Complex Catalysts Research Articles

Efficient formation of poly(ethylene glycol) polymer brushes on gold electrodes via surface-initiated electrochemically mediated ATRP (SI-eATRP)

Electrochemically mediated surface-initiated atom transfer radical polymerization (SI-eATRP) is an interesting grafting from technique used to grow, in a controlled way, polymeric films from a wide range of surfaces, including metal substrates. Aqueous SI-eATRP offers an appealing strategy for the preparation of polymers by using water-soluble monomers, employing an oxidatively stable catalyst complex. The goal of this work is to tailor compact poly-(oligo(ethylene glycol)methacrylate) (p-OEGMA) brushes on gold electrodes by combining the formation of self-assembled monolayers (SAMs) containing a suitable ATRP initiator molecule, and the SI-eATRP polymerization approach. We have investigated the effects of the SAM-initiator nature, the applied potential, and the duration of the polymerization process on the properties of the grafted p-OEGMA-brushes. In addition, we employed electrochemical quartz crystal microbalance measurements to evidence the living character of the polymerization. The final properties of the tailored p-OEGMA-brush were studied by cyclic voltammetry and electrochemical impedance spectroscopy, obtaining information on the electronic and ionic blocking behaviour of these films, under different experimental conditions. Further information on structural conformation, composition, and organization was obtained from different characterization techniques: IRRAS and XPS spectroscopies, contact angle measurements, and SEM and AFM microscopies. These techniques confirmed that an efficient formation of p-OEGMA-brushes via SI-eATRP was achieved. Thus, the designed SI-eATRP system offers an interesting option to functionalize a wide variety of substrates with water-soluble and biocompatible monomers. Such coatings present potential applications in various fields, including biomedical applications.

CO2 Activation with Manganese Tricarbonyl Complexes by an H-Atom Responsive Benzimidazole Ligand.

Herein, we report the synthesis and characterization of two manganese tricarbonyl complexes, MnI (HL)(CO)3 Br (1 a-Br) and MnI (MeL)(CO)3 Br (1 b-Br) (where HL=2-(2'-pyridyl)benzimidazole; MeL=1-methyl-2-(2'-pyridy)benzimidazole) and assayed their electrocatalytic properties for CO2 reduction. A redox-active pyridine benzimidazole ancillary ligand in complex 1 a-Br displayed unique hydrogen atom transfer ability to facilitate electrocatalytic CO2 conversion at a markedly lower reduction potential than that observed for 1 b-Br. Notably, a one-electron reduction of 1 a-Br yields a structurally characterized H-bonded binuclear Mn(I) adduct (2 a') rather than the typically observed Mn(0)-Mn(0) dimer, suggesting a novel method for CO2 activation. Combining advanced electrochemical, spectroscopic, and single crystal X-ray diffraction techniques, we demonstrate the use of an H-atom responsive ligand may reveal an alternative, low-energy pathway for CO2 activation by an earth-abundant metal complex catalyst.

Synergistic Effect between Fe<sup>4+</sup> and Co<sup>4+</sup> on Oxygen Evolution Reaction Catalysis for CaFe<sub>1−</sub><i><sub>x</sub></i>Co<i><sub>x</sub></i>O<sub>3</sub>

Chemical substitution is an effective way to improve electrocatalytic properties in transition metal oxides. We investigate the synergistic effect between Fe4+ and Co4+ ions on the catalytic activity for oxygen evolution reaction (OER) in the Fe-Co-mixed perovskite oxide CaFe1–xCoxO3. The OER activity of CaFe1–xCoxO3 is substantially increased by small amounts of Co (Fe) doping into CaFeO3 (CaCoO3), leading to the superiority compared to the pure Fe and Co perovskite oxides. The x dependences of the OER overpotential and specific activity for CaFe1–xCoxO3 (0.05 ≦ x ≦ 0.95) are expressed by constant offset from the weighted average between CaFeO3 and CaCoO3, which can be interpreted to be the synergistic effect between Fe4+ and Co4+ ions on OER activity. The absence of the optimum x for the highest activity for CaFe1–xCoxO3 contrasts with the volcano-like plots reported in various mixed-metal oxides. First-principle calculations using the special quasirandom structure models on CaFe1–xCoxO3 (x = 0.03–0.5) demonstrate that about half the amount of Fe4+ is electronically activated to possess smaller charge-transfer energies, corroborating the enhancement of catalytic activity in CaFe1–xCoxO3. These findings provide new insight into the synergistic effects in complex transition metal oxide catalysts.

Recent advancement in CO2 capturing: An electrochemical approach

Various methods have been suggested to address the problem of elevated levels of carbon dioxide (CO2) in the atmosphere. A potential technique among these strategies is the electrochemical reduction of CO2 (ERC), which can simultaneously solve issues like CO2-induced global warming and enable sustainable energy storage. The ERC procedure takes place within an electrochemical cell at the interface between an ionic conductor (the electrolyte) and an electron conductor (the cathode). In this process, the anode facilitates the oxidation of water, while the cathode enables CO2 reduction. However, a significant challenge in the ERC process lies in selecting an appropriate catalyst for the cathode material. In addition to explore the new transition metals, researchers have also investigated metal complex catalysts and nanostructures to enhance catalyst activity and product selectivity. Non-aqueous electrolytes have been employed to avoid the hydrogen evolution reaction. Ionic liquids have been proposed as a solution to address the challenge of low CO2 solubility in the reaction medium, offering significant potential for enhancing conversion rates to resolve the limitations faced by aqueous and non-aqueous systems solid polymer electrolytes are being considered. The cell configuration has been continuously improved to improve the mass transfer effects and to better separate the reaction products. This review paper provides a preface to the ERC process and an inclusive review of several decades of research work on ERC process by analyzing the adopted various and novel cathode materials, electrolytes and cell configuration.

Cationic Cobalt(II) Bisphosphine Hydroformylation Catalysis: In Situ Spectroscopic and Reaction Studies.

[HCo(CO)x(bisphosphine)](BF4), x = 1-3, is a highly active hydroformylation catalyst system, especially for internal branched alkenes. In situ infrared spectroscopy (IR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance studies support the proposed catalyst formulation. IR studies reveal the formation of a dicationic Co(I) paramagnetic CO-bridged dimer, [Co2(μ-CO)2(CO)(bisphosphine)2]2+, at lower temperatures formed from the reaction of two catalyst complexes via the elimination of H2. DFT studies indicate a dimer structure with square-pyramidal and tetrahedral cobalt centers. This monomer-dimer equilibrium is analogous to that seen for HCo(CO)4, reacting to eliminate H2 and form Co2(CO)8. EPR studies on the catalyst show a high-spin (S = 3/2) Co(II) complex. Reaction studies are presented that support the cationic Co(II) bisphosphine catalyst as the catalyst species present in this system and minimize the possible role of neutral Co(I) species, HCo(CO)4 or HCo(CO)3(phosphine), as catalysts.

The study of the catalytic reforming process of light resin of pyrolysis

The conditions enabling to obtain benzene and toluene by catalytic processing of light resin are studied. Such resin is an environmentally hazardous waste of hydrocarbon production. The process of catalytic reforming of pyrolysis light resin was studied under laboratory conditions. The use of an aluminum complex catalyst in the process of pyrolysis of light resin allows reducing the temperature and pressure in the system. At the same time, the mass fraction of valuable aromatic hydrocarbons (benzene, toluene and other substances) increases in the system. The distillation residue of light resin can be used in the preparation of paints and varnishes, and as an insulator in electrical lines. Thus, the use of an aluminum-containing catalytic complex enables to accelerate the production of benzene and toluene. In this case, other valuable products are also obtained due to the pyrolysis of difficult-to-recycle, environmentally hazardous liquid waste.

N-Heterocyclic Nitrenium-Catalyzed Photohomolysis of CF3SO2Cl for Alkene Trifluoromethylation.

N-Heterocyclic nitreniums (NHNs) have been utilized as Lewis acid catalysts to activate substrates with lone pairs. Alternative to their conventional applications, we have discovered that NHNs can also serve as charge transfer complex catalysts. Herein, we present another potential of NHNs by utilizing a weak interaction between NHNs and CF3SO2Cl. The method promotes CF3SO2Cl to undergo photohomolysis, resulting in the CF3 radical. Mechanistic studies suggested that the weak interaction could be due to the π-hole effect of NHNs.

Microwave-Assisted Palladium Acetate-Catalyzed C-P Cross-Coupling of Arylboronic Acids and >P(O)H Reagents in the Absence of the Usual Mono- and Bidentate P-Ligands: Mechanistic Insights.

A less-studied halogen-free variation of the Hirao reaction involving the coupling of arylboronic acids with >P(O)H reagents, such as diarylphosphine oxides, diethyl phosphite, and ethyl phenyl-H-phosphinate, was investigated in detail using Pd(OAc)2 as the catalyst precursor and applying some excess of the P-reagent to supply the ligand via its trivalent tautomeric (>P-OH) form. The optimum conditions (1.2 equiv of the P-reagent, 135-150 °C, and air) were explored for the synthesis of diaryl-phenylphosphine oxides, aryl-diphenylphosphine oxides, diethyl arylphosphonates, ethyl diphenylphosphinate, and two bisphosphinoyl derivatives. In the reaction of 4-chlorophenyl- or 3-chlorophenylboronic acid with Ph2P(O)H, triphenylphosphine oxide was also formed as a byproduct. Theoretical calculations suggested that the catalytic cycle of the P-C coupling of PhB(OH)2 with Ph2P(O)H is different from that of the usual cross-coupling reactions. It comprises the addition of a phenyl anion and then the tautomeric form >P-OH of the >P(O)H reagent to the Pd2+ catalyst complex. This is then followed by reductive elimination affording Ph3PO that is accompanied with the conversion of Pd2+ to Pd0. There is a need for a subsequent stoichiometric oxidation of Pd(0) by molecular oxygen. The spontaneous formation of the self-assembling ligands around the Pd2+ center from the >P(O)H reactant plays a crucial role in the mechanism and promotes the efficiency of the catalyst.

Catalytic pyrolysis of cellulose and hemicellulose: Investigation on furans selectivity with different zeolite structures at microporous scale

Furans, as high-value chemicals, are very important target products for catalytic pyrolysis of biomass. In this work, the comparative performance of several different zeolites (SAPO18, SAPO11, SAPO34, and SSZ13 with different Si/Al ratios) for catalytic pyrolysis of cellulose and hemicellulose to generate furan compounds was investigated. SSZ13 was firstly found to improve the selectivity of the catalytic conversion of cellulose and hemicellulose to furans with up to 51.83% and 70.03%, respectively. The high selectivity of SSZ-13 was attributed to its smaller crystal size and higher total acid. SAPO11 was found for the first time to have a positive effect on the conversion of 5-HMF to 5-MFF. Correlation analysis suggested that isomerization was the limiting step in converting the cellulose to furans, while dehydration may be the determining step for hemicellulose. GC-MS results indicated the compound in the inner surface after pyrolysis was dominated by polycyclic aromatics and no furans were detected, supporting the hypothesis that the furans were formed on the external surface of zeolite. The difference in mass transfer rates formed on the inner and outer surfaces of the microporous zeolites contributed to the generation and transfer of furans. Compared to complex multifunctional catalysts, simple zeolite structures may have more application potential.

Reaction-Induced Metal-Metal Oxide Interactions in Pd-In2 O3 /ZrO2 Catalysts Drive Selective and Stable CO2 Hydrogenation to Methanol.

Ternary Pd-In2 O3 /ZrO2 catalysts exhibit technological potential for CO2 -based methanol synthesis, but developing scalable systems and comprehending complex dynamic behaviors of the active phase, promoter, and carrier are key for achieving high productivity. Here, we show that the structure of Pd-In2 O3 /ZrO2 systems prepared by wet impregnation evolves under CO2 hydrogenation conditions into a selective and stable architecture, independent of the order of addition of Pd and In phases on the zirconia carrier. Detailed operando characterization and simulations reveal a rapid restructuring driven by the metal-metal oxide interaction energetics. The proximity of InPdx alloy particles decorated by InOx layers in the resulting architecture prevents performance losses associated with Pd sintering. The findings highlight the crucial role of reaction-induced restructuring in complex CO2 hydrogenation catalysts and offer insights into the optimal integration of acid-base and redox functions for practical implementation.

Chiral Organocatalysts in Enantioselective CO2 Utilization Reactions

AbstractThe development of efficient CO2 utilization reactions has gained a significant amount of attention in recent years. Although transformations of CO2 to produce basic chemicals have been extensively investigated, the development of catalytic enantioselective CO2 utilization reactions for the preparation of fine chemicals remains limited at this stage. Several excellent methods for catalytic enantioselective CO2 utilization using chiral metal complex catalysts have been reported. Many researchers have also focused on developing organocatalyzed approaches to enantioselective CO2 utilization, and several excellent examples have appeared in recent years. Herein, recent advances in catalytic enantioselective CO2 utilization reactions using chiral organocatalysts are reviewed to provide a forecast for organocatalyzed enantioselective CO2 utilization research.

Constructing a CrCoNi medium entropy alloy coordinated Pt ultrathin film as durable electrocatalysts for methanol oxidation reaction

As high-efficiency anode methanol oxidation catalysts, platinum-based materials have always occupied a dominant place since the inception of direct methanol fuel cells (DMFCs). However, because of the strong adsorption of intermediates, Pt-active centers are easily poisoned and induce serious activity loss. Here, we investigate the same commonality of the oxygen evolution reaction (OER) and methanol oxidation reaction (MOR) mechanism in Pt/CrCoNi complex catalysts and establish a relationship between them to help screen appropriate synergistic materials and reduce activity loss. With this strategy, the OER activity of Pt/CrCoNi is modified via temperature and oxygen pressure control, and the OER overpotential and MOR durability show a positive correlation. The MOR performance of the final optimized Pt/CrCoNi (7.5–225 Torr oxygen pressure and 400 °C annealing, OA) film reaches an advanced level in the Pt-based MOR catalysts and shows a mass activity of 3785 A gPt-1, as well as a good durability of 50000 s in the alkaline solution.

Titania Nanoparticle Catalysed N‐Alkylation of Amines by Hydrogen Auto‐Transfer Mechanisms

AbstractUnlike well‐defined homogenous metal complex catalysts, no distinct heterogeneous catalysts are available for hydrogen auto‐transfer or borrowing hydrogen methodologies. Alternatively, a variety of noble metals, base metals, and bimetallic systems supported on various metal oxides are scrutinized for this purpose, mainly by modulating the ability of metals and supports in a multistep reaction. Herein, we demonstrate that pristine defect‐induced titania nanoparticle catalyst is an efficient and recyclable heterogeneous catalyst for the preparation of N‐alkylated products (>90 %) in the absence of expensive hydrogen source, costly ligands, additives, or costly metals under greener conditions. Furthermore, the nano‐sized titania catalyst tolerates functional groups with high yields. In addition, the ‐alkylation of aniline affords the corresponding alkylated products in d yields with long aliphatic alcohols. Finally, the results of the control tests show that a process of Hydrogen Auto transfer is responsible for carrying out the reaction.

Highly efficient recyclable copper (I) chloride complex catalysts having photo-Fenton activity for methylene blue degradation by sunlight

Highly efficient recyclable copper (I) chloride complex catalysts having photo-Fenton activity for methylene blue degradation by sunlight

Catalysts for Liquid-Phase Oxidation of Organic Compounds by Hydrogen Peroxide: Homogeneous and Phase-Transfer Systems

The review presents a comparative analysis of promising homogeneous metal complex catalysts and presents the results of studies on the synthesis and determination of the structural characteristics of effective catalysts for the oxidation of organic compounds with hydrogen peroxide – Q3{PO4[WO(O2)2]4} (Q is a quaternary ammonium cation) – using methods EXAFS, SAXS, NMR, Raman and IR spectroscopy. The possibilities of using bifunctional homogeneous peroxopolyoxo complexes of metals in combination with organic cations having quaternized nitrogen under the conditions of interfacial catalysis are considered, using examples of reactions of oxidation of various classes of organic compounds with hydrogen peroxide to obtain popular products – aliphatic and aromatic epoxides, mono- and dicarboxylic acids, as well as biologically active compounds for medical and agro-industrial purposes.

A thermoregulated phase-transfer ruthenium nanocatalyst for the atmospheric hydrogenation of α,β-unsaturated ketones

The selective hydrogenation of chalcone to dihydrochalcone under atmospheric hydrogen pressure is investigated using a thermoregulated phase-transfer ruthenium nanocatalyst, representing the first ruthenium nanocatalyst used for this reaction. Under the optimal conditions, the conversion of chalcone into dihydrochalcone and the selectivity are 98% and >99%, respectively. The ruthenium catalyst can be easily separated from the product and recycled three times without any decrease in the conversion or selectivity. The turnover frequency is higher than that of a reported ruthenium complex catalyst. The thermoregulated phase-transfer ruthenium nanocatalyst is also applied for the atmospheric hydrogenation of other α,β-unsaturated ketones.

Catalytic Reaction with Benzoic Acid Phenylacetylene in the Presence of Complex Catalysts

In this work, we studied the processes of vinyl ester formation by the addition of benzoic acid to phenylacetylene in the presence of complex catalysts of diacetate-di (2-amino-5-methylthio-1,3,4-thiadiazolo) cobalt(II) ([Co(C3H5N3S2)2(CH3COO)2] and tetrakis(2-amino-5-ethylthio-1,3,4-thiadiazole)-zinc(II) dinitrate ([Zn(C4H7S2)4](NO3)2): 1-phenylvinyl benzoate and anti-Markovnikov coupling product - styryl benzoate were obtained. The effect of the nature of solvents on the yields of products in the presence of the [Zn(C4H7S2)4](NO3)2 complex catalyst was studied. The structure of the synthesized vinyl esters was proved by IR, 1H NMR, 13C NMR spectroscopies.

Pyrolysis-Free Synthesis of Bimetal Phthalocyanine Covalent Organic Polymers/Ordered Mesoporous Carbon Nanocomposites for an Efficient Oxygen Reduction Reaction

The development of non-noble metal–nitrogen ring complex (M–N–C) catalysts with high activity and stability for the oxygen reduction reaction (ORR) is of great significance. However, M–N–C catalysts are generally prepared from precursors under high-temperature pyrolysis, which leads to complex product structures and makes it difficult to identify the catalytic active center. Herein, we have successfully structured a novel bimetal phthalocyanine covalent organic polymer/ordered mesoporous carbon (CoFe-COP/OMC) electrocatalyst through a pyrolysis-free approach. Due to the well-defined active sites (Fe/CoN4), ordered COP structure, and highly conductive carrier materials, the CoFe-COP/OMC nanocomposite exhibits remarkable ORR electrocatalytic activity with a half-wave potential and initial potential of 0.908 and 0.932 V (vs reversible hydrogen electrode (RHE)), respectively, a limiting current density of 5.35 mA cm–2, and a nearly four-electron reduction pathway. In addition, the acquired hybrid catalysts show excellent methanol resistance and electrochemical stability compared with Pt/C. The outstanding performance confirms that the CoFe-COP/OMC nanocomposite is a promising high-efficiency ORR catalyst for metal–air batteries and fuel cells.

Electrochemical CO2 Reduction to Methane by Cu Complex‐Derived Catalysts in Non‐Aqueous Media

AbstractElectrochemical CO2 reduction using Cu complex catalysts in non‐aqueous media leads exclusively to carbon monoxide or formic acid. This study reports highly selective electrochemical CO2 reduction to methane using a 2,9‐dimethyl‐1,10‐phenanthroline Cu2+ complex in dimethylformamide. XRD reveals the formation of a Cu complex‐derived catalyst during the electrochemical testing, containing Cu and an organic phase when the electrolyte contained tetrabutylammonium cations. Insulating carbonates were preferentially deposited when the electrolyte contained alkali metal cations, leading to low Faradaic efficiency for methane. The dramatic electrolyte dependence is explained by the relative solubility of deposits in dimethylformamide under reductive potentials. The Faradaic efficiency for methane production varied from sample to sample, with underlaying variation in the morphology of the catalyst. Selective methane production using Cu complex‐derived catalysts in non‐aqueous media is intriguing, opening the possibility of combinatorial studies of the effect of ligand structures on product selectivity by involving ligands not soluble in water.

The "weak" C-H···S interaction drives enantioselectivity in cinchona alkaloid complex catalyzed thiocyanation.

The great success of asymmetric organocatalysis has made it one of the most important advancements made in chemistry in the past two decades. A significant achievement in this context is the asymmetric organocatalysis of the thiocyanation reaction. In the current study, computational studies with density functional theory have been done in order to understand an interesting experimental finding: the reversal of enantioselectivity from R to S when the electrophile is changed from b-keto ester to oxindole for the thiocyanation reaction with the cinchona alkaloid complex catalyst. The calculations reveal an unusual fact - the principal reason for the reversal is the presence of the C-H···S noncovalent interaction, which is present only in the major transition states in each of the two nucleophile cases. Only recently has it been realized that the supposedly weak C-H···S noncovalent interaction has the properties of a hydrogen bond, and the fact that this interaction is the cause of enantioselectivity has relevance, because of the large number of asymmetric transformations involving the sulphur heteroatom.