On the Role of Small Amines in Zeolite Synthesis. 2

摘要

A systematic study explored the ability of small amines to control the zeolites obtained at the high-pH levels typical of crystallizations at SiO_2/Al_2O_3 ratios between 20 and 40. Experiments in the absence of amine showed that mordenite (MOR) was the preferred product at the lowest ratio, and ZSM-5 was the preferred product at the highest SiO_2/Al_2O_3 ratio. However, the addition of C_2-, C_3-, and C_4-alkylamines changed the changed the product dramatically. On addition of ethylamine, ZSM-5 was produced at the lowest SiO_2/Al_2O_3 ratio, and ZSM-35 was the preferred product with C_3- and C_4-amines. On addition of the largest of the group, tert-butylamine, the product reverted to MOR. With polyamines such as diethylenetriamine and triethylenetetramine, ZSM-5 samples were produced having framework SiO_2/Al_2O_3 ratios below 20. The hexane cracking activity of these samples was consistent with their exceptionally high Al content. The cyclic C_4- to C_6-amines, at both SiO_2/Al_2O_3 extremes, yielded ZSM-35 and MCM-22. Again, with the largest of this amine group, 1,4-diazabicyclo[2.2.2]octane, the product reverted to mordenite (and ZSM-5). Despite the very high pH of the crystallizations, all of the ZSM-5, ZSM-35, and MCM-22 samples contained charge-balancing protonated amines and were low in Na; that is, they had Na/Al ratios well below unity. The MOR samples were similarly pore-filled with amine, but their Na/Al ratios did not indicate substantial amine protonation, which is consistent with mordenite being the limiting high-pH product in these experiments. Computer simulation was helpful in distinguishing, on the part of amine, between a generally beneficial but structurally nonspecific "pore-fill" role and a structurally specific, "pore-directing" role, the latter leading to preferred crystallization among alternative zeolite frameworks.