Platinum Single Atoms on Carbon Nanotubes as Efficient Catalyst for Hydroalkoxylation

Article abstract of DOI:10.1002/bkcs.11252

We report a facile synthesis of Pt single atoms on thiolated carbon nanotubes. To obtain Pt single atoms, it is crucial to treat thiol groups on carbon nanotubes. Pt single atoms on carbon nanotubes were used efficient catalyst for hydroalkoxylation of 3-buten-1-ol or 4-penten-1-ol. Hydroalkoxylation represents an atom-economic route to construct four or five- membered cyclic ethers through intramolecular addition of hydroxyl group. This catalyst exhibited higher catalytic activity than Pt complex and Pt nanoparticles on carbon nanotubes.

Full text of DOI:10.1002/bkcs.11252

Article
DOI: 10.1002/bkcs.11252
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H. Woo et al.
KOREAN CHEMICAL SOCIETY
Platinum Single Atoms on Carbon Nanotubes as Efficient Catalyst for
Hydroalkoxylation
Hyunje Woo,† Eun-Kyung Lee,‡ Su-Won Yun,‡ Shin-Ae Park,‡ Kang Hyun Park,§
and Yong-Tae Kim‡,
*
†Hybrid Materials Solution National Core Research Center (NCRC), Pusan National University, Busan
46241, Republic of Korea
‡Department of Energy System, Pusan National University, Busan 46241, Republic of Korea.
*E-mail: yongtae@pusan.ac.kr
§Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University,
Busan 46241, Korea
Received June 28, 2017, Accepted August 17, 2017
We report a facile synthesis of Pt single atoms on thiolated carbon nanotubes. To obtain Pt single atoms,
it is crucial to treat thiol groups on carbon nanotubes. Pt single atoms on carbon nanotubes were used effi-
cient catalyst for hydroalkoxylation of 3-buten-1-ol or 4-penten-1-ol. Hydroalkoxylation represents an
atom-economic route to construct four or five- membered cyclic ethers through intramolecular addition of
hydroxyl group. This catalyst exhibited higher catalytic activity than Pt complex and Pt nanoparticles on
carbon nanotubes.
Keywords: Platinum, Single atoms, Hydroalkoxylation, Catalyst
Introduction
dispersed single Pt atoms having a highly reduced state on
S-MWNTs as supports to overcome these problems.18
Pt-S-MWNTs catalyst was used as efficient catalyst for
intramolecular hydroalkoxylation by increasing activity
with O-donor and showed excellent performance and stabil-
ity compared with Pt nanoparticles and Pt complex.
Oxygen heterocyclic compounds have been much attention
as important structural motif in a wide range of biological
active molecules such as polyethers, antibiotics, marine
macrocycles, and flavor compounds.1–4 Among many syn-
thetic strategies, intramolecular addition of an O H bond
(intramolecular hydroalkoxylation) is an atom-economical,
and therefore attractive approach for synthesis of saturated
oxygen heterocycles.5,6 Until now, there have been many
efforts to develop efficient catalysts such as brønsted acids,
metal salts (Au, Ru, and Pt) and lanthanide complex to
remove use of toxic metal ions.7–11
Among them, in particular, the gold(I)-catalyzed intra-
molecular hydroalkoxylation has drawn extensive interest
owing to the potential for both stereospecific and enantiose-
lective transformations.12–14 Also, Pt catalysts are intro-
duced for the hydroalkoxylation of direct intramolecular
insertion of oxygen, exhibiting better activity and selectiv-
ity than Au catalysts,15–17 since oxo-philic early- and
middle- transition metals as well as Pt metal have strong
binding with O-donor ligand and compound containing
oxygen. In this paper, we report single Pt atoms on thio-
lated multiwalled carbon nanotubes (S-MWNTs) to maxi-
mize active surface area of catalyst for intramolecular
hydroalkoxylation. While transition metal nanoparticles
have been much attention for high surface area to volume
ratio and surface energy, they show low stability during
catalytic reaction due to leaching of metal atoms and mor-
phology change. Our group already reported highly
Experimental
Synthesis of Pt-S-MWNTs Catalysts18. MWNTs pre-
pared by a conventional CVD method were purchased from
Hanwha Nanotech. The MWNTs were purified by refluxing
at 70ꢀC for 12 h in 6 M hydrochloric acid (35–37%, Sam-
chun Chemical) after thermally treating raw soot containing
MWNTs at 400ꢀC for 2 h in static air. The thiolation of the
MWNTs was carried out using an amide bond formation
method. Carboxylated MWNTs were obtained by stirring
purified MWNTs in a concentrated H2SO4/HNO3 mixture
(3:1, 95% and 60% purity, respectively, Samchun Chemi-
cal) for 1 h and were chlorinated by refluxing at 70ꢀC for
12 h with SOCl2 (≥90 v/v%, Samchun Chemical). Then,
the remaining SOCl2 was removed by vacuum evaporation,
and the thiolated MWNTs, S-MWNTs, were obtained by
stirring at 70ꢀC for 24 h in dry toluene (99.8%, Aldrich,
Seoul, Korea) and C2H7NSClH (98.0%, Aldrich). A sus-
pension of S-MWNTs (20 mg) was prepared by ultrasoni-
cation in 40 mL of DI water and 3.125 mL of 10 mM
H2PtCl6Á6H2O (37.5% Pt basis, Aldrich) and 3.125 mL of
10 mM Pd(NH3)4Cl2ÁH2O (99.7%, Aldrich) (20 wt %)
were added to the suspension. The Pt precursors were
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Figure 1. TEM images of Pt SAs on the S-MWNTs (a) and
Pt/MWNTs (b).
Figure 2. (a) XRD data of Pt-S-MWNTs and Pt/MWNTs.
(b) EXAFS patterns showing 1NN Pt-heterogeneous atoms
(1.9 Å) of Pt-S-MWNT and 1NN Pt–Pt bond (2.8 Å) of Pt foil.
reduced and formed on the S-MWNTs by stirring them
with NaBH4 (98%, Acros, Seoul, Korea) for 12 h. Then,
they were filtered and washed with deionized water and
ethanol several times. After evaporation and drying, the
20 wt % Pt precursors supported on S-MWNT, referred to
as Pt-S-MWNTs, were obtained. Transmission electron
microscopy (TEM; JEM-2011, JEOL, Akishima, Japan),
angle-resolved X-ray photoelectron spectrometry (AR-XPS;
Theta Probe, cThermo Electron Corporation), and X-ray
diffraction (XRD; X’pert PRO MRD; Philips, Amsterdam,
Netherlands) were used to analyze the morphology of the
catalysts and to confirm the existence of the single atom
catalysts. Extended X-ray absorption fine structure
(EXAFS) data at the Pt-L3 edge were obtained in transmis-
sion mode with the synchrotron radiation of wide X-ray
absorption fine structure spectroscopy [10C beam line,
Pohang Light Source (PLS)] at room temperature. The
spectra were processed using the program IFEFFIT (version
1.2.11, IFEFFIT, Copyright 2008, Matthew Newville, Uni-
versity of Chicago, http://cars9.uchicago.edu/ifeffit/) with
background subtraction (AUTOBK) and normalization.
Fourier transform for EXAFS spectra was performed in the
range 2.5–12 Å in k-space and 1–3.5 Å in R-space with the
first-shell single scattering paths and k3 weighting. And the
powder XRD analysis was performed with an Advance X-
ray diffractometer (D8 ADVANCE) with CuKα radiation
(λ = 1.54056 Å). X-ray diffraction data were collected over
the 2θ range 10–90ꢀ with a step size of 0.02ꢀ and a count-
ing time of 0.2 s per step.
Intramolecular Hydroalkoxylation Catalyzed by Pt-S-
MWNTs. 4-Penten-1-ol (5 mmol, 99%, Sigma Aldrich,
Munich, Germany) and 3-buten-1-ol (6 mmol, 99%,
Aldrich) were dissolved in dioxane. Pt-S-MWNTs (2.5 mol
% with respect to the substrate concentration) were added
with triphenylphosphine (5 mol %, 98.5%, Sigma Aldrich).
At this time, the reaction was performed by refluxing at
80ꢀC for 24 h. Product yields from the reaction was meas-
ured using gas chromatography (GC, Vernier Mini GC,
start temperature 60ꢀC, ramp rate 5ꢀC/min, final tempera-
ture 140ꢀC, total length 10.0 min, and pressure 7.0 kPa).
Results and Discussion
Characterization of Pt-S-MWNTs. The synthetic process
of Pt-S-MWNTs have previously been reported by our
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Platinum Single Atoms on Carbon Nanotubes
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Table 1. Effect of catalyst and phosphine on the platinum-
catalyzed hydroalkoxylation.
Entry
PPh3 (mol%)
Catalyst
Yield (%)a
1
2
3
4
5
6
7
8
9
10
0
2.5
5
5
0
2.5
5
0
2.5
5
Pt-S-MWNTs
Pt-S-MWNTs
Pt-S-MWNTs
Pt-S-MWNTs
Pt(COD)Cl2
Pt(COD)Cl2
Pt(COD)Cl2
Pt/MWNTs
Pt/MWNTs
Pt/MWNTs
15
40
70
92b
<<5
<<5
10
<<10
<<10
20
Reaction condition: 4-penten-1-ol (5 mmol), catalyst (2.5 mol %),
dioxane (10 mL), 8 h, 80ꢀC.
a
GC detected yield (%).
7.5 mol % of catalyst was used at 100ꢀC.
b
presence of Pt single atoms, EXAFS was conducted
(Figure 2(b)). A peak at around 2.8 Å, corresponding to the
first nearest neighbor (1NN) Pt–Pt bond, shows the Pt sup-
ported on the untreated carbon nanotubes. However, in the
case of Pt-S-MWNT, there was no peak at around 2.8 Å,
while a clear peak at around 1.9 Å caused by 1NN Pt-
heterogeneous atoms. This demonstrates that the supported
particles are isolated Pt single atoms surrounded by hetero-
geneous atoms without any Pt–Pt bond. EXAFS parameters
were obtained by fitting the data to a model structure calcu-
lated by the FEFF (version 8.3). High-angle annular dark-
field scanning TEM (HAADF-STEM) image and elemental
mapping were also conducted for confirming Pt single
atoms on S-MWNT (Figure 3). However, Pt atoms were
aggregated each other when electron beam (200 kV) was
irradiated to Pt-S-MWNT sample, resulting in some Pt
nanoparticles on S-MWNT (Figure 3).
Figure 3. HAADF-STEM image (a) and elemental mapping (b) of
Pt-S-MWNT.
group,18 and therefore we briefly summarize the characteri-
zation of the single atom structure. We introduced thiol
groups, which have a high affinity for precious metals, on
carbon nanotubes to obtain the Pt single atoms on S-
MWNTs. The TEM image for Pt-S-MWNTs after the
reduction by sodium borohydride of H2PtCl6 on the S-
MWNT supports is shown in Figure 1(a). Agglomerated Pt
nanoparticles are clearly observed on carbon nanotubes
which is untreated by thiol groups (Figure 1(b)). In addi-
tion, X-ray diffraction measurements did not show any
characteristic Pt crystal lattice reflections (Figure 2(a)).
These results implies that Pt single atoms are successfully
loaded onto the thiolated nanotubes. To confirm the
Hydroalkoxylation (2-Methyloxetane, Tetrahydro-2-
methylfuran). To examine catalytic activity of catalysts,
hydroalkoxylation of 3-buten-1-ol or 4-penten-1-ol was
Table 2. Effect of catalyst concentration on the Pt-S-MWNTs
catalyzed hydroalkoxylation.
Entry
Catalyst (mol%)
Yield (%)a
1
2
3
4
1.25
2.50
5.00
7.50
20
40
70
80
Reaction condition: 4-penten-1-ol (5 mmol), PPh3 (5.0 mol %),
dioxane (10 mL), 8 h, 80ꢀC.
GC detected yield (%).
Scheme 1. Hydroalkoxylation of 3-buten-1-ol or 4-penten-1-ol
catalyzed by Pt-S-MWNTs.
a
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conducted. The higher yield of tetrahydro-2-methylfuran
(70%) was obtained than that of 2-methyloxetane (10%)
due to less ring strain effect (Scheme 1). Therefore, the cat-
alytic activity of Pt-S-MWNTs, Pt/MWNTs and Pt(COD)
Cl2 was investigated for the hydroalkoxylation of 4-penten-
1-ol (Table 1). Preliminary screening to determine the
amount of ligand (triphenylphosphine) revealed that
5.0 mol % could efficiently catalyze this reaction (Table 1).
The order of the conversion yield is: Pt-S-MWNTs
expected, 92% of yield was obtained when a reaction tem-
perature was increased to 100ꢀC (Table 1, entry 4). The fol-
lowing conditions proved to be optimal: Pt-S-MWNTs
(Pt base: 7.5 mol %), triphenylphosphine (5 mol %) in
dioxane at 100ꢀC for 8 h.
The possible origin of the high activity of the Pt-S-
MWNT is electron density and ligand effect of the sulfur in
the thiol groups. According to previous reports, if the elec-
tronic state of the central metal is close to zero, π-bond acti-
vation reactions could be easily occured.20,21 The oxidation
states of the tested catalysts can be summarized as follows:
0 for Pt/C, almost 0 for the Pt-S-MWNTs, and between +2
and +4 for the ligand complexes (Figure 4(a)). Even though
Pt-S-MWNTs are disadvantaged in comparison with
Pt/MWNTs regarding their electronic state, much higher
surface area of Pt-S-MWNTs for the reaction overcomes
this disadvantage. Therefore, for a highly active catalyst for
hydroalkoxylation, it is important that the electronic state
should be close to zero, and simultaneously the surface area
should be as large as possible.
(70%) > Pt/MWNTs
(20
%) > Pt(COD)Cl2
(10%)
(Table 1, entries 3,7,10). The Pt-S-MWNTs catalysts are
substantially more active than the Pt/MWNTs and Pt(COD)
Cl2, due to the effective activation of olefin in 4-penten-1-
ol compared with Pt/MWNTs and Pt(COD)Cl2.19 Gener-
ally, since platinum-catalyzed hydroalkoxylation involves
outer-sphere attack of the hydroxyl group on the platinum-
complexed olefin, activation of olefin is crucial to catalyze
hydroalkoxylation.19 The effects of quantity of catalyst (Pt-
S-MWNTs) in the reaction were also investigated, with the
highest yield obtained for 7.5 mol % (80%) (Table 2). As
Figure 4. XANES studies to verify the charge compensation process for Pt-S-MWNTs and Pt/MWNTs.
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In conclusion, we demonstrated that the Pt-S-MWNTs have
a remarkably high activity for the hydroalkoxylation. Pt-S-
MWNTs showed a high activity in comparison with con-
ventional catalysts; 3–7 times higher product yields than
Pt/MWNT, Pt complex, respectively. This high activity was
attributed to the effective activation of olefin in 4-penten-1-
ol compared with Pt/MWNTs. Hence, we firmly believe
that Pt-S-MWNTs catalyst can be applied to all catalytic
research field for organic reaction.
Acknowledgments. This work was supported by the
National Research Foundation (NRF) of Korea grant
(2015M1A2A2056556, 2015R1A2A1A10056156, 2013M-
1A8A1040703), Nano-Convergence Foundation (R20150-
0910), and Korea Institute of Energy Technology
Evaluation and Planning (KETEP) grant (2015303003-
1510).
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