Aluminum Incorporation and Interfacial Structures in MCM-41 Mesoporous Molecular Sieves

Abstract

Two-dimensional (2D) solid-state NMR spectroscopy has been used to identify interfacial species and establish framework locations of aluminum atoms incorporated in aluminosilicate MCM-41 mesophases and mesoporous solids. In these experiments, chemical shifts of protons in the material are correlated with the chemical shifts of nearby (ca. 1 nm) 13C, 27Al, or 29Si species via their respective heteronuclear dipole−dipole couplings. For aluminosilicate MCM-41 mesophases prepared at room temperature, 2D heteronuclear chemical shift correlation NMR spectra show that tetrahedrally coordinated aluminum and silicon species are in close spatial proximity to the trimethylammonium head groups of the cationic surfactant species in the as-synthesized materials and to ammonium cations following calcination and ion exchange. For MCM-41 materials synthesized under hydrothermal conditions, 2D heteronuclear correlation NMR measurements show that the appearance of six-coordinate aluminum species results from strongly bound water molecules coordinated to aluminum atoms that are also proximate to the surfactant species. The detection of couplings between 27Al or 29Si species and protons associated with the structure-directing surfactant molecules or exchangeable ammonium counterions establishes that a significant fraction of the aluminum atoms are present in the inorganic frameworks of these materials.