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 ¹⁹F 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 ¹⁹F 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 [4¹5²6²] cage. Periodic DFT calculations suggest that F in MFI resides always in the [4¹5²6²] cages, with the different ¹⁹F resonances observed being due to the different orientation of the closest imidazolium ring.