Towards the Ultimate Membranes: Two-dimensional Nanoporous Materials and Films

Abstract

The energy-efficient separation of molecules has been a popular topic in chemistry and chemical engineering as a consequence of the large energy-footprint of separation processes in the chemical industry. The Laboratory of Advanced Separations (LAS) at EPFL, led by Prof. Kumar Varoon Agrawal, is focused to develop next-generation, high-performance membranes that can improve the energy efficiency of hydrogen purification, carbon capture, hydrocarbon and water purification. For this, LAS is seeking to develop the ultimate nanoporous membranes, those with a thickness of 1 nm and possessing an array of size-selective nanopores. In this article, the research activities at LAS, especially in the bottom-up and top-down synthesis of chemically and thermally stable, nanoporous two-dimensional materials and membranes are discussed.

Exfoliating layered zeolite MFI into unilamellar nanosheets in solution as precursors for the synthesis of hierarchical nanocomposites and oriented films

“…23,29 Zeolite membrane preparation and testing are difficult and challenging tasks that require special equipment, expertise and skills. 25,44 Consequently, the development and evaluation of the potential presented by the current exfoliation approach in the area of membrane separation will require separate dedicated efforts.…”

“…There had been no indications nor proof of layer dispersion into a solution by direct or simple treatments, like ultrasonication, in any of the previous reports. 25–30 In one study, zeolite layers of MWW and MFI coated with surfactants were indeed dispersed into monolayer nanosheets in solution but through a multi-step process using twin-screw extrusion. It produced very low yields of the dispersed layers, which together with the surfactant coating and complicated preparation methodology presented very limited options for post-synthetic exploitation.…”

Exfoliated Ferrierite-Related Unilamellar Nanosheets in Solution and Their Use for Preparation of Mixed Zeolite Hierarchical Structures

“…This soft-chemical process − is one of the most effective delamination procedures that presents many practical benefits, unfeasible with bulk solids, for designing and synthesis of various materials using zeolite monolayers in solutions/liquid dispersion as building blocks. − Fundamentally, a complete exfoliation represents the ultimate manifestation of the two-dimensional (2D) nature for layered solids . It has not been shown and proven conclusively for 2D zeolites until now. ,− The tentative explanation for this lack of genuine high yield exfoliation is that suitable synthesis formulations (gel compositions), presumably giving low level of intergrowths, are needed for sufficiently high efficiency and yield of exfoliation, which in practice is very simple to carry out. This goal has been pursued previously by various strategies − since the discovery of layered zeolites three decades ago, − but no comparable outcomes, i.e., direct soft-chemical exfoliation in high yield has been documented. ,, At this point, the extension of this convenient and facile exfoliation to other zeolites depends on finding suitable synthesis formulations.…”

Current State and Perspectives of Exfoliated Zeolites

“…[ 51,52,54 ] The dispersed nanosheets were used to fabricate oriented zeolite films, which after repair to eliminate defects showed effective gas separation of small molecules. [ 44,45,48 ] This work was notable by demonstrating zeolite exfoliation and its effectiveness for the fabrication of oriented zeolite films but its practical side was limited. The multistep strategy based on a sequence of interlayer expansion, melt blending using a special equipment, dissolution, and purification has not been conducive to entice broader studies and applicability.…”