Molecular Diffusion in Hierarchical Zeolites with Ordered Mesoporosity: Pulsed Field Gradient Nuclear Magnetic Resonance Combined with Thermodynamic Modeling

The dynamics of fluids confined in hierarchical porous materials is gaining increasing attention. Here, using pulsed field gradient nuclear magnetic resonance, we report an experimental study of the self-diffusivity of cyclohexane confined at different temperatures in the bi-porous structure of hierarchical faujasite zeolites (i.e., combining the intrinsic zeolite microporosity of < 2 nm diameter and an ordered MCM-41-like mesoporosity of 4 nm diameter). For different mesoporous volumes, we consider cyclohexane self-diffusivity at chemical potentials where the porosity is either totally or only partially filled (in the latter case, the microporosity is completely filled, while the mesoporous surface is only covered by a molecular thin film). On one hand, in completely filled materials, as expected, the effective cyclohexane self-diffusivity is found to increase as the mesoporous volume increases. Moreover, in this regime, the self-diffusivity follows an Arrhenius behavior with an activation energy that is close toalthough slightly smaller thanthat for bulk cyclohexane. On the other hand, for partially filled materials, a striking behavior is observed as the measured self-diffusivity decreases upon increasing the loading and remains nearly constant upon increasing the temperature. We propose here that such a behavior can be rationalized by considering (1) the population redistribution between the microporosity and mesoporosity (including the gas phase in the mesopores) and (2) the effective number of molecules that contribute to spin echo attenuation upon increasing the temperature. In particular, while molecules diffuse faster overall with the increasing temperature, we show here that our measurements rely at each temperature on a slower and slower population (therefore leading to unexpected temperature variations).