Catalysts speed up reactions without being consumed. Homogeneous catalysts are in the same phase as reactants, while heterogeneous catalysts are exposed to a solid substrate. Organometallic compounds are used as homogeneous catalysts in complex chemistry, such as polymerization and pharmaceutical manufacturing. They are also being studied for artificial photosynthesis.
In chemistry, a catalyst is a substance that is added to speed up the rate of a reaction without being consumed in the process. The catalyst is often added in small amounts compared to the reactants and may be reused sequentially in future batches. A homogeneous catalyst is one that is in the same phase as the reactants—solid, liquid, or gas—during the reaction. Often the components are of different phases in their pure state but are dissolved in a common solvent. Under this definition, a gas that reacts with a liquid in the presence of a normally solid catalyst is also homogeneous if the three are all dissolved at that time.
Most industrial catalysis employs heterogeneous catalysts. In heterogeneous catalysis, there are two or more steps in the reaction and it often involves a liquid or gaseous component exposed to a solid catalyst component attached to a supporting substrate for convenience and to prevent loss. This could be because the catalyst is expensive, including a precious metal component. To increase efficiency, the surface area can be maximized with the catalyst being divided very definitively. An example is the catalytic converter found in most cars.
The use of a homogeneous catalyst in complex chemistry is of particular interest, in part due to the new uses of organometallic complexes. Early applications of organomagnesium and organolithium compounds were largely as reaction ingredients, rather than catalysts. Such compounds were unstable; their use required their dissolution in dangerous solvents such as ether or tetrahydrofuran (THF). Combining these with other liquid reactants placed those reactions, by definition, in the homogeneous category.
Today, many more organometallic compounds are known. Some of them can be included in the category of homogeneous catalysts. They are often more stable and easier to handle. This type of compound offers a wider scope of use and is often used as a homogeneous catalyst, rather than a reagent.
Some of the new reagents are useful in polymerization reactions. Others are suitable for pharmaceutical manufacturing due to their ability to impart chirality. This refers to the ability to control structural design so closely that polarized light rotates only one way.
A noteworthy application is the attempt to imitate the plant world by means of artificial photosynthesis. Another use of the term should not be confused with this: the splitting of water into hydrogen for the production of fuel. Rather, artificial photosynthesis in this case refers to the conversion of carbon dioxide and water into carbohydrates and oxygen. For some years now, organometallic catalysts have been studied with artificial photosynthesis in mind.
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