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