Hydroprocessing of Petroleum - Spent Hydroprocessing Catalysts - page 28

was greater than that of the -Al 2 O 3 support. It has been shown that the acidity control became
critical for achieving a high level of HDS (deep and/or ultra HDS) of distillates [75,83] . This
was confirmed by a much higher HDS activity of the CoMo catalysts supported on carbon
support compared with that of the corresponding catalysts supported on -Al 2 O 3 [75] . The
latter catalysts were more sensitive to poisoning by N-containing bases present in distillates.
Support interactions also play a key role in the dispersion and morphology of the active phases
(e.g., Co-Mo-S and Ni-Mo-S) [73,84] . Studies have shown that strong interactions between the
molybdate ions and support lead to the formation of low-active type I Co-Mo-S structures,
which are incompletely sulfided, and have some remaining Mo-O-Al linkages [53] . The
application of high-resolution electron microscopy has provided valuable information on the
degree of stacking in MoS 2 and Co-Mo-S structures prepared with different supports [85,86] .
Very weak support interaction resulted in the formation of multistacking of type II Co-Mo-S
phase. The degree of stacking can be controlled by carefully controlling support properties.
Formation of small stable single slabs MoS 2 crystallites on alumina support have been
observed. Such slabs will have a high MoS 2 edge concentration and dispersion and, as such,
can accommodate more Co and Ni atoms to form higher activity single slab type II Co-Mo-S
and Ni-Mo-S structures.
3.3.2 Physical Properties
The chemical composition of catalysts may not be so important unless suitable surface
properties have been established. This is desirable for maintaining a long life of catalyst during
the operation. Besides surface properties, the optimal size and shape of particles have to be
chosen to achieve optimal performance of catalyst. Furthermore, the catalyst utilization
usually increases with the decreasing size of catalyst particles. The influence of porosity as
well as that of the size and shape of catalyst particles is evident even for relatively light feeds,
such as AGO, VGO, and HGO [71] . Of course, for the asphaltenes and metals containing
feeds, the design and selection of the catalysts have become a much more challenging task.
Among the surface properties, pore volume and pore size distribution as well as the mean pore
diameter of the catalyst are much more important than surface area when heavy feeds are
considered. At the same time, for light feeds, surface area may be a reasonable indication of
the catalyst suitability. A high surface area and moderate pore volume catalysts are very active
for HDS because of the efficient dispersion of active metals in the pores. However, in the case
of heavy feeds, these pores become gradually unavailable because they are deactivated by pore
mouth plugging. On the other hand, the catalysts with a small surface area and a large pore
volume are less active because of the lower concentration of active sites. However, they are
more resistant to deactivation by pore mouth plugging and their metal storage capacity is
greater, therefore such catalysts may be suitable for HDM and HDAs. Apparently, the relation
between surface properties and catalyst activity is more complex as it is indicated by numerous