Roles of hydroxyls in the noncatalytic and catalyzed formation of levoglucosan from glucose

Density Functional Theory Studies on the Hydrolysis of Levoglucosenone to 5-Hydroxymethylfurfural

Selective conversion of lignocellulosic biomass-derived chemicals is of critical significance for sustainable fine and commodity chemical industries. Cellulose-derived levoglucosenone (LGO) has a promising potential for producing 5-hydroxymethylfurfural (HMF) with a substantial yield under acid conditions, but the mechanism is unidentified. Herein, we disclose the mechanism of LGO conversion to HMF in the aqueous phase without and with H₂SO₄ as a catalyst by density functional theory (DFT) calculations for the first time. Results showed that LGO first forms 6,8-dioxabicyclo[3.2.1]-octane-2,4,4-triol (DH) via two sequential hydration reactions occurring at the C═C bond and then the ketone group. The use of H₂SO₄ as a catalyst significantly reduced the free energy barriers of LGO and DH conversion to HMF, with a free energy barrier of 115 kJ/mol for LGO → HMF compared to that of 91 kJ/mol for DH → HMF, demonstrating that DH is easier for HMF formation.

Pyrolytic activation of cellulose: Energetics and condensed phase effects

Bottom-up design of lignocellulose pyrolysis to optimize the quality and yield of bio-oil is hindered by the limited knowledge of the underlying condensed phase biomass chemistry. The influence of condensed...

Cooperative Activation of Cellulose with Natural Calcium

Naturally occurring metals, such as calcium, catalytically activate the intermonomer β-glycosidic bonds in long chains of cellulose, initiating reactions with volatile oxygenates for renewable applications. In this work, the millisecond kinetics of calcium-catalyzed reactions were measured via the method of the pulse-heated analysis of solid and surface reactions (PHASR) at high temperatures (370-430 °C) to reveal accelerated glycosidic ether scission with a second-order rate dependence on the Ca²⁺ ions. First-principles density functional theory (DFT) calculations were used to identify stable binding configurations for two Ca²⁺ ions that demonstrated accelerated transglycosylation kinetics, with an apparent activation barrier of 50 kcal mol^-1 for a cooperative calcium-catalyzed cycle. The agreement of the mechanism with calcium cooperativity to the experimental barrier (48.7 ± 2.8 kcal mol^-1) suggests that calcium enhances the reactivity through a primary role of stabilizing charged transition states and a secondary role of disrupting native H-bonding.

Theoretical study of the pyrolysis of β-1,4-xylan: a detailed investigation on unimolecular concerted reactions

A theoretical study of the thermal decomposition of β-1,4-xylan, a model polymer of hemicelluloses, is proposed for the first time. A mechanism based on unimolecular concerted reactions is elaborated in a comprehensive way. Elementary reactions, such as dehydrations, retro-aldol, retro Diels-Alder, retro-ene, glycosidic bond fissions, isomerizations, etc., are applied to β-1,4-xylan, as well as to the fragments formed. At each stage of the construction of the mechanism, the fragments previously retained are decomposed and the low energy paths are selected to define new fragments. Energy barriers are computed at the CBS-QB3 level of theory and rate coefficients of important reactions are calculated. It is shown that the main reaction pathways can be modelled by reactions involving two specific fragments, which react in closed sequences, similarly to chain-propagating reactions. The proposed reaction scheme allows to predict important species observed during the pyrolysis of xylan, such as aldehydes or CO. In addition, we show that dehydrations require high activation energy and cannot compete with the other reactions. Therefore, it seems difficult to explain, by means of unimolecular homogeneous gas phase reactions, the significant formation of specific species such as furfural as reported by several authors.

Predicting Total Electron-Ionization Cross Sections and GC-MS Calibration Factors Using Machine Learning

Concentrations in GC-MS using electron-ionization mass spectrometry can be determined without pure calibration standards through prediction of relative total-ionization cross sections. An atom- and group-based artificial neural network (FF-NN-AG) model is created to generate EI cross sections and calibrations for organic compounds. This model is easy to implement and is more accurate than the widely used atom-additivity-based correlation of Fitch and Sauter (Anal. Chem. 1983). Ninety-two new measurements of experimental EI cross sections (70-75 eV) are joined with different interlaboratory datasets, creating a 396-compound cross-section database, the largest to date. The FF-NN-AG model uses 16 atom-type descriptors, 79 structural-group descriptors, and one hidden layer of 10 nodes, trained 500 times. In each cycle, 96% of the compounds in this database are freshly chosen at random, and then the model is tested with the remaining 4%. The resulting r² is 0.992 versus 0.904 for the Fitch and Sauter correlation, root mean square deviation is 2.8 versus 9.2, and maximum relative error is 0.30 versus 0.73. As an example of the model's use, a list of cross sections is generated for various sugars and anhydrosugars.

Selective preparation of 1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one by fast pyrolysis of cellulose catalyzed with metal-loaded nitrided HZSM-5

1-Hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one (LAC) is a valuable chiral compound, which can be prepared from catalytic fast pyrolysis (CFP) of cellulose. In this study, nitrided HZSM-5 (N-HZSM-5) and metal-loaded N-HZSM-5 were prepared for CFP of cellulose to selectively produce LAC. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and lab-scale experiments were conducted to explore LAC preparation affected by nitridation and metal modification of HZSM-5, pyrolytic reaction temperature and catalyst-to-cellulose (CA/CL) ratio. The Py-GC/MS experiments obtained the maximal LAC yield of 7.48 wt% with the corresponding selectivity of 30.33% under 5 wt% Mg loaded N-HZSM-5 (5%Mg/N-HZSM-5) with the CA/CL ratio of 6 at 350 °C, compared with those of 1.22 wt% and 2.87% in non-catalytic process. Moreover, lab-scale experiments resulted in the LAC yield and selectivity of 6.69 wt% and 26.18% under the conditions of 5%Mg/N-HZSM-5, CA/CL ratio of 4 and 400 °C. The results demonstrated the promising catalytic performance of Mg/N-HZSM-5 on LAC production from CFP of cellulose.