Synthesis of nanosheet SSZ-39 zeolite with enhanced catalytic performance in methanol-to-olefins reaction

In order to improve the mass transfer rate of SSZ-39 and prolong its catalytic lifetime, peculiar nanosheet SSZ-39 zeolites were prepared by hydrothermal synthesis using N,N-dimethyl-3,5-dimethylpiperidinium as an organic structure-directing agent and cetyltrimethylammonium bromide (CTAB) as a crystal growth regulator. The nanosheet SSZ-39 zeolites with different CTAB/SiO₂ ratios in the initial gels were synthesized and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), nitrogen adsorption–desorption, Fourier transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), ²⁷Al nuclear magnetic resonance (²⁷Al NMR), temperature programmed desorption of ammonia (NH₃-TPD) and thermogravimetric analysis (TGA). The addition of CTAB changed the morphology of SSZ-39 zeolite. Compared to conventional SSZ-39 without CTAB addition, the nanosheet SSZ-39 had a higher SiO₂/Al₂O₃ ratio (18.5 vs 16.7), a larger specific surface area (482 m² g⁻¹ vs 433 m² g⁻¹) and a higher strong acid density of (0.411 mmol g⁻¹ vs 0.317 mmol g⁻¹). Intensive research about the formation process of nanosheet SSZ-39 revealed that CTAB adsorbed on [001] crystal plane to inhibit the growth of the crystals along the c-axis, which played a crucial role in the formation of nanosheets. The nanosheet SSZ-39 with shorter diffusion path exhibited an enhanced performance in the methanol-to-olefins reaction. The sample with a CTAB/SiO₂ ratio of 0.01 in synthesis possessed a lower Hydrogen Transfer Index, indicating higher mass transfer efficiency, which endowed it a longer catalytic lifetime of 360 min and higher ethylene plus propylene selectivity than conventional SSZ-39.