Analysis of pore structure and gas adsorption in periodic mesoporous solids by in situ small-angle X-ray scattering

This paper describes a new method for characterizing the pore structure and gas adsorption in periodic mesoporous materials based on small-angle X-ray diffraction and scattering. For SBA-15 as a case study it is shown that the effective pore radius and volume of the ordered cylindrical pores and their corona can be determined accurately from the intensities of the Bragg reflections. The contribution of the intrawall porosity to the pore volume is then obtained by comparing this ordered pore volume with the total pore volume from nitrogen adsorption. Adsorption of a fluid in the ordered and disordered pores of SBA-15 is studied by analyzing scattering data measured in situ as a function of the vapor pressure of the fluid. In this way the different stages of pore filling can be monitored, and separate filling isotherms of the ordered and intrawall pores can be obtained for an adsorbate (dibromomethane) having a similar electron density as the solid matrix. Our analysis shows that the filling of the intrawall porosity has a pronounced effect on the evaluation of the adsorbed amount in the ordered pores. These finding emphasize the importance of disordered intrawall porosity in polymer-templated silicas like SBA-15 and open up new ways for an in-depth characterization of these materials.