Experimental Proof for Resonant Diffusion of Normal Alkanes in LTL and ZSM-12 Zeolites

Abstract

The intra-crystalline diffusion of normal alkanes in LTL and ZSM-12 zeolite was experimentally studied via gravimetric measurements performed at different temperatures. A periodic dependence of the diffusion coefficient on the number of carbon atoms in alkane was detected, which is an experimental proof for resonant diffusion. The present observations were described on the base of the existing theory of the resonant diffusion and several important parameters of the alkane-zeolite interaction and zeolite vibrations were obtained. In the considered temperature region the diffusion coefficient follows the Arrhenius law with periodic dependences of the pre-exponential factor and activation energy on the number of carbon atoms in alkanes. A compensation effect of simultaneous increases of the pre-exponential factor and the activation energy was also established.Diffusion of hydrocarbons in zeolites has been extensively studied both with experimental and theoretical approaches. This is due to the large number of applications of these materials in the refinery industry, e.g. as catalysts in hydroisomerization and cracking processes [1] and as molecular sieves in separation processes, such as pressure swing adsorption [2] and membranebased separation [3]. A significant factor for the efficiency of these processes is the rate of intracrystalline diffusion of species in zeolites. In all cases, the dynamic behavior of sorbed molecules is crucial in determining the performance of the considered catalytic or separation process. An interesting feature of the diffusion of normal alkanes through zeolites is the resonant diffusion, when the diffusion coefficient exhibits a periodic dependence on the number of carbon atoms of the alkane chain. This behavior was first reported by Gorring [4] who measured the macroscopic diffusivity for a series of normal alkanes in zeolite T. Other researchers [5], however, were unsuccessful to observe experimentally this sort of periodicity of the diffusion constant. This apparent discrepancy could be explained by the theory of resonant diffusion [6,7], which elucidates the necessary conditions for the zeolite structure and alkane-zeolite interaction in order to observe a periodic dependence of the diffusion coefficient on the number of carbon atoms in alkanes.In our previous paper [8], the diffusion of normal alkanes in one-dimensional zeolites is theoretically studied using the stochastic equation formalism. The calculated diffusion coefficient accounts for the vibrations of the diffusing molecule and zeolite framework, alkane-zeolite interaction, and specific zeolite structure. It is shown that if the interaction potential is modulated by the zeolite nano-pore structure, the diffusion coefficient varies periodically with the number of

Understanding cage effects in the n-alkane conversion on zeolites

“…Such a phenomenon was already known as "incommensurate diffusion" in physics [56], but was renamed "resonance diffusion" in catalysis [51]. In catalysis the existence of incommensurate diffusion remains controversial, highlighting the experimental difficulties in obtaining consistent diffusion rates [54,[57][58][59][60][61][62]. In this paper we show that incommensurate diffusion in ERI-type zeolites is a prerequisite to understanding these zeolites' remarkable reactant shape selectivity in n-alkane hydroconversion.…”

“…A window effect refers to the proportionality between the diffusion rate and the commensurateness of the shape of the adsorbent and the shape of the adsorbate [13,[49][50][51][52][53][54][55]. Such a phenomenon was already known as "incommensurate diffusion" in physics [56], but was renamed "resonance diffusion" in catalysis [51].…”

A review of common practices in gravimetric and volumetric adsorption kinetic experiments

“…As a second example consider the measurements of Yoo et al [5], who used a conventional flow asymmetric TGA system with carrier gas. A surprising feature of this paper is the fact that even though the title expressly mentions an experimental proof of the mechanism of resonant diffusion, experimental uptake curves are not reported.…”

Spectroscopic investigation into the design of solid–acid catalysts for the low temperature dehydration of ethanol

“…[ [36][37][38] The elastic incoherent scattering function (EISF) allows the intensity of the individual quasi-elastic components to be compared with the elastic component (Figure 8). The EISF for the component corresponding to jump diffusion of free ethanol (Figure 8a), decreases at higher temperatures, showing an increase in the relative intensity of this component with respect to the elastic component.…”