Influence of Morphology of Silica-Alumina Composites on ... - J-Stage

J. Jpn. Inst. Energy, Vol. 95, No. Journal of 6,the2016 Japan Institute of Energy , 95, 480-486（2016）

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Influence of Morphology of Silica-Alumina Composites on Their Activity for Hydrolytic Dehydrogenation of Ammonia Borane Naoki TOYAMA ※ 1, Shinobu OHKI ※ 2, Masataka TANSHO ※ 2, Tadashi SHIMIZU ※ 2, Tetsuo UMEGAKI ※ 1†, and Yoshiyuki KOJIMA ※ 1 （Received January 5, 2016）

In this work, we investigate the influence of the morphology of silica-alumina composites on their activity for hydrolytic dehydrogenation of ammonia borane. Three type of composites, hollow spheres, fine particles, and spherical particles are prepared by sol-gel method. The hollow spheres were prepared by using polystyrene particles as templates. The morphology of composites were observed by transmission electron microscopy. The activity of each type of composite for hydrolytic dehydrogenation of ammonia borane was compared. In the presence of the hollow spheres, the fine particles, and the spherical particles, 10, 2.5, and 1.5 mL of hydrogen was released with the completion times of the reaction being 12, 2, and 1 min, respectively. The amount of hydrogen evolution from the hollow spheres was much higher as compared to those from the fine particles and the spherical particles. Temperature-programmed desorption of ammonia suggested that the hollow spheres possess both weak and strong Brønsted acid sites, while the fine particles and the spherical particles possess only the weak Brønsted acid sites. These results indicate that the morphology of the silica-alumina composites influences their acidic properties, and the strong Brønsted acid sites are more effective for hydrolytic dehydrogenation of ammonia borane, as compared to the weak Brønsted acid sites. 本研究では，球状中空シリカ -アルミナ，シリカ -アルミナ微粒子および球状シリカ -アルミナをそれぞれ調製し，シリカ -アルミナの形態の違いがアンモニアボラン加水分解活性に与える影響について検討を行った。すべての試料はゾル - ゲル法によって調製し，球状中空シリカ -アルミナはポリスチレン粒子をテンプレートとする方法で調製した。透過型電子顕微鏡写真の結果から，球状中空体，微粒子および球状体が確認できた。これらの試料を用いてアンモニアボラン加水分解活性を行った。球状中空体，微粒子および球状体の存在下では，10，2.5，および 1.5 mL の水素発生が 12，2 および 1 min にそれぞれ完了した。この結果から，球状中空体の活性が微粒子および球状体と比較して高活性であった。またアンモニア昇温脱離測定の結果から，球状中空体は弱ブレンステッド酸点および強ブレンステッド酸点を有しており，微粒子および球状体は弱ブレンステッド酸点のみ有していた。以上の結果から，シリカ -アルミナの形態は酸性質に影響があり，強ブレンステッド酸点は，弱ブレンステッド酸点よりもアンモニアボラン加水分解に有効であることが示唆された。

Key Words Ammonia borane; Hydrolytic dehydrogenation; Morphology; Silica-alumina; Brønsted acid sites

1.　Introduction

developing efficient methods for hydrogen generation

Hydrogen is a clean energy carrier. It has been

and storage is a significant step toward future hydrogen

considered the most promising fuel, and can be a good

economy 3) ～ 5). In recent years, ammonia borane (NH3BH3)

. Hence,

has attracted significant attention as an efficient hydrogen

alternative to the conventional fossil fuels ※ 1 ※ 2 †

1) 2)

Department of Materials & Applied Chemistry, College of Science & Technology, Nihon University 1-8-14, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8308 National Institute for Materials Science 3-13, Sakura, Tsukuba, Ibaraki 305-0003 Corresponding Author

storage material because of its high hydrogen density (19.6 wt.%), low molecular weight (30.9 g mol-1), and nontoxicity 6)～ 9). Moreover, NH3BH3 can release hydrogen through hydrolysis in the presence of suitable acids or catalysts under mild conditions (Eq. 1) 10) ～ 20).

J. Jpn. Inst. Energy, Vol. 95, No. 6, 2016

NH3BH3 + 2H2O → NH4+ + BO2- + 3H2

(1)

481

heated at 343 K for 24 h under stirring at 250 rpm. The

Among various types of acids and catalysts, solid

final PS suspension was centrifuged at 6000 rpm for 5 min

acids have been investigated for hydrolytic dehydrogenation

and washed three times with ethanol (30 mL; Kanto Chem.

of NH 3 BH 3 10)

. However, only a few related studies

Co., >99.5%). The PS contents could be tailored through

have been published, and there is little information on the

the addition of ethanol. Aluminum isopropoxide (0.0057 g;

influence of important factors such as the structure and

Aldrich, >98.0%), aqueous ammonia solution (3 mL; 28 wt.%,

elemental compositions of the solid acids on their activity.

Kanto Chem. Co.), ethanol (40 mL), and tetraethoxysilane

～ 16)

Hollow spheres have been used in a wide range

(0.1551 mL; TEOS, Kanto Chem. Co., >99.9%) were added to

of applications such as catalysis, drug delivery, controlled

the PS suspension (15 g). The sol-gel reaction was carried

release, optoelectronics, microcavity resonance, and photonic

out at 323 K for 1.5 h. The composite was dried overnight

crystals 21) 22). Hollow spheres are prepared via the hard

in a desiccator. Then, the composites were calcined in air

template method using colloidal particles as templates,

at 873 K at a heating rate of 0.5 K min-1, and cooling down

as this technique allows the formation of monodisperse

immediately after the designated temperature was reached.

particles with a facile control of the shell and void structures 23) ～ 29).

Fine silica-alumina composite particles were prepared by sol-gel method without the PS template particles.

In recent years, microsized, monodisperse, hollow

Aluminum isopropoxide (0.0057 g), aqueous ammonia

silica 30) 31) and titania 32) spheres have been prepared using

solution (3 mL), and TEOS (0.1551 mL) were added to

polystyrene (PS) particles as templates. In this approach,

ethanol (40 mL). The sol-gel reaction was carried out at

positively charged PS particles were prepared by dispersion

323 K for 1.5 h. The composite was dried overnight in a

polymerization using the cationic precursor, 2-(methacryloyl)-

desiccator. Then, the composite was calcined under the

ethyltrimethylammonium chloride (MTC) as the co-

same conditions used for the hollow spheres.

monomer , or by emulsifier-free emulsion polymerization

Spherical silica-alumina composite particles were

using α,α’-azodiisobutyramidine dihydrochloride as the

prepared by sol-gel reaction. Ion-exchanged water (7 mL),

33)

initiator and poly(vinyl pyrrolidone) as the stabilizer 34) 35).

TEOS (1.615 mL), aqueous ammonia solution (1 mL), and

In previous study, we have reported that hollow silica-

aluminum isopropoxide (0.0593 g) were added to ethanol

alumina composite spheres show high activity for hydrolytic

(20 mL). The sol-gel reaction was carried out at 323 K for 1 h.

dehydrogenation of NH3BH3 14) 16). However, the influence of

The composite was dried overnight in a desiccator. Then,

morphology of silica-alumina composites on their activity is

the composite was calcined under the same conditions used

not clear. In this study, we prepared hollow silica-alumina

for the hollow spheres.

composite spheres, fine silica-alumina composite particles, and spherical silica-alumina composite particles, and investigated their activity for hydrolytic dehydrogenation of NH3BH3 .

　2.2　Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku MultiFlex X-ray diffractometer using Cu K α radiation (λ=0.15406 nm) operating at 30 kV and

2.　Experiments 　2.1　Preparation of composites

16 mA. The specific surface areas of the composites were measured by N2 sorption at 77 K using an ASAP 2010

Hollow silica-alumina composite spheres were

Automatic Physical Adsorption Instrument. Transmission

fabricated via the PS template method. Monodisperse

electron microscopy (TEM) observation was carried

PS particles were prepared by emulsifier-free emulsion

out using a Hitachi FE-2000 system operating at an

polymerization using the following procedure. Styrene

acceleration voltage of 200 kV. The elemental composition

(9.0 mL; Kanto Chem. Co., >99.0%), poly(vinyl pyrrolidone)

of the composites was determined using Hitachi TM-3000

K30 (1.5 g; Fluka, Mw ≈ 40000), and the cationic initiator

scanning electron microscope with an energy dispersive

2,2’-azobis(2-metylpropionamidine) dihydrochloride (0.26 g;

X-ray analysis (SEM/EDX). Solid-state

Al magic angle

27

Wako Pure Chemical, >97.0%) were dissolved in ion-

spinning (MAS) nuclear magnetic resonance (NMR) spectra

exchanged water (100 mL) inside a 250 mL three-necked

were recorded on a JEOL ECA-800 spectrometer (18.8 T);

flask. The flask was equipped with a mechanical stirrer, a

the relaxation delay time was 2 s, and the composites were

thermometer with a temperature controller, a nitrogen gas

spun at 20 kHz using a 3.2 mm ZrO2 rotor. Chemical shifts

inlet, and a Graham condenser, and it was placed in an oil

were referenced to 1.0 M aqueous aluminum chloride (Wako

bath for heating. The reaction solution was deoxygenated

Pure Chemical, >98.0%) solution. Temperature-programmed

by bubbling nitrogen gas at room temperature for 1 h, and

desorption of ammonia (NH 3 -TPD) was carried out on a


482

BELCAT-B instrument. The analysis was performed by

sizes. Particle agglomeration was observed in some parts

loading 50 mg of the composites into a quartz reactor and

of this composite. Fig. 1 (c) reveals the diameter of the

drying them under a flow of pure He at 783 K for 1 h

spherical particles to be about 260-280 nm.

followed by purging with pure He at the same temperature

The phase and composition of the hollow spheres, the

for 1 h. The composites were allowed to cool to 373 K under

fine particles, and the spherical particles were determined

the He flow, and then exposed to NH3 -He gas mixture (95

by powder XRD measurements. Fig. 2 shows the XRD

vol.% He) at 373 K for 1 h, to allow NH3 adsorption. The

patterns of the three type of silica-alumina composites.

composites were then purged using pure He, to allow for

The XRD patterns of all the composites showed a broad

the accurate detection of the desorbed NH3 . The NH3 -TPD

diffraction peak at around 2θ = 23°, indicating a typical

measurements were conducted by heating the composites

amorphous silica-alumina 36). The specific surface areas of

from 373 K to 773 K at a rate of 10 K min under a flow of

the hollow spheres, the fine particles, and the spherical

-1

pure He. The desorbed NH3 molecules were detected by a

particles were measured using the Brunauer-Emmett-

thermal conductivity detector (TCD).

Teller (BET) method. From the results, specific surface areas of the hollow spheres, the fine particles, and the

　2.3　Activity for hydrolytic dehydrogenation of NH3 BH3

spherical particles were found to be 393, 295, and 13 m2 g-1,

The composites (0.8 g) was placed in a two-necked

respectively. The result indicates that the specific surface

round-bottomed flask under air at room temperature. One

area of the hollow spheres was the highest of the three

of the necks was connected to a gas burette; the other was

composites. The elemental composition of the composites

connected to an addition funnel. The reaction was initiated

were determined by SEM/EDX analysis. The Si/Al molar

by adding aqueous NH 3 BH 3 solution (3.5 mL, 0.14 wt.%;

ratios of the hollow spheres, the fine particles, and the

Aldrich, 90%) from the addition funnel to the composites.

spherical particles were similar at 30.3, 32.1, and 29.7,

The evolution of gas from the reaction was monitored using

respectively.

the gas burette.

Activities of the silica-alumina composites for hydrolytic dehydrogenation of NH 3 BH 3 were compared.

3.　Results and discussion

Fig. 3 shows the time course of hydrogen evolution from

The morphology of the silica-alumina composites

aqueous NH 3 BH 3 solution in the presence of the silica-

was observed by TEM images as shown in Fig. 1. From

alumina composites. The hollow spheres, the fine particles,

the result of Fig. 1 (a), homogeneous hollow spheres were

and the spherical particles evolved 10, 2.5, and 1.5 mL of

observed in the TEM image of the composites prepared

hydrogen with the completion of the reaction in 12, 2, and

using PS templates. The shell thickness and diameter of the

1 min, respectively. The molar ratios of the hydrolytically

hollow spheres were 5-7 nm and 210-230 nm, respectively.

evolved hydrogen to the initial NH3BH3 were 2.6, 0.6, and 0.3

Fig. 1 (b) reveals that the fine particles are about 13 nm in

in the presence of the hollow spheres, the fine particles, and the spherical particles, respectively. These results indicate that the amount of hydrogen evolved in the presence of the

Fig. 1 TEM images of (a) hollow silica-alumina composite spheres, (b) silica-alumina composite fine particles, and (c) spherical silica-alumina composite particles

Fig. 2 Powder XRD pattern of (a) hollow silica-alumina composite spheres, (b) silica-alumina composite nano particles, and (c) spherical silica-alumina composite particles


483

hydrogen evolution than the original hollow spheres because the acidic protons might be exchange into ammonium ion on the Brønsted acid sites of the hollow spheres during hydrolytic dehydrogenation of NH3BH3 . Then, the recycled hollow spheres were calcined at 723 K at heating rate of 0.5 K min -1, and cooling down immediately after the designated temperature was reached. The recycled hollow spheres after calcination showed same activity as the recycled hollow spheres. These results suggest that almost all the acidic protons on Brønsted acid sites of the hollow Fig. 3 The H2/NH3BH3 molar ratios of hydrogen generated from aqueous NH 3 BH 3 solution (0.14 wt.%, 3.5 mL) in the presence of (a) hollow silica-alumina composite spheres (0.8 g), (b) silica-alumina composite fine particles (0.8 g), and (c) spherical silica-alumina composite particles (0.8 g)

spheres were consumed by the hydrolytic dehydrogenation reaction. To confirm the coordination number of the composites, solid-state 27Al MAS NMR spectra of the composites and the recycled composites was measured. All the composites have 4-, 5-, and 6-coordinated aluminum species, as evidenced

hollow spheres was significantly higher than the amount of

by the separate 27Al signals at around 66, 35, and 5 ppm

hydrogen evolved in the presence of the fine particles and

shown in Fig. 5. It has been reported that 4-coordinated

the spherical particles. It has been reported that the acidic

aluminum species are associated with Brønsted acid sites,

protons on Brønsted acid sites promote the dissociation

while 5- and 6-coordinated aluminum species are associated

of the B-N bond and the hydrolysis of BH 3 species to

with Lewis acid sites 37) ～ 39). We calculated the area under

produce borate ion species along with the hydrogen release

the peaks of the 4-, 5-, and 6-coordinated aluminum species

(Eq. 1) 10) 13) 14). However, the hydrolytic dehydrogenation

using a Gaussian function. The ratio of the peak area of

of NH 3 BH 3 in the presence of the hollow spheres was

4-coordinated aluminum species to the total area of the

incomplete. It is suggested that the H3BO3 produced from

peaks of 4-, 5-, and 6-coordinated aluminum species (I4/Iall)

the reaction of BO2- with the acidic protons on Brønsted

in the hollow spheres, the fine particles, and the spherical

acid sites shift the reaction shown in Eq. 2 to right 10).

particles was 0.26, 0.19, and 0.18, respectively. From the

H+ + BO2- + H2O ↔ H3BO3

(2)

result of Fig. 4 (B) and (C), the peaks of the recycled fine

Fig. 4 shows TEM images of the hollow spheres

particles and the recycled spherical particles were not

before and after the reaction. From the result, it was

significantly different from the original fine particles and

revealed that the hollow spheres almost maintain their

the original spherical particles. The

Al signals of the

27

original morphology even after the reaction. To determine

recycled hollow spheres were observed at around 56, 35,

the recycle ability of the composites, the activity of the

and 5 ppm as shown in the spectra in Fig. 5 (A). From the

recycled composites was compared. The recycled hollow

result, the chemical shift of the 4-coordinated aluminum

spheres evolved 1.5 mL of hydrogen with the completion

species of the recycled hollow spheres is red shifted as

of the reaction in 2 min. On the other hand, the recycled

compared to that of the native hollow spheres. It has been

fine particles and the spherical particles showed no activity.

reported that the peak of 4-coordinated aluminum species

The recycled hollow spheres were much lower amount of

at around 66 ppm is strongly related to hydroxide group . Based on these results, the recycled hollow spheres are

40)

suggested to have fewer Brønsted acid sites. The results also demonstrated the recycle ability of the hollow spheres. The acidic properties of the silica-alumina composites were measured using NH 3 -TPD. Fig. 6 shows NH 3 -TPD profiles of the hollow spheres, the fine particles, and the spherical particles. The NH 3 desorption from the hollow spheres showed two peaks: first peak at around 420 K (low temperature peak) and a broad peak at around 580 K (high Fig. 4 TEM images of hollow silica-alumina composite spheres (a) before and (b) after reaction for hydrolytic dehydrogenation of NH3BH3

temperature peak), while the NH 3 desorption from the fine particles and the spherical particles showed peaks at around 420 and 430 K (low temperature peaks), respectively.

484


Fig. 6　NH3 -TPD of (a) hollow silica-alumina composite spheres, (b) silica-alumina composite fine particles, and (c) spherical silica-alumina composite particles

and spherical particles possess only weak Brønsted acid sites. The amount of Brønsted acid sites calculated from the areas under the peak in the temperature range 400600 K 37)

～ 39)

for the hollow spheres, the fine particles,

and the spherical particles was 0.18, 0.10, and 0.06 mmol g-1, respectively. The result indicates that the amount of Brønsted acid sites in the silica-alumina composites depends on their morphology. Fig. 7 shows the H 2 /NH 3 BH 3 molar ratio for the hydrogen evolved from aqueous NH3BH3 solution versus the amount of Brønsted acid sites of the composites. According to the result, the amount of hydrogen evolution increases with increase of the amount of Brønsted acid sites. The amount of Brønsted acid sites in the fine particles is found to be 1.7 times higher than those in the spherical particles.

Fig. 5 Solid-state 27Al MAS NMR of (A) hollow silica-alumina composite spheres, (B) silica-alumina composite fine particles, and (C) spherical silica-alumina composite particles (a) before and (b) after reaction for hydrolytic dehydrogenation of NH3BH3

The low temperature peaks can be attributed to Brønsted acid sites with weakly adsorbed NH3 (weak Brønsted acid sites), while the high temperature peak observed for the hollow spheres can be attributed to Brønsted acid sites with strongly absorbed NH3 (strong Brønsted acid sites) 41) ～ 43). These results indicate that the hollow spheres possess both weak and strong Brønsted acid sites, while the fine particles

Fig. 7 The H2/NH3BH3 molar ratios of hydrogen generated from aqueous NH 3 BH 3 solution (0.14 wt.%, 3.5 mL) versus amount of Brønsted acid sites


Moreover, the amount of hydrogen evolved from the fine particles is 2 times higher than that from the spherical particles. On the other hand, the amount of Brønsted acid sites of the hollow spheres was 3 times higher than that of spherical particles, and the amount of hydrogen evolution of the hollow spheres is 8.7 times higher than those in the spherical particles. Based on the results of Figs. 3 and 5, it can be suggested that the morphology of silica-alumina composites influences their acidic properties, and that the strong Brønsted acid sites are more effective for hydrolytic dehydrogenation of NH3BH3 than the weak Brønsted acid sites.

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4.　Conclusion In this work, we investigated the influence of the morphology of silica-alumina composite on their activity for hydrolytic dehydrogenation of NH 3 BH 3 . TEM images were used to observe the morphology of the hollow spheres, the fine particles, and the spherical particles. The activity of each type of composite for hydrolytic dehydrogenation of NH 3 BH 3 was compared. According to the results, the amount of hydrogen evolved in the presence of the hollow spheres was significantly higher as compared to those evolved in the presence of the fine particles and the spherical particles. From the results of NH3 -TPD profiles, the hollow spheres are found to possess both weak and

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　Acknowledgement

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