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

Pyrolysis experiments were carried out with polymeric model compounds conta ining defined forms of bound nitrogen. The chosen compounds, polyvinylcarba zole, 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 st ructure was received from FT-IR, solid-state C-13-NMR and, in part, X-ray a bsorption near edge structure (XANES) spectroscopy. The application of diff erent analytical methods should result in a more reliable classification of the N Is 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 temperatur e products. In the case of the carbazole system it is still the dominant bo nding form. The high-temperature char from polyvinylpyridine contains nitro gen in both five- and six-membered rings. It may be concluded that the beha viour of the nitrogen during pyrolysis does not only depend on its function ality but also on its chemical environment. A comprehensible mechanism of t he transformation of N-6 into N-5 nitrogen is discussed on the basis of the FT-IR and solid-state C-13-NMR spectra of the low temperature chars. (C) 1 999 Elsevier Science Ltd. All rights reserved.