MOF-Supported Selective Ethylene Dimerization Single-Site Catalysts through One-Pot Postsynthetic Modification

The one-pot postfunctionalization allows anchoring a molecular nickel complex into a mesoporous metal–organic framework (Ni@(Fe)­MIL-101). It is generating a very active and reusable catalyst for the liquid-phase ethylene dimerization to selectively form 1-butene. Higher selectivity for 1-butene is found using the Ni@(Fe)­MIL-101 catalyst than reported for molecular nickel diimino complexes

Solubility of Gases in Water Confined in Nanoporous Materials: ZSM-5, MCM-41, and MIL-100

Oversolubility, which corresponds to large apparent gas solubilities in liquids confined in nanoporous solids, has been proposed as a means to develop novel adsorption, phase separation, or catalytic processes. We report a molecular simulation study to help design such hybrid adsorbents consisting of a solvent confined in host nanoporous materials. Water is selected as the confined solvent because of its ubiquity in industrial applications, while N₂, CH₄, and CO₂ are selected as they allow probing the effect of specific molecular interaction and polarity. For each system, we consider a zeolite, an ordered mesoporous silica, and a metal–organic framework as the confining host as they present different morphologies, porosities, and surface chemistries. We show that oversolubility is always observed because the apparent solubility in these materials surpasses the bulk solubility. In all cases, oversolubility is an enhanced bulklike solubility in which solubility is favored in the regions of low water density formed by the layering of the solvent. Such an oversolubility mechanism, which arises from the fact that the gas–solid interactions are weaker than the solvent–solid interactions, leads to large uptakes as high as a few hundred times that expected from bulk solubility

Gas Uptake in Solvents Confined in Mesopores: Adsorption versus Enhanced Solubility

Three molecular mechanisms for gas uptake in a solvent confined in mesopores are identified. On the one hand, CO₂ uptake is an adsorption-driven phenomenon that arises from the strong interaction between the gas molecules and the pore surface. On the other hand, H₂ uptake is a confinement-induced enhanced solubility in which solubility is favored in the regions of low solvent density formed by the layering of the solvent. In partially filled pores, adsorption at the gas/liquid solvent interface is a third mechanism that leads to large gas uptakes. This study, which sheds light on previously reported yet unclear oversolubility in pores, provides a guide to design hybrid porous catalysts consisting of a solvent confined in a porous solid

Metal/Acid Bifunctional Catalysis and Intimacy Criterion for Ethylcyclohexane Hydroconversion: When Proximity Does Not Matter

The apparent kinetics in metal/acid bifunctional catalysis is generally strongly affected by the metal to acid site ratio and their proximity. However, these two key parameters have not been systematically investigated in the scientific literature. Such a study is provided here for bifunctional catalysts using platinum as the metallic function and EU-1 zeolite as the acidic function. Two series of bifunctional catalysts with different metal to acid sites ratios and different metal to acid site distances were prepared and tested in ethylcyclohexane hydroconversion. By increasing the metal to acid sites ratio, the catalytic activity and isomerization selectivity increased until a plateau was reached, an observation that is in agreement with the classical bifunctional mechanism. At the same time, the intimacy criterion of Weisz was evaluated: strikingly, for a given metal to acid sites ratio, activities and selectivities are not affected by their distance (up to a micrometer scale). A dual-function kinetic model was successfully applied in order to quantify the effect of the metal to acid site ratio on the catalyst activity and isomerization properties. The application of this model showed that the metal to acid sites ratio needed to reach the catalytic activity plateau is higher than the ratio needed to reach the selectivity plateau. This was interpreted as a consequence of the lower kinetic rate constant for the naphthene ring-opening reaction in comparison to the naphthene isomerization reaction

Diffusion-Driven Selectivity in Oxidation of CO in the Presence of Propylene Using Zeolite Nano Shell as Membrane

The selective oxidation of CO over C₃H₆ is achieved in yolk-shell Pt@Silicalite-1 catalysts in which Pt nanoparticles are encapsulated in hollow silicalite-1 single crystals. The thin shell operates as a permselective membrane which limits Pt surface poisoning by C₃H₆. From adsorption measurements, we conclude that the catalytic selectivity arises from the fastest diffusion of CO over C₃H₆ through the silicalite-1 membrane

Role of Silver Nanoparticles in Enhanced Xenon Adsorption Using Silver-Loaded Zeolites

Molecular simulation is used to unravel the adsorption mechanisms of xenon on Ag-doped ZSM-5 zeolite. We show that silver nanoparticles, which form at the external surface of zeolite crystallites, are responsible for enhanced xenon physisorption at very low pressure. We also propose a simple model of adsorption on such composite materials made up of silver-exchanged zeolites and silver nanoparticles adsorbed at the zeolite surface. This model, which allows predicting the adsorption of other gases without any additional parameters, provides a tool to characterize the amount of reduced silver as well as the silver particle size distribution (in good agreement with transmission electron microscopy images). The presence of a majority of silver nanoparticles is further characterized by means of X-ray diffraction and X-ray Absorption Spectroscopy at the silver K edge

Xenon Capture on Silver-Loaded Zeolites: Characterization of Very Strong Adsorption Sites

The number and strength of adsorption sites for Xe in silver-modified zeolites are estimated from isotherm measurements at various temperatures over a broad range of pressure (from 1 ppm to atmospheric pressure). Fully and partially exchanged silver zeolites were synthesized starting from Na-ZSM-5(25), Na-ZSM-5(40), Na-Beta, NaX, and NaY. We have discovered that silver-modified zeolites may present one or two distinct adsorption sites depending on the nature of the material and silver loadings. The strongest adsorption sites are characterized by isosteric heat of adsorption in the order of −40 to −50 kJ·mol^–1. For Pentasil-type zeolites, we observe a linear 2:1 correlation between the total amount of silver and the number of strong sites. The highest concentration of strong sites is found for fully silver exchanged ZSM-5 (5.7 × 10^–4 mol/g), which presents the largest silver content for Pentasil-type zeolite. The equilibrium constant of Ag-ZSM-5 at low pressure is about 50 times larger than that of AgX. Qualitative correlations were established between Xe adsorption isotherms and Xe NMR signals. We show that Xe NMR could be used as a quantitative method for the characterization of the strength and of the number of strong Xe adsorption sites on silver-exchanged zeolites. The numbers of strong adsorption sites responsible for the Xe adsorption at 10–1000 ppm can be determined by the length of the plateau observed at low Xe uptake. In practice, our findings give guidelines for the discovery and optimization of silver-loaded zeolites for the capture of Xe at ppm levels. It appears that the amount of silver is a key parameter. Silver-modified ZSM-5 shows adsorption capacities 2–3 orders of magnitude larger than currently applied adsorbents for atmospheric Xe capture