Zeolites, delaminated

It is well know that the zeolite materials synthesized in alkaline systems usually have a high number of silanol groups (=SiOH) named defect groups [10] which possess a moderated Bronsted acidity [11]. Oppositely, Silicalite-1 synthesized in fluorine media are relatively defect-free [12] and the fluorine ions remain in the small cages of the MFI structure even after the calcination process [12]. The 29Si-NMR analyses carried out on samples Na-Silicalite-1 and F-Silicalite-1 confirm the presence of silanol groups only on the SI support surface (results not showed). Delaminated zeolites (ITQ-6) are obtained by exfoliation of as-synthesized lamellar precursor zeolites [13]. After this process, the final structure of the delaminated zeolite results in a completely hydroxylated and well-ordered external surface [13]. 

A delaminated zeolite with an Si/Al ratio of 29, derived from the layered zeolite Nu-6(1), was employed as catalyst for dehydration of xylose at 170 °C, using a water-toluene biphasic reactor-system.140 This material, designated del-Nu-6(l), proved to be efficient for this transformation, giving 47% selectivity to furfural at 90% xylose conversion. 

As was shown here in some examples, the field of catalysis over zeolites, although marnre, is still very much alive. The chemists who work with the synthesis zeolites continue to be very creative, the focus now being placed on the synthesis of materials that can catalyze reactions other than the acidic ones and/or reactions of bulkier molecules, that is, synthesis of zeolites with larger micropores or with a very large external surface, such as nanosize and delaminated zeolites. New concepts related to the mode of action of zeolite catalysts continue to emerge, as shown here with the shape selectivity of the external surface. These concepts are particularly useful to scientifically design selective and stable catalysts. 

A recent investigation has demonstrated the usefulness of ultrasonic irradiation in the preparation of delaminated zeolites, which are a particular type of modified oxides - microporous crystalline aluminosilicates with three-dimensional structures - having a greater catalytic activity than the layered structures (clays) and mesoporous catalysts. In an attempt to increase the pore size of zeolites, a layered zeolite precursor was... 

Two categories of mesoporous solids are of special interest M41S type materials and pillared or delaminated derivatives of layered zeolite precursors (pillared zeolites in short). The M41S family, first reported in early 1990 s [1], has been extensively studied [2,3]. These materials exhibit broad structural and compositional diversity coupled with relative ease of preparation, which provides new opportunities for applications as catalysts, sorption and support media. The second class owes its existence to the discovery that some zeolite crystallizations can produce a lamellar intermediate phase, structurally resembling zeolites but lacking complete 3-dimensional connectivity in the as-synthesized form [4]. The complete zeolite framework is obtained from such layered zeolite precursor as the layers become fused, e.g. upon calcination. The layers posses zeolitic characteristics such as strong acidity and microporosity. Consequently, mesoporous solids derived from layered zeolite precursors have potentially attractive characteristics different from M41S and the zeolite species... 

The successful preparation of pillared or delaminated zeolite is confirmed using X-ray diffraction and TEM, which also allow detection of M41S impurities if... 

Fraenkel et al. (54) were the first to propose that the external surface of zeolites could be responsible for shape selective catalysis. Acid sites located in the half cavities on the external surface of HMFI would be responsible for the selective formation of 2,6- and 2,7-dimethylnaphthalene during naphthalene methylation (nest effect). This explanation was afterwards rejected on the basis of adsorption experiments. However, a nest effect was recently proposed to be responsible for the shape selective properties of the MCM-22 (MWW) zeolite and its delaminated analog (ITQ-4) in aromatics alkylation (55). 

The triethoxysilyl endgroup is a popular functional group to bind the catalyst to a polymeric support [238]. Polymeric supports include silica gel, MCM-41 (mesoporous silica gel) and ITQ-2 (delaminated zeolite) [247]. Corma et al. used this approach to synthesise gold(I) and palladium(II) NHC complexes for Suzuki cross-coupling reactions between iodobenzene and various arylboronic acids (see Figure 4.78) [247]. The results were very modest at 35-80% dependent upon the substitution pattern of the arylboronic acid. Yields with gold(I) catalysts were marginally better than those for palladium(II) complexes. 

Zeolites are currently manu ctured as micron size crystals and compacted into millimeter size pellets for applications as packed beds. In many catalytic and adsorptive applications, mass and heat transfer properties could potentially be improved by structuring the zeolite in a different way. Research in this area led already to significant achievements. Alternatively structured zeolite matter are e.g. delaminated zeolites [1], supported zeolite films and membranes [2], hybrid structures with microporosity in walls of ordered mesoporous materials [3-6] and nanosized zeolites such as those synthesized in confined space [7]. The common property of these alternative zeolites is that at least in one direction, the zeolite framework has a dimension of around a nanometer. 

Most of the zeolite syntheses carried out under hydrothermal conditions directly results in the formation of three-dimensional crystalline frameworks. However, several zeolites, like MCM-22 or ferrierite, can be synthesized in the form of layered precursors, which can be transformed by further thermal treatment into the three-dimensional crystal structure. These layered solids arouse an interest due to their ability to intercalate guest molecules between two neighboring zeolite layers. Using a proper treatment, layered zeolite materials can be delaminated while the structure of layers is preserved, which makes accessible all active sites located on the external surface of such catalyst. By adding proper inorganic guest molecules functioning as pillars, the control of the interlayer distance can be achieved. Such materials... 

At present, MCM-22 and ferrierite represent the only two described examples of zeolites, which can be synthesized in the layered form and further transformed to 3-D crystal structures. Delamination of ferrierite led to the formation of ITQ-6 and ITQ-20 [52], while ITQ-2 can be prepared from the lamellar precursor of MCM-22 [15]. Not only aluminum but also titanium was incorporated into the siliceous ITQ-6 and evaluated in acid and oxidation reactions [52,53]. 

Delaminated zeolites like 1TQ-2, benefiting from greater accessibility of acid sites and shorter diffusion pathways, exhibited a higher catalytic activity in cracking of 1,3-diisopropylbenzene or vacuum oil compared with MCM-22 [15]. However, it seems that a high potential of delaminated catalysts can be found in synthesis of special chemicals. Corma and Fomes showed the synthesis of 2-melhyl-2-naphthyl-4-methyl-l,3-dioxolane, which has an orange blossom fragrance [110,111],... 

Delaminated zeolites as active catalysts for processing large molecules... 

MCM-41 type is that in the nanocrystalline and delaminated zeolites there is short-range order and consequently the active sites are of zeolitic nature. 

Up to now, we have delaminated four different lamellar zeolitic precursors which have generated the materials named as ITQ-2, ITQ-6, ITQ-18 and ITQ-20 (12,13). The first one, ITQ-2, was obtained by delaminating a MWW zeolite precursor (14) while ITQ-6 and ITQ-20 were prepared by delaminating laminar precursor of ferrierite (15) and MCM-47 (16,17), respectively, while the structure of ITQ-18 is still unknown to us. The artist view of the stmcture of the two first material is presented in Figure 1. 

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