Framework Lewis Acid Sites

In some microporous solids, framework cations in tetrahedral sites are able to expand their coordination by chemisorbing small molecules while remaining within the framework, so that removal of the adsorbed species permits their return to tetrahedral coordination. The most important example of this is the titanium in titanosilicates such as TS-1. The titanium cations in the framework of ETS-10 are only able to adopt octahedral coordination, and are unable to show this kind of behaviour. 

In the case of titanosilicate analogues to zeolites, the titanium has been nnequivocally shown to adopt tetrahedral coordination upon dehydration of the as-synthesised material. Upon adsorption of aqueous solutions of hydro- 

In the presence of excess water, the titanium loses its tetrahedral coordination and there is a modification of the first and second coordination shells of the 

Tetrahedral silicate frameworks also appear to be able to include small amounts of tin, which is then able to change coordination from tetrahedral to octahedral. Like titanium, variable coordination tin imparts activity in Lewis-acid-catalysed reactions (Section 9.2.2). 

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The effect of crystal size of these zeolites on the resulted toluene conversion can be ruled out as the crystal sizes are rather comparable, which is particularly valid for ZSM-5 vs. SSZ-35 and Beta vs. SSZ-33. The concentrations of aluminum in the framework of ZSM-5 and SSZ-35 are comparable, Si/Al = 37.5 and 39, respectively. However, the differences in toluene conversion after 15 min of time-on-stream (T-O-S) are considerable being 25 and 48.5 %, respectively. On the other hand, SSZ-35 exhibits a substantially higher concentration of strong Lewis acid sites, which can promote a higher rate of the disproportionation reaction. Two mechanisms of xylene isomerization were proposed on the literature [8] and especially the bimolecular one involving the formation of biphenyl methane intermediate was considered to operate in ZSM-5 zeolites. Molecular modeling provided the evidence that the bimolecular transition state of toluene disproportionation reaction fits in the channel intersections of ZSM-5. With respect to that formation of this transition state should be severely limited in one-dimensional (1-D) channel system of medium pore zeolites. This is in contrast to the results obtained as SSZ-35 with 1-D channels system exhibits a substantially higher... 

Conclusions, some of them contrary to the above, were reached more recently by Zhuang et al. (145) from a combination of 31P and 1H MAS NMR spectroscopy of adsorbed trimethylphosphine. These authors found not only Lewis acid sites (vide infra), but also Brpnsted acid sites in TS-1 (145). They claimed that the 1H, 29Si MAS NMR spectra and the resonance related to Brpnsted acid sites in the 31P MAS NMR demonstrated clearly that the presence of Ti in the framework results in the formation of a new OH group, titanols, which is more acidic than the silanols of silicalite-1 (145) . The peak at 4.3 ppm in the 31P MAS NMR spectra was assigned to a ((CH3)3P-H)+ complex arising from the interaction of (CH3)3P with Brpnsted acid sites present on TS-1. The origin of this proton is not clear at present, especially because the MAS NMR spectra of the same TS-1 samples did not differ significantly from those of silicalite-1 (145) the latter, when free from impurities, is not known to be a Brpnsted acid. 

III.B.3. Lewis Acid Sites and Extra-Framework Aluminum. 260... 

Pyridine sorption studies have shown the presence of both Bronsted and Lewis acid sites in USY zeolites, although to a lesser extent than in the corresponding HY zeolite (51,53). Acidity is maintained even after strong dehydroxylation of USY-B at 820°C. Rehydration of the calcined material did not regenerate significantly Bronsted acid sites, due to irreversible changes in the zeolite framework (51). 

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. 

Lewis acid sites may be formed following dehydroxylation of zeolite surface in H-form. At sufficiently high temperatures two Bronsted acid sites can drive off a water molecule and leave behind a coordinatively unsaturated Al site, as illustrated in Figure 13.16 [32]. Here not only the resulting tri-coordinated Al but also the tri-coordinated positively charged Si can act as a Lewis acid. Furthermore dehydroxylation may be followed by framework dealumination, leading to cationic extra-framework species like AlO AlfOHij that can act as Lewis acids [33-37]. 

It is generally accepted that Lewis acidity in zeolites is due mainly to extraframework aluminum (16,17,18). Consequently, Lewis acid sites measured by pyridine adsorption must correlate with extra-framework aluminum. In Table I, the amount of pyridine coordinated to Lewis sites decreases for samples with the lowest Si/Al ratio, showing that, after thermal treatment, the amount of extraframework aluminum decreases with Si/Al ratio of the Beta zeolite. 

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 ". ... 

The microcalorimetry of NH3 adsorption coupled with infrared spectroscopy was used to study the effect of the synthesis medium (OH or F ) on the nature and amount of acid sites present in Al,Si-MFl zeolites [103]. Both techniques revealed that H-MFl (F ) with Si/Al < 30 contained extra-framework aluminum species. Such species were responsible for the presence of Lewis acid sites and poisoning of the Brpnsted acidity. In contrast, MFl (F ) characterized by Si/Al > 30 presented the same behavior as H-MFl (OH ). 

The acidic/basic properties of zeolites can be changed by introdnction of B, In, Ga elements into the crystal framework. For example, a coincorporation of alnminnm and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [246] in boron-snbstitnted ZSM5 and ZSMll zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of abont 65 kJ/mol for H-B-ZSMll and showed that B-substituted pentasils have only very weak acidity [247]. Calcination at 800°C increased the heats of NH3 adsorption to about 170 kJ/mol by creation of strong Lewis acid sites as it can be seen in Figure 13.13. The lack of strong Brpnsted acid sites in H-B-ZSMll was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

The atom-planting method for the preparation of several metallosilicates with MFI structure was studied. By the treatment of silicalite or ZSM-5 type zeolite with metal chloride vapor at elevated temperatures, metal atom could be introduced into the zeolite framework. From the results of alumination of silicalite it is estimated that the metal atoms are inserted into defect sites, such as hydroxyl nests in zeolite framework. The metallosilicate prepared had both Bronsted and Lewis acid sites with specific acid strength corresponding to the kind of metal element. 

It has been reported that aluminium can be introduced into the framework of silicalite with MFI structure by the treatment with AICI3 vapor at elevated temperatures [4-8]. By such treatment, not only Bronsted acid sites but Lewis acid sites are also generated, because aluminium atoms are introduced not only into the framework sites but alkso into the non-framework sites [6-8]. It is expected that this method can be applied to prepare some metallosilicates with MFI structure. Namely, by treating silicalite with metal chloride vapor at... 

The simultaneous investigation of the methanol conversion on weakly dealuminated zeolite HZSM-5 by C CF MAS NMR and UV/Vis spectroscopy has shown that the first cyclic compounds and carbenium ions are formed even at 413 K. This result is in agreement with UV/Vis investigations of the methanol conversion on dealuminated zeolite HZSM-5 performed by Karge et al (303). It is probably that extra-framework aluminum species acting as Lewis acid sites are responsible for the formation of hydrocarbons and carbenium ions at low reaction temperatures. NMR spectroscopy allows the identification of alkyl signals in more detail, and UV/Vis spectroscopy gives hints to the formation of low amounts of cyclic compounds and carbenium ions. 

Side-Chain Alkylation. There is continued interest in the alkylation of toluene with methanol because of the potential of the process in practical application to produce styrene.430 Basic catalysts, specifically, alkali cation-exchanged zeolites, were tested in the transformation. The alkali cation acts as weak Lewis acid site, and the basic sites are the framework oxygen atoms. The base strength and catalytic activity of these materials can be significantly increased by incorporating alkali metal or alkali metal oxide clusters in the zeolite supercages. Results up to 1995 are summarized in a review.430... 

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. 

Pyridine adsorption experiments have showed that the nickel containing smectites have Lewis acid sites and do not have Bronsted acid sites [9]. The Ni2+-substituted smectite catalysts have large surface areas even after 873 K treatment because many small fragments with the same smectite structure are intercalated in the interlayer region. The activities of the Ni2+ substituted catalysts are derived from Ni2+ Lewis acid sites located on the edge framework. 

Depending on niobium location, the Nb-containing catalysts can reveal Bronsted acid, Lewis acid, or redox properties. Niobium oxide cationic species (NbOn(5-2n)+), which occupy the extra lattice cation positions, play the role of the Lewis acid sites and may exhibit the redox properties. Nb localized in the framework of mesoporous MCM-41 sieves provides the Lewis acidity [3,4] and the oxidizing properties [5,12]. 

Infrared spectral studies of pyridine adsorbed on alkali metal ion-exchanged faujasites have demonstrated the absence of Brpnsted acidity, as reported by Eberly (151), Ignat eva et al. (208), and Ward (156, 209-211). Pyridine is adsorbed weakly by coordination to the alkali metal ions (151, 156). Addition of small amounts of water does not result in formation of Br0nsted acid sites, indicating that the coordinate bound pyridine is not associated with Lewis acid sites in the zeolite framework (210). 

The formation of structural hydroxyl groups in the presence of divalent cations has been explained on the basis of a hydrolysis mechanism (148) involving water initially coordinated to the metal ions (210, 214-216). The formation of a nonacidic hydroxyl group on the metal ion and an acidic hydroxyl on the zeolite framework by dissociation of the water molecule is consistent with the observed IR spectra and pyridine adsorption experiments. Further calcination at higher temperatures results in dehydroxylation and formation of Lewis acid sites at tricoordinate aluminum atoms in the zeolite framework (149). 

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