An approach to the surface barrier concept in diffusion in zeolites by computer simulation

́-Maeder, F. Vigne; Amrani, Samir; Ge´lin, P.

doi:10.1016/0021-9517(92)90340-n

Cited by 24 publications

(16 citation statements)

References 7 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…10 There have been several molecular dynamics (MD) studies of the pore entrance problem also. Vigné-Maeder et al 11 used MD to examine the trajectories of argon and xenon atoms passing through the outer surface of MFI-and MOR-type zeolite crystals. They observed that the flux of the larger molecule, xenon, was significantly retarded relative to that of argon.…”

Section: Introductionmentioning

confidence: 99%

Effect of the Surface Energy Barrier on Sorbate Diffusion in AlPO₄-5

2001

View full text Add to dashboard Cite

The importance of pore exit effects on the diffusion of molecules in AlPO 4 -5 pores is evaluated using two molecular modeling techniques. In the first approach, a dual control volume grand canonical molecular dynamics technique is used to obtain molecular fluxes of methane out of the truncated crystal as a function of temperature and sorbate loading. The simulation results indicate the presence of a low-temperature surface barrier for diffusion, which retards the flux of methane relative to its apparent flux in the intracrystalline regions of the material. This pore exit barrier tends to diminish as temperature and loading increase. An explanation based on clustering phenomena is proposed to explain the latter. Next, a simple activated transport model is proposed to predict the relative importance of the surface barrier on the transport of sorbates in AlPO 4 -5. The potential of mean force for a single sorbate molecule along the pore axis of a truncated crystal provides the required activation energy barriers for the model. The model correctly predicts the reduction in the importance of exit effects with an increase in the temperature. It is also observed that exit effects become more important as the ratio of the size of the sorbate molecule to the pore size approaches unity. In particular, exit effects are significant in micrometer-thick AlPO 4 -5 crystals in the case of large molecules such as SnBr 4 and CCl 4 at room temperature.

show abstract

Section: Introductionmentioning

confidence: 99%

Effect of the Surface Energy Barrier on Sorbate Diffusion in AlPO₄-5

2001

View full text Add to dashboard Cite

show abstract

“…These geometric distortions become more prevalent as alkoxides become larger and more substituted, thus accounting for the unfavorable formation of bulkier alkoxides on TON relative to HPW measured experimentally. These crystalline framework distortions are likely to become more significant on nanometer-sized crystals and in near-surface regions, plausibly giving rise to observed diffusion and catalytic phenomena typically denoted as surface barriers (20)(21)(22) and pore mouth catalysis (23)(24)(25), respectively.…”

Section: Significancementioning

confidence: 99%

Stability of bound species during alkene reactions on solid acids

Sarazen

Iglesia

2017

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

This study reports the thermodynamics of bound species derived from ethene, propene, n-butene, and isobutene on solid acids with diverse strength and confining voids. Density functional theory (DFT) and kinetic data indicate that covalently bound alkoxides form C-C bonds in the kinetically relevant step for dimerization turnovers on protons within TON (0.57 nm) and MOR (0.67 nm) zeolitic channels and on stronger acids HPW (polyoxometalate clusters on silica). Turnover rates for mixed alkenes give relative alkoxide stabilities; the respective adsorption constants are obtained from in situ infrared spectra. Tertiary alkoxides (from isobutene) within larger voids (MOR, HPW) are more stable than less substituted isomers but are destabilized within smaller concave environments (TON) because framework distortions are required to avoid steric repulsion. Adsorption constants are similar on MOR and HPW for each alkoxide, indicating that binding is insensitive to acid strength for covalently bound species. DFT-derived formation free energies for alkoxides with different framework attachments and backbone length/structure agree with measurements when dispersion forces, which mediate stabilization by confinement in host-guest systems, are considered. Theory reveals previously unrecognized framework distortions that balance the C-O bond lengths required for covalency with host-guest distances that maximize van der Waals contacts. These distortions, reported here as changes in O-atom locations and dihedral angles, become stronger for larger, more substituted alkoxides. The thermodynamic properties reported here for alkoxides and acid hosts differing in size and conjugate-anion stability are benchmarked against DFT-derived free energies; their details are essential to design hostguest pairs that direct alkoxide species toward specific products.zeolites | alkene adsorption | heterogeneous catalysis | density functional theory H ydrocarbon reactions on solid Brønsted acids often involve bound alkene-derived intermediates. Their reactivity and selectivity in C-C bond formation and cleavage and in hydrogen transfer reactions depend on their thermodynamic properties, which reflect, in turn, the properties of the conjugate anion and of the confining voids in microporous solid acids. Weakly bound adsorbed alkanes interact with void walls via dispersion forces at low temperatures without molecular transformations; their unreactive nature allows accurate assessments of their binding properties (1-4). Alkenes interact with protons via H bonding or π-interactions, as well as by proton transfer from strong acids to form alkoxides; the intermediate between these states is a positively charged carbenium ion. The thermodynamics of alkoxide formation remain uncertain because of their very fast oligomerization and β-scission reactions (5, 6), thus requiring indirect inferences from kinetic, spectroscopic, or theoretical methods. This uncertainty is particularly prevalent in isobutene-derived species, for which steric effects may disfavor the f...

show abstract

“…This is whether there is local equilibrium at the particle boundary between adsorbed molecules and the molecules in the vapour phase at the edge of the particle. This assumption is frequently made by researchers in the ÿeld, while others suggest that there may be surface barrier e ects whereby molecules in the vapour or adsorbed on the external surface have to overcome an activation barrier to become adsorbed within the internal pore space (B ulow et al, 1980;VignÃ e-Maeder et al, 1992;Grenier et al, 1994Grenier et al, , 1995Chandross et al, 2001). For instance, uptake measurements of methanol in zeolite NaX are a ected by an apparent surface resistance to adsorption that changes during successive measurements; this has been attributed to a gradual build-up of coke on the crystal surface (Grenier et al, 1994(Grenier et al, , 1995.…”

Section: Transport Of Molecules At the Particle Boundary Between Adsomentioning

confidence: 99%