Surface Chemistry : Catalysis

“Catalyst is a substance which speeds up and speeds down a chemical reaction without itself being used up.”

‘or’

“A catalyst is a foreign substance the addition of which into the reaction mixture accelerates or retards the reaction.”

Berzelius (1836) introduced the term catalysis and catalyst.
Ostwald (1895) redefined a catalyst as, “A substance which changes the reaction rate without affecting the overall energetics of the reaction is termed as a catalyst and the phenomenon is known as catalysis.”

Catalytic reactions can be broadly divided into the following types,

(1) Homogeneous catalysis : When the reactants and the catalyst are in the same phase (i.e. solid, liquid or gas). The catalysis is said to be homogeneous. The following are some of the examples of homogeneous catalysis.

(i) Oxidation of sulphur dioxide into sulphur trioxide with oxygen in the presence of oxides of nitrogen as the catalyst in the lead chamber process.

2SO₂(g) + O₂(g) $\underrightarrow { \quad NO(g)\quad }$ 2SO₃(g)

The reactants, products and catalyst all are in gaseous state i.e. same phase.

(ii) Hydrolysis of methyl acetate is catalysed by H⁺ ions furnished by hydrochloric acid .

CH₃COOCH₃(l) + H₂O(l) $\underrightarrow { \quad HCl(l)\quad }$ CH₃COOH₃COOH(l) + CH₃OH(l)

(iii) Hydrolysis of sugar is catalysed by H⁺ ions furnished by sulphuric acid.

C₁₂H₂₂O₁₁(l) + H₂O(l) $\underrightarrow { \quad H_{ 2 }SO_{ 4 }(l)\quad }$ C₆H₁₂O₆(l) + C₆H₁₂O₆(l)

(Sucrose solution) (glucose solution) (Fructose solution)

(2) Heterogeneous catalysis : The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis. Some of the examples of heterogeneous catalysis are given below.

(i) Oxidation of sulphur dioxide into sulphur trioxide in the presence of platinum metal or vanadium pentaoxide as catalyst in the contact process for the manufacture of sulphuric acid. The reactants are in gaseous state while the catalyst is in solid state.

$2SO_{ 2 }(g)+{ O }_{ 2 }(g)\underrightarrow { \quad Pt(s)\quad } 2SO_{ 3 }(g)$

(ii) Combination between nitrogen and hydrogen to form ammonia in the presence of finely divided iron in Haber’s process.

$N_{ 2 }(g)+3H_{ 2 }(g)\underrightarrow { \quad Fe(s)\quad } 2NH_{ 3 }(g)$

(iii) Oxidation of ammonia into nitric oxide in the presence of platinum gauze as a catalyst in Ostwald’s process.

4NH₃(g) + 5O₂(g) $\underrightarrow { \quad Pt(s)\quad }$ 4NO(g) + 6H₂O(g)

(iv) Hydrogenation of vegetable oils in the presence of finely divided nickel as catalyst.

Vegetable oils(l) + H₂(g) $\underrightarrow { \quad Ni(s)\quad }$ Vegetable Ghee(g)

(3) Positive catalysis : When the rate of the reaction is accelerated by the foreign substance, it is said to be a positive catalyst and phenomenon as positive catalysis. Some examples of positive catalysis are given below.

(i) Decomposition of H₂O₂ in presence of colloidal platinum.

2H₂O₂(l) $\underrightarrow { \quad Pt\quad }$ 2H₂O(l) + O₂(g)

(ii) Decomposition of KClO₃ in presence of manganese dioxide.

2KClO₃(s) $\xrightarrow [ \quad 270^{ o }C ]{ MnO_{ 2 }(s) }$ 2KCl(s) + 3O₂(g)

(iii) Oxidation of ammonia in presence of platinum gauze.

4NH₃(g) + 5O₂(g) $\xrightarrow [ \quad 300^{ o }C ]{ Pt(s) }$ 4NO(g) + 6H₂O(g)

(iv) Oxidation of sulphur dioxide in presence of nitric oxide.

2SO₂(g) + O₂(g) $\underrightarrow { \quad NO(g)\quad }$ 2SO₃(g)

(v) Oxidation of sulphur dioxide in presence of platinised asbestos or vanadium pentaoxide.

2SO₂(g) + O₂(g) $\xrightarrow [ Pt(s)\quad ]{ { V }_{ 2 }{ O }_{ 5 }(s) }$ 2SO₃(g)

(vi) Oxidation of hydrochloric acid into chlorine by Deacon’s process in presence of .

4HCl(g) + O₂(g) $\xrightarrow [ 450^{ o }C\quad ]{ CuCl_{ 2 }(s) }$ 2Cl₂(g) + 2H₂O(g)

(vii) Formation of methane in presence of nickel.

CO(g) + 3H₂(g) $\underrightarrow { \quad Ni(s)\quad }$ CH₄(g) + 4H₂O(g)

(viii) Synthesis of ammonia by Haber’s process in presence of a mixture of iron and molybdenum.

N₂(g) + 3H₂(g) $\xrightarrow [ 450-500^{ o }C\quad ]{ Fe(s)\& Mo(s) }$ 2NH₃(g)

(ix) Hydrogenation of vegetable oil in presence of nickel.

Vegetable oil (l) + H₂(g) $\underrightarrow { \quad Ni(s)\quad }$ Ghee(s)

(x) Manufacture of methyl alcohol in presence of ZnO / Cr₂O₃.

CO(g) + 2H₂(g) $\xrightarrow [ Cr_{ 2 }O_{ 3 }(s) ]{ ZnO(g)250^{ o }C\quad }$ CH₃OH(g)

Note : Positive catalyst increases the rate by lowering activation energy of reaction. Catalyst changes the mechanism by changing the intermediate i.e. intermediate of low energy is formed. It increases the rate by converting some inactive molecule into active one.

(4) Negative catalysis : There are certain, substance which, when added to the reaction mixture, retard the reaction rate instead of increasing it. These are called negative catalyst or inhibitors and the phenomenon is known as negative catalysis. Some examples are as follows.

(i) The oxidation of sodium sulphite by air is retarded by alcohal. Alcohol acts as a negative catalyst

2Na₂SO₃(s) O₂(g) $\underrightarrow { \quad Alcohol(l)\quad }$ 2Na₂SO₄(s)

(ii) The oxidation of chloroform by air is retarded it some alcohol is added to it.

2CHCl₃(l) + O₂(g) $\underrightarrow { \quad Alcohol(l)\quad }$ 2COCl₂(g) + 2HCl(g)

(iii) The oxidation of benzaldehyde is retarded if some diphenyl amine is added. It acts as a negative catalyst.

2C₆H₅CHO(l) + O₂(g) $\xrightarrow [ Alcohol(l) ]{ Diphenyl\quad }$ 2C₆H₅COOH(l)

(iv) Addition of small amount of acetanilide or glycerine slow down the decomposition of hydrogen peroxide.

(v) Tetra ethyl lead (TEL) is added to petrol to retard the ignition of petrol vapours on compression in an internal combustion engine and thus minimise the knocking effect.

(5) Auto-catalysis : In certain reactions, one of the product acts as a catalyst. In the initial stages the reaction is slow but as soon as the products come into existences the reaction rate increases. This type of phenomenon is known as auto-catalysis. Some examples are as follows,

(i) The rate of oxidation of oxalic acid by acidified potassium permanganate increases as the reaction progresses. This acceleration is due to the presence of Mn²⁺ ions which are formed during reaction. Thus Mn²⁺ ions act as auto-catalyst.

5H₂C₂O₄ + 2KMnO₄ + 3H₂SO₄→ 2MnSO₄+ K₂SO₄ +10CO₂ + 8H₂O

(ii) When nitric acid is poured on copper, the reaction is very slow in the beginning, gradually the reaction becomes faster due to the formation of nitrous acid during the reaction which acts as an auto-catalyst.

(iii) In hydrolysis of ethyl acetate, acetic acid and ethyl alcohol are formed. The reaction is initially very slow but gradually its rate increases. This is due to formation of acetic acid which acts as an auto-catalyst in this reaction.

(6) Induced catalysis : When one reaction influences the rate of other reaction, which does not occur under ordinary conditions, the phenomenon is known as induced catalysis. Some examples are as follows,

(i) Sodium arsenite solution is not oxidised by air. If, however, air is passed through a mixture of the solution of sodium arsenite and sodium sulphite, both of them undergo simultaneous oxidation. The oxidation of sodium sulphite, thus, induces the oxidation of sodium arsenite.

(ii) The reduction of mercuric chloride (HgCl₂) with oxalic acid is very slow, but potassium permanganate is reduced readily with oxalic acid. If, however, oxalic acid is added to a mixture of potassium permanganate and HgCl₂ both are reduced simultaneously. The reduction of potassium permanganate, thus, induces the reduction of mercuric chloride.

(7) Acid-base catalysis : According to the Arrhenius and Ostwald H⁺ or H^– ion act as a catalyst.

(i) For example, Hydrolysis of an ester,

CH₃COOC₂H₅(l) + H₂O(l) $\xrightarrow [ OH^{ - } ]{ H^{ + } }$ CH₃COOH(l) + C₂H₅OH(l)

(ii) Inversion of cane sugar,

$\underset { Sugar }{ C_{ 12 }H_{ 22 }O_{ 11 } } (l)+H_{ 2 }O\underrightarrow { \quad H^{ + }\quad } \underset { Fructose }{ C_{ 6 }H_{ 12 }O_{ 6 } } +\underset { Glucose }{ C_{ 6 }H_{ 12 }O_{ 6 }(l) }$

(iii) Conversion of acetone into diacetone alcohol,

CH₃COCH₃(l) + CH₃COCH₃(l) $\underrightarrow { \quad OH^{ - }\quad }$ CH₃COCH₂.C(CH₃)₂OH(l)

(iv) Decomposition of nitramide, NH₂NO₂(l) $\underrightarrow { \quad OH^{ - }\quad }$ N₂O(g) + H₂O(l)

Note :

All Bronsted acids and bases act as acid base catalysts.
Catalytic converter for an automobile : The catalytic converter in the exhaust systems of cars, which converts polluting exhaust gases into non-toxic gases contains a heterogeneous catalyst. Mixtures of transition metals and their oxides embedded in inert supports act as catalyst. When the gases are passed through the catalyst bed, carbon monoxide (CO) and unburnt petrol are oxidised to carbon dioxide and water while nitric oxide (NO) is reduced to N₂gas,

$\underset { (Unburnt\quad petrol) }{ 2CO+O_{ 2 } } \underrightarrow { \quad catalyst\quad } 2CO_{ 2 };\quad Hydrocarbons\xrightarrow [ O_{ 2 } ]{ Catalyst } CO_{ 2 }+H_{ 2 }O;\quad 2NO\underrightarrow { \quad Catalyst\quad } N_{ 2 }+O_{ 2 }$

The following are the characteristics which are common to must of catalytic reactions.

(1) A catalyst remains unchanged in mass and chemical composition at the end of the reaction.

(2) A small quantity of the catalyst is generally sufficient to catalyses almost unlimited reactions

(i) For example, in the decomposition of hydrogen peroxide, one gram of colloidal platinum can catalyses 10⁸ litres of hydrogen peroxide.

(ii) In the some reaction the rate of the reaction is proportional to the concentration of the catalyst. For example the acid and alkaline hydrolysis of an ester, the rate of reaction is proportional to the concentration of H⁺or OH^–ions.

RCOOR(l) + H₂O(l) $\xrightarrow [ OH^{ - } ]{ H^{ + } }$ RCOOH(l) + ROH(l)

(iii) In Friedel – craft’s reaction, anhydrous aluminium chloride is required in relatively large amount to the extent of 30% of the mass of benzene,

(iv) C₆H₆ + C₂H₅Cl $\underrightarrow { \quad AlCl_{ 3 }\quad }$ C₆H₅C₂H₅ + HCl

(v) In certain heterogeneous reactions, the rate of reaction increases with the increase of area of the catalytic surface.

(3) The catalyst can not initiate the reaction: The function of the catalyst is to alter the speed of the reaction rather than to start it.

(4) The catalyst is generally specific in nature: A substance, which acts as a catalyst for a particular reaction, fails to catalyses the other reaction, different catalysts for the same reactant may for different products.

Examples :

(5) The catalyst can not change the position of equilibrium : The catalyst catalyse both forward and backward reactions to the same extent in a reversible reaction and thus have no effect on the equilibrium constant.

(6) Catalytic promoters : Substances which themselves are not catalysts, but when mixed in small quantities with the catalysts increase their efficiency are called as promoters or activators.

(i) For example, in Haber’s process for the synthesis of ammonia, traces of molybdenum increases the activity of finely divided iron which acts as a catalyst.

(ii) In the manufacture of methyl alcohol from water gas (CO + H₂), chromic oxide (Cr₂O₃) is used as a promoter with the catalyst zinc oxide (ZnO).

(iii) In the hydrogenation of oils, the activity of the catalyst nickel increases on adding small amount of copper.

(7) Catalytic poisons : Substances which destroy the activity of the catalyst by their presence are known as catalytic poisons.

(i) For example, the presence of traces of arsenious oxide (As₂O₃) in the reacting gases reduces the activity of platinized asbestos which is used as catalyst in contact process for the manufacture of sulphuric acid.

(ii) The activity of iron catalyst is destroyed by the presence of H₂S or CO in the synthesis of ammonia by Haber’s process.

(iii) The platinum catalyst used in the oxidation of hydrogen is poisoned by CO.

Note : The poisoning of the catalyst is probably due to the preferential adsorption of poison on the surface of the catalyst, thus reducing the space available for the adsorption of reacting molecules.

(8) Change of temperature alters the rate of catalytic reaction as it does for the same reaction in absence of catalyst : By increasing the temperature, there is an increase in the catalytic power of a catalyst but after a certain temperature its power begins to decrease. A catalyst has thus, a particular temperature at which its catalytic activity is maximum. This temperature is termed as optimum temperature.

(9) A positive catalyst lowers the activation energy

(i) According to the collision theory, a reaction occurs on account of effective collisions between the reacting molecules.

(ii) For effective collision, it is necessary that the molecules must possess a minimum amount of energy known as activation energy (E_a).

(iii) After the collision molecules form an activated complex which dissociate to yield the product molecules.

(iv) The catalyst provides a new pathway involving lower amount of activation energy. Thus,

large number of effective collisions occur in the presence of a catalyst in comparison to effective collisions at the same temperature in absence of a catalyst. Hence the presence of a catalyst makes the reaction to go faster.

(v) Figure shows that activation energy E_a, in absence of a catalyst is higher than the activation energy E_a, in presence of a catalyst.

(vi) E_R and E_P represent the average energies of reactants and products.

The difference gives the value of ∆G, i.e., ∆G = E_R – E_P

There are two theories of catalysis which is described as follows.

(1) Intermediate compound theory

(i) This theory was proposed by Clement and Desormes in 1806. According to this theory, the desired reaction is brought about by a path involving the formation of an unstable intermediate compound, followed by its decomposition into the desired end products with the regeneration of the catalyst.

(ii) The intermediate compound may be formed in either of two ways

(a) When the intermediate compound is reactive and reacts with the other reactants.

AB + X → $\underset { Intermediate }{ BX }$ + A

BX + C → CB+ X …….(i)

(b) When the intermediate is unstable and decomposes to give the final product.

A + B + X → $\underset { Intermediate }{ ABX }$ → AB + X …….(ii)

Where, A, B and C are the reactant molecules and X is the molecule of the catalyst. The first type of reaction sums up to, AB + C → CB + A

While the second to, A + B → AB in many cases, the intermediate compounds postulated to be formed are known compounds and often their presence is detected.

(2) Adsorption theory

(i) This theory is applicable to reactions between gases in the presence of a solid catalyst. Some typical examples are as follows.

(ii) The contact process for the oxidation of SO₂ to SO₃ with atmospheric oxygen in the presence of platinum as the catalyst.

(iii) The Haber’s process for the synthesis of ammonia with iron as the catalyst.

(iv) Adsorption results in the loosening of the chemical bonds in the reactant molecules, so that their rupture becomes easier. This is confirmed by the observed lower activation energies for heterogeneous catalytic reactions in the presence of the catalysts as compared to that for the same reaction in the absence of the catalyst.

(v) The metals copper and nickel are found particularly suitable for reactions involving hydrogen gas. These metals are known to strongly chemisorb hydrogen gas. Typical example includes the dehydrogenation of ethandol vapours when passed over heated metal at 350°C.

CH₃CH₂OH $\xrightarrow [ 350^{ o }C ]{ Ni }$ CH₃CHO + H₂

(vi) Aluminium oxide in some physical forms is a good adsorbent for water vapour. It is also a useful catalyst for reactions involving dehydration processes (i.e. processes involving the removal of water from molecules). For example, formation of ethene from ethyl alcohol,

CH₃CH₂OH $\xrightarrow [ \quad 350^{ o }C\quad ]{ \quad Al_{ 2 }O_{ 3 }\quad }$ CH₂ = CH₂ + H_2O

(1) Heterogeneous catalytic reactions generally proceed via adsorption of reactants on the surface of the catalyst.

(2) Mechanism of such surface reactions may be explained in terms of diffusion theory of catalysis. This theory postulates the following sequence for gaseous reactions on a solid surface.

Step: (i) Diffusion of the reactants to the surface.

Step: (ii) Adsorption of the reactant molecules onto the surface.

Step: (iii) Actual chemical reaction on the surface.

Step: (iv) Desorption of the products from the surface.

Step: (v) Diffusion of the products away from the surface.

In generally, Step (iii) determines the rate of reaction. However step (ii) and (iv) may be rate determining.

(3) According to Langmuir-Hinshelwood, the rate of a catalytic reaction is proportional to the concentration of the reacting species on the surface. For this, the reacting species must get adsorbed on the neighboring sites.

(4) Another way in which two reacting molecules may react on a solid surface is that one of them gets adsorbed and then the adsorbed molecules reacts with a molecule in the gas phase. This mechanism is called Rideal mechanism.

(1) Enzymes are complex nitrogenous substances secreted by low forms of vegetable animal organism.

(2) Enzymes are actually protein molecules of higher molecular mass.

(3) Enzymes form colloidal solutions in water and are very effective catalysts. They catalyse numerous reactions, especially those connected with natural processes.

(4) Numerous reactions occur in the bodies of animals and plants to maintain the life process. These reactions are catalysed by enzymes. The enzymes are thus, termed as bio-chemical catalysts and the phenomenon is known as bio-chemical catalysis.

(5) Nitrogenase an enzyme present in bacteria on the root nodules of leguminous plants such as peas and beans, catalyses the conversion of atmospheric N₂ to NH₃.

(6) In the human body, the enzyme carbonic anhydrase catalyses the reaction of CO₂ with H₂O,

CO₂(aq) + H₂O(l) → H⁺(aq.) + HCO₃^– (aq.)

The forward reaction occurs when the blood takes up CO₂ in the tissues, and the reverse reaction occurs when the blood releases CO₂ in lungs.

Catalysts in industry

Process	Catalyst
Haber’s process for the manufacture ammonia. N₂(g) + 3H₂(g) → 2NH₃(g)	Finely divided iron. Molybdenum as promoter and 200 atmospheric pressure and 450-500^oC temperature.
Ostwald’s process for the manufacture of nitric acid. 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g) 2NO(g) + O₂(g) → 2NO₂(g) 4NO₂(g) ₊2H₂O(l) + O₂(g) → 4HNO₃(l)	Platinised asbestos and temperature 300^o C.
Lead chamber process for the manufacture of sulphuric acid. 2SO₂(g) + O₂(g) → 2SO₃(g) SO₃(g) + H₂O(l) → H₂SO₄(l)	Nitric oxide
Contact process for the manufacture of sulphuric acid. 2SO₂(g) + O₂(g) → 2SO₃(g) SO₃(g) + H₂SO₄(l) → H₂S₂O₇(l) H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l)	Platinised asbestos or vanadium pentoxide (V₂O₅). Temperature 400-450⁰ C.
Deacon’s process for the manufacture of chlorine. 4HCl(g) + O₂(g) → 2H₂O(l) 2Cl₂(g)	Cupric chloride (CuCl₂). Temperature 500^o C.
Bosch’s process for the manufacture of hydrogen. $\underbrace { CO+H_{ 2 } }_{ water\quad gas } +H_{ 2 }O(g)\rightarrow CO_{ 2 }(g)+H_{ 2 }O(g)$	Ferric oxide (Fe₂O₃)+ chromic oxide as a promoter. Temperature 400-600^o C.
Synthesis of methanol. CO(g) + 2H₂(g) → CH₃OH(l)	Zinc oxide +chromic oxide as promoter. Pressure 200 atmopheres and temperature 250^o C.
Hydrogenation of vegetable oils. Oil(l) + H₂(g) → Vanaspati ghee (s)	Nickel (finely divide). Temperature 150-200^oC. High pressure.
Manufacture of ethyl alcohol by fermentation of molasses (sugar solution) $C_{ 12 }H_{ 22 }O_{ 11 }(l)+H_{ 2 }O(l)\underrightarrow { \quad Invertase\quad } \underset { glucose }{ C_{ 6 }H_{ 12 }O_{ 6 }(l) } +\underset { fructose }{ C_{ 6 }H_{ 12 }O_{ 6 }(l) }$ $C_{ 6 }H_{ 12 }O_{ 6 }(l)\underrightarrow { \quad Zymase\quad } 2C_{ 2 }H_{ 5 }OH(l)+2CO_{ 2 }(l)$	Invertase enzyme and zymase (yeast) enzyme. Temperature 25-30^o C. Conversion occurs in 2 or 3 days.
Manufacture of ethyl alcohol from starch. (a) Starch (1) $\underrightarrow { \quad Diastase\quad }$ Maltose (1) (b) Maltose $\underrightarrow { \quad Maltase\quad }$ Glucose $\underrightarrow { \quad Zyamase\quad }$ Alcohol	Germinated barley (diastase enzyme). Temperature 50- 60^o C. Yeast (maltase and zyamase enzymes). Temperature 25-30⁰ C.
Manufacture of acetic acid from ethyl alcohol C₂H₅OH(l) + O₂(g) → CHCOOH(l) + H₂O(l)	Mycoderma aceti. Temperature 25-30^o C.
Bergius process for the synthesis of petrol from coal. Coal + H₂(g) Mixture of hydrocarbons	Ferric oxide Fe₂O₃ . Temperature 475^oC and pressure 200 atmosphere.

(1) Activity : Activity is the ability of catalysts to accelerate chemical reaction, the degree of acceleration can be as high as 10¹⁰ times in certain reactions. For example reaction between H₂ and O₂ to form H₂O in presence of platinum as catalyst takes place with explosive violence.

In absence of catalyst, H₂ and O₂ can be stored indefinitely without any reaction.

(2) Selectivity : Is the ability of catalysts to direct reaction to yield particular products (excluding other).

Example :

(i) n – heptane $\underrightarrow { \quad Pt\quad }$

(ii) CH₃CH = CH₂ $\underrightarrow { \quad BiMoO_{ 4 }\quad }$ CH₂

(1) Zeolite are alumino–silicates of the general formula, M_x/n[AlO₂]_x.SiO₂)_y.mH₂O , where, M may be simple cation like Na⁺, K⁺ or Ca²⁺, n is the charge on the simple cation, m is the number of molecules of water of crystallization.

(2) Some well known zeolites are as follows,

ErioniteNa₂K₂CaMg(AlO₂)₂(SiO₂)₂.6H₂O

Gemelinite Na₂Ca(AlO₂)₂(SiO₂)₄.6H₂O

Faujasite (natural) Na₅₆(AlO₂)₅₆(SiO₂)₁₃₆.250H₂O

ZSM-5 H_x[(AlO₂)_x(SiO₂)_96–x].16H₂O

Linde-A (synthetic) [Na₁₂(AlO₂)₁₂(SiO₂)₁₂.27H₂O]₈

(3) The characteristic feature of zeolites is the openness of the structure, which permits cavities of different sizes.

(4) The open structure is provided by silica in which aluminium occupies x/(x+y) fraction of the telrahedral sites.

(5) The negative charge of the aluminosilicate framework is neutralized by the replaceable cations.

(6) The void space forms more than 50% of the total volume, which is occupied by water molecules.

(7) The reaction- selectivity of zeolites depends upon the size of cavities (cages), pores (apertures) and the distribution of pores in the structure. The pore size in zeolites generally varies from 260 pm to 740 pm.

(8) The building block of zeolites is a truncated octahedron. This is also called the sodalite cage (or β – cage).

(9) Tetrahedral atom denoted by open circles in fig (a) are present at the corners of polygons with the oxygen atoms approximately half way between them.

(10) Zeolite have high porosity due to the presence of one, two, or three dimensional networks of interconnected channels and cavities of molecular dimensions.

(11) Accordingly zeolite – A is formed by linking sodalite cages through double four-membered rings, Faujasite (Zeolite X and Y) is formed by linking the sodalite cages through double six-membred rings.

(12) Many Zeolites occur in nature and they can be readily prepared in laboratories.

(13) There is a new class of highly siliceous zeolites with an optimal pore diameter of 550pm. ZSM-5 is one such zeolite having the formula. [H_x(AlO₂)_x.(SiO₂)_96–x.16H₂O]

(14) The zeolite catalyst ZSM-5 converts alcohols to gasoline (petrol) by dehydrating the alcohol and producing a mixture of wide variety of hydrocarbons. The shape selectivity of this catalyst is demonstrated by data given in table.

	Input stock		Input stock
Product (in %)	methanol n-heptyl alcohol	Product (in %)	methanol n-heptyl alcohol
Methane	1.0 0.0	i- Pentane	7.8 8.7
Ethane	0.6 0.3	Benzene	1.7 3.7
i-butane	18.7 19.3	Toluene	10.5 14.3
n-butane	5.6 11.3	Xylenes	17.2 11.6

Surface Chemistry : Catalysis

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