基于DFT的半纤维素和木质素热解机理

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Herein, choline chloride/oxalic acid (ChCl/OA) and choline chloride/oxalic acid/ethylene glycol (ChCl/OA/EG) pretreatments of oil palm empty fruit bunches (EFB) and mesocarp fibers (MSF) were conducted to achieve protection of the lignin structure, while improving the enzymatic efficiency of the solid residues. Under the operating conditions of 90 °C and 6 h, ChCl/OA/EG demonstrated a higher lignin extraction selectivity and obtained solid residues with higher hemicellulose content compared to ChCl/OA. The digestibility of glucan and xylan in solid residues obtained using ChCl/OA/EG achieved 98.56 % and 95.63 %, respectively, for EFB and 75.95 % and 88.60 %, for MSF. Uncondensed lignin enriched with 71.79-81.61 % of β-O-4 bonds was obtained from EFB and MSF using ChCl/OA/EG. 2D HSQC NMR and the density functional theory calculation confirmed that substituting the lignin Cα position by ethylene glycol changed the local potentials of the β-O-4 bonds, impeding the attack of protons (H+). The higher β-O-4 linkage content in ChCl/OA/EG-Ls led to the formation of several oxygenated alkyl methoxy phenols and alkyl methoxy phenols were promoted during the pyrolysis. Moreover, molecular dynamics simulations showed that the main factor affecting lignin extraction and dissolution in this study was the diffusion coefficient of lignin in DESs.

Model compounds that represent important substructures in lignin have popularly been used to gain a better understanding of the behavior of lignin during thermal deconstruction, such as fast pyrolysis. The β-O-4 linkage of lignin has previously been the focus of many model compound studies as it is the most prevalent linkage found in native lignin. In this work, two lesser studied linkages, the β–β′ and 4-O-5, were investigated with density functional theory (DFT). Bond dissociation enthalpies (BDEs) were calculated for the relevant bonds along each interunit linkage for two model compounds containing these linkages. Conformational analysis of the first model oligomer has a relative enthalpy difference of 1.55 kcal mol−1. For the β–β′ linkage, the alpha carbons had the lowest BDEs of the ring opening reactions due to excessive electron delocalization around the aromatic rings. The bonds of the 4-O-5 linkage had similar BDEs but were appreciably higher than the BDEs for other ether linkages, such as β-O-4 and α-O-4. The higher BDEs of the 4-O-5 bonds is a result of the radical being formed on an aromatic carbon compared to an aliphatic carbon. Our results indicate the ring-opening reactions around the alpha-carbon of the β–β′ linkage would be a major reaction point during thermal deconstruction of the chosen oligomers. This work provides valuable information on the thermal deconstruction behavior of two lesser studied interunit linkages that builds on the authors' previous work, on β-O-4, α-O-4, and β-5 linkages, to develop a library of reaction information for various lignin interunit linkages.

This work elucidates Evans–Polanyi-like (EPL) relations to rapidly estimate the standard activation enthalpy of three ubiquitous reaction classes playing a central role in hemicellulose pyrolysis: ring-opening, ring contraction, and elimination. These models bypass computing the reaction enthalpy by leveraging computationally cheap local and global electron-density-based chemical reactivity descriptors, such as Fukui’s functions (f), electron population of C–O bonds (N), and the gross intrinsic strength bond index (Δg pair), evaluated for reactants solely. More than 270 reactions observed in twenty-eight functionalized β-d-xylopyranoses, the hemicellulose building block, are used under the 20–80% partition scheme for validating-deriving purposes. By using multilinear regression analysis, four EPL equations are proposed for informing barriers at the M06–2X/6–311++G(d,p), CBS-QB3, G4, and DLPNO–CCSD(T)-F12/cc-pVTZ-F12//M06–2X/6–311++G(d,p) levels. An adjusted coefficient of determination of 0.80 characterizes these parametric polynomials. Moreover, MAE and RMSE of ≈3.3 and ≈4.1 kcal mol–1 describe the performance of the best-fitting models at DFT and G4. Conversely, the highest values, MAE = 3.6 and RMSE = 4.7 kcal mol–1, are associated with the CBS-QB3 level. The benchmarking of the computed activation enthalpies at 298 K yields simple functions for high-level estimations from low levels of theory with R 2 ranging from 0.94 to 0.98. Extrapolating the DPLNO barriers to the complete basis set limit tends to lower them by 0.63 kcal mol–1. EPL expressions are tailored to facilitate the development of chemical kinetic models for hemicellulose pyrolysis, as the reactant structure is the only input required.

Computational studies on the pyrolysis of lignin using electronic structure methods have been largely limited to dimeric or trimeric models. In the current work we have modeled a lignin oligomer consisting of 10 syringyl units linked through 9 β-O-4' bonds. A lignin model of this size is potentially more representative of the polymer in angiosperms; therefore, we used this representative model to examine the behavior of hardwood lignin during the initial steps of pyrolysis. Using this oligomer, the present work aims to determine if and how the reaction enthalpies of bond cleavage vary with positions within the chain. To accomplish this, we utilized a composite method using molecular mechanics based conformational sampling and quantum mechanically based density functional theory (DFT) calculations. Our key results show marked differences in bond dissociation enthalpies (BDE) with the position. In addition, we calculated standard thermodynamic properties, including enthalpy of formation, heat capacity, entropy, and Gibbs free energy for a wide range of temperatures from 25 K to 1000 K. The prediction of these thermodynamic properties and the reaction enthalpies will benefit further computational studies and cross-validation with pyrolysis experiments. Overall, the results demonstrate the utility of a better understanding of lignin pyrolysis for its effective valorization.

It is anticipated that the insight into the demethylation and mechanism of CH4 formation from natural lignin using in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ FTIR) combined with two-dimensional perturbation correlation infrared spectroscopy (2D-PCIS) and density functional theory (DFT) calculation analysis would contribute to a deeper insight of bond cleavage mechanism of lignin pyrolysis. Herein, GS-type lignin (poplar MWL) was characterized by Fourier transform infrared spectroscopy (FTIR) and heteronuclear Single-Quantum Correlation Nuclear Magnetic Resonance (HSQC), and its pyrolysis at different temperatures was performed in a lab-scale fixed-bed reactor. The biochar, gaseous and liquid products were qualitative, and quantitative analysis of gases and bio-oil is demonstrated using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). The key of CH4 formation is the homolytic cleavage of the methoxyl functional group generating methyl radical and further verified via in-situ FTIR combined with 2D-PCIS and DFT calculation. The study established a new methodology based on multiple factor analysis to evaluate the CH4 formation mechanism in GS-type milled wood lignin at the molecular level, which is of positive significance for increasing lignin valorization and improving the environment.

Cinnamyl alcohols such as p-coumaryl alcohol ( p-CMA) are lignin models and precursors (monolignols) and the most important primary products of lignin pyrolysis. However, the detection of monomers is not straightforward since they either undergo secondary transformations or repolymerize to contribute to the char formation. Both concerted-molecular and free-radical pathways are involved in these processes. Our recent fundamentally based theoretical and low-temperature matrix-isolation-EPR studies of cinnamyl alcohols highlighted the role of side-chain reactivity in diversity of pyrolysis products and provided a network of the chemically activated H + p-CMA reactions ( Asatryan J. Phys. Chem. A, 2017 , 121 , 3352 - 3371 ). The readily available hydroxyl radicals also can trigger a cascade of free-radical processes. Here, we present a comprehensive potential energy surface (PES) analysis of the OH + p-CMA reaction using various DFT and ab initio protocols. Since the p-CMA involves both an alkyl OH-group and a side-chain double bond, the title reaction can also serve as a relevant model for reactions of unsaturated alcohols with hydroxyl radicals to form various oxygenates including polyhydric alcohols which are abundant in nature. The newly identified pathways suggest certain alternatives to the known radical reactions. Of particular interest are the roaming-like low-energy dehydration reactions to generate a variety of O- and C-centered intermediate radicals, which are primarily transformed into the phenolic compounds observed in pyrolysis experiments. Several concerted unimolecular decomposition pathways for p-CMA are also revealed, not considered previously, such as the migration of terminal OH-group, and/or its splitting over the ipso-C and ortho-C atoms of the benzene ring to form bicyclic oxispiro- and chromene compounds represented in natural lignin.

Lewis-acid catalyst Nb2O5 is first applied in catalytic fast pyrolysis (CFP) of enzymatic hydrolysis lignin (EHL) to produce aromatic hydrocarbons (AHs) that can be used as alternative liquid fuels. The catalyst exhibits a good talent to convert lignin into AHs with quite little polycyclic aromatic hydrocarbons (PAHs) formation. The yield of AHs reaches 11.2 wt% and monocyclic aromatic hydrocarbons (MAHs) takes up 94% under the optimized condition (Catalyst to Lignin ratio 9:1, 650 °C). No coke is generated during the reactions. The reaction sequence is proposed and verified by model compound reactions. Furthermore, DFT calculations are performed to understand the mechanisms of limitation of PAHs or char/coke formation and the efficient deoxygenation ability over catalyst. Nb2O5 with Lewis acid sites is proved to be a promising catalyst for the production of AHs from lignin. This work provides a new idea on choice of catalysts for CFP of lignin in future.

Catalytic fast pyrolysis of lignin with zeolite catalysts is a promising method to produce aromatic hydrocarbons. In this paper, alkali lignin was used as a model compound to pyrolyze with HZSM-5 (silica to alumina ratio, SAR = 23), HZSM-5(50), HZSM-5(80), HY and Hβ. Non-condensable vapours and condensable fractions were determined and quantified by GC/FID and GC/MS respectively. 7.63 wt% of aromatic hydrocarbons and 3.34 wt% of C1–C4 alkanes and alkenes were acquired. The effects of catalysts and pyrolysis parameters were studied in this work. Different reaction pathways were compared and discussed by combining density functional theory (DFT) calculations. Cyclization reactions to form aromatic hydrocarbons were thought to be the main reaction pathway, while direct demethylation, demethoxylation and dehydration reactions were the secondary reaction pathway to convert phenolic lignin monomers to non-oxygenated aromatic hydrocarbons.

CaO modified with acetic acid solution or sodium hydroxide (H-CaO/OH-CaO) was used to explore the relationship between the physical and chemical properties of CaO and the components of bio-oil during the pyrolysis of rice straw (RS) and model compounds via experiment and density functional theory(DFT) simulation. The results showed that the modification changed the properties of CaO, and thus the catalytic performance on production of bio-oil components. H-CaO with the larger number of strong basic sites (1.10 ∼ 2 times than commercial CaO) and the longer Ca-O bond length showed the better selectivity and performance on formation of ketones (the maximum relative content in bio-oil reached 43 %). The conversion pathway of cellulose/hemicellulose was changed by H-CaO, which promoted the formation of ketones. The easier combining of H-CaO with the pyrolysis primary products due to the longer Ca-O bond was the key to its better performance.

Chemical reaction neural networks (CRNN) and density functional theory (DFT) are gaining attention in biomass pyrolysis mechanism research. Reaction pathways are often speculated based on a single method, influenced by expert knowledge. To address this, the pyrolysis mechanism of xylose, a hemicellulose model compound, is studied using thermogravimetric-Fourier transform infrared spectroscopy (TG-FTIR), CRNN, and DFT. Based on the TG-FTIR measured products, seven main pyrolysis products of xylose are applied. A CRNN model of 8 species (7 + 1) and 10 reactions, including kinetic parameters, is established, achieving an MAE below 2 × 10-2. Furthermore, the detailed reaction pathways and potential energy surfaces of evolution into each species are studied through DFT calculations. The activation energy for the xylose ring-opening, ring-condensation, and dehydration reactions, as obtained from the CRNN model, are 152.78 kJ/mol, 485.81 kJ/mol, and 320.01 kJ/mol, respectively. The deviations from the theoretical DFT calculations are less than 37 %, demonstrating good agreement. Compared to dehydration and ring condensation reactions, xylose is most susceptible to ring-opening reactions to form d-xylose. d-xylose easily undergoes hemi-acetalization, isomerization and dehydration at the 3-OH + 2-H, 5-OH + 4-H and 4-OH + 3-H sites. Xylose is most susceptible to dehydration reactions at the 1-OH + 2-H and 2-OH + 3-H sites to generate dehydrated xylose. Finally, xylose can also generate furfural through a cyclic condensation reaction. The conclusion can promote the improvement and development of hemicellulose pyrolysis kinetics models, and the research process can provide experience for the pyrolysis mechanism of other materials.

Xylopyranose is the principal monosaccharide unit of hemicellulose, one of the three major biopolymers of lignocellulosic biomass. Understanding its decomposition mechanism is increasingly relevant for thermochemical biorefinery research such as pyrolysis. Significant efforts have been made to study its chemical and structural properties using both computational and experimental methods. However, due to its high structural flexibility and numerous hydroxyl groups, various metastable conformers arise. In this work, we performed a computational exploration of the conformational space of both anomeric forms, α and β, of D-xylopyranose using the semi-empirical GFN2-xTB method in conjunction with metadynamics and density functional theory simulations for structural optimization and vibrational analysis. Xylopyranose conformers free energy and enthalpy variations are analyzed across temperatures typical of fast biomass pyrolysis (298-1068 K), with the Boltzmann population distribution of the most populated conformers determined. This study provides a detailed computational analysis of the conformational space and thermochemistry of xylopyranose. Additionally, 44 and 59 conformers of the α and β anomers were found, for both of which a selection of 10 conformers based on Boltzmann population distribution analysis is performed to reduce the conformational space for ab initio studies of the pyrolysis reaction kinetics.

The conversion of inedible biomass by fast pyrolysis is a promising route for sustainable production of renewable fuels and value-added chemicals, but low selectivity toward desired products hampers its economic viability. Understanding the molecular-level reaction pathways of biomass fast pyrolysis could be the key to overcoming this challenge. However, the effects of intramolecular and interchain hydrogen bonds near the reaction center have not been thoroughly explored. In this work, the reaction pathways and kinetics of fast pyrolysis of cellulose, a major component of biomass, were investigated using the density functional theory. A new intramolecular hydroxyl-activated mechanism is presented for cellulose activation. Our calculations incorporating noncovalent interactions accurately captured the activation energy of 50.8 kcal mol-1, agreeable with the apparent activation energy measured experimentally. The findings of cellulose pyrolysis provide insights into the investigation of interactions during real-life biomass pyrolysis.

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In this paper, analytical pyrolyzer coupled with a gas chromatography–mass spectrometry set-up (Py-GC/MS) and density functional theory(DFT) theory was used to reveal the initial pyrolysis mechanism and product formation mechanism of cellulose pyrolysis. We demonstrated an experimentally benchmarked molecular simulation approach that delineates pyrolysis process of cellulose. Experimental results indicated that the cellulose pyrolysis products mostly incorporate levoglucosan (LG), glycolaldehyde (HAA), 5-hydroxyfurfural (5-HMF), and the like. The constituents of fast pyrolysis products of cellulose and cellobiose demonstrated the identical trend, although the contents of certain products are different. Laying the foundation of experimental analysis, the reaction pathways of four categories of cellulose pyrolysis were outlined using DFT theory; the pathways are those of generating LG, HAA, and 5-HMF and the dehydration reaction in the process of cellulose pyrolysis. Also, by comparing the energy barriers of various reactions, the optimal pathway of different reactions were summarized. The deduced cellulose pyrolysis reaction pathway opened up new ideas for studying the pyrolysis behavior of cellulose.

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In this study, functionalized graphene oxide-supported cerium oxide nanocatalysts (FGCe) with varying graphene oxide (GO) contents were prepared using an in-situ reflux method. The prepared nanocatalysts showcased improvement in the crystallinity and BET surface area values with increasing GO contents. The efficacies of prepared catalysts were investigated towards oxidative pyrolysis of alkali lignin in an ethanol-water system. Among various nanocatalyst samples, the best lignin conversion (93 %) and bio-oil yield (86 %) were achieved using 50 mg FGCe nanocatalyst (0.5 wt% GO) at 423 K and 60 min. GC-MS and 1HNMR analyses were used to identify significant lignin conversion products, including 2-pentanone-4-hydroxy-4-methyl, 2-methoxyphenol, nonylcyclopropane, vanillin, apocynin, homovanollic acid, and benzoic acid. Kinetic studies revealed that the activation energy for lignin conversion was 24.36 kJ/mol at 423 K. Mechanistic investigations by density functional theory analysis revealed that the lignin breakdown occurred at oxygen bonds producing aromatic.

Biomass pyrolysis is a promising thermochemical conversion pathway for producing renewable fuels, value-added chemicals, and carbon-based materials from sustainable feedstocks. However, the complex and highly sensitive nature of pyrolysis reactions, governed by biomass composition, operating conditions, and reactor design, continues to challenge predictive control and large-scale deployment. This review provides a comprehensive and critical synthesis of recent advances in biomass pyrolysis, with particular emphasis on feedstock characteristics; the thermal decomposition mechanisms of cellulose, hemicellulose, and lignin; and the influence of key operational parameters, such as temperature, heating rate, residence time, and particle size, on product distribution. Special attention is given to reaction intermediates and pathways identified through advanced analytical techniques, including Py-GC/MS, TG-FTIR, two-dimensional photoionization mass spectrometry, and complementary molecular-level simulations such as density functional theory and reactive molecular dynamics. By systematically integrating experimental observations with mechanistic insights, this review highlights current limitations, including the lack of unified kinetic models, weak coupling between experiments and simulations, and insufficient investigation of high-temperature pyrolysis regimes above 800 °C. Emerging opportunities for data-driven and machine-learning-assisted kinetic modeling are also discussed as a pathway to address biomass heterogeneity and complex reaction networks. The findings presented herein aim to support the development of predictive pyrolysis models, optimized reactor design, and the sustainable valorization of biomass within future bioenergy and biorefinery systems.

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Automatic potential energy surface (PES) exploration is important to a better understanding of reaction mechanisms. Existing automatic PES mapping tools usually rely on predefined knowledge or computationally expensive on-the-fly quantum-chemical calculations. In this work, we have developed the PESmapping algorithm for discovering novel reaction pathways and automatically mapping out the PES using merely one starting species is present. The algorithm explores the unknown PES by iteratively spawning new reactive molecular dynamics (RMD) simulations for species that it has detected within previous RMD simulations. We have therefore extended the RMD simulation tool ChemTraYzer2.1 (Chemical Trajectory Analyzer, CTY) for this PESmapping algorithm. It can generate new seed species, automatically start replica simulations for new pathways, and stop the simulation when a reaction is found, reducing the computational cost of the algorithm. To explore PESs with low-temperature reactions, we applied the acceleration method collective variable (CV)-driven hyperdynamics. This involved the development of tailored CV templates, which are discussed in this study. We validate our approach for known pathways in various pyrolysis and oxidation systems: hydrocarbon isomerization and dissociation (C4H7 and C8H7 PES), mostly dominant at high temperatures and low-temperature oxidation of n-butane (C4H9O2 PES) and cyclohexane (C6H11O2 PES). As a result, in addition to new pathways showing up in the simulations, common isomerization and dissociation pathways were found very fast: for example, 44 reactions of butenyl radicals including major isomerizations and decompositions within about 30 min wall time and low-temperature chemistry such as the internal H-shift of RO2 → QO2H within 1 day wall time. Last, we applied PESmapping to the oxidation of the recently proposed biohybrid fuel 1,3-dioxane and validated that the tool could be used to discover new reaction pathways of larger molecules that are of practical use.

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Lignin through pyrolysis results in a liquid byproduct that contains a significant proportion of phenolic compounds, which can be subsequently enhanced into aromatic hydrocarbons using hydrogenation methods. To comprehend the reaction mechanism of the lignin hydrogenation process, we conducted theoretical investigations on the hydrogenation reaction process of different phenolic lignin model compounds by theoretical calculation method M06-2X/6-31++G(d,p). Different potential reaction routes were developed for the hydrogenation process of lignin model compounds, and the thermodynamic and kinetic properties of key reaction steps in each pathway were computed. The calculated results indicate that the hydrogenation reaction process of lignin model compounds, the aromatic compound, CH2O, H2O, and CH3OH may be the products that emits firstly in lignin hydrogenation reaction process. And the presence of methoxyl and hydroxyl substituents on the benzene ring does not significantly impact the hydrogenation process of lignin model compounds. Furthermore, the hydrogenation reaction in lignin model compounds exhibits a high energy barrier of approximately 300.0 kJ/mol. Thus, the hydrogenation process of lignin necessitates either appropriate modifications to the depolymerization conditions or the introduction of a catalyst. This work offers a theoretical foundation for advancing the research on lignin of hydrogenation and achieving optimal resource utilization.

The alkaline aerobic oxidation of lignin to vanillin (4-hydroxy-3-methoxybenzaldehyde) has been employed as an industrial method for producing bio-based low-molecular-weight aromatic compounds. To deepen the molecular-level understanding of this reaction, we have been investigating the vanillin formation mechanism from native softwood lignin. One of the major reaction pathways involves oxidative degradation of β-O-4-type internal units, followed by alkaline-induced elimination of vanillin from the resulting vanillin end group. This study examined the reaction mechanism of a model compound, 4-[2-(3-ethoxy-4-methoxyphenyl)-2-hydroxy-1-(hydroxymethyl)ethoxy]-3-methoxybenzaldehyde, VEβ, which mimics the vanillin end group, in 4.0 mol L−1 aqueous NaOH, with particular focus on the formation pathways of vanillin and byproducts. VEβ rapidly formed an equilibrium mixture comprising various rearranged compounds, in which the vanillin residue had migrated to the α- and γ-positions of the side-chain via an acetal-type intermediate. Kinetic analysis based on a pseudo-first-order competitive reaction model revealed that this equilibrium mixture was consumed through two distinct pathways: vanillin elimination and side reaction accompanied by polymerization. The activation energy (Ea) for vanillin elimination was determined to be 17.0 kcal mol−1, which agreed moderately with the Ea value of 20.7 kcal mol−1 calculated by DFT(M06-2X) for the α-oxyanion-assisted elimination process. Although the details of the side reaction pathway remain unclear, the overall reaction followed pseudo-first-order kinetics despite the involvement of bimolecular steps, suggesting that the rate-determining step of the side reaction proceeds via a unimolecular process.

Lignin is a great sustainable source of fine aromatic compounds, which requires catalytic biomass pyrolysis (degradation) and subsequent reformation (upgrade) of oils into useful deoxygenated compounds. The adsorption and selective degradation of guaiacol, a predominant bio-oil monomer, and a model compound, was studied using the density functional theory (DFT) method on Cu (111) and its modified Ru and Ni bimetallic surfaces. The defect formation energies of Ru and Ni impurities with Cu (111) surfaces and guaiacol's binding energies were investigated. Comparing Cu (111) alloy with Ru and Ni, as well as with their deposited counterparts, at 1 ML impurity concentrations, our results show that alloying is favored over deposition. Ni-deposited surface forms chemical bonds with guaiacol, while physisorption is observed on the other surfaces. In most cases, dehydrogenation occurs via the loss of hydrogen from the hydroxyl group, except on the Ru-alloyed surface. Ni-alloy provides a special surface for overcoming the high catalytic generation of catechol from guaiacol by favoring anisole formation and hindering catecholate formation.

Fast pyrolysis is a promising technology for the production of renewable fuels and chemicals from lignocellulosic biomass. The product distribution (bio-oil, char) and the composition of bio-oil are significantly influenced by the presence of naturally occurring alkali and alkaline-earth metals (AAEMs). In this paper, we investigate, at the molecular level, the influence of Na(I), K(I), Ca(II), and Mg(II) ions on glycosidic bond breaking reactions using density functional theory. Glycosidic bond breaking reactions are categorized as direct C-O breaking mechanisms, namely, transglycosylation, glycosylation, and ring contraction and the two-step pathways, which include the mannose pathway, dehydration, and ring opening. Our calculations show that in the absence of metal, transglycosylation and dehydration pathways (activation barriers ∼55 kcal.mol-1) are kinetically most facile. The linkage type (α- or β-1,4) has an insignificant effect on kinetics of glycosidic bond cleavage. Mg(II) ions have a pronounced effect on lowering the activation barriers of glycosylation, ring contraction, and the mannose pathway, requiring activation enthalpies of 32-52 kcal.mol-1. Conversely, Mg(II) and Ca(II) ions inhibit the dehydration pathway. Na(I) and K(I) ions do not significantly influence the activation barriers of glycosidic bond cleavage reactions, as the reduction is only about 5-10 kcal.mol-1. Thus, AAEM ions exhibit different catalytic effects on glycosidic bond breaking reactions.

Biomass-derived degraded lignin and cellulose serve as possible alternatives to fossil fuels for energy and chemical resources. Fast pyrolysis of lignocellulosic biomass generates bio-oil that needs further refinement. However, as pyrolysis causes massive degradation to lignin and cellulose, this process produces very complex mixtures. The same applies to degradation methods other than fast pyrolysis. The ability to identify the degradation products of lignocellulosic biomass is of great importance to be able to optimize methodologies for the conversion of these mixtures to transportation fuels and valuable chemicals. Studies utilizing tandem mass spectrometry have provided invaluable, molecular-level information regarding the identities of compounds in degraded biomass. This review focuses on the molecular-level characterization of fast pyrolysis and other degradation products of lignin and cellulose via tandem mass spectrometry based on collision-activated dissociation (CAD). Many studies discussed here used model compounds to better understand both the ionization chemistry of the degradation products of lignin and cellulose and their ions' CAD reactions in mass spectrometers to develop methods for the structural characterization of the degradation products of lignocellulosic biomass. Further, model compound studies were also carried out to delineate the mechanisms of the fast pyrolysis reactions of lignocellulosic biomass. The above knowledge was used to assign likely structures to many degradation products of lignocellulosic biomass.

The catalytic pyrolysis mechanism of the initial lignin depolymerization products will help us develop biomass valorization strategies. How does isomerism influence reactivity, product formation, selectivities, and side reactions? By using imaging photoelectron photoion coincidence (iPEPICO) spectroscopy with synchrotron radiation, we reveal initial, short-lived reactive intermediates driving benzenediol catalytic pyrolysis over H-ZSM-5 catalyst. The detailed reaction mechanism unveils new pathways leading to the most important products and intermediates. Thanks to the two vicinal hydroxyl groups, catechol (o-benzenediol) is readily dehydrated to form fulvenone, a reactive ketene intermediate, and exhibits the highest reactivity. Fulvenone is hydrogenated on the catalyst surface to phenol or is decarbonylated to produce cyclopentadiene. Hydroquinone (p-benzenediol) mostly dehydrogenates to produce p-benzoquinone. Resorcinol, m-benzenediol, is the most stable isomer, because dehydration and dehydrogenation both involve biradicals owing to the meta position of the hydroxyl groups and are unfavorable. The three isomers may also interconvert in a minor reaction channel, which yields small amounts of cyclopentadiene and phenol via dehydroxylation and decarbonylation. We propose a generalized reaction mechanism for benzenediols in lignin catalytic pyrolysis and provide detailed mechanistic insights on how isomerism influences conversion and product formation. The mechanism accounts for processes ranging from decomposition reactions to molecular growth by initial polycyclic aromatic hydrocarbon (PAH) formation steps to yield, e.g., naphthalene. The latter involves a Diels–Alder dimerization of cyclopentadiene, isomerization, and dehydrogenation.

Transforming renewable lignin into high value‐added chemicals is a forward‐looking strategy to address the resource waste caused by insufficient utilization of biomass resources. On this basis, studying the efficient conversion of lignin to aldehydes/acids and their reaction mechanisms has become an attractive topic. A systematic investigation of the gas‐phase oxidation reaction mechanisms of the three model compounds initiated by O2 was carried out at the atomic and molecular levels by using density functional theory (DFT). Further revealing of oxidation behavior on two reaction sites of phenolic hydroxyl group and hydroxymethyl group were accomplished in detail. The potential energy surface information of 21 possible reaction channels of two pathways were obtained at B3LYP/6‐311+G(d,p) level. The influence of substituent effects on the reaction energy barrier was estimated. The calculation results showed that the reactivity of phenolic hydroxyl group is stronger than that of hydroxymethyl group, because the reaction Gibbs potential barriers are lower by about 4.9–8.7 kcal/mol. The reaction energy barriers on phenolic hydroxyl group site and hydroxymethyl group site decrease with the increase of the number of methoxy groups. Revealing the oxidation processes of lignin model compounds will provide a deeper understanding on the reaction mechanism and provide theoretical support for further experimental research on the conversion of lignin into high value‐added chemicals.

Hydrodeoxygenation (HDO) is a pivotal process in the efficient utilization of biomass, with ruthenium (Ru) emerging as a highly effective catalyst for this reaction. A dimer model compound, more representative of bio-oil oligomers than monomers, was used to explore the HDO mechanism over a Ru catalyst through both density functional theory (DFT) calculations and experimental studies. Initially, the adsorption of 2-Phenylethyl phenyl ether (PPE) was examined through DFT, leading to the determination of an optimized structure. Subsequent calculations of the HDO reaction pathways on the Ru (0001) surface revealed that the β -O-4 linkage cleavage occurred with significantly low activation energy. For the experimental study, a Ru/Nb 2 O 5 catalyst was synthesized using wet impregnation method. Characterization of this catalyst through scanning electron microscopy (SEM) and X-ray diffraction (XRD) confirmed its congruence with the DFT model. The catalytic performance of Ru/Nb 2 O 5 was evaluated in the PPE HDO process, where it demonstrated high efficiency. The applicability of the Ru/Nb 2 O 5 catalyst was extended to a real lignin bio-oil so as to further assess its effectiveness. This research provides a systematic study on PPE HDO over a Ru catalyst, illustrating the potential of using dimer model compounds in HDO mechanism investigations and the promising capabilities of Ru-based catalysts.

Lignin-carbohydrate complexes, in which lignin and polysaccharides are directly connected, have been identified and extensively analyzed. To date, however, the origin of these structures has not been unequivocally established. That notwithstanding, it has been found that delignification, whether by conventional pulping and bleaching processes or in the biorefinery context, is effected by the presence of lignin-carbohydrate complexes. Using density functional theory calculations, the current work has evaluated the thermodynamics of bond dissociation as a function of structure and chemical composition. Among the lignin-carbohydrate complexes that have been identified, the homolytic bond dissociation energy is highest for the α-benzyl ethers and γ-ester, with phenyl glycosides being markedly less endothermic. This is consistent with observations on the recalcitrance of these compounds. Heterolytic cleavage reactions of the α-benzyl ethers are less endothermic, due to water solvation of the ions. The latter observation may provide support for the proposed homolytic cleavage reaction, since if heterolysis were operative, the α-benzyl ethers would not exhibit the level of recalcitrance that is observed experimentally.

In view of elucidating the fragmentation patterns of aromatic systems induced by low-energy electron interactions, dissociative electron attachment (DEA) to gas-phase anisole was performed. Anionic fragments resulting from this DEA process were detected by a quadrupole mass spectrometer, and ion yields of those fragments as a function of incident electron energy were rendered. Our study showed the formation of CH3−, HCC−, and OCH3− fragments, suggesting that various dissociation channels proceed out of DEA to anisole. We employed density functional theory to compute thermodynamic threshold energies for each potential dissociation channel. Those theoretical calculations supported the prediction that the CH3−and OCH3−fragments form via mechanisms of single-bond cleavage; the HCC−fragments may form through two-, three-, or four-body dissociation channels that entail hydrogen transfers and the cleavage of multiple aromatic bonds. The experimental resonance energies that form the CH3−, HCC−, and OCH3−fragments were 6.0 eV, 5.8 and 9.7 eV, and 9.8 eV, respectively. Given the classification of anisole as a monosubstituted aromatic species, our results explain generalizable patterns of electron-mediated dissociation in aromatic systems.

Chemoselective C–C bond cleavage remains a challenge for the degradation of polymers because of the relatively high bond dissociation energy of C–C σ-bonds at room temperature. Organic molecular-based dye-sensitized photoelectrochemical cells (DSPECs) could offer a means of using renewable solar energy to drive energetically demanding chemoselective C–C bond cleavage reaction. This study reports the solar light-driven activation of a bicyclic aminoxyl mediator to achieve C–C bond cleavage in the aryl-ether linkage of a lignin model compound (LMC) at room temperature using a donor–π-conjugated bridge–acceptor (D–π–A) organic dye-based DSPEC system. The 5-[4-(diphenylamino)phenyl]thiophene-2-cyanoacrylic acid (DPTC) D–π–A organic dye was investigated along with a bicyclic aminoxyl radical mediator (9-azabicyclo[3,3,1]nonan-3-one-9-oxyl, KABNO) in solution and at the interface of a mesoporous structured TiO2 substrate in the presence of LMC. Photophysical studies of DPTC with KABNO were carried out to show intermolecular energy/electron transfer under 1 sun illumination (100 mW·cm− 2). The D–π–A type DPTC sensitized mesoporous TiO2 photoanode in the presence of KABNO can facilitate the generation of the reactive oxoammonium species KABNO+ as a strong oxidizing agent, which plays an important role in the photocatalytic oxidative C–C bond cleavage of LMC. The photoelectrochemical oxidative reaction in a complete DSPEC with KABNO afforded the chemoselective C–C bond cleavage products 2-(2-methoxyphenoxy)acrylaldehyde (94%) and 2,6-dimethoxy-1,4-benzoquinone (66%). This process provides a first report utilizing a D–π–A type organic dye in combination with a bicyclic nitroxyl radical mediator for heterogeneous photoelectrolytic oxidative cleavage of C–C σ-bonds, modeled on those found in lignin, at room temperature.

Fragmentation methods such as MIM (Molecules-in-Molecules) provide a route to accurately model large systems and have been successful in predicting their structures, energies, and spectroscopic properties. However, their use is often limited to systems at equilibrium due to the inherent complications in the choice of fragments in systems away from equilibrium. Furthermore, the presence of charges resulting from any heterolytic bond breaking may increase the fragmentation error. We have previously suggested EE-MIM (Electrostatically Embedded Molecules-In-Molecules) as a method to mitigate the errors resulting from the missing long-range interactions in molecular clusters in equilibrium. Here, we show that the same method can be applied to improve the performance of MIM to solve the longstanding problem of dependency of the fragmentation energy error on the choice of the fragmentation scheme. We chose four widely used acid dissociation reactions (HCl, HClO4, HNO3, and H2SO4) as test cases due to their importance in chemical processes and complex reaction potential energy surfaces. Electrostatic embedding improves the performance at both one and two-layer MIM as shown by lower EE-MIM1 and EE-MIM2 errors. The EE-MIM errors are also demonstrated to be less dependent on the choice of the fragmentation scheme by analyzing the variation in fragmentation energy at the points with more than one possible fragmentation scheme (points where the fragmentation scheme changes). EE-MIM2 with M06-2X as the low-level resulted in a variation of less than 1 kcal/mol for all the cases and 1 kJ/mol for all but three cases, rendering our method fragmentation scheme-independent for acid dissociation processes.

The efficient photocatalytic breakage of Cα-Cβ bonds has great significance for the valorization of lignin into value-added aromatic chemicals, but remains challenging owing to their demanding depolymerization conditions and high bond dissociation energies. In this study, the Z-scheme heterojunction H5PMo10V2O40/g-C3N4 (HPA/CN) photocatalyst was elaborately developed for the selective and efficient cleaving of Cα-Cβ bonds in real lignin and its β-O-4 models under mild conditions. The construction of Z-scheme heterojunction with irregular sheet micromorphology not only enhanced the charge separation and redox abilities, but also broadened the light absorption range and promoted charge-to-surface transfer in two redox components. Notably, 35 % HPA/CN could completely convert the 2-phenoxy-1-phenylethanol with Cα-Cβ bond cleavage selectivity of 97.4 %, achieving approximately 50.0- and 2.2-times higher conversion rates compared to HPA and CN, respectively. Meanwhile, this strategy also offered a wide substrate scope containing various β-O-4 model compounds and native lignin, leading to the generation of corresponding aromatics. The mechanism experiments revealed that photoinduced holes and superoxide radicals synergistically triggered the oxidative cleavage of Cα-Cβ bond. This study could provide a reference for photocatalytic production of value-added aromatic monomers by exploiting both renewable feedstocks and solar energy.

The catalytic depolymerization of lignin has long been challenged by the limited catalytic mass transfer and the complexity of its polymer structure. In this work, a series of hierarchical MFI nanosheet catalysts (named AL-MFI, M-BKC-MFI, M(metal)-AL-MFI and M(metal)-BKC-MFI, respectively) using biomass (lignin and lignin biochar) as a template were designed to realize the oxidative depolymerization of lignin and its derivatives efficiently and stably. The Zn element in the M-AL-MFI and M-BKC-MFI compensated for the acidity, while Ce could stimulate the production of O2- through redox and trigger a free radical pyrolysis reaction. The conversion rate of lignin was as high as 80.7 % and 82.5 %, respectively, with acetophenone as the main product in yields as high as 42.82 % and 47.13 %, respectively. DFT calculations revealed that the bulk sizes of alkali lignin and its derivatives (≤6.112 nm) were smaller than the average pore size of the catalysts (≥7.27 nm). And this finding provided direct evidence for the critical role of the mesoporous structure of the catalysts in lignin depolymerization. Specifically, the mesoporous structure at suitable acidity contributes to the mass transfer of lignin to the active sites of the catalyst, resulting in an efficient depolymerization process. What's more, the degradation pathways and mechanisms of lignin were analyzed with the help of DFT and GC-MS using 2-phenylethyl phenyl ether (PPE) and 2'-Phenoxyacetophenone (PTE) as model compounds. β-O-4 bonds were broken at a rate of more than 80 %, which is the primary mechanism of lignin cleavage. And the order of bond breaking was Cβ-O bond (253 J/mol) > Cα-Cβ bond (285.1 J/mol) > Caromatic-O bond (407.9 J/mol). M-MFIs promoted the cleavage of Cα-Cβ and Cα-O to a certain extent, which indicated that MFI nanosheets contributed to the cleavage of bonds with higher dissociation energies. This work not only helps to reveal the detailed process of lignin depolymerization, but also provides valuable theoretical guidance for further optimizing the catalyst design and improving the depolymerization efficiency.

Semiquinone (SQ) radicals play a critical role in the long‐lasting UV‐blocking application of lignin, while their origin and stable structure are unclear. Here, the organosolv lignin extracted from poplar (OL‐P) is self‐assembled into normal micelles (LNM) with more phenolic hydroxyl groups on the surface, and reverse micelles (LRM) with more methoxyl groups on the surface. After 12 h UV irradiation, the SQ radical contents in LNM and LRM increase 33% and 78% respectively. The performance of LNM based sunscreen keeps upswinging due to radical stabilization of phenolic hydroxyl groups. LRM based sunscreen experiences a gradual decrease after reaching maximum UV absorbance due to the quick generation and over oxidation of SQ radicals. Density functional theory (DFT) simulations reveal that methoxyl groups in OL‐P has bigger bond length and smaller bond dissociation enthalpy than phenolic hydroxyl groups, and are easy to form SQ radicals. The Gibbs free energy (ΔG) needed for SQ‐quinone transformation is above 26.10 kcal mol.−1, while that for SQ‐hydroquinone transformation is below −66.78 kcal mol.−1. Hydroquinone is the stable structure of SQ radicals. This work discloses the origin and stable structure of SQ radicals in lignin under UV irradiation, and provides an important guidance for its long‐lasting UV‐blocking application.

The interaction of fragments derived from lignin depolymerization with a heterogeneous palladium catalyst in methanol-water solution is studied by means of experimental and theoretical methodologies. Quantum chemistry calculations and molecular dynamics simulations based on the ReaxFF approach are combined effectively to obtain an atomic level characterization of the crucial steps of the adsorption of the molecules on the catalyst, their fragmentation, reactions, and desorption. The main products are identified, and the most important routes to obtain them are explained through extensive computational procedures. The simulation results are in excellent agreement with the experiments and suggest that the mechanisms comprise a fast chemisorption of identified fragments from lignin on the metal interface accompanied by bond breaking, release of some of their hydrogens and oxygens to the support, and eventual desorption depending on the local environment. The strongest connections are those involving the aromatic rings, as confirmed by the binding energies of selected representative structures, estimated at the quantum chemistry level. The satisfactory agreement with the literature, quantum chemistry data, and experiments confirms the reliability of the multilevel computational procedure to study complex reaction mixtures and its potential application in the design of high-performance catalytic devices.

Lignin conversion into high value-added chemicals is of great significance for maximizing the use of renewable energy. Ionic liquids (ILs) have been widely used for targeted cleavage of the C-O bonds of lignin due to their high catalytic activity. Studying the cleavage activity of each IL is impossible and time-consuming, given the huge number of cations and anions. Currently, the mainstream approach to determining the cleavage activity of one IL is to calculate the activation barrier energy (Ea) theoretically via transition state search, a process that involves the iterative determination of an appropriate "imaginary frequency". Machine learning (ML) has been widely used for catalyst design and screening, enabling accurate mapping from specified descriptors to target properties. To avoid complicated Ea calculations and to screen potential candidates, in this study, we selected nearly 103 ILs and guaiacylglycerol-β-guaiacyl ether (GG) as the lignin model and used the ML technology to train models that can rapidly predict the cleavage activity of ILs. Taking the easily accessible bond dissociation energy (BDE) of the β-O-4 bond in GG as the target, an ML model with r > 0.93 for predicting the catalytic activity of ILs was obtained. The change tendency of the BDE is consistent with the experimental yield of guaiacol, reflecting the reliability of the ML model. Finally, [C2MIM][Tyrosine] and [C3MIM][Tyrosine] as the optimal candidates for future applications were screened out. This is a novel strategy for predicting the catalytic activity of ILs on lignin without the need to calculate complicated reaction pathways while reducing time consumption. It is anticipated that the ML model can be utilized in future practical applications for targeted cleavage of lignin.

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.

Lignocellulosic biomass is an abundant renewable resource that can be upgraded to chemical and fuel products through a range of thermal conversion processes. Fast pyrolysis is a promising technology that uses high temperatures and fast heating rates to convert lignocellulose into bio-oils in high yields in the absence of oxygen. Hemicellulose is one of the three major components of lignocellulosic biomass and is a highly branched heteropolymer structure made of pentose, hexose sugars, and sugar acids. In this study, β-d-xylopyranose is proposed as a model structural motif for the essential chemical structure of hemicellulose. The gas-phase pyrolytic reactivity of β-d-xylopyranose is thoroughly investigated using computational strategies rooted in quantum chemistry. In particular, its thermal degradation potential energy surfaces are computed employing Minnesota global hybrid functional M06-2X in conjunction with the 6-311++G(d,p) Pople basis set. Electronic energies are further refined by performing DLPNO-CCSD(T)-F12 single-point calculations on top of M06-2X geometries using the cc-pVTZ-F12 basis set. Conformational analysis for minima and transition states is performed with state-of-the-art semiempirical quantum chemical methods coupled with metadynamics simulations. Key thermodynamic quantities (free energies, barrier heights, enthalpies of formation, and heat capacities) are computed. Rate coefficients for the initial steps of thermal decomposition are computed by means of reaction rate theory. For the first time, a detailed elementary reaction kinetic model for β-d-xylopyranose is developed by utilizing the thermodynamic and kinetic information acquired from the aforementioned calculations. This model specifically targets the initial stages of β-d-xylopyranose pyrolysis in the high-pressure limit, aiming to gain a deeper understanding of its reaction kinetics. This approach establishes a systematic strategy for exploring reactive pathways, evaluating competing parallel reactions, and selectively accepting or discarding pathways based on the analysis. The findings suggest that acyclic d-xylose plays a significant role as an intermediary in the production of key pyrolytic compounds during the pyrolysis of xylose. These compounds include furfural, anhydro-d-xylopyranose, glycolaldehyde, and dihydrofuran-3(2H)-one.

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Due to the rapidly growing interest in the use of biomass derived furanic compounds as potential platform chemicals and fossil fuel replacements, there is a simultaneous need to understand the pyrolysis and combustion properties of such molecules. To this end, the potential energy surfaces for the pyrolysis relevant reactions of the biofuel candidate 2-methylfuran have been characterized using quantum chemical methods (CBS-QB3, CBS-APNO and G3). Canonical transition state theory is employed to determine the high-pressure limiting kinetics, k(T), of elementary reactions. Rice-Ramsperger-Kassel-Marcus theory with an energy grained master equation is used to compute pressure-dependent rate constants, k(T,p), and product branching fractions for the multiple-well, multiple-channel reaction pathways which typify the pyrolysis reactions of the title species. The unimolecular decomposition of 2-methylfuran is shown to proceed via hydrogen atom transfer reactions through singlet carbene intermediates which readily undergo ring opening to form collisionally stabilised acyclic C5H6O isomers before further decomposition to C1-C4 species. Rate constants for abstraction by the hydrogen atom and methyl radical are reported, with abstraction from the alkyl side chain calculated to dominate. The fate of the primary abstraction product, 2-furanylmethyl radical, is shown to be thermal decomposition to the n-butadienyl radical and carbon monoxide through a series of ring opening and hydrogen atom transfer reactions. The dominant bimolecular products of hydrogen atom addition reactions are found to be furan and methyl radical, 1-butene-1-yl radical and carbon monoxide and vinyl ketene and methyl radical. A kinetic mechanism is assembled with computer simulations in good agreement with shock tube speciation profiles taken from the literature. The kinetic mechanism developed herein can be used in future chemical kinetic modelling studies on the pyrolysis and oxidation of 2-methylfuran, or the larger molecular structures for which it is a known pyrolysis/combustion intermediate (e.g. cellulose, coals, 2,5-dimethylfuran).

Quantum chemical methods have been intensively applied to study the pyrolytic conversion of glucose into hydroxymethylfurfural (HMF) and furfural (FF). Herein, we collect the most relevant mechanistic proposals from the recent literature and organize them into a single reaction network. All the transition structures (TSs) and intermediates are characterized using highly accurate ab initio methods and the possible reaction pathways are assessed in terms of the Gibbs energies of the TSs and intermediates with respect to β‐glucopyranose, selecting a 2D ideal‐gas standard state at 773 K to represent the pyrolysis conditions. Several pathways can lead to the formation of both HMF and FF passing through rate‐determining TSs that have ΔG‡ values of ~49–50 kcal/mol. Both water‐assisted mechanisms and nonspecific environmental effects have a minor impact on the Gibbs energy profiles. We find that the HMF → FF + CH2O fragmentation has a small ΔrxnG value and an accessible ΔG‡ barrier. Our computational results, which are in consonance with the kinetic parameters derived from lumped models, the results of isotopic labeling experiments and the reported HMF/FF molecular ratios, could be useful for modeling studies including on nonequilibrium kinetic effects that may render more information about product yields and the relevance of the various pathways.

Monolignols are precursor units and primary products of lignin pyrolysis. The currently available global (lumped) and semi-detailed kinetic models, however, are lacking the comprehensive decomposition kinetics of these key intermediates in order to advance toward the fundamentally based detailed chemical-kinetic models of biomass pyrolysis. para-Coumaryl alcohol (HOPh-CH=CH-CH2OH, p-CMA) is the simplest of the three basic monolignols containing a typical side-chain double bond and both alkyl and phenolic type OH-groups. The two other monomers additionally contain one and two methoxy groups, respectively, attached to the benzene ring. Previously, we developed a detailed fundamentally based mechanism for unimolecular decomposition of p-CMA (as well as its truncated allyl and cinnamyl alcohol models) and explored its reactivity toward H-radicals generated during pyrolysis. The reactions of p-CMA with pyrolytic OH-radicals is another set of key reactions particularly important for understanding the formation mechanisms of a wide variety of oxygenates in oxygen-deficit (anaerobic) conditions and the role of the lignin side groups in pyrolysis pathways. In Part I of the current study (Asatryan et al., J. Phys. Chem. A, 2019, 123, 2570-2585), we reported a detailed potential energy (enthalpy) surface analysis of the reaction OH + p-CMA with suggestions for a variety of chemically activated, unimolecular, and bimolecular reaction pathways. In this Part II of our study, we provide a detailed kinetic analysis of the major reaction channels to evaluate their significance and possible impacts on product distributions. Temperature- and pressure-dependent rate constants are calculated using the QRRK method and the master equation analysis for falloff and stabilization. Enthalpies of formation, entropies, and heat capacities are calculated using DFT and higher-level composite methods for stable molecules, radicals, and transition-state species. A significant difference between well depths for the chemically activated adduct radicals, [p-CMA-OH]*, is found for the α- and β-carbon addition reactions to generate the 1,3- and 1,2-diol radicals, respectively. This is due to the synergistic effect from conjugation of the proximal radical center with the aromatic ring and the strong H-bonding interaction between vicinal OH-groups in the β-adduct (1,2-diol radical). Both adducts undergo isomerization and low-energy transformations, however, with different kinetic efficiency due to the difference in stabilization energies. Reaction pathways include dissociation, intramolecular abstraction, atom and group transfers and elimination. Of particular interest is a roaming-like low-energy dehydration reaction to form O-centered intermediate radicals. The kinetic analysis demonstrated the feasible formation of various products detected in pyrolysis experiments, suggesting that the gas-phase reactions of OH-radicals can be a key process to form major products and complex oxygenates during lignin pyrolysis. Our preliminary experiments involving pyrolysis of the vaporized monomers support this basic statement. A novel mechanism for formation of benzofuran, identified in experimentation, is also provided based on the potential conversions of hydroxyphenylactaldehyde and corresponding isomers, which are kinetically favored products.

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Unveiling reaction mechanisms by isomer-selective detection of reactive intermediates requires advanced spectroscopic knowledge. We study the photoionization of fulvenone (c-C 5 H 4 =C=O), a reactive ketene species relevant in catalytic pyrolysis of lignin, which was generated by pyrolysis of 2-methoxy acetophenone. The highresolution threshold photoelectron spectrum (TPES) with vacuum ultraviolet synchrotron radiation revealed well-resolved vibrational transitions, assigned to ring deformation modes of the cyclopentadienyl moiety. The adiabatic ionization energy was determined to be 8.25 ± 0.01 eV and is assigned to the X +   2 A 2 ← X +   1 A 1 transition. A broad and featureless band arising at 9 eV is associated with the A + 2 B 1 ← X +   1 A 1 excitation. A conical intersection is responsible for the ultrafast relaxation of the fulvenone cation from the A + into the X +  state resulting in a featureless and lifetime broadened band. These insights will increase the detection capabilities for fulvenone and thereby help to elucidate reaction mechanisms in lignin catalytic pyrolysis.

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