Halogenation over solid catalysts

The use of solid catalysts in halogenation processes will avoid corrosion and disposal problems. Work-up procedures to isolate and recover the desired product will also be easier leading to simpler and cleaner process routes. In addition, the use of zeolites as the solid catalysts in nuclear aromatic halogenations might lead to enhanced yields of the para isomer, because of the shape selectivity of the zeolite. 

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Table 1. Continued 4.2 Halogenation over Solid Catalysts 137... 

The reactivity of a halogen compound in dehydrohalogenation over solid catalysts also depends on its steric arrangement. This was shown by studying the dehydrohalogenation of the rigid molecules l-bromo-1,2-diphenylethylene [188] and l-chloro-l,2-diphenylethylene [171] on catalysts of the type of metal salts and metal oxides the cis-compound was always more reactive than the trans-derivative. 

Halogenation. Fluorination, chlorination, and bromination of alkanes catalyzed by superacids have been reported.1,2 Reactions may be carried out in the liquid phase, or in the gas phase over solid superacids or supported noble metal catalysts. High selectivity and relatively mild reaction conditions are the main features of these transformations. 

The beneficial effects of solid surfaces on the rates of bromination reactions were observed as eai ly as 1923 [4-6], The polarity of the glass surface was found to enhance the rates of bromination of ethylene. Halogenation reactions over zeolite catalysts have been reviewed [1]. A summary of halogenations over non-zeolite (until mid-1999) and zeolite (1995-mid 1999) catalysts is given in Table 1. 

More progress has been reported on the halogenation of aromatic compounds over zeolite catalysts. Jang et al. [55] studied the vapor phase catalytic chlorination of chlorobenzene using solid-acid catalysts such as silica-alumina, alumina, zeolites and modified clay (bentonite) impregnated with FeClj Dichlorobenzene selectivity was higher over the zeolite catalysts. 

The reaction requires a dehydrating catalyst, which, for reactions in the liquid phase, is an acid substance this is usually a concentrated mineral acid, but may also be an organic sulfonic acid, acid salt, or halogen compound such as boron trifluoride, zinc chloride, or aluminum chloride. For etherification in the gaseous phase the alcohol vapor is passed over a solid catalyst such as A1203, Ti02, or dehydrated alum.656... 

The synthesis may be carried out as a batch process by bringing the olefin and halogen hydride together in a pressure vessel containing the. catalyst (1) suitably distributed over an absorbent material or (2) dissolved in an inert solvent. The synthesis may also be effected by passing the reactants as vapors over a solid catalyst, e.g., zinc chloride on charcoal at approximately 100°C. 

Halogenation—Hydrohalogenation. The most important intermediate is ethylene dichloride [107-06-2] (EDC) which is produced from ethylene either by direct chlorination or by oxychlorination. Direct chlorination is carried out in the liquid or vapor phase over catalysts of iron, aluminum, copper, or antimony chlorides, and at conditions of 60°C. Oxychlorination is carried out in a fixed or fluidized bed at 220°C with a suitable solid chloride... 

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