Impact of Pd-mordenite pretreatment on the heterogeneity of Heck catalysis

Article abstract of DOI:10.1039/b201180h

In Heck reactions with tributylamine as the base and in toluene, Pd(NH₃)₄²⁺-mordenite (0.4 wt% Pd) and Pd⁰-mordenite (0.4 and 4 wt% Pd) are not only active and selective, but also truly heterogeneous catalysts,

Full text of DOI:10.1039/b201180h

Impact of Pd-mordenite pretreatment on the heterogeneity of Heck
catalysis†
M. Dams, L. Drijkoningen, D. De Vos and P. Jacobs*
Kasteelpark Arenberg 23, 3001 Leuven, Belgium. E-mail: pierre.jacobs@agr.kuleuven.ac.be
Received (in Cambridge, UK) 31st January 2002, Accepted 25th March 2002
First published as an Advance Article on the web 18th April 2002
In Heck reactions with tributylamine as the base and in
yielding mordenite with 0.4 and 4 wt% Pd, respectively. Both
samples will be further denoted as 3PdNaMOR and 28PdNa-
MOR, respectively. A competitive ion exchange procedure with
2+
0
3 4
toluene, Pd(NH ) -mordenite (0.4 wt% Pd) and Pd -
mordenite (0.4 and 4 wt% Pd) are not only active and
selective, but also truly heterogeneous catalysts, while
+
2+
a 25-fold molar excess of Na w.r.t. Pd , is conducted in order
II
9
oxidized Pd species in an all-oxygen environment, i.e. ionic
to obtain an adequate Pd distribution through the crystals. The
2+
2+
Pd or PdO, are prone to leaching.
Pd(NH
3
)
4
cations absorb typically at approximately 299 nm in
the DRS spectra.
The nature of the Pd species obtained by drying and
II
The Pd-catalyzed arylation or vinylation of olefins, universally
known as the ‘Heck reaction’, is a versatile tool for the
2+
calcination of the Pd(NH
3
)
4
-mordenites under O
2
at 500 °C (1
1
21
generation of new C–C bonds in a single step. Advantages of
°C min , 8 h at 500 °C) largely depends on the Pd
2
+
Heck chemistry are the high selectivity, the tolerance of nearly
any functional group on the substrates and the mild reaction
conditions, associated with moderate toxicity and low cost of
the reagents.2 As an intriguingly flexible reaction it offers
considerable potential in the organic synthesis of polyfunctional
compounds, e.g. dienes, cinnamic esters and other variously
substituted olefinic compounds.3
concentration. In 0.4 wt% Pd-mordenite isolated Pd cations
are coordinated to lattice sites, as indicated by an absorption at
477 nm in the DRS spectrum; while at 4 wt% Pd loading, large
tobacco-brown PdO clusters are formed on the external surface
and cause diffraction at 2q = 34° (plane 101) in the X-ray
diffractogram.
2
Metallic Pd clusters are obtained by a H reduction at room
For economic reasons, industrial applications of homoge-
temperature, which results in a light grey and a black colour for
4
0
0
neous Heck reactions are limited. Development of a suitable
0.4 and 4 wt% Pd -mordenite, respectively. Pd -mordenite can
and re-usable heterogeneous catalyst remains therefore an
important goal. Since Pd switches between the zerovalent and
the divalent state in the catalytic Heck cycle, the choice of the
support is crucial for retaining both states. Ying and coworkers
reported on highly dispersed Pd in a mesoporous silica.5
Although this solid catalyst was characterized by excellent
activity and no Pd leaching, recycling was hampered by
degradation of the structure. From commercially available Pd/
C, Pd was found to leach into solution and to perform
also be obtained by autoreduction. A 4 wt% precalcined Pd-
2
mordenite heated under N at high temperature (12 h, 500 °C),
0
leads to a dark grey powder. Comparable Pd diffraction lines at
2q = 40° (plane 111) and at 2q = 47° (plane 200) are recorded
0
in the X-ray diffractograms of 4 wt% Pd -mordenite, irrespec-
tive of the reduction conditions. Despite these similarities, TEM
analysis visualizes the impact of the reduction conditions on the
0
Pd particle dimensions. TEM reveals the presence of spherical
0
Pd clusters with dimensions ranging from 50 to 150 nm for
6
0
homogeneous catalysis. Interestingly, almost all dissolved Pd
2
room temperature reduction under H , while an autoreduction
species can readsorb onto the surface of the carbon support after
completion of the reaction. Evidently, such a pseudo-heteroge-
neous catalyst would fail in a continuous reactor. Ionic as well
as neutral Pd species, entrapped in NaY and mordenite-type
zeolites, were found to leach only significantly from the Y
pretreatment results in the formation of a large number of small
0
Pd clusters in the 2 to 5 nm range, accompanied by some large
Pd metal clusters (450 nm). In any case (0.4 as well as 4 wt%
0
0
Pd -mordenite) SEM and TEM analyses prove that the large Pd
clusters clearly reside on the external surface of the zeolite.
7
zeolite. However, the severity of the leaching test used may
To evaluate the effect of the valence and the coordination
state of the metal on the heterogeneity and activity of the
supported Pd-mordenites, the prepared catalysts are tested in the
Heck reaction of 4-bromoacetophenone with n-butyl acrylate at
130 °C (Scheme 1). The reactions are performed in toluene and
with tributylamine as the base, since earlier research has proven
that Pd leaching occurs in an aprotic polar solvent, like DMA,
or with an acetate as a base.10 All Pd-mordenites yield n-butyl
trans-3-(4-acetylphenyl)acrylate (I) with a selectivity of more
than 98%, as evidenced by GC and liquid NMR of the isolated
product, with the cis-isomer as minor product (II). Typically,
the reaction‡ is followed until between 5 and 25% conversion is
reached. At this moment (split time), the hot suspension is
filtered (with 0.2 mm filter) and reaction progress is monitored
in filtrate and catalyst suspension.
seriously affect such conclusions.
The aim of this work is to find the relation between the nature
of Pd species in mordenite and their ability to catalyze truly
heterogeneous Heck chemistry. This implies that a clear
discrimination must be made between dissolved and zeolite-
associated activity. Neither recycling of the catalyst, nor
monitoring the filtrate activity after reaction completion, can
truly exclude the participation of dissolved Pd species in the
catalytic activity, as clearly evidenced by Arai and coworkers.6
Therefore, the filtrate should be collected from the suspension
while the reaction is still progressing, e.g. at 10 or 20%
8
conversion; moreover, the filtrate should be taken from a hot
suspension and under inert atmosphere. In this way, the filtrate
test becomes extremely sensitive, since it detects even sub-ppm
levels of active Pd. In addition, the total Pd content in the filtrate
is detected by AAS, including catalytically inactive Pd
species.
Na-mordenite (Norton Zeolon 100, Si/Al = 5.7, crystal size
2
1
1
.5–3.5 mm) with a CEC of 2.7 mequiv. g was exchanged
3 4 2
with an aqueous solution of Pd(NH ) Cl for 3 and 28%,
†
Electronic supplementary information (ESI) available: XRD patterns,
TEM and SEM images. See http://www.rsc.org/suppdata/cc/b2/b201180h/
Scheme 1 Standard Heck reaction.
1
062
CHEM. COMMUN., 2002, 1062–1063
This journal is © The Royal Society of Chemistry 2002

Catalysis with 100 mg of 3PdNaMOR is truly heterogeneous
Table 1, entry 1), since there is no catalytic activity in the
Table 2 Catalytic results of Heck reactions catalyzed by autoreduced
8PdNaMOR in the presence of 0.413 mmol Bun
NBr‡
2
4
(
filtrate after the split point and Pd leaching is not detected via
AAS ( < 1% of the total Pd content; less than 0.6 ppm). Besides,
the reaction is much faster than with 10 mg 28PdNaMOR (same
amount of Pd in the reactor, Table 1, entry 4). For the latter Pd
Conv.b
Time /h (%)
a
c
Aryl halide
Olefin
Sel. (%)
Iodobenzene
Bromobenzene
Benzyl chloride
n-Butyl acrylate
n-Butyl acrylate
n-Butyl acrylate
2
51
72
20
97
5
71
> 95
> 95
79
> 95
84
2+
3 4
leaching occurs, even though the amount of Pd(NH )
exchanged is still appreciably lower than the CEC of the
zeolite.
d
4-Bromoacetophenone n-Butyl acrylate
80
Whereas after calcination (Table 1, entries 2 and 5), an active
system is generated, neither the isolated Pd2 ions in 3PdNa-
MOR, nor the surface located PdO clusters in 28PdNaMOR
remain associated with the mordenite in the present reaction
conditions, as the filtrate shows a significant conversion. In
other reaction conditions, e.g. other solvents such as DMA or
other bases such as NaOAc, leaching from calcined Pd zeolites
4-Bromoacetophenone n-Butyl crotonate 144
4-Bromoacetophenone n-Butyl
> 90
+
methacrylate
18
133
15
59
75
80
4
-Bromoacetophenone Styrenee
a
Reaction time not optimized. b Conversion of the aryl halide. c Selectivity
NBr.
for the trans-compound. d 0.080 g catalyst. e no Bun
4
10
II
was even much more pronounced. Pd species with a
coordination sphere full of oxygen atoms are clearly prone to
leaching.
Bun
NI. Although quaternary ammonium compounds contrib-
4
4
ute to several accelerating mechanisms, it is probable that they
adsorb on the Pd clusters under the reaction conditions and
stabilize them against excessive sintering.12 Due to this
acceleration of Pd-mordenite catalysis, it is possible to broaden
the reaction scope to less active reactants (Table 2).
Hence, for the 3PdNaMOR catalysts, presence of amine
ligands is required for satisfactory retention of cationic Pd,
which can obviously be related to the higher affinity of
2
+
Pd(NH
3
)
4
complexes for the mordenite crystals than that of
2
+
II
This work highlighted the heterogeneity of Pd(NH
3
)
4
and
bare Pd ions, particularly at low exchange levels (compare to
0
II
Pd - mordenite in Heck catalysis, while oxidized Pd species in
an all-oxygen sphere clearly leach into solution. Further work
will prove the possibility to expand this well-defined catalytic
system to other zeolite supports and to use the catalysts in
related organic reactions.
the low but significant leaching for the untreated sample with
high exchange degree; Table 1, entry 4).11
Reduction of the precalcined Pd-mordenites leads again to
fully heterogeneous catalysts for reactions in toluene and with
tributylamine as base (Table 1, entries 3 and 6). Since residual
II
MD acknowledges FWO for a fellowship as Research
Assistant. This work was sponsored by the Belgian Federal IAP
programme and FWO project G.0355.99.
Pd species are expected to be very susceptible to leaching, this
reduction must be fairly complete. Meanwhile, too drastic
reduction and formation of large clusters must be avoided in
order to keep a considerable activity. Our preliminary kinetic
0
data show that the highly dispersed small Pd particles formed
Notes and references
‡
in the autoreduction exhibit a superior catalytic activity in
comparison with the hydrogen reduced catalyst 28PdNaMOR
Reaction conditions: 3 mmol aromatic compound, 4.5 mmol vinylating
agent, 3.3 mmol Bu N, Pd-mordenite (0.125 mol% of Pd based on the
3
n
(
Table 1, entries 6 and 7).
Thus, heterogeneous Heck catalysis is achieved by the
starting aryl compound) and 3 ml toluene in a well-stirred glass batch
reactor (8 ml, 700 rpm) at a temperature of 130 (± 1 °C). 1 ml samples are
analysed on a HP 5890A GC (10 m HP-1 Methyl Silicone Gum column and
a FID detector) and GC-MS (GC 8000 of Fisons Instruments, 30 m BPX5
SGE column and MD 800).
exchange of tetramine Pd complexes on mordenite, possibly
followed by an appropriate reduction, which avoids the
II
presence of all-oxygen coordinated Pd . In the case of untreated
3
PdNaMOR, TEM-examination of the zeolite samples during
0
1 W. Cabri and I. Candiani, Acc. Chem. Res., 1995, 28, 2; W. A.
Herrmann, C. Broßmer, K. Öfele, C. Reisinger, T. Riermeier, M. Beller
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763; H. Brunner, N. Le Cousturier de Courcy and J. P. Genêt,
catalytic reaction shows small external Pd particles, suggesting
that at least part of the amine ligands must be removed from the
2+
Pd(NH
As homogeneous Heck catalysis is promoted by the addition
of quaternary ammonium salts,4 the effect of Bun
NBr is
investigated in the reactions with 28PdNaMOR (N reduction)
3
)
4
during the reaction progress.
2
7
4
Tetrahedron Lett., 1999, 40, 4815.
2
3
M. Beller and K. Kühlein, Synlett, 1995, 441; F. Zhao, B. M. Bhanage,
M. Shirai and M. Arai, J. Mol. Catal. A: Chem., 1999, 142, 383.
4 I. P. Beletskaya and A. V. Cheprakov, Chem. Rev., 2000, 100, 3009.
5 C. P. Mehnert and J. Y. Ying, Chem. Commun., 1997, 2215; C. P.
Mehnert, D. W. Weaver and J. Y. Ying, J. Am. Chem. Soc., 1998, 120,
and the untreated 3PdNaMOR. In presence of the promoter the
reaction rate increases at least 5 times, the heterogeneity of the
catalyst remaining unaffected. Similar beneficial effects were
Br, Bun
4 4 4
observed with other added salts, such as NH NHSO or
1
2289.
6
F. Zhao, B. M. Bhanage, M. Shirai and M. Arai, Chem. Eur. J., 2000, 6,
843; F. Zhao, M. Shirai and M. Arai, J. Mol. Catal. A: Chem., 2000, 154,
39.
Table 1 Leaching tests in standard Heck reaction conditions‡ with
a
3PdNaMOR and 28PdNaMOR after different pretreatments
7
L. Djakovitch and K. Koehler, J. Am. Chem. Soc., 2001, 123, 5990; L.
Conv. (%) (time/h)
Djakovitch and K. Koehler, J. Mol. Catal. A: Chem., 2001, 142, 272.
Conv. (%)
split time/
Pretreatment h)
8 R. A. Sheldon, M. Wallau, I. W. C. E. Arends and U. Schuchardt, Acc.
Chem. Res., 1998, 31, 485.
9 J. F. Le Page, J. Cosyns, P. Courty, J. Limido and E. B. Miller, Applied
heterogeneous catalysts: design, manufacture, use of solid catalysts,
Technip Paris, France, 1987.
10 M. Dams, D. E. De Vos, L. Drijkoningen and P. A. Jacobs, Stud. Surf.
Sci. Catal., 2001, 135, 138.
11 R. M. Barrer and R. P. Townsend, J. Chem. Soc., Faraday Trans. 1,
1976, 72, 661; R. M. Barrer and R. P. Townsend, J. Chem. Soc.,
Faraday Trans. 1, 1976, 72, 2650.
(
Entry Catalyst
Suspension Filtrate
1
2
3
4
5
6
7
a
3PdNaMOR None
31 (22)
81 (47)
94 (26)
75 (66)
97 (164)
100 (23)
65 (48)
29 (47)
65 (26)
10 (65)
24 (164)
29 (79)
22 (48)
3PdNaMOR Calcination
3PdNaMOR Reduction
28PdNaMOR None
28 (7)
9 (19)
11 (72)
7 (8)
28PdNaMOR Calcination
28PdNaMOR Autoreduction 20 (24)
28PdNaMOR Reduction 32 (113)
See text.
12 M. Reetz and E. Westermann, Angew. Chem., 2000, 112, 170; M. Beller,
H. Fischer, K. Kühlein, C. P. Reisinger and W. A. Herrmann, J.
Organomet. Chem., 1996, 520, 257.
CHEM. COMMUN., 2002, 1062–1063
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