Intracrystalline Diffusion in Zeolites Studied by Neutron Scattering Techniques

Abstract

Résumé -Étude de la diffusion intracristalline dans les zéolithes par diffusion des neutrons -Les techniques de diffusion neutronique permettent d'étudier les différents mouvements des molécules adsorbées dans les zéolithes : vibrations, rotations et translation. En utilisant différents instruments, il est maintenant possible de mesurer des coefficients de diffusion dans la gamme 10 -7 -10 -14 m 2 s -1 . En outre, la diffusion incohérente, provenant de molécules hydrogénées, donne accès à l'auto-diffusion ; alors que la diffusion cohérente, en utilisant des molécules deutérées, permet d'obtenir le coefficient de diffusion de transport. Plusieurs applications des méthodes neutroniques sont décrites dans cette revue. Elles sont illustrées par différents exemples : xylènes dans des zéolithes de type X, alcanes dans des zéolithes de type MFI, n-alcanes dans la 5A, et eau dans des zéolithes de type A et X. Abstract -Intracrystalline Diffusion in Zeolites Studied by Neutron Scattering Techniques Synchrotron and Neutron Solutions to Oil Industry ProblemsUtilisation des grands instruments analytiques dans l'industrie pétrolière

Inverse Gas Chromatography Study of n-Alkane and 1-Alkene Adsorption on Pure-silica LTA (ITQ-29) and CHA

“…However, these kinetic experiments are disputed due to nonlinearity and nonisothermal artifacts. 67,68 Nevertheless, more recent experimental studies 58,60,68 using techniques as NMR show a nonmonotonous decrease of the diffusion coefficient. Beyond these kinetic "window" effects, nonmonotonic equilibrium effects have only been experimentally confirmed on a polar LTA zeolite (Si/Al 1.0), a polar CHA zeolite (Si/Al 2.5) and on the iso-structural SAPO-34 using n-alkanes, 59,69 but not using pure-silica variants, as typically employed in simulation studies.…”

“…Another remarkable phenomenon that has been studied using pure-silica zeolites, is the so-called “cage- and window effects” in small-pore zeolites. , In a series of detailed computational studies, Dubbeldam et al ,, elaborate on the existence of a nonmonotonic variation of the Henry and diffusion constants with carbon number for n -alkanes in CHA, ERI, and LTA frameworks. Generally, adsorption and diffusion properties of n -alkanes are monotonically chain-length dependent. − However, for small-pore zeolites in which the pore system consists of cages connected through narrow pores (sometimes called windows), an unusual and controversial nonmonotonic chain-length dependence behavior is reported. − Here, increasing the chain length forces the molecule to coil up, or, once the chain-length becomes too long, protrude the pore window into the adjacent cage. ,, This incommensurate fit between the adsorbate and adsorbent is proposed to lower their adsorption strength and increase their diffusion rate throughout the zeolite. The term “window effect” was presented by Gorring in the context of nonmonotonic diffusion coefficients, offering a “window of increased transmittance” at higher carbon numbers, also linked to catalytic performance.…”

“…These effects are mostly studied computationally on pure-silica zeolites using n -alkanes as probe molecules. ,, Gorring was the first to report on the existence of possible “window effects” in zeolites based on uptake experiments, focusing on diffusion coefficients. However, these kinetic experiments are disputed due to nonlinearity and nonisothermal artifacts. , Nevertheless, more recent experimental studies ,, using techniques as NMR show a nonmonotonous decrease of the diffusion coefficient. Beyond these kinetic “window” effects, nonmonotonic equilibrium effects have only been experimentally confirmed on a polar LTA zeolite (Si/Al 1.0), a polar CHA zeolite (Si/Al 2.5) and on the iso-structural SAPO-34 using n -alkanes, , but not using pure-silica variants, as typically employed in simulation studies.…”

Effect of Intracrystalline Silanol Defects on the Diffusivity of Benzene in Silicalite Zeolite

“…Jo ¨rg Ka ¨rger and Ju ¨rgen Caro presented a seminal paper which raised a decades-long debate about the large span of diffusivity values and types of diffusivities measured by the different techniques [40]. The study of complex systems can provide good insights into the mobility and transport phenomena of adsorbed molecules by application of these techniques [41]. Depending on the conditions of measurement provided by the host-guest system under study, the covered distances range from hundreds of nanometers to hundreds of micrometers.…”

Rotation dynamics of 2-methyl butane and n-pentane in MCM-22 zeolite: a molecular dynamics simulation study

“…The incoherent quasi-elastic neutron scattering technique is particularly suitable for investigating the self-dynamics of the hydrogen atoms in hydrocarbons adsorbed in zeolites because the incoherent scattering cross section of hydrogen is significantly larger than that of oxygen, silicon, aluminium and carbon. 10 The spectra of QENS are sensitive to both translational and rotational dynamics of hydrogen containing molecules. Using QENS, molecular diffusion from a few A ˚to several nm can be probed.…”