Regulation of framework Al distribution of high-silica hierarchically structured ZSM-5 zeolites by boron-modification and its effect on materials catalytic performance in methanol-to-propylene reaction

摘要

In methanol-to-propylene reaction, the catalytic performance of ZSM-5 zeolite is strongly related to the material textural properties and surface acidity. In this work, the regulation of framework Al distribution (origin of the active Bronsted acid sites) in high-silica hierarchically structured ZSM-5 zeolite has been realized by a boron-modification procedure. When the B/Al ratio of the gel compositions (Si/Al = 200) reaches 7 or 9, the resultant B-7- and B-9-HSZ materials possess the expected hierarchically micro/mesoporous structures and increased framework Al distribution in the narrow straight/sinusoidal channels of MFI-type zeolite, which have been confirmed by the complementary characterizations, i.e. XRD, N-2 sorption isotherms, ICP-OES, SEM, TEM, Al-27 MAS NMR and the probe reaction of 1-hexene cracking. As a result, compared to the unmodified sample B-0-HSZ and sample B-3-HSZ with lower B-doping amount in the gels, they demonstrate the improved target propylene selectivity (49.3 % and 48.9 %) and elongated catalyst lifetime (88 h and 91 h) in MTP reaction under the WHSV of 2.8 h(-1).