Incorporation of carbon dioxide into molecules of acetylene hydrocarbons on heterogeneous Ag-containing catalysts

The reaction between 2-(2-phenylethynyl)aniline and carbon dioxide on heterogeneous Ag-containing catalysts can lead either to benzoxazine-2-one or to 4-hydroxyquinoline-2(1Н)-one, depending on the reaction conditions (nature of the base, CO₂ pressure). The structures of the products were confirmed by ¹Н and ¹³С NMR spectroscopy. The maximum yield of the products (60 and 30% for benzoxazine-2-one and 4-hydroxyquinolin-2(1Н)-one, respectively) is achieved on the catalyst Ag(1%)/γ-Al₂O₃(F). According to the results of physicochemical studies, the high activity of the catalysts in the reactions proceeding via triple bond activation results from the combination of three factors. First, the catalyst contains metallic silver particles with the size >2 nm; second, metallic silver particles coexist with silver cations; and third, strong acid sites are present on the support surface.     

Molecular approaches to the electrochemical reduction of carbon dioxide

This article reviews recent progress in the exploitation of carbon dioxide as a chemical feedstock. In particular, the design and development of molecular complexes that can act as catalysts for the electrochemical reduction of CO(2) is highlighted, and compared to other biological, metal- and non-metal-based systems.     

Accelerating net-zero carbon emissions by electrochemical reduction of carbon dioxide

Electroreduction of CO₂ shows great potential for global CO₂ utilization and uptake when collaborated with renewable electricity. Recent advances have been achieved in fundamental understanding and electrocatalyst development for CO₂ electroreduction. We think this research area has progressed to the stage where significant efforts can focus on translating the obtained knowledge to the development of largescale electrolyzers, which have the potential to accelerat...     

Photocatalytic carbon dioxide reduction by photocatalyst innovation

The photocatalytic conversion of carbon dioxide into sustainable fuel methanol using carbon quantum dots is highlighted in this paper. The multifaceted roles of carbon quantum dots in photocatalytic reactions and future directions of CQD materials are outlined.     

Theoretical insight into the solvent effect on the stoichiometric reduction of carbonyl compounds by ammonia borane and N-methyl amine borane

For the traditional reduction of ketones and aldehydes, NH₃BH₃ ( AB ) and N-methyl amine borane ( M _n AB ) have been effective reducing agents. However, the reaction process is indefinite and different mechanisms have been proposed; also the solvent effect, which is closely related to the mechanism, has not been considered seriously. Here we employ density functional theory to carry out a comprehensive study on the mechanism. The calculated free energy of the concerted double hydrogen transfer process is lower than the hydroboration mechanism by 4.7 kcal/mol, which indicates that reduction of carbonyl by AB is likely due to be the concerted double hydrogen transfer in both aprotic (tetrahydrofuran) and protic (MeOH) solvents. For the reduction by M _n AB , the corresponding free energies of all reactions are higher than those of AB . Meanwhile, the reduction of benzaldehyde by M _n AB (n = 1, 2) also favors a concerted double hydrogen transfer rather than hydroboration.     

Investigation of main group promoted carbon dioxide reduction

The reduction of carbon dioxide (CO₂) is of interest to the chemical industry, as many synthetic materials can be derived from CO₂. To help determine the reagents needed for the functionalization of carbon dioxide this experimental and computational study describes the reduction of CO₂ to formate and CO with hydride, electron, and proton sources in the presence of sterically bulky Lewis acids and bases. The insertion of carbon dioxide into a main group hydride, generating a main group formate, was computed to be more thermodynamically favorable for more hydridic (reducing) main group hydrides. A ten kcal/mol increase in hydricity (more reducing) of a main group hydride resulted in a 35% increase in the main group hydride's ability to insert CO₂ into the main group hydride bond. The resulting main group formate exhibited a hydricity (reducing ability) about 10% less than the respective main group hydride prior to CO₂ insertion. Coordination of a second identical Lewis acid to a main group formate complex further reduced the hydricity by about another 20%. The addition of electrons to the CO₂ adduct of ^tBu₃P and B(C₆F₅)₃ resulted in converting the sequestered CO₂ molecule to CO. Reduction of the CO₂ adduct of ^tBu₃P and B(C₆F₅)₃ with both electrons and protons resulted in only proton reduction.     

Molecular insight into the high selectivity of double-walled carbon nanotubes

Combining experimental knowledge with molecular simulations, we investigated the adsorption and separation properties of double-walled carbon nanotubes (DWNTs) against flue/synthetic gas mixture components (e.g. CO(2), CO, N(2), H(2), O(2), and CH(4)) at 300 K. Except molecular H(2), all studied nonpolar adsorbates assemble into single-file chain structures inside DWNTs at operating pressures below 1 MPa. Molecular wires of adsorbed molecules are stabilized by the strong solid-fluid potential generated from the cylindrical carbon walls. CO(2) assembly is formed at very low operating pressures in comparison to all other studied nonpolar adsorbates. The adsorption lock-and-key mechanism results from perfect fitting of rod-shaped CO(2) molecules into the cylindrical carbon pores. The enthalpy of CO(2) adsorption in DWNTs is very high and reaches 50 kJ mol(-1) at 300 K and low pore concentrations. In contrast, adsorption enthalpy at zero coverage is significantly lower for all other studied nonpolar adsorbates, for instance: 35 kJ mol(-1) for CH(4), and 14 kJ mol(-1) for H(2). Applying the ideal adsorption solution theory, we predicted that the internal pores of DWNTs have unusual ability to differentiate CO(2) molecules from other flue/synthetic gas mixture components (e.g. CO, N(2), H(2), O(2), and CH(4)) at ambient operating conditions. Computed equilibrium selectivity for equimolar CO(2)-X binary mixtures (where X: CO, N(2), H(2), O(2), and CH(4)) is very high at low mixture pressures. With an increase in binary mixture pressure, we predicted a decrease in equilibrium separation factor because of the competitive adsorption of the X binary mixture component. We showed that at 300 K and equimolar mixture pressures up to 1 MPa, the CO(2)-X equilibrium separation factor is higher than 10 for all studied binary mixtures, indicating strong preference for CO(2) adsorption. The overall selective properties of DWNTs seem to be superior, which may be beneficial for potential industrial applications of these novel carbon nanostructures.     

Controlled/living heterogeneous radical polymerization in supercritical carbon dioxide

Supercritical carbon dioxide (scCO₂) is an inexpensive and environmentally friendly medium for radical polymerizations. ScCO₂ is suited for heterogeneous controlled/living radical polymerizations (CLRPs), since the monomer, initiator, and control reagents (nitroxide, etc.) are soluble, but the polymer formed is insoluble beyond a critical degree of polymerization (J_crit). The precipitated polymer can continue growing in (only) the particle phase giving living polymer of controlled well‐defined microstructure. The addition of a colloidal stabilizer gives a dispersion polymerization with well‐defined colloidal particles being formed. In recent years, nitroxide‐mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT) polymerization have all been conducted as heterogeneous polymerizations in scCO₂. This Highlight reviews this recent body of work, and describes the unique characteristics of scCO₂ that allows composite particle formation of unique morphology to be achieved. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3711–3728, 2009     

New insight into the effect of interface supercapacitance on the performance of titanium dioxide/carbon nanowire array for photoelectrochemical water oxidation

The electrode/electrlyte interface is of great signifance to photoelectrochemical (PEC) water oxidation as the reaction mainly occur here. Herein, we focus on the effect of supercapactance of the electrode/electrlyte interface on the performance of PEC. It is discovered that the supercapacitor on the interface is crucial because it links the charge transport and solution ion adsorption on its two sides. In this study, we demonstrate an approach to promote the performance of TiO₂ nanowire array (TiO₂ NWs) photoanode in photoelectrochemical cells (PECs) by increasing its supercapacitance. A 2−5 nm carbon layer was coated and the interface supercapacitance increases by about 150 times. This enhances the separation rate of electron-hole pairs by collecting more holes. Meanwhile, it also promotes the water oxidation rate by adsorbing more OH⁻ on its surface. As a result, the photocurrent density of C-TiO₂ NWs was about 8 times higher than that of its carbon-free counterpart. This approach of increasing the supercapacitance of photoanodes would be attractive for enhancement of the efficiency of PECs and this work demonstrate the importance of supercapacitance of the interface for PECs.     

Theoretical analysis of pyrrole anions addition to carbon disulfide and carbon dioxide

Quantum chemical analysis (MP2/6‐31+G*) of the pyrrole anions addition to carbon disulfide and the substitution effects therein shows that pyrrole‐2 (5)‐carbodithioates are thermodynamically the most stable compounds, while 1‐isomer obtained from the unsubstituted pyrrole is likely a kinetic product. Steric hindrances destabilize N‐adducts when a methyl substituent appears in a 2(5) position and the 2,5‐dimethyl‐1‐pyrrolecarbodithioate anion turns out to be even less stable than the 2,5‐dimethyl‐3‐pyrrolecarbodithioate anion. By contrast, pyrrole‐1‐carboxylates are calculated to be the most stable adducts of CO₂ with pyrrole anions. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

The facile reduction of carbon dioxide to carbon monoxide with an amido-digermyne

Taking the fizz out: A digermyne compound with a Ge?Ge single bond has been shown to quantitatively reduce CO(2) to CO at temperatures as low as -40?°C. The mechanism of this unprecedented reaction has been probed by spectroscopic and computational techniques and involves a metastable intermediate (see picture; Ar*=C(6) H(2) {C(H)Ph(2) }(2) Me-2,6,4).     

From carbon dioxide to methane: homogeneous reduction of carbon dioxide with hydrosilanes catalyzed by zirconium-borane complexes

A mixture of a zirconium benzyl phenoxide complex and tris(pentafluorophenyl)borane is reported that catalyzes the hydrosilation reaction of carbon dioxide to generate methane via a bis(silyl)acetal intermediate.     

Transition metal-nitrogen sites for electrochemical carbon dioxide reduction reaction

Electrochemical CO₂ reduction reaction (CO₂RR) powered by renewable electricity has emerged as the most promising technique for CO₂ conversion, making it possible to realize a carbon-neutral cycle. Highly efficient, robust, and cost-effective catalysts are highly demanded for the near-future practical applications of CO₂RR. Previous studies on atomically dispersed metal-nitrogen (M-N_x) sites constituted of earth abundant elements with maximum atom-utilization efficiency have demonstrated their performance towards CO₂RR. This review summarizes recent advances on a variety of M-N_x sites-containing transition metal-centered macrocyclic complexes, metal organic frameworks, and M-N_x-doped carbon materials for efficient CO₂RR, including both experimental and theoretical studies. The roles of metal centers, coordinated ligands, and conductive supports on the intrinsic activity and selectivity, together with the importance of reaction conditions for improved performance are discussed. The mechanisms of CO₂RR over these M-N_x-containing materials are presented to provide useful guidance for the rational design of efficient catalysts towards CO₂RR.     

Electrochemical reduction of aliphatic conjugated dienes in the presence of carbon dioxide

A voltammetric and electrolytic study involved in the electroreductive carboxylation of multi-substituted aliphatic coujugated dienes has been successfully conducted. With methyl sorbate as the modal compound, acceptable yields of carboxylation and dimerization were achieved, which were influenced by various reaction conditions such as the supporting electrolyte, cathode nature, current density, charge passed and temperature. A correlation was first established between distinct electronic effects of the dienes and the electrochemical characteristics of their reduction and the distribution law of target products.     

Theoretical insight into the spectroscopy and photochemistry of isoalloxazine, the flavin core ring

The electronic singlet-singlet and singlet-triplet electronic transitions of the isoalloxazine ring of the flavin core are studied using second-order perturbation theory within the framework of the CASPT2//CASSCF protocol. The main features of the absorption spectrum are computed at 3.09, 4.28, 4.69, 5.00, and 5.37 eV. The lowest singlet (S1) and triplet (T1) excited states are found to be both of pi character with a singlet-triplet splitting of 0.57 eV. On the basis of the analysis of the computed spin-orbit couplings and the potential energy hypersurfaces built for the relevant excited states, the intrinsic mechanism for photoinduced population of T1 is discussed. Upon light absorption, evolution of the lowest singlet excited state along the relaxation pathway leads ultimately to the population of the lowest triplet state, which is mediated by a singlet-triplet crossing with a state of npi* type. Subsequently a radiationless decay toward T1 through a conical intersection takes place. The intersystem crossing mechanism and the internal conversion processes documented here provide a plausible route to access the lowest triplet state, which has a key role in the photochemistry of the flavin core ring and is mainly responsible for the reactivity of the system.