The hydrothermal synthesis of hierarchical SAPO-34 with improved MTO performance

Abstract

The fabrication of intracrystalline secondary pores is an important approach to improve the catalytic performance of microporous zeolites. Herein, a two-step crystallization process using only TEAOH/TEA has been developed for the hydrothermal synthesis of hierarchical SAPO-34. When crystalline AlPO₄ characterized with four-coordinated aluminum and phosphorus atoms was applied, hierarchical SAPO-34 could hydrothermally be synthesized under the n_H2O : n_Al2O3 > 20 condition, and the crystallization process was analyzed. The crystalline AlPO₄ is beneficial for the SAPO-34 synthesis, and hierarchical SAPO-34 with abundant intracrystalline secondary porosity can be efficiently synthesized in 2 h. Although both the distribution range and volume of the mesopores decrease with time, the mesopore volume of hierarchical SAPO-34-31 achieved 0.069 cm³ g⁻¹. Furthermore, the silicon atoms in hierarchical SAPO-34-31 are mainly in the Si(OAl)₄ and Si(OSi)(OAl)₃ states, and the accessibility would be improved by the intracrystalline mesopores (16–20.3 nm). SAPO-34-31 exhibited more excellent MTO activity than the nano-sized SAPO-34, and the selectivity of ethylene and propylene achieved more than 80%, while the n_C2H4 : n_C3H6 is always about 0.8.