Microwave-assisted oxidation of alcohols by hydrogen peroxide catalysed by tetrabutylammonium decatungstate

Article abstract of DOI:10.2478/s11696-013-0386-9

This work deals with catalytic activity of tetrabutylammonium decatungstate(VI) in the oxidation of selected alcohols with hydrogen peroxide as an oxidant using 1,2-dichloroethane/water or acetonitrile/water as a solvent system. Different forms of heating were compared. The highest conversions of substrates were achieved in the two phase system acetonitrile/water using microwave irradiation combined with elevated pressure. Finally, optimum parameters for these reactions in a microwave pressurised reactor were established and discussed.

Full text of DOI:10.2478/s11696-013-0386-9

Chemical Papers 67 (9) 1240–1244 (2013)
DOI: 10.2478/s11696-013-0386-9
SHORT COMMUNICATION
Microwave-assisted oxidation of alcohols by hydrogen peroxide
‡
catalysed by tetrabutylammonium decatungstate
Mateusz Galica*, Wiktor Kasprzyk, Szczepan Bednarz, Dariusz Bogdaꢀl
Chair of Biotechnology and Renewable Materials, Faculty of Chemical Engineering and Technology,
Cracow University of Technology, Warszawska 24 St., 31-155 Krakow, Poland
Received 27 June 2012; Revised 5 February 2013; Accepted 6 February 2013
This work deals with catalytic activity of tetrabutylammonium decatungstate(VI) in the oxida-
tion of selected alcohols with hydrogen peroxide as an oxidant using 1,2-dichloroethane/water or
acetonitrile/water as a solvent system. Different forms of heating were compared. The highest con-
versions of substrates were achieved in the two phase system acetonitrile/water using microwave
irradiation combined with elevated pressure. Finally, optimum parameters for these reactions in a
microwave pressurised reactor were established and discussed.
ꢀ
c 2013 Institute of Chemistry, Slovak Academy of Sciences
Keywords: oxidations, alcohols, hydrogen peroxide, polyoxymetalate, microwaves
Oxidation of alcohols is a very important path of
organic syntheses. In this type of reactions, precursors
for synthesis of vitamins, drugs, and fragrances are
produced (Pybus & Sell, 1999; Singh et al., 1979; Fey
et al., 2001). Many processes of oxidation employ ox-
idising agents which are not environmentally friendly,
e.g. heavy metal salts or per acids. Consequently, more
and more recent studies have been focused on the de-
velopment of environmentally benign oxidation sys-
tems (Jamwal et al., 2008). Depending on the scale
and desired product of the reactions, many methods
for oxidation of alcohols can be identified. Alcohols can
be oxidised chemically and enzymatically. N-Halogen
reagents, peroxides, chromic acid, permanganate, and
oxygen oxidation are just some of the reactions. In
turn, biological oxidation takes place in the presence
of enzymes and requires very strict conditions. Labo-
ratory oxidation of alcohols is most often carried out
with chromic acid (H CrO ), which is usually pre-
at the aldehyde. Instead, the aldehyde is further ox-
idised to a carboxylic acid. Because of the toxicity
of chromium-based reagents, other methods for the
oxidation of alcohols have been developed. The per-
−
manganate ion, MnO , oxidises both primary and sec-
4
ondary alcohols in either basic or acidic solutions. In
case of primary alcohols, the product normally is the
carboxylic acid because the intermediate aldehyde is
oxidised rapidly by permanganate. The use of perox-
ide as the oxidising agent has its benefits as it is read-
ily available and inexpensive. Usually, the reactions
are carried out in the presence of d block metal ions
catalysts. This technique used to produce secondary
alcohols provides good results especially in combi-
nation with quaternary ammonium salts as a phase
transfer agent. Primary alcohols are rather not reac-
tive under these conditions. Nevertheless, a solvent-
free and halide-free procedure has been developed us-
ing [Na WO ][CH (n-C H ) N]HSO , which allowed
2
4
2
4
3
8
17 3
4
pared from chromic oxide (CrO3) or sodium dichro-
mate (Na2Cr2O7) in combination with sulphuric acid.
Primary alcohols are initially oxidised to aldehydes by
these reagents. The reaction, however, does not stop
Sato et al. (1999) to efficiently oxidise both primary
and secondary alcohols. We would like to point out
that there is a number of reports on the oxidation
of alcohols using very efficient but rather expensive
*
Corresponding author, e-mail: mateusz.galica@wp.pl
‡
Presented at the 39th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare,
1–25 May 2012.
2

M. Galica et al./Chemical Papers 67 (9) 1240–1244 (2013)
1241
catalysts e.g. silver nanoparticles supported on hy-
drotalcites (Mitsudome et al., 2008), iron nanopar-
ticles on MCM-41 (González-Arellano et al., 2008),
and palladium nanoparticles supported on metal ox-
ides (Enache et al., 2006). Therefore, new catalytic
systems, especially relatively inexpensive and effec-
tive systems, have to be developed. In this field, a
special and significant group of efficient catalysts of
the organic compounds oxidation are polyoxometa-
lates (POMs). These are polyatomic anions with metal
atoms from d or f blocks, most of which show high sol-
ubility in water and occasionally in polar organic sol-
vents such as acetonitrile, DMF, and dioxane (Pope
(2003) did not study the effect of solvents and pressure
on the oxidation of styrene and cyclohexene under mi-
crowave conditions with sodium tungstate, which was
studied earlier by Sato et al. (1996), and Bogda lꢀ &
Lꢀ ukasiewicz (2000).
In summary, knowledge of the polyoxotungstate
catalytic activity (e.g. tetrabutylammonium deca-
tungstate(VI) (TBADT)) in oxidation reactions of al-
cohols is limited.
The following reagents were purchased from
Sigma–Aldrich or Fluka: sodium tungstate dihydrate,
octan-1-ol, octan-2-ol, benzyl alcohol, 2-phenyletha-
nol, 1-phenylethanol, cyclohexanol, 2-methyl-1-cyclo-
hexanol, 2-pentanol, hexane-2,5-diol, 2-ethylhexane-
1,3-diol. Nitric acid, anhydrous magnesium sulphate,
1,2-dichloroethane, acetonitrile, and hydrogen perox-
ide (30 mass %) were supplied by POCH SA (Poland).
TBADT was prepared according to a published
method (Klemperer, 1990). The oxidation reactions
with a variety of alcohols were carried out in two
systems: acetonitrile/water and 1,2-dichloroethane/
water. The oxidising agent was hydrogen peroxide
(30 mass %), which was mixed with the other reac-
tants and the solvent in the amounts presented in Ta-
bles 1 and 2. The reactions were carried out under
conventional heating and microwave irradiation (Pro-
labo Synthewave 302) in open vessels with an upright
&
M u¨ ller, 1994). Studies concerning the application
of polyoxometalates as both Brønsted acid and oxida-
tion catalysts began in the late 1970s and they have
continued until today (Kozhevnikov, 2002). In litera-
ture, many examples of employing POMs as catalysts
for the oxidation of alcohols, cyclic and acyclic diols,
cyclic tertiary amines, aniline derivatives, as well as
for the epoxidation of olefins and allylic alcohols can
be found; wherein, the obtained yield and selectivity
were relatively high (Adam et al., 2002, 2003). Among
many POMs, polyoxotungstates containing the de-
4
−
catungstate anion (W O ) exhibit unique photo-
10
32
catalytic properties investigated also by Angioni et al.
2008), Protti et al. (2009), and Molinari et al. (2013).
(
◦
Some efforts have been devoted to conduct catalytic
oxidation of organic compounds with polytungstates
in both heterogeneous and homogeneous systems as
described by Guo et al. (2001) and Bonchio et al.
condenser at 70–80 C for 10–15 min; then, the mixture
was cooled. The oxidation reactions were also carried
out in a pressurised microwave reactor (Ertec Magnum
◦
II, Poland) at 135–140 C, under the pressure of 3.4–
(
2003).
3.6 MPa. Identification and calculation of the ratio of
products was done with the help of the HPLC–ESI–
MS analysis (Agilent 1200 Series LC 6210 TOF MS
with ESI source). The TBADT catalyst was prepared
as shown in the following equation:
Owing to the design and synthesis of polyoxo-
tungstate catalysts, researchers have managed to pre-
pare a new more efficient catalytic agent immobilised
on carriers like porous silica gel, aluminum oxide, and
appropriate polymers (Tzirakis et al., 2007; Fornal &
Giannotti, 2007). On the contrary, examples of ho-
mogeneous photocatalysis using decatungstate salts
are extensively presented in literature and not all
of them are presented here because it is not essen-
tial for the present study. Not only photocatalysis in
the presence of an anion was examined but also the
possibility of oxidation catalysis of salts built of an-
10Na2WO4 + 16HCl + 4(n-C4H9)NBr →
(1)
→ [(n-C4H9)N]4[W10O32] + 16NaCl + 4NaBr + 8H2O
The catalyst was regenerated as follows: distilled
water (20 mL) was added to the cooled reaction mix-
ture; the white precipitate (the catalyst) was sepa-
rated by centrifugation and dried. The IR spectra con-
firmed that chemical structure of the catalyst did not
change after recycling.
Compounds obtained by the oxidation reactions
were identified by comparison of FTIR and UV-
VIS spectral data with data presented by Klemperer
(1990). Catalytic activity of TBADT in the oxidation
reactions of selected alcohols is given in Table 1.
Oxidation carried out in a system of two immiscible
solvents proved not to be suitable for the selected alco-
hols. Even when microwave irradiation was applied, no
significant increase in the conversion rate of octan-1-ol
was observed. However, more efficient oxidation of sec-
ondary alcohols carried out in the microwave reactor
proved predomination of microwave irradiation over
4
−
ions (W O ) and substituted aminium cations (e.g.
10
32
aliphatic chains). Consequently, the most significant
but incomplete data on the oxidation of cyclohexanol,
octanol, and hexanol in the presence of the hexade-
cyltrimethylammonium decatungstate catalyst were
presented by Guo (2004) who stated that to achieve a
relatively high degree of conversion and selectivity of
above 90 %, the reaction has to be performed in water
at reflux for about 13 h. Therefore, modifications of
microwave heating as well as different solvents were
investigated. Moreover, reaction parameters such as
appropriate solvent, reaction time, temperature, pres-
sure, and the power of microwave irradiation can be
crucial for obtaining satisfactory results. Freitag et al.

1
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M. Galica et al./Chemical Papers 67 (9) 1240–1244 (2013)
Table 1. Oxidation of selected alcohols in the 1,2-dichloroethane/water systema
Conversion/mole % Products proportion/mole %
Substrate
CHb
14
8
MWc
CHb
MWc
Octanal
Octanoic acid
70
30
50
50
Octan-1-ol
18
43
74
Octan-2-ol
Octan-2-one
100
100
Benzaldehyde
Benzoic acid
75
25
80
20
Benzyl alcohol
44
2
-Phenylethanol
48
56
82
76
Phenylacetic acid
Cyclohexanone
100
100
100
100
Cyclohexanol
◦
a) Reaction conditions: 5 mmol of substrate, 3.5 mmol of H2O2 (30 mass %), 1 mole % of TBADT, 2 mL of H2O, 80 C; b) CH –
conventional heating for 30 min; c) MW – microwave heating for 15 min.
Table 2. Oxidation of selected alcohols in the acetonitrile/water systema
Conversion/mole %
Products proportion/mole %
MWb
Substrate
MWb
23
MWc
74
MWPc
Octanal
Octanoic acid
74
26
68
32
Octan-1-ol
Cyclohexanol
90
90
Cyclohxexanone
100
100
Benzaldehyde
Benzoic acid
95
5
96
4
Benzyl alcohol
64
21
100
93
2-Phenylacetaldehyde
87
13
13
87
2-Phenylethanol
Phenylacetic acid
1
-Phenylethanol
81
96
85
Acetophenone
100
100
100
100
2-Methylcyclohexanol
100
2-Methylcyclohexanone
Pentan-2-ol
Octan-2-ol
75
85
80
80
Pentane-2-one
Octan-2-one
100
100
100
100
Hexane-2,5-dione
90
10
72
28
Hexane-2,5-diol
80
92
100
100
3-Methyl-2-cyclopenten-1-one
2-Ethylhexane-1,3-diol
3-(Hydroxymethyl)-heptan-4-one
90
95
◦
a) Reaction conditions: 6 mmol of substrate, 18 mmol of H2O2 (30 mass %), 1 mole % of TBADT, 3 mL of MeCN, 80 C; b) MW
–
microwave heating for 30 min; c) MWP – microwave heating under higher pressure for 15 min.
conventional heating in this type of reactions. Also,
longer reaction time (30 min) did not give satisfac-
tory results. All the above mentioned results can be
explained by the fact that the reactions occurred in a
two phase system and thus the contact area between
the substrate and the oxidant was rather limited.
Optimum operating conditions of the pressurised
microwave reactor were chosen empirically, based on
the fact that at pressures below 3.4–3.6 MPa and tem-
mary alcohols’ oxidation, a significant increase in the
efficiency and the product ratio, with respect to the
corresponding acids, was observed. Finally, the data
presented in Table 2 show that the use of a pressurised
microwave reactor can reduce the reaction time by half
in comparison with the MW procedure.
In addition, the oxidation of benzyl alcohol was
carried out in the presence of the regenerated cata-
lyst. The results showed that TBADT preserved its
catalytic activity for two regeneration cycles (Table 3).
As it can be seen in Table 2, the reactivity of ster-
ically hindered hydroxyl group is generally lower than
that of the less hindered ones, surprisingly oxidation
◦
peratures below 135–140 C, the catalyst does not de-
compose. All oxidation reactions of the selected alco-
hols carried out under high pressure exhibited satisfac-
tory efficiency in the range of 80–100 %. In case of pri-

M. Galica et al./Chemical Papers 67 (9) 1240–1244 (2013)
1243
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Selectivity /%
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