Hydrogen—deuterium exchange reactions on ion-exchanged X-type zeolites

Abstract

The hydrogen—deuterium exchange reaction has been studied over a series of X-type zeolites. With CoX zeolites the activity depended linearly on the extent of ion-exchange showing that the Co 2+ ions do not enter preferentially the positions which are likely to be inaccessible to the reacting molecules. Enhanced activity was found after hydrogen treatment of CoX at 200 °C or CaX and MnX at 300 °C and to a lesser extent at temperatures about 50 K lower. This increase in activity was thought to arise from the displacement of water molecules from the active sites by chemisorbed hydrogen. Other evidence for the poisoning effects of residual water was obtained from the variation in the activity of a CoX zeolite with outgassing temperature. There was no correlation between activity and charge on the metal ions—CeX was only slightly more active than NaX and there was a large range in the activity of the zeolites containing different divalent ions (Ni ⪢ Zn > Co > Mn > Ca). It is possible that NiX, the only catalyst giving high rates below 0 °C, brings about the exchange by a mechanism involving the homolytic dissociation of hydrogen. The results show that the usefulness of deuterium for the study of the exchange reactions with hydrocarbons may be limited to a few of the more active zeolites such as NiX and possibly ZnX or CoX.