Selective Hydrogenations and Dechlorinations in Water Mediated by Anionic Surfactant-Stabilized Pd Nanoparticles

Article abstract of DOI:10.1021/acs.joc.8b00314

We report a facile, inexpensive, and green method for the preparation of Pd nanoparticles in aqueous medium stabilized by anionic sulfonated surfactants sodium 1-dodecanesulfonate 1a, sodium dodecylbenzenesulfonate 1b, dioctyl sulfosuccinate sodium salt 1c, and poly(ethylene glycol) 4-nonylphenyl-3-sulfopropyl ether potassium salt 1d simply obtained by stirring aqueous solutions of Pd(OAc)₂ with the commercial anionic surfactants further treated under hydrogen atmosphere for variable amounts of time. The aqueous Pd nanoparticle solutions were tested in the selective hydrogenation reactions of aryl-alcohols, -aldehydes, and -ketones, leading to complete conversion to the deoxygenated products even in the absence of strong Br?nsted acids in the reduction of aromatic aldehydes and ketones, in the controlled semihydrogenation of alkynes leading to alkenes, and in the efficient hydrodechlorination of aromatic substrates. In all cases, the micellar media were crucial for stabilizing the metal nanoparticles, dissolving substrates, steering product selectivity, and enabling recycling. What is interesting is also that a benchmark catalyst like Pd/C can often be surpassed in activity and/or selectivity in the reactions tested by simply switching to the appropriate commercially available surfactant, thereby providing an easy to use, flexible, and practical catalytic system capable of efficiently addressing a variety of synthetically significant hydrogenation reactions.

Full text of DOI:10.1021/acs.joc.8b00314

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Article
Selective Hydrogenations and Dechlorinations in Water
Mediated by Anionic Surfactant Stabilized Pd Nanoparticles
Giorgio La Sorella, Laura Sperni, Patrizia Canton, Lisa Coletti,
Fabrizio Fabris, Giorgio Strukul, and Alessandro Scarso
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00314 • Publication Date (Web): 18 May 2018
Downloaded from http://pubs.acs.org on May 18, 2018
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Selective Hydrogenations and Dechlorinations in Water
Mediated by Anionic Surfactant Stabilized Pd
Nanoparticles
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Giorgio La Sorella, Laura Sperni, Patrizia Canton, Lisa Coletti, Fabrizio Fabris, Giorgio Strukul,
,*
Alessandro Scarso
Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca' Foscari Venezia, via Torino
155, Mestre Venezia (ITALY). Tel:+39ꢀ041ꢀ2348569; Fax: +39ꢀ041ꢀ2340517; alesca@unive.it
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We report a facile, inexpensive and green method for the preparation of Pd nanoparticles in aqueous
medium stabilized by anionic sulfonated surfactants sodium 1ꢀdodecanesulfonate 1a, sodium
dodecylbenzenesulfonate 1b, dioctyl sulfosuccinate sodium salt 1c and poly(ethylene glycol) 4ꢀ
nonylphenylꢀ3ꢀsulfopropyl ether potassium salt 1d simply obtained by stirring aqueous solutions of
Pd(OAc) with the commercial anionic surfactants further treated under hydrogen atmosphere for variable
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times. The aqueous Pd nanoparticles solutions were tested in the selective hydrogenation reactions of
arylꢀalcohols, ꢀaldehydes and –ketones leading to complete conversion to the deoxygenated products even
in the absence of strong Brønsted acids in the reduction of aromatic aldehydes and ketones, in the
controlled semiꢀhydrogenation of alkynes leading to alkenes and in the efficient hydroꢀdechlorination of
aromatic substrates. In all cases the micellar media were crucial to stabilize the metal nanoparticles, to
dissolve substrates, to steer product selectivity and to enable recycling. What is interesting is also that a
benchmark catalyst like Pd/C can be often surpassed in activity and/or selectivity in the reactions tested
by simply switching to the appropriate commercially available surfactant, thereby providing an easy to
use, flexible and practical catalytic system capable of efficiently addressing a variety of synthetically
significant hydrogenation reactions.
Introduction
Thanks to their peculiar properties, metal nanoparticles (NP) are attracting much interest in the
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field of catalysis as witnessed by the numerous reviews on this subject. Usually metal NP in the
catalysis field are prepared in proper solvents from a metal salt precursor and a reducing agent in
the presence of a stabilizer in order to avoid aggregation. Examples of suitable stabilizers are
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numerous, ranging from classical solid supports such as carbon, metal oxides and zeolites, as
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common practice in heterogeneous catalysis, as well as organic supports like polymers,
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dendrimers and many others. In this general framework the stabilization of metal NP in water is
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likely to gain a central role aiming at the development of new catalytic systems able to work in
environmentally friendly solvents such as water.
In this respect however, one has to face two opposite features, on the one hand the use of
water as the solvent allows in principle an easier product separation and catalyst recycling, but
on the other hand, most organic substrates are insoluble in this solvent causing lower reaction
rates. While hydrogenation reactions in water for rather soluble substrates have been reported
using different Pd based supported heterogeneous catalysts displaying good selectivities and
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activities, more hydrophobic substrates require often the use of polar coꢀsolvents thus
decreasing the advantages in terms of better environmental impact due to the use of water as
solvent.
A possible solution to the problem is the use of surfactants that selfꢀaggregate into micelles or
larger assemblies generating apolar environments able to provide a separate phase for organics as
well as naked metal NP stabilization in water through a "ligand" effect. The capping of metal NP
with suitable selfꢀaggregating agents produces a sort of supramolecular catalytic environment
capable of driving the selectivity of these catalytic systems in a way that often outperforms
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traditional heterogeneous catalysts. Examples of this approach have been applied to different
metal species. Stabilization of Co NP based on the formation of water/isooctane or hexane
microemulsions in the presence of sodium di(ethylhexyl) sulfosuccinate with sodium
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tetraborohydride was described leading to nanoparticles in the size range of 2–5 nm. In other
cases, the role of the surfactant employed was twofold. This is the case of the non ionic
copolymers poly(ethylene oxide) – poly(propylene oxide) – poly(ethylene oxide) (PEOx–PPOy–
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PEOx) that acted both as a reductant and as stabilizer for the growth of Au NP, or the
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employment of polyaniline for the formation of PdꢀNP or the use of cucurbit[6]uril leading to
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active and recyclable PdꢀNP. Alternatively commercial or specifically synthesized cationic and
zwitterionic surfactants were employed for this purpose and have been applied to a wide range of
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chemical transformations. Lipshutz and coꢀworkers disclosed the micellar promoted semiꢀ
hydrogenation of alkenes mediated by PdꢀNP stabilized by the neutral designer surfactant TPGSꢀ
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50ꢀM in water with NaBH as reducing agent.
Herein we present a facile, inexpensive and green method to synthesize Pd
nanoparticles in water stabilized by commercial anionic sulfonated surfactants like
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sodium 1ꢀdodecanesulfonate 1a
,
sodium dodecylbenzenesulfonate 1b, dioctyl
sulfosuccinate sodium salt 1c and poly(ethylene glycol) 4ꢀnonylphenylꢀ3ꢀsulfopropyl
ether potassium salt 1d (Scheme 1) characterized by a range of particle size and stability
and catalytic activity. We tested these Pd systems in water in the hydrogenation reactions
of aromatic aldehydes, ketones and arylꢀalcohols, in the hydrodechlorination reaction of
aromatic substrates and in the chemoselective semiꢀhydrogenation reactions of alkynes to
the corresponding alkenes observing a modulation in catalytic activity, of the product
selectivity and recyclability by simply changing the surfactant.
SO -Na+
3
SO -Na+
3
1a
1b
O
SO -Na+
3
O
SO -
3
Na+
O
O
O
O
n
n=20
1c
1d
Scheme 1. Sulfonated anionic surfactants employed for the stabilization of PdꢀNP in water.
Results and discussion
Preparation and characterization of the Pd-NP in the presence of surfactants
The preparation of the solutions containing the PdꢀNP was carried out consisting in the simple
dissolution of Pd(OAc) in water in the presence of a given surfactant subsequently left under
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hydrogen atmosphere for a variable time observing the change of the solution color from orange
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to greyꢀblack as an evidence of the presence of PdꢀNP. The time required for this operation
varied from two minutes, as in the case of 1d, up to ten minutes for 1b. The concentration of
surfactant employed was in all cases at least one order of magnitude higher than the
corresponding c.m.c. and it was optimized in order to grant the system the highest stability using
the lowest possible amount of surfactant.
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Table 1: PdꢀNP average size determined by TEM analysis.
Surfactant Pd particle
size (nm)
1
a
4.4
5.0
5.1
6.1
1b
1c
1
d
The solutions of PdꢀNP obtained with the different series of anionic surfactants 1a-d were
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characterized by means of transmission electron microscopy (TEM) analysis. In all samples the
technique revealed the presence of pseudoꢀspherical nanoparticles with an average diameter of
about 4ꢀ6 nm (Table 1) and a relatively narrow distribution between 2 and 9 nm. High
Resolution Electron Microscopy (HREM) evidenced that in most cases the nanoparticles were
pure Pd single crystals. What was markedly different from one surfactant to another was the
degree of dispersion of the NP, as is clearly evident from Figure 1.
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Figure 1. TEM images of the PdꢀNP stabilized by sulfonated anionic surfactant obtained
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stirring for 1 h and treating with hydrogen a solution of 3 mL of water with Pd(OAc) 2.4
mM and A) 1a 80 mM; B) 1b 100mM; C) 1c 90mM; D) 1d 80mM.
In the presence of surfactant 1c we obtained wellꢀdispersed and rather smallꢀsized
nanoparticles, while with surfactant 1a and 1b similarlyꢀsized PdꢀNP were observed but either
confined at the external edge of a large organic area or present in large aggregates. In the sample
with surfactant 1d slightly larger NP were still visible but highly aggregated into clusters that
assumed a diameter of about 25 nm.
The observed differences are likely to be due to the different interaction between the
surfactant and Pd. In all cases the presence of the sulfonated group allowed an attractive
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interaction with the Pd(OAc) precursor favoring the formation of similarly sized primary PdꢀNP
upon reduction. A role on the subsequent aggregation seems to be played by the nature of the
hydrophobic tail of the surfactant. Where no donor heteroatoms are present, as in 1a and 1b, Pdꢀ
NP are forced to remain in close contact with the hydrophilic heads on the external surface of the
aggregates. With surfactant 1c bearing two hydrophobic tails the formation of vesicles is likely
to occur and this could be related to the lower aggregation phenomena observed for the Pd NPs.
Surfactants bearing extra oxygen atoms in the hydrophilic portion, as 1d, may interact with Pd
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favoring the dissolution of the metal more deeply in the micellar aggregate and the subsequent
aggregation of the NP in clusters stabilized by the long polyethylene glycol chains. Furthermore,
as will be clear afterwards, the catalytic behavior of the different systems is often hardly
ascribable to the large differences observed in the extent of aggregation, which raises the
question whether these differences apply also to the operating catalyst.
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The different aqueous NP solutions were tested in a series of hydrogenation reactions. In
particular, the reactions that attracted our interest were i) the reduction of aromatic ketones and
aldehydes to alcohols and of arylꢀalcohol to toluene derivatives, ii) the chemoselective semiꢀ
hydrogenation reaction of alkynes to alkenes and iii) the hydrodehalogenation of aromatic
chlorinated substrates.
Catalytic hydrogenation of aromatic carbonyl compounds and benzyl alcohols
As far as the hydrogenation of aromatic carbonyl compounds like ketones and aldehydes
is concerned, the reaction can be carried out with hydrogen donors such as e.g.
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polymethylhydrosiloxane (PMHS) or with molecular hydrogen. The reaction proceeds
stepwise leading initially to the corresponding benzyl alcohol that can be further reduced
to the corresponding alkyl substituted aryl derivative (Scheme 2). In this step, the
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presence of a source of acidity, provided by a heterogeneous support or by an acid coꢀ
1
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catalysts,
oxygenation when the reaction is carried out with molecular hydrogen. Preliminary
experiments showed that catalysts made of PdꢀNP in the presence of 1a 1c and 1d were
is generally necessary in order to obtain the complete substrate deꢀ
,
active in the hydrogenation of benzaldehyde derivatives and their catalytic activity was
compared with that of commercial 10% Pd/C in methanol (Table 2) taken as a benchmark
for this reaction. Under the same experimental conditions, formation of Pd black was
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observed during the hydrogenation reaction with benzaldehyde using surfactant 1b
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therefore this surfactant was excluded in the further catalytic aldehyde reduction studies.
R
1
O
R
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OH
R
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Pd, H O, surfactant
R
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R
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+
R
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H
2
1 atm, r.t.
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Scheme 2. Hydrogenation of aromatic aldehydes
2 and ketones catalyzed by PdꢀNP in
water.
As is evident from data reported in Table 2, the catalytic behavior of PdꢀNP systems is
only dependent on the surfactant present in solution. In our experimental conditions we
did not add any external source of acidity, nevertheless we observed, in some cases,
complete hydrogenation to the alkylbenzene derivative. In fact, the hydrogenation of
benzaldehyde 2a turned out to be possible with all Pd@surfactant systems developed,
observing always the initial formation of the benzyl alcohol derivative 3a that more
slowly converted to the corresponding alkylbenzene derivative 4a over time. In all cases
the Pd@surfactant systems showed a generally lower catalytic activity with respect to
Pd/C in methanol probably also because of the well known lower solubility of hydrogen
20
in water rather than in methanol. However the reduced catalytic activity enabled a better
chemoselectivity of the hydrogenation reaction leading to a better control on the
hydrogenation of individual benzaldehydes
2 selectively forming the corresponding
benzyl alcohols or alternatively leading to full deꢀoxygenation by simply changing the
3
surfactant and the reaction time.
Table 2. Hydrogenation reaction of aromatic aldehydes 2a-d and ketones 2e with different Pd/surfactant
1
a,c,d in water and with Pd/C in MeOH.
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a
b
a
#
Substrate
Pd@1a
Time (min) 50 350
Conv. (%) 97 >99
Pd@1c
Pd@1d
Pd/C
60 500 60 350 10 50
97 >99 99 >99 94 >99
O
H
1
2
3a (%)
4a (%)
88
9
1
92
5
1
96
3
80 73
ꢀ
2a
1
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3
3
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5
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0
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9
0
99
99
20 21 >99
Time (min) 40 120
Conv. (%) 95 >99
60 120 70 500 10 60
97 >99 99 >99 81 >99
O
H
3
4
b (%)
b (%)
62
ꢀ
76
ꢀ
96
3
3
68
97 13 >99
50
ꢀ
2
b
33 >99
21 >99
1
0
Time (min) 50 350
60 500 500
>
>
9
ꢀ
9
Conv. (%) 98
98
98
97 >99
O
3
99
1
H
2c
3
4
c (%)
c (%)
84
94
96
70
ꢀ
15 >99
4
99
2
27 >99
Time (min) 40 150
Conv. (%) 92 99
60 250 120 500 10 30
O
96
78
18
99
4
92 >99 97 >99
4
H
3
4
d (%)
d (%)
77
2
74
18
2
55
ꢀ
2
d
O
15 98
95
98 42 >99
60 1440
1
440
Time (min) 60 1440 1440
O
Conv. (%) 94 99
90
20
70
45
39
7
97 98
93 18
5
3e (%)
4e (%)
90
4
1
2e
98
4
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4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Experimental conditions: Pd/C 7.6 mg (0.0072 mmol Pd), substrate 0.72 mmol, H 1 atm, MeOH
2
3
mL, T 25°C. Pd(OAc)
2 2
0.0072 mmol, substrate 0.72 mmol, H 1 atm, water 3 mL, T 25°C. a)
[
surfactant 1a and 1d] = 80 mM; b) [surfactant 1c] = 100 mM.
Overall, 1a provided a catalytic system characterized by slightly higher activity with respect to 1c,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
although Pd with surfactant 1a showed the presence of more aggregated PdꢀNP. This may result by the
coordination of the oxygen atoms of 1c to the metal centers that provide stability to the PdꢀNP but at the
same time tend to reduce their activity. At variance, surfactant 1a binds less strongly the PdꢀNP and this
turns out into a slightly more active catalytic system despite the higher aggregation observed.
Observing the results obtained in the hydrogenation reactions catalyzed by Pd/C, a different reactivity
of the aldehydes investigated can be noticed. The presence of a methoxy group in the paraꢀ position in the
aromatic structure of 2d makes this substrate more active compared to benzaldehyde 2a in the
hydrogenation and in the deꢀoxygenation reactions. 2a and 2c showed a comparable reactivity, despite the
higher steric hindrance of the latter, while the hydrogenation reaction of 2b turned out to be slightly
slower. Conversely, the results obtained in the reactions catalyzed by Pd@1a and Pd@1c showed that the
order of reactivity was inverted. The more reactive substrate became 2b that showed the lowest activity
with Pd/C. The higher lipophilicity deriving from the presence of the tertꢀbutyl group in the paraꢀ position
in the molecular structure of 2b can favor its migration inside the micelles and could explain this effect.
The results obtained with Pd@1d deserve a separate discussion since a complete reversal of the
activity order was observed. 2a became the most reactive substrate followed by 2b. The hydrogenation
reaction of 2d proceeded at a lower rate, while 2c returned to be the least reactive substrate. The cause of
this particular reactivity might probably be found in the PEG oxygen donors present in surfactant 1d that
stabilize the catalytic system lowering its activity, a recurring effect with these systems (vide infra).
Systems Pd@1a and Pd@1c seemed quite active also in the subsequent deꢀoxygenation reaction. We
decided to stress this property performing the hydrogenation reaction using acetophenone 2e as an
1
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intrinsically less reactive substrate. With 2e the differences in catalytic behavior for the Pd@surfactant
NP with respect to Pd/C were more evident. In fact, Pd@1d proved to be the least active catalyst, while
Pd@1a and Pd@1c gave results comparable with those obtained with Pd/C. More specifically, comparing
the results obtained in the reactions catalyzed by Pd@1a and Pd/C, it seemed that Pd@1a in water had a
higher catalytic activity in the deꢀoxygenation reaction than Pd/C in methanol despite the higher solubility
of hydrogen in the latter medium.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
In order to investigate better this aspect, we decided to compare the Pd@surfactant and Pd/C
systems directly in the deꢀoxygenation reaction of primary, secondary and tertiary benzyl alcohols
using the conditions of the hydrogenation reaction of carbonyl groups (Scheme 3).
R1
R2
R1 R2
OH
Pd
H
6
5
H 1 atm, r.t.
2
Scheme 3. Deꢀoxygenation reaction of benzyl alcohols 5 to the corresponding alkylbenzene derivatives 6.
The results (Table 3) showed that it is possible to reach a catalytic activity greater than that
observed with Pd/C in methanol using the appropriate Pd/surfactant in water. When we performed
the deꢀoxygenation reaction of 5a, 5b and 5c with Pd/C, the difference in activity for the three
substrates was evident. In fact 5a reacted rather quickly (>99% conversion in 1.5 h), the reaction
of 5b did not provide a high conversion (70%) even after 24 hours, while the same time allowed
to obtain only 40% conversion of 5c even increasing up to 5 times the amount of Pd/C. With the
Pd@surfactant systems, the order of substrates reactivity was confirmed, but Pd@1a and Pd@1c
systems proved to be very active also in the deꢀoxygenation of secondary and tertiary benzyl
alcohols. With these substrates we observed that Pd@1a was the best catalyst. Pd@1c showed
slightly lower catalytic activity but both catalysts in water gave a quantitative conversion of the
substrates under the same mild conditions of Pd/C in methanol. The comparatively poor reactivity
1
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
observed with Pd@1d could be easily ascribed to the aggregation phenomena that causes the
formation of large metal cluster.
Table 3. Deꢀoxygenation reaction of benzyl alcohols 5a-c with Pd@surfactant 1a-d in water and Pd/C in
THF.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Time Conversion
#
Substrate
Catalyst
(h)
(%)
Pd@1a
Pd@1c
Pd@1d
Pd/C
6
>99
>99
50
OH
5a
10
24
1.5
20
24
24
24
24
24
24
24
1
>99
>99
>99
10
Pd@1a
Pd@1c
Pd@1d
Pd/C
OH
2
5
b
70
Pd@1a
Pd@1c
Pd@1d
Pd/C
>99
>99
8
OH
5c
a
3
40
Experimental conditions: Pd/C 7.6 mg (0.0072 mmol Pd), substrate 0.72 mmol, H
2
1 atm, MeOH 3 mL, T
25°C. Pd(OAc) 0.0072 mmol, substrate 0.72 mmol, H 1 atm, water 3 mL, T 25°C, [surfactant 1a and
2 2
1d] 80 mM, [surfactant 1c] 90 mM. a) Pd/C 38 mg (0.036 mmol Pd).
In no case was the addition of acid coꢀcatalyst necessary to promote the reaction. Commercial
activated carbons are generally obtained by an oxidation treatment that introduces in the structure
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acidic groups that favor metal adsorption and stability. In the micellar aggregates, the negatively
charged surface sulfonate groups present in the surfactant can polarize water causing the
development of an acidic layer on the micellar surface that may well coꢀcatalyze the deꢀ
oxygenation reaction of the substrate.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Catalytic semi-hydrogenation of alkynes to alkenes
The semiꢀhydrogenation reaction of alkynes to the corresponding alkenes is an extremely important
22
chemical transformation since the production of Zꢀalkenes starting from internal alkynes allows to
23
obtain important molecular building blocks for the synthesis of several fine chemicals. Apart from the
2
4
traditional Lindlar catalyst that makes use of toxic Pb salts, alternative heterogeneous approaches are
based on metal catalysts like Pd supported on a surface that impart preꢀorientation of the substrates thus
2
5
selectively leading to the Z products, even though sometimes possible isomerization to the E isomer was
observed. In the past years, most of the homogeneous catalytic systems developed for this reaction were
mainly based on transition metal complexes (Rh, Pd, Pt, etc.) in most cases operating in organic
22
solvents. However, the most common strategy is the use of heterogeneous catalysts with selective
2
3
poisoning in order to maximize the selectivity to alkenes and to limit the possible Z-E isomerization.
We selected phenylacetylene (Scheme 4) as a model substrate to optimize the reaction
7
conditions for the Pd@surfactant catalytic systems. Initially the reaction was carried out in
tetrahydrofuran with a commercial 10% Pd/C catalyst. In order to maximize the selective
conversion to styrene
that an interesting level of both activity and selectivity was achieved working with 0.04 mol% of
Pd compared to in THF.
8, the reaction was repeated with different amounts of catalyst observing
7
Pd
+
8
9
7
H2 1 atm, r.t.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Scheme 4. Hydrogenation reaction of phenylacetylene 7 leading to styrene 8 and eventually to
ethylbenzene 9.
The reaction was then extended to the Pd@surfactant systems under the same conditions.
Unfortunately Pd@1c and Pd@1d proved unsuitable for this selective reaction. In fact when the
conversion of 7 reached values in a range of 80ꢀ90%, the hydrogenation reaction of styrene became very
fast and high yields of ethylbenzene 9 were obtained (see e.g. Table 4, entry 6). Differently, the use of 1a
and 1b gave PdꢀNP micellar systems with both high catalytic activity and high alkene chemoselectivity.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 4. Semiꢀhydrogenation reaction of phenylacetylene 7 with Pd/surfactant 1a-c in water and with
10% Pd/C in THF.
[Surfactant] Cat/Subs
Time Conversion Selectivity to styrene
#
Catalyst
Solvent
(
mM)
ꢀ
(mol%)
(min)
10
(%)
94
88
96
94
86
84
99
96
85
(%)
93
97
95
97
98
62
98
97
98
1
2
3
Pd/C
Pd/C
1
0.1
0.04
1
THF
THF
THF
ꢀ
50
a
Pd/C
ꢀ
90
b
4 Pd@1a
80
H
2
O
50
b
5
6
7
8
Pd@1b
100
80
1
H O
2
25
b
Pd@1c
1
H
2
H
2
H
2
H
2
O
O
O
O
80
c
Pd@1b
100
100
120
0.5
0.33
0.5
27
d
Pd@1b
120
75
c
9 Pd@1b
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Experimental conditions: phenylacetylene (1.5 mmol), reaction volume 3 mL, H 1 atm, T 25°C.
2
a) phenylacetylene (3.5 mmol). b) phenylacetylene (0.7 mmol). c) phenylacetylene (1.4 mmol). d)
phenylacetylene (2.1 mmol).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The results reported in Table 4 show that the selectivity to 8 obtained using Pd@1a and Pd@1b in the
aqueous solvent in the presence of surfactants appeared to be even slightly higher than that in THF with
Pd/C under the same catalyst to substrate ratio, even if the productivity was lower because of the lower
substrate concentrations used. In this reaction, Pd@1b seemed to be the best option in terms of both
activity and selectivity observed. As a consequence we focused on this catalytic system for the
subsequent tests. Additional catalytic tests were carried out to optimize the amounts of surfactant 1b and
Pd. It was observed that the best results were obtained when the amount of Pd was lowered down to 0.5
mol% and the concentration of 1b was 100 mM (Table 4, entries 7ꢀ9) obtaining a very high selectivity
(98%) and high conversion (99%, Table 4, entry 7). A larger amount of surfactant or a further decrease in
the Pd amount led to an increase of the reaction time without any beneficial effect for the selectivity of
the reaction. With the best catalytic system at hand, we applied the latter experimental conditions to a
series of different alkynes, testing the Pd@1b selectivity and comparing the results observed with the
traditional Pd/C catalyst.
The results are reported in Table 5 where reaction times and conversions, as well as catalyst
amounts and substrate concentrations were chosen in order to maximize selectivities to the
intermediate alkenes in the consecutive reaction that are almost always in the ≥ 95% range with
some minor exceptions (entries 5, 10). In order to have comparable performance in terms of
conversion, the amount of Pd necessary with Pd@1b was about 5 times higher than that with
Pd/C. This compensates in part for the much lower solubility of hydrogen in water with respect to
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
THF. Under these conditions the measured turnover frequencies (TOF) for Pd@1b are not
dramatically lower than for Pd/C.
Table 5. Semiꢀhydrogenation of phenyl acetylene derivatives 7-13 and other alkynes 14ꢀ18 with Pd@1b
in water and with Pd/C in THF.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
#
Substrate
Catalyst Time Conversion TOF Selectivity
-1
(
min)
(%)
(h ) to Alkene
(%)
a
1
2
3
Pd/C
90
27
70
45
60
96
99
1490
434
95
98
95
95
95
b
b
7
Pd@1b
a
Pd/C
98.5
>99
88
1970
263
1
0
Pd@1b
a
Pd/C
2050
11
b
Pd@1b
20
70
70
83
91
94
491
1820
159
95
94
95
a
4
Pd/C
b
12
Pd@1b
a
5
6
Pd/C
60
50
80
80
77
98
1400
182
17
97
97
b
13
Pd@1b
a
Pd/C
94
14
20
b
Pd@1b
60
90
>99
68
197
10
97
95
a
7
Pd/C
HO
15
60
b
Pd@1b
80
62
92
93
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O
O
O
O
a
Pd/C
80
90
97
98
1700
129
90
88
16
b
Pd@1b
Experimental conditions: Reaction volume 3mL, H
Pd), substrate 3.5 mmol, solvent THF; b) Pd(OAc)
mmol, [1b] 100 mM, solvent H O.
2
1 atm, T 25°C. a) Pd/C 1.6 mg (0.0015 mmol
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
1.6 mg (0.0071 mmol Pd), substrate 1.4
2
If we analyze the behavior of the aryl alkynes reported in Table 5 with Pd/C it seems, as should be
expected, that the presence of an electronꢀdonating group in the para position increases the corresponding
TOF (compare entry 1 with entries 2ꢀ4). Substrate solubility in micelles is a widespread parameter to
rationalize the catalytic behavior of micellar catalysts with homologous substrates. For example we could
duly rationalize the substrate selectivity properties of Pd@1a in the C=C double bond hydrogenation in a
series of
α,β
ꢀunsaturated aldehydes bearing increasingly long aliphatic tails, establishing a clear
15
correlation between the lipophilicity of the substrate and its reactivity. In the present case this guideline
seems useless because the solubility of substrates in water decreases of about two orders of magnitude on
2
6
going from substrate 7 to substrate 12 which suggests that their lipophilicity, hence their solubility in 1b
should follow an opposite order.
The solubility in water of aryl alkynes 7, 10ꢀ12 is about four times higher than the corresponding
2
6
alkenes, which implies that in the course of the reaction the dissolution in the micelles of the alkene
semiꢀhydrogenation products are higher than the corresponding alkyne reactants, a situation that is
intrinsically unfavorable with respect to the same reaction carried out in e.g. THF. Nevertheless the
selectivity to alkene in the micellar medium is as good as with Pd/C in THF.
The same reaction carried out on aliphatic alkynes (Table 5, entries 5ꢀ8) showed a lower efficiency of
both catalytic systems as demonstrated by the generally lower TOFs and in one case (entry 7) the need of
1
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1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
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a lower conversion to achieve high selectivity to alkene. The presence of hydrophilic substituents as in 16
led to a lower selectivity, while the aromatic ring in 14 allowed a better performance with Pd@1b which
suggests the hypothesis of a possible πꢀπ interaction of the substrate with 1b explaining also the better
performance shown by aryl alkynes.
As a further application of the catalytic systems to a wider range of substrates, we tested the
activity of Pd@1b in the semiꢀhydrogenation reactions of internal triple bonds. Unfortunately
many substrates of this family were excluded because they are solid under mild conditions and
therefore they were hardly miscible in an aqueous micellar solution. For this reason we checked
only 17 and 18 that are liquid under the mild conditions applied and with this substrates we
investigated the properties of Pd@1b in the presence of internal alkynes characterized by different
lipophilicity.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The data reported in Table 6 show very different results for the two substrates. Substrate 19
reacted very rapidly in the hydrogenation reaction, thus leading to high conversions but low
selectivity to alkenes in the presence of both Pd@1b in water and Pd/C in THF. Elongating the
aliphatic tail of the substrate as in 18 significantly slowed down the reaction allowing a better
control and a better selectivity to alkene, even if in this case Pd@1b provided a less favorable E:Z
ratio with respect to Pd/C.
Table 6. Semiꢀhydrogenation reaction of internal alkynes 17 and 18 with Pd@1b in water and with Pd/C
in THF.
Time Conversion TOF Alkene Alkane
#
Substrate
Catalyst
Z:E
-
1
(min)
(%)
(h )
(%)
(%)
a
1
Pd/C
35
>99
4000
27
72
92 : 8
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b
Pd/1b
40
>99
84
296
713
57
79
42
5
95 : 5
97 : 3
a
Pd/C
165
2
b
18
Pd/1b
360
85
28
77
8
86 : 14
Experimental conditions: Reaction volume 3mL, H
Pd), substrate 3.5 mmol, solvent THF; b) Pd(OAc)
mmol, [1b] 100 mM, solvent H O.
2
1 atm, T 25°C. a) Pd/C 1.6 mg (0.0015 mmol
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
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0
2
1.6 mg (0.0071 mmol Pd), substrate 1.4
2
Catalytic hydrodechlorination of aromatic substrates
Many halogenated hydrocarbons are hazardous compounds that are quite persistent and
2
7
difficult to decompose by biological degradation. High amounts of these compounds are found
in ground water in some industrial areas and they tend to accumulate in the environment. It is also
known that their biological degradation can be considerably improved by previous deꢀ
halogenation. Several methods for the hydrogenolysis of chloroarenes using either stoichiometric
and/or catalytic reagents have been developed, but most of them suffer from incomplete
dechlorination, low catalytic activity, the need of low substrate/catalyst ratios or extreme
28
conditions. Nevertheless, catalytic hydrodehalogenation is still a convenient and promising
2
9
method for the decontamination of water from this kind of waste. Palladiumꢀbased catalysts
showed high activity in the hydrodehalogenation reaction and this prompted us to test the above
described Pd@surfactant systems as catalyst in the hydrodechlorination reactions of aromatic
compounds in water because the presence of the surfactant would improve the dispersion of the
aromatic substrate. Hence we investigated the catalytic activity of PdꢀNP in the presence of 1a,
1
b, 1c and 1d under conditions that could be of practical interest and verify their stability.
Chlorobenzene 19 was taken as a benchmark substrate and all the Pd@surfactant systems were
1
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6
tested at room temperature, in water under H at atmospheric pressure in the presence of 1
2
equivalent of sodium bicarbonate to quench the release of HCl during the reaction (Scheme 5).
Cl
Pd, H O, surfactant,
2
NaHCO3
0
1
2
3
4
5
6
7
8
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0
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9
H 1 atm, r.t.
2
20
Scheme 5. Hydrodechlorination reaction of chlorobenzene 19 leading to benzene 20.
When 1a, 1b and 1c were tested in this reaction, precipitation of PdꢀNP occurred rapidly and no
conversion of the substrate was observed. Conversely, in the presence of surfactant 1d the reaction led to
quantitative conversion of 19 into benzene 20 in 24 hours. We believe that the presence of the PEG
fragment in the structure of 1d, that in the previous reactions showed to reduce the activity of the
Pd@surfactant catalytic system, at the same time confers to the PdꢀNP improved stability and avoids Pd
aggregation under the experimental conditions. The latter catalytic system was further tested in the
hydrodechlorination of a polychlorinated aromatic substrate like 1,2,4ꢀtrichlorobenzene 21 (Scheme 6).
Cl
Cl
Cl
Cl
Pd, H O, surfactant,
2
Cl
NaHCO3
+
+
H 1 atm, r.t.
2
Cl
21
22
19
20
Scheme 6. Hydrodechlorination reaction of the 1,2,4ꢀtrichlorobenzene derivative 21 leading to
several possible products.
The reaction was optimized in order to obtain a quantitative conversion to 20. As reported in Table 7
this result was achieved under mild conditions (room temperature and 1 atm of H
2
) in 24 hours increasing
five times the initial amount of Pd and adding an excess of base (Table 7, entry 7). Upon recycling, after
2
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extracting the reaction products with an apolar solvent such as pentane, this catalyst maintained its
performance even reducing the base amount to stoichiometric (Table 7, entry 8). The catalytic system
Pd@1d showed stability to pH changes and maintained its activity even in basic solutions. When the
2
reaction was carried out under 5 atm of H (Table 6, entry 6), the Pd@1d system showed high stability
and a higher activity, observing a quantitative conversion in 24 hours even with a lower Pd loading.
In the liquidꢀphase hydrodehalogenation reactions of chlorinated aromatic compounds with
molecular hydrogen, supported palladium catalysts are usually strongly deactivated by HCl
formed during the reaction, but when the supports are doped with sodium hydroxide they lead to
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
30
increased activity in the hydrodehalogenation reaction. In this respect, Pd@1d showed two
peculiarities for this reaction: (i) the property to work directly in water and (ii) the ability to
maintain the catalytic activity simply by adding sodium bicarbonate.
Table 7. Hydrodechlorination reaction of 1,2,4ꢀtrichlorobenzene 21 with Pd@1d.
H2
Conversion Yield 22 Yield 19 Yield 20
#
Cat/Sub Sub/Base
(
atm)
(%)
83
(%)
30
36
33
24
17
ꢀ
(%)
(%)
46
1
2
3
4
5
6
7
1/100
1/100
2/100
2/100
2/100
2/100
5/100
5/100
1/3
1
1
1
1
1
5
1
1
7
5
7
6
3
ꢀ
a
1/3
1/3
1/6
1/9
1/6
1/6
1/3
71
30
90
50
90
60
95
75
>99
>99
>99
>99
99
>1
ꢀ
ꢀ
b
8
ꢀ
>99
2
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Experimental conditions: [Pd(OAc) ] 2.4 mM, water 3 mL, [surfactant 1d] 80mM, base =
2
NaHCO
3 2 3
, T 25°C. a) base K CO ; b) Recycling test after products extraction with 3 aliquots of 15
ml of pentane.
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Conclusions
In conclusion, we reported a straightforward preparation of PdꢀNP in aqueous medium
stabilized by commercial anionic sulfonated surfactants simply obtained by stirring micellar
1
1
1
1
1
1
1
1
1
1
2
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2
2
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0
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9
0
solutions of Pd(OAc) under hydrogen atmosphere. These PdꢀNP showed similar size range and
2
distribution but different morphology as a function of the surfactant employed as well as the
concentration and experimental conditions. This proved to be a facile, and inexpensive method to
synthesize catalysts with modulated activities simply by changing the surfactant that can operate
directly in water as the green solvent par excellence, thus avoiding the use of large amounts of
toxic organic solvents. This did not affect the performance of the surfactant stabilized PdꢀNP as
they showed in some cases better efficiency with respect to commercial Pd/C in various
hydrogenation reactions.
When catalyzed by Pd@1d, the hydrogenation reaction of benzaldehyde and some other
aromatic derivatives showed a very high chemoselectivity towards the formation of the
corresponding benzyl alcohols, while Pd/C as a benchmark comparison was more difficult to
control causing the formation of deꢀoxygenation reaction products. However, when we used 1a
and 1c we obtained catalytic systems with higher activity and their application in the deꢀ
oxygenation reactions of secondary and tertiary alcohols at room temperature and 1 atm of H in
2
the absence of acid coꢀcatalysts gave quantitative conversions. Much lower conversions were
obtained when these reactions were repeated with Pd/C in methanol. 1b gave a catalytic system
with controlled activity in the semiꢀhydrogenation reactions of alkynes to alkenes where a
particular affinity of the catalytic system for compounds endowed with aromatic rings was
observed probably due to πꢀπ supramolecular interactions between the surfactant molecular
structure and the substrates.
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6
To demonstrate the generality of the catalytic system developed in water, we have reported
also a possible application of these systems to the decomposition of hazardous chlorinated
aromatic compounds directly in water. With this purpose we studied the catalytic activity of Pdꢀ
NP stabilized by sulfonated anionic surfactants in the reaction of 1,2,4ꢀtrichlorobenzene. In this
way we demonstrated that Pd@1d could convert quantitatively the substrate under mild
conditions in water and recycling of the catalytic system was possible without detrimental effects
on its activity and selectivity.
0
1
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0
As is clear from the data reported, the role of the surfactant is crucial to ensure, first of all, the
necessary stability to PdꢀNP under the catalysis conditions applied. It is interesting that even if
the reaction environment is always the same (water, surfactant, hydrogen), the type of substrate
analyzed in the individual cases can completely change the stability of the metal system (see e.g.
the case of 1b that leads to Pd metal precipitation with aldehydes while it gives the best catalyst
with acetylenes) in a delicate balance between catalytic activity and stability that in some cases
can be quite significant as in the case of the hydrodechlorination reaction with Pd@1d that can
be even recycled.
In all these reactions, typical heterogeneous catalysis criteria like particle size/particle
distribution correlations were poorly helpful in rationalizing catalytic activity and
selectivity, rather, intriguing and somehow unpredictable "ligand" effect by the different
surfactants capping the PdꢀNP could be invoked to partly explain the sometimes large
differences in activity observed. What is interesting is also that a benchmark catalyst like
Pd/C can be often surpassed in activity and/or selectivity in the reactions tested by simply
switching to the appropriate cheap and commercially available surfactant, thereby
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providing an easy to use, flexible and practical catalytic system capable of efficiently
addressing a variety of synthetically significant hydrogenation reactions.
1
1
1
1
1
1
1
1
1
1
2
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Experimental
General
1
H NMR spectra were recorded at 298 K, unless otherwise stated, on a Bruker AVANCE 300
spectrometer operating at 300.15 MHz. δ values in ppm are relative to Si(CH ) . GC analyses
3
4
were performed on HP SERIES II 5890 equipped with a HP5 column (30 m, I. D. 0.25 m, film
.25 ꢁm) using He as gas carrier and FID as detector. GCꢀMS analyses were performed on a GC
0
Trace GC 2000 equipped with a HP5ꢀMS column (30 m, I.D. 0.25 mm, film 0.25 ꢀm) using He
gas carrier, coupled with a quadrupole MS Thermo Finnigan Trace MS with Full Scan method.
®
TLC analyses were performed on TLC Polygram
Sil G/UV254 of 0.25 mm thickness.
Pd(OAc) , 10% Pd/C, solvents, surfactants, reactants and products were all commercially
2
available products (Aldrich) and were used as received without any further purification.
Synthesis of Pd-NP in micellar media
In a 10 ml roundꢀbottomed flask, equipped with a magnetic stirrer, 1.6 mg of Pd(OAc) (2.4
2
mM), followed by the appropriate amount of surfactant and 3 mL of doubleꢀdistilled water were
introduced under nitrogen. The mixture was stirred for 1 hour until complete dissolution of the
metal complex to yield an orange homogeneous solution. Subsequently the solution was placed
under hydrogen flow for variable times as a function of the surfactant employed observing a net
color change from orange to grey as a proof of the formation of dispersed PdꢀNP.
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TEM Analysis
Size and morphology of the nanoparticles were studied through a JEOL JEM 3010
transmission electron microscope (TEM) operating at 300 kV; the powder specimens were
suspended in isopropyl alcohol and then sonicated, 5ꢀl of this suspension were deposited on a
copper grid (300 mesh) coated with holey carbon film. The copper grids were allowed to dry in
air.
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Catalytic Studies
Pd/C: In a 10 ml roundꢀbottomed flask, equipped with a magnetic stirrer were placed 3 ml
MeOH (or THF) and the appropriate amount of 10% Pd/C followed by H at atmospheric
2
pressure at room temperature. The mixture was vigorously stirred for 5 min then the required
amount of substrate was introduced. Substrate conversion over time was monitored by
periodically sampling with a microsyringe followed by GC analysis. Hydrogenation products
were identified by comparison with authentic samples and by GCꢀMS analysis.
Pd-NP: To a 10 ml roundꢀbottomed flask, equipped with a magnetic stirrer containing the
catalyst prepared according to the above procedure in 3 mL water was added the substrate in the
required amount. Substrate conversion over time was monitored by periodically sampling 100
ꢀL of mixture, extracted with 200 ꢀL of ethyl acetate and the organic phase analyzed by GC
analysis. Hydrogenation products were identified by comparison with authentic samples or by
GCꢀMS analysis.
Acknowledgements
The authors acknowledge Università Ca’ Foscari and MIUR for support.
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Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
XXXXX. Experimental details on Pd NPs preparation and characterization, catalytic tests and
characterization of compounds.
1
1
1
1
1
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Notes and references
1
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storage Chem. Soc. Rev. 2013, 42, 3127ꢀ3171; (b) Balanta A., Godard C., Claver C., Pd
nanoparticles for C–C coupling reactions Chem. Soc. Rev. 2011, 40, 4973ꢀ4985; (c) Sun Y.,
Controlled synthesis of colloidal silver nanoparticles in organic solutions: empirical rules for
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Rotello V.M., Nanomedicine Chem. Soc. Rev. 2012, 41, 2537ꢀ2538 (e) Doane T.L., Burda C.,
The unique role of nanoparticles in nanomedicine: imaging, drug delivery and therapy. Chem.
Soc. Rev. 2012, 41, 2885ꢀ2911.
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(a) Joo S.H., Choi S.J., Oh I., Kwak J., Liu Z., Terasaki O., Ryoo R., Ordered nanoporous
arrays of carbon supporting high dispersions of platinum nanoparticles Nature, 412, 169ꢀ172;
(b) Park K.H., Jang K., Kim H.J., Son S.U., NearꢀMonodisperse Tetrahedral Rhodium
Nanoparticles on Charcoal: The ShapeꢀDependent Catalytic Hydrogenation of Arenes Angew.
Chem. Int. Ed. 2007, 46, 1152ꢀ1155.
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(a) Herzing A.A., Kiely C.J., Carley A.F., Landon P., Hutchings G.J., Identification of active
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(
b) Akita T., Kohyama M., Haruta M., Electron Microscopy Study of Gold Nanoparticles
Deposited on Transition Metal Oxides Acc. Chem. Res. 2013, 46, 1773ꢀ1782.
0
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4 (a) Brito A., Garcıa F.J., AlvarezꢀGalvan M.C., Borges M.E., Diaz C., de la Pena O’Shea V.A.,
Catalytic behaviour of Pt or Pd metal nanoparticles–zeolite bifunctional catalysts for nꢀ
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Briend M. , Blanchard J. , Fajerwerg K. , Krafft J.ꢀM. , Breysse M. , Cattenot M. , Lacroix
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(a) S.Chen, L. Meng, B. Chen, W. Chen, X. Duan , X. Huang, B. Zhang , H. Fu, Y. Wan,
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Selective hydrogenation of
acetophenone
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with supported Pd and Rh catalysts in water, organic solvents, and CO ꢀdissolved expanded
2
liquids, Green Chem. 2016, 18, 4934ꢀ4940; (d) C.ꢀJ. Lin, S.ꢀH. Huang, N.ꢀC. Lai, C.ꢀM.
1
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