Direct Synthesis and Characterization of Amine-Functionalized Mesoporous Silica Materials and Their Applications as Formaldehyde Adsorbents

Amine-functionalized mesoporous materials were tailored through co-condensation of tetraethoxysilane and n-(2-aminoethyl)-3-aminopropyltrimethoxysilane varied in a range of 0-50 mol% in the presence of cetyltrimethyl ammonium bromide as template. Materials were characterized for textural properties by nitrogen adsorption methods and transmission electron microscopy (TEM) and chemical structure by Fourier transform infrared (FTIR) and silicon-29 nuclear magnetic resonance (Si-29-NMR) spectroscopies. Adsorptive capacity for gaseous formaldehyde was also investigated, using a desiccator method. Adsorption data was fit using Langmuir, Freundlich, and Temkin isotherms. BET surface area results showed the effect of n-(2-aminoethyl)-3-aminopropyltrimethoxysilane loading. As the concentration of n-(2-aminoethyl)-3-aminopropyltrimethoxysilane was increased, lower BET surface area was obtained. High concentration of n-(2-aminoethyl)-3-aminopropyltrimethoxysilane added into the initial mixture resulted in reduced surface area and pore volume. Solid-state NMR and FTIR demonstrated the incorporation of amine functional groups on pore walls of the amine-functionalized materials. In addition, FTIR spectra indicated that the amine site on pore surfaces reacted with formaldehyde during adsorption and showed that the surfactant template can be removed from silica matrix using an extraction method. Formaldehyde adsorption was well described by a Langmuir isotherm. The capacity of the adsorbent mainly depended on the amount of amine groups on the pore surface. Materials derived from 80 mol% of tetraethoxysilane and 20 mol% of n-(2-aminoethyl)-3-aminopropyltrimethoxysilane had lower surface area than that of the pure mesoporous silica adsorbent and shows the highest adsorptive capacity, which is around two times higher than that of the pure mesoporous silica.