Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 21

A catalyst alters the rate of a reaction without changing the reaction thermodynamic parameters through another route. Normally, the one in the energy of the transition state is lower. There are two types of catalysts which can affect the reaction pathway: (i) homogeneous catalysis and (ii) heterogeneous catalysts.

Since the acidity of the catalyst support reflects the reaction pathway and plays a key role in the catalyst performance, it is possible to differentiate between homogeneous and heterogeneous catalyst. The homogeneous catalyst has lower ability of acidic sites rather than heterogeneous catalysts and it requires higher temperature in its reaction system, whereas the solid structure and the surface of a heterogeneous catalyst give its ability for higher strength and locations for acidic sites.

Many mineral acid catalysts that are active in homogeneous catalysis can be made suitable for heterogeneous catalysis by supporting the catalyst on an inorganic oxide. Strongly acidic heterogeneous catalysts are prepared by supporting Brønsted acids such as trifluoro-sulfonic acid, sulfuric acid, phosphoric acid, and Lewis acids (such as such as boron trifluoride, BF₃, and antimony pentafluoride, SbF₅) on high surface area oxides such as silica, SiO₂, alumina, Al₂O₃, and zirconia, ZrO₂). In particular, supported phosphoric acid on silica is still widely used, and BF₃–γ-Al₂O₃ and H₂SO₄–ZrO₂ possess acidic sites that enable them to perform reactions that other solids are not strong enough acids to catalyze. Supported acids are difficult to characterize and are highly dependent on the methods and materials used in their preparation but do offer a suitable alternative for reactions that require strong and even super acidity.

See also: Catalysts.