by the Halcon process and the Sharpless dihydroxylation. Enzymes (including metalloenzymes) Enzymes are homogeneous catalysts that are essential for life but are also harnessed for industrial processes. A well-studied example is carbonic anhydrase, which catalyzes the release of CO2 into the lungs from the bloodstream. Enzymes possess properties of both homogeneous and heterogeneous catalysts. As such, they are usually regarded as a third, separate category of catalyst. Water is a common reagent in enzymatic catalysis. Esters and amides are slow to hydrolyze in neutral water, but the rates are sharply affected by metalloenzymes, which can be viewed as large coordination complexes. Acrylamide is prepared by the enzyme-catalyzed hydrolysis of acrylonitrile. US demand for acrylamide was as of 2007. Advantages and disadvantages Advantages Homogeneous catalysts are generally more selective than heterogeneous catalysts. For exothermic processes, homogeneous catalysts dump heat into the solvent. Homogeneous catalysts are easier to characterize precisely, so their reaction mechanisms are amenable to rational manipulation. Disadvantages The separation of homogeneous catalysts from products can be challenging. In some cases involving high activity catalysts, the catalyst is not removed from the product. In other cases, organic products are sufficiently volatile that they can be separated by distillation. Homogeneous catalyst have limited thermal stability compared to heterogeneous catalysts. Many organometallic complexes degrade <100 °C. Some pincer-based catalysts, however, operate near 200 °C. See also Concurrent tandem catalysis References Catalysis