Toward Earth-abundant Metals in Hydrodesulfurization Catalysts
Increasingly stringent environmental regulations for the sulfur content in fuels pose significant technological challenges for refineries. Current single-stage hydrodesulfurization (HDS) technologies are not efficient enough to achieve ultra-deep levels of sulfur, 10 ppmw S for transportation, under non-severe operating conditions. Platinum-group catalysts in the second stage HDS require high hydrogen pressure, which increases capital and operational expenses. Despite a great progress,
... nt of more efficient heterogeneous catalysts for both first and second stage HDS units using earth-abundant elements is still a challenge of paramount importance. Palladium (Pd) has shown outstanding performance in HDS of refractory sulfur compound 4,6-DMDBT through hydrogenation of aromatic ring. However, Pd is very scarce and sinters in high-temperature applications, which deactivates the catalysts and alters selectivity. The thermal stability of palladium was enhanced in this thesis through the formation of bimetallic nanostructures with yttrium, as a sintering-resistant element, via colloidal synthesis method. The addition of yttrium did not alter the overall HDS rate but doubled the ratio of direct desulfurization (DDS) to hydrogenation (HYD) selectivity and suppressed cracking twice as much as monometallic palladium catalyst did. This enabled the HDS process to be operated at a pressure as low as 1 MPa. The above-mentioned improvements were not achieved by the conventional impregnation synthesis method. As another alternative to the second-stage HDS catalysts, palladium species were reduced on the surface of colloidal iron (oxide) cores as nanosized islands using galvanic exchange reaction. The synergism between palladium species and iron oxide nanoparticles resulted in a four-fold iii enhancement in Pd-mass-based HDS activity at a reduced palladium loading. Pd dispersion and its HDS activity were maximized at the Pd/Fe molar ratio of 0.2. The incorporation of iron also improved the sulfur resistance of hydrogenation sites due to the higher affinity of sulfur to Fe as compared to Pd. For the first-stage HDS, niobium sulfide (NbS 2 ) is proposed that is more abundant and intrinsically more active than MoS 2 and WS 2 catalysts in HDS, hydrodenitrogenation, and hydrocracking reactions. However, the formation of NbS 2 via sulfidation of niobium oxides needs a high temperature above 700 °C due to the positive Gibbs free energy of sulfidation. It was shown for the first time that copper reduced the sulfidation/reduction temperature of Nb 2 O 5 dramatically that consequently enhanced the HDS activity. The highest HDS activity of bulk NbCu catalysts prepared via coprecipitation technique was achieved at Cu/Nb molar ratio of 0.3. Copper also promoted DDS and HYD selectivities and suppressed hydrocracking. The strong interaction between Nb 2 O 5 and oxide supports such as alumina makes the sulfidation of niobium oxide more difficult than that of bulk material. Among different support materials, carbon and alpha-alumina exhibited higher activities per mole of Nb and carbon support delivered the highest mass-based activity. Raman spectroscopy showed that various niobium oxides formed on the carbon support at different Nb loadings (from 2.0 to 12.0 wt%) exhibiting different sulfidation and catalytic behaviors. Niobium species at low loading (2.0 wt% Nb) showed the least sulfidation degree functioning as coordinatively unsaturated Lewis acid sites. This delivered the highest HDS activity per mole of Nb and an unprecedented hydrocracking selectivity of 71 % at 74 % conversion. Copper reduced sulfidation/reduction temperature of niobium oxide, improved the DDS selectivity, and reduced the hydrocracking selectivity to around 15 % over the entire Nb loadings. iv Colloidal synthesis methodology is a powerful technique to control the size and shape of nanostructures in a liquid phase. NbS 2 nanostructures in different shapes such as nanosheets, nanohexagons, nanobars, and nanospheres were prepared in the liquid phase at a low temperature of 300 °C in the presence of (non-)coordinating solvent and capping ligand. The developed NbS 2 nanostructures showed different catalytic behaviors in HDS of DBT depending on their shapes. The highest HDS activity and DDS selectivity were obtained on nanohexagons with abundant corner and edge active sites. This structure was twice more active than carbon-supported NbS 2 .