Surface area and microporosity of 1.8 nm pillared clays from the nitrogen adsorption isotherm

Nitrogen adsorption isotherms have been established over Al-, Ga-, and Ga-Al-pillared clays exhibiting 1.8 nm spacings and treated by the BET and Langmuir methods. It is shown that neither method yields a truly satisfying determination of the surface of pillared clays. This may be explained by the fact that multilayer adsorption required by the BET theory cannot occur within the micropores of these solids, whereas adsorption on the external surface is not ruled by the Langmuir equation. The external surfaces and micropore volumes have been determined by the t method. It is shown that the choice of the standard t function and the domain of the V-t curve considered for the linear fitting may influence the external surface and the micropore volume. A modified approach to treating the N-2 adsorption isotherm is proposed to establish the micropore volume and the external surface area of pillared clays. In this method, it is considered that (i) the BET theory is governing the adsorption on the external surface and (ii) the micropore volume is completely filled in the zone of validity of the BET method. This approach leads in the case of pillared clays to S-BET ext and C-BET ext values close to those obtained for non-pillared clays, and to micropore volumes consistent with those reported in the literature. For pillared clays with d(001) values near 1.8 nm, the micropore volume can be transformed into a microporous surface assuming that the adsorption on this surface is limited to a monolayer; the total surface area is obtained by summing up the microporous surface and S-BET ext. A comparison is made between the total surface calculated in this way and the surface obtained from the application of the BET and Langmuir methods on the experimental isotherm not corrected for the adsorption within the micropores.