English

Swelling Characteristics of g-C₃N₄ as Base Catalyst in Liquid-Phase Reaction

• Wu Jiequn ,
• Hua Weiming ,
• Yue Yinghong ^, ,
• Gao Zi

• Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200438, P. R. China

Corresponding author: Yue Yinghong, yhyue@fudan.edu.cn

• Received Date: 17 April 2019
  Accepted Date: 22 May 2019
  Revised Date: 05 May 2019
  Available Online: 15 January 2020
• Fund Project:

  The project was supported by the National Natural Science Foundation of China (91645201, 21273043), the National Key R&D Program of China (2017YFB0602204), and the Science & Technology Commission of Shanghai Municipality, China (13DZ2275200)

Abstract: Graphitic carbon nitrides (g-C₃N₄) with different surface areas were prepared by pyrolysis using different precursors including melamine, dicyandiamide, thiourea and urea, and subsequently characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA) and N2 adsorption. Their basicities were measured by temperature-programmed desorption of CO₂ (CO₂-TPD) and acid-base titration. The catalytic properties for the Knoevenagel condensation of benzaldehyde and malononitrile were investigated in various solvents. In non-polar toluene solution, the benzaldehyde conversions of the g-C₃N₄ catalysts were low and changed according to their respective surface areas and basicities. However, in polar ethanol solution, the benzaldehyde conversions of all catalysts were similar, and much higher than those in toluene. This could not be explained by the results obtained from either of the two conventional basicity measurements. Further experimental results proved that g-C₃N₄ catalysts swelled in polar solutions, and more basic sites were exposed on the surface of the swollen catalysts, leading to the imminent increase in catalytic activity. This was proved by the catalyst poisoning data, which showed that the g-C₃N₄ catalyst lost its activity completely in toluene by adding 40.9 mmol·g^-1 benzoic acid, while the same catalyst was still active in ethanol until the added amount exceeded 143.3 m·g^-1. Additionally, the reaction tests in various solutions showed that the swelling effect was enhanced according to the polarity of the solvent used. A similar conclusion could be reached for the Knoevenagel condensation of furfural and malononitrile in various solvents. The reusability of g-C₃N₄ catalyst in Knoevenagel condensation was also studied, which showed that g-C₃N₄ was stable in liquid-phase reactions, whose activity dropped from 74.2% to 63.8% after three regeneration processes.

Key words:
• g-C₃N₄
•  /  Base catalysis
•  /  Knoevenagel condensation
•  /  Swelling effect

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