Enhanced aromatic selectivity using the core–shell synergistic effects of HZSM-5/SAPO-5 zeolites in isobutane conversion. (October 2021) - Zhang, Ruizhen
Abstract: HZSM-5 molecular sieves are efficient catalysts in the aromatization of light hydrocarbons and the fabrication of HZSM-5-based core–shell composites has become an important strategy to improve their catalytic performance due to the simultaneous modification of the acidity and pore structures in the catalyst. In this paper, HZSM-5/SAPO-5 (CSZP and CSZC) were synthesized via a two-stage crystallization process using PDDA and CTAB as modifiers of HZSM-5, respectively. The structure, acidity, and the weight of carbon deposition of the resulting molecular sieves catalysts were characterized using various methods and their catalytic behavior in isobutane aromatization investigated. The results show that CSZP exhibits a perfect core–shell structure with a uniform and thin shell composed of nano-SAPO-5, while CSZC has an irregular coat configuration with HZSM-5 covered by a rough and thick shell consisting of flaky micro-SAPO-5 crystals. The introduction of SAPO-5 via an in-situ synthesis decreases the Brønsted acidity of the catalysts to different extents and provides a large number of intergranular mesopores due to the stacking of SAPO-5 on the surface of HZSM-5, which is favorable toward improving the BTEX selectivity (benzene, toluene, ethylbenzene, and xylene) and catalytic stability over CSZP and CSZC. Meanwhile, CSZP has more competitive advantages than CSZC in promoting the mass transfer efficiency due to its significantly thinner shell than CSZC, which combined with its moderate acidity and appropriate mesopore structure result in its excellent selectivity and stability during the aromatization of isobutane. Graphical abstract: ZSM-5 modified by CTAB and PDDA were respectively added into 403K- crystallization slurry of SAPO-5 and crystallized at 473 K to obtain HZSM-5/SAPO-5 (CSZC and CSZP). The reduction of strong acidity and the production of inter-granular mesopores were favorable to enhance the stability of catalysts and the selectivity of BTEX, and CSZP with perfect core-shell structure (thin shell, 10 nm mesopores) and morderate acidity had better aromatization performance than CSZC (thicker shell and 10–30 nm mesopores). Image 1 Highlights: HZSM-5/SAPO-5 was synthesized via a two-stage crystallization process using PDDA and CTAB as surface modifiers. A perfect HZSM-5/SAPO-5 core-shell structure was obtained using PDDA as a surface modifier. The thin shell consisted of SAPO-5 nanocrystals with 10 nm inter-granular mesopores and moderate acidity. HZSM-5/SAPO-5(PDDA) showed higher BTEX selectivity toward the aromatization of isobutane. The synergistic effects of the strong–weak acids and micro–meso pores enhanced the aromatic selectivity.