Synthesis of inter-crystalline mesoporous ZSM-5 generated by self-interlocked MFI nanosheet stacks

Abstract

Aiming to systematically investigate the possibility of using an amphiphilic surfactant, namely C_22-6-6Br₂, as a sole template to synthesise mesoporous ZSM-5 through the manipulation of synthesis conditions, the present work revealed that the NaOH concentration ([NaOH]) in the precursor solution played a predominant role in the formation of both microporous and mesoporous structures in the ZSM-5 products. A higher [NaOH] was required to facilitate the formation of micropores for ZSM-5 with lower Si/Al ratios under given synthesis conditions. The higher the [NaOH] was, the greater the crystallinity and micropore volume found in the product of any given Si/Al ratio. [NaOH] also played a crucial role in the formation of ZSM-5 nanosheet stacks, the structural ordering within these stacks, and the assembly of these stacks. Increasing [NaOH] at a given Si/Al ratio resulted in the formation of ordered nanosheets stacks, which could grow in a self-interlocked manner in an optimum [NaOH] range. All experimental evidence converged to prove that these self-interlocked ordered nanosheet stacks (SI-ONS's) were responsible for the inter-crystalline mesopores observed in the ZSM-5 products. This study showed that there is a common optimum [NaOH] range which supports the formation of both microporous and mesoporous structures in ZSM-5. The precursor composition domain for the ternary NaOH–SiO₂–AlO_1.5 system to produce mesoporous ZSM-5 was established, providing an important guiding tool to enable the synthesis of ZSM-5 with desired structural properties and Si/Al ratios ranging from 85 to 15.