On the relationship between the preparation method and the physicochemical and catalytic properties of the CoMo/gamma-Al(2)O(3) hydrodesulfurization catalysts

A series of CoMo/gamma-Al(2)O(3) catalysts have been prepared using various methodologies. One of them (EDF) was prepared by depositing the Mo species on the support via the equilibrium deposition filtration (EDF) technique and then the Co species by dry impregnation. Another catalyst (co-EDF) was prepared by depositing the Co and Mo species simultaneously via EDF. A third catalyst (co-WET) was prepared by depositing Mo and Co species simultaneously using the wet impregnation method. The fourth catalyst (WET) was prepared by depositing the Mo species through wet impregnation and then the Co species by dry impregnation. Finally, the fifth catalyst (s-DRY) was prepared by mounting the Mo species through successive dry impregnations and then the Co species by dry impregnation. In all cases the Mo and Co content was identical, giving a Co/(Co+Mo) ratio equal to 0.13. These catalysts were characterized using various physicochemical techniques (BET, NO chemisorption, DRS, LRS, TPR, and XPS), and their catalytic activity for the hydrodesulfurization of thiophene was determined. The trend observed for the HDS activity (namely, EDF>co-EDF>co-WET>s-DRY>WET) is attributed to similar trends observed for both the fraction of well-dispersed octahedral cobalt in the oxidic precursors and the concentration of the edge sulfur vacancies formed on the active phase of the sulfided samples. The EDF and co-EDF catalysts exhibited relatively low hydrogenating activity. The maximum HDS activity, achieved over the EDF catalyst, suggested the most suitable preparative strategy for the preparation of very active and less hydrogen-demanding CoMo/gamma-Al(2)O(3) HDS catalysts.     

Preparation, characterization, and catalytic activity of CoMo/gamma-Al2O3 catalysts prepared by equilibrium deposition filtration and conventional impregnation techniques

A CoMo/gamma-Al(2)O(3) catalyst, prepared by depositing on the Al(2)O(3) carrier first the Mo species via equilibrium deposition filtration (EDF) and then the Co species by dry impregnation, was compared to three CoMo/gamma-Al(2)O(3) samples prepared using various conventional impregnation methods. All samples had the same composition, corresponding to an atomic ratio Co/(Co+Mo) equal to 0.3. The above samples were characterized using various physicochemical techniques (AAS, BET, DRS, LRS, XPS, TPR, and NO chemisorption), and their catalytic activity was determined using the hydrodesulfurization (HDS) of thiophene as a probe reaction. The EDF-prepared catalyst was about 30-43% more active in HDS than those prepared with the conventional impregnation techniques at all reaction temperatures studied. In contrast, the EDF catalyst exhibited the lowest hydrogenation activity. The higher HDS activity of the EDF sample is attributed to the higher number of active HDS sites formed on its surface. It is concluded that the increased number of active sites is due to the fact that the deposition of the Mo species by EDF results to a higher coverage of the support surface by supported molybdenum phase, which in turn, inhibits the formation of the catalytically inactive CoAl(2)O(4) and favors the dispersion of octahedral cobalt on its surface.