An alternative pathway for production of acetonitrile: Ruthenium catalysed aerobic dehydrogenation of ethylamine

Article abstract of DOI:10.1039/c3gc36513a

The oxidative synthesis of acetonitrile from ethylamine was studied using a supported ruthenium catalyst. The reaction was conducted in both batch and flow processes and high conversions (over 85%) were achieved in both cases. Selectivity of both reactions was improved by optimisation of reaction conditions, achieving over 90% selectivity in the batch process and 80% selectivity in the continuous flow process. The use of a selective solid catalyst that utilises a feedstock that can be derived from biomass, dioxygen as the oxidant and water as the solvent represents a new, green route for the independent and efficient production of acetonitrile.

Full text of DOI:10.1039/c3gc36513a

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An alternative pathway for production of acetonitrile:
ruthenium catalysed aerobic dehydrogenation
of ethylamine
Cite this: Green Chem., 2013, 15, 928
Emily C. Corker,a Uffe V. Mentzel,a,b Jerrik Mielby,a Anders Riisagera and
Rasmus Fehrmann*a
The oxidative synthesis of acetonitrile from ethylamine was studied using a supported ruthenium catalyst.
The reaction was conducted in both batch and flow processes and high conversions (over 85%) were
achieved in both cases. Selectivity of both reactions was improved by optimisation of reaction conditions,
achieving over 90% selectivity in the batch process and 80% selectivity in the continuous flow process.
The use of a selective solid catalyst that utilises a feedstock that can be derived from biomass, dioxygen
as the oxidant and water as the solvent represents a new, green route for the independent and efficient
production of acetonitrile.
Received 25th September 2012,
Accepted 18th February 2013
DOI: 10.1039/c3gc36513a
www.rsc.org/greenchem
from ethanol and ammonia,9 rendering ethylamine a readily
available starting material. The present work focuses on the
Introduction
Acetonitrile is a bulk chemical with a large number of appli- development of the second reaction step, in which ethylamine
cations including its use as a solvent and an intermediate for is converted to acetonitrile.
pharmaceutical, agricultural and materials chemistry.1 Aceto-
Routes for the synthesis of nitriles from amines, such as
nitrile is currently produced as a by-product during the acetonitrile from ethylamine, using stoichiometric oxidants
ammoxidation of propylene to acrylonitrile.2–4 The recent have been known for some time.10–12 However, the use of
acetonitrile crisis of 2009,5 coupled with the reliance of aceto- atmospheric oxygen as an oxidant for the production of
nitrile production on petrochemical resources, highlights the nitriles from amines is attractive as only H2O is generated as a
need for an independent, efficient process for the production side-product (Scheme 1). There have been many studies
of acetonitrile from sustainable resources.
reported regarding the selective oxidation of alcohols using
Previously reported alternative routes to acetonitrile include molecular oxygen and heterogeneous catalysts, but the selec-
the ammoxidation of ethane5,6 and ethanol,7,8 though both tive oxidation of amines with O2 or air has been somewhat
routes appear to suffer from disadvantages such as high neglected. However, recently Yamaguchi et al. published a
ammonia to reactant ratios, low conversion or low selectivity to series of studies involving ruthenium hydroxide based catalysts
acetonitrile. A third option may involve the production of for a number of functional group transformations with O2 and
acetonitrile from ethanol or bioethanol via ethylamine ammonia,13–18 including the synthesis of aromatic and long
(Scheme 1). Ethylamine is presently produced on a large scale chain aliphatic nitriles from amines using O2 as the oxidant.19
The described Ru(OH)x/Al2O3 catalyst was efficient for the
tested transformations, though it was not applied to short
chain aliphatic nitriles and reactions were only conducted in
batch processes.
In recent years there has been an emergence of flow chem-
istry for functional group transformations, both within labora-
Scheme 1 Formation of acetonitrile from ethanol via ethylamine.
tories20,21 and industry.22 The use of continuous flow
processes offers advantages such as improved safety, efficient
catalyst separation, improved space economy, reproducibility,
automation and process reliability. For this reason, the work
on the conversion of ethylamine to acetonitrile presented here
aCentre for Catalysis and Sustainable Chemistry, Department of Chemistry,
Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark.
was performed in both a batch reactor and in a gas-phase, con-
tinuous flow reaction setup.
E-mail: rf@kemi.dtu.dk; Fax: +45 4588 3136; Tel: +45 4525 2389
bHaldor Topsøe A/S, Nymøllevej 55, DK-2800 Kgs. Lyngby, Denmark
928 | Green Chem., 2013, 15, 928–933
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The reactant liquid (10 wt% ethylamine in water) was introduced
by an HPLC pump and evaporated before reaching the reactor
(WHSVethylamine = 1.0 h−1). Air and helium were introduced by
Experimental
Catalyst synthesis and characterisation
The RuOx/Al2O3 catalyst was synthesised with a target Ru mass flow controllers and the total gas flow (including ethyl-
loading of 2.4 wt% by deposition precipitation using a method amine and water vapour) was kept constant at 150 cm3 min−1
analogous to the method previously reported by Yamaguchi in all experiments. All experiments were performed at a total
et al.16 0.25 g RuCl3 hydrate (38–42 wt% Ru) was dissolved in pressure of 1 bar and the reaction temperature was measured
143 ml of distilled water. 4.9 g Al2O3 (Saint-Gobain) was added inside the reactor, just below the catalyst bed. The product
to the solution to form a slurry which was stirred for mixture travelled in heated lines from the reactor to an online
15 minutes. 1 M NaOH solution was added to the slurry to GC (Agilent 6890) equipped with a flame ionization detector
adjust the pH to 13.5. The slurry was then stirred at room and an automated gas injection system. CO and CO2 were
temperature for 18 hours. The resulting dark green powder monitored continuously by a BINOS detector situated after
was filtered, washed with distilled water and dried at 140 °C.
The BET surface areas were analysed by N2 physisorption
the GC.
All chemicals for synthesis and activity testing were
using a Micromeritics ASAP 2020. XRPD analysis was carried obtained from Sigma-Aldrich unless otherwise stated and used
out using a Huber G670 diffractometer with an exposure time as received.
of 30 minutes and an incident wavelength of 1.54060 Å.
SEM images were obtained using a Quanta 2000 ESEM FEG
microscope, equipped with a field emission gun electron
Results and discussion
Catalyst characterisation
source, an Everhart–Thornley secondary electron detector and
an Oxford Instruments 80 mm2 X-max SDD EDS detector. The
catalyst was mounted on carbon tape and coated with Au
using a Cressington 208HR Sputter Coater, 40 mA current for
10 seconds, before SEM analysis. TEM images were obtained
using an FEI Tecnai T20 G2 microscope, equipped with a Ther-
mionic – LaB6 electron source, a Gatan 894 2K UltraScan 1000
camera and a 2K UltraScan 1000 FT camera and operated at
200 kV. The catalyst was dispersed on an amorphous carbon
film TEM grid prior to analysis.
XRF analysis was performed using a Panalytical Axios wave-
length dispersive X-ray fluorescence spectrometer. The sample
was mixed with micro-crystalline cellulose and pressed into a
solid pellet for analysis.
XRPD analysis of both the calcined and uncalcined RuOx/
Al2O3 catalysts showed the presence of highly amorphous
γ-Al2O3 23 and RuO2,24 as shown in Fig. 1. XRPD analysis of the
uncalcined catalyst showed only broad, undefined peaks, while
analysis of the 350 °C calcined catalyst showed sharper peaks
corresponding to the RuO2 phase, suggesting a larger average
crystallite size or a greater degree of crystallinity. Analysis
using the Debye–Scherrer equation suggested RuO2 nanoparti-
cles in the calcined catalyst with an average crystallite size of
20–25 nm.
The catalyst was imaged by SEM and analysed using EDS to
determine elemental composition. SEM images showed cata-
lyst particles of varying sizes of up to 20 μm, as seen in Fig. 2.
The ruthenium metal loading was found to be 1–3 wt% by
EDS. Small nano-scale features were observed on the surface of
the catalyst. These could be presumed to be ruthenium
Catalyst activity testing
Oxidation of ethylamine in the batch reactor was typically con-
ducted as follows: 0.425 g of RuOx/Al2O3 catalyst (<180 μm size
fraction) was loaded into a Teflon cup. 800 μl of aqueous 70%
ethylamine solution, 215 μl of dioxane (internal standard) and
2985 μl of water (or aqueous ammonia where appropriate) were
added to give a total volume of 4 ml. The Teflon cup was
loaded into a Parr autoclave reactor and pressurised to 10 bar
with O2. The autoclave was heated with stirring to the desired
reaction temperature (typically 80–100 °C) for the desired time
(1–24 hours). The reaction temperature was measured by a
thermocouple inside the Teflon cup. When the reaction time
was complete, the autoclave was allowed to cool. The reaction
mixture was filtered and analysed by GC (Agilent 6890).
Product concentrations were calculated by reference to the
internal standard. A similar process was used when THF was
used as the solvent in batch reactions with 5.05 ml of 2 M
ethylamine in THF.
Catalytic tests under flow conditions were carried out in a
fixed bed quartz reactor charged with 300 mg catalyst
(180–350 μm size fraction). Catalysts were calcined at
Fig. 1 XRPD analysis of the RuOx/Al2O3 catalyst uncalcined (A) and calcined in
air at 350 °C (B). Inserted lines correspond to 2θ values of major peaks of
150–350 °C in air or He in the reactor before the reaction. γ-Al2O3 23 and RuO2 24 obtained from the literature.
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Table 1 Physical properties of fresh, calcined and spent catalysts
BET surface
area (m2 g−1
Ru content (wt%)
determined by XRF
Catalyst
)
RuOx/Al2O3
247.8
241.8
189.6
1.33
2.06
1.22
RuOx/Al2O3 (350 °C calcined)
RuOx/Al2O3 (350 °C calcined –
Spent in EtNH2/H2O only)
RuOx/Al2O3 (350 °C calcined –
Spent in EtNH2/H2O/NH3)
210.0
1.25
Fig. 2 Secondary electron SEM images of the RuOx/Al2O3 catalyst.
in the form RuO2) is most likely due to hydration of the catalyst.
Both catalysts contained low levels (below 0.8 wt%) of Na and
Cl, indicating that the catalyst had been thoroughly washed
during preparation. Spent catalysts that were calcined and
then deactivated both with and without the presence of NH3
contained a Ru loading of 1.22–1.25 wt%, indicating rehydra-
tion of the catalysts during the reaction.
The XRF and TEM results related to the fresh uncalcined,
calcined and spent catalysts suggest that during the reaction
the catalyst reverts to a state similar to that occurred before cal-
cination, with a large degree of hydration and small Ru con-
taining areas.
Catalyst activity – batch reactor
The RuOx/Al2O3 catalyst was tested for the oxidative dehydro-
genation of ethylamine to acetonitrile in a similar manner to
the approach used by Yamaguchi et al. for the oxidative dehy-
drogenation of aromatic amines.19 The initial experiments
were conducted with water as the solvent and acetonitrile was
produced over a range of reaction times as shown in Fig. 4,
reaching 80% conversion after 18 hours at 120 °C.
In the absence of ruthenium, the alumina support was
found to be almost completely inactive, as shown in Table 2,
supporting the proposal that the active phase of the catalyst is a
ruthenium-containing species, whilst the alumina acts solely as
a support material. Calcination of the catalyst at 350 °C in air
prior to catalytic testing appeared to have no significant effect
Fig. 3 TEM images of the RuOx/Al2O3 catalyst. (A) Uncalcined catalyst, (B) cata-
lyst calcined at 350 °C, (C) catalyst calcined at 350 °C and deactivated by a
EtNH2, H2O reaction mixture in continuous flow, (D) catalyst calcined at 350 °C
and deactivated by a EtNH2, H2O and NH3 reaction mixture in continuous flow.
containing nanoparticles, though this could not be confirmed
by EDS.
Fresh and spent catalysts (obtained from activity tests
under flow conditions) were imaged using TEM, as shown in
Fig. 3. Darker patches shown in the images may be attributed
to areas containing higher atomic weight elements, i.e. Ru-con-
taining species. In agreement with the XRPD results, Ru con-
taining areas appear larger in the 350 °C calcined catalyst, in
comparison to the uncalcined catalyst. Both spent catalysts
appear similar to each other and the fresh, uncalcined catalyst.
The BET surface area of the as synthesised RuOx/Al2O3 cata-
lyst was 247.8 m2 g−1. Calcined and spent catalysts (also
obtained from activity tests under flow conditions) showed
slightly reduced surface areas, as would be expected following
exposure to high temperatures (see Table 1).
The fresh and 350 °C calcined catalysts were analysed using
XRF. The fresh catalyst was found to contain a Ru loading of
1.33 wt%, while the calcined catalyst contained a Ru loading
of 2.06 wt%. The lower Ru content by comparison to the
Fig. 4 The effect of reaction time on production of acetonitrile and by-pro-
ducts from ethylamine (0.010 mol) in a H2O solvent (4 ml) with 10 bar O2,
2.4 wt% (calculated on the basis of the presence of ruthenium 0.425 g RuOx/Al2O3 and temperature 120 °C in the batch reactor.
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Table 2 Products of catalytic activity tests of RuOx/Al2O3 and Al2O3 in the batch reactora
Catalyst
Reactant
Solvent
Conversion (%)
Acetonitrile (%)
Acetamide (%)
Acetic acid (%)
Other (%)
RuOx/Al2O3
RuOx/Al2O3
RuOx/Al2O3 (350 °C calcined)
Al2O3
Ethylamine
Ethylamine
Ethylamine
Ethylamine
H2O
THF
H2O
H2O
51
54
42
<1
32
41
31
0
11
2
2
7
0
3
0
<1
10b
6
0
<1
a Reaction conditions: 0.425 g catalyst, 0.010 mol reactant, 10 bar O2, 120 °C, 5 hours. b Mainly N-ethylethanimine.
on the activity during a 5 hour reaction, also shown in Table 2.
With water as the solvent, the acetonitrile selectivity and yield
was limited by production of acetamide and acetic acid by-pro-
ducts, suspected to be formed by hydration of the acetonitrile
product followed by hydrolysis, as shown in Scheme 2. The con-
version of ethylamine to acetonitrile and acetamide agrees with
the findings of Yamaguchi et al., who observed the synthesis of
aromatic nitriles and amides using a similar supported ruthe-
nium catalyst in an aqueous environment.13,19
Though ammonia may be thought to reduce the conversion
to acetic acid, the presence of aqueous ammonia was found to
have a detrimental effect on the catalyst activity, with signifi-
cantly reduced conversion, as shown in Fig. 5. Although good
selectivities were achieved in the presence of ammonia (over
90% for all ammonia ratios), yields of acetonitrile remained
Fig. 5 Effect of increasing the NH3 : EtNH2 molar ratio (0–1.4) on conversion of
ethylamine (0.010 mol) and selectivity to acetonitrile in the batch reactor with a
H2O solvent (4 ml), 10 bar O2 and 0.425 g RuOx/Al2O3, reaction time 5 hours,
reaction temperature 120 °C.
low (4–22%) due to the low conversion of ethylamine.
To prevent hydration of product acetonitrile to acetamide
and acetic acid, the solvent was changed to THF and aceto-
nitrile was again successfully produced over a range of reaction
times, as shown in Fig. 6. When THF was used as the solvent
there was little hydration of product acetonitrile and so selec-
tivities in excess of 90% were achieved. In an attempt to
prevent all hydration of the product acetonitrile, molecular
sieves (3 Å) were added to the reaction, though their addition
did not appear to yield any significant improvement in selec-
tivity. The lack of improvement in selectivity is possibly due to
the close proximity of the water by-product, the acetonitrile
product and the catalyst when the conversion to acetonitrile is
complete, increasing the likelihood of the subsequent reaction
to form acetamide before the by-product water can be
absorbed by the molecular sieves.
Catalyst activity – continuous flow
The oxidative dehydrogenation of ethylamine to acetonitrile
was also successful under gas-phase, continuous flow con-
ditions, as shown in Fig. 7. Water was used as a solvent for
introducing ethylamine and, contrary to the results seen in the
batch reactor, hydration of acetonitrile to acetamide and
hydrolysis to acetic acid were not observed under any tested
reaction conditions. However, under flow conditions the yield
Fig. 6 The effect of reaction time on production of acetonitrile and by-pro-
ducts from ethylamine (0.010 mol) in a THF solvent (5 ml) with 10 bar O2,
0.425 g RuOx/Al2O3 and reaction temperature 120 °C.
of acetonitrile was limited by the production of acetaldehyde
and N-ethylethanimine. Secondary imines such as N-ethyletha-
nimine are well known to be formed from amines and alde-
hydes,25 suggesting that N-ethylethanimine is formed by
reaction of the acetaldehyde by-product with the ethylamine
reactant, as shown in Scheme 3.
The selectivity to acetonitrile was slightly improved by
increasing the O2 to EtNH2 molar ratio, as shown in Fig. 7.
Scheme 2 Formation of acetonitrile from ethylamine, followed by hydration to
acetamide and hydrolysis to acetic acid.
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Fig. 8 Effect of time on stream on conversion of ethylamine at various NH3
:
EtNH2 molar ratios (0, 1 and 5 eq.). Temperature = 225 °C, O2 : EtNH2 = 3,
WHSVethylamine = 1.0 h−1, total gas flow = 150 cm3 min−1, 0.300 g RuOx/Al2O3
(350 °C calcined).
Fig. 7 Effect of the O2 : EtNH2 molar ratio on yield of acetonitrile, acet-
aldehyde, N-ethylethanimine and selectivity to acetonitrile during continuous
flow reaction. Temperature = 225 °C, WHSVethylamine = 1.0 h−1, total gas flow =
150 cm3 min−1, 0.300 g RuOx/Al2O3 (350 °C calcined).
Scheme 3 Formation of acetaldehyde and N-ethylethanimine by-products
from ethylamine during continuous flow reaction.
The increased selectivity to acetonitrile with higher O2 ratios is
hardly surprising when the stoichiometries of the formation of
acetonitrile and acetaldehyde from ethylamine are considered,
as outlined in Schemes 2 and 3.
Fig. 9 Effect of removal of ammonia (5 eq.) from the feedstock on conversion
of ethylamine after 70 hours on stream. Temperature = 225 °C, O2 : EtNH2 = 3,
WHSVethylamine = 1.0 h−1, total gas flow = 150 cm3 min−1, 0.300 g RuOx/Al2O3
(350 °C calcined).
In an attempt to decrease the synthesis rate of acetaldehyde
and N-ethylethanimine, ammonia was included in the reaction
mixture in various ratios. The presence of ammonia resulted in
a negligible effect on selectivity, with selectivity remaining at
approximately 75% for all ammonia ratios and times on stream.
However, the presence of ammonia caused the catalyst to show
reduced activity (Fig. 9). When the ammonia was removed from
the reactant mixture, activity increased significantly, as shown
in Fig. 8. It is therefore probable that the reduction in activity
caused by the presence of ammonia in the reactant mixture is
due to competitive binding of ammonia to the catalyst active
site, rather than degradation of the catalyst itself. Once
ammonia is removed from the feedstock, it no longer blocks
the catalyst active site and activity returns to a high level.
In the absence of ammonia, the catalyst proved to be sur-
prisingly stable, with only a small decrease in conversion after
40 hours on stream. However, the catalyst was only found to
have such a high stability when calcined in air, as shown in
Fig. 10. Catalysts that were calcined at lower temperatures or
calcined in He showed much more rapid deactivation.
Fig. 10 Effect of time on stream on conversion of ethylamine for catalysts cal-
cined in air or He at 350, 250 or 150 °C for 4 hours. Temperature = 225 °C,
O2 : EtNH2 = 3, WHSVethylamine = 1.0 h−1, total gas flow = 150 cm3 min−1
,
0.300 g RuOx/Al2O3.
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Consideration of the catalyst activity data from both batch XRPD and BET analysis, respectively. Our thanks also go to
and continuous flow processes, as well as characterisation Leon Van De Water and Dave Maclachlan from Johnson
data, suggests that the uncalcined catalyst contains a more Matthey Catalysts for their help with XRF analysis.
amorphous phase of the catalyst, whilst the calcined catalyst
contains relatively large (20–25 nm) RuO2 crystallites. Both the
amorphous and crystalline phases are active in both the batch
and continuous flow processes, while the calcined catalyst is
clearly superior in the continuous flow process, due to much
Notes and references
higher stability under reaction conditions. The higher stability
of catalysts calcined in air may be due to the presence of RuO2
nanoparticles of increased size and/or crystallinity, as observed
by XRPD. The proposed high activity and stability of the RuO2
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