Effect of steaming on acidity and catalytic performance of H-ZSM-5 and P/H-ZSM-5 as naphtha to olefin catalysts

The effect of steaming on the acidity and catalytic performance of H-ZSM-5 zeolites with various Si/Al atomic ratios and P/H-ZSM-5 as naphtha cracking catalysts to produce light olefin were investigated. The acid amount of H-ZSM-5(Si/Al atomic ratio = 51) was monotonously decreased with the steaming time at 873 K. However, the catalytic activity of H-ZSM-5(51) for heptane cracking was enhanced by the steaming for short times (similar to 0.5 h). The activation energy for H-ZSM-5(51) steamed for 0.5 h was lower than that for the parent H-ZSM-5(51). Moreover, the catalytic activity of H-ZSM-5(51) for cracking of cumene, from which the Bronsted acidity can be evaluated, was enhanced by the steaming for a short time of 0.5 h. From these results, the stronger Bronsted acid sites might be generated by the steaming for short times, resulting in the enhancement in the catalytic activity for cracking of heptane. The catalytic activity of H-ZSM-5(51) for cracking of heptane considerably decreased after the steaming for long times. On the other hand, only a little decrease in the rate constant for cracking of heptane over H-ZSM-5(200) by the steaming for long times was observed. These findings suggest that the high silica H-ZSM-5 as H-ZSM-5(200) may have a high percentage of aluminum atoms which is highly resistant to the dealumination by the steaming. The catalytic activity for cracking of heptane over P/H-ZSM-5(51) was also enhanced by the steaming for a short time. The P/H-ZSM-5(51) steamed for 10 h exhibited a much higher activity than the H-ZSM-5(51) steamed for 10 h. These results indicate that the phosphorus impregnation greatly suppresses the dealumination of H-ZSM-5. (C) 2014 Elsevier Inc. All rights reserved.