What are homogeneous catalysis and heterogeneous catalysis

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Homogeneous catalysis

It is typical of homogeneous catalysis that the reactants and the catalyst are in solution and the reaction temperatures are relatively low (20-150). Often, homogeneous catalysts are extremely sensitive to oxidation and / or hydrolysis, so that work must be carried out strictly under anaerobic conditions. Most organometallic complex catalysts are transition metal compounds with a defined structure and stoichiometry. In well-studied processes, the reaction sequence can be (almost) fully understood on a molecular basis, which is the basis for the targeted development of tailor-made catalysts. The reaction center is typically a metal atom (ion) to which ligands are coordinated that are not directly involved in catalysis (spectator ligands, control ligands). Variations of these ligands from the point of view of specifically influencing the electronic and / or steric conditions at the reaction center (ligand tuning) are the basis for catalyst optimization in terms of activity, selectivity and stability.

There are borderline cases between homogeneous and heterogeneous catalysis. The best known is the classic Ziegler catalyst (TiCl4/ AlEt3), which polymerizes ethene in an aliphatic hydrocarbon at normal pressure and room temperature at high speed. Today we know that initially TiCl3 which is insoluble in the aliphatic. The catalysis takes place on the surface of TiCl, which is chemically modified by the cocatalyst3. The reaction conditions are typical for homogeneously catalyzed processes, the elementary steps can in principle be understood on a molecular basis, and there are analogous catalysts that work in homogeneous solution. This justifies assigning such processes to homogeneous catalysis.

Probably the decisive advantage of homogeneously catalyzed processes is the fundamental mechanistic understanding as the basis for a targeted catalyst development and optimization. On the other hand, a decisive disadvantage is the sometimes expensive and laborious catalyst separation. Developments such as the immobilization of homogeneous catalysts on solid supports and two-phase catalysis take account of efforts to combine the advantages of homogeneous catalysis with those of heterogeneous catalysis in this aspect.

Conclusion

Both homogeneous and heterogeneous catalysts are technically important and will remain technically important.