Sulfur-mediated synthesis of N-doped carbon supported cobalt catalysts derived from cobalt porphyrin for ethylbenzene oxidation

Nitrogen-doped carbon supported cobalt catalysts are synthesized by a sulfur-mediated heat treatment. Cobalt(II) meso-tetraphenyl porphyrin (CoTPP) is used as a nitrogen-rich ligand for Co-N-x precursor complex formation and sublimed sulfur is designed as an external sulfur species. The catalysts have been investigated by techniques such as BET, TEM, HRTEM, SEM, XRD, Raman, FT-IR and XPS. The results reveal that the addition of sublimed sulfur not only changes the physical structure of the catalysts, such as specific surface area and pore volume, but also modifies the chemical properties by doping into the catalysts as well as changing the content of Co-N-x sites. When the mass ratio of adding sublimed sulfur is 0.2, the cobalt catalyst exhibits higher catalytic performance for ethylbenzene oxidation (i.e., 17.3% ethylbenzene conversion and 75.6% selectivity to acetophenone). This is ascribed to more Co-N-x active sites caused by the enhancement of sublimed sulfur. In addition, it could be easily recovered by centrifugation and reused 6 times without any significant loss in activity and selectivity (i.e., 16.3% ethylbenzene conversion and 73.9% selectivity to acetophenone) which may be attributed to the stabilization effect of S on Co-N-x by modifying the electronic density of the carbon matrix.