Catalytic efficiency and lifetime

Key points: A highly active catalyst, one that results in a fast reaction even in low concentrations, has a large turnover frequency. A catalyst must be able to survive a large number of catalytic cycles if it is to be of use. The turnover frequency, f, is often used to express the efficiency of a catalyst. For the conversion of A to B catalysed by Q and with a rate v,

then provided the rate of the uncatalysed reaction is negligible, the turnover frequency is

A highly active catalyst, one that results in a fast reaction even in low concentrations, has a high turnover frequency. In heterogeneous catalysis, the reaction rate is expressed in terms of the rate of change in the amount of product (in place of concentration) and the concentration of catalyst is replaced by the amount present. The determination of the number of active sites in a hetero- geneous catalyst is particularly challenging, and often the denominator [Q] in eqn 26.2 is replaced by the surface area of the catalyst. The turnover number is the number of cycles for which a catalyst survives. If it is to be economically viable, a catalyst must have a large turnover number. However, it may be destroyed by side reactions to the main catalytic cycle or by the presence of small amounts of impurities in the starting materials (the feedstock). For example, many alkene polymerization catalysts are destroyed by O₂, so in the synthesis of polyethene (polyethylene) and polypropene (polypropylene) the concentration of O₂ in the ethene or propene feedstock should be no more than a few parts per billion. Some catalysts can be regenerated quite readily. For example, the supported metal catalysts used in the reforming reactions that convert hydrocarbons to high-octane gasoline become covered with carbon because the catalytic reaction is accompanied by a small amount of dehydrogenation. These supported metal particles can be cleaned by interrupting the catalytic process periodically and burning off the accumulated carbon.

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مواضيع ذات صلة

Hydrogenation of alkenes

Alkene metathesis

Homogeneous and heterogeneous catalysts

Selectivity

Catalytic cycles

Energetics

Bionanocomposites

Natural and artificial nanomaterials: biomimetics

DNA and nanomaterials

Inorganic–organic nanocomposites

Metal–organic polyhedra and frameworks

Mesoporous materials