Synthesis of ferrierite by a new combination of co-structure-directing agents: 1,6-bis(N-methylpyrrolidinium)hexane and tetramethylammonium

Ferrierite zeolite was synthesized in fluoride medium using dicationic 1,6-bis(N-methylpyrrolidinium)hexane (MPH) as a bulky structure directing agent (SDA) in combination with tetramethylammonium (TMA) as a small co-structure-directing agent (co-SDA). When trimethylamine, pyrrole or tert-butylamine were used as co-SDA, amorphous and beta zeolite (BEA framework type) were obtained, suggesting that these amines do not play a strong role as co-SDA. Only when TMA was used as co-SDA in synthesis gels with moderate pH, ferrierite is obtained; in fact, we have found a dramatic influence of the pH of the synthesis gel on the ferrierite crystallization, since even using TMA as co-SDA, ferrierite was only obtained under moderate pH values. Interestingly, ICP and CHN chemical analyses suggest that the Al content is driven by the amount of SDAs able to be hosted within the ferrierite framework, more specifically, by the nitrogen (positive charge) content available to charge-balance the negative charge associated to the incorporation of Al. A computational study showed that the most stable configuration of the ferrierite system corresponds to the occupation of the FER cages by TMA, while MPH accommodates in the 10-membered ring channels of the FER structure; the packing arrangement of MPH determines the density of positive charges, and hence the Al incorporation. Moreover, the unit cell parameters of the as-made ferrierite samples indicate that the structure is slightly expanded along the [100] direction in order to accommodate the bulky MPH dication.