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

Abstract

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.