Isotopic exchange between 2,2-dimethylbutane and deuterium on rhodium/silica gel catalysts

Abstract Isotopic exchange between 2,2-dimethylbutane and deuterium has been investigated at 75°C on 10 rhodium on wide-pore silica gel catalysts with H(chemisorbed)/Rh ( D h ) ranging from 11 to 108% and subjected to a variety of pretreatment conditions. Some catalysts were prepared by ion exchange with Rh(NH 3 ) 5 (H 2 O) 3+ and some by impregnation to incipient wetness with Rh 4 (CO) 12 or Rh 6 (CO) 16 . The turnover frequency ( N t ) is affected both by D h and by conditions of pretreatment. The extreme variation was a factor of 330. Exchange into the ethyl group was about 10 times faster than into the t -butyl group. In general, the catalysts of largest D h gave the largest N t and the smallest degree of multiple exchange in the ethyl group. The pretreatment, H 2 ,450° gave the largest N t . Mere removal of H∗ from catalysts so treated by He,300° had little effect, but He,450° led to a substantial decrease in N t except for 11-Rh/SiO 2 . The various coupled values of N t and selectivity resulting from variations in D h and pretreatment require a minimum of eight different sites. N t 's and selectivities in the hydrogenolysis of methylcyclopropane on these same catalysts parallel those in the exchange of neohexane.