Robust Highly-Dispersed Heterogeneous Ruthenium Catalysts – Just Add a Pinch of Sodium (article review)

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The importance of heterogeneous catalytic hydrogenation reactions cannot be overemphasized and is one of the backbones of the chemical industry. Highly active and robust catalysts remain an objective particularly from an economical point of view. Catalyst deactivation, the loss over time of catalytic activity and/or selectivity, is a problem of great and continuing concern in the practice of industrial catalytic processes. The costs for the industry for catalyst replacement and process shutdown amount to billions of dollars per year. Time scales for catalyst deactivation vary considerably; for example, in the case of cracking catalysts, catalyst mortality may be on the order of seconds, while in ammonia synthesis the iron catalyst may last for 5–10 years.

Highly dispersed heterogeneous metal catalysts, such as single-atom or atomically dispersed metal catalysts that have been emerging as the next-generation heterogeneous catalysts, such deactivation processes are even more problematic. Analysis of these type of catalysts show that a large portion of catalytic metal centers are present in the form of cations with partial positive charges. These dispersed metals are easily reduced and sintered into large particles under hydrogenation conditions and the catalytic activity is rapidly lost. Still, new insights and developments are reported of which the recent article in Nature Catalysis (doi.org/10.1038/s41929-020-0481-6) by Gang Fu and co-workers is worth a highlight in this respect.
 

Figure 1. Comparison of the Na+-promoted Ru1/ gamma-alumina (Ru(Na)) with Ru// gamma-alumina (Ru(H))

Effect of sodium cations

The authors study the catalyst deactivation of atomically dispersed ruthenium(III) on Al2O3. Two different catalysts were prepared by deposition–precipitation of RuCl3 on γ-alumina using NaOH and NH4OH as precipitators yielding the so-called Ru(Na) and Ru(H) catalysts, respectively. Comparison of these two catalysts in the acetone hydrogenation at 150 °C and 30 bars showed that the Ru(Na) provided a stable and robust catalyst with lifetime of at least 32 hours whereas the Ru(H) quickly lost its activity.

To understand the atomic-scale functions of alkali ions, fresh and spent samples were analyzed using a large number of analytical techniques combined with DFT calculations. It was concluded that particularly the Ru(H) catalyst provide evidence for the Ru-aggregation into ruthenium nanoparticles whereas ruthenium species on the spend Ru(Na) were still present as atomically dispersed centers. The presence of sodium cation vicinal to the ruthenium species prevents the sintering and activates the Ru(III) catalytic species. Kinetic investigations (including kinetic isotope effects) show that the rate determining step for both (fresh) ruthenium catalysts was the heterolytic splitting of molecularly adsorbed H2 into Ru–H- and O–H+ reminiscent of the outer-sphere mechanism observed molecular Noyori-type ruthenium catalysts. Sintered ruthenium nanoparticles would only be able to activate hydrogen by a high energy homolytic process and thus would explain a lower catalyst activity for the sintered catalysts.

In summary, the article demonstrate that alkali cations play a versatile role in promoting catalytic hydrogenation of atomically dispersed metal catalysts. Decorating Ru1/Al2O3 with Na+ ions not only prevents the reduction induced sintering of isolated ruthenium(III) ions by limiting the proton transfer to a nearby oxygen. The sodium cations also promote the addition of activated hydrogen species to the substrates by stabilizing the negatively charged intermediates and transition states. Finally, the hydrogenation of industrially interesting substrates such as phenol, Guaiacol, di(2-ethylhexyl)phthalate, 4,4’-Methylenedianiline and pyridine are presented.

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Reference

Qin, R., Zhou, L., Liu, P. et al. Alkali ions secure hydrides for catalytic hydrogenation. Nat Catal (2020). https://doi.org/10.1038/s41929-020-0481-6
Published 13th of July 2020.