Methanol carbonylation catalyzed by TiO2-supported gold: An in-situ infrared spectroscopic investigation

Article abstract of DOI:10.1016/j.apcata.2015.06.020

TiO₂-supported gold samples prepared by deposition-precipitation were tested as catalysts for methanol carbonylation to give methyl acetate at atmospheric pressure in the presence of methyl iodide as cocatalyst. Infrared (IR) spectroscopy was used to investigate the reactions of methanol, CO and CH₃I on the surface of the samples. The results show the formation of methoxy species bonded to Ti⁴⁺ sites upon adsorption of methanol on the supported gold samples. Admission of CO to the flow reactor/IR cell led to the formation of gold carbonyls, which reacted with the surface methoxy species only when CH₃I was present to give methyl acetate. The data indicate that CH₃I is necessary to form gold-acetyl species, which are attacked by neighboring methoxy species to give the carbonylation product. Our results show that the methanol carbonylation catalyzed by TiO₂-supported gold in the presence of CH₃I occurs by a reaction mechanism that is similar to that occurring on supported Rh catalysts.

Full text of DOI:10.1016/j.apcata.2015.06.020

Accepted Manuscript
Title: Methanol Carbonylation Catalyzed by TiO2-Supported
Gold: An In-Situ Infrared Spectroscopic Investigation
Author: Z. Martinez-Ramirez G.A. Flores-Escamilla G.S.
˜
Berumen-Espana S.A. Jimenez-Lam B.E. Handy M.G.
Cardenas-Galindo Adan G. Sarmiento-Lopez J.C.
Fierro-Gonzalez
PII:
S0926-860X(15)30036-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.06.020
APCATA 15432
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
5-3-2015
4-6-2015
16-6-2015
Please cite this article as: Z.Martinez-Ramirez, G.A.Flores-Escamilla, G.S.Berumen-
˜
Espana, S.A.Jimenez-Lam, B.E.Handy, M.G.Cardenas-Galindo, Adan G.Sarmiento-
Lopez, J.C.Fierro-Gonzalez, Methanol Carbonylation Catalyzed by TiO2-Supported
Gold: An In-Situ Infrared Spectroscopic Investigation, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2015.06.020
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Methanol Carbonylation Catalyzed by TiO2-
Supported Gold: An In-Situ Infrared
Spectroscopic Investigation
Z. Martinez-Ramirez,1 G. A. Flores-Escamilla,1 G. S. Berumen-España,1 S. A. Jimenez-
Lam,1 B. E. Handy,2 M. G. Cardenas-Galindo2, Adan G. Sarmiento-Lopez1 and J. C.
Fierro-Gonzalez1*
1Departamento de Ingeniería Química, Instituto Tecnológico de Celaya, Av. Tecnológico
y Antonio García Cubas s/n. Celaya, Guanajuato, Mexico 38010.
2CIEP, Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí. San
Luis Potosí, Mexico. 78210.
* Corresponding author. Tel.: +52 4616117575; fax: +52 4616117744. E-mail address:
jcfierro@iqcelaya.itc.mx (J. C. Fierro-Gonzalez)
1

Research Highlights
 TiO2-supported gold is active for methanol carbonylation
 Methanol is activated on the support and CO on gold particles
 Methyl iodide promotes the reaction by favoring formation of gold-acetyl
intermediate
Abstract
TiO2-supported gold samples prepared by deposition-precipitation were tested as
catalysts for methanol carbonylation to give methyl acetate at atmospheric pressure in the
presence of methyl iodide as cocatalyst. Infrared (IR) spectroscopy was used to
investigate the reactions of methanol, CO and CH3I on the surface of the samples. The
results show the formation of methoxy species bonded to Ti4+ sites upon adsorption of
methanol on the supported gold samples. Admission of CO to the flow reactor/IR cell led
to the formation of gold carbonyls, which reacted with the surface methoxy species only
when CH3I was present to give methyl acetate. The data indicate that CH3I is necessary
to form goldacetyl species, which are attacked by neighboring methoxy species to give
the carbonylation product. Our results show that the methanol carbonylation catalyzed
by TiO2-supported gold in the presence of CH3I occurs by a reaction mechanism that is
similar to that occurring on supported Rh catalysts.
Keywords: gold catalysts; methanol carbonylation; infrared spectroscopy.
2

1. Introduction
Methanol carbonylation is the main route to produce acetic acid and methyl acetate
[1]. There are several successful industrial processes for the homogeneous methanol
carbonylation using various metals as catalysts, including Rh [1–3], Co [4,5] and Ir [4,5].
In the Monsanto process [2,3] the reaction is catalyzed by a Rh carbonyl complex in
solution in the presence of methyl iodide as cocatalyst. There is extensive spectroscopic
evidence of reaction intermediates formed during this processs [3,8,9], and the reaction
mechanism that has been proposed is widely accepted. However, because the costs
associated to the recovery and recycling of the catalyst in solution are relatively high,
there is a motivation to prepare solid catalysts that are able to achieve high conversion
and selectivity for heterogeneous methanol carbonylation processes.
Supported Rh [10,11], Ni [12,13] and Ir [14,15] catalysts have been used for
methanol carbonylation, and it was recently found that supported gold catalysts are also
active and selective for the reaction at relatively low temperatures and pressures [16–18].
Goguet et al. [17] investigated the methanol carbonylation catalyzed by carbon-supported
gold particles in the presence of methyl iodide as cocatalyst to give methyl acetate. They
characterized the samples by X-ray absorption spectroscopy under reaction conditions
and found that the presence of methyl iodide promoted the dispersion of gold particles on
the support to give (Au2I)δ+ or (Au3I2)δ+ surface species, which were proposed to be the
active sites for the reaction. Others [19,20] have also reported that methyl iodide favors
the dispersion of metal oxide supported gold particles. However, physical evidence of
methanol- and CO-derived surface species during methanol carbonylation catalyzed by
supported gold has not been reported.
3

Infrared (IR) spectroscopy has been a valuable tool to investigate the mechanism
of methanol carbonylation catalyzed by supported Rh and Ir complexes [11,14]. For
example, Haynes et al. [14] measured IR spectra of polymer-supported Rh and Ir
carbonyls under reaction conditions and found evidence of surface species that are
analogous to those observed during the homogeneous Monsanto process.
To identify possible reaction intermediates during methanol carbonylation we used IR
spectroscopy and mass spectrometry to investigate the reactions of methanol, CO and
methyl iodide with the surface of TiO2-supported gold catalysts. Our results indicate that
gold carbonyls, formed upon CO adsorption on the catalysts, react with neighboring
surface methoxy species bonded to Ti4+ sites of the support only in the presence of
methyl iodide to produce methyl acetate via an Auacetyl intermediate. The presence of
methyl iodide is essential for the reaction to take place at the conditions of our
experiments. Our data suggest that the methanol carbonylation catalyzed by TiO2-
supported gold proceeds by a reaction route that is analogous to those occurring on
supported Rh catalysts and in the homogeneous Monsanto process. These results indicate
that the TiO2-supported gold catalysts are bifunctional, with CO being activated on the
gold and methanol being activated on sites of the TiO2 support. The bifunctional
character of our catalyst resembles that of other supported metal catalysts for methanol
carbonylation [11,14], thus suggesting that it is possible to establish analogies between
the way in which supported gold and other supported metals catalyze specific reactions.
4

2. Experimental
2.1. Synthesis of TiO2-supported gold samples
Samples of TiO2-supported gold were prepared by deposition-precipitation [21].
TiO2 powder (Evonik P25) was dried over night at 110 oC prior to the samples syntheses.
A solution of HAuCl4 (Sigma Aldrich) was neutralized with NaOH 1M during stirring
o
and mixed with dry TiO2 until a pH value of 6 was reached at 60 C. The resulting
mixture was maintained at a pH of 6 and the solid was suction filtered and washed with
approximately 1 L of deionized water at 60 ºC to give approximately 3 g of TiO2-
o
supported gold samples. Finally, the powder was dried at 110 C for 24 h. The
concentration of the HAuCl4 solution was calculated to give a gold loading of 5.0 wt %.
2.2.
Transmission Electron Microscopy
Samples were characterized by high resolution scanning transmission electron
microscopy (STEM) on a Phillips TECNAI-F30 HRTEM operated at 300 kV and on a
JEOL JEM-2100F instrument operated at 200 kV. Fine sample powder was prepared for
analysis by ultrasonic dispersion in ethanol, depositing several drops of the suspension
onto copper grids coated with lacey carbon film (Ted Pella). Images were obtained
principally in high angle annular dark field (HAADF) mode, and EDX elemental analysis
was performed in several areas to confirm the presence of Au particles and estimate the
real content of gold on the surface of the samples.
5

2.3.
Temperature-Programmed Reduction
Temperature-programmed reduction (TPR) experiments were performed with a
ChemBET instrument (QuantaChrome) that was equipped with a thermal conductivity
detector (TCD). Typically, 0.3 g of the TiO2-supported gold samples were weighted,
loaded into a quartz tube, sealed, and transferred to the characterization system. Prior to
o
each experiment, the samples were pretreated at 110 C in flowing He [50 mL (NTP)
min-1] for 20 min to remove adsorbed water. The samples were then treated by heating
o
from 25 to 600 C at a rate of 10 K min-1 in a flow of 5 vol % H2 in Ar. The total gas
flow rate was 10 mL (NTP) min-1. The experimental parameters were chosen so that the
“K number” (defined as the ratio of reducible substance (mol) divided by the H2 flow rate
(mol s-1)) had a value in the range between 55 and 140 s. The TCD signal was calibrated
to determine H2 consumptions by using the complete reduction of CuO powder as a
standard (Aldrich, 99.995%) and by measuring the area under the TCD signal for known
H2 concentration. The H2 uptakes were determined with an accuracy of approximately
10%.
2.4.
Methanol carbonylation catalyzed by TiO2-supported gold
Methanol carbonylation catalysis experiments were carried out at atmospheric
pressure in a standard once-through nearly isothermal tubular packed-bed flow reactor.
Samples of the bare support and of supported gold (25 mg in each case) were loaded into
the reactor and treated in a flowing mixture of methanol (PMeOH = 109 Torr), CO (PCO
=
52 Torr), methyl iodide (PCH3I = 109 Torr) and He (total flow rate 50 ml (NTP) min-1 at
760 Torr) at room temperature for 30 min [22]. The temperature of the flow reactor was
6

o
increased linearly from room temperature to 400 C in the presence of the reactive
mixture. In other experiments, the TiO2-supported gold samples were tested as catalysts
o
for the reaction at constant temperature (200 C) in the presence of the same reactive
mixture. Mass spectra of the effluent gases from the reactor were measured with an on-
line Balzers OmniStar mass spectrometer running in multi-ion monitoring mode. The
changes in the signal intensities of the main fragments of methanol (m/e = 31), methyl
acetate (m/e= 43, 74), acetic acid (m/e = 60, 43, 45), dimethyl ether (m/e= 45, 46), CO2
(m/e = 16, 28, 30, 44) and water (m/e = 18) were recorded [23]. All signals are reported
relative to that of the He carrier gas at m/e = 4 to remove any effects of pressure
fluctuations. Quantitative analyses of the effluent gases were carried out by using the
method reported by Friend et al., [16] which consists of determining the molar fraction of
the various compounds as a function of the ion current recorded by the mass
spectrometer.
2.5.
IR spectroscopic characterization of TiO2-supported gold samples during
methanol adsorption and carbonylation
The TiO2-supported gold sample was loaded 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-1 with a Nicolet FTIR
6700 spectrometer equipped with a SpectraTech Collector DRIFT attachment fitted
with a high-temperature environmental chamber. Each spectrum corresponds to the co-
addition of 128 scans. KBr powder was normally used as a reference material. IR
spectra were recorded as the sample was exposed to a sequence of treatments in various
atmospheres, as follows. First, the sample was treated in a flowing mixture of methanol
7

(PMeOH = 109 Torr) and He (total flow rate 50 ml (NTP) min-1 at 760 Torr) for 5 min at
room temperature. Then, spectra were recorded as flowing CO (PCO= 76 Torr), was
admitted to the reactor mixture for 30 min, and finally as a mixture of CO, methanol and
CH3I was allowed to flow through the sample for 60 min at room temperature.
Difference IR spectra of the adsorbed species were obtained by absorption subtraction of
the cell and catalyst background spectra using the installed software.
3. Results and Discussion
3.1.
Methanol carbonylation catalyzed by TiO2-supported gold samples
Mass spectra of the effluent gases from the flow reactor recorded as a sample of
TiO2-supported gold was treated in the flowing mixture of methanol, CO and methyl
iodide at room temperature indicate the formation of methyl acetate, as evidenced by the
slight increase in the intensity of the mass fragment at m/e = 74 (Figure 1). The
formation of methyl acetate remained nearly constant during approximately 30 min time
on stream (TOS). Acetic acid was also formed during the first 5 min TOS, but its mass
spectral signal (m/e = 60) then decreased in intensity rapidly and after that time (Figure
1). Acetic acid is also a methanol carbonylation product, and a possible explanation for
the decrease of its production is that the partial pressure of methanol was higher than that
of the initially adsorbed water on the catalyst. These results indicate that methanol was
carbonylated to give methyl acetate and acetic acid even at room temperature in the
presence of our gold catalyst. As the temperature of the sample increased, the production
of methyl acetate and acetic acid also increased. The intensity of the mass spectral signal
o
of acetic acid reached a maximum value at approximately 120 C (Figure 1). Then, it
8

o
decreased and no acetic acid was detected at temperatures higher than 200 C. On the
other hand, the signal of methyl acetate reached a maximum value at approximately 200
oC (Figure 1). At higher temperatures the production of methyl acetate decreased and
dimethyl ether was formed, as evidenced by the increase in the intensity of the mass
fragment at m/e = 46 (Figure 1). The formation of dimethyl ether increased with
o
increasing temperature until the temperature reached 370 C, approximately. Above that
temperature, the intensity of the mass fragment of dimethyl ether decreased, together with
the slight increase in the intensity of the mass fragment of CO2 at m/e = 44 (Figure 1).
Dimethyl ether and CO2 were probably formed by methanol dehydration [24] and
oxidation, respectively. An alternative explanation for the formation of CO2 is that
adsorbed water (or water formed during methanol dehydration) might have reacted with
CO via the water-gas shift reaction. Indeed, others [25] have reported that TiO2-
supported gold catalysts are active for the water-gas shift reaction at temperatures that are
comparable to those at which CO2 formation was observed. When the same experiment
was performed in the presence of the bare TiO2 support, no formation of methyl acetate
nor acetic acid were observed and only dimethyl ether was produced (Figure 2).
Therefore, the results indicate that the presence of gold particles on the TiO2 favors the
methanol carbonylation. The data also show that the formation of dimethyl ether
occurred at lower temperature in the presence of the supported gold sample, thus
indicating that the gold particles also promote the methanol dehydration. The average
diameter of the supported gold particles in the initially prepared sample, as calculated
from TEM, was 4.2 nm (Supplementary Data).
9

To verify that the methanol carbonylation reaction was catalytic, the TiO2-supported
o
gold sample was treated in the presence of the reactive mixture at 200 C for 110 min.
Mass spectra of the effluent gases indicate the rapid production of methyl acetate during
the first minutes of the treatment, and its signal reached a maximum value after
approximately 20 minutes TOS (Figure 3). Then, the production of methyl acetate
decreased with increasing TOS until reaching a steady state after approximately 80 min.
-1
The calculated rate of production of methyl acetate at steady state was 5.7 mol h-1 gAu
[26]. This rate was determined by using the real content of gold on the surface of the
sample, as determined by EDX (i.e., 3.1 % w/w Au). Under the conditions of our
experiments, no acetic acid was detected in the effluent gases from the flow
reactor/DRIFT cell. The average diameter of the supported gold particles in a sample
o
after it had been used as catalyst for methanol carbonylation at 200 C for over 110 min
was 6.3 nm (Supplementary Data). These results indicate that the gold particles were
o
aggregated during the catalysis at 200 C. Furthermore, the particle size distributions of
the fresh and used catalysts were significantly different. Thus, it is possible that the
decrease in the catalytic activity with increasing TOS (Figure 3) might be associated with
the changes in the particle size and distribution of the supported gold particles. Although
other variables might also be important, such as build-up of surface iodo species, those
species were not detected by EDX characterization of the used catalysts.
3.2.
Methanol adsorption on TiO2 and TiO2-supported gold samples
IR spectra characterizing the bare TiO2 and TiO2-supported gold samples as they
were treated in flowing He at room temperature include bands at 3722, 3690, 3665 and
10

3630 cm-1 (Figure 4 a and b), assigned to O–H stretching (OH) vibration modes of
various types of isolated hydroxyl groups bonded to Ti4+ sites [27,28]. Spectra also
include a broad band centered at approximately 3380 cm-1 (Figure 4 a and b), assigned to
the OH vibration mode of hydrogen-bonded hydroxyl groups and/or adsorbed water
[27,28]. Admission of methanol to the flow reactor DRIFT cell led to disappearance of
the bands at 3690, 3665 and 3630 cm-1 (Figure 4 c and d), suggesting that the initially
present isolated hydroxyl groups participated in the adsorption of methanol [29].
Simultaneously, the broad OH band characteristic of hydrogen-bonded hydroxyl groups
centered at approximately 3380 cm-1 became even broader during methanol adsorption
(Figure 4 c and d). Thus, it is concluded that methanol was adsorbed molecularly on the
sample via hydrogen-bonds with the initially present isolated hydroxyl groups, leading to
the formation of more hydrogen-bonded hydroxyl groups on the surface of the sample, in
agreement with previous reports for the adsorption of methanol and other alcohols on
metal oxides [11,31].
Spectra in the C–H stretching (CH) region show bands at 2948 and 2841 cm-1 (Figure
4 c and d), attributed to the asymmetric and symmetric –CH3 stretching (aCH3 and sCH3
)
modes of molecularly adsorbed methanol [32–35]; and at 2922 and 2818 cm-1, assigned
to the same vibration modes of methoxy species bonded to TiO2 [32–35]. In fact, various
forms of surface methoxy species bonded to Ti4+ sites are identified during the
experiments, as evidenced by spectra at the region between 1600 and 950 cm-1 (Figure 5).
The bands at 1142 and 1119 cm-1 (Figure 5) can be assigned to the –CH3 rocking (CH3
)
and the O–C stretching (OC) vibration modes of linear methoxy species bonded to Ti4+
sites, respectively [34,35]. The band at 1438 cm-1 (Figure 5) is assigned to the –CH3
11

bending (CH3) mode of methoxy species [34,36] and the band at 1037 cm-1 can be
assigned to the CO vibrations of doubly-bonded methoxy species on Ti4+ sites [34].
Also, The bands at 1555 and 1357 cm-1 are assigned to the symmetric and asymmetric
COO stretching (sCOO and aCOO) vibration modes of adsorbed formate species,
respectively [32,37]. It is possible that formate species were produced by decomposition
of some methanol on the surface, as others have reported [38,39]. Finally, the band at
1050 cm-1 might be assigned to the OC vibration mode of either molecularly adsorbed
methanol [34] or surface methoxy species [40].
Therefore, our results indicate that methanol was adsorbed both molecularly and
dissociatively on the samples. Because spectra of the TiO2-supported gold and those of
the bare support include the same bands during methanol adsorption (Figures 4 and 5), it
is concluded that methanol-derived surface species were stabilized on sites of the support,
and not on the gold particles. This conclusion is consistent with previous reports on the
adsorption of various alcohols on supported metal catalysts [11,21,41,42]. A schematic
representation of methanol-derived surface species on the TiO2 and TiO2-supported gold
samples formed upon methanol adsorption is shown in Figure 6.
3.3.
Evidence of surface species formed during methanol carbonylation on
TiO2-supported gold samples at room temperature
With the aim of identifying surface species that were formed during methanol
carbonylation, IR spectra were recorded as a TiO2-supported gold sample was treated
sequentially in various atmospheres as follows: (a) in flowing methanol, (b) in a flowing
mixture of methanol and CO and (c) in a flowing mixture of methanol, CO and methyl
12

iodide at room temperature [43]. The results (Figures 7–9) show that admission of
methanol to the flow reactor/DRIFT cell led to the appearance of bands characteristic of
methoxy species bonded to Ti4+ sites on the TiO2-supported gold samples (i.e., at 2922,
2818,1438, 1142, 1119, and 1037 cm-1), as discussed above. When CO was admitted to
the cell, the bands assigned to the surface methoxy species did not change (Figures 7 and
8, light gray), thus indicating that the adsorbed alcohol did not react with flowing CO at
those conditions. However, a new band appeared at 1260 cm-1. The identity of the
species responsible for this band is not clear. Toledo-Antonio et al. [44] observed the
same band during CO adsorption on TiO2 nanotubes and assigned it to Ti–O vibrations.
Also, bands in the range between 1220 and 1270 cm-1 have been assigned to the asCOO
vibration mode of surface carbonates on metal oxides [45–47]. Those surface species
might be formed by the reactive adsorption of CO on the sample.
Simultaneously, spectra at the C–O stretching (CO) region showed the appearance of
bands at 2154 and 2105 cm-1 (Figure 9, light gray) when CO was admitted to the flow
reactor/DRIFT cell. The band at 2105 cm-1 can be attributed to the CO vibration mode of
CO bonded to zerovalent gold [48–50], and the band at 2154 cm-1 might be assigned to
CO bonded to cationic gold, as others [51–53] have attributed CO bands in the range
between 2150 and 2200 cm-1 to those species. It is observed that the intensity of the band
characteristic of CO bonded to cationic gold decreased rapidly with increasing TOS, as
the intensity of the band assigned to CO bonded to zerovalent gold increased. This result
indicates that part of the initially present cationic gold was reduced to zerovalent gold
when CO was present in the feed. Indeed, others have reported that CO is able to reduce
supported cationic gold at low temperatures [53,54]. To further investigate the changes
13

in the oxidation state of supported gold during the various treatments, TPR profiles were
measured for the initially prepared sample and for samples that had been exposed to
sequential treatments (a) in flowing methanol, (b) in a flowing mixture of methanol and
CO and (c) in a flowing mixture of methanol, CO and methyl iodide at room temperature.
The results (Figure 10) show the presence of two peaks of H2 consumption. A low-
o
temperature peak appeared with a maximum at approximately 240 C for all samples.
This peak might be attributed to the reduction of cationic gold present in the samples.
o
Indeed, others [55,56] have observed peaks in the range between 160 and 240 C in TPR
profiles of samples of gold supported on MgO and TiO2 and have attributed them to the
reduction of cationic gold. On the other hand, the high-temperature peak, with maximum
o
value at approximately 430 C could be attributed to the reduction of surface Ti4+ atoms
to Ti3+. Similar peaks have been observed by others [57,58] in TPR profiles of TiO2-
supported Pt [57] and Au [58] samples. In both cases, it was proposed that Au and Pt
particles promote the reduction of surface Ti4+ sites to Ti3+.
Our results indicate that cationic gold was present in the sample ever after it had been
exposed to the flow of the various gas mixtures, as the low-temperature peak was always
present in the TPR profiles (Figure 10). The area under the low temperature peak in the
TPR profile characterizing the sample after it had been treated in methanol (Figure 10b)
was essentially the same as that in the TPR profile of the fresh sample (Figure 10a).
Thus, it is concluded that methanol did not reduce the supported gold. This observation
is consistent with our IR data (Figure 4) showing that methanol was not adsorbed on the
gold particles, but on sites on the TiO2 support. In contrast, the area under the low-
temperature peak decreased significantly for the TPR profile characterizing the sample
14

that had been treated in the flowing mixture of CO and methanol (Figure 10c). This
result is also consistent with our IR data showing that part of the initially present cationic
gold was reduced in the presence of flowing CO. Interestingly, the TPR data
characterizing the sample after it had been exposed to the flowing mixture of CO,
methanol and methyl iodide show that the area under the low-temperature peak increased
in intensity (Figure 10d), indicating that the presence of methyl iodide led to the re-
oxidation of part of the gold that had been reduced in the presence of flowing CO.
Because methyl acetate was formed when methyl iodide was present in the feed, and the
TPR results indicate the presence of cationic gold, it could be proposed that cationic gold
was be involved in the methanol carbonylation. However, the presence of mixtures of
cationic and zerovalent gold sites complicates matters.
IR spectra recorded for the sample in the presence of methyl iodide, CO and methanol
indicate various changes (Figures 7–9, dark gray). Bands characteristic of methyl iodide
appeared initially at 2978, 2952, 1260 and 1240 cm-1 (Figures 7 and 8, dark gray). These
bands are assigned to the aCH3, sCH3, aCH3 and sCH3 vibration modes, respectively
[59,60]. When a TiO2-supported gold sample was treated in He saturated with methyl
iodide, the same bands appeared (Supplementary Data), thus confirming that they are
indeed related to the presence of methyl iodide.
Admission of methyl iodide also caused the rapid disappearance of the CO bands
characteristic of gold carbonyls, and only the doublet characteristic of gas-phase CO was
observed in that region of the spectra (Figure 9, dark gray). This observation indicates
the conversion of gold carbonyls in the presence of methyl iodide in the reactive mixture.
The consumption of gold carbonyls was simultaneous with the production of methyl
15

acetate, thus indicating that the gold carbonyls participated in the reaction. Although the
bands of both zerovalent and cationic gold carbonyls disappeared when methyl iodide
was present in the feed, it is impossible from our data to establish whether only one or
both species participated in the catalysis, as the changes in the CO region occurred so fast
that it only took the time relative to one IR spectrum for those bands to disappear. Our
results indicate that the catalyst is bifunctional, with methanol being activated on the
TiO2, in the form of methoxy species, and CO being activated on the gold. TPR profiles
of a sample that had been exposed to a mixture of methanol, CO and methyl iodide at
room temperature indicate a higher amount of cationic gold with respect to that observed
after the treatment in the flowing mixture of CO and methanol (Figure 10). These results
indicate that admission of methyl iodide led to the re-oxidation of part of the supported
gold and suggest that cationic gold might be involved in the catalysis.
Concomitant with the disappearance of the CO bands, various bands appeared in the
spectra in the region between 1600 and 1000 cm-1 when the sample was treated in the
flowing mixture of methanol, CO and methyl iodide (Figure 8, dark gray). The bands at
1465 and 1160 cm-1 could be assigned to the asymmetric bending and rocking of the
OCH3 group (aOCH3 and OCH3) of gas-phase methyl acetate [61–63], respectively.
Similarly, the band at 1380 cm-1 might be assigned to the sCH3 vibration mode of gas-
phase methyl acetate [61–63]. Zahidi et al. [60] assigned a band at 1302 cm-1 to methyl
acetate adsorbed on Ni(111). By analogy, we assign the band at 1300 cm-1 to methyl
acetate bonded to the supported gold particles.
These results are consistent with mass spectra of the effluent gases from the flow
reactor/DRIFT cell showing the formation of methyl acetate (Figure 1). Our data indicate
16

that the methanol carbonylation reaction only occurred on the samples when methyl
iodide was present in the feed. The need of methyl iodide for the occurrence of methanol
carbonylation in the presence of our TiO2-supported gold catalyst suggests that the
reaction mechanism might be similar to that occurring in the presence of Rh or Ir
catalysts, for which methyl iodide is also used as cocatalyst [3–7]. In those cases, it has
been proposed that methyl iodide reacts with the metal to give metal–methyl and metal–
iodide species [3–7]. Then, CO is inserted into the metal–methyl species to give metal–
acetyl species. There are reports [64–67] on the formation of surface Pd–acetyl and Pt–
acetyl complexes from the decomposition of various compounds on Pd(111) and Pt(111),
respectively. Those surface complexes are characterized by IR spectra showing CO,

sCH3 and CH3 bands in the ranges between 1560–1660, 1318–1355 and 1125–1132 cm-1,
respectively [64–67]. Our results, showing the appearance of bands at 1580, 1353 and
1130 cm-1 in the spectra of the supported gold sample as it was treated in the presence of
the reactive mixture of methanol, CO and CH3I (Figure 8, dark gray), are consistent with
the aforementioned examples. Thus, we assign the bands at 1580, 1353 and 1130 cm-1 to
the CO, sCH3 and CH3 vibration modes of Au–acetyl complexes formed during catalysis
[68]. The presence of these surface species suggests that there might be analogies
between methanol carbonylation catalyzed by gold and other metals.
3.4.
Role of methyl iodide on the methanol carbonylation catalyzed by
supported gold nanoparticles: Analogies between Rh and supported gold
catalysts
17

TEM images characterizing a TiO2-supported gold sample after it had been used for
methanol carbonylation at room temperature show the presence of gold particles with an
average diameter of 3.9 nm (Supplementary Material). This observation indicates that
the average size of the gold particles in the sample decreased slightly after it was used as
catalyst. Our results are consistent with previous reports showing that metal oxide
supported gold particles are re-dispersed when they are exposed to methyl iodide [17–
20]. This re-dispersion is thought to occur by the reaction between methyl iodide and
surface gold atoms to give segregated gold-iodide species. Chowdhury el al. [70]
reported that the reaction between gold films and methyl iodide led to the simultaneous
formation of Au–I and Au–CH3 surface species. Although it is not possible to identify
Au–CH3 surface species from our data, their existence might explain our IR results, as
CO bonded to the gold could be inserted rapidly into the goldmethyl bond to give
goldacetyl complexes (as evidenced by the rapid disappearance of the CO bands
characteristic of gold carbonyls (Figure 9) with the simultaneous appearance of the 1580-
, 1353- and 1130-cm-1 bands (Figure 8) once methyl iodide was admitted to the flow
reactor/DRIFT cell). The goldacetyl species then might undergo a nucleophillic attack
by neighboring methoxy species bonded to Ti4+ sites to give methyl acetate bonded to the
gold, which is then desorbed from the sample as the carbonylation product.
The Rh-catalyzed methanol carbonylation is thought to occur by an analogous
mechanism, in which RhI complexes undergo the oxidative addition of methyl iodide to
give RhIII complexes, which then undergo CO insertion to give Rh-acetyl complexes
[1,3,7,9]. The mechanism requires the presence of cycles between RhI and RhIII
complexes. In our case, TPR results show the presence of cationic gold in all samples,
18

before and after being used as catalysts for methanol carbonylation at room temperature.
One could speculate that the apparent analogies between our catalysts and the Rh catalyst
might be extended to propose that AuI and AuIII species participate in the methanol
carbonylation. Nevertheless, our samples are structurally complex, containing mixtures
of zerovalent and cationic gold.
4. Conclusions
In summary, we show that TiO2-supported gold catalysts are active for methanol
carbonylation to give methyl acetate in the presence of methyl iodide as cocatalyst. Our
IR results indicate that methanol is activated on the support (in the form of methoxy
species bonded to Ti4+ sites) and CO is activated on the gold particles. The presence of
methyl iodide is crucial to allow the formation of goldacetyl species, which might be
attacked by neighbouring surface methoxy species to give methyl acetate. These results
suggest that the methanol carbonylation on TiO2-supported gold occurs by a route that is
analogous to those occurring on other supported metal catalysts. Analogies between the
chemistry of gold catalysts and other metals have been proposed for other reactions
[71,72]. For example, the group of Corma [71] reported that CeO2-supported gold
catalysts function similarly to palladium catalysts for the Sonogashira cross-coupling
reactions. In that case, the origin of the similarities rested on the fact that AuI and Pd0
have both a d10 electronic configuration. In our case, the analogy between our catalysts
and Rh, Ir and Co catalysts for the methanol carbonylation rests on the ability of
19

supported gold to form carbonyls that react in the presence of methyl iodide to give
metal–acetyl species.
Acknowledgments. This research was supported by the Consejo Nacional de Ciencia y
Tecnología (CB-2006 61856) and DGEST (5272.14-P).
Appendix A. Supplementary Data.
Supplementary Data associated with this article can be found in the online
version.
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Figure Captions.
Figure 1. Changes in the intensities of the mass spectral signals of the effluent gases
from a flow reactor when the TiO2-supported gold sample was treated in (left) flowing
He at room temperature, (middle) the flowing reactive mixture of methanol, CO, CH3I
and He at room temperature and (right) the flowing reactive mixture at increasing
temperature. (●) methyl acetate (m/e = 74), (○) acetic acid (m/e = 60), () dimethyl ether
(m/e = 46), and (□) carbon dioxide (m/e = 44).
Figure 2. Changes in the intensities of the mass spectral signals of the effluent gases
from a flow reactor when the bare TiO2 support was treated in (left) flowing He at room
temperature, (middle) the flowing reactive mixture of methanol, CO, CH3I and He at
room temperature and (right) the flowing reactive mixture at increasing temperature. (●)
○) acetic acid (
methyl acetate (m/e = 74), (
m/e = 60), () dimethyl ether (m/e = 46), and
(□) carbon dioxide (m/e = 44).
Figure 3. Changes in the intensities of the mass spectral signals of the effluent gases
from a flow reactor when the TiO2-supported gold sample was treated in the flowing
reactive mixture of methanol, CO, CH3I and He at 200 oC. (●) methyl acetate (m/e = 74),
() methanol (m/e = 31) and (□) CO (m/e = 28).
27

Figure 4. IR spectra characterizing (a) the sample of TiO2 and (b) the sample of TiO2-
supported gold as they were treated in flowing He; (c) the sample of TiO2 and (d) the
sample of TiO2-supported gold as they were treated in methanol and He at room
temperature.
Figure 5. Subtraction IR spectra in the region between 1600 and 950 cm-1 characterizing
(a) the bare TiO2 support and (b) the TiO2-supported gold as they were treated in flowing
methanol and He at room temperature.
Figure 6. Schematic representation of (a) molecularly adsorbed methanol, (b) linear and
(c) doubly-brigded methoxy species bonded to Ti4+ sites.
Figure 7. IR spectra in the CH region recorded as a TiO2-supported gold sample was
sequentially treated in flowing mixtures of methanol and He (white); methanol, CO and
He (light gray); and methanol, CO, CH3I and He (dark gray) at room temperature.
Figure 8. Subtraction IR spectra in the region between 1600 and 1000 cm-1 recorded as a
TiO2-supported gold sample was sequentially treated in flowing mixtures of methanol
and He (white); methanol, CO and He (light gray); and methanol, CO, CH3I and He (dark
gray) at room temperature.
28

Figure 9. IR spectra in the CO region recorded as a TiO2-supported gold sample was
sequentially treated in flowing mixtures of methanol and He (white); methanol, CO and
He (light gray); and methanol, CO, CH3I and He (dark gray) at room temperature.
Figure 10. TPR profiles characterizing the TiO2-supported gold sample after it was
treated sequentially at room temperature in (a) flowing He for 5 min, (b) a flowing
mixture of methanol and He for 30 min, (c) a flowing mixture of methanol, CO and He
for 30 min, and (d) a flowing mixture of methanol, CO, methyl iodide and He for 60 min.
29