Efficient synthesis of niobium pentoxide nanowires and application in ethanolysis of furfuryl alcohol

Article abstract of DOI:10.1039/d0ra00085j

Nb₂O₅ nanowires with high specific surface area and crystallinity were prepared by using ammonium oxalate and an acetic acid solvent system. The nanomaterial was applied in ethanolysis of furfuryl alcohol (FA), and the yield of the product, 2-(ethoxymethyl)furan (FEE), achieved was up to 79.6%. Compared to mesoporous Nb₂O₅ materials and other porous materials, the residence time of FEE on the surface of the catalyst is shorter, and the yield of ethyl levulinate (EL) is lower. Furthermore, a high temperature calcination treatment can change the acid sites and acidity type distribution on the nanowire surface. By XRD, NH₃-TPD, IR, and TG-DTA determination methods, it was found that the weak and medium-strong acid sites on the surface of Nb₂O₅ nanowires were reduced after a 300 °C treatment, and the amount of strong acid was relatively higher. According to the catalytic performance test data and acidity determination, it was concluded that more weak acid and medium-strong acid sites improve the conversion of furfuryl alcohol to FEE, and the strong acid sites promote further conversion of FEE to EL.

Full text of DOI:10.1039/d0ra00085j

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Efficient synthesis of niobium pentoxide nanowires
and application in ethanolysis of furfuryl alcohol†
Cite this: RSC Adv., 2020, 10, 5690
a
b
a
a
a
Zhenwei Zhang, * Peng Wang,
Zeying Wu, Chuanjun Yue, Xuejiao Wei,
a
a
a
Jiwei Zheng, Mei Xiang
and Baoliang Liu
2 5
Nb O nanowires with high specific surface area and crystallinity were prepared by using ammonium
oxalate and an acetic acid solvent system. The nanomaterial was applied in ethanolysis of furfuryl alcohol
FA), and the yield of the product, 2-(ethoxymethyl)furan (FEE), achieved was up to 79.6%. Compared to
(
2 5
mesoporous Nb O materials and other porous materials, the residence time of FEE on the surface of
the catalyst is shorter, and the yield of ethyl levulinate (EL) is lower. Furthermore, a high temperature
calcination treatment can change the acid sites and acidity type distribution on the nanowire surface. By
XRD, NH
3
-TPD, IR, and TG-DTA determination methods, it was found that the weak and medium-strong
Received 4th January 2020
Accepted 27th January 2020
ꢀ
acid sites on the surface of Nb
O
2 5
nanowires were reduced after a 300 C treatment, and the amount of
strong acid was relatively higher. According to the catalytic performance test data and acidity
determination, it was concluded that more weak acid and medium-strong acid sites improve the
conversion of furfuryl alcohol to FEE, and the strong acid sites promote further conversion of FEE to EL.
DOI: 10.1039/d0ra00085j
rsc.li/rsc-advances
chemicals. Utilization of sulfuric acid and aluminum chloride
in the early stages has not conformed with environmental
1
. Introduction
In recent decades, with the growth of the global economy, protection rules, and the quality requirements of the reaction
nations require a higher level of environmental protection. equipment are higher. There is a wide variety of catalysts
2
Under increasing environmental pressures, the transformation currently used in alcoholysis. Sulfates and uorosulfonic
3,4
of traditional petrochemical raw materials to fuels and other acids have high catalytic activity, especially the long-chain
ne chemicals faces enormous challenges, and the disparity levulinate, which can reach a yield of more than 90%, but the

between energy supply and demand is increasingly signicant. equipment is highly corrosive, and catalyst recovery is difficult,
Transformation of ne chemicals production is currently one of leading to solid waste salts. The ionic liquid catalysts can
the most popular topics in the chemical industry. Compared sufficiently mix with the reactants to promote the reaction
5
–8
with petrochemical resources, biomass resources possess lower efficiently, but the ionic liquid catalysts have low reusability.
1
pollution, wider distribution, and are more abundant. Furfuryl Catalysts such as acidic resin Amberlyst-15 (ref. 9) and sulfonic
10,11
alcohol (FA) prepared from biomass is an important chemical acid-modied organic porous silicon materials
have high
platform compound, which can be used to synthesize other alcoholysis reaction efficiency due to sulfate modication on
important energy products and bio-based chemicals. Deter- the surface, and the conversion of FA can reach up to 100%.
mining how to convert FA into target chemicals or energy Other catalysts used in alcoholysis include heteropoly acid-MOF
1
2
13,14
15
substitutes efficiently and ecologically is a big challenge all over materials, molecular sieves,
the world.
multi-layer kaolin, graphe-
16
17
nes, and noble metals. However, the sulfate group is easily
Alcoholysis of FA to furfuryl alcohol ethers and alkyl levuli- lost aer long reaction time. MOFs and noble metal catalysts
nates have been a popular topic in catalysis. FA ethers are have high activity and selectivity, but their preparation
important additives in fuels, and alkyl levulinates are important processes are complicated, costly, and the scale-up to produc-
organic intermediates that can convert into many other tion is difficult. The molecular sieves and basic layered
compound catalyst have high specic surface areas, but the
humins produced by the alcoholysis reaction of FA blocks the
pores, and the diffusion rate is limited. Additionally, the water
produced in the alcoholysis reaction causes damage to the
porous structure. Although the graphene catalyst has high
a
School of Chemical Engineering and Materials, Changzhou Institute of Technology,
2
13022, People's Republic of China. E-mail: qianyangrun@163.com; Fax: +86 25
3587190; Tel: +86 25 83587190
8
b
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of
Petrochemical Engineering, Changzhou University, Gehu Road 1, Changzhou, selectivity, there is still a long way to industrial preparation.
1
8
Jiangsu 213164, People's Republic of China. E-mail: pengwang@cczu.edu.cn
Metal oxide catalysts such TiO
,
Al
2
O
, etc. exhibit higher
2
3
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra00085j
1
activity in the preparation of FEE. In contrast, Nb is an
2 5
O
1
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1
9
ꢀ
important class of solid acid catalyst characterized by high treatment at 180 C for 25 h. A white powder was obtained, most
20
ꢀ
stability, high acid strength, and good water resistance that of the organic impurities were washed in hot water at 80 C.
can be applied in ammoxidation, dehydration,
tion reactions. However, the low specic surface area of Nb O
21
22,23
and oxida- Then the white powder was ltered and dried in an oven at
24
ꢀ
100 C for 12 h. The obtained catalysts were denoted as 100-
2
5
leads to low catalytic activity. Mesoporous Nb
contain a large amount of Lewis acid (LA) sites and Brønsted
acid (BA) sites, which can effectively catalyze the preparation of wNb was calcinated in a tube furnace under different temper-
2
O
5
nanomaterials wNb.
To change acidity and surface properties of nanowires, 100-
ꢀ ꢀ ꢀ ꢀ
5
-hydroxymethylfurfural, and the conversion of glucose to HMF atures: 200 C, 300 C, 400 C, and 500 C. The programming
25
ꢀ
À1
from LA acid sites is more effective than BA acid sites.
rate was 5 C min to the target temperature, and the total time
However, the porous structure is easily blocked by carbon was 5 h. The obtained catalysts were denoted as 200-wNb, 300-
deposits in the alcoholysis reaction, thereby decreasing catalyst wNb, 400-wNb, and 500-wNb, respectively.
activity. The layered structure of Nb O has better catalytic
2
5
2 5
activity than mesoporous Nb O and performed well in prepa-
2
.3 Catalytic performance
26
ration of lactic acid by sugar. Thus, nanowire, nanorod, or
2 5
nanosheet morphologies can be good alternatives for Nb O
The alcoholysis experiments were carried out in a closed,
pressure-controlled vessel under continuous stirring, using
27
material in the alcoholysis of FA. However, the large scale
preparation of Nb materials with special morphology needs
further optimization.
250 mg of catalyst in a solution of 1 ml FA and 10 ml ethanol.
2 5
O
ꢀ
Once the set temperature was attained (160 C, 0.8 MPa) aer 5–
8
5
min, time was reset to zero and the reactions proceeded for
h. Aer the reaction, the mixture was cooled to room
In this study, by improving the preparation method, the
Nb O nanowire material can be produced on a large scale by
2
5
temperature (30 min) and ltered with 0.22 mm syringe lters.
The ltrate was analysed for GC-FID. All results (conversion and
yield) are expressed as molar percentages.
For catalyst stability experiments, the rst run was per-
formed with fresh catalysts, and the reaction carried out in nine
parallel reactions. Aer the reaction, the catalyst was separated
via centrifugation, washed with ethanol (10 ml per every time)
three times, then water (10 ml per every time) three times, and
using acetic acid and ammonium oxalate as directing agents in
an aqueous solution without using template agents; the added
organic impurities can be easily removed. The preparation
method is simple, and the prepared material has a high specic
2 5
surface area. A Nb O nanowire can be used as a solid acid for
the ethanolysis reaction of FA. Its wire morphology can effec-
tively increase the diffusion rate of reactants and products and
improve the catalytic reaction efficiency. The nanomaterials are
also easily recycled. Further studies have shown that by
controlling the temperature of calcination treatment, the
ꢀ
then dried in an oven under 80 C overnight. Finally, the cata-
lysts were transferred back to the reactor starting a new catalytic
run under standard reaction conditions.
acidity and surface properties of Nb
2 5
O nanowires can be
regulated, thereby allowing for selectivity tuning of FEE and EL.
2.4 Characterization
The crystallographic information on the obtained Nb
2 5
O
2
. Experimental
nanocrystals was established by X-ray diffraction (XRD, using
nickel-ltered Cu Ka radiation). The Nb O nanocrystal
2
.1 Chemical materials
Niobium oxalate (C10
Guanggong Wenjiang Chemical Reagent Co. Ltd. Acetic acid
>99%), ammonium oxalate ((NH , >99%), furfuryl
2
5
morphology was examined using transmission electron
microscopy (TEM, JEM-2100) and scanning electron microscopy
5
H NbO20, >99%) was purchased from
(SEM, supra55). BET specic surface area was determined using
(
4 2 2 4
) C O
a micromeritics Gemini instrument (type ASAP2010C). X-ray
photoelectron spectroscopy (XPS) was operated using
a Thermo Scientic Escalab 250 instrument under ultrahigh
alcohol (FA, 98%), and ethyl levulinate (EL, 99%) were
purchased from ShangHai LingFeng Company. Deionized water
was made from OKP equipment (Shanghai Lakecore Company).
The niobium oxalate was puried before use, and 30 g
niobium oxalate was dissolved in 100 ml deionized water, then
À9
vacuum with a pressure near 2 Â 10 mbar, and the compo-
2 5
sition of w-Nb O was determined from a single sampling.
ꢀ
3 3
NH temperature-programmed desorption (NH -TPD): the
heated to 80 C for 1 h, and nally ltered while hot. The
sample was heat treated under a He atmosphere at 393 K for 1 h
and then cooled. Aer that, it was processed to adsorb NH3
molecules at 373 K for 1 h, then purged with He to remove
solution cooled to room temperature, and the white powder was
collected. Then, the puried powder was used to prepare
a solution using deionized water, approximately 10 g/100 ml.
3
excess and weakly adsorbed NH molecules at 373 K. The
temperature was programmed to 973 K, and the heating rate
2.2 Synthesis process
À1
was 10 K min
.
The preparation method of Nb
2
O
5
nanowires was carried out as
Pyridine adsorption Fourier-transform infrared spectroscopy
follows: 50 ml of the niobium oxalate solution (as described in (Py-IR): the samples were formed into pellets and placed in
Section 2.1) and 15 ml acetic acid were mixed together. 2.5 g of a vacuum cell. Aer activation by a high temperature vacuum,
ammonium oxalate solid powder was added into the solution the background spectroscopy was taken. Pyridine adsorbed
ꢀ
and was stirred for 3 h at 30 C. Then the solution was poured aer cooling down to room temperature, and the spectrum was
ꢀ
into a Teon-lined autoclave, which underwent hydrothermal collected aer purged pyridine molecules at 160 C. The
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adsorption and desorption of pyridine was observed to reect reaction, Nb
2
O
5
nanomaterials showed obvious (001) crystal
the B acid type and L acid type on the nanowire surface.
planes. Aer 15 hours, the (001) crystal plane peak intensity
The possible intermediates produced during the catalytic increased and aer 25 h, the (001), (100), and (002) crystal plane
processes were identied by a HP6890GC-5973MSD mass strengths were all enhanced. It shows the that size of Nb O5
2
spectrometer coupled with gas chromatography with a HP-5 nanowire crystals gradually increase with increasing time.
ꢀ
capillary column: the temperature of the injection was 280 C,
According to the XRD analysis and TEM and SEM results in
ꢀ
and the temperature of the detector was 250 C. The tempera- Fig. S2 and S3,† Nb
2
O
5
materials with different morphologies
ꢀ
ture of the column was maintained at 90 C for ve minutes and can be obtained under different hydrothermal times. In the
ꢀ
ꢀ
À1
raised to 200 C with a ramp rate of 10 C min . Every sample early 5 h, niobium oxalate decomposed to form a small particle
size injected to the GC and GC-MS units is 1 ml by Agilent morphology. Aer 10 h, the Nb O particles were assembled to
2
5
automatic liquid sampler.
form a curve-rod morphology. Aer 15 h, a linear morphology
The yield of ethyl levulinate and other products were was formed by curve-rods, and aer 25 h, the Nb O nanowires
2
5
analyzed with gas chromatography. A HaiXin gas chromato- gradually grew to large scales. The SEM (b) and TEM (c) images
graph was tted with a HP-5 capillary column and ame ioni- clearly show the linear 100-wNb with a width of about 10 nm. It
ꢀ
zation detector. The temperature of the injection was 280 C, has a large difference compared to the morphology reported in
ꢀ
and the temperature of detector was 250 C. Temperature of the the literature, and its aspect ratio is large. The amino group on
ꢀ
ꢀ
column was maintained two min at 70 C, and raised to 200 C the ammonium oxalate molecule in the solution acts as
ꢀ
À1
with a ramp rate of 10 C min . The content of each substance a structure agent, and Nb O can form a linear morphology.
2
5
was quantied by using the FA standard curve method, using However, there is no macromolecular compound in the solu-
À1
30
different concentrations: 2, 5, 10, 20, and 40 mg ml
.
tion, no barrier effect exists, so the nanowire is further grown
along the length by several micrometers or more.
The nanowire morphology makes the structure uffy and
increases the specic surface area of the material. As shown in
Fig. 1(d), the specic surface area of 100-wNb was about 257.03
3
. Results and discussion
3.1 Preparation and characterization of Nb O nanowire
2 5
2
À1
m
g
2
by N physical adsorption desorption determination.
The crystal phase of the obtained 100-wNb nanowire deter-
When the relative pressure is 0.1–0.4, the adsorption amount
rapidly increases, indicating the surface of the material has
a macroporous or slit porous structure. When above 0.4, the
adsorption curve tends to be gentle and the hysteresis loop is
extremely small, but the adsorption amount continues to
increase, indicating the material has a microporous structure.
XPS was used to detect elemental content and the valence
state of Nb and O in the nanowires. According to the results, the
atomic ratio of Nb to O is 15.20 : 38.02, which is close to the
chemical formula. The signal peaks of Nb3d are shown in
mined by XRD is shown in Fig. 1(a). 2q values of planes 22.6,
28.3, 46.1, and 55.2 can be indexed as the (001), (100), (002), and
(
(
102) crystal planes of Nb O , according to the standard card
JCPDS no. 18-0911). In Fig. 1(a), the (001), (002) peak inten-
2
5
28
sities are in accordance with the standard PDF card, and the
other peaks are weak, which shows that the compound has
29
a special morphology. No other impurity peaks were detected,
indicating the high purity of the sample. Fig. S1† shows the
Nb O crystal change by hydrothermal reaction time. Aer 5 h,
2
5
amorphous Nb O particles were primarily formed. Aer 10 h of
2
5
Fig. 1 Characterization of the catalyst 100-wNb. (a) XRD patterns and standard card, (b) SEM image (scale bar, 200 nm), (c) N
adsorption desorption (d) TEM image (scale bar, 100 nm). (e) Raman spectroscopy and (f) X-ray Photoelectron Spectroscopy.
2
physical
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+
Fig. 1(e), and the strong peaks at 207.0 eV and 209.7 eV are Nb
in accordance with the literature.
Temperature is one of the most important factors in the
chemical reaction. Fig. 3 shows the effect of different temper-
31
The surface properties and valence bonds of 100-wNb were atures on the activity and selectivity of 100-wNb in the etha-
ꢀ
analyzed by Raman spectroscopy. According to literature, peaks nolysis reaction. When the temperature was 100 C, the
À1
at 500–700 cm represent symmetric tensile vibration Nb–O, conversion of FA was only 10.7%. Aer the temperature was
À1
ꢀ
and the vibration peak at 900–1200 cm
stretching of the Nb]O bond. The highly distorted octahedral the yields of FEE and EL were 38.4% and 3.3%, respectively. The
structure, NbO in Nb , has Nb]O chemical bonds and is lower temperature makes the side reaction less, and the
is symmetric raised to 120 C, the conversion of FA improved to 46.3%, and
6
2 5
O
associated with Lewis acids. The lowly distorted octahedral dimmer FA has less content. When the temperature was raised
ꢀ
structures, NbO and NbO , have Nb–O chemical bonds asso- to 140 C, the conversion of FA increased to 95.8%, and the yield
7
8
32
ꢀ
ciated with Brønsted acids. According to the analysis of the of FEE was 80.0%. At 160 C, the yield of FEE decreased slightly
00-wNb material in Fig. 1(f), the Nb O nanowire has a strong to 79.6%, and the yield of EL increased, but side reactions
peak around 500–800 cm . It shows that there is more increased, increasing the yield of diFA at temperatures higher
1
2
5
À1
ꢀ
ꢀ
Brønsted acid and scheelite-like structure in the 100-wNb.
than 140 C. When the temperature was increased to 180 C, the
side reaction increased signicantly, and the yields of both FEE
and EL decreased.
Fig. 4 shows 100-wNb catalyzed FA ethanolysis changing with
time at 160 C. The FA conversion rate is fast at the beginning of
3.2 Catalytic assessment
ꢀ
The main products and their contents in the ethanolysis reac-
tion of FA by 100-wNb nanomaterials can be detected by a gas
chromatography mass spectrometer and gas chromatography.
The HP-5 column is a non-polar column used in gas chroma-
tography equipment, so the compounds determined by the FID
detector are as follows in Fig. S4:† 2-(ethoxymethyl)furan (FEE),
a-angelica lactone (FO), ethyl levulinate (EL), and di(furan-2-yl)
methane (diFA) are the four main products (reaction 1). Fig. 2
shows the mass spectrum of the main product, FEE. The
molecular ion peak value of 126.1 is consistent with its molec-
ular weight. The base peak value m/z is 81, which could indicate
a fragment of 2-methylfuran anion. The m/z value of the other
fragments can be presumed to be FEE molecular structure. The
other three mass spectrometry analyses are shown in Fig. S5–
S7† to conrm the correct structures of FO, EL, and diFA,
respectively.
the reaction. When the reaction was carried out for 3 h, the
conversion of FA reached 95.7%, and then the reaction rate
slowed. The yield of FEE increased gradually over the rst 3 h,
and the highest yield was 83.0%, aer which the content
decreased. The intermediate product, FO, can rapidly convert
13
into other products. The content of the intermediate is below
.0% during the reaction, and EL and diFA contents increase
with reaction time. The yield of EL aer ve hours is about
2.8%.
Fig. 5 shows the activity of different catalysts in the etha-
nolysis reaction of FA. Acidic Al has a low specic surface
3
1
2 3
O
area, low acid content, and does not provide sufficient acidic
sites in the reaction, so FA cannot efficiently convert in the
reaction. Thus the conversion rate of FA is only 9.4%. The
acidity strength of Ta O is stronger than that of Al O , so the
2 5 2 3
conversion of FA is 36.7%. In the phosphoric acid-modied
Ta O catalyzed reaction, the Ta O surface has more acidic
2
5
2 5
sites, making the FA conversion increase to 65.8%, and the FEE
yield to 30.6%. Compared with Ta , hydrated niobium oxide
2 5
O
25
prepared by the literature method has an amorphous
Fig. 3 Effect of temperature on conversion and product yields in
ethanolysis, by using 250 mg 100-wNb, solvent ethyl alcohol 10 ml, FA
1.00 ml, reaction time 5 h.
Fig. 2 Mass spectrometry analysis of FEE.
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Fig. 6 Ethanolysis of FA at different times over 100-wNb, by using
Fig. 4 Ethanolysis of FA at different times over 100-wNb, by using
ꢀ
ꢀ
250 mg mesoporous Nb O , at 160 C, solvent ethyl alcohol 10 ml, FA
250 mg 100-wNb, at 160 C, solvent ethyl alcohol 10 ml, FA 1.00 ml,
2
5
1
.00 ml, reaction time 5 h.
reaction time 5 h.
the conversion rate of FA is slower, also the rate of FEE forma-
tion slower. The yield of EL improved.
2 5
To study the effects of surface acid properties of Nb O in the
ethanolysis reaction, 100-wNb nanowires calcined under
different high temperatures were used for ethanolysis reaction
assessment. Results in Fig. 7 show that since the acidic site on
the surface of the nanowire decrease as the calcination
temperature increases, the activity decrease leads to a gradual
decrease in the conversion of FA. However, the yield of EL rst
increased, then decreased and the EL yield of 300-wNb was the
highest. The FEE yield gradually decreased to 41.0% when using
500-wNb.
2 5
To further study the effects of surface acidity of Nb O
nanowires in the catalytic reaction, a calcination treatment
method in different (high) temperatures changed the acid site
and acidity distribution of 100-wNb nanowire surface. It can be
Fig. 5 Catalytic activity studies of various catalysts towards ethanolysis
of FA, by using 250 mg catalyst, at 160 C, solvent ethyl alcohol 10 ml,
FA 1.00 ml, reaction time 5 h.
ꢀ
seen from the TG-DTA analysis (in Fig. 8(a)) that the mass loss of
ꢀ
1
00-wNb starts around 200 C on the thermogravimetric curve,
ꢀ
and there is no lost weight at 300 C. The total mass loss is
about 23.6%. The DTA curve gradually increased from room
structure containing more acid sites. The conversion of FA was
80.5%, and the yield of FEE was 61.6%.
HY-molecular sieve and mordenite (MOR) have a high
specic surface area and a porous structure and efficiently
catalyze the FA with conversion rates of 98.4% and 92.3%,
respectively. The pore size of HY is larger than that of MOR, and
the mass transfer rate of reactants and products is higher.
Therefore, the yields of FEE and EL are higher than MOR, which
are 49.8% and 12.7%, respectively. Both m-NbOPO
4
and m-
4
has more
Nb have a mesoporous structure, but m-NbOPO
2
O
5
acidic sites on the surface and enables the conversion of FA up
to 100%. Compared with mesoporous Nb O , mesoporous
2
5
NbOPO has a higher EL yield by 20.4%. The conversion rate of
4
w-Nb O is 100%. Since the nanowire has a linear morphology,
2
5
high specic surface area and more acidic sites, making the
mass transfer rate fast, the yield of FEE can reach 79.6%. The
FEE remains too long in the pores of mesoporous Nb
NbOPO materials, further producing FO and EL. As shown in
Fig. 6, the catalytic reaction rate of mesoporous Nb
changes with time. Compared with Nb O nanowires in Fig. 3,
2 5
O and
2 5
Fig. 7 Catalytic activity studies of Nb O nanowire calcined under
4
different temperatures towards ethanolysis of FA, by using 250 mg
ꢀ
2
O
5
at 160 C
ꢀ
catalyst, at 160 C, solvent ethyl alcohol 10 ml, FA 1.00 ml, reaction
2
5
time 5 h.
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and (b) in the spectrum all contain the vibration absorption
À1
peak of Nb–O at 564 cm and the vibration absorption peak of
À1
Nb–O–Nb at 808 cm . In Fig. 8(c), the bending vibration
characteristic absorption peaks of water molecules at
À1
À1
À1
1
622 cm , 1378 cm , and 1250 cm are the characteristic
absorption peaks of the organic matter contained on the Nb
2
O
5
.
À1
3
406 cm is the –OH absorption peak caused by bound water,
À1
and 3240 cm is the –OH absorption peak of the water content
in Nb
33
2
O
5
.
Curve (c) shows that the peaks of organic matter at
À1
À1
1378 cm
and 1250 cm
in the infrared spectrum aer
calcination at 573 K disappeared. It was shown that aer Nb O5
2
calcined at 573 K, the organic matter is signicantly removed to
form pure phase nano Nb
A NH temperature-programmed desorption curve can
determine the total surface acidity properties of Nb aer
2 5
O .
3
2 5
O
different temperatures during calcination, and the acid amount
decreased with increasing calcination temperature. In Fig. 8(d),
Fig. 8 Characterization of the catalysts: 100-wNb, 200-wNb, 300-
ꢀ
wNb, 400-wNb, 500-wNb. (a) and (b) XRD patterns SEM image (scale the desorption peak at 100–400 C is weak and medium-strong
ꢀ
34
bar, 200 nm), (c) Fourier transform infrared spectroscopy analysis (I
00-wNb, II 200-wNb, III 300-wNb, IV 400-wNb, V 500-wNb), (d)
NH temperature programmed desorption (NH -TPD).
acid, and the desorption peak at 500–600 C is strong acid.
00-wNb has the largest acid amount, mainly weak acid and
1
1
3
3
medium-strong acid, and the selectivity of FEE is higher in the
catalytic reaction. However, as the calcination temperature
2 5
increases, the dehydration of Nb O causes the total acid
temperature and no exothermic peaks appeared. The results
show that the Nb O nanowire has no crystal phase transfer
amount to decrease. However, 300-wNb has the highest strong
2
5
ꢀ
acid desorption peak at 500–600 C, and the material has the
during high temperature calcination. From the XRD patterns (in
Fig. 8(b)) of ve samples: 100-wNb, 200-wNb, 300-wNb, 400-
wNb, and 500-wNb, it can be seen that as the temperature rises,
the peak intensities of the (001) and (002) crystal planes change
little and the improvement of crystallinity is not obvious. The
data proves no crystalline phase transformation, but the
morphological structures are similar. The cause of thermal
weight loss is desorption of hydroxyl groups on the surface and
decomposition of a small amount of organic matters. Aer
highest yield of EL in the ethanolysis reaction. According to the
pyridine infrared data analysis (in Fig. S8†), 100-wNb has
À1
a strong absorption peak at 1420–1440 cm , and there is
a larger amount of Brønsted acid. The peak of 300-wNb at
À1
À1
1
470 cm is stronger than 1440 cm , indicating more Lewis
35
2 5
acid sites on the Nb O than Brønsted acid. Therefore,
according to the data, in the ethanolysis reaction the weak acid
and the medium-strong acid are favorable for the formation of
FEE, and the strong acid can further catalyze the ring opening of
the intermediate product FEE to form EL.
Catalyst stability is an important indicator in measuring the
catalytic reaction. Fig. 9 shows the catalytic stability of 100-wNb.
Aer it was recycled eight times, the conversion rate of FA is still
high, only declining by 2.0%. The FEE yield decreased from the
initial 79.6% to 70.3% due to a certain amount of humin
ꢀ
a 300 C calcination treatment, there is almost no dehydration
on the surface of the Nb O material, and more Lewis acid sites
2 5
are exposed on the surface of the nanowire.
Fig. 8(c) shows infrared spectra of a Nb
2 5
O nanowire ob-
tained aer different temperatures of calcination. The curves (a)
adsorption on Nb
2 5
O during the recycle process, but the total
catalytic activity and selectivity remain essentially unchanged.
4. Conclusions
Acetic acid and ammonium oxalate form a successful hydro-
thermal system for large scale synthesis of Nb nanowire
2 5
O
materials. The advantage of this method is that no templating
agent is needed, and most of the organic impurities can be
removed easily via washing, to retain high quality Nb O . The
2 5
material has excellent activity in the ethanolysis reaction of FA.
Compared with microporous molecular sieves and mesoporous
materials, the morphology of Nb O nanowires reduces the
2
5
residence time of FEE on its surface and improves its catalytic
selectivity. Characterization analysis showed that the weak acid
Fig. 9 Reusability of 100-wNb towards ethanolysis of FA, by using
ꢀ
250 mg catalyst, at 160 C, solvent ethyl alcohol 10 ml, FA 1.00 ml,
reaction time 5 h.
2 5
and medium-strong acid sites on the surface of Nb O
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promoted efficient formation of FEE, and the strong acid site 14 P. Neves, S. Lima, M. Pillinger, S. M. Rocha, J. Rocha and
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Conflicts of interest
There are no conicts to declare.
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China (21802008), the Nature Science Foundation of the Jiangsu
Higher Education Institutions of China (19KJB530004), 2018
Changzhou Institute of Technology School Support Program for
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