Catalysis and Catalysts

Catalysis is an acceleration or retardation of the rate of a chemical reaction, brought about by the addition of a substance (the catalyst ) to the reaction medium. The catalyst, usually present in small amounts, is not consumed in the reaction. Catalysis today is almost always associated with rate acceleration, and is very important in industry because rate acceleration usually means that a chemical compound can be made more cheaply and cleanly. It is hard to envision what modern society would be like without its use of myriad chemicals, polymers, and pharmaceuticals, most of which are prepared industrially by catalytic chemistry.

The rate of reaction refers to the amount of reactant consumed or product formed per unit of time at a given temperature and pressure. Generally speaking, the rate of reaction goes up as the temperature of the reaction is raised. This is related to the fact that most reactants have to "climb" over one or more energy barriers to reach the product stage. This can be likened to one's climbing a hill. The taller the hill, the more energy one expends in reaching its top. Reactants must become energetic enough to "reach" the top of an energy barrier if a reaction is to occur. Raising the reaction temperature ultimately imparts more energy to the reactants, creating a greater probability that more of them will be energetic enough to traverse the barrier, and this results in a faster rate.

What does a catalyst do? First of all, a catalyst does not change the energetic characteristics of the reactants and products and the barriers between them. It instead finds an alternate reaction pathway that bridges reactants and products, and one that has lower (and thus easier-to-traverse) energy barriers. An alternate pathway means a faster reaction rate. Although a catalyst can itself be considered a reactant, it is regenerated, unchanged, at a later stage in the catalytic process. The regenerated catalyst can then be used to catalyze another like reaction. Thus, in principle, only a very small amount of catalyst is needed to generate copious amounts of product. This is desirable, as many catalysts that are used industrially are very expensive.

Homogeneous catalysis refers to catalytic reactions in which the catalyst is in the same phase as the reactant. This is most often the liquid phase, although gas phase examples are known. Ozone in the stratosphere , for example, is converted into oxygen via the catalytic action of chlorine atoms formed as a result of the photochemical destruction of chlorofluorocarbon refrigerants.

The Fischer esterification , named after the eminent German chemist Emil Fischer, is the Brønsted acid–catalyzed reaction of an alcohol with a carboxylic acid to form an ester (see Table 1). Sulfuric acid is often the catalyst. Esters are widely used in the soap, perfume, and food industries.

The Friedel–Crafts acylation reaction, named after the French and American chemists who discovered it, used to prepare aryl ketones , is catalyzed by the Lewis acid aluminum chloride. Although aluminum chloride is a catalyst, it must be used (in Friedel–Crafts reactions) in stoichiometric amounts, as the portion of aluminum chloride that is catalytically inactive strongly binds to the product. Because aluminum chloride is corrosive and difficult to handle and must be destroyed when the reaction is complete, chemists continue to seek more environmentally friendly catalysts for this reaction.

Transition metal salts and complexes also serve as homogeneous catalysts. In the Monsanto process, rhodium salts plus iodide convert methanol and carbon monoxide into an industrially useful carboxylic acid, acetic acid. The rhodium metal serves as the primary reaction site; it binds the reactants and subsequently unbinds the products. The key reactions at the metal reaction site are called oxidative addition and reductive elimination.

Heterogeneous catalysis describes reactions in which the catalyst and the reactants are in different phases. In these reactions the catalyst is most often an insoluble solid and the reactants are in the gaseous or liquid/solution phase. A key feature of this type of catalysis is that the reactants must adsorb to the catalyst's surface. Large catalyst surfaces, then, ensure that the desired reaction occurs rapidly.

In the Haber process, named after its German discoverer, gaseous hydrogen and nitrogen are converted into ammonia over an iron catalyst under pressure and elevated temperature. Ammonia is a feedstock for the synthesis of fertilizers, explosives, and dyes. L-dopa, a drug used in the treatment of Parkinson's disease, is prepared via a hydrogenation reaction (the addition of hydrogen) over a solid transition metal catalyst. Margarine is, likewise, synthesized via a hydrogenation reaction.

Crude oil is converted into gasoline in cracking reactions that transform large molecules into smaller ones. The catalytic reactions occur in the interiors of porous inorganic solids called zeolites. The actual catalysts are protons that exist on the interior walls of the zeolites. Titanium dioxide, a photocatalyst, is an interesting solid because it functions as a catalyst only when it is exposed to light. This is environmentally advantageous, as the combination of titanium dioxide and light can be used to catalyze the removal of pollutants from water via their oxidization with hydrogen peroxide or oxygen.

Most useful compounds require more than one reaction for their syntheses, all of which may be catalytic. A good illustration of this is the three step synthesis of the analgesic agent ibuprofen. The first reaction of the synthesis of ibuprofen is a homogeneous, hydrogen fluoride–catalyzed, Friedel–Crafts acylation reaction; the second, a heterogeneous, palladium–catalyzed, hydrogenation reaction; and the third, a homogeneous, palladium–catalyzed, carbonylation reaction (addition of carbon monoxide).

Enzyme catalysis, a form of homogeneous catalysis, refers to the catalytic chemistry that controls the rates of virtually all reactions occurring in living systems. Enzymes, which serve as biological catalysts, are high molecular weight polymers made up from twenty different monomers called amino acids. The linked amino acids have a common backbone, but differing side chains of great structural and chemical variety, and an overall confirmation of great complexity.

An enzyme, in general, functions by first binding the reactant to a site on its surface called the active site. It is here that the catalytic chemistry occurs. The bound reactant then interacts and reacts with the side chains of the amino acids that make up the enzyme, and it is this interaction that brings about the chemical transformation. When the reaction is complete, the bound product diffuses away from the active site. Enzyme reactions take place in water, the biological solvent, at ambient temperatures. They often occur at rates a million or more times faster than those of uncatalyzed reactions. Hundreds of thousands of reactions can occur at the site of a single enzyme in one second. Many enzymes require the assistance of a molecule called a coenzyme if the catalytic reaction is to occur. Other enzymes require metal cations , such as Zn⁺², at their active sites.

When we eat, our mouths very often experience a sweet taste. This sweetness is due to the conversion of starch polymer molecules into the sugar D-glucose by the enzyme α -glucosidase, present in saliva. The polymer cellulose, structurally related to starch, is converted into D-glucose by the enzyme β -glucosidase (but only in bacteria). There are enzymes in the stomach and gut that aid digestion. The liver produces enzymes that are instrumental in removing toxic compounds from blood. Yeast organisms contain enzymes that convert sugar into ethanol and carbon dioxide—important in the making of bread and alcoholic beverages. Even the genetic material DNA (and its ancillary material RNA ) is made by and functions by participation in enzyme-catalyzed reactions. Interestingly, DNA and RNA are also instrumental in synthesizing these very same enzymes.

see also Kinetics; Reaction Rates.

Richard Pagni

Bibliography

American Chemical Society Staff, eds. (1993). Chemistry in the Community, 2nd edition. Dubuque, IA: Kendall/Hunt.

Ball, P. (1994). Designing the Molecular World. Princeton, NJ: Princeton University Press.