Influence of Alkali and Alkaline-Earth Metals on the Cleavage of Glycosidic Bond in Biomass Pyrolysis: A DFT Study Using Cellobiose as a Model Compound

Identification and Parametrization of Key Factors Affecting Levoglucosan Emission During Solid Fuel Burning

Levoglucosan (LG) is a pyrolysis product of cellulose and hemicellulose at low combustion temperatures. However, LG release cannot be determined only by considering the contents of cellulose and hemicellulose exclusively due to the complexity of combustion processes and the physical-chemical properties of the fuel. This study detected the emission factors (EFs) of LG from 22 different solid fuel samples (including coal and biomass) by considering 18 different fuel properties and five combustion parameters. The average LGEFs during solid fuel burning varied in a range of 0.03-136 mg kg-1, with a magnitude difference of 1-4 orders. While the variations in cellulose (59.5-368 mg g-1) and hemicellulose (73.5-165 mg g-1) contents of fuel samples were only one- to 6-fold. A short combustion duration (<150 min) and a medium combustion temperature (200-400 °C) influenced by volatile and ash contents are crucial for the generation and accumulation of LG. A random forest coupled with the Akaike information criterion stepwise regression model successfully explained 96% of the total LG emission variation using three variables (ash content, cellulose content, and modified combustion efficiency). The ash content promoted coke formation and LG chain cracking by increasing the pyrolysis temperature and is considered the most important factor. The alkali metal in ash can reduce the energy barrier of intramolecular ring contraction reactions and inhibit the dehydration reactions, which led to additional heat being utilized by the competitive pathways of LG formation. This study provided a method to address the parametrization and release mechanisms of combustion source emissions.

Removal of crystal violet dye using a three-dimensional network of date pits powder/sodium alginate hydrogel beads: Experimental optimization and DFT calculation

Biodegradable and very low-cost adsorbent beads were prepared from date pits powder (DP) and sodium alginate (SA). DP to SA ratios was varied (1/2, 1/4 and 1/6) and used to eliminate Crystal violet (CV) a cationic dye. Adsorbents were characterized by FTIR, SEM-EDS, UV-vis DR, TGA and the point of zero charge (pHPZC). The optimal composite beads SA@6DP show high adsorption capacities of 83.565 mg/g toward CV than SA@2DP and SA@4DP. The kinetics investigation showed that the adsorption is well described by the pseudo-second-order kinetic (R2 = 0.998). The thermodynamics and isotherms studies exhibit that the adsorption phenomenon for SA@6DP adsorbent is endothermic and significantly fitted with the Redlich-Peterson model. The experimental adsorption tests were optimized by the Box-Behnken design (BBD) which led to conclude the maximal CV removal efficiency achieved by SA@6DP was 99.873 % using [CV] = 50 mg/L, adsorbent mass = 20 mg and 48 h of contact time. The theoretical calculation proved that the CV molecules favor the mode of attack due to their electrophilic character and can accept the SA@6DP adsorbent electrons more easily to form an anti-bonding orbital. SA@6DP hydrogel beads are therefore an exceptional bio-adsorbent that offers excellent adsorption performance.

Deciphering mechanistic implications of antimicrobial and antioxidant potentials of certain new dibutyltin(IV) formulations as possible therapeutic options based on DFT and hybrid materials paradigm

Mechanistic implications of antimicrobial and in vitro antioxidant potentials of a set of newly generated nonbridged mononuclear N,O-orthometallated and carboxylate bridged binuclear nonorthometallated dibutyltin(IV) formulations have been investigated. Some of these formulations were screened for their antibacterial and antifungal activities against Escherichia coli and Candida albicans, respectively whereas in vitro antioxidant potential was examined by Ferric reducing antioxidant power (FRAP) assay. Nonbridged mononuclear N,O-orthometallated dibutyltin(IV) formulations were generated by the reactions of Bu₂ SnCl₂ with sodium salts of 2-aminophenol/substituted 2-aminophenol and flexible N-protected amino acids in 1:1:1 molar ratio in refluxing dry THF. Plausible structures of these nonbridged mononuclear N,O-orthometallated dibutyltin(IV) formulations containing flexible N-protected amino acids have been suggested on the basis of spectroscopic and mass studies of some representative formulations. Plausible structures suggested on the basis of spectroscopic studies are corroborated by density functional theory (DFT/B3LYP method) (SPARTAN-20) investigation of a representative dibutyltin(IV) complex and the ligands involved in it. The presence of two different classes of organic ligands in this complex provides an opportunity to study optimized topologies, bonding, distortions, optimized energy, and stability of the complex.

Mechanistic Investigation into the Formation of Humins in Acid-Catalyzed Biomass Reactions

Humins are carbonaceous, polymeric byproducts formed during the acid-catalyzed condensed phase transformation of biomass-derived moieties and are responsible for significant carbon loss and catalyst deactivation. There exists very limited knowledge about their formation chemistry and composition. Infrared spectra of humins formed during the dehydration of glucose/fructose to 5-HMF show that the furan ring and the hydroxy methyl group of 5-HMF are present in humins, but the carbonyl group is not. Based on this, aldol addition and condensation between 5-HMF and other derived species are proposed as the main reactions that initiate humin formation. Hence, in this work, density functional theory (DFT)-based calculations are performed to compute the reaction pathways, activation barriers, and reaction free energies associated with all elementary reaction steps in the 5HMF-initiated, acid-catalyzed reactions leading to humin formation. The humin formation is initiated with the rehydration of HMF to form 2,5-dioxo-6-hydroxy-hexanal or DHH (key promoter of humin formation), followed by its keto-enol tautomerization and aldol addition and condensation with HMF. The rate-determining step in this pathway is the aldol-addition reaction between the DHH-derived enols with 5-HMF. Within the implicit solvation approximation, the formation of the 5-HMF-DHH dimer is slightly endergonic, whereas the 5-HMF rehydration leading to DHH is thermodynamically downhill. This mechanistic understanding of initiation reactions for humins could pave the way to screen and design solvent and catalyst systems to deter their formation.

New Perspectives into Cellulose Fast Pyrolysis Kinetics Using a Py-GC × GC-FID/MS System

Cellulose pyrolysis is reportedly influenced by factors such as sample size, crystallinity, or different morphologies. However, there seems to be a lack of understanding of the mechanistic details that explain the observed differences in the pyrolysis yields. This study aims to investigate the influence of particle size and crystallinity of cellulose by performing pyrolysis reactions at temperatures of 673–873 K using a micropyrolyzer apparatus coupled to a GC × GC-FID/TOF-MS and a customized GC-TCD. Over 60 product species have been identified and quantified for the first time, including water. Crystalline cellulose with an average particle size of 30–50 × 10^–6 m produced 50–60 wt % levoglucosan. Predominantly amorphous cellulose with an average particle size of 10–20 × 10^–6 m resulted in remarkably low yields (10–15 wt %) of levoglucosan complemented by higher yields of water and glycolaldehyde. A detailed kinetic model for cellulose pyrolysis was used to obtain mechanistic insights into the different pyrolysis product compositions. The kinetics of the mid-chain dehydration and fragmentation reactions strongly influence the total yields of low-molecular weight products (LMWPs) and are affected by cellulose chain arrangement. Levoglucosan yields are very sensitive to the activation of parallel cellulose decomposition reactions. This can be attributed to the mid-chain reactions forming smaller chains with the levoglucosan ends, which remain in the solid phase and react further to form LMWPs. Direct quantification of water helped to improve the description of the dehydration, giving further indications of the dominant role of mid-chain reaction pathways in amorphous cellulose pyrolysis.

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.

Unravelling the catalytic influence of naturally occurring salts on biomass pyrolysis chemistry using glucose as a model compound: a combined experimental and DFT study

Alkali and alkaline-earth metal loaded biomass pyrolysis highlights that different metal ions have different effects on bio-oil composition.