Kinetics and mechanism of the homogeneous oxidation of n-butenes to methyl ethyl ketone in a solution of mo-v-phosphoric heteropoly acid in the presence of palladium pyridine-2,6-dicarboxylate

Article abstract of DOI:10.1134/S0023158411060164

In catalytic two-step n-butene oxidation with dioxygen to methyl ethyl ketone, the first step is the oxidation of n-C₄H₈ with an aqueous solution of Mo-V-P heteropoly acid in the presence of Pd(II) complexes. The kinetics of n-butene oxidation with solutions of H₇PV ₄Mo₈O₄₀ (HPA-4) in the presence of the Pd(II) dipicolinate complex (H₂O)PdII(dipic) (I), where dipic²- is the tridentate ligand 2,6-NC₅H₃(COO^-)2, is studied. Calculation shows that, at the ratio dipic²- : Pd(II) = 1 : 1, the ligand decreases the redox potential of the Pd(II)/Pdmet system from 0.92 to 0.73-0.77, due to which Pd(II) is stabilized in reduced solutions of HPA-4. The reaction is first-order with respect to n-C₄H₈. Its order with respect to Pd(II) is slightly below unity, and its order with respect to HPA-4 is relatively low (~0.63). The activation energy of but-1-ene oxidation in the temperature range from 40 to 80°C is 49.0 kJ/mol, and that of the oxidation of but-2-ene is 55.6 kJ/mol. The mechanism of the reaction involving the cis-diaqua complex [(H₂O)2Pd^II(Hdipic)] ₊, which forms reversibly from complex I, is proposed. The reaction rate is shown to increase with an increase in the HPA-4 concentration due to an increase in the acidity of the solution. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S0023158411060164

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2011, Vol. 52, No. 6, pp. 828–834. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © V.F. Odyakov, E.G. Zhizhina, 2011, published in Kinetika i Kataliz, 2011, Vol. 52, No. 6, pp. 849–855.
Kinetics and Mechanism of the Homogeneous Oxidation
of nꢀButenes to Methyl Ethyl Ketone in a Solution
of Mo–V–Phosphoric Heteropoly Acid in the Presence
of Palladium Pyridineꢀ2,6ꢀDicarboxylate
V. F. Odyakov* and E. G. Zhizhina
Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
* eꢀmail: odkv@catalysis.ru
Received February 25, 2011
Abstract—In catalytic twoꢀstep
oxidation of ꢀC4H8 with an aqueous solution of Mo–V–P heteropoly acid in the presence of Pd(II) comꢀ
plexes. The kinetics of ꢀbutene oxidation with solutions of H7PV4Mo8O40 (HPAꢀ4) in the presence of the
nꢀbutene oxidation with dioxygen to methyl ethyl ketone, the first step is the
n
n
Pd(II) dipicolinate complex (H2O)PdII(dipic) ( ), where dipic2– is the tridentate ligand 2,6ꢀNC5H3(COO–)2
I ,
is studied. Calculation shows that, at the ratio dipic2– : Pd(II) = 1 : 1, the ligand decreases the redox potential
of the Pd(II)/Pdmet system from 0.92 to 0.73–0.77, due to which Pd(II) is stabilized in reduced solutions of
HPAꢀ4. The reaction is firstꢀorder with respect to
unity, and its order with respect to HPAꢀ4 is relatively low (~0.63). The activation energy of butꢀ1ꢀene oxiꢀ
dation in the temperature range from 40 to 80 C is 49.0 kJ/mol, and that of the oxidation of butꢀ2ꢀene is 55.6
kJ/mol. The mechanism of the reaction involving the cisꢀdiaqua complex [(H2O)2PdII(Hdipic)]+, which
forms reversibly from complex , is proposed. The reaction rate is shown to increase with an increase in the
nꢀC4H8. Its order with respect to Pd(II) is slightly below
°
I
HPAꢀ4 concentration due to an increase in the acidity of the solution.
DOI: 10.1134/S0023158411060164
Pd
Methyl ethyl ketone (MEK, butanꢀ2ꢀone) finds
wide use as a raw material for fine organic synthesis, as
a solvent for paints, enamels, and resins, and as a dewꢀ
axing agent for lubricating oils. In industry MEK is
2
nꢀC4H8 + HPA + H2O ⎯⎯⎯→ CH3COC2H5 +
m
(II)
2
+
m HmHPA,
2
1
2
(III)
m HmHPA + O2 → HPA + H2O,
2
m
produced from the
threeꢀstep technologies through butanꢀ2ꢀol ether
CH3CH(OSO2OH)C2H5 or CH3CH(OСOCH3)C2H5
nꢀbutene fraction via one of the
IV
m
V
where HmHPA (
reduced form of HPA, and
degree of reduction of HPA.
) is the
H
3+ x + mPMo
V
Vx − mO40
12 − x
,
m
= [VIV]/[HPA] is the
which is subjected to hydrolysis to butanꢀ2ꢀol, and the
latter is dehydrated to MEK [1, 2]. These multistep
technologies are energyꢀconsuming and produce large
amounts of acidic wastewater.
In step (II), which usually occurs at 40–70
vanadium(V) in the HPA solution oxidizes ꢀbutene
to MEK and is reduced to vanadium(IV). The resultꢀ
ing MEK is evaporated from the reduced catalyst Pd +
°C,
n
We have proposed a liquidꢀphase catalytic process
for the production of MEK by direct oxidation of
butenes with dioxygen via reaction (I) [3, 4].
nꢀ
HmHPA at 100
ated with dioxygen or air via reaction (III) at 140–
145 and a dioxygen pressure of about 3–4 atm
°C, after which the catalyst is regenerꢀ
°C
PO
Pd + HPA
1
2
(I)
nꢀC4H8 + O2 ⎯⎯⎯⎯⎯→ CH3COC2H5.
2
[5]. Target reaction (II), the separation of the resulting
MEK, and the catalyst regeneration step (III) constiꢀ
tute one technological cycle of reaction (I). The redox
potential of the Pd + HPA solution decreases and its
pH increases during reaction (II) [7, 8]. During reacꢀ
tion (III), these parameters return to their initial valꢀ
ues. Therefore, reaction (II) followed by catalyst
regeneration can be performed several times [9].
The homogeneous catalysts developed for this process
are aqueous solutions (Pd + HPA), where HPA is the
Keggin Mo–V–P heteropoly acid H3 + xPMo12 – xVxO40
[5, 6]. Such HPAs play the role of reversible oxidants in
which the vanadium atoms undergo redox transformaꢀ
tions vanadium(V)
concentration in these catalysts is (4–6)
vanadium(IV). The palladium
ꢀ
×
10–3 mol/l,
and the HPA concentration is 0.1–0.3 mol/l. To proꢀ
Reaction (II) is complicated and consists of steps
(IIa) and (IIb). In the first of them, the Pd(II) comꢀ
plex oxidizes nꢀbutene. In the second step, the comꢀ
vide explosion safety, process (I) is carried out in two
steps in different reactors.
828

KINETICS AND MECHANISM OF THE HOMOGENEOUS OXIDATION
829
plex of reduced Pd is oxidized with the vanadiumꢀconꢀ example, in mixtures of dioxygen with ozone [18] and,
taining heteropoly acid:
nꢀC4H8 + PdII + H2O → CH3COC2H5 + Pd0,
hence, it will withstand the action of the severe O2
atmosphere of reaction (III). H2dipic and pallaꢀ
(IIa)
dium(II) form the complexes (H2O)PdII(dipic) (
I
) and
2H2O [19]. However, only complex is
catalytically active in the oxidation of ꢀbutenes [12].
0
II
(IIb) PdII(Hdipic)2
⋅
I
2
2
Pd + m HPA2V V → Pd + HmHPA
.
2V IV
m
n
For reaction (II) to proceed rapidly and come to
completion, it is necessary that the following relationꢀ
ship be valid for the potentials of the redox systems
involved in this reaction:
It should be noted that dipicolinic acid is a rather
accessible chemical, being an intermediate in the proꢀ
duction of the drug Parmidinum [20].
In the present work, we have studied the kinetics of
ꢀbutene oxidation via reaction (II) in a solution of
EHPA H HPA > EPdII
> EMOEK/nꢀC H
.
(1)
Pd0
n
m
4
8
O
HPAꢀ4 in the presence of dipicolinate complex and
I
The value of
can easily be calculated using
E
MEK/nꢀC4H8
made some conclusions about the mechanism of this
reaction.
reference data [10]:
is constant and equal
E
HPA HmHPA
to 0.10 V. The values of
and
can be
Pd0
EHPA H
EPdII
mHPA
2−
varied. If PdSO4, in which the anion
forms an
SO4
EXPERIMENTAL
unstable complex with the cation Pd2+ [11], that is a
constituent of the catalyst, then palladium(II) in the
initial catalyst solution exists mainly as the aqua comꢀ
aq. In this case, the rate of reaction (II) at
40–70°C is nearly constant, and a dimeric redox sysꢀ
Aqueous solutions (0.20 M) of the Kegginꢀtype
HPA H7PV4Mo8O40 (HPAꢀ4) with pH0 ~ 0.45 were
synthesized from V2O5, MoO3, H3PO4, and a cool 5%
solution of H2O2 using the standard procedure [21].
The initial 30% H2O2 was specialꢀpurity grade, and the
three other reactants were highꢀpurity grade. Soluꢀ
tions with a lower HPAꢀ4 concentration were prepared
by dilution of the 0.20 M HPAꢀ4 solution with water.
plex Pd2+
⋅
2+
Pd2 Pd02
tem
having E ≈ 0.7 V works in the catalyst
[6]. However, in the MEK evaporation step, the vanaꢀ
dium(V) concentration in the solution of the reduced
The poorly waterꢀsoluble complex
I was synthesized
0
catalyst is insufficient for
oxidation via reaction
(IIb). Under these conditions, the dimeric complex
Pd2
from PdCl2 and dipicolinic acid (reagent grade) in a
0.2 M solution of HNO3 [19]. The Pd + HPAꢀ4 cataꢀ
×
Pd02
lysts with a Pd2+ content of (2–15)
10–3 mol/l conꢀ
can gradually polymerize to yield palladium
taining no foreign anions were prepared by introducꢀ
tion of various amounts of complex into the HPAꢀ4
solution. Note that the pH of the catalyst solution is
determined by the pH of the corresponding HPAꢀ4
solution [8], because the concentration of HPAꢀ4
metal, which will partially precipitate from the catalyst
solution [12]. This will yield the well known system
I
Pd2+
⋅
aq/Pdmet, which has the higher redox potential
of E0
≈ 0.92 V [13, 14].
Various methods for preventing this undesirable
phenomenon were proposed. In particular, a small
exceeds the concentration of complex
one order of magnitude.
I by more than
amount of HCl (up to [Cl–]
≈
0.035 mol/l) was introꢀ
We used 99.6% butꢀ1ꢀene (Budennovsk Chemical
Industrial Complex, Russia) and 91–94% butꢀ2ꢀene
prepared by dehydration of butanꢀ2ꢀol with H2SO4
[22]. Reaction (II) was carried out under atmospheric
duced into the catalyst [15], due to which the redox
potential of the system
2−/Pdmet decreased to
PdCl4
~0.70 V [16]. However, even the presence of this low
concentration of chloride ions in the catalyst solution
is undesirable, because the product undergoes partial
chlorination and the selectivity of reaction (II)
decreases [15]. For this reason, we have proposed to
decrease the redox potential of the system PdII/Pd0 by
pressure at 40–80 С in a shaken temperatureꢀconꢀ
°
trolled 170ꢀml glass reactor into which 40 ml of the
catalyst was introduced. The process was conducted
under kinetic control (400 reciprocations per minute).
In experiments at temperatures above 70 С, the reacꢀ
°
adding some pyridine derivatives to the catalyst comꢀ tor was additionally attached to a temperatureꢀconꢀ
position [3]. Chlorineꢀcontaining organic compounds trolled circulation device in which MEK vapor was
cannot form in the presence of pyridine derivatives.
It is known that the pyridine ring is very resistant to
oxidants. However, pyridine itself and its alkyl derivaꢀ
tives readily form insoluble salts with heteropolyanꢀ
absorbed with a 1.0 M aqueous solution of Girard’s
reagent (C5H5N+CH2CONHNH2⋅⋅⋅Cl–) recrystalꢀ
lized from 2ꢀmethoxyethanol [23].
The solubility of nꢀbutenes in the 0.20 M HPAꢀ4
ions and, hence, these compounds are unsuitable as solution in the absence of the palladium(II) complexes
stabilizers of palladium in Pd + HPA solutions. Some was preliminarily determined. At 40 , the solubility
pyridine derivatives containing acidic groups such as of butꢀ1ꢀene was 1.65
10–3 mol/l and that of butꢀ2ꢀ
COOH or SO3H were chosen for the stabilization of ene was 1.50 , their solubilities
10–3 mol/l. At 70
Pd [3, 12]. These are 3ꢀpyridinesulfonic acid and 2,6ꢀ were 0.42
10–3 and 0.39 10–3 mol/l, respectively.
pyridinedicarboxylic (dipicolinic) acid that form no The solubility of ꢀbutenes in the 0.20 M HPAꢀ4 soluꢀ
°
C
×
×
°
C
×
×
n
insoluble salts with heteropoly anions. Dipicolinic tion at other temperatures (50, 60, and 80
°
C) was
acid 2,6ꢀC5H3N(COOH)2, or H2dipic, is stable, for determined by interpolation or extrapolation.
KINETICS AND CATALYSIS Vol. 52
No. 6
2011

830
C H , ml
ODYAKOV, ZHIZHINA
Before each experiment,
(without bubbling) through the reactor with the cataꢀ
lyst (complex + HPAꢀ4) and also through the circuꢀ
lation device, if necessary. Thereafter, the reactor was
connected to a burette filled with ꢀbutene, and the
nꢀbutene was passed
V
4
8
I
А
200
150
100
50
n
first push of the shaken reactor was taken to be the iniꢀ
tial point of the run.
В
The typical dependences of the
absorbed by the + HPAꢀ4 solution on the reaction
time at 70 are presented in Fig. 1. The ꢀbutene
uptake rate gradually decreases during the run. The
rate of reaction (II), II, was estimated from the initial
segments of VC H curves in the range
and was expressed in units of (mol C4H8) l–1 min–1. Each
run was repeated two or three times with a fresh portion
of the catalyst, and the results obtained were used in the
construction of the plots shown in Figs. 2–4.
nꢀbutene volume
I
°
C
n
w
=
8
f
(
τ
)
0
≤
m 1
≤
4
In the runs in which the partial pressure of C4H8
(
PC H
)
was varied, the reactor was blown with C4H8 +
4
N2 ga8s mixtures, and pure
nꢀbutene was supplied from
0
20
40
60
80
100
Time, min
120
the burette in the course of the reaction.
Fig. 1. Butꢀ1ꢀene uptake by solutions of the catalyst (comꢀ
plex + HPAꢀ4) at HPAꢀ4 concentrations of ( ) 0.20 and
) 0.10 mol/l. Particular curves in groups and were
I
A
RESULTS AND DISCUSSION
(B
A
B
obtained in the repeated runs with a fresh portion of the
The logarithmic dependences of the rate of reacꢀ
catalyst. Reaction conditions: 40 ml of the 0.008 mol/l
tion (II) on the partial pressure of
presence of the catalytic system complex I
n
ꢀbutenes in the
+ HPAꢀ4
II
(
H O)Pd (dipic) + HPAꢀ4 solution, 70
°
C,
=
P C H +H O
2
(
)
2
4
8
1 atm, and
= 0.692 atm.
are shown in Fig. 2. It can be seen that, under all parꢀ
P
C4H8
tial pressures of butꢀ1ꢀene or butꢀ2ꢀene, wII is directly
logw
II [mol l–1 min–1]
proportional to PC H
.
4
8
1
The logarithmic dependences of wII on the concenꢀ
trations of complex II and HPAꢀ4 are shown in Fig. 3.
The apparent reaction order with respect to Pd is
~0.94. The dependence of wII on the HPAꢀ4 concenꢀ
tration is weaker: the apparent order is ~0.63.
–2.6
–2.8
–3.0
–3.2
–3.4
2
The temperature dependences of wII for the oxidaꢀ
tion of both
coordinates in Fig. 4. The rate wII for butꢀ2ꢀene at
40 is lower than that for butꢀ1ꢀene by approximately
1.75 times, whereas at 80
the range from 40 to 80
nꢀbutenes are presented in the Arrhenius
°
C
°
C
it is lower by 1.25 times. In
°
C, the apparent activation
energy of butꢀ1ꢀene oxidation is ~49.0 kJ/mol, while
that of butꢀ2ꢀene oxidation is ~55.6 kJ/mol.
The palladium complexes containing the dipicoliꢀ
2−
nate anion NC H
( ), having two carꢀ
dipic2–
COO
(
3
)
5
2
boxylate groups COO– , are important components of
the catalyst [12]. In neutral and weakly acidic media,
the dipic2– anion is a tridentate ligand and binds the
Pd2+ cation into the very strong monoaqua complex
–1.0
–0.8
–0.6
–0.4
–0.2 –0.0
logPC H [atm]
4
8
(
H2O)PdII(dipic) (
I) [19]. It is reasonable to assume
Fig. 2. Logarithmic plots of the oxidation rates of (
1ꢀene and (
tion conditions: 40 ml of a 0.008 mol/l (H O)Pd (dipic) +
1) butꢀ
that the COO– groups of this ligand are partially protoꢀ
2) butꢀ2ꢀene versus the partial pressure. Reacꢀ
II
nated in the acidic medium, due to which the ligand
2
becomes bidentate: NC5H3(COOH)(COO–) (Hdipic–
).
HPAꢀ4 solution, 70
°
C, and
= 1 atm,
=
P
C4H8
P C H +H O
(
)
2
4
8
0.692 atm.
In this case, complex I turns into the diaqua complex
KINETICS AND CATALYSIS Vol. 52
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2011

KINETICS AND MECHANISM OF THE HOMOGENEOUS OXIDATION
831
log[Pd] [mol/l]
–2.8 –2.6 –2.4 –2.2 –2.0 –1.8
[(H2O)2PdII(Hdipic)]+ (II), in which H2O molecules
are in cisꢀposition to each other:
H O PdII NC H COO + H+ + H O
(
)
(
)
]
[
1
2
5
3
2
2
I
–2.4
(IV)
+
ꢀ H O 2 PdII NC H COOH COO
.
(
)
(
)(
)
]
[
2
5
3
2
II
–2.6
–2.8
–3.0
Since in the course of reaction (II) the concentraꢀ
tion of H+ ions in the HPAꢀ4 solution decreases [8],
equilibrium (IV) gradually shifts to the left.
The redox potentials of the systems
(H2O)PdII(dipic)/Pdmet
and
HPAꢀ4/HmHPAꢀ4
should be used to determine the region of stability for
the catalyst examined (Pd complex + HPAꢀ4). The
values of
were determined earlier [8],
E
HPAꢀ4 HmHPAꢀ4
E
and the value of
can be calculated
H O Pd dipic Pdmet
(
)
(
)
2
taking into account equilibria (V) and (VI):
–1.4
–1.2
–1.0 –0.8
log[HPAꢀ4] [mol/l]
KC
←⎯⎯⎯
2+
H O PdII dipic
Pd ⋅ aq + dipic2−,
⎯⎯⎯→
(
)
(
)
2
(V)
I
Fig. 3. Logarithmic plots of the oxidation rate of butꢀ1ꢀene
versus the concentrations of ( ) complex and ( ) HPAꢀ4.
Reaction conditions: 40 ml of 0.008 mol/l
C, and
K1
+
−
⎯⎯⎯→
←⎯⎯⎯
H+
H3dipic
H2dipic
1
I
2
a
(VI)
K2
←⎯⎯⎯
K3
←⎯⎯⎯
+
−
2−
II
⎯⎯⎯→
⎯⎯⎯→
(
H O)Pd (dipic) + HPAꢀ4 solution, 70
°
=
P
C4H8
Hdipic
dipic .
2
+
H
H
0.692 atm.
In equilibrium (V) KC
(VI)
calculating
≈
10–16 [24], and in equilibria
K3 = 5.14 [25]. For
, one has to use material balances
pK
1 = –1.05, pK2 = 2.29, and
p
EPdII
logw
II [mol l–1 min–1]
Pdmet
for palladium(II) (2) and for free (noncomplexed)
dipicolinate anions (3) containing 0 to 3 protons:
–2.4
–2.8
–3.2
–3.4
II
II
2+
⎡
⎣
⎤
⎦
⎡
⎤
)
⎡
⎤
Pd
= H O Pd dipic + Pd ⋅ aq
2
⎣ ⎦ ⎣ ⎦
(
)
(
Σ
(2)
II
⎡
⎣
⎤
)
⎦
≈
H O Pd dipic ,
(
)
(
2
2−
−
⎡
⎣
⎤
⎦
⎡
⎣
⎤
⎦
dipic = dipic + Hdipic + H dipic
[
]
[
]
2
Σ
(3)
+
−
⎡
⎤
⎦
⎡
⎣
⎤
⎦
+ H3dipic ≈ Hdipic + H dipic .
[
]
2
⎣
The concentration of free Pd2+
⋅
aq ions at KC
≈
10–16
is negligibly low and, hence, can be neglected in
Eq. (2). In addition, in the pH 0.5–2.2 range the terms
[
dipic2–] and [H3dipic+] in equilibria (VI) can be
1
neglected. Under these conditions, the Hdipic– ion
should be the dominant form of free dipicolinate.
Since complex
cies Pd2+ aq, Hdipic– , and H2dipic are present in very
low concentrations in the solution of complex . They
are interrelated by the material balance equation
I is very stable, the noncomplexed speꢀ
2
⋅
I
–2.8
–2.9
–3.0
–3.1
T–1
–3.2
103, K–1
2+
−
⎡
⎣
⎤
⎦
⎡
⎣
⎤
⎦
(4)
Pd ⋅ aq = dipic ≈ Hdipic + H dipic .
[
]
[
]
2
×
Σ
Taking into account the equilibrium between Hdipic–
and H2dipic, we obtain the equation
Fig. 4. Dependences of logarithms of the oxidation rates of
) butꢀ1ꢀene and ( ) butꢀ2ꢀene on the inverse reaction
temperature. Reaction conditions: 40 ml of a 0.008 mol/l
2+
–
+
−1
⎡
⎣
⎤
⎦
⎡
⎣
⎤
⎦
⎡
⎣
⎤
⎦
(5)
Pd ⋅ aq ≈ Hdipic 1 + H K2
.
)
(
(
1
2
At pH
≤
2.2, there are almost no nonprotonated ions
II
(
H O)Pd (dipic) + 0.20 mol/l HPAꢀ4 solution, 70°C,
2
dipic2– in the catalyst solution, and equilibrium (V) is
and the pressure
was reduced to 0.692 atm.
P
C4H8
then transformed into (VII):
KINETICS AND CATALYSIS Vol. 52
No. 6
2011

832
ODYAKOV, ZHIZHINA
⋅ aq ions and
Calculation of the concentration of free Pd2+
the oxidation potential
acidic medi
The results of calculation of
are also given
EPdII
Pdmet
E
of complex (H2O)Pd(dipic) in
in the table. It can be seen that the potential
EPdII
a
Pdmet
decreases by 0.044 V as the pH of the solution
increases from 0.50 to 2.20. Nevertheless, at any pH
Conditions
Results of calculation
[PdII]
×
103,
[Pd2+]
×
106,
values the calculated values of
for the Pd(II)
EPdII
mΣol/l
mol/l
Pdmet
pH
E, V
dipicolinate complexes are much smaller than the
°
8.0
8.0
8.0
8.0
8.0
8.0
4.0
15.0
0.50
0.85
1.20
1.50
1.85
2.20
1.50
1.50
14.87
6.68
0.775
0.765
0.757
0.749
0.739
0.731
0.744
0.753
known value
= 0.92 V [14].
E
2+
Pd ⋅aq Pdmet
It was found earlier that the redox potentials
pH of the HPAꢀ4 solutions depend on the degree of
their reduction, , which increases in the course of
reaction (II) [7, 8]. Using the published data [8], we
have plotted for the homogeneous catalysts (
HPAꢀ4 complex) against their pH at 25 and various
HPAꢀ4 concentrations. It follows from Fig. 5, where
the arrow shows the direction of the shifts in and pH
during reaction (II), that the catalysts remain homoꢀ
geneous in the region above the (pH)
E and
3.04
1.59
m
0.765
0.394
1.36
E
I +
°
C
2.72
E
=
f
EPdII
Pdmet
H O PdII dipic + H
+
curve, according to inequality (1). But they become
unstable and begin to precipitate Pdmet in the region
under this curve. So, the acceptable lower boundary of
(
)
(
)
2
(VII)
K
Σ
⎯⎯⎯→
←⎯⎯⎯
2+
Pd ⋅ aq + Hdipic–,
2+
−
the potential
HPAꢀ4 solution is ~0.75 V. This corresponds to
E of the reduced catalyst based on the 0.2 M
⎡
⎣
⎤ ⎡
Pd ⋅aq Hdipic
⎦
⎤
KC
K3
⎦ ⎣
m
≈
2,
where
(6)
K =
Σ
=
.
+
II
⎡
⎣
⎤ ⎡
(H O)Pd (dipic)
⎦
⎤
i.e., to the reduction of approximately 50% of the vanaꢀ
dium(V) in the HPAꢀ4 solution to vanadium(IV) [8].
Further reduction of the catalyst can result in the preꢀ
cipitation of the metallic phase, which will decrease the
activity of the catalyst in the next cycle of reaction (I).
In our work [12], we began to study the problem of
palladium stabilization in the Pd + HPA catalytic
solutions. The palladium concentration in the reduced
H
2
⎦ ⎣
Expressing [Hdipic–] from Eq. (6), we obtain
+
II
⎡
⎣
⎤ ⎡
Pd
⎦
⎤
H
Σ
KC
−
⎦ ⎣
⎡
⎣
⎤
=
⎦
(7)
Hdipic
.
2+
K3
⎡
⎣
⎤
⎦
Pd ⋅ aq
Substituting [Hdipic–] from Eq. (7) into Eq. (5), we
have
solutions (complex
extraction colorimetric method using an
I
+ HPAꢀ5) was determined by our
ꢀbenzil
+
II
+
⎡
⎣
⎤ ⎡
Pd
⎦ ⎣
⎤⎛
⎦
⎡
⎣
⎤⎞
⎦
H
H
α
KC
2+
⎡
⎣
⎤
⎦
(8)
⎜
⎟
⎟
⎠
Pd ⋅ aq =
1 +
.
dioxime solution. It proved that palladium(II) is
retained in the solution of the catalyst based on HPAꢀ
5 until the reduction of ~60% of the vanadium(V) to
vanadium(VI). Thus, the regions of stability of pallaꢀ
dium in solutions of the homogeneous catalysts based
on HPAꢀ4 and HPAꢀ5 coincide rather closely.
2+
⎜
⎤
⎦ ⎝
K ⎡
K2
Pd ⋅ aq
3
⎣
Finally, combining both terms containing [Pd2+
⋅
aq]
on the leftꢀhand side of Eq. (8) and taking the square
root of [Pd2+
⋅
aq]2, we obtain
+
+
II
⎤
Thus, the ligands dipic2– and Hdipic indeed stabiꢀ
−
⎡
⎣
⎤
⎦
⎡
⎣
⎤ ⎡
Pd
⎦
KC
H
K + H
(
)
2
2+
⎦ ⎣
⎡
⎣
⎤
(9)
Pd ⋅ aq =
.
lize Pd(II) in the catalyst solution due to the strong
decrease in the redox potential of the
⎦
K2K3
The concentrations of free Pd2+
⋅
aq ions calculated
via Eq. (9) are given in the table. It can be seen that the
concentration of Pd2+
aq ions decreases with an
increasing in pH of the solution of complex , indicatꢀ
ing some strengthening of complex . Using the calcuꢀ
lated values of [Pd2+
aq], one can calculate the redox
potentials
(H2O)PdII(dipic)/Pdmet system compared to the Pd2+
⋅
aq/Pdmet system. According to our data, the increase
in temperature to ~90 does not appreciably shift the
redox potentials of the HPAꢀ4/HmHPAꢀ4 system. This
provides good reason to believe that the region of staꢀ
bility for the Pd + HPA catalysts at elevated temperaꢀ
tures will not differ substantially from the region of
their stability at room temperature.
⋅
°
C
I
I
⋅
in solutions of complex
I
using
EPdII
Pdmet
Nernst’s equation (10) [26]:
It seems likely that just complex II, not complex
exhibits some catalytic activity in reaction (II). In comꢀ
plex , only one coordination site is occupied by a water
molecule. But complex II formed from complex conꢀ
tains two H2O molecules, and one of them can readily
be replaced by an ꢀbutene molecule to form the mixed
cisꢀcomplex [(H2O)(
ꢀbutene)PdII(Hdipic)]+ (III
I,
2+
RT
nF
°
⎡
⎣
⎤
⎦
(10)
EPdII
= EPd2+
+
ln Pd ⋅ aq .
Pdmet
⋅aq Pdmet
I
For the valence transition Pd(II)/Pdmet
,
n
= 2. At
I
25
°
C
Eq. (10) simplifies to
n
2+
°
⎡
⎣
⎤
⎦
(11)
EPdII
= EPd2+
+ 0.02955log Pd ⋅ aq .
n
):
Pdmet
⋅aq Pdmet
KINETICS AND CATALYSIS Vol. 52
No. 6
2011

KINETICS AND MECHANISM OF THE HOMOGENEOUS OXIDATION
833
+
H O PdII Hdipic + nꢀbutene
E, V
(
)
(
)
2
2
2
II
1.0
0.9
0.8
0.7
(VIII)
3
+
ꢀ H O nꢀbutene PdII Hdipic + H2O.
(
)(
)
(
)
2
4
III
It is assumed that the further transformations of
complex III are analogous to the transformations of
complex (H2O)(olefin)PdCl2, which was described
many times as an intermediate in the oxidation of oleꢀ
fins with palladium(II) chloride complexes [27, 28].
These transformations lead to the complex
Pd0(H2dipic) (IV
) and the target product (MEK). It is
assumed that the Pd0…N bond in complex IV is
retained, but Pd0–OCOR bonds are absent [12]. Comꢀ
1
plex IV is then oxidized to the initial complex
I by
vanadium(V) contained in the HPAꢀ4 solution. Thus,
one turnover of palladium in the catalyst composition
is completed. Then all steps of reaction (II) are
repeated, and the HPAꢀ4 solution is gradually
reduced. Equilibrium (IV) is gradually shifted to the
left, and the concentration of reactive complex II
0.5
1.0
1.5
2.0
pH
decreases, inducing a decrease in the reaction rate wII
.
Fig. 5. Region of stability for solutions of the reduced
For the oxidation of butꢀ2ꢀene, the rate wII is lower
than that for the oxidation of butꢀ1ꢀene. It can be
assumed that this is due to the lesser shift of equilibꢀ
rium (VIII) to the right for the formation of mixed
complex III with butꢀ2ꢀene than for the formation of
the analogous complex with butꢀ1ꢀene.
homogeneous catalysts (complex
pH values at 25 C: ) complex
0.008 mol/l and (
) 0.20, ( ) 0.10, and (
I
+ HPAꢀ4) at various
°
(1
I in a concentration of
2–
4
) HPAꢀ4 in concentrations of
(
2
3
4) 0.05 mol/l.
change in the concentration of the active complex II at
different acidities of the HPAꢀ4 solutions.
In the presence of the Pd(II) chloride complexes,
the rate of reaction (II) decreases with an increase in
the acidity of the solution [27]. However, in the presꢀ
In recent years, we have developed methods for the
synthesis of modified nonꢀKegginꢀtype Mo–V–phosꢀ
phoric HPA solutions [31] that possess enhanced therꢀ
mal stability. The initial pH values of these solutions
are below zero. Probably, enhanced concentrations of
the reactive palladium(II) dipicolinate complexes can
be achieved in the catalysts based on these solutions.
ence of complex I, this reaction is accelerated with an
increase in the acidity of the HPAꢀ4 solution (Fig. 3).
This fact can be explained by the shift of equilibrium
(IV) to the right with an increase in the concentration
of the bidentate ligand Hdipic–. A similar acceleration
was earlier observed for the oxidation of allyl alcohol
by a complex Pd(II) with diethylenetetramine,
H2NCH2CH2NHCH2CH2NH2 [29]. When this soluꢀ
Thus, in the present work we have shown that the
tion is acidified, the tridentate ligand is also partially equilibrium mixture of dipicolinate complexes
I and II
protonated forming the respective bidentate ligand
is generated, if dipicolinate ion is used for stabilization
of Pd(II) in the aqueous solution of Pd(II) + HPAꢀ4
that is our homogeneous catalyst. In the course of
reaction (II), the concentration of the reactive comꢀ
plex II in the mixture decreases due to an increase in
pH during the reduction of the catalyst solution. It was
found earlier that all changes in the physicochemical
properties of solutions of Mo–V–P HPA during redox
reactions (II) and (III) are reversible [9]. Therefore,
the pH of the catalyst solution regenerated via reaction
(III) will be the same as the pH of the initial solution.
Hence, the initial rate of reaction (II) with the regenꢀ
erated catalyst should be equally high. Thus, the oxiꢀ
H3N+CH2CH2NHCH2CH2NH2, which has only two
Pd⋅⋅⋅N bonds with palladium(II).
In the present work, the reaction order with respect
to HPAꢀ4 is 0.63; i.e., it differs from the zero order,
which was observed in the presence of the dinuclear
2+
complex
[30]. This complex did not contain any
Pd2
ligands whose composition would change depending
on the acidity of the medium. Hence, the composition
of this complex remained invariable in the wide
medium acidity range from pH –0.34 to pH 0.81. Due
to this, the rate of alkene oxidation in the catalytic sysꢀ
2+
tem
+ HPA did not depend on the acidity of the
Pd2
medium [6]. Therefore, the dependence of
HPAꢀ4 concentration observed in the present study is
dation of nꢀbutenes in reaction (II) and the subseꢀ
quent regeneration of the catalyst can be carried out
several times with the retention of homogeneity of the
wII on the
only seeming. This dependence can be explained by a catalytic system (H2O)PdII(dipic) + HPAꢀ4.
KINETICS AND CATALYSIS Vol. 52
No. 6
2011

834
ODYAKOV, ZHIZHINA
17. Comprehensive Heterocyclic Chemistry, vol. 2, part 2A:
REFERENCES
SixꢀMembered Rings with One Nitrogen Atom, Boulton, A.J.
and McKillop, A., Eds., Oxford: Pergamon, 1984,
p. 324.
1. Chernyshkova, F.A. and Mushenko, D.V., Zh. Prikl.
Khim., 1980, vol. 53, no. 11, p. 2483.
2. Byulleten’ inostrannoi kommercheskoi informatsii: Metiꢀ
letilketon (Foreign Commercial Information Bulletin:
Methyl Ethyl Ketone), 08.10.2002.
18. Bailey, A., Griffith, W.P., Lueng, D.W., White, A.J.P.,
and Williams, D.J., Polyhedron, 2004, vol. 23, no. 17,
p. 2631.
3. USSR Inventor’s Certificate no. 1584200, Byull. Izoꢀ
bret., 1994, no. 22.
19. Odyakov, V.F., Russ. J. Inorg. Chem., 1999, vol. 44,
no. 2, p. 178.
4. RF Patent 2230612, 2004.
5. Zhizhina, E.G., Odyakov, V.F., Balashov, A.L., and
20. Yakhontov, L.N. and Karpman, Ya.S., Khim.
Geterotsikl. Soedin., 1981, no. 4, p. 435.
Matveev, K.I., Katal. v Promꢀsti, 2005, no. 5, p. 28.
6. Zhizhina, E.G., Simonova, M.V., Odyakov, V.F., and
21. Odyakov, V.F. and Zhizhina, E.G., Russ. J. Inorg.
Chem., 2009, vol. 54, no. 3, p. 361.
Matveev, K.I., Appl. Catal., A, 2007, vol. 319, no. 1, p. 91. 22. Rappoport, F.M. and Il’inskaya, A.A., Laboratornye
metody polucheniya chistykh gazov (Preparation of Pure
Gases in the Laboratory), Moscow: Goskhimizdat,
1963.
7. Zhizhina, E.G., Odyakov, V.F., and Matveev, K.I.,
Russ. J. Appl. Chem., 2000, vol. 73, no. 5, p. 824.
8. Odyakov, V.F., Zhizhina, E.G., and Matveev, K.I.,
23. Heuser, E., HoubenꢀWeil Methoden der organischen
Russ. J. Inorg. Chem., 2000, vol. 45, no. 8, p. 1258.
Chemie, vol. 2: Analytische Methoden, Stuttgart: Thiꢀ
eme, 1953.
24. Kragten, J. and DecnopꢀWeever, L.G., Talanta, 1980,
vol. 27, no. 8, p. 685.
25. Tissier, C. and Agoutin, M., J. Electroanal. Chem. Interꢀ
facial Electrochem., 1973, vol. 47, no. 3, p. 499.
26. Ewing, G.W., Instrumental Methods of Chemical Analyꢀ
sis, New York: McGrawꢀHill, 1987.
9. Zhizhina, E.G. and Odyakov, V.F., Appl. Catal., A
2009, vol. 358, no. 2, p. 254.
,
10. Stull, D.R., Westrum, E.F., Jr., and Sinke, H.C., The
Chemical Thermodynamics of Organic Compounds, New
York: Wiley, 1969.
11. Rudakov, E.S., Yaroshenko, A.P., Rudakova, V.I., and
Zamashchikov, V.V., Ukr. Khim. Zh., 1984, vol. 50,
no. 7, p. 680.
27. Moiseev, I.I.,
π
ꢀKompleksy v zhidkofaznom okislenii oleꢀ
12. Odyakov, V.F., Zhizhina, E.G., and Matveev, K.I.,
finov ꢀComplexes in LiquidꢀPhase Alkene Oxidaꢀ
(
π
Katal. Promꢀsti, 2007, no. 4, p. 19.
tion), Moscow: Nauka, 1970, pp. 75, 114.
13. Standard Potentials in Aqueous Solution, Bard, A.J.,
Parsons, R., and Jordan, J., Eds., New York: Marcel
Dekker, 1985, p. 343.
14. Nikolaeva, N.M. and Pogodina, L.P., Izv. Sib. Otd.
Akad. Nauk SSSR, Ser. Khim. Nauk, 1980, vol. 1, no. 2,
p. 130.
28. Henry, P.M., Palladium Catalyzed Oxidation of Hydroꢀ
carbons, Dordrecht: Reidel, 1980, vol. 2, p, 59.
29. Burgess, C., Hartley, F.R., and Searle, G.W., J. Orgaꢀ
nomet. Chem., 1974, vol. 76, no. 2, p. 247.
30. Zhizhina, E.G., Simonova, M.V., Odyakov, V.F., and
Matveev, K.I., React. Kinet. Catal. Lett., 2003, vol. 80,
no. 1, p. 171.
15. Patent WO 91/13852, 1991.
16. Levanda, O.G., Moiseev, I.I., and Vargaftik, M.N., Izv. 31. Odyakov, V.F., Zhizhina, E.G., and Maksimovskaua, R.I.,
Akad. Nauk SSSR, Ser. Khim., 1968, no. 10, p. 2368. Appl. Catal., A, 2008, vol. 342, no. 1, p. 126.
KINETICS AND CATALYSIS Vol. 52
No. 6
2011