Understanding the dehydrogenation mechanism over iron nanoparticles catalysts based on density functional theory

The conversion of chemical feedstock materials into high value-added products accompanied with dehydrogenation is of great value in the chemical industry.However,the catalytic dehydrogenation reaction is inhibited by a limited number of expensive noble metal catalysts and lacks understanding of dehydrogenation mechanism.Here,we report the use of heterogeneous non-noble metal iron nanoparticles(NPs) incorporated mesoporous nitrogen-doped carbon to investigate the dehydrogenation mechanism based on experiment observation and density functional theory(DFT) method.Fe NPs catalyst displays excellent performance in the dehydrogenation of 1,2,3,4-tetrahydroquinoline(THQ)with 100% selectivity and 100% conversion for 10-12 h at room temperature.The calculated adsorption energy implies that THQ prefers to adsorb on Fe NPs as compared with absence of Fe NPs.What is more,the energy barrier of transition state is relatively low,illustrating the dehydrogenation is feasible.This work provides an atomic scale mechanism guidance for the catalytic dehydrogenation reaction and points out the direction for the design of new catalysts.     

Density functional study on the mechanism of bicyclic guanidine-catalyzed Strecker reaction

As a direct and viable synthesis of amino acids, the small organic molecule catalyzed asymmetric Strecker reactions have been explored successfully in recent years. For these catalysts, the active sites may be a guanidine group or similarly a urea group. In an effort to elucidate the reaction mechanism, we have investigated the bicyclic guanidine-catalyzed Strecker reaction of HCN and methanimine using density functional theory with the B3LYP method. Assisted by guanidine, two competitive pathways to aminoacetonitrile were rationalized. The aminoisoacetonitrile may not form due to the instability of the product.     

Density functional study on the activation of methane over Pd2, PdO,and Pd2O clusters

Simple functional models for elementary steps in the total oxidation of methane over supported palladium catalysts were investigated using density functional theory. Three simple cluster models were proposed, namely, the palladium dimer and PdO diatomic and linear Pd₂O, to probe the mechanism of the methane activation on metallic and oxidized palladium phases, respectively. The strongest adsorption was found on Pd₂, where also the C(SINGLE BOND)H bond became easily activated; however, no stable product of the C(SINGLE BOND)H bond scission was indicated. Similar hydrogen activation took place on Pd₂O and, in addition, adsorbed methyl and OH species formed the most stable system after crossing a moderate energy barrier. The same product was previously found stable also in the case of PdO dimer but the activation barrier was high. On the Pd₂O cluster, the process of energy barrier crossing was accomplished in two steps: easy formation of a free hydrogen moiety and actual oxidation, which made the overall process less demanding energetically. © 1997 John Wiley & Sons, Inc.     

Comparison of the catalytic activity of Ziegler-type catalysts based on Cp2ZrCl2 and (Cp2ZrCl)2O

The rates of ethylene polymerization catalyzed by Cp₂ZrCl₂-polymethylalumoxane and (Cp₂ZrCl)₂O-polymethylalumoxane are equal. According to NMR and ESR spectral data, the same precatalyst, presumably Cp₂ZrMe₂, is formed in both systems by the action of AlMe₃. This accounts for the equal catalytic activity of the systems based on Cp₂ZrCl₂ and (Cp₂ZrCl)₂O. A scheme of reactions resulting in cleavage of the Zr-O-Zr bridge is proposed and confirmed by spectroscopic data.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2104–2107, December, 1993.     

甲基三氯硅烷水解反应机理的密度泛函研究

采用量子化学密度泛函方法研究了甲基三氯硅烷的结构性质和水解反应机理,发现水的O原子进攻Si原子取代Cl原子的同时,水的另一个H原子逐渐与Cl结合形成相应的羟基硅烷和HCl.甲基三氯硅烷水解按亲核取代--消除的协同反应机理完成.     

Density functional theory study of the mechanism of the proline-catalyzed intermolecular aldol reaction

Transition structures associated with the C-C bond-formation step of the proline-catalyzed intermolecular aldol reaction between acetone and isobutyraldehyde have been studies using density functional theory methods at the B3LYP/6-31G** computational level. A continuum model has been selected to represent solvent effects. For this step, which is the stereocontrolling and rate-determining step, four reactive channels corresponding to the syn and anti arrangement of the active methylene of the enamine relative to the carboxylic acid group of l-proline and the re and si attack modes to both faces of the aldehyde carbonyl group have been analyzed. The B3LYP/6-31G** energies are in good agreement with experiment, allowing us to explain the origin of the catalysis and stereoselectivity for these proline-catalyzed aldol reactions. Received: 2 April 2002 / Accepted: 18 July 2002 / Published online: 11 October 2002 Acknowledgements. This work was supported by research funds provided by the Ministerio de Educación y Cultura of the Spanish Government by DGICYT (project PB98–1429). All the calculations were performed on a Cray–Silicon Graphics Origin 2000 of the Servicio de Informática de la Universidad de Valencia. We are most indebted to this center for providing us with computer capabilities. Correspondence to: L. R. Domingo e-mail: domingo@utopia.uv.es     

Density functional theory study of the mechanism of the acid-catalyzed decarboxylation of pyrrole-2-carboxylic acid and mesitoic acid

The mechanisms of the acid-catalyzed decarboxylation of pyrrole-2-carboxylic acid and mesitoic acid have been investigated based on density functional theory calculations at the B3LYP/6-311G (d,p) level. A polarizable continuum model (PCM) has been established in order to evaluate the effects of solvents on these reactions. The results of the calculations indicate that the first step of the acid-catalyzed decarboxylation of the pyrrole-2-carboxylic acid has two possible pathways,that is,the proton of H3O+ a...     

Density functional study on the reaction mechanism of proton transfer in 2-pyridone: effect of hydration and self-association

The proton-transfer mechanism in the isolated, mono, dehydrated forms and dimers of 2-pyridone and the effect of hydration or self-assistance on the transition state structures corresponding to proton transfer from the keto form to the enol form have been investigated using B3LYP and BH-LYP hybrid density functional methods at the 6-311++G (2d, 2p) basis set level. The barrier heights for both H2O-assisted and self-assisted reactions are significantly lower than that of the bare tautomerization reaction from 2-pyridone to 2-hydroxypyridine, implying the importance of the superior catalytic effect of H2O and (H2O)2 and the important role of 2-pyridone itself for the intramolecular proton transfer. Long-range solvent effects have also been taken into account by using the continuum model (Onsager model and polarizable continuum model (PCM)) of water. The tautomerization energies and the potential energy barriers are increased both for the water-assisted and for the self-assisted reaction because of the bulk solvent, which imply that the tautomerization of PY becomes less favorable in the polar solvent.     

Electronic structure of the d1 bent-metallocene Cp2VCl2: A photoelectron and density functional study

The Cp₂VCl₂ molecule is a prototype for bent-metallocene complexes with a single electron in the metal d shell, but experimental measure of the binding energy of the d electron by photoelectron spectroscopy eluded early attempts due to apparent decomposition in the spectrometer to Cp₂VCl. With improved instrumentation, the amount of decomposition is reduced and subtraction of ionization intensity due to Cp₂VCl from the Cp₂VCl₂/Cp₂VCl mixed spectrum yields the Cp₂VCl₂ spectrum exclusively. The measured ionization energies provide well-defined benchmarks for electronic structure calculations. Density functional calculations support the spectral interpretations and agree well with the ionization energy of the d¹ electron and the energies of the higher positive ion states of Cp₂VCl₂. The calculations also account well for the trends to the other Group V bent-metallocene dichlorides Cp₂NbCl₂ and Cp₂TaCl₂. The first ionization energy of Cp₂VCl₂ is considerably greater than the first ionization energies of the second- and third-row transition metal analogues.     

Density functional study on the reaction mechanism of palladium-catalyzed addition of cyanoboranes to alkynes

The B3LYP hybrid density functional method has been carried out to study theoretically the mechanism of Pd(0)-catalyzed alkyne cyanoboration reaction. Both the intermolecular and intramolecular alkyne cyanoboration reactions were studied. For each reaction, three paths were proposed. In path A of each reaction, the first step is B-CN bond oxidative addition to bisphosphine complex Pd(PH(3))(2), in path B of each reaction, the first step is alkyne coordination to bisphosphine complex Pd(PH(3))(2), and in path C of each reaction, the first step is the PH(3) dissociation from Pd(PH(3))(2) to form monophosphine complex Pd(PH(3)). For both reactions, path B is favored. The dissociation and recoordination of phosphine ligand are found to be very important for the entire reaction, in agreement with the experiment. In both intermolecular and intramolecular cyanoboration reactions, cyano migration is preferred to take place compared with alkenylboryl migration for the formation of the final cis products. The rate-determining step for both intermolecular and intramolecular cyanoboration reactions is found to be the insertion of carbon-carbon triple bond into Pd-B bond with the activation energy of 38.4 and 34.3 kcal/mol relative to the initial reactants, respectively. These values suggest that intramolecular reaction is relatively easy to occur.     

Theoretical insights into the mechanism of photocatalytic reduction of CO2 over semiconductor catalysts

Photocatalytic reduction of CO₂ is one important approach to alleviate greenhouse gas emission and energy crisis, which has gained huge attention in the past decades. However, the lack of understanding complex reaction mechanism impedes new catalysts design. It is also very difficult to understand the mechanism by using only experimental approaches. For this concern, theoretical calculations can effectively supplement the experimental deficiency and thus play an important role. Recently theoretical calculations have been performed on adsorption, migration and reduction of CO₂ molecule on the photocatalyst surface, leading to useful information that have contributed greatly to this field. This review summarizes recent advances in first-principles calculations about CO₂ photoreduction over various semiconductor photocatalysts like metal oxides, sulfides and g-C₃N₄. The methods, models, adsorption and reaction pathways have been discussed in detail. The perspective about future investigation on the photocatalytic reduction of CO₂ using first principles calculations is also presented.     

活性炭纤维催化氧化NO的量子化学研究

利用密度泛函理论的B3LYP方法,6-31G(d)基组,在zigzag型的四并苯模型上对NO、O2分子在活性炭纤维(ACFs)表面的吸附行为进行研究,并探讨了ACFs催化氧化NO的主要机理路径。研究结果表明,环境气氛中的O2分子可以先吸附于ACFs表面形成两个半醌基(C-O),之后C-O和吸附态的NO(C-NO)发生氧化反应生成-NO2;游离态的O2也可以经过ACFs表面的催化作用形成活性氧原子(O*)从而直接和吸附态的NO反应生成-NO2。与NO相比,O2分子的吸附能大,在同NO的竞争吸附中占据优势,结合统计热力学分析,吸附态的NO和游离态的O2所产生的活性氧原子发生氧化反应是NO转化为NO2的主要途径。     

Density functional theory study of the reaction mechanism of the DNA repairing enzyme alkylguanine alkyltransferase

The reaction mechanism of human O⁶-alkylguanine-DNA alkyltransferase (AGT) is studied using density functional theory. AGT repairs alkylated DNA by directly removing the alkyl group from the O6 position of the guanine. A quantum chemical model of the active site was devised based on the recent crystal structure of the AGT–DNA complex. The potential energy curve is calculated and the stationary points are characterized. It is concluded that the previously proposed reaction mechanism is energetically plausible. In this mechanism, His146 first acts as a water-mediated general base to activate Cys145, which then performs a nucleophilic attack to dealkylate the guanine base.     

Density functional study on the mechanism of the vanadium-catalyzed oxidation of sulfides by hydrogen peroxide

A computational study with the Becke3LYP method is carried out on the mechanism of the reaction of complexes V(O)(L)(OOH) and V(O)(LH)(OO) (L = O(CH)(3)N(CH(2))(2)O) with CH(3)S-SCH(3), a system that stands as a model for experimental systems where the metal complex contains larger chelating Schiff bases and the substrate is bis(tert-butyl) disulfide. The different possible isomers of both the hydroperoxo V(O)(L)(OOH) and the peroxo V(O)(LH)(OO) forms of the catalyst are explored, and the reactivity of the most stable among them with the dimethyl disulfide substrate is studied through location of the corresponding transition states. A large variety of reactive paths happen to exist, though in all cases the reaction takes place through a direct transfer process, with the simultaneous formation of the S-O bond and breaking of the O-O bond being the rate-limiting step.     

Density functional theory study on the reaction mechanism of Ni+-catalysed cyclohexane dehydrogenation

Structural Chemistry - A detailed theoretical analysis of the mechanism of chemical bond activation in cyclohexane catalysed by the atomic transition-metal cation Ni+ was performed by density...     

The formation mechanism of Mo-methylidene species over Mo/HBeta catalysts for heterogeneous olefin metathesis: A density functional theory study

Density Functional Theory (DFT) calculations were performed to optimize the Mo active sites in HBeta zeolite catalysts as well as to locate the reaction pathways to form the Mo-methylidene species. Two different Mo active sites, i.e., the oxidized Mo^VIO₂ and its reduced form Mo^VO(OH), were developed and incorporated into HBeta zeolites by replacing a pair of Brønsted acidic sites. The Mo-methylidene species were found to be produced through two elementary reaction steps, and the Mo-oxametallacyclobutanes were identified as the intermediates. The activation barriers of the decompositions of the oxametallacyclobutane intermediates (Step 2) were estimated to be higher than those of the ethene addition on the Mo active sites (Step 1). The oxidation states of the Mo centers exerted marked influences on the stabilities of the intermediates as well as on the activation barriers and reaction heats of Steps 1 and 2, which were elucidated by the electronic properties of the O_b-ligands directly bonded to the Mo centers. Both free energy barriers and reaction heats have indicated that the whole processes of generating the Mo-methylidene species were preferred over the Mo(VI) rather than Mo(V) active site. Accordingly, the Mo(VI) active site was more efficient in catalyzing the formation of Mo-methylidene species in the heterogeneous Mo/HBeta catalytic systems.     

On the mechanism of propylene oxidation over gallium-molybdenum oxide catalysts

The products of selective oxidation are shown to be formed via a stepwise mechanism with participation of surface allylic complexes. A successive transformation of -allyl to -allyl and further to acrolein is possible. The products of complete oxidation may be formed through surface carbonate-carboxylate complexes via either a stepwise mechanism involving the catalyst's oxygen, or a concerted mechanism with the participation of molecular oxygen.

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