Abstract
KOH and RbOH are suitable bases to crystallize hierarchical ZSM-5 in one step using cetyltrimethylammonium as the only structure-directing agent.
Highlights
Hierarchical ZSM-5 zeolite can be obtained in a one-step synthesis approach using cetyltrimethyl–ammonium (CTA) as the sole organic template
The broad feature in the small-angle region of the X-ray diffraction (XRD) patterns, observed for all zeolites, is too diffuse to be correlated to any sort of symmetry it hints at partial order at the mesoscale
We explored the conditions under which hierarchical ZSM-5 zeolite can be obtained in a one-pot synthesis procedure using CTA as the only template
Summary
Performance due to severe mass transport limitations, as not the entire internal micropore space is effectively used and secondary reactions such as coking can prematurely deactivate the zeolite catalyst by pore blocking.[6,12] Optimizing catalytic performance through tailoring the pore texture of zeolites is currently a very important research topic in academia and industry.[8,13]. The resulting materials are invariably physical mixtures of MCM-41 and zeolite, which shows that two types of SDA do not generally work in concert to generate a hierarchical structure.[25,26] Recently, it has been reported that hierarchical ZSM-5 zeolite can be synthesized in the presence of CTA, albeit it that seeds are required likely to act as a crystal surface on which further zeolite growth occurs.[31,32,33,34,35] It remains unclear what obstructs obtaining mesoporous ZSM-5 with CTA directly.[36] Following the breakthrough finding that using the diquaternary ammonium surfactant (DQAS) SDAs developed by Ryoo's group can lead to the formation of ZSM-5 zeolite nanosheet.[17] Zhu et al recently showed that the difference between Ryoo's DQAS SDA and CTA resides that the CTA's quaternary ammonium head group fails to enter the inorganic matrix from the very onset of silica condensation, while the diquaternary ammonium head group in DAQS SDA shows enforced interaction with the silica matrix already during room temperature mixing of the zeolite synthesis ingredients.[37] Recently, we have demonstrated that replacement of NaOH by KOH facilitates dissolution of silicate from the initially amorphous condensed silica precursor, allowing to obtain highly crystalline hierarchical ZSM-5 zeolites in a singlestep synthesis procedure with CTA as the only organic SDA.[38] This approach emphasizes that CTA can act both as an SDA and as a mesoporogen towards hierarchical ZSM-5 formation. The catalytic performance of the hierarchical ZSM-5 zeolites was compared with a bulk ZSM-5 zeolite in the MTH reaction
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