Variability of the surface properties of sepiolite

Abstract

A comparative study of a wide collection of sepiolites (22 samples from different locations, including samples coming from the greatest deposits of sepiolite in the world) was made with the aim of understanding the real variability of the surface properties in sepiolite. The samples have been studied by XRD, Electron Microscopy (TEM and SEM) and the adsorption–desorption of N2. The influence of possible impurities, crystallinity and crystalline defects, and the textural variations, is taken into account to explain the high differences found in the specific surface area of sepiolites. The studied samples display a high variability in crystallinity, fibre length and texture. The BET surface varies between 77 and 399 m2 g−1, demonstrating the significant variation in the surface properties in natural sepiolites. Both the external and the microporous area vary between very wide extreme values, from 52 m2 g−1 to 246 m2 g−1 for the microporous area and 6 m2 g−1 to 178 m2 g−1 for the external area. The exceptionally low values of these properties found in some samples cannot be related to the presence of impurities, but rather to their structural and textural features. Two types of microporosity that can be described are structural microporosity and interfibre microporosity. The SSA and the porosity of each sepiolite is the result of the sum of the intracrystalline or structural microporosity and the textural porosity (interfibre microporosity and mesoporosity). As a consequence, there is a hierarchical distribution of pore sizes, which is different for each sepiolite. The lowest values of microporosity are related to the presence of very open pores and the sepiolites with higher SSA are those having a smaller length and a more closed porosity.