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

Abstract

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 1H, 23Na, 27Al, 29Si and 31P MAS (magic angle spinning) NMR and some double-resonance NMR methods using trimethylphosphine oxide (TMPO) as a probe molecule. XRD and 27Al 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. 1H and 31P MAS NMR results strongly supported the generation of Brønsted 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 31P/ 27Al TRAPDOR (Transfer Population in Double Resonance) and 29Si/ 31P REDOR (Rotational Echo in Double Resonance) NMR were performed to further correlate the TMPO probe molecule to the Brønsted acid sites in the silica framework. 31P/ 27Al TRAPDOR NMR experiments performed at different temperatures were able to establish the correlation between 31P and 27Al spins, further confirms the presence of Brønsted acid sites at 65 ppm in the 31P MAS NMR spectrum. Although the assignment of the Lewis acid sites was somehow unambiguous with 31P/ 27Al 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, 29Si/ 31P REDOR NMR results indicated that the protonated TMPO was highly mobile inside the mesoporous channels of Al-MCM-48 at the NMR time scale.