Diffusion of Sorbates in Zeolites Y and A: Novel Dependence on Sorbate Size and Strength of Sorbate-Zeolite Interaction

Abstract

A systematic investigation of monatomic spherical sorbates in the supercages of zeolites Y and A by molecular dynamics technique is presented. Rates of intercage diffusion, rates of cage visits, and the diffusion coefficients have been calculated as a function of the sorbatezeolite interaction strength. These properties exhibit markedly different dependences on interaction strength for the two zeolites. The observed behavior is shown to be a consequence of the two principal mechanisms of intercage diffusion and the energetic barrier associated with them. The diffusion coefficient and other properties associated with intercage diffusion are found to be directly proportional to the reciprocal of the square of the sorbate diameter when the sorbate diameter is significantly smaller than the window diameter. As the sorbate diameter increases, a peak is observed in all the transport properties investigated including the diffusion coefficient. We call this surprising effect as the ring or levitation effect and it explains several anomalous results reported in the literature and suggests a breakdown of the geometrical criterion for diffusion of sorbates. It shows that under certain conditions nongeometrical factors play a major role and geometrical factors become secondary in the determination of the molecular sieve property. A generalized parameter has been proposed which suggests conditions under which one can expect the ring or levitation effect in any porous medium. Inverse size selectivity becomes operative under these conditions.

Friction based modeling of multicomponent transport at the nanoscale

“…Figure 4 depicts the variation of this friction coefficient with pore diameter for H 2 at various temperatures, showing an interesting trend having both a minimum and maximum with respect to diameter. The minimum occurs at a pore diameter of about 0.75 nm, which is quantitatively consistent with a theoretical explanation from this laboratory 49 in which the diffusivity has a maximum at this pore size, following the floating molecule or levitation effect, first discussed by Derouane et al 50 and subsequently studied in detail by Yashonath and co-workers 51,52 for diffusion in zeolites. This maximum in the diffusivity, and the minimum in the friction coefficient, is best explained by considering the expression derived earlier…”

Non-monotonic size dependence of diffusion and levitation effect: A mode-coupling theory analysis

“…The simulation studies revealed that diffusion of small solutes show a diffusivity maximum as a function of the diffusant diameter, independent of the nature of the medium. This counter intuitive behaviour of solute diffusion through different types of porous structures was first reported by Yashonath and Santikary [6] [8] and later by Henson et al [12]. Yashonath et al have systematically varied the size of the solute by keeping the porous zeolite structure the same [8] [9].…”

“…Yashonath and co-workers have connected this behaviour to levitation effect [8][9][10][11][12][13][14]. They have found that when a solute size is approximately similar to the size of the zeolitic pore or in case of pure solvent, the size of the neck of the solvent cage then the solute particle feels a symmetric attraction from all directions and thus levitates through the pore and diffuses faster.…”

“…Hence even in system 2 the intermediate size particle levitates. Historically it is shown that when the solvent forms a cage and the solute-solvent interaction is attractive then there is a force balance and thus solutes of certain sizes levitate [7] [8]. However, we believe that the levitation is rather dependent on the relative size of the solute with respect to the neck of solvent cage rather than the interaction between the solute-solvent.…”

“…The motion of tagged particles in various mediums have been extensively studied using, microscopic theories, simulations and experiments [1][2][3][4][5][6][7][8][9][10][11][12]. These studies have predicted many interesting results.…”