Integrated CO2 Capture and Conversion to Formate and

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Integrated CO2 Capture and Conversion to Formate and Methanol: Connecting Two Threads Sayan Kar, Alain Goeppert, and G. K. Surya Prakash*

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Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, University Park, Los Angeles, California 90089-1661, United States CONSPECTUS: The capture of CO2 from concentrated emission sources as well as from air represents a process of paramount importance in view of the increasing CO2 concentration in the atmosphere and its associated negative consequences on the biosphere. Once captured using various technologies, CO2 is desorbed and compressed for either storage (carbon capture and storage (CCS)) or production of value-added products (carbon capture and utilization (CCU)). Among various products that can be synthesized from CO2, methanol and formic acid are of high interest because they can be used directly as fuels or to generate H2 on demand at low temperatures (700 °C) and is energy-intensive.9,72 In comparison, amino alcohols and amines can be regenerated at comparatively low temperatures (∼100 °C).

Figure 7. Regeneration of NaOH from Na2CO3.

Bicarbonate to Formate

Although the hydrogenation of captured product bicarbonate to give formate salts is well-documented, the integrated capture and conversion of CO2 to formate salts was not reported until 2018. Among notable catalytic systems for the hydrogenation of bicarbonate, Beller and co-workers used both iron- and ruthenium-based homogeneous complexes (Table 3, entries 1 and 2).66,67 The groups of Milstein, Prakash, and Peng also independently showed the facile synthesis of formate salts from bicarbonate salts using ruthenium and iron complexes (entries 3−5).68−70 Similarly, Gonsalvi and co-workers reported iron-based complexes bearing linear rac-tetraphos-1 ligands for formate synthesis from bicarbonate salts (entry 6).71 However, it should be noted that the above hydrogenations were carried out in the context of a “carbon-neutral H2 battery” rather than integrative CO2 capture and conversion to value-added products, and the synthesized formate salts were converted G

DOI: 10.1021/acs.accounts.9b00324 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research Table 3. Selected Examples of Homogeneous Hydrogenation of Bicarbonate To Give Formate

entry 1 2 3 4 5 6

cat.

product c

Fe pre + L-3 Ru pre + L-4c C-9 C-1 C-10 C-11

HCOONa HCOONa HCOONa HCOONa HCOONa HCOONa

PH2 (bar)a

solvent methanol THF/water THF/water THF/water THF/water methanol

60 80 8.3 40 30 60

(1:5)d (1:10)d (1:2)d (1:1)d

T (t) [°C (h)] 100 70 80 70 130 80

(20) (2) (16) (2) (24) (24)

% yieldb

TONb

ref

77 96 32 94 69 12

7546 1108 320 1175 1370 1229

66 67 68 69 70 71

a

At room temperature. bEntries with the highest TONs in the study were selected. cThe catalyst was formed in situ. dv/v.

Catalyst Molecular Structure and Reaction Rate. As in the previously discussed CO2 to methanol system, the substituents on the P atoms markedly affected the activity of the catalyst. Moreover, the formate salt countercation also influenced the TOF. Similar to Scheme 1 for the CO2 to ammonium formate system, the rate-determining step in the bicarbonate to formate hydrogenation is the detachment of the formate ligand from the metal center (from C-XC in Figure 10). The rate of this step depends primarily on two factors: the substituents on the P atoms and the counteranion present in the system. With regard to the P substituents, as the R group becomes more electron donating (Ph < i-Pr < tBu), the electron density at the ruthenium center increases. As a result, the resting state, ruthenium formate complex C-XC, becomes destabilized, facilitating faster release of formate. This is reflected in the increase in the TOF as the R group was changed from Ph (649 h−1) to i-Pr (1024 h−1) to t-Bu (2698 h−1). Similarly, the counteranion present in the system also influences the TOF through formate stabilization. The heat of formation of HCOOK is higher than that of HCOONa. Consequently, an increase in TOF was observed in the formation of HCOOK (> 5420 h−1) compared with HCOONa (2698 h−1) using catalyst C-7 (R = t-Bu). Regeneration of the Hydroxide Base. As mentioned above, regeneration of the hydroxide base is usually challenging and requires high temperatures. However, we have demonstrated that NaOH can be regenerated under much milder conditions from the aqueous HCOONa (aqueous layer after hydrogenation; see Figure 9B) in a cation exchange membrane direct formate fuel cell (CEMDFFC) at low temperature (80 °C) while simultaneously generating electricity. Cation-conducting direct formate fuel cells have only been reported very recently as an alternative to anion exchange membrane direct formate fuel

back to bicarbonate through dehydrogenation to release the stored H2 (Figure 8).

Figure 8. Bicarbonate−formate interconversion in a “H2 battery”.73

Integrated CO2 Capture and Conversion

In 2018 we presented the first example of integrated CO2 capture by hydroxide bases and subsequent conversion to formate salts (Figure 9).74 Among various hydroxide bases, NaOH, KOH, and CsOH performed efficiently to produce HCOONa, HCOOK, and HCOOCs, respectively, in a tandem approach. In the first step, CO2 was captured in an aqueous solution of the hydroxide base to form the bicarbonate salt. The produced bicarbonate was then hydrogenated with a ruthenium or iron pincer complex to produce the formate salt (Figure 9A). The use of a biphasic 2MTHF/water solvent system for hydrogenation allowed the separation of the catalyst and formate salt (Figure 9B). This separation in turn enabled easy recycling of the pincer catalysts, which were active over multiple cycles without a significant decrease in catalytic activity. H

DOI: 10.1021/acs.accounts.9b00324 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 9. (A) Integrated CO2 capture by aqueous hydroxide and conversion to formate salt. (B) The biphasic reaction system (2-MTHF/H2O) enables convenient catalyst recycling. (C) NaOH is regenerated in a sodium-conducting direct formate fuel cell (DFFC). (D) Plausible anode and cathode reactions occurring in the DFFC. Panel (B) reproduced from ref 74. Copyright 2018 American Chemical Society.

Figure 10. Dependence of the rate of hydrogenation on the P substituents (in blue) and the formate counteranion (in red).

cells (AEMDFFCs).75 As the name suggests, a CEMDFFC uses a cation-conducting membrane (Nafion 211 or F-1850 in our case) that allows the movement of positive charges instead of a hydroxide-conducting membrane (Figure 9C).76−78 Similar to AEMDFFCs, the formate salt is oxidized in the anode compartment to form CO2 (bicarbonate salt at the pH of the anode solution), while O2 is reduced at the cathode. The passage of Na+ ions through the membrane from the anode to cathode produces a NaOH solution on the cathode side (Figure 9D). It is important to note here that the basic nature of the aqueous formate salts (pH ∼8.5) plays a crucial role in the process, allowing enough hydroxide anion to be present in the anode solution through hydrolysis for effective charge movement. The regenerated NaOH in the cathode contains minute amounts of formate salts and carbonate salts due to crossover across the membrane. Our group is working on minimizing the crossover of these contaminants into the NaOH solution as well as the efficient regeneration of other hydroxide bases. In view of the vast deposits of Na2CO3 in Earth’s crust, this method can potentially serve as a less

energy-intensive substitute to the chlor-alkali process for the production of NaOH while at the same time allowing decoupling from the concomitant formation of chlorine gas. Thus, CO2 can be captured in the form of bicarbonate salts and converted to formate through hydrogenation. The hydroxide bases can be regenerated in a direct formate fuel cell. However, capture of CO2 from dilute sources such as ambient air by aqueous hydroxide bases produces carbonate salts rather than bicarbonates. The more electron-rich nature of the inorganic carbonate salts compared with bicarbonates makes their hydrogenation challenging. Most of the abovementioned bicarbonate to formate systems do not show high catalytic efficiency for carbonate hydrogenation, and in this regard further research is clearly required.

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CO2 CAPTURE BY HYDROXIDE SOLUTIONS AND CONVERSION TO METHANOL The synthesis of methanol from CO2 capture products (bicarbonate or carbonate) through hydrogenation using I

DOI: 10.1021/acs.accounts.9b00324 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 11. Difficulty in the hydrogenation of metal formate salts.

homogeneous catalysis has not been reported to date. The main difficulty lies in the nonreducibility of the formate salts that are generated in the solution. Unlike the alkylammonium formate salts of primary and secondary amines, which can form formamides at elevated reaction temperatures, metal formate salts cannot proceed through a similar route (Figure 11). Why are formate salts not hydrogenated by the pincer metal complexes? It is likely because the formate anion acts as a very good ligand to the metal center, and thus, unlike in the case of formamides, the hydrogenation does not proceed through an outer-sphere mechanism. Our lab is currently working on devising systems to achieve formate/bicarbonate hydrogenation to methanol (while at the same time regenerating the hydroxide base).

Alain Goeppert: 0000-0001-8667-8530 G. K. Surya Prakash: 0000-0002-6350-8325 Notes

The authors declare no competing financial interest. Biographies Sayan Kar obtained his B.Sc. and M.Sc. in 2015 from IIT Kanpur with general and advanced proficiency medals under the supervision of Prof. Sandeep Verma. In 2019 he received his Ph.D. from the University of Southern California (USC) for his work on homogeneous CO2 recycling in the group Prof. G. K. Surya Prakash. Alain Goeppert obtained his Ph.D. in 2002 from the University of Strasbourg. He is currently a Research Scientist in the Prakash group at the Loker Hydrocarbon Research Institute, USC. His research focuses on methane and CO2 activation and catalytic transformation to value-added products, including methanol, methyl formate, formic acid, and dimethyl ether. He is also involved in the catalytic decomposition of formic acid to hydrogen and CO2 as well as the development of regenerative sorbents for CO2 separation and capture from various sources, including air. He is a coauthor, with G. A. Olah and G. K. S. Prakash, of the book Beyond Oil and Gas: The Methanol Economy.

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CONCLUSIONS The systems for integrated CO2 capture and conversion can bypass the desorption and compression steps that are conventionally required to generate pure pressurized CO2. In the integrated approach, after CO2 is captured, the captured products are directly utilized to obtain value-added products like formate salts, methanol, urea, or oxazolidinones. Among them, both formate and methanol are valuable energy vectors and hydrogen carriers. In this Account, we have discussed the capture of CO2 by aqueous amine and hydroxide solutions and the in situ hydrogenation of the capture products (carbamate and/or bicarbonate) by homogeneous metal complexes to produce formate salts and methanol. Importantly, the amine or hydroxide solutions can also be regenerated for reuse in subsequent capture/ hydrogenation cycles. The developments in these nascent fields were divided into four subsections and discussed. Among them, integrated capture of CO2 by hydroxide and conversion to methanol has not been reported to date. On the other hand, CO2 capture by amines and conversion to methanol through an amine-assisted process looks very promising. Given the development of second-generation pincer complexes with improved catalytic turnovers, this process can potentially supplant the traditional synthesis of methanol from CO2 over heterogeneous catalysts. This is of particular importance for the continued development of the methanol economy that has been pioneered at our Institute. Mimicking nature’s carbon cycle, the recycling of CO2 directly from the atmosphere to fuels and materials using any renewable energy available will lead to a truly sustainable future for humankind.

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G. K. Surya Prakash received his B.Sc. (Honors) in 1972 from Bangalore University, his M.Sc. in 1974 from IIT Madras, and his Ph.D. in 1978 from USC. He joined the USC faculty in 1981 and is currently a Professor and Director of the Loker Hydrocarbon Research Institute, holding the Olah Nobel Laureate Chair in Hydrocarbon Chemistry. He is also the chair of the USC Chemistry Department. His research interests include fluorination and synthetic methods, mechanistic studies, superacid chemistry, electrochemistry, and the methanol economy. He is a very prolific author and has received several ACS national awards. He is a coproponent of the methanol economy concept with the late Prof. Olah, for which he and Prof. Olah shared the 2013 Eric and Sheila Samson Prime Minister’s Prize for Alternative Fuels for Transportation from the State of Israel.

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ACKNOWLEDGMENTS Support of our work by the Loker Hydrocarbon Research Institute, USC is gratefully acknowledged. REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sayan Kar: 0000-0002-6986-5796 J

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DOI: 10.1021/acs.accounts.9b00324 Acc. Chem. Res. XXXX, XXX, XXX−XXX