The HDS rate constants kHDS, kHYD,
kDS, kDDS,
kDDS/kHYD and TOF values are provided in
Table 7. The values of kHYD, kDS and
kDDS kinetic parameters could reflect the reactivity of
each reaction steps. kDDS/kHYD may be
applied to investigate the HDS selectivity. It shows that the
kHYD and kDDS present an increasing
tendency with the Ti and Al contents in the NiMo catalysts,
demonstrating that incorporation of Ti and Al into SBA-16 silica can
enhance the selectivity of HYD route and DDS route respectively.
Noticeably, the kDS value presents an increasing
tendency with the amount of B acidity for various catalysts. The
NiMo/AT-7.5 exhibits the highest kDS value of 2080
10-4 mol g-1 h-1may be due to its relative high amount of acidity and the highest amount
of S-edge sites.
The kHDS and TOF values could be applied to investigate
the intrinsic reactivity of DBT HDS reaction. The kHDSvalues for serial catalysts present the same tendency with their HDS
efficiencies from Fig. 1, confirming that the enhancement of acidity and
properties of active phases could improve the DBT HDS performance.
NiMo/SBA-16 catalyst shows the lowest kHDS and TOF
values of 6.2×10-4 mol g-1h-1 and 5.1 h-1, and NiMo/AT-7.5
catalyst presents the highest kHDS and TOF values of
16.3×10-4 mol g-1h-1 and 9.0 h-1 respectively. It is
wildly accepted that larger amount of vacant sites for active metals
will allow a better HDS performance.[59] However,
Vogelaar proposed that the fully sulfide catalyst with the lowest amount
of CUS sites presents the best HDS performance for thiophene
reactant.[54] Combining the CO-IR results and HDS
activity, our results show that the HDS efficiencies are mostly linked
with the total concentrations of Mo-edge and S-edge, but the NiMo/AT-7.5
exhibit the highest HDS performance even though its total concentration
of Mo-edge and S-edge is not the highest. This phenomenon should be
correlated with the special properties of -SH groups in S-edge. The
acidity of S-edge sites can be enhanced through easily accepting the
H+ from B acid site, which can significantly enhance
the DDS route of HDS reaction. Meanwhile, the -SH groups can also act as
the H2 source during the HDS
reaction.[8] For NiMo/AT-7.5 catalyst, the S-edge
concentration promoted by Al modification is the highest, and the
concentration of fully sulfide Mo-edge sites promoted by Ti modification
is relatively high. Therefore, the highest kHDS and TOF
values for NiMo/AT-7.5 catalyst can be ascribed to the synergistic
effect of the enhancing acidity and distribution and dispersion of
active metals. Above all, it can conclude that incorporating appropriate
contents of Al and Ti atoms into SBA-16 silica can maximize HDS
performance for the corresponding NiMo/Al-Ti-SBA-16 catalyst. The
principle is shown in Scheme 2.