Structural Studies of Anomalous Behavior in the Silica‐Alumina Gel System

Abstract

This paper compares the structure ofAl2O3‐SiO2 xerogels prepared with Al2O3 contents ranging from 21 to 75 wt% (13 to 64 mol%). The 47 wt% Al2O3 xerogel (Al/Si ≅ 1) exhibits anomalously low surface area (≅1 m2/g) and skeletal density compared with other Al2O3‐Si2 compositions. Based on the results of nitrogen adsorption/condensation, helium displacement, high‐resolution transmission electron microscopy (HRTEM), small‐angle X‐ray scattering (SAXS), and small‐angle neutron scattering (SANS), we attribute the low surface area and skeletal density (density of the solid phase which is inaccessable to helium) to the formation of closed micropores, whereas the higher surface area materials exhibit a slightly coarser texture comprising open pores with radii of ≅1 nm. X‐ray diffraction (XRD) and 29SI and 27Al magic‐angle spinning nuclear magnetic resonance (MASNMR) indicate no anomalous behavior in the 47% sample on molecular length scales. HRTEM indicates the presence of a small fraction of crystallites which is supported by the SAXS results, but it is unknown if this crystallinity is related to low surface area. Low‐field 1H NMR spin‐lattice relaxation measurements show that the physical structure of all of the ‘wet’ gels is similar implying that pore closure occurs during drying. Consistent with this idea, gel surface area and density increased significantly when the pore fluid (water) was replaced with a lower surface tension fluid (formamide, dioxane, ethanol. water/surfactant).