Nitric acid oxidation on carbon dispersion and suspension stability

In this study, we have investigated the effect of nitric acid oxidation on the dispersion and suspension stability of activated carbon powder. We treated activated carbon powder with nitric acid at different concentrations. We measured the content of functional groups, the zeta potential and the powder particle size of the activated carbon powder with the Boehm titration, Fourier transform infrared spectroscopy, a zeta potential analyzer, and scanning electron microscopy. The results show that with the increase of nitric acid molar concentration, the content of acid functional groups increased, whereas the content of alkali functional groups decreased. At a nitric acid concentration of 10 mol/l, the surface‐modified activated carbon powder displays features such as the highest functional group content and zeta potential value, the smallest particle size, and the best dispersion and suspension stability in the solution. Copyright © 2008 John Wiley & Sons, Ltd.     

The preferred reaction path for the oxidation of methanol by PQQ-containing methanol dehydrogenase: addition-elimination versus hydride-transfer mechanism

The catalytic oxidation of methanol to formaldehyde by pyrroloquinoline quinone (PQQ)-containing methanol dehydrogenase (MDH) was investigated at density functional B3LYP level. The still controversial addition-elimination and hydride-transfer reaction mechanisms were analysed. Computations performed in the gas phase and in the protein environment indicated that both suggested reaction sequences involve very high activation barriers. In this situation, the reactions should have scarce probability to occur and the preference for one of the two paths cannot be stated. Here, we will show how some corrections to the successive steps in the addition-elimination mechanism can sensibly decrease the activation barriers height, making possible the determination of the MDH-preferred catalytic path.     

A theoretical study of surface reduction mechanisms of CeO(2)(111) and (110) by H(2).

Reaction mechanisms for the interactions between CeO(2)(111) and (110) surfaces are investigated using periodic density functional theory (DFT) calculations. Both standard DFT and DFT+U calculations to examine the effect of the localization of Ce 4f states on the redox chemistry of H(2)-CeO(2) interactions are described. For mechanistic studies, molecular and dissociative local minima are initially located by placing an H(2) molecule at various active sites of the CeO(2) surfaces. The binding energies of physisorbed species optimized using the DFT and DFT+U methods are very weak. The dissociative adsorption reactions producing hydroxylated surfaces are all exothermic; exothermicities at the DFT level range from 4.1 kcal mol(-1) for the (111) to 26.5 kcal mol(-1) for the (110) surface, while those at the DFT+U level are between 65.0 kcal mol(-1) for the (111) and 81.8 kcal mol(-1) for the (110) surface. Predicted vibrational frequencies of adsorbed OH and H(2)O species on the surfaces are in line with available experimental and theoretical results. Potential energy profiles are constructed by connecting molecularly adsorbed and dissociatively adsorbed intermediates on each CeO(2) surface with tight transition states using the nudged elastic band (NEB) method. It is found that the U correction method plays a significant role in energetics, especially for the intermediates of the exit channels and products that are partially reduced. The surface reduction reaction on CeO(2)(110) is energetically much more favorable. Accordingly, oxygen vacancies are more easily formed on the (110) surface than on the (111) surface.     

Theoretical insights into enantioselective catalysis: the mechanism of the Kharasch-Sosnovsky reaction

The mechanism of the Kharasch-Sosnovsky reaction has been investigated using B3 LYP/6-31G* calculations on a chiral reaction model [cyclohexene+tert-butyl perbenzoate-->cyclohex-2-enyl benzoate+tert-butyl alcohol, catalyzed by a chiral bisoxazoline-copper(I) complex]. Although two previous reaction mechanisms have been considered, the results are consistent with a new mechanistic pathway. This path involves ligand exchange between the catalyst-cyclohexene complex with tert-butyl perbenzoate to give a catalyst-perester complex, which undergoes an (either one- or two-step) oxidative addition reaction to yield a copper(III) complex. The limiting step of the Kharasch-Sosnovsky reaction consists of an intramolecular step involving the abstraction of an allylic hydrogen from cyclohexene [which is pi-bound to the copper(III) complex]. The resulting allyl-copper(III) complex (subsequent to the loss of tert-butanol) can undergo a haptotropic rearrangement by means of an eta1-allyl/eta3-allyl equilibrium, leading to scrambling between vinylic and allylic positions when an isotopically labeled substrate is used. The allyl-copper(III) ion undergoes a stereospecific reductive elimination involving the pi-bond migration to yield a reaction product-catalyst complex, which can regenerate the alkene-copper(I) complex by ligand exchange. The proposed reaction mechanism is consistent with all known experimental results (including enantioselectivity data).     

Sustainable Heck–Matsuda Reaction with Catalytic Amounts of Diazonium Salts: An Experimental and Theoretical Study

The palladium‐catalyzed Heck–Matsuda reaction with a catalytic amount of an in‐situ‐generated diazonium salt proceeded under mild and sustainable conditions. The reaction proceeded at room temperature, under base‐free conditions, and only generated tBuOH, H₂O, and N₂ as by‐products. Ortho‐substituted diazonium salts were more‐efficiently coupled to methyl acrylate than their corresponding para isomers, which required the addition of anisole as an additive. In support of these experimental data, we carried out theoretical studies to gain a deeper understanding of these reaction outcomes.     

Addition of carbenes to the sidewalls of single-walled carbon nanotubes

The addition of carbenes CX(2) (X=H, Cl) to single-walled carbon nanotubes (SWNTs) was investigated by density functional theory and finite, hydrogen-terminated nanotube clusters or periodic boundary conditions in conjunction with basis sets of up to polarized triple-zeta quality. For armchair [(3,3) to (12,12)] and zigzag tubes [(3,0) to (18,0)], reaction of CH(2) with the C--C bond oriented along the tube axis (A) is less exothermic than with those C--C bonds having circumferential (C) orientation. This preference decreases monotonically with increasing tube diameter for armchair, but not for zigzag tubes; here, tubes with small band gaps have a very low preference for circumferential addition. Axial addition results in cyclopropane products, while circumferential addition produces "open" structures for both armchair and zigzag tubes. The barriers for addition of dichlorocarbene to a (5,5) SWNT, studied for a finite C(90)H(20) cluster, are higher than that for addition to C(60), in spite of similar diameters of the carbon materials. Whereas addition of CCl(2) to [60]fullerene proceeds in a concerted fashion, addition to a (5,5) armchair SWNT is predicted to occur stepwise and involve a diradicaloid intermediate according to B3LYP, PBE, and GVB-PP computations. Addition to C bonds of (5,5) armchair tubes resulting in the thermodynamically more stable insertion products is kinetically less favorable than that to A bonds yielding cyclopropane derivatives.     

Catalytic Water Splitting with an Iridium Carbene Complex: A Theoretical Study

Catalytic water oxidation at Ir (OH)⁺ ( Ir =IrCp*(Me₂NHC), where Cp*=pentamethylcyclopentadienyl and Me₂NHC=N,N′‐dimethylimidazolin‐2‐ylidene) can occur through various competing channels. A potential‐energy surface showing these various multichannel reaction pathways provides a picture of how their importance can be influenced by changes in the oxidant potential. In the most favourable calculated mechanism, water oxidation occurs via a pathway that includes four sequential oxidation steps, prior to formation of the O?O bond. The first three oxidation steps are exothermic upon treatment with cerium ammonium nitrate and lead to formation of Ir ^V(?O)(O ^. )⁺, which is calculated to be the most stabile species under these conditions, whereas the fourth oxidation step is the potential‐energy‐determining step. O?O bond formation takes place by coupling of the two oxo ligands along a direct pathway in the rate‐limiting step. Dissociation of dioxygen occurs in two sequential steps, regenerating the starting material Ir (OH)⁺. The calculated mechanism fits well with the experimentally observed rate law: v=k_obs[ Ir ][oxidant]. The calculated effective barrier of 24.6 kcal mol^?1 fits well with the observed turnover frequency of 0.88 s^?1. Under strongly oxidative conditions, O?O bond formation after four sequential oxidation steps is the preferred pathway, whereas under milder conditions O?O bond formation after three sequential oxidation steps becomes competitive.     

Mechanisms of and effect of coadsorption on water dissociation on an oxygen vacancy of the MgO(100) surface

The dissociation mechanism of a water molecule at an oxygen vacancy on the MgO(100) surface was studied by using the embedded cluster method at the DFT/B3 LYP level, while the energetic information was refined by using the IMOMO method at the CCSD level. We found that a water molecule initially adsorbs on one of the magnesium ions surrounding the vacancy site with a binding energy of 15.98 kcal mol(-1). It then can dissociate on the MgO(100) surface along two possible dissociation pathways. One pathway produces a hydroxyl group bonded to the original magnesium with a proton filling the vacancy via a transition state with a barrier of 4.67 kcal mol(-1) relative to the adsorbed water configuration. The other pathway yields two hydroxy groups; the hydroxy group originally belonging to the water molecule fills the vacancy, while the hydrogen atom binds with the surface oxygen to form the other hydroxy group. Hydrogen atoms of these hydroxy groups can recombine to form a hydrogen molecule and the surface is healed. Although the barrier (14.09 kcal mol(-1)) of the rate-controlling step of the latter pathway is higher than that of the former one, the energies of all of its stationary points are lower than that of the separated reactants (H(2)O+cluster). The effects of water coadsorption are modeled by placing an additional water molecule near the active center, which suggests that the more coadsorbed water molecules further stabilize the hydroxy species and prevent the hydrogen molecule formation through the latter pathway. The results support the photoemission spectral evidence of water dissociation on the defective MgO(100) surface at low water coverage.     

Kinetics and Mechanisms for the Reactions of Phenyl Radical with Ketene and its Deuterated Isotopomer: An Experimental and Theoretical Study

Kinetics and mechanism for the reaction of phenyl radical (C₆H₅) with ketene (H₂C_β?C_α?O) were studied by the cavity ring‐down spectrometric (CRDS) technique and hybrid DFT and ab initio molecular orbital calculations. The C₆H₅ transition at 504.8 nm was used to detect the consumption of the phenyl radical in the reaction. The absolute overall rate constants measured, including those for the reaction with CD₂CO, can be expressed by the Arrhenius equation k=(5.9±1.8)×10¹¹ exp[?(1160±100)/T] cm³ mol^?1 s^?1 over a temperature range of 301–474 K. The absence of a kinetic isotope effect suggests that direct hydrogen abstraction forming benzene and ketenyl radical is kinetically less favorable, in good agreement with the results of quantum chemical calculations at the G2MS//B3LYP6‐31G(d) level of theory for all accessible product channels, including the above abstraction and additions to the C_α, C_β, and O sites. For application to combustion, the rate constants were extrapolated over the temperature range of 298–2500 K under atmospheric pressure by using the predicted transition‐state parameters and the adjusted entrance reaction barriers E_α=E_β=1.2 kcal mol^?1; they can be represented by the following expression in units of cm³ mol^?1 s^?1: k_α=6.2×10¹⁹ T^?2.3 exp[?7590/T] and k_β=3.2×10⁴ T^2.4 exp[?246/T].     

Photochemistry of molecules relevant to the atmosphere: photodissociation of nitric acid in the gas phase.

This Minireview gives an account of the photochemical decay of nitric acid HNO3 in the gas phase, which has been well investigated under bulk and molecular-beam conditions. Due to the importance of this molecule in atmospheric chemistry, attention was paid to the irradiation regions around 300 and 200 nm, where solar photolysis of HNO3 is expected to be particularly efficient. While the low-energy region is characterized by the products OH and NO2, the high-energy region gives rise to a variety of photochemical decay pathways, dominated by channels which lead to the products HONO + O in different electronic states.     

A Theoretical Study on the Mechanism of C2H4 Oxidation over a Neutral V3O8 Cluster

Density functional theory (DFT) calculations are used to investigate the reaction mechanism of V₃O₈+C₂H₄. The reaction of V₃O₈ with C₂H₄ produces V₃O₇CH₂+HCHO or V₃O₇+CH₂OCH₂ overall barrierlessly at room temperature, whereas formation of hydrogen‐transfer products V₃O₇+CH₃CHO is subject to a tiny overall free energy barrier (0.03 eV), although the formation of the last‐named pair of products is thermodynamically more favorable than that of the first two. These DFT results are in agreement with recent experimental observations. The (O_b)₂V(O_tO_t)^. (b=bridging, t=terminal) moiety containing the oxygen radical in V₃O₈ is the active site in the reaction with C₂H₄. Similarities and differences between the reactivities of (O_b)₂V(O_tO_t)^. in V₃O₈ and the small VO₃ cluster [(O_t)₂VO_t^.] are discussed. Moreover, the effect of the support on the reactivity of the (O_b)₂V(O_tO_t)^. active site is evaluated by investigating the reactivity of the cluster VX₂O₈, which is obtained by replacing the V atoms in the (O_b)₃VO_t support moieties of V₃O₈ with X atoms (X=P, As, Sb, Nb, Ta, Si, and Ti). Support X atoms with different electronegativities influence the oxidative reactivity of the (O_b)₂V(O_tO_t)^. active site through changing the net charge of the active site. These theoretical predictions of the mechanism of V₃O₈+C₂H₄ and the effect of the support on the active site may be helpful for understanding the reactivity and selectivity of reactive O^. species over condensed‐phase catalysts.     

Formic acid catalyzed gas-phase reaction of H2O with SO3 and the reverse reaction: a theoretical study

The formic acid catalyzed gas‐phase reaction between H₂O and SO₃ and its reverse reaction are respectively investigated by means of quantum chemical calculations at the CCSD(T)//B3LYP/cc‐pv(T+d)z and CCSD(T)//MP2/aug‐cc‐pv(T+d)z levels of theory. Remarkably, the activation energy relative to the reactants for the reaction of H₂O with SO₃ is lowered through formic acid catalysis from 15.97 kcal mol^?1 to ?15.12 and ?14.83 kcal mol^?1 for the formed H₂O ??? SO₃ complex plus HCOOH and the formed H₂O ??? HCOOH complex plus SO₃, respectively, at the CCSD(T)//MP2/aug‐cc‐pv(T+d)z level. For the reverse reaction, the energy barrier for decomposition of sulfuric acid is reduced to ?3.07 kcal mol^?1 from 35.82 kcal mol^?1 with the aid of formic acid. The results show that formic acid plays a strong catalytic role in facilitating the formation and decomposition of sulfuric acid. The rate constant of the SO₃+H₂O reaction with formic acid is 10⁵ times greater than that of the corresponding reaction with water dimer. The calculated rate constant for the HCOOH+H₂SO₄ reaction is about 10^?13 cm³ molecule^?1 s^?1 in the temperature range 200–280 K. The results of the present investigation show that formic acid plays a crucial role in the cycle between SO₃ and H₂SO₄ in atmospheric chemistry.     

Theoretical Studies on the Asymmetric Baeyer–Villiger Oxidation Reaction of 4‐Phenylcyclohexanone with m‐Chloroperoxobenzoic Acid Catalyzed by Chiral Scandium(III)–N,N′‐Dioxide Complexes

The mechanism and enantioselectivity of the asymmetric Baeyer–Villiger oxidation reaction between 4‐phenylcyclohexanone and m‐chloroperoxobenzoic acid ( m ‐CPBA ) catalyzed by Sc^III–N,N′‐dioxide complexes were investigated theoretically. The calculations indicated that the first step, corresponding to the addition of m ‐CPBA to the carbonyl group of 4‐phenylcyclohexanone, is the rate‐determining step (RDS) for all the pathways studied. The activation barrier of the RDS for the uncatalyzed reaction was predicted to be 189.8 kJ mol^?1. The combination of an Sc^III–N,N′‐dioxide complex and the m ‐CBA molecule can construct a bifunctional catalyst in which the Lewis acidic Sc^III center activates the carbonyl group of 4‐phenylcyclohexanone while m ‐CBA transfers a proton, which lowers the activation barrier of the addition step (RDS) to 86.7 kJ mol^?1. The repulsion between the m‐chlorophenyl group of m ‐CPBA and the 2,4,6‐iPr₃C₆H₂ group of the N,N′‐dioxide ligand, as well as the steric hindrance between the phenyl group of 4‐phenylcyclohexanone and the amino acid skeleton of the N,N′‐dioxide ligand, play important roles in the control of the enantioselectivity.     

Density functional characterization of the chemoselective oxidation of catechol by using molecular oxygen: thermodynamics of the reaction between [(triphos)Ir(dtbc)]+ and O2

The catalytic mechanism of intra and extra catechol cleaving dioxygenases is still a matter of debate, because the intermediate transient oxygen adduct has never been isolated in these enzymes. The complex [(triphos)Ir(dtbc)](+) (triphos=MeC(CH(2)PPh(2))(3), DBTC(2-)=3,5-di-tert-butylcatecholate), reacts with molecular oxygen and exhibits a ring-cleaving catalytic activity, and a stable oxygen intermediate appears; this stimulates an opportunity for obtaining information that can be transferable to the above-mentioned enzymatic catalytic centers, therefore, giving a breakthrough into the strategy of oxygen activation in these natural systems. Here, we present the results of our computational approach, based on density functional theory, focusing on the following aspects: to explain the interaction with molecular oxygen and how to avoid the existing spin problem, to understand the formation of the bridging moiety, to clarify, if any, the possible influence on the structure of the formed adduct of the bidentate versus monodentate binding of the catechol ligand, and to disclose, eventually, the nature of final decomposition products with the aim to rationalize the intra versus extra cleavage of the catechol molecule itself.     

Intramolecular [2+2+2] cycloaddition reactions of yne-ene-yne and yne-yne-ene enediynes catalysed by Rh(I): experimental and theoretical mechanistic studies

N-tosyl-linked open-chain yne-ene-yne enediynes 1 and 2 and yne-yne-ene enediynes 3 and 4 have been satisfactorily synthesised. The [2+2+2] cycloaddition process catalysed by the Wilkinson catalyst [RhCl(PPh(3))(3)] was tested with the above-mentioned substrates resulting in the production of high yields of the cycloadducts. Enediynes 1 and 2 gave standard [2+2+2] cycloaddition reactions whereas enediynes 3 and 4 suffered β-hydride elimination followed by reductive elimination of the Wilkinson catalyst to give cycloadducts, which are isomers of those that would be obtained by standard [2+2+2] cycloaddition reactions. The different reactivities of these two types of enediyne have been rationalised by density functional theory calculations.