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 pha se over oxide catalysts have been surveyed. The reactions include oxid ative 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 pl ot was constructed with data from a variety of catalysts and a range o f operating conditions. There was a clear upper limit in terms of sele ctivity-conversion beyond which experimental studies have not advanced for each reaction studied. The object of this study is to elucidate t he reasons for the observed limitations. A correlation was observed be tween the limiting selectivities at fixed conversions and the function : (DHC-H (reactant)-H-0) - (DHC-H or C-C (product)-H-0) where (DHC-H ( reactant)-H-0) is bond dissociation enthalpy of the weakest C-H bond i n the reactant and (DHC-H or C-C (product)-H-0) is the bond dissociati on enthalpy of the weakest bond in the selective oxidation product. Th e results show that if this difference is less than 30 kJ/mol a very h igh selectivity is achievable at all conversions, whereas for differen ces greater than 70 kJ/mol poor selectivity is always recorded. This f inding characterizes the degree to which active sites in oxide catalys ts are capable of selectively activating target bonds.