Hierarchical SAPO-34 templated by amphiphilic single-quaternary-ammonium surfactants

The introduction of mesopores and downsizing of the crystals of SAPO-34 can enhance its diffusion efficiency, leading to high catalytic activity in methanol-to-olefin (MTO) reactions. Amphiphilic single-quaternary-ammonium molecules with different lengths of carbon chains ([N+(CH2CH3)(3)-CnH2n+1][Br-], Cn-1N (n=4, 6, 8)) were designed and synthesized as co-structure directing agents. Due to the electrostatic interaction, amphiphilic molecules acted as crystal growth inhibitors or pore-forming agents, which resulted in the formation of hierarchical SAPO-34 (HS) zeolite with a smaller crystal size. When the carbon chain length was four (C4-1N), the hierarchical SAPO-34 zeolite (HS-4) was synthesized with a smaller crystal size and without other crystal phases. SAPO-34 zeolites with high mesopore volume (HS-6 and HS-8) were synthesized with C6-1N and C8-1N as co-structure directing agents. Without adding a co-structure directing agent, conventional SAPO-34 (CS) was synthesized. There were many tetrahedral particles in CS, which is the early morphology of the cubic particles. The results of the X-ray fluorescence spectrometer (XRF) and Si-29 magic angle spinning nuclear magnetic resonance (Si-29 MAS NMR) indicated that Cn-1N molecules restrain Si atoms from incorporating into the AlPO4 framework. The results of NH3-temperature-programmed desorption (NH3-TPD) proved the strong acid gradually decreases in the order of CS > HS-8 > HS-6 > HS-4. The diffusion behaviors of methanol molecules in HS-4, HS-6, and HS-8 were better than in CS. HS-4 showed excellent catalytic performance in MTO reaction: the selectivity of ethylene (C2H4) and propylene (C3H6) (84.4%) has improved by about 4% compared to that of CS (80.5%), and the catalyst lifetime of HS-4 was about two times the lifetime of CS. HS-6 also exhibited high selectivity of C2H4 and C3H6 (83.5%), and its lifetime was almost the same as HS-4. HS-8 had a high mesopore volume, but the higher strong acidity promoted hydrogen transfer reactions, leading to alkanes, aromatics, and coke, so the lifetime of HS-8 in MTO reaction was slightly prolonged.

Automated summary

The introduction of mesopores and downsizing of SAPO-34 crystals enhance its diffusion efficiency and catalytic activity in methanol-to-olefin reactions. Amphiphilic molecules were designed and used as co-structure directing agents, resulting in the formation of hierarchical SAPO-34 zeolites with smaller crystal sizes. These zeolites showed improved selectivity and catalytic performance compared to conventional SAPO-34.