Versatile CO₂ Hydrogenation–Dehydrogenation Catalysis with a Ru–PNP/Ionic Liquid System

Abstract

High catalytic activities of Ru–PNP [Ru = ruthenium; PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine] complexes in ionic liquids (ILs) were obtained for the reversible hydrogenation of CO2 and dehydrogenation of formic acid (FA) under exceedingly mild conditions and without sacrificial additives. The novel catalytic system relies on the synergic combination of Ru–PNP and IL and proceeds with CO2 hydrogenation already at 25 °C under a continuous flow of 1 bar of CO2/H2 (1:5), leading to 14 mol % FA with respect to the IL. A pressure of 40 bar of CO2/H2 (1:1) provides 126 mol % of FA/IL corresponding to a space-time yield (STY) of FA of 0.15 mol L–1 h–1. The conversion of CO2 contained in imitated biogas was also achieved at 25 °C. Furthermore, the Ru–PNP/IL system catalyzes FA dehydrogenation with average turnover frequencies up to 11,000 h–1 under heat-integrated conditions for proton-exchange membrane fuel cell applications (<100 °C). Thus, 4 mL of a 0.005 M Ru–PNP/IL system converted 14.5 L FA over 4 months with a turnover number exceeding 18,000,000 and a STY of CO2 and H2 of 35.7 mol L–1 h–1. Finally, 13 hydrogenation/dehydrogenation cycles were achieved with no sign of deactivation. These results demonstrate the potential of the Ru–PNP/IL system to serve as a FA/CO2 battery, a H2 releaser, and a hydrogenative CO2 converter.

Ruthenium-Catalyzed Formic Acid/Formate Dehydrogenation and Carbon Dioxide/(bi)carbonate Hydrogenation in Water

“…38 presence of an ionic liquid, 1-ethyl-3 methylimidazolium acetate (EMIM OAc). 39 Recently, Fischmeister et al developed an arene-based Ru complex for FA dehydrogenation (TOF of 84 h −1 and TON of 173) at 90 °C, and CO 2 hydrogenation was performed (TON of 160) at 60 °C in DMSO. 40 It is evident from these reports that Cp*Ir-based catalysts are the most active catalysts, and these Cp*Ir catalysts are also active in aqueous conditions.…”

“…Under the same reaction conditions, CO 2 was hydrogenated efficiently to formate (TON of 75) . Nielsen et al used a Ru-PNP pincer complex for the dehydrogenation of FA with a TON of 18,000,000 and TOF of 11,134 h –1 at 80 °C and CO 2 hydrogenation at 25 °C, achieving a TON > 190 in the presence of an ionic liquid, 1-ethyl-3 methylimidazolium acetate (EMIM OAc) . Recently, Fischmeister et al developed an arene-based Ru complex for FA dehydrogenation (TOF of 84 h –1 and TON of 173) at 90 °C, and CO 2 hydrogenation was performed (TON of 160) at 60 °C in DMSO …”

High-pressure hydrogen generation from dehydrogenation of formic acid

“…13–21 This has drawn the attention of many researchers and several very active homogeneous as well as heterogeneous catalysts have been developed over the past 15 years which efficiently dehydrogenate FA to H 2 and CO 2 selectively. 22–50 In this context, iridium based catalytic systems were found to be most effective. For instance, in 2015, Li et al developed a Cp*-Ir( iii ) catalyst containing a bisimidazoline ligand ( 1 in Fig.…”

“…1) in combination with an ionic liquid (1-ethyl-3-methylimidazolium acetate, EMIM OAc) for FADH, achieving an overall TON exceeding 18 million. 50 In 2014, Hazari and co-workers reported a non-noble metal Fe based PNP-pincer complex ( 11 in Fig. 1) for FADH to achieve TONs of up to 983 642 using LiBF 4 as an additive.…”

Tuning Ethanol Upgrading toward Primary or Secondary Alcohols by Homogeneous Catalysis

“…We commenced our studies by investigating the ruthenium PNP pincer complexes Ru - 1 to Ru - 5 shown in Tables and for the catalytic upgrading of ethanol. These complexes are robust catalysts for a wide range of sustainable chemical transformations under mild reaction conditions − However, in support of the findings made by Wass and co-workers, Liauw and co-workers recently suggested that consumption of ethoxide additive, via formation of NaOAc by ethyl acetate saponification, is a major reason for low yields of ethanol upgrading to 1-butanol catalyzed by Ru - 2 . Indeed, NaOEt is often used for Guerbet reactions promoting the aldol condensation of acetaldehyde and selective formation of 1-butanol and higher primary alcohols. ,, Thus, we set to study the outcome in the solid, liquid, and gas phases of the upgrading reaction of ethanol containing 20 mol % NaOEt using Ru - 1 to Ru - 5 as catalysts.…”