Ethanol gas-phase ammoxidation to acetonitrile: The reactivity of supported vanadium oxide catalysts

Article abstract of DOI:10.1039/c6cy01275b

New insights on the gas-phase ammoxidation of ethanol to acetonitrile over supported vanadia catalysts were obtained by means of reactivity experiments (in ethanol ammoxidation and oxidation) as well as in situ Raman and DRIFT spectroscopy. It was found that the rate-determining step during the redox process depends on the support type. In the case of V₂O₅/ZrO₂, the V oxidation state under reaction conditions is closer to V⁵⁺, whereas with V₂O₅/TiO₂, the reduction of V⁵⁺ is faster than the re-oxidation of the corresponding reduced V species by O₂; thus, the V oxidation state under steady state conditions is lower than for V₂O₅/ZrO₂. In the latter catalyst, the more oxidized V species is responsible for ammonia activation and reaction with the intermediate acetaldehyde, leading in the end to a better acetonitrile yield than with V₂O₅/TiO₂. It was also found that V₂O₅/ZrO₂ is more selective to acetaldehyde than V₂O₅/TiO₂. With the former catalyst, ethanol is able to reduce V₂O₅ only to a limited extent. Conversely, V₂O₅/TiO₂ is readily reduced by ethanol but this reduced V species is responsible for an unselective oxidation of the alcohol, giving more CO and CO₂.

Full text of DOI:10.1039/c6cy01275b

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Ethanol gas-phase ammoxidation to acetonitrile:
the reactivity of supported vanadium oxide
catalysts†
Cite this: DOI: 10.1039/c6cy01275b
a
a
ab
L. Ottc and M. Janssenc
*
F. Folco, J. Velasquez Ochoa, F. Cavani,
New insights on the gas-phase ammoxidation of ethanol to acetonitrile over supported vanadia catalysts
were obtained by means of reactivity experiments (in ethanol ammoxidation and oxidation) as well as in
situ Raman and DRIFT spectroscopy. It was found that the rate-determining step during the redox process
depends on the support type. In the case of V2O5/ZrO2, the V oxidation state under reaction conditions is
closer to V5+, whereas with V2O5/TiO2, the reduction of V5+ is faster than the re-oxidation of the corre-
sponding reduced V species by O2; thus, the V oxidation state under steady state conditions is lower than
for V2O5/ZrO2. In the latter catalyst, the more oxidized V species is responsible for ammonia activation and
reaction with the intermediate acetaldehyde, leading in the end to a better acetonitrile yield than with
V2O5/TiO2. It was also found that V2O5/ZrO2 is more selective to acetaldehyde than V2O5/TiO2. With the
former catalyst, ethanol is able to reduce V2O5 only to a limited extent. Conversely, V2O5/TiO2 is readily re-
duced by ethanol but this reduced V species is responsible for an unselective oxidation of the alcohol, giv-
ing more CO and CO2.
Received 10th July 2016,
Accepted 30th November 2016
DOI: 10.1039/c6cy01275b
www.rsc.org/catalysis
This awoke the pharma industry to the fact that a dedicated
process for the synthesis of acetonitrile is needed. Possible
Introduction
Acetonitrile is a very versatile compound. Its unique chemical
properties, such as polarity, miscibility with water, low boiling
point, low acidity and low UV cut-off, make it a good solvent
for the synthesis of pharmaceuticals and intermediates, oligo-
nucleotides and peptides.1 It is also used as a reactant in
chemical syntheses such as the production of malononitrile,
pesticides or pharmaceuticals. Unlike other solvents, commer-
cial acetonitrile is not the result of a direct synthesis but is
mostly obtained as a by-product of the industrial-scale pro-
duction of acrylonitrile. Acrylonitrile is the primary product of
propylene ammoxidation, in which 2–4% acetonitrile is also
formed.2 In some cases after acrylonitrile purification the
waste (mostly acetonitrile) is incinerated. In a few cases, how-
ever, the acetonitrile is isolated by distillation; depending on
the waste stream and the distillation capability, different ace-
tonitrile qualities are obtained.3
routes for the synthesis of acetonitrile include:
(a) Reaction between CO, NH3 and H2; the reaction is car-
ried out in the gas phase using a molybdenum or iron cata-
lyst supported over silica, as described in Monsanto pat-
ents.4,5 Various mechanisms have been proposed for this
reaction in the literature.6,7
(b) Hydrocyanation of C1–C2 compounds; in this process
methane reacts with HCN without a catalyst, at 900 °C, to
produce acetonitrile and hydrogen.8,9 In an alternative pro-
cess, acetylene reacts with ammonia in the presence of a cat-
alyst to produce acetonitrile.
(c) Ethane2,10–19 or ethylene20–23 ammoxidation; catalysts
for ethane ammoxidation include mixed Nb/Sb oxides, Nb-
promoted Ni oxides, Co-exchanged zeolites, Cr-exchanged ze-
olites and zeolite-supported Sb oxides. Because of the better
yields obtained, ethane ammoxidation is the preferred route.
Late in 2008 and throughout 2009 the chemical industry
experienced a severe acetonitrile shortage, because several ac-
rylonitrile production plants were temporarily shut down.
Recently Alzchem claimed to have started a gas-phase
ammoxidation process for the production of acetonitrile.24
An alternative acetonitrile process based on a renewable
source might make use of ethanol as the reactant. In general,
the ammoxidation of alcohols has been much less discussed
in the literature than other substrates, with the very few ex-
amples limited to the liquid-phase ammoxidation of benzyl
alcohol,25–29 glycerol and ethanol, and to the gas-phase
ammoxidation of glycerol30 and ethanol.31–34 Indeed, ethanol
ammoxidation can be carried out in the same reactor and
a Dipartimento di Chimica Industriale e dei Materiali, ALMA MATER STUDIORUM
Università di Bologna, Viale del Risorgimento 4, 40136 Bologna, Italy.
E-mail: fabrizio.cavani@unibo.it
b Consorzio INSTM, Research Unit of Bologna, Firenze, Italy
c Lonza Ltd, 3930 Visp, Switzerland
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c6cy01275b
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process where propylene is ammoxidized into acrylonitrile.
The conditions for the ammoxidation of ethanol are claimed
to be not very different from those necessary for conducting
propylene ammoxidation, while the co-feeding of ethanol and
propylene has been proposed as a tool to increase the aceto-
nitrile/acrylonitrile selectivity ratio.35,36
nation at 450 °C) or TiO2 (anatase supplied by CristalACTiV™
DT-51, 22.0 m2 g−1) (see Table S2†). The suspension was
stirred for 1 h and then dried in a rotavapor. Afterwards it
was heated to 120 °C for 3 h and then calcined at 450 °C for
5 h with a 10 °C min−1 rate. The amount of vanadia loaded (7
wt%) corresponds to approximately 2 and 3 monolayers for
V2O5–ZrO2 (catalyst code V/Zr/O, surface area 24.2 m2 g−1) and
V2O5–TiO2 (catalyst code V/Ti/O, surface area 21.3 m2 g−1),
respectively.
The tubular type quartz reactor had an internal diameter
of 0.8 cm and an overall length of 46 cm. The catalytic bed
was positioned at mid-height, corresponding to the isother-
mal zone of the furnace. The catalyst was held on a porous
septum of sintered glass. A stainless steel internal tube with
a diameter of 1/16″ contained a thermocouple which made it
possible to measure the temperature at different heights in-
side the catalytic bed. The temperature at the reactor exit was
maintained at 200 °C by means of a heating strip. The W/F
ratio (W = catalyst weight; F = overall inlet gaseous flow rate,
calculated at normal conditions) was typically in the range of
0.1–1.0 g s mL−1.
Table S1 (in ESI†) summarizes the results reported in the
literature on the gas and liquid-phase ammoxidation of pri-
mary alcohols to nitriles. In some cases the mechanism pro-
posed includes the formation of carboxylic acid, then amide,
which, in turn, is dehydrated to nitrile.33 In other cases the
mechanism proposed is either via aldehyde, imine and final
oxidehydrogenation to nitrile27–29,31,32 or via an exchange be-
tween water and ammonia with the formation of amine and
subsequent oxidehydrogenation to nitrile.31 Mizuno et al.
have recently published several papers discussing synthetic
procedures for the aerobic preparation of nitriles and the di-
rect synthesis of primary amides (where the latter are formed
by hydration of nitriles). The catalysts described are based
not only on RuIJOH)3-alumina, but also on manganese oxide-
based octahedral molecular sieves (KMn8O16: OMS-2).37 Man-
ganese oxide is also
a
catalyst for the oxidative
The ethanol used for the reactivity experiments was an
azeotropic mixture, in order to simulate a process in which
the azeotrope, and not pure ethanol, would be used.
The analysis of the effluent stream was carried out via on-
line sampling and injection into a HP 5890A gas chromato-
desulphurization of primary thioamides into the correspond-
ing nitriles,38 while Ru hydroxide is a catalyst for the oxida-
tive transformation of primary azides into nitriles.39 Ishida
et al.25 reported that in the liquid-phase ammoxidation of
benzyl alcohol catalyzed by CeO2-supported Au, a key inter-
mediate is hemiaminal, which is obtained by the addition of
ammonia to the carbonyl bond of benzaldehyde. The
hemiaminal can either be oxidehydrogenated to an amide or
dehydrated to an imine and then oxidehydrogenated to ni-
trile. Other procedures for the ammoxidation of alcohols (es-
pecially aryl alcohols) to give nitriles entail conventional
methods of organic synthetic chemistry; see, for instance, the
several methods cited in ref. 40 and 41, which use properly
selected oxidants and reagents as N sources.
graph with two columns. The first column was
a
semicapillary HP Plot U, 30 m long, with an internal diameter
of 0.53 mm, a 20 μm thick fixed phase and a maximum
allowed temperature of 190 °C. This column was used to sep-
arate ethanol, ammonia, carbon dioxide, water, acetaldehyde,
acetonitrile and other C-containing by-products (hydrocyanic
acid and other nitriles). The second column was
a
semicapillary HP Molesieve, 30 m long, with an internal di-
ameter of 0.53 mm and a maximum allowed temperature of
300 °C. This column was used to separate oxygen, nitrogen
and carbon monoxide. The two columns were joined to a
splitter/union and the combined flow was conveyed to the
thermal-conductivity detector. The GC oven temperature was
programmed as follows: 6.8 min at 60 °C, heating rate 40 °C
min−1 up to 100 °C, isothermal operation for 8.5 min, heating
rate 60 °C min−1 up to 130 °C, isothermal operation for 5
min, final heating rate 60 °C min−1 up to 170 °C, final iso-
thermal operation for 8 min.
Raman studies were performed using a Renishaw 1000 in-
strument, equipped with a Leica DMLM microscope and a la-
ser source Argon ion (514 nm) with a power of 35 mW. In situ
Raman experiments were carried out by loading 5–10 mg of
the catalyst into the Raman cell (Linkam TS1500) under a
stream of ethanol vapour in He or air.
Finally, it is worth mentioning the gas-phase ammonolysis
of ethanol, carried out over Ni-based catalysts.42
In this paper, we report on the chemistry of the gas-phase
ammoxidation of ethanol to acetonitrile, by means of cata-
lysts based on supported vanadium oxide. These catalysts
were chosen because they had already been described in the
past as excellent systems for gas-phase ammoxidation of
alkylaromatics to aromatic nitriles, but were never tested for
alcohol ammoxidation.43–48 Another aim of this work is to
contribute to the understanding of the interaction between
the substrate and the V species and the role of the rate deter-
mining step on the catalytic performance of V oxide-based
catalysts for selective oxidation reactions.
For in situ diffuse-reflectance-Fourier-transform-spectro-
scopy (DRIFTS) experiments, the sample was pre-treated at
450 °C in a He flow (10 mL min−1) for 45 min, in order to re-
move any molecules adsorbed on the material. Then the sam-
ple was cooled to 85 °C, and ethanol was fed at 0.6 μL min−1
for 15 min using He or air as the carrier. Following this, the
Experimental
Catalysts were prepared by means of wet impregnation: the
appropriate amount of NH4VO3 was dissolved in water at 40
°C and the support was added to the solution. The support
was either ZrO2 synthesized in the lab (26.5 m2 g−1 after calci-
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carrier gas was left to flow until weakly adsorbed ethanol was
evacuated; then the temperature was raised to 400 °C at 5 °C
min−1. Spectra were recorded every 50 °C using the spectrum
of the catalyst without adsorbed ethanol as a background. IR
Spectra were recorded using an MCT detector with 128 scans
and 2 cm−1 resolution on a Bruker Vertex 70 using a Pike
DiffusIR cell attachment. The desorbed products were moni-
tored by on-line mass spectrometry using an GeneSys Evolu-
tion from European Spectrometry Systems.
Surface area of the catalysts was measured with a Micro-
meritics ASAP2020 instrument. TPR experiments were carried
out using a TPD/TPR/TPO Micromeritics instrument. 30–50
mg of sample were pretreated at 200 °C under a He flow. Af-
ter cooling down to 50 °C, a mixture of 5% H2 in He was
used and after 20 min the temperature program started: from
50 to 800 °C at 5 °C min−1, 800 °C for 30 min.
Results and discussion
Catalytic behaviour of V2O5/ZrO2 and V2O5/TiO2 catalysts in
ethanol ammoxidation
Experiments were carried out with two supported catalysts
containing the same amount of V2O5 (7 wt%) dispersed on ei-
ther ZrO2 (monoclinic) (catalyst code V/Zr/O) or TiO2 (ana-
tase) (catalyst code V/Ti/O). The two catalysts had comparable
surface areas (see Table S2†). The amount of vanadia loaded
corresponds to approximately 2 and 3 monolayers for V/Zr/O
and V/Ti/O, respectively. First of all, it should be noted that
V/Ti/O catalysts with approximately 1 monolayer of V2O5
displayed a lower activity than the catalysts containing more
V, but the distribution of products was similar and the maxi-
mum acetonitrile yield was identical. It was thus decided to
investigate the performance of the two catalysts containing
7% V2O5 in detail.
Moreover, it should be mentioned that H2-TPR experi-
ments (Fig. S1 in ESI†) showed different reduction profiles
for both V/Ti/O and V/Zr/O compared to the corresponding
profiles of samples obtained by mixing 7% V2O5 with either
the TiO2 or the ZrO2 support and then calcining the corre-
sponding powder mixtures at 450 °C. Despite the relatively
high amount of V2O5 (ca. 2–3 monolayers of V oxide), a modi-
fication of the redox properties of V5+ due to the interaction
between the dispersed V species and the support was ob-
served compared to bulk V2O5.
Fig. 1 shows the catalytic behaviour of V/Zr/O at 0.1 g s
mL−1 W/F ratio using an ethanol/ammonia/oxygen feed com-
position (mol%) of 5/6/6, i.e. a reactant ratio close to the stoi-
chiometric ratio for acetonitrile synthesis (1/1/1), but with a
slight excess of both oxygen and ammonia. This component
ratio was chosen to maximize the reactant conversion, while
limiting the formation of over-oxidized products (CO, CO2
and N2) and the consumption of ammonia.
The data show that complete ethanol and oxygen conver-
sions were obtained at 350–370 °C with an acetonitrile yield
of 73–74%. By-products of the reaction were ethylene, CO,
CO2 and HCN. At 330 °C acetaldehyde selectivity was very
Fig. 1 Effect of temperature on reactant conversion (top figure) and
on selectivity to products (bottom figure). Reaction conditions: W/F
ratio 0.1 g s mL−1, feed composition (molar%): ethanol (azeotrope)/
ammonia/oxygen 5/6/6. Symbols: ethanol conversion (♦), ammonia
conversion (▲) and oxygen conversion (■). Selectivity to: acetonitrile
(◊), acetaldehyde (*), ethylene (Δ), CO (○), CO2 (×), HCN (□) and N2
(calculated with respect to converted ammonia) (●). Catalyst V/Zr/O.
high, but it decreased when the temperature was raised
with a concomitant increase of selectivity to acetonitrile.
The carbon balance was close to 100%, which indicates that
no heavy compounds were formed. The achieved yield to
acetonitrile was lower than the yields reported in the litera-
ture with vanadyl pyrophosphate and SAPO/VAPO cata-
lysts.31,33 It is worth mentioning, however, that we tried to
replicate the catalysts and conditions reported in those pa-
pers, but obtained selectivity values far lower than those
claimed.
Another set of experiments was conducted by increasing
the partial ammonia pressure in an attempt to increase ace-
tonitrile yield (ethanol/ammonia/oxygen mol% 5/13/6). Never-
theless, the maximum yield achieved was still 73–74% at
370–390 °C and at total ethanol conversion (see Fig. S2†). In
this case, at low temperature the ethanol conversion was sig-
nificantly less than in the tests carried out at lower ammonia
partial inlet pressure. This clearly indicates that ammonia
and ethanol compete for adsorption and activation over the
same type of active sites. The distribution of products was
not different from that shown in Fig. 1. In the low tempera-
ture range (T < 350 °C), the selectivity to by-products, with
the exception of CO2, was zero; at higher temperatures,
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selectivity to CO, ethylene, HCN and N2 (the latter deriving
from ammonia combustion) increased, whereas that to CO2
decreased slightly.
der any of the reaction conditions investigated. It can be in-
ferred that acetaldehyde reacts with activated ammonia to
yield ethanimine, which is then oxidehydrogenated to nitrile.
Experiments conducted using different contact times will
confirm the role of acetaldehyde as the key reaction interme-
diate (see below).
Fig. 3 shows the catalytic performance of V/Ti/O under the
same conditions as shown in Fig. 1 (W/F 0.1 g s mL−1; etha-
nol/ammonia/oxygen feed composition 5/6/6 mol%). Results
show that this catalyst was less active than V/Zr/O, a phenom-
enon which may in part be attributed to the lower surface
area of V/Ti/O. The main difference was in the distribution of
products. The best selectivity to acetonitrile was lower (55%
at 90% ethanol conversion), with a higher selectivity to all by-
products, especially CO and CO2. The combustion of ammo-
nia to N2 was also much greater than with V/Zr/O.
In another set of experiments, the partial pressure of all
reactants was increased in order to increase acetonitrile pro-
duction. Results are reported in Fig. S3.† In this case, the best
yield to acetonitrile was slightly lower, 70–71% at 400 °C, and
the same was true for ethanol conversion (compare Fig. 1, S2
and S3†), probably due to a surface saturation effect; never-
theless, the production of acetonitrile was considerably en-
hanced: 2.5 kgEtOH kgcat h−1 for the conditions of Fig. 1 and
S2† compared to 4.8 kgEtOH kgcat h−1 for the conditions of
Fig. S3.†
To check the stability of the catalytic performance, we
tested the short-term lifetime of the catalyst, using ethanol-
rich conditions at 400 °C (temperature at which the best yield
was obtained). Results are shown in Fig. 2. There was a slow
initial deactivation effect, with a slight decline in ethanol
conversion during the first 15–20 h; afterwards, the perfor-
mance was stable. The selectivity to acetonitrile remained sta-
ble at approximately 70%.
Even when conditions were drastically changed, for exam-
ple, by increasing both ammonia and oxygen concentrations
as well as the contact time, the best acetonitrile yield
achieved was still only approximately 50% (Fig. S4†). Since V/
Ti/O was more selective than V/Zr/O to CO, CO2 and HCN
The results obtained indicate that acetaldehyde is the key
reaction intermediate, since no acetic acid was detected un-
Fig. 2 Effect of reaction time on reactant conversion (top figure) and
on selectivity to products (bottom figure). Reaction conditions: W/F
ratio 0.1 g s mL−1, feed composition (molar%): ethanol (azeotrope)/
ammonia/oxygen 10/12/10, temperature 400 °C. Symbols: ethanol
conversion (♦), ammonia conversion (▲) and oxygen conversion (■).
Selectivity to: acetonitrile (◊), acetaldehyde (*), ethylene (Δ), CO (○),
CO2 (×) and HCN (□). Catalyst V/Zr/O.
Fig. 3 Effect of temperature on reactant conversion (top figure) and
on selectivity to products (bottom figure). Reaction conditions: W/F
ratio 0.1 g s mL−1, feed composition (molar%): ethanol (azeotrope)/
ammonia/oxygen 5/6/6. Symbols: ethanol conversion (♦), ammonia
conversion (▲) and oxygen conversion (■). Selectivity to: acetonitrile
(◊), acetaldehyde (*), ethylene (Δ), CO (○), CO2 (×), HCN (□) and N2
(calculated with respect to converted ammonia) (●). Catalyst V/Ti/O.
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and less selective to acetonitrile, we hypothesised that oxygen
might play an important role in determining the selectivity.
We therefore investigated the effect of partial oxygen pressure
with V/Ti/O, with the aim of limiting oxidative degradation by
the use of a lower O2 partial pressure in the feed. The results
of these experiments are shown in Fig. 4; the other condi-
tions were the same as those used in Fig. S4† at 320 °C.
It is shown that ethanol was converted even in the absence
of oxygen, producing acetaldehyde and ethane by means of a
disproportionation reaction, known to occur with reduced Va-
nadium oxide catalysts:49,50
These experiments indicate that in the absence of oxygen,
but in the presence of ammonia, the catalyst converts ethanol
into acetaldehyde. The latter, however, does not react with
ammonia (ammonia conversion in the absence of oxygen was
negligible), thus indicating that ammonia needs oxygen in or-
der to be activated. We thus conclude that the activation of
ammonia most likely occurs on oxidized V sites. Further ex-
periments (discussed below) show that in the absence of oxy-
gen the V species in V/Ti/O is very quickly reduced.
Therefore, the presence of oxygen has several effects: (i) it
contributes to ethanol conversion by providing an alternative
route to acetaldehyde, via oxidehydrogenation; (ii) it catalyzes
the combustion of acetaldehyde into CO2; the rate of CO2 for-
mation is almost directly proportional to the oxygen content
in the inlet feed; (iii) it generates active sites for the reaction
between acetaldehyde and activated ammonia to produce
acetonitrile.
With this catalyst, however, the best selectivity to acetoni-
trile was still no higher than 50%, at both very low (2%) and
high (13%) O2 partial pressure. Therefore, a low (but not
zero) partial oxygen pressure permits a rapid conversion of
acetaldehyde into both acetonitrile and CO2. A further in-
crease in O2 concentration, however, only increased the rate
of acetaldehyde transformation into CO2 and, to a lesser ex-
tent, into CO and HCN. This can be attributed to the limited
number of active sites which are capable of activating ammo-
nia. In fact, the experiments conducted with different contact
times (Fig. S5†) confirmed that acetaldehyde undergoes con-
secutive transformations into both acetonitrile and CO2 (and
to a lesser extent into CO and HCN).
2CH3CH2OH → CH3CHO + C2H6 + H2O
Adding oxygen led to a progressive decrease in the forma-
tion of acetaldehyde and ethane and to a corresponding in-
crease in CO2 (conversely, the yield to CO remained very low).
The selectivity to acetonitrile showed a steep rise when 2%
oxygen was added in the feed, but remained constant over
further increases in oxygen content.
The results discussed above show that the superior perfor-
mance of V/Zr/O, compared to V/Ti/O, is due in part to its
greater efficiency in the conversion of acetaldehyde into ace-
tonitrile, together with lower formation of both CO2 from ac-
etaldehyde combustion and N2 from ammonia combustion.
Another difference between the two systems may be the dif-
ferent efficiencies in the conversion of ethanol into acetalde-
hyde. In order to gain more information on this step of the
process, we compared the reactivity of the catalysts in ethanol
oxidation.
It is also worth noting that zirconia-supported vanadium
oxide catalysts have been reported to give better performance
in ammoxidation reactions, specifically for toluene and
picoline.51–53 The same type of catalyst was also used in sev-
eral oxidation reactions, such as ethanol to acetaldehyde,54,55
methanol to formaldehyde56–58 and alkane oxidehydro-
genation to olefins.58–64 In general, various catalysts based on
supported vanadium oxide have been described for ethanol
oxidation.65–69
Fig. 4 Effect of oxygen partial pressure on reactant conversion (top
figure) and on selectivity to products (bottom figure). Reaction
conditions: temperature 320 °C, W/F ratio 0.8
g s , feed
mL−1
Catalytic behaviour in ethanol oxidation
composition (molar%): ethanol (azeotrope)/ammonia/oxygen/inert
5/13/variable/remainder. Symbols: ethanol conversion (♦) and
ammonia conversion (▲). Selectivity to: acetonitrile (◊), acetaldehyde
(*), ethylene (Δ), CO (○), CO2 (×), HCN (□), ethane (+) and N2
(calculated with respect to converted ammonia) (●). Catalyst V/Ti/O.
Fig. 5 compares the performance of the two catalysts as a
function of time, at 400 °C with a feed stream containing 5%
ethanol and 6% oxygen. These conditions were chosen to
simulate the best conditions for ammoxidation (feed
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catalyst. However, the final selectivity to acetaldehyde was
only 35%.
With both catalysts, some minor products were formed
(not shown in figure): ethane, diethylether, acetone and
propylene, with an overall selectivity between 10 and 15% for
V/Zr/O and approximately 5% for V/Ti/O.
These experiments demonstrate that the two catalysts dif-
fer greatly in regard to the redox behaviour. The fresh V/Ti/O
is unselective, but this changes notably during reaction in
the presence of ethanol and oxygen, and it becomes progres-
sively more selective to acetaldehyde. The considerable
change in the observed catalytic behaviour may be related to
a change in the nature of the active V sites; the fully oxidized
(and unselective) V sites become progressively more reduced
and thus more selective to the formation of acetaldehyde and
CO and less to the formation of CO2. However, even under
best conditions, this catalyst is less selective than V/Zr/O.
Conversely, with V/Zr/O, catalyst changes have less effect on
the product distribution, which indicates that the active V
sites do not undergo important modifications on exposure to
the reactant stream. These differences may be explained by
taking into account a different type of rate-determining step
in the redox process. In V/Ti/O, V5+ reduction (by ethanol) is
faster than the re-oxidation of the reduced V species, giving
rise to a working V species which, under steady-state condi-
tions, is strongly reduced. The opposite is true for V/Zr/O,
where the rate-determining step is the reduction of the V spe-
cies, and the working V species is fully or almost fully
oxidized.
Fig. 5 Effect of reaction time on ethanol conversion and on yields to
products, with V/Ti/O (top) and V/Zr/O (bottom). Reaction conditions:
temperature 400 °C, W/F ratio 1.0
g s , feed composition
mL−1
The nature of active sites may be very different in the ab-
sence or presence of ammonia, meaning that this result can-
not be used directly to interpret ammoxidation results. Never-
theless, it is reasonable to assume that the different
performance shown during ethanol oxidation to acetaldehyde
may also be one reason for the lower selectivity to acetonitrile
observed during ethanol ammoxidation with V/Ti/O. In fact,
acetaldehyde is the key intermediate in this reaction. Com-
paring Fig. 1 (V/Zr/O) and Fig. 3 (V/Ti/O), it can be seen that
for an ethanol conversion of around 60% (achieved at 330 °C
for the former catalyst and at 350 °C for the latter), the over-
all selectivity to acetaldehyde + acetonitrile is 75% for V/Zr/O
and 65% for V/Ti/O.
It is also worth noting that both bare supports, used at
the same conditions as for Fig. 5, led to ethanol conversions
close to 90%, with the following distribution of products (reg-
istered after 6 h time-on-stream):
a) TiO2: 26% selectivity to acetaldehyde, 18% to CO2, 9%
to ethylene, 36% C loss, and minor formation of croton-
aldehyde, ethylacetate, butene and acetone.
(molar%): ethanol (azeotrope)/oxygen/inert 5/6/remainder. Symbols:
ethanol conversion (♦). Yield to: acetaldehyde (*), ethylene (Δ), CO (○)
and CO2 (×).
concentrations) and for yield to acetonitrile (temperature).
Variation in the catalytic behaviour of the catalyst under
these conditions would indicate a progressive change in the
nature of V sites during reaction.
Using both catalysts, the conversion of ethanol was total
or almost total throughout the entire reaction time. Consider-
able differences, however, were seen in the selectivity. In the
case of V/Zr/O (Fig. 5 bottom), acetaldehyde selectivity in-
creased from 23% (for the freshly oxidized catalyst) up to a
stable value of 48% for the “equilibrated” catalyst; minor
changes were seen over time for CO, CO2 and also for ethyl-
ene. The formation of H2 (4% yield, decreasing over time)
was also observed. Along with the increase in acetaldehyde
formation, an improved C balance was observed after longer
reaction times. This indicates that some products were
retained on the fresh catalyst surface and were responsible
for the poor C balance observed.
b) ZrO2: 23% selectivity to acetaldehyde, 25% to CO2, 8%
to ethylene, 30% C loss, and minor formation of acetone,
crotonaldehyde, butene and ethylacetate.
In the case of V/Ti/O, the fresh catalyst was highly selective
to CO2, with no other products detected in the outlet stream;
over time the selectivity to CO2 decreased, whereas that to
CO and acetaldehyde increased; H2 did not form using this
Therefore, it seems that the role of V is not only that one
of facilitating ethanol conversion, but also of improving the
selectivity to acetaldehyde, while decreasing the formation of
heavy compounds responsible for the C loss registered.
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Catalytic behaviour in the anaerobic oxidation of ethanol
only reaction occurring on the completely reduced catalyst. It
is worth noting that this latter reaction occurs with no oxygen
ion exchange between ethanol and the catalyst.
The reducibility of V/Zr/O and V/Ti/O by alcohol was studied
via experiments at a fixed temperature (400 °C) and a feed
stream of 5% ethanol/N2. As with the O2 co-feeding experi-
ments, the relative concentrations of products as a function
of time were measured (Fig. 6). This enabled us to calculate
the extent of V reduction, assuming that all V in the freshly
calcined V/Zr/O and V/Ti/O was present as V5+.
With both catalysts, ethanol conversion was total or al-
most total throughout the entire experiment time. In the case
of V/Ti/O, initially only CO2 and H2O were produced, with a
CO2 yield close to 60%, which decreased close to 0% after ap-
proximately 15 min reaction time. Based on the stoichiometry
of ethanol combustion, the extent of V5+ reduction can be cal-
culated from the amount of O2− released from V2O5. It was
found that all the V5+ present in the fresh catalyst was re-
duced to (almost) metallic V under the conditions investi-
gated. At the same time, the formation of CO2 decreased
while acetaldehyde and ethane became the predominant
products, with formation of small amounts of ethylene (selec-
tivity 3%). Thus, the disproportionation of ethanol was the
In the case of V/Zr/O, a similar phenomenon occurred.
However, the initial formation of CO2 (the only product at
the very beginning of the reaction) lasted only a few minutes
– after just 6 min it had totally disappeared. This indicates a
limited extent of reduction of V5+, no higher than 40%. Eth-
ane and acetaldehyde were the predominant products after a
few minutes' reaction time, with minor formation of ethylene
(selectivity < 1%) and acetone (3%). After the initially un-
steady behaviour, the reaction was shown to be stable for 2
h. With both catalysts, the formation of H2 was very low
(yield lower than 1%).
These experiments demonstrate that ethanol is able to re-
duce V5+ both in V/Ti/O and V/Zr/O. In the absence of oxygen,
the oxidized V species selectively forms CO2, but upon reduc-
tion CO2 formation is prevented and the disproportionation
of ethanol becomes the main reaction. However, the extent of
V5+ reduction in the two catalysts is quite different: it is to-
tally reduced with V/Ti/O, whereas with V/Zr/O only a partial
reduction occurs.
Raman spectroscopy study of catalysts
In order to support the hypothesis proposed above, we char-
acterized the systems by means of in situ Raman and DRIFT
spectroscopies.
The Raman spectra of the two catalysts recorded under N2
flow at increasing temperature are shown in Fig. 7. After
heating, the samples were maintained at 400 °C for several
hours, first under N2 and then under air. For both samples,
no further changes in the Raman spectra were observed after
the end of the temperature ramp.
It is well known that the structure of vanadia species
supported on zirconia and titania depends on VOx surface
density. Supported VOx species form polyvanadate domains
of increasing size as well as monolayers and clusters as the
vanadium surface density increases. Crystalline V2O5 appears
when vanadia loading exceeds an apparent surface density of
7–8 V atoms per nm2.60,61,70–72
In the case of V/Zr/O, the broad band at around 995 cm−1
recorded at room temperature is attributable to bulk vana-
dium oxide; other Raman bands of bulk V2O5 were shown at
around 700, 525, 405, 285 and 145 cm−1. However, when
heating the sample, a band at 1023 cm−1 became predomi-
nant (not shown at room temperature), which may be attrib-
uted to the symmetric stretch of a terminal mono-oxo VO
bond present in dispersed and dehydrated polyvanadate spe-
cies (this band shifts from 1017 cm−1 for tetrahedral mono-
meric vanadate species to 1036 cm−1 for polyvanadate spe-
cies).53,54,56,60,61,70,72,73 At the same time, the intensity of
bands attributable to bulk V2O5 progressively decreased. It is
important to note that when the sample was left for several
hours at high temperature, no changes in the spectrum
appeared. If the heating was carried out in air (instead of in
Fig. 6 Effect of reaction time on ethanol conversion and on yields to
products, with V/Ti/O (top) and V/Zr/O (bottom) under anaerobic
conditions. Reaction conditions: temperature 400 °C, W/F ratio 1.0 g s
mL−1
,
feed composition (molar%): ethanol (azeotrope)/inert
5/remainder. Symbols: ethanol conversion (♦). Yield to: acetaldehyde
(*), ethylene (Δ), CO (○), CO2 (×) and ethane (+).
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bution of V–O stretching modes within polyvanadate do-
mains, such as V–O–V and V–O–Zr modes. The number of
these domains is affected by changes in the coordination
around V atoms, which in turn affects the bond order.60 The
broad band recorded between 700 and 800 cm−1 might be
assigned to a ZrV2O7 phase; however, this compound is typi-
cally characterized by sharp bands at 775 and 975–982 cm−1
and the latter is not seen in our spectra. ZrV2O7 is known to
form by reaction between monoclinic zirconia and vanadium
oxide, especially for high V2O5 loadings and at high tempera-
tures, as a consequence of zirconium migration into V2O5
crystallites.60,71,75 The other bands are attributable to zirco-
nia. For example, the Raman band at around 340 cm−1 is typ-
ical of monoclinic ZrO2.54
In the case of V/Ti/O, bands attributable to bulk vanadium
oxide were also observed at room temperature at 995, ca. 700,
525, 405 (these two latter are overlapped by TiO2 bands), 285
and 145 cm−1. The intensity of some of these bands declined
when the temperature was raised, with a concomitant in-
crease in the intensity of the peak assigned to the terminal
VO bond in dehydrated and dispersed polyvanadate species
at 1030 cm−1. Surface polyvanadate functionalities (broad Ra-
man band at around 930–940 cm−1) were also observed, albeit
very weak.72
The Raman spectra were also recorded while heating sam-
ples under an ethanol/air feed, with an equimolar alcohol :
oxygen ratio (Fig. 8). The spectrum of V/Zr/O remained the
same as that recorded under a N2 flow (even after several
hours at 400 °C under reaction conditions), whereas V/Ti/O
Fig. 7 In situ Raman spectra recorded under N2 flow at increasing
temperature for V/Zr/O (top) and V/Ti/O (bottom) (● bands
attributable to ZrO2 or TiO2; ■ bands attributable to V oxide).
N2) there was again a progressive disappearance of bands at-
tributable to bulk V oxide, albeit at higher temperatures than
with N2. Xie et al. reported that changes in the IR spectra in
a 12% V2O5/SiO2 catalyst were due to the loss of oxygen and
the consequent formation of nonstoichiometric V suboxides,
an event which may occur even in an oxidizing environ-
ment.74 On the other hand, it cannot be ruled out that a par-
tial spreading of vanadium oxide onto the ZrO2 surface might
also have contributed to the change of spectrum. However, it
should be mentioned that in our case the phenomena
recorded were quite reversible upon the cooling and rehydra-
tion of the sample, an observation which contrasts with the
formation of V suboxides.
A very broad peak occurred between 950 and 700 cm−1. In
this range, various bands could be seen which shift and
change intensity upon heating. For example, Raman bands at
around 825, 860 and at 920–950 cm−1 have been attributed to
the presence of polyvanadate species, and specifically to V–
O–V stretching modes;60,61 broad Raman bands at around
800 and 930 cm−1 were associated with the vibration of bridg-
ing V–O–V bonds and bridging V–O–Zr bonds, respectively.70
In general, however, a broad peak in the 950–700 cm−1 range
represents a wide set of configurations, arising from a distri-
Fig. 8 In situ Raman spectra recorded under 5% ethanol/air flow at
increasing temperature for V/Zr/O (top) and V/Ti/O (bottom).
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showed the progressive disappearance of all bands attribut-
able to V2O5, possibly as a result of the reduction of vana-
dium oxide.
b) In the absence of oxygen, on V/Ti/O the adsorbed etha-
nol easily desorbs into the gas phase under a nitrogen feed,
but in doing so O2− ion from the V–O site is picked up and
the V ion reduced, while ethanol is oxidized. Therefore, un-
der the conditions used, ethanol acts as a reductant towards
V ions. In contrast, on V/Zr/O, desorption occurs with no (or
only partial) V reduction.
This difference in the behaviour of the two catalysts is at-
tributable to the different rate-limiting steps in the redox re-
action to form the key intermediate, acetaldehyde. With V/Zr/
O, the rate-limiting step seems to be the reduction of the V5+
species by ethanol, whereas with V/Ti/O, the rate-limiting step
appears to be V reoxidation. In other words, under reaction
conditions V/Ti/O shows a constant level of V oxidation that
is lower than that shown by V/Zr/O. In agreement with these
findings, Christodoulakis et al.60 observed some significant
changes of all bands attributable to V–O under reduction
with propane at 500 °C in case of supported vanadium oxide
catalysts. However, significantly higher degrees of reduction
of VOx species were observed with TiO2- than with ZrO2-
supported catalysts. Wachs et al.72 compared different
vanadia-supported catalysts for n-butane oxidation. With
V2O5/ZrO2, the Raman intensities of the terminal VO bond
and polymeric vanadate functionalities decreased somewhat
due to the reduced surface vanadium oxide species, while the
bulk vanadium oxide was not affected. Conversely, in the
case of V2O5/TiO2, the Raman intensities of surface vanadium
oxide functionalities decreased. Burcham et al.56 reported
that upon the introduction of a methanol/O2/He gas stream,
both the terminal VO and bridging V–O–V bands decreased
in intensity and shifted to lower wavenumbers for all
supported vanadia catalysts. Su and Bell71 reported that for
V2O5/ZrO2 catalysts, the partial reduction of the dispersed
vanadia species alters the Raman spectra; the ratio of inten-
sity of the zirconia bands to intensity of vanadia bands in-
creased. This phenomenon was not observed in our V/Zr/O
sample, thus supporting the hypothesis that vanadium oxide
was less reduced under our conditions.
When the same experiment was carried out with ethanol
only (diluted in He), the two catalysts showed similar behav-
iours (Fig. S6†); both showed a progressive decrease in the
intensity of all bands – those attributed to V oxide and also
those attributed to the support (zirconia or titania) – which
almost disappeared at high temperature. At the same time
strong bands attributable to carbonaceous compounds
appeared at around 1340 and 1585 cm−1. This phenomenon,
however, was quite reversible (Fig. S7†). In fact, carbonaceous
compounds desorbed completely when the ethanol stream
was replaced by a N2 stream at 400 °C and the spectrum
returned almost to that of the original catalyst. In the case of
V/Zr/O, bands attributable to V oxide as well as to the support
appeared again, whereas in case of V/Ti/O, only bands attrib-
utable to anatase were observed.
It is evident that the support plays a key role and greatly
affects the catalytic behaviour of the catalyst. The role of sup-
ports in V oxide catalysts has been highlighted by several au-
thors in the literature.55,60,72,76 Tian et al. reported that for
various vanadia-supported catalysts used for alkane oxida-
tion, the support cation is a potent ligand that directly influ-
ences the reactivity of the V–O-support species, the catalytic
active site, by controlling its basic character via the electro-
negativity of the support.70 Even the acid-basic features of
the support can affect reactivity. For example, Enache
et al.63,64 reported that in ethane oxidation over titania- and
ZrO2-supported V oxides, the greater selectivity to acetic acid
observed with the former system was due to the stronger re-
tention of acetic acid over the more basic ZrO2 support.
In conclusion, Raman experiments made it possible to
highlight the different redox properties of V species in V/Zr/O
and V/Ti/O catalysts. In the case of V/Zr/O, the V species is
less easily reduced by ethanol and the catalyst works, on aver-
age, in a highly oxidized state during ethanol oxidation. Con-
versely, the V2O5 in V/Ti/O is more easily reduced and the V
species works, on average, in a more reduced state. The pres-
ence of more oxidized V sites in V/Zr/O may be important in
the activation of ammonia during ethanol ammoxidation, as
hypothesized in the discussion above.
After ethanol oxidation experiments, used catalysts were
characterized by Raman spectroscopy; the spectra are shown
in Fig. S8.† It can be seen that V/Ti/O was indeed strongly re-
duced, whereas V/Zr/O appeared to be mostly in its oxidized
form; bands attributable to V2O5 were clearly seen. This fur-
ther supports the hypothesis that in case of V–Zr–O, V sites
still were oxidised after reaction and were not covered by
coke.
Experiments carried out using DRIFT spectroscopy under
oxidation conditions with ethanol provide further support for
the mentioned hypothesis and are discussed, see below.
DRIFT spectroscopy study of catalysts
Temperature-programmed ethanol desorption was recorded
by in situ DRIFT spectroscopy; the spectra are shown in
Fig. 9. In these experiments, the carrier was air.
In the low temperature range (85–150 °C), the band for
chemically bound ethanol (1380 cm−1δs CH3) was dominant
in the spectrum of V/Ti/O, while the spectrum of V/Zr/O
showed bands corresponding to ethoxy species (dissociated
ethanol, 1057, 1097 and 1143 cm−1, ν(as) C–Obident, ν(as) C–C
and ν(as) C–Omonodent, respectively)77 and a negative band at
1630 cm−1 (δ H–O–H), indicating that water is desorbed from
the catalyst. The desorption of water suggests an oxidative
From these results it can be concluded that:
a) In the absence of oxygen, ethanol is strongly retained
on the catalyst surface, forming adsorbed species which are
easily desorbed at 400 °C under a nitrogen feed. Conversely,
when oxygen is present, no compounds are retained on the
catalyst surface; they are transformed into products and rap-
idly desorb into the gas-phase.
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been reduced, the reoxidation process is more difficult, lead-
ing to the decomposition of the acetates.
Scheme 1 depicts the proposed routes for aerobic ethanol
oxidation over the two catalysts.
Experiments were also conducted with V/Zr/O in the ab-
sence of O2 (anaerobic conditions), i.e., with a feed of ethanol
and He as the carrier gas. Ethanol was first adsorbed at room
temperature and then the temperature was increased while
recording DRIFT spectra (Fig. 10). It can be seen that at low
temperatures ethanol dissociated forming the ethoxy species.
At 150 °C, however, a band at 1632 cm−1 appeared. The pres-
ence of this band, attributable to the CO stretching of acet-
aldehyde, and the lack of a negative band for water desorp-
tion confirm that under anaerobic conditions the aldehyde is
formed mainly by disproportionation. Interestingly, the spec-
trum of V/Zr/O under inert atmosphere is similar to that
shown for V/Ti/O in air, confirming that the latter catalyst re-
duces quickly but is not easily reoxidised.
Fig. 11 compares the main m/z signals recorded during
the ethanol TPD measurements conducted with and without
oxygen for V/Zr/O.
Under anaerobic conditions at low temperature, the de-
sorption of some unconverted ethanol and the formation
of CO2 and water were observed, in agreement with the
results for the fresh catalyst in the reactivity test (see
Fig. 6). The concomitant formation of aldehyde and hydro-
gen was also observed. This confirms that V/Zr/O is not
able to release the oxygen ion in the form of water and
perform the oxidative dehydrogenation of the alcohol in
the absence of an external oxygen source. At high temper-
ature there was a CO2 desorption that was not due to to-
tal oxidation (because there was no water formation), but
to a decomposition of the intermediates. Conversely, in
the presence of air the concomitant desorption of acetal-
dehyde and water in similar relative amounts at low tem-
perature and the absence of hydrogen formation indicate
Fig.
9 DRIFT spectra recorded during ethanol temperature-
programmed-desorption using air as carrier (10 mL min−1) for V/Zr/O
(top) and V/Ti/O (bottom).
dehydrogenation over V/Zr/O which is not observed in the
case of V/Ti/O. With increasing temperature, several broad
bands appeared in the spectra of both catalysts. The bands at
1350–1355, 1438–1442 and 1531–1536 cm−1 correspond to the
stretching of acetate-like moieties (δs CH3, ν(as) O–C–O, ν(s) O–
C–O, respectively);78 they seem to be the predominant surface
species in the case of V/Ti/O at 200 °C, but at higher tempera-
tures the intensity of the bands simply decreased. However,
in the case of V/Zr/O there are additional bands especially in
the CO region, indicating the formation of more oxidized
products.
Even if an unambiguous assignment is not possible, the
evolution of bands at 1402, 1670 and 1689 cm−1 suggests the
formation of acetic acid or ethylacetate, and the band at 1562
cm−1 is typical for a surface carbonate79,80 (see in Fig. S9† the
deconvoluted spectrum for both catalysts at 350 °C). Thus,
from this analysis it can be inferred that in the case of V/Zr/
O, the formation of acetaldehyde results mainly from oxida-
tive dehydrogenation of activated ethanol (ethoxy species).
The resulting reduced V species is promptly reoxidized by O2
and is able to continue the oxidation process towards the for-
mation of acetic acid, ethylacetate, carbonates and so on.
Conversely, in the case of V/Ti/O, acetaldehyde forms mainly
by disproportionation of ethanol (with co-formation of eth-
ane), and then it evolves to acetates. However, after V5+ has
Scheme
1 Main pathways for aerobic ethanol oxidation on the
surface of V/Zr/O (left) and V/Ti/O (right) catalysts.
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Conclusions
Two vanadium oxide catalysts, one supported on titania, the
other on zirconia, were tested for the gas-phase
ammoxidation of ethanol to acetonitrile. The two showed dif-
ferent catalytic behaviours, with V/Zr/O being more active
and more selective to acetonitrile than V/Ti/O. The higher se-
lectivity gave rise to the formation of less CO2 and N2 from
ammonia combustion.
These different behaviours were attributed to the fact that
V/Zr/O is more selective than V/Ti/O in the oxidative dehydro-
genation of ethanol to acetaldehyde, which is a key reaction
intermediate in ammoxidation to acetonitrile. This is because
the nature of V sites under steady-state conditions is different
in the two cases: with V/Ti/O, the rate-determining step of the
redox process is the reoxidation of V by O2, whereas with V/
Zr/O it is the reduction by ethanol. This means that the V in
V/Ti/O is more reduced. The different reducibility of V5+ by
ethanol has also been demonstrated by specifically designed
reducibility experiments. It is also important to note that the
fully oxidized V5+ is intrinsically unselective under anaerobic
conditions, giving rise to the formation of CO2 and H2O only.
The reduced catalyst catalyzes the disproportionation of etha-
nol to acetaldehyde and ethane, a reaction which, conversely,
occurs to a negligible extent in the presence of molecular
oxygen. V/Zr/O is also more efficient in the subsequent step
of acetaldehyde reaction with ammonia, which is the step
needed to form imine, the precursor for nitrile formation. In
this step, not only does V/Ti/O produce a greater amount of
CO2, but ammonia combustion to yield N2 is also increased,
which is clearly detrimental to the production of acetonitrile.
The more efficient activation of ammonia with V/Zr/O has
been attributed to the greater availability of oxidized V sites
in this catalyst.
Fig. 10 DRIFT spectra of V/Zr/O recorded during temperature-
programmed-ethanol desorption using He as the carrier gas.
Acknowledgements
Lonza is acknowledged for financial support.
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