Alkene separation from alkane/alkene mixtures is regarded as one of the most challenging engineering problems, as traditional distillation methods are energy-intensive and expensive. In 2001, Wang and Stiefel proposed alkane/alkene electroseparation as an energy-efficient and sustainable alternative to distillation¹. The method leverages selective alkene binding to bis(dithiolene) transition metal complexes, followed by electroreduction and alkene release from the reduced adduct state. While the experimental results have been promising, questions remain about the thermodynamic and kinetic factors governing alkene binding and release and their sensitivity to metal and ligand variations.

In this study, we aim to develop a universal, predictive tight binding model that captures thermodynamic trends of alkene binding and release using a minimal number of physically justified and transferable parameters. The development of such a model will help us discover the promising separation agents for a follow-up experimental study.

In our initial analysis, we derived an analytical expression to describe the interaction between the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) of a complex and an alkene. The mathematical form was inspired by the non-empirical tight binding theory² introduced by our group and approximates the interaction energy as the sum of attractive hybridization and repulsive orthogonalization terms. The model effectively captures binding energy trends of a Pd (II) complex with varying ligand identities, using only three parameters. Further improvements toward a more generalizable and accurate model include accounting for electrostatic interactions for all atom pairs, refining the functional forms of resonance integrals, and incorporating an electronegativity equalization mechanism into the model. We anticipate that once fully developed, the model will facilitate screenings of ligand/transition metal combinations, paving the way toward the systematic design of efficient agents for alkene separation.

References

(1) Wang, K.; Stiefel, E. I. Toward Separation and Purification of Olefins Using Dithiolene Complexes: An Electrochemical Approach. Science 2001, 291 (5501), 106–109. 10.1126/science.291.5501.106.

(2) Mironenko, A. V. Analytical and Parameter-Free Hückel Theory Made Possible for Symmetric Hx Clusters. Journal of Physical Chemistry A 2023, 127 (37), 7836–7843. https://doi.org/10.1021/acs.jpca.3c03646.