Oxidative dehydrogenation of 2,3,5-trimethyl-1,4-hydroquinone in the presence of titanium dioxide hydrogel

Article abstract of DOI:10.1023/B:RUGC.0000025176.95256.b2

Liquid-phase oxidative dehydrogenation of 2,3,5-trimethyl-1,4-hydroquinone in the presence of titanium dioxide hydrogel was studied by a kinetic method. Associative interactions between the substrate, oxidant, and gel were detected by voltammetry and ESR and IR spectroscopy.

Full text of DOI:10.1023/B:RUGC.0000025176.95256.b2

Russian Journal of General Chemistry, Vol. 74, No. 1, 2004, pp. 101 104. Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 1, 2004,
pp. 110 113.
Original Russian Text Copyright
2004 by Kharchuk, Buldakova, Shishmakov, Kuznetsova, Kovaleva, Koryakova, Molochnikov, Yanchenko,
Petrov.
Oxidative Dehydrogenation of 2,3,5-Trimethyl-1,4-hydroquinone
in the Presence of Titanium Dioxide Hydrogel
V. G. Kharchuk, L. Yu. Buldakova, A. B. Shishmakov, O. V. Kuznetsova, E. G. Kovaleva,
O. V. Koryakova, L. S. Molochnikov, M. Yu. Yanchenko, and L. A. Petrov
Institute of Solid-State Chemistry, Ural Division, Russian Academy of Sciences, Yekaterinburg, Russia
Institute of Organic Synthesis, Ural Division, Russian Academy of Sciences, Yekaterinburg, Russia
Ural State Academy of Forestry Engineering, Yekaterinburg, Russia
Received April 24, 2001
Abstract Liquid-p[hase oxidative dehydrogenation of 2,3,5-trimethyl-1,4-hydroquinone in the presence of
titanium dioxide hydrogel was studied by a kinetic method. Associative interactions between the substrate,
oxidant, and gel were detected by voltammetry and ESR and IR spectroscopy.
1
We showed in our previous papers how active
polydisperse highly ordered oxide systems can affect
liquid-phase oxidation processes as catalytically active
structuring ingredients [1, 2]. Here we consider oxi-
dative dehydrogenation of 2,3,5-trimethyl-1,4-hydro-
quinone I (a model hydroxyarene) in the presence of
titanium dioxide gel II.
to that of the reference (CH3) band at 1464 cm (A)
1
decreases from 0.932 to 0.772; the band at 1412 cm
changes in the shape and intensity, with its A decreas-
ing from 0.969 to 0.597. Furthermore, the bending
vibration band of the OH group of I ( 1216 cm )
transforms into a doublet at 1232 and 1220 cm . The
hydroxonium absorption band at 1730 1680 cm ,
1
1
1
characteristic of gel II [6], disappears upon sorption
Electrophilic liquid-phase oxidation of I with at-
mospheric oxygen in a neutral medium occurs selec-
tively by the concerted molecular mechanism and
yields 2,3,5-trimethyl-1,4-benzoquinone III [3]:
1
of I. The maxima of the stretching ( 3400 cm ) and
1
bending ( 1620 cm ) bands of hydration water in II
1
shift to 3280 and 1640 cm , respectively. Weak
1
Ti OH bending bands of II ( 1192, 1168, 1090 cm )
OH
O
are not observed in the spectrum of the sample with
sorbed I.
H3C
CH3
CH3
H3C
CH3
CH3
[O]
These changes in the IR spectra are apparently due
to binding of the hydroxonium proton and hydrogen
atoms of water molecules localized on the surface of
gel II with the -electron system and hydroxyl oxygen
atoms of I. Formation of hydrogen-bonded complexes
decreases the electron density on the oxygen atoms of
the hydroxy groups in I [7, 8], which is responsible
for the decreased reactivity of I in electrophilic oxi-
dation.
OH
O
I
III
The reaction was performed in aqueous methanol at
50 C in air. Its progress was monitored by GLC [4].
Hydrogel II, when present in the sphere of reaction
of I with atmospheric oxygen, decreases the initial
1
reaction rate v0 to 0.10 mol l 1 h (v0 without gel II
Addition of gel II to the reaction system in oxida-
tion of I with Cu2+ ions in the presence of oxygen
accelerates the reaction: v0 becomes as high as
1
is 0.26 mol l 1 h ). This result suggests the occur-
rence of competing interparticle interactions involving
gel II and reactants. Indeed, an IR study of substrate I
sorbed on gel II reveals significant changes in the
1
1
1.05 mol l 1 h , against 0.54 mol l 1 h in the ab-
sence of gel II. In this system, substrate I is involved
in two parallel reactions, one of which is inhibited by
gel II (see above). Intensification of the reaction in-
volving Cu2+ ions is apparently due to their associa-
tive interaction with fragments of the added hydro-
gel II.
1
absorption bands at 1344 and 1412 cm , which are
superpositions of the C O stretching bands and vibra-
tion bands of the aromatic ring [5]. In going from free
1
to sorbed I, the band maximum at 1344 cm shifts to
1
1324 cm , and the ratio of the intensity of this band
1070-3632/04/7401-0101 2004 MAIK Nauka/Interperiodica

102
KHARCHUK et al.
dI/d ,
40
dI/d ,
A s
1
2
1
1
A s
40
0
3
2
0
3
4
1
40
40
80
80
120
1
0
1
0
1
E, V
E, V
Fig. 2. Influence of adsorption on the surface of TiO
2
2+
Fig. 1. Oxidation of substrate I on carbon paste electrode:
gel on the electrochemical behavior of Cu
2+
ions:
3
2+
(1) no additions, (2) in the presence of 5 10 M Cu
(1) solution of Cu ions and carbon paste electrode,
2+
2+
solution, (3) with introduction of TiO Cu
into the
(2) Cu
ions adsorbed on the surface of TiO gel
2
ads
2
electrode, and (4) with introduction of TiO into the
electrode in the presence of 5 10 M Cu solution.
introduced into carbon p[aste electrode, and (3) carbon
paste electrode with introduced TiO (background).
2
2
3
2+
Oxidation of hydroquinone I in the presence of
Cu2+ ions was also studied electrochemically with a
carbon paste electrode into which air-dry gel II was
introduced. Upon addition of Cu2+ ions into solution,
or upon preliminary adsorption of Cu2+ on gel II
followed by introduction of the gel into the carbon
paste electrode, the course of oxidation of I changes
compared to the process performed on the carbon
paste electrode without introduced gel II (Fig. 1).
The current of the oxidation peak of I increases, and
the shift of the peak potential reaches 0.15 0.16 V,
which is indicative of increased oxidation rate; also,
an additional oxidation peak appears at a lower poten-
tial (0.35 0.40 V).
oxidation (at +0.5 V) and by an ill-defined oxidation
peak at +0.8 V (Fig. 2, curve 2).
An ESR study of Cu2+-containing hydrogels II
based on TiO2 showed that Cu2+ ions, depending on
2+
their content in the sorbent phase (
), form mono-
nuclear complexes IV, associates Cuof mononuclear
complexes V with increased local concentration of
metal ions, and Cu2+ compounds VI giving no ESR
signal at the given frequency and temperature. Such
structures were also revealed on the surface of nano-
structured films and powders based on TiO2 [9]. We
believe that a separate phase of insoluble Cu(OH)2
can play the role of compound VI at relatively low
content of sorbed Cu2+ ions, and at their high content
ESR-inactive bridged polynuclear Cu2+ compounds
(clusters with strong exchange coupling or polynu-
clear structures with bridging OH groups) can also
form, by analogy with the related structures formed
in ion exchangers [10, 11].
Study of the electrochemical behavior of Cu2+ ions
adsorbed on gel II shows that reduction of Cu2+ to
Cu+ and of Cu+ to Cu0 is facilitated (Fig. 2, curves 1,
2). Whereas the reduction of Cu2+ from solution on a
carbon paste electrode is characterized by a common
two-electron wave, reduction of Cu2+ ions adsorbed
on gel II occurs in several steps. In adsorption on
gel II, the waves of copper oxidation shift, and the
process of copper oxidation/reduction becomes more
reversible (the difference between the peaks of
Cu2+/Cu0 reduction/oxidation is 1.27 and 0.86 V,
respectively). These data suggest that, upon adsorption
of Cu2+, its reduction to Cu+ and reverse oxidation to
Cu2+ are appreciably facilitated, and the Cu2+ and Cu+
ions can form with gel II two strongly bound and one
weakly bound complexes. Formation of the latter is
suggested by broadening of the peak of copper anodic
Cu2+
By varying
, we evaluated the quantitative
relationships between mononuclear complexes IV,
their associates V, and Cu2+ compounds VI. As
2+
is increased, the content of mononuclear compleCxues
IV (nIV) decreases, and that of compounds VI (nVI)
grows. The content of associates V (nV) passes
0.37 mmol Cu2+ per
gram of gel II. In this case, the three types of Cu2+
species IV VI are present in the phase of gel II in
Cu2+
through a maximum at
Cu2+
approximately equal amounts. At
0.57 mmol
Cu2+ per gram of gel II, the ratio n : nV : nVI was
IV
0.06 : 0.20 : 0.74. The reactivities of species IV VI
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 1 2004

OXIDATIVE DEHYDROGENATION OF 2,3,5-TRIMETHYL-1,4-HYDROQUINONE
103
were judged from the initial reaction rates v0(IV VI)
,
where v0i is the initial reaction rate at a Cu2+ content
in the gel phase of i; v0IV, initial reaction rate in the
presence of mononuclear Cu2+ complexes IV; v0V,
initial reaction rate in the presence of associates V of
mononuclear complexes; v0VI, initial reaction rate in
the presence of Cu2+ hydroxide VI; n(IV VI)i, relative
contents of species IV VI at a total Cu2+ concentra-
tion in the gel phase of i. The quantities v0(IV VI)
were determined with an error of 15%, which in-
cludes the error of determining n(IV VI) by ESR.
estimated at 0.35, 1.00, and 1.20 mol l 1 h , respec-
tively. That is, the Cu2+ ions bound in magnetic asso-
ciates V and polynuclear compounds VI are more
reactive than those in mononuclear complexes IV.
This is apparently due to the mobility of electrons in
systems V and VI, in which the Cu2+ ions are linked
to each other and the electron density transfer is facili-
tated. Previous measurements of the microwave con-
ductivity [12, 13] revealed a correlation between the
electron mobility and catalytic activity of Cu2+ ions.
Acceleration of the dehydrogenation is due to the fact
that oxidation of substrate I and reduction of oxygen
can be in this case separated in space.
1
Quantitative determination of I was performed by
GLC on a Chrom-4 chromatograph similarly to [4].
The IR spectra were recorded on a Specord M-80
1
spectrometer in the range 4000 400 cm in KBr and
Thus, we can conclude from our results that the
oxidative transformation of I in the presence of gel II
occurs in the sphere of homo- and heteroassociative
interparticle interactions involving the gel, substrate,
and oxidant.
mineral oil. The spectra were taken in the optical den-
sity scale; the error of indication of the optical density
was 0.0001. Samples for measurements were prepared
by sorption of hydroquinone I from solution on gel II,
followed by drying in an inert gas flow. The band
assignment was based on [5].
EXPERIMENTAL
The ESR spectra of Cu2+ in the gel phase were
recorded on a PS-100Kh ESR radiospectrometer in the
3 cm wavelength range at room temperature in thin-
walled round quartz ampules (d 4 mm). The spectra
were processed with special programs from the spec-
trometer software. The integral intensioties of ESR
signals were analyzed using a reference sample and
software for spectrum processing, following the pro-
cedures suggested in [14 16]. Samples for studies
were prepared by sorption of Cu2+ ions on gel II from
CuCl2 2H2O solutions, followed by drying at room
temperature. The concentration of Cu2+ ions in the gel
phase was determined by atomic absorption spectros-
copy on a Perkin Elmer-403 spectrometer.
Titanium dioxide gel II with the specific surface
1
area Ssp 260 13 m2 g (as determined by adsorption
of an inert gas) was prepared by hydrolysis of an
alcoholic solution of tetrabutoxytitanium with water
at room temperature with vigorous stirring.
Experiments were performed in a temperature-
controlled reactor equipped with a reflux condenser
and a bubbler for air supply. The mixture was stirred
1
1
at a rate of 2 s ; the air flow rate was 6.2 l h . These
conditions ensure kinetic control of the reaction. The
reaction was performed in aqueous methanol (water :
methanol = 1 : 1 by volume) at 50 0.2 C. Oxygen
and copper dichloride dihydrate in the presence of
atmospheric oxygen were used as oxidants. The sub-
strate was dissolved in aqueous methanol to obtain a
The procedure of voltammetric experiments was
described in [17]. The supporting electrolyte was
0.025 M HCl in aqueous methanol (1 : 1 by volume);
2
1
6.6 10 M solution, and the gel (0.13 mol l )
2
and CuCl2 2H2O (0.59 10 M) were added. This
3
the Cu2+ concentration in solution was 5 10 M,
amount of CuCl2 2H2O was completely sorbed on the
2
and the concentration of I in solution, 10 M. The
gel within 45 s (content of sorbed Cu2+ ions on the
1
3
1
potential sweeping rate was 20 mV s .
Cu2+
gel
0.57 10 mol g gel).
Kinetic measurements were performed by taking
ACKNOWLEDGMENTS
aliquot samples and determining the content of the
starting compound. The functions obtained were
approximated by polynomials. The initial reaction
rates v0 were determined by numerical differentiation
and interpolation; their error did not exceed 10%.
The study was financially supported by the Russian
Foundation for Basic Research (project no. 01-03-
96521).
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n
+ v0V Vi
n
+ v0VI VIi
n
,
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104
KHARCHUK et al.
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