October 15, 2012
Make propylene oxide by generating hydrogen peroxide in situ. As I have frequently noted in this column, directly oxidizing propylene to propylene oxide is one of the “holy grails” of the petrochemical industry. This goal, however, remains elusive.
The historical route for making propylene oxide (PO) on an industrial scale is based on the reaction of propylene with HClO to give the corresponding chlorohydrin. The chlorohydrin is treated with base to give PO. About 40% of the world’s PO is made with this old chlorine-based technology.
In 1969, an alternative process was commercialized. It was based on the reaction of t-BuOOH (made by air-oxidizing isobutane) with propylene to give PO and coproduct t-BuOH. (t-BuOH is typically converted to MTBE.) In 1974, a more useful coproduct method was commercialized: the famous PO–styrene monomer (POSM) process. In this process, ethylbenzene hydroperoxide reacts with propylene to give PO and hydroxyethylbenzene, which is easily reconverted to styrene.
The POSM technology has proliferated, and more than 12 of these plants are in commercial operation. They all generate styrene at a rate 2.3 times that of PO.
In 2008, two more processes, which make PO with no coproducts, were commercialized. One method, developed by Sumitomo, treats propylene with cumene hydroperoxide to give PO. The byproduct is converted back to cumene for reuse. In the second, developed independently by Dow–BASF JV and Evonik, propylene reacts with H2O2 to give PO, with only water as the byproduct. The downside to this method is that H2O2 is relatively expensive.
A potential way to decrease the impact of the H2O2 cost would be to oxidize propylene with a H2–O2 mixture, essentially making H2O2 in situ. T. Kawabata and H. Abekawa disclose reaction conditions to accomplish this, as illustrated by the following example.
A gas mixture consisting of propylene, oxygen, hydrogen, and nitrogen in a 4:1:8:87 volume ratio is fed to a 500-mL autoclave at the rate of 16 L/h. In addition, a 1:4 w/w H2O–MeCN solvent mixture containing 0.7 mmol/kg NH4H2PO4 is supplied to the reactor at the rate of 108 mL/h. During the reaction, 131 g of the reaction solvent containing 0.133 g titanosilicate Ti-MWW and 0.03 g Pd/carbon black catalyst mixture is retained in the reactor.
The reaction is run at 60 °C and a pressure of 0.8 MPa (gauge). After 5 h, analysis of the product mixture shows that PO is produced at the rate of 3.28 mmol/(g Ti-MWW·h). PO selectivity based on propylene is 72%; selectivity based on hydrogen consumed is 35%. (Sumitomo Chemical [Tokyo]. US Patent 8,273,907, Sept. 25, 2012; Jeffrey S. Plotkin)