DFT‑D Study of ¹⁴N Nuclear Quadrupolar Interactions in Tetra‑<i>n</i>‑alkyl Ammonium Halide Crystals

The density functional theory-based method with periodic boundary conditions and addition of a pair-wised empirical correction for the London dispersion energy (DFT-D) was used to study the NMR quadrupolar interaction (coupling constant <i>C</i>_<i>Q</i> and asymmetry parameter η_<i>Q</i>) of ¹⁴N nuclei in a homologous series of <i>tetra</i>-<i>n</i>-alkylammonium halides (C_<i>x</i>H_2<i>x</i>+1)₄N⁺X^– (<i>x</i> = 1–4), (X = Br, I). These ¹⁴N quadrupolar properties are particularly challenging for the DFT-D computations because of their very high sensitivity to tiny geometrical changes, being negligible for other spectral property calculations as, for example, NMR ¹⁴N chemical shift. In addition, the polarization effect of the halide anions in the considered crystal mesophases combines with interactions of van der Waals type between cations and anions. Comparing experimental and theoretical results, the performance of PBE-D functional is preferred over that of B3LYP-D. The results demonstrated a good transferability of the empirical parameters in the London dispersion formula for crystals with two or more carbons per alkyl group in the cations, whereas the empirical corrections in the tetramethylammonium halides appeared to be inappropriate for the quadrupolar interaction calculation. This is attributed to the enhanced cation–anion attraction, which causes a strong polarization at the nitrogen site. Our results demonstrated that the ¹⁴N <i>C</i>_<i>Q</i> and η_<i>Q</i> are predominantly affected by the molecular structures of the cations, adapted to the symmetry of the anion arrangements. The long-range polarization effect of the surrounding anions at the target nitrogen site becomes more important for cells with lower spatial symmetry

ZSM‑5 Zeolite: Complete Al Bond Connectivity and Implications on Structure Formation from Solid-State NMR and Quantum Chemistry Calculations

Al site distribution in the structurally complex and industrially important ZSM-5 zeolite is determined by studying the spectroscopic response of Al­(OSi)₄ units and using a self-consistent combination of up-to-date solid-state NMR correlations (²⁹Si–²⁷Al and ¹H–²⁷Al <i>D</i>-HMQC) and quantum chemistry methods (DFT-D). To unravel the driving forces behind specific Al sitting positions, our approach focuses on ZSM-5 containing its more efficient OSDA, tetrapropylammonium

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

Recrystallization on Alkaline Treated Zeolites in the Presence of Pore-Directing Agents

In previous works aiming at understanding the mesoporous network after alkaline treatment in the presence of organic additives, conventional bulk characterization techniques led to the conclusion that the dissolved zeolite does not undergo any kind of recrystallization [Verboekend, D., Cryst. Growth. Des. 2013, 13, 5025−5035]. Here for the first time, we demonstrate using the data obtained from ¹H and ¹²⁹Xe NMR spectroscopy that such recrystallization does occur, which leads to the formation of a very thin coating of the mesopore walls. This demonstration is done on a beta (BEA) zeolite treated in the presence of TPA⁺ in an alkaline solution. The formation of a small amount of nanosized crystals or embryonic phases of silicalite-1 (MFI) zeolite is evidenced, as well as their homogeneous dispersion on the mesoporous surface of the beta zeolite. We think that these results may explain why a homogeneous mesopore size distribution is obtained, when organic pore-directing agents are used in the zeolite hierarchization process performed in an alkaline medium