Selective oxidation of ethanol over Ag, Cu and Au nanoparticles supported on Li2O/Γ-Al2O3

Article abstract of DOI:10.1016/j.jcat.2018.05.011

In an effort to verify a previous striking report that ethanol could be oxidized selectively to ethylene oxide, ethanol oxidation on Ag, Cu, or Au nanoparticles supported on Li₂O/γ-Al₂O₃ or γ-Al₂O₃ was examined between 100 and 400 °C. Ag and Cu catalysts were found to be highly selective to acetaldehyde (>95% on Ag below 325 °C and on Cu below 250 °C). On Au, selectivities to acetaldehyde were lower, with higher selectivity to ethyl acetate and acetic acid. No ethylene oxide was observed under any conditions. Our results, including selectivity variations among these metals, are consistent with previous studies of ethanol oxidation over coinage metals supported on γ-Al₂O₃, with no changes in primary product identity and minor changes in selectivity upon addition of Li₂O. Unfortunately, these results are in direct contradiction to previous work reporting the desirable direct conversion of ethanol to ethylene oxide on Ag, Cu, and Au on Li₂O/γ-Al₂O₃.

Full text of DOI:10.1016/j.jcat.2018.05.011

Journal of Catalysis 364 (2018) 40–47
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Selective oxidation of ethanol over Ag, Cu and Au nanoparticles
supported on Li O/c-Al O
2
2 3
Trent L. Silbaugh a,1, Pierre Devlaminck , John A. Sofranko , Mark A. Barteau
b
b
a,
⇑
a
Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA
b
Bio2Electric LLC, Monmouth Junction, NJ 08852, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
In an effort to verify a previous striking report that ethanol could be oxidized selectively to ethylene
oxide, ethanol oxidation on Ag, Cu, or Au nanoparticles supported on Li O/ -Al or -Al was exam-
ined between 100 and 400 °C. Ag and Cu catalysts were found to be highly selective to acetaldehyde
>95% on Ag below 325 °C and on Cu below 250 °C). On Au, selectivities to acetaldehyde were lower, with
Received 12 January 2018
Revised 14 April 2018
Accepted 13 May 2018
2
c
2
O
3
c
2 3
O
(
higher selectivity to ethyl acetate and acetic acid. No ethylene oxide was observed under any conditions.
Our results, including selectivity variations among these metals, are consistent with previous studies of
Keywords:
Selective oxidation
Copper
Silver
Gold
ethanol oxidation over coinage metals supported on
and minor changes in selectivity upon addition of Li
diction to previous work reporting the desirable direct conversion of ethanol to ethylene oxide on Ag, Cu,
and Au on Li O/ -Al
c-Al
2 3
O , with no changes in primary product identity
2
O. Unfortunately, these results are in direct contra-
2
c
2 3
O .
Acetaldehyde
Lithium oxide
Gamma alumina
Ó 2018 Published by Elsevier Inc.
1
. Introduction
With the growing emphasis on fuels and chemicals from renew-
2
ethylene oxide (EO) and, with the addition of Li O to the support,
can do so with EO selectivities above 95% at complete conversion
[6,7]. EO is an important industrial chemical and its direct produc-
tion from ethanol at high conversion and selectivity would be a
very attractive alternative to ethylene epoxidation.
In spite of the myriad potential benefits, including economic
ones, of such a process, as far as we are aware there have been
no published studies to date that have tried to reproduce the Lip-
pits results. We therefore set out to do so. Unfortunately, we find
no evidence for the direct oxidation of ethanol to ethylene oxide.
able resources, much attention has been devoted to catalytic con-
version of biologically derived feedstocks. Of the possible chemical
intermediates, ethanol is attractive because of its large-scale pro-
duction for transportation fuel. While ethanol can be dehydrated
to produce ethylene that can be utilized in the existing petrochem-
ical production chain, direct conversion of ethanol to higher value
chemical products, e.g., in integrated biorefineries [1,2], is highly
desirable.
Gas-phase oxidation of alcohols over copper and silver catalysts
for commercial production of aldehydes dates back to the 19th
century. Studies in the scientific literature since that time have
examined the performance of these catalysts and defined the
mechanism of the reaction. Recent work has shown that ethanol
can selectively produce acetaldehyde over nanoparticles of the
2. Experimental
Two sets of catalyst materials were prepared following the pro-
cedures described by Lippits and Nieuwenhuys [6,7]. The first set
used commercially available pseudo-boehmite (BASF, surface area
2
coinage metals (Ag, Cu, Au) supported on
therefore quite surprising when Lippits and Nieuwenhuys reported
in 2010 that Ag, Cu, and Au supported on -Al can produce
c-Al
2
O
3
[3–5]. It was
ꢀ300 m /s) calcined to 550 °C for 1 h. The resulting gamma alu-
2
mina phase, verified with XRD, had a BET surface area of 260 m /
c
2
O
3
g. LiNO3 (1.38 g) was dissolved in 6.25 ml of ultrapure water and
2 3
added dropwise, at room temperature, to 5.0 g of c-Al O to obtain
a Li/Al ratio of ꢀ3/15. The support was then dried at 110 °C for 8 h
⇑
Corresponding author at: Department of Chemical Engineering, Texas A&M
University, College Station, TX 77843, USA.
followed by calcination at 350 °C (using a 10 °C/min. ramp rate) for
4
h. Following calcination at 350 °C, the Cu, Ag, or Au were depos-
ited on the prepared Li O/ -Al support using homogeneous
deposition-precipitation with urea as precipitating agent. Cu
E-mail address: barteau@tamu.edu (M.A. Barteau).
1
c
2 3
O
Current address: Department of Chemical Engineering, Washington University, St.
Louis, MO 63130, USA.
2
https://doi.org/10.1016/j.jcat.2018.05.011
0
021-9517/Ó 2018 Published by Elsevier Inc.

T.L. Silbaugh et al. / Journal of Catalysis 364 (2018) 40–47
41
(
NO
3
)
2
, AgNO
3
, or HAuCl
4
(all purchased from Alfa-Aesar) was
O/ -Al support material in a
species and a reference (ethanol). GC calibration experiments using
a gas stream containing both ethylene oxide and acetaldehyde pro-
vided retention time separation of ꢀ1 min using the DB-1 column,
allowing for unambiguous resolution of these product species. Spe-
cies assignments were confirmed by gas chromatography with
mass spectrometry (GC–MS (Agilent)) on a separate reactor sys-
tem. The effluent stream was sampled every 30 min, yielding 1
data point during the initial transient observed after reaching a
new stable temperature and 1 data point at steady state per tem-
perature ramp step. Product signals reached steady state after
10–15 min at a new temperature as determined by continuous
sampling with mass spectrometry for all catalysts investigated.
No differences in steady state signals from one ramp cycle to the
next were observed. Data in figures below represent an average
of at least 3 ramp cycles.
added to a suspension of the Li
2
c
2 3
O
ratio of 50 ml of ultrapure water per 1 g support. Urea (3/2 g
urea/g support) was then added to the rapidly stirred suspension,
then heated to 80 °C and held for ꢀ18 h until the solution reached
a pH of 8–8.5. The material was then filtered, washed, and dried
overnight at 80 °C. The intended metal to aluminum ratio was
ꢀ
1/75 for all prepared materials (i.e., 2.7 wt% Ag, 1.6 wt% Cu and
.8 wt% Au). The urea co-deposition step results in the leaching
of lithium from the support, so a higher than intended loading
3/15 Li/Al) was used as the starting material. The intended Li/Al
4
(
ratio following metal deposition is 1/15. Actual Li and metal load-
ings were determined using ICP-OES (Varian) with calibration
standards prepared by diluting relevant standards (Inorganic Ven-
tures). BET surface areas of all final catalysts prepared using the
pseudo-boehmite support (i.e., Me/Li
2
O/
c
-Al
2
O
3
) were 218–231
Conversion was determined by comparing the measured etha-
nol signal to those measured at 100 °C through the reactor (negli-
gible conversion) and through the bypass. This ethanol signal was
collected both before and after running the temperature program.
Using this conversion as a basis, a carbon balance indicated that a
small fraction of ethanol (<10%) decomposed to deposit solid car-
bon or to produce unidentified secondary products at tempera-
tures above 350 °C for all supported nanoparticle catalyst
materials. Below 350 °C, total carbon was balanced within 5% or
better. Selectivity was calculated on a per carbon basis by dividing
by the ethanol conversion and the ratio of the number of carbon
atoms in ethanol and the respective product species.
2
m /g. A comparison of the physical properties of the lithium-
promoted catalysts used in this study with those reported by Lip-
pits and Nieuwenhuys [6,7] is contained in Table S1 of the Supple-
mental Information.
Materials were also synthesized with a lower surface area
gamma alumina support (Inframat Advanced Materials 26R-
2
0
804UPG, surface area of 70–100 m /g). Following calcination at
3
2 3
50 °C, the Cu, Ag, or Au were deposited on the c-Al O support
using homogeneous deposition precipitation with urea as the pre-
cipitating agent. Cu(NO , AgNO , or HAuCl (all purchased from
Alfa-Aesar) was added to a suspension of the -Al support
3
)
2
3
4
c
2 3
O
material (9.98 g) in approximately 50 ml ultrapure water. The urea
precipitation and subsequent treatment steps were carried out as
above. The intended metal to aluminum ratio was ꢀ1/75 as above.
Actual Li and metal loadings were verified by Galbraith Laborato-
ries (ICP-OES).
3. Results
3.1. Characterization of
c-Al
2 3
O , Li
2
O/c
2
-Al O
3 2
, and Metal/Li O/c
2
-Al O
3
Reactions were carried out in either a quartz tube reactor or a
reactor cell (supplied by Harrick) using 40–100 mg (tube reactor)
or 20–30 mg (Harrick cell) of catalyst. The material was loaded into
the reactor and exposed to a pure helium gas stream at 298 K for
catalysts and ethanol selective oxidation over
c
-Al , Li O/
2
O
3
2
c
-Al
2 3
O
Fig. 1 shows the X-ray diffraction patterns of the pseudo-
boehmite (PAL-M 300), calcined for 1 h at 550 °C, and lithium
deposited on the calcined pseudo-boehmite support material. Both
the calcined PAL-M 300 and lithium-deposited support (denoted
1
5 min. The catalyst was then reduced in hydrogen (4 vol%, balance
helium) at 400 °C for 2 h followed by cooling to 100 °C. Upon
reaching a steady temperature of 100 °C (ꢀ1 h), the reactant
stream was switched from bypass to the reactor. The reactant
hereon as Li
the gamma alumina (
2
O/
c
-Al
2
O
3
) exhibited broad features consistent with
-Al ) phase. Following the urea co-
c
2 3
O
stream was prepared by passing a stream of 3 vol% O
2
in He
deposition of Ag and Cu and subsequent reduction of the final cat-
through a bubbler containing absolute ethanol held in a recirculat-
ing cooling bath. The temperature of the bath (ꢀ15 °C) was
adjusted to give an ethanol to oxygen ratio of 1:1. For all experi-
alyst at 400 °C, as performed prior to reaction studies, this material
exhibited an XRD pattern consistent with
2 3
c-Al O with no addi-
tional diffraction peaks, indicating the majority of metal resides
in particles with sizes below the instrument detection limit. Upon
Au deposition, broad diffraction peaks consistent with small crys-
talline gold nanoparticles were observed (Fig. 1). Additional char-
À1
ments carried out in the tubular reactor, a GHSV of 2500 h was
used. These conditions we chosen to match those of the Lippits
experiments [6,7]. For all experiments performed in the Harrick
À1
cell, a GHSV of 10,000 h was used. All experiments were per-
acterization of these Au/Li
2
O/
c-Al
2
O
3
materials with SEM
formed at atmospheric pressure at the catalyst bed outlet.
Following the preparation procedure above and the introduc-
tion of the reactant stream, the temperature was held for 1 h at
provided an average particle size of 4.5 nm. SEM of the Cu and
Ag containing materials found that, relative to the Au materials, a
significantly smaller number of nanoparticles per unit area were
visible with larger diameters of 5–25 nm (See supplemental infor-
mation for representative SEM images). As the molar Metal/Al ratio
is the same for all materials, this smaller number of visible parti-
cles is consistent with the XRD results indicating that the majority
of Cu and Ag does not reside in crystalline nanoparticles >3 nm in
diameter. Based on the XRD and SEM results for the Cu and Ag cat-
alysts, it is clear that urea co-deposition is not an effective synthe-
sis method for producing uniform nanoparticle catalysts using Cu
or Ag Nitrates as precursors. Actual lithium, Au, Ag, and Cu load-
ings for each material used in reaction studies were determined
using ICP-OES and are presented in Table 1.
1
00 °C. Experiments were then carried out with two cycles of the
following temperature program: Increase temperature by 25 °C
at a rate of 5 °C/min, hold 55 min at a constant temperature, repeat
up to 400 °C, lower temperature back to 100 °C. As noted below,
the hold time at each temperature step was sufficient for the
reaction to reach steady state, as determined by mass spectrome-
try. The bed temperature was monitored and controlled using a
k-type thermocouple inserted into the inlet side of the reactor tube
and placed in direct contact with quartz wool at the top of the cat-
alyst bed.
The effluent stream was analyzed using an on-line gas chro-
matograph (Agilent) with a DB-1 column and a molecular sieve
and Hayesep Q column. All measured peaks were identified by
comparison to peak elution times determined using custom gas
mixtures and liquid injections of mixtures containing relevant
Ethanol oxidation data is organized into fractional conversion,
fractional yield and fractional selectivity plots for all materials as
a function of reactor temperature ranging from 100 to 400 °C.
The conversion of ethanol and oxygen, Fig. 2(a), over the calcined

4
2
T.L. Silbaugh et al. / Journal of Catalysis 364 (2018) 40–47
Fig. 1. X-ray diffraction patterns of
-Al , and Cu/Li O/ -Al
2 3 2 2 3 2 2 3 2
c-Al O , Li O/c-Al O , Au/Li O/c-Al O , Ag/Li O/
c
2
O
3
2
c
2 3
O .
Table 1
Catalyst metal and lithium loading as determined by ICP-OES. Materials in rows 1–5
used the pseudo-boehmite material following 1 h calcination step. Rows 6–8 used the
lower surface area
c
-Al
2
O
3
.
wt% metal loading Li/Al (1/15
int.)
1
2
3
4
5
6
7
8
Li
Li
Au/Li
Ag/Li
Cu/Li
2 3
Au/c-Al O
2
O/
O/
c
c
-Al
-Al
c
c
c
2
O
O
-Al
-Al
-Al
3
3
(for reaction studies)
(for synthesis of 3–5)
–
–
1/15
3.6/15
1.3/15
1.3/15
1.5/15
–
2
2
2
O/
2
O
3
4.5
1.9
1.6
4.3
2.1
1.4
2
O/
2
O
O
3
2
O/
2
3
Ag/
Cu/
c
-Al
2
O
3
–
–
c
-Al
2
O
3
Fig. 2. (a) Conversion of ethanol and oxygen, (b) fractional yield, and (c) fractional
selectivity as a function of reactor temperature for the selective oxidation of ethanol
over
c
-Al
2
O
3
. Reactant stream was 3% EtOH with 1:1 EtOH:O
2
and a GHSV of 2500
2 3
pseudo-boehmite (c-Al O ) material shows limited conversion
À1
h
.
below 200 °C followed by a sharp increase up to 0.80 at 300 °C.
Above 300 °C, the ethanol conversion continues to increase more
slowly with temperature. Concurrently with this ethanol conver-
sion increase, oxygen conversion increases linearly from 0.02 at
2 3 2 2 3
3.2. Ethanol selective oxidation over Au/c-Al O and Au/Li O/c-Al O
3
00 °C to 0.25 at 400 °C. Fractional yield, Fig. 2(b), and selectivity,
Fig. 2(c), indicate that the dehydration product diethyl ether dom-
inates at low temperature, with ethylene and acetaldehyde pro-
duction increasing above 225 °C at the expense of diethyl ether.
Along with the products mentioned, a number of unidentified
products (not plotted in Fig. 2(a) and (b) were detected in gas chro-
matography in small amounts at elevated temperature, accounting
for ꢀ20% of the observed conversion at 400 °C based on closure of
the carbon balance.
Fig. 4(a) plots the conversion of EtOH and O
2
versus tempera-
ture over Au/ -Al . Small increases in conversion of both species
c
2 3
O
occurred up to 175 °C, followed by larger changes in conversion at
temperatures up to 300 °C. Above 300 °C, conversion of oxygen
remained unchanged at 0.45, while conversion of ethanol
increased slowly from 0.68 at 300 °C to 0.85 at 400 °C. The EtOH
to O
range.
The fractional yield plotted versus temperature over Au/
, Fig. 4(b), shows that the dominant product is acetaldehyde,
2
conversion ratio ranged from 2:1 to 3:2 over this temperature
The addition of Li
2
O to the alumina support shows an overall
c-
suppression of the catalytic activity, Fig. 3, with conversion begin-
2 3
Al O
ning only above 225 °C and reaching a maximum of 0.45 at 400 °C.
with a maximum yield of 0.62 at 325 °C. Ethyl acetate was pro-
duced, with a maximum yield also at 325 °C. The shapes of the
yield versus temperature curves were very similar for acetalde-
hyde and ethyl acetate. As the production of both of these species
began to decrease above 325 °C, the production of diethyl ether
(labelled as ether in relevant figures) and ethylene began to
increase.
In contrast to
of ethanol, with EtOH:O
temperature to 1.2:1 at 400 °C. Also in contrast to
mary product on Li O/ -Al was acetaldehyde, with a selectivity
c-Al
2
O
3
, the oxygen conversion roughly tracked that
conversion ratios between 2:1 at low
-Al , the pri-
2
c
2 3
O
2
c
2 3
O
above 0.85 over the entire temperature range of 100–400 °C. Selec-
tivities of all additional products, including diethyl ether and ethyl
acetate, were below 0.1 for all temperatures. The low diethyl ether
yield, particularly at low temperature indicates that the surface
coverage of Li O at these loadings is sufficient to remove the
2
majority of acid activity. The small amount of diethyl ether
observed is likely due to some residual acid sites.
2 3
The maximum selectivity to acetaldehyde over Au/c-Al O ,
0.95, was observed between 175 and 250 °C. As shown in Fig. 4
(c), above 250 °C the selectivity to acetaldehyde began to decrease
linearly as selectivity to diethyl ether and ethylene increased to
maxima of 0.18 and 0.12, respectively, at 400 °C. The selectivity

T.L. Silbaugh et al. / Journal of Catalysis 364 (2018) 40–47
43
Fig. 4. (a) Conversion of ethanol and oxygen, (b) fractional yield, and (c) fractional
Fig. 3. (a) Conversion of ethanol and oxygen, (b) fractional yield, and (c) fractional
selectivity as a function of reactor temperature for the selective oxidation of ethanol
selectivity as a function of reactor temperature for the selective oxidation of ethanol
2 3 2
over Au/c-Al O . Reactant stream was 3% EtOH with 1:1 EtOH:O and a GHSV of
2500 h .
À1
over Li
2
O/
c
-Al
2
O
3
. Reactant stream was 3% EtOH with 1:1 EtOH:O
2
and a GHSV of
À1
2
500 h .
duction of acetic acid increased linearly. Coinciding with the pla-
teau of ethyl acetate production at and above 250 °C, a larger
linear increase in the acetic acid yield was observed, with a maxi-
mum of 0.32 at 350 °C, followed by a sharp decrease above 350 °C.
The maximum selectivity to acetaldehyde over these Au catalysts,
to ethyl acetate was greatest at 100 °C (0.16), at ethanol fractional
conversions less than 0.02. Ethyl acetate selectivity then decreased
to 0.05 at 200 °C, followed by a slow increase up to 0.10 at 325 °C.
Very little CO or CO were produced over this temperature range,
2
with selectivities below 0.02 for both species. No ethylene oxide
was observed under any conditions.
0
.88, was observed at 150 °C. As shown in Fig. 5(c), the selectivity
to acetaldehyde and ethyl acetate both decrease at the expense of
acetic acid as temperature increases up to 350 °C. The selectivity to
ethyl acetate was greatest at 100 °C (0.38), at ethanol conversions
less than 0.02. Selectivity to acetic acid was maximized at 0.35 at
The conversion of EtOH and O
O/ -Al is shown in Fig. 5(a). Conversion of ethanol began at
00 °C and increased linearly up to a maximum of 0.95 at 350 °C,
2
versus temperature over Au/
Li
1
2
c
2 3
O
followed by a leveling off and subsequent decrease up to the max-
imum temperature of 400 °C investigated here. Oxygen conversion
increased linearly with temperature starting at 150 °C, with a slope
slightly lower than that of ethanol, up to a temperature of 250 °C.
Over the range 250–350 °C, the change in oxygen conversion with
temperature roughly mirrored that of ethanol. As a result, the ratio
of ethanol to oxygen conversion decreases with increasing temper-
ature, falling from 4:1 at 175 °C to 1.2:1 at 350 °C.
The fractional yield versus temperature, Fig. 5(b), shows that
the dominant product throughout is acetaldehyde, with maxima
in the yield profile of 0.55 at 325 °C and 0.57 at 400 °C. Ethyl acet-
ate was produced, with yield plateauing at 0.11 at 250 °C. The
shapes of the yield versus temperature curves were very similar
for acetaldehyde and ethyl acetate. Between 200 and 250 °C, pro-
3
50 °C. No ethylene oxide was observed under any conditions.
3.3. Ethanol selective oxidation over Ag on
Al O
2 3 2
c-Al O and Ag on Li O/c-
2
3
Ethanol oxidation on Ag and Cu catalysts showed even higher
selectivities to acetaldehyde than on gold catalysts. Fig. 6(a) shows
the ethanol and oxygen conversion over Ag on Li O/ -Al O . Con-
version of both reactants was negligible at 100 °C and increased
slowly up to an ethanol (EtOH) conversion of 0.07 at 175 °C.
Between 175 and 325 °C, EtOH conversion increased from 0.07 to
0.94, with complete conversion achieved by 350 °C. Oxygen con-
version was approximately half that of ethanol up to a temperature
2
c
2 3

4
4
T.L. Silbaugh et al. / Journal of Catalysis 364 (2018) 40–47
Fig. 5. (a) Conversion of ethanol and oxygen, (b) fractional yield, and (c) fractional
selectivity as a function of reactor temperature for the selective oxidation of ethanol
Fig. 6. (a) Conversion of ethanol and oxygen, (b) fractional yield, and (c) fractional
selectivity as a function of reactor temperature for the selective oxidation of ethanol
2 2 3 2
over Au Li O/c-Al O . Reactant stream was 3% EtOH with 1:1 EtOH:O and a GHSV
2 2 3 2
over Ag Li O/c-Al O . Reactant stream was 3% EtOH with 1:1 EtOH:O and a GHSV
À1
of 2500 h
.
À1
of 2500 h
.
of 225 °C, as expected for stoichiometric reaction with ethanol to
form acetaldehyde plus water. Oxygen conversion increased
roughly linearly from 0.14 at 225 °C up to a conversion of 0.96 at
3
Al
.4. Ethanol selective oxidation over Cu on Li
O
2 3
2 2 3
O/c-Al O and Cu on c-
4
00 °C.
The fractional product yield is shown in Fig. 6(b), with the for-
mation of acetaldehyde (labeled aldehyde in all relevant figures) as
the major product up to 325 °C. Above 325 °C, CO began to
The conversion of EtOH and O
ature over Cu/Li O/ -Al is shown in Fig. 7(a). Conversion of
EtOH increased roughly linearly from ꢀ0 at 100 °C to 0.1 at 275
C. Oxygen conversion was approximately half the EtOH conver-
2
as a function of reactor temper-
2
c
2 3
O
2
°
increase steadily with temperature. Other products included acetic
acid, ethylene and ethyl acetate, although yields of all of these spe-
cies were below 0.02. Unaccounted-for carbon (i.e., the discrepancy
in mass balance closure) increased above 325 °C to 0.02 at 400 °C.
However, no appreciable loss of activity was observed upon repe-
tition of experiments using the same catalyst. Fractional selectivity
versus temperature is plotted in Fig. 6(c). Selectivity to acetalde-
hyde was above 0.95 between 100 and 350 °C. Coupled with the
steep increase in conversion occurring above 200 °C, an operating
range of 325–350 °C yielded both high conversion > 0.90 and
sion over this temperature range. A sharp increase in EtOH conver-
sion with temperature was then observed between 275 and 400 °C.
A sharp increase in O
2
conversion was also seen between 300 and
4
00 °C, with a conversion of 0.8 reached at 400 °C. The ratio of
2
EtOH to O conversion changed from approximately 2 at 275 °C
to 1.1 at 400 °C.
A plot of the fractional product yield versus temperature, Fig. 7
(
b), shows a linear increase of acetaldehyde from 150 to 275 °C,
roughly tracking EtOH conversion. Above 275 °C, a greater increase
in acetaldehyde yield with temperature was observed along with
selectivity > 0.95. A roughly linear increase in selectivity to CO
and carbon) was seen in the range 350–400 °C at the expense of
acetaldehyde. All other species exhibited selectivities less than
.02. No ethylene oxide was detected. Reaction studies of Ag/
Al exhibited comparable yield and selectivity to acetaldehyde
as those observed with the Li O-containing materials described
above (See Supporting Information for Ag/ -Al data).
2
the formation of diethyl ether, ethyl acetate, CO
2
and ethylene.
(
Beginning at 300 °C, the yields of CO and CO increased linearly
2
with increasing temperature. Ethylene yield continued to increase
above 325 °C while acetaldehyde yield leveled off and decreased at
0
c-
2
O
3
3
75 °C.
The fractional selectivity is plotted in Fig. 7(c) versus reactor
temperature. Selectivity to acetaldehyde was >0.98 between 100
2
c
2 3
O

T.L. Silbaugh et al. / Journal of Catalysis 364 (2018) 40–47
45
Fig. 7. (a) Conversion of ethanol and oxygen, (b) fractional yield, and (c) fractional
selectivity as a function of reactor temperature for the selective oxidation of ethanol
over Cu Li O/ -Al . Reactant stream was 3% EtOH with 1:1 EtOH:O and a GHSV
Fig. 8. (a) Conversion of ethanol and oxygen, (b) fractional yield, and (c) fractional
selectivity as a function of reactor temperature for the selective oxidation of ethanol
2
À1
c
2
O
3
2
of 2500 h
.
2 3 2
over Cu/c-Al O . Reactant stream was 3% EtOH with 1:1 EtOH:O and a GHSV of
À1
2
500 h .
and 225 °C. Diethyl ether and ethyl acetate were formed at 200–
50 °C, with peak selectivities of 0.07 and 0.4 respectively at 300
C. No ethylene oxide was detected.
The conversion of EtOH and O as a function of reactor temper-
ature over Cu/ -Al is shown in Fig. 8(a). Conversion of EtOH
3
°
325 °C, with a peak selectivity of 0.06 at 275 °C. Again, no ethylene
oxide was detected.
2
c
2 3
O
increased roughly linearly from ꢀ0 at 125 °C to 0.1 at 225 °C. Oxy-
4. Discussion
gen conversion was about half the EtOH conversion over this tem-
perature range.
temperature was then observed between 250 and 325 °C followed
by a leveling off from 350 to 400 °C at ꢀ0.95. A sharp increase in O
conversion was also seen between 250 and 325 °C, with complete
conversion of O achieved by 350 °C. The ratio of EtOH to O con-
A
sharp increase in EtOH conversion with
Direct conversion of ethanol to ethylene oxide with high selec-
tivity (>90%) and high conversion under mild conditions (near
atmospheric pressure and 200 °C), as reported in work by Lippits
et al., would be a very attractive alternative to the industrial ethy-
lene epoxidation process over a promoted Ag catalyst supported on
2
2
2
version changed from approximately 2 at 250 °C to 1.25 at 300 °C
to 0.9 at 325 °C.
a
-Al
2
O
3
. To this end, the present work was carried out in order to
containing materials
reproduce the experiments using Li
2
O
A plot of the fractional product yield versus temperature, Fig. 8
b), shows a linear increase of acetaldehyde from 125 to 225 °C,
roughly tracking EtOH conversion. Above 250 °C, a greater increase
reported by Lippits [6,7]. A lower surface area gamma alumina
was also used in order to directly compare materials prepared
via urea co-deposition (present work) to other ethanol oxidation
(
in acetaldehyde yield with temperature was observed along with
studies available in the literature for metals deposited on
prepared using other methods.
2 3
c-Al O
the formation of CO
2
, CO and ethylene. Beginning at 325 °C, the
yields of CO and CO became nearly constant with increasing tem-
perature. Ethylene yield continued to increase above 325 °C while
acetaldehyde yield leveled off and decreased at 350 °C.
The fractional selectivity is plotted in Fig. 8(c) versus reactor
temperature. Selectivity to acetaldehyde was >0.98 between 100
and 225 °C. A small amount of ethyl acetate was formed at 200–
2
It was found that the results obtained in the present work dif-
fered greatly from the original reports for the lithium oxide-
containing materials, with high selectivity to acetaldehyde
observed here on all investigated materials rather than high selec-
tivity to ethylene oxide as previously reported [6,7]. The catalysts
used here were synthesized using the same procedures communi-

4
6
T.L. Silbaugh et al. / Journal of Catalysis 364 (2018) 40–47
cated in the earlier publications [6,7] and experiments were car-
ried out in a tubular flow reactor with a GHSV of 2500 h to match
ilarly, the partial oxidation to acetaldehyde was suppressed over
the lithium-containing material, with yields of roughly one third
À1
the previous work. Carrying out experiments with the same mate-
rial in a Harrick cell reactor with a GHSV of 10,000 h led to much
that of the Cu/
the lower oxidation activity with the addition of Li
mum yield over the Cu/Li O/ -Al (0.63) occurs at 375 °C, while
the very similar maximum (in terms of magnitude (0.60)) on Cu/
Al occurs at 300 °C.
Based on the present results and above discussion, it is clear
that the addition of Li O has a negligible effect on the identity of
the primary selective oxidation product, acetaldehyde. Comparison
of the present M/ -Al results to previous literature shows
c
-Al
2
O
3
material at 300 °C. Further underscoring
À1
2
O, the maxi-
lower conversions (<10%), but did not appreciably change the pro-
duct distribution. A high selectivity to acetaldehyde was main-
tained up to higher temperatures in the Harrick cell, but no
ethylene oxide was observed. The discrepancy between the present
and previous work is therefore not due to slight differences in
experimental conditions, as no ethylene oxide was produced under
any experimental conditions investigated here.
2
c
2 3
O
c-
2 3
O
2
c
2 3
O
The selective oxidation of ethanol to acetaldehyde in the pres-
ence of oxygen has been previously observed on Ag, Cu and Au par-
ticles supported on a range of oxide supports exhibiting basic,
neutral and acidic properties [3–5,8,9]. Screening of 23 separate
oxide support materials for ethanol oxidation over supported Au
nanoparticles by Takei et al. [5] found that the support affected
whether the catalyst produced primarily acetaldehyde (mild
oxidative activity), acetaldehyde and acetic acid (deep oxidative
excellent agreement in both overall conversion and product selec-
tivity for all three metals investigated. Our results are consistent
with numerous previous studies that have shown acetaldehyde
as either a major product derived from ethanol or an intermediate
species that further reacts to form acetic acid or ethyl acetate over
supported Ag, Cu and Au catalysts. The addition of Li
2
O oxide
serves only to suppress acid catalysis by the -Al support and
c
2 3
O
to alter the selectivity of secondary oxidation reactions. To our
knowledge, no studies, other than those of Lippits, have reported
ethylene oxide formation.
activity), or complete oxidation to CO
ditions. It was found that Au/ -Al was 99% selective to acetalde-
hyde at 120 °C and produced acetaldehyde, CO and diethyl ether
2
under similar reaction con-
c
2 3
O
2
In spite of our best efforts to reproduce the catalysts described in
papers by Lippits and Nieuwenhuys [6,7], it is possible that there is
some unique feature of the synthesis or catalyst properties that we
have failed to reproduce adequately. However, one must also enter-
tain the possibility that the discrepancy lies in issues with product
identification. The gas chromatograph used in the Lippits work was
outfitted with both a CTR-1 column and a molecular sieve in series
with a Hayesep Q column. The CTR-1 column is primarily used for
at 280 °C. The only other minor products reported to have been
detected in that study were acetone and ethyl acetate.
The similarity between the present results and those presented
by Takei et al. [5] confirms that the urea co-deposition synthesis
method does not lead to appreciably different product selectivities
2 3
for the Au/c-Al O catalyst, with only slightly differences in selec-
tivities to diethyl ether and lack of ethylene formation at elevated
temperature in the Takei et al. report. The increase in selectivity to
ethyl acetate and the appearance of acetic acid production over the
2 2 2 2
the separation of CO, CO , H , O and N and no information on its
capability for ethylene oxide/acetaldehyde separation could be
found in the literature. The Hayesep Q column cannot separate
acetaldehyde and ethylene oxide based on the following two pieces
of information: (1) relative retention time data reported on the
Hayesep product website [11] for vinyl chloride and ethylene oxide
from Hayesep Q, R and S columns (6.04 and 6.06, 9.04 and 8.78, and
9.7 and 9.7 min respectively referenced to an ethane retention time
of 1 min) show that vinyl chloride and ethylene oxide cannot be
separated and (2) comparison of Poropak R and S (equivalent to
Hayesep R and S) columns found an overlap of vinyl chloride, ethy-
lene oxide, and acetaldehyde peaks [12]. Although Lippits [6,7]
reported verification of ethylene oxide production by mass spec-
trometry and by bubbling the product stream through a NaOH solu-
tion to form of ethylene glycol, (indicative of the presence of
ethylene oxide), descriptions of the quantitative analysis of these
results are lacking in the original works.
Li
et al. for a support exhibiting deeper oxidative activity. This is
unsurprising as reactor studies for the Li O/ -Al support pre-
2
O-containing materials is consistent with the results from Takei
2
c
2 3
O
sented here already indicated greater oxidation activity and the
suppression of acid-catalyzed products.
The results from the present work on Ag/
with previous work on ethanol oxidation over Ag/
McCabe and Mitchell [3] that also identified acetaldehyde and
CO as principal products. That work focused on catalysts exhibit-
ing complete oxidation of ethanol to CO for automotive exhaust
c-Al
2
O
3
are consistent
2 3
c-Al O by
2
2
applications and therefore their experiments were conducted at
lower ethanol concentrations (0.1 vol%) in excess oxygen and
À1
much higher space velocities (52,000 h ). Nonetheless, observa-
2
tions of an increase in acetaldehyde yield at 175 °C and CO yield
beginning above 300 °C from those experiments are consistent
with the present work. The similarities between the present work
It is not surprising that the oxidation of ethanol would result in
the more thermodynamically stable acetaldehyde product. The
on Ag/Li
indicates that suppression of Al acid sites has very little effect on
the oxidation product selectivity over Ag supported on -Al
2 2 3 2 3
O/c-Al O and the present and past studies on Ag/c-Al O
3+
conversion of ethylene to ethylene oxide on
2 3
a-Al O supported
c
2
O
3
,
Ag, the incumbent technology for EO production, has been shown
to take place on supported silver particles and preferentially on
extended Ag(1 0 0) facets [10]. The key intermediate in this reac-
suggesting a limited support effect on the oxidation of ethanol by
silver. This is in contrast with the support effects discussed above
for the oxidative activity of gold nanoparticles.
2 2
tion has been identified as a surface oxametallacycle (–CH CH -
Ethanol oxidation over Cu/
gen ratio has also been reported by Poulopoulos et al. [4] with sim-
ilar results to those found here for Cu on -Al . In that work, the
authors reported a rise in conversion beginning at ꢀ225 to 250 °C,
with complete conversion achieved by 350 °C. The principal pro-
duct was also found to be acetaldehyde, with a maximum yield
occurring at the same temperature, 350 °C, as found here. A linear
decrease in yield of acetaldehyde between 350 and 400 °C and the
c
-Al
2
O
3
using a 1:1 ethanol to oxy-
O–) bound through both the terminal oxygen and carbon atoms
[13]. While this intermediate can be made directly from ethylene
and atomic oxygen, creating an oxametallacycle intermediate on
a surface from ethanol would require the preferential breaking of
c
2 3
O
a
c-H bond in an adsorbed ethoxy species. However, b-hydride
elimination from ethoxy, resulting in acetaldehyde formation, has
a much lower barrier and should proceed more readily on all three
metal surfaces investigated here (Ag, Cu and Au) [14,15]. Even if an
oxametallacycle were formed, the barrier on an unpromoted Ag
surface for the formation of acetaldehyde from the oxametallacyle
is comparable to the barrier to make EO [16], making the high EO
selectivities and complete absence of acetaldehyde reported previ-
ously [6,7] unlikely.
nearly constant yield of CO
our experiments, were also observed. The addition of Li
Al support largely serves to suppress oxidation activity. The
complete oxidation of ethanol to CO was greatly decreased with
the Li O addition, particularly at temperatures below 375 °C. Sim-
2
above 300 °C, depicted in Fig. 8(b) for
2
O to the
c-
2 3
O
2
2

T.L. Silbaugh et al. / Journal of Catalysis 364 (2018) 40–47
47
The
addition of Li
c
-Al
2
O
3
support was shown to exhibit residual acidity upon
O in the previous reports by Lippits, as evidenced by
5. Conclusion
2
the appearance of esterification and dehydration products upon
exposure to ethanol [6]. Acid sites on the support can also catalyze
Ethanol is selectively oxidized to acetaldehyde over Ag, Cu, and
Au nanoparticles supported on Li O/ -Al O , consistent with most
2
c
2 3
opening of epoxide rings. This is one of the reasons that
rather than -Al is used for supported silver catalysts for ethy-
lene epoxidation. High ethylene oxide yields and selectivities from
a-Al
2
O
3
previous reports on ethanol oxidation over nanoparticle catalysts
supported on -Al O and many other oxides materials. The addi-
tion of Li O to the
c
2
O
3
c
2
3
2
2 3
c-Al O support was not observed to shift in
any feedstock over a
improbable.
c-Al
2
O
3
support with residual acidity are thus
selectivity from acetaldehyde to ethylene oxide as the primary pro-
duct, in contrast to the reports by Lippits and Nieuwenhuys [6,7].
A final point of discussion is the relatively high selectivity to
ethyl acetate (EA) observed in the present work on Au/ -Al
and acetic acid and EA on Au/Li2O/ -Al . Takei et al. [5] reported
formation of EA only at 120 °C over Au/ -Al . Figs. 4(c) and 5(c)
show EA selectivity at all temperatures investigated in the present
work over both supports. It is known that EA is produced on Au
surfaces through the coupling of ethoxy and acetaldehyde moieties
Rather, Li O serves to remove support acid activity and alter the
selectivity to secondary oxidation products.
2
c
2
O
3
c
2 3
O
c
2 3
O
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jcat.2018.05.011.
[
17]. Oxygen adatoms are required to initially remove the hydroxyl
hydrogen of ethanol to make ethoxy and may also participate in
the b-H elimination to form acetaldehyde [18]. As bulk Au is unable
to dissociate molecular oxygen, oxygen adatoms must be supplied
either through the addition of promoters or via a support material.
Through very careful surface science studies using oxygen precov-
ered Au(1 1 1), it has been confirmed that providing a source of
atomic oxygen is all that is required for selective cross-coupling
of alcohols [18]. Nanoporous Au (npAu) has been shown to be very
selective toward EA and acetaldehyde, producing equal parts of
each [19]. While the formation of acetaldehyde and EA is believed
to take place exclusively on the Au surface, the source of atomic
oxygen has been attributed to the presence of small amounts of
Ag (ꢀ1%) left behind during the dealloying process used to make
the material that can activate molecular oxygen [20]. The selectiv-
ity to EA was found to be relatively insensitive to oxygen fraction
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