Influence of the Particle Sizes on the Energetic Performances of MFI-Type Zeolites

Nanoporous materials have an important role in addressing some of the major energy and environmental-related problems facing society. Herein, four MFI-type zeosils with crystallite size ranging from nanometer to micrometer were synthesized (nanosheets, nanocrystals, honeycombs, and big crystals) in order to establish a relation between the crystal size of these zeosils and their energetic performances under high-pressure intrusion extrusion experiments (mechanical energy storage). The intrusion-extrusion behavior of water and concentrated LiCl aqueous solution (20 M) in these four zeosils was evaluated at room temperature. Whatever the crystal size, the "Silicalite-1-water" systems displayed a spring behavior, whereas "Silicalite-1-LiCl aqueous solution" systems moved slightly toward a shock-absorber behavior with an increase in the intrusion and extrusion pressures (273-285 MPa) compared to "Silicalite-1-water" systems (88-96 MPa). Therefore, in the case of the LiCl aqueous solution (20 M), the energetic performance was tripled. Compared to the big crystal sample, both the honeycomb and the nanocrystal samples showed a slight decrease of the intrusion and extrusion pressures. A decrease of the intrusion and extrusion volumes was observed in the case of nanocrystal sample compared to both big crystal and honeycomb samples, which is attributed to the noncrystallized silica regions infused within the nanocrystals. Contrary to these three samples, liquid intrusion occurred at atmospheric pressure for the nanosheet sample, which is likely due to both the presence of a high number of surface defects and the low thickness of the zeolite nanosheets (2 nm). Solid-state NMR spectroscopy and thermogravimetric analyses provided evidence on the presence of local defects on the nonintruded samples and the breaking of some siloxane bridges after the intrusion extrusion step.