Design and Synthesis of Microporous and Micro/Mesoporous Silica Materials with Excellent Adsorption Properties via Self-Assembly of Silica Species with Tetraethyl Ammonium in Acidic Aqueous Media

Abstract

The concentration dependence of tetraethyl ammonium cations (TEA+) in strongly acidic media was investigated through a pulsed field gradient nuclear magnetic resonance (PFG-NMR) technique. The results indicated that TEA+ is monodispersed in aqueous solution at relatively low concentration (<6 wt %), whereas they partially aggregate with each other at relatively high concentration (6−40 wt %). Different states of TEA+ cations are expected to be used as possible templates for design of micropores or micro/mesopores mixed materials. Accordingly, a series of microporous silica materials and hierachically micro/mesoporous silica materials have been synthesized via self-assembly of silica species with TEA+ cations in acidic solutions by simply controlling TEA+ concentration. N2 isotherms show that the samples synthesized at low TEA+ concentration (<6 wt %) have uniform micropores (0.64−0.65 nm) and the samples synthesized at high TEA+ concentration (6−16 wt %) exhibit both micropores (0.66−0.68 nm) and mesopores (3.5−3.6 nm). Furthermore, differential thermal analysis and thermogravimetric (DTA−TG) curves and infrared (IR) spectra indicated that TEA+ species were certainly included in as-synthesized samples, confirming that TEA+ species were the templates for the formation of porosity. Very importantly, these porous silica materials showed higher adsorption capacity of volatile organic compounds (VOCs) such as benzene and cyclohexane at low relative pressure than conventional zeolites (Y and ZSM-5) and mesoporous materials (SBA-15 and MCM-41), which would be potentially important for the removal of VOCs in the future.