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.