Study of zeolite acidity by broad-line

Simulation of 4K broad-line 1H NMR spectra of solids, with Brönsted acid properties and loaded with known amounts of water, allows the determination of the concentrations of the species: H3O+, H2O...HO and of the remaining initial species: OH and H2O. This is an original method for studying the acid strength of solids. The following results concern HY zeolite taken as example. A parallel study using 1H HR-NMR (MAS) at ambient temperature confirms the results obtained by using both methods on HZSM-5 (1)

Highly fluorinated silica obtained by direct F₂-gas fluorination: stability and unprecedented fluorosilicate species revealed by solid state NMR investigations

The direct F2-gas fluorination of mesoporous silica is a unique method leading to high fluorinated (up to 13 wt % F) and homogeneous powder with a controlled amount of grafted fluorine. Thermogravimetric analysis coupled with mass spectrometry for water, hydroxyl, and fluorine groups have allowed concluding that low fluorine-grafted silicas are thermically stable up to 550 °C whereas high fluorine-grafted silicas start to decompose from 250 °C with departure of SiF4 species. Water adsorption measurements have demonstrated the hydrophobic character of fluorinated silica and proved that direct fluorination is a way to control the hydrophilic−hydrophobic balance of silica. Pristine and fluorinated silicas have been studied by 19F and 1H MAS, and 1H−29Si and 19F−29Si CP-MAS NMR spectroscopies. NMR measurements have revealed various tetrahedral (O2/2SiF2, O3/2SiF) and pentahedral (O4/2SiF) fluorosilicate species previously observed in moderately fluorinated silica, and two unprecedented pentahedral species occurring in these highly fluorinated silica: O3/2SiF2 and O2/2SiF3 (δiso(19F) = −143.5 and −136.5 ppm; δiso(29Si) = −124 and −132 ppm, respectively). The occurrence of these unusual species, thanks to the coupling between the redox mechanism and an etching phenomenon provoked by direct F2-gas fluorination, should explain the thermal stability as well as the water affinity of fluorinated silica

Force field and a surface model database for silica to simulate interfacial properties in atomic resolution

Silica nanostructures find applications in drug delivery, catalysis, and composites, however, understanding of the surface chemistry, aqueous interfaces, and biomolecule recognition remain difficult using current imaging techniques and spectroscopy. A silica force field is introduced that resolves numerous shortcomings of prior silica force fields over the last 30 years and reduces uncertainties in computed interfacial properties relative to experiment from several 100% to less than 5%. In addition, a silica surface model database is introduced for the full range of variable surface chemistry and pH (Q2, Q3, Q4 environments with adjustable degree of ionization) that have shown to determine selective molecular recognition. The force field enables accurate computational predictions of aqueous interfacial properties of all types of silica, which is substantiated by extensive comparisons to experimental measurements. The parameters are integrated into multiple force fields for broad applicability to biomolecules, polymers, and inorganic materials (AMBER, CHARMM, COMPASS, CVFF, PCFF, INTERFACE force fields). We also explain mechanistic details of molecular adsorption of water vapor, as well as significant variations in the amount and dissociation depth of superficial cations at silica–water interfaces that correlate with ζ-potential measurements and create a wide range of aqueous environments for adsorption and self-assembly of complex molecules. The systematic analysis of binding conformations and adsorption free energies of distinct peptides to silica surfaces will be reported separately in a companion paper. The models aid to understand and design silica nanomaterials in 3D atomic resolution and are extendable to chemical reactions