Introduction
Recently, with the increasingly strict environmental regulations,
various novel supports for hydrogenation catalysts are studied to remove
the sulfur in the oil product.[1, 2] For designing
good-performance hydrotreating catalysts, it is significant to
understand the HDS reaction mechanisms and kinetics of sulfur-containing
molecules. According to the structure model proposed by Topsøe, the
catalytic active sites of Co-Mo-S are located on the edge of
MoS2 slabs.[3] It was reported
that MoS2 slabs principally including two types of
edges: the sulfur-terminated edge (S-edge) and Mo-terminated edge
(M-edge). [4] Prins reported that DDS reaction may
be likely to occur on the metal edges of the Co-MoS2crystallites, and the HYD reaction occurs on the brim sites close to the
edges of the MoS2 plane and Co sites located at the
metal edge.[5] Moses et al. report that the HYD
and DDS routes could occur on the Mo-edge and S-edge, but the breakage
of C-S bond is easier to take place on S-edge sites, which is also
considered as the rate limiting step in HDS
reaction.[6] Oliviero et al reveal that S-edge is
intrinsically more active than Mo-edge due to its activation of
adsorbing thiophene reactant for subsequent C-S bond breakage in HDS
reaction. The adsorption of H on S-edge is stronger than
M-edge.[7] Topsoe et al propose that the S-H
groups with high hydrogenolysis activity are mainly associated with
S-edge and B acidic sites.[8] Han et al disclose
that B acid sites not only can promote the formation of coordinatively
unsaturated sites (CUS) sites, but also increase the acidity of -SH
sites and further improve the hydrogenolysis
activity.[9] Meanwhile, the fully sulfide Mo-edge
sites can promote the HYD route.[10]
Besides understanding the effect of different active sites, the supports
of hydrotreating catalysts with excellent physico-chemical properties
for promoting the dispersion of active metals and reactant diffusion
should also be developed. Noticeably, mesoporous materials have been
wildly applied in the fields of catalysis, adsorption and separation due
to their high pore size, pore volume, surface area and structural order
degree.[11, 12] Among these materials, SBA-16
silica with highly ordered degree possesses large cage-like mesopores
arranged in cubic body-centered Im3m symmetry, which will lead to
lower mass transfer resistance and further facilitate the catalytic
reactions.[13] However, SBA-16 pure silica
presents low acidity and poor dispersion of active metals, which is
adverse to HDS reaction. Therefore, it is necessary to incorporate other
atoms into the SBA-16 material for increasing the acidity of catalysts
and dispersion of active metals. Typically, Al modification into pure
silica materials can strongly improve the amount of
acidity.[14] Meanwhile, the incorporation of Ti or
Zr atoms into pure silica materials can improve the properties of active
metals.[15-18] Mere incorporation of single atoms
into SBA-16 pure silica shows advantages and disadvantages. Therefore,
it is necessary to incorporate dual metals into SBA-16 materials for
obtaining high-efficiency catalysts.
In this study, serial Al-Ti-SBA-16 composites with different acidities
and physico-chemical properties were successfully prepared. Before
introducing Al species, the Ti-SBA-16 precursors were prepared by a
two-step method through pre-hydrolysis of TEOS before the addition of
tetrabutyl titanate for protecting the original structure of SBA-16
silica. Various characterization methods were carried out on
Al-Ti-SBA-16 composites and corresponding NiMo/Al-Ti-SBA-16 catalysts to
investigate the influence of Al and Ti species on their properties.
Moreover, the HDS performances of NiMo/SBA-16 and NiMo/Al-Ti-SBA-16
catalysts were also evaluated to correlate the relationship between the
HDS activity and selectivity and properties of support and catalysts. An
appropriate composition of Al and Ti species in the NiMo/Al-Ti-SBA-16
catalysts, which will allow the highest HDS efficiency, were obtained.
The relationship between S-edge and Mo-edge sites and HDS performance
and selectivity was also proposed. Finally, the kinetic and
thermodynamic analyses were applied to investigate the influence of
S-edge and Mo-edge on the intrinsic HDS reactivity for various
catalysts.