Host–Guest Stabilization of a Zeolite Strained Framework: In Situ Transformation of Zeolite MTW into the Less Dense and More Strained ITW

The new organic structure-directing
agent 1-ethyl-2,3-dimethylimidazolium,
in conjuction with fluoride anions, shows selectivity toward pure
silica zeolite ITW. At low water contents this zeolite crystallizes
directly, while at higher water contents, the denser and more stable
in the absence of occluded species MTW crystallizes first and then
transforms in situ into ITW. A detailed physicochemical and structural
characterization and a periodic density functional theory analysis
are provided, and we show crystallographic and DFT evidence for a
significant distortion of the SiO4 tetrahedra, attributable
to a polarization of the Si–O bond that helps relax the otherwise
strained silica framework. A comparative analysis of five closely
related imidazolium cations suggests the importance of both hydrophilicity
and conformational flexibility in determining their selectivity as
structure-directing agents.