Framework zeolite Beta

Elucidation of the structure of a solid catalyst is paramount to any understanding of its activity. Without such information, inferences about its activity would be speculation. Often it is instructive to determine the structure of a catalyst after a treatment such as oxidation, reduction, or exposure to a reactant, or with the catalyst in a particular state it may be helpful to compare a fresh catalyst with a spent or a regenerated catalyst. XAFS spectroscopy used in this manner is "static the structure of the catalyst is determined in a specific well-defined state determined by the treatment and gas environment during the measurement. Two such examples are discussed here the determination of the location of the isomor-phous substitution of a heteroatom (tin) into a zeolite framework (zeolite beta), and the structure of dispersed rhenium oxide supported on y-Al203. 

Electron crystallography offers an alternative approach in such cases, and here we describe a complete structure determination of the structure of polymorph B of zeolite beta [3] using this technique. The clear advantage of electron microscopy over X-ray powder diffraction for elucidating zeolite structures when they only occur in small domains is demonstrated. In order to test the limit of the structural complexity that can be addressed by electron crystallography, we decided to re-determine the structure of IM-5 using electron crystallography alone. IM-5 was selected for this purpose, because it has one of the most complex framework structures known. Its crystal structure was solved only recently after nine years of unsuccessful attempts [4],... 

As has been confirmed by XRD, the framework of montmorillonite has been partly destroyed due to the calcination under high temperature. Most diffraction peaks of montmorillonite are faint. After hydrothermal crystallization the characteristic Bragg reflections for zeolite Beta structure at 7.7° and 22.42° 20 are detected in the composite, indicating the presence of the Beta phase. 

The isomorphous replacement of aluminum by gallium in the framework structure of zeolites (beta, MFI, offretite, faujasite) offers new opportunities for modified acidity and subsequently modified catalytic activity such as enhanced selectivity toward aromatic hydrocarbons [249,250]. The Ga + ions in zeolites can occupy tetrahedral framework sites (T) and nonframework cationic positions. 

Zeolite beta and its mineral analog tschernichite are intergrown materials and BEA represents the framework of a hypothetical end member. Simulated powder patterns for... 

Zeolites are not typically used in Lewis acid type catalysis due to the absence of Lewis acid centers in zeolites. This is due to the coordination of the Al-site to four lattice-oxygens in a perfect zeolite framework. It has, however, been shown for zeolite Beta that the aluminum atom can reversibly move between a framework Brpnsted acid site and a framework-grafted Lewis-acid site.70 Accordingly, Creyghton et al. showed that zeolite Beta is active in the Meerwein-Ponndorf-Verley reduction (MPV) of ketones (scheme 4).71 In this reaction a hydrogen hydride transfer reaction between an alcohol and a ketone takes place. 

One approach is to incorporate Lewis acids into, for example, zeolites or me-soporous silicas [141]. For example, incorporation of Sn(IV) into the framework of zeolite beta afforded a heterogeneous water-tolerant Lewis acid [142]. It proved to be an effective catalyst for the intramolecular carbonyl-ene reaction of citronellal to isopulegol [143] (Fig. 2.43) in batch or fixed bed operation. Hydrogenation of the latter affords menthol (Fig. 2.43). 

The incorporation of Ti into various framework zeolite structures has been a very active research area, particularly during the last 6 years, because it leads to potentially useful catalysts in the oxidation of various organic substrates with diluted hydrogen peroxide [1-7]. The zeolite structures, where Ti incorporation has been achieved are ZSM-5 (TS-1) [1], ZSM-11 (TS-2) [2] ZSM-48 [3] and beta [4]. Recently, mesoporous titanium silicates Ti-MCM-41 and Ti-HMS have also been reported [5]. TS-1 and TS-2 were found to be highly active and selective catalysts in various oxidation reactions [6,7]. All other Ti-modified zeolites and molecular sieves had limited but interesting catalytic activities. For example, Ti-ZSM-48 was found to be inactive in the hydroxylation of phenol [8]. Ti-MCM-41 and Ti-HMS catalyzed the oxidation of very bulky substrates like 2,6-di-tert-butylphenol, norbomylene and a-terpineol [5], but they were found to be inactive in the oxidation of alkanes [9a], primary amines [9b] and the ammoximation of carbonyl compounds [9a]. As for Ti-P, it was found to be active in the epoxidation of alkenes and the oxidation of alkanes and alcohols [10], even though the conversion of alkanes was very low. Davis et al. [11,12] also reported that Ti-P had limited oxidation and epoxidation activities. In a recent investigation, we found that Ti-P had a turnover number in the oxidation of propyl amine equal to one third that of TS-1 and TS-2 [9b]. As seen, often the difference in catalytic behaviors is not attributable to Ti sites accessibility. 

Based on our experimental data, the following conclusions may be drawn (i) the incorporation of titanium into the framework of beta zeolite was achieved by treating Al-beta zeolite with ammonium titanyl oxalate solution and calcining the resultant material at 833 K for 6 h, (ii) the presence of Ti in tetrahedral framework positions was evidenced by various techniques, particularly UV-Vis, XPS and catalytic properties and (iii) Ti-P and Ox-Ti-P samples were active in the epoxidation of olefins. 

Titanium-containing pure-silica ZSM-48 (e.g., [71, 72]), a unidimensional medium pore zeolite, and titano-aluminosilicates with the structure of zeolite Beta [72-74] are materials which are currently scrutinized in catalytic oxidation reactions [75]. In the latter case, however, residual acidity created by framework aluminum leads to undesired side reactions. Since, so far, the direct synthesis of Al-free pure titaniumsilicate Beta was not successful, van Bekkum et al. [76] developed a special post-synthesis modification technique. The three-step procedure... 

The framework aluminium in zeolite beta is very sensitive to calcination and ion-exchange and dealumination occurs even under very mild conditions [5-7]. 27a1 NMR shows that octahedrally coordinated aluminium is often present in calcined H-betas, although it has been suggested that at least some of this aluminium is still attached to the framework and can be reincorporated under the right conditions [6,7]. 

Zeolite beta has been widely studied as a Bronsted acid catalyst and has been shown to be highly active for reactions such as alkylation and acylation [8,9]. The effect of the crystallite size on the catalytic activity of beta has been investigated [10] but since betas often have very high external surface areas, it is possible that acid sites associated with framework aluminium close to the outer surface will contribute to the overall catalytic activity of the zeolite. This may adversely affect the shape-selectivity of the reaction. In this study a series of beta zeolites with differing Si/Al ratio and ESA were investigated by means of non-contact AFM and N2 absorption measurements and the catalytic activity was tested by an acylation reaction capable... 

Zeolite beta samples with different framework and extraframework composition have been prepared by submitting the acid form of a commercial TEA-beta sample to different post-synthesis treatments, i.e. steam calcination, acid (HCl) leaching, and ammonium hexafluorosilicate (HFS) treatment. The samples were characterized by XRD, adsorption of N. at 77 K. i.r. spectroscopy with adsorbed pyridine, Si and Al MAS-NMR and XPS. Bifunctional catalysts were obtained by impregnation with 0.3 wt% Pt, and the catalytic activity for the isomerization of a simulated LSR feed (n-Cj/n-Cj, 60/40 wt%) was measured under different reaction conditions. 

Acid form of zeolite beta (Hp) was obtained from a commercial TEA-beta (Valfor CP806B-25) by calcination at 773 K for three hours to remove the template, followed by ion exchange with a 2M aqueous solution of NH4CI at 353 K for two hours, and a final calcination at 773 K for three hours. Then, the HP was dealuminated using three different procedures, i.e. steam calcination at 873 K during three hours (sample HPs,), acid treatment with HCl O.IM at reflux for two hours (sample HP, ), and ammonium hexafluorosilicate (HFS) treatment (sample HP ps2)- The latter two procedures produced almost EFAL-free beta samples. Moreover, the EFAL formed in HP during the calcination steps was also extracted with the required amount of ammonium hexafluorosilicate in order to avoid framework dealumination (sample HP fsi)- The HFS treatments were carried out in an ammonium acetate buffer at 348 K with slow addition of a 3M hexafluorosilicate solution (12 cm /h). Afterwards,... 

ZSM-5 only offers a very low activity but exhibits a very high selectivity of 29. This complies well with the fact that ZSM-5 is the zeolite with the strongest acidic sites used in this study and was expected to exhibit the experimentally found behaviour. A comparison with zeolite beta which has a similar aliuninium content but a more spacious and moreover a 3-D channel system indicates that the low activity is caused by slower diffusion in the much smaller channels of ZSM-5. Whether the high selectivity is only due to the high acidity of the framework or if there is an additional shape selectivity for the para -isomer (3) is not absolutely clear. Compared to para-and meta-xylene, the widths of the corresponding isomers (3) and (4) are only slightly smaller (=0.lA) but the difference between the isomers is nearly the same as between the two xylenes. This seems to be a strong indication of shape selectivity. 

In order to prove this we have located the toluene molecule in the center of the "cavity" formed in zeolite Beta when the channels cross (Fig. 5) and PM3 calculations show that the energy of the HOMO has increased in 2.5 e.v. with respect to the value of the HOMO of toluene in gas phase. Furthermore, it has been observed, that a further increase in 0.8 e.v. occurs when the A1 content of the zeolite structure given above passes to have 3 to 1 framework A1 [37]. 

Thermal analysis is an appropriate technique to investigate the precise nature of the organic molecules occluded in zeolite frameworks (41). For a series of zeolite Beta samples synthesized under various conditions (Table VII) DTA provides evidence for presence of both TEA+ ionic species (DTA sharp peak near 460°C) and TEAOH ionic pairs (weak broader DTA peak recorded near 345°C) (61). Similar conclusions were proposed by Perez-Pariente et al. (31) for a number of Beta samples prepared under slightly different conditions TEA+ ions undergo decomposition above 350°C while the neutral TEAOH species are released between 220 and 350°C. Our TG-DTA combined system allowed a quantitative determination of both species (Table VII). 

---