Mordenite metal cations

Kucherov and Slinkin reported solid-state reactions of H-mordenite and HZSM-5 zeolites with metallic oxides such as CuO (13), Cr2O-, Mo03, and V205 (14-17). The resulting samples were studied by EPR (electron paramagnetic resonance) spectrometry. The authors have shown that the metal cations migrate to cationic sites, where they are coordinately unsaturated. 

By means of ion exchange using metal cations of different size and specific charge, the geometrical restrictions, the number and the strength of the Bronsted acid sites, as well as the adsorption properties of the zeolite material can be influenced. Investigations of this kind have been reported in the literature, for example for ZSM-5 and mordenite catalysts [20, 105]. 

The framework charge-compensating cations in a zeolite, which for synthetic zeolites are normally sodium ions, can be exchanged for other cations of different type and/or valency. However, care must be taken during ion exchange to avoid strongly acidic solutions which can lead to proton exchange with the zeolite metal cations or even structure collapse. For example, zeolites A, X, and Y decompose in 0.1 N HCI. The more silica-rich zeolites such as mordenite are, however, stable under such conditions. Acidity can be introduced into a zeolite in a number of different ways ... 

Dealumination and Heat of Immersion. Treatments of mordenite, clinoptilolite, and ferrierite with dilute acid in the first instance remove metallic cations and yield the hydrogen forms 4, 12, 13). Such forms also result by heating the ammonium zeolites (24). Treatment with stronger acid solutions removes increasing amounts of A1 4, 13, 26) (Table IV). The marked effect this has upon qn is shown in the table. 

Influence of Dealumination. Clinoptilolite and mordenites free of metallic cations and with Al/Si ratios which were progressively reduced gave heats of immersion which diminished with the A1 content (Table IV). Each A1 atom removed reduces the framework charge by 1, according to the reaction (4)... 

The infrared spectrum of adsorbed nitrogen can also be used to probe cation sites in zeolites. Zecchina et al [34] compared vibrational frequencies of CO and N2 adsorbed at low temperatures in mordenite containing different alkali metal cations. In both cases the vibrational frequencies could be correlated with (Rx + Rm) > where Rx is the cation radius and Rm the radius of the adsorbed molecule, suggesting a simple electrostatic field explanation for the frequency shifts between different cations. The appearance of a band due to N2 interacting with a particular zeolite cation will also mean that that particular cation is located in sites accessible to the N2 molecule. 

While the ion exchange with alkaline metal cations does not result in catalysts active in acid-catalysed hydrocarbon reactions and, in contrast, may be carried out to remove any residual activity [1], the incorporation of alkaline earth cations by solid-state reaction should lead to active catalysts. It was shown, however, that the solid-state ion exchange had to be followed by contact with water vapour in order to obtain calcium or magnesium mordenites which are sufficiently active in, for instance, disproportionation of ethylbenzene. This is in full agreement with the IR spectroscopic results which indeed showed that only upon interaction of the heat-treated CaCl2/H-MOR mixture with water vapour were acidic OH groups generated. 

As well as readily exchanging cations in aqueous solution, it has been found that ion exchange also occurs upon bringing a cationic zeolite in contact with a metal salt, typically a chloride, and heating the intimate mixture. In particular, Beyer and co-workers have followed the reaction of protonated or ammonium forms of zeolites with metal halides (particularly volatile halides) at elevated temperatures (Scheme 6.5)." This results in hydrogen chloride and ammonium chloride removal and proton exchange by the metal cations. Many such examples have been reported, including zinc, iron(II) and lanthanum into zeolites Y, mordenite and ZSM-5. 

ABSTRACT. Early observations and more recent systematic studies of solid-state ion exchange in zeolites, which is a possible way of zeolite mo fication, are reviewed. Particular attention is paid to the presentation of important experimental techniques which are appropriate to prove solid-state reaction in zeolites and determine the degree of such solid-state ion exchange. Examples are provided for the introduction of alkaline, alkaline earth, rare earth and transition metal cations into zeolites such as A, X, Y, mordenite and ZSM-5. Techniques of investigation are IR, ESR, MAS NMR, XRD, XPS, TPD and chemical analysis. 

The siting of cations in mordenite is generally less well understood than that in the zeolites described above. Smith and co-workers (12) have, however, in recent years carried out a number of single-crystal X-ray analyses on various cation-exchanged forms of mordenite. These workers correctly emphasize, however, that the cation population densities are subject to unknown errors due to pseudosymmetry. The alkali metal ions are distributed over four major sites, namely ... 

The catalytic properties of H-, Li-, Na-, K-, Mg-, Ca-, Zn-, Cd-, and Al-forms of synthetic mordenite in the reactions of cyclohexane and n-pentane isomerization and benzene hydrogenation have been studied. The cation forms of mordenite that do not involve the metals of column VIII of the Mendeleyev Table show high activity in these reactions. To elucidate the mechanism of n-pentane isomerization, the kinetics of the reaction on H-mordenite have been studied. Carbonium ion is supposed to result from splitting off hydride ion from hydrocarbon molecule. Na-mordenite catalytic activity in benzene hydrogenation reaction decreases linearly with the increase of decationization. This indicates that cations are responsible for the catalytic activity of zeolite. The high activity of cations of nontransition metals in oxidation-reduction reactions seems to be quite unexpected and may provide evidence for some uncommon mechanism of benzene hydrogenation. 

To compare the hydrogenating activity of the cation forms of mordenite with that of H-form which contains the metals of column VIII, we have studied benzene hydrogenation on the catalysts 0.5% Pd/HM and 5% Ni/HM. Under the conditions indicated in Table II, the extent of benzene hydrogenation on these catalysts is 85 and 95%, respectively. Thus, the hydrogenating activity of certain cation forms of mordenite is not inferior to that of H-mordenite, which contains palladium and nickel. Benzene hydrogenation on these catalysts is accompanied by a considerable hydroisomerization to yield methylcyclopentane 30-40%. 

Among the various applications of natural zeolites in sectors of technical significance, the use in the control and abatement of environmental pollution is gaining increasing interest all over the world. Copious references on this are found in the introductory plenary lectures of the three International Conferences on natural zeolites held in the last years (1-3) Focusing our attention on the wastawaters pollution abatement, it may be observed that not more than two or three zeolites (clinoptilolite, mordenite and phillipsite) have been considered until now for removal of cationic pollutants from waters, and that the recourse to ion exchange processes has been limited to the treatment of few cations, substantially ammonium, cesium and strontium. Lack of data is found for instance in the literature on the use of natural zeolites for the removal of heavy metals (4-5), even if these are common... 

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