Preparation of active HDS catalysts by controlling the dispersion of active species

It is demonstrated that the structural control of the metal ion precursors in the impregnating solution by adding the chelating agents is effective to prepare the higher active CoMo supported on alumina catalysts (View the MathML source) for hydrodesulfurization (HDS). Coordination structures of the Co and Mo complexes in the CoMo impregnating solution and distributions of the Co and Mo complexes were evaluated by spectroscopic characterization techniques and by using a computational calculation, respectively. An addition of a chelating agent, such as NTA (nitrilotriacetic acid) and Glu (L-glutamic acid), in the CoMo solution results in the selective formation of the Co complexes, while the amount of the Mo complex is negligibly small at the practical pH of 9.2. The addition of the chelating agent increases the thiophene HDS activity of the sulfided catalysts typically by 50%, compared with that prepared without the chelating agent. Dispersion results of Co and Mo species on both oxidic and sulfided catalysts indicate that the higher HDS activity is explained by the higher degree of surface exposure of Co sites (namely the dispersion of Co) rather than that of Mo sites. The selective formation of the Co-chelate complexes keeps Co ions stable in solution up to high concentration. Furthermore, the Co complexes are estimated to be stable on the support even in the initial step of calcination, which would depress the formation of crystalline Co compounds, such as CoAl2O4 and CoMoO4. These effects result in the higher dispersion of the active Co surface species.