Mobility of adsorbed species in zeolites: methane, ethane, and propane diffusivities

Abstract

A from C3 on the ( 2 1 x 3 axis (Figure 4). The crystallographic analyses of parabanic acid and alloxan show that the C.-O-C angles corresponding to the closest intermolecular distances are 157.4 and 154.7O, re~pectively;~.~' the corresponding projected positions of the oxygens on the molecular planes of XI1 and XI11 are 1.06 A from C2 and C3 for the former and 1.19 A from C3 for the latter. Thus these oxygens, which are negative ( Figure 4), are near the most positive regions of the surface potentials, and the resulting electrostatic interactions can accordingly account for the short internuclear distances.Finally, it should be noted that the calculated atomic charges listed in Table I bear little or no relationship to the Vs,c values.For example, the dinitro derivative IX, which has the largest V S ,~ in the table, has only an average acyl carbon charge. Even more striking are the ordering and range of magnitudes of the acyl carbon charges in parabanic acid and alloxan, which do not show at all the same trends as do the surface potentials. In alloxan, the carbon with the largest Vs,c is C3, which has the lowest charge in all of Table I. These observations are consistent with the absence of a reliable correlation between calculated atomic charges and chemical r e a c t i~i t y ? *~~*~~ SummaryWe have investigated the susceptibilities toward nucleophilic attack, e.g. hydrolysis, of a group of cyclic ureides. ,We examined the surface electrostatic potentials of these molecules, focusing upon the regions above the acyl carbons, to determine the effects of various chemical and structural modifications, including the presence of NO2 and/or NF2 substituents. The relative hydrolytic stabilities of these systems have been predicted. For the polycarbonyl molecules parabanic acid and alloxan, the magnitudes and locations of the maxima in their surface potentials are fully consistent with observed unusually short intermolecular distances in their crystalline forms.Acknowledgment. We thank Dr. Jorge M. Seminario and Mrs.Monica Concha for computational assistance. We greatly appreciate the support of this work by the Office of Naval Research through Contract No. N00014-85-K-0217. Molecular dynamics simulations were performed in order to study the influence of the zeolite structure on molecular migration in the zeolitic void space. The migration of methane in various zeolitic environments was examined by using the all-silica polymorphs of zeolite EU-I , mordenite, and silicalite. Furthermore, diffusion simulations of ethane and propane in silicalite were carried out and computed diffusion characteristics were compared with experimental data.

Diffusion measurements through embedded zeolite crystals

“…The MC simulation provides information on equilibrium properties, whereas the MD simulation also provides information on dynamic properties such as diffusivities. Numerous MD simulations have been performed on the transport of hydrocarbons in zeolites (Nicholas et al, 1993;Nowak et al, 1991;June et al, 1992;Goodbody et al, 1991). The diffusivities by MD simulations agree well with the microscopic data obtained by PFG-NMR measurement.…”

Sorbate properties and cage-to-cage diffusion of argon in NaCaA: a molecular dynamics study

“…Figure 6 shows the variation of mean square displacement, ( u 2 ( t ) ) , as a function of time for various temperatures. Diffusion coefficients at various temperatures obtained from the Einstein relationf9 D = (u2(t))/2dit ( 6 ) valid for long times are listed in Table 111. Here di is the dimensionality of the system, taken as 3 in the present case.…”

“…Figure 10 shows the variation of the total energy (U) = (Ugh) + ( Ugg) with d, the perpendicular distance between the sorbate and the plane of the eight-ring window. We know that the sorbate has to pass through the eight-ring window to migrate 6 . 0 to another cage.…”

Computational study of molecular hydrogen in zeolite Na-A. I. Potential energy surfaces and thermodynamic separation factors for ortho and para hydrogen

“…Often the use of a rigid lattice is based on the intuition that the dynamical behavior of fairly small guests will not be altered if a flexible framework is used. 37 In one study by Demontis et al, 38 both flexible and rigid frameworks were used to evaluate the diffusion rate of methane in silicalite. It was found that properties like diffusion constants, activation energies, and radial distribution functions were almost unaffected by framework vibrations.…”