Seed-assisted synthesis of ZSM-48 zeolite with low SiO2/Al2O3 ratio for n-hexadecane hydroisomerization

Abstract

Synthesis of highly-crystallized ZSM-48 zeolite with low SiO2/Al2O3 ratio was successfully performed via seed-assisted route. The influence of various factors on the seed-assisted synthesis was investigated systematically, including the seeds dosages, different seeds species, adding order of seeds, hydrothermal temperature and time, silica and aluminum sources, and gel compositions. The crystallinity, morphology, textural and acidic properties of ZSM-48 were characterized by various techniques, such as XRD, SEM, FTIR, Py-IR, N2 physisorption, 29Si and 27Al MAS NMR. Aluminum sources posed a critical influence on the crystallinity with NaAlO2 more beneficial to obtain well-crystallized ZSM-48, whereas the crystallinity seemed slightly susceptible to silica sources. SEM results revealed that the crystal size was significantly reduced from ca. 20 μm to around 2 μm after introduced the seeds. Kinetic analysis suggested that the added seeds can significantly reduce the apparent activation energy of nucleation and crystallization, encourage the formation of crystal nuclei and promote crystallization. The Pt-supported seed-assisted samples exhibited considerably higher density of acid sites than that loaded on seed-free sample. The hydroisomerization of n-hexadecane was investigated over various Pt-supported catalysts, and the Pt-supported seed-assisted samples exhibited the highest i-C16 yield of ca. 75 wt% at conversion over 87 wt%.