CHL727(3-0-2):Heterogeneous Catalysis and Reactor Design

Prof. K.K.PantDepartment of Chemical Engineering

IIT Delhi.kkpant@chemical.iitd.ac.in

L 3

Why R& D in catalysis is important

• 90 % of chemical industry involve products made

using catalysts (food, fuels, polymers, textiles,

pharma/ agrochemicals,etc)

• For discovery/use of alternate sources of energy/fuels/

raw material for chemical industry.

• For Pollution control-Global warming.

• For preparation of new materials (organic & inorganic-

eg: Carbon Nanotubes).

3

WHAT DOES A CATALYST LOOK LIKE

4

5

6

Turnover Frequency TOF

The turnover frequency TOF quantifies the specific activity of a catalytic center for a special reaction under defined reaction conditions by the number of molecular reactions or catalytic cycles occurring at the center per unit time.For most relevant industrial applications the TOF is in the range

10–2 to 102 s–1 (enzymes 103–107 s–1).

TOF (time -1) = volumetric rate of reaction/number of centers /volume

• The reaction activation energy is altered

• The intermediates formed are different from those formed in non-catalytic reaction

• The rates of reactions are altered (bothdesired and undesired ones)

– Reactions proceed under less demanding conditions

• Allow reactions occur under a milder conditions, e.g. at lower temperatures for those heat sensitive materials

• It is important to remember that the use of catalyst DOES NOT vary DG & Keq values of the reaction concerned, it merely change the PACE of the process.

7

Action of Catalysts

oWhether a reaction can proceed or not and to what extent a reaction can proceed is solely determined by the reaction thermodynamics, which is governed by the values of dG & Keq, NOT by the presence of catalysts.

o The reaction thermodynamics provide the driving force for a rxn; the presence of catalysts changes the way how driving force acts on that process.

8

e.g CH4(g) + CO2(g) = 2CO(g) + 2H2(g)

DG°373=151 kJ/mol (100 °C) & DG°973

=-16 kJ/mol (700 °C)

=>At 100°C, DG°373=151 kJ/mol > 0. There is no thermodynamic driving force, the reaction won’t proceed with or without a catalyst

9

=>At 700°C, DG°373= -16 kJ/mol < 0.

The thermodynamic driving force is required. However,

simply putting CH4 and CO2 together in a reactor does not

mean they will react. Without a proper catalyst heating the

mixture in reactor results no conversion of CH4 and CO2 at

all. When Pt/ZrO2 or Ni/Al2O3 is present in the reactor at

the same temperature, equilibrium conversion can be

achieved (<100%).

10

Action of Catalysts• Catalysis action - Reaction kinetics and

mechanism Catalyst action leads to the rate of a reaction to change. This is realised by changing the course of reaction (compared to non-catalytic reaction)

• Forming complex with reactants/ products, controlling the rate of elementary steps in the process.

11

ACTIVITY MEASURE FOR CATALYST: For comparative measurements, such as catalyst screening, determination of process para-meters, optimization of catalyst production conditions, and deactivation studies – Conversion under constant reaction conditions– Space velocity for a given, constant conversion– Space–time yield– Temperature required for a given conversion

12

13

The suitability of a catalyst for an industrial process depends mainly on the followingthree properties:

– Activity– Selectivity– Stability (deactivation behavior)

Which of these functions is the most important is difficult toanswer because the demands made on the catalyst are different for each process.

Activity : how fast one or more reactions proceed in the presence of the catalyst. Activity can be defined in terms of kinetics or from a more practically oriented viewpoint. In a formal kinetic treatment, it is appropriate to measure reaction rates in the temperature and concentration ranges that will be present in the reactor.

Mode of Action of Catalysts

Heterogeneous Catalysis:right specieswith the right coverageat the right temperatureon the right surface

Fundamental Concepts of Catalysis

• Catalysis is a cycle; Sabatier’s Principle•Catalysts break bonds!….How?•Elementary surface reactions (NO + CO reaction)•CO oxidation is structure sensitive (every reaction is…)•Dissociation is the key step•Elementary rate parameters •Lateral interactions affect the kinetics•Reaction design?

Popular Representation of Catalysis

optimum interaction catalyst -adsorbate:not too strongnot too weak

optimum coverage at right surface

The Sabatier Effect

Applications 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

21

Applications 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

22

Research in Catalysis

• 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…

23

Industrialecology

Greenengineering

Greenchemistry

Renewableenergy

Practical approaches

Sustainabledevelopment

Strategic goal

Life-cycleassessment

Catalysis

Wastemanagement

E-factor,atom economy

Process intensification

Operational tools

Monitoring tools

The picture of green catalysis