Host-Guest Silicalite-1 Zeolites: Correlated Disorder and Phase Transition Inhibition by a Small Guest Modification

We have investigated the nature and extent of nanoscale disorder in prototypical host-guest zeolites, made of silicalite-1 (host) and organic structure-directing agent (OSDA, guest). The four different selected OSDA-silicalite-1 differ in: the mineralizing agent used (F- vs OH-), the synthesis method (hydrothermal vs solvent-free), and the OSDA (tetrapropylammonium (TPA) vs tripropylethylammonium TPEA). The comparison between TPA and TPEA, chemically similar but differing in their symmetry, is examined in great detail owing to the novel relationship found between the geometrical disorder and the monoclinic-orthorhombic (m-o) phase transition occurring at low temperatures. Long- and short-range organization and ordering are characterized by complementary X-ray diffraction (XRD), Raman analysis, and multinuclear NMR spectroscopy (C-13, N-14, Si-29). The possibility of the m-o transition is studied by all of these techniques at variable low T values. An in-depth study of the disorder is carried out by X-ray structure determination and two-dimensional (2D) NMR Si-29-Si-29 INADEQUATE correlations, including an up-to-date analysis of anisotropic atomic displacement parameters and a new fitting approach to estimate correlated disorder from 2D NMR data sets. The collected results allow us to demonstrate how the disorder created by the positioning of the less symmetric TPEA guest leads to a correlated geometrical disorder for half of the atom sites in the host framework that completely inhibits the m-o phase transition.

Automated summary

The study characterized the long- and short-range organization and ordering of different OSDA-silicalite-1 materials using XRD, Raman analysis, and multinuclear NMR spectroscopy. The research focused in detail on the comparison between TPA and TPEA, revealing how the disorder caused by the less symmetric TPEA guest inhibits the m-o phase transition.