Homogeneous Reduction of Carbon Dioxide with Hydrogen

Aerobic Oxidative Synthesis of Formamides from Amines and Bioderived Formyl Surrogates

Herein we present a new strategy for the oxidative synthesis of formamides from various types of amines and bioderived formyl sources (DHA, GLA and GLCA) and molecular oxygen (O2) as oxidant on g-C3N4 supported Cu catalysts. Combined characterization data from EPR, XAFS, XRD and XPS revealed the formation of single CuN4 sites on supported Cuphen/C3N4 catalysts. EPR spin trapping experiments disclosed ⋅OOH radicals as reactive oxygen species and ⋅NR1R2 radicals being responsible for the initial C-C bond cleavage. Control experiments and DFT calculations showed that the successive C-C bond cleavage in DHA proceeds via a reaction mechanism co-mediated by ⋅NR1R2 and ⋅OOH radicals based on the well-equilibrated CuII and CuI cycle. Our catalyst has much higher activity (TOF) than those based on noble metals.

Recent Advances in Reversible Liquid Organic Hydrogen Carrier Systems: From Hydrogen Carriers to Catalysts

Liquid organic hydrogen carriers (LOHCs) have gained significant attention for large-scale hydrogen storage due to their remarkable gravimetric hydrogen storage capacity (HSC) and compatibility with existing oil and gas transportation networks for long-distance transport. However, the practical application of reversible LOHC systems has been constrained by the intrinsic thermodynamic properties of hydrogen carriers and the performances of associated catalysts in the (de)hydrogenation cycles. To overcome these challenges, thermodynamically favored carriers, high-performance catalysts, and catalytic procedures need to be developed. Here, significant advances in recent years have been summarized, primarily centered on regular LOHC systems catalyzed by homogeneous and heterogeneous catalysts, including dehydrogenative aromatization of cycloalkanes to arenes and N-heterocyclics to N-heteroarenes, as well as reverse hydrogenation processes. Furthermore, with the development of metal complexes for dehydrogenative coupling, a new family of reversible LOHC systems based on alcohols is described that can release H2 under relatively mild conditions. Finally, views on the next steps and challenges in the field of LOHC technology are provided, emphasizing new resources for low-cost hydrogen carriers, high-performance catalysts, catalytic technologies, and application scenarios.

Formic acid stability in different solvents by DFT calculations

CONTEXT

New technologies have been developed toward the use of green energies. The production of formic acid (FA) from carbon dioxide (CO[Formula: see text]) hydrogenation with H[Formula: see text] is a sustainable process for H[Formula: see text] storage. However, the FA adduct stabilization is thermodynamically dependent on the type of solvent and thermodynamic conditions. The results suggest a wide range of dielectric permittivity values between the dimethyl sulfoxide (DMSO) and water solvents to stabilize the FA in the absence of base. The thermodynamics analysis and the infrared and charge density difference results show that the formation of the FA complex with H[Formula: see text]O is temperature dependent and has a major influence on aqueous solvents compared to the FA adduct with amine, in good agreement with the experiment. In these conditions, the stability thermodynamic of the FA molecule may be favorable at non-organic solvents and dielectric permittivity values closer to water. Therefore, a mixture of aqueous solvents with possible ionic composition could be used to increase the thermodynamic stability of H[Formula: see text] storage in CO[Formula: see text] conversion processes.

METHODS

Using the Quantum ESPRESSO package, density functional theory (DFT) calculations were performed with periodic boundary conditions, and the electronic wave functions were expanded in plane waves. For the exchange-correlation functional, we use the vdW-DF functional with the inclusion of van der Waals (vdW) forces. Electron-ion interactions are treated by the projector augmented wave (PAW) method with pseudopotentials available in the PSlibrary repository. The wave functions and the electronic densities were expanded employing accurate cut-off energies of 6.80[Formula: see text]10[Formula: see text] and 5.44[Formula: see text]10[Formula: see text] eV, respectively. The electronic density was computed from the wave functions calculated at the [Formula: see text]-point in the first Brillouin-zone. Each structural optimization was minimized according to the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm, with force and energy convergence criteria of 25 meV[Formula: see text]Å[Formula: see text] and 1.36 meV, respectively. The electrostatic solvation effects were performed by the [Formula: see text] package with the Self-Consistent Continuum Solvation (SCCS) approach.

Synergistic Catalysis in Heterobimetallic Complexes for Homogeneous Carbon Dioxide Hydrogenation

Two heterobimetallic Mo,M’ complexes (M’ = IrIII, RhIII) were synthesized and fully characterized. Their catalytic activity in homogeneous carbon dioxide hydrogenation to formate was studied. A pronounced synergistic effect between the two metals was found, most notably between Mo and Ir, leading to a fourfold increase in activity compared with a binary mixture of the two monometallic counterparts. This synergism can be attributed to spatial proximity of the two metals rather than electronic interactions. To further understand the nature of this interaction, the mechanism of the CO2 hydrogenation to formate by a monometallic IrIII catalyst was studied using computational and spectroscopic methods. The resting state of the reaction was found to be the metal-base adduct, whereas the rate-determining step is the inner-sphere hydride transfer to CO2. Based on these findings, the synergism in the heterobimetallic complex is beneficial in this key step, most likely by further activating the CO2.

Single-Site Iridium Picolinamide Catalyst Immobilized onto Silica for the Hydrogenation of CO2 and the Dehydrogenation of Formic Acid

The development of an efficient heterogeneous catalyst for storing H₂ into CO₂ and releasing it from the produced formic acid, when needed, is a crucial target for overcoming some intrinsic criticalities of green hydrogen exploitation, such as high flammability, low density, and handling. Herein, we report an efficient heterogeneous catalyst for both reactions prepared by immobilizing a molecular iridium organometallic catalyst onto a high-surface mesoporous silica, through a sol–gel methodology. The presence of tailored single-metal catalytic sites, derived by a suitable choice of ligands with desired steric and electronic characteristics, in combination with optimized support features, makes the immobilized catalyst highly active. Furthermore, the information derived from multinuclear DNP-enhanced NMR spectroscopy, elemental analysis, and Ir L₃-edge XAS indicates the formation of cationic iridium sites. It is quite remarkable to note that the immobilized catalyst shows essentially the same catalytic activity as its molecular analogue in the hydrogenation of CO₂. In the reverse reaction of HCOOH dehydrogenation, it is approximately twice less active but has no induction period.

We report the synthesis of a heterogeneous immobilized catalyst (Ir_PicaSi_SiO₂) and its successful application in aqueous CO₂ hydrogenation and FA dehydrogenation. The information derived from multinuclear DNP-enhanced NMR spectroscopy, elemental analysis, and XAS indicates the presence of cationic iridium sites in Ir_PicaSi_SiO₂. The latter shows essentially the same catalytic activity as its molecular analogue in the hydrogenation of CO₂. In the reverse reaction of HCOOH dehydrogenation, it is approximately twice less active but has no induction period.

Substituent’s Effects of PNP Ligands in Ru(II)-Catalyzed Hydrogenation of CO2 to Formate: Theoretical Analysis Considering Steric Hindrance and Promotion of Hydrogen Bonding

This paper investigates the effects of substituents in PNP-type ruthenium complexes in the catalytic hydrogenation of CO2 to formate using the DFT method. Six groups were considered as substituents linked to the P atom of the PNP ligand: hydrogen, methyl, iso-propyl, tert-butyl, cyclopentyl, and cyclohexyl. The substituent effects were analyzed from the perspectives of steric hindrance and promotion of hydrogen bonding. With the joint functions of steric hindrance and hydrogen bonding promotion during the CO2 coordination step, hydride addition step, and HCOO− rotation step, these groups exhibited very different substituent effects. The results showed that the methyl group was the most favorable substituent when the solvent’s effects were not included, as it formed hydrogen bonding with relatively weak steric hindrance. The second favorable substituent was the iso-propyl group, while the tert-butyl group was the most unfavorable one, due to remarkable steric hindrance. When the substituent was cyclopentyl or cyclohexyl, the complex provided a wider open space for the reaction compared with the tert-butyl-substituted complex, because cyclopentyl and cyclohexyl are cyclic groups. Therefore, the principle for choosing the substituent in PNP-type complexes allowing the design of highly efficient catalysts for CO2 hydrogenation indicates that more hydrogen atoms but wider open space are ideal. In addition, the substituent’s effects can be markedly impacted by the solvent used.

Metal-Free Hydroxymethylation of Indole Derivatives with Formic Acid as an Alternative Way to Indirect Utilization of CO2

The selective N-alkylation of indole substrates remains an ongoing research challenge for the relative attenuated nucleophilicity toward nitrogen. Herein, we developed the hydroxymethylation of indole derivatives to afford N-alkylated indole products with formic acid. This metal-free process was promoted by the organic base 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) using phenylsilane as the reductant under mild conditions. Besides, this strategy represents an alternative way for indirect utilization of CO₂, considering the facile hydrogenation of CO₂ to produce HCOOH.

Supported CuII Single-Ion Catalyst for Total Carbon Utilization of C2 and C3 Biomass-Based Platform Molecules in the N-Formylation of Amines

The shift from fossil carbon sources to renewable ones is vital for developing sustainable chemical processes to produce valuable chemicals. In this work, value‐added formamides were synthesized in good yields by the reaction of amines with C₂ and C₃ biomass‐based platform molecules such as glycolic acid, 1,3‐dihydroxyacetone and glyceraldehyde. These feedstocks were selectively converted by catalysts based on Cu‐containing zeolite 5A through the in situ formation of carbonyl‐containing intermediates. To the best of our knowledge, this is the first example in which all the carbon atoms in biomass‐based feedstocks could be amidated to produce formamide. Combined catalyst characterization results revealed preferably single Cu^II sites on the surface of Cu/5A, some of which form small clusters, but without direct linking via oxygen bridges. By combining the results of electron paramagnetic resonance (EPR) spin‐trapping, operando attenuated total reflection (ATR) IR spectroscopy and control experiments, it was found that the formation of formamides might involve a HCOOH‐like intermediate and ^.NHPh radicals, in which the selective formation of ^.OOH radicals might play a key role.

Recent advances in pincer-nickel catalyzed reactions

Organometallic catalysts have played a key role in accomplishing numerous synthetically valuable organic transformations that are either otherwise not possible or inefficient. The use of precious, sparse and toxic 4d and 5d metals are an apparent downside of several such catalytic systems despite their immense success over the last several decades. The use of complexes containing Earth-abundant, inexpensive and less hazardous 3d metals, such as nickel, as catalysts for organic transformations has been an emerging field in recent times. In particular, the versatile nature of the corresponding pincer-metal complexes, which offers great control of their reactivity via countless variations, has garnered great interest among organometallic chemists who are looking for greener and cheaper alternatives. In this context, the current review attempts to provide a glimpse of recent developments in the chemistry of pincer-nickel catalyzed reactions. Notably, there have been examples of pincer-nickel catalyzed reactions involving two electron changes via purely organometallic mechanisms that are strikingly similar to those observed with heavier Pd and Pt analogues. On the other hand, there have been distinct differences where the pincer-nickel complexes catalyze single-electron radical reactions. The applicability of pincer-nickel complexes in catalyzing cross-coupling reactions, oxidation reactions, (de)hydrogenation reactions, dehydrogenative coupling, hydrosilylation, hydroboration, C-H activation and carbon dioxide functionalization has been reviewed here from synthesis and mechanistic points of view. The flurry of global pincer-nickel related activities offer promising avenues in catalyzing synthetically valuable organic transformations.

Ni pincer complex catalytic hydroboration of CO2: a DFT study on the influence of borane reductants on selective reduction

An insight into the influence of borane reductants on the selectivity of hydroboration of CO₂ catalyzed by (tBuPCP)NiH.

Ph2PCH2CH2B(C8H14) and Its Formaldehyde Adduct as Catalysts for the Reduction of CO2 with Hydroboranes

We study two metal-free catalysts for the reduction of CO₂ with four different hydroboranes and try to identify mechanistically relevant intermediate species. The catalysts are the phosphinoborane Ph₂P(CH₂)₂BBN (1), easily accessible in a one-step synthesis from diphenyl(vinyl)phosphine and 9-borabicyclo[3.3.1]nonane (H-BBN), and its formaldehyde adduct Ph₂P(CH₂)₂BBN(CH₂O) (2), detected in the catalytic reduction of CO₂ with 1 as the catalyst but properly prepared from compound 1 and p-formaldehyde. Reduction of CO₂ with H-BBN gave mixtures of CH₂(OBBN)₂ (A) and CH₃OBBN (B) using both catalysts. Stoichiometric and kinetic studies allowed us to unveil the key role played in this reaction by the formaldehyde adduct 2 and other formaldehyde-formate species, such as the polymeric BBN(CH₂)₂(Ph₂P)(CH₂O)BBN(HCO₂) (3) and the bisformate macrocycle BBN(CH₂)₂(Ph₂P)(CH₂O)BBN(HCO₂)BBN(HCO₂) (4), whose structures were confirmed by diffractometric analysis. Reduction of CO₂ with catecholborane (HBcat) led to MeOBcat (C) exclusively. Another key intermediate was identified in the reaction of 2 with the borane and CO₂, this being the bisformaldehyde-formate macrocycle (HCO₂){BBN(CH₂)₂(Ph₂P)(CH₂O)}₂Bcat (5), which was also structurally characterized by X-ray analysis. In contrast, using pinacolborane (HBpin) as the reductant with catalysts 1 and 2 usually led to mixtures of mono-, di-, and trihydroboration products HCO₂Bpin (D), CH₂(OBpin)₂ (E), and CH₃OBpin (F). Stoichiometric studies allowed us to detect another formaldehyde-formate species, (HCO₂)BBN(CH₂)₂(Ph₂P)(CH₂O)Bpin (6), which may play an important role in the catalytic reaction. Finally, only the formaldehyde adduct 2 turned out to be active in the catalytic hydroboration of CO₂ using BH₃·SMe₂ as the reductant, yielding a mixture of two methanol-level products, [(OMe)BO]₃ (G, major product) and B(OMe)₃ (H, minor product). In this transformation, the Lewis adduct (BH₃)Ph₂P(CH₂)₂BBN was identified as the resting state of the catalyst, whereas an intermediate tentatively formulated as the Lewis adduct of compound 2 and BH₃ was detected in solution in a stoichiometric experiment and is likely to be mechanistically relevant for the catalytic reaction.

Computational prediction of pentadentate iron and cobalt complexes as a mimic of mono-iron hydrogenase for the hydrogenation of carbon dioxide to methanol

A series of amidate-ligated pentadentate iron and cobalt complexes with N-heterocyclic pyridinol groups were proposed and computationally screened as potential catalysts for CO₂ reduction. Density functional theory calculations reveal a ligand assisted heterolytic H₂ cleavage mechanism with a total free energy barrier of 23.3 kcal mol^-1 for the hydrogenation of CO₂ to methanol catalysed by a pentadentate Co complex with a 2-[bis(pyridine-2-ylmethyl)]amino-N-3,9-purin-2-one ligand.

Recent advances in nucleophile-triggered CO2-incorporated cyclization leading to heterocycles

CO₂, as a sustainable, feasible, abundant one-carbon synthon, has been utilized in carboxylative cyclization, carbonylative cyclization, and reductive cyclization.

Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment

CO₂ conversion covers a wide range of possible application areas from fuels to bulk and commodity chemicals and even to specialty products with biological activity such as pharmaceuticals. In the present review, we discuss selected examples in these areas in a combined analysis of the state-of-the-art of synthetic methodologies and processes with their life cycle assessment. Thereby, we attempted to assess the potential to reduce the environmental footprint in these application fields relative to the current petrochemical value chain. This analysis and discussion differs significantly from a viewpoint on CO₂ utilization as a measure for global CO₂ mitigation. Whereas the latter focuses on reducing the end-of-pipe problem "CO₂ emissions" from todays' industries, the approach taken here tries to identify opportunities by exploiting a novel feedstock that avoids the utilization of fossil resource in transition toward more sustainable future production. Thus, the motivation to develop CO₂-based chemistry does not depend primarily on the absolute amount of CO₂ emissions that can be remediated by a single technology. Rather, CO₂-based chemistry is stimulated by the significance of the relative improvement in carbon balance and other critical factors defining the environmental impact of chemical production in all relevant sectors in accord with the principles of green chemistry.

Chemical reduction of CO2facilitated by C-nucleophiles

This feature article describes recent advances in chemical reduction of CO₂facilitated by carbon-based molecular nucleophiles.