Other Catalysts
Other catalysts that have been used in hydrotreating are supported NiCoMo
(still in use for hydrotreating) and NiW (very rarely in use for hydrotreating).
NiCoMo catalysts attempt to combine the benefits of CoMo and NiMo at intermediate pressures, achieving sufficient hydrodesulfurization (primarily) at
the minimum required hydrogen consumption. NiW catalysts are more widely
practiced in hydrocracking applications than hydrotreating applications. Historically, sulfided NiW exhibits hydrocracking activity surpassing that of both
CoMo and NiMo. Increasing the activity of the support material with promoters,
amorphous silica alumina, or zeolite can further enhance the hydrocracking
activity.
Type I vs. Type II Hydrotreating Catalysts
The designations “Type I” and “Type II,” as originally applied by Topsøe
et al. refer to the nature of the bonding between the active phase (e.g., CoMoS,
NiMoS) and the support, the extent of stacking of the active phase as dispersed
MoS 2 nanosheets, and the coordination of the promoter metal at the edges of the
nanosheets. The promoted active phase along the edges, as Co(Ni)-Mo-S structures,
is believed to be responsible for the relatively greater hydrotreating activity of Type
II catalysts. Catalysts of the former type consist predominantly of an active phase
that is more strongly bound to the support, as in Mo-O-Al where the support is
γ-alumina, with relatively shorter stacks of MoS 2 nanosheets, and wherein a
relatively smaller portion of the promoter metal is coordinated into the MoS 2
nanosheets’ edges. Catalysts of the latter type consist predominantly of an active
phase that is more weakly bound to the support via dipole-dipole (van der Waals)
interactions, with relatively taller stacks of MoS 2 nanosheets, and wherein a
relatively greater portion of the promoter metal is coordinated into the MoS 2
nanosheets’ edges. The active phase is, in general, fully sulfidable (Topsøe
et al. 1996; Hensen et al. 2002; Topsøe and Clausen 1984). This description may
require some revision, especially as regards the necessity for taller stacks of MoS 2
nanosheets.
Unsupported or Bulk Transition Metal Sulfide (TMS) Catalysts
The quantum improvement in hydrotreating catalyst performance can be observed
in the activity of unsupported TMS catalysts. Unsupported catalysts appear to
represent a hearkening back to the very early history of hydrotreating, when most
hydrotreating catalysts were unsupported (C ˇ ejka 2003). In principle, today’s
unsupported catalysts are comprised only of active phase, representing a much
higher population density of active phase per unit volume than Type I and Type II
supported catalysts, without the limitation of active phase-support interactions.
416
P. Kokayeff et al.
Other catalysts that have been used in hydrotreating are supported NiCoMo
(still in use for hydrotreating) and NiW (very rarely in use for hydrotreating).
NiCoMo catalysts attempt to combine the benefits of CoMo and NiMo at intermediate pressures, achieving sufficient hydrodesulfurization (primarily) at
the minimum required hydrogen consumption. NiW catalysts are more widely
practiced in hydrocracking applications than hydrotreating applications. Historically, sulfided NiW exhibits hydrocracking activity surpassing that of both
CoMo and NiMo. Increasing the activity of the support material with promoters,
amorphous silica alumina, or zeolite can further enhance the hydrocracking
activity.
Type I vs. Type II Hydrotreating Catalysts
The designations “Type I” and “Type II,” as originally applied by Topsøe
et al. refer to the nature of the bonding between the active phase (e.g., CoMoS,
NiMoS) and the support, the extent of stacking of the active phase as dispersed
MoS 2 nanosheets, and the coordination of the promoter metal at the edges of the
nanosheets. The promoted active phase along the edges, as Co(Ni)-Mo-S structures,
is believed to be responsible for the relatively greater hydrotreating activity of Type
II catalysts. Catalysts of the former type consist predominantly of an active phase
that is more strongly bound to the support, as in Mo-O-Al where the support is
γ-alumina, with relatively shorter stacks of MoS 2 nanosheets, and wherein a
relatively smaller portion of the promoter metal is coordinated into the MoS 2
nanosheets’ edges. Catalysts of the latter type consist predominantly of an active
phase that is more weakly bound to the support via dipole-dipole (van der Waals)
interactions, with relatively taller stacks of MoS 2 nanosheets, and wherein a
relatively greater portion of the promoter metal is coordinated into the MoS 2
nanosheets’ edges. The active phase is, in general, fully sulfidable (Topsøe
et al. 1996; Hensen et al. 2002; Topsøe and Clausen 1984). This description may
require some revision, especially as regards the necessity for taller stacks of MoS 2
nanosheets.
Unsupported or Bulk Transition Metal Sulfide (TMS) Catalysts
The quantum improvement in hydrotreating catalyst performance can be observed
in the activity of unsupported TMS catalysts. Unsupported catalysts appear to
represent a hearkening back to the very early history of hydrotreating, when most
hydrotreating catalysts were unsupported (C ˇ ejka 2003). In principle, today’s
unsupported catalysts are comprised only of active phase, representing a much
higher population density of active phase per unit volume than Type I and Type II
supported catalysts, without the limitation of active phase-support interactions.
416
P. Kokayeff et al.