ABSTRACT
Expanded clays bipillared with [Al(13)O(4)(OH)(24)(H(2)O)(12)](7+) ions and with hexameric Cu complexes such as M[(&mgr;-OH)Cu(&mgr;-OCH(2)CH(2)NEt(2))](6)(ClO(4))(3), or with M[(&mgr;-OH)Cu(&mgr;-OCH(2)CH(2)NEt(2))](6)(PF(6))(3) where M = Fe, Al, Ga, form microporous materials whose stability and microporosity depend mainly on the identity of the hexamer central metal atom. In fact, a general decrease in thermal stability, interlamellar heights, surface areas, and pore volumes was noted when, in the (Cu,M) hexamer, M changed from gallium to aluminum to iron. Mossbauer results have indicated that only Fe(3+) in octahedral coordination is present in the iron-containing bi-PILC samples (bi-PILC = bipillared interlayered clays). It is believed that metals such as Fe(3+) and Cu(2+) can interact with the interlamellar Keggin ions thereby decreasing the stability of the alumina pillars. In contrast, the intermediate Al(13)-PILC structure is least affected when the more stable Cr complex is used. Bi-PILC materials containing 2.7-3.4% Cr stable to 500 degrees C have been obtained. The low polarity of the chosen solvent (acetonitrile) appears to inhibit the back-exchange of the intermediate PILC's Keggin ions with the hexameric Cu complexes. Elemental analysis together with XRD results suggests that the primary intercalation pathway was diffusion or ion exchange when Cr[(&mgr;-OCH(3))(&mgr;-OCH(2)CH(2)NEt(2))CuCl](3) or M[(&mgr;-OH)Cu(&mgr;-OCH(2)CH(2)NEt(2))](6)(ClO(4))(3), respectively, was used. In all preparations, bi-PILC were produced containing complexes that suffered ligand losses during the synthesis reaction. Molecular scale AFM images have shown that these complexes can be found also outside the clay interlamellar space.
ABSTRACT
A new synthesis procedure is reported for the preparation of mixed Al,Ga-offretites over the entire solid solution range 0 < or = Ga/(Ga + Al) < or = 1. The resulting materials are characterized by X-ray powder diffraction, adsorption microcalorimetry and multinuclear solid state NMR. The 29Si MAS-NMR data are consistent with statistical occupancy of the T1 and T2 sites by aluminum and gallium, and also show no positive evidence for preferential siting effects between both framework metals. Isotropic chemical shifts and nuclear electric quadrupolar coupling constants for 27Al and 71Ga have been obtained from a field-dependent analysis of the center of gravity in the MAS-NMR spectra. H-Al,Ga-offretites produced by ammonium exchange and subsequent calcination reveal evidence of partial demetallation of the framework with formation of extra-lattice metal species.
