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. 2021 Jan 25;1(2):130–136. doi: 10.1021/jacsau.0c00091

Alcohol-Assisted Hydrogenation of Carbon Monoxide to Methanol Using Molecular Manganese Catalysts

Akash Kaithal †, Christophe Werlé †,‡, Walter Leitner †,§,*
PMCID: PMC8395606  PMID: 34467278

Abstract

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Alcohol-assisted hydrogenation of carbon monoxide (CO) to methanol was achieved using homogeneous molecular complexes. The molecular manganese complex [Mn(CO)2Br[HN(C2H4PiPr2)2]] ([HN(C2H4PiPr2)2] = MACHO-iPr) revealed the best performance, reaching up to turnover number = 4023 and turnover frequency 857 h–1 in EtOH/toluene as solvent under optimized conditions (T = 150 °C, p(CO/H2) = 5/50 bar, t = 8–12 h). Control experiments affirmed that the reaction proceeds via formate ester as the intermediate, whereby a catalytic amount of base was found to be sufficient to mediate its formation from CO and the alcohol in situ. Selectivity for methanol formation reached >99% with no accumulation of the formate ester. The reaction was demonstrated to work with methanol as the alcohol component, resulting in a reactive system that allows catalytic “breeding” of methanol without any coreagents.

Keywords: carbon monoxide, hydrogenation, methanol, homogeneous catalysis, manganese, pincer ligands, liquid phase, alcohols

Methanol is a central pivot in the chemical supply chain and an important energy carrier for combustion engines or fuel cells.1−8 The synthetic potential ranges from the production of base chemicals such as propylene or aromatics to application as a C1 building block to produce fine chemicals and pharmaceuticals.9−11 Global methanol production is on the order of 100 million metric tons annually, using mainly coal and natural gas as primary feedstocks. Worldwide efforts are directed toward the change of the raw material basis toward renewable or recycled carbon sources (biomass, CO2, waste).12−16

The industrial production of methanol is based on gas-phase processes over heterogeneous catalysts operating at elevated temperatures (>200 °C) and pressures (35–55 bar). The gas feed is a mixture of H2 and CO (Syngas) with the addition of defined amounts of CO2. The CO2 is used to balance the H/C ratio and, at the same time, is essential for catalytic conversion. Over typically supported metal oxide catalysts such as the industrial standard Cu/ZnO/Al2O3, the reaction network comprises a combination of water gas shift (WGS) and CO2 hydrogenation involving surface formate species as intermediates.17−22 The structural and dynamic interplay of active sites at the various catalyst components is still a matter of fundamental investigation even after nearly a century of industrial practice, demonstrating the complexity of the intricate bond breaking and bond forming events.

While the conversion of CO to methanol over heterogeneous catalysts is well-established, molecular complexes have been studied very rarely for this particular transformation. One of the biggest challenges for the hydrogenation of CO via homogeneous molecular complexes is the high binding affinity of CO to metal complex and its reluctance for migratory insertion into metal hydride bonds,23−26 often leading to deactivation of the metal complex for further hydrogenation reactions. Consequently, the direct hydrogenation of CO to methanol by homogeneous catalysts has proven very inefficient. Cobalt carbonyl complexes as studied in the late 1970s by the group of Feder27 resulted in low turnover numbers (TON < 10) even at very high pressures and temperature (p = 300 atm, T = 180–200 °C) with only marginal improvements by later studies employing ruthenium carbonyl complexes.28,29

Lately, the groups of Prakash30 and Beller31 reported the use of amines as coreagents to circumvent this limitation via the formation of formamides as intermediates (Scheme 1). The Prakash group used the ruthenium-MACHO catalyst in the presence of diethylenediamine (DETA) and K3PO4 as base, leading to a TON of 539. The Beller group employed Mn-MACHO in the presence of pyrrols or indols together with K3PO4, resulting in a TON of up to 3170. The formamide intermediates were typically not fully converted to methanol, and trace amounts of N-methylated product were sometimes observed, leading to significant side products. Therefore, an effective system to produce methanol from hydrogen and CO with a clean or ultimately even without any coreagent remains a challenge in homogeneous catalysis.

Scheme 1. Alcohol-Assisted Hydrogenation of CO to Methanol Using the Mn-MACHO Catalyst Enables Clean and Efficient CO Hydrogenation.

Scheme 1

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Results and Discussion

Herein, we report the manganese-catalyzed hydrogenation of CO to methanol via an alcohol-assisted pathway exploiting the in situ formation of formate ester intermediates. Previously, the groups of Marchionna,32 Ohyama,33,34 and Mahajan35,36 investigated the catalyst system Ni(CO)4/base for the hydrogenation of CO in the presence of methanol, demonstrating that the resulting Ni species were operating as homogeneous catalysts. Our group recently demonstrated that the formate ester pathway opens a possible entry into a CO hydrogenation manifold for catalytic methylation reactions using Syngas as a C1 source.37

Intrigued by these reports, we investigated the possibility for methanol formation from H2 and CO in the presence of alcohol and base using manganese complexes as well as other noble and non-noble metal molecular complexes with known activity for related reduction reactions (Table 1). For the catalyst screening, a mixture of EtOH and toluene (2:1) was used as the reaction medium, employing 10 μmol of the metal complex with 10 equiv of NaOtBu at a temperature of 150 °C and a pressure of 5 bar of CO and 50 bar of H2. Product analysis was carried out by 1H NMR spectroscopy using mesitylene as the internal standard. Only methanol and formate esters (ethyl formate and small amounts of butyl formate from transesterification) were detected as products from CO conversion in the liquid phase (see the Supporting Information for details).

Table 1. Alcohol-Assisted Hydrogenation of CO Using Different Metal Complexes [M]a.

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# [M] temp (°C) MeOH (mmol) formate (mmol) selectivity (%) to CH3OH TON (CH3OH)
1 1 150 2.61 0 >99 261
2 2 150 2.75 0 >99 275
3 3 150 2.97 0 >99 297
4 4 150 0 0.28 0 0
5 5 150 0 0.29 0 0
6 6 150 0 0.25 0 0
7 7 150 0.66 0.14 83 66
8 8 150 0 0.27 0 0
9 9 150 0 0.28 0 0
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a

All reactions were performed in a 10 mL autoclave.

Initially, the well-established hydrogenation catalysts38,39 Ru-MACHO (1) and Ru-MACHO-BH (2) were examined, and methanol formation was confirmed in quantities corresponding to a TON of 261 and 275, respectively. Motivated by their diagonal relationship in the periodic table, the noble metal Ru2+ was replaced with its 3d congener Mn1+, and the Mn-MACHO-iPr complex 3(40,41) affirmed the formation of methanol in even higher yield with a TON of 297. Interestingly, the closely related MACHO-type complex 8(42) comprising a Si-bridge in the backbone did not result in measurable catalytic activity, and only formate ester was detected. Likewise, lutidine-type manganese pincer43,44 complexes 4 and 5 did not show any methanol formation. Triazine-type pincer complexes45,46 proved inactive with Mn1+ (6) as well as with the Co2+ (9) as the central metal. Only the indole-based PNN ligand in complex 7(47) yielded another active Mn1+ catalyst for methanol formation with a TON of 66.

As manganese is cheap, earth-abundant, and environmentally friendly, further investigations on the influence of different reaction parameters were carried out using complex 3 (Table 2). Reducing the reaction time from 24 to 8 h did not lead to a significant decrease in TON and was therefore set as standard reaction time for the other variations. Increasing the temperature to 170 °C gave a significantly reduced methanol yield, indicating probably a combination of thermal instability of the catalyst and reduced equilibrium concentration of formate ester (Table 2, entry 1). The reaction temperature could be reduced to 120 °C and 100 °C maintaining a TON of 274 and 258, respectively (Table 2, entries 2, 3). At 80 °C, the conversion of formate ester was incomplete, and the TON reduced to 60 (Table 2, entry 4). Several organic and inorganic bases such as KOtBu and NaH revealed similar reactivity as compared to NaOtBu, yielding TONs of 294 and 283, respectively (Table 2, entry 5, 6), while KOH and Cs2CO3 resulted in a strongly reduced formation of methanol as well as formate ester (Table 2, entry 7, 8).

Table 2. Influence of Reaction Temperature and Basea.

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# base temp (°C) MeOH (mmol) formate (mmol) selectivity (%) to CH3OH TON (CH3OH)
1 NaOtBu 170 0.89 0 >99 89
2 NaOtBu 120 2.74 0 >99 274
3 NaOtBu 100 2.58 0 >99 258
4 NaOtBu 80 0.60 0.93 39 60
5 KOtBu 150 2.94 0 >99 294
6b NaH 150 2.83 0 >99 283
7 KOH 150 0.09 0.03 75 9
8 Cs2CO3 150 0.16 0.09 64 16
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a

Conditions: CO (5 bar), H2 (50 bar), 3 (10 μmol), Base (0.1 mmol), EtOH (1 mL), toluene (0.5 mL), T, 8 h. All reactions were performed in a 10 mL autoclave.

b

1H NMR spectroscopy of the reaction mixture, using mesitylene as an internal standard, confirmed that ethanol is recovered with a yield of >99%.

Subsequently, the influence of the partial pressures of the reactive gases and the solvent was investigated (Table 3). Increasing the CO pressure at constant H2 pressure of 50 bar resulted in a continuing increase in methanol yield up to a TON of 760 at p(CO) = 15 bar (Table 3, entry 1, 2). Performing the reaction in the absence of toluene lowered the methanol production and resulted in a TON of 314 (Table 3, entry 3). Further improvement was not achieved, and the methanol formation even decreased significantly to TON 398 at 25 bar of CO (Table 3, entry 4, 5). This can be attributed to inhibition of the hydrogenation activity as evidenced by the buildup of formate ester under these conditions. Increasing the H2 pressure again above the stoichiometric ratio of 1:2 compensated at least partly for this effect (Table 3, entry 6, 7). Consequently, the reaction was performed with 20 bar of CO pressure, and the pressure of hydrogen was increased to 65 bar, resulting in the most considerable amount of methanol formation with 10.22 mmol corresponding to a TON of 1022 (Table 3, entry 6). Carrying out the reaction at 25 bar of CO and 70 bar of hydrogen led to a TON of 851 (Table 3, entry 7).

Table 3. Pressure Optimization for the Hydrogenation of COa.

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# CO (bar) H2 (bar) MeOH (mmol) formate (mmol) selectivity (%) to CH3OH TON (CH3OH)
1 10 50 4.69 0 >99 469
2 15 50 7.60 0 >99 760
3b 15 50 3.14 0.25 94 314
4 20 50 7.34 0 >99 734
5 25 50 3.98 0.68 85 398
6 20 65 10.22 0 >99 1022
7 25 70 8.51 0.47 96 851
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a

Conditions: CO, H2, 3 (10 μmol), NaOtBu (0.1 mmol), EtOH (1 mL), toluene (0.5 mL), 150 °C, 8 h. All reactions were performed in a 10 mL autoclave.

b

Reaction was performed in the absence of toluene.

We then turned our attention to the productivity and activity of the catalyst. Decreasing the catalyst loading from 10 μmol to 2.5 μmol and performing the reaction at 15 bar of CO and 50 bar of H2 resulted in methanol formation with a yield of 4.76 mmol and TON of 1904 (Table 4, entry 1). When the pressure of CO was increased to 20 bar, the TON for methanol formation was decreased to 745 at the expense of remaining formate, indicating the inhibiting effect of CO on the hydrogenation activity again (Table 4, entry 2). With the knowledge from Table 3, this was overcome by increasing p(H2) to 65 bar, resulting in the formation of methanol with a TON of 1949 (4.87 mmol methanol) (Table 4, entry 3). To increase the amount of available CO gas without increasing the CO/Mn ratio, the reactor volume was increased from 10 to 20 mL. As expected, this improved the yield of methanol significantly, demonstrating the robustness and potential for high productivity of the catalyst under the given conditions. The amount of methanol was 6.64 mmol, corresponding to a TON of 2654 when employing 2.5 μmol of complex 3 in 1.5 mL of solvent (EtOH:toluene = 2:1) under 10 bar of CO and 50 bar of H2 at 150 °C for 12 h (Table 4, entry 4). Analysis of the reaction mixture under the same conditions after 1 h revealed a high turnover frequency (TOF) of 857 h–1, indicating a very high activity of the catalytic system for the overall CO conversion. Further optimization of the CO pressure and the catalyst loading resulted in a TON of 4023 at >99% selectivity for MeOH formation (Table 4, entry 5, 6).

Table 4. Turnover Number Optimization for the Hydrogenation of COa.

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# CO (bar) H2 (bar) reactor volume (mL) 3 (μmol) MeOH (mmol) selectivity (%) to CH3OH TON (CH3OH)
1 15 50 10 2.5 4.76 93 1904
2 20 50 10 2.5 1.86 72 745
3 20 65 10 2.5 4.87 >99 1949
4 10 50 20 2.5 6.64 >99 2654
5 10 50 20 1 3.77 96 3766
6 5 50 20 1 4.02 >99 4023
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a

Conditions: CO, H2, 3 (1–2.5 μmol), NaOtBu (0.1 mmol), EtOH (1 mL), toluene (0.5 mL), 12 h. All reactions were performed in a 10 mL autoclave.

A number of control experiments were conducted to elucidate the role of the base and to confirm the formate ester as a true intermediate (Scheme 2). First, the generally presumed active species I formed by reacting the precursor 3 with base was generated and isolated independently (see Supporting Information for details).41 Only small amounts of methanol corresponding to TON 8 were formed when complex I was subjected to standard reaction conditions but in the absence of NaOtBu (Scheme 2a). This clearly indicates that the base is required not only for catalyst formation but also for CO activation. The catalytic effect of the base on the carbonylation of ethanol under the reaction conditions was also ascertained (Scheme 2b). The requirement of the alcohol as coreagent was probed by conducting a catalytic reaction in the absence of ethanol. A TON of 89 was obtained under these conditions, which is an order of magnitude lower than in the presence of EtOH, indicating the essential role of the alcohol as coreagent (Scheme 2c). The catalytic activity of complex 3 for hydrogenation of ethyl formate under typical reaction conditions in the presence of CO was also ascertained (Scheme 2d).

Scheme 2. Control Experiments to Probe the Role of Base and Alcohol.

Scheme 2

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Based on these results, the overall reaction sequence can be rationalized by combining two catalytic processes (Scheme 3). The first process is the activation of CO by base-catalyzed carbonylation of the alcohol to the corresponding formate ester.36,48 The base also activates the precursor complex [Mn(MACHO-iPr)(CO)2Br] by the elimination of HBr to the active hydrogenation catalyst I. A putative catalytic cycle for the hydrogenation of the formate ester to methanol is illustrated in Scheme 3 via the widely inferred metal–ligand cooperation (MLC) mechanism.40,41,49−51 Plausible alternatives are, however, also conceivable, involving, for example, activation of the C=O unit by Na+ instead of H+ at the ligand amide group or heterolytic H2 cleavage at a cationic Mn1+ complex after carboxylate, alcoholate, or CO dissociation (see the Supporting Information for a schematic representation).9,52−65

Scheme 3. Mechanistic Rationale for the Alcohol Mediated Hydrogenation of CO by Mn1+ Catalyst 3 Illustrated for an MLC Cycle.

Scheme 3

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Irrespective of the mechanistic details of the ester hydrogenation, the alcohol-assisted catalytic system results in a very clean way to activate and hydrogenate CO to methanol. As shown in Scheme 3, the individual pathways are fully closed, resulting in the net conversion of CO + 2 H2 = CH3OH. Most intriguingly, the scheme implies the possibility to employ methanol itself as the alcohol mediator. To quantify the methanol formation from CO hydrogenation, 13C-labeled carbon monoxide was employed (p(13CO) = 5 bar, p(H2) = 50 bar, Vreactor = 20 mL). The amount of 13CH3OH formed by the catalytic reaction was determined as 2.81 mmol (TON 2807). Thus, the amount of new CH3OH “bred” in this reaction corresponds to 13.5% of the originally charged methanol even under these nonoptimized conditions. As product and mediator are identical under these conditions, the process represents de facto a catalytic system that enables the liquid phase formation of MeOH from CO and H2 without any coreagents (Scheme 4).

Scheme 4. Mn-Catalyzed Hydrogenation of 13CO Using Complex 3 with the Product Methanol as a Mediator, Resulting in a Homogeneously Catalyzed System without Additional Coreagent.

Scheme 4

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In conclusion, we established an active catalytic system that performs the alcohol-assisted hydrogenation of CO to methanol by employing a molecular manganese complex together with a catalytic amount of base (base:Mn = 10:1). The manganese pincer complex [Mn(CO)2Br[HN(C2H4PiPr2)2]] 3 achieved a TON for methanol synthesis up to 4023 and a TOF of 857 h–1 without accumulation of the formate ester intermediates. When using methanol as the alcohol component, the reactive system comprises only the reactive gases and the product, opening the possibility for a continuous-flow operation of liquid-phase methanol synthesis. The metal-catalyzed hydrogenation of CO or CO2 via formate esters to methanol requires essentially two elementary processes as key steps: hydride transfer to an activated C=O unit and heterolytic cleavage of the H2 molecule. It appears that Mn1+ embedded in a pincer ligand environment provides a privileged coordination framework to accommodate various pathways for the efficient combination of these two steps. Thus, the defined molecular structure provided through the ligand framework offers further possibilities to optimize the catalytic performance of the earth-abundant 3d metal manganese for COx hydrogenation.

Acknowledgments

We gratefully acknowledge financial support from the Max Planck Society. The studies were performed as part of our activities in the framework of the “Fuel Science Center” funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy–Exzellenzcluster 2186, The Fuel Science Center “ID: 390919832”. Open access funding was provided by the Max Planck Society.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.0c00091.

  • General considerations, experimental methods, synthetic details, and copies of NMR spectra (PDF)

The authors declare no competing financial interest.

Supplementary Material

au0c00091_si_001.pdf (2.2MB, pdf)

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