HPM-2, the Layered Precursor to Zeolite MTF

HPM-2 is a new organosilicate layered material synthesized by the fluoride route using 2-ethyl-1,3,4-trimethylimidazolium. The layers are of a new structural type (denoted mtf) and are held together by strong hydrogen bonds giving rise to a ¹H magic angle spinning (MAS) nuclear magnetic resonance (NMR) at around 16 ppm and by Coulombic interactions between silanolates and the organic cations residing in the interlayer space. Upon calcination HPM-2 transforms into the pure silica MTF zeolite by topotactic condensation, a process that is essentially completed at 400 °C. Attempts to apply known methods to derive new materials (swelling, delamination, interlayer expansion) are described. In the case of the interlayer expansion reaction, a nonmicroporous dense phase is obtained, which is likely due to an unfavorable disposition of silanols in close couples within each layer. Silanol condensation between the newly incorporated silicon species occurs across the main window producing narrower (6 member ring, 6MR) rather than wider (10MR) windows

Zeolite Structure Direction by Simple Bis(methylimidazolium) Cations: The Effect of the Spacer Length on Structure Direction and of the Imidazolium Ring Orientation on the ¹⁹F NMR Resonances

A series of doubly charged structure-directing agents based on two methylimidazolium moieties linked by a linear bridge of <i>n</i> = 3,4,5, or 6 methylene groups has been used in the synthesis of pure silica zeolites in the presence of fluoride. All of them yielded zeolite TON while only the one with <i>n</i> = 4 was able to produce also zeolite MFI at highly concentrated conditions. In this MFI zeolite, two distinct ¹⁹F MAS NMR resonances with about equal intensity were observed, indicating two different chemical environments for occluded fluoride. With the singly charged 1-ethyl-3-methylimidazolium cation, which can be formally considered as the “monomer” of the bis-imidazolium cation with <i>n</i> = 4, TON and MFI were also obtained, and again two ¹⁹F MAS NMR resonances now with largely dissimilar intensities were observed in MFI. Molecular mechanics simulations support a commensurate structure-direction effect for <i>n</i> = 4 in MFI, with each imidazolium ring, in two different orientations, sitting close to the [4¹5²6²] cage. Periodic DFT calculations suggest that F in MFI resides always in the [4¹5²6²] cages, with the different ¹⁹F resonances observed being due to the different orientation of the closest imidazolium ring

Synthesis, Structure, and Optical Activity of HPM-1, a Pure Silica Chiral Zeolite

2-Ethyl-1,3,4-trimethylimidazolium is a poor organic structure-directing agent in the synthesis of pure silica zeolites using fluoride as a mineralizer at 150 °C. Under these conditions only ill-crystallized solids are obtained after long hydrothermal treatments (several weeks). It disappoints despite its relatively large size, conformational rigidity, and intermediate hydrophilic/hydrophobic character, attributes which would qualify it as a promising structure-directing agent, according to prior investigations. By raising the crystallization temperature to 175 °C under otherwise identical conditions, crystallization is dramatically accelerated. Depending on the water/silica ratio and crystallization time, two different materials are obtained: the recently reported pure silica polymorph of the chiral STW-type zeolite, HPM-1, and the new layered organosilicate, HPM-2. Prolonged heating transforms these phases into the small-pore ITW-type zeolite, while no signs of the SOF-type zeolite (formally built from the same layers as STW) was found. A complete physicochemical and structural characterization of the as-made chiral HPM-1 zeolite is provided, and the proposed stabilization of this zeolite by polarization of the Si–O bond is supported by the observed deviation from tetrahedrality. HPM-1 is optically active, and a study of several crystallites by Mueller matrix microscopy shows that their optical activity can be individually measured and that this technique could be useful for the assessment of the enantiomeric purity of a microcrystalline powder

Zeolite Synthesis in Fluoride Media: Structure Direction toward ITW by Small Methylimidazolium Cations

Pure silica ITW zeolite can be synthesized using 1,2,3-trimethylimidazolium and 1,3-dimethylimidazolium cations and fluoride anions as structure-directing agents (SDAs). Similarly to the previously reported 1,3,4-trimethylimidazolium, the dimethyl cation can also produce the zeolite TON, but this higher framework density phase finally transforms <i>in situ</i> into ITW. The structures of the as-made and calcined phases prepared with the new cations show a unit cell doubling along <i>z</i>, and the refined structures are reported. Periodic Density Functional Theory calculations provide the energies of the six SDA-ITW and SDA-TON zeolites, and their relative stabilities fully agree with the experimental observations. Structure-direction in this system is discussed from experimental and theoretical results that give strong support to the idea that strained silica frameworks are made possible in fluoride media by decreasing the covalent character of the Si–O bond. This decreased covalency is enhanced with the 1,2,3-trimethyl isomer, which is shown to be the strongest SDA for ITW and, at the same time, is the more hydrophilic of the three SDAs tested. Our observations with the three SDAs agree with the so-called Villaescusa’s rule, i.e., the low framework density phase is favored at higher concentrations, but at the same time question the supersaturation hypothesis that has been proposed to explain this rule, since here the low-density phase is the most stable one

Host–Guest Stabilization of a Zeolite Strained Framework: In Situ Transformation of Zeolite MTW into the Less Dense and More Strained ITW

The new organic structure-directing agent 1-ethyl-2,3-dimethylimidazolium, in conjuction with fluoride anions, shows selectivity toward pure silica zeolite ITW. At low water contents this zeolite crystallizes directly, while at higher water contents, the denser and more stable in the absence of occluded species MTW crystallizes first and then transforms in situ into ITW. A detailed physicochemical and structural characterization and a periodic density functional theory analysis are provided, and we show crystallographic and DFT evidence for a significant distortion of the SiO₄ tetrahedra, attributable to a polarization of the Si–O bond that helps relax the otherwise strained silica framework. A comparative analysis of five closely related imidazolium cations suggests the importance of both hydrophilicity and conformational flexibility in determining their selectivity as structure-directing agents

Magnesium-Enhanced Reactivity of Boron Particles: Role of Mg/B₂O₃ Exothermic Surface Reactions

Boron offers great promise as a candidate fuel in high-energy composites as a result of its high gravimetric and volumetric energy content; however, its oxidation rate is limited by sluggish diffusion of reactive species across its low-melting oxide shell. On the other hand, Mg nanoparticles (NPs) have been shown recently to undergo fast oxidation following rapid vaporization (∼100 μs at high heating rates of ∼105 °C/s). This release of vapor-phase Mg can potentially be exploited to react exothermically (ΔHr = −420 kJ/mol) with the B2O3 layer of boron, inducing surface disrputions and promoting its combustion. In this paper, we explore this effect by evaluating Mg NPs as additive fuel to B/CuO nanoenergetic composites. We observe that incorporating Mg as an additive fuel in B/CuO composites results in a ∼6-fold enhancement in reactivity with a ∼60% reduction in burn time. Through thermal and reaction product analysis along with high-speed time-of-flight mass spectrometry (T-jump/TOFMS) and ignition characterization, we investigate the reaction mechanism of Mg/B2O3 particles as a simulant system for the interaction of Mg with the B2O3 shell of boron. These characterizations reveal that exothermic heterogeneous reactions occur between vapor-phase Mg and the molten B2O3 shell of boron at ∼500–650 °C. The role of these exothermic surface reactions in inducing surface modifications and reactivity enhancement of boron particles is discussed