Abstract
Hierarchical zeolites attract people’s attention due to their enhanced mass transfer properties in catalytic reactions. The research over obtaining these zeolites in green and efficient methods is of great significance for the current post-treatments and templating strategies tend to be costly for hierarchical zeolite synthesis. In this research, nanosized mesoporous ZSM-5 (SN-ZSM-5) zeolites have been synthesized without the addition of mesoscale templates under highly concentrated conditions. The physicochemical characteristics were systematically investigated by XRD, SEM, N2 sorption, TEM, and NMR. The SEM images showed ZSM-5 crystals with sizes between 50–110 nm were obtained. N2 sorption and high-resolution TEM images gave direct evidence for the hierarchical structure of SN-ZSM-5. The forming mechanism for the hierarchical structure was proposed that the etching effect of the highly alkaline environment for the starting gels played a critical role in the formation of hierarchical structure. Catalytic tests in methanol-to-olefins (MTO) showed the SN-ZSM-5 performed a longer catalyst lifetime and higher propylene selectivity than the conventional ZSM-5 zeolites (C-ZSM-5) obtained from a traditional hydrothermal method. The features of hierarchical structure in the SN-ZSM-5 crystals and the sustainability for synthesis method could show a promising choice for wide applications of these SN-ZSM-5 zeolites in the future.
Highlights
Zeolitic materials have always been drawing people’s attention due to their important applications in catalysis and adsorption [1,2,3,4,5]
It is worth noting that an apparent gradual increase can be observed at the relative pressure of 0.01–0.9, suggesting there may be mesopores in these samples. It has been reported the hierarchical structure in zeolite crystals would give advantages to the mass transfer and longer catalyst lifetime in catalytic reactions [6,7,8]
The BET and micropore volume values of the obtained zeolite crystals are relatively small compared to standard ZSM-5 zeolites, which may be caused by the effect of nano size and internal mesopores, as previously reported [38,39]
Summary
Zeolitic materials have always been drawing people’s attention due to their important applications in catalysis and adsorption [1,2,3,4,5]. In these cases, the construction of nanosized zeolite usually needs more organic template consumptions to produce more crystal nuclei, leading to the cost increase and large water consumption for zeolite synthesis [1,21,22,23]. Scientists reported that we could prepare hierarchical zeolites from templating strategies using organic directing agents such as nanosized carbon, polymers, and amphiphilic surfactants [27,28]. Both post-treatments and templating strategies are confirmed to be successful for the preparation of hierarchical zeolite crystals, but the relatively high cost still hinders their wide applications in industrial processes. The convenient and efficient synthesis for hierarchical nano ZSM-5 zeolites combined with the excellent catalytic performance provides a sustainable methodology for efficient zeolite catalysts in the future
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