The isomorphous substitution of Si(4Al) with P in FAU zeolite and its stabilization effect

It's a dream to make zeolites more stable so as to keep the activity and stability of the catalyst. Phosphorus modification is an effective method, however, the known post-modification predominantly presents as non-framework P, and the P-loss is a big trouble in application. We have discovered that in the reaction of PCl3 or SiCl4 + PCl3 with NaY zeolite, a portion of P enters the framework of the zeolite, forming a PO4-AlO4 structure. Further investigations revealed that P cannot be incorporated into the framework through P for Al isomorphous substitution, but rather through isomorphous substitution with Si(4Al). PCl3 exhibits two effects in reaction: at lower quantities, it mainly undergoes isomorphous substitution with Si(4Al); at higher quantities, it leads to zeolite dealumination generating aluminum vacancies, which disrupt the zeolite framework or promotes Si for Al isomorphous substitution. After hydrothermal aging at 800 °C for 4 h, the stable presence of framework PO4-AlO4 implies that the weakest site Si(4Al) becomes the more stable site P(4Al) in the Y zeolite framework. Compared with the JPY (prepared by impregnating (NH4)2HPO4 solution to high silica Y zeolite) sample, after 10 cycles of hydrothermal and water washing treatment, the P2O5 loss rate of DPY (prepared by SiCl4 + PCl3 with NaY) zeolite ranged from 0.4 % to 3.5 %, significantly lower compared to the range of 18.7 % to 25.9 % observed in the JPY sample. This indicates that in DPY zeolite, the interaction between P and the zeolite is stronger and less prone to loss. Moreover, P is more uniformly distributed on the surface and bulk phase of DPY than JPY sample.