Synthesis of Sodalite Precursor Nanosheets and Facile Assembly for Hydrogen Purification

Zeolite membranes have been studied for energy-efficient gas separation under thermally and chemically harsh conditions for several decades [1]. However, their implementation in the gas and vapor separations has been hampered because of the reproducibility issue arising from the complex hydrothermal synthesis route and the defect formation during the activation step [2]. The synthesis of molecular-sieving zeolitic membranes by the assembly of high-aspect ratio and crystalline nanosheets as building-blocks can help the scalability and reproducibility since the hydrothermal step is not required [3]. Two-dimensional zeolite nanosheets based-membranes have been successfully synthesized for the xylene isomers separation [4-5]. However, the intersheet gaps, intrinsic to the stacked nanosheets films, tend to dominate the overall transport leading to poor performance unless the undesired secondary-growth step is performed. Zhang et al. achieved a secondary-growth-free synthesis for the separation of butane isomers where a separation factor of 5.4 was achieved by processing MFI nanosheets into thin films using filtration [6].

Herein, we report the exfoliation of the layered zeolite precursor of sodalite, RUB-15, into single 0.8 nm-thick nanosheets hosting hydrogen-sieving six-membered (6MR) rings of SiO₄ tetrahedra and their assembly by simple filtration into thin films for H₂ sieving (Fig. 1a) yielding H₂/CO₂ over 100 [7]. RUB-15 layers were synthesized via hydrothermal synthesis route using a modified method reported by Gies and co-workers [8] and Okubo and co-workers [9]. The as-made material was confirmed to be RUB-15 using X-ray diffraction, ²⁹Si MAS NMR and SAED. Swelling of RUB-15 with a C₁₆ cationic surfactant was performed to increase the interlayer spacing and weaken the interlayer interactions resulting in a shift of the (002) towards small scattering angles (Fig. 1b). Finally, to overcome the electrostatic binding energy, which holds together the nanosheets, and obtain the desired nm-thick single layer RUB-15 nanosheets we used the melt compounding technique [4-6]. Exfoliated nanosheets crystallinity was confirmed by TEM (Fig. 1c and 1c inset).

Membrane fabrication was performed by filtration of the nanosheets dispersed in ethanol. As-filtered membranes showed a periodical arrangement of the nanosheets along the z-axis with a d-spacing of 11.4 Å which translates in a gallery spacing of 3.4 Å. Intersheet gaps dominated the overall transport leading to a cut-off in the kinetic diameter of 3.6 Å yielding H₂/N₂ selectivities over 20, while CO₂ was able to pass between the gallery spacings. The presence of reactive terminal silanol groups in the RUB-15 nanosheets presented a unique opportunity for the elimination of the nanosheets gaps. The neighboring silanol groups can be condensed by simple heating to form covalent Si-O-Si linkages, which can reduce the intersheet gaps, thereby blocking the molecular transport along these gaps. Indeed, calcination of the as-filtered nanosheets film in air at 500 ºC for 1 h led to a new periodical configuration with a decreased interlayer d-spacing of ~7.4 Å (Fig. 1d). The calcined films preserved their crystallinity as confirmed by the in-plane XRD where the (020) and (022) plane reflections were still visible suggesting the in-plane order of the new nanosheets configuration (Fig. 1e).

Upon calcination, the dominated transport was through the 6-MR yielding H₂/CO₂ selectivities in the range 20-100 and H₂ permeance in the range 41-424 GPU at 250-300 ºC (Fig. 1f). The high selectivity was exclusively from the transport across 6-MR which was confirmed by a good agreement between the experimentally-determined apparent activation energy of H₂ and that computed by ab initio calculations.

Fig. 1. a) The structure of RUB-15 layer. Si and O are shown in yellow and red, respectively. H₂ and interlayer guests are omitted for clarity. b) Powder XRD pattern from the surfactant-swollen and as-synthesized RUB-15. The inset shows corresponding illustrations. c) High-magnification TEM image of the exfoliated nanosheets, inset: SAED pattern of an exfoliated nanosheet along the [100] zone axis. Comparison of the d) out-of-plane and e) in-plane XRD patterns (Cu Kα λ=1.5406 Å) from the nanosheet film before and after the calcination step. f) Single-gas permeation data from the calcined nanosheets membranes as a function of temperature. Scale bar in (c) is 100 nm, inset is 3 nm^-1.

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