Alkane metathesis by tandem alkane dehydrogenation and olefin metathesis: Modeling the selectivity for carbon-number

A tandem system for the metathesis of n-alkanes, comprising pincer-ligated alkane-dehydrogenation catalysts and either Schrock-type or heterogeneous olefin-metathesis catalysts, was recently reported by the groups of Brookhart and Goldman. While other (heterogeneous) systems for alkane metathesis have been previously reported, a unique and very attractive aspect of this system is selectivity for the formation of C(2n-2) product and ethane from C(n) alkane; for example, n-decane and ethane from two mol n-hexane. However, this selectivity is only partial and is only obtained with certain catalyst combinations. In order to design more selective systems we wish to understand the factors that determine this selectivity; to that end we have conducted a modeling study using GEPASI and COPASI kinetics simulation software. The distribution of n-alkane product (from C2 to C10 in the case of n-hexane reactant) yields a significant number of concentration constraints. In addition the relative kinetic parameters used by the model are consistent with known thermodynamic values (e.g. for olefin isomerization and metathesis), and with those absolute rate constants that have been experimentally determined. Our model successfully reproduces the observed product distributions with a relatively small set of reactions and rate constants. The model yields several intriguing predictions that will be subject to future experimental testing.