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.

Acid‐Functionalized Mesostructured Aluminosilica for Hydrophilic Proton Conduction Membranes

“…Following calcination or solvent extraction to remove the structure-directing block-copolymer species, post-synthesis grafting of aluminosilica moieties onto the interior mesopore walls introduces both hydrophilic and acidic surface properties. [38,39] While mesoporous silica or aluminosilica alone have relatively poor proton conduction properties, [40] incorporation of strong acid moieties, such as perfluorosulfonic acid species, onto their interior mesopore surfaces and channels results in films with high and stable proton conductivities, even at temperatures above 150°C. The preparation of a heterogeneous membrane material that combines high mesoscale ordering, crack-free inorganic film integrity, high hydrophilicity, and high proton conductivity, however, is exceedingly difficult to achieve in a single processing step.…”

Towards the Loewenstein limit (Si/Al=1) in thermally stable mesoporous aluminosilicates

“…Thus, on the basis on 2-D NMR data, Janicke et al 11 have recently concluded that both four-and sixcoordinate Al centers can coexist in the inorganic walls. In any case, it seems clear that Al atoms in octahedral environments do not contribute to the Brønsted acidity.…”

Biosurfactant-mediated one-step synthesis of hydrophobic functional imogolite nanotubes

“…In our system, the amount of MG is not enough to justify the formation of a liquid crystal. Nevertheless, in the field of surfactant-templated mesostructured silica and aluminosilica materials, the formation of lyotropic hybrid organic/inorganic phases occurs both under a liquid crystal like approach (high concentration of surfactant) 21 and under dilute conditions, where self-organisation and precipitation is driven in solution by the cooperative formation of inorganic-organic interfaces, as proposed by Monnier et al 22 Chmelka and co. also reported on the formation of mesostructured aluminosilicate by the same approach and, interestingly, their advanced solid state NMR study showed that incorporation of aluminum in the inorganic framework did not induce a 29 Si chemical shift at -78 ppm, 23 which supports the fact that in our case the formation of a classical mesoporous aluminosilicate is most-likely excluded.…”