In a chemical reaction there are five basic parameters one can control; temperature, pressure, concentration of species, contact time and pattern. While early attempts at improving reaction rates
and conversions were successful by relying on high temperature and high pressure processes these conditions are energy intensive, corrosive and result in undesirable side products.
Catalysis is a longstanding proposed application of supra molecular chemistry and the production of supra molecular systems capable of mimicking the catalytic ability of natural enzymes is one of the ultimate goals of self assembly research. The total amount of catalyst is small compared to the amount of reactants and products made during the life of the catalyst. The turnover frequency of the cycle is the quantity that defines the activity of a catalyst.
"A catalyst is a substance that transforms reactants into products through an uninterrupted and repeated cycle of elementary steps, until the last step in the cycle regenerates the catalyst in its original form."
Many types of materials can sere as catalysts. These include metals, compounds, organo metallic complexes and enzymes. Because not all portions of a catalyst participate in the transformation of reactants to products those portions that do are referred to as active sites. Types of catalysis are broadly classified into two categories.
- Homogeneous catalysis
"One of the most basic definitions of a catalyst is "a material that enhances the rate and selectivity of a chemical reaction and in the process is cyclically regenerated."
The definition of a catalyst is the same - a material that changes the rate of a reaction without itself being consumed. Catalysts have no effect on the position of equilibrium , and one cannot make a reaction proceed that is forbidden by the laws of thermodynamics
A catalyst acts to lower the activation energy barrier for reactions that have a net decrease in free energy. The alternate reaction paths provided by catalysts within the laws of thermodynamics and chemistry add value to feedstock materials in the refining and chemical processing industry.
The essential role of enzymes in almost all physiological processes stems from two key features of enzymatic catalysis.
- Enzymes greatly accelerate the rates of chemical reactions and
- Enzymes act on specific molecules referred to as substrates to produce specific reaction products.
Enzyme catalysis can be regarded as an alternative to chemical catalysis or to fermentation. Applying strict economic principles an enzyme must be able to catalyze a given conversion more cheaply but in practice there are areas where the three methods do not compete. Similarly there are many areas of chemical technology where no one can envisage enzyme applications as yet.
Enzyme catalysis provides excellent examples of how the structure and dynamics of proteins relate to their activity.
Catalysis is homogeneous when the catalyst is soluble in the reaction medium and heterogeneous when the catalyst is existing in a phase distinctly different from the phase of the reaction medium. In most instants of heterogeneous catalysis the catalyst is a solid that is brought into contact with gas or liquid reactants to bring about a transformation.
From the heterogeneous catalyst the expression "contact catalysis" frequently used to designate heterogeneous catalysis. Those transformation catalyzed by enzymes have a special classification independent of their homogeneous or heterogeneous nature.
A heterogeneous catalyst exist in a physical state different from that of the reactant. It is important to recognize the subjective nature of this definition when applied too nano catalysis. The traditional definition of a heterogeneous catalyst may be evolved in 1985 in which the nature of catalyst was classified based on the number of different catalytically active sites available on a single catalyst. So the catalyst with various types of active sites was termed as heterogeneous.
Covalent catalysis refers to any catalytic rate enhancement gained from transient formation of covalent reaction intermediates. That thousands of enzymes form the covalent intermediates confirms that significant advantages must be gained from their formation.
Enzymes organize covalent intermediate formation and turnover into discrete stages.
- First there is nucleophilic stage in which a catalytic functional group attacks the substrate to form a covalent bond.
- Second electrons are withdrawn by a now electrophilic catalyst and
- Third rupture of the covalent bond permits further reaction and regenerates the enzyme based nucleophile.
Decarboxylation of acetoacetate catalyzed by primary amines is an example of covalent catalysis.
Acid-base plays important role in the enzyme catalysis. Acid catalysis can be defined as a process of partial proton transfer from an acid which lowers the free energy of the transition state. A reaction is said to be base catalyzed when reaction rate is increased by partial proton abstraction by a base.
Many amino acid residues have pKs near the physiological pH and hence act as acid and base catalyst eg. Asp, Glu, His, Cys, Tyr and Lys. Thus protonated form of histidine acts as an acid.
RNaseA (a digestive enzyme secreted by pancreas) which hydrolyses RNA to its component nucleotides is an example of enzymatically mediated acid base catalysis.
Enzymes which require the presence of metal ions for the catalytic activity can be classified into two groups.
- Metalloenzymes: These enzymes contain tightly bound metal ions as co-factors. eg. Fe2+, Fe3+, Cu2+, Zn2+ or Mn2+.
- Metal-activated enzymes: These enzymes contain loosely bound metal ions. eg. Na+, K+, Mg2+ or Ca2+.
Metal ions may help catalytic process either by
- Binding to substrate for proper orientation or
- Change its own oxidation state thereby carrying out oxidation-reduction reaction or
- Electrostatically stabilize the charges.
Carbonic anhydrase contains an essential Zn2+ ion and it is considered to play important role in the enzymes catalytic mechanism.
Phase Transfer Catalysis allows substrates to come together without the need for a mutual solvent. Phase transfer agents can catalyze a variety of organic reactions including simple anion exchange and aromatic halogen exchange and the generation of carbenes.
"A phase transfer catalyst can be defined as a substance which increase the rate of reaction between substrates present in separate phases."
Simple and cheap inorganic reagents can be used, which when coupled with its versatility and environmental advantages makes the phase transfer catalysis a powerful tool for the organic chemist and a clean method for industry.