Catalysts are diverse, classified by state into liquid catalysts and solid catalysts; by phase of the reaction system into homogeneous catalysts and heterogeneous catalysts. Homogeneous catalysts include acid, base, soluble transition metal compound, and peroxide catalysts. Heterogeneous catalysts include solid acid catalysts, organic base catalysts, metal catalysts, metal oxide catalysts, complex catalysts, rare earth catalysts, molecular sieve catalysts, biocatalysts, and nanocatalysts. They are also classified by reaction type into catalysts for polymerization, condensation, esterification, acetalization, hydrogenation, dehydrogenation, oxidation, reduction, alkylation, and isomerization; and by their effectiveness into main catalysts and co-catalysts.
A reaction in which the catalyst and reactants exist in the same phase without a phase boundary is called homogeneous catalysis. Catalysts that can perform homogeneous catalysis are called homogeneous catalysts. Homogeneous catalysts include liquid acid and base catalysts, solid acid and base catalysts, and soluble transition metal compounds (salts and complexes). Homogeneous catalysts act independently at the molecular or ionic level, with uniform active centers, exhibiting high activity and high selectivity.
