Anchored Pd complex in MCM-41 and MCM-48: Novel heterogeneous catalysts for hydrocarboxylation of aryl olefins and alcohols

Article abstract of DOI:10.1021/ja025991q

We report here, for the first time, synthesis of anchored Pd complexes in mesoporous supports such as MCM-41 and MCM-48 as true heterogeneous catalysts for hydrocarboxylation of aryl olefins and alcohols to give excellent conversion (～100%) and regioselectivity (～99%) for 2-arylpropionic acids. The catalysts were characterized by powder-XRD, ³¹PCP-MAS NMR, FT-IR, TEM, XPS and ICP-AES. Recycle studies with these anchored Pd mesoporous catalysts were performed to confirm true heterogeneity. Copyright

Full text of DOI:10.1021/ja025991q

Published on Web 07/25/2002
Anchored Pd Complex in MCM-41 and MCM-48: Novel Heterogeneous
Catalysts for Hydrocarboxylation of Aryl Olefins and Alcohols
Kausik Mukhopadhyay, Bibhas R. Sarkar, and Raghunath V. Chaudhari*
Homogeneous Catalysis DiVision, National Chemical Laboratory, Pune - 411008, India
Received February 21, 2002
Carbonylation of aryl olefins and alcohols provide highly
promising and eco-friendly routes for the synthesis of aryl propanoic
acids having applications as nonsteroidal, antiinflammatory drugs.1
This is considered as one of the best examples of the role of catalysis
in developing cleaner, environmentally benign routes replacing
Scheme 1. Hydrocarboxylation of Alkenes or Alcohols to Acids
with Pd-pyca-MCM-41 and Pd-pyca-MCM-48 Catalysts
2
stoichiometric organic synthesis as evidenced by the commercial
success of the Hoechst-Celanese process for the synthesis of
3
ibuprofen, which involves mainly a Pd catalyst with 10% HCl as
Table 1. Comparison of Activity and Selectivity of Various
Catalyst Systems for Hydrocarboxylation
4
a promoter. High regioselectivity for ibuprofen (>95%) is achieved
at high pressures (16-35 MPa) while, the selectivity reduces to
selectivity (%)
-
1
6
7% with a TOF of 50-70 h at lower pressures (6-7 MPa). In
TOF (h-1
catalysts
substrate
conversion (%)
iso
n
)
time (h)
5
recent reports, a significant enhancement in the catalytic activity
A
B
C
A
B
C
A
B
C
A
B
C
Sa
S
S
97.00
98.12
98.37
95.06
97.80
98.11
95.00
93.40
95.10
99.00
95.60
95.00
99.01 0.98
99.31 0.68
99.03 0.96
99.12 0.81
98.60 1.38
99.10 0.88
99.00 0.98
99.31 0.67
99.23 0.75
99.00 0.99
97.50 2.45
97.90 1.98
2600
463
417
1173
406
367
1313
286
262
804
450
439
0.18
12.0
12.0
0.83
12.0
12.0
0.35
12.0
12.0
0.60
12.0
11.0
-
1
(TOF ) 800-2600 h ) and regioselectivity (99%) for ibuprofen
at lower pressures has been demonstrated with modified promoters,
5c,d
4-Me-S
4-Me-S
4-Me-S
different phosphorus ligands, and Pd complexes.
These homogeneous catalysts often pose a serious threat on the
practical utility due to difficulties in catalyst-product separation and
t
4- Bu-S
6
reuse. Using supported Pd catalysts for carbonylation of aryl
t
4- Bu-S
halides6 and p-isobutylphenylethanol (IBPE), high activity (TOF
)
a
6b
t
4- Bu-S
1675-3375 h-1) and selectivity (99%) for branched carboxylic
IBPE
IBPE
IBPE
acid derivatives was achieved for IBPE; however, in both cases it
was concluded that the catalytic activity was due to Pd leaching
into the solution under reaction conditions. Pd-impregnated catalysts
a S ) styrene; catalyst: 50 mg (10 mg for A); substrate: 4.8 mmol;
(
Pd on SiO
2
, Al
2
O
3
, and clay) have also been used for hydrocar-
LiCl: 0.5 mmol; TsOH: 0.5 mmol; PPh3: 0.095 mmol; H2O: 0.01 mmol;
solvent: MEK; PCO: 3.06 MPa; agitation speed: 18.34 Hz; temperature:
7a
7b
boxylation of olefins and IBPE wherein leaching of Pd metal
after repeated reuse and use of HCl as a corrosive acidic promoter
are the major drawbacks. Therefore, the problem of developing a
true heterogeneous Pd catalyst for such carbonylation reactions still
remains an open challenge.
-
5
3
3
88 K; total liquid volume: 2.5 × 10 m ; Pd content: 0.18 wt % in B
(
Pd-pyca-MCM-41); 0.20 wt % in C (Pd-pyca-MCM-48); TOF ) turn
over frequency.
3
treatment of these functionalized supports with Pd(pyca)(PPh )-
OTs) (A),5 stable heterogeneous catalysts were obtained. Fol-
c,d
(
lowing this approach, Pd-pyca-MCM-41 (catalyst B) and Pd-
pyca-MCM-48 (catalyst C) were prepared (see Supporting
Information, for syntheses of catalysts B and C).
These catalysts were characterized by FT-IR, powder-XRD (for
details see Supporting Information, Table 1), TEM, XPS, and solid-
state P CP-MAS NMR. Powder-XRD patterns and TEM images
We report here for the first time, novel heterogeneous catalysts
containing a Pd complex anchored in mesoporous supports such
31
8
of the parent materials and anchored catalysts showed the typical
hexagonal phase (p6mm) of MCM-41 with the main (100) Bragg
reflection and cubic phase (Ia3d) of MCM-48 with distinct (211)
and (220) reflections indicating a high degree of ordered mesopo-
rosity (see Supporting Information, Figures 1 and 2). Catalysts B
and C showed no changes in the porous structures after function-
alization and incorporation of complex A inside the mesopores.
We imaged parent MCM-41 and MCM-48 materials and Pd-
anchored materials by TEM to direct the location of the grafted
complex A inside the mesopores. We could observe a striking
difference in the images of the materials, where the exterior surface
as MCM-41 and MCM-48 for hydrocarboxylation of aryl olefins
and alcohols with high regioselectivity, activity, and recyclability
without the leaching of Pd complex from the supports. Unlike silica
gel supports, Si-MCM-41 and Si-MCM-48 are highly ordered
mesoporous silica having high surface areas, high porosity, well-
defined porous structures, and controllable narrow pore-size
9
distribution. The stoichiometric reactions involved in hydrocar-
boxylation of aryl olefins and alcohols are presented in Scheme 1.
For preparation of the anchored Pd complex catalysts, 3-ami-
nopropyltrimethoxysilane (APTS) was used to functionalize MCM-
1
0
4
1 and MCM-48 by a procedure described elsewhere. On
10
retains strong image contrast probably due to complex A inside
the mesopores (treated with Ph
2 2
SiCl ), in comparison to the complex
*
5
Corresponding author. E-mail: rvc@ems.ncl.res.in. Tel/Fax: 0091-20-
893260.
anchored (without Ph SiCl treatment) both on the interior as well
2
2
9692
9
J. AM. CHEM. SOC. 2002, 124, 9692-9693
10.1021/ja025991q CCC: $22.00 © 2002 American Chemical Society

C O MMU N I C A T I O N S
of total Pd content in the system) by ICP-AES, which showed
almost no leaching of Pd metal from catalysts B and C in the liquid
phase during reactions. The filtrates also showed no hydrocarboxy-
lation activity, when tested without any addition of fresh catalysts.
The catalysts were found to be very stable, restoring high activity
and selectivity even after three recycles, as has been presented for
hydrocarboxylation of styrene with catalysts B and C (see Sup-
porting Information, Figures 4 and 5). Comparison of catalysts B
and B-3 (recycle 3 with catalyst B) analyzed by 31P CP MAS NMR
(Figure 1-c), showed no change in δiso values. Thus, the geometry
of the Pd-pyca anchored inside the MCM-41 remains intact even
after the third recycle. This observation provides evidence that
complex A is encapsulated inside the mesopores and does not leach
out under reaction conditions. In contrast to this, the catalyst
prepared by anchoring the complex A on silica gel as a support
showed a considerable amount of leaching of Pd metal (∼15%)
during reaction (similar to that observed by Fraile et al. for
epoxidation reactions ). The anchored Pd catalysts in MCM-41
and MCM-48 reported here represent the first case of true
heterogeneous Pd catalysts for hydrocarboxylation of aryl olefins
and alcohols.
Figure 1. 31P CP MAS NMR chemical shifts of (a) Pd-pyca complex, (b)
Pd-pyca-MCM-41 before reaction, (c) Pd-pyca-MCM-41 after 3rd recycle,
at 500 MHz (* in NMR patterns denote sidebands at 8 kHz).
as exterior walls of MCM-41 (see Supporting Information, Figure
2
B and C). Catalyst C, as an example, was chosen for X-ray
photoelectron spectra (XPS), which showed typical Pd(II) oxidation
state (Pd 3d5/2 ) 337.0 eV, Pd 3d3/2 ) 343.0 eV) and no cluster
formation of Pd metal in the catalyst (see Supporting Information,
Figure 3 and Table 2). FT-IR of the catalysts B and C showed
9a
-
1
-1
both the stretching vibrations at 1329 cm (νOdC-O) and 1669 cm
-
1
(ν
CdO), similar to those of complex A (1330 and 1668 cm
respectively). The Pd-N stretching frequencies of catalysts B and
C at 564 cm (νPd-N) varied from that of complex A (νPd-N
68 cm ), which reflects a possible coordination between NAPTS
and Pd-atom. P CP MAS NMR of catalysts A and B (Figure 1 a
and b) showed only one major 31P signal (δiso ) 33.13 ppm) for
Acknowledgment. K.M. and B.R.S. thank Council of Scientific
and Industrial Research (CSIR), India for fellowships. We thank
Dr. S. Ganapathy, Dr. A. B. Mandale, and Mr. A. Ghosh for their
help in characterizations.
5d
-
1
)
-
1 5d
5
31
Supporting Information Available: Syntheses, powder-XRD,
TEM, FT-IR, XPS data, and recycle-runs of Pd-pyca-MCM-41 (B)
and Pd-pyca-MCM-48 (C) (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
complex A, as envisaged from the structure, while that of catalyst
B has a shifted 31P signal (δiso ) 17.968 ppm). We presume APTS
(
tethered inside the mesopore walls) when anchored with Pd-pyca
complex donates an electron pair from NAPTS to the Pd atom
Scheme 1). This in turn increases the electron density on the P
atom of PPh by dπ (P) T dπ (Pd) bonding. This is consistent
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-arylpropanoic acids. The TOF values for the anchored catalysts
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(
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1
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-
4
filtrates were analyzed for Pd content (0.7 ppm, ∼3.5 × 10
%
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J. AM. CHEM. SOC.
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VOL. 124, NO. 33, 2002 9693