Controllable preparation and hydrodesulfurization performance of eggshell type Co/Mo catalyst

The catalytic cracking gasoline with high sulfur and high olefin content accounts for about 60% of the gasoline pool in China. Therefore, ultra-deep selective hydrodesulfurization of catalytic cracking gasoline and the protection of olefins from hydrogenation saturation are still the key technical problems to be solved in the production of high octane gasoline for a long time in the future. During the decades of the development of fluid catalytic cracking gasoline hydrodesulfurization, researchers have systematically investigated the distribution of sulfur species and hydrocarbon composition, process technology, active phase structure, promoter effect and mass transfer. From the view of mass transfer, this paper innovatively adopted the method of adjusting aging time, prepared a series of eggshell type catalysts in a controllable way. The performance of these catalysts for ultra-deep selective hydrodesulfurization of fluid catalytic cracking gasoline was investigated. A new concept was provided for the design and development of fluid catalytic cracking gasoline hydrodesulfurization catalysts. The catalysts were characterized by SEM-EDX, XRD, H₂-TPR, N₂ adsorption/desorption, XPS and HRTEM. The SEM-EDX results showed that the Mo element was an eggshell type distribution in the catalyst while the Co element evenly distributed in the catalyst. The thickness of eggshell increased gradually with the extension of aging time and Mo elements tended to be evenly distributed until the aging time is 95 min. The reason was that the Co element existed as Co²⁺ while the Mo and P element formed Mo₁₂O₄₀P³⁻ complex in the solution. The migration rate of Mo₁₂O₄₀P³⁻ complex in the catalyst pores was slow during impregnation while the Co²⁺ migrating fast. The SEM-EDX results indicated that the eggshell thickness of catalyst could be adjusted by controlling aging time. The XRD and H₂-TPR results showed that the eggshell distribution of Mo did not change the dispersibility of the active metals. The N₂ adsorption/desorption results showed that the aging time had little effect on the pore structure of the catalysts. The XPS results showed that the content of Mo⁴⁺ and Co-Mo-S on catalyst surface decreased with the increasing of eggshell thickness, it may be due to the migration of Mo element into the interior of the catalyst, resulting in a decrease in the concentration of Mo element on the surface of the catalyst. The HRTEM results showed that the eggshell thickness had a slight effect on the length and number of layers of MoS₂, but with the increase of eggshell thickness, the proportion of f_e/f_c in the catalyst decreases slightly. Finally, model catalytic cracking gasoline was used to investigate the desulfurization performance of the catalyst. The results showed that the eggshell type catalyst had little effect on desulfurization activity, but the eggshell type catalyst had lower olefin saturation activity. At different reaction temperatures, with the increase of the eggshell thickness, the desulfurization selectivity of the catalyst gradually decreased which mainly due to the fact that the eggshell catalyst had a higher f_e/f_c, while the lower mass transfer resistance made the olefin molecules stay in the shell containing the Mo active phase for a shorter time.