Transforming Volatile Amines into Stable Solid Sorbents via Crosslinking for Direct Air Capture of Carbon Dioxide

Abstract

Supported amines are popular solid sorbents for direct air capture (DAC) of carbon dioxide (CO2). However, those sorbents have several drawbacks such as leaching, volatility, oxidative degradation and corrosivity, among others. In this work, solid crosslinked (CL) polymers without the need of support were synthesised from several short chain amines that would be otherwise unviable for DAC due to their high volatility. This was achieved utilising an epoxide crosslinker N,N,N’,N’-Tetrakis(2,3-epoxypropyl)-m-xylene-alpha,alpha’-diamine (TEP-MXDA), and small amines including ethylenediamine (EtDA), diaminobutane (DAB), hexamethylenediamine (HMD), diethylenetriamine (DET), triethylenetetramine (TETA) and m-xylylenediamine (mXDA) through a one-pot procedure at room temperature for 12 hours. The reaction yielded no wastes, no side products and required no separation and purification, adhering to 11 of the 12 principles of green chemistry overall. Under realistic DAC conditions, CO2 absorption capacities up to 5.6 wt.% were achieved for CL-DET polymers. Additionally, the CL-TETA and CL-HMD polymers retained a greater CO2 capacity compared to their silica supported counterparts after accelerated ageing tests conducted in air at 75℃ for 24 hours, corresponding to approximately 150 absorption/desorption cycles. Novel insight into the effect of amine chain length on resulting polymer mesh size and CO2 absorption capacity is provided by positron annihilation lifetime spectroscopy (PALS) analysis, and reveals the beneficial role PEG plays in hydrogel-like amine-based crosslinked sorbents. This study demonstrates the viability of crosslinked short-chain amines as solid sorbents for DAC applications, and provides novel insight into the effects variables such as amine choice, amine chain length and crosslinking ratio have on the performance of the resulting materials in CO2 absorption.