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

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 pos-sess different affinities for various adsorbate

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 ion-exchanger 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 ionTYPES�Cation exchange resin (R -H +):with acidic functional gp(-COOH, -SO3H, etc) ex. Styrene-divinyl benzene

�Anion Exchange Resin (R+OH-)With basic functional gps(-NH2, =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

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

Zeolite Process �Zeolite: (Ze) Hydrated Sodiumaluminiumsili-cate

�Cation Exchanger

�Exchange reversibly its sodium ions for hardness producing ions, Ca2+ , Mg 2+ etc

�Na2Ze + Ca(HCO3)2

CaZe+ 2NaHCO3

�Regeneration: CaZe + NaCl

Na2Ze + CaCl2

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+

• CaCl2 +2 RH R2Ca + 2HCl

• NaCl + RH RNa + HClRegeneration: when the cation exchange resin is exhausted, it can

be regenerated by passing a solution of dil. HCl or H2SO4: RNa + HCl RH + NaCl

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 amine-formaldehyde 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 -

• SO42- + 2ROH R2SO4 + 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- H2O (Pure water)

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- H2O

• 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.

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.

Mixed-bed Deioniser

• Deionizers are categorized as mixed-bed, 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 cation-exchange 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------

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 1ppmLimitation: • 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.

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 RSO3

- or

RCOO- reject anions

• Anion selective membrane permits only anion because of positively charged fixed ions R4N

+

• The rate of diffusion of ion is increased by passing electric current through the electrode placed near the membrane

» Contd------

Electro Dialysis (ED)

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------

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------

Role of Adsorbent in Chromatography

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.

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

Role of Adsorbent in Chromatography

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.

.

Preparation and PropertiesActivated carbon is produced from carbonaceous source materials like nutshells, wood and coal.Preparation

1) Direct Process: Heating carbon at 850C in a current of su-perheated steam2) 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 or-ganic compounds(VOC) at lower temperature and can be re-leased on heating to a high temperature

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 materialsGranular activated carbon (GAC)●Diameter of greater than 0.1nm●Granular activated carbon are fine granules has a relative-ly larger particle size and presents a smaller external sur-face has large internal surface area due to porosity.●Produced from coconut shells, fruit pits , charcoal and bri-quetted 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, de-odourisation and separation of components of flow system. ●GAC can be either in the granular form or extruded form

Classification-contd

●Impregnated carbon●Porous carbons containing several types of inorgan-ic 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

Types of Different Activated Carbon

PACGAC Exrtuded GAC

Adsorption in GAC

➲ GAC uses the physical adsorption process whereby attractive van der Waals forces pull the so-lute 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

Role of Activated carbon In air pollution abatement

➲ Activated carbon has very high surface area and it ad-sorb odorous, gaseous, and liquid contaminants form-ing a strong chemical bond or attraction.

➲ In producing high vacua - For this partly evacuated appa-ratus 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 air-conditioning process inlarge restaurants, auditoriums and in refrigerators, in shoe insole

➲ Role in Activated carbon In air pollution abate-ment-Contd

● An air purifier removes contaminants from air and particularly bene-ficial 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 un-desirable or noxious components like Benzene. It also selective-ly removes residual Hydrogen Sulphide, Napthalene, Cyanogen compounds, Resins, Gums and Nitrogen Oxides.

● · Purification of feed or exhaust air from unhealthy or annoy-ing 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 Pow-er Plants

●

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 at-mosphere.

➲ VOC's are often used in paint, carpet backing, plastics, cos-metics 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

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, dis-solved chemicals in water flowing over the surface of carbon get ad-sorbed on the surface while the water passes over.

➲ Activated carbons used in waste water treatment are Granular Activat-ed carbon(GAC), Extruded solid carbon black (CB) and powdered Ac-tivated 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.

Activated Carbon in Domestic Water Treatment

➲ Activated carbon (AC) filters have been used in home water purifica-tion 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 organ-ics 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 car-bon can purify about one thousand litres of water in an hour)The PAC adsorbs contaminants and is then removed by sedimentation or filtra-tion

➲ In soft drink industries and in breweries, the potable water used is ster-ilized with chlorine. The excess chlorine and its odour, dissolved organ-ic contaminants, the trihalomethanes and phenolics are removed by treatment with activated carbon

Removal metals from waste water solution➲ ·In purifying waste water, most of the contamination in the waste wa-

ter 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 car-bon.

PAC

Primary water treatment – filtration &sedimentation insettling tank

coagulants

Sand filtrationGAC

GAC for filtration &adsorption

Flocculation and Settling

Biological Treatment

Disinfection/chlorination

Disposal into water bodies

Industrial effluent treatment

➲ In industrial contamination of ground water reserves, acti-vated 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 Hy-drocarbons, Polymers, Ethylene and etc.

➲ Activated bituminous C is used for the reduction of high chlorine content of water