From alkyl aromatics to aromatic esters: Efficient and selective C-H activation promoted by a bimetallic heterogeneous catalyst

Article abstract of DOI:10.1002/cssc.201200611

Oxidizing aromatics: We report an operationally simple and green catalytic oxidative esterification approach that selectively converts methyl aromatics to aromatic carboxylates utilizing a highly stable, active, and reusable heterogeneous bimetallic Au-Pd catalyst and molecular oxygen as benign oxidant without requiring any additives.

Full text of DOI:10.1002/cssc.201200611

DOI: 10.1002/cssc.201200611
From Alkyl Aromatics to Aromatic Esters: Efficient and Selective CÀH
Activation Promoted by a Bimetallic Heterogeneous Catalyst
Hongli Liu,[a] Gongzhou Chen,[a] Huanfeng Jiang,[a] Yingwei Li,*[a] and Rafael Luque*[b]
Esters (e.g., aromatic carboxylates) are an important class of
chemicals, widely utilized in fine chemicals, natural products,
pharmaceuticals, agrochemicals, and food additives.[1] , Esters
are traditionally prepared by reacting carboxylic acids or acti-
vated carboxylic-acid derivatives (e.g., acid anhydrides, acyl
chlorides) with alcohols (Scheme 1, path A). The synthetic pro-
inexpensive and abundant feedstocks can offer a number of
advantages to produce and market high-added-value aromatic
esters. Such a method should avoid the use of environmentally
unfriendly catalysts and oxidants as well as basic promoters.
Alkanes (as bulk chemicals) are a highly attractive type of
esterification reagent because of their availability and low cost
compared to their oxidation products, including alcohols, alde-
hydes, and acids. However, alkanes are rarely utilized as oxida-
tive esterification agents owing to the low reactivity of sp3 CÀ
H bonds.[5] Aromatic carboxylates are mostly prepared by the
oxidative esterification of aromatic acids, aldehydes, or alco-
hols.[2–4] To date, only two very recent reports describe the se-
lective oxidative esterification of methyl aromatics with alco-
hols to yield aromatic methyl esters, employing homogeneous
organophotocatalysis (CBr4 or anthraquinone-2,3-dicarboxylic
acid) under light irradiation.[6] Nevertheless, the reported proto-
cols have a relatively restricted scope of substrates as well as
low efficiencies [with turnover numbers (TONs) <10] related to
the inherent drawbacks of photocatalysis. The homogeneous
system is not particularly environmentally friendly or cheap,
which significantly limits its application to more useful and
broad catalytic processes.
Scheme 1. Esterification methodologies for the synthesis of aromatic-derived
methyl esters from various aromatic compounds with alcohols.
As a continuation of our recent development of useful and
environmentally sound alternative methodologies in oxidation
chemistries, we report here an operationally simple and green-
er catalytic oxidative esterification approach that efficiently
converts methyl aromatics to aromatic carboxylates utilizing
a highly stable, active, and reusable heterogeneous bimetallic
Au–Pd catalyst and molecular oxygen as benign oxidant
(Scheme 1). The proposed catalytic system features a broad
substrate scope for both methyl aromatics and alcohols, pro-
viding a high selectivity to target products under mild solvent-
free conditions without the addition of any base additives.
Au–Pd bimetallic nanoparticles have attracted considerable
interest due to their high activities and selectivities in a range
of oxidation reactions.[7] Bimetallic Au–Pd nanoparticles on
carbon or TiO2 have been recently shown to be an efficient
system in the selective oxidation of toluene to symmetric
esters (e.g., benzyl benzoate).[8] Benzyl alcohol and benzalde-
hyde were detected as intermediates for the formation of
esters.[8] Based on these precedents, Au–Pd nanoparticles
could possibly be active in the oxidative esterification of tolu-
ene with alcohols to synthesize unsymmetric aromatic esters.
We selected MIL-101 (a representative metal–organic frame-
work) as support for Au–Pd nanoparticles owing to its high
surface area and large pore size. MIL-101 proved to be an ex-
cellent support for the preparation of highly dispersed metal
nanoparticles as well as in a range of catalytic applications.[9]
MIL-101 supported Au–Pd catalysts were prepared by using
a simple colloidal deposition method with poly(vinyl alcohol)
cedure involves multiple steps, and suffers from inherent prob-
lems such as the formation of undesired byproduct and time-
and resource-consuming steps to isolate or separate catalysts
and/or products.[2] The oxidative esterification of aldehydes
with alcohols (path B) has recently attracted much attention as
alternative for traditional protocols.[3] However, such methodol-
ogies generally require stoichiometric amounts of transition-
metal oxidants or peroxy salts (e.g., KMnO4, CrO3, Oxone, and
sodium perborate). Further research efforts have been devoted
to the direct synthesis of esters from alcohols (path C),[4] in
which a stoichiometric amount (or excess) of base additives is
generally required to achieve high ester yields.[4a–f]
In view of these settings, the development of a suitable,
more efficient, and environmentally sound alternative catalytic
methodology for the direct oxidative synthesis of esters from
[a] H. Liu, G. Chen, Prof. H. Jiang, Prof. Y. Li
Key Laboratory of Fuel Cell Technology of Guangdong Province
School of Chemistry and Chemical Engineering
South China University of Technology
Guangzhou 510640 (PR China)
E-mail: liyw@scut.edu.cn
[b] Prof. R. Luque
Departamento de Quꢀmica Orgꢁnica
Universidad de Cꢂrdoba, Edif. Marie Curie, Ctra Nnal IVa
Km 396, 14014 Cꢂrdoba (Spain)
E-mail: q62alsor@uco.es
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200611.
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(PVA) as protecting agent (for experimental details, see the
Supporting Information). Different Au–Pd molar ratios were uti-
lized for the preparation of a series of Au–Pd/MIL-101 materi-
als. After the incorporation of metal nanoparticles, the frame-
work of MIL-101 was well preserved, as confirmed by powder
X-ray diffraction (XRD) and N2 adsorption measurements (Sup-
porting Information, Table S1 and Figure S1). Pd and Au/MIL-
101 analogous systems were also synthesized and character-
ized accordingly. XPS results showed that the Pd0 3d and Au0
4f peaks for the Au–Pd/MIL-101 were shifted to lower binding
energies by approximately 0.5 eV and 0.3 eV, respectively, com-
pared to the monometallic Pd/MIL-101 and Au/MIL-101 sam-
ples (Supporting Information, Figures S2 and S3). This effect is
particularly noticeable in the Pd XPS (Figure S3) in which peaks
intensities are also different. Such observed shifts for both Pd
3d and Au 4f may be attributed to the flow of electron density
into Au (higher Au s or p electron densities) and Pd (gaining d
electrons from Au), and could be indicative of the formation of
Au–Pd bimetallic alloys, in good agreement with previous re-
ports.[10]
uct, with small quantities of benzaldehyde, benzaldehyde di-
methyl acetal, and benzylbenzoate being co-generated in the
reaction. We also detected small amounts of formaldehyde
and formic acid produced from the oxidation of methanol. Re-
sults of the oxidation esterification pointed to an optimized
performance of Au–Pd/MIL-101 with a Pd/Au 1.5:1 molar ratio
in terms of activity and selectivity (entry 5). A further increase
in Pd content led to reduced conversions and selectivities to
methyl benzoate (entries 6 and 7). These results indicate a re-
markable synergistic effect of the Au–Pd catalysts in the con-
ducted oxidative esterification reactions.
A further optimization of reaction parameters was subse-
quently conducted. The temperature and pressure of O2 in the
reaction were found to strongly influence activities of the bi-
metallic systems in the proposed reaction. An increase in O2
pressure resulted in remarkably reduced selectivities to methyl
benzoate (Table 1, entry 8). However, increasing the tempera-
ture to 1208C improved toluene conversions while maintaining
high selectivities to methyl benzoate (entry 9). We also in-
creased the amount of methanol and found that the reaction
proceeded with enhanced activity and selectivity when 5 mL
of methanol were added in the system (entries 5 and 10). The
effects of substrate/metal molar ratio were further investigated
(entries 11–13), and a maximum of 96.9% conversion of tolu-
ene could be achieved with a 96.6% selectivity to methyl ben-
zoate at a lower substrate/metal ratio and 1208C within 48 h
(entry 12). The oxidative esterification reaction also proceeded
well at a high substrate/metal molar ratio (6000:1), giving
a TON of 1386 with a good selectivity to methyl benzoate
(entry 13).
Initially, we chose toluene and methanol as model sub-
strates, with molecular oxygen as oxidant, to screen reaction
parameters. Reactions in the absence of catalyst or using the
parent MIL-101 as catalyst gave no activity in the systems after
48 h (Table 1, entries 1 and 2). Interestingly, Au/MIL-101 exhibit-
ed a very low activity for the oxidative esterification of toluene
(entry 3), despite previously reported high activities and selec-
tivities in the aerobic oxidation of alcohols under base-free
conditions.[9h]
Conversions were remarkably improved by the addition of
Pd (Table 1, entries 4–6), and no CO2 was observed for all reac-
tions. Methyl benzoate was obtained as main oxidation prod-
The scope of aromatic hydrocarbons amenable to be con-
verted into esters via the proposed Au–Pd/MIL-101 catalyzed
aerobic oxidative esterification
protocol was also investigated.
As shown in Table 2, all reactions
Table 1. Results of the direct oxidative esterification of toluene with methanol.[a]
proceeded smoothly with excel-
lent selectivities to the desired
methyl aromatic esters under
optimized reaction conditions.
The electronic and steric factors
Entry
Pd/Au molar
ratio
T
[8C]
PO
[MP]
Conv.
[%]
Selectivity to
products [%]
2
of the substituents played an im-
portant role in determining the
reactivity of the aromatic sub-
stance. In general, electron-defi-
cient aromatic hydrocarbons
provided the corresponding
ester products in lower yields
(e.g., entries 2 vs. 7). An increase
in steric hindrance led to a slight
decrease in product yields (en-
tries 3–5).
3a
4a
5a
6a
1[b]
2[c]
3
4
5
6
7
8
9
10[d]
11[d]
12[d,e]
13[d,f]
14[g]
–
–
0:1
1:3
100
100
100
100
100
100
100
100
120
100
120
120
120
100
1
1
1
1
1
1
1
2
1
1
1
1
1
1
–
–
1.4
–
–
–
–
–
–
–
–
–
–
4.2
12.0
–
12.1
6.2
0.7
1.6
0.8
5.1
4.0
17.2
77.1
87.8
71.9
13.4
71.5
84.2
96.4
94.7
96.6
80.1
88.5
46.0
4.9
1.8
2.9
62.6
9.3
3.6
1.4
1.9
1.4
6.2
1.7
36.8
18.0
6.2
13.2
24.0
7.1
6.0
1.1
1.8
1.2
8.6
5.8
24.7
34.3
33.8
1.5
50.5
50.3
43.6
62.6
96.9
23.1
35.2
1.5:1
3:1
1:0
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
The application of the newly
developed oxidative esterifica-
tion protocol was then extended
to the use of various alcohols.
Various aliphatic alcohols includ-
ing methanol, ethanol, n-propa-
[a] Reaction conditions: toluene (1 mL), methanol (3 mL), 1 wt% Au–Pd/MIL-101, substrate/metal=2000, 48 h.
[b] No catalyst used. [c] Catalyst: MIL-101. [d] Methanol (5 mL). [e] Substrate/metal=1000. [f] Substrate/metal=
6000. [g] Third run to test the reusability of Au–Pd/MIL-101 under the conditions of entry 5.
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effect in Au–Pd nanoparticles is
suggested to be responsible for
the improved activity and selec-
tivity compared to monometallic
catalysts. Such a molecular-scale
synergy of Au–Pd alloys has also
been observed in a range of
other oxidation reactions.[8,10b,11]
The enhanced surface electron
density of the bimetallic Au–Pd
catalyst (see XPS data, Support-
ing Information) would be favor-
able for the activation of O2 to
generate oxygen species, proba-
bly in a superoxo-like form.[12]
Subsequently, the CÀH bond in
toluene could be oxidized to
form benzyl alcohol by the
active oxygen species. The
formed benzyl alcohol then un-
dergoes a b-hydride elimination
Table 2. Oxidative esterification of various methyl aromatics with methanol.[a]
Entry
Substrate
Product
Conv.[b]
[%]
Sel.[b]
[%]
Yield[c]
[%]
1
1a
1b
1c
1d
1e
3a
3b
3c
3d
3e
96.9
93.9
92.4
88.4
80.2
96.6
98.0
90.6
90.0
87.3
94 (90)
92 (90)
84 (80)
80 (75)
70 (66)
2
3[d]
4[d]
5[d]
6[d]
1 f
3 f
73.3
85.5
63 (58)
to
generate
benzaldehyde,
7[e]
1g
1h
3g
3h
75.3
22.9
88.2
80.0
66 (60)
18 (12)
which directly reacts with meth-
anol to form the corresponding
hemiacetal species.[13] Ester prod-
ucts are believed to be finally
formed by the direct oxidation
of hemiacetals.[13] A proof that
supports the proposed mecha-
nism is the observation of ben-
zaldehyde dimethyl acetal as
product in the reaction (Table 1).
8
9[e]
1i
3i
78.6
89.5
70 (65)
[a] Reaction conditions: methyl aromatic (10 mmol), methanol (5 mL), 1 wt% Au–Pd/MIL-101, substrate/metal=
1000, 1208C, 1.0 MPa O2, 48 h. [b] Determined by GC-MS analysis. [c] GC yield (isolated yield in parenthesis).
[d] 72 h. [e] 1408C.
nol, and isopropyl alcohol smoothly underwent esterification
with toluene and provided excellent selectivities to the desired
methyl aromatic esters under optimized conditions using Au–
Pd/MIL-101 (Table 3, entries 1–4). A remarkable reduction in
product yield was observed as the carbon chain length and
steric hindrance of the alcohol increased (e.g., tert-butanol).
The reaction results of different aromatic hydrocarbons with al-
Scheme 2. Possible pathway for the oxidative esterification reaction.
cohols (Tables 2 and 3) demonstrated the general applicability
of this catalytic system in the proposed selective oxidative
esterification.
Kinetic studies revealed that the amount of benzaldehyde
dimethyl acetal decreased with time, accompanied by a gradual
increase in the quantity of methyl benzoate in the course of
reaction (Supporting Information, Figure S4). On the basis of
these results, it might be speculated that a dynamic equilibri-
um between the hemiacetal species and benzaldehyde di-
methyl acetal is likely to be present in the system under the in-
vestigated reaction conditions.[13f] The equilibrium may be
driven to the production of hemiacetal species from benzalde-
hyde dimethyl acetal, which undergo oxidation to form methyl
benzoate.
To ascertain the potential presence of radical intermediates,
we added a typical radical scavenger (p-benzoquinone) to a re-
action run. The addition of p-benzoquinone did not significant-
ly affect the reactivity of the oxidative esterification of toluene
with methanol (Supporting Information, Table S2), which indi-
cates a rather improbable radical intermediate formation in
this transformation using Au–Pd catalysts and under the inves-
tigated conditions. In view of these findings, we propose a pos-
sible pathway for the one-pot aerobic conversion of aromatic
hydrocarbons into methyl aromatic esters, which principally in-
volves four steps (Scheme 2). Firstly, toluene is oxidized to
benzyl alcohol via primary CÀH bond activation over the bi-
metallic Au–Pd nanoparticles.[8] In this process, the synergistic
Reusability studies of Au–Pd/MIL-101 indicate that the cata-
lyst can be reused upon simple separation without a significant
loss of efficiency (Table 1, entry 14). The crystalline structure of
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In conclusion, we have developed an innovative, highly effi-
cient, and reusable heterogeneous catalyst system for the
direct esterification of alkyl aromatics with alcohols catalyzed
by porous MOF-supported Au–Pd bimetallic nanoparticles and
molecular oxygen as the oxidant. Our catalytic oxidative
esterification method exhibits a broad substrate scope, as well
as a high selectivity to target products without requiring any
co-catalysts or additives. The use of less-expensive and readily
available methyl aromatics as starting materials and O2 as
benign oxidant as well as a recyclable catalyst in the absence
of solvents constitutes a significant step towards the develop-
ment of novel and more efficient industrially relevant and ap-
plicable green and sustainable catalytic processes. Studies
aimed at understanding the reaction mechanism and further
improvements and optimization of the catalytic system are
currently underway in our laboratories.
Table 3. Oxidative esterification of toluene with various aliphatic alco-
hols.[a]
Entry
Product
Conv.[b]
[%]
Sel.[b]
[%]
Yield[c]
[%]
1
3a
3j
96.9
99.0
94.5
92.8
80.2
71.9
69.0
45.2
96.6
95.6
89.3
84.6
80.0
70.1
86.9
70.3
94 (90)
95 (90)
84 (80)
79 (73)
64 (60)
50 (45)
60 (56)
32 (25)
2
3
3k
3l
4
5[d]
6[e]
7[e]
8[d]
3m
3n
3o
3p
Acknowledgements
This work was supported by the NSFC (20803024, 20936001,
21073065), FRFCU (2011ZG0009), NSF of Guangdong
(S2011020002397, 10351064101000000), and the program for
NCETU (NCET-08-0203). R.L. gratefully acknowledges MICINN for
the concession of a Ramon y Cajal Contract (RYC-2009-04199)
and projects CTQ2011-28954-C02-02 (MEC) and P10-FQM-6711
(Consejeria de Ciencia e Innovacion, Junta de Andalucia)
[a] Reaction conditions: toluene (10 mmol), aliphatic alcohol (0.13 mol),
1 wt% Au–Pd/MIL-101, substrate/metal=1000, 1208C, 1.0 MPa O2, 48 h.
[b] Determined by GC-MS analysis. [c] GC yield (isolated yield in parenthe-
sis). [d] 72 h. [e] 96 h.
Keywords: CÀH activation · esters · heterogeneous catalysis ·
metal–organic frameworks · oxidation
the catalyst also remained mostly unchanged after three cata-
lytic cycles (Supporting Information, Figure S1). No metal was
detected in the filtered reaction solution (after 9 h reaction),
and the liquid phase did not exhibit any further reactivity (Sup-
porting Information, Figure S5). In good agreement with these
findings, transmission electron microscopy (TEM) images of the
reused catalysts did not reveal any appreciable changes in
nanoparticle size (average particle size around 3.0 nm) in the
materials as compared to the fresh catalysts prior to reaction
(Figure 1). These results confirm the stability of the described
heterogeneous system under the investigated conditions.
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Received: August 18, 2012
Published online on && &&, 0000
ChemSusChem 0000, 00, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
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5
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These are not the final page numbers!

COMMUNICATIONS
H. Liu, G. Chen, H. Jiang, Y. Li,* R. Luque*
&& – &&
From Alkyl Aromatics to Aromatic
Esters: Efficient and Selective CÀH
Activation Promoted by a Bimetallic
Heterogeneous Catalyst
Oxidizing aromatics: We report an op-
erationally simple and green catalytic
oxidative esterification approach that
selectively converts methyl aromatics to
aromatic carboxylates utilizing a highly
stable, active, and reusable heterogene-
ous bimetallic Au–Pd catalyst and mo-
lecular oxygen as benign oxidant with-
out requiring any additives.
&6
&
www.chemsuschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
ÝÝ
These are not the final page numbers!