Hydrogen-exchanged zeolite

Truong T N 1997 Thermal rates of hydrogen exchange of methane with zeolite a direct ab initio dynamics study on the importance of quantum tunneling effects J. Rhys. Chem. B 101 2750... 

A rare-earth-exchanged zeolite increases hydrogen transfer reactions. In simple terms, rare earth forms bridges between two to three acid sites in the catalyst framework. In doing so, the rare earth protects... 

The next homologues are 1- and 2-butyne, where similar isomerizations have been observed [20] a recent report describes the reaction on a basic, alkali metal-exchanged zeolite [21]. As an unexpected product, an allene was obtained in reactions with hydrogen and a samarium catalyst [16, 22]. 

Metal ion discharge, chemical identity of adsorbed intermediates, 38 19-20 Metal ion-exchanged zeolites, 31 13 Metallacycle mechanism, 41 323-324 Metallathiabenzenes formation of, 42 421 reaction with hydrogen gas, 42 420 Metallic catalysts, 27 3 see also specific catalysts... 

Fig. 12. Minima and transition states on the reaction path of hydrogen exchange for methanol on a zeolite cluster. The upper and lower diagrams shown the equivalent neutral complexes, and the middle figure illustrates the transition state. Reprinted with permission from Ref. 221. Copyright 1995 American Chemical Society.

Fig. 16. Reaction coordinate for hydrogen exchange between methane and an acidic zeolite cluster. Arrows represent displacement vectors along the reaction coordinate. Reprinted with permission from Ref. 248. Copyright 1994 American Chemical Society.

DNP level found no evidence for a stable benzenium cation in contact with a cluster modeling the zeolite conjugate base site. We were able to locate a transition state for benzene H/D exchange as shown in Fig. 20, which is similar to the transition state for methane H/D exchange on zeolites (121). These transition states clearly show that hydrogen exchange is a concerted process. 

The crystal structure analysis of palladium-exchanged zeolite allows the determination of initial cation positions in the dehydrated porous framework. Similar studies after reduction by hydrogen at various temperatures should permit the observation of palladium removal from the cation sites and thus the estimation of the reduction level. Moreover, the presence of metal on the external surface is easily detected. Hence, x-ray diffraction techniques should give a good picture of hydrogen reduction of palladium in Y zeolites. 

We were interested in the change in the oxidation state of Pd (II), incorporated in the zeolite, during heat treatment in oxygen or in vacuo. Hydrogen and carbon monoxide interactions were also studied. The experiments involved two techniques ESR, which provides direct identification of palladium in an ionic state, and IR spectroscopy, which gives information on the superficial structure of the exchanged zeolite and on the adsorbed species. 

The spectra of alkaline earth ion-exchanged samples, with the exception of the barium form (211), have hydroxyl absorption bands at 3645 and 3540 cm-1, similar to those found in H—Y zeolite. The barium form behaves like the alkali-exchanged zeolites. The similarity of the spectra of the alkaline earth forms with that of the hydrogen form suggests that the acidic hydroxyls are associated with the same structural features (151). Band frequencies in the region of 3600 to 3560 cm-1 vary with the cations and are thought to result from hydroxyl groups associated with the divalent cations (211). They are weakly acidic or inaccessible to adsorbate molecules since the band intensity is not affected by adsorption of pyridine (209). 

Ward measured the o-xylene isomerization activities of Na, Mg, RE, and H—Y zeolites and found the rare earth form to be intermediate in activity between the magnesium and hydrogen forms as shown in Table IX (212). The sodium form was essentially inactive. He interpreted the activity relationship RE—Y > Mg—Y to result from the formation of two acidic structural hydroxyl groups per trivalent rare earth cation. The formation of acidic structure hydroxyl groups by exchange of sodium ions with protons in the rare earth solution, as proposed by Bolton (218), may also account for the greater activity of the rare earth-exchanged zeolite. 

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