Isotopic exchange between deuterium and 2,2-dimethylbutane on platinum/silica and palladium/silica

Isotopic exchange between deuterium and 2,2-dimethylbutane (neohexane) has been investigated on a series of Pt/SiO2 (percentage exposed of metal, D/sub h/ = 6.3 to 81%) and Pd/SiO2 (D/sub h/ = 13.8 to 96.7%) catalyst at temperatures in the vicinity fo 1000C. Catalysts were investigated after three different pretreatments: standard: O2,3000; H2,3000; He,4500 and H2,4500: O2,3000; H2,4500 and H2,1000: O2,3000; H2,1000. One might expect exchange into the methylene group to be subject to substantial steric hindrance from the adjacent t-butyl group and the degree of steric hindrance to be less at edge atoms than at atoms in (100) and (111) faces, i.e., relatively less on small crystallites than on larger ones. In general, the effect of D/sub h/ upon the relative yield of ethyl-d5 in neohexane did not accord with this expectation. A possible explanation is that the bounding faces of the crystallites are defective and contain many vacancies which generate sites about equivalent to edge sites in the degree of hindrance. If so, usual tests of structure sensitivity could be seriously affected