Study On Preparation And Hydrodesulfurization Properties Of Phosphide Monolithic Catalysts

Transition metal phosphides have shown excellent catalytic activity in hydrodesulfurization(HDS) and hydrodenitrogenation(HDN) reactions. Monolithic catalysts as new-type catalysts have honeycomb structure usually using metal or ceramic materials as supports.Compared with the conventional pellet catalysts,monolithic catalysts have much better mass transfer performance and much lower pressure drop.They have been widely applied in the environmental field.But there are few reports in HDS using monolithic catalysts.In this paper,the treatment conditions for monolithic catalyst supports were investigated.First,the nickel phosphide precursors were directly coated on the cordierite by impregnating alumina sol containing nickel nitrate and phosphoric acid,and then nickel phosphide/Al₂O₃/cordierite catalysts were prepared by temperature-programmed reduction(TPR) method.The obtained catalysts were characterized by X-ray diffraction(XRD) and evaluated by HDS of dibenzothiophene(DBT) using 1 wt%DBT in decalin as model oil. Secondly,the nickel phosphide precursors were directly coated on the cordierite with different opening ratios by impregnating silica sol containing nickel nitrate and phosphoric acid,and then nickel phosphide/SiO₂/cordierite catalysts were prepared by TPR method.The structure of catalysts was characterized,and the activity of catalysts for HDS of DBT was evaluated.The HDS test results showed that the monolithic catalysts prepared with alumina sol had poor HDS activity.The catalytic activity increased with the increase of Ni contents and reaction temperature.But the Highest DBT conversion achieved by these catalysts was lower than 30%.In contrast,the monolithic catalysts prepared with silica sol have good HDS activity.The HDS test results also showed that the catalytic activity increased with the increase of Ni contents and reaction temperature.For the nickel phosphide/SiO₂/cordierite catalysts,when the Ni contents were lower than 3.2 wt%,nickel phosphides were well dispersed on the surface of cordierite. When the Ni contents were more than 6.4 wt%,the active phase of catalysts was Ni₂P.The pore size of the obtained catalysts was about 3.6 nm.The thickness of the active layer was about 20μm.The catalyst with Ni content of 12.8 wt%had the best HDS activity at a liquid hourly space velocity(LHSV) of 1.9 h^-1.At 360℃,the DBT conversion was 92.0%,and the selectivity for biphenyl(BP) and cyclohexylbenzene(CHB) was 69.8%and 30.2%, respectively.The HDS of DBT was mainly the direct desulfurization(DDS) route on the nickel phosphide/SiO₂/cordierite catalysts. The effects of P/Ni molar ratios and the preparation methods for active sol on catalytic activity were also investigated.With the increase of P/Ni molar ratio,the DBT conversion increased first and then decreased.The XRD results showed that the intensity of the diffraction peaks for Ni₂P increased first and then decreased.The catalyst with P/Ni molar ratio of 0.8 had the highest intensity of diffraction peaks for Ni₂P,and had the best HDS activity. The HDS activity of monolithic catalysts prepared by indirect preparation method was lower than those prepared by direct preparation method.