Spectroscopic characterisation of the strength and stability of the acidic sites of Al-rich microporous micelle-templated silicates

A micelle-templated material with Si/Al = 2.5 has been synthesised, then either calcined to provide the H form of the sample or partially exchanged with Li+ and Na+ ions. After surfactant removal, all samples were microporous, with average diameter around 18 Angstrom. In the H form, sizeable formation of octahedral and pentacoordinated Al was found, whereas, with Na sample, Al-27 NMR spectra showed minimal occurrence of octahedral Al, so indicating that Na+ cations, in contrast with protons, do compensate tetrahedral Al in the silicate structure. All samples have been characterised via FT-IR spectroscopy of surface hydroxyl species and adsorbed probe molecules acting as Lewis bases (carbon monoxide, ammonia, propene). Although the Si/Al ratio is close to that of Y zeolites, these systems showed definitely weaker acidity, similar to that of amorphous silica-aluminas of low Al content. With H sample, no zeolite-like Bronsted site has been observed, the more acidic hydroxyls being those absorbing at 3660 cm(-1), ascribed to Si-O-Al(OH)-O-Si groups, able to transfer a proton to ammonia, but not to propene, and undergoing with CO a shift of about 220 cm(-1). With Na sample, the nearly complete exchange caused the disappearance of more acidic hydroxyls and of Al3+ Lewis sites; a fraction of Na+ cations resulted not accessible to probe molecules, being probably buried into the thick walls. With Li sample, some AI-OH species and Al3+ Lewis sites are at the surface, as a consequence of the lower degree of exchange. (C) 2003 Elsevier Inc. All rights reserved.