**Recent Advances in Transition Metal-Catalysed Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media**

Wan-Hui Wang and Yuichiro Himeda

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48658

### **1. Introduction**

248 Hydrogenation

178). ISSN: 0021-9517.

4073). ISSN: 0897-4756.

Yang, S. H., & Satterfield, C.N. (1983). Some Effects of Sulfiding of a NiMoAl2O3 Catalyst on its Activity for HDN of quinoline. *J. of Catalysis*, Vol. 81, No. 1, (May 1983), pp. (168-

Zepeda, T.A., Fierro, J.L.G., Pawelec, B., Nava, R., Klimova, T., Fuentes, G.A., & Halachev, T. (2005). Synthesis and Characterization of Ti-HMS and CoMo/Ti-HMS Oxide Materials with Varying Ti Content. *Chemistry of Materials.* Vol. 17, No. 16, (May 2005), pp. (4062–

> The excessive combustion of fossil fuels leads to enormous emissions of carbon dioxide which is the major greenhouse gases and significantly contributes to global warming. Since the middle of 20th century, the atmospheric concentration of CO2 has risen remarkably. With the development of human society and increase in energy demand, emissions of CO2 are increasing dramatically. To reduce the server environmental impact, scientists have paid considerable effort to prohibiting the increase of atmospheric CO2 concentration. CO2 is an attractive C1 resource because it is non-toxic, nonflammable, and abundant. Transforming of carbon dioxide to useful chemicals, fuels, and materials have attracted increasing attention because it could reduce the dependence on diminishing fossil oil as well as mitigate CO2 increase. However, utilizing carbon dioxide is still a challenge research field due to its high stability (∆*G*° 298 = −394.36 kJ mol−1). In the last decades, the homogeneous catalytic hydrogenation of CO2 has been widely studied. There are some reviews related to this subject.(Leitner et al., 1998; Jessop et al., 2004; Himeda, 2007; Jessop, 2007; Federsel et al., 2010b; Wang et al., 2011) Besides formic acid, theoretically, CO2 can be hydrogenated to multiple compounds such as, formamides, formaldehyde, methanol, and methane (Eq 1). However, generation of these compounds typically require harsher conditions which make most homogeneous catalysts deactivating, increase the energy cost and make these reductions economically unfavourable. In this chapter, we will focus on the hydrogenation of CO2 to formic acid or formate which is relatively easy to achieve. Especially, formic acid has recently been recognized as a feasible hydrogen vector. Hydrogenation of CO2 to formic acid, combined with the reverse reaction (ie. decomposition of formic acid) is considered as one promising method of hydrogen storage (Eq 1, step 1).

© 2012 Wang and Himeda, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Wang and Himeda, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

$$\text{ClO}\_{2} \xrightarrow[\text{H}\_{2}\text{O}^{2-}]{\text{H}\_{2}} \text{H}\_{2}\text{O} \xrightarrow[\text{H}\_{2}\text{O}^{2-}]{\text{H}\_{2}} \text{H}\_{2}\text{O} \xrightarrow[\text{H}\_{2}\text{O}^{2-}]{\text{H}\_{2}} \text{O} \\ \text{HO} \xrightarrow[\text{H}\_{2}\text{O}^{2-}]{\text{H}\_{2}} \text{O}\_{2} \xrightarrow[\text{H}\_{2}\text{O}^{2-}]{\text{H}\_{2}} \text{O}\_{2} \quad \text{(1)}$$

Recent Advances in Transition Metal-Catalysed

Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media 251

**2. Hydrogenation with Ru and Rh complexes** 

this system is the bicarbonate anion.(Laurenczy et al., 2000)

and their performance are listed in Table 1.

**2.1. Phosphorous ligands** 

In the pioneering work of Inoue et al., the famous Wilkinson catalyst (RhCl(PPh3)3) and the Ru analogue (RuCl(PPh3)3) were used and showed much better results than other catalysts of Pd, Ni, and Ir.(Inoue et al., 1976) Following this work, a variety of Rh and Ru catalyst based on various phosphorus ligands were developed and applied in the hydrogenation of CO2. Recently, N-based ligands have been also investigated for this purpose and achieved great success. Most of the highly efficient Ru and Rh catalysts as well as some Ir complexes

In 1993, Leitner et al. reported the first homogenous hydrogenation of CO2 with water soluble rhodium–phosphane complexes in aqueous solutions.(Gassner & Leitner, 1993) The reaction was carried out in an aqueous solution of amine at room temperature under 40 atm of H2/CO2 (1/1). Using dimethylamine as an additive, the Rh complex [RhCl(tppts)3]/tppts (tppts = tris(3-sulfonatophenyl)phosphine) can provide 1.76 M of formic acid with TON of 3440 which is the highest at that time. In 1999, Joó et al. have reported the hydrogenation using inorganic base such as NaHCO3 and CaCO3 instead of the organic amine as additive.(Joó et al., 1999) Among the different series of Ru and Rh catalysts, [RhCl(tppms)3] (tppms = 3-sulfonatophenyldiphenylphosphine) exhibited the better activity than others, and gave a turnover frequency (TOF) of 262 h−1. Based on the equilibrium of CO2 + H2O HCO3<sup>−</sup> + H+, they proposed that HCO3<sup>−</sup> may be the real substrate in the catalytic cycle. In 2000, Laurenczy et al. reported hydrogenation with moderate activity using [RuCl2(PTA)4] (PTA = 1,3,5-triaza-7-phosphaadamantane) in an aqueous solution at 25-80 °C under 20 bar CO2 and 60 bar H2. In contrast to the other works, they found that slightly acidic and neutral conditions are preferable for the reaction rate. In case of 10%HCO3−/90%CO2, they obtained the maximum TOF of 807 h−1 and they suggested that the real substrate of hydrogenation in

In 2003, Joó et al. reported the hydrogenation of bicarbonate with [RuCl2(tppms)2]2 (tppms = sodium diphenylphosphinobenzene-3-sulfonate) at 50 °C and 10 bar H2, a TOF of 54 h−1 was obtained.(Elek et al., 2003) Their results suggest that bicarbonate is more reactive than hydrated CO2. Interestingly, in presence of 5 bar CO2 the reaction was about 10% slower than that without CO2. This result is in contrast to the reaction with [RuCl2(PTA)4], which increased significantly with increasing of CO2 pressure. Using this complex [RuCl2(tppms)2]2, they have achieve a TOF of 9600 h−1, the highest rate in pure aqueous

solutions at that time, at 80 °C under H2/CO2 (60/35 bar) in a 0.3 M NaHCO3 solution.

Most recently, Beller et al. reported hydrogenation of bicarbonate in H2O/THF with in situ catalyst of [RuCl2(C6H6)]2/dppm (dppm = 1,2-bis(diphenylphosphino)methane).(Boddien et al., 2011) In the presence of CO2, the reaction gave higher TON than that in the absence of CO2. In addition, the catalyst can also catalyse the dehydrogenation of formate. Consequently, they pronounced the first hydrogen storage based on interconversion of formate and bicarbonate. Soon after that, Joó et al. used [RuCl2(tppms)2]2/tppms to catalyse

Complexes based on most of group VIII transition metals such as Pd, Ni, Rh, Ru, Ir et al. can be used to catalysis CO2 hydrogenation. Among these catalysts, Rh, Ru, and most recently Ir complexes were found to be most effective. Besides transition-metal catalyst, solvent is also important for optimizing the reaction rate. The homogeneously catalytic hydrogenation of CO2 to formic acid was firstly reported in 1976 by Inoue et al.(Inoue et al., 1976) They found the reaction was accelerated by adding small amounts of H2O. However, in the early years, water-insoluble phosphine ligands are generally employed. Due to the insolubility of the phosphorous complexes in water, the homogeneous hydrogenation of CO2 generally proceeded in organic solvents, such as DMSO, with water less than 20%. Until 1993, Leitner et al. reported the first water soluble rhodium catalysts which achieve the high turnover number (TON) of 3440 under relatively mild conditions.(Gassner & Leitner, 1993) Noyori and Jessop et al. have demonstrated supercritical CO2 is an effective solvent due to the enormous concentration of CO2 and H2 and obtained highest catalytic performance at that time.(Jessop et al., 1994; Jessop et al., 1996; Munshi et al., 2002) Compared to reduction of CO2 in organic solvent and supercritical CO2, the homogeneous hydrogenation of CO2 to formic acid in the green solvent—water has recently achieved great success and attracted much more attention. Despite H2 is less soluble in water, it is still considered to be a preferred solvent because water is abundant, inexpensive, and eco-friendly. More importantly, hydrogenation of CO2 in water is considerably favoured (G° = −4 kJ mol−1) compared to the reaction in gas phase (G° = +32.9 kJ mol−1). In addition, excellent activity usually requires basic additives, such as NaOH, NaHCO3, Na2CO3 and amines, which can absorb the generated proton and make the reaction thermodynamically favourable (Scheme 1).


**Scheme 1.** The thermodynamics of hydrogenation of carbon dioxide to formic acid/formate.

In this chapter, we review the state-of-the-art in homogeneous CO2 hydrogenation to formic acid or formate in water; discuss the design and synthesis of highly effective water soluble complexes, as well as the catalytic mechanism. We also present the latest strategy for recycle and reuse of homogenous catalyst.

### **2. Hydrogenation with Ru and Rh complexes**

In the pioneering work of Inoue et al., the famous Wilkinson catalyst (RhCl(PPh3)3) and the Ru analogue (RuCl(PPh3)3) were used and showed much better results than other catalysts of Pd, Ni, and Ir.(Inoue et al., 1976) Following this work, a variety of Rh and Ru catalyst based on various phosphorus ligands were developed and applied in the hydrogenation of CO2. Recently, N-based ligands have been also investigated for this purpose and achieved great success. Most of the highly efficient Ru and Rh catalysts as well as some Ir complexes and their performance are listed in Table 1.

### **2.1. Phosphorous ligands**

(1)

250 Hydrogenation

1).

and reuse of homogenous catalyst.

Complexes based on most of group VIII transition metals such as Pd, Ni, Rh, Ru, Ir et al. can be used to catalysis CO2 hydrogenation. Among these catalysts, Rh, Ru, and most recently Ir complexes were found to be most effective. Besides transition-metal catalyst, solvent is also important for optimizing the reaction rate. The homogeneously catalytic hydrogenation of CO2 to formic acid was firstly reported in 1976 by Inoue et al.(Inoue et al., 1976) They found the reaction was accelerated by adding small amounts of H2O. However, in the early years, water-insoluble phosphine ligands are generally employed. Due to the insolubility of the phosphorous complexes in water, the homogeneous hydrogenation of CO2 generally proceeded in organic solvents, such as DMSO, with water less than 20%. Until 1993, Leitner et al. reported the first water soluble rhodium catalysts which achieve the high turnover number (TON) of 3440 under relatively mild conditions.(Gassner & Leitner, 1993) Noyori and Jessop et al. have demonstrated supercritical CO2 is an effective solvent due to the enormous concentration of CO2 and H2 and obtained highest catalytic performance at that time.(Jessop et al., 1994; Jessop et al., 1996; Munshi et al., 2002) Compared to reduction of CO2 in organic solvent and supercritical CO2, the homogeneous hydrogenation of CO2 to formic acid in the green solvent—water has recently achieved great success and attracted much more attention. Despite H2 is less soluble in water, it is still considered to be a preferred solvent because water is abundant, inexpensive, and eco-friendly. More importantly, hydrogenation of CO2 in water is considerably favoured (G° = −4 kJ mol−1) compared to the reaction in gas phase (G° = +32.9 kJ mol−1). In addition, excellent activity usually requires basic additives, such as NaOH, NaHCO3, Na2CO3 and amines, which can absorb the generated proton and make the reaction thermodynamically favourable (Scheme

**Scheme 1.** The thermodynamics of hydrogenation of carbon dioxide to formic acid/formate.

In this chapter, we review the state-of-the-art in homogeneous CO2 hydrogenation to formic acid or formate in water; discuss the design and synthesis of highly effective water soluble complexes, as well as the catalytic mechanism. We also present the latest strategy for recycle In 1993, Leitner et al. reported the first homogenous hydrogenation of CO2 with water soluble rhodium–phosphane complexes in aqueous solutions.(Gassner & Leitner, 1993) The reaction was carried out in an aqueous solution of amine at room temperature under 40 atm of H2/CO2 (1/1). Using dimethylamine as an additive, the Rh complex [RhCl(tppts)3]/tppts (tppts = tris(3-sulfonatophenyl)phosphine) can provide 1.76 M of formic acid with TON of 3440 which is the highest at that time. In 1999, Joó et al. have reported the hydrogenation using inorganic base such as NaHCO3 and CaCO3 instead of the organic amine as additive.(Joó et al., 1999) Among the different series of Ru and Rh catalysts, [RhCl(tppms)3] (tppms = 3-sulfonatophenyldiphenylphosphine) exhibited the better activity than others, and gave a turnover frequency (TOF) of 262 h−1. Based on the equilibrium of CO2 + H2O HCO3<sup>−</sup> + H+, they proposed that HCO3<sup>−</sup> may be the real substrate in the catalytic cycle. In 2000, Laurenczy et al. reported hydrogenation with moderate activity using [RuCl2(PTA)4] (PTA = 1,3,5-triaza-7-phosphaadamantane) in an aqueous solution at 25-80 °C under 20 bar CO2 and 60 bar H2. In contrast to the other works, they found that slightly acidic and neutral conditions are preferable for the reaction rate. In case of 10%HCO3−/90%CO2, they obtained the maximum TOF of 807 h−1 and they suggested that the real substrate of hydrogenation in this system is the bicarbonate anion.(Laurenczy et al., 2000)

In 2003, Joó et al. reported the hydrogenation of bicarbonate with [RuCl2(tppms)2]2 (tppms = sodium diphenylphosphinobenzene-3-sulfonate) at 50 °C and 10 bar H2, a TOF of 54 h−1 was obtained.(Elek et al., 2003) Their results suggest that bicarbonate is more reactive than hydrated CO2. Interestingly, in presence of 5 bar CO2 the reaction was about 10% slower than that without CO2. This result is in contrast to the reaction with [RuCl2(PTA)4], which increased significantly with increasing of CO2 pressure. Using this complex [RuCl2(tppms)2]2, they have achieve a TOF of 9600 h−1, the highest rate in pure aqueous solutions at that time, at 80 °C under H2/CO2 (60/35 bar) in a 0.3 M NaHCO3 solution.

Most recently, Beller et al. reported hydrogenation of bicarbonate in H2O/THF with in situ catalyst of [RuCl2(C6H6)]2/dppm (dppm = 1,2-bis(diphenylphosphino)methane).(Boddien et al., 2011) In the presence of CO2, the reaction gave higher TON than that in the absence of CO2. In addition, the catalyst can also catalyse the dehydrogenation of formate. Consequently, they pronounced the first hydrogen storage based on interconversion of formate and bicarbonate. Soon after that, Joó et al. used [RuCl2(tppms)2]2/tppms to catalyse the hydrogenation of bicarbonate as well as the dehydrogenation of formate, and constructed a simple, rechargeable hydrogen storage device.(Papp et al., 2011) Similar with the system reported by Beller et al., but Joó's system is in pure aqueous solutions and require no organic solvent.

Recent Advances in Transition Metal-Catalysed

Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media 253

**Figure 1.** Ru and Rh catalysts with nitrogenous ligands.

**Table 1.** Hydrogenation of CO2 to formic acid/formate. *a*. The data in parenthesis are initial TOF.

### **2.2. Nitrogenous ligands**

In 2003, Himeda et al. announced the homogenous hydrogenation of CO2 in water with a series of 2,2'-bipyridine- and 1,10-phenathroline-based Ru and Rh catalysts including [Cp\*Rh(bpy)Cl]Cl (Cp\* = pentamethylcyclopentadinyl; bpy = 2,2'-bipyridine), [Cp\*Rh(4,4'- Me-bpy)Cl]Cl (4,4'-Me-bpy = 4,4'-dimethyl-2,2'-bipyridine), [(6-C6Me6)Ru(phen)Cl]Cl (phen = 1,10-phenathroline) etc.(Himeda et al., 2007a) Among these catalysts, complex **4** based on 4,7-dihydroxyl-1,10-phenanthroline (DHPT) exhibited high activity and reached a TON of 2400 in a 1 M KHCO3 solution under 4 MPa H2/CO2 (1/1) at 80 °C after 21 h. Soon after that, Ogo et al. reported a mechanistic study of the hydrogenation with similar complexes under acidic conditions. They synthesized a water-soluble ruthenium hydride complex [(6- C6Me6)RuII(bpy)H](SO4) from the reaction of an aqua complex [(6- C6Me6)RuII(bpy)(OH2)](SO4) with NaBH4 in water.(Hayashi et al., 2003) The hydride complex was found to be active in reaction with CO2, but the reaction rate obtained by UV spectroscopy was demonstrated to be very slow. One year later, they achieved the hydrogenation of CO2 to HCOOH in acidic solutions (pH 2.5-5.0) under H2 (5.5 MPa) and CO2 (2.5 MPa) at 40 °C with ruthenium complexes [(6-C6Me6)RuII(bpy)(OH2)](SO4) and [(6- C6Me6)RuII(4,4'-OMe-bpy)(OH2)](SO4) (4,4'-MeO-bpy = 4,4'-dimethoxyl-2,2' bipyridine).(Hayashi et al., 2004) The TON was over 50 after 70 h. The reaction rate reached a maximum value at 40 °C and decreased with further increasing of temperature due to the decomposition of HCOOH at higher temperature. In contrast to the inactivity of this kind of complexes, Himeda et al. achieved significantly higher activity with **1**-**4** (Figure 1 and Table 1) by introducing two strong electron-donating groups into the bipyridine ligands. (Himeda et al., 2004, 2006, 2011) More interestingly, much higher activity was obtained with the iridium analogue (vide infra).

In 2010, Peris et al. used strong electron-donating bis-NHCs ligand (**5** and **6**) to mimic the bipyridine ligand and achieved a high TON of 23,000 with complex **6** at 40 atm H2/CO2 (1/1) and 200 °C in a 1 M KOH solution for 75 h.(Sanz et al., 2010a) It is worth note that they also achieved transfer hydrogenation of CO2 with *i*PrOH using the Ru complex **6**, and obtained the highest TON of 874 so far reported for this type of reaction.

### **3. Hydrogenation with Ir complexes**

Although research into iridium catalysts dates back to 1976(Inoue et al., 1976), the promising catalytic activity of iridium complexes has only recently been discovered. In the pathbreaking work of Inoue et al., the iridium complex H3Ir(PPh3)3 exhibited a lower activity than Rh and Ru analogues. About 20 years later, an iridium catalyst was again applied to CO2 hydrogenation and similar result was observed.(Joó et al., 1999) Rhodium(I) and

#### Recent Advances in Transition Metal-Catalysed Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media 253


252 Hydrogenation

require no organic solvent.

**2.2. Nitrogenous ligands** 

iridium analogue (vide infra).

the hydrogenation of bicarbonate as well as the dehydrogenation of formate, and constructed a simple, rechargeable hydrogen storage device.(Papp et al., 2011) Similar with the system reported by Beller et al., but Joó's system is in pure aqueous solutions and

In 2003, Himeda et al. announced the homogenous hydrogenation of CO2 in water with a series of 2,2'-bipyridine- and 1,10-phenathroline-based Ru and Rh catalysts including [Cp\*Rh(bpy)Cl]Cl (Cp\* = pentamethylcyclopentadinyl; bpy = 2,2'-bipyridine), [Cp\*Rh(4,4'- Me-bpy)Cl]Cl (4,4'-Me-bpy = 4,4'-dimethyl-2,2'-bipyridine), [(6-C6Me6)Ru(phen)Cl]Cl (phen = 1,10-phenathroline) etc.(Himeda et al., 2007a) Among these catalysts, complex **4** based on 4,7-dihydroxyl-1,10-phenanthroline (DHPT) exhibited high activity and reached a TON of 2400 in a 1 M KHCO3 solution under 4 MPa H2/CO2 (1/1) at 80 °C after 21 h. Soon after that, Ogo et al. reported a mechanistic study of the hydrogenation with similar complexes under acidic conditions. They synthesized a water-soluble ruthenium hydride complex [(6- C6Me6)RuII(bpy)H](SO4) from the reaction of an aqua complex [(6- C6Me6)RuII(bpy)(OH2)](SO4) with NaBH4 in water.(Hayashi et al., 2003) The hydride complex was found to be active in reaction with CO2, but the reaction rate obtained by UV spectroscopy was demonstrated to be very slow. One year later, they achieved the hydrogenation of CO2 to HCOOH in acidic solutions (pH 2.5-5.0) under H2 (5.5 MPa) and CO2 (2.5 MPa) at 40 °C with ruthenium complexes [(6-C6Me6)RuII(bpy)(OH2)](SO4) and [(6- C6Me6)RuII(4,4'-OMe-bpy)(OH2)](SO4) (4,4'-MeO-bpy = 4,4'-dimethoxyl-2,2' bipyridine).(Hayashi et al., 2004) The TON was over 50 after 70 h. The reaction rate reached a maximum value at 40 °C and decreased with further increasing of temperature due to the decomposition of HCOOH at higher temperature. In contrast to the inactivity of this kind of complexes, Himeda et al. achieved significantly higher activity with **1**-**4** (Figure 1 and Table 1) by introducing two strong electron-donating groups into the bipyridine ligands. (Himeda et al., 2004, 2006, 2011) More interestingly, much higher activity was obtained with the

In 2010, Peris et al. used strong electron-donating bis-NHCs ligand (**5** and **6**) to mimic the bipyridine ligand and achieved a high TON of 23,000 with complex **6** at 40 atm H2/CO2 (1/1) and 200 °C in a 1 M KOH solution for 75 h.(Sanz et al., 2010a) It is worth note that they also achieved transfer hydrogenation of CO2 with *i*PrOH using the Ru complex **6**, and obtained

Although research into iridium catalysts dates back to 1976(Inoue et al., 1976), the promising catalytic activity of iridium complexes has only recently been discovered. In the pathbreaking work of Inoue et al., the iridium complex H3Ir(PPh3)3 exhibited a lower activity than Rh and Ru analogues. About 20 years later, an iridium catalyst was again applied to CO2 hydrogenation and similar result was observed.(Joó et al., 1999) Rhodium(I) and

the highest TON of 874 so far reported for this type of reaction.

**3. Hydrogenation with Ir complexes** 


**Table 1.** Hydrogenation of CO2 to formic acid/formate. *a*. The data in parenthesis are initial TOF.

ruthenium(II) complexes with a water-soluble phosphine ligand, tppms, showed a TOF up to 262 h−1 in an aqueous solution under mild conditions. However, the iridium complex, [IrCl(CO)(tppms)2], gave no formate product under the same conditions. In 2008, Gonsalvi and Laurenczy et al. reported a half-sandwich iridium complex, **7**, bearing a water-soluble phosphine ligand PTA.(Erlandsson et al., 2008) Compared to the ruthenium and rhodium analogous,(Horváth et al., 2004) it gave a much lower TOF of 22.6 h−1 at 100 °C. These preliminary studies implied that iridium complexes only provide low catalytic activity for the hydrogenation of CO2. However, recent research made a breakthrough and the high catalytic ability of iridium complexes was demonstrated resulting in renewed attention on iridium complexes. The representative iridium catalysts, **7**–**15**, are presented in Figure 2 and the catalytic results are listed in Table 2.

Recent Advances in Transition Metal-Catalysed

/h−<sup>1</sup> TON

Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media 255

TOF*<sup>a</sup>*

/MPa t/h Initial

Catalyst Solvent Additive T/°C Pressure

parenthesis are average TOF.

H2O - 100 10 - 23 - H2O KOH 120 6 57 42,000 190,000 H2O KOH 60 0.1 50 33 376 H2O KOH 60 0.1 50 32 444 H2O KOH 30 0.1 30 3.5 81 H2O KOH 120 6 48 33,000 222,000 H2O/THF KOH 200 5 2 (150,000) 300,000 H2O/THF KOH 120 6 48 (73,000) 3,500,000 H2O KOH 185 5.5 24 (14,500) 348,000 H2O KOH 185 5.5 1 (18,780) 18,780 H2O KOH 80 6 18 - 1600 H2O KOH 200 4 20 - 9500 H2O KOH 200 6 75 (2500) 190,000

**Table 2.** Hydrogenation of CO2 to formic acid/formate using iridium catalysts. *a*. The data in

Under basic conditions, the hydroxyl group can be deprotonated to generate an oxyanion, which is a much stronger electron donor. Therefore, high catalytic activity was achieved by introducing two electron-donating hydroxyl groups onto the bipyridine ligand. Table 3 shows the effect of the hydroxyl group in the half-sandwich bipyridine catalyst, [(CnMen)M(L)Cl]+ (M = Rh, Ir, n = 5; M = Ru, n = 6), on the hydrogenation of CO2. Significant activation of the catalysts was observed. The TONs of the iridium catalysts with hydroxyl groups were 52–103 times greater than those of the unsubstituted catalysts. The electronic substituent effect was investigated using [(CnMen)M(4,4'-R2-2,2'-bpy)Cl]+ (M = Ir, Rh, Ru; R = OH, OMe, Me, H). Note that under basic conditions the hydroxyl group (Hammett constant, p+ = −0.92) was deprotonated to generate an oxyanion, which is a much stronger donor and has a p+ of −2.30. The Hammett plots show a good correlation between the initial TOFs and the p+ values which indicate their electron donating ability (Figure 4). This result suggests that strong donating ability of the substituents lead to high activity of the complexes. On the other hand, the substituent effects on the rhodium and ruthenium complexes, **1** and **2**, were moderate compared to the effect on iridium complex **8** (Figure 4).(Himeda et al., 2011) It is

**Figure 3.** Acid-base equilibrium between hydroxyl and oxyanion forms.

**Figure 2.** Representative iridium catalysts for the hydrogenation of CO2.

Himeda and co-workers achieved a highly efficient iridium catalyst for the hydrogenation of CO2 in H2O through sophisticated ligand design.(Himeda et al., 2004, 2005, 2006, 2007b; Himeda, 2007) At first, they focused on a half-sandwich bipyridine (bpy) rhodium complex, [Cp\*Rh(bpy)X]+, as a prototype catalyst.(Himeda et al., 2003) Preliminary studies showed that this catalyst successfully hydrogenated CO2 in water but in low rate. Based on the rationale that electron-donating ligands should improve the catalytic activity of the complex, a tunable dihydroxylbipyridine (DHBP) ligand was introduced. The acid-base equilibrium between the hydroxyl and oxyanion forms enabled switching of the polarity and electron-donating ability of the ligand thus affecting the catalytic activity and watersolubility of the complex (Figure 3).

Recent Advances in Transition Metal-Catalysed Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media 255


**Table 2.** Hydrogenation of CO2 to formic acid/formate using iridium catalysts. *a*. The data in parenthesis are average TOF.

**Figure 3.** Acid-base equilibrium between hydroxyl and oxyanion forms.

254 Hydrogenation

the catalytic results are listed in Table 2.

**Figure 2.** Representative iridium catalysts for the hydrogenation of CO2.

solubility of the complex (Figure 3).

Himeda and co-workers achieved a highly efficient iridium catalyst for the hydrogenation of CO2 in H2O through sophisticated ligand design.(Himeda et al., 2004, 2005, 2006, 2007b; Himeda, 2007) At first, they focused on a half-sandwich bipyridine (bpy) rhodium complex, [Cp\*Rh(bpy)X]+, as a prototype catalyst.(Himeda et al., 2003) Preliminary studies showed that this catalyst successfully hydrogenated CO2 in water but in low rate. Based on the rationale that electron-donating ligands should improve the catalytic activity of the complex, a tunable dihydroxylbipyridine (DHBP) ligand was introduced. The acid-base equilibrium between the hydroxyl and oxyanion forms enabled switching of the polarity and electron-donating ability of the ligand thus affecting the catalytic activity and water-

ruthenium(II) complexes with a water-soluble phosphine ligand, tppms, showed a TOF up to 262 h−1 in an aqueous solution under mild conditions. However, the iridium complex, [IrCl(CO)(tppms)2], gave no formate product under the same conditions. In 2008, Gonsalvi and Laurenczy et al. reported a half-sandwich iridium complex, **7**, bearing a water-soluble phosphine ligand PTA.(Erlandsson et al., 2008) Compared to the ruthenium and rhodium analogous,(Horváth et al., 2004) it gave a much lower TOF of 22.6 h−1 at 100 °C. These preliminary studies implied that iridium complexes only provide low catalytic activity for the hydrogenation of CO2. However, recent research made a breakthrough and the high catalytic ability of iridium complexes was demonstrated resulting in renewed attention on iridium complexes. The representative iridium catalysts, **7**–**15**, are presented in Figure 2 and

> Under basic conditions, the hydroxyl group can be deprotonated to generate an oxyanion, which is a much stronger electron donor. Therefore, high catalytic activity was achieved by introducing two electron-donating hydroxyl groups onto the bipyridine ligand. Table 3 shows the effect of the hydroxyl group in the half-sandwich bipyridine catalyst, [(CnMen)M(L)Cl]+ (M = Rh, Ir, n = 5; M = Ru, n = 6), on the hydrogenation of CO2. Significant activation of the catalysts was observed. The TONs of the iridium catalysts with hydroxyl groups were 52–103 times greater than those of the unsubstituted catalysts. The electronic substituent effect was investigated using [(CnMen)M(4,4'-R2-2,2'-bpy)Cl]+ (M = Ir, Rh, Ru; R = OH, OMe, Me, H). Note that under basic conditions the hydroxyl group (Hammett constant, p+ = −0.92) was deprotonated to generate an oxyanion, which is a much stronger donor and has a p+ of −2.30. The Hammett plots show a good correlation between the initial TOFs and the p+ values which indicate their electron donating ability (Figure 4). This result suggests that strong donating ability of the substituents lead to high activity of the complexes. On the other hand, the substituent effects on the rhodium and ruthenium complexes, **1** and **2**, were moderate compared to the effect on iridium complex **8** (Figure 4).(Himeda et al., 2011) It is

apparent that the remarkable activation of the iridium DHBP catalyst can be attributed to the strong electron-donating ability of the oxyanion. The maximal catalytic activity (TOF = 42,000 h−1, TON = 190,000) of the iridium DHBP catalyst was obtained at 6 MPa and 120 °C. Moreover, the catalyst **8** allowed the reaction proceeding at atmospheric pressure. These results indicate that the corresponding hydride complex can easily be generated as an active species at atmospheric pressure.

Recent Advances in Transition Metal-Catalysed

Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media 257

experimentally isolated air- and moisture-stable complex **12**. When **12** was used for the hydrogenation of CO2, maximum TON of 348,000 and TOF of 18,780 h−1 were obtained,

N-Heterocyclic carbenes (NHCs), which have a high electron-donating ability, have also been introduced as ligands in iridium complexes for the hydrogenation of CO2. Most recently, Peris and co-workers reported a series of IrCp\*(NHC) complexes. A bis-NHC Ir complex, **13**, showed modest activity (TON of 1600) for the hydrogenation of CO2 to HCOOK.(Sanz et al., 2010b) To improve the water solubility of the complex, hydroxyl groups were introduced to the side carbon chains. Consequently, complex **14** gave a higher TON of 9500 under optimized conditions.(Sanz et al., 2010a) Furthermore, blocking the C2 position of imidazole with a methyl group and coordinating to the C5 position led to a higher electron-donating ability of the ligand. In addition, the introduction of sulfonate groups into the bis-NHC ligand increased the water solubility of the complex. As a result, a TON of 190,000 was achieved with complex **15**.(Azua et al., 2011) Interestingly, these complexes also succeeded in the transfer hydrogenation of CO2 to formate using *i*PrOH as a

As mention above, the homogenous catalysts for hydrogenation of CO2 into formic acid are typically restricted to complexes of the precious or platinum-group metals Rh, Ru and Ir. Other metals are less investigated due to the low efficiency. Hence the development of nonprecious metal based homogeneous catalyst is limited. Most catalysis using this kind of complexes were carried out in organic solvent and only few examples were in aqueous media, but not pure water. In the original work of Inoue, a non-platinum-group metal catalyst, Ni(dppe)2 (dppe = 1,2-bis(diphenylphosphino)ethane), have been studied. It was proved to be inefficient with only a low TON of 7. Two year later, Evans and Newell studied the homogeneous catalytic reduction of CO2 to formate esters in alcohols with [HFe3(CO)11]−, but only obtained a low TOF (0.06 h−1) and TON (< 6). (Evans & Newell, 1978) In 1994, Yamamoto et al. have studied the Pd based complex. They have synthesized and characterized the first carbon dioxide coordinated palladium(0) complex, Pd(2- CO2)(PMePh2)2. In addition, using the Pd complexes, PdCl2L2 (L = PMe3; PMePh2; PPh3), they obtained formic acid in 12% yield in benzene/H2O under 100 atm H2/CO2 (1/1) at room

Nonprecious metal was almost not concerned in the following years until 2003. Jessop and co-workers investigated a number of inexpensive metals such as Cu, Fe, Mn, Mo, Ni, and Zn with a high-throughput screening method in the hydrogenation of CO2 in DMSO.(Tai et al., 2003) They found the combination of FeCl3 and NiCl2 with dcpe ligand (dcpe = Cy2PCH2CH2PCy2) gave better results (TON up to 117, TOF up to 15.6 h−1) than other metals. In 2010, Beller and Laurenczy et al. have reported different iron precursors and various nitrogen- and phosphine-ligands for the homogeneous hydrogenation of CO2 and bicarbonate to formate in MeOH. The best iron catalyst, Fe(BF4)2/PP3 (PP3 =

which is comparable to the best system reported by Nozaki.

**4. Hydrogenation with other metal complexes** 

hydrogen donor.

temperature.(Sakamoto et al., 1994)


**Table 3.** Substituent effect of the ligand on the TON for hydrogenation of CO2. The reaction was carried out with the catalyst (0.1 mM) in a 1 M KOH solution under 4 MPa (CO2/H2 = 1:1) at 80 °C for 20 h. *a*. Rh-L = [Cp\*Rh(L)Cl]Cl, Ru-L = [(C6Me6)Ru(L)Cl]Cl, Ir-L = [Cp\*Ir(L)Cl]Cl. *b*. [Catalyst] = 0.2 mM.

**Figure 4.** Correlation between initial TOFs and p+ values of substituents (R) for the hydrogenation of CO2 catalyzed by [(CnMen)M(4,4'-R2-2,2'-bpy)Cl]Cl. a) M = Ir, n = 5; b) M = Rh, n = 5; c) M = Ru, n = 6; R = OH, OMe, Me, H. The reactions were carried out in an aqueous 1 M KOH solution under 4 MPa (CO2:H2 = 1:1) at 80 °C for 20 h.

In 2009, Nozaki and co-workers designed Ir(III) complexes **10** in which alkylphosphinebased pincer ligands were employed as efficient electron donors. These complexes were used for the hydrogenation of CO2 in H2O/THF. The PNP-Ir trihydride complex, **10**, showed the highest TON (3,500,000) and TOF (150,000 h−1) to date.(Tanaka et al., 2009) In 2011, Hazari and co-workers investigated CO2 insertion into PNP-Ir hydrides using a computational method.(Schmeier et al., 2011) They evaluated the nucleophilicity of the hydride through its calculated NBO charge and found a strong correlation between the NBO charge of the hydride and the thermodynamics of CO2 insertion. Using this simple model, they predicted that complex **11** is favorable for CO2 insertion. Furthermore, they experimentally isolated air- and moisture-stable complex **12**. When **12** was used for the hydrogenation of CO2, maximum TON of 348,000 and TOF of 18,780 h−1 were obtained, which is comparable to the best system reported by Nozaki.

N-Heterocyclic carbenes (NHCs), which have a high electron-donating ability, have also been introduced as ligands in iridium complexes for the hydrogenation of CO2. Most recently, Peris and co-workers reported a series of IrCp\*(NHC) complexes. A bis-NHC Ir complex, **13**, showed modest activity (TON of 1600) for the hydrogenation of CO2 to HCOOK.(Sanz et al., 2010b) To improve the water solubility of the complex, hydroxyl groups were introduced to the side carbon chains. Consequently, complex **14** gave a higher TON of 9500 under optimized conditions.(Sanz et al., 2010a) Furthermore, blocking the C2 position of imidazole with a methyl group and coordinating to the C5 position led to a higher electron-donating ability of the ligand. In addition, the introduction of sulfonate groups into the bis-NHC ligand increased the water solubility of the complex. As a result, a TON of 190,000 was achieved with complex **15**.(Azua et al., 2011) Interestingly, these complexes also succeeded in the transfer hydrogenation of CO2 to formate using *i*PrOH as a hydrogen donor.

### **4. Hydrogenation with other metal complexes**

256 Hydrogenation

species at atmospheric pressure.

Catalyst*a* TON

Rh-L 216*<sup>b</sup>*

Ir-L 105*<sup>c</sup>*

(CO2:H2 = 1:1) at 80 °C for 20 h.

Ru-L 68 4400 78c

apparent that the remarkable activation of the iridium DHBP catalyst can be attributed to the strong electron-donating ability of the oxyanion. The maximal catalytic activity (TOF = 42,000 h−1, TON = 190,000) of the iridium DHBP catalyst was obtained at 6 MPa and 120 °C. Moreover, the catalyst **8** allowed the reaction proceeding at atmospheric pressure. These results indicate that the corresponding hydride complex can easily be generated as an active

**Table 3.** Substituent effect of the ligand on the TON for hydrogenation of CO2. The reaction was carried out with the catalyst (0.1 mM) in a 1 M KOH solution under 4 MPa (CO2/H2 = 1:1) at 80 °C for 20 h. *a*. Rh-L = [Cp\*Rh(L)Cl]Cl, Ru-L = [(C6Me6)Ru(L)Cl]Cl, Ir-L = [Cp\*Ir(L)Cl]Cl. *b*. [Catalyst] = 0.2 mM.

**Figure 4.** Correlation between initial TOFs and p+ values of substituents (R) for the hydrogenation of CO2 catalyzed by [(CnMen)M(4,4'-R2-2,2'-bpy)Cl]Cl. a) M = Ir, n = 5; b) M = Rh, n = 5; c) M = Ru, n = 6; R = OH, OMe, Me, H. The reactions were carried out in an aqueous 1 M KOH solution under 4 MPa

In 2009, Nozaki and co-workers designed Ir(III) complexes **10** in which alkylphosphinebased pincer ligands were employed as efficient electron donors. These complexes were used for the hydrogenation of CO2 in H2O/THF. The PNP-Ir trihydride complex, **10**, showed the highest TON (3,500,000) and TOF (150,000 h−1) to date.(Tanaka et al., 2009) In 2011, Hazari and co-workers investigated CO2 insertion into PNP-Ir hydrides using a computational method.(Schmeier et al., 2011) They evaluated the nucleophilicity of the hydride through its calculated NBO charge and found a strong correlation between the NBO charge of the hydride and the thermodynamics of CO2 insertion. Using this simple model, they predicted that complex **11** is favorable for CO2 insertion. Furthermore, they

L: bpy DHBP phen DHPT

1800 220 2300

5500 59 6100

5100

As mention above, the homogenous catalysts for hydrogenation of CO2 into formic acid are typically restricted to complexes of the precious or platinum-group metals Rh, Ru and Ir. Other metals are less investigated due to the low efficiency. Hence the development of nonprecious metal based homogeneous catalyst is limited. Most catalysis using this kind of complexes were carried out in organic solvent and only few examples were in aqueous media, but not pure water. In the original work of Inoue, a non-platinum-group metal catalyst, Ni(dppe)2 (dppe = 1,2-bis(diphenylphosphino)ethane), have been studied. It was proved to be inefficient with only a low TON of 7. Two year later, Evans and Newell studied the homogeneous catalytic reduction of CO2 to formate esters in alcohols with [HFe3(CO)11]−, but only obtained a low TOF (0.06 h−1) and TON (< 6). (Evans & Newell, 1978) In 1994, Yamamoto et al. have studied the Pd based complex. They have synthesized and characterized the first carbon dioxide coordinated palladium(0) complex, Pd(2- CO2)(PMePh2)2. In addition, using the Pd complexes, PdCl2L2 (L = PMe3; PMePh2; PPh3), they obtained formic acid in 12% yield in benzene/H2O under 100 atm H2/CO2 (1/1) at room temperature.(Sakamoto et al., 1994)

Nonprecious metal was almost not concerned in the following years until 2003. Jessop and co-workers investigated a number of inexpensive metals such as Cu, Fe, Mn, Mo, Ni, and Zn with a high-throughput screening method in the hydrogenation of CO2 in DMSO.(Tai et al., 2003) They found the combination of FeCl3 and NiCl2 with dcpe ligand (dcpe = Cy2PCH2CH2PCy2) gave better results (TON up to 117, TOF up to 15.6 h−1) than other metals. In 2010, Beller and Laurenczy et al. have reported different iron precursors and various nitrogen- and phosphine-ligands for the homogeneous hydrogenation of CO2 and bicarbonate to formate in MeOH. The best iron catalyst, Fe(BF4)2/PP3 (PP3 = P(CH2CH2PPh2)3), could reduce bicarbonate to formate in a TON of 610 which was comparable with that using [RuCl2(C6H6)2]/PP3. It also could transform CO2 and H2 to formate esters and formamide in the presence of the corresponding alcohols and amines with TON up to 292 and 727, respectively.(Federsel et al., 2010a) Very recently, Beller et al. reported hydrogenation of sodium bicarbonate and CO2 with in situ generated cobalt catalyst. They obtained a high TON of 3877 using the Co(BF4)2·6H2O and PP3 in a sodium formate at 120 °C for 20 h. This catalytic productivity is six times as high as the TON for the iron catalyst. They also found other phosphine ligands, such as triphenylphosphine, xantphos, 1,2-bisdiphenylphosphinoethane, and 1,1,1-tris(diphenylphosphinomethyl) ethane, showed no activity.(Federsel et al., 2012)

Recent Advances in Transition Metal-Catalysed

Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media 259

**Figure 5.** Water effect in the hydrogenation of CO2.

facilitates CO2 insertion (Figure 6).

ligand in activating the complex.

Lau et al. have demonstrated the intramolecular N-H---H-Ru hydrogen bond in the Ru complexes catalysed hydrogenation.(Chu et al., 1998) Although the reaction rate was very slow, their research gave insight into the mechanism of hydrogen activation: intramolecular heterolytic cleavage of the dihydrogen was aided by the pendant amino group. The design principle has been employed by Hazari et al. in the designing of PNP Ir(III) catalyst and demonstrated the feasibility of such activation method.(Schmeier et al., 2011) With DFT calculations, they demonstrated that the intramolecular hydrogen bond in complex **12**

**Figure 6.** Reaction mechanism of CO2 hydrogenation proposed by Hazari et al.

Sasaki's theoretical calculation(Ohnishi et al., 2005) and Jessop's experimental results(Tai et al., 2002) have demonstrated the strong electron-donating power of the ligand resulting in high activity of the complexes. The following design of complexes generally applied this principle. As abovementioned, Himeda et al. developed DHBP catalyst by introducing the hydroxyl group to bpy; Nozaki et al. designed the complex with PNP-based pincer ligand as a strong donor; Peris et al. used the NHC as a strong electron-donating ligand for new catalyst design. All the examples have verified the importance of the donor powder of the

In 2009, Nozaki et al. reported the PNP pincer ligated Ir(III) complexes, **10**, and achieved the highest TON (3,500,000) and TOF (150,000 h−1) to date.(Tanaka et al., 2009) They also

Inspired by the iridium pincer complexes, in 2011, Milstein et al. reported the most active iron(II) pincer complex trans-[(*t*Bu-PNP)Fe(H)2(CO)] which showed similar activity to noble metal catalysts.(Langer et al., 2011) Hydrogenation of sodium bicarbonate to formate in H2O/THF (10/1) have achieved a TON of 320 at 80 °C under 8.3 bar H2. Hydrogenation of CO2 in a 2 M NaOH solution gave a TON of 788 and TOF of 156 h−1 at total pressure of 10 bar (H2/CO2 = 6.7/3.3) for 5 h.

### **5. Mechanism of CO2 hydrogenation**

To understand the reaction process of CO2 hydrogenation and design better catalyst, the study of the mechanism has always been the focus for chemists.(Hutschka et al., 1997; Getty et al., 2009) In the CO2 hydrogenation, there are several aspects need to pay attention to, such as activation of dihydrogen and CO2 involving ligand and metal, as well as the important effect of solvent and additive.

Along with the development of reaction in water, the exploration of the water effect has been on-going. The accelerating effect of small amounts of added water in organic solvents has been observed in active Pd(Inoue et al., 1976), Rh(Tsai & Nicholas, 1992)] and Ru(Jessop et al., 1996) etc. systems. Note that in some case adding small amount of water to organic solvent could not improve the performance of the reaction system, even showed prohibit effect due to the deactivation of the hydrophobic catalyst.(Leitner et al., 1994) Therefore understanding the mechanism and then developing appropriate catalyst that can be applied in water is essential to high effective catalytic system. Nicholas et al. (Tsai & Nicholas, 1992) have proposed that water acts as an ancillary ligand and form hydrogen bond with oxygen of the CO2 to facilitate CO2 insertion (Figure 5A). Lau et al. found the reaction rate is enhanced by adding 20% water in THF by using TpRu(PPh3)(CH3CN)H [Tp = hydrotris(pyrazolyl)borate] as a catalyst.(Ng et al., 2004) They also have studied the promoting effect of water with the same Ru catalyst and proposed a mechanism to illustrate the water effect (Figure 5B). As suggested by their calculation, the incorporation of water could activate the CO2 molecule and significantly reduce the reaction barrier.(Yin et al., 2001) In the most recent work of Nozaki et al. they investigated the reaction mechanism by density functional theory (DFT) calculation. They found that adding one or two water molecules the reaction barrier is markedly lowered compared to that in gas phase.

**Figure 5.** Water effect in the hydrogenation of CO2.

ethane, showed no activity.(Federsel et al., 2012)

**5. Mechanism of CO2 hydrogenation** 

important effect of solvent and additive.

bar (H2/CO2 = 6.7/3.3) for 5 h.

P(CH2CH2PPh2)3), could reduce bicarbonate to formate in a TON of 610 which was comparable with that using [RuCl2(C6H6)2]/PP3. It also could transform CO2 and H2 to formate esters and formamide in the presence of the corresponding alcohols and amines with TON up to 292 and 727, respectively.(Federsel et al., 2010a) Very recently, Beller et al. reported hydrogenation of sodium bicarbonate and CO2 with in situ generated cobalt catalyst. They obtained a high TON of 3877 using the Co(BF4)2·6H2O and PP3 in a sodium formate at 120 °C for 20 h. This catalytic productivity is six times as high as the TON for the iron catalyst. They also found other phosphine ligands, such as triphenylphosphine, xantphos, 1,2-bisdiphenylphosphinoethane, and 1,1,1-tris(diphenylphosphinomethyl)

Inspired by the iridium pincer complexes, in 2011, Milstein et al. reported the most active iron(II) pincer complex trans-[(*t*Bu-PNP)Fe(H)2(CO)] which showed similar activity to noble metal catalysts.(Langer et al., 2011) Hydrogenation of sodium bicarbonate to formate in H2O/THF (10/1) have achieved a TON of 320 at 80 °C under 8.3 bar H2. Hydrogenation of CO2 in a 2 M NaOH solution gave a TON of 788 and TOF of 156 h−1 at total pressure of 10

To understand the reaction process of CO2 hydrogenation and design better catalyst, the study of the mechanism has always been the focus for chemists.(Hutschka et al., 1997; Getty et al., 2009) In the CO2 hydrogenation, there are several aspects need to pay attention to, such as activation of dihydrogen and CO2 involving ligand and metal, as well as the

Along with the development of reaction in water, the exploration of the water effect has been on-going. The accelerating effect of small amounts of added water in organic solvents has been observed in active Pd(Inoue et al., 1976), Rh(Tsai & Nicholas, 1992)] and Ru(Jessop et al., 1996) etc. systems. Note that in some case adding small amount of water to organic solvent could not improve the performance of the reaction system, even showed prohibit effect due to the deactivation of the hydrophobic catalyst.(Leitner et al., 1994) Therefore understanding the mechanism and then developing appropriate catalyst that can be applied in water is essential to high effective catalytic system. Nicholas et al. (Tsai & Nicholas, 1992) have proposed that water acts as an ancillary ligand and form hydrogen bond with oxygen of the CO2 to facilitate CO2 insertion (Figure 5A). Lau et al. found the reaction rate is enhanced by adding 20% water in THF by using TpRu(PPh3)(CH3CN)H [Tp = hydrotris(pyrazolyl)borate] as a catalyst.(Ng et al., 2004) They also have studied the promoting effect of water with the same Ru catalyst and proposed a mechanism to illustrate the water effect (Figure 5B). As suggested by their calculation, the incorporation of water could activate the CO2 molecule and significantly reduce the reaction barrier.(Yin et al., 2001) In the most recent work of Nozaki et al. they investigated the reaction mechanism by density functional theory (DFT) calculation. They found that adding one or two water

molecules the reaction barrier is markedly lowered compared to that in gas phase.

Lau et al. have demonstrated the intramolecular N-H---H-Ru hydrogen bond in the Ru complexes catalysed hydrogenation.(Chu et al., 1998) Although the reaction rate was very slow, their research gave insight into the mechanism of hydrogen activation: intramolecular heterolytic cleavage of the dihydrogen was aided by the pendant amino group. The design principle has been employed by Hazari et al. in the designing of PNP Ir(III) catalyst and demonstrated the feasibility of such activation method.(Schmeier et al., 2011) With DFT calculations, they demonstrated that the intramolecular hydrogen bond in complex **12** facilitates CO2 insertion (Figure 6).

**Figure 6.** Reaction mechanism of CO2 hydrogenation proposed by Hazari et al.

Sasaki's theoretical calculation(Ohnishi et al., 2005) and Jessop's experimental results(Tai et al., 2002) have demonstrated the strong electron-donating power of the ligand resulting in high activity of the complexes. The following design of complexes generally applied this principle. As abovementioned, Himeda et al. developed DHBP catalyst by introducing the hydroxyl group to bpy; Nozaki et al. designed the complex with PNP-based pincer ligand as a strong donor; Peris et al. used the NHC as a strong electron-donating ligand for new catalyst design. All the examples have verified the importance of the donor powder of the ligand in activating the complex.

In 2009, Nozaki et al. reported the PNP pincer ligated Ir(III) complexes, **10**, and achieved the highest TON (3,500,000) and TOF (150,000 h−1) to date.(Tanaka et al., 2009) They also proposed a mechanism for the catalytic reaction (Figure 7): the insertion of CO2 into **10** gives formato complex **16**, which undergoes dissociation of the formato ligand under basic conditions. Simultaneously, deprotonative dearomatization of the PNP ligand by OH− leads to intermediate **17**, which is hydrogenated to regenerate the trihydride complex **10**.

Recent Advances in Transition Metal-Catalysed

Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media 261

The catalytic hydrogenation mechanism of nitrogen-based complexes has been less investigated. In 2006, Ogo et al. determined that the different rate-determining step for bpybased Ru and Ir complexes by the observation of the saturation behaviour of the TON with increasing PH2 and PCO2 respectively.(Ogo et al., 2006) The rate-determining step of [(6- C6Me6)Ru(bpy)(OH2)](SO4) and [(6-C6Me6)Ru(4,4'-OMe-bpy)(OH2)](SO4) was suggested to be the reaction of aqua complexes with H2. In contrast, the Ir analogous was supposed to be the CO2 insertion into the iridium hydride complexes which were isolated and characterized by NMR, ESI-MS, and IR. The different mechanism of Ru and Ir complexes may help to

Homogeneous catalyst has exhibited high catalytic activity in the hydrogenation of CO2. For further practical application, the recycle and reuse of the catalyst is an important issue that

In 2011, Baffert et al. reported a series of silica supported ruthenium-N-heteroclyclic carben species for hydrogenation of CO2.(Baffert et al., 2011) Using pyrrolidine as an additive, Rucym and M-Rucym (Figure 9) showed low catalytic activity. By introducing basic phosphorous ligand PMe3, the activity of M-RuP (Figure 9) is improved and showed comparable TON with the parent catalyst [RuCl2(PMe3)4]. However, the supported catalyst are unstable due to

Zheng et al. have reported the ruthenium immobilized on functionalized silica could be used as catalyst precursor for hydrogenation of CO2 in organic solvent with adding triphenylphosphine.(Zhang et al., 2004) In light of this result, in 2008, Han et al. prepared the silica supported catalyst "Si"-(CH2)3NH(CSCH3)-[RuCl3(PPh3)] and used it to the hydrogenation of CO2 in a mixture solvent of H2O and ionic liquid. The ionic liquid, 1-(N,Ndimethylaminoehtyl)-2,3-dimethylimidazolium trifluoromethanesulfonate ([mammin][TfO]), has a tertiary amino group which makes it can acts as a basic additive as well as a solvent. After the reaction, the immobilized catalyst could be simply recycled by filtration. The filtrate was first warmed to 110 °C to remove the water and then heated to 130 °C to separate the formic acid. Since the ionic liquid is non-volatile and stable below 220 °C, it can be separated and reused after the distillation. The catalyst and ionic liquid could be reused four times without decrease of TOF (~44 h−1). With ICP-AES analysis, they found no significant loss of Ru

understand the excellent performance of other iridium complexes.

needs to be resolved because most of the catalysts contain noble metal.

**Figure 9.** Silica supported Ru catalysts. (Mes: Mesityl; TMS: trimethylsilyl)

during the recycling process. (Zhang et al., 2008)

the weak Ru-NHC linkage, 50% of Ru was found leached into reaction solution.

**6. Catalyst immobilization and recycle** 

**Figure 7.** Catalytic mechanism for the hydrogenation of CO2 proposed by Nozaki.

After Nozaki and co-workers reported the excellent PNP Ir(III) complexes, several groups have investigated the mechanism of CO2 hydrogenation with these catalysts. Ahlquist et al. used a simple (PNP)IrH3 structure to study the mechanism with DFT calculation.(Ahlquist, 2010) Their research suggested that the deprotonation by the hydroxide is the rate-limiting step (Figure 7). This calculation agreed with the experimental observation that higher basicity leads to a higher rate. Most recently, Yang reinvestigated this mechanism using the DFT method and proposed a different reaction pathway (Figure 8).(Yang, 2011) He suggested that direct H2 cleavage by OH− is more favourable than the Nozaki-postulated H2 cleavage model. Using this new reaction pathway, the calculation gave a low overall enthalpy barrier of 77.8 kJ mol−1 for the formation of HCOOH from H2 and CO2.

**Figure 8.** Catalytic mechanism of CO2 hydrogenation proposed by Yang.

The catalytic hydrogenation mechanism of nitrogen-based complexes has been less investigated. In 2006, Ogo et al. determined that the different rate-determining step for bpybased Ru and Ir complexes by the observation of the saturation behaviour of the TON with increasing PH2 and PCO2 respectively.(Ogo et al., 2006) The rate-determining step of [(6- C6Me6)Ru(bpy)(OH2)](SO4) and [(6-C6Me6)Ru(4,4'-OMe-bpy)(OH2)](SO4) was suggested to be the reaction of aqua complexes with H2. In contrast, the Ir analogous was supposed to be the CO2 insertion into the iridium hydride complexes which were isolated and characterized by NMR, ESI-MS, and IR. The different mechanism of Ru and Ir complexes may help to understand the excellent performance of other iridium complexes.

### **6. Catalyst immobilization and recycle**

260 Hydrogenation

proposed a mechanism for the catalytic reaction (Figure 7): the insertion of CO2 into **10** gives formato complex **16**, which undergoes dissociation of the formato ligand under basic conditions. Simultaneously, deprotonative dearomatization of the PNP ligand by OH− leads

to intermediate **17**, which is hydrogenated to regenerate the trihydride complex **10**.

**Figure 7.** Catalytic mechanism for the hydrogenation of CO2 proposed by Nozaki.

enthalpy barrier of 77.8 kJ mol−1 for the formation of HCOOH from H2 and CO2.

**Figure 8.** Catalytic mechanism of CO2 hydrogenation proposed by Yang.

After Nozaki and co-workers reported the excellent PNP Ir(III) complexes, several groups have investigated the mechanism of CO2 hydrogenation with these catalysts. Ahlquist et al. used a simple (PNP)IrH3 structure to study the mechanism with DFT calculation.(Ahlquist, 2010) Their research suggested that the deprotonation by the hydroxide is the rate-limiting step (Figure 7). This calculation agreed with the experimental observation that higher basicity leads to a higher rate. Most recently, Yang reinvestigated this mechanism using the DFT method and proposed a different reaction pathway (Figure 8).(Yang, 2011) He suggested that direct H2 cleavage by OH− is more favourable than the Nozaki-postulated H2 cleavage model. Using this new reaction pathway, the calculation gave a low overall Homogeneous catalyst has exhibited high catalytic activity in the hydrogenation of CO2. For further practical application, the recycle and reuse of the catalyst is an important issue that needs to be resolved because most of the catalysts contain noble metal.

In 2011, Baffert et al. reported a series of silica supported ruthenium-N-heteroclyclic carben species for hydrogenation of CO2.(Baffert et al., 2011) Using pyrrolidine as an additive, Rucym and M-Rucym (Figure 9) showed low catalytic activity. By introducing basic phosphorous ligand PMe3, the activity of M-RuP (Figure 9) is improved and showed comparable TON with the parent catalyst [RuCl2(PMe3)4]. However, the supported catalyst are unstable due to the weak Ru-NHC linkage, 50% of Ru was found leached into reaction solution.

**Figure 9.** Silica supported Ru catalysts. (Mes: Mesityl; TMS: trimethylsilyl)

Zheng et al. have reported the ruthenium immobilized on functionalized silica could be used as catalyst precursor for hydrogenation of CO2 in organic solvent with adding triphenylphosphine.(Zhang et al., 2004) In light of this result, in 2008, Han et al. prepared the silica supported catalyst "Si"-(CH2)3NH(CSCH3)-[RuCl3(PPh3)] and used it to the hydrogenation of CO2 in a mixture solvent of H2O and ionic liquid. The ionic liquid, 1-(N,Ndimethylaminoehtyl)-2,3-dimethylimidazolium trifluoromethanesulfonate ([mammin][TfO]), has a tertiary amino group which makes it can acts as a basic additive as well as a solvent. After the reaction, the immobilized catalyst could be simply recycled by filtration. The filtrate was first warmed to 110 °C to remove the water and then heated to 130 °C to separate the formic acid. Since the ionic liquid is non-volatile and stable below 220 °C, it can be separated and reused after the distillation. The catalyst and ionic liquid could be reused four times without decrease of TOF (~44 h−1). With ICP-AES analysis, they found no significant loss of Ru during the recycling process. (Zhang et al., 2008)

Soon after that, they report another type of ionic liquid, 1,3-di(N,N-dimethylaminoethyl)-2 methylimidazolium trifluoromethanesulfonate ([DAMI][TfO]), which has two tertiary amino groups on the side chain of the cation. Using the silica supported Ru catalyst, "Si"- (CH2)3NH(CSCH3)-[RuCl3(PPh3)], CO2 was hydrogenated to formic acid in the presence of water and [DAMI][TfO]. They found TOF increased with increasing the amount of water added, and a weight ratio of water to ionic liquid is suitable between 1 and 2.5. Under the optimal conditions, a TON of 1840 and a TOF of 920 h−1 was obtained at 80 °C under 18 MPa of H2/CO2 (1/1) for 2h. The ionic liquid and catalyst can be reused at least over four cycles without significant decrease of TOF.(Zhang et al., 2009)

Recent Advances in Transition Metal-Catalysed

Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media 263

**Figure 11.** Recycling of proton-responsive catalyst with tunable solubility.

disadvantages of homogeneous catalysis.

60 °C for 2 h. *a*. Determined by ICP-MS analysis.

**7. Conclusion** 

Cycle Loaded/recovered cat./ppm Recovery

The recovered catalyst retained a high catalytic activity across four cycles, as shown in Table 4. It is clear that the three components (i.e., catalyst, product, and solvent) can be easily separated without significant waste. The sophisticated design of the catalyst provided a proton-responsive catalyst with pH-tunable catalytic activity and water solubility. These results suggest that by carefully considering reaction profiles, the design and use of innovative homogeneous catalytic systems such as tailor-made catalysts can overcome the

efficiency/%

**Table 4.** Catalyst recycling studies for the conversion of CO2 into formate using iridium-DHPT catalyst **9**. Reaction conditions: DHPT catalyst (2.5 mol), 6 MPa of H2/CO2 (1:1), 0.1 M KOH solution (50 mL),

After decades of research, chemists have achieved great success in homogeneous hydrogenation of carbon dioxide. With appropriate catalysts and optimum conditions, some of the results are close to commercialization. However, to hydrogenate carbon dioxide efficiently, economically, and eco-friendly, several critical problems remain to be solved. The first is the high activity of the catalyst, which is generally required in order to lower the overall energy barrier for the conversion of thermodynamically stable CO2. Present catalysts usually need high temperature and pressure to achieve high activity. Consequently, the energy cost is increased. The highly efficient catalyst that can work under mild conditions is highly requisite. Therefore, further research and understanding the mechanism and delicate design of the catalyst with multi-functional ligand are necessary. The second is the

1 9.0 - 0.11 0.105 2 8.4 93 0.22 0.104 3 7.7 92 0.42 0.103 4 7.0 91 0.61 0.103

Leaching iridium*<sup>a</sup>*

/ppm

Final conc. of formate/M

The catalyst recycle usually require a solid supporter, and suffer from loss of catalytic activity due to the insolubility of the catalyst in the reaction solution. Himeda et al. reported an interesting method for catalyst recycling without a supporter. It was realized by utilising the tunable solubility of the complex along with changing pH of the reaction solution. The catalyst **9** based on DHPT showed a similar TOF and a slightly improved TON(Himeda et al., 2005) than DHBP catalyst **8** (Table 2). The abovementioned acid-base equilibrium not only changes the electronic properties of the complex but also affects its polarity and thus its water solubility. As shown in Figure 10, DHPT catalyst **9** exhibited negligible solubility (ca. 100 ppb) in a weakly acidic formate solution. Therefore, recycling of **9** was investigated in batch-wise cycles based on the concept shown in Figure 11. When the added KOH was completely consumed, the catalyst precursor spontaneously precipitated due to its decreased water solubility as a result of the lower pH value. Thus, the reaction was terminated and formed a heterogeneous system that could be filtered to recover the precipitated catalyst. The iridium complex remaining in the filtrate was found to be less than 2% of the catalyst loading (0.11 ppm). Since the catalytic action was "turned off", the reverse reaction (i.e., the decomposition of formic acid) was prevented in the separation step. Additionally, the pure product (i.e., the formate salt) could be isolated simply by evaporating the filtrate.

**Figure 10.** Solubility of a) DHBP catalyst **8** and b) DHPT catalyst **9** in a 1 M aqueous formate solution.

**Figure 11.** Recycling of proton-responsive catalyst with tunable solubility.

The recovered catalyst retained a high catalytic activity across four cycles, as shown in Table 4. It is clear that the three components (i.e., catalyst, product, and solvent) can be easily separated without significant waste. The sophisticated design of the catalyst provided a proton-responsive catalyst with pH-tunable catalytic activity and water solubility. These results suggest that by carefully considering reaction profiles, the design and use of innovative homogeneous catalytic systems such as tailor-made catalysts can overcome the disadvantages of homogeneous catalysis.


**Table 4.** Catalyst recycling studies for the conversion of CO2 into formate using iridium-DHPT catalyst **9**. Reaction conditions: DHPT catalyst (2.5 mol), 6 MPa of H2/CO2 (1:1), 0.1 M KOH solution (50 mL), 60 °C for 2 h. *a*. Determined by ICP-MS analysis.

### **7. Conclusion**

262 Hydrogenation

evaporating the filtrate.

Soon after that, they report another type of ionic liquid, 1,3-di(N,N-dimethylaminoethyl)-2 methylimidazolium trifluoromethanesulfonate ([DAMI][TfO]), which has two tertiary amino groups on the side chain of the cation. Using the silica supported Ru catalyst, "Si"- (CH2)3NH(CSCH3)-[RuCl3(PPh3)], CO2 was hydrogenated to formic acid in the presence of water and [DAMI][TfO]. They found TOF increased with increasing the amount of water added, and a weight ratio of water to ionic liquid is suitable between 1 and 2.5. Under the optimal conditions, a TON of 1840 and a TOF of 920 h−1 was obtained at 80 °C under 18 MPa of H2/CO2 (1/1) for 2h. The ionic liquid and catalyst can be reused at least over four cycles

The catalyst recycle usually require a solid supporter, and suffer from loss of catalytic activity due to the insolubility of the catalyst in the reaction solution. Himeda et al. reported an interesting method for catalyst recycling without a supporter. It was realized by utilising the tunable solubility of the complex along with changing pH of the reaction solution. The catalyst **9** based on DHPT showed a similar TOF and a slightly improved TON(Himeda et al., 2005) than DHBP catalyst **8** (Table 2). The abovementioned acid-base equilibrium not only changes the electronic properties of the complex but also affects its polarity and thus its water solubility. As shown in Figure 10, DHPT catalyst **9** exhibited negligible solubility (ca. 100 ppb) in a weakly acidic formate solution. Therefore, recycling of **9** was investigated in batch-wise cycles based on the concept shown in Figure 11. When the added KOH was completely consumed, the catalyst precursor spontaneously precipitated due to its decreased water solubility as a result of the lower pH value. Thus, the reaction was terminated and formed a heterogeneous system that could be filtered to recover the precipitated catalyst. The iridium complex remaining in the filtrate was found to be less than 2% of the catalyst loading (0.11 ppm). Since the catalytic action was "turned off", the reverse reaction (i.e., the decomposition of formic acid) was prevented in the separation step. Additionally, the pure product (i.e., the formate salt) could be isolated simply by

**Figure 10.** Solubility of a) DHBP catalyst **8** and b) DHPT catalyst **9** in a 1 M aqueous formate solution.

without significant decrease of TOF.(Zhang et al., 2009)

After decades of research, chemists have achieved great success in homogeneous hydrogenation of carbon dioxide. With appropriate catalysts and optimum conditions, some of the results are close to commercialization. However, to hydrogenate carbon dioxide efficiently, economically, and eco-friendly, several critical problems remain to be solved. The first is the high activity of the catalyst, which is generally required in order to lower the overall energy barrier for the conversion of thermodynamically stable CO2. Present catalysts usually need high temperature and pressure to achieve high activity. Consequently, the energy cost is increased. The highly efficient catalyst that can work under mild conditions is highly requisite. Therefore, further research and understanding the mechanism and delicate design of the catalyst with multi-functional ligand are necessary. The second is the prevention of waste generation (e.g., organic solvents and additives) during the reaction. The third is the recycle and reuse of the catalyst, which is important to increase the cost efficiency. The research of the catalyst recycling is still in the preliminary stage and suffers from a lot of problems. Better performance can be expected with the development of new immobilizing method, such as using ionic liquid.

Recent Advances in Transition Metal-Catalysed

Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media 265

ruthenium(II) phosphine complexes. *Appl. Catal. A-Gen.,* Vol.255, No.1, pp. 59-67, ISSN

Erlandsson, M.; Landaeta, V. R.; Gonsalvi, L.; Peruzzini, M.; Phillips, A. D.; Dyson, P. J. & Laurenczy, G. (2008). (Pentamethylcyclopentadienyl)iridium-PTA (PTA = 1,3,5-Triaza-7-phosphaadamantane) Complexes and Their Application in Catalytic Water Phase Carbon Dioxide Hydrogenation. *Eur. J. Inorg. Chem.,* Vol.2008, No.4, pp. 620-627, ISSN

Evans, G. O. & Newell, C. J. (1978). Conversion of CO2, H2, and alcohols into formate esters using anionic iron carbonyl hydrides. *Inorg. Chim. Acta,* Vol.31, pp. L387-L389, ISSN

Federsel, C.; Boddien, A.; Jackstell, R.; Jennerjahn, R.; Dyson, P. J.; Scopelliti, R.; Laurenczy, G. & Beller, M. (2010a). A well-defined iron catalyst for the reduction of bicarbonates and carbon dioxide to formates, alkyl formates, and formamides. *Angew. Chem. Int. Ed.,*

Federsel, C.; Jackstell, R. & Beller, M. (2010b). State-of-the-art catalysts for hydrogenation of carbon dioxide. *Angew. Chem. Int. Ed.,* Vol.49, No.36, pp. 6254-6257, ISSN 1521-3773

Federsel, C.; Ziebart, C.; Jackstell, R.; Baumann, W. & Beller, M. (2012). Catalytic hydrogenation of carbon dioxide and bicarbonates with a well-defined cobalt dihydrogen complex. *Chem.-Eur. J.,* Vol.18, No.1, pp. 72-75, ISSN 1521-3765 (Electronic) Gassner, F. & Leitner, W. (1993). CO2 Activation 3. Hydrogenation of Carbon Dioxide to Formic Acid Using Water-Soluble Rhodium Catalysts. *J. Chem. Soc.-Chem. Commun.,*

Getty, A. D.; Tai, C.-C.; Linehan, J. C.; Jessop, P. G.; Olmstead, M. M. & Rheingold, A. L. (2009). Hydrogenation of Carbon Dioxide Catalyzed by Ruthenium Trimethylphosphine Complexes: A Mechanistic Investigation Using High-Pressure NMR Spectroscopy. *Organometallics,* Vol.28, No.18, pp. 5466-5477, ISSN 0276-7333 Hayashi, H.; Ogo, S.; Abura, T. & Fukuzumi, S. (2003). Accelerating effect of a proton on the reduction of CO2 dissolved in water under acidic conditions. Isolation, crystal structure, and reducing ability of a water-soluble ruthenium hydride complex. *J. Am. Chem. Soc.,*

Hayashi, H.; Ogo, S. & Fukuzumi, S. (2004). Aqueous hydrogenation of carbon dioxide catalysed by water-soluble ruthenium aqua complexes under acidic conditions. *Chem.* 

Himeda, Y. (2007). Conversion of CO2 into formate by homogeneously catalyzed hydrogenation in water: Tuning catalytic activity and water solubility through the acidbase equilibrium of the ligand. *Eur. J. Inorg. Chem.,* No.25, pp. 3927–3941, ISSN 1434-

Himeda, Y.; Miyazawa, S. & Hirose, T. (2011). Interconversion between Formic Acid and H2/CO2 using Rhodium and Ruthenium Catalysts for CO2 Fixation and H2 Storage.

Himeda, Y.; Onozawa-Komatsuzaki, N.; Sugihara, H.; Arakawa, H. & Kasuga, K. (2003). Transfer hydrogenation of a variety of ketones catalyzed by rhodium complexes in

Vol.49, No.50, pp. 9777-9780, ISSN 1521-3773 (Electronic)

No.19, pp. 1465-1466, ISSN 0022-4936

Vol.125, No.47, pp. 14266-14267, ISSN 0002-7863

*Commun.,* No.23, pp. 2714-2715, ISSN 1359-7345

*ChemSusChem,* Vol.4, No.4, pp. 487-493, ISSN 1864-5631

0926-860X

14341948

0020-1693

(Electronic)

1948

Since much more effort has been paid to homogeneous hydrogenation of carbon dioxide over the last decade, we can expect more exciting results in the near future. We also believe transformation and utilization of carbon dioxide, especially for fuels production, will decrease its emission and reduce the reliance on fossil sources.

### **Author details**

Wan-Hui Wang and Yuichiro Himeda *National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan* 

### **Acknowledgement**

We thank the Japanese Ministry of Economy, Trade, and Industry for financial support.

### **8. References**


ruthenium(II) phosphine complexes. *Appl. Catal. A-Gen.,* Vol.255, No.1, pp. 59-67, ISSN 0926-860X

Erlandsson, M.; Landaeta, V. R.; Gonsalvi, L.; Peruzzini, M.; Phillips, A. D.; Dyson, P. J. & Laurenczy, G. (2008). (Pentamethylcyclopentadienyl)iridium-PTA (PTA = 1,3,5-Triaza-7-phosphaadamantane) Complexes and Their Application in Catalytic Water Phase Carbon Dioxide Hydrogenation. *Eur. J. Inorg. Chem.,* Vol.2008, No.4, pp. 620-627, ISSN 14341948

264 Hydrogenation

**Author details** 

**Acknowledgement** 

**8. References** 

13811169

1521-3765 (Electronic)

ISSN 1864-564X (Electronic)

pp. 2768-2777, ISSN 0276-7333

prevention of waste generation (e.g., organic solvents and additives) during the reaction. The third is the recycle and reuse of the catalyst, which is important to increase the cost efficiency. The research of the catalyst recycling is still in the preliminary stage and suffers from a lot of problems. Better performance can be expected with the development of new

Since much more effort has been paid to homogeneous hydrogenation of carbon dioxide over the last decade, we can expect more exciting results in the near future. We also believe transformation and utilization of carbon dioxide, especially for fuels production, will

*National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan* 

We thank the Japanese Ministry of Economy, Trade, and Industry for financial support.

Ahlquist, M. S. G. (2010). Iridium catalyzed hydrogenation of CO2 under basic conditions— Mechanistic insight from theory. *J. Mol. Catal. A: Chem.,* Vol.324, No.1-2, pp. 3-8, ISSN

Azua, A.; Sanz, S. & Peris, E. (2011). Water-soluble IrIII N-heterocyclic carbene based catalysts for the reduction of CO2 to formate by transfer hydrogenation and the deuteration of aryl amines in water. *Chem.-Eur. J.,* Vol.17, No.14, pp. 3963-3967, ISSN

Baffert, M.; Maishal, T. K.; Mathey, L.; Coperet, C. & Thieuleux, C. (2011). Tailored ruthenium-N-heterocyclic carbene hybrid catalytic materials for the hydrogenation of carbon dioxide in the presence of amine. *ChemSusChem,* Vol.4, No.12, pp. 1762-1765,

Boddien, A.; Gärtner, F.; Federsel, C.; Sponholz, P.; Mellmann, D.; Jackstell, R.; Junge, H. & Beller, M. (2011). CO2-"Neutral" Hydrogen Storage Based on Bicarbonates and

Elek, J.; Nadasdi, L.; Papp, G.; Laurenczy, G. & Joó, F. (2003). Homogeneous hydrogenation of carbon dioxide and bicarbonate in aqueous solution catalyzed by water-soluble

Formates. *Angew. Chem. Int. Ed.,* Vol.50, No.28, pp. 6411-6414, ISSN 1521-3773 Chu, H. S.; Lau, C. P.; Wong, K. Y. & Wong, W. T. (1998). Intramolecular N−H···H−Ru Proton−Hydride Interaction in Ruthenium Complexes with (2- (Dimethylamino)ethyl)cyclopentadienyl and (3- (Dimethylamino)propyl)cyclopentadienyl Ligands. Hydrogenation of CO2 to Formic Acid via the N−H···H−Ru Hydrogen-Bonded Complexes. *Organometallics,* Vol.17, No.13,

immobilizing method, such as using ionic liquid.

Wan-Hui Wang and Yuichiro Himeda

decrease its emission and reduce the reliance on fossil sources.


aqueous solution and their application to asymmetric reduction using chiral Schiff base ligands. *J. Mol. Catal. A-Chem.,* Vol.195, No.1-2, pp. 95–100, ISSN 1381-1169

Recent Advances in Transition Metal-Catalysed

Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media 267

Langer, R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J.; Ben-David, Y. & Milstein, D. (2011). Low-Pressure Hydrogenation of Carbon Dioxide Catalyzed by an Iron Pincer Complex Exhibiting Noble Metal Activity. *Angew. Chem. Int. Ed.,* pp. 9948-9952, ISSN 1521-3773

Laurenczy, G.; Joó, F. & Nadasdi, L. (2000). Formation and characterization of water-soluble hydrido-ruthenium(II) complexes of 1,3,5-triaza-7-phosphaadamantane and their catalytic activity in hydrogenation of CO2 and HCO3− in aqueous solution. *Inorg. Chem.,*

Leitner, W.; Dinjus, E. & Gassner, F. (1994). Activation of Carbon Dioxide 4. Rhodium-Catalyzes Hydrogenation of Carbon Dioxide to Formic Acid. *J. Organomet. Chem.,*

Leitner, W.; Dinjus, E. & Gassner, F., In *Aqueous-Phase Organometallic Catalysis, Concepts and Applications*, Cornils, B.; Herrmann, W. A., Eds. Wiley-VCH: Weinheim, 1998; pp 486-

Munshi, P.; Main, A. D.; Linehan, J. C.; Tai, C. C. & Jessop, P. G. (2002). Hydrogenation of carbon dioxide catalyzed by ruthenium trimethylphosphine complexes: The accelerating effect of certain alcohols and amines. *J. Am. Chem. Soc.,* Vol.124, No.27, pp.

Ng, S. M.; Yin, C. Q.; Yeung, C. H.; Chan, T. C. & Lau, C. P. (2004). Ruthenium-catalyzed hydrogenation of carbon dioxide to formic acid in alcohols. *Eur. J. Inorg. Chem.,* No.9,

Ogo, S.; Kabe, R.; Hayashi, H.; Harada, R. & Fukuzumi, S. (2006). Mechanistic investigation of CO2 hydrogenation by Ru(II) and Ir(III) aqua complexes under acidic conditions: two catalytic systems differing in the nature of the rate determining step. *Dalton Trans.,*

Ohnishi, Y. Y.; Matsunaga, T.; Nakao, Y.; Sato, H. & Sakaki, S. (2005). Ruthenium(II) catalyzed hydrogenation of carbon dioxide to formic acid. theoretical study of real catalyst, ligand effects, and solvation effects. *J. Am. Chem. Soc.,* Vol.127, No.11, pp. 4021-

Papp, G.; Csorba, J.; Laurenczy, G. & Joó, F. (2011). A Charge/Discharge Device for Chemical Hydrogen Storage and Generation. *Angew. Chem. Int. Ed.,* Vol.50, No.44, pp. 10433-

Sakamoto, M.; Shimizu, I. & Yamamoto, A. (1994). Synthesis of the first carbon dioxide coordinated palladium(0) complex, Pd(2-CO2)(PMePh2)2. *Organometallics,* Vol.13, No.2,

Sanz, S.; Azua, A. & Peris, E. (2010a). '(6)-arene)Ru(bis-NHC)' complexes for the reduction of CO2 to formate with hydrogen and by transfer hydrogenation with *i*PrOH. *Dalton* 

Sanz, S.; Benítez, M. & Peris, E. (2010b). A New Approach to the Reduction of Carbon Dioxide: CO2 Reduction to Formate by Transfer Hydrogenation in *i*PrOH.

Schmeier, T. J.; Dobereiner, G. E.; Crabtree, R. H. & Hazari, N. (2011). Secondary coordination sphere interactions facilitate the insertion step in an iridium(III) CO2

*Trans.,* Vol.39, No.27, pp. 6339-6343, ISSN 1477-9234 (Electronic)

*Organometallics,* Vol.29, No.1, pp. 275-277, ISSN 0276-7333

(Electronic)

498.

Vol.39, No.22, pp. 5083-5088, ISSN 0020-1669

Vol.475, No.1-2, pp. 257-266, ISSN 0022-328X

No.39, pp. 4657-4663, ISSN 1477-9226 (Print)

7963-7971, ISSN 0002-7863

4032, ISSN 0002-7863

10435, ISSN 1521-3773

pp. 407-409, ISSN 0276-7333

pp. 1788-1793, ISSN 1434-1948


Langer, R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J.; Ben-David, Y. & Milstein, D. (2011). Low-Pressure Hydrogenation of Carbon Dioxide Catalyzed by an Iron Pincer Complex Exhibiting Noble Metal Activity. *Angew. Chem. Int. Ed.,* pp. 9948-9952, ISSN 1521-3773 (Electronic)

266 Hydrogenation

0276-7333

Patent 3968431 (filed on Jan. 21, 2003), 2007a.

pp. 306–309, ISSN 1010-6030

ISSN 0276-7333

pp. 863-864, ISSN

pp 489-511.

7345

aqueous solution and their application to asymmetric reduction using chiral Schiff base

Himeda, Y.; Onozawa-Komatsuzaki, N.; Sugihara, H.; Arakawa, H. & Kasuga, K. (2004). Half-sandwich complexes with 4,7-dihydroxy-1,10-phenanthroline: Water-soluble, highly efficient catalysts for hydrogenation of bicarbonate attributable to the generation of an oxyanion on the catalyst ligand. *Organometallics,* Vol.23, No.7, pp. 1480–1483, ISSN

Himeda, Y.; Onozawa-Komatsuzaki, N.; Sugihara, H.; Arakawa, H. & Kasuga, K. Japan

Himeda, Y.; Onozawa-Komatsuzaki, N.; Sugihara, H. & Kasuga, K. (2005). Recyclable catalyst for conversion of carbon dioxide into formate attributable to an oxyanion on the catalyst ligand. *J. Am. Chem. Soc.,* Vol.127, No.38, pp. 13118–13119, ISSN 0002-7863 Himeda, Y.; Onozawa-Komatsuzaki, N.; Sugihara, H. & Kasuga, K. (2006). Highly efficient conversion of carbon dioxide catalyzed by half-sandwich complexes with pyridinol ligand: The electronic effect of oxyanion. *J. Photochem. Photobiol. A-Chem.,* Vol.182, No.3,

Himeda, Y.; Onozawa-Komatsuzaki, N.; Sugihara, H. & Kasuga, K. (2007b). Simultaneous tuning of activity and water solubility of complex catalysts by acid-base equilibrium of ligands for conversion of carbon dioxide. *Organometallics,* Vol.26, No.3, pp. 702–712,

Horváth, H.; Laurenczy, G. & Kathó, Á. (2004). Water-soluble (6-arene)ruthenium(II) phosphine complexes and their catalytic activity in the hydrogenation of bicarbonate in aqueous solution. *J. Organomet. Chem.,* Vol.689, No.6, pp. 1036-1045, ISSN 0022328X Hutschka, F.; Dedieu, A.; Eichberger, M.; Fornika, R. & Leitner, W. (1997). Mechanistic aspects of the rhodium-catalyzed hydrogenation of CO2 to formic acid - A theoretical and kinetic study. *J. Am. Chem. Soc.,* Vol.119, No.19, pp. 4432-4443, ISSN 0002-7863 Inoue, Y.; Izumida, H.; Sasaki, Y. & Hashimoto, H. (1976). Catalytic fixation of carbon dioxideto formic acid by transition-metal complexes under mild conditions. *Chem. Lett.,*

Jessop, P. G., Homogeneous hydrogenation of carbon dioxide. In *Handbook of Homogeneous Hydrogenation*, De Vries, J. G.; Elsevier, C. J., Eds. Wiley-VCH: Weinheim, 2007; Vol. 1,

Jessop, P. G.; Hsiao, Y.; Ikariya, T. & Noyori, R. (1996). Homogeneous catalysis in supercritical fluids: Hydrogenation of supercritical carbon dioxide to formic acid, alkyl formates, and formamides. *J. Am. Chem. Soc.,* Vol.118, No.2, pp. 344-355, ISSN 0002-7863 Jessop, P. G.; Ikariya, T. & Noyori, R. (1994). Homogeneous Catalytic-Hydrogenation of Supercritical Carbon-Dioxide. *Nature,* Vol.368, No.6468, pp. 231-233, ISSN 0028-0836 Jessop, P. G.; Joó, F. & Tai, C.-C. (2004). Recent advances in the homogeneous hydrogenation of carbon dioxide. *Coord. Chem. Rev.,* Vol.248, No.21-24, pp. 2425-2442, ISSN 00108545 Joó, F.; Laurenczy, G.; Nadasdi, L. & Elek, J. (1999). Homogeneous hydrogenation of aqueous hydrogen carbonate to formate under exceedingly mild conditions - a novel possibility of carbon dioxide activation. *Chem. Commun.,* No.11, pp. 971-972, ISSN 1359-

ligands. *J. Mol. Catal. A-Chem.,* Vol.195, No.1-2, pp. 95–100, ISSN 1381-1169


reduction catalyst. *J. Am. Chem. Soc.,* Vol.133, No.24, pp. 9274-9277, ISSN 1520-5126 (Electronic)

**Chapter 11** 

© 2012 Sheshko and Serov, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Sheshko and Serov, licensee InTech. This is a paper distributed under the terms of the Creative Commons

**Hydrogenation of Carbon Oxides on Catalysts** 

One way of obtaining synthetic liquid fuels and valuable chemical compounds on the basis of non-oil raw materials (coal, natural gas, biomass) is the synthesis of hydrocarbons from CO and H2, which takes place with the participation of catalysts containing transition metals of Group VIII, known as the Fischer-Tropsch synthesis [1-3]. Although there are other methods for hydrocarbon mixtures of non-oil raw materials (for example, hydrogenation of coal and biomass pyrolysis and semi-coking coal), the priority development of the Fischer-Tropsch process clearly demonstrates its vitality and promise, as determined by an enormous source of raw materials - coal in the energy equivalent an order of magnitude

In addition to carbon monoxide for the hydrogenation reaction is possible, and repeatedly described the synthesis of hydrocarbons from mixtures containing carbon dioxide [3-6] and

nCO2 + (2n + m) H2 CnH2m + 2n H2O The use of carbon dioxide is one of the most promising directions of development of efficient catalytic systems that allow atmospheric pressure to convert the process emissions

Global trend, most pronounced in industrialized countries, it became tougher environmental legislation. It is aimed primarily at reducing harmful emissions, which led to a sharp increase in the number of works connected with the search for technologies that could be returned to the commercialization of gas by-products. Process emissions include both mono-and carbon dioxide, and hydrogen. Develop and implement new and improved catalysts, allowing atmospheric pressure to convert these emissions into olefins is one of the

**Bearing Fe, Co, Ni, and Mn Nanoparticles** 

T.F. Sheshko and Yu. M. Serov

http://dx.doi.org/10.5772/47743

**1. Introduction** 

higher than the oil.

Additional information is available at the end of the chapter

hydrogen according to the equation of general form:

containing both CO and CO2 in the olefins.

