ISOTOPIC EXCHANGE BETWEEN 2,2-DIMETHYLBUTANE AND DEUTERIUM ON RHODIUM/SILICA GEL CATALYSTS

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 (Dh) ranging from 11 to 108% and subjected to a variety of pretreatment conditions. Some catalysts were prepared by ion exchange with Rh(NH3)5(H2O)3+ and some by impregnation to incipient wetness with Rh4(CO)12 or Rh6(CO)16. The turnover frequency (Nt) is affected both by Dh 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 Dh gave the largest Nt and the smallest degree of multiple exchange in the ethyl group. The pretreatment, H2,450° gave the largest Nt. Mere removal of H* from catalysts so treated by He,300° had little effect, but He,450° led to a substantial decrease in Nt except for 11-Rh/SiO2 . The various coupled values of Nt and selectivity resulting from variations in Dh and pretreatment require a minimum of eight different sites. Nt's and selectivities in the hydrogenolysis of methylcyclopropane on these same catalysts parallel those in the exchange of neohexane. © 1992.