the production of thymoquinone from thymol and carvacrol

CHAPTER 1
INTRODUCTION
The development of efficient catalysts for the oxidation of organic compounds in
environmental friendly conditions is an active field of research. Although homogeneous
metal complex catalysts exhibit good activity and selectivity, heterogenization of
homogeneous metal-complex catalysts by encapsulating them inside the zeolite
possesses the advantages of both homogeneous and heterogeneous counterpart like easy
separation, rigidity, site isolation effect (Maurya et al. 2002a). Site isolation of redox
active metal centers in zeolite cages can afford the catalysts higher activity by
separating redox metal centers in inorganic matrix (Sheldon et al. 1998). Heterogeneous
catalysts can be prepared by using ship in a bottle method (Arends et al. 2001). This
method involves assembling a metal complex in intrazeolite space such that the
complex, once formed, is too large to diffuse out. If leaching of zeolite-encapsulated
metal complexes does not occur under the reaction conditions, catalyst is said to be
completely heterogeneous. Recently, metal complexes of porphyrins, salen, salpn and
phthalocyanines have been encapsulated into zeolitic matrix for the development of
efficient oxidation catalysts (Skrobot et al. 2003, Nakagaki et al. 2000, Maurya et al.
2002, Shevade et al. 1999).However, the most important question of metal leaching was
not considered in these studies.
Chemical transformation of abundant and cheap products into novel and more
valuable compounds can be achieved by liquid-phase oxidation reactions using
hydrogen peroxide as clean oxidant (Martin et al. 1999). Hydrogen peroxide is one of
the preferred oxidants because it is easy to handle and its reaction produces only water
as by-product (Arends et al. 2001). Catalytic oxidation of aromatic monoterpenes with
hydrogen peroxide is a reaction of industrial importance (Monteiro et al. 2004).
Carvacrol is the example of p-menthane type aromatic monoterpene, which can be
found in the essential oils of several aromatic plants. Carvacrol and its geometrical
isomers thymol can be oxidized to thymoquinone, which has a commercial value
considerably higher than its precursor’s thymol and carvacrol. Thymoquinone has
antitumor and hepatoprotective activity (Badary et al. 1999). Since the natural resources
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