Catalyst Deactivation - Spent Hydroprocessing Catalysts - page 75

polarization inversion at the low or moderate magic angle spinning was used. The analysis was
complemented by using the proton-gated decoupled single-pulse excitation and by the results
of elemental analysis. With this approach, the limitations of the technique, used by Fonseca
et al. [235-237] , were minimized. The spent catalysts were extracted either by toluene or THF
before their characterization. After 1 h on stream, the H/C ratios of both coke and asphaltenes
in the feed were similar. This suggests that fouling of the catalyst by asphaltenes was the main
cause of the coke formation. This would indicate little involvement of the catalyst surface
during very early stages. However, the NMR analysis of the toluene insolubles (TIS)- and
tetrahydrofuran insolubles (THFIS)-coke indicated that already after 1 h on stream, the
structure of asphaltenes changed after being deposited on the catalyst. This included the loss of
long chains in particular. As it is shown in Fig. 4.20 , after 1 h, the TIS- and THFIS-coke
structures differed from that of asphaltenes. It consisted of the less polycondensed aromatic
rings with shorter but heavily branched alkyl substituents attached to them. Between 1 and
12 h, the coke deposition slowed down. In this region, a simultaneous accumulation of
aromatic carbon in the coke, some HYD of aromatic rings, isomerization and dealkylation
were occurring simultaneously. For the THFIS-coke, a ring condensation occurred as well.
Figure 4.20: Effect of time on stream on structure of TIS and THFIS of coke on catalyst
(Mo/Al 2 O 3 , atmospheric residue [AR], 653 K, 12MPa) [From ref. 239 . Reprinted with
permission].