Structural effects and catalytic performance of Pd catalysts with micro-mesoporous structure on coal tar hydrodesulfurization

Abstract

Abstract A Pd composite catalyst with a micro-mesoporous structure was successfully prepared by hydrothermal crystallization and applied to the hydrodesulfurization (HDS) of coal tar. The mesoporous Pd/γ-Al2O3 and microporous Pd/ZSM-5 catalysts were compared to study the effects of the catalyst pore structure on the HDS activity. A tetrahydronaphthalene solution with 2 wt% dibenzothiophene (DBT) was used as the model compound for coal tar. The reaction products were detected via gas chromatography/mass spectrometry, while the reaction path and mechanism of HDS on the catalyst were speculated. The results show that the mesopores were sulfur poisoned after irreversible adsorption with SO42− species and had high deactivation rates in HDS reactions due to a weak carbon deposition resistance. However, the micro-mesoporous composite structure has anti-coking deposition and sulfur resistance due to its high mass transfer efficiency, suitable acid distribution, and Pd dispersion. This special material with the micro-mesoporous structure played a dual role, including a strong sulfur (especially H2S) resistance in its micropores and desulfurization capacity in its mesopores. And the DBT conversion over FC reached 85.5% in the HDS reaction. After 10 h of the FC catalyst used in the HDS reaction, Pb loss rate and acid loss were not less than 5.64% and 2.98%, respectively. At the same time, direct desulfurization is the primary route of HDS reactions in the micro-mesoporous structure catalyst. The free diffusion of overflow hydrogen on the micro-mesoporous interface and the desorption of H2S have important effects to maintain a constant number of active centres for the catalyst.