Quantitative NMR spectroscopic studies
(27Al, 31P, and 1H magic-angle spinning
NMR) were combined with
temperature-programmed desorption (TPD) to investigate the impacts
of sequential treatment procedures, i.e., calcination and hot water
washing, of phosphorus-modified ZSM-5 samples on their local structural
changes for aluminum and phosphorus as well as on the acidities. Introduction
of phosphorus resulted mainly in the transformation of tetrahedrally
coordinated framework aluminum Altetra‑frame to
distorted aluminum Altetra‑dist with a tetrahedral
coordination. The lowered Altetra‑frame concentration
accounted for the decrease in Brønsted acidity for the phosphorus-modified
zeolites. High-temperature calcination of P HZSM-5 resulted in formation
of occluded condensed polyphosphates and Al–O–P complexes
(Altetra‑dist–O–P and Alocta–O–P species where Al remained at the original framework
positions), which were not present in the standard HZSM-5. Washing
with hot water removed most of the introduced P and partly restored
framework Altetra‑frame sites from these Al–O–P
complexes, consequently increasing the corresponding Brønsted
acidity. This could be explained by the partial reversibility of the
modification process of HZSM-5 with phosphorus anions. Moreover, the
treatments changed also the ratio of the framework Al sites, i.e.,
B1 and B2 sites. The larger extent of restoration of B2 compared to
B1 acid sites increased the relative abundance of B2. The modified
acidities were correlated to their catalytic performances in the methanol-to-olefins
reaction.
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