Thermal Conversion of Guanylurea Dicyanamide into Graphitic Carbon Nitride via Prototype CN_xPrecursors

Guanylurea dicyanamide, [(H 2 N)C( O)NHC(NH 2 ) 2 ][N(C⋮N) 2 ], has been synthesized by ion exchange reaction in aqueous solution and structurally characterized by single-crystal X-ray diffraction ( C 2/ c, a = 2249.0(5) pm, b = 483.9(1) pm, c = 1382.4(3) pm, β = 99.49(3)°, V = 1483.8(5) × 10 6 pm 3, T = 130 K). The thermal behavior of the molecular salt has been studied by thermal analysis, temperature-programmed X-ray powder diffraction, FTIR spectroscopy, and mass spectrometry between room temperature and 823 K. The results were interpreted on a molecular level in terms of a sequence of thermally induced addition, cyclization, and elimination reactions. As a consequence, melamine (2,4,6-triamino-1,3,5-triazine) is formed with concomitant loss of HNCO. Further condensation of melamine yields the prototypic CN x precursor melem (2,6,10-triamino- s -heptazine, C 6 N 7 (NH 2 ) 3 ), which alongside varying amounts of directly formed CN x H y material transforms into layered CN x H y phases without significant integration of oxygen into the core framework owing to the evaporation of HNCO. Thus, further evidence can be added to melamine and its condensation product melem acting as “key intermediates” in the synthetic pathway toward graphitic CN x H y materials, whose exact constitution is still a point at issue. Due to the characteristic formation process and hydrogen content a close relationship with the polymer melon is evident. In particular, the thermal transformation of guanylurea dicyanamide clearly demonstrates that the formation of volatile compounds such as HNCO during thermal decomposition may render a large variety of previously not considered molecular compounds suitable CN x precursors despite the presence of oxygen in the starting material.