Search results for "ZEOLITE"

Enhancement of D 2 /H 2 Selectivity in Zeolite A through Partial Na–K Exchange: Single-Gas and Coadsorption Studies at 45–77 K

2020

International audience; We report D2/H2 adsorption selectivities under cryogenic temperatures (45-77 K) in Na and K-Na exchanged zeolites A (LTA structural code) measured by co-adsorption technique. These values are systematically compared with Ideal Adsorbed Solution Theory (IAST) predictions based on the single gas adsorption isotherms. For NaA zeolite the evolution of the selectivity as a function of total pressure and gas mixture composition at 77 K shows ideal behavior. In contrast, as temperature decreases, D2/H2 selectivity rises exponentially and its values can no longer be predicted by IAST. We found that in these conditions the evolution of the selectivity can be described by a si…

Identification of New Templates for the Synthesis of BEA, BEC, and ISV Zeolites Using Molecular Topology and Monte Carlo Techniques

2020

The presence of organic structure directing agents (templates) in the synthesis of zeolites allows the synthesis to be directed, in many cases, toward structures in which there is a large stabilization between the template and the zeolite micropore due to dispersion interactions. Although other factors are also important (temperature, pH, Si/Al ratio, etc.), systems with strong zeolite-template interactions are good candidates for an application of new computational algorithms, for instance those based in molecular topology (MT), that can be used in combination with large databases of organic molecules. Computational design of new templates allows the synthesis of existing and new zeolites …

Isomerization and disproportionation of m-xylene in a zeolite with 9- and 10-membered ring pores: Molecular dynamics and catalytic studies

2006

The unique pore topology of zeolite ITQ-13 with 9-membered ring (MR) channels intersected by perpendicular 10-MR channels and larger void space with 10-MR cross-sections was studied for meta-xylene isomerization and disproportionation. Product distribution was interpreted on the basis of pore topology and compared with zeolites with 12-MR (β), 10-MR, and intersecting 12-MR cavities (NU-87), as well as 10-MR (ZSM-5) zeolites. The presence of cavities allows more space for bulky intermediates and/or products and also provides room for molecules to drive consecutive reactions toward thermodynamic equilibrium. Channels, on the other hand, allow diffusion without trapping if their free diameters…

Decalin and Tetralin as Probe Molecules for Cracking and Hydrotreating the Light Cycle Oil

2001

Abstract Cracking of tetralin and decalin was carried out over several zeolites to establish the effect of the pore topology of the catalyst on product distribution. These molecules were chosen as probe molecules, because they indicate which catalyst is the best for cracking or hydrotreating the light cycle oil (LCO) fraction, which is obtained directly from fluid catalytic cracking units. A set of zeolites with medium-sized (ZSM-5, MCM-22, ITQ-2), large (USY, Beta), and ultralarge pores (UTD-1), as well as a mesoporous MCM-41, were used as catalysts at 723 K. The results demonstrate that pore size and topology have a strong influence on diffusion, and consequently, on activity and selectiv…

Synthesis and Structure Determination of the Hierarchical Meso-Microporous Zeolite ITQ-43

2011

[EN] The formation of mesopores in microporous zeolites is generally performed by postsynthesis acid, basic, and steam treatments. The hierarchical pore systems thus formed allow better adsorption, diffusion, and reactivity of these materials. By combining organic and inorganic structure-directing agents and high-throughput methodologies, we were able to synthesize a zeolite with a hierarchical system of micropores and mesopores, with channel openings delimited by 28 tetrahedral atoms. Its complex crystalline structure was solved with the use of automated diffraction tomography.

Recovering ammonium by treated and untreated zeolitic mixtures: A comprehensive experimental and modelling study

2023

The recovery of ammonium (NH4+) from aqueous solutions by zeolite is attractive. In this study, the physicalchemistry of NH4+ adsorption process from aqueous solution by two zeolitic mixtures, either treated or not treated with NaCl, was assessed. Results suggested that the zeolitic mixture richer in mordenite and with high specific surface area adsorbed more NH4+ than the one richer in clinoptilolite and heulandite showing a lower specific surface area. NaCl treatment increased the amount of NH4+ adsorbed by the zeolitic mixtures. The higher amount of NH4+ adsorbed by the zeolitic mixtures treated with NaCl was explained by the low/high density water model accounting for cation exchange am…

Thermal Effects of Water Intrusion in Hydrophobic Nanoporous Materials.

2009

Liquid water intrusion in hydrophobic nanoporous silicalite-1, a pure siliceous zeolite, in isothermal conditions under high pressure produces an endothermic effect. After intrusion, confined water in zeolite pores is in a different state from that of the liquid bulk water. Such forced intrusion also chemically modifies the material and tends to render it slightly more hydrophilic.

Location of Water and Xylene Molecules Adsorbed on Prehydrated Zeolite BaX. A Low-Temperature Neutron Powder Diffraction Study

1999

PFG n.m.r. study of diffusion anisotropy in oriented ZSM-5 type zeolite crystallites

1991

ZSM-5 zeolite crystallites are oriented by introducing them into a system of parallel capillaries. In this way, by applying pulsed-field gradient (PFG) n.m.r., a direct measurement of the orientation dependence of diffusion in ZSM-5 crystals has become possible. Using methane as a diffusant, the ratio D xy /D z between the diffusivities in the xy plane and in the z direction has been found to be of the order of 4.5. This value is in satisfactory agreement with the behavior expected from both MD calculations and a random walk model of molecular propagation in the two-channel network of ZSM-5-type zeolites.

H−ZSM-5 Modified Zeolite:  Quantum Chemical Models of Acidic Sites

2007

A ZSM-5 fragment, containing 52 tetrahedral moieties, each of them formed by one silicon or one aluminum atom surrounded by four oxygen atoms, was employed to model (52T systems) by quantum chemical calculations (i) the influence of the positions of the acidic sites on the energetics of 22 aluminum monosubstituted and bisubstituted 52T acidic zeolite (H-ZSM-5) systems and (ii) the local adsorption properties and acidic strength of the corresponding -OH sites. The energetics and the structural properties of simpler acid H-ZSM-5 systems containing only five Tetrahedral moieties (5T systems) were also modeled for comparison. B3LYP/6-31G(d,p) partial geometry optimization routines were performe…