Zeolites hydrogen form

Thermal dealumination. The method involves calcination of the ammonium (or hydrogen) form of the zeolite at relatively high temperatures (usually over 500°C) in the presence of steam. This results in the expulsion of tetrahedral aluminum from the framework into non-framework positions, but does not remove the aluminum from the zeolite. The process consists essentially in a high-temperature hydrolysis of Si-O-Al bonds and leads to the formation of neutral and cationic aluminum species (Figure 1A). 

Barrer first described the preparation of hydrogen forms of zeolites by oxidative degradation of ammonium zeolites (12)... 

Uytterhoeven, Christner, and Hall, in an elegant study of the thermal decomposition products of ammonium zeolite Y, proposed a scheme to explain the loss of chemical water from the hydrogen form of the zeolite (IS). 

The chemistry and structure of the hydrogen form of zeolite Y have been thoroughly investigated 82) and are not considered further. The structure of the dehydroxylated zeolite proposed by Uytterhoeven, Christ-ner, and Hall 15) remains unchanged. Recently Ward, on the basis of infrared studies, suggested that this form may be amorphous 27). The extreme instability of dehydroxylated zeolite Y to moisture complicates detailed study 19). The elucidation of the detailed nature of this material lies in the future. At present, completely dehydroxylated Y is little understood and presents a challenging void in our knowledge of the nature of ammonium zeolite Y thermal decomposition products. 

Direct Conversion of Ammonium Zeolite Y. The procedures of McDaniel and Maher 20), Hansford 27), and Kerr 26) appear to have in common reaction conditions which effect hydrolysis and removal of a portion of the tetracoordinate aluminum ions from the framework of the hydrogen form during decomposition of the ammonium form at temperatures of 400° C and above. 

Y. Kerr showed that about one-third of the ammonium and aluminum could be removed from ammonium Y using H4EDTA 25). Carefully controlled calcination of this material (under conditions which yield the relatively unstable, normal hydrogen form from the normal ammonium form) yielded a hydrogen zeolite of very high stability. Kerr proposed the following reaction steps to explain the stability 23,25). 

The relationship between acid site density and effective acidity may account for the interesting observation of Hopkins that maximum cracking activity of n-hexane was obtained over a partially dehydroxylated hydrogen zeolite Y (45). While the normal hydrogen form would contain a greater overall concentration of acid sites, the partially dehydroxylated form may have a greater overall acid activity because of the increased effective acidity of the remaining sites. 

Palladium ions were reduced by hydrogen at room temperature. The zeolite thus formed has hydroxyl groups identical to those found in de-cationated Y zeolites and probably has a Bronsted acid character. Furthermore, hydrogen reduction produces metallic palladium almost atomically, dispersed within the zeolite framework as demonstrated by our IR, volumetric, and x-ray (23) results. Palladium atoms are located near Lewis acid sites which have a strong electron affinity. Electron transfer between palladium atoms and Lewis acid sites occurs, leaving some palladium atoms as Pd(I). Reduction by hydrogen at higher temperatures leads to a solid in which metal palladium particles are present. The behavior of these particles for CO adsorption seems to be identical to that of palladium on other supports. 

Hydrogen forms of the ammonium zeolites were obtained by heating at 380°C for 15 hours in a stream of dry air. Further heating from 550° up to 1000°C under the same conditions provided samples for studying thermal stability. Such treatment avoided the formation of ultrastable zeolite. The chemical compositions of the samples are listed in Table I. 

Ammonia decomposes on zeolites (9), and the effect of this decomposition on the chlorobenzene reaction may be important. Thus, the activity of CuY zeolite for ammonia decomposition was studied. Helium was used as a carrier gas, 1 ml of ammonia was injected, and the extent of ammonia decomposition was determined as a function of temperature. The decomposition was 2.4% at 350°C, 7.8% at 450° C, and 24% at 550° C. The apparent activation energy of ammonia decomposition was estimated at 13 kcal/mole. The activation energy of ammonia decomposition is close to that of benzene formation from chlorobenzene and ammonia. Thus, benzene formation results from the reaction of chlorobenzene and hydrogen formed by the decomposition of ammonia. 

A study is presented of the synthesis and properties of the novel synthetic zeolite omega. The synthesis variables and kinetics of formation are discussed, as well as the ion exchange, sorption, and thermal properties. By decomposition of imbibed tetra-methylammonium ions and exhaustive treatments of the zeolite with ammonium ions, a pure hydrogen form can be obtained which is a suitable substrate for the preparation of hydrocarbon conversion catalysts. Several catalysts were prepared and utilized to isomerize n-hexane, and to hydrocrack a heavy gas oil. 

This requires a second zeolite tank that has a zeolite resin in the hydrogen form in addition to the usual tank with the resin in the sodium form. The two tanks are operated in parallel. In one tank, calcium and magnesium ions are replaced by hydrogen ions. The effluent from this tank with the resin in hydrogen form is on the acid side and has a lower total-solids content. The total flow can be proportioned between the two tanks to produce an effluent with any desired alkalinity as well as excellent hardness removal. When the hydrogen resin is exhausted, it is regenerated with acid. 

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. 

The hydrogen forms of the ZSM-5 and ZSM-11 samples were prepared from the parent as-synthesized zeolites ( ) by drying at 125°C, calcining in nitrogen... 

The revolutionary zeolite cracking catalyst (synthetic Linde X and Y) was introduced commercially over 28 years ago, but considerable effort is still being expended on the improvement of its stability and catalytic properties. Decreasing the aluminum content of the zeolite framework and the replacing the rare-earth with the hydrogen form have greatly increased activity at the expense of stability. The thermal stability of the faujasites is fairly well understood, while the reasons for the increased catalytic activitity are still not fully known. 

The hydrogen forms of zeolite X and Y are generally more active, but less selective, than their cation-exchanged forms. Side reactions include hydrogen transfer (resulting in the formation of coke and paraffinic products), double-bond migration and disproportionation. 

Hence, the hydrogen forms of zeolites should contain only several definite types of structural hydroxyls. In addition, each of them should predominate for a distinct crystal structure and chemical composition (Si/Al ratio). The exceptions include high-silica-containing zeolites, where the presence of only OH-I groups should be anticipated. 

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