Oxidation of propylene by means of SnO 2-MoO 3 catalysts : II. Hydrogendeuterium exchange between propylene and water

Abstract

It has been found that in the presence of SnO 2 covered with MoO 3 and at a temperature of 360 °C, propylene and D 2O rapidly exchange hydrogen against deuterium. The distribution of the deuterium over the propylene molecules proved to be approximately binomial. One of the hydrogen atoms in the propylene molecule participated only slightly in the exchange; this must have been the atom bonded to the central carbon atom. Infrared absorption measurements revealed that deuterium appears preferably in the methylene group (as was confirmed by a proton magnetic resonance measurement), and for the greater part in the cis position with respect to the methyl group. An explanation of the exchange which accounts for the availability of five hydrogen atoms in the propylene molecule for the exchange while being compatible with the abovementioned preference, is represented by a reversible reaction between propylene and acid surface hydroxyl groups yielding surfacebonded isopropyl groups (or isopropoxy groups, if the surface oxygen atoms are considered to be included). In our view, these groups also constitute the intermediate species in the oxidation of propylene to acetone and acetic acid. MoO 3 gave a low exchange rate, while SnO 2 proved to hold an intermediate position between MoO 3 and SnO 2 MoO 3 . The same applies to the rate of oxidation of propylene. With SnO 2 MoO 3 , isopropyl alcohol was very rapidly dehydrated to propylene at 370 °C; also during oxidation of propylene over this catalyst, the off gas showed the low equilibrium concentration of isopropyl alcohol—even at a very high space velocity. In the dehydration of isopropyl alcohol, MoO 3 showed a fairly high activity. The catalytic action of SnO 2 on isopropyl alcohol, however, was directed almost exclusively towards the oxidative dehydrogenation to acetone. This is one of the arguments to be raised against the view that the above-mentioned isopropyl (isopropoxy) group is also an intermediate in the dehydration of isopropyl alcohol. All experiments were done in the presence of air and water vapor.