Communication Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
pubs.acs.org/JACS
Catalytic Homogeneous Hydrogenation of CO to Methanol via Formamide Sayan Kar, 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
Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on August 6, 2019 at 01:59:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
Dombek in 1980 independently reported homogeneous CO hydrogenation to methanol catalyzed by ruthenium carbonyl or acetate complexes, although high temperatures (230−275 °C) and pressures (320−1300 atm) were employed.11,12 Later, Mahajan and co-workers and others demonstrated sequential CO hydrogenation to methanol through methyl formate at lower temperatures (90−140 °C) (Figure 1A), but the systems
ABSTRACT: A novel amine-assisted route for low temperature homogeneous hydrogenation of CO to methanol is described. The reaction proceeds through the formation of formamide intermediates. The first amine carbonylation part is catalyzed by K3PO4. Subsequently, the formamides are hydrogenated in situ to methanol in the presence of a commercially available ruthenium pincer complex as a catalyst. Under optimized reaction conditions, CO (up to 10 bar) was directly converted to methanol in high yield and selectivity in the presence of H2 (70 bar) and diethylenetriamine. A maximum TON of 539 was achieved using the catalyst Ru-Macho-BH. The high yield, selectivity, and TONs obtained for methanol production at low reaction temperature (145 °C) could make this process an attractive alternative over the traditional high temperature heterogeneous catalysis.
M
ethanol is a versatile chemical that has many useful applications including as a fuel, fuel additive, a feedstock for the production of small chain hydrocarbons, a C1 precursor in organic synthesis, an energy storage molecule and a convenient liquid hydrogen carrier.1−5 Currently, methanol is produced industrially in huge quantities (100 billion liters annually) mainly from coal, natural gas, and other hydrocarbons.6 Typically, these hydrocarbons are first converted to synthesis gas (CO + 2H2), which is further converted to methanol over heterogeneous (Cu/ZnO/Al2O3) catalysts at high temperatures (250 °C) and pressures (>50 bar) (Scheme 1).7−10 The development of homogeneous
Figure 1. Previous CO and CO2 to methanol studies as compared to present amine-assisted CO hydrogenation to methanol process.
suffered from excessive use of moisture-sensitive methoxide base and the frequent use or in situ generation of toxic nickel carbonyl complexes.13−21 Recently, Jens and co-workers have reported copper catalyzed systems for CO to methanol (through methyl formate) that again required a highly caustic medium and afforded low TONs.22,23 Here, we have developed a novel chemical route for the homogeneous low-temperature hydrogenation of CO to methanol that proceeds through the formation of formamide (Figure 1C). This amine-assisted CO hydrogenation to methanol consists of two parts(i) in the first anchoring step, CO is anchored onto the amine as a formamide, (ii) subsequently the formamide is hydrogenated in situ to methanol in the hydrogenation step. The impetus for this came from our recent works on amine-assisted CO2 hydrogenation to methanol, where CO2 is sequentially reduced employing a similar route (Figure 1B).24−27 The potential advantages of an amine-assisted CO to methanol process include(i) lower temperature required, (ii) the absence of corrosive, difficult to handle alkali-metal alkoxide bases, (iii)
Scheme 1
catalytic systems able to catalyze CO hydrogenation at lower temperature would be a significant progress toward increasing the energy efficiency of the CO to methanol process. Furthermore, CO hydrogenation to methanol being an exothermic reversible reaction (ΔH = −90.6 kJ/mol), a lower reaction temperature would increase CO conversion which in current processes is thermodynamically limited to around 30%. One of the key challenges in the hydrogenation of CO using organometallic complexes as catalysts is the high binding affinity of CO with different metal centers. Bradley in 1979 and © XXXX American Chemical Society
Received: June 20, 2019
A
DOI: 10.1021/jacs.9b06586 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
Table 2. Hydrogenation of 1a to 1 and MeOHa
use of inexpensive K3PO4 as carbonylation catalyst, (iv) high TONs achieved, (v) potential for improved turnovers by rational ligand design, (vi) inexpensive high-boiling polyamines can be used, making the whole process cost-effective, and (vii) high catalytic longevity of the system (tested for 1 week). In the initial stages, we explored the anchoring amine carbonylation step. Several compounds, including ruthenium carbonyl, alkoxide bases, and ionic liquids, were reported in the literature as being active catalysts for amine carbonylation.28−31 In 2014, Kim and co-workers reported the synthesis of formamides through amine carbonylation using K3PO4 as the catalyst.32 For our intended amine-assisted CO to methanol process, K3PO4 is an ideal carbonylation catalyst, as the typically employed formamide hydrogenation catalysts (for the second step) also display facilitated catalytic activities in the presence of K3PO4. Thus, we started our investigation with the K3PO4 catalyst and piperidine as the model substrate, as it was one of the most active substrates for the carbonylation (Table 1).32 In the absence of any solvent, the carbonylation of
cat.
solvent
yield (%)b
1 2 3 4 5
K3PO4 K3PO4 K3PO4 K3PO4 K3PO4
− Tol MeOH EtOH Tol/EtOH
10 13 96 95 74
solvent
conversion (%)b
MeOH (%)b,c
1 (%)b
1 2 3 4 5
− Tol MeOH EtOH Tol/EtOH (3/1)
99 97 9 60 82
88 84 e 46 73
92 91 10 35d 68d
a
Reaction conditions: 1-formylpiperidine (10 mmol), H2 (60 bar), C1 (0.25 mol %), K3PO4 (10 mol %), solvent (5 mL), 140 °C, 24 h. b Conversion and yields calculated from 1H NMR using TMB as an internal standard. cThe slightly lower yields of methanol are presumably due to its volatility. dYield is lower than conversion due to N-ethylation of the product under reaction conditions. eYield could not be measured. Conversion and yield calculations error: ±5%.
Table 1. N-Carbonylation of Piperidinea
entry
entry
solution due to a side reaction between the product piperidine and solvent ethanol through hydrogen autotransfer (Figure S3).34−36 The conversion and methanol yield increased further to 82% and 73%, respectively, when a 3/1 v/v mixture of toluene and ethanol was used as solvent (entry 5). From Table 1 and Table 2, it is evident that the first carbonylation step is favored in polar protic solvents (MeOH, EtOH), whereas the second hydrogenation step is favored in a relatively nonpolar solvent like toluene. Thus, for the sequential CO to methanol reaction via carbonylation and hydrogenation, we decided to carry out the first carbonylation step in ethanol as the solvent. Afterward, the in situ formed formamides were hydrogenated after the addition of the hydrogenation catalyst and toluene in the presence of 60 bar of H2 (Table 3). However, when the reaction solution from Table 1, entry 4 was hydrogenated following this protocol using C-1 as the hydrogenation catalyst, no appreciable amount of methanol was observed after 24 h (Table 3, entry 1). This is most probably due to the residual CO dissolved in the solution deactivating the catalyst C-1. Recently, we reported the deactivation pathway of ruthenium pincer hydrogenation catalysts in the presence of CO through the formation of monohydride biscarbonyl cationic complexes, which are unable to catalyze the hydrogenation of formamides to amine and methanol (Scheme S1).33 In the same study, we also found that complex Ru-Macho-BH (C-2) and Ru-Macho (C-3) are more resistant to CO poisoning than complex C-1, due to the electron-withdrawing nature of the phenyl groups. Subsequently, we attempted the hydrogenation using C-2 as a catalyst. To our delight, the hydrogenation proceeded smoothly with C-2 under 60 bar of H2 at 140 °C, and after 24 h, a 75% methanol yield was observed (entry 2). The MeOH yield increased further to 80% when the amine was changed from piperidine to diethylenetriamine (DETA) (entry 3).37 The lower vapor pressure, high amine content per unit of mass and volume, and the absence of the foul smell of DETA as compared to 1 make DETA a more attractive amine for this process. Similar to C-2, Ru-Macho (C-3) was also active for this sequential hydrogenation to obtain methanol from CO (entry 4).
a
Reaction conditions: piperidine (10 mmol), CO (30 bar at rt), cat. (10 mol %), solvent (5 mL), 140 °C, 24 h. bYields calculated from 1H NMR using 1,3,5-trimethoxybenzene (TMB) as internal standard. Yield calculations error: ±5%.
piperidine was sluggish; after 24 h of reaction at 140 °C at a CO pressure of 30 bar (at rt) and a K3PO4 loading of 10 mol %, only 10% of the intended product 1-formylpiperidine (1a) was formed as observed through 1H NMR (entry 1). Addition of a nonpolar aprotic solvent, toluene, did not increase the yield (entry 2). On the other hand, the yield of 1a increased significantly when the solvent was changed to polar protic ones, and 96% and 95% of 1a were observed in methanol and ethanol, respectively (entries 3−4). The reaction also proceeded when a 1:1 v/v mixture of toluene and ethanol was used as solvent, although the rate was slower compared to pure ethanol (entry 5). In subsequent studies, the hydrogenation of 1a to methanol and piperidine by a reported formamide hydrogenation catalyst, RuHClPNPiPr(CO) (C-1), in different solvents was explored (Table 2).33 The hydrogenation proceeded rapidly at 140 °C in the absence of any solvent, and after 24 h of reaction, 92% of piperidine was formed along with a 99% conversion (entry 1). The reaction also proceeded rapidly in toluene (entry 2). On the other hand, when methanol was used as the solvent, the rate of hydrogenation slowed down and only 9% conversion of 1a was observed after 24 h (entry 3). The conversion increased to 60%, with a methanol yield of 46%, when ethanol was used as a solvent (entry 4). A minor amount of N-ethylpiperidine (1b) (∼20%) was observed in the B
DOI: 10.1021/jacs.9b06586 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society Table 3. Sequential Two-Step CO to Methanola
entry
amine
cat.
amide (%)b
MeOH (%)b
amine (%)b
1 2 3d 4d
1 1 DETA DETA
C-1 C-2 C-2 C-3
95 21 0 0
0 75 80 77
7 57c e e
CO (Figure S7) as compared to 4.1% before the reaction, corresponding to an ∼91% CO conversion. From the product NMR analysis, 10.6 mmol of MeOH (yield: 77%) were observed, corresponding to a TON of 211. The intermediate formamide products were also observed in 13% yield.38 The direct hydrogenation proceeded even at a higher CO pressure of 10 bar, although expectedly, the reaction took longer for completion. After 168 h, 88% of CO gas was consumed as observed through the GC analysis of the unreacted gas mixture. The solution contained 27.0 mmol of MeOH (∼1.1 mL; 59% yield), with a C-2 TON of 539, along with 15% of the formamide intermediates. The excess amine present in the system was crucial to obtain methanol in high yield. When a lower amount of DETA was used, the methanol yield dropped to 12% (5.4 mmol) (entry 3). Thus, it was demonstrated that CO can be hydrogenated to methanol at 145 °C through an amine-assisted pathway with high selectivity and yield. In conclusion, we have developed a novel route for lowtemperature hydrogenation of CO to methanol that involves formamide intermediates. The amine works as a shuttle where CO is initially anchored, becoming amenable to hydrogenation in a subsequent step. The first anchoring step is catalyzed by K3PO4, which also assists the hydrogenation catalyst in the second step. Using the combination of diethylenetriamine as the amine and Ru-Macho-BH as the catalyst, CO was directly converted to methanol in high yield (∼77%), selectivity, and catalytic turnovers (539). Compared to the other homogeneous CO to methanol systems, this route does not use any toxic carbonyl complexes and excess amounts of difficult-tohandle alkoxide bases and is active for long periods of time (∼1 week). Our next focus in this context is toward minimizing alkylation side reactions as well as developing catalysts for this hydrogenation with higher turnover numbers and frequencies through judicious ligand tuning.
Reaction conditions: The carbonylation step was carried out first. After that, the autoclave was opened in a N2 chamber and toluene (15 mL) and the hydrogenation catalyst were added. The reaction mixture was heated under H2 at 140 °C for 24 h. bYields after hydrogenation step. cN-Ethylpiperidine (18%) was observed. d3.33 mmol of DETA used; carbonylation reaction for 36 h (formamide yield ∼85%). e Could not be accurately determined from 1H NMR spectra. Conversion and yield calculations error: ±5%. a
After identifying the suitable combination of amine and catalyst from the sequential stepwise reaction, we pursued the direct hydrogenation of CO to methanol in one step (Table 4). Table 4. Direct CO Hydrogenation to MeOHa
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b06586. General information and experimental details (PDF) entry
DETA (mL)
1 2e 3
5 5 0.36
CO H2 (bar) (bar) 3 10 10
70 70 70
t (h)
conv (%)b
Amide (%)c
MeOH (%)c
TONd
40 168 168
91 88 16
13 15 3
77 59 12
211 539 107
■
AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
a
Reaction conditions: CO (as specified), H2 (70 bar), DETA (as specified), C-2 (50 μmol), K3PO4 (1 mmol), Tol/EtOH (5 mL/5 mL), 145 °C for specified amount of time. bConversions were calculated from GC analysis of the unreacted gas mixture. cYields calculated from 1H NMR. dTON = per mole of methanol formed per mole of catalyst. eMinor amounts of N-methylation of amine was observed.
Sayan Kar: 0000-0002-6986-5796 Alain Goeppert: 0000-0001-8667-8530 G. K. Surya Prakash: 0000-0002-6350-8325 Notes
The authors declare no competing financial interest.
■
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.
A 1/1 v/v mixture of ethanol and toluene was used as the reaction solvent. In the first attempt, 3 bar of CO (∼13.7 mmol) was heated under excess H2 (70 bar) in the presence of 10 mL of solvent, 0.36 mol % of C-2 (with respect to CO), 5 mL of DETA (amine content ∼140 mmol), and 1 mmol K3PO4 at 145 °C for 40 h. A decrease in pressure inside the reactor during the reaction was observed (Figure S6), revealing CO and H2 consumption. After the reaction, GC of the unreacted gas mixture showed the presence of only 0.4% of
■
REFERENCES
(1) Olah, G. A.; Goeppert, A.; Prakash, G. K. S. Beyond Oil and Gas: The Methanol Economy, Third, Updated and Enlarged ed. 2018; Wiley-VCH Verlag GmbH & Co. KGaA: 2018.
C
DOI: 10.1021/jacs.9b06586 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society (2) Olah, G. A.; Goeppert, A.; Prakash, G. K. S. Chemical Recycling of Carbon Dioxide to Methanol and Dimethyl Ether: From Greenhouse Gas to Renewable, Environmentally Carbon Neutral Fuels and Synthetic Hydrocarbons. J. Org. Chem. 2009, 74, 487−498. (3) Olah, G. A.; Prakash, G. K. S.; Goeppert, A. Anthropogenic chemical carbon cycle for a sustainable future. J. Am. Chem. Soc. 2011, 133, 12881−12898. (4) Goeppert, A.; Czaun, M.; Jones, J.-P.; Prakash, G. K. S.; Olah, G. A. Recycling of carbon dioxide to methanol and derived products − closing the loop. Chem. Soc. Rev. 2014, 43, 7995−8048. (5) Natte, K.; Neumann, H.; Beller, M.; Jagadeesh, R. V. TransitionMetal-Catalyzed Utilization of Methanol as a C 1 Source in Organic Synthesis. Angew. Chem., Int. Ed. 2017, 56, 6384−6394. (6) https://www.methanol.org/the-methanol-industry/ (Accessed on June 5th, 2019). (7) Galadima, A.; Muraza, O. From synthesis gas production to methanol synthesis and potential upgrade to gasoline range hydrocarbons: A review. J. Nat. Gas Sci. Eng. 2015, 25, 303−316. (8) Venvik, H. J.; Yang, J. Catalysis in microstructured reactors: Short review on small-scale syngas production and further conversion into methanol, DME and Fischer−Tropsch products. Catal. Today 2017, 285, 135−146. (9) Grabow, L. C.; Mavrikakis, M. Mechanism of Methanol Synthesis on Cu through CO2 and CO Hydrogenation. ACS Catal. 2011, 1, 365−384. (10) Somorjai, G. A. The Catalytic Hydrogenation of Carbon Monoxide. The Formation of C1 Hydrocarbons. Catal. Rev.: Sci. Eng. 1981, 23, 189−202. (11) Bradley, J. S. Homogeneous carbon monoxide hydrogenation to methanol catalyzed by soluble ruthenium complexes. J. Am. Chem. Soc. 1979, 101, 7419−7421. (12) Dombek, B. D. Hydrogenation of carbon monoxide to methanol and ethylene glycol by homogeneous ruthenium catalysts. J. Am. Chem. Soc. 1980, 102, 6855−6857. (13) Mahajan, D. Atom-economical reduction of carbon monoxide to methanol catalyzed by soluble transition metal complexes at low temperatures. Top. Catal. 2005, 32, 209−214. (14) Ohyama, S. Low-temperature methanol synthesis in catalytic systems composed of nickel compounds and alkali alkoxides in liquid phases. Appl. Catal., A 1999, 180, 217−225. (15) Marchionna, M.; Basini, L.; Aragno, A.; Lami, M.; Ancillotti, F. Mechanistic studies on the homogeneous nickel-catalyzed low temperature methanol synthesis. J. Mol. Catal. 1992, 75, 147−151. (16) Li, K.; Jiang, D. Methanol synthesis from syngas in the homogeneous system. J. Mol. Catal. A: Chem. 1999, 147, 125−130. (17) Mahajan, D.; Krisdhasima, V.; Sproull, R. D. Kinetic modeling of homogeneous methanol synthesis catalyzed by base-promoted nickel complexes. Can. J. Chem. 2001, 79, 848−853. (18) Chen, Y.-Z.; Liaw, B.-J.; Chen, B.-J. One-step synthesis of methanol from CO/H2 at low temperature over ultrafine CuB catalysts. Appl. Catal., A 2002, 236, 121−128. (19) Mahajan, D.; Sapienza, R. S.; Slegeir, W. A.; O’Hare, T. E. Homogeneous Catalyst Formulation for Methanol Production. U.S. Patent 4,935,395, June 19, 1990. (20) Ohyama, S. A comparison of the catalytic performance for lowtemperature methanol synthesis in a liquid medium. In Studies in Surface Science and Catalysis; Corma, A., Melo, F. V., Mendioroz, S., Fierro, J. L. G., Eds.; Elsevier: 2000; Vol. 130, pp 3753−3758. (21) Mahajan, D. Liquid phase low temperature method for production of methanol from synthesis gas and catalyst formulations therefor. U.S. Patent 6,921,733, July 26, 2005. (22) Li, B.; Jens, K. J. Liquid-Phase Low-Temperature and LowPressure Methanol Synthesis Catalyzed by a Raney Copper-Alkoxide System. Top. Catal. 2013, 56, 725−729. (23) Li, B.; Jens, K.-J. Low-Temperature and Low-Pressure Methanol Synthesis in the Liquid Phase Catalyzed by Copper Alkoxide Systems. Ind. Eng. Chem. Res. 2014, 53, 1735−1740.
(24) Rezayee, N. M.; Huff, C. A.; Sanford, M. S. Tandem Amine and Ruthenium-Catalyzed Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2015, 137, 1028−1031. (25) Kar, S.; Goeppert, A.; Kothandaraman, J.; Prakash, G. K. S. Manganese-Catalyzed Sequential Hydrogenation of CO2 to Methanol via Formamide. ACS Catal. 2017, 7, 6347−6351. (26) Kar, S.; Sen, R.; Goeppert, A.; Prakash, G. K. S. Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy. J. Am. Chem. Soc. 2018, 140, 1580−1583. (27) Kar, S.; Goeppert, A.; Prakash, G. K. S. Combined CO2 Capture and Hydrogenation to Methanol: Amine Immobilization Enables Easy Recycling of Active Elements. ChemSusChem 2019, 12, 3172−3177. (28) Choi, Y.-S.; Shim, Y. N.; Lee, J.; Yoon, J. H.; Hong, C. S.; Cheong, M.; Kim, H. S.; Jang, H. G.; Lee, J. S. Ionic liquids as benign catalysts for the carbonylation of amines to formamides. Appl. Catal., A 2011, 404, 87−92. (29) Tsuji, Y.; Ohsumi, T.; Kondo, T.; Watanabe, Y. Dodecacarbonyltriruthenium catalysed carbonylation of amines and hydroamidation of olefins. J. Organomet. Chem. 1986, 309, 333−344. (30) Gerack, J. C.; McElwee-White, L. Formylation of Amines. Molecules 2014, 19, 7689. (31) Gerack, C. J.; McElwee-White, L. Oxidative carbonylation of amines to formamides using NaIO4. Chem. Commun. 2012, 48, 11310−11312. (32) Kim, Y. J.; Lee, J. W.; Lee, H. J.; Zhang, S.; Lee, J. S.; Cheong, M.; Kim, H. S. K3PO4-catalyzed carbonylation of amines to formamides. Appl. Catal., A 2015, 506, 126−133. (33) Kar, S.; Sen, R.; Kothandaraman, J.; Goeppert, A.; Chowdhury, R.; Munoz, S. B.; Haiges, R.; Prakash, G. K. S. Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2019, 141, 3160−3170. (34) Guillena, G.; Ramón, D. J.; Yus, M. Hydrogen Autotransfer in the N-Alkylation of Amines and Related Compounds using Alcohols and Amines as Electrophiles. Chem. Rev. 2010, 110, 1611−1641. (35) Enyong, A. B.; Moasser, B. Ruthenium-Catalyzed N-Alkylation of Amines with Alcohols under Mild Conditions Using the Borrowing Hydrogen Methodology. J. Org. Chem. 2014, 79, 7553−7563. (36) Elangovan, S.; Neumann, J.; Sortais, J.-B.; Junge, K.; Darcel, C.; Beller, M. Efficient and selective N-alkylation of amines with alcohols catalysed by manganese pincer complexes. Nat. Commun. 2016, 7, 12641. (37) DETA is one of the active amines that can assist the production of methanol through CO2 hydrogenation. See ref 33. (38) Little amounts of N-CH2CH3 product was also detected in the solution through NMR, which formed via the coupling of amine with ethanol solvent in the presence of C-2 (Figures S3 and S8).
D
DOI: 10.1021/jacs.9b06586 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
