Carbenium Ion Dynamics in Lewis Acid Catalyzed Isopentane Disproportionation

The Science

Isopentane disproportionation catalyzed by a chloroaluminate ionic liquid is a known route to the low temperature and pressure forming and breaking of C–C and C–H bonds. However, a detailed understanding of the underlying mechanism and kinetics has remained unknown. New research revealed that the carbenium ion-AlCl₄⁻ ion-pairs facilitates hydride transfer, while an off-cycle equilibrium between carbenium ion and alkene intermediates leads to kinetic regime changes from a transient to a steady-state. This work provides a simple kinetic framework for evaluating catalyst hydride transfer ability, which is useful across a broad range of reactions.

The Impact

Alkane conversion typically requires severe conditions such as high temperatures and pressures due to the stability of C–C and C–H bonds. The low temperatures and pressures required for alkane disproportionation with Lewis acidic chloroaluminate ionic liquid makes understanding the mechanism behind the process important for developing new approaches to forming and breaking C–C and C–H bonds. This work provides insight into the mechanisms central to these conversions. Pairing these insights with the newly developed core kinetic model, this reaction can be a probe for benchmarking a catalyst’s hydride transfer ability to develop routes to more efficient and reliable hydrocarbon conversion.

Summary

Alkane disproportionation proceeds catalytically at ambient temperature and pressure using a Lewis acidic chloroaluminate ionic liquid, a reaction observed decades ago. Despite this history, a detailed mechanism of the reaction has remained unclear. A research team elucidated the disproportionation mechanism and captured it in a simple kinetic model with just three governing rate constants.

An alkene intermediate, key to the proposed mechanism, was observed via in situ ¹H nuclear magnetic resonance spectroscopy combined with Raman, 2D heteronuclear multiple bond correlation spectroscopies and density-functional calculations, further confirming the mechanism. The team developed a comprehensive kinetic model to describe the reaction, but the full model risked overfitting. The researchers identified intrinsic connections between specific rate constants and systematically reduced parameters through sensitivity analysis, arriving at a minimal model with only three governing rate constants.

The simplicity of the reaction and its kinetic model together offer an ideal probe for evaluating a catalyst’s hydride transfer ability. Hydride transfer is the rate-determining step in many hydrocarbon conversions central to the petrochemical industry. A clean probe reaction paired with a straightforward kinetic model is necessary for screening and benchmarking catalyst performance for more efficient and reliable hydrocarbon conversion processes.

Contact

Sungmin Kim, Pacific Northwest National Laboratory, sungmin.kim@pnnl.gov 

Zdenek Dohnalek, Pacific Northwest National Laboratory, Zdenek.Dohnalek@pnnl.gov 

Funding

This work was performed at Pacific Northwest National Laboratory, which is a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy (DOE) under contract number DE-AC06-76RL0183. This work was primarily supported by DOE, Office of Science (SC), Basic Energy Sciences (BES) program, Division of Chemical Sciences, Geosciences and Biosciences (Towards a polyolefin-based refinery: understanding and controlling the critical reaction steps, FWP 78459). NMR measurement and COPASI modeling analysis were supported by DOE/SC/BES Chemical Sciences, Geosciences, and Biosciences Division, (Advancing key catalytic reaction steps for achieving carbon neutrality, FWP 47319). This research used resources of the National Energy Research Scientific Computing Center (NERSC), a DOE SC user facility supported by the Office of Science of the DOE under Contract No. DE-AC02-05CH11231 using NERSC award BES-ERCAP0032413.