Niobic acid nanosheets synthesized by a simple hydrothermal method as efficient Bronsted acid catalysts

Article abstract of DOI:10.1021/cm400192q

This paper reports a novel bottom-up hydrothermal route for the synthesis of niobic acid nanosheets. This route is simpler and greener than the conventional top-down and multistep route for the synthesis of hydrated metal oxide nanosheets via exfoliation of layered compounds, which typically requires the use of bulky organic cations. We have clarified that the pH of the suspension for hydrothermal treatment, the hydrothermal temperature and time, and the presence of NH₄⁺ play roles in determining the morphology of the product. We propose that the nanosheet is formed from amorphous niobic acid nanoparticles through a dissolution-crystallization mechanism. The obtained niobic acid nanosheets are uniform with a thickness of ～2 nm and uniquely possess mainly Bronsted acid sites. As compared to the conventional amorphous niobic acid and several other typical solid acid catalysts, the niobic acid nanosheet synthesized by our bottom-up method exhibits significantly higher activity and selectivity for the Friedel-Crafts alkylation of anisole with benzyl alcohol. We have further demonstrated that our niobic acid nanosheet is a water-tolerant and efficient catalyst for the hydrolysis of inulin, a polysaccharide-based biomass, into fructose.

Full text of DOI:10.1021/cm400192q

Subscriber access provided by UNIV OF SOUTHERN CALIFORNIA
Article
Niobic Acid Nanosheets Synthesized by a Simple
Hydrothermal Method as Efficient Brønsted Acid Catalysts
Wenqing Fan, Qinghong Zhang, Weiping Deng, and Ye Wang
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm400192q • Publication Date (Web): 28 Jul 2013
Downloaded from http://pubs.acs.org on August 1, 2013
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth
Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 14
Chemistry of Materials
1
2
3
4
5
6
7
Niobic Acid Nanosheets Synthesized by a Simple Hydrother-
mal Method as Efficient Brønsted Acid Catalysts
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Wenqing Fan, Qinghong Zhang,* Weiping Deng, Ye Wang*
State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Maꢀ
terials, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Department of
Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
ABSTRACT: This paper reports a novel bottomꢀup hydrothermal route for the synthesis of niobic acid nanosheets. This route is
simpler and greener than the conventional topꢀdown and multiꢀstep route for the synthesis of hydrated metal oxide nanosheets via
exfoliation of layered compounds, which typically requires the use of bulky organic cations. We have clarified that the pH of the
suspension for hydrothermal treatment, the hydrothermal temperature and time, and the presence of NH4+ play roles in determining
the morphology of the product. We propose that the nanosheet is formed from amorphous niobic acid nanoparticles through a disꢀ
solutionꢀcrystallization mechanism. The obtained niobic acid nanosheets are uniform with a thickness of ~2 nm and uniquely posꢀ
sess mainly Brønsted acid sites. As compared to the conventional amorphous niobic acid and several other typical solid acid cataꢀ
lysts, the niobic acid nanosheet synthesized by our bottomꢀup method exhibits significantly higher activity and selectivity for the
FriedelꢀCrafts alkylation of anisole with benzyl alcohol. We have further demonstrated that our niobic acid nanosheet is a waterꢀ
tolerant and efficient catalyst for the hydrolysis of inulin, a polysaccharideꢀbased biomass, into fructose.
KEYWORDS: nanosheet; niobic acid; hydrothermal synthesis; Brønsted acid; green chemistry
Nb2O5ꢁnH2O is complicated, and this makes it difficult to furꢀ
ther tune the acidity by structure manipulation.
INTRODUCTION
The development of new solidꢀacid catalytic materials to reꢀ
place the liquid acids has attracted much attention in both
green chemistry and materials chemistry.1ꢀ3 Moreover, solid
acid catalysts have recently been expected to play crucial roles
in the development of new green chemical processes such as
the conversion of renewable biomass into chemicals or fuels.4
For this purpose, solid acid catalysts are usually required to be
capable of working in water medium under hydrothermal conꢀ
ditions.4 Niobic acid, also known as hydrated niobium oxide
(denoted as Nb2O5ꢁnH2O), is a unique waterꢀtolerant solid
acid.1,5 While most of the solid acid metal oxides would lose
acidity in the presence of water, Nb2O5ꢁnH2O can function
well in water medium at least under temperatures ≤ 373 K.1,5
In addition to functioning as an efficient acid catalyst for many
reactions including hydration, dehydration, and hydrolysis in
water medium,5ꢀ7 Nb2O5ꢁnH2O could also work in the presence
of various liquid media with different polarities and proticities
such as decane, cyclohexane, toluene, methanol, and isoproꢀ
panol.6
However, the nature of the acid sites on Nb2O5ꢁnH2O is not
fully understood yet. Both Brønsted and Lewis acid sites are
known to coꢀexist on Nb2O5ꢁnH2O, and these sites likely corꢀ
respond to the OH groups associated with highly polarized
NbO6 octahedra and the NbO4 tetrahedra.5,8 Nb2O5ꢁnH2O genꢀ
erally functions as an acid catalyst in amorphous state, and the
crystalline Nb2O5 after complete dehydration only possesses
very weak Lewis acidity. The structure of amorphous
Nanosheets represent a new form of materials, and the high
anisotropy of nanosheets with ultraꢀthin thickness can provide
unique physicochemical properties.9 Recent studies demonꢀ
strated that some nanosheets of composite transition metal
oxides or hydrated metal oxides with bridged OH groups such
as HTiNbO5, HNbWO6, HNbMoO6, HTaWO6, and HNb3O8
functioned as efficient solid acid catalysts for FriedelꢀCrafts
alkylation, esterification, hydrolysis, and dehydration.10ꢀ13 The
twoꢀdimensional structure of nanosheets can not only enhance
the accessibility of bulky reactant molecules to almost all the
acid sites, increasing the catalytic performance, but also may
afford acid sites with unique properties owing to the anisoꢀ
tropic feature. The nanosheets of Nbꢀbased composite metal
oxides have been prepared mainly by a topꢀdown route, which
is a multiꢀstep process via exfoliation of lamellar compounds
and typically includes: (1) the synthesis of the parent lamellar
compounds by a conventional solidꢀstate reaction at high temꢀ
peratures, (2) the protonation of the layered compounds in
acidic solution, (3) the addition of bulky organic cations (e.g.,
tetrabutylammonium cations) to expand the interlayer spaces
and to cause exfoliation, and (4) the addition of acid solution
to the nanosheet solution to form aggregated nanosheets.9ꢀ13
This complicated route is not environmentally benign and not
costꢀeffective because of the use of expensive organic comꢀ
pounds. It is highly desirable to develop a simpler, cheaper,
and greener route for the synthesis of nanosheet materials.
ACS Paragon Plus Environment

Chemistry of Materials
Page 2 of 14
1
2
3
4
5
6
7
8
In the present paper, we report a novel bottomꢀup route for
quartz reactor was exposed to NH3 for adsorption at 373 K for
the synthesis of niobic acid nanosheets (denoted as
Nb2O5ꢁxH2O) by a simple hydrothermal method from NbCl5
without using expensive bulky quarternary alkyl ammonium
cations. The structure and acidity of the obtained Nb2O5ꢁxH2O
nanosheets have been characterized by various techniques.
This paper will show that our Nb2O5ꢁxH2O nanosheets with
thickness of ~2 nm display a unique acid property, possessing
mainly Brønsted acid sites. Furthermore, we will demonstrate
that the Nb2O5ꢁxH2O nanosheet is a superior catalyst for the
FriedelꢀCrafts alkylation of anisole with benzyl alcohol, the
hydrolysis of ethyl acetate, and the hydrolysis of inulin, which
is a kind of inedible polysaccharideꢀbased biomass, into frucꢀ
tose.
1 h after the pretreatment at 393 K for 1 h in He gas flow. The
gas phase or the weakly adsorbed NH3 was then purged by
highꢀpurity He at 373 K. NH3ꢀTPD was performed in the He
flow by raising the temperature to 900 K at a rate of 10 K
min−1, and the desorbed NH3 molecules were detected with a
ThermoStar GSD 301 T2 mass spectrometer by monitoring the
signal of m/e = 16. The amount of NH3 adsorbed on each cataꢀ
lyst was evaluated by quantifying the peaks of the desorbed
NH3 molecules. Fourierꢀtransform infrared (FTꢀIR) studies of
adsorbed pyridine were performed with a Nicolet 6700 inꢀ
strument equipped with an MCT detector. The sample was
pressed into a selfꢀsupported wafer and was placed in an in
situ IR cell. After pretreatment under vacuum at 423 K for 1 h,
the sample was cooled down to 323 K. Then, pyridine was
adsorbed onto the sample for a sufficient time at the same
temperature. FTꢀIR spectra were recorded after the gaseous or
weakly adsorbed pyridine molecules were removed by evacuaꢀ
tion at 323 K. 1H magicꢀangle spinning nuclear magnetic resoꢀ
nance (MAS NMR) spectra were measured at room temperaꢀ
ture using a Bruker AVIIIꢀ400M spectrometer.
Catalytic Reaction. The catalyst was evacuated at 353 K
for 1 h prior to catalytic reactions. The FriedelꢀCrafts alkylaꢀ
tion was performed in a batchꢀtype reactor. Typically, 0.10 g
of catalyst was added into a reaction vessel preꢀcharged with
benzyl alcohol (10 mmol) and anisole (100 mmol). After the
air in the reactor was replaced by N2, the reactor was placed in
an oil bath and was heated to the reaction temperature (typicalꢀ
ly 373 K) in about 10 min. We confirmed that the conversion
of benzyl alcohol was <2 % even for the most active catalyst
during the heatingꢀup interim. The reaction was started by
vigorous stirring and the reaction temperature was kept for a
fixed time (typically 2 h). The products including benzyl aniꢀ
sole and dibenzyl ether were quantified by a gas chromatogꢀ
raphy (Shimadzu GCꢀ14B) equipped with a RTXꢀ1 capillary
column and an FID detector. We employed nꢀdecane as an
internal standard.
The hydrolysis of ethyl acetate was performed in a batchꢀ
type reactor. Typically, the catalyst (0.20 g) was added into the
reactor preꢀcharged with ethyl acetate (15 mmol) and deionꢀ
ized water (10 mL). The temperature was increased to 343 K
in less than 10 min, and no conversion of ethyl acetate was
detected during the heatingꢀup interim. The reaction was perꢀ
formed at 343 K with vigorous stirring for 3 h. After the reacꢀ
tion, the products were analyzed by highꢀperformance liquid
chromatography (HPLC, Shimazu LCꢀ20A) equipped with a
RI detector and a Shodex SUGARSHꢀ1011 column (8×300
mm) using deionized water as a mobile phase.
The conversion of inulin was performed in a Teflonꢀlined
stainlessꢀsteel autoclave. Typically, the catalyst (0.10 g) was
added into the reactor preꢀcharged with inulin purchased from
Alfa Aesar (0.10 g) and distilled H2O (10 mL). The air in the
reactor was purged by Ar for several times. After the introducꢀ
tion of Ar with a pressure of 0.5 MPa, the reactor was placed
in an oil bath, and was heated to 363 K in 10 min. Almost no
reaction occurred during the heatingꢀup interim. The reaction
was initiated by vigorous stirring. After 2 h, the reaction was
quickly terminated by cooling the reactor to room temperature
in cold water. The liquid products were analyzed by HPLC
(Shimazu LCꢀ20A) equipped with a RI detector and a Shodex
SUGARSHꢀ1011 column (8×300 mm) using dilute H2SO4
aqueous solution as a mobile phase.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
EXPERIMENTAL SECTION
Synthesis of Nb2O5·xH2O Nanosheets. The Nb2O5ꢁxH2O
nanosheets were synthesized by a hydrothermal method. Typiꢀ
cally, NbCl5 (1.0 g) was first dissolved in ethanol (10 mL).
Then, an NH3ꢁH2O aqueous solution (4 wt%, 50 mL) was addꢀ
ed into the NbCl5 ethanol solution under continuous stirring.
After the mixture was aged for 2 h at ambient temperature, the
precipitates were separated by centrifugation and were transꢀ
ferred to a Teflonꢀlined autoclave (100 mL) preꢀcharged with
distilled water (50 mL). The pH value of the suspension was
about 8.0 because of the NH3ꢁH2O remaining on the precipiꢀ
tate. The hydrothermal treatment was performed typically unꢀ
der 513 K for 24 h. After the hydrothermal treatment, the solid
product was recovered by centrifugation, washed repeatedly
by distilled water, and finally dried at 353 K under vacuum for
10 h.
The amorphous Nb2O5ꢁnH2O was synthesized by the hyꢀ
drolysis of NbCl5 as typically adopted in the literature.8 In
brief, NbCl5 was added into distilled water, and the mixture
was stirred at ambient temperature for 3 h. The resulting preꢀ
cipitate was washed with distilled water until the filtrate beꢀ
came neutral, followed by drying at 353 K under vacuum for
10 h to obtain the amorphous Nb2O5ꢁnH2O powders.
Materials Characterization. Xꢀray diffraction (XRD)
measurements were carried out on a Panalytical X’pert Pro
Super Xꢀray diffractometer with CuꢀKα radiation (40 kV and
30 mA). Nitrogen physisorption at 77 K was performed with a
Micromeritics Tristar 3000 Surface Area and Porosimetry
analyzer to examine the surface areas of samples. Thermogravꢀ
imetric (TG) analysis was carried out using a SDT Q600 analꢀ
ysis instrument (TA Instruments). Scanning electron microsꢀ
copy (SEM) was carried out using a Sꢀ4800 or LEO1530
scanning electron microscope with 10 kV or 20 kV acceleratꢀ
ing voltage. Transmission electron microscopy (TEM) measꢀ
urements were performed on a JEMꢀ2100 electron microscope
operated at an acceleration voltage of 200 kV. Samples for
TEM measurements were suspended in ethanol and dispersed
ultrasonically. Drops of suspensions were applied on a copper
grid coated with carbon. Atomic force microscopy (AFM) was
performed with an Agilent 5500 ILM. Raman spectroscopic
measurements were carried out with a Renishaw Inva Raman
System 1000. A laser output of 40 mW was used, and the
maximum incident power at the sample was approximately 4
mW in each measurement.
NH3 temperatureꢀprogrammed desorption (NH3ꢀTPD) was
performed on a Micromeritics AutoChem 2920 II instrument
to measure the acidity. Typically, the sample loaded in a
2
ACS Paragon Plus Environment

Page 3 of 14
Chemistry of Materials
1
2
3
4
5
6
7
8
9
We performed the same experiment at least in triplicate for
temperature rose to 483 K. The combination of these observaꢀ
tions (Figure 3) with the SEM results (Figure 2) suggests that
the diffraction peaks at 2θ of ~6°, ~25°, and ~52° correspond
to the Nb2O5ꢁxH2O sample with the sheetꢀlike morphology.
These peaks cannot be ascribed to any crystalline phase of
dehydrated Nb2O5, suggesting that the Nb2O5ꢁxH2O nanosheets
are not composed of dehydrated Nb2O5 crystallites. The difꢀ
fraction peak at 2θ of ~6° reveals a d value of ~14.5 Å, which
is characteristic of layered or sheetꢀlike structures. H3Nb3O9 or
HNb3O8ꢁH2O, a layered Nb compound, showed main XRD
peaks at 2θ of ~8°, ~12°, and ~25°, corresponding to (020),
(120) and (240) reflections, respectively.14 This compound
could be exfoliated into nanosheets.10,12,13 The single
nanosheet may not possess distinct XRD peaks because the
sheets are not stacked in parallel to induce the interference of
Xꢀrays, but the increase in the thickness of the nanosheet can
cause weaker XRD peaks.9 Actually, weak XRD peaks at 2θ
of 6ꢀ7° and ~25° were reported for HNb3O8ꢁxH2O nanosheet
aggregates, which were prepared by the exfoliation of layered
HNb3O8ꢁH2O.10 Thus, we speculate that our Nb2O5ꢁxH2O
nanosheet may resemble the HNb3O8ꢁH2O nanosheet in strucꢀ
ture prepared by the exfoliation method.
We have measured the content of H2O in each sample by the
TG analysis. As shown in Figure S1 in the Supporting Inforꢀ
mation, gradual decreases in the weight with increasing temꢀ
perature indicate the gradual loss of water during the heating
of the sample. A bigger decrease in the weight up to ~700 K
was observed for the amorphous Nb2O5ꢁnH2O, and the n value
in this sample was evaluated to be 2.5. On the other hand, for
the Nb2O5ꢁxH2O samples prepared by the hydrothermal treatꢀ
ment, the loss of H2O was relatively smaller and proceeded via
two steps; the first step was from ~300 to ~473 K, while the
second step was from ~473 to ~700 K. We quantified the x
value in the Nb2O5ꢁxH2O samples prepared by the hydrotherꢀ
mal method. With prolonging the hydrothermal time at a fixed
temperature of 513 K or increasing the hydrothermal temperaꢀ
ture at a fixed time of 24 h, x value decreased gradually to 1.1
(Table 1). Thus, the composition of the nanosheet prepared by
the hydrothermal treatment at 513 K for 24 h was
Nb2O5ꢁ1.1H2O. The molar ratios of Nb/H and Nb/O in this
Nb2O5ꢁxH2O nanosheet were quite close to those in the
HNb3O8ꢁH2O. Because of the twoꢀstep feature of weight loss,
we further divided the x value in each Nb2O5ꢁxH2O sample
into two values, i.e., x1 and x2, which corresponded to the
amounts of H2O lost in the lower and the higher temperature
regions, respectively. The result summarized in Table 1 shows
that the x1 and x2 values in the Nb2O5ꢁxH2O sample after the
hydrothermal treatment at 513 K for 24 h were 0.34 and 0.75,
respectively.
each run of catalytic reactions, and the relative errors were
<5% for both reactant conversions and product selectivities.
We performed the repeated uses of the Nb2O5ꢁxH2O nanosheet
and Amberlystꢀ15 catalysts for the hydrolysis of inulin. The
used catalyst was first separated through centrifugation from
the reaction mixture after the reaction. Then, the separated
catalyst was washed thoroughly with distilled water and dried
at 353 K under vacuum for 10 h before being used for the next
reaction cycle.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
RESULTS AND DISCUSSION
Formation of Nb2O5·xH2O Nanosheets. We found that niꢀ
obic acid nanosheets (Nb2O5ꢁxH2O) could be synthesized by
simply treating the precursor, which was obtained from the
hydrolysis of NbCl5 dissolved in ethanol solution with
NH3ꢁH2O, under hydrothermal conditions. The SEM measꢀ
urement clearly shows the uniform sheetꢀlike morphology of
the Nb2O5ꢁxH2O sample obtained after the hydrothermal
treatment at 513 K for 24 h, although the arrangement of the
nanosheets lacks regularity (Figure 1). On the other hand, the
Nb2O5ꢁnH2O, which was synthesized by the conventional hyꢀ
drolysis of NbCl5,8 was composed of the aggregates of small
particles.
We have clarified that the hydrothermal conditions signifiꢀ
cantly affect the morphology of the obtained sample. Figure 2
shows the SEM images of niobic acids after hydrothermal
treatment at 513 K for different times. The sample synthesized
by the same procedure but without hydrothermal treatment
(time = 0) resembled the conventional Nb2O5ꢁnH2O in morꢀ
phology, composed of aggregates of small particles (Figure
2a). When the hydrothermal time increased from 0 to 12 h, the
morphology changed gradually from the aggregates of partiꢀ
cles to nanosheets. Perfect sheetꢀlike morphology was obꢀ
tained after 12 h of hydrothermal synthesis at 513 K, and there
was no significant change in morphology with further proꢀ
longing the hydrothermal time. The dependence of the morꢀ
phology of samples on hydrothermal temperature is also disꢀ
played in Figure 2. At lower hydrothermal temperatures, i.e.,
423 and 453 K, the sample remained aggregates of particles in
morphology, and a higher temperature (≥ 483 K) was required
for the formation of nanosheets. These observations clearly
demonstrate the evolution of the morphology of niobic acid
from aggregates of nanoparticles to nanosheets by increasing
the hydrothermal temperature or prolonging the hydrothermal
time.
The effect of the hydrothermal treatment on the crystalline
structure of niobic acid has been investigated by XRD. Only a
very broad diffraction peak at 2θ of ~29° was observed for the
Nb2O5ꢁnH2O prepared by the hydrolysis of NbCl5, confirming
the amorphous feature of the conventional Nb2O5ꢁnH2O (Figꢀ
ure 3, curve a). For the niobic acid samples synthesized by the
hydrothermal treatment at 513 K for a shorter time (≤ 1 h)
(Figure 3, curves b and c) or at a lower temperature (≤ 453 K)
(Figure 3, curves h and i), also only the broad peak at 2θ of
~29° was observed. The increase in the hydrothermal time to 3
h at 513 K resulted in the appearance of three distinct diffracꢀ
tion peaks at 2θ of ~6°, ~25°, and ~52°. The intensity of these
peaks increased with the hydrothermal time and became alꢀ
most saturated when the hydrothermal time reached 24 h. At a
fixed hydrothermal time of 24 h, the diffraction peaks at 2θ of
~6°, ~25°, and ~52° began to appear when the hydrothermal
To gain more information on the hydrothermal process, we
further investigated the effect of pH value of the suspension
for hydrothermal treatment. Under the standard conditions, the
pH value of the suspension was 8 because of the NH3ꢁH2O
remaining over the precipitate. We regulated the pH value of
the suspension by adding HCl or further adding NH3ꢁH2O into
the suspension. SEM measurements for the samples obtained
by regulating the pH values of the suspensions to 6 and 7, i.e.,
acidic and neutral conditions, showed nanoꢀpillar and nanoꢀ
rod morphologies, respectively (Figures 4a and 4b). XRD patꢀ
terns of these two samples showed diffraction peaks at 2θ of
22.5°, 28.5°, 36.5°, 45.9°, 50.5°, 54.9°, 55.9°, 58.9° and 63.8°
(Figure 5), and these peaks could be attributed to crystalline
Nb2O5 with hexagonal structure (TꢀNb2O5, Powder Diffraction
3
ACS Paragon Plus Environment

Chemistry of Materials
Page 4 of 14
1
2
3
4
5
6
7
8
File No. 00ꢀ027ꢀ1312). On the other hand, the samples obꢀ
forming twoꢀdimensional structure of niobic acid. It is worth
tained at pH values of 10 and 12, i.e., stronger alkaline condiꢀ
tions, were thicker plates in morphology (Figures 4c and 4d).
XRD patterns for these two samples showed the appearance of
several sharp diffraction peaks besides the broad peaks beꢀ
longing to the nanosheets prepared at a pH value of 8 (standꢀ
ard conditions). These sharp diffraction peaks at 2θ degrees of
~8, 12.9, 16.5, 23.9, 24.8, 25.8, 23.6, 31.2, 32.8 and 47.9 were
attributable to the orthorhombic H3Nb3O9 or (H3O)Nb3O8
(Powder Diffraction File No. 00ꢀ0044ꢀ0672), which was a
layered compound.14,15 Thus, stronger alkaline conditions led
to the formation of thicker plateꢀlike layered compounds. It
should be mentioned that the exfoliation of this layered comꢀ
pound could also afford niobic acid nanosheets but the proceꢀ
dure was more complicated and bulky organic amines were
required.12,13 It is of interest that the sheetꢀlike material or the
layered compound can be obtained by simply adjusting the pH
of the suspension used for hydrothermal treatment.
mentioning that the use of bulky ammonium cations can lead
to the exfoliation of layered compounds such as H3Nb3O9 to
form niobic acid nanosheets in the topꢀdown route. It is of
interest that the organic ammonium cations can also function
for the formation of niobic acid nanosheets in our bottomꢀup
+
route. More significantly, even cheaper NH4 can induce the
assembly of nanosheet structure under our hydrothermal
treatments.
9
Structure of Nb2O5·xH2O Nanosheets. Figure 8 shows the
TEM images of the Nb2O5ꢁxH2O nanosheets synthesized by
the hydrothermal treatment at 513 K for 24 h. The faint conꢀ
trast of the sample lying on the grid (top view) confirms the
ultraꢀthin feature of the Nb2O5ꢁxH2O nanosheet (Figure 8a).
From the topꢀview TEM images, we have estimated the lateral
dimensions by counting ~100 nanosheets. The result shows
that the average lateral size of the Nb2O5ꢁxH2O nanosheets is
130 nm (see Figure S2 in the Supporting Information).
The sideꢀview TEM image of the Nb2O5ꢁxH2O sample (Figꢀ
ure 8b) suggested that our sample was not a complete monoꢀ
layer but was composed of a few layers of single sheets. The
distance between single sheets was evaluated to be 14.6 Å
(Figure 8c), which was close to that estimated from XRD (2θ
≈ 6°). We could see the crystalline fringe from the high resoluꢀ
tion TEM (HRTEM) image of a nanosheet in the center part
(Figure 8d), suggesting the crystalline feature of the nanosheet.
A lattice spacing of 3.6 Å was obtained (Figure 8e). The edge
part of the nanosheet was less crystalline than the center part
(Figure 8f). We speculate that this may imply the growth of
the crystalline nanosheet from the center towards the edge part.
To gain quantitative information about the thickness of the
Nb2O5ꢁxH2O nanosheet, we have performed AFM measureꢀ
ments. The representative AFM images are shown in Figure 9.
From the roughness profile displayed in Figure 9, the average
thickness of the Nb2O5ꢁxH2O nanosheets was estimated to be
2.0 nm. Considering the thickness of a single sheet and the
distance between the nanosheets (14.6 Å), we speculate that
this thickness corresponds to two layers of single sheets. In
other words, our Nb2O5ꢁxH2O nanosheet is mostly composed
of two layers of single sheets.
Raman spectroscopic measurements have been performed to
gain information about the coordination of Nb atoms in the
Nb2O5ꢁxH2O nanosheet. Figure 10 shows that the crystalline
Nb2O5 exhibits Raman bands at 500ꢀ750, 800ꢀ900, and ~990
cmꢀ1 (curve a), which could be attributed to NbꢀOꢀNb, NbO6
octahedra, and NbO4 tetrahedra with Nb=O, respectively.8,15
For the amorphous Nb2O5ꢁnH2O sample, only broad Raman
bands at 500ꢀ700 and 800ꢀ900 cmꢀ1 were observed (curve b).
It is known that the Nb2O5ꢁnH2O contains both NbO6 octaheꢀ
dra and NbO4 tetrahedra, and the absence of the Raman band
at ~990 cmꢀ1 may be due to the formation of NbO4ꢀH2O adꢀ
ducts.8 We observed that the evacuation of the Nb2O5ꢁnH2O at
473 K caused the appearance of the Raman band belonging to
NbO4 tetrahedra at 988 cmꢀ1 (curve c), confirming the removal
of water from the NbO4ꢀH2O adducts. However, the Raman
band at ~990 cmꢀ1 could not be observed for the Nb2O5ꢁxH2O
nanosheets even after the evacuation at 473 K (curve e), sugꢀ
gesting the essential absence of NbO4 tetrahedra in the strucꢀ
ture of Nb2O5ꢁxH2O nanosheets. Thus, NbO6 octahedra are the
dominant building units in our Nb2O5ꢁxH2O nanosheets.
Acidic Properties of Nb2O5·xH2O Nanosheets. The acid
strength and acid density are two key factors in determining
the acidic properties of solid acid materials. It is known that
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
+
We further investigated the possible effect of NH4 on the
formation of nanosheets. Before the hydrothermal synthesis,
the precipitates obtained from the hydrolysis of NbCl5 with
NH3ꢁH2O in ethanol solution were completely washed with
water. To keep the pH of the suspension used for hydrotherꢀ
mal treatment at 8, a small amount of NaOH aqueous solution
was added. However, the hydrothermal treatment of this susꢀ
pension at 513 K for 24 h led to the formation of spherical
particles (Figure 6a). XRD measurements revealed that these
particles were Nb2O5 crystallites (Figure 7a). On the other
hand, the regulation of the pH by adding NH3ꢁH2O into the
suspension containing thoroughly washed precipitates resulted
in the formation of the Nb2O5ꢁxH2O nanosheets (Figures 6b
+
and 7b). This indicates the key role of NH4 in the formation
of the Nb2O5ꢁxH2O nanosheets. Furthermore, we found that
the addition of nꢀbutyl amine or tertꢀbutylammonium hydroxꢀ
ide (TBAOH) to control the pH value of the suspension conꢀ
taining completely washed precipitates at 8 could also yield
sheetꢀlike products (Figures 6c and 6d) after hydrothermal
treatment, which exhibited similar XRD patterns to the
Nb2O5ꢁxH2O nanosheet (Figures 7c and 7d).
On the basis of the results described above, we speculate
that the Nb2O5ꢁxH2O nanosheet is formed by a dissolutionꢀ
crystallization mechanism. Without hydrothermal treatment,
the precipitates obtained from hydrolysis of NbCl5 with
NH3ꢁH2O were amorphous Nb2O5ꢁnH2O particles. It is known
that the amorphous Nb2O5ꢁnH2O can dissolve in alkaline meꢀ
dium and different types of niobic oxide anionic species such
as Nb12O3612−, Nb6O198−, and HxNb6O19(8−x)− can be formed deꢀ
pending on the pH of the medium.5d,16,17 Moreover, single
7−
octahedron NbO6 anions can be formed under hydrothermal
conditions.18,19 It can be expected that the formation of the
niobic oxide anionic species is determined by both the pH
value and the temperature. Our result that higher hydrothermal
temperatures and longer hydrothermal times favor the forꢀ
mation of nanosheets (Figures 2 and 3) suggest the crucial role
7−
of the hydrothermal process. The NbO6 anions may function
as the elementary species for the formation of the Nb2O5ꢁxH2O
+
nanosheets. We have further demonstrated that NH4 ions or
organic ammonium cations also play key roles in the forꢀ
mation of nanosheets. Although the detailed functioning
+
mechanism of the NH4 ions or organic ammonium cations is
+
still unclear at this moment, we speculate that the NH4 or
organic ammonium cations may interact with the niobic oxide
anionic species and may exert influences on the crystal growth,
4
ACS Paragon Plus Environment

Page 5 of 14
Chemistry of Materials
1
2
3
4
5
6
7
8
the acid strength can be determined by the color change of
this unique acidity arises from the peculiar structure of the
Hammett indicators with different pKa values,20 while the acid
density can be estimated by amine titration in combination
with the Hammett indicators.21 Our experimental results obꢀ
tained for the crystalline Nb2O5, the amorphous Nb2O5ꢁnH2O,
and the Nb2O5ꢁxH2O nanosheet are summarized in Table 2. As
expected, the crystalline Nb2O5 only exhibits very weak acidity
Nb2O5ꢁxH2O nanosheet. The NbO4 tetrahedra has been proꢀ
posed to function as Lewis acid sites in the amorphous
Nb2O5ꢁnH2O.8 Our Raman spectroscopic result (Figure 10)
suggests that the Nb2O5ꢁxH2O nanosheet contains NbO6 octaꢀ
hedra as the main building units other than NbO4 tetrahedra.
The properties of hydrogen species in the Nb2O5ꢁxH2O
nanosheet, which may be related to its Brønsted acidity, were
with
a
lower acid density, whereas the amorphous
1
Nb2O5ꢁnH2O sample possesses relatively stronger acidity and a
larger acid density. As compared to the amorphous
Nb2O5ꢁnH2O, the Nb2O5ꢁxH2O nanosheet has demonstrated a
further stronger acidity. The solid acid with Hammett function
(H0) < −3.0 corresponds to 48% H2SO4.20 The acid density for
the Nb2O5ꢁxH2O nanosheet was also larger than that for the
amorphous Nb2O5ꢁnH2O, although the specific surface area of
the Nb2O5ꢁxH2O nanosheet (87 m2 gꢀ1) was lower than that of
the amorphous Nb2O5ꢁnH2O (151 m2 gꢀ1).
investigated by H MAS NMR spectroscopy. A broad peak
9
with a chemical shift (δ) of 5.3 ppm together with a shoulder
at δ of ~7.0 ppm was observed for the amorphous
Nb2O5ꢁnH2O (Figure 13). The peaks with lower and higher
chemical shifts may be assigned to the isolated NbꢀOH and the
OH groups shared by two Nb5+ [i.e., Nb(OH)Nb], respectively,
and it has been proposed that the Nb(OH)Nb functions as a
strong Brønsted acid site in Nb2O5ꢁnH2O.10 The peaks became
sharper for the Nb2O5ꢁxH2O nanosheet. Moreover, the intensity
of the peak at the higher chemical shift became significantly
stronger. This suggests that the fraction of the strong Brønsted
acid sites with δ > 7.0, which probably correspond to the
Nb(OH)Nb, over the Nb2O5ꢁxH2O nanosheet is larger than that
over the amorphous Nb2O5ꢁnH2O. This is in agreement with
the results obtained from the measurements using the indicator
method (Table 2) and the NH3ꢀTPD technique (Figure 11).
Catalytic Behaviors of Nb2O5·xH2O Nanosheets. The
FriedelꢀCrafts alkylation is a kind of important acidꢀcatalyzed
reactions, and niobic acid has been demonstrated to be an acꢀ
tive catalyst for this kind of reactions.23 Recent studies sugꢀ
gested that the FriedelꢀCrafts alkylation of anisole with benzyl
alcohol to benzyl anisole (Scheme 1) was catalyzed by the
Brønsted acid sites, whereas the Lewis acid sites favored the
intermolecular dehydration of benzyl alcohol, forming dibenꢀ
zyl ether as the main byproduct.11b,24 Thus, this reaction may
be employed as a model reaction to examine the unique cataꢀ
lytic properties of the Nb2O5ꢁxH2O nanosheet, which possesses
mainly Brønsted acid sites.
Table 3 shows that the crystalline Nb2O5 is inactive and the
amorphous Nb2O5ꢁnH2O only exhibits low activity and selecꢀ
tivity at 373 K for the FriedelꢀCrafts reaction. Significantly
higher activity and benzyl anisole selectivity were obtained
over the Nb2O5ꢁxH2O nanosheet at 373 K. Dibenzyl ether was
formed as the only byproduct over our catalysts. The time
course for the FriedelꢀCrafts alkylation of anisole with benzyl
alcohol at 403 K showed that the yield of benzyl anisole inꢀ
creased almost linearly in the initial 1 h over the Nb2O5ꢁxH2O
nanosheet (Figure S3 in the Supporting Information). The
selectivity of benzyl anisole did not undergo significant
changes with the reaction time. Figure S3 further confirmed
that the Nb2O5ꢁxH2O nanosheet was significantly more active
and selective for the formation of benzyl anisole than the
amorphous Nb2O5ꢁnH2O and crystalline Nb2O5. At 403 K, the
benzyl anisole selectivity and yield over the Nb2O5ꢁxH2O
nanosheet were 96% and 91%, respectively, after 2 h of reacꢀ
tion. The Nb2O5ꢁxH2O nanosheet was also more efficient than
several other typical solid Brønstedꢀacid catalysts including
zeolites HꢀZSMꢀ5 and Hꢀbeta, and Amberlystꢀ15, a polymeric
resin containing SO3H groups. We propose that, in addition to
the acidity, the accessibility of the acid sites also plays an imꢀ
portant role in determining the catalytic behaviors. The twoꢀ
dimensional open structure of the Nb2O5ꢁxH2O nanosheet can
allow the reactant molecules to access the acid sites more facꢀ
ilely. On the other hand, the acid sites in zeolites are mostly
located inside the micropores and this may limit the converꢀ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
We also investigated the acidity by using the NH3ꢀTPD
technique. As shown in Figure 11, almost no desorption of
NH3 could be observed from the crystalline Nb2O5, confirming
that the crystalline Nb2O5 does not possess acid sites strong
enough. The amorphous Nb2O5ꢁnH2O showed a main desorpꢀ
tion peak of NH3 at ~440 K together with a shoulder peak at
~540 K. On the other hand, the main desorption peak of NH3
for the Nb2O5ꢁxH2O nanosheet occurred at ~600 K along with
a weaker shoulder peak at ~500 K. This provides further eviꢀ
dence that the acidity of the Nb2O5ꢁxH2O nanosheet is stronger
than that of the amorphous Nb2O5ꢁnH2O. We can further calꢀ
culate the total adsorption amount of NH3 by quantifying the
NH3ꢀTPD peaks. The NH3 adsorption amounts on the
Nb2O5ꢁxH2O nanosheet and the amorphous Nb2O5ꢁnH2O were
evaluated to be 0.51 and 0.24 mmol gꢀ1, respectively. The
NH3ꢀTPD profiles for some typical solid acids are also disꢀ
played in Figure 11 for comparison. HꢀZSMꢀ5, Hꢀbeta, and
mesoporous HꢀZSMꢀ5 exhibited two NH3 desorption peaks at
400ꢀ550 K and 550ꢀ800 K. The higherꢀtemperature peak,
which corresponds to the stronger acid sites accounts for
~30% of the total acid sites. For Amberlystꢀ15, the main NH3
desorption peak was observed at ~440 K. The total amounts of
NH3 desorption for these samples have been quantified and
are listed in Table 3.
To gain insights into the nature of the acid sites, we perꢀ
formed Fourierꢀtransform infrared (FTꢀIR) studies of adsorbed
pyridine. It is known that IR bands at ~1540 and ~1450 cmꢀ1
can be attributed to the Brønsted and the Lewis acid sites,
respectively, while that at ~1490 cmꢀ1 arises from both
Brønsted and Lewis acid sites.22 As displayed in Figure 12, no
IR band was observed in the region of 1600ꢀ1400 cmꢀ1 for
Nb2O5. This is in agreement with the result that the crystalline
Nb2O5 only possesses very weak acidity. The amorphous
Nb2O5ꢁnH2O showed a distinct IR band at 1445 cmꢀ1 ascribed
to Lewis acid sites, while the IR band at the position for the
Brønsted acid sites (~1540 cmꢀ1) appeared as a shoulder of a
broad band at ~1560 cmꢀ1. On the other hand, the Nb2O5ꢁxH2O
nanosheet displayed a distinct IR band at 1540 cmꢀ1 attributaꢀ
ble to the Brønsted acid sites, whereas the IR band belonging
to the Lewis acid sites were not observed. We also performed
pyridineꢀadsorbed FTꢀIR studies for HꢀZSMꢀ5, a typical solid
acid with strong Brønsted acidity. IR bands at 1545 and 1455
cmꢀ1 were both observed, suggesting that both Brønsted and
Lewis acidity existed on HꢀZSMꢀ5. Therefore, the
Nb2O5ꢁxH2O nanosheet, which possesses predominantly
Brønsted acid sites, is quite unique. It can be speculated that
5
ACS Paragon Plus Environment

Chemistry of Materials
Page 6 of 14
1
2
3
4
5
6
7
8
sion or formation of bulky reactants or products. As expected,
Hꢀbeta, which possessed larger sizes of micropores than Hꢀ
ZSMꢀ5, showed higher activity than HꢀZSMꢀ5. The exploitaꢀ
tion of mesoporous HꢀZSMꢀ5, which was prepared by treating
HꢀZSMꢀ5 with an aqueous solution of NaOH followed by ion
ate for the production of valuable chemicals and liquid fuels.27
The hydrolysis of inulin could be catalyzed by enzymes under
mild conditions but the process required long reaction times
up to more than 10 days.26 Few studies have been focused on
the hydrolysis of inulin to fructose by using solid acid cataꢀ
lysts. In this work, we found that the Nb2O5ꢁxH2O nanosheet
was a highly efficient and waterꢀtolerant catalyst for the hyꢀ
drolysis of inulin under relatively mild conditions.
As displayed in Table 5, the Nb2O5ꢁxH2O nanosheet showed
significantly higher conversion of inulin and higher yield of
fructose than the amorphous Nb2O5ꢁnH2O and zeolites. At 363
K, the yield of fructose reached 87% after 2 h of reaction.
Amberlystꢀ15 was also a highly efficient catalyst for the hyꢀ
drolysis of inulin and a fructose yield of 90% could be obꢀ
tained over Amberlystꢀ15. This is likely because Amberlystꢀ15
possesses a markedly larger acid density. However, the TOF
for the formation of fructose, i.e., the rate per acid site, over
Amberlystꢀ15 was lower than that over the Nb2O5ꢁxH2O
nanosheet.
We investigated the effect of reaction temperature on the
catalytic performance of the Nb2O5ꢁxH2O nanosheet for the
hydrolysis of inulin. The result showed that the conversion of
inulin was >99% at temperatures ≥363 K (Figure S4 in the
Supporting Information). The yield of fructose was 87% at
363 K, and a further increase in reaction temperature rather
decreased the yield of fructose due to the formation of humin,
carbonaceous species, resulting from the side reactions of
dehydration and oligomerization.8 Thus, the optimum reaction
temperature for the hydrolysis of inulin to fructose was 363 K.
We further performed repeated uses of the Nb2O5ꢁxH2O
nanosheet for the hydrolysis of inulin. Figure 14 shows that
the inulin conversion and the fructose yield only decrease
lightly from 90% to 86% and from 78% to 73%, respectively,
after the initial three recycles. The inulin conversion of 86%
and fructose yield of 73% could be sustained in the subseꢀ
quent recycling uses. On the other hand, significant decreases
in catalytic performances were observed during the repeated
uses of Amberlystꢀ15 for the hydrolysis of inulin (Figure 15).
The inulin conversion and the fructose yield decreased graduꢀ
ally from >99% and 88% to 62% and 53%, respectively, after
five reaction cycles. This is possibly because of the leaching
of SO3H groups from Amberlystꢀ15 to the aqueous solution
during the reaction. These results demonstrate that the
Nb2O5ꢁxH2O nanosheet is stable under our reaction conditions
and is an efficient waterꢀtolerant Brønstedꢀacid catalyst.
+
exchange using NH4 and subsequent calcination,25 increased
the conversion of benzyl alcohol and the yield of benzyl aniꢀ
sole. Amberlystꢀ15 exhibited higher selectivity and yield of
benzyl anisole than the zeolites but was inferior to the
Nb2O5ꢁxH2O nanosheet. Particularly, the Nb2O5ꢁxH2O
nanosheet exhibits significantly higher selectivity of benzyl
anisole. This clearly demonstrates the favorable catalytic funcꢀ
tion of the Nb2O5ꢁxH2O nanosheet in the reaction requiring
Brønsted acid sites. It should be noted that the formations of
two isomers, i.e., oꢀbenzyl anisole and pꢀbenzyl anisole, were
observed over our catalysts. The ratios of oꢀbenzyl anisole to
pꢀbenzyl anisole over different catalysts were similar, being in
a range of 37/63 to 45/55 (Table 3).
To make a better comparison of the activity among these
solid acids, we have further evaluated the turnover frequency
(TOF) for the formation of benzyl anisole by calculating the
moles of benzyl anisole produced per mole of surface acid site
per hour. The acid density for each catalyst is expressed by
using the adsorption amount of NH3, which has been measꢀ
ured by NH3ꢀTPD. The result summarized in Table 3 clearly
demonstrates that the Nb2O5ꢁxH2O nanosheet possesses the
highest TOF among the solid acid catalysts examined in this
work. The TOF over the Nb2O5ꢁxH2O nanosheet was about one
order of magnitude higher than that over Amberlystꢀ15, which
was also an excellent Brønstedꢀacid catalyst. The TOF obꢀ
tained at a lower conversion by shortening the reaction time to
20 min over the Nb2O5ꢁxH2O nanosheet was also significantly
higher than that over Amberlystꢀ15. This further suggests that,
besides the stronger acidity, the easier accessibility of the acid
sites located on the twoꢀdimensional Nb2O5ꢁxH2O nanosheet
by the bulky reactant molecules should contribute to its higher
catalytic efficiency.
We also performed the hydrolysis of ethyl acetate at 343 K,
a catalytic reaction in water medium with smaller reactant
molecules, over the Nb2O5ꢁxH2O nanosheet and the amorꢀ
phous Nb2O5ꢁnH2O catalysts together with several typical solid
acids. In this case, the TOF for ethyl acetate conversion over
HꢀZSMꢀ5 was higher than that over Hꢀbeta (Table 4), although
the latter zeolite possessed a larger pore size. The use of mesꢀ
oporous HꢀZSMꢀ5 only slightly increased the TOF for ethyl
acetate conversion. This is likely because the smaller reactant
molecules can also access the acid sites located in the miꢀ
cropores of zeolites easily. Table 4 reveals that the TOFs over
the amorphous Nb2O5ꢁnH2O and the Nb2O5ꢁxH2O nanosheet
are higher than those over other solid acid catalysts examined.
This suggests that niobic acids are more efficient catalysts in
water medium. Moreover, the Nb2O5ꢁxH2O nanosheet demonꢀ
strates higher catalytic performance than the amorphous
Nb2O5ꢁnH2O.
Inulin is an inedible polysaccharide consisting of fructose
units linked by βꢀ2,1ꢀglycosidic bonds with the polymer chains
terminating in a glucose unit (Scheme 2). Inulin exists as the
main component in many plants such as helianthus tuberosus
and chicory, which are coldꢀ and heatꢀresistant and are easy to
cultivate even in desert areas. The hydrolysis of inulin can
produce mainly fructose, which can be used in food and
pharmaceutical industries26 or can be exploited for the producꢀ
tion of 5ꢀhydroxymethylfurfural (HMF), a versatile intermediꢀ
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CONCLUSIONS
We succeeded in synthesizing niobic acid nanosheets via a
novel bottomꢀup route by simply treating the precursor obꢀ
tained from the hydrolysis of NbCl5 dissolved in ethanol with
NH3ꢁH2O under hydrothermal conditions. The nanosheet was
suggested to be formed from amorphous niobic acid nanoparꢀ
ticles via a dissolutionꢀcrystallization mechanism. The pH of
the suspension, the hydrothermal temperature and time were
key factors for the formation of nanosheets. We further clariꢀ
fied that the NH4+ ions played a crucial role in determining the
morphology of the product. Organic ammonium cations could
also be used to direct the assembly of niobic acid nanosheets
+
instead of the NH4 ions. Our characterizations revealed that
the Nb2O5ꢁxH2O nanosheet possessed a thickness of ~2.0 nm
with an average lateral size of ~130 nm. The crystalline strucꢀ
ture of the Nb2O5ꢁxH2O nanosheet was similar to that of the
6
ACS Paragon Plus Environment

Page 7 of 14
Chemistry of Materials
1
2
3
4
5
6
7
8
(4) (a) Rinaldi, R.; Schüth, F. Energy Environ. Sci. 2009, 2, 610. (b)
Hara, M. Enery Environ. Sci. 2010, 3, 601. (c) Shimizu, K.; Satsuma,
A. Energy Environ. Sci. 2011, 4, 3140.
nanosheet synthesized by the exfoliation of the layered
HNb3O8ꢁH2O. The characterization further suggested that the
Nb2O5ꢁxH2O nanosheet was composed of two layers of single
sheets. NbO6 octahedra were found to be the predominant
building units without NbO4 tetrahedra present in our
Nb2O5ꢁxH2O nanosheet. As compared to the conventional
amorphous Nb2O5ꢁnH2O particles, the Nb2O5ꢁxH2O nanosheet
possessed enhanced acid strength and acid density. Furtherꢀ
more, the Nb2O5ꢁxH2O nanosheet contained mainly Brønsted
acid sites and almost no Lewis acid sites. The Nb2O5ꢁxH2O
nanosheet showed unique catalytic behaviors in the Friedelꢀ
Crafts alkylation of anisole with benzyl alcohol, the hydrolysis
of ethyl acetate, and the hydrolysis of inulin to fructose. As
compared to several typical solid acids, the Nb2O5ꢁxH2O
nanosheet showed higher yields and TOFs for the target prodꢀ
ucts for these reactions. It is proposed that the stronger acidity
and the twoꢀdimensional structure of the nanosheet, which
makes the acid sites easier to be accessed by the bulky reactant
molecules, contribute to its superior catalytic performances.
The Nb2O5ꢁxH2O nanosheet also demonstrated a particularly
higher selectivity for the FriedelꢀCrafts alkylation because of
the lack of Lewis acid sites. Furthermore, the Nb2O5ꢁxH2O
nanosheet displayed higher stability during the repeated uses
than Amberlystꢀ15, which also showed excellent activity for
the conversion of inulin to fructose.
(5) (a) Tanabe, K. Catal. Today 1990, 8, 1. (b) Tanabe, K.; Okazaki,
S. Appl. Catal. A Gen. 1995, 133, 191. (c) Tanabe, K. Catal. Today
2003, 78, 65. (d) Nowak, I.; Ziolek, M. Chem. Rev. 1999, 99, 3603.
(6) Carniti, P.; Gervasini, A.; Biella, S.; Auroux, A. Chem. Mater.
2005, 17, 6128.
(7) Nakajima, K.; Fukui, T.; Kato, H.; Kitano, M.; Kondo, J. N.;
Hayashi, S.; Hara, M. Chem. Mater. 2010, 22, 3332.
(8) Nakajima, K.; Baba, Y.; Noma, R.; Kitano, M.; Kondo, J. N.;
Hayashi, S.; Hara, M. J. Am. Chem. Soc. 2011, 133, 4224.
(9) Ma, R.; Sasaki, T. Adv. Mater. 2010, 22, 5082.
(10) Takagaki, A.; Tagusagawa, C.; Hayashi, S.; Hara, M.; Domen,
K. Energy Environ. Sci. 2010, 3, 82.
(11) (a) Takagaki, A.; Sugisawa, M.; Lu, D.; Kondo, J. N.; Hara, M.;
Domen, K.; Hayashi, S. J. Am. Chem. Soc. 2003, 125, 5479. (b)
Tagusagawa, C.; Takagaki, A.; Hayashi, S.; Domen, K. J. Phys.
Chem. C 2009, 113, 7831.
(12) Takagaki, A.; Lu, D.; Kondo, J. N.; Hara, M.; Hayashi, S.;
Domen, K. Chem. Mater. 2005, 17, 2487.
(13) Yang, Z. J.; Li, L. F.; Wu, Q. B.; Ren, N.; Zhang, Y. H.; Liu, Z.
P.; Tang, Y. J. Catal. 2011, 280, 247.
(14) Yang, G.; Hou, W.; Feng, X.; Xu, L.; Liu, Y.; Wang, G.; Ding,
W. Adv. Funct. Mater. 2007, 17, 401.
(15) Li, X.; Kikugawa, N.; Ye, J. Adv. Mater. 2008, 20, 3816.
(16) Pittman, R. M.; Bell, A. T. J. Phys. Chem. 1993, 97, 12178.
(17) Jehng, J. M.; Wachs, I. E. Catal. Today 1990, 8, 37.
(18) Magrez, A.; Vasco, E.; Seo, J. W.; Dieker, C.; Setter, N.; Forró,
L. J. Phys. Chem. B 2006, 110, 58.
(19) Liang, S.; Wu, L.; Bi, J.; Wang, W.; Gao, J.; Li, Z.; Fu, X.
Chem. Commun. 2010, 46, 1446.
(20) (a) Walling, C. J. Am. Chem. Soc. 1950, 72, 1164. (b) Benesi, H.
A. J. Am. Chem. Soc. 1956, 78, 5490.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ASSOCIATED CONTENT
Supporting Information
TG curves for several niobic acid samples, distribution of latꢀ
eral sizes of the Nb2O5ꢁxH2O nanosheets, time courses for the
FriedelꢀCrafts alkylation of anisole with benzyl alcohol, and
dependence of catalytic performances of the Nb2O5ꢁxH2O
nanosheets for the hydrolysis of inulin on reaction temperature.
This material is available free of charge via the Internet at
http://pubs.acs.org
(21) Benesi, H. A. J. Phys. Chem. 1957, 61, 970.
(22) Zaki, M. I.; Hasan, M. A.; AlꢀSagheer, F. A.; Pasupulety, L.
Colloids Surf. A 2001, 190, 261.
(23) Morais, M.; Torres, E. F.; Carmo, L. M. P. M.; Gonzalez, W. A.;
dos Santos, A. C. B.; Lachter, E. R. Catal. Today 1996, 28, 17.
(24) (a) Shishido, T.; Kitano, T.; Teramura, K.; Tanaka, T. Catal.
Lett. 2009, 129, 383. (b) Shishido, T.; Kitano, T.; Teramura, K.;
Tanaka, T. Top. Catal. 2010, 53, 672.
(25) Kang, J.; Cheng, K.; Zhang, L.; Zhang, Q.; Ding, J.; Hua, W.;
Lou, Y.; Zhai, Q.; Wang, Y. Angew. Chem. Int. Ed. 2011, 50, 5200.
(26) Singh, R. S.; Dhaliwal, R.; Puri, M. J. Ind. Microbiol.
Biotechnol. 2008, 35, 777.
AUTHOR INFORMATION
Corresponding Author
Eꢀmail: zhangqh@xmu.edu.cn, wangye@xmu.edu.cn, Tel: +86ꢀ
592ꢀ2186156, Fax: +86ꢀ592ꢀ2183047
(27) Zhao, H.; Holladay, J. E.; Brown, H.; Zhang, Z. C. Science 2007,
316, 1597.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the National Basic Research Program of
China (Nos. 2013CB933100 and 2010CB732303), the Natural Sciꢀ
ence Foundation of China (Nos. 21173172, 21103143, 21161130522,
21033006), and the Program for Changjiang Scholars and Innovative
Research Team in Chinese University (No. IRT1036).
REFERENCES
(1) (a) Tanabe, K.; Hölderich, W. F. Appl. Catal. A: Gen. 1999, 181,
399. (b) Okuhara, T. Chem. Rev. 2002, 102, 3641. (c) Busca, G.
Chem. Rev. 2007, 107, 5366.
(2) Clark, J. H. Acc. Chem. Res. 2002, 35, 791.
(3) (a) Harmer, M. A.; Farneth, W. E.; Sun, Q. Adv. Mater. 1998, 10,
1255. (b) Xing, R.; Liu, N.; Wu, H.; Jiang, Y.; Chen, L.; He, M.; Wu,
P. Adv. Funct. Mater. 2007, 17, 2455.
7
ACS Paragon Plus Environment

Chemistry of Materials
Page 8 of 14
1
2
3
4
5
6
7
8
Table 1. The content of H2O in the Nb2O5·xH2O samples
prepared by the hydrothermal method under different
conditions.
Table 3. Catalytic behaviors of some solid acid catalysts
for the Friedel-Crafts alkylation of anisole by benzyl alco-
hol.a
Conversion
[%]
Benzyl anisole
Yield
[%]
Acid
densityc
[mmol gꢀ1]
0
TOFd
[hꢀ1]
Hydrothermal
temperature
[K]
Hydrotherꢀ
mal time
[h]
Catalyst
selectivityb
x in
Nb2O5ꢁxH2O
x1 in
x2 in
Nb2O5ꢁxH2Oa
Nb2O5ꢁxH2Ob
[%]
−
Crystalline
Nb2O5
0
0
0
513
513
513
513
513
513
423
453
483
0
2.5
1.6
1.5
1.2
1.1
1.1
2.1
1.6
1.2
n.d.c
0.54
0.59
0.51
0.34
0.31
n.d.
n.d.c
1.05
0.93
0.67
0.75
0.76
n.d.
Amorphous
Nb2O5ꢁnH2O
0.61
23 (43:57)
39 (45:55)
85 (37:63)
96 (40:60)
81 (37:63)
0.14
0.24
0.24
0.51
0.51
0.51
0.3
1
9
3
Amorphous
8.5
79
95
16
3.3
67
91
13
7.1
65
88
83
12
24
72
24
24
24
Nb2O5ꢁnH2Oe
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Nb2O5ꢁxH2O
nanosheet
Nb2O5ꢁxH2O
nanosheete
Nb2O5ꢁxH2O
0.75
0.49
0.83
0.73
nanosheet f
HꢀZSMꢀ5
21
38
52 (40:60)
56 (39:61)
11
21
1.3
1.1
4.2
9.5
(16.5)g
ax1 denotes the number of H2O in Nb2O5ꢁxH2O desorbed in the
Mesoporous
b
HꢀZSMꢀ5
lower temperature region. x2 denotes the number of water in
Hꢀbeta (25)g
43
56 (43:57)
24
0.96
12
c
Nb2O5ꢁxH2O desorbed in the higher temperature region. Not
Hꢀbeta (25)e,g
Amberlystꢀ15
Amberlystꢀ15f
87
85
16
66 (45:55)
68 (44:56)
62 (45:55)
76
58
0.96
4.7
38
6.2
7.0
detected.
9.9
4.7
aReaction conditions: anisole, 100 mmol; benzyl alcohol, 10
mmol; catalyst, 0.10 g; reaction temperature, 373 K; reaction
Table 2. The strength and density of acid sites determined
by Hammett indicators and titration using n-butyl amine.
b
time, 2 h. The number in the parentheses denotes the ratio of
Solid acid (acid density [mmol gꢀ1])a
oꢀbenzyl anisole/pꢀbenzyl anisole. The byproduct was dibenzyl
ether. cThe acid density was evaluated by NH3ꢀTPD. dTOF was
evaluated by the rate of benzyl anisole formation per acid site.
Hammett indicator
pKa
Amorphous
Nb2O5ꢁnH2O
Nb2O5ꢁxH2O
nanosheet
Nb2O5
f
g
eReaction temperature, 403 K. Reaction time, 20 min. The
Neutral red
+6.8
+4.8
+3.3
+1.5
−3.0
−8.2
+ (0.11)
+ (0.95)
+ (0.36)
+ (0.16)
+ (0.10)
−
+ (1.30)
+ (0.71)
+ (0.40)
+ (0.33)
+ (0.06)
−
Methyl red
+ (0.01)
number in the parenthesis denotes the Si/Al ratio.
Methyl yellow
−
−
−
−
Benzeneazodiphenylamine
Dicinnamalacetone
Anthraquinone
Table 4. Catalytic behaviors of some solid acid catalysts
for the hydrolysis of ethyl acetate.a
−
a“+” and “−” represent occurrence and no occurrence of color
change, respectively. The number in the parenthesis is the acid
density measured by titration using nꢀbutyl amine.
Catalyst
Rate
Acid densityb
[mmol gꢀ1]
TOFc
[hꢀ1]
[ꢂmol gꢀ1 minꢀ1]
Amorphous Nb2O5ꢁnH2O
Nb2O5ꢁxH2O nanosheet
5.2
0.24
1.3
18
0.51
2.1
HꢀZSMꢀ5 (16.5)d
15
16
1.3
1.1
0.68
0.89
Mesoporous HꢀZSMꢀ5
Hꢀbeta (25)d
6.0
63
0.96
4.7
0.37
0.80
Amberlystꢀ15
aReaction conditions: ethyl acetate, 15 mmol; H2O, 10 mL;
catalyst, 0.20 g; reaction temperature, 343 K. bThe acid density
was evaluated by NH3ꢀTPD. cTOF was evaluated by the rate of
d
ethyl acetate conversion per acid site. The number in the paꢀ
renthesis denotes the Si/Al ratio.
8
ACS Paragon Plus Environment

Page 9 of 14
Chemistry of Materials
1
2
3
Table 5. Catalytic behaviors of some solid acid catalysts
for the hydrolysis of inulin.a
4
5
6
Selectivityb [%]
Acid
Conv.
Catalyst
[%]
TOFd
densityc
Fructose
yield [%]
[hꢀ1]
[mmol gꢀ1]
Fructose
Glucose
7
8
9
Amorphous
Nb2O5ꢁnH2O
23
67
87
81
69
14
16
87
55
37
0.24
0.51
0.51
1.3
2.1
5.3
6.7
0.88
Nb2O5ꢁxH2O
nanosheet
>99
68
2.3
5.7
7.4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Nb2O5ꢁxH2O
nanosheete
HꢀZSMꢀ5
(16.5)f
54
Mesoporous
HꢀZSMꢀ5
89
79
63
90
80
3.5
8.8
3.3
5.5
70
8.6
90
38
1.1
0.96
4.7
2.0
Hꢀbeta (25)f
14
0.28
0.59
2.5
Amberlystꢀ15
>99
48
Amberlystꢀ
15g
4.7
aReaction conditions: inulin, 0.10 g; H2O, 10 mL; catalyst,
0.10 g; Ar, 0.5 MPa; reaction temperature, 363 K; reaction
b
time, 2 h. The residual byproducts were sucrose and humin.
cThe acid density was evaluated by NH3ꢀTPD. dTOF was evalꢀ
e
uated by the rate of fructose formation per acid site. Catalyst,
0.050 g. fThe number in the parenthesis denotes the Si/Al ratio.
gCatalyst, 0.010 g.
9
ACS Paragon Plus Environment

Chemistry of Materials
Page 10 of 14
1
2
3
Scheme 1. Friedel-Crafts alkylation of anisole by benzyl
alcohol to benzyl anisole.
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. Representative SEM images. (a) Nb2O5ꢁxH2O
nanosheets, (b) amorphous Nb2O5ꢁnH2O.
Scheme 2. Hydrolysis of inulin to fructose and glucose.
Figure 2. SEM images of niobic acids synthesized by the hydroꢀ
thermal treatment under different conditions. At 513 K for differꢀ
ent times: (a) 0, (b) 1 h, (c) 3 h, (d) 12 h, (e) 48 h, (f) 72 h. At
different temperatures for 24 h: (g) 423 K, (h) 453 K, (i) 483 K.
Figure 3. XRD patterns of niobic acids synthesized by the hydroꢀ
thermal method under different conditions together with the
amorphous Nb2O5ꢁnH2O.
10
ACS Paragon Plus Environment

Page 11 of 14
Chemistry of Materials
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Figure 6. SEM images of niobic acids synthesized by the hydroꢀ
thermal method with the suspension controlled at a pH of 8 by
adding different bases. (a) NaOH, (b) NH3ꢁH2O, (c) nꢀbutyl amine,
(d) TBAOH.
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. SEM images of niobic acids synthesized by the hydroꢀ
thermal method with the suspension controlled at different pH
values. (a) pH = 6, (b) pH = 7, (c) pH = 10, pH = 12.
Figure 7. XRD patterns of niobic acids synthesized by the hydroꢀ
thermal method with the suspension controlled at a pH of 8 by
adding different bases. (a) NaOH, (b) NH3ꢁH2O, (c) nꢀbutyl amine,
(d) TBAOH.
Figure 5. XRD patterns of niobic acids synthesized by the hydroꢀ
thermal method with the suspension controlled at different pH
values.
11
ACS Paragon Plus Environment

Chemistry of Materials
Page 12 of 14
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 10. Raman spectra. (a) Nb2O5 crystallites, (b) amorphous
Nb2O5ꢁnH2O, (c) amorphous Nb2O5ꢁnH2O after dehydration by
evacuation at 473 K for 2 h, (d) Nb2O5ꢁxH2O nanosheet, (e)
Nb2O5ꢁxH2O nanosheet after dehydration by evacuation at 473 K
for 2 h.
Figure 8. TEM images of the Nb2O5ꢁxH2O nanosheets syntheꢀ
sized by the hydrothermal method at 513 K for 24 h. (a) Top view,
(b) side view, (c) enlarged image of (b), (d) HRTEM of the center
part, (e) enlarged image of (d), (f) HRTEM of the edge part.
Figure 11. NH3ꢀTPD profiles for crystalline Nb2O5, amorphous
Nb2O5ꢁnH2O, and Nb2O5ꢁxH2O nanosheet together with some
typical solid acids.
Figure 9. AFM image and the corresponding roughness profile
for the Nb2O5ꢁxH2O nanosheets synthesized by the hydrothermal
method at 513 K for 24 h.
12
ACS Paragon Plus Environment

Page 13 of 14
Chemistry of Materials
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 12. Pyridineꢀadsorbed FTꢀIR spectra for the Nb2O5 crysꢀ
tallites, amorphous Nb2O5ꢁnH2O, Nb2O5ꢁxH2O nanosheet, and Hꢀ
ZSMꢀ5.
Figure 14. Repeated uses of the Nb2O5ꢁxH2O nanosheet for hyꢀ
drolysis of inulin to fructose. Reaction conditions: inulin, 0.10 g;
H2O, 10 mL; Ar, 0.5 MPa; catalyst, 0.075 g; temperature, 363 K;
time, 2 h.
Figure 13. 1H MAS NMR spectra for the amorphous
Nb2O5ꢁnH2O and the Nb2O5ꢁxH2O nanosheet.
Figure 15. Repeated uses of Amberlystꢀ15 for hydrolysis of inuꢀ
lin to fructose. Reaction conditions: inulin, 0.25 g; H2O, 10 mL;
Ar, 0.5 MPa; catalyst, 0.010 g; temperature, 363 K; time, 2 h.
13
ACS Paragon Plus Environment

Chemistry of Materials
Page 14 of 14
1
2
Table of Contents Graphic
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
14
ACS Paragon Plus Environment