Eić, Mladen

Iliyas, Abduljelil

Vasenkov, Sergey

Zahedi-Niaki, M. Hassan

Qucosa

Towards observation of single-file diffusion using TZLC

HSSS - Hochschulschriftenserver der SLUB

Qucosa - Publikationsserver der Universität Leipzig

 Towards Observation of Single-File Diffusion Using TZLC  Abduljelil Iliyas1, Mladen Eić 1* , M. Hassan Zahedi-Niaki2 and  Sergey Vasenkov3 1Dept. of Chemical Engineering, University of New Brunswick, Fredericton, Canada 2Department of Chemical Engineering, Laval University, Québec, Canada 3Chemical Engineering Dept., University of Florida, Gainesville, USA *Corresponding author: meic@unb.ca 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).  2. Experimental The detailed synthesis procedure for ZSM-12 and SAPO-5 samples used in this study is described elsewhere [6]. Textural and morphological characterizations were performed using BET, SEM and XRD methods. The tracer exchange profiles of acetylene, water and toluene were carried out using TZLC technique [7] and continuously monitored with a quadrupole mass spectrometer (Dycor Dymaxion Quadrupole MS). 3. Results and Discussion. The SEM, XRD and N2-adsorption results confirmed that the as-synthesized samples are of very good quality with high crystallinity. Toluene molecules in SAPO-5 and ZSM- 1The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and Application© 2007, M. EicDiffusion Fundamentals 6 (2007) 63.1 - 63.2 12 exhibit a significant deviation in the shape of the measured tracer exchange curves from monoexponential behaviour (Fig.1). The monoexponential behaviour is expected in the limit of large times for normal, i.e. Fickian diffusion in channel systems with one and the same channel length.  The results in Fig.1 are in agreement with the expectation of the existence of the distribution over the channel lengths in sorbent particles. For the case of the center-of-gravity diffusion, which we expect for toluene, the influence of the distribution over the channel lengths on the measured curves should be further enhanced in comparison to the normal diffusion. Such an enhancement is expected because the center-of-gravity diffusivity is inversely proportional to the channel length. Hence, the distribution over the channel lengths should lead to a larger gap between the rates of tracer exchange in channels with different lengths for the case of the center-of-gravity (single file) diffusion than for the case of normal diffusion.  0 500 1000 15001E-41E-30.010.11Time (s)AC/C00 500 1000 15001E-41E-30.010.11C/C0Time (s)B Fig 1. Experimental tracer ZLC exchange profiles of toluene at P = 0.34 torr and 30oC showing deviation from Fickian diffusion (i.e., solid lines). (A) ZSM-12; (B) SAPO-5. To verify the existence of such additional distribution over diffusivities leading to an enhancement of deviations of the tracer exchange curves in Fig.1 from the monoexponential behaviour, tracer exchange profiles will be measured with much smaller molecules. In particular, we plan to study acetylene/SAPO-5, water/SAPO-5 and water/ZSM-12 systems, which are expected to follow the well-established Fickian-type diffusion because the kinetic diameters of both sorbates are smaller than the radius of host channels.  4. Conclusion This study aims to provide first-time TZLC evidence of the occurrence of anomalous, single-file diffusion in microporous sorbents. 5. References [1] Hahn, K., J. Karger, and V. Kukla, Phys. Rev. Lett., , 76 (1996) 2762-2765. [2] Jobic, H., et al., J.  Phys. Chem. B, 101 (1997) 5834-5841. [3] Iliyas, A. Zahedi-Niaki., M. H.; Eic, M., AIChE Annual Meeting, 2006. [4]  Hahn, K.; Kärger, J. J. Phys. Chem., 102 (1998) 5766-5771. [5]  Nelson, P. H.; Auerbach, S. M., J. Chem. Phys., 110 (1999) 9235-9243. [6] Iliyas, A. Zahedi-Niaki., M. H.; Eic, M.; Kaliaguine, S. Micro. Meso. Mater.,  102 (2007) 171-177. [7] Brandani, S., Z. Xu, and D. Ruthven. Micro. Mater.,  7 (1996) 323-331.  2

English

https://ul.qucosa.de/api/qucosa%3A14241/attachment/ATT-0/

Towards observation of single-file diffusion using TZLC

Abstract

Similar works

Full text

Available Versions

Qucosa

HSSS - Hochschulschriftenserver der SLUB

Qucosa - Publikationsserver der Universität Leipzig