Catalytic activity of CoMo catalysts supported on boron-modified alumina for the hydrodesulphurization of dibenzothiophene and 4,6-dimethyldibenzothiophene

Boria-alumina mixed oxides were prepared by impregnation and used as supports for CoMo catalysts (MoO₃: 13 wt%; Co: 2.3 wt%) for the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). A detailed characterization of B₂O₃-Al₂O₃ carriers showed that boric acid reacts with surface Al-OH groups to form a boria over-layer, stabilized via Al-O-B bridges. Using X-ray diffraction, we showed that the addition of B to the alumina carrier leads to a reduction in the formation rate of bulk α-CoMoO₄, initially present on the surface of CoMo/Al₂O₃ oxide precursors prepared by co-impregnation. This can be attributed to the strong interaction between Co and the B-modified carrier, as demonstrated using X-ray photoelectron spectroscopy (XPS). Further investigations using XPS showed that, because of this seemingly stronger interaction of Co-oxide species with the B₂O₃(x)-Al₂O₃ supports, the surface concentration of Mo was also affected for both oxide and sulfide CoMo catalysts. In the meantime, the degree of reducibility of Mo (XPS fraction of Mo⁴⁺/Mo_total) did not seem to be significantly affected for boria loadings below 5 wt%. However, for higher boria loadings (7 and 10 wt%), the Mo reducibility decreases with boron content due very probably to the presence of less reducible bulk MoO₃ in CoMo oxide precursors. The HDS activity of DBT and 4,6-DMDBT over CoMo/B₂O₃(x)-Al₂O₃ exhibit a maximum for B₂O₃ loadings around 3-5 wt%. To a first approximation, this increase in the HDS activity can be attributed to the stronger interaction of Co-species with the B-modified carrier suppressing the formation of α-CoMoO₄, which is known to yield the formation of less active Co₉S₈, after sulfurization, instead of generating the more active "CoMoS" phase. For boria loadings above 3-5 wt%, the DBT and 4,6-DMDBT HDS activity of CoMo/B₂O₃-Al₂O₃ catalysts decreases due to a decrease in the dispersion and reducibility/sulfidability of Mo-supported species, as evidenced using XPS.