ZSM-12 nanocrystals with tunable acidity directed by rigid diquats: Synthesis and catalytic applications

ZSM-12 is an important zeolite used as a catalyst in several industrial hydrocarbon conversion reactions. Facile synthesis of nanocrystalline ZSM-12 in a wide Si/Al compositional range remains a challenge. Here we report the successful synthesis of nanosized ZSM-12 from flexible aluminosilicate initial gels (Si/Al = 20, 50 and 100) using rigid diquaternary ammonium compounds (p-xylene-bridged bis-methylpyrrolidinium, -methylpiperidinium and −1,2-dimethylimidazolium -) as the organic structure-directing agents (OSDAs). The effective structure-directing ability of these OSDAs for nanocrystalline ZSM-12 synthesis is due to the strong interaction between the OSDA and the growing zeolite framework. The resulting nanocrystalline ZSM-12 zeolites with proper acidity exhibit substantially improved catalytic performance in hydroconversion of n-hexadecane (n-C₁₆) and methanol-to-hydrocarbons reactions. TGA analysis of used catalysts and in situ IR spectroscopy of intra-zeolite organic species during the MTH reaction revealed that the smaller crystals result in lighter aromatics and less coke deposition due to the higher rate of desorption attributed to the larger external surface area.