Effect of desilication of H-ZSM-5 by alkali treatment on catalytic performance in hexane cracking

• Published in: Applied catalysis. A, General Vol. 449; pp. 188 - 197
• Main Authors: Mochizuki, Hiroshi, Yokoi, Toshiyuki, Imai, Hiroyuki, Namba, Seitaro, Kondo, Junko N., Tatsumi, Takashi
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier B.V 27-12-2012; Elsevier
• Subjects: Alkali treatment; Catalysis; Catalysts; Chemistry; Coke; Colloidal state and disperse state; Crystallites; Deposition; Exact sciences and technology; Fracture mechanics; General and physical chemistry; Hexane cracking; Hexanes; Ion-exchange; Lewis acid site; Mesoporous zeolite; Parents; Porous materials; Surface physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Zeolites: preparations and properties; ZSM-5; Hexane cracking; Mesoporous zeolite; Lewis acid site; ZSM-5; Alkali treatment; Micropore; Coke; Acidic site; Selectivity; High temperature; Porous material; Precursor; Crystallites; Xylene; Surface properties; Hexane; Diffusion; Lewis acid; Catalytic reaction; Lewis site; Deactivation; Hydrocarbon; Toluene; Hydride transfer; Conversion; Coke deposition; Mesoporosity; Heterogeneous catalysis; Aromatization; Microporosity; Dehydrogenation; Benzene; Benzenic compound; Surface area; Catalyst
• ISSN: 0926-860X; 1873-3875
• DOI: 10.1016/j.apcata.2012.10.003

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► The enlargement of the external surface area of ZSM-5 suppressed the pore blocking by coke formation. ► Lewis acid sites are generated by the alkali treatment and removed by the subsequent acid treatment. ► Lewis acid sites accelerated the aromatization and the coke formation in hexane cracking.
The effects of external surface and acid properties of desilicated H-type ZSM-5 zeolites (H-ZSM-5) on their catalytic performance in hexane cracking were investigated. H-ZSM-5 with two different crystallite sizes of 100nm and 1μm were treated with NaOH solution of different concentrations. The external surface area (SEXT) was increased with an increase in the NaOH concentration, because of the formation of mesopores inside the H-ZSM-5 crystallites as a result of desilication. The increase in the SEXT of the H-ZSM-5 catalysts contributed to mitigating the catalyst deactivation during the hexane cracking. Although the amount of coke deposited on the alkali-treated H-ZSM-5 was larger than that on the parent H-ZSM-5, the micropore volume of the alkali-treated H-ZSM-5 decreased less due to coke deposition than that of the parent. The deactivation rate and the decrease in the micropore volume of the small-sized H-ZSM-5 catalysts were smaller than those of the large-sized catalysts, because they had shorter average diffusion path lengths. Thus the activity of the alkali-treated H-ZSM-5, especially small-sized one is less sensitive to coke deposition. Lewis acid sites (LASs) were generated by treating with NaOH of high concentrations. The selectivities to benzene, toluene and xylene (BTX) in the hexane cracking were increased with an increase in the LASs amount at high reaction temperatures (≥873K). The LASs on the alkali-treated H-ZSM-5 were selectively removed by acid treatment. The resultant H-ZSM-5 exhibited a slightly lower hexane conversion and a lower selectivity to BTX but a small amount of coke compared to the parent and alkali-treated H-ZSM-5 catalysts, suggesting that LASs on alkali-treated H-ZSM-5 accelerated the dehydrogenation including hydride transfer and aromatization, forming BTX, which would be precursors of coke.