Fully dispersed Pt entities on nano-Au dramatically enhance the activity of gold for chemoselective hydrogenation catalysis

Article abstract of DOI:10.1039/c0cc03790g

Adding a small amount of fully dispersed Pt entities onto the Au surface in Au/SiO₂ catalyst is found to be an efficient approach to improve the catalytic activity of Au (up to 70-fold) for the hydrogenation of α,β-unsaturated carbonyl compounds, without alternating its selectivity towards CO or CC bond hydrogenation.

Full text of DOI:10.1039/c0cc03790g

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Fully dispersed Pt entities on nano-Au dramatically enhance the activity
of gold for chemoselective hydrogenation catalysisw
Yong-Chun Hong, Ke-Qiang Sun, Gui-Rong Zhang, Ru-Yi Zhong and Bo-Qing Xu*
Received 10th September 2010, Accepted 1st November 2010
DOI: 10.1039/c0cc03790g
m4
Adding a small amount of fully dispersed Pt entities onto the Au
surface in Au/SiO catalyst is found to be an efficient approach to
of fully dispersed Pt entities in SiO -immobilized Pt Au
2
4
2
catalysts (Pt
m
2
Au/SiO , m o 0.1) can enhance up to 70-fold
improve the catalytic activity of Au (up to 70-fold) for the hydro-
genation of a,b-unsaturated carbonyl compounds, without alternat-
ing its selectivity towards CQO or CQC bond hydrogenation.
the activity of Au NPs for the chemoselective hydrogenation of
a,b-unsaturated carbonyl compounds, without changing the
selectivity propensity of Au NPs for the reactions. A possible
explanation for this surprising chemistry is presented.
4
Chemoselective hydrogenation of a,b-unsaturated carbonyl
compounds to their semi-hydrogenated products (i.e., satu-
rated carbonyl compounds from hydrogenation at the CQC
bond conjugated with the carbonyl group and unsaturated
alcohols from hydrogenation at the CQO bond) is a critical
step in organic syntheses of fine chemicals such as fragrances,
A series of colloidal Pt
m
Au samples was synthesized by
reductive deposition of Pt atoms onto nearly mono-disperse
8
,9
PVA-stabilized Au NPs with sizes of 3.0 ꢀ 0.6 nm, the
atomic Pt/Au ratio was controlled at m = 0.005–0.2. The
4
dispersion or percentage exposed of Pt in each Pt
m
Au sample
was measured electrochemically according to our previous
4
1
8,9
procedures. Immobilization of these colloidal Pt
drugs and others. Supported Au NPs were found to be highly
m
Au with
selective towards the hydrogenation of either CQO or CQC
bonds in the a,b-unsaturated carbonyl compounds such as
cinnamaldehyde, acrolein, crotonaldehyde, etc., depending
a non-interacting SiO
4
2
(Degussa, Aerosil 90) support produced
the Pt
m
Au/SiO
2
catalysts (Au loading = 1 wt%) for the
2,3
4
5
6
study of the chemoselective hydrogenation reactions (see ref. 3
2–4
on the size and/or morphology of Au NPs, the nature of the
2
and ESIw). The average size of the immobilized Au and
6
4
support material as well as the reaction conditions. However,
the hydrogenation activity of Au NPs was often much lower
Pt
m
Au particles became slightly larger than their colloidal
counterparts (TEM images and size distributions of the
4
5
than those of the platinum-group metals, most probably
,6
immobilized Pt
m
Au nanoparticles are presented as Fig. S1
4
due to an ‘‘intrinsic’’ nobleness of Au for H activation; the
2
in ESIw), but the features of Au and Pt
changed during the immobilization process.
m
Au NPs were not
3
,8,9
dissociative adsorption of H on Au is an activated process and
2
is restricted at the edge and corner sites or at the interface
7
perimeter around Au NPs in contact with oxide support. Such
The chemoselective hydrogenation of CAL is a well
studied ‘‘standard’’ reaction among the hydrogenation of
2
,3,6,10
low hydrogenation activity of the Au NPs has been a major
obstacle for achieving practical applications.
a,b-unsaturated carbonyl compounds.
As schemed in
Table 1, the hydrogenation at the CQC bond produces
HCAL, an important intermediate for the syntheses of
11
We have previously employed nearly monodisperse Au NPs to
4
construct Pt-on-Au nanostructures (coded as Pt
m
m4
Au, m refers
pharmaceuticals for the treatment of HIV,
while
to the atomic Pt/Au ratio) for electrocatalysts (Pt Au/C) with
significantly improved Pt utilization, by chemical deposition of
8
Pt entities/flecks on the Au surface. When the Au NPs were of
hydrogenation at the CQO bond produces cinnamyl alcohol
(COL), a pharmaceutical intermediate for the syntheses of
cinnarizine, naftifine, toremifene, etc. Summarized in Table 1
4
ca. 3.0 nm, the Pt entities were obtained with 90+% dispersions
at m r 0.1 and showed distinctly high activity for the
electrooxidation of methanol as well as formic acid; compared
are the catalytic data of the Pt
m
Au/SiO
2
catalysts (0 o m r 0.2)
for the hydrogenation of CAL. The Au NPs carrying no Pt
4
(i.e., Au/SiO
2
or Pt
m
Au/SiO
2
of m = 0) produced a low CAL
with the commercial Pt/C catalyst, the mass-specific activity of
4
conversion (11%) in a reaction period as long as 12 h. When
the same Au NPs were loaded with a very small amount of Pt
(m = 0.005), however, a dramatically higher CAL conversion
Pt in Pt
m
Au/C was enhanced by up to one order of magnitude
8
for methanol electrooxidation and two orders of magnitude for
9
formic acid electrooxidation. When Au NPs of varying sizes
(20%) was achieved in a reaction period of only 4 h. Increasing
4
(
3–10 nm), but carrying no Pt deposits, were immobilized
the amount of Pt up to m = 0.2 in the Pt
m
Au/SiO
2
catalyst
on SiO , they showed high selectivity for the formation
2
resulted in continued shortening of the reaction period for
achieving the CAL conversion between 11–20%. For instance,
of hydrocinnamaldehyde (HCAL) in the hydrogenation of
3
cinnamaldehyde (CAL). We now report that a small amount
a reaction period of 15 min was found long enough to produce a
4
CAL conversion of 15% over the Pt0.05 Au/SiO
2
catalyst; the
CAL conversion increased up to 23, 33, 44 and 63% when the
reaction period was prolonged, respectively, to 0.5, 1.0, 2.0, and
4.0 h over this catalyst of m = 0.05 (see Table S1 of ESIw).
Innovative Catalysis Program, Key Lab of Organic Optoelectronics
& Molecular Engineering, Department of Chemistry, Tsinghua
University, Beijing, 100084, China.
E-mail: bqxu@mail.tsinghua.edu.cn; Fax: +86 10 6279 2122;
Tel: +86 10 6279 2122
w Electronic supplementary information (ESI) available: Preparation
procedures, experimental details for catalytic testing, TEM images and
detailed catalytic data. See DOI: 10.1039/c0cc03790g
To make a rigorous comparison of the Au activity in these
4
Pt
m
Au/SiO
2
catalysts, the mass specific activity of Au
ꢁ ꢁ1
1
(
MASAu, the CAL consumption rate by mol h gAu ) was
calculated using the reaction data at CAL conversion levels
1
300 Chem. Commun., 2011, 47, 1300–1302
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2
Table 1 Selective hydrogenation of cinnamaldehyde at 150 1C under 1.0 MPa H (CAL/Au (molar) = 1600; stirring speed: 900 rpm)
Sel. (%)
ꢁ1
Auꢁ1
a
ꢁ1
Catalyst
SiO
Au/SiO
Pt dispersion (%) Rxn. time/min CAL conv. (%) HCAL COL HCOL MSAAu/mol h
g
TOFPt /s
2
—
—
240
720
240
120
15
0
11
20
19
15
16
15
14
—
73
77
79
72
71
75
84
—
23
15
17
19
21
19
13
—
4
—
0.07 (1)
0.40 (6)
—
—
b
2
4
Pt0.005 Au/SiO
2
499
499
499
90.4
62.1
25.0
8
4
9
8
6
4
4.42
4.42
5.24
3.11
2.12
0.13
4
Pt0.01 Au/SiO
2
0.76 (11)
4.77 (68)
5.09 (73)
4.77 (68)
—
4
Pt0.05 Au/SiO
2
4
Pt0.1 Au/SiO
2
15
15
120
4
Pt0.2 Au/SiO
2
5
wt% Pt/SiO
2
a
b
Obtained by assuming that the exposed Pt atoms were solely responsible for the catalyst activity. Data in the parentheses give the numbers
catalyst.
relative to the Au/SiO
2
ꢁ
1
very similar TOFPt numbers (4.4–5.2 s ) that were
between 11–20% (obtained by adjusting the length of the
reaction period). The MSAAu, number listed in the column
second to the last in Table 1, increased remarkably with m for
significantly higher than the Pt with lower dispersions
4
(Pt
m
Au/SiO
2
4
at m Z 0.1). This explains why the MSAAu
4
4
4
all Pt
SiO
3, respectively. It should be emphasized that this dramatic
m
Au/SiO
2
catalysts such that the MSAAu for Pt0.05 Au/
for the Pt0.1 Au/SiO
2
and Pt0.2 Au/SiO
2
catalysts cannot be
2
and Pt
0.14
Au/SiO
2
was enhanced by a factor of 68 and
much higher than that for Pt
4
0.054
Au/SiO
2
, though, on the
7
basis of Pt0.05 Au/SiO
2
, the overall amount of Pt was doubled
4
4
enhancement of the Au activity was achieved without
in Pt0.1 Au/SiO2 and redoubled in Pt0.2 Au/SiO . This
2
modifying the selectivity of Au/SiO for HCAL. This point is
2
discussion identifies that the fully dispersed Pt entities
well demonstrated in Fig. 1, which correlates the product
showed the highest efficiency for the activity enhancement of
4
selectivity with the conversion level of CAL using all of the
4
their underlying Au NPs in Pt
The two catalysts Au/SiO
m
Au/SiO
2
catalysts.
4
reaction data over every Pt
m
Au/SiO
2
catalyst (Table S1 in
2
and Pt0.05 Au/SiO
2
were
ESIw). The selectivity for HCAL was kept at the levels of
also compared for the hydrogenation of several other a,b-
unsaturated carbonyl compounds, including heterocyclic
7
0–80%z over a wide range of CAL conversion, while the fully
0
hydrogenated product (HCOL) increased slightly with
increasing CAL conversion, at the expense of COL.
aldehyde (furfural) and aromatic ketones (chalcone and 4 -
methoxy-4-methylchalcone). As shown in Table 2, the Au/SiO
2
As it would be expected, the reference 5 wt% Pt/SiO
STREM, dPt = ca. 4 nm) catalyst appeared more active
2
catalyst without Pt was hardly active for these reactions but
showed very specific selectivity (499%) for the hydrogenation
at the CQO bond of furfural and the CQC bond of the
ketones. The activity became one order of magnitude higher
(
than Au/SiO with no Pt and showed some higher selectivity
2
for HCAL. The numbers in the last column of Table 1 show the
turn-over frequency data of CAL over Pt (TOF ), obtained by
Pt
(10–20 fold) with the presence of the fully dispersed Pt entities
4
assuming that the exposed Pt atoms were solely responsible for
the catalyst activity using the CAL hydrogenation data in
(i.e., over Pt0.05 Au/SiO
2
) while the specific product selectivity
remained unchanged (see Table S2 of ESIw). These results
demonstrate again that the fully dispersed Pt entities enhanced
the activity of their underlying Au NPs without disturbing the
selectivity propensity of the hydrogenation catalysis by Au
NPs. Therefore, hydrogenation both at the CQC and CQO
bonds can be dramatically accelerated by the presence of the Pt
entities. To be noted, the product of the furfural hydrogenation
in this study, furfuryl alcohol, is an important starting
Table 1 (CAL conversion 11–20%). Compared with that for
4
the reference Pt/SiO , the TOFPt for every Pt
2
m
m4
Au/SiO
2
catalyst
2
catalysts,
was surprisingly higher. For the Pt in the Pt Au/SiO
the fully dispersed Pt (99+% dispersion at m r 0.05) showed
1
2
molecule for the manufacture of resins, the addition of
a very small amount of Pt entities (m = 0.05) improved its
productivity by 10-fold, which would have important
implication for application development.
It should be noted that the Pt entities on Au NPs did not
4
improve the catalytic activity of Au NPs in the Pt
m
Au/SiO
2
catalysts during the transfer-hydrogenation of CAL using
benzyl alcohol as the H-donor (see Table S3 of ESIw). Thus,
a change of the H-source from molecular H to benzyl alcohol
2
4
significantly ‘‘suppressed’’ the catalytic activity of Pt
m
Au/
SiO catalysts. Interestingly, the selectivity for COL was as
2
Fig. 1 Selectivity–conversion curve for CAL hydrogenation over
m4
high as 63% over Au/SiO but it decreased to ca. 30% over the
2
4
Pt Au/SiO
2
catalysts at 150 1C.
Ptm Au/SiO2 catalysts (m = 0.005–0.2) in the transfer
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Chem. Commun., 2011, 47, 1300–1302 1301

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Table
2
Catalytic hydrogenation of a,b-unsaturated carbonyl
governed by the adsorption mode of the substrate molecules on
compounds at 150 1C under 1.0 MPa H
2
(stirring speed: 900 rpm)
the Au surface.
4
b
Sel. (%)
Recycling of the Pt0.05 Au/SiO catalyst for CAL hydrogena-
2
tion was also attempted. After the reaction, the reacted catalyst
was separated, washed with toluene, dried under vacuum and
then reused for the same reaction. The CAL conversion and
product selectivity data obtained over this recovered catalyst
were found similar to those over the fresh one, proving that the
catalyst was reusable.
a
Conv. (%)
Substrate
Catalyst
CQC
CQO
499
Au/SiO
2
1
—
0.054
2
2
2
Pt
Au/SiO
10
3
—
499
In summary, a small amount of fully dispersed Pt entities
loaded on Au NPs resulted in dramatic activity enhancement
of the Au NPs (up to 70-fold) for the chemoselective
hydrogenation of a,b-unsaturated carbonyl compounds,
without changing the selectivity propensity of Au NPs for
the reactions. A synergic effect between Pt and Au in the
Au/SiO
2
499
—
0.054
Pt
Au/SiO
20
3
499
499
—
—
Au/SiO
2
4
Ptm Au/SiO2 catalysts is proposed to explain this
0.054
observation, in which the Pt sites function to activate the H2
Pt
Au/SiO
62
499
—
a
molecules while the Au sites serve to activate the unsaturated
For the reaction of furfural, substrate/Au (molar) = 1600, rxn. time
240 min; for the reaction of chalcones, substrate/Au (molar) =
substrate molecules by chemisorption.
=
1000, rxn. time
b
=
60 min. CQC: product of CQC bond
We thank Dr Sun Wei (Lanzhou Institute of Chemical
hydrogenation; CQO: product of CQO bond hydrogenation.
Physics, Lanzhou, China) for the syntheses of chalcone and
0
4
-methoxy-4-methylchalcone. This work was financially
supported by NSF of China (grants: 20773074, 20921001 and
1033004).
2
Notes and references
z The selectivity for HCAL was 92% at 150 1C using a home made
3
reactor in our earlier work. This difference could arise from system-
2
associated errors in the H pressure and reaction temperature. The Parr
reactor employed in this study is equipped with a well-calibrated
pressure gauge (ꢀ1 psi) and reports the temperature with a thermal
3
couple inside a Hastelloy autoclave reactor, while in the earlier work
the temperature was recorded by a thermocouple in the heating unit
2
outside a stainless autoclave reactor and the error for H pressure
could be ꢀ30 psi.
1
J. Grolig, Ullmann’s Encyclopedia of Industrial Chemistry, 7th edn,
Wiley-VCH, Weinheim, 2003.
Fig. 2 Pt promotion of the hydrogenation catalysis of Au NPs.
2
C. Milone, C. Crisafulli, R. Ingoglia, L. Schipilliti and S. Galvagno,
Catal. Today, 2007, 122, 341; E. Bus, R. Prins and J. A. van
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hydrogenation reaction, which is in contrast to the case when
high pressure H
2
was the H-source (Table 1). These differences
4
3
H. Shi, N. Xu, D. Zhao and B. Q. Xu, Catal. Commun., 2008, 9,
1
in the reactions over Pt
m
Au/SiO
2
using H
2
and benzyl alcohol
949.
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promoted H activation by the Pt sites is the main cause for the
4 P. Claus, A. Bruckner, C. Mohr and H. Hofmeister, J. Am. Chem.
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2
dramatic activity enhancement of Au NPs in the conventional
chemoselective hydrogenation reactions (Tables 1 and 2).
Therefore, the activity enhancement of Au NPs by the
deposition of Pt entities for the chemoselective hydrogenation
of a,b-unsaturated carbonyl compounds could be explained by a
synergic effect of Pt and Au (Fig. 2). It seems reasonable that the
5
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Pt sites were responsible for the dissociative activation of H
2
8
9
molecules while the Au sites for the adsorption-activation of the
substrate molecules. The activated H atoms would then migrate
by spill over toward nearby Au sites for a fast reaction with the
adsorbed substrate. The dramatic enhancement of the Au
activity by the fully dispersed Pt entities would suggest that
1
0 S. Galvagno, G. Capannelli, A. Donato and R. Pieteropaol, J. Mol.
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2
or supply of H atoms. On the other hand, the well maintained
selectivity propensity of the Au NPs in the hydrogenation
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2
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1
302 Chem. Commun., 2011, 47, 1300–1302
This journal is c The Royal Society of Chemistry 2011