Zeolite aluminium

A series of ZSM-5 samples with differing framework aluminium contents (containing tetrapropylammonium cations, [TPA]+) have been characterised by Raman spectroscopy. Difference Raman spectra reveal evidence for two distinct occluded species in samples with non-zero framework aluminium content. These species have been Identified as [TPA]+ cations associated with framework anionic sites and non-framework anions such as Br or OH-, on the basis of correlations between the integrated intensities of difference spectra and zeolite aluminium content. The relative abundance of the two forms have been determined semi-quantitatlvely and empirical evidence for [TPA]+ disordering is reported. 

For all three samples of ZSM-5 a band appears at ca. 1780 cm 1 when coke in ZSM-5 is exposed to air at conversion temperatures and above (see Fig. 1 (a-c)). This is accompanied by the loss of some IR bands due to coke between 1700 and 1300 cm, especially at temperatures above 500°C. This band is stable up to 450°C and at conversion temperatures takes a few hours to reach maximum intensity. Heating above 550°C produces a clean spectrum of the zeolite which only contains a pair of bands at 3740 and 3610 cm"1 due to the hydroxyl groups of the zeolite. The rate of growth of the 1780 cm"1 band is independent of zeolite aluminium content for similar coke contents. 

TS-1 and Ti-beta, with structure MFI and BEA respectively, have been used as catalysts. TS-1 was provided by the Groupement de Recherche de Lacq Ti-beta was prepared in this laboratory according a recently reported procedure [15]. The composition of these zeolites (aluminium free) was Ti / (Ti+Si)= 0.011 and 0.008 for TS-1 and Ti-beta respectively. Crystal size as determined by scanning electron microscopy (Cambridge Stereocam 260) was... 

Under these conditions the concentration of Si in solution is too small for observation by NMR, so only Al-NMR can give as much information about solution as solid phase. The results are shown in Fig. 2. In solution, only one sharp line belonging to monomeric tetrahedral aluminium exists (the same is true for Si not shown). The intensity of this aluminum is decreasing, following the patterns in Fig. 2. The very broad peak (59 ppm) belongs to amorphous tetrahedral Al-from the gel. In the course of the action, this peak became narrower and shifted to 58.3 ppm, which is typical for tetrahedral zeolite aluminium. During the whole process (except maybe immediately after Si increase in solution) alumino-silicate species in solution were not found. It confirms that the process is going within gel phase and if solution crystallization exists, it amounts to only 2—5% of total yield. 

Many catalysts have been used but the standard catalysts are generally mixtures of silica and alumina or natural or synthetic aluminium silicate zeolites. 

Figure C2.12.1. Origin of ion exchange capacity in zeolites. Since every oxygen atom contributes one negative charge to the tetrahedron incoriDorated in the framework, the silicon tetrahedron carries no net charge while the aluminium tetrahedron carries a net charge of-1 which is compensated by cations M.

Besides stmctural variety, chemical diversity has also increased. Pure silicon fonns of zeolite ZSM-5 and ZSM-11, designated silicalite-l [19] and silicahte-2 [20], have been synthesised. A number of other pure silicon analogues of zeolites, called porosils, are known [21]. Various chemical elements other than silicon or aluminium have been incoriDorated into zeolite lattice stmctures [22, 23]. Most important among those from an applications point of view are the incoriDoration of titanium, cobalt, and iron for oxidation catalysts, boron for acid strength variation, and gallium for dehydrogenation/aromatization reactions. In some cases it remains questionable, however, whether incoriDoration into the zeolite lattice stmcture has really occurred. 

Figure C2.12.8. Schematics of tlie dealumination of zeolites. Water adsorbed on a Br( msted site hydrolyses tire Al-O bond and fonns tire first silanol group. The remaining Al-0 bonds are successively hydrolysed leaving a silanol nest and extra-framework aluminium. Aluminium is cationic at low pH.

The product distribution in the reaction of benzene with dodecene was determined for a number of catalysts (Table 5.1-4). As can be seen, the reaction with the zeolite H-Beta gave predominantly the 2-phenyldodecane, whereas the reaction in the pure ionic liquid gave a mixture of isomers, with selectivity similar to that of aluminium chloride. The two supported ionic liquid reactions (H-Beta / IL and T 350 / IL) again gave product distributions similar to aluminium(III) chloride (T350 is a silica support made by Degussa). 

Two kinds of solution were prepared in advance. Solution A was a water solution containing an Si source, which was obtained by hydrolyzing metal alkoxide (tetraethylorthosilicate, TEOS) with a dilute tetrapropylammoniumhydroxide (TPA-OH)/water solution at room temperature. The molar ratio of Si to the template was 3. In peparation of ZSM-S zeolite nanoerystals, aluminium isopropoxide as an A1 source and sodium chloride were added into solution A. Solution B was an oi mic solution containing surfectant Nonionie surfactants, poljraxyethylene (15) cxslylether (C-15), polyoxyethylene (15) nonylphenylether (NP-15), and polyoxyethylene (15) oleylether (O-15), and ionic surfoctnnts, sodium bis(2-ethylhexyl) sulfosucdnate (AOT) and... 

In order to prepare ZSM-5 zeolite nanocrystals, an A1 source of aluminium isopropoxide was added into solution A, and hydrothermal synthesis of the solution A containing Si and A1 sources was carried out in an 0-15/cyclohexane solution at 120 degree C for 50 h. Figures 4 show ac-NHj-TPD spectra and a SEM photograph of the ZSM-5 zeolite nanocrystals. Nanocrystals with a diameter of approximately 150 nm were observed, and the NH3-TPD spectrum showed desorption of NHj above 600 K, indicating that the nanocrystals possessed strong acid sites. 

Zeolites are prepared by the linking of basic structural units around a template molecule. The structural units are typically based on oxides of silicon and aluminium, and the templates are usually individual small molecules. Under the right conditions, the silicon and aluminium oxide precursors will link up around the template to form a crystalline three-dimensional matrix containing the template molecules. The template... 

Zeolites based on silicon and aluminium are acidic catalysts and are extremely thermally stable. This makes them ideal for use in the petrochemical industry, where some of the largest scale and most high energy transformations are carried out. These transformations are carried out in... 

Catalysts can be metals, oxides, sulfides, carbides, nitrides, acids, salts, virtually any type of material. Solid catalysts also come in a multitude of forms and can be loose particles, or small particles on a support. The support can be a porous powder, such as aluminium oxide particles, or a large monolithic structure, such as the ceramics used in the exhaust systems of cars. Clays and zeolites can also be solid catalysts. 

The assignment given is such (see Table III) that the peak at -88ppm is associated with silicon linked (within the tetrahedral manifold) via oxygen to 2 silicons and 1 aluminium whilst the peak at -93ppm is associated with silica linked exclusively to other silicons (25-27) Using an analysis based on Loewenstein s rule and widely used in zeolite struct jral analyses (28) we have shown elsewhere (20) that for this particular synthetic heidellite the (Si/Al). ratio is 11.5 ... 

Many standard reactions that are widely applied in the production of fine chemicals employ. strong mineral or Lewis acids, such as sulphuric acid and aluminium chloride, often in stoichiometric quantities. This generates waste streams containing large amounts of spent acid, which cannot easily be recovered and recycled. Replacement of these soluble mineral and Lewis acids by recyclable. solid acids, such as zeolites, acid clays, and related materials, would represent a major breakthrough, especially if they functioned in truly catalytic quantities. Consequently, the application of solid acids in fine chemicals synthesis is currently the focus of much attention (Downing et al., 1997). 

Many inorganic oxides can be manufactured to provide granular, porous materials with high surface areas, which can readily adsorb organic liquids. Preliminary screening of a range of oxides, namely aluminium oxides, titanium dioxides, zinc oxide, hydrotalcites, zeolites and silicas, indicated that the latter two materials were able to retain the largest quantities of biocide. 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

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