Infrared spectroscopic evidence of adsorbed species during the oxidation of 2-propanol catalyzed by γ-Al2O3-supported gold: Role of gold as a hydrogen-subtractor

Article abstract of DOI:10.1016/j.molcata.2011.05.001

Samples of gold supported on γ-Al₂O₃ were prepared by deposition-precipitation and tested as catalysts for the aerobic oxidation of 2-propanol to give acetone. IR spectra recorded during catalysis show that the alcohol is first dissociatively adsorbed on the catalysts to form predominantly 2-propoxide species bonded to coordinatively unsaturated Al ³⁺ sites. It is proposed that gold particles in the proximity of the 2-propoxide species are capable of subtracting hydrogen from the β-C-H bond to give acetone bonded to Al³⁺ sites, as evidenced by the appearance of a band at 1704 cm^-1 at a similar temperature as that at which the onset of formation of acetone occurred. Other possible surface reaction pathways are suggested, indicating the interactions of the alcohol with support and metal sites. These results (with a model reaction) might be general and explain the activity of supported gold for the oxidation of other alcohols.

Full text of DOI:10.1016/j.molcata.2011.05.001

Journal of Molecular Catalysis A: Chemical 344 (2011) 47–52
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Infrared spectroscopic evidence of adsorbed species during the oxidation of
2
-propanol catalyzed by ␥-Al O -supported gold: Role of gold as a
2 3
hydrogen-subtractor
∗
Z. Martinez-Ramirez, S.A. Jimenez-Lam, J.C. Fierro-Gonzalez
Departamento de Ingeniería Química, Instituto Tecnológico de Celaya, Av. Tecnológico y Antonio García Cubas s/n, Celaya 38010, Guanajuato, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Samples of gold supported on ␥-Al O were prepared by deposition–precipitation and tested as catalysts
2
3
Received 24 March 2011
Received in revised form 27 April 2011
Accepted 1 May 2011
for the aerobic oxidation of 2-propanol to give acetone. IR spectra recorded during catalysis show that
the alcohol is first dissociatively adsorbed on the catalysts to form predominantly 2-propoxide species
3+
bonded to coordinatively unsaturated Al sites. It is proposed that gold particles in the proximity of the
Available online 10 May 2011
2
-propoxide species are capable of subtracting hydrogen from the ␤-C–H bond to give acetone bonded to
3
+
−1
Al sites, as evidenced by the appearance of a band at 1704 cm at a similar temperature as that at which
the onset of formation of acetone occurred. Other possible surface reaction pathways are suggested,
indicating the interactions of the alcohol with support and metal sites. These results (with a model
reaction) might be general and explain the activity of supported gold for the oxidation of other alcohols.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Supported gold
Alcohol oxidation
Infrared spectroscopy
Mass spectrometry
1
. Introduction
The selective oxidation of alcohols to aldehydes and ketones
zaldehyde [13], and the oxidation of glucose to give gluconate [14],
among other reactions [15].
Although there are numerous reports [5–7] attempting to
explain the origin of the catalytic activity of supported gold for CO
oxidation, the investigation of reaction mechanisms for the oxi-
dation of alcohols catalyzed by supported gold has only recently
received attention. Some authors [16] have suggested that the
rate-determining step in the oxidation of alcohols catalyzed by sup-
ported gold is the subtraction of hydrogen from the ␤-C–H bond (␣
to the O atom) of the alcohol.
is one of the most important reactions in organic chemistry and
the search for cost-effective and environmentally benign processes
that avoid the use of stoichiometric oxidants, such as permanganate
and dichromate has gained great attention [1–3]. An attractive
alternative is the use of solid catalysts that are able to perform
this reaction in the presence of oxygen at low temperatures and
atmospheric pressure.
The discovery by the group of Haruta et al. [4] in the 1980s that
gold finely dispersed on metal oxides is highly active for CO oxida-
tion at low temperatures led to a drastic increase in the research
of the origin of the activity of a metal that was widely regarded as
essentially catalytically inactive. Since then, it has been reported
that supported gold catalysts are active for many reactions [5–7],
including the selective oxidation of alcohols to ketones and aldehy-
des [8–11]. Rossi, Prati and coworkers [8,9] were the first to show
that supported gold was active for the oxidation of alcohols. They
reported that carbon-, Al O - and TiO -supported gold samples
Because some authors have reported that even unsupported
gold films and single crystals are active for the oxidation of alcohols
[17–19], and because it has been observed that the presence of gold
nanoparticles on supports is necessary for the catalysts to be active,
it has been assumed that reaction intermediates derived from the
alcohol must be bonded to surface gold atoms in supported gold
catalysts. Various authors have proposed that during catalysis the
alcohol is first dissociated on gold particles to form a surface gold
alkoxide, which then reacts to give the oxidation product [1,14,20].
Nevertheless, there is no direct physical evidence of such surface
species in typical supported gold catalysts and there is no discus-
sion in the literature regarding the need of their presence for the
catalysts to be active.
2
3
2
were excellent catalysts for the oxidation of various diols and carbo-
hydrates [8,9,12]. Since then, it has been reported that supported
gold is catalytically active for the selective oxidation of a variety
of alcohols, including the oxidation of benzyl alcohol to give ben-
In the absence of evidence of alcohol-derived species bonded to
the supported metal, the debate about the origin of the catalytic
activity of supported gold for the oxidation of alcohols has focused
on the investigation of the influence of structural variables, includ-
ing the electronic state of the gold [21], the size of the supported
∗ Corresponding author. Tel.: +52 4616117575; fax: +52 4616117744.
E-mail address: jcfierro@iqcelaya.itc.mx (J.C. Fierro-Gonzalez).
1
381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.05.001

4
8
Z. Martinez-Ramirez et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 47–52
Fig. 1. IR spectra characterizing (a) the sample of ␥-Al2O3 and (b) the sample of ␥-Al2O3-supported gold as they were treated in flowing He; (c) the sample of ␥-Al2O3 and
d) the sample of ␥-Al2O3-supported gold as they were treated in flowing 2-propanol, O2 and He at room temperature.
(
particles [22,23] and the nature of the support [24]. However, even
investigations that analyze the influence of a single variable pro-
vide apparently contradictory results. For example, whereas some
authors have suggested that the presence of small gold particles
is crucial for the catalysts to be active [22,23], others have found
that samples containing relatively large gold particles (i.e., average
diameter > 15 nm) are also active [25].
der was normally used as a reference material. Difference IR spectra
of the adsorbed species were obtained by absorption subtraction
of the cell and catalyst background spectra using the installed soft-
ware.
In the experiments, IR spectra were recorded as each sam-
−
1
ple (ca. 25 mg) was treated in flowing He (50 ml (NTP) min ) at
atmospheric pressure and room temperature for 30 min. Then, as a
In this work, we report the selective aerobic oxidation of 2-
propanol to produce acetone catalyzed by ␥-Al O -supported gold.
flowing mixture of 2-propanol (PiPrOH = 50 Torr), O (PO2 = 76 Torr)
2
−
1
and He (total flow rate 50 ml (NTP) min at 760 Torr) was allowed
to enter in contact with the sample at room temperature for 40 min.
Finally, the temperature of the cell was increased from room tem-
2
3
We focused on the characterization of adsorbed species on the func-
tioning catalysts by means of infrared (IR) spectroscopy. Our results
show that 2-propanol is dissociatively adsorbed in the form of 2-
propoxide species that are predominantly bonded to Al sites on the
surface of ␥-Al O . Those species can be oxidized to produce ace-
◦
perature to 300 C in the presence of the reactive mixture.
2
3
2.3. Mass spectra characterizing the effluent gases from the flow
tone when gold is present on the ␥-Al O surface. Our data suggest
2
3
reactor/DRIFTS cell
that the 2-propoxide species can be oxidized in the proximity of
gold particles, which provide sites for hydrogen subtraction from
the 2-propoxide species. Other possible reaction pathways on the
surface of the catalysts are discussed.
Mass spectra of the effluent gases from the reactor/DRIFTS cell
TM
were measured with an on-line Balzers OmniStar
mass spec-
trometer running in multi-ion monitoring mode. The changes in
the signal intensities of the main fragments of hydrogen (m/e = 2),
H2O (m/e = 16, 17, 18), O2 (m/e = 16, 32), 2-propanol (m/e = 27, 29,
2
. Experimental
4
3, 45), propene (m/e = 27, 39, 41, 42, 43), acetone (m/e = 15, 27, 43,
2
.1. Synthesis of ꢀ-Al O -supported gold samples
58) and CO2 (m/e = 16, 28, 30, 44) were recorded. All signals are
reported relative to that of the He carrier gas at m/e = 4 to remove
any effects of pressure fluctuations.
2
3
Samples of ␥-Al O -supported gold were prepared by
2
3
deposition–precipitation. A solution of HAuCl4 (Sigma–Aldrich)
◦
was neutralized slowly with vigorous stirring to pH 7 at 60 C with
3
. Results and discussion
NaOH 1 M and mixed with ␥-Al O (Degussa), which had been
dried over night at 120 C. The resulting mixture was maintained
at a pH of 8.0. The mixture was filtered and the solid was washed
with deionized water at 60 C. The solid was suction filtered and
then it was dried at 110 C for 24 h. The concentration of the
2
3
◦
3.1. Adsorption of 2-propanol on ꢀ-Al O and ꢀ-Al O -supported
2
3
2
3
gold samples at room temperature
◦
◦
IR spectra characterizing the bare ␥-Al O3 and the ␥-Al O -
2
2
3
HAuCl4 solution was calculated to give a gold loading of 5.0 wt%.
supported gold samples as they were treated in flowing He at
room temperature show a shoulder at 3730 cm and a band at
3692 cm assigned to two types of (Al) OH groups (Fig. 1a and
−
1
−
1
2
.2. Infrared spectra characterizing ꢀ-Al O3 and
2
2
ꢀ
-Al O -supported gold
b) [26,27]. Also, the IR spectra of both samples include a broad
2
3
−
1
band centered at approximately 3530 cm , assigned to hydrogen-
−
1
Samples of the bare support and of supported gold were loaded
bonded OH groups [28], and a band at 1640 cm , assigned to
adsorbed water.
into a diffuse reflectance Fourier transform (DRIFT) IR cell that was
closed and isolated with two standard three-way vacuum valves.
IR spectra were recorded with a resolution of 4 cm with a Nicolet
Exposure of the samples to a flowing mixture of 2-propanol,
O2 and He at room temperature led to the immediate disappear-
−
1
TM
−1
−1
FTIR 6700 spectrometer equipped with a SpectraTech Collector
ance of the shoulder at 3730 cm
and the band at 3692 cm .
DRIFT attachment fitted with a high-temperature environmental
chamber. Spectra were recorded by coadding 128 scans. KBr pow-
This observation is consistent with an interaction of the alcohol
with isolated hydroxyl groups of the support. Indeed, the disap-

Z. Martinez-Ramirez et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 47–52
49
Scheme 1. Schematic representation of (a) 2-propanol molecularly adsorbed on
-Al2O3 and (b) 2-propoxide species bonded to cus Al3+ sites of the support.
␥
pearance of the shoulder at 3730 cm−1 and the band at 3692 cm−1
was accompanied by the appearance of a broad band centered at
−1
approximately 3440 cm , which can be attributed to either (a)
H-bonded hydroxyl groups on the support [28] formed by inter-
action of the alcohol with the initially present isolated OH groups
and/or (b) ꢁOH vibrations of the adsorbed alcohol (Fig. 1c and d)
[
26]. Concomitant with these changes, ꢁ
bands at 2974, 2935
CH
−1
and 2887 cm appeared (Fig. 1c and d). These bands are assigned
to ꢁCH3as, 2× ␦ and ꢁCH3 vibrations of the adsorbed alcohol,
CH3as
respectively [26]. These results are consistent with the presence of
-propanol molecules H-bonded to surface Al–OH groups on both,
the bare support and the ␥-Al O -supported gold sample.
2
2
3
Bands characteristic of 2-propoxide species bonded to coordi-
natively unsaturated (cus) Al3 sites also appeared in the IR spectra
of both samples. Frequencies and assignments of these bands are
+
−
1
−1 −1
; 1379 cm , ␦CH3s ; 1341 cm , ıCH;
as follows: 1466 cm , ␦
CH
3
as
and 1165 and 1131 cm 1, ꢁCO/ꢁCC vibrations (Fig. 1c and d) [26,29].
The appearance of two bands for the ꢁCO/ꢁCC vibrations in the IR
spectra of the samples is indicative of the presence of linearly- and
bridge-bonded 2-propoxide species on cus Al3 sites on the surface,
as demonstrated by Rossi et al. [30].
−
+
These results are in agreement with previous reports [26,29,31]
showing the reaction of 2-propanol with surface OH groups on
Fig. 2. Changes in the intensity of the mass spectral signals of the effluent gases
from the flow reactor/DRIFTS cell when (A) the bare support and (B) the sample of
Al2O3-supported gold were treated in a mixture of flowing O2, 2-propanol and He
at increasing temperatures. (ꢀ) 2-Propanol, (♦) oxygen, (᭹) acetone, (ꢀ) propene
and (ꢁ) carbon dioxide.
␥
-Al O to produce surface 2-propoxide species at room temper-
2 3
ature. Our data are consistent with the presence of two species
on the bare support and the ␥-Al O -supported gold sample: (a)
2
3
2
-propanol molecules H-bonded with OH groups on the support
3+
and (b) 2-propoxide species bonded to cus Al sites (Scheme 1).
There are linearly- and bridge-bonded 2-propoxide species on the
◦
mately 150 C, as evidenced by a slight increase in the intensity of
␥
-Al O surface.
2 3
the signal of the fragment at m/e = 43 (Fig. 2A). The intensity of the
signal of acetone increased slowly during the rest of the experi-
ment until the set point (300 C) was reached. At approximately
20 C propene started to form, as evidenced by the increase in the
intensity of the signal of a fragment at m/e = 41. The intensity of the
signal of propene increased rapidly from that temperature until the
set point was reached (Fig. 2A). At approximately 280 C, formation
of CO2 started, as evidenced by a slight increase in the intensity of
the signal at m/e = 44 (Fig. 2A). O2 and 2-propanol were consumed
as the various products formed (Fig. 2A).
These results are in agreement with previous reports showing
that 2-propanol can be converted to acetone, propene and CO2
on the surface of ␥-Al O3 [26,32]. Our data show that ␥-Al O3 is
Because we identify the same IR bands upon adsorption of
-propanol on the bare support and on the ␥-Al O -supported
2
◦
2
3
gold sample, we conclude that the species observed in this work
are bonded to Al atoms of the support and not to Au atoms of
the supported particles. The lack of evidence of alcohol-derived
species bonded to the gold might be explained by (a) their fast
reaction to give oxidation or dehydration products, (b) their rapid
migration to give surface species bonded to the support or (c)
their absence. Because no formation of acetone or propene was
observed by mass spectra of the effluent gases from the flow
reactor/DRIFTS cell at room temperature, the first possibility is
ruled out.
◦
2
◦
2
2
selective towards the formation of acetone only in the tempera-
◦
3.2. Reactions of 2-propanol and oxygen with the surfaces of
ture range between 150 and 220 C under the reaction conditions
ꢀ
-Al O and ꢀ-Al O -supported gold at increasing temperature
used in this work. At higher temperatures, 2-propanol reacts to give
predominantly the dehydration product (propene) and the total
2
3
2
3
Mass spectra characterizing the effluent gases from the reac-
tor/DRIFTS cell that contained a sample of the bare ␥-Al O as it was
oxidation product (CO ). These results are analogous to those of
2
Ermini et al. [32], who observed that 2-propanol decomposed on
2
3
◦
treated in a flowing mixture of 2-propanol, O and He at increasing
the surface of ␥-Al O to form acetone at approximately 150 C and
2
2
3
◦
propene at approximately 200 C.
temperature show the formation of acetone starting at approxi-

5
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Z. Martinez-Ramirez et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 47–52
Mass spectra characterizing the effluent gases from the reac-
tor/DRIFTS cell that contained a sample of ␥-Al O -supported gold
2
3
as it was treated in a flowing mixture of 2-propanol, O2 and He at
increasing temperatures are shown in Fig. 2B. In contrast with the
results obtained with the bare support, these data show the for-
◦
mation of acetone starting at approximately 120 C (Fig. 2B). The
formation of acetone increased rapidly with increasing tempera-
ture without appreciable formation of propene or CO2 (Fig. 2B). At
◦
approximately 250 C, the signal of acetone reached a maximum
and it then decreased with increasing temperature until the set
point was reached. The decrease of the intensity of the signal of
acetone was accompanied by the increase of intensity of the signal
◦
of propene (Fig. 2B). At approximately 280 C, CO2 also started to
form (Fig. 2B).
Because acetone formed at lower temperatures when the exper-
iment was done in the presence of the supported gold sample than
when it was done in the presence of the bare support, it is concluded
that the gold favors the oxidation of 2-propanol to give acetone.
3.3. IR spectra characterizing surface species formed during the
reactions of 2-propanol and oxygen with the surfaces of ꢀ-Al O
2
3
and ꢀ-Al O -supported gold at increasing temperature
2
3
To investigate the origin of the differences in the catalytic
activity of the bare support and the supported gold sample, both
samples were characterized by IR spectroscopy during the thermal
treatments in the presence of the flowing reactive mixture. The
evolution with increasing temperature of IR bands in the spectra
recorded as the bare support and the ␥-Al O -supported gold sam-
2
3
ple were treated in the flowing mixture of 2-propanol, O2 and He
−
1
is shown in Fig. 3. It is observed that bands at 1735 and 1695 cm
appeared in the IR spectra of the bare ␥-Al O at approximately
2
3
◦
1
50 C (Fig. 3A). Because this temperature is the same as that at
which mass spectra of the effluent gases from the reactor/DRIFTS
cell indicated the beginning of acetone formation (Fig. 2A), it is
expected that they are related to the production of acetone. Indeed,
−
1
the band at 1735 cm
phase acetone [26], whereas bands in the range between 1690
is assigned to the ꢁCO vibration of gas-
−1
and 1702 cm
have been assigned to the ꢁ
vibration of ace-
Fig. 3. IR subtraction spectra in the 1800–1100 cm−1 region characterizing (A) the
␥-Al2O3 and (B) the ␥-Al2O3-supported gold sample as they were treated in flowing
CO
tone molecules bonded to cus Al3 sites on aluminum-containing
+
2-propanol, O2 and He at increasing temperature.
supports [33].
In contrast, when the experiment was done in the presence
of the ␥-Al O -supported gold sample, bands attributed to ace-
2
3
−1
tone (1735 and 1704 cm ) also appeared in the spectra (Fig. 3B),
but at lower temperature (approximately 125 C) than when the
◦
experiment was done with the bare support. Again, the appear-
ance of these bands occurred at almost the same temperature as
that at which formation of acetone was detected by mass spectra
of the effluent gases (Fig. 2B). Together with the appearance of the
−1
bands at 1704 and 1735 cm in the IR spectra of the supported
−1
gold sample, sharp bands at 1570 and 1463 cm , assigned to sur-
face acetate species appeared (Fig. 3B) [34,35]. A possible pathway
for the formation of surface acetates has been proposed to occur
through a nucleophilic attack of a surface hydroxyl group on nearby
molecules of adsorbed acetone [35].
To test whether the formation of acetone is a true catalytic pro-
cess, a sample of ␥-Al O -supported gold was treated in the flowing
2
3
◦
mixture of 2-propanol, O2 and He at 200 C for 2 h. The results
show the almost immediate formation of acetone when the sam-
ple was exposed to the reactive mixture, as evidenced by the rapid
increase of the mass spectral signal of acetone (Fig. 4). After its ini-
tial increase, the intensity of the signal of acetone decreased with
increasing time on stream (TOS) and then it stabilized at a nonzero
value after approximately 80 min.
Fig. 4. Changes in the intensity of the mass spectral signals of the effluent gases
from the flow reactor/DRIFTS cell when a sample of ␥-Al2O3-supported gold was
◦
treated in flowing 2-propanol, O2 and He at 200 C. (ꢀ) 2-Propanol, (♦) oxygen, and
(
᭹) acetone.
Although it is clear that the presence of gold favors the catalytic
formation of acetone, our data do not provide any evidence of sur-

Z. Martinez-Ramirez et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 47–52
51
Scheme 2. Schematic representation of the catalytic cycle for the oxidation of 2-
propanol over ␥-Al2O3-supported gold.
◦ ◦
200 C) was reached. After approximately 20 min TOS at 200 C
(
in the presence of flowing 2-propanol and He, the signals of ace-
tone and H2 remained nearly constant. These results indicate that
the gold facilitates the cleavage of the ␤-C–H bond of the sur-
face 2-propoxide species to form acetone and H2 in the absence
Fig. 5. Changes in the intensity of the mass spectral signals of the effluent gases from
the flow reactor/DRIFTS cell when a sample of ␥-Al2O3-supported gold was treated
in flowing 2-propanol and He as the temperature increased from room temperature
◦
◦
to 200 C, and in the presence of 2-propanol, O2 and He at 200 C. (ꢀ) 2-Propanol,
of O . When the same experiment was done in the presence of the
(᭹) acetone, (ꢂ) hydrogen and (ꢃ) water.
2
bare ␥-Al O , no formation of acetone or hydrogen were observed
2
3
(
Supplementary Data). Cleavage of the ␤-C–H bond could lead
face species derived from the alcohol or the acetone directly bonded
to the gold. Instead, the IR results show that the alcohol and the ace-
tone are adsorbed on cus Al3 sites (Fig. 3B). The lack of evidence of
alcohol- and acetone-derived species bonded to the gold during
catalysis might be explained by (a) the fast reaction of alcohol-
derived species bonded to gold to give acetone molecules, which
then migrate from gold sites to the support, or (b) the absence
of such species. Thus, a question arises for the role of gold in the
formation of acetone.
to the formation of a gold hydride. Although we have no direct
evidence of such species, a recent report provides electron param-
agnetic resonance (EPR) spectroscopic and spin trapping evidence
of gold hydrides during the oxidation of secondary alcohols cat-
alyzed by supported gold [43]. In the absence of O , hydrogen atoms
bonded to gold might recombine to give H , which explains the
+
2
2
detection of H in mass spectra of the effluent gases from the reactor
2
(Fig. 5).
When flowing O2 was admitted to the reactor/DRIFTS cell at
◦
00 C, the concentration of acetone in the effluent gases increased
2
3.4. Role of gold as hydrogen-subtractor
drastically, and the concentration of H2 decreased. Concomitant
with these changes, formation of H O was observed (Fig. 5). These
2
Various authors have proposed that metals serve as sites for H-
results suggest that O2 reacts with the gold hydrides to give H2O,
leaving surface gold atoms ready to remove more hydrogen atoms
from the ␤-C–H bond of the surface 2-propoxide species. Possi-
ble pathways for the formation of acetone from 2-propanol in the
absence and in the presence of flowing O2 are shown in Scheme 2.
It is emphasized that this proposal does not require the alcohol to
be bonded to the gold, but to cus Al3+ sites, which are expected
to be in the proximity of gold particles. The role of gold is crucial,
providing sites for the removal of hydrogen from the ␤-C–H bond
of the surface 2-propoxide species. Communication between the
support and the gold sites might be accomplished by surface dif-
fusion of hydrogen atoms [36], as it has been proposed for many
examples in which other supported metals subtract hydrogen from
C–H bonds [39–42].
subtraction from C–H bonds of adsorbed molecules [36–38], as well
as for the recombinatory desorption of H2 [36]. There are many
examples showing that metals are capable of subtracting hydro-
gen from organic species that are bonded to neighboring sites on
supports. Some examples include the conversion of cyclohexane to
benzene catalyzed by Al O -supported MoOx [39], aldol condensa-
2
3
tion reactions of ethanol catalyzed by NiO- and MgO-supported
Cu [40,41], and the dehydrogenation of n-alkanes catalyzed by
carbon-supported Pt [42]. In all cases, it was shown that C–H acti-
vation occurs on the support, whereas H-adatoms are removed
by neighboring metal sites that catalyze their recombinative des-
orption to form H2 molecules. Thus, one can hypothesize that a
similar reaction pathway might occur during the oxidation of 2-
propanol catalyzed by ␥-Al O -supported gold. Such a pathway
2
3
might explain the absence of alcohol- and acetone-derived species
bonded to gold during catalysis.
3.5. Other possible reaction pathways for the oxidation of
2-propanol catalyzed by ꢀ-Al O -supported gold
2
3
To test whether the gold facilitates hydrogen subtraction by
cleavage of the ␤-C–H bond of the surface 2-propoxide species
Because there are reports [17–19] showing that unsupported
gold is active for the oxidation of alcohols, the possibility of the exis-
tence of alcohol- and acetone-derived species bonded to the gold
in our ␥-Al O -supported gold samples during catalysis cannot be
completely ruled out. Some experiments done with unsupported
gold have shown that the oxidation of alcohols proceeds on gold
surfaces when they are precovered with atomic oxygen [17,18].
Although no physical evidence of surface species has been reported,
this observation has been explained in terms of the Brønsted-base
(
Scheme 1), a sample of ␥-Al O -supported gold was treated (in
2 3
the absence of O ) with a flowing mixture of He and 2-propanol
2
as the temperature increased linearly from room temperature to
2
3
◦
2
00 C. Mass spectra characterizing the effluent gases from the flow
reactor/DRIFTS cell show that acetone started to form during the
◦
thermal treatment at approximately 135 C, with the concomitant
formation of H2 (Fig. 5). The intensities of the signals of acetone
and H2 increased with increasing temperature until the set point

5
2
Z. Martinez-Ramirez et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 47–52
character of oxygen adatoms on the group 1B metals, which induces
the abstraction of acidic hydrogen from alcohols to give metal
alkoxides. The alkoxide species might then be oxidized to produce
ketones or aldehydes [19,44]. In principle, this rationale could be
extended to explain what might happen on typical supported gold
catalysts.
Therefore, in addition to the reaction pathway shown in
Scheme 2, oxygen activated on gold atoms at the perimeter of
the supported gold particles could oxidize molecularly adsorbed 2-
propanol and/or 2-propoxide species bonded to either gold atoms
or to the support to give acetone.
vation on supported gold catalysts for oxidation reactions still need
resolution.
Acknowledgments
This research was supported by the Consejo Nacional de Cien-
cia y Tecnología (CB-2006 61856) and the Consejo de Ciencia y
Tecnología del Estado de Guanajuato (FOMIX-GTO2006).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2011.05.001.
3
.6. Implications for oxidation of other alcohols catalyzed by
supported gold
References
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4
. Conclusions
We prepared a ␥-Al O -supported gold catalyst that is active
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