Synthesis, Characterization And Catalytic Properties Of Mesoporous MFI Zeolite With Self-Stacked Morphology

Recently, mesoporous zeolites have been paid much attentions both in fundamental research and industrial applications due to their combining advantages of mesoporous materials (i.e. large surface area, fast mass transfer and bulky molecular conversion etc.) and zeolite crystals (i.e. high thermal and hydrothermal stabilty, strong acidity and oxidation ability etc.). Typical successful examples of mesoporous zeolites are prepared by addition of mesoscale templates into the starting aluminosilicate gels, and the types of mesoscale templates have been extended to carbons nanoparticles, aerogels, polymers, organosilanes and inorganic nanoparticles. On the other hand, it is well known that well-controlled morphology of zeolites is of great importance as devices and catalysts in many applications. However, most of mesoporous zeolites reported show poor crystal morphology. Mesoporous zeolites with controlled morphology are scarce. Therefore, the development of synthetic strategies for the control of crystal size and morphology of mesoporous zeolites with intracrystal mesopores is increasingly important. Here, we show a facile method for preparation of mesoporous MFI zeolite (including silicalite-1 and ZSM-5 ) with intracrystal mesopores and self-stacked morphology via hydrothermal synthesis by using a cationic polymer ( PDDA , poly (diallyldimethylammonium chloride) ) as template.Catalytic activities of M-ZSM-5( Mesoporous ZSM-5 zeolites )in carcking of cumene and 1,3,5-triisopropylbenzene (TIPB) are showed in my work. M-ZSM-5 showed similar conversion (42%) in cumene cracking compared with conventional ZSM-5 with similar ratio of Si/Al, but much high conversion in TIPB cracking while conventional ZSM-5 is nearly inactive in this reaction. This phenomenon can be explained by that TIPB is much larger than cumene and the former cannot enter in the pore of ZSM-5 (ca. 0.55 nm). The presence of mesopores in M-ZSM-5 sample offers the opportunity to react with the active sites in the sample. Furthermore, the catalytic performance have also been checked in methanol to olefins (MTO) reaction. The conversion of methanol is almost keep constant (100%) even after steam for 6 hours. The selectivity for propylene and ethylene are 34 and 12%, respectively. And the ratio of propylene to ethylene is 2.83. Compared with conventional ZSM-5, the catalyst life and the ratio of propylene to ethylene are improved, which is attributed to the presence of mesopores.Further investigation reveals that conventional silicalite-1 or ZSM-5 rather than M-S-1 or M-ZSM-5 with intracrystal mesopores and self-stacked morphology are obtained in the absence of polymers. Moreover, the amount of polymer should be well controlled, otherwise samples with individual crystals in stead of self-stacked morphology are produced, although containing mesopores. These results suggest that polymers play important roles in the preparation of mesoporous MFI zeolites with self-stacked morphology.Cationic polymers templated mesoporous zeolites has been confirmed that it is a versatile method for preparation various mesoporous zeolite. However, the morphology control of mesoporous zeolite by cationic polymers has not been reported yet. We believe that this method is not limited to preparation of mesoporous MFI zeolite. Other zeolites such as TS-1,Beta,Y with intracrystal mesopores and controlled morphology could also be obtained by using suitable cationic polymers.