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

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Atomic force microscopy (AFM), nitrogen and argon porosimetry, high resolution electron microscopy (HREM) together with powder X-ray diffraction (XRD) have been used to characterize a heat stable pillared interlayered clay (PILC). The clay catalyst was prepared by replacing the charge compensating cations in a Texas Na-montmorillonite with [Al 13O 4(OH) 24(H 2O) 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 product was extensively washed with deionized water to ensure a complete hydrolysis of the interlamellar Al species and formation of stable Keggin ions. Molecular scale AFM images have shown the absence of adsorbed surface Al species indicating that, at the synthesis conditions used, the added Keggin ions are located in the clay interlamellar space where they are believed to hydrogen bond with basal oxygens. Model calculations have been used to estimate its dimensions to be 0.97 nm×(0.89 nm×1.09 nm). After heating in air at 500°C, the Keggin ions lose their water ligands forming shorter Al 13 blocks 0.84 nm in height that become the structure supporting pillars. Thermal and hydrothermal treatments can transform ACH powders into gamma-alumina, a transition phase having a spinel-like structure, as identified by XRD. However gamma-alumina formation could not be observed by XRD in similarly 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 microporous material with a pore volume (PV) in the 0.10–0.12 cm 3 g −1 range and with a Langmuir surface area (SA) of 300–320 m 2 g −1, well in agreement with SA values measured from density functional theory methods based on cylindrical-like pore geometry. Discrepancies between calculated and measured SA 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°C with 100% steam decomposes the clay aluminosilicate layers, thus providing evidence that during this hydrothermal treatment the Al 13-PILC structure will collapse irrespective of the stability of its Al 13 pillars.