Revealing long- and short-range structural modifications within phosphorus-treated HZSM-5 zeolites by atom probe tomography, nuclear magnetic resonance and powder X-ray diffraction.

摘要: The average and the local structure of phosphorus-treated HZSM-5 zeolites were investigated by means atom probe tomography, powder X-ray diffraction (at ambient cryogenic temperatures) 1H, 29Si, 27Al, 31P magic angle spinning (MAS) solid state nuclear magnetic resonance (NMR) spectroscopy. Phosphatation to yield a product with P/Al ≤ 1 followed thermal treatment leads breaking Si–OH–Al bridging groups, subsequent partial dealumination zeolite framework, as shown contraction orthorhombic unit-cell volume loss tetrahedral framework Al, observed in 27Al Multiple Quantum (MQ) MAS NMR spectrum. Most Al is present an electronic environment distorted presence phosphorus appears not be involved classic Bronsted acid sites. signals indicate that interacts zeolitic locally form silico-aluminophosphate (SAPO) domains new kind acidic site was confirmed at ∼8.6 ppm 1H spectra, attributed P–OH groups. Increasing loading (P/Al ≫ 1) promotes further cross-dehydroxylation between Si–OH species, leading formation crystalline silicon orthophosphate phase. With decreasing content, monoclinic becomes preferred, especially 85 K where strain relaxation higher. However, higher amount silicophosphate impurities hinders low-temperature release indicating some these species are localized pores contribute build up.