural and electronic properties of the accessible Cu(I) site of a faujasite-type zeolite have been studied, by use of large cluster models and a density functional theory-based methodology. We demonstrate that the local ideal C(3) symmetry of the Cu(I) site II is broken. The Cu(I) cation is bonded to the zeolite framework by one bond of about 2.26 A and two shorter ones of 2.07 A. We demonstrate that only one cation position exists at this site. This result is also confirmed by a molecular electrostatic potential analysis. We show that local properties at site II, as well as the global properties of the solid (frontier orbitals), do not depend on the Al and cation distribution and only slightly on the cocation nature. Taking into account the present results and well-known experimental data, we propose that specific catalytic behaviors are correlated with local response properties, such as the local acid strength or, in other reactions, specific local architecture or confinement.

Comparison of the (17)O NMR spectra of zeolites LTA and LSX

17O NMR studies of various cation-exchanged LTA and LSX zeolites have shown similarities between the two systems. LSX samples containing divalent cations contain resonances with similar chemical shifts to those previously assigned to 'bare' framework oxygen atoms in Ca-LTA and Sr-LTA. The assignments are consistent with the trends seen in the spectra of monovalent cation-containing LSX and LTA zeolites, which show an increase in the average chemical shift with increasing cationic radius. The spectrum of Li-LSX, like Na-LSX, can be assigned based on the T-O-T bond angles. Gas sorption studies on Li-LSX are used to help identify the framework oxygen atoms that form the beta-cages and demonstrate the sensitivity of the (17)O shifts to gas loading.

Probing the Location and Distribution of Paramagnetic Centers in Alkali Metal-Loaded Zeolites through 7Li MAS NMR

The nature and surroundings of lithium cations in lithium-exchanged X and A zeolites following loading with the alkali metals Na, K, Rb, and Cs have been studied through (7)Li solid-state NMR spectroscopy. It is demonstrated that the lithium in these zeolites is stable with respect to reduction by the other alkali metals. Even though the lithium cations are not directly involved in chemical interactions with the excess electrons introduced in the doping process, the corresponding (7)Li NMR spectra are extremely sensitive to paramagnetic species that are located inside the zeolite cavities. This sensitivity makes (7)Li NMR a useful probe to study the formation, distribution, and transformation of such species.

Bridging Methoxy Groups in NaY, NaX and NaLSX Zeolites

Distribution of chemisorbed methyl groups and sodium cations in the structure of NaY, NaX and NaLSX zeolites was estimated by neutron diffraction. Chemisorbed methyl groups were prepared in the structure by reaction of methyl iodide with reactive sodium cations available in SII and SIII positions of faujasites. Methyl cations CH3+, formed in the reaction, react immediately with the lattice oxygen forming surface bonded methyl groups in bridging configuration. 13C NMR signals of chemisorbed surface species and their linear dependence on the intermediate electronegativity of the zeolite lie in the interval from 53 ppm for most basic CsLSX to 58 ppm TMS for stabilized and acid leached sample of H,NaY-St. Changes in the distribution of structural sodium cations in the lattice after chemisorption of methyl cations have been detected. C-O distances in surface methoxy groups in void cavities were longer than in ordinary crystalline organometallic compounds with bridging methoxy groups. The location of chemisorbed methyl groups at the O1 lattice oxygen type was most probable for NaY. Nuclear densities of chemisorbed methyl groups were detected in NaX at O1 and at O4 lattice oxygens. The origin of the split signal at 58 ppm on NaX and NaLSX samples has been discussed.

Simultaneous occupation of SI and SI' cation sites in dehydrated zeolite LSX

Simultaneous occupation of adjacent SI (or SIa) and SI' sites is calculated to be favourable in dehydrated zeolite K-LSX (supporting the experimental work of Paillaud et al.), although such a configuration is unlikely in other dehydrated LSX zeolites.

Investigation on the nature of the adsorption sites of pyrrole in alkali-exchanged zeolite y by nuclear magnetic resonance in combination with infrared spectroscopy

Multinuclear solid-state NMR and infrared spectroscopy have been applied to investigate the host-guest interactions and the nature of the adsorption sites of pyrrole on alkali-exchanged zeolites Y (LiNaY, NaY, KNaY, and CsNaY). The presence of pyrrole provokes changes in the MAS NMR spectra of (23)Na, (7)Li, and (133)Cs to a degree dependent upon the amount adsorbed. The decrease in the quadrupolar coupling constant for (23)Na as well as the shift for (7)Li and (133)Cs signals are attributed to the interaction of the cation with the pyrrole ring system. The adsorption of pyrrole induces the displacement of cations located at SI' and SII sites toward the supercage to bind the guest molecules. In this way, the distribution of the cations at nonframework sites depends on the amount of adsorbate in the zeolite. At low loadings, pyrrole molecules bind preferentially to more electropositive cation in partially exchanged zeolites Y. Quantitative analysis by (1)H NMR shows that the cation-pyrrole complexes formed possess a stoichiometry of 1:1. The origin of the basic site heterogeneity, evidenced by the presence of several components in the -NH infrared stretching band, is investigated assuming that the heterocycle of pyrrole interacts with cations at SII sites in the supercage and the -NH group forms a hydrogen bond with a basic oxygen atom placed in the framework six-member ring. Making use of the information derived from NMR, it is concluded that the main source of basic site heterogeneity comes from the number of aluminum atoms in the six-member rings of the SII site where the alkaline cation is located.