A Carbon-Neutral CO2 Capture, Conversion, and Utilization Cycle

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A Carbon-Neutral CO2 Capture, Conversion, and Utilization Cycle with Low-Temperature Regeneration of Sodium Hydroxide Sayan Kar, Alain Goeppert, Vicente Galvan, Ryan Chowdhury, Justin Olah, and G. K. Surya Prakash* Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, University Park, Los Angeles, California 90089-1661, United States

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S Supporting Information *

demonstrated the tandem capture and conversion of CO2 to ammonium formate salts and CH3OH with amines as capturing agents.5g,6 However, given the low CO2 concentration in air, inorganic hydroxide bases provide a more effective system for integrated CCU. Advantages of employing aqueous hydroxide solutions for CCU compared to amine systems include (i) easy availability of the hydroxides via electrolysis of aqueous salts, (ii) higher carbon capture efficiency from ambient air, (iii) negligible vapor pressure thus avoiding atmospheric contamination, (iv) low toxicity, and (v) regenerability of the hydroxide bases. Furthermore, unlike ammonium formate salts, metal formate salts can be utilized directly in fuel cells and achieve a carbon-neutral cycle. Herein, we describe such a carbon-neutral, amine-free integrated system utilizing the captured CO2 as C1 source to produce formate salts through hydrogenation. The formate salt is subsequently used directly without purification in a direct formate fuel cell (DFFC) to produce electricity and regenerate sodium hydroxide (Figure 1). While the reduction of

ABSTRACT: A highly efficient recyclable system for capture and subsequent conversion of CO2 to formate salts is reported that utilizes aqueous inorganic hydroxide solutions for CO2 capture along with homogeneous pincer catalysts for hydrogenation. The produced aqueous solutions of formate salts are directly utilized, without any purification, in a direct formate fuel cell to produce electricity and regenerate the hydroxide base, achieving an overall carbon-neutral cycle. The catalysts and organic solvent are recycled by employing a biphasic solvent system (2-MTHF/H2O) with no significant decrease in turnover frequency (TOF) over five cycles. Among different hydroxides, NaOH and KOH performed best in tandem CO2 capture and conversion due to their rapid rate of capture, high formate conversion yield, and high catalytic TOF to their corresponding formate salts. Among various catalysts, Ru- and Fe-based PNP complexes were the most active for hydrogenation. The extremely low vapor pressure, nontoxic nature, easy regenerability, and high reactivity of NaOH/KOH toward CO2 make them ideal for scrubbing CO2 even from lowconcentration sourcessuch as ambient airand converting it to value-added products.

T

he recent increase in atmospheric CO2 concentration has prompted scientists to develop processes for capturing CO2 from point and diffused sources. Various CO2 capture materials have been reported, including aqueous hydroxides, amine or amino alcohol solutions, metal organic frameworks, and silica amine hybrid adsorbents.1 Metal hydroxides have long been known to capture atmospheric CO2 and were extensively studied with different capture methods such as sprays, towers, pools, etc.2,3 The regeneration of hydroxides is challenging and generally achieved through a series of steps (causticization, calcination, slaking) at high temperature (750 °C). The CO2 released in the process can be sequestered underground. Alternatively, the captured CO2 can be converted into valueadded products and fuels such as formic acid, formate salts, and methanol.4 With recent progress in low-temperature formic acid and CH3OH synthesis through homogeneous CO2 hydrogenation, carbon capture and utilization (CCU) can be a powerful tool to utilize atmospheric CO2 as a chemical feedstock.5 The integration of carbon capture with utilization is an area of immense importance. Recently, our group and others have © XXXX American Chemical Society

Figure 1. Previously reported integrated CCU systems compared to the present study.

bicarbonate salts has been reported previously,7 its integration with CO2 capture and the recycling of the capturing hydroxide base are achieved for the first time in this study in the context of integrated CCU systems. Among different metal hydroxide bases screened for CO2 capture, NaOH, KOH, and CsOH were found most effective. Received: August 29, 2018

A

DOI: 10.1021/jacs.8b09325 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

Table 2. Hydrogenation of the Captured CO2a

Under the capture conditions (Table 1), an aqueous NaOH solution captured 13.8 mmol of CO2 after 3 h, corresponding to Table 1. CO2 Capture by Aqueous Hydroxide Solutionsa entry

base

CO2 captured (mmol)b

CO2/ OHc

CO2/ gd

time (min)

1 2 3 4 5e

NaOH KOH LiOH CsOH Ca(OH)2

13.8 13.8 6.9 13.7 9.0

∼1 ∼1 ∼0.5 ∼1 ∼0.65

25.0 17.8 20.8 6.7 17.6

167 123 161 109 186

a

Capture conditions: base (13.8 mmol), water (10 mL), stirring (800 rpm), rt. See SI for details. bCalculated through gravimetric analysis. Calculations error ±5%. cMoles of CO2 captured per mole of hydroxide. dMillimoles of CO2 captured per gram of base. e6.9 mmol of Ca(OH)2 was used.

1 mole of CO2 per mole of OH− (Table 1, entry 1). The formation of NaHCO3 was observed through 13C NMR. In the KOH solution, CO2 capture was completed within 2 h, and KHCO3 formation was observed (entry 2). LiOH captured only 0.5 mol of CO2 per hydroxide group, indicating the formation of Li2CO3, which precipitated from the aqueous solution (Table 1, entry 3). CsOH captured 1 CO2/OH−, similar to NaOH and KOH, with the formation of CsHCO3. Finally, with Ca(OH)2, mainly the formation of CaCO3 was observed after 3 h. Also, due to the low solubility of Ca(OH)2 in water, its initial CO2 capture rate was significantly lower.8 Following CO2 capture, in situ hydrogenation of the CO2loaded aqueous solutions was performed at 80 °C under 50 bar of H2 with different catalysts to obtain the formate salts (Table 2). The reactions were conducted in a 2-MTHF/H2O biphasic system (5/10 mL), which allows for efficient recovery of the catalyst. When the CO2-loaded NaOH solution (containing NaHCO3) was hydrogenated under these conditions with 5 μmol of Ru-Macho-BH (C-1), 93% yield in HCOONa (turnover number (TON) = 2560) was observed after 12 h (Table 2, entry 1). The hydrogenation progress was followed by monitoring the pressure decrease inside the reactor, and a reaction completion time of 132 min was observed (turnover frequency (TOF) = 1164 h−1). Analysis of the reaction mixture showed that the aqueous layer contained HCOONa and the organic layer contained the catalyst, thus allowing for easy recycling. A lower TOF was obtained with catalyst RuHClPNPPh(CO) (C-2, Ru-Macho; Table 2, entry 2). The TOF improved slightly with complexes RuHClPNPiPr(CO) (C-3) and RuHClPNP Cy (CO) (C-4) to 1024 and 811 h −1 , respectively. However, a significant increase in TOF was observed (2698 h−1) with RuHClPNPtBu(CO) (C-5), and the reaction was completed within 62 min (Table 2, entry 5). Thus, the substitution on the P atoms in the pincer ligand markedly affects the hydrogenation rate. Among base-metal catalysts, manganese-based catalysts, MnBrPNPiPr(CO)2 (C-6) and MnBrPNPCy(CO)2 (C-7), were not very effective, with low TOFs of 31 and 33 h−1 observed, respectively (Table 2, entries 6 and 7). On the other hand, the iron-pincer complex, FeHBrPNPiPr(CO) (C-8), displayed a high hydrogenation rate with a TOF of 428 h−1 (Table 2, entry 8). Thus, C-8 could be a cost-effective catalyst for a large-scale implementation of this process. Finally, complex C-9, the N-Me analogue of C-2, was equally active in the hydrogenation (Table 2, entry 9),

entry

M(OH)n

CO2 (mmol)

cat.

time (min)

yield (%)b

TONc

TOF (h−1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14d

NaOH NaOH NaOH NaOH NaOH NaOH NaOH NaOH NaOH KOH CsOH LiOH Ca(OH)2 NaOH

13.8 13.8 13.9 13.8 13.7 13.7 13.8 13.8 13.6 13.8 13.7 6.9 9.0 13.7

C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-5 C-5 C-5 C-5 C-5

132 256 159 188 62 2419 4142 379 258 5420 2548 0 3 1595

a

Reaction conditions: solutions from Table 1 (as specified) were hydrogenated after addition of organic solvent and catalyst. Catalyst (5 μmol), 2-MTHF (5 mL), H2 (50 bar), 80 °C, 12 h. bYields determined by 1H NMR with imidazole as internal standard. cTON = moles of formate produced per mole of catalyst. dCPME (1 mL) used as co-solvent. Yield calculations error ±5%.

indicating that the N−H moiety may not be actively involved in the reaction mechanism. Next, CO2-loaded solutions of different bases were hydrogenated with C-5 to identify the most suitable base. Compared to HCOONa, the formation of HCOOK was faster and completed within 30 min (TOF > 5420 h−1; Table 2, entry 10). The improved rate can be explained by the higher standard enthalpy of formation of HCOOK (−679.7 kJ/mol) compared to HCOONa (−666.5 kJ/mol), which facilitates the release of the formate group from the catalytic resting state (vide infra). With a CO2-loaded solution of CsOH, the hydrogenation also proceeded rapidly (∼1 h), and 198% HCOOCs yield was observed, giving a TOF of 2548 h−1 (Table 2, entry 11). Surprisingly, CO2-loaded LiOH and Ca(OH)2 solutions were not hydrogenated efficiently under our reaction conditions, with only traces of formate salt observed in either case (Table 2, entries 12 and 13). This is likely due to the electron-rich nature of the carbonates, as compared to the bicarbonate salts, and similar findings were reported recently by Huang et al.5m Besides 2-MTHF, cyclopentyl methyl ether (CPME) was also a good cosolvent for the biphasic system, with 10% CPME (v/v) being sufficient for effective hydrogenation (Table 2, entry 14). From B

DOI: 10.1021/jacs.8b09325 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society the hydrogenation yields and TOFs observed, catalyst C-5 is clearly the most suitable for hydrogenation, with NaOH and KOH being the most judicious choice of bases. Subsequently, the recycling of the active elements involved was investigated. As mentioned earlier, easy recycling of the catalyst and organic solvent can be achieved by utilizing a biphasic solvent system (Figure 2). Following this recycling

Figure 2. Recycling scheme for the catalyst, 2-MTHF, and hydroxide base.

Figure 4. (A) Polarization curve of the fuel cell used in this study. (B) voltage vs time at a constant current of 10 mA at 80 °C. See SI for details.

protocol, catalyst C-5 was recycled for five cycles. A catalytic activity similar to that in the first cycle was retained even in the fifth cycle, with over 90% HCOONa yield in each cycle (Figure 3). Thus, catalyst C-5 is an efficient and robust catalyst for this hydrogenation reaction.

crossover, which can be substantially mitigated by using an F1850 sodium conducting membrane (see Supporting Information (SI)). The obtained cathode solution containing NaOH was able to capture CO2 again (Figure S16). After 13 h, a total of 2.1 mmol of NaOH was collected, as quantified by the amount of captured CO2, corresponding to a 43% current efficiency in NaOH regeneration. Importantly, even though the previous report by He et al.9d predicted the theoretical formation of NaOH based on the current and potential observed in the fuel cell, its presence in the cathodic feed was not directly detected. Thus, this is the first study reporting the regeneration of NaOH as verified though pH analysis and CO2 capture experiment. Consequently, it is demonstrated that HCOONa can be recycled in a DFFC to regenerate NaOH, unlike other available studies where HCOONa was converted, instead, to NaHCO3 through dehydrogenation.7c,d,f,g Alternatively, HCOONa can be recycled to regenerate NaOH either by direct electrodialysis10 or through acidification of HCOONa with HCl to form HCOOH, followed by the production of NaOH and HCl from the resultant NaCl through the chlor-alkali process.11 The efficiency of the system for direct air capture and conversion of CO2 to formate was also tested. CO2 was captured by bubbling ambient air through a 1 N NaOH (15 mL) solution using a pump, and after 64 h, 3.0 mmol of CO2 had been captured in the form of carbonate salt (Figure S10). After hydrogenation of the resulting solution using C-5, 2.2 mmol of HCOONa was observed, giving a 73% formate yield.12 The catalytic resting state was also investigated with catalyst C-2, and the presence of the ruthenium formate species [RuH(OOCH)PNPPh(CO)] was detected, indicating its role as a catalytic resting state under the reaction conditions (see SI). In conclusion, an amine-free system was developed that captures CO2 from concentrated or ultradilute sources, including air, and is easily integrated with a subsequent hydrogenation step to synthesize formate salts. Easy availability, high CO2 capture efficiency, lower vapor pressure, and regenerability make the hydroxide base systems promising

Figure 3. TOF in consecutive cycles of catalyst recycling.

Regeneration of the hydroxide solution was explored next. In this regard, DFFCs present the most convenient way to harness the chemical energy of the formed HCOONa and regenerate the hydroxide base.9 Recently, He et al. described a sodium ion conducting DFFC that produced electricity with simultaneous theoretical regeneration of NaOH.9d We improved upon this previously reported fuel cell by using Pt/C as the cathode catalyst for O2 reduction instead of Pd/C to obtain a 1.5-fold increase in peak power density at 80 °C (Figure S14). The aqueous layer obtained from the hydrogenation containing HCOONa (∼1 M) was directly fed to the DFFC anode (Pd/C) without purification. As shown in Figure 4B, at 10 mA constant current, a relatively steady voltage of about 0.7 V was observed for more than 13 h (Figure 4A represents the polarization curve). The pH of the collected cathodic solution after 2 h showed that it was highly caustic (pH ∼12.6) compared to the parent formate solution (pH ∼8.7), indicating the formation of NaOH, while at the anode, formation of NaHCO3 was observed by 13C NMR (Figure S15). Minor amounts of HCOONa and Na2CO3 were also detected in the cathode solution due to C

DOI: 10.1021/jacs.8b09325 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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alternatives to the formerly reported amine-based systems. With a suitable choice of base (KOH) and catalyst (C-5), a TOF as high as 5420 h−1 was obtained. The biphasic system also enabled effective recycling of the precious metal catalyst and the hydroxide base as well as convenient utilization of the produced formate salts in a DFFC. Our next efforts in this context will be directed toward developing amine-free integrated CO2 capture and conversion to methanol systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b09325. General information and experimental details, including Figures S1−S17 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

G. K. Surya Prakash: 0000-0002-6350-8325 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support of our work by the Loker Hydrocarbon Research Institute, USC, is gratefully acknowledged. S.K. thanks the Carolyn C. Franklin and Morris S. Smith Foundations for providing endowed Graduate Fellowships.

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REFERENCES

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DOI: 10.1021/jacs.8b09325 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX