Absence Of Mass Transport Limitations Research Articles

Tuning aldol condensation product selectivity by controlling diffusion and reaction rates in zeolites

This study presents a method for quantifying mass-transfer limitations in microporous acid zeolites that combines a titration technique and a reaction–diffusion model. The method involves partially exchanging H+ sites with Na+ sites to adjust the site density and observing the resulting reaction rates for the aldol condensation of acetone and cyclopentanone. When the intrinsic activity per site is uniform throughout the zeolite and in the absence of mass-transport limitations, a linear decrease in the reaction rate is expected as the density of sites is systematically lowered by gradual titration. Conversely, if mass-transport limitations are present, the initial decrease in rate with increasing number of titrated sites is much lower than that observed when a large fraction of the active sites have been titrated. This non-linear variation of observed rate as a function of the density of titrated sites indicates the presence of mass-transport limitations in the system. Near the limit where the density of remaining active sites is low enough to avoid generating a concentration gradient inside the zeolite crystallite, the variation in rate becomes linear. The titration results can be coupled with reaction–diffusion modeling to fit and calculate the Thiele modulus, which provides quantitative information on the extent of mass-transport limitations in the studied porous acidic domains. In the case of acetone-cyclopentanone cross-condensation reaction, the selectivity towards the different products was found to be influenced by the degree of mass transfer limitations. That is, kinetically driven systems favor the cyclopentanone self-condensation product while diffusion-limited environments favor products of the reactant with faster diffusion rates. Accordingly, the manipulation of site density was found to be useful in tuning product selectivity towards specific desired products, which demonstrates the potential of this method for developing and optimizing microporous catalysts with controllable acid site density and provide valuable insight into the relationship between mass-transfer limitations and product selectivity.

Direct Observation of Carbon Dioxide Electroreduction on Gold: Site Blocking by the Stern Layer Controls CO2 Adsorption Kinetics.

Directly observing active surface intermediates represents a major challenge in electrocatalysis, especially for CO2 electroreduction on Au. We use in-situ, plasmon-enhanced vibrational sum frequency generation spectroscopy, which has detection limits of <1% of a monolayer and can access the Au/electrolyte interface during active electrocatalysis in the absence of mass transport limitations. Measuring the potential-dependent surface coverage of atop CO confirms that the rate-determining step for this reaction is CO2 adsorption. An analysis of the interfacial electric field reveals the formation of a dense cation layer at the electrode surface, which is correlated to the onset of CO production. The Tafel slope increases in conjunction with the field saturation due to active site blocking by adsorbed cations. These findings show that CO2 reduction is extremely sensitive to the potential-dependent structure of the electrochemical double layer and provides direct observation of the interfacial processes that govern these kinetics.

Multivalency Effects on the Immobilization of Sucrose Phosphorylase in Flow Microchannels and Their Use in the Development of a High‐Performance Biocatalytic Microreactor

AbstractMicrostructured reactors are emerging engineering tools for the development of biocatalytic conversions in continuous flow. A promising layout involves flow microchannels that are wall‐coated with enzyme. As protein immobilization within closed microstructures is challenging, we suggested a confluent design of enzyme and microreactor: fusion to the silica‐binding module Zbasic2 is used to engineer enzymes for high‐affinity oriented attachment to the plain wall surface of glass microchannels. In this study of sucrose phosphorylase, we examined the effects of multiple Zbasic2 modules in a single enzyme molecule on the activity and adsorption stability of the phosphorylase immobilized in a glass microchannel reactor. Compared to the “monovalent” enzyme, two Zbasic2 modules, present in tandem repeat at the N‐terminus, separated at the N‐ and C‐terminus of an enzyme monomer, or arranged N‐terminally in a protein homodimer, boosted the effectiveness of the immobilized phosphorylase by up to twofold. They attenuated (up to 12‐fold) the elution of the wall‐coated enzyme during continuous reactor operation. The divalent phosphorylase was distributed uniformly on the microchannel surface and approximately 70 % activity could still be retained after 690 reactor cycles. Reaction–diffusion regime analysis in terms of the second Damköhler number (DaII≤0.02) revealed the absence of mass transport limitations on the conversion rate. The synthesis of α‐d‐glucose 1‐phosphate occurred with a productivity of ∼14 mm min−1 at 50 % substrate conversion (50 mm). The use of wall‐coated enzyme microreactors in high‐performance biocatalytic reaction engineering is supported strongly.

Gasification kinetics during entrained flow gasification – Part II: Intrinsic char reaction rate and surface area development

The reaction behaviour of two different coals is analysed in a pressurised entrained flow reactor (PiTER) and a pressurised thermogravimetric analyser (PRETA). The entrained flow reactor is used to produce char samples under conditions relevant to larger scale entrained flow gasifiers, i.e. at temperatures up to 1600°C and pressures up to 2.5MPa. The conversion rate of these char samples is measured under defined conditions in the PRETA using CO2 and H2O atmospheres.Furthermore, the development of char surface area during devolatilisation and char gasification is analysed in the PiTER. The initial surface area after devolatilisation is dependent on pressure. In the later stages of conversion a significant loss of surface area is detected. The decrease in surface area is attributed to a melting of mineral matter as the PiTER is operated above the ash melting temperature of the coals. The development of the mass specific surface area during gasification cannot be described by the Random Pore Model probably because of the ash melting, but by an empirical correlation.The surface area data are combined with the thermogravimetric analysis to derive the intrinsic reaction rates at the char surface. The intrinsic rate is modelled by nth order and Langmuir–Hinshelwood rate equations. The temperature influence on the char–CO2 reaction can be described by an activation energy of 200kJ/mol, and on the char–H2O reaction by an activation energy of 212kJ/mol. The activation energy is found to be independent of the char preparation conditions, whereas the pre-exponential factor significantly decreases for a higher heat treatment severity of the char sample.The model equations can be used to predict the intrinsic gasification behaviour in the absence of mass transport limitations.

Low temperature catalytic methane steam reforming over ceria–zirconia supported rhodium

This paper presents an experimental study of the catalytic steam reforming of methane over newly developed ceria–zirconia supported rhodium as an active candidate catalyst for low temperature sorption enhanced hydrogen production from methane. The kinetic experiments are performed in a tubular fixed bed reactor over a temperature range of 475–725 °C and a total pressure of 1.5 bar in the absence of mass transport limitations. The over all reaction orders in methane and steam are determined to be less than 1 from 475 to 625 °C. At low temperature, most of the gas product is composed of CO 2 and H 2 due to the pronounced influence of the water–gas shift reaction. At higher temperature and low steam/carbon ratio (S/C), this influence is diminished. Inhibitory effects of H 2, CO, and CO 2 on the methane conversion rates are detected. Temperature-programmed steam reforming experiments over ceria–zirconia support revealed insignificant methane adsorption on the surface from 550 to 725 °C. Catalyst deactivation and steady state stability over time were examined. A molecular reaction mechanism is proposed to qualitatively explain the kinetic observations. Two distinct sites are thought to be responsible for the dissociative adsorption of methane and steam on the catalyst and the support surfaces. Methane is dissociatively adsorbed on the rhodium active metal sites and steam is dissociatively adsorbed on the support surface. Surface reactions of carbon containing methane precursors on the interface between the active metal and the support are considered to be the rate determining steps.

The Nucleoside Diphosphate Kinase D (NM23-H4) Binds the Inner Mitochondrial Membrane with High Affinity to Cardiolipin and Couples Nucleotide Transfer with Respiration

Nucleoside diphosphate kinase (NDPK/Nm23), responsible for intracellular di- and triphosphonucleoside homeostasis, plays multiple roles in cellular energetics, signaling, proliferation, differentiation and tumor invasion. The only human NDPK with a mitochondrial targeting sequence is NDPK-D, the NME4 gene product, which is a peripheral protein of mitochondrial membranes. Subfractionation of rat liver and HEK 293 cell mitochondria revealed that NDPK-D is essentially bound to the inner membrane. Surface plasmon resonance analysis of the interaction using recombinant NDPK-D and model liposomes showed that NDPK-D interacts electrostatically with anionic phospholipids, with highest affinity observed for cardiolipin. Mutation of the central arginine (Arg-90) in a surface-exposed basic RRK motif unique to NDPK-D strongly reduced interaction with anionic phospholipids. Due to its symmetrical hexameric structure, NDPK-D was able to cross-link anionic phospholipid-containing liposomes, suggesting that NDPK-D could promote intermembrane contacts. Latency assays with isolated mitochondria and antibody binding to mitoplasts indicated a dual orientation for NDPK-D. In HeLa cells, stable expression of wild type but not of the R90D mutant led to membrane-bound enzyme in vivo. Respiration was significantly stimulated by the NDPK substrate TDP in mitochondria containing wild-type NDPK-D, but not in those expressing the R90D mutant, which is catalytically equally active. This indicates local ADP regeneration in the mitochondrial intermembrane space and a tight functional coupling of NDPK-D with oxidative phosphorylation that depends on its membrane-bound state.

Advanced Concepts for the Kinetic Modeling of Fatty Acid Methyl Esters Hydrogenation

Kinetic studies of the catalytic hydrogenation of vegetable oils and fatty acid methyl esters in liquid-phase are commonly performed in the framework of the Langmuir-Hinshelwood-Hougen-Watson (LHHW) formalism using the competitive and non-competitive adsorption models, which are certainly extreme. Based on the advanced concepts of multicentered adsorption and semi-competitive adsorption, mechanistic kinetic models including a distinction between occupied-sites and covered-sites by the large molecules of FAMEs were formulated without expressing an opinion a priori on whether the adsorption regime is competitive or non-competitive. The theoretical basis of the advanced kinetic modeling is described and successfully applied to three application examples of increasing complexity, including: (a) the hydrogenation of methyl oleate without cis-trans isomerization distinction, (b) the cis-trans isomerization and hydrogenation of the methyl oleate, and (c) the methyl linoleate hydrogenation including the cis-trans isomerization of the methyl oleate. The kinetic studies were carried out using a Ni/?-Al2O3, at 398, 413, 428 and 443 K, under isobaric conditions at hydrogen pressures of 370, 510, and 650 kPa, in the absence of mass-transport limitation. After model discrimination based on statistical analysis and taking into account the physical meaning of the estimated parameters, semi-competitive adsorption models were found to be more realistic than the classical LHHW competitive and non-competitive ones, mainly because they give additional information indicating that the adsorbed molecules of methyl linoleate and methyl oleate could cover up to 12 and 7 surface sites, respectively. These values are in adequate agreement with those expected from a rough computational simulation and seem to be the most interesting result, since they are factual and unattainable from the classical LHHW approaches.

Methyl oleate isomerization and hydrogenation over Ni/α-Al 2O 3: A kinetic study recognizing differences in the molecular size of hydrogen and organic species

The kinetics of the hydrogenation and cis/ trans isomerization of methyl oleate on a Ni/α-Al 2O 3 catalyst was studied in the absence of mass-transport limitation, at 398 K ≤ T ≤ 443 K and 370 kPa ≤ P H 2 ≤ 650 kPa . On the basis of the Horiuti–Polanyi mechanism, involving a σ half-hydrogenated surface intermediate, a kinetic model was derived in the framework provided by the Langmuir–Hinshelwood–Hougen–Watson formalism, using the advanced concept of semi-competitive adsorption. The classical LHHW rate equations for competitive and non-competitive adsorption between the hydrogen and large organic species were matched as asymptotic cases. Statistical results clearly demonstrated the inadequacy of the model approaching non-competitive adsorption to describe the experimental data, but the residual sum of squares between experimental data and model predictions was insufficient to discriminate between the kinetic models based on competitive and semi-competitive adsorption. However, the model considering semi-competitive adsorption gave additional indication that the adsorbed molecules of cis- and trans-methyl oleate could cover up to eleven surface sites, which is in excellent agreement with a rough estimate from primary molecular modeling. This feature seems to be the most fascinating result, since it is factual and unattainable from the classical LHHW approaches. Results and distinctive features characterizing this advanced approach are highlighted. Some insights to improve parameter estimation and adsorption model discrimination are also pointed out.

Liquid-phase hydrogenation of methyl oleate on a Ni/α-Al 2O 3 catalyst: A study based on kinetic models describing extreme and intermediate adsorption regimes

The kinetics of the hydrogenation of methyl oleate on a Ni/α-Al 2O 3 catalyst was studied in the absence of mass-transport limitation, at 398 ≤ T ≤ 443 K and 3.7 ≤ P H 2 ≤ 6.5 bar. The kinetic modeling was performed on the basis of elementary step mechanisms involving different regimes of competition between hydrogen and methyl oleate. Admitting a distinction between occupied-sites and covered-sites by the large molecule of methyl oleate, a rigorous proposal was made to link the seemingly separate kinetic models corresponding to the extreme modes of competitive and non-competitive adsorption, without having to draw the common distinction between two types of surface sites. General rate equations were formulated without expressing opinion a priori on whether the adsorption regime is competitive or non-competitive. Then, typical LHHW rate equations for both extreme adsorption regimes were straightforwardly derived as special cases. Statistical results demonstrated the inadequacy of the models approaching non-competitive adsorption to describe the experimental data but results did not allow a definite discrimination between rival models with competitive and semi-competitive adsorption. A mechanistic model featuring dissociative adsorption of hydrogen, molecule of methyl oleate interacting with a single atom of Ni, and second insertion of hydrogen as RDS, proved to be the best candidate to describe the experimental data satisfactorily with physically reasonable parameters. As a distinctive feature, the model considering semi-competitive adsorption gave additional indication that the adsorbed molecule of methyl oleate could cover up to seven surface sites. From this finding, the semi-competitive model seems to be more realistic than the competitive one.

Kinetic Monte Carlo simulations of electrodeposition: Crossover from continuous to instantaneous homogeneous nucleation within Avrami’s law

The influence of lateral adsorbate diffusion on the dynamics of the first-order phase transition in a two-dimensional Ising lattice gas with attractive nearest-neighbor interactions is investigated by means of kinetic Monte Carlo simulations. For example, electrochemical underpotential deposition proceeds by this mechanism. One major difference from adsorption in vacuum surface science is that under control of the electrode potential and in the absence of mass-transport limitations, local adsorption equilibrium is approximately established. We analyze our results using the theory of Kolmogorov, Johnson and Mehl, and Avrami (KJMA), which we extend to an exponentially decaying nucleation rate. Such a decay may occur due to a suppression of nucleation around existing clusters in the presence of lateral adsorbate diffusion. Correlation functions prove the existence of such exclusion zones. By comparison with microscopic results for the nucleation rate I and the interface velocity of the growing clusters v, we can show that the KJMA theory yields the correct order of magnitude for Iv 2. This is true even though the spatial correlations mediated by diffusion are neglected. The decaying nucleation rate causes a gradual crossover from continuous to instantaneous nucleation, which is complete when the decay of the nucleation rate is very fast on the time scale of the phase transformation. Hence, instantaneous nucleation can be homogeneous, producing negative minima in the two-point correlation functions. We also present in this paper an n-fold way Monte Carlo algorithm for a square lattice gas with adsorption/desorption and lateral diffusion.

Three-phase hydrogenation of d-glucose over a carbon supported ruthenium catalyst—mass transfer and kinetics

The catalytic hydrogenation of d-glucose to d-sorbitol over a 5% Ru/C catalyst was studied in a semi-batch slurry autoclave operating at 373–403 K and 4.0–7.5 MPa hydrogen pressure. The d-glucose concentration was varied between 0.56 and 1.39 mol/l. The kinetic experiments were carried out in the absence of mass transport limitations, which was verified by using measured gas–liquid mass transfer coefficients and estimated diffusion and liquid–solid mass transfer coefficients. Many literature reports suffer from transport limitations. In the operating regime studied the reaction rate showed a first order dependency with respect to hydrogen. A shift in the order of d-glucose was observed. At low d-glucose concentrations (up to ca. 0.3 mol/l) the reaction showed a first order dependency, while at higher concentrations this changed to zero order behavior. No inhibition by sorbitol or mannitol was observed. The kinetic data were modeled using three plausible rate models based on Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetics assuming that the surface reaction is rate-determining. Model 1 involves non-competitive adsorption of hydrogen and d-glucose. Hydrogen adsorption is either molecular or dissociative, but due to the weak adsorption it results in both cases in a linear hydrogen pressure dependency, i.e. the same rate expression; Model 2 is based on competitive adsorption of molecular hydrogen and d-glucose; and Model 3 assumes competitive adsorption of dissociatively chemisorbed hydrogen and d-glucose. All three models described the data satisfactorily and further statistic discrimination between these models was not possible.

Real-time Monitoring of the Interactions of Transforming Growth Factor-β (TGF-β) Isoforms with Latency-associated Protein and the Ectodomains of the TGF-β Type II and III Receptors Reveals Different Kinetic Models and Stoichiometries of Binding

Mature transforming growth factor-beta (TGF-beta) is proteolytically derived from the C terminus of a precursor protein. Latency-associated protein (LAP), the N-terminal remnant of the TGF-beta precursor, is able to bind and neutralize TGF-beta. Mature TGF-beta exerts its activity by binding and complexing members of two subfamilies of receptors, the type I and II receptors. In addition to these signaling receptors, TGF-beta can also interact with an accessory receptor termed the type III receptor. Using a surface plasmon resonance-based biosensor (BIAcore), we determined the mechanisms of interaction of four binding proteins (LAP, the type II and III receptor ectodomains (EDs), and a type II receptor ED/Fc chimera) with three TGF-beta isoforms, and we quantified their related kinetic parameters. Using global fitting based on a numerical integration data analysis method, we demonstrated that LAP and the type II receptor/Fc chimera interacted with the TGF-beta isoforms with a 1:1 stoichiometry. In contrast, the type II ED interactions with TGF-beta were best fit by a kinetic model assuming the presence of two independent binding sites on the ligand molecule. We also showed that the type III ED bound two TGF-beta molecules. Further experiments revealed that LAP was able to block the interactions of TGF-beta with the two EDs, but that the two EDs did not compete or cooperate with each other. Together, these results strongly support the existence of a cell-surface complex consisting of one type III receptor, two TGF-beta molecules, and four type II receptors, prior to the recruitment of the type I receptor for signal transduction. Additionally, our results indicate that the apparent dissociation rate constants are more predictive of the neutralizing potency of these TGF-beta-binding proteins (LAP, the type II and III receptor EDs, and the type II receptor/Fc chimera) than the apparent equilibrium constants.

Photocatalytic Reactors. 3. Kinetics of the Decomposition of Chloroform Including Absorbed Radiation Effects

The rate of decomposition of water pollutants by means of photocatalytic processes can be quantitatively evaluated by using a fully irradiated photoreactor in which the rate of energy absorption is precisely quantified. Using a suspension of titanium dioxide as the active photocatalyst, excess oxygen over the stoichiometric demand, and a mercury arc lamp, we have found that (i) The rate of photocatalytic decomposition of chloroform in water is of first order with respect to [HCCl3], zero order with respect to [O2], and about one-half order with respect to the volumetric rate of energy absorption (VREA). (ii) The effects due to catalyst concentration are accounted for within the VREA dependence. (iii) These findings coincide with a reaction pathway in which, at moderate to high irradiation conditions and in the absence of mass transport limitations, the recombination rate of the photogenerated charge carriers is much faster than the hole trapping by hydroxyl groups attached to surface TiIV atoms (and/or th...

Implications of reaction-diffusion theory for the disinfection of microbial biofilms by reactive antimicrobial agents

A theoretical framework was developed for analyzing the efficacy of antimicrobial agents when applied to microbial biofilms with which they react. Reaction-diffusion theory was adapted to investigate the potential for transport limitation of the overall rate of biofilm disinfection and the rate of antimicrobial penetration into the biofilm. Disinfection efficacy was investigated with simulations that assumed catalytic reaction of the antimicrobial agent with live and dead cells in a uniformly thick slab with simultaneous transformation of live to dead cells by an independent rate process (disinfection). The intrinsic rate of disinfection was assumed to follow first-order dependence on antimicrobial concentration. Zero- and first-order reaction kinetics of antimicrobial agent with biomass were analyzed. Microbial growth and external mass transfer resistance were neglected. Results show that antimicrobial efficacy, defined as the ratio of the observed rate of biofilm disinfection to the rate that would prevail in the absence of mass transport limitation, decreases sharply as the Thiele modulus exceeds one. The reduction in efficacy worsens when the antimicrobial dose is more concentrated or longer. A second case examined the penetration of an antimicrobial agent into a biofilm with which it reacts stoichiometrically, as would be expected with an oxidizing biocide such as chlorine or ozone. The antimicrobial agent eventually penetrates the biofilm by depleting the reactive biomass constituent, but the time scale for penetration can exceed the time scale for transient diffusion in the absence of reaction by orders of magnitute. These results provide a theoretical basis for explaining experimentally observed resistance of biofilms to chemical disinfectants.

On mass transport limitation to the rate of reaction of gases in liquid droplets

Abstract Expressions are derived to readily compare contributions to overall mass transport limitation by gas-phase diffusion, interfacial transport and aqueous-phase diffusion to the overall rate of reaction in liquid droplets. The three criteria previously derived by Schwartz (1986, in Chemistry of Multiphase Atmospheric Systems , pp. 415–471, Springer) for the absence of mass transport limitation are shown to be special cases of the more general criterion obtained here. Sensitivity of the characteristic time to achieve steady state to the accommodation coefficient is examined. Inequalities are established that give lower and upper bounds of mass transport limitation in cases of non-first-order chemical kinetics.