MFI-type materials

The products used for the calorimetric measurements and for the synthesis of the MFI-type materials were high purity reagents 40% aqueous HF ( Prolabo) (n-C Hy NBr ( Fluka, purum, 99%) (0-0387)3N... 

Shape selectivity is not confined to reactions of hydrocarbons in the absence of polar functional groups. MFI type materials have been reported to catalyze the isomerisation of cresols, chlorotoluenes, toluonitriles and toluidines [259]. In the isomerization of aniline derivatives the reaction temperatures have to be relatively mild as under severe reaction temperatures isomerization to methylpyridine would occur [260]. For dimethylanilines it could be shown that only the isomers with the smallest minimum kinetic diameter reacted (reactant selectivity), and that those with a larger kinetic diameter did not form (product selectivity). The isomerization is concluded to occur via a 1,2 methyl shift which is interpreted to indicate transition state selectivity [261]. 

The products used for the calorimetric measurements and for the synthesis of the MFI-type materials were high purity reagents 40%... 

The X-ray diffractograms of calcined samples ZSM-5, ZSM-5/10 and ZSM-5/IMP are shown in Fig. 1. As diffractograms of samples ZSM-5/3 and ZSM-5/6 are similar, it can be concluded that all samples are completely crystalline and contain exclusively MFI type material. 

Kofke et al. (168) determined the concentration of gaUium in the framework of MFI-type materials by adsorption methods. KUk et al. (76) investigated the gaUium incorporation by using high-field Ga MAS NMR spectroscopy, showing that gallium preferentiaUy occupies the framework positions in Ga-MFI materials that are prepared by direct synthesis from a gel. 

Single crystals of 500 jim x 350 ]im x 350 ]im and incomplete single crystals of up to 750 pm in the c-direction of MFI with a Si/Al ratio of 300 have been made [39,40]. The synthesis formulation and procedure developed [41] deviate significantly from the regular synthesis recipes used for the preparation of MFI-type materials as illustrated in the compositional diagram of reactants in Fig. 10. 

The zeolite ZSM-5 has the MFI-type structure and can be obtained with many different Si/Al ratios typically ranging from about 10 to If no aluminum is present (Si/Al = ), a pure siliceous structure is obtained and the resulting material is then called Silicalite-1. The unit cell composition of an MFI-type zeolite can be written as (Na+, H A Si, xOl92-/ H20 withx < 27 (in most cases <9) and n < 16 (the more siliceous a zeolite gets, the higher is its hydrophobicity). 

Commercially significant zeolites include the synthetic zeolites type A (LTA), X (FAU), Y (FAU), L (LTL), mordenite (MOR), ZSM-5 (MFI), beta ( BEA/BEC), MCM-22 (MTW), zeolites E (EDI) andW (MER) and the natural zeolites mordenite (MOR), chabazite (CHA), erionite (ERl) and clinoptiloUte (HEU). Details of the structures of some of these are given in this section. Tables in each section lists the type material (the common name for the material for which the three letter code was established), the chemical formula representative of the unit cell contents for the type material, the space group and lattice parameters, the pore structure and known mineral and synthetic forms. 

There are many different zeolite structures but only a few have been studied extensively for membrane applications. Table 10.1 lists some of these structures and their basic properties. One of the most critical selection criterion when choosing a zeolite for a particular application is the pore size exhibited by the material. Figure 10.1 compares the effective pore size of the different zeolitic materials with various molecule kinetic diameters. Because the pores of zeolites are not perfectly circular each zeolite type is represented by a shaded area that indicates the range of molecules that may stiU enter the pore network, even if they diffuse with difficulty. By far the most common membrane material studied is MFI-type zeolite (ZSM-5, Al-free siUcahte-l) due to ease of preparation, control of microstructure and versatility of applications [7]. 

The amount of MFI- and MCM-41-type material were estimated from XRD peak intensities. For the samples containing only small amounts of MFI, the presence of non-XRD detectable structures were identified for all samples by weak intensities in the infrared spectra near a frequency of 550 cm-1, which is characteristic for a skeleton bending mode for the MFI type samples [7]. The hydrothermal stability obtained also varied considerably for the samples, in the following, only three samples revealing the best hydrothermal stability will be discussed in detail. Synthesis and characterization data for the selected samples A, B and C, are summarized in Tables 1 and 2. The corresponding data for a siliceous pure MCM-41 sample synthesized by the same procedure [4,5], have been included for comparison reasons. 

Previous characterisation (SEM, HREM) of the purely siliceous composite materials [4] revealed the formation of fairly complex aggregates of MFI and MCM-41 type material. The data were not sufficient to assess a more intimate integration of the two structures, although the enhanced hydrothermal stability obtained [5] would indicate the presence of a closer interaction between the two phases than just a physical mixture. The actual structure of the present samples has not been studied in further detail so far. Taking into account the considerable potential within catalytic applications for structures which may possess unique integrations of acidity and structural properties, the synthesis, modification, catalytic activity and structure of these materials should be studied closer. 

Titanosilicalite (TS-1)[165,166], a highly siliceous MFI type zeolite in which 0.1 to 2.5% of the Si atoms are replaced by Ti, is the most successful example for the use of isomorphously substitited zeolites. As a consequence of the high Si/Al ratio of TS-1 the material contains only a negligible concentration of strong Bronsted acid sites. In fact, the presence of acid sites is detrimental to the selectivity of the catalysts, as discussed below. TS-1 has been found to be a selective oxidation catalyst for a wide variety of reactions such as the conversion of alkenes to epoxides [167], alcohols to aldehydes [168], alkanes to secondary alcohols and ketones [169,170], phenol to hydroquinone and catechol [171] and amines to hydroxylamines [ 172]. A schematic representation of the chemistry is given in Fig. 7 which is adapted from ref [17]. 

Y.-C. Long, X. Chen, Z.-H. Ping, S.-K. Fu, and Y.-J. Sun, MFI-type zeolite filled silicone rubber membranes Preparation, composition, and performance. Zeolites and related microporous materials State of the art 1994 Part B. Proc. lOth Int. Zeol. Conf., Garmisch-Panenkirchen (J. Weitkamp, H.G. Karge, H. Pfeifer, and W. Holderich, eds.), Elsevier, Amsterdam, 1994, p. 1083. 

R. Kumar, P. Mukheijee, R. Pandey, P. Rajmohanan, and A. Bhaumik, Role of Oxyanions as Promoter for Enhancing Nucleation and Crystallization in the Synthesis of MFI-type Microporous Materials. Microporous Mesoporous Mater., 1998, 22, 23-31. 

Membranes synthesized on y- AI2O3 have various morphology (Figs. 1, 3a and 4). The typical results obtained are shown in Table 1 together with synthesis conditions. The materials obtained are amorphous, MFI type zeolite or MSU mesoporous materials with a disordered wormhole like structure, depending on the support pretreatment and synthesis conditions. 

GIS). A three letter code (e.g. GIS) is assigned to confirmed framework types by the Structure Commission of the International Zeolite Association according to rules set up by an 1UPAC Commission on Zeolite Nomenclature [3,4]. The codes are normally derived from the name of the zeolite or type material , e.g. FAU from the mineral faujasite, LTA from Linde Type A, and MFI from ZSM-5 (Zeolite Socony Mobil - five). Information pertinent to these framework types is published in the Atlas of Zeolite Framework Types [5] and on the internet at http //www.iza-strncture.org/databases/. As new codes are approved, they arc announced on the IZA Structure Commission s WWW pages (http //www.iza-structure.org/) and included in the internet version of the Atlas. As of January 2005, 161 zeolite framework types had been confirmed by the Structure Commission. In this chapter, all references to materials whose framework types are known will be accompanied by the appropriate three letter code in boldface type. 

Porous materials can also be coated with zeolite films by direct synthesis. For example, microcellular SiOC ceramic foams in the form of monoliths were coated on their cell walls with thin films of silicalite-1 and ZSM-5 using a concentrated precursor solution for in situ hydrothermal growth (Fig. 9).[62] The zeolite-coated monoliths show a bimodal pore system and are thermally stable to at least 600 °C. A related strategy is based on the conversion of macroporous Vycor borosilicate glass beads, having pores of about 100 nm, to MFI-type zeolite-containing beads retaining the same macroscopic shape.[63] This conversion was achieved by hydrothermal treatment with an aluminium source and a template such as TPABr. 

Structural and Sorptive Properties of Some MFI- and AEL-Type Materials... 

Pentasil MFI (ZSM-5) type materials exhibiting different crystallite sizes and AEL (SAPO-11) type materials have been studied. Structural investigations using X-ray diffraction techniques, developed for polycrystalline powdered samples, allow one to determine precisely the structure and crystallinity of the samples and the effect of adsorbates (e.g., p-xylene, n-hexane, etc) on the framework topology (monoclinic towards orthorhombic for MFI) and unit-cell dimensions. 

Another feature that one must also keep in mind, is the possible crystal structure modifications induced by hydrocarbon adsorption, which might result in changes in some catalytic properties. Therefore, it appeared very important to us, to characterize the structural properties of zeolitic-type materials versus chemical, hydrothermal or adsorptive modifications, and subsequently to determine the changes in acidic and catalytic properties. The aim of this paper is to present some examples of our own, in order to illustrate the changes observed for the ZSM-5 (MFI) and SAP0-11 (AEL) type materials (11). 

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