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

Abstract

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 Å. In the H form, sizeable formation of octahedral and pentacoordinated Al was found, whereas, with Na sample, 27Al 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 Brønsted 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 Al 3+ Lewis sites; a fraction of Na + cations resulted not accessible to probe molecules, being probably buried into the thick walls. With Li sample, some AlOH species and Al 3+ Lewis sites are at the surface, as a consequence of the lower degree of exchange.