An amino acid based system for CO2 capture and catalytic utilization to produce formates

Herein, we report a novel amino acid based reaction system for CO2 capture and utilization (CCU) to produce formates in the presence of the naturally occurring amino acid l-lysine. Utilizing a specific ruthenium-based catalyst system, hydrogenation of absorbed carbon dioxide occurs with high activity and excellent productivity. Noteworthy, following the CCU concept, CO2 can be captured from ambient air in the form of carbamates and converted directly to formates in one-pot (TON > 50 000). This protocol opens new potential for transforming captured CO2 from ambient air to C1-related products.


Introduction
Carbon dioxide concentration in the atmosphere and global warming is ever-increasing with the enormous global energy demand supplied by consuming fossil fuels (mainly coal, oil, and natural gas). 1,2 CO 2 capture and storage (CCS) enable the use of fossil fuels with signicantly lower CO 2 emissions than usual. 3 CCS is based on the separation of CO 2 from energy conversion or other industrial processes, followed by compression, transport, and storage. However, CCS processes are meanwhile energy intensive as the electricity burden with amine scrubbing (113 kW h per mt CO 2 removed) constitutes the minimum work to separate and compress CO 2 (150 bar). Indeed, in two demonstration units, Boundary Dam and Thompsons, 210-220 kW h per mt were required for this purpose. 4 Developing novel CO 2 capture and utilization (CCU) methods for converting CO 2 from air or ue gas not only saves energy from CCS (mainly CO 2 desorption and compression steps) but also provides C1-related products (Scheme 1a). [5][6][7][8][9][10][11][12] It's thus an important opportunity for developing a sustainable economy. [13][14][15] In nature, inorganic carbon (particularly CO 2 ) is converted to organic compounds by living organisms, which is known as carbon xation, with photosynthesis as the most prominent example. 16 It is estimated that approximately 258 billion tons of CO 2 are converted into biomass by photosynthesis annually. 17 As the most abundant protein on the Earth, ribulose 1,5bisphosphate carboxylase/oxygenase (RuBisCO) is involved in the rst major step of carbon xation by plants and other photosynthetic organisms. 18,19 L-Lysine (Lys) is one of the six crucial amino acids (AAs) that are part of the active site of RuBisCO and it stabilizes CO 2 in the form of carbamate for subsequent enzyme catalysis. 20 By contrast, in industry e.g., power plants, the most common process for capturing CO 2 relies on the use of aqueous amine solutions (Scheme 1b). 3,21,22 However, the maximum CO 2 absorption capacity for an amine system varies based on which products are formed. When carbamates are the preferred products, this capacity is 50 mol% per amines at most. If bicarbonates are mainly formed, this capacity could reach up to 100 mol% per amines. Alkanolamines have been extensively investigated as chemical absorbents; 23 however, their largescale use also created some environmental concerns. Substituting such conventional amine absorbents with high boiling and innocuous natural AAs in combining CO 2 capture and catalysis is therefore highly relevant. Noteworthy, CO 2 capture with aqueous AAs, [24][25][26][27] including Lys 25 was already reported, but not its direct valorization. Based on the infusive phenomenon of carbon xation by RuBisCO and our long-term interest in CO 2 reduction, we report herein a CCU process which enables CO 2 capture from ambient air and its conversion to formate in the presence of L-lysine. Moreover, to the best of our knowledge, there exists no example of catalytic hydrogenation of CO 2 assisted by AAs.
Scheme 1 (a) Schematic CCU concept for CO 2 hydrogenation to C1 products. (b) Reaction pathways for CO 2 absorption with amines under aqueous conditions. Several Rh-and Ru-based homogeneous catalysts have been previously reported for CO 2 capture and in situ hydrogenation to C1 products (Fig. 1). 8,28 In 2013, pioneering work was performed by the group of He utilizing RhCl 3 $3H 2 O and phosphine ligands, for instance CyPPh 2 , DPEphos, and PPh 3 , as catalysts where gaseous CO 2 was absorbed by polyethyleneimine (PEI), 29 amidines, 30 and potassium phthalimide 31 as well as hydrogenated in situ to formates or formic acid.
In addition, ruthenium complexes have also been proven to be suitable catalysts for the hydrogenation of captured CO 2 to formate or methanol. In 2014, Heldebrant and co-workers captured CO 2 by DBU in methanol to form the methyl carbonate, which then was hydrogenated to formates catalyzed by [RuCl 2 (PPh 3 ) 3 ]. 32 One year later, Milstein and co-workers reported a CCU approach, where CO 2 reacted with aminoethanols yielding oxazolidinones which were hydrogenated to CH 3 OH in 78-92% yield with a Ru-PNN pincer catalyst. 33 In the same year, the Sanford group reported the CO 2 capture with NHMe 2 to form carbamate and subsequent hydrogenation to a mixture of DMF and CH 3 OH catalyzed by Ru-MACHO-BH complex. 34 Employing the same catalyst and tetramethylguanidine (TMG), 35 metal hydroxides, 36 pentaethylenehexamine (PEHA), 37-39 a mixture of metal hydroxides, 40 or a tertiary amine 41 with ethylene glycol as CO 2 absorbent systems, Prakash and his colleagues combined CO 2 capture from air with subsequent hydrogenation to produce formates or methanol. Recently, the group of Heldebrant reported a method where epoxides reacted with CO 2 leading to cyclic carbonates. Then, in situ hydrogenation took place into methanol and glycol, with Ru-MACHO as catalyst. 42 Compared to methanol, no hydrogen is lost in the form of water when formic acid or formate salts are produced by CO 2 hydrogenation. Currently, formic acid is industrially produced by carbonylation of methanol to methyl formate and subsequent hydrolysis. 43 It is mainly used as a preservative and antibacterial agent in livestock feed, e.g. silage and winter feed for cattle. In addition, formic acid is utilized in the production of leather and in dyeing and nishing textiles. More recently, it also gained interest as hydrogen storage medium as it contains 4.4 wt% of hydrogen with 53 g H 2 per L of volumetric storage density. 7

CO 2 capture with amino acids
For the development of a CCU concept to produce formic acid or formates, suitable CO 2 absorbents must be used. Inspired by the carbon xation pattern in nature, specically RuBisCO, we considered applying AAs for this purpose. [24][25][26][27] Thus, at the start of our investigations, we evaluated the ability of 12 different AAs, including the 6 ones involved in the active site of RuBisCo and some analogues to capture CO 2 . For this purpose, CO 2 (2 bar) was charged into an aqueous solution of the respective AAs (5 M) and stirred at r.t. for 2-18 h.
As shown in Table S1, † most of the tested systems such as Lproline, L-glutamine, and L-histidine achieved only small to moderate amounts of CO 2 capture, around 0.1 mol of CO 2 per mol of AA (CO 2 /AA), (Table S1, entries 1-11 †). Interestingly, in the presence of L-lysine (Lys), a signicantly improved performance (3.63 mmol of captured CO 2 , corresponding to 0.73 CO 2 /Lys) was obtained in 18 h (Table S1, entry 12 †). Such high CO 2 capture efficiency could be attributed to the basic side chain of Lys, as its pK a value is 10.7.
Thus, we investigated the effect of Lys for CO 2 absorption under various conditions ( On the other hand, L-cysteine, L-histidine, L-serine, and Lthreonine led to formates in much lower yields (up to 13%), while other AAs, such as glycine, L-proline, and L-glutamine showed no activity at all in the presence of catalyst Ru-1, (Table   S2, entries 2-12 †). Taking Lys as a benchmark CO 2 absorbent, the TON of formate can be considerably increased from 2187 to 197 559 when decreasing the loading of Ru-1 from 400 ppm (based on Lys) to 4 ppm ( Table 2, entries 1-3). With 4 ppm of Ru-MACHO (Ru-2) as catalyst, the highest TON 212 139 was achieved ( Table 2, entry 4). Interestingly, in these reactions, CO 2 was selectively converted to formate in up to 87% yield with less than 1% of formamide. Next, several ruthenium pincer complexes were tested at 4 ppm loading for the hydrogenation of gaseous CO 2 in the presence of Lys within 3 h ( Table 2, entries 6-10). Ru-1 and Ru-2 gave formate in 55% and 58% yields, respectively, whereas Ru-MACHO iPr (Ru-3) was less active leading to formate in only 6% yield. With Milstein's Ru-PNP complex (Ru-4) as catalyst, formate was obtained in 47% yield. However, no formate can be detected in the reaction catalyzed by Fe-MACHO iPr -BH complex (Fe-1).
Several blank reactions were also carried out (Table S3 †): in the absence of either Lys, Ru-1, or CO 2 , no formate was detectable. These results clearly demonstrate that Lys and Ru-1 are both crucial to promote the hydrogenation of CO 2 from air to formate. Reactions with other solvents, for example, triglyme, methanol, ethylene glycol or their 1 : 1 mixture with water could not improve the reaction efficiency (Table S4 †). When replacing THF with the more eco-friendly green solvent 2-methyltetrahydrofuran (2-MTHF), 45 a comparable yield of formate (86%) was observed. Lowering the temperature from 145 to 105 C, the yield of formate decreased only slightly from 79% to 64% (Table  S5 †).

Development of a general CCU concept
Aer having studied the individual processes of (a) CO 2 absorption and (b) CO 2 reduction in the presence of Lys, the overall CCU concept was demonstrated by combining CO 2 capture and in situ hydrogenation to formate (Table 3 and Fig. S14-S16 †).
Using captured CO 2 (2.42 mmol) as substrate in the presence of Ru-1 (2.0 mmol) as catalyst, 46% formate yield (based on  were obtained with the same amount of Ru-2 (Table 3, entry 7). Finally, some Lys analogues and derivatives as well as selected benchmark amines 35,37,39 were applied according to our overall protocol (Fig. 2). In the presence of 6-aminohexanoic acid and 1,5-diaminopentane, 0.12 and 0.82 CO 2 /amine were achieved and formates were obtained in yields of 25% and 34%, respectively. Noticeably, 2,3-diaminopropanoic acid and the simplest amino acid glycine did not show any activity in both CO 2 absorption and hydrogenation processes. In the case of TMG and PEHA, CO 2 was captured with 0.86 and 0.83 CO 2 /TMG or PEHA, respectively. However, the presence of TMG inhibited the hydrogenation of CO 2 , whereas PEHA led to formate and formamides in 38% and 8% yield, respectively. Applying the inorganic base NaOH 36 resulted in a CO 2 /base ratio of 1.08 and 23% formate yield. All these experiments demonstrate the superiority of using Lys for carbon dioxide capture and direct valorizations. It also indicates the crucial presence of an aamino acid moiety and an additional amine function in the side chain of AA.
To rationalize the perfect selectivity towards formates in the current study, we conducted further experiments by heating up the mixture of formic acid and Lys or PEHA in H 2 O at 145 C (Table S6 †). Indeed, Lys led to formate in quantitative yield without any formamide detectable aer 12 h, whereas PEHA gave 28% yield of formamide along with 71% formate. Obviously, the less basic conditions applying Lys (pH 10.2 for a 5 M aqueous solution) prevented the formation of formamides taking place in the presence of PEHA (pH 13.4).

Conclusions
In conclusion, we described an amino acid based catalyst system for the highly relevant CO 2 capture and utilization (CCU) process to produce formates in one-pot. The naturally occurring amino acid L-lysine affords formate generation with a high efficiency. Among the investigated catalysts, the most active ones are identied with Ru-MACHO complexes (Ru-1 and Ru-2) for the hydrogenation of gaseous CO 2 (TON > 210 000) and the in situ hydrogenation of captured CO 2 (TON > 50 000). Noteworthy, in the present CCU concept, CO 2 can be captured from ambient air in the form of carbamates and hydrogenated to formate directly.    Table 1, entry 4 and Table 2 entry 6, respectively. CO 2 / amine (mols of CO 2 captured per mol of amine) are shown with yield and TON of formates in parentheses; n.d. ¼ not detectable.