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 center dot center dot center dot 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 Bri nsted 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".
