Efficient co-upcycling of glycerol and CO2 into valuable products enabled by a bifunctional Ru-complex catalyst

Glycerol and CO2 are largely produced as by-products in modern industry. Herein, three Ru complexes bearing air- and water-stable NHC-nitrogen-nitrogen ligands are designed as bifunctional catalysts to upcycle glycerol and CO2 simultaneously. Among them, Ru complex 2 featuring an N-H structure shows the highest efficiency with TONs up to 300 000 and 387 000 for formate and lactate, respectively. 13C labelling experiments clearly manifest that formate is primarily derived from CO2. Furthermore, in situ FTIR spectra suggest that glyceraldehyde from glycerol might serve as a key intermediate to form lactate, while DFT calculations indicate that Ru complex 2 possesses the lowest reaction barriers and can form the RuN intermediate, contributing to its higher activity.

Nanosilica polyamidoamine dendrimers for enhanced direct air CO2 capture

Exploring efficient systems to recover CO2 from the atmosphere could be a way to address the global carbon emissions issue. Herein, we report the synthesis of nanosilica (NS) functionalized with polyamidoamine (PAMAM) dendrimers (NS-PAMAM) as efficient adsorbents for CO2 capture under simulated direct air capture (DAC) (400 ppm CO2 in helium at 30 °C) and indoor air (≥400 ppm, 50 ± 3% RH at 30 °C) conditions. The results inferred that the 1st (NS-G1.0), 2nd (NS-G2.0), 3rd (NS-G3.0), and 4th (NS-G4.0) generations of the NS-PAMAM dendrimers exhibited excellent performance for CO2 capture. Compared to the other generations, NS-G3.0 demonstrated superior CO2 adsorption capacities of 0.50 mmol g-1 under simulated dry CO2 conditions (400 ppm in He), 1.02 mmol g-1 under indoor air (dry) CO2 conditions (≥400 ppm, 26 ± 3% RH), and 1.54 mmol g-1 under indoor air (humid) CO2 conditions (≥400 ppm, 50 ± 3% RH). The study included the evaluation of CO2 adsorption-desorption performance of the NS-PAMAM dendrimers under varying structural and chemical parameters, kinetics, regeneration at low temperature (80 °C), as well as CO2 adsorption under humid conditions. Additionally, NS-G3.0 displayed a substantially superior performance with stable CO2 capture displayed during ten short temperature swing adsorption (TSA) cycles, making it a promising candidate for CO2 capture from ambient air. Finally, we demonstrated the recovery and reutilization of the captured CO2 for both the synthesis of formate via carbonate hydrogenation and for the production of calcium carbonate pellets.

The Impact of Metal Centers in the M-MOF-74 Series on Formic Acid Production

The confinement effect of porous materials on the thermodynamical equilibrium of the CO2 hydrogenation reaction presents a cost-effective alternative to transition metal catalysts. In metal-organic frameworks, the type of metal center has a greater impact on the enhancement of formic acid production than the scale of confinement resulting from the pore size. The M-MOF-74 series enables a comprehensive study of how different metal centers affect HCOOH production, minimizing the effect of pore size. In this work, molecular simulations were used to analyze the adsorption of HCOOH and the CO2 hydrogenation reaction in M-MOF-74, where M = Ni, Cu, Co, Fe, Mn, Zn. We combine classical simulations and density functional theory calculations to gain insights into the mechanisms that govern the low coverage adsorption of HCOOH in the surrounding of the metal centers of M-MOF-74. The impact of metal centers on the HCOOH yield was assessed by Monte Carlo simulations in the grand-canonical ensemble, using gas-phase compositions of CO2, H2, and HCOOH at chemical equilibrium at 298.15-800 K, 1-60 bar. The performance of M-MOF-74 in HCOOH production follows the same order as the uptake and the heat of HCOOH adsorption: Ni > Co > Fe > Mn > Zn > Cu. Ni-MOF-74 increases the mole fraction of HCOOH by ca. 105 times compared to the gas phase at 298.15 K, 60 bar. Ni-MOF-74 has the potential to be more economically attractive for CO2 conversion than transition metal catalysts, achieving HCOOH production at concentrations comparable to the highest formate levels reported for transition metal catalysts and offering a more valuable molecular form of the product.

Confinement Effects in Well-Defined Metal-Organic Frameworks (MOFs) for Selective CO2 Hydrogenation: A Review

Decarbonization has become an urgent affair to restrain global warming. CO₂ hydrogenation coupled with H₂ derived from water electrolysis is considered a promising route to mitigate the negative impact of carbon emission and also promote the application of hydrogen. It is of great significance to develop catalysts with excellent performance and large-scale implementation. In the past decades, metal-organic frameworks (MOFs) have been widely involved in the rational design of catalysts for CO₂ hydrogenation due to their high surface areas, tunable porosities, well-ordered pore structures, and diversities in metals and functional groups. Confinement effects in MOFs or MOF-derived materials have been reported to promote the stability of CO₂ hydrogenation catalysts, such as molecular complexes of immobilization effect, active sites in size effect, stabilization in the encapsulation effect, and electron transfer and interfacial catalysis in the synergistic effect. This review attempts to summarize the progress of MOF-based CO₂ hydrogenation catalysts up to now, and demonstrate the synthetic strategies, unique features, and enhancement mechanisms compared with traditionally supported catalysts. Great emphasis will be placed on various confinement effects in CO₂ hydrogenation. The challenges and opportunities in precise design, synthesis, and applications of MOF-confined catalysis for CO₂ hydrogenation are also summarized.

Nitrogen–nitrogen-functionalized N-heterocyclic carbene ruthenium(ii) complexes realized efficient CO2 hydrogenation to formate

Three new Ru–CNN complexes are synthesized for the hydrogenation of CO2 to formate. The Ru–CNN complex exhibits a long lifetime of over 400 h at 170 °C with a high TON of 6.5 × 105.

Reduction of Carbon Dioxide with Ammonia-Borane under Ambient Conditions: Maneuvering a Catalytic Way

In the development of alternatives to the traditional catalytic hydrogenation of CO₂ with gaseous H₂, employing nongaseous H₂ storage compounds as potential reductants for catalytic transfer hydrogenation of CO₂ is promising. Ammonia-borane, due to its high hydrogen storage capacity (19.6 wt %), has been used for catalytic transfer hydrogenation of several organic unsaturated compounds. However, a similar protocol involving catalytic transfer hydrogenation of less reactive CO₂ with NH₃BH₃ is yet to be realized experimentally. Herein, we demonstrate the first catalytic CO₂ transfer hydrogenation process for generating formate salt with NH₃BH₃ under ambient conditions (1 atm and 30 °C) employing a cationic "Ir(III)-abnormal NHC" catalyst via an electrophilic NH₃BH₃ activation route. It exhibited an initial turnover frequency of 686 h^-1 and a high turnover number (TON) of ≈1300 in just 4 h. Most significantly, the catalyst was durable enough to maintain long-term activity, and upon only periodic recharging of NH₃BH₃, it furnished a total TON of >4200 in 10 h.

Iridium(NHC)-Catalyzed Sustainable Transfer Hydrogenation of CO2 and Inorganic Carbonates

Iridium(NHC)-catalyzed transfer hydrogenation (TH) of CO2 and inorganic carbonates with glycerol were conducted, demonstrating excellent turnover numbers (TONs) and turnover frequencies (TOFs) for the formation of formate and lactate. Regardless of carbon sources, excellent TOFs of formate were observed (CO2: 10,000 h−1 and K2CO3: 10,150 h−1). Iridium catalysts modified with the triscarbene ligand showed excellent catalytic activity at 200 °C and are a suitable choice for this transformation which requires a high temperature for high TONs of formate. On the basis of the control experiments, the transfer hydrogenation mechanism of CO2 was proposed.

Sustainability in Ru- and Pd-based catalytic systems using N-heterocyclic carbenes as ligands

This review is a critical presentation of catalysts based on palladium and ruthenium bearing N-heterocyclic carbene ligands that have enabled a more sustainable approach to catalysis and to catalyst uses.

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

(2,2-Bipyridine-κ2 N,N′)chlorido[η6-1-methyl-4-(propan-2-yl)benzene]ruthenium(II) tetraphenylborate

The title complex, [RuCl(C10H14)(C10H8N2)](C24H20B), has monoclinic (P21) symmetry at 100 K. It was prepared by the reaction of the dichlorido[1-methyl-4-(propan-2-yl)benzene]ruthenium(II) dimer with 2,2′-bipyridine, followed by the addition of ammonium tetraphenylborate. The 1-methyl-4-(propan-2-yl)benzene group, the 2,2′-bipyridine unit and a chloride ion coordinate the ruthenium(II) atom, with the 1-methyl-4-(propan-2-yl)benzene ring and bipyridine moieties trans to each other. In the crystal, the complex cations are linked by C—H...Cl hydrogen bonds, forming chains parallel to [010]. These chains are linked by a number of C—H...π interactions, involving the phenyl rings of the tetraphenylborate anion and a pyridine ring of the bpy ligand, resulting in the formation of layers parallel to (10\overline{1}).

Generation and Quantification of Formate Ion Produced from Aqueous Sodium Bicarbonate in the Presence of Homogeneous Ruthenium Catalyst

Formic acid and its salts are an alternative source for hydrogen generation. In this study, we store hydrogen using the formate-bicarbonate cycle. Aqueous sodium bicarbonate is hydrogenated to form sodium formate, which can then be decomposed to release hydrogen and sodium bicarbonate. The hydrogenation step is carried out under mild conditions in the presence of a homogeneous ruthenium catalyst. Hydrogen charge is realized at 70 °C under a hydrogen pressure of 20 bar, achieving yields > 80% and turnover number > 610. The catalyst is stable and robust through numerous cycles of the hydrogenation reaction. The formate ion formed during the bicarbonate hydrogenation is assayed and quantified by ion chromatography.

Transfer hydrogenation of carbon dioxide and bicarbonate from glycerol under aqueous conditions

Catalytic transfer hydrogenation of CO₂ from glycerol to afford formic and lactic acid is an attractive path to valorizing two waste streams. The process is significantly more thermodynamically favorable than direct CO₂ hydrogenation.

Ruthenium (II) and Iridium (III) Complexes of N-Heterocyclic Carbene and Pyridinol Derived Bidentate Chelates: Synthesis, Characterization, and Reactivity

We report the synthesis and characterization of new ruthenium(II) and iridium(III) complexes of a new bidentate chelate, NHCR'-pyOR (OR = OMe, OtBu, OH and R' = Me, Et). Synthesis and characterization studies were done on the following compounds: four ligand precursors (1-4); two silver complexes of these NHCR'-pyOR ligands (5-7); six ruthenium complexes of the type [η6-(p-cymene)Ru(NHCR'-pyOR)Cl]X with R' = Me, Et and R = Me, tBu, H and X = OTf-, PF6- and PO2F2- (8-13); and two iridium complexes, [Cp*Ir(NHCMe-pyOtBu)Cl]PF6 (14) and [Cp*Ir(NHCMe-pyOH)Cl]PO2F2 (15). The complexes are air stable and were isolated in moderate yield. However, for the PF6- salts, hydrolysis of the PF6- counter anion to PO2F2- during t-butyl ether deprotection was observed. Most of the complexes were characterized by 1H and 13C-NMR, MS, IR, and X-ray diffraction. The ruthenium complexes [η6-(p-cymene)Ru(NHCMe-pyOR)Cl]OTf (R = Me (8) and tBu (9)) were tested for their ability to accelerate CO2 hydrogenation and formic acid dehydrogenation. However, our studies show that the complexes transform during the reaction and these complexes are best thought of as pre-catalysts.

Synthesis of Carboxylic Acids and Esters from CO2

The achievements in the synthesis of carboxylic acids and esters from CO₂ have been summarized and discussed.

Steric and Electronic Effects of Bidentate Phosphine Ligands on Ruthenium(II)-Catalyzed Hydrogenation of Carbon Dioxide

The reactivity difference between the hydrogenation of CO2 catalyzed by various ruthenium bidentate phosphine complexes was explored by DFT. In addition to the ligand dmpe (Me2 PCH2 CH2 PMe2 ), which was studied experimentally previously, a more bulky diphosphine ligand, dmpp (Me2 PCH2 CH2 CH2 PMe2 ), together with a more electron-withdrawing diphosphine ligand, PN(Me) P (Me2 PCH2 N(Me) CH2 PMe2 ), have been studied theoretically to analyze the steric and electronic effects on these catalyzed reactions. Results show that all of the most favorable pathways for the hydrogenation of CO2 catalyzed by bidentate phosphine ruthenium dihydride complexes undergo three major steps: cis-trans isomerization of ruthenium dihydride complex, CO2 insertion into the Ru-H bond, and H2 insertion into the ruthenium formate ion. Of these steps, CO2 insertion into the Ru-H bond has the lowest barrier compared with the other two steps in each preferred pathway. For the hydrogenation of CO2 catalyzed by ruthenium complexes of dmpe and dmpp, cis-trans isomerization of ruthenium dihydride complex has a similar barrier to that of H2 insertion into the ruthenium formate ion. However, in the reaction catalyzed by the PN(Me) PRu complex, cis-trans isomerization of the ruthenium dihydride complex has a lower barrier than H2 insertion into the ruthenium formate ion. These results suggest that the steric effect caused by the change of the outer sphere of the diphosphine ligand on the reaction is not clear, although the electronic effect is significant to cis-trans isomerization and H2 insertion. This finding refreshes understanding of the mechanism and provides necessary insights for ligand design in transition-metal-catalyzed CO2 transformation.

Insight into the electronic effect of phosphine ligand on Rh catalyzed CO2 hydrogenation by investigating the reaction mechanism

The effect of the outer coordination sphere of the diphosphine ligand on the catalytic efficiency of [Rh(PCH₂X^RCH₂P)₂]⁺ (X^R = CH₂, N–CH₃, CF₂) catalyzed CO₂ hydrogenation was studied. It was found that the hydricity of the metal hydride bond determined the activation energy of the rate determining step of the reaction.