Brønsted acid property of alumina-based mixed-oxides-supported tungsten oxide

Journal Pre-proof Brønsted Acid Property of Alumina-Based Mixed-Oxides-Supported Tungsten Oxide Mizuki Saito (Investigation) (Writing - original draft), Takeshi Aihara (Investigation)Data Curation), Hiroki Miura (Methodology) (Writing - review and editing), Tetsuya Shishido (Conceptualization) (Supervision) (Project administration) (Writing review and editing)

PII:

S0920-5861(20)30062-6

DOI:

https://doi.org/10.1016/j.cattod.2020.02.009

Reference:

CATTOD 12677

To appear in:

Catalysis Today

Received Date:

2 October 2019

Revised Date:

11 January 2020

Accepted Date:

6 February 2020

Please cite this article as: Saito M, Aihara T, Miura H, Shishido T, Brønsted Acid Property of Alumina-Based Mixed-Oxides-Supported Tungsten Oxide, Catalysis Today (2020), doi: https://doi.org/10.1016/j.cattod.2020.02.009

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Brønsted Acid Property of Alumina-Based Mixed-Oxides-Supported Tungsten Oxide Running title Acid Property of Supported Tungsten Oxide Mizuki Saitoa, Takeshi Aiharaa, Hiroki Miuraa,b,d, and Tetsuya Shishido a,b,c,d,* a

Department of Applied Chemistry for Environment, Graduate School of Urban

Tokyo 192-0397, Japan b

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Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji,

Research Center for Hydrogen Energy-Based Society, Tokyo Metropolitan University,

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1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan c

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Research Center for Gold Chemistry, Tokyo Metropolitan University, 1-1

Minami-Osawa, Hachioji, Tokyo 192-0397, Japan d

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Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, 1-30

Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan Corresponding author: [email protected]

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Graphical abstract

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*

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Highlights

WO3/Al2O3–TiO2 showed high activity for Brønsted acid-catalyzed reactions.

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Brønsted acids were formed at the boundaries between amorphous WO3 and

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Surface Lewis acid and OH groups are key to form Brønsted acid on

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

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Al–Ti support.

Abstract

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Al2O3-based supports.

The effect of alumina-based supports (Al2O3, Al2O3–TiO2, Al2O3–ZrO2, SiO2–Al2O3,

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and SiO2) on the structure and acid properties of supported tungsten oxide catalysts was

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investigated. Among the tested supported tungsten oxide catalysts, WO3/Al2O3–TiO2 (Al2O3/TiO2 = 9, Al–Ti9) showed the highest activity for reactions catalyzed by Brønsted acid sites and the largest Brønsted acidity. Structural characterization revealed that Brønsted acid sites on WO3/Al–Ti9 were generated at the boundaries between domains of amorphous monolayer WO3 and Al2O3–TiO2 support and WO3/Al2O3 reported previously, and that Al2O3 and Al–Ti9 with a high density of Lewis acid and

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surface hydroxyl groups resulted in the formation of Brønsted acid sites. Crystalline WO3 species formed mainly on SiO2–Al2O3 and SiO2 with a low density of Lewis acid sites and a high density of surface hydroxyl groups. These results suggested that a high density of Lewis acid sites and surface hydroxyl groups are important factors to form amorphous WO3 monolayer domains and to generate Brønsted acid sites on

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alumina-based supports.

Keywords: Brønsted acid, Tungsten oxide, Surface hydroxyl group, Lewis acid

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1. Introduction

Supported metal oxides have been studied widely to control their catalytic

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properties and the structure of surface metal oxides, by using different supports, precursors, and calcination temperatures [1-18]. For example, tungsten oxides that are

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supported on zirconia, titania, and iron oxide (WO3/ZrO2, WO3/TiO2, and WO3/Fe2O3) are termed solid superacid catalysts. The acid strength of WO3/ZrO2, WO3/TiO2, and

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WO3/Fe2O3 is estimated to be H0 < –14.6, –13.1, and –12.5, respectively. These catalysts promote acid reactions that require superacid sites, such as skeletal

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isomerization of butane [5-18]. The acid properties of tungsten oxide on alumina (WO3/Al2O3) have also been investigated. Arata et al. reported that 5 and 10 wt%

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WO3/Al2O3 exhibited a high activity for cumene cracking when the catalyst was calcined above 1273 K [5-7]. Soled et al. suggested that high-temperature calcination increased the fraction of Brønsted acidity to total acidity for 10 wt% WO 3/Al2O3 calcined at 1173 K [9]. However, the relationship between the structure and acidic

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properties of WO3/Al2O3 remains unclear and the structure of the acid sites has not been clarified. Recently, we characterized structural changes in a series of alumina-supported tungsten oxide catalysts with various WO3 loadings to clarify the relationship between their acidic properties and the local structures around the tungsten species. WO3/Al2O3 (20 wt%) exhibited the highest reactivity for acid-catalyzed reactions (benzylation of

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anisole and isomerization of α-pinene) and Brønsted acidity [18]. Tungsten oxide was loaded as two-dimensional monolayer domains below 20 wt% and these domains

covered most of the alumina surface at 20 wt%. Additional WO 3 loading causes the

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formation of inert WO3 crystals and a decrease of Brønsted acidity. Brønsted acid sites

are probably generated at tungsten oxide monolayer domain boundaries. A proposed

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generation mechanism of Brønsted acid sites is as follows. At a low loading, tungsten

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oxide monolayer domains are dispersed on alumina and few boundaries are present between domains. As the WO3 loading increases, the number of tungsten oxide monolayer domains increases and boundaries between domains form Brønsted acid sites.

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When tungsten oxide monolayer domains cover most of the alumina surface (20 wt% WO3/Al2O3), the number of boundaries between tungsten oxide monolayer domains is

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maximized, which generates the largest number of Brønsted acid sites. In the proposed

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model, the formation of tungsten oxide monolayer domains that are stabilized on the support has an important role to generate Brønsted acid sites. However, the relationship between the support surface property, the structure of the supported tungsten oxide, and the acidic properties of the supported tungsten oxide catalysts remains unclear. We investigated the effect of alumina-based supports (Al2O3, Al2O3–TiO2, Al2O3–ZrO2, SiO2–Al2O3, and SiO2) on the structure of tungsten oxide and the acid

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property of supported tungsten oxide catalysts. We characterized structural changes in a series of tungsten oxides that were supported on alumina-based supports with various WO3 loadings to clarify the relationship between their acidic properties and the local structures around tungsten species.

2. Experimental

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2.1 Materials

α-Pinene, camphene, limonene, β-pinene, α-terpinene, γ-terpinene, and terpinolene were from Wako Pure Chemical Industries and Tokyo Chemical Industry,

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Japan. Anisole and benzylalcohol were from Wako Pure Chemical Industries, Japan.

Products, such as p-benzylanisole and o-benzylanisole, were synthesized, and dibenzyl

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ether was from Tokyo Chemical Industry, Japan. Cumene and benzene were from Wako

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Pure Chemical Industries, Japan. γ-Al2O3 (JRC–ALO–8), Al2O3-SiO2 (JRC–SAH–1) and SiO2 (JRC-SIO-10) were provided by the Catalysis Society of Japan. (NH4)10W12O41 ･5H2O, aluminum sec-butoxide, titanium isopropoxide, ethanol and

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Industry, Japan.

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ethyl acetoacetate were from Wako Pure Chemical Industries and Tokyo Chemical

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2.2 Catalyst preparation

A series of Al2O3-based mixed oxides supports was prepared by the sol–gel

method [19, 20]. Aluminum sec-butoxide was dissolved in ethanol solvent followed by the addition of ethyl acetoacetate. After vigorously stirring of the solution for 30 min, titanium isopropoxide or zirconium propoxide was added to obtain a solution with a final composition with a weight% TiO2 or ZrO2. Distilled water in the following molar

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ratio, H2O/alkoxide = 5, was added dropwise to the mixed solution to complete the hydrolysis reaction. The mixed solution was stirred continuously for 2 h and heated at 353 K to transform the solution to a dried gel. The dried gel was dried in air at 353 K for 1 h, and calcined at 1123 K for 3 h in air. We abbreviate Al2O3–MeO2 (Me: metal, Al2O3/MeO2 = X) as Al–MeX. The BET surface areas of the alumina-based support are summarized in Table 1.

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A series of WO3/Al2O3 catalysts was prepared by impregnation of supports with an aqueous solution of (NH4)10W12O42, drying at 353 K for 6 h, and then calcining

at 1123 K for 3 h in flowing air. A series of supported WO3 catalysts was prepared by

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the impregnation method and calcined at 773 K for 3 h in flowing air. The WO3 loading

was estimated at 100% coverage with a WO3 monolayer by using the cross-sectional

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area of a WO6 octahedral unit (0.22 nm-2) [21]. The BET surface areas and measured

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WO3 coverages of the supported WO3 catalysts are summarized in Table 2. Table 1 Properties of alumina-based supports

Abbreviation

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Support

/ m2 g-1

JRC-ALO-8

Al2O3

146

Al2O3/TiO2=9

Al-Ti9

241

Al2O3/TiO2=7

Al-Ti7

253

Al2O3/TiO2=5

Al-Ti5

217

Al2O3/TiO2=3

Al-Ti3

239

Al2O3/ZrO2=9

Al-Zr9

249

Al-Si

432

SiO2

192

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Al2O3

SBET

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Al2O3-TiO2

Al2O3-ZrO2 Al2O3-SiO2 SiO2

JRC-SAH-1 (Al2O3/SiO2=0.5) JRC-SIO-10

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a) Japan Reference Catalysts

Table 2 Properties of supported WO3 catalysts

amount (wt%)

SBET

Surface

/ m2 g-1

coverage (%)

20

115

99

WO3/Al-Ti9

30

171

100

WO3/Al-Ti7

31

168

106

WO3/Al-Ti5

28

159

101

WO3/Al-Ti3

29

167

99

WO3/Al-Zr9

30

154

113

WO3/Al-Si

44

123

204

WO3/SiO2

25

128

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WO3/Al2O3

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loading

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WO3

Catalyst

112

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2.3 Catalytic activity test

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a) Estimated coverage with WO3 monolayer

Isomerization of α-pinene, benzylation of anisole (Friedel–Crafts alkylation of

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anisole with benzylalcohol), and cumene cracking were used as test reactions to examine the acid–base properties of WO3-supported catalysts.

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Isomerization of α-pinene was performed in a dry nitrogen atmosphere by

using a stirred batch reactor. Prior to each run, sample (100 mg) was pretreated at 673 K in an oxygen atmosphere for 1 h and evacuated at the same temperature for 1 h. α-Pinene (12.5 mmol) was added to the reactor and stirred at 323 K for 3 h. The products were determined by gas chromatography (GC-2014 with a flame ionization detector, Shimadzu) using a CBP20 column. 7

Anisole benzylation was examined in the liquid phase. Catalyst (0.5 g) was pretreated in a nitorogen flow at 473 K for 1 h and then added to a mixture of benzyl alcohol (6.25 mmol) and anisole (92.5 mmol) in a 100-mL flask. The reaction was performed at 353 K for 1 h, and the products were analyzed by gas chromatography (GC-2014 with a flame ionization detector, Shimadzu, Kyoto, Japan) using a CBP10 column.

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Cumene Cracking was performed in a pulse reactor with a 6-mm inner

diameter at ambient helium pressure. Prior to the reaction, 50 mg of catalyst (25/50 mesh) was placed in the catalyst bed, and the catalyst was pretreated at a 623 K for 1 h.

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Cumene (0.3 μl) was fed through the top of the reactor at 433 K with a carrier gas flow

of 40 mL min-1. The cumene pulse of five times was carried out at a 623 K. Liquid

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effluents that were collected in a liq. N2 trap (77 K) were analyzed by gas

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chromatography (Shimadzu GC-8A, PEG-20M column) equipped with a thermal conductivity detector (TCD).

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2.3 Characterization

X-ray diffraction (XRD) patterns of the catalyst were recorded by using a

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Rigaku SmartLab with Cu–Kα radiation. The samples were scanned from 2θ= 10−70° at 10° min-1 and a resolution of 0.01°. X-ray photoelectron spectroscopy (XPS) analysis

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of the catalysts was performed using a JEOL JPS-9010 MX instrument. The spectra were measured using Mg–Kα radiation. All spectra were calibrated using C 1s (284.5 eV) as a reference. The Brunauer–Emmett–Teller (BET) specific surface area was estimated from nitrogen isotherms that were obtained using a BELSORP-mini II (BEL Japan, Osaka, Japan) at 77 K. The analyzed samples were evacuated at 573 K for 3 h

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prior to the measurement. Fourier transfer infrared (FTIR) were recorded using a FT/IR-4200 typeA (JASCO, Japan) with a resolution of 4 cm-1. Each sample (30 mg) was pressed into a self-supporting 20-mm-diameter wafer. Catalysts were pretreated under 40 kPa of oxygen at 773 K for 1.5 h and then evacuated. To determine the number of Brønsted and Lewis acid sites on WO3 supported catalysts, the wafer was exposed to pyridine (0.5 kPa) at 303 K for 30 min, followed by evacuation at 423 K for

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30 min. Deuterium exchange of OH in the support surface was tested using D2O (99.8% D from Wako Pure Chemical Industries, Japan.) at 773 K for 20 min and then evacuated

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at the same temperature for 15 min. This procedure was repeated three times.

3. Results and discussion

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α-Pinene isomerization was used to investigate the acid–base properties of the catalyst. Figure 1 shows the results of the reaction over WO3 catalysts that were

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supported on various metal oxides at 100% coverage with a WO3 monolayer. A series of products was formed during α-pinene isomerization, which can be divided into three

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groups, 1) bicyclic compounds, such as α-fenchene and camphene, 2) monocyclic compounds, such as limonene and α-terpinene, and 3) β-pinene (Scheme S1). Bicyclic

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and monocyclic compounds are produced on Brønsted acid sites, and β-pinene is produced on base sites. Ohnishi et al. [22] and Yamamoto et al. [23,24] reported that the

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ratio of monocyclic and bicyclic compounds would be dominated by each catalyst acid strength. We also demonstrated that the camphene and limonene yields correlate with the Brønsted acidity of the WO3/Al2O3 catalyst [18]. The catalyst support affected their activity significantly. Among the catalysts tested, WO3/Al–Ti9 showed the highest

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activity. Therefore, the effect of loading amount of WO3 in WO3/Al–Ti9 was investigated. The results of the isomerization of α-pinene over WO3/Al–Ti9 catalysts with different WO3 loadings are shown in Figure 2a. The camphene and limonene yields increased with an increase in WO3 loading up to 30 wt% (100% coverage), and then decreased. The main products of the isomerization of α-pinene on WO3/Al–Ti9 were camphene

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and limonene, and their selectivities were approximately 68% and 23%, respectively (Figure S1). This result suggests that the isomerization of α-pinene to camphene and limonene occurred at Brønsted acid sites on the WO3/Al–Ti9 catalysts.

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The results of the benzylation of anisole over WO3/Al–Ti9 catalysts with various WO3 loadings are shown in Figure 2b. The benzylation of anisole reaction is reported to

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take place at Brønsted acid sites. [25–31]. The main reaction products were p- and

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o-benzyl anisole, and dibenzyl ether was a minor product. No double-alkylated product was formed. Dibenzyl ether is known to be produced on Lewis acid sites by the dehydration of two benzyl alcohol molecules. The yield of dibenzyl ether was less than

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0.18 mmol in all reactions. The ratio of p-benzyl anisole to o-benzyl anisole was approximately 2:1 regardless of the catalyst supports (Figure S1). Such a constant

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selectivity showed that the acid strength was independent of the WO 3 loading amount,

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but the amount of acid sites varied. The catalytic activity of WO3/Al–Ti9 depended strongly on the loading amount of

WO3. The Brønsted acidity of WO3/Al–Ti9 was measured by pyridine adsorbed FT-IR spectra. Figure S2 shows IR spectra of adsorbed pyridine on Al–Ti9 and 30 wt% ~

WO3/Al–Ti9 calcined at 773 K. Four bands at v = 1447, 1490, 1578, and 1609 cm-1 were observed in the IR spectrum of Al–Ti9 [32-35]. These four bands correspond to

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pyridine species that were adsorbed on Lewis acid sites. In contrast, WO3/Al–Ti9 ~

exhibited new bands at v = 1543 and 1640 cm-1, which corresponding to pyridine species that were adsorbed on Brønsted acid sites, and indicate that the introduction of WO3 to the surface of Al2O3 generated Brønsted acid sites. The specific amount of Brønsted acid sites was estimated by the amount of pyridinium ions with an integrated ~

molar adsorption coefficient value of ε = 1.67 cm μmol-1 for the band area at v = 1545

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cm-1 of pyridinium ions [34, 35]. The yield of camphene, limonene, and benzyl anisole

exhibited a linear correlation with the Brønsted acidity of the catalyst (Figure 3), wjocj indicates that the benzilation of anisole and the isomerization of α-pinene occurred at

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the Brønsted acid sites.

Figure 4 shows the XRD patterns of WO3/Al–Ti9 with various loading amounts of

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WO3. No diffraction peak from monoclinic WO3 (m-WO3) was observed in the XRD

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patterns of WO3/Al–Ti9 with a loading amount of WO3 that was less than 30 wt%. In contrast, m-WO3 appeared for 35 wt% WO3/Al–Ti9 and its contribution grew as the WO3 loading increased because m-WO3 has no Brønsted acid sites and did not promote

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α-pinene isomerization and anisole benzylation. The Brønsted acidity on WO3/Al–Ti9 decreased and m-WO3 was formed above a 30 wt% loading amount of WO3, which

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suggests that Brønsted acid sites should be generated on amorphous tungsten oxide.

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The surface atomic ratios of W, Ti, and Al of WO3 supported catalysts were estimated by XPS analysis. Peaks that were attributed to W 4f7/2 and W 4f5/2 appeared at binding energies of 36.0 and 38.1 eV, respectively. These binding energies were constant regardless of the loading amount of WO3. Therefore, the valence of the tungsten cation of WO3 supported catalyst was W6+, regardless of the WO3 loading amount (Figure S3). Figure 5 shows the surface W/Al ratio of WO3/Al2O3 and

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W/(Al+Ti) of WO3/Al–Ti9 with various loading amounts of WO3. The surface atomic ratio of WO3/Al2O3 and WO3/Al–Ti9 increased linearly with WO3 loading up to 20 and 30 wt% respectively, and then increased gradually. Therefore, the catalyst surface was covered completely with WO3 when WO3 was loaded on alumina and Al–Ti9 as 20 wt% and 30 wt%, respectively. Because the maximum Brønsted acidity was observed for 20 wt% WO3/Al2O3 and 30 wt% WO3/Al–Ti9, respectively, a monolayer of tungsten

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oxide generates Brønsted acid sites. Structural analyses by XRD and XPS indicated that

an amorphous WO3 monolayer was formed on the Al–Ti9 surface up to a 30 wt% loading amount of WO3. These results indicate that a Brønsted acid site was formed at

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the boundaries between the WO3 domains as well as WO3/Al2O3. The amount of Brønsted acid site on amorphous WO3 monolayer is related to the surface area and

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support properties.

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Cumene cracking occurred over supported WO3 catalysts on various supports at a 100% WO3 monolayer coverage. Cumene cracking was used as a test reaction to investigate the Brønsted acid properties of the catalyst. The main products that were

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formed in the cracking of cumene on Brønsted acid sites was benzene and propylene. Among the catalysts tested, WO3/Al–Ti9 showed the highest activity. The yields of

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benzene were correlated finely with the Brønsted acidity of the catalyst (Figure 6),

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which indicates that the cumene cracking occured at the Brønsted acid sites. Structural analysis by XRD indicated that an amorphous WO3 monolayer was formed

on the surface of Al2O3, Al–Ti9, and Al–Zr9 support surfaces (Figure S4). In contrast, crystalline WO3 species was formed on Al–Si and SiO2. The cumene cracking assumed that amorphous tungsten oxide is the source of the Brønsted acid sites even for supported WO3 catalysts on various supports.

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In the following section, we investigated the effect of supports for the acidity on ~

supported WO3 catalysts. The Lewis acidity was estimated by the band area at v = 1455 cm-1 that corresponded to the coordinated pyridine and its integrated molar adsorption coefficient value of ε = 2.22 cm μmol-1 [34, 35]. The density of the Lewis acidity on various supports is shown in Table 3. Al2O3 and Al–Ti9 that supported WO3 catalysts showed a high Brønsted acidity and a high Lewis acid density. These results

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indicate that the high density of Lewis acids has an important role in Brønsted acid site generation when WO3 is loaded on the support.

FT-IR spectra of various supports at the νOH region after calcination under O2 are

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shown in Figure 7. Bands that corresponded to the various hydroxyl groups of each

support were observed. Three types of the isolated hydroxyl groups of Al2O3 were

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observed at 3690−3775 cm-1 [36]. Two types of isolated hydroxyl groups of ZrO2 were observed at 3668−3720 cm-1 [37, 38]. An isolated hydroxyl groups of SiO2 appeared at

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3747 cm-1 [39]. However, these bands imply the presence of hydroxyl groups in the bulk and support surface. Therefore, we carried out deuterium exchange of OH groups

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on the support surfaces by using D2O. Figure S5 shows different spectra of various supports after and before deuterated exchange. A new band at 2650−2820 cm-1

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corresponds to the OD stretching vibration of vOD [40]. The surface hydroxyl density

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was estimated by using the area of vOD region (Table S1).

Table 3 Lewis acid density of alumina-based support Lewis acidity / μmol g-1 SBET / m2 g-1

Lewis acid density / μmol m-2

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0.75

Al-Ti9

124/241

0.51

Al-Ti7

152/253

0.60

Al-Ti5

146/217

0.67

Al-Ti3

191/239

0.80

Al-Zr9

31/249

0.13

Al-Si

137/432

0.32

SiO2

6/192

0.03

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110/146

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Al2O3

Figure 8 shows the relationship between the Lewis acid density and surface hydroxyl

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groups density of supports on the Brønsted acid density. Al2O3 and Al–Ti9 with a high Lewis acid density and surface hydroxyl groups density possessed a high Brønsted acid

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density. In contrast, Al–Ti3 had a high Lewis acid density and a low surface hydroxyl groups density, and SiO2 and Al–Si had a low Lewis acid density and a high surface

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hydroxyl groups. The Brønsted acid density on these supports was much lower than that on Al2O3 and Al–Ti9. These results suggest that a high density of the Lewis acid and

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surface hydroxyl groups are important for the formation of amorphous WO3 monolayer

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domains, which generate Brønsted acid sites. It is likely that a high density of hydroxyl groups is required to generate a highly dispersed tungsten complex in the impregnation step to avoid tungsten species aggregation that form crystalline WO3 species. Besides, it is likely that a high density of Lewis acid sites is necessary to stabilize WO 3 monolayer domains in the calcination step to avoid the growth of tungsten species in the c-axis because of the strong interaction between WO3 monolayers and supports.

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4. Conclusions The density of Lewis acid sites and surface hydroxyl groups of supports affected 1) the structure of supported tungsten oxide and 2) the Brønsted acidity. For Al2O3–TiO2 and Al2O3 with a high density of Lewis acid sites and a high density of surface hydroxyl groups, amorphous monolayer WO3 domains were dispersed and Brønsted acid sites were generated. Crystalline WO3 species formed mainly on SiO2–Al2O3 and SiO2 with a

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low density of Lewis acid sites and a high density of surface hydroxyl groups. We

propose that a high density of Lewis acid sites and surface hydroxyl groups are

Brønsted acid sites on alumina-based supports. Credit_author_statement

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important factors that form amorphous WO3 monolayer domains and to generate

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Acknowledgement

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Mizuki Saito: Investigation, Writing - Original Draft Kenji Aihara: Investigation, Data Curation Hiroki Miura: Methodology, Writing - Review & Editing Tetsuya Shishido: Conceptualization, Supervision, Project administration, Writing - Review & Editing

This study was supported in part by the Program for Element Strategy Initiative for

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Catalysts & Batteries (ESICB) commissioned by the Ministry of Education, Culture,

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Sports, Science and Technology (MEXT) of Japan (JPMXP0112101003).

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Figure captions Figure 1. Isomerization of α-pinene over supported WO3 catalysts. ■: camphene and □: limonene. Pretreatment temperature: 673 K.

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Figure 2. Activity of WO3/Al-Ti9 calcined at 773 K with various loadings for a) the isomerization of α-pinene (●: camphene and ○: limonene) and b) the Friedel–Crafts alkylation (■: benzyl anisole and □: dibenzyl ether). Figure 3. Correlation between the catalytic activities of WO3/Al-Ti9 with various loadings and their Brønsted acidities. a) the isomerization of α-pinene and b) the Friedel–Crafts alkylation (●: camphene and ○: limonene).

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Figure 4. XRD patterns of WO3/Al-Ti9. a:0 wt% , b:10 wt%, c:17 wt%, d:24 wt%, e:27 wt%, f:30 wt%, g:35 wt%, h:39 wt% , i:50 wt%, j:60 wt%, k:100 wt%. γ = γ-Al2O3; m = m-WO3.

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Figure 5. Effect of surface atomic ratio on WO3 supported catalysts. a) WO3/Al2O3 and b) WO3/Al-Ti9. ◆: Brønsted acidity, □: surface atomic ratio.

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Figure 6. Correlation between the activities of WO3/support catalysts and their Brønsted acidity. The cumene cracking, cat. amount: 50 mg, reaction temp.: 623 K, cumene pulse (0.3 μl): 5th time.

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Figure 7. IR spectra of a) Al2 O3, b) Al-Ti9, c) Al-Ti7, d) Al-Ti5, e) Al-Ti3, f) Al-Zr9, g) Al-Si, h) SiO2.

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Figure 8. Effects of Lewis acidity and surface hydroxyl density. The number in a parenthesis is the Brønsted acid density [μmol m-2] of the WO3/Support.

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WO3/Al2O3 WO3/Al-Ti9 WO3/Al-Ti7 WO3/Al-Ti5

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WO3/Al-Ti3 WO3/Al-Zr9

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WO3/Al-Si WO3/SiO2

2 3 Yield / mmol

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Figure. 1 Isomerization of α-pinene over supported WO3 catalysts.

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■: camphene and □: limonene. Pretreatment temperature: 673 K.

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Figure. 2. Activity of WO3/Al-Ti9 with various loadings for a) the isomerization of α-pinene (●: camphene and ○: limonene) and b) the Friedel–Crafts alkylation (■: benzyl anisole and □: dibenzyl ether).

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Figure. 3. Correlation between the catalytic activities of WO3/Al-Ti9 calcined with various loadings and their Brønsted acidities. a) the isomerization of α-pinene and b) the

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Friedel–Crafts alkylation (●: camphene and ○: limonene).

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Figure. 4. XRD patterns of WO3/Al-Ti9. a:0 wt% , b:10 wt%, c:17 wt%, d:24 wt%, e:27 wt%, f:30 wt%, g:35 wt%, h:39 wt% , i:50 wt%, j:60 wt%, k:100 wt%. γ = γ-Al2O3 ; m = m-WO3.

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Figure 5. Effect of surface atomic ratio on WO3 supported catalysts. a) WO3/Al2O3 and b) WO3/Al-Ti9. ◆: Brønsted acidity, □: surface atomic ratio.

Figure 6. Correlation between the activities of WO3/support catalysts and their Brønsted acidity. The cumene cracking, cat. amount: 50 mg, reaction temp.: 623 K, cumene pulse (0.3 μl): 5th time.

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Figure 7. IR spectra of a) Al2 O3, b) Al-Ti9, c) Al-Ti7, d) Al-Ti5, e) Al-Ti3, f) Al-Zr9, g) Al-Si, h) SiO2.

Figure 8. Effects of Lewis acidity and surface hydroxyl density. The number in a parenthesis is the Brønsted acid density [μmol m-2] of the WO3/Support.

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