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CRE II Heterogeneous Catalysis

L 8

Prof. K.K.Pant

Department of Chemical EngineeringIIT Delhi.

kkpant@chemical.iitd.ac.in

mailto:kkpant@chemical.iitd.ac.inmailto:kkpant@chemical.iitd.ac.in

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SOLID ACID CATALYSTS

Examples:

Zeolites

SAPOs

Clays; pillared clays Ion-exchange resins

Oxides; X, SO4-oxides

Mixed oxides; amorphous

Heteropoly acids

ACID CATALYSIS

Two types of acid sites are recognized

- Brnsted

- Lewis

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Most active Metals: Co, Ni, Rh, Ru, Pd, Ir, Pt,

Approximately one vacant d orbital per atom.

Whereas, V, Cr, Mo, W etc. each metal has a

large number of vacant d orbital and are less

active as a result of strong adsorption for the

reactants or products or both.

However oxides of Mo (MoO2) and Cr (Cr2O3) are

quite active for most rexns involving H2.

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Cracking /

hydrocrac

king

Crack large molecules in

petroleum oils

FCC additives for more C3and

octane

Silica-alumina;

ZeoliteY ZSM-5

Dewaxing Crak n-paraffins (waxes) in

petroleum oils

ZSM-5

Isode-waxing

Isomerization of waxymolecules.

SAPO-11

Xylene

isomeri-

sation

p- and o-xylenes from m-

xylene.

ZSM-5;

Mordenite

Reactions / processes based on

acid catalysisName of

reactionDescr ipt ion Solid-acid

catalyst u sed

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Pore Size and Shape

Pore Diameter micropores (< 2 nm)

mesopores (250 nm)

macropores (> 50 nm)

Pore Shape

cylinder

slit

ink-bottle

wedge

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TerminologySubstrate- Frequently used to describe the solid

surface onto which adsorption can occur; the

substrate is also occasionally (although not here)

referred to as the adsorbent.

Adsorbate- The general term for the atomic orMolecular species which are adsorbed (or are

capable of being adsorbed) onto the substrate.

Adsorption- The process in which a molecule

becomes adsorbed onto a surface of anotherPhase.

(different from Absorption which is used

when describing uptake into the bulk of a solid

or liquid phase )

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Adsorptive Gas to be adsorbed

Adsorbent Material on which gas adsorbs

Adsorbate Adsorbed gas

Micropore Pore width 2 nm

Mesopore Pore width 250 nm

Macropore Pore width 50 nm

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Coverage-A measure of the extent of

adsorption of a species onto a surface.

Exposure-A measure of the amount of gas

which as surface has seen; more specifically it

is the product of the pressure and time of

exposure.

(normal unit is the langmuir ,where 1 L=10-6

Torr *s).

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Item Physical Adsorption Chemisorption

Forces ofattraction

WeakVanderWaalsforces

Strong valency forces

Specificity Low High

Quantity Large Small

Heat Effects Exothermic, 1-15 kCal/mol Exothermic, 10-100 kCal/mol

Activation

energy

Low High

Effect of

Temp.

Rapid at low temperatures

& reach equilibrium

quickly. Beyond TCof the

gas, no ads.

Slow at low temp., Rate

increases with temp.

Effect of

Pressure

Increases with increase in

pressure

Little effect

Surface Whole surface active Fraction of surface only

Layers Multi-layer adsorption Mono-layer adsorption

Physical Adsorption Vs. Chemisorption

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Physical Adsorption

Texture and morphology specific surface area of catalyst

pore size

pore shape pore-size distribution

(same size or various sizes?)

pore volume

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Chemisorption

Surface Characterization

Specific surface area of phases

Types of active sites Number of active sites

Reactivity of active sites

Stability of active sites

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Chemisorption

Metal Dispersion

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Supported Metal Particles

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Mechanism of Heterogeneous Catalysis:

1. Bulk Diffusion of reacting molecules to the

surface of the catalyst2. Pore Diffusion of reacting molecules into

the interior pores of the catalyst

3. Adsorption of reactants (chemisorption) onthe surface of the catalyst

4. Reaction on the surface of the catalystbetween adsorbed molecules

5. Desorption of products

6. Pore Diffusion of product molecules to thesurface of the catalyst

7. Bulk Diffusion of product molecules

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Mechanism of Heterogeneous Catalysis:

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Mechanism of Heterogeneous Catalysis:

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Pore and film resistances in a catalyst particle

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Rate-Determining Step (rds)In a kinetics scheme involving more than one step,it may be that one change occurs much faster or

much slower than the others (as determined byrelative magnitudes of rate constants).

In such a case, the overall rate, may be determined

almost entirely by the slowest step, called the rate-determining step (rds).

The rate of the rdsis infinitesimal when comparedto the rates of other steps.

Alternately the rates of other steps are infinite

compared to the rate of rds.

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Desorption:

Desorption of a product could also be rate

controlling in a few cases

Complexities:

Theoretically more than one step can berate controlling

Too many possible mechanisms

Experimental data is normally fitted to anysingle rate controlling step, which is thencalled the most plausible mechanism

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Item Physical Adsorption Chemisorption

Forces of

attraction

Weak VanderWaals forces Strong valency forces

Specificity Low High

Quantity Large Small

Heat Effects Exothermic, 1-15 kCal/mol Exothermic, 10-100 kCal/mol

Activationenergy

Low High

Effect of Temp. Rapid at low temperatures &reach equilibrium quickly.

Beyond TCof the gas, no ads.

Slow at low temp., Rate increaseswith temp.

Effect ofPressure

Increases with increase inpressure

Little effect

Surface Whole surface active Fraction of surface only

Layers Multi-layer adsorption Mono-layer adsorption

Physical Adsorption Vs. Chemisorption

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Chemisorption:

A chemical bond involving substantialrearrangement of electron density, is

formed between the adsorbate and

substrate.

The Nature of this bond may lie

anywhere between the extremes of

virtually complete ionic or completecovalent character.

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Chemisorption:There is some sort of interaction

between the surface of the catalyst and the

reactant molecules which makes them more

reactive. This might involve an actual reaction

with the surface, or some weakening of the

bonds in the attached molecules.

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Bonding strength between H2 & metal surface

increases with increase in vacant d orbital.

Maximum catalytic activity will not be realized

if the bonding is too strong and the productsare not easily desorbed from the surface.

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Adsorption at gas/solid interface.Adsorption: Term used to describe the process

Whereby a molecule(the adsorbate) forms aBond to a solid surface(an adsorbent).

Fractional surface coverage

N number of sites occupied by adsorbates=N total number of adsorption sites

When =1,NS=Nand an adsorbed monolayer

is formed.The fractional coverage dependson pressure of adsorbing gas phase species.

This =(P) relationship is called an adsorption

isotherm.

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Langmuir Adsorption Isotherm

Simple approach toquantitatively describe

an adsorption process

at the gas/solid interface

N = N + N number of vacant sitesVS

Assumptions:

Solids surface is homogeneous and contain a

number of equivalent sites,each of which is

occupied by a single adsorbate molecule.

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A dynamic equilibrium exist between gas

phase reactant and adsorbed species.

No interactions between adsorbed species.

Adsorbed species localised,Hadsisindependent of coverage ..

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Chemisorption rates:

Adsorption data is reported in the form of

isothermsChemisorption may be considered as a

reaction between a reactant molecule and anactive site resulting in an adsorbed molecule

A + A (or) A + S AS

Turnover Frequency (N): defined as the number

of molecules reacting per active site persecond at the conditions of the experiment a measure for the activity of the catalyst

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Langmuir Isotherm - Assumptions:

Surface is uniformly active

All sites are identical

Amounts of adsorbed molecules will notinterfere with further adsorption

Uniform layer of adsorption

Site balance:

t

v

v sitestotal

sitesvacantofNo

sitesvacantofFraction

.

t

AA

sitestotal

sitesoccupiedofNoAbyoccupiedsitesofFraction

.

1Av

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Alternately:

numbersAvogadro

massunitsitesactiveofNositesactiveofconcMolarCt

'

/..

numbersAvogadro

massunitsitesvacantofNositesvacantofconcMolarCv

'

/..

numbersAvogadro

massunitAbysitesofNo

AbysitesofconcMolarCAS '

/..

tASv CCC

Though other isotherms account for non-uniform surfaces, they

have primarily been developed for single adsorbing components.

Thus, the extensions to interactions in multi-component systems is

not yet possible, as with the Langmuir isotherm.

Langmuir isotherms are only used for developing kinetic rate

expressions.