Dehydrogenation Pathway Research Articles

Unveiling a unique outer-sphere pathway in manganese-catalyzed acceptorless dehydrogenation reaction.

First-row transition metals are widely used in acceptorless dehydrogenative coupling reactions, making the detailed investigation of their catalytic mechanisms crucial for the rational design of efficient catalysts. In this study, density functional theory (DFT) was employed to explore the mechanism of an acceptorless dehydrogenation reaction catalyzed by a bifunctional complex based on a high-spin Mn(II) center. The computational results reveal that the Mn(II) catalyst follows a novel dehydrogenation pathway, where the external base KOH first coordinates to the metal center, and dehydrogenation proceeds via an outer-sphere mechanism. The hydride transfer step is identified as the rate-determining step, with an energy barrier of 26.3 kcal mol-1, consistent with experimental results. To further investigate the selectivity of the mechanism, energy decomposition analysis (EDA) and extended transition state-natural orbitals for chemical valence (ETS-NOCV) analysis were conducted on key transition states. The results show that the small steric hindrance and strong orbital interactions of the external base KOH are the key factors contributing to the selectivity of this mechanism. These findings not only deepen our understanding of the reaction mechanism but also provide valuable theoretical insights for the design and optimization of future acceptorless dehydrogenation catalysts.

Exploring tribochemical transduction pathways for dehydrogenation of molecular hydrides

Operando investigation of ethane-1,2-diamineborane (EDAB): no tribochemical dehydrogenation but possible secondary reactions of bond cleavage.

Engineering Topological and Chemical Disorder in Pd Sites for Record-Breaking Formic Acid Electrocatalytic Oxidation.

Designing palladium-based formic acid oxidation reaction (FAOR) catalysts to achieve significant breakthroughs in catalytic activity, pathway selectivity, and toxicity resistance is both urgent and challenging. Here, these challenges are addressed by pioneering a novel catalyst design that incorporates both topological and chemical disorder, developing a new class of PdCuLaYMnW high-entropy amorphous alloys with a porous network (Net-Pd-HEAA) as a highly active, selective, and stable FAOR electrocatalyst. This novel Net-Pd-HEAA demonstrates record-breaking FAOR performance, achieving the mass and specific activities of 5.94 A mgPd -1 and 8.94mA cm-2, respectively, surpassing all previously reported Pd-based catalysts and showing strong competitiveness against advanced Pt-based catalysts. Simulataneously, Net-Pd-HEAA exhibits extraordinary stability in accelerated durability tests (ADT) and chronoamperometry (CA) tests. Advanced characterization and in situ, spectral analysis reveal that the extremely disordered atomic structure effectively regulates the geometric and electronic structure of the Pd sites, enhancing active intermediate coverage, facilitating dehydrogenation pathway, and inhibiting the production/adsorption of CO. Furthermore, when employed as the anode catalyst in proton exchange membrane water electrolysis (PEMWE), Net-Pd-HEAA only requires a potential of 1.28V to obtain a current density of 1 A cm-2, and operates stably in a highly corrosive electrolyte for over 100 h.

Surface reactions of ethanol on UO2 thin film. Dehydrogenation and dehydration pathways

The reaction of ethanol has been investigated on a UO2 thin film by temperature programmed desorption. Two channels for ethanol desorption are identified. The first, in the 250–500 K region, is coverage dependent while the second with a maximum peak temperature (Tp) at ca. 630 K is not. The desorption energy, Ed, of the second channel is found to be equal to ca. 150 kJ/mol with a prefactor of 1012 s−1 and a desorption order n = 2. This is attributed to surface ethoxides re-combinative desorption. This second desorption channel is accompanied by the desorption of acetaldehyde (and hydrogen) and ethylene (and water). Acetaldehyde (CH3CHO) desorption, produced by the dehydrogenation of ethoxides, was sensitive to surface coverage. Its Tp changed from 640 K at θ = 0.06 to 610 K at θ = 1, while ethylene (CH2CH2) desorption Tp, produced by the dehydration of ethoxides did not shift. The molar ratio of these two products (CH3CHO/CH2CH2) of 0.8 is similar to that previously found on a UO2(1 1 1) single crystal and fits with the UO bonding nature that contains a non-negligible fraction of covalency.

Metabolic pathways, pharmacokinetic, and brain neurochemicals effects of capsaicin: Comprehensively insights from in vivo studies

BackgroundCapsaicin (CAP), a prominent component of chili pepper known for its potent agonistic effects on TRPV1, has attracted significant attention for its diverse physiological effects. Nevertheless, there remains a paucity of data concerning its in vivo distribution, metabolism, pharmacodynamic properties, and influence on the metabolic profile of the brain. MethodsStable isotope tracing, in vitro enzyme incubation, microdialysis coupled with UHPLC-MS/MS techniques were employed to investigate the in vivo metabolic pathways, distribution, and pharmacokinetic properties of CAP, and the potential biases in metabolic pathways was elucidate through molecular docking. Furthermore, the effect of CAP on brain metabolic profiles was assessed using untargeted metabolomics, and spatial visualization analysis was conducted through mass spectrometry imaging. ResultsCAP was distributed predominantly in the kidneys, with lower content in the liver, heart, lungs, brain, and spleen following peripheral administration, and the absorption half-life in the body was about 20 min. CAP primarily underwent alkyl terminal dehydrogenation, hydroxylation, and macrocyclization metabolic pathways under the action of CYP2C9, CYP2C19 and CYP2D6, resulting in at least four metabolites. Among them, the hydroxylation products were main metabolites and the dehydrogenation product 16,17-dihydrocapsaicin could interact with the key binding sites Leu515 and Thr550 of TRPV1 like CAP. CAP quickly diffused to various brain regions and the metabolic characteristics in the striatum were relatively different from that in the blood. The distribution of CAP in the brain primarily triggered the release of neurotransmitters in areas associated with reward, cognition, and memory. Both acute and chronic exposure to CAP elevated amino acid levels in cortical regions, while producing contrasting effects on nucleotide metabolites. ConclusionThis study offers an initial in-depth analysis of the distribution patterns, metabolic pathways and pharmacodynamic properties of CAP in the body and brain. These findings established a basis for further studies on CAP's pharmacology properties and its influence on the central nervous system.

Tandem Methanolysis and Catalytic Transfer Hydrogenolysis of Polyethylene Terephthalate to p‐Xylene over Cu/ZnZrOx Catalysts

We demonstrate a novel approach of utilizing methanol (CH3OH) in a dual role for (1) the methanolysis of polyethylene terephthalate (PET) to form dimethyl terephthalate (DMT) at near‐quantitative yields (~97%) and (2) serving as an in‐situ H2 source for the catalytic transfer hydrogenolysis (CTH) of DMT to p‐xylene (PX, ~63% at 240 °C and 16 h) on a reducible ZnZrOx supported Cu catalyst (i.e., Cu/ZnZrOx). Pre‐ and post‐reaction surface and bulk characterization, along with density functional theory (DFT) computations, explicate the dual role of the metal‐support interface of Cu/ZnZrOx in activating both CH3OH and DMT and facilitating a lower free‐energy pathway for both CH3OH dehydrogenation and DMT hydrogenolysis, compared to Cu supported on a redox‐neutral SiO2 support. Loading studies and thermodynamic calculations showed that, under reaction conditions, CH3OH in the gas phase, rather than in the liquid phase, is critical for CTH of DMT. Interestingly, the Cu/ZnZrOx catalyst was also effective for the methanolysis and hydrogenolysis of C‐C bonds (compared to C‐O bonds for PET) of waste polycarbonate (PC), largely forming xylenol (~38%) and methyl isopropyl anisole (~42%) demonstrating the versatility of this approach toward valorizing a wide range of condensation polymers.

Tandem Methanolysis and Catalytic Transfer Hydrogenolysis of Polyethylene Terephthalate to p-Xylene Over Cu/ZnZrOx Catalysts.

We demonstrate a novel approach of utilizing methanol (CH3OH) in a dual role for (1) the methanolysis of polyethylene terephthalate (PET) to form dimethyl terephthalate (DMT) at near-quantitative yields (~97 %) and (2) serving as an in situ H2 source for the catalytic transfer hydrogenolysis (CTH) of DMT to p-xylene (PX, ~63 % at 240 °C and 16 h) on a reducible ZnZrOx supported Cu catalyst (i.e., Cu/ZnZrOx). Pre- and post-reaction surface and bulk characterization, along with density functional theory (DFT) computations, explicate the dual role of the metal-support interface of Cu/ZnZrOx in activating both CH3OH and DMT and facilitating a lower free-energy pathway for both CH3OH dehydrogenation and DMT hydrogenolysis, compared to Cu supported on a redox-neutral SiO2 support. Loading studies and thermodynamic calculations showed that, under reaction conditions, CH3OH in the gas phase, rather than in the liquid phase, is critical for CTH of DMT. Interestingly, the Cu/ZnZrOx catalyst was also effective for the methanolysis and hydrogenolysis of C-C bonds (compared to C-O bonds for PET) of waste polycarbonate (PC), largely forming xylenol (~38 %) and methyl isopropyl anisole (~42 %) demonstrating the versatility of this approach toward valorizing a wide range of condensation polymers.

High purity H2 resource from methanol steam reforming at low-temperature by spinel CuGa2O4 catalyst for fuel cell

Methanol steam reforming (MSR) is an effective way to provide high purity hydrogen for PEMFCs, however the main challenge is the toxic effect of CO. In this study, CuGa2O4 spinel catalyst was applied first for MSR at low temperature. The properties of catalysts were characterized by various techniques, the MSR catalytic performance was also studied in deeply. The surface of the catalyst is composed of particle chains that are interconnected, forming high-volume pores that increase the specific surface area. The catalyst also has a rich porous structure, exposing more active sites. Furthermore, the presence of oxygen vacancies facilitates the adsorption of reactive oxygen species, reducing CO generation. A possible pathway for methanol dehydrogenation has been determined using DFT calculations. The relatively low overall energy barrier on the catalyst surface facilitates methanol activation. Among the steps, formaldehyde dehydrogenation is the rate-determining step in the methanol dehydrogenation process. The CuGa2O4 catalyst exhibits good gas selectivity, with a hydrogen selectivity of 98 % and CO selectivity of 0. and no deactivation occurred within 50 h, demonstrating outstanding durability. These features allow it to serve as an efficient catalyst for MSR online hydrogen production and direct supply to PEMFCs.

Mechanism regulation over dual-atom catalyst enables high-performance oxidative alcohol esterification

The development of heterogeneous catalysts with well-defined uniform isolated or multiple active sites is of great importance for understanding catalytic performances and studying reaction mechanisms. Herein, we present a CoCu dual-atom catalyst (CoCu-DAC) where bonded Co–Cu dual-atom sites are embedded in N-doped carbon matrix with a well-defined Co(OH)CuN6 structure. The CoCu-DAC exhibits higher catalytic activity and selectivity than the Co single-atom catalyst (Co-SAC) and Cu single-atom catalyst (Cu-SAC) counterparts in the catalytic oxidative esterification of alcohols and a variety of methyl and alkyl esters have been successfully synthesized. Kinetic studies reveal that the activation energy (29.7 kJ mol−1) over CoCu-DAC is much lower than that over Co-SAC (38.4 kJ mol−1) and density functional theory (DFT) studies disclose that two different mechanisms are regulated over CoCu-DAC and Co-SAC/Cu-SAC in three-step esterification of alcohols. The bonded Co–Cu and adjacent N species efficiently catalyze the elementary reactions of alcohol dehydrogenation, O2 activation and ester formation, respectively. The stepwise alkoxy pathway (O–H and C–H scissions) is preferred for both alcohol dehydrogenation and ester formation over CoCu-DAC, while the progressive hydroxylalkyl pathway (C–H and O–H scissions) for alcohol dehydrogenation and simultaneous hemiacetal dehydrogenation are favored over Co-SAC and Cu-SAC. Characteristic peaks in the Fourier transform infrared spectroscopy analysis may confirm the formation of the metal-C intermediate and the hydroxylalkyl pathway over Co-SAC.

Sequential Synthesis of Alkenylated Quinazolines Through sp3 C−H Functionalization and Their Photophysical Properties

AbstractQuinazolines are biologically potent heterocyclic compounds; however, their synthesis poses high challenges. In this present study, we have developed an inexpensive cobalt metal‐catalyzed acceptorless dehydrogenative pathway for the sequential synthesis of alkenylated quinazolines from benzhydrol via dehydrogenation, cyclization, and subsequent dehydrogenative sp3 C−H functionalization. This transformation holds potential owing to its commercially available key starting materials, good functional tolerance, moderate reaction conditions, environmental friendliness, and applicability for gram‐scale synthesis. Furthermore, the synthetic utility of the synthesized derivatives has been explored through various reactions, including reduction, halogenation, methoxylation, Michael addition, and spiro cyclization. Additionally, the photophysical properties indicate that the nitrogen atom of the synthesized derivatives readily undergoes a reversible protonation, leading to significant changes in colour. This characteristic presents an opportunity for the creation of pH sensors with colourimetric capabilities.

Molecular Iridium Catalyzed Electrochemical Formic Acid Oxidation: Mechanistic Insights.

Electrochemical formic acid oxidation reaction (FAOR) is a pivotal model for understanding organic fuel oxidation and advancing sustainable energy technologies. Here, we present mechanistic insights into a novel molecular-like iridium catalyst (Ir-N4-C) for FAOR. Our studies reveal that isolated sites facilitate a preferential dehydrogenation pathway, circumventing catalyst poisoning and exhibiting high inherent activity. In situ spectroscopic analyses elucidate that weakly adsorbed intermediates mediate the FAOR and are dynamically regulated by potential-dependent redox transitions. Theoretical and experimental investigations demonstrate a parallel mechanism involving two key intermediates with distinct pH and potential sensitivities. The rate-determining step is identified as the adsorption of formate via coupled or sequential proton-electron transfer, which aligns well with the observed kinetic properties, pH dependence, and hydrogen/deuterium isotope effects in experiments. These findings provide valuable insights into the reaction mechanism of FAOR, advancing our understanding at the molecular level and potentially guiding the design of efficient catalysts for fuel cells and electrolyzers.

Molecular Iridium Catalyzed Electrochemical Formic Acid Oxidation: Mechanistic Insights

Electrochemical formic acid oxidation reaction (FAOR) is a pivotal model for understanding organic fuel oxidation and advancing sustainable energy technologies. Here, we present mechanistic insights into a novel molecular‐like iridium catalyst (Ir‐N4‐C) for FAOR. Our studies reveal that isolated sites facilitate a preferential dehydrogenation pathway, circumventing catalyst poisoning and exhibiting high inherent activity. In‐situ spectroscopic analyses elucidate that weakly adsorbed intermediates mediate the FAOR and are dynamically regulated by potential‐dependent redox transitions. Theoretical and experimental investigations demonstrate a parallel mechanism involving two key intermediates with distinct pH and potential sensitivities. The rate‐determining step is identified as the adsorption of formate via coupled or sequential proton‐electron transfer, which aligns well with the observed kinetic properties, pH dependence, and hydrogen/deuterium isotope effects in experiments. These findings provide valuable insights into the reaction mechanism of FAOR, advancing our understanding at the molecular level and potentially guiding the design of efficient catalysts for fuel cells and electrolyzers.

CO2 reduction via oxidative dehydrogenation and dry reforming of ethane over Fe3Ni1 nanoparticles: The influence of the oxide support

The effect of the CO2:C2H6 feed ratio, the relative Lewis acidity of reducible and unreducible catalytically active metal oxide supports with and without Fe3Ni1 alloy nanoparticles on the activity, selectivity and stability for the CO2-mediated oxidative dehydrogenation of ethane (CO2-ODHE) is investigated. To circumvent the influence of the typically dissimilar textural properties of the supports in bulk form, overlayer oxide supports of V, Cr, Ga, Ti or Sm coated on a common γ-Al2O3 carrier were employed. Separately, (Ni0.75Fe0.25)Fe2O4 precursor nanoparticles were synthesized via a nonaqueous surfactant-free method, sonication-deposited onto supports and reduced in situ into an Fe3Ni1 alloy microstructure of bcc and fcc mixed phases captured with in situ XRD. When exposed to carbon dioxide at 255 °C, a selective re-oxidation of the bcc phase via CO2 dissociation is observed, while the fcc phase stays stable and only partially re-oxidizes above 525 °C. Upon exposure to CO2-ODHE conditions, the initial activity of the bare supports increases with increasing acid site strength, but this activity is rapidly lost in case of the strongly acidic supports. Comparison of the C2H4 and CO selectivity indicate direct dehydrogenation is preferred over the oxidative dehydrogenation pathway and is initially occurring in combination with some CO-forming routes, possibly the dry reforming of C2H6. This CO forming route is significant over the most acidic and reducible VOx@Al2O3 support in the early stages of operation. The addition of the Fe3Ni alloy increases the conversions of both C2H6 and CO2 across all supports, with a notably stronger effect observed on CO2 conversion especially over the highly acidic and reducible VOx@Al2O3 and CrOx@Al2O3. As a result, the CO selectivity is increased due to ethane dry reforming activity over the latter supports while CO2-ODHE activity is observed over the supports with intermediate and weak acid sites.

Dehydrogenation of primary alcohols to carboxylic acids in water catalyzed by PNHN-manganese(I) carbonyl complexes

Three classes of manganese(I) complex cation, [(PNHN)Mn(CO)3]Br (Mn1), [(SNHN)Mn(CO)3]Br (Mn2) and [(NRNHN)Mn(CO)3]Br (Mn3 (NMe), Mn4 (NEt), Mn5 (NiPr)), all incorporating a chelating 5,6,7,8-tetrahydro-8-quinolinamine (NHN) but distinct in their third donor atom, have been assessed as catalysts for the dehydrogenative oxidation of primary alcohols to form their corresponding carboxylic acids. Using water as solvent and NaOH as base, PNHN-containing Mn1 proved the most effective allowing both aromatic and aliphatic alcohols to be converted at 160 °C with excellent functional group tolerance (25 examples disclosed). A possible mechanism for this process, that makes use of an acceptorless dehydrogenation pathway, has been proposed and supported by targeted experimentation. Overall, these catalysts show great promise for applications in atom-economic carboxylic acid synthesis as well as in the development of organic hydride-based hydrogen storage systems.

CO2 reforming of ethane using Ni-La intermetallic sites within a nanocapsule framework

CO2 reforming of light alkanes from unconventional gas resources (shale gas/coalbed gas) presents an attractive route to achieve CO2 utilization and valuable chemical production. However, it remains challenging in both high stability and product selectivity due to the inevitable strong competition between dry reforming and oxidative dehydrogenation pathways. We report here a La modified Ni@SiO2 nanocapsule as the thermal-catalyst for CO2 reforming of ethane, achieving complete ethane conversion and remarkable CO2 conversion of 86.5 %, with stability for more than 200 h experiencing no carbon deposition. Detailed kinetic study, in situ characterizations and DFT calculation results revealed that the formation of NiLa intermetallic sites improved the adsorption of active oxygen species and activation of C2H6, which stabilized the key intermediate CH3CH2O* as well as the subsequent C2* cracking to CHx(O)*, leading to high selectivity toward syngas. Meanwhile, the removal of carbon deposits on Ni-La2O3 interfaces are faster than Ni surface. The synergism of spatial confinement structure provided by the nanocapsule with sufficient mechanical strength together with the chemical Ni-La2O3 interface of the enwrapped Ni-La mixed metal (oxide) NPs can timely eliminate C2* intermediates, which results in the most effective anti-coking ability during ethane reforming. This work highlights a tangible process towards unconventional (C2+) gas utilization with fine-tunable nanoreactor catalysts.

The reaction of nonacarbonyldiiron with 8‐alkynyloxyquinolines: Alkynyl activation, cyclization via C‐N bond formation, and C‐H bond cleavage

In this work, the reactions of nonacarbonyldiiron with 8‐alkynyloxyquinoline ligands (L1‐L3) and their resultant iron carbonyl complexes (1a, 1b, 2, and 3) are described. The complexes including the ligands were fully characterized by using a variety of spectroscopic techniques. The four complexes were also crystallographically determined. In the reactions, the quinoline N atom underwent cyclization to form a quinolinium skeleton while the C‐H bond of the quinoline at position 2 or the substituted methyl group was cleaved to form Fe‐C bond(s). Meanwhile, the alkynyl triple bond was reduced to a single bond (1a) or double bonds (1b, 2, and 3) to further coordinate the iron in σ‐bonds or η‐bonds. Particularly, pathways of hydrogenation (1a and 2) and/or dehydrogenation (1b and 3) were accompanied by the reactions. Moreover, the oxidation states of the iron centers in these complexes were identified through analyses of their bond formation, FTIR and NMR spectroscopic signals (1H NMR and DEPT‐135) in combination with their diamagnetic nature, that is, iron (0) for 2 and iron (I) for the rest of the diiron complexes.

KOtBu Mediated Alcohol Dehydrogenation Strategy: Synthesis of 2‐Aryl Quinazolinones

AbstractHerein we report an atom‐economical, transition metal‐free method for synthesizing 2‐aryl quinazolinones through a cascade annulation of 2‐amino benzamide and benzyl alcohol. The reaction proceeds via KOtBu‐mediated acceptorless alcohol dehydrogenation pathways. The procedure tolerates a wide variety of functional groups and provides a convenient method for the synthesis of 2‐aryl quinazolinones. Mechanistic insights by experiments and DFT calculations lead to unveil the reaction mechanism.

Molybdenum Catalyzed Acceptorless Dehydrogenation of Alcohols for the Synthesis of Quinolines

AbstractThe first molybdenum triazolylidene complexes catalyzing the atom‐economical synthesis of quinolines through acceptorless dehydrogenative coupling of alcohols is reported. A new family of Mo(0) complexes bearing chelating bis‐1,2,3‐triazolylidene, pyridyl‐1,2,3‐triazolylidene, and bis‐triazole ligands have been prepared and applied as catalysts for the synthesis of quinolines. Interestingly, Mo complexes bearing bis‐1,2,3‐triazolylidene ligands with alkyl groups (Et, n‐Bu) displayed superior catalytic activities than those containing aryl substituents on the triazolylidene rings. Control experiments corroborated that the catalytic reaction involves the dehydrogenation pathway.

Detailed kinetic modeling of catalytic oxidative coupling of methane

The oxidative coupling of methane (OCM) at high temperature over monolithic Pt-based catalysts operated at short contact times is investigated as an attractive method for methane upgrading to higher value products like ethylene and acetylene. An experimental measurement campaign aiming at elucidating the effect of operation parameters on the catalyst performance revealed that lower N2 dilution, lower CH4/O2 ratio, and higher space velocities promote high C2 yields. A maximum C2 yield of 10 % with 94 % CH4 conversion was obtained at a CH4/O2 ratio of 1.1, 50 % N2 dilution, and a space velocity of 4.5 × 105 h−1. Since both heterogenous and homogenous gas-phase chemistry together are required for determining C2 formation pathways, a detailed OCM surface reaction mechanism over Pt is presented consisting of 26 species and 86 reactions that are consistent from the view of thermodynamics and micro-kinetic reversibility. The combination of this OCM surface mechanism with a detailed gas-phase mechanism allows a numerical micro-kinetic description of the experimental measurements. The simulations presented in this study suggest that C2 formation takes place in the gas-phase at temperature above 1200 K with both oxidative and pyrolytic pathways for methane dehydrogenation to form CH3 radicals, whose coupling results in C2H6, C2H4 and C2H2 formation. Furthermore, the simultaneous presence of sufficient oxygen content and heat are vital for high C2 species yields.

Electronic perturbation-promoted interfacial pathway for facile C–H dissociation

Interface has often played significant role in the behavior of metal-based catalysts during various heterogeneous reactions. Herein, we reported a novel interfacial pathway for propane dehydrogenation on Pt-GaOx dual sites. The active ensemble, composing Pt particles and Ga2O3 clusters, facilitates the facile dissociation of C–H bonds (energy barrier < 0.30 eV) through H-abstraction by O sites, coupled with alkyl stabilization on the Pt surface. Notably, the electronic perturbation of the Pt slab induces higher-lying O 2p states at the interface, accompanied by a substantial density of states around fermi level. This is in stark contrast to the O species present in Ga2O3 crystals or clusters alone, leading to an exceptionally strong affinity for H and a robust ability for C–H scission. The Pt/Ga-Al2O3 catalysts prepared with Pt nanoparticles decorated by Ga oxide exhibit superior performance compared to Pt/Al2O3 and PtSn/Al2O3 catalysts. This insight into interfacial catalysis contributes to a broader understanding of the origin of the high catalytic efficiency observed in metal-based catalysts.