Using Trimethylphosphine as a Probe Molecule to Study the Acid Sites in Al−MCM-41 Materials by Solid-State NMR Spectroscopy

Using trimethylphosphine (TMP) as a probe molecule, acid sites in a series of Al−MCM-41 materials with Si/Al ratios ranging from 16 to 80 were investigated by various solid-state NMR techniques including 29Si, 31P, 27Al, 1H magic-angle spinning (MAS), and some double- or triple-resonance methods such as 31P → 1H cross polarization (CP), 1H/27Al, and 1H/31P/27Al TRAPDOR (transfer of population in double resonance). By means of the 1H/27Al TRAPDOR technique, which has the ability to establish a correlation between 1H and 27Al, the hydroxyl groups associated with Al could be discriminated from the silanol groups. Two signals at 3.5 and 1.9 ppm were observed in the 1H/27Al TRAPDOR spectra; they are likely due to the bridging hydroxyl (SiOHAl) and aluminum hydroxyl groups, respectively. 31P MAS NMR strongly supports the formation of zeolitelike Brönsted acid sites in the mesoporous material after the incorporation of aluminum. The concentration of the Brönsted sites determined from 31P MAS NMR is about 2 orders of magnitude lower than that of total SiOH groups, which renders their direct observation by 1H MAS NMR spectroscopy difficult. However, these sites can be well studied by 31P → 1H CP/MAS and various triple-resonance TRAPDOR NMR techniques, which provide a more detailed description of the interactions between TMP and the different hydroxyl groups in the Al−MCM-41 materials. Although the acid strength of the Brönsted sites in the Al−MCM-41 materials is lower than that of microporous zeolites such as HY, these sites can protonate TMP molecules. Some of the framework T-site aluminum atoms are not associated with the bridging hydroxyl groups and are probably present as Al−OH. No Lewis acid sites were found in our samples, though the 27Al MAS spectra show the existence of five-coordinate extraframework aluminum.