Chemical News

Wednesday, February 27, 2008

Direct Oxidation of Propylene to PO

Ethylene oxide is produced commercially by the vapor-phase oxidation of ethylene over a silver-based catalyst. Selectivity to ethylene oxide is 80–86 mol%, depending on the catalyst used. But for direct propylene oxidation, the challenge has been to find a catalyst that gives sufficiently high selectivity to propylene oxide by suppressing the competing combustion reaction. The two competing reactions are shown in the figure.

Process and catalyst developers have been successful at minimizing overoxidation of ethylene to CO2. This success, however, has been achieved in a slow, evolutionary manner over the past 40 years. The challenge to developers is even greater in the case of direct oxygen– or air-based selective oxidation of propylene to propylene oxide.

Propylene, unlike ethylene, contains allylic hydrogen atoms that are highly prone to oxidation; this feature is the basis of the successful commercialization of acrolein, acrylic acid, and acrylonitrile processes. To achieve high selectivities to propylene oxide, oxidation of the allylic hydrogen atoms must be avoided or at least minimized.

M. Yako and M. Yamamoto disclose catalysts and operating conditions for the direct oxidation of propylene to propylene oxide that give good propylene conversion and propylene oxide selectivity. The basis of the invention is that it is essential to feed water vapor along with the oxygen and propylene.

In one example, 2 mL of a specially prepared silver catalyst was charged into a fixed-bed reactor. Propylene (360 mL/h), air (360 mL/h), and water (1 mL/h) were fed at 200 °C and 0.1 MPa absolute pressure. Propylene conversion was 6.3% and propylene oxide was produced at the rate of 107 ┬Ámol/h. When this experiment was repeated without adding water, propylene conversion dropped to 0.7% and propylene oxide productivity fell to 14 ┬Ámol/h. This is an exciting development, but there is still a long way to go before such a process is commercialized