Synthesis of STW zeolites using imidazolium-based dications of varying length

Abstract

Dications consisting of 1,2-dimethylimidazolium units linked by chains of 4, 5 or 6 methylene groups display structure direction towards the chiral zeolite STW as racemic conglomerates with decreasing strength as the length of the linker increases. A phase transformation from the denser and less strained MTW to the less dense and more strained STW reveals significant stabilization in the case of the 4-methylene dication, which is able to produce STW even in the absence of germanium (i.e., for a pure silica composition), and to a lesser extent for the 5-methylene derivative. When Ge is introduced in the synthesis the crystallization is easier and the crystallization field is wider, with Ge preferentially occupying positions at the double four ring units of the STW framework, as revealed by Rietveld refinement from synchrotron data. Quantum mechanics calculations show that the interaction energy diminishes as the length of the linker increases, in agreement with the experimental findings. In addition, calculation of the ¹³C NMR chemical shieldings and comparison with experimental spectra allowed us to identify different conformations and orientations of the imidazolium rings in the asymmetric STW cavities, depending on the spacer length. An exploratory study strongly suggests that an asymmetric derivative with additional methyl groups at both sides of the tetramethylene chain could afford the crystallization of the homochiral zeolite even as a pure silica material.