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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e 11.6% with a small amount <strong>of</strong> THQ. Zn, Ni <strong>and</strong> Bi catalysts were more selective for<br />
<strong>the</strong> TQ formation compared to Fe <strong>and</strong> Cr based catalysts. However, <strong>the</strong> leached metal<br />
ions detected by ICP technique in <strong>the</strong> reaction medium catalyzed <strong>the</strong> <strong>carvacrol</strong> oxidation<br />
reaction homogeneously. Therefore, it was difficult to call <strong>the</strong>se catalysts as complete<br />
heterogeneous catalysts due to significant metal ion leaching <strong>from</strong> catalysts to reaction<br />
medium. For Zn, Ni <strong>and</strong> Bi, Fe, catalysts <strong>the</strong> presence <strong>of</strong> metal ions in <strong>the</strong> reaction<br />
mixture after 5 hours was detected by ICP analysis. The percentage <strong>of</strong> leached metal<br />
ions to <strong>the</strong> reaction medium was 25.2%, 19%, 64%, 10% for Zn, Ni <strong>and</strong> Bi, Fe catalysts<br />
respectively (reaction conditions: <strong>carvacrol</strong>/H2O2 molar ratio=3, 0.1 g catalyst, 60 ºC).<br />
For Cr catalyst no metal ion was detected by ICP technique. Since Cr <strong>and</strong> Fe-based<br />
catalysts have shown better catalytic activity than <strong>the</strong> o<strong>the</strong>rs we decided to examine <strong>the</strong><br />
heterogeneities <strong>of</strong> <strong>the</strong>se catalysts (in section 7.2.3) by hot filtration technique fur<strong>the</strong>r<br />
<strong>and</strong> <strong>the</strong> results were presented later in <strong>the</strong> text. As seen <strong>from</strong> Figure 7.8 conversion <strong>of</strong><br />
<strong>carvacrol</strong> increased with time for all catalysts, but reached a steady state at different<br />
times. The maximum conversion <strong>of</strong> about 14.2% was observed with Cr(salpn)-NaY<br />
after a period <strong>of</strong> 1.5 hours. For Fe(salpn)-NaY 12.7% <strong>carvacrol</strong> conversion was<br />
observed within 2 hours. Rest <strong>of</strong> <strong>the</strong> catalysts recorded lower conversions than<br />
Fe(salpn)-NaY, Cr(salpn)-NaY catalysts.<br />
As seen <strong>from</strong> Table 7.6, maximum conversions were achieved by using<br />
Cr(salpn)-NaY <strong>and</strong> Fe(salpn)-NaY catalyst for both <strong>carvacrol</strong>-to-hydrogen peroxide<br />
molar ratios <strong>of</strong> 3 <strong>and</strong> 1. When <strong>the</strong> <strong>carvacrol</strong>-to-hydrogen peroxide molar ratio was<br />
decreased to 1 (i.e.; increasing hydrogen peroxide) again, <strong>the</strong> oxidation <strong>of</strong> <strong>carvacrol</strong><br />
gave <strong>thymoquinone</strong> with close to 100% selectivity for Zn, Ni <strong>and</strong> Bi catalysts. The<br />
selectivity towards <strong>thymoquinone</strong> decreased with increasing <strong>carvacrol</strong> conversion for<br />
Fe(salpn)-NaY <strong>and</strong> Cr(salpn)-NaY catalysts. The decrease in selectivity was due to <strong>the</strong><br />
two more components observed in <strong>the</strong> reaction mixture <strong>and</strong> no attempts were made to<br />
identify <strong>the</strong>m. Some <strong>of</strong> <strong>the</strong>m were probably higher molecular weight compounds<br />
(7.1)<br />
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