In situ growth-etching approach to the preparation of hierarchically macroporous zeolites with high MTO catalytic activity and selectivity

Silicoaluminophosphate zeolite SAPO-34 with CHA topology is known as one of the best catalysts for methanol-to-olefin (MTO) conversion. In this work, we demonstrate a facile one-step hydrothermal synthesis of hierarchically macroporous SAPO-34 through the etching effect of hydrofluoric acid. The highly crystalline hierarchically macroporous SAPO-34 is prepared as central-holed rhombohedral crystals with particle size of ca. 5-10 μm that comprise intracrystalline parallel macrochannels of ca. 100 nm. The formation of macroporous structures via in situ growth-etching can be directly imaged by SEM. Strikingly, a particular crystal configuration consisting of eight pyramidal parts has been first observed during the zeolite crystal growth process, which eventually grows to form a perfect rhombohedral shape. The HF etching effect is further elucidated by the analysis of changes of pH values as well as of solid and liquid compositions following the evolution of crystallization. The texture properties, chemical environments of framework atoms, and acidity of the synthesized SAPO-34 are characterized by N₂ adsorption/desorption, MAS NMR and NH₃-TPD measurements. The hierarchically macroporous SAPO-34 shows larger micropore volume, slightly stronger acid strength, and lower external surface acidity than its conventional counterpart synthesized without using HF. Consequently, the hierarchically macroporous SAPO-34 exhibits excellent MTO catalytic performance, showing much higher selectivity to ethylene and propylene as well as longer lifetime than the conventional counterpart. In comparison with previously reported methods for the generation of hierarchical porosity, this one-step HF-assisted in situ growth-etching synthetic route is simple, straightforward and cost-effective, which offers a new approach to prepare hierarchically porous zeolites with improved catalytic activity.