Cyclohexane oxidation with an O2–H2 mixture in the presence of a two-component Pt/C–heteropoly acid catalyst and ionic liquids

Article abstract of DOI:10.1134/S0023158417050147

Approaches to increase the efficiency of Pt/C–heteropoly acid catalyst in a liquid-phase oxidation of cyclohexane using an O₂–H₂ mixture were studied. It was shown that small additives of ionic liquid (BMImBr, Bu₄NBr, or Bu₄NHSO₄) significantly improve the catalytic effect of the Pt/C–H₃PMo₁₂O₄₀–CH₃CN system at 35°C, by slowing the rate of side reactions resulting in water formation, increasing the rate of oxygenate formation, and inhibiting their secondary oxidation reactions. The efficiency of H₂ consumption increases from 2 to 18–25%, while the selectivity of cyclohexane conversion is 92–98%. The substitution of one or two Mo(VI) ions by V(V) in the structure of the heteropoly acid decreases these parameters. In the presence of Bu₄NHSO₄, a Pt/C catalyst can be used many times. During the reaction, the heteropoly acid present in the solution is in a reduced state under the action of the reaction medium and undergoes reversible redox transformations. The nature of the catalytic action of the studied system is explained from the viewpoint of the effect of ionic liquids on the properties of a Pt/C catalyst in activating O₂, heteropoly molybdate chemistry, and the known mechanisms of the peroxide oxidation of hydrocarbons.

Full text of DOI:10.1134/S0023158417050147

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 5, pp. 522–532. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © L.I. Kuznetsova, N.I. Kuznetsova, 2017, published in Kinetika i Kataliz, 2017, Vol. 58, No. 5, pp. 540–550.
Cyclohexane Oxidation with an O –H Mixture in the Presence
2
2
of a Two-Component Pt/C–Heteropoly Acid Catalyst
and Ionic Liquids
L. I. Kuznetsova* and N. I. Kuznetsova
Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
*
e-mail: livkuzn@catalysis.ru
Received January 17, 2017
Abstract—Approaches to increase the efficiency of Pt/C–heteropoly acid catalyst in a liquid-phase oxidation
of cyclohexane using an O –H mixture were studied. It was shown that small additives of ionic liquid
2
2
(
BMImBr, Bu NBr, or Bu NHSO ) significantly improve the catalytic effect of the Pt/C–H PMo O –
4 4 4 3 12 40
CH CN system at 35°C, by slowing the rate of side reactions resulting in water formation, increasing the rate
3
of oxygenate formation, and inhibiting their secondary oxidation reactions. The efficiency of H consump-
2
tion increases from 2 to 18–25%, while the selectivity of cyclohexane conversion is 92–98%. The substitution
of one or two Mo(VI) ions by V(V) in the structure of the heteropoly acid decreases these parameters. In the
presence of Bu NHSO , a Pt/C catalyst can be used many times. During the reaction, the heteropoly acid
4
4
present in the solution is in a reduced state under the action of the reaction medium and undergoes reversible
redox transformations. The nature of the catalytic action of the studied system is explained from the viewpoint
of the effect of ionic liquids on the properties of a Pt/C catalyst in activating O , heteropoly molybdate chem-
2
istry, and the known mechanisms of the peroxide oxidation of hydrocarbons.
Keywords: cyclohexane oxidation, oxygen, hydrogen, ionic liquid, heteropoly acid, Pt catalyst
DOI: 10.1134/S0023158417050147
The reactions of hydrocarbon oxidation with a industrial process of cyclohexane oxidation with
mixture of O and H gases meet the requirements of molecular oxygen [11].
2
2
green chemistry because they occur at a low tempera-
The aim of this work is to study approaches to
ture with a high selectivity level of the substrate’s increase the efficiency of a two-component Pt/C–
transformation and the lack of any concomitant prod- heteropoly acid catalyst in the liquid-phase oxidation
of cyclohexane with the use of an O –H gas mixture.
ucts except for water [1–4]. Oxidation catalysts
include metals capable of activating molecular oxygen
2
2
A problem that prevents the wide application of an
by an incomplete reduction to hydrogen peroxide and O –H oxidizing agent is the excessive consumption
2
2
the second component that fixes the active forms of of O –H gases to form water, which is not related to
2
2
peroxide oxygen for a reaction with the substrate. organic substrate oxidation. A solution is sought
Two-component combinations of platinum metals among the new catalyst formulations, the selection of
and gold with redox-active transition metal com- the reaction conditions, and the use of promoters. A
pounds are known: Pd/Al O along with V(acac) or liquid-phase reaction provides a possibility to involve
2
3
3
soluble promoters in the system, for example, ionic
liquids (ILs) soluble in the organic media salts of
organic cations. It is known at present that the cations
and anions of ILs can affect or even participate in the
oxidation processes. A number of liquid-phase oxida-
tion systems containing ILs as catalyst components are
known [12–18]. In this context, the works related to
hydrogen peroxide are of special interest. Thus, it was
found that ILs increase the yield of hydrogen peroxide
FeCl [5], Pt, Rh, Ir, Pd or Ru with V O , Y O ,
Nb O , WO , La O or MoO /SiO [6], Au/Ti-
2
2
5
2
3
2
5
3
2
3
3
2
MCM-41 [7], and Au/TS-1 [8]. The studies on the
reaction are focused on the most practically important
oxidation reactions in the gas phase or liquid, such as
propylene epoxidation on Au/Ti-MCM-41 [7] and
Au/TS-1 [8], benzene hydroxylation on Pd/TS-1 [9],
and propane oxidation into acetone and isopropanol
on Au/TS-1 [10]. Recently, attention has also been in the reaction of O and H on a Pd catalyst [19]. A
2
2
focused on the reaction of cyclohexane with an O –H
2
2
work was published recently on the oxidation of cyclo-
oxidizing agent as an alternative to the important hexane with hydrogen peroxide where organic cations
522

CYCLOHEXANE OXIDATION WITH AN O –H MIXTURE
523
2
2
were involved in the catalyst composition as counteri-
ons for anions of heteropoly acid (HPA) H PMo V O
Catalytic Experiments
5
10
2
40
The catalytic reaction was carried out in a 35-mL
thermostated glass reactor at atmospheric pressure
and 35°C. The reactor was charged with a Pt/C cata-
lyst, HPA, and acetonitrile containing dissolved ionic
liquid. The reactor with an attached gas burette was
purged with an O : H = 1 : 2 (vol/vol) mixture and
[
20]. Since these two reactions are directly related to
the studied reaction of cyclohexane oxidation with an
O –H gas mixture in the presence of Pt/C–HPA, the
2
2
use of ILs as possible promoters of catalytic system
seems reasonable. The study of promoting properties
2
2
of several ILs was directed at the efficient use of H as
2
equilibrated with atmospheric pressure. Next, cyclo-
hexane was injected via a syringe through a side stop-
cock and the solution was vigorously magnetically
stirred. The consumption of the gas mixture was mea-
sured using a gas burette. After completion of the reac-
tion, the solution was cooled to 15°C and analyzed on
a Khromatek Kristall 2000M chromatograph (Russia)
equipped with an SPB-1000 capillary column (30 m ×
the most valuable component of the O –H gas mix-
2
2
ture for the selective transformation of cyclohexane
into oxidation products.
We used a catalyst containing a heterogeneous
component of Pt/C and a solution of heteropoly acid
(
HPA) in acetonitrile. Previously, we studied two-
component catalysts that were close to each other in the
liquid-phase reactions of hydrocarbons oxidation with
an O –H mixture [11]. We performed propane dehy-
0
.53 mm) and a flame ionization detector. The prod-
ucts were identified by retention times in comparison
with the standard compounds and by GC–MS on an
2
2
drogenation on the applied catalyst (Pt–H PMo O –
H PO )/SiO in the gas phase and observed high
3
12 40
7
000 GC/MS instrument (ZB-WAX column (30 m ×
3
4
2
0.25 mm), Agilent, United States). The amounts of
cyclohexane, cyclohexanol, and cyclohexyl bromide
were determined using calibration plots. After determin-
ing the initial and final amount of cyclohexane, the con-
version was calculated by the formula
selectivity level of propylene formation [2]. All these
catalytic systems contained HPA as a key component.
In this work, we characterized the state of
H PMo O acid in cyclohexane oxidation under cat-
3
12 40
alytic reaction conditions and tested other HPAs with
various metal ions within the heteropoly anion. We
explained the properties of a catalytic system, includ-
ing the effect of IL, in terms of the action of a Pt/C
catalyst, the chemical properties of HPA, and the
known mechanisms of peroxide oxidation.
C H
− С H
6 12fin
6
12init
CC6H12
=
×100%.
C H
6
12init
To calculate the H consumption efficiency for
2
cyclohexanol and cyclohexanone formation, we used
the equation for the formation of hydrogen peroxide
(
I) and its consumption for cyclohexane oxidation (II)
and (III), reduction (IV), and degradation (V) [24]:
Н + О = Н О , (I)
EXPERIMENTAL
2
2
2
2
Materials and Reagents
cyclo-C H + H O
2
6
12
2
(
II)
In the work, we used cyclohexane cyclo-C H
6
12
= cyclo-C H (OH) + H O,
6
11
2
(
(
analytical grade, Reakhim), acetonitrile CH CN
3
grade 0, Kriokhrom), 1-butyl-3-methylimidaz-
cyclo-C H12 + H O
6 2 2
cyclo-C H (O) + 2H O,
6 10 2
(
III)
olium bromide BMImBr (Aldrich), 1-butyl-3-
=
methylimidazolium tetrafluoroborate BMImBF
4
Н О + Н = 2Н О,
(IV)
(V)
(
Aldrich), tetrabutylammonium hydrogen sulfate
2
2
2
2
Bu NHSO (PS), and tetrabutylammonium bromide
4
4
Н О = Н О + 0.5О .
2 2 2 2
Bu NBr (Reakhim).
4
Taking into consideration the stoichiometry of
HPA H PMo O ⋅ 9H O (analytical grade, Reakhim) reactions (I)–(V), the efficiency of hydrogen con-
3
12 40
2
was purified by extraction with diethyl ether. HPA sumption was calculated by the formula
H PMo VO ⋅ 10H O, H PMo V O ⋅ 5H O, and
4
11
40
2
5
10
2
40
2
H2 prod
H PW VO ⋅ 4.5H O were obtained as described in
4
11
40
2
SH2
=
×100%,
the works [21, 22].
H
2 prod + H2H O
2
A Pt/C catalyst was prepared by the fast hydrolysis where H2 prod and H2H2O are the number of hydrogen
of chloroplatinic acid H PtCl followed by reduction
2
6
moles consumed for the formation of water and prod-
with sodium formate on a Sibunit carbon material
ucts, respectively.
2
(
1
d002 = 3.49 Å, S = 426 m /g, d = 3.9 nm and 30–
0 pore
00 nm, V
= 0.57 mL/g, particle size 5–50 μm).
The platinum content in the catalyst was 5%. The plat-
inum dispersity measured by CO chemisorption was
pore
Analysis of Reaction Solutions
The reaction solutions were analyzed after separat-
ing the Pt/C catalyst by centrifugation. To determine
48% [23]. The obtained catalyst was stored in air.
KINETICS AND CATALYSIS Vol. 58 No. 5 2017

5
24
L. I. KUZNETSOVA, N. I. KUZNETSOVA
Table 1. Absorption of O –H gas mixture and oxidation of cyclohexane in the presence of IL*
2
2
Cyclo-C H
12
Products, mmol (selectivity, %)
cyclohexyl
Gas
absorption,
mmol
6
Time,
min
SH2,
%
Entry
IL, mmol
–
taken, conver-
mmol sion, %
cyclohexanol cyclohexanone
bromide
1
2
3
4
5
6
7
24
90
150
110
25
1.70
1.97
1.83
1.82
1.59
1.80
1.87
1.85
1.85
0.84
0.89
1.85
1.85
0.89
~1
0.021 (98.6)
0.0003 (1.4)
0.022 (7.7)
0.0157 (7.8)
0.0025 (3.0)
0.0002 (0.9)
0.0025 (1.3)
0.003 (2.4)
~0
1.85
25.1
18.9
7.2
Bu NBr 0.056
15.0 0.262 (91.9)
24.0 0.184 (91.6)
9.5 0.080 (97.0)
0.0011 (0.4)
0.0012 (0.6)
4
BMImBr 0.036
traces
–
BMImBF 0.47
≈1
0.022 (99.1)
2.1
4
Bu NHSO 0.056 30
11.0 0.196 (98.7)
14.8 0.123 (97.6)
–
–
17.3
10.7
4
4
40
*
Reaction conditions: 20 mg of Pt/C, 20 mg of H PMo O ⋅ 9H O, 2 mL of CH CN, 35°C, O : H = 1 : 2 (vol/vol).
3 12 40 2 3 2 2
the Pt content in the reaction solution, an aliquot of ture; however, the cyclohexane conversion was low.
the solution was evaporated to dryness and the residue We expected the Pt/C–HPA catalytic system to
–
was refluxed in an HNO –HCl mixture. Analysis was improve by the addition of ionic liquids containing Br ,
3
+
+
performed by inductively coupled plasma–atomic H , and BMIm ions, which were previously used as
emission spectroscopy (ICP–AES) on a Perkin Elmer promoters in the synthesis of H O [19, 24, 26]. The
2
2
Optima 4300 DV spectrometer (United States).
addition of small amounts of BMImBr or Bu NBr (1–
4
The quantities of IL and HPA in the solutions after 2 wt %) could not significantly change the reagent’s
contact with a catalyst and completion of a catalytic solubility; however, nonetheless, they decreased the
reaction were calculated from the results of determin- absorption rate of the gas mixture. In contrast, the
ing C, N, and Mo. Samples for these analyses were amount of cyclohexane oxidation products related to
prepared as follows: a sample of the solution (usually the amount of absorbed gases and the efficiency of H
2
1
.5 mL) was added to 0.25 g of SiO calcined at 500°C,
2
consumption (S ) increased (Table 1, entries 2–4).
H2
the solvent was carefully evaporated in air while being
stirred, and the sample was heated at 170°C for 1 h.
The content of C and N was determined on an
EuroVector Euro EA 3000 CHNS-analyzer (Italy).
The content of Mo in the samples was determined by
X-ray fluorescent analysis on a Thermo Scientific
ARL ADVANT’X instrument (United States).
The UV-Vis spectra of the solutions were recorded
on a Shimadzu UV-2501 PC spectrometer (Japan) in
the region of 200–900 nm in a closed quartz cuvette
Figure 1 (curves 1, 2) displays the dependence of SH2
on the concentration of bromide ILs. The optimal
concentrations of the bromides are within 0.01–
0
.05 mol/L. At these concentrations, the value of SH2
is 18–25% and decreases at a higher IL concentration.
A similar effect of halide ions was observed in the syn-
thesis of H O from O and H on Pd or PdAu catalysts
2
2
2
2
in acidic aqueous solutions [24, 26] when the addition
of bromide increased the H O yield. The promoting
2
2
effect of bromide is explained by the increase in plati-
num’s positive charge, which stabilizes peroxide oxy-
gen by preventing its complete reduction to water and
(
0.06 mm).
31
The P NMR spectrum of the solution was obtained
as described in the work [25]. The chemical shifts were
determined relative to concentrated H PO .
peroxide decomposition. As a result, the rate of O and
2
3
4
H absorption decreased in the presence of bromide-
2
containing ILs. At the same time, the probability of
cyclohexane oxidation with a reactive peroxide spe-
cies increased and the efficiency of hydrogen con-
sumption rose.
RESULTS AND DISCUSSION
Cyclohexane Oxidation with the Use
of an O –H Mixture in the Presence of IL
2
2
Ionic liquids not containing bromide (BMImBF
4
Platinum applied on carbon suspended in CH CN
3
and Bu NHSO ) had little effect on gas absorption.
4
4
catalyzed the absorption of an O –H gas mixture to
2
2
BMImBF had no effect on the yield of oxidation
4
form H O. The incomplete reduction of oxygen with
2
products (Table 1, entry 5), whereas Bu NHSO
4
4
hydrogen occurred in an HPA solution, and partially
reduced oxygen formed peroxide intermediates, which
oxidized the cyclohexane into cyclohexanol and cyclo-
hexanone (Table 1, entry 1). The combined oxidation
of hydrogen and cyclohexane in the presence of a Pt/C
increased the yield (Table 1, entries 6, 7). The reaction
solution contained 0.0046 mol/L H PMo O acid. A
3
12 40
higher acidity level of the solution in the presence of
the acidic salt Bu NHSO favored the enhanced sta-
4
4
catalyst proceeded at close to the ambient tempera- bility of the free H
O
and peroxide intermediates par-
2
2
KINETICS AND CATALYSIS
Vol. 58
No. 5
2017

CYCLOHEXANE OXIDATION WITH AN O –H MIXTURE
525
2
2
tion with an O –H mixture without using ILs and the
SH2
2
2
same ketone : alcohol ratio was obtained in the reac-
tion with H O [20]. We have found that, under the
2
4
2
0
2
2
2
2
conditions of the reaction on a Pt/C catalyst, the
hydrogenation of cyclohexanone to cyclohexanol does
not take place (0.965 mmol of the ketone was used for
the hydrogenation). Therefore, cyclohexanol should
be considered as a primary product of cyclohexane
oxidation. We assume that one of the reasons of the
predominance of cyclohexanol in products is the abil-
ity of ionic liquids to inhibit the free-radical reactions
responsible for the secondary oxidation of the alcohol.
A similar effect of the inhibition of the H O radical
3
2
1
8
1
6
1
4
1
2
1
0
1
8
6
4
2
2
2
0
0.05
0.10
IL], mol/L
0.15
0.20
degradation was observed in the synthesis of hydro-
[
gen peroxide in the presence of BMImBF [19].
4
Indeed, alkylimidazolium cations can behave as rad-
ical traps [27]. Other ILs (Bu NHSO and Bu NBr)
Fig. 1. Efficiency of H2 consumption in presence of
BMImBr (1), Bu NBr (2), and Bu NHSO (3). Reaction
conditions: 20 mg of Pt/C, 20 mg of H PMo O ⋅ 9H O,
4
4
4
4
4
4
can also decrease alcohol oxidation by decreasing the
rate of the radical degradation of peroxide.
3
12 40
2
2
mL of CH CN, 0.2 mL (1.85 mmol) of cyclo-C H ,
3 6 12
The reaction characteristics in the presence of the
O : H = 1 : 2 (vol/vol), 35°C.
2
2
optimal Bu NBr amount were studied depending on
4
the conditions (Fig. 2). The number of moles of the
absorbed gas mixture and the resulting products of
cyclohexane oxidation increased proportionally with
the reaction time (curves 1–3), while the efficiency of
ticipating in the oxidation of cyclohexane. As a result,
the efficiency of H consumption increased to 19% at
2
a Bu NHSO concentration above 0.05 mol/L (Fig. 1,
4
4
H consumption remained unchanged (entries 1–3 in
2
curve 3). In the light of published data on hydrocarbon
the table under Fig. 2). At a longer reaction time, a
marked acceleration of the gas mixture absorption is
detected on curve 3. This is due to water accumula-
tion, as seen from the experiment with the addition of
the same amount of water to the initial mixture before
the reaction. In this case, the absorption rate of the gas
mixture was higher (Fig. 2, curve 4). The reducing
oxidation with the use of an O –H gas mixture [3],
2
2
the obtained values of H efficiency in cyclohexane
2
oxidation in the presence of ILs are high. The highest
selectivity was observed previously in the gas-phase
reactions. Thus, the efficiency of H was usually ~10–
2
2
0% on Au/Ti silicates in propylene epoxidation [7, 8]
and in propane oxidation into acetone and 2-propanol
10]. Let us note that the H O consumption efficiency
treatment of the Pt/C catalyst in an H flow at 300°C
2
[
2
2
also led to the enhanced absorption of the gas mixture
in cyclohexane oxidation in the presence of salt of
H PMo V O HPA was also at the level of 20%, as
(
Fig. 2, curve 5). In experiments with the addition of
5
10
2
40
water and reduction of the platinum catalyst, the value
follows from the results reported in the work [20].
of SH2 decreased (entries 4 and 5 in the table under
Cyclohexane oxidation results in the preferable for- Fig. 2) due to the increase in the nonproductive H
2
mation of cyclohexanol and small amounts of cyclo-
hexanone (Table 1). Small admixtures of cyclohexyl
bromide were detected in the presence of bromide-
containing ILs. The total amount of the products
determined by gas chromatography agrees closely with
the amount of consumed cyclohexane. A small loss of
cyclohexane (not more than 4%) is explained by its
high volatility. Trace amounts of peroxide compounds
in the reaction solutions were determined only quali-
tatively. Selective alcohol formation is a feature of
consumption for water formation.
Effect of HPA Composition on Catalysis
As shown in this and our previous works [2, 11], the
catalytic action in the cyclohexane oxidation with an
O –H mixture can be dependent on the modification
2
2
of the applied Pt particles, the nature of the support,
the medium’s acidity, and the composition of the het-
eropoly compounds. When the promoting additive
cyclohexane oxidation in the presence of Bu NBr,
4
Bu NHSO was used at a rather high concentration
BMImBr, and Bu NHSO . Table 1 displays the selec-
4
4
4
4
(
0.052 mol/L), we could observe the effect of the het-
tivity values of cyclohexanol, cyclohexanone, and
cyclohexyl bromide relative to the sum of these prod-
ucts. During the cyclohexane conversion of 10–24%,
the selectivity of the product formation is 92–98% and
eropoly acid composition, which varied on the substi-
tution of redox-active metal ions within the structure
of the heteropoly anion. The results of testing are given
in Table 2. On passing from H PMo O (I) to vana-
8
–2% for alcohol and ketone, respectively. For com-
3
12 40
dium-containing HPAs H PMo VO
(II) and
H PMo10V O40 (III), the yield of the main product
5 2
parison, the authors of the work [6] obtained 25% of
ketone against 75% of alcohol in cyclohexane oxida-
4
11
40
KINETICS AND CATALYSIS Vol. 58 No. 5 2017

5
26
L. I. KUZNETSOVA, N. I. KUZNETSOVA
(
cyclohexanol) decreased at the same amount of
H + O gas mixture consumption, mL
2 2
absorbed gas; i.e., the efficiency of H consumption
2
5
4
4
3
0
decreased on an increase in the number of vanadium
atoms in the anion (Table 2, entries 1–3). A further
5
0
5
3
decrease of the efficiency of SH2 occurred on the use of
H PW VO (IV). The share of ketone increased to
4
11
40
30
5
1
6% (Table 2, entry 4).
2
5
0
4
2
2
In the absence of the Bu NHSO acid, the yield of the
4
4
15
products and SH2 were low in all cases (Table 2,
entries 5–8). In the series of HPAs, these characteristics
were the highest for H PMo VO probably due to the
higher H concentration compared with H PMo O ;
however, they further decreased in the same order.
10
1
5
0
4
11
40
+
3
12 40
0
20
40
60
80
100
t, min
After completion of the reaction, we were able to
Curve
Time, (H + O ), Products,
SH2,
%
evaluate the average degree of HPA reduction (n ) by
2
2
e
number
min
mmol
mmol
titrating the reaction solutions not containing
Bu NHSO with KMnO (Table 2, entries 5–8). The
4
4
4
1
2
3
4
5
28
42
90
22
18
0.46
0.88
2.00
0.92
0.98
0.070
0.140
0.300
0.100
0.087
26.5
25.1
25.1
18.7
14.5
largest n of ~3 was found for HPA I; i.e., ~3 Mo(VI)
e
ions were reduced to Mo(V). Heteropoly acids II and
III were also significantly reduced; i.e., they contained
V(IV) and Mo(V) ions. Only the V(V) ion was reduced
in HPA IV. Along with the main reaction, the vana-
dium ions are active in the radical decomposition of
hydrogen peroxide. Therefore, the decreased effi-
Fig. 2. Time dependence for volume of reacted gas mix-
ture: 1–3, on increase of reacted gas amount; 4, in pres-
ence of moisture; 5, after preliminary reductive treatment
of catalyst. Reaction conditions: 20 mg of Pt/C, 20 mg of
H PMo O ⋅ 9H O, 2.0 mL of CH CN, 18 mg
ciency of H consumption and selectivity toward alco-
2
hol occurred in the oxidation of cyclohexane in the
presence of vanadium-substituted HPAs.
3
12 40
2
3
(
0.056 mmol) of Bu NBr, 0.2 mL (1.85 mmol) of cyclo-
4
C H , O : H = 1 : 2 (vol/vol), 35°C, 0.02 mL of H O
(4). Reaction products and hydrogen consumption effi-
ciency SH2 are given in table under figure.
6
12
2
2
2
The State of Catalytic System Components
The content of IL and HPA H PMo O in the
3
12 40
solutions was determined based on the C, N, and Mo
analysis of specially prepared solid samples [28]. The
analysis results for the samples prepared from the ini-
tial solutions (Table 3, entries 1 and 4) agree closely
with the theoretical element content. A comparison of
the analysis results for the initial solutions and the
solutions after contact with a Pt/C catalyst (Table 3,
entries 2, 5, and 7) showed that IL adsorption on the
catalyst surface is comparable with the experimental
error; it was less than ~4% and not larger ~0.14 mmol
per g of the catalyst. After the catalytic reaction, the
The state of HPA in the presence of IL was charac-
terized by the UV–Vis spectra. The spectrum of
the H PMo O solution in CH CN did not alter on
3
12 40
3
the addition of Bu NHSO or BMImBF (Fig. 3,
4
4
4
spectra 1–3). This means that HPA does not react
with ionic liquid cations. After contact with the Pt/C
catalyst, the solution’s absorption decreased by ~20%
(
spectra 4, 5), which corresponds to HPA adsorption.
A similar picture was observed for HPA solutions with
Bu NBr and BMImBr, except for the weak absorption
4
content of IL in the solution remained unchanged
band of reduced HPA with a maximum at 725 nm,
which appeared after contact with Pt/C (Fig. 4, spec-
–
(
Table 3, entries 3, 6, and 8). The content of Br ions
on the catalyst seems to be similarly low for bromide-
containing ILs. However, even such a low bromide
content was sufficient for promoting the effect on the
platinum surface [24]. The determination of Mo
–
trum 1). This indicates that Br ions favor platinum
oxidation with heteropoly acid due to the stabilization
of Pt(II) as bromide complexes.
(
Table 3) showed that the reaction solution after con-
Figure 4 displays the change in the H PMo O
3
12 40
tact with the catalyst and after the completion of the spectrum resulting from a catalytic reaction. A com-
reaction contained on average ~80% of the initial parison of spectra 1 (before the reaction) and 2 (1 h
amount of HPA. Thus, the adsorption of HPA on the after catalytic testing) shows that HPA reduction
catalyst was ~0.1 mmol per g of the catalyst, which is occurs during the reaction, which agrees closely with
close to the adsorption (mainly reversible) of the titration of the HPA solution (Table 2). The reduc-
H PW O on the Sibunit support from polar organic tion leads to a decrease of the absorption intensity in
3
12 40
solvents [29].
the long-wavelength region with a maximum at 315 nm,
KINETICS AND CATALYSIS
Vol. 58
No. 5
2017

CYCLOHEXANE OXIDATION WITH AN O –H MIXTURE
527
2
2
Table 2. Effect of redox-active metals within heteropoly anion on yield and product selectivity of cyclohexane oxidation
with O –H mixture*
2
2
Product yields
Bu NHSO ,
4
4
Entry
HPA
S
%
nē**
H2,
mol/L
cyclohexanol cyclohexanone
mmol
mol %
1
2
3
4
0.052
–
I
II
III
IV
0.2160
0.1350
0.0973
0.0458
0.0039 (1.8)
0.0051 (3.6)
0.0071 (6.8)
0.0105 (19)
19.4
11.9
8.9
–
5.2
5
6
7
8
I
II
III
IV
0.0240
0.0702
0.0582
0.0400
0.00058 (2.3)
0.0019 (2.6)
0.0016 (2.7)
0.0077 (16)
2.0
5.7
4.7
4.4
3
1.7
2.5
0.7
*
3
Reaction conditions: 20 mg of 5% Pt/C, 2 mL of CH CN, 0.2 mL of cyclo-C H , 0.01 mmol of HPA (entries 1 and 5, 20 mg of
3 6 12
H PMo O ⋅ 9H O; 2 and 6, 20 mg of H PMo VO ⋅ 10H O; 3 and 7, 19 mg H PMo V O ⋅ 5H O; 4 and 8, 28 mg of
12 40
2
4
11
40
2
5
10
2
40
2
H PW VO ⋅ 4.5H O); 35°C, O : H = 1 : 2 (vol/vol).
4
11
40
2
2
2
*
*Average number of electrons transferred upon reduction and calculated per HPA molecule.
Table 3. Analysis results for N, C, and Mo in IL and HPA solutions [28]*
Found/calculated, wt %
C
IL in initial
solutions**, g
Entry
IL
Procedure***
N
Mo
1
2
3
Bu NBr
0.0180
0.0180
0.0179
A
B
C
0.197/0.193
0.188/0.193
0.192/0.192
2.69/2.65
2.55/2.65
2.65/2.64
2.85/2.86
2.20/2.86
2.72/2.86
4
4
5
6
Bu NHSO4
0.0194
0.00975
0.0189
A
B
C
0.201/0.198
0.106/0.102
0.176/0.183
2.71/2.71
1.40/1.39
2.83/2.52
2.91/2.85
2.52/2.92
4.13/5.45
4
7
8
BMImBr
0.0140
0.0129
B
C
0.430/0.442
0.375/0.411
1.51/1.55
1.38/1.41
2.36/2.89
2.48/2.90
*
Initial solutions: IL weighed sample, 20 mg of HPA (entry 6, 40 mg); 2.2 mL (entries 4 and 6, 4.4 mL) of CH CN : C H = 10 : 1 (vol/vol).
3 6 12
*
* Solution sample taken for analysis: 1.5 mL (3.0 mL for entries 4 and 6) per 0.25 g SiO .
2
*
** A, initial solution; B, after contact with 20 mg of Pt/C, 1 h, air, 25°C; C, after catalytic reaction.
which is caused by a charge transfer from the molecu- absorption at 315 nm (spectrum 3), which is associated
lar orbital (MO) composed of p orbitals of bridging with the process of the heteropoly anion’s reoxidation
oxygen atoms to the lower antibonding MO of the with oxygen.
molybdenum atom in the heteropoly anion structure
The composition of heteropoly anions in the reac-
[
30], and strong absorption appeared with maxima at
31
tion solution was analyzed by P NMR (Fig. 5). After
the reaction, the spectrum of the catalyst solution sep-
arated from Pt/C exhibited two signals: the
x−5
7
00 and 855 nm due to the charge transfer transitions
Mo(V) → Mo(VI) in the HPA structure [31]. The
emergence of these two absorption bands indicates the
formation of a deeply reduced protonated form of the
heteropoly anion upon the reduction of HPA cata-
H PMo O anion (–6.8 ppm) reduced by two elec-
x
12 40
3
−
40
trons and the oxidized PMo O (–4 ppm) anion
1
2
lyzed by Pt/C [32]. In the presence of air, the spec- [25] with an intensity ratio of 20 and 80%. The share
trum changes in the long-wavelength region to form a of reduced HPA decreased during the spectrum
distinct maximum at ~760 nm and an increase in recording due to the contact with air (spectra 2, 3),
KINETICS AND CATALYSIS Vol. 58 No. 5 2017

5
28
L. I. KUZNETSOVA, N. I. KUZNETSOVA
А
А
4
2.5
2
.0
.5
.0
3
2
1
1
315
1
760
3
11
700
855
1
0.5
418
2
3
1
2
3
725
4
5
0
2
00 300 400 500 600 700 800 900
3
00 400 500 600 700 800 900
Wavelength, nm
Wavelength, nm
Fig. 4. UV–Vis absorption spectra for reaction solution of
Fig. 3. UV–Vis absorption spectra for solutions of
H PMo O ⋅ 9H O (0.005 mol/L) and IL (0.026 mol/L)
H PMo O ⋅ 9H O (0.0046 mol/L) and 18 mg of
3
4
12 40
2
3
12 40
2
Bu NBr (0.026 mol/L) in contact with Pt/C: (1) before
in CH CN: (1) HPA; (2) HPA and BMImBF ; (3) HPA
3
4
catalysis; (2) after catalytic reaction; (3) solution 2 was
and Bu NHSO ; (4 and 5) solutions 2 and 3, respectively,
4
4
kept in cuvette with access to air.
after contact with Pt/C in air for 30 min at 35°C.
trons, also led to reduced by four electrons, protonated
while no other phosphorus signals appeared. Thus, in
y−7
the acetonitrile solution as in the other organic polar heteropoly anions H PMo ,O , which could be
y
12
40
3
1
solvents [25], the reduction of H PMo O led to the
3
12 40
observed in the P NMR spectrum at ~–13 ppm [25].
formation of HPAs, which underwent reversible oxida- But the latter seem to undergo rapid oxidation in air. The
tion with oxygen. According to the absorption spectra reversible transformations of the heteropoly anion in
(
Fig. 4, spectra 2, 3), we can assume that the reaction in CH CN, close to the solid state of the HPA [33], are
3
the solution, along with the anions reduced by two elec- shown in Scheme 1.
Pt/C, H2
Pt/C, H2
3
−
(V) x−5
(V) y−7
⎯
⎯ ⎯ ⎯ ⎯→
⎯ ⎯ ⎯ ⎯ ⎯→
PMo O
H PMo Mo O
H PMo Mo O
40 ←⎯ ⎯ ⎯ ⎯⎯
y 8 4 40
1
2
40 ←⎯ ⎯ ⎯ ⎯⎯
x
10
2
1
2O2
1 2O2
Scheme 1. The formation of reduced anions from HPA H PMo O under the reaction conditions.
3
12 40
We also studied the Pt/C catalyst’s stability under alytic reaction was repeated. After the triple repetition
reaction conditions. Platinum did not transfer to the of the catalytic testing and reduction treatment (Table 4,
reaction solution in the absence of IL (Table 4, entry 1).
The platinum dissolved when the catalyst came into
contact with the bromide ionic liquid solution at a
entries 4b–4d), the solution contained less platinum
than after first testing; this denotes the possibility of
reducing platinum directly in the reaction solution.
molar ratio of Bu NBr : Pt = 11, which is optimal for
4
However, the efficiency of H consumption decreased
2
attaining the maximum selectivity for H (Table 4,
2
in the course of this testing, mainly due to the water
accumulation in the solution.
entry 2). During the cyclohexane oxidation, more
than half of the platinum passed into the solution
(
Table 4, entry 3a). This indicates that platinum read-
The platinum was not dissolved from the surface of
the Pt/C catalyst in the presence of Bu NHSO . In
ily undergoes oxidation and dissolution in the pres-
4
4
–
ence of a strongly binding Br ligand. On repeated
repeated testing (Table 4, entries 5a–5d), the catalyst
testing of the used catalyst in a fresh solution, the H –
2
was separated each time and tested again in a fresh
O gas absorption slowed to half its pace, while the
2
solution. The efficiency of the H consumption
2
efficiency SH2 remained rather high (Table 4, entry
remained almost constant, a slight decrease of SH2
seems to have resulted from the water accumulation on
the catalyst surface, which was not dried between the
3
b). In the solution separated from the Pt/C catalyst,
the reaction of O with H , as well as the cyclohexane
2
2
oxidation, does not take place. This indicates that H2
oxidation and O activation proceed on the platinum trials. Thus, the Pt/C catalyst does not undergo deac-
2
surface. In another experiment, the entire reaction tivation on multiple usage in the presence of an ionic
mixture after catalysis was treated with H and the cat- Bu NHSO liquid not containing bromide.
2
4
4
KINETICS AND CATALYSIS
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2017

CYCLOHEXANE OXIDATION WITH AN O –H MIXTURE
529
2
2
–4
–
6.8
1
2
3
~
20%
NS 80
Scanning continued
NS 240
~
16%
New scanning
NS 200
~
4%
1
–1
–3
–5
–7
–9
–11
–13
Chemical shift, ppm
Fig. 5. 31P NMR spectra of reaction solution after reaction interruption and catalyst separation recorded at 298 K on contact with
air: (1) 8 min after start of recording; (2) when scanning continued; (3) new scanning. Reaction mixture composition: 20 mg of
Pt/C, 20 mg of H PMo O ⋅ 9H O, 18 mg of Bu NBr, 2 mL of CH CN, 0.2 ml of cyclo-C H .
3
12 40
2
4
3
6
12
On the Mechanism of Cyclohexane Oxidation
the selective formation of alcohol in the presence of
deeply reduced H PMo O indicates a different
mechanism of hydrocarbon oxidation with the partic-
ipation of redox-active heteropoly molybdates.
3
12 40
According to the current notions [11, 34–36],
cyclohexane oxidation in the presence of variable-
valence metal ions can be initiated by OH radicals.
Next, the cyclohexyl radicals react with molecular
In the studied catalytic system (in the absence of
oxygen to give cyclohexyl hydroperoxide. Decomposi- added hydrogen peroxide), reactive peroxide interme-
tion of the hydroperoxide leads to oxidation products: diates are produced under the action of a two-compo-
alcohol and ketone. Such a mechanism was men- nent Pt/C–HPA catalyst (Scheme 2). The activation
tioned in [35] for the reaction of cyclohexane oxida- of molecular oxygen on the platinum surface is a key
tion with hydrogen peroxide catalyzed by metal-sub- stage. Other components, HPA and IL (in particular,
stituted heteropoly tungstates and, judging from the bromide IL), distributed between the solid catalyst
obtained alcohol to ketone ratio (Table 4, entries 4 and 8), and the solution promote the surface of the Pt/C cat-
it can be realized for H PW VO in reaction with the alyst, thus providing the conditions for the formation
4
11
40
participation of the formed in situ peroxide. However, of a peroxide species on platinum. The acidic HPA
KINETICS AND CATALYSIS Vol. 58 No. 5 2017

5
30
L. I. KUZNETSOVA, N. I. KUZNETSOVA
Table 4. State of Pt/C catalyst and H consumption efficiency in cyclic tests*
2
[Pt]soln
mg/mL
,
SH2,
%
Gas absorption,
mmol
Reaction time,
min
Entry
IL
–
1
3.44
–
29
60
0.002
0.15
1.9
–
21
Bu NBr
4
3
3
a
b
Bu NBr
1.82
70
150
0.30
0.035
23.2
19.8
4
2
1.65
4
4
a
Bu NBr
1.82
.72
70
100
–
0.16
–
7.3
4
b–4d3
5
5
5
5
5
a
b
c
d
Bu NHSO4
1.85
.83
1.88
.82
38
36
38
32
0.0008
0.0008
0.002
17.3
18.3
14.8
15.5
4
2
1
2
1
0.0005
2
*
Reaction conditions: 40 mg of Pt/C, 40 mg of HPA, 4 mL of CH CN, 38 mg of IL, 0.4 mL of C H , 35°C, O : H = 1 : 2 (vol/vol).
3 6 12 2 2
1
2
3
In air. On repeated testing of Pt/C catalyst in fresh reaction solution. Catalytic test, treatment of reaction mixture with H and
2
repeated testing (b, c, and d are single, double, and triple, respectively).
and Bu NHSO components are necessary for the sta- with O –H gases on the surface of catalysts supported
4
4
2
2
bilization of the peroxide by protonation. Further per- by SiO [1, 2]. As for the catalysts used under the gas-
2
oxide transformations involve redox-active HPAs, phase conditions, the conclusion was drawn that sub-
among which H PMo O is the most efficient in strate oxidation with the participation of active oxygen
3
12 40
cyclohexane oxidation. The HPA adsorption on the proceeds on Pt/HPA contacts [1]. In the liquid-phase
surface of the Pt/C catalyst provides a possibility to reaction, the reaction of the HPAs with the peroxide
produce active forms of oxygen, as it took place in the species formed on platinum can proceed most likely
gas-phase reactions of benzene or propane oxidation through the solution.
Pt(0) O2
(
VI)
(V)
cyclo-C H OH, H O
6 11
2
H PMo Mo2
x
10
Pt(II)(O )
2−
2H+
2
H2
Pt(II)
Scheme 2. Cyclohexane oxidation with an O –H mixture
(VI)
(V)
cyclo-C H
6
12
H PMo Mo H O
2
x
10
2
2
2
2
in a Pt/C–H PMo O –Bu NHSO –CH CN catalytic system.
3
12 40
4
4
3
The features of the mechanism of the reaction occurs upon the reduction due to the protonation, and
under study are caused by the reversible redox trans- peroxide oxygen can coordinate on the Mo(V) or
formations of molybdophosphoric acid (as well as 1-
and 2-vanadium-substituted HPAs). Due to the action
of hydrogen and the Pt/C catalyst, heteropoly molyb-
date anions are in a rather deeply reduced state.
Anions reduced by two or even four electrons as shown
in Scheme 1 can be an active form of molybdophos-
V(IV) ions in the active center of the heteropoly
molybdate anion as shown in Scheme 3. The presence of
one-electron reducing agents (Mo(V) or V(IV)) in the
heteropoly anion and the fast electron exchange between
metal ions in the heteropoly anion [37] cause the forma-
phoric HPA. A weakening of the bridging Mo–O–Mo tion of radical forms of oxygen reactive in the cyclohex-
(
or V) bonds in the anion of the Keggin structure ane oxidation into cyclohexanol [38] (Scheme 3).
KINETICS AND CATALYSIS
Vol. 58
No. 5
2017

CYCLOHEXANE OXIDATION WITH AN O –H MIXTURE
531
2
2
+
+
H H
O
2 2
H O
OH OOH
OH O O
2
2
Mo(V) Mo(V)
Mo(VI)]
Mo(V)
Mo(V)
Mo(V)
Mo(V)
[
H+
[Mo(VI)]
H+
[Mo(V)]
+
+
cyclo-C H
H H
O
6
11
2H+
cyclo-C6H12
OH O OH
2
Mo(V)
Mo(V) Mo(V)
[Mo(VI)]
Mo(V)
[Mo(V)]
6 11 2
cyclo-C H OH, H O
Scheme 3.
The suggested mechanism of oxygen activation
This work was conducted within the framework of
budget project no. 0303-2016-0006 for Boreskov Insti-
tute of Catalysis.
i
implies that active OH radicals do not enter the solu-
tion. At the same time, the free-radical processes in
the solution responsible for the secondary oxidation of
cyclohexanol and decomposition of hydrogen perox-
ide are inhibited by the ionic liquids, which is the rea-
son of their positive effect on the efficient hydrogen
consumption and the selective conversion of cyclo-
hexane into cyclohexanol.
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Translated by I. Kudryavtsev
KINETICS AND CATALYSIS
Vol. 58
No. 5
2017