Modification of sepiolite by treatment with fluorides: Structural and textural changes

In the search for new applications of natural silicates, various F- treatme nts have been applied to sepiolite to increase its acidic properties and fo r use as a catalyst in reactions occurring via carbonium ions. Two types of treatments including hydrofluoric acid (HF) at different concentrations an d 2 N NH4F have been utilized and the physicochemical characteristics of th e resulting materials studied using standard techniques. The X-ray diffract ogram (XRD) patterns indicate a decrease in crystallinity of the original m aterial as well as the appearance of amorphous silica. SEM micrographs show ed a shortening and aggregation of the sepiolitic fibers. X-ray photoelectr on spectroscopy (XPS), Fourier transform infrared (FTIR), thermogravimetric analysis/differential thermal analysis (TGA/DTA), N-2 adsorption-desorptio n isotherms and Hg intrusion were used to study the changes occurring in th e structure, surface area and pore distribution of samples and acidity was evaluated by IR and thermoprogrammed desorption (TPD) of adsorbed ammonia a nd pyridine. It was found that acidity increased in most of the samples aft er anionic and cationic interchange between the activating agents and the s urface sites, or extralattice cations. Additionally, structural changes ind uced by treatments modified the Bronsted and Lewis acidity. Mild treatments with ammonium fluoride are more effective than HF treatments in acidity ge neration.