Zeolite structure direction by simple bis(methylimidazolium) cations: the effect of the spacer length on structure direction and of the imidazolium ring orientation on the 19F NMR resonances.

A series of doubly charged structure-directing agents based on two methylimidazolium moieties linked by a linear bridge of n = 3,4,5, or 6 methylene groups has been used in the synthesis of pure silica zeolites in the presence of fluoride. All of them yielded zeolite TON while only the one with n = 4 was able to produce also zeolite MFI at highly concentrated conditions. In this MFI zeolite, two distinct (19)F MAS NMR resonances with about equal intensity were observed, indicating two different chemical environments for occluded fluoride. With the singly charged 1-ethyl-3-methylimidazolium cation, which can be formally considered as the "monomer" of the bis-imidazolium cation with n = 4, TON and MFI were also obtained, and again two (19)F MAS NMR resonances now with largely dissimilar intensities were observed in MFI. Molecular mechanics simulations support a commensurate structure-direction effect for n = 4 in MFI, with each imidazolium ring, in two different orientations, sitting close to the [4(1)5(2)6(2)] cage. Periodic DFT calculations suggest that F in MFI resides always in the [4(1)5(2)6(2)] cages, with the different (19)F resonances observed being due to the different orientation of the closest imidazolium ring.