Deep eutectic solvents as both precursors and structure directing agents in the synthesis of nitrogen doped hierarchical carbons highly suitable for CO2 capture

Abstract

Deep eutectic solvents (DESs) have been used in the synthesis of nitrogen-doped carbons exhibiting a hierarchical porous structure. The CO₂ sorption capacity of these solid sorbents was extraordinary because of their relatively high nitrogen content and their bimodal porous structure where micropores provide high surface areas (ca. 700 m² g⁻¹) and macropores provide accessibility to such a surface. DESs were composed of resorcinol, 3-hydroxypyridine and choline chloride in 2 : 2 : 1 and 1 : 1 : 1 molar ratios. Polycondensation of resorcinol and 3-hydroxypyridine (with formaldehyde) promoted DES segregation in a spinodal-like decomposition process by the formation of a polymer rich phase and a depleted polymer phase. Thus, DESs played a multiple role in the synthetic process; the liquid medium that ensured reagents homogenization, the structure-directing agent that is responsible for the achievement of the hierarchical structure, and the source of carbon and nitrogen of the solid sorbent obtained after carbonization. Interestingly, the homogeneous incorporation of nitrogen at the solution stage of the synthetic process (rather than by post-treatment of the preformed carbon) allowed the achievement of significant nitrogen contents even in carbons obtained at relatively high temperatures (e.g. 8–12 at% for 600 °C and ca. 5 at% for 800 °C). It is worth noting that, despite thermal treatments at high temperatures tend to decrease the nitrogen content, the high surface area of the solid sorbents obtained at 800 °C contributed to a significant enhancement of CO₂ capture while providing superior selectivity, recyclability and stability.