First-Formed Framework Species and Phosphate Structure Distributions in Phosphorus-Modified MFI Zeolites

Zeolite ZSM-5 catalysts were modified with varying amounts of H₃PO₄ to yield a suite of catalysts with phosphorus loadings ranging from 0.3 to 5 wt %, corresponding to the P/Al molar ratio ranging from ca. 0.25 to 4.2 based on the Si/Al ratio = 40. A combination of ³¹P, ²⁷Al, ²⁹Si, and ¹H NMR experiments at multiple field strengths suggests that discrete phosphate structures form at Al lattice sites at low H₃PO₄ loadings, with increased phosphorus structural heterogeneity at high loadings. Al–O–P bond formation is detected for H₃PO₄ loadings corresponding to a P/Al ratio as low as 1:4. Control experiments with similar phosphorus treatment on silicalite, the silica analogue of ZSM-5, show only free phosphate and small oligomeric phosphate formation, with none of the spectroscopic signatures indicative of proximate Al and P atoms that are observed in similar ZSM-5 catalysts. Two-dimensional heteronuclear multiple quantum coherence experiments involving multiple nuclei (²⁷Al–³¹P, ²⁷Al–¹H, ²⁷Al–²⁹Si, and ¹H–³¹P) indicate that framework-bound phosphates and a distribution of acidic hydroxyl groups form at the expense of original Brønsted acid sites (BAS) in the catalyst. However, even at the highest loadings investigated, some residual BASs remain. Prior to any hydrothermal treatments, multiple-quantum magic-angle spinning experiments indicate that P-modified catalysts contain several different Al sites, versus only two that are observed in the untreated ZSM-5. Density functional theory calculations provide further support for the formation of framework Al–O–P species, in addition to strongly hydrogen-bonded H₃PO₄/BAS complexes.