Towards Observation of Single-File Diffusion Using TZLC

1. Introduction The growing interest in the application of one-dimensional (1D) molecular sieves both as solid catalysts and selective adsorbents has stimulated considerable attention in the study of diffusive transport of guest molecules in the 1D pores. It has been established that normal Fickian diffusion is not necessarily applicable to describe the diffusion mechanism of guest-host interaction in 1D microporous solids, particularly when the diameter of the diffusion molecule is greater than channel radius [1]. For these systems, single-file diffusion mechanism has been employed to interpret the data from experimental studies using techniques such as PFG NMR [1] and QENS [2] to reasonable degree of correlation. Recently, we have compared the tracer exchange curves obtained with 3-D toluene/β-zeolite with those of 1-D toluene/ZSM-12 system under similar conditions [3]. A non-linear behavior was obtained for the long-time asymptote in the 1-D system. This behavior can be explained by the existence of a distribution over the channel lengths in spherical particles of ZSM-12. The objective of this study is to provide evidence of the suitability of tracer ZLC technique in observing molecular transport under conditions of anomalous, single-file diffusion. More specifically, we aim to observe the center-of-gravity diffusion mechanism, which is expected for sufficiently large diffusion times in single-file systems of finite length [4, 5]. In relation to this, the tracer exchange profiles of large probe molecules, i.e., toluene having kinetic diameter of 0.61nm in a host circular channeled SAPO-5 (0.73nm dia.) and elliptical channeled ZSM-12 (0.56 x 0.6 nm dia.) is compared to the corresponding tracer exchange profiles of smaller guest molecules, e.g., water (D ≈ 0.26nm) and/or acetylene (D ≈ 0.33nm).