Inorganic & Coordination Chemistry, Short talk
IC-013

Synthesis and structural characterization of sodalite nanosheets and their films

M. Dakhchoune1, K. V. Agrawal1*
1Laboratory of Advanced Separations (LAS), ISIC, EPFL

The synthesis of zeolite membranes 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]. However, the intersheet gaps tend to dominate the overall transport leading to poor performance unless the undesired secondary-growth step is performed.

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 (6-MR) rings of SiO4 tetrahedra and their assembly by simple filtration into thin films for H2 sieving (Fig. 1d) yielding H2/CO2 over 100 [4]. RUB-15 layers were synthesized via hydrothermal synthesis route using a modified method reported by Gies and co-workers [5]. Swelling of RUB-15 with a C16 cationic surfactant was performed to increase the interlayer d-spacing and weaken the interlayer interactions. Finally, to overcome the electrostatic binding energy, which holds the nanosheets together, melt-compounding technique was employed (Fig. 1a) [6]. As-filtered membranes showed a periodical arrangement of the nanosheets along the z-axis with a d-spacing of 11.4 Å (Fig. 1b and 1e blue). 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. Neighboring silanol groups (Si-OH) could be condensed by heating to form covalent Si-O-Si bonds, reducing the d-spacing to 7.4 Å and blocking the molecular transport along these gaps (Fig. 1c and 1e red). Upon calcination, the dominated transport was through the 6-MR yielding H2/CO2 in the range 20-100 and H2 permeance in the range 41-1000 GPU at 25-300 ºC and were stable for 100 h under the presence of 4% volume of water vapor at 200 °C (Fig. 1f).