Synthesis of Carboxylic Acid by 2-hexenal oxidation using gold catalysts Supported on MnO2

Article abstract of DOI:10.1155/2016/2016407

Synthesis of carboxylic acid can be achieved by the oxidation of aldehyde using air as an oxidant in the presence of a potential catalyst. We demonstrated that 2-hexenal can be oxidized to carboxylic acid by Au, Pd, and Au-Pd catalysts and investigated the effects of catalyst support (graphite, TiO₂, MgO, SiC, MnO₂, CeO₂, and Al₂O₃), preparation method for supported catalyst (sol immobilization, impregnation, and deposition precipitation), and choice of catalyst components. Analysis of conversion% and selectivity% for 2-hexenoic acid showed that MnO₂-supported gold nanoparticles are the best catalysts for 2-hexenal oxidation. Moreover, catalysts prepared by sol immobilization are the most active possibly due to the much smaller gold nanoparticle size. Selectivity for 2-hexenoic acid is a major pathway of oxidation of 2-hexenal.

Full text of DOI:10.1155/2016/2016407

Hindawi Publishing Corporation
Journal of Chemistry
Volume 2016, Article ID 2016407, 7 pages
http://dx.doi.org/10.1155/2016/2016407
Research Article
Synthesis of Carboxylic Acid by 2-Hexenal Oxidation Using
Gold Catalysts Supported on MnO2
Hamed Alshammari
Department of Chemistry, Faculty of Science, University of Hail, P.O. Box 2440, Hail 81451, Saudi Arabia
Correspondence should be addressed to Hamed Alshammari; h.alshammari@uoh.edu.sa
Received 6 September 2016; Revised 3 November 2016; Accepted 8 November 2016
Academic Editor: Sedat Yurdakal
Copyright © 2016 Hamed Alshammari. is is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Synthesis of carboxylic acid can be achieved by the oxidation of aldehyde using air as an oxidant in the presence of a potential
catalyst. We demonstrated that 2-hexenal can be oxidized to carboxylic acid by Au, Pd, and Au-Pd catalysts and investigated the
effects of catalyst support (graphite, TiO , MgO, SiC, MnO , CeO , and Al O ), preparation method for supported catalyst (sol
2
2
2
2
3
immobilization, impregnation, and deposition precipitation), and choice of catalyst components. Analysis of conversion% and
selectivity% for 2-hexenoic acid showed that MnO -supported gold nanoparticles are the best catalysts for 2-hexenal oxidation.
2
Moreover, catalysts prepared by sol immobilization are the most active possibly due to the much smaller gold nanoparticle size.
Selectivity for 2-hexenoic acid is a major pathway of oxidation of 2-hexenal.
1. Introduction
above 120∘C. In addition, supported gold has been used as a
catalyst for the oxidation of 5-hydroxymethylfurfural (HMF)
to 5-hydroxymethyl-2-furandicarboxylic acid (HFCA) [10–
13]. Moreover, Davis et al. [14] used supported metal catalysts,
such as Pt/C, Pd/C, and Au/C, for oxidation of HMF at a
temperature of 22∘C and with 690 KPa of oxygen, and they
demonstrated that gold supported on carbon had a higher
reaction rate and that HFCA was the major product. Pasini
et al. investigated the influence of bimetallic gold-copper
on HMF oxidation [13] and showed that Au-Cu catalyst
Oxidation plays a key role in the synthesis of chemical inter-
mediates for the manufacture of high-value fine chemicals,
agrochemicals, pharmaceuticals, and high-tonnage com-
modity chemicals. e oxidation of aldehyde to carboxylic
acid is an important synthetic transformation [1], and it is
considered to be a commonly encountered organic chemistry
reaction [2]. ese oxidation reactions ofen use stoichio-
metric oxygen donors such as chromate or permanganate,
which results in the production of a large amount of environ-
mental toxins. erefore, finding environmentally friendly
alternatives is crucial, and the ultimate goal is to efficiently use
molecular oxygen from air with an oxidation catalyst. Oxida-
tion with molecular oxygen under green oxidation methods
has been a very attractive proposition in recent years, and
gold catalysts are effective for the oxidation of alcohols [3],
oxidation of hydrogen to hydrogen peroxide, and oxidation of
alkenes [4–7]. Later, it has been found that the supported gold
nanoparticles are efficient catalysts for several oxidation pro-
cesses. Marsden et al. [8] reported that gold nanoparticles can
oxidize an aldehyde, which had been dissolved in a primary
alcohol, to ester, while Choudhary and Dumbre [9] showed
that MgO-supported nanogold catalyzes the oxidation of
aldehyde in the presence of molecular oxygen at temperatures
supported on TiO was more active than monometallic gold.
2
Biella et al. [15] showed that gold was a highly active catalyst
for propanal and butanal oxidation under 300 KPa of oxygen
pressure at 363 K in water. More recently, Wan et al. reported
that carbon nanotube-supported Au-Pd alloy nanoparticles
were highly efficient and recyclable heterogeneous catalysts
for the aerobic oxidation of HMF in water under base-free
conditions [16].
e present study evaluated whether aldehyde oxida-
tion can be catalyzed by supported gold or gold-palladium
nanoparticles to the corresponding carboxylic acid using
molecular oxygen from the air. is study also investi-
gated whether 2-hexenal can be oxidized selectively under
mild, solvent-free green conditions with high selectivity to
the corresponding carboxylic acid. Moreover, the reaction

2
Journal of Chemistry
Table 1: Effect of support on 2-hexenal oxidation.
conditions, namely, the support, reaction temperature, cat-
alyst preparation methods, and choice of metal, were opti-
mized.
Selectivity%
2-Hexenoic acid
Catalyst
Conversion%
Dimer
0
4.0
0
2.9
0
6.2
0
4.1
0
8.3
0
4.3
0
Graphite
Au/G
0.7
74.5
0.9
68.2
1.1
54.0
0.7
69.3
0.8
81.3
1.2
72.8
0.4
93.1
86.0
96.5
88.4
89.3
85.9
92.1
82.8
95.6
89.1
2. Experimental
TiO
Au/TiO
MgO
2.1. Materials. PdCl and HAuCl ⋅3H O were purchased
2
2
4
2
from Johnson Matthey. All other chemicals were purchased
from Sigma-Aldrich.
2
Au/MgO
SiC
2.2. Catalyst Preparation. Catalysts were prepared by sol
immobilization, impregnation, and deposition precipitation
methods using standard procedures. For all catalysts, the
concentration of metal is expressed as weight percent of the
combined metal and solid support material.
Au/SiC
MnO
2
Au/MnO
2
CeO
90.5
84.6
89.0
2
Au/CeO
2
2.2.1. Sol Immobilization. Monometallic (Au, Pd) and bime-
tallic (Au-Pd) catalysts were prepared using sol-immobiliza-
tion method. e detailed procedure for the preparation of 1%
Al O
2
3
Au/Al O
65.4
4.8
84.2
2
3
∘
Reaction conditions: 4.23 g of 2-hexenal, 0.06 g of catalyst, 50 C, 24 h,
atmospheric pressure.
(w/w) Au-Pd/MnO catalyst is explained here for an example.
2
Aqueous solutions of PdCl (1.1698 mL from the stock of 6 mg
2
Dimer: 4,6-dipropyl-3,4,5,6-tetrahydropyran-2-one-5-carboxaldehyde.
in 1 mL), HAuCl ⋅3H O (1.06003 mL from the stock solution
4
2
of 12.25 g in 1 L), and polyvinyl alcohol (1% solution, MW =
10000, 80% hydrolysed) were prepared and mixed together
by stirring for 15 min. To this solution, freshly prepared 0.1 M
of NaBH (>96%, NaBH /Au, mol/mol = 5) was added and
2.4. Transmission Electron Microscopy. To prepare the sam-
ples, the powder catalyst was dispersed in ethanol, and
the suspension was dropped onto a lacey carbon coated
300-mesh copper grid. Sample analysis was carried out by
transmission electron microscope (JEOL 2100) equipped
with a lanthanum hexaboride filament (200 kV).
4
4
stirred for 30 min. Afer adjusting the pH to 1, by adding
sulphuric acid, manganese dioxide (MnO , 1.98 g) was added
2
and stirred for 1 h to make a slurry. e slurry was filtered, and
the recovered catalyst was washed with 2 L distilled water and
dried (110∘C, 12 h).
2.5. X-Ray Powder Diffraction (XRD). e catalysts were
ground into powder and then placed in a sample holder. e
XRD analysis was performed using a PANalytical X’Pert Pro
2.2.2. Impregnation. Preparation of 1% (w/w) Au/MnO
catalyst was done by impregnation. In short, 0.816 mL of
2
with CuK X-ray source operated at 40 kV and 40 mA fitted
ꢀ
HAuCl ⋅3H O solution (12.25 g in 1 L) was added to MnO
with an X’Celerator detector.
4
2
2
(0.99 g) and stirred until it formed a paste. e paste was dried
(110∘C, 16 h) and calcined in static air (400∘C, 3 h).
3. Results and Discussion
2.2.3. Deposition Precipitation Using NaOH. Deposition pre-
cipitation (DP) method was also used to prepare 1% (w/w)
Au/MnO catalyst as follows: MnO solution was first pre-
Activity of catalyst can be affected by several factors, includ-
ing the nature of the catalyst support. Catalyst supported by
different materials showed different activities and selectivity,
even under the same conditions [17, 18]. is research work
was performed to examine the effect of the support on
2-hexenal oxidation. e reactivity of the pure supports
alone in the absence of gold was fairly minimal with 2-
hexenoic acid being the major product. e addition of gold
supported on graphite or metal oxide, however, leads to a
significant increase in the conversion of the 2-hexenal as
shown in Table 1. Five different oxides were analyzed for
their potential to be a catalyst support in the liquid-phase
oxidation of 2-hexenal and compared with graphite and
silicon carbide (Table 1). Among the five different supports
2
2
pared by mixing MnO with distilled water (0.99 g in 150 mL)
2
and stirring at 60∘C. 0.816 mL of HAuCl ⋅3H O solution
4
2
(12.25 g in 1 L) was added and stirred for 1.5 h. e pH was
adjusted to 9 by the dropwise addition of NaOH solution
(0.1 M). At the end, filtration method was used to recover the
catalyst. en, the catalyst was washed with distilled water
(1 L) to remove impurities, dried (110∘C, 16 h), and calcined
in static air (400∘C, 3 h).
2.3. Catalyst Testing and Characterization. 2-Hexenal oxida-
tion was carried out using air in a glass reactor, which consists
of a round-bottom flask and a reflux condenser and heating
source. In short, the substrate (4.23 g) was mixed with the
catalyst (0.06 g) and stirred for 24 h at 50∘C. Once the reaction
was completed, the reaction mixture was analyzed using gas
chromatography (Varian Star CP-3800) with a CP-wax 32
column and a flame ionisation detector.
tested in the study, Au catalyst supported on MnO showed
2
greater activity. ese supports can be divided into reducible
(TiO , CeO , and MnO ) and irreducible (Al O and MgO)
2
2
2
2
3
supports. e latter have a low capacity for adsorbing or
storing oxygen [19], which may explain the higher activity
that was observed when using reducible supports, relative to

Journal of Chemistry
3
20
40
60
80
20
40
60
80
2 (deg.)
2 (deg.)
(a)
(b)
Figure 1: e XRD pattern of (a) 1% Au/MnO and (b) MnO .
2
2
100
95
Table 2: Effect of bimetal on 2-hexenal oxidation.
90
70
50
30
10
Selectivity%
Catalyst
Conversion%
Dimer
8.3
8.0
2-Hexenoic acid
Au/MnO
81.3
83.3
73.6
89.1
89.9
89.2
2
Au-Pd/MnO
2
90
Pd/MnO
7.5
2
∘
Reaction conditions: 4.23 g of 2-hexenal, 0.06 g of catalyst, 50 C, 24 h,
atmospheric pressure.
Dimer: 4,6-dipropyl-3,4,5,6-tetrahydropyran-2-one-5-carboxaldehyde.
85
Table 3: Effect of preparation methods on 2-hexenal oxidation using
80
1% Au/MnO .
2
Selectivity%
75
50
Method
Conversion%
60
70
80
Dimer
8.3
7.8
2-Hexenoic acid
∘
Temperature ( C)
Sol
81.3
76.1
77.6
89.1
89.1
Impregnation
DP
Figure 2: Effect of temperature on 2-hexenal oxidation using
Au/MnO . Reaction conditions: 4.23 g of 2-hexenal, 0.06 g of cat-
6.2
90.3
2
alyst, 24 h, atmospheric pressure.
∘
Reaction conditions: 4.23 g of 2-hexenal, 0.06 g of catalyst, 50 C, 24 h,
atmospheric pressure.
Dimer: 4,6-dipropyl-3,4,5,6-tetrahydropyran-2-one-5-carboxaldehyde.
Al O and MgO, at 50∘C. e graphite showed similar activity
2
3
to silicon carbide in the oxidation of 2-hexenal. Overall, the
MnO support showed the highest activity and selectivity
Table 2 compares the activity of MnO -supported Au, Pd,
2
2
for 2-hexenoic acid. erefore, Au/MnO (1 wt.%) catalyst
and Au-Pd catalysts for 2-hexenal oxidation under solvent-
free conditions using oxygen from air. e highest conversion
was observed when using the bimetallic catalyst, while the
Pd catalyst showed the lowest activity. It is clear that the
bimetallic catalysts have a synergistic effect, as the catalytic
activity was higher than the corresponding monometallic
catalysts at 50∘C. In addition, it was observed that monogold
gave a higher conversion rate, compared to Pd only. ese
results are in accordance with the previous studies on the
redox reactions, namely, the oxidation of primary alkyl
alcohols [21] and the synthesis of H O from H oxidation
2
was used for further studies. An XRD pattern of MnO and
2
Au/MnO catalyst is shown in Figure 1. Comparison with
2
pure support reveals that most of the reflections stem from
the support. e main reflections of gold are expected at 2ꢀ
= 38.1803∘ (100%), 44.4∘ (52%), 64.6∘ (32%), and 77.4∘ (36%),
while the reflection of MnO phase can be identified by the
2
major peaks at 28.88∘, 37.4∘, 42.62∘, 56.65∘, 59.10∘, 64.96∘,
67.15∘, and 72.30∘.
e oxidation of 2-hexenal was analyzed using MnO -
2
supported Au catalyst (Au/MnO ) atmospheric air at a wide
2
2
2
2
range of temperatures (Figure 2). It can be observed that
an increase in the temperature led to a significant increase
in the oxidation of 2-hexenal. e selectivity data for 2-
hexenal oxidation at four different reaction temperatures
(50–80∘C) is also shown in Figure 2. Increase in the reac-
tion temperature is associated with a slight decrease in
the selectivity for 2-hexenoic acid. is may be due to
the catalyst-facilitated dimerisation of the oxidation product
2-hexenal to 4,6-dipropyl-3,4,5,6-tetrahydropyran-2-one-5-
carboxaldehyde via the Diels-Alder reaction [20].
[23], whereby very pronounced synergistic effects on activity
and selectivity were found.
e preparation method is a commonly used parameter
with regard to affecting the catalyst activity [24]. SI method,
DP method, and the impregnation method (IM) were used
to prepare 1% Au/MnO and the activities were summarized
2
in Table 3. Catalysts prepared by impregnation and DP
showed similar activities. However, catalyst prepared by SI
method showed enhanced activity, which may be due to the
much smaller gold nanoparticle size and its higher dispersion
For the oxidation of alcohols, the activity of gold catalysts
can be increased by alloying gold with another metal [21, 22].
[25]. e particle size distributions for Au/MnO , prepared
using SI, DP, and IM, are shown in Figure 3. It is apparent
2

4
Journal of Chemistry
30
25
20
15
10
5
0
100 nm
12
14
16
18
20
22
24
26
28
Particle size (nm)
(a)
(b)
35
30
25
20
15
10
5
0
200 nm
14
16
18
20
22
24
26
28
Particle size (nm)
(c)
(d)
30
25
20
15
10
5
0
20 nm
1
2
3
4
5
6
7
8
Particle size (nm)
(e)
(f)
Figure 3: Particle size distributions and TEM image of Au/MnO using different preparation methods, respectively: DP (a) and (b); IM (c)
2
and (d); SI (e) and (f).

Journal of Chemistry
5
90
80
70
60
50
40
30
20
0
10
0
5
10
15
20
25
30
Time (h)
Figure 4: Effect of reaction time for the conversion of 2-hexenal using Au/MnO . Reaction conditions: 4.23 g of 2-hexenal, 0.06 g of catalyst,
2
atmospheric pressure.
O
O
O
H
1% Au/MnO2
50∘C
H
O
H2O
+
HO
O
O
O
2-Hexenal
[O]
H
O
O
4,6-Dipropyl-3,4,5,6-tetrahydropyran-2-one-5-carboxaldehyde
Figure 5: Diels-Alder formation of product 4,6-dipropyl-3,4,5,6-tetrahydropyran-2-one-5-carboxaldehyde from oxidation product 2-
hexenal.
that the DP and IM show very similar distribution, which
is in accordance with the activity for those two catalysts.
Figure 3 showed the particle size distribution of the catalysts
prepared by DP and IM (10–30 nm). is is consistent with
the activities of the gold catalysts that were prepared by DP
and IM, which gave the same activity, as shown in Table 3. In
contrast, the particle size distribution of the catalyst prepared
using the SI method comprises much smaller particles with
the diameter of 2-3 nm. is gave the catalysts enhanced
activity, possibly due to higher dispersion of much smaller
gold nanoparticles, as shown in Figure 3. Several previous
studies [19–22] noted that gold catalyst activity has an inverse
relationship with the gold particle size. ese results suggest
that SI can be used to prepare effective catalysts of this type
with regard to 2-hexenal oxidation.
time. is increase was linear, indicating that there was
no apparent deactivation of the catalysts in this reaction
under our conditions. Table 4 shows that the selectivity for
carboxylic acids was observed at the beginning of the reac-
tion. However, the selectivity for carboxylic acids decreased
with increasing time, indicating that the partial oxidation
of 2-hexenal is stable for a short time. In contrast, the
selectivity for 4,6-dipropyl-3,4,5,6-tetrahydropyran-2-one-5-
carboxaldehyde (dimer) increased with increasing time. e
catalyst may facilitate the dimerisation of the oxidation of
2-hexenal to the dimer via the Diels-Alder reaction [20], as
shown in Figure 5. is was previously reported by Hutchings
et al. who found that a dimer could be formed by formal
Diels-Alder dimerisation of crotonaldehyde.
In conclusion, the present study showed that gold-
supported catalysts were an effective catalyst for 2-hexenal
oxidation under green reaction conditions. Gold nanoparti-
cles supported on MnO , CeO , SiC, graphite, MgO, Al O ,
Figure 4 showed the results of time online study of the 2-
hexenal oxidation, which was performed using the optimized
1% Au/MnO SI catalyst. Under optimal conditions, there
2
2
2
3
2
and TiO exhibited different activities in 2-hexenal oxidation,
was a steady increase in 2-hexenal oxidation with reaction
2

6
Journal of Chemistry
Table 4: Effect of reaction time on the selectivity during the
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oxidation of 2-hexenal using 1% Au/MnO .
2
Selectivity%
Reaction time
Dimer
3.5
4.4
4.8
5.3
2-Hexenoic acid
2 h
4 h
8 h
16 h
24 h
95.2
94.1
94.0
92.3
8.3
89.1
∘
Reaction conditions: 4.23 g of 2-hexenal, 0.06 g of catalyst, 50 C, atmo-
spheric pressure.
Dimer: 4,6-dipropyl-3,4,5,6-tetrahydropyran-2-one-5-carboxaldehyde.
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2
2
2
2
3
MgO. erefore, it was concluded that MnO was the support
2
of choice. e oxidation pathway was the main reaction
under almost all conditions. Furthermore, it was shown that
a significant amount of the dimer arose from a dimerisation
reaction. e highest activity of gold catalysts was obtained
when SI method was used for catalyst preparation, which gave
smaller gold nanoparticles.
Competing Interests
[13] T. Pasini, M. Piccinini, M. Blosi et al., “Selective oxidation
of 5-hydroxymethyl-2-furfural using supported gold-copper
nanoparticles,” Green Chemistry, vol. 13, no. 8, pp. 2091–2099,
2011.
e author declares that there are no competing interests
regarding the publication of this paper.
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conditions for the preparation of highly active gold catalysts,”
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the support on the activity of gold catalysts for CO oxidation,”
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[20] D. W. Cameron and P. E. Schutz, “Dihydropyran derivatives
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Acknowledgments
Research reported in this publication was supported by the
University of Hail, Kingdom of Saudi Arabia (Grant no.
0150411).
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