Mesoporous Titanosilicate Molecular Sieves Prepared at Ambient Temperature by Electrostatic (S⁺I^-, S⁺X^-I⁺) and Neutral (S°I°) Assembly Pathways:  A Comparison of Physical Properties and Catalytic Activity for Peroxide Oxidations

Abstract

Hexagonal mesoporous titanosilicates with distinguishable framework charges and textural mesoporosity, namely, Ti-MCM-41 and Ti-HMS, were prepared at ambient temperature by electrostatic and neutral assembly processes, respectively. Titanium incorporation at the 2 mol % level for both materials was accompanied by increases in lattice parameters and wall thicknesses, but the framework pore sizes remained unaffected. Cross-linking of the anionic framework of as-synthesized Ti-substituted MCM-41 prepared by electrostatic S+I- and S+X-I+ assembly pathways (where S+ is a quaternary ammonium surfactant and I- and I+ are ionic silicon precursors) was enhanced significantly by Ti substitution, as judged by 29Si MAS NMR. The neutral framework of as-synthesized Ti-HMS formed by S°I° assembly (where S° is a primary amine and I° is a neutral silicon precursor) exhibited the same high degree of cross-linking as the unsubstituted silica analog. UV−vis and XANES spectra for the calcined forms of Ti-MCM-41 and Ti-HMS indicated (i) the presence of site-isolated Ti species in the framework, (ii) predominantly tetrahedral coordination for Ti, along with some rehydrated five- and six-coordinated sites, and (iii) Ti siting that was virtually independent of the framework assembly pathway. All mesoporous molecular sieves exhibited catalytic activities superior to that of titanium silicalite for the liquid phase peroxide oxidations of methyl methacrylate, styrene, and 2, 6-di-tert-butylphenol. The exceptional catalytic activity in the case of Ti-HMS, especially toward larger substrates, was attributable to the small crystallite size and complementary textural mesoporosity that facilitates substrate access to framework Ti sites.

Mesoporous titania photocatalysts: preparation, characterization and reaction mechanisms

“…Hu et al, 105 have prepared Ti-MCM-41 mesoporous molecular sieves using TEOS and TPOT as the starting materials and CTAB as a structure directing agent. 106,107 The photocatalytic reactivity of these catalysts for the decomposition of NO into N 2 and O 2 was found to strongly depend on the local structure of the Ti-oxide species including their coordination and distribution, i.e., the charge transfer excited state of the highly dispersed isolated tetrahedrally coordinated Ti-oxides act as the active sites for the photocatalytic decomposition of NO. Ti-MCM-41 showed higher photocatalytic reactivity than Ti-HMS for the decomposition of NO.…”

コロイド状および担持ナノ粒子の設計─その合成，構造解析と触媒作用─

“…It has been reported that the large 2,6-DTBP can not diffuse into the small micropore of Ti-containing zeolite (TS-1) and therefore can not be transformed 78) . Upon completion of the oxidation reaction, the magnetic properties of the γ-Fe2O3@Ti-HMS can afford a straightforward way of isolating the catalyst.…”

On the Possibility of Mo Substitution in Mesoporous Materials - Cluster Model Calculations

“…TS-1 has been shown to selectively catalyze a variety of organic substrates with 30% hydrogen peroxide under mild conditions provided the molecule could enter the relatively small pores of the zeolite [1,[5][6][7][8][9]. An important development in this field is the synthesis of transition metal-containing mesoporous (M41S) molecular sieves [10][11][12][13]. The benefits of the larger pores are manifested in the oxidation of bulkier substrates with hydrogen peroxide as well as the possibility of using hydroperoxides as the oxygen source.…”