Supported catalysts

3815 11 00 to 3815 19 90 Supported catalysts These are catalysts of a widely used type which are deposited on a support, generally by impregnation, coprecipitation or mixing. They usually consist either of one or more active substances deposited on a support or of mixtures with a basis of active substances. The active substances are usually very finely divided metals, metallic oxides or other metallic compounds. The metals most commonly used are those belonging to group VIII of the periodic table (especially cobalt, nickel, palladium and platinum) and molybdenum, chromium, copper and zinc. The support generally consists of alumina, silica gel, fossil meal, whether or not activated, ceramic materials, etc. They are used in industrial processes for the production of organic and inorganic compounds and in petroleum refining (for example, in ammonia synthesis, hydrogenation of fats, hydrogenation of olefins). This category of catalysts also includes the following products: 1. certain preparations, based on compounds of transition metals, the function of which is to facilitate oxidation, and consequently elimination in the form of carbon dioxide in the course of combustion, of carbon residues (as in boilers and burners); 2. ‘post-combustion’ catalysts intended to be introduced into the exhaust systems of motor vehicles in order to reduce the polluting effect of the exhaust gases by converting the carbon monoxide into carbon dioxide and by converting other toxic products (for example, heterocyclic compounds) resulting from the combustion of petrol. 3815 90 10 and 3815 90 90 Other These subheadings cover mixtures based on compounds, the nature and proportions of which vary according to the chemical reaction to be catalysed. They are frequently used in the manufacture of plastics, often under the name of initiators, transfer agents, terminators or telomers, and reticulating (or cross-linking) agents. They include the following: 1. ‘radical’ catalysts These are preparations, based on organic substances, which decompose slowly under reaction conditions and produce fragments that, by colliding with the initial monomers, favour the establishment of a bond and the formation of new free radicals capable of repeating the process and propagating the chain. Amongst these are: (a) preparations based on organic peroxides of the type R–O–O–R′ (organic solutions of peroxides such as acetyl and dibenzoyl peroxides). During the reaction, RO· and R′O· radicals are formed and act as activators; (b) preparations, based on azo-compounds (such as azobisisobutyronitrile), which decompose in the course of the reactions, releasing nitrogen and forming free radicals; (c) redox preparations (for example, a mixture of potassium peroxide and dodecylmercaptan), in which the formation of activating radicals is due to a redox reaction; 2. ionic catalysts These are generally organic solutions of compounds which generate ions capable of attaching themselves to the double bond and reproducing active sites in the resulting product. They include: (a) ziegler-type catalysts for the production of polyolefins (for example, a mixture of titanium and triethylaluminium); (b) ziegler-natta-type catalysts (stereocatalysts, orientating catalysts), such as a mixture of titanium trichloride and trialkylaluminium, for the preparation of isotactic polypropylene and ethylene-olefin block copolymers; (c) catalysts for the preparation of polyurethanes (for example, a mixture of triethylenediamine and tin compounds); (d) catalysts for the preparation of amino-plastics (for example, phosphoric acid in an organic solvent); 3. catalysts for polycondensation reactions These are preparations based on various compounds (for example, a mixture of calcium acetate, antimony trioxide, titanium alcoholates, etc.).