Large-Grain, Oriented, and Thin Zeolite MFI Films from Directly Synthesized Nanosheet Coatings

Directly synthesized zeolite MFI nanosheets are promising building blocks for MFI thin films with large and oriented grains. The secondary growth of MFI nanosheets on Si wafers in tetraethylammonium hydroxide (TEAOH) silica sols was investigated, and conditions that result in well-oriented and intergrown film microstructure were established. This has enabled the fabrication of thin (∼300 nm) <i>b</i>-oriented MFI films with large grain-size (>2 μm) from seed-removed nanosheet monolayer coatings. Moreover, the faceted and anisotropic shape of MFI nanosheets allowed the measurement of MFI growth in different crystallographic directions and confirmed the twinning-free preferential growth along the <i>c</i>-axis (a lateral direction of the nanosheet), compared to the <i>b</i>-axis (the direction normal to the nanosheet basal plane), with ratios in a range between 4 and 11

TraPPE-zeo: Transferable Potentials for Phase Equilibria Force Field for All-Silica Zeolites

The transferable potentials for phase equilibria (TraPPE) force field is extended to all-silica zeolites. This novel force field is parametrized to match the experimental adsorption isotherms of <i>n</i>-heptane, propane, carbon dioxide, and ethanol with the Lennard-Jones parameters for sorbate–framework interactions determined in a consistent manner using the Lorentz–Berthelot combining rules as for other parts of the TraPPE force field. The TraPPE-zeo force field allows for accurate predictions for both adsorption and diffusion of alkanes, alcohols, carbon dioxide, and water over a wide range of pressures and temperatures. In order to achieve transferability to a wider range of molecule types, ranging from nonpolar to dipolar and hydrogen-bonding compounds, Lennard-Jones interaction sites and partial charges are placed at both the oxygen and the silicon atoms of the zeolite lattice, which allows for a better balance of dispersive and first-order electrostatic interactions than is achievable with the Lennard-Jones potential used only for the oxygen atoms. The use of the Lorentz–Berthelot combining rules for unlike interactions makes the TraPPE-zeo force field applicable to any sorbate as long as the relevant TraPPE sorbate–sorbate parameters are available. The TraPPE-zeo force field allows for greatly improved predictive power compared to force fields that explicitly tabulate the individual cross-interaction parameters

One-Pot Synthesis of 5-(Ethoxymethyl)furfural from Glucose Using Sn-BEA and Amberlyst Catalysts

5-(Ethoxymethyl)­furfural (EMF) was produced from glucose in ethanol in a single reactor at 90 °C. The reaction proceeds via the isomerization of glucose to fructose with zeolite Sn-Beta, a Lewis acid catalyst. Fructose is converted to 5-(hydroxymethyl)­furfural, which is then etherified to EMF using a Brønsted acid catalyst, Amberlyst 131. An EMF yield of 31% was achieved

Development of the Transferable Potentials for Phase Equilibria Model for Hydrogen Sulfide

The transferable potentials for phase equilibria force field is extended to hydrogen sulfide. The pure-component and binary vapor–liquid equilibria with methane and carbon dioxide and the liquid-phase relative permittivity are used for the parametrization of the Lennard–Jones (LJ) and Coulomb interactions, and models with three and four interaction sites are considered. For the three-site models, partial point charges are placed on the sites representing the three atoms, while the negative partial charge is moved to an off-atom site for the four-site models. The effect of molecular shape is probed using either only a single LJ interaction site on the sulfur atom or adding sites also on the hydrogen atoms. This procedure results in four distinct models, but only those with three LJ sites can accurately reproduce all properties considered for the parametrization. These two are further assessed for predictions of the liquid-phase structure, the lattice parameters and relative permittivity for the face-centered-cubic solid, and the triple point. An effective balance between LJ interactions and the dipolar and quadrupolar terms of the first-order electrostatic interactions is struck in order to obtain a four-site model that describes the condensed-phase properties and the phase equilibria with high accuracy

Combining Pre- and Post-Nucleation Trajectories for the Synthesis of High FAU-Content Faujasite Nanocrystals from Organic-Free Sols

The effects of synthesis conditions on the FAU/EMT content and the size of nanocrystals, formed from inorganic aluminosilicate sols, were investigated. High-resolution transmission electron microscopy imaging and comparison of experimental X-ray diffraction patterns with simulations demonstrated that all materials made starting from synthesis mixtures in the composition range (1.8–33) SiO₂:1 Al₂O₃:(2.7–33) Na₂O:(41–1000) H₂O contain FAU/EMT intergrowths. Compositions with low water content increase the FAU fraction up to 0.8 but the crystal size exceeds 100 nm. Extension of the higher FAU purity to nanocrystals was achieved only by first mixing the sol at high water content compositions that favor nanocrystal formation and thenafter a certain timelowering by freeze-drying the water to levels favoring the formation of FAU. Cryogenic transmission electron microscopy and small-angle X-ray scattering from representative optically clear and colloidally stable precursor sols (aged and crystallized at ambient temperature) reveal the formation of amorphous aggregates before the detection of crystals, in agreement with earlier findings and an existing model for the aggregative growth of the zeolite MFI. The presence of these amorphous aggregates coincides with the aforementioned state of sol that preserves the original trajectory toward nanocrystals after the pronounced reduction of water content by freeze-drying. If water reduction by freeze-drying is applied earlier (before the detection of amorphous aggregates), the sol follows the low water content trajectory toward larger crystals. Despite this memory effect, the sol at this stage is still agnostic toward FAU or EMT formation, the relative content of which is dominantly determined by the final water content. These findings demonstrate that it is possible to combine the effects of pre- and post-nucleation sol composition to steer crystal size and crystal structure, respectively. They confirm precursor nanoparticle evolution, while they emphasize the importance of solution phase composition at both pre- and post-nucleation stages of aggregative crystal growth

Direct Synthesis of 7 nm-Thick Zinc(II)–Benzimidazole–Acetate Metal–Organic Framework Nanosheets

Direct Synthesis of 7 nm-Thick Zinc(II)–Benzimidazole–Acetate Metal–Organic Framework Nanosheet

Understanding Diffusion in Hierarchical Zeolites with House-of-Cards Nanosheets

Introducing mesoporosity to conventional microporous sorbents or catalysts is often proposed as a solution to enhance their mass transport rates. Here, we show that diffusion in these hierarchical materials is more complex and exhibits non-monotonic dependence on sorbate loading. Our atomistic simulations of <i>n</i>-hexane in a model system containing microporous nanosheets and mesopore channels indicate that diffusivity can be smaller than in a conventional zeolite with the same micropore structure, and this observation holds true even if we confine the analysis to molecules completely inside the microporous nanosheets. Only at high sorbate loadings or elevated temperatures, when the mesopores begin to be sufficiently populated, does the overall diffusion in the hierarchical material exceed that in conventional microporous zeolites. Our model system is free of structural defects, such as pore blocking or surface disorder, that are typically invoked to explain slower-than-expected diffusion phenomena in experimental measurements. Examination of free energy profiles and visualization of molecular diffusion pathways demonstrates that the large free energy cost (mostly enthalpic in origin) for escaping from the microporous region into the mesopores leads to more tortuous diffusion paths and causes this unusual transport behavior in hierarchical nanoporous materials. This knowledge allows us to re-examine zero-length-column chromatography data and show that these experimental measurements are consistent with the simulation data when the crystallite size instead of the nanosheet thickness is used for the nominal diffusional length

Density Functional Theory Study on the Adsorption of H₂S and Other Claus Process Tail Gas Components on Copper- and Silver-Exchanged Y Zeolites

The potential use of Cu- and Ag-exchanged Y zeolites as selective adsorbents for hydrogen sulfide (H₂S) from Claus process tail gas was investigated with density functional theory (DFT). The adsorption energies of H₂S and other Claus tail gas components (CO, H₂O, N₂, and CO₂) were computed for these zeolites as well as for Li–Y, Na–Y, and K–Y on a cluster model. Comparison of adsorption energies for H₂S versus the other components indicated that Ag–Y has potential for selective adsorption of H₂S, whereas Cu–Y is subject to strong adsorption of CO, and alkali metal-exchanged Y zeolites are subject to H₂O adsorption. Comparison with alkali metal-exchanged Y zeolites was performed to clarify the role of d electrons, while the influence of the zeolite framework was assessed by comparing adsorption energies on the cluster model with those on bare cations. Absolutely localized molecular orbital energy decomposition analysis (ALMO EDA) revealed that for Cu- and Ag-containing systems, transfer of electrons between the cation and the adsorbate, i.e., the donation of d electrons and the acceptance of electrons in the unoccupied orbitals of the cation, plays an important role in determining the adsorption energy. On the other hand, for alkali metals-containing systems, charge transfer is negligible and adsorption energies are dominated by interactions due to electrostatics, polarization, and structural distortions

Probing the Relationship between Silicalite‑1 Defects and Polyol Adsorption Properties

The relationship between polyol adsorption affinity and silanol defect density was investigated through the development of vapor and aqueous adsorption isotherms on silicalite-1 materials which vary in structural and surface properties. Silicalite-1 crystals prepared through alkaline synthesis, alkaline synthesis with steaming post-treatment, and fluoride synthesis routes were confirmed as crystalline mordenite framework inverted (MFI) by SEM and XRD and were shown to contain ∼8.5–0 silanol defects per unit cell by ²⁹Si MAS, ¹H MAS, and ¹H–²⁹Si CPMAS NMR. A hysteresis in the Ar 87 K adsorption isotherm at 10^–3 <i>P</i>/<i>P</i>₀ evolved with a decrease in silanol defects, and, through features in the XRD and ²⁹Si MAS NMR spectra, it is postulated that the hysteresis is the result of an orthorhombic–monoclinic symmetry shift with decreasing silanol defect density. Gravimetric and aqueous solution measurements reveal that propylene glycol adsorption at 333 K is promoted by silanol defects, with a maximum 20-fold increase observed for aqueous adsorption at ∼10^–3 g/mL with an increase from ∼0 to 8.5 silanols per unit cell. A comparison of vapor and aqueous propylene glycol adsorption isotherms on defect-free silicalite-1 at 333 K, both of which exhibit the Type-V character, indicates that water enhances adsorption by a factor of ∼2 in the Henry’s Law regime. Henry’s constants for aqueous C₂–C₄ polyol adsorption (concentrations below 0.004 g/mL) at 298 K are shown to have a linear dependence on the silanol defect density, demonstrating that these molecules preferentially adsorb at silanol defects at dilute concentrations. This systematic study of polyol adsorption on silicalite-1 materials highlights the critical role of defects on adsorption of hydrophilic molecules and clearly details the effects of coadsorption of water, which can guide the selection of zeolites for separation of biomass-derived oxygenates

Coating of Open Cell Foams

The interior surfaces of three-dimensional open cell foams were coated by a combination of dip coating and spin coating. Glycerol/water solutions were used as model Newtonian liquids, and the coating processes were studied on open cell carbon foams with 10 or 30 pores per inch (PPI). The amount of liquid retained in the foam structures after dip coating increased with withdrawal speed and coating viscosity, as expected from the conventional understanding of dip coating onto nonporous substrates such as flat plates and rods. However, the liquid retention and hence average coating thickness increased with surface tension, a result counter to the observation with coating onto nonporous substrates. Pockets of liquid were observed after dip coating and results with coatings of alumina suspension showed that after drying, the trapped liquid can block pore windows. Spinning the foams after dip coating resulted in uniform liquid distribution and uniform coatings. Foams were placed in a special apparatus and rotated using a commercial spin coater. The liquid layer thickness decreased with spinning time and rotational speed, and increased with the liquid viscosity, results consistent with spin coating theory. The coating thickness after spinning was not affected by the initial dip coating procedure. The dip and spin process was also used to create γ-alumina and zeolite coatings, which are of interest for catalysis applications