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Structural Changes In Nanoporous Solids Due to Fluid Adsorption: Thermodynamic Analysis and Monte Carlo Simulations

Alain H. Fuchs1, Anne Boutin2, and Marie Jeffroy2. (1) Enscp, Paris Graduate School of Chemical Engineering, Paris, France, (2) Chemistry, University of Paris-Sud, Batiment 349, Orsay, France

Open framework nanoporous materials are gaining increasing interest on account of their exceptional adsorption properties. Zeolites are now widely used in industry as molecular sieves and catalysts. More recently, metal-organic framework (MOF) materials were shown to be very promising for such applications as carbon dioxide or hydrogen capture and storage.Molecular simulations can help in understanding the adsorption process at the molecular level, and contribute to the design of new materials.

There is ample literature on fluid adsorption in zeolites. In most cases, a rigid framework was assumed for the zeolitic adsorbent. Framework flexibility is generally assumed to play a role in transport properties, but not much on thermodynamics. However, a few examples were reported of “stepped” adsorption isotherms, instead of the usual continuous Langmuir (or type I) isotherm. This was attributed to a structural transition of the host framework taking place upon fluid adsorption. With the advent of “soft” hybrid adsorbents like MOFs, it is likely that framework-adsorbate coupling will become more frequent than in the relatively stiff zeolite solids

The need exists then to devise a method that enables the description and prediction of structural changes in adsorbent upon fluid adsorption. A direct route would of course be to simulate the fully flexible solid in presence of various amounts of fluid adsorbate. Unfortunately the existing forcefields are not always appropriate for such simulations.

We propose here a thermodynamic analysis of the framework-adsorbate coupling, based on the so-called “osmotic” statistical ensemble. This method makes use of standard Grand Canonical Monte Carlo simulations only. It allows to compute the thermodynamic potential of the combined host+fluid system, and thus to predict the occurrence of a structural change upon fluid adsorption. The inputs of this method are the crystallographic structures of the host solid phases and their relative lattice energies.

Our case study is silicalite-1 zeolite which has been observed in three different crystalline structures: MONO (the stable monoclinic structure for the bare zeolite at room temperature) ORTHO (observed at high temperature and also when filled by some guest molecules) and PARA (observed at high loading of aromatic molecules). A stepped adsorption isotherm has been recently reported in the case of tetrachloroethene (4CE). Structural changes of the host framework were reported but no conclusion was drawn on which transition was responsible for this step. Interestingly enough, the adsorption isotherm for a similar molecule: 1,1,2 trichloroethene (3CE), was smooth, although zeolite structural changes were also reported We have thus studied in some detail the 4CE/3CE adsorption process in silicalite-1, and were able to predict structural changes, and to reproduce the jump resulting from one of these transitions. The proposed methodology is very general and can be applied to any type of guest/host systems.