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.
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.  Many alkylamines are prepared by amination of alcohols, catalyzed by solid acids.
Solid acids can be used as electrolytes in fuel cells.