C-H bond activation of propane on Ga_(2)O_(2)^(2+) in Ga/H-ZSM-5 and its mechanistic implications

Propane dehydrogenation(PDH)on Ga/H-ZSM-5 catalysts is a promising reaction for propylene production,while the detail mechanism remains debatable.Ga_(2)O_(2)^(2+) stabilized by framework Al pairs have been identified as the most active species in Ga/H-ZSM-5 for PDH in our recent work.Here we demonstrate a strong correlation between the PDH activity and a fraction of Ga_(2)O_(2)^(2+) species corresponding to the infrared GaH band of higher wavenumber(GaHHW)in reduced Ga/H-ZSM-5,instead of the overall Ga_(2)O_(2)^(2+) species,by employing five H-ZSM-5 supports sourced differently with comparable Si/Al ratio.This disparity in Ga_(2)O_(2)^(2+) species stems from their differing capacity in completing the catalytic cycle.Spectroscopic results suggest that PDH proceeds via a two-step mechanism:(1)C-H bond activation of propane on H-Ga_(2)O_(2)^(2+) species(rate determining step);(2)β-hydride elimination of adsorbed propyl group,which only occurs on active Ga_(2)O_(2)^(2+) species corresponding to GaHHW.

A Mechanistic Switch in C-H Bond Activation by Elusive Fe^(V)(O)(TAML)Reaction Intermediate:A Theoretical Study

The divergent behavior of C-H bond oxidations of aliphatic substrates compared to those of aromatic substrates shown in Gupta’s experiment was mechanistically studied herein by means of density functional theory calculations.Our calculations reveal that such difference is caused by different reaction mechanisms between two kinds of substrates(the aliphatic cyclohexane,2,3-dimethylbutane and the aromatic toluene,ethylbenzene and cumene).For the aliphatic substrates,C-H oxidation by the oxidant Fe^(V)(O)(TAML)is a hydrogen atom transfer process;whereas for the aromatic substrates,C-H oxidation is a proton-coupled electron transfer(PCET)process with a proton transfer character on the transition state,that is,a proton-coupled electron transfer process holding a proton transfer-like transition state(PCET(PT)).This difference is caused by the strongπ-πinteractions between the tetra-anionic TAML ring and the phenyl ring of the aromatic substrates,which has a“pull”effect to make the electron transfer from substrates to the Fe=O moiety inefficient.

Photocatalytic C-H Bonds Activation:Ambient Carbonylation of Cyclohexane by Co(acac)_2 Under Ultraviolet Irradiation

A high turnover number was achieved in the photocatalytic carbonylation of C - H bonds of cyclohexane catalyzed by Co (acac)2 under ambient conditions (1 atm,25℃) to give mainly cyclohexanecarboxaldehyde.

Competition between C-C and C-H Activation in Reactions of Neutral Nickel Atom with Cycloalkanes （n = 3-7）

A theoretical investigation of the reaction mechanisms for C-H and C-C bond activation processes in the reaction of Ni with cycloalkanes C,,H2. （n = 3-7） is carried out. For the Ni ＋ CnH2, （n = 3, 4） reactions, the major and minor reaction channels involve C-C and C-H bond activations, respectively, whereas Ni atom prefers the attacking of C-H bond over the C-C bond in CnH2n （n = 5=7）. The results are in good agreement with the experimental study. In all cases, intermediates and transition states along the reaction paths of interest are characterized, It is found that both the C-H and C-C bond activation processes are proposed to proceed in a one-step manner via one transition state. The overall C-H and C-C bond activation processes are exothermic and involve low energy barriers, thus transition metal atom Ni is a good mediator for the activity of cycloalkanes CnH2n （n = 3 -7）.

Synthesizing active and durable cubic ceria catalysts(<6 nm)for fast dehydrogenation of bio-polyols to carboxylic acids coproducing green H_(2)

Dehydrogenation is considered as one of the most important industrial applications for renewable energy.Cubic ceria-based catalysts are known to display promising dehydrogenation performances in this area.Large particle size(>20 nm)and less surface defects,however,hinder further application of ceria materials.Herein,an alternative strategy involving lactic acid(LA)assisted hydrothermal method was developed to synthesize active,selective and durable cubic ceria of<6 nm for dehydrogenation reactions.Detailed studies of growth mechanism revealed that,the carboxyl and hydroxyl groups in LA molecule synergistically manipulate the morphological evolution of ceria precursors.Carboxyl groups determine the cubic shape and particle size,while hydroxyl groups promote compositional transformation of ceria precursors into CeO_(2) phases.Moreover,enhanced oxygen vacancies(Vo)on the surface of CeO_(2) were obtained owing to continuous removal of O species under reductive atmosphere.Cubic CeO_(2) catalysts synthesized by the LA-assisted method,immobilized with bimetallic PtCo clusters,exhibit a record high activity(TOF:29,241 h^(-1))and Vo-dependent synergism for dehydrogenation of bio-derived polyols at 200℃.We also found that quenching Vo defects at air atmosphere causes activity loss of PtCo/CeO_(2) catalysts.To regenerate Vo defects,a simple strategy was developed by irradiating deactivated catalysts using hernia lamp.The outcome of this work will provide new insights into manufacturing durable catalyst materials for aqueous phase dehydrogenation applications.

A density functional theory study of methane activation on MgO supported Ni_(9)M_(1) cluster:role of M on C-H activation

Methane activation is a pivotal step in the application of natural gas converting into high-value added chemicals via methane steam/dry reforming reactions.Ni element was found to be the most widely used catalyst.In present work,methane activation on MgO supported Ni–M(M=Fe,Co,Cu,Pd,Pt)cluster was explored through detailed density functional theory calculations,compared to pure Ni cluster.CH_(4)adsorption on Cu promoted Ni cluster requires overcoming an energy of 0.07 eV,indicating that it is slightly endothermic and unfavored to occur,while the adsorption energies of other promoters M(M=Fe,Co,Pd and Pt)are all higher than that of pure Ni cluster.The role of M on the first C–H bond cleavage of CH_(4)was investigated.Doping elements of the same period in Ni cluster,such as Fe,Co and Cu,for C–H bond activation follows the trend of the decrease of metal atom radius.As a result,Ni–Fe shows the best ability for C–H bond cleavage.In addition,doping the elements of the same family,like Pd and Pt,for CH_(4)activation is according to the increase of metal atom radius.Consequently,C–H bond activation demands a lower energy barrier on Ni–Pt cluster.To illustrate the adsorptive dissociation behaviors of CH_(4)at different Ni–M clusters,the Mulliken atomic charge was analyzed.In general,the electron gain of CH_(4)binding at different Ni–M clusters follows the sequence of Ni–Cu(–0.02 e)<Ni(–0.04 e)<Ni–Pd(–0.08 e)<Ni–Pt(–0.09 e)<Ni–Co(–0.10 e)<Ni–Fe(–0.12 e),and the binding strength between catalysts and CH_(4)raises with the CH_(4)electron gain increasing.This work provides insights into understanding the role of promoter metal M on thermal-catalytic activation of CH_(4)over Ni/MgO catalysts,and is useful to interpret the reaction at an atomic scale.

Photocatalytic Methane Conversion:Insight into the Mechanism of C(sp^(3))-H Bond Activation

Mild and direct conversion of methane into high value-added products is a desired goal for chemistry,energy,and environment.The active species generated in the photocatalytic reaction system under mild conditions activate the inert C-H bond in methane and promote methane conversion.This review focuses on the developed mechanisms for C(sp^(3))-H bond activation in photocatalytic methane conversion,including radical and active site mechanisms.Particular emphasis is placed on the detailed summary of mechanism,characterization method,and corresponding application in photocatalytic methane conversion.We also discuss the challenges and prospects for future direction on C(sp^(3))-H bond activation mechanism in photocatalytic methane conversion.This review aims to promote the development of photocatalytic methane conversion and provides guidance for the design of high-efficiency photocatalysts.

甲烷C-H键催化活化研究新进展

简要综述了近年来甲烷C-H键活化的一些研究进展。重点评述了该领域理论和实验研究的进展及前景。参考文献46篇。

气相中二氧化钒活化甲烷C-H键的理论研究

运用密度泛函B3LYP/6-311++G(3df,3pd)//6-311G(2d,p)法研究了VO+2(1A1/3A')+CH4生成P1[V(OCH2)++H2O]和P2[(OCH2)++H2]的气相反应,揭示了VO+2活化甲烷的微观机理,研究结果表明生成P2的反应是主反应通道.并对影响反应机理和反应速率的势能面交叉现象进行了讨论.

Catalytic C-H activation-initiated transdiannulation:An oxygen transfer route to ring-fluorinated tricyclic γ-lactones

Catalytic C–H activation-initiated annulation reactions have emerged as a versatile strategy for the efficient construction of diverse ring structural units and complex cyclic molecules in synthetic chemistry.Herein,we describe a new Rh(Ⅲ)-catalyzed C–H activation-initiated transdiannulation reaction of N,Ndimethyl enaminones with gem-difluorocyclopropenes in the presence of H_(2)O,enabling a facile and oxygen transfer access to ring-fluorinated tricyclicγ-lactones with a 6-5 ring-junction tetrasubstituted stereocenter.This approach features bond-forming/annulation efficiency,good functional group tolerance and complete regioselectivity,which may include a complex process consisting of Rh(Ⅲ)-catalyzed C(sp2)–H activation,cyclic alkene insertion,defluorinated ring-opening of gem-difluorocyclopropane,intramolecular oxygen transfer,intramolecular cyclization and oxidative hydration.

Theoretical studies and industrial applications of oxidative activation of inert C-H bond by metalloporphyrin-based biomimetic catalysis

High costs and low catalytic efficiency of metalloporphyrins, which are an analogue of cytochrome P450 enzyme, are the bot-tlenecks in the industrialization of biomimetic hydrocarbon oxidation reactions. The basic principle and research technique of physical organic chemistry were applied to the process of biomimetic oxidation of hydrocarbon catalyzed by metalloporphyrins. This biomimetic technology could be adapted to bulk chemicals production by developing the new methods for efficient scale-up synthesis of metalloporphyrins, new pathways for molecular oxygen activation on an industrial scale and new approaches to elevate the catalytic efficiency of metalloporphyrins. This review mainly focuses on research carried out in our group.

Palladium-Catalyzed Formal [4＋2] Cycloaddition of Benzoic and Acrylic Acids with 1,3-Dienes via C-H Bond Activation： Efficient Access to 3,4-Dihydroisocoumarin and 5,6-Dihydrocoumalins

We report a palladium-catalyzed formal intermolecular [4＋2] cycloaddition of benzoic and acrylic acids with 1,3-dienes including the stock chemicals 1,3-butadiene and isoprene leading to synthetically useful 3,4-dihydroisocoumarins and 5,6-dihydrocoumalins. Stepwise C-H bond cleavage and annulation are likely involved in the reaction pathway. The synthetic potential of the methodology was demonstrated by two short derivatizations and total synthesis of natural product Clausamine B.

Palladium-catalyzed late-stage mono-aroylation of the fully substituted pyrazoles via aromatic C-H bond activation

The palladium-catalyzed late-stage aroylation of 4-methyl-1,5-diaryl-lH-pyrazole-3-carboxylates has been developed via direct and exclusive mono-Csp^2-H bond activation with broad substrate scope and good functional group tolerance. A dual-core dimeric palladacycle is confirmed by X-ray single crystal crystallography, and probably serves as an active species in the catalytic cycle.

Direct C-H Bond Activation of Benzoxazole and Benzothiazole with Aryl Bromides Catalyzed by Palladium（Ⅱ）-N-heterocyclic Carbene Complexes

Herein, we report that a series of novel palladium（II）-NHC complexes （NHC=N-heterocyclic carbene） were synthesized. The structures of all novel complexes were characterized by ^1H NMR, ^13C NMR, FT-IR spectroscopy and elemental analysis techniques. These palladium（ll）-NHC complexes were tested as efficient catalysts in the direct C-H bond activation of benzoxazole and benzothiazole with aryl bromides in the presence of 1 mol% catalyst loading at 150 ℃ for 4 h. Under the given conditions, various aryl bromides were successfully applied as the arylating reagents to achieve the 2-arylbenzoxazoles and 2-arylbenzothiazoles in acceptable to high yields.

Proposal of an Amide-Directed Carbocupration Mechanism for Copper-Catalyzed meta-Selective C-H Arylation of Acetanilides by Diaryliodonium Salts

We examined the puzzling mechanism for Cu-catalyzed meta-C-H arylation reaction of anilides by diaryliodonium salts through systematic theoretical analysis. The previously proposed anti-oxy-cupration mechanism featuring anti-1,2- or anti-1,4-addition of cuprate and oxygen to the phenyl ring generating a meta-cuprated intermediate was excluded due to the large activation barriers. Alternatively, a new amide-directed carbocupration mechanism was proposed which involves a critical rate- and regio-determining step of amide-directed addition of the Cu（III）-aryl bond across the phenyl C2=C3 double bond to form an orthocuprated, meta-arylated intermediate. This mechanism is kinetically the most favored among several possible mechanisms such as ortho-or para-cupration/migration mechanism, direct meta C-H bond cleavage mediated by Cu（III） or Cu（I）, and Cu（III）-catalyzed ortho-directed C-H bond activation mechanism this mechanism has been shown to Furthermore, the predicted regioselectivity based on favor the meta-arylation that is consistent with the experimental observations.

Insight into the photoexcitation effect on the catalytic activation of H_(2) and C-H bonds on TiO_(2)(110)surface

Semiconductor photocatalysis holds great promise for breaking the inert chemical bonds under mild condition;however,the photoexcitation-induced modulation mechanism has not been well understood at the atomic level.Herein,by performing the DFT+U calculations,we quantitatively compare H_(2) activation on rutile TiO_(2)(110)under thermo-versus photo-catalytic condition.It is found that H2 dissociation prefers to occur via the heterolytic cleavage mode in thermocatalysis,but changes to the homolytic cleavage mode and gets evidently promoted in the presence of photoexcited hole(h^(+)).The origin can be ascribed to the generation of highly oxidative lattice O-radical(O_(br)^(·−))with a localized unoccupied O-2p state.More importantly,we identify that this photo-induced promotion effect can be practicable to another kind of important chemical bond,i.e.,C-H bond in light hydrocarbons including alkane,alkene and aromatics;an exception is the C(sp^(1))-H in alkyne(HC≡CH),which encounters inhibition effect from photoexcitation.By quantitative analysis,the origins behind these results are attributed to the interplay between two factors:C-H bond energy(E_(bond))and the acidity.Owing to the relatively high E_(bond) and acidity,it favors the C(sp^(1))-H bond to proceed with the heterolytic cleavage mode in both thermo-and photo-catalysis,and the photoexcited O_(br)^(·−)is adverse to receiving the transferred proton.By contrast,for the other hydrocarbons with moderate/low E_(bond),the O_(br)^(·−)would enable to change their activation mode to a more favored homolytic one and evidently decrease the C-H activation barrier.This work may provide a general picture for understanding the photocatalytic R-H(R=H,C)bond activation over the semiconductor catalyst.

Partial oxidation of methane by photocatalysis

Methane chemistry is one of the“Holy Grails of catalysis”.It is highly desirable but challenge to transform methane into value-added chemicals,because of its high C-H bonding energy(435 kJ/mol),lack ofπbonding or unpaired electrons.Currently,commercial methane conversion is usually carried out in harsh conditions with enormous energy input.Photocatalytic partial oxidation of methane to liquid oxygenates(PPOMO)is a future-oriented technology towards realizing high efficiency and high selectivity under mild conditions.The selection of oxidant is crucial to the PPOMO performance.Hence,attentions are paid to the research progress of PPOMO with various oxidants(O_(2),H_(2)O,H_(2)O_(2)and other oxidants).Moreover,the activation of the selected oxidants is also highly emphasized.Meanwhile,we summarized the methane activation mechanisms focusing on the C-H bond that was broken mainly by·OH radical,O-specie or photogenerated hole(h+).Finally,the challenges and prospects in this subject are briefly discussed.

Ab initio molecular dynamics simulation reveals the influence of entropy effect on Co@BEA zeolite-catalyzed dehydrogenation of ethane

The C–H bond activation in alkane dehydrogenation reactions is a key step in determining the reaction rate.To understand the impact of entropy,we performed ab initio static and molecular dynamics free energy simulations of ethane dehydrogenation over Co@BEA zeolite at different temperatures.AIMD simulations showed that a sharp decrease in free energy barrier as temperature increased.Our analysis of the temperature dependence of activation free energies uncovered an unusual entropic effect accompanying the reaction.The unique spatial structures around the Co active site at different temperatures influenced both the extent of charge transfer in the transition state and the arrangement of 3d orbital energy levels.We provided explanations consistent with the principles of thermodynamics and statistical physics.The insights gained at the atomic level have offered a fresh interpretation of the intricate long-range interplay between local chemical reactions and extensive chemical environments.

气相中CrO_2^+活化甲烷C—H键的理论研究

用密度泛函UB3LYP/6-311++G**方法计算研究了气相中CrO+2(2A1/4A")活化甲烷C—H键的微观机理,找到了四条反应通道.对其中涉及的两态反应(TSR)进行了分析,并对影响反应机理和反应速率的势能面交叉现象(potentialenergysurfacescrossing)进行了详细讨论,进而运用Hammond假设和Yoshizawa等的内禀坐标单点垂直激发计算的方法找出了一系列势能面交叉点[crossingpoints(CPs)],并作了相应的讨论.进一步用碎片分子轨道理论[fragmentmolecularorbital(FMO)]对TS1中的轨道相互作用进行了分析,解释了CrO2+活化甲烷C—H键的机理.

甲烷选择氧化和氧化官能团化的研究进展

在当今国际能源和资源供求竞争异常激烈的背景下,有关甲烷催化活化和选择转化的研究,对于优化天然气、煤层气等作为非油基能源和化工原料,具有重要的战略意义。针对甲烷选择氧化和氧化官能团化直接制甲醇、甲醛等重要含氧化合物这一催化领域的高难挑战,评述了近几年来在均相和多相催化体系方面取得的重要研究进展。