Zeolite ligand fields

A notable exception are chemisorbed complexes in zeolites, which have been characterized both structurally and spectroscopically, and for which the interpretation of electronic spectra has met with a considerable success. The reason for the former is the well-defined, although complex, structure of the zeolite framework in which the cations are distributed among a few types of available sites the fortunate circumstance of the latter is that the interaction between the cations, which act as selective chemisorption centers, and the zeolite framework is primarily only electrostatic. The theory that applies for this case is the ligand field theory of the ion-molecule complexes usually placed in trigonal fields of the zeolite cation sites (29). Quantum mechanical exchange interactions with the zeolite framework are justifiably neglected except for very small effects in resonance energy transfer (J30). ... 

The individual Cu emissions reflect the energy difference between the lowest d s and the d level, controlled by the ligand field strength and symmetry of the Cu ion. For the Cu ions coordinated to framework oxygens, the ligand field strength is supposed to be controlled by its symmetry. It follows that the Cu emission spectra reflect different coordination of the Cu ions. This assumption is supported by the excitation spectrum for the 480 nm band (for Cu-zeolites, where this band does not overlap with the others), which was identical for Cu located in both erionite and ZSM-5 matrix, see Fig. 2B. 

The individual chemical species with chiral catalytic properties, such as complex, organometallic compounds, organic ligands or molecules, anchored or grafted into the channels of microporous and mesoporous materials, and some microporous compounds possessing chiral channels or their pore structures composed of the chiral motifs, all promise further development and potential application in microporous chiral (asymmetric) catalysis and separations. It is an important frontier direction in the zeolite catalytic field at present. Therefore, the synthesis and assembly of chiral microporous compounds and materials are of particular interest for researchers engaged in porous materials. This is a research field in rapid development. 

Quantum chemical calculations on metal clusters in zeolite A [12] and semi-empirical ligand field interpretations of spectroscopic data of transition metal ions [6] have proven to be successful in structural characterizations of molecular sieves and their guest species. The present tendency in catalysis towards a more fundamental approach justifies the expectation that ESR, combined with other spectroscopic techniques, will become important. However, this requires an accurate and unambiguous parameterization of the ESR spectra. The parameter set thus obtained forms a firm basis for a theoretical investigation of the coordination environment of the paramagnetic entity. 

Table 4. Sites, d-d band positions of some bare 3d TMls in zeolite LTA and their ground and excited electronic states in D,], ligand fields [52,79]...

In the case of small clusters entrapped in zeolites or of zeolites with exchanged metal cations the complexity of the XPS data interpretation increases. Here the effects must be considered of the zeolite lattice field and of the coordination to ligands on a photoionized metal atom, which should have strong effects on the relaxation of the core hole, thus influencing the final state of the photoionized atom. The unusual chemical situation is demonstrated, for example, by the fact that intermediate Me + states of transition metals can be stabi-... 

In the field of transition metal catalysis, zeolites may offer opportunities for uniform active sites. With the discovery of both aluminosilicate and aluminophosphate, zeolites with a variety of transition-metal ions in tetrahedral firework positions may offer new possibilities. On the basis of existing zeolite chemistry dealing vrith aluminum hydrolysis and the formation of adsorption adducts in the zeolite pores, chemists may envision strategies aimed at the activation of tetrahedral transition metal ions, either by lattice oxide replacement or by the application of strong donor ligands. The demonstrated... 

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