Deep eutectic assisted synthesis of carbon adsorbents highly suitable for low-pressure separation of CO2–CH4 gas mixtures

Abstract

Ionic liquids and deep eutectic solvents (a new class of ionic liquids obtained by complexation of quaternary ammonium salts with hydrogen bond donors such as acids, amines, and alcohols among others) have been recently used as solvents and even as precursors in the synthesis of carbonaceous materials. The use of long-alkyl-chain derivatives of ionic liquids that, playing the role of structure directing agents, were capable of designing the mesoporous structure of the resulting carbons is of particular interest. Meanwhile, deep eutectic solvents proved efficient in tailoring the structure comprised between large mesopores and small macropores, but the control over the smaller ones (e.g. small mesopores and micropores) is still a challenge as compared to ILs. Herein, we have used deep eutectic solvents composed of resorcinol, 4-hexylresorcinol and tetraethylammonium bromide for the synthesis of carbon monoliths with a tailor-made narrow microporosity. Behaving as molecular sieves, these carbons exhibited not only good capacities for CO₂ adsorption (up to 3 mmol g⁻¹) but also an outstanding – especially at low pressures – CO₂–CH₄ selectivity.