May 30, 2011
These cracking catalysts give ethylene and propylene equally. Catalytically cracking hydrocarbon feedstocks to give light olefins has long been a goal of industry. The conventional process for manufacturing the two largest-volume petrochemical building blocks, ethylene and propylene, is noncatalytic steam cracking of natural gas liquids, naphtha, or gas oil.
Historically, the goal of steam cracking was to maximize the yield of ethylene; propylene was always considered a byproduct. For example, naphtha steam cracking typically gives an ethylene/propylene ratio of 2:1. Over the last few years, however, propylene demand has been strong, and its price is often higher than ethylene. Consequently, techniques to increase propylene yield from steam crackers are now of keen interest.
J.-s. Choi and colleagues disclose a family of catalysts that produce ethylene and propylene in almost equal amounts in good yield. The catalysts have excellent thermal stability and can be regenerated by burning off coke deposits. The catalysts consist of chromium, zirconium, and in some cases phosphorus oxides; some of the catalysts also contain cesium or titanium. Seemingly, the best results were obtained from catalyst composition CrZr4Ce4P11.13Ox.
To test the catalysts, a quartz reactor tube (¼” outside diam) is filled with catalyst to a height of 5 cm. The hexane feedstock is pumped into a vaporizer at a rate of 2.75 mL/h. Water is pumped into a separate vaporizer at 0.92 mL/h. The vaporizer temperatures are maintained at 400 and 500 °C, respectively. The two gases are mixed well and fed to the reactor tube at 800 °C. Hexane conversion is 83.09%, the ethylene/propylene ratio is 1.1:1, and the combined yield of ethylene and propylene is 45.67%.
A key challenge to commercializing this type of catalytic process is demonstrating sufficiently long-lasting catalytic activity and life. The inventors state that the catalysts can be reused after burning the coke off the catalyst, but they supply no data. (LG Chem [Seoul]. US Patent 7,935,654; May 3, 2011; Jeffrey S. Plotkin)