Multi-step Reaction Mechanism Research Articles

Understanding the uneven phase distribution and multi-step reaction mechanism of carbonated γ-C2S-based foam concrete

γ-C2S has been attracting much attention as the role of exclusive or primary binder to fabricate carbonated materials due to its high carbonation reactivity. However, the very limited hydration reactivity of γ-C2S makes it difficult in the production of casting-formed materials such as foam concrete, and this is exacerbated by the presence of bursting-prone foams in the mixture. Given the highly cementitious property of Portland cement (PC), 10 wt% of PC was added to γ-C2S-based foam concrete (CFC) as the supplemented binder to maintain its cellular structure while providing demoulding strength. The compressive strength of the CFC (600 kg/m3), carbonated for 2 h at ambient conditions, impressively peaks at 4.49 MPa, comparable to that of autoclaved aerated concrete with the same density grade, and is three times higher than the standard strength of foam concrete. This is partly attributed to the more uniform air-void size distribution formed by the enrichment of cement hydration products on the void-wall. Furthermore, the presence of cement hydration products positively promotes the dissolution of calcium ions from γ-C2S, forming a mixture of calcium and silicon products. This paper aims to understand the carbonation mechanisms of composite CFC, and also provide guidance for further realizing the reaction process associated with multiple carbonatable phases.

Pyrolysis characteristics and kinetic analysis of coating pitch derived from ethylene tar using model-free and model-fitting methods

This work conducted the pyrolysis of coating pitch derived from the ethylene tar through thermogravimetry analysis. The thermogravimetry-mass spectrometry and solid-state 13C nuclear magnetic resonance spectroscopy were performed to correlate the structural characteristics with the pyrolysis behavior. Meanwhile, the pyrolysis kinetics were estimated with model-free and model-fitting methods. The TG-DTG results showed that the pyrolysis process was generally divided into drying, fast pyrolysis, and polycondensation stages, accompanied by the decomposition of active functional groups like the aliphatic carbons bonded to oxygen (falO), the pyrolysis of relatively stable groups like the non-protonated aromatic carbon (faH), and the condensation of high bonding-energy aromatic structure like aromatic bridgehead carbon (faB). The activation energy obtained from model-free analysis ranged from 78.12 to 150.92kJ/mol with the increasing conversion, indicating the pyrolysis process as a multi-step reaction mechanism. Furthermore, the reaction model A1/3 (gα=[−ln(1−α)]3) was identified as the most suitable model for the pyrolysis of coating pitch in the conversion range of 0.25-0.8. The modeling values matched well with the experimental data, indicating the accuracy and feasibility of the results.

Computational design of serine hydrolases.

Enzymes that proceed through multistep reaction mechanisms often utilize complex, polar active sites positioned with sub-angstrom precision to mediate distinct chemical steps, which makes their de novo construction extremely challenging. We sought to overcome this challenge using the classic catalytic triad and oxyanion hole of serine hydrolases as a model system. We used RFdiffusion1 to generate proteins housing catalytic sites of increasing complexity and varying geometry, and a newly developed ensemble generation method called ChemNet to assess active site geometry and preorganization at each step of the reaction. Experimental characterization revealed novel serine hydrolases that catalyze ester hydrolysis with catalytic efficiencies (k cat /K m ) up to 3.8 x 103 M-1 s-1, closely match the design models (Cα RMSDs < 1 Å), and have folds distinct from natural serine hydrolases. In silico selection of designs based on active site preorganization across the reaction coordinate considerably increased success rates, enabling identification of new catalysts in screens of as few as 20 designs. Our de novo buildup approach provides insight into the geometric determinants of catalysis that complements what can be obtained from structural and mutational studies of native enzymes (in which catalytic group geometry and active site makeup cannot be so systematically varied), and provides a roadmap for the design of industrially relevant serine hydrolases and, more generally, for designing complex enzymes that catalyze multi-step transformations.

Synergistic effects of kinetics and lyophilicity stimulated by Mg2+/Li+ co-insertion in Bi2-xSbxS3 toward reversible hybrid batteries

Co-intercalation type hybrid magnesium-lithium-ion batteries (MLIBs) have received widespread attention in recent years for higher electrolyte utilization and conspicuous insertion kinetics. However, the co-intercalation strategy is rarely explored in replaceable anode host structures. We propose a cation exchange strategy to optimize metal sulfide, endowing MLIBs with high specific capacity, prominent cycle stability and rate performance. Kinetics analyses combined with density functional theory (DFT) calculations demonstrate the preferential absorption of Mg2+/Li+ in Bi-Sb sites and fast migration kinetics induced by Mg2+/Li+ co-insertion. COMSOL Multiphysics simulations further prove the prominent lyophilic effect of the bimetallic sulfide in facilitate ion diffusion. In addition, the multi-step reaction mechanisms and structural evolutions are profoundly revealed by a series of ex-situ investigations. Our findings provide inspirations on exploring novel electrode materials with outstanding electrochemical performance toward large-scale energy storage applications.

Kinetic and thermodynamic behavior of co-pyrolysis of olive pomace and thermoplastic waste via thermogravimetric analysis

This work represents the first attempt to analyze kinetics, thermodynamics and reaction mechanism of olive pomace (OP) and waste plastic materials (PM) co-pyrolysis. Among PM, polypropylene (PP), polystyrene (PS), high density polypropylene (HDPE), polyvinyl chloride (PVC) and poly (ethylene terephthalate) glycol (PETG) were selected. Non-isothermal TG experiments were carried out under inert conditions at four heating rates, namely 5, 10, 20 and 40 °C/min. The kinetic triplet for raw materials and their blends was determined using Starink, Kissinger-Akahira-Sunose and Ozawa-Flynn-Wall iso-conversional models. Pyrolysis mechanism reactions were explained by diverse models, depending on thermal degradation progress. Results shown that co-pyrolysis followed a complex multi-step reaction mechanism. A synergistic effect was detected during co-pyrolysis of OP/PM mixtures. The addition of 50 % (w/w) OP biomass to PM waste decreased the energy of activation (Ea) from 50 to 25 % for all blends, except for PVC/OP. Thermodynamic analysis reveals that adding OP generally reduces the energy barrier (ΔH), except for PS-OP, and improves energy efficiency (ΔG) by facilitating radical formation and molecular chain cleavage.As a conclusion, this study may open up new avenues for waste valorization and resource recovery. Thus, it may contribute to the transition towards a circular and sustainable economy, through zero waste goal.

Numerical Research of Flame Propagation Process in a Supersonic Combustor with Multi-Strut

ABSTRACT The flame processes and the influence of different factors on the cross flame propagation width in a supersonic combustor equipped with multi-strut fueled with kerosene were analyzed to deepen the understanding of supersonic flame characteristics. Under the conditions of T t = 1680K, P t = 1.64Mpa, and Ma = 2.7, the numerical calculations using the multi-step combustion reaction mechanism were carried out. The mixing performance and flame propagation width could be improved by increasing the number of struts. In the multi-strut injection method, the cross flame was observed outside the central flame. The mechanism of cross flame formation was revealed. The establishment of the cross flame had a maximum spacing requirement between injection struts, but this limit could be expanded by the high temperature zone formed by the narrow-distance injection strut. The effects of igniter energy and ignition strut equivalence ratio on the flame diffusion process were studied. Then the boundary conditions required for the cross flame propagation were obtained. The results of this study are highly significant for enhancing the performance of large-scale supersonic combustors in the future.

Analyzing multi-step reaction mechanisms: Unveiling non-linear dynamics with advanced reduction techniques

This study aims to achieve the computational results for the oxidation of carbon monoxide using the spectral quasi-equilibrium manifold and the intrinsic low-dimensional manifold techniques, enabling the identification of the slow invariant manifold and subsequent simulation of the solutions. In addition, the application of Gibbs’ rule facilitates the reduction of the system, thereby effectively transforming higher dimensions into lower dimensions, ultimately revealing the underlying 1D manifold dynamics. This research serves to elucidate the intricacies of phase-shifting behaviors in the context of key species within the overarching and interconnected multi-step reaction mechanisms, contributing significantly to the advancement of understanding in this complex field.

Molecular Mechanism for Converting Carbon Dioxide Surrounding Water Microdroplets Containing 1,2,3-Triazole to Formic Acid.

Spraying water microdroplets containing 1,2,3-triazole (Tz) has been found to effectively convert gas-phase carbon dioxide (CO2), but not predissolved CO2, into formic acid (FA). Herein, we elucidate the reaction mechanism at the molecular level through quantum chemistry calculations and ab initio molecular dynamics (AIMD) simulations. Computations suggest a multistep reaction mechanism that initiates from the adsorption of CO2 by Tz to form a CO2-Tz complex (named reactant complex (RC)). Then, the RC either is reduced by electrons that were generated at the air-liquid interface of the water microdroplet and then undergoes intramolecular proton transfer (PT) or switches the reduction and PT steps to form a [HCO2-(Tz-H)]- complex (named PC-). Subsequently, PC- undergoes reduction and the C-N bond dissociates to generate COOH- and [Tz-H]- (m/z = 69). COOH- easily converts to HCOOH and is captured at m/z = 45 in mass spectroscopy. Notably, the intramolecular PT step can be significantly lowered by the oriented electric field at the interface and a water-bridge mechanism. The mechanism is further confirmed by testing multiple azoles. The AIMD simulations reveal a novel proton transfer mechanism where water serves as a transporter and is shown to play an important role dynamically. Moreover, the transient •COOH captured by the experiment is proposed to be partly formed by the reaction with H•, pointing again to the importance of the air-water interface. This work provides valuable insight into the important mechanistic, kinetic, and dynamic features of converting gas-phase CO2 to valuable products by azoles or amines dissolved in water microdroplets.

PH-dependent reaction triggering in PmHMGR crystals for time-resolved crystallography

Serial crystallography and time-resolved data collection can readily be employed to investigate the catalytic mechanism of Pseudomonas mevalonii 3-hydroxy-3-methylglutaryl (HMG)-coenzyme-A (CoA) reductase (PmHMGR) by changing the environmental conditions in the crystal and so manipulating the reaction rate. This enzyme uses a complex mechanism to convert mevalonate to HMG-CoA using the co-substrate CoA and cofactor NAD+. The multi-step reaction mechanism involves an exchange of bound NAD+ and large conformational changes by a 50-residue subdomain. The enzymatic reaction can be run in both forward and reverse directions in solution and is catalytically active in the crystal for multiple reaction steps. Initially, the enzyme was found to be inactive in the crystal starting with bound mevalonate, CoA, and NAD+. To observe the reaction from this direction, we examined the effects of crystallization buffer constituents and pH on enzyme turnover, discovering a strong inhibition in the crystallization buffer and a controllable increase in enzyme turnover as a function of pH. The inhibition is dependent on ionic concentration of the crystallization precipitant ammonium sulfate but independent of its ionic composition. Crystallographic studies show that the observed inhibition only affects the oxidation of mevalonate but not the subsequent reactions of the intermediate mevaldehyde. Calculations of the pKa values for the enzyme active site residues suggest that the effect of pH on turnover is due to the changing protonation state of His381. We have now exploited the changes in ionic inhibition in combination with the pH-dependent increase in turnover as a novel approach for triggering the PmHMGR reaction in crystals and capturing information about its intermediate states along the reaction pathway.

Metal-Free Photocatalytic CO2 Reduction to CH4 and H2 O2 under Non-sacrificial Ambient Conditions.

Photocatalytic CO2 reduction to CH4 requires photosensitizers and sacrificial agents to provide sufficient electrons and protons through metal-based photocatalysts, and the separation of CH4 from by-product O2 has poor applications. Herein, we successfully synthesize a metal-free photocatalyst of a novel electron-acceptor 4,5,9,10-pyrenetetrone (PT), to our best knowledge, this is the first time that metal-free catalyst achieves non-sacrificial photocatalytic CO2 to CH4 and easily separable H2 O2 . This photocatalyst offers CH4 product of 10.6 μmol ⋅ g-1 ⋅ h-1 under non-sacrificial ambient conditions (room temperature, and only water), which is two orders of magnitude higher than that of the reported metal-free photocatalysts. Comprehensive in situ characterizations and calculations reveal a multi-step reaction mechanism, in which the long-lived oxygen-centered radical in the excited PT provides as a site for CO2 activation, resulting in a stabilized cyclic carbonate intermediate with a lower formation energy. This key intermediate is thermodynamically crucial for the subsequent reduction to CH4 product with the electronic selectivity of up to 90 %. The work provides fresh insights on the economic viability of photocatalytic CO2 reduction to easily separable CH4 in non-sacrificial and metal-free conditions.

Ab Initio Study of Glycine Formation in the Condensed Phase: Carbon Monoxide, Formaldimine, and Water Are Enough

Glycine is considered to be crucial in the formation of proteins and prebiotic substances. Nevertheless, the mechanism of spontaneous glycine formation under prebiotic Earth conditions or within the interstellar medium (ISM) remains a topic of debate, given the changing geochemical environment over Earth’s history and the difficulty of detecting it within the ISM. Yet it is believed that its formation could be possible in interstellar water-rich ice. In this study, using molecular dynamics (MD) simulations at the ab initio level of theory enhanced with modern free energy calculations, we modeled the chemical reaction between carbon monoxide, formaldimine, and water to produce glycine. We estimated under what conditions, in condensed phase at 50, 70, 100, and 300 K, glycine is formed. We also explored the effect of different electric fields on this process. Our results show that glycine could be formed with energy barriers as low as 0.5 kcal mol−1 at 50 K. We discuss whether this reaction could be a suitable candidate for explaining the mechanism of glycine formation under conditions that resemble various astrophysical environments, such as planets, exoplanets, and Earth. This study is relevant to finding a consensus among various proposals for glycine formation. Moreover, it highlights the importance of metadynamics and Car–Parrinello MD methods as tools in finding unknown complex, multistep reaction mechanism pathways, possibly important to the astronomical phenomena.

Preparation and Use of Iron on Carbon Foam for Removal of Organic Dye from Water: Batch Studies.

The presence of dyes in effluents from textile industries has a detrimental effect on aquatic ecosystems as it hinders the process of photosynthesis by reducing the penetration of sunlight. The adsorption capacity of a carbon foam-based iron oxide sorbent obtained from natural sources for the removal of organic methylene blue (MB) dye from water was investigated. The adsorption capacities were examined by batch experiments, wherein the impacts of varying iron content, sorbent dosage, contact time, dye concentration, and characterization were assessed. The physical characteristics and surface morphology of the synthesized carbon foam were also investigated. The carbon precursor and iron oxide precursor were coalesced within a singular container and subjected to carbonization process. This resulted in the formation of a porous structure that is capable of effectively providing support to the iron oxide particles. The carbon foam produced is a self-assembled formation that possesses the characteristic shape and underlying network structure reminiscent of bread. As the number of nanoparticles went up, so did the number of active sites. At elevated temperatures, the interactions between the dye molecules were enhanced, resulting in a more efficient process of dye removal. The magnetite sample exhibited endothermic adsorption, and all other samples exhibited exothermic adsorption. The adsorption of MB onto iron supported by carbon foam did not exhibit intraparticle diffusion as the only rate-limiting step for all samples. The adsorption rate was governed by a multistep elementary reaction mechanism in which multiple processes occurred simultaneously. The experimental data in this study may be accurately modeled by the pseudo-second-order kinetic model (R2 > 0.96). Additionally, the Freundlich isotherm best describes the adsorption equilibrium, which is supported by the outstanding fit of data to the model (R2 > 0.999). The findings suggest that the utilization of a natural carbon foam as a support for an immobilized iron oxide sorbent demonstrates considerable effectiveness in the removal of methylene dye from industrial effluent.

Flame Surface Density and Artificially Thickened Flame Combustion Models Applied to a Turbulent Partially-Premixed Flame

The Sydney Flame provides an excellent framework for systematic analysis of premixed, partially premixed and non-premixed methane/air combustion. It represents (depending on the axial position from the fuel inlet) non-premixed, partially premixed and fully premixed combustion. We simulate the test case FJ200-5GP-Lr75-57, which represents the partially premixed mode. Large-eddy simulations (LES) are conducted using a flame surface density combustion model (FSD) using one-step chemistry and an artificially thickened flame model (ATF) with a global two-step and an analytically reduced multi-step chemistry reaction mechanism. Unity Lewis numbers are assumed (Le=1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Le=1$$\\end{document}) at first. The FSD and ATF models are compared using the same computational mesh, numerical scheme and boundary conditions. Both models will be analysed regarding their accuracy and computational efficiency in the regime of partially premixed combustion. A comparison of the results points out the strengths and weaknesses of the FSD models. The FSD model with inclusion of flame stretch effects yields good agreement with mean experimental temperature, CO2 and H2O mass fraction distributions in contrast to the ATF model using the global two-step mechanism, which overestimates downstream temperature, CO2 and H2O mass fractions. The FSD model performs well in all regions, which are dominated first by premixed, then partially premixed and finally non-premixed combustion along the flame. The ATF multi-step chemistry shows good results only in the premixed mode region, while mean temperature, CO2 and mass fractions are overestimated in the non-premixed mode regions at higher mixture fraction values. Including differential diffusion into the transport equations improved the ATF model results in comparison with experiments. A mesh study revealed, that the ATF model performs only after mesh refinement. In particular results are improved in the non-premixed combustion region. In contrast, the FSD model is less resolution sensitive and performs very well for both meshes. An evaluation of the Wasserstein metric provides a quantitative assessment of different ATF model setups on simulation accuracy. Finally, computational times for all simulation setups are compared.

Electrocatalytic reduction of Cr(VI) on gold-based electrodes in acidic medium: A systematic approach to chromium detection

In the present research, gold immobilized on different substrates (Au, GC, Pt) was examined to fabricate a Cr(VI) sensor in an acidic medium via reduction reactions. Comparative analysis revealed that Au-Pt electrode exhibited better electrocatalytic activity towards the reduction of Cr(VI) compared to Au, Au-Au or Au-GC electrode, which is emphatically discussed in this article. Specifically, the inner α-Au2O3 species of Au matrix on the catalytic surface played a key role in catalyzing Cr(VI) in the acidic medium. Voltammetric investigations revealed that the concerned electrode process is proton-dependent. From hydrodynamic voltammetric studies, it was found that the reduction reaction follows a multistep reaction mechanism with a single electron transfer rate-determining step over the Au-Pt surface. The Cr(VI) reduction reaction follows 1st order kinetics at the Au-Pt electrode surface, where the standard rate constant (k) was found to be 1.4×10−2 cm−1. The excellent improvement of reduction current over the Au-Pt surface was exploited in the fabrication of the Cr(VI) sensor. The developed sensor detects Cr(VI) ions with a linear dynamic range of 10 to 130 μM, whereas the sensitivity and limit of detection values are 3.21 μAμM−1cm−2 and 2.97 μM, respectively. Overall, the findings highlight the promising performance of the Au-Pt electrode in detecting Cr(VI) ions and provide valuable insights into the underlying catalytic diagnosis.

Chemical kinetics of two-step thermochemical decomposition of hydrogen sulfide over nickel sulfide

The study develops a multistep reaction mechanism for a two-step thermochemical cycle that produces hydrogen from H2S decomposition, using nickel sulfide as a catalyst. Two mechanisms were investigated: 1) a gas-phase mechanism that occurs without a Ni catalyst to determine the contribution of thermal decomposition to the overall reaction without a catalyst, and 2) a microkinetic model of the catalytic Ni surface phase to examine the key parameters affecting the overall cycling process and H2S conversion efficiency. Both mechanisms were validated using CHEMKIN-Pro software to analyze the experimental data and trends. The study showed that gas-phase H2S conversion was not kinetically significant below 850 °C. The microkinetic model showed that most H2S was transformed into H2 and NiS products when the sulfurization temperature reached 500 °C. Additionally, the site fraction of NiS decreased while the number of free active nickel sites increased as the regeneration temperature increased from 500 °C to 800 °C. A temperature limit of 700 °C was set for the regeneration step due to the agglomeration behavior of nickel sulfide particles.

Study on biofuel efficiency of tropical banana leaf biomass using spectroscopy, kinetic and thermodynamic parameters

Bio-oils and biochar are some of the valuable products, which are usually harnessed through the pyrolysis process. Under the current investigation, three popular edible banana species were subjected to the pyrolysis process in order to decipher their bioenergy potential. Thermo-gravimetric analysis was utilized to determine the kinetic parameters. The levels of Eα were estimated to be 206 kJ/mol for Musa acuminata, 179 kJ/mol Musa chinichampa and 180 kJ/mol for Musa paradisiaca. Values of A in the range of 1012–1015 s−1, 1012–1015 s−1,1013–1015 s-1, and 1013–1015 s−1 were observed under FWO, KAS, Starink, and Tang model, respectively. From the master plot analysis, it was observed that the reaction mechanism involved in the thermal decay of major cell wall polymers was quite complex and followed a multi-step reaction mechanism. Analysis of the thermodynamic parameters, validated the endothermic nature of pyrolysis. Overall, combined with the different physical and chemical characteristics, it is observed that the leaf biomass of tropical banana species Musa chinichampa is a better feed stock for bioenergy production.

Multi-Step Nucleation of A Crystalline Silicate Framework via A Structurally Precise Prenucleation Cluster.

Hierarchical nucleation pathways are ubiquitous in the synthesis of minerals and materials. In the case of open framework lattices such as zeolites and metal-organic frameworks, pre-organized multi-ion "secondary building units" (SBUs) have long been proposed as fundamental building blocks of the forming crystals. However, detailing the progress of multi-step reaction mechanisms in going from monomeric species to stable crystals and defining the structures of the intermediate SBUs remains an unmet challenge. Combining in situ nuclear magnetic resonance, small-angle X-ray scattering, and atomic force microscopy, we show that crystallization of the framework silicate, cyclosilicate hydrate, occurs through an assembly of cubic octameric Q38polyanions formed through cross-linking and polymerization of smaller silicate monomers and other oligomers. These Q38 are stabilized by hydrogen bonds with surrounding H2O and tetramethylammonium ions (TMA+). When Q38 levels reach a threshold of~32% of the total silicate species nucleation within these clathrates occurs. Further growth proceeds through the incorporation of [(TMA)x(Q38)·nH2O](x-8) clathrate complexes into step edges on the crystals. These findings provide a clear picture of the multi-step nucleation process by which SBUs build a framework silicate lattice with implications for the synthesis of both functional materials and natural minerals.

Lead-Free Perovskite Cs2 SnBr6 /rGO Composite for Photocatalytic Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran.

Severe poisonousness and prolonged instability existing in organic-inorganic lead-based perovskite are two matters seriously hindering its potential future application in photocatalysis. Therefore, it is particularly important to explore ecology-friendly, air-stable and highly active metal-halide perovskites. Herein, a new and stable lead-free perovskite Cs2 SnBr6 decorated with reduced graphene oxide (rGO), is synthesized and employed in the photocatalytic organic conversion. The as-prepared Cs2 SnBr6 is ultrastable, exhibiting no clear changes after being placed in the air for six months. The Cs2 SnBr6 /rGO composite shows excellent photocatalytic activity in photo-driven-oxidation of 5-hydroxymethylfurfural (HMF) to high value enclosed 2,5-diformylfuran (DFF), achieving>99.5 % conversion of HMF and 88 % DFF selectivity in the presence of green oxidant O2 . Comprehensive characterizations disclose a multistep reaction mechanism, demonstrating that the molecular oxygen, photogenerated carriers, ⋅O2 - and 1 O2 altogether synergistically participate in the effective photo-driven conversion of HMF to DFF. This work expands the material gallery towards selective organic conversion and environmentally friendly perovskite options for photocatalytic application.

Thermal degradation study of natural fibre through thermogravimetric analysis

Thermogravimetric analysis was performed to understand the thermal degradation characteristics of natural fibre (bhimal). The objective of the study was to evaluate the suitability of fibre as an alternative to synthetic fibre for reinforcement in polymer composite synthesis. The thermal degradation study was conducted at three heating rates, 10, 15 and 20 °C/min from room temperature to 800 °C. The highest weight loss (65–71 wt%) was observed in the temperature range of 200–435 °C. Three model-free isoconversional methods, FWO, KAS, and Friedman, were applied to estimate the kinetic parameters (activation energy and pre-exponential factor). Average activation energies of 115.96 ± 30.89, 111.08 ± 33.01, and 112.67 ± 41.87 kJ/mol were calculated by FWO, KAS, and FR methods, respectively. The respective estimated pre-exponential factors were 1.41E + 11, 6.62E + 10, and 4.49E + 13 min−1. The kinetic parameters varied substantially with the conversion level, which shows the multi-step reaction mechanism of bhimal fibre thermal degradation. The present study suggests the apparent activation energy of bhimal fibre in the range of 108–120 kJ/mol for polymer composite synthesis.

Site-Averaged Ab Initio Kinetics: Importance Learning for Multistep Reactions on Amorphous Supports

Single-atom centers on amorphous supports include catalysts for polymerization, partial oxidation, metathesis, hydrogenolysis, and more. The disordered environment makes each site different, and the kinetics exponentially magnifies these differences to make ab initio site-averaged kinetics calculations extremely difficult. This work extends the importance learning algorithm for efficient and precise site-averaged kinetics estimates to ab initio calculations and multistep reaction mechanisms. Specifically, we calculate site-averaged proton transfer relaxation rates on an ensemble of cluster models representing Brønsted acid sites on silica-alumina. We include direct and water-assisted proton transfer pathways and simultaneously estimate the water adsorption and activation enthalpies for forward and backward proton transfers. We use density functional theory (DFT) to obtain a site-averaged rate, somewhat like a turnover frequency, for the proton transfer relaxation rate. Finally, we show that importance learning can provide orders-of-magnitude acceleration over standard sampling methods for site-averaged rate calculations in cases where the rate is dominated by a few highly active sites.