Homogeneous Catalysis in Supercritical Fluids: Hydrogenation of Supercritical Carbon Dioxide to Formic Acid, Alkyl Formates, and Formamides

Enhancing Carbon Nanotube Yarns via Infiltration Filling with Polyacrylonitrile in Supercritical Carbon Dioxide

Carbon nanotube (CNT) fibers are renowned for their exceptional axial tensile strength and modulus. However, in yarn form, they frequently encounter transverse loading in practical applications, which exposes their suboptimal mechanical attributes rooted in inadequate inter-tube interactions and yarn surface defects. Efforts to mitigate micro-slippage among CNTs have encompassed gap-filling methodologies with varied materials, yet the outcomes have fallen short of expectations. This work aimed to enhance the mechanical properties of CNT yarns via infiltration with polyacrylonitrile (PAN) under supercritical carbon dioxide (sc-CO2) conditions. PAN was strategically chosen for its capability to undergo pre-oxidation and subsequent carbonization, leading to robust graphitic reinforcement. Leveraging sc-CO2's swelling and high permeability properties, the infiltration process effectively plugged interstitial spaces, elevating the yarn's tensile strength to 277.50 MPa and Young's modulus to 5094.05 MPa. Additional enhancements were realized after pre-oxidation, conferring a dense, reinforced shell structure that augmented tensile strength by 96.93% and Young's modulus by 298.80%. Scanning electron microscopy (SEM) analyses revealed a homogeneous PAN distribution within the yarn matrix, corroborated by X-ray photoelectron spectroscopy (XPS) evidence of C-N bonding, indicative of a successfully interlaced network. Consequently, this investigation introduces a novel strategy to tackle micro-slippage in CNT yarns, thereby achieving substantial improvements in their mechanical resilience.

Unraveling the role of substrate materials in governing the carbon/carbide growth of molten carbonate electrolysis of CO2

The interface interaction between deposited carbon and metallic electrode substrates in tuning the growth of CO2-derived products (e.g., amorphous carbon, graphite, carbide) is mostly unexplored for the high-temperature molten-salt electrolysis of CO2. Herein, the carbon deposition on different transition-metal cathodes was performed to reveal the role of substrate materials in the growth of cathodic products. At the initial stage of electrolysis, transition metals (e.g., Cr, Fe, Ni, and Co) that exhibit appropriate carbon-binding ability (in range of -30 to 60 kJ mol-1) allow carbon diffusing into and then dissociating from metal to form graphite, as the carbon-binding ability can be determined by the Gibbs free energy of formation of metallic carbides. The catalytic cathodes showing super strong (e.g., Ti, V, Mo, and W) or weak (e.g., Cu) carbon-binding ability produce stable carbides or amorphous carbon, respectively. However, the subsequent deposited carbon is immune to the catalysis of the substrate, forming amorphous carbon nanoparticles and nanofibers on the surface of carbides and graphite, respectively. This paper not only highlights the role of the catalytic cathodes for carbon deposition, but also offers a material selection principle for the controllable growth of CO2-derived products in molten salts.

Theoretical Prediction of Electrocatalytic Reduction of CO2 Using a 2D Catalyst Composed of 3 d Transition Metal and Hexaamine Dipyrazino Quinoxaline

Transition metals and organic ligands combine to form metal-organic frameworks (MOFs), which possess distinct active sites, large specific surface areas and stable porous structures, giving them considerable promise for CO2 reduction electrocatalysis. In the present study, using spin polarisation density-functional theory, a series of 2D MOFs constructed from 3d transition metal and hexamethylene dipyrazoline quinoxaline(HADQ) were investigated. The calculated binding energies between HADQ and metal atoms for the ten TM-HADQ monolayers were strong sufficient to stably disperse the metal atoms in the HADQ monolayers. Of the ten catalysts tested, seven (Sc, Ni, Cu, Zn, Ti, V and Cr) exhibited high CO2 reduction selectivity, while Mn, Fe and Co required pH values above 2.350, 6.461 and 6.363, respectively, to exhibit CO2 reduction selectivity. HCOOH was the most important producer for Sc, Zn, Ni and Mn, while CH4 was the main producer for Ti, Cr, Fe and V. Cu and Co were less selective, producing HCHO, CH3 OH, and CH4 simultaneously at the same rate-determining step and limiting potential. The Cu-HADQ catalyst had a high overpotential for the HCHO product (1.022 V), while the other catalysts had lower overpotentials between 0.016 V and 0.792 V. Thus, these results predict TM-HADQ to show excellent activity in CO2 electrocatalytic reduction and to become a promising electrocatalyst for CO2 reduction.

Molten salt electro-preparation of graphitic carbons

Graphite has been used in a wide range of applications since the discovery due to its great chemical stability, excellent electrical conductivity, availability, and ease of processing. However, the synthesis of graphite materials still remains energy-intensive as they are usually produced through a high-temperature treatment (>3000°C). Herein, we introduce a molten salt electrochemical approach utilizing carbon dioxide (CO2) or amorphous carbons as raw precursors for graphite synthesis. With the assistance of molten salts, the processes can be conducted at moderate temperatures (700-850°C). The mechanisms of the electrochemical conversion of CO2 and amorphous carbons into graphitic materials are presented. Furthermore, the factors that affect the graphitization degree of the prepared graphitic products, such as molten salt composition, working temperature, cell voltage, additives, and electrodes, are discussed. The energy storage applications of these graphitic carbons in batteries and supercapacitors are also summarized. Moreover, the energy consumption and cost estimation of the processes are reviewed, which provides perspectives on the large-scale synthesis of graphitic carbons using this molten salt electrochemical strategy.

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.

Microwave-Assisted, Metal-Free, Chemoselective N-Formylation of Amines using 2-Formyl-3-methyl-1H-imidazol-3-ium Iodide and In Situ Synthesis of Benzimidazole and Isocyanides

An efficient, environmentally benign, chemoselective, microwave-assisted N-formylation protocol of aromatic, aliphatic, alicyclic, benzylic amines, inactivated aromatic amines and sterically demanding heterocyclic amines using 2-formyl-1,3-dimethyl-1H-imidazol-3-ium iodide has been developed. This affords a series of N-substituted formamides with good to excellent yields (23 examples, 53–96% yield) and can be readily scaled. The methodology can be further extended to synthesize benzimidazole and isocyanide derivatives.

Homogeneous Catalytic CO2 Hydrogenation by [Fe]-Hydrogenase Bioinspired Complexes: A Computational Study

Computational modeling at the DLPNO-CCSD(T)/CBS//M06-L/def2-TZVP level of theory was used to propose four different iron catalysts whose structures were inspired on the [Fe]-hydrogenase active site: [Fe(^MeP^NNH^NP)(acmp)] (C(1), ^MeP^NNH^NP = 2,6-bis(dimethylphosphine), acmp = acylmethylpyridine), [Fe(C^NNH^NC)(acmp)] (C(2), C^NNH^NC = 2,6-bis(methylimidazol-2-ylidene)), [Fe(^MeP^NN^NP)(acmp)] (D(1), ^MeP^NN^NP = 2,6-bis((dimethylphosphine)pyridine)), and [Fe(C^NN^NC)(acmp)] (D(2), C^NN^NC = 2,6-bis((methylimidazol-2-ylidene) pyridine)). Through these electronic structure calculations, the catalytic mechanism of the reaction was explored. The intermediates and transition states present along the reaction coordinate were identified and described as to their equilibrium geometries, vibrational frequencies, and energies. Quasi-harmonic corrections were performed considering conditions analogous to those used experimentally. To compare the catalytic activities of the studied catalysts, turnover frequencies (TOFs) were calculated. Based on the explored catalytic cycles and TOF values (D(1) > C(1) > D(2) > C(2)), the most suitable iron catalysts are those with tridentate phosphine pincer-type ligands coordinated to the metal center. These systems are new promising iron catalysts to promote the CO₂ hydrogenation to formic acid without any use of bases or additives.

Formamidation of a wide range of substituted and functionalized amines with CO and a base

We have developed a base mediated formamidation of amines with CO under mild conditions, which allows for the synthesis of a wide range of aromatic and aliphatic formamides in high yields and gram amounts in the absence of a transition metal.

Formic Acid as H2 Storage System: Hydrogenation of CO2 and Decomposition of Formic Acid by Solid Molecular Phosphine Catalysts

The synthesis and decomposition of formic acid (FA) in aqueous triethylamine (NEt3) with solid molecular phosphine catalysts is demonstrated. Ru-catalyst based on the polymeric analog of 1,2-bis(diphenylphosphino)ethane presented the highest...

Hydroxycarbonylation of alkenes with formic acid using a rhodium iodide complex and alkyl ammonium iodide

Hydroxycarbonylation of alkenes using formic acid (HCOOH) is ideal for the synthesis of various carboxylic acids as a means to develop a sustainable reaction system with lower environmental impact. In this study, we developed a new catalytic system for hydroxycarbonylation of alkenes with HCOOH using a Vaska-type Rh complex with an iodide ligand, RhI(CO)(PPh3)2 (1), as the catalyst, and a quaternary ammonium iodide salt as the promoter for the catalyst. In comparison with similar reaction systems using Rh catalysts, our reaction system is safer and more environmentally friendly since it does not require high-pressure conditions, explosive gases, or environmentally unfriendly CH3I and extra PPh3 promoters. In addition, we also experimentally clarified that the catalytic reaction proceeds via RhHI2(CO)(PPh3)2 (2), which is formed by the reaction of 1 with a quaternary ammonium iodide salt and p-TsOH. Furthermore, the Rh(iii) complex 2 can catalyze hydroxycarbonylation of alkenes with HCOOH without any promoters.

Chromium-catalysed efficient N-formylation of amines with a recyclable polyoxometalate-supported green catalyst

A simple and efficient protocol for the formylation of amines with formic acid, catalyzed by a polyoxometalate-based chromium catalyst, is described. Notably, this method shows excellent activity and chemoselectivity for the formylation of primary amines; diamines have also been successfully employed. Importantly, the chromium catalyst is potentially non-toxic, environmentally benign and safer than the widely used high valence chromium catalysts such as CrO₃ and K₂Cr₂O₇. The catalyst can be recycled several times with a negligible impact on activity. Finally, a plausible mechanism is provided based on the observation of intermediate and control experiments.

Supercritical carbon dioxide application using hydantoin acrylamide for biocidal functionalization of polyester

Biocidal functionalization in polyester fibers is a really tough challenge because of the lack of tethering groups. This study indicated supercritical carbon dioxide application using N-halamine would be an alternative solution for obtaining antibacterial function on the polyester surface. Firstly, N-(2-methyl-1-(4-methyl-2,5-dioxo-imidazolidin-4 yl)propan-2 yl)acrylamide was synthesized and applied to the polyester in supercritical carbon dioxide medium, at 120 °C, 30 MPa for different processing times. The addition of N-halamine on the surface significantly brought antibacterial activity against E. coli. The chlorine loadings showed that 6 -h exposure time was critical to obtain sufficient antibacterial activity. This treatment caused a reasonable and tolerable loss in color and mechanical properties. But, the durability to abrasion, stability, and rechargeability of oxidative chlorine, and the durability of N-halamine on the surface were remarkably good. Conclusively, it can be available to work on polyester surfaces with resource-efficient and eco-friendly supercritical carbon dioxide technique for getting more functionalization and modification.

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.

Experimental and modeling evidence for structural crossover in supercritical CO_{2}

The physics of supercritical states is understood to a much lesser degree compared to subcritical liquids. Carbon dioxide, in particular, has been intensely studied, yet little is known about the supercritical part of its phase diagram. Here, we combine neutron scattering experiments and molecular dynamics simulations and demonstrate the structural crossover at the Frenkel line. The crossover is seen at pressures as high as 14 times the critical pressure and is evidenced by changes of the main features of the structure factor and pair distribution functions.

An Efficient and Practical System for the Synthesis of N,N-Dimethylformamide by CO2 Hydrogenation using a Heterogeneous Ru Catalyst: From Batch to Continuous Flow

In the context of CO₂ utilization, a number of CO₂ conversion methods have been identified in laboratory-scale research; however, only a very few transformations have been successfully scaled up and implemented industrially. The main bottleneck in realizing industrial application of these CO₂ conversions is the lack of industrially viable catalytic systems and the need for practically implementable process developments. In this study, a simple, highly efficient and recyclable ruthenium-grafted bisphosphine-based porous organic polymer (Ru@PP-POP) catalyst has been developed for the hydrogenation of CO₂ to N,N-dimethylformamide, which affords a highest ever turnover number of 160 000 and an initial turnover frequency of 29 000 h^-1 in a batch process. The catalyst is successfully applied in a trickle-bed reactor and utilized in an industrially feasible continuous-flow process with an excellent durability and productivity of 915 mmol h^-1  g_Ru ^-1 .

Identification of two novel trace impurities in mobile phases prepared with commercial formic acid

While liquid chromatography/high-resolution mass spectrometry (LC/HRMS) is a versatile analytical technique, it is also sensitive to trace impurities. These impurities may come from a variety of sources, including reagents, solvents, and the sample matrix itself. Impurities in reagents may become concentrated and elute as peaks when a gradient method is used, and these peaks may cause suppression of peaks of interest both in the electrospray source, as well as in the C-trap in systems that contain one.

METHODS

We observed a notable increase in the size of several impurity peaks in a reversed-phase gradient method upon switching suppliers of formic acid. We used LC/HRMS to separate and fragment these impurity compounds and assign probable formulae.

RESULTS

The mass spectra were compared with those of compounds found in the literature with the same formulae, and the observed peaks were matched to two amine compounds not previously reported as impurities in LC/MS systems: trihexylamine and N-methyldihexylamine. The identities were confirmed by high-resolution accurate mass and retention time matching against commercially available standards of these compounds.

CONCLUSIONS

To the best of our knowledge, this is the first time that trihexylamine and N-methyldihexylamine have been reported in such systems. We hypothesize that these are derived from the formic acid manufacturing process and recommend that users monitor purchased formic acid for the presence of impurities.

Copper-Catalyzed Formylation of Amines by using Methanol as the C1 Source

Cu/TEMPO catalyst systems are known for the selective transformation of alcohols to aldehydes, as well as for the simultaneous coupling of alcohols and amines to imines under oxidative conditions. In this study, such a Cu/TEMPO catalyst system is found to catalyze the N-formylation of a variety of amines by initial oxidative activation of methanol as the carbonyl source via formaldehyde and formation of N,O-hemiacetals and oxidation of the latter under very mild conditions. A vast range of amines, including aromatic and aliphatic, primary and secondary, and linear and cyclic amines are formylated under these conditions with good to excellent yields. Moreover, paraformaldehyde can be used instead of methanol for the N-formylation.

Structural inhomogeneity as a factor promoting the homogenous catalysis of CO2 hydrogenation by (PMe3)4RuH2

During homogenous catalysis by organometallic complexes, the dissociation of a ligand to produce an unsaturated site on the metal center is often invoked as the first step of activation, especially when photo-excitation is involved. In this theoretical study, we demonstrated that under mild conditions, a thermodynamically unstable yet dynamically favorable active intermediate could be produced by the inhomogeneity of the solvent distribution around the catalyst rather than by ligand dissociation. This occurred at the end of the first catalytic cycle when the product was eliminated. The empty site was immediately filled by one of the additive molecules aggregated around the reaction center even when the intermediate complex was unstable, producing a transient and more active catalyst. This process accounted for the accelerated reaction rate observed in the landmark CO₂ hydrogenation catalyzed by (PMe₃)₄RuH₂ in supercritical CO₂ when H₂O, MeOH, or HNMe₂ was added. This also suggests a new way to exploit the structural inhomogeneity around an organometallic complex for the design of superior catalysts.

Molecular Catalysis for Utilizing CO2 in Fuel Electro-Generation and in Chemical Feedstock

Processes for the conversion of CO2 to valuable chemicals are highly desired as a result of the increasing CO2 levels in the atmosphere and the subsequent elevating global temperature. However, CO2 is thermodynamically and kinetically inert to transformation and, therefore, many efforts were made in the last few decades. Reformation/hydrogenation of CO2 is widely used as a means to access valuable products such as acetic acids, CH4, CH3OH, and CO. The electrochemical reduction of CO2 using hetero- and homogeneous catalysts recently attracted much attention. In particular, molecular CO2 reduction catalysts were widely studied using transition-metal complexes modified with various ligands to understand the relationship between various catalytic properties and the coordination spheres above the metal centers. Concurrently, the coupling of CO2 with various electrophiles under homogeneous conditions is also considered an important approach for recycling CO2 as a renewable C-1 substrate in the chemical industry. This review summarizes some recent advances in the conversion of CO2 into valuable chemicals with particular focus on the metal-catalyzed reductive conversion and functionalization of CO2.

Direct Synthesis of Methyl Formate from CO2 With Phosphine-Based Polymer-Bound Ru Catalysts

Methyl formate was produced in one pot through the hydrogenation of CO₂ to formic acid/formate followed by an esterification step. The route offers the possibility to integrate renewable energy into the fossil-based chemical value chain. In this work, a phosphine-polymer-anchored Ru complex was shown to be an efficient solid catalyst for the direct hydrogenation of CO₂ to methyl formate. The 1,2-bis(diphenylphosphino)ethane-like polymer presented the highest activity with a turnover number (TON) of up to 3401 at 160 °C. The reaction parameters were systemically investigated to optimize the reaction towards the formation of methyl formate. This catalyst could be reused seven times without a significant decrease in activity. Evolution of the catalytic Ru center during the reaction was revealed, and a possible reaction mechanism was proposed.

Electrocatalytic Reduction of CO2 to Methanol by Iron Tetradentate Phosphine Complex Through Amidation Strategy

The iron complex of tetradentate tris[2-(diphenylphosphino) ethyl]phosphine (PP₃ ), [Fe(PP₃ )(MeCN)₂ ](BF₄ )₂ , was able to electrocatalytically reduce CO₂ to formate with a Faradaic efficiency (FE) of approximately 97.3 % in acetonitrile. Upon addition of diethylamine as a cocatalyst, electrocatalytic reduction to methanol was achieved with an FE of 68.5 %, and other products were formamide and formate. A mechanistic study suggested that the [FeH(PP₃ )](BF₄ ) hydride complex was the active species in the electrocatalysis. Added amine as cocatalyst could react with CO₂ to form carbamate, which could then be reduced to formamide and further to methanol. By contrast, free CO₂ could only be reduced to formate as the end-product.

Liquid phase hydrogenation of CO2 to formate using palladium and ruthenium nanoparticles supported on molybdenum carbide

We report the development of palladium nanoparticles supported on Mo₂C as an active catalyst for the liquid-phase hydrogenation of CO₂ to formate under mild reaction conditions (100 °C and 2.0 MPa of a 1 : 1 CO₂ : H₂ mixture).

CO2 hydrogenation by phosphorus–nitrogen PN3P-pincer iridium hydride complexes: elucidation of the deactivation pathway

PN³P–Ir pincer hydride complexes were synthesized and characterized as catalysts and key intermediates in the direct hydrogenation of CO₂ to formate under mild conditions.