Characterization of aluminum species in alumina multilayer grafted MCM-41 using ²⁷Al FAM(II)-MQMAS NMR

Grafting a 4-fold alumina multilayer inside the nanotubes of a wide-pore MCM-41 material, by consecutive reactions with Al(O-sec-Bu)₃ followed by hydrolysis-calcination, changes the surface chemical functionality of the material from silica to alumina. This procedure leads to partial filling of the MCM-41 mesopores with an amorphous alumina phase. In this publication the multi-grafting process and the structure of the produced aluminum-containing phases are characterized by performing FAM-II enhanced ²⁷Al 3QMAS and 5QMAS NMR experiments. A reference γ-alumina sample, prepared from Al(O-sec-Bu)₃ by a sol-gel method and calcined at 500°C, with a surface area of 460 m²/g and a domain diameter of 1.5-2 nm, showed a similar short range order as commercially obtained well-crystalline γ-alumina. In the Al₂O₃/MCM-41 sample grafted in one step (Si/Al = 4.9), Al-species were observed in O_h and T_d positions, belonging to alumina clusters inside the pores (∼65%), and in T_d sites implanted into the silica framework (∼35%). After successive grafting (4 steps, Si/Al = 1.6) the relative population of silica-substituted Al sites amounted to about 25%. The aluminum atoms in the grafted phase consisted of O_h (59%), T_d (31%), and stable pentacoordinated sites (10%)-the last at a significantly higher concentration than is generally observed in transition aluminas. This result, together with the large presence of T_d surface sites, can explain the high acidity and catalytic activity of the alumina multilayer inside the pores of the MCM-41.