Solvent-free selective oxidation of primary alcohols-to-aldehydes and aldehydes-to-carboxylic acids by molecular oxygen over MgO-supported nano-gold catalyst

Article abstract of DOI:10.1016/j.catcom.2011.07.001

Magnesium oxide supported nano-gold catalyst (prepared by the homogeneous deposition precipitation technique) showed high activity/selectivity and excellent reusability in the oxidation of different primary alcohols and aldehydes to corresponding aldehydes and carboxylic acids, respectively, by molecular oxygen (under atmospheric pressure) in the absence of any solvent. Influence of the catalyst calcination temperature (400-900 °C), reaction temperature (50-120 °C) and use of different solvents (viz. toluene, p-xylene, DMF or DMSO) on the oxidation reaction has also been studied.

Full text of DOI:10.1016/j.catcom.2011.07.001

Catalysis Communications 13 (2011) 82–86
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Solvent-free selective oxidation of primary alcohols-to-aldehydes and
aldehydes-to-carboxylic acids by molecular oxygen over MgO-supported
nano-gold catalyst
⁎
Vasant R. Choudhary , Deepa K. Dumbre
Chemical Engineering & Process Development Division, National Chemical Laboratory, Pune, 411 008 India
a r t i c l e i n f o
a b s t r a c t
Article history:
Magnesium oxide supported nano-gold catalyst (prepared by the homogeneous deposition precipitation
technique) showed high activity/selectivity and excellent reusability in the oxidation of different primary
alcohols and aldehydes to corresponding aldehydes and carboxylic acids, respectively, by molecular oxygen
(under atmospheric pressure) in the absence of any solvent. Influence of the catalyst calcination temperature
(400–900 °C), reaction temperature (50–120 °C) and use of different solvents (viz. toluene, p-xylene, DMF or
DMSO) on the oxidation reaction has also been studied.
Received 13 May 2011
Received in revised form 1 July 2011
Accepted 1 July 2011
Available online 8 July 2011
Keywords:
Alcohols
© 2011 Published by Elsevier B.V.
Aldehydes
Au/MgO
Molecular oxygen
Solvent-free oxidation
1. Introduction
talcite [9], Au/TiO2 [10], Au/MgO [11] and Au/U3O8 [12]. Use of a few
noble metal (Pd or Ru) containing solid catalysts for the alcohol
Liquid phase primary alcohols-to-aldehydes and aldehydes-to-
carboxylic acid oxidations are important organic transformation
reactions. Particularly, the benzyl alcohol-to-benzaldehyde oxidation
is commercially important for the production of chlorine-free
benzaldehyde, required for perfumery and pharmaceutical industries.
To be a cost effective and environmentally-friendly (or green) process,
the alcohol or aldehyde oxidation with high product selectivity and
yield must be accomplished under solvent-free condition, using
molecular oxygen (which is not only a clean but also the cheapest
oxidizing agent) as an oxidant and also using a highly active solid
catalyst (which is easily separable and also reusable) in the process.
A few studies have been reported earlier for the benzyl alcohol-to-
benzaldehyde oxidation by clean oxidizing agents (hydrogen perox-
ide or oxygen) but in the presence of organic solvent, using solid
catalysts, such as Pd/C [1], Pd-hydrotalcite [2], Pd-Ag/pumice [3], Ru-
Co-Al-hydrotalcite [4], Ni-hydrotalcite [5,6] and NiO2 [7]. Solvent-free
oxidation of benzyl alcohol-to-benzaldehyde by tert-butyl hydroper-
oxide (which leaves tert-butanol as a co product and hence it is not a
clean oxidizing agent) has also been reported making use of solid
catalysts, such as MnO4−1-exchanged hydrotalcite [8], transition metal
containing LDH (Layered Double Hydroxides) and/or mixed hydro-
oxidation has also been reported [13–20].
Aldehydes can be oxidized to carboxylic acids by atmospheric
oxygen but with very low product yield [21]. Several other oxidizing
reagents have been reported for obtaining good product yields in the
oxidation of aldehydes. However, the commonly used oxidants like
chromic acid, potassium permanganate (in acidic, basic and neutral
solution), bromine and nitric acid are not suitable for the aldehyde-to-
carboxylic acid oxidation because of the formation of hazardous
waste. A few studies have been reported on the oxidation of aldehyde
using homogeneous or heterogeneous catalysts/reagents, such as Co-
type ion-exchange membrane [22], transition metal ions [23] and
silica gel supported potassium permanganate [24] in stoichiometric
amounts. The stoichiometric reactions have severe limitations, such as
the formation of large amounts of liquid and solid wastes and the
corrosive nature of the reaction medium. Recently, we have observed
that MnO4−1-exchanged hydrotalcite shows high activity for the
oxidation of benzaldehyde to benzoic acid by tert-butyl hydroperox-
ide [8]. A use of Ni-acetate catalyst in ionic liquid for the aromatic
aldehyde oxidation has also been reported [25]. Our earlier prelim-
inary studies on the solvent-free oxidation of benzyl alcohol to
benzaldehyde by molecular oxygen over a number of supported nano-
gold catalysts revealed that the Au/MgO catalyst, prepared by the
homogeneous deposition precipitation method, shows very good
activity and high selectivity in the alcohol oxidation [26]. In this paper,
we report our investigation on the solvent-free selective oxidation of
different primary alcohols and aldehydes to corresponding aldehydes
⁎
Corresponding author at: NASI Sr. Scientist, Chemical Engineering and Process
Development Division, National Chemical Laboratory, Pune, 411 008, India. Tel.: +91
20 25902318; fax: +91 20 25902612.
E-mail addresses: vr.choudhary@ncl.res.in, vrc0001@yahoo.co.in (V.R. Choudhary).
1566-7367/$ – see front matter © 2011 Published by Elsevier B.V.
doi:10.1016/j.catcom.2011.07.001

V.R. Choudhary, D.K. Dumbre / Catalysis Communications 13 (2011) 82–86
83
Table 1
reactant were analyzed by gas chromatograph with a flame ionization
detector, using a SE-30 column and N2 as a carrier gas. While in case of
aldehyde oxidation, after the completion of reaction, the reaction
mixture was treated with ethyl acetate (30 ml). The catalyst was
separated by filtration and washed with ethyl acetate. The filtrate was
then treated with water and the organic layer was separated. The
product from the organic layer was purified by column chromatography
using silica gel withpetroleum ether/ethyl acetate aseluent. Thecatalyst
was further washed with acetone, dried and reused. The reaction
product isolated by column chromatography, was confirmed by NMR
and IR spectroscopy.
Performance of the Au/MgO catalyst in the oxidation of different primary alcohols by
molecular oxygen (reaction conditions: alcohol=30 mmol, pressure=0.95 atm,
catalyst=0.1 g, bath temperature=120 °C and reaction time=5 h).
Entry Alcohol
Product
Conversion of
alcohol (%)
Selectivity (%)
aldehyde ester
1
2
3
4
5
6
1
2
3
4
5
6
7
8
Ph–CH2OH
Ph–CHO
55.5
47.5
48.5
52.0
53.0
68.0
46.0
42.0
95.0
98.5
98.0
90.0
90.5
95.0
60.5
85.5
5.0
1.5
2.0
10.0
9.4
5.0
39.5
14.4
3NO2–Ph–CH2OH 3NO2–Ph–CHO
3OPh–Ph–CH2OH 3OPh–Ph–CHO
3MeO–Ph–CH2OH 3MeO–Ph–CHO
2MeO–Ph–CH2OH 2MeO–Ph–CHO
4MeO–Ph–CH2OH 4MeO–Ph–CHO
3. Results and discussion
Ph–CH2–CH2OH
Ph–CH2–CH2–
CH2OH
Ph–CH2–CHO
Ph–CH2–CH2–
CHO
3.1. Oxidation of alcohols and aldehydes over Au/MgO
Results of the oxidation of different primary alcohols by molecular
O2 over the Au/MgO catalyst at near atmospheric pressure are
presented in Table 1. From the results following important observa-
tions can be made:
9
4MeO–Ph–CH2–
CH2–CH2OH
4t-(CH3)3–Ph–
CH2OH
4MeO–Ph–CH2– 30.2
CH2–CHO
78.5
88.0
21.4
12.0
10
4t–(CH3)3–Ph–
60.0
CHO
– In the oxidation of primary alcohols, the products observed are only
the corresponding aldehyde and ester. A corresponding carboxylic
acid is not detected in the products. The formation of significant
amount of ester, however, indicates that a corresponding acid is
formed in a significant amount in the alcohol oxidation but, as soon as
formed, it is reacted with the alcohol, leading to the ester formation.
The consecutive reactions involved in the alcohol oxidation are given
in Scheme 1. The reaction step 3 (i.e. esterification) is expected to be
much faster than the reaction steps 1 and 2.
and carboxylic acids, respectively, by molecular O2 (at atmospheric
pressure) over the Au/MgO catalyst. Influence of the catalyst
calcination temperature, reaction temperature and use of different
solvents on the process performance has also been studied. The
catalyst showed high activity/selectivity and excellent reusability in
both oxidation processes.
2. Experimental
– The catalyst showed very good activity in the oxidation of all the
alcohols, depending upon the nature of substituent group (R′)
and/or the length of alkyl chain [(CH2)n] in the alcohol. It shows
highest activity for the oxidation of 4-methoxy benzyl alcohol
(entry 6) and lowest activity for the oxidation of 4-MeO-(C6H4)-
(CH2)-CH2OH (entry 9). In most cases, the aldehyde selectivity was
quite high (90–98%), A lower aldehyde selectivity (consequently
higher ester selectivity) was observed for the oxidation of alcohols
with longer alkyl chain (CH2)n- with n≥0 (entry 7 to 9).
– For the different isomers of methoxy benzyl alcohol, the catalyst
showed the highest activity and selectivity in the oxidation of 4-
methoxy benzylalcohol (entry 6). Its performance for the oxidation
of other two isomers was comparable (entries 4 and 5).
2.1. Catalyst preparation and characterization
The preparation and characterization of Au/MgO catalyst (Au
loading=0.38 mmol/g, surface area=47 m2/g and Au particle
size=9.0 nm) have been described elsewhere [11,27]. The gold on
MgO was deposited by the homogeneous deposition–precipitation
technique [28,29]. For studying the influence of catalyst calcination
temperature on the process performance, the Au/MgO catalyst after its
preparation was calcined in muffle furnace at different temperatures
(200 °C, 400 °C, 700 °C or 900 °C) for a period of 2 h. Unless otherwise
mentioned, the catalyst calcination temperature was 400 °C.
2.2. Catalytic reactions
Result showing the catalyst performance in the oxidation of different
aldehydes [represented by a general formula: R-(CH2)n-CHO where
R=alkyl or aryl (with or without substitution) and n=0 or 1] to
corresponding carboxylic acids by molecular O2 at near atmospheric
pressure are presented in Table 2. Importantobservationsmade from the
result are as follows:
The catalyst showed a very good activity for the oxidation of
benzaldehyde with high benzoic acid yield of 95% (entry 1). However,
the product yield in the oxidation of substituted benzaldehydes is
significantly lower (69–89%) than that observed for the benzaldehyde
oxidation (entry 2–8). Also, the product yields for the oxidation of
aliphatic aldehyde (75%) is also lower (entry 10). Nevertheless, the
catalyst showed good activity even for the oxidation of substituted
The oxidation of alcohols or aldehyde by molecular O2 over the
MgO supported gold catalyst was carried out in a magnetically stirred
round bottom flask (capacity: 25 cm3), provided with a mercury
thermometer and a reflux condenser connected to O2 filled rubber
balloon, under near atmospheric pressure. Unless otherwise men-
tioned, the oxidation was carried out in the absence of solvent at the
following reaction conditions: reaction mixture=30 mmol alcohol or
10 mmol aldehyde, catalyst= 0.1 g, pressure= 0.95 atm., bath
temperature=120 °C, reaction time=5 h. In case of alcohol oxidation,
after the completion of reaction, the catalyst was removed from the
reaction mixture by filtration. The reaction products and unconverted
Scheme 1. Reaction scheme for the oxidation of primary alcohols.

84
V.R. Choudhary, D.K. Dumbre / Catalysis Communications 13 (2011) 82–86
Table 2
3.2. Influence of reaction temperature
Performance of the Au/MgO catalyst in the oxidation of different aldehydes by molecular
oxygen (reaction conditions: aldehyde=10 mmol, catalyst=0.1 g, pressure=0.95 atm,
bath temperature=120 °C, reaction time=5 h).
Results showing the influence of reaction temperature on the conver-
sion and selectivity or product yield in the solvent-free 4-methoxy
benzyl alcohol-to-4-methoxy benzaldehyde and benzaldehyde-to-
benzoic acid oxidation reactions over the catalyst are presented in Fig.1.
Up to the temperature of 80 °C, there was almost no reaction. With
increasing the temperature above 80 °C, the conversion in the alcohol
oxidation and the isolated product yield in the benzaldehyde
oxidation are increased exponentially. In the alcohol oxidation, the
selectivity for aldehyde is decreased while that for ester is increased
with increasing the temperature.
Entry Aldehyde
Product
Isolated yield
(%)
1
2
3
4
5
6
7
8
9
Ph–CHO
Ph–COOH
95
83
89
80
75
85
82
69
83
3-MeO–Ph–CHO
4-MeO–Ph–CHO
Ph–CH2–CHO
4t-(CH3)3–Ph–CHO
2-Br–Ph–CHO
4Me–Ph–CHO
3Me-Ph-CHO
(2-NC5H4)–CHO
CH3-CH2-CH2-CH2-CH2-
CH2-CHO
CH3–CH2–CH2–CH2–CH2–
CH2–CHO
3-MeO–Ph–COOH
4-MeO–Ph–COOH
Ph-CH2-COOH
4t–(CH3)3–Ph–COOH
2-Br–Ph–COOH
4Me–Ph–COOH
3Me–Ph–COOH
(2-NC5H4)–COOH
CH3-CH2-CH2-CH2-CH2-CH2- 75
COOH
CH3–CH2–CH2–CH2–CH2–
CH2–COOH
3.3. Influence of catalyst calcination temperature
10
Results showing the influence of calcination temperature of the
Au/MgO catalyst on its performance in the 4-methoxy benzyl alcohol-
to-4-methoxy benzaldehyde and benzaldehyde-to-benzoic acid oxi-
dation reactions under solvent-free conditions are presented in Fig.2.
In both oxidation reactions, the catalyst showed best performance
when it was calcined at 400 °C; the product yield in both cases passes
through a maximum at the catalyst calcinations temperature of
400 °C. A similar observation was made earlier for the benzyl alcohol-
to-benzaldehyde oxidation by tert-butyl hydroperoxide over the
same catalyst [11]. The poor performance of the catalyst calcined at
200 °C may be due to incomplete decomposition of the catalyst
precursor and/or because of the presence of moisture in the catalyst.
The decrease in the catalyst performance with increasing the calcina-
tions temperature above 400 °C (i.e. at 700 °C and 900 °C) is expected
mostly because of the sintering of gold particles in the catalyst. The Au
particle size of the catalyst is increased from 9 nm to 36 nm with
increasing the temperature from 400 °C to 900 °C [27,30]. The increased
Au particle size has, however, no significant effect on the aldehyde
selectivity in the alcohol oxidation. The role of MgO in the catalyst is to
separate gold particles from each other and thereby to avoid their
growth through Au particles — MgO (support) interactions. The high
11
93a
12
3Me–Ph–CHO
3Me–Ph–COOH
90a
a
Reaction time was 9 h.
benzaldehydes and aliphatic aldehydes to corresponding carboxylic
acids. It also showed good activity for the oxidation of pyridine-2-
carboxaldehyde to pyridine-2-carboxylic acid (entry 9). Higher
carboxylic acid yield could be obtained by increasing the reaction
period (entries 11 and 12).
In both the oxidation reactions, the catalyst can be reused several
times. In the 4-methoxy benzyl alcohol-to-4methoxy benzyaldehyde
oxidation, the alcohol conversion in the 5th reuse of the catalyst was
67.5% with the aldehyde selectivity of 95%. Also, in the benzaldehyde
oxidation, the isolated yield of benzoic acid in the 4th reuse of the
catalyst was 95%.
Influence of the reaction temperature, catalyst calcination tem-
perature and use of different solvents in the 4-methoxy benzyl
alcohol-to-4-methoxy benzaldehyde and benzaldehyde-to-benzoic
acid oxidations over the catalyst have also been studied.
120
a) 4-methoxy benzylalcohol Oxidation
100
Aldehyde selectivity
80
60
40
20
0
Conversion
Ester selectivity
100
b) Benzaldehyde oxidation
80
60
40
20
0
50
60
70
80
90
100
110
120
130
140
Reaction temperature (oC)
Fig. 1. Influence of reaction temperature on the solvent-free a) 4-methoxy benzyl alcohol-to-4-methoxy benzaldehyde oxidation and b) benzaldehyde-to-benzoic acid oxidation.

V.R. Choudhary, D.K. Dumbre / Catalysis Communications 13 (2011) 82–86
85
120
100
80
a) 4-methoxy benzyl alcohol oxidation
Aldehydde seleectiviity (%)
60
Converrsion (%)
Ester selecctiviity (%)
40
20
0
100
b) Benzaldehyde oxidation
80
60
40
20
100
200
300
400
500
600
700
800
900
1000
Catalyst calcination temperature (oC)
Fig. 2. Influence of catalyst calcination temperature on the solvent-free a) 4-methoxy benzyl alcohol-to-4-methoxy benzaldehyde oxidation and b) benzaldehyde-to-benzoic acid
oxidation.
specificity of the oxidation reactions is attributed to the nano gold
particles — MgO (support) interactions.
4. Conclusions
Nano-gold supported on MgO (prepared by homogeneous deposi-
tion precipitation method) shows very good catalytic activity/selectivity
in the primary alcohols-to-aldehydes and aldehydes-to-carboxylic
acid oxidations by molecular oxygen at near atmospheric pressure
under solvent-free condition. Both the oxidation protocols are green/
environmentally friendly. In both cases, the catalyst can be easily
removed from the reaction mixture and reused several times without a
significant loss of its catalytic activity. The catalyst shows inferior
performance in the presence of different solvents and its activity in both
the processes is strongly influenced by its calcination temperature and
also by the reaction temperature. The catalyst shows its optimum
performance in both processes when it is calcined at 400 °C.
3.4. Influence of the presence of solvent
Results showing the influence of the use of different solvents in the
oxidation of 4-methoxy benzyl alcohol to 4-methoxy benzaldehyde
on the catalyst performance in the alcohol oxidation reaction are
presented in Table 3.
The results clearly reveal that the catalyst shows best performance
in the alcohol-to-aldehyde oxidation process in the absence of any
solvent. The alcohol conversion was the highest in the absence of
solvent. The selectivity for aldehyde was also the highest in the
absence of solvent, except in the case of DMSO as the solvent. The
observed much lower aldehyde yield in the presence of solvent may
be a consequence of a competitive adsorption of solvent molecules on
the catalyst surface preventing ready access to the active sites of the
catalyst.
Acknowledgments
VRC and DKD are grateful to the National Academy of Sciences,
India, for the NASI Senior Scientist Platinum Jubilee Fellowship and
Project Assistantship, respectively.
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