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

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.