Development of 2D carbon membranes with ordered iso-ultramicroporosity

Abstract

2D material-based thin-film composite (TFC) membranes has been considered as an efficient separation platform for low-cost production of water, fuel, pharmaceuticals and petrochemicals. Most of all, graphene, a 2D carbon material with atomic thickness and excellent physicochemical properties (e.g., mechanical strength, chemical inertness, thermal/electrical conductivity) has been considered as an ideal starting material for high-performance membranes. Since the pristine graphene is extremely impermeable to any molecules due to its densely arranged carbon lattice (sp2-hybridized aromatic framework), creation of molecular transport pathways through the materialis essential for membrane applications. There have been several attempts to introduce micropores (< 2 nm) into the basal plane of graphene via ion bombardment or UV-irradiation, however, it was difficult to simultaneously achieve narrow pore-size distribution and high pore densities because those methods are based on random defect-generation mechanism. In this thesis, zeolite-templated synthesis of novel 2D carbon materials with extremely regular micropore size and high pore density and its membrane application will be discussed. Most zeolites have 3D microporous structures, but very few have 2D microporous structures and sufficiently large micropore sizes which is suitable for the carbon-growth inside the pores. It was found that high-quality 2D microporous carbon materialcan be obtained via carbon deposition inside the 2D porous channel of zeolite followed by the etching treatment of zeolite framework. This novel material possesses extremely regular micropore size (<1 nm) and unexpectedly high pore density (~70 pores/100 nm2), which is approximately several orders of magnitude higher than that of perforated graphene materials prepared through conventional top-down methods (~1 pore/100 nm2). This material can be fabricated into high-performance TFC membranes with the thickness of ~ 500 nm (selective layer) via solution-phase chemical exfoliation followed by vacuum-assisted self-assembly on porous nylon support. Thismembrane showed ultra-high solvent permeance which is up to 21-fold higher than that of conventional graphene oxide-based membrane, with high selectivity (>90% rejection) toward organic molecules larger than 300 g mol-1. This is originated from the unique porous structure of the carbon material, and it is expected that the paradigm-changing membrane which overcomes existing permeability/selectivity trade-off relationship can be realized through further optimization of membrane fabrication method.