Highly Efficient Gas-Phase Oxidation of Renewable Furfural to Maleic Anhydride over Plate Vanadium Phosphorus Oxide Catalyst

Article abstract of DOI:10.1002/cssc.201701866

Maleic anhydride (MAnh) and its acids are critical intermediates in chemical industry. The synthesis of maleic anhydride from renewable furfural is one of the most sought after processes in the field of sustainable chemistry. In this study, a plate vanadium phosphorus oxide (VPO) catalyst synthesized by a hydrothermal method with glucose as a green reducing agent catalyzes furfural oxidation to MAnh in the gas phase. The plate catalyst—denoted as VPO_HT—has a preferentially exposed (200) crystal plane and exhibited dramatically enhanced activity, selectivity and stability as compared to conventional VPO catalysts and other state-of-the-art catalytic systems. At 360 °C reaction temperature with air as an oxidant, about 90 % yield of MAnh was obtained at 10 vol % of furfural in the feed, a furfural concentration value that is much higher than those (<2 vol %) reported for other catalytic systems. The catalyst showed good long-term stability and there was no decrease in activity or selectivity for MAnh during the time-on-stream of 25 h. The high efficiency and catalyst stability indicate the great potential of this system for the synthesis of maleic anhydride from renewable furfural.

Full text of DOI:10.1002/cssc.201701866

Accepted Article
Title: Highly Efficient Gas Phase Oxidation of Renewable Furfural to
Maleic Anhydride over Plate VPO Catalyst
Authors: Yugen Zhang, Xiukai Li, and Jogie Ko
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Highly Efficient Gas Phase Oxidation of Renewable Furfural to
Maleic Anhydride over Plate VPO Catalyst
Xiukai Li,[a] Jogie Ko[a] and Yugen Zhang*[a]
Abstract: Maleic anhydride (MAnh) and its acids are critical
intermediates in chemical industry. The synthesis of maleic
anhydride from renewable furfural is one of the most sought after
processes in the field of sustainable chemistry. We report a plate
vanadium phosphorous oxide (VPO) catalyst synthesized by a
hydrothermal method with glucose as a green reducing agent for
furfural oxidation to MAnh in gas phase. The plate VPOHT catalyst
has a preferentially exposed 200 crystal plane and exhibited
dramatically enhanced activity, selectivity and stability as compared
to conventional VPO catalysts and other state-of-the-art catalytic
systems. At 360 ºC reaction temperature with air as an oxidant,
about 90% yield of MAnh was achieved at 10 vol% of furfural in the
feed, a furfural concentration value that is much higher than those (<
2 vol%) reported for other catalytic systems. The catalyst showed
good long term stability and there was no decrease in activity and
selectivity for MAnh in the time-on-steam duration of 25 h. The high
efficiency and catalyst stability established this system very
prominent for the synthesis of maleic anhydride from renewable
furfural.
product isolation, and better catalyst reutilization. There have
been few reports on the gas phase fixed-bed catalytic oxidation
of furfural to MAnh [7]. However, current state-of-the-art systems
are suffered from the low selectivity, low feed concentration
(typically < 2 vol%) and low catalyst stability issues [7c]. For
instance, VOx/Al2O3 catalyst gave 73% of MAnh yield at a
furfural concentration of 1.6 vol% yet the long term stability of
the catalyst was not clear [7c]. In light of the high demand for the
synthesis of MAnh from renewable resource, more catalytic
processes merit to be explored for the potential practical
application.
Scheme 1. Processes for the synthesis of maleic anhydride and maleic acid.
Vanadium phosphorous oxide (VPO) containing vanadyl
pyrophosphate ((VO)2P2O7) as the main active phase is a well-
Introduction
known catalyst in n-butane oxidation to MAnh, giving product
yield of up to 90%
[2,
8]. Previous mechanism studies have
Maleic anhydride (MAnh) and its acids (i.e., maleic acid and
fumaric acid) are critical chemical intermediates used for the
large volume production of lubricant additives, unsaturated
proposed that n-butane and furfural oxidation to MAnh share the
[4h, 7c,
same furan-type intermediates
9]. We therefore envision
polyester resins, vinyl copolymers, plasticizers, and
that VPO-type catalysts might also be effective in the catalysis of
furfural oxidation to MAnh. However, there has been no open
report on VPO catalyst for furfural oxidation in gas phase though
various methods have been developed for the synthesis of
VPOs [10]. In this work, we report a two-step hydrothermal
method to synthesize plate VPO catalyst (VPOHT) by using
glucose as a green reducing agent. The plate VPOHT catalyst
was applied in the gas-phase oxidation of furfural to MAnh and it
exhibited dramatically enhanced activity, selectivity and stability.
At 360 °C, about 90% yield of MAnh was achieved at 10 vol% of
furfural in the feed. The catalyst also demonstrated good long
term stability and there was no decrease in activity for the
duration of 25 h. This study provides a prominent catalytic
system for the synthesis of maleic anhydride from renewable
furfural.
[1]
pharmaceuticals
.
Maleic
anhydride
is
industrially
manufactured from the gas phase oxidation of benzene and n-
butane [2]. Further hydrolysis of maleic anhydride leads to maleic
acid (MA). In view of renewable resources and chemical
sustainability [1, 3], biomass derived platform molecules such as
furfural have been extensively studied as starting materials for
the synthesis of maleic anhydride/acid [4]
.
Various systems have been developed for the transformation
of furfural to MAnh/MA, generally with molecular oxygen or
hydrogen peroxide as oxidants (Scheme 1). The homogeneous
catalytic systems usually offer the desired product in less than
[4a-c,
50% yields
5]. Heterogeneous catalytic systems (batch)
could give higher yields of product, for examples, a titanium
silicate-1 (TS-1) catalyst gave 78% yield of MA from furfural with
[4g]
H2O2 (7.5 eq.) as an oxidant
and a Mo-V-O catalyst gave
65% yield of MAnh from furfural with pressurized O2 (20 bar) [4h]
.
However, these catalysts suffered from critical stability issue as
significant metal leaching was observed during the reaction.
Besides the catalytic batch conversions, formic acid-mediated
[a]
Dr. X. Li, Miss J. Ko and Dr. Y. Zhang
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way,
The Nanos #04-01, Singapore 138669 (Singapore)
Fax: (+65) 6478 9080
oxidation of furfural to MA with H2O2 in a catalyst-free system
[6]
Email: ygzhang@ibn.a-star.edu.sg.
has also been reported and 95% yield of MA was achieved
.
Supporting information for this article is given via a link at the end of
the document.
The continuous flow gas phase reactions are important
processes in chemical industry established by the many
advantages such as higher operation temperature, easier
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Results and Discussion
A
Catalyst characterization
The preparation chemistry for the industrial VPO catalysts
has long been a topic of research. VPO catalysts can be
synthesized in either aqueous phase or organic (alcohol) phase.
The VPO catalysts generated from the organic medium
generally possess larger surface area and higher activities for n-
butane oxidation than the catalysts generated from the aqueous
medium. A major issue in the organic medium case is the use of
large volumes of fatty alcohol and/or aromatic alcohol as
reducing agent/solvent. There have also been reports on the
successful hydrothermal synthesis of VPO catalysts starting
from V2O4 and H3PO4, or from V2O5 and H3PO3, as the
vanadium and phosphorus sources [11]. However, V2O4 is an
expensive reagent and H3PO3 is highly toxic to handle. Herein,
the VPOHT catalyst was synthesized by using a modified
hydrothermal method starting from inexpensive and easily
available V2O5 and H3PO4 as starting materials and
VHPOHT
VHPOorg
VHPOaq
10
20
30
40
50
2
B
act.
VPOHT
VPOHT
carbohydrate glucose as
a
green reducing agent. For
comparison, VPOaq and VPOorg catalysts have also been
synthesized from respective aqueous and organic medium.
The structure and the morphology of the (VO)2P2O7 (VPO)
catalysts are determined mainly by the precursor samples, viz.
VOHPO4·0.5H2O (VHPO). Figure 1 shows the SEM images of
VPO samples prepared by different methods. The VPOaq sample
contains irregular particles (Figure 1a). The VPOorg sample
contains agglomerated small plates with irregular particle sizes
(Figure 1b). The VHPOHT sample (precursor of VPOHT) from
hydrothermal synthesis contains dispersed thin plates with
smooth surface (Figure 1c). The VHPOHT sample was
dehydrated to VPOHT in nitrogen gas at 400 °C for 12 h and its
morphology was well retained as thin plates (Figure 1d). The
VPOHT plates have thickness of approximately 100 nm, and the
particles are smooth and highly crystallized. BET surface area
values of VPOaq, VPOorg and VPOHT catalysts are 2.6, 20.1, and
9.7 m2 g-1, respectively.
VPOorg
VPOaq
20
30
40
50
2
Figure 2. The XRD patterns of (A) the VHPO precursors synthesized by
different methods, and (B) the VPO catalysts derived from the dehydration of
VHPO precursors. VPOaq, VPOorg and VPOHT catalysts were dehydrated from
act.
the corresponding precursors at 400 °C for 12 h in flowing N2. VPOHT was
the VPOHT catalyst further activated in the reactant mixture (6% furfural in 10%
O2/He, 20 mL min-1) at 340 °C for 8 h.
Figure 2 shows the XRD patterns of the VHPO precursors
derived from different methods. All the samples exhibited the
diffraction peaks assignable to crystalline VOHPO4·0.5H2O. For
the VHPOaq and VHPOorg samples, the 220 peaks (2θ = 30.8°)
are more intense than the 001 peaks (2θ = 16.1°). In contrast,
VHPOHT displays a much stronger 001 peak, indicating a
different crystallinity or preferential growth of crystal faces. The
001 peak broadened the most significantly for VHPOorg. There
have been reports that for VPO precursors synthesised in mixed
alcohol solvent, the intercalation of organic molecules may result
in structure disorder and the broadening of the 001 peak.[2b]
Thus, the VHPOHT from hydrothermal synthesis was better
crystallized. After dehydration, the VPOHT catalyst exhibits the
highest relative intensity of the 200 peak to the 024 peak,
indicating the 200 planes of the VPOHT catalyst are well exposed.
act.
Figure 1. SEM images of (a) the VPOaq; (b) VPOorg; (c) VHPOHT; (d) VPOHT
.
Such XRD features were retained for VPOHT
which was
All VPO catalysts were dehydrated from the corresponding precursors at
400 °C for 12 h in flowing N2.
further activated in the reactant mixture (6% furfural in 10%
O2/He, 20 mL min-1) at 340 °C for 8 h. It is therefore deduced
that the better crystallinity and the preferential growth of the 001
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10.1002/cssc.201701866
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plane for the VHPO precursor favours the exposure of the 200
the lattice oxygen of VPOaq is the least active. The VPOHT and
VPOHT samples showed moderate reducibility among the four
act.
plane of VPO [9b, 12]. It has been reported that 200 is the active
[9a, 12a, b]
face of VPO catalyst in n-butane oxidation to MAnh
.
samples. Factors like phase composition, particle size, and
crystal structure may affect the onset reduction temperature. It
was evidenced by the Raman spectroscopy that V5+ species
related to VOPO4 phases exist in some of the samples (Figure
3). Reduction of V5+ to V4+ species occurs at lower temperature
and deeper reduction of V4+ to V3+ species occurs at higher
temperature. The VPOorg sample that possesses the smallest
particle size and contains the VOPO4 phases exhibited the
lowest onset reduction temperature.
Thus, highly exposed 200 plane of VPO catalyst is an important
character.
Vanadyl pyrophosphate ((V(IV)O)2P2O7) and vanadyl
phosphate (V(V)OPO4) are the two major phases for vanadium
phosphorous oxide (VPO) catalysts. The components of VPO
catalyst are determined by both the precursor synthesis method
and also the dehydration atmosphere. Raman spectroscopy is
very sensitive to the small amount of V5+ phases exist in those
VPO samples (Figure 3). The signals at around ~930, ~1130,
and ~1180 cm-1 are the characteristics of the (VO)2P2O7 phase,
among them the signal at ~930 cm-1 could be assigned to the νas,
825
826
2-
in the P2O7 unit [2, 13]. Various V5+ phases coexist in the
P–O–P
VPOaq, VPOorg, and VPOHT samples, evidenced by the ~590,
~997, ~1017, ~1035, and ~1083 cm-1 signals assignable to
767
d
[2,
VOPO4 phases including -, -, -VOPO4 and VOPO4·2H2O
12b, 14]. The V5+ phases exist in larger proportion in the VPOHT
sample that was activated in air. The additional signals at ~997
and ~1034 cm-1 indicate the formation of more -VOPO4 due to
764
828
c
740
812
act.
the oxidation in air. The Raman spectrum of the VPOHT
b
a
777
sample first activated in nitrogen then in situ treated in the
reactant mixture (Figure 3e) differs remarkably to the spectrum
of the sample activated only in nitrogen (Figure 3c). The V5+
phases with Raman signal at ~1017 and ~1083 cm-1 almost
completely disappeared owing to the reduction by furfural.
600
650
700
750
800
850
900
Temperature / C
Figure 4. Temperature-programmed reduction (TPR) profiles of VPO catalysts
(a) VPOaq, (b) VPOorg, (c) VPOHT, and (d) VPOHTact.. Catalysts were activated
under the same conditions indicated in Figures 2&3.
928
Activity evaluation
1182
a
b
1128
1083
1035
1017
For n-butane oxidation to MAnh, it is recognized that
crystalline (VO)2P2O7 is the active phase while VOPO4 is
responsible for COx formation [14-15]. However, for furfural
oxidation reaction, the role of different phases of VPO remains
unknown. Thus, various vanadium catalysts were evaluated for
furfural oxidation, with the results shown in Table 1. At the
reaction temperature of 340 °C, simple vanadium oxides (V2O5
and V2O4) gave almost full furfural conversions, but the
selectivities for COx were around 80%. The selectivities for
possible polymerized products were included in “others”. The
V(V)OPO4 catalyst showed relatively low activity (27.7%
conversion) but good selectivity (87.4%) for MAnh. 2-Furoic acid
(FA) was detected as the main side product with 11.3%
selectivity. The formation of COx was insignificant (< 2%
selectivity), and this is different to the case of n-butane oxidation
over V(V)OPO4 where COx was the major side product [14-15]. The
VPOaq catalyst with (V(IV)O)2P2O7 as the major phase gave the
lowest furfural conversion (18.5%) among VPOs probably owing
to the smallest surface area value of 2.6 m2 g-1. Maleic
anhydride and 2-furoic acid was produced at the selectivities of
56.1% and 37.8%, respectively. The VPOorg catalyst exhibited
60.4% furfural conversion and 79.8% selectivity toward MAnh,
while the selectivity for 2-furoic acid decreased to 8.2%. The
VPOHT catalyst showed the best activity and selectivity among
the three VPOs, with 80% furfural conversion and 95.3%
selectivity for MAnh. 2-Furoic acid was still observed yet at a
930
931
590
1017
1083
c
928
932
1178
1089
997
1034
d
e
500
600
700
800
900
1000
1100
1200
1300
Raman shiftcm-1
Figure 3. Raman spectra of the VPO catalysts derived from different methods.
Samples: (a) VPOaq, N2 activated at 400 °C for 12 h; (b) VPOorg, activated in
N2 at 400 °C for 12 h; (c) VPOHT, activated in N2 at 400 °C for 12 h; (d)
VPOHTair, activated in air at 400 °C for 12 h; (e): VPOHTact., activated in N2 at
400 °C for 12 h, then further treated in the reactant mixture (6% furfural in 10%
O2/He, 20 mL min-1) at 340 °C for 8 h.
Figure 4 depicts the temperature programmed reduction
(TPR) behaviours of the VPO catalysts. All samples show one
main reduction peak in the temperature range of 750–850 °C,
attributable to the removal of the lattice oxygen in the VPO
catalysts. The onset reduction temperature, which indicates the
activity of lattice oxygen of VPO catalysts, are 777 °C for VPOaq,
740 °C for VPOorg, 764 °C for VPOHT, and 767 °C for VPOHT
The VPOaq sample showed the highest onset reduction
temperature and consumed the least amount of hydrogen. Thus,
act.
.
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10.1002/cssc.201701866
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much lower selectivity of 1.6%. Although the VPOHT catalyst has
an intermediate surface area value among the three VPOs, it
showed the best catalytic performance. It is obvious that the
catalyst activity is not only related to the total surface area but
also the active surface area. The high catalytic activity of plate
VPOHT should be related to its highly exposed 200 plane (Figure
2). This is also correlated to the results of temperature
programmed reduction (TPR) analysis of the VPO catalysts
(Figure 4). Though it seems that the lattice oxygen of VPOorg is
more reactive than VPOHT, VPOHT is practically more active for
furfural oxidation which is due to its unique morphology and the
preferentially exposed 200 plane.
The phase component of VPO catalyst is highly related to
the conditions for catalyst precursor (viz. VOHPO4) dehydration
and activation. Generally, the dehydration of VPO precursor in
oxidizing atmosphere resulted in a catalyst with certain amount
of V5+ species [16]. In Table 1, when the VPOHT catalyst was
dehydrated in air, the furfural conversion increased notably to
97.9% but the selective for MAnh dropped to 86.7%,
accompanied by the selectivity for COx increased remarkably to
11.5%. When the VPOHT catalyst dehydrated in N2 was further
treated in the reactant mixture (6% furfural in 10% O2/He) at
340 °C for 8 h, it turned out that almost full furfural conversion
and 97.9% selectivity for MAnh was achieved. Thus, for a VPOHT
that is freshly activated in N2, an initiation period is necessary to
induce the suitable active sites/phases by removing the
undesired V5+ phases.
Conv.
5%O2
Sel.
10%O2
21%O2(Air)
100
80
60
40
20
0
340 360
340 360
340 360
Temperature / °C
act.
Figure 5. Furfural conversion and Manh selectivity over the VPOHT catalyst
act.
with 5%, 10%, 21% oxygen in the carrier gas. Reaction conditions: VPOHT
catalyst, 0.4 g; carrier gas, 20 mL min-1; furfural concentration, 2 vol% in the
feed.
act.
In pursuit of a high output system, the present VPOHT
catalyst was evaluated at furfural concentrations of 2 vol%, 6
vol% and 10 vol% in the feed, with cheap synthetic air as the
oxidant (Figure 6). At a reaction temperature below 360 °C, the
furfural conversion decreased with its concentration in the feed.
With the temperature rise, the furfural conversions increased
and reached almost full at 360 °C (Figure 6A). With increasing
reaction temperature, the selectivity for MAnh also increased
notably while the selectivity for 2-furoic acid decreased
dramatically (Figure 6B). At the reaction temperature of 360 °C,
the selectivities for MAnh were 87.6%, 98.4%, and 90.8%,
Table 1. Furfural gas phase oxidation over different phases of vanadium
catalysts.
Sel. [%]
Catalyst
V2O5
Conv. [%]
MAnh
FA
Others
COx
98.8
99.6
27.7
18.5
60.4
17.5
2.6
87.4
56.1
79.8
0.0
0.0
11.3
37.8
8.2
4.0
12.0
0.0
3.8
7.9
78.6
85.4
1.3
2.2
4.2
V2O4
VOPO4
VPOaq
VPOorg
VPOHT
respectively, at furfural concentrations of 2 vol%, 6 vol% and 10
vol%. Thus, the present VPOHT
act.
catalyst is efficient for the
80.0
97.9
99.2
95.3
86.7
97.9
1.6
0.7
0.5
2.5
1.0
0.9
0.6
oxidation of furfural at the concentration up to 10 vol%, a value
that is much higher than those (< 2 vol%) in literature reports.
Further increase the furfural concentration in the feed would
require higher reaction temperature or larger loading amount of
catalyst.
air
VPOHT
VPOHT
11.5
0.7
act.
Reaction conditions: catalyst, 0.4 g; furfural concentration, 2 vol%; carrier gas,
10% O2/He, 20 mL min-1; reaction temperature, 340 °C. VPOaq, VPOorq and
VPOHT: dehydrated in N2 at 400 °C for 12 h. VPOHTair: dehydrated in air at
400 °C for 12 h. VPOHTact.: first dehydrated in N2 at 400 °C for 12 h, then
treated in the reactant mixture (6% furfural 10% O2/He, 20 mL min-1) at 340 °C
for 8 h. MAnh: maleic anhydride; FA: 2-furoic acid; COx: CO and CO2. “Others”
include possible polymerized products and other unidentified products.
The effect of the contact time was studied for 6 vol% furfural
in the feed by changing the flow rate of the carrier gas (Figure
7A). At the flow rates of 20, 40, and 60 mL min-1, the contact
time was 1.5, 0.75, and 0.5 seconds, respectively. At the
reaction temprature of 340 °C, both the furfural conversion and
Manh selectivity decreased linearly with the flow rate of the
carrier gas, while the selectivity for 2-furoic acid increased. The
conversion and selectivity can be further improved by increasing
the reaction temperature when the contact time was short (or
higher flow rate of the carrier gas). It is shown in Figure 7B,
92.8% conversion of furfural and 94.4% selectivity for MAnh
could be achieved at 360 °C for the reaction carried out at
furfural concentration of 6 vol% and a flow rate of 60 mL min-1
(GHSV: 7200 h-1). When the reaction temperature exceeded
360 °C, the selectivity for COx increased and the selectivity for
MAnh dropped accordingly.
The catalyst performance is also influenced by the carrier
gas for the reaction. The effect of oxygen concentration in the
carrier gas was therefore studied (Figure 5). The furfural
conversion generally increased with oxygen concentration in the
carrier gas. An oxygen concentration around 10% is favourable
for the best MAnh selectivity of 98% at 340 °C. When synthetic
air (21%O2/N2) was used as an oxidant, over 91% and 87%
MAnh selectivities were achieved at 340 °C and 360 °C,
respectively. In view of the cost, air would be a better oxidant for
practical application.
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A
100
90
80
70
60
50
40
30
20
10
0
A
100
80
60
40
20
Conv.
Sel. MAnh
Sel. FA
Furfural concentration:
2 vol%
6 vol%
10 vol%
20
30
40
50
60
300
310
320
330
340
350
360
Flow rate / mL min-1
Temperature / C
B
100
80
60
40
20
0
B
100
90
80
70
60
50
40
30
20
10
0
Conv.
Sel. MAnh
Sel. FA
Sel. CO
x
Furfural
2 vol%
6 vol%
10 vol%
Sel. Manh Sel. FA
300 310 320 330 340 350 360 370 380
300
310
320
330
340
350
360
Temperature / C
Temperature / C
Figure 7. (A) Furfural conversion and MAnh/FA selectivities at the carrier
gas of 20, 40, and 60 mL min-1 and the reaction temprature of 340 °C, and
(B) temperature dependence of furfural oxidation at carrier gas of 60 mL min-
Figure 6. (A) Furfural conversion and (B) MAnh and FA selectivities as a
function of reaction temperature at 2%, 6%, and 10 vol% furfural
act.
concentrations in the feed. Reaction conditions: VPOHT
catalyst, 0.4 g;
1
act.
(GHSV: 7200 h-1). Other reaction conditions: VPOHT
catalyst, 0.4 g;
carrier gas, syntheic air (21% O2/N2), 20 mL min-1. MAnh: maleic anhydride;
furfural concentration, 6 vol% in feed; carrier gas, 21% O2/N2.
FA: 2-furoic acid.
The stability of the VPOHT catalyst was tested as a function
of time on stream at 6 vol% of furfural concentration and a flow
rate of 40 mL min-1 (GHSV: 4800 h-1). As shown in Figure 8, the
initial conversion and Manh selectivity were 92.9% and 94.5%,
respectively. Furfural conversion increased gradually to 99% in
the first 7 hours, indicating an initiation period for the in situ
activation of VPOHT. Within the duration of 25 hours, the catalyst
was stable and there was no appreciable decrease in activity
and selectivity for MAnh. The MAnh product was collected as
white solid from the outlet of the reactor by simple condensation
in air (Supporting Information, Figure S1). NMR analysis
confirmed that the collected MAnh was in high purity (Supporting
Information, Figure S2). XRD analysis for the used catalyst
indicates that the structure of the catalyst was intact after activity
testing (Supporting Information, Figure S3). The used catalyst
was also subjected to thermogravimetric analysis (TGA)
(Supporting Information, Figure S4). The weight loss in the
temperature range of 300–500 °C was less than 2%, indicating
the coke deposition on sample surface is insignificant. The
superb stability enables the VPOHT catalyst a prominent one for
furfural oxidation to MAnh.
From Figures 6B and 7B, it is seen that 2-furoic acid was the
major side product for furfural oxidation over the VPOHT catalyst.
With temperature rise, the selectivities for 2-furoic acid
decreased while the selectivity for Manh increased accordingly,
suggesting that 2-furoic acid is a key intermediate in the current
reaction and that Manh was derived from the consecutive
oxidation of 2-furoic acid. On the basis of product distribution,
the reaction pathway for furfural oxidation to Manh with VPO
catalyst was proposed (Scheme 2), which is also consistent with
previous reported mechanism study [4h, 7c, 17]. Because furan was
detected in very trace amount or was not detected, we propose
that the oxidation of furfural to 2-furoic acid is the rate-
determining step for furfural oxidation over the VPOHT catalysts,
while the decarboxylation of 2-furoic acid to furan and the
subsequent oxidation of furan to Manh are fast.
Scheme 2. The proposed reaction pathway for furfural oxidation to maleic
anhydride with VPO catalysts.
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Catalyst evaluation
Catalytic performances of catalysts (typically 0.4 g, 35–60 mesh) for the
gas phase oxidation of furfural to maleic anhydride was evaluated in a
quartz fixed-bed microreactor (Φ = 6 mm) with a continuous reactant
down flow. All catalysts were pressed into pellets and then crushed and
sieved to 35–60 mesh before activity evaluation. Furfural was charged in
a saturator that was submerged in an oil bath, and it was brought to the
reactor by the bubbles of carrier gas (10%O2/He or synthetic air, flow rate
20-60 mL min-1). The concentration of furfural in the gas phase was
controlled by the temperature of the oil bath. On-line gas chromatography
(Shimadzu GC2014) equipped with FID and TCD was used to analyse
the outlet effluent after the system was stabilized for 1 h at each
temperature point. A capillary column (HP FFAP, 30 m × 0.32 mm, 0.25
µm) was used to separate the organic components, and a packed carbon
column (3 mm × 300 mm) was used to separate CO and CO2.
Conversion
Selectivity
100
80
60
40
20
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26
Time / h
Characterization techniques
Figure 8. Time on stream test for furfural oxidation to MAnh over the VPOHT
catalyst. Reaction conditions: catalyst, 0.4 g; furfural concentration, 6 vol%;
carrier gas, 21% O2/N2, 40 mL min-1 (GHSV: 4800 h-1), 350 °C.
Powder X-ray Diffraction (XRD) was conducted using a Bruker automatic
diffractometer (Bruker D8 discover GADDS) with monochromatized Cu-
Kα radiation (λ = 0.15406 nm) at a setting of 30 kV.
The BET surface areas and pore structures were measured by
Micromeritics ASAP 2020. The samples were degassed at 250 °C for 3 h
before N2 adsorption. The SEM images were obtained with a JEOL JSM
7500 scanning electron microscope.
Conclusions
We demonstrate here a plate vanadium phosphorous oxide
(VPO) as efficient catalysts for furfural oxidation to maleic
anhydride in gas phase. The present VPOHT catalyst
synthesized by a hydrothermal method with glucose as a green
reducing agent has novel plate morphology, preferentially
exposed 200 crystal plane which played key role in the catalytic
reaction, and exhibited superb catalytic performance as
compared to other state-of-the-art catalytic systems. About 90%
yield of MAnh was achieved at furfural concentration of 10 vol%
and air as an oxidant. MAnh can be collected as high purity solid
by simple air condensation. The catalyst showed good stability
and there was no decrease in activity and selectivity in the
duration of 25 h on time stream. The high efficiency and catalyst
stability established this system very prominent for the synthesis
of maleic anhydride from renewable furfural.
Temperature-programmed reduction (TPR) was carried out in the
temperature range of 100–800 ºC. The sample (50 mg) was reduced in a
feed of 5% H2/Ar (30 mL min-1) at a heating rate of 5 ºC min-1.
Raman analysis was conducted using Horiba Jobin Yvon Modular
Raman Spectrometer. The laser excitation wavelength: Stellar Pro
Argon-ion laser at 514 nm (Green). Laser power used: 50 mW. The
Raman system was calibrated using a silicon reference before the
measurement.
Acknowledgements
This work was supported by the Institute of Bioengineering and
Nanotechnology (Biomedical Research Council, Agency for
Science, Technology and Research (A*STAR), Singapore),
Biomass-to-Chemicals Program (Science and Engineering
Research Council, A*STAR, Singapore).
Experimental Section
VPOHT catalyst preparation
Keywords: biomass • furfural • VPO catalyst • maleic anhydride
• selective oxidation
The VPOHT catalyst was synthesized by a two-step hydrothermal method
in a Teflon-lined autoclave. Firstly, a mixture of V2O5 (2.73 g), glucose
(0.81 g), and water (90 mL) was heated at 180 ºC for 12 h in the
autoclave. After the autoclave was cooled to room temperature, the black
solid (V2O4) was separated by filtration, washed twice with water, and
then dried at 100 ºC overnight. In the second step, the mixture of V2O4
(0.83 g), H3PO4 (85%, 0.82 mL), PEG (0.5 g), and water (30 mL) was
heated in the autoclave at 160 ºC for 24 h. The blue product was
collected by filtration, washed with ethanol and water, and then dried at
100 ºC overnight. The catalyst precursor was dehydrated at 400 ºC for 12
h in flowing N2 (100 mL min-1) to generate the VPOHT catalyst.
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Entry for the Table of Contents
A
plate VPOHT catalyst
Xiukai Li, Jogie Ko and Yugen Zhang*
synthesized by
a
Highly
Efficient
Gas
Phase
hydrothermal method has
preferentially exposed 200
crystal planes and exhibited
Oxidation of Renewable Furfural to
Maleic Anhydride over Plate VPO
Catalyst
excellent
activity
and
selectivity for furfural oxidation
to maleic anhydride.
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