Mechanism and theory of d-glucopyranose homogeneous acid catalysis in the aqueous solution phase

Steady states and kinetic modelling of the acid-catalysed ethanolysis of glucose, cellulose, and corn cob to ethyl levulinate

Ethyl levulinate is a promising advanced biofuel and platform chemical that can be derived from lignocellulosic biomass by ethanolysis processes. It can be blended with both diesel and gasoline and, thus, used in conventional engines and infrastructure. Previously, it has been shown that alkyl levulinate/alcohol/alkyl ether mixtures exhibit significantly enhanced fuel properties relative to any of the individual fuel components, particularly when blended with conventional hydrocarbon liquid fuels. Consequently, this study specifically quantifies the three primary components of the alcoholysis reaction mixture: ethyl levulinate, diethyl ether, and ethanol. The steady state and kinetic phase fractions of ethyl levulinate and diethyl ether produced from glucose, cellulose, and corn cob with 0.5-2 mass% sulphuric acid in ethanol are determined for 5, 10, and 20 mass% of feedstock at 150 °C. Knowledge of the steady state equilibrium mixture fraction is specifically targeted due to its importance in assessing commercial-scale production and in modelling analysis as: (i) it defines the maximum yield possible at a given condition, and (ii) it is equitable to the minimum free energy state. Maximum steady state yields (mass%) of ethyl levulinate of (46.6 ± 3.7), (50.2 ± 5.4), and (27.0 ± 1.9)% are determined for glucose, cellulose, and corn cob, respectively. The conversion of glucose and cellulose to ethyl levulinate in the presence of ethanol and sulphuric acid is shown to be a catalytic process, where the ethyl levulinate yield is not dependent on the acid concentration. For corn-cob biomass, in a new and contrasting finding, the ethyl levulinate yield is shown to strongly depend on the acid concentration. This effect is also observed in the fractions of diethyl ether formed, providing strong evidence that the hydrogen cation is not being replenished in the ethanolysis process and the overall reaction with corncob is not wholly catalytic. Thus, for the acid catalysed alcoholysis of lignocellulosic biomass, acid concentration must be scaled with feedstock concentration. The critical corn cob-to-acid ratio that maximises ethyl levulinate yields while minimizing the formation of undesired co-products (diethyl ether) is in the range 10-20 : 1 at 150 °C. A detailed, hierarchical, mass-conserved chemical kinetic model capable of accurately predicting the relative abundance of the three primary components of the ethanolysis reaction: ethyl levulinate, diethyl ether, and ethanol, from the biochemical composition of the feedstock, is elucidated and validated.

Hemicellulose pyrolysis: mechanism and kinetics of functionalized xylopyranose

This work analyzes the thermochemical kinetic influence of the most prominent functionalizations of the β-D-xylopyranose motif, specifically 4-methoxy, 5-carboxyl, and 2-O-acetyl, regarding the pyrolytic depolymerization mechanism. The gas-phase potential energy surface of the initial unimolecular decomposition reactions is computed with M06-2X/6-311++G(d,p), following which energies are refined using the G4 and CBS-QB3 composite methods. Rate constants are computed using the transition state theory. The energies are integrated within the atomization method to assess for the first time the standard enthalpy of formation of β-D-xylopyranose, 4-methoxy-5-carboxy-β-D-xylopyranose, and 2-O-acetyl-β-D-xylopyranose: -218.2, -263.1, and -300.0 kcal mol-1, respectively. For all isomers, the activation enthalpies of ring-opening are considerably lower, 43.8-47.5 kcal mol-1, than the ring-contraction and elimination processes, which show higher values ranging from 61.0-81.1 kcal mol-1. The functional groups exert a notable influence, lowering the barrier of discrete elementary reactions by 1.9-8.3 kcal mol-1, increasing thus the reaction rate constant by 0-4 orders of magnitude relative to unsubstituted species. Regardless of the functionalization, the ring-opening process appears to be the most kinetically favored, characterized by a rate constant on the order 101 s-1, exceeding significantly the values associated with ring-contraction and elimination, which fall in the range 10-4-10-10 s-1. This analysis shows the decomposition kinetics are contingent on the functionalization specificities and the relative orientation of reacting centers. A relatively simple chemical reactivity and bonding analysis partially support the elaborated thermokinetic approach. These insights hold significance as they imply that many alternative decomposition routes can be quickly, yet accurately, informed in forthcoming explorations of potential energy surfaces of diverse hemicellulose motifs under pyrolysis conditions.

Unlocking the Diagnostic Potential of Saliva: A Comprehensive Review of Infrared Spectroscopy and Its Applications in Salivary Analysis

Infrared (IR) spectroscopy is a noninvasive and rapid analytical technique that provides information on the chemical composition, structure, and conformation of biomolecules in saliva. This technique has been widely used to analyze salivary biomolecules, owing to its label-free advantages. Saliva contains a complex mixture of biomolecules including water, electrolytes, lipids, carbohydrates, proteins, and nucleic acids which are potential biomarkers for several diseases. IR spectroscopy has shown great promise for the diagnosis and monitoring of diseases such as dental caries, periodontitis, infectious diseases, cancer, diabetes mellitus, and chronic kidney disease, as well as for drug monitoring. Recent advancements in IR spectroscopy, such as Fourier-transform infrared (FTIR) spectroscopy and attenuated total reflectance (ATR) spectroscopy, have further enhanced its utility in salivary analysis. FTIR spectroscopy enables the collection of a complete IR spectrum of the sample, whereas ATR spectroscopy enables the analysis of samples in their native form, without the need for sample preparation. With the development of standardized protocols for sample collection and analysis and further advancements in IR spectroscopy, the potential for salivary diagnostics using IR spectroscopy is vast.

Base-Free Aerobic Oxidation of Furfuralcohols and Furfurals to Furancarboxylic Acids over Nitrogen-Doped Carbon-Supported AuPd Bowl-Like Catalyst

Upgrading furfuralcohols and furfurals to furancarboxylic acids is of great significance for high value-added downstream chemicals synthesis and biomass conversion. Developing an efficient catalyst is the key to acquiring a completely sustainable process. Herein, nitrogen-doped carbon-supported bimetallic AuPd bowl-like catalysts were synthesized. The surface wettability of nitrogen-doped carbon was well adjusted by the nitrification process. Benefiting from the alloying effect of bimetallic AuPd catalyst and the formation of hydroxyl radical initiated by H₂ O dissociation on the hydrophilic surface of nitrated nitrogen-doped carbon, base-free aerobic oxidation of 5-hydroxymethylfurfural (HMF) could produce the highest 2,5-furandicarboxylic acid (FDCA) yield of 93.9 %. In-situ infrared spectroscopy uncovered adsorption configuration of HMF, and the nitrated carbon surface was favorable for HMF and intermediates to enter the active sites, greatly promoting the catalytic oxidation process. Employing other furfuralcohols (furfuryl alcohol, furan-2,5-diyldimethanol, 2,5-bishydroxymethylfuran) as well as furfural and 5-methylfurfural as starting materials, 35.6-95.4 % yield of furancarboxylic acids (FDCA, 2-furoic acid, 5-methyl-2-furoic acid) were also obtained. Moreover, the developed catalysts could maintain excellent stability and activity after four successive runs. This deep insight into the role of bimetallic synergy and surface wettability provides a basis for the rational design of the highly efficient catalysts for the oxidation of furfuralcohols and furfurals and related reactions.

Crystal Faces-Tailored Oxygen Vacancy in Au/CeO2 Catalysts for Efficient Oxidation of HMF to FDCA

Developing an efficient catalyst to upgrade 5-hydroxymethylfurfural (HMF) to high-value-added downstream chemicals is of great significance in biomass conversion. Nanorod (110)-, nanocube (100)-, and nanooctaheron (111)-CeO₂ -supported Au nanoparticles were prepared to investigate the intrinsic effect of CeO₂ crystal faces on the oxidation of HMF to 2,5-furandicarboxylic acid (FDCA). The experimental results and density functional theory calculation revealed that the concentration of oxygen vacancy (V_O ) for exposed specific crystal faces was crucial to the oxygen adsorption ability, and Au/nanorod-CeO₂ with the highest V_O concentration promoted the formation of more oxygen active species (superoxide radical) on CeO₂ (110) crystal face than (100) and (111) crystal faces. Besides, the higher V_O concentration could provide a strong adsorption ability of HMF, greatly boosting the activation of HMF. Thus, these results led to a superior catalytic activity for HMF oxidation over Au/nanorod-CeO₂ (FDCA yield of 96.5 %). In-situ Fourier-transform (FT)IR spectroscopy uncovered the HMF oxidation pathway, and the possible catalytic mechanism was proposed. The deep insight into the role of regulation for crystal faces provides a basis for the rational design of highly active facets for the oxidation of HMF and related reactions.

Experimental and theoretical studies on glucose conversion in ethanol solution to 5-ethoxymethylfurfural and ethyl levulinate catalyzed by a Brønsted acid

The fundamental understanding of glucose conversion to 5-ethoxymethylfurfural (EMF) and ethyl levulinate (EL) (value-added chemicals from biomass) in ethanol solution catalyzed by a Brønsted acid is limited at present. Consequently, here, the reaction pathways and mechanism of glucose conversion to EMF and EL catalyzed by a Brønsted acid were studied, using an experimental method and quantum chemical calculations at the B3LYP/6-31G(D) and B2PLYPD3/Def2TZVP level under a polarized continuum model (PCM-SMD). By further verification through GC/MS tests, the mechanism and reaction pathways of glucose conversion in ethanol solution catalyzed by a Brønsted acid were revealed, showing that glucose is catalyzed by proton and ethanol, and ethanol plays a bridging role in the process of proton transfer. There are three main reaction pathways: through glucose and ethyl glucoside (G/EG), through fructose, 5-hydroxymethylfurfural (HMF), levulinic acid (LA), and EL (G/F/H/L/EL), and through fructose, HMF, EMF, and EL (G/F/H/E/EL). The G/F/H/E/EL pathway with an energy barrier of 20.8 kcal mol^-1 is considered as the thermodynamic and kinetics primary way, in which the reaction rate of this is highly related to the proton transfer in the isomerization of glucose to fructose. The intermediate HMF was formed from O5 via a ring-opening reaction and by the dehydration of fructose, and was further converted to the main product of EMF by etherification or by LA through hydrolysis. EMF and LA are both unstable, and can partially be transformed to EL. This study is beneficial for the insights aiding the understanding of the process and products controlling biomass conversion in ethanol solution.

Salivary molecular spectroscopy: A sustainable, rapid and non-invasive monitoring tool for diabetes mellitus during insulin treatment

Monitoring of blood glucose is an invasive, painful and costly practice in diabetes. Consequently, the search for a more cost-effective (reagent-free), non-invasive and specific diabetes monitoring method is of great interest. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy has been used in diagnosis of several diseases, however, applications in the monitoring of diabetic treatment are just beginning to emerge. Here, we used ATR-FTIR spectroscopy to evaluate saliva of non-diabetic (ND), diabetic (D) and insulin-treated diabetic (D+I) rats to identify potential salivary biomarkers related to glucose monitoring. The spectrum of saliva of ND, D and D+I rats displayed several unique vibrational modes and from these, two vibrational modes were pre-validated as potential diagnostic biomarkers by ROC curve analysis with significant correlation with glycemia. Compared to the ND and D+I rats, classification of D rats was achieved with a sensitivity of 100%, and an average specificity of 93.33% and 100% using bands 1452 cm^-1 and 836 cm^-1, respectively. Moreover, 1452 cm^-1 and 836 cm^-1 spectral bands proved to be robust spectral biomarkers and highly correlated with glycemia (R² of 0.801 and 0.788, P < 0.01, respectively). Both PCA-LDA and HCA classifications achieved an accuracy of 95.2%. Spectral salivary biomarkers discovered using univariate and multivariate analysis may provide a novel robust alternative for diabetes monitoring using a non-invasive and green technology.