Reversibility of the Modification of HZSM-5 with Phosphate Anions

Quantitative NMR spectroscopic studies (Al-27, P-31, and H-1 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 Bronsted 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 Al-octa-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 Bronsted 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.