Direct Synthesis and Characterization of Hydrophobic Aluminum-Free Ti−Beta Zeolite

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Abstract

Incorporation of Ti into the framework of aluminium-free zeolite Beta has been achieved in F- medium and has produced hydrophobic selective oxidation catalysts. The Ti−Beta(F) materials have been characterized by X ray diffraction, infrared, Raman, ultraviolet, XANES, EXAFS, 29Si MAS NMR, and 1H→29Si CP MAS NMR spectroscopies, adsorption microcalorimetry, and catalytic testing. At near neutral pH the incorporation of Ti into the framework appears to present an upper limit of ca. 2.3 Ti/uc, beyond which anatase is detected in the calcined materials. However, at higher pH (ca. 11) larger amounts of Ti can be incorporated without anatase formation. After calcination, Ti incorporation in the framework is characterized by an increase in the unit cell volume, the appearance of one Raman band and three infrared bands in the region near 960 cm-1 and the presence of a strong absorption band in the 205−220 nm ultraviolet spectrum. By 29Si MAS NMR, 1H→29Si CP MAS NMR, and infrared spectroscopies it is concluded that upon contact with ambient humidity there is no hydrolysis of Si−O−Ti bonds in Ti−Beta zeolites prepared by the fluoride route, while it is probably a major feature of those synthesized in OH- medium. XANES and EXAFS spectroscopies of calcined dehydrated Ti−Beta zeolites unambiguously demonstrate the tetrahedral coordination of Ti with a Ti−O bond length of ca. 1.80 Å. Upon hydration, the changes in the XANES and EXAFS spectra are consistent with a change in the coordination of Ti to reach a state which depends on the composition and synthesis route and which ranges from a 5-fold coordination for Al-free Ti−Beta synthesized by the F- method to a highly distorted 6-fold coordination in Ti,Al−Beta synthesized in OH- medium. Adsorption microcalorimetry experiments show the strict hydrophobic nature of pure SiO2 zeolite Beta synthesized in F- medium while evidencing a slight increase in the hydrophilicity of the material upon incorporation of Ti to the framework. This is due to the relatively strong adsorption of precisely one H2O molecule per Ti site. On the contrary, the materials synthesized in OH- medium show an enhanced hydrophilicity. Finally, Ti−Beta(F) is an active and selective catalyst for oxidation of organic substrates with H2O2. A comparison of the activities and selectivities of Ti−Beta(F), Ti−Beta(OH) and TS-1 in the epoxidation of 1-hexene using acetonitrile and methanol as solvents demonstrates that the major differences between Ti−Beta and TS-1 catalysts are intrinsic to each zeolitic structure. Because of its high hydrophobicity, Ti−Beta(F) catalyst can advantageously replace Ti−Beta(OH) in the epoxidation of substrates, like unsaturated fatty acids or esters, which contain a polar moiety.

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