Boron doped g-C3N4 as an effective metal-free solid base catalyst in Knoevenagel condensation

In this paper, boron, sulfur and phosphor doped g-C3N4 materials (defined as CNBF, CNSF and CNPF, respectively) were successfully prepared by thermal copolymerization of dicyandiamide and imidazole ionic liquids. The CNBF catalysts exhibited significantly enhanced activity in various Knoevenagel condensation of aldehyde with malononitrile, which were typical base-catalyzed reactions, comparing with CNSF and CNPF catalysts. The detailed characterizations of Fourier transform infrared spectra (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), High angle annular dark field-scanning transmission electron microscopy (HADDF-STEM) and B-11 solid-state MAS NMR showed that boron was evenly doped in tri-s-triazine rings of g-C3N4 by substituting the carbon sites. In addition, CO2 Temperature programed desorption (CO2-TPD) characterizations indicated that the basic site concentration of CNBF catalysts was proportional to the doped boron content. Excluding the effect of specific surface area, the catalytic activity of CNBF catalysts followed the order CNBF-0.1 < CNBF-0.3 < CNBF-0.5 < CNBF-0.7, which was the same as that of boron content and basic site concentration. The catalytic performance revealed that boron-doping was an effective strategy to enhance the basicity of g-C3N4. This study will put forward the further application of boron-doped or boron-dominant two dimensional materials towards metal-free heterogeneous base catalysis.