Kinetics and mechanism of catalytic oxidation of alcohols to carbonyl compounds with dioxygen in the Pd-containing aqua system

Article abstract of DOI:10.1007/s11172-007-0132-y

The oxidation of lower aliphatic alcohols C₁-C₄ with dioxygen to form the corresponding carbonyl compounds in the presence of the PdII tetraaqua complexes and Fe^II-Fe^III aqua ions in an aqueous medium was studie

Full text of DOI:10.1007/s11172-007-0132-y

Russian Chemical Bulletin, International Edition, Vol. 56, No. 5, pp. 875—882, May, 2007
875
Kinetics and mechanism of catalytic oxidation of alcohols to carbonyl
compounds with dioxygen in the Pdꢀcontaining aqua system
V. V. Potekhin
St. Petersburg State Technological Institute (Technical University),
2
6 Moskovsky prosp. 198013 St. Petersburg, Russian Federation.
Fax: +7 (812) 712 7791. Eꢀmail: potechin@mail.admiral.ru
The oxidation of lower aliphatic alcohols C —C with dioxygen to form the corresponding
1
4
II
II
III
carbonyl compounds in the presence of the Pd tetraaqua complexes and Fe —Fe aqua ions
in an aqueous medium was studied at 40—80 °C. The introduction of an aromatic compound
II
(
acetophenone, benzonitrile, phenylacetonitrile, oꢀcyanotoluene, nitrobenzene) and Fe aqua
III
ion instead of the Fe aqua ion into the reaction system increases substantially the catalytic
activity and the yield of the carbonyl compound. The key role of the Pd species in the
intermediate oxidation state stabilized by the aromatic additive in the catalytic cycle of alcohol
oxidation with dioxygen to the carbonyl compound was shown. An increase in the kinetic
isotope effect with an increase in the temperature of methanol oxidation indicates a change in
II
II
III
the rateꢀdetermining step of alcohol oxidation with dioxygen in the presence of Pd —Fe —Fe
and the aromatic compound. At temperatures below 60 °C, the catalytically active palladium
II
II
species are mainly formed upon the reduction of the Pd tetraaqua complex with the Fe aqua
II
ion, whereas at higher temperatures the reaction between the alcohol and Pd predominates.
The mechanism and kinetic equation of the process were proposed.
Key words: tetraaquapalladium(II) complex, iron(III)/(II), oxygen, oxidation of alcohols.
The catalytic oxidation of aliphatic alcohols С —С
valent state of palladium. It has been shown7,8 that the
palladium species in an intermediate oxidation state act
1
4
to the corresponding carbonyl compounds in the presꢀ
II
III
II
ence of the Pd tetraaqua complex, Fe , and dioxygen
as the catalyst in the oxidation of the Fe aqua ion with
1
II
in an aqueous medium has been studied earlier
dioxygen in the presence of the Pd tetraaqua complex.
The activity of these species increases in the presence of
additives of some aromatic compounds, in particular, aroꢀ
matic nitriles. For instance, in the presence of benzoꢀ
nitrile, no precipitation of palladium black occurs and
1
2
1 2
2
R R CHOH + O
2 R R CO + 2 H O.
(1)
2
2
Based on the obtained kinetic regularities, we conꢀ
I
cluded that Pd species formed upon the reduction of the
II
III 8
Fe is oxidized with dioxygen to Fe
.
II
Pd tetraaqua complex with the substrate are involved in
This experimental fact can be used to determine the
conditions providing an increase in the yield of carbonyl
compounds and the selectivity of alcohol oxidation with
I
the catalytic reaction. In this case, the Pd species play
both a positive role, providing selective conversion of
alcohols to carbonyl compounds, and a negative role. The
latter is related to a side reaction removing the Pd species
from the catalytic cycle as inactive palladium black
through the interaction between the Pd tetraaqua comꢀ
plex and Fe aqua ion. The Fe ion is accumulated
during reaction (1), because the oxidation of the Fe
aqua ion to Fe with dioxygen in a perchloric acid meꢀ
II
III
dioxygen in the presence of the Pd —Fe aqua ions in
an aqueous medium. At the same time, the additives of
aromatic compounds in the catalytic system can suppress
I
II
II
II
the side reaction between Pd and Fe with palladium
black formation. In the present work we studied the
II
2
II
II
II
II/III
Pd —Fe
—PhX catalytic system (PhX is an aromatic
III
compound) in the oxidation of alcohols С —С with diꢀ
1
4
dium occurs in low yield. Therefore, one can expect that
in a wide temperature range under the conditions of quanꢀ
titative oxidation of the Fe aqua ion with dioxygen in the
oxygen.
II
Experimental
II
presence of the Pd tetraaqua complex the yield of the
carbonyl compounds upon alcohol oxidation in the
The PdII tetraaqua complex was synthesized according to an
earlier described procedure. Alcohols (reagent grade) were used.
The initial aromatic compounds (benzonitrile, phenylacetoniꢀ
trile, acetophenone, oꢀcyanotoluene, and nitrobenzene) were
II
II
III
Pd —Fe /Fe —O system should increase.
9
2
II
A special role in the redox reactions involving Pd
3
—8
belongs to the aromatic derivatives
stabilizing the lowꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 842—848, May, 2007.
066ꢀ5285/07/5605ꢀ0875 © 2007 Springer Science+Business Media, Inc.
1

876
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 5, May, 2007
V. V. Potekhin
purchased from Aldrich. The aromatic compounds were used as
.02—0.04 М aqueous solutions corresponding to their limiting
V(O )/mL
2
0
solubility in water. To obtain specified concentrations of subꢀ
stances, a calculated amount of a solution of the Pd tetraaqua
4.0
3.0
2.0
1.0
0
2
3
4
II
complex and oxidized alcohol were added to an aqueous soluꢀ
III
tion of the aromatic compound. The Fe ion was introduced
into the reaction by adding a solution of Fe (SO ) •9H O in
2
4 3
2
II
perchloric acid. A specified concentration of Fe in the reaction
mixture was created by the dissolution of a weighed sample of
Moore´s salt. In all runs, the volume of the reaction solution was
0 mL, and the concentration of perchloric acid was 0.7 mol L–
1
.
1
1
The reaction was carried out using a volumetric setup and a
temperatureꢀcontrolled reactor of the "catalytic duck" type at
0—80 °C, and the amount of consumed dioxygen was meaꢀ
4
50
100
150
200 250
τ/min
sured at a constant pressure of 0.1 MPa. In preliminary experiꢀ
ments we determined the shaking frequency above which the
reaction rate remained unchanged, i.e., the kinetic regime was
provided. The partial oxygen pressure was varied by dilution of
Fig. 1. Effect of the aromatic additives ([PhX] = 12 mmol L–1
)
on the rate of methanol oxidation with dioxygen: 1, no arene
additive; 2, benzonitrile; 3, acetophenone; 4, nitrobenzene;
II
–1
III
–1
[
Pd ] = 5 mmol L , [Fe ]0 = 30 mmol L , [MeOH] =
oxygen with argon.
0
–
1
–1
The concentration of PdII was measured spectrophotometꢀ
4 mol L , [HClO4] = 0.7 mol L , 65 °C; hereinafter V(O2) is
the volume of absorbed oxygen.
rically from the intensity of the green color of the complex
II
formed by the addition of excess Sn chloride solution to the
analyzed sample.10 The concentration of FeIII was determined
1
1
In all cases, the induction period caused by the formaꢀ
tion (accumulation) of an active species followed by the
region of the developed reaction in which the rate of
oxygen absorption is maximum and constant, are observed.
As should be expected, in the presence of an aromatic
compound, the duration of the developed reaction of alꢀ
cohol oxidation is much higher than that in the solution
containing no additive. In the latter case, the reaction is
rapidly retarded (see Fig. 1, curve 1) because of the reꢀ
by spectrophotometry as a complex with sulfosalicylic acid.
The amount of formaldehyde formed was measured spectroꢀ
photometrically in the presence of chromotropic acid sodium
1
2
salt. The concentrations of other carbonyl compounds were
determined by the gravimetrically from the formation of hydraꢀ
zone with a solution of 2,4ꢀdinitrophenylhydrazone.
The conversion products of the aromatic compounds were
analyzed by LCꢀMS (Agilent 6890/5973 Network, HPꢀ5
Crosslinked 5% PH Siloxane column with a length of 100 m).
The organic substances were extracted from the reaction soluꢀ
tion with chlorobenzene that, as found for model systems, comꢀ
pletely extracts aromatic compounds from an aqueous solution.
In the case of benzonitrile calibration was used.
II
0
duction of Pd to Pd and precipitation of palladium
black. In the case of the areneꢀcontaining additive (benꢀ
zonitrile, acetophenone, nitrobenzene), no Pd reduction
occurs in the solution.
Results and Discussion
When methanol is oxidized in the presence of the
areneꢀcontaining additives, the number of formaldehyde
formed and the conversion of Fe increase (Table 1).
As can be seen from the data in Table 1, the amount of
the aromatic derivative introduced into the reaction soluꢀ
III
The kinetic curves of oxygen absorption during methaꢀ
II
III
nol oxidation in the system of Pd —Fe aqua ions in the
presence of some aromatic compounds are shown in Fig. 1.
Table 1. Effect of the aromatic compounds (PhX) on the oxidation rate of methanola
τb
tc
[FeIII
]
[PdII]
[O2]d
[HCHO]
wmax(O )•10
/
6
PhX
2
mol L– s–1
1
min
mmol L–1
No additives
PhCN
40
60
65
70
100
170
170
200
300
27
10
10
10
7
0
5.75
18.8
18.8
19.1
16
18
48
48
49
44
2
2.6
2.6
4.6
4.6
4.2
4.6
PhCH CN
2
PhC(O)CH3
PhNO2
2.2
121 (60)e
1.6 (2.3)e
a
Pd ] = 5 mmol L–1, [FeIII]0 = 30 mmol L–1, [PhX] = 12 mmol L–1, [MeOH] = 4 mmol L–1, [HClO ] =
II
[
0
4
–
1
0
b
.7 mol L , 65 °C.
Induction period.
Reaction duration.
Concentration of reacted oxygen.
PhNO ] = 5 mmol L .
с
d
e
–1
[
2

Tetraaquapalladium(II) as oxidation catalyst
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 5, May, 2007
877
tion exerts no substantial effect on the rate of the develꢀ
oped reaction. However, the induction period changes: it
increases with the introduction of the additive and has the
maximum duration in the case of nitrobenzene. A deꢀ
crease in the initial nitrobenzene concentration in the
reaction mixture decreases the induction period, which is
probably caused by the oxidation properties of nitrobenꢀ
zene, because no such effect is observed for other aroꢀ
matic derivatives.
Pd (PhX)(O )] + 2 Fe2+
2+
[
2 2 aq
2
Pd2+ + 2 Fe3+ + PhX + 2 H O,
(10)
aq
aq
2
Pd22+ + R R CHOH
1
2
1
2
0
+
R R CO + 2 Pd + 2 H ,
(11)
(12)
(13)
(14)
(15)
Pd 2+ + 2 Fe2+
2
2 Pd0 + 2 Fe3+aq
,
aq
For all the aromatic compounds used, there is some
Pd2+ + Fe2+
aq
Pd + Fe3+aq,
+
–
1
critical concentration, ∼ 3 mmol L , below which alcoꢀ
hol is oxidized in the same way as in the absence of an
additive.
+
2+
0
Pd + Fe3+aq,
Pd + Fe
aq
The effect of the aromatic compounds on the
Pdꢀcatalyzed oxidation of alcohols in argon has been reꢀ
0
2+
Pd22+.
Pd + Pd
aq
6
ported earlier. The determining role of the benzene ring
The main reactions proceeding via parallel routes can
be distinguished from an array of steps (2)—(15). Reacꢀ
tions (2), (4), (5) and (2), (6)—(10) compose the catalytic
cycle of alcohol oxidation with iron(III) and dioxygen,
respectively, and reactions (3) and (11)—(14) producing
palladium black refers to chain termination in the cycle of
the Pdꢀcatalyzed alcohol transformation into the carbonyl
compound.
rather than the electronic properties of the functional
group of the substituent in the aromatic ring was shown.6
It is most likely that the mechanisms of the influence of
the arene compounds in the alcohol oxidation and the
II
oxidation of Fe aqua ion with dioxygen in the presence
II
of the Pd tetraaqua complex have a common feature: the
active form of an intermediate palladium species is stabiꢀ
lized and its transformation into palladium black is supꢀ
pressed.
Note that the palladium complex with dioxygen is also
2
+
formed by the reaction of dioxygen with the Pd2 species
containing no coordinated aromatic ligand. However, the
data on the kinetics of alcohol oxidation with dioxygen in
the absence of PhX suggest that in this case the reactions
producing palladium black (reactions (11) and (15)) proꢀ
ceed more rapidly than the formation of the palladium
complex with the active dioxygen. Therefore, alcohol oxiꢀ
Based on the results of several studies,1,2,6—8,13,14 we
can assume that the oxidation of alcohols with dioxygen
II
III
in the catalytic Pd —Fe aqua ions system in the presꢀ
ence of the PhX aromatic compounds includes the folꢀ
lowing steps:
1
2
2+
R R CHOH + [Pd(H O) ]
II
III
2
4
dation with dioxygen in the presence of the Pd —Fe
1
2
+
+
[
R R C(OH)Pd(H O) ] + H O ,
(2)
(3)
aqua ions but without an aromatic additive is characterꢀ
ized by a considerably lower number of catalytic cycles
2
3
3
II
1
2
+
(2.5 with respect to Pd ) compared to the system conꢀ
[
R R C(OH)Pd(H O) ]
2 3
taining the PhX additive (more than 60).
The key role in the mechanism (see reacꢀ
tions (2)—(15)) belongs to the palladium species in the
intermediate oxidation state (Pd ) and the Pd
The formation of the cluster palladium species during
aliphatic alcohol oxidation with the Pd tetraaqua comꢀ
plex was proved by UV spectrophotometry from the apꢀ
pearance of an absorption band with a maximum at 312 or
1
2
0
+
R R CO + Pd + H O ,
3
1
2
+
3+
+
[
R R C(OH)Pd(H O) ] + Fe
2
3
aq
I
2+
clusters.
2
1
2
+ Fe2+ + H O ,
+
R R CO + Pd
(4)
(5)
aq
aq
3
II
Pd+ + Fe3+
Pd2+ + Fe2+aq,
aq
aq
aq
1
2
+
2+
13
[
R R C(OH)Pd(H O) ] + Pd
316 nm, depending on the nature of alcohol.
2
3
aq
2
+
Pd22+ + R R CO + H O ,
1
2
+
(6)
(7)
The Pd2 species manifest the enhanced reactivity
3
2
toward dioxygen. It has previously been found that the
III
Fe aqua ion does not oxidize the cluster palladium but
Pd22+ + PhX
[Pd (PhX)]2+,
2
has a substantial effect on the step of their formation. In
I
III
II
excess Fe aqua ion over Pd , no cluster palladium speꢀ
cies are formed in concentrations sufficient for the progress
[
[
Pd (PhX)]2+ + O2
[Pd (PhX)(O )]2+,
(8)
(9)
2
2
2
III
of reactions (7)—(10) until the rate of Fe consumption
II
in reaction (4) becomes lower than the rate of reaction (6).
This time determines the induction period in the kinetic
curves of oxygen absorption. Therefore, the duration of
the induction period of alcohol oxidation with dioxygen
2+
1 2
Pd (PhX)(O )] + R R CHOH
2
2
1
2
2+
R R CO + 2 Pd + PhX + 2 H O,
2

878
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 5, May, 2007
V. V. Potekhin
in the presence of the Pd —FeIII aqua ions elongates
II
tion can be unconvincing, because the decrease in the
III
with an increase in the initial concentration of the
initial concentration of the Fe aqua ion itself shortens
III
III
Fe aqua ion.
the induction period (see above). In addition, the Fe
The duration of the induction period (τ) for methanol
ion has an effect on the rate of the reaction between the
II
–1
II
II
oxidation with dioxygen ([Pd ] = 5 mmol L
,
Pd and Fe aqua ions, which decreases with an increase
0
–
1
–1
III
[
1
MeOH] = 4 mol L , [HClO ] = 0.7 mol L , [PhCN] =
in the initial concentration of Fe
due to reversible
4
aq
–
1
2
2 mmol L , 65 °C) at different initial concentrations
reaction (13). If this suggestion is valid, the increase in
the rate of formation of the active Pd2 species and,
III
2+
of Fe is presented below.
correspondingly, shortening of the induction period are
[
FeIII]0/mmol L–1
τ/min
10
20
20
40
30
60
II
possible when the Fe aqua ion is introduced in the abꢀ
III
sence of the Fe aqua ion in the initial reaction solution
of alcohol oxidation with dioxygen.
On going of the process from the induction period to
the developed reaction of alcohol oxidation with dioxyꢀ
II
Indeed, the addition of the Fe aqua ion to the reacꢀ
III
II
tion solution in the absence of Fe results in the disapꢀ
gen, a certain ratio of concentrations of the formed Fe
III
pearance of the induction period (Fig. 3).
and unreacted Fe is established in the reaction solution.
III
–1
If the solution contains no aromatic compound, palꢀ
ladium black precipitates immediately due to the reaction
When the initial Fe concentration is 30 mmol L , the
solution at the end of the induction period contains the
II
II
FeII aqua ion in a concentration of 20 mmol L–1 correꢀ
between the Pd and Fe aqua ions.
When alcohol is oxidized with dioxygen in the presꢀ
sponding to the 2 : 1 ratio. It is most likely that this ratio is
close to equilibrium value because it remains unchanged
II
II
ence of the Pd —Fe aqua ions, palladium black is comꢀ
pletely formed in the presence of all aromatic compounds
used, except for aromatic nitrile such as benzonitrile. At
until the rate of oxygen consumption is constant.
If the initial reaction solution contains the FeII and
III
FeIII aqua ions in concentrations of 20 and 10 mmol L
–1
the same time, in excess Fe aqua ion, all studied aroꢀ
,
matic additives exert a similar effect. Therefore, we studꢀ
respectively, methanol oxidation with dioxygen has a conꢀ
siderably shorter induction period (Fig. 2).
II
ied the effect of the Fe aqua ion on the rate of reacꢀ
tion (1) in the presence of benzonitrile.
The rate of oxygen consumption is constant and
slightly changes with the variation of the initial Fe conꢀ
centration in a range of 5—60 mmol L . The material
balance of methanol oxidation and the reaction rates unꢀ
der different initial conditions are given in Table 2.
The data in Table 2 show that the rate of oxygen
consumption in the catalytic system containing the Fe
aqua ion in the initial solution instead of Fe , under
It should be mentioned that if the catalytic alcohol
oxidation with dioxygen is related to the formation of
palladium cluster species, then the introduction of a reꢀ
ducing agent into the reaction solution should shorten the
induction period.
II
–
1
The formation of the cluster palladium species is a
II
result of reduction of the Pd tetraaqua complex with the
II
II
initial organic substrate and also with the Fe aqua ion
III
2
(
see reactions (13)—(15)). Therefore, for the combined
II
III
other equivalent conditions, is higher than that in the
addition of the Fe and Fe aqua ions in the presence of
the aromatic compounds, one can expect a decrease in
the induction period (see Fig. 2). However, this explanaꢀ
III
catalytic system prepared with the Fe aqua ion (see
–
6
–6
– –1
Table 1): 3.6•10 and 2.6•10 mol L s , respectively.
This suggests that the rate increase is caused by the conꢀ
V(O )/mL
2
V(O )/mL
2
4
2
1
1
2
3
4
4
3
2
1
0
5
3
2
1
0
5
0
100
150
τ/min
2
0
40
60
80
τ/min
II
II
III
Fig. 2. Effect of Fe on methanol oxidation in the Pd —Fe
system: 1, [Fe ]0 = 30 mmol L , [Fe ] = 0; 2, [Fe ]0
0 mmol L , [Fe ] = 20 mmol L ; [Pd ] = 5 mmol L
MeOH] = 4 mol L , [PhCN] = 12 mmol L , [HClO ] =
III
–1
II
III
II
=
Fig. 3. Effect of the initial concentration of the Fe aqua ion on
methanol oxidation: [Fe ] = 5 (1), 10 (2), 20 (3), 40 (4), and
60 mmol L (5); [Pd ] = 5 mmol L , [MeOH] = 4 mol L ,
0
[PhCN] = 12 mmol L , [HClO ] = 0.7 mol L , 65 °C.
4
0
II
–
1
II
–1
–1
II
1
[
0
,
0
–
0
0
1
–1
–1
II
–1
–1
4
–
1
–1
–1
.7 mol L , 65 °C.

Tetraaquapalladium(II) as oxidation catalyst
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 5, May, 2007
879
Table 2. Material balance of methanol oxidation with dioxygena
[PdII]aq,0 [FeII]aq,0
mmol L–1
t/min
[PdII]aqb [FeIII]aqb [CH O]
∆[O2]
Σw(O )•10
6
[
/
MeOH]0
mol L–1
2
2
/mol L s–1
–1
mmol L–1
1
2
4
7
1
4
4
4
4
4
4
4
4
4
4
4
4
.5
.5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
2.5
8
20
20
20
20
20
5
10
20
20
20
20
30
40
60
100
20
20
100
90
90
65
50
75
75
90
55
25
15
100
90
90
70
130
55
5
5
5
4.8
4
4.7
4.8
4.8
5
5
5
4
4
4
3.6
2.1
6.2
7
7
6
2.1
1.5
1.4
2.8
6
6
6
5
10
13
16
48
6.2
5.7
21
16
32
36
37
35
36
33
16
7
11
11
18
19
19
18
19
18
10
5
1.8
2.9
3.6
6.4
9.1
3.6
3.6
3.6
3.6
3.6
3.6
3.8
3.8
4.2
9.7
2.2
5.6
0
3
3
28
27
26
16
27
35
18
18
17
19
15
19
a
b
PhCN] = 12 mmol L–1, [HClO ] = 0.7 mol L , p(O ) = 0.1 MPa, 65 °C.
–1
[
4
2
II
III
Concentration of the Pd and Fe aqua ions at the end of the reaction.
II
II
FeIII concentration decrease due to reaction (9). The fact
of establishment of the steadyꢀstate Fe concentration
tribution of the reaction between Pd and Fe producing
2
+
III
the catalytically active palladium species Pd2 (see reacꢀ
tions (13)—(15)) that catalyze the oxidation of the alcoꢀ
indicates that the alcohol is oxidized with both the dioxyꢀ
II
III
hol and Fe with dioxygen.
gen and Fe aqua ion.
II
The values of the rate of oxygen consumption (Σw(O ))
presented in Table 2 are equal to the sum of the oxidation
rates of Fe and alcohol with dioxygen. The contribution
of each reaction to the total rate of oxygen consumption
depends on the initial conditions. The higher the methaꢀ
nol concentration, the higher the formation rate and the
larger the amount of formaldehyde formed. If an insignifꢀ
icant amount of the palladium black formed is neglected,
the material balance with respect to dioxygen obeys the
following equation:
The change in the Fe concentration in the range
2
–
1
5—40 mmol L induces no noticeable effect on the total
rate of oxygen absorption, which is more sensitive to a
change in the alcohol concentration and increases proꢀ
portionally to the latter.
II
The linear dependence of the rate on the alcohol conꢀ
centration indicates that the catalytically active palladiꢀ
um species are mainly formed in reaction (6), which is
preceded by the rateꢀdetermining step of reduction of the
II
Pd tetraaqua complex with the alcohol. The rate of reacꢀ
tion (6) depends on the initial concentrations of the alcoꢀ
III
II
∆
[O ] = [F ]/4 + [CH O]/2.
hol and Pd . The dependence of the total rate of oxygen
2
2
II
absorption on the initial concentration of the Pd tetraꢀ
As follows from the data in Table 2 (see the dynamics
aqua complex is linear, indicating the first order with
respect to the Pd concentration.
The dependence of the total rate of oxygen consumpꢀ
tion on the partial oxygen pressure in the 0.02—0.1 MPa
interval is characterized by the curve with saturaꢀ
tion (Fig. 4).
III
II
of changing the Fe concentration), the steadyꢀstate conꢀ
III
centration of Fe is established in the solution rather
rapidly, during the first minutes of the reaction. Its value
II
depends on the initial concentrations of Fe and alcohol
II
and is independent of the initial Pd concentration. With
II
an increase in the initial Fe concentration, the rate of
It is known that the plots of this type are observed for
the processes that proceed via the mechanism involving
the formation of an intermediate catalytic complex in the
equilibrium reaction between the initial reactant and cataꢀ
lyst. This complex is rather stable kinetically and is not
transformed irreversibly into the reaction products. In
this case, this is caused by reactions (7) and (8) in which
III
Fe formation in reactions (10) and (13)—(15) increasꢀ
es, resulting in a proportional increase in the steadyꢀstate
III
II
Fe concentration. In this case, the ratio of the Fe
concentration in a solution of the oxidant to the steadyꢀ
III
state Fe concentration remains constant, being ∼ 2.
When the initial alcohol concentration increases, the rate
III
14
of formation of the Fe aqua ion and the steadyꢀstate
complexes I and II, respectively, are formed.

880
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 5, May, 2007
V. V. Potekhin
w(O )/mol L s–1
–1
V(O2)/mL
2
6
7
5
4
3
2
1
0
3
2
1
0
4
3
2
0
.02
0.04
0.06
0.08 p(O )/MPa
2
1
Fig. 4. Plot of the total rate of oxygen consumption (Σw(O ))
vs. oxygen pressure (p(O )) during methanol oxidation; [Pd ] =
mmol L , [Fe ]0 = 20 mmol L , [MeOH] = 4 mol L ,
PhCN] = 12 mmol L , [HClO ] = 0.7 mol L , 65 °C.
4
2
II
2
0
–1
–
1
II
–1
5
[
2
0
40
60
80
100
τ/min
–
1
–1
Fig. 5. Kinetic curves of oxygen consumption during methanol
oxidation at temperatures 40 (1), 50 (2), 55 (3), 60 (4), 65 (5),
The data on the dependence of the rate of oxygen
II
–1
II
7
2
0 (6), and 80 °C (7); [Pd ] = 5 mmol L , [Fe ]
=
,
consumption on the partial oxygen pressure during methaꢀ
nol oxidation at 65 °C are satisfactorily described by the
Michaelis—Menthen equation
0
0
0 mmol L– , [MeOH] = 4 mol L , [PhCN] = 12 mmol L
1
–1
–1
–
1
[
HClO ] = 0.7 mol L
.
4
Σw(O ) = 6.6•10–6•p(O )/[K + p(O )].
2
2
2
ln(Σw(O ))
2
–
0.6
Based on the revealed kinetic regularities, we can
present the equation of the rate of oxygen consumption in
the interval of Fe concentrations from 5 to 40 mmol L
II
–1
–1.0
1.4
in the following form:
–
II
Σw(O ) = k[Pd ][MeOH]p(O )/[K + p(O )],
2
2
2
where k = 3.3•10–4 L mol–1 s–1, and K = 0.06 MPa
at 65 °C.
–1.8
–2.2
At the initial FeII concentration of 0.1 mol L the
–1
total rate of oxygen consumption is 9.7•10 mol L–1
–
6
s
–1
II
and close to the rate of oxidation of the Fe aqua ion with
II
dioxygen in the presence of the Pd tetraaqua complex
0.35
0.36
0.37
0.38 1000/RT
and benzonitrile (8.2•10 mol L s–1) at 65 °С. The
difference in the rates corresponds to the rate of oxygen
consumption for alcohol oxidation. This change in the
total rate of oxygen absorption with the change in the
–
6
–1
8
Fig. 6. Arrhenius plot of the rate of oxygen consumption in a
methanol solution; [Pd ] = 5 mmol L , [Fe ] = 20 mmol L
[MeOH] = 4 mol L , [PhCN] = 12 mmol L , [HClO ] =
0.7 mol L–
II
–1
II
–1
0
0
–
1
–1
4
1
.
II
–1
–1
Fe concentration from 5—40 mmol L to 0.1 mol L
can indicate a change in the step that causes the formaꢀ
tion of the active palladium species. It can be assumed
that the rate of oxygen consumption, in this case, will be
distinctly two temperature regions with different activaꢀ
tion energies: 40—55 and 60—80 °С, where the activaꢀ
–
1
tion energies are is 24.8±3 and 99.9±9 kꢁ mol , respecꢀ
tively.
II
determined by the kinetics of the reaction of Pd with
II
Fe in which the key role also belongs to the cluster
Similar character of the temperature dependence of
the rate of oxygen absorption was found for the oxidation
of other alcohols under study. The found values of the
activation energy are given in Table 3.
palladium species.
It follows from the data obtained by studying the temꢀ
perature effect on the rate of oxygen absorption that the
rateꢀdetermining step depends on the conditions of
process.
The kinetic curves of oxygen consumption during
methanol oxidation at different temperatures are shown
in Fig. 5. The Arrhenius plot of the total rate of oxygen
absorption on the inverse temperature (Fig. 6) indicates
The data in Table 3 show that at 40—55 °С the energy
of the reaction is independent of the alcohol nature. In
this case, the activation energy is close to that of oxidaꢀ
II
II
tion of the Fe aqua ion in the presence of the Pd
tetraaqua complex in the presence of the benzonitrile,
–
1 7
which is equal to 31.7±3 kꢁ mol . In the temperature

Tetraaquapalladium(II) as oxidation catalyst
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 5, May, 2007
881
Table 3. Activation energies for alcohol oxidation at
Table 4. Oxidation of the aliphatic alcohols with dioxygen in the
presence of the Pd —Fe —Fe aqua ions*
II
II
III
4
0—55 (I) and 60—80 °C (II)*
E /kꢁ mol–1
Alcohol
[R R CHOH] [R R CHO]
1
2
1
2
Yield (%)
Alcohol
a
0
I
II
mmol L–1
Methanol
24.8±2.5
24.8±2.5
26.5±2.6
25.4±2.5
34±3
99.9±9
53.4±5
70.1±7
48±4
Methanol
Ethanol
Propanꢀ1ꢀol
Propanꢀ2ꢀol
Butanꢀ1ꢀol
Butanꢀ2ꢀol
4
0.3
0.4
0.3
0.5
0.3
0.5
7
25
60
13
66
25
Propanꢀ1ꢀol
Propanꢀ2ꢀol
Butanꢀ1ꢀol
Butanꢀ2ꢀol
1.5
0.5
4
0.5
2
76±7
[PdII ] = 5 mmol L , [FeII ] = 20 mmol L
–1
–1
–1
,
,
*
aq 0
aq 0
–
1
* [PdII]0 = 5 mmol L–1, [FeII]0 = 20 mmol L–1, [FeIII]0
=
5 mmol L– , [PhCN] = 12 mmol L , [HClO ] = 0.7 mol L
[
PhCN] = 12 mmol L , [HClO ] = 0.7 mol L
4
1
–1
–1
p(O ) = 0.1 MPa.
,
2
4
6
5 °C, p(O ) = 0.1 MPa.
2
range 60—80 °С the activation energy depends on the
dioxygen molecule and the formation of a rather stable
alcohol nature, changes according to the energy of alcoꢀ
1
5
II
complex similar to the dꢀmetal peroxide complexes are
thus created. Thus, the "strongly bound" dioxygen and
activated dioxygen selectively oxidize the alcohol to the
hol oxidation with the Pd tetraaqua complex, and deꢀ
1
creases on going from methanol to butanꢀ1ꢀol.
Thus, the palladium species exhibiting the catalytic
II
II
carbonyl compound. The Fe aqua ion favors the formaꢀ
activity in the oxidation of the alcohols and Fe aqua ion
tion of the catalytically active cluster species, which exert
a positive effect on the increase in the rate of alcohol
with dioxygen are formed via two routes. According to
one route, the active intermediate palladium species are
III
II
oxidation with dioxygen. At the same time, the Fe aqua
formed in the reaction between the alcohol and Pd tetraꢀ
ion that formed control the redox process of alcohol conꢀ
version to the carbonyl compounds, restricting complete
aqua complex, whereas the second route involves the reꢀ
II
II
duction of the Pd tetraaqua complex with the Fe aqua
ion. The contribution of each route depends on the temꢀ
perature. At an elevated temperature the first route with
the rateꢀdetermining step of the interaction of alcohol
II
0
reduction of the Pd tetraaqua complex to Pd and pallaꢀ
dium black formation.
Based on the proposed mechanism, we determined
the conditions for the highly selective synthesis of carboꢀ
nyl compounds by the oxidation of alcohols involving the
II
with Pd (reaction (6) is predominant). In the 40—55 °С
temperature interval, the rate of the reaction is deterꢀ
mined by another rateꢀdetermining step: the reaction beꢀ
II
II
III
catalytic system of the Pd —Fe —Fe —O aqua ions.
2
II
II
The yields of the carbonyl compounds in alcohol oxidaꢀ
tion with dioxygen in this catalytic system for 24 h at
the selectivity of the process close to 100% are given in
Table 4.
tween the Fe aqua ion and Pd .
The change in the rateꢀdetermining step during the
temperatureꢀdependent oxidation with dioxygen in the
II
II
ROH—Pd aq—Fe aq—PhCN system is indicated by the
kinetic isotope effect (KIE) found for methanol oxidaꢀ
tion. At 50 °C the KIE is kCH3OH/kCD3OH = 1, and at
The reaction byꢀproducts are the products of oxidaꢀ
tion and acid hydrolysis of benzonitrile, in particular,
8
benzoic acid, which has earlier been discussed. Benzoꢀ
6
5 °C the KIE is 2. In the latter case, the KIE corresponds
III
nitrile is oxidized at the aromatic ring to form phenol
derivatives, (in particular, hydroxybenzonitrile isomers),
to the value found for methanol oxidation with the Fe
aqua ion in the presence of the Pd tetraaqua complex in
an inert atmosphere.1
II
II
which are readily coordinated with the Pd tetraaqua comꢀ
II
plex. Due to ligand substitution, the initial labile Pd
Thus, the proposed reactions (2)—(15) of the compliꢀ
cated multiꢀroute oxidation of alcohols with dioxygen in
the presence of the aromatic compound in the catalytic
tetraaqua ion is transformed into the kinetically lowꢀacꢀ
II
tivity Pd complex and the catalytic process of alcohol
II
III
II
oxidation ceases.
system of the Pd —Fe —Fe aqua ions describe satisꢀ
factorily the oxidation process and agree with the obꢀ
served kinetic regularities. As a whole, the system behaves
itself as an interrelated harmonically working machine in
which each component supplements and enhances the
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2
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II
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882
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V. V. Potekhin
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