Nanosheets as highly active solid acid catalysts for green chemical syntheses

Abstract

Solid acid catalysts offer the opportunity to reduce environmental impact owing to such advantages as ease of product separation and recyclability of the catalyst, which contribute considerably to green chemistry. Nanosheets, crystalline two-dimensional metal oxide sheets prepared from cation-exchangeable layered metal oxides through exfoliation and aggregation, are a novel class of potential solid acid catalysts for replacing liquid acids, such as sulfuric acid. This article reviews the acid strength and acid catalysis of several types of nanosheets, which are strongly dependent on novel strong Brønsted acid sites attributed to bridged OH groups formed only on nanosheets. An efficient acid catalysis of layered protonated niobium molybdate in Friedel–Crafts alkylation, esterification and hydrolysis, owing to its unique intercalation availability, is also discussed.

---

Atsushi Takagaki

Atsushi Takagaki received a BSc in chemistry from the Tokyo University of Science in 2001 and a MSc (2003) and PhD (2006) in chemistry from the Tokyo Institute of Technology supervised by Professor Kazunari Domen and Michikazu Hara. After graduating, Dr Takagaki had moved to The University of Tokyo as a postdoc in 2006–2008. In 2008, Dr Takagaki joined Japan Advanced Institute of Science and Technology (JAIST) as Assistant Professor. His research interests are the development of heterogeneous catalysts, especially solid acids for green and sustainable chemistry, including biomass utilization.

Caio Tagusagawa

Caio Tagusagawa received a BSc in chemistry from The University of São Paulo in Brazil in 2005. After moving to Japan, he received a MSc in chemistry from The University of Tokyo in 2007. He is currently a PhD candidate at The University of Tokyo under the supervision of Professor Kazunari Domen. His research interests focus on the development of novel classes of solid acid catalysts based on transition metal oxides.

Shigenobu Hayashi

Shigenobu Hayashi was employed at the National Chemical Laboratory for Industry (currently National Institute of Advanced Industrial Science and Technology) in 1978, after leaving a graduate school of science in The University of Tokyo midcourse. He obtained his PhD degree in chemistry from The University of Tokyo in 1983. He has conducted extensive research on the application of solid-state nuclear magnetic resonance to materials science, especially to hydrogen-storage materials, proton-conducting materials, and microporous and mesoporous materials.

Michikazu Hara

Michikazu Hara received his PhD degree (1992) in chemistry from the Tokyo Institute of Technology. After four years as a researcher at the Toshiba Corporation he joined the Tokyo Institute of Technology in 1997 and was promoted to Associate Professor in 2000. In 2006, as a Professor, he moved to the Materials and Structures Laboratory in the same university and concurrently holds the post of a project leader at Kanagawa Academy of Science and Technology. His research interests are in the development of solid catalysts for the environmentally benign production of chemicals and energy. He was awarded Scientific American 50 (2006).

Kazunari Domen

Kazunari Domen received BSc (1976), MSc (1979), and PhD (1982) honors in chemistry from the University of Tokyo. Dr Domen joined the Chemical Resources Laboratory, Tokyo Institute of Technology in 1982 as Assistant Professor and was subsequently promoted to Associate Professor in 1990 and Professor in 1996. Moving to the University of Tokyo in 2004, his research interests now include heterogeneous catalysis and materials chemistry, with particular focus on surface chemical reaction dynamics, photocatalysis, solid acid catalysis, and mesoporous materials.

---

Broader context

Most chemicals and fuels are currently synthesized by catalytic reactions. Over 15 million tons of sulfuric acid is annually consumed for the production of industrially important chemicals. However, there are inherent drawbacks to the use of liquid acids as catalysts, such as the cost of separating the acid from the reaction solution and the generation of large amounts of waste. Solid acid catalysts, which are reusable and readily separable from the reaction product, offer the opportunity to reduce the impact on the environment and increase profits. The chemical industry is currently searching for a highly active and stable solid acid to improve the environmental safety of the production of chemicals and energy. Here, we describe metal oxide nanosheets, a novel class of potential solid acid catalysts. Exfoliated nanosheets have strong Brønsted acid sites on the two-dimensional sheets, and exhibit remarkable performances for several acid-catalyzed reactions.

1. Introduction

Catalytic technologies are crucial for the current chemical industry. More than 90% of all chemicals are produced through at least one catalytic process.¹ Acids are the most widely used catalysts for the production of a variety of fuels and chemicals. Until the first half of the 20th century, the most commonly used acids were mineral acids, such as HF and H₂SO₄. These acids, however, have serious drawbacks: they necessitate the costly and inefficient separation of the catalyst from homogeneous reaction mixtures, resulting in a huge waste of energy and a large amount of waste products. The use of solid acid catalysts generally offers several advantages over liquid acids.^1–9 These include reduced equipment corrosion, ease of product separation and recyclability of the catalyst. The principles of “green chemistry” and “green technology” dictate that production should have a minimal adverse effect on the environment and human health,^5,6 stimulating the replacement of these acid catalysts with recyclable, nontoxic solid acids with strong acid sites. From the 1940s onwards, liquid acids have been replaced by solid acids for the production of fuels and bulk chemicals in the fields of oil refining and petrochemicals on an enormous scale.⁴ Most of these reactions are carried out in the gas phase, and zeolites are widely used because of their excellent thermal and chemical stability, acid properties and shape selectivity.^1–7 At present, more than 180 industrial processes, such as alkylation, isomerization, dehydration, condensation, cracking and etherification, employ solid acid catalysts, including zeolites, clays, metal oxides and ion-exchange resins.⁴

However, in liquid phase reactions, most processes are still catalyzed by homogeneous acids, such as H₂SO₄, HF and AlCl₃. This disagreeable situation is due to the lack of suitable highly active solid acid catalysts. Thus, the development of solid acids as heterogeneous catalysts is desirable from the viewpoint of environmentally sustainable chemistry. One of the most attractive classes of acid-catalyzed reactions is chemical synthesis in water. Organic synthesis using water as a solvent has several advantages, such as non-flammability, non-toxicity and safety, which make it a highly enviromentally-benign process. These reactions require the replacement of liquid acids with water-tolerant solid acids.⁸ Several types of solid acids, such as high-silica zeolite,^10–13 niobium oxide,^14–18 ion-exchange resin^9,19–21 and functionalized mesoporous materials,^22,23 have been reported. It should be noted that the development of water-tolerant solid acids is also an important issue in the field of biorefinery.^24–30 In contrast to petroleum, biomass is already highly functionalized and condensed to form a complexed structure. As a major biomass, lignocellulose, which is composed of cellulose, hemicellulose and lignin, should be depolymerized by hydrolysis for use in the production of fuels and chemicals. Monosaccharides, such as glucose, fructose and xylose, obtained from hydrolysis of polysaccharides, are further transformed into furan derivatives, such as 5-hydroxymethylfurfural and furfural, as key intermediates for the production of fuels and chemicals by dehydration of water.^{24–28,31–37} Such essential biorefinery processes, including hydrolysis and dehydration, are also catalyzed by acids. In biorefinery, too, then, sulfuric acid is consumed. Thus, from the petroleum industry to biorefinery, there is a strong demand for the replacement of liquid acids by solid acid catalysts in order to ensure green chemical synthesis.

2. Exfoliated nanosheets

Ion-exchangeable layered metal oxides offer several attractive features owing to their wide variety of structural and electronic properties. Cation-exchangeable layered metal oxides consist of polyanion sheets of metal oxides with intercalated alkaline cations. The metal oxide polyanion sheets are negatively charged, and the high negative charge density of these metal oxide sheets allow them to function as host structures for ion-exchange reactions with various cations. The sheets of the material often exhibit electrical conductivity and photoresponses based on bandgap transitions beneficial to highly active photocatalysts.^38–63

From the viewpoint of heterogeneous catalysis, these layered materials offer several attractive features. For example, the large inner surface area is potentially applicable to various catalytic reactions if the reactant molecules can be intercalated. Another interesting property is the solid acidity, which the H⁺-exchanged form of the layered compound is expected to exhibit. In the case of clay minerals, such catalytic properties have already been examined extensively.^64–69 However, for layered transition-metal oxides, applications have yet to be explored because of the difficulty of intercalation of reactant molecules. This resistance to intercalation is attributed to the higher charge density of the sheets. Exfoliation of the layered sheets may overcome this disadvantage and allow the use of layered materials in catalytic reactions.

It is well known that some clay minerals spontaneously undergo delamination in water, producing single-layer colloidal suspensions. In the last decade, it has been reported that a similar exfoliation can be artificially achieved for several classes of layered materials by a number of soft chemical processes.^70–72 Exfoliated single layers are expected to behave as a new kind of inorganic macromolecule, which may exhibit unique properties different from those of bulk materials.

A variety of layered metal oxides can be exfoliated into single nanosheets. Layered metal oxides which could be exfoliated to date include Cs_xTi2−x_/4□_x/4O₄ (□: vacancy),^73–83 Na₂Ti₃O₇,⁸⁴ KCa₂Nb₃O₁₀,^85–94 KSr₂Nb₃O₁₀,^93,95,96 KLaNb₂O₇,^93,97 KLa₂Ti₃O₁₀,^97,98 KTiNbO₅,^92,99 CsTi₂NbO₇,¹⁰⁰ KNb₃O₈,^99,101 K₄Nb₆O₁₇,^{82,94,100,102–111} RbTaO₃,¹¹² Bi₂SrTa₂O₉,¹¹³ LiNbWO₆,¹¹⁴ LiTaWO₆,¹¹⁴ H₂W₂O₉,¹¹⁴ Cs₆₊xW₁₁O₃₆,¹¹⁵ manganese oxide^83,116–120 and ruthenium oxide.¹²¹

Typically, layered transition-metal oxides are exfoliated by the addition of bulky quarternary alkylammonium ions (tetrabutylammonium, TBA) into aqueous solutions containing protonated layered transition-metal oxides. The resulting exfoliated nanosheets are inorganic macromolecules with a two-dimensional crystalline lattice. Exfoliated nanosheets are characterized by unique properties, such as single-crystalline quality, high crystallinity and a well-defined composition. Most exfoliated nanosheets are unilamellar and retain their high crystallinity after multiplex synthesis processes, including ion-exchange, intercalation and exfoliation, as determined by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM).

Exfoliated nanosheets are used for photochemical and photocatalytic applications. Aggregated nanosheets and dye-nanosheet hybrid materials have photocatalytic activity for hydrogen production under ultraviolet and visible irradiation.^{86–88,90,91,93,94,102–104,109,110} They are also utilized for electrochemical energy storage.

Here, we introduce the application of exfoliated transition metal oxide nanosheets as solid acid catalysts. The exfoliated sheets are regarded as two-dimensional macropolyanions. Heteropolyanions, such as SiW₁₂O₄₀⁴⁻ and PW₁₂O₄₀³⁻, are known to exhibit strong Brønsted acidity when the counter cations are protons, and these materials have already been applied to several industrial catalytic processes.^122,123 A notable difference between the two systems is that macropolyanion sheets are heterogeneous catalysts, while heteropolyanions are essentially homogeneous catalysts in aqueous solution. Another important aspect of exfoliated macropolyanion sheets is their two-dimensional single-crystal structure. For most solid acid materials of transition-metal oxides, the surface structures are as spontaneously defined as in most heterogeneous catalysts. However, exfoliated sheets maintain their two-dimensional crystal structure, forming a precisely defined atomic structure. This facilitates examination of the origin of solid acidity based on an atomic scale and quantum chemical levels, which are not well understood for most solid acid materials. Moreover, a wide variety of cation-exchangeable transition metal layered oxides have been reported, which will be helpful in an analysis of the strength of the solid acidity.

3. Nanosheets as solid acid catalysts

3.1. Exfoliated aluminosilicates

Before we describe the application of exfoliated transition metal oxides as potential solid acid catalysts, it should be mentioned that delaminated aluminosilicates are highly promising materials as solid acids. Corma and co-workers have conducted extensive research on the exfoliated zeolites, such as ITQ-2,^124–131 ITQ-6^130,132,133 and ITQ-18,^130,134 which have been successfully used for a variety of chemical syntheses, including cracking,^{124–126,132} acetalization,¹²⁷ isomerization,¹²⁸ condensation,¹³⁰ and Beckmann rearrangement.¹³¹

3.2. Acid properties of exfoliated transition metal oxide nanosheets

Acid properties of exfoliated transition metal oxide nanosheets depend strongly on the surface OH groups on the two-dimensional sheets. The crystal structure of the parent layered metal oxides affects the formation of acidic OH groups on exfoliated nanosheets, resulting in different acid properties.

A distinct difference can be observed between HSr₂Nb₃O₁₀ nanosheets and HTiNbO₅ nanosheets.¹³⁵Fig. 1 schematizes the structures of these two types of nanosheets, [Sr₂Nb₃O₁₀]⁻ and [TiNbO₅]⁻. Sr₂Nb₃O₁₀ nanosheets are slabs of layered perovskite. In HTiNbO₅, the two-dimensional anion sheets are composed of TiO₆ and NbO₆ octahedra where TiO₆ and NbO₆ are randomly located.

Fig. 1 Schematic structures of [Sr₂Nb₃O₁₀]⁻ nanosheet and [TiNbO₅]⁻ nanosheet (green ball: Nb, white ball: Sr, blue ball: Ti or Nb).

Esterification of acetic acid with ethanol on HTiNbO₅ nanosheets and HSr₂Nb₃O₁₀ nanosheets in the liquid phase indicated that HTiNbO₅ nanosheets exhibited higher activity for the reaction than two protonated conventional zeolites (H-ZSM-5 and HMOR), whereas HSr₂Nb₃O₁₀ nanosheets showed no activity. Layered HTiNbO₅ and HSr₂Nb₃O₁₀ also hardly catalyzed the reaction, confirming that the interlayer H⁺ ions in layered metal oxides are not available because the reactant molecules are unable to penetrate the interlayer space to utilize the H⁺ ions as catalyst. Similar results were obtained for cumene cracking and dehydration of 2-propanol in gas phase reactions.¹³⁵ HTiNbO₅ nanosheets exhibited high turnover frequency comparable to SiO₂–Al₂O₃ for cumene cracking. The dehydration of 2-propanol also proceeded on HTiNbO₅ nanosheets, whereas HSr₂Nb₃O₁₀ nanosheets did not show activity for either reaction.

Coloration using indicator reagents and ¹H magic angle spinning (MAS) NMR measurements were examined to determine the acidities of two different nanosheets. The coloration indicated that the acidity of HTiNbO₅ nanosheets corresponds to 90% sulfuric acid or stronger (Hammet acidity function H₀ < −8.2), whereas no coloration was observed for HSr₂Nb₃O₁₀ nanosheets even in chalcone (H₀ > −5.6). The properties of hydrogen species in the interlayer space and on the nanosheets were examined in detail by ¹H MAS NMR spectroscopy. The ¹H chemical shift reflects acidity, and hydrogen species with a large chemical shift are expected to possess strong acidity. A peak at 10.0 ppm, ascribed to strongly acidic OH groups fixed in the interlayer space, was observed for layered HSr₂Nb₃O₁₀ after dehydration (Fig. 2). However, this peak disappeared after exfoliation and aggregation. For HSr₂Nb₃O₁₀ nanosheets, two peaks were observed at 5.1 and 1.6 ppm. The latter main peak was ascribed to isolated OH groups without hydrogen bonds, and the former to OH groups with hydrogen bonds. CD₃CN adsorption indicated that there were no strong acid sites on HSr₂Nb₃O₁₀ nanosheets. In the original layered oxide, strong hydrogen bonds were formed in the narrow interlayer clearance, which made the OH groups strongly acidic. These strong hydrogen bonds disappeared with the formation of HSr₂Nb₃O₁₀ nanosheets.

Fig. 2 ¹H MAS NMR spectra for layered HSr₂Nb₃O₁₀ and HSr₂Nb₃O₁₀ nanosheets.

In the case of HTiNbO₅ nanosheets, four peaks are observed at 12, 7.6, 4.9 and 1.6 ppm (Fig. 3). The peak at 12 ppm is ascribed to OH groups in an environment similar to that of layered HTiNbO₅, suggesting that some of the exfoliated sheets aggregate in a regular stacking pattern. Layered HTiNbO₅ exhibits no activity for acid catalysis reactions, indicating that the OH groups are not available for these reactions in this case. Fig. 3 also schematizes the structure of the HTiNbO₅ nanosheets. In [TiNbO₅]⁻ sheets obtained by the exfoliation of layered HTiNbO₅, a negative charge is expected to be localized on the TiO₆ octahedra. It is considered that there are two types of OH groups on the nanosheets: isolated Ti–OH and bridged Ti(OH)Nb. By analogy to isolated Nb–OH and hydrogen-bonded Nb–OH in HSr₂Nb₃O₁₀ nanosheets, the peaks at 1.6 and 4.9 ppm can be assigned to isolated Ti–OH and hydrogen-bonded Ti–OH, respectively. The peak at 7.6 ppm is not observed in the ¹H MAS NMR spectrum of HSr₂Nb₃O₁₀ nanosheets, suggesting that this peak is due to strong acid species on HTiNbO₅ nanosheets. Thus, it seems reasonable to assign the peak at 7.6 ppm to Ti(OH)Nb. CD₃CN-adsorbed HTiNbO₅ nanosheets are comparable to CD₃CN-adsorbed strong Brønsted acid sites in zeolites,¹³⁶ which is consistent with the fact that HTiNbO₅ nanosheets function as a strong solid acid. Ti(OH)Nb are regarded as strong Brønsted acid sites in HTiNbO₅ nanosheets.

Fig. 3 ¹H MAS NMR spectrum for HTiNbO₅ nanosheets.

The acid catalytic activity of HTiNbO₅ nanosheets is attributable to the strong Brønsted acid sites, Ti(OH)Nb. In contrast, HSr₂Nb₃O₁₀ nanosheets exhibited no or slight acid catalytic activity for these reactions. These nanosheets host only isolated and hydrogen-bonded OH groups, which do not have strong acidity.

3.3. Exfoliated metal oxide nanosheets with strong Brønsted acid sites

3.3.1. Strong Brønsted acid sites formed on nanosheets. Strong Brønsted acid sites such as Ti⁴⁺(OH)Nb⁵⁺ appear only on nanosheets prepared from layered compounds.¹³⁷Fig. 4 shows the amount of ethyl acetate formed by esterification of acetic acid for 6 h in the presence of two kinds of metal oxides containing Ti⁴⁺ and Nb⁵⁺ (Ti⁴⁺ : Nb⁵⁺ = 1 : 1) (an amorphous oxide and a mixture of crystalline TiO₂ (rutile), Nb₂O₅ and TiNb₂O₇). Neither sample exhibited catalytic activity for the reaction, whereas both titanium niobate nanosheets (HTiNbO₅ and HTi₂NbO₇) gave high activity. It was confirmed that the peak due to Ti(OH)Nb was absent in the ¹H MAS NMR spectrum for the bulk samples. Thus, it appears that Ti(OH)Nb cannot be formed on bulky metal oxides prepared simply by mixing Ti⁴⁺ and Nb⁵⁺, suggesting that nanosheets of TiO₆ and NbO₆ octahedra mixed at an atomic level are essential for the generation of such strong acid sites.

Fig. 4 Production of ethyl acetate on titanium niobate nanosheets and Ti–Nb mixed oxides (343 K, catalyst: 0.2 g, acetic acid: 0.10 mol, ethanol: 0.10 mol, reaction time: 6 h).

The formation of this kind of strong Brønsted acid site on pristine niobium oxide nanosheets would be helpful for understanding the unique acid properties of niobic acid.¹³⁸ Niobic acid, which is amorphous niobium oxide with incorporated water molecules, Nb₂O₅·nH₂O, is sufficiently stable and remains active for acid catalysis in the presence of water. Nb₂O₅·nH₂O is already being used for the production of industrially important chemicals, such as methyl tertiary butyl ether (MTBE), methyl methacrylate and 2,5-dimethyl-2,4-hexadiene.^4,15 However, the properties of Nb₂O₅·nH₂O have yet to be examined thoroughly, and although it has been reported that Nb₂O₅·nH₂O hosts several types of acid sites,^14,139–142 the details remain unclear, presenting an obstacle to the further development of active Nb₂O₅·nH₂O catalysts. The lack of investigation can largely be attributed to the complexity of amorphous Nb₂O₅·nH₂O.^140–144 Niobium oxide nanosheets (HNb₃O₈) can be readily synthesized from layered KNb₃O₈ through exfoliation and aggregation. The two-dimensional single-crystal nanosheets enable a simpler characterization of the surface structure and surface functional groups. The hydrolysis of ethyl acetate in the presence of a large amount of water showed that the HNb₃O₈ nanosheets functioned as a water-tolerant solid acid catalyst and exhibited high catalytic performance, reaching a level twice that of niobic acid. The HNb₃O₈ nanosheets were also superior to niobic acid for other acid-catalyzed reactions, including esterification and Friedel–Crafts alkylation. As mentioned above, HSr₂Nb₃O₁₀ nanosheets did not function as a solid acid. Although Nb₂O₅·nH₂O, HNb₃O₈ nanosheets and HSr₂Nb₃O₁₀ nanosheets are all Nb⁵⁺-containing oxides, there appears to be a definite distinction in catalysis and acidity between these materials, suggesting that the appearance of strong Brønsted acid sites is largely structure-dependent. Fig. 5 shows ¹H MAS NMR spectra for HSr₂Nb₃O₁₀ nanosheets, HNb₃O₈ nanosheets and niobic acid, along with the schematic structures of the sheets. On HSr₂Nb₃O₁₀ sheets, which consist of corner-sharing NbO₆ octahedra,^145–147 H is located only on oxygen atoms at the vertices of the NbO₆ octahedra. The two peaks at 5.1 and 1.6 ppm in the NMR spectrum for the HSr₂Nb₃O₁₀ nanosheets can be assigned to Nb–OH groups with and without hydrogen bonds, respectively.¹²⁴ In the NMR spectrum for HNb₃O₈ nanosheets, three peaks appear at 1.3, 5.5 and 8.5 ppm. As the HNb₃O₈ sheets are composed of edge-sharing NbO₆ octahedra,^148–150 it is thought that there are OH groups shared by two Nb⁵⁺ (Nb(OH)Nb) in addition to isolated Nb–OH groups. As a result, the chemical shift at 8.5 ppm can be attributed to Nb(OH)Nb. This peak has a large chemical shift and is not observed in the ¹H MAS NMR spectrum of HSr₂Nb₃O₁₀ nanosheets (not an acid catalyst), indicating that Nb(OH)Nb functions as a strong Brønsted acid site. The ¹H MAS NMR spectrum for dehydrated Nb₂O₅·nH₂O reveals a broad peak centered around 6.9 ppm with a shoulder peak at 1.7 ppm. Nb₂O₅·nH₂O is formed by the random condensation of H₈Nb₆O₁₉ clusters,^141,142 resulting in a large variety of hydroxyl groups; hence the broad peak in the ¹H MAS NMR spectrum. Thus, Nb(OH)Nb functions as a strong Brønsted acid site in Nb₂O₅·nH₂O as well.

Fig. 5 ¹H MAS NMR spectra for HSr₂Nb₃O₁₀ nanosheets, HNb₃O₈ nanosheets and niobic acid.

3.3.2. Acid strength order of metal oxide nanosheets. Exfoliation-aggregation of protonated layered compounds does not always result in strong solid acids, and the formation of bridging hydroxyl groups, M(OH)M′, which are intrinsic to the crystal structure of the two-dimensional sheets, is essential for the preparation of nanosheet catalysts with strong acidity.^{135,137,138,151} There is a wide variety of cation-exchangeable layered metal oxides, and nanosheets capable of forming bridged OH groups (i.e., Ti⁴⁺(OH)Nb⁵⁺) function as strong solid acids. The strong Brønsted acid sites are ascribed to M(OH)M′, and the acid strength is significantly influenced by the metal cations, M and M′, in the order (Ti⁴⁺, Nb⁵⁺) < (Nb⁵⁺, Nb⁵⁺) < (Nb⁵⁺, W⁶⁺).¹⁵¹

Fig. 6 shows SEM images of the aggregated HTiNbO₅, HNb₃O₈ and HNbWO₆ nanosheets. The addition of H⁺ to solutions containing colloidal nanosheets causes the nanosheets to aggregate randomly as precipitates with the expected composition ratios, as confirmed by energy dispersive X-ray spectroscopy, and the resultant precipitates are free of the tabular particles observed in the original layered compounds.¹⁵¹ The Brunauer-Emmett-Teller (BET) surface areas of the aggregated nanosheet precipitates are 153 m² g⁻¹ for HTiNbO₅, 101 m² g⁻¹ for HNb₃O₈ and 66 m² g⁻¹ for HNbWO₆, all much higher than that of the original protonated layered oxides (about 1 m² g⁻¹). This indicates that the exfoliation-aggregation process provides access to the surface of the nanosheets.

Fig. 6 SEM images of aggregated HTiNbO₅, HNb₃O₈ and HNbWO₆ nanosheets.

The XRD pattern of HTiNbO₅ nanosheets shows a weaker (002) diffraction peak and a complete absence of (004) and (008) peaks, in contrast to the original sample, indicating a much poorer periodic layer structure than that of the original HTiNbO₅ (Fig. 7). However, diffraction peaks such as (200) and (020), corresponding to in-plane diffraction, are preserved in the XRD pattern of HTiNbO₅ nanosheets, which means the in-plane structure is maintained after precipitation. The HNb₃O₈ precipitate also has a weaker diffraction peak at low angle (2θ < 10°), and the in-plane diffraction peaks (e.g., (101)) are similarly retained upon exfoliation-aggregation. HNbWO₆ nanosheets form the same poorly periodic structure with a partial absence of diffraction peaks at low angle (<10°) and the preservation of in-plane diffraction peaks ((110) and (200)). For these nanosheets, the XRD patterns displayed much weaker diffraction peaks compared to those of the original layered compounds, while the in-plane diffraction peaks such as (410), (200) and (210) persisted even after exfoliation-aggregation.

Fig. 7 XRD patterns for layered- and nanosheets metal oxides. (a) HTiNbO₅, (b) HNb₃O₈, and (c) HNbWO₆.

NH₃ temperature programmed desorption (NH₃-TPD) is a basic method for measuring the acid amount and acid strength from the desorbed NH₃ quantity and desorbed temperature. Fig. 8 shows the NH₃-TPD (m/z = 16) results for metal-oxide aggregated nanosheets. The NH₃-TPD profiles for HTiNbO₅ and HNb₃O₈ nanosheets exhibit three and two distinct peaks, respectively, at 430, 482 and 535 K for HTiNbO₅, and at 440 and 550 K for HNb₃O₈. HNbWO₆ nanosheets exhibit similar peaks at 455 K and 560 K. The peaks at 535–560 K are attributable to strong acid sites. The order of acid strength in these solid acids increases in the order HTiNbO₅ < HNb₃O₈ ≤ HNbWO₆. The strong acid amount calculated from the NH₃-TPD results for the aggregated nanosheets is 0.24 mmol g⁻¹ for HTiNbO₅, 0.28 mmol g⁻¹ for HNb₃O₈ and 0.34 mmol g⁻¹ for HNbWO₆.

Fig. 8 NH₃-TPD (m/z 16) spectra for HTiNbO₅, HNb₃O₈, and HNbWO₆ nanosheets.

The acid properties of the aggregated nanosheets were also evaluated by ³¹P MAS NMR, using trimethylphosphine oxide (TMPO) as a probe molecule. As the ³¹P chemical shifts of protonated TMPO (i.e., TMPOH⁺) tend to move downfield, higher chemical shifts indicate higher protonic acid strength. Fig. 9 shows ³¹P MAS NMR spectra for 50 mol% TMPO adsorbed aggregated nanosheets. The peaks below 50 ppm are ascribed to physisorbed or crystalline TMPO, which are excluded from the following discussion. The HNb₃O₈ nanosheet catalyst exhibits a main broad ³¹P peak centered at ca. 70 ppm, while the HTiNbO₅ nanosheet catalyst exhibits a main peak at ca. 63 ppm. The HNbWO₆ nanosheet catalyst exhibits a main broad peak at ca. 71 ppm, with a shoulder peak at 65 ppm. The larger chemical shift corresponds to the stronger acidity. The results in Fig. 9 demonstrate that stronger acidic sites are present in the order HNbWO₆ ≥ HNb₃O₈ > HTiNbO₅, which correlates with the trend in maximum temperature of the NH₃-TPD curves assigned to M(OH)M′ acid sites. The peak positions in the ³¹P MAS NMR spectra indicate that the strong acid sites in the HNbWO₆ and HNb₃O₈ nanosheet precipitates (70–71 ppm) are comparable in strength to those in HY zeolites (65 ppm).¹⁵²

Fig. 9 ³¹P MAS NMR spectra for trimethylphosphine oxide adsorbed HTiNbO₅, HNb₃O₈, and HNbWO₆ nanosheets.

The acid catalytic activity of the aggregated nanosheets is examined through liquid-phase Friedel–Crafts alkylation (Table 1). The original layered metal oxides (HTiNbO₅, HNb₃O₈ and HNbWO₆) do not catalyze these reactions, with alkylated products absent or afforded at yields of less than 1%. However, the exfoliated and aggregated materials, with the exception of HTiNbO₅, exhibit significant activity for this reaction. The HNb₃O₈ aggregated nanosheets have a relatively low activity for this reaction at 373 K, but exhibit remarkable activity at 423 K. The catalytic activity of the nanosheet precipitates decreases in the order HNbWO₆ > HNb₃O₈ ≫ HTiNbO₅, which is consistent with the acid strength determined by NH₃-TPD. The nanosheet materials consisting of the group-5 element (Nb⁵⁺) and group-6 element (W⁶⁺) afford higher benzylanisole yields and higher NH₃ desorption temperatures than the nanosheet precipitate bearing the group-4 element (Ti⁴⁺). Catalysts with the aggregated nanosheet structure have higher acid strength, depending on the combination of metal oxides, decreasing in the order Nb⁵⁺(OH)W⁶⁺ > Nb⁵⁺(OH)Nb⁵⁺ > Ti⁴⁺(OH)Nb⁵⁺.

Table 1 Friedel–Crafts alkylation of anisole with benzyl alcohol over metal oxide nanosheets^a

Catalyst	Yield (%)^b	Selectivity (%)^b	Turnover rate /h^−1
a Reaction conditions: anisole (100 mmol), benzyl alcohol (10 mmol), catalyst (0.2 g), 373 K, 2 h. b benzylanisole yield and selectivity. c 423 K, 4 h.
HTiNbO5 nanosheets	0 (0.3 ^c)	—	0 (0.2)
HNb3O8 nanosheets	1.8 (94^c)	23	1.6 (42)
HNbWO6 nanosheets	8.6	42	6.3
HTaWO6 nanosheets	58.4	66	104
H-ZSM5	10.5	37	13.1
H-Beta	36.6	56	9.2
Amberlyst-15	57.9	58	3.0
Nafion NR50	63.0	63	17.5

---

Substitution of niobium by tantalum is also effective in generating strong acid sites on nanosheets.^137,151 HTiTaO₅ and HTaWO₆ are known to form layered structures similar to HTiNbO₅ and HNbWO₆. The BET surface areas of these tantalum oxide nanosheets are 92 m² g⁻¹ for HTiTaO₅ nanosheets and 47 m² g⁻¹ for HTaWO₆ nanosheets, lower than that of niobium-based oxides (153 m² g⁻¹: HTiNbO₅ nanosheets; 66 m² g⁻¹: HNbWO₆ nanosheets). NH₃-TPD measurement revealed that tantalum-based nanosheets possess much stronger acid sites than the corresponding niobium-based nanosheets. HTaWO₆ nanosheets exhibit a broad TPD profile with peaks at 455 K, 555–595 K, and 667 K (Fig. 10). The highest peak at 667 K is due to preserved acid sites in the layered structure that cannot contribute to acid catalysis. The peaks at 555–595 K for HTaWO₆ nanosheets are assigned to strong acid sites, higher than that for HNbWO₆ nanosheets (560 K). The difference of acidity between tantalum- and niobium-based nanosheets strongly affects acid catalysis. HTaWO₆ nanosheets exhibit remarkable activity for Friedel–Crafts alkylation of anisole with benzyl alcohol, giving a yield six times higher than that for HNbWO₆ nanosheets and comparable to that for Amberlyst-15 and Nafion NR50 (Table 1). The turnover rate for the HTaWO₆ nanosheets are estimated to be 104 h⁻¹, higher than that for Nafion NR50 (17.5 h⁻¹) and H-ZSM5 (13.1 h⁻¹). A similar behavior was observed for HTiTaO₅ and HTiNbO₅ nanosheets. For hydrolysis of ethyl acetate and esterification of acetic acid, HTiTaO₅ nanosheets function as a water-tolerant solid acid superior to HTiNbO₅ nanosheets. The catalytic activity of HTiTaO₅ nanosheets reached about twice that of HTiNbO₅ nanosheets and Nb₂O₅·nH₂O.

Fig. 10 NH₃-TPD (m/z 16) spectrum for HTaWO₆ nanosheets.

3.4. Layered niobium molybdate

During our investigation, we have discovered that protonated niobium molybdate, HNbMoO₆, exhibits a significantly high catalytic activity.^153–156 Layered HNbMoO₆·nH₂O can be obtained easily from the Li form by a proton-exchange reaction. The resultant material consists of layers formed of randomly sited MO₆ (M = Nb and Mo) octahedra with H₂O in the interlayer. This layered protonated niobium molybdate is highly active for several acid-catalyzed reactions, including Friedel–Crafts alkylation, acetalization, esterification, hydrolysis, and hydration. Fig. 11 shows the catalytic performance results for the Friedel–Crafts alkylation of toluene with benzyl alcohol. HNbMoO₆ exhibits remarkable activity for the reaction, much higher than that of the other solid acids tested. HNbMoO₆ also catalyzes the alkylation of anisole or benzene, achieving high yields: the yield of benzyl anisole reached 99% after 30 min, whereas those for ion-exchange resins only reached ca. 42% even after 1 h. The results of acetalization of cyclohexanone, esterification of lactic acid, hydrolysis of ethyl lactate, and hydration of 2,3-dimethyl-2-butene over HNbMoO₆ are summarized in Table 2. For these reactions, HNbMoO₆ showed remarkable activity, comparable or superior to conventional ion-exchange resins (Nafion NR50 and Amberlyst-15). Even in the presence of water (hydrolysis and hydration), HNbMoO₆ functioned as an efficient catalyst. The HNbMoO₆ was recoverable by filtration and washing with water and acetone to remove the residue within the interlayer, and the material was confirmed to be reusable with no change in crystal structure or activity after three reuse cycles.

Fig. 11 Yield of benzyltoluene for Friedel–Crafts alkylation over layered HNbMoO₆. Reaction conditions: toluene (100 mmol), benzyl alcohol (10 mmol), catalyst (0.2 g), 353 K, 4 h.

Table 2 The results for acetalization, esterification, hydrolysis and hydration over layered HNbMoO₆

Catalyst	Acetalization^a reaction rate /mmol h^−1	Esterification^b reaction rate /mmol h^−1	Hydrolysis^c reaction rate /mmol h^−1	Hydration^d Yield (%)
a Cyclohexanone (20 mmol), methanol (5 mL), catalyst (0.1 g), 323 K. b Lactic acid (100 mmol), ethanol (1 mol), catalyst (0.2 g), 343 K. c Ethyl lactate (50 mmol), H2O (9 mL), catalyst (0.1 g), 343 K. d 2,3-dimethyl-2-butene (12.5 mmol), H2O (7.6 mL), catalyst (0.2 g), 343 K, 5 h.
HNbMoO6	158	5.7	0.9	3.4
Amberlyst-15	66	4.5	1.22	3.7
Nafion NR50	58	6.0	1.06	2.2
—	0	1.4	0	0.4

---

This high catalytic activity of layered HNbMoO₆ is attributed to the efficient intercalation of substrates within the interlayer with strong acidity. Substrates such as benzyl alcohol are readily intercalated into HNbMoO₆. In order to validate this process, the adsorption of benzyl alcohol onto HNbMoO₆ was investigated in isolation by immersing HNbMoO₆·nH₂O (n = 1.23) in benzyl alcohol solution under constant shaking for 30 min. Fig. 12 shows the XRD pattern of the resultant material after drying at room temperature. Immersion in benzyl alcohol shifts the (001) peak due to HNbMoO₆ to lower angles, corresponding to an increase in basal spacing from 13.7 to 16.3 Å, which confirms the intercalation of benzyl alcohol into the layered HNbMoO₆. Given the interlayer spacing of the fully dehydrated sample (10.8 Å),¹⁵⁷ the total expansion is estimated to be 5.5 Å. In the Friedel–Crafts alkylation of toluene in the presence of benzyl alcohol, the interlayer spacing of the layered HNbMoO₆ was increased to 16.6 Å. A preliminary benzyl alcohol-intercalated sample, placed in anisole solution, then heated at 373 K for 1 h, indicated that alkylated products (benzyl anisole) were detected, and the basal spacing of HNbMoO₆ was found to decrease to 14.1 Å. These results indicate that intercalated benzyl alcohol is consumed in the reaction and that the interlayer sites of HNbMoO₆ function as active sites. Other substrates, including n-alkyl alcohols and ketones, could also be successfully intercalated into the interlayer gallery of HNbMoO₆.

Fig. 12 XRD patterns of HNbMoO₆: (a) dehydrated, (b) after immersion in benzyl alcohol for 30 min, (c) after reaction of anisole, (d) during Friedel–Crafts alkylation.

The intercalation mechanism greatly affects the catalytic activity of this layered catalyst. A difference in intercalation behavior was observed between carboxylic acid and hydroxycarboxylic acid. The latter could easily intercalate, whereas the former did not, resulting in a unique behavior of acid catalysis for esterification.¹⁵⁴ Layered HNbMoO₆ exhibits negligible activity for the esterification of carboxylic acid, and remarkable activity for the esterification of hydroxycarboxylic acid. Fig. 13 shows the results of the esterification of three carboxylic acids (acetic, propionic, butyric) and lactic acid with n-butanol. Amberlyst-15, Nafion NR50 and layered HNbMoO₆ were compared. For the three carboxylic acids, the ester yield over the three catalysts examined decreases in the order Amberlyst-15 > Nafion NR50 ≫ HNbMoO₆, and the activity decreases with increasing carbon number of the carboxylic acid. The two ion-exchange resins exhibit similar yields for propionic acid and lactic acid, which have the same carbon number. However, layered HNbMoO₆ exhibits a very high activity for lactic acid, with a yield of 35.1%, as compared to the 6.2% for propionic acid. The catalytic activity of ion-exchange resins is influenced only by the carbon number of the carboxylic acid, and is unaffected by the presence of OH groups in lactic acid. In contrast, the OH groups of lactic acid appear essential for allowing intercalation into the layered HNbMoO₆. Given that activation of carboxylic acid by a proton is necessary for the reaction, esterification of the three carboxylic acids did not occur due to the difficulty of intercalation. However, lactic acid, which has an OH group adjacent to the carboxyl group, could intercalate into the oxide, resulting in a high catalytic activity for the reaction.

Fig. 13 The results of esterification of carboxylic acids (acetic, propionic, butyric) and lactic acid with n-butanol over HNbMoO₆, Amberlyst-15 and Nafion NR50. Reaction condition: carboxylic acid (0.05 mol), n-butanol (0.25 mol), catalyst (0.1 g), 343 K, 5 h.

The acid properties of HNbMoO₆ were determined by ³¹P MAS NMR using TMPO as a probe molecule.¹⁵⁵ HNbMoO₆ exhibits distinct peaks at 86.4 and 81 ppm, attributable to strong acid sites. The peak positions indicate that the acid sites of layered HNbMoO₆ are stronger than those of both H-type zeolites (65 ppm for HY,¹⁵² 78 ppm for H-Beta¹⁵⁸) and ion-exchange resin (81 ppm for Amberlyst-15), and comparable in strength to those on strongly acidic zeolites (86 ppm for HZSM-5¹⁵⁹ and HMOR¹⁵⁸). It was also confirmed by XRD that TMPO intercalates into HNbMoO₆, indicating that the strong acid sites of HNbMoO₆, detectable in the NMR spectrum, are present in the interlayer.

3.5. Biorefinery applications

Exfoliated nanosheets and layered HNbMoO₆ function as efficient solid acid catalysts even in the presence of water. This remarkable feature can be exploited in biorefinery applications. Valente and co-workers reported that exfoliated metal oxide nanosheets could dehydrate D-xylose to produce furfural in a water–toluene solvent mixture.¹⁶⁰ HTiNbO₅ nanosheets catalyzed this reaction to achieve a furfural yield of 55% at 92% conversion, whereas the furfural yield in the presence of niobic acid (Nb₂O₅·nH₂O) was 12%. Other nanosheets, including HTi₂NbO₇, H₂Ti₃O₇, HNb₃O₈, and H₄Nb₆O₁₇, also gave a high furfural yield. These nanosheets could be reused at least three times without loss of activity.

Layered HNbMoO₆ exhibits remarkable performance for hydrolysis of saccharides.¹⁵⁶ Polysaccharides such as starch and cellulose can be converted into monosaccharides such as glucose, a major platform for the synthesis of a variety of chemicals, by a homogeneous acid-catalyzed reaction using sulfuric acid and/or by enzymatic reactions. The development of a reusable and readily separable solid acid catalyst for the hydrolysis of saccharides is considered essential for converting biomass into bioethanol and useful chemicals with the lowest environmental impact. Fig. 14 shows the results of the hydrolysis of sucrose and cellobiose. The hydrolysis of cellobiose was carried out at 373 K using 0.2 g of catalyst, 1.0 g of cellobiose, and 10 mL of water. The catalytic performance of these catalysts are compared with that of a range of conventional solid acids, including ion-exchange resins, H-type zeolite and niobic acid. For the hydrolysis of sucrose, an equivalent amount of glucose and fructose was produced. H-ZSM5 zeolite was inactive for this reaction, and niobic acid, although known to be water-tolerant, did not achieve appreciable activity for this reaction. The activity of layered HNbMoO₆, however, substantially exceeded the maximum performance of any of the other solid materials tested. This high activity is also found for hydrolysis of cellobiose. Cellobiose, a subunit of cellulose, consists of β-1,4-glycosidic bonds, which are much more stable than the α-1,2-glycosidic bonds comprising sucrose, and thus is more resistant to hydrolysis. The layered HNbMoO₆ catalyst also exhibited the highest performance in this hydrolysis reaction, producing glucose at twice the rate of the ion-exchange resins and achieving a turnover rate comparable to that of Nafion NR50. It should be noted that the turnover rate of HNbMoO₆ was much higher than that of Amberlyst-15 and twice that of sulfuric acid. This layered oxide could produce glucose from starch and cellulose in the presence of water.

Fig. 14 Hydrolysis of disaccharides (sucrose and cellobiose) over solid acid catalysts. Sucrose hydrolysis: sucrose (2.92 mmol), H₂O (20 mL), catalyst (0.2 g), 353 K. Cellobiose hydrolysis: cellobiose (2.92 mmol), H₂O (10 mL), 373 K.

Conclusions

Strong solid acid catalysts based on nanosheets obtained through protonation, exfoliation and aggregation of cation-exchangeable layered metal oxides have been developed. Exfoliation-aggregation of protonated layered oxides does not always result in strong solid acids, and the formation of bridging hydroxyl groups, M(OH)M′, which are intrinsic to the crystal structure of the two-dimensional sheets, is essential for the preparation of nanosheet catalysts with strong acidity. The acid strength of the strong Brønsted acid sites is significantly influenced by metal cations M and M′ in the order (Ti⁴⁺, Nb⁵⁺) < (Nb⁵⁺, Nb⁵⁺) < (Nb⁵⁺, W⁶⁺). In addition, a unique method has been developed for the acid catalysis of layered niobium molybdate, exploiting its facile intercalation availability for substrate within the interlayer gallery with strong acidity. Exfoliated nanosheets and layered HNbMoO₆ functioned as efficient solid acid catalysts even in the presence of water, and were exploited in biorefinery applications, including the formation of monosaccharides by hydrolysis and furan-derivatives by dehydration.

Notes and references

1. W. F. Hölderich, J. Röseler, G. Heitmann and A. T. Leibens, Catal. Today, 1997, 37, 353.
2. A. Corma, Chem. Rev., 1995, 95, 559.
3. A. Corma, Chem. Rev., 1997, 97, 2373.
4. K. Tanabe and W. F. Hölderich, Appl. Catal., A, 1999, 181, 399.
5. J. H. Clark, Acc. Chem. Res., 2002, 35, 791.
6. J. H. Clark, Green Chem., 1999, 1, 1.
7. R. A. Sheldon and R. S. Downing, Appl. Catal., A, 1999, 189, 163.
8. T. Okuhara, Chem. Rev., 2002, 102, 3641.
9. A. M. Harmer and Q. Sun, Appl. Catal., A, 2001, 221, 45.
10. S. Namba, N. Hosonuma and T. Yashima, J. Catal., 1981, 72, 16.
11. H. Ishida, Catal. Surv. Jpn., 1997, 1, 241.
12. H. Ogawa, T. Koh, K. Taya and T. Chihara, J. Catal., 1994, 148, 493.
13. P. Botella, A. Corma, J. M. Lopez Nieto, S. Valencia, M. E. Lucas and M. Sergio, Appl. Catal., A, 2000, 203, 251.
14. I. Nowak and M. Ziolek, Chem. Rev., 1999, 99, 3603.
15. K. Tanabe and S. Okazaki, Appl. Catal., A, 1995, 133, 191.
16. K. Ogasawara, T. Iizuka and K. Tanabe, Chem. Lett., 1984, 645.
17. O. Okazaki and H. Harada, Chem. Lett., 1988, 1313.
18. K. Hanaoka, T. Takeuchi, T. Matsuzaki and Y. Sugi, Catal. Today, 1990, 8, 123; T. Hanaoka, K. Takeuchi, T. Matsuzaki and Y. Sugi, Catal. Lett., 1990, 5, 13.
19. G. A. Olah, P. S. Lyer and G. K. S. Prakasn, Synthesis, 1986, 513.
20. M. A. Harmer, Q. Sun and W. E. Farneth, J. Am. Chem. Soc., 1996, 118, 7708.
21. K. Okuyama, X. Chen, K. Takata, D. Odawara, T. Suzuki, S. Nakata and T. Okuhara, Appl. Catal., A, 2000, 190, 253.
22. R. L. Dhepe, M. Ohashi, S. Inagaki, M. Ichikawa and A. Fukuoka, Catal. Lett., 2005, 102, 163.
23. J. A. Bootsma and B. H. Shanks, Appl. Catal., A, 2007, 327, 44.
24. G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006, 106, 4044.
25. A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107, 2411.
26. J. N. Chheda, G. W. Huber and J. A. Dumesic, Angew. Chem., Int. Ed., 2007, 46, 7164.
27. G. W. Huber and A. Corma, Angew. Chem., Int. Ed., 2007, 46, 7184.
28. R. Rinaldi and F. Schüth, Energy Environ. Sci., 2009, 2, 610.
29. A. Onda, T. Ochi and K. Yanagisawa, Green Chem., 2008, 10, 1033.
30. S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, H. Kato, S. Hayashi and M. Hara, J. Am. Chem. Soc., 2008, 130, 12787.
31. Y. Román-Leshkov, J. N. Chheda and J. A. Dumesic, Science, 2006, 312, 1933; J. N. Chheda, Y. Román-Leshkov and J. A. Dumesic, Green Chem., 2007, 9, 342.
32. H. Zhao, J. E. Holladay, H. Brown and Z. C. Zhang, Science, 2007, 316, 1597; Y. Su, H. M. Brown, X. Huang, X.-D. Zhou, J. E. Amonette and Z. C. Zhang, Appl. Catal., A, 2009, 361, 117.
33. J. B. Binder and R. T. Raines, J. Am. Chem. Soc., 2009, 131, 1979.
34. D. Mercadier, L. Rigal, A. Gaset and J.-P. Gorrichon, J. Chem. Technol. Biotechnol., 1981, 31, 489.
35. Y. Nakamura and S. Morikawa, Bull. Chem. Soc. Jpn., 1980, 53, 3705.
36. C. Moreau, R. Durand, S. Razigade, J. Duhamet, P. Faugeras, P. Rivalier, P. Ros and G. Avignon, Appl. Catal., A, 1996, 145, 211.
37. C. Carlini, M. Giuttari, A. M. R. Galletti, G. Sbrana, T. Armaroli and G. Busca, Appl. Catal., A, 1999, 183, 295.
38. K. Domen, A. Kudo, A. Shinozaki, A. Tanaka, K.-I. Maruya and T. Onishi, J. Chem. Soc., Chem. Commun., 1986, 356.
39. K. Domen, A. Kudo, M. Shibata, A. Tanaka, K.-I. Maruya and T. Onishi, J. Chem. Soc., Chem. Commun., 1986, 1706.
40. A. Kudo, A. Tanaka, K. Domen, K.-I. Maruya and T. Onishi, J. Catal., 1988, 111, 67.
41. A. Kudo, K. Sayama, A. Tanaka, K. Asakura, K. Domen, K. Maruya and T. Onishi, J. Catal., 1989, 120, 337.
42. K. Sayama, A. Tanaka, K. Domen, K. Maruya and T. Onishi, Catal. Lett., 1990, 4, 217.
43. K. Sayama, A. Tanaka, K. Domen, K. Maruya and T. Onishi, J. Catal., 1990, 124, 541.
44. K. Sayama, A. Tanaka, K. Domen, K. Maruya and T. Onishi, J. Phys. Chem., 1991, 95, 1345.
45. J. Yoshimura, A. Tanaka, J. N. Kondo and K. Domen, Bull. Chem. Soc. Jpn., 1995, 68, 2439.
46. T. Takata, Y. Furumi, K. Shinohara, A. Tanaka, M. Hara, J. N. Kondo and K. Domen, Chem. Mater., 1997, 9, 1063.
47. S. Ikeda, M. Hara, J. N. Kondo, K. Domen, H. Takahashi, T. Okubo and M. Kakihana, Chem. Mater., 1998, 10, 72.
48. S. Ikeda, M. Hara, J. N. Kondo, K. Domen, H. Takahashi, T. Okubo and M. Kakihana, J. Mater. Res., 1998, 13, 852.
49. J. Yoshimura, Y. Ebina, J. Kondo, K. Domen and A. Tanaka, J. Phys. Chem., 1993, 97, 1970.
50. K. Domen, Y. Ebina, T. Sekine, A. Tanaka, J. Kondo and C. Hirose, Catal. Today, 1993, 16, 479.
51. Y. Ebina, A. Tanaka, J. N. Kondo and K. Domen, Chem. Mater., 1996, 8, 2534.
52. Y. Yamashita, K. Hyuga, V. Petrykin, M. Kakihana, M. Yoshimura, K. Domen and A. Kudo, J. Ceram. Soc. Jpn., 2007, 115, 511.
53. H. Hata, Y. Kobayashi, V. Bojan, W. J. Youngblood and T. E. Mallouk, Nano Lett., 2008, 8, 794.
54. T. Mitsuyama, A. Tsutsumi, T. Hata, K. Ikeue and M. Machida, Bull. Chem. Soc. Jpn., 2008, 81, 401.
55. K. Shimizu, Y. Tsuji, M. Kawakami, K. Toda, T. Komada, M. Sato and Y. Kitayama, Chem. Lett., 2002, 1158.
56. K. Shimizu, S. Itoh, T. Hatamachi, T. Komada, M. Sato and K. Toda, Chem. Mater., 2005, 17, 5161.
57. H. Takahashi, M. Kakihana, Y. Yamashita, K. Yoshida, S. Ikeda, M. Hara and K. Domen, J. Alloys Compd., 1999, 285, 77.
58. H. Takahashi, M. Kakihana, Y. Yamashita, K. Yoshida, S. Ikeda, M. Hara and K. Domen, Phys. Chem. Chem. Phys., 2000, 2, 4461.
59. M. Machida, J. Yabunaka and T. Kijima, Chem. Commun., 1999, 1939.
60. M. Machida, J. Yabunaka and T. Kijima, Chem. Mater., 2000, 12, 812.
61. M. Machida, K. Miyazaki, S. Matsushima and M. Arai, J. Mater. Chem., 2003, 13, 1433.
62. Y. I. Kim, S. J. Atherton, E. S. Brigham and T. E. Mallouk, J. Phys. Chem., 1993, 97, 11802.
63. T. Nakato, K. Kusunoki, K. Yoshizawa, K. Kuroda and M. Kaneko, J. Phys. Chem., 1995, 99, 17896.
64. T. J. Pinnavaia, Science, 1983, 220, 365.
65. P. Laszlo, Acc. Chem. Res., 1986, 19, 121.
66. Y. Izumi and M. Onaka, Adv. Catal., 1992, 38, 245.
67. K. Motokura, N. Fujita, K. Mori, T. Mizugaki, K. Ebitani and K. Kaneda, Angew. Chem., Int. Ed., 2006, 45, 2605.
68. K. Motokura, N. Nakagiri, K. Mori, T. Mizugaki, K. Ebitani, K. Jitsukawa and K. Kaneda, Org. Lett., 2006, 8, 4617.
69. K. Motokura, N. Nakagiri, T. Mizugaki, K. Ebitani and K. Kaneda, J. Org. Chem., 2007, 72, 6006.
70. A. J. Jacobson, Mater. Sci. Forum, 1994, 152–153, 1.
71. R. E. Schaak and T. E. Mallouk, Chem. Mater., 2002, 14, 1455.
72. M. Osada and T. Sasaki, J. Mater. Chem., 2009, 19, 2503.
73. T. Sasaki, M. Watanabe, H. Hashizume, H. Yamada and H. Nakazawa, J. Am. Chem. Soc., 1996, 118, 8329.
74. T. Sasaki and M. Watanabe, J. Am. Chem. Soc., 1998, 120, 4682.
75. R. Abe, K. Shinohara, A. Tanaka, M. Hara, J. N. Kondo and K. Domen, Chem. Mater., 1998, 10, 329.
76. F. Kooli, T. Sasaki, V. Rives and M. Watanabe, J. Mater. Chem., 2000, 10, 497.
77. T. Sasaki, Y. Ebina, T. Tanaka, M. Harada, M. Watanabe and G. Decher, Chem. Mater., 2001, 13, 4661.
78. N. Sakai, Y. Ebina, K. Takada and T. Sasaki, J. Am. Chem. Soc., 2004, 126, 5851.
79. M. Osada, Y. Ebina, K. Takada and T. Sasaki, Adv. Mater., 2006, 18, 295.
80. M. Osada, Y. Ebina, H. Funakubo, S. Yokoyama, T. Kiguchi, K. Takada and T. Sasaki, Adv. Mater., 2006, 18, 1023.
81. T. Shibata, K. Fukuda, Y. Ebina, T. Kogure and T. Sasaki, Adv. Mater., 2008, 20, 231.
82. S. Ida, C. Ogata, D. Shiga, K. Izawa, K. Ikeue and Y. Matsumoto, Angew. Chem., Int. Ed., 2008, 47, 2480.
83. N. Sakai, K. Fukuda, Y. Omomo, Y. Ebina, K. Takada and T. Sasaki, J. Phys. Chem. C, 2008, 112, 5197.
84. N. Miyamoto, K. Kuroda and M. Ogawa, J. Mater. Chem., 2004, 14, 165.
85. Y. S. Han, I. Park and J. H. Choy, J. Mater. Chem., 2001, 11, 1277.
86. Y. Ebina, T. Sasaki, M. Harada and M. Watanabe, Chem. Mater., 2002, 14, 4390.
87. Y. Ebina, N. Sakai and T. Sasaki, J. Phys. Chem. B, 2005, 109, 17212.
88. O. C. Compton, E. C. Carroll, J. Y. Kim, D. S. Larsen and F. E. Osterloh, J. Phys. Chem. C, 2007, 111, 14589.
89. E. C. Carroll, O. C. Compton, D. Madsen, F. E. Osterloh and D. S. Larsen, J. Phys. Chem. C, 2008, 112, 2394.
90. O. C. Compton, C. H. Mullet, S. Chiang and F. E. Osterloh, J. Phys. Chem. C, 2008, 112, 6202.
91. O. C. Compton and F. E. Osterloh, J. Phys. Chem. C, 2009, 113, 479.
92. M. Fang, C. H. Kim, G. B. Saupe, H.-N. Kim, C. C. Waraksa, T. Miwa, A. Fujishima and T. E. Mallouk, Chem. Mater., 1999, 11, 1526.
93. K. Maeda and T. E. Mallouk, J. Mater. Chem., 2009, 19, 4813.
94. K. Maeda, M. Eguchi, S. A. Lee, W. J. Youngblood, H. Hata and T. E. Mallouk, J. Phys. Chem. C, 2009, 113, 7962.
95. K. Saruwatari, H. Sato, T. Idei, J. Kameda, A. Yamagishi, A. Takagaki and K. Domen, J. Phys. Chem. B, 2005, 109, 12410.
96. K. Okamoto, H. Sato, K. Saruwatari, K. Tamura, J. Kameda, T. Kogure, Y. Umemura and A. Yamagishi, J. Phys. Chem. C, 2007, 111, 12827.
97. S. Ida, C. Ogata, M. Eguchi, W. J. Youngblood, T. E. Mallouk and Y. Matsumoto, J. Am. Chem. Soc., 2008, 130, 7052.
98. R. E. Schaak and T. E. Mallouk, Chem. Mater., 2000, 12, 3427.
99. T. Nakato, N. Miyamoto and A. Harada, Chem. Commun., 2004, 78.
100. S. W. Keller, H.-N. Kim and T. E. Mallouk, J. Am. Chem. Soc., 1994, 116, 8817.
101. K. Akatsuka, G. Takanashi, Y. Ebina, N. Sakai, M. Haga and T. Sasaki, J. Phys. Chem. Solids, 2008, 69, 1288.
102. K. Domen, Y. Ebina, S. Ikeda, A. Tanaka, J. N. Kondo and K. Maruya, Catal. Today, 1996, 28, 167.
103. R. Abe, K. Shinohara, A. Tanaka, M. Hara, J. N. Kondo and K. Domen, Chem. Mater., 1997, 9, 2179.
104. R. Abe, K. Shinohara, A. Tanaka, M. Hara, J. N. Kondo and K. Domen, J. Mater. Res., 1998, 13, 861.
105. R. Abe, M. Hara, J. N. Kondo, K. Domen, K. Shinohara and A. Tanaka, Chem. Mater., 1998, 10, 1647.
106. G. B. Saupe, C. C. Waraksa, H. N. Kim, Y. J. Han, D. M. Kaschak, D. M. Skinner and T. E. Mallouk, Chem. Mater., 2000, 12, 1556.
107. N. Miyamoto, H. Yamamoto, R. Kaito and K. Kuroda, Chem. Commun., 2002, 2378.
108. G. H. Du, Q. Chen, Y. Yut, S. Zhang, W. Z. Zhou and K. M. Peng, J. Mater. Chem., 2004, 14, 1437.
109. K. Maeda, M. Eguchi, W. J. Yongblood and T. E. Mallouk, Chem. Mater., 2008, 20, 6770.
110. R. Z. Ma, Y. Kobayashi, W. J. Youngblood and T. E. Mallouk, J. Mater. Chem., 2008, 18, 5982.
111. N. Miyamoto, Y. Yamada, S. Koizumi and T. Nakato, Angew. Chem., Int. Ed., 2007, 46, 4123.
112. K. Fukuda, I. Nakai, Y. Ebina, R. Ma and T. Sasaki, Inorg. Chem., 2007, 46, 4787.
113. S. Ida, C. Ogata, U. Unal, K. Izawa, T. Inoue, O. Altuntasoglu and Y. Matsumoto, J. Am. Chem. Soc., 2007, 129, 8956.
114. R. E. Schaak and T. E. Mallouk, Chem. Commun., 2002, 706.
115. K. Fukuda, K. Akatsuka, Y. Ebina, R. Ma, K. Takada, I. Nakai and T. Sasaki, ACS Nano, 2008, 2, 1689.
116. Y. Omomo, T. Sasaki, L. Z. Wang and M. Watanabe, J. Am. Chem. Soc., 2003, 125, 3568.
117. L. Z. Wang, Y. Omomo, N. Sakai, K. Fukuda, I. Nakai, Y. Ebina, K. Takada, M. Watanabe and T. Sasaki, Chem. Mater., 2003, 15, 2873.
118. X. Yang, Y. Makita, Z. Liu, K. Sakane and K. Ooi, Chem. Mater., 2004, 16, 5581.
119. N. Sakai, Y. Ebina, K. Takada and T. Sasaki, J. Phys. Chem. B, 2005, 109, 9651.
120. K. Kai, Y. Yoshida, H. Kageyama, G. Saito, T. Ishigaki, Y. Furukawa and J. Kawamata, J. Am. Chem. Soc., 2008, 130, 15938.
121. W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami and Y. Takasu, Angew. Chem., Int. Ed., 2003, 42, 4092.
122. N. Mizuno and M. Misono, Chem. Rev., 1998, 98, 199.
123. T. Okuhara, N. Mizuno and M. Misono, Appl. Catal., A, 2001, 222, 63.
124. A. Corma, V. Fornes, S. B. Pergher, Th. L. M. Maesen and J. G. Buglass, Nature, 1998, 396, 353.
125. A. Corma, V. Fornés, J. Martínez-Triguero and S. B. Pergher, J. Catal., 1999, 186, 57.
126. A. Corma, U. Diaz, V. Fornés, J. M. Guil, J. Martínez-Triguero and E. J. Creyghton, J. Catal., 2000, 191, 218.
127. I. Rodriguez, M. J. Climent, S. Iborra, V. Fornés and A. Corma, J. Catal., 2000, 192, 441.
128. A. Corma, V. Fornés, J. M. Guil, S. Pergher, Th. L. M. Maesen and J. G. Buglass, Microporous Mesoporous Mater., 2000, 38, 301.
129. A. Corma, A. Martínez and V. Martínez-Soria, J. Catal., 2001, 200, 259.
130. A. Corma, P. Botella and C. Mitchell, Chem. Commun., 2004, 2008.
131. P. Botella, A. Corma, S. Iborra, R. Monton, I. Rodriguez and V. Costa, J. Catal., 2007, 250, 161.
132. A. Corma, U. Diaz, M. E. Domine and V. Fornés, Angew. Chem., Int. Ed., 2000, 39, 1499.
133. A. Corma, U. Diaz, M. E. Domine and V. Fornés, J. Am. Chem. Soc., 2000, 122, 2804.
134. A. Corma, V. Fornés and U. Díaz, Chem. Commun., 2001, 2642.
135. A. Takagaki, M. Sugisawa, D. Lu, J. N. Kondo, M. Hara, K. Domen and S. Hayashi, J. Am. Chem. Soc., 2003, 125, 5479.
136. C. Pazé, A. Zecchina, S. Spera, G. Spano and F. Rivetti, Phys. Chem. Chem. Phys., 2000, 2, 5756.
137. A. Takagaki, T. Yoshida, D. Lu, J. N. Kondo, M. Hara, K. Domen and S. Hayashi, J. Phys. Chem. B, 2004, 108, 11549.
138. A. Takagaki, D. Lu, J. N. Kondo, M. Hara, S. Hayashi and K. Domen, Chem. Mater., 2005, 17, 2487.
139. Z. Chen, T. Iizuka and K. Tanabe, Chem. Lett., 1984, 1085.
140. T. Ushikubo, Y. Koike, K. Wada, L. Xie, D. Wang and X. Guo, Catal. Today, 1996, 28, 59.
141. B. K. Sen, A. V. Saha and N. Chatterjee, Mater. Res. Bull., 1981, 16, 923.
142. T. Ikeya and M. Senna, J. Non-Cryst. Solids, 1988, 105, 243.
143. J. M. Jehng and I. E. Wachs, Chem. Mater., 1991, 3, 100.
144. L. J. Burcham, J. Datka and I. E. Wachs, J. Phys. Chem. B, 1999, 103, 6015.
145. M. Dion, M. Ganne and M. Tournoux, Mater. Res. Bull., 1981, 16, 1429.
146. A. J. Jacobson, J. T. Lewandowski and J. W. Johnson, J. Less Common Met., 1986, 116, 137.
147. X. Wang, J. Adhikari and L. J. Smith, J. Phys. Chem. C, 2009, 113, 17548.
148. P. M. Gasperin, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1982, 38, 2024.
149. K. Nassau, J. W. Shiever and J. L. Bernstein, J. Electrochem. Soc., 1969, 116, 348.
150. R. Nedjar, M. M. Borel and B. Raveau, Mater. Res. Bull., 1985, 20, 1291.
151. C. Tagusagawa, A. Takagaki, S. Hayashi and K. Domen, J. Phys. Chem. C, 2009, 113, 7831.
152. E. F. Rakiewicz, A. W. Peters and R. F. Wormsbecher, J. Phys. Chem. B, 1998, 102, 2890.
153. C. Tagusagawa, A. Takagaki, S. Hayashi and K. Domen, J. Am. Chem. Soc., 2008, 130, 7230.
154. A. Takagaki, R. Sasaki, C. Tagusagawa and K. Domen, Top. Catal., 2009, 52, 592.
155. C. Tagusagawa, A. Takagaki, S. Hayashi and K. Domen, Catal. Today, 2009, 142, 267.
156. A. Takagaki, C. Tagusagawa and K. Domen, Chem. Commun., 2008, 5363.
157. N. S. P. Bhuvanesh and J. Gopalakrishinan, Inorg. Chem., 1995, 34, 3760.
158. H. M. Kao, C. Y. Yu and M. C. Yeh, Microporous Mesoporous Mater., 2002, 53, 1.
159. Q. Zhao, W. H. Chen, S. J. Huang, Y. C. Wu, H. K. Lee and S. B. Liu, J. Phys. Chem. B, 2002, 106, 4462.
160. A. S. Dias, S. Lima, D. Carriazo, V. Rives, M. Pillinger and A. A. Valente, J. Catal., 2006, 244, 230.

---