Clay active sites Acid-activated

Vaccari (1983,1999) has given a state-of-the-art account of the preparation and catalytic properties of cationic and anionic clays. Some examples of industrial importance have also been reported. Clays exhibit many desirable features, such as low cost, wide range of preparation variables, ease of set-up and wOrk-up, high selectivity, and environmental friendliness. Cationic clays are widespread in nature, whereas anionic clays are rarely found in nature, but they can be synthesized cheaply. Cationic clays are prepared from the minerals but industrial anionic clays are generally synthetic. Smectite clays exhibit both Brpnsted and Lewis acid sites on the edges of the crystals. Hammet s acidity function values are as follows Na -montmorillonite (M), -3 to t- 1.5 NH4VM -3 to 1.5 H M -8.2 to -5.6 acid activated clay less than -8.2. Laporte also has a synthetic version of cationic clays, Laponite. The acid... 

Suzuki and Suga reported the use of clays as solid acids to support and activate metallocene catalysts for olefin polymerization. They were able to use much less alkylaluminmn cocatalyst relative to solution polymerization conditions. The clays were slurried with AlMeg in toluene, then treated with a solution containing zirconocene dichloride, II, and AIMeg. The metallocenium cation was presumed formed via abstraction of chloride and/or methyl ligands by acidic sites on the surface of the clay, and the low basicity of the clay smface was proposed to stabilize the coordinatively unsaturated cation. Propylene was copolymerized with 250 psi ethylene at 70°C. For acid-treated KIO montmorillonite, an activity of 3300 X 10 kg polymer/(g Zr h) was obtained. Catalysts based on vermiculite, kaolin, and synthetic hectorite all showed lower but still appreciable activities. In this brief report, the Al/Zr ratio was not specified, and the clay dispersion was not reported. 

Case 2 - The Hyde Park Landfill site, located in an industrial complex in the extreme northwest corner of Niagara, New York, was used from 1953 to 1975 as a disposal site for an estimated 80,000 tons of chemical waste, including chlorinated hydrocarbons. A compacted clay cover was installed in 1978 over the landfill and a tile leachate collection system was installed in 1979. Hazardous compounds such as ortho-, meta- and para-chlorobenzoic acid toluene ortho- and meta-chlorotoluene 3,4-dichlorotoluene and 2,6-dichlorotoluene were detected in the leachate (Irvine et al., 1984). Since 1979, the existing leachate treatment system has used activated carbon as the technology for removing organic carbon. Although... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

The British Non-Ferrous Metals Research Association carried out two series of tests, the results of which have been given by Gilbert and Gilbert and Porter these are summarised in Table 4.12. In the first series tough pitch copper tubes were exposed at seven sites for periods of up to 10 years. The two most corrosive soils were a wet acid peat (pH 4-2) and a moist acid clay (pH 4-6). In these two soils there was no evidence that the rate of corrosion was decreasing with duration of exposure. In the second series phosphorus-deoxidised copper tube and sheet was exposed at five sites for five years. Severe corrosion occurred only in cinders (pH 7 1). In these tests sulphides were found in the corrosion products on some specimens and the presence of sulphate-reducing bacteria at some sites was proved. It is not clear, however, to what extent the activity of these bacteria is a factor accelerating corrosion of copper. 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

Dimerization of unsaturated fatty acids, to. so-called dimer acids, is widely practised in industry, where acid-treated clays are invariably used as a catalyst. In the case of oleic acid the major products are dimers, trimers, and isosteric acid. Koster et al. (1998) have investigated the relative importance of the various acid sites as well as structural and textural parameters of montmorrilonite. The interlamellar space dominates the oleic acid dimerization and the active site is the tetrahedrol substitution site. 

Circulation flow system, measurement of reaction rate, 28 175-178 Clausius-Clapeyron equation, 38 171 Clay see also specific types color tests, 27 101 compensation behavior, 26 304-307 minerals, ship-in-bottle synthesis, metal clusters, 38 368-379 organic syntheses on, 38 264-279 active sites on montmorillonite for aldol reaction, 38 268-269 aldol condensation of enolsilanes with aldehydes and acetals, 38 265-273 Al-Mont acid strength, 38 270-271, 273 comparison of catalysis between Al-Mont and trifluorometfaanesulfonic acid, 38 269-270... 

The activity advantage of zeolite catalysts over amorphous silica-alumina has well been documented, Weisz and his associates [1] reported that faujasite Y zeolite showed 10 to 10 times greater activity for the cracking of n-hexane than silica-alumina. Wang and Lunsford et al. [2] also noted that acidic Y zeolites were active for the disproportionation of toluene while silica-alumina was inactive. The activity difference between zeolite and silica-alumina has been attributed to their acidic properties. It is, however, difficult to explain the superactivity of zeolite relative to silica-alumina on the basis of acidity, since the number of acid sites of Y-type zeolite is only about 10 times larger than that of silica-alumina. To account for it, Wang et al. [2] proposed that the microporous structure of zeolite enhanced the concentration of reactant molecules at the acid sites. The purpose of the present work is to show that such a microporous effect is valid for pillared clay catalysts. 

Mesoporous solids including silicas and acid-treated clays can be functionalised at their surfaces so as to provide high local concentrations of active sites. These sites can be introduced by post-modification or via sol-gel preparations. In this way a range of novel materials with useful catalytic and other properties can be prepared. One of the most valuable applications for these materials is as replacements for environmentally hazardous reagents including corrosive mineral and Lewis acids, caustic bases and toxic metallic compounds. 

Infrared spectra of pyridine adsorbed on kaolinite indicated that the dry clay (110°C) contained both Brtfnsted and Lewis acid sites (235). At 1% water content only protonic acid sites were observed. It was not possible to assign the polymerization activity to either type of acid site, since both were present on samples which were catalytically active. 

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