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

Abstract

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 31 P 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 31 P-27 Al internuclear distance measurement and a comprehensive set of two-dimensional (2D) heteronuclear correlation (HETCOR) ( 1 H-31 P, 31 P-27 Al, and 27 Al-1 H) 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(CH 3 ) 3 . Further evidence including postsynthetic treatments and 31 P-31 P 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 1 H NMR spectroscopy, new insights of Al(IV)-2 (essentially Brønsted sites) and framework-bonded Lewis sites are provided. Finally, 31 P-31 P 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".

Water structures on acidic zeolites and their roles in catalysis

“…In general, the aluminum ions with threecoordinated, tetrahedral, penta-coordinated and octahedral coordination environment are the potential precursor species for the formation of Lewis acidic sites in zeolite structures. [102][103][104][105][106][107][108] Lewis acidic aluminum sites on the aluminosilicates can be originated from framework, frameworkassociated or extra-framework species. It calls for detailed structural understandings about these sites by employing various instrumental techniques, e.g., MAS NMR.…”

Enhancement of Propylene Metathesis Performance of MoO3/Al2O3 with Promotion of Phosphoric Acid

“…Due to its large chemical shift range, high sensitivity, and relatively straightforward spin À 1/2 nature in 31 P NMR, TMP proves to be an effective probe molecule for characterizing the acidity properties of solid surfaces. [18] When TMP is surfaceloaded on molybdenum oxide materials, it undergoes oxidation to form trimethylphosphine oxide (TMPO). [19] Since TMPO exhibits solid-like properties, it promptly adsorbs onto sites near the molybdenum oxide, thereby providing valuable in- sights into the surrounding environment of molybdenum oxide.…”