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.
Results from a molecular dynamics simulation of xenon in silicalite at 298 K and 4 atoms per unit cell (A&, = -26.9 kJ/mol, D = 1.86 X m2/s) are in good agreement with the experimental value of -24.5 kJ/mol and the diffusion coefficient derived from the NMR pulsed field-gradient method (4.00 X m2/s). The diffusivity is predicted to be negligible at temperatures around 77 K and then increases over the investigated range to D = 3.25 X m2/s at 450 K, yielding an activation energy of 5.5 kJ/mol. Increasing the concentration from 4 to 16 atoms per unit cell results in a decreased internal energy of adsorption (-28.1 kJ/mol) and a decreased diffusion coefficient (D = 0.37 X lo4 m2/s). The anisotropy of diffusion is also examined. IntroductionComputer simulations of zeolite/adsorbate systems are currently of increasing importance in the context of understanding the catalytic properties of zeolites. They are used in conjunction with experimental techniques but possess the significant advantage that a wide range of conditions may be simulated, conditions which may be difficult to obtain in an experimental setup. Early based upon molecular mechanics procedures, employed simple atom-atom potentials to calculate heats of adsorption and adsorption isotherms and to estimate diffusion coefficients. More recently, adsorption sites and potential energy maps were calculated for xenon in zeolite rho,3 pyridine in zeolite L,4 and benzene in silicalite and zeolite theta-1.5 Monte Carlo simulations have been employed as a useful means of probing the potential energy space of zeolite/adsorbate systems as a function of temperature and sorbate concentration. Yashonath et al. examined the temperature dependence of the behavior of methane in zeolite Y6 and Smit and den Ouden the adsorption of methane in mordenite with variable zeolite composition.' More recently the molecular dynamics (MD) technique has been applied to such systems with a view to obtaining time-dependent data such as diffusion coefficients for methane and benzene in zeolite NaY8s9 and water in ferrierite.I0 These studies have been based upon the approximation of a rigid zeolitic framework, but MD framework simulations of natrolite" and zeolite AIz have been reported recently by Demontis et al.The emergence of '29Xe N M R as a means of probing the internal structure of zeolitesI3 and of exploring xenon as an adsorbate14 prompted us to investigate the silicalite/xenon system using the MD technique. There is also the added value that the diameter of xenon is close to that of methane, a molecule of increasing importance in heterogeneous catalysis. We developed three separate computer programs which were first used to investigate the test system, xenon in silicalite, at 298 K and a concentration of 4 xenon atoms per unit cell, followed by the use of the separate programs for varying the system temperature from 77 to 450 K and the adsorbate concentration from 4 to 16 ad-
In order to understand the high selectivities observed in adsorption processes and catalytic conversions of hydrocarbons using zeolites, it is necessary to study the behavior of molecules inside the pores of a zeolite. This kind of information is difficult to obtain from direct experiments. We report Monte Carlo computer simulations on the adsorption of methane in the zeolites faujasite, mordenite, and ZSM-5. For all compounds our simulations yield good agreement with the available experimental data on the heat of adsorption. In addition, the present calculations predict that the heat of adsorption of methane in mordenite should decrease steeply with increasing Al/Si ratio. Analysis of the distribution of CHI in mordenite suggests a simple explanation for this peculiar effect, which may appear to be of great importance for the catalytic activity of mordenite.Living organisms, employing enzymes, can synthesize molecules with essentially 100% selectivity. Compared to this elegance, chemical reactions in the process industries are rather crude ways of producing useful chemicals. However, recent improvements in the design of zeolite catalysts suggest that much higher selectivities may be achieved in the near future.'Zeolites are microporous crystalline materials usually consisting of silicon, aluminum, oxygen, and sodium. Certain positions on the inner walls of the micropores appear to behave as active sites, where catalytic conversions can take place. Selectivity can be obtained by adjusting the size of the micropores or cavities and by modifying the location of the active sites in such a way that, ideally, only one type of molecular species can reach these active sites. Of course, the performance of currently available zeolites is still far removed from this "enzyme-like" selectivity, but it is clear that a better understanding of the microscopic basis for the catalytic action of zeolites may help us approach this ultimate goal of zeolite chemistry. One of the first factors that must be understood is the relation between the molecular structure of a zeolite and the adsorption of specific molecules. One particularly promising tool to obtain such information is computer simulation .2-J Recently, Yashonath et aL2 have reported a simulation study on the adsorption of methane in faujasite, with emphasis on the influence of the temperature on the adsorption. In this work we have used the methods pioneered by Yashonath et aL2 to gain a better understanding of the effect of the chemical composition of mordenite, which is indicated by the Al/Si ratio, on the adsorption process. The latter parameter is known to have a pronounced effect on the catalytic activity of zeolitese6We have carried out simulations at low coverage of methane (zero filling). This allows us to ignore methane-methane interactions. In our simulations (cf. we assume that 3 , can be written as the sum of a Lennard-Jones potential and a Coulombic interaction where qi is the charge of atom i and rij the distance between the atoms i and j . The values of A...
Fourier-transform pulsed-field-gradient NMR m e a s u r e m e n t s h a v e been used to analyse the diffusion of some n-alkanes (methane, n-butane and n-pentane) in zeolite ZSM-5. The intention of t h e NMR study was to compare results obtained by molecular dynamics calculations and by uptake measurements, using t h e same systems. Methane clearly exhibits a bi-exponential spin-echo attenuation. This indicates two types of diffusion : intracrystalline diffusion and long-range diffusion, which is a combination of intera n d intra-crystalline diffusion. In t h e case of small crystals the diffusion of methane into t h e macropores between t h e crystals dominates t h e decay. However, a s expected, for larger crystals, t h e contribution of intracrystalline (or micropore) diffusion increases significantly. From t h e curves, t h e coefficient of intracrystalline diffusion of methane in ZSM-5 has been determined (3.8 x lo-' m2 s-' at 25°C). The NMR methane diffusion data are in good agreement with values obtained by molecular dynamics calculations. Subsequent NMR measurements of n-butane and npentane diffusion in ZSM-5 indicate that t h e diffusion decreases sharply with increasing chain length of t h e hydrocarbons (11 x lo-'' a n d 4.4 x lo-'' m2 s-', respectively, at 25°C and 20 kPa loading). To allow for a comparison with t h e diffusivities obtained independently by other techniques, t h e concentration dependence of t h e NMR self-diffusion coefficient of n-pentane in ZSM-5 was determined and was found to decrease with increasing sorbate concentration. In addition, from t h e temperature dependence of the diffusion rates the activation energies for t h e n-butane and n-pentane diffusion in ZSM-5 have been determined (8.4 and 12.6 kJ mol-' at 20 kPa, respectively). The PFG NMR and M D results for the diffusion of light n-alkanes in ZSM-5 have also been compared with relevant diffusion data from t h e literature (obtained using other techniques, i.e. uptake methods, ZLC, MT). The microscopic self-diffusivity (from PFG N M R and MD) differs systematically by ca. two orders of magnitude from t h e much slower, macroscopic diffusion observed by uptake, ZLC and MT methods. On t h e other hand, t h e r e is satisfactory agreement between t h e self-diffusivity of n-butane obtained with PFG NMR and t h e transport diffusivity of t h e same system measured using t h e frequency response method.
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