Zeolite catalysts pave the road to decentral chemical processes Confined space increases reactivity
Nature as a modelNature provided the reference for the development of the new process. In biological systems, enzymes with small pockets in their surface accelerate chemical processes. “We thought about how we could apply theses biological functions to organic chemistry,” explains Lercher. “While searching for suitable catalysts that accelerate the reaction, we stumbled upon zeolites – crystals with small cavities in which the reactions take place under cramped conditions comparable to those in enzyme pockets.”
Cornered hydronium ionsBut, do cramped quarters really increase the reactivity? To answer this question, Lercher’s team compared the reactions of carbon compounds with acids in a beaker to the same reactions in zeolites. The result: In the crystal cavities, where the reacting molecules, for example alcohols, meet upon the hydronium ions of the acids, reactions run up to 100 times faster and at temperatures just over 100 °C. “Our experiments demonstrate that zeolites as catalysts are similarly effective as enzymes: Both significantly reduce the energy levels required by the reactions,” reports Lercher. “The smaller the cavity, the larger the catalytic effect. We achieved the best results with diameters far below one nanometer.”
Geckos, wax and zeolitesBut why do tight spaces foster the reactivity of molecules? “The force that improves the reaction path is the same as the one that causes wax to stick to a tabletop and that allows geckos to walk on ceilings,” replies Lercher. “The more contact points there are between two surfaces, the larger the adhesion. In our experiments, the organic molecules, which are in an aqueous solution, are literally attracted to the pores in the zeolites.” Thus, the hydronium ions within the cavities have a significantly greater likelihood of bumping into a reaction partner than those outside. The result is an acid catalyzed chemical reaction that takes place faster and with lower energy input.
From garbage to fuelWhen they come into contact with hydronium ions, organic molecules such as alcohols lose oxygen. This makes the process suitable to converting bio-oil obtained from organic waste into fuel. It will take some time, of course, before the new process can be deployed in the field. “We are still working on the fundamentals,” emphasizes Lercher. “We hope to use these to create the conditions required for new, decentral chemical production processes that no longer require large-scale facilities.” The work was developed in a cooperation of the Chair for Technical Chemistry II and the Catalysis Research Institute at the Technical University of Munich with the Pacific Northwest National Laboratory (PNNL). They were funded by the U.S. Department of Energy (DOE). Some of the NMR experiments were performed at the PNNL’s Environmental Molecular Science Laboratory (EMSL). PNNL’s National Energy Research Scientific Computing Center (NERSC) provided simulation time.
- Enhancing the catalytic activity of hydronium ions through constrained environments. Y. Liu, A. Vjunov, H. Shi, S. Eckstein, D. M. Camaioni, D. Mei, E. Barath, J. A. Lercher; Nat. Comm., 8, 14113 (2017) – DOI: 10.1038/ncomms14113, https://www.nature.com/articles/ncomms14113
- Tailoring nanoscopic confines to maximize catalytic activity of hydronium ions. H. Shi, S. Eckstein, A. Vjunov, D.M. Camaioni, J.A. Lercher; Nat. Comm., 8, 14113 (2017) – DOI: 10.1038/ncomms15442,https://www.nature.com/articles/ncomms15442