Effect of Impregnating Fe into P-Modified HZSM-5 in the Coupling Cracking of Butene and Pentene

Iron-containing phosphorus-modified HZSM-5 catalysts were prepared and used for the coupling cracking of butene and pentene (mole ratio of butene to pentene = 4:1). The impact of iron content on the structure and acidity of catalysts was studied by XRD, N-2 adsorption, UV-vis spectroscopy, NMR, and FTIR of adsorbed pyridine. The results revealed that iron interacted with phosphorus and increased the content of tetrahedral framework aluminum after impregnating iron into P/HZSM-5. Such interaction recovered partial Bronsted acid sites which were previously neutralized by phosphorus modification, and introducing iron created some Lewis acid sites, significantly promoting the concentration and strength of acid sites. In the coupling cracking of butene and pentene, the addition of iron into phosphorus-modified HZSM-5 significantly increased the conversions of butene and pentene. As iron loading increased from 0 to 2 wt %, butene conversion and pentene conversion increased by 26.8% and 45.8% at 550 degrees C, weight hourly space velocity = 6 h1, steam ratio = 0.36 g/g, and time-on-stream = 4 h. With increasing iron loading in phosphorus modified HZSM-5, ethylene selectivity increased while propylene selectivity decreased. The catalytic performance of catalyst was strongly influenced by reaction conditions in the cracking. Proper reaction conditions could inhibit the secondary reactions and enhance propylene selectivity.