Change of chemical bonding of nitrogen of polymeric N-heterocyclic compounds during pyrolysis Original Research Article

Pyrolysis experiments were carried out with polymeric model compounds containing defined forms of bound nitrogen. The chosen compounds, polyvinylcarbazole, polyvinylpyridine and polyvinylpyrrolidone, were pyrolysed in a fixed bed annular reactor at 873 and 1173 K. The functionalities of the nitrogen in the precursors as well as that in the derived chars were determined by X-ray photoelectron spectroscopy (XPS). Additional information about the structure was received from FT-IR, solid-state 13C-NMR and, in part, X-ray absorption near edge structure (XANES) spectroscopy. The application of different analytical methods should result in a more reliable classification of the N 1s electron binding energies than is possible by the sole use of XPS. It is interesting to note that the nitrogen in five-membered rings (N-5) of the N-heterocyclic compounds remains in existence in the high temperature products. In the case of the carbazole system it is still the dominant bonding form. The high-temperature char from polyvinylpyridine contains nitrogen in both five- and six-membered rings. It may be concluded that the behaviour of the nitrogen during pyrolysis does not only depend on its functionality but also on its chemical environment. A comprehensible mechanism of the transformation of N-6 into N-5 nitrogen is discussed on the basis of the FT-IR and solid-state 13C-NMR spectra of the low temperature chars.