Industrial catalysis

Lec 8 week 11

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.

What is catalysis

• 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.

– 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 or 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.

Turnover Frequency or Specific Activity

• where NAv is Avogadro’s number (6.023 *1023 molecules/g mole) and S represents the number of sites in the experimental system and can be represented as:

S=A* L• where L is the number 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

• 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

Activity(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 equilibria.

Activity

(3) Activation energy Ea

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

Selectivity• The selectivity (Sp) of a reaction: 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 crucial 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 catalystsare 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 catalysisPetroleum, 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- 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