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ARTICLE IN PRESS
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A. Wróblewska et al. / Catalysis Today xxx (2015) xxx–xxx
formation of 1,2-epoxylimonene. During the secondary reactions
the formation of the following by-products is observed (very often
at the appropriate conditions these reactions can be the main reac-
tions of this process): 1,2-epoxylimonene diol – the product of the
hydration of the epoxide ring in 1,2-epoxylimonene molecule, 8,9-
epoxylimonene and its diol (epoxidation and next hydration of the
epoxide ring at the position 8–9), carveol – the product of the allylic
oxidation at the position 6, carvone – the product of the oxida-
tive dehydrogenation of carveol molecule, 1–2,8–9-diepoxide and
products of the oxidation of perillyl alcohol [4,5,7–11]. However,
the formation of perillyl alcohol as the predominant product was
observed mainly during oxidation of limonene in the presence of
acyclic allylic hydrogen abstraction during the oxidation of ter-
penes (3-carene and ␣-pinene – compounds with the structure
similar to limonene) and especially limonene was described by
Rothenberg [16] and Wróblewska [17], respectively. Not only 1,2-
are very valuable intermediates which are applied in the production
of: flavors, perfumes, cosmetics, food additives [4,6,7], agrochemi-
cals, special fragrant polymers [18,19] and drugs used in the curing
were used: hydrogen peroxide and TBHP [25,26]. The reaction was
performed in acetonitrile as the solvent and for the reaction time to
24 h. The following molar ratios of limonene/oxidizing agent were
used in these studies: the molar ratio of limonene/H2O2 = 1:1.3
and the molar ratio of limonene/TBHP = 1:1.6. For the reaction
with H2O2 the temperature of 70 ◦C and for TBHP the temperature
of 80 ◦C were applied. For all investigated oxidizing agents the
selectivity of 1,2-epoxylimonene was 100 mol%, but the conversion
of limonene amounted to 40 mol% for H2O2, and 60 mol% for TBHP.
The preliminary studies on the oxidation of limonene over
the TS-1 and Ti-SBA-15 catalysts (both catalysts were hydrother-
mally synthesized by the sol–gel method) were performed in the
glass reactor with the capacity of 25 cm3 equipped with the reflux
condenser, the thermometer and the magnetic stirrer and at the
following constant starting conditions: the temperatures of 0 ◦C,
40 ◦C, 80 ◦C and 120 ◦C, the molar ratio of limonene/H2O2 = 1:2,
methanol (solvent) concentration 80 wt% and the catalyst content
3 wt% [17]. The reaction time changed from 0.5 to 24 h. These
studies were only the preliminary studies which showed the pos-
sibility to perform the oxidation of limonene over the microporous
TS-1 catalyst (in spite of small pore size of this catalyst) and
over the Ti-SBA-15 mesoporous catalyst (it was possible to obtain
high selectivity of 1,2-epoxylimonene at the low selectivity of 1,2-
epoxylimonene diol). Moreover, these studies allow to propose the
possible mechanism of the formation of the appropriate products
of this process. It was observed during these studies that in almost
catalysts in the liquid phase oxidation of limonene with hydrogen
peroxide or t-butyl hydroperoxide (TBHP). Until now the following
titanium silicalite catalysts were used in this process: Ti-MCM-41
[8,22–24], Ti-MMM-2 [10], Ti-SBA-15 [17,25,26] and TS-1 [17,26].
catalyst were performed with such oxidizing agents as: hydrogen
peroxide and TBHP. Ti-MCM-41 catalyst used in the oxidation
of limonene was synthesized by the direct sol–gel method [8],
and also by grafting (Ti-grafted MCM-41) [22] or wetness and
studies over the Ti-MCM-41 catalysts were performed at the
temperatures in the range of 70–85 ◦C, for the molar ratio of
limonene/H2O2 = 3.7:1 and for the reaction time in the range
of 0.5–7 h (in one case from 1 h to 24 h [22]). Over the direct
synthesized Ti-MCM-41 catalyst the selectivity of the epoxide
compounds (the sum of 1,2- and 8,9-epoxylimonene) was about
60 mol%. Carveol and carvone were formed with the selectivity of
from 36 to 80 mol%. Over the silylated Ti-MCM-41 catalyst the
results of limonene epoxidation were very close, and the only one
difference was a higher value of the hydrogen peroxide efficiency
[23]. The two other methods of Ti-MCM-41 preparation (wetness
and vet impregnation) do not cause important changes in the
catalyst activity, only a slight decrease in the epoxides selectivity
was observed [24]. The utilization of TBHP in the oxidation of
limonene caused that after the reaction time of 24 h the conversion
of limonene was 62 mol% and the selectivity of 1,2-epoxylimonene
amounted to 75 mol%. Over the Ti-MMM-2 catalyst the oxidation of
limonene was performed in acetonitrile as the solvent, at the tem-
perature of 60 ◦C and for the molar ratio of limonene/H2O2 = 1:2
[10]. These studies showed that the amount of the obtained epox-
ide compound was three times higher than the other products
and the amount of carvone was higher than carveol. Moreover,
in the post-reaction mixtures also perillyl alcohol was detected
[10]. During the studies over the Ti-SBA-15 catalyst synthesized
by grafting titanium on a SBA-15 structure two oxidizing agents
The aim of this work is the detailed studies on limonene oxida-
tion with 60 wt% hydrogen peroxide over the TS-1 catalyst – the
titanium silicate zeolite type material. This work is a continuation
of the paper [17], where were presented the preliminary studies for
the process of limonene oxidation over TS-1 catalysts but over this
catalyst the broad studies on limonene oxidation have not been per-
formedyet. Itwillbeveryinterestingtodescribethefullinfluenceof
the following technological parameters on the course of limonene
oxidation: the temperature, the molar ratio of limonene/H2O2,
methanol (solvent) concentration, the TS-1 catalyst content, and
the reaction time. It will be also helpful in the establishing of the
most beneficial conditions of 1,2-epoxylimonene obtaining, tak-
ing into account the values of the main functions describing this
process. Also very important is the presentation the conditions of
the formation the by-products of this process, because these by-
products very often can be the main products of limonene oxidation
process and they have a lot of applications. Also the possibility of
the reuse of the TS-1 catalyst was examined on this work.
2. Material and methods
The TS-1 catalyst was prepared according to the method
described by Thangaraj et al. [27]. The molar ration of Si/Ti in the
gel before crystallization was 64:1. The Ti content in this catalyst
was 3.01 wt%, the specific surface area amounted to 372 m2/g, the
size of the pores achieved 0.5 nm and the crystal size amounted to
0.5 m. The full characteristic of this catalyst was presented in our
previous articles [17,28].
In the studies on the oxidation of limonene the following raw
materials were used: R-(+)-limonene (97%, Sigma), hydrogen per-
oxide (60 wt% water solution, Chempur), and methanol (solvent)
(analytical grade, Chempur). During these studies the influence
of the following parameters was tested: the temperature in the
range of 0–120 ◦C (the concentration of limonene amounted to
about 13.7 wt% and hydrogen peroxide 3.4 wt%), the molar ratio
of limonene/H2O2 = 1:2–5:1 (the values of limonene and hydro-
gen peroxide concentration in relation to the appropriate molar
Please cite this article in press as: A. Wróblewska, et al., The studies on the limonene oxidation over the microporous TS-1 catalyst,