Reversibility of the Modification of HZSM-5 with Phosphate Anions

Abstract

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 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 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 ...