Modification of Sepiolite by Treatment with Fluorides: Structural and Textural Changes

María Jesús Belzunce1, †, Sagrario Mendioroz1 and Jerzy Haber2
1 Instituto de Catálisis y Petroleoquímica, CSIC, Campus de la UAM, Cantoblanco, 28049 Madrid, Spain
2 Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek, 30239 Kraków, Poland
Present Address: Dpto de Oceanografía y Medio Ambiente Marino (AZTI), Satrústegi 8, 20008 San Sebastian, Spain.

Abstract: In the search for new applications of natural silicates, various F treatments have been applied to sepiolite to increase its acidic properties and for use as a catalyst in reactions occurring via carbonium ions. Two types of treatments including hydrofluoric acid (HF) at different concentrations and 2 N NH4F have been utilized and the physicochemical characteristics of the resulting materials studied using standard techniques. The X-ray diffractogram (XRD) patterns indicate a decrease in crystallinity of the original material as well as the appearance of amorphous silica. SEM micrographs showed a shortening and aggregation of the sepiolitic fibers. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), thermogravimetric analysis/differential thermal analysis (TGA/DTA), N2 adsorption-desorption isotherms and Hg intrusion were used to study the changes occurring in the structure, surface area and pore distribution of samples and acidity was evaluated by IR and thermoprogrammed desorption (TPD) of adsorbed ammonia and pyridine. It was found that acidity increased in most of the samples after anionic and cationic interchange between the activating agents and the surface sites, or extralattice cations. Additionally, structural changes induced by treatments modified the Brönsted and Lewis acidity. Mild treatments with ammonium fluoride are more effective than HF treatments in acidity generation.

Key Words: Activation • Fluoride Activation • Sepiolite • Silicate Acidity

Clays and Clay Minerals; December 1998 v. 46; no. 6; p. 603-614; DOI: 10.1346/CCMN.1998.0460601
© 1998, The Clay Minerals Society
Clay Minerals Society (www.clays.org)