Limiting factors in the selective oxidation of C₄ hydrocarbons

A series of selectivity-conversion plots have been constructed from open literature data for selective oxidation reactions of C₄ hydrocarbons and oxygenated species occuring in the gas phase over oxide catalysts. For each reaction there was an upper limit in terms of selectivity-conversion beyond which experimental studies have not yet advanced. 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 : DH_{C-H(reactant)} - DH_{C-H or C - C(product)} where DH_{C-H(reactant)} is the bond dissociation enthalpy of the weakest C - H bond at the reactant and DH_{C - H or C - C (product)} is the bond dissociation enthalpy of the weakest bond at 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.