Communication Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy Sayan Kar, Raktim Sen, Alain Goeppert, 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 S Supporting Information *
condition in the presence of dimethylamine (Figure 1).7 The presence of an amine provides the opportunity to capture and
ABSTRACT: Herein we report an efficient and recyclable system for tandem CO2 capture and hydrogenation to methanol. After capture in an aqueous amine solution, CO2 is hydrogenated in high yield to CH3OH (>90%) in a biphasic 2-MTHF/water system, which also allows for easy separation and recycling of the amine and catalyst for multiple reaction cycles. Between cycles, the produced methanol can be conveniently removed in vacuo. Employing this strategy, catalyst Ru-MACHO-BH and polyamine PEHA were recycled three times with 87% of the methanol producibility of the first cycle retained, along with 95% of catalyst activity after four cycles. CO2 from dilute sources such as air can also be converted to CH3OH using this route. We postulate that the CO2 capture and hydrogenation to methanol system presented here could be an important step toward the implementation of the carbon neutral methanol economy concept.
Figure 1. Amine assisted CO2 hydrogenation to CH3OH.
hydrogenate CO2 in tandem, as was demonstrated by our group.8 Since Sanford et al.’s initial report, multiple studies have been published using various metal complexes for amine assisted hydrogenation of CO2 to CH3OH.8,9 However, the integration of CO2 capture with subsequent hydrogenation to CH3OH with easy recycling of the active elements had not been explored. Most of the reported metal complexes catalyzing CO2 hydrogenation to CH3OH are soluble in organic solvents, whereas the capturing amines are soluble in water. Aqueous solutions of amines have long been utilized for scrubbing CO2 from industrial gas streams.10 Water is a desirable solvent due to its benign nature and ability to enhance the amines’ CO2 absorption capacity. Thus, a biphasic system was envisioned, where after the hydrogenation step, the amine and catalyst can be easily separated and recycled from the aqueous and organic layer, respectively (Figure 2). The formed CH3OH can be extracted through distillation. Similar biphasic systems were
T
he rise of atmospheric CO2 concentration and associated global warming have prompted researchers to develop strategies for capturing CO2 from both emission point sources and diffuse sources like ambient air.1 Whereas the captured CO2 can be sequestered underground in geological formations, a more sustainable approach is to utilize the CO2 to produce fuels and other value-added products.2 CH3OH in particular can be used as a fuel, fuel additive or precursor in organic synthesis.3 The utilization of CO2 to produce CH3OH through hydrogenation, followed by the use of CH3OH as fuel results in an overall carbon neutral cycle, and represents an area of interest in the context of carbon footprint reduction.4 Development of integrated CO2 capture and utilization (CCU) systems, wherein the captured CO2 can be directly converted to value-added products (in this case CH3OH), is an area of enormous interest as it can bypass the otherwise intermediary and energy intensive desorption and compression steps to produce pure CO2. For practical implementation, recycling of the catalyst and capture material is essential to keep the entire process cost-effective. Traditional catalysts for CO2 hydrogenation to CH3OH are heterogeneous and require high temperatures and pressures.5 Over the past decade, however, significant advances were made in both indirect and direct onepot homogeneous catalytic CO2 to CH3OH synthesis under much milder conditions.6 In 2015, Sanford et al. demonstrated a direct CO2 hydrogenation system to CH3OH under basic © XXXX American Chemical Society
Figure 2. Schematic representation of biphasic CO2 to methanol system with recyclable catalyst and amine. Received: November 17, 2017
A
DOI: 10.1021/jacs.7b12183 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
Following the capture, the formed aqueous solutions were hydrogenated at 145 °C in the presence of homogeneous catalysts and 70 bar of H2, after addition of 5 mL 2-MTHF (Table 2). When the catalyst Ru-MACHO-BH (C-1) (10 μmol) was used along with PEHA as the capture material, 47% CH3OH yield was observed (5.2 mmol) after 72 h (entry 1). No concomitant CO/CH4 formation was observed through GC analysis of the reaction gas mixture. 1H and 13C NMR revealed that 14% of formed CH3OH was present in the upper organic layer, whereas the remaining CH3OH along with PEHA, formamide and formate intermediates, was in the bottom aqueous layer (Figure S6 and S7). The catalyst remained in the organic layer as observed by 31P NMR (Figure S5), indicating the possibility of easy catalyst separation from the biphasic mixture. Increasing the catalyst loading to 20 μmol increased the methanol yield to 79% (8.7 mmol) (entry 2). Next, various amine solutions after CO2 capture (from Table 1) were hydrogenated to identify the most promising amine for an integrated CO2 capture/hydrogenation system. Switching from PEHA to BPEI800 or BPEI25k decreased both the amounts of CH3OH formed (4.5 and 5.2 mmol, respectively) and the hydrogenation yield (45% and 50%) (entry 3 and 4). For LPEI2.5k, LPEI100k, and PAA10k, CH3OH yields decreased drastically to 0.9, 0.9, and 0.1 mmol, respectively (entry 5− 7). Surprisingly, with MEA, no CH3OH was formed (entry 8), but increased amounts of formamide and formate intermediates were observed. We surmise that in the presence of primary amines, such as PAA10k and MEA, the second hydrogenation step of formamide to methanol becomes more challenging. Indeed, when DEEDA, a secondary analogue of MEA, was used, methanol was obtained in 46% yield after 72 h (3.3 mmol; entry 9). Thus, among various amines, PEHA was the most efficient for the overall CO2 capture and conversion to CH3OH. The low vapor pressure and easy availability of inexpensive PEHA (Table 1) make it promising for a large scale CCU process. Next, known hydrogenation catalysts were screened to investigate their efficacy. Ru-MACHO (C-2), expectedly, was almost equally effective to Ru-MACHO-BH, in the presence of an additional base K3PO4 (entry 10). The P-substituent in the PNP ligand heavily influenced the CH3OH yield. With RuHClPNPiPr(CO) (C-3), a meager 5% methanol yield was observed (entry 11). Complex MnBrPNPiPr(CO)2 (C-4), recently reported by us to catalyze sequential CO2 hydrogenation to methanol, was only capable of producing CH3OH in 5% yield (0.5 mmol; entry 12). No methanol formed with FeHBrPNPiPr(CO) (C-5) (entry 13). Accumulation of formamide and formate intermediates in the case of C-3 to C-5 suggests lower activities of these catalysts for the effective hydrogenation of formamides and formates to CH3OH under the present conditions. The most suitable organic solvent for the biphasic hydrogenation system was subsequently explored. A slight decrease in methanol formation was observed when switching from 2MTHF to cyclopentyl methyl ether (CPME) or p-xylene (52% and 57%, respectively) (entry 14 and 15). The reason behind the decrease in methanol yield with more hydrophobic solvents is not entirely clear, but is most probably a combination of different catalyst/H2/CO2/CH3OH solubility in different organic solvents (see SI). Also, the higher hydrophobicity of these solvents compared to 2-MTHF resulted in an accumulation of the produced methanol exclusively in the aqueous layer (Figure S13). However, the lower solubility of C-
demonstrated recently by us and Leitner et al. for integrated CO2 capture and conversion to formate salts.11 For the CO2 capture, amines with low vapor pressures are desirable to avoid atmospheric amine contamination. High boiling polyamines were therefore selected for capture, along with two ethanolamines (Table 1). Among various polyamines, Table 1. CO2 Capture by Aqueous Amine Solutionsa
Entry
Amine
$/kgb
CO2 (mmol)/gc
CO2/Nd
1 2 3 4e 5f 6f 7 8
PEHA BPEI800 BPEI25k PAA10k LPEI2.5k LPEI100k MEA DEEDA
105 (S) 352 (S) 320 (S) 7533 (P) 56,000 (P) 24,500 (P) 35 (S) 820 (T)
11.0 10.2 10.4 6.2 5.5 6.1 11.7 7.2
0.43 0.46 0.47 0.36 0.25 0.28 0.71 0.53
a
Capture conditions: Amine (1g), water (3 mL), stirring (800 rpm), rt. Aqueous amine solutions stirred in CO2 atmosphere at a constant pressure of 1 psi. Captured CO2 amounts calculated through gravimetric analysis. Calculations error ± 5%. bPrices from SigmaAldrich (S), Polysciences (P) or TCI America (T), as of Nov 14, 2017. c CO2 captured per gram of amine. dmols of CO2 captured per mol of nitrogen. eCommercial 15 wt % PAA10k aqueous solution used directly. f 10 mL water, capture at 70 °C
pentaethylenehexamine (PEHA), and branched polyethylenimines (BPEI) were found efficient for CO2 capture. The aqueous PEHA solution captured 11.0 mmol of CO2 per g of PEHA after 4 h, corresponding to 0.43 mol of CO2 captured per mol of amino group (CO2/N), (Table 1, entry 1). The 13C NMR of the CO2 loaded aqueous PEHA solution revealed the presence of carbamate and carbonate/bicarbonate (Figure S2). Similarly, BPEI800 and BPEI25k captured 0.46 and 0.47 CO2/N, respectively (10.2 and 10.4 mmol of CO2/g, respectively) (entry 2−3). CO2 capture by aqueous PAA10k solution was slower, as after 4 h only 6.2 mmol/g of CO2 was captured, corresponding to 0.36 CO2/N (entry 4). Linear polyethylenimines (LPEI2.5k, LPEI100k) displayed limited solubility in water at room temperature, making them less convenient for aqueous CO2 capture. With more water (10 mL), and a higher temperature (70 °C), LPEI2.5k and LPEI100k captured 5.5 and 6.1 mmol of CO2/g, respectively. Monoethanolamine (MEA), which has long been used industrially for scrubbing CO2 and H2S from flue gases, was the most effective for CO2 capture both by mass and efficiency of amine utilization (11.7 mmol/g; 0.71 CO2/N) (entry 7). Similarly, 7.2 mmol/g CO2 (0.53 CO2/N) was captured by diethanolethylenediamine (DEEDA) (entry 8). B
DOI: 10.1021/jacs.7b12183 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society Table 2. Tandem Homogeneous Hydrogenation of CO2 Captured by Aqueous Amine Solutionsa
Entry
Amine
Captured CO2 (mmol)
Catalyst (μmol)
Formate (%)b
Formamide (%)b
MeOH (mmol)b
Yield (%)b
PMeOHc
TON
1 2 3 4 5 6 7 8 9 10d 11d 12d 13d 14e 15f 16g 17h
PEHA PEHA BPEI800 BPEI25k LPEI2.5k LPEI100k PAA10k MEA DEEDA PEHA PEHA PEHA PEHA PEHA PEHA PEHA PEHA
11.0 11.0 10.2 10.4 5.5 6.1 6.2 11.7 7.2 11.0 11.0 11.0 11.0 11.0 11.0 11.0 5.4
C-1(10) C-1(20) C-1(20) C-1(20) C-1(20) C-1(20) C-1(20) C-1(20) C-1(20) C-2(20) C-3(20) C-4(20) C-5(20) C-1(20) C-1(20) C-1(50) C-1(50)
11 5 16 10 30 21 32 26 15 6 15 18 20 12 9 3 5
10 2 13 7 15 44 38 17 6 13 20 19 18 11 6 0 0
5.2 8.7 4.5 5.2 0.9 0.9 0.1 0 3.3 7.4 0.5 0.5 0.0 5.7 6.3 10.4 4.8
47 79 45 50 16 15 2 0 46 67 5 5 0 52 57 95 89
0.16 0.17 0.18 0.13 0.11 0.11 0 nd 0.14 0.15 0 0 nd 0 0 0.17 0.11
520 435 225 260 45 45 5 0 165 370 25 25 0 285 315 208 96
a
Reaction conditions: Solutions from Table 1 (as specified) were hydrogenated after adding organic solvent and catalyst. 2-MTHF (5 mL), H2 (70 bar), 145 °C, 72 h. bYields based on 1H NMR with 1,3,5-trimethoxybenzene (TMB) and imidazole (Im) as internal standards for organic and aqueous layer, respectively. cPMeOH = methanol in organic layer/methanol in aqueous layer. dK3PO4 (1 mmol) added. eCPME used as organic solvent. fP-xylene used as organic solvent. gH2 (80 bar). hCO2 captured from simulated air (CO2 concentration: 408 ppm) with 0.79 g PEHA. Yield calculations error ± 5%. TON = mols of methanol formed per mol of catalyst. nd = nondefinable
1 in these solvents at room temperature caused some of the catalyst to precipitate out from the solution during workup, making its complete recycling challenging. Hence, 2-MTHF was identified as the most convenient solvent for repeated capture and utilization studies. Using 2-MTHF, a CH3OH yield as high as 95% was obtained with a higher C-1 loading of 50 μmol (entry 16). Finally, CO2 from air was also captured and hydrogenated to CH3OH in high yields following this protocol (entry 17). With the optimized selection of amine (PEHA), catalyst (C1), and organic solvent (2-MTHF), two recycling studies were conducted. In a first study, only the catalyst was recovered from the organic layer and reused for successive hydrogenation cycles (see SI). After four cycles, 95% of C-1’s catalytic efficiency of the initial cycle was retained, with a total of 40.5 mmol of CH3OH formed, demonstrating the high recyclability of the catalyst, enabled by this biphasic system (Figure 3A). In a second study (Figure 3B), both the catalyst and capturing amine were recovered and reused. 89% of the CO2 capture efficiency of the amine was retained in the third cycle, along with 87% of methanol productivity. The slight loss in capture is most probably due to the presence of formate species after the
Figure 3. Methanol formation with catalyst recycling (A) and catalyst and amine recycling (B). Reaction conditions: After capture with 1 g PEHA in 3 mL water, H2 (80 bar), C-1 (50 μmol), 2-MTHF (10 mL), 145 °C, 72 h. Methanol yields calculated from 1H NMR with Ph−CH3 and Im (A)/ t-BuOH (B) as internal standards for organic and aqueous layer, respectively. Error in yield calculations ±5%.
reaction and the loss of amine while transferring PEHA solution between glassware. In conclusion, a tandem system for CO2 capture (even from air) in aqueous amine solution and subsequent hydrogenation to methanol is described where the catalyst and amine can be recycled multiple times without significant loss in effectiveness. C
DOI: 10.1021/jacs.7b12183 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
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Among the catalysts tested, a well-defined and commercially available complex, Ru-MACHO-BH (C-1) was found most effective, whereas, among various amines, high boiling polyamine, PEHA, provided the best CH3OH yields. Our next focus in the context of integrated CO2 capture and hydrogenation will be toward developing a continuous CO2 to CH3OH flow system.
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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.7b12183. General information and experimental details (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. REFERENCES
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DOI: 10.1021/jacs.7b12183 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
