PNNL

The Science

Zeolite catalysts are used in numerous industrial processes. Thus, finding ways to make the reactions catalyzed within their pores more efficient has broad utility. Researchers described how the high ionic strength in zeolite micropores leads to an impressive increase in reaction rates for acid catalyzed reactions in the aqueous phase. The reaction rates are capped by the energy required to partially separate ion pairs in such a constrained environment. These results allow for the prediction of the free energy barrier for acid catalyzed reactions involving charged transition states and provide an understanding of their temperature dependence on the reaction rate.

The Impact

Developing next-generation catalysts with targeted reactivities requires understanding the fundamentals of catalyst behavior. By linking quantitative insights about reaction rates with material and solvents properties of microporous catalysts, researchers are one step closer to predicting how different materials will behave during acid catalyzed reactions. This new, enhanced understanding provides a window into how data science and artificial intelligence could eventually guide material synthesis for tailored rates and selectivities. While the insights from this work focus on zeolites, evidence suggests that they are generalizable to other systems.

Summary

The acid catalyzed elimination of water from alcohols is an important class of reactions for removing oxygen from molecules. Researchers combined advanced kinetic measurements with molecular spectroscopy and calorimetry to dissect the roles that increasing hydronium ion concentration and the environment of zeolite micropores play in the reaction. The micropores help minimize energy differences by stabilizing the transitions between molecular states. However, microporous environments also impose an upper limit on how much ion concentrations can enhance reaction rates given the energy required to separate ion pairs by reorganizing the solvent and reactant molecules in the constrained environment. Insight into the quantitative link between pore size for the zeolite BEA and hydronium ion concentration in the presence of water, the thermodynamic state of the reacting substrates, and the substrate conversion path allows scientists to predict maxima and temperature dependence of reaction rates. Strong evidence suggests that this is a generalizable insight, enabling the design of new catalysts for acid catalyzed reactions.

Contact

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

Johannes Lercher, Pacific Northwest National Laboratory, johannes.lercher@pnnl.gov 

Funding

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences program, Division of Chemical Sciences, Geosciences, and Biosciences (Impact of catalytically active centers and their environment on rates and thermodynamic states along reaction paths, FWP 47319).