Promoting effect of water for aliphatic primary and secondary alcohol oxidation over platinum catalysts in dioxane/aqueous solution media

Article abstract of DOI:10.1016/j.cattod.2011.02.058

In the selective oxidation with air of 1-octanol and 2-octanol in 1,4-dioxane at 100 °C and 10 bar in the presence of carbon supported platinum catalysts, the catalytic activity could be impressively boosted by substitution of pure dioxane by increasing amounts of water. Changing the polarity of the solvent strongly influences the adsorption equilibrium of substrates and products at the catalyst surface and hence plays an influential role on the reaction rate.

Full text of DOI:10.1016/j.cattod.2011.02.058

Catalysis Today 173 (2011) 81–88
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Promoting effect of water for aliphatic primary and secondary alcohol oxidation
over platinum catalysts in dioxane/aqueous solution media
Antonio Frassoldati, Catherine Pinel, Michèle Besson∗
IRCELYON (Institut de recherches sur la catalyse et l’environnement de Lyon), UMR5256 CNRS-Université Lyon1, 2 avenue Albert Einstein 69626, Villeurbanne Cedex, France
a r t i c l e i n f o
a b s t r a c t
In the selective oxidation with air of 1-octanol and 2-octanol in 1,4-dioxane at 100 ◦C and 10 bar in the
presence of carbon supported platinum catalysts, the catalytic activity could be impressively boosted by
substitution of pure dioxane by increasing amounts of water. Changing the polarity of the solvent strongly
influences the adsorption equilibrium of substrates and products at the catalyst surface and hence plays
an influential role on the reaction rate.
Article history:
Received 17 December 2010
Received in revised form 23 February 2011
Accepted 28 February 2011
Available online 9 April 2011
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Selective oxidation
Air
Aliphatic alcohols
Supported platinum catalysts
Water effect
1. Introduction
Pd nanoclusters (1 mol.%) were able to oxidize 1-phenylethanol
to completion in water at 100 ◦C after 6 h, but 2-octanone yield
Liquid phase oxidation of alcohols is an important transfor-
mation since the corresponding aldehyde, ketone or carboxylic
derivatives serve as important and versatile intermediates for the
synthesis of fine and specialty chemicals [1–3]. For environmental
and economic significance, use of molecular oxygen as a green, effi-
cient and selective oxidant has received growing attention; water
is then the only by-product of the process. Accordingly, many
homogeneous [3–9] and heterogeneous [10–41] catalyzed selec-
tive oxidations of alcohols, using molecular oxygen as oxidant, have
been reported. Because of their facile handling, separation and recy-
cling, solid catalysts, in particular a wide range of supported and/or
immobilized noble metals [for reviews, see 14,39–41,49] attracted
much attention. Various supported platinum [15,16,19–21], palla-
dium [22–24], ruthenium [13,25–28], gold [29–37], Pd–Au [36,37]
or Pt–Au [42,43] catalysts have been proposed, in some cases with
modifier (such as Bi) [44,45]. Also, differently stabilized soluble
metal nanoparticles are in development as efficient catalysts in
water [29,50–54] or aqueous/organic media [55].
from 2-octanol under the same conditions was only 7% after
24 h [51]. Over a silver-based catalyst, different alcohols could
be converted with high conversion and selectivity at room tem-
perature under molecular oxygen atmosphere while 2-octanol
afforded only moderate yields of 2-octanone [56]. In the pres-
ence of polymer-incarcerated, carbon-stabilized gold nanoclusters
(1 mol.%), whereas several aromatic and allylic alcohols were oxi-
dized smoothly to afford the corresponding ketones in quantitative
yields, secondary aliphatic alcohols were less reactive [57]. A
Ru/MnOx/CeO2 catalyst showed high activity at room tempera-
ture and atmospheric O2 for the oxidation of different alcohols
in trifluorotoluene, and 2-octanol was oxidized to 2-octanone in
83% yield. However, longer reaction time than other activated
benzylic and allylic alcohols were required [58]. Uozumi et al.
[52,53] prepared amphiphilic polystyrene-polyethylene glycol (PS-
PEG) resin-supported nanoparticles of palladium and platinum
which catalyzed oxidation of alcohols in refluxing water at 100 ◦C
with atmospheric pressure of molecular oxygen. In the presence
of Pd (1 mol.%), secondary ␣-arylated alcohols, such as 1-phenyl-
ethanol, diphenylmethanol, 1-hydroxyindane gave acetophenone,
benzophenone, and indanone in 99, 85, and 95% yield, respectively.
In contrast, oxidation of alicyclic and aliphatic alcohols was inade-
quate [52]. On the other hand, using the Pt nanocatalyst, 2-, 3- and
4-octanol were efficiently converted to the corresponding ketones
in 81–93% yield [53]. This is also the case if Pt nanoclusters syn-
thesized via reduction by glycol and stabilization by PV are used.
They showed high efficiency to oxidize 2-octanol in water at 80 ◦C
In the class of alcohols surveyed, non-activated aliphatic
alcohols are usually much more demanding substrates for this
kind of reaction compared with aromatic or allylic alcohols
[15,25,46–48,51–54,56–58]. The following few examples illustrate
the poor reactivity of such alcohols. Microgel-stabilized soluble
∗
Corresponding author. Tel.: +33 0 472445358; fax: +33 0 472445399.
E-mail address: michele.besson@ircelyon.univ-lyon1.fr (M. Besson).
0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.02.058

82
A. Frassoldati et al. / Catalysis Today 173 (2011) 81–88
under 1 bar O2 with a 94.2% yield of 2-octanone after 24 h and were
stable [54]. Pd nanoparticles encapsulated in a hollow porous car-
bon sphere or Pd supported catalysts did not show any catalytic
activity for oxidation of 1-octanol in K2CO3 aqueous solution at
80 ◦C, while excellent catalytic activity was observed for oxidation
of various primary benzylic or allylic alcohols [59].
the potential of classical platinum supported on activated carbon
catalysts.
2. Experimental
2.1. Catalyst preparation
As concerns the solvent, water is the preferred one. It has
been widely used for water-soluble substrates, in particular for
carbohydrates [36,37]. However, most alcohols, among them
octyl alcohols, are poorly soluble in water even at high tem-
peratures or very few catalysts are active for a wide range of
alcohols in water. Therefore, many experiments in the literature
for hydrophobic substrates were achieved in the presence of
organic flammable solvents (toluene, xylene, trifluorotoluene,
acetonitrile, cyclohexane. . .) which ensure dissolution of the
substrate [15,23,60]. Excellent properties of various catalysts were
shown. The experiments in organic solvents are usually performed
at lab-scale in very small volumes of reactor (typically only a few
mL solvent). However, despite these progresses, there are still
serious safety issues associated with the use of dioxygen with these
oxidisable organic solvents [61]. A solution proposed to overcome
the low water solubility was the use of an amphiphilic insoluble
polymer support to achieve a local favourable environment for
adsorption of the alcohol [52,53]. Supercritical CO2 has also been
proposed as an alternative solvent [62–66]. Thus, over Pd/Al2O3,
2-octanol (5 mol.%) was oxidized to 2-octanone in almost 100%
selectivity at 120 ◦C under 95 bar total pressure and a contact time
of 1.02 g h mol−1 in a continuous reactor. However, conversions
were moderate because of the difficulties to dissipate the heat
produced by the exothermic reaction [62]. Also, in a miniature
reactor, 75% yield of 2-octanone was achieved with a > 90% mass
balance over a 5 wt.%Pt–1 wt.%Bi/Al2O3 catalyst [66].
The supported metallic catalysts were prepared on a synthetic
carbon from MAST Carbon Technology Ltd. The mesoporous car-
bon was prepared by carbonisation at 800 ◦C of a porous polymeric
resin prepared from a phenolic resin and hexamethylenetetramine
in ethyleneglycol and activation using CO2 at 850 ◦C. The catalysts
were prepared by impregnation of the carbon with an aqueous solu-
tion of hexachloroplatinic acid followed by liquid phase formalde-
hyde reduction [20,21]. The aqueous solutions of the precursors
used for the preparation were in the appropriate concentrations to
obtain Pt loadings of 3–5 wt.%. The Pt–Bi catalysts were prepared by
redox surface reaction from an aqueous BiONO3 glucose solution,
according to previously reported procedure [11]. By this way, Bi is
directly and preferentially deposited on the noble metal.
2.2. Characterization of materials
The surface area and pore volume of the carbon were deter-
mined from analyses of nitrogen gas adsorption isotherms at
−196 ◦C performed with a Micromeritics ASAP 2020. The sample
was degassed at 350 ◦C for 3 h prior to analysis. X-ray diffrac-
tion characterizations were performed using a Siemens D5005
diffractometer with CuK␣ radiation at 0.154184 nm. Transmission
electron microscopy (TEM) direct observations of the catalysts
were conducted using a JEOL 2010 microscope (200 kV, resolution
0.19 nm).
The nature of the solvent on the performances of the cat-
alyst for alcohol oxidation may be important and has scarcely
been studied. With a Ru/C catalyst (5 mol.%) at 50 ◦C under atmo-
spheric molecular oxygen, 1-phenyl-1-pentanol was for instance
efficiently oxidized in toluene to valerophenone (93% conversion
after 24 h), whereas the conversion was moderate in acetoni-
trile (57% conversion) and very low in t-BuOH or DMSO (13 and
4%, respectively) [67]. High throughput screening of platinum
group metal catalysts for the selective oxidation of several alco-
hols under mild conditions (<70 ◦C and <5 bar) concluded that
toluene and xylene were the most effective solvents [15]. In the
oxidation of 2-octanol with a Pt–Bi/C catalyst, the reaction rate
was very dependent on the composition of dioxane/heptane mix-
tures [68]. It was also demonstrated that water may play an
influential role. The catalytic activity in the oxidation of benzy-
lalcohol in xylene over Au/TiO2 [69] or in toluene over carbon
nanotube-supported Ru catalysts [70] could be significantly pro-
moted by water. In these multiphase reaction systems, the solid
catalyst formed emulsion droplets at the interface of the two liquid
phases.
Dioxane is very hydrophilic and a water-miscible organic sol-
vent largely used in oxidation reactions in the fine chemicals and
pharmaceuticals industry. We previously reported on the use of
dioxane as solvent for the oxidation of benzyl alcohol derivatives
using Pt/C catalysts [20,21]. By changing the solvent from diox-
ane to dioxane/alkaline aqueous solution mixtures, it was possible
to tune the selectivity of benzylalcohol derivatives to the alde-
hyde or to the carboxylic acid. Under the conditions used (<100 ◦C,
<10 bar air), this solvent did not undergo oxidations and no solvent-
derived by-products were observed. The reaction rate of oxidation
of the alcohol was also greatly enhanced. In line with our previous
work [20,21] we now present the catalytic oxidation of 1-octanol
and 2-octanol selected as model substrates for aliphatic alcohols
in dioxane and dioxane/water solvent with the aim to improve
2.3. Catalyst evaluation in the aerobic oxidation of alcohols
The reaction tests were performed under a set of realistic con-
ditions in a stirred autoclave reactor of 300 mL made of Hastelloy
in batch mode equipped with a magnetically driven stirrer set. In a
typical oxidation reaction, the reactor was loaded with a mixture of
alcohol (15 mmol) and solvent (150 mL) and the supported catalyst,
to get a 1 mol.% Pt with respect to the alcohol. After purging with
argon, the reactor was heated to the desired reaction temperature,
and then air was introduced up to the pressure of 10 bar. Efficient
stirring was started and this time was considered to be zero time
for the reaction.
Liquid samples were periodically withdrawn from the reactor
and analysed along the reaction course. The quantitative analysis
of the reactants and products was carried out by Gas Chromatog-
raphy (GC) equipped with a Flame Ionisation Detector (FID) and
a DBWax column (30 m × 0.25 mm, 0.25 ␮m film thickness) using
acetophenone as external standard. In the case of high amounts of
water in dioxane which do not ensure good solubility of the sub-
strate and the products, only the final solution was analysed; at the
end of the reaction, dioxane was added to the reaction medium, the
catalyst was filtered off and the sample was analysed. Recoveries
were always very good with this procedure. In some experiments,
the isolated yield was calculated. After the given reaction time, the
reaction solution was transferred into a separation funnel, diluted
with water (25 mL), and extracted with an equal volume of EtOAc
(3 aliquots of 25 mL). The organic layer was analysed by 1H NMR to
determine the content in reactant and products.
1H NMR data were conform to the literature:
2-Octanol (1H NMR, 250 MHz, CDCl3): ı 3.80 (m, 1 H); ı 1.83 (s,
1 H); ı 1.40 (m, 10 H); ı 1.18 (d, 3 H, Jab = 6.1 Hz); ı 0.86 (t, 3 H,
Jab = 6.2 Hz).

A. Frassoldati et al. / Catalysis Today 173 (2011) 81–88
83
2-Octanone (1H NMR, 250 MHz, CDCl3): ı 2.40 (t, 2 H, Jab = 6.8 Hz);
ı 2.13 (s, 3 H); ı 1.57 (m, 2 H); ı 1.30 (m, 6 H); ı 0.86 (t, 3 H,
Jab = 6.7 Hz).
1-Octanol (1H NMR, 250 MHz, CDCl3): ı 3.61 (t, 2 H, Jab = 6.7 Hz);
ı 1.77 (s, 1 H); ı 1.60 (m, 2 H); ı 1.26 (m, 10 H); ı 0.86 (t, 3 H,
Jab = 6.5 Hz).
Octanal (1H NMR, 250 MHz, CDCl3): ı 9.76 (t, 1 H, J1 = 2.1 Hz); ı
2.42 (td, 2 H, J1 = 2.1 Hz, J2 = 6.7 Hz); ı 1.63 (m, 2 H); ı 1.30 (m, 8
H); ı 0.87 (t, 3 H, J2 = 6.7 Hz).
Octanoic acid (1H NMR, 250 MHz, CDCl3): ı 11.6 (s, 1 H); ı 2.34 (t,
2 H, Jab = 6.7 Hz); ı 1.63 (m, 2 H); ı 1.29 (m, 8 H); ı 0.88 (t, 3 H,
Jab = 6.7 Hz).
The platinum contents in the reaction medium were checked by
inductively coupled plasma-optical emission spectroscopy (ICP-
OES) analysis of the filtrates at the end of the reactions. All measures
were below the detection limit (<0.1 ppm), showing no leaching of
the catalyst. Similarly, when Bi promoted catalysts were used, no
leaching of this metal was detected.
3. Results and discussion
3.1. Characterization of the platinum catalysts
The specific surface area of the support was 1265 m2 g−1 with
average pore sizes of 3.4 and 11 nm.
Several batches of Pt catalysts with different loadings in the
range of 2.5–4.7 wt.% were prepared during this study. XRD of all
the solids demonstrated that no diffraction peak characteristic of
the Pt phase could be observed, indicating that the Pt crystallites
were probably well dispersed. Fig. 1 shows typical TEM images of
a 2.8%Pt/C. The sample consisted of relatively small Pt particles
homogeneously distributed throughout the carbon surface with a
metal particle size around 2 nm. Some aggregates of these small
particles were also observed.
3.2. Influence of the solvent in the oxidation of 1-octanol and
2-octanol in the presence of Pt/C catalyst
The catalytic activity of the Pt catalysts was investigated in the
oxidation of 1-octanol at 100 ◦C and 10 bar air. The tests were con-
ducted with a substrate/metal ratio of ca. 100. Fig. 2 shows the
concentrations vs. reaction time profiles for experiments in dioxane
(a), in dioxane/water 90%/10% (b) and 50%/50% (c), using a Pt/C cat-
alyst. 1-octanal and 1-octanoic acid were the sole products under
the present reaction conditions. It was observed that no signifi-
cant conversion occurred during heating under inert atmosphere
(time < 0).
Fig. 1. TEM representative images of 2.8%Pt/C catalyst.
The results clearly show that catalyst activity is strongly depen-
dent on the nature of the solvent.
des is expected, screening of the literature suggests that Ru-based
catalysts would be a better option [15,45].
In dioxane (Fig. 2a), the catalyst was very active initially and
the conversion reached 36% within one hour. Then, the reaction
rate slowed down progressively and the conversion stopped at
ca. 60% after 6–8 h of reaction, indicating severe deactivation of
the catalyst. Under these conditions, both the aldehyde and the
acid were formed from the beginning of the reaction, and the final
selectivity was slightly in favour of the aldehyde (54%). Previously,
we have shown that the use of dioxane as solvent for substituted
benzylalcohol afforded the aldehyde very selectively with com-
plete conversion [21,22]. This tendency toward further oxidation
of the aldehyde was expected, because of the easier hydration of
the aliphatic aldehyde and formation of the gem-diol compared to
an aromatic aldehyde. Water formed in the reaction can react with
the aldehyde to form very easily an aldehyde hydrate, which further
reacts to give acid. Indeed, when high selectivity to the aldehy-
When dioxane/water 90%/10% and 50%/50% (v/v) were
employed as the solvent, the initial reaction rate of oxidation of
1-octanol was clearly greatly enhanced (Fig. 2b and c). Conversion
after one hour was 73% and 67%, respectively, compared to 36% in
pure dioxane. However, a severe deactivation was also observed
and 1-octanol conversion was rapidly limited to ca. 70%. The selec-
tivity of each oxidation reaction was also determined. We noted
that while the major product in dioxane was 1-octanal, the major
product was 1-octanoic acid in both dioxane/aqueous solutions.
The selectivity to octanoic acid shifted from 46% in dioxane to 69% in
dioxane/water 50%/50% in the final solutions. Water can react with
the aldehyde to form the aldehyde hydrate, which further reacts to
form 1-octanoic acid. Note that octyloctanoate was never detected
and that no total selectivity toward the aldehyde was obtained at
that point of the study.

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A. Frassoldati et al. / Catalysis Today 173 (2011) 81–88
0.12
0.12
0.1
a
0.1
0.08
0.06
0.04
0.02
0
0.08
0.06
0.04
0.02
0
-1
0
1
2
3
4
5
6
7
8
-1
0
1
2
3
4
5
6
7
8
time (h)
time (h)
0.12
0.1
Fig. 3. Promotion effect of water on the catalytic oxidation of 2-octanol to
2-octanone in dioxane/water mixtures in the presence of 2.8%Pt/C. Reaction condi-
tions: 2-octanol (0.1 mol L−1), Pt/C (1 mol.% Pt), solvent (150 mL), 100 ◦C, 10 bar, (᭹)
dioxane, (ꢁ) dioxane/water 75%/25%, (ꢂ) dioxane/water 66%/34%, (ꢀ) dioxane/water
50%/50%.
b
0.08
0.06
0.04
0.02
0
An experiment was also performed, using a dioxane/water
25%/75% ratio. However, under these conditions, total solubility
of 2-octanol was not obtained and the reaction was performed in
biphasic conditions without following the concentration profiles.
After 1 h, after cooling and depressurization, dioxane was added to
the reaction medium to obtain complete solubilization. Analysis of
the final solution demonstrated total conversion of 2-octanol and
total selectivity to the ketone, which suggests that oxidation can be
carried out even if the reactant is not totally soluble. The higher the
water content, the higher the activity.
-1
0
1
2
3
4
5
6
7
8
time (h)
0.12
0.1
c
To deeper understand the possible role of water on the catalytic
activity and the deactivation of the catalyst, another set of experi-
ments was performed at the lower temperature of 60 ◦C. The effect
of substitution of a fraction of dioxane with water and the influence
of sodium hydroxide addition was also examined.
The results for 1-octanol are illustrated in Fig. 4. As expected a
decrease of the reaction temperature leads to a strong decrease in
catalyst activity, but the reaction could go up to complete conver-
sion. In dioxane/water 50%/50% with addition of 0.03 equiv. base,
although a reaction time of 25 h was needed at that temperature,
conversion was total with a complete selectivity to the carboxylic
acid. However, a significant lower activity was obtained with the
addition of a too large amount of NaOH (0.5 equiv.). As observed
at 100 ◦C, the replacement of some dioxane by water progressively
shifted the selectivity from the aldehyde to the acid. After 1 h, the
selectivity to the aldehyde was 90%, 68%, 54% and 6%, in diox-
ane, dioxane/water 50%/50% or dioxane/alkaline aqueous solutions,
0.08
0.06
0.04
0.02
0
-1
0
1
2
3
4
5
6
7
8
time (h)
Fig. 2. Oxidation of 1-octanol in dioxane (a), dioxane/water 90%/10% (b) and
50%/50% (c) in the presence of 2.9%Pt/C (a) or 4.7%Pt/C (b and c). Reaction conditions:
150 mL 0.1 mol L−1 1-octanol, 100 ◦C, 10 bar air, 1-octanol/Pt molar ratio = 100, (ꢀ)
1-octanol, (ꢁ) 1-octanal, (ꢂ) 1-octanoic acid.
Similarly, the oxidation of 2-octanol to 2-octanone was carried
out in dioxane and in dioxane/water mixtures. The effect of water
on the catalytic activity of the platinum catalyst is shown in Fig. 3.
It appears from these data that addition of water to dioxane
also plays a key role in the oxidation rate of 2-octanol. In dioxane,
the Pt/C catalyst showed low activity and it afforded a conversion
of only 29% within 6 h. When a dioxane/water 66%/34% mixture
was used, a 60% conversion was achieved. A further increase in the
water concentrations to 50 vol.% led to a jump to 100% after only
1 h reaction. In all these experiments, the reaction gave rise to the
formation of 2-octanone in quantitative yields. No other products
beside 2-octanol, 2-octanone and the internal standard could be
detected by GC. The purity of the product was checked by 1H NMR
analysis.
The reaction profiles in dioxane or dioxane/water mixtures for
2-octanol are slightly different from the one described by Moun-
zer et al. [68]. Upon oxidation of 2-octanol in heptane or xylene
(0.248 M, 3 g of 5%Pt–1%Bi/C, 70 ◦C, atmospheric pressure), the
reaction rate decreased dramatically shortly after the start of the
reaction.
0.12
0.1
0.08
0.06
0.04
0.02
0
0
5
10
15
20
25
30
time (h)
Fig. 4. Oxidation of 1-octanol in dioxane/water solvent in the presence of Pt/C at
60 ◦C. Reaction conditions: 1-octanol 0.1 mol L−1, Pt 1 mol.%, 150 mL solvent, 10 bar,
(᭹) dioxane, (ꢂ) dioxane/water 50%/50%, (ꢀ) dioxane/water 50%/50% and 0.03eq.
NaOH, (ꢃ) dioxane/water 50%/50% and 0.5 eq. NaOH.

A. Frassoldati et al. / Catalysis Today 173 (2011) 81–88
85
0.12
0.1
oxidation. Further experiments were performed using the diox-
ane/water 50%/50% solvent and adding two different quantities of
NaOH (0.03 equiv. or 0.5 equiv. based on 1-octanol initial concen-
tration). The conversion profiles of 1-octanol were not significantly
modified (Fig. 4). The selectivity to the acid was still higher (>90%),
since the hydration of the aldehyde is favoured under basic condi-
tions.
0.08
0.06
0.04
0.02
0
Another source of deactivation is decarbonylation of the car-
bonyl formed upon alcohol oxidation, affording adsorbed CO and
a hydrocarbon residue. Adsorbed CO or hydrocarbon residues are
likely responsible for site blocking and deactivation. Many stud-
ies have demonstrated this possible decarbonylation over Pd or
Pt, either using model catalysts [71,72] or real powder catalysts
[73]. Applying attenuated total reflection (ATR) spectroscopy, infor-
mation could be obtained on the species adsorbed to the catalyst
surface. It was revealed that CO was present on the surface in
the absence of oxygen, though it was not observed under air over
Pt/Al2O3 [41]. In our experiments, consistent with the observation
that the ketones are more resistant to decarbonylation than are
aldehydes [72], lower deactivations were observed in the oxida-
tion of the secondary alcohol, 2-octanol (compare Figs. 2 and 3). On
the other hand, no formation of hydrocarbons could be detected
in solution by GC analysis during 1-octanol or 2-octanol reaction,
which suggests that decarbonylation does not occur during this
reaction, unless the carbonaceous residues remained adsorbed.
Deactivation of Pt catalysts by overoxidation of the particle sur-
face, i.e. oxygen atoms penetration into the most external layers
of metal crystals thus forming “subsurface” oxygen is often men-
tioned [44,45]. Alcohol oxidation has been shown to be regularly
faster under mass-transport limiting conditions. Oxygen dissolved
from the gas phase should be just present to react with hydrogen
present at the surface but not to oxidize the metal surface. Oxygen
solubility changes with the composition of the solvent. In particu-
lar, it is known that the oxygen solubility is much lower in water
than in dioxane and other organic solvents [74]. Hence, the amount
of oxygen should be lower in water-containing dioxane. If overox-
idation was due to an excess dissolved oxygen concentration in
pure dioxane, the catalyst should be less deactivated as far as water
is added to the reaction medium and decreasing the solubility of
oxygen. Indeed, this is what was observed upon replacing dioxane
with water, which suggests that some mass transport limitation
was then generated for the reaction.
To further understand the reason of deactivation during oxida-
tion of 1-octanol, a supplementary experiment was performed in
dioxane/water 50%/50%. After 5 h, when the conversion attained a
plateau at 64% conversion, the reactor was cooled to room tem-
perature and depressurized. After flushing under argon at room
temperature, the reaction medium was again heated to the reac-
tion temperature of 100 ◦C and pressurized to 10 bar. The reaction
immediately started again. However, after 2 h more reaction, con-
version reached a new plateau at 93%. Similarly, when 1-octanol
was added in the reaction medium after cooling and purging with
argon and before re-heating and adding air pressure, the catalyst
was able to oxidize half of the 1-octanol before new deactivation.
Typically, these results show that the supported platinum catalysts
may suffer from deactivation caused by reversible overoxidation
of active sites. The above findings may also show that restarting
the reaction with fresh octanol may partly remove the adsorbed
carbonaceous fragments and regenerate the activity.
0
5
10
15
20
25
time (h)
Fig. 5. Oxidation of 2-octanol in dioxane (᭹) and dioxane/water 50%/50% (ꢂ) solvent
in the presence of 2.8%Pt/C at 60 ◦C. Reaction conditions: 2-octanol 0.1 mol L−1, Pt
1 mol.%, 150 mL solvent, 10 bar.
respectively. At the end of the reactions, selectivity to the alde-
hyde was 88% in dioxane while it was nearly total to the acid in the
aqueous solvents. These results also confirm that the acid formed,
even with high selectivity, does not poison the catalyst, under these
reaction conditions.
Similarly, 2-octanol was oxidized in the presence of 2.8%Pt/C at
60 ◦C (Fig. 5). In the absence of water, the conversion of 2-octanol
was less than 12% after 24 h, whereas in the dioxane/water 50%/50%
solvent, conversion reached 85% at that time. Again, a much higher
activity was observed in the dioxane/water system.
Considering the results obtained with 1- and 2-octanol, we ten-
tatively examined the possible reasons why the combination of
dioxane and water led to a substantial increase in activity, and why
deactivation of the platinum catalyst was sometimes observed. We
evaluated a range of parameters on the conversion of these alcohols
to deeper understand the mechanism.
3.3. Nature of catalyst deactivation
According to the most accepted model, the actual mechanism
of such heterogeneous oxidation is an oxidative dehydrogenation
mechanism, in which the substrates are adsorbed and dehydro-
genated on the metal surfaces. The adsorbed hydrogen atoms react
with oxygen to form water [17] (Scheme 1).
However, deactivation is a problem which is often encountered
in this type of oxidation over Pd or Pt nanoparticles and remains a
challenge. Various processes for deactivation have been mentioned
[44,45]: overoxidation (oxygen poisoning), sintering of metal par-
ticles, metal leaching, blocking of active sites by strongly adsorbed
reactant, products or degradation by-products. We examined the
different possible causes.
The possibility that deactivation was due to metal leaching or
sintering in the present conditions was disregarded. As mentioned
in the experimental part, analysis of the final solutions showed
no observable loss of metal. Also, XRD observations of the used
catalysts did not detect Pt diffraction peak, as in the fresh catalyst.
We supposed that during oxidation of 1-octanol, deactivation
could be due to the production of 1-octanoic acid which strongly
adsorbs on the catalytic sites, lowering their availability for alcohol
Scheme 1. Classical dehydrogenation mechanism of alcohol oxidation over Pt metal.

86
A. Frassoldati et al. / Catalysis Today 173 (2011) 81–88
0.12
0.12
0.1
a
0.1
0.08
0.06
0.04
0.02
0
0.08
0.06
0.04
0.02
0
-1
0
1
2
3
4
5
6
7
8
-1
0
1
2
3
4
5
6
7
8
time (h)
time (h)
Fig. 7. Promotion effect of water on the catalytic oxidation of 2-octanol to 2-
octanone in dioxane/water mixtures in the presence of Pt–Bi/C. Reaction conditions:
2-octanol (0.1 mol L−1), Pt–Bi/C (1 mol.% Pt), solvent (150 mL), 100 ◦C, 10 bar. (᭹)
dioxane, (ꢁ, ꢃ) dioxane/water 75%/25%, (ꢂ, ꢀ) dioxane/water 50%/50%, ( ) 2.5%Pt-
0.9%Bi/C, (—-) 2.7%Pt–0.9%Bi/C.
0.12
0.1
b
0.08
0.06
0.04
0.02
0
be excluded in our case, since no Bi ions could be detected by ICP-
OES analysis of the final reaction medium.
3.4. Promoting effect of water
-1
0
1
2
3
4
5
6
7
8
time (h)
Fig. 8 gives the initial reaction rate (expressed as the percentage
of conversion of 2-octanol after 1 h) as a function of the volume per-
centage of water in the solvent, whether Pt/C or Pt–Bi/C catalysts
were used. As mentioned before, Bi promotion of the Pt catalyst
did not really improve the activity. Under identical reaction condi-
tions, when water is added into dioxane, a substantial increase in
the conversion is observed. The highest activity with total conver-
sion within 1 h could be reached from 50 vol.% water, using Pt/C as
catalyst.
Similarly, under the same conditions, conversion after 1 h in
dioxane for 1-octanol oxidation was 36 and 43% in the presence of
Pt/C or Pt–Bi/C catalyst, respectively. Addition of 10 and 50 vol.% of
water enhanced rapidly this conversion to 67–73% (Figs. 2 and 6). It
should also be pointed out that the primary alcohol reacted initially
faster than did the secondary alcohol.
Fig. 6. Oxidation of 1-octanol in dioxane (a) or dioxane/water 50%/50% with addition
of 0.03 equiv. NaOH (b) in the presence of 2.5%Pt–0.9%Bi/C. Reaction conditions:
150 mL 0.1 mol L−1 1-octanol, 100 ◦C, 10 bar air, 1-octanol/Pt molar ratio = 100, (ꢀ)
1-octanol, (ꢁ) 1-octanal, (ꢂ) 1-octanoic acid.
Numerous reports have been published on the improvement
of performances of supported Pd and Pt catalysts by promotion
with bismuth in terms of overoxidation and stability [44,45]. One
reason is that Bi is oxidized more easily than the noble metal,
thus maintaining it reduced and active [41]. Thereby a 2.5%Pt-
0.9%Bi/C catalyst prepared by redox deposition on platinum was
tested. Fig. 6 gives the results for the reactions performed in diox-
ane or dioxane/water 50%/50% mixtures with addition of 0.03 equiv.
NaOH.
Oxidation of the alcohol produces water. One may think that
the water will accumulate on the catalyst surface resulting in a
decrease of accessible active sites. However, the carbon surface is
rather hydrophobic, since the activation procedure of the used car-
bon did not significantly introduce polar groups such as carboxylic
or phenolic groups. Also, the solvent chosen, dioxane is able to
carry large quantities of water. Thus, water when formed should be
rapidly removed from the surface and be transferred to the solvent.
In dioxane, substantial oxidation of the 1-octanol was observed
under these conditions, with conversion of 43% within 1 h. Then, the
oxidation slowed considerably over time. As in the case of Pt/C, the
replacement of part of dioxane by water and addition of 0.03 equiv.
NaOH increased the initial reaction rate and favoured the forma-
tion of the acid. However, though conversion was slightly improved
some important slow down of the reaction was still observed and
conversion did not go to completion.
Both the unpromoted and promoted catalysts behave similarly,
significant improvement of the initial activity being observed with
addition of water. However, the catalyst activity of the unpromoted
and promoted catalysts was comparable and conversion reached
rapidly a stable value.
We also carried out experiments on 2-octanol oxidation in the
presence of Pt catalysts promoted by Bi, using two different batches
2.5%Pt–0.9%Bi/C and 2.7%Pt–0.9%Bi/C. The results are shown Fig. 7.
The bimetallic 2.5%Pt–0.9%Bi/C catalyst was slightly more active
in dioxane than the unpromoted monometallic 2.8%Pt/C catalyst
(40% conversion after 6 h in comparison to 29%). Replacement of
25 vol.% of dioxane by water increased only slightly the initial reac-
tion rate. On the other hand, addition of 50 vol.% of water to the
medium increased dramatically the activity of the catalyst, what-
ever the batch of catalyst.
100
80
60
40
20
0
0
20
40
60
80
100
water in solvent (vol. %)
Fig. 8. Conversion of 2-octanol after 1 h reaction as a function of the volumic
percentage of water in solvent in the presence of Pt/C (ꢀ) or Pt–Bi/C (ꢀ) cata-
lysts. Reaction conditions: 2-octanol (0.1 mol L−1), Pt or Pt–Bi/C (1 mol.% Pt), solvent
(150 mL), 100 ◦C, 10 bar.
It has also been reported in the literature that Bi promoter can
partially be leached out during reactions performed in acidic solu-
tions, resulting in irreversible catalyst deactivation [45]. This could

A. Frassoldati et al. / Catalysis Today 173 (2011) 81–88
87
0.12
0.1
forming the reaction in dioxane/water solvent at 60 ◦C instead
of 100 ◦C, probably by reducing overoxidation or by suppress-
ing decarbonylation and thus strong adsorption of by-products.
2-octanol oxidation was very selective to 2-octanone and gave bet-
ter yields than 1-octanol, since decarbonylation of ketone is less
important. Promotion by Bi does not eliminate the deactivation of
Pt.
For both substrates, with increasing water contents in a diox-
ane/water mixture, a dramatic enhancement of the catalytic
activity was observed. The enhancement is likely to be ascribed
to the relative affinity of the substrate for the catalyst surface (sup-
port) as a function of the polarity of the solvent.
0.08
0.06
0.04
0.02
0
-1
0
1
2
3
4
time (h)
Acknowledgements
Fig. 9. Oxidation of 2-octanol in the presence of 2-octanone added from the begin-
ning of the reaction. Reaction conditions: 150 mL 0.1 mol L−1 2-octanol, 100 ◦C,
10 bar, 2-octanol/Pt molar ratio = 100, (ꢀ, ꢁ, ꢂ) dioxane/water 66%/34% with addition
of 0, 15 or 30 mmol L−1 2-octanone, (ꢃ, ꢀ) dioxane/water 50%/50% with addition of
0 or 30 mmol L−1 2-octanone.
Antonio Frassoldati held a fellowship from Région Rhône-
Alpes through the Cluster Chimie. The financial support is greatly
acknowledged. The authors thank MAST Carbon for providing the
support used in this study.
A rapid deactivation induced by adsorption of intermediate and
by-products has also been reported for oxidation of alcohols. The
carbonyl products formed upon oxidation of 1-or 2-octanol could
strongly adsorb on the catalyst surface. We examined the possi-
bility of adsorption of the acid (vide supra). We also checked the
effect of 2-octanone. Mounzer et al. [68] showed that 2-octanone
was strongly adsorbed on a 5%Pt–1%Bi/C, very rapidly inhibiting
the reaction in heptane or xylene. A 16–18% (v/v) dioxane/heptane
solvent corresponding to a minimum in the adsorption coefficient
of 2-octanone was suitable for a better desorption of the ketone
and a longer activity of the catalyst. In the present case, one may
assume that substitution of dioxane with some water will mod-
ify the hydrophilicity properties of the solvent, hence facilitating
desorption of the ketone from the catalyst surface. Several oxida-
tion reactions of 2-octanol were carried out in which 2-octanone
was added in the solution from the beginning of the reaction in
dioxane/water to check if the reaction was affected by the formed
2-octanone. The results are shown in Fig. 9 for two compositions of
the solvent.
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