Meng
Cui
ab,
Qingli
Qian
*a,
Zhenhong
He
a,
Zhaofu
Zhang
a,
Jun
Ma
a,
Tianbin
Wu
a,
Guanying
Yang
a and
Buxing
Han
*a
aBeijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: qianql@iccas.ac.cn; hanbx@iccas.ac.cn; Tel: +86-10-62562821
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
First published on 18th April 2016
Iodides are commonly used promoters in C2+OH synthesis from CO2/CO hydrogenation. Here we report the highly efficient synthesis of C2+OH from CO2 hydrogenation over a Ru3(CO)12–Co4(CO)12 bimetallic catalyst with bis(triphenylphosphoranylidene)ammonium chloride (PPNCl) as the cocatalyst and LiBr as the promoter. Methanol, ethanol, propanol and isobutanol were formed at milder conditions. The catalytic system had a much better overall performance than those of reported iodide promoted systems because PPNCl and LiBr cooperated very well in accelerating the reaction. LiBr enhanced the activity and PPNCl improved the selectivity, and thus both the activity and selectivity were very high when both of them were used simultaneously. In addition, the catalyst could be reused for at least five cycles without an obvious change of catalytic performance.
Alcohols are important bulk chemicals. The synthesis of alcohols from CO2 hydrogenation has received much attention, but research progress was mainly focused on the synthesis of methanol.3 Higher alcohols (C2+OH) are more desirable in many cases, especially as fuel and fuel additives. However, the synthesis of C2+OH via CO2 hydrogenation is obviously a challenge. The acquired results for this topic are mostly focused on heterogeneous catalysis. For example, a CoMoS based catalyst produced 35.6% of C2+OH in alcohol products at 340 °C.4 An alkali-promoted Mo/SiO2 catalyst could generate alcohols at 250 °C with a C2+OH selectivity of 75.6%.5 A [Rh10Se]/TiO2 catalyst could catalyze the reaction at 350 °C with ethanol selectivity of 83%.6 A combined Rh–Fe–Cu based catalyst could produce ethanol with ethanol selectivity of about 70%.7 A C2+OH selectivity of 87.1% could be reached when modified K/Cu–Zn–Fe catalysts were used at 300 °C.8 It was found that water could promote C2+OH generation when a Pt/Co3O4 catalyst was used in a mixed solvent of water and DMI.9 In general, the activity and selectivity of C2+OH over heterogeneous catalysts were low and harsh reaction conditions were required.
Homogeneous catalysis is known for its higher catalytic efficiency compared to heterogeneous catalysis. But it has rarely been reported in CO2 hydrogenation to C2+OH, because the metal complexes are usually unstable in the reaction conditions. In the limited cases of homogeneous catalysis, iodides were used as a promoter and played a key role in the formation of C2+OH.10 But the catalytic systems suffer from low selectivity and/or low activity of C2+OH formation. For example, in the Ru–Co–KI system, only methanol and ethanol were generated and the ethanol selectivity was low (26.4%).10a When noble Rh was used to replace Co, the selectivity of C2+OH was improved but the activity was still low.10b Moreover, it is well known that iodides are the most commonly used promoters in C2+OH synthesis from CO2/CO hydrogenation because of their stronger nucleophilicity, which is favourable for generating larger alcohols.10–12 Bromides are much more stable and cheaper than iodides, but poor performances for generating higher alcohols limit their application in the reaction. Obviously, exploring more efficient and cheaper catalytic systems for the reaction is an interesting topic.
Herein we report the highly efficient synthesis of C2+OH from CO2 hydrogenation promoted by bromide using a Ru–Co bimetallic catalyst with PPNCl as the cocatalyst (Scheme 1). Methanol, ethanol, propanol and isobutanol were generated at milder conditions. The catalytic system had both a high activity and selectivity of C2+OH compared to those of iodide promoted reactions. In addition, the catalytic system could be recycled and reused at least five times without an obvious change of catalytic performance. As far as we know, this is the first work to use PPNCl as a cocatalyst in C2+OH synthesis via CO2 hydrogenation and we found that LiBr is a better promoter than LiI because PPNCl and LiBr cooperate effectively in enhancing the activity and selectivity.
Entry | Catalyst precursors | Promoter | Cocatalyst | Solvent | STYb [C mmol L−1 h−1] | C2+OH Sel. [%] | ||||
---|---|---|---|---|---|---|---|---|---|---|
Methanol | Ethanol | Propanol | Isobutanol | Total | ||||||
a Reaction conditions: 40 μmol Ru catalyst and 20 μmol Co catalyst (based on the metal), 4 mmol promoter, 0.15 mmol cocatalyst, 2 mL solvent, 3 MPa CO2 and 6 MPa H2 (at room temperature), 200 °C, 12 h. b STY stands for space time yield (C mmol L−1 h−1). The STY was determined by GC analysis using toluene as the internal standard. c Black precipitate was observed after the reaction. Sel.: selectivity. | ||||||||||
1 | Ru3(CO)12, Co4(CO)12 | LiBr | PPNCl | DMI | 3.1 | 29.5 | 0.6 | 0.5 | 33.7 | 90.8 |
2 | Ru3(CO)12, Co4(CO)12 | — | PPNCl | DMI | 10.2 | 0.7 | 0.1 | 0 | 11.0 | 7.3 |
3 | Ru3(CO)12, Co4(CO)12 | LiBr | — | DMI | 9.5 | 19.2 | 0.4 | 0.3 | 29.4 | 67.7 |
4c | Ru3(CO)12, Co4(CO)12 | — | — | DMI | 0.5 | 0 | 0 | 0 | 0.5 | 0 |
5 | Ru3(CO)12, Co4(CO)12 | LiCl | PPNCl | DMI | 35.5 | 13.1 | 1.0 | 0 | 49.6 | 28.4 |
6 | Ru3(CO)12, Co4(CO)12 | LiI | PPNCl | DMI | 0.4 | 1.9 | 3.9 | 0 | 6.2 | 93.5 |
7c | Ru3(CO)12, Co4(CO)12 | LiBF4 | PPNCl | DMI | 2.7 | 0.3 | 0 | 0 | 3.0 | 10.0 |
8 | Ru3(CO)12, Co4(CO)12 | NaBr | PPNCl | DMI | 42.4 | 2.5 | 0 | 0 | 44.9 | 5.6 |
9 | Ru3(CO)12, Co4(CO)12 | KBr | PPNCl | DMI | 47.5 | 2.6 | 0 | 0 | 50.1 | 5.2 |
10 | Ru3(CO)12, Co4(CO)12 | KI | PPNCl | DMI | 44.4 | 4.5 | 0 | 0 | 48.9 | 9.2 |
11 | Ru3(CO)12 | LiBr | PPNCl | DMI | 12.1 | 20.8 | 0 | 0 | 32.9 | 63.2 |
12 | Co4(CO)12 | LiBr | PPNCl | DMI | 0.3 | 0 | 0 | 0 | 0.3 | 0 |
13 | Ru3(CO)12, Co4(CO)12 | LiBr | LiCl | DMI | 1.3 | 5.9 | 0.5 | 0.2 | 7.9 | 83.5 |
14 | Ru3(CO)12, Co4(CO)12 | LiBr | TBACl | DMI | 8.9 | 23.4 | 0.8 | 0.7 | 33.8 | 73.7 |
15 | Ru3(CO)12, Co4(CO)12 | LiBr | TPPTS | DMI | 10.1 | 13.3 | 0.2 | 0 | 23.6 | 57.2 |
16 | Ru3(CO)12, Co4(CO)12 | LiBr | PPh3 | DMI | 10.9 | 13.7 | 0.3 | 0 | 24.9 | 56.2 |
17 | Ru3(CO)12, Co4(CO)12 | LiBr | Imidazole | DMI | 5.1 | 11.5 | 0 | 0 | 16.6 | 69.3 |
18 | Ru3(CO)12, Co4(CO)12 | LiBr | PPNCl | NMP | 8.7 | 13.6 | 4.7 | 4.0 | 31.0 | 71.9 |
19 | Ru3(CO)12, Co4(CO)12 | LiBr | PPNCl | DMF | 8.2 | 0 | 0 | 0 | 8.2 | 0 |
20 | Ru3(CO)12, Co4(CO)12 | LiBr | PPNCl | [Bmim]NTf2 | 1.1 | 0 | 0 | 0 | 1.1 | 0 |
21 | Ru3(CO)12, Co4(CO)12 | LiBr | PPNCl | 1-Methylpiperidine | 0 | 0 | 0 | 0 | 0 | 0 |
22 | Ru3(CO)12, Co4(CO)12 | LiBr | PPNCl | THF | 71.4 | 2.3 | 0 | 0 | 73.7 | 3.1 |
23 | Ru3(CO)12, Co4(CO)12 | LiBr | PPNCl | Cyclohexane | 1.2 | 0.1 | 0 | 0 | 1.3 | 7.7 |
24c | Ru3(CO)12, Co4(CO)12 | LiBr | PPNCl | H2O | 2.1 | 1.1 | 0.1 | 0 | 3.3 | 36.4 |
25 | Ru3(CO)12, Co2(CO)8 | LiBr | PPNCl | DMI | 10.0 | 22.6 | 1.1 | 0 | 33.7 | 70.3 |
26c | RuBr3, CoBr2 | LiBr | PPNCl | DMI | 4.2 | 8.5 | 0 | 0 | 12.7 | 66.9 |
27 | (PPh3)3RuCl2, (PPh3)3CoCl | LiBr | PPNCl | DMI | 3.1 | 4.2 | 0 | 0 | 7.3 | 57.5 |
The LiBr promoter played a crucial role in accelerating the reaction. Without LiBr, both the activity and selectivity of the C2+OH synthesis were very low (entry 2). The LiBr also enhanced the stability of the catalyst. When LiBr was used without PPNCl, the catalyst was also very active, but the selectivity to C2+OH was much lower (entry 3). This indicates that LiBr and PPNCl cooperated very well for producing C2+OH. LiBr enhanced the activity and PPNCl improved the selectivity. Thus, both the activity and selectivity were very high when both of them were present. A black metal precipitate was observed if both LiBr and PPNCl were absent (entry 4). We also tested the promoters with other cations (Na+ and K+) and anions (Cl−, I− and BF4−), but the activity and/or selectivity of the catalyst were poor (entries 5–10). The contribution of the lithium halide to C2+OH selectivity followed the order of: LiI > LiBr > LiCl, while their contribution to the activity (STY) follows the reverse order (entries 1, 5 and 6). The data show that the selectivities of the catalytic systems with LiBr and LiI were similar, but the activity of the catalytic system with LiBr was much higher (entries 1 and 6). The excellent performance of the catalytic system with LiBr can be attributed mainly to the presence of PPNCl, which enhanced the selectivity significantly, whilst retaining the high activity (entries 1 and 3). Thus LiBr was the best promoter for the above Ru–Co–PPNCl catalyst in this reaction. In the previous work, the single Ru catalyst using an iodide promoter had very poor performance for producing C2+OH.10 While in this work, the ethanol selectivity could reach 63.2% when the Ru–PPNCl catalyst was promoted by LiBr (entry 11). The Co catalyst itself showed very poor catalytic performance (entry 12). But, when it was combined with Ru catalyst, propanol and isobutanol were produced and the selectivity of C2+OH increased to 90.8% (entry 1). Hence a synergistic effect existed in the Ru–Co–PPNCl catalysts. The PPNCl was important for the catalytic properties. Without PPNCl, the catalytic performance, especially the C2+OH selectivity, was much lower (entry 3). We also tried other cocatalysts, but the results were not satisfactory (entries 13–17). The X-ray photoelectron spectroscopy (XPS) study revealed the coordination between Ru3(CO)12 and Cl− in PPNCl (Fig. S2†). The coordination increased the electron density of the Ru atom and could promote the oxidative addition of alkyl halides to the active center, which is usually a key step in chain growth reactions.12 The superiority of PPNCl over other chlorides (entries 1, 13 and 14) may be due to the big steric hindrance of the substituents around the N atom, which weakened the electrostatic attraction between PPN+ and Cl− and the nucleophilicity of the Cl− was enhanced accordingly.13,14 After screening the solvents, we found that DMI was the best for the reaction (entries 18–24). We also tried other Ru–Co combinations, but the efficiency was lower than that of Ru3(CO)12 and Co4(CO)12 (entries 1 and 25–27). The Co2(CO)8 adopted in the literature10a was not suitable here (entry 25).
The impact of catalyst dosage and gas pressure on the reaction was studied and the results are given in Table 2. When the total dosage of Ru and Co catalysts was fixed, the optimized ratio of Ru/Co was 2:1 (entries 1–4 of Table 2). As expected, the increase of the catalyst dosage enhanced the catalytic efficiency, but it was less sensitive when the dosage was large enough (entries 3, 5 and 6 of Table 2). At a fixed ratio of CO2 and H2, the STY of the C2+OH increased rapidly with elevating pressure (entries 3, 7 and 8 of Table 2). The optimal ratio of CO2 and H2 was 1:2 at a given total pressure (entries 3 and 9–11 of Table 2).
Entry | Ru/Co [μmol] | pCO2 [MPa] | pH2 [MPa] | STY [C mmol L−1 h−1] | C2+OH Sel. [%] | ||||
---|---|---|---|---|---|---|---|---|---|
Methanol | Ethanol | Propanol | Isobutanol | Total | |||||
a Reaction conditions: Ru3(CO)12 and Co4(CO)12 were used as catalyst precursors and their dosage was based on the metal, 4 mmol LiBr, 0.15 mmol PPNCl, 2 mL DMI, 200 °C, 12 h. Sel.: selectivity. | |||||||||
1 | 20/40 | 3 | 6 | 7.5 | 18.8 | 0.2 | 0.1 | 26.6 | 71.8 |
2 | 30/30 | 3 | 6 | 7.3 | 22.3 | 0.5 | 0.3 | 30.4 | 76.0 |
3 | 40/20 | 3 | 6 | 3.1 | 29.5 | 0.6 | 0.5 | 33.7 | 90.8 |
4 | 45/15 | 3 | 6 | 9.5 | 23.5 | 0.6 | 0.4 | 34.0 | 72.1 |
5 | 20/10 | 3 | 6 | 9.9 | 17.9 | 0.4 | 0.2 | 28.4 | 65.1 |
6 | 60/30 | 3 | 6 | 2.9 | 30.5 | 0.8 | 0.7 | 34.9 | 91.7 |
7 | 40/20 | 1 | 2 | 0.4 | 0 | 0 | 0 | 0.4 | 0 |
8 | 40/20 | 2 | 4 | 1.6 | 12.8 | 0.6 | 0.6 | 15.6 | 89.7 |
9 | 40/20 | 2.25 | 6.75 | 13.2 | 25.9 | 0.7 | 0.4 | 40.2 | 67.2 |
10 | 40/20 | 4.5 | 4.5 | 3.4 | 17.8 | 1.0 | 0.4 | 22.6 | 85.0 |
11 | 40/20 | 6 | 3 | 1.7 | 6.1 | 0.3 | 0.2 | 8.3 | 79.5 |
Fig. 1a shows that the STY and selectivity of C2+OH were enhanced significantly by increasing the LiBr dosage from 0–4 mmol. When LiBr usage was further increased the STY of C2+OH decreased notably. Thus, the suitable dosage of LiBr was 4 mmol. In contrast, the STY of methanol increased evidently when the LiBr usage increased from 0–2 mmol, whereas it decreased drastically with further increasing of the LiBr dosage. It is obvious that LiBr played a key role in generating methanol and transforming it into C2+OH. In addition, Br− would occupy the active sites of the catalyst and inhibit the reaction when its dosage was high enough. The effect of PPNCl dosage on the reaction is depicted in Fig. 1b. With the increase of PPNCl dosage, the STY of C2+OH improved gradually, but it dropped when the dosage exceeded 0.15 mmol, which may be due to occupation of the active sites. In contrast, the STY of methanol always decreased with increasing the dosage of PPNCl. Hence the appropriate dosage of PPNCl was 0.15 mmol. The above results also support the conclusion that the PPNCl promotes the transformation of methanol into C2+OH.
Fig. 1 Effect of reaction conditions (a–d) and results of recycling tests (e) over 40 μmol Ru3(CO)12 and 20 μmol Co4(CO)12 (based on the metal) in DMI under 9 MPa of the initial pressure (CO2/H2 = 1/2): (a) effect of LiBr dosage, 0.15 mmol PPNCl, 200 °C, 12 h; (b) effect of PPNCl dosage, 4 mmol LiBr, 200 °C, 12 h; (c) effect of reaction temperature, 4 mmol LiBr, 0.15 mmol PPNCl, 12 h; (d) effect of reaction time, 4 mmol LiBr, 0.15 mmol PNNCl, 200 °C; (e) the reaction condition is the same as that of entry 1 in Table 1. |
The impact of reaction temperature is demonstrated in Fig. 1c. Methanol and ethanol began to emerge at 140 °C. A minor amount of propanol and isobutanol appeared at 160 °C. With the increase of reaction temperature, the STY and selectivity of C2+OH enhanced evidently, but it became insensitive when the temperature was above 200 °C. So a suitable reaction temperature was 200 °C. The time course of the reaction is shown in Fig. 1d. Methanol and ethanol were formed in 1 h. At 3 h and 6 h, propanol and isobutanol began to appear, respectively. With time going on, the yield and selectivity of C2+OH increased rapidly, while the methanol content kept decreasing. After 12 h, the change of methanol content was not evident and growth of the ethanol yield became slower, and at the same time, the yield of propanol and isobutanol increased. As a whole, the variation of C2+OH selectivity after 12 h was not obvious. Fig. 1d also demonstrates that the amount of C2+OH showed nearly a linear increase with reaction time, suggesting that the water generated in situ had no considerable influence on the activity of the catalyst.
We also studied the recyclability of the catalytic system. After the reaction, the alcohols generated in the reaction were removed under vacuum, which was confirmed by gas chromatography. Then the catalytic system was reused directly for the next run. The results of the recycling test are shown in Fig. 1e. The STY and selectivity of C2+OH did not decrease obviously after five cycles (12 h each cycle), indicating that the catalytic system had excellent stability and reusability.
As is shown in Fig. 1d, methanol is firstly generated and gradually consumed. At the same time, the C2+OH increased accordingly. These phenomena suggest that methanol was first formed from CO2 hydrogenation, and then acted as an intermediate to produce larger alcohols. To support this assumption, we conducted tracer experiments by adding a small amount of 13CH3OH into the reaction. The GC-MS data indicated that 13C appeared in all of the target C2+OH (Fig. S3†), supporting the above argument. We also tried 13C2H5OH and obtained a similar result (Fig. S4†). Thus it can be concluded that in CO2 hydrogenation to generate the alcohols, the small alcohols acted as building blocks for the larger ones.
The possible mechanism for the synthesis of C2+OH from CO2 hydrogenation is depicted in Scheme 2. Methanol and CO were generated via Ru catalyzed CO2 hydrogenation (Step 1). The methanol generated in situ was further converted into ethanol via a hydrocarbonylation reaction (Steps 2–5). The generation of propanol from ethanol should follow similar steps. Minor isobutanol was formed via the Guerbet reaction between methanol and propanol.15 The Ru–halide catalyzed synthesis of methanol and CO from CO2 hydrogenation has been reported elsewhere.16 The mechanism of methanol hydrocarboxylation using Ru–Co–iodide systems has been extensively investigated.12 The Co catalyst was mainly responsible for the generation of ethanol and acetaldehyde from methanol hydrocarbonylation, and the Ru catalyst further hydrogenated the acetaldehyde into ethanol. However, in this work, the bromide promoted Ru catalyst predominated the production of ethanol from methanol (entry 11 of Table 1), while the single Co complex could not effectively catalyze the reaction (entry 12 of Table 1). The Co catalyst ([Co]) in this work mainly accelerated the generation of C2+OH in the reaction. The coordination between the active Ru center (Ru*) and Cl− from PPNCl enhanced the electron density of the metal center, which would expedite the oxidative addition step (Step 3).17 Meanwhile, the increase of the electron density on the Ru* could promote the hydrogenation step (Step 5).12
To test the reusability of the catalytic system, the alcohols formed in the reaction were removed under vacuum at 80 °C for 2 h, and then the catalytic system was reused directly for the next run.
Footnote |
† Electronic supplementary information (ESI) available: GC-MS and GC data. See DOI: 10.1039/c6sc01314g |
This journal is © The Royal Society of Chemistry 2016 |