Fabricating ZSM-23 with reduced aspect ratio through ball-milling and recrystallization: Synthesis, structure and catalytic performance in N-heptane hydroisomerization

Unidimensional zeolites often form rod-shaped crystals that suffer from low catalytic selectivity or catalyst lifetime owing to their poor diffusion properties. Combined postsynthetic ball-milling and recrystallization methods were investigated to produce unidimensional ZSM-23 crystals with reduced aspect ratio. The influences of ball-milling in the presence/absence of structure directing agent (SDA) and diverse recrystallization media, including H2O, Na2Si3O7 solution, synthetic mixture and mother liquor, over crystallinity, crystal size, textural properties as well as acidity, were intensively explored using characterization techniques such as XRD, SEM, N-2 physisorption, Al-27 MAS NMR, NH3-TPD, pyridine IR spectroscopy and diffusion measurement. The results showed that crystal size of ZSM-23 was considerably reduced after ball-milling treatment, however, at the expense of crystallinity, acidity and porosity. Recrystallization in the presence of mother liquor was identified as the most effective and economic way to restore crystallinity. The optimized ZSM-23-M-m sample derived from ball-milling in the presence of SDA and a subsequent recrystallization in separated synthesis mother liquor exhibited higher isomer yield (ca. 53%) than the parent sample (ca. 34%) in n-heptane hydroisomerization reaction owing to the enhanced diffusion property and regained acidity after recrystallization.