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

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• 作者：Hsien-Ming Kao ; Pai-Ching Chang ; Yun-Wen Liao ; Lung-Ping Lee ; Chu-Hua Chien
• 关键词：Mesoporous silica Al-MCM-48 ; Solid-state NMR ; TRAPDOR ; REDOR
• 刊名：Microporous and Mesoporous Materials
• 出版年：2008
• 期刊代码：78_13871811
• 类别：ch
• 出版时间：1 September 2008
• 卷：114
• 期：1-3
• 页码：352-364
• 文件大小：401 K

摘要

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.