Activation of a sepiolite with dilute solutions of HNO₃ and subsequent heat treatments: II. Determination of surface acid centres

Abstract

A sepiolite from Toledo, Spain, was activated with dilute solutions of HNO3 (0.25, 0.50 and 0.75 M) followed by heat treatment at 110, 200, 300, 400 and 500~ for 10 h. The retention of n-butylamine in cyclohexane solution was used to follow the variation of surface acid centres of the sepiolite as a function of the acid and heat treatments, these reaching a maximum for the product treated with 0.75 r~ HNO3 and heated at 300~ Acid treatments within the range 0-50-0-75 M and subsequent heat treatment at 200-300~ for 10 h yielded products with the highest surface activity.The catalytic activity of a solid is related to the total number of surface acid centres present and their relative strengths. The number of such surface acid centres may be modified by acid and heat treatment (Kayser & Bloch, 1952). This paper describes the variation in surface acidity of a sepiolite from Toledo, Spain, following acid (0.25, 0-50 and 0.75 M HNO3) and subsequent heat (10 h at temperatures between 110 and 500~ treatments. The increase in surface area of this sepiolite following these treatments has been described in a previous paper (L6pez-Gonzalez et al., 1981). EXPERIMENTALMaterial used for this and the previous (L6pez- Gonzalez et al., 1981) This formula was derived from the chemical analysis of a carbonate-free sample which had been ignited at 1000~The natural sepiolite, crushed to a particle size between 2.38 and 4.76 mm, was boiled with HNO3 (0.25, 0-50 and 0.75 M) for 1 h. Air-dried portions of the resulting products were treated at different temperatures (room temperature, 110, 200, 300, 400 and 500~ for 10 h. The products are designated by an S followed by two subscripts; the first refers to the concentration of the acid used and the second to the temperature at which the product was subsequently heated.

Physical and catalytic characterization of smectites from Boa-Vista, Paraíba, Brazil

“…This behaviour is governed by the extent and nature of their external surface, which can be modified by suitable acid and thermal treatments. Acid and thermal treatments increase the catalytic and adsorbent activity of certain clay minerals, but further and stronger treatments decrease this activity [11,[15][16][17][18].…”

Structural and Textural Modifications of Palygorskite and Sepiolite Under Acid Treatment

“…The behavior of sepiolite when treated with acid solutions has been studied from different points of view (Bonilla et al 1981;Corma et al 1986;Cetisli and Gedikbey 1990;Vicente et al 1994). However, the literature contains few references to acid treatment of palygorskite as compared with other clays (Singer 1976;Gonz~lez et al 1989;Corma et al 1990;Su~ez et al 1995).…”

Effect of Pretreatment on Ammonium Adsorption Properties of Sepiolite: Equilibrium, Kinetics and Thermodynamic Studies

“…Several works have shown that the acid treatment modifies the number of acid sites by disaggregation of particles, elimination of mineral impurities, removal of metal exchange cations and proton exchange [9,12]. There are three different acid species which have been described to activate natural sepiolite: HCl [13], HNO 3 [14] and H 2 SO 4 [15]. Although the variation of sepiolite surface area under various HNO 3 concentrations (1 − 5N) and heat treatment conditions (110 − 900°C, for 100 hours) had been performed by Jimenez-Lopez et al [16], the effect of these modified sepiolite samples on ammonium removal efficiency has not been studied.…”