the production of thymoquinone from thymol and carvacrol

7.2.2. Oxidation of Carvacrol
Catalytic activities of prepared catalysts were evaluated for the oxidation of
carvacrol with hydrogen peroxide by using HPLC analysis technique. Table 7.6
summarizes the results i.e., percentage of carvacrol conversion and products formed
along with their yield. Yield and conversion values were determined from carvacrol and
thymoquinone calibration curves which were shown in Appendix B. It was known that
encapsulation of metal ions in zeolites could result in unusual oxidation states/electronic
configurations and consequent catalytic activity. Hydrogen peroxide has advantage of
high mobility in the pores of zeolite system due to its smaller size. Moreover hydrogen
peroxide is cheaper and sufficiently environment-friendly. Hydrogen peroxide alone
was unable to oxidize carvacrol to a significant extent. With NaY zeolite, no significant
carvacrol oxidation reactions were investigated in this study, indicating that Y zeolite
was inactive under the reaction conditions (Table 7.6). Oxidation of carvacrol cannot
occur unless a catalyst promotes the reaction. This observation was in agreement with
the observations made by several researchers (Skrobot et al. 2003; Xavier et al. 2004).
To determine the performance of the catalysts, the percentage of carvacrol conversion
was plotted as a function of time at two different carvacrol-to-hydrogen peroxide molar
ratios. The results of carvacrol oxidation reactions at carvacrol-to-hydrogen peroxide
molar ratios of 3 and 1 were seen in Figures 7.8 and 7.9, respectively. As seen from
these figures all catalysts have shown activity for the oxidation of carvacrol.
The catalytic oxidation of carvacrol was expected to give mixture of
benzoquinones (Santos et al. 2003) as shown by Eq. (7.1). According to the literature,
the quinone may result from the oxidation of a hydroquinone intermediate (Martin et al.
1999, Milos et al. 2001, Santos et al. 2003, Martin et al. 2001). The formation of TQ
from carvacrol was a result of the selective hydroxylation of the aromatic ring in the
para position relatively to the OH group, followed by the subsequent oxidation of the
resulting hydroquinone to the quinone stage. This hydroquinone derivative might arise
from a protonated intermediate, the p-hydroxylated species. Such intermediate was
stabilized by the OH present in the carvacrol (Eq 7.1) (Martin et al. 1999).
For the catalytic systems investigated here thymoquinone (TQ),
thymohydroquinone (THQ) and other benzoquinones (BQ) appeared successively as
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