Framework of zeolite Beta

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). 

Iron in the framework of zeolite beta has been thoroughly characterised using RT and 4.2 K Mossbauer spectroscopy. An external appUed field (4.13 T) was also applied at 4.2 K. The IS of 0.22 mm s at RT and 0.32 mm s" at 4.2 K and the narrow, sharp hyperfine sextet at 4.2 K imder the applied field demonstrated that most of the Fe + ions were present as tetrahedral iron in the framework [34]. 

The post-synthetic incorporation of titanium into the framework of zeolite beta has been achieved by liquid-phase treatment employing ammonium titanyl oxalate [59]. Ti-beta has been also prepared by treating Al-containing beta with a concentrated solution of perchloric or nitric acid in the presence of dissolved titanium [60]. Although Ti(OBu)4 and Tip4 were efficient sources for the incorporation of tetrahedral Ti, the use of 1102 as the Ti source gives rise to octahedral Ti as well as tetrahedral Ti. In this case, extraction of Al from the framework occurred simultaneously, giving almost Al-free Ti-beta. 

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],... 

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. 

Competitive roles of Na+ and TEA+ ions in forming and stabilizing frameworks of zeolites ZSM-20 and Beta... 

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... 

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,... 

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). 

A. Corma, M.T. Navarro, F. Rey, J. Rius, and S. Valencia, Pure Polymorph C of Zeolite Beta Synthesized by using Framework Isomorphous Substitution as a Structure-directing Mechanism. Angew. Chem., Int. Ed., 2001, 40, 2277-2280. 

Recently Creyghton et al. [6,7] reported the use of zeolite beta in the MPVO reduction of 4-t-butylcyclohexanone. ITie high selectivity towards the thermodynamically less favoured ds-alcohol is explained by a restricted transition-state around a Lewis-acidic aluminium in the zeolite pores. When using an aluminium-free zeolite, titanium beta, in the epoxidation of olefins, we have shown that Ti-beta has acidic properties when alcoholic solvents were employed [8], This was ascribed to the Lewis-acidic character of titanium in the zeolite framework. As we reported very recently [9], Ti-beta is found to be an excellent catalyst in MPVO reactions with a tolerance for water. Here, results are presented on the high selectivity, stability and low by-product formation of the catalyst, Ti-beta, in both the liquid-phase and gas-phase MPVO reactions. 

To explain this competition we hypothesized that Ti could play a role similar to that of A1 in the crystallization of zeolite Beta, that is, the creation of negative charges in the framework and thus its stabilization by interaction with the TEA+ templating cations. This hypothesis was also supported by the fact that the amount of TEA+ cations decomposing at T>620K in air (as determined by thermal analysis) was dependent on the total amount of A1 + Ti, rather than only on A1 (ref. 14). This hypothesis required the ability of zeolitic Ti to change its coordination number, something which obtained substantial support from XANES and EXAFS measurements (ref. 15). 

Zeolite beta was among the first zeolites which underwent successful replacaaoent of boixrn for aluminium (1). The main grx>und for inserting boron in zeolitic frameworks is the modulation of the strength of the acid sites (2-5), but structural boron proved to be less stable than aluminium In the activation treatments, especially in hydrothermal conditions (6, 7). Ihis drawback may be turned into advantage Wien a network quite unstable under dealuminating conditions is concerned, as in the case of zeolite beta (8). The milder conditions required for deboration are likely to affect to a lesser extent the lattice stability. B-beta could then represent a suitable precursor of the activated form of the zeolite (9). Moreover the different kinetics of incorporation of boron and aluminium are likely to influence other properties of the solid, like the size and habit of the crystals and the defect patterns (10-12). 

A novel method has been developed for hydrothermal synthesis of zeolite beta with high chromium content and low aluminum content under static conditions. The crystalline phase and spectroscopic property of this material were characterized by means of XRD, XRF, IR, UV-Vis DRS, ESR and SEM. The results revealed that part of the chromium ions were incorporated into the zeolite framework during crystallization. Using tris(acetylacetonato) chromium(III), [Cr(C5H702)3], as a chromium source, Cr/Si molar ratio can be up to 1/48 in the calcined and ion-exchanged sample. The chromium aluminosilicate exhibited a high oxidative activity in the presence of dilute H2O2. 

Samples of zeolite Beta with various degrees of dealumination were prepared via various treatments by Yang and Xu [263]. These authors provided framework spectra of their samples, obtained in the region 1400-400 cm through the conventional FTIR/KBr pellet technique. A band at about 950 cm was attributed to Si-O vibrations in structural vacancies (cf. also [264]) and monitored in dependence on the treatment. 

The FTIR/KBr method was, inter aha, applied by Maache et al. [306] for the characterization of various samples of H-Beta dealuminated by acid leaching. In the case of framework vibrations of zeolite Beta, a good Hnear correlation was found between the wavenumber of the asymmetric O-T-O vibration, Vas(O T-o)> and the aluminum mole fraction, nAi/(nsj+nAi) [263]. Ponthieu et al. [307] attempted to find a correlation between the wavenumbers of framework vibrations of ojfretite and zeolite Omega and the degree of dealumination (aluminum mole fraction) similar to those observed with faujasites and mordenites. However, Hnear correlations were hardly obtained. 

Higgins JB, LaPierre RB, Schlenker JL, Rohmian AC, Wood JD, Kerr GT, et al. The framework topology of zeolite beta. Zeolites 1988 8 446-52. 

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

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 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... 

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