Templateless Synthesis of Ultra-Microporous 3D Graphitic Carbon from Cyclodextrins and Their Use as Selective Catalyst for Oxygen Activation

Pyrolysis of α-, β-, and γ-cyclodextrins at 900 °C gives rise to the formation of crystalline graphitic porous nanoparticles (G_CD), where the dimensions of the pores are uniform in the range from 0.63 to 0.97 nm, from G_α-CD to G_γ-CD, as determined by transmission electron microscopy. It is found that, while for G_β-CD and G_γ-CD, the surface area measured by N₂ adsorption is about 330–550 m² g⁻¹, respectively, no area can be measured for G_α-CD with N₂ or Ar due to its small pore dimensions. However, CO₂ adsorption reveals for G_α-CD the presence of ultra-microporosity and a surface area of 727 m² g⁻¹. G_CD exhibits activity as metal-free catalysts for the aerobic oxidation of alcohols and the activity increases as the pore dimension decreases. Density functional theory calculations indicate that this high catalytic activity for O₂ activation derives from confinement effects that favor charge transfer from the graphitic walls to O₂. Studies on the formation mechanism shows that the key step leading to the formation of the channels is the melting of cyclodextrin precursors that makes possible the assembly of these capsules before their transformation into microporous graphitic particles.