Binder, Tomas

Chmelik, Christian

Kärger, Jörg

Schmidt, Wolfgang

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Microscopic analysis of phase transition effects during benzene sorption in MFI type zeolites

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The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and Application Fig. 1: Silicalite-1 crystal, top: building scheme with 90° rotated subunits, bottom: picture of the x-z-plane in normal optical mode (transmission). Microscopic Analysis of Phase Transition Effects during Benzene Sorption in MFI Type Zeolites Tomas Binder1, Christian Chmelik1, Jörg Kärger1, Wolfgang Schmidt2 1 Universität Leipzig, Fakultät für Physik und Geowissenschaften, Abteilung GFP, Linnéstr. 5, 04103 Leipzig, Germany, E-Mail: tomas@uni-leipzig.de 2 Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany 1. Introduction Adsorption of aromatic hydrocarbons in MFI zeolites has been intensively studied due to the importance of ZSM-5 and the aluminium-deficient analogue, Silicalite-1, in catalysis and separation. Applying different macroscopic techniques, e.g. gravimetric sorption isotherms, X-ray diffraction or solid-state NMR, several groups reported not only a phase transition of sorbate molecules under confinement of the zeolite lattice, but also minor changes of the framework structure itself depending on temperature and loading. Microscopic studies of individual crystals by means of interference microscopy (IFM) and IR microscopy (IRM) now reveal deeper insights into the sorption behaviour and phase changes of the aromatic-MFI system. 2. Materials and Methods The present work is mainly based on the recently established interference microscopy as a powerful tool to monitor time-dependent intracrystalline concentration profiles with a two-dimensional special resolution of 0.45 µm [1, 2]. IFM basically detects the change of the refractive index of a large individual crystal, being directly correlated to the intracrystalline concentration of sorbate molecules. The sample consists of large, coffin-shaped Silicalite-1 crystals with a size of 200 × 25 × 25 µm³ [3]. Individual crystals are known to be generally built up from six building blocks with a 90° rotated intergrowth structure (fig. 1) [4]. The zigzag and straight channels are running in a- and b-direction, respectively, and the diffusion of sorbate molecules in c-direction is strongly hindered due to the lack of a connection in this last case. 3. Experimental Results IFM and IRM experiments were carried out in the range from 0.1 to 100 mbar pressure of benzene. The region of interest with the clearly visible phase transition could be found between 5 and 15 mbar depending on very small changes in the surrounding © 2011, Tomas Binder diffusion-fundamentals.org 16 (2011) 64, pp 1-21 Fig. 2: IRM and IFM isotherm data compared with literature values. Fig. 3: Scheme of diffusion front (top) and phase transition front (bottom) after a pressure step from 5 to 10 mbar benzene.  temperature. This can be related to an intracrystalline loading above 4 molecules per unit cell. The obtained isotherm is shown in fig. 2 in comparison with gravimetric uptake data from literature [5]. The more interesting outcome of IFM experiments, namely intracrystalline concentration profiles, reveals a two-step adsorption-phase-transition behaviour when applying only one pressure step, e.g., 5 to 10 mbar. During the first minutes a common adsorption process can be identified with the diffusion front moving along the a and b directions from the long crystal faces towards the center (fig. 3 top). Without further changes to the pressure a second step can be observed during the next two to three hours, clearly indicating a phase transition. The front of this process moves from the short crystal faces along the c-direction to the center and occurs only in the pyramidal segments first (fig. 3 bottom). Further investigations have shown a strong dependence on the applied pressure step and revealed several metastable states, in which the second step, i.e., the phase transition, occurs only in parts of the crystal.  4. Conclusion and Outlook Interference microscopy has shown to yield deep insights into the phase transition behaviour of Silicalite-1 crystals upon benzene adsorption. The present study indicates a two-step process of adsorption/diffusion and phase transition with the option to follow both the kinetics and the position dependence within an individual crystal. References [1] T. Binder, C. Chmelik, L. Heinke, F. Hibbe, J. Kärger, T. Titze, D. Tzoulaki,  diffusion-fundamentals.org 11 (2009) 1-17 [2] C. Chmelik, J. Kärger, Chem. Soc. Rev. 39 (2010) 4864. [3] D. Tzoulaki, L. Heinke, W. Schmidt, U. Wilczok, J. Kärger, Angew. Chem., Int. Ed., 47 (2008) 3954-3957  [4] E. R. Geus, J. C. Jansen, H. van Bekkum, Zeolites 14 (1994) 82. [5] C.-K. Lee, A. Chiang, J. Chem. Soc., Faraday Trans. 92 (1996) 3445-3451. 0.01 0.1 1 10 100012345678910θ / molecules per uc  Gravimetric, 293K, [5]  Gravimetric, 303K, [5]  IRM, 295K  IFM, 295Kp / mbar© 2011, Tomas Binder diffusion-fundamentals.org 16 (2011) 64, pp 1-22

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Microscopic analysis of phase transition effects during benzene sorption in MFI type zeolites

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