Abstract
Hierarchical single-crystal ZSM-5 zeolites with different Si/Al ratios (Hier-ZSM-5-x, where x = 50, 100, 150 and 200) were synthesized using an ordered mesoporous carbon-silica composite as hard template. Hier-ZSM-5-x exhibits improved mass transport properties, excellent mechanical and hydrothermal stability, and higher catalytic activity than commercial bulk zeolites in the benzyl alcohol self-etherification reaction. Results show that a decrease in the Si/Al ratio in hierarchical single-crystal ZSM-5 zeolites leads to a significant increase in the acidity and the density of micropores, which increases the final catalytic conversion. The effect of porous hierarchy on the diffusion of active sites and the final catalytic activity was also studied by comparing the catalytic conversion after selectively designed poisoned acid sites. These poisoned Hier-ZSM-5-x shows much higher catalytic conversion than the poisoned commercial ZSM-5 zeolite, which indicates that the numerous intracrystalline mesopores significantly reduce the diffusion path of the reactant, leading to the faster diffusion inside the zeolite to contact with the acid sites in the micropores predominating in ZSM-5 zeolites. This study can be extended to develop a series of hierarchical single-crystal zeolites with expected catalytic performance.
Highlights
Natural evolution has produced optimized hierarchically structured organisms in both plant and animal form in which the pore size decreases regularly and terminates in size-invariant units [1,2]
The external active sites were poisoned by DTBP and the hierarchical single-crystal ZSM-5 zeolites still exhibited higher catalytic performance as compared with Com-ZSM5
The introduction of more aluminum into the ZSM-5 framework produces more defects, which generates a large number of micropores and active sites, so the decrease in the Si/Al ratio resulted in significantly increases the acidity and the density of micropores
Summary
Natural evolution has produced optimized hierarchically structured organisms in both plant and animal form in which the pore size decreases regularly and terminates in size-invariant units [1,2]. Note that 2,6-di-tert-butylpyridine (DTBP) may be added in another BA self-etherification reaction to poison the active sites on the surface of the zeolites, so that only the active sites inside the zeolitic micropores could contact the BA and participate in the reaction The comparison of these reactions can be used to demonstrate the benefits of hierarchical single-crystal ZSM-5 zeolites and verify how the Si/Al ratio affects its catalytic performance. The external active sites were poisoned by DTBP and the hierarchical single-crystal ZSM-5 zeolites still exhibited higher catalytic performance as compared with Com-ZSM5 This improved catalytic performance is further explained by the introduction of mesopores, which greatly reduce the diffusion path so that reactants and products can more diffuse in and out of the zeolitic framework vis-à-vis Com-ZSM-5, which have only micropore present in the framework. The main reason for this increase is that a decreased Si/Al ratio renders hierarchical single-crystal ZSM-5 zeolites more acidic and increases the density of micropores, allowing the use of more acid sites in the microporous framework of ZSM-5 zeolite
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