Physicochemical characterization of a Texas montmorillonite pillared with polyoxocations of aluminum Part I: the microporous structure

Atomic force microscopy (AFM), nitrogen and argon porosimetry, high resolut ion electron microscopy (HREM) together with powder X-ray diffraction (XRD) have been used to characterize a heat stable pillared interlayered clay (P ILC). The clay catalyst was prepared by replacing the charge compensating c ations in a Texas Na-montmorillonite with [Al13O4(OH)(24)(H2O)(12)](7+) (Al -13) ions from an aluminum chlorhydroxide (ACH) solution using an ACH/clay and water/clay (wt/wt) ratio of 1.0 and 100, respectively. The reaction pro duct was extensively washed with deionized water to ensure a complete hydro lysis of the interlamellar Al species and formation of stable Keggin ions. Molecular scale AFM images have shown the absence of adsorbed surface Al sp ecies indicating that, at the synthesis conditions used, the added Keggin i ons are located in the clay interlamellar space where they are believed to hydrogen bond with basal oxygens. Model calculations have been used to esti mate its dimensions to be 0.97 nm x (0.89 nm x 1.09 nm). After heating in a ir at 500 degrees C, the Keggin ions lose their water ligands forming short er Al,, blocks 0.84 nm in height that become the structure supporting pilla rs. Thermal and hydrothermal treatments can transform ACH powders into gamm a-alumina: a transition phase having a spinel-like structure, as identified by XRD. However gamma-alumina formation could not be observed by XRD in si milarly treated Al-13-PILCs. Pore size distribution data from nitrogen and argon porosimetry experiments have indicated that the Al-13-PILC under study is essentially a microporou s material with a pore volume (PV) in the 0.10-0.12 cm(3) g(-1) range and w ith a Langmuir surface area (SA) of 300-320 m(2) g(-1), well in agreement w ith SA values measured from density functional theory methods based on cyli ndrical-like pore geometry. Discrepancies between calculated and measured S A and PV values have been attributed mainly to the incomplete expansion of all the Na-montmorillonite platelets, as observed in HREM images. Moreover, AFM and HREM images have shown that steam aging for 5 h at 760 degrees C w ith 100% steam decomposes the clay aluminosilicate layers, thus providing e vidence that during this hydrothermal treatment the Al-13-PILC structure wi ll collapse irrespective of the stability of its Al-13 pillars. (C) 2000 El sevier Science B.V. All rights reserved.