Lennox E. Iton

The EPR spectra of Gd3+ and Eu2+ in glassy systems

A critical analysis is given of the EPR spectrum exhibited by the rare‐earth S‐state ions, Gd3+ and Eu2+, in glassy and disordered polycrystalline materials. The analysis of this spectrum and of its previous interpretations is based on (a) a set of criteria derived from a wide range of experimental EPR and optical data, and (b) a first principles computer simulation method which explicitly incorporates broad distributions in the crystal field interaction parameters. It is found that all four previous interpretations of the glassy spectrum are unsatisfactory, each failing to satisfy two or more of the criteria imposed by the full range of data. The correct general solution to the spectrum is unequivocally established and shown to be a convolution of (a) a broad and essentially unimodal distribution of second‐order crystal field parameters, b02, with a maximum in the approximate range 0.051≲b02 ≲0.056 cm−1, and (b) a broad distribution of asymmetry parameters, λ’=b22/b02, with appreciable probability over t...

EPR study of europium ions in type A zeolite. The general classification of the EPR spectra of S‐state rare‐earth ions in disordered polycrystalline or glassy matrices

A study of polycrystalline A zeolite exchanged with Eu(OH)2 solution has been performed using EPR spectroscopy. Detailed computer simulation techniques have been used to simulate EPR spectra from first principles, with the explicit incorporation of random distributions in the crystal field interaction parameters. This method is equally well applicable to the analysis of the spectra of glasses. Two components in the spectrum of the dehydrated material have been distinguished. The first corresponds to three‐ coordinate Eu2+ ions at regular six‐ring sites in the zeolite, giving rise to an axial EPR spectrum with a large (b02)avg value of 0.32 cm−1. The second corresponds to an orthorhombic EPR spectrum with a (b02)avg value of similar magnitude, and a rhombic component b22≂0.34 (b02)avg. This latter feature is attributed to three‐coordinate Eu2+ ions located at six‐ring sites, where one of the oxide ions in the ring has been protonated. The magnitudes of the axial components of these spin Hamiltonian crystal...

Stabilization of Co (III) in aluminophosphate molecular sieve frameworks

Abstract Electronic absorption, electron paramagnetic resonance (e.p.r.), and infrared spectroscopic measurements have provided definitive evidence for the stabilization of Co(III) in framework positions of calcined, cobalt-substituted aluminophosphate molecular sieves. Calcining of the templated form of Co(II)APO-34, Co(II)APSO-34, and Co(II)APO-5 leads to significantly reduced intensity of the Co(II)-associated d-d electronic transitions and e.p.r. signal strength. Reduction of the Co(III) to Co(II) with H2 produces new protonated sites in the molecular sieve framework structure, whereas reduction with NO leads to the appearance of bands due to NO+ and coordinated NO in the infrared spectrum of the molecular sieve. The redox character of these cobalt-substituted aluminophosphates suggests that potential may exist for incorporation of an oxidative coupling step into the shape-selective catalysis properties exhibited by cobalt-substituted AlPO4s.

Predicted proton affinities of H3SiO−, H3SiOH, H3SiOSiH3, and H3SiOAlH−3

Abstract The proton affinities of H 3 SiO − , H 3 SiOH, H 3 SiOSiH 3 , and H 3 SiOAlH − 3 are predicted from ab initio molecular orbital theory using the recently developed GAUSSIAN-1 (G1) theory for computing accurate molecular energies. The calculated proton affinities (PA 0 ) are 356.0, 177.4, 179.0, and 302.4 kcal/mol, respectively. The proton affinities are also reported at lower levels of theory for comparison with the G1 results. Finally, a comparison is presented of the trends in the OH stretching frequencies and proton affinities.

A new approach to the generation of metal-bearing, medium-pore, shape-selective zeolites for fischer-tropsch catalysis: spectroscopic studies of zeolites

Abstract We have combined two recent developments in zeolite synthesis and modification to demonstrate a new approach to the generation of metalbearing, medium-pore, shape-selective zeolites for use as catalysts in Fischer-Tropsch conversions. The sequence of steps is: (1) synthesis of an alumino-ferrisilicate zeolite having the ZSM-5-type structure; (2) removal of the organic base template incorporated in the channel system during synthesis; (3) formation of a polycyano inclusion compound in the AFS zeolite; and (4) reduction of the inclusion compound by hydrogen at ~ 400 °C. PAS and EPR spectroscopy have been used to establish that the as-synthesized AFS zeolite contains Fe 3+ ions in both framework and non-framework sites. Estimates of the orthorhombic crystal field interaction parameters for the two kinds of Fe 3+ are obtained from the EPR spectra. The non-framework Fe 3+ ions react with complex polycyano anions in solution to precipitate the inclusion compounds in the zeolite. PAS has been used to monitor the reaction chemistry and establish the formation of the following inclusion compounds in the AFS zeolite: (1) KFe III [Ru II (CN) 6 ]; (2) KFe III [Co II (CN) 6 ]; and (3) Fe III [Fe II (CN) 5 (NO)]. The latter two compounds are the products of slow, secondary one-electron reductions of the starting anionic complex. It is shown that these inclusion compounds must occur in grain boundaries of the AFS zeolite microcrystallites, rather than in the intracrystalline channel structure. FMR spectroscopy has been used to confirm the formation of the metallic (Fe) and bimetallic (Fe/Ru and Fe/Co) particles as products of the reduction of the inclusion compounds by hydrogen. The application of some other spectroscopic techniques in recent studies of cations, complexes, and metal particles in zeolites is reviewed in a brief appendix. The examples cited illustrate the use of high-resolution solid-state NMR, nuclear spin relaxation studies, FMR, EXAFS, and XANES.

An assessment of density functional methods for studying molecular adsorption in cluster models of zeolites

Abstract The performance of six density functional theory (DFT) methods has been tested for a zeolite cluster containing three tetrahedral atoms (3T) and the complexes it forms with water and methanol molecules. The DFT methods (BLYP, BP86, BPW91, B3LYP, B3P86, B3PW91) give results in good agreement with second-order perturbation theory (MP2). The results in this paper provide evidence of the suitability of DFT methods for studying hydrogen-bonded adsorption complexes in zeolites. Generally, the hybrid DFT methods are in closer agreement with experiment and MP2 than the pure DFT methods for geometrical parameters. The only exception is the Z − geometry, perhaps due to its anionic character. All DFT methods give results in good overall agreement with MP2 for intramolecular geometrical parameters of the adsorption complexes, intramolecular vibrational frequencies, and adsorption energies. The B3LYP method gives intermolecular geometries and intermolecular vibrational frequencies which are closest to those obtained from the MP2 method. Thus, the B3LYP method seems to be the best choice for a density functional treatment of molecular adsorption in zeolite systems.

Room temperature aging of a ZSM-5 preparation detected by small angle X-ray and neutron scattering and n.m.r. spectroscopy

The room temperature aging process in a solution phase ZSM-5 preparation has been studied by scattering and n.m.r. techniques. Small-angle X-ray scattering detected particles growing in the 1-10 nm size domain, and their evolution was followed over a period of 4 days; the most dramatic changes occurred in the first 24 h. A contrast variation small-angle neutron scattering experiment was performed to determine if the particles that formed contained the template, the tetrapropylammonium cation. The nature of the bonds formed during the aging process was investigated by liquid phase {sup 29}Si, {sup 13}C, and {sup 27}Al n.m.r. experiments.

Designing intermediate-range order in amorphous materials

Amorphous materials are commonly understood to consist of random organizations of molecular-type structural units. However, it has long been known that structural organizations intermediate between discrete chemical bonds and periodic crystalline lattices are present even in liquids. Numerous models—including random networks and crystalline-type structures with networks composed of clusters and voids—have been proposed to account for this intermediate-range order. Nevertheless, understanding and controlling structural features that determine intermediate-range order in amorphous materials remain fundamental, yet presently unresolved, issues. The most characteristic signature of such order is the first peak in the total structure factor, referred to as the first sharp diffraction peak or ‘low Q’ structure. These features correspond to large real-space distances in the materials, and understanding their origin is key to unravelling details of intermediate-range order. Here we employ principles of crystal engineering to design specific patterns of intermediate-range order within amorphous zinc-chloride networks. Using crystalline models, we demonstrate the impact of various structural features on diffraction at low values of Q. Such amorphous network engineering is anticipated to provide the structure/property relationships necessary to tailor specific optical, electronic and mechanical properties.

EXAFS as a probe of local coordination environments of transition metal ions in polycrystalline zeolite catalysts. Co(ii) in hydrated zeolite –A and –Y

Synthetic zeolites exchanged with transition metal or rare earth ions are of immense importance in catalysis, both commercially and in basic research; however, difficulty in growing single crystals and problems of long range disorder make crystallographic studies formidable tasks. In this study, extended x‐ray absorption fine structure (EXAFS) spectroscopy has been successfully applied to characterize the local coordination of Co(ii) ions in polycrystalline zeolite –A and –Y, thereby establishing the utility of this technique for structural study of intrazeolitic complexes. The Co(ii) ions in Co18.2Na20.7–Y⋅xH2O are found to be ’’solutionlike’’, six‐coordinate with Co–O bond distances of 2.06(1) A, but in Co4.3Na3.4–A⋅xH2O and Co5.5Na1.0–A⋅xH2O, they are five coordinate with Co–O distances of 2.07(1) A. The latter result is compatible with several published spectroscopic measurements, but is in conflict with a previous single crystal x‐ray diffraction study; this discrepancy has been critically analyzed. ...

Metallic cobalt and iron particles in large and medium pore zeolites. Methods of generation and ferromagnetic resonance characterization

Abstract Spectroscopic studies utilizing photoacoustic electronic spectroscopy, electron paramagnetic resonance and ferromagnetic resonance have been made on the formation of (1) Co metal particles from Co(NH3)3+6 exchanged into Y-zeolite and (2) Fe metal particles from non-framework Fe3+ ions in an aluminoferrisilicate analogue of ZSM-5 zeolite. Decomposition of the Co(NH3)3+6 complex is accompanied by autoreduction to the Co2+ state, observed by EPR at 7 K, but no Co metal is formed. The Co2+ ions migrate from the supercage locations of the parent complexes into the hexagonal prisms and are subsequently very difficult to reduce, even with H2 at 823 K. High-temperature FMR data suggest that the small amount of Co metal particles which are formed exist with the f.c.c. crystal structure yielding g = 2.17 at 508 K, consistent with an empirically calculated value for f.c.c. Co. Hexagonal close-packed Co particles are expected to exhibit much larger magnetic anisotropy than was observed. The Fe3+ ions in the mediumpore pentasil aluminoferrisilicate zeolite can be reduced to the metallic state following either (a) precipitation of the ‘inclusion’ compound, FeIII[FeII(CN)5(NO)], in the zeolite, or (b) generation of superparamagnetic oxidic ferric ion clusters. High temperature FMR data establish that smaller Fe particles can be obtained by the latter method.