Coke characterisation in aged residue hydrotreating catalysts by solid-state C-13-NMR spectroscopy and temperature-programmed oxidation

A long term (7400 h) petroleum residue hydroprocessing study was conducted with a view to gain a clear understanding of the nature of the coke that de posits on the catalyst surface during the operation and to investigate the influence of processing time on coke build-up. Elemental analysis of the catalysts showed a rapid coke build-up on the cat alyst surface during the early hours of the run, reaching as high as 12.4 w t.% within 100 h, and a maximum value between 1100 and 2100 h. From 2100 h, it was showed a significant decrease of the carbon content reaching a stab le value from 3100 h. The enhanced nitrogen content of the coke deposits su ggested that asphaltene fractions containing nitrogen were concentrated on the catalyst in the initial operation period. Additionally, the coke deposits on the spent catalysts were investigated us ing C-13 solid-state nuclear magnetic resonance techniques of cross-polaris ation with magic-angle spinning (CP-MAS) for characterising the aromatic an d aliphatic carbon types. Nuclear magnetic resonance (NMR) spectra revealed an increase of the aromaticity with a further increase of time on stream a nd the removal of aliphatic reversible carbon from the catalysts. Temperature-programmed oxidation (TPO) study showed the existence of differ ent profiles, which characterised the reactivity of the deposits on the cat alysts. (C) 2001 Elsevier Science B.V. All rights reserved.