A theoretical study on the hydrogenation of CO2 to methanol catalyzed by ruthenium pincer complexes

First-Principles Insights into the Mechanism of CO2 Hydrogenation Reactions by Fe-PNP Pincer Complex

Using the state of the art theoretical methods, we have provided a comprehensive mechanistic understanding of the CO2 hydrogenation into HCOOH, H2CO, and CH3OH by 2,6-bis(diisopropylphosphinomethyl)pyridine (PNP)-ligated Fe pincer complex, featuring one CO and two H as co-ligands. For the computational investigation, a verified structural model containing methyl groups in place of the experimental isopropyl groups was used. Three catalytic conversions involving hydrogenation of CO2 into formic acid (HCOOH), HCOOH into formaldehyde and methanol were studied in different solvent medium. Our modelled complex appears to be a viable base-free catalyst for the conversion of CO2 into HCOOH and HCOOH into H2CO, based on the free energy profiles, which show apparent activation energy barriers of 16.28 kcal/mol and 23.63 kcal/mol for the CO2 to HCOOH and HCOOH to H2CO conversion, respectively. However, the computed results show that, due to the huge energy span of H2CO to CH3OH conversion, complete hydrogenation of CO2 into methanol could not occur under moderate conditions. Morpholine co-catalyst, which can lower the hydrogenation barrier by taking part in a simultaneous H-atom donation-acceptance process, could have assisted in completing this step.

Cyclopentadienone Diphosphine Ruthenium Complex: A Designed Catalyst for the Hydrogenation of Carbon Dioxide to Methanol

The development of homogeneous metal catalysts for the efficient hydrogenation of carbon dioxide (CO2) into methanol (CH3OH) remains a significant challenge. In this study, a new cyclopentadienone diphosphine ligand (CPDDP ligand) was designed, which could coordinate with ruthenium to form a Ru-CPDDP complex to efficiently catalyze the CO2-to-methanol process using dihydrogen (H2) as the hydrogen resource based on density functional theory (DFT) mechanistic investigation. This process consists of three catalytic cycles, stage I (the hydrogenation of CO2 to HCOOH), stage II (the hydrogenation of HCOOH to HCHO), and stage III (the hydrogenation of HCHO to CH3OH). The calculated free energy barriers for the hydrogen transfer (HT) steps of stage I, stage II, and stage III are 7.5, 14.5, and 3.5 kcal/mol, respectively. The most favorable pathway of the dihydrogen activation (DA) steps of three stages to regenerate catalytic species is proposed to be the formate-assisted DA step with a free energy barrier of 10.4 kcal/mol. The calculated results indicate that the designed Ru-CPDDP and Ru-CPDDPEt complexes could catalyze hydrogenation of CO2 to CH3OH (HCM) under mild conditions and that the transition-metal owning designed CPDDP ligand framework be one kind of promising potential efficient catalysts for HCM.

The theoretical design of manganese catalysts with a Si-N-Si-C-Si-C six-membered ring core-based bowl-shaped quadridentate ligand for the hydrogenation of CO/CN bonds

Herein, a new series of bowl-shaped quadridentate ligands with a Si-N-Si-C-Si-C six-membered ring core and their manganese catalysts were designed using the density functional theory (DFT) method for the hydrogenation of unsaturated CX (XN, O) bonds. The frameworks of these ligands named by LYG (LYG = P(R1)2CH2Si(CH2)(CH3)NSi(CH3)(CH2Si(CH3)CH2P(R3)2)CH2P(R2)2) have a Si-N-Si-C-Si-C six-membered ring core at the bottom of the bowl structure and each Si atom links with one phosphorus arm (-CH2PR2). The Mn catalyst Mn(CO)-LYG was constructed to catalyze the hydrogenation of CO/CN bonds. The calculated results indicate that due to the bowl-shaped structure of LYG quadridentate ligands, these Mn catalysts could be advantageous not only in the tuneup of catalytic activity and stereoselectivity by modifying three phosphorus arms but also in the homogeneous catalyst immobilization by linking with the Si-N-Si-C-Si-C six-membered ring core using different supports. This work might provide theoretical insights to design new framework transition-metal catalysts for the hydrogenation of CX bonds.

Nanomaterials as catalysts for CO2 transformation into value-added products: A review

Carbon dioxide (CO₂₎ is the most important greenhouse gas (GHG), accounting for 76% of all GHG emissions. The atmospheric CO₂ concentration has increased from 280 ppm in the pre-industrial era to about 418 ppm, and is projected to reach 570 ppm by the end of the 21^st century. In addition to reducing CO₂ emissions from anthropogenic activities, strategies to adequately address climate change must include CO₂ capture. To promote circular economy, captured CO₂ should be converted to value-added materials such as fuels and other chemical feedstock. Due to their tunable chemistry (which allows them to be selective) and high surface area (which allows them to be efficient), engineered nanomaterials are promising for CO₂ capturing and/or transformation. This work critically reviewed the application of nanomaterials for the transformation of CO₂ into various fuels, like formic acid, carbon monoxide, methanol, and ethanol. We discussed the literature on the use of metal-based nanomaterials, inorganic/organic nanocomposites, as well as other routes suitable for CO₂ conversion such as the electrochemical, non-thermal plasma, and hydrogenation routes. The characteristics, steps, mechanisms, and challenges associated with the different transformation technologies were also discussed. Finally, we presented a section on the outlook of the field, which includes recommendations for how to continue to advance the use of nanotechnology for conversion of CO₂ to fuels.

The Origin of Stereoselectivity in the Hydrogenation of Oximes Catalyzed by Iridium Complexes: A DFT Mechanistic Study

Herein the reaction mechanism and the origin of stereoselectivity of asymmetric hydrogenation of oximes to hydroxylamines catalyzed by the cyclometalated iridium (III) complexes with chiral substituted single cyclopentadienyl ligands (Ir catalysts A1 and B1) under acidic condition were unveiled using DFT calculations. The catalytic cycle for this reaction consists of the dihydrogen activation step and the hydride transfer step. The calculated results indicate that the hydride transfer step is the chirality-determining step and the involvement of methanesulfonate anion (MsO^-) in this reaction is of importance in the asymmetric hydrogenation of oximes catalyzed by A1 and B1. The calculated energy barriers for the hydride transfer steps without an MsO^- anion are higher than those with an MsO^- anion. The differences in Gibbs free energies between TSA5-1fR/TSA5-1fS and TSB5-1fR/TSB5-1fS are 13.8/13.2 (ΔΔG^‡ = 0.6 kcal/mol) and 7.5/5.6 (ΔΔG^‡ = 1.9 kcal/mol) kcal/mol for the hydride transfer step of substrate protonated oximes with E configuration (E-2a-H⁺) with MsO^- anion to chiral hydroxylamines product R-3a/S-3a catalyzed by A1 and B1, respectively. According to the Curtin-Hammet principle, the major products are hydroxylamines S-3a for the reaction catalyzed by A1 and B1, which agrees well with the experimental results. This is due to the non-covalent interactions among the protonated substrate, MsO^- anion and catalytic species. The hydrogen bond could not only stabilize the catalytic species, but also change the preference of stereoselectivity of this reaction.

Challenges and Prospects in the Catalytic Conversion of Carbon Dioxide to Formaldehyde

Formaldehyde (HCHO) is a crucial C₁ building block for daily-life commodities in a wide range of industrial processes. Industrial production of HCHO today is based on energy- and cost-intensive gas-phase catalytic oxidation of methanol, which calls for exploring other and more sustainable ways of carrying out this process. Utilization of carbon dioxide (CO₂ ) as precursor presents a promising strategy to simultaneously mitigate the carbon footprint and alleviate environmental issues. This Minireview summarizes recent progress in CO₂ -to-HCHO conversion using hydrogenation, hydroboration/hydrosilylation as well as photochemical, electrochemical, photoelectrochemical, and enzymatic approaches. The active species, reaction intermediates, and mechanistic pathways are discussed to deepen the understanding of HCHO selectivity issues. Finally, shortcomings and prospects of the various strategies for sustainable reduction of CO₂ to HCHO are discussed.