preparation of hetrogenous catalyst lec 6

8/2/2019 Preparation of Hetrogenous Catalyst Lec 6

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Preparation of heterogeneous

catalyst

lec 6


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Industrial catalysts are generallyshaped bodies of various forms, e.g., rings, spheres, tablets, pellets.

The production of heterogeneouscatalysts consists of numerousphysical and chemical steps.

The conditions in each step have adecisive influence on the catalystproperties.

the main physical properties of acatalyst that are influenced by the

production Conditions are: Activesurface area; pore structure;mechanical strength.


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Components of a Typical

Heterogeneous Catalyst

A. Active phase - metal that provides active sites where the

chemical 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


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Bulk catalysts

Bulk catalysts also known asprecipitated catalysts are

mainly produced when theactive components are cheap.

The preferred method ofproduction is precipitation.

One or more components inthe form of aqueous solutionsare mixed and then coprecipitated as hydroxides orcarbonates. An amorphous orcrystalline precipitate or a gel

is obtained, which is washedthoroughly until salt free. Thisis then followed by furthersteps: drying, shaping,calcination, and activation


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Preparation of bulk catalyst


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Typical examples of precipitated catalysts

are:


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Impregnated catalyst ( support catayst)

One of the best known methods for producingcatalysts is the impregnation of poroussupport materials with solutions of active

components. Especially catalysts with expensive active

components such as noble metals areemployed as supported catalysts.

A widely used support is Al2O3, Activatedcarbon and silica gel.


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Preparation of catalyst by

impregnation

Impregnation as a means of supported catalystpreparation is achieved by filling the pores of asupport with a solution of the metal salt.

The catalyst is prepared either by spraying thesupport with a solution of the metal compoundor by adding the support material to a solution ofa suitable metal salt, This is then followed by

drying and subsequent decomposition of the saltat an elevated temperature, either by thermaldecomposition or reduction.


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impregnation

The support is immersed ina solution of the activecomponent under preciselydefined conditions

(concentration, mixing,temperature, time).Depending on theproduction conditions,selective adsorption of the

active component occurs onthe surface or in the interiorof the support. The result isnon uniform distribution.


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Supported metal catalyst


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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.


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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


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Dry mixing

Physically mixed, grind, and calcination

preparation of hetrogenous catalyst lec 6

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