Resorcinol-Based Deep Eutectic Solvents as Both Carbonaceous Precursors and Templating Agents in the Synthesis of Hierarchical Porous Carbon Monoliths

Deep eutectic solvents are a new class of ionic liquids obtained via the complexion of quaternary ammonium salts with hydrogen-bond donors (such as acids, amines, and alcohols, among others). The charge delocalization that occurs through hydrogen bonding between the halide anion with the hydrogen-donor moiety is responsible for the decrease in the freezing point of the mixture, relative to the melting points of the individual components. We have recently reported on the use of deep eutectic solvents as suitable solvents, to carry out the polycondensation of resorcinol−formaldehyde. [Chem. Mater. 2010, 22, 2711−2719.] Herein, we describe the synthesis of deep eutectic solvents (DESs) based on resorcinol, the use of which as both carbonaceous precursors and structure-directing agents allowed the preparation of hierarchical porous (bimodal, with micropores and mesopores) carbon monoliths via formaldehyde polycondensation and subsequent carbonization. The performance of resorcinol-based DESs as carbonaceous precursors was remarkable, with carbon conversions of ∼80%. Moreover, the use of DESs as structure-directing agents resulted in the achievement of hierarchical porous carbon monoliths with pore surface areas up to 600 m2/g and narrow mesopore diameter distributions. The mechanism governing the formation of mesopores was based on a spinodal-decomposition-like-process via resorcinol polycondensation and subsequent segregation of the resorcinol counterpart that is forming the DESs. Thus, the use of resorcinol-based DESs that have different counterparts (e.g., either choline chloride or a mixture of choline chloride and urea) allowed the preparation of hierarchical carbons with tailored mesopore diameters of ca. 23 nm and ca. 10 nm.