effectsofsteamtreatmentonacidityofzsm5anditscatalyticperformanceinbutenecracking

Abstract

Propylene is one of the fastest growing petrochemicals, driven primarily by the high growth rate of polypropylene. The traditional methods to produce propylene cannot satisfy its demand. To increase the production of propylene, the catalytic cracking of low value C4+ hydrocarbons, by-products of petrochemical industry, to propylene and ethylene was investigated. The ZSM-5 zeolite was modified by steam treatment at different temperatures and for different time, and its acidity was measured by NH3-TPD technique. With increasing treatment temperature and time, the acid density and the acid strength of ZSM-5 decreased. When used as a catalyst for butene cracking, the ZSM-5 with low acid density and weak acid strength was favorable to enhance the yield of propylene plus ethylene, besides decreased the production of aromatics and low carbon paraffins. By leaching out the non-skeletal Al produced by the steam treatment with citric acid, the pore volume of ZSM-5 was enlarged and had a better capacity of accommodating the carbon deposit so that the catalyst life was prolonged. The yield of propylene plus ethylene could be enhanced if the reaction conditions were properly selected, as well as the yield of aromatics and paraffins could be decreased. The optimum conditions were as follows: WHSV = 3.5similar to8.8 h(-1), p = 0.06similar to0.1 MPa and theta = 600similar to620 degreesC