PE333 Chemical reaction &industrial catalysis

(Part 2)

Dr/Marwa Abdelfattah

Fall 2013-2014

Part (2)Industrial catalysis

Lec.1

Recommended text books:• Industrial Catalysis, A practical approach, Second edition. By

Jens Hagen• Kinetics of catalytic reactions, by M. Albert Vannice, Springer,

2005.

 

Course Description:

 

Principles of the industrial utilization of heterogeneous catalysis, topics include adsorption phenomena, methodology in catalyst preparation, characterization and evaluation of catalysts, diffusion and reaction in porous catalysts, and a survey of major industrial processes.

Course Objectives:

This course is designed to give the students:

• Understanding the Mode of Action of Catalysts, classification of Catalysts and Comparison of Homogeneous and Heterogeneous Catalysis

• Knowledge of the fundamental of heterogamous catalysts.  • Knowledge of the adsorption phenomena. • Knowledge of the methodology in catalyst preparation.• Knowledge of the characterization and evaluation of catalysts• Knowledge of the diffusion and reaction in porous catalysts.• Focus on the major industrial processes

Course outcomes:

At the end of this course, the students should be able to:• Classify the catalysts and Compare between Homogeneous

and Heterogeneous Catalysis.• Describe the adsorption phenomena and determine the

adsorption isotherm.• Define the different methods used for preparation of catalyst.• Describe the different techniques used to characterize the

catalyst.

What is catalysis?• The term “catalysis” was introduced as early as

1836 by Berzelius in order to explain various decomposition and transformation reactions that includes catalyst.

• Catalysis is the phenomenon in which a relatively small amount of a foreign material, called a catalyst, increases the rate of a chemical reaction without itself being consumed.

• A catalyst leads the rate of reaction to change, so we can say that catalyst accelerates the chemical reaction.

The catalysis is a cyclic process:

The reactants are bound to one form of the catalyst, and the products are released from another, regenerating the initial state.

Mode of Action of Catalysts• General requirements for a good catalyst

Activity –being able to promote the rate of desired reactions

Selectivity- being able to promote only the rate of desired reaction and also retard the undesired reactions.

Stability – a good catalyst should resist to deactivation caused by:

• The presence of impurities in feed• Thermal deterioration, volatility and hydrolysis of active

components• Alteration due to mechanical movement or pressure shock.

Note: A solid catalyst should have reasonable large surface area needed for reaction (active sites). This is usually achieved by making the solid into porous structure

Turnover Frequency(TOF) (Specific Activity)

• When the reaction rate is normalized to the surface area of the active component in the catalyst, such as the metal surface area, it is frequently referred to as the specific activity.

• If the reaction rate is normalized to the number of surface metal atoms present, or to another specified type of site that has been counted by some stated method, then a turnover frequency (TOF), based usually on a specified reactant, is obtained.

• where :• NAv is Avogadro’s number (6.023 *1023 molecules/g mole)

• S represents the number of sites in the experimental system and can be represented as: S=A* L

Where[ L is the density of sites (per unit area, such as cm2)]. • A TOF has units of reciprocal time and is typically expressed

as [s-1]

Turn over number(TON)• The turn over number (TON) specifies the maximum

use that can be made of a catalyst for a special reaction under defined conditions.

• The relation between TOF and TON is as follows:

Activity• Activity is a measure of how fast one or more

reactions proceed in the presence of the catalyst.

• There are three possibilities for expressing catalyst activity:

(1) Reaction rate

(2) Rate constant (k)

Kinetic activities are derived from the fundamental rate laws, for example, for a simple irreversible reaction

A P

k = rate constantf (cA) is a concentration term that can exhibit a first- or higher order

dependence on adsorption equilibrium.

(3) Activation energy( Ea)

The temperature dependence of rate constants is given by the Arrhenius equation:

Selectivity(Sp)• The selectivity of a reaction is

the fraction of the starting material that is converted to the desired product P.

• It is expressed by the ratio of the amount of desired product to the reacted quantity of a reaction partner A and therefore gives information about the course of the reaction.

Stability• The chemical, thermal, and mechanical stability of a

catalyst determines its lifetime.

• In industrial reactors. Catalyst stability is influenced by numerous factors, including: decomposition, coking, and poisoning.

• Catalysts that lose activity during a process can often be regenerated before they ultimately have to be replaced. The total catalyst lifetime is of the most importance for the economics of a process.

Types of catalysts• Classification based on its physical state.

The catalyst can be :

- Gas

- Liquid

- Solid• Classification based on the substances from which a catalyst is made

- Inorganic

- Organic• Classification based on the ways catalysts work

- Homogenous

- Heterogeneous• Classification based on the catalysts action

- Acid base catalysts

- Enzymatic

- Photocatalysis

- Electro-catalysis

Homogeneous catalysis

• Catalytic processes that take place in a uniform gas or liquid phase are classified as homogeneous catalysis.

• It includes only one phase.• Examples are: mineral acids

Heterogeneous catalysis

• Heterogeneous catalysis takes place between several phases. Generally the catalyst is a solid, and the reactants are gases or liquids.

• Examples of heterogeneous catalysts

are Pt/Rh nets for the oxidation of ammonia to nitrous gases (Ostwald process).

Applications of catalysis• Industrial applications

Almost all chemical industries have one or more steps employing catalysis

Petroleum, energy sectors, fertilizer, pharmaceutical

• Advantages of catalytic process:- Achieving better process (economics and productivity)

- Increase reaction rate (fast)

- Simplify the reaction steps (low investments cost)

- Carry out reaction under mild conditions (low T, P)

- Reducing wastes by:

- Improving selectivity toward desired products-less raw materials required- less unwanted wastes

- Replacing harmful materials with readily available ones

- Producing certain products that may not be possible without catalysts

- Having better control of process (safety,)

Comparison between homogenous and heterogeneous catalysts