Catalytic hydrogenation of carbon dioxide to methanol using molecular catalysts

• Katalytische Hydrierung von Kohlenstoffdioxid zu Methanol unter Verwendung molekularer Katalysatoren

Wesselbaum, Sebastian; Leitner, Walter (Thesis advisor); Klankermayer, Jürgen (Thesis advisor); Herres-Pawlis, Sonja (Thesis advisor)

Aachen (2016, 2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2016

Abstract

This thesis deals with the development and detailed investigation of the very first organometallic catalyst for the selective hydrogenation of CO2 to methanol. Organometallic catalysts were believed to allow the conversion of CO2 to methanol only via 1) the intermediate formation of CO via reverse watergas shift reaction (RWGS), leading to an unselective raction, or via 2) the intermediate formation of alkyl formates as stable intermediates, requiring a mixture of catalysts.The neutral complex [Ru(TMM)(Triphos)] (2) (TMM = trimethylenmethane, Triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane) was shown to be active in the selective hydrogenation of CO2 to methanol without the need for stable intermediates, when applied in combination with an acidic additive (e.g. HNTf2 = bis(trifluoromethane)sulfonimide). The activity per metal centre of this system was comparable with the activity per metal centre of classical heterogeneous methanol catalysts. Based on mechanistic investigations, the cationic complex [Ru(OAc)(Triphos)(S)]NTf2 (14) (S = solvent or free coordination site) was developed as molecular catalyst for a system free of any additives. Under reaction conditions, complex 2/HNTf2 as well as complex 14 formed the cationic formate complex [Ru(O2CH)(Triphos)(THF)]NTf2 (8a) which was characterised spectroscopically and shown to be an intermediate. DFT calculations supported the assumption of a stepwise reduction of CO2 within the coordination sphere of a ruthenium-Triphos centre: A series of hydride transfer and protonolysis steps lead to the reduction of CO2 to formic acid, formaldehyde, and finally methanol. The facial coordination of the Triphos ligand as well as the high thermal stability of Ru-Triphos complexes are crucial factors for the unprecedented activity of the investigated complexes. Recycling of the catalytic system 2/HNTf2 was demonstrated using a biphasic system based on 2 methyltetrahydrofurane and water. Finally, immobilisation of 2/HNTf2 in an ionic liquid allowed its application in a continuous-flow process for the hydrogenation of CO2 to methanol.