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 27 Al 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 m 2 /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 Al 2 O 3 /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.