Designing ordered mesoporous aluminosilicates under acidic conditions via an intrinsic hydrolysis method

Abstract

Acid-mediated synthesis of ordered mesoporous aluminosilicates (OMAS) with medium-to-strong Brønsted acid sites and trivalent aluminium exclusively in a tetrahedral framework structure is realized by a newly devised intrinsic hydrolysis method. In this way, we have synthesized a series of well-ordered OMAS, e.g., H-AlSBA-15 and H-AlIITM-56, which are otherwise difficult to distinguish under acidic conditions owing to very different hydrolysis rates of both silicon and aluminium sources used for preparation as well as easy dissociation of thus formed Al–O–Si bonds. This novel intrinsic hydrolysis approach, however, relies mainly on similar hydrolysis rates of constituent inorganic species leading to efficient condensation. Thus, the innovative methodology using tetraethyl orthosilicate and aluminium citrate, respectively, as tetravalent silicon and trivalent aluminium as precursors facilitates the formation of high quality OMAS with a narrow pore size distribution, thicker walls, and trivalent aluminium in a tetrahedral framework structure with a high aluminium content, as evidenced by a battery of characterization techniques, viz., XRD, XRF, SEM, TEM and BET. The resulting materials, in turn, generate Brønsted acid sites in the aluminosilicate matrix, with the absence of the usually encountered Lewis acid sites, viz., extra-framework and/or non-framework species, as confirmed by both ²⁷Al MAS-NMR and NH₃-TPD studies. All the prepared catalysts exhibit excellent activity towards the tertiary butylation of phenol, and the high activity of the catalysts is attributed to the unique and exclusive presence of medium-to-strong acid sites in the OMAS matrix.