CATALYSIS Course Syllabus Definitions Characteristics of Catalytic - - PowerPoint PPT Presentation

CATALYSIS

Chapter 2

Definitions Characteristics of Catalytic Materials Types of Catalysis Homogeneous Catalysis Theories of Acid – Base Catalysts Heterogeneous Catalysis Different Theories of Heterogeneous Catalytic Reactions

Course Syllabus

• CATALYSIS comes from two Greek words,

the prefix CATA- meaning down and the verb LYSEIN, meaning to split or break. “A catalyst breaks down the normal forces which inhibit the reactions of molecules”

Life cycle on the earth

 Catalysts (enzyme) participates most part of life cycle (e.g. forming, growing, decaying)  Catalysis contributes great part in the processes of converting sun energy to various other forms of energies  Catalysis plays a key role in maintaining our environment (e.g. photosynthesis by plant) CO2 + H2O=HC + O2

Catalysis & Catalysts

Fa Facts a s and Fi Figures a abou bout Ca t Catal talysi sis

Chemical Industry

• ca. $2 bn annual sale of catalysts
• ca. $200 bn annual sale of the chemicals that are related products
• 90% of chemical industry has catalysis-related processes
• Catalysts contributes ca. 2% of total investment in a chemical process

Catalyst Is a substance which accelerates the rate of approach to equilibrium of a chemical reaction without being substantially consumed in the reaction. Catalyst is a substance which speeds up a reaction, but is chemically unchanged at the end of the reaction. A catalyst is defined as a substance which alters the rate of a chemical reaction, itself remaining chemically unchanged at the end of the reaction. The process is called Catalysis. A catalyst which enhances the rate of a reaction is called a Positive catalyst and the process Positive catalysis or simply Catalysis. A catalyst which retards the rate of a reaction is called a Negative catalyst and the process Negative catalysis. Inhibitor Is a substance that decreases the rate of the reaction; it was formerly called a negative catalyst.

What is Catalysis: Definitions

Threekeyas pectso fcatalys tactio n

 Taking part in the reaction

it will change itself during the process by interacting with other reactant/product molecules

 Altering the rates of reactions

in most cases the rates of reactions are increased by the action

• f catalysts; however, in some situations the rates of undesired

reactions are selectively suppressed

 Returning to its original form

After reaction cycles a catalyst with exactly the same nature is ‘reborn’ In practice a catalyst has its lifespan - it deactivates gradually during use

appliCation of Catalysis

Industrial applications

Almost all chemical industries have one or more steps employing catalysts

Petroleum, energy sector, fertiliser, pharmaceutical, fine chemicals, …..

Advantages of catalytic processes

 Achieving better process economics and productivity

 Increase reaction rates - fast  Simplify the reaction steps - low investment cost  Carry out reaction under mild conditions (e.g. low T, P) - low energy consumption

 Reducing wastes

Improving selectivity toward desired products - less raw materials required, less unwanted wastes Replacing harmful/toxic materials with readily available ones

 Producing certain products that may not be possible without catalysts  Having better control of process (safety, flexible etc.)  Encouraging application and advancement of new technologies and materials  And many more …

Environmental applications

 Pollution controls in combination with industrial processes

 Pre-treatment - reduce the amount waste/change the composition of

emissions

 Post-treatments - once formed, reduce and convert emissions  Using alternative materials

 Pollution reduction

 gas - converting harmful gases to non-harmful ones  liquid - de-pollution, de-odder, de-colour etc  solid - landfill, factory wastes

 And many more …

Other applications

Catalysis and catalysts play one of the key roles in new technology development.

• 1. The Catalyst is unchanged chemically at the

end of the reaction (Stability)

The amount of the catalyst should not be changed at the end of the process, unchanged chemically although it frequently changes physically. Example: Manganese dioxide catalyzes the potassium chlorate decomposition; changes from large crystals to fine powder (Physical change).

General CharaCteristiCs of Catalysts

A good catalyst should resist to deactivation, caused by

• the presence of impurities in feed (e.g. lead in petrol poison TWC.
• thermal deterioration, volatility and hydrolysis of active components
• attrition due to mechanical movement or pressure shock

• 2. A small amount of catalyst is often sufficient to

bring about a considerable extent of reaction

A small amount of catalyst will often cause large quantities of material to react. Example: Cupric ions (Cu2+) at a concentration of 1g ion in 109

liters accelerate the oxidation of sodium sulphite solution by atmospheric oxygen.

Example: Some catalysts need to be present in relatively large

amount to be effective. (Friedel-Crafts reaction), anhydrous AlCl3 functions as a catalyst effectively when present to the extent of 30% of the mass of benzene.

HCl H C H C Cl H C H C

AlCl

+   →  +

5 2 5 6 5 2 6 6

3

• 3. A catalyst does not affect the final position of

equilibrium, although it shortens the time required to establish the equilibrium

Example: Sulphur trioxide vapor decomposes readily in the presence of platinum, which is also the better catalyst for the combination

• f

sulphur dioxide and oxygen.

• A catalyst does not alter the

position

• f

equilibrium for a reversible process.

• It

must affect the direct and reverse reactions to the same extent.

• The ratio of the rates of two
• pposing

reactions i.e., the equilibrium constant, remains unchanged. Example: Haber Process

3 2 2

2 3 NH H N

Fe

→ ← +

• 4. The catalyst is specific in its action

While a particular catalyst works for one reaction, it will not necessarily work for another reaction. Different catalysts, moreover, can bring about completely different reactions for the same substance.

OH CH H CO O H CH H CO

ZnO Cu Ni 3 / 2 2 4 2

2 3    →  + + →  +

Example 1

nation) (dehydroge

• n)

(dehydrati

2 3 hot 5 2 2 2 2 5 2

3 2

H CHO CH OH H C O H CH CH OH H C

Cu O Al hot

+    →  + =    → 

Example 2

• 5. A catalyst provides an alternative route for the

reaction with lower activation energy

"A catalyst provides an alternative route for the reaction with lower activation energy“ It does not "lower the activation energy of the reaction"

Without cat With cat

Y

Product X

React

Energy Reaction coordinate

EA EA,cat

Catalysis Action: Reaction Kinetics and Mechanism

Catalyst action leads to change of the rate of the reaction by A)Forming complex with reactants/products, controlling the rate of elementary steps in the process. This is evidenced by the facts that

• The reaction activation energy

is altered

• The intermediates formed are

different from those formed in non-catalytic reaction

• The rates of reactions are

altered (both desired and undesired ones) B) Reactions proceed under less demanding conditions . Allow reactions occur under a milder conditions, e.g. at lower temperatures for those heat sensitive materials

• 6. A catalyst is more effective when finely divided

A solid catalyst should have reasonably large surface area needed for reaction (active sites). This is usually achieved by making the solid into a porous structure. In heterogeneous catalysis, the solid catalyst is more effective when in a state of fine subdivision than it is used in bulk. A lumpﺔﻌﻄﻗةﺮﯿﺒﻛ

• f platinum will have much less

catalytic activity than colloidal or platinised asbestos. Finely divided nickel is a better catalyst than lumps of solid nickel.

Typ ypes o es of Ca Catalyst

A catalyst can be gas, liquid or solid

CLASSIFICATION BASED ON THE PHYSICAL STATE CLASSIFICATION BASED ON THE SUBSTANCES FROM WHICH A CATALYST IS MADE

Inorganic (gases, metals, metal oxides, inorganic acids, bases etc.) Organic (organic acids, enzymes etc.)

CLASSIFICATION BASED ON THE CATALYSTS ACTION

Acid – base Catalysts Enzymatic Catalysts Photocatalysts Electrocatalysts etc.

Classification of Catalytic Reactions Classification based on the ways catalysts work

Heterogeneous Homogeneous Enzymatic

No phase boundary exists Phase Boundary separates catalyst from the reactants Complex organic molecules, usually protein, which form a lyophilic colloid

both catalyst and all reactants / products are in the same phase (gas or liq) reaction system involves multi- phase (catalysts + reactants / products)

Research in catalysis involve a multi-discipline approach

Reaction kinetics and mechanism

 Reaction paths, intermediate formation & action, interpretation of results obtained

under various conditions, generalising reaction types & schemes, predict catalyst performance

Catalyst development

 Material synthesis, structure properties, catalyst stability, compatibility

Analysis techniques



Detection limits in terms of dimension of time & size and under extreme conditions (T, P) and accuracy of measurements, microscopic techniques, sample preparation techniques

Reaction modelling

 Elementary reactions and rates, quantum mechanics/chemistry, physical chemistry

Reactor modelling



Mathematical interpretation and representation, the numerical method, micro- kinetics, structure and efficiency of heat and mass transfer in relation to reactor design

Catalytic process

 Heat and mass transfers, energy balance and efficiency of process

Resea esearch in in Ca Catalysis is

 A catalytic reaction can be operated in a batch manner

• Reactants and catalysts are loaded together in reactor and

catalytic reactions (homo- or heterogeneous) take place in pre-determined temperature and pressure for a desired time / desired conversion

• Type of reactor is usually simple. The basic requirements

 Withstand required temperature & pressure  Some stirring to encourage mass and heat transfers  Provide sufficient heating or cooling  Catalytic reactions are commonly operated in a continuous manner

• Reactants, are mostly in gas phase or liquid phase

 reactant fed to reactor in steady rate (e.g. mol/h, kg/h, m3/h)  easy transportation  The heat & mass transfer rates in gas phase is much faster

than those in liquid

Ca Catalytic ytic Rea eactio ion P Processes cesses

• Usually a target conversion is set for the reaction, based on this

target  required quantities of catalyst is added  required heating or cooling is provided  required reactor dimension and characteristics are designed.

• Catalysts are pre-loaded, when using a solid catalyst, or fed

together with reactants when catalyst & reactants are in the same phase and pre- mixed

• It is common to use solid catalyst because of its easiness to

separate catalyst from unreacted reactants and products

• With pre-loaded solid catalyst, there is no need to transport

catalyst which is then more economic and less attrition of solid catalyst

• In some cases catalysts has to be transported because of need of

regeneration  In most cases, catalytic reactions are carried out in a fixed-bed reactor (fluidised-bed in case of regeneration being needed), with the reactant being gases or liquids

 Active phase . Where the reaction occurs (mostly metal/metal oxide)  Promoter

• Textual promoter (e.g. Al - Fe for NH3 production)
• Electric or Structural modifier
• Poison resistant promoters

 Support / carrier

• Increase mechanical strength
• Increase surface area (98% surface area is supplied within the

porous structure)

• may or may not be catalytically active

Support / Carrier

Solid id Ca Catalysts Co Compositio sition

Catalyst

 Alumina

• Inexpensive
• Surface area: 1 ~ 700 m2/g
• Acidic

 Silica

• Inexpensive
• Surface area: 100 ~ 800 m2/g
• Acidic

 Zeolite

• mixture of alumina and silica
• often exchanged metal ion present
• shape selective
• Acidic

Active carbon (SBET up to 1000 m2/g) Titania (S BET 10 ~ 50 m2/g) Zirconia (S BET 10 ~ 100 m2/g) Magnesia (S BET 10 m2/g) Lanthana (S BET 10 m2/g)

Active site pore

porous solid

Type pes o

• f

f So Soli lid Supp Support / / Car arrier

Some common solid support / carrier materials Other supports

 Precipitation To form non-soluble precipitate by desired reactions at certain pH and temperature  Adsorption & ion-exchange Cationic: S-OH+ + C+ → SOC+ + H+ Anionic: S-OH- + A- → SA- + OH- Ion-exch.:S-Na+ + Ni 2+ → S-Ni 2+ + Na+  Impregnation Fill the pores of support with a metal salt solution of sufficient concentration to give the correct loading.  Dry mixing Physically mixed, grind, and fired

precipitate

• r deposit

precipitation filter & wash the resulting precipitate

Drying & firing

precursor add acid/base solution with pH control Support Support

Drying & firing

• Soln. of metal Pore saturated

precursor pellets

Pre reparation

• n Me

Methods hods of

• f Sol
• lid

d Catalysts

Amount adsorbed Concentration

Drying & firing

Support

• Catalysts need to be calcined

(fired) in order to decompose the precursor and to received desired thermal stability.

• Catalysts need to Pre-treatments. The commonly used Pre-

treatments

• Reduction: if elemental metal is the active phase
• Sulphidation: if a metal sulphide is the active phase
• Activation: some catalysts require certain activation steps in
• rder to receive the best performance.
• Typical catalyst life span can be

many years or a few mins.

75 50 25 100 500 600 700 800 900

T emperature °C BET S.A. m2/g

40 10

Time / hours BET S.A. Activity Time Normal use Induction period dead

Preparation of Catalysts

• Reactants and the catalyst of different phases
• The catalyst is mostly solid

1.1 Catalyst Preparation

• The higher the active surface area of the catalyst, the greater the

number of product molecules produced per unit time.

• Transition metal ions or atoms are deposited in the micropores of

suitable support (surface area = 100-400 m2/g), which are then heated and reduced to produce small metal particles 10-102 A in size with virtually all the atoms located on the surface (unity dispersion).

• Additives that are usually electron donors (alkali metals) or

electron acceptors (halogens) are adsorbed on the metal or on the

• xide to act as bonding modifiers for the coadsorbed reactants.
• 1. Heterogeneous Catalysis

Monolith substrate

Al2O3 washcoat Pore 100A

Monolith wall

Pt catalytic sites

 The long journey for reactant molecules to

• 1. travel within gas phase
• 2. cross gas-liquid phase boundary
• 3. travel within liquid phase/stagnant

layer

• 4. cross liquid-solid phase boundary
• 5. reach outer surface of solid
• 6. diffuse within pore
• 7. arrive at reaction site
• 8. be adsorbed on the site and activated
• 9. react with other reactant molecules

 Product molecules must follow the same track in the reverse direction to return to gas phase  Heat transfer follows similar track

1 7 8 9

gas phase

pore

porous solid liquid phase / stagnant layer

2 3 4 5 6

gas phase reactant molecule

Heterogeneous Catalytic Reaction Process

Diffusion from the surface Diffusion of the reactants to the surface

E1=2-4

A surface reaction between reactants to give a desired product

E3>10 E5=2-4

Monolith substrate

CO+O2 C O O O CO2

E = Kcal/mol

CO2 1.2. Kinetics of Heterogeneous Catalytic reactions