Insights on the synthesis mechanism of green phenolic resin derived porous carbons via a salt-soft templating approach

A combined salt-soft template approach to synthesize porous carbon materials is reported along with their synthesis mechanism. This consists in the evaporation induced self-assembly (EISA) of aqueous solutions containing green phenolic resins, a triblock copolymer template and a metallic salt, followed by thermal treatment and washing. The increase of pH up to 5 using NaOH, induces significant improvement in the carbon microporosity but in the detrimental of mesoporosity. As suggest by 13C and 1H NMR, the mesoporosity lost is caused by the decrease of H-bonding and self-assembly between the phenolic resin and the template due to the strong “salting-out” effect of OH ions. For higher pH (pH-9), the porosity start to decrease and graphene-sheet like morphology is formed. The microporosity varies with the salt in the following order: KCl > NaCl > LiCl, while the mesoporosity in the opposite way. The structure changes as well from smooth turbostatic (KCl) to defective graphitic one (NaCl, LiCl). These textural and structural modifications are explained in terms of cation hydration enthalpy and cation-π binding energy and by the competition between the metal salt cations and the Na ions (used to regulate the pH) for water or phenolic resin aromatic ring sites.