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Piperazine as a versatile organic structure-directing agent for zeolite synthesis: effect of SiO2/Al2O3 ratio on phase selectivity

Bai, Luyi, Xiong, Zhiping, Zhan, Ensheng, Li, Shuang, Shen, Wenjie
Journal of materials science 2019 v.54 no.10 pp. 7589-7602
crystals, gels, piperazine, texture, zeolites
The synthesis of aluminosilicate zeolites by using piperazine as the organic structure-directing agent was studied. Under optimized conditions, zeolites ZSM-4 (MAZ), mordenite (MOR), ZSM-35 (FER), ZSM-5 (MFI) and ZSM-12 (MTW) were obtained in pure phase. The effect of SiO₂/Al₂O₃ ratio in the initial gel on zeolite phase selectivity was systematically investigated. The SiO₂/Al₂O₃ ratio ranges for the formation of each zeolite had been established. At low SiO₂/Al₂O₃ ratios, ZSM-4 (SiO₂/Al₂O₃ = 9) and mordenite (SiO₂/Al₂O₃ = 12.8) could be obtained, while ZSM-35 (14.3 ≤ SiO₂/Al₂O₃ ≤ 29.3), ZSM-5 (SiO₂/Al₂O₃ = 58.7) and ZSM-12 (SiO₂/Al₂O₃ = 117.2) gradually evolved as the SiO₂/Al₂O₃ ratio increased. Piperazine was considered to have a charge-compensatory role when the amine group was protonated; its overall effectiveness depended on the zeolite structure. Piperazine fitted best with the FER structure, and the obtained FER zeolite had a hierarchical house of card texture which was assembled by the primary plate-shaped crystals. The ZSM-4, mordenite and ZSM-35 zeolites (in acidic form) were tested for the dimethyl ether carbonylation reaction; the essential role of 12-member ring (12MR) pore in mordenite for molecule transportation to the 8MR pore was proved by comparing to ZSM-4 that lacks interconnectivity between 12MR and 8MR pores.