Simultaneous HDS of DBT and 4,6-DMDBT over single-pot Ti-SBA-15-NiMo catalysts: influence of Si/Ti ratio on the structural properties, dispersion and catalytic activity

Abstract

A series of Ti-SBA-15-NiMo catalysts with Si/Ti ratios of 1, 2.5, 5, and 10 were prepared by a single-pot method. The dispersion and performance of the catalysts were characterized by N₂ adsorption, temperature programmed techniques (NH₃-TPD and H₂-TPR), X-ray diffraction (XRD), and X-ray fluorescence (XRF) as well as by Raman and Fourier transform infrared (FTIR) spectroscopy. The catalytic activity was determined by simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) in a batch autoclave reactor. Variations in the Si/Ti ratio resulted in noticeable differences in the characteristics of the catalysts. XRD diffractograms show that Ti is well incorporated into the SBA-15 support at Si/Ti ratios of 10 and 5. However, at Si/Ti ratios below 5, the Ti phases predominate and peaks corresponding to MoO₃ phases were not observed. As the Ti incorporation is increased, the moderate surface acidity decreases while the contribution from strong acidity becomes evident. TPR results indicate that a higher amount of Ti increases the metal-support interaction. The HDS rates for DBT and 4,6-DMDBT over a catalyst with an Si/Ti ratio of 10 increased by 33% and 49%, respectively. The enhancement in HDS can be attributed to a lower metal-support interaction. However, a further increase in the Si/Ti ratio removed the enhancement in the HDS rates. A regenerated catalyst with an Si/Ti ratio of 10 exhibited 91% and 86% HDS compared to the fresh catalyst for DBT and DMDBT, respectively. Direct desulfurization was the preferred route for both DBT and 4,6-DMDBT. The HDS rate for DDS is enhanced by the addition of Ti while the HDS rate for the hydrogenation route remains almost constant. The divergent effects on the DDS and HYD pathways indicate that the active sites for hydrogenolysis and hydrogenation are different and are influenced differently by titania addition.