Hierarchical ZSM-5 nanosheets for production of light olefins and aromatics by catalytic cracking of oleic acid†

Abstract

The bolaform surfactant C₆H₁₃-N⁺(CH₃)₂-C₆H₁₂-N⁺(CH₃)₂-C₆H₁₂-O-C₆H₄-C₆H₄-O-C₆H₁₂-N⁺(CH₃)₂-C₆H₁₂-N⁺(CH₃)₂-C₆H₁₃ (BC_PH-6-6-6) was synthesized and used to guide the synthesis of hierarchical ZSM-5 nanosheets (HZN) for use as zeolite catalysts. The number of acidic sites and the Al distribution in the zeolite pores were varied by changing the amounts of sodium sulfate and Al in the initial gel. The X-ray diffraction, Fourier transform infrared spectroscopy, scanning electronic microscopy, NH₃ temperature programmed desorption, ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy and N₂ adsorption/desorption were used to characterize these materials. These zeolites each had a well-developed hierarchical system and specific surface areas as high as 631 m² g⁻¹, indicating that the BC_PH-6-6-6 formed lamellar micelles based on π–π stacking. Interconnected hierarchical pore structures were retained after the templating agent was removed. The HZN sample exhibited a 90° rotational intergrowth structure and retained a large number of lamellae. The same material had a high concentration of acidic sites and contained Al in a tetrahedral coordination framework. The catalytic cracking of oleic acid using this zeolite gave light olefin yields up to 52.2% at 500 °C, exceeding the performance of conventional ZSM-5 (38.9%), and the catalyst remained active for up to 60 h. At 450 °C, the BTX selectivity of 6.72% obtained with this material also exceeded that from the conventional ZSM-5. These results were attributed to the connected hierarchical pores of the HZN, which promoted diffusion and provided higher carbon resistance.