Clean synthesis of furfural oxime through liquid-phase ammoximation of furfural over titanosilicate catalysts

Article abstract of DOI:10.1039/c7gc02069d

The clean synthesis of furfural oxime (FO) has been realized through titanosilicate-catalyzed liquid-phase ammoximation of furfural with ammonia and hydrogen peroxide. A detailed investigation of furfural ammoximation over three representative titanosilicates Ti-MOR, TS-1 and Ti-MWW reveals that the reaction involves the hydroxylamine route and the imine route. The hydroxylamine route accounts for the formation of the target product (FO), while the imine route leads to the formation of undesired products such as 2-furylamide and 2-furoic acid. With a high efficiency for hydroxylamine formation, Ti-MOR proves to be superior to TS-1 and Ti-MWW. The catalytic performance of Ti-MOR depends greatly on the operating conditions of the reaction, which is closely related to its activity in catalyzing hydroxylamine decomposition. The decomposition of hydroxylamine and the non-catalytic oxidation of furfural can be effectively suppressed in Ti-MOR-catalyzed ammoximation when employing water as the solvent and adding H₂O₂ dropwise into the reaction system. Under optimized conditions, Ti-MOR is capable of providing furfural conversion and oxime selectivity both above 97%.

Full text of DOI:10.1039/c7gc02069d

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Clean synthesis of furfural oxime through liquid-phase
ammoximation of furfural over titanosilicate catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Xinqing Lu, Yejun Guan*, Hao Xu, Haihong Wu and Peng Wu*
Clean synthesis of furfural oxime (FO) has been realized through titanosilicate-catalyzed liquid-phase ammoximation of
DOI: 10.1039/x0xx00000x
furfural with ammonia and hydrogen peroxide.
A detailed investigation of furfural ammoximation over three
www.rsc.org/
representative titanosilicates of Ti-MOR, TS-1 and Ti-MWW reveals that the reaction involves the hydroxylamine route and
the imine route. The hydroxylamine route answers for the formation of objective product (FO), while the imine route leads
to the formation of undesired products such as 2-furylamide and 2-furoic acid. With a high effeciency for the
hydroxylamine formation, Ti-MOR proves to be superior to TS-1 and Ti-MWW. The catalytic performance of Ti-MOR
depends greatly on the operating conditions of the reaction, which is closely related to its activity in catalyzing
hydroxylamine decomposition. The decomposition of hydroxylamine and the non-catalytic oxidation of furfural can be
effectively suppressed in Ti-MOR-catalyzed ammoximation when employing water as the solvent and adding H2O2
dropwise into the reaction system. Under optimized conditions, Ti-MOR is capable of providing furfural conversion and
oxime selectivity both above 97%.
The conventional processes for manufacturing FO are of
multi-steps including the synthesis of hydroxylamine
1. Introduction
derivatives like (NH2OH)2·H2SO4, the non-catalytic oximation of
Renewable biomass resources have the potential to serve as a
sustainable and alternative carbon supply of modern chemical
industry that is currently based on fossil resources to large
extent.1 Furfural, produced by acid-catalyzed dehydration of
cellulose biomass, have attracted much attention recently for
the production of biofuels and downstream chemicals.2-10 The
annual global production of furfural can be amounted up to
300 Kton according to a report in 2008.4 A number of valuable
chemicals and biofuels can be produced from furfural, such as
furfuryl alcohol, tetrahydrofuran, methyl-tetrahydrofuran,
pentanediol, and cyclopentanone.11 Besides these oxygenates,
furfural derivatives containing nitrogen atom have also
attracted remarkable interest. For instance, furfurylamine,
produced by the selective reductive amination of furfurals with
ammonia, possesses many potential applications in
pharmaceutical synthesis or resin industry.12-15 Another
furfural derivative, furfural oxime (FO) is also an important
intermediate with a constantly increasing world market for the
production of biologically active compounds with a wide range
of medicinal properties. Moreover, FO has gained attention as
furfural with hydroxylamine sulfate and the recovery of FO by
distilling after neutralizing the formed acid with ammonia.
These processes are encountering serious disadvantages, such
as using poisonous hydroxylamine salts and producing large
amounts of ammonium sulphate byproducts together with
nitrogen oxides, leading to environmental problems. Thus, it is
urgent to develop environmentally friendly routes for
synthesizing FO as well as economically. In this regard, one-pot
production of FO using NH3 and H2O2 as the ammoximation
agents shows obvious advantages compared with conventional
methods.
a
versatile platform chemical for the production of 2-
furylamide, 2-furonitrile, 2-furfurylamine, tetrahydro-2-
furfurylamine (Scheme 1). In addition, FO and its derivatives
are used in agronomy as nitrification inhibitors that are
effective against nitrobacteria species, thereby retarding the
nitrification rate.16, 17
Scheme 1. Downstream chemicals and potential applications of furfural oxime.
The ammoximation of several carbonyl compounds such as
cyclohexanone has been widely studied and has been
commercialized by EniChem based on the TS-1/H2O2 system
since 1990s.18, 19 Many other titanosilicates, such as TS-2,20 Ti-
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Beta,21 Ti-MOR and Ti-MWW,22-28 have been developed for the 0.18 TPAOH : 18 H2O under rotation (10 rpm) at 443 K for two
aldehydes or ketones ammoximation. These titanosilicates days.30
show structure- and substrate-dependent activities and 2.2.Characterization methods
selectivities due to the varying hydrophilic/hydrophobic
The X-ray diffraction (XRD) patterns were recorded on a Rigaku
characteristics of the reaction system. In this sense,
Ultima IV diffractometer using Ni-filtered Cu Kα radiation (λ =
environmental and selective synthesis of furfural derived
0.1541 nm) in a scanning range of 2θ = 5 - 35 ° to confirm the
oxime remains to be a challenging task because furfural
structure and crystallinity of the titanosilicates. Morphologies
contains several reactive functional groups.
and crystal sizes were examined by a Hitachi S-4800 scanning
In this study, the clean synthesis of FO has been attempted
electron microscope. The coordination states of Ti species
from the ammoximation of furfural on three representative
were examined by UV-visible spectroscopy on a Shimadzu
titanosilicates, Ti-MOR, TS-1 and Ti-MWW. By thoroughly
2700PC spectrophotometer using BaSO4 as
a reference.
investigating the furfural ammoximation in particular the
parameters controlling the oxime selectivity, Ti-MOR is found
to be the best catalyst with the highest reaction activity and
FO selectivity in the presence of green solvent of water. The
preliminary study on the reaction mechanism of furfural
ammoximation suggests that the unique performance of Ti-
MOR is due to the highly efficient production of hydroxylamine,
which does not undergo significant decomposition on Ti-MOR
but quickly reacts with furfural when it forms. Our report
Infrared spectra were collected by a Nicolet Nexus 670 FT-IR
spectrometer at a resolution of 2 cm-1. The spectra were
obtained using a KBr pellet technique (3 wt. % diluted in KBr).
In order to avoid the influence of absorbed water, the KBr
diluted wafer placed in a quartz IR cell sealed with KBr
windows, was evacuated at 723 K for 2 h before measurement.
The amounts of Si, Ti, B and Al were determined by inductively
coupled plasma (ICP) on a Thermo IRIS Intrepid II XSP after the
samples were dissolved in aqueous HF solution. The textural
properties of the titanosilicates were determined by N2
adsorption at 77 K using a BEL SORP instrument. Prior to the
adsorption measurements, the samples were degassed in
vacuum at 573 K at least for 6 h.
undoubtedly provides
production.
a much cleaner process for FO
2. Experimental
2.3.Catalytic reactions
2.1.Catalyst preparation
Furfural with a purity of 99 % was procured from J & K
Chemicals and it was used without further purification. The
ammoximation of furfural to FO was carried out under
According to the procedures reported previously,29 Ti-MOR
was post-synthesized through a solid-gas reaction between
dealuminated mordenite (Si/Al = 196) and TiCl4 vapor at
elevated temperature. A dealuminated MOR sample (2 g)
placed in a quartz tube reactor (Ø 3 cm) was pretreated at 773
K for 4 h in a dry N2 stream (50 mL min-1). The TiCl4 vapor was
carried by N2 flow into the reactor, which contacted the zeolite
bed to induce Ti incorporation. After TiCl4 treatment at 773 K
for 2 h, the sample was purged with pure N2 at the same
temperature for 1 h to remove any residual TiCl4 species from
the zeolite powder. After cooling down to the room
temperature under N2 atmosphere, the TiCl4-treated sample
was washed with deionized water and dried at 353 K overnight
(denoted as Ti-MOR).
vigorous stirring in a three-neck flask equipped with
a
condenser. The catalyst (50 - 150 mg), solvent (12 g), furfural
(20 mmol), and NH3 (25 wt. % aqueous solution, 20 - 40 mmol)
were added into the flask, and the mixture was heated to the
desired temperature (303 - 343 K). The reaction was then
initiated by adding 20 - 40 mmol of H2O2 (31 wt.% aqueous
solution) into the reactor at once or feeding at a constant rate
continuously with a micropump for a certain period of time (15
- 60 min).
The hydroxylamine formation reactions were also
proceeded in the above-mentioned three-neck flask. In a
typical run, a mixture of Ti-MOR (0.1 g), H2O (12 g), NH3 (25
wt. % aqueous solution, 30 mmol) was added in the flask and
heated to 323 K. Then, 25 mmol of H2O2 (31 wt. %) was fed
into the slurry at once or dropwise at a constant rate with a
micropump for a certain period of time (0.25 - 1 h). After the
catalyst solid was removed by filtration, an excessive amount
of furfural was added into the filtrate and stirred at 323 K for 1
h. FO thus formed under non-catalytic conditions was
determined by gas chromatography, and its amount was taken
as that of hydroxylamine formed in the catalytic oxidation step
of ammonia with H2O2 over Ti-MOR catalyst. With respect to
the hydroxylamine decomposition reaction, commercially
available hydroxylamine chloride was used instead of free
hydroxylamine considering the safety to handle. A mixture of
Ti-MOR (if used, 0.1 g), H2O (12 g), and hydroxylamine chloride
(20 mmol) was added in the flask and heated to 323 K. Then, a
certain amount of H2O2 (31 wt. %) was fed into the slurry at
Ti-containing MWW lamellar were hydrothermally
synthesized using boric acid and piperidine (PI) as
crystallization-supporting agent and structure-directing agent
(SDA), respectively. The gels, with a molar composition of 1.0
SiO2 : 0.05 TiO2 : 1.4 PI : 0.67 B2O3 : 19 H2O, was crystallized
under rotation (10 rpm) at 443 K for 7 days in a Teflon-lined
stainless-steel autoclave. After crystallization, the obtained
precursor was refluxed with 2 M aqueous HNO3 solution at a
solid-to-liquid weight ratio of 1:30 with the purpose of
removing extra-framework Ti species as well as part of
framework boron. The acid-treated products were filtered,
washed with deionized water and dried at 353 K overnight.
Subsequently, the sample was calcinated at 823 K for 6 h,
denoted as Ti-MWW. TS-1 was hydrothermally synthesized
from the gel with a molar composition of 1.0 SiO2 : 0.03 TiO2 :
2 | J. Name., 2012, 00, 1-3
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once or dropwise at a constant rate with a micropump for a
certain period of time (1 - 5 min). After the catalyst was
removed by filtration, an excessive amount of furfural was
added into the filtrate and stirred at 323 K for 1 h. From the
amount of furfural converted, the amount of remaining
hydroxylamine was then determined.
The products were separated from the reaction mixture by
centrifugation and determined by
a gas chromatograph
(Shimadzu 2014) equipped with an Rtx®-Wax capillary column
(30 m × 0.25 mm × 0.25 μm) and FID detector. The amounts of
H2O2 and NH3 remaining in the mixture were determined by
iodometry and acid-base titration method, respectively. In
case of water solvent, the reaction mixture was converted into
a homogeneous phase by adding 15 g of ethanol, and then it
was subject to GC analysis. The products formed were
identified by a GC-MS (Agilent 6890 series GC system, 5937
network mass selective detector). The result of the furfural
ammoximation reaction was given using these criteria:
Fig. 1. SEM images of Ti-MOR (a), TS-1 (b), Ti-MWW (c) and XRD patterns of
titanosilicates (d).
molesof furfuralcomsumed
Furfuralconversion =
×100 %
Table 1 Physicochemical properties of various titanosilicates.
molesof furfuralin the feed
molesof FO produced
molesof furfuralin the feed
Catalyst
Structure
Ti contenta
(mmol g-1)
0.31 (53)
0.33 (50)
0.37 (44)
Crystal sizeb
(μm)
SAc
(m2 g-1)
552
FO yield=
×100 %
molesof a defined product
molesof furfuralcomsumed
Ti-MOR
TS-1
MOR
MFI
0.2 - 0.4
Product selectivity =
×100 %
0.2 - 0.3
537
Ti-MWW
MWW
0.6 × 0.6 × 0.1
545
a
Determined by ICP analysis. The values in parentheses indicate the Si/Ti molar
ratios.
3. Results and discussion
3.1. Characterization of titanosilicate catalysts
b Evaluated by SEM.
Fig. 1 shows the SEM images and XRD patterns of the
titanosilicates. It can be seen that all materials show
characteristic morphology of MOR, MFI and MWW structures.
Ti-MOR and TS-1 crystals were aggregates of nanocrystals
while Ti-MWW crystals showed a typical platelet-shaped
morphology with the thickness of ~50 nm. The XRD patterns
clearly suggest that these materials are of high crystallinity and
no impurities of other zeolite phase or amorphous phase are
detected. The specific surface areas (Langmuir) determined
from N2 adsorption at 77 K were in the range of 537 - 552 m2 g-
1 (Table 1). As shown in Fig. 2, the band at 210 nm and 963 cm-
c Specific surface area (Langmuir) measured by N2 adsorption at 77 K.
210
963
B
A
a
330
b
c
a
b
c
1
200
300
400
500
in UV-vis and IR spectra, respectively, indicating the
1000 900 800 700 600 500
Wavelength (nm)
Wavenumber (cm-1)
incorporation of the tetrahedral Ti species in the zeolite
framework.29,
Fig. 2. UV-vis spectra (A) and FT-IR spectra (B) of Ti-MOR (a), TS-1 (b) and Ti-
MWW (c).
31
Meanwhile, the band around 330 nm was
nearly negligible, indicating the absence of anatase-like phase
in these titanosilicates.
3.2.Comparison of furfural ammoximation among various
titanosilicates
Table 2 shows the results of ammoximation of furfural over
various titanosilicate catalysts in different solvents. It should
be noted that all the furfural converted goes to the detected
products in all cases. The reaction of furfural in the absence of
heterogeneous catalysts is also listed for comparison. To
exclude the effect of un-catalysed reactions, we started the
blanks tests by following the conversion of furfural with either
NH3 or H2O2 in the homogeneous conditions and the results
suggested that no reaction occurred in neither case. For
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homogeneous oxidation in solution containing both NH3 and that the catalytic ammoximation of furfural to FO occurred
H2O2 in the absence of titanosilicate catalysts, 15.5% of simultaneously or competed with the non-catalytic oxidation
furfural conversion was found, producing 2-furylamide and 2- of furfural to 2-furylamide and 2-furoic acid, meanwhile the
furoic acid with selectivity of 86.9% and 13.1%, respectively dehydration of FO to 2-furonitrile was inevitable. It was found
(Table 2, Entry 1). It is well known that imine can be easily that a higher furfural conversion tended to be accompanied by
formed when ketones react with ammonia, which is shown as a higher FO selectivity, similar to the common phenomena well
imine route in Scheme 2. However, the formed imine is observed in the processes of titanosilicate-catalyzed
19, 22-28
unstable and tends to decompose back to a ketone through ammoximation of various ketones and aldehydes.18,
the reverse reaction in the absence of H2O2,22 resulting in no Two findings can be drawn from the data shown in Table 2. On
visible conversion of furfural with NH3. In presence of H2O2, the one hand, the catalytic ammoximation and the non-
the formed imine can then be oxidized to 2-furylamide and 2- catalytic oxidation takes place in parallel and compete with
furoic acid. It should be noted that no FO was detected in each other. On the other hand, the product distribution shows
these non-catalyzed reactions. We then tried the non-catalytic clear structure-dependent and solvent-dependent trend.
oximation of furfural with hydroxylamine chloride. This time Generally, reactions in methanol whatever the catalyst used
we observed a good FO yield of 92.3%, with selectivity of always gave moderate furfural conversion about 40% while
97.6% at a high furfural conversion of 94.6% (Table 2, Entry 2). low selectivity to FO (<30%). When the solvent is water, iso-
The main byproduct was 2-furonitrile that formed due to acid- propanol or t-BuOH, the FO selectivity after reaction for 5 min
catalyzed dehydration of FO (Scheme 2). 2-furylamide and 2- on Ti-MOR is all higher than 92% (entries 3, 6 and 7) at furfural
furoic acid were not detected in this non-catalytic oximation, conversion around 20%. These results suggest that Ti-MOR
confirming that they are produced through the imine route.
shows much better performance than TS-1 and Ti-MWW in
In Table 2, entries 3-17 show us the furfural conversion and ammoximation of furfural. Under similar reaction conditions,
product distribution in the presence of titanosilicate catalysts. Ti-MOR showed the highest FO yield of 94.8 % together with
It can be seen that in all cases FO was formed with yield range the highest FO selectivity of 96.1 % in water. These results
from 5.1 to 94.8% depending on the catalysts and solvents suggest that the undesired byproduct formation pathways can
used. Byproducts such as 2-furonitrile, 2-furamide, and 2- be effectively retarded by choosing
a highly active
furoic acid were also found in all cases. These results suggest titanosilicate such as Ti-MOR for catalytic ammoximation.
Table 2 A comparison of ammoximation of furfural in various solventsa
Entry
Cat.
Solvent
Furfural conv. (%)
FO yield (%)
Products selectivity (%)
FO
-
2-furonitrile
2-furylamide
86.9
2-furoic acid
13.1
-
1
2
None
Noneb
Ti-MOR
H2O
H2O
15.5
94.6
-
92.3
-
2.4
97.6
-
3
H2O
98.6 (24.7)c
94.8 (24.0)
9.0
96.1 (97.0)
19.9
2.4 (1.5)
1.4
1.3 (1.2)
75.5
0.2 (0.3)
3.2
4
MeOH
EtOH
45.0
5
46.5
32.9
70.7
2.2
25.0
2.1
6
iso-propanol
t-BuOH
H2O
90.9 (22.0)c
83.8 (21.3)c
28.5
84.5 (20.4)
79.1 (20.1)
7.0
93.0 (92.8)
94.4 (94.2)
24.6
6.2 (6.5)
4.2 (4.1)
1.9
0.5 (0.7)
1.2 (1.7)
64.9
0.0 (0.0)
0.0 (0.0)
8.6
7
8
TS-1
9
MeOH
EtOH
47.9
13.7
28.6
0.8
65.8
4.8
10
11
12
13
14
15
16
17
43.4
10.6
24.4
0.5
69.4
5.7
iso-propanol
t-BuOH
H2O
28.6
12.2
42.6
2.6
52.7
2.1
25.1
6.8
27.1
0.1
66.1
6.7
Ti-MWW
23.9
5.1
21.4
1.6
65.2
11.8
4.1
MeOH
EtOH
43.1
6.9
16.1
7.0
72.8
40.3
5.8
14.4
1.0
83.1
1.5
iso-propanol
t-BuOH
25.6
7.1
27.9
1.2
68.1
2.8
24.2
6.3
26.1
1.0
68.0
4.9
a
Reaction conditions: cat. (if), 0.1 g; solvent, 12 g; furfural, 20 mmol; H2O2 (31 wt. %), 25 mmol; NH3 (25 wt. %), 30 mmol; temp., 323 K; total time, 1 h. H2O2 added
dropwise at a constant rate within 1 h.
b Reaction conditions: NH2OH·HCl, 25 mmol; solvent, 12 g; furfural, 20 mmol; temp., 323 K; time, 1 h.
c The values in parentheses are results from studies at the reaction conditions as follows: Ti-MOR, 0.1 g; solvent, 12 g; furfural, 20 mmol; H2O2 (31 wt. %), 2.08 mmol;
NH3 (25 wt. %), 30 mmol; temp., 323 K; total time, 5 min. H2O2 added dropwise at a constant rate within 5min.
4 | J. Name., 2012, 00, 1-3
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conversion of 78.5 % along with a FO selectivity of 72.6 %.
While prolonging the feeding time of H2O2 periodically, both
the furfural conversion and the FO selectivity increased and
finally reached 98.6 % and 96.1 % at a feeding time of 1 h,
respectively. In our earlier work, it has been demonstrated
that the ammoximation of cyclohexanone over Ti-MWW
depended greatly on the method of adding substrates.27 We
believe that when H2O2 was charged into the reactor too
quickly the unreacted H2O2 would accelerate the consecutive
oxidation and decomposition of hydroxylamine to nitrogen
oxides before it participates the oximation of ketone to
corresponding oxime. Thus, a high conversion of ketone to
oxime is only achieved when H2O2 is fed slowly. In the case of
furfural ammoximation, the addition of H2O2 not only played a
significant role on the furfural conversion but also obviously
altered the product distribution.
Scheme 2. Possible reaction pathways in furfural ammoximation.
3.3.2. Effect of catalyst amount
Early studies have systematically investigated the Fig. S1C shows the effect of catalyst amount on the
ammoximation of ketones (e.g. cyclohexanone and methyl ammoximation of furfural over Ti-MOR. Reasonably, the
ethyl ketone) on TS-1, Ti-MWW and Ti-MOR.18, 19, 22, 26-28 These furfural conversion increased gradually with an increasing Ti-
catalysts have been proved to be effective and selective for MOR amount, and it reached the maximum of 98.8 % at 0.125
the ammoximation of these commonly-used ketones. For g catalyst loading. Additionally, the effect of catalyst amount
furfural, it shows rather high reactivity for imine formation and on the FO selectivity has also been investigated. As shown in
thus readily results in byproducts other than oxime. In this Fig. S2A, the FO selectivity increased with the increase of
regard, selective ammoximation relies on a good choice of catalyst amount. The above results imply the reaction should
titanosilicate to a huge extent.
be operated using a sufficient amount of catalyst to fasten the
main catalytic step of ammoximation while suppressing the
step of non-catalytic oxidation.
3.3.Effects of reaction parameters on the ammoximation over Ti-
MOR
3.3.3. Effect of reaction temperature
Our preliminary results have undoubtedly shown that Ti-MOR The effect of the reaction temperature on the performance of
serves as an appropriate and efficient catalyst for the selective furfural ammoximation was investigated in the temperature
formation of FO in the furfural ammoximation when H2O is range of 303 - 333 K (Fig. S1D). With increasing the reaction
chosen as a solvent. We next investigated the effect of temperature from 303 to 323 K, the furfural conversion
reaction parameters such as the reaction time, the adding increased and reached a maximum, and they remained almost
method of substrates, the catalyst amount, the reaction unchanged from 323 to 333 K. The byproducts of 2-furylamide
temperature and H2O2/furfural and NH3/furfural molar ratios, and 2-furoic acid were hardly detected, only a small amount of
on the ammoximation behaviour over Ti-MOR.
3.3.1. Effect of reaction time and adding method of substrates
2-furonitrile (< 3 %) was by-produced in the temperature range
of 323 - 343 K (Fig. S1D). The effect of reaction temperature on
the FO selectivity has also been shown in Fig. S2B. It can be
seen that the FO selectivity increased with the increase of
reaction temperature, indicating that the non-catalytic
oxidation could also be suppressed by increasing the reaction
temperature.
The effect of reaction time on the performance of furfural
ammoximation over Ti-MOR was investigated while H2O2 was
fed at once into the reactor. As shown in Fig. S1A, both the
furfural conversion and the FO selectivity increased with
prolonging the reaction time on stream, however, a large
amount of the byproducts of 2-furylamide and 2-furoic acid (>
20 %) were by-produced from the non-catalytic oxidation of
furfural due to the presence of excess free H2O2.
3.3.4. Effect of H2O2/furfural and NH3/furfural molar ratios
As shown in Fig. S1E, the effect of H2O2/furfural ratio on the
performance of furfural ammoximation was investigated in the
range from 1 to 2 while fixing the NH3/furfural molar ratio at
1.5, reaction temperature at 323 K and total reaction time at 1
Fig. S1B compares the results of furfural ammoximation
over Ti-MOR with various H2O2 feeding rate. When H2O2 was
fed at once into the reactor, the reaction gave a furfural
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h. The H2O2/furfural ratio was varied by changing the added other side reactions.32,
Journal Name
33
The non-catalytic oximation of
H2O2 amount. The stoichiometric furfural ammoximation ketone with hydroxylamine rate constant (k3) is independent
requires equal moles of H2O2 and furfural. However, when the of the type of titanosilicate used, but the hydroxylamine
reaction performed at a H2O2/furfural molar ratio of 1, the formation rate constant (k1) and the hydroxylamine
furfural conversion was only 90.7 % due to the unproductive decomposition rate constant (k2) relies closely on the catalytic
decomposition of H2O2. The furfural conversion increased to ability of the Ti species.27, 34
98.8 % at a H2O2/furfural ratio of 1.25, and almost leveled off
The kinetics was firstly studied for the formation of
at the ratios > 1.25. The FO selectivity reached a plateau (> hydroxylamine with H2O2 over Ti-MOR, TS-1 and Ti-MWW in
97 %) at H2O2/furfural ratios of 1 - 1.5. When further increasing the presence of water (Fig. S4). It is well reasonable to assume
the H2O2/furfural ratio, the FO selectivity decreased slightly that the hydroxylamine formation pathway proceeds always
due to the acceleration of the non-catalytic oxidation of along with that of the hydroxylamine decomposition. As
furfural to 2-furylamide and 2-furoic acid with the existence of shown in Fig. S4, the hydroxylamine yield over Ti-MOR is
excess amount of free H2O2 in the reaction system.
comparable to the added H2O2 amount, indicating that the
The effect of NH3/furfural ratio on the furfural decomposition of hydroxylamine was effectively retarded
ammoximation was also investigated while fixing the when H2O2 was supplied controllably under dropwise
H2O2/furfural ratio at 1.25 (Fig. S1F). The furfural conversion conditions. Thus, the reaction rate constant (k1) of Ti-MOR in
and turnover number (TON) per Ti site increased with the hydroxylamine formation determined by the CNH2OH versus
increasing the NH3/furfural ratio and reached 98.7 % and 510 reaction time is estimated to be 27.7 mmol L-1 min-1 (Table 3).
at 1.5, respectively, while the utilization efficiencies of H2O2 As the hydroxylamine yield over TS-1 or Ti-MWW is far below
and NH3 were always > 79.0 % and 66.7 %, respectively, along the added H2O2 amount, its reaction rate constant (k1) in the
with a FO selectivity of > 97.0 %. No further changes in either hydroxylamine formation could not be accurately determined.
furfural conversion or oxime selectivity were observed at As shown in Fig. S5, the kinetics were also studied for the
NH3/furfural ratios > 1.5. The high furfural conversion obtained decomposition of hydroxylamine (chloride salt) with H2O2 in
in an excessive amount of NH3 was probably due to a partial the presence of water. The reaction rate constant (k2) of TS-1
loss of NH3 via vaporization and the fact that a suitable basic in the decomposition of hydroxylamine is about 13.9 mmol L-1
environment benefits the ammoximation.
A
similar min-1, which is far below the reaction rate constant (k3) of non-
phenomenon has already been discovered in the catalytic furfural oximation with hydroxylamine chloride (69.1
ammoximation of other aldehydes or ketones over mmol L-1 min-1) (Fig. S6, Table 3). It means that the formed
titanosilicate catalysts.23, 24, 26, 27
hydroxylamine prefers to react with furfural rather than being
decomposed by H2O2 or TS-1/H2O2. Hence, the low activity of
TS-1 in the catalytic furfural ammoximation is possibly due to
the weak ability for the formation of hydroxylamine. With
respect to Ti-MWW, it showed the highest reaction rate
constant (k2) of 69.8 mmol L-1 min-1 in the decomposition of
hydroxylamine, which is comparable to the reaction rate
constant (k3) of the furfural ammoximation with
hydroxylamine (69.1 mmol L-1 min-1) (Fig. S6, Table 3). Thus,
the high hydroxylamine decomposition activity could account
for the low activity of Ti-MWW in the furfural ammoximation.
Based on the above results, we propose that the best catalytic
performance of Ti-MOR in the production of FO among these
titanosilicates is due to its high hydroxylamine formation
efficiency and low hydroxylamine decomposition rate.
3.4.Stability and regeneration of Ti-MOR
The stability and reusability of a heterogeneous catalyst was
the essential factor for actual application in industrial
processes. We have checked the stability and reusability of Ti-
MOR in the ammoximation of furfural (Fig. 3). The used Ti-
MOR was regenerated by washing with acetone and by further
calcination at 823 K for 6 h, and then it was subjected to
repeated ammoximation of furfural at a constant ratio of
catalyst-oxidant-NH3-solvent. After five recycle of reaction-
regeneration, the catalyst maintained high furfural conversion
and FO selectivity of 98.7 % and 97.3 %, respectively. ICP
analysis indicated that Ti leaching was within 5% after the
repeated cycling. Furthermore, no significant difference was
found in XRD patterns, N2 adsorption isotherm and UV-vis
spectra between the fresh Ti-MOR and the regenerated one
(Fig. S3), indicating that Ti-MOR is extremely stable and serves
as a reusable catalyst for the ammoximation of furfural.
In order to prove whether the above assumption works or
not for furfural ammoximation over Ti-MOR, we have studied
the dependence of FO yield and hydroxylamine yield as the
function of H2O2 amount added in the furfural ammoximation
and the hydroxylamine formation reaction, respectively. As
seen in Fig. 4, when the amount of H2O2 added was less than
10 mmol, the FO yield obtained in the furfural ammoximation
reaction well equalled to hydroxylamine yield in the
hydroxylamine formation reaction, both of which are
equivalent to the amount of H2O2 added. This result firmly
agrees with our previous finding that the crucial step in this
process is the catalysed formation of hydroxylamine on Ti-
MOR, while the next ammoximation step proceeds very fast.
We can safely conclude that FO is formed from the
3.5 Investigation into the reaction mechanism of furfural
ammoximation
According to the hydroxylamine route (Scheme 3), the furfural
ammoximation includes several reaction pathways: (1) the
hydroxylamine formation by the catalytic oxidation of
ammonia with H2O2 on the Ti sites inside the zeolite channels,
(2) subsequent non-catalytic oximation of furfural with
hydroxylamine intermediate to FO, (3) the nonproductive
oxidation of hydroxylamine by H2O2 on Ti sites to NOx, and (4)
6 | J. Name., 2012, 00, 1-3
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hydroxylamine route and the decomposition of hydroxylamine
could be effectively retarded at the initial reaction stage when
the amount of H2O2 fed dropwise was consumed off. Further
increasing the amount of H2O2, the FO yield and the
hydroxylamine formation reaction both increased but was
lower than the added H2O2 amount.
25
20
15
10
5
25
20
15
10
5
Furfural oxime
2-Furonitrile
2-Furamide
2-Furoic acid 100
Furfural conversion
100
80
60
40
20
0
80
60
40
20
0
25
0
0
5
10
15
20
H2O2 amount (mmol)
Fig. 4. Hydroxylamine yield in ammonia oxidation with H2O2 (left) and FO yield in
furfural ammoximation with NH3 and H2O2 (right) as the function of H2O2 amount
added. Hydroxylamine formation condition: Ti-MOR, 0.1 g; H2O, 12 g; NH3 (25
wt. %), 30 mmol; H2O2 (31 wt. %), 24 mmol; temp., 323 K; total time, 1 h. Furfural
ammoximation condition: Ti-MOR, 0.1 g; H2O, 12 g; furfural, 20 mmol; NH3 (25
wt. %), 30 mmol; H2O2 (31 wt. %), 24 mmol; temp., 323 K; total time, 1 h. H2O2
was dropwise added at a constant rate within 1 h.
0
5
0
1
2
3
4
Recycle number
Fig. 3. Changes of furfural conversion and products selectivity with the reaction-
regeneration cycles on Ti-MOR. Reaction conditions for the first run: Ti-MOR, 1.25 g;
H2O, 120 g; furfural, 0.2 mol; H2O2 (31 wt. %), 0.25 mol; NH3 (25 wt. %), 0.3 mol; temp.,
323 K; total time, 1 h. H2O2 was added within 1 h. The used catalyst was regenerated
after every catalytic run. All the catalytic runs proceed at a constant ratio of catalyst-
oxidant-NH3-solvent. Regeneration condition: washed with acetone and dried at 353 K
for 8 h, and further calcined at 823 K in air for 6 h.
100
80
60
40
20
0
a
b
c
Fig. 5. The decomposition of hydroxylamine in the presence of Ti-MOR but
without H2O2 (a), with H2O2 but without Ti-MOR (b), and in the presence of both
Ti-MOR and H2O2 (c). Reaction conditions: Ti-MOR (if added), 0.1 g; H2O2 (if
Scheme 3. The reaction steps involved in furfural ammoximation.
Table 3 The rate constants of reaction steps involved in the ammoximation.
added), 25 mmol; H2O, 12 g; NH2OH·HCl, 20 mmol; temp., 323 K; time, 1 h.
Catalyst
Reaction rate constant (mmol L-1 min-1)
In addition, the decomposition of hydroxylamine by
oxidation with H2O2 was further investigated by using a
common hydroxylamine derivative of hydroxylamine chloride
instead of free hydroxylamine that is commercially unavailable
and not easy for safe handling. Hydroxylamine hardly
decomposed over Ti-MOR while in the absence of H2O2 (Fig.
5a). For comparison, ca. 30% decomposition of H2O2 was found
in the presence of H2O2 without catalyst (Fig. 5b), indicating
that the decomposition took place partially by the non-
catalytic route. However, the Ti-MOR/H2O2 system showed a
much higher activity for hydroxylamine decomposition (74.4 %)
than the non-catalytic reaction (Fig. 5c). We surmise that Ti-
OOH species formed through interaction of H2O2 molecules
with Ti species in the framework may account for the above
result, as Ti-MOR solely is inactive in the hydroxylamine
decomposition when H2O2 is absent. This phenomenon well
explains that a lower yield of hydroxylamine is noticed than
that of FO at high H2O2 amount in Fig. 4. Therefore a relative
low H2O2 concentration benefits to the clean synthesis of FO in
this process.
a
b
c
k1
k2
k3
Ti-MOR
TS-1
27.7
31.3
13.9
69.8
69.1
69.1
69.1
d
-
d
Ti-MWW
-
a The formation of hydroxylamine. Reaction conditions: cat., 0.1 g; NH3, 30 mmol;
H2O2, 2.4 mmol; H2O, 12 g; temp., 323 K. H2O2 was dropwise added at a constant
rate within 6 min.
b
The decomposition of hydroxylamine. Reaction conditions: cat., 0.1 g;
NH2OH·HCl, 20 mmol; H2O2, 2.4 mmol; H2O, 12 g; temp., 323 K. H2O2 was
dropwise added at a constant rate within 6 min.
c The furfural oximation with hydroxylamine. Reaction conditions: NH2OH·HCl, 20
mmol; H2O, 12 g; temp., 323 K.
d Hardly detectable for TS-1 and Ti-MWW.
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Journal Name
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4. Conclusions
Clean synthesis of furfural oxime can be achieved by using
titanosilicate-catalyzed liquid-phase ammoximation. Ti-MOR is
capable of converting furfural to corresponding oxime at a
conversion and selectivity both above 97 % in aqueous
solution under optimized reaction conditions, and it proves to
be a stable and reusable catalyst. The hydroxylamine route
and the imine route are both found in the furfural
ammoximation, with the former one leading to formation of
furfural oxime while the latter one resulting in formation of 2-
furylamide and 2-furoic acid. The key role of Ti-MOR in this
process is that it efficiently catalyse the formation of
hydroxylamine, thereby greatly accelerates the hydroxylamine
route. Though the decomposition pathway of hydroxylamine
in H2O2 on Ti-MOR is inevitable, feeding H2O2 dropwise into
the reaction system can effectively retard this undesired step.
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Acknowledgements
The authors gratefully acknowledge financial support from the
NSFC of China (21533002, 21373089, 21603075), and China
Ministry of Science and Technology under contract of
2016YFA0202804.
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