DOI: 10.1002/cssc.200800023
New Reaction: Conversion of Glycerol into Acrylonitrile
M. Olga Guerrero-Prez and Miguel A. BaÇares*[a]
Glycerol is a major by-product from methanolysis during the
production of biodiesel. Thus, it is an increasingly important
molecule in the context of renewable biomass resources to
provide energy and chemical intermediates. However, the development of selective glycerol-based catalytic processes is a
major challenge as a result of their low selectivity.[1–4] In several
approaches, this broad product distribution has been narrowed. Bioconversion of glycerol to 1,3-propanediol is of major
interest, but it still exhibits a limited metabolic efficiency.[5–6]
From an applied point of view, the direct catalytic production
of syngas from glycerol under mild conditions is a major achievement.[7] The selective oxidation of alcohols to carbonyl
compounds[8] is an important transformation for the synthesis
of fine chemicals and intermediates. Recently, the telomerization of glycerol with 1,3-butadiene was described to obtain C8chain mono-, di-, and triethers of glycerol with potential applications in surfactant chemistry.[9]
The glycerol-to-acrolein process is very attractive,[10–12] and
complete dehydration of glycerol to acrolein has been reported.[10–11] However, the application of glycerol dehydration to
acrolein is typically limited by selectivity.[4, 13] Acrolein is an intermediate in the formation of acrylonitrile during the ammoxidation of propane and propene.[14–15] The use of an additional
reactant to narrow the product distribution has proven an attractive option,[4] for example, affording glycerol carbonate by
reaction with CO2.[16] Thus, it makes sense that feeding ammonia during the reaction of glycerol may concentrate the product distribution towards nitriles. Herein, we report a reaction of
glycerol with ammonia to form nitriles that minimizes side reactions and affords a value-added product, namely, the reaction of glycerol with ammonia to acrylonitrile under moderate
reaction conditions. Acrylonitrile is probably the nitrile that is
manufactured on the largest scale as it is used mainly as a monomer in the manufacture of synthetic polymers, especially
polyacrylonitrile for acrylic fibers.[17] Acrylonitrile is industrially
produced by ammoxidation of propylene,[18–19] which is obtained by steam cracking or catalytic cracking of petroleum
fractions. It is thus necessary to develop a process for acrylonitrile synthesis based on renewable materials, such as glycerol.
Figure 1 shows Raman spectra of dehydrated vanadium-containing catalysts, V/Al, VSb/Al, and VSbNb/Al. Both Sb/Al and
Nb/Al have been characterized elsewhere and present Sb2O3
crystallites[20] and dispersed niobium oxide species,[21] respectively. The spectra exhibit a Raman band near 1030 cm 1 which
is sensitive to hydration, typical of dispersed surface vanadium
oxide species. The broad bands in the 800–900 cm 1 region
[a] Dr. M. O. Guerrero-P rez, Dr. M. A. BaÇares
Instituto de Catlisis y Petroleoqumica (CSIC)
Marie Curie 2, 28049 Madrid (Spain)
Fax: (+ 34) 585-4760
E-mail: banares@icp.csic.es
ChemSusChem 2008, 1, 511 – 513
Figure 1. In situ Raman spectra of dehydrated V/Al, SbV/Al and SbVNb/Al
supported catalysts.
are characteristic of bridging oxygen vibrations of surface polymeric vanadia species.[22] V/Al also exhibits Raman bands at
996, 285, and 144 cm 1 which are characteristic of bulk V2O5
crystallites. These must be no larger than 4 nm as otherwise
they would generate no X-ray diffraction pattern. The Raman
section of V2O5 crystallites is more than an order of magnitude
in intensity higher than that of dispersed surface vanadium
oxide species,[23] thus most vanadium sites are present as dispersed surface vanadium oxide species. The Raman spectrum
of SbV/Al exhibits a broad Raman band near 800 and 880 cm 1
owing to the formation of defective rutile VSbO4 phase.[24–26]
This broad Raman band shifts to 915 cm 1 on VSbNb/Al; this
band belongs to incipient V–Nb–O mixed phases.[27] The binary
and tertiary supported oxides exhibit no Raman bands of
Sb2O3, Sb2O4, or V2O5 crystalline phases. The Raman spectra
show that the presence of Sb promotes the defective rutile
VSbO4 phase and that mixed V–Nb–O phases dominate in the
presence of Nb and Sb additives.
Table 1 shows the conversions of glycerol and selectivities to
main products on alumina-supported Sb, V, Nb, SbV, and
SbVNb oxide catalysts. Alumina-supported Sb and Nb oxides
are significantly less active than vanadium-containing supported oxides, which display negligible selectivity to acrylonitrile,
with alcohols, aldehydes, and ketones being the dominant
products. Alumina-supported Sb oxide exhibits significant selectivity to a nitrile product (acetonitrile). Alumina-supported
vanadia is most active, but it produces propanal, 1,2-propanediol, acrolein, and cracking products, where dehydration is important. Alumina-supported antimony and niobium oxide catalysts are quite inactive, but note that both produce acrolein,
which is an intermediate to the formation of acrylonitrile,[13–14]
and most interestingly Sb/Al exhibits a capacity to form
carbon–nitrogen bonds (acetonitrile).
B 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
511
Table 1. Conversions of glycerol and selectivities to main products at 400 8C.
Catalyst
Sb/Al
Nb/Al
V/Al
VSb/Al
VSbNb/Al
Conv.
[%]
CO
CO2
CH4
C2H6
C2H4
propane
Selectivity to products [%]
propylene
1,2-propanediol
acetonitrile
propanal
acrolein
acrylonitrile
10.4
16.2
87.2
71.6
82.6
0.5
0.2
0.6
0.7
1.1
7.7
6.8
7.7
2.9
4.4
0.0
0.0
0.1
0.5
0.2
–
–
1.5
–
–
–
–
1.2
–
–
–
–
0.1
–
–
–
–
0.1
–
–
19.2
1.5
1.1
1.0
0.8
14.4
34.7
53.5
4.3
2.1
28.9
26.7
6.9
28.9
26.2
1.2
1.3
–
56.0
58.3
Among the supported oxides, vanadium-containing catalysts
afford significantly higher conversions of glycerol. V/Al is rather
unselective. Modification of supported vanadium oxide by the
addition of Sb and Nb modulates their selectivity. The glycerol
conversions are high, and the catalysts are more selective to
acrolein and acrylonitrile, with acrylonitrile obtained as the
main product. Thus, VSb/Al is slightly less active than aluminasupported vanadium oxide but it produces acrylonitrile with
56 % selectivity at 71.6 % conversion. When the catalyst is further promoted with the acidic dopant Nb (VSbNb/Al), both the
conversion of glycerol and selectivity to acrylonitrile increase.
The catalysts exhibit some deactivation during time on
stream, and deactivation becomes apparent after 2 h, in parallel with the formation of carbon deposits. The dehydration reaction of glycerol typically exhibits side reactions leading to
polyaromatic compounds, which are the cause of the formation of coke on the catalysts;[10] similar side reactions may
occur in presence of ammonia that may lead to polyacrylonitrile-derived deposits.
The details of the reaction mechanism are yet to be determined. The reaction schemes of classical ammoxidation reaction and those of glycerol oxidative dehydration are to be considered. Acrolein appears as an intermediate in the ammoxidation of C3 hydrocarbons to produce acrylonitrile.[13–14] Acrolein
is produced from glycerol too.[10–13] The junction of these two
reaction mechanisms appears critical to understand this new
reaction. The presence of ammonia as a co-reactant appears to
concentrate the product distribution towards acrylonitrile
(combined selectivity of acrolein and acrylonitrile over 84.9 %).
A possible reaction scheme would involve a combination of
the glycerol dehydration mechanism proposed by Nimlos
et al.,[28] followed by a subsequent C N bond formation (as illustrated in Scheme 1).
The results show that vanadium sites are necessary for the
activation of glycerol but that they are not selective to acrylonitrile, whereas Sb sites endow the catalyst with the capacity
to form C N triple bonds. Sb/Al does not afford acrylonitrile
but instead a mixture of oxygenates and acetonitrile. It appears that the interaction of Sb and V is necessary to obtain a
Scheme 1. The glycerol-to-acrylonitrile reaction.
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28.1
28.8
27.1
5.7
6.9
catalyst formulation that is efficient for acrylonitrile formation.
The Raman spectra confirm the existence of V–Sb oxide interactions (VSbO4 ; Figure 1). Nb in Nb/Al would provide acidity
that facilitates the dehydration of glycerol to acrolein. The
presence of Nb in the VSbNb/Al catalyst would increase acidity,
which would enhance the activation of ammonia. This would
be consistent with trends reported for the ammoxidation of
propane to acrylonitrile over similar catalytic formulations.[21]
In summary, the data reported here present a new process
for valorization of glycerol to acrylonitrile with very promising
selectivities at high conversions and moderate reaction temperatures. Such a process is of high potential for industrial application.[29] A systematic study is currently underway to assess
the molecular structure–activity relationships and reaction
mechanism, and to optimize the catalyst formulation and reaction conditions to further improve the total yield to acrylonitrile under mild conditions and also to minimize the formation
of coke to obtain catalysts that are selective over a longer
time.
Experimental Section
Catalyst preparation: VSb/Al and VSbNb/Al were prepared by a
slurry method. Two batches were prepared where Sb2O3 (Aldrich,
p.a.) was added to an aqueous solution of NH4VO3 (Sigma, p.a.),
and to one of the batches ammonium niobium soluble complex
(Niobium Products) was added. Sb/Al catalyst was prepared by the
slurry method using a suspension of Sb2O3. V/Al and Nb/Al catalysts were prepared by impregnation of the corresponding salt indicated above. Details of the synthesis can be found elsewhere.[24]
The catalysts were prepared so that a total coverage of V, Sb, Nb,
V + Sb, or V + Sb + Nb oxides would correspond to the dispersion
limit on alumina. The dispersion limit was determined by Raman
spectroscopy as the maximum surface loading that remain dispersed, with no crystalline phases.[24] Atomic ratios for Sb/V and
Nb/V were 1. The Brunauer–Emmett–Teller (BET) surface area
values obtained for Sb/Al, Nb/Al, V/Al, VSb/Al, and VSbNb/Al catalysts were 105, 122, 130, 117, and 126 m2 g 1, respectively.
Catalyst characterization. Nitrogen adsorption isotherms ( 196 8C)
were recorded on an automatic Micromeritics ASAP-2000 apparatus. Raman spectra were recorede with a single monochromator
Renishaw System 1000 equipped with a cooled CCD detector
( 73 8C) and holographic super-Notch filter. The samples were excited with the 488-nm Ar line; spectral resolution was about
4 cm 1, and acquisition of the spectrum consisted of 10 accumulations of 10 s. The spectra were obtained under dehydrated conditions (ca. 450 8C) in a hot stage (Linkam TS-1500). Hydrated samples (not shown) were obtained at room temperature after and
B 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2008, 1, 511 – 513
during exposure to a stream of humid synthetic air. X-ray diffraction analysis (not shown) was run on a Siemen Krystalloflex D500
diffractometer.
Glycerol ammoxidation reaction: The catalyst with a particle size of
0.250–0.125 mm (50 mg) was diluted with CSi (250 mg) to minimize exothermic effect, and the mixture was situated inside a
fixed-bed reactor made of quartz. The dead volumes upstream and
downstream in the catalyst bed were minimized to prevent the
contribution of the gas-phase reaction. A thermocouple was situated in the catalytic bed for measurement of the temperature. The
catalyst was activated by feeding a helium/oxygen mixture (70:30
volumetric percentages) at a rate of 20 mL min 1 (STP) and increasing the temperature from ambient to 300 8C at 3 8C min 1 and
maintaining that temperature for 1 h. Glycerol was introduced at
30 mL h 1 by a syringe for liquids and then heated to 300 8C to vaporize it before its dilution in the preheated reaction gas feed. The
gas reaction feed consisted of 25 % oxygen and 8.6 % ammonia,
with helium accounting for the rest (volumetric percentages). The
gases were introduced by mass flow controllers, and the reaction
was run at 400 8C. The reaction products were analyzed with a gas
chromatograph after 30 min operation. The gas chromatograph
was equipped with two detectors, a flame ionization detector and
the other a thermal conductivity detector. The line was maintained
at 250 8C to avoid condensation of glycerol and other reaction
products.
Acknowledgements
This research was funded by the Spanish Ministry of Education
and Science (CTQ2005–02802/PPQ) and the ESF (COST Action
D36–006–06). M.O.G.-P. thanks the CSIC for an I3PDR-8–02 postdoctoral position.
Keywords: acrylonitrile · ammoxidation ·
heterogeneous catalysis · sustainable chemistry
glycerol
·
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Received: January 4, 2008
Revised: March 23, 2008
Published online on April 24, 2008
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