High-Temperature Synthesis of Stable Ordered Mesoporous Materials Using Fluorocarbon Surfactants

摘要

The hydrothermal stability of mesoporous materials is currently of great interest because of the requirement for potential applications. A number of successful examples of mesoporous materials with good hydrothermal stability have been reported recently. Notably, these mesostructured materials are prepared at room temperature or relatively low temperatures (80-150℃). This is quite different from the higher temperatures (150-250℃) used for the syntheses of many microporous zeolites or phosphates and is because surfactant molecules will not be able to direct mesoporous structure formation due to unfavorable conditions for micelle formation at the higher temperatures. As with silica-based materials, a critical factor in increasing Kydrothermal stability is to have more silica condensation on the pore walls, but low synthetic temperatures result in imperfectly condensed mesoporous walls with large amounts of terminal hydroxyl groups which make the mesostructure unstable. It can be expected that the level of silica condensation will be enhanced by increasing the crystallization temperature. Fluorocarbon surfactants are a kind of stable surfactants which are widely used at high temperatures (> 200℃). However, due to their rigidity and the strong hydrophobicity of the fluorocarbon chains, fluorocarbon surfactants are not suitable as templates for the preparation of well-ordered mesoporous mateials. We demonstrate here that when a fluorocarbon surfactant (C_3F_7O(CFCF_3CF_2O)_2CFCF_3CONH(CH_2)_3N~+(C_2H_5)_2 CH_3I~-, FC-4) is mixed with a triblock copolymer surfactant (EO_(20)PO_(70)EO_(20), Pluronic P123) to form a surfactant mixture and this mixture is used as the template, highly ordered mesoporous silica-based materials (JLU-20) with unusual hydrothermal stability are successfully synthesized at high temperatures (160-250℃).