HZSM-5

with good surface acidity and shape selectivity

was reported as hydrocarbon cracking catalyst for multiple decades

however the hydrothermal stability

especially dealumination of tetrahedrally coordinated framework aluminum (TFAl)

has been proved extensively as one of the major challenges during reactionregeneration process. Phosphorus was proposed to stabilize TFAl and indeed it enhanced the hydrothermal stability. Unfortunately

most of the phosphorus species would remain outside of the zeolite pore

mainly as polyphosphate species

and block the micropore severely

with only a limited portion introduced into the channel being able to interact with TFAl. Enlarging the pore size by alkali treatment (desilication) is one of the most convinced methods

but the details about specific P species during alkali treatment and its transformation upon hydrothermal activation is not acquired

thus the mechanism has not been fully understood. Herein

the P-containing species and its transformation during direct P modification and acid/alkali treatment followed by P modification have been studied

and the mechanism on t

he interaction between P and Al species has been investigated

using several analytical methods

especially Solid-state nuclear magnetic resonance (SSNMR) spectroscopy. It was found that the combination of desilication and subsequent phosphorus modification can enhance the activity of the ZSM-5 for the cracking of ethylcyclohexane

due to the better hydrothermal stabilization of acid sites by the enhanced interaction between phosphorus and TFAl

resulting from the improved accessibility of TFAl because of the successful generation of mesoporosity. Whereas the acid treatment followed by phosphorus modification

with declined retention of crystallinity and P/Al ratio

monoclinic/orthorhombic transition during steam activation

and the failed generation of mesopores

would cause obvious aggregation of the phosphorus species and could not improve the hydrothermal stability of the ZSM-5effectively

and the direct phosphatation turned out much worse. Finally

a specific index that the intensity of the signal at 39 in

27

Al MAS NMR spectra before steam activation was proposed as the indicator for determining the efficiency of phosphorus modification. And the proposed mechanism on the interaction between phosphorus and TFAl during the phosphorus modification could also be applicable in other zeolites.