Catalyst Deactivation - Spent Hydroprocessing Catalysts - page 74

with increasing temperature [36] . Then, at temperatures approaching 700 K, the SH groups
could donate proton and initiate the formation of carbocations. Therefore, even for
carbon-supported catalysts, the involvement of carbocations during coke formation cannot be
entirely ruled out.
The development of the mechanism of coke formation benefits from the advancements in
spectroscopic techniques (e.g., proton nuclear magbetic resonance [1H NMR], carbon 13
nuclear magnetic resonance [ 13 C NMR], laser desorption mass spectroscopy [LD-MS],
Fourier transfer infrared [FTIR], etc.) and other analytical methods, which allowed more
detailed analysis of both coke as well as corresponding feed and products. This allowed the
determination of various structural parameters of the feed, products and the coke, which was
deposited on the catalyst surface. With the availability of such information, the mechanism of
coke formation could be defined more accurately and in more details.
The CH 2 Cl 2 soluble and insoluble parts of the deposit on two spent catalysts from
hydroprocessing of VGO were characterized by Sahoo et al. [232] . The structural parameters
of the former, termed as a “soft” coke, were similar as those of the heavy components of the
VGO feed. At the same time, the “hard” coke was more aromatic but less aromatic than the
similar “hard” coke on the spent catalysts from hydroprocessing of residues. This is not
surprising because the latter require more severe conditions (e.g., higher temperatures) to
attain desirable level of conversions. Also, in the case of VGO, the HYD of some coke
components could occur because of the less severe conditions. Then, the factors, which
dominate coke formation using the distillate feeds, may differ from those for the residues.
Seki and Kumata [233,234] carried out the extensive characterization of asphaltenes and resins
in the products by spectroscopic techniques. The study involved the HDM and HDS of the
Kuwait atmospheric residue over the Mo/A 2 O 3 and NiMo/Al 2 O 3 catalysts, respectively. In this
case, the molecular weight of both asphaltenes and resins in products progressively decreased
in the course of HDM reactions. The rate of coke build-up significantly increased above 673 K.
This was accompanied by the removal of alkyl chains from asphaltenes molecules. Therefore,
the aromaticity of asphaltenes was increased. Such change facilitated the adsorption of
asphaltenes on the catalyst surface and increased deactivation. In the presence of alkyl chains,
the adsorption of asphaltenes was diminished because of the steric interference between the
coke molecules and catalyst surface, provided mainly by aliphatic chains.
Fonseca et al. [235-237] recognized that the solid-state 13 C NMR could be a useful tool for the
characterization of coke deposits on catalyst surface. The investigated CoMo/Al 2 O 3 catalyst
(0.7 wt.% CoO; 4.5 wt.% MoO 3 ) was used in the three-stage ebullated-bed pilot plant. The
spent catalyst was withdrawn after four, 21 and 120 days on stream from the first and third
reactor. The feed was the blend of Khafji vacuum residue and a diluent. Less than 69% of the
coke carbons could be observed by the NMR technique employed. In the study of Hauser et al.
[238,239] , the solid-state 13 NMR with the application of the cross polarization with