Adsorption and Diffusion of Hydrocarbons

Smit B, Loyens L D J C and Verbist G L M M 1997 Simulation of adsorption and diffusion of hydrocarbons in zeoWtes Faraday Disc. Ohem. See. 106 93-104... 

Malka-Edery, A., Abdullah, K., Grenier Ph., and Meunier F., Influence of traces of water on adsorption and diffusion of hydrocarbons in NaX zeolite, Adsorption, 1, 17-25, 2001. 

Ab Initio force field CFF91-CZEO, PCFF Force field for zeolite frameworks 25, 26, 29 31, 32 Adsorption and diffusion of hydrocarbons in zeolites Structure and spectra of silica and aluminum-phosphates 30... 

Atom-atom pair potentials for zeolite-molecule interactions 20 Monte Carlo and molecular dynamics simulation of adsorption and diffusion of hydrocarbons and rare gases in zeolites 34-37... 

The concept of shape selectivity relies on the control of the adsorption and diffusion of the molecule in the pore of the catalytic material. Linear paraffins have the smallest critical diameter among hydrocarbons and can therefore penetrate smaller pore structures. In the figure, for example, the n-heptane molecule penetrates the tubular... 

Lin, D., Ducarme, V., and Vedrine, J.C. Adsorption and diffusion of different hydrocarbon in MFI zeolite of varying crystallite sizes. in Zeolites as Catalysts, Sorbents and Detergent Builders (H.G. Karge and J. Weitkamp, Eds.), Elsevier, Amsterdam, p. 615, 1989. 

Using the accumulated data obtained from in-situ and high resolution electron microscopy studies, Baker and co-workers (ref. 22) developed a model to account for the growth of filamentous carbon resulting from metal catalyzed decomposition of selected hydrocarbons. The main steps in the mechanism are adsorption and decomposition of hydrocarbons at the leading face of catalyst particle, followed by dissolution of carbon in the metal and diffusion to the trailing faces where carbon is precipitated from solution to form the filament. Deactivation of the catalyst occurs... 

Silicalite is a microporous crystalline silica molecular sieve with remarkable hydrophobic properties ( 1) and has been considered to offer practical applications in the clean-up of water contaminated with hydrocarbons and the separation of ethanol from dilute fermentation aqueous solutions (2 ii> 2) Many studies have been reported on the properties of adsorption and diffusion of gases in silicalite (e.g., 6, 8, , HI) However, despite the many potential applica-... 

The current work indicates the strong effect of acid sites on the interaction and diffusivity of hydrocarbons. To further study this effect, we determined the single-component diffusion coefficients and specifically the activation energy for diffusion. Activated diffusion is described by the Arrhenius-type Eq. 8. The pre-exponential factor Djnf is related to the jump frequency between adsorption sites in the zeolite lattice, while the exponential expresses the chance that the molecules are able to overcome the free energy barrier - act between these sites. The loadings of n-hexane and 2-methylpentane in H-ZSM-5 and silicalite-1 have been measured at temperatures between 373 and 533 K at intervals of 20 K. The hydrocarbon pressure was taken identical... 

Song, L.J., Sun, Zi., and Rees, L.V.C., Experimental and molecular simulation studies of adsorption and diffusion of cyclic hydrocarbons in silicalite-1, Microp. Mesop. Mater., 55, 21 49, 2002. 

To diminish methanol crossover, PEMs have been researched for decreasing three stages of methanol crossover (adsorption, diffusion, and desorption). Rigidity and lower swelling of hydrocarbon-based polymers can induce minimization of adsorption and diffusion of methanol. Hydrocarbon-based PEM is needed for optimization in mobility and stiffness of ionic groups. Control of ionic and non-ionic phases also can be developed for satisfactory ionic conductivity and methanol crossover at the same time. A smaller number of ionic groups reduce methanol adsorption to PEM. Distribution of ionic sites in PEM affects the morphology... 

High sensitivity, fast response, and well-defined flow patterns make the TEOM an excellent tool for determining diffusivities of hydrocarbons in zeolites. Moreover, the TEOM has provided a unique capability for gaining knowledge about the effects of coke deposition on adsorption and diffusion under catalytic reaction conditions. An application of the TEOM in zeolite catalysis by combining several approaches mentioned above can lead to a much more detailed understanding of the catalytic processes, including the mechanisms of reaction, coke formation, and deactivation. 

A theoretical understanding of the diffusion of hydrocarbons through the porous catalyst layer (see Fig. 2.45) may be obtained by simulations using semi-classical molecular dynamics (as in Fig. 2.3). Such calculations have been performed for the penetration of various hydrocarbons through AljOj catalysts with and without Pt insertions (Szczygiel and Szyja, 2004). As indicated in Fig. 2.46, it is found that fuel transport depends on both cavity structure and the adsorption on internal catalyst walls. 

We now report how theoretical methods can be used to provide information on the adsorption, diffusion, and reactivity of hydrocarbons within acidic zeolite catalysts. In Section A, dealing with adsorption, the physical chemistry of molecules adsorbed in zeolites is reviewed. Furthermore, in this section the results of hydrocarbon diffusion as these data are obtained from the use of the same theoretical methods are described. In Section B we summarize the capability of the quantum-chemical approaches. In this section, the contribution of the theoretical approaches to the understanding of physical chemistry of zeolite catalysis is reported. Finally, in Section C, using this information, we study the kinetics of a reaction catalyzed by acidic zeolite. This last section also illustrates the gaps that persist in the theoretical approaches to allow the investigation of a full catalytic cycle. 

The carbon-deposition process of EUV optics has been described to involve adsorption, diffusion, and dissociation of hydrocarbons at the surface of the... 

Abstract Neutron scattering was first used to derive the self-diffusivities of hydrocarbons in zeolites, but transport diffusivities of deuterated molecules and of molecules which do not contain hydrogen atoms can now be measured. The technique allows one to probe diffusion over space scales ranging from a few A to hundreds of A. The mechanism of diffusion can, thus, be followed from the elementary jumps between adsorption sites to Lickian diffusion. The neutron spin-echo technique pushes down the lower limit of diffusion coefficients, traditionally accessible by neutron methods, by two orders of magnitude. The neutron scattering results indicate that the corrected diffusivity is rarely constant and that it follows neither the Darken approximation nor the lattice gas model. The clear minimum and maximum in diffusivity observed by neutron spin-echo for n-alkanes in 5A zeolite is reminiscent of the controversial window effect . 

E. J. M. Hensen, A. M. de Jong, and R. A. van Santen have written Chapter 7, which introduces the tracer exchange positron emission profiling (TEX-PEP) as an attractive technique for in-situ investigations, for example, in a stainless steel reactor, of the adsorption and diffusive properties of hydrocarbons in zeolites under chemical steady-state conditions. Self-diffusion coefficients of hydrocarbons, labeled by proton-emitting C at finite loadings and even in the presence of another imlabeled alkane, may be extracted. The method is illustrated by adsorption and diffusion measurements of linear (n-hexane) and branched (2-methylpentane) alkanes in Fl-ZSM-5 and silicalite-1. 

The ability of microporous solids to act as high-capacity molecular sieves has long been exploited in a wide range of applications in adsorption and separation. The electrostatic interactions of the traditional cationic forms of aluminosilicates are well suited for the uptake of polar molecules (such as H2O) and are also able to separate oxygen from air. The development of microporous solids with varied chemistry has enabled adsorption and diffusion properties to be finely tuned for particular technologies. Pure silica zeolite polymorphs such as silicalite have particular importance, because they enable separation on the basis of a different range of polarity and on molecular size the absence of aluminium in the framework also prevents the presence of unwanted acidity, so adsorbed hydrocarbons do not undergo any catalytic transformation. 

Here, we present the application of these techniques to the diffusion of hydrocarbons (benzene, n-hexane, paraxylene), pure and mixed, in a fixed bed of HZSM-5 zeolite during their adsorption at room temperature. 

Choudhary, V.R. Akolekar, D.B.. and Singh. A.P.. Single- and multicomponent sorption/diffusion of hydrocarbons from their iso-oclane solution in H-ZMS-5 zeolite. Chem. Eng. Sci.. 44(5). 1047-1060 (1989). Davini, P., Adsorption of sulphur dioxide on thermally treated active carbon. Fuel. 68(2), 145-148 (1989). Frimpong, S. Plank, C.A., and Laukhuf, W.L.S.. Evaluation of sorption and transport of sulfur dioxide in polycarbonate, Chem. Eng. J.. 42( I). 25-36 (1989)... 

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