Zeolite pore confinement in adsorption and skeletal isomerization of n-hexane

The zeolite pore confinement can stabilize desired chemical pathways but may also introduce mass-transfer limitations, which greatly affect catalytic performance and remain not fully understood. Herein, pore confinement effect in n-hexane hydroisomerization is clarified by investigating the adsorption and skeletal isomerization of n-hexane over Ni/zeolite catalysts with various pore sizes and dimensions (USY, Beta, MOR, ZSM-5 and MCM-22). The 12-MR pores are more favorable for the adsorption of n-hexane and iso-hexane compared with 10-MR pores, due to less diffusion limitation and more adsorbed sites. The zeolite with pore size of similar to 0.68 nm has an optimal match for the n-hexane adsorption and formation of iso-hexane, in which a balance between n-hexane molecular confinement stabilization and diffusion limitation is achieved. Oversized pores weaken the interaction of n-hexane with pore walls and smaller pores exhibit great resistance to migration of n-hexane, neither of them is conductive to the adsorption and skeletal isomerization of n-hexane. Excellent adsorption capacity and more accessible active sites endow Ni/Beta and Ni/MOR catalysts with the highest conversion and isomers selectivity. Pore dimensions show no perfect match to n-hexane and mono-branched iso-hexane but a significant match to larger sized di-branched iso-hexane. 2,2-DMB are commensurate with straight channels and are easily adsorbed in them compared with 3-D channels, resulting in a higher 2,2-DMB/2,3-DMB ratio for 1-D and 2-D zeolites. This work provides theoretical support for the pore structure design of efficient alkane hydroisomerization catalysts.