All-silica Zeolite Research Articles

Synthesis of (h0l)-oriented zeolite Fe-beta membrane

The continuous and intergrown zeolite Fe-beta membrane was synthesized on the porous α-Al2O3 substrate by fluoride route in combination with a secondary seeded growth method. XRD and SEM results show that the synthesized zeolite Fe-beta membrane and all-silica zeolite beta membrane are (h0l)-oriented. The thickness of zeolite Fe-beta membrane is just 5.6μm, which is thinner than that of the all-silica zeolite beta membrane (8.5μm). Leaking tests confirm that non-zeolitic pores in the synthesized membranes are negligible. XRD patterns of the powdery samples which were collected from the bottom of the autoclave after secondary growth indicated that most of iron is incorporated in the zeolite beta framework.

A comparative study of the adsorption of water and methanol in zeolite BEA: a molecular simulation study

Grand canonical Monte Carlo simulations were carried out to study the equilibrium adsorption concentration of methanol and water in all-silica BEA zeolite and HBEA zeolites with different Si/Al ratios over a wide range of temperatures and loadings. These zeolites have oval-shaped channels with one side longer than the other. Water sorption into the hydrophobic BEA zeolite had a sharp transition with its sorption going from zero to near full capacity over a very small pressure range. Methanol sorption was much more gradual with respect to pressure. With the addition of hydrophilic sites for the HBEA zeolites by decreasing the Si/Al ratio, adsorption at lower pressures increased significantly for water and methanol. At higher loadings, water and methanol adsorption were found to behave in fundamentally different ways. Water structures in the zeolite channels formed hydrogen-bonded chains while maximising contact with the surfaces on the longer edges of the zeolite channels. Methanol molecules, in contrast, formed very few hydrogen bonds between themselves, with their hydroxyl groups primarily binding with surface of the shorter edge of the zeolite channels and their methyl groups located near the middle of the zeolite channels. The addition of hydrophilic groups in the HBEA zeolites strongly influenced positions of the methanol hydroxyl groups at high loadings, but did not have a significant effect on water structure.

Novel surfactant-free route to delaminated all-silica and titanosilicate zeolites derived from a layered borosilicate MWW precursor

Layered borosilicate zeolite precursor ERB-1P (Si/B = 11) is delaminated via simultaneous deboronation and SDA removal, to yield material DZ-1 consisting of silanol nests, using a simple aqueous Zn(NO3)2 treatment. Characterization of this synthesis process by PXRD shows loss of long-range order, and transmission electron microscopy (TEM) demonstrates transformation of rectilinear layers in the layered zeolite precursor to single and curved layers in the delaminated material. N2 physisorption confirms the expected decrease of micropore volume and increase in external surface area for delaminated materials relative to their calcined 3D zeolite counterpart. Elemental analysis shows loss of B and absence of Zn in the delaminated material. Resonances corresponding to silanol nests are evident via (29)Si solid-state NMR spectroscopy in DZ-1, which should be located within 12-MR pockets near the external surface. We have successfully utilized these nests as tetrahedral recognition sites for incorporation of Ti within an isolated framework coordination environment in material Ti-DZ-1. Diffuse-reflectance ultraviolet (DR-UV) spectroscopy of Ti-DZ-1 confirms isolated framework Ti sites, which are assigned to bands in the range of 210 nm-230 nm. Infrared spectra of Ti-DZ-1 consist of a distinct absorption band at 960 cm(-1), which is absent in DZ-1 prior to Ti incorporation and has been previously correlated with the presence of framework Ti species. Infrared spectra after pyridine adsorption demonstrate bands consistent with Lewis-acid sites in the resulting Ti-substituted delaminated zeolite. The accessibility of these Lewis-acid sites is confirmed when using Ti-DZ-1 as a catalyst for cyclohexene epoxidation using tert-butyl hydroperoxide as the organic oxidant - a reaction for which both DZ-1 and TS-1 are inactive.

What makes zeolitic imidazolate frameworks hydrophobic or hydrophilic? The impact of geometry and functionalization on water adsorption

We demonstrate, by means of Grand Canonical Monte Carlo simulation on different members of the ZIF family, how topology, geometry, and linker functionalization drastically affect the water adsorption properties of these materials, tweaking the ZIF materials from hydrophobic to hydrophilic. We show that adequate functionalization of the linkers allows one to tune the host–guest interactions, even featuring dual amphiphilic materials whose pore space features both hydrophobic and hydrophilic regions. Starting from an initially hydrophobic material (ZIF-8), various degrees of hydrophilicity could be obtained, with a gradual evolution from a type V adsorption isotherm in the liquid phase to a type I isotherm in the gas phase. This behavior is similar to what was described earlier in families of hydrophobic all-silica zeolites, with hydrophilic “defects” of various strength, such as silanol nests or the presence of extra-framework cations.

Quasi All‐Silica Zeolite Obtained by Isomorphous Degermanation of an As‐Made Germanium‐Containing Precursor

AbstractGe‐containing ITQ‐22 zeolites have been almost completely degermanated under strong acidic conditions without modifications of the framework topology. Simultaneous to Ge extraction, the framework was partially dissolved; mesopores were formed but the structure was maintained through the re‐incorporation of some of silicon species at vacant sites. The presence of many defects in the degermanated framework enabled the incorporation of tetrahedral aluminum, opening the way to the preparation of new and stable acid catalysts with original topologies.

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 n-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-Jo...

Modeling CO2 Adsorption in Zeolites Using DFT-Derived Charges: Comparing System-Specific and Generic Models

For most guest molecules, electrostatic interactions have a non-negligible impact on the adsorption properties of microporous materials, such as zeolites or metal–organic frameworks. In force-field-based simulations of adsorption, partial charges located at atomic sites are most commonly used to account for electrostatics. These charges are either derived empirically or obtained from electronic structure calculations. In previous work addressing the adsorption of CO2 in all-silica zeolites, we have used a first-principles approach to derive system-specific charges from density functional theory (DFT) calculations. While this approach has been shown to perform very well, it has the drawback that it requires a separate DFT calculation for every system. In this work, we develop a set of generic charges that reproduces interaction energies from dispersion-corrected DFT calculations equally well as the initial, system-specific set. The performance of this set of charges is then assessed using grand-canonical Monte Carlo simulations of CO2 adsorption and CO2/N2 mixture adsorption for a total of 24 zeolite frameworks. The results are compared to analogous simulations using the system-specific charges. While the qualitative features are reproduced very well, the quantitative deviations are often non-negligible, amounting to more than 20% in a number of cases. Therefore, the generic charge set can be recommended for screening studies, but system-specific charges should be employed for more detailed investigations.

On the synthesis and characterization of all-silica CHA zeolite particles

Mono-dispersed anisometric zeolite particles are desirable for forming a uniform, oriented layer. The epitaxial growth of this uniform layer is expected to provide an easier method for manufacturing continuous, oriented zeolite films than other approaches, including in situ methods. However, the inhomogeneous environment of the solid-like precursor required for the synthesis of all-silica CHA (Si-CHA) zeolites impedes the formation of mono-dispersed Si-CHA particles; instead, the precursor generally produces nearly cubic Si-CHA particles with a wide size distribution (∼1–10μm). Accordingly, elucidation of the effects of synthetic conditions on size distribution, morphology, and crystal-phase formation or transition of Si-CHA zeolites is necessary to achieve desired Si-CHA particles. In this study, the reaction temperature, molar composition, and synthesis time were systematically varied to investigate their effects on the resultant Si-CHA particles. Along with the nearly cubic Si-CHA particles, plate-like Si-CHA particles, which are promising for forming a uniformly oriented layer, were found to be synthesized as a minor product. The H2O/SiO2 molar ratio, which was less than ∼3 in the synthesis precursor, was key to the co-synthesis of plate-like Si-CHA particles. The plate-like Si-CHA particles could be observed by SEM and indirectly confirmed via selective deposition. Thermogravimetric analysis effectively recognized the minor presence of the plate-like Si-CHA particles presumably due to the shorter pathways for template removal during calcination. Though considerably minor, EAB-like particles, which were possibly generated due to a fault in stacking sequence, were discovered in addition to the above-mentioned nearly cubic and plate-like Si-CHA particles.

A computational study of the adsorption of n-perfluorohexane in zeolite BCR-704

Monte Carlo simulations in the grand canonical and Gibbs ensembles were carried out to predict the adsorption isotherms of argon, nitrogen, and n-perfluorohexane in BCR-704, a faujasite-type calcium aluminosilicate with a Si/Al ratio of 1.6. Starting from existing force fields for the sorbate molecules and a transferable force field for all-silica zeolites, attempts were made to develop a force field that would reproduce the experimentally determined argon and nitrogen adsorption isotherms in BCR-704. However, good agreement for the nitrogen adsorption isotherm could only be obtained either for a set of force field parameters with a greatly reduced partial charge on the calcium cation or using a larger partial charge and assuming that co-adsorbed water molecules are present at the temperature used for the argon and nitrogen adsorption isotherms. Predictions of the adsorption isotherm for n-perfluorohexane in BCR-704 using the latter force field parameters and the FAU-X structure with a water/calcium ratio of 1 yield good agreement with the experimental benchmark data.

Modeling Adsorption Properties on the Basis of Microscopic, Molecular, and Structural Descriptors for Nonpolar Adsorbents

We propose a method for analytically predicting single-component adsorption isotherms from molecular, microscopic and structural descriptors of the adsorbate-adsorbent system and concepts of statistical thermodynamics. Expressions for Henry's constant and the heat of adsorption at zero coverage are derived. These functions depend on the pore size, pore shape, chemical composition, and density of the adsorbent material. They quantify the strength of the solid-fluid interaction, which governs the low-pressure part of the adsorption isotherm. For intermediate and high pressures, the fluid-fluid interactions must also be taken into account. Both solid-fluid and fluid-fluid interactions are combined within the framework of the Ruthven statistical model (RSM). The RSM thus constructs theoretical adsorption isotherms that are entirely based on microscopic molecular and structural descriptors. The theoretical results that we obtained are compared with experimental data for the adsorption of pure CO2 and CH4 on all-silica zeolites. The developed methodology allows for the estimation of the optimum properties of a nonpolar adsorbent for the adsorption of CO2 in cyclic adsorption processes.

Insights on the Molecular Mechanisms of Hydrogen Adsorption in Zeolites

Physisorption on microporous materials has emerged as a promising strategy to store hydrogen for use as an energy carrier. Here, we investigate adsorption of hydrogen in all-silica zeolites ITQ-29 and MFI at low temperatures using molecular simulations. Out of five existing models for hydrogen and its interactions with the zeolite, we determine that a model with hydrogen as a single uncharged Lennard-Jones center best reproduces experimental adsorption isotherms. We present a new set of Lennard-Jones parameters for this model and find that small variations in the parameters have a large impact on computed hydrogen adsorption at 77 K. Preferential adsorption sites of hydrogen in the two zeolite structures are analyzed. We investigate the effect of incorporating quantum corrections via use of a Feynman–Hibbs effective interaction potential and determine that quantum corrections are important at 25 K.

Highly Selective Lewis Acid Sites in Desilicated MFI Zeolites for Dihydroxyacetone Isomerization to Lactic Acid

Desilication of commercial MFI-type (ZSM-5) zeolites in solutions of alkali metal hydroxides is demonstrated to generate highly selective heterogeneous catalysts for the aqueous-phase isomerization of biobased dihydroxyacetone (DHA) to lactic acid (LA). The best hierarchical ZSM-5 sample attains a LA selectivity exceeding 90 %, which is comparable to that of the state-of-the-art catalyst (i.e., the Sn-beta zeolite); this optimized hierarchical catalyst is recyclable over three runs. The Lewis acid sites, which are created through desilication along with the introduction of mesoporosity, are shown to play a crucial role in the formation of the desired product; these cannot be achieved by using other post-synthetic methods, such as steaming or impregnation of aluminum species. Desilication of other metallosilicates, such as Ga-MFI, also leads to high LA selectivity. In the presence of a soluble aluminum source, such as aluminum nitrate, alkaline-assisted alumination can introduce these unique Lewis acid centers in all-silica MFI zeolites. These findings highlight the potential of zeolites in the field of biomass-to-chemical conversion, and expand the applicability of desilication for the generation of selective catalytic centers.

Adsorption isotherms and ideal selectivities of hydrogen sulfide and carbon dioxide over methane for the Si-CHA zeolite: comparison of carbon dioxide and methane adsorption with the all-silica DD3R zeolite

Adsorption isotherms of H2S, CO2, and CH4 on the Si-CHA zeolite were measured over pressure range of 0–190 kPa and temperatures of 298, 323, and 348 K. Acid gases adsorption isotherms on this type of zeolite are reported for the first time. The isotherms follow a typical Type-I shape according to the Brunauer classification. Both Langmuir and Toth isotherms describe well the adsorption isotherms of methane and acid gases over the experimental conditions tested. At room temperature and pressure of 100 kPa, the amount of CO2 adsorption for Si-CHA zeolite is 29 % greater than that reported elsewhere (van den Bergh et al. J Mem Sci 316:35–45 (2008); Surf Sci Catal 170:1021–1027 (2007)) for the pure silica DD3R zeolite while the amounts of CH4 adsorption are reasonably the same. Si-CHA zeolite showed high ideal selectivities for acid gases over methane at 100 kPa (6.15 for H2S and 4.06 for CO2 at 298 K). Furthermore, H2S adsorption mechanism was found to be physical, and hence, Si-CHA can be utilized in pressure swing adsorption processes. Due to higher amount of carbon dioxide adsorbed and lower heats of adsorption as well as three dimensional channels of Si-CHA pore structure, this zeolite can remove acid gases from methane in a kinetic based process such as zeolite membrane.

Synthesis of all-silica zeolites from highly concentrated gels containing hexamethonium cations

A pure and highly crystalline all-silica EU-1 zeolite has been obtained from the crystallization of gels containing very low water contents in the presence of hexamethonium cations. Decreasing the water content in the gel down to H2O/Si<1 inhibited the formation of ZSM-48, which is usually observed under more diluted standard crystallization conditions. Moreover, addition of NH4F to the synthesis led to the formation of “half-fluorinated” ITQ-13 in which fluoride anions occupied only the center of D4R cages. In larger cages, the charge of the template was compensated by framework connectivity defects, clearly demonstrating once more the essential role of F− in the formation of D4R units. The formation of such hybrid (F,OH) is particularly interesting from a synthesis point of view, particularly for understanding the respective roles of fluoride and hydroxide anions in the crystallization process.

Molecular-Dynamics Studies of the Diffusion of H&lt;sub&gt;2&lt;/sub&gt; in All-Silica ZSM-5

Molecular dynamics (MD) techniques were employed to simulate the diffusion properties of molecular hydrogen on all-silica ZSM-5 zeolite. The results indicated that in the temperature range of 77-293K and the pressure range of 14-3360 kPa, the diffusion coefficients are found to range from 1.2×10-9m2/s to 3.8×10-7m2/s, and increase with increasing temperature, and decrease with increasing pressure. The activated energy for hydrogen diffusion determined from the simulation is pressure-dependent. The diffusion of the molecular hydrogen in all-silica ZSM-5 zeolite channels is anisotropic duo to the increase of the temperature. The diffusion coefficients in straight channels are higher than that in zigzag channels. The difference results from the structural difference of the two channels and the different loadings of hydrogen. Keywords: Hydrogen; Zeolite; Diffusion; Molecular-dynamics simulation

Adsorption and diffusion in zeolites: the pitfall of isotypic crystal structures

The influence of isotypic crystal structures on adsorption and diffusion of methane in all-silica LTA, SAS and ITE zeolites is studied. Results obtained with the experimental structures are compared with structure predictions and approximations that are commonly employed. The results indicate that diffusion coefficients are much more affected than Henry coefficients. In fact, orders of magnitude deviations in the diffusivity can be observed and a systematic parameter study finally gives rise to the correlation between structure sensitivity and diffusion-window size.

ChemInform Abstract: Nonlinear Optical Zeolite Films for Second and Third Harmonic Generation

AbstractReview: ≈60 refs.

Nonlinear Optical Zeolite Films for Second and Third Harmonic Generation

Methods to prepare novel second-order nonlinear optical (2O-NLO) materials composed of all-silica zeolite (silicalite-1) and a series of 2O-NLO molecules having high second order hyperpolarizability constants ( values) are reviewed. These methods include the development of novel methods to incorporate a series of hemicyanine (HC) molecules into the channels of silicaite-1 films in uniform orientations. The first method is to incorporate HC molecules tethered with long alkyl chains (octadecyl or longer) into the silicalite-1 channels with the long alkyl chain side first through the hydrophobic-hydrophobic interaction between the long alky chains and the silicalite-1 channels. The second method is to incorporate the HC molecule tethered with a medium length alkyl chain (nonyl) into the silicalite-1 channels with the medium length alkyl chain side first through hydrophobic-hydrophobic interaction between the medium length alky chain in the photoexcited state and the silicalite-1 channels. The third method is to incorporate the HC molecule tethered with propionic acid into the silicalite-1 channels with the propionic acid side last mediated by a tetrabultylammonium cation ion-paired to the propionate unit. A method to prepare a novel third-order nonlinear optical (3O-NLO) material composed of zeolite-Y and PbS or PbSe quantum dots is also reviewed. This Account thus describes a promising new direction to which the search for highly sensitive 2O-NLO and 3O-NLO materials has to be conducted and a new direction to which zeolite research and applications have to be expanded.

Solvothermal Synthesis and Characterization of Large-Crystal All-Silica, Aluminum-, and Boron-Containing Ferrierite Zeolites

Large crystals of all-silica, aluminum-, and boron-containing ferrierite zeolites were synthesized using a solvothermal synthesis method and a fluoride medium. Alkylamines, namely, propylamine, butylamine, and pentylamine were employed as templates in the syntheses. The products were characterized by powder X-ray diffraction (XRD), inductively coupled plasma-optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), elemental (CHN) analyses, scanning electron microscopy (SEM), N2-adsorption, and 1H, 11B, 27Al, as well as 29Si MAS NMR spectroscopy. The 27Al and 11B MAS NMR spectroscopic investigations revealed that aluminum and boron were successfully incorporated into the FER-type framework. SEM indicated the occurrence of crystals with a size of up to about 600 μm. Furthermore, for all-silica and boron-containing ferrierite zeolites, the shape, size, and thickness of the crystals could be controlled by increasing the chain length of the alkyl group in the alkylamine and the crystallizatio...

On the nucleation and crystallization of silicalite-1 from a dilute clear sol

Nucleation and crystallization of silicalite-1, the all-silica zeolite with MFI framework, is typically studied using the tetrapropylammonium (TPA)-silica–water system. Here, TPA-silica sols were studied by a combination of small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and cryogenic TEM (cryo-TEM). silicalite-1 crystals, as small as 20 nm, were successfully imaged by employing high-resolution cryo-TEM. The results are consistent with the hypothesis that non-crystalline aggregates form prior to silicalite-1 crystallization, and they indicate that at least some of the aggregates contain crystalline fragments, which is suggestive of zeolite nucleation within the aggregates. It was also determined that the typical TPA-silicalite-1 crystal morphology develops when nanoparticle concentration is substantially reduced suggesting that solution mediated processes rather than nanoparticle attachment are responsible for the development of the typical faceted crystals. It was also confirmed that silicalite-1 crystal morphology as well as nucleation and crystallization kinetics are strongly dependent on the structure directing agents present. In experiments using a dimer of TPA (Bis-1,6-(tripropyalammonium)hexamethylene dihydroxide), the time at which the first evidence of crystallinity was observed and the crystal growth rate were substantially delayed. However, the sequence of events observed follow that of TPA-silicalite-1.