Saccharide formation by sustainable formose reaction using heterogeneous zeolite catalysts.

The formose reaction is a unique chemical reaction for the preparation of saccharides from formaldehyde, a single carbon compound. We applied zeolite materials as heterogeneous catalysts to the formose reaction. The simple addition of Linde type A zeolite containing calcium ions (Ca-LTA) to an aqueous solution of formaldehyde and glycolaldehyde produced saccharides at room temperature. A quantitative analysis performed by high-performance liquid chromatography revealed that triose, tetrose, pentose, and hexose saccharides were produced with few byproducts. Ca-LTA was recovered from the reaction mixture by filtration, and the retrieved zeolite was found to be reusable under the same conditions. The catalytic activity of Ca-LTA was higher than those of conventional calcium catalysts and other solid materials such as silica, alumina, and hydroxyapatite. Several other types of zeolites with different crystal structures and alkali/alkali-earth metal ions also showed catalytic activity for saccharide formation. Based on the analytical results obtained by infrared spectroscopy, temperature-programmed desorption profiles and NMR measurements, we propose a reaction mechanism in which C-C bond formation is promoted by the mild basicity of the oxygen atoms and acidity on the metal ions of the aluminosilicate on the zeolite surfaces with low SiO2/Al2O3 ratios.

Enhanced performance of molecular electrocatalysts for CO2 reduction in a flow cell following K+ addition.

Electrocatalytic CO2 reduction is a key aspect of artificial photosynthesis systems designed to produce fuels. Although some molecular catalysts have good performance for CO2 reduction, these compounds also suffer from poor durability and energy efficiency. The present work demonstrates the improved CO2 reduction activity exhibited by molecular catalysts in a flow cell. These catalysts were composed of a cobalt-tetrapyridino-porphyrazine complex supported on carbon black together with potassium salt and were both stable and efficient. These systems were found to promote electrocatalytic CO2 reduction with a current density of 100 mA/cm2 and generated CO over at least 1 week with a selectivity of approximately 95%. The optimal catalyst gave a turnover number of 3,800,000 and an energy conversion efficiency of more than 62% even at 200 mA/cm2.

Construction of an autocatalytic reaction cycle in neutral medium for synthesis of life-sustaining sugars.

Autocatalytic mechanisms in carbon metabolism, such as the Calvin cycle, are responsible for the biological assimilation of CO2 to form organic compounds with complex structures, including sugars. Compounds that form C-C bonds with CO2 are regenerated in these autocatalytic reaction cycles, and the products are concurrently released. The formose reaction in basic aqueous solution has attracted attention as a nonbiological reaction involving an autocatalytic reaction cycle that non-enzymatically synthesizes sugars from the C1 compound formaldehyde. However, formaldehyde and sugars, which are the substrate and products of the formose reaction, respectively, are consumed in Cannizzaro reactions, particularly under basic aqueous conditions, which makes the formose reaction a fragile sugar-production system. Here, we constructed an autocatalytic reaction cycle for sugar synthesis under neutral conditions. We focused on the weak Brønsted basicity of oxometalate anions such as tungstates and molybdates as catalysts, thereby enabling the aldol reaction, retro-aldol reaction, and aldose-ketose transformation, which collectively constitute the autocatalytic reaction cycle. These bases acted on sugar molecules of substrates together with sodium ions of a Lewis acid to promote deprotonation under neutral conditions, which is the initiation step of the reactions forming an autocatalytic cycle, whereas the Cannizzaro reaction was inhibited. The autocatalytic reaction cycle established using this abiotic approach is a robust sugar production system. Furthermore, we found that the synthesized sugars work as energy storage substances that sustain microbial growth despite their absence in nature.

Computational Analysis of Chemical Reactions Using a Variational Quantum Eigensolver Algorithm without Specifying Spin Multiplicity.

The analysis of a chemical reaction along the ground-state potential energy surface in conjunction with an unknown spin state is challenging because electronic states must be separately computed several times using different spin multiplicities to find the lowest energy state. However, in principle, the ground state could be obtained with just a single calculation using a quantum computer without specifying the spin multiplicity in advance. In the present work, ground-state potential energy curves for PtCO were calculated as a proof-of-concept using a variational quantum eigensolver (VQE) algorithm. This system exhibits a singlet-triplet crossover as a consequence of the interaction between Pt and CO. VQE calculations using a statevector simulator were found to converge to a singlet state in the bonding region, while a triplet state was obtained at the dissociation limit. Calculations performed using an actual quantum device provided potential energies within ±2 kcal/mol of the simulated energies after error mitigation techniques were adopted. The spin multiplicities in the bonding and dissociation regions could be clearly distinguished even in the case of a small number of shots. The results of this study suggest that quantum computing can be a powerful tool for the analysis of the chemical reactions of systems for which the spin multiplicity of the ground state and variations in this parameter are not known in advance.

Molecular Structure Optimization Based on Electrons-Nuclei Quantum Dynamics Computation.

A new concept of the molecular structure optimization method based on quantum dynamics computations is presented. Nuclei are treated as quantum mechanical particles, as are electrons, and the many-body wave function of the system is optimized by the imaginary time evolution method. The numerical demonstrations with a two-dimensional H2 + system and a H-C-N system exemplify two possible advantages of our proposed method: (1) the optimized nuclear positions can be specified with a small number of observations (quantum measurements) and (2) the global minimum structure of nuclei can be obtained without starting from any sophisticated initial structure and getting stuck in the local minima. This method is considered to be suitable for quantum computers, the development of which will realize its application as a powerful method.

Calculation of Core-Excited and Core-Ionized States Using Variational Quantum Deflation Method and Applications to Photocatalyst Modeling.

The possibility of performing quantum-chemical calculations using quantum computers has attracted much interest. Variational quantum deflation (VQD) is a quantum-classical hybrid algorithm for the calculation of excited states with noisy intermediate-scale quantum devices. Although the validity of this method has been demonstrated, there have been few practical applications, primarily because of the uncertain effect of calculation conditions on the results. In the present study, calculations of the core-excited and core-ionized states for common molecules based on the VQD method were examined using a classical computer, focusing on the effects of the weighting coefficients applied in the penalty terms of the cost function. Adopting a simplified procedure for estimating the weighting coefficients based on molecular orbital levels allowed these core-level states to be successfully calculated. The O 1s core-ionized state for a water molecule was calculated with various weighting coefficients, and the resulting ansatz states were systematically examined. The application of this technique to functional materials was demonstrated by calculating the core-level states for titanium dioxide (TiO2) and nitrogen-doped TiO2 models. The results demonstrate that VQD calculations employing an appropriate cost function can be applied to the analysis of functional materials in conjunction with an experimental approach.

Ab initio study on the excited states of pyrene and its derivatives using multi-reference perturbation theory methods.

Low-lying singlet excited states of pyrene derivatives originated from the 1La and 1Lb states of pyrene have decisive influences on their absorption and fluorescence emission behaviors. Calculation of these excited states with quantitative accuracy is required for the theoretical design of pyrene derivatives tailored to target applications; this has been a long-standing challenge for ab initio quantum chemical calculations. In this study, we explore an adequate computational scheme through calculations of pyrene and its phenyl-substituted derivatives using multi-reference perturbation theory (MRPT) methods. All valence π orbitals on the pyrene moiety were assigned to the active orbitals. Computational load was reduced by restricting the electron excitations within the active orbitals in the preparation of reference configuration space. A generalized multi-configuration quasi-degenerate perturbation theory (GMCQDPT) was adopted to treat the reference space other than the complete active space. The calculated 1La and 1Lb excitation energies of pyrene are in good agreement with the experimental values. Calculations of 1,3,6,8-tetraphenyl pyrene suggest that the energetic ordering of 1La and 1Lb is inverted through tetraphenyl substitution and its lowest singlet excited state is the 1La parentage of pyrene, which is consistent with the experimentally deduced scheme. These results are not readily obtained by MRPT calculations with a limited number of active orbitals and single-reference theory calculations. Diphenyl pyrenes (DPPy) were also calculated at the same level of theory to investigate the dependence on the substitution positions of phenyl groups.

Heterogeneous water oxidation photocatalysis based on periodic mesoporous organosilica immobilizing a tris(2,2'-bipyridine)ruthenium sensitizer.

A periodic mesoporous organosilica (PMO) containing 2,2'-bipyridine groups (BPy-PMO) has been shown to possess a unique pore wall structure in which the 2,2'-bipyridine groups are densely and regularly packed. The surface 2,2'-bipyridine groups can function as chelating ligands for the formation of metal complexes, thus generating molecularly-defined catalytic sites that are exposed on the surface of the material. We here report the construction of a heterogeneous water oxidation photocatalyst by immobilizing several types of tris(2,2'-bipyridine)ruthenium complexes on BPy-PMO where they function as photosensitizers in conjunction with iridium oxide as a catalyst. The Ru complexes produced on BPy-PMO in this work were composed of three bipyridine ligands, including the BPy in the PMO framework and two X2bpy, denoted herein as Ru(X)-BPy-PMO where X is H (2,2'-bipyridine), Me (4,4'-dimethyl-2,2'-bipyridine), t-Bu(4,4'-di-tert-butyl-2,2'-bipyridine) or CO2Me (4,4'-dimethoxycarbonyl-2,2'-bipyridine). Efficient photocatalytic water oxidation was achieved by tuning the photochemical properties of the Ru complexes on the BPy-PMO through the incorporation of electron-donating or electron-withdrawing functionalities. The reaction turnover number based on the amount of the Ru complex was improved to 20, which is higher than values previously obtained from PMO systems acting as water oxidation photocatalysts.

Re(bpy)(CO)3 Cl Immobilized on Bipyridine-Periodic Mesoporous Organosilica for Photocatalytic CO2 Reduction.

This paper describes the physicochemical properties of a rhenium (Re) complex [Re(bpy)(CO)3 Cl] immobilized on a bipyridine-periodic mesoporous organosilica (BPy-PMO) acting as a solid support. The immobilized Re complex generated a metal-to-ligand charge transfer absorption band at 400 nm. This wavelength is longer than that exhibited by Re(bpy)(CO)3 Cl in the polar solvent acetonitrile (371 nm) and is almost equal to that in nonpolar toluene (403 nm). The photocatalytic activity of this heterogeneous Re complex was lower than that of a homogeneous Re complex due to the reduced phosphorescence lifetime resulting from immobilization. However, the catalytic activity was enhanced by the co-immobilization of the ruthenium (Ru) photosensitizer [Ru(bpy)3 ]2+ on the PMO pore surfaces. Quantum chemical calculations suggest that electron transfer between the Ru and Re complexes occurs through interactions between the molecular orbitals in the pore walls. These results should have applications to the design of efficient heterogeneous CO2 reduction photocatalysis systems.

Photocatalytic CO2 Reduction by Periodic Mesoporous Organosilica (PMO) Containing Two Different Ruthenium Complexes as Photosensitizing and Catalytic Sites.

A periodic mesoporous organosilica (PMO) containing 2,2'-bipyridine (bpy) ligands within the framework (BPy-PMO) has great potential for designing novel catalysts by modifying metal complexes. A photosensitizing site (Ru(PS)) was introduced by treating cis-[Ru(bpy)2 (dimethylsulfoxide)Cl]Cl with BPy-PMO. Then a catalytic site (Ru(Cat)) was brought in Ru(PS)x -BPy-PMO by reaction with a ruthenium polymer [Ru(CO)2 Cl2 ]n . The stepwise modification of BPy-PMO successfully affords a novel photocatalyst Ru(PS)x -Ru(Cat)y -BPy-PMO. The molar fractions (x, y) of Ru(PS) and Ru(Cat) were determined by energy dispersive X-ray (EDX) measurement and quantification of the amount of CO emitted in the photo-decarbonylation of Ru(Cat), respectively. Photochemical CO2 reduction (λex >430 nm) by Ru(PS)x -Ru(Cat)y -BPy-PMO in a CO2 -saturated N,N-dimethylacetamide/water solution containing 1-benzyl-1,4-dihydronicotinamide catalytically produced CO and formate. The total turnover frequency of CO and formate reached more than 162 h-1 on x=0.11 and y=0.0055. The product selectivity (CO/formate) became large when the ratio of Ru(PS)-to-Ru(Cat) (x/y) was increased. The photocatalysts can be recycled at least three times without losing their catalytic activity, demonstrating that the Ru(PS) and Ru(Cat) units were strongly immobilized on the BPy-PMO framework.

Superior Low-Temperature Power and Cycle Performances of Na-Ion Battery over Li-Ion Battery.

The most simple and clear advantage of Na-ion batteries (NIBs) over Li-ion batteries (LIBs) is the natural abundance of Na, which allows inexpensive production of NIBs for large-scale applications. However, although strenuous research efforts have been devoted to NIBs particularly since 2010, certain other advantages of NIBs have been largely overlooked, for example, their low-temperature power and cycle performances. Herein, we present a comparative study of spirally wound full-cells consisting of Li0.1Na0.7Co0.5Mn0.5O2 (or Li0.8Co0.5Mn0.5O2) and hard carbon and report that the power of NIB at -30 °C is ∼21% higher than that of LIB. Moreover, the capacity retention in cycle testing at 0 °C is ∼53% for NIB but only ∼29% for LIB. Raman spectroscopy and density functional theory calculations revealed that the superior performance of NIB is due to the relatively weak interaction between Na+ ions and aprotic polar solvents.

Computational Evidence of Inversion of (1)La and (1)Lb-Derived Excited States in Naphthalene Excimer Formation from ab Initio Multireference Theory with Large Active Space: DMRG-CASPT2 Study.

The naphthalene molecule has two important lowest-lying singlet excited states, denoted (1)La and (1)Lb. Association of the excited and ground state monomers yields a metastable excited dimer (excimer), which emits characteristic fluorescence. Here, we report a first computational result based on ab initio theory to corroborate that the naphthalene excimer fluorescence is (1)La parentage, resulting from inversion of (1)La and (1)Lb-derived dimer states. This inversion was hypothesized by earlier experimental studies; however, it has not been confirmed rigorously. In this study, the advanced multireference (MR) theory based on the density matrix renormalization group that enables using unprecedented large-size active space for describing significant electron correlation effects is used to provide accurate potential energy curves (PECs) of the excited states. The results evidenced the inversion of the PECs and accurately predicted transition energies for excimer fluorescence and monomer absorption. Traditional MR calculations with smaller active spaces and single-reference theory calculations exhibit serious inconsistencies with experimental observations.

Electrochemical reactions of lithium-sulfur batteries: an analytical study using the organic conversion technique.

This investigation elucidates the electrochemical reaction process occurring within lithium-sulfur battery cells in detail, which has been unclear even after a half century of study primarily due to the very high reactivity of the polysulfide species. The polysulfide intermediates were deactivated by organic conversion - benzylization, and LC/MS and NMR analyses were first applied. The results demonstrate that the second voltage plateau in the discharge profile, which is the most important step in practical use because of its constant voltage, is dominated by the reduction of the Li2S3 intermediate. The first voltage plateau and the transition state between the plateaus, in which the voltage varies with the capacity, are associated with multiple reactions including the decomposition of S8 into Li2Sx (x = 1 to 7) and the transformation of Li2Sy (y = 4 to 8) into Li2Sz (z = 1 to 3). It is also revealed that longer polysulfide species, Li2Si (i = 6 to 8), are responsible for the redox shuttle phenomenon, which causes serious capacity degradation.

Boron clusters as highly stable magnesium-battery electrolytes.

Boron clusters are proposed as a new concept for the design of magnesium-battery electrolytes that are magnesium-battery-compatible, highly stable, and noncorrosive. A novel carborane-based electrolyte incorporating an unprecedented magnesium-centered complex anion is reported and shown to perform well as a magnesium-battery electrolyte. This finding opens a new approach towards the design of electrolytes whose likelihood of meeting the challenging design targets for magnesium-battery electrolytes is very high.

A solid chelating ligand: periodic mesoporous organosilica containing 2,2'-bipyridine within the pore walls.

Synthesis of a solid chelating ligand for the formation of efficient heterogeneous catalysts is highly desired in the fields of organic transformation and solar energy conversion. Here, we report the surfactant-directed self-assembly of a novel periodic mesoporous organosilica (PMO) containing 2,2'-bipyridine (bpy) ligands within the framework (BPy-PMO) from a newly synthesized organosilane precursor [(i-PrO)3Si-C10H6N2-Si(Oi-Pr)3] without addition of any other silane precursors. BPy-PMO had a unique pore-wall structure in which bipyridine groups were densely and regularly packed and exposed on the surface. The high coordination ability to metals was also preserved. Various bipyridine-based metal complexes were prepared using BPy-PMO as a solid chelating ligand such as Ru(bpy)2(BPy-PMO), Ir(ppy)2(BPy-PMO) (ppy = 2-phenylpyridine), Ir(cod)(OMe)(BPy-PMO) (cod = 1,5-cyclooctadiene), Re(CO)3Cl(BPy-PMO), and Pd(OAc)2(BPy-PMO). BPy-PMO showed excellent ligand properties for heterogeneous Ir-catalyzed direct C-H borylation of arenes, resulting in superior activity, durability, and recyclability to the homogeneous analogous Ir catalyst. An efficient photocatalytic hydrogen evolution system was also constructed by integration of a Ru-complex as a photosensitizer and platinum as a catalyst on the pore surface of BPy-PMO without any electron relay molecules. These results demonstrate the great potential of BPy-PMO as a solid chelating ligand and a useful integration platform for construction of efficient molecular-based heterogeneous catalysis systems.

Ab initio molecular orbital study on the excited states of [2.2]-, [3.3]-, and siloxane-bridged paracyclophanes.

Paracyclophanes are simple idealized model molecules for the study of interacting π-stacking systems. In this study, the excited states of [2.2]paracyclophane ([2.2]PCP), [3.3]paracyclophane ([3.3]PCP), and siloxane-bridged paracyclophane (SiPCP) are systematically investigated using the multiconfiguration quasi-degenerated perturbation theory (MCQDPT) method. The excited states of the alkyl- and silyl-substituted benzene monomers and benzene dimer, which can be regarded as the building blocks of paracyclophanes, are also examined at the same level of theory for more detailed understanding. The accuracy of the time-dependent density functional theory (TD-DFT) method required for excited state geometry optimization of the paracyclophanes is confirmed from calculations of the benzene dimer. The equilibrium distances between the benzene rings of the paracyclophanes in the first excited states are shorter than those in the ground state, and the benzene rings at the excited state optimized geometries are in an almost eclipsed parallel configuration, which indicates excimer formation. The calculated transition energies and oscillator strengths are generally in good agreement with the corresponding experimental results. A clear correlation between the excited state properties and the molecular structures is systematically demonstrated based on the calculation results for the substituted benzene monomers and benzene dimer. The transition energies of SiPCP are close to the corresponding absorption and fluorescence energies of the experimentally studied phenylene-silica hybrids, which indicates that the electronic properties of organic-silica hybrids, which is a new class of material with potential in photofunctional applications, can be approximated by simple siloxane-bridged cyclophane derivatives.

Ab initio studies of aromatic excimers using multiconfiguration quasi-degenerate perturbation theory.

The aromatic excimers of benzene, naphthalene, anthracene, pyrene, and perylene are systematically investigated using the multiconfiguration quasi-degenerate perturbation theory (MCQDPT) method, which is one of high-level ab initio quantum chemical methods. The reference configuration space for MCQDPT is carefully designed for an appropriate description of the target electronic state with a tractable computational cost. The dimers with eclipsed parallel arrangement are investigated. The basis set dependence of the selected spectroscopic parameters is examined for the benzene and naphthalene dimers, and that of the excimer binding energy is found to be significant. In contrast, the equilibrium intermolecular distance and excimer fluorescence energy are less sensitive to the size of the basis sets used, and they agree with the corresponding experimental values, even with a nonextensive basis set size. The calculated spectroscopic parameters for anthracene, pyrene, and perylene dimers are also in good agreement with the experimental results. The electronic properties of the excimers are discussed in relation to those of the corresponding monomers. The wave functions of the excimers are analyzed in detail to clarify the origin of the attractive nature between the two monomers.

Theoretical studies on Si-C bond cleavage in organosilane precursors during polycondensation to organosilica hybrids.

Molecular orbital theory calculations were carried out to predict the occurrence of Si-C bond cleavage in various organosilane precursors during polycondensation to organosilica hybrids under acidic and basic conditions. On the basis of proposed mechanisms for cleavage of the Si-C bonds, the proton affinity (PA) of the carbon atom at the ipso-position and the PA of the carbanion generated after Si-C cleavage were chosen as indices for Si-C bond stability under acidic and basic conditions, respectively. The indices were calculated using a density functional theory (DFT) method for model compounds of organosilane precursors (R-Si(OH)(3)) having organic groups (R) of benzene (Ph), biphenyl (Bp), terphenyl (Tph), naphthalene (Nph), N-methylcarbazole (MCz), and anthracene (Ant). The orders for the predicted stability of the Si-C bond were Ph > Nph > Bp > Ant > Tph > MCz for acidic conditions and Ph > MCz > Bp > Nph > Tph > Ant for basic conditions. These behaviors were primarily in agreement with experimental results where cleavage of the Si-C bonds occurred for Tph (both acidic and basic), MCz (acidic), and Ant (basic). The Si-C bond cleavage of organosilane precursors during polycondensation is qualitatively predicted from these indices based on our theoretical approach.

Silicon nanosheets and their self-assembled regular stacking structure.

Silicon nanomaterials are encouraging candidates for application to photonic, electronic, or biosensing devices, due to their size-quantization effects. Two-dimensional silicon nanosheets could help to realize a widespread quantum field, because of their nanoscale thickness and microscale area. However, there has been no example of a successful synthesis of two-dimensional silicon nanomaterials with large lateral size and oxygen-free surfaces. Here we report that oxygen-free silicon nanosheets covered with organic groups can be obtained by exfoliation of layered polysilane as a result of reaction with n-decylamine and dissolution in an organic solvent. The amine residues are covalently bound to the Si(111) planes. It is estimated that there is ca. 0.7 mol of residue per mole of Si atoms in the reaction product. The amine-modified layered polysilane can dissolve in chloroform and exfoliate into nanosheets that are 1-2 microm wide in the lateral direction and with thicknesses on the order of nanometers. The nanosheets have very flat and smooth surfaces due to dense coverage of n-decylamine, and they are easily self-assembled in a concentrated state to form a regularly stacked structure. The nanosheets could be useful as building blocks to create various composite materials.

Accurate calculation of core-electron binding energies: multireference perturbation treatment.

Multireference perturbation theory (MRPT) with multiconfigurational self-consistent field (MCSCF) reference functions is applied to the calculations of core-electron binding energies (CEBEs) of atoms and molecules. Orbital relaxations in a core-ionized state and electron correlation are both taken into account in a conventional MCSCF-MRPT procedure. In the MCSCF calculation, the target core ionized state is directly optimized as an excited state and this treatment can completely prevent a variational collapse. Multireference Moller-Plesset perturbation theory and multiconfigurational self-consistent field reference quasidegenerated perturbation theory were used to treat electron correlation. The present method quite accurately reproduced the 1s CEBEs of CH4, NH3, H2O, and FH; the average deviation from the experimental data is 0.11 eV using Ahlrichs' VTZ basis set. The C 1s and O 1s CEBEs of formic acid and acetic acid were calculated and the results are consistent with the bonding characters of the atoms in these molecules. The present procedure can also be applied to CEBEs of higher angular momentum orbitals by including spin-orbit coupling. The calculated CEBEs of Ar 2p, HCl 2p, Kr 3d, and HBr 3d are in reasonable agreement with the available experimental values. In the calculation of the 3d CEBEs, a relativistic correction significantly improves the agreements. The effect of polarization functions is also discussed.