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 beaks. 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 version of the Kelvin equation for nitrogen adsorption in cylindrical pores. Statistical film thickness curves (t curves) for nitrogen adsorbed on the surface of MCM-41 pores were also evaluated and compared with the t curve for a macroporous silica gel. It was shown that although all these t curves were very similar to one another in the low-pressure range, the statistical film thickness for MCM-41 was slightly enhanced as the primary mesopore size decreased. At higher pressures, an increase in the statistical film thickness was observed as the capillary condensation pressure range was approached, especially for the material with the smallest primary mesopore diameter. Thus, the current study of highly ordered MCM-41 materials demonstrates a self-consistency of pore size analysis methods based on geometrical considerations and on the modified Kelvin equation.