Novel ionic liquid-templated ordered mesoporous aluminosilicates: Synthesis, characterization and catalytic properties

Highly crystalline ordered mesoporous aluminosilicates with hexagonal AlIITM-41 and cubic AlIITM-48 pore symmetries with varying silicon-to-aluminium ratios were hydrothermally synthesized, for the first time, using long-chain ionic-liquid (IL) such as 1-hexadecyl-3-methylimidazolium chloride (HDMIC), as structure directing agent. The prepared materials were systematically characterized by various analytical, spectroscopic and imaging techniques, viz., XRD, BET, TEM and XRF. In addition, the extent of aluminium incorporation and the nature of acid sites were deduced from Al-27 MAS-NMR and NH3-TPD measurements. The structural and physicochemical properties of both AlIITM-41 and AlIITM-48 were compared with analogous AIMCM-41 and AIMCM48, respectively prepared using hexadecyltrimethylammonium bromide (CTAB). It was found that all the ionic liquid-templated materials, viz., AlIITM-41 and AlIITM-48, and exhibited large surface area, high crystallinity and thicker pore walls. The observed superior orderness of AlIITM-41 and AlIITM-48 could attribute to a well distributed positive charge and pi-pi stacking of aromatic imidazolium head group of ionic-liquid. In addition, it also assisted in the incorporation of all the aluminium species exclusively in the tetrahedral silicate framework even after the calcination as confirmed by Al-27 MAS NMR. AlIITM-41 and AlIITM-48 possess more medium and strong Bronsted acid sites due to their thicker walls, high crystallinity and presence of tetrahedral aluminium in the framework, due to which they exhibited higher catalytic activity towards tertiary butylation of phenol with a good Para tertiary butyl phenol selectivity compared to commonly used aluminosilicates, H-AlMCM-41 and H-AlMCM-48.