Lamellar zeolites

FIG. 23.24. The structure of a lamellar zeolite (phillipsite). The double tetrahedral unit shown at (a) projects in the plan (b) as a single (equilateral) triangle. 

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. 

Soil removal (%) Global detergency LABS Nonionic C12-70E Silicate R2, Spray-dried Silicate R2, solution Silicate, lamellar Zeolite Soda ash silicate R2, spray-dried silicate R2 spray-dried... 

Roth WJ, Shvets OV, Shamzhy M, Chlubna P, Kubu M, Nachtigall P, et al. Postsynthesis transformation of three-dimensional framework into a lamellar zeolite with modifiable architecture. J Am Chem Soc 2011 133 6130-3. 

When supported complexes are the catalysts, two types of ionic solid were used zeolites and clays. The structures of these solids (microporous and lamellar respectively) help to improve the stability of the complex catalyst under the reaction conditions by preventing the catalytic species from undergoing dimerization or aggregation, both phenomena which are known to be deactivating. In some cases, the pore walls can tune the selectivity of the reaction by steric effects. The strong similarities of zeolites with the protein portion of natural enzymes was emphasized by Herron.20 The protein protects the active site from side reactions, sieves the substrate molecules, and provides a stereochemically demanding void. Metal complexes have been encapsulated in zeolites, successfully mimicking metalloenzymes for oxidation reactions. Two methods of synthesis of such encapsulated/intercalated complexes have been tested, as follows. 

The synthesis procedure led to a highly crystalline MCM-22 zeolite, as indicated by XRD data (Fig. 1A, a). The introduction of platinum by ion exchange procedure does not modify the crystallinity of the zeolite (Fig. 1A, b). SEM micrographs show that the sample obtained in static hydrothermal conditions is characterized by the presence of homogeneous round-shaped particles of ca. 10 pm in diameter formed by aggregation of lamellar particles [7,8], The morphology of the zeolite is not modified after platinum introduction (Fig. IB). 

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]. 

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... 

Furthermore, aluminosilicates are available in a variety of different structural types including lamellar clays and three-dimensional microcrystalline zeolites. Such solids can be useful... 

For Further Reading M. Jacoby, Iron-rich zeolite stands up to water, Chemical and Engineering News. September 15, 1997, pp. 8-9. P. T. Tanev and T. J. Pinnavaia, Biomimetic templating of porous lamellar silicas by vesicular surfactant assemblies, Science 271 1267-1269. J. M. Thomas, Solid acid catalysts, Scientific American, April 1992, pp. 112-118. 

Phosphates having these types of open structures can act as shape-selective acid catalysts, for example, for the cracking and isomerization of hydrocarbons. For examples of lamellar materials, see Section 5.3 and see Intercalation Chemistry). Microporous catalysts are described above and in (see Porous Inorganic Materials and Zeolites). Mesoporous AlPO materials have larger pores within a matrix of amorphous A1P04. ... 

We now report the first extension of the concept of pillaring to the realm of zeolites. By exploiting the lamellar nature of the precursor to zeolite MCM-22 ( as-synthesized MCM-22 ) [8] we have synthesized the first zeolite-based pillared molecular sieve, designated MCM-36 [9]. The following describes physical characterization of MCM-36, which unequivocally establishes its existence as a novel large pore pillared material with zeolite properties. 

The preparation of MCM-36 involves a lamellar Intermediate, designated MCM-22 precursor, produced in a hydrothermal process. The layers in MCM-22 precursor are approximately 25 A thick and upon calcination condense with the formation of T-O-T moieties producing the zeolite MCM-22. This relationship indicates that the internal structure of the MCM-22 precursor layers matches that of zeolite MCM-22, which is also reflected in the similarity of their X-ray diffraction patterns (XRD) shown in Figure 1. 

A characteristic feature of zeolites is the existence of tunnels (or systems of interconnected polyhedral cavities) through the structures, and we may therefore expect three main types in which the tunnels are parallel to (a) one line, giving the crystals a fibrous character, (b) two lines and arranged in planes, so that the crystals have a lamellar nature, or (c) three non-coplanar lines, such as cubic axes, when the crystals have no pronounced fibrous or lamellar structure. The most symmetrical... 

Catalytic behaviour of Rh-supported catalysts on lamellar and zeolitic structures by anchoring of organometallic compound... 

Rhodium catalysts were prepared by hydrogen reduction at atmospheric pressure of a cationic organometallic rhodium complex and anchored onto lamellar and zeolitic products. The effect of the structure and characteristics of the support on metal load and dispersion was studied in the heterogeneous catalysts prepared. The new rhodium catalysts were applied in the hydrogenation of acetone. The reaction was carried out under milder conditions. 

The SEM photomicrographs (Fig. 2) showed the change from a lamellar structure in the original sample to a spherical granular structure in the zeolitic materials obtained with alkaline treatment. The spherical units formed are attributed to the Na cation [8], which controls the crystalline phase, while the unit particle size depends on the characteristics of the medium used (distilled or sea water) because of the presence of ions in the latter. 

In most studies on the use of clays as catalysts or catafyst supports, the surface acidity of the clays has been considered a determinant of their catalytic activity [9,10]. All the supports (of lamellar and zeolitic structures) have acid centers, whose strength and number varied depending on the peculiar structure of the synthesized constituent materials [11]. 

Table 2 also shows that the resulting Rh dispersion is not a fimction of the metal loading but of the structure of the support [12,13], The natme of the support influences dispersion and therefore the size of the metallic particles. There was no correlation between metallic load and dispersion. Lamellar structures (BENa and BENPIL) incorporated larger amounts of rhodium complex but had lower dispersion than the catalysts synthesized on zeolitic products. The influence of the support structure is also reflected in the results for the zeolitic product synthesized with different treatment conditions or in different media, which determine the channel dimensions and the number of anchoring centres in the resulting samples. Higher dispersion was achieved in the more transformed samples, ZE— P, than in ZE—X, and in those synthesized in distilled water (ZEDI-) than in sea water medium (ZESE-). 

The PIL-clay and zeolitic materials synthesized are suitable as support for preparing heterogeneous catalysts by anchoring an organometallic complex by ion exchange. The structure of the materials used as supports had a great influence on the catalyst prepared. A higher metal content was achieved in the supports with lamellar structures, while better dispersion was shown by the catalysts supported on zeolitic structures. 

Activity, selectivity and durability of the new heterogeneous rhodium catalysts depend on the nature of the support used in the preparation of the catalyst. The lamellar structures mainly yield methyl isobutyl ketone (MIBK), whereas the zeolitic structures, in general, yield isopropanol (IPA). The activity remains steady throughout the period tested (7h), except when the support is the BENPIL indeed, the deactivation of Rh/BENPIL is relatively rapid particularly during the first hour of reaction. The turnover numbers frequencies (TOP) obtained with these catalysts might be attributed to the structure of the metal no sensitive reaction the hydrogenation acetone. 

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]. 

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