Maximum Rate Enhancement Research Articles

Combination of Best Promoter and Micellar Catalyst for Chromic Acid Oxidation of D-Mannitol to Mannose in Aqueous Media

Abstract Chromic acid oxidation of D-mannitol to mannose has been studied in aqueous media. The effect of promoter (PA, phen and bpy), micellar catalyst (SDS, TX-100 and CPC) and their combination is studied. All the reactions were performed under the condition [D-mannitol]T ≫ [Cr(VI)]T. All the promoters accelerate the reaction rate and the rate is highest in presence of phen. In absence of a promoter the anionic surfactant SDS increases the rate followed by Triton TX-100. The cationic surfactant CPC retards the reaction in comparison to the reaction in aqueous media. Although phen is the best promoter in absence of any surfactant the catalyst combination of bpy and SDS produce a maximum rate enhancement.

Influence of cationic gemini and conventional CTAB on the interaction of [Cr(III)-Gly-Tyr]2+ complex with ninhydrin

The kinetics of reaction between ninhydrin and [Cr(III)-Gly-Tyr]2+ complex have been studied spectrophotometrically in aqueous and aqueous–micellar media of CTAB and geminis (16-s-16, s=4, 5, 6) at 353K and pH 5.0. The reaction follows first- and fractional-order kinetics with respect to [Cr(III)-Gly-Tyr]2+ and [ninhydrin], respectively, both in the absence and presence of micelles. The observed rate constant is affected by [CTAB] changes and the maximum rate enhancement is nearly 4-fold. The catalytic role of [CTAB] can be related to the extent of incorporation or association of the [Cr(III)-Gly-Tyr]2+ and ninhydrin into the micelles. Dicationic gemini surfactants provide much better environment for the reaction as compared to the corresponding monocationic counterpart CTAB. Also, whereas typical rate constant (kΨ) increase and leveling-off regions, just like CTAB, are observed with the geminis, the latter produce a third region of increasing kΨ at higher concentrations. The results obtained in micellar media are explained in terms of the pseudo-phase model of the micelles (proposed by Menger and Portnoy and developed by Bunton and Romsted). The kinetic data have been used to evaluate the micellar binding constants KS for [Cr(III)-Gly-Tyr]2+complex and KN for ninhydrin.

Combination of best promoter and catalyst for hypervalent chromium oxidation of l-sorbose to lactone of C5 aldonic acid in aqueous media at room temperature

Chromic acid oxidation of l-sorbose to lactone of C5 aldonic acid has been studied in aqueous media. The effect of promoter (PA, phen and bpy), micellar catalyst (SDS, TX-100 and CPC) and their combination is studied. All the reactions were performed under the condition [l-sorbose]T≫[Cr(VI)]T. All the promoters accelerate the reaction rate. But the rate is highest in the presence of phen. In absence of promoter anionic surfactant SDS increases the rate followed by TX-100 whereas cationic surfactant CPC retards the reaction in comparison to aqueous media. Although phen is the best promoter in the absence of catalyst combination of bpy and SDS produces maximum rate enhancement.

Electrochemical activation of a non noble metal catalyst for the water–gas shift reaction

A novel electrochemical catalyst (Ni/K-βAl2O3/Au) has been prepared, characterized and tested under electrochemical promotion conditions for the water gas shift reaction. At 350 °C, under CO/H2O: 1%/3% composition, negative potential applications in the single chamber cell, i.e., K+ supplied ions to the active Ni catalyst film, caused a strong increase in the catalytic reaction rate for H2 production (maximum rate enhancement ratio ρ = 2.7 at optimal potential conditions). The promotional phenomenon was found to be reversible due to the activation of H2O molecules chemisorption vs. CO with the consequent formation of active OH species. The achieved results show potential for the practical application of the phenomenon of electrochemical promotion of catalysis in the activation of base metal catalysts for this kind of processes.

Briefly Bound to Activate: Transient Binding of a Second Catalytic Magnesium Activates the Structure and Dynamics of CDK2 Kinase for Catalysis

We have determined high-resolution crystal structures of a CDK2/Cyclin A transition state complex bound to ADP, substrate peptide, and MgF(3)(-). Compared to previous structures of active CDK2, the catalytic subunit of the kinase adopts a more closed conformation around the active site and now allows observation of a second Mg(2+) ion in the active site. Coupled with a strong [Mg(2+)] effect on in vitro kinase activity, the structures suggest that the transient binding of the second Mg(2+) ion is necessary to achieve maximum rate enhancement of the chemical reaction, and Mg(2+) concentration could represent an important regulator of CDK2 activity in vivo. Molecular dynamics simulations illustrate how the simultaneous binding of substrate peptide, ATP, and two Mg(2+) ions is able to induce a more rigid and closed organization of the active site that functions to orient the phosphates, stabilize the buildup of negative charge, and shield the subsequently activated γ-phosphate from solvent.

ChemInform Abstract: Cornerstone Works for Catalytic 1,3‐Dipolar Cycloaddition Reactions

AbstractReview: 42 refs.

Selectivity enhancement by hydrogen addition in selective area metal‐organic vapor phase epitaxy of GaN and InGaN

AbstractSelective area metal‐organic vapor phase epitaxy (SA‐MOVPE) allows in‐plane control of layer composition and thickness. The use of relatively wide (>10 µm) selective masks brings about large wavelength modulation, taking advantage of vapor‐phase diffusion of precursors. The wide masks, however, tend to induce nucleation on them, leading to much reduced and uncontrollable wavelength modulation. To obtain deposition selectivity between masks and crystal surface, hydrogen addition was proved to be effective by preventing GaN nucleation on masks. Consequently, the maximum growth rate enhancement of 4.8 and the photoluminescence peak shift of 54 nm (nominally 11% shift in the indium composition) were achieved for InGaN bulk layer.

Cornerstone Works for Catalytic 1,3-Dipolar Cycloaddition Reactions

This review covers the following subjects: (1) The chemistry of N-metalated azomethine ylide as a new 1,3-dipole containing a metal atom is studied. This 1,3-dipole, generated from (N-alkylideneamino)alkanoates upon treatment with a Lewis acid and amine base, undergoes rapid 1,3-dipolar cycloadditions toward electron-deficient alkenes. Especially, α,β-unsaturated carbonyl substrates show high rate acceleration to give pyrrolidine-2,4-dicarboxylates in a highly regio- and endo-selective manner. (2) This 1,3-dipolar cycloaddition reaction of (N-alkylideneamino)acetate can be switched into anti-selective Michael addition reaction by modification of substrate structures as well as reaction conditions. (3) Two types of heterocyclic chiral auxiliaries are demonstrated for the asymmetric 1,3-dipolar cycloadditions. One types are attached at the β-position of α,β-unsaturated esters. The resulting chiral α,β-unsaturated esters are successfully applied to the stereoselective asymmetric cycloadditions of N-metalated azomethine ylides. The other types include 4-benzyl-2,2,5,5-tetramethyloxazolidine-3-acrylamides which are based on the conformational control of the acrylamide reaction site and that of steric shielding of 4-benzyl amoiety. These are applied to the absolutely asymmetric nitrile oxide cycloadditions. (4) The nitrone cycloadditions to electron-deficient alkenes is attained in the presence of a Lewis acid catalyst. This provides the first catalytic stereo control of 1,3-dipolar cycloadditions. (5) The extremly effective rate enhancement of nitrile oxide 1,3-dipolar cycloadditions to the magnesium alkoxides of allylic alcohols. Maximum rate enhancement observed is 16,000 fold rate acceleration of the uncatalyzed reaction. This method offers the first successful rate acceleration in nitrile oxide 1,3-dipolar cycloadditions. (6) Use of magnesium alkoxide of allylic alcohols is also highly effectve in nitrone 1,3-dipolar cycloadditons, resulting in high stereo- and regioselectivities. (7) Catalytic enantioselective 1,3-dipolar cycloadditions are achieved by use of nitrones, nitronates, diazomethane, and nitrile oxides in the presence of tolerant chiral catalysts based on our R,R-DBFOX/Ph ligand.

Micellar and salt effects on the interaction of [Cu(II)‐Gly‐Gly]+ with ninhydrin

AbstractThe effect of cationic micelles of cetyltrimethylammonium bromide (CTAB) on the kinetics of interaction of copper dipeptide complex [Cu(II)‐Gly‐Gly]+ with ninhydrin has been studied spectrophotometrically at 70°C and pH 5.0. The reaction follows first‐ and fractional‐order kinetics, respectively, in complex and ninhydrin. The reaction is catalyzed by CTAB micelles, and the maximum rate enhancement is about twofold. The results obtained in the micellar medium are treated quantitatively in terms of the kinetic pseudophase and Piszkiewicz models. The rate constants (kobs or kΨ), micellar‐binding constants (kS for [Cu(II)‐Gly‐Gly]+, kN for ninhydrin), and index of cooperativity (n) have been evaluated. A mechanism is proposed in accordance with the experimental results. The influence of different inorganic (NaCl, NaBr, Na2SO4) and organic (NaBenz, NaSal) salts on the reaction rate has also been seen, and it is found that tightly bound/incorporated counterions are the most effective. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 556–564, 2007

Enhanced heat transfer in inclined solar chimneys by electrohydrodynamic technique

Heat transfer enhancement of natural convection inside the inclined solar chimneys is investigated using electrohydrodynamic technique. The interactions between electric field, flow field, and temperature field are analyzed. The ranges of parameters considered are 10 4⩽ Ra⩽10 7, 7.5 kV⩽ V 0⩽17.5 kV, 30°⩽ θ⩽120°, and 2⩽aspect ratio⩽14. Flow and heat transfer enhancements are significantly influenced at low Rayleigh number. The optimum inclined angle which obtains maximum volume flow rate and heat transfer is found to be at θ=60°. A maximum volume flow rate enhancement is expressed in relation with the number of electrodes. The relation between aspect ratio of chimney and number of electrodes that performs the optimum condition between efficiency and economy is analyzed incorporating with all concerning parameters.

Omnidirectional light emission via surface plasmon polaritons

The simulations and experiments on light emitters within a planar, metal/insulator/metal cavity demonstrate an omnidirectional resonance. This is a resonance for which the maximum decay rate enhancement in the cavity occurs at a well-defined wavelength independent of emission angle. It is shown numerically that the resonance occurs when the cavity thickness is a quarter of the surface plasmon wavelength. Experiments on a gold/blue-emitting polymer/gold cavity support this. Further simulations show that low loss omnidirectional cavities have an emission intensity that is largely independent of angle, which may find application for solid state light sources that require a broad emission cone.

Kinetics and mechanism of interaction of dipeptide (glycyl–glycine) with ninhydrin in aqueous micellar media

AbstractThe rates of reaction between ninhydrin and dipeptide glycyl–glycine (Gly–Gly) have been determined by studying the reaction spectrophotometrically at 70°C and pH 5.0 in aqueous and in aqueous cationic micelles of cetyltrimethylammonium bromide (CTAB). The reaction follows first‐ and fractional‐order kinetics, respectively, in [Gly–Gly] and [ninhydrin]. The observed rate constant is affected by [CTAB] changes and the maximum rate enhancement is ca. three‐fold. As the kψ − [CTAB] profile shape is characteristic of bimolecular reactions catalyzed by micelles, the catalysis is explained in terms of the pseudo‐phase model of the micelles (proposed by Menger and Portnoy and developed by Bunton and Romsted). The presence of inorganic salts (NaCl, NaBr, Na2SO4) does not reveal any regular effect but the data with organic salts (NaBenz, NaSal) show an increase in the rate followed by a decrease. The kinetic data have been used to calculate the micellar binding constants KS for Gly–Gly and KN for ninhydrin and the respective values are 317 and 69 mol−1 dm3. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 643–650, 2006

Probing the onset of dense shell packing by measuring the aminolysis rates for a series amine terminated dendrimers

In an effort to probe dendritic structure, the initial rate of an aminolysis reaction was measured for a series of dendrimers with 4, 8, 16, 32 and 64 terminal amine groups (1st–5th generation dendrimers, respectively). From these experiments, a relationship between size and rate was observed. In all cases, the rate was faster than the control experiment using a simple amine ( N-acetylethylene diamine), with the 4th generation dendrimer displaying the maximum rate enhancement (28-fold with respect to the control experiment). The observed increase in rates was due to: (i) hydrophobic binding and (ii) transition state stabilization, (dependent on dendrimer generation). These experimental results supported a preliminary surface density analysis, which predicted the onset of a closed shell/globular structure at or around the 4th generation PAMAM dendrimer.

Nonmonotonic composition dependence of vibrational phase relaxation rate in binary mixtures

We present here isothermal-isobaric N-P-T ensemble molecular dynamics simulations of vibrational phase relaxation in a model system to explore the unusual features arising due to concentration fluctuations which are absent in one component systems. The model studied consider strong attractive interaction between the dissimilar species to discourage phase separation. The model reproduces the experimentally observed nonmonotonic, nearly symmetric, composition dependence of the dephasing rate. In addition, several other experimentally observed features, such as the maximum of the frequency modulation correlation time tau(c) at mole fraction near 0.5 and the maximum rate enhancement by a factor of about 3 above the pure component value, are also reproduced. The product of mean square frequency modulation [<Delta omega(2)(0)>] with tau(c) indicates that the present model is in the intermediate regime of inhomogeneous broadening. The nonmonotonic composition chi(A) dependence of the dephasing time tau(v) is found to be primarily due to the nonmonotonic chi dependence of tau(c), rather than due to a similar dependence in the amplitude of Delta omega(2)(0). The probability distribution of Delta omega shows a markedly non-Gaussian behavior at intermediate composition (chi(A) approximately =0.5). We have also calculated the composition dependence of the viscosity in order to explore the correlation between the composition dependence of viscosity eta(*) with that of tau(v) and tau(c). It is found that both the correlation time essentially follow the composition dependence of the viscosity. A mode coupling theory is presented to include the effects of composition fluctuations in binary mixture.

A common speed limit for RNA-cleaving ribozymes and deoxyribozymes.

It is widely believed that the reason proteins dominate biological catalysis is because polypeptides have greater chemical complexity compared with nucleic acids, and thus should have greater enzymatic power. Consistent with this hypothesis is the fact that protein enzymes typically exhibit chemical rate enhancements that are far more substantial than those achieved by natural and engineered ribozymes. To investigate the true catalytic power of nucleic acids, we determined the kinetic characteristics of 14 classes of engineered ribozymes and deoxyribozymes that accelerate RNA cleavage by internal phosphoester transfer. Half approach a maximum rate constant of approximately 1 min(-1), whereas ribonuclease A catalyzes the same reaction approximately 80,000-fold faster. Additional biochemical analyses indicate that this commonly encountered ribozyme "speed limit" coincides with the theoretical maximum rate enhancement for an enzyme that uses only two specific catalytic strategies. These results indicate that ribozymes using additional catalytic strategies could be made that promote RNA cleavage with rate enhancements that equal those of proteins.

Electrochemical Promotion of NO Reduction by Hydrogen on a Platinum/Polybenzimidazole Catalyst

The electrochemical promotion of catalytic NO reduction by hydrogen was studied using a (NO, Ar), Pt polybenzimidazole Ar) fuel cell at 135°C. A mixture of was used as the working mixture at one electrode and a mixture of was used as reference and counter gas at the other electrode. Products of NO reduction and were analyzed by an on-line mass spectrometer. At high flow rate (17 mL/min; 17 and 354 mL/min, respectively, at atmospheric pressure) the maximum rate enhancement ratio was 4.65. At low flow rate (17 mL/min; 17 and 140 mL/min, respectively), NO reduction increased 20 times even without polarization compared to the high gas flow rate. The electrochemical promotion effect occurs at positive polarization with a maximum increase at approximately 0.08 V and with 1.5 times the zero polarization value. The promotion at the negative polarization can be attributed to the electrochemical production of the promoters. At low gas flow rates, a charge-induced change of the strength of chemisorptive bonds can take place. © 2003 The Electrochemical Society. All rights reserved.

Kinetics and Mechanism of the Reaction of Ninhydrin with Chromium(III)-tryptophan Complex in Absence and Presence of Surfactants

The kinetics of the title reaction have been investigated in both aqueous and micellar cetyltrimethylammonium bromide(CTAB) solutions. Rate data indicate that the reaction follows the template mechanism in both the media. The observed rate constant is affected by [CTAB] changes and the maximum rate enhancement is ca. 4 -fold. The following parameters have been calculated at pH 5.0: for aqueous, kobs = 5.6 χ 10-5 s-1, ΔH* = 61 k J mol-1, ΔS* = -122 J K-1mol-1 ; for CTAB , kΨ = 24.7 χ 10-5 s-1 , ΔH* = 35 k J mol-1, ΔS* = -200 J K-1 mol-1, which reveal CTAB behaving as a catalyst. To explain the dependence of the reaction rate on [CTAB], pseudo-phase model proposed by Menger and Portnoy and modified by Bunton and Rodenas was used. The catalytic role of CTAB can be related to the extent of incorporation or association of the chromium(III)-tryptophan and ninhydrin into the micelles. Various factors which affect km, Ks KN and are discussed taking into account the micellar surface solvent properties.

The effect of size on the rate of an aminolysis reaction using a series of amine terminated PAMAM dendrimers

When compared to an equivalent amine concentration of ethylenediamine, the initial rate of a simple aminolysis reaction in water (at pH 8.5) was found to be greatly enhanced by the use of amine terminated PAMAM dendrimers of generations 1–5 (with 4, 8, 16, 32 and 64 terminal amines, respectively). The fourth generation dendrimer gives the maximum rate enhancement, even when compared to the larger 64 amine terminated dendrimer. The observed increase in aminolysis rates is due to hydrophobic binding of the substrate within the outer region of the larger dendrimers, which results in the substrate being held in close proximity to the reactive amine groups on the surface. Another contributory factor is transition state stabilisation by the internal amide groups within the dendrimers. The trend in reactivities of dendrimers is discussed and provides strong evidence that both these effects are responsible for the enhanced rates of aminolysis.

Kinetics and Mechanism of the Reaction of Copper(II)-tryptophan Complex with Ninhydrin in Aqueous and Micellar Media

Kinetics of the title reaction has been investigated spectrophotometrically in aqueous solution as well as in the presence of cetyltrimethylammonium bromide (CTAB) cationic micelles. The reaction rate is first- and fractional-order with respect to [substrate] and [ninhydrin], respectively, both in the absence and presence of cationic micelles. Whereas SDS addition shows formation of a different product, the reaction of copper(II)-tryptophan complex with ninhydrin is catalyzed by CTAB: the maximum rate enhancement being ca . 5-fold. The data are interpreted in terms of the pseudo-phase model (assuming the association/incorporation of both the reactants at the micellar surface). The binding constants of copper(II)-tryptophan ( K S ) and ninhydrin ( K N ) with CTAB micelles have been calculated. The presence of inorganic salts (NaCl, NaBr, NaNO 3 and Na 2 SO 4 ) leads to a decrease in the rate followed by an increase and it is found that tightly bound/incorporated anions are most effective.

Electrochemical Promotion and Metal–Support Interactions

Ethylene oxidation on IrO2, which is a metal-type conducting metal oxide, and on Pt and Rh was used as a model reaction to compare the rate enhancement and kinetic modification induced by (i) electrochemical promotion (non-Faradic electrochemical modification of catalytic activity effect) via electrochemical O2− supply from YSZ (Y2O3-stabilized ZrO2) and TiO2 and by (ii) metal–support interactions obtained by interfacing IrO2 with TiO2 (with and without electrochemical promotion) or by depositing dispersed Rh on TiO2 and YSZ porous supports. It was found that the addition of TiO2, which is catalytically inactive, to IrO2 submicrometer particles enhances the activity of IrO2 for C2H4 oxidation by a factor of 12 and that the same maximum rate enhancement is obtained via electrochemical promotion of the IrO2 catalyst (i.e., via electrochemical O2− supply to the IrO2 catalyst from a YSZ solid electrolyte). Furthermore it was found that the IrO2–TiO2 catalyst mixtures can only be marginally promoted electrochemically. These observations show conclusively that the mechanism of metal (IrO2)–support (TiO2) interaction in the system IrO2–TiO2 is identical to that of the electrochemically promoted IrO2–YSZ system (i.e., continuous O2− supply to the IrO2 catalyst surface). This conclusion is also corroborated by independent kinetic and XPS studies of electrochemical promotion of Pt utilizing TiO2 (instead of YSZ) as the O2− donor. The kinetics of C2H4 oxidation were investigated on Rh films interfaced with YSZ at various imposed potentials and thus imposed work function values, Φ (i.e., under conditions of electrochemical promotion and also on finely dispersed Rh catalysts deposited on various doped and undoped TiO2 and YSZ porous supports of measured work function Φ). Again it was found that the reaction kinetics are affected in the same way upon varying the work function of the Rh film via electrical polarization of the Rh/YSZ interface or upon varying the work function of the support of the dispersed Rh catalysts. This observation confirms the equivalence of the promoting mechanism of metal–support interactions and electrochemical promotion (i.e., O2− migration onto the catalyst surface). The results show conclusively that electrochemical promotion is an electrically controlled metal–support interaction and that at least certain types of metal–support interactions are induced by reverse spillover of oxygen anions from the carrier onto the surface of the metal crystallites.