Reverse hydrogen spillover on and hydrogenation of supported metal clusters: insights from computational model studies

"Reverse" spillover of hydrogen from hydroxyl groups of the support onto supported transition metal clusters, forming multiply hydrogenated metal species, is an essential aspect of various catalytic systems which comprise small, highly active transition metal particles on a support with a high surface area. We review and analyze the results of our computational model studies related to reverse hydrogen spillover, interpreting available structural and spectral data for the supported species and examining the relationship between metal-support and metal-hydrogen interactions. On the examples of small clusters of late transition metals, adsorbed in zeolite cavities, we showed with computational model studies that reverse spillover of hydrogen is energetically favorable for late transition metals, except for Au. This preference is crucial for the chemical reactivity of such bifunctional catalytic systems because both functions, of metal species and of acidic sites, are strongly modified, in some cases even suppressed - due to partial oxidation of the metal cluster and the conversion of protons from acidic hydroxyl groups to hydride ligands of the metal moiety. Modeling multiple hydrogen adsorption on metal clusters allowed us to quantify how (i) the support affects the adsorption capacity of the clusters and (ii) structure and oxidation state of the metal moiety changes upon adsorption. In all models of neutral systems we found that the metal atoms are partially positively charged, compensated by a negative charge of the adsorbed hydrogen ligands and of the support. In a case study we demonstrated with calculated thermodynamic parameters how to predict the average hydrogen coverage of the transition metal cluster at a given temperature and hydrogen pressure.     

Spectroscopic evidence for a persistent benzenium cation in zeolite H-beta

Experimental evidence of protonation of an aromatic ring by a zeolite is hereby presented for the first time. The changes in vibrational properties and electronic transitions of the highest polymethylbenzene homologue, that is, hexamethylbenzene, were investigated after introducing the compound directly into a H-beta zeolite. Protonation of the aromatic ring, and thus the loss of symmetry, activated a vibrational ring mode at 1600 cm(-)(1). Furthermore, an electronic transition around 26 000 cm(-)(1), which was totally absent for the neutral species, was an obvious consequence of protonation. A parallel study of hexamethylbenzene adsorbed on a beta zeolite virtually free from protons did not show those distinctive spectral features. On the basis of the gas-phase proton affinity of hexamethylbenzene, a complete proton transfer from the zeolite framework to the molecule is, according to conventional considerations, not expected. The hexamethylbenzenium ion is stable in the zeolite cavities at least up to 200 degrees C. The remarkable persistence of this carbenium ion may be attributed to spatial constraints imposed by the tight fit of the cation inside the zeolite channels. Hexamethylbenzene is a relevant reaction intermediate in the methanol-to-hydrocarbons reaction and also plays a central role as a coke precursor in zeolite-catalyzed reactions that involve polymethylbenzenes.     

Theoretical study of modes of adsorption of water dimer on H-ZSM-5 and H-Faujasite zeolites

Modes of adsorption of water dimer on H-ZSM-5 and H-Faujasite (H-FAU) zeolites have been investigated by a quantum embedded cluster approach, using the hybrid B3LYP density functional theory. The results indicate that there are two possible adsorption pathways, namely the stepwise process where only one water binds strongly to the (-O)3-Al-O(H) tetrahedral unit while the other weakly binds to the zeolite framework and the concerted process where both water molecules form a large ring of hydrogen-bonding network with the Br?nsted proton and an oxygen framework. With inclusion of the effects of the Madelung potential from the extended zeolite framework, for adsorption on H-ZSM-5 zeolite, both the neutral and ion-pair complexes exist with adsorption energies of -15.13 and -14.73 kcal/mol, respectively. For adsorption on the H-FAU, only the ion-pair complex exists with the adsorption energy of -14.63 kcal/mol. Our results indicate that adsorption properties depend not only on the acidity of the Br?nsted acidic site but also on the topology of the zeolite framework, such as on the spatial confinement effects which lead to very different adsorption structures for the ion-pair complexes in H-ZSM-5 and H-FAU, even though their adsorption energies are quite similar. Our calculated vibrational spectra for these ion-pair complexes support previous experimental IR interpretations.     

Adsorption of organic matter at mineral/water interfaces. 6. Effect of inner-sphere versus outer-sphere adsorption on colloidal stability

The effects of the adsorption modes of several low molecular weight (LMW) organic anions (maleate, oxalate, and citrate) on the colloidal stability of corundum-water suspensions have been examined using electrokinetic and shear yield stress (tau(y)) measurements over a broad range of pH conditions and LMW organic anion concentrations. Consistent with previous studies, increasing concentrations of maleate, oxalate, and citrate progressively shift the electrokinetic isoelectric point and pH of the maximum shear yield stress (tau(y,max)) to more acidic conditions. Due to its predominant electrostatic driving force for adsorption, outer-spherically adsorbed maleate possesses a very limited ability to charge reverse the corundum-water interface or bind to the negatively charged corundum surface. By contrast, inner-spherically adsorbed oxalate and citrate can significantly charge reverse the corundum-water interface, with the extent of charge reversal being related to the relative binding strengths of the oxalate and citrate anions. Adsorbed maleate, oxalate, and citrate generate steric barriers to interparticle approach, leading to substantial reductions in the magnitude of tau(y,max) at low to intermediate concentrations of those LMW anions. At the highest anion concentrations investigated, however, increases in tau(y,max) are observed, and can be attributed to the formation of bridging Al(III)-organic surface precipitates, as suggested by in situ attenuated total reflectance Fourier transform infrared spectroscopic measurements of corundum-oxalate suspensions at high oxalate concentrations. The extent of precipitate formation is greatest for the corundum-oxalate system due to the strong dissolution-enhancing properties of the inner-spherically adsorbed oxalate anion (i.e., its ability to generate enhanced concentrations of dissolved Al(III) which can then participate in precipitate formation). The effects of the LMW organic anion adsorption modes on both the forms of the measured tau(y) versus pH data, and the ability to quantitatively compare tau(y) and zeta potential data measured at different corundum concentrations, are also discussed.     

Voltammetry of copper species in estuarine waters induced adsorption of copper on the hanging mercury drop electrode in complexing ligand/surfactant/chloride media

The adsorption of copper species on a positively charged hanging mercury drop electrode in complexing ligand/surfactant/chloride solution is discussed. Techniques used were differential pulse voltammetry of the copper in the adsorbed film, and potential-step reduction of adsorbed copper followed by different pulse anodic stripping voltammetry of Cu(Hg). The CuCl^?₂ species is shown to be the most important copper moiety adsorbed on the electrode and the adsorption is enhanced by organic films. This can be a critical pathway in the reduction of copper(II) in estuarine waters. The induced adsorption of copper in organic layers has biogeochemical implications associated with the nature of organic films and their influence on the Cu(II)/Cu(I) redox couple. There are also analytical applications, e.g., the compositional assay of organic monolayers by utilising Cu(II) and Cu(I) adsorption as electoractive probes and the determination of solution copper-organic binding.     

Adsorption of Benzene and Propylene in Zeolite ZSM-5： Grand Canonical Monte Carlo Simulations

The adsorption behavior of benzene and propylene in zeolite ZSM-5 was studied by Grand Canonical Monte Carlo(GCMC) simulations. It could be found that benzene and propylene molecules showed different adsorption behavior in the zeolite cavities. The loadings of propylene were significantly larger than those of benzene at 100 kPa. From the figures of potential energy distribution,the potential energy of benzene/zeolite was more negative than that of propylene/zeolite,so benzene could be adsorbed more stably t...     

Identification of an adsorption complex between an alkane and zeolite active sites

Direct observation of the Bronsted acid site signal in an active zeolite catalyst following adsorption of stoichiometric quantities of isobutane reveals the presence of a specific adsorption complex. Independent polarization transfer experiments in which magnetization originates with either the catalyst or the adsorbed isobutane confirm this assignment. The initial steps in alkane reactivity are poorly defined, and this experimentally verified complex is proposed as a route to C-H bond activation in solid acids.     

A density functional theory study of molecular and dissociative adsorption of H2 on active sites in mordenite

Adsorption and chemisorption of H2 in mordenite is studied using ab initio density functional theory (DFT) calculations. The geometries of the adsorption complex, the adsorption energies, stretching frequencies, and the capacity to dissociate the adsorbed molecule are compared for different active sites. The active centers include a Br?nsted acid site, a three-coordinated surface Al site, and Lewis sites formed by extraframework cations: Na+, Cu+, Ag+, Zn2+, Cu2+, Ga3+, and Al3+. Adsorption properties of cations are compared for a location of the cation in the five-membered ring. This location differs from the location in the six-membered ring observed for hydrated cations. The five-membered ring, however, represents a stable location of the bare cation. In this position any cation exhibits higher reactivity compared with the location in the six-membered ring and is well accessible by molecules adsorbed in the main channel of the zeolite. Calculated adsorption energies range from 4 to 87 kJ/mol, depending on electronegativity and ionic radius of the cation and the stability of the cation-zeolite complex. The largest adsorption energy is observed for Cu+ and the lowest for Al3+ integrated into the interstitial site of the zeolite framework. A linear dependence is observed between the stretching frequency and the bond length of the adsorbed H2 molecule. The capacity of the metal-exchanged zeolite to dissociate the H2 molecule does not correlate with the adsorption energy. Dissociation is not possible on single Cu+ cation. The best performance is observed for the Ga3+, Zn2+, and Al3+ extraframework cations, in good agreement with experimental data.     

硅沸石完美骨架上的吸附Ⅱ.乙胺、正戊烷、乙醇在 FAU型沸石上的吸附热

用测定吸附等温线法研究乙胺、正戊烷和乙醇在疏水高硅FAU沸石上的吸附热效应。根据Clapeyron-Clausius方程,处理吸附等温线,得到不同覆盖度C(C=被吸附分子数/晶胞)的等量吸附热Qst(C)(一定覆盖度C时由Clapeyron-Clausius方程计算的吸附热)及平均等量吸附热Qst^*(一定温度区间里等量吸附热Qst的平均值),以及Qst^*与沸点蒸发热△Hv的差值△H1(定义为相互作用强度△H1=Qst^*-△Hv)。所研究的三种有机分子的△H1的次序为△H1(乙胺)>△H1(正戊烷)>>△H1(乙醇)。这与AT值(定义为脱附温度Td与吸附质的沸点温度Tb的差值,无需单位)有正相关关系。由AT值观察到的高硅FAU沸石Si-O骨架与被吸附乙胺之间可能存在的强相互作用、为本研究测定的热力学定量数据△H1值所证明。     

Electrochemical study of insulin at the polarized liquid-liquid interface

This paper reports on the electrochemical behavior of bovine insulin at the interface between two immiscible electrolyte solutions (ITIES). The voltammetric ion-transfer response obtained in the presence of insulin was dependent on the aqueous phase pH conditions and on the nature of the organic phase electrolyte employed in experiments. Optimal detection was obtained at acidic pH below the isoelectric point of insulin where it was positively charged. A shift in transfer potentials to lower potential values was observed with decreasing hydrophobicity of the anion of the organic phase electrolyte. No ion-transfer response was observed at pH values of the aqueous phase above the isoelectric point, where insulin was negatively charged. These results suggest that the voltammetric response is due to ion-pairing interactions at the ITIES between positively charged insulin and the hydrophobic anion of the organic phase electrolyte, together with adsorption of the ion-pair at the interface. The voltammetric response was obtained for insulin at concentrations down to 1 muM. These results show that electrochemistry is useful in studying the behavior of this important protein molecule at the polarized water-1,2-DCE interface and provides an alternative detection mode for bioanalytical applications.     

Meisenheimer complexes bonded at carbon and at oxygen

The carbon-bonded gas-phase Meisenheimer complex of 2,4,6-trinitrotoluene (TNT) and the nitromethyl carbanion CH(2)NO(2)(-) (m/z 60) is generated for the first time by chemical ionization using nitromethane as the reagent gas. Collision-induced dissociation (CID) of the Meisenheimer complex furnishes deprotonated TNT, a result of the higher gas-phase acidity of TNT than nitromethane. The formation of Meisenheimer complexes with CH(2)NO(2)(-) in the gas phase is selective to highly electron-deficient compounds such as dinitrobenzene and trinitrobenzene and does not occur with organic molecules with lower electron-affinity such as methanol, methylamine, propionaldehyde, acetone, ethyl acetate, chloroform, toluene, m-methoxytoluene, and even nitrobenzene and p-fluoronitrobenzene. As such, the reaction allows selective detection of TNT in mixtures. Meisenheimer complexes between CH(2)NO(2)(-) and the three dinitrobenzene isomers display distinctive fragmentations. The oxygen-bonded sigma-complex of TNT with the deprotonated hemiacetal anion CH(3)OCH(2)O(-) (m/z 61), represents a different type of Meisenheimer complex. It displays characteristic fragmentation involving loss of HNO(2) upon CID. The combination of a selective ion/molecule reaction (Meisenheimer complex formation) followed by a characteristic CID process provides a second novel and highly selective approach to the detection of TNT and closely related compounds in mixtures. The assay is readily implemented using neutral loss scans in a triple quadrupole mass spectrometer. Gas-phase reactions of denitrosylated TNT with benzaldehyde produce the corresponding dihydrofuran in an aldol condensation, a result that parallels the corresponding condensed-phase reaction.     

UV-Vis spectroscopic study on effects of pressure for adsorption ofp-nitrotoluene at liquid-solid interface

The behavior ofp-nitrotoluene adsorbed at zeolite/n-heptane interface has been investigated by the electronic spectroscopy under pressure up to 300 MPa. The uv-vis absorption bands of adsorbedp-nitrotoluene were deconvoluted into ones for the species adsorbed on the cation sites, and one for that on the pore wall of zeolite. The peak of adsorbed species on the cation site red-shifted by 20–80 nm from the position of the same species in the liquid phase, and their magnitudes of shift depended on the strength of electric field generated by the cation in zeolites. The peak intensities of adsorbed species on the cation site were enhanced but these or the pore wall site were reduced with the increase in pressure, suggesting that a part ofp-nitroluene molecules on the pore wall site desorbed and the adsorption on the cation site was enhanced by compression. The pressure dependence of peak intensity indicated that the behavior of this adsorption system was strongly governed by the solvation structure of the adsorbate in the zeolite pore. In particular, it was found that the adsorption of solvent molecules on the cation site strongly affected the volume change of the adsorption system.     

The study of thiophene adsorption onto La(III)-exchanged zeolite NaY by FT-IR spectroscopy

Zeolites NaY and LaNaY (ion-exchanged with aqueous lanthanum nitrate solution) were used as adsorbents for removing organic sulfur compounds from model gasoline solutions (without and with toluene) and fluid catalytic cracked gasoline in fixed-bed adsorption equipment at room temperature and atmosphere pressure. The adsorptive selectivity for organic sulfur compounds was significantly increased when Na(+) ions in zeolite NaY were exchanged with lanthanum ions. IR spectra of thiophene adsorption indicate that thiophene is adsorbed onto La(3+) ions via direct S-La(3+) interaction and Na(+) ions via pi-electronic interaction for La(3+)-exchanged zeolite NaY, but only via pi-electronic interaction with Na(+) ions for NaY. The amount of adsorbed thiophene on La(3+)-exchanged zeolite Y was slightly decreased by coadsorption of benzene, but greatly reduced on NaY. The adsorption of thiophene via interaction with La(3+) on La(3+)-exchanged zeolite Y is hardly replaced by benzene coadsorption. The direct S-La(3+) interaction might be the essential reason for the evidently improved adsorptive selectivity of LaNaY for removing organic sulfur compounds from solutions containing large amount of aromatics.     

UV-Vis spectroscopic study on effects of pressure for adsorption ofp-nitrotoluene at liquid-solid interface

The behavior ofp-nitrotoluene adsorbed at zeolite/n-heptane interface has been investigated by the electronic spectroscopy under pressure up to 300 MPa. The uv-vis absorption bands of adsorbedp-nitrotoluene were deconvoluted into ones for the species adsorbed on the cation sites, and one for that on the pore wall of zeolite. The peak of adsorbed species on the cation site red-shifted by 20–80 nm from the position of the same species in the liquid phase, and their magnitudes of shift depended on the strength of electric field generated by the cation in zeolites. The peak intensities of adsorbed species on the cation site were enhanced but these or the pore wall site were reduced with the increase in pressure, suggesting that a part ofp-nitroluene molecules on the pore wall site desorbed and the adsorption on the cation site was enhanced by compression. The pressure dependence of peak intensity indicated that the behavior of this adsorption system was strongly governed by the solvation structure of the adsorbate in the zeolite pore. In particular, it was found that the adsorption of solvent molecules on the cation site strongly affected the volume change of the adsorption system.     

Formation of stable Meisenheimer adduct ion pairs in apolar solvents: implications for stereoselective reactions

[reaction: see text] A detailed study concerning the formation of Meisenheimer adducts in biphasic solvent systems is described. The process relies on utilizing a significantly lipophilic quaternary ammonium salt to transfer a nucleophile (e.g., hydroxide ion) between an aqueous and organic layer containing the electron-deficient aromatic substrate. Provided that the organic layer is sufficiently apolar, the resultant Meisenheimer adduct is considerably stable, likely the result of a strong ion-pairing interaction between the large polarizable anionic complex and the diffusively charged tetraalkylammonium cation. Using the diethylamide of 3,5-dinitrobenzoic acid as a model compound, the influence of ion-pairing reagents and solvents on adduct formation was investigated. Dramatically increased equilibrium formation of the Meisenheimer adduct is observed in the biphasic medium (e.g., benzene/2 M NaOH) relative to the same adduct generated in single-phase systems. Spectroscopic studies on this adduct are consistent with those conducted in single-phase polar or dipolar aprotic solvents. The methodology is extended to performing highly enantioselective biphasic kinetic resolutions of several racemic electron-deficient amides.     

Distribution of adsorbed molecules along zeolitecavities

Conclusions From the experimental isotherm of adsorption on a zeolite, on the assumption of a quasiindependence of the molecules found in different cavities, it is possible to determine the distribution of the adsorbed molecules along the zeolite cavities. If the adsorption isotherm satisfies the Henry equation, then this distribution coincides with the Poisson distribution, while if it satisfies the Langmuir equation then the distribution coincides with the Bernoulli distribution.Translated from. Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 452–454, February, 1973.     

Adsorption of small organic molecules on anatase and rutile surfaces: a theoretical study

The adsorption of several small organic molecules on rutile (110) and (100) as well as on anatase (101) surfaces was investigated by Car-Parrinello molecular dynamics in aqueous solution and a new approach to the calculation of adsorption energies is proposed, taking into account the potential energy fluctuation of larger systems. Acetylene and ethylene insert into twin oxygen vacancies in the surface and form polarized covalent Ti-C bonds. In one case spontaneous coupling of two acetylene molecules to a C(4)H(3) molecule with a structure similar to trans-butadiene was observed. Neutral catechol and the singly charged anion were not reactive on any titanium dioxide surface, but the twofold-charged anion attained stable mono- and bidentated geometries on anatase. Methanol, ethanol, formaldehyde and acetaldehyde adsorbed with their functional groups. Very stable geometries provide a Ti-O bond and have adsorption energies of 60-200 kJ/mol. The adsorbates compete with water molecules for similar adsorption sites in point defects as well as on perfect surfaces.     

Understanding the solubility of triamino-trinitrobenzene in hydrous tetramethylammonium fluoride: a first principles molecular dynamics simulation study

With the aim to understand the relatively high solubility of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), an important energetic material with a high degree of inter- and intra-molecular hydrogen bonding, in fluoride anion containing ionic liquids (ILs), first principles molecular dynamics simulations in the isobaric-isothermal ensemble were carried out for a system using hydrous tetramethylammonium fluoride as the prototypical solvent. Simulations initiated from both molecular TATB and its Meisenheimer complex (i.e., a σ-complex of the fluoride and the electrophilic ring of TATB) yield a Zundel-type complex where a proton is shared between an amino group and an F(-) ion, whereas the Meisenheimer complex is found to be only transiently stable. An analysis of the electronic structure probing the Wannier function centers supports the finding of a proton-sharing complex with a three-center four-electron like bond. The Zundel-type complex also yields an electronic absorption spectrum consistent with the experimentally observed color change. This study provides evidence that the remarkable solubility of otherwise hard-to-dissolve molecular crystals in ILs can be aided by chemical modification of the solute.     

Medium and interfacial effects in the multistep reduction of binuclear complexes with robson-type ligand

We present a combined experimental and computational approach to the modeling and prediction of reactivity in multistep processes of heterogeneous electron transfer. The approach is illustrated by the study of a Robson-type binuclear complex (-Cu(II)-Cu(II)-) undergoing four-electron reduction in aqueous media and water-acetonitrile mixtures. The observed effects of solvent, pH, buffer capacity, and supporting electrolyte are discussed in the framework of a general reaction scheme involving two main routes; one of them includes protonation of intermediate species. The main three problems are addressed on the basis of modern charge transfer theory: (1) the effect of the nature of reactant and intermediate species (protonated/deprotonated, bare or associated with supporting anion/solvent molecule) on the standard redox potential, the electronic transmission coefficient, and the intramolecular reorganization; (2) possible effect of protonation on the shape of the reaction free energy surfaces which are built using the Anderson Hamiltonian; (3) electron transfer across an adsorbed chloride anion. Quantum chemical calculations were performed at the density functional theory level.     

Evidence for the intermediacy of Wheland-Meisenheimer complexes in SEAr reactions of aminothiazoles with 4,6-dinitrobenzofuroxan

Reactions of DNBF with a series of 2-aminothiazoles (1 a-f) to afford thermodynamically stable C-bonded sigma-adducts have been investigated in acetonitrile. A most significant finding emerged on recording NMR spectra immediately after mixing of equimolar amounts of DNBF and the unsubstituted 2-aminothiazole (1 a) in Me2SO: namely, that the formation of 9 a is preceded by that of a short-lived intermediate species X. From the 1H NMR parameters characterizing this intermediate, as well as the dependence of its lifetime on the experimental conditions-the presence of excess DNBF over 1 a increases the lifetime of X while an excess of base (1 a) accelerates its conversion into 9 a--it is convincingly demonstrated that the structure of X combines the presence of a positively charged Wheland complex moiety (with regard to the thiazole ring) with that of a negatively charged Meisenheimer complex moiety (with regard to the benzofuroxan system). So far, only one intermediate of this type (noted WM) has been successfully characterized, in the reactions of DNBF with 1,3,5-tris(N,N-dialkylamino)benzenes. Among the key features supporting the intermediacy of X along the reaction coordinate leading to 9 a is the fact that the reactions of DNBF with 1 a in the presence of an alcohol (MeOH, EtOH, nPrOH) produce new adducts arising from the addition of an alcohol molecule to the thiazole moiety of WM-1 a. Reflecting the presence of three chiral centres, these species are formed as mixtures of several diastereomers that could all be characterized in their racemic forms in ethanol. These findings generalize the previous report on the formation of Wheland-Meisenheimer carbon-carbon complexes in homocyclic series.