Ethylene oligomerization to liquid fuels over steam and phosphorus modified MFI zeolites

A series of steam and phosphorus modified ZSM-5 samples were prepared to investigate the effect of pore structure and acid property on ethylene oligomerization. Characterization results showed that steam treatment contributed to generate mesopores with pore sizes of 2-3 nm and 10-20 nm, while progressive aggregation of phosphorus species on the exterior surface resulted in blockage of channels. Dealumination of framework aluminum led to a gradual decline in total acidity and acid strength with an increase of steam temperature or P/Al ratio. Sufficient strong acid sites and enhanced accessibility to the inner active sites were crucial factors in conversion of ethylene to liquid fuels. The steam modified sample at 700 °C (HZ-700) obtained the highest yield of liquid fuels up to 78.1%, which demonstrated an increase of 40% in comparison with the unmodified sample. Due to improved diffusional capacity originating from mesopores, HZ-700 showed a higher deactivation stability than the phosphorus modified sample at similar conversion levels. This work will provide some insight into the development of zeolite-based catalyst for ethylene oligomerization to liquid fuels.