Effect of Ligand Electronics on the Reversible Catalytic Hydrogenation of CO₂ to Formic Acid Using Ruthenium Polyhydride Complexes: A Thermodynamic and Kinetic Study

Abstract

Hydrogenation of CO2 to formic acid or formates is often carried out using catalysts of the type H4Ru­(PR3)3 (1). These catalysts are also active for the reverse reaction, i.e., the decomposition of formic acid to H2 and CO2. While numerous catalysts have been synthesized for reactions in both directions, the factors controlling the elementary steps of the catalytic cycle remain poorly understood. In this work, we synthesize a series of compounds of type H4Ru­(P­(C6H4R)3)3 containing both electron-donating and electron-withdrawing groups and analyze their influence on the kinetic and thermodynamic parameters of CO2 insertion and deinsertion. The data are correlated with the catalytic performance of the complexes through linear free-energy relationships. The results show that formic acid dissociation from the catalyst is rate-determining during CO2 hydrogenation, while deinsertion is critical for the decomposition reaction.

Probing the Interactions of Immobilized Ruthenium Dihydride Complexes with Metal Oxide Surfaces by MAS NMR: Effects on CO2 Hydrogenation

“…This is consistent with previous studies showing that the rate-determining step of Ru catalyzed CO 2 hydrogenation to formate esters is often esterification of the formate intermediate rather than insertion of CO 2 into the metal hydride. 29,49 Sanford and co-workers studied the relative rates of the two steps (1) hydrogenation of CO 2 to formic acid and (2) esterification of formic acid to methyl formate. 29 Indeed, esterification to methyl formate was very slow even at 135 °C but could be accelerated by the addition of Lewis acids such as Sc(OTf) 3 .…”

Reaction Network Analysis of the Ruthenium‐Catalyzed Reduction of Carbon Dioxide to Dimethoxymethane

“…It can be concluded that, with Lewis acid Al(OTf) 3 present, the rate of esterification to MF is several orders of magnitude higher, making the reduction step rate-determining and allowing the two steps to be combined as one. The reverse reaction was considered in our reaction network because ester hydrolysis [18] as well as the decomposition of formic acid to hydrogen and carbon dioxide gas [15,19] have already been reported in literature. The reversibility of CO 2 reduction and the following esterification was confirmed in a separate experiment, in which MF was employed as substrate instead of CO 2 at 60 bar H 2 and otherwise identical catalytic conditions with CO 2 formation being observed via in situ IR spectroscopy (peak at 2341 cm À 1 , for detailed documentation see the SI).…”

“…Even though catalyst modification, reaction parameter optimization as well as recyclability studies were conducted to dramatically increase turnover numbers and selectivities, the kinetic parameters of the reaction network are not well investigated. First advances in this direction mainly focused on methanol production and compared the complexes used [14] or investigated only a part of the entire cascade [15] . We envisioned that more detailed studies on the reaction network may provide crucial insights that can further contribute to the optimization of this promising process and its potential industrial realization.…”

“…First advances in this direction mainly focused on methanol production and compared the complexes used [14] or investigated only a part of the entire cascade. [15] We envisioned that more detailed studies on the reaction network may provide crucial insights that can further contribute to the optimization of this promising process and its potential industrial realization.…”

Unsupported Nanoporous Palladium Catalyst for Highly Selective Hydrogenation of Carbon Dioxide and Sodium Bicarbonate into Formate

“…Among these fine chemicals, formic acid (FA) is widely used for antibacterial, pharmaceutical and preservative [6–8] . Moreover, FA has been recognized as a liquid organic hydrogen carrier (LOHC) owing to its high stability, low toxicity and convenient storage and transportation [9–12] . Accordingly, great efforts have been exerted to generate FA via CO 2 hydrogenation and achieve economically feasible CO 2 ‐mediated hydrogen cycles [13] …”