M.A. Vicente Rodríguez

Acid activation of a palygorskite with HCl : development of physico-chemical, textural and surface properties

Abstract A palygorskite from Bercimuel (Segovia, Spain) was treated with 1.0, 3.0, 5.0 and 7.0 N solutions of HCI for 1 hour under reflux. The solids obtained were characterized by XRD, FT-IR spectroscopy, thermal analyses, TEM and nitrogen adsorption-desorption isotherms at 77 K. The acid treatment runs with the removal of the octahedral cations—Mg(II), Al(III) —and the formation of amorphous silica from the tetrahedral sheet. An important increase in the specific surface area was observed during treatment, reaching a maximum of 286 m2/g in the sample treated with 5.0 N HCI. The silica obtained after the treatments maintains the fibrous morphology of natural palygorskite. No creation of microporosity is observed during the treatment.

Preparation of microporous solids by acid treatment of a saponite

Abstract A ferrous saponite (griffithite) from Griffith Park (CA, USA) was treated with solutions of HCl (0.62, 1.25 and 2.5%, w/w) at 25°C for 2, 6, 24 and 48 h. The characterization of the resulting solids shows the destruction of the silicate structure and generation of free silica by the treatments. The specific surface areas increase greatly in activated solids, with values up to 300 m 2 /g, ten times higher than the surface area of natural saponite (35 m 2 /g). The analyses of the corresponding isotherms by using t , f and α s plots show that the creation of microporosity has a big influence on the increase of surface area. The texture of the free silica, generated by destruction of the tetrahedral sheet of the silicate, conditioned the properties of the acid-activated solids. If the free silica is digested, a sudden decrease in specific surface areas was observed, which indicates that free silica makes a very important contribution to the surface area of leached samples.

ACID ACTIVATION OF A FERROUS SAPONITE (GRIFFITHITE): PHYSICO-CHEMICAL CHARACTERIZATION AND SURFACE AREA OF THE PRODUCTS OBTAINED

A ferrous saponite (griffithite) from Griffith Park (California, USA) was treated with solutions of HC1 (0.62, 1.25 and 2.5% by weight) at 25~ for 2, 6, 24 and 48 hours. The resulting solids were characterized by XRD, FT-IR spectroscopy, thermal analyses, SEM, TEM and nitrogen adsorption iso- therms at 77 K, showing the destruction of silicate structure by the treatments. The free silica generated by these treatments was digested and determined in all samples. Several samples had specific surface areas up to 250 m2/g, with maximum values which are 10 times higher than the surface area of natural saponite (35 m2/g). A sudden decrease in specific surface areas was observed when free silica was digested, which indicates that free silica makes a very important contribution to the surface area of leached samples.A ferrous saponite (griffithite) from Griffith Park (California, USA) was treated with solutions of HCl (0.62, 1.25 and 2.5% by weight) at 25°C for 2, 6, 24 and 48 hours. The resulting solids were characterized by XRD, FT-IR spectroscopy, thermal analyses, SEM, TEM and nitrogen adsorption isotherms at 77 K, showing the destruction of silicate structure by the treatments. The free silica generated by these treatments was digested and determined in all samples. Several samples had specific surface areas up to 250 m2/g, with maximum values which are 10 times higher than the surface area of natural saponite (35 m2/g). A sudden decrease in specific surface areas was observed when free silica was digested, which indicates that free silica makes a very important contribution to the surface area of leached samples.

Influence of the free silica generated during acid activation of a sepiolite on the adsorbent and textural properties of the resulting solids

A sepiolite from Vallecas (Spain) has been treated with 1.25 and 2.5 wt.% HCl aqueous solutions, at 25 °C for 2, 6, 24 and 48 h. The resulting solids were characterized using XRD, FTIR, TEM and nitrogen adsorption–desorption isotherms at 77 K and the free silica formed during the acid attack was digested and determined in all samples. The solids obtained after free silica digestion were also characterized and compared with the same solids before digestion. The influence of the unattacked sepiolite and of the free silica generated during acid activation is thus proved. Free silica has little influence on the properties of the solids obtained when acid treatment of sepiolite has been mild, because these solids are mainly formed by unattacked sepiolite. This influence becomes very important when solids obtained after more intense acid treatments are considered.

Acid activation of a Spanish sepiolite. II: Consideration of kinetics and physico-chemical modifications generated

Abstract Sepiolite from Vallecas (Spain) was treated with HCl solutions (1.25; 2.5; 5.0; 10.0 and 20.0 wt%) at 25°C for 2, 6, 24 and 48 h. The solutions obtained after washing the acid treated samples were analysed and a kinetic model was applied to the cation dissolution data. The development of porosity and number of acid centres was studied. The experimental conditions which yield products with the highest surface activity were established. The size of the silica fibres, obtained after the treatment, decreases when the intensity of the treatments is increased. These characteristics are important for the adsorptive properties and the catalytic potential of a solid.