Evaluating pore characteristics and acid site locations in hierarchical SAPO-11 by catalytic model reactions

Abstract

Hierarchical SAPO-11 molecular sieves were synthesized with three different mesopore structure directing agents (meso-SDAs): cetyltrimethylammonium bromide (CTAB), polyvinyl alcohol (PVA) and [3-(trimethoxysilyl) propyl] dimethyloctadecylammonium chloride (TPOAC). Two model reactions, methanol-to-hydrocarbons (MTH) and the Beckmann rearrangement (BMR) of cyclohexanone oxime, were employed to evaluate the pore topology and acid site locations of the hydrothermally synthesized hierarchical SAPO-11s. Initially, the modified porosity of the hierarchical SAPO-11s was thoroughly probed by employing a set of general characterization methods and by comparing the results to the conventional microporous C-SAPO-11. The nitrogen physisorption results revealed that CTAB-11 had a uniform distribution of mesopores centered at 2.8 nm, whereas the presence of mesopores in PVA-11 could not be convincingly resolved through conventional methods. Instead, the pore topology of PVA-11 was determined by utilizing model reactions, where the shape selective MTH model reaction revealed that the sample had mesopores present through an increased production of large products compared to the conventional C-SAPO-11. Additionally, the MTH model reaction showed that while PVA shifted the location of the Brønsted acid sites (BAS) towards the mesopores, CTAB did not affect the BAS location of SAPO-11. Finally, the BMR model reaction elucidated the excellent intrapore connectivity of the hierarchical SAPO-11s through an increased lifetime compared to the conventional C-SAPO-11. • Hierarchical SAPO-11 (H-S11) with interconnected micro- and mesopores was synthesized with CTAB and PVA as meso-SDAs. • Model reactions are crucial for understanding the topological differences between microporous SAPO-11 (µ-S11) and H-S11. • CTAB significantly alters the textural properties of the resulting H-S11, while PVA shows similar properties to µ-S11. • PVA directs formation of Brønsted acid sites to mesopores, whereas CTAB forms silanols in mesopores. • Superior lifetime for the isomerization of cyclohexanone oxime revealed the intraconnected pore topology for H-S11.