Lewis protonic zeolites

In aU three cases the protonated zeolite, HZ, is created containing Bronsted sites, which, when heated above 550 °C, lose water to prodnce Lewis sites, viz. 

The structure work consists of synthesizing the faujasite material and characterizing it by X-ray diffraction. The mechanism of the synthesis has been studied and an investigation has been made of the nature of the replacement of the sodium ion by the ammonium ion and of the details of the process of the decomposition of the ammonium ion into the protonic zeolite (Bronsted acid) and decationated zeolite (Lewis acid). (We shall call the material in which the cation has been displaced by a proton and then heated to remove the proton as water, the decationated material.)... 

This work will be continued by examining the X-ray patterns of materials that had been subjected to the heat treatment. Undoubtedly the Lewis acid form (decationated zeolite) has a different X-ray pattern than the Bronsted (protonated zeolite) because of the charge separation... 

The role of Lewis acidity is particularly clear in the reaction of H-[A1]-MCM-41 with propene (Bonelh B, Garrone E, unpublished results). This reaction was studied to assess whether Bronsted acidity was strong enough to promote polymerization, a reaction clearly visible in the IR. Basically, the result was that, imlike protonic zeolites, H-[A1]-MTS systems do not polymerize propene. Traces of polymerization products observed increased with temperature of pretreatment, thus suggesting that the reaction occurs on Lewis and not on Bronsted sites. 

The external surface of protonic zeolites can be relevant in acid catalysis. Several data suggest that nonshape-selective catalysis can occur at these sites, like in the case of alkylaromatics conversions over H-MFI [225,226]. On the other hand, H-zeolites also catalyze reactions of molecules, which do not enter the cavities due to their bigger size. Therefore, the external surface of zeolites is certainly active in acid catalysis. Additionally, the bulk versus surface Si/Al composition of a zeolite could be different and different preparation procedures can allow to modify this ratio [226]. Corma et al. [84] reported data on the accessibility of protonic sites of different zeolites to 2,6-di-ter-butyl-pyridine (DTBP). This molecule has been considered selective for Brpnsted sites, due to its impossible interaction with Lewis sites for steric hindrance. According to these authors, however, the interpretation of the data is not straightforward, for several reasons such as the presence of different cavities and the big size of the probe itself. Surprisingly, Corma et al. found a complete accessibility of the sites of beta zeolite to DTBP. This contrasts the data of Trombetta et al. [197], who showed that protonic sites exist also on the smaller channels of beta, whose access to DTBP seems very unlikely. In a more recent publication, Farcasiu et al. [227] reported an accessibility of 90% of the protons of H-BEA to DTBP, much higher than the 36% for H-MOR and 31% for H-USY. 

The presence of wolframate species on both titania and zirconia causes a little increase of the Lewis acid strength of the residual Lewis sites, an almost full disappearance of the surface anions acting as basic sites and the appearance of a very strong Brpnsted acidity [268-271]. Strong Brpnsted and Lewis acidity are also found on pure WO3 [272], Similar effects have been found for sulfated titanias [261]. The Brpnsted acid strength of these materials, measured by the olefin oligomerization method (Table 9.8) is superior to that of silica-alumina and comparable to that of protonic zeolites [268]. 

The room temperature adsorption of CO on protonic zeolites allowed Lewis acidic sites to be calorimetrically discriminated from the Brpnsted acidic ones Al(III) CO adducts of relative stability were formed at the cus Al(III) Lewis sites, whereas only very weak H-bonding adducts were formed at the Brpnsted Si(OH)+Al acidic sites [73],... 

The next step is the ahstraction of a hydride ion hy a Lewis acid site from the zeolite surface to form the more stable allylic carhocation. This is again followed hy a proton elimination to form a cyclohexadiene intermediate. The same sequence is followed until the ring is completely aromatized. 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

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. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

The rich variety of active sites that can be present in zeolites (i) protonic acidic sites, which catalyze acid reactions (ii) Lewis-acid sites, which often act in association with basic sites (acid-base catalysis) (iii) basic sites (iv) redox sites, incorporated either in the zeolite framework (e.g., Ti of titanosHicates) or in the channels or cages (e.g., Pt clusters, metal complexes). Moreover, redox and acidic or basic sites can act in a concerted way for catalyzing bifunctional processes. 

It is generally admitted that over zeolites, acetylation of aromatic substrates with acetic anhydride (AA) is catalyzed by protonic acid sites. The direct participation of Lewis sites was excluded by using two BEA samples with similar protonic acidities, but with very different Lewis acidities indeed, these samples were shown to have quasi-similar activities. The currently accepted mechanism is shown in Figure 12.6 for the anisole acetylation example. The limiting step of the process is the attack of anisole molecules by acylium ions. 

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... 

Theoretical studies on the Beckmann rearrangement mechanism over zeolite catalyst supported by experimental data have increased. The catalytic activity of the zeohte is determined by Brpnsted and Lewis acid sites created by protonation or activation by metallic cations. The reactivity of the acid sites is strongly influenced by the geometry and flexibility of the zeolite framework ". ... 

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