Organosilane surfactant-directed synthesis of nanosheet-assembled SAPO-34 zeolites with improved MTO catalytic performance

Hierarchical SAPO-34 zeolites with a special nanosheet-assembled morphology were hydrothermally synthesized under dynamic condition by using the commercial quaternary ammonium-type organosilane surfactant [3-(trimethoxysilyl) propyl] octadecyldimethyl-ammonium chloride (TPOAC) for the mesoscopic aggregation and tetraethylammonium hydroxide as the micropore structure-directing agent. The growth evolution during the crystallization process and governing factors including TPOAC content and dynamic hydrothermal condition in the synthesis were discussed in detail, and the possible nucleation and growth process were proposed as well. With a systematic structure characterization by XRD, FTIR, N₂ adsorption–desorption, ICP, SEM, TEM, NH₃-TPD and pyridine-adsorbed IR measurements, the nanosheet-assembled SAPO-34 (NA-SP34) prepared under the optimized synthesis condition presented the pure phase of CHA framework, high degree of crystallinity, interconnected hierarchical meso-/microporosity and relatively weak acidity comparing with the conventional microporous SAPO-34 (CM-SP34) with micron size and the nanosheet-like SAPO-34 (NL-SP34) synthesized in the absence of TPOAC. Moreover, the catalytic performances evaluated by the methanol-to-olefin reaction indicated that the optimal NA-SP34 catalyst presented remarkable improvements of not only much longer catalytic lifetime (300 min) and slower coke formation rate (0.24 mg g⁻¹ min⁻¹) but also higher ethylene and propylene selectivity (81.93%) comparing with those (95 min, 0.55 mg g⁻¹ min⁻¹ and 80.75%, respectively) of CM-SP34, which can be attributed to the integrated balance of well-remained microporosity, considerable mesoporosity and suitable weak acidity.