The role of reactant and product bond energies in determining limitations to selective catalytic oxidations

A wide range of selective oxidation reactions occurring in the gas phase over oxide catalysts have been surveyed. The reactions include oxidative dehydrogenation of alkanes, oxidation of alkenes and alcohols to aldehydes and the oxidation of alkanes to acids. The literature data was gathered into a series of selectivity-conversion plots and each plot was constructed with data from a variety of catalysts and a range of operating conditions. There was a clear upper limit in terms of selectivity-conversion beyond which experimental studies have not advanced for each reaction studied. The object of this study is to elucidate the reasons for the observed limitations. A correlation was observed between the limiting selectivities at fixed conversions and the function: D⁰H_{C - H (reactant)}-D⁰H_{C - H or C - C (product)} where D⁰H_{C - H (reactant)} is the bond dissociation enthalpy of the weakest C-H bond in the reactant and D⁰H_{C - H or C - C (product)} is the bond dissociation enthalpy of the weakest bond in the selective oxidation product. The results show that if this difference is less than 30 kJ/mol a very high selectivity is achievable at all conversions, whereas for differences greater than 70 kJ/mol poor selectivity is always recorded. This finding characterizes the degree to which active sites in oxide catalysts are capable of selectively activating target bonds.