Development of highly active hydrodesulfurization catalysts - Effect of the Addition of CyDTA and Boria -

Introduction To meet the severe regulation for reductions of SOx, NOx, and SPM contained in the exhaust gases from diesel vehicles, the improvement of catalyst performances especially for hydrodesulfurization (HDS) is one of the most important subjects. van Veen et al. reported that CoMo catalysts modified with nitric triacetic acid (NTA) show higher activities for HDS of thiophene at atmospheric pressure. We have investigated effects of other chelating reagents such as ethylenediamine-N, N, N', N'-tetra-acetic acid (EDTA) and trans-1, 2-Cyclohexanediamine-N, N, N', N'-tetraacetic acid (CyDTA) and found that CyDTA improves the activity of CoMo/Al2O3 catalyst for HDS of dibenzothiophene (DBT) at high pressure more more effectively than NTA. Besides, CyDTA is found to be effective for hydrogenation of o-xylene and tetralin over NiW/Al2O3 catalyst. Since CyDTA forms stable complexes with Co or Ni in the impregnating solution, it is suggested that CyDTA retards the sulfidation of Ni species, leading to the effective formation of active phases, namely Co(Ni)-Mo-S or Ni-W-S phases. On the other hand, the addition of boric acid to NiMo/Al2O3 catalyst has been reported to improve the activities for HDS of DBT. From EXAFS analysis of NiMo/B2O3/Al2O3 catalyst, the addition of boric acid improves the sulfidation degree of Mo sulfide species. Since Decanio et al. showed that the addition of boric acid to γAl2O3 affects the distribution of hydroxyl groups on γ-Al2O3 surface by means of IR spectroscopy, it is suggested that boria weakens the interaction between Mo sulfide species and γ-Al2O3. Since the additions of CyDTA and boric acid affects the structure of sulfide phases in different ways, it is expected that the simultaneous addition of both CyDTA and boric acid improves the HDS activities of hydrotreating catalysts than CyDTA and boric acid alone. Thus, in the present study, we have prepared the catalysts modified with both CyDTA and boric acid and investigated their activities for HDS of 4, 6-dimethyldibenzothiophene (4,6-DMDBT) and their surface fine structures.