Zeolite diffusion, simulations hydrocarbons

In a first series of simulations, the force field description of the system was the same as the one used for diffusion. In order to study the effect of the force field on the adsorption energies, a second set of simulations was performed using Dreiding n to represent both the zeolite and the hydrocarbon molecules. 

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... 

Some experimental studies point out that the diffusion rate of pure hydrocarbons decreases with the coke content in the zeolite [6-7]. Theoretical approaches by the percolation theory simulate the accessibility of active sites, and the deactivation as a function of time on stream [8], or coke content [9], for different pore networks. The percolation concepts allow one to take into account the change in the zeolite porous structure by coke. Nevertheless, the kinetics of coke deposition and a good representation of the pore network are required for the development of these models. The knowledge of zeolite structure is not easily acquired for an equilibrium catalyst which contains impurity and structural defects. 

The results obtained for the set of molecules studied follow the expected trend, based on the differences of mass and shape of the molecules. Moreover, the results, when compared to the available experimental data, indicate that the simulations can indeed provide a realistic representation of the microscopic process of the diffusion of light hydrocarbons in the pores of zeolites. However, the fact that simulations conducted in very different conditions gave similar results also indicate that a more systematic study is required to establish the influence of the different factors (cluster size, force field, trajectory length, flexibility of the zeolite lattice) in the final results. 

Studies of adsorption, diffusion and molecular simulation of cyclic hydrocarbons in MFI zeolites... 

The diffusion of gaseous benzene and paraxylene during their adsorption in a fixed bed of HZSM-5 zeolite crystallite has been studied by Xe NMR of adsorbed xenon used as a probe. The equations of diffusion in the macropores and micropores have been analytically solved, giving the hydrocarbon concentration profiles against time in both types of pores and allowing the simulation of the Xe NMR spectra. The comparison of simulated and experimental spectra leads to the value of the intracrystallite diffusion coefficients which are in good agreement with the literature. 

The discrepancies between the values of the activation energies provided by different authors can be attributed to the different alkane partial pressures. Several theoretical and experimental studies indicate a significant concentration dependence of diffusion in zeohtes [4,19,77,78,80]. Coppens et al. [19] have shown for MFI zeolite with Monte-Carlo simulations that the diffusivity can drop by a factor of ten when the occupancy is close to saturation. In this work we performed our experiments under a hydrocarbon partial pressure of 6.6 kPa, which is higher than the pressures in TEOM, gravimetric and volumetric measurements. 

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