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.

The variations in surface area of a sepiolite (Toledo, Spain) after acid and heat treatments (1 n to 5 n, HNO3, 110°C to 900°C for 100 h) have been studied. Surface areas have been determined from N2 adsorption isotherms at 77 K using B.E.T., Langmuir and Dubinin-Radushkevich equations. SBET values range from 65 up to 391 m2 g−1, the maximum value corresponding to the sample activated with 1 n HNO3 solution and treated at 300°C. SL and SD-R values are similar, but slightly larger than the corresponding SBET values.A correlated study has been carried out with samples activated with 1 n and 2 n HNO3 solutions and heat treated at 110, 200, 300, 400 and 500°C for 10 h. SBET values of these samples are rather similar to those of the corresponding samples treated during 100 h.

Porosity development in a sepiolite from Toledo, Spain, has been studied as a function of acid (0-25, 0.50, 0.75 and 1-0 M HNO3) and subsequent heat treatments (1 lff400~