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 Bronsted 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.