Identity, Evolution, and Acidity of Partially Framework-Coordinated Al Species in Zeolites Probed by TMP ³¹P-NMR and FTIR

Increasing research has shown that active sites in zeolite catalysts are structurally and spatially complex, which poses challenges to effective characterization methods, especially for the high demand in pursuing molecular-level understanding of the nature of the active sites. Herein, using trimethylphosphine (TMP) as a probe molecule, the species giving rise to 31P NMR resonance at −58 ppm, which is typically recognized as TMP physically adsorbed on unreactive species, is found to possess more catalytic meanings as the TMP bindings are proven to be strong. NMR-assisted 31P-27Al internuclear distance measurement and a comprehensive set of two-dimensional (2D) heteronuclear correlation (HETCOR) (1H-31P, 31P-27Al, and 27Al-1H) NMR experiments explicitly demonstrate that the TMP-binding site is neither a bridging acid site (BAS) nor a Lewis acid site (LAS), but special Al–OH groups, i.e., Al–OH···P(CH3)3. Further evidence including postsynthetic treatments and 31P-31P homonuclear NMR correlation experiments exclusively shows that these Al–OH groups originate from the partially bonded framework Al(IV)-2 species recently reported. By linking IR and 1H NMR spectroscopy, new insights of Al(IV)-2 (essentially Brønsted sites) and framework-bonded Lewis sites are provided. Finally, 31P-31P homonuclear correlation experiment was capable of ruling out chemical exchange from spin diffusion and thereby exclusively demonstrates that the “BAS and Al(IV)-2” is in shorter spatial distance than that of “BAS and LAS”.