Titanium Dioxide-Surfactant Mesophases and Ti-TMS1

Several attempted syntheses of Ti-TMS1, a hexagonal mesoporous TiO2 reported by Antonelli and Ying,1 have resulted in a lamellar structure as determined by twodimensional powder X-ray diffraction and transmission electron microscopy (TEM). Conventional one-dimensional powder X-ray diffraction patterns of the lamellar materials are similar to those reported for Ti-TMS1.1 Regions, of similar size as those reported to be hexagonal by Antonelli and Ying,1 of our partially calcined lamellar materials when observed by TEM can be mistaken for hexagonal material. In no cases have we produced specimens that were unambiguously hexagonal. It is concluded that the hexagonal material exists, if at all, only as a minor component of a larger lamellar structure when phosphate surfactants are used, hexagonal Ti-TMS1 therefore remains elusive. For many years it has been known that some organic compounds, especially surfactants, self-assemble into micellar phases when mixed with water.2 These micellar phases include lamellar, hexagonal, and cubic structures. In 1992, researchers at Mobil discovered a new family of SiO2-based molecular sieves (M41S),3 formed in a coassembly process4-7 with a surfactant, which contain hierarchical structures based on the micellar phases formed by the surfactant.8-10 Since the Mobil discovery, a new hexagonal mesoporous TiO2 analogue of M41S called Ti-TMS1, also formed by coassembly with a surfactant, has been reported by Antonelli and Ying.1 Additional, related, materials are continuing to be announced11-15 including a hexagonally packed mesoporous semiconductor, CdS.16 The energetics of the M41S and related SiO2 materials have been studied recently.17-21 A similar study of Ti-TMS1 materials and other TiO2 polymorphs would provide data useful for models and generalizations describing the coassembly process and the role of the organic/inorganic interactions. Thus, we have attempted to synthesize the Ti-TMS1 hexagonal phase following the procedure reported by Antonelli and Ying.1 We discuss here our findings that the hexagonal form has not been reproduced but that the lamellar form is synthesized under a wide variety of conditions. Antonelli and Ying1 used tetradecyl phosphate (a 14carbon chain) as the surfactant in forming their hexagonal mesoporous TiO2 material. This surfactant was synthesized using well-known reactions.22 Our work has shown that using commercially available dodecyl phosphate (Alfa/Æsar, a 12-carbon chain) as a surfactant produces conventional powder X-ray diffraction patterns very similar to those published by Antonelli and Ying1 with the principal peak shifted slightly to higher diffraction angle (2θ) due to the decreased chain length of the surfactant (Figure 1). Such a substitution would have a similar but benign effect in the MCM41 system reported by Beck et al.3