Apparent Anomalous Temperature Dependence of Self-Diffusion Studied by Pulsed-Field Gradient Nuclear Magnetic Resonance and Thermodynamic Modeling.

The self-diffusivity of cyclohexane and n-octane adsorbed in hierarchical zeolite monoliths has been investigated by using PFG-NMR. In these samples, the intrinsic FAU-X zeolite microporosity combines with a complex macroporous network composed of aggregated zeolite nanocrystals. As temperature is increased, cyclohexane self-diffusivity apparently decreases, reaches a minimum, and then starts increasing upon further increasing the temperature. Such striking, i.e., non-Arrhenius, temperature dependence is not observed for n-octane in the same samples and for cyclohexane adsorbed in purely microporous FAU-X. Through thermodynamic modeling, we show that this anomalous behavior can be rationalized by considering the evolution in the adsorbate populations when changing the temperature. In more detail, we show that the slow and fast diffusing species present in the microporosity and secondary porosity arising from the packing of zeolite nanocrystals vary significantly with a strong impact on the effective diffusivity. Applying the temperature evolution of their relative fractions to a simple two-phase diffusion model helps obtain insights into the physicochemical factors responsible for the complex behavior of effective self-diffusivity in hierarchical zeolites.

Silica based ionogels: interface effects with aprotic and protic ionic liquids with lithium.

In the frame of the development of solid ionogel electrolytes with enhanced ion transport properties, this paper investigates ionogel systems constituted by ∼80 wt% of ionic liquids (ILs) confined in meso-/macroporous silica monolith materials. The anion-cation coordination for two closely related ILs, either aprotic (AIL) butylmethylpyrrolidinium or protic (PIL) butylpyrrolidinium, both with bis(trifluoromethylsulfonyl)imide (TFSI) anions, with and without lithium cations, is studied in depth. The ILs are confined within silica with well-defined mesoporosities (8 to 16 nm). The effects of this confinement, onto melting points, onto conductivity followed by impedance spectroscopy, and onto lithium-TFSI coordination followed by Raman spectroscopy, are presented. Opposite effects have been observed on the melting temperature: it increased for the AIL (+2 °C) upon confinement, while it decreased for the PIL (-2 °C). With lithium, the confinement led to an increase of the melting temperature (+1 °C) for the PIL and AIL. Regarding ionic conductivities, a relative maximum was observed at 40 °C for a mesopore diameter of 10 nm for the AIL with 0.5 M lithium, while it was not clearly visible for the PIL. These differences are discussed in view of the charge balance at the interface between silanols and ILs: the presence of a PIL, contrary to an AIL, is expected to modify the acidity of the silica. Raman data showed that the coordination number of lithium by TFSI is reduced upon AIL confinement, although this was not observed for PILs. At last, this work highlights the impact of the acidity of a PIL on the chemistry occurring at the interface of the host network within ionogels.