ReviewRecent advances in the homogeneous hydrogenation of carbon dioxide
Introduction
It has been almost a decade since the field of homogeneously-catalyzed CO2 hydrogenation has been reviewed by one of us [1] and by Leitner [2]. Advances that have been reported since then include the discovery of new catalysts and more effective alcoholic co-catalysts, increases in mechanistic understanding, the greater use of alternative media, and the development of methods for the preparation of formamides other than DMF by CO2 hydrogenation. The authors have therefore prepared this review which covers advances published or in press since 1995, in addition to a more comprehensive review of the topic of aqueous-phase hydrogenation of CO2 or carbonates. Recent advances in the homogeneously-catalyzed hydrogenation of carbon dioxide to formic acid, formate esters, formamides, CO, methanol and other products are reviewed. Unanswered questions still remain even in the relatively well understood case of reduction to formic acid, including the role of the water or alcohol co-catalysts found to be helpful in many systems, and the kinetic role, if any, of added amine.
Section snippets
General considerations on the use of water as solvent
The highest volume process involving carbon dioxide as a raw material is its reduction and utilization as a C1 building block in green plant photosynthesis which takes place in a bulk aqueous environment. Several attempts have been made to achieve the homogeneous hydrogenation of CO2 in aqueous solutions. The possible primary products of this reaction are formic acid, formaldehyde, methanol, and methane, however, in general the exclusive formation of formic acid was observed (Eq. (1)):
Catalysis by ruthenium complexes bearing pendant amines
Placement of a basic amine group on a pendant arm of a ligand has been investigated by Lau’s group as a strategy for promoting the hydrogenation of CO2 [25]. The tethered amine could influence the heterolytic activation of H2 by replacing the external base. Alternatively or additionally, the protonated amine group could influence the CO2 insertion step by hydrogen bonding to the CO2 oxygen atoms. Complex 1 (n = 2 or 3) was found to catalyze the hydrogenation of CO2 in the absence of added base,
Synthesis of formamides
At the time of the previous review, by far the most efficient method for preparing dimethylformamide (DMF), using a homogeneous catalyst and CO2, was by reacting Me2NH, H2, and supercritical CO2 using RuCl2(PMe3)4 as catalyst (Eq. (10)). Not long afterward, this method was used to prepare DMF with a TON of 420,000 and with complete selectivity [21]. The reaction was found, by spectroscopic measurements of the reaction mixture at partial conversion, to proceed via CO2 hydrogenation to the
Synthesis of other products
Carboxylic acids have been prepared from methyl iodides, CO2 and H2 (Eq. (12), formation of [BH]I assumed). Fukuoka et al. [86] found that the hydrogenation with a 10:1 ratio of Co2(CO)8 and Ru3(CO)12 as catalysts gave up to 17 TON of acetic acid (with respect to the moles of Ru atoms). Similarly, benzoic acid and propanoic acid could be obtained in low yield from PhI and EtI, respectively. Byproducts from these reactions were CO from CO2 hydrogenation and methane, benzene, or ethane from RI
Conclusions
In summary, highly efficient homogeneous catalysts are now known for the hydrogenation of CO2 to formic acid and derivatives such as formamides. Active catalysts have been found for CO2 hydrogenation in water, organic solvents, supercritical CO2 and ionic liquids. The range of formamides that can be produced in high yield has been expanded greatly. However, there are still no high activity and selectivity homogeneous catalysts for the reduction of CO2 to methyl formate, acetic acid, methanol,
Acknowledgements
F.J. is grateful for the financial support of the National Research Fund of Hungary (OTKA T043365). P.G.J. and C.-C.T. acknowledge the support of the Division of Chemical Sciences, Office of Basic Energy Sciences, US Department of Energy (grant number DE-FG02-99ER14986). P.G.J., Canada Research Chair in Green Chemistry, also acknowledges the support of the Canada Research Chairs program.
References (95)
- et al.
J. Organomet. Chem.
(1994) - et al.
Inorg. Chim. Acta
(1997) - Y. Inoue, H. Izumida, Y. Sasaki, H. Hashimoto, Chem. Lett. (1976)...
- et al.
Organometallics
(1998) - et al.
Appl. Catal. A: Gen.
(2003) - Y. Himeda, N. Onozawa-Komatsuzaki, H. Sugihara, H. Arakawa, K. Kasuga, Proceedings of the 50th Symposium on...
- Y. Kayaki, T. Suzuki, T. Ikariya, Chem. Lett. (2001)...
- et al.
J. Organometal. Chem.
(2004) - et al.
Appl. Organometal. Chem.
(2000) - et al.
J. Organomet. Chem.
(1972)