Concentration Of Active Sites Research Articles

Effect of Chitosan Addition on NiMo/Al2O3 Catalysts for Dibenzothiophene Hydrodesulfurization

Abstract A series of supported NiMo catalysts were prepared by impregnating ammonium heptamolybdate and nickel nitrate (at 12 and 3 wt% of Mo and Ni, respectively) over chitosan (Chi) modified Al2O3 carrier. Alumina substrate was first impregnated with the organic additive (in acidic aqueous HNO3 solution, given the insolubility of chitosan at neutral pH) at concentrations corresponding to Chi/Ni mol ratios of 0.5, 1 and 2. Lower Mo dispersion in materials prepared over Chi-modified alumina was observed by Raman spectroscopy. Also, in those samples formation of hardly reducible aluminum molybdates (due to acidic conditions used during chitosan deposition) was evidenced by thermal analysis. Sulfided (under H2S/H2 flow, at 400 °C) catalysts were tested in dibenzothiophene hydrodesulfurization (HDS, at 72.4 kg/cm2, 320 °C, batch reactor, n-hexadecane as solvent) where no improved activity (in pseudo first order kinetic constant basis) was registered for chitosan-modified samples. However, increased selectivity to the product from direct desulfurization route (biphenyl) suggested enhanced promotion of MoS2 phase (by Ni) in catalysts prepared with organic additive. CO adsorption at -173 °C (followed by infrared spectroscopy) showed lower concentration of NiMoS active sites over catalysts prepared over chitosan-impregnated alumina carrier pointing out to the existence of highly efficient sites, in spite of their lower surface concentration. The present investigation opens the possibility of using chitosan, a sub-product from seafood industry, as efficient HDS catalyst additive.

Oxide Catalysts for Rechargeable High‐Capacity Li–O2 Batteries

AbstractNano‐crystalline mixed metal oxides with an expanded pyrochlore structure are synthesized by a chemical precipitation route in alkaline media. The high concentration of surface active sites afforded by their high surface area, intrinsically variable oxidation state and good electron transport lead to promising electrocatalytic properties for oxygen evolution in Li‐O2 cells, yielding rechargeable discharge capacities over 10 000 mAh g−1 and lowering anodic overpotentials significantly. The discharge capacity of the Li‐O2 cell is increased further when a small amount of gold is deposited on the pyrochlore oxide, which serves as a more efficient oxygen reduction catalyst. The amount of catalyst necessary for oxygen evolution performance is reduced to as little as 5 wt% by supporting the highly‐divided pyrochlore oxide crystallites on carbon using in‐situ deposition methods.

Kinetic Modeling of 1-Hexene Polymerization Catalyzed by Zr(tBu-ONNMe2O)Bn2/B(C6F5)3

Kinetic modeling using a population balance approach has been performed in order to identify a mechanism and a set of rate constants that describe the batch polymerization of 1-hexene by the homogeneous single-site catalyst Zr(tBu-ONNMe2O)Bn2 activated by B(C6F5)3 in toluene. The mechanism and rate constants were determined by making use of a multiresponse data set, including (i) monomer concentration versus time for various initial concentrations of monomer and catalyst, (ii) the time evolution of the molecular weight distribution, (iii) active site concentrations versus time, and (iv) vinyl end group concentrations versus time. The overall mechanism requires slow chain initiation compared to propagation, 2,1-misinsertion and recovery, and two chain transfer pathways—one forming vinylidene end groups and the other forming vinylene end groups. The quantitative analysis of kinetic data clearly shows that a significant fraction of the catalyst does not participate in the chain growth process. The quantitati...

Catalytic cracking of alkanes on FAU: Single‐event microkinetic modeling including acidity descriptors

AbstractSingle‐event microkinetic (SEMK) modeling is applied to catalytic cracking of 2,2,4‐trimethylpentane on a series of faujasites with Si/Al ratio ranging from 2.6 to 30. Standard activation entropies of the various elementary reaction families are calculated a priori from transition state theory and statistical thermodynamics, while activation energies are estimated on a reference faujasite by regression to experimental kinetic data. The SEMK model is then extended with two acidity descriptors. The concentration of active sites is available from independent NH3‐TPD measurements, while the change in standard protonation enthalpy, relative to the reference faujasite, is obtained by regression to experimental kinetic data. The latter parameter accounts for the effect of the zeolite average acid strength both on the stability of the intermediates and on the activation energies of the protonation and protolytic scission reactions. For these five commercially available faujasites, a variation in standard protonation enthalpy of 29 kJ mol−1 was found. © 2012 American Institute of Chemical Engineers AIChE J, 2012

Assembly, Purification, and Pre-steady-state Kinetic Analysis of Active RNA-dependent RNA Polymerase Elongation Complex

NS5B is the RNA-dependent RNA polymerase responsible for replicating hepatitis C virus (HCV) genomic RNA. Despite more than a decade of work, the formation of a highly active NS5B polymerase·RNA complex suitable for mechanistic and structural studies has remained elusive. Here, we report that through a novel way of optimizing initiation conditions, we were able to generate a productive NS5B·primer·template elongation complex stalled after formation of a 9-nucleotide primer. In contrast to previous reports of very low proportions of active NS5B, we observed that under optimized conditions up to 65% of NS5B could be converted into active elongation complexes. The elongation complex was extremely stable, allowing purification away from excess nucleotide and abortive initiation products so that the purified complex was suitable for pre-steady-state kinetic analyses of polymerase activity. Single turnover kinetic studies showed that CTP is incorporated with apparent K(d) and k(pol) values of 39 ± 3 μM and 16 ± 1 s(-1), respectively, giving a specificity constant of k(pol)/K(d) of 0.41 μM(-1) s(-1). The kinetics of multiple nucleotide incorporation during processive elongation also were determined. This work establishes a novel way to generate a highly active elongation complex of the medically important NS5B polymerase for structural and functional studies.

Mass transfer modeling of a consecutive reaction in rotating foam stirrer reactors: Selective hydrogenation of a functionalized alkyne

The external and internal mass transfer for a rotating foam stirrer reactor has been studied using the hydrogenation of 3-methyl-1-pentyn-3-ol as a model reaction. Catalytic foam stirrers with different number of pores per linear inch (ppi) and washcoating thicknesses are used and their performance are described in terms of activity and selectivity. The liquid–solid mass transfer does not limit the overall reaction rate when a 20ppi foam is used. For this catalytic foam stirrer, a kLSaLS value of 0.2s−1 is estimated at 300rpm. The use of finer pore foam structures is not convenient due to a drastic decrease of catalyst surface refreshment. As a result, the lower kLS value negates the benefit of a larger area (aLS) resulting in a lower liquid–solid mass transfer. The catalyst accessibility is a controlling factor in this fast reaction. The optimal washcoat thickness to avoid diffusion limitations is 5μm. Gas–liquid mass transfer rate is not a limiting factor. A kGLaGL of 0.6s−1 is measured at 300rpm by the physical absorption method. When compared to the conventional slurry reactor, the liquid–solid and the gas–liquid mass transfer rates are 17 and 6 times higher, respectively. However, the intrinsic activity of the foam catalyst is 4.6 times less than the slurry catalyst prepared with the same palladium loading. This is explained by a difference in palladium dispersion caused by a non-homogenous concentration of active sites along the washcoat layer. On the other hand, the less active foam catalyst leads to higher selectivity. The production rate is then increased by mounting more active blades on the stirrer shaft, thus creating more surface area for catalyst deposition while still keeping the high accessibility of a 4μm washcoat layer. Hence, rotating foam reactors have promising applications for multiphase processes. High mass transfer rates can be easily achieved, catalyst filtering is not longer needed and reusability of the foam catalyst is possible.

Adsorbent ability of lignin-based activated carbons for the removal of p-nitrophenol from aqueous solutions

This paper reports the p-nitrophenol (PNP) removal from aqueous solutions by adsorption onto active carbons (ACs). Nine ACs were prepared from acid-precipitated eucalyptus kraft lignin following a two-step process consisting in CO2 partial gasification (750–850°C) after carbonization (350–800°C) in N2 atmosphere. The amount adsorbed ranged from 1 to 4.4mmol/g, and it is related to the initial concentration of adsorbate, temperature, pH, burn off of the activated carbons, and contact time. The equilibrium results were fitted by the Temkin, Dubinin-Radushkevich, and Guggenheim-Anderson-de Boer equations. The main thermodynamic magnitudes were estimated as well, and their values indicated that the adsorption processes were spontaneous and exothermic. The kinetic study showed that the processes are of apparent second order related to the concentrations of the empty active sites on the ACs surface. The values of the effective diffusivities have been calculated and they have suggested that the internal diffusion controls the net mass transfer. The results obtained in the present work can be for the benefit of the preparation of new adsorbents, as well as the primary design of the adsorption equipment with separation or environmental purposes.

Synthesis of quinolines via Friedländer reaction catalyzed by CuBTC metal–organic-framework

Friedländer condensation between 2-aminoaryl ketones and different carbonyl compounds, catalyzed by CuBTC was investigated by a combination of various experimental techniques and by density functional theory based modelling. CuBTC exhibiting hard Lewis acid character showed highly improved catalytic activity when compared with other molecular sieves showing high concentraion of Lewis acid sites, e.g. in BEA and (Al)SBA-15. Polysubstituted quinolines were synthesized via a Friedländer reaction catalyzed by CuBTC under the solvent-free conditions. High concentration of active sites in CuBTC together with the concerted effect of a pair of adjacent Cu(2+) coordinatively unsaturated active sites are behind a very high quinoline yield reached within a short reaction time. Results reported here make CuBTC a promising catalyst for other Lewis acid-promoted condensations, including those leading to biologically active compounds with a particular relevance for the pharmaceutical industry. The mechanism of a catalyzed Friedländer reaction investigated computationally is also reported.

Functionalized Ionic Liquids for the Synthesis of Metal Nanoparticles and their Application in Catalysis

AbstractColloidal suspensions of metal nanoparticles (NPs) in non‐functionalized ionic liquids (ILs) are active catalysts for a wide variety of organic transformations. The weak ionic interaction between the IL and the metal surface provides a bare particle with a high concentration of active metal sites. However, the long‐term stabilities can be affected as the IL cannot provide sufficient stabilization and aggregation of the NPs under catalytic conditions limits the reuse of NP:IL catalysts. ILs functionalized with a metal‐binding moiety (FILs) can alleviate this agglomeration problem and provide catalysts with improved activities and recyclabilities. The enhanced stability is provided by the covalent attachment of the stabilizer to the metal surface, which holds the electrostatic stabilizing IL headgroup at the outer ligand sphere of the NP. FILs have been used in the synthesis of NPs as soluble ligands in aqueous, organic or IL solution, as neat solvents, as anchors to immobilize NPs onto a solid support and incorporated into polymeric stabilizers. This Minireview discusses the synthesis and application of NP:FIL systems, with an emphasis on catalysis.

Abstract C99: Beyond bortezomib: Development of Millennium's next-generation proteasome inhibitors.

Abstract Bortezomib (Velcade®) is a proteasome inhibitor that has been approved by the U.S. Food and Drug Administration for the treatment of patients with multiple myeloma or relapsed mantle cell lymphoma. It is currently available in more than 90 countries worldwide. The molecule is an N-capped dipeptidyl boronic acid and its molecular mechanism involves slow-tight binding to the chymotrypsin-like (β5) sites of the 26S proteasome. In the development of Millennium's next generation inhibitors, we have investigated several parameters affecting the drug's activity in biological systems. These include tissue proteasome concentration, blood/plasma partitioning and the kinetics of proteasome inhibition in cultured cells. We find that the abundance of the proteasome (approx. 1–5 μM β5 active site concentration) in cells and tissues together with the slow rate of dissociation of bortezomib from the proteasome (110 min. half-life) led to partitioning of the inhibitor in red blood cells, thereby limiting its distribution to potential sites of therapeutic action. Based on the hypothesis that more rapid dissociation from the proteasome should improve tissue distribution, we have developed an investigational, dipeptidyl boronic acid proteasome inhibitor (MLN9708). MLN9708 displays comparable potency (i.e. similar Ki) to bortezomib for the β5 site of the proteasome but a shorter (18 min.) half-life of dissociation. This affects rapid recovery of proteasome activity in tissue culture cells upon washout of the drug as well as reduced blood/plasma partitioning in mice, supporting the hypothesis that a more rapid equilibrium proteasome inhibitor can improve tissue distribution. MLN9708 is currently in phase 1 clinical trials. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C99.

Associating in vitro target binding and in vivo CNS occupancy of serotonin reuptake inhibitors in rats: The role of free drug concentrations

The present study aimed at investigating the theory that free (unbound) active site concentrations are the best predictors of target binding of compounds blocking the serotonin transporter (Sert) in the central nervous system (CNS).Thirteen serotonin reuptake inhibitors were evaluated for their Sert-binding affinities in vitro and in vivo in rats together with their unbound fractions in plasma and brain. Cortical Sert occupancy was used in vivo to acquire EC50-estimates from total plasma, free plasma, whole brain, and free brain concentrations after acute drug administration.The in vitro–in vivo Sert occupancy analyses showed that the best correlation was achieved when unbound brain concentrations were employed. Unbound brain concentrations also provided a better correlation when compared with unbound plasma concentrations, which could be related to lack of equilibrium between plasma and brain at time of measurements or involvement of active brain efflux processes. In addition, brain-free fractions were shown to be directly correlated to the lipophilicity of the compounds.These data emphasize the use and impact of applying free fraction data in assessment of pharmacological in vitro–in vivo correlations and demonstrates its use to validate in vivo Sert occupancy as pharmacodynamic marker for serotonin reuptake inhibitors in rats.

The Y-Procedure methodology for the interpretation of transient kinetic data: Analysis of irreversible adsorption

This paper deals with the analysis of transient kinetic data obtained in the Thin-Zone (TZ) configuration of the Temporal Analysis of Products (TAP) reactor. The Y-procedure that reconstructs the reaction rate and gas composition in the catalytic zone with no assumptions on the reaction mechanism ( Yablonsky et al., 2007) was applied to irreversible adsorption with coverage-dependent kinetic coefficients. We have used the Y-Procedure analysis to develop a novel framework for the interpretation of non-steady-state kinetic data. This framework was illustrated by multiple numerical examples concerning irreversible adsorption, and by experimental data corresponding to oxygen adsorption on polycrystalline platinum. The total concentration of active sites was estimated as 6 × 10 − 7 mol / g cat . It was found that for the surface coverage up to 0.8, kinetics followed the dissociative adsorption model with an intrinsic adsorption constant of 1.12 × 10 6 m 3 / mol / s . Kinetic parameters estimated via the Y-procedure agreed well with the values obtained via the moment analysis. The methodology developed in this paper will guide the systematic application of the Y-Procedure analysis to more complex catalytic systems.

ChemInform Abstract: Variable Temperature IR Spectroscopy in the Studies of Oxide Catalysts

AbstractReview: 25 refs.

Rectangular MgO microsheets with strong catalytic activity

Rectangular hydromagnesite microsheet is prepared by simple solution route through controlled hydrolysis-condensation of magnesium chloride in urea solution using ethylene glycol (EG) as a bifunctional cosolvent. Calcination of the hydromagnesite sheets at 450 °C produces MgO without any alteration in the morphology. The MgO sheets were of 2–3 μ side length and 4–5 nm thickness, and made of highly crystalline MgO nano-particles, which show reasonably high BET surface area of 106 m 2 g −1. These MgO microsheets showed very strong activity as heterogeneous base catalyst in the solvent free Claisen–Schmidt condensation of benzaldehyde with acetophenone giving 99.0% conversion in less than 4 h with almost no loss of activity in three successive cycles. The strong activity is correlated with its large surface area and presence of oxide ions in low coordination (LC) O LC 2− (where LC = 5C, 4C and 3C for terrace, edge, corner and kink sites, respectively) along with defects, which was revealed by photoluminescence (PL) emission studies. Presence of basic sites of different strength, determined by temperature programmed desorption of carbon dioxide (TPD-CO 2) also supports the strong catalytic activity of the MgO micro-sheets. Increase in calcination temperature to 650 °C destroys the open sheet morphology and enhances crystallite size, which causes decrease in catalytic activity due to low surface area, constrain in mass transport from the core structure and low concentration of active basic sites.

Influence of the structural and surface characteristics of activated carbon on the catalytic decomposition of hydrogen peroxide

Hydrogen peroxide decomposition has been studied with three activated carbons before and after different modifications upon HCl lixiviation, oxidation with HNO 3 and heat treatment in inert atmosphere. The surface of activated carbon promotes H 2O 2 decomposition to an extent depending on related-structural features and the amount and nature of surface oxygen groups. A lower ordering of the carbon structure with less developed graphene layers leads to a higher decomposition rate due to a higher surface concentration of unpaired electron active sites. In activated carbons with a similar structural ordering, the presence of acidic surface oxygen groups inhibits the decomposition of hydrogen peroxide. Taking into account these two phenomena, empirical equations based on the most reactive carbon fraction by TPO, defects aliphatic carbon fraction, assessed by XPS, and the content of surface oxygen groups, determined by TPD, have been developed for describing the hydrogen peroxide decomposition ability of activated carbons.

Modeling of a multizone gas‐phase polyethylene reactor with a cluster‐based approach

AbstractA circulating fluidized reactor of polyethylene was modeled with the proper hydrodynamics for a riser and downer and combined with a kinetic model based on the moment equations. The hydrodynamic model was able to predict the profiles of the following parameters through the riser and downer: cluster velocity, bed porosity, concentration of potential active sites, active sites, gas‐phase components, molecular weights, and reactor temperature. It was shown that one could control the monomer consumption and molecular weight, which are crucial in the reactor behavior and production properties, respectively, by setting different operating hydrodynamic conditions, such as the gas velocity in the riser and the solid circulation rate. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

On the mechanism of nascent site deactivation in graphene

When studying the ubiquitous and practically significant phenomena of annealing or aging of carbon materials, it is instructive to distinguish the reduction of site (re)activity from a decrease in the concentration of (re)active sites. While the latter has been analyzed extensively for different carbon precursors and heating rates, the former – being more complex even though it can occur at constant temperature – has not received sufficient attention. Here we use computational quantum chemistry to define more precisely and quantify the phenomenon of nascent site deactivation (NSD). The NSD process, which determines the fate of so-called “dangling bonds” in graphene, takes place within a relatively short time (e.g., seconds) and is a consequence of radical stabilization during formation or reactions of sp 2-hybridized carbon materials, both flat and curved. Stabilization of reactive zigzag sites results in the formation of carbene-like edges. Free armchair sites can also rehybridize, to form carbyne-like edges. Adjacent zigzag edges can reconstruct to form pentagon–heptagon pairs that preserve planarity but convert the reactive sites from zigzag to armchair. We report the thermodynamic driving forces for these fundamental processes by determining the optimized geometries of ground states, as well as charge and spin density distributions, in prototypical graphene molecules.

The effect of iron nanocrystallites’ size in catalysts for ammonia synthesis on nitriding reaction and catalytic ammonia decomposition

Abstract Iron catalyst for ammonia synthesis of various mean sizes of iron nanocrystallites were nitrided with ammonia in a differential reactor equipped with systems that made it possible to conduct both thermogravimetric measurements and hydrogen concentration analyses in the reacting gas mixture. The nitriding process was investigated under atmospheric pressure at the temperature of 475°C. It was found that along with an increase of mean size of iron nanocrystallites, with a decrease of specific surface area of the samples, nitriding degree of solid samples increased. At the same time the rate of surface reaction of catalytic ammonia decomposition decreased. Along with an increase of the samples’ specific surface area an increase of the catalyst’s activity was observed. However, it was also observed that the concentration of active sites on the catalysts’ surface decreased along with an increase of specific surface area.

Removals of fly ash and NO in a fluidized-bed reactor with CuO/activated carbon catalysts

This study investigates the effects of fly ash compositions (SiO2 and Al2O3), particle sizes (4–10μm and 40μm), and concentrations on the simultaneous removals of fly ash and NO using a fluidized-bed catalyst reactor. Experimental results show that the removal efficiencies of fly ash and NO at particle concentrations of 968–11,181mgm−3 are 71–97% and 42–57%, respectively. SiO2 particles have more influences than Al2O3 particles on the performances of fluidized-bed CuO/AC catalyst. As the concentration of fine particle increases, the pores and active sites on catalyst surface are obstructed and therefore the activities of catalysts are depressed.

Optically Transparent Cathode for Dye-Sensitized Solar Cells Based on Graphene Nanoplatelets

Commercial graphene nanoplatelets in the form of optically transparent thin films on F-doped SnO(2) (FTO) exhibited high electrocatalytic activity toward I(3)(-)/I(-) redox couple, particularly in electrolyte based on ionic liquid (Z952). The charge-transfer resistance, R(CT), was smaller by a factor of 5-6 in ionic liquid, compared to values in traditional electrolyte based on methoxypropionitrile solution (Z946). Optical spectra and electrochemical impedance confirm that the film's absorbance scales linearly with R(CT)(-1). Electrocatalytic properties of graphene nanoplatelets for the I(3)(-)/I(-) redox reaction are proportional to the concentration of active sites (edge defects and oxidic groups), independent of the electrolyte medium. Dye-sensitized solar cell (DSC) was assembled with this material as a cathode. Semitransparent (>85%) film of graphene nanoplatelets presented no barrier to drain photocurrents at 1 Sun illumination and potentials between 0 and ca. 0.3 V, but an order of magnitude decrease of R(CT) is still needed to improve the behavior of DSC near the open circuit potential and, consequently, the fill factor. We predict that the graphene composite is a strong candidate for replacing both Pt and FTO in cathodes for DSC.