Current commercial processes for making ethanol from ethylene operate at very high temperatures around 475.degree.-600.degree. F. in order to obtain suitable reaction rates. The catalyst used commercially is a phosphoric acid catalyst impregnated on a diatomaceous support, and the ethanol yields are around 6 percent.
The problem with vapor-phase hydration reactions of olefins to alcohol is that they are equilibrium limited. There is a maximum amount of alcohol which the vapor phase can hold before the reverse reaction, that is alcohol to olefin plus water, equals the alcohol formation. This maximum is usually about 5 to about 25 percent, depending on the temperature and pressure of the hydration reaction. The yields per pass are lower than the equilibrium amount, which makes the yields even smaller. To obtain an appreciable yield for the overall process, commercial vapor-phase hydration processes use recycle. This is expensive because of compressor costs and the larger reaction vessels which are required. Some processes use a mixed-phase reaction with liquid water so that the alcohol produced is continuously absorbed and the reaction is not equilibrium limited. Such processes get good yields per pass but require high water/olefin rates plus the alcohol/water solution in the reactor tends to dissolve the acid catalyst.
It would be desirable to have a process for the vapor-phase hydration of olefins to alcohols in which high yields of product can be obtained without the disadvantages of the existing commercial processes.