March 7, 2011
Produce ethylene and propylene with a durable zeolite catalyst. Catalytic cracking of low-value olefins such as butenes to higher value olefins such as ethylene and propylene would be economical if robust catalysts were developed. The challenge to developing such catalysts is that when activity is high, catalyst activity often quickly degrades because coke deposits on the catalyst surface. When activity is low, ethylene and propylene yields are poor. Y. Takamatsu and K. Nomura developed catalysts that can crack butenes to ethylene and propylene in good yield and maintain fairly high activities over time.
The catalysts in this invention are based on hydrogen ion–exchange types of ZSM-5/SiO2 zeolites with 230 ppmw sodium. The zeolite is treated with AgNO3 and ion-exchanged for 2 h to add 0.084 wt% silver to the catalyst. The catalyst is placed in a tubular reactor, and C4 raffinate-2 (the C4 fraction from a naphtha cracker after butadiene and isobutylene are removed) is passed through the catalyst bed at a weight hourly space velocity of 7.25 h–1, a temperature of 550 °C, and a gauge pressure of 0.1 MPa. After 2 h, C4 olefin conversion is 67.97%, and ethylene and propylene yields are 4.95% and 23.1%, respectively. After 48 h, C4 olefin conversion is 60.01%, and ethylene and propylene yields are 3.07% and 21.34%, respectively.
In a test of a similar catalyst with 2200 ppmw sodium, C4 olefin conversion is 57.74%, and ethylene and propylene yields are 2.79% and 19.44%, respectively after 2 h. After 48 h, however the catalyst degrades substantially, and C4 olefin conversion is only 31.63% with very low ethylene and propylene yields. (Asahi Kasei Chemicals [Tokyo]. US Patent 7,893,311, Feb. 22, 2011; Jeffrey S. Plotkin)