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.