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

Abstract

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 (Fvs. 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 temperature. Long-and short-range organization and ordering are characterized by complementary XRD, Raman and multinuclear NMR ( 13 C, 14 N, 29 Si). The possibility of the m-o transition is studied by all these techniques at variable low T. An in-depth study of the disorder is carried out by x-ray structure determination and 2D NMR 29 Si-29 Si 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 datasets. The collected results allow 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.

Solvent-free synthesis of zeolites: New insights into the mechanism and non-mechanochemical route

“…Therefore, the local order of the silica zeolite framework as well as the OSDA conformation and positioning in this framework do not present any significant variations with grinding that will otherwise modify the spectra. This is different from what is observed by 29 Si NMR when varying the thermal duration with a fixed grinding time (vide infra).…”

“…The above-mentioned results were completed with NMR experiments. We observed that the grinding period does not affect the spectral resolution of 29 Si (if not considering the broad peaks of the amorphous particles, Fig. S4) and 13 C NMR spectra (Fig.…”

“…We monitor the changes in the synthesis powder during the thermal treatment for periods comprised between 1 h and up to 28 days (beyond reaching the fully crystalline phase). A combination of characterization techniques (XRD, SEM, 13 C, 19 F and 29 Si solid-state NMR) brings new information on the mechanisms of formation. Besides that, we show for the first time that the solvent-fee method can be carried out without any mechanochemical activation of the initial precursor.…”

“…From Fig. 7, we observe the evolution of the 29 Si peaks after the induction period. The Q 3 peak keeps decreasing with increasing the thermal treatment duration.…”

Regression Machine Learning Models Used to Predict DFT-Computed NMR Parameters of Zeolites

“…We built the MFI-OSDA structures from the available crystallographic data for TPA (ETPA) [59] and ZSM-5-TPA [60] zeolites. These structures were optimised in our previous studies [61,62] using the same level of DFT theory. We thus constructed a more heterogeneous data set that contains Si, Al, N, C, H, Li, and F atomic environments.…”