Synthesis of acetic acid via methanol hydrocarboxylation with CO2 and H2

Abstract

Acetic acid is an important bulk chemical that is currently produced via methanol carbonylation using fossil based CO. Synthesis of acetic acid from the renewable and cheap CO₂ is of great importance, but state of the art routes encounter difficulties, especially in reaction selectivity and activity. Here we report a route to produce acetic acid from CO₂ , methanol and H₂ . The reaction can be efficiently catalysed by Ru-Rh bimetallic catalyst using imidazole as the ligand and LiI as the promoter in 1,3-dimethyl-2-imidazolidinone (DMI) solvent. It is confirmed that methanol is hydrocarboxylated into acetic acid by CO₂ and H₂ , which accounts for the outstanding reaction results. The reaction mechanism is proposed based on the control experiments. The strategy opens a new way for acetic acid production and CO₂ transformation, and represents a significant progress in synthetic chemistry.

ARTICLE

Received 9 Dec 2015 | Accepted 1 Apr 2016 | Published 11 May 2016

Acetic acid is an important bulk chemical that is currently produced via methanol carbonylation using fossil based CO. Synthesis of acetic acid from the renewable and cheap CO2 is of great importance, but state of the art routes encounter difculties, especially in reaction selectivity and activity. Here we report a route to produce acetic acid from CO2,

methanol and H2. The reaction can be efciently catalysed by RuRh bimetallic catalyst using imidazole as the ligand and LiI as the promoter in 1,3-dimethyl-2-imidazolidinone (DMI)

solvent. It is conrmed that methanol is hydrocarboxylated into acetic acid by CO2 and H2, which accounts for the outstanding reaction results. The reaction mechanism is proposed based on the control experiments. The strategy opens a new way for acetic acid production and CO2 transformation, and represents a signicant progress in synthetic chemistry.

DOI: 10.1038/ncomms11481 OPEN

Synthesis of acetic acid via methanol hydrocarboxylation with CO2 and H2

Qingli Qian1, Jingjing Zhang1, Meng Cui1 & Buxing Han1

1 Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Beijing 100190, China. Correspondence and requests for materials should be addressed to Q.Q. (email: mailto:[email protected]

Web End [email protected] ) or to B.H. (email: mailto:[email protected]

Web End [email protected] ).

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Acetic acid is an important bulk chemical1,2 that is currently produced via methanol carbonylation (CH3OH CO-CH3COOH), such as Monsanto

process3. CO2 is a greenhouse gas4 and its xation into value-added chemicals is highly desirable for a sustainable society5. So far, CO2 has been utilized to synthesize various chemicals611, such as alcohols, urea, carbonates, polymers and carboxylic acids. In the eld of carboxylic acids syntheses using CO2, major advance has been focused on hydrogenating CO2 into formic acid or its derivatives1219 and hydrocarboxylating unsaturated hydrocarbons or nucleophiles into ne chemicals2024. Synthesis of acetic acid utilizing CO2 is of great importance, but is challenging. The reported routes suffer from obvious disadvantages, such as low selectivity, low activity, higher reaction temperature and use of expensive and/or toxic reactants2529. For example, acetic acid could form slowly with low selectivity when CO2 was reduced by iron nanoparticles25.

Synthesis of acetic acid from CO2 and CH4 is thermodynamically unfavourable, thus high temperature is required and the acetic acid yield is very low26,27. Trace acetic acid in CO2 hydrogenation was detected where CO accounted for 96% of the total product28. When methyl iodide (CH3I), CO2 and H2 were used as reactants acetic acid was formed at low rate and selectivity (acetic acid10.7%, CO 58.4%, and CH4 30.9%)29. In addition, the reactant CH3I is toxic and expensive.

Here we show a protocol to produce acetic acid from CO2,

methanol and H2 (Fig. 1). The reaction could proceed very efciently by homogeneous catalysis under mild condition. Interestingly, the CO2 (not via CO) participates in acetic acid formation with H2, accounting for the outstanding reaction results. The strategy represents a signicant progress in synthetic chemistry. Because the reported routes of hydro-carboxylation use other substrates, such as alkenes, alkynes, arenes and/or organic halides, and the metallic reducing agents are generally utilized2024. This work opens a practical way to x CO2 into bulk chemicals using easily available and cheap feedstocks, which is a promising countermeasure for mankind to solve the ever-increasing crisis in environment and resources.

ResultsCatalytic system for acetic acid synthesis. The target reaction was catalysed effectively by RuRh bimetallic catalyst using imidazole as the ligand and LiI as the promoter in 1,3-dimethyl-2-imidazolidinone (DMI) at milder conditions (Table 1). Acetic acid was the predominant product and other products being negligible in the reaction solution (Supplementary Fig. 1a). The turnover frequency (TOF) of acetic acid reached 30.8 h 1 and the yield of acetic acid based on methanol was 70.3% (Entry 1).

The rest of the methanol was converted into CH4. Very interestingly, the CO was hardly detectable in the gaseous sample (Supplementary Fig. 1b).

The ligand was crucial to the catalytic performance. Without ligand, the catalyst was unstable with much lower activity and selectivity (Entry 2). We also tried other ligands, but the results

were not satisfactory (Entries 36). So imidazole was the suitable ligand for the reaction. The high efciency of imidazole in this reaction should be due to its good coordination capability with the active center, which will be discussed in detail in the following paragraph. The promoter was also indispensable in this reaction. Without promoter, no acetic acid was formed and the catalyst was unstable (Entry 7). When the promoters with other cations (Na , K and Sn4 ) or anions (Cl and Br ) were utilized, the results were poor (Entries 813). Therefore, LiI was the best promoter in catalysing the target reaction. The better performance of lithium cation may be due to its stronger Lewis acidity and proper ion size, which could render appropriate coordination sites during the reaction. The superiority of the iodide anion could be attributed to its stronger nucleophilicity, which would facilitate the CC bond formation in the generation of acetic acid.

We tested Ru3(CO)12 as single catalyst but no acetic acid formed (Entry 14). When we tried Rh2(OAc)4 separately, acetic acid formed at a lower rate (Entry 15). Thus, Rh complex was the major catalyst and Ru complex was the co-catalyst. We have combined Rh2(OAc)4 with other Ru compounds, such as RuO2 or

Ru(PPh3)3Cl2, but the reaction results were poor (Entries 16, 17). We also combined Ru3(CO)12 with other Rh compounds, such as RhCl3 3H2O or Rh(CO)H2(PPh3)3, but the efciencies were also

not satisfactory (Entries 18, 19). Obviously, synergistic effect existed between the RuRh catalysts in accelerating the reaction (Entry 1). The superiority of the Ru3(CO)12/Rh2(OAc)4 in producing acetic acid could be ascribed to their tness in triggering the synergistic effect.

The solvent effect is also important for the reaction. On the basis of Ru3(CO)12/Rh2(OAc)4, imidazole and LiI, other solvents were tested, but the catalytic performances were poor (Entries 2024). When other solvents, such as DMF, tetrahydrofuran, cyclohexane and water, were used, the metal complex decomposed during the reaction and evident black precipitates were observed. The results indicate that DMI could stabilize the catalyst. As a weak Lewis base, DMI may also help to absorb and activate acidic CO2. Moreover, the DMI is stable under H2 atmosphere and the generation of acetic acid from acetate in Rh2(OAc)4 was excluded because the reaction did not occur when only H2 was used as the reactant (Entry 25). Hence, the catalytic system consisting of Ru3(CO)12, Rh2(OAc)4, imidazole, LiI, and DMI was the best for the target reaction.

Effect of reaction parameters. On the basis of the optimized catalytic system, we studied the effects of reaction temperature, pressure and dosage of each catalyst component on the reaction. Figure 2 shows the TOF of acetic acid at different temperatures. The acetic acid was not detectable when the reaction was carried out at 170 C, and it emerged with remarkable amount when the temperature was elevated to 180 C. The activity grew steadily with the increase of temperature until 200 C. The TOF of acetic acid at 200 C reached 30.8 h 1 and increased slowly when the temperature was further increased.

The results in Fig. 2 suggest that 200 C is a suitable temperature. We further studied the effects of other parameters on the reaction at this temperature, and the results are given in Table 2. The pressure of the reaction gases (CO2 and H2)

evidently affected the reaction. At xed ratio of CO2 and H2 (1:1), the yield of acetic acid increased markedly as the total pressure was raised from 2 to 10 MPa (Entries 15). At a xed total pressure of 8 MPa, the ratio of CO2 and H2 also inuenced the reaction and highest yield of acetic acid was obtained at the ratio of 1:1 (Entries 4, 6, 7). In the absence of CO2 or H2, the reaction did not occur (Entries 8, 9). Hence both CO2 and H2 are

Imidazole, LiI Ru3(CO)12/Rh2(OAc)4

CH3OH + CO2 + H2 [H33356]180 C, in DMI CH3COOH + H2O

H298K = 137.6 kJ .mol

1

G298K = 66.4 kJ .mol

1

Figure 1 | Synthesis of acetic acid by reaction of methanol with CO2 and H2. In the reaction CO2 participates in acetic acid formation with H2, and does not via CO.

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Table 1 | Methanol hydrocarboxylation using different catalytic systems.

Entry Catalyst precursors Ligand Promoter Solvent TOF* (h 1) Yieldw (%) 1 Ru3(CO)12, Rh2(OAc)4 Imidazole LiI DMI 30.8 70.32z Ru3(CO)12, Rh2(OAc)4 LiI DMI 5.5 12.73z Ru3(CO)12, Rh2(OAc)4 Pyridine LiI DMI 3.7 8.34z Ru3(CO)12, Rh2(OAc)4 PPNCl LiI DMI 12.1 27.75z Ru3(CO)12, Rh2(OAc)4 2,20-Bipyridine LiI DMI 1.1 2.36z Ru3(CO)12, Rh2(OAc)4 PPh3 LiI DMI 0.2 0.37z Ru3(CO)12, Rh2(OAc)4 Imidazole DMI 0 08 Ru3(CO)12, Rh2(OAc)4 Imidazole NaI DMI 2.2 5.09 Ru3(CO)12, Rh2(OAc)4 Imidazole KI DMI 1.1 2.310z Ru3(CO)12, Rh2(OAc)4 Imidazole SnI4 DMI 0 011z Ru3(CO)12, Rh2(OAc)4 Imidazole LiCl DMI 0 012z Ru3(CO)12, Rh2(OAc)4 Imidazole LiBr DMI 0.4 1.013z Ru3(CO)12, Rh2(OAc)4 Imidazole KBr DMI 0 014 Ru3(CO)12 Imidazole LiI DMI 0 015z Rh2(OAc)4 Imidazole LiI DMI 1.9 4.316z RuO2, Rh2(OAc)4 Imidazole LiI DMI 6.1 14.0 17z Ru(PPh3)3Cl2, Rh2(OAc)4 Imidazole LiI DMI 0 018z Ru3(CO)12, RhCl3 3H2O Imidazole LiI DMI 0.2 0.3

19z Ru3(CO)12, Rh(CO)H2(PPh3)3 Imidazole LiI DMI 0.4 1.020 Ru3(CO)12, Rh2(OAc)4 Imidazole LiI NMP 20.0 45.6 21z Ru3(CO)12, Rh2(OAc)4 Imidazole LiI DMF 0 0 22z Ru3(CO)12, Rh2(OAc)4 Imidazole LiI THF 0 0 23z Ru3(CO)12, Rh2(OAc)4 Imidazole LiI Cyclohexane 0 0 24z Ru3(CO)12, Rh2(OAc)4 Imidazole LiI water 0 0 25z,y Ru3(CO)12, Rh2(OAc)4 Imidazole LiI DMI 0 0

TOF, turnover frequency.

Reaction conditions: 40 mmol Ru catalyst and 40 mmol Rh catalyst (based on metals); 0.75 mmol ligand; 3 mmol promoter; 2 ml solvent; 12 mmol MeOH; 4 MPa CO and 4 MPa H (at room temperature);

200 C; and 12 h.*TOF denotes moles of acetic acid produced per mole of Rh catalyst per hour in the steady state.

wYield is based on methanol feedstock (100 moles of acetic acid product per mole of methanol feedstock). zBlack precipitate was observed after the reaction. yOnly H was used as reactant.

0 170 180 190 200 210

40

35

30

25

TOF (h1 )

20

15

10

5

T (C)

Figure 2 | The TOF of acetic acid at different temperatures. Condition: 40 mmol Ru3(CO)12 and 40 mmol Rh2(OAc)4 (based on metals), 0.75 mmol imidazole, 3 mmol LiI, 2 ml DMI, 12 mmol MeOH, 4 MPa CO2 and 4 MPa H2 (at room temperature), and 12 h. TOF denotes moles of acetic acid produced per mole of Rh catalyst per hour in the steady state.

necessary for the formation of acetic acid. These results demonstrated that acetic acid was not generated from the CO in Ru3(CO)12 via methanol carbonylation, and DMI was stable at the reaction condition.

The dosages of imidazole and LiI also inuenced the reaction signicantly. The yield of acetic acid was the highest when 750 mmol of imidazole was used (Entries 4, 10, 11), and the highest yield occurred at LiI dosage of 3 mmol (Entries 4, 12, 13). The results indicate that excess amount of imidazole or LiI was not favourable to the reaction. The main reason may be that the

active sites were occupied by the excess imidazole or iodide anions due to their good coordination capability, and the reaction was inhibited accordingly. The atom ratio of the Ru and Rh also affected the yield of the reaction. At the same total amount of Ru and Rh (80 mmol), 40 mmol Ru 40 mmol Rh gave the highest

yield of acetic acid (Entries 4, 14, 15). As expected, the total yield of acetic acid increased with increasing catalyst dosage (Entries 4, 16, 17), but it was less sensitive when the amount of catalyst was large enough. The above results reveal that the reaction condition in Entry 1 of Table 1 was the optimal.

Recyclability. To study the reusability of the catalytic system, the acetic acid generated in the reaction system was removed in a vacuum oven at 85 C for 5 h, and GC analysis showed that the acetic acid remained in the reactor was negligible after the evacuation process, then the catalytic system was used directly for the next run. The results indicated that the catalytic activity did not change considerably after ve cycles and the TON of acetic acid reached 1,022 in the ve cycles. (Fig. 3).

Time course of the reaction. Figure 4 presents the time course of the reaction. The amount of acetic acid increased slowly at the beginning (03 h) mainly because the acetic acid reacted with methanol to form methyl acetate. After that time the amount of acetic acid increased steadily (39 h). The reaction slowed down when methanol feedstock was gradually used up (912 h). As expected, with consumption of methanol, the methyl acetate formed initially was converted into acetic acid because of reverse esterication. The CO2 consumption directly correlated with the production of acetic acid. The amount of CH4 generated in the

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Table 2 | Effect of reaction parameters on methanol hydrocarboxylation.

Entry Ru/Rh (lmol) Imidazole (lmol) LiI (mmol) CO2/H2(MPa) TOF (h 1) Yield (%) 1 40/40 750 3 1/1 0.1 0.22 40/40 750 3 2/2 0.3 0.63 40/40 750 3 3/3 4.8 11.04 40/40 750 3 4/4 30.8 70.35 40/40 750 3 5/5 32.8 75.06 40/40 750 3 2/6 1.8 4.07 40/40 750 3 6/2 6.3 14.38 40/40 750 3 4/0 0 09 40/40 750 3 0/4 0 010 40/40 450 3 4/4 18.8 43.011 40/40 1050 3 4/4 30.4 69.312 40/40 750 2 4/4 13.4 30.713 40/40 750 4 4/4 20.0 45.614 20/60 750 3 4/4 24.5 55.915 60/20 750 3 4/4 7.2 16.316 20/20 750 3 4/4 9.2 21.017 60/60 750 3 4/4 22.5 77.0 18* 40/40 0 3 4/4 0 019* 40/40 750 3 4/4 0 0 20w 40/40 0 3 4/4 0 021w 40/40 750 3 4/4 0 0

TOF, turnover frequency.

Reaction conditions: Ru (CO) /Rh (OAc) were used as catalysts and their dosage was based on metal; imidazole was used as ligand; LiI was used as promoter; 12 mmol MeOH; 2 ml DMI; 200 C; and 12 h.*CO and H were used as reactants.

wCO and H were used as reactants.

0 1 2 3 4 5

250

a

14

Methanol Ethanol Methyl acetate Ethyl acetate Acetic acid

200

12

Liquid content (mmol)

10

150

TON

8

100

6

50

4

2

Recycle times

Figure 3 | The results of the recycling test. Condition: 40 mmol Ru3(CO)12 and 40 mmol Rh2(OAc)4 (based on metals), 0.75 mmol imidazole, 3 mmol

LiI, 2 ml DMI, 12 mmol MeOH, 4 MPa CO2 and 4 MPa H2 (at room temperature), 200 C, and 12 h. TON denotes moles of acetic acid produced per mole of Rh catalyst.

0 0 3 6 9 12

Time (h)

b

40

35

30

Gas content (mmol)

25

reaction was minor. Surprisingly, in the whole process, CO was nearly undetectable and alcohols formation was negligible.

Role of the imidazole. To understand the above results, we studied the hydrogenation of CO using the catalytic system. The results showed that plenty of alcohols and CH4 were generated, and imidazole had no obvious impact on the reaction (Supplementary Figs 2 and 3). When we tried CO2 hydrogenation without imidazole, considerable amounts of CO, alcohols and CH4 were formed in the reaction (Supplementary Fig. 4). Because

CO is a well-known intermediate in CO2 hydrogenation to generate alcohols and alkanes30, we could deduce that in the absence of imidazole the CO2 was rstly transformed into CO, then alcohols and CH4 were produced via CO hydrogenation. However, CO and liquid product formed were negligible when

H2 CO2

CO CH4

20

15

10

5

0 0 2 4 6 8 10 12

Time (h)

Figure 4 | Time course of the methanol hydrocarboxylation. (a) Liquid content, (b) gas content. Condition: 40 mmol Ru3(CO)12 and40 mmol Rh2(OAc)4 (based on metals), 0.75 mmol imidazole, 3 mmol LiI, 2 ml DMI, 12 mmol MeOH, 4 MPa CO2 and 4 MPa H2 (at room temperature), 200 C.

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the imidazole was used in the CO2 hydrogenation reaction (Supplementary Fig. 5). Hence, we conclude that the imidazole inhibited hydrogenation of CO2 into CO, which is the origin for the excellent selectivity of acetic acid in this work. As we have mentioned in the former paragraph, the imidazole also played a key role in catalytic activity and stability. The X-ray photoelectron spectroscopy study revealed the facile coordination of imidazole with the Ru and Rh catalysts, which accounted for the role of imidazole in the reaction (Supplementary Fig. 6).

Reaction pathway. Production of acetic acid from CO and methanol, that is, methanol carbonylation, is a well-known reaction3. So there are two possible pathways of acetic acid synthesis from CO2, methanol and H2. The rst is the CO pathway, that is, the CO2 was hydrogenated to CO, then acetic acid was formed by methanol carbonylation. The second is the CO2 pathway, namely, the methanol was hydrocarboxylated into acetic acid by CO2 and H2. All the above experimental results support the second pathway. To get further evidence to support the above argument, we studied the time course of the reaction of CO, methanol and H2 (Fig. 5). At the beginning, methanol was mostly homologated into ethanol by CO and H2, accompanying with gradual accumulation of CO2 because CO2 is a common byproduct in the methanol homologation, especially in the presence of amines31. At 6 h, CO in the reactor decreased to3.5 mmol and CO2 increased to 12.5 mmol accordingly. At this point, the ethanol generation ceased and minor acetates formed. With time going on, the CO2 played a key role in the reaction.

After 9 h, the CO2 content dropped obviously and considerable

acetic acid and acetates emerged accordingly. These results rule out the possibility of rst pathway (via CO). The CO2 pathway was further conrmed by tracer experiments using CH3OD, CH318OH and 13CH3OH, respectively (Supplementary Figs 7, 8 and 9). To our knowledge, this is the rst work on methanol hydrocarboxylation with CO2 and H2. It is an important contribution to synthetic chemistry.

DiscussionOn the basis of all the results above, we proposed the possible mechanism of the reaction (Fig. 6). There are ve major steps in the reaction cycle. First, methanol is in situ converted into methyl iodide, which is similar to the Monsanto process. (Step 1). It is known that CH3I could form spontaneously from methanol and iodine compounds at elevated temperature32, which would be promoted by the Lewis acidic cation (Li )33. The CH3I is a commonly used promoter or intermediate in organic reactions3,8,32,33. The tracer experiments using CH3OD and

CH318OH afrmed that the OH broke away from CH3OH during the reaction, supporting the formation of CH3I (Supplementary Figs 7 and 8). The NMR spectra of the reaction solution using

13CH3OH as reactant (Supplementary Fig. 10) also veried that the CH3 group of methanol is transferred into the acetic acid molecule, which is consistent with the proposed mechanism. Secondly, CH3Rh*I was formed via oxidative addition of CH3I to the active Rh species (Rh*) (Step 2). The oxidative addition is a basic step in organic synthesis and has been well studied3,32,33. In addition, the tracer experiment and NMR spectra using

13CH3OH supported the transfer of CH3 group during the reaction (Supplementary Figs 9 and 10). The third step was the insertion of CO2 into CH3Rh*I to form CH3COORh*I (Step 3).

Rh catalyst was responsible for generating acetic acid (Entry 15 of Table 1). The insertion of CO2 into Rh-alkyl bond (including

CH3Rh bond) has been well studied34, which could be accelerated by enhancing the electron density of the Rh atom. The coordination with imidazole may increase the electron density of the Rh*, which explains the role of imidazole in facilitating the catalytic activity. During the insertion of CO2 into the CH3Rh bond, the CH3COORh*I formed and the O atom of the C-O adsorbed on the catalyst before further reaction34. Next step was reductive elimination of acetic acid from the CH3COORh*I in the presence of H2, which was promoted by the active Ru species (Ru*) (Step 4). The tracer experiment and NMR spectra using 13CH3OH indicated that the CH3 group of methanol nally entered into the acetic acid molecule (Supplementary Figs 9 and 10). The tracer experiments using CH3OD afrmed that the H in the COOH group of acetic acid was from the reactant H2 (Supplementary Fig. 7). The catalytic data showed that acetic acid generation was remarkably promoted by the Ru catalyst (Entries 1, 15 of Table 1). The promoting effect of Ru complex on hydrogenating intermediate into product has

a

14

Methanol Ethanol Methyl acetate Ethyl acetate Acetic acid

12

Liquid content (mmol)

10

8

6

4

2

0 0 2 4 6 8 10 12

Time (h)

b

40

35

30

Gas content (mmol)

25

20

15

CH3COOH

10

H2O

1

H2

4

5

HI

5

[Ru*]

CH3COORh*I

CO2

0 0 2 4 6 8 10 12

LiOH

Time (h)

H2 CO2

CO CH4

Rh*

3

Figure 5 | Time course of the reaction of methanol with CO and H2.

(a) Liquid content, (b) gas content. Condition: 40 mmol Ru3(CO)12 and 40 mmol Rh2(OAc)4 (based on metals), 0.75 mmol imidazole, 3 mmol LiI, 2 ml DMI, 12 mmol MeOH, 4 MPa CO and 4 MPa H2 (at room temperature), 200 C.

LiI

CH3Rh*I

CH3OH CH3I

2

Figure 6 | Proposed mechanism. The Ru* and Rh* represent the active Ru and Rh species in the reaction, respectively.

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been reported in other Rh catalysed reactions35. Finally, the LiOH and HI generated in situ neutralized spontaneously to form LiI and H2O (Step 5). At this time, all the catalytic species were regenerated for the next cycle.

In summary, we have developed a route of acetic acid synthesis from methanol, CO2 and H2. The reaction is efciently promoted by RuRh bimetallic catalyst. The acetic acid can be generated in large amount at above 180 C, and the TON of acetic acid exceeds 1,000 after ve cycles. The ligand imidazole plays a key role for the high catalytic stability, activity and selectivity of the catalyst. The reaction does not proceed via CO pathway. This route has great potential of application because cheap, easily available starting materials are used and the efciency is high. Future work is to study the detailed reaction mechanism and design catalytic systems of better performance for industrial application.

Methods

Chemicals. Ruthenium carbonyl (Ru3(CO)12, 98%) and potassium bromide (KBr, 99.9%) were purchased from Adamas Reagent, Ltd. Ruthenium(IV) oxide (RuO2, 99.9%, metal basis), Dichlorotris (triphenylphosphine) ruthenium(II)

(Ru(PPh3)3Cl2, 97%), Carbonylhydridotris (triphenylphosphine)rhodium(I) (Rh(CO)H2(PPh3)3, Rh410%), Rhodium(III) chloride hydrate (RhCl3 3H2O,

Rh438.5%), imidazole (99%), lithium bromide (LiBr, 99%), lithium iodide(LiI, 99.95%), sodium iodide (NaI, 99.5%), potassium iodide, (KI, 99.9%), Tin(IV) iodide (SnI4, 99.998%), Triphenylphosphine (PPh3, 99%), 2,20-Bipyridine (99%)

and Bis(triphenylphosphoranylidene) ammonium chloride (PPNCl, 97%) were obtained from Alfa Aesar China Co., Ltd. Rhodium acetate dimer (Rh2(OAc)4), lithium chloride (LiCl, 98%) and 1,3-Dimethyl-2-imidazolidinone (DMI, 99%) were purchased from TCI Shanghai Co., Ltd. N-Methyl-2-pyrrolidone (NMP,99.5%), N,N-dimethylformamide (DMF, 99.5%), cyclohexane (99.5%) and pyridine (99%) were provided by Sinopharm Chemical Reagent Co., Ltd. Methanol (99.5%), tetrahydrofuran (A.R. grade) was obtained from Beijing Chemical Company. Toluene (99.8%, HPLC) was obtained from Xilong Chemical Co., Ltd. Methanol13C (13CH3OH, 99 atom% 13C) and Methanol18O (CH318OH, 95 atom% 18O) were purchased from Sigma-Aldrich Co. LLC. MethanolD1 (CH3OD, 99.5 atom% D) was provided by Beijing InnoChem Science & Technology Co., Ltd. The CO2 (99.99%) and H2 (99.99%) were purchased Beijing Analytical Instrument Company.

Catalytic reaction. All the reactions were conducted in a 16 ml Teon-lined stainless steel batch reactor equipped with a magnetic stirrer. The inner diameter of the reactor was 18 mm. In a typical experiment, known amounts of Ru and/or Rh catalysts, imidazole or another ligand, LiI or another promoter, methanol or (13CH3OH, CH318OH or CH3OD if used), and 2 ml DMI or another solvent were loaded sequentially into the reactor. The reactor was purged two times with CO2 of

1 MPa in ice-water. At room temperature, CO2 in the cylinder was charged into the reactor to desired pressure, and the inlet valve of CO2 was closed. Then H2 was charged into the reactor until suitable total pressure was reached. The reactor was placed in an air bath of constant temperature, and the magnetic stirrer was started at 800 r.p.m. After reaction, the reactor was cooled in an ice-water bath for 1 h, the residual gas was released slowly and collected in a gasbag. The liquid mixture was analysed by GC (Agilent 7890B) equipped with a ame ionization detector and an HP-5 capillary column (0.32 mm in diameter, 30 m in length) using toluene as the internal standard. Identication of the liquid products was done using a GCMS (SHIMADZU-QP2010) as well as by comparing the retention times of the standards in the GC traces. The yields of the products were calculated from the GC data. The gaseous samples were analysed using a GC (Agilent 4890D) equipped with a TCD detector and a packed column (Carbon molecular sieve TDX-01, 3 mm in diameter and 1 m in length) using Argon as the carry gas.

Recycling test. After reaction, the reactor was cooled down using an ice bath and the residual gas was released. The amount of product was determined as discussed above. Then the acetic acid formed and the unreacted methanol in the reactor were removed in a vacuum oven at 85 C for 5 h. GC analysis conrmed the complete removal of the acetic acid at this condition. The catalytic system (catalyst

promoter DMI) was used directly for the next run.

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Acknowledgements

We thank the National Natural Science Foundation of China (21373234, 21533011, 21133009, 21321063) and the Chinese Academy of Sciences (KJCX2.YW.H30). We are grateful for the help of Dr Junfeng Xiang in the NMR analysis.

Author contributions

Q.Q. and B.H. proposed the project, designed and conducted the experiments and wrote the manuscript, J.Z. and M.C. performed some experiments and discussed the work.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/naturecommunications

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How to cite this article: Qian, Q. et al. Synthesis of acetic acid via methanol hydrocarboxylation with CO2 and H2. Nat. Commun. 7:11481 doi: 10.1038/

ncomms11481 (2016).

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