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Elucidation of Metal-Sugar Complexes: When Tungstate Combines with d-Mannose
Abstract
The control of metal-sugar complexes speciation in solution is crucial in an energy transition context. Herein, the formation of tungstate-mannose complexes is unraveled in aqueous solution using a multitechnique experimental and theoretical approach. 13C nuclear magnetic resonance (NMR), as well as 13C-1H and 1H-1H correlation spectra, analyzed in the light of coordination-induced shift method and conformation analysis, were employed to characterize the structure of the sugar involved in the complexes. X-ray absorption near edge structure spectroscopy was performed to provide relevant information about the metal electronic and coordination environment. The calculation of 13C NMR chemical shifts for a series of tungstate-mannose complexes using density functional theory (DFT) is a key to identify the appropriate structure among several candidates. Furthermore, a parametric study based on several relevant parameters, namely, pH and tungstate concentration, was carried out to look over the change of the nature and concentrations of the complexes. Two series of complexes were detected, in which the metallic core is either in a ditungstate or a monotungstate form. With respect to previous proposals, we identify two new species. Dinuclear complexes involve both α- and β-furanose forms chelating the metallic center in a tetradentate fashion. A hydrate form chelating a ditungstate core is also revealed. One monotungstate complex appears at high pH, in which a tetrahedral tungstate center is bound to α-mannofuranose through a monodentate site at the second deprotonated hydroxyl group. This unequalled level of knowledge opens the door to structure-reactivity relationships.
... Significant shifts in the position of the first-shell peak in j χ(R) j are not observed and WÀ O interatomic distances are 1.78 � 0.01 Å at all temperatures, similar to WÀ O bond lengths in other tetrahedrally coordinated compounds [42] and consistent with the 1.783 Å WÀ O spacing of cubic Na 2 WO 4 measured at ambient temperature. [43] The XANES spectra shown in Figure 4a demonstrate that the catalyst at T � 873 K has characteristics similar to solvent-separated Na + and WO 4 2-, [44][45][46] rather than crystalline Na 2 WO 4 · 2 H 2 O. Given that XAS measures contributions of all W atoms while XRD measures crystalline contributions only, the XANES and second-shell EXAFS results in Figure 4 and Figure S11 suggest that a significant fraction of W atoms are not present in the cubic Na 2 WO 4 lattice even though a bulk crystalline phase exists prior to melting according to XRD. ...
... Collectively, these results demonstrate that the Na 2 WO 4 / SiO 2 formulation studied herein forms a stable, disordered, and highly selective catalyst for aerobic hydrogen combus- Figure 4. Comparison of W L III -edge (a) XANES and (b) k 2 -weighted j χ(R) j spectra for Na 2 WO 4 /SiO 2 samples held in He at varying temperatures. XANES spectra of Na 2 WO 4 standards are given for comparison; frozen Na 2 WO 4 solution spectra were obtained from Ref. [46]. Normalized μ(E) are offset in increments of 0.1. ...
... The authors have cited additional references within the Supporting Information. [24,[44][45][46][47][48][49][50][51][52] ...
Na2WO4/SiO2, a material known to catalyze alkane selective oxidation including the oxidative coupling of methane (OCM), is demonstrated to catalyze selective hydrogen combustion (SHC) with >97 % selectivity in mixtures with several hydrocarbons (CH4, C2H6, C2H4, C3H6, C6H6) in the presence of gas‐phase dioxygen at 883–983 K. Hydrogen combustion rates exhibit a near‐first‐order dependence on H2 partial pressure and are zero‐order in H2O and O2 partial pressures. Mechanistic studies at 923 K using isotopically‐labeled reagents demonstrate the kinetic relevance of H−H dissociation and absence of O‐atom recombination. In situ X‐ray diffraction (XRD) and W LIII‐edge X‐ray absorption spectroscopy (XAS) studies demonstrate, respectively, a loss of Na2WO4 crystallinity and lack of second‐shell coordination with respect to W⁶⁺ cations below 923 K; benchmark experiments show that alkali cations must be present for the material to be selective for hydrogen combustion, but that materials containing Na alone have much lower combustion rates (per gram Na) than those containing Na and W. These data suggest a synergy between Na and W in a disordered phase at temperatures below the bulk melting point of Na2WO4 (971 K) during SHC catalysis. The Na2WO4/SiO2 SHC catalyst maintains stable combustion rates at temperatures ca. 100 K higher than redox‐active SHC catalysts and could potentially enable enhanced olefin yields in tandem operation of reactors combining alkane dehydrogenation with SHC processes.
Na2WO4/SiO2, a material known to catalyze oxidative coupling of methane, is demonstrated to catalyze selective hydrogen combustion (SHC) with >97% selectivity in mixtures with several hydrocarbons (CH4, C2H6, C2H4, C3H6, C6H6) in the presence of gas‐phase dioxygen at 883‐983 K. Hydrogen combustion rates exhibit a near‐first‐order dependence on H2 partial pressure and are zero‐order in H2O and O2 partial pressures. Mechanistic studies using isotopically‐labeled reagents demonstrate the kinetic relevance of H‐H dissociation and absence of O‐atom recombination. In situ X‐ray diffraction and W LIII‐edge X‐ray absorption spectroscopy studies demonstrate, respectively, a loss of Na2WO4 crystallinity and lack of second‐shell coordination with respect to W6+ cations below 923 K; benchmark experiments show that alkali cations must be present for the material to be selective for hydrogen combustion, but that materials containing Na alone have much lower combustion rates (per gram Na) than those containing Na and W. These data suggest a synergy between Na and W in a disordered phase during SHC catalysis. The Na2WO4/SiO2 SHC catalyst maintains stable combustion rates at temperatures ca. 100 K higher than redox‐active SHC catalysts and could potentially enable enhanced olefin yields in tandem operation of reactors combining alkane dehydrogenation with SHC processes.
Conversion of naturally occurring sugars, the most abundant biomass resources on Earth, to fuels and chemicals provides a sustainable and carbon-neutral alternative to the current fossil resource–based processes. Tungsten-based catalysts (e.g., WO 3 ) are efficient for selectively cleaving C-C bonds of sugars to C 2,3 oxygenate intermediates (e.g., glycolaldehyde) that can serve as platform molecules with high viability and versatility in the synthesis of various chemicals. Such C-C bond cleavage follows a mechanism distinct from the classical retro-aldol condensation. Kinetic, isotope ¹³ C-labeling, and spectroscopic studies and theoretical calculations, reveal that the reaction proceeds via a surface tridentate complex as the critical intermediate on WO 3 , formed by chelating both α- and β-hydroxyls of sugars, together with the carbonyl group, with two adjacent tungsten atoms (W-O-W) contributing to the β-C-C bond cleavage. This mechanism provides insights into sugar chemistry and enables the rational design of catalytic sites and reaction pathways toward the efficient utilization of sugar-based feedstocks.
Various hydrous ammonium metatungstate phases (NH4)6[H2W12O40]·XH2O (AMT-X) have been obtained in the form of single crystals, using supersaturated solutions, antisolvent crystallization, and partial dehydration as strategies for crystal growth. On the basis of laboratory X-ray diffraction data, the crystal structures of the hydrous phases with X = 22, 12.5, 9.5, 8.5, 6, 4, and 2 as well of the cocrystals with X = 12-ethanol and X = 4-2acetone have been determined for the first time. AMT-22 forms the solid solution series (NH4)6–xKx[H2W12O40]·22H2O with the potassium salt (PMT-22) without a miscibility gap. Its tetragonal crystal structure shows a distorted cubic close-packed arrangement of the spherical α-Keggin-type [H2W12O40]6– anions with disordered ammonium N and water O atoms in the voids. The crystal structures of the other, less-hydrated AMT phases can be derived from distorted hexagonal rod packings of the α-Keggin anions, likewise with the cations and crystal water molecules in the voids. The thermal behavior of AMT-22 crystals reveals a quick dehydration, with AMT-9.5 as an intermediate and AMT-4 as a stable hydrate phase under ambient conditions. Upon further heating, the material decomposes with stepwise formation of AMT-2, AMT-1, AMT-0, and an amorphous phase, before orthorhombic WO3 forms above 400 °C. One of the commercially available AMT phases, “(NH4)6[H2W12O40]·XH2O” (with X = unspecified) has a water content of X = 4 but crystallizes in a structure other than AMT-4. Consequently, the 4-hydrate shows polymorphic behavior.
The conversion of abundant hexoses (e.g. glucose, mannose and galactose) and pentoses (e.g. xylose and arabinose) to 5-hydroxymethylfurfural (5-HMF) and 2-furfural (2-F) is subject to intensive research in the hope of achieving competitive production of diverse materials from renewable resources. However, the abundance of literature on this topic as well as the limited number of studies systematically comparing numerous monosaccharides hinder progress tracking. Herein, we compare and rationalize reactivities of different ketoses and aldoses. Dehydration mechanisms of both monosaccharide types are reviewed regarding the existing experimental evidence. Ketose transformation to furan derivatives likely proceeds through cyclic intermediates and is hindered by side-reactions such as isomerization, retro-aldol reactions and polymerization. Different strategies can improve furan derivative synthesis from ketoses: limiting the presence of water, improving the dehydration rate, protecting 5-HMF and 2-F reactive moieties with derivatization or solvent interactions and extracting 5-HMF and 2-F from the reaction medium. In contrast to ketoses, aldose conversion to furan derivatives is not favored compared to polymerization reactions because it involves their isomerization or a ring contraction. Enhancing aldose isomerization is possible with metal catalysts (e.g. CrCl3) promoting a hydride shift mechanism or with boric/boronic acids promoting an enediol mechanism. This catalysis is however far more challenging than ketose dehydration because catalyst activity depends on numerous factors: Brønsted acidity of the medium, catalyst ligands, catalyst affinity for monosaccharides and their accessibility to several chemical species simultaneously. Those aspects are methodically addressed to support the design of new monosaccharide dehydration systems.
Biomass is currently the most widespread form of renewable energy and its exploitation is further increasing due to the concerns over the devastative impacts of fossil fuel consumption, i.e., climate change, global warming and their negative impacts on human health. In line with that, the present articles reviews the different sources of biomass available, along with their chemical composition and properties. Subsequently, different conversion technologies (i.e., thermo-chemical, biochemical, and physicochemical conversions) and their corresponding products are reviewed and discussed. In the continuation, the global status of biomass vs. the other renewable energies is scrutinized. Moreover, biomass-derived energy production was analyzed from economic and environmental perspectives. Finally, the challenges faced to further expand the share of biomass-derived energy carriers in the global energy market are presented.
Consistent basis sets of double- and triple-zeta valence with polarization quality for the fifth period have been derived for periodic quantum-chemical solid-state calculations with the crystalline-orbital program CRYSTAL. They are an extension of the pob-TZVP basis sets, and are based on the full-relativistic effective core potentials (ECPs) of the Stuttgart/Cologne group and on the def2-SVP and def2-TZVP valence basis of the Ahlrichs group. We optimized orbital exponents and contraction coefficients to supply robust and stable self-consistent field (SCF) convergence for a wide range of different compounds. The computed crystal structures are compared to those obtained with standard basis sets available from the CRYSTAL basis set database. For the applied hybrid density functional PW1PW, the average deviations of calculated lattice constants from experimental references are smaller with pob-DZVP and pob-TZVP than with standard basis sets.
Lactic acid is a naturally occurring organic acid that can be used in a wide variety of industries, such as the cosmetic, pharmaceutical, chemical, food, and, most recently, the medical industries. It can be made by the fermentation of sugars obtained from renewable resources, which means that it is an eco-friendly product that has attracted a lot of attention in recent years. In 2010, the U.S. Department of Energy issued a report that listed lactic acid as a potential building block for the future. Bearing the importance of lactic acid in mind, this review summarizes information about lactic acid properties and applications, as well as its production and purification processes.
Converting saccharides into 5-hydroxymethylfurfural (5-HMF) has attracted more and more research interest under the background of global energy shortage. A lot of effort has been devoted to the conversion of fructose and glucose. However, other underused sugars receive very limited attention, although some of them have considerable reserves in nature as well. In this work, mannose, a major component from hemicellulose, was effectively converted into 5-HMF under mild conditions. AlCl3·6H2O exhibited superior activity among the tested catalysts, achieving a maximum 5-HMF yield of 60% in a dimethyl sulfoxide (DMSO)/water mixed solvent at 130 °C within 45 min. Adding an appropriate amount of water in DMSO could suppress some side reactions while preserving the reactivity of mannose dehydration. Mannose showed a comparable reactivity to fructose in the employed catalytic system. The studied system was also effective in the conversion of di/trisaccharides such as cellobiose and melezitose into 5-HMF. A number of control experiments with different catalysts and additives were conducted to elucidate the preliminary mechanism.
The quest for sustainable sources of fuels and chemicals to meet the demands of a rapidly rising global population represents one of this century’s grand challenges. Biomass offers the most readily implemented, and low cost, solution for transportation fuels, and the only non-petroleum route to organic molecules for the manufacture of bulk, fine and speciality chemicals and polymers. Chemical processing of such biomass-derived building blocks requires catalysts compatible with hydrophilic, bulky substrates to facilitate the selective deoxygenation of highly functional bio-molecules to their target products. This chapter addresses the challenges associated with carbohydrate utilisation as a sustainable feedstock, highlighting innovations in catalyst and process design that are needed to deliver high-value chemicals from biomass-derived building blocks.
Succinic acid, derived from fermentation of agricultural carbohydrates, has a specialty chemical market in industries producing
food and pharmaceutical products, surfactants and detergents, green solvents and biodegradable plastics, and ingredients to
stimulate animal and plant growth. As a carbon-intermediate chemical, fermentation-derived succinate has the potential to
supply over 2.7 × 108 kg industrial products/year including: 1,4-butanediol, tetrahydrofuran, γ-butyrolactone, adipic acid, n-methylpyrrolidone and linear aliphatic esters. Succinate yields as high as 110 g/l have been achieved from glucose by the
newly discovered rumen organism Actinobacillus succinogenes. Succinate fermentation is a novel process because the greenhouse gas CO2 is fixed into succinate during glucose fermentation. New developments in end-product recovery technology, including water-splitting
electrodialysis and liquid/liquid extraction have lowered the cost of succinic acid production to U.S. 0.55/kg at the 75 000
tonne/year level and to 2.20/kg at the 5000 tonne/year level. Research directions aimed at further improving the succinate
fermentation economics are discussed.
Levulinic acid (LA) is one of the most promising biomass derived platform chemicals owing to its wider convertibility to a large number of commodity chemicals. Numerous LA derivatives have paramount importance in the global economy. In this article, we have comprehensively reviewed various processes that have been developed to produce LA and its derivatives from different sugars and cellulosic feedstocks. These designs are discussed in order to provide comparative information on their chemical mechanism, process merits, demerits, and scale-up potentials. Monosacharides such as fructose, glucose and xylose and disaccharides such as sucrose are good feedstocks for LA production with Brønsted or Lewis acids as the catalysts in ether homogenous or heterogeneous reaction systems. LA yield is in the range of 2-90% which is greatly dependent on the reaction conditions. Polysaccharides such as cellulose and even lignocellulose are also employed for LA production. Brønsted acids especially mineral acids appear to be more efficient than Lewis acids to catalyze the conversion of polysaccharides and lignocellulosic biomass to LA. The important LA derivatives and their preparation reactions such as aminolevulinic acid, diphenolic acid, γ-valerolactone, various alkyl esters, valerate etc. also have been reviewed. These derivatives have extended utilization in modern industries due to the emergent environmental concerns. Furthermore, challenges arise out during these lab-scale processes are critically unveiled to pave the way for process selection and scale-up study for the production of LA and different derived chemicals. It has been recommended that to improve the efficiency of LA and its derivatives production, efforts should be made to develop robust catalysts and reaction system in order to improve the reaction selectivity. The mechanisms of LA formation from various feedstocks are also significantly important to guide the intensification of the process. Reactors with potential of industrial application are one of the crucial steps for scaling up of LA production.
The DFT calculation of 1H and 13C NMR chemical shifts in a series of ten classically known Strychnos alkaloids with a strychnine skeleton were performed at the PBE0/pcSseg‐2//pcseg‐2 level. It was found that calculated 1H and 13C NMR chemical shifts provided a markedly good correlation with experiment characterized by a mean absolute error of 0.08 ppm in the range of 7 ppm for protons and 1.67 ppm in the range of 150 ppm for carbons, so that a mean absolute percentage error was as small as ca. 1% in both cases.
The Basis Set Exchange (BSE) has been a prominent fixture in the quantum chemistry community. First publicly available in 2007, it is recognized by both users and basis set creators as the de facto source for information related to basis sets. This popular resource has been rewritten, utilizing modern software design and best practices. The basis set data has been separated into a separate library, and the website updated to use the current generation of web development libraries. The general layout and workflow of the website is preserved, while small but helpful features have been added. Overall, this design should increase adaptability and lend itself well into the future as a dependable resource for the computational chemistry community. This article will discuss the decision to rewrite the BSE, the new architecture and design, and the new features that have been added.
Periodic density functional theory (DFT) calculations with long range corrections were used to analyze the opening of the fructose and glucose rings catalyzed by metal-substituted beta zeolites (M-BEA). The reaction mechanisms were systematically analyzed on BEA substituted with tin (Sn), titanium (Ti), zirconium (Zr), and hafnium (Hf). Here, we proposed a mechanism for the conversion of fructose to dihydroxyacetone (DHA) and glyceraldehyde (GLA) and a novel mechanism for the glucose ring opening. The preferential site of substitution of the metals in BEA were reported. The adsorption energies of fructose and glucose through their different oxygen atoms on M-BEA were also reported. The transition state energies were calculated using the nudge elastic band and dimmer methods. Among the zeolites studied, Sn-BEA displays the lowest energies barriers for the conversion of the fructose to its trioses and for the glucose ring opening.
A wide variety of commodity chemicals can be produced from the catalytic oxidation of carbohydrates or carbohydrate derived molecules including formic acid, acetic acid, glycolic acid, gluconic acid, glucaric acid, malonic acid, oxalic acid, 2,5-diformylfuran (DFF), 5-hydroxymethyl-2-furancarboxylic acid (HFCA), 5-formyl-2-furancarboxylic acid (FFCA), and 2,5-furandicarboxylic acid (FDCA). This review will highlight the recent research progress in the development of new routes for the production of organic acids and furan compounds via catalytic oxidation reactions. Particular attention will be paid to these one-pot reactions with the requirements of an acidic site and a metal site. For the one-pot transformation of cellobiose or lignocellulose into gluconic acid, these reactions were performed via a one-step strategy using a single catalyst containing an acidic site and a metal site. However, a two-step strategy was adopted for the oxidative transformation of carbohydrates into DFF or FDCA in order to avoid the oxidation of the carbohydrates. The first step was performed for the dehydration of carbohydrates into 5-hydroxylmethylfuran (HMF) in the presence of an acid catalyst, and the second step was performed for the oxidation of HMF into DFF or FDCA with a metal catalyst.
This study demonstrated that glucose could be isomerized to fructose in the concentrated aqueous solution of lithium bromide (LiBr) under mild conditions. The isomerization mechanisms were studied via isotopic labeling experiments. It was found that both the cation (Li+) and the anion (Br‾) in the system catalyzed the isomerization of glucose to fructose. The Br‾ catalyzed the isomerization through the proton transfer mechanism via enediol intermediate, while the Li+ did through the intramolecular hydride shift mechanism from C2 to C1. The estimation using quantitative 13C-NMR analysis indicated that Br‾ catalyzed approximately 85% of the isomerization, while Li+ was responsible for the rest 15%. The results indicated that 31% of fructose was produced from glucose in LiBr trihydrate at 120 °C for 15 min. The outcomes of this study provided not only better understanding and insights of the sugar transformations in the LiBr solution but also an alternative approach to produce fructose from glucose.
Catalytic conversion of cellulose to ethylene glycol (EG) or 1,2-propylene glycol (1,2-PG) represents an attractive approach in the valorization of biomass, due to the high atom economy of the reaction process and large market demand of the diol products. The one-pot catalytic conversion of cellulose is a complex reaction network, comprising hydrolysis, retro-aldol condensation, hydrogenation, isomerization, de-hydrogenation, thermal side reactions, etc. Besides EG and 1,2-PG, a variety of by-products such as sorbitol, mannitol, erythritol, 1,2-butanediol and glycerol might be co-produced. The key point for obtaining high selectivity for EG or 1,2-PG lies in effective control of major reaction steps in the reaction network proceeding at matching rates. In this prospective, we depict the general reaction route for glycols production from cellulose, and summarize the active elements for retro-aldol condensation reaction, which is the determinant step for the formation of C2 and C3 intermediates. The in-situ or operando methods for the catalyst characterization are discussed. Then the reaction kinetics over one representative example, i.e. the tungstenic catalyst, is summarized briefly, and the approaches to control the products selectivity are suggested. After an overview of progresses and challenges in catalytic conversion of lignocellulose for application, we present an outlook for cellulose conversion to diols from aspects of catalyst development, reaction mechanism study and practical applications.
Albeit the isomerization mechanism of glucose to fructose catalyzed by M(IV)-incorporated zeolites is widely studied, scant attention has been given to the adsorption of related sugars that is critical to catalysis. Here p-DFT calculations are conducted to have a comprehensive understanding within this context, considering the effects of adsorption modes, identity of framework-M(IV) ions, pore topology and conformational states of glucose. Monodentate rather than bidentate adsorption structures of glucose are the most energetically favorable within all investigated zeolites except Sn-CHA. Adsorption performances of different M(IV)-incorporated BEA zeolites decline as Zr > Sn > Ti > Ge, where Ti-and Ge-BEA, especially the latter, is obviously inferior for sugar adsorption and catalysis. Pore topology of zeolites plays an even more pronounced effect during glucose adsorption. Non-covalent interactions contribute significantly to the adsorption processes. Dispersion effects of different framework-M(IV) ions, although close to each other, show a clear opposite trend as adsorption energies. FER rather than other zeolites shows surprisingly high dispersion effects (e. g.; -218 kJ/mol for SnFER vs. -123 kJ/mol for Sn-BEA). It also shows that dispersion effects for the various conformational states of glucose are closely related with structural flexibilities.
Fructose is regarded as a key intermediate for transformation of cellulosic biomass to downstream products. In gas phase, D-fructofuranose conformers that predominate in biochemically relevant polysaccharides are strongly disfavoured, and the condition improves slightly by inclusion of bulk solvent effects. The various O sites in four selected fructose conformers are accessible to the Sn Lewis acidic site of Sn-BEA zeolite except five cases mainly as a result of steric hindrances from the adjacent groups. The adsorption energies of fructose in Sn-BEA zeolite are calculated within -73.3 ~ -161.5 kJ/mol that may differ substantially for the various conformers and various O sites. D-fructofuranose rather than D-fructopyranose conformers are significantly preferred in Sn-BEA zeolite and this fundamentally alters the conformational preference of gas phase, as corroborated by the computational relative stabilities ranking as F4 ≤ F3 < F1 < F2. Accordingly, D-fructofuranose conformers that predominate in biochemically relevant polysaccharides exist favorably in Sn-BEA zeolite. Dispersion interactions in gas phase can alter the relative stabilities of fructose conformers not more than 5.6 kJ/mol; however, dispersion interactions play a considerably larger role during the interaction with Sn-BEA zeolite that are averaged at -123.8, -123.4, -125.6 and -127.5 kJ/mol for F1, F2, F3 and F4, respectively.
The direct hydrogenolysis of cellulose represents an attractive and promising route for green polyol production. Designing a catalyst system that could control the selectivity of polyols of this process is highly desirable. In this work, we realized the selectivity-switchable production of ethylene glycol (EG) and 1,2-propylene glycol (1,2-PG) by using Sn species with different valences in combination with Ni catalysts. The combination of Ni/AC and metallic Sn powders exhibited a superior activity toward EG (57.6%) with up to 86.6% total polyol yield, while the combination of Ni/AC and SnO favored the formation of 1,2-PG (32.2%) with a 22.9% yield of EG. The Sn species in NiSn alloy in situ formed from metallic Ni and Sn powders was found to be the active sites for the high selectivity of EG as evidenced by control experiments and characterizations including X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, energy dispersive X-ray mapping, and 119Sn Mössbauer spectroscopy. The effects of Sn loading, reaction temperature, reaction time, and the concentration of cellulose were investigated for Ni/AC + Sn powders. Because of the formation of NiSn alloy, the Ni-Sn catalyst showed good stability during repeated use. Experimental results disclosed that the Sn species with different valence possessed distinct catalytic functions. Both SnO and the alloyed Sn species could catalyze the retro-aldol condensation of glucose to glycolaldehyde, and meanwhile, SnO was also active for the isomerization of glucose to fructose. Therefore, controlling the glycol products distribution could be realized using SnO or the alloyed Sn species as catalysts.
Mechanisms of tungstate sorption on the mineral boehmite (γ-AlOOH) were studied using batch uptake experiments and X-ray absorption spectroscopy. Batch uptake experiments over the pH range 4-8 and [W]=50-2000μM show typical oxyanion behavior, and isotherm experiments reveal continued uptake with increasing tungstate concentration without any clear uptake maximum. Desorption experiments showed that sorption is irreversible at pH 4 and partly reversible at pH 8. Tungsten L1- and L3-edge XANES spectroscopy indicates that all sorbed tungstates are octahedrally coordinated, even though the dominant solution species at pH 8 is a tetrahedral monotungstate. Tungsten L3-edge EXAFS analysis shows that sorbed tungstate occurs as polymeric form(s), as indicated by the presence of corner- and edge-sharing of distorted tungstate octahedra. The occurrence of polymeric tungstate on the surface at pH 8 indicates that sorption is accompanied by polymerization and a coordination change from tetrahedral (in solution) to distorted octahedral (on the surface). The strong tendency for tungstate polymerization on boehmite can explain the continued uptake without an apparent maximum in sorption, and the limited desorption behavior. Our results provide the basis for a predictive model of tungstate uptake by boehmite, which can be important for understanding tungstate mobility, toxicity, and bioavailability.
We investigate the metal catalyzed intramolecular 1,2 carbon shift reaction, usually associated with the epimerization of sugars, using the density functional theory. Metals like molybdenum (Mo), tungsten (W) and vanadium (V), in their oxoanion complex forms (molybdate, tungstate, vanadate) are investigated. The experimentally observed difference in the activity of these metal complexes, in catalyzing 1,2 carbon shift in glucose, at different pH is explained. Formation of larger (polynuclear) metal oxoanions ("metalates") at higher pH reduces the structural flexibility of the metal complex. Thus, upon binding with glucose, their ability to rearrange the carbon backbone in sugar molecules decreases, resulting in lower activity towards 1,2 carbon shift at higher pH. For example, the activation barrier for 1,2 carbon shift catalyzed by "active" di-nuclear molybdate (exists at pH 2.5-3.5) is 21.1 kcal/mol; whereas it is 26.8 kcal/mol for the tetra-nuclear molybdate (exists at pH 4-5.5). Bonding interaction analysis has been carried out to quantify the flexibility/rigidity of the metal oxoanion complexes. Comparison of catalytic activities of Mo, W and V (as dimetalate) showed that vanadate is more active than the popular molybdate complex and W is the least active of them all. The trend in the catalytic activities of these metals is explained on the basis of the participation of higher orbitals in the complexation of metal oxoanions with glucose. Additionally, the flexibility/rigidity of the metal complex is also shown to be a descriptor of its catalytic activity for the 1,2 carbon shift reaction.
Using cellulosic biomass to synthesize bulk quantities of high-value chemicals is of great interest for developing a sustainable biobased society. Especially, direct catalytic conversion of cellulose to glycols, important building blocks for polymers, remains a grand challenge. Herein, we report the development of a versatile binary nickel-lanthanum(III) catalyst for the conversion of cellulose to both ethylene glycol (EG) and propylene glycol (1,2-PG) in a yield of 63.7%, which is one of the best performances reported for this catalytic reaction. Especially, lanthanum(III) exhibited a high level of activity toward the degradation of cellulose (TON = 339) at a very low concentration (0.2 mmol/L). On the basis of density functional theory calculations and experimental analyses, we addressed a dual route for this catalytic mechanism: a major route involving the selective cracking of sugars into C2 molecules and a minor route involving the hydrogenolysis of sugar alcohols. Lanthanum(III) catalyzes the cleavage of the C2-C3 bond in glucose via sequential epimerization and 2,3-hydride shift reactions to form glycolaldehyde, the precursor of EG.Keywords: cellulose; ethylene glycol; propylene glycol; nickel−lanthanum catalyst; dual route; theoretical calculation
Sn-beta zeolite, in combination with borate salts, is a potential inorganic catalyst for sugars epimerization. We investigate, at molecular level, the catalytic mechanism of glucose epimerization to mannose, using density functional theory. Our calculations suggest that the tetrahedral borate ion forms a complex with glucose and inhibits the competitive isomerization reaction. The Lewis-acidic stannanol group of Sn-beta catalyzes glucose ring opening, which is followed by the silanol group (Brønsted acid site) catalyzed enolization. The epimerization then proceeds via an intramolecular 1,2 carbon shift and is found to be the rate-limiting step with an activation enthalpy of 26.3 kcal/mol. Catalytic activities of different tetravalent metal centers are compared, and Sn is found to be the most active metal. Additionally, it was found that the proximity of silanol group to the stannanol group, within the zeolitic framework, plays a key role in enhancing the catalytic activity of the silanol group. Hence, it is crucial to perform calculations with the entire ring structure of Sn-beta that opens up due to the hydrolysis of Sn–O–Si bridge.
We present the first DFT-based microkinetic model for the Brønsted acid-catalyzed conversion of glucose to 5-hydroxylmethylfurfural (HMF), levulinic acid (LA), and formic acid (FA) and perform kinetic and isotopic tracing NMR spectroscopy mainly at low conversions. We reveal that glucose dehydrates through a cyclic path. Our modeling results are in excellent agreement with kinetic data and indicate that the rate-limiting step is the first dehydration of protonated glucose and that the majority of glucose is consumed through the HMF intermediate. We introduce a combination of 1) automatic mechanism generation with isotopic tracing experiments and 2) elementary reaction flux analysis of important paths with NMR spectroscopy and kinetic experiments to assess mechanisms. We find that the excess formic acid, which appears at high temperatures and glucose conversions, originates from retro-aldol chemistry that involves the C6 carbon atom of glucose.
New tetra-iron(III) (K4[1]·25H2O·(CH3)2CO and K3[2]·3H2O·(OH)) and di-copper(II) (Na3[3]·5H2O) complexes as carbohydrate binding models have been synthesized and fully characterized used several techniques including single crystal X-ray crystallography. Whereas K4[1]·25H2O·(CH3)2CO and Na3[3]·5H2O are completely water-soluble, K3[2]·3H2O·(OH) is less soluble in all common solvents including water. The binding of substrates, such as d-mannose, d-glucose, d-xylose, and xylitol with the water-soluble complexes in different reaction conditions were investigated. In aqueous alkaline media, complexes K4[1]·25H2O·(CH3)2CO and Na3[3]·5H2O showed coordination ability toward the applied substrates. Even in the presence of stoichiometric excess of the substrates, the complexes form only 1:1 (complex/substrate) molar ratio species in solution. Apparent binding constants, pKapp, values between the complexes and the substrates were determined and specific mode of substrate binding is proposed. The pKapp values showed that d-mannose coordinates strongest to K4[1]·25H2O·(CH3)2CO and Na3[3]·5H2O. Syntheses, characterizations and detailed substrate binding study using spectroscopic techniques and single crystal X-ray diffraction are reported.
Conversion of renewable, non-edible and resource-abundant lignocellulose to fuels, chemicals and materials has received significant attention for it holds the possibility of carbon neutral technologies as an efficient way to combat global changes. Considering the relatively high oxygen content in cellulose, it is more desirable to be transformed into oxygenated chemicals rather than hydrocarbon fuels in view of the atom efficiency. Among the oxygen-rich chemicals from biomass, polyols, such as ethylene glycol and propylene glycol, are widely used in polymer synthesis, food industry and pharmaceutical manufacture. Hydrolysis, coupled with hydrogenation and hydrogenolysis serves as an effective approach to transform biomass to polyols. This review summarizes recent advances of biomass upgrading reactions for the production of polyols with special emphasis on the formation of glycols.
A series of mononuclear six-coordinate tungsten compounds spanning formal oxidation states from 0 to +VI, largely in a ligand environment of inert chloride and/or phosphine, was interrogated by tungsten L-edge X-ray absorption spectroscopy. The L-edge spectra of this compound set, comprised of [W(0)(PMe3)6], [W(II)Cl2(PMePh2)4], [W(III)Cl2(dppe)2][PF6] (dppe = 1,2-bis(diphenylphosphino)ethane), [W(IV)Cl4(PMePh2)2], [W(V)(NPh)Cl3(PMe3)2], and [W(VI)Cl6], correlate with formal oxidation state and have usefulness as references for the interpretation of the L-edge spectra of tungsten compounds with redox-active ligands and ambiguous electronic structure descriptions. The utility of these spectra arises from the combined correlation of the estimated branching ratio of the L3,2-edges and the L1 rising-edge energy with metal Zeff, thereby permitting an assessment of effective metal oxidation state. An application of these reference spectra is illustrated by their use as backdrop for the L-edge X-ray absorption spectra of [W(IV)(mdt)2(CO)2] and [W(IV)(mdt)2(CN)2](2-) (mdt(2-) = 1,2-dimethylethene-1,2-dithiolate), which shows that both compounds are effectively W(IV) species even though the mdt ligands exist at different redox levels in the two compounds. Use of metal L-edge XAS to assess a compound of uncertain formulation requires: (1) Placement of that data within the context of spectra offered by unambiguous calibrant compounds, preferably with the same coordination number and similar metal ligand distances. Such spectra assist in defining upper and/or lower limits for metal Zeff in the species of interest. (2) Evaluation of that data in conjunction with information from other physical methods, especially ligand K-edge XAS. (3) Increased care in interpretation if strong π-acceptor ligands, particularly CO, or π-donor ligands are present. The electron-withdrawing/donating nature of these ligand types, combined with relatively short metal-ligand distances, exaggerate the difference between formal oxidation state and metal Zeff or, as in the case of [W(IV)(mdt)2(CO)2], exert the subtle effect of modulating the redox level of other ligands in the coordination sphere.
The structure of a molybdenum(VI) complex of the sugar, lyzose, has been established by the X-ray method.
Perseitol (d-glycero-d-galacto-heptitol) forms dinuclear complexes with tungstate that have a stability higher than those of galactitol and d-mannitol, and all of the formation constants are three orders of magnitude higher than those for molybdate. The 13C-n.m.r. data showed that the tungstate and molybdate complexes had similar structures that involved four vicinal hydroxyl groups. The sites of chelation involved the galacto group in galactitol and perseitol, and the arabino group HO-3,4,5 and HO-6 in d-mannitol. d-Mannitol and perseitol formed pairs of isomeric complexes that involved the same site of chelation but in reversed orientations.
The isomerization of glucose to fructose in the presence of Sn-containing zeolite BEA (beta polymorph A) was studied by periodic DFT calculations. Focus was placed on the nature of the active site and the reaction mechanism. The reactivities of the perfect lattice Sn(IV) site and the hydroxylated SnOH species are predicted to be similar. The isomerization activity of the latter can be enhanced by creating an extended silanol nest in its vicinity. Besides the increased Lewis acidity and coordination flexibility of the Sn center, the enhanced reactivity in this case is ascribed to the reaction environment that promotes activation of the confined sugar intermediates through hydrogen bonding. The resulting multidentate activation of the substrate favors the rate-determining hydrogen-shift reaction. These findings suggest the important role of defect lattice sites in Sn-BEA for catalytic glucose isomerization.
Cellulose is the most abundant source of biomass in nature, and its selective conversion into polyols provides a viable route towards the sustainable synthesis of fuels and chemicals. Here, we report the marked change in the distribution of polyols in the cellulose reaction with the Sn/Pt atomic ratios in a wide range of 0.1–3.8 on the SnOx-modified Pt/Al2O3 catalysts. Such a change was found to be closely related to the effects of the Sn/Pt ratios on the activity for the hydrogenation of glucose and other C6 sugar intermediates involved in the cellulose reaction as well as to the notable activity of the segregated SnOx species for the selective degradation of the sugar intermediates on the Pt–SnOx/Al2O3 catalysts. At lower Sn/Pt ratios of 0.1–1.0, there existed electron transfer from the SnOx species to the Pt sites and strong interaction between the catalysts, as characterized by temperature-programmed reduction in H2 and infrared spectroscopy for CO adsorption, which led to their superior hydrogenation activity (per exposed Pt atom), and in-parallel higher selectivity to hexitols (e.g. sorbitol) in the cellulose reaction, as compared to Pt/Al2O3. The hexitol selectivity reached the greatest value of 82.7% at the Sn/Pt ratio of 0.5, nearly two times that of Pt/Al2O3 at similar cellulose conversions (20%). As the Sn/Pt ratios exceeded 1.5, the Pt–SnOx/Al2O3 catalysts exhibited inferior hydrogenation activity (per exposed Pt atom), due to the formation of the crystalline Pt–Sn alloy, which led to the preferential conversion of cellulose to C2 and especially C3 products (e.g. acetol) over hexitols, most likely involving the isomerization of glucose to fructose and retro-aldol condensation of these sugars on the segregated SnOx species, apparently in the form of Sn(OH)2. These findings clearly demonstrate the feasibility for rational control of the cellulose conversion into the target polyols (e.g. acetol or propylene glycol), for example, by the design of efficient catalysts based on the catalytic functions of the SnOx species with tunable hydrogenation activity.
Graphene, graphene oxide, sulfonated graphene, and sulfonated graphene oxide (SGO) have been prepared, characterized and tested for the dehydration of xylose to furfural in water. In particular, SGO was proven to be a rapid and water-tolerant solid acid catalyst even at very low catalyst loadings down to 0.5wt.% vs xylose, maintaining its initial activity after 12 tested repetitions at 200°C, with an average yield of 61% in comparison to 44% for the uncatalyzed system. Raman spectroscopy, energy dispersive X-ray spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, 13C solid state nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy and surface area analysis suggested that the aryl sulfonic acid groups were the key active sites for high temperature production of furfural in water. They were more thermally stable under the reaction conditions and acidic than other functional groups attached to the graphene surface.
The compositions of complexes between molybdate and some acyclic polyhydroxy-compounds have been determined. Ionophoresis of reduced oligosaccharides in molybdate solution has been shown to be a useful method for determining the position of the glycosidic linkage to the reducing group of the original oligosaccharide.
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Dinuclear tungstate complexes of alditols have been studied in aqueous solution by C-13 and W-183 NMR spectroscopies. Ligands possessing hydroxyl groups in erythro configuration (erythritol, galactitol) formed a series of tetradentate complexes homologous to the known corresponding molybdate erythro compounds. In contrast, ligands with threo hydroxyl groups (threitol, xylitol) formed tridentate complexes different from the tetradentate molybdate threo species. For all alditols, the W-183 NMR spectra showed two sharp tungsten signals separated by 3-7 ppm (erthro series) or congruent-to 60 ppm (threo series). In the erythro complexes, large 3J(W,H) coupling constants (8-10 Hz) were measured and allowed the estimation of the corresponding dihedral angles. A structure that accounts for the asymmetry of the chelating site is proposed for the tridentate threo complexes. The structural difference between the molybdate and tungstate complexes of threo-alditols is discussed in connection with their stabilities and reactivities.
The formation of dinuclear tungstate and molybdate complexes of d-glycero-l-manno-heptose was studied in aqueous solution by 13C and 183W NMR spectroscopy. The chelating aldose is always tetradentate and occurs exclusively in furanose or acyclic hydrated forms, the proportions of which depend on the pH and nature of the metal ion. In the tungstate species, two types of major complexes, noted M (O-2,3,5,6) and L (O-1,2,3,5) according to the site of chelation, involve the ligand in furanose form. In one of the minor tungstate complexes, chelation occurs at the galacto (O-3,4,5,6) site of the acyclic heptose. In the molybdate species, the complex of type M does not exist, and besides the complex of type L, the major species involve the acyclic ligand with either the galacto (O-3,4,5,6) or arabino (O-1,2,3,4) sites of chelation. Multinuclear NMR data are provided for the identification of the various types of complexes. Marked differences were noticed with respect to the complexes of d-mannose, in which species of type L prevailed with both molybdate and tungstate.
The formation of tungstate complexes of aldoses and ketoses with lyxo configuration was studied in aqueous solution by 13C and 183W NMR spectroscopy. Two series of complexes were structurally characterized, in which the sugars adopt the furanose form and chelate a ditungstate group at a tetradentate site. All sugars form a major complex (type L for lyxo) similar to the molybdate species, in which the sites of chelation, O-1,2,3,5 for aldoses or O-2,3,4,6 for ketoses, involve the anomeric oxygen atom and the side chain atom O-5 or O-6. A second type of complex was identified (type M for manno), in which the sites of chelation are O-2,3,5,6 for d-mannose (tungstate species only) and O-3,4,6,7 for d-manno-heptulose (tungstate and molybdate complexes). The overall equilibrium constants for the formation of the complexes are reported and show that ketoses form stronger complexes than aldoses.
We analyzed the X-ray absorption fine structure (XAFS) spectra of W L-1- and L-3-edges in order to determine the structure of various W species on TiO2. Two 2p -> 5d transitions were observed in the second derivative of the W L-3-edge X-ray absorption near-edge structure (XANES) spectra of all samples containing W (WO3, Na2WO4, Cr2WO6, Ba2NiWO6, (NH4)(10) W12O41 center dot 5H(2)O, Sc2W3O12, H3PW12O40 center dot 13H(2)O, and TiO2-loaded WO3 samples). These two transitions can be assigned to 5d orbitals split by the ligand field. The split in the 5d orbital has a linear relationship with the area of pre-edge peak of the W L-1-edge XANES spectrum. This linear relationship is supported by density functional theory (DFT) calculations of the octahedral and tetrahedral W models. On the basis of this linear relationship, we estimated the structure of various W species on TiO2. The obtained structure is in accordance with the curve fitting analysis of W L-3-edge extended X-ray absorption fine structure (EXAFS). Additionally, we found that the octahedral WO3 species loaded on TiO2 sample exhibits the high activity for the photo-oxidation of NH3 (photo-SCO).
Following our previous report on the selective transformation of cellulose to ethylene glycol (EG) over a binary catalyst composed of tungstic acid and Ru/C, we herein report a new low-cost but more effective binary catalyst by using Raney nickel in place of Ru/C (Raney Ni+H2 WO4 ). In addition to tungstic acid, other W compounds were also investigated in combination with Raney Ni. The results showed that the EG yield depended on the W compound: H4 SiW12 O40 <H3 PW12 O40 <WO3 <H2 WO4 , but all the investigated W compounds were selective towards EG. Moreover, both WO3 and H2 WO4 were dissolved partially under the reaction conditions and transformed into Hx WO3 , which is the genuinely active species for the CC bond breakage of cellulose. This result further confirmed that the reaction that involves the selective breakage of the CC bonds of cellulose with W species is homogenous. Among various binary catalysts, the combination of Raney Ni and H2 WO4 gave the highest yield of EG (65 %), which could be attributed to the high activity of Raney Ni for hydrogenation and its inertness for the further degradation of EG. Moreover, Raney Ni+H2 WO4 showed good reusability; it could be reused at least 17 times without any decay in the EG yield, which shows its great potential for industrial applications.
Natural-abundance 13C NMR spectra (at 67.9 MHz) of aqueous D-mannose (4 M in H2O, 36 °C) yield identifiable resonances of five carbons of α-D-mannofuranose and three carbons of β-D-mannofuranose. Integrated intensities indicate the presence of 0.6 ± 0.1% α-D-mannofuranose and 0.3 ± 0.1% β-D-mannofuranose.
A new method for the calculation of NMR shielding tensors in the framework of density functional theory (DFT) is introduced, and results of shielding calculations are presented. The implementation is based on recent achievements in DFT, especially nonlocal exchange-correlation (XC) functionals, and on gauge-including atomic orbitals (GIAO). The calculated shielding constants and tensors are in good agreement with experiment and with other DFT-based methods. An explanation for the strong influence of the XC functional on the results is suggested.
Tungstate complexes of alditols have been studied in aqueous solution by W-183 and C-13 NMR spectroscopies. The ligands used in this work may chelate a ditungstate group through several different sites having erythro or threo configurations and therefore yield mixtures of complexes. The structural type of each complex was defined by the characteristic pattern of its W-183 NMR spectrum, and the sites of chelation were identified by C-13 NMR spectroscopy. Alditols which possess an asymmetrical site of erythro configuration (D-arabinitol, D-mannitol, D-glucitol) form two isomeric complexes, as this tetradentate site can be occupied in two reversed orientations. Ribitol forms a single complex of the same type. A single additional complex is formed when another tridentate site of chelation of threo configuration is available (D-arabinitol, D-glucitol). Perseitol (D-glycero-D-galacto-heptitol) affords a pair of erythro complexes involving the tetradentate HO-2,3,4,5 galacto site, together with a third complex of a novel ''mixed'' bis-dinuclear type. For the latter species, four sharp signals in the W-183 NMR spectrum characterized two ditungstate groups bound respectively to an erythro site (delta-74.2 and -81.4) and a threo site (delta-55.4 and -117.6). These sites were assigned from the C-13 NMR spectrum respectively to the HO-4,5,6,7 and the HO-1,2,3 systems, indicating that the ligand was heptadentate.
Applied Catalysis A: General j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p c a t a a b s t r a c t An efficient procedure for the conversion of unutilized abundant sugar derivatives, such as d-mannose, d-galactose and lactose, into platform molecule 5-hydroxymethylfurfural in aqueous and organic medium is reported. This process involves biopolymer sodium alginate templated porous TiO 2 nanocatalysts con-taining strong Lewis acidic sites as determined by pyridine-IR and temperature programmed desorption (TPD) of ammonia studies. Porous TiO 2 materials were characterized by XRD, SEM, HR-TEM and N 2 sorption techniques. Biopolymer templating pathway provided effective synthetic route for the TiO 2 nanomaterials, which contain considerable mesoscopic void space as revealed from the HR-TEM and N 2 sorption studies. Using this TiO 2 catalyst, improved HMF yields were obtained without using expensive ionic liquids as solvents. Hydrothermally obtained TiO 2 nanocatalysts showed maximum activity in d-mannose conversion to HMF, whereas nonporous TiO 2 is inactive in this reaction. Porosity (surface area), small particle size and strong acidic sites generated through biopolymer templating pathway are crucial for high catalytic activity.
Chopped up: The success of the title reaction strongly depends on the domain size of the WO(3) crystallites, and the type of support. Structurally stable WO(3) plays a bifunctional role in the promotion of the hydrolysis of cellulose into sugar intermediates, and more significantly in the selective cleavage of the CC bonds in these sugars.
Testing of the commonly used hybrid density functional B3LYP with the 6-31G(d), 6-31 G(d,p), and 6-31+G(d,p) basis sets has been carried out for 622 neutral, closed-shell organic compounds containing the elements C, H, N, and O. The focus is comparison of computed and experimental heats of formation and isomerization energies. In addition, the effect of an empirical dispersion correction term has been evaluated and found to improve agreement with the experimental data. For the 622 compounds, the mean absolute errors (MAE) in the heats of formation are 3.1, 2.6, 2.7, and 2.4 kcal/mol for B3LYP/6-31G(d), B3LYP/6-31G(d,p), B3LYP/6-31+G(d,p), and B3LYP/6-31+G(d,p) with the dispersion correction. A diverse set of 34 isomerizations highlights specific issues of general interest, such as performance on differences in steric effects, conjugation, and bonding. The corresponding MAEs for the isomerizations are 2.7, 2.4, 2.2, and 1.9 kcal/mol. Improvement is obtained for isomerizations of amines and alcohols when both polarization and diffuse functions are used, but the overstabilization of linear alkanes compared to branched isomers can be relieved only with the dispersion correction. Besides the insights on DFT methods, the study also aimed to quantify the gains in accuracy that can be achieved by replacing energetics from NDO-based semiempirical methods with DFT results. Since the MAEs obtained with the PDDG/PM3 method for the 622 heats of formation and 34 isomerizations are 2.8 and 2.3 kcal/mol, negligible advantage in accuracy for the B3LYP-based methods emerged in the absence of the dispersion corrections.
Gaussian basis sets of quadruple zeta valence quality for Rb-Rn are presented, as well as bases of split valence and triple zeta valence quality for H-Rn. The latter were obtained by (partly) modifying bases developed previously. A large set of more than 300 molecules representing (nearly) all elements-except lanthanides-in their common oxidation states was used to assess the quality of the bases all across the periodic table. Quantities investigated were atomization energies, dipole moments and structure parameters for Hartree-Fock, density functional theory and correlated methods, for which we had chosen Møller-Plesset perturbation theory as an example. Finally recommendations are given which type of basis set is used best for a certain level of theory and a desired quality of results.
The structural and coordination properties of complexes formed upon the interaction of copper(II) and chromium(II) chlorides with dialkylimidazolium chloride (RMIm(+)Cl(-)) ionic liquids and glucose are studied by a combination of density functional theory (DFT) calculations and X-ray absorption spectroscopy (XAS). In the absence of the carbohydrate substrate, isolated mononuclear four-coordinated MeCl(4)(2-) species (Me = Cu, Cr) dominate in the ionic liquid solution. The organic part of the ionic liquid does not directly interact with the metal centers. The interactions between the RMIm(+) cations and the anionic metal chloride complexes are limited to hydrogen bonding with the basic Cl(-) ligands and the overall electrostatic stabilization of the anionic metal complexes. Exchange of Cl(-) ligands by a hydroxyl group of glucose is only favorable for CrCl(4)(2-). For Cu(2+) complexes, the formation of hydrogen bonded complexes between CuCl(4)(2-) and glucose is preferred. No preference for the coordination of metal chloride species to specific hydroxyl group of the carbohydrate is found. The formation of binuclear metal chloride complexes is also considered. The reactivity and selectivity patterns of the Lewis acid catalyzed reactions of glucose are discussed in the framework of the obtained results.
We present a new continuum solvation model based on the quantum mechanical charge density of a solute molecule interacting with a continuum description of the solvent. The model is called SMD, where the "D" stands for "density" to denote that the full solute electron density is used without defining partial atomic charges. "Continuum" denotes that the solvent is not represented explicitly but rather as a dielectric medium with surface tension at the solute-solvent boundary. SMD is a universal solvation model, where "universal" denotes its applicability to any charged or uncharged solute in any solvent or liquid medium for which a few key descriptors are known (in particular, dielectric constant, refractive index, bulk surface tension, and acidity and basicity parameters). The model separates the observable solvation free energy into two main components. The first component is the bulk electrostatic contribution arising from a self-consistent reaction field treatment that involves the solution of the nonhomogeneous Poisson equation for electrostatics in terms of the integral-equation-formalism polarizable continuum model (IEF-PCM). The cavities for the bulk electrostatic calculation are defined by superpositions of nuclear-centered spheres. The second component is called the cavity-dispersion-solvent-structure term and is the contribution arising from short-range interactions between the solute and solvent molecules in the first solvation shell. This contribution is a sum of terms that are proportional (with geometry-dependent proportionality constants called atomic surface tensions) to the solvent-accessible surface areas of the individual atoms of the solute. The SMD model has been parametrized with a training set of 2821 solvation data including 112 aqueous ionic solvation free energies, 220 solvation free energies for 166 ions in acetonitrile, methanol, and dimethyl sulfoxide, 2346 solvation free energies for 318 neutral solutes in 91 solvents (90 nonaqueous organic solvents and water), and 143 transfer free energies for 93 neutral solutes between water and 15 organic solvents. The elements present in the solutes are H, C, N, O, F, Si, P, S, Cl, and Br. The SMD model employs a single set of parameters (intrinsic atomic Coulomb radii and atomic surface tension coefficients) optimized over six electronic structure methods: M05-2X/MIDI!6D, M05-2X/6-31G, M05-2X/6-31+G, M05-2X/cc-pVTZ, B3LYP/6-31G, and HF/6-31G. Although the SMD model has been parametrized using the IEF-PCM protocol for bulk electrostatics, it may also be employed with other algorithms for solving the nonhomogeneous Poisson equation for continuum solvation calculations in which the solute is represented by its electron density in real space. This includes, for example, the conductor-like screening algorithm. With the 6-31G basis set, the SMD model achieves mean unsigned errors of 0.6-1.0 kcal/mol in the solvation free energies of tested neutrals and mean unsigned errors of 4 kcal/mol on average for ions with either Gaussian03 or GAMESS.
Most polyols (L = alditol or carbohydrate) form dinuclear tungstate complexes according to the over-all equilibrium 2WO4(2-) + 2H+ + L equal equilibrium [W2O7L]2- + H2O. When the reaction is fast and complete, it allows the acidimetric titration of tungstate. The formation constants of the complexes of a series of polyols were determined by potentiometry. Their values were higher at low ionic strengths. Opposite structural factors govern the stabilities and the formation rates of complexes: alditols of threo configuration react with tungstate faster than those of erythro configuration, but the stability order is erythro greater than threo. Of the polyols investigated, only xylitol and D-glucitol (sorbitol) allowed a fast and accurate potentiometric titration. Using 0.02 M HCl as titrand, 0.04 mmol of tungstate was determined in a volume of 100 cm3. The interference of molybdate is discussed in detail.