"Problems associated with hydrotreating of heavy and extra-heavy crude oils in conventional fixed-bed reactors have led to the design of more innovative reactors such as slurry reactors. Dispersed fine catalysts are commonly used in slurry reactors. However, separation of the fine particles with sizes in micrometers or nanometers from the liquid phase represents a challenge. In this work, novel magnetically recyclable catalysts are designed to address this issue. Two different magnetic core materials, Fe3O4 and Fe3S4, were studied. Core-and-shell composite catalysts MoS2/Fe3O4 and MoS2/Fe3S4 were designed. Both catalysts showed high activity in the hydrodesulfurization (HDS) of dibenzothiophene (DBT). Former one showed high affinity towards the direct desulfurization (DDS) pathway while the later one presented a balanced selectivity between DDS and hydrogenation (HYD) pathways. This provides the refineries an option to adjust the hydrotreating process based on their needs using each of the catalysts or a combination of both. Catalyst MoS2/Fe3S4 was further promoted by nickel and cobalt. The activity of the catalysts could be ranked as NiMoS/Fe3S4 > CoMoS/Fe3S4 > MoS2/Fe3S4. Addition of Ni to MoS2 catalyst forms the Ni-Mo-S phase with increased accessible sulfur edge which is responsible for the enhancement in both hydrogenation and desulfurization. The catalyst MoS2 with greigite core and its promoted catalyst NiMoS/CoMoS were also applied to the hydrodeoxygenation (HDO) of stearic acid. HDO was the dominant pathway for all of the catalysts. Catalysts MoS2/Fe3O4 and MoS2/Fe3S4 were used to investigate the roles of H2 and H2S and the involved active sites in the HDS of DBT. Two different active sites for hydrogenation (HYD) and hydrodesulfurization (HDS) were identified. The MoS2 with magnetite core demonstrated high selectivity towards the DDS pathway, which was attributable to the differences in adsorption energy of hydrogen and DBT over hydrogenation and desulfurization sites. H2S is favored being adsorbed on the S-edge vacancies to transfer the active sites to HYD/Isomerization sites. Hydrogen plays three distinctive roles: 1) When adsorbed at Mo edge, hydrogen promotes hydrogenation; 2) Strongly bonded at S-edge it accounts for direct desulfurization; 3) while loosely bonded to S-edge it favors HYD and isomerization."--Pages ii-iii.