INTRODUCTORY LECTURE ON CATALYSIS

INTRODUCTORY LECTURE ON CATALYSIS

A.V. RAMASWAMYNATIONAL CENTRE FOR CATALYSIS RESEARCHINDIAN INSTITUTE OF TECHNOLOGY MADRAS

CHENNAI 600036Email: avram@iitm.ac.in, av_ram@hotmail.com

IMPORTANCE OF CATALYSIS

PETROLEUM REFININGINDUSTRY

CHEMICAL INDUSTRY

FUELS

LUBRICANTS

PETROCHEMICALS

SPECIALITY CHEMICALS

FINE CHEMICALS/PHARMACEUTICALS

(VALUE = $ 10 TRILLION)

- CATALYSIS IS KEY ENABLER- 80% OF ALL INDUSTRIAL CHEMICAL PROCESSES ARE CATALYZED

WHY USE CATALYSTS ?

- IMPROVED CHEMICAL EFFICIENCY

- IMPROVED ENERGY EFFICIENCY

- SUSTAINABLE TECHNOLOGY

- ENVIRONMENTALLY BENIGN

- FASTER PRODUCT DEVELOPMENT

- REDUCED CAPITAL COST

THE CATALYST INDUSTRY

CATALYSTINDUSTRY

PETROLEUM REFINING

PETROCHEMICALS

POLYMERS

BULK CHEMICALS

FINE CHEMICALS

ENVIRONMENTAL

TOTAL WORLD MARKET (2000) > $10 BILLION GROWTH OF > 5% PER ANNUM IS PROJECTED

AREAS OF CATALYST DEVELOPMENT

HOMOGENEOUS HETEROGENEOUS

HYDROGENATION

OXIDATION

ACID/BASE

DEHYDROGENATION

BIOCATALYSIS

OTHERS

STRATEGIC RESEARCH AIMED AT

INCREASE YIELDIMPROVE QUALITY OF THE PRODUCTSINCREASE ATOM EFFICIENCYENVIRONMENTAL BENEFITSMINIMIZE ENERGY

TARGETS

- DEVELOP NOVEL CATALYSTS/MATERIALS- IMPROVEMENT OF EXISTING CATALYTIC PROCESSES- MAKING NON-CATALYTIC ROUTES CATALYTIC- INCORPORATION OF CATALYTIC TECHNOLOGY INTO FINE CHEMICALS/PHARMACUETICALS PRODUCTION

RESEARCH DRIVERS

COMBINATORIAL High throughput screening for identification and optimization of

catalysts

SURFACE SCIENCE Better fundamental understanding of reaction mechanisms

MODELING Improve ability to model and predict catalytic processes

DEACTIVATION Ability to predict lifetimes underoperating conditions

FUTURE TRENDS IN CATALYSIS

EVOLUTIONARY - Continuous improvement of existing catalysts

REVOLUTIONARY - Replacement of non-catalytic routes with catalytic routes- Combined heat & power fuel cells for power generation- No NOx ; Lower CO2 emission- Asymmetric catalysis for pharmaceutical products- Catalytic membranes; use of membranes in selective oxidation- Bio-inorganic/organic materials in bio-catalysis

Surface Science-Atoms-Interactions

Inorganic Chemistry-Metals, non-metals-Organometallics

Material Science-Properties-Phases

Organic Chemistry-Reaction mechanism

CATALYSISPhysical Chemistry-Kinetics-Thermodynamics

Biochemistry-Enzyme specificity

Theoretical Chemistry-Quantum mechanics-Modeling

Chemical Engineering-Heat & Mass transfer-Reactor design-Process technology

Catalysis has left the realm of alchemy and entered the field of scienceIt is still pretty much of an art to design and optimize new catalysts, but it is no longer a black artThe eventual hope is to tailor-make catalysts to fit each of the developing needs

CATALYSIS COURSE WORK 2010

REACTION KINETICS

- SYSTEM APPROACH TO KINETICS AND CATALYSIS & CHEMICAL KINETIC THEORY

- TRANSIENT STATES IN CATALYSIS AND INTERMEDIATES

- ARRHENIUS EQUATION, ACTIVATION ENERGY AND METHODS OF THEIR ESTIMATION

BASIC PRINCIPLES OF CATALYSISChemical reactions are

governed bya) Thermodynamics andb) Chemical Kinetics

A B

What will be the yield?

If the equilibrium constant is low, even if the reaction is fast, the yield will be low.For a system with a large equilibrium constant, where a high yield is potentially attainable, if the rate is low, it will take a long time to attain economic yield.

a) Chemical Equilibrium

aA + bB cC + dD (gaseous reaction)

rf = kf PAa PB

b ; rr = krPCcPD

d and

kf /kr = PCc PD

d / PAa PB

b = Kp

Gibbs free energy, G

For a reaction to occur spontaneously, G must be negative.The magnitude of - G will determine how far the reaction will go.The more negative the value of G, the larger will be the value of Kp

Reaction isotherm

Standard free energy change, - Gø = RT ln Kp

Reaction isochore

d ln Kp/dT = Hø/RT2

where Hø is the standard enthalpy (or heat content) change atconstant pressure.

Entropy is given by,

G = H - T S

b. Chemical Kinetics

A + B C

The rate, r = k PAm PB

n, where ‘m’ and ‘n’ are the order of reaction, in A and B, respectively and k is the rate co-efficient.

The order of the reaction gives some indication as to how the reactants enter the rate-controlling step,but the equilibrium rates, rf and rr are influenced by the fast steps as well as slow steps, so that a, b, etc., are not equal to the order of the reaction.

Arrhenius EquationThe rate constant (coefficient) is related to absolute temperature,

k = A exp (-E/RT)

where A = pre-exponential factor, E = the activation energy andR = gas constantFrom collision theory,

k = ZAB exp (-E/RT)(higher rates than observed)

k = PZAB exp (-E/RT) (unsatisfactory)where ZAB = collision number and P = steric factor

Transition State theory or the theory of absolute reaction rates

A + B AB# C + D

The activated complex, AB# falls apart when a vibrational mode changes to translational mode, since the latter is an irreversible vibration.We can now apply the thermodynamic relationship to the Equilibirum constant, Kp

# The free energy of activation is given by

-G# = RT ln Kp#

And further, G# = H# - TS#

where, H# is standard enthalpy of activation, and S# is standard entropy of activation.

Then,k = kT/h exp (-G# /RT)

= kT/h exp (S# /R) exp(-H# /RT)

Comparing this with Arrhenius equation, k = A exp (-E/RT)

-H# replaces the activation energy, E and

A = kT/h exp (S# /R)

CATALYST

Jons Jacob Berzelius (1835)The experiments “constitute a sufficient number of Examples to establish the existence of catalytic power”

This catalytic power comes from substances “able to awaken affinities which are asleep at this temperature by their mere presence and by their own affinity”

Ostwald (1895) introduced the term, “rate” of a reaction.“A catalyst is a substance that changes the rate of a chemical reaction without itself appearing into the products”(It means that the catalyst can also slow down the reaction)

Present definition:“A catalyst is a substance which increases the rate at which a chemical reaction approaches equilibrium, without becoming itself permanently involved”

Catalysts help reactions to occur at lower temperatures.

Catalysts do not shift equilibrium of a reaction.

The primary effect of a catalyst on a chemical reaction,-increase the rate, i.e., increase its rate coefficientAccording to collision theory, the difference in –E between the uncatalyzed and catalyzed reactions will be at best 65 kJmol-1

For efficient catalysis, the activation energy difference typically must exceed 100 kJmol-1

According to the transition-state theory,the effect of a catalyst must be to decrease the free energy of activation, G#

This in turn composed of an Entropy of activation and an Enthalpy of activation.

What about the entropy term?Will usually be less than in the corresponding non-catalyzed reactionHence, to compensate for this, there should be a corresponding decrease in the enthalpy of activation or to overcompensate for efficient catalysis.

By either theory, the activation energy for a catalyzed reaction must be less than for the same uncatalyzed reaction.The lowering of the activation energy is a fundamental principle of catalysis.

}

YX

HeterogeneousCatalyzed reaction

Gas-phase uncata-lyzed reactionAhet

Ahom

103/T

Log

10 (r

ate

cons

tant

), ar

bitr

ary

units

Measurable range

Arrhenius diagram for uncatalysed and catalysed reactions: X and Y are the ranges of 1/T in which the respective reactions can be observed

Kinetics and Catalysis

Acceleration of a reaction in presence of a catalyst, as compared to non-catalyzed reaction:

A + B P

d[P]/dt = k1[A][B]

For a catalyzed reaction,

d[P]/dt = k2[Cat][A][B] or d[P]/dt = k3[Cat][A], etc

- The order of the reaction is different, even in the simplest case.- Comparison of the two rate constants has little meaning. Comparisons are meaningful only if the catalysts follow the same mechanism and if the product formation can be expressed by the same rate equation.- Rate enhancements of catalysts relative to another is valid.- The rate equations are normally more complicated than the ones above.- The rate equations are important for understanding the mechanism of a chemical reaction

Definitions

Activity:Turn-over number (TON) is the total number of substrate molecules that a catalyst converts into product molecules.

Turn-over frequency (TOF) is the number in a certain period of time.For A B in presence of a catalyst of Q moles,

rate, r = d[B]/dt,the turn-over rate will be r/QA figure of merit to compare the ‘activity’ of different catalysts.For a solid catalyst, Q may be specific surface area, metal area, etc.

Note: Only a fraction of the catalyst surface is active during the reaction, this fraction is usually unknown. Hence, specific rate provides a lower limit of the catalyst’s activity

Selectivity:

Among many products that can form in a chemical reaction, the ratio of the moles of the desired product to the total number of moles of all the products formed in a specific time frame.

Reaction systemsFollow the course of the following sequence of reactions:1) A B2) A B3) A B C4) A B C5) A B C

B6) A

C

B7) A

C

INTRODUCING SELECTIVITY

Catalyst activity is defined by TON Selectivity to a particular product among many products can be defined by atom efficiency A + B C (There is no selectivity issue) e.g., 3H2 + N2 2NH3

k1 k2

Consider, A B C (consecutive reactions) k1 k2

B A C (simultaneous reactions)

The rate constants k1 & k2 will be important. B is an intermediate or a product of importance

k1 k2

e.g., CH2=CH–CH3 + O2 CH2-CH-CH3 CO2 + H2O O

CLASSIFICATION OF SELECTIVITY

1. Chemoselectivity:

B A C Microscopic or chemical nature of catalyst

2. Regioselectivity:

Positional isomers Spatial configuration of the catalyst 3. Diastereoselectivity:

Substrate has a stereogenic center Catalyst to direct the mode

4. Enantioselectivity:

Achiral substrate Enantio-pure Catalysts

Propylene Oxidation

C3H6 + O 2

productsIn term ediate

CO 2 + H2O

Reaction coordinate

E2

E1

Pot

entia

l ene

rgy

Potential energy profile for a multi stage oxidation reaction

The intermediate is metastable, can be isolated only when the contact time of the reactants with catalyst is relatively short. A catalyst can produce more of the desired product by catalyzing only that reaction.

Steady state approximation: S + H2 Product

M + S (k-1) (k⇋ 1)MS (1)MS + H2 (k2) M + product (2)

The rate of product formation, r = k2 [MS][H2] (3)

At steady state, d[MS]/dt = 0 = k1[M][S] – k-1[MS][H2] (4)

Amount of catalyst added, [Mt] = [M] + [MS] (5)

The rate can now be described from measurable quantities, r = k1k2[Mt][S][H2] / k1[S] + k2[H2] + k-1 (6)

Equation (6) can be simplified,If k2 >> k-1 >> k1, then r = k1[M][S]

If reaction (1) is the rate determining step, the rate of the reaction is independent of the H2 concentration.If there is a fast pre-equilibrium, and the rate-determining step is (2),Then, k1 , k-1 >> k2 , then r = k2 [MS][H 2]Substituting for [MS], the equilibrium fraction of the catalyst,

r = k1 k2[Mt][S][H2] / k1 [S] + k-1

Modify, 1/r = 1/k2 [Mt][H2] + k-1 /k1k2 [Mt][H2][S]Plot 1/r vs. 1/[S] gives a straight line to get k2 and equilibrium constant.

Typical free energy profiles

Case Awhere k2 >>k-1 >>k1

r = k1[M][S]The order in S and M = 1The order in H2 = 0

Case Bwhere k1,k-1>>k2, r = k2[MS][H2]The order in S = between 0 and 1The order in M = 1The order in H2 = 1Saturation kineticsMichaelis-Menten kinetics

M + S (k-1) ⇋ (k1) MS (1)MS + H2 (k2) M + product (2)

Michaelis-Menten Kinetics (Enzyme catalysed reactions) Saturation kinetics

A complex is formed between the substrate and the catalyst by a rapid equilibrium reaction. The equilibrium constant of this reaction is K, and it is followed by the rate-determining step with a rate constant, k.Increasing the substrate concentration will increase the rate initially, followed by more or less constant rate high substrate concentration, when

K[substrate] ~ 1 + K[substrate]

at constant catalyst concentration, a plot of (1/rate) vs. (1/(substrate)

will give a straight line.

Rate = k.K[substrate][catalyst]/1 + K[substrate]

Case CReaction 2 is rate-determiningRate equation (6)Fractional order in H2 and SFirst order in MCase DBarrier bet k-1 and k2 are similarr = k1[Mt][S]x k2[H2]/k2[H2]+k-1

Fractional order in H2 and SFirst order in M

M + S (k-1) (k⇋ 1) MS (1)MS + H2 (k2) M + product (2)

Case EIntermediate MS has a lower energythan the starting catalyst M

r = k2[Mt][H2]

Rate is independent of [S]

M + S (k-1) (k⇋ 1) MS (1)MS + H2 (k2) M + product (2)

Selectivity is determined after rate-controlling step

Product selectivity

Selectivity is determined in steps 1, while steps 2 are rate-controlling

Catalytic cycle and intermediates

Homogeneous CatalysisMLn+1 MLn + LMLn + H2 H2MLn

H2MLn + alkene H2MLn(alkene)H2MLn(alkene) HMLn(alkyl)HMLn(alkyl) MLn + alkane

Rate equationa) Empirical models: The power rate law for a bimolecular reaction, r = kmnAmBn

b) Mechanistic models:When the catalytic reaction mechanism is known or can be speculated, it is possible to derive rate equations, assuming one of the steps in the catalytic sequence as a rate-determining step.

Heterogeneous Catalysis

a) Diffusion of reactants to the surface (surface diffusion)

b) Adsorption of reactants at the surface

a) Chemical reaction on the surface (molecular rearrangement at active surface sites)

d) Desorption of products from the surface

e) Diffusion of products away from the surface

These are consecutive steps and the slowest step, therefore,determines the rate of the reaction

Ehet

Ehom

Pote

ntia

l Ene

rgy

Activatedstate forgas reaction

Activated state for surface reaction

Gaseous reactants

GaseousproductsAdsorbed

products

Adsorbed reactants

Reaction coordinates

Kinetics of heterogeneous catalytic reactions

1. Gas-solid reactions2. Liquid-solid reactions3. Gas-Liquid-solid reactions

The overall rate is dependent on:a) External andb) Intraparticle mass transferc) Intrinsic kinetics

The mechanism is primarily developed based on the concepts of adsorption.The role of the surface of a solid catalyst should beunderstood.

Coordinative unsaturation of a metal center (in metal complexes) in homogeneous catalysis, and the so-called active site in solidsin heterogeneous catalysis is a common principle for initiatingan ‘interaction’ between the substrate and the catalyst

Consider the reaction,A + B P

The power law model,rate, r = kmnCA

mCBn

Langmuir-Hinshelwood modelsa) Single site L-H modelFraction of the surface covered = ; uncovered = 1-

= Kp/1+KpFor a reaction, A B,

A + surface site Aads

Aads Bads

Bads B + surface siteThe rate is proportional to and hence,

r = k = kKp/(1+Kp) or = kCA/1+CA

b) Dual site L-H modelr = kCA CB /(1+KCA + K’B )2

Experimental Methods: Data Collection

Dependence of rate on the concentrations of reactantsExcess of one of the substrates (ten fold) – under pseudo-first-order conditions

1) Initial rates from the rate at zero to low conversion (10-15%) (Differential method)

2) Monitor reaction rate over a longer period of time Rate parameters by integrating relevant rate equations

for time-concentration data

-Rate data to be collected under constant intrinsic catalytic activity- Significance of mass transfer limitations

Reactors:- A batch reactor with good mixing- A plug-flow reactor (no mixing, residence time determines the degree of conversion)-A CSTR, steady state obtained based on the residence time

Importance of Diffusion in Catalysis

Heterogeneous catalysis/Porous solids/Particle size and shape

1. External or inter particle diffusionTransport of reactants through the fluid phase to the solid catalyst

2. Internal or intra particle diffusion (Pore diffusion)Diffusion of reactants through the pores of the solid to the interior

Introduce an effectiveness factor, = r/r’, where r = observed rate and r’ = rate without the limitation of intra-particle diffusion.

= tanh mL/mLwhere mL is a dimensionless quantity known as Thiele modulus And can be related to the rate constant per unit volume of the catalystpellet, k and the diffusion coefficient D, as

mL = L k/Dint

When mL = <0.5, then = 1, there is no resistance to reaction by porediffusion. When mL > 5, = 1/mL, pore diffusion strongly influencesthe reaction; the observed rate = 1/mL x intrinsic rate

Strong pore resistance and order of a reaction

Limitation Eact Order of the Effect of particle Effect ofreaction diameter superficial linear

velocity

Intrinsic Kinetics E n Nil NilInternalDiffusion E/2 (n+1)/2 1/dP NilExternalDiffusion 1-5 kcal 1 Depends Depends

Homogeneous reactionpredominates

Slope = -Ehomo/R

Mass transfer to outside of particle controls

Slope ~ -(1 to 2) /R

D

CB

A

Significant porediffusion

Slope ~ -Ehetero/2R

Intrinsic surfaceReaction controls

Slope = -Ehetero/R

Rat

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rea

ctio

n (lo

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ale)

1/T

REFERENCE BOOKS

1. “Heterogeneous Catalysis” G.C. Bond 2nd EditionOxford University Press, 1987 Chapter 1

2. “Principles and Practice of Heterogeneous Catalysis” J.M. Thomas and W.J. Thomas VCH Publication, NY, 1996

3. “Homogeneous Catalysis: Understanding the Art”, P.W.N.M. van Leeuwen, Kluwer Acad. Pub., 2004, Chapter 3

4. “Catalysis: Principles and Application” Edited by B.Viswanathan, S. Sivasanker and A.V. RamaswamyNarosa Pub.Co., New Delhi, 2002, Chapters 11 and 26

5. “The role of Diffusion in Catalysis”, C.N.Satterfield and T.K. Sherwood, Addison-Wesley Pub., 1963

6. “Handbook of Heterogeneous Catalysis”, Edited byG. Ertl, H. Knozinger and J. Weitkamp, Wiley-VCH, 1997