科研之友
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摘要
Nanosized H-ZSM-5 has been proven to be an efficient way to improve mass transport properties with shape selectivity in many catalytic reactions. Generally, the synthesis of very fine nanosized H-ZSM-5 always suffers from low product yield and requires a complicated centrifugal separation process, both of which severely hinder its large-scale preparation and industrial applications. Herein, we report a centrifugation-free and high yield synthesis route for hierarchically nanosized ZSM-5 with a wide Si/Al ratio range by a combination of pre-aging process and steam-assisted conversion method using alkalis-free powder as the ZSM-5 precursor. This facile route not only avoids the energy-intensive centrifugal separation and ion-exchange process, but also significantly increases the crystallization efficiency along with a high yield. The obtained nanosized ZSM-5 possesses an ultrafine uniform size, high surface area, high total pore volumes, tunable Si/Al molar ratio, and high crystallinity. As a result, the nanosized ZSM-5 shows excellent catalytic performance when used in the catalytic conversion of methanol to aromatics. Notably, the nanosized ZSM-5 with a Si/Al-th of 60 (NZS-60) shows an almost 25-fold longer catalytic lifetime, as well as up to 16% higher total aromatic selectivity when compared with conventional ZSM-5. Furthermore, the selectivity of 1,2,4-trimethylbenzene over this catalyst can be up to 44% in all products and 64% in aromatics products. Characterization results of the spent samples reveal that the most-improved catalytic performance and high selectivity of 1,2,4-trimethylbenzene over the nanosized ZSM-5 could be attributed to its small crystal size and hierarchical structure, which not only prevents the deposition of polyaromatic hydrocarbon in the microspores, but also sharply increases the reaction efficiency of bulky intermediate products on the surface of the catalyst.
