Stabilization of palladium catalysts for the heck reaction by support functionalization and solvent selection

Article abstract of DOI:10.1002/cctc.201402523

Pd nanoparticles were synthesized on nonfunctionalized and functionalized SBA-15 grafted with thiol or amine groups. The resulting materials were used as catalysts to study the influence of these functional groups for a similar Pd particle size of approxi

Full text of DOI:10.1002/cctc.201402523

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DOI: 10.1002/cctc.201402523
Stabilization of Palladium Catalysts for the Heck Reaction
by Support Functionalization and Solvent Selection
Rafael L. Oliveira,[a] Jasper B. F. Hooijmans,[a] Petra E. de Jongh,[a]
Robertus J. M. Klein Gebbink,[b] and Krijn P. de Jong*[a]
Pd nanoparticles were synthesized on nonfunctionalized and
functionalized SBA-15 grafted with thiol or amine groups. The
resulting materials were used as catalysts to study the influ-
ence of these functional groups for a similar Pd particle size of
approximately 2 nm on the Heck reaction that uses iodide sub-
strates. The nonfunctionalized catalysts lost their activity quick-
ly because of Pd leaching and particle growth. However, if
a mixed solvent that consisted of toluene and DMF was used
and thiol groups were grafted on SBA-15, we were able to
reduce the Pd leaching and particle growth to thus obtain
a more stable catalyst in which an important role of the li-
gands was to recapture Pd. However, heterogeneity tests, such
as a hot filtration test and poisoning experiments, gave
a strong indication that Pd still leached from the support and
that homogeneous Pd species were mainly responsible for the
activity. Pd on S-functionalized SBA-15 was able to catalyze the
Heck reaction using different substrates to achieve good activi-
ty and selectivity in most cases.
Introduction
CÀC bond formation reactions are among the most important
transformations in organic chemistry. Traditionally, these reac-
tions are performed using strongly basic reagents such as
Grignards and lithiated carbon nucleophiles, however, these re-
agents lead to a considerable amount of waste and low selec-
tivity.[1] In 1968, Heck invented a new route to form CÀC bonds
using homogeneous Pd catalysts.[2] The Heck reaction offers
several advantages, such as reactants available commercially,
mild reaction conditions, high selectivity, and the use of small
amounts of Pd. Nonetheless, the homogeneous catalysts suffer
from low stability, high cost, and poor recyclability.
the catalytic activity is influenced by the nature of the sup-
port.[4] Among these supports, ordered mesoporous silica has
been applied for CÀC coupling reactions.[5] These materials
have large specific surface areas, uniform pore sizes, and tuna-
ble periodic pore arrangements. All these properties make
them very interesting for metal particles encapsulation and
catalysis application.[6] For example, Bao et al. showed that
confined particles in SBA-15 were more stable and did not
grow as severely as unconfined particles on silica gel.[7]
In general, nanoparticles supported on mesoporous silica
are confined in mesopores, which prevent their agglomeration.
Traditionally, supported metal nanoparticles have been encap-
sulated in mesoporous materials by incipient wetness impreg-
nation (IWI) and sol–gel processes. However, these methodolo-
gies display a lack of control of the particle size and in many
cases result in particles with a bimodal size distribution or
a mixture of nanowires and nanoparticles.[8] The methodolo-
gies used to deposit metal particles on different supports have
been reviewed.[9]
In the last decades, the application of heterogeneous Pd cat-
alysts for CÀC coupling reactions has received a lot of atten-
tion because these can be separated easily from the reaction
mixture to result in their more efficient reuse.[3] However,
leaching, poisoning, and particle growth have remained a chal-
lenge for heterogeneous catalysts, which result in a loss of ac-
tivity and limited recyclability in some cases.
The most common heterogeneous catalysts are metal nano-
particles supported on carbon, alumina, or silica. In general,
The synthesis of nanoparticles by ligand-assisted methods
has been used as an alternative to produce heterogeneous cat-
alysts for liquid-phase reactions.[10] In this methodology, materi-
als such as silica and polymers are modified with N-, S-, and P-
containing groups that are able to anchor metal complexes or
nanoparticles.[11] Moreover, the substitution of silanol by other
functional groups facilitates the adsorption of metal precursors
and the control of the particle size.[12]
[a] R. L. Oliveira, J. B. F. Hooijmans, Prof. Dr. P. E. de Jongh,
Prof. Dr. K. P. de Jong
Inorganic Chemistry and Catalysis
Debye Institute for Nanomaterials Science
Utrecht University
Universiteitweg 99, 3584CG, Utrecht (The Netherlands)
E-mail: k.p.dejong@uu.nl
These ligand-assisted methods have been applied to pro-
duce catalysts active for CÀC coupling reactions.[13] Shimizu
et al. immobilized Pd on FSM-16 modified with 3-mercaptopro-
pylsiloxane and reported a detailed characterization of this ma-
terial using extended X-ray absorption fine structure (EXAFS)
analysis, which showed the interaction between the ligand
[b] Prof. Dr. R. J. M. Klein Gebbink
Organic Chemistry and Catalysis
Debye Institute for Nanomaterials Science
Utrecht University
Universiteitweg 99, 3584CG, Utrecht (The Netherlands)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402523.
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and the metal.[14] Kaliaguine et al. functionalized the surface of
SBA-16 and KIT-6 with thiol groups and used these solids to
immobilize Pd complexes; the obtained materials presented
a high activity for CÀC coupling, however, the silica structure
collapsed after some cycles.[15] Dutta and Sarkar modified the
surface of silica gel with polyethylene glycol to produce a cata-
lyst that could be recycled five times without loss of activity.[16]
Ji et al.[17] and Richardson and Jones[18] used different types
of modified silica to study the active species of the catalyst,
that is, leached or supported metal, using different techniques,
such as hot filtration, poly(4-vinylpyridine) (PVPy) poisoning,
and three-phase tests, and concluded that leached species
were responsible for the activity. However, Crudden et al.[19]
used the same catalysts and tests and concluded that the
Heck reaction takes place at the surface of the catalysts. The
true nature of the active species continues to be a controversial
topic of discussion.
Figure 1. N2 physisorption of SBA-15 and functionalized SBA-15.
The influence of the nature of the solvent in the
Heck reaction was reported previously.[20] In general,
polar solvents such as DMF and N-methylpyrrolidone
(NMP) give a higher yield than nonpolar solvents for
both homogeneous and heterogeneous catalysts.
However, if DMF and NMP are used in heterogeneous
systems, severe Pd leaching is often reported. On the
other hand, nonpolar solvents often bring about
longer reaction times and require higher amounts of
Pd.
Table 1. Porosity of synthesized materials from N2 physisorption.
Materials
SBET
[m2 gÀ1
Pore volume
Pore diameter,
D [nm][a]
Density of S or N
[mmolmÀ2
]
]
[cm3 gÀ1
]
SBA-15
SBA-15SH
SBA-15NH2
774
588
394
0.77
0.62
0.45
6.8
6.6
5.6
–
1.0
5.4
[a] BJH analysis from the adsorption branch.
Despite the development of these new catalysts, no rigorous
comparison has been reported of the activity of Pd nanoparti-
cles with similar sizes synthesized on unmodified and modified
supports to further understand the influence of functional
groups on the support surface. In this article, we report the
synthesis of Pd nanoparticles on nonfunctionalized SBA-15
using IWI and ion adsorption. Furthermore, Pd nanoparticles of
a similar size immobilized on SBA-15 grafted with amino or
thiol groups have been studied. We compared the activity, re-
cyclability, and stability in the Heck reaction. Moreover, Pd par-
ticle growth, the nature of the active species (leached or sup-
ported), and the influence of the solvent were examined.
step, the amine-functionalized sample showed a greater de-
crease of the surface area. This phenomenon was probably
caused by amine groups that autocatalyze the functionaliza-
tion reaction.[22] Elemental analysis showed that SBA-15NH2
contained 3.0 wt% of N and SBA-15SH contained 1.9 wt% of S.
FTIR spectroscopy and thermogravimetric analysis (TGA) were
performed to confirm the incorporation of functional groups
on the silica surface (Figures S1 and S2).The low-angle XRD
pattern (Figure S3) confirmed that all samples exhibited the
characteristic diffraction lines of P6mm, which indicates no loss
of the structure, but in the case of SBA-15NH2 a lower intensity
of the peaks was observed. SEM analysis (Figure S4) of SBA-15
before and after functionalization shows that the material re-
tains the same morphology. Two other functional groups were
grafted onto the SBA-15 surface (diphenylphosphino and sul-
fonic acid groups). However, diphenylphosphino groups were
a poor Pd scavenger, and the acid nature of sulfonic groups in-
hibited the catalytic performance (Supporting Information).
The Pd nanoparticles were synthesized on nonfunctionalized
SBA-15 by IWI or ion adsorption (Experimental Section) fol-
lowed by an oxidation treatment and a reduction procedure.
The resulting Pd nanoparticles were observed by dark-field
scanning transmission electron microscopy (STEM; Figure 2). In
the case of IWI, the particles had a relatively broad particle size
distribution with an average size of 4.5 nm (Pd-4.5). The ion ad-
sorption method gave particles with a narrower particle size
distribution with an average size of 1.9 nm (Pd-1.9). Pd was
also deposited on N- or S-functionalized SBA-15 using a modi-
fied methodology described by Ying et al.[23] For both supports,
particles with a relatively narrow size distribution were ob-
Results and Discussion
Catalyst synthesis and characterization
SBA-15 was selected as a support because it has high porosity,
surface area, stability, and a high concentration of silanol
groups, which permits us to obtain a good metal dispersion
and distribution of functional groups during the functionaliza-
tion step. A postsynthetic functionalization technique[21] was
used to graft two different functional groups that contain
amine (SBA-15NH2) or thiol (SBA-15SH) on the silica surface. N2
physisorption isotherms of samples before and after the func-
tionalization are shown in Figure 1. The functionalized samples
displayed a decrease in the specific surface area (Table 1) and
pore volume, which confirms the functionalization of external
and internal walls of SBA-15. Although we used the same
molar amount of ligand precursor during the functionalization
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Figure 2. Dark-field STEM images and histograms of catalysts A) Pd-4.5, B) Pd-1.9, C) Pd-N-1.5, and D) Pd-S-1.8; scale bars 20 nm.
served. In the case of SBA-15SH particles with an average size
of 1.8 nm (Pd-S-1.8) were observed, and for SBA-15NH2 parti-
cles of 1.5 nm (Pd-N-1.5) were seen. A Gaussian function was
used to fit the particle size distribution for all materials
(Figure 2 and Table 2).
Scheme 1. The Heck reaction between iodobenzene and butylacrylate.
cies that resulted from Pd leaching. Elemental analysis showed
a high concentration of the Pd in the liquid after the catalyst
separation, and values that correspond to 71% (DMF) and
80% (NMP) of the Pd present in the solid originally were ob-
tained.
Table 2. Metal loading and particle size distribution of synthesized mate-
rials.
Catalyst
Pd
[wt%][a]
Pd particle size
[nm]
If toluene was used as a solvent, a colorless solution was ob-
served after the reaction and a much lower conversion of iodo-
benzene was obtained (Figure S5; Table 3, entry 3). The hot fil-
tration test showed a negative result, which implies that there
are no active Pd species in the solution. However, inductively
coupled plasma (ICP) analysis still revealed Pd in low concen-
Pd-1.9
Pd-4.5
Pd-S-1.8
Pd-N-1.5
1.55
1.52
1.54
1.57
1.9Æ0.5
4.5Æ1.0
1.8Æ0.5
1.5Æ0.4
[a] Determined by AAS.
Catalyst test for Heck reaction
Table 3. Results of the Heck coupling reaction of iodobenzene and butylacrylate
using different bases and solvents.
Influence of solvent and detection of leached species
Entry
Solvent
Base
Conversion
[%]
t
[h]
Pd leaching
[%]
The Heck reaction between iodobenzene and butyl-
acrylate was used as a model reaction (Scheme 1).
The influences of solvents and bases were examined
first using catalyst Pd-4.5 (Table 3). The highest con-
version was obtained if DMF and NMP were used as
solvents, and triethylamine was a more suitable base
than piperidine. However, if we used DMF or NMP
and triethylamine, a black solution was obtained
after the catalysis, which indicates the formation of
Pd black in solution (Figure S5). The hot filtration test
confirmed that the black liquid contained active spe-
1
2
3
4
5
6
7
DMF
NMP
triethylamine
triethylamine
triethylamine
triethylamine
piperidine
90
90
44
60
40
74
35
1
1
71
80
6
19
72
68
5
toluene
toluene+10% v/v DMF
DMF
NMP
24
12
1
piperidine
piperidine
1
toluene
24
[a] Reaction conditions: 2.25 mmol of iodobenzene, 3.4 mmol of butyl acrylate, 20 mg
of supported Pd-4.5 (0.13 mol% of Pd relative to iodobenzene), 2.14 mmol of base,
2.2 mL of solvent, 1008C; determined by GC (hexamethylbenzene, internal standard).
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tration in the solution (Table 3). Probably, the leached species
were deactivated during the test; the limitation of the hot fil-
tration test and its limited reproducibility have been reported
before.[17,24]
tionalized catalysts provided a direct influence of the function-
al groups in the Heck reaction, which excludes the effect of Pd
particle size and distribution.
If DMF is used as the solvent, the nonfunctionalized solids
and Pd-S-1.8 presented very high conversion (ꢀ90%) and the
reaction rate was almost the same in all cases (Figure S7),
which shows no effect of particle size and thiol groups but
a much stronger effect of the solvent on Pd leaching. However,
a slightly lower conversion was observed for Pd-N-1.5. Possibly
the higher amount of amine groups overcoordinated the
metal and decreased its catalytic performance. In all cases,
a severe loss of metal was observed, however, functionalized
samples showed a lower concentration of metal in the solution
(Table 4).
A mixture of toluene with 10% v/v DMF (Table 3, entry 4)
was also tested as a solvent. The mixture with DMF enhanced
the conversion of iodobenzene compared to that in pure tolu-
ene. The hot filtration test showed a positive result, and ele-
mental analysis showed the presence of a lower concentration
of Pd in the liquid than that for pure DMF (Table 3, entries 1
and 4).
Insoluble PVPy (2% cross-linked) was used by Yu et al.[25] as
a trap to confirm the presence of leached Pd. In the current
work, we used insoluble PVPy to complement the hot filtration
test and elemental analysis. For all cases studied, the catalytic
activity was ceased by the addition of PVPy (350 equivalents of
pyridine sites to total Pd).This indicates that Pd was liberated
from the surface of the support and that these leached species
were responsible for most of the catalytic performance.
A kinetic study of the reaction using DMF, toluene, and
a mixture of solvents is shown in Figure S6. The plots indicate
that 90% of iodobenzene was converted in 10 min if DMF is
the solvent. However, if toluene was the solvent, a long induc-
tion period and a much slower reaction rate were observed. In
the case of a mixture of DMF and toluene, no induction time
was observed and the reaction rate was higher than that in
pure toluene. Some authors suggested that the higher activity
for the Heck reaction if a DMF and NMP mixture is used as the
solvent is related to more severe Pd leaching because of the
stronger coordination of leached Pd.[20] The mixture of solvents
used in this work suppressed the leaching of Pd with conse-
quent moderation of the reaction rate.
If a mixed solvent was applied, Pd-S-1.8 and Pd-1.9 showed
the highest conversion. Probably, the small particles are dis-
solved more easily than larger particles of Pd-4.5, which causes
the faster formation of Pd species in the solution through oxi-
dative addition. The kinetics studies revealed almost the same
reaction rate for Pd-S-1.8 and Pd-1.9 and a much lower rate for
Pd-4.5 and Pd-N-1.5 (Figure S7). The hot filtration tests showed
positive results in all cases studied, and the conversion de-
creased tremendously if PVPy was added to the reactor
(Table 4), which confirms the contribution of leached species
to the conversion of iodobenzene.
Elemental analysis showed the presence of Pd in the liquid
in all cases. However, the metal leaching was less severe for
the mixed solvents than for pure DMF. Again, the nonfunction-
alized samples faced a higher loss of Pd than functionalized
solids (Table 4). Probably, functional groups on SBA-15 were
able to recapture the Pd leached to the solution, which thus
avoids a large amount of metal in the solution.
The recyclability of Pd-4.5, Pd-1.9, and Pd-S-1.8 is shown in
Figure 3. If DMF was used as the solvent, a gradually decreased
activity was observed after each cycle. The decreasing conver-
sion can be explained by leaching of metal to the solution
after each cycle. Pd-S-1.8 had a higher stability than the non-
functionalized catalysts because of a lower degree of Pd leach-
ing after catalysis. If the mixed solvent system was used
(Figure 3, bottom), the nonfunctionalized solids lost their activ-
ities gradually as observed previously. On the other hand, Pd-
S-1.8 could be recycled ten times with only a slight
Effect of Pd particle size and functional groups on activity
and recyclability
The catalytic tests for all Pd catalysts were performed in DMF
and in a mixture of toluene and DMF as described previously
(Table 4). The comparison between nonfunctionalized samples
was used to analyze the influence of Pd particle size in the
Heck reaction, and the comparison between Pd-1.9 and func-
decrease in activity after six cycles, which shows that
Table 4. Heck reaction performance for synthesized catalysts.
the combination of functional groups and the mixed
solvent decreased the metal leaching and enhanced
Catalyst
Solvent
Conversion
[%]
Pd leaching
[%]
Conversion (PVPy test)
[%]
the lifetime of this catalyst.
STEM images of the recovered catalysts after the
first cycle are shown in Figure 4. For all catalysts, the
structure of SBA-15 was still present, and no consid-
erable damage to its structure is observed. However,
the Pd particle size and distribution changed drasti-
cally for nonfunctionalized samples (Figure 4A and B)
as large metal particles were observed on the outer
surface of SBA-15 particles, which shows a redistribu-
tion of particles through Ostwald ripening. In this rip-
ening process, atoms or clusters are detached from
Pd-4.5
Pd-1.9
Pd-S-1.8
Pd-N-1.5
Pd-4.5
Pd-1.9
Pd-S-1.8
Pd-N-1.5
DMF
DMF
DMF
toluene+DMF
toluene+DMF
toluene+DMF
toluene+DMF
toluene+DMF
90
91
95
83
60
89
91
54
71
64
33
30
6
5.1
0.5
0.7
3.0
2.0
2.4
1.2
2.0
3.0
2.8
0
[a] Reaction conditions: 2.25 mmol of iodobenzene, 3.4 mmol of butyl acrylate, 20 mg
of supported catalyst (0.13 mol% of Pd), 2.14 mmol of base, 2.2 mL of solvent, 1008C,
1 hour (DMF) and 12 h (toluene+DMF); determined by GC (hexamethylbenzene, inter-
nal standard).
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Figure 4. Dark-field STEM images of recovered catalysts A) Pd-1.9, B) Pd-4.5,
and C) Pd-S-1.8.
ate the corresponding cross-coupling products with no forma-
tion of byproducts. The compounds with an electron-with-
drawing group, such as cyano, amine, and carbonyl groups,
gave a higher conversion. 1-Iodo-2-methylbenzene had the
lowest conversion, which shows the influence of the steric
effect (entry 2).
Figure 3. Recyclability of Pd-S-1.8, Pd-4.5, and Pd-1.9 using A) DMF and
B) toluene+10% v/v DMF. Reaction conditions: 2.25 mmol of iodobenzene,
3.4 mmol of butyl acrylate, 20 mg of supported catalyst (0.13 mol% of Pd),
2.14 mmol of base, 2.2 mL of DMF or 2.0 mL toluene+0.2 mL DMF, 1008C,
0.5 h (DMF) and 12 h (toluene+DMF); determined by GC (hexamethylben-
zene, internal standard).
Conclusions
Solvents were shown to have a large effect on metal leaching
and on the reaction rates of the Heck reaction. In general,
a high degree of metal leaching was related to fast substrate
conversion. The hot filtration test, poly(4-vinylpyridine) poison-
ing, and Pd analysis gave a strong indication that soluble spe-
cies influenced the final conversion of iodobenzene considera-
bly.
the surface of small Pd particles and the deposition of these
species takes place on larger particles.
Particle growth was less severe in the case of Pd-S-1.8. Some
Pd nanoparticles were observed at the outer surface but most
of the particles were confined in the SBA-15 mesopores, al-
though particle growth inside the mesopores was observed
(Figure 4C). These results indicate clearly that thiols on the
SBA-15 surface are crucial to decrease the metal leaching and
to slow particle growth by controlling Pd redeposition.
The Pd particle size had little effect on the activity and recy-
clability of the Heck reaction especially if DMF was used as the
solvent. A mixed solvent of toluene and DMF was important to
decrease metal leaching. The combination of a mixed solvent
and thiol groups grafted on SBA-15 was very efficient to limit
Pd leaching and particle growth, which permitted this catalyst
to be recycled ten times with only a slight loss of its activity.
Clearly, the functionalized catalysts were more stable than the
nonfunctionalized catalysts, which lost their activity gradually
because of metal leaching and Pd particle growth. Further-
more, the Pd on S-functionalized SBA-15 catalyst (Pd-S-1.8) was
able to catalyze the Heck reaction with different substrates to
achieve good activity and selectivity in almost all cases.
Tolerance of organic functional groups
The Heck reaction has high organic functional group tolerance
especially if homogeneous catalysts are used. This tolerance is
very important for the application of catalysts in fine chemistry
in which molecules with different organic functional groups
are used. The ability of Pd-S-1.8 to convert different substrates
was studied (Table 5). Electron-deficient, electron-neutral, and
electron-rich aryl iodides reacted with butylacrylate to gener-
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Catalysts preparation and catalysis experiments
Table 5. Heck reaction of aryl iodides with butyl acrylate promoted by
Pd-S-1.8.[a]
SBA-15 synthesis
Entry
Compound
Product
Conversion
[%]
SBA-15 was synthesized following the procedure described previ-
ously.[26] In a 250 mL polypropylene bottle P123 block copolymer
(2.0 g, 0.35 mmol) was dissolved in a mixture of water (15 g) and
HCl (60 g, 2m). This solution was stirred at 358C overnight. TEOS
(4.0 g, 19.2 mmol) was added dropwise to this solution under vigo-
rous stirring. After 5 min of stirring, the mixture was kept under
static conditions at 358C for 20 h then 24 h at 808C. The solid
product was collected by filtration, washed with distilled water,
dried at 608C for 24 h, and calcined at 5508C in static air for 6 h.
1
2
3
86
41
85
Functionalization of the silica surface
SBA-15 (500 mg) was dried at 1408C under vacuum for 12 h. The
solid was dispersed in dried toluene (20 mL) under nitrogen. Then,
(3-aminopropyl)triethoxysilane (0.93 mL, 4 mmol) or (3-thiopropyl)-
triethoxysilane (1.2 mL, 4 mmol) was added to the mixture drop-
wise over 5 min under vigorous stirring. Then, the mixture was
heated at 1108C and stirred for 24 h under nitrogen. The obtained
materials were collected by filtration and washed once with tolu-
ene and twice with ethanol and dried at 608C for 24 h.
4
5
85
86
IWI
6
95
In this methodology, the metal precursor is dissolved in an aque-
ous solution, which is used to fill the pores of the support, fol-
lowed by drying and calcination. In this procedure, SBA-15
(500 mg) was dried at 1408C under vacuum for 12 h. Then, an
aqueous solution of [Pd(NH3)4(NO3)2] (0.59 mL, 0.12m, 0.071 mmol)
was added to the solid with stirring. The obtained material was
dried at 608C for 24 h in static air. After drying, the catalyst precur-
sor was inserted in a microreactor and heated to 2008C at a heat-
ing rate of 108CminÀ1. During this period, the solid was exposed
to an O2 flow (30 mLminÀ1) for 30 min. The reduction of particles
was performed by heating the sample to 4008C at a rate of
108CminÀ1 and flow rate of 30 mLminÀ1 of a mixture of N2 and H2
(1:1, v/v).
7
8
86
100
9
99
[a] Reaction conditions: 2.25 mmol of iodobenzene, 3.4 mmol of butyl ac-
rylate, 20 mg of supported catalyst (0.13 mol% of Pd), 2.14 mmol of tri-
ethylamine, 2.0 mL of toluene and 0.2 mL of DMF, 1008C, 12 h; deter-
mined by GC (hexamethylbenzene, internal standard).
Ion adsorption
In this methodology, an attractive electrostatic interaction between
the metal precursor and support prevailed.[27] The solid was synthe-
sized using a modification of the methodology described by Zou
et al.[28] In this process, SBA-15 (500 mg) was added to deionized
water (1 mL), and the pH of the solution was adjusted to 8 through
the dropwise addition of a solution of NH4OH (1m). Then, an aque-
ous solution of [Pd(NH3)4(NO3)2] (0.59 mL, 0.12m, 0.071 mmol) was
added to this solid with stirring. The pH was readjusted to 8 using
NH4OH(aq), and the mixture was stirred for 12 h. The solid was re-
covered by centrifugation and washed with water.
Experimental Section
Chemicals
The same calcination treatment described above for IWI was used
for this material.
Triblock copolymer poly(ethylene oxide)-poly(propylene oxide)-
poly(ethylene oxide) (P123), tetraethyl orthosilicate (TEOS) 99%, (3-
mercaptopropyl) triethoxysilane 80%, (3-aminopropyl)triethoxysi-
lane 97%, 2% cross-linked PVPy, butyl acrylate 99%, piperidine
99%, triethylamine 99%, and tetraamminepalladium(II) nitrate solu-
tion 10 wt% in water were purchased from Sigma–Aldrich. Palladi-
um acetate (Pd(OAc)2, 47.1% Pd) was purchased from Degussa,
and iodobenzene 98% and other aryl halides were purchased from
Acros Organics.
Synthesis of Pd nanoparticles on the functionalized support
The protocol was based on the modification of a procedure report-
ed by Ying et al.[23] Functionalized SBA-15 (250 mg) was dried
under vacuum at 1408C for 12 h. After drying, the solid was dis-
persed in dry toluene (5 mL) under N2. Palladium(II) acetate (8 mg,
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0.036 mmol) was dissolved in dichloromethane (1 mL) and then
added slowly to the dispersion. The dispersion was then heated to
608C and stirred for 24 h. The solids were collected by centrifuga-
tion, washed once with toluene and once with ethanol, and dried
at 608C. To reduce the Pd, Pd-loaded SBA-15 (100 mg) was dis-
persed in water (1 mL). Then, NaBH4 (1 mL, 0.2m, 0.2 mmol) was
added rapidly under vigorous stirring. After 20 min, the dispersion
was diluted with water to 35 mL, centrifuged, washed thoroughly
with water, and dried at 608C.
water after cutting were picked up by a Pt loop, put on a carbon-
coated polymer grid, and left to dry.
The histograms of particle sizes were obtained from the measure-
ment of approximately 500 particles found using representative
images.
N2 physisorption
N2 physisorption measurements were performed at 77 K using a Mi-
cromeritics Tristar 3000. The samples were dried before measure-
ment under an N2 flow at 2508C (SBA-15) and 1508C (functional-
ized materials) for at least 12 h. The pore size distribution of the
mesoporous silica materials was calculated from the adsorption
branch of the isotherm by Barrett–Joyner–Halenda (BJH) analysis.
The maximum of the pore size distribution was reported.
Catalyst test
In a typical Heck reaction, a mixed solution of iodobenzene
(2.25 mmol), butylacrylate (3.47 mmol), base (2.15 mmol), hexame-
thylbenzene (internal standard for GC analysis, 1.1 mmol), and sol-
vent (2.2 mL) was prepared under nitrogen (Schlenk technique),
and catalyst (20 mg, 0.1 mol% of Pd relative to iodobenzene) was
added. The mixture was stirred for a given time and at a given
temperature.
Elemental analysis
S contents of the silica were determined by inductively coupled
plasma atomic emission spectroscopy (ICP-AES) by using a Met-
rohm IC Plus 883. Pd contents of the solid samples and in the liq-
uids from catalysis experiments were analyzed by atomic absorp-
tion spectroscopy (AAS) by using an AAS AANALYST200 instrument
(PerkinElmer). N contents were measured by using a Vario-EL (Ele-
mentar) system equipped with a CHNOS-Analyzer.
After the catalytic reaction, the solid catalysts were recovered by
centrifugation, washed with ethanol (35 mL), and dried under
vacuum. For recycling experiments, a new solution mixture de-
scribed above was added to the solid.
A small fraction of the liquid samples were taken, diluted with tolu-
ene, and analyzed by GC with flame ionization detection (FID). GC
analysis was performed by using
a PerkinElmer Clarus 500
equipped with a 30 m capillary column with 5% phenyl/95%
methylpolysiloxane as the stationary phase (AT5) using the follow-
ing parameters: initial temperature 508C, temperature ramp
108CminÀ1, final temperature 2508C, injection volume 0.5 mL.
TGA
TGA was performed by using a PerkinElmer Pyris 1 apparatus, a typ-
ical S/D sample quantity of 5 mg under an air flow of 10 mL minÀ1
.
A heating rate of 58CminÀ1 from 50–7008C was used.
Hot filtration test and PVPy poisoning test
FTIR spectroscopy
A Heck reaction was allowed to run for 30 min in DMF, 3 h for
a mixture of solvents, and 14 h for toluene. Then, the solids were
collected by filtration under a static vacuum by using a swivel frit
filter connected to an empty flask, and the filtrate was kept at reac-
tion temperature. The conversion was monitored by GC.
IR spectra were measured by using a PerkinElmer System 2000
FTIR instrument. The functionalized samples were dried at 1308C
and bare SBA-15 was dried at 2108C under vacuum before the
measurement.
The catalyst poison was added to the Schlenk flask before the ad-
dition of the reaction solution. PVPy was used at 350 equiv. of pyri-
dine sites to total of Pd.
Acknowledgements
The authors are grateful to the Dutch National Research School
Combination Catalysis (NRSC-Catalysis) for financial support and
Marjan Versluijs-Helder and Hans Meeldijk for SEM and TEM
analysis, respectively.
Characterization
XRD
Long-range pore ordering was confirmed with by low-angle XRD.
Patterns were obtained at RT from 2q=0.5–88 by using a Bruker-
AXS D2 Phaser powder X-ray diffractometer in Bragg–Brentano
mode equipped a Lynxeye detector using CoKa12 radiation, l=
1.79026 ꢁ, operated at 30 kV and 10 mA.
Keywords: Heck reaction · immobilization · nanoparticles ·
palladium · supported catalysts
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Received: July 7, 2014
Published online on && &&, 0000
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A cheer for the supporters! Functional-
ized and nonfunctionalized SBA-15 are
used as supports for Pd catalysts for the
Heck reaction. The nonfunctionalized
catalysts suffer from leaching and
severe particle growth. However, the
functionalized SBA-15 was able to mini-
mize the metal leaching and particle
growth to produce a catalyst that can
be recycled 10 times without significant
activity loss.
R. L. Oliveira, J. B. F. Hooijmans,
P. E. de Jongh, R. J. M. Klein Gebbink,
K. P. de Jong*
&& – &&
Stabilization of Palladium Catalysts for
the Heck Reaction by Support
Functionalization and Solvent
Selection
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