ISOTOPIC EXCHANGE BETWEEN ALKANES AND DEUTERIUM ON RHODIUM SILICA-GEL CATALYSTS

Isotopic exchange between deuterium and cyclopentane and between pentane and ethane on the Rh/SiO2 catalysts of the preceeding paper has been investigated after several different pretreatments. Although a more restricted number of catalysts was examined, as with 2,2-dimethylbutane, increased percentage exposed (Dh) appears to lead to increased turnover frequency (Nt). After the standard pretreatment, H2,300°; He,450°, the rates in alkane(g) ⇌ alkane(ads) increase only slowly as Dh increases, but the rates in monoadsorbed alkane ⇌ diadsorbed alkane decline relatively rapidly. As with 2,2-dimethylbutane, the pretreatment H2,450° leads to the largest values of Nt. He,450° following H2, 450° removes surface hydrogen atoms and leads to reconstruction to a surface of much lower activity for exchange. Cyclopentane exchanges much faster than pentane on these catalysts. It appears that adsorption and desorption of cyclopentane involve single-step processes to and from diadsorbed cyclopentane + 2H*. Adsorption is faster than that with pentane because cyclopentane is essentially in the eclipsed conformation needed for this process. The rate of monoadsorbed ⇌ diadsorbed cyclopentane is similarly augmented. Adsorbate-surface strain reduces the rate constant for adsorption of 2,2-dimethylbutane and increases that for desorption. When coverages by the unhindered hydrocarbon are large, the relative rate of exchange of the hindered hydrocarbon vs that of the unhindered hydrocarbon will be larger than one would conclude merely from the relative rate constants of adsorption. © 1992.