Characterization of coke by MAS 13C-NMR

The molecular architecture of coke is difficult to determine. Nuclear magnetic resonance spectroscopy allows one to understand the chemical environment or types of hydrogen and carbon in the sample. It is also possible to estimate the proportion of atoms in the various environments. Here, liquid-state and solid-state NMR have been used to characterize different series of feedstocks and coke samples. Liquid phase 13C NMR was used to analyse feeds and products separated by HPLC (PEW) in fractions called aliphatics, mono-, di-, tri-, and polyaromatics. An internal lock with deuterated chloroform was used due to the small amount of material present in each fraction. The NMR analysis gives the aromaticity and gives a nice correlation with the results obtained by GC/MS (average no of C-atoms in each fraction, chainlength, no. of substituents etc.). Structural features were gained by comparison of different fractions, while differences were seen, when feed and product were compared. For liquid samples, a model has been proposed in which the feed is supposed to be described as a mixture of six model compounds. Results concerning three series of feedstocks and the structural paramaters deduced from 13C and 1H NMR spectra are discussed and compared with the experimental data obtained from elemental analysis. In the solid state, by using Cross Polarization (CP) and Magic Angle Spinning (MAS), it is possible to obtain information on different carbon structural types. Structural parameters, such as the aromaticity fa or the H/C ratio can be evaluated with a good reliability. (AB) (23 refs.)