Characterization of Highly Ordered MCM-41 Silicas Using X-ray Diffraction and Nitrogen Adsorption

Abstract

Highly ordered MCM-41 silicas were synthesized in the presence of alkyltrimethylammonium surfactants and characterized using powder X-ray diffraction and nitrogen adsorption at 77 K. Samples prepared in the presence of hexadecyltrimethylammonium (HTMA+) and octadecyltrimethylammonium surfactants using the repeated pH adjustment method were found to exhibit a very high degree of structural ordering and (100) interplanar spacings of 4.1 and 4.6 nm, respectively. Using a high-temperature (438 K) synthesis procedure involving HTMA+ surfactants, it was also possible to prepare MCM-41 with the (100) spacing above 5 nm, which exhibited as many as five distinct XRD peaks. The MCM-41 materials had appreciable amounts of secondary mesopores, but no micropores. Pore sizes evaluated from (100) interplanar spacings and primary mesopore volumes using geometrical considerations were found to be in excellent agreement with those calculated using the Barrett−Joyner−Halenda method with the recently reported modified versio...