Restricted diffusion of model sulfides over a NiMo/BK catalyst under hydrodesulfurization reaction conditions

Abstract

Micro–mesoporous composite Beta-KIT-6 (BK) material with cubic Iad mesoporous structure was synthesized, and then a NiMo/BK hydrodesulfurization (HDS) catalyst was prepared using BK as a support. Four kinds of model sulfides with different structures and molecular sizes, including thiophene, benzothiophene, dibenzothiophene, and 4,6-dimethyldibenzothiophene, were chosen as the reactant probe molecules. The intra-particle diffusion effects on HDS of these four different model sulfides were investigated systematically over the novel NiMo/BK micro–mesoporous-supported catalyst under HDS reaction conditions. The changing rules of the catalyst effective factor (η) and effective diffusion coefficient (D_e) of model sulfides were investigated. The results showed that the η values of NiMo/BK catalyst in the HDS of different reactant molecules are higher than those of the traditional NiMo/Al₂O₃ owing to excellent mass transfer ability of the well-ordered NiMo/BK catalyst, and the D_e values decreased with the increasing molecular sizes of model sulfur-containing compounds. The relationship of effective diffusion coefficient (D_e) with molecule diameter to pore diameter ratio (λ) was further determined. Restrictive factors were correlated by F(λ) = (1 − λ)^3.15 for NiMo/BK and F(λ) = (1 − λ)^3.67 for NiMo/Al₂O₃ catalyst, indicating that large pore size is beneficial to the diffusion of macromolecular reactants.