HYDRATION OF PROPYLENE (Isopropanol [CH₃CHOHCH₃])

Isopropanol (2-propanol) is an important alcohol of great synthetic utility. It is the second-largest volume alcohol after methanol (1998 U.S. production was approximately 1.5 billion pounds) and it was the 49^th ranked chemical. Isopropanol under the name “isopropyl alcohol” was the first industrial chemical synthesized from a petroleum-derived olefin (1920).

The production of isopropanol from propylene occurs by either a direct hydration reaction (the newer method) or by the older sulfation reaction followed by hydrolysis. In the direct hydration method, the reaction could be effected either in a liquid or in a vapor-phase process. The slightly exothermic reaction evolves 51.5 KJ/mol.

In the liquid-phase process, high pressures in the range of 80–100 atmospheres are used. A sulfonated polystyrene cation exchange resin is the catalyst commonly used at about 150°C. An isopropanol yield of 93.5% can be realized at 75% propylene conversion. The only important byproduct is diisopropyl ether (about 5%). Figure 1.1 is a flow diagram of the propylene hydration process.

Gas phase hydration, on the other hand, is carried out at temperatures above 200°C and approximately 25 atmospheres. The ICI process employs WO3 on a silica carrier as catalyst.

Figure 1.1. A flow diagram for the hydration of propylene to isopropanol:16 (1) propylene recovery column, (2) reactor, (3) residual gas separation column, (4) aqueous - isopropanol azeotropic distillation column, (5) drying column, (6) isopropyl ether separator, (7) isopropyl ether extraction.

Older processes still use the sulfation route. The process is similar to that used for ethylene in the presence of H₂SO₄, but the selectivity is a little lower than the modern vapor-phase processes. The reaction conditions are milder than those used for ethylene. This manifests the greater ease with which an isopropyl carbocation (a secondary carbonium ion) is formed than a primary ethyl carbonium ion:

Table 1.1 compares sulfuric acid concentrations and the temperatures used for the sulfation of different light olefins

Table 1.1 : Acid concentration and temperatures used for the sulfation of various olefins