H-faujasite

In the faujasite framework there are also four crystallographically distinct oxygens. The experimental proton occupation in H-faujasite is 8 2 4 for the... 

M. Brandle and J. Sauer, J. Mol. Catal. A, 119,19 (1997). Combining Ab Initio Techniques with Analytical Potential Functions. A Study of Zeolite-Adsorbate Interactions for NH3 on H-Faujasite. 

For the conversion of m-xylene, the activity of H-faujasite depends strongly on its degree of dealumination. Maximum catalytic activity is obtained for aluminum T-atom fractions equal to 0.10. Surprisingly, in contrast to both theoretical predictions and to the behaviour of H-ZSM-5, for dealuminated H-faujasites the turnover frequency per protonic aluminum site exhibits a pronounced maximum when the aluminum T-atom fraction is 0.09. The present results can be rationalized if, besides the classical predictions on zeolite acidity, a new concept of "hidden acid sites" is handled. Changes of the m-xylene isomerisation and disproportionation selectivities with the degree of dealumination of faujasite are in agreement with this concept. 

For the last two decades the acidity changes of H-faujasite upon removal of Alp. atoms have received continuous research interest (3.) Essentially two... 

In this paper we will provide another example of the occurrence of a maximum catalytic turnover frequency for a specific composition of the faujasite catalysts. We also will present further evidence that for every Bronsted acid catalysed reaction in H-faujasites, this is the generally expected behaviour and will introduce a new concept to rationalize this. 

Figure 1. Literature data on the evolution of the catalytic turnover frequency (TOF) per framework aluminum site of H-faujasite (a), cumene cracking (4) (b), hexane cracking (6) and (c), cumene cracking (3). ...

The "Hidden Site" Concept in Faujasite. The infrared active hydroxyl vibrations in H-faujasite absorb at two distinct frequency regions, commonly denoted as high-frequency (HF) and low-frequency (LF) hydroxyl bands (17.). In the structure of faujasite, there are four crystallographically distinct oxygen atoms. The HF band vibrating in the 3680-3625 cm--1- range is generally... 

Unfortunately, there is no reference structure free of cations, though perhaps a faujasite-type framework containing only Si02 ultimately will be synthesized. The dehydrated H-faujasite produced by heating ammonium-exchanged faujasite (48) is the closest to a cation-free fauja-site, but even it has protons condensed with framework oxygens to form hydroxyls. 

Table III shows significant trends. In dehydrated H-faujasite, the 4-rings are approximately square. In dehydrated Ni-faujasite, occupation of I by the small Ni ion causes the 1-03 distance to fall from 2.68 in dehydrated H-faujasite to 2.29A. The 1-02 distance does not change significantly, but the bridging 4-ring (Figure 1) distorts tremendously with 03-03 rising from 3.58 to 4.27, and 04-04 falling from 3.52 to 3.03A. Reciprocally, occupation of I by the large cations K or Ba causes the 1—03 and 04-04 distances to increase, and the 03-03 distance to fall. In fact, there is an excellent correlation between the 3 distances for...

The clearest data are for H-forms of faujasite-type zeolites. The ammonium-exchanged form is heated, driving off ammonia and water, and thereby producing the H-form. It would be inconceivable chemically for the protons not to condense with framework oxygens. Olson and Dempsey (48) determined the crystal structure of H-faujasite held in the dehydrated form at room temperature. Unfortunately, there were no electron density peaks ascribable to the protons, probably because... 

The Diels-Alder cycloaddition reaction of dihydropyran with acrolein was performed in the presence of various H-form zeolites such as H-Faujasites, H-p, H-Mordenites which differ both in their shape selective as well as their acidic properties. The activity of the different catalysts was determined and the reaction products were identified. High 3delds in cycloadduct were obtained over dealuminated HY (Si/Al=15) and Hp (Si/Al=25) compared to HM (Si/Al=10). These results were accounted for in terms of acidity, shape selectivity and microporosity vs mesoporosity properties. The activity and the regioselectivity were then discussed in terms of frontier orbital interactions on the basis of MNDO calculations for thermal and catalyzed reactions by complexing the diene and the dienophile with Bronsted and Lewis acidic sites. From these calculations, Bronsted acidic sites appeared to be more efficient than Lewis acidic sites to achieve Diels-Alder reactions. 

In order to get more information on these two effects we have performed the Diels-Alder reaction between dihych opyi an and acrolein in the presence of various H-form zeolites such as H-faujasites, H-(t and H-mordenites. This reaction was reported to proceed with difficulty under thermal conditions in the absence of catalyst (11). The catalysts tested in this work differ both in their shape selective as well as their acidic properties. On the other hand, molecular orbital calculations (MNDO) have been performed to account for the experimental results on the uncatalyzed and catalyzed reactions. 

Mixing decreases rate for C5 conversion less with Pt/H/Mordenite than with Pd/H-Faujasite Mixing increases rate for Cg conversion with Pt/H/Mordenite, but it decreases with Pd/H/Faujasite... 

Polymerization behaviour of ethyl vinyl ether and isobutyl vinyl ether on H-mordenite and H-faujasite (H-Y) reveals certain distinctive features in comparison with that of n-butyl vin>d ether on these catalysts. 

The physical implication of Eq. (3) is that the catalytic centres in H-faujasite (H-Y) are progressively inactivated by polymer growth, possibly as a result of the formation of immobilized polymer clogging the intracrystalline diannel of H-Y more and more so that access of monomer to the intracrystalline sites becomes increaan y difficult. However, little or no immolnle intracrystalline polymer forms on H-mordenite surface. Consequently surface sites can be active and monomer can always displace polymer segments on an external surface, hence being available at the reaction ates. Further evidence on this interpretation of the kinetics over H-Y as con ared with ITmordenite is provided from the observed effect of water on the kinetics and OTG curves. 

The atomic multipole expansion of the BI electrostatic potential is extremely useful, when the long-range, purely point-multipolar part of the potential yields an important contribution. This is the case in crystals, where the multipolar sums (up to the quadrupolar potential) are conditionally convergent lattice sums. Special techniques, like Ewald summation method [130, 131] and its generalizations [132] are needed to handle properly these infinite sums. Recently we applied the multipolar BI method, coupled with Ewald summation for the evaluation of electrostatic potentials and fields in zeolite cavities [133] and for the prediction of the IR frequency sequence of the different acidic sites in H-faujasite [134]. 

Lonyi, F., Valyon, J., and Pal-Borb ly, G. A DRIFT spectroscopic study of the N2 adsorption and acidity of H-faujasite, Micropor. Mesopor. Mater. 2003, 66, 273-282. 

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