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Lotsch, Bettina Valeska (2006): From Molecular Building Blocks to Condensed Carbon Nitride Networks: Structure and Reactivity. Dissertation, LMU München: Fakultät für Chemie und Pharmazie



The scope of this thesis was defined by three major issues, which have been arising from the requirement to further extend and to deepen research on carbon nitride chemistry from a materials chemistry point of view. Access to highly condensed CNx species and, ultimately, binary carbon nitride C3N4, was primarily sought by using suitable carbon nitride precursor species, based on the following leitmotifs: 1. Gaining a deeper understanding of the reactivity of precursors and the mechanisms governing solid-state reactions, the latter being the key to the directed synthesis of novel precursor systems as well as to extended carbon nitride solids with tailored properties. 2. Developing novel CNx precursors based on the evaluation of reactivity principles and solid-phase reaction trajectories thus established. 3. Providing an experimental basis for the predominantly speculative discussion centered on the structure of graphitic carbon nitride-type systems, with the major focus being on the nature of the structural building-blocks of polymeric CNxHy solids. The interplay between structural requirements of suitable CNx precursors and their thermal reactivity was demonstrated by a combined 2H solid-state NMR and neutron diffraction study of ammonium dicyanamide, as well as by a comprehensive spectroscopic study of the thermal decomposition of ammonium cyanoureate. Various novel non-metal dicyanamides and tricyanomelaminates were synthesized, structurally characterized and screened for potential thermally induced solid-state reactivity. Their suitability as CNx precursors for the synthesis of graphitic carbon nitride g-C3N4 was evaluated, leading to the observation that the formation of melamine C3N3(NH2)3 is favored in all systems at elevated temperatures. Therefore, particular emphasis was placed on the study of the thermal behavior of the prototypic precursor melamine, whose pyrolysis, including the identities of the intermediates, has been a highly controversial issue during the past decades. The present work provides the structures of the two “missing links” in melamine condensation, melam [(H2N)2C3N3]2NH and melon [C6N7NH(NH2)]n. In addition, the identities of two further intermediates were resolved, which could be identified as co-crystallisates made up from melamine and melem in the distinct ratios 2:1 and 1:2, respectively. Ultimately, the identity of a CNxHy polymer obtained by pyrolysis of melamine at T = 893 -913 K was resolved by a concerted approach based on electron diffraction, solid-state NMR spectroscopy, and theoretical investigations. It was demonstrated that the material commonly associated with a hydrogen-contaminated graphitic carbon nitride material is in fact melon, a 1D polymer composed of NH-bridged heptazine rings first described by Liebig in 1834. Melon represents a so far unique example of a structurally characterized, 1D polymeric carbon nitride material, and at the same time sheds new light on the present discussion regarding the identity and structure of graphitic carbon nitride.