SYNTHESIS OF METATUNGSTATE PILLARED LAYERED DOUBLE HYDROXIDES WITH VARIABLE LAYER COMPOSITION - EFFECT OF THE MG-AL RATIO ON THE MICROPOROUS STRUCTURE

A series of layered double hydroxides (LDHs) pillared with the Keggin ion H2W12O406- has been synthesized with final Mg:Al ratios of 1.75:1, 2.31:1 and 3.51:1 by ion exchange reaction of the LDH-hydroxide and L DH-adipate precursors. In each case, the pillared product was characte rized by specific BET N-2 surface areas of more than 110 m(2) g(-1) an ti micropore volumes in excess of 0.026 mi g(-1). The micropore size d istribution plots obtained from the argon adsorption isotherms indicat e that the micropore diameters become smaller as the surface charge de nsity of the LDH increased. A study was also performed on each of the LDH precursors in order to determine any problematic steps in the over all synthesis. Significant differences were noticed in the POXRD patte rns of the LDH-adipate precursor; these are believed to be due to diff erences in the orientation and degree of order of the adipate anions i n the gallery space of the LDH. It was found that adipate anions orien tated with the long axis perpendicular to the inorganic layers were ea sier to exchange than those where the long axis was aligned parallel t o the inorganic layers. (C) 1998 Elsevier Science B.V.