Reversibility of the Modification of HZSM-5 with Phosphate Anions

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• 作者：Miroslaw Derewinski ; Priit Sarv ; Xianyong Sun ; Sebastian M眉ller ; Andre C. van Veen ; Johannes A. Lercher
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：March 27, 2014
• 年：2014
• 卷：118
• 期：12
• 页码：6122-6131
• 全文大小：484K
• 年卷期：v.118,no.12(March 27, 2014)
• ISSN：1932-7455

文摘

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 Al_tetra-frame to distorted aluminum Al_tetra-dist with a tetrahedral coordination. The lowered Al_tetra-frame concentration accounted for the decrease in Br酶nsted acidity for the phosphorus-modified zeolites. High-temperature calcination of P HZSM-5 resulted in formation of occluded condensed polyphosphates and Al鈥揙鈥揚 complexes (Al_tetra-dist鈥揙鈥揚 and Al_octa鈥揙鈥揚 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 Al_tetra-frame sites from these Al鈥揙鈥揚 complexes, consequently increasing the corresponding Br酶nsted 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.