Zeolites Bronsted sites

The preparation was performed on a commercial microcrystalline beta zeolite. The zeolite was treated with the Fenton s reagent and less than 0.3 wt% of carbon remained after the treatment. The porosity was fully developed as revealed by the pore-size distribution. Elemental analysis combined with TPR did confirm the high degree of Fe-exchange (98%) on the Bronsted sites. 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

Figure 2. (A) IR spectra of CO adsorbed on the Fe-TON zeolites of different Si/Fe ratio at very low CO coverage required for the detection of the Lewis sites., (B) Difference spectra of CO adsorbed on Fe-TON of different Si/Fe ratio recorded upon the saturation of all Bronsted sites.

A very convenient method to quantitatively determined the number of Bronsted add sites in the often used photochemical nano-vessels, zeolites X and Y, is available.28 This method take advantage of indicator/probe molecules which undergo an intense color change upon protonation within the zeolite pore network. The amount of a base necessary to quench the color change gives a direct measure of the concentration of acidic sites. The base used to titrate the Bronsted sites must be more basic than the probe molecule and sufficiently basic to be completely protonated. 

As mentioned above, an acidic zeolite can provide both protonic (Bronsted) and aprotonic (Lewis) sites. The Bronsted sites are typically structural or surface hydroxyl groups and the Lewis sites can be charge compensating cations or arise from extra-framework aluminum atoms. A basic (proton acceptor) molecule B will react with surface hydroxyl groups (OH ) via hydrogen bonding... 

The infrared stretching frequency of the hydroxyl associated with the Bronsted sites in decationized zeolites, fails in the range 3600 to 3660 cm. As the Si/Al ratio in the framework increases, this frequency tends to decrease. What does this suggest about the acidity of the highly siliceous zeolites ... 

The bands at 1454 and 1542 cm-1 were chosen to measure the quantities of PyL and PyH+ respectively. In Figure 3, the optical densities plotted are relative to the same number of unit cells of zeolite. This figure shows that the number of Bronsted sites capable of chemisorbing Py at... 

C is constant on the three La zeolites heated at 900° C in dry air whereas the number of Lewis sites decreases with increasing La content. The Na-8.7 sample heated at 900° C in dry air has no acidity. Accordingly, the Bronsted and Lewis acid sites on the Na-8.7 sample heated at only 800° C are plotted in Figure 3 for comparison with the 900°C-pretreated La zeolites. The plots indicate that the introduction of La3+ ions in the lattice increases the number of Bronsted sites even for a higher temperature... 

Evidence that something more than a proper Alf distribution is needed to create strong acidity first came from the experiments of Beyerlein et al. (7). Using the acid-catalyzed conversion of isobutane as a measure of strong acidity, they found that a dealuminated zeolite prepared by treatment with ammonium hexafluorosilicate (AHF) exhibited much less carbonium ion activity than might be expected, based on the number of Alf atoms. This treatment leaves very little extraframework Al in the zeolite. When the sample was mildly steamed, the activity became considerably greater. The authors concluded that the enhanced acidity was a result of a synergism between the framework Bronsted sites and the Lewis sites associated with extraframework aluminum. 

When the chemical treatment is applied to a sample previously steamed at a higher temperature (725 C), which would simulate the zeolite in an equilibrium catalyst, only a very small, if any, decrease in activity is observed (Fig. lb). The fact that when the steaming temperature is high (> 700flC), the Bronsted sites of enhanced acidity (bands at 3600 and 3525 cm"1) are not observed (8), and only the HF and LF bands are present (8), will explain the activity behaviour observed for samples USY-2 and U2F-35. 

The alpha values of synthetically prepared mordenltes are within the range of 10 -10°. There Ls only a small variation In a values of siliceous mordenlte as the aluminum content of the material varies. This Ls In contrast to dealumlnated mordenltes, which exhibit a much larger variation Ln alpha values as the aluminum content varies. These catalytic results on synthetic and acid dealumlnated mordenltes Indicate that factors other than the total aluminum content must contribute to the variation In activity of the catalysts. The linear correlation of alpha versus aluminum content reported for some zeolites (26) does not apply to the acid dealumlnated samples. Evidence has been presented for the presence of both Bronsted sites and Lewis sites enhancing the strong acidity of zeolite catalysts (27). The presence of extra-lattice aluminum Ln acid dealumlnated mordenlte samples was confirmed by 2 A1 NMR spectroscopy. The presence of both framework aluminum and extra-lattice aluminum Ln the acid treated materials may account for the wide variation Ln alpha values as a function of aluminum content. 

In our opinion, the main factor which governs the acidity of bridged OH groups in zeolites is the chemical one. If the local nature of Bronsted sites is taken into account, the following rational classification can be proposed for the bridged OH groups of zeolites with regard to their acidity and Si/Al ration in the framework (34). 

Extraframework aluminum species created by mild steaming were shown to increase the catalytic activity of zeolites. This increase in activity was ascribed to the creation of sites exhibiting enhanced acidity through interaction of bridging hydroxyl groups (BrOnsted sites) and neighboring small extraframework aluminum species (Lewis acid sites) (16). Their exact nature is still a matter of debate, but is schematically represented as ... 

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