Synthesis of mesoporous zeolites in fluoride media with structure-directing multiammonium surfactants

Abstract

Mesoporous zeolite beta in pure-silica form and its analogues, including aluminosilicate, stannosilicate, and titanosilicate, were synthesized at near-neutral pH in the presence of fluoride ion as a mineralizer and piperidinium-functionalized multiammonium surfactant as a zeolite structure-directing agent (SDA). During hydrothermal synthesis, the surfactant headgroup generated zeolite beta frameworks with a thickness of 9 nm while numerous surfactant molecules were self-organized to form a nanosponge-like mesostructure in a disordered manner. The mesopores generated after calcination to remove the surfactant were highly uniform and further tailored with diameters in the range of 3.2–4.5 nm according to the surfactant tail lengths. MFI zeolite was also synthesized with a nanosheet morphology using C16H33N+(CH3)2C6H12N+(CH3)2C6H13 as the SDA in fluoride media. The zeolite samples obtained in this manner exhibited remarkably enhanced thermal stability, Brønsted acid strength, and hydrophobic nature compared to those synthesized at a high pH using OH− as the mineralizer due to the reduced defect sites, as confirmed by 29Si MAS NMR and FT-IR measurements. Due to these advantages, the mesoporous zeolite beta obtained in the fluoride media exhibited high catalytic activity in esterification of oleic acid with methanol.