The Relationship between the Structure of NiMo/SiO2 Catalyst Precursors Prepared in the Presence of Chelating Ligands and the Hydrodesulfurization Activity of the Final Sulfided Catalysts

The structure of nickel and molybdenum in silica-supported NiMo catalyst precursors, modified by the addition of various chelating ligands, was studied by means of X-ray, Raman, and UV–VIS spectroscopy. Thiophene hydrodesulfurization tests showed that a wide variety of ligands have a beneficial effect on the performance of the sulfided catalysts. The activity of catalysts prepared with nitrilotriacetic acid (NTA) and ethylenediamine (EN) was measured as a function of the ligand to Ni ratio in the catalyst precursors as well. EN gradually causes an improvement of the catalytic activity when the EN/Ni molar ratio increases from 0 to 4, while the best result for NTA is obtained when the NTA/Ni ratio is 1.5. UV–VIS spectra of NiMoEN impregnation solutions and dried catalysts showed that some EN is removed from the first coordination sphere of Ni during the drying procedure. Ni K-edge extended X-ray absorption fine structure (EXAFS) proved the presence of a Ni–Si shell and, thus, of Ni–SiO2 interactions in the catalyst precursors prepared without ligands. These interactions disappeared gradually with increasing amounts of the ligand. Raman spectroscopy and Mo K-edge EXAFS showed that Mo is present on the support as a mixture of MoO42− and polymolybdate clusters. Raman spectroscopy also suggested that these polymolybdate clusters interact with the support. Mo K-edge EXAFS spectra showed that EN has very little effect on Mo, whereas at NTA concentrations higher than 1 the [MoO3(NTA)]3− complex is formed. The increase in activity obtained with EN and NTA is attributed to the elimination of Ni–SiO2 interactions and to the presence of the ligands in the Ni coordination sphere, whereas the decrease in activity observed in catalysts with NTA/Ni ratios above 2 is explained by the formation of [MoO3(NTA)]3−.