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

Amir Goldbourt, Miron V. Landau, Shimon Vega

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42 Scopus citations

Abstract

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 Oh and Td positions, belonging to alumina clusters inside the pores (∼65%), and in Td 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 Oh (59%), Td (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 Td surface sites, can explain the high acidity and catalytic activity of the alumina multilayer inside the pores of the MCM-41.

Original languageEnglish
Pages (from-to)724-731
Number of pages8
JournalJournal of Physical Chemistry B
Volume107
Issue number3
DOIs
StatePublished - 23 Jan 2003

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