Force Field Molecular Dynamics Research Articles

Low-temperature degradation of waste epoxy resin polymer improved by swelling-assisted pyrolysis

The disposal of waste epoxy resin matrix composite becomes an urgent issue, and pyrolysis is an effective method to decompose the epoxy resin polymer, but the high operating temperature hinders its application. In this study, a swelling-assisted pyrolysis method was proposed for the low-temperature decomposition of epoxy resin. It was found that the degradation ratio of epoxy resin increased from 59.13 % to 79.10 % at 300 °C after the swelling treatment with acetic acid. Compared with traditional pyrolysis, the cracking of epoxy resin was promoted in the swelling-assisted pyrolysis, increasing the yields of pyrolysis gas and oil, as well as enriching the light components in pyrolysis oil and raising the graphite degree of pyrolysis char. Following the characterizations and reactive force field molecular dynamics (ReaxFF-MD) simulation, the key mechanisms of swelling-assisted pyrolysis were determined as the interaction of the enhanced heat transfer and the accelerated ring-open reactions. After the swelling treatment, epoxy resin became porous, improving the heat transfer in the pyrolysis process. Meanwhile, abundant O radicals were generated with the decomposition of acetic acid, accelerating the ring-open reactions of benzene rings. The results could provide promising guidance for the pyrolysis recovery of epoxy resin matrix composites towards the circular economy.

Mechanism of initial activation of carbon–oxygen bonds for deoxidation of acetic acid

Producing valuable fuel-ranged hydrocarbons from cellulosic biomass is an effective approach to addressing potential energy issues. However, the cellulose-derived bio-oil may contain carboxylic acid components that contribute to the unfavorable acidic and corrosive nature, which necessitates upgrading through the hydrodeoxygenation (HDO) treatment using heterogeneous catalysis. Here, we performed reactive force-field molecular dynamics (ReaxFF-MD) to study the hydrogenolysis, dehydrogenation and decarboxylation mechanisms of the acetic acid model compound on the Ni-based catalyst. Comparative analysis of the activation of C–O and C=O bonds was presented, followed by an examination of the associative desorption of H2 and the dynamic evolution of main intermediates. Furthermore, the conversion pathways of key intermediates were determined and four main pathways for ethanol generation and the temperature-response mechanism for coking formation were proposed. This mechanism diagram explains the strong effects of a hydrogen environment in biomass-related reactions, highlighting the dynamic mechanism of selective hydrogenation and deoxygenation of carbon–oxygen bonds.

Impact of Single-Walled Carbon Nanotube Functionalization on Ion and Water Molecule Transport at the Nanoscale.

Nanofluidics has a very promising future owing to its numerous applications in many domains. It remains, however, very difficult to understand the basic physico-chemical principles that control the behavior of solvents confined in nanometric channels. Here, water and ion transport in carbon nanotubes is investigated using classical force field molecular dynamics simulations. By combining one single walled carbon nanotube (uniformly charged or not) with two perforated graphene sheets, we mimic single nanopore devices similar to experimental ones. The graphitic edges delimit two reservoirs of water and ions in the simulation cell from which a voltage is imposed through the application of an external electric field. By analyzing the evolution of the electrolyte conductivity, the role of the carbon nanotube geometric parameters (radius and chirality) and of the functionalization of the carbon nanotube entrances with OH or COO- groups is investigated for different concentrations of group functions.

On the Na+ transport and electrochemical stability window in NaTFSI:NMA deep eutectic solvent

Deep eutectic solvents (DES), such as the ▪:▪ (sodium bistrifluoromethane sulfonyl imide - N-methyl acetamide) system, have been considered as promising electrolytes for sodium-ion (▪) batteries. However, our current atomistic understanding of those systems is still in a state of evolution and remains incomplete. In this context, we combined classical force field molecular dynamics (MD) simulations and periodic density functional theory (DFT) calculations to enhance our atomistic understanding on the ▪ transport mechanisms, DES structure organization, hydrogen-bond network formation, and electrochemical stability windows (ESWs). Firstly, we obtained a substantial improvement in the description of the DES transport and structural properties using self-consistent MD/DFT calculations to assign effective charges on all atoms, while the remaining OPLS parameters were kept constant. From our analyses, an increasing in the ▪ mole fraction leads to different chemical environments, reflecting an electrolyte with larger aggregates, low conductivity, and high viscosity. The liquid structure seems to remain unchanged as a function of temperature. However, higher temperatures reduce the lifetime of ionic pairs and increase the mobility of ▪ ions. The disruption of the hydrogen-bonds formed between the ▪ species increases with the ▪ concentration. In addition, our periodic DFT calculations reveals the substitution of atomic sites from ▪ to ▪ during the reduction process. Furthermore, the ESW results provided evidence supporting the safeguarding of the solvent from degradation at higher salt concentrations.

Computational insight of lithium adsorption and intercalation in bilayer TiC3

Lithium-ion batteries (LIBs) have gained significant attention owing to their long lifespan. However, these batteries offer unmatched energy storage capacity and suffer from restricted lithium-ion mobility within the electrodes. Here, we employ first-principles calculation to investigate the two-dimensional TiC3 bilayer material. The results exhibit a remarkably high specific capacity of 1277 mAh/g and a low diffusion energy barrier of 0.12 eV. The TiC3 bilayer is anticipated to show high electrical conductivity, maintaining its metallicity due to strong bonding with four Li atoms. Additionally, its high thermal and dynamic stabilities are expected to significantly enhance the battery performance. Notably, the AB stacking bilayer TiC3 experiences a mere 6.01 % increase in volume, considerably smaller compared to the 28 % increase observed in the SiC bilayer. This suggests that TiC3 bilayers remain intact even at the highest concentration of lithium adsorptions. We also explored the solid-electrolyte interface (SEI) formation at the outset of battery operation using reactive force field molecular dynamics simulation. The reactive products of SEI are nicely matched with previous experimental and theoretical findings. All these intriguing properties position the TiC3 bilayer as an exceptionally promising material for use in LIBs.

Hydrogen diffusion in zirconium hydrides from on-the-fly machine learning molecular dynamics

Reactor pressure vessels and fuel cladding tubes have repeatedly failed due to zirconium hydrides. Zirconium hydride precipitation and growth are directly affected by hydrogen atom transport properties, which would make nuclear fuel storage less safe over long periods of time. Herein, we employ first-principles calculations to investigate the hydrogen diffusion mechanism in zirconium hydrides, utilizing on-the-fly machine learning force field molecular dynamics. It is verified that the machine learning force field can accurately describe the hydrogen atomic diffusion properties in zirconium hydrides at several temperatures and compositions. The atomic migration paths of hydrogen in zirconium hydrides as well as their barriers and pre-factors are also calculated. According to our results, the temperature and composition of zirconium hydrides affect the microscopic dynamical behavior of hydrogen.

Engineering MOF/carbon nitride heterojunctions for effective dual photocatalytic CO2 conversion and oxygen evolution reactions.

Photocatalysis appears as one of the most promising avenues to shift towards sustainable sources of energy, owing to its ability to transform solar light into chemical energy, e.g. production of chemical fuels via oxygen evolution (OER) and CO2 reduction (CO2RR) reactions. Ti metal-organic frameworks (MOFs) and graphitic carbon nitride derivatives, i.e. poly-heptazine imides (PHI) are appealing CO2RR and OER photo-catalysts respectively. Engineering of an innovative Z-scheme heterojunction by assembling a Ti-MOF and PHI offers an unparalleled opportunity to mimick an artificial photosynthesis device for dual CO2RR/OER catalysis. Along this path, understanding of the photophysical processes controlling the MOF/PHI interfacial charge recombination is vital to fine tune the electronic and chemical features of the two components and devise the optimum heterojunction. To address this challenge, we developed a modelling approach integrating force field Molecular Dynamics (MD), Time-Dependent Density Functional Theory (TD-DFT) and Non-Equilibrium Green Function DFT (NEGF-DFT) tools with the aim of systematically exploring the structuring, the opto-electronic and transport properties of MOF/PHI heterojunctions. We revealed that the nature of the MOF/PHI interactions, the interfacial charge transfer directionality and the absorption energy windows of the resulting heterojunctions can be fine tuned by incorporating Cu species in the MOF and/or doping PHI with mono- or divalent cations. Interestingly, we demonstrated that the interfacial charge transfer can be further boosted by engineering MOF/PHI device junctions and application of negative bias. Overall, our generalizable computational methodology unravelled that the performance of CO2RR/OER photoreactors can be optimized by chemical and electronic tuning of the components but also by device design based on reliable structure-property rules, paving the way towards practical exploitation.

Exploring mechanochemistry of pharmaceutical cocrystals: effect of incident angle on molecular mixing during simulated indentations of two organic solids.

The solid-state reaction of the active pharmaceutical ingredient theophylline with citric acid is a well-established example of a mechanochemical reaction, leading to a model pharmaceutical cocrystal. Here, classical force field molecular dynamics was employed to investigate the molecular mixing and structural distortion that take place on the mechanically driven indentation of a citric acid nanoparticle on a slab of crystalline theophylline. Through non-equilibrium molecular dynamics simulations, a 6 nm diameter nanoparticle of citric acid was introduced onto an open (001) surface of a theophylline crystal, varying both the angle of incidence of the nanoparticle between 15° and 90° and the indentation speed between 1 m s-1 and 16 m s-1. This theoretical study enabled the evaluation of how these two parameters promote molecular mixing and overall structural deformation upon the mechanical contraction of theophylline and citric acid, both of which are important parameters underlying mechanochemical cocrystallisation. The results show that the angle of incidence plays a key role in the molecular transfer ability between the two species and in the structural disruption of the initially spherical nanoparticles. Changing the indentation speed, however, did not lead to a discernible trend in molecular mixing, highlighting the importance of the incident angle in mechanochemical events in the context of supramolecular chemistry, such as the disruption of the crystal structure and molecular transfer between molecular crystals.

Differentiable simulation to develop molecular dynamics force fields for disordered proteins.

Implicit solvent force fields are computationally efficient but can be unsuitable for running molecular dynamics on disordered proteins. Here I improve the a99SB-disp force field and the GBNeck2 implicit solvent model to better describe disordered proteins. Differentiable molecular simulations with 5 ns trajectories are used to jointly optimise 108 parameters to better match explicit solvent trajectories. Simulations with the improved force field better reproduce the radius of gyration and secondary structure content seen in experiments, whilst showing slightly degraded performance on folded proteins and protein complexes. The force field, called GB99dms, reproduces the results of a small molecule binding study and improves agreement with experiment for the aggregation of amyloid peptides. GB99dms, which can be used in OpenMM, is available at https://github.com/greener-group/GB99dms. This work is the first to show that gradients can be obtained directly from nanosecond-length differentiable simulations of biomolecules and highlights the effectiveness of this approach to training whole force fields to match desired properties.

Prediction of toluene/water partition coefficients of SAMPL9 compounds: comparison of the molecular dynamics force fields GAFF/RESP and GAFF/IPolQ-Mod + LJ-fit.

The SAMPL9 blind challenge aims to predict the toluene/water partition coefficient of 16 active pharmaceutical ingredients. In this work, the transfer free energy between the solvation in water and toluene is predicted by molecular dynamics simulations using the MBAR method [M. R. Shirts and J. D. Chodera, J. Chem. Phys., 2008, 129, 123105] with replica exchange molecular dynamics [Y. Sugita, A. Kitao and Y. Okamoto, J. Chem. Phys., 2000, 113, 6042-6051]. Thereby, simulation results using the force field GAFF/IPolQ-Mod + LJ-fit [A. Mecklenfeld and G. Raabe, ADMET and DMPK, 2020, 8, 274-296] are compared to simulations with the standard GAFF/RESP model [J. Wang, R. M. Wolf, J. W. Caldwell, P. A. Kollman and D. A. Case, J. Comput. Chem., 2004, 25, 1157-1174]. By statistical evaluation of RMSD and R2, we compare the results with other participants of the blind challenge. Furthermore, we provide a detailed analysis of solvation structures using the combined distribution function for simulations in water and the plane projection analysis for simulations in toluene, and we work out differences and similarities of the two force fields. These studies allow to gain important insights to increase the understanding of the mechanism of interactions between the drugs and the solvent.

PES and transport properties of the He⋯HBr complex from kinetic theory and molecular dynamics simulations.

The ab initio intermolecular potential of the He⋯HBr van der Waals (vdW) complex was calculated at the CCSD(T)/a5zBF level of theory and expanded in terms of the orthogonal Legendre polynomials. The PES then was implemented to calculate the interaction viscosity (η12) and diffusion (D12) coefficients through classical Mason-Monchick approximation (MMA), quantum mechanical close-coupling (CC), and molecular dynamics (MD) simulations. Energy-dependent Senftleben-Beenakker (SB) cross-sections were calculated using rotationally averaged cross-sections, and Boltzmann averaging was used to reveal the temperature dependence of the SB cross-sections over the temperature range of T = 50-1000 K. The calculated transport properties from MMA are in close agreement with the CC results, especially for temperatures lower than T = 900 K. The ab initio potential data then were used to derive LJ (12,6) and Vashishta MD force fields, and the equilibrium MD simulation methods were implemented to extract η12 and D12 coefficients using Einstein formulas. It was found that the Vashishta 3-body interaction potential model shows better accuracy than the LJ (12,6) model in MD simulations of D12. For η12, however, both MD potential models are successful, and an average absolute deviation of lower than 1% was obtained when compared to the quantum mechanical CC method.

Atomic surface achieved through a novel cross-scale model from macroscale to nanoscale.

Chemical mechanical polishing (CMP) is widely used to achieve an atomic surface globally, yet its cross-scale polishing mechanisms are elusive. Moreover, traditional CMP normally employs toxic and corrosive slurries, resulting in potential pollution to the environment. To overcome these challenges, a novel cross-scale model from the millimeter to nanometer scale is proposed, which was confirmed by a newly developed green CMP process. The developed CMP slurry consisted of hydrogen peroxide, sodium carbonate, sodium hydroxycellulose, and silica. Prior to CMP, fused silica was polished by a ceria slurry. After CMP, the surface roughness (Sa) was 0.126 nm, the material-removal rate was 88.3 nm min-1, and the thickness of the damaged layer was 8.8 nm. The proposed model was built by fibers, through integrating Eulerian and Lagrangian models and reactive force field-molecular dynamics. The results predicted by the model were in good agreement with those of CMP experimentally. A model for large-sized fibers revealed that a direct contact area of 11.12% was obtained for a non-woven polishing pad during the CMP experiments. Another model constructed via combining Eulerian and Lagrangian functions showed that the stress at the intersections of the fibers varied mainly from 0.1 to 0.01 MPa and was higher than the stress at other parts. An increase in viscosity led to a decrease in the areas with low stress, demonstrating that viscosity enhanced the stress and facilitated the removal of material. At the microscale and nanoscale, the stress of the abrasive surface exposed to the workpiece changed from 2.21 to 6.43 GPa. Stress at the interface contributed to the formation of bridging bonds, further promoting the removal of material. With increasing the compressive stress, the material-removal form was transformed from a single atom to molecular chains. The proposed model and developed green CMP offer new insights to understand the cross-scale polishing mechanism, as well as for designing and manufacturing novel polishing slurries, pads, and setups.

Understanding electrochemical interfaces through comparing experimental and computational charge density-potential curves.

Electrode-electrolyte interfaces play a decisive role in electrochemical charge accumulation and transfer processes. Theoretical modelling of these interfaces is critical to decipher the microscopic details of such phenomena. Different force field-based molecular dynamics protocols are compared here in a view to connect calculated and experimental charge density-potential relationships. Platinum-aqueous electrolyte interfaces are taken as a model. The potential of using experimental charge density-potential curves to transform cell voltage into electrode potential in force-field molecular dynamics simulations, and the need for that purpose of developing simulation protocols that can accurately calculate the double-layer capacitance, are discussed.

Mechanisms of microexplosion-accelerated pyrolysis and oxidation of lithium-containing droplets: an atomistic perspective.

Microexplosion has been extensively studied in the context of fuel spray and droplet evaporation in engines, while its existence, impact and atomistic insight have rarely been explored in the context of flame synthesis of nanoparticles. In this study, reactive force-field molecular dynamics simulations are performed to elucidate the mechanisms of pyrolysis and oxidation of an isolated lithium nitrate nanodroplet. During the pyrolysis process, the nucleation and growth of a bubble are observed inside the droplet, which should be ascribed to the release of nitrogen and oxygen gases from the decomposition of lithium nitrate, ultimately leading to rapid droplet fragmentation (microexplosion). To demonstrate the role of microexplosion with various intensities, via altering ambient temperature and addition of oxygen gas into the environment, thorough analyses of bond reactions, droplet morphology and compounds of the synthesized lithium nanoparticles are carried out. With elevated ambient temperature, the droplet substantially expands due to bubble growth and the time required for droplet disruption is shortened, which implies the enhanced strength of microexplosion. Simultaneously, the connection between the lithium and other atoms becomes weak, as evidenced by a decrease in the number of lithium bonds. These give rise to a reduction in the quantity of large-sized lithium agglomerates and simultaneously an increase in the amount of fine lithium nanoparticles. To further clarify the reaction mechanism for a lithium-containing droplet under various ambient conditions, three reaction modes, i.e., core-shell diffusion-controlled, microexplosion-accelerated and microexplosion-dominated, are distinguished based on the intensity of microexplosion and the quality of synthesized lithium nanoparticles. Fine lithium-containing nanoparticles are expected to be produced in the microexplosion-dominated mode under high temperature conditions.

Theoretical study of the structural and energetic properties of Ce1-xZrxO2 nanoparticles via molecular dynamics simulations.

The combination of ceria (CeO2) with different metal oxides (MO2), e.g. Ce1-xMxO2, has been strategically used to enhance its intrinsic properties. Moreover, the controlled synthesis of mixed oxide nanoparticles (NPs) opens the opportunity to explore the size dependence and chemical composition of the physical-chemical properties. However, our atomic-level understanding of how the physical-chemical and thermodynamic characteristics change with particle size and composition remains far from satisfactory. Here, we used force-field molecular dynamics simulations to investigate the effects of composition (x) and size on the physical-chemical properties of Ce1-xZrxO2 NPs with diameter from 1 (32 cations) up to 3 nm (256 cations), where x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0. We found abrupt changes in potential energy versus temperature for NPs with more than 108 cations, indicating a structural phase transition from disordered to ordered structures, which was confirmed by the radial distribution function. We found a linear relationship between the phase transition temperature (Tpt) and the size and composition of the NPs: the increase in the molar fraction of Zr4+ and the reduction in particle size are related to lower Tpt temperature and less defined decays of potential energy versus temperature. NPs larger than 56 cations show a radial distribution function with several peaks, which is related to the order of cations and anions in these structures. These peaks gradually disappear as the size decreases and the fraction of Zr4+ increases. Similar trends were observed with X-ray diffraction analysis; for example, fluorite-like motifs occur even with 56 cations in the case of ceria, but only for NPs with 108 cations for zirconia. Common neighbor analysis confirmed that NPs with well-defined values of the temperature Tpt have face-centered cubic (fcc)-like domains in the core region. The number of ordered fcc cations increases with increasing NP size and decreasing Zr4+ concentration. Finally, we observed that ceria nucleate first during simulated annealing and occupy highly coordinated sites within the core, while Zr4+ prefers the lowest coordinated sites on the surface.

New 5-alkyl-1,2,4-triazole-3-thione surfactants with antifungal and silver nanoparticles stabilization activity

New 5-alkyl-1,2,4-triazole-3-thiones containing structural fragments of anionic surfactants have been synthesized and examined as stabilizers in the wet chemistry synthesis of silver nanoparticles (AgNPs). The synthesis of the new 1,2,4-triazole-3-thiones was done via the cyclization reaction of 4-acyl-thiosemicarbazides in basic conditions. The structure of the new surfactant was analyzed by spectroscopic (infrared and nuclear magnetic resonance) and theoretical DFT methods. The surface activity characteristic, such as critical micelle concentration, was estimated using the conductivity method and the 1H NMR technique. The structural peculiarities of the new compounds testify to their surfactant properties, which is why they were explored as stabilizers in the chemical synthesis of silver nanoparticles from silver nitrate in an aqueous solution. The UV–VIS spectroscopy technique confirmed the formation of AgNPs in aqueous solutions of 5-alkyl-1,2,4-triazole-3-thiones. The stable AgNPs were obtained only using 1,2,4-triazole-3-thiones with longer alkyl chains (pentadecyl and heptadecyl), whereas the 5-pentyl-1,2,4-triazole-3-thione and the 5-heptyl-1,2,4-triazole-3-thione have not shown considerable stabilization activity. To better understand the interaction of the 1,2,4-triazole-3-thiones with AgNPs, Raman spectra analysis, quantum chemical modeling, and force field molecular dynamics were performed. The new triazoles were studied for their antifungal activity by the determination of minimal inhibition concentrations and docking studies.

Molecular analysis of hydrogen-bond structures in polymer electrolyte membrane in polymer electrolyte fuel cells below freezing temperatures

Fuel cells that generate electricity by using hydrogen and oxygen have been attracted to achieve carbon neutralization in the world because fuel cells don’t produce the greenhouse gases like carbon dioxide. The Polymer Electrolyte Fuel Cell (PEFC) has been used for many applications, such as cars and home stationary power supplies. Protons are transported in the polymer electrolyte membrane (PEM) and the transportation efficiency affects power generation efficiency. The internal state of the PEM has a great impact on the proton transport; however, the state is still not unclear below freezing temperatures because contained water might be frozen. Below freezing temperatures, proton-hopping, which is the mediation of proton conduction by water molecules, is important for proton transport. Proton-hopping is the mediation of proton conduction by water molecules. Therefore, we aim to understand the internal state of PEM below freezing temperatures. We performed reactive force-field molecular dynamics (ReaxFF MD) simulations to incorporate the proton-hopping. Our simulations consisted of two parts. First, bulk water was simulated at different temperatures to find the melting point of water by ReaxFF MD simulation. Radial distribution function (RDF) was used to find out whether the water is frozen or not. RDF of bulk water have a peak. When water is frozen, RDF of water have two distinct peaks and water molecules do not have translational or rotational motion. The size of the simulation box was 30.0 Å×30.0 Å×30.0 Å and the number of water molecules is 865. The temperature of the system was set at 1 K, 100 K, 200 K and 300 K. The time step was set at 0.50 fs and the sampling interval was 0.50 fs and the sampling interval was 100 steps. Second, the water condition in Nafion membrane was simulated. We used 4 Nafion polymer chains, 40 hydronium ions and 120 water molecules (water content: λ=4). The time step was set at 0.25 fs and the sampling interval was 100 steps. System of hydronium ions, water and Nafion changes faster than bulk water system and requires shorter timesteps. The temperature of the system was set at 300 K and 200 K. The annealing process was applied in order to equilibrate the system. After annealing, the production run was 250 ps using NVT ensemble. We performed the same analysis of the first simulation, and we analyzed the condition of water in the Nafion membrane. The RDF from the first simulation is shown in left figure. This figure shows that the two peaks were clearly visible at 1 K, 100 K and 200 K, although the second peak was barely visible at 300 K. Based on these results, the melting point of water in the ReaxFF potential was expected between 200 K and 300 K. The results of the second simulation are shown on the right figure. The results show that the heights of the two peaks are higher at 200 K than at 300 K. From these results, we could not find out whether the water in Nafion membrane was frozen or not only by these analysis methods. In this study, the melting point of water in the ReaxFF potential was found, and the state of water in polymer below freezing temperatures was analyzed. It was discussed in a paper by Qiangu Yan et al. According to them, water inside the Nafion membrane can be divided into three states. These are free water, bound water, and antifreeze water. While free water freezes, bound water has a lower melting point than normal water, and antifreeze water is not freeze. The specific heat of water in polymers should be analyzed in the future. Proton diffusion considering proton-hopping should also be analyzed in the future. Figure 1

Co-pyrolysis behavior and synergistic mechanism of sub-bituminous coal and polypropylene: A ReaxFF molecular dynamics simulation

The co-pyrolysis behavior of coal and polypropylene (PP) was investigated by reactive force field molecular dynamics (ReaxFF MD) simulation in the present study. In addition to analyzing the co-pyrolysis behavior at the atomic scale, the trajectories of different elements from different material sources were first observed. The weight loss of coal and PP is consistent with previous experimental studies, demonstrating the reliability of the ReaxFF MD simulation. The analysis of pyrolysis products showed that all blended systems contributed to tar and H2 yields at high temperatures compared to the calculated values. Among them, the coal/PP (6:4) system had the most significant effect on improving tar yield. H2 can be divided into three types according to different sources of H·, and with the increase of PP proportion, H· from PP is more willing to produce HPP2 at 3000 K. Then, the kinetics of the co-pyrolysis systems with multiple heating rates were studied, and only the activation energy of the coal/PP (4:6) system was reduced. Subsequently, we gave the mechanism by combining the pyrolysis process and the change in the number of free radicals. The blended systems promoted the production of more H· through PP, which made heavy compounds in coal further decomposition.

Microscopic mechanism for CO2-assisted co-gasification of polyethylene and softwood lignin: A reactive force field molecular dynamics study

Co-gasification of biomass and waste plastic to produce syngas and value-added products is an attractive technology for renewable energy utilization and waste disposal. CO2 as a gasifying agent has received much attention as it can act as carbon source and oxidant in the reaction. In this paper, the feasibility and characteristics of CO2-assisted co-gasification of polyethylene and softwood lignin were explored. Simulation results showed that the lignin macromolecule began to decompose through C–O–C bond breaking and the PE chain gradually decomposed through C–C bond breaking. CO2-assisted co-gasification showed a negative synergistic effect on gas yield at the early stage, which delayed the reaction, but showed a positive synergistic effect at the late stage. The reaction between CO2 and carbon-containing fragments greatly promoted CO production. The hydrogen and hydrocarbon radicals from PE were actively involved in H2 and hydrocarbon gas production. Compared with O2-assisted co-gasification, the CO2-assisted and CO2/O2-assisted co-gasification yielded more CO, hydrocarbon gases and combustible gases and increased the lower heating value of gas product. This work sheds light on the underlying mechanisms of CO2-assisted co-gasification of polyethylene and softwood, and would be helpful for the development of this technology.

Molecular level structure development of Indian coal using experimental, ML and DFT techniques

The nature of interaction between coal surface and reagent affects fine coal flotation yield. Hence, it is crucial to understand the nature of coal surface to improve flotation yield. Therefore, determination of coal surface at the molecular level is important to optimize the flotation performance. But coal is a complex material, and its molecular presentations depend highly on its geographical origin. In this work, various experimental techniques such as ultimate analysis, proximate analysis, TGA, FTIR and solid state 13C NMR were used to characterize the structure of low-rank Indian coal. The data extracted from these experiments were then used to develop molecular level presentation of coal using machine learning (ML) and DFT. Five stable molecular level structures were proposed for this Indian coal. DFT calculated FTIR spectra matches reasonably with the experimental FTIR data. Finally, a 3D model of the coal sample was developed, and reactive force field (ReaxFF) molecular dynamics simulations were performed for thermal decomposition analysis.