Support effect on the catalytic performance of trimetallic NiMoW catalysts prepared with citric acid in HDS of dibenzothiophenes

In the present work, a series of trimetallic NiMoW hydrodesulfurization (HDS) catalysts supported on mesoporous SBA-15 silica and conventional -y-Al203 was prepared with and without the addition of citric acid (CA). Acid (pH = 1) and basic (pH = 9) aqueous impregnation solutions were used in the preparation of the catalysts with CA. The aim of this study was to investigate if the support's nature has an influence on the effect of using citric acid in the preparation of trimetallic catalysts on their activity in HDS of dibenzothiophene-type compounds. Supports and catalysts were characterized by nitrogen physisorption, small-angle and powder XRD, thermogravimetric analysis (TGA/DTG), temperature programmed reduction, UV-vis DRS, HRTEM, and tested in the simultaneous HDS of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). Nitrogen physisorption and XRD results showed that the principal characteristics of the SBA-15 and gamma-Al2O3 supports were maintained in the trimetallic catalysts. According to the characterization of the alumina-supported catalysts by TPR, DRS and HRTEM, the addition of CA, regardless of the pH of the impregnation solution used, does not allow to modify significantly the dispersion of Mo and W species in comparison with the NiMoW/gamma-Al203 catalyst prepared without CA. In contrast, the use of citric acid in the preparation of trimetallic catalysts supported on SBA-15 allowed to obtain better dispersed Ni, Mo and W oxide species in comparison with the reference sample prepared without CA (XRD, TPR). HRTEM characterization of the sulfided catalysts confirmed that the best dispersion of the active Mo(W)52 phase was in the trimetallic NiMoW(1)/SBA-15. In general, the use of CA improved catalytic performance of the trimetallic catalysts supported on SBA-15, in contrast to their counterparts supported on gamma-Al2O3. The trimetallic NiMoW(1)/SBA-15 catalyst, prepared from an acid impregnation solution with CA, resulted to be the most active in HDS of both dibenzothiophenes tested. Its activity in HDS of 4,6-DMDBT was at least twice higher than that of the most active catalyst of the alumina-supported series (NiMoW/gamma-Al2O3). In addition to the best dispersion of oxide and sulfide Mo and W species in the NiMoW(1)/SBA-15 sample, it showed the highest hydrogenation ability among all the prepared catalysts. The above results indicate that the effect obtained by using citric acid in the preparation of trimetallic catalysts strongly depends on the support used and it's interaction with the deposited Ni, Mo and W species. When the interaction is strong (gamma-Al2O3), citric acid is not able to improve catalytic performance. However, when the metal-support interaction is not very strong (SBA-15), the addition of CA results in a noticeable improvement of catalytic performance. (C) 2014 Elsevier B.V. All rights reserved.