Synthesis of ZSM-5 zeolite from metakaolinite: Effects of the SiO2/Al2O3 molar ratio, the initial precursor and the presence of organic template agent

Abstract

Highly crystalline ZSM-5 zeolites are successfully prepared from kaolinite as low cost precursor. The effects of SiO₂/Al₂O₃ molar ratio, the presence of tetrapropylammonium bromide (TPABr) and the initial precursor on the textural properties of the final products have been investigated. Crystallized samples have been characterized by scanning electron microscopy and nitrogen adsorption in addition to X-ray diffraction. The results show that getting pure ZSM-5 zeolite with a high crystallinity degree is directly dependent on the starting precursor as well as on the presence of the organic template highlighting the role of SiO₂/Al₂O₃ molar ratio. The directed-template ZSM-5 sample prepared frommetakaolinite with the smallest particles (crystal size of 700 nm) and a maximum crystallinity of 98% is obtained at a SiO₂/Al₂O₃ molar ratio of 31.69. Increasing the SiO₂/Al₂O₃ molar ratio to 41.13 isrequired to prepare an organic-template free ZSM-5 from metakaolinite with a relative crystallinity of 81%. In order to synthesize ZSM-5 zeolite from the acid-activated metakaolinite as the only silica and alumina sources, a SiO₂/Al₂O₃ molar ratio of 76.19 is used, the maximal crystallinity degree is 79%, with the largest ZSM crystals of about 3000 nm.