MECHANISTIC STUDIES OF HYDROCARBON CH/D2 EXCHANGE OVER THERMALLY ACTIVATED MAGNESIUM-OXIDE

The CH/D2 exchange reaction over thermally activated solid base catalyst MgO, where CH represents a hydrocarbon (carbon acid), has been investigated from a mechanistic point of view. Extensive kinetic studies of the exchange, analyses of hydroxyl content of MgO activated at various temperatures, spectroscopic studies, and consideration of pore sizes and possible reaction sites on MgO have led to the following conclusions: (1) the active site on MgO involves basic O2-ions, which abstract protons from carbon acids, as well as acidic Mg2+ ions, which stabilize resulting carbanions (2) the active site is probably a defect site of high electron density (perhaps a cation vacancy) and contains an Oz.sbnd;H (or Oz.sbnd;D) group; (3) the transition state in the RH(g) + D2(g) → MgO R-D(g) + HD(g) exchange reaction probably involves O2-D+ attack on adsorbed R- - H+ in the active site. A reaction coordinate diagram is proposed that explains our findings that exchange rates roughly correlate with literature solution-phase pKa values for RH. © 1990.