Secondary Reaction Pathways Research Articles

Operando Spectroelectrochemistry Unravels the Mechanism of CO2 Electrocatalytic Reduction by an Fe Porphyrin.

Iron porphyrins are molecular catalysts recognized for their ability to electrochemically and photochemically reduce carbon dioxide (CO2). The main reduction product is carbon monoxide (CO). CO holds significant industrial importance as it serves as a precursor for various valuable chemical products containing either a single carbon atom (C1), like methanol or methane, or multiple carbon atoms (Cn), such as ethanol or ethylene. Despite the long-established efficiency of these catalysts, optimizing their catalytic activity and stability and comprehending the intricate reaction mechanisms remain a significant challenge. This article presents a comprehensive investigation of the mechanistic aspects of the selective electroreduction of CO2 to CO using an iron porphyrin substituted with four trimethylammonium groups in the para position [(pTMA)FeIII-Cl]4+. By employing infrared and UV/Visible spectroelectrochemistry, changes in the electronic structure and coordination environment of the iron center can be observed in real-time as the electrochemical potential is adjusted, offering new insights into the reaction mechanisms. Catalytic species were identified, and evidence of a secondary reaction pathway was uncovered, potentially prompting a re-evaluation of the nature of the catalytically active species.

Integrated supercritical carbon dioxide extraction for efficient furfural production from xylose using formic acid as a catalyst

This study investigates the production of furfural via formic acid-catalyzed dehydration of xylose and the effect of simultaneous extraction of furfural using supercritical carbon dioxide (Sc-CO2). The addition of Sc-CO2 results in a secondary reaction pathway comprised of two steps: CO2-catalyzed isomerization of xylose into the reactive intermediate xylulose, followed by furfural production from xylulose, catalyzed by formic acid. Xylose dehydration with CO2 in both batch and semi-batch systems yielded a higher furfural yield and selectivity compared with systems without CO2. The Sc-CO2 extraction in a semi-batch system prevents furfural degradation by maintaining high productivity, even with increased initial xylose concentration. A maximum furfural yield of 68.5% (71.4% selectivity and 99% separation efficiency) was achieved after 5 h at 140 °C and 20 MPa with a constant flow rate of 5 g/min of CO2 and initial concentrations of 10 g/L of xylose and 10 wt% of formic acid.

Enhancing the high-spin reactivity in C-H bond activation by Iron (IV)-Oxo species: insights from paclitaxel hydroxylation by CYP2C8.

Previous theoretical studies have revealed that high-spin states possess flatter potential energy surfaces than low-spin states in reactions involving iron(IV)-oxo species of cytochrome P450 enzymes (P450s), nonheme enzymes, or biomimetic complexes. Therefore, actively utilizing high-spin states to enhance challenging chemical transformations, such as C-H bond activation, represents an intriguing research avenue. However, the inherent instability of high-spin states relative to low-spin states in pre-reaction complexes often hinders their accessibility around the transition state, especially in heme systems with strong ligand fields. Counterintuitively, our investigation of the metabolic hydroxylation of paclitaxel by human CYP2C8 using a hybrid quantum mechanics and molecular mechanics (QM/MM) approach showed that the high-spin sextet state exhibits unusually high stability, when the reaction follows a secondary reaction pathway leading to 6β-hydroxypaclitaxel. We thoroughly analyzed the factors contributing to the enhanced stabilization of the high-spin state, and the knowledge obtained could be instrumental in designing competent biomimetic catalysts and biocatalysts for C-H bond activation.

Particle-scale heat and mass transfer processes during the pyrolysis of millimeter-sized lignite particles with solid heat carriers

• Devolatilization kinetics is coupled with the heat and mass transfer processes. • Evolution of pore structure in coal pyrolysis with solid heat carrier is included. • Multi-physical fields inside a lignite particle during pyrolysis is elaborated. • The intraparticle mass transfer mechanism influences the secondary reaction. The study of mesoscale solid fuel pyrolysis behavior is a key bridge between the microscopic reaction kinetics and macroscopic multiphase reaction flow in a reactor. In this work, a one-dimensional nonstationary model was developed in spherical coordinate system for the pyrolysis of millimeter-scale lignite particles with solid heat carriers, in which a modified multistep kinetic model (MSM) was coupled with a series of correlative transient heat and mass conservation equations in conjunction with the dusty gas model (DGM). The MSM for coal pyrolysis was modified by adding a set of kinetic equations of pseudo-elementary secondary reactions, making the kinetic model comparable to the Chemical Percolation Devolatilization model in terms of accuracy. In addition, a semiempirical submodel for the evolution of pore structure parameters (porosity, pore size, and permeability) was incorporated to obtain the precise mechanism of volatile species transport in the porous coal matrix. The modified MSM and pore evolution submodel were validated against literature data. The coupling effects between intraparticle transport processes and transient devolatilization kinetics were investigated. It was concluded that notable multiphysical fields (pressure, concentration, and velocity) emerged within lignite particles during pyrolysis, governed by the heating history. In addition, it should be noted that the direction and mechanism of volatiles transfer within porous lignite particles varied during pyrolysis, which can significantly affect secondary reactions. Flow reversal and shifts in the dominant mass transfer mode influenced the secondary reaction pathway, in addition to the impact of volatiles flow velocity on the reaction degree.

Hydrothermal carbonization of glucose: Secondary char properties, reaction pathways, and kinetics

Hydrothermal carbonization (HTC) is a promising green route to synthesize carbon materials and valorize biomass waste. The intricacy behind the conversion is enormous, requiring investigations of the mechanisms starting from the fundamentals. Intending to contribute to the rudiments of HTC, this work provides a systematic insight into the decomposition of glucose during HTC. The interest in glucose stems from its nature, indeed it is: one of the building blocks of biomass, a substrate for producing advanced carbon materials, and a precursor of secondary char - the solid phase deriving from re-polymerization and condensation reactions from the liquid phase back onto solid. The work approaches the glucose HTC holistically, focusing on kinetics and characterization of the products. A 1.1 M glucose solution was hydrothermally carbonized over several operating conditions (180–270 °C and 0–8 h) with analysis of the solid and liquid phases. Results show that hydrochar changes significantly with time only at 180 °C. Above 180 °C, the solid yield stabilizes at 47–50 % and the carbon content at 67–70 %. The time-evolution of liquid intermediates (like HMF and small carboxylic acids) detected through NMR enabled the deduction of formation pathways and calibration of a higher-order kinetics model. Meanwhile, SEM and Raman spectra show hydrochars characterized by typical nano/microspheres, having a size distribution dependent on operating conditions and mainly an amorphous structure, with a tendency towards graphitization as the HTC severity increases. All the hydrochars show photoluminescence, a functionality that opens to several application possibilities.

Dynamic pyrolytic reaction mechanisms, pathways, and products of medical masks and infusion tubes

Given the COVID-19 epidemic, the quantity of hazardous medical wastes has risen unprecedentedly. This study characterized and verified the pyrolysis mechanisms and volatiles products of medical mask belts (MB), mask faces (MF), and infusion tubes (IT) via thermogravimetric, infrared spectroscopy, thermogravimetric-Fourier transform infrared spectroscopy, and pyrolysis-gas chromatography/mass spectrometry analyses. Iso-conversional methods were employed to estimate activation energy, while the best-fit artificial neural network was adopted for the multi-objective optimization. MB and MF started their thermal weight losses at 375.8 °C and 414.7 °C, respectively, while IT started to degrade at 227.3 °C. The average activation energies were estimated at 171.77, 232.79, 105.14, and 205.76 kJ/mol for MB, MF, and the first and second IT stages, respectively. Nucleation growth for MF and MB and geometrical contraction for IT best described the pyrolysis behaviors. Their main gaseous products were classified, with a further proposal of their initial cracking mechanisms and secondary reaction pathways.

Spatially-resolved reaction profiles in Fischer-Tropsch synthesis – influence of operating conditions and promotion for iron-based catalysts

The complex reaction mechanism of iron-based Fischer-Tropsch synthesis comprises of manifold primary and secondary reaction pathways. Within this work, an experimental setup for spatially-resolved measurements on promoted iron-based Fischer-Tropsch catalysts is presented, allowing for a general study of the influence of operating conditions and promotion as well as of the influence of secondary reactions on product selectivity. It was found that, depending on catalyst composition, considerable hydrogenation of 1-alcohols and olefins occurs, while no considerable influence of reinitiation of chain growth on product distribution could be found.

A novel approach in revealing mechanisms and particular step predictors of pH dependent tartrazine catalytic degradation in presence of Oxone®

The degradation of tartrazine in the presence of cobalt activated Oxone® (potassium peroxymonosulfate) was investigated at different initial pH values. Aluminum pillared clay had the role of a support for catalytically active cobalt oxide species. The degradation of tartrazine and the formation of a mixture of degradation products were monitored using the Ultraviolet–Visible (UV–Vis) spectroscopy and gas chromatography–mass spectrometry (GC-MS). The exact qualitative composition of this mixture and the determination of the most probable mechanism of degradation (the primary goal) were obtained using GC-MS. Besides, the main reaction pathway (reaction with SO4˙ˉ radical anion) and secondary pathways were proposed depending on the pH value. At pH = 6 the reaction with HO˙ radical was proposed. At pH = 11 decarboxilation was suggested as the first step of the secondary proposed reaction pathway. The combination of results acquired from the deconvolution of UV–Vis spectra and the theoretical UV–Vis spectra of degradation products, whose occurrence was predicted by quantum-chemical calculations, was proven to be beneficial for the identification of tartrazine degradation products and for defining UV–Vis predictors of particular degradation steps. An additional contribution of this paper, from the reactivity aspect, was the establishment of the critical structural demand for the radical degradation of any diazo compound. The existence of a hydrogen atom bound to a diazo group was found to be the essential prerequisite for the radical cleavage of diazo compounds.

A Secondary Reaction Pathway for the Alumina Atomic Layer Deposition Process with Trimethylaluminum and Water, Revealed by Full-Range, Time-Resolved In Situ Mass Spectrometry

A method to obtain full mass over charge (m/z), time-resolved quadruple mass spectrometry (QMS) spectra of an atomic layer deposition (ALD) cycle is proposed. This method allows one to circumvent t...

Pyrolysis chemistry of n-propylcyclohexane via experimental and modeling approaches

Pyrolysis of hydrocarbons has a variety of practical applications in energy utilization and conversion. The pyrolysis study on n-propylcyclohexane (n-PCH) was conducted in a flow reactor by using the synchrotron VUV photoionization mass spectrometry. The temperature in this work ranged from 950 to 1300 K, and the pressures were controlled at 30 and 760 Torr with the initial molar concentration of the fuel kept at 0.5% in gaseous mixture. Quantitative measurements were conducted to obtain the mole fractions for over thirty pyrolytic species. The experimental data were compared with the predictions from the newly developed n-PCH kinetic model and previous models. The present model well predicts the chemistry behavior of the n-PCH pyrolysis, including fuel conversion, production of olefins and diolefins, and growth of aromatic species. The primary decomposition of the fuel and the secondary reaction pathways for the cyclic C9H17 and alkylcyclohexyl radicals with their accompanying reaction rates were discussed. Ethylene, propene and 1,3-butadiene were the highest produced intermediates under the investigated pyrolysis conditions. In addition, the high concentrations of light species and the six-membered structure of n-PCH have large effects on the formation of aromatic species, such as benzene, toluene and ethylbenzene.

Representative model compounds for understanding the pyrolytic behavior of pre-oxidized β-ether-type lignin

To achieve a better understanding of the pyrolysis behavior of pre-oxidized β-ether-type lignin, three Cα=O type dimers with different substituent groups on the aromatic ring were synthesized and analyzed by a simultaneous thermal analysis instrument (STA), in-situ Fourier transform infrared spectroscopy (in-situ FTIR), and pyrolysis-gas chromatography/ mass spectrometry (Py-GC/MS). The results showed that major primary pyrolysis reactions of Cα=O type models normally occurred at 200 to 400 °C, and connecting bridge structures of models were completely destroyed, causing the emission of abundant volatiles. Substituent groups of aromatic rings played direct roles in thermal stability of models, volatiles emission, product characteristics, and secondary reaction pathways of major primary products. Particularly, the aryl–OCH3 group clearly enhanced the reactivity of intramolecular linkages and was an important active functional group for secondary reactions. As major primary products and intermediates, guaiacol and 2-methoxy-benzaldehyde were formed via the cleavage of Cα–O and Cα–Cβ bonds and could also be converted into phenol, benzaldehyde, and 2-methylphenol via rearrangement of aryl–OCH3 into an aryl–CH3 group or –OCH3 group removal. Oxidization of benzylic alcohol to benzylic ketone not only simplified depolymerization pathways, but also resulted in better selectivity of phenolic monomers and a predictable product distribution.

Kinetics of Secondary Reactions Affecting the Organosolv Lignin Structure.

Many valorization approaches for lignin rely on its organic solvent (organosolv) extraction. However, the severity of the extraction conditions required to obtain high lignin extraction generally results in low-quality lignin for downstream processing. To better understand the secondary reaction pathways and kinetics related to molecular alterations that result from organosolv extraction under extreme conditions, extractions were conducted at temperatures of 150, 180, and 210 °C. Lignin was collected at residence times between 0.25 and 18 h and analyzed by NMR techniques to quantify the concentrations of key chemical moieties that appear or disappear upon reactions of lignin molecules during and after their fractionation from biomass. The kinetics of chemical moiety evolution was modeled as processes in-series. In these models, pseudo first-order kinetics were used to describe the change in concentration of chemical moieties on extracted lignin as a function of residence time.

Untangling the Formation of Methoxymethanol (CH3OCH2OH) and Dimethyl Peroxide (CH3OOCH3) in Star-forming Regions

Abstract Methoxymethanol (CH3OCH2OH) was recently detected toward the MM1 core in the high-mass star-forming region NGC 6334I. However, the underlying formation mechanisms of this complex organic molecule (COM) as well as its structural isomers ethylene glycol (HOCH2CH2OH) and the hitherto unobserved dimethyl peroxide (CH3OOCH3) are still elusive. Here, we report the very first confirmed synthesis of dimethyl peroxide—at various deuteration levels within interstellar analogous ices of D3-methanol (CD3OH) exposed to ionizing radiation at ultralow temperatures of 5 K. The discrimination of specific isomers is achieved by exploiting reflectron time-of-flight mass spectrometry coupled with isomer-selective photoionization of the subliming molecules in the temperature programmed desorption phase of the experiment. Based on the distribution of the identified species at distinct mass-to-charge ratios, we reveal primary and secondary reaction pathways to methoxymethanol, ethylene glycol, and dimethyl peroxide involving radical–radical recombination of methoxy (CH3O) and hydroxymethyl (CH2OH). Our findings help to constrain the formation mechanism of COMs detected within star-forming regions (methoxymethanol, ethylene glycol) and propose that the hitherto elusive dimethyl peroxide isomer represents an excellent candidate for future astronomical searches.

Experimental and theoretical studies on the thermal decomposition of metformin

This work is a first attempt to understand the mechanism of metformin thermal decomposition under inert conditions. Thermal gravimetric analysis coupled with mass spectrometry was used to probe the thermal degradation reactions. Density functional theory and second-order perturbation (MP2) theoretical calculations were used to construct a reaction mechanism for metformin decomposition. It was evident that the reactions are initiated via formation of methyl radicals, and ammonia via 1.3-H shift, followed by a series of different secondary reaction pathways. The formation of cyanamide, dimethylamine and HCN were among the main secondary products. The proposed mechanism is important for future treatment of wastewater containing metformin and similar drugs formulations, and their possible conversion to useful commodities.

Direct synthesis of H2O2 on PdZn nanoparticles: The impact of electronic modifications and heterogeneity of active sites

The direct synthesis of H2O2 (H2 + O2 → H2O2) would reduce the cost of H2O2, but few catalytic materials demonstrably provide the necessary combination of high H2O2 selectivity and stability over long periods of continuous reaction. Here, we show that silica supported PdZnx nanoparticles (where x indicates the bulk composition, 0 ≤ x ≤ 30) are stable and selective catalysts for the continuous production of H2O2 in the absence of liquid-phase promoters. Increasing the ratio of Zn to Pd precursors in the synthesis lead to distributions of nanoparticles that contain greater fractions of β1-Pd1Zn1 and lower fractions of face-centered cubic (fcc) Pd nanoparticles. A combination of the measured functional dependence of steady-state H2O2 and H2O formation rates on H2 and O2 pressures and the need for protic solvents indicate that H2O2 forms on all members of the PdZnx series by proton-electron transfer steps to dioxygen and hydroperoxy surface intermediates that saturate active sites during catalysis. Primary H2O2 selectivities increase systematically with the Zn to Pd ratio and range from 26% on fcc Pd nanoparticles to 69% on PdZn30 catalysts, which contains β1-Pd1Zn1 nanoparticles and lacks detectable amounts of fcc Pd. The differences in H2O2 selectivities among the PdZnx catalysts reflect changes in activation enthalpies (ΔH‡) for H2O2 (from -3 to 14 kJ mol−1) and H2O (11 to 44 kJ mol−1) formation. These results demonstrate that pathways for H2O2 formation are less sensitive than H2O formation to electronic differences between active sites on fcc Pd and intermetallic β1-Pd1Zn1 nanoparticles, whose densities of states differ significantly near the Fermi level. Comparisons of the ratios of H2O2 and H2O formation rates to ΔH‡ values among Pd and PdZnx catalysts show that the increases in H2O2 selectivities are less than predicted from the differences in ΔH‡ values alone. This analysis shows that the current synthesis method introduces at least two distinguishable catalytically active sites, and a portion of the sites formed predominantly cleave O-O bonds in primary and secondary reaction pathways that form H2O and lead to lower H2O2 selectivities than would be achieved in their absence. These findings demonstrate that differences in electronic structure of active sites cause β-Pd1Zn1 nanoparticles to give greater H2O2 selectivities than Pd catalysts and suggest that a synthesis procedure that uniformly creates β-Pd1Zn1 and alloys all Pd on the support may lead to increasingly selective conversion of O2 and H2 to H2O2.

Thermal degradation of medical plastic waste by in-situ FTIR, TG-MS and TG-GC/MS coupled analyses

In this paper, thermal degradation of medical plastic waste (the blends of medicinal plastic bottles and plastic infusion bag) that mainly composed of polystyrene (PS) and polypropylene (PP) is studied under both inert and oxidative atmospheres using in-situ FTIR, TG-MS and TG-GC/MS coupled analyses. Meanwhile, the gas evolution profiles as well as the function groups of the decomposition residues during medical plastic waste thermal degradation are also discussed. The aliphatic CH, aromatic CC and aromatic CH exhibit the dramatically vary with temperature, indicating the medical plastic waste begins vitrifying at about 100 °C, starts degrading around 300 °C and reaches to the maximum near 400 °C in inert atmosphere, produces mainly styrene, benzene, toluene, and small amounts of C1–C4 aliphatic hydrocarbons as the initial pyrolysis products. The aromatic compounds are mainly ascribed to PS degradation, and alkanes and alkenes are mainly originated from PP creaking. It is also found that the gaseous evolution profiles are well consistent with DTG curves in terms of appearance of peaks and relevant stages in the whole temperature range. Compared with thermal degradation of medical plastic waste in inert atmosphere, the initial degradation temperature for the medical plastic waste is shifted to lower temperature, while the degradation rate is reduced significantly in the oxidative atmosphere that produces oxygenated hydrocarbons such as acetic acid, phenol and benzoic acid due to the O-atom attack. Lastly, the initial creaking mechanism together with the secondary reaction pathways of the primary volatiles produced from medical plastic waste thermal degradation are also proposed.

A density functional theory study of the decomposition mechanism of nitroglycerin.

The detailed decomposition mechanism of nitroglycerin (NG) in the gas phase was studied by examining reaction pathways using density functional theory (DFT) and canonical variational transition state theory combined with a small-curvature tunneling correction (CVT/SCT). The mechanism of NG autocatalytic decomposition was investigated at the B3LYP/6-31G(d,p) level of theory. Five possible decomposition pathways involving NG were identified and the rate constants for the pathways at temperatures ranging from 200 to 1000K were calculated using CVT/SCT. There was found to be a lower energy barrier to the β-H abstraction reaction than to the α-H abstraction reaction during the initial step in the autocatalytic decomposition of NG. The decomposition pathways for CHOCOCHONO2 (a product obtained following the abstraction of three H atoms from NG by NO2) include O-NO2 cleavage or isomer production, meaning that the autocatalytic decomposition of NG has two reaction pathways, both of which are exothermic. The rate constants for these two reaction pathways are greater than the rate constants for the three pathways corresponding to unimolecular NG decomposition. The overall process of NG decomposition can be divided into two stages based on the NO2 concentration, which affects the decomposition products and reactions. In the first stage, the reaction pathway corresponding to O-NO2 cleavage is the main pathway, but the rates of the two autocatalytic decomposition pathways increase with increasing NO2 concentration. However, when a threshold NO2 concentration is reached, the NG decomposition process enters its second stage, with the two pathways for NG autocatalytic decomposition becoming the main and secondary reaction pathways.

Effects of Atmospheric Water on ·OH-initiated Oxidation of Organophosphate Flame Retardants: A DFT Investigation on TCPP.

Tris(2-chloroisopropyl) phosphate (TCPP), a widely used organophosphate flame retardant, has been recognized as an important atmospheric pollutant. It is notable that TCPP has potential for long-range atmospheric transport. However, its atmospheric fate is unknown, restricting its environmental risk assessment. Herein we performed quantum chemical calculations to investigate the atmospheric transformation mechanisms and kinetics of TCPP initiated by ·OH in the presence of O2/NO/NO2, and the effects of ubiquitous water on these reactions. Results show the H-abstraction pathways are the most favorable for the titled reaction. The calculated gaseous rate constant and lifetime at 298 K are 1.7 × 10-10 cm3molecule-1 s-1 and 1.7 h, respectively. However, when considering atmospheric water, the corresponding lifetime is about 0.5-20.2 days. This study reveals for the first time that water has a negative role in the ·OH-initiated degradation of TCPP by modifying the stabilities of prereactive complexes and transition states via forming hydrogen bonds, which unveils one underlying mechanism for the observed persistence of TCPP in the atmosphere. Water also influences secondary reaction pathways of selected TCPP radicals formed from the primary H-abstraction. These results demonstrate the importance of water in the evaluation of the atmospheric fate of newly synthesized chemicals and emerging pollutants.

Alkaline‐Earth‐Metal‐Catalyzed Thin‐Film Pyrolysis of Cellulose

AbstractThe conversion of lignocellulosic biomass to “bio‐oils” by thermochemical pyrolysis is a promising reactor technology for renewable chemicals and biofuels. Although the fundamental understanding of relevant catalysts within reacting biomass particles is only in its infancy, it is known that inorganic materials naturally present within biomass act as catalysts that limit the yield of bio‐oil and alter the product distribution. In this work, the effect of alkaline earth metals on cellulose pyrolysis chemistry was investigated to determine the catalytic effect on primary (transport‐free) and secondary (diffusion‐limited) reaction pathways. The catalytic materials included homogeneous metal ions Ca2+ and Mg2+ from their inorganic salts, Ca(NO3)2 and Mg(NO3)2, and their corresponding heterogeneous metal oxides, CaO and MgO. Although the oxides had a limited impact on cellulose pyrolysis chemistry, the metal ions altered the secondary reaction pathways of cellulose significantly under diffusion‐limited conditions common to lignocellulosic particles within industrial reactors.

DetectTLC: Automated Reaction Mixture Screening Utilizing Quantitative Mass Spectrometry Image Features.

Full characterization of complex reaction mixtures is necessary to understand mechanisms, optimize yields, and elucidate secondary reaction pathways. Molecular-level information for species in such mixtures can be readily obtained by coupling mass spectrometry imaging (MSI) with thin layer chromatography (TLC) separations. User-guided investigation of imaging data for mixture components with known m/z values is generally straightforward; however, spot detection for unknowns is highly tedious, and limits the applicability of MSI in conjunction with TLC. To accelerate imaging data mining, we developed DetectTLC, an approach that automatically identifies m/z values exhibiting TLC spot-like regions in MS molecular images. Furthermore, DetectTLC can also spatially match m/z values for spots acquired during alternating high and low collision-energy scans, pairing product ions with precursors to enhance structural identification. As an example, DetectTLC is applied to the identification and structural confirmation of unknown, yet significant, products of abiotic pyrazinone and aminopyrazine nucleoside analog synthesis. Graphical Abstract ᅟ.