Unseeded organotemplate-free hydrothermal synthesis of heteroatomic MFI zeolite poly-nanocrystallites

Abstract

A facile one-pot method without organic templates and seeds is reported for the fabrication of framework-substituted heteroatomic zeolites for the first time. Such a breakthrough is achieved by the acidic co-hydrolysis of tetraethylorthosilicate (TEOS) with metal salts, followed by basic aging and hydrothermal crystallization. Fe(Mo, Ti, Co, Cr) MFI zeolites can be obtained with high crystallinity, thermal stability and tunable chemical composition, in which Fe³⁺ and Al³⁺ are proposed to act as co-structure-directing agents, inducing the growth of the zeolite crystal and their special poly-nanocrystalline morphology. Structural characterizations and Knoevenagel condensations were conducted to evidence the incorporation of Fe³⁺ into the MFI framework. The catalytic activity on the as-synthesized framework-substituted Fe-ZSM-5 is not only tens of times higher than that of Fe³⁺-exchanged or ferric oxide impregnated analogues, but also much higher than that of its as-calcined analogue, suggesting the unique ability of our method to inhibit the transfer of heteroatoms out of the framework. This easy, controllable and green new strategy may be extended to other heteroatomic MFI zeolites, and is potentially important for practical utilization.