Brønsted and Lewis acid sites of Sn-beta zeolite, in combination with the borate salt, catalyze the epimerization of glucose: A density functional theory study

Bioproduction of Rare d-Allulose from d-Glucose via Borate-Assisted Isomerization

d-Allulose is a low-calorie functional rare sugar with excellent processing suitability and unique physiological efficacy. d-Allulose is primarily produced from d-fructose through enzymatic epimerization, facing the constraints of a low conversion yield and high production cost. In this study, a double-enzyme cascade system with tetraborate-assisted isomerization was constructed for the efficient production of d-allulose from inexpensive d-glucose. With the introduction of sodium tetraborate (STB), capable of forming complexes with diol-bearing sugars, the conversion yield of d-allulose from d-glucose substantially escalated from the initial 17.37% to 44.97%. Furthermore, d-allulose was found to exhibit the most pronounced binding affinity for STB with an association constant of 1980.51 M-1, notably surpassing that of d-fructose (183.31 M-1) and d-glucose (35.37 M-1). Additionally, the structural analysis of the sugar-STB complexes demonstrated that d-allulose reacted with STB via the cis 2,3-hydroxyl groups in the α-furanose form. Finally, the mechanism underlying STB-assisted isomerization was proposed, emphasizing the preferential formation of an allulose-STB complex that effectively shifts the isomerization equilibrium to the allulose side, thereby resulting in high yield of d-allulose. Such an STB-facilitated isomerization system would also provide a guidance for the cost-effective synthesis of other rare sugars.

Elucidation of Metal-Sugar Complexes: When Tungstate Combines with d-Mannose

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.

MoOx Nanoparticle Catalysts for d-Glucose Epimerization and Their Electrical Immobilization in a Continuous Flow Reactor

Activity and immobilization of catalysts in liquid-phase reactions seem not to coexist. We report here the excellent activity of an MoO_x nanoparticle (NP) catalyst for d-glucose epimerization to d-mannose and the electrical immobilization of NPs in a flow reaction. Prior to that, a green and one-pot method to synthesize the MoO_x NPs (3.05 nm) via oxidizing metal Mo by hydrogen peroxide was presented. The NPs overwhelmed the reported catalysts including epimerase for d-glucose epimerization, originating from a strong interaction between the NPs and the reactant that was demonstrated by ex situ and in situ characterizations and theoretical calculations. The electrically charged feature of NPs inspired us to find a convenient way to "immobilize" them inside an activated carbon bed, and thereby, a flow reactor was assembled. The continuous epimerization was run under 24 V for 16 days with an almost unchanged activity, and only 3.2% of total Mo was lost.

Active Sites of M(IV)-incorporated Zeolites (M = Sn, Ti, Ge, Zr)

M(IV)-incorporated zeolites have recently aroused wide interest due to outstanding catalytic effects while their active sites remain largely elusive. Here periodic density functional theory calculations are conducted finding that active sites are determined jointly by identity of M(IV) ions, topology of zeolites, type of framework species and choice of T sites. All M2(IV) active sites in BEA zeolites are penta-coordinated with chemisorption of one water while subsequent water molecules that form only H-bonds promote chemisorption of the first water, especially the second water possessing comparable or even higher adsorption strengths as the first water; Ti(IV) and Ge(IV) active sites at the intersection remain penta-coordinated and Sn(IV) and Zr(IV) active sites prefer to hexa-coordination although potentially expanded to hepta-coordination. Different from other zeolites, Ti(IV) active sites in FER zeolites are hexa-coordinated as Sn(IV) active sites, due to the promoting effect of the first water. Lewis acidic defects expand Ti(IV) active sites to hexa-coordination while inhibit the formation of hepta-coordinated Sn(IV) species. Two forms of Brϕnsted acidic defects exist for Sn(IV) sites instead of only one for Ti(IV) sites, and all M(IV) Brϕnsted acidic defects, regardless of different acidic forms and M(IV) ions, can chemisorb only one water.

Catalytic transfer hydrogenation of levulinic acid to γ-valerolactone over a bifunctional tin catalyst

Efficient conversion of levulinic acid to γ-valerolactone over a bifunctional Sn catalyst under catalytic transfer hydrogenation.

Radicalization and Radical Catalysis of Biomass Sugars: Insights from First-principles Studies

Ab initio and density functional calculations are conducted to investigate the radicalization processes and radical catalysis of biomass sugars. Structural alterations due to radicalization generally focus on the radicalized sites, and radicalization affects H-bonds in D-fructofuranose more than in D-glucopyranose, potentially with outcome of new H-bonds. Performances of different functionals and basis sets are evaluated for all radicalization processes, and enthalpy changes and Gibbs free energies for these processes are presented with high accuracy, which can be referenced for subsequent experimental and theoretical studies. It shows that radicalization can be utilized for direct transformation of biomass sugars, and for each sugar, C rather than O sites are always preferred for radicalization, thus suggesting the possibility to activate C-H bonds of biomass sugars. Radical catalysis is further combined with Brønsted acids, and it clearly states that functionalization fundamentally regulates the catalytic effects of biomass sugars. In presence of explicit water molecules, functionalization significantly affects the activation barriers and reaction energies of protonation rather than dehydration steps. Tertiary butyl and phenyl groups with large steric hindrances or hydroxyl and amino groups resulting in high stabilities for protonation products drive the protonation steps to occur facilely at ambient conditions.

Catalytic Isomerization of Biomass-Derived Aldoses: A Review

Selected aldohexoses (D-glucose, D-mannose, and D-galactose) and aldopentoses (D-xylose, L-arabinose, and D-ribose) are readily available components of biopolymers. Isomerization reactions of these substances are very attractive as carbon-efficient processes to broaden the portfolio of abundant monosaccharides. This review focuses on the chemocatalytic isomerization of aldoses into the corresponding ketoses as well as epimerization of aldoses at C2. Recent advances in the fields of catalysis by bases and Lewis acids are considered. The emphasis is laid on newly uncovered catalytic systems and mechanisms of carbohydrate transformations.

Efficient Epimerization of Aldoses Using Layered Niobium Molybdates

Both non-acidic LiNbMoO6 and strongly acidic HNbMoO6 efficiently catalyze the epimerization of sugars including glucose, mannose, xylose, and arabinose in water. The reactions over these oxides reached almost equilibrium within a few hours where yields of corresponding epimers from glucose, xylose, and arabinose were 24-29%. The layered mixed oxides functioned as heterogeneous catalysts and could be reused without loss of activity, whereas bulk molybdenum oxide MoO3 was completely dissolved during the reaction. A (13)C substitution experiment showed that the reaction proceeds through a 1,2-rearrangement mechanism. The surface Mo octahedra were responsible for the activity. The layered HNbMoO6 could also afford mannose from cellobiose through hydrolysis and successive epimerization.

Design of Lewis-acid centres in zeolitic matrices for the conversion of renewables

We review preparative approaches to introduce Lewis-acid centres in zeolites, underlining the structure–performance relationship and highlighting directions to design improved materials of practical relevance.