Physicochemical characterization of a Texas montmorillonite pillared with polyoxocations of aluminum. II. NMR and microcalorimetry results

Solid state magic angle spinning nuclear magnetic resonance (MAS NMR), together with microcalorimetry and XRD data, have been used to characterize a pillared interlayered day (PILC) containing 1.3 x 10(-4) mol Keggin ion/g PILC, The clay catalyst (Al-13-PILC) was prepared by reacting a Texas Na-montmorillonite with Keggin ions, [Al-13(O)(4)(OH)(24)(H2O)(12) ](7+), from an aluminum chlorhydroxide (ACH) solution. Initially, Keggin ions H-bond with the clay silicate layers causing an interlamellar expansion that is consistent with the size of the Keggin ion and its water ligands. Model calculations have been used to compute the pillar dimensions to be 0.98 nm x 1.09 nm x 0.97 nm. Al-27 MAS NMR spectra of Al-13-PILC samples contain the expected resonances due to Al(VI) and Al(IV) sites. After air drying at 100 degrees C, the Al-13-PILC's extra-framework Al had an Al(VI)/Al(IV) ratio of 12.3, consistent with the presence of Keggin ions. Loss of water ligands and dehydroxylation reactions induced by heating in air at T greater than or equal to 500 degrees C, result in the loss of the Keggin ion structure and the formation of blocks of 13 Al atoms, containing Al(IV), Al(VI) as well as Al(V) sites that disappear when the sample is allowed to re-hydrate. Although on calcination ACH powders produce a gamma-alumina phase containing Al(V)-sites, gamma-alumina formation could not be observed by XRD in thermally and hydrothermally treated Al-13-PILC samples. The Si-29 NMR spectrum for the sample of Texas Na-montmorillonite contains a resonance near -93.6 ppm attributed to T(3Si,1Al) sites and a less intense one near -110.8 ppm attributed to silica impurities. In the corresponding Al-13-PILC calcined at 500 degrees C, the main resonance is observed at the same chemical shift, but the spectrum is broadened considerably. Spectral changes in the calcined Al-13-PILC sample have been attributed to the formation of Si-O-Al-p, bonds between the clay silicate layers and the aluminum (Al-p) in the pillars. Microcalorimetry experiments with ammonia indicate that these microporous materials have initial heat values similar to those measured for AAA-alumina and that the number of strong acid sites present is similar to that observed in a commercial sample of alumina (pseudoboehmite) used in hydrotreating catalysts preparation. However, acid (B + L) site density in Al-13-PILCs is lower than in any of the other solid acids tested. (C) 2000 Elsevier Science B.V. All rights reserved.