NMR Insights on the Properties of ZnCl₂ Molten Salt Hydrate Medium through Its Interaction with SnCl₄ and Fructose

The solvent properties of ZnCl₂ molten salt medium and its synergic effect with the Lewis acid catalyst, Sn⁴⁺, for biomass conversion, were investigated by nuclear magnetic resonance. The tautomeric distribution of fructose in the ZnCl₂ molten salt medium was examined, and its effect for humins formation during the biomass conversion was evaluated. The ion complex composed by Sn⁴⁺ and Zn²⁺ indicated that there is a synergic catalytic effect between these two Lewis acid ions. ¹³C NMR spectra of fructose in different ZnCl₂ molten salt hydrate concentrations revealed that the concentration of β-furanose and α-furanose tautomers, which lead to 5-HMF, were significantly decreased with increased salt concentration. Meanwhile the β-pyranose tautomer, which is correlated with humins formation, was increased significantly

Graphene Oxide: A Convenient Metal-Free Carbocatalyst for Facilitating Aerobic Oxidation of 5‑Hydroxymethylfurfural into 2, 5‑Diformylfuran

By thermal treatment in vacuum, graphite oxide prepared by Hummer’s method was exfoliated and partially reduced. This procedure imparts to the graphene oxide (GO) high reactivity with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) as cocatalyst for selective oxidation of 5-hydroxymethylfrufural (HMF) to 2,5-diformylfuran (DFF) under certain conditions (100% HMF conversion with HMF selectivity 99.6% at 80 wt % GO loading, 1 atm air pressure). This study found that GO could function as an oxidant for anaerobic oxidation of HMF during which carboxyl groups in GO were reduced. Importantly, the partially reduced GO material could continue to activate molecular oxygen during aerobic oxidation. Further study showed that oxygen functionalities in the GO material had a crucial effect on the catalytic oxidation of HMF. By control experiments and molecular analogues tests, a plausible mechanism was proposed in which the high reactivity was attributed to the synergistic effect of the carboxylic acid groups and unpaired electrons at GO edge defects, with TEMPO as the cocatalyst and oxygen as the terminal oxidant

Insight into the Mechanism of Water-Promoted Hydrogenation of Maleic Acid to Succinic Acid on Pd/C Catalyst

Solvent provides additional degrees of freedom to regulate catalyst reactivity in liquid-phase heterogeneous catalysis, but it is still a challenge to have insight into the multifaceted solvent effects. Herein, a remarkable promotional effect of water in maleic acid (MAc) hydrogenation to succinic acid (SAc) was observed. Kinetic studies showed that the apparent activation energy in water was much lower than in organic solvents. A series of isotope-labeling experiments were designed, and the products were analyzed by NMR (1H, 13C, 2H, and DEPT135 spectra). The results showed that D2O participated in MAc CC hydrogenation and 34.7% of SAc was deuterated. The structures of these deuterated compounds were further confirmed by electrospray mass spectrometry (ESI-MS). The detailed mechanism of water participating in MAc CC hydrogenation was studied by quasi-in situ mass spectrometry experiments. The results showed that H2 exchanged with D2O and formed the HD2O* transition state over the active site of Pd. Quantitative 13C NMR demonstrated that 46.2% of SAc was generated through the HD2O* transition state pathway. Based on these results, a rational mechanism of MAc hydrogenation in aqueous solution was proposed. Finally, a recyclability experiment showed that Pd/C had much better stability in water than in organic solvents

NMR Study of the Hydrolysis and Dehydration of Inulin in Water: Comparison of the Catalytic Effect of Lewis Acid SnCl₄ and Brønsted Acid HCl

Various NMR techniques were employed to study the catalytic performance of the Lewis acid SnCl₄ and the Brønsted acid HCl in the conversion of inulin to value-added compounds by hydrolysis and subsequent dehydration. The hydrolysis of inulin was examined to reveal the catalytic abilities of SnCl₄ besides its intrinsic acidity by in situ ¹H and ¹³C NMR at 25 °C. The dehydration reaction of inulin with SnCl₄ as catalyst was followed by high temperature in situ ¹H NMR at 80 °C. The fructose moieties were dehydrated to 5-(hydroxy­methly)­furfural (5-HMF), but the glucose fragment of inulin was inactive for dehydration reaction under this condition. The formation of 5-HMF and its transformation into formic acid and levulinic acid through a rehydration reaction could be monitored by in situ NMR spectroscopy. Moreover, diffusion ordered spectroscopy NMR revealed that the Lewis acid ion, Sn⁴⁺ interacts with the inulin model compounds, i.e., sucrose and fructose. The synergistic effects of complexation and acidity from the hydrolysis of SnCl₄ results in a higher catalytic ability of this Lewis acid catalyst compared with a Brønsted acid

Efficient C–C Bond Formation between Two Levulinic Acid Molecules To Produce C₁₀ Compounds with the Cooperation Effect of Lewis and Brønsted Acids

An original route for levulinic acid (LA) conversion was achieved via C–C bond formation in a co-catalysis system containing Lewis and Brønsted acids. Different from conventional base-catalyzed processes, this acidic reaction system inhibits the carboxylic acid from deactivating the base catalyst, additionally simplifies the technical processes. The synergistic effect of the two types of acids effeciently catalyzed the dimerization reaction of LA to generate two C₁₀ compounds, tetrahydro-2-methyl-5,γ-dioxo-2-furanpentanoic acid and 3-(2-methyl-5-oxo-tetrahydrofuran-2-yl)-4-oxopentanoic acid, with the total yield of 50.9% at 59.7% conversion. These products present high added value as fuel or fine chemical precursors

Deep Eutectic Solvents: Green Solvents and Catalysts for the Preparation of Pyrazine Derivatives by Self-Condensation of d‑Glucosamine

Deep eutectic solvents (DESs) exhibit similar physicochemical properties to the ionic liquids. They are inexpensive, renewable, nontoxic, and environmentally benign solvents and have gradually attracted attention in several fields, for example, biorefinery. Here choline chloride-based DESs have been used as solvents and catalysts for the preparation of deoxyfructosazine (DOF) through a self-condensation reaction of d-glucosamine (GlcNH₂). The catalytic performances of a “green cocatalyst”, amino acids, and the reaction mechanism were also studied. The results displayed that choline chloride/urea was capable to convert GlcNH₂ efficiently, with a 13.5% yield of DOF at low temperature and with a short reaction time (100 °C, 150 min). Among the screened amino acids, arginine showed the highest activity and gave the highest yield of DOF (30.1%) under the optimized reaction conditions. Nuclear magnetic resonance (NMR) studies revealed a strong hydrogen bond interaction between GlcNH₂ and arginine. Moreover, a detectable intermediate, namely dihydrofructosazine, in the condensation of GlcNH₂ to DOF/fructosazine (FZ) was captured by in situ NMR technique

Chemical Recycling of Carbon Fiber Reinforced Epoxy Resin Composites via Selective Cleavage of the Carbon–Nitrogen Bond

An efficient strategy is developed for chemical recycling of cured epoxy resin (CEP) from its carbon fiber reinforced polymer composites (CFREP) using AlCl₃/CH₃COOH as the degradation system. Acetic acid swells the dense structures of CEP, facilitating the penetration of the aluminum ion catalyst into the polymer matrix. The weakly coordinating aluminum ions in CH₃COOH solution selectively cleave the C–N bond while leaving the C–C, C–O (aryl alkyl ether) bonds intact. This process recovers valuable oligomers and carbon fibers from CFREP

Product Distribution Control for Glucosamine Condensation: Nuclear Magnetic Resonance (NMR) Investigation Substantiated by Density Functional Calculations

Selective conversion of glucosamine (GlcNH₂) to deoxyfructosazine (DOF) and fructosazine (FZ) with additives was investigated. Significantly enhanced yield of DOF can be improved to 40.2% with B­(OH)₃ as the additive. Chemical shift titration (via one-dimensional nuclear magnetic resonance (1D ¹H and ¹³C NMR)) and two-dimensional nuclear magnetic resonance (2D NMR) including ¹H–¹³C HSQC and ¹H–¹H COSY are used to investigate intermolecular interactions between B­(OH)₃ and GlcNH₂. Diffusion-ordered NMR spectroscopy (DOSY) was further employed to identify intermediate species. Mechanistic investigation by NMR combined with electron spray ionization–mass spectroscopy (ESI-MS) discloses that a mixed 1:1 boron complex was identified as the major species, shedding light on the promotional effects of B­(OH)₃, which is substantiated by density functional theory (DFT). Boron coordination effects make ring-opening and subsequent dehydration reaction thermodynamically and kinetically more favorable. Dehydration of dihydrofructosazine is a key step in controlling overall process (49.7 kcal/mol). Interestingly, chelating effect results in substantial reduction of this free-energy barrier (31.5 kcal/mol). Notably, FZ was gradually becoming the main product (yield up to 25.3%), with H₂O₂ as the oxidant

X-radiographs of SD rat spines in vitro.

<p>The Representative images showed that visible fusion occurred in both G2 and G3 groups at 16 weeks after the implantation (coronal view (C) and sagittal view (S)). The intervertebral space in G3 was hid under the new bone. A great quantity of new bone could be found and the processus transversus were joined to form a whole from a posterior lateral aspect between L4 and L5. There was a similar, but slightly imperfect result in G2.</p

X-Radiographs of SD rat spines in vivo.

<p>Representative images of rats in each group are shown (coronal view). 8 weeks after the implantation, the spines appeared fused in G3, partially fused in G2 and no fused in G1.</p