Comparison of Sugar Molecule Decomposition through Glucose and Fructose: A High-Level Quantum Chemical Study

Efficient chemical conversion of biomass is essential to produce sustainable energy and industrial chemicals. Industrial level conversion of glucose to useful chemicals, such as furfural, hydroxymethylfurfural, and levulinic acid, is a major step in the biomass conversion but is difficult because of the formation of undesired products and side reactions. To understand the molecular level reaction mechanisms involved in the decomposition of glucose and fructose, we have carried out high-level quantum chemical calculations [Gaussian-4 (G4) theory]. Selective 1,2-dehydration, keto–enol tautomerization, isomerization, retro-aldol condensation, and hydride shifts of glucose and fructose molecules were investigated. Detailed kinetic and thermodynamic analyses indicate that, for acyclic glucose and fructose molecules, the dehydration and isomerization require larger activation barriers compared to the retro-aldol reaction at 298 K in neutral medium. The retro-aldol reaction results in the formation of C2 and C4 species from glucose and C3 species from fructose. The formation of the most stable C3 species, dihydroxyacetone from fructose, is thermodynamically downhill. The 1,3-hydride shift leads to the cleavage of the C–C bond in the acyclic species; however, the enthalpy of activation is significantly higher (50–55 kcal/mol) than that of the retro-aldol reaction (38 kcal/mol) mainly because of the sterically hindered distorted four-membered transition state compared to the hexa-membered transition state in the retro-aldol reaction. Both tautomerization and dehydration are catalyzed by a water molecule in aqueous medium; however, water has little effect on the retro-aldol reaction. Isomerization of glucose to fructose and glyceraldehyde to dihydroxyacetone proceeds through hydride shifts that require an activation enthalpy of about 40 kcal/mol at 298 K in water medium. This investigation maps out accurate energetics of the decomposition of glucose and fructose molecules that is needed to help find more efficient catalyts for the conversion of hexose to useful chemicals

Exploring Meerwein–Ponndorf–Verley Reduction Chemistry for Biomass Catalysis Using a First-Principles Approach

Liquid phase catalytic hydrogenation of decomposition products of sugar molecules is challenging, but essential to produce platform chemicals and green chemicals from biomass. The Meerwein–Ponndorf–Verley (MPV) reduction chemistry is an excellent choice for the hydrogenation of keto compounds. The energy landscapes for the liquid phase catalytic hydrogenation of ethyl levulinate (EL) and furfural (FF) by Sn­(IV) and Zr­(IV) zeolite-like catalytic sites utilizing the hydrogen atoms from an isopropanol (IPA) solvent are explored using quantum chemical methods. The computed apparent activation free energy for the catalytic hydrogenation of EL by a Sn­(IV) zeolite-like catalyst model site is (21.9 kcal/mol), which is close to the Al­(III)-isopropoxide catalyzed (20.7 kcal/mol) EL hydrogenation indicating the similar efficiency of the Sn­(IV) zeolite-like catalyst compared with the Al­(III) catalyst used in the traditional MPV reactions. The catalytic efficiency of metal isopropoxides for the catalytic hydrogenation of EL is computed to be Al­(III) > Sn­(IV) > Zr­(IV) in IPA solution, in agreement with experiment. Calculations were also performed with furfuryl alcohol as the source for hydrogen for the conversion of EL to γ-valerolactone using the Sn­(IV) catalytic site. The barrier (22.7 kcal/mol) suggests a hydrogenation using aromatic primary alcohol as a hydrogen donor and using a Sn­(IV) catalyst is feasible. In terms of reaction mechanisms, an intramolecular hydride transfer through a six membered transition state was found to be the turnover controlling transition state of liquid phase catalytic hydrogenation of carbonyl compounds considered in this study

Investigation of the Redox Chemistry of Anthraquinone Derivatives Using Density Functional Theory

Application of density functional calculations to compute electrochemical properties such as redox windows, effect of substitution by electron donating and electron withdrawing groups on redox windows, and solvation free energies for ∼50 anthraquinone (AQ) derivatives are presented because of their potential as anolytes in all-organic redox flow batteries. Computations suggest that lithium ions can increase (by ∼0.4 V) the reduction potential of anthraquinone due to the lithium ion pairing by forming a Lewis base–Lewis acid complex. To design new redox active species, the substitution by electron donating groups is essential to improve the reduction window of AQ with adequate oxidative stability. For instance, a complete methylation of AQ can improve its reduction window by ∼0.4 V. The quantum chemical studies of the ∼50 AQ derivatives are used to derive a relationship that connects the computed LUMO energy and the reduction potential that can be applied as a descriptor for screening thousands of AQ derivatives. Our computations also suggest that incorporating oxy-methyl dioxolane substituents in the AQ framework can increase its interaction with nonaqueous solvent and improve its solubility. Thermochemical calculations for likely bond breaking decomposition reactions of unsubstituted AQ anions suggest that the dianions are relatively stable in the solution. These studies provide an ideal platform to perform further combined experimental and theoretical studies to understand the electrochemical reversibility and solubility of new quinone molecules as energy storage materials

Influence of Electronic Type Purity on the Lithiation of Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) have emerged as one of the leading additives for high-capacity nanocomposite lithium ion battery electrodes due to their ability to improve electrode conductivity, current collection efficiency, and charge/discharge rate for high power applications. However, since as-grown SWCNTs possess a distribution of physical and electronic structures, it is of high interest to determine which subpopulations of SWCNTs possess the highest lithiation capacity and to develop processing methods that can enhance the lithiation capacity of underperforming SWCNT species. Toward this end, SWCNT electronic type purity is controlled <i>via</i> density gradient ultracentrifugation, enabling a systematic study of the lithiation of SWCNTs as a function of metal <i>versus</i> semiconducting content. Experimentally, vacuum-filtered freestanding films of metallic SWCNTs are found to accommodate lithium with an order of magnitude higher capacity than their semiconducting counterparts, which is consistent with <i>ab initio</i> molecular dynamics and density functional theory calculations in the limit of isolated SWCNTs. In contrast, SWCNT film densification leads to the enhancement of the lithiation capacity of semiconducting SWCNTs to levels comparable to metallic SWCNTs, which is corroborated by theoretical calculations that show increased lithiation of semiconducting SWCNTs in the limit of small SWCNT–SWCNT spacing. Overall, these results will inform ongoing efforts to utilize SWCNTs as conductive additives in nanocomposite lithium ion battery electrodes

Electronic Structure of Lithium Peroxide Clusters and Relevance to Lithium–Air Batteries

The prospect of Li–air­(oxygen) batteries has generated much interest because of the possibility of extending the range of electric vehicles due to their potentially high gravimetric density. The exact morphology of the lithium peroxide formed during discharge has not been determined yet, but the growth likely involves nanoparticles and possibly agglomerates of nanoparticles. In this article, we report on density functional calculations of stoichiometric lithium peroxide clusters that provide evidence for the stabilization of high spin states relative to the closed shell state in the clusters. The density functional calculations indicate that a triplet state is favored over a closed shell singlet state for a dimer, trimer, and tetramer of lithium peroxide, whereas in the lithium peroxide monomer, the closed shell singlet is strongly favored. Density functional calculations on a much larger cluster, (Li₂O₂)₁₆, also indicate that it similarly has a high spin state with four unpaired electrons located on the surface. These results have been confirmed by higher level G4 theory calculations that indicate that the singlet and triplet states of the dimer are nearly equal in energy and that the triplet state is more stable than the singlet for clusters larger than the dimer. The high spin states of the clusters are characterized by O–O moieties protruding from the surface, which have superoxide-like characteristics in terms of bond distances and spin. The existence of these superoxide-like surface structures on stoichiometric lithium peroxide clusters may have implications for the electrochemistry of formation and decomposition of lithium peroxide in Li–air batteries including electronic conductivity and charge overpotentials

Structure-Specific Reactivity of Alumina-Supported Monomeric Vanadium Oxide Species

Oxidative dehydrogenation (ODH) catalysts based on vanadium oxide are active for the production of alkenes, chemicals of great commercial importance. The current industrial practice for alkene production is based on energy-intensive, dehydrogenation reactions. UV resonance and visible Raman measurements, combined with density functional studies, are used to study for the first time the structure–reactivity relationships for alumina-supported monomeric vanadium oxide species. The relationship between the structure of three vanadium oxide monomeric surface species on a θ-alumina surface, and their reducibility by H₂ was determined by following changes in the vanadia’s UV Raman and resonance Raman spectra after reaction with H₂ at temperatures from 450 to 650 °C. The H₂ reducibility sequence for the three monomeric species is bidentate > “molecular”> tridentate. The reaction pathways for H₂ reduction on the three vanadium oxide monomeric structures on a θ-alumina surface were investigated using density functional theory. Reduction by H₂ begins with reaction at the VO bond in all three species. However, the activation energy, Gibbs free energy change under reaction conditions, and the final V oxidation state are species-dependent. The calculated ordering of reactivity is consistent with the observed experimental ordering and provides an explanation for the ordering. The results suggest that synthesis strategies can be devised to obtain vanadium oxide structures with greatly enhanced activity for ODH resulting in more efficient catalysts

Rapid Ether and Alcohol C–O Bond Hydrogenolysis Catalyzed by Tandem High-Valent Metal Triflate + Supported Pd Catalysts

The thermodynamically leveraged conversion of ethers and alcohols to saturated hydrocarbons is achieved efficiently with low loadings of homogeneous M­(OTf)_<i>n</i> + heterogeneous Pd tandem catalysts (M = transition metal; OTf = triflate; <i>n</i> = 4). For example, Hf­(OTf)₄ mediates rapid endothermic ether ⇌ alcohol and alcohol ⇌ alkene equilibria, while Pd/C catalyzes the subsequent, exothermic alkene hydrogenation. The relative C–O cleavage rates scale as 3° > 2° > 1°. The reaction scope extends to efficient conversion of biomass-derived ethers, such as THF derivatives, to the corresponding alkanes

Interactions of Dimethoxy Ethane with Li₂O₂ Clusters and Likely Decomposition Mechanisms for Li–O₂ Batteries

One of the major problems facing the successful development of Li–O₂ batteries is the decomposition of nonaqueous electrolytes, where the decomposition can be chemical or electrochemical during discharge or charge. In this paper, the decomposition pathways of dimethoxy ethane (DME) by the chemical reaction with the major discharge product, Li₂O₂, are investigated using theoretical methods. The computations were carried out using small Li₂O₂ clusters as models for potential sites on Li₂O₂ surfaces. Both hydrogen and proton abstraction mechanisms were considered. The computations suggest that the most favorable decomposition of ether solvents occurs on certain sites on the lithium peroxide surfaces involving hydrogen abstraction followed by reaction with oxygen, which leads to oxidized species such as aldehydes and carboxylates as well as LiOH on the surface of the lithium peroxide. The most favorable site is a Li–O–Li site that may be present on small nanoparticles or as a defect site on a surface. The decomposition route initiated by the proton abstraction from the secondary position of DME by the singlet cluster (O–O site) requires a much larger enthalpy of activation, and subsequent reactions may require the presence of oxygen or superoxide. Thus, pathways involving proton abstraction are less likely than that involving hydrogen abstraction. This type of electrolyte decomposition (electrolyte with hydrogen atoms) may influence the cell performance including the crystal growth, nanomorphologies of the discharge products, and charge overpotential

Reaction Pathways and Energetics of Etheric C–O Bond Cleavage Catalyzed by Lanthanide Triflates

Efficient and selective cleavage of etheric C–O bonds is crucial for converting biomass into platform chemicals and liquid transportation fuels. In this contribution, computational methods at the DFT B3LYP level of theory are employed to understand the efficacy of lanthanide triflate catalysts (Ln­(OTf)₃, Ln = La, Ce, Sm, Gd, Yb, and Lu) in cleaving etheric C–O bonds. In agreement with experiment, the calculations indicate that the reaction pathway for C–O cleavage occurs via a C–H → O–H proton transfer in concert with weakening of the C–O bond of the coordinated ether substrate to ultimately yield a coordinated alkenol. The activation energy for this process falls as the lanthanide ionic radius decreases, reflecting enhanced metal ion electrophilicity. Details of the reaction mechanism for Yb­(OTf)₃-catalyzed ring opening are explored in depth, and for 1-methyl-<i>d</i>₃-butyl phenyl ether, the computed primary kinetic isotope effect of 2.4 is in excellent agreement with experiment (2.7), confirming that etheric ring-opening pathway involves proton transfer from the methyl group alpha to the etheric oxygen atom, which is activated by the electrophilic lanthanide ion. Calculations of the catalytic pathway using eight different ether substrates indicate that the more rapid cleavage of acyclic versus cyclic ethers is largely due to entropic effects, with the former C–O bond scission processes increasing the degrees of freedom/particles as the transition state is approached

Ni-Doping Effects on Oxygen Removal from an Orthorhombic Mo₂C (001) Surface: A Density Functional Theory Study

Density functional theory (DFT) calculations were used to investigate the effect of Ni dopants on the removal of chemisorbed oxygen (O*) from the Mo-terminated (T_Mo) and C-terminated (T_C) Mo₂C­(001) surfaces. The removal of adsorbed oxygen from the catalytic site is essential to maintain the long-term activity and selectivity of the carbide catalysts in the deoxygenation process related to bio-oil stabilization and upgrading. In this contribution, the computed reaction energetics and reaction barriers of O* removal were compared among undoped and Ni-doped Mo₂C­(001) surfaces. The DFT calculations indicate that selected Ni-doped surfaces such as Ni adsorbed on T_Mo and T_C Mo₂C­(001) surfaces enable weaker binding of important reactive intermediates (O*, OH*) compared to the undoped counterparts, which is beneficial for the O* removal from the catalyst surface. This study thus confirms the promoting effect of the Ni dopant on O* removal reaction on the T_Mo Mo₂C­(001) and T_C Mo₂C­(001) surfaces. This computational prediction has been confirmed by the temperature-programmed reduction profiles of Mo₂C and Ni-doped Mo₂C catalysts, which had been passivated and stored in an oxygen environment