Solvent controlled catalysis: Synthesis of aldehyde, acid or ester by selective oxidation of benzyl alcohol with gold nanoparticles on alumina

Article abstract of DOI:10.1016/j.apcata.2014.08.003

As shown herein, selectivity of aerobic oxidation of benzyl alcohol with gold nanoparticles dispersed on Al₂O₃ support is solvent-controlled. Using different solvents, the reaction was directed toward a variety of products: benzaldehyde, benzoic acid or benzoic esters were selectively produced from benzyl alcohol depending on the conditions. In toluene, benzaldehyde was produced with high selectivity, while quantitative conversion to benzoic acid was achieved in alkaline water. In methanol, methyl benzoate was produced with very high yield. Moreover, activity of the catalyst was dramatically improved without loss of selectivity in biphasic media; with a toluene/alkaline water system the highest TOF for benzoic acid synthesis was close to 5200 h^-1, selectivity being 98%.

Full text of DOI:10.1016/j.apcata.2014.08.003

Applied Catalysis A: General 485 (2014) 202–206
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Solvent controlled catalysis: Synthesis of aldehyde, acid or ester by
selective oxidation of benzyl alcohol with gold nanoparticles on
alumina
Sari Rautiainena, Olga Simakovab, Hongfan Guoa, Anne-Riikka Leinoc, Krisztián Kordásc,
Dmitry Murzinb, Markku Leskeläa, Timo Repoa,∗
a Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, A.I. Virtasen aukio 1, P.O. Box 55, 00014 University of Helsinki, Finland
b Laboratory of Industrial Chemistry and Reaction Engineering, Åbo Akademi University, Biskopsgatan 8, 20500 Turku, Finland
c Microelectronics and Materials Physics Laboratories, EMPART Research Group of Infotech Oulu, University of Oulu, P.O. Box 4500, 90014 Oulu, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
As shown herein, selectivity of aerobic oxidation of benzyl alcohol with gold nanoparticles dispersed
on Al2O3 support is solvent-controlled. Using different solvents, the reaction was directed toward a
variety of products: benzaldehyde, benzoic acid or benzoic esters were selectively produced from benzyl
alcohol depending on the conditions. In toluene, benzaldehyde was produced with high selectivity, while
quantitative conversion to benzoic acid was achieved in alkaline water. In methanol, methyl benzoate
was produced with very high yield. Moreover, activity of the catalyst was dramatically improved without
loss of selectivity in biphasic media; with a toluene/alkaline water system the highest TOF for benzoic
acid synthesis was close to 5200 h−1, selectivity being 98%.
Received 27 February 2014
Received in revised form 28 July 2014
Accepted 4 August 2014
Available online 12 August 2014
Keywords:
Solvent-controlled
Selective oxidation
Alcohol
© 2014 Elsevier B.V. All rights reserved.
Gold catalysts
Biphasic solvent
1. Introduction
As reported for benzyl alcohol oxidations, majority of nano-gold
catalysts in solvent free conditions favor selectivity toward benz-
The oxidation of alcohols to the corresponding carbonyl com-
pounds is a fundamental reaction of great industrial importance.
Beside conventional stoichiometric oxidants, catalytic processes
using environmentally friendly air or dioxygen are developed to
reduce both costs and environmental impact [1,2]. Gold-based
catalysis has become lately one of the key points of interest in this
area [3–5]. Due to their catalytic properties, gold and especially
supported gold nanoparticles are attracting increasing attention
in a variety of transformations [6–8]. In terms of green chem-
istry, catalysts based on gold nanoparticles are promising in alcohol
oxidation because they provide high catalytic activities and often
high selectivities [9]. The oxidation of primary alcohols is espe-
cially challenging as it can yield several products. Using different
gold catalysts, the production of aldehydes [10,11], carboxylic acids
[12,13] or esters [14,15] is reported. Being industrially relevant
compounds, a catalyst is focused and optimized for one of these
oxidation products.
aldehyde [11,14,16]. As shown for gold on mesoporous silica and
titania, similar high selectivity toward benzaldehyde occurs also in
nonpolar solvents like toluene and xylene [16,17].
Addition of water to Au/TiO2 markedly improves the catalytic
activity (TOF up to 300 h−1) but reduces selectivity and as con-
sequence only 67% selectivity to benzaldehyde is obtained [17].
Addition of base (Na2CO3) increases the activity of Au/TiO2 further
but the product distribution shifts and quantitative yield of ester,
benzyl benzoate, is reported. Further on, performing the oxidation
with Au/TiO2 in methanol and presence of a base, methyl benzoate
is produced in high yield [18]. Reports on the third appreciated
product, benzoic acid are rather sparse. High to quantitative yields
are obtained with polymer-stabilized nano-gold catalysts in aque-
ous K2CO3 or KOH solutions, respectively, but with low activities
[12,13].
In general, the activity and selectivity of gold catalysis are con-
sidered to depend on several factors including size of the gold
particles and the catalyst support. However, the marked examples
above underline the sensitivity of the catalyst selectivity also to
changes in reaction conditions. This opens a way to develop “one
for all” catalytic system; using one catalyst at different reaction
∗
Corresponding author. Tel.: +358 294150194; fax: +358 294150198.
E-mail address: timo.repo@helsinki.fi (T. Repo).
http://dx.doi.org/10.1016/j.apcata.2014.08.003
0926-860X/© 2014 Elsevier B.V. All rights reserved.

S. Rautiainen et al. / Applied Catalysis A: General 485 (2014) 202–206
203
products were analyzed using GC (Agilent 6890N) equipped with a
HP-Innowax column and a flame ionization detector. Molar selec-
tivity and conversion were determined using acetophenone as a
standard. GC-MS was used to identify the products. In control reac-
tions without the catalyst, no oxidation occurred without added
base. With 1 equiv. NaOH, 10% benzaldehyde was formed. No oxi-
dation of toluene was observed under our reaction conditions.
Turn-over frequencies (TOF) were calculated from the conversion
of the substrate using shorter reaction times and higher substrate
to gold ratios (total Au) than in the selectivity studies (see Table 2).
3. Results
Fig. 1. Oxidation of benzyl alcohol with Au/Al2O3 is solvent-controlled; depending
on the solvent the catalyst can produce either benzaldehyde, benzoic acid or methyl
benzoate with high selectivity and activity.
The Au/Al2O3 catalyst was prepared using direct ion exchange
method (DIE) which has the advantage of producing small gold par-
ticles with a narrow size distribution [21]. This type of catalyst has
previously been studied, e.g., in oxidation of carbohydrates [21],
hydroxymatairesinol [22] and carbon monoxide [23,24]. Herein,
the Au/Al2O3 catalyst is studied in the oxidation of benzyl alcohol
in various solvents with and without the addition of base.
Water is an attractive solvent for catalytic transformations, but
first experiments with 0.38 mol% Au/Al2O3 showed that the oxida-
tion of benzyl alcohol is unselective in pure water (Table 1, entry 1).
Additional bases can be beneficial for the oxidation [25], so in order
to improve productivity and selectivity of the catalyst, the effect of
bases was studied. The addition of K2CO3 resulted in full conver-
sion and the subsequent oxidation of benzaldehyde to benzoic acid
(Table 1, entry 2). The optimal amount of K2CO3 was 50 mol%, as
lower and higher amounts resulted in both decreased selectivity
and conversion. Under these conditions benzoic acid was formed
with 82% yield along with a significant amount of benzyl benzoate
as a side product (18%). The addition of NaOH instead of K2CO3 had
even higher positive effect on the reaction; equimolar ratio of NaOH
to the alcohol gave quantitative conversion to benzoic acid in one
hour (entry 3).
Along with excellent selectivity, the presence of base markedly
improved the catalyst activity; the highest activity in water
(TOF = 3069 h−1) was achieved using NaOH (Table 2, entry 3). This
is nearly 10-fold increase of the TOF value in comparison to the
results in pure water. To study the differences in activity in dif-
ferent solvents, higher substrate to gold ratio and shorter reaction
times were used for TOF value measurements (Table 2) than in the
selectivity studies presented in Table 1.
When the solvent was changed from water to toluene, Au/Al2O3
produced benzaldehyde with 89% selectivity and with increased
activity (Tables 1 and 2, entry 4 vs. 1). Similar activities were
reported for gold on mesoporous silica in toluene (364 h−1) and
solvent-free oxidation (377 h−1) [16]. The effect of alkali was stud-
ied by adding solid K2CO3 or NaOH to the toluene solution. Like
in water, the addition of base increased the catalytic activity
conditions to selectively switch from one reaction product to
another. Already known example is the epoxidation of cyclohex-
ene with Au/C and its dependence on the solvent [19]. Inspired by
this idea we focus on developing “one for all” Au/Al2O3 catalyst
to convert benzyl alcohol selectively to corresponding aldehyde,
carboxylic acid and esters (Fig. 1).
2. Experimental
According to our preliminary studies with different catalysts,
the most interesting results were obtained with Au/Al2O3. The
Au/Al2O3 catalyst was prepared with direct ion exchange method
[20]. An aqueous solution of HAuCl4 (99.9%, ABCR) of concentra-
tion 5 × 10−4 M was prepared corresponding to final Au loading of
2 wt%. The solution was heated to 70 ◦C and powdered Al2O3 (UOP,
A-201, SBET = 200 m2/g) was added. The slurry was mixed for 1 h,
washed with 100 ml 4 M ammonia for 1 h, filtered, dried overnight
at 70 ◦C and calcined in air at 300 ◦C for 4 h. Based on transmis-
sion electron microscopy (TEM) the average gold particle size was
1.0 0.3 nm and dispersion 77% [21]. Actual gold loading of the cat-
alyst was 1.5 wt% according to ICP determination. Detailed catalyst
characterization is reported in Ref. [21].
The oxidation experiments were carried out in glass liners
loaded into a pressurized steel autoclave. For a typical run,
0.97 mmol of benzyl alcohol, 0.38 mol% Au/Al2O3 (48 mg) and the
solvents were measured into the glass liners. Additional base was
added as a solid when only organic solvent was used. The auto-
clave was pressurized with 10 bar oxygen, heated to 100 ◦C and the
mixtures were stirred with magnetic stirring at 1000 rpm.
After the reaction the autoclave was depressurized and the
mixtures were extracted twice with ethyl acetate. After the
first extraction the reaction mixtures were acidified with HCl to
transfer benzoate salt to the organic phase as benzoic acid. Cen-
trifuge (3000 rpm, 2 min) was used to separate the phases. The
Table 1
Oxidation of benzyl alcohol in different solvents with 0.38 mol% Au.
Entry
Solvent
Time (h)
Base (mol%)
Conversion (%)
Selectivity (%)
Benzaldehyde
Benzoic acid
Benzyl benzoate
1
2
3
4
5
6
Water
Water
Water
Toluene
Toluene
Toluene
Biphasic
Biphasic
Biphasic
2
2
1
2
2
1
2
2
–
50
100
100
86
100
100
79
53
0
0
89
57
29
86
0
23
82
100
9
40
66
11
90
98
24
18
0
2
3
5
3
10
2
K2CO3 (50)
NaOH (100)
–
K2CO3 (50)
NaOH (100)
–
7a
8a
9a
K2CO3 (50)
NaOH (100)
100
100
0.5
0
Reaction conditions: 0.97 mmol benzyl alcohol, 100 ◦C, 10 bar O2, 0.38 mol% Au/Al2O3 (48 mg), 2 ml solvent.
a
1 ml toluene and 1 ml water.

204
S. Rautiainen et al. / Applied Catalysis A: General 485 (2014) 202–206
Table 2
Turn-over frequencies of benzyl alcohol oxidation in different solvent with reduced catalyst loading.a,b
.
Entry
Solvent
Base (mol %)
Conversion (%)
Main product
Selectivity (%)
TOF (h−1
)
1a
2b
3b
4a
5b
6b
7a
8b
9b
Water
Water
Water
Toluene
Toluene
Toluene
Biphasic
Biphasic
Biphasic
–
27
52
58
35
45
79
27
93
97
Benzaldehyde
Benzoic acid
Benzoic acid
Benzaldehyde
Benzaldehyde
Benzoic acid
Benzaldehyde
Benzoic acid
Benzoic acid
67
45
95
94
86
58
90
49
93
361
2747
3069
471
2376
4228
360
K2CO3 (50)
NaOH (100)
–
K2CO3 (50)
NaOH (100)
–
K2CO3 (50)
NaOH (100)
4967
5188
Reaction conditions: 0.97 mmol benzyl alcohol (BA), 100 ◦C, 10 bar O2, 2 ml solvent.
a
30 min reaction, 0.15 mol% Au/Al2O3.
15 min reaction, 0.08 mol% Au/Al2O3.
b
TOF value calculated from the conversion: molBA*conv./(molAu total*hours).
Fig. 2. The effect of the volume ratio of solvents on oxidation of benzyl alcohol using 0.38 mol% Au/Al2O3 (a) without base in 2 h reaction and (b) with 1 equiv. of NaOH in
15 min reaction.
remarkably (entries 5–6). However, more benzoic acid formed as
a side product which makes the addition of base unfavorable in
toluene.
achieved already in 30 min (entry 9). Also, the increase in the cat-
alytic activity is remarkable; the TOF values measured in alkaline
biphasic media were higher than in either solvent alone, and under
optimized conditions the highest TOF value (5188 h−1) in this work
was achieved (Table 2, entry 9 vs. 3 and 6). Compared to alkaline
water, the increase in activity was notable already at 25 vol% of
toluene, though the optimal solvent ratio was found to be 1:1 in
terms of both conversion and selectivity (Fig. 2b).
When the oxidation was carried out using aliphatic alcohols as
solvents, oxidative esterification of benzyl alcohol with the sol-
vent to the corresponding benzoic esters occurred. In methanol
without any additive, methyl benzoate formed with 49% selectivity
(Table 3, entry 1). It has been shown, that the addition of 10 mol%
of sodium methoxide to the reaction to increases the selectivity to
benzoic ester [18]. Instead of sodium methoxide, we used NaOH
as such to deprotonate the alcohol in situ. The addition of 5 mol%
NaOH resulted in very high selectivity to methyl benzoate (entry
2). The oxidative esterification of benzyl alcohol was also stud-
ied in ethanol, isopropanol and cyclohexanol. With these alcohols
longer reaction times were needed to reach high conversions, and
In further experiments we studied the behavior of the Au/Al2O3
catalyst in biphasic water-toluene systems. Ideally, aqueous
biphasic systems have the advantage of combining water, a cheap
and safe solvent, with the good solvation properties of organic sol-
vents [26]. Compared to the reaction in pure water, already 25 vol%
of toluene in the solvent significantly increased the conversion and
selectivity toward aldehyde (Fig. 2a). The use of more toluene in
the mixture resulted only in minor improvement both in the selec-
tivity and conversion. In a 1:1 mixture of water and toluene, benzyl
alcohol was oxidized with 79% conversion and 86% selectivity to
benzaldehyde (Table 1, entry 7). In general, the oxidation results
in biphasic solvent resemble the results in pure toluene (Table 1,
entry 4 vs. 7).
When K2CO3 (50 mol%) was added into 1:1 mixture of water
and toluene, selectivity toward benzoic acid was 90%, which is
higher than in water (Table 1, entry 8 vs. 2). Notably, with equimo-
lar amount of NaOH, very high selectivity to benzoic acid 98% was
Table 3
Oxidative esterification of benzyl alcohol in alcohol into corresponding benzoic esters.
Entry
Time (h)
Solvent
Base (mol%)
Conversion (%)
Selectivity (%)
1a
2a
3
4
5
2
1
20
20
20
20
Methanol
Methanol
Ethanol
Ethanol
Isopropanol
Cyclohexanol
0
100
100
90
100
73
49
96
NaOH (5)
NaOH (5)
NaOH (20)
NaOH (5)
NaOH (5)
70b
81b
23c
16d
6
77
Reaction conditions: 0.97 mmol benzyl alcohol, 80 ◦C, 10 bar O2, 0.38 mol% Au/Al2O3, 2 ml solvent.
a
Temperature 100 ◦C, selectivity to methyl benzoate.
Selectivity to ethyl benzoate.
Selectivity to isopropyl benzoate.
Selectivity to cyclohexyl benzoate.
b
c
d

S. Rautiainen et al. / Applied Catalysis A: General 485 (2014) 202–206
205
b) 100
a) 100
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
Conversion
Aldehyde
Acid
Conversion
Aldehyde
Acid
Ester
Ester
0
0.5
1
1.5
2
0
0.5
1
1.5
2
Reaction time (h)
Reaction time (h)
Fig. 3. The effect of time on oxidation of benzyl alcohol with 0.38 mol% Au/Al2O3 in biphasic solvent (a) without base and (b) with 1 equiv. of NaOH. The data points represent
separate reactions.
high selectivity was achieved only in ethanol with the addition of
20 mol% NaOH (entries 3–6).
In addition, as illustrated in Fig. 4b, base prevents the agglom-
eration of the Au/Al2O3 catalyst in aqueous media. As a result, the
addition of base has significant effect on both the product distribu-
tion and the activity of the catalyst, and over 10-fold increases in the
activity were observed. Of the two bases studied, NaOH increased
both selectivity and activity of the oxidation more than K2CO3. With
NaOH, the selectivity toward benzoic acid was very high already
at incomplete conversion, indicating that the oxidation of benz-
aldehyde to benzoic acid is very fast (Table 2, Figs. 2b and 3b).
With K2CO3 however, longer reaction time was needed to achieve
high selectivity to acid (Table 1 and Figure S1). Also, formation of
benzyl benzoate as a side product was more pronounced with car-
bonate, which has also been reported with steel fiber supported
gold catalysts [4]. The addition of base also prevents benzoic acid
from poisoning the catalyst [25]. The alumina surface is nega-
tively charged, as pH is above the point of zero charge of the
support [33].
Remarkably, the highest oxidation activities were observed by
using alkaline biphasic solvent. Under the applied conditions, the
catalyst is evenly dispersed in the aqueous phase (Fig. 4a). The sig-
nificantly improved activity compared to alkaline water could be
accounted partly for the increased solubility of oxygen in toluene
as well as the better dispersion of the catalyst in the aqueous
phase.
4. Discussion
As shown here, the solvent and the presence of additional base
have a crucial role in the Au/Al2O3 catalyzed oxidation of benzyl
alcohol. In water, low selectivity toward benzaldehyde (53%) is
obtained due to over-oxidation into benzoic acid and subsequent
reaction to benzoic benzoate. However, in nonpolar organic sol-
vents over-oxidation is markedly reduced and the catalyst favors
selectivity toward aldehyde [27]. In toluene, both the catalyst activ-
ity and selectivity toward benzaldehyde are higher than in water.
As the oxidation passes through a polar transition state, the non-
polar solvent does not necessarily enhance the reaction rate, but the
higher activity observed herein is most probably related to higher
solubility of oxygen and benzyl alcohol in toluene than in water
[28,29]. In biphasic toluene/water solution the selectivity of the cat-
alyst toward benzaldehyde (86%) is similar to that in toluene and
thus considerably higher than in water. Selectivity toward aldehyde
is high throughout the reaction (Fig. 3a). The product is more solu-
ble in toluene than in water, and also the catalyst is dispersed in the
organic phase (Fig. 4a), which might account for the low amount of
side products.
In the case of oxidative esterification with aliphatic alcohols, the
reaction is proposed to proceed through hemiacetal formation as
opposed to condensation between carboxylic acid and the solvent
alcohol [34]. In our studies, no benzoic acid was detected which
supports the hemiacetal pathway.
The additional base has several roles in the oxidation [30].
In the first step of the catalytic cycle, hydroxide ion assists the
dehydrogenation of the alcohol [27]. Also, the subsequent oxida-
tion of benzaldehyde was promoted leading to very high yields of
benzoic acid in aqueous and biphasic solvents. This step is pro-
posed to proceed through a geminal diol formed by reaction of
the aldehyde with hydroxide from water [31]. Thus increasing the
hydroxide ion concentration results in increased acid formation.
However, the presence of water is essential for high selectivity
toward acid. As reported by Davis et al. on the oxidation of 5-
hydroxymethylfurfural, the oxygen atoms present in the produced
acid are from water, and not dioxygen [32]. Based on our results,
similar mechanism can be proposed here.
5. Conclusions
In the present work, we have introduced a new “one for all”
concept for selective oxidation of benzyl alcohol into a variety
of the products. The Au/Al2O3 catalyst, prepared by direct ion
exchange, is versatile and able to oxidize benzyl alcohol selectively
into benzaldehyde, benzoic acid or benzoic esters, depending on
the reaction media used. Moreover, in the present work it was
shown that the use of biphasic alkaline media instead of a single sol-
vent can remarkably enhance the catalytic activity of the Au/Al2O3
system.
Acknowledgement
Fig. 4. Au/Al2O3 after the reaction (a) in biphasic solvent without and with 50 mol%
K2CO3, from left to right; (b) in water without and with 50 mol% K2CO3, from left to
right.
Academy of Finland is gratefully acknowledged for funding
(project no. 124187).

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