Manganese Promoted (Bi)carbonate Hydrogenation and Formate Dehydrogenation: Toward a Circular Carbon and Hydrogen Economy

We report here a feasible hydrogen storage and release process by interconversion of readily available (bi)carbonate and formate salts in the presence of naturally occurring α-amino acids. These transformations are of interest for the concept of a circular carbon economy. The use of inorganic carbonate salts for hydrogen storage and release is also described for the first time. Hydrogenation of these substrates proceeds with high formate yields in the presence of specific manganese pincer catalysts and glutamic acid. Based on this, cyclic hydrogen storage and release processes with carbonate salts succeed with good H2 yields.


Standard procedure for catalytic hydrogenation of potassium bicarbonate
Potassium bicarbonate (KHCO 3 , 10 mmol), Mn-2 (0.1 mg, 0.18 μmol), THF (5 mL) and H 2 O (5 mL) were added to a 100 mL autoclave equipped with a magnetic stir bar. After pressurizing the reactor with H 2 (60 bar), the reaction mixture was heated and stirred on a pre-heated oil bath for 12 h. Then the reactor was cooled to r.t. and the inside pressure was carefully released.
A biphasic reaction mixture was obtained containing a transparent organic upper layer and an aqueous yellow lower layer. Addition of DI water (ca. 3 mL) to the above mentioned mixture resulted in a homogeneous solution. 11 DMF (500 μL, 6.48 mmol) was added as an NMR internal standard to the reaction mixture, which was then analyzed by 1        Hydrogen yield is calculated with the following equation: The turnover number (TON) of H 2 is calculated with the following equation: Where:  is the gas evolution volume of catalytic reaction measured in the manual burettes. S14  is the gas evolution volume of the blank reaction measured in the manual burettes. Calculation of CO 2 molar volume :  h. Volume and content of the gas phase from manual burettes were analyzed by gas chromatography (GC) after correction by the blank volume. CO is not detectable in all cases (below the CO quantification limit of 10 ppm). Yield of H 2 is calculated by (mmol H 2 )/(mmol KHCO 2 )×100%. The dotted lines serve as guides to the eye. S17 Figure S11. GC chromatogram of blank reaction for the dehydrogenation of FA. Only argon was reported at retention time 8.122 min. S18 Figure S12. Typical GC chromatogram of potassium formate dehydrogenation in the absence of additives. S19 Figure S13. Typical GC chromatogram of potassium formate dehydrogenation in the presence of L -lysine (Lys). Figure S14. Typical GC chromatogram of potassium formate dehydrogenation in the presence of L -glutamic acid (Glu).

Standard procedure for H 2 storage-release cycles starting from formate salts/Lys
The H 2 storage-release cycles starts from the dehydrogenation of formate salt (H 2 release): Mn-2 (5 μmol), formate salt (5.0 mmol), Lys, THF (5 mL) and H 2 O (5 mL) were added to a 100 mL autoclave equipped with a magnetic stir bar. The reaction mixture was then heated and stirred on a pre-heated oil bath at 90 °C for 12 h. The reactor was cooled to r.t. and the inside pressure was released carefully to the manual burettes and the content of the gas phase was analyzed by gas chromatography (GC). CO is not detectable in all cases (below the CO quantification limit of 10 ppm). The autoclave was then filled with 60 bar of H 2 , heated and stirred on a pre-heated oil bath at 90 °C for 12 h (H 2 storage). After the completion of H 2 storage, the reactor was cooled to r.t. and the inside pressure was released carefully. Then the autoclave was subjected to the H 2 release procedure again. Following such process, the H 2 evolution in the H 2 storage-release cycles were implemented with various formate salts.

Standard procedure for H 2 storage-release cycles starting from carbonate salts/Glu
The H 2 storage-release cycles starts from the hydrogenation of carbonate salt (H 2 storage): Mn-2 (5 μmol), carbonate salt (5.0 mmol), Glu, THF (5 mL) and H 2 O (5 mL) were added to a 100 mL autoclave equipped with a magnetic stir bar. After pressurizing the reactor with H 2 (60 bar), the reaction mixture was heated and stirred on a pre-heated oil bath for 12 h.
Afterwards, the reactor was cooled to r.t. and the inside pressure was released carefully. The autoclave was then subjected to H 2 release process at 90 °C for 12 h. After the completion of the H 2 release step, the reactor was cooled to r.t. and the inside pressure was released carefully to the manual burettes and the content of the gas phase was analyzed by gas chromatography (GC). CO is not detectable in all cases (below the CO quantification limit of 10 ppm). Then the autoclave was subjected to the H 2 storage procedure again. Following such process, the H 2 evolution in the H 2 storage-release cycles were tested with various carbonate salts.    Figure S20. 13