Green Synthesis of Robust Imine-Linked Two-Dimensional Covalent Organic Frameworks in Supercritical Carbon Dioxide

Abstract

The traditional vacuum activation of covalent organic framework (COF) materials typically erodes the materials' crystallinity significantly and then reduces the surface area of isolated materials. Developing new methods for synthesizing robust COFs which could survive the duration of the vacuum activation process is of great practical significance. In this work, a robust imine-linked 2D COF TAPB-TA was creatively constructed in supercritical CO 2 (ScCO 2 ) with γ-valerolactone (GVL) as a cosolvent instead of previously widely reported reaction conditions. Upon vacuum activation, the as-prepared COF (denoted as Sc-COF) via this green synthesis could still possess a high surface area, while preventing the pores from undesirable collapse and the loss of material quality could be avoided successfully. Further, Sc-COF showed excellent stability toward solvent immersion−reactivation cycles. Furthermore, this method is universal and can be facilely extended to the synthesis of other iminelinked 2D COFs and is expected to be transferable to a broader range of customized COFs. Theoretic calculations imply that ScCO 2 serves to attenuate the COF−COF interactions during the activation process preventing the collapse of the structure. It decreases the viscosity of the solution as compared to pure GVL, enabling fast diffusion kinetics of monomers to the reactive sites. Moreover, thanks to the low dielectric constant, usage of the ScCO 2 solvent diminishes the energy barriers of imine bond formation, thus facilitating ordered COF growth. These insights have important implications for studying and understanding how more robust 2D COFs could form and thus greatly expand their practical applications.

Organic electrodes based on redox-active covalent organic frameworks for lithium batteries

“…108 Most COFs are synthesised using solvothermal conditions (typically between 120 and 150 °C), acid catalysts and high boiling point organic solvents such as mesitylene, 1,4-dioxane, or N , N -dimethylacetamide. 105 More sustainable and environmentally friendly syntheses have already been proposed by replacing organic solvents with water, 109,110 using supercritical CO 2 , 111 or solvent-free methods. 112 Microwave-assisted synthesis of COFs has also been extensively explored to considerably reduce reaction times.…”

Supercritical CO2 Directional‐Assisted Synthesis of Low‐Dimensional Materials for Functional Applications

“…Li et al used SC CO 2 and γ-Valerolactone (GVL) as a reaction medium to obtain iminelinked 2D COF 1,3,5-tris(4-aminophenyl)benzene/terephthalaldehyde (TAPB-TA). [216] During the synthesis process, SC CO 2 is proven to transform the fragile COF into a robust COF with maintaining high crystallinity and high surface area, compared to after vacuum activation, only postprocessing of SC CO 2 and solvent immersion-reactivation cycles. It is found that the sharp and more peaks intense XRD patterns and Raman spectra imply the higher crystallinity of Sc-COF, according to Figure 24b,c.…”

“…Reproduced with permission. [216] Copyright 2022, American Chemical Society. illustration of phase transformation from bulk Cu-BTC to 2D topological structure.…”

“…h) Mechanism of acetic-acid catalyzed imine formation under different dielectric environments: in GVL solution with a dielectric constant of ε = 34.47 and in a GVL + SC CO 2 mixture with a dielectric constant close to vacuum with ε = 1. Reproduced with permission [216]. Copyright 2022, American Chemical Society.…”

Green and Facile Preparation of Covalent Organic Frameworks Based on Reaction Medium for Advanced Applications

“…In an investigation by Xue et al, robust imine-linked 2D COFs were creatively fabricated by a green method using 1,3,5tris(4-aminophenyl)benzene/terephthalaldehyde monomers in supercritical CO 2 with γ-valerolactone as a co-solvent. [73] γ-Valerolactone is derived from biomass and has great features low toxicity, biodegradability, low vapor pressure, and good stability in both acidic and basic media. Also, supercritical CO 2 is a green solvent with zero surface tension, gas-like diffusivity, and viscosity as well as liquid-like density and solvating properties.…”