Catalytic Conversion of Ethylene to Propylene and Butenes over H−ZSM-5

Among 11 kinds of molecular sieves examined, H−ZSM-5 exhibited the highest activity for the direct conversion of ethylene to propylene. The conversion of ethylene was 58% and the selectivities to propylene and butenes were 42% and 21%, respectively, over H−ZSM-5 at 723 K. Our studies demonstrate that the conversion of ethylene increases with the degree of H + exchange in the H−Na−ZSM-5 series of samples with different H + exchange degrees and the Al content in the H−ZSM-5 samples with different Si/Al ratios. The strong Brønsted acid sites are proposed to account for the conversion of ethylene. The modification of H−ZSM-5 with phosphorus or boron could enhance the selectivity of propylene but decreased the conversion of ethylene due to the decreased acidity. In situ FT-IR studies confirm the reaction of ethylene molecules with the Brønsted acid sites associated with −Si−(OH)−Al− groups. In situ FT-IR results further suggest the occurrence of oligomerization of ethylene on the surface of H−ZSM-5. We speculate that the cracking of the oligomeric intermediates may lead to the formation of propylene.