Selective aerobic oxidation of furfural to maleic anhydride with heterogeneous Mo-V-O catalysts

Article abstract of DOI:10.1039/c6gc00508j

A heterogeneous catalytic system using binary Mo-V metal oxides as catalysts was demonstrated for the selective aerobic oxidation of furfural to maleic anhydride (MA). Aspects such as the solvent for the reaction, the phase composition and the Mo/V ratio for the catalyst, and other reaction conditions were investigated in detail. Up to 65% yield of MA was achieved over the Mo₄VO₁₄ catalyst in an acetic acid solvent under optimal conditions. The catalyst could be recycled. The reaction mechanism and the role of the acetic acid solvent were also discussed.

Full text of DOI:10.1039/c6gc00508j

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DOI: 10.1039/C6GC00508J
Journal Name
ARTICLE
Selective Aerobic Oxidation of Furfural to Maleic Anhydride with
Heterogeneous Mo-V-O Catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
*
Xiukai Li, Ben Ho and Yugen Zhang
DOI: 10.1039/x0xx00000x
Heterogenous catalytic system using binary Mo-V metal oxides as catalysts were demonstrated for the selective areobic
oxidation of furfural to maleic anhydride (MA). Aspects such as the solvent for the reaction, the phase composition and
the Mo/V ratio for the catalyst, and other reaction conditions were investigated in detail. Up to 65% yield of MA was
www.rsc.org/
achieved over the Mo
4
VO14 catalyst in acetic acid solvent under optimal conditions. The catalyst could be recycled. The
reaction mechanism and the role of the acetic acid solvent were also discussed.
Chemicals synthesis from renewable, non-petroleum based biomass still in their infancy. MA could be achieved from furfural at 70%
[
1]
resources has attracted increasing interest in the past decade.
yield in continuous gas phase reaction and 54% yield in batch
[8]
Maleic anhydride (MA), an important starting material for the reaction. For the gas phase oxidation of furfural to MA, the high
synthesis of unsaturated polyester resins, is currently manufactured MA yield (70%) was achieved at the furfural concentration of 1.6%
in large capacity from the gas phase oxidation of n-butane and in the feed; thus, the productivity was low. As to the oxidation of
benzene over vanadium-based catalysts. In view of the limited furfural to MA in batch reaction, the catalytic system was complex,
reservation yet the fast depletion of petroleum, MA production employing H PMo V O heteropolyacid in combination with
from renewable biomass resources is highly desired. It has been Cu(CF SO ) as the catalyst and acetonitrile/acetic acid mixture as
reported that MA can be produced from 5-Hydroxymethyl furfural the solvent. The homogeneous feature also makes it difficult for
HMF), a C6 platform chemical that could be derived from the catalyst separation and product isolation. Besides HMF and furfural,
cellulosic biomass resources. Yields of MA from HMF were MA can also be produced from the aerobic oxidation of lignin and
[8b]
[2]
5
10 2 40
[3]
3
3 2
[
8a]
(
[4]
[3, 9]
achieved at respectively 50% and 64% over a VO(acac) catalyst and its derivatives; however, the overall yields were also suffered.
2
[5]
a vanadium-containing heteropolyacid catalyst. Recently, we have Thus, it is very intriguing to explore more efficient and simple
demonstrated that up to 79% yield of MA was achieved over a catalytic systems for furfural conversion to MA.
simple V O catalyst from pure HMF and 50% overall yield of MA
2
5
[6]
was achieved starting from fructose.
The costs for the starting materials are the crucial point when
biomass-based processes compete with the petroleum-based
processes. Despite the fact that the yield of MA from pure HMF is
good, the tedious isolation and purification processes in HMF
synthesis limit the scale production of MA with costly HMF as the
raw material. Alternatively, furfural has similar furan ring as HMF
and could serve as another precursor for MA synthesis (Scheme 1).
Scheme 1. Conversion of furfural to maleic anhydride and maleic
acid.
In this work, we demonstrate a heterogeneous catalytic system
for the conversion of furfural to MA using simple metal oxides as
the catalysts and sole acetic acid as the solvent. In our previous
report, we have disclosed that the vanadium-based metal oxides
are efficient for the conversion of HMF to MA under mild
[4]
Contrasting to the lab scale synthesis of HMF, the industrially
production of furfural from a variety of agricultural by-products
(
e.g., sawdust, corncobs, wheat bran, and oat) has been well
[7]
developed. It is therefore considered that furfural is a better
starting material than HMF for MA synthesis from biomass.
Although the transformation of furfural to a variety of downstream
chemicals has been extensively studied, there were very limited
reports on furfural conversion to MA and the related researches are
[6]
conditions. Thus, several vanadium-based oxides as well as WO3
and MoO were evaluated for the oxidation of furfural under the
3
conditions identical to those (10 bar O , 100 °C, 4 h) for HMF
2
conversion to MA. It turned out that the MoO sample is poorly
3
active (12% conversion) and nonselective for MA, while WO and
3
the vanadium-based oxides showed moderate activities (46 ~ 66%
conversion) yet rather low selectivity (7 ~ 15%) for MA (Table 1). It
indicates that the furfural substrate is less reactive than HMF. When
Dr. X. Li, Mr. B. Ho, Dr. Y.G. Zhang.
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos,
Singapore 138669, Singapore. E-mail: ygzhang@ibn.a-star.edu.sg
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
the intensified reaction conditions (20 bar O , 120 °C, 16 h) were
2
applied, furfural conversion notably increased to > 90% over all the
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Journal Name
The solvent also plays a key role in the aerobic oxidation of HMF
catalysts except MoO . The selectivity for MA also increased over
3
View Article Online
DOI: 10.1039/C6GC00508J
[10]
WO and the vanadium-based oxides. The best MA yield (62%) was and furfural.
Acetic acid and acetonitrile or their mixture are
3
achieved over the Mo VO mixed metal oxides. This result is better good solvents for the conversion of HMF and furfural to MA under
4
14
[5, 8a]
than that (54% yield) achieved in the homogeneous catalytic homogenous system,
while acetic acid is the suitable solvent
[6]
systems using H PMo V O in combination with Cu(CF SO ) as for HMF conversion to MA over the heterogeneous V O catalyst.
5
10
2
40
3
3 2
2 5
catalyst. V O and VOHPO showed good selectivity (> 75%) for MA Thus, several common solvents were tested for the current system
2
5
4
[6]
formation from HMF; however they are less selective for MA (Table 2). The best yield for MA (65%) was achieved in the acetic
formation from furfural. The VO catalyst showed similar activity acid solvent. In the acetonitrile solvent the conversion of furfural
2
and selectivity as compared to V O , signifying that the valence was high (90%), however, the selectivity for MA was negligible (4%).
2
5
state of V is not crucial for this system. Maleic acid was also In ethanol, formic acid, and water solvents the conversions were ≤
detected and which was hydrolysed from MA with the in situ 10% and MA was not detected. As maleic anhydride is the more
generated water and the moisture existed in the solvent (Scheme 1). preferred product over maleic acid, acetic anhydride was used as
The ratio of maleic anhydride to maleic acid was approximately the solvent instead of acetic acid to suppress the hydrolysis of MA.
1
.8:1 right after the reaction (Fig. S1) and the value decreased with The furfural conversion and MA selectivity decreased slightly in the
the storage time of the reaction mixture. After hydrolysis with hot acetic anhydride solvent and the ratio of maleic anhydride to maleic
water, maleic anhydride would be fully converted to maleic acid (Fig. acid increased from 1.8:1 to 2.6:1. Addition of other stronger
S2). Formic acid was observed as the major by-product (Fig. S1) and dehydration reagents is expected to further improve the amount of
it might be originated from the dissociation of the aldehyde group MA in products.
of furfural. Some unidentified by-products were observed in trace
amounts. There was also small amount of black humin substances
in the system after the reaction.
Mo VO
5
17
Table 1. Furfural oxidation over various heterogeneous catalysts.
Mo VO
[c]
4
14
11
Entry
Catalyst
Conv. [%] Sel. [%]
Yield [%]
[
a]
Mo VO
1
2
3
4
5
6
7
8
9
1
1
1
V2O5
V2O5
VO2
VO2
66
96
56
90
58
93
46
100
55
97
12
18
8
40
7
40
9
20
15
62
7
28
0
5
38
4
34
5
19
7
62
4
27
0
3
[
b]
2
Mo VO8
[a]
[b]
[a]
[b]
[a]
[b]
[a]
MoVO5
VOHPO4
VOHPO4
Mo VO
10
20
30
40
50
60
70
80
90
2
4
14
Mo VO
4
14
Fig. 1. XRD patterns of serial Mo-V-O catalysts.
WO3
WO3
MoO3
MoO3
[b]
0
1
2
[
a]
b]
[
0
0
[
1
a] Reaction conditions: furfural 2 mmol, catalyst 30 mg, acetic acid
0 ml, O 10 bar, 100 °C, 4 h. [b] O 20 bar, 120 °C, 16 h. [c] Total
2
2
yield for maleic anhydride and maleic acid.
Table 2. Furfural oxidation over the Mo VO catalyst in different
4
14
[a]
solvents.
[
b]
Entry
Solvent
AcOH
Acetic anhydride 84
Conv. [%] Sel. [%]
Yield [%]
1
2
3
4
5
6
100
65
56
4
65 (1.8:1)
47 (2.6:1)
Acetonitrile
EtOH
90
10
10
6
4
0
0
0
0
0
0
Fig. 2. SEM image for the Mo
VO14 catalyst.
4
Formic acid
As the Mo VO catalyst is efficient for furfural oxidation to MA
H O
2
4
14
in acetic acid, serial binary Mo-V metal oxides with various Mo/V
ratios were therefore prepared and evaluated. The XRD patterns
[
a] Reaction conditions: furfural 2 mmol, catalyst 2.8 mol%, acetic
acid 10 ml, O 20 bar, 120 °C, 16 h. [b] Total yield for maleic
anhydride and maleic acid. The value in the bracket indicates the
ratio of maleic anhydride to maleic acid.
2
(
Fig. 1) depict that all samples were well crystallized in the single
hexagonal MoO -type phase except that MoVO was crystallized in
3
5
[11]
the triclinic (V,Mo) O -type phase. All the samples possessed the
2
5
2
| J. Name., 2016, 00, 1-3
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agglomeration of micron sized irregular particles (Fig. 2). The serial
Mo-V-O catalysts were evaluated for furfural oxidation to maleic
anhydride with the results shown in Fig. 3. The conversion and MA
yield increased with the Mo/V ratio. The full conversion and 65%
MA yield maximized at the Mo VO and Mo VO catalysts with
A
View Article Online
DOI: 10.1039/C6GC00508J
1
00
8
6
4
0
0
0
3
11
4
14
Mo/V ratios of 3 and 4. Further increasing the Mo/V ratio to 5 led to
a catalyst with low activity and the lower MA yield. Thus, Mo VO11
3
Yield
Conv.
and Mo VO are the most active phases for furfural oxidation to
4
14
MA. Mo VO and Mo VO are also the active phases for a variety
20
0
3
11
4
14
of partial oxidation reactions such as acrolein and allyl alcohol to
[12]
acrylic acid. It was proposed that the shear structure of Mo VO
3
11
0
1
2
3
4
5
6
7
8
and Mo VO is favourable for oxygen transition from the corner-
4
14
[13]
Catalyst loading / mol%
linked octahedral into the edge-sharing regions.
B
1
00
8
6
4
2
0
0
0
0
0
1
00
Conv.
Yield
8
6
4
2
0
0
0
0
0
Yield
Conv.
6
0
80
100
120
140
Temperature / C
C
MoVO5 Mo VO8 Mo VO Mo VO Mo VO
17
100
2
3
11
4
14
5
Fig. 3. Furfural oxidation to maleic anhydride over serial Mo-V-O
catalysts. Furfural 2 mmol, catalyst 2.8 mol%, acetic acid 10 ml, O2
8
6
4
0
0
0
2
0 bar, 120 °C, 16 h.
The reaction conditions for the Mo VO catalyst were varied in
4
14
Yield
Conv.
terms of the amount of catalyst loading, reaction temperature,
reaction time, and oxygen pressure. Both the conversion of furfural
and the yield for MA increased with the loading amount of catalyst
20
(
Fig. 4A). Full furfural conversion and 64% yield for MA were
0
1
2
3
4
5
6
7
8
9
10 11
achieved at the loading amount of 5 mol%; further increasing the
loading amount of catalyst led to decreased yield for MA. The
influence of the reaction temperature is significant (Fig. 4B). The
conversion of furfural increased gradually with the reaction
temperature and full conversion was achieved at 140 °C. The yield
for MA reached 64% at 100 °C and the value does not change
appreciably with the further temperature rise. As to the reaction
time (Fig. 4C), the conversion of furfural improved rapidly to 93% in
the first 4 h and then increased slowly after 4 h. The yield for MA
increased to 65% at 4 h and then decreased at longer reaction time
probably owing to the oxidative degradation of product to small
molecules. The oxygen pressure also showed impacts on the
oxidation reaction (Fig. 4D). Both the conversion of furfural and the
yield for MA increased with oxygen pressure and the 93%
conversion and 65% yield was observed at 20 bar. Further increase
in the oxygen pressure does not affect the conversion and yield
notably. Thus, the optimal reaction conditions for the current
system are 5 mol% of catalyst loading, 100 °C of reaction
Time / h
D 100
9
8
0
0
70
6
5
4
3
0
0
0
0
Yield
Conv.
10
15
20
25
30
Pressure / bar
Fig. 4. Furfural oxidation to maleic anhydride over the Mo VO
catalyst. Reaction conditions: furfural 2 mmol, acetic acid 10 ml. (A)
O 20 bar, 100 °C, 8 h. (B) O 20 bar, 8 h, catalyst 5 mol%. (C) O 20
4
14
2
2
2
temperature, 4 h of reaction time, and 20 bar of O pressure.
2
bar, 100 °C, catalyst 5 mol%. (D) 100 °C, 4 h, catalyst 5 mol%.
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The recyclability for the Mo VO catalyst was tested (Fig. 5), as oxidation is a free radical reaction.
Journal Name
[5, 8a]
In our previous report on
4
14
View Article Online
DOI: 10.1039/C6GC00508J
the heterogeneous feature of this system enables an easier catalyst HMF oxidation to MA over heterogeneous V O catalyst, we have
2
5
separation and product purification. The solid catalyst after each demonstrated that the addition of a free radical initiator in the
run was recovered by centrifuge separation, washed with ethanol, system could induce the reaction while the addition of a free radical
[6]
and dried in air and then in argon. Over 90% of furfural conversion inhibitor would suppress the reaction. In order to understand the
and over 60% of MA yield was retained in the first three runs. The reaction mechanism for the current system, we did similar control
furfural conversion and the yield for MA dropped to 75% and 45% experiments and observed quite similar results. MA still could be
in the fourth run and then to 52% and 23% in the fifth run. The produced when H O (free radical initiator) was added in the
2
2
recovered catalyst was characterized by XRD and there was no absence of any catalyst, while the reaction was substantially
appreciable change observed in catalyst structure (Fig. S3). suppressed when 4-tert-butylphenol was added into the reaction
However, the morphology of the catalyst changed notable after 5 system as a free radical inhibitor in the presence of the Mo VO14
4
runs of the hydrothermal reactions (Fig. S4). The recovered catalyst catalyst (Table S3). It is likely that the current reaction also
was also characterized by FT-IR (Fig. S5). There was no signal of proceeds as a free radical reaction. The free radicals may be
[8a, 14]
organic polymer observed, although it cannot exclude the generated from the aldehyde groups of furfural and HMF.
possibility that the polymer compounds presented on sample Using acetic acid as the solvent might also be beneficial for the
surface in trace amounts. The filtrate of the reaction mixture of reaction as acetic acid is a free radical acceptor and could be
each run was also checked by ICP analysis with the results shown in oxidized to acetic acid peroxide. This might explain why acetic acid
Table S1. It turned out that certain amounts of the vanadium and is a more efficient solvent for the oxidation of HMF and furfural to
the molybdenum components leaked into the solution. The extent MA.
of vanadium leak is more remarkable than that of molybdenum leak.
The hot filtration experiment was conducted for the fresh Mo VO14
catalyst. The reaction was terminated at 1 h at 31% conversion and
4
Conclusions
1
1% of MA yield. The catalyst was separated and the filtrate was
heated for another 1 h, and the conversion and the MA yield were
5% and 11%, respectively. Thus, it is likely that V and Mo leaked as
inactive species into the solution.
We have demonstrated the selective aerobic oxidation of
furfural to MA over heterogeneous Mo-V-O catalysts. The phase
composition and the type of the reaction solvent are crucial for
catalyst performance. The Mo-V binary metal oxides performed
better than simple V O and MoO , and up to 65% yield of MA was
3
2
5
3
100
Conv.
Yield
achieved over the Mo VO catalyst in acetic acid. The catalyst
4
14
could be recycled for more than three times and the heterogeneous
feature of this system enables easier catalyst separation and
product purification. The mechanism study indicates that furfural
oxidation to MA proceeds probably through the free radical
reaction.
8
6
4
2
0
0
0
0
0
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).
1
2
3
4
5
Recycle sequence
Fig. 5. Recyclability of the Mo VO catalyst in furfural oxidation to
4
14
MA. Reaction conditions: furfural 2 mmol, catalyst 5 mol%, acetic
acid 10 ml, O 20 bar, 100 °C, 4 h. Catalyst was washed with ethanol
2
and heated in air (275 °C, 4 h) and argon (325 °C, 4 h) before reuse.
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4
Table 1 have demonstrated that MO is almost an inactive phase for
3
furfural conversion; V O is active but less selective to MA than the
2
5
Mo VO catalyst. The mechanical mixture of MO with V O or VO
4
14
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5
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14
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14
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| J. Name., 2016, 00, 1-3
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Green Chemistry
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DOI: 10.1039/C6GC00508J
Journal Name
ARTICLE
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A binary Mo-V metal oxide catalyzed selective areobic oxidation
of furfural to maleic anhydride (MA) with up to 65% yield.
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