Basic catalysts aluminosilicates

Cation-exchanged aluminosilicates can also act as bases, particularly when the extra lattice cation is large. A low Si Al ratio produces a more basic catalyst. A Cs+ exchanged amorphous aluminosilicate can catalyze the liquid phase aldol condensation of benzaldehyde with cyclooctanone (Eqn. lO.lb). Both mono-and di-benzylidene products are formed over this amorphous basic catalyst. As discussed later, with the crystalline aluminosilicates, the zeolites, selectivity for raono-benzylidene product formation is increased. 

Furthermore, the acidity of the OH group on the solid surface was also suggested to affect the activity of immobilized IL catalysts. Sakai et al. prepared immobilized phosphonium IL catalysts for the CO2 addition to epoxyhexane using silica, aluminosilicate, and basic alumina as the support materials [40]. The conversion was in the order of silica > aluminosilicate > basic alumina. Because this order is the same with that of the acidity of the... 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

The state and capabilities of quantum chemical modeling of silicon dioxide framework structures (silica, aluminosilicates, zeolites) are discussed by Zhido-mirov and Kazansky. Here are basic theoretical approaches to a class of catalytic compositions which contribute the probably most massive quantities of catalysts used by man s (petroleum and chemical) industries. 

As described above, zeolites can also act as solid base catalysts when the Si Al ratio is low and the extra framework cation is a large one such as The most basic common exchanged zeolite is CsX. With this material the aldol condensation of cyclooctanone with benzaldehyde gave only the monobenzylidene compound (Eqn. 10.23) 5 while reactions using piperidine, m nesium oxide or amorphous cesium aluminosilicate gave both the mono- and di-benzylidene products(Eqn. 10.12). The smaller ketones, cyclohexanone and... 

Solid catalysts such as zeolite (aluminosilicate), heteropolyacids and their alkali or alkaline earth metal salts on a metal oxide support and basic resins have been used as catalysts to manufacture polysulfides. 

Catalysts. - The catalysts and sources of HCHO that appeared in patents are listed in Table 6. Solid bases such as hydroxides of alkali and alkaline earth metal supported on a support such as silica gel or aluminosilicate have mainly been claimed in patents to be effective as the catalysts. In addition, another type of compounds, which possess acidic property as well as basic property, are also claimed in patents, for example, PbO, Mn02, AI2O3, metal phosphates, metal borates, multicomponent oxides containing V, Nb, W, and Mo. 

Silica-aluminas (amorphous aluminosilicates) are widely used as catalyst supports due to their high acidity and surface area. The behaviour of silica-alumina surfaces is similar to that of zeolites, concerning the initial differential heats of ammonia and pyridine, but the total number of acidic sites varies with the preparation method and the Si/Al ratio. The basicity of silica-alumina surfaces, as determined by CO2 adsorption [94,95], appears to be weaker than that of pure alumina. 

The acid forms of aluminosilicate zeolites have found wider use as acid catalysts than any other materials. Their outstanding utility derives from their relatively high acid strength, their high hydrothermal stability, their ability to impart shape selectivity to product distributions and the reproducibility with which they can be synthesised and modified. Each of these advantages stems directly from their crystalline structure. The two basic types of acid site types in microporous solids are Bronsted, which are protons located at bridging sites (Si-O-Al in zeolites, M-O-P in aluminophosphates) and Lewis, usually incompletely coordinated metal cations (especially aluminium in zeolites) in... 

Mesoporous materials can also be used as base catalysts— for example, when the negative charge on a mesoporous aluminosilicate is compensated by metal ions such as sodium (Na) or cesium (Cs). Amines anchored on mesoporous silica can also be used as base catalysts. Successful utilization of basic sites requires the total absence of acid sites since the two functions tendto drive reactions through different pathways. There are, however, some cases where adjacent acid/base sites are desirable. 

In the case of solid catalysts, any atomic (ionic) group at the surface that can donate a proton is a Brdnsted acid while any place where one empty electron orbital exists is Lewis acid. For example, in the case of zeolites, Brdnsted acid site is a part of microporous aluminosilicate framework—a bridging [= Si (OH) A1 =] configuration which is able to donate a proton to an acceptor while Lewis acid site is either tri-coordinated A1 atom or charge-balancing cation Me " " which are able to accept the electron pair. Accordingly to the same theories, any place at the solid surface which can accept proton is a Brdnsted base while any place which can donate electron(s) is a Lewis basic site. For example, in the case of MeOjt (metallic oxides), the oxygen ions (0 ) behave as Brpnsted bases (because they are proton acceptors) while cations at the surface possess Lewis acidity (they are electron acceptors) [27, 28],... 

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