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

Abstract

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 Si-29, P-31, Al-27, H-1 magic-angle spinning (MAS), and some double- or triple-resonance methods such as P-31 --> H-1 cross polarization (CP), H-1/Al-27, and H-1/P-31/Al-27 TRAPDOR (transfer of population in double resonance). By means of the H-1/Al-27 TRAPDOR technique, which has the ability to establish a correlation between 1H and Al-27, 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 H-1/Al-27 TRAPDOR spectra; they are likely due to the bridging hydroxyl (SiOHAl) and aluminum hydroxyl groups, respectively. P-31 MAS NMR strongly supports the formation of zeolitelike Bronsted acid sites in the mesoporous material after the incorporation of aluminum. The concentration of the Bronsted sites determined from P-31 MAS NMR is about 2 orders of magnitude lower than that of total SiOH groups, which renders their direct observation by H-1 MAS NMR spectroscopy difficult. However, these sites can be well studied by P-31 --> H-1 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 Bronsted 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 Al-27 MAS spectra show the existence of five-coordinate extra framework aluminum.