Significant promoting effects of Lewis acidity on Au-Pd systems in the selective oxidation of aromatic hydrocarbons

Article abstract of DOI:10.1039/c2cc32024j

An unprecedented synergistic effect, obtained for rationally designed Au-Pd alloy nanoparticles supported on an acidic metal-organic framework (MOF), in the aerobic oxidation of the primary C-H bonds in toluene and derivates is reported.

Full text of DOI:10.1039/c2cc32024j

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Significant promoting effects of Lewis acidity on Au–Pd systems in the
selective oxidation of aromatic hydrocarbonsw
Hongli Liu,a Yingwei Li,*a Huanfeng Jiang,a Carolina Vargasb and Rafael Luque*c
Received 20th March 2012, Accepted 22nd May 2012
DOI: 10.1039/c2cc32024j
An unprecedented synergistic effect, obtained for rationally
designed Au–Pd alloy nanoparticles supported on an acidic
metal–organic framework (MOF), in the aerobic oxidation of
the primary C–H bonds in toluene and derivates is reported.
Based on the aforementioned fundamental understanding of
redox processes and catalyst design, we have recently proved that
a bifunctional system comprising Pd nanoparticles supported on a
MOF, MIL-101, support was highly effective for a series of
reactions.6,7 The significant Lewis acidity of the MIL-101 support
has been demonstrated to play significant roles in promoting the
reactivity of aromatic substrates.6,7
The selective oxidation of primary carbon–hydrogen bonds is
an important academic and industrial process.1 For example,
toluene can be converted into oxidation products such as
benzyl alcohol, benzaldehyde, benzoic acid and benzyl benzoate.
These compounds are in fact industrially valuable chemicals
and are intermediates in the synthesis of fine chemicals,
agrochemicals, pharmaceuticals, and polymers.1,2 However,
the selective oxidation of primary C–H bonds also represents
one of the greatest challenges in catalysis due to the relatively
high energy of C–H bonds, and the selectivity issues arising
from the increased reactivity of products as compared to
starting materials.3 Industrially, such catalytic oxidation
processes are often associated with poor selectivities, low
energy efficiencies and issues in catalyst recycling as well as a
significant negative environmental impact.4
Au–Pd alloy nanoparticles have attracted considerable attention
because of their high activity and selectivity in a range of
selective oxidation reactions including toluene oxidation.5c,8 In
this work, we report an unprecedented synergistic effect,
obtained for rationally designed Au–Pd alloy nanoparticles
supported on acidic MIL-101 material (Au–Pd/MIL-101),
in the aerobic oxidation of toluene and derivates. The multi-
functional system is able to provide excellent activities with a
high selectivity for esters (or aldehydes), even at a temperature
as low as 120 1C. Au–Pd/MIL-101 exhibited TONs 3–100 times
greater than those of previously reported catalysts under milder
or at least comparable conditions.
The solvent-free oxidation of toluene was carried out at
120 1C and under 1.0 MPa of O2. Blank runs gave no activity
in the systems (even with the MIL-101 support) (Table 1,
entries 1 and 2). In view of the reported interesting synergy of
Au–Pd alloy nanoparticles in selective oxidations,5c,8 Au–Pd/
MIL-101 catalysts with different Au–Pd molar ratios were screened.
A great deal of research effort has been devoted to finding
alternative and more environmentally sound methodologies
to achieve the selective oxidation of primary C–H bonds.
Heterogeneous catalysis can play a key role in the development
of sustainable processes for the oxidation of primary C–H
bonds. For the oxidation of aromatic hydrocarbons, a number
of catalysts have been explored in this regard.5 These include
copper manganese oxides,5a Mn/Si mixed oxides,5i Au–Pd/C5c
and chromium silicalite-1 molecular sieves,5d etc. Nevertheless,
high selectivities at (almost) quantitative conversion values
still remains a challenge. Moreover, most selective oxidations
of toluene are performed with turnover numbers (TONs: mole
of product per mole of metal catalyst) lower than 100, even at
temperatures 4190 1C.5
Table 1 Results of the oxidation of toluene in the absence of solventa
Selectivity (mol%)
C–C
BZ BZBZ PhBZ prod.
Pd : Au Conv.
Entry molar ratio (mol%) TON
1b
2c
3
–
–
1.0
1.2
75.6
98.6
32.0
73.9
–
–
61
65
4
–
–
89
93
–
–
4
3
3
4
–
–
3
2
3
2
1.5 : 1
1.5 : 1
1.5 : 1
1.5 : 1
1663
986
2080 13 81
1626 89
4d
5e
6f
2
5
a School of Chemistry and Chemical Engineering, South China
University of Technology, Guangzhou 510640, China.
E-mail: liyw@scut.edu.cn
a
Toluene (5 ml), 1 wt% Au-Pd/MIL-101, substrate/metal = 2200,
b
120 1C, 1 MPa O2, 48 h. No catalyst used. Other products: benzyl
alcohol (39.0%) and benzoic acid (0.5%). Catalyst: MIL-101. The
d
other product is benzyl alcohol (35%). Substrate/metal = 1000,
c
b Universidad Rey Juan Carlos, Tecnologia Quimica y Ambiental,
C/Tulipan s/n, 28933, Mostoles, Spain
c
e
f
´
Departamento de Quımica Organica, Universidad de Cordoba,
Ctra Nnal IVa, Km 396, E14014, Cordoba, Spain.
´
´
36 h. Substrate/metal = 6500. Third run of Au–Pd/MIL-101 under
the conditions of entry 3. Products: benzaldehyde (BZ); benzylbenzoate
(BZBZ); phenyl benzoate (PhBZ); C–C coupling products from 2 toluene
molecules (C–C prod.).
´
E-mail: q62alsor@uco.es
w Electronic supplementary information (ESI) available: Catalyst
characterization and reaction results. See DOI: 10.1039/c2cc32024j
c
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Chem. Commun.

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Results of the toluene oxidation pointed to an optimised
performance of Au–Pd/MIL-101 with a 1.5 : 1 Pd : Au molar
ratio (Table S3, ESIw). Benzyl benzoate was the predominant
oxidation product, with small amounts of benzaldehyde
and phenyl benzoate being co-generated (Table 1, entry 3).
Additionally, neither benzyl alcohol nor benzoic acid were
detected in Au–Pd/MIL-101 catalysed toluene, in constrast
with previous reports on Au–Pd systems on carbon or TiO2.5c
These results suggest a significant effect of the support on the
catalytic efficiency of the Au–Pd catalysts.
A plausible explanation for the excelling catalytic activities
is further proposed after careful characterization of the
designed catalysts. Interestingly, TEM micrographs showed
that the highly active Au–Pd systems exhibited almost
identical nanoparticle sizes as compared to Pd- and Au-MIL-101
(Fig. S4, ESIw) which pointed out that there is no significant
nanoparticle size effect in the different activity of the systems.
A similar Au–Pd/C catalyst was also prepared and included in
Fig. S4w for comparative purposes. In view of these results,
and based on our previous studies with bifunctional catalysts
in redox chemistries,6,9 we believed the presence of Lewis
acid sites in the materials could potentially influence the
activity of the proposed systems in aromatic oxidations. To
further unravel the proposed potential involvement of Lewis
acid sites on the observed improved catalytic efficiency in
toluene oxidation, a series of experiments were carefully
designed.
Using the optimised Au–Pd catalyst, we investigated the
oxidation of toluene at a lower substrate : metal molar ratio,
and a maximum of 98.6% conversion was observed with 93%
selectivity to benzyl benzoate at 120 1C in a shorter reaction
time (i.e., 36 h) (Table 1, entry 4). The reaction also proceeded
smoothly at a high substrate : metal ratio, exhibiting a TON of
2080 with a good selectivity for benzyl benzoate (Table 1,
entry 5). Most remarkably, the recently reported Au–Pd/C
catalyst, among the most active catalysts for toluene oxidation
in the literature, showed a TON of 690 under the same reaction
conditions.5c We also investigated the effects of temperature
and oxygen pressure and found that the reaction proceeded well
even at 0.6 MPa of O2 and at 120 1C (Table S4, ESIw).
The Au–Pd/MIL-101 system was also found to be highly
efficient in the oxidation of a range of aromatics including
xylenes, 2-, 3- and 4-methoxytoluene and 4-fluorotoluene
(Table 2). Xylenes have been reported to be difficult compounds
to activate using Au–Pd nanoparticles supported on carbon or
TiO2, providing a low conversion of 6.8% (TON = 171) after
48 h, even at higher temperatures (160 1C).5c In the present study,
a TON of 1206 was obtained in the oxidation of p-xylene under
milder reaction conditions (120 1C) using Au–Pd/MIL-101
(Table 2). In the oxidation of substituted toluenes, the main
products were also esters (overall selectivity 480%) with no
observed acids and/or CO2. Au–Pd/MIL-101 was also efficient
in the oxidation of ethylbenzene providing the corresponding
aldehyde and ketone as main products (Table S5, ESIw),
further demonstrating the general applicability of our catalytic
system.
Firstly, ethylenediamine (EN) was employed as reagent
to interact with the Lewis acid sites in MIL-101 via grafting
of one amine group from EN onto a Cr(III) coordinatively
unsaturated site.10 Au–Pd nanoparticles were subsequently
deposited onto the EN-MIL-101 using a previously reported
method.11
Fig. 1 clearly points out a significantly decreased conversion
and selectivity of Au–Pd/EN-MIL-101 as compared to
Au–Pd/MIL-101. In addition to this experiment, we also
prepared a Au–Pd catalyst by employing another representa-
tive MOF, ZIF-8, as the support, in which the metal ions
are all coordinatively saturated. A low activity with a poor
selectivity for benzyl benzoate was also observed (Fig. 1).
Au–Pd/C also provided a significantly reduced activity in the
reaction. The observed decrease in activity (between 50–80%)
can be well correlated with the Lewis acidity of the systems, as
is clearly noticeable in Fig. 1 (see also the ESIw for DRIFT
data). Pyridine (PY) titration, diffuse reflectance infrared
fourier transform spectroscopy (DRIFTS)12,13 and X-ray
photoelectron spectroscopy (XPS) (ESIw) demonstrate the
synergy of Au–Pd in oxidations of aromatic substrates
recently proposed by Hutchings et al.,5c as well as an unpre-
cedented Au–Pd-MIL-101 Lewis acidity promotion effect in
the selected process, in good agreement with activity data.
Interestingly, these findings also support previous findings of
the group related to a synergetic effect observed in iron oxide
nanoparticles and Lewis acid sites in aluminosilicates for
oxidation of alcohols and alkenes9 and highlight the strong
Furthermore, the Au–Pd/MIL-101 is stable and reusable
under the investigated conditions (Table 1, entry 6), shows
negligible metal leaching or aggregration and maintains the
crystalline structure after three catalytic cycles (see the ESIw).
Table 2 Solventless aerobic oxidation of various aromatic hydro-
carbons using 1 wt% Au–Pd/MIL-101a
Selectivity (mol%)
Conv.
sBZ sBZBZ sPhBZ Others
(mol%) TON
Substrate
p-Xylene
o-Xylene
m-Xylene
4-Methoxytoluene 45.5
3-Methoxytoluene 32.0
2-Methoxytoluene 38.2
40.2
35.8
31.6
1206
1074 10
948 11
9
65
64
70
66
67
62
71
18
17
12
20
14
18
10
8
9
7
8
7
6
9
1365
6
960 12
1146 14
765 10
p-Fluorotoluene
25.5
a
Substrate (47 mmol), 1 wt% Au–Pd/MIL-101 (1.5 : 1 Pd : Au molar
ratio), substrate/metal = 3000, 1 MPa O2, 120 1C, 48 h. Products sBZ,
sBZBZ and sPhBZ are the corresponding substituted benzaldehyde,
benzylbenzoate and phenyl benzoate, respectively.
Fig. 1 Influence of the Lewis acidity (normalised values with respect
to MIL-101, which accounts for 100) in the conversion and selectivity
to benzyl benzoate (mol%) for a range of investigated catalysts.
c
Chem. Commun.
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methyl group of toluene more active, while Au–Pd activates
O2, and benzaldehyde is formed quickly. Subsequent acid–
base interaction between benzaldehyde and the Lewis centre
inhibits further oxidation to benzoic acid, and makes the
carbonyl group in benzaldehyde more active to undergo an
aldol condensation reaction to produce benzyl benzoate.
This work was supported by the NSFC (20803024, 20936001,
21073065), FRFCU (2011ZG0009), NSF of Guangdong
(S2011020002397, 10351064101000000), and NCETU (NCET-
08-0203). RL gratefully acknowledges MICINN for the
concession of a Ramon y Cajal Contract (ref. RYC-2009-
04199) and projects CTQ2011-28954-C02-02 (MEC) and
P10-FQM-6711 (Consejeria de Ciencia e Innovacion, Junta de
Andalucia).
Scheme 1 Plausible mechanism for the selective aerobic oxidation of
toluene using Au–Pd/MIL-101.
promoting effects of existing Lewis acidity (in this case in the
MIL-101 support) on the selective oxidation of toluene.
In view of these findings, we have proposed a plausible
reaction mechanism for the production of benzyl benzoate
from toluene oxidation (Scheme 1). Lewis acids have been
reported to adsorb aromatic rings.6,14 Therefore, an inter-
action between the aromatic ring of toluene with the Lewis
acidic Cr sites of the MIL-101 support should be present in the
mechanism. Such an interaction may affect the electron
distribution of the methyl group. As a consequence, the carbon
atom of the methyl group is relatively electron-deficient and
should be more easily attacked by the activated oxygen species,
which is adsorbed on the surface of the Au–Pd nanoparticles,
probably in a superoxo-like form.15 This process might be
facilitated by the electron donation effects of the aryl rings of
the MIL-101 support.16 The transformation of benzyl alcohol
to benzaldehyde should be very fast on Au–Pd/MIL-101 under
the investigated conditions as indicated by the fact that mono-
metallic Au/MIL-101 can achieve a complete conversion of
benzyl alcohol within 1 h even under much milder conditions
(e.g., 1 atm of O2, 80 1C).16
An apparent difference in product distribution from
previous literature reports on Au–Pd alloy catalysts5c is the
complete absence of benzoic acid produced in toluene oxidation
experiments using Au–Pd/MIL-101 (Table 1). However, heating
benzaldehyde alone in O2 without the catalyst (Table S6, ESIw)
gave benzoic acid. These results indicate that further oxidation of
benzaldehyde to benzoic acid was effectively suppressed over
Au–Pd/MIL-101, which could also be related to the Lewis acidity
of the support, as it has been reported that Lewis basic CQO
groups can coordinate to Lewis acids (Scheme 1).17 Such an
interaction makes the carbon atom of the carbonyl group
in benzaldehyde highly electrophilic, thereby facilitating a
nucleophilic attack on the carbon atom by benzyl alcohol to
form the corresponding hemiacetal. In the final step, the
hemiacetal is further oxidized to benzyl benzoate.5c There are
some other possible pathways to form the ester (Scheme S1,
ESIw), nevertheless the control experiments seemed to rule out
these mechanisms (see the ESIw).
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On the basis of the above discussions, we can conclude that
the high activity and selectivity of Au–Pd/MIL-101 in toluene
oxidation can be correlated to an unprecendented and unique
double synergetic effect due to the presence of bimetallic
Au–Pd nanoparticles as well as the existing Lewis acidity of
the MIL-101 support. Lewis acid coordination makes the
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.