Zeolite cation effect

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

Fig. 20 Cation effect on an intrazeolite singlet oxygen ene reaction in Y-zeolite.

Barrer showed these hydrogen zeolites, mordenite and chabazite, to be crystalline using x-ray diffraction, and stated, Hydrogen zeolites are effectively crystalline aluminosilicic acids, the salts of which are their diverse cation exchange products." Szymanski, Stamires, and Lynch (13) used simple thermal decomposition of an ammonium zeolite X in an attempt to prepare the hydrogen zeolite... 

Preliminary results for NaY- and KY-supported Ru catalysts demonstrate significant effects of the nature of the zeolite cation for the selectivity of 3-methyl crotonaldehyde hydrogenation. It was suggested that increased basicity of the zeolite resulted in increased selectivity toward the unsaturated alcohol product. These results agree with earlier suggestions that the nature of the support can have significant influence on the product distribution in the hydrogenation of < ,/3-unsaturated aldehydes. 

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). ... 

Changing the cations in a zeolite may effectively enlarge the pore openings by diminishing the cation population and/or a resiting of cations which are normally located near these openings. In the zeolite A, divalent ion exchange opens the... 

Sadeghpoor, R., Ghandi, M., Najafi, H.M., and Farzaneh, F. (1998) The oxa-di-jt-methane rearrangement of p.y-unsaturated ketones induced by the external heavy atom cation effect within a zeolite. Chemical Communications, (3), 329-330. 

Figure 8. Cation effect on XRD powder patterns. Calculated XRD patterns for isostructural zeolites with different inorganic cations- harmotome (Ba) and phillipsite (K).

Zeolitic materials have been widely used in the last decades in the chemical and petrochemical industries. This increasing interest on these materials is based in their unique properties a uniform intra-crystalline microporosity that provides aceess to a large and well-defined surface, the molecular sieve effect, and the electrostatic field centered at zeolite cations. Furthermore, some properties of zeolites can be tailored by changing the nature of the compensating cation located in the inner part of the cavities by means of their ion-exchange capability. In this way, the pore accessibility of some zeolites used in gas separation processes, as well as the adsorbent-adsorbate interactions, can be tailored by the introduction of cations with different size and chemical nature. Similarly, different cations can be used to introduce new chemical properties (acid-base, redox, etc.), which are needed for a given application in catalytic processes. 

Any basic or alkaline material can react with a zeolite to effectively neutralize the acidic active sites, which generally results in irreversible loss of catalyst activity. Basic compounds found in the ethylene or benzene feedstocks can include amines, amides, nitriles, and trace metal cations such as sodium and potassium. Of particular concern are nitrogen-containing organic compormds typically present in the benzene feed. 

It is known that the number and size of cations in the channels and cavities as well as the polarizability of the sorbate molecules influence the sorption properties of zeolites. This effect must also be considered in the case of R-exchanged zeolites. 

If templating is important, key ratios will define the cation distribution, Na Si02, K Si02, R4N Si02, and so forth, ratios which can range (individually) from zero to more than two. In Table III, examples are selected from the literature to illustrate cation effects in crystallization experiments. The Na-series examples show a shift in product from Y to mazzite-related structures (ZSM-4, Omega) on addition of only small amounts of TMA ion. When potassium replaces sodium, the L structure results but limited addition of TMA produces yet another structural type, the offretite-type zeolites. 

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