Preparation of heterogeneous catalyst

lec 11 week 14

• Industrial catalysts are generally shaped bodies of various forms, e. g., rings, spheres, tablets, pellets.

• The production of heterogeneous catalysts consists of numerous physical and chemical steps.

• The conditions in each step have a decisive influence on the catalyst properties.

• the main physical properties of a catalyst that are influenced by the production Conditions are: Active surface area; pore structure; mechanical strength.

Components of a Typical Heterogeneous Catalyst

A. Active phase - metal that provides active sites where thechemical reaction takes place

B. Support or Carrier - high surface area oxide whichdisperses and stabilizes the active phase

(adds efficiency, physical strength, sometimes selectivity)

C. Promoter(s) - additive which improves catalyst properties, e.g. activity, selectivity, catalyst life

Bulk catalysts• Bulk catalysts also known as

precipitated catalysts are mainly produced when the active components are cheapactive components are cheap.

• The preferred method of production is precipitation.

• One or more components in the form of aqueous solutions are mixed and then co precipitated as hydroxides or precipitated as hydroxides or carbonatescarbonates. An amorphous or crystalline precipitate or a gel is obtained, which is washed thoroughly until salt free. This is then followed by further steps: drying, shaping, calcination, and activation

Preparation of bulk catalyst

Typical examples of precipitated catalysts are:

Impregnated catalyst ( support catayst)

• One of the best known methods for producing catalysts is the impregnation of porous support materials with solutions of active components.

• Especially catalysts with expensive active with expensive active components such as noble metals are components such as noble metals are employed as supported catalysts.employed as supported catalysts.

• A widely used support is Al2O3, Activated carbon and silica gel.

Preparation of catalyst by impregnation

• Impregnation as a means of supported catalyst preparation is achieved by filling the pores of a support with a solution of the metal salt.

• The catalyst is prepared either by spraying the support with a solution of the metal compound or by adding the support material to a solution of a suitable metal salt, This is then followed by drying and subsequent decomposition of the salt at an elevated temperature, either by thermal decomposition or reduction.

impregnation• The support is immersed in

a solution of the active component under precisely defined conditions (concentration, mixing, temperature, time). Depending on the production conditions, selective adsorption of the active component occurs on the surface or in the interior of the support. The result is non uniform distribution.

Supported metal catalyst

The advantages of impregnated catalysts compared with precipitated catalysts

• Pore structure and surface of the catalyst can be controlled.

• More economic, since the content of expensive active components is often low.

• The distribution and crystallite size of the active components can generally be varied over a wide range.

• Multiple impregnation is possible.

Examples of industrial impregnated catalysts are:

– Ethylene oxide catalysts in which a solution of a silver salt is applied to Al2O3

– Catalysts in the primary reformer of ammonia synthesis, with 10–20 % Ni on Al2O3

– Catalysts for the synthesis of vinyl chloride from acetylene and HCl: HgCl2/activated carbon; HgCl2 is applied from aqueous solution

Dry mixing

• Physically mixed, grind, and calcination