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Effects of Steam-Assisted Crystallization Synthesis Conditions on the Mesopore Properties of Hierarchical ZSM-5
Zeolites are crystalline microporous aluminosilicates that are widely used because of their repeating microporous structure. However, diffusion of molecules throughout the micropores is hindered by mass transfer limitation. Hierarchical zeolites have been synthesized as a means to eliminate these mass transfer limitations while maintaining shape selectivity due to the micropores. Widely reported post synthesis methods have been used for introducing mesopores within the zeolite crystals by desilication and dealumination by steaming. However, the major drawback to these methods is the inability to control mesopore formation and a limitation in SiO2/Al2O3 ratios. Subsequently, constructive techniques have employed structure-directing agents (dual templating) in direct or two-step synthesis pathways. However, because of a phase-separation phenomena, single step syntheses are difficult, where during hydrothermal synthesis the surfactant is expelled from the aluminosilicate domain thus producing non-mesoporous zeolites. Consequently, steam-assisted crystallization methods employing a mesopore template have been used to resolve this issue of dual phase products. Dry gel conversion (DGC) methods, avoid phase separation difficulties by first completely drying the gel. Subsequently, the gel is steamed under autogenous pressure without directly contacting liquid water in the container. In the present work, effects on material physical properties, total acidity, acid site accessibility and aluminum distribution were investigated after varying the crystallization time for this steam assisted crystallization method using soft templates.
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