Water‐Tolerant, Highly Active Solid Acid Catalysts Composed of the Keggin‐Type Polyoxometalate H₃PW₁₂O₄₀ Immobilized in Hydrophobic Nanospaces of Organomodified Mesoporous Silica

Abstract

Since the discovery of mesoporous silica, [1][2][3] new organicinorganic nanocomposites based on mesoporous silica materials have been extensively investigated in the development of functional materials in various fields.[4] The grafting of organic groups onto the pore walls of the silica [5] has provided novel materials for catalysis, [6] heavy-metal ion adsorption, [7] photocontrollable molecular storage, [8] gas separation, [9] and molecular recognition. [10][11][12] Since the chemical functionalities of these materials have been ascribed mainly to the organic moiety, a promising strategy toward new functions is to design an inorganic-organic cooperative mechanism in nanostructured materials. [11,12] Solid acid catalysts have served as important functional materials in about 180 industrial processes in the petroleum refinery industry and in the production of chemicals.[13] In contrast, a significant number of acid-catalyzed reactions, such as Friedel-Crafts reactions, esterification, and hydrations, are still carried out by using conventional acids, such as H 2 SO 4 and AlCl 3 . In particular, for the reactions in which water participates as a reactant or product, such as hydrolysis, hydration, and esterification, only a few solid acids show acceptable performances. [14][15][16] The development of new water-tolerant solid acids is expected to have a major impact in industrial applications as well as in scientific aspects. One of the major difficulties concerned with the use of solid acids is the severe deactivation of the acid sites by water, and in fact, most solid acids lose their catalytic activity in aqueous solutions.We have overcome this difficulty by designing acid catalysts comprising polyoxometalate (hetero-polyacid) molecules and organografted mesoporous silica. We found that the acidic protons in the hydrophobic environment of organomodified mesoporous silica show extremely high catalytic activity for ester hydrolysis in water. Figure 1 illustrates the concept of the nanostructured catalyst. Two kinds of organic groups, n-octyl and 3-aminopropyl, were grafted onto the pore walls of mesoporous silica SBA-15. [3] The aminopropyl groups immobilize the H 3 PW 12 O 40 polyoxometalate anions on the pore walls, while the octyl groups (ca. 1 nm in length) form hydrophobic regions around the polyanions. It was found that water and reactant molecules can penetrate into the nanospaces through the remaining spaces at the centers of the SBA-15 pores. In the preparation of the catalyst, first alkyl groups and then 3-aminopropyl groups (AP groups) were grafted on SBA-15 (pore diameter 7.5 nm, BET surface area 458 m 2 g À1 ) to obtain organomodified materials, denoted by C n -AP-SBA, where n is the number of carbon atoms of the alkyl group. After neutralization of the amino groups with hydrochloric acid, Figure 1. Schematic illustration outlining the preparation and structure of the catalysts. Octyl and 3-aminopropyl groups were subsequently grafted on the pore walls of mesoporous silica SBA-15, followed by immobil...

Polyoxotungstate-Surfactant Layered Crystal toward Conductive Inorganic-Organic Hybrid

“…Combination of polyoxometalates (POMs) and surfactant cations [9][10][11] leads to promising functional inorganic-organic layered hybrids. POMs can add various physicochemical properties to the hybrids as inorganic components [12][13][14][15][16], while surfactants enable the control of layered structures as structure-directing organic components [17][18][19].…”

“…POMs can add various physicochemical properties to the hybrids as inorganic components [12][13][14][15][16], while surfactants enable the control of layered structures as structure-directing organic components [17][18][19]. While there are many types of POM-surfactant hybrid [9][10][11][20][21][22][23][24][25], POM-surfactant single crystals are rare [26][27][28][29][30][31][32]. These POM-surfactant hybrids can allow fine tuning of the structures and functions, and are promising conducting materials as precedented inorganic-organic hybrid conductors [2,3].…”

Chemically stable magnetic nanoparticles for metal adsorption and solid acid catalysis in aqueous media

“…H-ZSM-5, Naon, Cs 2.5 H 0.5 PW 12 O 40 ). [58][59][60] The reactions became stable aer a second run, suggesting that Co@SiO 2 -SPS nanoparticles can be reused without any signicant loss of activity. The negligible difference in the cation exchange capacity of Co@SiO 2 -SPS suggests that the active catalytic active sites were not degraded by repeated reactions, and that there was no signicant erosion of Co from the catalyst support which poisoned the active sites.…”

Polyoxometalate (POM)-Layered Double Hydroxides (LDH) Composite Materials: Design and Catalytic Applications

“…In another case, Inumaru et al demonstrated the efficiency of a POM-SiO 2 heterogeneous catalyst where H 3 PW 12 O 40 species immobilized the channels of mesoporous silica decorated with hydrophobic alkyl groups. The produced catalyst not only revealed exceptional stability but also showed exceptionally high catalytic activity for ester hydrolysis in aqueous medium [33]. Various elegant approaches have been adopted for the design of appropriate components for the development of new POM-based heterogeneous catalytic systems.…”