Investigation of the medium range order of polyhedra forming the walls of MCM-41 — An X-ray diffraction study

Abstract

Pure silica MCM-41, Al-MCM-41 (Si/Al=30, 20, 10) and Ti-MCM-41 (Si/Ti=40) have been synthesized and characterized by transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). The calcined materials have been investigated by wide-angle X-ray scattering (WAXS), in order to elucidate the structural arrangement of the non-crystalline walls with regard to the medium-range order (MRO). The experimental radial distribution functions (RDF) of all samples under investigation indicate a considerable degree of MRO within the examined length scale of 8 Å. Even though, the addition of aluminium sulfate as source for aluminium, or titanium-isopropylate as source for titanium to the synthesis gel is known to affect the regularity of the pores, the MRO is not significantly affected. Modelling studies have been performed to describe the average atomic arrangement of the inorganic material of pure silica MCM-41. Best agreement between experimental and calculated data with regard to the MRO has been obtained using a structural arrangement of tetrahedra, which is also found in the structure of α-cristobalite.