Fe3O4 nanoparticles-supported palladium-bipyridine complex: Effective catalyst for suzuki coupling reaction

Article abstract of DOI:10.1007/s10562-010-0293-4

Fe₃O₄ nanoparticles-supported palladium-2, 2-bipyridine complex was synthesized and used as catalyst for Suzuki coupling reactions between different aryl iodides and bromides with phenyl boronic acid under mild conditions. This catalyst can be separated magnetically. However, due to the aggregation of catalyst with the increase in reaction times, the activity dropped dramatically from >99% for the first time use to 45% for the fourth time use and then kept constant subsequently.

Full text of DOI:10.1007/s10562-010-0293-4

Catal Lett (2010) 135:256–262
DOI 10.1007/s10562-010-0293-4
Fe3O4 Nanoparticles-Supported Palladium-Bipyridine Complex:
Effective Catalyst for Suzuki Coupling Reaction
•
Yan-Qi Zhang Xian-Wen Wei Rui Yu
•
Received: 23 May 2009 / Accepted: 1 February 2010 / Published online: 17 February 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Fe3O4 nanoparticles-supported palladium-2,
20-bipyridine complex was synthesized and used as catalyst
for Suzuki coupling reactions between different aryl
iodides and bromides with phenyl boronic acid under mild
conditions. This catalyst can be separated magnetically.
However, due to the aggregation of catalyst with the
increase in reaction times, the activity dropped dramati-
cally from[99% for the first time use to 45% for the fourth
time use and then kept constant subsequently.
potential in the synthesis of natural products, herbicides,
pharmaceuticals, and conducting polymers [1, 2]. The most
frequently used catalyst for the Suzuki coupling reaction is
triphenylphosphine/Pd complexes but often a number of
by-products are formed in the reaction [3, 4]. Recently,
new types of ligands, such as carbenes [5, 6], pincer ligands
[7], and palladacycles [8, 9], etc., all binding to palladium
have emerged as catalyst for the Suzuki reaction with
excellent catalytic results in terms of yield and selectivity.
However, these catalysts, in general, suffer from the defi-
cits in recovery and recycling of homogeneous palladium
catalysts and the separation of catalysts and products. The
heterogenization of homogeneous catalysts can facilitate
their use for practical processes to overcome these problems.
Palladium complexes immobilized on various supports, such
as polystyrene [10], silica [11, 12], or mesoporous materials
[13, 14] were found to be suitable catalysts for the Suzuki–
Miyaura coupling reaction.
Keywords Fe3O4 nanoparticles-supported ꢀ
Palladium-bipyridine complex ꢀ Catalyst ꢀ
Suzuki coupling reactions
1 Introduction
The palladium-catalyzed Suzuki–Miyaura cross-coupling
reaction of aryl halides with arylboronic acids has become
a common and convenient synthetic method in organic
chemistry for biaryl compounds, offering considerable
Nevertheless, one primary problem of the heterogeni-
zation is still the separation of the reaction products asso-
ciated with the reuse of the catalyst after nanofiltration
through the membrane. Thus the development of an easily
recoverable and recyclable heterogeneous catalyst is still in
high demand. To meet the challenge, magnetic nanoparti-
cles (MNPs)-supported catalysts may be the better choice
because the magnetic separation prevents loss of catalyst
and increases its reusability as compared to filtration or
centrifugation [15, 16]. Additionally, as MNPs have a high
surface area, MNPs-supported catalysts also show high
dispersion and reactivity with a high degree of chemical
stability. The various types of organic reactions using the
MNPs-supported catalysts that have emerged recently
include C–C coupling reactions [17–24], hydroformylation
[25, 26], hydrogenation [27–35], C–N coupling reaction
[36, 37], oxidation [38–43], cleavage of allyl esters and
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-010-0293-4) contains supplementary
material, which is available to authorized users.
Y.-Q. Zhang ꢀ X.-W. Wei (&) ꢀ R. Yu
College of Chemistry and Materials Science, Anhui Key
Laboratory of Functional Molecular Solids, Anhui Normal
University, 241000 Wuhu, People’s Republic of China
e-mail: xwwei@mail.ahnu.edu.cn
Y.-Q. Zhang
e-mail: yqz1967@126.com
Y.-Q. Zhang
Department of Chemistry and Biology, Chuzhou College,
239012 Chuzhou, People’s Republic of China
123

Fe3O4 Nanoparticles-Supported Palladium-Bipyridine Complex
257
ethers (deallylation catalyst) [44], enantioselective acyla-
tion [45], multicomponents Aza-Sakurai reaction [46], the
Paal–Knorr reaction [47], CO2 cycloaddition reactions
[48], asymmetric hydrosilylation of ketones [49], and
esterification [50]. Other reports of MNPs-supported cata-
lysts include O-Alkylation reaction [51], halogen exchange
reaction [52], polymerization reactions [53], enzymes for
carboxylate resolution [54], amino acids for ester hydro-
lysis [55], organic amine catalysts promoting Knoevenagel
[56], one-pot reaction cascades [57], and the various acid-
catalyzed reactions (deprotection reaction of benzaldehyde
dimethylacetal) [58]. Concerning the Suzuki coupling
reactions, palladium complexes over MNPs include: Fe2O3/
G3-melamine dendrimers/Pd-phosphine [17], c-Fe2O3/poly-
mer/ Pd–NHC [19], Fe3O4/SiO2/Pd-N-heterocyclic carbenes
(NHC) [20], and Fe2O3/SiO2/polystyrene resins/Pd-NHC
[21]. These catalysts exhibit good activity and selectivity.
Here we report the first example of the preparation and
characterization of a magnetically separable palladium-2,
20-bipyridine (Bpy) (air stable, less expensive, and non-toxic
bidentate pyridyl ligand) complex, and its application for the
filtration-free, recyclable Suzuki coupling reaction between
a range of aryl halides and phenylboronic acid. The objec-
tive is not only to easily separate the catalyst from the
reactant by means of the simple application of an external
magnetic field, but at the same time also, as MNPs have a
high surface area, to immobilize palladium-2, 20-bipyridine
complex on MNPs with high dispersion and higher activity
than the palladium-2, 20-bipyridine complex itself.
7.466 g of K2Cr2O7 (26.052 mmol, 6 equiv) were dis-
solved in 7.5 mL of deionized water under magnetic stir-
ring. After the solution was cooled at 0 °C, 10 mL of
concentrated H2SO4 was added in small portions for
30 min. The mixture which turned blue-green was heated
to 75 °C for 4 h, stirred 10 h at room temperature, and
finally poured into a mixture of ice/water. The green pre-
cipitate was separated by centrifugation and washed sev-
eral times with water. This green powder was then
suspended in water, and KOH was added under vigorous
stirring until the solution was basic. The blue insoluble
residual powder was filtered and washed with water. The
aqueous solution was acidified with HCl to precipitate the
diacid which was filtered, washed with water, methanol,
and diethyl ether, and dried under vacuum.
2.3 Synthesis of Fe3O4-Bpy
4, 4-Diethoxycarbonyl-2, 2-bipyridine (Bpya) was linked to
the surface of the Fe3O4 nanoparticles via the amide covalent
bond [61, 62]. 16.3 mg (0.058 mmol) of 4, 4-Diethoxycar-
bonyl-2, 2-bipyridine, 20 mg of N-hydroxysuccinimide
(NHS), 30 mg of N, N0-dicyclohexylcarbodiimide (DCC)
and 12.7 mg of dopamine hydrochloride (DOPA) were dis-
solved in a mixture solvent containing 20 mL of CHCl3,
10 mL of DMF, and 50 mg of anhydrous Na2CO3. The
solution was stirred at room temperature for 2 h before
6.8 mg (0.029 mmol) of Fe3O4 nanoparticles were added.
The resulting solution was stirred 24 h at room temperature
under Ar protection. The modified Fe3O4 nanoparticles were
precipitated by adding hexane, collected by a permanent
magnet, washed with deionized water and ethanol several
times and dried under vacuum.
2 Experimental
2.1 Preparation of Fe3O4
2.4 Synthesis of Fe3O4-Bpy-Pd(OAc)2
Magnetic nanoparticles of 6–15 nm in size were prepared
by a coprecipitation method [59]. Briefly, FeSO4ꢀ7H2O
(1.213 g, 4.35 mmol) and FeCl3ꢀ6H2O (2.35 g, 9 mmol)
were dissolved in distilled and deionized water (20 mL)
under Ar and vigorous stirring. As the solution was heated
to 60 °C, 5 mL of NH3ꢀH2O (28% (w/w)) was added
quickly. The solution was aged at 60 °C for 2 h and then
cooled down to room temperature. Black precipitates were
collected by filtration and carefully washed with deionized
water and ethanol several times before finally drying under
vacuum.
The synthetic Fe3O4-Bpy and Pd(OAc)2 (26 mg, 0.116 mmol)
were suspended in 20 mL of DMF and refluxed under dry
oxygen-free argon for 8 h and stirred at room temperature for
12 h [63]. The product was precipitated from the mixture by
adding a large amount of chloroform and hexane (DMF:
chloroform: hexane v/v/v = 1:3:3) and collected by a perma-
nent magnet. After the solid product had been washed several
times with a mixed solution of chloroform and hexane (v/v =
1:1) [26], the product wasdriedunder vacuum for 24 h to obtain
the catalyst.
2.2 Preparation of 4, 4-Diethoxycarbonyl-2,
2-Bipyridine (Bpya)
2.5 General Reaction Conditions for Suzuki Coupling
Reactions
This reagent was prepared and characterized with little
modification as compared to the article described in [60].
0.8 g of 4, 40-Dimethyl-2, 20-bipyridine (4.342 mmol) and
In a 50 mL glass flask were placed aryl halide (0.5 mmol),
boronic acid (0.75 mmol), K2CO3 (1.5 mmol), and a
123

258
Y.-Q. Zhang et al.
catalyst (15 mg) in toluene (5 mL) at 80 °C for 12 h. The
reaction was monitored by TLC, and after completion of
the reaction, the mixture was extracted three times with
ethyl acetate and then a magnet was used to separate the
catalyst from the reaction mixture. The combined organic
extracts were evaporated under reduced pressure, and the
mixture was then purified by column chromatography over
silica gel to give a product with high purity and confirmed
with 1H NMR and 13C NMR. The catalyst was then washed
several times with chloroform and dried at room temper-
ature under vacuum before being used again for the next
reaction.
even more seriously after six runs in contrast to the
behavior observed before the reaction.
Figure 2a shows Fourier transformed infrared (FTIR)
spectroscopy of 4, 4-Diethoxycarbonyl-2, 2-bipyridine.
Bands at 1,717 cm-1 (C=O stretching of COOH group),
1,605, 1,562, 1,462 cm-1 (bipyridine), 3,113, 3,180 cm-1
(O–H of COOH group) are observed. Figure 2b shows the
FT-IR spectrum of Fe3O4-Bpy with bands at 1,643 cm-1
(C=O stretching of a secondary amide), 1,566, 1,555 and
1,485 cm-1 (bipyridine), 598 cm-1 (Fe3O4) and 3,457 cm-1
(O–Hfromwaterremaindersinthesample).Allofthesebands
indicate that 4, 4-Diethoxycarbonyl-2, 2-bipyridine is suc-
cessfully linked to the surface of the Fe3O4 nanoparticles.
Figure 2c shows the FT-IR spectrum of Fe3O4-Bpy-
Pd(OAc)2. Bands at 1,643 cm-1 (C=O stretching of a sec-
ondary amide), 1,620, 1,555 and 1,485 cm-1 (bipyridine),
586 cm-1 (Fe3O4) and 3,387 cm-1 (O–H from water
remainders in sample). Notably, the peaks at 1,605 cm-1 in
Fig. 2a, and 1,566 cm-1 in Fig. 2b which are due to the C=N
peakofbipyridine, disappear oncomplexationwithPdandare
replaced by a band at 1,620 cm-1 in Fig. 2c. The increase in
frequncy of the C=N peak indicate that a metal–ligand bond
forms and that the Pd indeed pumps electrons into bonding
orbitals of the bipyridine.
3 Results and Discussion
The IR spectra data of Bpya are as follow: 3,180, 3,113
(OH), 1,717 (C=O), 1,605, 1,562, 1,462, 764 cm-1
(bipyridine), 1,292 and 1,242 cm-1 (C–O), which are
identical to those reported in the literature [60, 64].
The catalyst Fe3O4-Bpy-Pd(OAc)2 was prepared by the
concise route outlined in Scheme 1. Dopamine (DOPA)
was first linked with one of the carboxylic groups in the 4,
4-Diethoxycarbonyl-2, 2-bipyridine (Bpya) via the EDC/
NHS chemistry. The DPA moiety proved to have high
affinity to the Fe3O4 surface [65]. Through DOPA, the
Bpya was covalently anchored on the surface of the Fe3O4
particles.
X-ray photoelectron spectroscopy (XPS) studies were
carried out to establish the oxidation state of Pd in the
catalyst (Fig. 3). The binding energy of Pd 3d5/2 in the
catalyst is found to be 337.8 eV (Fig. 3b), which is lower
than that in Pd (OAc)2 (338.2 eV) (Fig. 3a). This result
suggests that the metal (Pd(II) ions) has coordinated to the
ligand (Bpy) to form the coordination bond and there is a
charge transfer from bipyridine to Pd(II) leading to a
reduction in its oxidation state.
The transmission electron microscopy (TEM) images of
bare Fe3O4 are illustrated in Fig. 1a. It is observed that the
bare Fe3O4 nanoparticles are spherical, about 6–15 nm in
size, and nearly monodisperse. Figure 1b illustrates the
TEM image of the Fe3O4-Bpy-Pd(OAc)2 nanocomposite.
Accordingly, slightly aggregated nanoparticles between
6–20 nm are obtained. Agglomeration of the Fe3O4-Bpy-
Pd(OAc)2 particles after two and six runs (inset) can be
observed in Fig. 1c. In particular, it can be seen that the
catalyst already aggregates after two runs and aggregates
Thermal gravimetric (TG) analysis (Fig. 4) shows that
the organic content in the samples of Fe3O4-bpy is 7.26 wt.
%,. This confirms that bpy has been linked to the surface of
Fe3O4, and the order of thermal stability is: Fe3O4 [
Fe3O4-bpy [ Fe3O4-Bpy-Pd(OAc)2 [ Bpya. The weight
O
O
HOOC
K2Cr2O7
COOH
NHS, DCC, DOPA, Na2CO3
CH3
H3C
N C
C
N
H2SO4
r.t, 2h, Ar
r.t, 24 h, Ar
CHCl3, DMF,
N
N
N
N
N
N
O
O
O
O
COOH
N C
N C
C
N
COOH
Pd(OAc)2
DMF, Ar, 120oC, 8h
N
C
or
or
N
N
N
N
N
N
Pd
Pd
AcO
OAc
AcO
OAc
=Fe3O4
Scheme 1 Schematic representation of the preparation of Fe3O4-Bpy-Pd(OAc)2 complex
123

Fe3O4 Nanoparticles-Supported Palladium-Bipyridine Complex
259
200
180
160
140
120
100
80
(b)
(c)
60
40
20
(a)
0
4000
3500
3000
2500
2000
1500
1000
500
Wave numbers (cm-1)
Fig. 2 FT-IR spectra of a 4, 4-Diethoxycarbonyl-2, 2-bipyridine b
Fe3O4-Bpy c Fe3O4-Bpy-Pd(OAc)2
337.8 3d5/2
Pd 3d
338.2 3d5/2
343.2 3d3/2
343.6 3d3/2
(b)
(a)
332
334
336
338
340
342
344
346
348
350
Binding Energy (eV)
Fig. 3 XPS spectra of a Pd(OAc)2 b Fe3O4-Bpy-Pd(OAc)2
100
95
90
85
80
75
70
65
Fig. 1 TEM images of a bare Fe3O4 particles b Fe3O4-Bpy-
Pd(OAc)2 particles c Fe3O4-Bpy-Pd(OAc)2 particles after two runs
and after six runs (inset)
(a)
(b)
(c)
loss of Fe3O4-Bpy-Pd(OAc)2 is 4 wt. % at 100 °C, which
indicates that the catalyst has good thermal stability below
100 °C.
The magnetization curves measured at 300 K show that
the Fe3O4 nanoparticles-supported palladium-bipyridine
complex is superparamagnetic (Fig. 5). Fe3O4-Bpy-Pd
(OAc)2 nanoparticles have a saturation magnetization of
41.4 emu/g (Fig. 5b). The value is slightly smaller than that
of Fe3O4 nanoparticles (60.5 emu/g, Fig. 5a), which indi-
cates the presence of surface-bound Pd catalysts.
(d)
50
100
150
200
250
300
350
Temperature(οC)
The element analysis of catalyst Fe3O4-Bpy-Pd(OAc)2
before reaction by flame atomic absorption spectroscopy
indicates that the content of palladium is about 7.75 wt. %.
Fig. 4 TGA curves for a bare Fe3O4 b Fe3O4-Bpy c Fe3O4-Bpy-
Pd(OAc)2 d Bpya
123

260
Y.-Q. Zhang et al.
80
60
Table 1 Optimization of solvent for Suzuki coupling reaction
Entry
Solvent
Yield (%)a
(a)
(b)
1
2
3
4
5
6
7
Toluene
Dioxane
Ethanol
Acetone
CH2Cl2
DMF
[99
51
40
20
9
15
0
Trace
Trace
43
-20
-40
-60
-80
DME
Reaction conditions: 0.5 mmol of 4-bromoanisole, 0.75 mmol of
phenylboronic acid, 1.5 mmol of K2CO3, 15 mg of catalyst, 5 mL of
solvent, 80 °C under Ar, 12 h
a
-15000 -10000
-5000
0
5000
10000
15000
Isolated yield (%)
H(Oe)
Table 2 Optimization of base for Suzuki coupling reaction
Fig. 5 Magnetization hysteresis loops of a bare Fe3O4 b Fe3O4-Bpy-
Pd(OAc)2
Entry
Base
Yield (%)a
1
2
3
4
5
6
7
K2CO3
Na2CO3
NaOH
[99
Trace
Trace
89
The catalyst was tested for the Suzuki cross-coupling
reactions of aryl halides with phenyl boronic acid. The
coupling of 4-bromoanisole with phenylboronic acid was
studied as a model reaction and several bases and solvents
were chosen to monitor the best possible combination. The
reaction conditions were systematically optimized, and the
results are presented in Tables 1 and 2. When using K2CO3
as the base along with toluene, dioxane, ethanol, acetone,
CH2Cl2, DMF and DME as solvents, we observe that the
reaction runs to near completion with a yield [99% in
toluene systems, while in the other solvents a lower yield is
achieved (Table 1). With toluene as the solvent system,
and KOH or NaOAc as the base instead of K2CO3, the
yield is 89 or 58%, respectively, which is lower than for
K2CO3 ([99%). When using triethylamine, K3PO4,
Na2CO3 and NaOH as the base, the yield is very low
(Table 2).
KOH
NaOAc
N(C2H5)3
K3PO4
58
Trace
10
Reaction conditions: 0.5 mmol of 4-bromoanisole, 0.75 mmol of
phenylboronic acid, 1.5 mmol of base, 15 mg of catalyst, 5 mL of
toluene, 80 °C under Ar, 12 h
a
Isolated yield (%)
resins/Pd-NHC [21] and Fe2O3/G3-melamine dendrimers/
PdII-phosphine [17], since the yields are larger by 12–21%
and 5–27% for iodo-, bromo-derivatives, respectively.
In addition, we have compared the catalytic activity
of Fe3O4-Bpy-Pd(OAc)2 with non-supported catalysts Pd
(OAc)2(Bpy) and Pd(OAc)2(Bpya) which only produce 51%
yields and 10% yields, respectively, while [99% yield is
obtained by Fe3O4-Bpy-Pd(OAc)2 (reactant:4-bromoani-
sole). This result proves that the catalytic activity of Fe3O4
-supported catalyst is much higher than that of the non-
supported catalyst. The reason may be that Fe3O4-supported
catalyst has high dispersion in toluene and therefore shows
much higher reactivity than Pd(OAc)2(Bpy).
Based on these observations the reaction conditions
were systematically optimized and the best composition for
the reaction read as follows: 15 mg of catalyst, 0.5 mmol
of aryl halides and 0.75 mmol of phenyl boronic acid in
5 mL of toluene, in combination with 1.5 mmol of K2CO3
within 12 h at 80 °C, delivering high isolated yield ([99%)
of the product 4-methoxy-biphenyl (Scheme 2). Table 3
gives the summary of the Suzuki coupling reactions
with bromo, iodo and chloro derivatives catalyzed by the
as-obtained catalyst. The bromo and iodo derivatives can
react with phenyl boronic acid and give the corresponding
products with good yields, while the chloride derivatives
produce very low yields.
The heterogeneity of the catalyst systems was confirmed
by atomic absorption spectroscopy (AAS) analysis and a
hot filtration test [66, 67]. Accordingly, the reaction mix-
ture was filtered and treated with concentrated hydrochloric
acid to extract any leached palladium. After dilution to
50 mL the acid was analyzed by AAS and no palladium
was detected. Furthermore a hot filtration test was also
performed. Filtration after 1 h at 80 °C and allowing the
filtrate to react for additional 7 h, no conversion was
Experimental results also demonstrate that Fe3O4-Bpy-
Pd(OAc)2 is more active than Fe3O4/SiO2/Pd-NHC [20],
c-Fe2O3/polymer/Pd-NHC [19], Fe2O3/SiO2/polystyrene
123

Fe3O4 Nanoparticles-Supported Palladium-Bipyridine Complex
261
Scheme 2 Suzuki coupling
reactions for a range of aryl
halides
Fe3O4-Bpy-Pd(OAc)2
K2CO3, Toluene, 80 oC
B(OH)2
R
X
+
R
Consequently, it is believed that the strong interaction
between PdII and modified Fe3O4 nanoparticles plays a
significant role in preventing metal leaching during the
reactions.
Table 3 Summary of Suzuki coupling reaction of aryl halides with
boronic acid using Fe3O4-Bpy-Pd(OAc)2
Entry
Halide (R/X)
Time (h)
Yield (%)a
1
H/I
6
[99
[99
[99
[99
[99
[99
[99
80.3
17
The above results indicate that the aggregation of the
catalyst is responsible for the loss of the activity of Fe3O4-
Bpy-Pd(OAc)2 in reuse.
2
4-CH3/I
4-OCH3/I
4-NO2/I
H/Br
6
3
6
4
6
5
12
12
12
12
12
12
6
6
4-CH3/Br
4-OCH3/Br
4-NO2/Br
H/Cl
4 Conclusion
7
8
In summary, we have synthesized a Fe3O4 nanoparticles-
supported palladium-2, 20-bipyridine complex and used it
as a heterogeneous catalyst for the Suzuki coupling reac-
tions. This catalyst has shown high catalytic activity for
iodo and bromo derivatives with phenylboronic acid under
mild conditions. More importantly, this catalyst can be
easily separated from the reaction by decantation of the
reaction mixture in the presence of an external magnetic
field making the separation of catalyst and products very
simple: using a magnet of 105 mT helps reach the goal.
Unfortunately, the activity of the catalyst decreases with its
reuse. One of the important reasons for this is that the
catalyst aggregates seriously so that the dispersion of the
active phase decreases with time of reaction. At present,
we are performing further studies to develop a method
which prevents the catalyst particles from aggregating
during reaction.
9
10
11
12
4-NO2/Cl
2-CH3/I
2-CH3/Br
35
[99
99
12
Reaction conditions: 0.5 mmol of 4-bromoanisole, 0.75 mmol of
phenylboronic acid, 1.5 mmol of K2CO3, 15 mg of catalyst, 5 mL of
toluene at 80 °C under Ar
a
Isolated yield (%)
Table 4 Recycling experiment
with Fe3O4-Bpy-Pd(OAc)2
Run
Conversion (%)
1
2
3
4
5
6
[99
82
57
45
42
Acknowledgments We thank National Natural Science Foundation
(Nos. 20671002, 20490217, and 20271002) of PR China, Science and
Technological Fund of Anhui Province for Outstanding Youth (No.
08040106906), the State Education Ministry (EYTP, SRF for ROCS,
SRFDP20070370001), and the Education Department (No. 2006K-
J006TD) of Anhui Province for financial support.
43
observed and no palladium was detected. On the basis of
these results we conclude that the observed catalysis is
heterogeneous in nature [28].
The recyclability of the catalyst was studied in the
coupling reaction forming 4-Methoxy-biphenyl (Table 3
entry 7) at 80 °C under Ar and with 15 mg of Fe3O4-Bpy-
Pd(OAc)2 catalyst. The activity of the catalyst dropped
dramatically from[99% in the first time use to 45% in the
fourth time use and then kept constant in any further use
(Table 4). This may be caused by Pd leaching or aggre-
gation of the catalyst with its continual use in several
reaction cycles.
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