Glycometallate surfactants Part 2: non-aqueous synthesis of mesoporous titanium, zirconium and niobium oxides

Abstract

Cetyltrimethylammonium glycotitanate(IV), glycozirconate(IV) and glyconiobate(V), CTA_1,2[M(OCH₂CH₂)₃] where M=Ti(IV), Zr(IV), Nb(V), have been synthesized under non-aqueous conditions by reacting Na_1,2[M(OCH₂CH₂)₃] with CTACl in ethylene glycol. The glycometallates all have a structure based upon an octahedral metal center containing three chelated glycolate ligands. In ethylene glycol all of the CTA_1,2[M(OCH₂CH₂)₃] moieties are found to self-assemble into a lamellar mesophase with a structure based upon bilayers of cationic CTA⁺ that are charge-balanced by [M(OCH₂CH₂)₃]^2– counter-anions. Hydrolysis of the lamellar glycometallate mesophase in ethylene glycol leads to well-ordered hexagonal phases of mesoporous zirconia and niobia but partially ordered for mesoporous titania. In all cases it is found that the extent of condensation-polymerization of the transition metal oxide in the as-synthesized mesoporous materials is insufficient to sustain the integrity of the structure after the surfactant template is removed by a thermal treatment. Structure re-enforcement of the metal oxide framework can however be achieved by silanation post-treatment of the dehydrated mesoporous metal oxide with Si₂H₆ at 100 °C. All of the mesoporous transition metal oxides that emerge from this procedure are found to be extensively polymerized and stable to the removal of surfactant. Moreover, they all contain silica in the structure at levels that are tunable over a much wider range as compared to that previously reported in the literature. The method described in this paper is a novel approach to the synthesis of stable high surface area mesoporous transition metal oxides with an adjustable level of silica incorporation into the structure.

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