Modifications in the HZSM-5 zeolite for the selective transformation of ethylene into propylene

• K, P and mild in situ steaming decreases acidity and acid strength of HZSM-5 zeolite.
• The decrease in acid strength attenuates secondary reactions and coke deposition.
• Ethylene conversion needs an intermediate acidity for selectively obtaining propylene.
• HZSM-5 (SiO2/Al2O3 = 80) modified by mild in situ steaming yields 37% of propylene.

Propylene production in the transformation of ethylene has been studied on HZSM-5 zeolite catalysts of different SiO2/Al2O3 ratio (30, 80 and 280), doped (1 wt.%) with K or P or treated by mild in situ steaming, in order to attenuate their acidity. The experiments have been carried out in a fixed bed reactor at 500 °C, space time of 3.2 (gcatalyst h)(molCH2)−13.2 (gcatalyst h)(molCH2)−1 and time on stream up to 15 h. The results highlight the effect catalyst acidity has on ethylene conversion, catalyst stability (deactivation by coke deposition) and propylene yield. The balance between these indices entails the use of a zeolite of moderate acidity subjected to a mild treatment in order to maintain a suitable activity for propylene formation by oligomerization-cracking reactions, and thus minimizing secondary reactions of hydrogen transfer and aromatization. A propylene yield of 37% is obtained for a time on stream of 11.3 h with the HZSM-5 zeolite catalyst of SiO2/Al2O3 = 80 modified by mild in situ steaming treatment.

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