STRUCTURAL CHARACTERIZATION OF CATALYTIC COKE BY SOLID-STATE C-13-NMRSPECTROSCOPY

After reviewing some recent studies on the characterisation of coke de posits on fluid catalytic cracking (FCC) and hydroprocessing catalysts by solid state C-13-MMR, the quantitative structural information that has been obtained through the use of demineralisation of FCC catalyst s to provide coke concentrates for analysis will be described. The dea ctivated catalysts investigated contain only approx. 1% (w/w) carbon a nd were obtained both from refinery units operating with heavy feeds a nd from laboratory fluidised-bed tests with n-hexadecane. As for other carbonaceous materials, the use of a low-field field strength in conj unction with the single pulse excitation (SPE or Bloch decay) techniqu e has enabled most of the carbon to be detected and, therefore, NMR-in visible graphitic layers are not thought to be major structural featur es of the cokes. Although stripping the catalysts gives rise to highly aromatic cokes (aromaticity>0.95), even for n-hexadecane, differences in feedstock composition are still reflected in the structure of the resultant cokes with those derived from n-hexadecane containing less c ondensed aromatic nuclei than those from heavy feeds.