Aerobic selective oxidation of 5-hydroxymethyl-furfural over nitrogen-doped graphene materials with 2,2,6,6-tetramethylpiperidin-oxyl as co-catalyst†
N-doped graphene materials with various types and quantities of N species were prepared via thermal treatment of graphene oxide in flowing NH3, and their catalytic performance was tested in aerobic selective oxidation of 5-hydroxymethyl-furfural (HMF). A full HMF conversion and nearly 100% selectivity to DFF can be obtained under relatively mild reaction conditions (6 h, 100 °C, 1 atm air pressure) with 2,2,6,6-tetramethylpiperidin-oxyl (TEMPO) as co-catalyst. The reduction degree of graphene oxide in NH3 was characterized by visible Raman spectroscopy. The amount and nature of doped nitrogen species were examined by X-ray photoelectron spectroscopy to reveal the genesis of active species by nitrogen doping. The graphitic nitrogen species doped into the graphene lattice were demonstrated to be responsible for the activation of molecular oxygen. In the selective oxidation of HMF, N-doped graphene showed a much lower apparent activation energy (ca. 13.5 kJ mol−1) compared with conventional active carbon supported metal catalysts (Ru/C, Pt/C, Pd/C, Au/C, 51–77 kJ mol−1). Based on current results and previous reports, a possible reaction pathway with the graphitic N species as active sites was proposed. This study examines the origin of the enhanced catalytic activity, which can be linked to the synergistic effect of TEMPO, N-doped graphene and molecular oxygen. This kind of synergistic effect makes the oxidation of HMF run smoothly. The present study demonstrates the potential of functionalized nitrogen-doped graphene materials as an efficient and alternative material to metal-based catalysts for organic synthetic reactions.