Solid-state NMR characterization of the acid sites in cubic mesoporous Al-MCM-48 materials using trimethylphosphine oxide as a 31P NMR probe

The acidic properties of Al-MCM-48 with Si/Al ratios ranging from 10 to 67, synthesized with Gemini surfactant as the template, have been characterized by a combination of multinuclear solid-state H-1, Na-23, Al-27, Si-29 and P-31 MAS (magic angle spinning) NMR and some double-resonance NMR methods using trimethylphosphine oxide (TMPO) as a probe molecule. XRD and Al-27 MAS NMR results indicated that aluminum has been successfully incorporated into the framework of MCM-48 materials up to Si/Al = 17.9 by direct synthesis. H-1 and P-31 MAS NMR results strongly supported the generation of Bronsted acid sites in the cubic MCM-48 mesoporous material after the incorporation of aluminum, even without ion-exchange treatment. Double-resonance NMR techniques such as P-31/Al-27 TRAPDOR (Transfer Population in Double Resonance) and Si-29/P-31 REDOR (Rotational Echo in Double Resonance) NMR were performed to further correlate the TMPO probe molecule to the Bronsted acid sites in the silica framework. P-31/Al-27 TRAPDOR NMR experiments performed at different temperatures were able to establish the correlation between P-31 and Al-27 spins, further confirms the presence of Bronsted acid sites at 65 ppm in the P-31 MAS NMR spectrum. Although the assignment of the Lewis acid sites was somehow unambiguous with P-31/Al-27 TRAPDOR NMR, the FT-IR observation of the calcined samples adsorbed with pyridine did reveal the presence of Lewis acid sites. In contrast to the pore size constraints of zeolites, Si-29/P-31 REDOR NMR results indicated that the protonated TMPO was highly mobile inside the mesoporous channels of Al-MCM-48 at the NMR time scale. (C) 2008 Elsevier Inc. All rights reserved.