Solid acid

Solid acids are acids that are insoluble in the reaction medium. They are often used as heterogeneous catalysts. Many solid acids are zeolites.^[1] A variety of techniques are used to quantify the strength of solid acids.^[2]

Examples

Crystalline benzoic acid shown here is a solid and an acid, but, in the context of this article, it is not a "solid acid", which are polymeric materials and typically stronger acids.

Examples of inorganic solid acids include silico-aluminates (zeolites, alumina, silico-aluminophosphate), and sulfated zirconia. Many transition metal oxides are acidic, including titania, zirconia, and niobia.^[3] Such acids are used in cracking.^[1] Many solid Brønsted acids are also employed industrially, including polystyrene sulfonate, solid phosphoric acid, niobic acid, and heteropolyoxometallates.^[4]

Applications

Solid acids are used in catalysis in many industrial chemical processes, from large-scale catalytic cracking in petroleum refining to the synthesis of various fine chemicals.^[5]

One large scale application is alkylation, e.g., the combination of benzene and ethylene to give ethylbenzene. Another application is the rearrangement of cyclohexanone oxime to caprolactam.^[6][7][8] Many alkylamines are prepared by amination of alcohols, catalyzed by solid acids.

Zeolite, ZSM-5 is widely used as a solid acid catalyst.

Acylations are also catalyzed by solid acids.^[9][10]

Solid acids can be used as electrolytes in fuel cells.^[3]