Ultrasmall Platinum Stabilized on Triphenylphosphine-Modified Silica for Chemoselective Hydrogenation

Article abstract of DOI:10.1002/chem.201700980

Chemoselective hydrogenation of substrates with more than one functional group that could be hydrogenated is quite challenging and is of fundamental importance. Here, the enhanced chemoselectivity of ultrasmall (<1 nm) platinum nanoparticles (NPs) stabili

Full text of DOI:10.1002/chem.201700980

A Journal of
Accepted Article
Title: Ultrasmall Pt Stabilized on Triphenylphosphine Modified Silica for
Chemoselective Hydrogenation
Authors: Qihua Yang, Sanjeevi Jayakumar, Arindam Modak, Miao
Guo, He Li, and Xiangping Hu
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Ultrasmall Pt Stabilized on Triphenylphosphine Modified Silica for
Chemoselective Hydrogenation
Sanjeevi Jayakumar[a],[b],§ Arindam Modak[a],§, Miao Guo[a],[c], He Li[a], Xiangping Hu[d] and Qihua Yang[a]*
Abstract: Chemoselective hydrogenation of substrates bearing
more than one functional groups that could be hydrogenated is quite
challenging and has fundamental importance. Herein, we report the
enhanced chemoselectivity of ultrasmall ( 1 nm) Pt nanoparticles
(NPs) stabilized on silica modified with triphenylphosphine (PSiO2) in
hydrogenation reactions. Pt NPs on PSiO2 exhibits much higher
selectivity than those on SiO2 in the hydrogenation of acetophenone
to 1-phenyl ethanol (99.9% versus 36%) and hydrogenation of
phenylacetylene to styrene (85% versus 52%). The results of NMR,
XPS and in-situ CO adsorption FT-IR spectroscopy indicate the
existence of strong interaction among triphenylphosphine and Pt
NPs. Consequently, Pt NPs on PSiO2 has smaller particle size and
more positive Pt 4f binding energy than Pt/SiO2, which mainly
contribute to the superior chemoselectivity of Pt NPs. We believe
that the organically modified silica could act as an efficient solid
ligand for tuning the catalytic performance of metal NPs.
hydroxylanilines.[5b] Very recently, Glorious and co-workers
reported that the chemoslectivity of Ru/K-Al2O3 could be greatly
enhanced with N-heterocyclic carbene (NHC) as modifier in
obtaining partial hydrogenation products of acetophenone,
stilbene, and phenylacetylene.[6] Triphenylphosphine (PPh3) as a
quite popular and versatile ligand in homogenous catalysis has
been employed to stabilize Pd, Ru, Ir, Rh, Au NPs and obvious
ligand effect on chemoselectivity in partial hydrogenation of
substituted arene was observed.[7] The above work
demonstrates that the electron donating and/or withdrawing
characteristics of ligands are quite useful in modifying the
catalytic performance of metal NPs.
However,
organic
ligands
modified
on
metal
NPs/nanowires may break from the support and/or are generally
consumed during the reaction process, which requires
continuous addition of ligands to keep stable performance of
metal NPs. Also, the ligand-stabilized “soluble” metal NPs face
the difficulties in product separation and catalyst recycling.
Cross-linking of organic groups on solid supports may partly
solve the above problems. Previously, thiol (-SH) functionalized
porous polymers and amine (-NH2) functionalized silica have
been employed to stabilize and tune the activity of metal NPs
(Au, Pt, and Pd etc.) on account of the promotional effect by thiol
and amine groups,[8] however such investigation has been rarely
done with PPh3-functionalized solid supports. Only some
nonporous styrene-divinylbenzene copolymers with pendant
PPh3 groups are used for stabilizing Pd, Rh NPs, but the
interaction between NPs and PPh3 support has not been
thoroughly investigated.[9]
Herein, we report that the silica modified with high surface
area triphenylphosphine polymer (PSiO2) is an efficient solid
support for stabilizing ultrasmall Pt NPs for tuning its catalytic
performance. Pt NPs supported on PSiO2 exhibits high
chemoselectivity for hydrogenation of acetophenone (99.9% sel.
to 1-phenylethanol at 100% conv.) and phenylacetylene (87.5%
sel. to styrene at 72% conv.). Our preliminary results
demonstrate that the silica modified with phosphine polymers
are beneficial in controlling the catalytic performances of Pt NPs
for the accomplishment of selective hydrogenation reactions.
Introduction
Chemoselective hydrogenation catalyzed by supported metal
nanoparticles (NPs) is an important area of research for
sustainable production of fine chemicals.[1] However, high
“chemoselectivity” in hydrogenation is quite challenging with
substrates bearing more than one functional groups (C=C, C=O,
C≡C etc.) that could be easily hydrogenated.[2] Several
approaches have been employed for adjusting the
chemoselectivity of hydrogenation reactions, such as by addition
of additives, poisoning some of the most active sites of metal
NPs, and modifying spacial environment, size and electronic
properties of metal NPs.[3]
In homogeneous catalysis, the utility of organic “ligands” in
modifying spatial or electronic properties of metal complexes
and/or organometallic intermediates has been a widely used
strategy.[4] However, the tuning of electronic properties of metal
NPs by organic ligand is not well investigated in heterogeneous
catalysis and is so far limited to a few reports.[5-7] Zheng and co-
workers found that ethylenediamine-coated Pt nanowires show
high selectivity for thermodynamically unfavorable N-
Results and Discussion
[a]
Sanjeevi Jayakumar, Dr. Arindam Modak, Miao Guo, Dr. He Li, Prof.
Qihua Yang
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, 457-Zhongshan Road,
Dalian 116023, China
E-mail: yangqh@dicp.ac.cn
International College, University of Chinese Academy of Sciences,
Beijing 100049, China
1. Synthesis and Characterization
The hybrid material with PPh3 cross-linked on silica was
prepared by one-pot facile polymerization method as illustrated
in Scheme 1. Based on TG analysis, the polymer content on
PSiO2 is ca. 25 wt%, corresponding to ca. 99% monomer in the
initial mixture was cross-linked on silica by polymerization
(Figure S1). ICP analysis shows that the PSiO2 has the P
content of 2.34 wt%. The Fourier-transform infrared (FT-IR), 13C
CP-MAS and 31P MAS NMR spectra of PSiO2 are summarized in
Figure 1. In addition to the large and broad peak at 1100 cm-1
[b]
[c]
[d]
University of Chinese Academy of Sciences, Beijing 100049, China
Prof. Xiangping Hu, Laboratory of Fine Chemicals, Dalian Institute of
Chemical Physics.
§Both the authors contributed equally
Supporting information for this article is given via a link at the end of
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AIBN, THF, 80 oC
Fumed silica
H2PtCl6
NaBH4
Scheme 1. Schematic illustration for the preparation of PSiO2 and Pt/PSiO2 (via AIBN initiated polymerization of tris(4-vinylphenyl)phosphine in the presence of
fumed SiO2) and loading of Pt NPs on PSiO2.
assigned to -Si-O stretching vibration, the peak at 830 cm-1 for
C-P stretching,[10] 1600 cm-1 for aromatic C=C stretching, and
2923 cm-1 for C-H stretching are observed, which clearly shows
the successful incorporation of PPh3 on the silica. The
characteristic carbon peak for C-P appears at 148 ppm along
with other aromatic C=C and =C-H bands at 136, 132, 128, 112
ppm and aliphatic CH2 at 42 ppm.[10] Additionally, the 31P MAS
NMR spectrum shows important insights regarding the presence
of PPh3 polymer. The peak at -5.6 ppm corresponds to free
phosphorous in tertiary state and the relatively small peak at
25.8 ppm could be assigned to quarternary “P” atom, mainly
because of the oxidation of PPh3 moiety.[10,11] The combined
results of TGA, FT-IR, and NMR confirm the successful
modification of silica with cross-linked PPh3 ligand.
The porous structure of PSiO2 and SiO2 were
characterized with N2 adsorption-desorption isotherm (Figure 2)
and the physical parameters are summarized in Table 1. The N2
sorption isotherms of PSiO2 and SiO2 show type I isotherm
pattern characteristics of microporous materials. The sharp N2
uptake at P/P0 (~0.9) suggests the existence of intervoid space
due to particle aggregation. The N2 uptake at P/P0 (<0.1) is
sharper in PSiO2 than that in SiO2, signifying that PSiO2 has
more amount of micropores than SiO2. The previous study
suggests that the polymerization of PPh3 containing precursor
results in the formation of microporous polymers.[11-12]
Consequently, the BET surface area, pore volume and
micropore volume of PSiO2 are higher than SiO2. The N2
sorption results further confirm the existence of PPh3 polymer on
PSiO2.
Pt NPs were loaded onto PSiO2 through impregnation-
reduction method. Compared with PSiO2, slight decrease in BET
surface area and micropore volume were observed with
Pt/PSiO2 (Table 1). Similar tendency was also observed for
Pt/SiO2. The FT-IR spectrum of Pt/PSiO2 is almost identical to
that of PSiO2, indicating that Pt loading did not disturb the
composition of PSiO2 (Figure 1a).
HAAD STEM technique was used to characterize the
particle size of Pt on Pt/PSiO2 and Pt/SiO2 (Figure 3). The STEM
image of Pt/SiO2 clearly shows that Pt NPs have average size of
1-3 nm but for Pt/PSiO2, it is hard to clearly estimate the particle
size of Pt NPs from STEM due to the poor conductivity of the
sample (possibly because of the existence of thick polymers).
However, existence of Pt NPs with particle size less than 1 nm
could still be observed from HAAD STEM image of Pt/PSiO2.
The formation of small Pt clusters on Pt/PSiO2 is possibly due to
the coordination ability of phosphorous ligand to Pt NPs. The
formation of ultrasmall metal NPs, especially for noble metals, is
difficult but quite important to improve the usage efficiency of
expensive metal nanoparticles.
Figure 1. (a) FT-IR spectra of PSiO2 and Pt/PSiO2, (b) 13C CP-MAS NMR spectrum of PSiO2 and (c) 31P MAS NMR spectra of PSiO2 and Pt/PSiO2 (*indicates
spinning side bands)
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1200
catalyzed by solid catalyst and no leaching of Pt occurs during
the catalytic process (Figure 4a).
,
SiO2
,
PSiO2
900
600
300
0
0.0
0.2
0.4
P/P0
0.6
0.8
1.0
Figure 2. N2 adsorption (open circles and triangles), desorption (closed circles
and triangles) isotherms of SiO2 and PSiO2 respectively.
(b)
Table 1. Physical and chemical parameters of PSiO2 and SiO2
[b]
[a]
Vtotal
P
(wt%)
Pt[c]
(wt%)
Pt[d]
(nm)
Sample
SBET
Smicro
(m2 g-1)
45
17
38
(cm3 g-1)
(m2 g-1)
490
-
-
PSiO2
SiO2
Pt/PSiO2
Pt/SiO2
1.7
0.6
1.5
0.7
2.3
-
2.3
-
215
478
167
-
-
1.65
0.50
 1
1-3
-
[a]BET surface area was measured from N2 sorption isotherm at 77 K; [b]Total
pore volume was measured at P/P0 = 0.98; [c]measured from ICP analysis; [d]
Pt size distribution from STEM analysis.
5 nm
Figure 3. (a) HAAD STEM image of Pt/SiO2 and (b) Pt/PSiO2
2. Chemoselective hydrogenation
Hydrogenation of acetophenone could produce 1-phenyl ethanol
(A), 1-cyclohexyl ethanone (B), and 1-cyclohexyl ethanol (C), as
shown in Table 2.[13] With Pt/SiO2 as catalyst, the selectivity to
desired product, 1-phenyl ethanol, is only 36% at conversion of
17% and the main product is fully hydrogenated 1-cyclohexyl
ethanol. Commercial Pt/C gives selectivity to A of 63.5% with
conversion of 92%. This suggests that PtSiO2 and Pt/C has very
poor chemoselectivity.
Under similar reaction conditions, Pt/PSiO2 could obtain
99.9% selectivity to A with conversion of 100%, suggesting that
carbonyl bond is preferred for hydrogenation. The control
sample, H2PtCl6/PSiO2 (prepared by impregnation of H2PtCl6 on
PSiO2 without reduction) shows no activity in acetophenone
hydrogenation reaction, suggesting that Pt NPs are the active
sites for this reaction. The kinetic curve of Pt/PSiO2 indicates
that the reaction rate is fast in the initial 20 min without any
induction time (Figure 4a). Within 2 h, 100% conversion has
been achieved. Hot filtration test was also performed by
separation of Pt/PSiO2 from the reaction system and the filtrate
was further subjected for reaction. After 80 minutes no further
increase in acetophenone conversion could be found,
demonstrating that the hydrogenation reaction is indeed
Importantly, Pt/PSiO2 retains high chemoselectivity to 1-
phenylethanol all throughout the reaction process. Under more
harsh conditions, e.g. higher pressure, higher temperature and
higher Substrate/Catalyst ratio (S/C), the selectivity to 1-phenyl
ethanol maintains as high as 99% (Table 2, Figure 4b). The
above results signify that Pt/PSiO2 has high selectivity to
carbonyl hydrogenation even under harsh reaction conditions.
Under solvent free conditions, Pt/PSiO2 could also smoothly
catalyze acetophenone hydrogenation reaction to afford turnover
frequency (TOF) of 2304 h-1 and selectivity to 1-phenyl ethanol
of 99.5%. The H2 pressure has no influence on the selectivity to
1-phenyl ethanol but affects the activity of Pt/PSiO2.
The TOF of Pt/PSiO2 and H2 pressure follows the volcano
curve. During the catalytic process, the H2 dissociates on Pt NPs
and dissociated H reacts with substrates for the formation of
product.[14] Thus the TOF increases with H2 pressure. However,
too much H2 may inhibit the adsorption of acetophenone on Pt.
Consequently, the TOF decreases at high H2 pressure. When
comparing with Pt/C and Pt/SiO2, Pt/PSiO2 shows both higher
selectivity and higher activity, signifying the superior
performance of Pt/PSiO2.
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The catalytic performance of Pt/PSiO2 was also tested in the
hydrogenation of phenylacetylene (PA) to styrene because
styrene or substituted styrene derivatives are important
intermediate/partial hydrogenation product of PA or
alkylphenylacetylene, which is difficult to isolate but it has much
importance in industry (Table 3). In this regard, high
chemoselective synthesis of styrene without formation of
undesired ethylbenzene could be scientifically challenging.[15]
Although Pt/PSiO2 and Pt/SiO2 could smoothly catalyze this
reaction, but the selectivity to styrene is lower (52%) on Pt/SiO2
in contrast to Pt/PSiO2 (85.2%) for almost similar conv. of PA.
The low styrene selectivity of Pt/SiO2 could be attributed to its
high hydrogenation activity than Pt/PSiO2, as evidenced that
Pt/SiO2 shows 92% conv. of PA within 20 min. On the other
hand, PPh3 modified catalyst shows relatively sluggish
hydrogenation of PA and at 72% conv. of PA, highest styrene
selectivity (87.5%) can be achieved after 120 min reaction. This
result suggests that Pt/SiO2 has faster hydrogenation activity
and relatively lower selectivity than Pt/PSiO2. Also initial TOF of
Pt/SiO2 is almost 5 times higher that of Pt/PSiO2, signifying the
remarkable effect of phosphor ligands in controlling the catalytic
performance of Pt NPs.
Table 2. Catalytic performance of Pt/SiO2 and Pt/PSiO2 in the hydrogenation
of acetophenone.[a]
Sel (%)
others
TOF[b]
(h-1)
Catalyst
T
S/C
Conv.
(%)
(oC)
A
36
>99.9
-
63.5
98.6
99.5
64
0
-
36.5
1.4
0.5
Pt/SiO2
60
60
60
60
100
100
200
200
200
200
500
1000
17
100
0
92
96.2
99.1
17
297
-
Pt/PSiO2
H2PtCl6/PSiO2
Pt/C
Pt/PSiO2
Pt/PSiO2
-
[c]
2030
2304
[c]
[a]0.2 mmol acetophenone, 2 mL ethanol, 20 bar H2, 120 min; [b]Initial TOF at
10 min of reaction; [c]Under solvent free conditions at 20 bar H2, 120 min.
Table 3. Selective hydrogenation of PA to styrene on Pt/PSiO2 and
Pt/SiO2.[a]
H2
H2
.
120
100
80
60
40
20
0
120
100
80
60
40
20
0
(a)
Ethylbenzene
Desired product
(Styrene)
Filtration
Catalyst
Time
(min)
Conv.
(%)
Sel (%)
TOF[b]
(h-1)
Styrene
Ethylbenzene
X
X
X
X
X
35
92
42
72
88
85
52
88
87.5
85.2
Pt/SiO2
10
20
40
120
140
15
48
12
2050
396
Pt/PSiO2
12.5
14.7
0
20
40
60
80
100
120
Time / min
[a]Reactions were carried out at 45 oC, 20 bar H2, phenyl acetylene (50 mg),
S/C = 1000; [b]Initial TOF was calculated at 10 min reaction.
(b)
300
200
100
0
90
60
30
0
% Sel.
TOF
The recycle stability of Pt/PSiO2 was also investigated. As
shown in Figure 5, both conversion and selectivity did not exhibit
any considerable decrease in comparison with the fresh catalyst,
demonstrating that Pt/PSiO2 is highly stable and durable.
The acetophenone hydrogenation results show that
Pt/PSiO2 is highly chemoselective to partial hydrogenation
products such as 1-phenylethanol. This demonstrates the
unique role of phosphor ligand in modifying the catalytic
performance of Pt NPs. For understanding the influence of PPh3
ligand on catalytic performance of Pt NPs, 31P MAS NMR was
firstly employed for characterizing the interaction between Pt
NPs and PPh3 ligand (Figure 1c). For Pt/PSiO2, the peak with
chemical shift at -5.6 ppm assigned to free tertiary phosphorus-
0
5
10
15
20
25
30
35
Pressure / bar
Figure 4. (a) Reaction kinetics for Pt/PSiO2 (ethanol as solvent), (b) Selectivity
and TOF of acetophenone hydrogenation to 1-phenyl ethanol on Pt/PSiO2 at
various H2 pressure considering S/C = 200.
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Conv.
Sel.
Linear
Linear
Bridging
100
80
60
40
20
0
Pt/PSiO2
Pt/SiO2
H2PtCl6/PSiO2
1
2
3
4
Number of Cycles
2200 2150 2100 2050 2000 1950 1900
Wave number / cm-1
Figure 5. Recycling stability of Pt/PSiO2 for acetophenone hydrogenation
under solvent free condition at 20 bar H2, 120 min reaction time, 100 oC,
S/C=1000.
Figure 7. In-situ CO adsorption FT-IR spectra of Pt/PSiO2 (red line), Pt/SiO2
(black line), H2PtCl6/PSiO2 (blue line).
-atom decreases in intensity, accompanying by an increase in
peak intensity with chemical shift at 28.5 ppm assigned to the
phosphorous atom coordinated with Pt NPs. This shows the
existence of strong interaction of Pt NPs with PPh3.
For understanding the difference in Pt NPs on SiO2 and on
PSiO2, we measured in-situ CO adsorption FT-IR of Pt/SiO2 and
Pt/PSiO2 because the stretching frequency of CO is very
sensitive to the electronic structure of the metal surface onto
which it is chemisorbed (Figure 7).[19] In-situ CO adsorption FT-
IR spectrum of Pt/SiO2 clearly shows CO adsorption band at
2083 cm-1 corresponding to linear bound signal for CO. However
the band is slightly broad due to strong dipolar coupling
interaction between Pt and CO.[20] Interestingly, no bridging CO
adsorption signal was observed in Pt/SiO2. On the other hand,
H2PtCl6/PSiO2 did not show any characteristics CO adsorption
signal. Pt/PSiO2 shows distinguishable and broad CO band at
2081 cm-1, 2019 cm-1, and 1968 cm-1, further confirming the
existence of Pt(0) on Pt/PSiO2. The band at 2081 cm-1 and 2019
cm-1 could be assigned to linearly adsorbed CO on Pt/PSiO2. It
is evident that there are two types of CO linearly adsorbed for
Pt/PSiO2, indicating the presence of two different adsorption
sites, Pt(0) and PtPPh3(0). For Pt/PSiO2, the additional band at
1968 cm-1 may be assigned to the bridging bound signal of CO,
which is responsible to several multimetallic carbonyl
complexes.[21] Interestingly for Pt/PSiO2, the IR band at 2019 cm-
Furthermore, XPS analysis of Pt/PSiO2 was used to probe
the chemical valence state of Pt, which is summarized in Figure
6 and compared with that of Pt/SiO2 and H2PtCl6/PSiO2. For
H2PtCl6/PSiO2, the binding energy (BE) of Pt4f7/2 and Pt4f5/2
appears respectively at 72.6 eV and 75.9 eV, which is consistent
with Pt(IV) as observed in several Pt complexes.[16] While
Pt/SiO2 gives Pt4f7/2 BE at 70.8 eV and Pt4f5/2 BE at 74.2 eV,
which could be assigned to Pt(0) NPs.[17] However, Pt4f BE for
Pt/PSiO2 falls at 72.1 eV (4f7/2) and 75.5 eV (4f5/2). Therefore, Pt
on PSiO2 shows positive binding energy shift (+1.28 eV)
compared with Pt on silica. This is possibly due to the strong
interaction of Pt NPs with organophosphor ligands and also the
ultrasmall particle size of Pt NPs, as previously reported.[18]
Pt4f5/2
Pt4f7/2
Pt/SiO2
1
red shifts from that of Pt/SiO2 (2083 cm-1). It is worthy to
mention, red shift in CO adsorption is responsible to electron
rich Pt surfaces, which is justified from the interaction between
ultrasmall Pt NPs and basic PPh3 sites.[22] Therefore from in-situ
CO adsorption studies it is clear that Pt/PSiO2 has at least two
types of CO binding site, that is Pt without direct interaction with
PPh3P and Pt directly coordinated with PPh3P.
H2PtCl6 /PSiO2
Pt/PSiO2
Based on NMR, XPS and in-situ CO adsorption FT-IR
characterizations, we could see that there are strong interactions
among PPh3 ligand and Pt NPs, which may change the
electronic properties of Pt NPs and also results in the formation
of ultrasmall Pt NPs. These alterations make Pt/PSiO2 to exhibit
unique chemoselectivity in hydrogenation reactions.
80
78
76
74
72
70
68
Binding energy / eV
Figure 6. XPS spectra of Pt 4f electrons of Pt/SiO2 (black), H2PtCl6/PSiO2
(blue) and Pt/PSiO2 (brown)
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Conclusions
under vacuum at room temperature (25 oC). The material was denoted as
PSiO2.
Loading of Pt on PSiO2
In summary, the tuning of chemoselectivity of Pt NPs was
achieved by using silica modified with cross-linked PPh3 ligand.
The PPh3 ligand not only stabilizes the ultrasmall Pt NPs but
also forms strong interaction with Pt NPs. In the hydrogenation
of acetophenone and phenylacetylene, Pt/PSiO2 shows superior
chemoselectivity to Pt/SiO2, which could be attributed to the
ligand effect of PPh3. The solid support modified with organic
ligand therefore provides a new approach for tailoring the
catalytic performance of metal NPs.
PSiO2 (0.2 g) was dispersed in THF (1.5 mL), followed by the addition of
208 μL of H2PtCl6 solution (0.1 M). After the mixture was stirred for 6 h at
o
25 C, the solid material was filtered, washed with water and dried under
vacuum. The solid was re-dispersed in deionized water (2 mL), followed
by the addition of NaBH4 (20 equiv with respect to Pt). The suspension
was again stirred for 15 h at 25 oC under Ar and thereafter filtered,
washed with water, ethanol and dried under vacuum. The black solid was
denoted as Pt/PSiO2, where Pt loading is 2 wt%. For comparison, Pt/SiO2
with Pt loading of 1 wt% was prepared in a similar method to Pt/PSiO2
with the exception that fumed silica was used instead of PSiO2. A control
sample H2PtCl6/PSiO2 was prepared in a similar method to Pt/PSiO2 with
the exception that no reduction of Pt complex was done.
Experimental Section
Hydrogenation reactions
Chemicals and reagents
Hydrogenation of acetophenone
All reagents were of analytical grade and used as received unless
otherwise stated. H2PtCl6 was purchased from Sigma-Aldrich Company,
Ltd. (USA). Fumed silica (specification, 7-40 nm size) was obtained from
Aladdin-reagent Company, China. Tris(4-vinylphenyl)phosphine was
prepared according to the literature report as mentioned elsewhere.[10]
Acetophenone, sodium borohydride, Pt/C (1 wt%) and all other chemicals
as required for catalysis were bought from Aladdin-reagent Company,
China.
A desired amount of catalyst (12.3 mg, 0.5 mol% Pt; 1.04 x 10-6 mmol)
was added to an ampoule tube, followed by the addition of acetophenone
(25 mg, 2.08×10-4 mol) and ethanol (2 mL). The ampoule tube was then
transferred into a stainless-steel autoclave and sealed, flushed with H2
three times to remove air from inside the autoclave and finally
pressurized to 2 MPa H2, which was then heated in an oil bath at 60 oC
under stirring. After 2 h, the remaining H2 was carefully discharged and
the solid catalyst was separated by centrifugation, the filtrate was
collected, diluted with ethanol, and analyzed by using an Agilent 7890
GC instrument equipped with an Agilent J&W GC HP-5 capillary column
(30 m x 0.32 mm x 0.25 mm). The products are also characterized by 1H
NMR measurement, as given in Figure S2.
Instrumentation
N2 adsorption-desorption isotherm at -196 oC has been measured from
Micromeritics ASAP2020 adsorption analyzer with the samples degassed
at 100 oC for 6 h. X-ray photoelectron spectroscopy (XPS) was recorded
on a VG ESCALAB MK2 apparatus by using AlKα (hλ = 1486.6 eV) as
the excitation light source. HAAD STEM has been performed on JEOL
JEM ARM200F instrument. Thermogravimetry analysis (TGA) was
measured by using a NETZSCH STA 449F3 analyzer from 30 oC to 900
oC with a heating rate of 10 oC min-1 in air. Solid state 31P NMR was
carried out at 9.4 T on a Varian Infinity Plus 400 spectrometer with a 31P
frequency of 161.83 MHz using a 5 mm Chemagnetic probe. The
chemical shifts were referenced to (NH4)2HPO4. Solid state 13C NMR
spectra were obtained with a Bruker 500 MHz spectrometer equipped
with a magic-angle spin probe using a 4 mm ZrO2 rotor. 13C signals were
referenced to glycine (C2H5NO2). The experimental parameters were as
follows: 8 kHz spin rate, 5 s pulse delay, and 2500 scans. 1H liquid NMR
of the products were measured from Bruker AvanceIII 400 MHz
instrument. Fourier transform infrared spectra (FT-IR) were recorded on
a Nicolet Nexus 470 IR spectrometer (KBr pellets were prepared). ICP
elemental analysis was performed on PLASAMSPEC-II inductively
coupled plasma atomic emission spectrometry (ICP-AES). CO adsorption
FT-IR was measured by Nicolot iS50 FT-IR Spectrometer, Thermo Fisher
Scientific at 25 oC.
The solvent free reaction was carried out according to the similar
procedure as mentioned above, except that 100 oC, Pt/PSiO2 (6.15 mg,
0.1 mol% Pt; 5.2 x 10-7 mmol) and acetophenone (62.5 mg, 5.2 x 10-4
mol) was used.
Hydrogenation of phenylacetylene
The catalyst (5 mg, 4.23 x 10-7 mmol Pt) was added to an ampoule tube,
along with phenyl acetylene (50 mg; 0.5 mmol) without any solvent. The
tube was then transferred into a stainless-steel autoclave and sealed,
flushed with H2 three times to remove air from inside the autoclave and
finally pressurized to 2 MPa H2, which was then heated in an oil bath at
45 oC under stirring. The remaining procedure is exactly similar as
mentioned for the hydrogenation of acetophenone. In Figure S3 we have
given the characterization of products by 1H NMR.
Acknowledgements
Methods
This work was supported by the NSFC (No. 21321002,
21232008, 21325313) and the Strategic Priority Research
Program of the Chinese Academy of Sciences Grant No.
XDB17020200. S. J thanks for the award of CAS-TWAS
President’s Research Fellowship. A.M thanks Chinese Academy
of Science (CAS) Post doctoral fellowship Grant No.
2017PC0049 and also to China Post doctoral Science
Foundation Grant No. 2016M590236.
Synthesis of PSiO2
Fumed silica (1 g) in a round bottom flask was degassed at 120 oC for 3
h. After cooling to room temperature (25 oC) under Ar flow, 40 mL of
anhydrous THF was added. Then, tris(4-vinylphenyl)phosphine (PPh3V,
0.37 g; 1.08 mmol) and azobisisobutyronitrile (AIBN) (3 wt% with respect
to PPh3V) were added to the suspension of silica. The resulting mixture
was refluxed at 67 oC for 24 h under Ar atmosphere. Finally, the solid
product was filtered and thoroughly washed with THF, ethanol and dried
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10.1002/chem.201700980
Chemistry - A European Journal
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Keywords: Triphenylphosphine modified fumed silica •
Ultrasmall Pt nanoparticles • Chemoselective Hydrogenation
reaction • Acetophenone • Phenylacetylene
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10.1002/chem.201700980
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Ultrasmall Pt NPs stabilized by
triphenylphosphine modified Pt/SiO2
shows high activity and good
Sanjeevi Jayakumar, Dr. Arindam
Modak, Miao Guo, Dr. He Li, Prof.
Xiangping Hu, Prof. Qihua Yang*
selectivity in acetophenone and
phenyl acetylene hydrogenation
reactions than bare Pt/SiO2, because
of strong metal-support interaction.
Page No. – Page No.
Ultrasmall Pt Stabilized on
Triphenylphosphine Modified Silica
for Chemoselective Hydrogenation
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