This thesis describes a development of novel solid acid catalysts. A number of metal oxide solid acids were evaluated in order to study the acid catalytic activity of various mixed metal oxides with different structures and compositions. Layered protonated metal oxides, exfoliated nanosheets and mesoporous structured metal oxides were studied as novel solid catalysts. The metal oxides examined in this study were mainly composed of group 5 (Nb, Ta) and group 6 (Mo, W) elements. The thesis consists of 13 chapters all written in English.

Chapter 1: A general introduction about solid acid catalysts is described.

Chapter 2: Nanosheet aggregates prepared from protonated layered tungstates HMWO6 (M=Nb, Ta) were examined as potential solid acid catalysts. The nanosheet aggregates were formed by soft chemical processing of the layered compound using tetra(n-butylammonium) hydroxide, and the catalytic activity and acid strength of the aggregates were compared with those for HTiNbO5, HNb3O8, and a range of conventional solid acids. The catalytic activity for the Friedel-Crafts alkylation of anisole in the presence of benzyl alcohol increased in the order HTiNbO5 < HNb3O8 < HMWO6 (M=Nb, Ta), consistent with the acid strengths determined by desorption measurements and nuclear magnetic resonance spectroscopy. Nuclear magnetic resonance spectroscopy indicated that the acid catalytic activity of the nanosheet aggregates was attributable to strong Bronsted acid sites, presumably M(OH)M' (M=Ti, Nb, Ta; M'=Nb, W).

Chapter 3: Layered HNbMoO6 was found to function as a strong solid acid catalyst, exceeding the activity of zeolites and ion-exchange resins for Friedel-Crafts alkylation, acetalization and hydrolysis of saccharides. HNbMoO6 also exhibited high catalytic activity for esterification of hydrocarboxylic acid and hydration. The catalytic performance of layered HNbMoO6 is attributed to the intercalation of reactants into the interlayer and the development of strong acidity.

Chapter 4: The acid properties and catalytic activity of layered and nanosheet aggregates of HNbMoO6 for liquid-phase Friedel-Crafts alkylation were examined. 31P MAS NMR spectroscopy using trimethylphosphine oxide as a probe molecule revealed that HNbMoO6 possesses strong acid sites in the interlayer region. It was determined that benzyl alcohol was intercalated into the HNbMoO6 interlayer during alkylation to form a monolayer configuration, allowing the strong interlayer acid sites to participate in the reaction.

Chapter 5: The intercalation and reaction process of layered HNbMoO6 for liquid-phase Friedel-Crafts alkylation and esterification of lactic acid are examined. Different sized layered HNbMoO6 with same acid properties, examined by NH3-TPD and FT-IR, reveal that not all of the strong acid sites in the interlayer region of layered HNbMoO6 are efficiently used during reaction. It is determined that the reactions occur at the edge of the layered structures.

Chapter 6: Protonated, layered transition metal oxides H1-xNb1-xMo1+xO6 with various Nb and Mo ratios were examined as solid acid catalysts. Layered Nb-Mo oxides exhibited remarkable catalytic activity in Friedel-Crafts alkylation of toluene with benzyl alcohol and hydrolysis of cellobiose exceeding that of ion-exchange resins, and H-type zeolites, and this was attributed to the intercalation behavior and changes on solid acid proprieties for different ratios of Nb and Mo.

Chapter 7: Layered and nanosheet aggregates of HTaMoO6 were examined as solid acid catalysts. The HTaMoO6 aggregated nanosheets were formed by soft chemical processing of layered HTaMoO6 using tetra(n-butylammonium) hydroxide. The catalytic activity and acid properties of the HTaMoO6 compounds were compared with those of layered HNbMoO6 and a range of conventional solid acids. The catalytic activity of HTaMoO6 for Friedel-Crafts alkylation with benzyl alcohol, hydrolysis of disaccharides, and esterification of acetic acid and lactic acid increased after exfoliation and aggregation, consistent with the acid strengths determined by NH3 temperature-programmed desorption and 31P magic-angle spinning nuclear magnetic resonance spectroscopy measurements. HTaMoO6 nanosheets possess additional strong acid sites on the oxide formed by exposure of a single layer, along with strong acid sites within the interlayer of the layered aggregate structure, resulting in higher acid catalytic activity than that of the original layered oxide.

Chapter 8: Mesoporous NbxW(10-x) mixed oxides with different Nb and W concentrations prepared from NbCl5 and WCl6 in the presence of poly block copolymer surfactant Pluronic P-123 were examined as potential solid acid catalysts. Amorphous wormhole-type mesopores were observed for samples from x=3 to 10 whereas W-rich samples (x=0 to 2) formed a non-mesoporous structure with presence of crystallized tungsten oxide (WO3). The acid-catalytic activity, acid strength and mesopore structure of mesoporous Nb-W oxides changed in order of W concentrations, exhibiting a very high activity for both Friedel-Crafts alkylation of anisole and hydrolysis of sucrose. The results were compared with those for non-porous Nb2O5-WO3 and a range of conventional solid acids. Mesoporous Nb-W oxides obtained higher turnover rate than that of non-porous Nb2O5-WO3 led to the strong acid sites and a mesoporous structure with a high surface area and easy reactant accessibility.

Chapter 9: Mesoporous Nb3W7 oxides were studied by changing the mesopore structure, using different block copolymers as structural directing agent and changing the alcohol solutions and calcination temperatures. The acid-catalytic activity and mesopore structure of mesoporous Nb3W7 oxides changed in order of pore size observed by both X-ray diffractions and N2 desorption. The mesoporous Nb3W7 oxides with bigger pore size exhibited higher activity for Friedel-Crafts alkylation of anisole, hydrolysis of disaccharides and esterification of lactic acid led to the liquid diffusion limit, influencing negatively on acid reaction rates for small sized mesopores. And the mesoporous Nb3W7 oxides samples synthesized at different calcination temperatures (673-873 K) exhibited different acid property for NH3 temperature-programmed desorption, forming stronger acid sites for mesoporous Nb3W7 oxides calcinated at higher temperatures obtaining higher reaction rate for both alkylation and hydrolysis reactions.

Chapter 10: Several mesoporous TaxW10-x mixed oxides prepared from TaCl5 and WCl6 in the presence of poly block copolymer surfactant Pluronic P-123 were examined as potential solid acid catalysts. Mesoporous Ta-W oxides (Ta:W=3:7) with an amorphous wormhole-type mesoporous were found to be a highly active solid acid, exceeding the results obtained from non-porous Ta2O5-WO3, HTaWO6 nanosheets, ion-exchange resins (Nafion NR50 and Amberlyst-15) and zeolites (H-ZSM5 and H-Beta) in Friedel-Crafts alkylation and hydrolysis reactions. Mesoporous Ta-W oxides exhibited a higher turnover rate than non-porous Ta2O5-WO3 and HTaWO6 nanosheets, indicating that the mesoporous structure is an advantageous environment for the strong acid sites, because of the high surface area and easy reactant accessibility.

Chapter 11: Several porous NbxMo10-x mixed oxides prepared from NbCl5 and MoCl5 in the presence of block copolymer surfactant Pluronic P-123 were examined as potential solid acid catalysts. Amorphous mesoporous structures were observed in samples with x from 10 to 9, whereas samples with higher Mo concentrations (x=3-8) formed large pores by inter-particle voids and a non-porous structures with crystallized molybdenum oxide (MoO3) were observed in samples with x from 0 to 2. The acid strength increased with increasing the Mo content in NbxMo10-x oxides, which was determined by NH3 temperature-programmed desorption (NH3-TPD) and Fourier transform infrared (FT-IR) spectroscopy using pyridine as a probe molecule. Porous Nb3Mo7 oxide exhibited the highest acid-catalytic activity for the Friedel-Crafts alkylation of anisole due not only to mole ratio of Nb/Mo but also to pore structure.

Chapter 12: The results described in the Chapters 2-11 are summarized.

Chapter 13: General Conclusion of thesis is described.

Novel mixed transitional metal oxide solid acids were prepared in layered, aggregated and mesoporous structures. Each solid acid had a unique nature. Aggregated nanosheets make possible the usage of acid sites present in the interlayer. Layered HNbMoO6 is the first case of a layered metal oxide with excellent solid acid reaction rates due to its intercalation ability. Mesoporous NbxW(10-x) oxide is the first mixed metal oxide mesoporous solid acid with high catalytic activity due to its high surface area mesoporous reaction field. This investigation is very important as a pioneering study on the development and characterization of different metal oxide based structured solid acids with unique characteristics with potential use in both the fine and commodity chemical sectors.

Therefore, this thesis is recognized as a Doctoral Thesis for Chemical System Engineering Course of The University of Tokyo..

本論文は、メソポーラス及び層状構造を有する新規固体酸触媒の研究について記述されている。本論文は全部で13の章から構成されている。

第1章では、本研究の意義、固体酸触媒の種類、金属酸化物からなる固体酸の事例、固体酸触媒の現状と課題、博士論文の概要が記載されている。

第2章では, 層状構造のHNbWO6とHTaWO6酸化物をソフトケミカルプロセスを用いて層剥離し、更に硝酸添加によりナノシート凝集体を合成し、その酸性質や酸触媒活性等を検討している。類似のソフトケミカルプロセスを用いて得られたHTiNbO5, HNb3O8ナノシート凝集体と比較し酸強度、酸触媒活性共にHTiNbO5 < HNb3O8 < HNbWO6 < HTaWO6の順に高くなっていることを確認している。これらの結果よりナノシートを構成する金属イオンが4価と5価、5価と5価、5価と6価の順に酸触媒活性が向上すると推測している。

第3章では、高い固体酸触媒能を示す層状HNbMoO6の物性、酸性質、酸触媒活性について詳細に検討している。層状HNbMoO6は従来の層状金属酸化物と異なり層間に反応物をインターカレートし、層間内の強い酸点上で反応が進行するユニークな固体酸であると報告している。フリーデルクラフツ反応、エステル化反応、アセタール反応、水和反応、加水分解反応において高い活性が得られ、中でも水を溶媒にした糖類の加水分解反応で高い活性が得られたことよりバイオリファイナリーの分野でも有望な固体酸触媒であると述べている。

第4章では層状およびナノシート凝集体HNbMoO6の酸性質や触媒活性を評価することにより、同層状体の酸点や活性点の詳細な評価を行っている。31P MAS NMRの結果と触媒活性の評価を比較検討した結果、層状HNbMoO6は反応物を層間にインターカレートする際、モノレイヤー又はバイレイヤー構造を形成することを見出している。酸強度を評価する際にモノレイヤー構造を用いた場合、上下両方の酸点が同時に作用するためバイレイヤー構造より高い酸強度を示すと推測している。酸触媒反応中のHNbMoO6においてもモノレイヤー構造であることを確認し、それに起因して高い触媒活性が得られたと結論している。

第5章では、層状HNbMoO6の反応場が層空間全域であるのか、あるいはエッジの部分のみであるのかを検討している。粒子サイズの異なるHNbMoO6を合成し評価した結果、HNbMoO6の反応場は主にエッジ付近に存在していると結論付けている。

第6章では、層状H1-xNb1-xMo1+xO6のNbとMoの比率を変えた際の酸性質、酸触媒活性を評価している。電気陰性度のより高いMoの比率を増やすことで酸強度、そしてNbを増やす事で酸量が増加することを見出している。この結果、従来のHNbMoO6よりも高活性なH1-xNb1-xMo1+xO6を得ることに成功している。

第7章では、層状HTaMoO6の物性、酸性質、酸触媒活性の評価について記述している。層状HTaMoO6はHNbMoO6と同じインターカレーション能を有しており高い触媒活性と酸強度を有していると結論している。

第8章では、メソポーラスNbxW(10-x)酸化物のNb/W比を変えて物性、酸性質、酸触媒活性の評価を行なっている。メソポーラス金属酸化物は単体の金属酸化物の研究が主流であり、二つの金属を混合した場合の評価は初の試みである。Wの比率が増すにつれ酸強度とブレンステッド酸点の量が増加し、また次第にメソポーラス構造が失われることを見出している。酸強度の上昇はWの高い電気陰性度に由来するものであり、同時に酸触媒活性の上昇も確認できるが、Wの比率がNb3W7を超えるとメソポーラ構造が失われ、酸触媒活性も急落することを見出している。つまりメソポーラスNbxW(10-x)酸化物の酸触媒活性を向上させるには酸強度と共にメソポーラス構造の制御が重要であると結論している。

第9章では、界面活性剤とアルコール溶媒を変えることでメソポーラスNbxW(10-x)酸化物のポアサイズを制御した触媒に付いて議論されている。メソポーラスNbxW(10-x)のポアサイズを制御することで触媒反応の選択性をコントロール出来る事を示している。

第10章では、メソポーラスTaxW(10-x)酸化物の評価を行っている。メソポーラスTaxW(10-x)酸化物はメソポーラスNbxW(10-x)酸化物と類似の性質を持っており、Wの比率を増やすことで酸強度を上げられることが記載されている。

第11章では、メソポーラスNbxMo(10-x)酸化物の評価が行なわれている。メソポーラスNbxMo(10-x)は前章にて評価されたメソポーラスNbxW(10-x)、メソポーラスTaxW(10-x)と異なり、Moの親水性が熟成過程におけるメソポーラス構造形成の障害となることを述べている。

第12章では、第2章から第11章までの研究成果が簡潔にまとめられている。

第13章では、論文全体の結論、研究結果の評価、今後の酸触媒開発の課題や展望が述べられている。

以上のように、本論文は層状HNbMoO6やメソポーラスNbxW(10-x)固体酸及びその類縁体など、様々な構造を有する金属酸化物固体酸触媒の開発について述べられており、新規固体酸触媒の開発に関する多くの成果を報告している。一連の研究成果は固体酸触媒の新しい可能性を示すもので、化学システム工学及び触媒化学に貢献するところが大きく、今後工業的にも応用される可能性を有している。

よって本論文は博士(工学)の学位請求論文として合格と認められる。