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 <sup>19</sup>F NMR Resonances
2012-02-29T00:00:00Z (GMT) by
A series of doubly charged structure-directing agents based on two methylimidazolium moieties linked by a linear bridge of <i>n</i> = 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 <i>n</i> = 4 was able to produce also zeolite MFI at highly concentrated conditions. In this MFI zeolite, two distinct <sup>19</sup>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 <i>n</i> = 4, TON and MFI were also obtained, and again two <sup>19</sup>F MAS NMR resonances now with largely dissimilar intensities were observed in MFI. Molecular mechanics simulations support a commensurate structure-direction effect for <i>n</i> = 4 in MFI, with each imidazolium ring, in two different orientations, sitting close to the [4<sup>1</sup>5<sup>2</sup>6<sup>2</sup>] cage. Periodic DFT calculations suggest that F in MFI resides always in the [4<sup>1</sup>5<sup>2</sup>6<sup>2</sup>] cages, with the different <sup>19</sup>F resonances observed being due to the different orientation of the closest imidazolium ring.