Hydrogenation of carbon dioxide to methanol using a homogeneous ruthenium–Triphos catalyst: from mechanistic investigations to multiphase catalysis

Hydrogenation of carbon dioxide to methanol using a homogeneous ruthenium–Triphos catalyst: from mechanistic investigations to multiphase catalysis

Type	Journal Article
Author	Sebastian Wesselbaum
Author	Verena Moha
URL	Free Full Text Source:  http://pubs.rsc.org/en/content/articlehtml/2015/sc/c4sc02087a
Volume	6
Issue	1
Pages	693-704
Publication	Chem. Sci.
Date	2015
Abstract	The hydrogenation of CO2 to methanol can be achieved using a single molecular organometallic catalyst. Whereas homogeneous catalysts were previously believed to allow the hydrogenation only via formate esters as stable intermediates, the present mechanistic study demonstrates that the multistep transformation can occur directly on the Ru–Triphos (Triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane) center. The cationic formate complex [(Triphos)Ru(η2-O2CH)(S)]+ (S = solvent) was identified as the key intermediate, leading to the synthesis of the analogous acetate complex as a robust and stable precursor for the catalytic transformation. A detailed mechanistic study using DFT calculations shows that a sequential series of hydride transfer and protonolysis steps can account for the transformation of CO2via formate/formic acid to hydroxymethanolate/formaldehyde and finally methanolate/methanol within the coordination sphere of a single Ru–Triphos-fragment. All experimental results of the systematic parameter optimisation are fully consistent with this mechanistic picture. Based on these findings, a biphasic system consisting of H2O and 2-MTHF was developed, in which the active cationic Ru-complex resides in the organic phase for recycling and methanol is extracted with the aqueous phase. The depletion of fossil carbon sources together with the increasing global energy consumption demand alternative ways for the sustainable production of fuels and chemicals. In this context, the usage of carbon dioxide (CO2) as an alternative carbon source has seen renewed and increasing interest at the interface of the chemical and energy sectors, as it is a readily available, non-toxic by-product of various large scale industrial processes.1–11 In particular, the effective hydrogenation of carbon dioxide to methanol could play an important role in supply chains with reduced carbon footprint economies, as methanol can serve as an energy carrier and a versatile basic chemical.12–15 Today, methanol is produced on a megaton scale from fossil feedstock-based syngas (CO/H2).15,16 These processes utilize heterogeneous catalysts at elevated temperatures (200–300 °C) and pressures (50–100 bar). A certain percentage of CO2 is added to the feedstock stream to balance the C/H ratio. The heterogeneously catalyzed hydrogenation of pure CO2 to methanol has been implemented, capitalizing on the specific regional energy and feedstock supply in Iceland, for example.17 A detailed picture of the elementary steps and the role of the multi-component catalyst material have been elucidated for the classical Cu/ZnO/Al2O3 systems, mapping out the complex series of bond cleavage and bond forming processes on the catalyst surface that enable the seemingly simple overall transformation of CO or CO2 and hydrogen to methanol.18 In sharp contrast, the hydrogenation of CO2 to methanol using a molecularly defined, single-site catalyst has remained elusive up to now. Tominaga et al. reported the formation of methanol, together with methane and CO, from CO2 hydrogenation using Ru3(CO)12 in the presence of alkaline iodides under harsh reaction conditions (240 °C, 80 bar). Under these conditions, CO2 was reduced to CO, followed by the hydrogenation of CO to methanol and methane.19 Later, the catalytic formation of methanol from CO2 was reported with organometallic complexes using high energy reduction reagents such as boranes.20 With hydrogen, indirect routes via the conversion of CO2-derived intermediates like organic carbonates, carbamates, formate esters and ureas were proposed (see Scheme 1, upper pathway, for formate esters). The viability of this concept was first demonstrated in the seminal work by Milstein et al., who developed highly efficient ruthenium(II) pincer complexes for the hydrogenation of these challenging substrates.21,22 Huff and Sanford reported a three-step, one-pot hydrogenation of CO2 to methanol via methyl formate as an intermediate using a combination of the Milstein catalyst with two other catalysts.23.