Probing Disorder in Al-ZSM‑5 Zeolites by ¹⁴N NMR Spectroscopy

¹⁴N solid-state NMR spectroscopy is used to investigate and quantify the nanometer scale disorder promoted by Al/Si substitution in ZSM-5 zeolites. After a preliminary characterization by SEM, XRD, and multinuclear (¹H, ¹³C, ¹⁹F, ²⁷Al, ²⁹Si) solid-state NMR, the ¹⁴N MAS NMR spectra of a series of as-synthesized ZSM-5 zeolites containing various amounts of Al are analyzed. The ¹⁴N spinning sideband patterns are shown to evolve with the Si/Al ratio. The modeling of the NMR spectra allows one to estimate the local disorder arising from the Al site distribution within the tetrahedral sites of the zeolites, the variations of F locations, and the presence of silanol defects. The influence of the zeolite framework modifications due to Al/Si substitution on ¹⁴N NMR parameters is discussed on the basis of the results obtained with the Density Functional Theory periodic quantum chemical calculations augmented with an empirical London dispersion term. Analysis of the results highlighted the influence of CNC angle variations on the ¹⁴N quadrupole coupling constant distributions

Ionic Liquid Mediated Sol-Gel Synthesis in the Presence of Water or Formic Acid: Which Synthesis for Which Material?

Sol-gel syntheses involving either neutral water or formic acid as a reactant have been investigated (1) to determine the best conditions to confine a maximum of ionic liquid (IL) inside silica-based matrixes and (2) to reach the highest porosity after removing the IL from the ion gels (washed gels). Several sets of ionogels were prepared from various 1-butyl-3-methylimidazolium ILs and various silica or organosilica sources. The study evidenced a critical effect of the anion on the morphology (monolith, powder) and texture of the resulting washed gels. Particularly, tetrafluoroborate anion led to monolith ionogels by a simple hydrolytic method, affording highly condensed mesoporous silicas with some fluorinated surface sites. Such sites have never been reported before and were evidenced by ¹⁹F NMR. On the other hand, formic acid solvolysis turned out to be the only method to get non-exuding, crack-free, and transparent monoliths from ILs containing bis­(trifluoromethylsulfonyl)­imide [NTf₂] anion, with promising applications in photochemistry or photosensing. With bulky imidazolium and pyridinium cations, removal of the IL led to highly porous silicas with pore diameters and pore volumes as high as 10–15 nm and 3 cm³ g^–1, respectively. These silicas could find applications as supports for immobilizing bulky molecules