Study Of Reaction Mechanism Research Articles

In situ Überwachung photokatalytischer Prozesse mit polymeren Kohlenstoffnitrid‐Dünnschichtfilmen

AbstractPolymere Kohlenstoffnitridmaterialien sind aufgrund ihrer Anregung mit sichtbarem Licht zunehmend in den Mittelpunkt der heterogenen Photokatalyse gerückt. In dieser Arbeit berichten wir über die Verwendung von Kohlenstoffnitrid beschichteten NMR‐Röhrchen in Kombination mit in situ Beleuchtung, die zur detaillierten Aufklärung photokatalytischer Reaktionsprozesse beitragen. Zunächst verwenden wir die mit Kohlenstoffnitrid beschichteten Crimp‐Vials als Photoreaktoren für die photokatalytische Fluorierung unaktivierter C(sp3)−H Bindungen mit sichtbarem Licht, die jeweils mäßige bis ausgezeichnete Ausbeuten liefern und über mehrere Zyklen wiederverwendet werden können. Zudem wurden Kohlenstoffnitrid‐beschichtete NMR‐Röhrchen als Photoreaktor verwendet, indem sie mit einer optischen Glasfaser gekoppelt und direkt im Inneren des Spektrometers bestrahlt werden. Diese Methode ermöglicht es, Reaktionen mittels in situ NMR‐Spektroskopie zu detektieren und reaktive Intermediate nachzuweisen, die in konventionellen Analysemethoden schwer nachzuweisen sind. Dieses Konzept bietet erhebliche Vorteile für die Untersuchung komplexer photokatalytischer Mechanismen und erspart den Einsatz zusätzlicher Analysemethoden für den Nachweis reaktiver Intermediate.

In situ Monitoring of Photocatalysis on Polymeric Carbon Nitride Thin Films.

Polymeric carbon nitride has attracted significant interest in heterogeneous photocatalysis due to its activity under visible-light irradiation. Herein, we report on using carbon nitride-coated NMR tubes for in situ studies of photocatalytic reaction mechanisms. In a first step, we exploited carbon nitride-coated crimp vials as batch photoreactors for visible photocatalytic fluorinations of unactivated C(sp3)-H bonds, with moderate to excellent yields and reusability over multiple cycles. Eventually, carbon nitride-coated NMR tubes were used as a photoreactor by coupling them with optical fiber irradiation directly inside the spectrometer. This enabled us to follow the reaction with in situ NMR spectroscopy identifying reactive intermediates otherwise elusive in conventional analyses. The method provides advantages for the study of photocatalytic mechanisms of complex reactions and substantially reduces the need of comparative tests for depicting reaction intermediates and conversion pathways.

Reaction mechanism study of calcium carbide residue with graphene oxide in aqueous environment: Adsorption properties and mechanical potentials

Graphene Oxide (GO) is widely used, but its hydrophilic properties make it difficult to remove once it enters water and soil environments. In this paper, the adsorption effect of calcium carbide residue (CCR) as adsorbent on GO was investigated through a series of adsorption tests. Adsorption thermodynamics, kinetics, isotherm models, and various characterization techniques were employed to explore the adsorption mechanism. Additionally, the study assessed CCR’s ability to stabilize GO-contaminated soils through unconfined compressive strength tests. The results showed that (1) at T = 303 K, with a pH of 11 and an initial GO concentration of 80 mg/L, CCR demonstrated excellent adsorption performance. (2) The adsorption process followed the Langmuir isotherm and a quasi-second-order kinetic model, indicating chemical adsorption with spontaneous heat adsorption. (3) CCR not only acts as an effective adsorbent for removing GO from wastewater but also has the potential to strengthen GO-contaminated soils. In addition, due to its favorable environmental benefits, this study has a wide range of potential applications in industrial fields such as wastewater treatment, air purification, and energy storage and conversion. This study not only proposes an effective method for removing graphene oxide from aqueous environments, but also provides a new idea for waste resource utilization, which helps to achieve the dual goals of environmental protection and resource reuse.

Effective abatement of naphthalene from flue gas by V-W/Ti catalyst: Study of vanadium species and reaction mechanism

The removal of naphthalene is important but deficient, so that the available V-W/Ti catalysts (1.0 wt% V2O5/TiO2) were prepared for its efficient abatement in this work. Especially, influences of different existence status of vanadium species on naphthalene elimination were investigated via changing the pH value of precursor solution. With the increase of precursor solution acidity, more highly polymeric vanadium species were formed on the catalyst surface. The redox capability was enhanced markedly as a consequence of the stronger electron transfer between V/W and Ti that induced more surface active oxygen and V5+, whereas the catalytic activity was obviously boosted and almost 90 % conversion of naphthalene was achieved at 293 °C. Moreover, the weaker chemisorption between naphthalene and V-W/Ti catalyst allowed the deeper conversion to COx. GC–MS manifested that naphthalene seemed to be an organic component that was difficult to be totally degraded. Naphthalene oxidation was in the following pathway of naphthalene → 1,4-naphthoquinone → phthalic anhydride → phthalates → benzene → benzoquinone → maleic anhydride → COx/H2O. The opening of aromatic ring in phthalic anhydride and benzene ring in benzene were proven to be the rate-controlling step based on in-situ DRIFTS results. The key of naphthalene oxidation was the complete degradation of various intermediates.

Hyperconjugation-Driven Isodesmic Reaction of Indoles and Anilines: Reaction Discovery, Mechanism Study, and Antitumor Application.

Isodesmic reactions, in which chemical bonds are redistributed between substrates and products, provide a general and powerful strategy for both biological and chemical synthesis. However, most isodesmic reactions involve either metathesis or functional-group transfer. Here, we serendipitously discovered a novel isodesmic reaction of indoles and anilines that proceeds intramolecularly under weakly acidic conditions. In this process, the five-membered ring of the indole motif is broken and a new indole motif is constructed on the aniline side, accompanied by the formation of a new aniline motif. Mechanistic studies revealed the pivotal role of σ→π* hyperconjugation on the nitrogen atom of the indole motif in driving this unusual isodesmic reaction. Furthermore, we successfully synthesized a diverse series of polycyclic indole derivatives; among quinolines, potential antitumor agents were identified using cellular and in vivo experiments, thereby demonstrating the synthetic utility of the developed methodology.

Hyperconjugation‐Driven Isodesmic Reaction of Indoles and Anilines: Reaction Discovery, Mechanism Study, and Antitumor Application

AbstractIsodesmic reactions, in which chemical bonds are redistributed between substrates and products, provide a general and powerful strategy for both biological and chemical synthesis. However, most isodesmic reactions involve either metathesis or functional‐group transfer. Here, we serendipitously discovered a novel isodesmic reaction of indoles and anilines that proceeds intramolecularly under weakly acidic conditions. In this process, the five‐membered ring of the indole motif is broken and a new indole motif is constructed on the aniline side, accompanied by the formation of a new aniline motif. Mechanistic studies revealed the pivotal role of σ→π* hyperconjugation on the nitrogen atom of the indole motif in driving this unusual isodesmic reaction. Furthermore, we successfully synthesized a diverse series of polycyclic indole derivatives; among quinolines, potential antitumor agents were identified using cellular and in vivo experiments, thereby demonstrating the synthetic utility of the developed methodology.

Magnesiothermic reduction of beryllium fluoride: Reaction mechanism and kinetic study

Beryllium (Be) is mainly produced by magnesiothermic reduction of beryllium fluoride (BeF2). This research aims to improve the extraction rate of Be by investigating the reaction mechanism and kinetics during the magnesiothermic reduction of BeF2. It was found that the solid product layer composed of MgF2 and Be metal produced during the magnesiothermic reduction process is the main reason hindering the further improvement of the reduction rate. Kinetic study on the magnesiothermic reduction of BeF2 shows that it was controlled by volume diffusion. An apparent activation energy of 66.01 kJ/mol was obtained for the magnesiothermic reduction in the temperature range of 850–950 °C. Aiming to extract Be from BeF2 with a high efficiency, granular-shaped Mg (particle size 0.2–5 mm) and BeF2 powder (particle size < 0.83 mm) were used as raw materials for magnesiothermic reduction at 900 °C for 30 min, protected using Ar atmosphere. This was followed by further heating to 1300 °C and holding for 10 min, and the highest extraction rate of Be was achieved at 90.1 wt% with the Be purity of 94.2 wt%.

Experimental and reaction mechanism study on laminar burning velocity and characteristics of OH/NH generation in ammonia co-combustion

Developing fuels without carbon, such as ammonia (NH3), is crucial for achieving carbon reduction. Laminar burning velocity (LBV) is investigated, and distribution characteristics of OH and NH radicals are identified through planar laser-induced fluorescence. Rate of production of main free radicals and sensitivity analysis are also studied. The addition of NH3 to CH4-air mixtures results in a decrease in the LBV, a rise in preheating zone height, a reduction in OH radical fluorescence intensity, and an enhancement in NH radical fluorescence intensity. Simulation reveals that the high NH3 ratio enhance the negative impact of nitrogen-containing elementary reactions on LBV. The addition of NH3 enhances the initial consumption of more OH via reaction R248:NH2+OH=NH+H2O, leading to increased NH production, and indicating a negative effect on LBV. The decrease in OH concentration is the main factor leading to a decrease in LBV. The reactions affecting the OH generation and consumption are R39:H+O2=O+OH and R85:OH+H2=H+H2O, respectively. The addition of NH3 leads to a more significant reduction in R39.

Hazard reduction of heavy metals by co-pyrolysis of modified vermiculite with paper mill sludge/municipal solid waste: Characterization, risk and reaction mechanism study in pyrolytic environment

The increasing volume of paper mill sludge (PM) and municipal solid waste (MW) poses environmental problems associated with heavy metals. Co-pyrolysis of PM/MW with additives effectively reduces the risk of heavy metal contamination. In this study, co-modified vermiculite (LMV) was prepared using the modification methods of intercalation-exfoliation, Mg2+ impregnation, and thermal activation. PM and MW were mixed at different doping ratios, and a non-doping experiment was performed to compare the pyrolysis processes and the retention rates for different heavy metals (Zn, Cr, Cu, Cd, and Pb), as well as the potential ecological risks, and the form distribution of the heavy metals. The addition of LMV reduced the risk posed by heavy metals. In the absence of doping, a higher pyrolysis temperature is conducive to an increase in the oxidizable and residual fractions of heavy metals without higher retention rates. Among these heavy metals, Zn, Cr, and Cu had higher retention rates and low risks, whereas Cd had a relatively high risk with low retention rates. Simulations showed that there was a transfer of the Mulliken charge after the reaction, indicating that all the heavy metals can interact with O, Al, and Si in the LMV; this reaction mainly occurs around the No. 17 O atom of the LMV. The monomers, oxides, and chlorides of Zn/Cr/Cu exhibited better adsorption on vermiculite compared to that of Pb/Cd.

A first principles study of the oxygen reduction reaction mechanism on porous Ag and Ag-TM nanostructure

Porous Ag has good electron conductivity and is one of the typical oxygen reduction reaction(ORR) catalysts. In order to investigate the mechanism of porous Ag for ORR, the relaxed structure and detailed partial density of states are determined using density-functional theory. Among multiple possible active sites, the overpotential of porous Ag is 0.50 V, which is better than that of Ag (111) at 0.62 V. After doping Pt and Pd, the overpotentials are 0.47 V and 0.49 V, respectively. Furthermore, the introduction of a transition metal has led to changes in the charge distribution on the catalyst surface, which has resulted in improved catalytic performance. By investigating the synergistic effects between doped transition metals and ORR intermediates, this can facilitate the development of catalysts with higher activity and better stability.

A novel lanthanum ion-assisted fluorapatite precipitation approach for efficient fluoride removal from water

Excess amounts of fluoride ions (F-) in water/wastewater bring great hazards to human health and the environment. The traditional method such as precipitation by calcium ions does not work well for low-concentration F- removal. Herein, we developed a lanthanum ion (La3+)-assisted fluorapatite precipitation approach to treat F--containing water, aiming at high removal efficiency. Through an orthogonal experimental design and single-factor experiment, the optimized reaction conditions were obtained as the molar ratio of (Ca2++La3+): PO43-: F-=7.5: 4: 1, Ca2+: La3+ = 90: 10, and reaction pH of 9. Under such reaction conditions, F- in water (10 mg L−1) could be well controlled under 1 mg L−1, fulfilling relevant environmental standards. No remarkable F- removal efficiency loss was found in the presence of typical anions (e.g., halides, bicarbonate, sulfate). The developed F- removal approach also worked well in a wide F- concentration range (10–200 mg L−1) and reaction temperature range (5–45 °C). Further reaction mechanism study proved that the formation of LaFx(OH)3-x colloids was the key step, which could accumulate F- ions in water into colloids. The subsequently added Ca2+ and PO43- destroyed the stable colloids, and then promoted the formation of fluorapatite with simultaneous La3+ doping, which existed as precipitate and allowed the easy separation from the reaction solution. As a by-product, the as-produced La3+-doped fluorapatite was evaluated as an adsorbent for organic pollutant removal, which possessed a high adsorption capacity (763.4 mg g−1) for the model pollutant of Congo red. Considering the facile operation process, high F- removal efficiency, wide operation temperature window, and anti-inference for co-existing anions, the La3+-assisted fluorapatite precipitation approach possessed great potential for F--containing water/wastewater treatment.

Biocatalytic Carbenoid N-H Insertions Via Engineered Myoglobins: A Systematic Reaction Mechanism Study

Recently engineered myoglobins have demonstrated excellent biocatalytic activities for a broad range of carbenoid N-H insertion reactions with up to >99% yield. Such biocatalysts offer various tuning points beyond protein residues, such as metal center, and axial ligand, to regulate different chemical reactivities. However, there is no computational mechanistic study to reveal its basic reaction mechanism, which could be utilized to study effects of different catalyst component for rational biocatalyst design. The reported computational mechanistic papers of other heme protein catalyzed N-H insertion have limited information without a careful comparison of all possible pathways and just focused on certain steps. Building on our group’s recent mechanistic work on a number of heme carbene transfer reactions, we performed a quantum chemical study to systematically study all possible reaction pathways for myoglobin catalyzed N-H insertions, which starts from the reactants to final released products and thus constitute a complete reaction pathway study. We examined ylide (proton transfer), hydrogen atom transfer, and hydride transfer pathways. In the favored ylide pathways, we calculated the direct proton transfer from amine to carbene and indirect paths with both concerted and stepwise proton transfer and dissociation components. For the indirect pathways in which the proton resides on the carbonyl oxygen of the original carbene substituent, subsequent water-assisted proton transfer from this carbonyl oxygen to carbene’s carbon to form the final product was also investigated. The most favorable pathway from this systematic reaction mechanism study was further supported by new experimental work from our collaborators, the original developer of myoglobin-based biocatalysts. These novel mechanistic results provide important mechanistic information for future biocatalyst design with enhanced reactivities.

Multi-Level Protocol for Mechanistic Reaction Studies Using Semi-Local Fitted Potential Energy Surfaces.

In this work, we propose a multi-level protocol for routine theoretical studies of chemical reaction mechanisms. The initial reaction paths of our investigated systems are sampled using the Nudged Elastic Band (NEB) method driven by a cheap electronic structure method. Forces recalculated at the more accurate electronic structure theory for a set of points on the path are fitted with a machine learning technique (in our case symmetric gradient domain machine learning or sGDML) to produce a semi-local reactive potential energy surface (PES), embracing reactants, products and transition state (TS) regions. This approach has been successfully applied to a unimolecular (Bergman cyclization of enediyne) and a bimolecular (SN2 substitution) reaction. In particular, we demonstrate that with only 50 to 150 energy-force evaluations with the accurate reference methods (here complete-active-space self-consistent field, CASSCF, and coupled-cluster singles and doubles, CCSD) it is possible to construct a semi-local PES giving qualitative agreement for stationary-point geometries, intrinsic reaction coordinates and barriers. Furthermore, we find a qualitative agreement in vibrational frequencies and reaction rate coefficients. The key aspect of the method's performance is its multi-level nature, which not only saves computational effort but also allows extracting meaningful information along the reaction path, characterized by zero gradients in all but one direction. Agnostic to the nature of the TS and computationally economic, the protocol can be readily automated and routinely used for mechanistic reaction studies.

Chemoselective C(sp2)–C(sp2) doubly decarbonylative cross-electrophile coupling enabled by palladium catalysis: Reaction development and mechanism studies

Transition metal-catalyzed cross-electrophile coupling of two electrophiles has proved to be a powerful and step-economic strategy for the construction of C–C bonds in organic synthesis. Despite the considerable advances, cross-electrophile coupling of sp2-hybridized carboxylic acid derivatives via a decarbonylative fashion remains a formidable challenge. Herein, we disclose a chemoselective C(sp2)–C(sp2) doubly decarbonylative cross-electrophile coupling of aroyl fluorides with a wide range of aryl and alkenyl thioesters by palladium catalysis. The catalytic protocol provides an alternative route for the efficient synthesis of biaryls and stilbene derivatives owing to wide accessibility, natural abundance and stability of carboxylic acid and relevant derivatives. The combination of a series of experiments and theoretical calculations reveals that conversion of aroyl fluorides into the corresponding thioesters, chloride anion in catalyst PdCl2 and in situ generated Zn(II) reagents play the crucial roles in the current cross-coupling reactions.

Study of the Dynamic Reaction Mechanism of the Cable-Stayed Tube Bridge under Earthquake Action

In order to explore the failure mode of the cable-stayed pipe bridge under earthquake action, taking the structural system of an oil and gas pipeline–cable-stayed pipe bridge as the research object, the full-scale finite element calculation model of the cable-stayed pipe bridge–oil and gas pipeline structural system as well as the finite element calculation model considering the additional mass of the oil and gas medium and the fluid–structure interaction effect were established by using ANSYS Workbench finite element software. The stress and displacement of the cable under the earthquake action were analyzed in the time history, as were the response characteristics of the cable when subjected to both methods. The calculation results show that the overall failure of the pipeline is basically the same under the two methods. Compared with the additional mass method, the solution for the fluid–structure coupling method can be derived through a comprehensive analysis of the flow field and structure, respectively, avoiding the sudden change caused by model simplification or calculation error so that the analysis results can better simulate the actual situation. In summary, the fluid–structure interaction method enables a more precise prediction of the dynamic response of the structure, and the findings of this research can provide a theoretical foundation and technical guidance for optimizing the seismic performance of cable-stayed pipe bridges.

Manganese-rhodium nanoparticles: Adsorption on titanium oxide surfaces and catalyst for syngas reactions.

Manganese-rhodium (Mn-Rh) nanoparticles have emerged as a promising candidate for catalytic applications in the production of syngas, a critical precursor for a wide range of industrial processes. This study employs a comprehensive, theoretical, and computational approach to investigate the structural and electronic properties of Mn-Rh nanoparticles, with a specific focus on their interaction with titanium oxide (TiO2) surfaces and their potential as catalysts for syngas reactions. The density functional theory calculations are employed to explore the adsorption behavior of Mn-Rh nanoparticles on TiO2 surfaces. By analyzing the adsorption energies, geometries, and electronic structure at the nanoscale interface, we provide valuable insights into the stability and reactivity of Mn-Rh nanoparticles when immobilized on TiO2 supports. Furthermore, the catalytic performance of Mn-Rh nanoparticles in syngas production is thoroughly examined. Through detailed reaction mechanism studies and kinetic analysis, we elucidate the role of Mn and Rh in promoting syngas generation via carbon dioxide reforming and partial oxidation reactions. The findings demonstrate the potential of Mn-Rh nanoparticles as efficient catalysts for these crucial syngas reactions. This research work not only enhances our understanding of the fundamental properties of Mn-Rh nanoparticles but also highlights their application as catalysts for sustainable and industrially significant syngas production.

Fischer-Tropsch Synthesis for the Production of Sustainable Aviation Fuel: Formation of Tertiary Amines from Ammonia Contaminants.

Fischer-Tropsch synthesis combined with product workup is a promising route toward synthetic aviation fuel from renewable hydrogen and carbon sources like biomass, CO2, and waste. Cost savings can be achieved by reducing the number of gas treatment steps in new plants, but the consequence of contaminants in the feed needs investigation. While feeding 2.6 ppmV ammonia to a Fischer-Tropsch reactor, it was shown that ammonia was predominantly chemically converted into organic amines, with most nitrogen found in the water phase (89%), followed by heavy wax (7%) and light wax (1%). The concentration difference between water and light wax was shown to be due to the post-condensation separation of amines on polarity. Amines up to a chain length of 120 were detected in the heavy wax with MALDI-FT-ICR-MS, which, in combination with the high nitrogen content, suggests that amines have a similar chain growth probability compared to the main hydrocarbon products. Detailed product analysis with three independent analytical techniques showed that tertiary N,N-dimethylalkylamines were by far the most abundant amine class. This suggests that ammonia is decomposed on the cobalt surface and, potentially as a dimethylamine fragment, incorporated in the growing chain. Further evidence was obtained from the abundance of trimethylamine and from the reconciled nitrogen product analysis up to C100, which showed that the amine product distribution followed from naphtha onward the same ASF kinetics as alkanes and oxygenates while being distinctively different from the alkene distribution. The presented findings provide further avenues for studies of the Fischer-Tropsch reaction mechanism and indicate the opportunity of cost saving on gas treatment, while further validation is required to assess the impact on hydrocracking and product quality.

Insights into the role of Mo in boosting CHx* oxidation for CO2 methane reforming

CHx* oxidation is one of the most vital routes to alleviate the carbon deposition problem of CO2 reforming of methane (DRM) reaction. Whereas, little experimental evidence has been observed on NiMo catalysts where the CHx* oxidation was dominant over its dissociation reaction. Herein, to experimentally unveil the CHx* oxidation route of NiMo catalysts, we design three catalysts with different particle sizes and structures. Among them, Mo/Niphy@SiO2 core shell catalyst demonstrated the dominant CHx* oxidation route over its dissociation based on in-situ diffuse reflectance infrared Fourier transform spectroscopy experiments. This was attributed to the confinement effect of SiO2 and the formation of Ni–Mo alloy, inhibiting the CHx* dissociation reaction. It exhibited relatively stable CH4 and CO2 conversions (77 % and 75 % respectively) within 180 h. By contrast, on Mo/Niphy catalyst which has a big Ni size, CHx* was mainly dissociated to C* and oxidized to CO which further underwent a disproportion reaction to produce CO2 and C*, leading to the severe carbon deposition and unstable DRM performance. The strategy to unveil the dominant role of CHx* oxidation via design catalysts with different sizes and structures sheds light on the study of reaction mechanism of other reactions.

Vicinal effect on chlorination of diols

The vicinal effect on chlorination reactions of diols was investigated by computational methods. Applying density functional theory (DFT) combined with polarizable continuum model (PCM) and explicit solvent molecules, we demonstrated the effect of hydration on the leaving and vicinal hydroxyl group in SN2 reactions of selected alcohols. Our results point out to the importance of intermolecular hydrogen bonds as a stabilizing factor in reaction pre-complexes and alcohols with vicinal hydroxyl groups, highlighting the importance of solute-solvent network formation. This indicates the significance of adequately describing the solvent in the study of reaction mechanisms, in special when the interaction with solute molecules and ions are strong. Also, we found a “vinical effect” on the reactivity, in other words, alcohols with vicinal hydroxyl groups exhibit higher activation energies when compared to related monoalcohols. Moreover, 1,3-propanediol and 1,3-butanediol present the highest activation energies.

Reaction mechanism and kinetic study on the methanol dominated gas-based reduction process

Alcohols, represented by methanol, have received attention as a new type of reducing agent in various fields, including carbide synthesis, surface carburization etc. In order to clarify the reduction routes, reaction mechanisms and influencing factors on the morphology of the products during the methanol dominated reduction process, this work examined the preparation of molybdenum hemicarbide by reducing and carbonizing MoO3 with methanol. We used a combination of thermodynamic calculations, thermogravimetric analysis and XRD to clarify the reduction path of MoO3 at different temperatures, and characterized the morphology evolution of the product using SEM. Furthermore, a novel kinetic mathematical model was proposed to account for the intricate reaction involving methanol and multivalent oxide, which incorporated numerous parameters with practical physical significance, enhancing its predictive precision. In summary, our findings provide valuable insights into thermodynamics, kinetics, and experimental aspects of reactions involving methanol, offering guidance for controlled carbide particle synthesis.