The Effect of Heating In Vacuo on the Microporosity of Sepiolite

Abstract

Abstract--The t-plot method has been applied to the results of nitrogen adsorption at 76~ on sepiolite first heated in vacuo at various temperatures. Heating sepiolite samples in vacuo at 427~ results in a large decrease in surface area compared with samples outgassed at 373~ A change in structure and a consequent collapse of micropores is postulated. However, the t-plots indicate that some microporosity remains. Outgassing at 623~ appears to destroy completely the micropores. INTRODUCTIONThe t-plot method (Lippens et al., 1964;Lippens and de Boer, 1965;de Boer et al., 1966) for the calculation of the surface areas of solids has been extended by Mikhail et al. (1968a) to the determination of both internal and external surface areas of microporous solids, i.e. solids with pores of radii 1.(L1.5 nm or less (Dubinin, 1966). According to the authors a single gas adsorption isotherm can yield information concerning the extent of the micropore system and the external surface area of solids which is not obtainable by methods such as the BET method (Brunauer et al., 1938). Complete pore structure analyses have been made using the t-plot approach by Mikhail et al. (1968b).In the t-plot method, the adsorption isotherm data (volumes, V~ (STP), or weights, xa, of gas adsorbed/g adsorbent at a series of equilibrium pressures) is converted into a plot of V, (or x,) against t, the statistical thickness of the adsorbed layer, by means of a "master" t-curve for non-porous adsorbents, several of which have been published (Shull, 1948;Cranston and Inkley, 1957;de Boer et al., 1966). Such curves have been derived for a large number of non-porous solids by calculating the statistical thickness, t, from V~/S~I values (where Sj~,~ is the specific BET surface area), assuming close packing of adsorbate molecules, and plotting these against the corresponding relative pressures at equilibrium. Smoothed-out ("universal") t-curves are obtained for groups of solids. Non-porous solids yield linear t-plots (Va vs t) which pass through the origin if no capillary condensation takes place. A surface area, S,, may be calculated from the slope of such a linear plot. However, the choice of the master t-curve influences the results obtained. The interpretation of the results continues to be a matter of controversy (Sing, 1973).

Surface Properties of Sepiolite from Amboseli, Tanzania, and Its Catalytic Activity for Ethanol Decomposition

“…Approximately 55% of the surface area of sepiolite 2 is due to the 'internal' (micropore) area. This result is similar to that obtained for Amboseli sepiolite 1 (Dandy and Nadiye-Tabbiruka, 1975). Serna and Vanscoyoc (1979) quoted a similar (theoretical) ratio for Vallecas sepiolite which has a measured SBE~ (N2) value of 321 m2/g.…”

“…The t-plot surface area, St (de Boer et al, 1966) is 323 m2/ g, in good agreement. These values are smaller than those obtained for Amboseli sepiolite 1, which are SBET = 364 m2/g (Dandy,197t)and St = 380 m2/g (Dandy and Nadiye-Tabbiruka, 1975). This difference may be a result of the poorer crystallinity of sepiolite 2 compared with sepiolite 1, and/or of the dolomite impurity.…”

“…On heating the structure loses water, resulting in a decrease in the surface area and micropore accessibility (Hayashi et al, 1969;Martin Vivaldi and Fenoll Hach-Ali, 1969;Nagata et al, 1974;Dandy and Nadiye-Tabbiruka, 1975;Fernandez-Alvarez, 1978;Serratosa, 1979). Sepiolite has a high adsorptive capacity for many gases and vapors (Barrer et al, 1954;Dandy, 1968Dandy, , 1969Dandy, , 1971Fenoll Hach-Ali and Martin Vivaldi, 1970;Serna and Fernandez-Alvarez, 1975;Serna and Vanscoyoc, 1979).…”

Hydroxy-Chromium Smectite: Influence of Cr Added

“…The slope of the linear branch at high pressure can be used to calculate the external surface area of the solid, Se (Gregg and Sing 1991). The slope of the line drawn from the origin to the "monolayer point" (t = 0.35 nm, point R, Figure 4), could be used to calculate the surface area of the micropores and the external surface, Sme (Dandy and Nadiye-Tabbiruka 1975). The Table 2 shows the surface area calculated with the BET equation and from t-plot.…”

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

“…Barrer and Mackenzie (1954) and Dandy (1968Dandy ( , 1971 suggested that the availability of these structural channels to the nitrogen molecules is limited. The loss in BET-N2 surface area observed on outgassing at >200~ has been attributed to a sintering of the external fiber surface (Dandy and Nadiye-Tabbiruka, 1975). On the other hand, Moiler and Kolterrnan (i 965), Delon and Cases (1970), Fernandez Alvarez (1970, 1978, and JimenezLopez et al (1978) suggested that the nitrogen molecule may easily enter the structural channels after the zeolitic water is removed.…”