Amine-Functionalized Lignin for CO₂ CapturePart 2: A Double Amine Grafting Strategy

Global warming has been exacerbated by the escalating emissions of greenhouse gases from fossil fuel power plants and other industrial and transportation sources. Postcombustion CO₂ capture by amine-containing materials has evolved into a burgeoning industry for mitigating point-source emissions. Nevertheless, the deployment of sustainable and biodegradable solid-supported amine adsorbents remains underexplored despite their tunable surface chemistry that can be leveraged for CO₂ adsorption. Therefore, the purpose of this study is to valorize lignin for CO₂ capture using a double amine functionalization method. Lignin was subjected to two separate grafting procedures: one with diethylenetriamine (DETA) to produce DETA-aminated lignin (DAL) and another procedure involving 3-aminopropyltrimethoxysilane (APTMS) and triaminosilane (TRI) to make APTMS- and TRI-grafted lignin. Subsequently, DAL was further grafted by TRI (TRI/DAL) to increase amine content and improve CO₂ capture. TRI/DAL was found to achieve a considerably higher CO₂ uptake than that of individually modified materials. In the presence of 15% dry CO₂/N₂ at 25 °C, CO₂ uptake reached 1.31 mmol/g, representing a 72% increase compared to DAL (0.76 mmol/g) and more than 250% of TRI/Lignin-1 (0.37 mmol/g). Humidity further promoted CO₂ capture, with a CO₂ uptake of 1.84 mmol/g and an amine efficiency of 0.43 mol CO₂/mol N at 55% relative humidity (RH). In addition to enhancing CO₂ uptake, the presence of moisture improved the cyclic stability of the material, with TRI/DAL retaining 97.5 and 95% of its initial capacity after ten cycles at 25 and 50 °C in the presence of 15% CO₂/N₂ with 55% RH.