`color{purple}(✓✓)color{purple} " DEFINITION ALERT"`
`text(Definition) :` The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis.

`=>` Zeolites are good shape-selective catalysts because of their honeycomb-like structures.

`=>` They are microporous aluminosilicates with three dimensional network of silicates in which some silicon atoms are replaced by aluminium atoms giving `Al–O–Si` framework.

`=>` The reactions taking place in zeolites depend upon the size and shape of reactant and product molecules as well as upon the pores and cavities of the zeolites.

`=>` They are found in nature as well as synthesised for catalytic selectivity.

`=>` Zeolites to be used as catalysts are heated in vacuum so that the water of hydration is lost. Because of this zeolites become porous. The size of the pores varies between 260 pm and 740 pm. So only those molecules can enter these cavities whose size is small and can even leave easily.

`=>` So these zeolites act as selective adsorbents called molecular sieves.

`text(Uses of Zeolites :)`

● Zeolites are being very widely used as catalysts in petrochemical industries for cracking of hydrocarbons and isomerisation.

● An important zeolite catalyst used in the petroleum industry is ZSM-5.

● It converts alcohols directly into gasoline (petrol) by dehydrating them to give a mixture of hydrocarbons.

`=>` Enzymes are complex nitrogenous organic compounds which are produced by living plants and animals.

`=>` They are actually protein molecules of high molecular mass and form colloidal solutions in water.

`=>` They are very effective catalysts; catalyse numerous reactions, especially those connected with natural processes.

`=>` Numerous reactions that occur in the bodies of animals and plants to maintain the life process are catalysed by enzymes.

`=>` The enzymes are, thus, termed as biochemical catalysts and the phenomenon is known as biochemical catalysis.

`=>` Many enzymes have been obtained in pure crystalline state from living cells.

`=>` The first enzyme was synthesised in the laboratory in 1969.

`=>` The following are some of the examples of enzyme-catalysed reactions :

(i) Inversion of Cane Sugar : The invertase enzyme converts cane sugar into glucose and fructose.

`undersettext(Cane sugar) (C_(12) H_(22) O_(11) (aq)) + H_2O (l) oversettext(Invertase)→ undersettext(Glucose)(C_6H_(12) O_6 (aq))+undersettext(Fructose)(C_6H_(12)O_6 (aq))`

(ii) Conversion of Glucose into Ethyl Alcohol : The zymase enzyme converts glucose into ethyl alcohol and carbon dioxide.

`undersettext(Glucose)(C_6H_(12) O_6(aq)) oversettext(Zymase)→ undersettext(Ethyl alcohol)(2C_2H_5OH(aq)) +2CO_2 (g)`

(iii) Conversion of Starch into Maltose : The diastase enzyme converts starch into maltose.

`undersettext(Starch) (2 (C_6H_(10) O_5)_n(aq)) +n H_2O(l) oversettext(Distance) → undersettext(Maltose)(nC_(12)H_(22)O_(11)(aq))`

(iv) Conversion of Maltose into Glucose : The maltase enzyme converts maltose into glucose.

`undersettext(Maltose)(C_(12)H_(22)O_(11)(aq))+H_2O(l) oversettext(Maltose) → undersettext(Glucose)(2C_6H_(12)O_6(aq))`

(v) Decomposition of Urea into Ammonia and Carbon Dioxide : The enzyme urease catalyses this decomposition.

`NH_2CONH_2(aq) + H_2O (l) oversettext(Urease)→ 2NH_3(g) +CO_2 (g)`

(vi) In stomach, the pepsin enzyme converts proteins into peptides while in intestine, the pancreatic trypsin converts proteins into amino acids by hydrolysis.

(vii) Conversion of Milk into Curd : It is an enzymatic reaction brought about by lacto bacilli enzyme present in curd.

Enzyme catalysis is unique in its efficiency and high degree of specificity. The following characteristics are exhibited by enzyme catalysts :

(i) Most Highly Efficient : One molecule of an enzyme may transform one million molecules of the reactant per minute.

(ii) Highly Specific Nature : Each enzyme is specific for a given reaction, i.e., one catalyst cannot catalyse more than one reaction.

● For example, the enzyme urease catalyses the hydrolysis of urea only. It does not catalyse hydrolysis of any other amide.

(iii) Highly Active under Optimum Temperature : The rate of an enzyme reaction becomes maximum at a definite temperature, called the optimum temperature.

● On either side of the optimum temperature, the enzyme activity decreases.

● The optimum temperature range for enzymatic activity is `298-310K`. Human body temperature being `310 K` is suited for enzyme-catalysed reactions.

(iv) Highly Active under Optimum pH : The rate of an enzyme-catalysed reaction is maximum at a particular `pH` called optimum pH, which is between `pH` values `5-7`.

(v) Increasing Activity in Presence of Activators and Co-enzymes : The enzymatic activity is increased in the presence of certain substances, known as co-enzymes.

● It has been observed that when a small non-protein (vitamin) is present along with an enzyme, the catalytic activity is enhanced considerably.

● Activators are generally metal ions such as `Na^+`, `Mn^(2+)`, `Co^(2+)`, `Cu^(2+)`, etc. These metal ions, when weakly bonded to enzyme molecules, increase their catalytic activity.

● Amylase in presence of sodium chloride i.e., `Na^+` ions are catalytically very active.

(vi) Influence of Inhibitors and Poisons : Like ordinary catalysts, enzymes are also inhibited or poisoned by the presence of certain substances. The inhibitors or poisons interact with the active functional groups on the enzyme surface and often reduce or completely destroy the catalytic activity of the enzymes. The use of many drugs is related to their action as enzyme inhibitors in the body.

`=>` There are a number of cavities present on the surface of colloidal particles of enzymes.

`=>` These cavities are of characteristic shape and possess active groups such as `-NH_2, -COOH, -SH, -OH`, etc. These are actually the active centres on the surface of enzyme particles.

`=>` The molecules of the reactant (substrate), which have complementary shape, fit into these cavities just like a key fits into a lock.

`=>` On account of the presence of active groups, an activated complex is formed which then decomposes to yield the products.

`=>` Thus, the enzyme-catalysed reactions may be considered to proceed in two steps :

Step 1 : Binding of enzyme to substrate to form an activated complex.

`E + S → ES`

Step 2 : Decomposition of the activated complex to form product.

`ES → E+ P`

Some of the important technical catalytic processes are listed in Table to give an idea about the utility of catalysts in industries.