Highly efficient and robust Au/MgCuCr2O4 catalyst for gas-phase oxidation of ethanol to acetaldehyde

Article abstract of DOI:10.1021/ja406820f

Gold nanoparticles (AuNPs) supported on MgCuCr₂O ₄-spinel are highly active and selective for the aerobic oxidation of ethanol to acetaldehyde (conversion 100%; yield ～95%). The catalyst is stable for at least 500 h. The unprecedented catalytic performance is due to strong synergy between metallic AuNPs and surface Cu⁺ species. X-ray photoelectron spectroscopy shows that Cu⁺ is already formed during catalyst preparation and becomes more dominant at the surface during ethanol oxidation. These Cu⁺ species are stabilized at the surface of the ternary MgCuCr₂O₄-spinel support. Further kinetic measurements indicate that the Cu⁺ species act as sites for O ₂ activation.

Full text of DOI:10.1021/ja406820f

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Communication
Highly Efficient and Robust Au/MgCuCr2O4 Catalyst
for Gas-Phase Oxidation of Ethanol to Acetaldehyde
Peng Liu, and Emiel J. M. Hensen
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja406820f • Publication Date (Web): 06 Sep 2013
Downloaded from http://pubs.acs.org on September 10, 2013
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Highly Efficient and Robust Au/MgCuCr O Catalyst for Gas-
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Phase Oxidation of Ethanol to Acetaldehyde
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,†
,†
Peng Liu,* and Emiel J. M. Hensen*
†
Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eind-
hoven, The Netherlands
Supporting Information Placeholder
12
catalyst gradually deactivated and selectivity was below 80%.
Among various unitary oxide supported AuNP catalysts,
ABSTRACT: Gold nanoparticles (AuNP) supported on spinel
MgCuCr are highly active and selective for the oxidation of
2
O
4
Au/MoO showed the highest acetaldehyde yield (>90% at 240
3
ethanol with molecular oxygen to acetaldehyde (conversion 100%;
yield ~95%). The catalyst is shown to be stable for at least 500 h.
The unprecedented catalytic performance is understood in terms
o
14
C), but without evidence of catalyst stability. More recently,
pre-oxidized bimetallic Au-Cu/SiO catalysts were reported to be
active and stable for ethanol conversion (~90%) with ~85% acet-
aldehyde selectivity at 200 C for 50 h on-stream. However,
these catalyst systems are far from practical application, mainly
because of the low ethanol concentration (< 1 vol%) , the low
2
+
of strong synergy between metallic AuNPs and surface Cu spe-
o
15
+
cies. X-ray photoelectron spectroscopy shows that Cu is already
formed during catalyst preparation and becomes more dominant
14
+
at the surface during the ethanol oxidation reaction. These Cu
10-13,15
acetaldehyde selectivity (< 90%)
space velocity (GHSV < 10,000 mL gcat h )
and/or the low gas hourly
species are stabilized at the surface of the ternary MgCuCr O -
-1 -1 10,12,13,15
2
4
.
+
spinel support. Further kinetic measurements indicate that the Cu
In this report, we present a ternary spinel (MgCuCr
ported AuNP catalyst that is capable of achieving ~100% ethanol
2
4
O ) sup-
species act as sites for O activation.
2
o
conversion with ~95% acetaldehyde selectivity at 250 C of a
-
1
-1
feed of 1.5 vol% ethanol at a GHSV of ~ 100,000 mL gcat h .
The catalyst is very stable for at least 500 h (Figure 1). The un-
Production of chemicals and fuels from abundant, renewable
biomass and its derivatives provides a viable route to alleviate our
strong dependence on depleting fossil fuels. For this purpose,
precedented catalytic performance relates to synergy between
+
1
AuNP and surface Cu species, which facilitates O
2
activation.
+
The MgCuCr O spinel stabilizes Cu formed during catalyst
2
4
ethanol is particularly attractive because of its facile synthesis by
biomass fermentation and expected increased availability and
preparation and ethanol oxidation and also ensures high and sta-
ble AuNP dispersion.
2
reduced cost. Selective oxidation of ethanol to acetaldehyde
Chromite-spinels were chosen primarily because of the poten-
tial shown in our previous work on hydrotalcite supported AuNP
catalysts for liquid-phase aerobic oxidation of alcohols. The
strong synergy between MgCr-hydrotalcite and AuNP motivated
us to explore the gas-phase ethanol oxidation on MgCr-based
(
CH CHO), acetic acid and ethyl acetate is of great interest for
3
the chemical industry to decrease the use of petrochemical re-
1
7
3
serves. In particular, acetaldehyde is an important bulk chemical
for the production of peracetic acid, pentaerythritol, pyridine
bases, butylene glycol and chloral, with a worldwide production
6
4
oxides (typically MgCr O spinel) supported gold catalysts. To
over 10 tons/year. Currently, production of acetaldehyde is
mainly by the Wacker process, via oxidation of ethylene in strong
2
4
date, no chromite-spinel supported AuNP catalysts were reported
18
5
for ethanol oxidation. We prepared several Mg0.75M0.25Cr O
acidic solutions catalyzed by palladium and copper chlorides.
2
4
spinels (denoted as MgMCr O , M = Co, Ni, Cu) with the same
Here, a promising opportunity arises to replace petroleum-based
ethylene by renewable ethanol for more green and sustainable
acetaldehyde production.
2
4
structure as MgCr O by a coprecipitation-calcination method,
2
4
which was also used for the preparation of other reference spinel
supports (see the Supporting Information for details). All gold
catalysts were prepared by the deposition-precipitation method
Recently, the use of supported gold nanoparticles (AuNP) to
catalyze aerobic oxidation of alcohols has become the center of
19
6
with urea. The surface area, mean AuNP size and Au loading of
attention. AuNP catalysts have, in particular, shown promise for
7-15
the catalysts are collected in Table 1. Under identical preparation
conditions, all chromite spinels showed much lower surface area
than aluminate spinels. The very low surface area of copper-
containing chromite spinels is consistent with the presence of
large crystals (see Figure S1, S2 for XRD patterns and SEM im-
ages). Evidently, the difference in spinel surface area has little
influence on gold dispersion (Figure S3), with mean AuNP sizes
always being between 2-4 nm.
selective oxidation of ethanol with molecular oxygen.
Liquid-
phase ethanol oxidation mainly yields acetic acid or ethyl acetate,
but supported AuNP catalysts suffer deactivation upon reuse due
7
to gold sintering under hydrothermal conditions. A more attrac-
tive process for industrial application would be gas-phase ethanol
oxidation because of the convenience of catalyst separation, sol-
1
6
vent-free conditions and facile continuous process operation.
The most significant disadvantage of current gold-based catalysts
would appear to be the high reaction temperature, which tends to
cause low selectivity and deactivation. For instance, Stucky re-
ported that 6.3 nm Au/SiO showed an ethanol conversion of 45%
2
o
10
with 75% acetaldehyde selectivity at 200 C. Au/TiO (~2 nm)
was able to convert 60% of an ethanol feed at ~120 C; full con-
version was reached at 280 C, but under these conditions the
2
o
o
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Table 1. Textural and physicochemical properties and cat-
alytic activity results for various supported Au catalysts.
1
2
3
4
5
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8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
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2
2
2
2
2
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a
b
b
S
dAu [Au]
(m /g) (nm) (wt%) (%) (%) (h )
X
S
TOF
BET
Catalyst
2
-1 c
1
2
3
4
5
6
Au/MgCr
2
O
4
16
26
17
5
3.3 0.93
3.2 0.97
3.3 0.96
3.1 0.90
3.6 0.93
2.5 0.83
2.8 0.93
3.4 0.78
6.3 1.87
2.2 0.98
4.5 0.90
3.1 0.90
6
9
7
96
98
96
207
293
Au/MgCoCr
2
O
4
Au/MgNiCr
Au/MgCuCr
2
O
4
233
2
O
4
68 99
40 99
30 98
20 97
2351
1556
898
Au/CuCr
2
O
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
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0
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2
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0
1
2
3
4
5
6
7
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0
Au/MgCuAl
2
O
4
158
165
152
287
50
5
d
7
Au/CuO-SiO
Au/MgAl
Au/SiO
Au/TiO
Au/MgCuCr O
2
592
8
2
O
4
4
99
178
e
9
2
12 92
13 91
38 99
73 99
378
1
0
2
264
f
1
1
1909
2539
2
4
4
g
1
2
Au/MgCuCr
2
O
5
a
b
Determined by ICP-OES. Ethanol conversion (X) and acetal-
o
dehyde selectivity (S) at 200 C (catalyst 0.1 g, ethanol/O /He =
1
based on acetaldehyde yield and gold dispersion (D = 1.3/d) ,
and given in molaldehyde mol
2
-1
-1
c
/3/63, GHSV = 100,000 mL gcat h ). Turnover frequency
11
Figure 1. TEM images of a) fresh Au/MgCuCr
2
O
4
catalyst, b)
-1 -1
d
surface Au
h . By using CuO/SiO
2
used Au/MgCuCr O catalyst after 500 h on-stream; c) ethanol
e
f
2
4
(
26.2 wt% Cu) as support. By using Au(en) Cl as precursor.
2 3
o g
conversion and acetaldehyde selectivity vs time on stream using
Au/MgCuCr O (Reaction conditions: catalyst 0.1 g, GHSV =
Catalyst was calcined at 500 C in air for 5 h. Pretreated in H at
3
2
2
4
o
00 C for 2 h.
-1
-1
1
00,000 mL gcat h , ethanol/O /He = 1/3/63).
2
o
Figure 1c. The catalyst was found to be stable at 200 C for the
Initially, we investigated the effect of the chromite-spinels on
o
first 20 h, and also at 250 C for the following 100 h with conver-
the catalytic activity of Au/MgMCr O in gas-phase aerobic oxi-
2
4
sion increasing to 95% and acetaldehyde selectivity slightly de-
creasing to 97%. When the temperature was further increased to
o
dation of ethanol (Table 1, entries 1-4). At 200 C, acetaldehyde
is the predominant product (selectivity > 95%) in all cases.
Au/MgCuCr O outperforms the other non-Cu containing cata-
o
275 C, full conversion was achieved during 50 h with selectivity
2
4
decreasing to 90%. Interestingly, after decreasing the temperature
lysts (Table 1, entries 1-3) by nearly an order of magnitude,
pointing to a strong synergy between AuNP and the Cu-
containing spinel. To identify the beneficial effect of Cu-
containing oxide supports, Au/CuCr O , Au/MgCuAl O and
o
to 250 C, the conversion and selectivity remained at 100% and
9
5%, respectively, for more than 300 h. On the contrary, the etha-
o
nol conversion of Au/MgCuAl O catalyst at 200 C decreased
2
4
2
4
2
4
from 30 to 25% after 5 h. To our delight, after 500 h on-stream,
the structure and morphology of the Au/MgCuCr O catalyst was
Au/CuO-SiO catalysts were also evaluated (Table 1, entries 5-7).
2
2
4
2
These catalysts showed much lower activity than Au/MgCuCr O4
largely retained (see Figure S2 for SEM image), with the average
AuNP size only slightly increased from 3.1 to 3.4 nm (Figure 1a,
1b). Several other reference catalysts showed significant increases
in the AuNP size after 50 h on-stream (see Figure S5), pointing to
the stabilizing effect of MgCuCr O on AuNP size. These results
catalyst, despite their higher Cu content, higher surface area and
smaller AuNP size. Moreover, Au/MgCuCr O also outperforms
2
4
7
a
10,11
previously reported preferred Au/MgAl O , Au/SiO2
and
2
4
12
Au/TiO2 catalysts for ethanol oxidation (Table 1, entries 8-10).
These results clearly point to a specific gold-support interaction in
the Au/MgCuCr O catalyst.
2
4
unequivocally demonstrate the highly efficient and robust nature
of the Au/MgCuCr O catalyst for the gas-phase ethanol oxida-
2
4
2
4
Figure 2 shows the temperature dependence of ethanol conver-
sion and acetaldehyde selectivity over representative catalysts.
Au/MgCuCr O exhibited higher activity than other catalysts
tion to acetaldehyde. Considering that prior to reaction the cata-
o
lyst was pre-treated in O at 300 C and the decreased activity for
2
2
4
catalyst with larger AuNP size (Table 1, entry 11, also see Figure
S6), we surmised that the activity increase may be due to ethanol-
induced changes to the active sites.
o
o
below 300 C, with a dramatic activity increase after 150 C. The
selectivity to acetaldehyde of Au/MgCuCr O (see Table S1),
2
4
o
interestingly, showed a maximum between 150-250 C, with ethyl
acetate as by-product at lower temperature and acetic acid and
CO2 as main by-products at higher temperature. Although
Au/TiO typically achieves moderate conversion at low tempera-
2
o
12
ture (~120 C), its performance is inferior to Au/MgCuCr O at
2
4
the same low temperature, mainly due to the much higher GHSV
applied here (see Figure S4 for catalytic performance at lower
GHSV). MgCr-spinel oxide supported AuNP (Au/MgCr O )
2
4
o
without Cu showed low activity even at 300 C, indicating the
16a
importance of Au-Cu interactions
2 4
. Au/MgCuAl O and
Au/CuO-SiO with much higher surface area and smaller AuNP
Figure 2. Temperature-dependent ethanol oxidation over various
catalysts showing a) ethanol conversion, b) acetaldehyde selectiv-
ity. (●) Au/MgCuCr O , (★) Au/MgCuAl O , (■) Au/TiO , (▲)
2
size still showed inferior activity. These results further point to
the pivotal role of the chromite-spinel support.
2
4
2
4
2
To investigate the stability of Au/MgCuCr O , the catalyst was
2 4 2
Au/MgCr O and (▽) Au/CuO-SiO .
2
4
run for 500 h in ethanol oxidation and the results are shown in
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Figure 4. H
2 2 4
consumption traces (TPR) of a) Au/MgCuCr O , b)
Au/CuCr O , and c) Au/MgCuAl O catalysts. The dashed line
2
4
2
4
represents the trace of the corresponding spinel support.
containing catalysts (see Table S2). The somewhat decreased
Au/Cu ratio (~0.6) in the spent catalyst after 500 h can be due to
the small increase of AuNP size from 3.1 to 3.4 nm. These find-
0
+
ings point to Au -Cu interactions decreasing AuNPs sintering.
To understand the reducibility of the spinel supported gold
catalysts, we performed temperature-programmed reduction (TPR)
study (Figure 4). The amount of reducible species of the spinel
supports and the gold catalysts derived thereof was found to be
consistent with the Cu content (see Table S3). For the
Au/MgCuCr O and Au/CuCr O catalysts, only 5.2 and 6.3% of
2 4
Figure 3. XP spectra of the Au/MgCuCr O catalyst before and
after 500 h on stream. a) Au 4f XP spectra, b) Cr 2p XP spectra, c)
Cu 2p3/2 and Cu LMM (inset) XP spectra.
To obtain further insight into the gold-support interactions, we
studied the oxidation state of Au, Cr and Cu of the fresh
Au/MgCuCr O catalyst and after catalyzing ethanol oxidation for
2
4
2
4
o
the Cu species were reduced below 300 C, respectively. In the
case of Au/MgCuAl O , however, over 80% of Cu species was
+
2
4
2
4
500 h by X-ray photoelectron spectroscopy (XPS, Figure 3). The
binding energy (BE) was corrected for surface charging by taking
the C 1s peak of contaminant carbon as a reference at 284.5 eV.
The Au 4f7/2 BE remains constant at 84.0 eV, the value for metal-
lic gold. Similarly, the BE of Cr 2p was ~576.2 eV, which is
o
2+
reduced below 300 C, indicating the lower stability of Cu /Cu
+
in the MgCuAl O spinel. Although the surface Cu fraction of
fresh Au/MgCuAl O catalyst is as high as 54%, the facile reduc-
tion of these Cu species to Cu in the presence of ethanol (see
Figure S9 for XPS results) results in inferior activity, presumably
because of the absence of desirable Au -Cu interactions. On the
2
4
2
4
0
3
/2
3+ 20
typically attributed to Cr . The Cu 2p3/2 XP spectra (Figure 3c)
are different before and after use of Au/MgCuCr O as a catalyst.
0
+
2
4
0
contrary, the Au/MgCuCr O catalyst contains no Cu , demon-
2
4
Two contributions are discerned at 935 and 932 eV. The higher
2
+
strating the advantage of MgCr
2
O
4
-type spinels in stabilizing
Cu /Cu species. On the basis of these observations, the unique
promotional effect of MgCuCr spinel support can be attributed
to (i) the possibility of reducing Cu to Cu without Cu forma-
BE peak at ~935 eV is assigned to Cu in the spinel, accompa-
2
+
+
2
+
nied by the characteristic Cu shake-up satellite peaks (938-945
2
1
2 4
O
eV). The lower BE peak at ~932 eV suggests the presence of
2+
+
0
+
0
20,21
0
Cu or Cu species.
Because Cu 2p3/2 XPS cannot differentiate
0
+
+
tion, leading to development of Au -Cu synergy beneficial for
ethanol oxidation and, (ii) strong metal-support interactions, thus
rendering robust Au/MgCuCr O catalyst with stable AuNP size.
between Cu and Cu , Auger Cu LMM spectra were used to con-
+
22
firm the presence of Cu at BE ~570 eV. This is consistent with
the fact that both the spinel support and the gold catalyst were
prepared by adopting a calcination step in air, so that the presence
of metallic Cu in the fresh catalyst is not likely. Interestingly,
significant changes were observed when comparing Cu XP spec-
tra of the fresh and spent catalysts, with the Cu LMM transition
2
4
0
+
In an attempt to further understand the nature of Au -Cu
synergy and reaction mechanism, we determined the contribution
of non-oxidative ethanol dehydrogenation to the total activity of
aerobic ethanol oxidation. Indeed, CuO·CuCr O (copper chro-
2 4
+
+
showing only Cu species. The surface Cu fraction, derived from
Cu 2p3/2 XP spectra, increased from 26 to 55% during 500 h time
mite) is known to be an active catalyst for ethanol dehydrogena-
23
tion. Figure 5(a, b) shows the activity and selectivity compari-
2+
on stream, indicating that part of surface Cu species are reduced
to Cu species during ethanol oxidation. To further prove the
selective reduction of Cu to Cu in the MgCuCr O spinel, the
son of MgCuCr
version in the presence and absence of oxygen in the feed. Over
the bare MgCuCr spinel, negligible oxidation took place at
temperatures below 200 C, but at higher temperature acetalde-
hyde was gradually produced accompanied by formation of ethyl
acetate and CO (see Table S4). The MgCuCr O spinel, however,
exhibited very poor activity for non-oxidative dehydrogenation of
ethanol even at 350 C, which we attribute to the absence of Cu
sites for the initial O-H bond cleavage in our catalyst. Figure 5c
2 4 2 4
O spinel and Au/MgCuCr O for ethanol con-
+
2
+
+
2 4
O
2
4
o
Au/MgCuCr O catalyst and the MgCuCr O support were pre-
2
4
2
4
o
treated in ethanol vapor or H at 300 C. XPS confirms the signif-
icantly increased surface Cu content following these treatments
and the absence of bulk Cu species (see Figure S7). The surface
Cu fraction is 63% for H -reduced Au/MgCuCr O . The pre-
2
+
2
2
4
0
+
o
0
2
2
4
24
reduced catalyst showed higher activity than the pre-oxidized one
under identical conditions (Table 1, entry 12, also see Figure S8).
These results further support that the synergy is related to the co-
shows that Au/MgCuCr O deactivates during ethanol dehydro-
genation and the more so at higher temperatures, with eth-
ylene/ethyl acetate as main by-products. Compared with the ex-
2 4
0
+
existence of Au and Cu species, and the increased catalytic
o
o
performance at 250 C after running for 50 h at 275 C (Figure 1c)
cellent activity of Au/MgCuCr
2 4
O in aerobic oxidation, its low
0
+
is due to enhanced Au -Cu synergy. It is noteworthy that the
surface Au/Cu atomic ratio in the pre-oxidized and pre-reduced
Au/MgCuCr O catalysts is similar (~0.7) and much higher than
activity in dehydrogenation demonstrates the importance of O
2
0
+
for the Au -Cu synergy. The absence of activated oxygen species
as H acceptor to remove the surface adsorbed Au-H hydride and
2
4
in the other Cu-
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ASSOCIATED CONTENT
Supporting Information
Experimental details, preparation and characterization of the cata-
lysts. These materials are available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
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pliu503@hotmail.com; e.j.m.hensen@tue.nl
ACKNOWLEDGMENT
This research was financially supported by Programme Strategic
Scientific Alliances between the Netherlands and China (Grant
No. 2008DFB5-130). We thank the Cryo-TEM Research Unit of
Eindhoven University of Technology for access to TEM facilities.
Figure 5. a) ethanol conversion, b) acetaldehyde selectivity for
temperature-dependent aerobic oxidation of ethanol over (●)
Au/MgCuCr O and (■) MgCuCr O , and non-oxidative ethanol
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2
4
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O and (□) MgCuCr O ; c)
(c) Gallezot, P. Chem. Soc. Rev. 2012, 41, 1538.
(2) (a) Rass-Hansen, J.; Falsig, H.; Jørgensen, B.; Christensen, C. H. J.
2
4
Chem. Technol. Biotechnol. 2007, 82, 329. (b) Sun, J.; Zhu, K.; Gao, F.;
Wang, C.; Liu, J.; Peden, C. H. F.; Wang, Y. J. Am. Chem. Soc. 2011, 133,
-
1
-1
GHSV = 100,000 mL gcat h , ethanol/He = 1/66).
1
1096.
0
to release free Au sites may be the reason for activity decreasing
(3) Takei, T.; Iguchi, N.; Haruta, M. Catal. Surv. Asia 2011, 15, 80.
with the temperature and time increasing. XPS (see Figure S7)
(4) Caro, C.; Thirunavukkarasu, K.; Anilkumar, M.; Shiju, N. R.;
+
excludes the reduction of Cu during non-oxidative ethanol dehy-
Rothenberg, G. Adv. Synth. Catal. 2012, 354, 1327.
(5) (a) Jira, R. Angew. Chem. Int. Ed. 2009, 48, 9034. (b) Keith, J. A.;
Henry, P. M. Angew. Chem. Int. Ed. 2009, 48, 9038.
drogenation and temperature-programmed oxidation (TPO, Figure
S10) confirms that rapid coking is the cause of the deactivation.
(6) (a) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem. Int. Ed. 2006,
On the contrary, in the presence of O , coking is suppressed and
2
4
5, 7896. (b) Pina, C. D.; Falletta, E.; Prati, L.; Rossi, M. Chem. Soc. Rev.
stable performance can be achieved.
2008, 37, 2077. (c) Corma, A.; Garcia, H. Chem. Soc. Rev. 2008, 37, 2096.
(d) Zhang, Y.; Cui, X.; Shi, F.; Deng, Y. Chem. Rev. 2012, 112, 2467.
(7) (a) Christensen, C. H.; Jørgensen, B.; Rass-Hansen, J.; Egeblad, K.;
Madsen, R.; Klitgaard, S.; Hansen, S.; Hansen, M.; Andersen, H.; Riisager,
A. Angew. Chem. Int. Ed. 2006, 45, 4648. (b) Jørgensen, B.; Christiansen,
S. E.; Thomsen, M. L.; Christensen, C. H. J. Catal. 2007, 251, 332. (c)
Tembe, S. M.; Patrick, G.; Scurrell, M. S. Gold Bull. 2009, 42, 321.
Based on the above observations, we propose that O activation
2
+
occurs on Cu sites instead of AuNP. This is supported by the
finding that Au/MgCr O is significantly less active for ethanol
2
4
oxidation than Au/MgCuCr O . The resulting active oxygen spe-
2
4
-
-
cies (O or O ) are thought to act as basic sites to facilitate O-H
bond cleavage and metal-alcoholate formation (Scheme S1).
2
6
(
(
8) Sun, K. Q.; Luo, S. W.; Xu, N.; Xu, B. Q. Catal. Lett. 2008, 124, 238.
9) Biella, S.; Rossi, M. Chem. Commun. 2003, 378.
AuNP in close proximity to such centers will act as the sites for
C-H cleavage, which is believed to be the most difficult step in
(10) Zheng, N.; Stucky, G. D. J. Am. Chem. Soc. 2006, 128, 14278.
11,13a,15
2+
alcohol oxidation.
We speculate that the oxidized Cu -OH
(11) Guan, Y.; Hensen, E. J. M. Appl. Catal. A 2009, 361, 49.
intermediates can be reduced by the proximate Au-H hydride
formed by C-H cleavage of adsorbed Au-alcoholate, accompanied
by water formation and removal, thereby recovering the initial
(12) (a) Simakova, O. A.; Sobolev, V. I.; Koltunov, K. Y.; Campo, B.;
Leino, A.-R.; Kordás, K., Murzin, D. Y. ChemCatChem 2010, 2, 1535. (b)
Sobolev, V. I.; Koltunov, K. Y.; Simakova, O. A.; Leino, A.-R.; Murzin,
D. Y. Appl. Catal. A 2012, 433-434, 88.
+
0
Cu and free Au active centers. This novel synergistic effect
+
(
13) (a) Gong, J.; Mullins, C. B. J. Am. Chem. Soc. 2008, 130, 16458. (b)
between Cu and Au provides a more efficient route for ethanol
oxidation to acetaldehyde than using previously reported Cu -
containing AuCu alloy catalysts.
0
Kong, X. M.; Shen, L. L. Catal. Commun. 2012, 24, 34.
(14) Takei, T.; Iguchi, N.; Haruta, M. New J. Chem. 2011, 35, 2227.
1
5, 16a
(15) Bauer, J. C.; Veith, G. M.; Allard, L. F.; Oyola, Y.; Overbury, S. H.;
In summary, we report for the first time an approach to achieve
highly efficient, selective and stable oxidation of ethanol to acet-
aldehyde by using MgCuCr O -spinel supported gold nanoparti-
Dai, S. ACS Catal. 2012, 2, 2537.
(16) (a) Pina, C. D.; Falletta, E.; Rossi, M. J. Catal. 2008, 260, 384. (b)
Fan, J.; Dai, Y.; Li, Y.; Zheng, N.; Guo, J.; Yan, X.; Stucky, G. D. J. Am.
Chem. Soc. 2009, 131, 15568.
(17) (a) Liu, P.; Guan, Y.; van Santen, R. A.; Li, C.; Hensen, E. J. M.
Chem. Commun. 2011, 47, 11540. (b) Liu, P.; Li, C.; Hensen, E. J. M.
Chem. Eur. J. 2012, 18, 12122.
2
4
cles. This significant progress is based on the identification of a
novel and potentially broader applicable Au-Cu synergy in alco-
hol oxidation, likely being based on the interaction of AuNPs
+
+
with Cu which activate O . The Cu species are stabilized in a
2
(18) Sobczak, I.; Szrama, K.; Wojcieszak, R.; Gaigneaux, E. M.; Ziolek,
chromite-spinel phase and become more dominant at the surface
M. Catal. Today 2012, 187, 48.
(19) Zanella, R.; Giorgio, S.; Henry, C. R.; Louis, C. J. Phys. Chem. B
2002, 106, 7634.
(20) Deutsch, K. L.; Shanks, B. H. J. Catal. 2012, 285, 235.
(21) Severino, F.; Brito, J. L.; Laine, J.; Fierro, J. L. G.; Agudo, A. L. J.
Catal. 1998, 177, 82.
(22) Platzman, I.; Brener, R.; Haick, H.; Tannenbaum, R. J. Phys. Chem.
C 2008, 112, 1101.
+
during the ethanol oxidation. Through interactions with the Cu -
containing chromite spinel the AuNPs are stable during reaction.
We have already demonstrated stable ethanol oxidation operation
o
-1 -1
at 250 C (1.5 vol% ethanol and a GHSV of 100,000 mL g
h )
cat
for 500 h. Under these conditions, the ethanol conversion is com-
plete and the acetaldehyde selectivity is 95%. It is therefore rea-
sonable to state that the novel catalyst has potential for acetalde-
hyde production from bioethanol.
(
(
23) Prasad, R. Mater. Lett. 2005, 59, 3945.
24) Zhang, M.; Li, G.; Jiang, H.; Zhang, J. Catal. Lett. 2011, 141, 1104.
D
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