Effect of Fe substitution over ZSM-48 on performance of n-dodecane hydroisomerization and distribution of isomers

A series of Fe substituted ZSM-48 nanocrystal aggregates were synthesized by the dynamic hydrothermal method. Through incorporation of Fe into ZSM-48 framework, the acidity of ZSM-48 zeolites has been modified. The hydroisomerization performances of n-dodecane were investigated on Pt supported ZSM-48 and Fe incorporated ZSM-48 zeolites. Fe incorporated ZSM-48 catalysts show higher n-C12 hydroisomerization selectivity compared with parent sample due to weaker acid strength of Si–OH–Fe than Si–OH–Al. High temperature favor the generation of the multi-branched isomers (mainly dimethyl isomers) following the “Key-Lock” mode. Meanwhile, 5-methylundecane is the main product in the monomethyl isomers (MOB) and 2,6-dimethyldecane is the main product in the multi-branched isomers (MUB), which suggests the excellent shape selectivity of ZSM-48 and the “Key-Lock” mode is preferred on ZSM-48 catalysts regardless Fe substituted or not. The relationship between the mono-methyl isomers and the butene-TPD results has been correlated and investigated. Four desorption peaks corresponding to butene weakly adsorbed over Si–OH, Fe–OH or Lewis acid sites, the “Key-Lock” mode adsorbed butene, the “Pore-Mouth” mode adsorbed butene and the strongly adsorbed butene respectively, can be distinguished. ZLC results indicate that Fe modified ZSM-48 can effectively increase n-hexane diffusion rate and reduce the activation energy of diffusion of n-hexane, which is benefit for the hydroisomerization performance. SMOB/SMUB can be altered by Fe modification, however, the distribution of the mono-methyl isomers can hardly be affected.