Easy-handling and low-leaching heterogeneous palladium and platinum catalysts via coating with a silicone elastomer

Article abstract of DOI:10.1016/j.tetlet.2019.02.035

We have developed a practical protocol for coating of commercial Pd/Al₂O₃ and Pt/Al₂O₃ catalysts in micro-powders with a silicone elastomer. Compared to original catalysts, the treated catalysts are easier to weight and transfer, and they are easier to recover by simple filtration. More importantly, the metal leaching of treated catalysts was significantly reduced. The treated catalysts worked very well in diverse hydrogenation reactions.

Full text of DOI:10.1016/j.tetlet.2019.02.035

Accepted Manuscript
Easy-handling and Low-leaching Heterogeneous Palladium and Platinum Cat-
alysts via Coating with a Silicone Elastomer
Mi Zhou, Tingting Li, Bo Xu
PII:
S0040-4039(19)30167-4
https://doi.org/10.1016/j.tetlet.2019.02.035
TETL 50629
DOI:
Reference:
Tetrahedron Letters
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Revised Date:
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17 December 2018
16 January 2019
19 February 2019
Please cite this article as: Zhou, M., Li, T., Xu, B., Easy-handling and Low-leaching Heterogeneous Palladium and
Platinum Catalysts via Coating with a Silicone Elastomer, Tetrahedron Letters (2019), doi: https://doi.org/10.1016/
j.tetlet.2019.02.035
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Easy-handling and Low-leaching
Heterogeneous Palladium and Platinum
Catalysts via Coating with a Silicone
Elastomer
Leave this area blank for abstract info.
Mi Zhou, Tingting Li, Bo Xu*

1
Tetrahedron Letters
Easy-handling and Low-leaching Heterogeneous Palladium and Platinum Catalysts
via Coating with a Silicone Elastomer
Mi Zhou, Tingting Li, Bo Xu*
Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua
University, Shanghai 201620, China.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
We have developed a practical protocol for coating of commercial Pd/Al2O3 and Pt/Al2O3
catalysts in micro-powders with a silicone elastomer. Compared to original catalysts, the
treated catalysts are easier to weight and transfer, and they are easier to recover by
simple filtration. More importantly, the metal leaching of treated catalysts was
significantly reduced. The treated catalysts worked very well in diverse hydrogenation
reactions.
Available online
Keywords:
Heterogeneous catalysts
Palladium
Platinum
Silicone elastomers
2009 Elsevier Ltd. All rights reserved.
Heterogenous palladium or platinum catalysts such as Pd/C,
Pt/Al2O3 are extremely important catalysts for production of fine
chemicals in hydrogenations and many other reactions.1 However,
these catalysts are usually supplied as very fine powders, which
cause many problems such as difficulty of recovery by filtration,
leaching of heavy metals, and hazardous nature during handling
of these fine catalyst particles in solution phase chemistry.
also used the high permeability of silicone elastomer to
encapsulate nano-catalysts.5j The high permeability of silicone
coating permits the incorporation and diffusion of organic
starting materials and products from reaction solution to catalyst
surface through the silicone coating (Figure 1). Because both the
inorganic supports such as Al2O3 and supported metal
nanoparticles such as Pd nanoparticles are both imbedded in the
silicone coating, it is very likely this coating can reduce the metal
leaching in solution phase chemistry.
Recently, we have developed a new product - silicone
elastomer-coated glass powder (‘FastWoRX’) as an absorbent to
extract organic products from an aqueous mixture.2 In this
method, the glass powder is coated with a thin layer of permeable
silicone elastomer, which can be used to extract organic products.
We envision a similar strategy could be used to coat the fine
powders of heterogeneous palladium and platinum catalysts such
as Pd/Al2O3. The coated layer of permeable silicone elastomer
could be used to reduce leaching of precious metals. Also, by
fine tuning the amount of coating material, coated particles may
aggregate to form relatively bigger particles, the bigger size may
make the treated catalysts easier to handle and easier to recover
through filtrations (Figure 1).
Figure 1. Coating of heterogeneous catalysts for easy-handling
and low-leaching.
We chose a commercially available and inexpensive silicone
elastomer (silicone rubber) precursor 13 for coating the
heterogenous metal catalyst with inorganic supports such as
Al2O3. 1 was widely used as a sealant in industry and household.
Precursor 1 has the poly(dimethylsiloxane) (PDMS) backbone
with reactive acetoxy sites for cross-linking. 1 reacts with the
surface hydroxy groups on the Al2O3 powders and cross-links by
reacting with the moisture in the air, releasing the by-product
acetic acid to form a strong coating (Figure 2).3 Silicone
elastomers are generally inert and highly permeable.4 And they
have been used in catalyst immobilizations5 and solid-phase
microextraction6 based on their high permeability.4a Recently, we
Figure 2. Coating on the surface of Al2O3 support.

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Tetrahedron
Our proof of concept application is the coating of widely
(Figure 4a). The slight slowdown could be rationalized by the
used and commercially available Pd/Al2O3 (5% Pd content).
Commercial Pd/Al2O3 is supplied as very fine powders (few m
in sizes) (Figure 3a). This causes many problems during
weighting, transferring and filtrating. Our coating method is very
simple: Pd/Al2O3 in micro powders was mixed with a
fact the additional mass transfer process was needed in the case
of Pd/Al2O3@Si. Due to easy recovery by simple filtration,
Pd/Al2O3@Si could be recycled and reused for 7 times without
significant loss of reactivity (Figure 4b). Due to the difficult in
the filtration of Pd/Al2O3, after filtration by a filtration paper, the
reaction mixture still contained 12.88 ppm of palladium
(determined by ICP-MS) (Figure 4c). To our delight, our treated
catalyst could significantly reduce the palladium leaching (Figure
4b). After simple filtration through filter paper, the palladium
leaches in reaction mixture of each cycle were all well below 0.7
ppm. After further purification, the leaching of Pd in the final
product should be further reduced.
concentrated solution of 1 in toluene, after toluene was
evaporated, 1 was coated to the surface of catalysts particles and
the coated catalyst was let to cure for 24 h (see supporting
information for more detail). The particles of our treated catalyst
(named as Pd/Al2O3@Si) are bigger in size (ca. 100 m) (Figure
3a). The original Pd/Al2O3 catalyst is hydrophilic due to the
nature of inorganic support Al2O3, but our treated catalyst is
highly hydrophobic (Figure 3b). This indicates that the coating is
successful. Our treated catalyst is much easier to weight and
transfer due to its bigger particle size, and it is also much easier
to recover by filtrations. As a comparison, both original and
treated catalyst was stirred in ethanol. After stirring for 6 h, the
original Pd/Al2O3 catalyst formed a relative stable suspension and
the suspension did not precipitate in 10 min (Figure 3c, left). On
the other hand, the treated catalyst Pd/Al2O3@Si precipitated
immediately after the stirring was stopped (Figure 3c, left).
Pd/Al2O3 was difficult to filter, after a vacuum filtration using a
filtration paper, significant amount the catalyst particles passed
through the filtration paper. In the contrary, the coated
100
a)
80
Pd/Al2O3
60
Pd/Al2O3@Si
40
20
0
Pd/Al2O3@Si catalyst was very easy to filter, and no visible
particle was present in the filtrate (Figure 3c, right).
0.0
0.5
1.0
1.5
2.0
a)
reaction time (h)
b)
100
80
60
40
20
0
Pd/Al2O3
Pd/Al2O3@Si
b)
c)
1
2
3
4
5
6
7
cycles
Pd/Al2O3 in water
Pd/Al2O3 @Si in water
Pd/Al2O3
c)
12
10
8
Pd/Al2O3 Pd/Al2O3@Si
Pd/Al2O3
Pd/Al2O3@Si
6
4
Pd/Al2O3@Si
2
After stirring in ethanol for 6 h
After filtration
0
0
1
2
3
4
5
6
7
8
Figure 3. Comparison of original Pd/Al2O3 and t Pd/Al2O3@Si.
cycles
We used the hydrogenation of nitro-compound 1a as our model
reaction to study the reactivity of our treated catalyst -
Figure 4. Pd catalyzed reduction of nitro-compound 1a.
Pd/Al2O3@Si. Using hydrogen gas of 1 atmosphere (H2 balloon),
both original Pd/Al2O3 and Pd/Al2O3@Si reduced 1a to the amine
product 2a smoothly at room temperature, and reaction of
Pd/Al2O3@Si was only slightly slower than original Pd/Al2O3

3
Figure 5. Applicability of Pd/Al2O3@Si in diverse hydrogenation reactions.
Encouraged by the success in the hydrogenation of nitro-
compound, we explored the applicability of our treated catalyst in
diverse hydrogenation reactions (Figure 5). First, we explored the
scope of nitro-compounds hydrogenation (Figure 5a) using 1 atm
of hydrogen gas at room temperature. Both aryl and heterocyclic
nitro-compounds worked very well, high yields of reduced
products (amines) were obtained. We also explored the
high yields. Similarly, our treated catalyst also worked very well
for reductions of carbonyl compounds such as aldehydes and
ketones (Figure 5c). The same condition also worked for
reduction of imine (Figure 5d). Pd/Al2O3@Si was also ideal
catalyst for hydrogenation of alkenes and alkynes (Figure 5e),
good yields of reduced products were obtained regardless. Lastly,
we investigated the applicability of our catalyst in deprotection
reactions (debenzylations) (Figure 5f). In all cases, good isolated
hydrogenations of organo azides (Figure 5b), diverse organo
azides were smoothly reduced to corresponding amines in very

4
Tetrahedron
yields of deprotection products were obtained without any
filtration paper) (Figure 6c). To our delight, our treated catalyst
exception.
could significantly reduce the Pt leaching (Figure 6b). After
simple filtration through filter paper, the Pt leaches in reaction
mixture of each cycle were all well below 1 ppm.
a)
a)
100
80
60
40
20
0
Pt/Al2O3
Pt/Al2O3@Si
0
1
2
3
4
b)
time(h)
10
8
b)
98
100
50
0
96
94
93
90
88
6
4
2
0
1
2
3
4
5
6
cycles
Pt/Al2O3
c)
c)
14
12
10
8
6
4
2
Pt/Al2O3@Si
0
0
1
2
3
4
5
6
cycles
Figure 7. Microscopic images of Pt/Al2O3 and Pt/Al2O3@Si. a)
Figure 6. Pt catalyzed selective reduction of nitro-compound 1d.
Original Pt/Al2O3; b) Pt/Al2O3@Si; c) Further magnification of b.
We also investigated the microscopic images of original and
treated catalysts (Figure 7). The original Pt/Al2O3 catalyst
appeared to be very fine powders (few m in sizes) (Figure 7a),
this led to difficult recovery by filtration. The treated catalyst
Pt/Al2O3@Si was much larger in sizes (ca 100 m) (Figure 7b).
The Pt/Al2O3@Si particles could be considered as an aggregate of
coated Pt/Al2O3 particles. The bigger size led to easy handling
and filtration, and the silicone coating and aggregation did not
change the kinetic of hydrogenation reaction significantly. So, in
the end, the silicone treatment gave the catalysts of easy handling,
low metal leaching with a only small reduction of reaction rate
(see also Figure 4a).
The same coated process could be also used in treatment of
commercial Pt/Al2O3 catalyst (see SI for detail). The obtained
catalyst Pt/Al2O3@Si was also a good catalyst for selective
reduction of nitro-compound 1d (Figure 6). Using hydrogen gas
of 1 atmosphere (H2 balloon), both original Pt/Al2O3 and
Pt/Al2O3@Si reduced 1d to the amine product 2b smoothly with
the ketone group untouched, and reaction of Pt/Al2O3@Si was
only slightly slower than original Pt/Al2O3 (Figure 6a). Due to
easy recovery by simple filtration, Pt/Al2O3@Si could be
recycled and reused for 6 times without significant loss of
reactivity (Figure 6b). Due to the difficult in the filtration of
Pt/Al2O3, after filtration by a filtration paper, the reaction mixture
still contained 10-20 ppm of Pt (depending on the types of

5
3.
Brassard, D. M., The Silicone Elastomer Handbook : A guide to
In conclusion, we have developed a practical coating protocol
for treatment of commercial Pd/Al2O3 and Pt/Al2O3 catalysts.
The silicone coating treatment gave the catalysts of easy handling,
easy filtration. More importantly, the metal leaching of treated
catalysts was significantly reduced. More application of coating
method for heterogenous catalyst are under investigations in our
laboratory.
applied silicone elastomer technology. First Edition. ed.; Silicone Solutions:
Twinsburg, Ohio, 2010; p xix, 350 pages.
4.
Rubber Chem. Technol. 1979, 52, 437-525; (b) Watson, J. M.; Payne, P. A.
Journal of Membrane Science 1990, 49, 171-205.
5.
P.; Baceiredo, A. Angew. Chem., Int. Ed. 2007, 46, 8632-8635; (b)
Motoyama, Y.; Abe, M.; Kamo, K.; Kosako, Y.; Nagashima, H. Chem.
Commun. 2008, 5321-5323; (c) Motoyama, Y.; Kamo, K.; Nagashima, H.
(a) Warrick, E. L.; Pierce, O. R.; Polmanteer, K. E.; Saam, J. C.
(a) Bonnette, F.; Kato, T.; Destarac, M.; Mignani, G.; Cossio, F.
Acknowledgments
Org. Lett. 2009, 11, 1345-1348; (d) Mwangi, M. T.; Schulz, M. D.; Bowden,
N. B. Org. Lett. 2009, 11, 33-36; (e) Motoyama, Y.; Kamo, K.; Yuasa, A.;
Nagashima, H. Chem. Commun. 2010, 46, 2256-2258; (f) Yang, H.; Xu, M.;
Guo, L.-X.; Ji, H.-F.; Wang, J.-Y.; Lin, B.-P.; Zhang, X.-Q.; Sun, Y. RSC
Adv. 2015, 5, 7304-7310; (g) Zhang, W.; Hu, Y.; Ge, J.; Jiang, H.-L.; Yu, S.-
H. J. Am. Chem. Soc. 2014, 136, 16978-16981; (h) Vankelecom, I. F. J.; Tas,
D.; Parton, R. F.; Van de Vyver, V.; Jacobs, P. A. Angew. Chem. Int. Ed.
1996, 35, 1346-1348; (i) Mwangi, M. T.; Runge, M. B.; Bowden, N. B. J.
Am. Chem. Soc. 2006, 128, 14434-14435; (j) Xu, L.; Xu, B. Tetrahedron Lett.
2017, 58, 2542-2546.
We are grateful to the National Science Foundation of China for
financial support (NSFC-21672035).
References and notes
1.
(a) Roberts, S. M.; Whittall, J., Regio- and stereo-controlled
oxidations and reductions. John Wiley: Chichester, England ; Hoboken, NJ,
2007; p xxii, 312 p; (b) Kudaibergenov, S. E.; Tatykhanova, G. S.; Selenova,
B. S. J. Inorg. Organomet. Polym. Mater. 2016, 26, 1198-1211; (c) Navalon,
S.; Garcia, H. Nanomaterials 2016, 6, 123/1-123/3.
6.
(a) Baltussen, E.; Sandra, P.; David, F.; Janssen, H.-G.; Cramers,
C. Anal. Chem. 1999, 71, 5213-5216; (b) Lee, J. N.; Park, C.; Whitesides, G.
M. Anal. Chem. 2003, 75, 6544-6554.
2.
Shi, Z.; Hammond, G. B.; Xu, B. ACS Omega 2018, 3, 6748-6756.
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Tetrahedron
Highlights
1. Coating of commercial Pd/Al2O3 and Pt/Al2O3 catalysts with a
silicone elastomer.
2. Easy-handling and easy to filter.
3. Low metal leaching.