Efficient microwave-promoted acrylonitrile sustainable synthesis from glycerol

Article abstract of DOI:10.1039/b904689e

Solvent-free microwave-activation, in the liquid phase using an alumina supported V-Sb-O catalyst, affords highly efficient conversion (47%) of glycerol into acrylonitrile under mild conditions, short reaction times and in the absence of any solvent; in a

Full text of DOI:10.1039/b904689e

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Efficient microwave-promoted acrylonitrile sustainable synthesis from
glycerol
Vanesa Calvino-Casilda,*a M. Olga Guerrero-Pe´rezb and Miguel A. Ban˜aresa
Received 6th March 2009, Accepted 16th April 2009
First published as an Advance Article on the web 22nd April 2009
DOI: 10.1039/b904689e
Solvent-free microwave-activation, in the liquid phase using
an alumina supported V-Sb-O catalyst, affords highly effi-
cient conversion (47%) of glycerol into acrylonitrile under
mild conditions, short reaction times and in the absence
of any solvent; in addition, it increases selectivity (>80%)
compared to conventional thermal activation.
Commodity chemicals derived from fossil resources at present
would be producible in future biorefineries from renewable
resources, such as plant-derived sugar and other compounds.1
Glycerol is a main by-product in biodiesel production; sub-
sequently, its effective utilization will contribute to biodiesel
promotion.2–6
Several processes to transform glycerol into valuable chem-
icals have been described,7–11 but the reactivity of its three
Scheme 1 Possible products in the catalytic reaction of glycerol with
hydroxyl groups is quite similar, making it difficult to control
selectivity. In this sense, the use of an additional reactant may
narrow the product distribution. For example, the reaction of
glycerol with ammonia and oxygen can yield acrylonitrile.12
Acrylonitrile is nowadays produced using propylene as a starting
material; with this new process, propylene, a fossil raw material,
would be replaced by glycerol, a renewable source. In addition,
this process can be carried out under mild conditions. The
novel glycerol ammoxidation process has been run in the vapor
phase; liquid-phase ammoxidation of glycerol would be a more
desirable route to produce acrylonitrile, but its yield is too low.
We describe, for the first time, the microwave-assisted liquid-
phase reaction of glycerol with ammonia and hydrogen peroxide,
which is run in the absence of any solvent (Scheme 1).
The short reaction time, and unique reactive features of
microwave-assisted syntheses are ideally suited to an increasingly
demanding chemical industry affording high conversions with
peculiar selectivity. The use of catalysts in dry media (solvent-
free conditions) is highly interesting because it may combine
its catalytic effect with mild conditions and good selectivity.13
This method thus offers a practical alternative to conventional
heating in traditional catalysis.
ammonia and hydrogen peroxide under microwave activation.
50 min, then, g-Al2O3 was added. The resulting solution was
dried in a rotatory evaporator at 80 ◦C at 0.3 atm. The resulting
solid was dried at 115 ◦C for 24 h and then calcined at 400 ◦C for
4 h in air. For the second preparation method (Sb1V/Al2O3**
series), the same procedure was used but Sb was added as soluble
tartrate complex.15 The catalysts were prepared so that a total
coverage of V + Sb would correspond to their dispersion limit
loading on alumina, typically known as monolayer coverage.
The microwave equipment employed in this work is a Multi-
mode MicroSYNTH Labstation provided with magnetic stirring
and a PRO-6 Rotor reactor vessel. The temperature control
during the reaction was carried out by an IR sensor. Milestone
quartz vessels, with a capacity of 5 mL, were used to carry out
the microwave activated reaction. These vessels support high
pressure and are provided by a pressure relief spring in case
the increasing pressure due to the ammonia vapour overcomes
the spring pressure. Glycerol (0.5 mmol) and 200 mg of the
catalyst were blended in the quartz vessel and hydrogen peroxide
(15 mmol) and ammonia (57 mmol) were added. The mixture
was introduced in the microwave equipment and irradiated with
a power high enough (0–100 W) to reach 100 ◦C of temperature
with a heating ramp of 10 ◦C min-1. The reactor vessel was sealed
and held in the microwave for 60 min under continuous stirring
while temperature and power were registered by easyCONTROL
software. After cooling, the reaction products were extracted
and filtered. The reaction was followed using a HP5890 gas
chromatograph (GC) equipped with a 50 m long Ultra2–5%
phenyl methyl siloxane capillary column and a flame ionization
detector (FID).
The alumina-supported Sb-V oxide catalysts were prepared by
two different methods, depending on the antimony precursor;
as described elsewhere.14,15 In the first preparation method
(Sb1V/Al2O3* series),14 Sb2O3 was added to an aqueous solution
◦
of NH4VO3, this solution was kept under stirring at 80 C for
aCatalytic Spectroscopy Laboratory Instituto de Cata´lisis y
Petroleoqu´ımica (CSIC) Marie Curie, 2, E-28049, Madrid, Spain.
E-mail: vcalvino@icp.csic.es; Fax: +34 915 85 47 60
bDepartamento de Ingenier´ıa Qu´ımica Universidad de Ma´laga, E-29071,
Ma´laga, Spain
For comparative purposes, the reaction was also run under
conventional thermal activation. Similar to microwave-activated
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Table 1 Characterization results obtained for the catalysts
Wt.
(%Sb) (%V)
Wt.
BET area/ Phases determined by
Catalyst
m2 g-1
Raman spectroscopy
V/Al
Sb/Al*
Sb/Al**
Sb1V/Al* 10.1
Sb1V/Al** 9.4
—
31.4
29.1
8.4
—
—
4.2
3.8
130
122
105
118
139
VOx
a-Sb2O4
Amorphous SbOx species
VOx
VOx, VSbO4
Catalyst preparation: *Sb2O3 precursor/**Sb-tartrate complex precur-
sor. Wt % obtained by ICP.
runs, a mixture of glycerol (0.5 mmol), hydrogen peroxide
(15 mmol) and ammonia (57 mmol) was heated in an oil
batch reactor at 100 ◦C while stirring and in absence of any
solvent. After 5 min, the catalyst (200 mg) was added and
the reaction time started. The reaction was monitored by gas
chromatography, as described above.
Fig. 1 In situ Raman spectra of the catalysts.
The catalysts have been characterized by XRD, in situ
Raman spectroscopy (hydrated and dehydrated), and nitrogen
physisorption at -196 ◦C to determine the BET area values.
Table 1 summarizes the characterization data. 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
phase17 (Fig. 1). The absence of diffraction peaks indicates that
VSbO4 crystals are nanoscaled, typically smaller than 4 nm.
Vanadium-containing catalysts exhibit a Raman band near
1030 cm-1, which is sensitive to hydration and broad bands in the
800–900 cm-1 region, are characteristic of bridging oxygen vi-
brations of surface polymeric vanadia species.18 1Sb/Al* exhibit
an intense Raman band at 190 cm-1, characteristic of segregated
Sb2O3 crystallites.19 1Sb/Al** exhibit broad features at 780, 600,
468 and 210 cm-1, characteristic of surface amorphous antimony
oxide.19
under conventional thermal activation in the liquid phase
at 100 ◦C shows low performance and only Sb1V/Al2O3**
catalysts affords some glycerol conversion (13.9%) and 62.5%
selectivity to acrylonitrile after 20 h of reaction (Fig. ◦2), which
is much worse than its performance after 1 h at 100 C under
microwave activation (46.8% conversion and 83.8% selectivity).
The rest of catalysts are not active under conventional thermal
activation.
Blank microwave-activated◦reaction reaches ca. 38.2% glyc-
erol conversion in 1 h at 100 C, and produces acrolein as the
main reaction product; thus, under these reaction conditions the
dehydration of glycerol to yield acrolein can occur without any
catalyst. The presence of alumina moderately shifts selectivity
towards acrylonitrile and propanal. Antimony doping does not
have a strong effect on total conversion values, but affects
catalyst structure and selectivity. Nanoscaled a-Sb2O4 forms
if a suspension of Sb2O3 is used as an antimony precursor
(Sb/Al*); its performance is essentially like that of bare alumina.
Molecularly dissolved antimony tartrate complex precursor
(Sb/Al**) produces dispersed amorphous SbOx species on
Fig. 2 shows activity/selectivity values for glycerol conversion
under microwave and conventional thermal activations. Blank
runs under thermal activation yield negligible conversion after
1 h (not shown), and very little conversion after 20 h. Tests
Fig. 2 Glycerol conversion (%) and selectivity (%) to main products under conventional thermal activation (oil batch, 100 ◦C) for 20 h and under
microwave (MW) activation for 1 h (Power = 0–100 W, heating ramp = 10 ◦C min-1) Catalyst preparation: * Sb2O3 precursor/**Sb-tartrate complex
precursor.
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alumina and shift product distribution towards 1,2-propanediol.
The presence of vanadia on alumina (V/Al) slightly promotes
acrylonitrile. Neither of the single-oxide doped aluminas become
significantly more efficient for acrylonitrile formation, since
these samples (V/Al, Sb/Al** and Sb/Al*) are essentially
selective to oxygenates. The addition of antimony modulates
the selectivity of alumina-supported vanadium oxide catalysts,
which is consistent with the trends for vapor-phase conversion
of glycerol to acrylonitrile under thermal activation.12 The
antimony precursor (Sb2O3 suspension vs. molecularly dissolved
antimony tartrate complex) results in significantly different
structures and catalytic behavior for the V-Sb-oxide system.
Sb1V/Al* catalyst, in which rutile VSbO4 species have not
been detected (Table 1), produces acrylonitrile and acrolein with
similar selectivity values at a rather low glycerol conversion,
lower than that afforded by V/Al. Rutile VSbO4 forms on
alumina when antimony is added as a dissolved tartrate complex,
(Table 1); its performance for glycerol conversion to acrylonitrile
is dramatically increased, reaching more than 80% selectivity
to acrylonitrile at 47% conversion. This is consistent with
the relevance of the rutile VSbO4 phase for ammoxidation
of propane to acrylonitrile,16 and suggests that propane am-
moxidation and glycerol conversion to acrylonitrile exhibit
similar mechanistic reaction steps. Acrolein is the main reaction
product obtained under microwave activation in the absence of
a catalyst. Acrolein appears to be a critical intermediate in the
ammoxidation of C3 hydrocarbons (propane and propylene) to
produce acrylonitrile.16 Thus, the microwave activation would
transform glycerol into acrolein and the rutile VSbO4 phase
would be efficient to form carbon–nitrogen bonds transforming
acrolein into acrylonitrile with a very high selectivity. This
reactive system is significantly more efficient than conventional
gas-phase thermal activation, which demands much higher
temperatures (400 ◦C) to afford 58.3% selectivity to acrylonitrile
at 82.6% glycerol conversion.12
This microwave-assisted process is a novel cost-effective and
solvent-free route to produce acrylonitrile.
Acknowledgements
This research was funded by Spanish Ministry of Education
and Science (CTQ2008–04261/PPQ) and ESF COST Action
D36–006-06. The authors thank also Prof. Mart
´ın-Aranda from
Department of Inorganic Chemistry and Technical Chemistry
(UNED, Madrid) for her help with the Microwave equipment.
Alumina was provided by Girdler-Su¨d-Chemie.
Notes and references
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In summary, microwave activation dramatically enhances se-
lectivity and activity values to acrylonitrile drastically reducing
reaction time. These results underline the advantage of using
microwave activation for a selective and energetically efficient
valorization of glycerol and the relevance of the rutile VSbO4
phase to transform glycerol into acrylonitrile. These results
present a selective new process for valorization of glycerol—a
renewable raw material—to acrylonitrile under mild conditions.
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The Royal Society of Chemistry 2009
Green Chem., 2009, 11, 939–941 | 941
©