Rapid “Mix-and-Stir” Preparation of Well-Defined Palladium on Carbon Catalysts for Efficient Practical Use

Article abstract of DOI:10.1002/cctc.201700738

A facile direct deposition approach for the preparation of recyclable Pd/C catalysts simply by stirring a solution of tris(dibenzylideneacetone)dipalladium(0) with a suitable carbon material was evaluated. An extraordinarily rapid catalyst preparation procedure (<5 min) under mild conditions and its excellent performance in cross-coupling and hydrogenation reactions were demonstrated. The key point for catalyst design is the direct deposition of Pd⁰ centers onto the highly accessible surface area and the avoidance of ill-defined Pd^II/Pd⁰ states.

Full text of DOI:10.1002/cctc.201700738

Accepted Article
Title: Rapid 'Mix-and-Stir' Preparation of Well-defined Pd/C Catalysts
for Efficient Practical Usage
Authors: Sergey Yakukhnov, Evgeniy Pentsak, Konstantin Galkin,
Roman Mironenko, Vladimir Drozdov, Vladimir Likholobov,
and Valentine P. Ananikov
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: ChemCatChem 10.1002/cctc.201700738
Link to VoR: http://dx.doi.org/10.1002/cctc.201700738
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ChemCatChem
10.1002/cctc.201700738
ARTICLE
Rapid ‘Mix-and-Stir’ Preparation of Well-defined Pd/C Catalysts
for Efficient Practical Usage
[
a],
[a],
[a]
[b]
Sergey A. Yakukhnov, Evgeniy O. Pentsak, Konstantin I. Galkin, Roman M. Mironenko,
[
b]
[b]
[a]
Vladimir A. Drozdov, Vladimir A. Likholobov and Valentine P. Ananikov*
Abstract: A facile direct deposition approach for the preparation of
recyclable Pd/C catalysts simply by stirring a solution of Pd dba
Common methods
Direct Deposition
2
3
with a suitable carbon material was evaluated. An extraordinary
rapid catalyst preparation procedure (< 5 min) under mild conditions
and its excellent performance in cross-coupling and hydrogenation
reactions were demonstrated. The key point for catalyst design was
to directly deposit Pd(0) centers onto highly accessible surface area
and to avoid ill-defined Pd(II)/Pd(0) states.
limitations:
- multi-step process
advantages:
- fast one-step approach
- only routine equipment
- mild reaction conditions
- Pd(0) precursor
-
-
-
-
special equipment
harsh reaction conditions
incomplete reduction
Supported Pd/C catalysts have been extensively
developed and ubiquitously applied in numerous reactions in
Pd(II)/Pd(0) ratio difficult to control - visual process control
[1-4]
organic synthesis.
Indeed, in the current research practice
Pd/C catalysts have become an indispensable synthetic tool.
The most common techniques for preparation of Pd/C systems
include deposition-reduction, deposition-precipitation and
Figure 1. Preparation of Pd/C catalysts (a - deposition-precipitation, b -
deposition-reduction, c - impregnation, [Red] - reducing agent).
[
1-3]
impregnation (Figure 1).
The deposition-reduction method
involves treatment of suspension of a carbon material and a
Pd(II) salt with reductants (additional chemicals), which may
lead to contamination of carbon surface due to adsorption of the
additives. In the deposition-precipitation method, Pd(II) salts
serve as a source of Pd(OH) in the presence of bases. The
2
impregnation method involves wetting of a support material by a
solution of Pd(II) salt and subsequent solvent removal. In the
Application of commercially available catalysts is accompanied
with a series of well-known problems: large batch-to-batch
differences, ill-defined active centers with varying Pd(II)/Pd(0)
ratio and variations in composition and properties. Even within
the same research group, sometimes it is difficult to reproduce
own previous results when a new batch of commercial catalyst
arrives. Such critical parameters as catalyst selectivity and
efficiency are especially sensitive.
2
latter two methods, treatment under H atmosphere is usually
carried out for reduction of Pd(II) species. This requires special
equipment for treatment with the hydrogen gas at elevated
pressure (up to 10 bar) and temperature (200-500°C).
In the present study, we report a fast and efficient
approach for preparation of Pd/C catalysts based on direct
deposition (Figure 1). A very simple protocol was developed
involving only standard laboratory glassware and a magnetic
stirrer. The whole procedure of catalyst preparation can take < 5
min. The Pd(0) complex is used for the catalyst preparation; thus,
the reduction of metal species is not required. The scope of the
developed approach was investigated for various carbon
materials, and its efficiency was tested for a variety of substrates
in two most representative synthetic transformations – cross-
coupling and hydrogenation.
[
2a-c,3]
In overall, preparation of Pd/C catalysts is technically
rather difficult, and it is very rarely done in regular organic
synthesis laboratories. Moreover, incomplete reduction to Pd(0)
may occur when Pd(II) precursors are used as a source of
[
5]
palladium. Even in the presence of an excess of reducing
[
1a-c,2d,4f-g]
agent, the amount of unreacted Pd(II) may exceed 25%.
Thus, such catalysts may contain a combination of Pd(II)/Pd(0)
centers, where the ratio is difficult to control. Preparation of Pd/C
catalysts starting with zero-valent metal complexes was
previously studied; however, it required a lengthy procedure or
2 3
We utilized commercially available Pd dba as a metal
precursor for deposition of Pd nanoparticles. Easy generation of
reactive Pd(0) species and convenient determination of purity by
[
6a-d]
special equipment.
Usage of microwave heating during
1
[7]
catalyst preparation may initiate modification of carbon surface
H NMR are important advantages of Pd
2
dba
3
.
Several
[
6e]
depending on the conditions and particles morphology.
different carbon materials were used to prepare Pd/C catalysts:
graphite, multiwall carbon nanotubes (MWCNT), Vulcan XC72
(
Vulcan) and nanoglobular carbon (NGC). In the designed
procedure (Figure 1), the catalyst preparation was performed as
a single-stage process in a fairly simple procedure: mixing of
carbon material with the solution of Pd dba , heating of the
2 3
obtained mixture, and filtration and washing of the Pd/C catalyst.
The overall procedure including preparation and isolation could
take less than 5 min and the deposition itself could take 2 min
(video is available in the Supporting Information) and could be
performed using routine laboratory equipment.
[
[
a]
b]
Prof. V. P. Ananikov, S. A. Yakukhnov, Dr. E. O. Pentsak, Dr. K. I.
Galkin
Zelinsky Institute of Organic Chemistry
Russian Academy of Sciences
Leninskiy prospect 47, 119991 Moscow, Russia
E-mail: val@ioc.ac.ru
R. M. Mironenko, V. A. Drozdov, Prof. V. A. Likholobov
Institute of Hydrocarbon Processing
Russian Academy of Sciences, Siberian Branch
Neftezavodskaya street 54, 644040 Omsk, Russia
Supporting information for this article is given via a link at the end of
the document.
1
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ChemCatChem
10.1002/cctc.201700738
ARTICLE
Carbon
Surface area,
V
micro
,
NPs Size,
nm
2
-1
-1
F
support
This work:
m ·g
cc·g
G
NGC
116
194
228
4
0
2.1±0.6
1.4±0.3
1.9±0.6
6.2±2.2
MWCNT
Vulcan XC72
Graphite
0.010
0.060
0
Commercial catalyst:
Charcoal
1157
0.422
2.8±0.7
Figure 2. TEM images and particle size distribution histograms of prepared catalysts: Pd/NGC (A), Pd/MWCNT (B), Pd/Vulcan (C), Pd/graphite (D), and
commercially available Pd/charcoal catalyst (E); structural data of these Pd/C catalysts (F) and comparison of catalytic activity (G) in the Suzuki-Miyaura reaction
(evaluated as an average value from three independent experiments).
At the beginning of the procedure, the solution of the Pd
2
dba
3
pronounced tendency to agglomerate. Agglomeration was not
observed in the case of the other studied carbon materials with
larger surface area. For comparative purpose, we also included
a commercial Pd/C catalyst made of charcoal as a carbon
support (Figure 2E).
complex was dark red, and complete discoloration occurred at
the end of the deposition process. Thus, the process of Pd/C
formation can be visually controlled. As a result, all the
palladium species were attached to the carbon surface
quantitatively as confirmed by ICP-AES and elemental analysis
to form catalysts with a content of the metal of 1wt.%.
Next, we evaluated the performance of the Pd/C catalysts
in the Suzuki-Miyaura cross-coupling as a model reaction. The
reaction between 4-bromoanisole 1a and phenylboronic acid 2a
was carried out in the presence of 0.1mol% of a catalyst. The
process was stopped after 3h for a reliable comparison of
catalytic activity (at longer time or higher temperature, the
complete conversion is achieved, but comparison of relative
activity is not possible). The commercial Pd/C catalyst showed
yield of the product of 23%, and a close value of 21% was
obtained in case of Pd/graphite. Much better performance was
observed for Pd/Vulcan and Pd/MWCNT (33% and 37%,
respectively); the highest catalytic performance was found for
the nanoglobular carbon-derived catalyst (Figure 2G). The
Pd/graphite catalyst exhibited low activity owing to the larger
size of Pd nanoparticles and to its agglomeration. Although
small metal nanoparticles were present in the Pd/charcoal
catalyst, some of them might be confined into micropores (Vmicro
The prepared supported Pd/C catalysts were characterized
by transmission and scanning electron microscopy (Figure 2A-E,
3
and Supporting Information). Results of BET analysis of the
carbon materials are shown in Figure 2F. The microscopy study
showed the formation of spherical metal nanoparticles dispersed
across the surface of the carbon material. Excellent
morphologies of Pd/C consisting of small nanoparticles with
narrow particle size distribution were found for nanoglobular
carbon, MWCNT and Vulcan (Figure 2A-C). Averaged
nanoparticle diameters of 2.1±0.6nm, 1.4±0.3nm and 1.9
±
0.6nm were found for these carbon materials, respectively
(
Figure 2F). Larger average particle size and wider size
distribution of 6.2±2.2nm were observed in the case of graphite.
A relatively low surface area of graphite led to high arrangement
density, and the metal particles became relatively larger with a
2
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ChemCatChem
10.1002/cctc.201700738
ARTICLE
-1
=
0.42cc g ; Figure 2F), where mass-transfer is hindered. The
presence of diffusion-limited micropores in the carbon material
can significantly decrease the number of accessible active sites
in the Pd/charcoal catalyst. The highly accessible surface of
-1
Vulcan and MWCNT (Vmicro = 0.06 and 0.01cc g , respectively)
gave these Pd/C catalysts with better activity. The Pd/C catalyst
with an outstanding morphology was prepared using
nanoglobular carbon as a support (Figure 3).
Figure 3. SEM image of Pd/C catalyst prepared on nanoglobular carbon
support (A) and STEM image of the selected region showing Pd NPs (B).
All the metal nanoparticles were located on the surface of the
carbon material and were easily accessible for catalysis. The
spherical morphology of the carbon units ensured a large
surface area and the absence of non-uniform surface defects
Scheme 1. Pd/C catalyzed Suzuki-Miyaura reaction. Isolated yields are shown.
For 3a-3w Hal = Br; for 3x, 4a-b Hal = I. A part of the biaryl structure, derived
from boronic acid, is shown in blue. Pyrazolone derivatives 4a-b were
obtained according to the same procedure.
(
Figure 3).
After optimization of the reaction conditions (Table S1 in
the Supporting Information), we studied the performance of the
Pd/C catalyst with the nanoglobular carbon support for a large
scope of substrates. A variety of aryl iodides, as well as
electron-rich and electron-poor aryl bromides, reacted smoothly
to provide biaryl products in excellent yields (Scheme 1).
Irrespective of electronic properties and steric hindrance, 26
different cross-coupling products were synthesized in > 94%
isolated yields. The cross-coupling reaction was also
successfully carried out with the iodantipirin drug as a substrate
To further evaluate the performance of the prepared
catalysts, we examined their activity in the hydrogenation
reaction. Hydrogenation is the process of paramount importance
in modern organic synthesis, and about 75% of these
transformations are carried out using Pd/C catalysts. A well-
known and convenient technique of transfer hydrogenation using
formic acid was chosen as a model reaction (Figure 4). Again,
the Pd/C catalyst with the nanoglobular carbon support showed
the best activity. It is notable that similar relative activities were
observed for different carbon supports, as compared to the
cross-coupling reaction (cf. Figure 2G and Figure 4A). Under
optimized conditions, alkenes 5a-g were fully hydrogenated into
corresponding products 6a-g (Figure 4B).
[
8]
(
4a and 4b).
3
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ChemCatChem
10.1002/cctc.201700738
ARTICLE
A
recycled five times simply by separation from reaction mixture
after the reaction. Excellent conversion of 100% was observed
in all runs (see Supporting Information). After fifth cycle the
catalysts were studied with electron microscopy. Remarkably,
the size of Pd NPs remained very small (2.1±0.4 nm and 2.0±0.5
nm for MWCNT and NGC, respectively); and, to our great
delight, NPs sintering did not take place and no palladium
agglomerates were observed. Thus, the activity of the catalysts
was fully retained after five successive runs without an indication
of catalyst degradation.
To summarize, direct deposition of Pd(0) from the solution
2 3
of Pd dba represents a convenient approach for preparation of
B
recyclable Pd/C catalysts. Demanding Pd/C catalysts can be
made in regular organic synthesis laboratories without usage of
dedicated equipment and harsh reaction conditions. The scope
of the developed approach was studied for various carbon
supports, and the utility of the prepared catalysts was confirmed
5
a, 99%
c, 99%
5b, 99%
for
a wide range of substrates in cross-coupling and
hydrogenation reactions. Specific morphology of nanoglobular
carbon made it an excellent choice for preparation of Pd/C
catalysts with maximized accessibility of catalytic active centers
on the carbon surface.
5
5d, 99%
5g, 99%
5e, 99%
5
f, 99%
Figure 4. Comparison of activity of catalysts in styrene hydrogenation (A) and
scope of hydrogenation reaction (B). Unsaturated C-C bonds, which are
reduced during the reaction, are shown in blue.
Experimental Section
Preparation of Pd
2
dba
3
as source of Pd(0) species
Table 1. Performance of prepared Pd/C catalysts under different
conditions.
The quality of Pd
2
dba
3
is important for correct determination of the
amount of Pd(0) centers available for deposition onto a carbon support.
For commercially obtained samples, purity of Pd dba should be
can also be synthesized as
2
3
Entry
Conditions
Pd/NGC
Pd/MWCNT
Pd/Graphite
1
[7b]
evaluated by H NMR.
2 3
Fresh Pd dba
[
7b]
[
a]
described previously.
Cross-coupling
1
2
NMP/H
2
O, 3h,
Experimental procedure for the preparation of Pd/C catalysts
>
>
98%
99%
69-92%
> 99%
68-74%
> 99%
70°C
Pd
a magnetic stirrer were loaded into a screw-capped reaction vessel and
3
placed into an oil bath at 80 °C under stirring. After dissolution of Pd dba ,
2 3 3 3
dba •CHCl (25.0 mg), carbon material (500 mg), CHCl (25 mL) and
EtOH/H
2
O, 3h,
100°C
2
[
b]
a dark red solution was formed, and stirring was carried out until the red
color of the solution disappeared. The Pd/C catalyst was separated from
the solution by filtration or centrifugation and was washed with sufficient
amount of acetone (30 mL) to remove free dba, followed by drying under
reduced pressure (10-2 mbar, 100 °C). Detailed step-by-step procedure
is provided in the dedicated video movie (video is available in the
Supporting Information).
Hydrogenation
> 99%
3
4
EtOH, 2h, 50°C
EtOH, 4h, 70°C
35-85%
> 99%
10-20%
> 99%
> 99%
[
a] Cross-coupling of phenylboronic acid with 1c, 1d, and 1i [b]
Hydrogenation of 5a – 5c. See Supporting Information.
More information about experimental procedures, NMR monitoring of
deposition process, catalyst characterization, analytical data, catalyst
recycling, cross-coupling and hydrogenation reactions is provided in the
Supporting Information.
The catalyst derived from nanoglobular carbon showed the
highest activity and could be suggested as the catalyst of choice.
However, for regular synthetic transformations Pd nanoparticles
supported on MWCNT or even on graphite could also be used.
Applying higher temperature and longer reaction time resulted in
excellent yields in the studied representative cross-coupling (cf.
entries 1 and 2; Table 1) and hydrogenation (cf. entries 3 and 4;
Table 1) reactions. Thus, by using the developed catalyst
preparation approach efficient Pd/C catalysts can be prepared
from a variety of carbon supports.
ACKNOWLEDGMENTS
This work was supported by the Russian Science Foundation
(
RSF Grant 14-13-01030).
Keywords: Pd/C, catalyst preparation, cross-coupling,
Stability and recycling are important properties of
heterogeneous catalysts. Recycling of Pd/MWCNT and Pd/NGC
catalysts, prepared in the present study, was evaluated in
successive reaction runs. Both catalysts were successfully
hydrogenation, catalyst recycling
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10.1002/cctc.201700738
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