Forward Reaction Rate Research Articles

Kinetics of esterification of methanol and acetic acid with mineral homogeneous acid catalyst

In this work, esterification of acetic acid and methanol to synthesize methyl acetate in a batch stirred reactor is studied in the temperature range of 305.15–333.15K. Sulfuric acid is used as the homogeneous catalyst with concentrations ranging from 0.0633mol·L−1 to 0.3268mol·L−1. The feed molar ratio of acetic acid to methanol is varied from 1:1 to 1:4. The influences of temperature, catalyst concentration and reactant concentration on the reaction rate are investigated. A second order kinetic rate equation is used to correlate the experimental data. The forward and backward reaction rate constants and activation energies are determined from the Arrhenius plot. The developed kinetic model is compared with the models in literature. The developed kinetic equation is useful for the simulation of reactive distillation column for the synthesis of methyl acetate.

Monitoring solid oxide CO2 capture sorbents in action.

The separation, capture, and storage of CO2 , the major greenhouse gas, from industrial gas streams has received considerable attention in recent years because of concerns about environmental effects of increasing CO2 concentration in the atmosphere. An emerging area of research utilizes reversible CO2 sorbents to increase conversion and rate of forward reactions for equilibrium-controlled reactions (sorption-enhanced reactions). Little fundamental information, however, is known about the nature of the sorbent surface sites, sorbent surface-CO2 complexes, and the CO2 adsorption/desorption mechanisms. The present study directly spectroscopically monitors Na2 O/Al2 O3 sorbent-CO2 surface complexes during adsorption/desorption with simultaneous analysis of desorbed CO2 gas, allowing establishment of molecular level structure-sorption relationships between individual surface carbonate complexes and the CO2 working capacity of sorbents at different temperatures.

Explaining unexpected data via competitive equilibria and processes in radical reactions with reversible deactivation.

In the game rock-paper-scissors, each of the three options, or pathways, has an equal chance of dominating, that is, rock beats scissors, scissors beats paper, and paper beats rock. In the classical form of the game, there is no one dominant pathway to follow, but they are all balanced in likelihood. However, in chemical reactions with several competing reagents, the question must be asked, are all competing reactions and pathways accessible? A related question is, if there are two, or more reversible processes that compete for the same reagent, will both processes equilibrate simultaneously, or will one process dominate system? Can these competing processes shed light on otherwise puzzling data? Several unexpected and counterintuitive experiments have been reported in radical reactions with reversible deactivation. These unexpected observations can be illustrated by the near absence of the products of conventional bimolecular radical coupling in the radical transformation of methylcobaltamine to acetylcobaltamine. Another counterintuitive observation is a difference in the copolymer composition in some copolymerizations proceeding via reversible-deactivation radical polymerization (RDRP) vs conventional radical polymerization (RP), for example, nitroxide mediated polymerization of styrene and methyl methacrylate. A similarly puzzling phenomenon is the reduction in the branching fraction in poly(acrylates) polymerized by RDRP vs conventional RP. In the three previously mentioned cases, the radicals formed in reversible deactivation radical reactions are identical to those formed in the corresponding conventional radical process; therefore an explanation for the discrepancy in the outcomes of the reaction is needed. Other unexpected observations include the presence of initial periods of slower monomer consumption in RDRP reactions initiated by a conventional radical initiator with certain chain transfer agents, while changing the nature of chain transfer agent can lead to an acceleration of the reaction during the initial period. Similarly, in Cu mediated atom transfer radical polymerization (ATRP) with initiators for continuous radical regeneration (ICAR) initiated by a conventional radical initiator, the rate of polymerization should not depend on either the alkyl halide concentration or the Cu concentration. However, experiments show that the rate could be 10 times faster with alkyl halide than without alkyl halide. Finally, in aqueous media, the presence of active alkyl halides can appear to stop the disproportionation of Cu(I) complexes. These unusual data point to a more complex mechanism than originally envisioned, and in fact all these counterintuitive observations can be explained by the concept of competing equilibria and processes. In these cases, the presence of two or more reactions competing for the same reagent typically causes one pathway to dominate, while the rate of the other pathways are diminished. Alternatively, the competing pathways and processes can cause one or more reversible or pseudoreversible reactions to be imbalanced and lead to products distinct from the case where rates of forward and reverse reactions are balanced. In this Account, the concept of competitive processes and equilibria is developed and used to explain each of the unusual observations highlighted above.

Feasibility of mapping unidirectional Pi-to-ATP fluxes in muscles of the lower leg at 7.0 Tesla.

To assess the feasibility of mapping the kinetics and unidirectional fluxes of inorganic phosphate (Pi) to adenosine triphosphate (ATP) reactions in the entire volume of the lower leg muscles using a three-dimensional saturation transfer (ST) phosphorus (31 P) imaging sequence. We imaged the lower leg muscles of five healthy subjects at 7.0 Tesla. The total experimental time was 45 min. We quantified muscle-specific forward reaction rate constants (k'f ) and metabolic fluxes (Vf ) of the Pi-to-ATP reaction in the tibialis anterior, the gastrocnemius, and the soleus. In the tibialis anterior, k'f and Vf were 0.11 s-1 ± 0.03 (mean ± standard deviation) and 0.34 mM s-1 ± 0.10, respectively. In the gastrocnemius, k'f was 0.11 s-1 ± 0.04 and Vf was 0.37 mM s-1 ± 0.11, while in the soleus muscle k'f was 0.10 s-1 ± 0.02 and Vf was 0.36 mM s-1 ± 0.14. Our results suggest that mapping the kinetics and unidirectional fluxes from Pi-to-ATP in both the anterior and posterior muscles of the lower leg is feasible at ultra-high field and may provide useful insights for the study of insulin resistance, diabetes and aging. Magn Reson Med 74:225-230, 2015. © 2014 Wiley Periodicals, Inc.

Abstract 177: Promotion Of Nitroso-redox Balance By Beta 3 Adrenoceptor Agonism: Therapeutic Implications For Cardiovascular Complications Of Diabetes

Rationale: Disrupted balance between NO and O2.- is central in pathobiology of diabetes-induced cardiomyopathy and vascular dysfunction. We examined if stimulation of β3 adrenergic receptors (β3 ARs), coupled to endothelial nitric oxide synthase (eNOS) activation, would re-establish NO/O2.- balance, relieve oxidative inhibition of key caveolar proteins and protect against diabetes-induced cardiovascular dysfunction. Methods/Results: A hyperglycemic, hyperinsulinemic state was established in male White New Zealand rabbits by infusion of the insulin receptor antagonist S961 (12 μg/kg/h). Diabetes induced NADPH oxidase-dependent glutathionylation (GSS-) of the caveolar proteins Na+-K+ pump’s β1 subunit and eNOS in cardiac myocytes and aorta, an oxidative modification that inhibits the pump and uncouples eNOS. Consistent with this, diabetes was associated with reduced electrogenic Na+-K+ pump current in voltage-clamped cardiac myocytes and impaired endothelium-dependent vasorelaxation. Selective β3 AR agonist CL316243 (CL, 40 μg/kg/h) restored NO levels analysed by spin-trapping of NO-Fe(DETC)2 complexes; decreased diabetes-induced elevation in O2.- measured by HPLC analysis of dihydroethidium oxidation products, improved endothelium-dependent vasorelaxation, and restored the Na+-K+ pump function in cardiac myocytes. These effects were mediated by CL abolishing diabetes-induced increase in eNOS-GSS and β1-GSS through a decrease in forward reaction rate for glutathionylation by suppressing diabetes-induced NADPH oxidase activation, which was further amplified by promotion of de-glutathionylation via enhancement in association of glutaredoxine-1, the enzyme catalysing de-glutathionylation, with eNOS and Na+-K+ pump. Conclusion: β3 AR activation re-established nitroso-redox balance and relieved oxidative inhibition of key caveolar proteins in diabetes. β3 AR agonists are promising in treatment of diabetes-induced cardiovascular complications.

Microkinetic modeling of CO2 hydrolysis over Zn-(1,4,7,10-tetraazacyclododecane) catalyst based on first principles: Revelation of rate-determining step

Density functional theory (DFT) calculations were used to study the mechanism of CO2 hydrolysis by Zn-(1,4,7,10-tetraazacyclododecane), also referred to as zinc–cyclen, and evaluate the associated thermodynamic and kinetic parameters. A microkinetic model was then built based on the kinetics and thermodynamics derived from first principles. The model describes the sigmoidal dependence of the observed reaction rate constant on pH, which is similar to behavior observed for CO2 hydrolysis catalysts in nature, i.e., human carbonic anhydrase. The inflection point is identical to the pKa value of the zinc–cyclen complex, in good agreement with previous results. The overall reaction rate constant was calculated to be 3055M−1s−1, which is in excellent agreement with the range of experimental values reported (2691.5–3300M−1s−1). Importantly, though, the microkinetic model quantifies how the reaction rate and the associated overall rate constant varies as a function of time, underscoring that observed overall rate constants are a function of the species’ concentrations used to extract them, which may not be the initial conditions. The decrease in initial reaction rate over 0–12ms was ascribed to the decrease in the concentration of the catalytic form, Zn-OH−, which was primarily converted to Zn-HCO3−. Through calculating the ratio of the net rate to the forward rate of each elementary reaction, the rate-limiting step of the catalytic cycle was identified as the adsorption of CO2. To aid in the interpretation of the complex microkinetic model, an analytical rate expression was derived based on a simplified potential energy surface comprised of three major regions: the adsorption of CO2, the release of HCO3− via displacement by water, and the deprotonation of Zn-H2O. Analysis showed quantitatively how the observed reaction rate constant depends not only on how fast a CO2 molecule is captured but also on how deep the Zn-HCO3− sits.

Influence of divalent cations adsorption on the performance of electromembrane process

In this study, the adsorption equilibrium and kinetics of divalent cations were studied to describe transport phenomena occurring in an electromembrane process. The adsorption isotherms of divalent cations over cation-exchange resins were fitted to the Langmuir and Freundlich isotherms. Adsorption equilibrium experiments of divalent cations showed the preferential adsorption between adsorbent and adsorbate and Ca2+ showed about five times higher adsorption capacity than Mg2+. Adsorption kinetics was studied using the reversible first-order model and the forward and reverse reaction rate constants were estimated for divalent cations adsorption. The forward reaction rate constants for the removal of divalent cations were much higher than the reverse reaction rate constants, indicating that the rate of adsorption was dominant in ion-exchange process. In this study, the higher removal efficiency of Ca2+ than Mg2+ in an electromembrane process could be understood by the adsorption equilibrium and kinetic study.

Mixed disulfide formation at Cys141 leads to apparent unidirectional attenuation of Aspergillus niger NADP-glutamate dehydrogenase activity.

NADP-Glutamate dehydrogenase from Aspergillus niger (AnGDH) exhibits sigmoid 2-oxoglutarate saturation. Incubation with 2-hydroxyethyl disulfide (2-HED, the disulfide of 2-mercaptoethanol) resulted in preferential attenuation of AnGDH reductive amination (forward) activity but with a negligible effect on oxidative deamination (reverse) activity, when monitored in the described standard assay. Such a disulfide modified AnGDH displaying less than 1.0% forward reaction rate could be isolated after 2-HED treatment. This unique forward inhibited GDH form (FIGDH), resembling a hypothetical ‘one-way’ active enzyme, was characterized. Kinetics of 2-HED mediated inhibition and protein thiol titrations suggested that a single thiol group is modified in FIGDH. Two site-directed cysteine mutants, C141S and C415S, were constructed to identify the relevant thiol in FIGDH. The forward activity of C141S alone was insensitive to 2-HED, implicating Cys141 in FIGDH formation. It was observed that FIGDH displayed maximal reaction rate only after a pre-incubation with 2-oxoglutarate and NADPH. In addition, compared to the native enzyme, FIGDH showed a four fold increase in K0.5 for 2-oxoglutarate and a two fold increase in the Michaelis constants for ammonium and NADPH. With no change in the GDH reaction equilibrium constant, the FIGDH catalyzed rate of approach to equilibrium from reductive amination side was sluggish. Altered kinetic properties of FIGDH at least partly account for the observed apparent loss of forward activity when monitored under defined assay conditions. In sum, although Cys141 is catalytically not essential, its covalent modification provides a striking example of converting the biosynthetic AnGDH into a catabolic enzyme.

Production of High-Purity Hydrogen and Carbon Dioxide Capture by Sorption Enhanced WGS Reaction Process

Global warming has become more and more serious, which is caused by greenhouse gases. Cutting down the emission of CO2 has already become one of the major research target in the world. This study is numerically investigating Thermal Swing Sorption Enhanced Reaction Process on water gas shift (WGS) reaction by Na2O-promoted alumina. According to Le Chateliers law, the forward reaction rates and conversion can be increased by removing some products selected. Therefore, this concept can be used to generate product of high-purity hydrogen. The purified H2 can be sent to gas turbine for generating electrical power or can be used for other energy source. Carbon dioxide can also be recovered and sequestrated to reduce greenhouse gas effects. The method of lines is utilized in simulation, combined with upwind differences, cubic spline approximation and LSODE of ODEPACK software to solve the problem. The concentration, temperature, and adsorption quantity in the bed are integrated with respect to time by LSODE of ODEPACK software. The simulation is stopped when the system reaches a cyclic steady state. In this study, we first simulate breakthrough curve of Na2O-promoted alumina cited from literatures to prove the accuracy of simulation program. The optimal operating conditions of the WGS TSA (temperature swing adsorption) single-bed six-process is obtained by varying operating variables, such as feed time and rinse time. Furthermore, WGS TSA single-bed six-process could achieve 99.89% purity of H2 (dry-basis) as the top product and 90.95% purity and 98.22% recovery of CO2 (dry-basis) as the bottom product.

Reactivities of the sulphydryl groups of horse (Equus caballus) haemoglobin (1013.11)

The kinetics of the reaction of Ellman’s reagent (DTNB) with CysF9[93]β sulphydryl group of the horse haemoglobins were studied at neutral and physiological pH (6.8 < pH > 7.6) ranges under pseudo first order conditions. The reaction is of first order with respect to the DTNB concentration. The reactions are pH dependent of the observed rate constant gave a complex trend. The observed rate shows that at neutral pH, the presence of inositol hexakisphosphate (inositol‐P6) increases the pseudo first order rate constant. For the first time, inositol‐P6 increases rate of forward reaction, kF at neutral pH values by increasing the mean value of the transition constant, Krt3. The Krt3 for the haemoglobin without inositol‐P6 gave the value of 0.138 ± 0.1 while the haemoglobin in the presence of inositol‐P6 gave the Krt3 value of 0.325 ± 0.2. The results show that inositol‐P6 increases the relative population of the t tertiary conformation. So, it increases the reactivity of CysF9[93]β by changing the relative distribution of two protein conformations.

On the Kinetics of the Absorption of Nitric Oxide into Ammoniacal Cobalt(II) Solutions

Experiments were conducted using a custom double-stirred tank reactor to determine the rate constants of reactions between nitric oxide (NO) and both pentaaminecobalt(II) and hexaaminecobalt(II) at temperatures of 298.2 and 303.2 K and pH levels between 8.50 and 9.87 under atmospheric pressure. The NO concentration of simulated flue gas stream ranged from 400 to 1400 ppmv. Ammoniacal cobalt(II) solutions were prepared by adding aqueous ammonia into a cobalt(II) nitrate solution in the presence of concentrated ammonium nitrate. The reaction rate constants were calculated with an enhancement factor for gas absorption associated with parallel chemical reactions. The results showed that the reaction between NO and pentaaminecobalt(II) was first order with respect to both the NO and the pentaamminecobalt(II) ion. Similarly, the reaction between NO and hexaamminecobalt(II) was first order with respect to both the NO and the hexaamminecobalt(II) ion. The forward reaction rate constants of these two reactions were 6.43 × 10(6) and 1.00 × 10(7) L · mol(-1) · s(-1) at 298.2 K, respectively, and increased to 7.57 × 10(6) and 1.12 × 10(7) L · mol(-1) · s(-1) at 303.2 K, respectively. Ammoniacal cobalt(II) solutions also have the potential to simultaneously remove CO2, SO2, and NOx from postcombustion flue gas.

Modeling and CFD‐PBE simulation of an airlift bioreactor for PHB production

ABSTRACTIn the present attempt, a 3D simulation using a commercially available computational fluid dynamics package (FLUENT 6.3.26), and a metabolic model were used to investigate hydrodynamics and production of polyhydroxybutyrate (PHB) in an airlift bioreactor, performed by Tavares et al. (L. Z. Tavares, E. S. da Silva, J. G. C. Pradella. Biochem. Eng. J., 2004; 18, 21–31). An Eulerian approach was applied to model the gas–liquid interactions. In order to account for the combined effect of bubble breakup and coalescence in the bioreactor, a population balance model implemented in the software was used. Biosynthesis of PHB in the bioreactor was examined and maximum forward reaction rate values of thiolase, reductase, and synthase steps were determined as V1(thiolase) = 1.8 mM/min, V1(reductase) = 400 mM/min, and V1(synthase) = 380 mM/min to predict the experimental PHB production rate by Tavares et al. suitably. A simplified reaction was considered for PHB production. Moreover, gas holdup, liquid velocity vectors, shear stress, and volumetric oxygen transfer coefficient were investigated. Also, molar concentration profiles of PHB and glucose within the bioreactor were obtained. © 2014 Curtin University of Technology and John Wiley & Sons, Ltd.

Non-invasive In-cell Determination of Free Cytosolic [NAD+]/[NADH] Ratios Using Hyperpolarized Glucose Show Large Variations in Metabolic Phenotypes

Accumulating evidence suggest that the pyridine nucleotide NAD has far wider biological functions than its classical role in energy metabolism. NAD is used by hundreds of enzymes that catalyze substrate oxidation and, as such, it plays a key role in various biological processes such as aging, cell death, and oxidative stress. It has been suggested that changes in the ratio of free cytosolic [NAD(+)]/[NADH] reflects metabolic alterations leading to, or correlating with, pathological states. We have designed an isotopically labeled metabolic bioprobe of free cytosolic [NAD(+)]/[NADH] by combining a magnetic enhancement technique (hyperpolarization) with cellular glycolytic activity. The bioprobe reports free cytosolic [NAD(+)]/[NADH] ratios based on dynamically measured in-cell [pyruvate]/[lactate] ratios. We demonstrate its utility in breast and prostate cancer cells. The free cytosolic [NAD(+)]/[NADH] ratio determined in prostate cancer cells was 4 times higher than in breast cancer cells. This higher ratio reflects a distinct metabolic phenotype of prostate cancer cells consistent with previously reported alterations in the energy metabolism of these cells. As a reporter on free cytosolic [NAD(+)]/[NADH] ratio, the bioprobe will enable better understanding of the origin of diverse pathological states of the cell as well as monitor cellular consequences of diseases and/or treatments.

A Molecular Modeling Study of N2 Desorption from NO Heterogeneous Reduction on Char

Mechanisms of N2 desorption from NO heterogeneous reduction on char surface were investigated by molecular modeling using density functional theory. All geometric optimization and single point energy calculations were performed at B3LYP/6-31G(d) level. Adsorption of NO on a char surface was a strongly exothermic process. Two different reaction pathways for N2 desorption were obtained by calculations, both of which released more than 1,000 kJ/mol. The energy barrier of the rate-limiting step for N2 desorption was 95 kJ/mol by reaction of surface carbon-nitrogen species with another NO molecule and was 205 kJ/mol by combination of two neighboring surface carbon-nitrogen species. The forward reaction rate of each intermediate reaction was calculated employing conventional transition state theory. Predicted rate constants of rate-limiting steps were 3.17 × 1011 s−1 and 7.52 × 107 s−1 at 1,400 K for the two foregoing reactions, respectively.

The Viedma Deracemization of Racemic Conglomerate Mixtures as a Paradigm of Spontaneous Mirror Symmetry Breaking in Aggregation and Polymerization

Simulations of a chemical kinetics model, based on the free-energy relationships of classical primary nucleation theory, show that the deracemization phenomenon in systems of achiral or fast racemizing compounds yielding enantiopure crystals as the more stable solid phase is a true spontaneous mirror symmetry breaking process (SMSB). That is, the achievement of a stationary chiral state is more stable than the racemic one. The model translates the free-energy relationships determined by the existence of a critical size cluster to a chemical kinetics model, in which the consideration of forward and backward reaction rate constants avoids the misuse of network parameters that violate thermodynamic constraints (microreversibility principle), which would lead to apparent in silico SMSB. The model provides qualitative agreement for deracemizations by mechanical attrition of visible crystals, as well as for those obtained under temperature gradients. The analysis of the effect of the system parameters to obtain a SMSB scenario shows that the network possesses the principal characteristics of SMSB networks: 1) an enantioselective autocatalytic stage, corresponding to the non-linear kinetics of enantioselective (homochiral) cluster-to-cluster growth, and 2) the mutual inhibition step originating in the backward flow of chiral clusters towards smaller achiral clusters, or even to a racemizing monomer. The application of such a SMSB kinetic model to enantioselective polymerizations and to chiral biopolymers is discussed.

Pore diffusion model to predict the kinetics of heterogeneous catalytic esterification of acetic acid and methanol

A novel pore diffusion model in combination with a homogeneous reaction in the bulk solution is proposed to model the kinetic behavior of conversion of acetic acid during its esterification with methanol using ion exchange resin catalyst. The model parameters are determined based on the experimental data of a batch reactor with reactant quantities in stoichiometric ratio. Experimental kinetic data of conversion of acetic acid is obtained for various ranges of parameters such as catalyst loading in g/cc of reactant mixture volume, temperature in the range of 323.15–353.15K, average diameter of catalyst particle varied from 425μm to 925μm and RPM of magnetic stirrer. A multi time scale model is adopted in updating the conversion of acetic acid in the bulk liquid where the measurement is carried out. Since the homogeneous part of catalytic esterification is slow reaction, a large time step is used for updating bulk concentration or conversion, whereas a small time step is used to find the concentration distribution and inward flux of acetic acid inside the catalyst particle in a quasi steady state mode. This multi time scale approach leads to utilizing only the initial time kinetics of reaction for predicting two reaction constants namely kf1, the forward reaction rate constant for homogeneous reaction in bulk solution and kf2, the forward reaction rate constant for reaction inside the catalyst particle. The equilibrium conversion data provides the reaction equilibrium constant. The model predicts the overall kinetics of heterogeneous esterification reaction accurately.

On the photostationary state of the flavylium network of chemical reactions

Abstract Calculation of the quantum yields from the photostationary state, in the cases where the thermal back reaction competes efficiently with the photoproducts formation is described. The system can be mathematically treated as a single equilibrium involving two opposite reactions, one driven by light of constant intensity and the other due to the thermal back reaction. By analogy with excited state redox potential calculations, the photostationary state can be defined by a constant, which is function of the number of absorbed photons by the cis and trans forms, the respective reaction quantum yields, the light intensity, irradiated volume, as well the rates of the thermal forward and backward thermal reactions (in the absence of light). Application of this concept to the flavylium network of chemical reactions is presented. The model compound 4′,7-dihydroxyflavylium was used to test the model.

1H NMR and hyperpolarized 13C NMR assays of pyruvate–lactate: a comparative study

Pyruvate-lactate exchange is mediated by the enzyme lactate dehydrogenase (LDH) and is central to the altered energy metabolism in cancer cells. The measurement of exchange kinetics using hyperpolarized (13) C NMR has provided a biomarker of response to novel therapeutics. However, the observable signal is restricted to the exchanging hyperpolarized (13) C pools and the endogenous pools of (12) C-labelled metabolites are invisible in these measurements. In this study, we investigated an alternative in vitro (1) H NMR assay, using [3-(13) C]pyruvate, and compared the measured kinetics with a hyperpolarized (13) C NMR assay, using [1-(13) C]pyruvate, under the same conditions in human colorectal carcinoma SW1222 cells. The apparent forward reaction rate constants (kPL ) derived from the two assays showed no significant difference, and both assays had similar reproducibility (kPL = 0.506 ± 0.054 and kPL = 0.441 ± 0.090 nmol/s/10(6) cells; mean ± standard deviation; n = 3); (1) H, (13) C assays, respectively). The apparent backward reaction rate constant (kLP ) could only be measured with good reproducibility using the (1) H NMR assay (kLP = 0.376 ± 0.091 nmol/s/10(6) cells; mean ± standard deviation; n = 3). The (1) H NMR assay has adequate sensitivity to measure real-time pyruvate-lactate exchange kinetics in vitro, offering a complementary and accessible assay of apparent LDH activity.

Effects of Cu–ZnO Content on Reaction Rate for Direct Synthesis of DME from Syngas with Bifunctional Cu–ZnO/γ-Al2O3 Catalyst

The effects of Cu–ZnO content on the performance of bifunctional Cu–ZnO/γ-Al2O3 catalysts for dimethyl ether (DME) synthesis from syngas were investigated by varying the weight ratios of Cu–ZnO/γ-Al2O3 prepared by the coprecipitation of Cu–ZnO in a slurry of γ-Al2O3. A higher rate of DME production with CO conversion of 47.6 % and DME selectivity of 61.1 % was observed with the bifunctional catalyst at an optimal weight ratio of CuO/γ-Al2O3 of two, providing a higher surface area of metallic copper and an abundance of weak acid sites. The number of acidic sites on solid-acid γ-Al2O3 is a more crucial factor to enhance DME yield, due to the faster dehydration rate of methanol to DME than that of CO hydrogenation to methanol. Although the first step of methanol synthesis on active copper sites is a rate-limiting step with a low equilibrium value, the second step of the dehydration of methanol to DME on acid sites adjusts the overall rate by enhancing the forward reaction rate of CO hydrogenation to methanol with a simultaneous formation of surplus hydrogen by a water–gas shift reaction. Therefore, the proper design of a high surface area of metallic copper with larger acid sites on the bifunctional CuO–ZnO/γ-Al2O3 catalysts at an optimal ratio, produced by adjusting the weight ratio of CuO/γ-Al2O3, is an important factor for improved catalytic performance.

Experimentally determined dissolution kinetics of SON68 glass at 90 °C over a silica saturation interval: Evidence against a linear rate law

Flow-through dissolution experiments were carried out on the SON68 glass—the inactive analog of the French high-level waste glass—at 90°C and pH=9 over a silica saturation interval [0–150ppm Si(aq)]. Two types of specimens were subjected to dissolution studies: powders (249–150, 149–75 or 74–45μm diameter size fractions) or monoliths (∼1cm×1cm×4mm). At each Si concentration interval individual coupon and glass powder experiments were run at the same flow-to-surface area (q/S) conditions. Under dilute solution conditions, the dissolution rate becomes independent of q/S and define the forward rate of reaction of 0.30g/(m2d). Dissolution rates of powders normalized on either a geometric or a B.E.T. surface area basis were compared to rates determined on monoliths. In every case, the geometric surface area normalized rates matched those of the monoliths, conclusively showing that B.E.T. normalized rates are too low by a factor of ∼3×. In Si-free solutions changes in flow rates resulted in differences in Al released to solution (∼50 to 3140ppb), correlating inversely with dissolution rates. Si was added to input solutions and kept constant (“direct”), but in select experiments Si concentrations were above target concentrations, and then dropped to their target values (“indirect”). Both “direct” and “indirect” Si addition experiments yielded identical dissolution rates within experimental uncertainty. Progressive addition of Si to the input solution caused a decrease in rates from 0.30 to 0.0020g/(m2d), or a factor of ∼150×, but the decrease is strongly non-linear with respect to Si. These data are inconsistent with simple models relating glass dissolution to the chemical affinity of reaction.