Mechanistic insights to drive catalytic hydrogenation of formamide intermediates to methanol via deaminative hydrogenation

Amine-promoted hydrogenation of CO2 to methanol typically proceeds via a formamide intermediate when amines are used as additives or if the hydrogenation is performed in carbon capture solvents. The catalysts used for the hydrogenation of the formamide intermediate dictate the selectivity of the products formed: 1) Deoxygenative hydrogenation (C–O bond cleavage) resulting in N-methylation of amine and deactivation of the solvent, 2) Deaminative hydrogenation (C–N bond cleavage) resulting in formation of methanol and regeneration of the solvent. To date, catalytic reductions of CO2 with amine promoters suffer from poor selectively for methanol which we attribute to the limiting formamide intermediate, though to date, the conditions that favor C–N cleavage have yet to be fully understood. To better understand the reactivity of the formamide intermediates, a range of heterogenous catalysts were used to study the hydrogenation of formamide. Well-known gas phase CO2 hydrogenation catalysts catalyze the hydrogenation of formamide to N-methyl product via C–O bond cleavage. However, the selectivity can be readily shifted to selective C–N bond cleavage by addition of an additive with sufficient basicity for both homogenous and heterogeneous catalytic systems. The base additive shifts the selectivity by deprotonating a hemiaminal intermediate formed in situ during the formamide hydrogenation. This prevents dehydration process leading to N-methylated product, which is a key capture solvent deactivation pathway that hinders amine use in carbon capture, utilization, and storage (CCUS). The findings from this study provide a roadmap on how to improve the selectivity of known heterogenous catalysts, enabling catalytic reduction of captured CO2 to methanol

Hydrogen storage in liquid hydrogen carriers: recent activities and new trends

Efficient storage of hydrogen is one of the biggest challenges towards a potential hydrogen economy. Hydrogen storage in liquid carriers is an attractive alternative to compression or liquefaction at low temperatures. Liquid carriers can be stored cost-effectively and transportation and distribution can be integrated into existing infrastructures. The development of efficient liquid carriers is part of the work of the International Energy Agency Task 40: Hydrogen-Based Energy Storage. Here, we report the state-of-the-art for ammonia and closed CO2-cycle methanol-based storage options as well for liquid organic hydrogen carriers

Manganese-Catalyzed Sequential Hydrogenation of CO₂ to Methanol via Formamide

Mn­(I)-PNP pincer catalyzed sequential one-pot homogeneous CO₂ hydrogenation to CH₃OH by molecular H₂ is demonstrated. The hydrogenation consists of two partsN-formylation of an amine utilizing CO₂ and H₂, and subsequent formamide reduction to CH₃OH, regenerating the amine in the process. A reported air-stable and well-defined Mn-PNP pincer complex was found active for the catalysis of both steps. CH₃OH yields up to 84% and 71% (w.r.t amine) were obtained, when benzylamine and morpholine were used as amines, respectively; and a TON of up to 36 was observed. In our opinion, this study represents an important development in the nascent field of base-metal-catalyzed homogeneous CO₂ hydrogenation to CH₃OH

Conversion of CO₂ from Air into Methanol Using a Polyamine and a Homogeneous Ruthenium Catalyst

A highly efficient homogeneous catalyst system for the production of CH₃OH from CO₂ using pentaethylenehexamine and Ru-Macho-BH (<b>1</b>) at 125–165 °C in an ethereal solvent has been developed (initial turnover frequency = 70 h^–1 at 145 °C). Ease of separation of CH₃OH is demonstrated by simple distillation from the reaction mixture. The robustness of the catalytic system was shown by recycling the catalyst over five runs without significant loss of activity (turnover number > 2000). Various sources of CO₂ can be used for this reaction including air, despite its low CO₂ concentration (400 ppm). For the first time, we have demonstrated that CO₂ captured from air can be directly converted to CH₃OH in 79% yield using a homogeneous catalytic system

Efficient Reversible Hydrogen Carrier System Based on Amine Reforming of Methanol

A novel hydrogen storage system based on the hydrogen release from catalytic dehydrogenative coupling of methanol and 1,2-diamine is demonstrated. The products of this reaction, <i>N</i>-formamide and <i>N</i>,<i>N</i>′-diformamide, are hydrogenated back to the free amine and methanol by a simple hydrogen pressure swing. Thus, an efficient one-pot hydrogen carrier system has been developed. The H₂ generating step can be termed as “amine reforming of methanol” in analogy to the traditional steam reforming. It acts as a clean source of hydrogen without concurrent production of CO₂ (unlike steam reforming) or CO (by complete methanol dehydrogenation). Therefore, a carbon neutral cycle is essentially achieved where no carbon capture is necessary as the carbon is trapped in the form of formamide (or urea in the case of primary amine). In theory, a hydrogen storage capacity as high as 6.6 wt % is achievable. Dehydrogenative coupling and the subsequent amide hydrogenation proceed with good yields (90% and >95% respectively, with methanol and <i>N</i>,<i>N</i>′-dimethylethylenediamine as dehydrogenative coupling partners)

Formic Acid As a Hydrogen Storage Medium: Ruthenium-Catalyzed Generation of Hydrogen from Formic Acid in Emulsions

Formic acid is decomposed to H₂ and CO₂ in the presence of RuCl₃ and triphenylphosphines in an emulsion. In situ formed ruthenium carbonyls, such as [Ru­(HCO₂)₂(CO)₂(PPh₃)₂] (<b>1</b>), [Ru­(CO)₃(PPh₃)₂] (<b>2</b>), and [Ru₂(HCO₂)₂(CO)₄(PPh₃)₂] (<b>3</b>), and a large cluster, involving a Ru₁₂ core, were identified and structurally characterized from the reaction mixtures. The catalytic activity of the mono and binuclear complexes was also investigated and it was found that [Ru₂(HCO₂)₂(CO)₄(PPh₃)₂] (<b>3</b>) shows high stability even at elevated temperatures and pressures and its activity is 1 order of magnitude lower than those measured for the mononuclear complexes. It was also attempted to use [Ru­(HCO₂)₂(CO)₂(PPh₃)₂] (<b>1</b>) as a catalyst for the hydrogenation of CO₂ to formic acid under neutral conditions. Although the reduction of CO₂ did not take place, the conversion of [Ru­(HCO₂)₂(CO)₂(PPh₃)₂] (<b>1</b>) to an unexpected carbonate, [Ru­(CO₃)­(CO)₂(PPh₃)₂]·H₂O was observed

Iridium-Catalyzed Continuous Hydrogen Generation from Formic Acid and Its Subsequent Utilization in a Fuel Cell: Toward a Carbon Neutral Chemical Energy Storage

This study represents a notable step toward a potentially carbon neutral energy storage solution based on formic acid as a hydrogen/energy carrier. A catalytic system derived from IrCl₃ and 1,3-<i>bis</i>(2′-pyridyl-imino)-isoindoline (IndH) in the presence of aqueous sodium formate showed high selectivity and robustness for hydrogen generation from formic acid (FA) at 90–100 °C under both high and moderate pressure conditions suppressing the formation of CO impurity. Being a solid substance, the catalyst can be recovered by a simple filtration, if necessary. Furthermore, addition of neat formic acid is sufficient to reuse the catalyst and maintain a constant flow of H₂ and CO₂ mixture and the stable performance of a coupled fuel cell. The easy to recycle catalyst did not show any loss of activity after 20 days of continuous use, and similar activity was observed even a year after the original preparation. The reactor for formic acid decomposition provided a one to one ratio of a H₂/CO₂ mixture that was coupled to a hydrogen/air proton exchange membrane (PEM) fuel cell to demonstrate a stable and continuous conversion of chemical energy to electricity. This integrated system embodies the first example of an indirect formic acid fuel cell, which can function, without the requirement of applying inert conditions and feed gas purification, for extended periods of time