Exploiting H-transfer as a tool for the catalytic reduction of bio-based building blocks: The gas-phase production of 2-methylfurfural using a FeVO4 catalyst

Article abstract of DOI:10.1039/c7gc01749a

Over the past decade, a great deal of effort has been devoted to developing reductive processes in the field of biomass valorisation for the sustainable production of bio-fuel additives and chemicals. Catalytic transfer hydrogenation, which uses alcohol as the hydrogen source, is an interesting approach that avoids the use of both high H₂ pressure and precious metal catalysts. In this work, the vapour-phase production of 2-methylfuran from biomass-derived furfural (FU), using methanol as the H-transfer agent and FeVO₄ catalyst, was studied. At a temperature of 320°C it was possible to achieve 80% yield of 2-methylfuran, with small amounts of 2,5-dimethylfuran and 2-vinylfuran as by-products. Catalyst characterization highlighted that FeVO₄ reduction took place under the studied conditions, leading to the in situ development of a true active phase. The study of the reaction network permitted us to infer on the relative contribution of H-transfer and hydrogenation, the latter from the in situ generated formaldehyde and H₂, to 2-methylfuran, formation. The reported results indicate the potential application of H-transfer with FeVO₄ catalysts as an efficient process for the selective de-oxygenation of biomass-derived molecules.

Full text of DOI:10.1039/c7gc01749a

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ARTICLE
Exploiting H-transfer as a tool for the catalytic reduction of bio-
based building blocks: the gas-phase production of 2-
methylfurfural using a FeVO4 catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
L. Graziaa,b, D. Bonincontroa, A. Lollia,b, T. Tabanellia, C. Lucarellib,c, S. Albonettia,b,* and F. Cavania,b,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Over the past decade, great efforts have been devoted in the field of biomass valorisation to the development of reductive
processes for the sustainable production of bio-fuel additives and chemicals. Catalytic transfer hydrogenation, which uses
alcohols as the hydrogen source, offers an interesting approach that avoids the use of both high H2 pressure and precious
metal catalysts. In this work, the vapour-phase production of 2-methylfuran from biomass-derived furfural (FU), using
methanol as the H-transfer agent and FeVO4 catalyst, was studied. At a temperature of 320°C it was possible to achieve
80% yield to 2-methylfuran, with small amounts of 2,5-dimethylfuran and 2-vinylfuran as by-products. Catalyst
characterization highlighted that FeVO4 reduction took place under the studied conditions, leading to the in-situ
development of the true active phase. The study of the reaction network permitted us to infer on the relative contribution
of H-transfer and hydrogenation, the latter from the in-situ generated formaldehyde and H2, to 2-methylfuran formation.
The reported results indicate the potential application of H-transfer with FeVO4 catalysts as an efficient process for the
selective de-oxygenation of biomass-derived molecules.
1. Introduction
superior properties as compared to bio-ethanol because of its
In recent decades the continuing growing global demand for higher energy density and octane values 13. The octane
petrochemical products and fossil fuels, with the associated number of MF is higher than that of gasoline (103 vs. 97 RON),
increase in CO2 emission, has forced researchers to seek some while their energy densities are very close (28.5 MJ L−1 vs. 31.9
renewable alternative feedstock for the production of MJ L−1), which means that with the same volume of fuel, MF
chemicals and fuels 1,2. In this field, the conversion of non-food contains 34% more energy than the market-leading biofuel
lignocellulosic second-generation biomasses such as wood ethanol 14. For the synthesis of MF from FU, the selective
chips and agricultural and municipal wastes into chemicals and hydrodeoxygenation of the formyl group is necessary, avoiding
fuels has gained increasingly more importance. Indeed, the opening or hydrogenation of the furan ring. Indeed, one of
lignocellulose can be converted into different chemicals and the key challenges for upgrading FU is product selectivity, since
liquid fuels via biological and chemical pathways 3. Furan the hydrogenation of FU often results in a mixture of side-
derivatives play an important role in the transformation of this chain and ring-hydrogenated products along with ring-opening
renewable feedstock 4-7. For instance, furfural (FU), which is products. Nickel-, copper-, and noble metal-based catalysts
available on an industrial scale via the hydrolysis–dehydration have been reported to be active in the conversion of FU to MF
of the hemicellulose part of lignocellulosic biomass 8,9, is a key 15,16. Zheng et al. reported an 87% yield to MF with a catalyst
precursor for the synthesis of derivatives with applications in of composition Cu/Zn/Al/Ca/Na=59:33:6:1:117.
A
SiO2-
the fuel and polymer industries 10. The FU upgrade processes supported Cu/Fe bimetallic catalyst at 1 bar H2 is reported to
involve the selective hydrogenation of the carbonyl group into produce a 98% yield to MF 18. On the other hand, a
the corresponding unsaturated furfuryl alcohol (FAL), which comparison of silica-supported monometallic Ni and bimetallic
finds application in industry for the production of resins and Ni-Fe catalysts for the conversion of FU at 1 bar of H2 and 210-
fine chemicals 11,12, and into 2-methylfuran (MF) (Scheme 1), 250°C showed that FAL is the primary reduction product
used as a renewable fuel. Indeed, this furan compound has obtained in high yield through the monometallic system. The
introduction of Fe leads to an increased MF yield; indeed, Fe
suppresses the decarbonylation activity of Ni and promotes
a.“Toso Montanari” Department of Industrial Chemistry, University of Bologna,
the hydrogenation-hydrogenolysis of the FU carbonyl group 19
.
Vale Risorgimento 4, 40136 Bologna, Italy). Email: stefania.albonetti@unibo.it,
fabrizio.cavani@unibo.it
OH
O
OH
O
O
O
H2
H2
+
H
2O
b.Consorzio INSTM - Via G. Giusti, 9 - 50121 Florence, Italy.
c. Department of Science and High Technology, University of Insubria, Via Valleggio
11, 22100, Como, Italy.
O
-
3H2
OH
CAT
CAT
O
OH
OH
Furfural
(FU)
Furfuryl alcohol
(FAL)
2-methylfuran
(MF)
Xylose
Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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J. Name., 2013, 00, 1-3 | 1
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is ViewthAerticle Ornelinael
(hydroxyl)alkylating agent. Furthermore, Andersson and co-
workers reported that FeVO4 is very active and selective in
Scheme 1. Production of 2-methylfuran (MF) from xylose and furfural quantity
(FU).
of
formaldehyde,
which
DOI: 10.1039/C7GC01749A
An alternative approach for the reduction of the FU carbonyl methanol oxidation to formaldehyde 32
.
group to produce FAL and MF, bypassing the need for high Herein, we report on the vapour-phase reduction of FU to MF.
hydrogen pressure and noble metal-based catalysts, is the The reaction was carried out in a continuous reactor by means
Meerwein–Ponndorf–Verley (MPV) reaction, in which an of H-transfer from methanol to FU and FeVO4 as the catalyst.
alcohol is used as the hydrogen source. Indeed, under Only 2,5-dimethylfuran and 2-vynilfuran were formed as by-
appropriate conditions, the reaction can be highly chemo- products, and catalyst pre-treatment under reducing
selective towards carbonyl groups. 20-22
conditions was found to be fundamental for increasing MF
Catalysts studied for this reaction may be divided into three selectivity, while limiting heavy compounds production.
main groups: Lewis basic, Lewis acid, and metal-supported Moreover, the study of the reaction network was conducted
catalysts23-25
.
by feeding reaction intermediates and various H-transfer
Di Cosimo and co-workers reported on the use of MgO as molecules and molecular hydrogen, as well.
selective catalyst for the conversion of a wide range of
unsaturated ketones to the corresponding alcohols using 2. Experimental
isopropanol as the hydrogen source 26
.
Hermans et al. have reported that Cu, Ni and Pd supported on 2.1. Catalyst preparation
Fe2O3 are active in the catalytic transfer hydrogenation of FU
A FeVO4 catalyst was prepared by co-precipitation from an
to produce a mixture of FAL, MF and 2-methyltetahydrofuran
(MTHF) both in batch and continuous flow reactor, using 2-
propanol as the hydrogen donor. They demonstrated that the
high activity of Pd/Fe2O3 is due to strong metal-support
aqueous solution containing the corresponding metal
precursors, to obtain an atomic ratio Fe3+/V5+ equal to 1. For
the synthesis of 20 g FeVO4, a solution containing 47.32 g of
Fe(NO3)3*9H2O (Sigma Aldrich, 99% purity) in 105 ml of
distilled water was prepared. A second solution containing
13.70 g NH4VO3 (Sigma Aldrich, 99% purity) in 105 ml of
distilled water was prepared and added dropwise under
vigorous stirring to the Fe solution. The pH of the resulting
solution was adjusted to the value of 6.8 with an ammonia
solution (14%) to precipitate the FeVO4 precursor. After 1 h of
aging under stirring, the resulting precipitate was filtered and
washed with an excess of water and dried overnight at 110°C.
The dried solid was ground and calcined at 650°C in static air
for 3 h.
interaction 27
.
Additionally, Vlachos and co-workers reported on the vapour-
phase hydrodeoxygenation of FU to MF with 50-60% selectivity
using MoC as the catalyst 28. They also described a liquid-phase
catalytic transfer hydrogenation of FU over Ru/C and Ru/RuO
catalysts, using various alcohols as hydrogen sources. 29
Nevertheless, there are only a few studies in literature
concerning the vapour-phase production of MF from FU with
heterogeneous catalysts, and most of them require noble
metal-supported catalysts, together with high hydrogen
pressure and reaction temperature. Therefore, the
development of a continuous catalytic process based on non-
noble metal catalysts for the hydrodeoxygenation of FU to MF
is a very attractive topic. Moreover, the use of H-transfer
2.2. Catalyst characterization
BET specific surface area. The BET surface area of catalysts
instead of classical hydrogenation could make it possible to was determined by means of N2 absorption–desorption at
use bio-alcohols as hydrogen sources, thus increasing the liquid N2 temperature, using a Sorpty 1750 Fison instrument.
sustainability of the entire process.
In this specific field, we recently reported on the use of sample was outgassed at 150°C before N2 absorption.
methanol as a clean and efficient H-transfer reactant for
X-ray diffraction analyses (XRD). XRD powder patterns of
0.3 g of the sample was used for the measurement, and the
carbonyl reduction in the liquid phase 30. Compared to other the catalysts were recorded with Ni-filtered Cu Kα radiation (λ
molecules used in H-transfer, methanol showed the advantage = 1.54178 Å) on a Philips X'Pert vertical diffractometer
of producing gaseous components as the only co-products. equipped with a pulse height analyser and a secondary curved
Using high-surface area MgO as a simple, easily recoverable graphite-crystal monochromator.
and reusable catalyst for FU reduction, a 100% FAL yield was
obtained.
Thermogravimetric/differential thermal analyses (TGA/
DTA). TGA/DTA analyses of fresh and spent catalysts were
As an alternative, mixed iron-vanadium mixed oxides could be carried out using a SDT Q 600 instrument, to identify the
used for the catalytic transfer hydrogenation on FU. Indeed, amount of heavy compounds absorbed on the catalyst surface.
these materials were reported to be able to activate methanol 5-10 mg of sample were typically used, from room
in several reactions, for potential industrial applications. For temperature up to 700°C, with a heating rate of 10°C min-1 in
example, Asahi Company reported that FeVO4 is a very active air.
and stable catalyst for the gas-phase methylation of phenol to
Raman spectroscopy. Laser Raman spectra were recorded
o-cresol using methanol as the alkylating agent 31. It was at room temperature using a Renishaw 1000 spectrometer
demonstrated that the strong de-hydrogenating properties of equipped with a Leica DLML microscope (5x, 20x, 50x lenses
the catalyst make possible the in-situ formation of a high were used) and a CCD detector. Samples were excited with a
2 | J. Name., 2012, 00, 1-3
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diode laser beam (782 nm or 514 nm). Raman spectra were mesh column. FU Conversion, product selectivity, and C loss
View Article Online
recorded in the spectral window 500-2000 cm-1.
were expressed as follows:
DOI: 10.1039/C7GC01749A
ꢔꢎꢕ
ꢍꢎꢏꢍꢎꢏꢐꢑ
ꢈꢒꢓ
̇
ꢍꢎꢏꢍꢎꢏꢐꢑ − ꢈ̇
Atomic absorption. The elemental analysis was carried out
with a VARIAN SpectrAA 100 (equipped with a graphite tube
atomizer); the solutions resulting from the mineralization
process performed with hot aqua regia on both fresh and
spent catalysts were analysed to determine the real Fe/V ratio.
In-situ DRIFTS (Diffuse Reflectance Infrared Fourier
ꢀꢁꢂꢃꢁꢂꢄꢅ ꢆꢇꢈꢉꢊꢂꢋꢌꢇꢈ =
ꢒꢓ
ꢍꢎꢏꢍꢎꢏꢐꢑ
ꢈ̇
ꢔꢎꢕ
ꢈ̇
ꢜꢏꢔꢝꢎꢞꢕ
ꢔꢎꢕ
ꢍꢎꢏꢍꢎꢏꢐꢑ ꢍꢎꢏꢍꢎꢏꢐꢑ
ꢖꢂꢇꢗꢁꢘꢙꢋ ꢚꢊꢅꢊꢘꢙꢌꢉꢌꢙꢛ =
ꢒꢓ
ꢈ̇
− ꢈ̇
ꢆ − ꢟꢇꢋꢋ = 100 − � ꢖꢂꢇꢗꢁꢘꢙ ꢚꢊꢅꢊꢘꢙꢌꢉꢌꢙꢛ
ꢒ
Transform
Spectroscopy)
experiment
with
mass
Preliminary tests were conducted, making it possible to
exclude any problems originating from interparticle and
intraparticle diffusion limitations. More specifically, some
catalytic tests were conducted by changing the catalyst
particle size for the same catalyst weight/inlet flow ratio; no
effect on catalyst performance was observed. The catalytic
tests performed while keeping the contact time constant but
doubling or halving the total volumetric flow and catalyst
volume, compared to the conditions typically used, showed no
performance changes.
spectrometer. The IR apparatus used was a Bruker Vertex 70
with a Pike DiffusIR cell attachment. Spectra were recorded
using a MCT detector after 128 scans and 2 cm-1 resolution.
The mass spectrometer used was an EcoSys-P from European
Spectrometry Systems. In most cases the catalyst was loaded
into the sample holder and the cell was closed and inserted
into the DRIFT apparatus. A pretreatment up to 320-450°C in
helium flow was then performed to remove any molecules
adsorbed on the material, mainly carbon dioxide and water.
The sample was then cooled down to 85°C and the spectra of
the pure catalyst was recorded and used as a background for
the following measurement. A first set of experiments was
performed at 85°C by feeding methanol in a helium flow and
vaporising it using heating strips. Then methanol was stopped
and only helium was flowed inside the IR cell. This way, the
low-temperature adsorption and desorption process was
monitored. The second set of experiments was performed
3. Results and Discussion
3.1. Bulk catalyst features
The catalyst was characterised via different techniques such as
BET surface area, atomic absorption, X-ray diffraction, and
Raman spectroscopy. After calcination at 650°C, the catalyst
showed a specific surface area of 12 m2g-1, a value in
with
a low-temperature adsorption and a programmed
temperature desorption during which the following
temperatures were monitored: 125°C, 175°C, 225°C, 275°C,
and 320°C.
agreement with that reported by other authors 33,34
.
XRD patterns of the dried precursor and the calcined material
are compared in Figure S1. The dried sample showed an
amorphous structure: no reflection was registered, while the
calcined catalyst presented a well-defined FeVO4 triclinic
structure with traces of segregated iron oxide phase
(hematite).
The Raman analysis confirmed these results. The spectrum
reported in Figure 1 can be divided into four spectral regions
which agree well with those reported in literature for
crystalline FeVO4 33: (i) at Raman shift 1050-880 cm-1, there are
bands attributable to terminal V=O bond stretching; (ii) at 880-
700 cm-1, to V—O--Fe bond stretching; (iii) at 700-550 cm-1 to
V—O--Fe and V--O--Fe stretching; and (iv) at < 550 cm-1, to the
deformation of V—O—V bonds and Fe—O stretching. In
addition to these bands, at 1300 cm-1 a weak broad band can
be attributed to α-Fe2O3 35, indicating the possible segregation
of a small quantity of hematite.
2.3. Vapor-Phase catalytic tests
Catalytic tests were conducted in a continuous-flow fixed-bed
micro-reactor (Pyrex, length 38 cm, internal diameter ⅓ inch).
The catalyst (30-60 mesh particles) was placed into the reactor
in a quantity appropriate for changing the contact time from
0.1 to 1.0 s, and then heated to the desired reaction
temperature (250-400°C) under N2 flow (54 ml min−1). The
catalytic reaction was started by the vaporisation of methanol
(WWR Chemicals) and FU (Sigma-Aldrich) in a 10/1 molar ratio
using nitrogen as the carrier gas (54 ml min−1). FU was purified
via distillation prior to being fed into the flowing gas stream.
The total volumetric inlet flow rate was held constant at 60 ml
min−1 and the molar concentrations of FU, methanol, and
nitrogen were respectively 1, 10, and 89%. In all cases, results
were taken after 1 h reaction time. Analyses of reactants and
products were carried out as follows: the outlet stream was
scrubbed for 1 h in cold acetonitrile, which was maintained
at -26°C by a F32 Julabo thermostat. The condensed products
were analysed by means of HPLC, using an Agilent
Technologies 1260 Infinity instrument equipped with a DAD
UV-Vis detector and an Agilent POROshell 120 C-18 column.
Non-condensable gases (CO, CO2, CH4 and H2) were analysed
on-line with a PerkinElmer Clarus 500 gas chromatograph
equipped with a TCD detector and a Carbosphere® 80/100
The elemental analysis indicated the presence of a Fe/V molar
ratio equal to 1.09±0.02 in fresh catalyst. This value is slightly
higher than the theoretical one, thus highlighting the possible
loss of some vanadium during catalyst synthesis. Indeed, the
analysis of the solution obtained after filtration and washing of
the catalyst showed the presence of very low amount of
vanadium thus justifying the slight excess of iron in the solid.
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to the presence of both methoxy and molecular physi-
View Article Online
DOI: 10.1039/C7GC01749A
adsorbed methanol were identified. For prolonged adsorption-
desorption times, the appearance of broad bands at 1670-
38
1500 cm-1, 1350-1300 cm-1 and 1713-1772 cm-1
could be
attributed to C-O vibration of molecular formaldehyde,
providing evidence for the formation of chemi-adsorbed
formate and formaldehyde species, thus confirming the ability
of the catalyst to activate and dehydrogenate methanol at this
temperature. Furthermore, the intensity of the IR bands
remained unchanged during desorption, indicating that the
interaction between activated methanol and catalyst was
strong.
We first studied the influence of the reaction temperature. In
these catalytic tests, MF was the main product detected, with
small amounts of 2,5-dimethylfuran (DMF) and 2-vinylfuran
(VINFU).
Since the product selectivity changed significantly during the
first hour of reaction (Figures S3_A and S3_B), catalyst
performances were compared at the second hour. Figure 3
shows FU conversion and product selectivity based on
temperature (range 250-400°C).
Figure 1. Raman spectrum of calcined FeVO4.
(a)
(b)
At a low temperature (250°C), the hydrogenation of FU to MF
occurred with very low efficiency. Indeed, the conversion of FU
was lower than 50% and the overall selectivity to the identified
products was around 25% (≈ 20% selectivity to MF). The low
selectivity to MF and the high C loss indicate the poor ability of
the catalyst to activate methanol for temperatures lower than
300°C. Indeed, the C loss could be ascribed to the formation of
heavy carbonaceous compounds deriving from the
degradation of FU.
When the temperature was increased up to 300-350°C, the
efficiency of the H-transfer increased significantly. In this
temperature range, we registered a notable increase in FU
conversion and a remarkable enhancement of the overall
selectivity to reduction products to up to approximately 75%
(≈ 65% selectivity to MF). The formation of heavy compounds,
due to FU degradation, was less than 20-30%.
Ads 10 min
Ads 20 min
Des 10 min
Des 20 min
Des 30 min
Ads 30 min
Ads 40 min
Ads 50 min
Des 40 min
Des 50 min
Des 60 min
Des 70 min
Des 80 min
Ads 60 min
2000
1500
1000
Wavenumber cm-1
2000
1500
1000
Figure 2. DRIFT spectra recorded after methanol adsorption
and desorption at the temperature of 320°C. During adsorption
methanol was continuously fed over the catalyst while, during
desorption, only He was fluxed. Legend: (a) DRIFT spectra
registered during methanol adsorption, (b) DRIFT spectra
registered during methanol desorption.
100
80
60
40
20
0
100
80
60
40
20
0
3.2. Hydrodeoxygenation of FU with FeVO4 catalyst: effect
of reaction temperature
The gas phase hydrodeoxygenation of FU was carried out using
methanol as the hydrogen source.
The strong interaction between methanol and FeVO4 was
demonstrated by means of in-situ DRIFTS (Diffuse Reflectance
Infrared Fourier Transform Spectroscopy) experiments.
Indeed, by feeding continuously the alcohol during the
adsorption at low temperature (Figure S2), we demonstrated
that methanol was mainly physi-adsorbed as methoxy species
(IR bands at 2930 and 2828 cm-1) 36,37. The same behaviour was
also confirmed at high temperature, but for very short time
only (Figure 2); during adsorption, the main IR bands relating
250
300
320
350
400
Temperature (°C)
Figure 3. Effect of reaction temperatures on FU conversion and
product selectivity in the second hour of reaction, catalyst
FeVO4. Feed composition: FU 1%, CH3OH 10%, N2 89%;
Pressure 1 atm, overall gas residence time 1.0 s. Symbols:
conversion, MF selectivity, DMF selectivity, VINFU
selectivity, C loss.
♦ FU
■
■
■
■
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The presence of the oligomeric compounds adsorbed on the With the aim of studying the effect of reaction conditions on
View Article Online
DOI: 10.1039/C7GC01749A
used catalyst surface was confirmed by Raman spectroscopy the amount of carbonaceous deposits, TGAs were carried out
(Figure S4), with the characteristic D3 band at 1600cm-1, which on the spent catalysts (Figure 4).
39
is ascribable to the presence of amorphous carbon species
.
Fresh
100
95
No products of ring hydrogenation or decarbonylation were
formed.
Used at 400°C
Used at 320°C
The ability of FeVO4 to activate methanol for temperatures
higher than 300°C was confirmed by the results in Figure S5,
which shows the number of moles of light compounds formed,
based on time, at 250°C and 320°C. The negligible amount of
CO, CO2, CH4, and H2 produced at 250°C agrees with the results
in Figure 3. As a matter of fact, in addition to the main reaction
involving the H-transfer to MF, the process was accompanied
by the decomposition of formaldehyde, which is produced by
methanol dehydrogenation, into light gaseous compounds, CO,
CO2, CH4, and H2. In a previous work, we demonstrated that in
similar reaction conditions formaldehyde was decomposed
following the formal set of reactions reported in Scheme 2
90
Used at 250°C
85
0
100
200
300
400
500
600
700
Temperature (°C)
Figure 4. Thermogravimetric analysis (TGA) in nitrogen: (
fresh FeVO4 calcined at 650°C; FeVO4 used in catalytic test (
―
―
)
)
at 250°C, (―) at 320°C and (―) at 400°C.
40,41
.
Thermograms were obtained by heating up the samples in a
nitrogen flow, instead of air, in order to avoid the re-oxidation
of the catalyst that would invalidate the results. Thus, it is
necessary to take into account that some carbonaceous
compounds could remain on the catalyst after TGA due to the
anaerobic condition utilized (see the section concerning the
effect of reaction time on catalyst structure).
CH2O → CO + H2
(1)
(2)
(3)
(4)
(5)
(6)
(7)
CO + H2O → CO2 + H2
2 CH2O → CH3O-C(O)H
CH3OC(O)H → CO2 + CH4
HCOOH → CO2 + H2
HCOOH → CO + H2O
CH3OH + H2O → CO2 + 3H2
As expected, the fresh catalyst calcined at 650°C (black line)
did not show any weight loss. Conversely, all the used catalysts
Scheme 2. Summary of main reactions involving methanol in
the catalytic H-transfer of FU 40,41
showed
a weight loss that could be related to the
.
desorption/decomposition of the amorphous carbon
deposited during reactivity tests; however, the weight loss was
a function of the temperature used for FU reduction. In fact,
after heating up to 150-170°C, samples showed a common and
marginal initial weight loss (2-3 wt %) which is related to the
desorption of some physisorbed water. However, upon
increasing the temperature, an evident weight loss (≈ 10 wt
%), in the range 200-450°C, was observed for the sample used
at 250°C; a further and limited weight loss (≈ 3-4 wt %) was
then registered while heating up the sample to 700°C. This
latter weight loss could be related to the removal of the
heavier oligomeric carbonaceous species, whereas that
registered at lower temperatures could be connected to the
desorption of lower molecular mass deposits. It may be
hypothesized that the catalyst oxidises carbonaceous residua,
while being itself reduced because of anaerobic conditions.
It is also shown that, in the case of the sample used at 250°C,
the formation of the lighter carbonaceous deposits was largely
improved. The sample used at 320°C showed a lower weight
loss (≈5-6 wt %) in the range 200-450°C, and a similar weight
loss in the high temperature region. These results can be
correlated to the trend of C loss observed in function of
reaction temperature (Figure 3). Indeed, the formation of
heavy carbonaceous deposits (oligomeric species deriving from
the condensation of several furanic species) could represent
the main contribution to C loss.
The formaldehyde generated as the co-product of H-transfer
decomposes, leading to the formation of CO and H2 (Reaction
1). Alternatively, two adsorbed CH2O molecules may
disproportionate to formate and a methoxy species, yielding
methylformate (reaction 3) (the latter can also be formed by
Tishchenko dimerization), which decomposes at high
temperatures to CH4 and CO2 (reaction 4). Formic acid may
also form through the oxidation of formaldehyde by Fe3+, and
decompose to CO2 and H2 (reaction 5) or CO and H2O (reaction
6). Moreover, water gas shift (WGS) (reaction 2) or methanol
reforming (reaction 7) cannot be disregarded.
The high amount of CH4 produced, higher than that of CO2,
suggests the occurrence of a disproportionation involving two
methanol molecules to yield an equimolar amount of
methane, formaldehyde, and water. Ueda et al. have recently
reported similar results over vanadium and molybdenum
oxides, using various alcohols, including methanol and ethanol,
to produce the corresponding alkanes and aldehydes in
equimolar ratio 42,43
.
Lastly, when FU was reduced with methanol at 400°C, the total
conversion of the substrate was registered. The main product
detected was MF with a selectivity of 62%. However, a lower
carbon balance was registered due to the degradation of FU to
carbonaceous compounds.
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Journal Name
Lastly, it should be pointed out that increasing the reaction In order to gain better insight into this hypothesis, we
View Article Online
temperature from 250°C to 400°C significantly increased the performed a catalytic test in which onlyDmOIe: t10h.a10n3o9l/Cw7aGsC0fe17d49aAt
temperature at which carbonaceous deposits were eliminated. 320°C for 1h. Figure S6 shows the number of moles of light
This could be connected to the formation of compounds with compounds formed, based on the time; methanol and furfural
more ordered structures.
were then fed under usual conditions.
These results confirm that the catalyst has the best The trend of light compounds, obtained by feeding methanol
performance in the temperature range between 300 and alone, was very similar to that registered in the test performed
350°C. In particular, at 320°C a conversion higher than 95% by co-feeding methanol and FU (Figure S5). Therefore, during
was registered, with a selectivity to MF of around 70%; the first hour of reaction, methanol preferentially reacts with
moreover, a fairly good C balance was obtained (C loss 25%).
the catalyst, thus decreasing the amount of methanol available
as the H source for the reduction of FU. In these conditions, FU
3.3. Hydro-deoxygenation of FU with FeVO4 catalyst: effect was mainly degraded, leading to the high C loss registered
of reaction time
(Figure 5).
In order to further validate this hypothesis and evidence
modifications to FeVO4 occurring during the first hours of
reaction, we characterised samples subjected to different
treatments by means of XRD and elemental analysis. The
analysis of the catalysts used, either during FU reduction or
with methanol alone, showed a very similar Fe/V atomic ratio
(1.08), a value very close to that of the fresh sample (1.09),
thus ruling out any loss of vanadium or iron during the first
hour of reaction.
Furthermore, a test carried out by feeding methanol only over
a fresh FeVO4 catalyst was performed in order to evaluate the
reduction degree of the sample; the latter was calculated by
taking into account reaction stoichiometries and yields to all
products. In fact, the O content in the outlet stream was
higher than that contained in the inlet methanol stream, an
evidence that confirmed the release of O2- from the catalyst
because of metal ions reduction. The experimental reduction
degree estimated with this method was 30% (Figure S7 A and
B). The patterns of the two used samples, after reaction with
either methanol alone or with the FU/methanol mixture
(Figure 6), were very similar and, at the same time, different
from that one of the fresh catalyst (Figure S1).
The analysis of the diffraction pattern confirms the formation
of a pure spinel structure. However, the only two known
spinels containing iron and vanadium are Fe2VO4 and FeV2O4
(so called coulsonites), and neither of them completely match
with the pattern of our spent catalyst; in coulsonites, iron can
be present either as Fe3+ or Fe2+, and vanadium as V3+ or V2+.
Overall, the analysis of the used catalyst confirmed the same
Fe/V ratio as for the fresh one, and showed the formation of a
single spinel structure with a reduction extent of 30%. All this
The effect of reaction time on catalyst performance was
examined by conducting tests for 6 h at the optimised reaction
temperature (320°C) (Figure 5). FU conversion slightly
decreased during the first 3 hours of reaction; then, starting
from the 4th hour, a rapid FU conversion decline was observed
and a final value of 55% was reached. This drop in conversion
may be correlated to the previously discussed accumulation of
C residues (Figure 4 and S4).
With regard to the products formed, a notable difference in
selectivity was observed between the first and following hours
of reaction, thus highlighting a different catalyst behavior over
time. Indeed, in the first hour of reaction, a C loss of 63% and a
selectivity to MF of 32% only were registered; starting from
the second hour, the C loss dropped to 25%, with a significant
increase in MF (66%), DMF and VINFU yields.
A further evidence of the change in catalytic behaviour
occurring during the first hour can be inferred from Figure S5,
showing the light gaseous compound amounts. CO and CO2
exhibited an initial rapid increase up to a maximum value at 15
and 30 minutes, respectively. After this maximum, their
concentration rapidly decreased. The high amounts of CO and
CO2 initially produced are probably due to the oxidation of
methanol by the catalyst.
100
80
60
40
20
0
100
80
60
40
20
0
led us to conclude that the composition of the spinel in the
III
used catalyst was FeIIFe0,5
V
IIIO4. Nevertheless, further in-
1,5
situ studies are currently being carried out in order to confirm
this hypothesis.
●
*
1
2
3
4
5
6
●
Time on stream (h)
●
●
*
●
●
*
●
*
*
*
●
*
*
●
● ●
*
Treated in CH3OH at 320°C
Figure 5. Effect of reaction time on FU conversion and product
selectivity for FeVO4 catalyst. Feed composition: FU 1%, CH3OH
10%, N2 89%; Pressure 1 atm, 320°C, overall gas residence time
Used in reaction at 320°C
70
10
20
30
40
50
60
Position [°2Theta] (Copper (Cu))
1.0 s. Legend:
♦
FU conversion,
■
MF selectivity, ■ DMF
Figure 6. XRD patterns of reduced FeVO4 samples. Reference
patterns: (●) Fe2VO4, (*) FeV2O4.
selectivity, VINFU selectivity,
■
■
C loss.
6 | J. Name., 2012, 00, 1-3
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contact time was set at 1 s. At the same time aVlieowwAertricleCOlnolisnes
was observed, from 63% down to 18%. DDOMI:F10a.1n0d39V/CIN7GFCU01w74e9rAe
produced in low amounts (≈ 5%) while FAL was no longer
formed at contact times higher than 0.01 s, due to its
consecutive transformation.
3.4. Effect of catalyst pre-reduction
Since catalyst characterisation demonstrated that FeVO4 was
reduced by methanol during the first hour of reaction, the
effect of catalyst pre-reduction was studied in order to check
whether it can affect catalytic properties. In particular, catalyst
pre-reduction was obtained by feeding methanol alone for 1 h
at 320°C; then FU and methanol were fed again and activity
100
80
60
40
20
0
2,5
2
was monitored as usual for
6 h. Results (Figure 7)
1,5
1
demonstrated that catalyst pre-reduction improved the
performance. In fact, it was not just a higher MF selectivity
compared to the untreated catalyst that was registered during
the first hour of reaction, but also a greater stability with lower
deactivation extent during the 6 h reaction time.
Accordingly, a notably lower C loss was observed and a final
value of 80% conversion was registered after 6 h. With the
fresh catalyst, final conversion was just 55%. The improved
catalyst stability was due to the lower amount of heavy
carbonaceous compounds formed with the pre-reduced
sample during the initial period, the latter being due, in turn,
to the catalyst’s higher efficiency in methanol activation for H-
transfer.
0,5
0
0
0,2
0,4
0,6
0,8
1
Contact time (s)
Figure 8. Effect of contact time on FU conversion and product
selectivity for FeVO4 catalyst. Feed composition: FU 1%, CH3OH
10%, N2 89%; Pressure 1 atm, Temperature 320°C, overall gas
residence time 1.0 s. Legend:
MF selectivity, ● DMF selectivity, ● VINFU selectivity, ● C
loss.
♦ FU conversion, ■ FAL selectivity
▲
100
80
60
40
20
0
Some catalytic tests were conducted using different substrates
(FU, FAL, MF, and DMF) as starting reagents, at 320°C, with
both pre-reduced and fresh FeVO4 (Table 2).
Entry 1 is the result obtained in the standard catalytic test
after 1 h reaction time, using the pre-reduced catalyst. This
can be compared with the result obtained by feeding FAL with
methanol (entry 2). Also with FAL, the total conversion of the
substrate was registered; MF and DMF were produced with
60% and 4% selectivity, respectively: values very similar to
those obtained by feeding FU. This further supports the
hypothesis that in FU reduction to MF, FAL is the reaction
intermediate.
FU and FAL were then made to react, in the absence of
methanol, on the pre-reduced catalyst (Table 2 – entries 3 and
5). These experiments demonstrate that, in the absence of the
H-transfer reactant, these substrates undergo degradation;
indeed, the C loss registered was 70% or higher in both cases.
However, also in this case we observed the formation of MF
from FU and FAL, despite the absence of methanol. The
analysis of the gas stream (Figure S8) showed the presence of
1
2
3
4
5
6
Time on stream (h)
Figure 7. Effect of reaction time on FU conversion and product
selectivity for pre-reduced FeVO4 catalyst, feeding only
methanol at 320°C for 1 h (bold line), and non-pre-reduced
catalyst (dotted line). Feed composition: FU 1%, CH3OH 10%, N2
89%; Pressure 1 atm, overall gas residence time 1.0 s. Legend:
♦
FU conversion,
selectivity, ● C loss.
▲ MF selectivity, ■ sum of DMF and VINFU
3.5. Study of the reaction network
We conducted experiments aimed at elucidating the reaction CO2, CH4, and H2. Therefore, it can be hypothesized that during
network. More specifically, we wanted to investigate the the first half hour of reaction some adsorbed methanol,
reasons why we never detected FAL, which is supposed to be deriving from the previous catalyst reduction treatment, was
the intermediate for MF formation.
available as the H source for the reduction of FU or FAL to MF.
First, we studied the effect of contact time (Figure 8). Results The same catalytic tests were performed by feeding FU and
were taken at the optimised temperature (320°C), having pre- FAL, in the absence of methanol, with the fresh FeVO4 (Table 2
reduced the catalyst with methanol and changing the contact – entries 4 and 6). In these conditions, only small amounts of
time from 0.01 to 1.0 s. FU conversion increased from 16% at FU and MF formed from FAL (both with 6% selectivity) and C
0.01 s contact time up to 100%. At 0.01 s, MF was the main loss was very high. The equimolar formation of FU and MF
product with 32% selectivity; however VINFU and FAL, the could be due to FAL disproportionation; this reaction might
latter with 2% selectivity, were also formed. Upon increasing contribute to MF and FU formation also with the pre-reduced
FU conversion, MF selectivity increased up to 75% when catalyst (entry 5). TG/DT analysis of the spent catalyst (Figure
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Entry
Journal Name
View Article Online
DOI: 10.1039/C7GC01749A
Products Selectivity (%)
FAL
FU
DMF
MF
VINFU
Conversion
(%)
Substrate
O
C loss
O
O
O
O
O
OH
1
2
FU + CH3OH a
FAL + CH3OH a
FU a
99
100
31
35
85
56
21
0
-
0
-
75
60
21
0
5
4
2
0
0
0
0
0
0
0
0
0
18
34
79
100
70
88
58
0
2
-
3
0
0
-
0
4
FU b
-
0
5
FAL a
5
6
0
0
0
0
25
6
0
6
FAL b
-
0
7
MF + CH3OH a
MF a
0
0
0
0
-
42
0
8
-
9
DMF + CH3OH a
DMF a
13
0
0
0
100
0
10
0
-
Table 2. Reactivity experiments carried out by feeding different reactants: FU or FAL or MF or DMF 1%, CH3OH 10% , N2
89% or 99%; pressure 1 atm, temperature 320°C, overall gas residence time 1,0 s, reaction time 1 h.
a. Catalyst pre-reduced by feeding methanol at 320°C for 1 h.
b. Catalyst calcined in static air at 650°C
S9) in air showed a weight loss of ≈ 3% in the temperature desorbing in the gas phase; indeed, it was observed only in
range 290-360°C which, coupled with an exothermic DTA peak, traces at a very low contact time (0.01s); ii) MF can be
demonstrated the combustion of carbonaceous deposits. The transformed into DMF by reacting with the in-situ-generated
absence of MF
formaldehyde;
the
intermediate
2-methyl-5-
in FU reduction with the fresh catalyst confirms the hypothesis hydroxymethylfuran, deriving from the electrophilic attack of
that the formation of MF with the pre-reduced catalyst formaldehyde, is also rapidly reduced to DMF; iii) VINFU
(entries 3 and 5) was due to the presence of pre-adsorbed formed only from FU in the presence of methanol (Table 2 –
methanol.
entry 1).
The catalytic tests performed by feeding MF and methanol We then tested the reactivity of FU and FAL on the pre-
(Table 2 – entry 7) showed that the reduced catalyst is able to reduced catalyst with different H sources: formaldehyde,
convert MF into DMF through a hydroxyalkylation process that isopropanol, acetone, and hydrogen. Table 3 shows the results
involves MF and formaldehyde, the latter having been formed of these experiments, collected during the first hour of
by methanol dehydrogenation. Furthermore, some reaction and compared with the results obtained with
experiments carried out by feeding MF and DMF without methanol as the H source.
methanol (Table 2 – entry 8 and 10) demonstrated that both It is interesting to note that the use of methanol (Table 3 –
compounds are stable and did not undergo consecutive entry 1) led to the formation of MF with a yield (74%) which
degradation reactions.
was even higher than that registered with isopropanol (Table 3
Indeed, no substrate conversion was registered in the catalytic – entry 2), despite the latter being considered a much more
tests performed by feeding either MF or DMF. However, in the active H-transfer reactant than methanol. It may be
presence of methanol, DMF was converted by the 13% (Table hypothesized that formaldehyde could also be directly
2
– entry 9) and completely transformed into heavy involved in the mechanism of MF formation from FU.
carbonaceous compounds adsorbed over the catalyst surface; Conversely, with the much more reactive FAL, similar yields to
this behaviour was probably ascribable to the in-situ formation FU were obtained with methanol and isopropanol (Table 3 –
of formaldehyde that favours consecutive reactions on DMF.
Based on this study, it is possible to conclude that: i) FAL is the The direct involvement of formaldehyde in MF formation with
intermediate for the transformation of FU into MF; however, the co-production of CO2 through
disproportionation
FAL is very reactive, and is very rapidly reduced to MF without involving the two aldehydes, in a way similar to that previously
entries 6 and 7).
a
8 | J. Name., 2012, 00, 1-3
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View Article Online
Conversion, Selectivity (%)
DOI: 10.1039/C7GC01749A
FU
FAL
DMF
MF
VINFU
Entry Substrate H-source
MF Yield (%)
C
loss
O
O
O
O
O
O
OH
1
2
CH3OH
99
93
0
0
75
60
5
0
2
0
18
40
74
56
2-
propanol
FU
3
4
5
6
acetone
CH2O (a)
H2
58
79
55
2
0
0
8
0
4
0
4
0
0
0
0
92
26
83
34
5
55
9
70
17
60
0
CH3OH
100
60
2-
7
2
97
58
0
0
40
56
propanol
FAL
8
9
CH2O (a)
H2
1
1
97
60
33
8
0
0
0
31
66
58
33
100
(a) CH2O from formalin solution in water: 37% w/w CH2O, 7-8% w/w CH3OH.
Table 3. FU/FAL conversion and product distribution as a function of the hydrogen source used. Reaction conditions: 1% FU or FAL, 10% H
source, 89% N2, 1 atm, temperature 320°C, overall gas residence time 1.0s, reaction time 1h. Pre-reduced catalyst.
reported for salicylic aldehyde reduction41 was demonstrated analysis of the products revealed the formation of VINFU and
by reacting FU and FAL with formaldehyde (entries 4 and 8); ethylfuran; the latter may form by hydrogenolysis of the
the same reaction occurred only at a minor extent with alcohol. When the same experiment was conducted without
acetone, the co-product of FU reduction with isopropanol methanol, no VINFU formed.
(entry 3).
The formation of the hypothesized ketone was demonstrated
hemiacetal, trifluoroacetaldehyde methyl
The results of the experiments carried out with H2 (Table 3 – by reacting
a
entries 5 and 9) confirmed the higher reactivity of FAL, and hemiacetal (unfortunately, the hemiacetal formed by reaction
showed that substrate reduction may also occur with H2; the between furfural and methanol is not commercially available),
yield to MF, however, was much lower than that obtained with without methanol. Hemiacetal was readily dehydrated to the
methanol. Nevertheless, these results demonstrate that a
contribution to MF formation deriving from the in-situ
generated H2 should be also taken into account.
ketone (in this case, trifluoroacetone).
OH
O
O
O
It is also worth noting that DMF formed in experiments
conducted with methanol and formaldehyde, whereas VINFU
formed exclusively with methanol. This confirms that the
formation of the former compound involves the aldehyde, via
an hydroxyalkylation step, whereas the latter forms by
reaction between FU and methanol.
A possible mechanism for VINFU formation might occur by the
formation of the intermediate furfural methyl hemiacetal, as
shown in Scheme 3 30. Hemiacetal could then be dehydrated
into the corresponding enol, which would rapidly rearrange to
furylmethyl ketone; the latter is reduced by methanol to the
corresponding alcohol, which is finally dehydrated to VINFU.
CH3OH
-H
CH2O
OH
2O
O
OH
O
O
O
+
-
CH3OH
CH3OH
O
CH
3
OH
FU
O
CH
2
2
-H
2
-H
O
O
O
O
VINFU
2-ethylfuran
In order to support the mechanism proposed, we conducted Scheme 3. Possible reaction pathways for the formation of
an experiment entailing a direct feed of furylmethyl ketone VINFU from FU in the presence of methanol on FeVO4 catalyst.
and methanol onto the FeVO4 catalyst at 320°C. The GC-MS
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These tests support the hypothesized mechanism for the to the classic H-transfer mechanism, the in-siVtiuew-gAertnicelerOantleinde
formation of VINFU.
formaldehyde and H2 played a direct roDleOiIn: 10th.1e03f9o/Crm7GaCt0io17n49oAf
The overall network for the vapour-phase reduction of FU with MF.
methanol and the FeVO4 catalyst is summarised in Scheme 4. Furthermore, it has to be highlighted that the mixture of MF,
MF can form via three different routes:
i) The classic H-transfer two-step mechanism with the to the high energy content, high RON (higher than that of
reduction of FU to FAL and of FAL to MF. Indeed, under ethanol and 97-RON commercial gasoline), excellent auto-
DMF and VINFU could be directly used as fuel-additive thanks
normal conditions FAL is not isolated as an intermediate,
ignition behavior 13
.
O
OH
-
+
H2 H2O
CH3OH
CH2O
CH2O
CH2O
O
CH3OH
O
O
O
O
OH
-H
2O
FAL
MF
DMF
FU
-H
-
-H
+
2
CH2O CO2
H2O
2
-
+
CH2O CO2
-
+
2H2 H2O
OH
O
O
+
-
CH3OH
CH3OH
O
VINFU
Scheme 4. Overall reaction pathways for the transformation of FU on FeVO4 catalyst in the presence of CH3OH as H
source.
because of its very high reactivity, also because of the
possible involvement of formaldehyde (co-product of the
MPV reaction) in the immediate reduction of FAL to FU
Acknowledgements
(see the following point);
INSTM is acknowledged for co-financing the project.
ii) The disproportionation reaction involving the in-situ-
generated formaldehyde, with reduction of FU to MF and
co-production of CO2;
iii) The hydrogenation of FU and FAL to MF involving the in-
situ-generated H2.
DMF forms by the hydroxyalkylation of MF with formaldehyde,
and the reduction of the alcohol. VINFU forms by reaction
between FU and methanol to the corresponding hemiacetal,
which is then dehydrated to furylmethyl ketone and finally
reduced again via H-transfer.
Notes and references
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2
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FeVO4 is a very active catalyst in the gas-phase production of
2-methylfuran from biomass-derived furfural using methanol
as the H-transfer reactant. 320°C was the optimum reaction
temperature at which the selectivity to MF was the highest
and, at the same time, the formation of heavy carbonaceous
residues was minimised. The deposition of the latter was the
main cause of catalyst deactivation.
8
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