As the production of fuels and chemicals shifts from fossil-based sources to renewables, challenges related to the diversity and variability of energy and feedstock supplies are starting to become evident. Consequently, catalysis research must pursue innovative solutions to address the dynamic nature of alternative energy sources and establish post-fossil value chains. This is particularly important when utilizing "green" molecular hydrogen (H₂) to convert renewable carbon feedstocks into essential value-added products, including fuels, fine chemicals, agrochemicals, and pharmaceuticals. With increasing emphasis on flexibility, there is a growing demand for catalysts with adaptive reactivity that can be modulated on demand.

The Multifunctional Catalytic Systems team led by Dr. Alexis Bordet in the Molecular Catalysis Department of Prof. Leitner at the Max Planck Institute for Chemical Energy Conversion recently developed adaptive hydrogenation catalysts composed of ruthenium nanoparticles (NPs) immobilized on amine-functionalized polymer grafted silica supports. The catalysts’ hydrogenation selectivity could be controlled using CO₂ as a molecular trigger through the reversible formation ammonium formate species [Nat. Chem. 2021, Angew. Chem. 2023]. Elusive mechanism, complex catalyst design, and ammonium formate stability were limiting the system's practical application, however.

In this new study, Dr. Yuyan Zhang – first author of the paper and postdoctorate researcher in Dr. Bordet’s team – uses mechanistic studies and theoretical calculations to investigate the modulation of catalyst performance from the adjustment of the H₂ + CO₂ = HCOOH equilibrium. Results reveal that adsorption of formic acid at the silica-supported metal nanoparticles generates surface formate species which compete with functional groups in the substrates for hydrogenation at the active sites. Thus, the hydrogenation of the competing functional group is shut down upon addition of CO₂ to the hydrogen feed gas and fully restored immediately upon returning to pure H₂.

Based on this new mechanistic understanding, Ru NPs were immobilized on a simple guanidinium-based supported ionic liquid phase (SILP_GB). The resulting Ru@SILP_GB catalyst was characterized and applied to the hydrogenation of biomass-derived furfuralacetone and related ketone substrates, confirming the practically instantaneous on/off switching of the C=O hydrogenation with Ru@SILP_GB under H₂ or H₂/CO₂. The guanidinium-based surface molecular modifier was found essential to stabilize concentrations of formic acid sufficient to observe a selectivity switch, which was not observed using other Ru-based reference catalysts. The selectivity switch was found robust and fully reversible in real time, providing either full hydrogenation under H₂ or partial hydrogenation under H₂/CO₂.

This study reveals that the major prerequisite for design of adaptive catalytic systems based on CO₂ as trigger is the ability to shift the H₂ + CO₂ = HCOOH equilibrium sufficiently to exploit competing adsorption of surface formate and targeted functional groups. Thus, the concept can be expected to be more generally applicable beyond ruthenium as the active metal paving the way for next-generation adaptive catalytic systems in hydrogenation reactions more broadly. 

Original Paper:
Yuyan Zhang, Natalia Levin, Liqun Kang, Felix Müller, Mirijam Zobel, Serena DeBeer, Walter Leitner* and Alexis Bordet*. Design and Understanding of Adaptive Hydrogenation Catalysts Triggered by the H₂/CO₂–Formic Acid Equilibrium. J. Am. Chem. Soc. 2024, https://doi.org/10.1021/jacs.4c06765

For further information on the design of multifunctional and adaptive catalysts:
- Bordet, A., Leitner, W. Metal Nanoparticles Immobilized on Molecularly Modified Surfaces: Versatile Catalytic Systems for Controlled Hydrogenation and Hydrogenolysis. Acc. Chem. Res. 2021, 54, 2144-2157. https://doi.org/10.1021/acs.accounts.1c00013
- Zhang, Y., Bordet, A. (2024). Metal Nanoparticles on Molecularly Modified Surfaces and Their Application in Catalysis. In: Topics in Organometallic Chemistry. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3418_2024_121
- Bordet, A., Leitner, W. Adaptive Catalytic Systems for Chemical Energy Conversion. Angew. Chem. Int. Ed. 2023, e202301956. https://doi.org/10.1002/anie.202301956