Bimetallic Single-Site Heterogeneous Catalysts: A New Paradigm for Enhancing Reactivity in Olefin Metathesis

The global commodity chemical market is responding to a shift away from traditional petroleum refining (driven primarily by decreased oil refining) to greater reliance on natural gas. Simultaneously, more energy-efficient and targeted chemical manufacturing processes are needed to ensure the supply of high-volume chemical feedstocks during the clean-energy transition to net-zero carbon emissions.

Those trends have created a gap in the production of propylene – a vital olefinic hydrocarbon critical to the production of plastics and other polymeric materials. The project addresses the so-called propylene gap by researching novel catalyst designs to lower the energy requirements (i.e. decreased carbon emissions) while simultaneously increasing the reaction efficiency of producing propylene from other olefinic hydrocarbons in a chemical conversion process known as olefin metathesis.

To those ends, the project contributes to U.S. energy security, net-zero emissions goals, and the sustained supply of high-volume chemicals that drive the economy.

Monometallic single-site heterogeneous catalysts based on tungsten (W) and Molybdenum (Mo) on silica supports have been widely used for olefin metathesis of ethene and 2-butene into propylene (i.e., propene). In a series of breakthrough studies, the investigators have shown that the presence of small amounts of a second metal, such as niobium (Nb) (e.g.,1 wt% of Nb loading for 20 wt% of W/Mo) in the silicate support matrix, significantly increases the propene yield compared to the monometallic catalyst.

While increased metal (M) dispersion on the Nb-based silicate support, and the formation of additional dioxo [(O=)2M(-O-Nb)(-O-Si)] pre-catalyst sites, clearly contribute to the enhancement, a fundamental understanding for the enhancement effect is still lacking. The project will explore two specific thrusts to fill the existing knowledge gaps and provide testable predictions for the improvement of the catalytic activity: 1) determine the complete reaction mechanism for the metathesis of ethene and 2-butene to form propene on silica-supported W or Mo catalyst in the presence of a second metal such as Nb, and 2) investigate the structural and electronic effects of varying the second metal.

Quantum chemistry and machine learning simulations will be combined with experimental catalyst synthesis and characterization to identify chemical descriptors that will lead to rational design principles for better-performing catalyst materials. Although the immediate benefits of the project are predicated on propylene synthesis utilizing ethene and 2-butene from natural gas, the generic nature of the catalytic olefin metathesis mechanism opens the door to olefins produced from biorenewable feedstocks.

For example, the repurposing of ethanol-based biorefineries to make chemicals with significantly more value (compared to its use as a transportation fuel, which will steadily decline as electrification of the passenger car industry takes hold) has potential to facilitate profitability and rejuvenate rural agro-based economies, while significantly promoting decarbonization and sustainability in the chemical industry.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.