Synergistic effect of gold and copper in the catalytic conversion of ethanol to linear Α-alcohols

Article abstract of DOI:10.1134/S0965544116080120

The reaction features of the direct conversion of ethanol to butanol-1 and hexanol-1 in the presence of mono- and bimetallic active components based on Au and Cu supported on γ-Al₂O₃ have been studied. It has been found that under conditions providing the supercritical state of ethanol, the reaction rate and selectivity in the presence of the Au–Cu/Al₂O₃ catalyst abruptly increases. In addition, a synergistic effect is observed: the yield of desired products over the Au–Cu catalyst is 6 or 14 times that over the Au or Cu monometallic counterpart, respectively. Differences in the catalytic behavior of the Au–Cu, Au-, and Cu-based systems have been discussed taking into account their structural features and the reaction mechanism.

Full text of DOI:10.1134/S0965544116080120

ISSN 0965-5441, Petroleum Chemistry, 2016, Vol. 56, No. 8, pp. 730–737. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © S.A. Nikolaev, A.V. Chistyakov, P.A. Zharova, M.V. Tsodikov, I.N. Krotova, D.I. Ezzgelenko, 2016, published in Neftekhimiya, 2016, Vol. 56, No. 5,
pp. 502–508.
Synergistic Effect of Gold and Copper in the Catalytic Conversion
of Ethanol to Linear α-Alcohols
S. A. Nikolaeva, b, *, A. V. Chistyakovc, d, P. A. Zharovac, M. V. Tsodikovc, d
I. N. Krotovaa, and D. I. Ezzgelenkoa
,
a Moscow State University, Moscow, Russia
b Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, Russia
c Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, Russia
d Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia
*e-mail: serge2000@rambler.ru
Received February 11, 2016
Abstract—The reaction features of the direct conversion of ethanol to butanol-1 and hexanol-1 in the pres-
ence of mono- and bimetallic active components based on Au and Cu supported on γ-Al2O3 have been stud-
ied. It has been found that under conditions providing the supercritical state of ethanol, the reaction rate and
selectivity in the presence of the Au–Cu/Al2O3 catalyst abruptly increases. In addition, a synergistic effect is
observed: the yield of desired products over the Au–Cu catalyst is 6 or 14 times that over the Au or Cu mono-
metallic counterpart, respectively. Differences in the catalytic behavior of the Au–Cu, Au-, and Cu-based
systems have been discussed taking into account their structural features and the reaction mechanism.
Keywords: gold, copper, nanoparticles, synergism, butanol, ethanol, catalysis
DOI: 10.1134/S0965544116080120
The search for alternative sources of hydrocarbons activity in the low-temperature oxidation of CO [6].
and the development of methods for producing energy Later, it was found that Au nanoparticles are effective
carriers and organic monomers are largely focused on catalysts for other reactions: hydrogenation [7, 8],
the conversion of biomass products [1, 2]. The most isomerization [9, 10], hydrodechlorination [11, 12],
available and common product derived from biomass and hydrocarbon conversion [13, 14]. Recently, it has
is ethanol. In 2013, the production of ethanol was been reported that oxide-supported bimetallic com-
85.3 million tons per annum [3]. Along with ethanol, posites of copper-promoted gold clusters exhibit
other lower (C1 and C3–C6) alcohols are being seen to high activity and selectivity in the steam reforming of
methanol [15] and the reduction of p-nitrophenol to
p-aminophenol [16]. Thus, the development of Au–
Cu catalysts for ethanol conversion to linear α-alco-
hols seems to be extremely promising.
In this paper, we present the results of study of the
basic features of the catalytic behavior of alumina-
supported Au–Cu, Au, and Cu nanocomposites in the
conversion of ethanol to butanol-1 and hexanol-1
under conditions providing the supercritical state of
ethanol and under conventional subcritical conditions
under which organic substrates are in the gaseous
state.
have a rapid growth in production now. In the past
decade, chemistry has been significantly enriched in
novel reactions for conversion of ethanol and its
homologues to hydrocarbons of various classes, which
are important components of motor fuels and valuable
petrochemical monomers and can be used as solvents
and surfactant components [1–4]. A promising pro-
cess, which is currently enjoying a surge of attention, is
a direct catalytic reaction yielding higher alcohols
owing to the condensation of the hydrocarbon skele-
ton of ethanol and/or the alkylation (cross-coupling)
of ethanol with other lower alcohols [1–5]. However,
it should be noted that the research in this area largely
concerns homogeneous catalysis; therefore, the
implementation of the research results in industry is
complicated by the known difficulties of isolation and
regeneration of catalyst systems.
EXPERIMENTAL
A Cu/Al2O3 catalyst containing 0.07 wt % Cu was
prepared by incipient wetness impregnation of
the support [17]. To this end,
a
calculated
Gold had long been disregarded as a catalytic
metal—until 1989 when Japanese chemist M. Haruta
amount of Cu((NO3)2 ⋅ 6H2O (Sigma-Aldrich, 98%)
showed that Au particles of a 2–5 nm size exhibit high was dissolved in water. The resulting solution was
730

SYNERGISTIC EFFECT OF GOLD AND COPPER
731
added to γ-Al2O3 pellets calcined at 400°C for 3 h (AO
Angarsk Catalysts and Organic Synthesis Plant,
160 m2/g, pellet diameter of 0.5 mm). After that, the
wet pellets were dried at 25°C for 24 h and calcined at
400°C for 6 h.
The gaseous reaction products were analyzed by
gas chromatography using a Kristall-4000M instru-
ment (flame ionization detector, HP-PLOT column)
for gaseous C1–C5 hydrocarbons and a Kristall-4000
chromatograph (thermal conductivity detector, SKT
column) for CO, CO2, and H2. The qualitative com-
position of the liquid organic reactants was deter-
mined by gas chromatography–mass spectrometry on
Agilent MSD 6973 (HP-5MS column) and Delsi Ner-
mag Automass-150 instruments (CPSil-5 column) at
EI = 70 eV. The quantitative determination of the liq-
uid reactants was performedby gas–liquid chromatog-
raphy on a Varian 3600 instrument (Chromatec col-
umn; SE-30; 0.25 × 250 cm; Df = 0.3 mm; 50°C
(5 min); 10 °C/min; 280°C; tinj = 250°C; Pinj = 1 bar;
split ratio, 1/200; flame ionization detector; internal
standard, n-octane).
The metal loading of the catalysts was determined
by atomic absorption spectrometry on a Thermo iCE
3000 instrument [17]. The relative error in the mea-
surement of the metal content by this method does not
exceed 1%. X-ray diffraction (XRD) patterns of the
catalysts were recorded by XRD analysis on a Rigaku
D/MAX2500 instrument using a CuKα radiation
source. Transmission electron microscopy (TEM)
images of the catalysts were taken with a JEOL JEM
2100F/UHR instrument with a resolution of 0.1 nm.
Prior to examination, 0.1 g of the sample was placed in
30 mL of C2H5OH and sonicated for 300 s. A droplet
of the resulting mixture was placed on a standard car-
bon-coated TEM grid, dried for 1 h, placed under the
microscope, and studied. Particle size was determined
as the maximum linear size. The average particle size
was determined by statistical processing of data on
250–300 particles. The chemical composition of the
particles was determined by energy dispersive X-ray
spectroscopy (EDS) analysis on a JED-2300 instru-
ment.
An Au/Al2O3 catalyst containing 0.2 wt % Au was
prepared by ion exchange [18, 19]. To this end, a cal-
culated amount of HAuCl4 (Sigma-Aldrich, 98%) was
dissolved in water. The solution pH was adjusted to 7.0
using NaOH (Reakhim, 0.1 M); the solution was
poured to Al2O3 pellets in water; the resulting mixture
was stirred at 50°C for 1 h. After that, the pellets were
separated from the mother liquor, washed with 4 L of
water, dried at 25°C for 24 h, and calcined at 400°C for
3 h. After the first calcination, a portion of the
Au/Al2O3 precursor was used to prepare an Au–
Cu/Al2O3 catalyst; the remaining portion was calcined
again at 400°C for 3 h to prepare an Au/Al2O3 catalyst.
An Au–Cu/Al2O3 catalyst containing 0.2 and
0.07 wt % of Au and Cu, respectively, was prepared by
incipient wetness impregnation. To this end, a portion
of the Au/Al2O3 precursor obtained after the first cal-
cination at 400°C was impregnated with a copper
nitrate solution in water, dried at 25°C for 24 h, and
calcined at 400°C for 3 h.
The catalysts were tested on a Parr 5000 Series mul-
tireactor autoclave-type setup with the reactor volume
of 45 mL. In a standard test on the conversion of eth-
anol in the supercritical state, the reactor was charged
with 25 mL of ethanol and 5.6 g of the catalyst. The
reactor was sealed and purged with argon to remove
oxygen. Next, argon was pumped out to a residual
pressure of 0.1 atm. The reaction was conducted at a
temperature of 245, 275, and 295°C for 5 h. The reac-
tion mixture was agitated with a magnetic stirrer (rota-
tion speed of 1200 rpm). For the above conditions, the
initial ethanol pressure in the reactor was 100 atm;
during the experiment (5 h), the pressure was
increased to 110–120 atm.
RESULTS AND DISCUSSION
In a standard test on the conversion of ethanol in
the subcritical state, the reactor was charged with 5 mL
of ethanol and 1.1 g of the catalyst. The reactor was
sealed and purged with argon; after that, argon was
bled to a residual pressure of 0.1 atm. Next, the reactor
was heated to 275°C and the stirring of the reaction
mass was turned on. The reaction time was 5 h. For the
above conditions, the initial ethanol pressure in the
reactor was 50 atm; during the experiment (5 h), the
pressure was increased to 55–60 atm.
Features of Catalytic Behavior of Au–Cu, Au,
and Cu Catalysts
At 275°C in the presence of the Au–Cu catalyst,
ethanol in the supercritical state (partial pressure of
120 atm) is converted to butanol-1 and hexanol-1 with
2–3 times higher selectivities than those achieved in
the conversion of gaseous ethanol (Table 1). It should
also be noted that in this study, the product yield over
the Au–Cu catalyst is an order of magnitude above
that of the desired alcohols in the presence of other
heterogeneous catalysts studied in [20–22].
Upon the completion of each catalytic test, the
reactor was cooled with water to room temperature;
the entire amount of the reaction gas was withdrawn
Table 1 shows that the ethanol conversion and the
into a gas holder to conduct qualitative and quantita- alcohol selectivity over the Au–Cu catalyst signifi-
tive analyses of the gas mixture. After that, the reactor cantly depend on temperature. The maximum selec-
was unsealed to withdraw the liquid reaction mixture tivity for butanol-1 (74.4%) and hexanol-1 (17.8%) is
for analysis.
achieved at 275°C. An increase in the reaction tem-
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732
NIKOLAEV et al.
Table 1. Ethanol conversion and selectivity for desired products in the conversion of ethanol in the presence of mono- and
bimetallic catalysts. The data are derived under conditions providing the supercritical state of ethanol (ethanol partial pres-
sure in the reactor of 100–120 atm)
Selectivity, %
Ethanol
conversion, %
Catalyst
Time, h
T, °C
butanol-1
hexanol-1
Au–Cu/Al2O3
Au–Cu/Al2O3
Au–Cu/Al2O3
Au–Cu/Al2O3 *
Au–Cu/Al2O3**
Cu/Al2O3
5
5
245
275
295
275
275
275
275
4.6
33.4
35.1
15.1
32.9
11.5
30
57.9
74.4
57.2
25.3
75.1
0.2
3
17.8
6.4
7.2
17.5
0
5
5
50
5
Au/Al2O3
5
15.9
0.5
* Data for gaseous ethanol (ethanol partial pressure in the reactor of 50–60 atm). ** Data derived in ten consecutive 5-h catalytic cycles.
Table 2. Qualitative and quantitative composition of the ethanol conversion products over the Au–Cu catalyst under con-
ditions providing the supercritical state of ethanol (ethanol partial pressure in the reactor of 100–120 atm)
Temperature, °C
245
4.6
275
295
Ethanol conversion, %
Substance
33.4
35.1
Product selectivity, %
Acetaldehyde
Butene
n-Butane
3
4
1
0
0
0
0
5
1
Diethyl ether
Butanone-2
Butanol-2
5
0
0
1
0
0
16
1
1
Ethyl acetate
CxHy olefins (x = 6+)
Butanol-1
2-Butene-1-ol
Ethyl butyl ether
2-Ethylbutanol-1
Hexanol-1
Ethyl hexyl ether
Ethyl caproate
The rest
5
0
58
17
0
0
3
0
2
0
0
74
0
0
2
18
1
1
1
2
1
57
0
3
2
6
0
1
3
4
Σ
100
100
100
perature from 275 to 295°C leads to a slight increase in
A decrease in temperature from 275 to 245°C leads
the ethanol conversion from 33.4 to 35.1% and a to an abrupt decrease in the ethanol conversion and
decrease in the selectivity for butanol-1 and hexanol-1 the selectivity for butanol-1 and hexanol-1 by 16.5 and
by 17.2 and 11.4%, respectively. Analysis of the ethanol 14.8%, respectively (Table 1). It is significant that, at
conversion products (Table 2) shows that the decrease 245°C, the products contain a fairly high amount of
in selectivity at 295°C is primarily attributed to the assumed intermediates of butanol-1 formation
development of both intramolecular and intermolecu- (scheme), such as acetaldehyde and 2-buten-1-ol
lar alcohol dehydration reactions, which lead to an (Table 2). Their accumulation in the reaction mixture
increase in the yield of olefins and ethers (primarily, suggests that the first two stages shown in the scheme
diethyl ether).
rapidly occur and that, apparently, the rate-limiting
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SYNERGISTIC EFFECT OF GOLD AND COPPER
733
is due to weakening of diffusion limitations. According
to [23, 24], another cause of the increase in the etha-
nol conversion can be an increase in the polarization
of the O–H bonds and a decrease in the number of
hydrogen bonds in supercritical ethanol, changes that
facilitate ethanol adsorption and activation on the
active sites of the catalyst.
V EtOH res
critical point EtOH
VBuOHconv
25
20
15
10
5
100
S BuOH
X EtOH
90
80
70
60
50
40
30
20
10
0
The degrees of conversion and selectivities of the
monometallic counterparts of the Au–Cu catalyst in
the conversion of ethanol in the supercritical state are
collated in Table 1. It is evident that at 275°C in the
presence of the Cu/Al2O3 catalyst, the main product
of ethanol conversion is diethyl ether. The ethanol
conversion and the selectivity for the desired
alcohol are 11.5 and 0.2%, respectively (Table 1). In
the presence of the Au/Al2O3 catalyst, the ethanol
conversion is 30% and the selectivities for butanol-1
and hexanol-1 are 15.9 and 0.5%, respectively
(Table 1). Thus, the bimetallic catalyst exhibits sig-
nificantly higher activity and selectivity in the ethanol
conversion to linear α-alcohols. To determine
the probable causes of the different behavior of the
Au–Cu, Au, and Cu systems, the structural features of
the mono- and bimetallic catalysts were studied by
XRD, X-ray photoelectron spectroscopy (XPS), and
TEM. The results are described below.
0
0
1
2
3
4
5
6
7
Reaction time, h
Fig. 1. Dynamics of change in the amounts of ethanol and
butanol-1 during the reaction (T = 275°C, ethanol initial
partial pressure of 100 atm). V is the amount of ethanol
cr*
in the reactor required to provide the ethanol pressure
above the critical value (63 atm).
step of the entire process is the hydrogenation of 2-
butene-1-ol.
−2H2
2
H3C
O
2
H3C
OH
+H2
+H2
H3C
OH
H3C
OH
Structural Features of the Au–Cu, Au, and Cu Catalysts
The XRD patterns of the Au–Cu, Au, and Cu cat-
alysts are shown in Fig. 2. Curve 1 in Fig. 2 shows that
the XRD pattern of Cu/Al2O3 exhibits reflections at
2θ = 32.5°, 37.6°, 39.5°, 46.0°, 61.1°, and 66.8°, which
correspond to the reflections of the (220), (311),
(222), (400), (511), and (440) faces of γ-Al2O3 [17].
The absence of reflections of copper-containing
phases in the XRD pattern of Cu/Al2O3 is most prob-
ably attributed to the low copper concentration. The
XRD pattern of Au/Al2O3 (Fig. 2, curve 2) exhibits, in
addition to reflections of Al2O3, reflections at 2θ =
38.1°, 44.4°, and 64.6°, which correspond to the
reflections of the (111), (200), and (222) faces of gold
crystallites [19]. In comparison with Au/Al2O3 (Fig. 2,
curve 2), the diffraction peaks of gold in Au–
Cu/Al2O3 are broadened (Fig. 2, curve 3); this finding
indicates a higher dispersion of the Au-containing
phase in Au–Cu/Al2O3.
Scheme 1. Formation of butanol-1 from ethanol [1–3].
In ten consecutive 5-h tests, the activity and selec-
tivity of the Au–Cu/Al2O3 system remain unchanged
(Table 1). The high stability of the catalyst is appar-
ently due to the stability of Au–Cu particles against
sintering, as evidenced, in particular, by the small par-
ticle size preserved in Au–Cu/Al2O3 compared with
the Au/Al2O3 monometallic counterpart (see Struc-
tural Features of the Catalysts).
Figure 1 shows the dynamics of change in the
amount of consumed ethanol and butanol-1 produced
in the reactor. It is evident from Fig. 1 that ethanol is
consumed during the entire experiment; however, the
amount of it is sufficient to maintain the supercritical
state; that is, ethanol is actually in the fluid state
throughout the experiment. It should be noted that the
initial partial pressure of ethanol—100 atm—is equiva-
lent to the total pressure in the system; during the
experiment, the total pressure increases by 10–20 atm.
Some examples of the formation of new phases
composed of bimetallic alloys on the surface of Au–
A kinetic study of ethanol conversion over the Au– Cu, Au–Ni, and Pd–Ce catalysts were described in
Cu catalyst showed that the reaction is satisfactorily [25, 26]. The authors of the cited studies observed that
described by the first-order rate equation. Data pro- the formation of the new alloys is accompanied by the
cessing in Arrhenius coordinates (ln(X)–1/T) in the occurrence of new reflections or by a shift of reflec-
range of 245–280°C showed that the apparent activa- tions of the supported phases of noble metals toward
tion energy for ethanol conversion is 72 or 13 kJ/mol larger angles on a 2θ scale. Our XRD data show that
for ethanol in the supercritical or gaseous state, the diffraction patterns of the Au–Cu sample do not
respectively. These values suggest that the increase in exhibit new reflections or shifts of Au reflections; this
the ethanol conversion under supercritical conditions finding suggests that the formation of solid-solution
PETROLEUM CHEMISTRY Vol. 56 No. 8 2016

734
NIKOLAEV et al.
alloys or intermetallic compounds with a regular
structure can be excluded with a high degree of proba-
bility.
The electronic state of metals in Au, Cu, and
Au‒Cu catalysts was previously studied by XPS; the
results are described in [17]. It was found that copper
present on the surface of Cu/Al2O3 is in the form of
CuO (80 at %) and Cu2O (20 at %). Au/Al2O3 com-
10000
9500
9000
8500
8000
7500
7000
6500
6000
а
b
c
prises gold in the form of Au0 (100 at %). The electron
binding energy of Au 4f7/2 in the XPS spectrum of
Au‒Cu/Al2O3 is 84.4 eV [17]. This result shows that,
along with the originally zero-valent gold, Au(+n) cat-
ions (0 < n < 1) are formed in Au–Cu/Al2O3. The
Au(+n) content is low—on the order of 10–20 at %.
Analysis of the Cu 2p core-level XPS spectra [17] sug-
gests that, in Au–Cu/Al2O3, the fraction of surface
oxide CuO decreases and the fraction of
Cu2O increases. The above results indicate that, in the
Au–Cu catalyst, a contact between the Au(0) and
CuO phases leads to electron transfer from the former
to the latter. This process results in a partial reduction
of CuO to Cu2O; the evolved oxygen is bound to
Au(+n) to form AuOy. This model is consistent with
the structural data on Au–Cu catalysts described in
[27, 28].
25 30 35 40 45 50 55 60 65 70 75
2θ, deg
Fig. 2. XRD patterns of (1) Cu/Al O , (2) Au/Al O , and
(3) Au–Cu/Al O .
2
3
2 3
2
3
Causes of Different Behaviors of the Au–Cu, Au,
and Cu Catalysts
Representative TEM micrographs of mono- and
bimetallic catalysts are shown in Figs. 3a–3c. It is evi-
dent from Fig. 3a that the Cu/Al2O3 sample surface
comprises regions of ordered atoms, which, according
to EDS, correspond to Cu-containing particles with
an average size of 3 1 nm. The small size of the sup-
ported particles in Cu/Al2O3, which was determined
by TEM, is consistent with the X-ray amorphous state
of the copper phases in Cu/Al2O3, which was revealed
by XRD (Fig. 2, curve 1). The Au/Al2O3 surface com-
prises regions of ordered atoms, which, according to
EDS, correspond to gold particles with an average size
of 10 2 nm (Fig. 3b). The formation of relatively
large gold particles in Au/Al2O3 is attributed to the
weak metal–support interaction, which leads to the
sintering of noble metal particles during the calcina-
tion of the catalyst precursor [19].
TEM data suggest that the active phase in
Cu/Al2O3 and Au/Al2O3 is represented by particles of
3 and 10 nm in size, respectively. It is known that the
heat of adsorption of ethanol on the Cu surface is
higher than that for Au [29]. It is also known that the
heat of adsorption of alcohol abruptly increases with a
decrease in the particle size from 4 to 1 nm [26].
Hence, it is reasonable to assume that the number of
alcohol molecules strongly bound to the catalyst sur-
face should be higher in Cu/Al2O3 and lower in
Au/Al2O3. In addition, the deactivation of the alco-
hol-blocked active sites and a decrease in the ethanol
conversion should occur. Comparison of the data on
catalysis and morphology of active particles is consis-
tent with this hypothesis: all other test conditions
being equal, the ethanol conversion over Au/Al2O3
and Cu/Al2O3 is 30 and 11.5%, respectively.
The average particle size in Au–Cu/Al2O3 is
smaller than that of Au/Al2O3: it is 5 nm (Fig. 3c). The
shift of the average particle size in Au–Cu/Al2O3 sug-
gests that copper oxides contribute to the stabilization
of small Au particles on the surface of the support. An
example of identification of the chemical composition
of particles in the Au–Cu/Al2O3 catalyst is shown in
Figs. 3c–3e. TEM–EDS analysis of a sample of
180 particles located both in particle clusters and at a
considerable distance from each other showed that
about 20% of particles in Au–Cu/Al2O3 are individual
Au particles with a size of 12 nm and copper oxide par-
ticles with a size of 3 nm, while the remaining 80% are
The fact that the particle size in Au/Al2O3 is
larger than that in Cu/Al2O3 provides an adequate
explanation to the high selectivity for butanol-1 and
hexanol-1. It is obvious that a high rate of growth of
the hydrocarbon skeleton of the products should be
observed in the case of coordination of two ethanol
molecules on two free and closely spaced active sites
(Scheme 1), the number of which should be larger in
the 10-nm Au particles.
According to our reckoning, the increase in the
ethanol conversion over Au–Cu/Al2O3 in comparison
with the most active counterpart—Au/Al2O3—is
bimetallic composites consisting of copper oxide par- attributed to two factors. First, a contact between cop-
ticles (3 nm) and small gold particles (5 nm). per oxide and gold leads to an increase in the degree of
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SYNERGISTIC EFFECT OF GOLD AND COPPER
(a)
735
(b)
2 nm
5 nm
(c)
1800
1600
1400
1200
1000
800
600
400
200
0
(d)
Au
1
2
3
8
9
keV
1800
1600
1400
1200
1000
800
600
400
200
0
(e)
10 nm
1
2
3
8
9
keV
Fig. 3. Representative TEM micrographs of (a) Cu/Al O , (b) Au/Al O , and (c) Au–Cu/Al O . (d, e) EDS spectra of locations
2
3
2
3
2 3
marked in (c). It is evident that the locations comprise either a bimetallic composite or individual metal particles.
dispersion of the gold clusters from 10 to 5 nm; there- Au(+n) sites (0 < n < 1) in Au–Cu/Al2O3. The scheme
fore, at identical amounts of gold, the specific surface
area of gold should be higher in Au–Cu/Al2O3.
Apparently, the degree of dispersion of gold in the
Au–Cu sample has not yet become sufficiently high to
correspond to the case where the contribution of the
mechanism of catalyst inhibition by hydrocarbon
molecules strongly bound to the metal surface is a lim-
iting factor for the entire process.
shows that the growth of the hydrocarbon skeleton of
the target alcohols passes through the dehydrogena-
tion–hydrogenation stages. It is known that these pro-
cesses over metal-containing catalysts are accompa-
nied by a change in the oxidation state from M(n) to
M(n + 2) [30]. For Au/Al2O3, changes in the oxidation
state of the metal can be represented by the following
cycle: Au(0) → Au(+2) → Au(0), which includes the
oxidation state of (+2), which is not characteristic of
gold. Therefore, the dehydrogenation–hydrogenation
of hydrocarbons over zero-valent Au clusters in
A certain contribution to the high activity of
Au‒Cu/Al2O3 can come from the formation of new
PETROLEUM CHEMISTRY Vol. 56 No. 8 2016

736
NIKOLAEV et al.
Au/Al2O3 should proceed at a relatively low rate. In the formation of butanol-1 and hexanol-1 from etha-
nol under supercritical conditions over the Au–Cu
catalyst. In this case, the supercritical state of ethanol
apparently provides the most favorable conditions for
the occurrence of the reaction because the ethanol
conversion products formed under the experimental
conditions transit into a gaseous state and thus facili-
tate the access to the surface active sites for ethanol
fluids.
fact, according to [30], zero-valent gold exhibits a low
activity in the hydrogenation reaction. The formation
of individual atoms in the oxidation state close to (+1)
on the surface of the gold particles provides the occur-
rence of a high-rate cycle consisting of oxidation states
characteristic of gold: Au(+1) → Au(+3) → Au(+1).
In this case, according to [19, 31], the presence of cat-
ionic gold leads to an increase in the activity of gold
catalysts in acetylene hydrogenation and allylbenzene
isomerization occurring through a dehydrogenation–
hydrogenation cycle. Therefore, it is reasonable to
assume that the dehydrogenation–hydrogenation of
hydrocarbons (and hence the formation of high-
molecular-weight alcohols according to the scheme)
over gold cations in Au–Cu/Al2O3 can also occur at a
high rate. It is certainly necessary to take into account
the presence of hydrogen in the reaction sphere and
the relatively high reaction temperature; therefore, it
would be only logical to expect the reduction of Au
cations to Au0 and the subsequent decrease in the con-
version to the level achieved over Au/Al2O3. However,
the data on the stability of the Au–Cu catalyst suggest
otherwise—the product yield does not change for at
least the first ten cycles.
According to our reckoning, an increase in the
selectivity for butanol-1 and hexanol-1 over Au–
Cu/Al2O3 in comparison with the most selective
counterpart—Au/Al2O3—can be attributed to a
change in the morphology of the active site of the cat-
alyst. Figure 3 shows that extended agglomerates of
Aun–Cun–Aun–Cun clusters are formed in the bime-
tallic catalyst. Apparently, these agglomerates contrib-
ute to the close coordination of a few ethanol mole-
cules at once and, thereby, facilitate the growth of a
more extended hydrocarbon skeleton of the alcohols.
This hypothesis is supported by the fact that of all the
tested samples, the Au–Cu catalyst alone provides the
formation of trace amounts of a higher alcohol (octa-
nol-1).
ACKNOWLEDGMENTS
This work was supported by the Russian Science
Foundation (project no. 14-13-00574) and performed
using the equipment purchased in accordance with the
Development Program of the Moscow State Univer-
sity.
S.A. Nikolaev acknowledges the support of the
Russian Foundation for Basic Research (project
no. 16-03-00073) in the synthesis of the catalysts and
analysis of their structure.
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