Synthesis of fluoride-containing high dimensionally structured nb oxide and its catalytic performance for acid reactions

Oxide and Its Catalytic Performance for Acid Reactions

Article

Synthesis of Fluoride-Containing High Dimensionally Structured Nb

Satoshi Ishikawa,* Mai Shinoda, Yuta Motoki, Shota Tsurumi, Momoka Kimura, Norihito Hiyoshi,

Akihiro Yoshida, and Wataru Ueda*

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c00949

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sı Supporting Information

ABSTRACT: High dimensionally structured niobium oxide (HDS-NbO) containing

−

ﬂuoride (F ) was prepared by a hydrothermal synthesis. F could be introduced into HDS-

−

NbO by replacing lattice oxygen up to a solid F /Nb ratio of 0.55. The introduction of an

appropriate amount of F⁻promoted the crystal growth of HDS-NbO, while niobium

oxyﬂuoride having the hexagonal tungsten bronze structure (HTB-Nb(F,O) ) was

−

concomitantly formed by excess F addition. HAADF-STEM analysis suggested that the

number of micropores (hexagonal and heptagonal channels) in HDS-NbO was increased

−

by the introduction of an appropriate amount of F . The catalytic activity for Brønsted acid

reactions was evaluated by Friedel−Crafts alkylation. The catalytic activity was signiﬁcantly

−

increased by the introduction of F , while excess introduction of F signiﬁcantly decreased

the activity. Catalytic activity for the Lewis acid reaction in the presence of water was

evaluated by the transformation of pyruvaldehyde into lactic acid. The catalytic activity was

−

changed by the introduction of F in a manner similar to that observed in the Friedel−

Crafts alkylation. On the basis of the results obtained, we propose that the local catalyst

structure around the micropores of HDS-NbO is crucial for the acid reactions.

. INTRODUCTION

(Figure 1). HDS-NbO showed outstanding catalytic activity

for Brønsted acid reactions, as evaluated by Friedel −Crafts

Niobium oxide (NbO) is well-known as a unique acid catalyst

showing signiﬁcantly strong Brønsted acidity comparable to

−3

0% sulfuric acid solution.

Many eﬀorts have been made to

−8

identify the origin of its unique acid property. However, the

catalytic properties over NbO are still a matter of debate

despite tremendous eﬀorts. There are two main reasons for the

diﬃculty in clarifying its acid properties. The ﬁrst reason is the

amorphous nature of NbO. The amorphous nature of NbO

makes it diﬃcult to investigate the catalytically active sites,

since only limited information can be obtained due to the

inherent heterogeneity of the catalytically active sites. The

second reason is methods for characterization of the acid

properties. Although many methods for characterization have

been proposed so far, it is still diﬃcult to clearly evaluate the

nature of true active sites for catalytic reactions since

conventional characterization methods sometimes detect acid

sites that are inactive for catalysis. These problems have been

masking the true catalytically active sites over NbO.

Figure 1. HAADF-STEM images of HDS-NbO in a side section (A)

2−

and a cross section (B) of the rod. The {Nb O } pentagonal unit

is marked as a yellow dotted circle. A hexagonal channel and a

heptagonal channel are represented as blue and red enclosures,

respectively.

We recently reported the synthesis and characterizations of

,10

high dimensionally structured niobium oxide (HDS-NbO).

This material is constituted by structural arrangements based

Received: April 1, 2020

2−

on {Nb O }

pentagonal units and {NbO } octahedra to

form a rod-shaped crystal by stacking of the above units along

the rod direction with an interval distance of ca. 0.4 nm. Due

to its structural arrangement, micropores (hexagonal and

heptagonal channels) are formed in the cross section of the rod

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 2. (A) XRD patterns of as-synthesized xNbFO. Changes in the lattice parameter to the c axis (B) and crystallite size (C) as a function of F/

Nb ratio in as-synthesized xNbFO determined by the XRD peak derived from the (001) plane of HDS-NbO. (D) XRD patterns of xNbFO after air

calcination at 400 °C for 2 h. Solid circles in (A) and (D) indicate the XRD peaks of HTB-Nb(F,O)_x.

alkylations, and for Lewis acid reactions in the presence of

water, as evaluated by the transformation of pyruvaldehyde to

preparative F/Nb ratio. Prior to the catalytic reactions, xNbFO was

calcined at 400 °C for 2 h under static air.

,10

2.2. Characterization of Materials. The materials obtained were

characterized by the following techniques. Powder XRD patterns were

recorded with a diﬀractometer (RINT Ultima+, Rigaku) using Cu Kα

radiation (tube voltage, 40 kV; tube current, 20 mA). Diﬀractions

lactic acid.

Since the activity was much higher than that of

other Nb oxide catalysts even on a surface area basis, the local

crystal structure over HDS-NbO was suggested to be

responsible for its catalysis. However, the true catalytically

−1

were recorded in the range of 4−60° with a 5° min scan speed.

active sites over HDS-NbO were still unclear, since the

contributions of various structural sites (e.g., the pentagonal

SEM-EDX analysis was carried out with an electron microscope

(SU8010, Hitachi) equipped with an EDX detector (EMAX

Evolution X-MAX, Horiba). XPS (JPC-9010MC, JEOL) with

nonmonochromatic Mg Kα radiation was used for measuring the

^e^l^e^m^eⁿ^t^a^l^c^o^m^p^o^sⁱ^tⁱ^oⁿ^o^v^e^r^t^h^e^c^a^t^a^l^y^s^t^s^u^r^f^a^c^e^.^N2 adsorption

isotherms at liquid N₂temperature were obtained using an

autoadsorption system (BELSORP MAX, BEL JAPAN). Prior to

the N₂adsorption, the samples were pretreated under vacuum

conditions at 400 °C for 2 h. The surface area was determined by a

multipoint Brunauer−Emmett−Teller (BET) method using the

2−

{

Nb O } unit, {NbO } octahedra, and micropores) to its

6 21 6

catalysis have not been elucidated.

Here, we synthesized HDS-NbO containing F inside the

−

crystal structure (HDS-NbFO). The introduction of an

−

appropriate amount of F increased the number of micropore

−

sites in the HDS-NbO structure, while excess addition of F

caused concomitant formation of a niobium oxyﬂuoride having

the hexagonal tungsten bronze structure (HTB-Nb(F,O)_x).

The catalytic activity for Brønsted acid reactions (Friedel−

Crafts alkylation) and Lewis acid reactions in the presence of

water (pyruvaldehyde transformation) was signiﬁcantly in-

creased on the basis of the surface area by the introduction of

adsorption range of 0.05 ≤ P/P ≤ 0.30. Aberration-corrected

STEM images were obtained using ARM-200F (JEOL Ltd., Japan)

equipped with a cold ﬁeld emission gun at an acceleration voltage of

200 kV. Temperature-programmed desorption (TPD) was carried out

by using an autochemisorption system (BEL JAPAN). The sample

−

(ca. 50 mg) was sandwiched by two layers of quartz wool. The

an appropriate amount of F , while the activity was decreased

−1

temperature was increased from 40 to 600 °C with 50 mL min of

He ﬂow. Pyridine adsorption FT-IR analysis was carried out using a

gas ﬂow system equipped with a spectrometer (FT/IR-6100, JASCO).

For analysis, a sample disk (ca. 40 mg, disk diameter 20 mm) was set

at the IR cell, and the temperature was increased with a ramp of 10

−

by excess addition of F . On the basis of the results obtained,

we propose that a speciﬁc structural part in HDS-NbO is

crucial for the above reactions and that the acid sites formed

around the micropore sites are responsible for the catalysis of

these reactions.

−

°C/min to 400 °C under 50 mL min of He ﬂow. After the

temperature was kept at 400 °C for 10 min, the temperature was

decreased to 100 °C under the same gas ﬂow condition. Then 0.2 μL

of pyridine was injected from the top of the IR cell and the injection

was repeated until the IR band intensity became constant. IR spectra

were obtained on an MCT detector by integration of 128 scans with a

. EXPERIMENTAL SECTION

2.1. Synthesis of High Dimensionally Structured Niobium

Oxide Containing Fluoride. High dimensionally structured

niobium oxide containing ﬂuoride (HDS-NbFO) was prepared by a

conventional hydrothermal method using an aqueous ammonium

niobium oxalate (ANO, NH (NbO(C O ) (H O) ·nH O; Sigma-

−

resolution of 4 cm . After the pyridine adsorption IR experiment, 10

mg of the sample was subjected to TG measurement (Thermo plus

−1

EVO2, Rigaku) under 100 mL min of air ﬂow. The amount of

adsorbed pyridine was estimated on the basis of the weight loss from

ca. 150 °C to ca. 500 °C. The amounts of Brønsted acid sites and

Lewis acid sites were separately calculated by taking into account (1)

the area of IR bands assignable to the pyridine adsorbed on Bronsted

9,10

Aldrich) solution. First, 6 mmol of ANO was dissolved in 40 mL of

deionized water. To the ANO solution was added a mmol (a = 0−6)

of NH F (Wako), and the solution was stirred for 10 min at room

temperature. The resulting solution was transferred to a Teﬂon inner

tube (50 mL), and then hydrothermal synthesis was conducted at 175

−

−1

acid sites (1540 cm ) or on Lewis acid sites (1445 cm ), (2) the

11,12

°C for 72 h. The obtained white precipitate was washed with 500 mL

molar extinction coeﬃcients for infrared adsorption bands,

and

of deionized water and then dried at 80 °C overnight. The materials

obtained are abbreviated as xNbFO, where x represents the

(3) the amount of adsorbed pyridine measured by the TG

experiment. The acid strength was evaluated using the Hammett

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Table 1. Physicochemical Properties of xNbFO

F/Nb ratio

−1

catalyst

crystallite size /nm

BET surface area /m g

EDX

XPS

amt of NH desorbed /mmol g

NbFO

19.9 (18.5)

22.0 (20.9)

25.6 (22.3)

26.8 (23.4)

30.6 (25.6)

33.2 (27.6)

34.9 (31.5)

33.8 (24.0)

233

206

187

176

175

150

131

880

968

1058

997

1113

1038

1343

2003

.1NbFO

.2NbFO

.3NbFO

.4NbFO

.5NbFO

.7NbFO

.0NbFO

0.09 (0.02)

0.18 (0.06)

0.32 (0.09)

0.46 (0.12)

0.55 (0.10)

0.62 (0.29)

0.59 (0.47)

0.11 (0.06)

0.15 (0.08)

0.18 (0.10)

0.24 (0.04)

0.17 (0.12)

0.43 (0.37)

1.78 (0.72)

Crystallite size of HDS-NbO determined by using the XRD peak at 2θ = 22.7° before air calcination is shown. Crystallite size after the calcination

is shown in parentheses. Measured by N adsorption at liquid N temperature and determined by the BET method. F/Nb ratios before air

calcination are shown and F/Nb ratios after air calcination at 400 °C are shown in parentheses. Measured by TPD.

indicator. Catalysts were immersed in a liquid solution containing 4-

nm in 0.5NbFO as estimated by SEM images. In the case of

(

−

phenylazo)diphenylamine (pK = 1.5), dicinnamal acetone (pK =

a a

1.0NbFO, rod-shaped crystals formed from the hexagonal-

shaped particles were observed (Figure S2D) as well as crystals

having a crystal morphology similar to that of 0.5NbFO

3.0), chalcone (pK = −5.6), and anthraquinone (pK = −8.1), and

the solution color was checked.

.3. Catalytic Reaction. 2.3.1. Friedel−Crafts Alkylation. First, a

(

Figure S2C). Elemental compositions of xNbFO were

mixture of benzyl alcohol (1 mmol, TCI), anisole (45 mmol, Wako),

and decane (an internal standard, 0.5 mmol, Wako) was placed in a

measured by EDX (bulk) and XPS (surface). In the case of

EDX, the elemental composition of the rod-shaped crystals

derived from HDS-NbFO was measured. The measured

elemental compositions are shown in Table 1. In both cases,

the F/Nb ratio increased with an increase in x. Figure 2B

shows the relationship between the lattice parameter to the c

axis and the F/Nb ratio in the solid. The lattice parameter to

the c axis increased proportionally with an increase in the F/

Nb ratio in the solid up to an F/Nb ratio of 0.55. The increase

in the lattice parameter implies the replacement of constituent

elements with another element having a diﬀerent ion radius. In

20 mL ﬂask. Then 20−50 mg of the catalyst and a Teﬂon-coated

magnetic stir bar were loaded into a reactor. The ﬂask was set with the

reactor preheated at 100 °C, and stirring with the magnetic stir bar

was performed at a rate of 600 rpm. The reaction time was measured

from the time when the ﬂask was placed in the reactor. Aliquots (0.2

mL each) were collected at intervals. The amounts of the reactant and

the product were measured by gas chromatography using a ﬂame

ionization detector (GC-2010, Shimadzu) with a TC-WAX column

(

0.25 mm × 30 m).

.3.2. Transformation of Pyruvaldehyde to Lactic Acid. First, a

mixture of pyruvaldehyde (0.5 mmol, Sigma-Aldrich) and distilled

water (5 mL) was placed in a 20 mL ﬂask. Then 25 mg of the catalyst

and a Teﬂon-coated magnetic stir bar were loaded into the reactor.

The ﬂask was set with the reactor preheated at 100 °C, and stirring

with the magnetic stir bar was performed at a rate of 600 rpm. The

reaction time was measured from the time when the ﬂask was placed

in the reactor. Aliquots (0.2 mL each) were collected at intervals. The

amounts of the reactant and the product were measured by high-

performance liquid chromatography with a refractive index detector

2−

the present case, O in {NbO } octahedra is thought to be

replaced with F up to an F/Nb ratio of 0.55. In addition to

−

the increase in the lattice parameter, the crystallite size of

HDS-NbFO estimated by the Scherrer equation (2θ = 22.8°)

increased linearly with an increase in the F/Nb ratio in the

solid (Figure 2C). The results of an N adsorption experiment

supported this observation, since the BET surface area of

xNbFO decreased in the order of the crystallite size (Table 1).

(

RID-20A, Shimadzu) and with a HPX-87H column (300 mm × 7.8

The addition of NH F above 0.7 ≤ x had a slight eﬀect on the

mm) by using citric acid as the external standard.

crystallite size and lattice parameter of the HDS-NbO

structure, and new XRD peaks appeared at 2θ = 13.6, 23.2,

. RESULTS AND DISCUSSION

.1. Synthesis and Characterizations of xNbFO. XRD

patterns of as-synthesized xNbFO are shown in Figure 2A.

3.7, 26.9, 27.4, 33.4, and 36.1°. These peaks are attributed to

the conventional hexagonal tungsten bronze (HTB) struc-

16,17

ture.

Therefore, the materials showing the above XRD

NbFO shows characteristic XRD peaks at 2θ = 22.8 and

6.8°. These peaks are commonly observed in the materials

peaks are hereafter denoted as HTB-Nb(F,O) . Further

addition of NH F increased the XRD peak intensities of

formed on the basis of the stacking of octahedra (e.g.,

crystalline Mo VO , high dimensionally structured MoFeO),

HTB-Nb(F,O) without changing the crystallite size and lattice

parameter of the HDS-NbO structure. We concluded that the

−

indicating that HDS-NbO is comprised of the stacking of

introduction of F signiﬁcantly inﬂuenced the crystal

,10,14,15

octahedra with an interval distance of ca. 0.39 nm.

formation of HDS-NbO.

.1 ≤ x ≤ 0.5, XRD peak intensities at 2θ = 22.8 and 46.8°

Then xNbFO was calcined at 400 °C for 2 h under an air

−

were increased and downward shifts in these peaks were

observed. Since the XRD pattern of HDS-NbO was almost the

same despite the addition of NH₄by using (NH ) SO

atmosphere. The amount of F was signiﬁcantly decreased

after air calcination (Table 1), and the F/Nb ratio measured by

EDX was 0 ≤ F/Nb ≤ 0.12 at 0 ≤ x ≤ 0.5. Although

4 2

−

(preparative (NH ) SO /Nb molar ratio 0.25), F was found

desorption species related to F were diﬃcult to identify, HF is

to increase the crystallinity of HDS-NbO (Figure S1 ). SEM-

EDX measurements were carried out for 0NbFO, 0.5NbFO,

and 1.0NbFO (Figure S2 ). These materials were rod-shaped

the possible desorbed species. The F/Nb ratio at 0.7 ≤ x was

considerably higher than those at 0 ≤ x ≤ 0.5 after air

calcination, and the ratios were 0.29 and 0.47 for 0.7NbFO and

−

crystals derived from the stacking of {NbO } octahedra, as we

1.0NbFO, respectively. F species might be stably accom-

,10

reported previously. The diameter of the rods was increased

by the introduction of F from ca. 6 nm in 0NbFO to ca. 15

modated at the hexagonal moiety in HTB-Nb(F,O) that was

−

concomitantly formed with HDS-NbFO, and such F is

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

and 0.5NbFO: Figure 4 ). In fact, the numbers of hexagonal

Article

thought to be hardly removed by calcination. The F/Nb ratio

evaluated by XPS analysis was also decreased by calcination in

a manner similar to that measured by EDX, indicating that

signiﬁcant amounts of F⁻species were removed from the

catalyst surface by air calcination. XRD patterns of xNbFO

after air calcination at 400 °C for 2 h are shown in Figure 2D.

XRD patterns were almost unchanged after air calcination at 0

observed in the HAADF-STEM image is roughly proportional

to the atomic number of the elements, the white spots in the

image indicate the location of Nb and the dark spots indicate

the pores produced by the arrangement of {NbO } octahedra.

The diameter of the cross section of 0.5NbFO was ca. 11 nm,

which is quite consistent with the average diameter of the rods

estimated by the SEM images. Therefore, we believe that the

crystal we measured is the representative crystal as 0.5NbFO.

The cross section of 0.5NbFO was constituted by the

≤

x ≤ 0.5. In fact, the crystallite size of xNbFO was almost the

same regardless of air calcination (Table 1). However, the

lattice parameter to the c axis was changed after the calcination

and the lattice parameters became comparable regardless of the

2−

arrangement of {Nb O }

pentagonal units and {NbO₆}

octahedra, forming micropores such as hexagonal and

heptagonal pores in the structure. In the case of 0.5NbFO,

−

value of x (Figure S3). The removal of F from the crystal

2−

structure should be related to its lattice parameter change. The

the {Nb O }

pentagonal units were isolated from each

XRD pattern at 0.7 ≤ x was very diﬀerent after air calcination,

other by {NbO } octahedral linkers, and the number of

and the XRD peak intensities related to HTB-Nb(F,O) were

micropores in the crystal structure seems to be increased in

comparison with that in 0NbFO (structural images of 0NbFO

signiﬁcantly decreased, possibly due to the amorphization as

shown later, while the XRD peaks derived from HDS-NbFO

were almost unchanged.

TEM and HAADF-STEM analyses were then conducted for

.5NbFO and 1.0NbFO in order to investigate the local crystal

structure. These analyses were conducted for samples after air

calcination at 400 °C for 2 h. Figure 3 A shows a TEM image of

Figure 4. Structural images of 0NbFO (A) and 0.5NbFO (B) created

by using their HAADF-STEM images (Figures 1 and 3B). Color

2−

scheme: yellow, {Nb O }

pentagonal units; blue, {NbO₆}

2−

octahedra. {Nb O }

pentagonal units were well isolated in

.5NbFO due to the change in the coordination state around Nb

−

atoms by replacing O with F .

and heptagonal channels estimated by HAADF-STEM images

were 0.26/nm and 0.18/nm for 0NbFO and 0.50/nm and

.50/nm for 0.5NbFO, respectively. TPD measurement was

then carried out in order to support these experimental

observations (Table 1). In the TPD experiment, an NH₃

desorption proﬁle centered at ca. 320 °C was observed in all

of the xNbFO samples (Figure S4). The amount of desorbed

NH estimated from the NH desorption peak area was 880

−

μmol g in 0NbFO. This amount was increased by the

Figure 3. TEM (A) and HAADF-STEM images (B−D) of 0.5NbFO

−

−1

introduction of F , and the amount was 968−1113 μmol g at

0.1 ≤ x ≤ 0.5. Theoretically, this amount should be decreased

by an increase in x, since a decrease in the solid anionic charge

caused by the replacement of O with F should decrease the

number of countercations (NH ). Therefore, the observed

(

A, B) and 1.0NbFO (C, D). The insert in (D) is a magniﬁed image

2−

of the red square. The {Nb O } pentagonal unit is marked by a

yellow dotted circle in (B).

−

.5NbFO. This material was a rod-shaped crystal showing a

increase in the amount of NH desorption suggests an increase

in the structural sites where NH₄can be accommodated. We

lattice fringe for the rod direction with an internal distance of

ca. 0.4 nm, being consistent with the XRD results. The

observed lattice fringe indicates periodic stacking of {NbO₆}

octahedra with a corner-sharing manner along the rod

direction. The diameter of the rod estimated from this image

was ca. 11 nm, which was clearly larger than that of 0NbFO

have reported that NH desorption takes place around 300 °C

from the heptagonal channel of crystalline Mo VO material

(MoVO) that is constituted by pentagonal units and octahedra

18−20

in a manner similar to that for HDS-NbO.

On the basis of

the structural similarity between HDS-NbO and MoVO, the

observed TPD proﬁle should be derived from the NH₃

desorption from micropores of HDS-NbFO. Accordingly, the

(

ca. 3 nm, Figure 1), implying an increase in the diameter by

−

the introduction of F , as was also seen in the SEM images.

Then the local crystal structure in a cross section was measured

from the (001) direction, and the obtained HAADF-STEM

image of 0.5NbFO is shown in Figure 3B. Since the contrast

increase in the amount of NH desorption by the introduction

−

of F should indicate an increase in the number of micropores

in the solid. On the basis of these results, we concluded that

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

the number of micropores in HDS-NbFO was increased by the

−

introduction of an appropriate amount of F . The increase in

the number of micropores might be caused by local structural

changes derived from the changes in the sharing state of

2−

−

{

NbO } octahedra. The replacement of O with F is

expected to increase the coordination number of Nb atoms

due to the decreased anionic charge, which should promote the

changes in the sharing state of the octahedra from an edge or

21−23

face sharing to corner sharing.

This local coordination

state change would contribute to the isolation of pentagonal

2−

{

Nb O } units to promote the formation of micropores, as

6 21

can be seen in the HAADF-STEM images. The increase in

crystallite size is also explainable, since corner sharing is

normally energetically more stable than are edge sharing and

face sharing, which cause an unfavorable repulsion between

cations due to the small distance.

Figure 5. (A) IR spectra of 0NbFO and 0.5NbFO after pyridine

adsorption. (B) Amount of acid at the Lewis site (L site, black bars)

and Brønsted acid site (B site, white bars) over xNbFO calculated on

the basis of surface area.

HAADF-STEM images of 1.0NbFO are shown in Figure

C,D. This material was also a rod-shaped material showing a

pyridinium ions formed on the B site and at 1445 cm⁻

assignable to coordinated pyridine on the L site. The amounts

of B and L sites were calculated on the basis of the weight loss

derived from pyridine (Figure S6) and the molar extinction

coeﬃcients for IR bands of the B site and L site (ε /ε = 1.33),

lattice fringe for the rod direction with an internal distance of

ca. 0.4 nm, indicative of the stacking of octahedra (Figure 3C).

Figure 3D shows a cross section of the rod-shaped crystal

having a hexagonal-shaped morphology. In the hexagonal-

^s^h^a^p^e^d^c^r^y^s^t^a^l^,^aⁿ^H^T^B^-^N^b⁽^F^,^O⁾x texture was partially

observed, as can be seen in the magniﬁed image in Figure

11,12

respectively.

The acid amounts of 0NbFO estimated

according to this method were comparable to the reported

−

D, while a large part of the crystal was disordered. The

observed disordered structure might be caused by an

values (reported values, B site 0.09 ± 0.01 mmol g , L site

−

−1

.20 ± 0.01 mmol g ; estimated values, B site 0.13 mmol g ,

−

amorphization of the HTB-Nb(F,O) structure by the air

L site 0.25 mmol g ). The environment around the Lewis

calcination conducted prior to the HAADF-STEM measure-

ment, as could be seen in XRD analysis.

acid in xNbFO should be similar, as can be seen in almost the

same shape of the absorption band at 1610 cm , which is

sensitively changed in a reﬂection of the environment around

−

On the basis of the results described above, it was found that

−

29,30

the introduction of an appropriate amount of F into HDS-

the Lewis acid site.

Figure 5B shows the amounts of B and

NbO enhanced the crystal growth and increased the number of

micropores in the crystal structure. However, the introduction

L sites on the basis of the surface area. The amounts of both B

and L sites were increased by an increase of x up to 0.5, while

the amounts of both sites were slightly decreased at x = 0.7. A

local structural change over the catalyst surface might be

related to the observed changes. The amount of acid over

1.0NbFO was diﬃcult to estimate, since it was quite diﬃcult to

make an IR disk of 1.0NbFO itself unless a binder (KBr) was

−

of an excess amount of F induced the formation of HTB-

Nb(F,O) . In the next section, acid properties over xNbFO will

be discussed.

.2. Acid Properties over xNbFO. Here, acid properties

over the catalyst surface of xNbFO are evaluated. Prior to

characterizations, xNbFO samples were calcined at 400 °C

−

mixed. The band area ratios of 1540 and 1445 cm , A₁₅₄₀/

A₁₄₄₅, over xNbFO are shown in Figure S7. The A₁₅₄₀/A₁₄₄₅

ratios were comparable at 0 ≤ x ≤ 0.7, indicating that the

ratios of the B site and L site over the catalyst surface are

comparable. However, the A₁₅₄₀/A₁₄₄₅ratio over 1.0NbFO was

slightly smaller than those at 0 ≤ x ≤ 0.7. The coformation of

under static air. Almost all of the NH₄was removed from the

structure of xNbFO, since almost no IR band derived from

−1

NH₄deformation (1401 cm ) was observed in HDS-NbO

with 0.1 ≤ x ≤ 0.5 after air calcination (Figure S5), while a

small band derived from NH₄was observed at 0.7 ≤ x.

Nevertheless, Brønsted acid sites (B sites) are expected to be

HTB-Nb(F,O) moieties in 1.0NbFO might be related to this

formed over xNbFO.

change.

NH adsorption TPD measurements are often conducted for

The acid strength over xNbFO was evaluated by the

Hammett indicator. The results obtained are summarized in

Table S1. xNbFO showed comparable acid strengths at 0 ≤ x

24−26

the characterization of acid properties.

This is quite a

useful method to evaluate the acid properties over a catalyst.

However, this method is considered not to be appropriate for

xNbFO, since this material has micropores that are capable of

≤ 0.7, and they were in the range of −8.2 < pK ≤ −5.6.

However, the acid strength over 1.0NbFO was smaller than

those of other xNbFO samples, and the strengths were −5.6 <

pK ≤ −3.0. The amorphized HTB-Nb(F,O) texture might

adsorbing NH and the amount of acid from the bulk would be

evaluated as well as that from the catalyst surface. Therefore,

we needed to use the base prove molecule inaccessible inside

the micropores in order to evaluate the acid properties over the

surface of xNbFO. For this purpose, a pyridine adsorption IR

experiment was carried out. This method is useful for

distinguishing the type of acid site (either Brønsted acid site

form weaker acid sites in comparison to those formed over

HDS-NbO. On the basis of the above results, we concluded

that the amount of acid over the catalyst surface of HDS-

NbFO was increased by the introduction of an appropriate

−

amount of F , while the acid strength was not changed

(

B site) or Lewis acid site (L site)), since pyridine adsorbed

drastically other than for 1.0NbFO. In the next section, the

catalytic activities over xNbFO for Brønsted acid reactions and

Lewis acid reactions will be discussed.

over the B site and that adsorbed over the L site give diﬀerent

IR bands.

7,28

The IR spectra of 0NbFO and 0.5NbFO after

pyridine adsorption are shown in Figure 5A. Both of the

3.3. Acid Reactions over xNbFO. As described above, i₋t

catalysts showed IR bands at 1540 cm⁻assignable to

was found that the addition of an appropriate amount of F

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 6. (A) Benzyl anisole yields on the basis of benzyl alcohol as a function of reaction time over A-NbO, 0NbFO, and 0.5NbFO. Reaction

conditions: benzyl alcohol/anisole/decane (internal standard), 1.0/45/0.5 mmol; catalyst amount, 0.050 g; reaction temperature, 100 °C. (B)

Initial formation rate of benzyl anisole over xNbFO. Reaction conditions: benzyl alcohol/anisole/decane (internal standard), 1.0/45/0.5 mmol;

catalyst amount, 0.020−0.033 g; reaction temperature, 100 °C; reaction time, 0−30 min.

1,32

modiﬁed the local crystal structure of HDS-NbO and increased

the number of micropore sites in the crystal structure. The acid

properties evaluated by pyridine adsorption IR and Hammett

indicator experiments revealed that the acid amount was

catalyzed the reaction.

Figure 6A shows the benzyl anisole

yields on the basis of benzyl alcohol as a function of reaction

time over 0NbFO, 0.5NbFO, and A-NbO. A-NbO was almost

inactive for this reaction, and almost no benzyl anisole was

formed. However, benzyl anisole was formed over 0NbFO and

the benzyl anisole yield increased with an increase in reaction

time. The reaction was almost complete at 2 h from the start of

the reaction. Interestingly, the benzyl anisole yield over

0.5NbFO was much higher than that over 0NbFO and the

reaction was almost complete within 1.0 h. Although the acid

properties, including the acid densities and acid strengths, were

similar over 0NbFO, 0.5NbFO, and A-NbO, the catalytic

activities were very diﬀerent. The initial formation rate of

benzyl anisole was then measured over xNbFO. Note that the

reaction rate was obtained in the range in which the yield of

benzyl anisole increased linearly with reaction time (Figure S9;

benzyl anisole yield below 15%). Figure 6B shows the initial

formation rate of benzyl anisole over xNbFO on the basis of

the surface area. In the case of 0NbFO, the initial formation

−

changed by the introduction of F , while the strengths were

comparable at 0 ≤ x ≤ 0.7. Catalytic activities for Brønsted

acid reactions (B reactions) and Lewis acid reactions (L

reactions) were evaluated over xNbFO.

As a B reaction, Friedel−Crafts alkylation was carried out by

using anisole and benzyl alcohol as substrates. This reaction is

widely used as a model reaction to elucidate the catalytic

activity for a B reaction, since a B site can promote the

,31,32

formation of an alkylation product (benzyl anisole).

xNbFO samples after calcination at 400 °C for 2 h under an air

atmosphere were used as catalysts for this reaction. Amorphous

Nb O ·2.1H O (A-NbO) calcined at 250 °C for 2 h under an

air atmosphere was used for comparison with xNbFO, since it

is well-known to be an eﬀective catalyst for various acid

reactions. The acid amount and acid strength over A-NbO

after the calcination were also evaluated by pyridine adsorption

IR and Hammett indicator. Figure S8A shows the results of

pyridine adsorption IR over A-NbO. IR bands derived from

rate on the basis of the surface area was 0.31 μmol min⁻m .

−2

This rate continuously increased with an increase in x, and a

−

−2

rate of 0.92 μmol min

was achieved over 0.5NbFO, 3

−

the adsorption of pyridine on the B site (1540 cm ) and on

times higher than that over 0NbFO. On the other hand, the

rate was signiﬁcantly decreased by a further increase in x and

the rate was 0.52 μmol min

activity changed with x in a manner similar to the amount of

acid estimated by pyridine adsorption IR. We reported that

HDS-NbO showed clearly higher catalytic activity for Friedel−

Crafts alkylation in comparison to the catalytic activities of

−

the L site (1445 cm ) were observed, and the amounts of B

and L sites were calculated by TG analysis (Figure S8B); and

the molar extinction coeﬃcients for IR bands of B and L sites

−

−2

over 1.0NbFO. The catalytic

were 1.0 and 0.4 μmol/m , respectively (BET surface area: 130

m /g). The ratio of the areas of IR bands derived from

pyridine adsorption over B and L sites (A₁₅₄₀/A₁₄₄₅) was 2.6,

comparable with that of xNbFO (0 ≤ x ≤ 0.7). The acid

other Nb-based oxides such as T-Nb O and pyrochlore. On

strength estimated by the Hammet indicator was −8.2 < pK ≤

the basis of these results, we suggested that the local catalyst

structure over HDS-NbO is responsible for the catalysis. In the

present study, local structural changes caused by the

introduction of F were suggested by the results of HAADF-

STEM analysis, and it was found that the number of

−

5.6 and was similar to that of xNbFO.

In the Friedel−Crafts alkylation, all of the catalysts tested

−

aﬀorded benzyl anisole as a main product and only a tiny

amount of benzyl ether was formed (less than 3% selectivity in

all cases), indicating that the B site over xNbFO dominantly

−

micropores was increased by the introduction of F . On the

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 7. (A) Lactic acid yields as a function of reaction time over A-NbO, 0NbFO, and 0.5NbFO. (B) Initial formation rate of lactic acid over

xNbFO.

area was 0.66 μmol min⁻m . This rate continuously

−2

basis of these facts, we here propose that the micropore sites

can work as B sites to catalyze this reaction. The local catalyst

structure around the heptagonal channel might be crucial for

the catalysis, since a signiﬁcant decrease in activity was

observed at 0.7 ≤ x, in which hexagonal textures derived from

increased with an increase in x, and the rate became 1.12

−

−2

μmol min

over 0.5NbFO. On the other hand, the rate

was signiﬁcantly decreased by a further increase in x and the

−

−2

rate was 0.35 μmol min

over 1.0NbFO. The catalytic

the HTB-Nb(F,O) moieties was observed by XRD patterns

activity changed with x in a manner similar to the B reaction,

suggesting that the micropore sites over the HDS-NbO catalyst

provided the water-tolerant Lewis acid sites related to the

catalysis. We reported that the local structure around the

(

Figure 2D) and HAADF-STEM images (Figure 3). Arrhenius

plots over 0NbFO and 0.5NbFO in the temperature range of

0−110 °C were obtained (Figure S10). While these catalysts

34−36

showed comparable activation energies (0NbFO, 129.3 kJ/

heptagonal channel site can form oxygen defect sites.

mol; 0.5NbFO, 127.9 kJ/mol), an almost 3-fold diﬀerence was

According to that report, it can be speculated that the oxygen

defect sites around micropore sites form coordinatively

unsaturated sites workable as L sites even in the presence of

water. Furthermore, the local catalyst structure around the

heptagonal channel site should be crucial for this catalytic

reaction, since the catalytic activity decreased at 0.7 ≤ x

−

observed in their frequency factors (0NbFO, 0.99 μmol min

−

−1

−2

m ; 0.5NbFO, 2.44 μmol min m ), implying that the

activity diﬀerence between 0NbFO and 0.5NbFO is derived

from the increase in the number of catalytically active sites.

The decrease in the catalytic activity at 0.7 ≤ x is most

probably caused by the concomitant formation of the HTB-

possibly due to the coformation of the HTB-Nb(F,O)

moieties that is constituted by the arrangement of the

hexagonal channels.

On the basis of the above facts, we suggest here that the B

site and L site generated over the micropore sites, particularly

over the heptagonal channel site, can work as the catalytically

active sites. A crucial role of the local catalyst structure in acid

reactions was clariﬁed (Figure 8).

Nb(F,O) phase with HDS-NbO.

Nakajima et al. discovered that Nb O ·nH O has water-

tolerant Lewis acidity that can catalyze acid reactions relating

to the Lewis acid site even in the presence of water. We

recently reported that 0NbFO showed water-tolerant Lewis

acidity that can catalyze the transformation of pyruvaldehyde

to lactic acid and the dehydration of xylose to form furfural

even in the presence of water. Here, we tested the

transformation of pyruvaldehyde to lactic acid in order to

evaluate the catalytic activity over the xNbFO catalyst for

Lewis acid reaction in the presence of water. Figure 7A shows

the changes in lactic acid yield with reaction time over 0NbFO,

4. CONCLUSION

High dimensionally structured Nb oxide (HDS-NbO)

−

containing F was synthesized by the addition of NH F in

−

hydrothermal synthesis. F was located by replacing the lattice

oxygen in the HDS-NbO structure, and the amount of F

could be increased up to an F /Nb ratio of 0.55. Interestingly,

−

.5NbFO, and A-NbO. As we reported previously, 0NbFO and

−

A-NbO promoted the formation of lactic acid. The catalytic

activity of 0NbFO was clearly superior to that of A-NbO, and

the lactic acid yields at 3 h from the start of the reaction were

the local catalyst structure was changed by the introduction of

−

so as to increase the number of micropores including

2.7% over A-NbO and 75.5% over 0NbFO. 0.5NbFO showed

hexagonal and heptagonal channels. The introduction of an

−

catalytic activity comparable to that of 0NbFO, and the lactic

acid yield at 3 h was 85.4%. Figure 7B shows the initial

formation rate of lactic acid over xNbFO on the basis of

surface area. The reaction rate was obtained in the range in

which lactic acid yield increases linearly with reaction time

appropriate amount of F substantially increased the catalytic

activity for Brønsted reactions, as evaluated by Friedel−Crafts

alkylation. However, the catalytic activity was decreased by

−

excess addition of F . Almost the same trend was observed in

the water-tolerant Lewis acid reaction, as evaluated by the

transformation of pyruvaldehyde to lactic acid. On the basis of

(Figure S11 ). In 0NbFO, the initial reaction rate per surface

Inorg. Chem. XXXX, XXX, XXX−XXX

orcid.org/0000-0001-7855-8643

Article

Wataru Ueda − Department of Material and Life Chemistry,

Faculty of Engineering, Kanagawa University, Yokohama 221-

686, Japan; Email: uedaw@kanagawa-u.ac.jp

Authors

Mai Shinoda − Department of Material and Life Chemistry,

Faculty of Engineering, Kanagawa University, Yokohama 221-

686, Japan

Yuta Motoki − Department of Material and Life Chemistry,

Faculty of Engineering, Kanagawa University, Yokohama 221-

686, Japan

Shota Tsurumi − Department of Material and Life Chemistry,

Faculty of Engineering, Kanagawa University, Yokohama 221-

686, Japan

Momoka Kimura − Department of Material and Life

Chemistry, Faculty of Engineering, Kanagawa University,

Yokohama 221-8686, Japan

Norihito Hiyoshi − National Institute of Advanced Industrial

Science and Technology (AIST), Sendai 983-8551, Japan;

Akihiro Yoshida − Institute of Regional Innovation, Hirosaki

Figure 8. Image of the B reaction and L reaction over the heptagonal

channel of HDS-NbO. Color scheme: Nb, light blue; O, red; C, gray;

H, white.

University, Aomori 030-0813, Japan

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c00949

Author Contributions

The manuscript was written through contributions by all

authors. All authors have given approval to the ﬁnal version of

the manuscript.

the local catalyst structure and the catalytic activity, we suggest

that the Brønsted acid sites and Lewis acid sites formed around

the heptagonal channel work as the catalytically active sites for

these reactions. We conclude that the local catalyst structure

formed in the crystal structure is crucial for the catalysis of the

above acid reactions.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

This work was supported by JSPS KAKENHI Grant Number

15H0-2318.

■

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00949.

■

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■

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Satoshi Ishikawa − Department of Material and Life Chemistry,

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