Clay properties Lewis acid sites

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... 

Protons are released upon heating which in part balance the negative charge of the host clay layers. A number of review articles have recently appeared which summarize the synthesis and physical properties of metal oxide pillared days derived fix>m the intercalation of polyoxocations of aluminum, zirconium, chromium and many other metals [10-12]. The Lewis acid sites provided by coordinatively unsaturated metal ion sites on the pillar and the Bronsted addity formed upon thermolysis imparts novel chemical catalytic properties [13,14]. Since the pores between pillars often are larger than those foimd in conventional zeolites, there is considerable interest in the use of metal oxide pillared clays for the processing of large organic molecules, espedally petroleum [14-17]. 

For cracking reactions, combinations of zeolites, alumina, clay, and silica are used as the catalyst. These acidic materials, which contain both Br0nstead and Lewis acidic sites, initiate a complex set of carbonium- and carbenium ion-based reactions. Note that carbonium ions are protonated alkyl groups (e.g., C Hg ), while carbenium ions refer to alkyl cations (e.g., To enhance the acidic properties, rare... 

Br0nsted acid sites 286, 288, 289 catalytic behavior 284, 288, 289 deactivation 289 interlayers 279, 281, 284, 286 Lewis acid sites 288, 289 molecular sieve properties 281-283 pore size 271, 281, 282, 284 sorption of organic molecules 281-283 surface area 281, 282, 286, 288, 291, 292 thermal stability 278, 288, 292 zirconium polymer, in 286 pillared clays (see also PILCS ) 20, 271-290 pinacolato complexes 190, 191 platinum... 

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. 

The presence of Lewis and Bronsted acid sites gives rise to a variety of chemical reactions. Amines, or other Lewis bases, readily adsorb and the use of fatty amines to render the clay organophilic has been applied for many years to modify properties in a number of applications [19,20]. Because of these reactive sites, kaolin will enter into organic reactions and, of particular interest for plastics, will catalyse the polymerisation of certain monomers. Sometimes depolymerisation can occur and it can promote the dehydrochlorination reaction of PVC [21]. 

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