540
3.1.1 Catalysts Synthesis by Wet Synthesis Methods
Conventionally catalysts are synthesized by wet synthesis methods. These methods
include precipitation, co-precipitation, impregnation, sol gel, incipient wetness and
hydrothermal synthesis. The precipitation method of catalyst manufacture involves
the formation of a new solid phase (catalyst particle precipitate) from a homogeneous liquid solution by nucleation and subsequent agglomeration of particles. The
precursor (usually nitrate salts of the active metal) is dissolved in a solvent (usually
deionised water) to form a homogeneous solution. The predominant methods
employed to create supersaturation conditions necessary to form catalyst particle
precipitates include change in pH by addition of acids or bases, addition of complexing agents and solvent evaporation [41, 42]. The co-precipitation method refers
to the preparation of multi-component precipitates from a homogeneous solution of
several precursors. The precipitate obtained can be a single solid compound consisting of the several metallic components in the precursor solution which may be the
active catalyst or the precursor to the active catalyst which is formed by subsequent
treatment [41, 42]. Several component precipitates which are highly inter-dispersed
in a homogeneous solid phase (solid solution) are also formed by co- precipitation [43].
Impregnation involves the deposition of dissolved aqueous precursors on a preformed support by contacting the preformed support and the active component precursor solution. This can be achieved by immersion of the catalyst support in the
precursor solution or spraying of the precursor solution onto the catalyst support.
After some contact time, the support is separated from the precursor solution and
the excess solvent imbibed by the support is removed by drying. When the volume
of the precursor solution contacted with the porous support is equal or less than the
pore volume of the porous support, the catalyst preparation method is called incipient wetness method [41, 42].
In contrast with the co-precipitation method, which is a discontinuous transformation, the sol-gel method is a homogeneous process which results in a continuous
transformation of a precursor solution into a hydrated porous solid structure. The
sol-gel method is based on the controlled hydrolysis and condensation of typically
metal alkoxide precursors in an organic medium, usually, by addition of water and
carefully controlling pH and temperature to form a sol, and subsequent polymerisation to form a porous solid gel with entrapped solutions of condensation by- products
(typically water or alcohol) [41, 44–46]. The gel is further treated to obtain the catalytic material.
The hydrothermal synthesis method involves precipitate or gel formation from a
mixture of dissolved precursors with additives such as structure directing agents at
elevated temperatures under autogenous pressure in an autoclave for a determined
time. This method is successful for the synthesis of metal oxides, mixed metal oxide
and zeolite catalysts [47, 48].
Subsequent treatment steps which may include ageing, filtration, washing, drying, forming and calcination are implemented to obtain the catalytic material in its
useful form. These final treatment steps also impact useful characteristics to the
catalyst. Ageing involves stirring the precipitate or the gel in its mother liquor
O. Okonkwo and P. Biswas
3.1.1 Catalysts Synthesis by Wet Synthesis Methods
Conventionally catalysts are synthesized by wet synthesis methods. These methods
include precipitation, co-precipitation, impregnation, sol gel, incipient wetness and
hydrothermal synthesis. The precipitation method of catalyst manufacture involves
the formation of a new solid phase (catalyst particle precipitate) from a homogeneous liquid solution by nucleation and subsequent agglomeration of particles. The
precursor (usually nitrate salts of the active metal) is dissolved in a solvent (usually
deionised water) to form a homogeneous solution. The predominant methods
employed to create supersaturation conditions necessary to form catalyst particle
precipitates include change in pH by addition of acids or bases, addition of complexing agents and solvent evaporation [41, 42]. The co-precipitation method refers
to the preparation of multi-component precipitates from a homogeneous solution of
several precursors. The precipitate obtained can be a single solid compound consisting of the several metallic components in the precursor solution which may be the
active catalyst or the precursor to the active catalyst which is formed by subsequent
treatment [41, 42]. Several component precipitates which are highly inter-dispersed
in a homogeneous solid phase (solid solution) are also formed by co- precipitation [43].
Impregnation involves the deposition of dissolved aqueous precursors on a preformed support by contacting the preformed support and the active component precursor solution. This can be achieved by immersion of the catalyst support in the
precursor solution or spraying of the precursor solution onto the catalyst support.
After some contact time, the support is separated from the precursor solution and
the excess solvent imbibed by the support is removed by drying. When the volume
of the precursor solution contacted with the porous support is equal or less than the
pore volume of the porous support, the catalyst preparation method is called incipient wetness method [41, 42].
In contrast with the co-precipitation method, which is a discontinuous transformation, the sol-gel method is a homogeneous process which results in a continuous
transformation of a precursor solution into a hydrated porous solid structure. The
sol-gel method is based on the controlled hydrolysis and condensation of typically
metal alkoxide precursors in an organic medium, usually, by addition of water and
carefully controlling pH and temperature to form a sol, and subsequent polymerisation to form a porous solid gel with entrapped solutions of condensation by- products
(typically water or alcohol) [41, 44–46]. The gel is further treated to obtain the catalytic material.
The hydrothermal synthesis method involves precipitate or gel formation from a
mixture of dissolved precursors with additives such as structure directing agents at
elevated temperatures under autogenous pressure in an autoclave for a determined
time. This method is successful for the synthesis of metal oxides, mixed metal oxide
and zeolite catalysts [47, 48].
Subsequent treatment steps which may include ageing, filtration, washing, drying, forming and calcination are implemented to obtain the catalytic material in its
useful form. These final treatment steps also impact useful characteristics to the
catalyst. Ageing involves stirring the precipitate or the gel in its mother liquor
O. Okonkwo and P. Biswas