Dr. Gonzalez Eva (CSIC)
Zeolites are materials with a well-defined microporous geometry which make them attractive for many industrial applications, for example, in catalysis or in the separation of mixtures. Understanding their adsorption behaviour is therefore an issue of major relevance from a practical point of view, but also from a fundamental one, as it is common that the properties of the adsorbed fluid are different from those in the bulk. One intriguing finding in this context is the observation that the adsorption isotherm of some simple gases (such as argon) on silicalite-1 exhibits a sub-step at intermediate loadings whereas others (such as methane) do not. Even though considerable experimental and theoretical efforts have been made, the origin of this sub-step is not clear. Some authors claim that this behaviour is a result of a fluid-like to solid-like transition of the adsorbed fluid, whereas others attribute it to a zeolite structural change. With the aim of providing more information that may aid to understand the appearance of sub-steps in the adsorption of some gases in silicalite-1, we have carried out a comprehensive experimental and simulation study of the adsorption of argon and toluene on the structurally similar zeolite silicalite-2. Both zeolites exhibit a similar structure consisting on a three-dimensional network of fairly narrow cylindrical channels with diameters in the range of 5.0 to 5.6 Å. The essential difference lies in the fact that silicalite-1 consists of an array of parallel cylindrical pores intersected by sinusoidal channels, whereas in silicalite-2 all the pores are linear (see Figure 1). First we performed volumetric experiments that confirm that the adsorption of argon and toluene on silicalite-2 also exhibits a sub-step at half loading (see Figure 2), suggesting that this behaviour does not depend on the specific structural details of the pores. Subsequently, the microscopic origin of this sub-step was investigated by means of molecular simulations. However, the agreement between the experimental and simulated adsorption isotherm was only qualitative, evidencing deficiencies of the models used to describe the interactions between the different components in the system. Thus the structure of the adsorbate/adsorbent system was further investigated by performing powder diffraction experiments at three different loads: empty, at half-load (before the sub-step) and at high load (after the sub-step). These data were used as input of N-Reverse Monte Carlo simulations to obtain atomic structural models compatible with the experimental diffractograms. In both instances, namely adsorption of argon and toluene, a good fit of the experimental data was only obtained when incorporating the zeolite flexibility, which shows that the structure of the zeolite can change at high loads or when the size of the adsorbed molecules is comparable to that of the pores. In the case of argon, after the sub-step, a considerable order of the fluid also builds up, suggesting that the sub-step might be attributed to a fluid structural change facilitated by a slight deformation of the zeolite. Interestingly, the structural models obtained from Monte Carlo and N-Reverse Monte Carlo simulations are significantly different, even at half loading. We ascribe these discrepancies to deficiencies in the adsorbent-adsorbate interatomic potential.
Dr. Gonzalez Eva (CSIC)
Dr. Guil José María (CSIC) Prof. Lomba Enrique (CSIC) Dr. Sánchez-Gil Vicente (CSIC)