The impact of “empty-corner” tetrahedra in the synthesis of MFI type zeolites: unveiling selenium stereoactivity

Abstract

This study investigates the impact of selenium in different oxidation states, specifically Se⁶⁺ and Se⁴⁺, on the structural features of MFI-type zeolites. Se⁴⁺ is characterized by trigonal pyramidal coordination, forming a [Se⁴⁺O₃E]²⁻ entity (E being an active lone pair of electrons). The three-coordinated [Se⁴⁺O₃E]²⁻ entity represents an “empty-corner tetrahedron”, analogous to four-coordinated [Se⁶⁺O₄]²⁻ and [SiO₄]⁴⁻ tetrahedra. Aiming to understand the impact of lone pair bearing complexes on MFI, Se⁴⁺ and Se⁶⁺ were introduced through a one-pot synthesis approach giving rise to Se(IV)-MFI and Se(VI)-MFI samples. Both the Se(IV)-MFI and Se(VI)-MFI samples exhibit monoclinic symmetry at room temperature, transitioning into orthorhombic symmetry at temperatures above 100 °C and reverting to monoclinic symmetry upon cooling. A significant difference was observed in terms of the silanol content, i.e. Se(IV)-MFI shows a lower concentration of SiOH compared to Se(VI)-MFI. These observations are coherently supported by ²⁹Si and ⁷⁷Se NMR, Se K-edge XAS, FT-IR, and Raman spectroscopy, suggesting that Se⁴⁺ oxycomplexes with a lone pair of electrons are more favorable for incorporation into the MFI framework compared to Se⁶⁺. ⁷⁷Se NMR analyses revealed similar Se tetrahedral coordination in both Se(VI)-MFI and Se(IV)-MFI, indicating the stabilizing role i.e. oxidation of Se⁴⁺ to Se⁶⁺ directly during crystallization of MFI. Considering the differences in the silanol content and the similarities in Se coordination states in Se(IV)-MFI and Se(VI)-MFI, the transformation of “empty-corner tetrahedra” into regular tetrahedra i.e. selenite to selenate favors the introduction of Se in the MFI framework. These results provide valuable insights into the tunability of zeolite frameworks using selenium in different oxidation states.