Mass Transfer in Boronate Ester 2D COF Single Crystals

Abstract

Unlike graphene and similar structures, 2D covalent organic frameworks (2D COFs) exhibit intrinsic porosity with a high areal density of well‐defined and uniform openings. Given the pore size adjustability, 2D COFs are likely to outperform artificially perforated inorganic layers with respect to their prospects in membrane separation. Yet, exploring the mass transport in 2D COFs is hidden by the lack of laterally extended free‐standing membranes. This work reports on direct molecular permeation measurements with single crystals of an interfacially synthesized boronate ester 2D COF. In accordance with the material topography, the atmospheric and noble gases readily pass the suspended nanosheets while their areal porosity is quantified to be almost 40% exceeding that in any 2D membranes known. However, bulkier aromatic hydrocarbons are found to deviate substantially from Graham's law of diffusion. Counterintuitively, the permeation rate is demonstrated to rise from benzene to toluene and further to xylene despite the increase in the molecular mass and dimensions. The results are interpreted in terms of adsorption‐mediated flow that appears to be an important transport mechanism for microporous planar nanomaterials.

Molecular Permeation in Freestanding Bilayer Silica

“…However, this does not imply the pores to be too small, as far greater transmembrane fluxes were witnessed when bilayer silica was exposed to saturated vapors of HW, PA, and MIBK. The behavior is reminiscent of the properties of carbon nanomembranes (CNMs) and 2D covalent organic frameworks (COFs) and can be explained by the permeation of adsorbed molecules as opposed to the backscatter of inert particles. , The surface-mediated diffusion is also evident from the steep pressure dependence of the permeation rates measured (Figure b), which reflects a nonlinear increase in the number of adsorbed molecules. Thus the probability of gas-phase molecules to get right to the middle of the pores at the normal incidence angle seems to be extremely low, whereas the adsorbates possess enough residence time to adjust their configuration.…”

Surface coverage of alcohols on carbon nanomembranes under ambient conditions

“…18 In a recent ACP study, single-crystalline 2D covalent organic frameworks (COFs) exposed to He, Ne, N 2 , Ar, CO 2 , Kr, and Xe demonstrated unimpeded molecular effusion, whereas the permeation of aromatic hydrocarbons was found to deviate from Graham's law of diffusion. 19 Despite the increasing molecular dimensions, the membrane appeared more selective towards xylene than toluene and benzene. This was attributed to favourable gas-membrane interactions, and in line with adsorption isotherms for condensable substances, the surface flux under given conditions was shown to depend on the saturation vapour pressure.…”

Thickness-Varied Carbon Nanomembranes from Polycyclic Aromatic Hydrocarbons

“…3 Perforated graphene and related single-layer materials have demonstrated encouraging performance, but controlling the size of artificial pores upon membrane upscaling has been proven challenging. 4−6 Fortunately, there exist planar nanostructures with intrinsic porosity, such as 2D covalent organic frameworks (COFs), 7 tetrahedral bilayer oxides, 8 and carbon nanomembranes (CNMs), 9 which feature high areal densities of uniform openings and become available in macroscopic quantities. In particular, CNMs prepared from self-assembled monolayers (SAMs) via electron irradiation exhibit spectacular characteristics with respect to water transport and promise benefits for various applications from desalination to dehydration of organic solvents.…”