the production of thymoquinone from thymol and carvacrol

homogeneous internal structures which consisting of uniform, well-defined cavities and
channels of molecular dimensions. Confinement of the redox active site in channels or
cavities can endow the catalyst with over higher activity than the conventional catalyst
(Arends et al. 2001, Sheldon et al. 1998).
Confinement of the redox active sites in channels provides site-isolation of
active metal ions or complexes in inorganic matrices. This prevents their dimerization
or oligomerization to less reactive species. Site-isolation of different redox metal centres
in inorganic matrices can afford oxidation catalysts with higher activities (Arends et al.
2001).
A major disadvantage of this approach is the mobility of the metal ion (leaching
into solution). This generally involves solvolysis of M-O-surface bonds by reaction with
polar solvents. Since the products of oxidation reactions are usually polar molecules,
leaching will be a problem. And also the usage of mesoporous molecular sieves, such as
USY zeolites and MCM-41 suffers from leaching as an effect of the pores being open to
the external surface. The synthetic utility of solid catalysts stands or falls with their
stability towards leaching. (Arends et al. 2001, Sheldon et al. 1998, Kaduk et al. 1995).
Various methods as described in Figure 2.2 can be employed for immobilizing
redox active elements in a solid inorganic matrix. These are; (a) substitution of metals in
framework of silicalites, zeolites, aluminaphosphate (AlPOs) and silica-
aluminaphosphates (SAPOs) (b,c) grafting and tethering of metal complexes to the
internal surface of the molecular sieves (d) encapsulation of metal complexes in
intrazeolite space, ship in a bottle catalyst as seen in Figure 2.2.
(a) (b) (c) (d)
Figure 2.2. Strategies for heterogenization of metal centre (M) (a) Framework
substituted (b) Grafted (c) Tethered (d) Ship in a bottle
(Source: Arends et al. 2001)
8