Deposition of gold clusters on porous coordination polymers by solid grinding and their catalytic activity in aerobic oxidation of alcohols

Article abstract of DOI:10.1002/chem.200800980

The direct deposition of Au clusters onto various kinds of porous coordination polymers (PCP) including MOF-5 by solid grinding with a volatile organogold complex without using organic solvents was reported. The results show that the PCP supports, Al-MIL53 is able to support Au clusters with the smallest size of below 1 nm, resulting in a mean diameter of 1.5 nm with a standard deviation of 0.7 nm. The size of Au is found to be strongly dependent on the metallic properties, porous structures, and pore size of PCPs. The Au/PCP exhibit high catalytic activity for the aerobic oxidation of alcohols in the liquid phase, while Au/MOF-5 shows the highest catalytic activity to obtain methyl benzoate. Au clusters on Cu-PCPs are found to exhibit high selectivity towards benzaldehyde and the catalytic activity order of Au/PCPs in the oxidation of 1-phenylethanol is similar to the oxidation of benzyl alcohol.

Full text of DOI:10.1002/chem.200800980

DOI: 10.1002/chem.200800980
Deposition of Gold Clusters on Porous Coordination Polymers by Solid
Grinding and Their Catalytic Activity in Aerobic Oxidation of Alcohols
Tamao Ishida,[a, b] Megumi Nagaoka,[a] Tomoki Akita,[b, c] and Masatake Haruta*[a, b]
Gold has turned out to be one of the most attractive ele-
ments in catalysis research since the discovery of CO oxida-
tion at À708C over Au nanoparticles (NPs) supported on
base metal oxides.[1] During the last decade, Au NPs sup-
ported on metal oxides[2] and activated carbons (AC)[3] have
been widely studied for liquid phase reactions.[4] In liquid
phase reactions, catalytic performance of Au particles is
mainly defined by two major factors: i) the nature of sup-
ports and ii) the size of Au particles.In particular, the size
of 2 nm appears to be a critical point, where the Au particles
dramatically change their catalytic and physicochemical
properties.[5] However, the conventional deposition–precipi-
tation method is not applicable to AC due to the acidic
nature of AC.Mixing Au colloids with AC could hardly give
Au clusters with a diameter smaller than 2 nm.A latest
trend in the selection of supports is the use of organic poly-
mers.[6–8] Recently, some organic polymers were found to be
effective to stabilize colloidal Au in a cluster size, which
showed high catalytic activity for aerobic oxidation of alco-
hols at room temperature.[7] However, a constraint is that
sophisticated synthetic techniques are usually required to
obtain Au clusters.
On the other hand, porous coordination polymers (PCPs)
consisting of metal ions and organic ligands with highly reg-
ular nanometer-sized cavities or channels are an emerging
class of porous materials.[9] They are expected to be efficient
supports for metal clusters to control size and shape by
means of their cavities.[10–12] In addition, PCPs have a wide
variety of porous structures, various kinds of components,
and surface properties which would lead to tailor-made cata-
lysts for the desired reactions.Thus, investigations of the
preparation methods for PCP supported Au clusters and the
support effect of PCPs would offer a new frontier in cataly-
sis by Au.Fischer and co-workers have reported the prepa-
ration of Pd, Cu, Ru clusters, and Au NPs stabilized by Zn-
containing PCP, MOF-5 ([Zn4O(bdc)3]n (bdc=benzene-1,4-
N
dicarboxylate)[13] by chemical vapour deposition (CVD).[11]
Although Au particles could be hardly deposited in cluster
size, they were obtained as NPs in the range of 5 to 20 nm
on MOF-5 due to weak interaction.[11,12] Therefore, the gen-
eration of Au clusters that fit in and/or on the cavities of
PCPs is still a challenging research target.
Herein we report a very simple but the most effective
method for the direct deposition of Au clusters onto several
kinds of PCPs including MOF-5 by solid grinding with a vol-
atile organogold complex without using organic solvents.We
also investigated their catalytic properties for the liquid
phase alcohol oxidation with molecular oxygen.To the best
of our knowledge, catalysis of PCP supported Au clusters in
liquid phase has not yet been studied.
[a] Dr.T.Ishida, M.Nagaoka, Prof.Dr.M.Haruta
Department of Applied Chemistry
Graduate School of Urban Environmental Sciences
Tokyo Metropolitan University, 1-1 Minami-osawa
Hachioji, Tokyo 192-0397 (Japan)
Fax : (+81)42-677-2851
E-mail: haruta-masatake@center.tmu.ac.jp
The PCP supports used were one-dimensionally channel-
led PCPs such as CPL-1 ([Cu
zine-2,3-dicarboxylate, pyz=pyrazine),[14] CPL-2 ([Cu2-
(pzdc)2(bpy)]n, Al-MIL53
bpy=4,4’-bipyridine),[14]
([Al(OH)
(bdc)]n)[15] with pores of 4”6, 6”8, 8.5”8.5 Š2, re-
spectively, and three-dimensional PCPs such as MOF-5 and
Cu-BTC ([Cu3
(btc)2]n (btc=benzene-1,3,5-tricarboxylate)[16]
with pores of 15”15 and 11”11 Š2, respectively.Volatile or-
ganogold complex, Me2Au(acac) (acac=acetylacetonate)
A
G
[b] Dr.T.Ishida, Dr.T.Akita, Prof.Dr.M.Haruta
Japan Science and Technology Agency (JST)
CREST, 4-1-8 Hon-cho, Kawaguchi
Saitama 322-0012 (Japan)
A
ACHTREUNG
AHCTREUNG
[c] Dr.T.Akita
AHCTREUNG
Research Institute for Ubiquitous Energy Devices
National Institute of Advanced Industrial Science
and Technology (AIST), 1-8-31 Midorigaoka
Ikeda, Osaka 563-8577 (Japan)
AHCTREUNG
and PCPs were ground in an agate mortar in air for 20 min
at room temperature.Then the mixture was treated in a
stream of 10 vol% H2 in N2 at 1208C for 2 h to obtain Au/
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800980.
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COMMUNICATION
PCPs by solid grinding (SG)).[17] For comparison, supported
Au catalysts were also prepared by CVD[18] and by the dep-
osition reduction from an aqueous solution containing
HAuCl4 and NaBH4.[19]
As typically shown by Au/CPL-2 in Figure 1a, SG gave
small Au particles in a nearly uniform cluster size with a
mean diameter of 2.2 nm and a standard deviation of 0.3 nm
(Figure 1b).On the contrary, CVD yielded larger Au NPs in
addition to Au clusters.The average diameter was 31. nm
with much wider standard deviation of 1.9 nm (Figure 1c, d).
In the SG method, the sublimation of Me2Au(acac) might
G
occur during grinding and the rapid diffusion of the vapour
yielded uniform distribution of the Au precursors.However,
CVD took longer time (ca.6 h) for the complete vaporiza-
tion of Me2Au
N
G
2
through the support was not uniform during CVD without
agitation of powder support.This might cause the agglomer-
ation of Au precursor and the particle growth at densely ad-
sorbed sites to form larger NPs.Liquid-phase deposition
(deposition–reduction method) caused the formation of Au
colloids both on the PCP surfaces and in aqueous solution
yielding only Au aggregates on PCPs.
Figure 2.a) HAADF-STEM image (the scale bar indicates 10 nm) and b)
size distribution of 1 wt% Au/Al-MIL53 (SG).
peaks at around 520 nm which can be ascribed to Au NPs
larger than 2 nm were not observed for 1 wt% Au/Al-
MIL53, showing that majority of the Au particles were in
cluster size.[19]
The specific surface area obtained by the N2 adsorption
isotherms of 0.5 wt% Au/CPL-2 prepared by SG and CVD
were reduced to 64 and 123 m2 gÀ1, respectively, from about
500 m2 gÀ1 of as-synthesized CPL-2.[19] From TEM observa-
tions, Au clusters were found to be larger than the pore size
and were mostly placed on the outer surfaces of PCPs.
Therefore, the appreciable decrease in specific surface area
observed indicates the pore blocking by Au clusters and the
partial degradation of porous structure.However, the XRD
(X-ray diffraction) patterns of PCPs did not change after
the treatment with Me2Au(acac) and the deposition of
U
Au.[19] This indicates that the majority of regular pore struc-
tures remained after the deposition of Au.The peak at 38 8
Figure 1.TEM images of Au/CPL-2 prepared by a) SG (Au 1 wt%) and
c) CVD (Au 0.5 wt%) and b) the size distributions of Au/CPL-2 pre-
pared by b) SG (Au 1 wt%), and d) by CVD (Au 0.5 wt%).
corresponding to Au(111) was not detected for 1 wt% Au/
N
CPL-2 (SG) due to the small size of Au.Analyses by energy
dispersive X-ray spectroscopy (EDX) and the absence of
Cu
(111) peak at 448 (2q) in XRD proved that CuII in the
G
High-angle annular dark-field scanning transmission elec-
tron microscopy (HAADF-STEM) image more clearly
shows the presence of small Au clusters less than 2 nm (Fig-
ure 2a).Among the PCP supports used in this work, Al-
MIL53 could support Au clusters with the smallest size of
below 1 nm, resulting in a mean diameter of 1.5 nm with a
standard deviation of 0.7 nm (Figure 2). Surface plasmon
framework of Cu-PCPs was not reduced to large metallic
Cu0 particles during the H2 treatment at 1208C except for
Cu-BTC.
The size of Au strongly depended on the kind of PCPs;
metallic species, porous structures, and pore size.[19] Gates
and co-workers have investigated the mechanism of the
ligand exchange of Me2Au(acac) with the surface hydroxyl
A
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8457

M.Haruta et al.
groups of Al2O3 and adsorbed H2O molecules.[20] Okumura
et al.previously reported the possibility of hydrogen bond-
ing between oxygen atoms of Me2Au(acac) and hydroxyl
G
groups at the surface of SiO2.[18] A similar ligand exchange
can be assumed to occur during the diffusion of Me2Au-
A
N
and uncoordinated carboxylate groups, metal sites at the
crystal surfaces of PCPs, and surface adsorbed H2O.Since
porous frameworks fix the position of both metal-coordinat-
ing sites and carboxylate linker groups with high regularity
even at the outer surfaces, metallic species, porous struc-
tures, and pore size of PCPs could mainly determine the size
of Au particles and prevent them from aggregation.Genera-
tion of Au clusters at the outer surfaces of PCPs is not dis-
couraging but beneficial in catalytic applications that involve
diffusion-controlled mechanisms owing to the rapid diffu-
sion of substrates to the Au catalytic sites.
Catalytic performance of Au/CPL-2 was first tested for
the gas phase CO and H2 oxidation.[19] Even though the size
of Au clusters is as small as 2.1 nm, Au/CPL-2 was not cata-
lytically active in the oxidation of neither CO nor H2.This
result was surprising but could be predicted based on the re-
action mechanism of CO oxidation.We thus propose that
the oxygen activation takes place at the perimeter interfaces
around Au NPs with the metal oxide supports.[5,21] Porous
coordination polymers, carbons, and conventional organic
polymers will not generate oxygen vacancy sites on the pe-
rimeter interfaces, resulting in poor catalytic activity in the
gas phase oxidation.
Scheme 1.Oxidation of benzyl alcohol in methanol.
tivity of Au/PCPs was superior to Au/AC and Au/SiO2
under base-free condition.
Firstly, Au/CPL-2 catalysts gave benzaldehyde as the main
product and Au/CPL-2 (SG) (entry 10) was more active
than Au/CPL-2 (CVD) (entry 9) owing to the smaller size of
Au clusters.In contrast, Au/MOF-5 showed the highest cata-
lytic activity to obtain methyl benzoate in a yield of 91% at
a full conversion in the presence of K2CO3 within 3 h in
spite of the largest size of Au particles (4.8 nm) (entry 3).
The remaining 9% might be due to the adsorption of benzyl
alcohol.From these results, the selection of PCP supports
and the preparation method to obtain small size of Au clus-
ters appeared to be the important factors.The turnover fre-
quency (TOF) of Au/MOF-5 at 808C was calculated to be
82 hÀ1 per loaded Au atoms from the results of entry 2.
Biffis et al.reported a high TOF value as high as 960 h
608C for the initial 15 min in the benzyl alcohol oxidation
over Au/polymer gel in the presence of NaOH in water.[8a]
However, selectivity was low as 36% to benzoic acid and
49% to benzaldehyde, respectively.
À1
at
In contrast to gas phase, Au/PCPs exhibited high catalytic
activity for the aerobic oxidation of alcohols in the liquid
phase.Table 1 shows the results of benzyl alcohol oxidation
in methanol with Au/PCPs to yield benzaldehyde and
methyl benzoate (Scheme 1).Two characteristic features
are: i) the selection of PCP supports significantly affected
catalytic activity and product selectivity, and ii) catalytic ac-
A
surprising product selectivity of benzaldehyde or
methyl benzoate was observed, which dramatically changed
with the PCP support.The two catalysts Au/MOF-5 and Au/
Al-MIL53 were selective to methyl benzoate, which is gen-
[a]
Table 1.Oxidation of benzyl alcohol catalyzed by Au/PCPs.
Run Catalyst
Preparation[b] Base
Au [mol%] t [h] Conversion[c] [%]
Yield [%][c]
PhCHO PhCO2Me PhCO2Bn PhCO2H PhCH(OMe)2
G
1
2
3
4
5
6
7
8
MOF-5[d]
Au/MOF-5
Au/MOF-5
Al-MIL53[d]
–
K2CO3
0
1
1
0
1
0
1
0
1
1
0
1
1
1
1
0.5
18
1
3
0
0
3
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
8
0
SG
SG
–
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
none
82
>99
0
66
91
0
77
0
0
0
1
1
23
23
23
23
23
23
23
23
3
1
3
3
3
Au/Al-MIL53 SG
98
41
45
37
51
55
26
70
>99
69
12
25
2
trace
0
0
0
0
0
0
trace
0
trace
trace
trace
CPL-1[d]
–
SG
–
17
26
18
37
43
6
48
0
31
4
Au/CPL-1
CPL-2[d]
9
Au/CPL-2
Au/CPL-2
Cu-BTC[d]
Au/Cu-BTC
Au/AC
CVD
SG
–
SG
SG
SG
SG
SG
10
11
12
13
14
15
16
0
3
86
23
trace
8
Au/MOF-5
Au/AC
none
none
[e]
Au/SiO2
12
[a] Reaction conditions: 1 mmol benzyl alcohol, 0.5 mmol base, 99 mg 2 wt% Au catalytst (Au 1 mol%), 3 mL MeOH, 14 mL n-pentadecane, pO2
0.5 MPa, 808C.[b] SG: solid grinding, CVD: chemical vapour deposition.[c] GC yield by using n-pentadecane as an internal standard.[d] Only support
(99 mg) was added.[e] 1 wt% Au catalyst (198 mg) was used.
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Chem. Eur. J. 2008, 14, 8456 – 8460

Aerobic Oxidation of Alcohols
COMMUNICATION
erally the main product in the presence of base.[2] In con-
trast, Au clusters on Cu-PCPs exhibited high selectivity to-
wards benzaldehyde (entries 6–12).In alcohol oxidations,
the addition of a base accelerates the reaction by deprotona-
tion of the alcohol.However, Cu-PCPs were regarded as
acidic supports by the measurement of the pH of Cu-PCPs
dispersed in water.Therefore, weakly acidic Cu-PCP surfa-
ces reduced the deprotonation of benzyl alcohol (step i in
Scheme 1) and, in particular, formation of the hemiacetal in-
termediate (step ii).This would result in lower conversions
than for neutral MOF-5 and Al-MIL53 surfaces, where ben-
zaldehyde would be the main product.Another interesting
feature is that Cu-PCPs were active as heterogeneous cata-
lysts in the oxidation of benzyl alcohol without Au deposi-
tion (entries 6, 8, 11).However, catalytic activity was obvi-
ously enhanced by the incorporation of Au clusters.
Secondly, the oxidation reaction also proceeded over Au/
MOF-5 without base (entry 14).Gold clusters and NPs were
successfully deposited directly onto acidic supports, such as
AC and SiO2, by SG with Me2Au(acac) followed by the re-
N
duction of AuIII to Au0 with H2 or calcination,[22] however,
over Au/AC[3d] and Au/SiO2 the yields were extremely low
without base (entries 15, 16).Rossi and co-workers pro-
posed a reaction mechanism for the oxidation of glucose
over Au/AC that the Au surfaces deposited on AC could
not adsorb alcohol and base was requisite to form the alco-
holate before the adsorption onto Au.Then, oxygen could
adsorb onto the AudÀ sites[23] (Scheme 2) because anionic
Au clusters are considered to be favourable for the activa-
tion of oxygen molecule on the Au surfaces.[24] Concerning
the structure of oxygen adsorbed, calculations by density
functional theory (DFT) have suggested that oxygen mole-
cule may adsorb perpendicularly onto gold clusters.[24] In
contrast, the surfaces of Au clusters on PCPs were suffi-
ciently active to adsorb alcohol to form Au–alcoholate with-
out base or were negatively charged to adsorb oxygen
(Scheme 2).This implies that the PCP might electronically
affect Au clusters, whereas AC did not.Although SG
method could directly deposit small Au clusters onto both
of AC and SiO2 as well as PCP, PCP supports are advanta-
geous in the base-free alcohol oxidation.
Scheme 2.Plausible pathways for benzyl alcohol oxidation over Au/PCPs
and Au/AC.
Scheme 3.Oxidation of 1-phenylethanol.
[a]
Table 2.Oxidation of 1-phenylethanol catalyzed by Au/PCPs (SG).
Run
Catalyst[b]
Yield [%][c]
1
2
3
4
Au/MOF-5
Au/MIL-53
Au/CPL-2
Au/AC
79
56
13
77
[a] Reaction conditions: 1 mmol 1-phenylethanol, 0.5 mmol K2CO3,
1 mol% [Au], 3 mL MeOH, pO2 0.5 MPa, 808C, 23 h.[b] 2 wt% Au cata-
lysts were prepared by SG.[c] GC yield of acetophenone by using n-pen-
tadecane as an internal standard.
Catalytic activity order of Au/PCPs in the oxidation of 1-
phenylethanol was almost similar to that the oxidation of
benzyl alcohol (Scheme 3 and Table 2).Moreover, Au/
MOF-5 (entry 1) exhibited the highest catalytic activity
among all the Au catalysts including Au/AC (entry 4).
sence of base and showed unique product selectivity which
can be tuned by the selection of PCP supports, in particular,
by the degree of surface acidity, for benzyl alcohol oxida-
tion.
In summary, the SG method using Me2Au(acac) as an Au
A
precursor was proved to be the simplest and the most effi-
cient method to deposit Au clusters directly onto PCPs.The
SG method can be applied not only to PCPs but also to
other materials, especially, acidic supports such as SiO2,
WO3, and AC on which Au NPs can not be directly deposit-
ed by the conventional deposition–precipitation method.In
addition, SG needs neither organic solvents nor careful
Acknowledgements
We thank Prof.S.Kitagawa of Kyoto University for providing CPLs for
our preliminary experiments and Dr.U.Müller of BASF for supplying
MOF samples.We are also grateful for Prof.Y.Chujo of Kyoto Universi-
ty and Prof.K.Kanamura of Tokyo Metropolitan University who allowed
us to use the XRD apparatus.This work was financially supported by a
washing after the vapour deposition of Me2Au(acac).Gold
A
clusters and NPs on PCPs exhibited noticeably high catalytic
activity in liquid phase alcohol oxidation even in the ab-
Chem. Eur. J. 2008, 14, 8456 – 8460
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8459

M.Haruta et al.
Grant-in-Aid for Scientific Research in the Priority Area “Chemistry of
Coordination Space” (no.434) of the Ministry of Education, Culture,
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Keywords: alcohols
heterogenous catalysis · oxidation
· coordination polymers · gold ·
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