Role of Adsorbents. In Catalytic Reaction: Heterogeneous catalysis

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1 Role of Adsorbents In Catalytic Reaction: Heterogeneous catalysis Catalysis: The phenomenon of change in the rate of reaction with the help of a catalyst Adsorption or Contact Theory: Due to the close proximity of adsorbed reactants the molecules react faster and the product formed desorbed due to decreased affinity with the adsorbent surface StepI : Diffusion of reactants to surface of catalyst Step II: Adsorption of reactant molecules at the surface of the catalyst StepIII: Chemical reaction at the surface StepIV: Desorption of products from the surface StepV: Diffusion of products away from the surface

2 Role of Adsorbent(Catalyst) Finely divided state of catalyst is more efficient as it increases the surface area Rough surface enhances the activity as the rough surface posses more active sites like cracks, corners and peaks Promoters increase the rate of the reaction by either increase active sites or by increase the lattice space Catalytic poison destroys the activity of catalyst as it preferentially gets adsorbed st the active sites Catalyst is specific as different adsorbents will possess different affinities for various adsorbate

3 Role of adsorbent in ion-exchange adsorption The process of releasing the cation or anion and adsorbing another like ion Synthetic resins are used as ion-exchange resin or ionexchanger in ion-exchange process Insoluble cross-linked, long-chain org polymers with the functional gp responsible for ion-exchanging properties They posses one adsorbed ion on it They release this ion and adsorbs another like ion TYPES Cation exchange resin (R - H + ):with acidic functional gp(- COOH, -SO 3 H, etc) ex. Styrene-divinyl benzene Anion Exchange Resin (R + OH-)With basic functional gps (-NH 2, =NH as hydrochloride) Styrene-divinyl benzene contain amino or quaternary ammonium or quarternary phosphonium or sulphonium gp as an integral part of the resin matrix

4 Applications of Ion- exchange Adsorption Softening of Water Zeolite Process Deionization or Demineralization of water Mixed bed Deioniser Electrical Demineralization of water Medical use: Ex. Dialysis Ion-Exchange Chromatography

5 Zeolite Process Zeolite: (Ze) Hydrated Sodiumaluminiumsilicate Cation Exchanger Exchange reversibly its sodium ions for hardness producing ions, Ca 2+, Mg 2+ etc Na 2 Ze + Ca(HCO 3 ) 2 CaZe+ 2NaHCO 3 Regeneration: CaZe + NaCl Na 2 Ze + CaCl 2

6 Cation Exchanger Capable of exchanging their cation (H + ) with other cations present in the water. Cation exchange resin is represented as R - H + Resins containing acidic fn group(- COO - H +, -SO3 - H + ) are capable of exchanging their H+ ions with other cations of hard water. Mainly styrene (divinyl) benzene copolymers which on sulphonation or carboxylation become cabable to exchange their hydrogen ions with cations in water Process: The hard water is first passed through a cation exchange column, which removes all the cations like Calcium, Magnesium from it and equivalent amount of H+ ions are released. M + + R - H + RM + H + CaCl 2 +2 RH R 2 Ca + 2HCl NaCl + RH RNa + HCl Regeneration: when the cation exchange resin is exhausted, it can be regenerated by passing a solution of dil. HCl or H2SO4: RNa + HCl RH + NaCl

7 Anion Exchanger Resins containing basic functional groups ( NH2 ) or quaternary ammonium groups are capable of exchanging their OH- ions with other anions of hard water. Anion exchange resin is represented as R + OH - Anion exchange resin Styrene-divinyl benzene or amineformaldehyde copolymers which contain amino or quarternary amm or quarternary phosphonium or quarternary sulphonium gp as integral part of resin matrix When water is then passed through an anion exchange column, which removes all the anions like chlorides, sulphates etc. and equivalent amount of OH- ions are released from the column. X - + ROH RX + OH - SO ROH R 2 SO 4 + 2OH - Regeneration The exhausted anion exchanger dil.naoh can be used. RCl + NaOH ROH + NaCl Net result on passing through both cation and anion exchanger: H + + OH - H 2 O (Pure water)

8 Softening of water As the metallic ions (cations) in the water affix themselves to the exchange material, the latter releases its hydrogen ions on a chemically equivalent basis. Because of the relatively high concentration of hydrogen ions, the solution is very acid. deionization process is just half complete. While the positive metallic ions have been removed, the water now contains positive hydrogen ions, and the anions originally in the raw water. The partially treated water now flows through a second unit, this time an anion exchange material normally consists of replaceable hydroxyl anions and fixed irreplaceable cations. The negative ions in solution (the anions) are absorbed into the anion exchange material. Released in their place are hydroxyl anions. Contains the positive hydrogen ions released in the initial exchange plus the negative hydroxyl ions released in the second exchange. They have combined (positive to negative) to produce water molecules. H + + OH - H 2 O The result of this two-stage ion exchange process is completely free from cations and anions. This water is known as Demineral water or deionised water.

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10 Ion Exchange Process / Demineralization process / de-ionisation process Advantages: 1) Used to soften highly acidic or alkaline water 2) Produces water of very low hardness(2ppm) Disadvantages: 1) The equipment is costly and expensive 2) If water contains turbidity, then the output of the process is reduced. The turbidity must be below 10ppm.If it is more it has to be removed by coagulation and filtration.

11 Mixed-bed Deioniser Deionizers are categorized as mixedbed, containing both cation and anion resin in a single vessel, or dual-bed, where each resin type is in a separate vessel Mixed-bed deionisers: The cationexchange and anion-exchange resins are intimately mixed and contained in a single pressure vessel. The two resins are mixed by agitation with compressed air, so that the whole bed can be regard as an infinite number of anion and cation exchangers in. Contd------

12 Mixed-bed Deioniser Regeneration: Seperation of both ion exchanger: When the resins are exhausted the mixed bed is back washed by forcing water in the upward direction as the lighter anion exchanger get displaced to form an upper layer above the heavier cation exchanger Anion exchanger is regenerated by passing caustic soda from the top The lower cation exchange resin is regenerated by acid from the bottom The two beds are then rinsed thoroughly, mixed again by forcing compressed air Advantages: More convenient to use More efficient removal of dissolved salts- Dissolved salts can be reduced to less than 1ppm Limitation: More expensive to regenerate Mixed-bed plants are more sensitive to impurities in the water supply and involve a more complicated regeneration process. Mixed-bed deionizers are normally used to polish the water to higher levels of purity after it has been initially treated by either a two-bed deionizer or a reverse osmosis unit.

13 Electrical demineralization of water Under the influence of the strong direct current field applied, charged ions are pulled off the resin and drawn towards the respective, oppositely-charged electrodes Ion exchange resins are supported on a paper or a fibre and used as membranes. Ion-selective membranes Cation selective membrane permits only cations not anions because the presence of charge fixed (inside the membrane) fn gps such as RSO 3 - or RCOO - reject anions Anion selective membrane permits only anion because of positively charged fixed ions R 4 N + The rate of diffusion of ion is increased by passing electric current through the electrode placed near the membrane» Contd------

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15 Electro Dialysis (ED)

16 Role of Adsorbent in Chromatography Based on the differences in the adsorption coefficients of substances on solid Process: StepI: Separation of various compounds When the solution containing different solutes(mobile phase-adsorbate) is poured down a column, filled with finely divided adsorbent(stationary phase-adsorbent) partial separation takes place due to the difference in adsorption coefficient The separation is improved by passing suitable solvent(developer) slowly through the column A number of horizontal bands of different colours are produced in the column Chromatogram: The banded column of the adsorbed substance contd------

17 Role of Adsorbent in Chromatography StepII: Identification f compounds Colored compounds produce colored rings, zones or bands Colorless compounds are observed by either exposing UV light on the column or by spraying suitable chemical reagent in order to make them colored. StepIII: Separation and estimation The various zones are dissolved separately in suitable solvents and estimated The process of recovery of various substances is elution and the solvent is called eluent. contd------

18 Ion Exchange Chromatography Charged substances are separated by differential adsorption on adsorbents carry an opposite charge Example : Separation of proteins At strong acidic ph, proteins are present as cations as a result of dissociation of carboxylic acid group and protonation of amino group At ph values above 12, they present as anions because the amino groups exist as free base Adsorption and desorption of proteins on charged stationary phase is brought by changing ph.

19 Role of Adsorbent in Chromatography Advantages of Chromatography Separation of a mixture of similar molecules is possible only in adsorption chromatography Large volume of samples can be separated Applications of chromatography Quantitative separation of two or more substances Purification of substances Identification of product

20 Activated carbon Activated carbon, also called activated charcoal or activated coal, is extremely porous carbon which had a very large surface area available for adsorption or chemical reactions. It was first used as an adsorbent in gas masks in world war I..

21 Preparation and Properties Activated carbon is produced from carbonaceous source materials like nutshells, wood and coal. Preparation 1) Direct Process: Heating carbon at 850C in a current of superheated steam 2) Briquetting process: Powdered carbon is mixed with wood, tar and caustic soda,compressed in small briquettes and then heated in steam at 800C. 3) Chemical Process: Wood is impregnated with the solution of zinc chloride or phosphoric acid and then carbonized. Properties Activated carbon has enormous surface area of the order of 1000 m2 / g. Activated carbon is able to adsorb practically any volatile organic compounds(voc) at lower temperature and can be released on heating to a high temperature

22 Classification -Based on the size of particle Powdered activated carbon (PAC) Diameter of less than 200mesh Used in purification of liquids Saw dust and lignite are raw materials Granular activated carbon (GAC) Diameter of greater than 0.1nm Granular activated carbon are fine granules has a relatively larger particle size and presents a smaller external surface has large internal surface area due to porosity. Produced from coconut shells, fruit pits, charcoal and briquetted coal Diffusion of the adsorbate is an important factor. These carbons are preferred for all adsorption of gases and vapours as their rate of diffusion are faster. Granulated carbons are used for water treatment, deodourisation and separation of components of flow system. GAC can be either in the granular form or extruded form

23 Classification-contd Impregnated carbon Porous carbons containing several types of inorganic impregnant such as iodine, silver, cation such as Al, Mn, Zn, Fe, Li, C or application in air pollution control especially in museums and galleries Polymers coated carbon This is a process by which a porous carbon can be coated with a bio-compatible polymer to give a smooth and permeable coat without blocking the pores

24 Types of Different Activated Carbon PAC GAC Exrtuded GAC

25 Adsorption in GAC GAC uses the physical adsorption process whereby attractive van der Waals forces pull the solute out of solution and onto its surface Adsorption proceeds through 3 basic steps Substances adsorb to the exterior of the carbon granules Substances move into the carbon pores Substances adsorb to the interior walls of the carbon

26 Role of Activated carbon In air pollution abatement Activated carbon has very high surface area and it adsorb odorous, gaseous, and liquid contaminants forming a strong chemical bond or attraction. In producing high vacua - For this partly evacuated apparatus is connected to a vessel containing activated carbon, cooled in liquid air. At this temperature carbon adsorbs residual air very effectively. In gas masks or respirators - it adsorbs poisonous or foul smelling and other harmful gases and vapour more readily than it adsorbs air.. Thus the air getfiltered on passing thro the gas mask before breathing. Remove offensive odor from the air (deodorizer) in airconditioning process inlarge restaurants, auditoriums and in refrigerators, in shoe insole

27 Role in Activated carbon In air pollution abatement-contd An air purifier removes contaminants from air and particularly beneficial to allergy sufferers and asthmatics, and at reducing second-hand tobacco smoke. In room air purifiers, the activated carbon is often combined with zeolite. In gas purification, activated carbon is used to remove undesirable or noxious components like Benzene. It also selectively removes residual Hydrogen Sulphide, Napthalene, Cyanogen compounds, Resins, Gums and Nitrogen Oxides. Purification of feed or exhaust air from unhealthy or annoying substances like Hydrogen Sulphide and Carbon Disulphide For the fine purification Carbon Dioxide In the gas phase application the carbon is used to trap any undesirable gases such as Xenon, Krypton in the Nuclear Power Plants

28 Role of Activated Carbon in indoor air pollution -Volatile Organic Compounds Volatile Organic Compounds (VOCs) are organic chemical compounds that have high enough vapor pressures under normal conditions to significantly vaporize and enter the atmosphere. VOC's are often used in paint, carpet backing, plastics, cosmetics and wood preservatives, contribute to sick building syndrome because of their high vapour pressure Air purifiers are used to reduce the concentration of these airborne contaminants and are especially useful for people who suffer from allergies and asthma. Activated carbon because of porous nature absorb volatile chemicals and changing contaminants from a gaseous phase to a solid phase, VOC can be recovered later by desorption

29 In abatement of water pollution The fresh water available on the earth is only 2%. Hence proper maintenance,conservation and use of water resources is required to avoid chances of water scarcity forfuture generation. It is therefore necessary to treat water and reuse wastewater, so as to increase and improve the quality of water and make it for use. Activated carbon is used in water treatment process. Theory: It is both hydrophobic and oleophilic (ie) it hates water but has affinity for oil and other chemicals. When the flow condition is suitable, dissolved chemicals in water flowing over the surface of carbon get adsorbed on the surface while the water passes over. Activated carbons used in waste water treatment are Granular Activated carbon(gac), Extruded solid carbon black (CB) and powdered Activated carbon (PAC). GAC is used because of its porous structure and high surface area. It is cheaper and can be reused For drinking water, the solid carbon block filters are most efficient and effective. GAC s can be regenerated in a furnace by heating it in the presence of limited amount of water vapour, flue gas and oxygen. Some carbon (5-10%) is destroyed during regeneration process.

30 Activated Carbon in Domestic Water Treatment Activated carbon (AC) filters have been used in home water purification systems primarily to remove taste and odor. AC is most effective at removing organic compounds such as volatile organic compounds, pesticides and benzene. It can also remove some metals, chlorine and radon. GAC (granular activated carbon), can be used after coagulation and sedimentation as a layer, or used after sand filtration to remove organics from the water. PAC (powdered activated carbon), is more commonly used than GAC to control taste and odor in drinking water treatment. PAC can be added directly to the water prior to coagulation or just sand filtration. Purification of drinking water (5-20 grams of powdered activated carbon can purify about one thousand litres of water in an hour)the PAC adsorbs contaminants and is then removed by sedimentation or filtration In soft drink industries and in breweries, the potable water used is sterilized with chlorine. The excess chlorine and its odour, dissolved organic contaminants, the trihalomethanes and phenolics are removed by treatment with activated carbon

31 Removal metals from waste water solution In purifying waste water, most of the contamination in the waste water is broken down biologically by micro organisms. Toxic contaminants in the water can upset the biology badly, unless the toxins are made harmless through adsorption onto activated carbon. Primary water treatment filtration & sedimentation in settling tank GAC PAC Sand filtration coagulants Flocculation and Settling Disinfection /chlorination GAC for filtration &adsorption Biological Treatment Disposal into water bodies

32 Industrial effluent treatment In industrial contamination of ground water reserves, activated carbon is made use to reduce total organic halogens (TOX) and adsorbable organic halogens (AOX) including chloroform, tetrachloroethylene and trichloroethane to meet environmental legislation Recovery of fuels from synthesis gases like Motor fuel Hydrocarbons, Polymers, Ethylene and etc. Activated bituminous C is used for the reduction of high chlorine content of water