Reaction Mechanism Research Articles

Plasmon-Mediated Organic Photoelectrochemistry Applied to Amination Reactions.

Organic electrochemistry is currently experiencing an era of renaissance, which is closely related to the possibility of carrying out organic transformations under mild conditions, with high selectivity, high yields, and without the use of toxic solvents. Combination of organic electrochemistry with alternative approaches, such as photo-chemistry was found to have great potential due to induced synergy effects. In this work, we propose for the first time utilization of plasmon triggering of enhanced and regio-controlled organic chemical transformation performed in photoelectrochemical regime. The advantages of the proposed route is demonstrated in the model amination reaction with formation of C-N bond between pyrazole and substituted benzene derivatives. Amination was performed in photo-electrochemical mode on the surface of plasmon active Au@Pt electrode with attention focused on the impact of plasmon triggering on the reaction efficiency and regio-selectivity. The ability to enhance the reaction rate significantly and to tune products regio-selectivity is demonstrated. We also performed density functional theory calculations to inquire about the reaction mechanism and potentially explain the plasmon contribution to electrochemical reaction rate and regioselectivity.

Mechanistic insight into heteroatom removal from vacuum gas oil blended with PMMA or PET waste.

This work analyzes vacuum gas oil (VGO) and hydrocracking products of feed blended with polymethylmethacrylate (PMMA) or polyethylene terephthalate (PET) in depth to clarify the oxygen, nitrogen, and sulfur removal pathways in these complex mixtures. Hydrocracking reactions are conducted in a semi-batch reactor with a Pt-Pd/HY bifunctional catalyst at 400 °C and 80 bar for 300 min with 10 wt% waste plastic using 0.1 catalyst/feed weight ratio. The samples are analyzed using various techniques, including high-resolution mass spectrometry, providing an improved, more detailed analytical representation. The results demonstrate the synergistic effect of cofeeding oxygenated plastics to the VGO, altering the preferential reaction pathways of heteroatom-containing species in the following order: nitrogen, oxygen, and sulfur. We assess the chemical structures from the gathered data, establish plausible reaction mechanisms, and evaluate the catalyst's role.

Enhancing the combustion of nAl with AlF3 coating: gas-solid reaction mechanism for reducing combustion agglomeration of Al powder.

The combustion agglomeration of nano-aluminum (nAl) powder leads to incomplete combustion, which seriously hinders its application as metal fuel. In this work, nAl@AlF3 composites were produced by coating nAl with AlF3via a facile chemical deposition method. TEM and SEM analyses indicated that the AlF3 layer was evenly coated on the surface of nAl with a thickness of 4.6-9.1 nm, thereby varying the quantity of AlF3 applied. Experimental results from combustion indicated that the prepared nAl@AlF3 composites exhibit superior combustion efficiency, a higher combustion rate, and reduced combustion agglomeration as compared to raw nAl. Contrary to the widely accepted explanation that volatilization of AlF3 hinders Al combustion agglomeration, we proved that the gas-solid reaction between nAl and AlF3 plays an important role in inhibiting the sintering of nAl particles produced. The gaseous intermediate (i.e., AlOF and HF) released from the hydrolysis of AlF3 could reduce the diffusion barrier of Al2O3 to facilitate the reaction of Al core, which enhances the combustion reaction kinetics. More importantly, these gaseous products actively participate in the reaction cycle to continuously exert their catalytic effects.

Structural basis of the Meinwald rearrangement catalysed by styrene oxide isomerase.

Membrane-bound styrene oxide isomerase (SOI) catalyses the Meinwald rearrangement-a Lewis-acid-catalysed isomerization of an epoxide to a carbonyl compound-and has been used in single and cascade reactions. However, the structural information that explains its reaction mechanism has remained elusive. Here we determine cryo-electron microscopy (cryo-EM) structures of SOI bound to a single-domain antibody with and without the competitive inhibitor benzylamine, and elucidate the catalytic mechanism using electron paramagnetic resonance spectroscopy, functional assays, biophysical methods and docking experiments. We find ferric haem b bound at the subunit interface of the trimeric enzyme through H58, where Fe(III) acts as the Lewis acid by binding to the epoxide oxygen. Y103 and N64 and a hydrophobic pocket binding the oxygen of the epoxide and the aryl group, respectively, position substrates in a manner that explains the high regio-selectivity and stereo-specificity of SOI. Our findings can support extending the range of epoxide substrates and be used to potentially repurpose SOI for the catalysis of new-to-nature Fe-based chemical reactions.

Advances in the Removal of Organic Pollutants from Water by Photocatalytic Activation of Persulfate: Photocatalyst Modification Strategy and Reaction Mechanism.

Environmental pollution caused by persistent organic pollutants has imposed big threats to the health of human and ecological systems. The development of efficient methods to effectively degrade and remove these persistent organic pollutants is therefore of paramount importance. Photocatalytic persulfate-based advanced oxidation technologies (PS-AOTs), which depend on the highly reactive SO4·- radicals generated by the activation of PS to degrade persistent organic pollutants, have shown great promise. This work discusses the application and modification strategies of common photocatalysts in photocatalytic PS-AOTs, and compares the degradation performance of different catalysts for pollutants. Furthermore, essential elements impacting photocatalytic PS-AOTs are discussed, including the water matrix, reaction process mechanism, pollutant degradation pathway, singlet oxygen generation, and potential PS hazards. Finally, the existing issues and future challenges of photocatalytic PS-AOTs are summarized and prospected to encourage their practical application. In particular, by providing new insights into the PS-AOTs, this review sheds light on the opportunities and challenges for the development of photocatalysts with advanced features for the PS-AOTs, which will be of great interests to promote better fundamental understanding of the PS-AOTs and their practical applications.

Cysteine Radical and Glutamate Collaboratively Enable C-H Bond Activation and C-N Bond Cleavage in a Glycyl Radical Enzyme HplG.

Hydroxyprolines are abundant in nature and widely utilized by many living organisms. Isomerization of trans-4-hydroxy-d-proline (t4D-HP) to generate 2-amino-4-ketopentanoate has been found to need a glycyl radical enzyme HplG, which catalyzes the cleavage of the C-N bond, while dehydration of trans-4-hydroxy-l-proline involves a homologous enzyme of HplG. Herein, molecular dynamics simulations and quantum mechanics/molecular mechanics (QM/MM) calculations are employed to understand the reaction mechanism of HplG. Two possible reaction pathways of HplG have been explored to decipher the origin of its chemoselectivity. The QM/MM calculations reveal that the isomerization proceeds via an initial hydrogen shift from the Cγ site of t4D-HP to a catalytic cysteine radical, followed by cleavage of the Cδ-N bond in t4D-HP to form a radical intermediate that captures a hydrogen atom from the cysteine. Activation of the Cδ-H bond in t4D-HP to bring about dehydration of t4D-HP possesses an extremely high energy barrier, thus rendering the dehydration pathway implausible in HplG. On the basis of the current calculations, conserved residue Glu429 plays a pivotal role in the isomerization pathway: the hydrogen bonding between it and t4D-HP weakens the hydroxyalkyl Cγ-Hγ bond, and it acts as a proton acceptor to trigger the cleavage of the C-N bond in t4D-HP. Our current QM/MM calculations rationalize the origin of the experimentally observed chemoselectivity of HplG and propose an H-bond-assisted bond activation strategy in radical-containing enzymes. These findings have general implications on radical-mediated enzymatic catalysis and expand our understanding of how nature wisely and selectively activates the C-H bond to modulate catalytic selectivity.

Catalysts for C-N coupling in urea electrosynthesis under ambient conditions from carbon dioxide and nitrogenous species.

Urea is an indispensable nitrogen-containing organic compound in modern human life. However, the current industrial synthesis of urea involves ammonia, which is produced through the Haber-Bosch process under harsh reaction conditions, causing huge energy consumption and heavy environmental pollution. Electrochemical reduction of carbon dioxide (CO2) and nitrogenous species (N2, NOx- and NO) have achieved significant progress, offering a promising approach for the electrochemical C-N coupling to produce urea under ambient conditions. Urea synthesis driven by renewable electricity represents a suitable alternative to the traditional process, contributing to the goal of carbon neutrality and nitrogen cycles. However, challenges such as low yield rate, poor selectivity and unveiled reaction mechanisms still need to be addressed. This review provides a summary of the latest catalysts utilized in urea electrosynthesis, aiming to provide guidance and prospects for the development of high-performance catalysts.

New Avenues for the Synthesis of Oxa‐Cycles Using Lewis/Brønsted Acid Mediated Carboxygenation and Carboalkoxylation of Alkynes

AbstractThe cascade functionalizations of alkynes have provided many elegant methods for the synthesis of useful scaffolds. Amongst these functionalizations, the ones comprising C−C and C−O bond formation have been utilized extensively due to the abundant presence of oxygenated motifs in various bio‐active natural as well as unnatural products. Transition‐metal catalysis is a mainstay for these tandem functionalizations as they are very efficient. However, their utilization does typically involve stoichiometric additives like other metal salts and/or Lewis acids/Brønsted acids (LA/BA). Thus, approaches that rely solely on using LA/BA, make these vital transformations not only eco‐friendly and atom economical but also cost beneficial are fast emerging as complementary alternatives. The present review addresses the gap in the literature by summarizing recent developments in LA/BA mediated 1,1/1,2‐carboxygenation and carboalkoxylation of alkynes for the synthesis of oxa‐cycles. The reaction mechanisms are particularly emphasized to bring out understanding needed from the point of future developments in this domain.

Multistate Dynamics and Kinetics of CO2 Activation by Ta+ in the Gas Phase: Insights into Single-Atom Catalysis.

The activation of carbon dioxide (CO2) by a transition-metal cation in the gas phase is a unique model system for understanding single-atom catalysis. The mechanism of such reactions is often attributed to a "two-state reactivity" model in which the high-energy barrier of a spin state correlating with ground-state reactants is avoided by intersystem crossing (ISC) to a different spin state with a lower barrier. However, such a "spin-forbidden" mechanism, along with the corresponding dynamics, has seldom been rigorously examined theoretically, due to the lack of global potential energy surfaces (PESs). In this work, we report full-dimensional PESs of the lowest-lying quintet, triplet, and singlet states of the TaCO2+ system, machine-learned from first-principles data. These PESs and the corresponding spin-orbit couplings enable us to provide an extensive theoretical characterization of the dynamics and kinetics of the reaction between the tantalum cation (Ta+) and CO2, which have recently been investigated experimentally at high collision energies using crossed beams and velocity map imaging, as well as at thermal energies using a selected-ion flow tube apparatus. The multistate quasi-classical trajectory simulations with surface hopping reproduce most of the measured product translational and angular distributions, shedding valuable light on the nonadiabatic reaction dynamics. The calculated rate coefficients from 200 to 600 K are also in good agreement with the latest experimental measurements. More importantly, these calculations revealed that the reaction is controlled by intersystem crossing, rather than potential barriers.

Structural Evolution of Liquid Metals and Alloys.

Low-melting liquid metals are emerging as a new group of highly functional solvents due to their capability to dissolve and alloy various metals in their elemental state to form solutions as well as colloidal systems. Furthermore, these liquid metals can facilitate and catalyze multiple unique chemical reactions. Despite the intriguing science behind liquid metals and alloys, very little is known about their fundamental structures in the nanometric regime. To bridge this gap, this work employs small angle neutron scattering and molecular dynamics simulations, revealing that the most commonly used liquid metal solvents, EGaIn and Galinstan, are surprisingly structured with the formation of clusters ranging from 157Å to 15.7Å. Conversely, non-eutectic liquid metal alloys of GaSn or GaIn at low solute concentrations of 1, 2, and 5wt%, as well as pure Ga, do not exhibit these structures. Importantly, the eutectic alloys retain their structure even at elevated temperatures of 60 and 90°C, highlighting that they are not just simple homogeneous fluids consisting of individual atoms. Understanding the complex soft structure of liquid alloys will assist in comprehending complex phenomena occurring within these fluids and contribute to deriving reaction mechanisms in the realm of synthesis and liquid metal-based catalysis. This article is protected by copyright. All rights reserved.

BiVO4-based heterojunction nanophotocatalysts for water splitting and organic pollutant degradation: a comprehensive review of photocatalytic innovation

Abstract The novel semiconductor photocatalytic material bismuth vanadate (BiVO4) is gaining significant attention in research due to its unique characteristics, which include a low band gap, good responsiveness to visible light, and non-toxic nature. However, intrinsic constraints such as poor photogenerated charge transfer, slow water oxidation kinetics, and fast electron–hole pair recombination limit the photocatalytic activity of BiVO4. Building heterojunctions has shown to be an effective strategy for enhancing charge separation and impeding electron–hole pair recombination over the last few decades. This review covers the state-of-the-art developments in heterojunction nanomaterials based on BiVO4 for photocatalysis. It explores heterojunction design, clarifies reaction mechanisms, and highlights the current developments in applications including photocatalytic water splitting and organic matter degradation. Finally, it offers a preview of the development paths and opportunities for BiVO4-based heterojunction nanomaterials in the future. This comprehensive assessment of BiVO4-based heterojunctions provides insightful knowledge to researchers in materials science, chemistry, and environmental engineering that will drive advances and breakthroughs in these important fields.

Application of Graphdiyne Confined Gold Nanoparticles Modified Electrode as A Robust Sensing Platform for The Detection of 7-Hydroxy-6-methoxy-hydrocoumarin

Abstract A graphdiyne (GDY) and gold nanoparticles (AuNPs) nanocomposite was formed with AuNPs growing on the surface and between the layers of GDY, which was used as the sensing interface on carbon ionic liquid electrode (CILE) to detect 7-hydroxy-6-methoxy-hydrocoumarin (7-6-HDC) that isolated from the noni juice. The GDY-Au nanocomposite was characterized by different spectroscopic techniques, which exhibited corrugated and folded morphology with large specific surface area. The GDY-Au nanocomposite modified electrode displayed excellent performance which is attributed to the highly conductive nanocomposite, where GDY structure with defects could immobilize more AuNPs to decrease the electron transfer resistance. In addition, the reaction mechanism of 7-6-HDC at GDY-Au surface was deduced based on the electrochemical kinetics investigations. Combined with density functional theory calculation, it was verified that the active site of 7-6-HDC and the presence of AuNPs enhanced the electrocatalytic capacity of GDY-Au nanocomposite. Under optimal conditions, the modified electrode displayed a wider linear range from 10.0 nM to 1.0 mM for 7-6-HDC analysis with a detection limit of 3.22 nM (3S0/S). This work provides a promising material to realize accurate analysis for 7-6-HDC, and the discovered sensing mechanism provides method guidance for the exploration of other novel electrochemical sensors.

Competing Reaction Mechanisms of Peptide Bond Formation in Water Revealed by Deep Potential Molecular Dynamics and Path Sampling.

The formation of an amide bond is an essential step in the synthesis of materials and drugs, and in the assembly of amino acids to form peptides. The mechanism of this reaction has been studied extensively, in particular to understand how it can be catalyzed, but a representation capable of explaining all the experimental data is still lacking. Numerical simulation should provide the necessary molecular description, but the solvent involvement poses a number of challenges. Here, we combine the efficiency and accuracy of neural network potential-based reactive molecular dynamics with the extensive and unbiased exploration of reaction pathways provided by transition path sampling. Using microsecond-scale simulations at the density functional theory level, we show that this method reveals the presence of two competing distinct mechanisms for peptide bond formation between alanine esters in aqueous solution. We describe how both reaction pathways, via a general base catalysis mechanism and via direct cleavage of the tetrahedral intermediate respectively, change with pH. This result contrasts with the conventional mechanism involving a single pathway in which only the barrier heights are affected by pH. We show that this new proposal involving two competing mechanisms is consistent with the experimental data, and we discuss the implications for peptide bond formation under prebiotic conditions and in the ribosome. Our work shows that integrating deep potential molecular dynamics with path sampling provides a powerful approach for exploring complex chemical mechanisms.

Influence of Ti-incorporated Zeolite Topology and Pore Condensation on Vapor Phase Propylene Epoxidation Kinetics with Gaseous H2O2.

Vapor-phase propylene (C3H6) epoxidation kinetics with hydrogen peroxide (H2O2) strongly reflects the physical properties of Ti-incorporated zeolite catalysts and the presence of spectating molecules ("solvent") near active sites even without a bulk liquid phase. Steady-state turnover rates of C3H6 epoxidation and product selectivities vary by orders of magnitudes, depending on the zeolite silanol ((SiOH)x) density, pore topology (MFI, *BEA, FAU), and the quantity of condensed acetonitrile (CH3CN) molecules nearby active sites, under identical reaction mechanisms sharing activated H2O2 intermediates on Ti surfaces. Individual kinetic analyses for propylene oxide (PO) ring-opening, homogeneous diol oxidative cleavage, and homogeneous aldehyde oxidation reveal that secondary reaction kinetics following C3H6 epoxidation responds more sensitively to the changes in zeolite physical properties and pore condensation with CH3CN. Thus, higher PO selectivities achieved in hydrophilic Ti-MFI at steady-state reflect the preferential stabilization of transition states for C3H6 epoxidation (a primary reaction) relative to PO ring-opening and oxidative cleavage (secondary reactions) that solvation effects that reflect interactions among condensed CH3CN within pores and the extended pore structure.

Combined Computational and Experimental Study Reveals Complex Mechanistic Landscape of Brønsted Acid-Catalyzed Silane-Dependent P═O Reduction.

The reaction mechanism of Brønsted acid-catalyzed silane-dependent P═O reduction has been elucidated through combined computational and experimental methods. Due to its remarkable chemo- and stereoselective nature, the Brønsted acid/silane reduction system has been widely employed in organophosphine-catalyzed transformations involving P(V)/P(III) redox cycle. However, the full mechanistic profile of this type of P═O reduction has yet to be clearly established to date. Supported by both DFT and experimental studies, our research reveals that the reaction likely proceeds through mechanisms other than the widely accepted "dual activation mode by silyl ester" or "acid-mediated direct P═O activation" mechanism. We propose that although the reduction mechanisms may vary with the substitution patterns of silane species, Brønsted acid generally activates the silane rather than the P═O group in transition structures. The proposed activation mode differs significantly from that associated with traditional Brønsted acid-catalyzed C═O reduction. The uniqueness of P═O reduction originates from the dominant Si/O═P orbital interactions in transition structures rather than the P/H-Si interactions. The comprehensive mechanistic landscape provided by us will serve as a guidance for the rational design and development of more efficient P═O reduction systems as well as novel organophosphine-catalyzed reactions involving P(V)/P(III) redox cycle.

Reaction Mechanism of Hydrogen Generation and Nitrogen Fixation at Carbon Nitride/Double Perovskite Heterojunctions

Photocatalytically active heterojunctions based on metal halide perovskites (MHPs) are drawing significant interest for their chameleon ability to foster several redox reactions. The lack of mechanistic insights into their performance, however, limits the ability of engineering novel and optimized materials. Herein, a report is made on a composite system including a double perovskite, Cs2AgBiCl6/g‐C3N4, used in parallel for solar‐driven hydrogen generation and nitrogen reduction, quantified by a rigorous analytical approach. The composite efficiently promotes the two reactions, but its activity strongly depends on the perovskite/carbon nitride relative amounts. Through advanced spectroscopic investigation and density function theory (DFT) modeling the H2 and NH3 production reaction mechanisms are studied, finding perovskite halide vacancies as the primary reactive sites for hydrogen generation together with a positive contribution of low loaded g‐C3N4 in reducing carrier recombination. For nitrogen reduction, instead, the active sites are g‐C3N4 nitrogen vacancies, and the heterojunction best performs at low perovskites loadings where the composites maximize light absorption and reduce carrier losses. It is believed that these insights are important add‐ons toward universal exploitation of MHPs in contemporary photocatalysis.

A review on Fe2O3‐based catalysts for toluene oxidation: catalysts design and optimization with the formation of abundant oxygen vacancies

Volatile organic compounds (VOCs) are typical pollutants with hazards for humans and the environment, and can be efficiently mitigated by catalytic combustion. Fe2O3‐based catalysts are a promising choice due to their low cost and strong redox ability. Several attempts have been made to promote the catalytic performance of Fe2O3 at low temperatures. This review summarizes the research progress on Fe2O3‐based catalysts for the oxidation of toluene, one of the most common and harmful VOC. Firstly, the properties and catalytical performance for Fe2O3‐based catalysts were summarized, and the reaction mechanisms for toluene oxidation on the surface of Fe2O3 were detailed to comprehend the role of oxygen vacancy. Then, the modification for single Fe2O3 catalysts, including synthesis parameters, structure and morphology control, has been introduced to reveal the correlation between physicochemical properties of catalysts and their activity. In addition, composite Fe2O3 catalysts, which can promote catalytic performance significantly due to the synergetic effect between different components, were comprehended. Finally, waste‐derived Fe2O3 catalysts with sustainable merit as converting waste into worth have been discussed. Moreover, the advanced machine learning tools, which are helpful in accelerating catalyst design, configuration optimization and reactivity prediction, have been introduced as an emerging research opportunity for the future.

Influence of Ti‐incorporated Zeolite Topology and Pore Condensation on Vapor Phase Propylene Epoxidation Kinetics with Gaseous H2O2

Vapor‐phase propylene (C3H6) epoxidation kinetics with hydrogen peroxide (H2O2) strongly reflects the physical properties of Ti‐incorporated zeolite catalysts and the presence of spectating molecules (“solvent”) near active sites even without a bulk liquid phase. Steady‐state turnover rates of C3H6 epoxidation and product selectivities vary by orders of magnitudes, depending on the zeolite silanol ((SiOH)x) density, pore topology (MFI, *BEA, FAU), and the quantity of condensed acetonitrile (CH3CN) molecules nearby active sites, under identical reaction mechanisms sharing activated H2O2 intermediates on Ti surfaces. Individual kinetic analyses for propylene oxide (PO) ring‐opening, homogeneous diol oxidative cleavage, and homogeneous aldehyde oxidation reveal that secondary reaction kinetics following C3H6 epoxidation responds more sensitively to the changes in zeolite physical properties and pore condensation with CH3CN. Thus, higher PO selectivities achieved in hydrophilic Ti‐MFI at steady‐state reflect the preferential stabilization of transition states for C3H6 epoxidation (a primary reaction) relative to PO ring‐opening and oxidative cleavage (secondary reactions) that solvation effects that reflect interactions among condensed CH3CN within pores and the extended pore structure.

Extracting Accurate Parameters from a Proton Exchange Membrane Fuel Cell Model Using the Differential Evolution Ameliorated Meta-Heuristics Algorithm

The electrochemical proton exchange membrane fuel cell (PEMFC) is an electrical generator that utilizes a chemical reaction mechanism to produce electricity, serving as a sustainable and environmentally friendly energy source. To thoroughly analyze and develop the features and performance of a PEMFC, it is essential to use a precise model that incorporates exact parameters to effectively suit the polarization curve. In addition, parameter extraction plays a crucial role in the simulation analysis, evaluation, optimum control, and fault detection of the proton exchange membrane fuel cell (PEMFC) system. Despite the development of many algorithms for parameter extraction in PEMFC, obtaining accurate and trustworthy results rapidly remains a challenge. This study presents a hybridized algorithm, namely differential evolution ameliorated (DEA) for reliably estimating PEMFC model parameters. To evaluate the proposed DEA-based parameter identification, a comparison analysis with previously published methods is conducted using MATLAB/SimulinkTM (R2016b, MathWorks, Natick, MA, USA) in terms of system correctness and convergence process. The proposed DEA algorithm is tested to extract the parameters of two PEMFC models: SR-12,500 W and 250 W. The sum of the squared errors (SSE) between the experimental and the obtained voltage data is defined as an objective function. The simulation results prove that the suggested DEA algorithm is capable of identifying the optimal PEMFC parameters rapidly and accurately in comparison with other optimization algorithms.

Insights into the Origin of Activity Enhancement via Tuning Electronic Structure of Cu2O towards Electrocatalytic Ammonia Synthesis

The insight of the activity phase and reaction mechanism is vital for developing high-performance ammonia synthesis electrocatalysts. In this study, the origin of the electronic-dependent activity for the model Cu2O catalyst toward ammonia electrosynthesis with nitrate was probed. The modulation of the electronic state and oxygen vacancy content of Cu2O was realized by doping with halogen elements (Cl, Br, I). The electrocatalytic experiments showed that the activity of the ammonia production depends strongly on the electronic states in Cu2O. With increased electronic state defects in Cu2O, the ammonia synthesis performance increased first and then decreased. The Cu2O/Br with electronic defects in the middle showed the highest ammonia yield of 11.4 g h−1 g−1 at −1.0 V (vs. RHE), indicating that the pattern of change in optimal ammonia activity is consistent with the phenomenon of volcano curves in reaction chemistry. This work highlights a promising route for designing NO3−RR to NH3 catalysts.