the production of thymoquinone from thymol and carvacrol

is the simplest and most direct method. Pores are filled with a solution of metal salt of
sufficient concentration to give correct loading. Ion exchange is the reversible
interchange of ions between a solid and a liquid. There is no permanent change in the
structure of the solid, which is the ion exchange material. Usually, contacting a zeolite
with a salt solution of different cation performs ion exchange; one type of cation is
replaced with other.
The ion exchange process:
zAB zB (z)+ zBA zA (s)+ zAB zB (s)+ zBA zA (z)
Where zA and zB are the charges of the exchange cation A and B and the subscript z and
s refer to the zeolite and solution, respectively.
A widely recognized modification of catalyst is to replace the Na + ions with
other metal ions and then reduce them in situ so that the metal atoms are deposited
within the framework. The resultant material displays the properties associated with a
supported metal catalyst. Ion exchanged cation can induce new catalytic features in
many reactions especially in liquid phase reactions, by their different size and chemical
structure (Sheldon et al. 1998).
2.5 Design of Stable Heterogeneous Catalysts
Liquid phase oxidation is widely used in bulk chemical manufacture. It is
becoming increasingly important in the synthesis of fine chemical (Arends et al. 2001).
Catalytic oxidations in the liquid phase generally employ soluble metal salts or
complexes in combination with clean, inexpensive oxidants such as O2, H2O2, or RO2H.
Heterogeneous catalysts are widely used in liquid phase oxidation reactions because of
their advantages.
For heterogeneous catalysts; incorporation of the redox metal centre into the
surface of an oxide support is the conventional methods for the immobilizing metal
catalyst. Such as heterogeneous Ti IV /SiO2 catalysts which is used for the epoxidation of
the propylene (Arends et al. 2001; Sheldon et al. 1998).
Another approach to design stable solid catalysts is confinement of redox metal
ions or complexes into the framework or cavities of zeolites, redox molecular sieves.
Unlike conventional supported catalysts; they have a regular microenvironment with
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