Catalytic behavior of copper(II) chelate complexes sterically held in zeolite large cavities and fixed on its outer surface by a topological anchor

Article abstract of DOI:10.1134/S1070363209120019

Catalytic properties of copper(II) tricyclic chelate compounds based on enamino ketone of 3-acetyl- 2,4-pentanedione and 1,6-hexametylenediamine fixed on NaY and CaA zeolites by the methods of topological and topologically-anchor retention, respectively,

Full text of DOI:10.1134/S1070363209120019

ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 12, pp. 2563–2573. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © A.N. Zakharov, N.S. Zefirov, 2009, published in Zhurnal Obshchei Khimii, 2009, Vol. 79, No. 12, pp. 1937–1947.
Catalytic Behavior of Copper(II) Chelate Complexes
Sterically Held in Zeolite Large Cavities and Fixed
on Its Outer Surface by a Topological Anchor
A. N. Zakharov and N. S. Zefirov
Moscow State University, Vorob’evy gory 1, Moscow, 119992 Russia
e-mail:zakh-alex@yandex.ru
Received July 3, 2008
Abstract—Catalytic properties of copper(II) tricyclic chelate compounds based on enamino ketone of 3-acetyl-
2,4-pentanedione and 1,6-hexametylenediamine fixed on NaY and СаА zeolites by the methods of topological
and topologically-anchor retention, respectively, were compared. The mode of fixation of chelates on a support
affects the character of the liquid-phase catalytic oxidation of cyclohexene by molecular oxygen. At the
topological fixation by a steric retention of a chelate compound in a large cavity of NaY zeolite, the reaction
rate related to one reaction center of a metal complex is not proportional to the filling degree that points to
inaccessibility for a substrate of catalyst molecules localized in inner cavities of crystallites. In an alternative
mode of the catalyst topologically-anchor fixation, when only a ligand fragment is held in СаА zeolite, and
catalytically active metal center is turned to the side of the reaction medium, the linear dependence of the
cyclohexene oxidation rate on the amount of the chelate on the support is retained within the whole studied
range of the surface catalyst concentrations. The catalytic activity of the topologically-anchor fixed Cu(II)
chelate compound coincides with its activity in the zeolite absence, which points to a pseudo-homogeneous
mode of cyclohexene oxidation. At topologically-anchor fixation of a Cu(II) chelate compound on CaA zeolite
diffusion limitations characteristic for the fixation by steric retention are completely eliminated.
DOI: 10.1134/S1070363209120019
Chelate compounds of transition metals with
organic ligands are used in various fields of prepa-
rative chemistry and chemical technology as effective
homogeneous catalysts of organic reactions. At the
same time, fixation of metal chelates on a solid support
makes it possible to carry out the process in a hetero-
geneous-catalytic mode more favorable technologically.
zeolites of the faujasite type) with their unique struc-
ture, chemical inertness, thermal stability, and rather
high mechanical strength. The fixation of metal chelate
compounds on alkaline forms of zeolite supports has
been realized by so-called topological (steric) retention
[2], the sense of which consists in the steric hindrances
arising because of differences in sizes of chelate com-
plexes and effective diameters of entrance windows
(Fig. 1a). Metal chelates synthesized in situ in zeolite
pores and having sizes greater than the effective
diameter of entrance windows cannot leave the support
without destruction and are sterically held inside large
cavities, not forming chemical bonds with their
“walls.”
However, the deposition of metal complexes on a
solid phase encounters significant obstructions, as
chelates, in particular of nonionic type, are poorly held
on a surface. For some substances (polymeric
compounds and silica gels) this problem is solved by
fixing metal chelate compounds due to chemical
bonding with “grafted” surface functional groups
(anchor fixation) [1]. The anchor fixation on metal
oxides or on aluminosilicates possessing more
favorable chemical-mechanical properties is extremely
problematic.
As zeolites are molecular sieves, the arrangement
of catalyst molecules in their hollows do not help to
solve the problem of diffusion to a catalytically active
center, but even more aggravates this problem owing
to the appearance of the secondary sieve effect [3]
arising due to additional blocking of pores with the
A special case is represented by alkaline forms of
crystalline aluminosilicates (for example, synthetic
2563

2564
ZAKHAROV, ZEFIROV
“help” of synthetically injected molecules of coor-
dination compounds. Bulk reagents or reaction pro-
ducts are especially sensitive to the secondary sieve
effect. In the latter case a chelate compound is im-
mediately removed from the catalysis, as the con-
version products are irreversibly held in large zeolite
cavities in the same way as the catalyst itself is
captured at its own synthesis in situ.
not the whole chelate compound molecule suffers the
steric retention, as is the case with the topological
fixation, but only its part belonging to a ligand and
practically not affecting the catalytic properties of the
complex (Fig. 1b). In this case catalytically active
metal centers of a chelate compound are forced out of
zeolite matrix pores and become accessible to
substrates of any sizes. Thus, molecules of reagents
can contact with catalytically active centers, not
penetrating into zeolite pores.
To eliminate the negative secondary sieve effect,
we offered a topologically-anchor mode of fixing
metal chelate compounds on synthetic zeolites X and
Y and on zeolite A [4]. The essence of the
topologically-anchor fixation consists in the fact that
At present direct quantitative evidences confirming
advantages of the topologically-anchor homogeneous
catalysts are scarce. Therefore in this work we have
(a)
0.7–0.8 nm
4
2
1.0–1.1 nm
1
4
(b)
2
3
4
1
Fig. 1. (a) Scheme of topological (steric) retention (1) of complex I in (2) large cavity of NaY zeolites; (b) schematic diagram of (1)
fixing chelate metal complex in СаА zeolite by means of (3) anchor fragment topologically kept in (2) large cavity; (4) entrance
windows.
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CATALYTIC BEHAVIOR OF COPPER(II) CHELATE COMPLEXES
CH2
2565
CH3
CH3
CH3
CH3
CH3
CH3
O
CH
O
2
CH3
CH
2N
O
O
O
O
O
O
CH2
Cu
Cu
CH2
N
O
CH3
O
CH3
CH2
CH3
CH3
CH3
I
II
studied the catalytic behavior of heterogenized chelate
compound I as a function of the mode of its deposition
on zeolite. The aim of the work was to compare
catalytic activities of thus prepared heterogeneous
catalysts and to confirm quantitatively the existence of
the secondary sieve effect for the case of compound I
sterically held in large zeolite cavities and also the
absence of reaction inhibition when a topological
anchor is used for fixing chelate compound I. To reach
these aims, we have chosen the reaction of the liquid-
phase cyclohexene oxidation by molecular oxygen,
which we have earlier studied in detail in homo-
geneous conditions in the presence of related cop-
per(II) chelates [5–7].
cavities of zeolite NaY [2] (Fig. 1a). In sample B
complex I is fixed on zeolite СаА by means of a
topological anchor [4] (Fig. 2).
Effective sizes of large cavities of zeolites NaY and
CaA are approximately equal (~1.1 nm), whereas
effective diameters of entrance windows are essentially
different and equal 0.7–0.8 and 0.5 nm, respectively
[8]. The maximal size of complex I does not exceed
the effective diameter of large cavities of NaY and
CaA zeolites and hence for topological (steric) fixation
any of them is suitable. However, in this case it is
more expedient to use zeolite NaY (sample A) with
wider entrance windows (~0.8 nm). For the matrix
synthesis of complex I, chelate compound II was
inserted as a predecessor in large cavities of zeolite
NaY, as the size of compound II is smaller than the
effective diameter of entrance windows of zeolite
To fix chelate compound I, we used NaY (sample
A) and CaA (sample B) zeolites. Sample A was
obtained by a steric retention of complex I in large
(a)
(b)
CH3
CH2
CH2
CH3
CH3
CH2
CH3
N
O
O
N
CH2 Cu
CH2
N
CH3
N
O
CH3
CH2
CH3
CH3
Fig. 2. Topologically-anchor fixation of a Cu(II) chelate compound on zeolite СаА. (a) Phase of CaA support and (b) liquid phase.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 12 2009

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ZAKHAROV, ZEFIROV
NaY, but exceeds the effective diameter of entrance
windows of zeolite СаА. Thus, according to geo-
metrical reasons, zeolite СаА is not suitable for the
topological fixation, as compound II cannot penetrate
from solution into pores of the support. Therefore
zeolite СаА was chosen for the alternative topo-
logically-anchor fixation, as narrow entrance windows
of this zeolite guarantee more effective retention of a
topological anchor (Fig. 1b, position 2) and,
consequently, chelate I. Furthermore large cavities of
zeolite СаА with less wide entrance windows are
impermeable for cyclohexene, which makes senseless
the arrangement of catalyst molecules in its cavities.
in all considered cases it proceeds by a common
mechanism.
Chelate compound I is sparingly soluble in
nonpolar and low-polar organic solvents (benzene,
toluene, etc.) and in their mixtures with cyclohexene.
In the course of the homogeneous-catalytic RH2
oxidation the following products are formed: 2-
cyclohexen-1-one (RO), 2-cyclohexen-1-ol (HROH),
and epoxycyclohexane (RH2 > O) in the molar ratio
RO : HROH : RH2 > O = 2 : 1: 1, which corresponds to
stoichiometric Eq. (1).
Below the composition and the main characteristics
of samples A and B containing chelate compound I
sterically held in large cavities of zeolite NaY and
fixed on zeolite СаА by means of a topological anchor,
respectively, are presented (m is the weight of
dehydrated zeolite and y is Cu(II) specific concentra-
tion in a sample).
OOH
2
+ 2
OH
+ O2
(1)
O
2
+
+
O + 2 H2O
Cu(II) content,
Carrier
Sample
m, g
y, μmol g–1
mg
NaY
CaA
5.55
5.01
1.012
0.385
2.87
1.21
А
A similar ratio between products of the liquid-phase
cyclohexene oxidation by molecular oxygen was
observed for related Cu(II) chelates [5–7], which
confirms the similarity of reaction (1) mechanisms in
these catalytic systems. The identity of the mechanisms
of cyclohexene oxidation in various catalytic systems
allows us to eliminate a possible influence of this
factor (mechanism) on the catalytic effect on passing
from one catalyst to another.
B
The procedure of fixing a copper(II) chelate
compound on zeolite СаА has been considered in
detail in [3]. Here we present the main characteristics
of samples A and B, which were used for comparison
of their catalytic activity in the reaction of cyclohexene
liquid-phase oxidation by molecular oxygen.
Let us note the main singularities of the behavior of
the system under consideration in the liquid-phase RH2
oxidation. First, О2 absorption by cyclohexene does
not occur (exclusive of the background reaction) if at
least one of reagents, HROOH or complex I, is absent
from the system. In this case HROOH in the presence
of chelate I is only consumed. Second, in the case of
the liquid-phase cyclohexene oxidation in the presence
of chelate compound I, free radicals were not detected
in the bulk liquid phase, which agrees with the
assumption about the role of HROOH as a mediator of
molecular oxygen transfer on a substrate molecule as a
result of the coordination with a metal atom of the
catalyst. The assumed mechanism of the cyclohexene
liquid-phase oxidation in the presence of chelate com-
pound I is shown in the following scheme.
Homogeneous oxidation. Catalytic activity of
homogeneous catalyst I and heterogeneous samples A
and B (see above) was studied in the reaction of
cyclohexene liquid-phase oxidation by molecular
oxygen. It was found beforehand that zeolite NaY or
CaA and their decationated forms are inactive in the
liquid-phase cyclohexene oxidation under the experi-
mental conditions.
The liquid-phase oxidation of cyclohexene (RH2)
by molecular oxygen in the presence of cyclohexenyl
hydroperoxide (HROOH) was studied in detail earlier
[5–7] for copper(II) chelates close in the chemical
structure. It is convenient as a model reaction for
studying catalytic behavior of chelate compound I, as
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CATALYTIC BEHAVIOR OF COPPER(II) CHELATE COMPLEXES
2567
H
H
O O
O
O
H
O
N
O
N
O
CuII
CuI
N
N
HO
H
O
H
H
O
O
O
H
C
D
+RH2
+HROOH
O
O
+
H2O
H
H
H
O
O
O
H
O
O
O
+O2
O
N
O
N
O
N
O
O
O
N
CuI
CuII
CuI
CuII
N
N
N
N
HO
HO
H
H
O
O
O
H
I
H
O
H
H
F
E
OH
H
O H
O
O
+
H2O
O
N
CuII
N
H
O
O
H
G
It is known [9] that the coordination of HROOH
with a metal atom in coordination compounds occurs
through the O atom bound to a radical, therefore we
can consider the formation of adducts H and C (see the
scheme) as the key moment in the cyclohexene
oxidation by molecular oxygen in the presence of
chelate compound I. The distinctive feature of the
adduct structure is the two-center HROOH coordina-
tion with the formation of a strongly distorted five-
membered cycle, in which the axial extraligand is
bound to the metal atom and to the coordinated O atom
of the chelating ligand by a hydrogen bond. A cyclic
transition state of a similar type was considered, for
example, when studying the oxygen transfer on an al-
kene from an oxovanadate complex with HROOH [10].
The assumed scheme reflects not only the
qualitative catalyst composition taking into account the
formation of three main products of the liquid-phase
cyclohexene oxidation by molecular oxygen, but also
the above-enumerated singularities of the process. For
example, it considers the possibility of the catalytic
cyclohexene oxidation directly by cyclohexenyl
hydroperoxide (route H–C–D–E–G), though, accord-
ing to the experimental data, the contribution of this
route seems to be insignificant.
It is shown in the considered scheme that the
absorption of molecular oxygen, which is possible only
under conditions of formation of the above-named
adducts H and C, becomes “involved” in the process in
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2568
ZAKHAROV, ZEFIROV
the stage of the complex E formation. Thus, the minor
cycle (E–G) of the oriented graph presented by the
scheme can be considered as the conjugate process of
the catalytic cyclohexene oxidation by molecular
oxygen and cyclohexenyl hydroperoxide.
The observed initial rate r0, effective rate constant
keff of the liquid-phase cyclohexene oxidation in
toluene (1:1) by molecular oxygen in the presence of
0.2 M HROOH, chelate compound I (0.51 M), and
samples A and B are given below; the volume of
solution 0.02 l; Т 293 K; m is the weight of a sample,
and y is the specific Cu(II) concentration in the
sample; the constant was calculated by Eqs. (3)–(5).
The observed rate of the catalytic cyclohexene
oxidation in the presence of chelate complex I is
independent of the partial pressure of oxygen in the
range of 0.006–0.06 MPa and linearly depends on the
analytical concentration of homogeneous catalyst I and
concentrations of substrate RH2 and cyclohexenyl
hydroperoxide НROOH. Kinetic curves of the oxygen
absorption at the liquid-phase RH2 oxidation in the
presence of chelate compound I are given in Fig. 3. In
all considered cases the observed initial rate r0 of the
homogeneous cyclohexene oxidation by molecular
oxygen agrees with empirical Eq. (2).
keff, l2 mol–2 s–1
robs×106,
m,
y,
Catalyst
mol l–1 s–1
g
μmol g–1
khom
3.0×10–2
khet
–
7.50
0.39
1.48
–
–
I
1.10
1.89
2.87
1.21
–
–
2.5×10–3
2.6×10–2
А
B
Heterogeneous oxidation. Now we shall consider
how the catalytic activity of chelate compound I varies
after its fixation on zeolite during the liquid-phase
oxidation of cyclohexene. In the absence of chelate
compound I zeolites NaY and CaA are inactive under
conditions of this experiment. Therefore the observed
catalytic effect, which was detected in the presence of
heterogeneous catalysts A and B (Fig. 3), is connected
exclusively with the catalytic activity of the fixed
chelate compound.
r0 = khom[RH2]0[I][HROOH]0.
(2)
Here khom is the effective rate constant of the homo-
geneous oxidation, [RH2]0 and [HROOH]0 are initial
concentrations of cyclohexene and cyclohexenyl hyd-
roperoxide, respectively, and [I] is the analytical
concentration of a homogeneous catalyst.
n, mmol l–1
0.20
Kinetic curves of the О2 absorption by cyclohexene
solutions in toluene in the presence of samples A and
B, and also in the presence of homogeneous catalyst I,
are shown in Fig. 3. In both cases after hetero-
genization of homogeneous catalyst I absorption of О2
on the solid support occurs with a rate exceeding the
rate of the background process by several orders.
Therewith in the presence of heterogeneous catalysts A
and B the mechanism of cyclohexene oxidation does
not change. It is confirmed by the qualitative and
quantitative catalysate composition, in which 2-cyclo-
hexen-1-one, 2-cyclohexen-1-ol, and epoxycyclo-
hexane in the molar ratio 2:1:1 were detected.
1
0.15
0.10
0.05
3
2
The initial rate of О2 absorption by cyclohexene in
a heterogeneous system containing catalysts A or B is
described by Eq. (3).
0
0
5
10
15
20
25
Time, min
r0 = k* [RH2]0[HROOH]0.
(3)
het
Fig. 3. Absorption of О2 (n, mmol) by cyclohexene
solutions (20 ml) in toluene as a function of time in the
presence of: (1) chelate I (1×10-4 M) at [HROOH] 0.05 M,
[RH2] 1 M, and (2) heterogeneous catalysts A (m = 1.1 g;
y = 2.87 μmol g–1) and (3) B (m 1.89 g; y 1.21 μmol g–1) at
[HROOH] 0.2 M, [RH2] 4.94 M, normal atmospheric
pressure, and 293 K.
Here k*het is the effective rate constant of the hetero-
geneous oxidation of cyclohexene in the presence of
samples A and B; [RH2]0 and [HROOH]0 are initial
concentrations of cyclohexene and cyclohexenyl
hydroperoxide, respectively.
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CATALYTIC BEHAVIOR OF COPPER(II) CHELATE COMPLEXES
2569
The effective rate constant k* depends on the
amount of a homogeneous catalyst fixed on zeolite.
However this assertion is not valid practically for any
catalyst concentrations in zeolite.
het
content of an active component in the solid catalyst
support, sample weight, and the technique of
depositing complex I on zeolite. It was found that the
length of a linear section of the rate of cyclohexene
oxidation by molecular oxygen in the presence of
samples A and B is different. An apparent decrease in
Let θ be a degree of pores filling of the catalyst
support by molecules of compound I. Then θ = N1/N2,
where N1 and N2 are the amounts of catalyst molecules
and large cavities, respectively, in a sample of
dehydrated zeolite NaY. The dependence of the ef-
fective rate constant of cyclohexene heterogeneous
oxidation khet (1) calculated by Eqs. (3)–(5) on the
degree of the sample A filling by the catalyst is shown
in Fig. 4. The effective rate constant khet is calculated
for an initial time instant. It is seen that only at low
filling degrees khet is almost independent of the chelate
compound concentration in zeolite. In this case the
maximal value of khet (~2.5×10–3 mol–2 l2 s–1) does not
coincide with the value of khom. As the filling degree
increases further, the effective rate constant of
cyclohexene heterogeneous oxidation khet decreases.
k* depending on time of the catalyst operation takes
het
place first for sample A. The time of “working-out” of
the catalysts B and I is much longer, and an apparent
decrease in their activity takes place at high conversion
degrees.
Thus, the interval of the cyclohexene stationary
oxidation by molecular oxygen in the presence of
sample B is retained for higher conversion degrees as
compared to sample A. It seems to be connected with a
diffusion inhibition of reaction (1) due to selective
adsorption of water formed upon cyclohexene
oxidation. Zeolite wetting results in the displacement
of a reagent from large cavities and to the apparent
decrease in the effective rate constant.
The maximal concentration of chelate compound I
in a sample, at which khet is conditionally constant,
corresponds to filling of no more than 1–3% of large
cavities of NaY zeolite with chelate compound. For
geometrical reasons the molecule of chelate compound
I can be arranged only in one large cavity of zeolite, in
Catalytic behavior of sample A. Let us assume
that all molecules of chelate compound I fixed on
zeolite participate in the catalytic process. We shall
assume also that the concentration of chelate I in a
heterogeneous system ([I]het) can be presented as ratio
(4) of the compound I content in a solid phase (a, mol)
to the total volume of liquid and solid phases (v, l).
log khet
–2.6
[I]het = a/v.
(4)
When a solid phase is used in a volume comparable
with a solution volume, the system is intensively
stirred, and also rather effective permeability of zeolite
for reagents is taken into account, we can neglect the
concentration gradient on the surface of crystallites. In
this case we can consider the solid phase as a peculiar
quasi-solution with boundaries moved apart conven-
tionally up to the total volume of the heterogeneous
system. Furthermore, the above-mentioned assump-
tions also do not contradict the common sense because
sterically fixed metal chelates in large zeolite cavities
are in the state of monomolecular dispersion. Then the
effective rate constant khet can be presented by Eq. (5).
1
–2.8
2
–3.0
khet = k* /[I]het.
(5)
het
0.03
0.06
0.09
θ
By physical meaning the value khet is the
heterogeneous analog of the constant khom and repre-
sents the effective rate constant of the heterogeneous
cyclohexene oxidation related to one catalytic center of
sample A. By definition, khet should not depend on the
Fig. 4. Dependence of effective rate constant khet of liquid-
phase cyclohexene oxidation on specific density of filling
large cavities y of sample A by chelate compound I in the
system: cyclohexene–toluene, 1:1; [HROOH] 0.2 M:
(1) “direct” run and (2) “inverse” run. Т = 293 K.
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2570
ZAKHAROV, ZEFIROV
1 g of which the number of large cavities is ~0.61 mmol.
Therefore the specific concentration of copper(II)
chelate compound in sample A, at which khet continues
to be conventionally independent of the degree of
filling large cavities with the catalyst, does not exceed
~0.02 mmol g–1 NaY.
dependence is presented in Fig. 4 by curve 2. It is seen
from Fig. 4 that the “reverse” run of khet does not
coincide with the “direct” run and lays lower for all
values 0.01 < θ < 0.09.
The incongruity of the khet(θ) dependences seems to
be connected with several reasons. First, with the fact
that the molecules of the catalyst obtained by the
“direct” and “inverse” modes are differently arranged
in cavities. The extraction of an active component
from sample A by the destruction of the complex
begins from opening cavities on an outer surface of
crystallites. This is the place where catalyst molecules
accessible to a reagent are arranged. In the course of
all subsequent extraction procedures the large cavities
disposed further from the external surface of
crystallites are sequentially depleted of catalyst
molecules. Compound I is extracted from sample A
nonuniformly over the volume of crystallites, and the
most screened molecules of chelate compound I are
extracted from zeolite in the latest time. As chelate
compound I is extracted from a sample, i.e. as the total
amount of copper in zeolite decreases, the fraction of
the most hardly accessible molecules of the catalyst
can decrease also due to a redistribution of catalyst
molecules in the volume of crystallites and due to
more uniform filling of large cavities on the border
with the solid phase outer surface.
The constancy of the effective rate constant in the
interval of low degrees of filling zeolite (0.01 < θ <
0.02) points to the fact that in this case all molecules of
the chelate compound deposited on zeolite are
accessible to molecules of reagents and consequently
take part in the heterogeneous catalytic process.
The decrease in the effective rate constant with
increasing filling degree of large zeolite cavities by
catalyst molecules, which is observed at high filling
degrees, can be caused by different reasons. As the
effective rate constant was calculated for an initial time
instant, deactivation of the catalyst is improbable. The
decrease in the effective rate constant khet is probably
connected with increasing diffusion resistance of a
sample with respect to cyclohexene molecules.
The concentration dependence of the effective rate
constant khet is a typical evidence of the secondary
sieve effect, which is caused by the fact that catalyst
molecules localized in large cavities inside crystallites
are removed from the catalytic process. At a high
filling of zeolite large interior cavities become
inaccessible for the reagent RH2 owing to blocking
pores by catalyst molecules. Therefore, when we use
chelate compound I topologically fixed on zeolite, a
self-screening of an active component due to blocking
pores and increasing diffusion resistance is observed.
Thus, the increase in the fraction of chelate I
molecules fixed on the zeolite and taking no
participation in the catalytic cyclohexene oxidation
because of steric hindrances, leads to the fact that the
effective constant khet becomes dependent on the total
amount of the catalyst in a solid phase.
Thus, the apparent increase in the activity of the
heterogeneous catalyst in the course of the extraction
of copper(II) chelate compound is caused by the
increase in the fraction of accessible catalyst molecules
on the background of the decrease in the total
concentration [I]het [relation (4)].
So, in the case of the topological (steric) fixation of
catalyst I in zeolite NaY the maximal activity is
reached at low θ degrees of filling large cavities. At
the same time the reaction rate depends on the value of
θ, the decrease of which is extremely inexpedient as it
results in a drop in the catalyst efficiency. Though khet
is independent of a sample weight for any filling
degree by definition, the rate r0 of the heterogeneous
cyclohexene oxidation at a fixed filling degree depends
linearly on the sample weight A (Fig. 5).
This explanation of the effect of the inhibition of
the liquid-phase cyclohexene oxidation by molecular
oxygen with increasing specific concentration of the
catalyst in a solid phase is confirmed by a rise of the
catalyst A activity after partial destruction of chelate
compound I by a dilute acid solution and extraction of
destruction products from the sample. In this case only
the trend to “recovery” of the effective rate constant
khet value is retained as the active component is
extracted from zeolite. No adequate recovery of khet
takes place even at a very high extraction degree. This
Catalytic properties of sample B. Sample B
contains chelate compound I fixed on zeolite СаА by
the method of topologically-anchor retention. The
concentration of complex I in a heterogeneous system
and the effective rate constant khet were calculated by
Eqs. (3)–(5) in the same manner as before. The value
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CATALYTIC BEHAVIOR OF COPPER(II) CHELATE COMPLEXES
2571
of the effective rate constant of cyclohexene oxidation
by molecular oxygen in the presence of sample B is
given above.
(effective rate constant) for homogeneous and hetero-
geneous processes points to the fact that in the
presence of heterogeneous catalyst B the liquid-phase
cyclohexene oxidation occurs in a mode close to
homogeneous. Therefore, when we use sample B, in
which catalytically active centers of copper(II) chelate
compound are taken away from zeolite pores and
turned to the side of the reaction medium, the behavior
of the heterogeneous system reminds the behavior of
homogeneous catalyst I. In other words, in the
presence of sample B the pseudo-homogeneous mode
is realized. In sample B complexes I are not adsorbed
on the outer surface of crystallites, but are in the liquid
phase volume, not losing a topological connection with
the catalyst support, and are completely accessible to
reagents. Therefore at effective solution stirring the
cyclohexene oxidation in the presence of sample B
occurs in the kinetic mode. The dependence of the
initial rate r0 of cyclohexene liquid-phase oxidation in
toluene (1:1, 0.02 l) by molecular oxygen on the
specific density y of chelate compound I fixed on
zeolite СaA is given below; [HROOH] 0.2 M; Т =
293 K, the presented data were calculated from the
parameters of dependences 1–3 (Fig. 6).
First of all it is necessary to note that the values of
effective rate constants khet for samples A and B do not
coincide. The values of khet calculated for sample B is
not only higher than the maximal khet value for sample
A, but within the limits of an experimental error
coincides with the effective rate constant khom of the
liquid-phase cyclohexene oxidation in the presence of
homogeneous catalyst I.
The difference in properties of samples A and B is
not limited only to the formal incongruity of effective
rate constants khet. In the case of the liquid-phase
cyclohexene oxidation involving sample B the
observed reaction rate linearly depends on specific
concentration (y) of chelate compound I in a solid
phase (Fig. 6), whereas the effective rate constant khet
is practically constant within the experimental error
limits.
Thus, the catalytic behavior of chelate compound I
depends on the mode of applying the catalyst on a
solid phase. The coincidence of kinetic parameters
2.5
3
r0×108, M s–1
100
3
2.0
80
60
2
1.5
1.0
40
1
2
0.5
20
1
0.6
0.8
1.0
1.2
0
y, μmol g–1
0
0.4
0.8
m, g
Fig. 6. Dependence of initial rate r0 of cyclohexene
oxidation on specific concentration (y) of chelate
compound I fixed by topologically-anchor retention in
zeolite СаА. System cyclohexene–toluene (1:1, 0.02 l);
[HROOH] 0.2 M; Т = 293 K, samples m, g: (1) 0.5, (2) 1.0,
and (3) 1.5.
Fig. 5. Dependence of initial rate r0 of cyclohexene
oxidation on weight m of sample A in the system
cyclohexene–toluene (1:1, 0.02 l); at [HROOH] 0.2 M, Т =
293 K, and various filling degrees: (1) 0.005, (2) 0.047, and
(3) 0.07.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 12 2009

2572
ZAKHAROV, ZEFIROV
EXPERIMENTAL
dr0/dy,
g l–1 s–1
khet,
l2 mol–2 s–1
Curve
m, g
Chelate compound I was synthesized by the method
described in [12] from freshly prepared copper(II)
hydroxide and a corresponding ligand. The hetero-
genization of complex I in synthetic zeolite NaY
(fraction of 0.5–1 mm, SiO2:Al2O3 = 5.1) (sample A)
was carried out by a matrix synthesis from copper(II)
triacetylmethanate complex and hexametylenediamine
fixed in large cavities [2, 13]. Sample B was prepared
by topologically-anchor retention on zeolite CaA [4].
The samples were characterized by diffuse reflection
spectra and elemental analysis.
1
2
3
0.5
1.0
1.5
0.64
1.28
1.91
0.0267
0.0259
0.0257
The same complexes in sample A are fixed in
zeolite pores, and the realization of the kinetic mode is
impossible even at infinitesimal concentration of
chelate I in the catalyst support, as is indicated by a
low limiting value of khet.
Coincidence of the khet value in the presence of
sample B with the khom value for the liquid-phase
cyclohexene oxidation in the presence of compound I
confirms the independence of the catalytic activity
from a detailed structure of distant substituents in γ-
position of a quasi-aromatic chelate metal cycle de-
tected earlier [11] for the instance of related copper(II)
chelate compounds. In fact, the topologically-anchor
fixation of chelate compounds on zeolites is impos-
sible without a ligand modification that generally does
not guarantee the conservation of catalytic properties
of the initial chelate compound. However, as our data
show, the topologically-anchor fixation of chelate
compound I practically does not affect its catalytic
properties.
Zeolite СаА (ALDRICH) was used in the form of a
commercial granular preparation with size of granules
of 1–1.5 mm. Zeolite СаА was calcinated in air by a
standard technique within 5 h at 550°С before the
synthesis of the catalyst.
Chelate compound I was partially extracted from a
matrix by a dilute solution of an acid with the
subsequent determination of copper(II) in the extract
by the iodometric titration. After partial extraction of
copper(II) from a sample, zeolite was multiply washed
out with water and held in a sodium sulfate solution for
the inverse ion exchange to restore the cationic form.
Cyclohexene was obtained by a dehydration of
cyclohexanol, purified from cyclohexenyl hydro-
peroxide by boiling with metal sodium, and stored in
an argon atmosphere. The concentration of cyclo-
hexenyl hydroperoxide was determined by the
iodometric titra-tion in an alcohol solution.
In conclusion it is necessary to note that the
comparison of two related methods (steric and
topologically-anchor fixation of chelate compounds of
metals on zeolites) points to the fact that the latter
method has a number of advantages. The matrix
synthesis yielding a catalytically active copper(II)
chelate compound sterically held in zeolite is
connected with the appearance of a secondary sieve
effect and, hence, with a decrease in the effectiveness
of using a heterogenized catalyst. The secondary sieve
effect and self-screening of a catalyst negatively
influence the value of the effective reaction rate
constant.
The kinetics of liquid-phase oxidation of cyclo-
hexene were studied by a static method in a thermally-
controlled glass reactor at atmospheric pressure and
room temperature. Toluene, which relatively readily
dissolves homogeneous catalyst I, was used as a
solvent. The qualitative and quantitative composition
of the catalysate was determined by the TLC method
and by the use of a Сhrom-5 Separon gas chromato-
graph (column 1.5 mm × 3m).
At the same time the effective rate constant of
cyclohexene oxidation in the presence of chelate
compound I fixed on zeolite СаА within the
experimental error limits coincides with the value of
khom that allows us to consider the method of the
retention of metal coordination compounds by
topological anchor as one of ways for solving the
actual problem of connections between homogeneous
and a heterogeneous catalyses.
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