Catalysis by Solid Acids

AbstractThis article is an introduction to catalysis by solid acids, with an emphasis on principles and a selection of industrially important examples chosen to illustrate them. Acid catalysts often provide a proton to a reactant to initiate a catalytic cycle, but often the chemistry is more complex. Brønsted acid sites (usually surface OH groups) and Lewis acid sites (such as Al3+ions at a surface) are often important, and other groups, such as metals and redox groups on the catalyst surface, may also play a role, for example, in initiation of catalysis. Subtleties in the catalytic chemistry are associated with various acid–base interactions, such as adsorption of a basic product on the proton donors that are catalytic sites. The best‐understood solid acid catalysts are those with the simplest structures, crystalline solids such as zeolites. Methods are available for counting the proton‐donor sites in them. In one example of catalysis by an acidic zeolite, catalytic cracking of an alkane, there is a clear linear dependence of the catalytic activity on the number of proton‐donor groups in the zeolite. Theory is beginning to have an impact on the understanding of the simplest catalytic reactions catalyzed by solid acids. Mass transfer effects may be important, as in shape‐selective catalysis by some zeolites, and other phenomena related to the catalyst pore structure may be important, as in restricted transition state shape‐selective catalysis of alkane cracking in some zeolites.