17
active metals are supported on a carrier in HDT catalysts, and the most commonly
used support or carrier material is γ-Al 2 O 3 due to its favorable characteristics in
terms of textural/mechanical properties and relatively low cost. γ-Al 2 O 3 carrier usually provides high surface area for maximum dispersion of active phases and good
mechanical strength to the catalyst. Alumina does not act as an inert carrier material
in hydrotreating catalysts but plays a significant role in the catalyst performance
through interaction with the active metals [30]. Interaction of support and active
metals plays an important role in enhancing the activity of hydrotreating catalysts.
For example, silica-supported Ni-Mo or Co-Mo hydrotreating catalysts exhibit relatively lower activity compared to alumina-supported catalysts due to weak interactions between the silica support and active metals [10]. Support–metal interaction
affects the catalytic activity by strongly influencing the formation and dispersion of
active catalytic phases such as Co-Mo-S and Ni-Mo-S on the surface of the catalyst.
Although γ-alumina has favorable characteristics in terms of mechanical strength
resulting in longer catalyst life and high surface area and pore volume, it was
reported to have strong interaction with transition metal oxides leading to incomplete sulfidation and lower HDS performance of the final catalyst. This fact resulted
in the search for the development of better supports to overcome the limitations of
traditional alumina support. Some of the support materials investigated by researchers for their suitability in HDT/HDS applications include carbon, TiO 2 , ZrO 2 , zeolites (e.g., USY), MgO, clays, silica, zirconia, titania, and mesoporous materials
(e.g., MCM-41).
Acidic supports such as zeolites and amorphous silica-alumina were also considered for HDS catalysts, and studies were performed for their effectiveness using
CoMo, NiMo, or Mo metallic phases. The acidic zeolites and amorphous silicaalumina- based catalysts were found to improve the HDS of recalcitrant sulfur species such as 4,6-DMDBT due to enhanced isomerization of the alkyl groups
resulting in the suppression of the steric hindrance effect. Among the acidic components, Y zeolite has been found to be effective for the removal of refractory sulfur
species from diesel due to its favorable physical properties, acid strength, acidity,
and hydrogen transfer activity. Incorporation of supports with high acidity in HDT/
HDS catalysts could enhance the rate of HDS of refractory sulfur compounds by
promoting reactions such as isomerization and dealkylation, which may favorably
remove the steric hindrance of these molecules [31–36]. Isoda et al. [31, 35, 36]
studied deep HDS pathways for 4,6-DMDBT with the addition of 5 wt% Y-zeolite
in CoMo/Al 2 O 3 catalyst and found favorable HDS activity through modification of
the molecular structure of the refractory species through isomerization and demethylation of methyl groups. Pawelec et al. [37], Tanga et al. [38], and Sun et al. [39]
studied bifunctional HDS catalysts with incorporation of different acidic components such as β zeolite, ZSM-5, and Y zeolites in the support material. The Brønsted
acidity of these catalysts was responsible for the isomerization activity, which
enabled removal of steric hindrance for sulfur removal [33].
As already discussed, metal–support interactions play a key role in the formation
of Type I and Type II catalytic active sites and result in differences in the catalyst
activity. In addition to this, support interactions are also found to affect the degree
Recent Advances in Hydrotreating/Hydrodesulfurization Catalysts: Part I: Nature…
active metals are supported on a carrier in HDT catalysts, and the most commonly
used support or carrier material is γ-Al 2 O 3 due to its favorable characteristics in
terms of textural/mechanical properties and relatively low cost. γ-Al 2 O 3 carrier usually provides high surface area for maximum dispersion of active phases and good
mechanical strength to the catalyst. Alumina does not act as an inert carrier material
in hydrotreating catalysts but plays a significant role in the catalyst performance
through interaction with the active metals [30]. Interaction of support and active
metals plays an important role in enhancing the activity of hydrotreating catalysts.
For example, silica-supported Ni-Mo or Co-Mo hydrotreating catalysts exhibit relatively lower activity compared to alumina-supported catalysts due to weak interactions between the silica support and active metals [10]. Support–metal interaction
affects the catalytic activity by strongly influencing the formation and dispersion of
active catalytic phases such as Co-Mo-S and Ni-Mo-S on the surface of the catalyst.
Although γ-alumina has favorable characteristics in terms of mechanical strength
resulting in longer catalyst life and high surface area and pore volume, it was
reported to have strong interaction with transition metal oxides leading to incomplete sulfidation and lower HDS performance of the final catalyst. This fact resulted
in the search for the development of better supports to overcome the limitations of
traditional alumina support. Some of the support materials investigated by researchers for their suitability in HDT/HDS applications include carbon, TiO 2 , ZrO 2 , zeolites (e.g., USY), MgO, clays, silica, zirconia, titania, and mesoporous materials
(e.g., MCM-41).
Acidic supports such as zeolites and amorphous silica-alumina were also considered for HDS catalysts, and studies were performed for their effectiveness using
CoMo, NiMo, or Mo metallic phases. The acidic zeolites and amorphous silicaalumina- based catalysts were found to improve the HDS of recalcitrant sulfur species such as 4,6-DMDBT due to enhanced isomerization of the alkyl groups
resulting in the suppression of the steric hindrance effect. Among the acidic components, Y zeolite has been found to be effective for the removal of refractory sulfur
species from diesel due to its favorable physical properties, acid strength, acidity,
and hydrogen transfer activity. Incorporation of supports with high acidity in HDT/
HDS catalysts could enhance the rate of HDS of refractory sulfur compounds by
promoting reactions such as isomerization and dealkylation, which may favorably
remove the steric hindrance of these molecules [31–36]. Isoda et al. [31, 35, 36]
studied deep HDS pathways for 4,6-DMDBT with the addition of 5 wt% Y-zeolite
in CoMo/Al 2 O 3 catalyst and found favorable HDS activity through modification of
the molecular structure of the refractory species through isomerization and demethylation of methyl groups. Pawelec et al. [37], Tanga et al. [38], and Sun et al. [39]
studied bifunctional HDS catalysts with incorporation of different acidic components such as β zeolite, ZSM-5, and Y zeolites in the support material. The Brønsted
acidity of these catalysts was responsible for the isomerization activity, which
enabled removal of steric hindrance for sulfur removal [33].
As already discussed, metal–support interactions play a key role in the formation
of Type I and Type II catalytic active sites and result in differences in the catalyst
activity. In addition to this, support interactions are also found to affect the degree
Recent Advances in Hydrotreating/Hydrodesulfurization Catalysts: Part I: Nature…