Application of hydrogen peroxide encapsulated in silica xerogels to oxidation reactions

Article abstract of DOI:10.3390/molecules17078068

Hydrogen peroxide was encapsulated into a silica xerogel matrix by the sol-gel technique. The composite was tested as an oxidizing agent both under conventional and microwave conditions in a few model reactions: Noyori′s method of octanal and 2-octanol oxidation and cycloctene epoxidation in a 1,1,1-trifluoroethanol/Na₂WO₄ system. The results were compared with yields obtained for reactions with 30% H₂O₂ and urea-hydrogen peroxide (UHP) as oxidizing agents. It was found that the composite has activity similar to 30% H₂O₂ and has a several advantages over UHP such as the fact that silica and H₂O are the only products of the composite decomposition or no contamination by urea or its derivatives occurs; the xerogel is easier to heated by microwave irradiation than UHP and could be used as both an oxidizing agent and as solid support for microwave assisted solvent-free oxidations.

Full text of DOI:10.3390/molecules17078068

Molecules 2012, 17, 8068-8078; doi:10.3390/molecules17078068
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
Application of Hydrogen Peroxide Encapsulated in Silica
Xerogels to Oxidation Reactions
Szczepan Bednarz *, Barbara Ryś and Dariusz Bogdał
Chair of Biotechnology and Renewable Materials, Cracow University of Technology, Warszawska 24,
31-155 Krakow, Poland; E-Mails: rysq86@o2.pl (B.R.); pcbogdal@cyf-kr.edu.pl (D.B.)
* Author to whom correspondence should be addressed; E-Mail: sbednarz@pk.edu.pl;
Tel.: +48-12-628-25-72; Fax: +48-12-628-29-47.
Received: 13 May 2012; in revised form: 18 June 2012 / Accepted: 25 June 2012 /
Published: 4 July 2012
Abstract: Hydrogen peroxide was encapsulated into a silica xerogel matrix by the sol-gel
technique. The composite was tested as an oxidizing agent both under conventional
and microwave conditions in a few model reactions: Noyori’s method of octanal and
2-octanol oxidation and cycloctene epoxidation in a 1,1,1-trifluoroethanol/Na2WO4 system.
The results were compared with yields obtained for reactions with 30% H2O2 and
urea-hydrogen peroxide (UHP) as oxidizing agents. It was found that the composite has
activity similar to 30% H2O2 and has a several advantages over UHP such as the fact that
silica and H2O are the only products of the composite decomposition or no contamination
by urea or its derivatives occurs; the xerogel is easier to heated by microwave irradiation
than UHP and could be used as both an oxidizing agent and as solid support for microwave
assisted solvent-free oxidations.
Keywords: oxidation; epoxidation; microwave irradiation; silica xerogel; hydrogen
peroxide; sol-gel; fluoroalcohols; urea-hydrogen peroxide
1. Introduction
One of the methods for the preparation of porous materials is sol-gel processing [1]. The method is
characterized by the formation of stable colloidal solutions (sol) in the first step followed by
anisotropic condensation of colloidal particles producing a polymeric network with an entrapped
solution. After washing and removal of the solvent xero- or aerogels are formed depending on the

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drying mode. The sol-gel technique is employed for the production of porous solids, such as silica and
alumina, widely used as catalyst supports. Various metal substituted silica matrices were synthesized
and catalytic activity in oxidation reactions was demonstrated. For example titania-silica xerogels and
aerogels were used as a catalyst for oxidation reactions with H2O2 or tert-butyl hydroperoxide as
oxidants [2]. The oxidation of some arenes with alkyl side groups by H2O2 with tungstoboric acid
supported on SiO2 xerogels was reported [3]. Additionally, a xerogel composite of V2O5 was evaluated
as a catalyst for the epoxidation of styrene and cyclooctene using iodosylbenzene, H2O2 and m-chloro-
perbenzoic acid as oxidants [4]. Metallosilicate compounds, prepared by dispersion of metal oxide
(TiO2, MoO3 or WO3) in amorphous silica, were catalytically active in the oxidation of alkenes and
alcohols with 30% H2O2 solution [5].
Sol-gel processed porous silica glasses also have considerable potential as carriers for controlled
drug and temperature-sensitive molecule release [6]. Recently, a sol-gel method was successfully
applied for incorporation of H2O2 into a silica xerogel matrix to obtain a new antimicrobial composite
for medical applications [7]. It was demonstrated that the xerogel was biologically active to nearly the
same extent as H2O2 solutions [7]. The composite containing up to 68% of H2O2 was stable because of
the formation of strong hydrogen bonds between siloxane oxygens and H2O2 [7,8]. The studies have
shown that H2O2 was quickly and fully released from the composite into water [7].
To the best of our knowledge, silica xerogel-H2O2 composites have not been used in organic
synthesis as an oxidant. This paper demonstrates a successful application of these xerogels as a H2O2
source for the oxidation of aldehydes, secondary alcohols and epoxidation of cycloolefins, under both
conventional and microwave conditions.
2. Results and Discussion
Drying of a mixture of H2O2 in silicic acid sol results in the evaporation of H2O and partially of
H2O2. Simultaneously, silicic acid undergoes condensation and polymerization (crosslinking) followed
by an aggregation. Further drying caused the transformation of the silica gel into a silica xerogel and
entrapment of both H2O and H2O2 inside the silica network (Figure 1). The IR spectrum (Figure 2A) of
the composite is similar to described in the literature [8] and shows characteristic peaks of hydrated
silica (broad band 3700–3000 cm−1 O–H stretching frequencies, 1630 cm−1 molecular bending
vibrations of water, 1400–1000 cm−1 deformation vibrations of the hydroxyl of Si-OH) and H2O2
(1350 cm−1 H–O–O bending band). Additionally, the Raman spectrum [Figure 2B] shows a peak at
876 cm−1 clearly indicating presence of the peroxide. The amount of encapsulated H2O2 and the mass
ratio of H2O2/H2O depend on drying time and temperature, which was monitored by weighing the
composite during drying [7]. The determined average composition of the obtained composites was
40% H2O2, 43% H2O and 17% SiO2. It was found that loading efficiency of H2O2 was c.a. 50%, thus
half of the initial amount of H2O2 in a sol was encapsulated in the final product and the rest was lost
during drying of the sol.

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Figure 1. A simplified structure of silica xerogel-hydrogen peroxide composite [7,8].
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Figure 2. Spectroscopic characterization of silica xerogel-hydrogen peroxide composite:
(A) FTIR spectrum, (B) Raman spectrum.
To investigate the activity of H2O2 encapsulated in silica xerogel, some model reactions were
examined such as Noyori’s procedures for solvent-free aldehyde [9] and alcohol [10,11] oxidation and
epoxidation of cyclic olefins in fluorinated alcohol as a solvent [12] (Scheme 1). For comparison the
reactions were also carried out with 30% H2O2 solution and urea-hydrogen peroxide adduct (UHP) as
oxidant. Additionally, microwave irradiation was employed to shorten the reaction times and to
increase their yields.
Scheme 1. The model reactions: oxidation of octanal (a), 2-octanol (b) and epoxidation of
cyclooctene (c).

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The oxidation of octanal (Table 1) both by the 30% of H2O2 and H2O2 encapsulated in the xerogel
gave very good yield of the acid (>80%). Application of UHP resulted in lower conversion of
octanal (49%) and, additionally, under microwave condition side reactions took place-most probably
condensation of urea with the aldehyde [14]. For that reason, in spite of high conversion, the reaction
product was not detected by GC.
Table 1. Effect of a source of H2O2 on the oxidation of octanal under conventional and
microwaves conditions.
Yield of octanoic acid, %
H2O2 source
Conventional conditions,
Microwave irradiation,
90 °C, 120 min
90–95 °C, 45 min
30% H2O2
UHP
The xerogel
86
49
83
81
0 a
73
a
Several undefined by-products were detected by GC and the product (octanoic acid) was
not affirmed.
2-Octanol was successfully oxidized in nearly quantitative yield both by the composite and 30%
aqueous H2O2 (Table 2). The application of UHP as an oxidant lowers the yield of the reaction to 69%.
Moreover, when the reaction temperature was decreased to 70 °C, UHP exhibited low reactivity
manifested only 6% yield of 2-octanone with comparison to 64% and 56% for aqueous H2O2 and the
xerogel, respectively. Oxidation under microwave conditions gave very good results only when 30%
H2O2 was applied.
Table 2. Effect of the source of H2O2 on the oxidation of 2-octanol under conventional and
microwaves conditions.
Yield of 2-octanone, %
H2O2 source
Conventional conditions,
80 °C, 30 min
92 (64 a)
Microwave irradiation,
80–90 °C, 15 min
30% H2O2
UHP
The xerogel
99
<1
46
69 (6 a)
98 (56 a)
a Reactions carried out at 70 °C (below melting point of UHP).
In further studies, the yield of cyclooctene oxide was highest for reaction with UHP (94%) as an
oxidant, while slightly lower yield (88%) was observed for reactions with 30% H2O2. However, in the
presence of the xerogel, 58% conversion of the olefine was observed. Elongation of the reaction time
or increasing the amount of the xerogel improved the yield.

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Table 3. Effect of the source of H2O2 on the epoxidation of cyclooctene under conventional
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and microwaves conditions.
Yield of cyclooctene oxide, %
H2O2 source
Conventional conditions,
Microwave irradiation,
3 h 60 °C
1 h 60–70 °C
30% H2O2
UHP
The xerogel
88
94
90
95
71
58 (69 a, 74 b)
a Reaction time was 4 h. b Highest amount of the xerogel was used (equal 14 mmol H2O2).
Silica xerogel–hydrogen peroxide composites stabilized by addition of small amounts of H3PO4
can be stored at 3 °C for 2 months without significant loss of H2O2 [7]. However, we have used
unstabilized composites. We have found that after 2 weeks of storing at 5 °C, the composite loses
about a half amount of H2O2 probably because slow decomposition of H2O2 catalyzed by heavy metals
impurities presented in the silica matrix (probably, traces of Fe2+ from “water glass”). Relative
instability of the composite could be a reason of its activity in oxidation reactions. In contrast, UHP is
a stable H2O2 complex and such a high stability could be considered, to be a drawback to its potential
chemical reactivity. An oxidation yield might by higher if dissociation of the adduct, i.e., dissolving or
melting (m.p. 82 °C [15]) took place. UHP/hexafluoro-2-propanol (HFIP) system was investigated in
epoxidation of olefins, and it was concluded that the fluoroalcohol has the unique ability to combine
the two requirements: dissociation (dissolving) of the UHP adduct and activation of H2O2 [16]. We
have observed similar effect of TFE on the epoxidation yield of cyclooctene with UHP as an oxidant
(Table 3). The temperature effect on UHP and the silica xerogel–H2O2 composite activity was
investigated in 2-octanol oxidation (Table 2). At the temperature 70 °C (below melting point of UHP)
a conversion of the alcohol was very low and was significant increased when the temperature risen to
80 °C. In opposition, oxidation with 30% H2O2 and also with the composite did not show such high
yield–temperature dependency.
We have found that silica xerogel–H2O2 composite is more suitable oxidant to microwave assisted
reactions than UHP. The composite is easily heated by microwave irradiation probably because they
contain c.a. 80% of polar components: H2O2 and H2O. These molecules are relatively weakly bonded
to silica matrix and can undergo reorientation in microwave field, resulting in heat generation. Silica
matrix is almost transparent for microwave irradiation (” < 0.001 2.5 GHz [17]) and does not heated
directly by the radiation. For this reason, undesired superheating effect of the xerogel under microwave
condition is much less probably than for solids strongly interacted with microwaves such as chromium
dioxide (Magtrieve™) [18]. UHP is a crystalline complex formed by hydrogen bonding between urea
and H2O2 [15], thus the molecules are locked together in a crystal structure. From that reason solid
UHP weakly interacts with microwave irradiation in contrast to the melted one, which strongly absorbs
microwave energy. It could cause overheating, thermal decomposition of H2O2 and even thermal
degradation of urea. This might explain rather poor yields of reactions conducted under microwave
condition where UHP were used as an oxidant in the solid state (Tables 1 and 2). A presence of polar
solvent could overcome mentioned limitations, e.g., high yields of oxidation with UHP in methanol [19]
or 1,4-dioxane-water [20] were obtained in microwave assisted reactions.

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H2O2 encapsulated in silica matrix shown similar activity in the investigated oxidation reactions as
30% H2O2 (Tables 1 and 2). However, the composite exhibited lower activity in epoxidation of
cyclooctene carried out in TFE (Table 3), an elongation of the reaction time or increasing of the
composite amount improves the yield. On the other hand, fluorous alcohols were found to be excellent
activators of H2O2 and a several mechanisms based on formation of hydrogen bonds between fluorine
and H-O-O hydrogen atom were proposed to explain the phenomenon of the H2O2 activation [16,21–25].
Additional, hydrogen bonds found to be responsible for stablility of H2O2-silica xerogels. FTIR studies
and DFT and B3LYP calculations shown that there are complex interactions between H2O2, H2O and
silica network because of hydrogen bonds formation [8]. It may be possible that H2O2 molecules
involved in formation of silica complex are not activated by TFE. This might be a reason of the small
decrease of the yield of the epoxidation carried out in TFE with silica xerogel–H2O2 composite with
comparison to 30% H2O2 or UHP.
Tentative mechanisms of oxidation with H2O2 encapsulated in silica xerogel could be proposed.
However, further investigations are needed to prove which of supposed mechanisms (below)
taking place.
(1) H2O2 and H2O are fast released from the xerogel to a reaction mixture (Figure 3A). The silica
xerogel acts as inert ‘vessel’ for H2O2 and H2O molecules and does not take a part in an
oxidation reaction.
(2) Small amounts of H2O2 are slowly released from the xerogel to a reaction mixture due to poor
solubility of H2O2 and H2O in the solution (Figure 3B). From that reason, the reaction is carried out
inside pores of the silica xerogel or on its surface thus additional interactions between the solid support
and reagents might occur.
(3) A combination of the two above mechanisms. H2O2 and H2O is released from the composite to a
reaction mixture and the silica xerogel acts both as a solid support for the oxidant and also takes a part
in an oxidation reaction, because of a specific interactions silica matrix–reagents.
Figure 3. Proposed mechanisms for the xerogel mediated oxidation: S→P. S: substrate,
P: oxidation product. (A) H2O2 and H2O are fast released from the xerogel to a reaction mixture.
(B) Small amounts of H2O2 are slowly released from the xerogel to a reaction mixture.
It seems that the composites may be modified to enhance their properties. It is well-know that silica
xerogel dopped with metals could be prepared by drying a mixture of aqueous H2O2 and silicic acid or
silica sol (obtained by e.g., cation exchange method or by hydrolysis of tetraethoxysilane) with a metal

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salt or a metal sol (e.g., Mg, Al, Ti, Zr, Sn) [26,27]. If the supposed mechanism 2 or 3 is valid, the
metallosilica xerogels could be considered as a matrix for H2O2 encapsulation with potential catalytic
sites active in oxidation reactions. Moreover, silica xerogels might be modified by addition of
water-soluble polymers forming adducts with H2O2 such as poly(N-vinylpyrrolidinone) [28–30] or
poly(N-vinylcaprolactam) [31,32] to develop tailor-made composites, e.g., with tuned H2O2 release
rate, which is especially important if the hypothetical oxidation mechanism 1 is correct.
3. Experimental Section
3.1. Equipment
The yield of the reactions was determined by a gas chromatograph coupled with flame ionization
detector-GC-FID 6850, Agilent (Santa Clara, CA, USA). Microwave assisted reactions were carried
out in a multimode microwave reactor Magnum, Ertec (Wrocław, Poland). FT-IR spectra were
obtained by a Biorad STS 165 spectrometer using the KBr pellet technique. FT-Raman spectra were
collected by means of an EZRaman-M spectrometer in a range from 250–2340 cm−1, using 785 nm
excitation diode laser.
3.2. Materials
Amberlite IR-120 (H+ form), octanal, octanoic acid, 2-octanol, 2-octanone, cis-cyclooctene,
cyclooctene oxide, 1,1,1-trifluoroethanol (TFE), Na2WO4H2O, UHP, tetrabutylammonium hydrogen
sulfate (TBAHS) and 1,6-dibromohexane were purchased from Sigma-Aldrich and used as received.
30% solution of H2O2 was from POCH (Gliwice, Poland) and “glass water” R-145 was obtained from
Cazet (Łazy, Poland).
3.3. Preparation of Silica Xerogel-Hydrogen Peroxide Composites by the Sol-Gel Method
The composites were prepared according to the literature procedure with some modifications [7].
Silicic acid was obtained from water glass by the cation-exchange method. Water glass (4 g) was
diluted with distilled water (16 g). The solution was passed through a bed of cation-exchange resin
(Amberlite, 80 g) in a glass column (O.D. 25 mm × 600 mm). The bed was washed with distilled water
(40 mL) and effluents were combined and mixed with 30% of H2O2 (20 mL) without addition of any
H2O2 decomposition inhibitor. The sol was poured onto a Petri dish and dried under hood at room
temperature for 48h to give 5–10g of the composite (xerogel). Then the hydrogen peroxide content
(%H2O2) in the composite was determined by iodometric titration and silica content (%SiO2) was
estimated by drying of the xerogel at 150 °C to a constant weight. Water content was approximated as:
%H2O = 100-%H2O2-%SiO2, while loading efficiency (LE) was calculated from the equation:
LE(%) = (mx/ms) × 100 where mx and ms are weight of H2O2 in sol and xerogel, respectively [7].

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3.4. Model Reactions
3.4.1. Octanal Oxidation
Octanal (1.05 g, 10 mmol), TBAHS (0.02 g, 0.06 mmol), 1,6-dibromohexane as an internal standard
(100 µL) and the oxidant–equivalent of 12 mmol H2O2 (30% H2O2 1.36 g; UHP 1.13 g; or the xerogel
1.02 g) were mixed in a vial and heated at 90 °C for 2 h. After that, the reaction mixture was diluted by
ethyl acetate (3 mL), dried over MgSO4 and the solution was and passed through a short pad of MnO2
to decompose residue of H2O2. The solution was analyzed by GC-FID to determining octanoic acid
yield. Retention times of the substrate and the product were compared with authentic samples.
3.4.2. 2-Octanol Oxidation
2-Octanol (0.98 g, 7.5 mmol), Na2WO4·H2O (0.1 g, 0.3 mmol), TBAHS (0.1 g, 0.3 mmol),
1,6-dibromohexane as an internal standard (100 µL) and the oxidant–equivalent of 12 mmol H2O2
(30% H2O2 1.36 g; UHP 1.13 g; or the xerogel 1.02 g) were mixed in a vial and heated at 80 °C for 30
min. After that, the reaction mixture was diluted with ethyl acetate (3 mL), dried over MgSO4, and
passed through a short pad of MnO2 to decompose the residue of H2O2. The solution was analyzed by
GC-FID to determining 2-octanone yield. Retention times of the substrate and the product were
compared with authentic samples.
3.4.3. Cyclooctene Epoxidation
cis-Cyclooctene (0.45 g, 4 mmol), TFE (3.3 mL), Na2WO4·2H2O (0,013 g, 0.04 mmol),
1,6-dibromohexane and as internal standard (100 L) and the oxidant–equivalent of 7 mmol H2O2
(30% H2O2 0.80 g; UHP 0.65 g; or the xerogel 0.60 g) were mixed in a vial and heated at 60 °C for 3h.
After that, the reaction mixure was diluted with dichloromethane (2 mL), dried over MgSO4 and
passed through a short pad of MnO2 to decompose the residue of H2O2. The solution was analyzed by
GC-FID to determining cyclooctene oxide yield. Retention times of the substrate and the product were
compared with authentic samples.
3.5. Microwave Assisted Reactions
Microwave assisted reactions were carried out in the microwave reactor equipped with a magnetic
stirrer and upright condenser. Temperature was monitored by a universal fiber-optic sensor (FTI-10,
FISO, Canada) immersed directly in a reaction mixture. Reactions time and temperature are indicated
in Tables 1–3.
4. Conclusions
In summary, the results presented in this paper clearly show that silica xerogel–hydrogen peroxide
composites obtained by the sol-gel technique are useful oxidizing agents, with an activity similar to
30% H2O2. Moreover, they have several advantages: they are easy to handle, cheap solids, made from
readily available reagents (water glass and 30% H2O2). Additionally, unlike other solidified H2O2
forms such as sodium percarbonate or sodium perborate, the composite does not change the pH of a

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reaction mixture. Silica xerogel–hydrogen peroxide composite exhibits also advantages over UHP. For
instance silica (easy to remove by filtration) and H2O are the only by-products of the silica based
material decomposition. Undesired contamination by urea, its decomposition products or other urea
side-reactions can be avoided if the silica xerogel is used. In opposition of UHP complex, H2O2 is not
strongly bonded in the silica matrix thus can be easier released to the reaction solution without heating.
Additionally, the xerogel is more easily heated by microwave irradiation than UHP and could be used
both as oxidizing agent and as a solid support for microwave assisted solvent free reactions.
Acknowledgments
This work was undertaken as part of the EU sponsored D40 COST Program (Innovative Catalysis:
New Processes and Selectivities). We would like to thank Jacek Żegliński, for receiving his Ph.D.
thesis available to study.
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Sample Availability: Samples of the compounds xerogels are available from the authors.
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