Modification of the Porous Structure and Surface Area of Sepiolite under Vacuum Thermal Treatment

Abstract

Abstract--Modifications of the external surface area and the two types of microporosity of sepiolite (structural microporosity and inter-fiber porosity) were examined as a function of the temperature of a vacuum thermal treatment to 500~ The methods used included: reciprocal thermal analysis, N 2 and Ar low-temperature adsorption microcalorimetry, gas adsorption volumetry (for N2, mr, and Kr at 77 K and CO2 at 273 and 293 K), water-vapor adsorption gravimetry, and immersion microcalorimetry into liquid water at 303 K. If the sample was not heated > 100~ only 20% of the structural microporosity was available to N2, whereas 52% was available to CO: at 293 K. In both experiments, the channels filled at very low relative pressures. At > 350~ the structure transformed to anhydrous sepiolite, which showed no structural microporosity. The inter-fiber microporosity decreased from 0.031 to 0.025 cmVg (as seen with N2), and the external specific surface area decreased from 120 to 48 m2/g. The water adsorption isotherms showed a lower and lower affinity of the external surface of fibers for water as the temperature of thermal treatment increased. The thickness of the bound water on the external surface was estimated to be -<3.5 monolayers, i.e., less than l0/~.

Mechanism of Adsorption and Desorption of Water Vapor by Homoionic Montmorillonites: 2. The Li+, Na+, K+, Rb+ and Cs+-Exchanged Forms

“…Controlled Transformation-Rate Thermal Analysis (CTRTA) (Rouquerol 1989, Grillet et al 1988. This process allows one to overcome partial overlaps of successive dehydration steps when using conventional thermal analysis.…”

“…Immersion microcalorimetry at 30~ was used to determine the enthalpy of immersion in water of the montmorillonite sample (about 100 mg of clay, outgassed under the conditions given above for the gravimetric adsorption of water vapor, in a cell of about 6 cm 3) versus the pre-equilibrium relative pressure of water vapor following a method described by Grillet et al (1988). The enthalpy curve was then used to derive the external specific surface area of quasi-crystals immersed in water (final state) by applying a modified Harkins and Jura procedure (Partyka et al 1979, Cases and Francois 1982, Fripiat et al 1982.…”

Evolution of the Porous Structure and Surface Area of Palygorskite Under Vacuum Thermal Treatment

“…The method was described under the name of "reciprocal thermal analysis" by Grillet et aL, 1988and Rouquerol (1970, 1989. In conventional thermal analysis, a physical property of a substance is measured as a function of temperature.…”

“…Generally, individual fibers are separated substantially from one another, and typically the fibers form bundles with frayed ends. The association of fibers produces an interfiber microporosity (Grillet et al, 1988). The external surface consists predominantly of {011 ) crystal corrugated faces (Fenoll Hach-Ali and Martin Vivaldi, 1968;Martin Vivaldi and Fenoll Hach-Ali, 1969).…”

“…Theoretical average external and internal surfaces have been estimated as 300 m2/g external and 600 mZ/g internal for palygorskite from Attapulgus, Georgia . In palygorskite, the surface area available depends mainly on the nature of the molecules (size, shape, polarity) used as an adsorbate to penetrate the intra-or structural microporosity, the pretreatment (temperature and extent of outgassing), and the method used for adsorption (Grillet et al, 1988;Michot et al, 1990).…”