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

Structure-Directing Agent Governs the Location of Silanol Defects in Zeolites

Structure-Directing Agent Governs the Location of Silanol Defects in Zeolite

Role of Al Distribution in CO₂ Adsorption Capacity in RHO Zeolites

Tailoring the CO2 adsorption performance of high-aluminum-containing zeolites is typically considered from the perspective of controlling the type and location of extra-framework cations. In this work, using solid-state 29Si nuclear magnetic resonance (NMR), we show that local order, i.e., the aluminum distribution within the framework of Na,Cs-RHO type zeolites with different Al contents, plays a fundamental role in governing the CO2 adsorption capacity and structural flexibility. From this analysis, the cation type and location within the RHO structure as a consequence of the framework Al distribution are not the only parameters that deserve consideration. This is despite their paramount importance in optimizing the adsorption capacity of samples with a fixed Al content. In addition, we observe strong correlations between the 29Si NMR barycenter and ellipticity of the eight-ring pore apertures and the nearest neighbor and next-nearest neighbor framework atom distances. From this analysis, we rationalize that the zeolite framework flexibility can be viewed as a consequence of the distribution of Si species rather than being exclusively a consequence of the type of cation loading only

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

Engineering RHO Nanozeolite: Controlling the Particle Morphology, Al and Cation Content, Stability, and Flexibility

The engineering of RHO nanozeolite is demonstrated by synthesis from a colloidal precursor suspension using only inorganic structure-directing agents (Na+, Cs+), whereby the particle morphology, Si/Al ratio, cation content, stability, and flexibility are tailored. RHO nanozeolite with a higher Si/Al ratio (2.0) and superior thermal stability (up to 700 °C) compared to previous reports is synthesized. Optimization of the synthesis procedure by introducing additional Si precursors facilitated the targeted improvement in the Si/Al ratio while maintaining the nanosized dimensions of the discrete zeolite crystals with well-defined rhombic dodecahedral morphology. The structural properties of the RHO nanozeolites are characterized by in situ variable-temperature X-ray powder diffraction (XRPD) experiments showing that the nanozeolites possess a single structural phase up to 740 °C; further heating to 760 °C induces a symmetry change from noncentrosymmetric to centrosymmetric associated with a large increase in the anisotropic displacement parameter of the Cs+ extra-framework cations. The structural behavior is unique compared to more siliceous Na+ and Cs+-containing RHO zeolites (Si/Al ≥ 3), which possess a centrosymmetric structure when hydrated. These experiments reveal a delineation, based on the Si/Al ratio and content of the extra-framework cations between the as-synthesized Na+ and Cs+-containing RHO zeolites that possess centrosymmetric or noncentrosymmetric symmetry when hydrated, as well as single or coexisting structural phases, expanding the scope of intelligently designed nanozeolites with tailored properties for precise applications

Engineering RHO Nanozeolite: Controlling the Particle Morphology, Al and Cation Content, Stability, and Flexibility

The engineering of RHO nanozeolite is demonstrated by synthesis from a colloidal precursor suspension using only inorganic structure-directing agents (Na+, Cs+), whereby the particle morphology, Si/Al ratio, cation content, stability, and flexibility are tailored. RHO nanozeolite with a higher Si/Al ratio (2.0) and superior thermal stability (up to 700 °C) compared to previous reports is synthesized. Optimization of the synthesis procedure by introducing additional Si precursors facilitated the targeted improvement in the Si/Al ratio while maintaining the nanosized dimensions of the discrete zeolite crystals with well-defined rhombic dodecahedral morphology. The structural properties of the RHO nanozeolites are characterized by in situ variable-temperature X-ray powder diffraction (XRPD) experiments showing that the nanozeolites possess a single structural phase up to 740 °C; further heating to 760 °C induces a symmetry change from noncentrosymmetric to centrosymmetric associated with a large increase in the anisotropic displacement parameter of the Cs+ extra-framework cations. The structural behavior is unique compared to more siliceous Na+ and Cs+-containing RHO zeolites (Si/Al ≥ 3), which possess a centrosymmetric structure when hydrated. These experiments reveal a delineation, based on the Si/Al ratio and content of the extra-framework cations between the as-synthesized Na+ and Cs+-containing RHO zeolites that possess centrosymmetric or noncentrosymmetric symmetry when hydrated, as well as single or coexisting structural phases, expanding the scope of intelligently designed nanozeolites with tailored properties for precise applications

Unraveling the Effect of Silanol Defects on the Insertion of Single-Site Mo in the MFI Zeolite Framework

The preparation of defect-free MFI crystals containing single-site framework Mo through a hydrothermal postsynthesis treatment is reported. The insertion of single Mo sites in the MFI zeolite samples with different crystal sizes of 100, 200, and 2000 nm presenting a diverse concentration of silanol groups is revealed. The nature of the silanols and their role in the incorporation of Mo into the zeolite structure are elucidated through an extensive spectroscopic characterization (29Si NMR, 1H NMR, 31P NMR, and IR) combined with X-ray diffraction and HRTEM. In addition, a DFT-based theoretical modeling of a large Si154O354H92 nanoparticle containing 600 atoms is carried out to understand the expansion of the unit cell volume measured by X-ray diffraction. An accurate quantification of the silanols in the MFI crystals with different particle sizes and the insertion of Mo in the zeolitic framework is reported for the first time. The results confirmed that the non-H-bonded silanols seem to be the gateway for the insertion of single Mo atoms in the zeolite structure. Such materials with single metal sites present high crystallinity and perfect structure, thus providing great stability in catalytic applications

Host–Guest Silicalite‑1 Zeolites: Correlated Disorder and Phase Transition Inhibition by a Small Guest Modification

We have investigated the nature and extent of nanoscale disorder in prototypical host–guest zeolites, made of silicalite-1 (host) and organic structure-directing agent (OSDA, guest). The four different selected OSDA-silicalite-1 differ in: the mineralizing agent used (F– vs OH–), the synthesis method (hydrothermal vs solvent-free), and the OSDA (tetrapropylammonium (TPA) vs tripropylethylammonium TPEA). The comparison between TPA and TPEA, chemically similar but differing in their symmetry, is examined in great detail owing to the novel relationship found between the geometrical disorder and the monoclinic–orthorhombic (m–o) phase transition occurring at low temperatures. Long- and short-range organization and ordering are characterized by complementary X-ray diffraction (XRD), Raman analysis, and multinuclear NMR spectroscopy (13C, 14N, 29Si). The possibility of the m–o transition is studied by all of these techniques at variable low T values. An in-depth study of the disorder is carried out by X-ray structure determination and two-dimensional (2D) NMR 29Si–29Si INADEQUATE correlations, including an up-to-date analysis of anisotropic atomic displacement parameters and a new fitting approach to estimate correlated disorder from 2D NMR data sets. The collected results allow us to demonstrate how the disorder created by the positioning of the less symmetric TPEA guest leads to a correlated geometrical disorder for half of the atom sites in the host framework that completely inhibits the m–o phase transition