Effect of chelating agents on HDS and aromatic hydrogenation over CoMo-and NiW/Al2O3

Hydrotreatment catalysts (CoMo-, NiMo- and NiW/Al2O3) were prepared by an impregnation method with solutions containing a chelating agent (nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA) or cyclohexanediaminetetraacetic acid (CyDTA)), and were subjected to some test reactions: hydrodesulfurization (HDS) of benzothiophene and dibenzothiophene, and hydrogenation (HGN) of o-xylene and 1-methylnaphthalene. Chelating agent modified CoMo-and NiW/Al2O3 showed higher activities in both HDS and HGN than the catalysts without the modification. The chelating agents had little effect on the activity of NiMo/Al2O3. CyDTA was the most effective for HDS activity of CoMo/Al2O3 and HGN activity of NiW/Al2O3. The chelating agents had no effect on the activity of each component catalysts (i.e., Co-, Ni-, Mo-, and W/Al2O3). The activity of the CyDTA-modified catalysts depended strongly on sulfiding temperature. CyDTA exhibited the improving effect at higher sulfiding temperatures, while working as an inhibitor at lower temperatures. Our previous study had indicated that the pre-formation of the MoS2-like structure was necessary to induce the intrinsic promoting effect of Co on the activity of Mo/Al2O3. The chelating agent, forming Co complex which decomposes at a rate depending on sulfiding temperature, was considered to adjust timing when Co ions interact with the MoS2-like structure.