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Theory of adsorption and diffusion in zeolites

This project was finished with a set of publications, and a successful Ph.D. thesis defence.

Zeolites nowadays play an important role in chemistry. For example, they are used in gas separation processes, in washing powder, and as catalyst for a vast number of different reactions. One of the most important applications is in the field of oil refining, in which they are applied in cracking and hydro-isomerization reactions. For these reactions, not only the presence of Brønsted acid sites or other reactive sites is important for it's performance, but also adsorptive and diffusive properties of products and reactants are of vital importance to the catalytic properties of these materials.

Due to the fact that the pores inside a zeolite are of molecular dimensions, diffusion inside these channels is completely different from gas-phase diffusion. The mobility of molecules inside these pores therefore greatly depend on the structure of the channels, the pore topology and the bulkiness of the diffusing molecule itself. The large interactions between molecules and the zeolite lattice also results in the fact that molecules are strongly adsorbed inside these channels. Both adsorption and diffusion can be studied using forcefield-based simulations like configurational-bias Monte Carlo and molecular dynamics simulations. These methods have been used to study the diffusion of alkanes ranging from methane to n-dodecane in a number of different zeolites.^[1]

A different project is focussed on another peculiar behaviour in zeolites, called single-file diffusion. This behaviour can be observed when the zeolite pores are so narrow, that molecules cannot pass each other, and results in a strong decrease of the mobility. The characteristic behaviour that the mean-squared displacement of a particle becomes proportional to the square root of time, depends on the structure of the channels in which the molecules move. Using a very simple system with hard spheres inside a structured tube, this dependence is being studied.^[2]

The study of diffusion and adsorption is not only focussed on theoretical work, but also on linking the theoretical results with the experimental world. For this purpose, there has been cooperation with the NMR group, where a new technique has been developed to measure hopping rates of alkanes in zeolites. This 1D ¹³C exchange technique makes use of the specific cage structure of some zeolites, that results in different chemical environments (and thus different NMR signals) when a molecule is adsorbed in the different cages of the zeolite. This technique has been used to study the hopping rate and activation energy of n-pentane in zeolite ZK-5.^[3]

Last, but not least, work is currently being done on interpreting Positron Emission Profiling (PEP)^[4,5] data. For this purpose, a program to simulate and analyse diffusion in a catalyst bed containing zeolite crystals has been developed. Tracer EXchange (TEX)-PEP in combination with large zeolite crystals ensures that micropore diffusion really is the dominant mechanism and that the system is at equilibrium during the entire experiment.^[6] The results found with this technique match well with microscopic techniques such as PFG-NMR and QENS, and is being used to study mixtures of linear and branched alkanes in Silicalite.^[7] Furthermore, enhancements to the macroscopic model are being studied to improve the link between the parameters obtained from studies performed on a molecular scale and the ones obtained in these large-scale systems.

Publications:

1. D. Schuring, A.P.J. Jansen and R.A. van Santen, "Concentration and Chainlength Dependence of the Diffusivity of Alkanes in Zeolites Studied with MD Simulations", J. Phys. Chem. B 104(5), 941-948 (2000).

2. D. Schuring and R.A. van Santen, "Properties of Single-File Systems Studied With Molecular and Hard-Sphere Dynamics", to be published.

3. P.C.M.M. Magusin, D. Schuring, E.M. van Oers, J.W. de Haan and R.A. van Santen, "n-Pentane Hopping in Zeolite ZK-5 Studied with ¹³C NMR", Magn. Reson. Chem. 37, S108-S117 (1999).

4. B.G. Anderson, N.J. Noordhoek, D. Schuring, F.J.M.M. de Gauw, A.M. de Jong, M.J.A. de Voigt and R.A. van Santen, Imaging of Hydrocarbon Reactions in Zeolite Packed-Bed Reactors using Positron Emission Profiling, Catal. Lett. 56, 137-144 (1998).

5. R.A. van Santen, B.G. Anderson, F.J.M.M. de Gauw, N.J. Noordhoek, D. Schuring, A.M. de Jong, and M.J.A. de Voigt, "Imaging of hydrocarbon reactions in zeolite packed-bed reactors using positron emission profiling", in: "Abstracts of the 215^th ACS Meeting", Dallas, March 29- April 2, 1998.

6. A.O. Koriabkina, D. Schuring, A.M. de Jong, J. van Grondelle, and R.A. van Santen, "Adsorption and Diffusion of Alkanes and their Mixtures in Silicalite Studied with the Positron Emission Profiling Technique", to appear in: "Studies in Surface Science and Catalysis", Elsevier, Amsterdam, 2001

7. D. Schuring, A.O. Koriabkina, A.M. de Jong, B. Smit, and R.A. van Santen, "Adsorption and Diffusion of n-Hexane/2-Methylpentane Mixtures in Zeolite Silicalite: Experiments and Modeling", J. Phys. Chem. B. 105(32), 7690-7698 (2001).

These pages are maintained by Bouke Bunnik (B.S.Bunnik@tue.nl). Comments and suggestions are welcome.