Absence Of Diffusional Limitations Research Articles

Selective hydrogenation of α‐pinene on a nickel supported aluminophosphate catalyst: Process optimization and reaction kinetics

Abstractcis‐Pinane, the hydrogenation product of pinene, is a key chemical intermediate and its production is an important forest chemical process. In this study, α‐pinene was hydrogenated using a nickel‐supported aluminophosphate catalyst (Ni/APO‐PT) in the batch mode of operation. The pore structure of the catalyst was characterized, the process parameters were studied, the hydrogenation process was optimized, and the kinetic study was conducted. The results showed that the conversion of pinene was positively correlated with temperature, duration, and catalyst dosage. In contrast, the selectivity of cis‐pinane was negatively correlated with temperature and catalyst dosage. An optimization was performed using the response surface methodology (RSM). A selectivity of 96.2% was obtained with α‐pinene conversion of 98.5% under optimal conditions, that is, a temperature of 405 K, reaction time of 60 min, hydrogen pressure of 3 MPa, and catalyst loading of 3.32 wt%. The reusability of the catalyst was studied, and the catalyst was found to maintain efficient activity for seven cycles. The kinetics of hydrogenation were investigated using both linear and nonlinear methods in the absence of diffusional limitation. In the temperature range from 403 to 433 K, the hydrogenation of α‐pinene to pinane was compatible with the pseudo‐first‐order process, and the activation energies for the formation of cis‐pinane and trans‐pinane were 41.8 and 56.1 kJ/mol, respectively. This study shows that the performance of the Ni/APO‐PT catalyst is similar to that of noble metals and the catalyst has the potential for industrial applications.

Biocatalyst engineering of Thermomyces Lanuginosus lipase adsorbed on hydrophobic supports: Modulation of enzyme properties for ethanolysis of oil in solvent-free systems

Different immobilized biocatalysts of Thermomyces lanuginosus lipase (TLL) exhibited different properties for the ethanolysis of high oleic sunflower oil in solvent-free systems. TLL immobilized by interfacial adsorption on octadecyl (C-18) supports lost its 1,3-regioselectivity and produced more than 99% of ethyl esters. This reaction was influenced by mass-transfer limitations. TLL adsorbed on macroporous C-18 supports (616 Å of pore diameter) was 10-fold more active than TLL adsorbed on mesoporous supports (100–200 Å of pore diameter) in solvent-free systems. Both derivatives exhibited similar activity when working in hexane in the absence of diffusional limitations. In addition, TLL adsorbed on macroporous Purolite C-18 was 5-fold more stable than TLL adsorbed on mesoporous Sepabeads C-18. The stability of the best biocatalyst was 20-fold lower in anhydrous oil than in anhydrous hexane. Mild PEGylation of immobilized TLL greatly increased its stability in anhydrous hexane at 40 °C, fully preserving the activity after 20 days. In anhydrous oil at 40 °C, PEGylated TLL-Purolite C-18 retained 65% of its initial activity after six days compared to 10% of the activity retained by the unmodified biocatalyst. Macroporous and highly hydrophobic supports (e.g., Purolite C-18) seem to be very useful to prepare optimal immobilized biocatalysts for ethanolysis of oils by TLL in solvent-free systems.

Stable Fe/ZSM-5 Nanosheet Zeolite Catalysts for the Oxidation of Benzene to Phenol.

Fe/ZSM-5 nanosheet zeolites of varying thickness were synthesized with di- and tetraquaternary ammonium structure directing agents and extensively characterized for their textural, structural, and catalytic properties. Introduction of Fe3+ ions in the framework of nanosheet zeolites was slightly less effective than in bulk ZSM-5 zeolite. Steaming was necessary to activate all catalysts for N2O decomposition and benzene oxidation. The higher the Fe content, the higher the degree of Fe aggregation was after catalyst activation. The degree of Fe aggregation was lower when the crystal domain size of the zeolite or the Fe content was decreased. These two parameters had a substantial influence on the catalytic performance. Decreasing the number of Fe sites along the b-direction strongly suppressed secondary reactions of phenol and, accordingly, catalyst deactivation. This together with the absence of diffusional limitations in nanosheet zeolites explains the much higher phenol productivity obtainable with nanostructured Fe/ZSM-5. Steamed Fe/ZSM-5 zeolite nanosheet synthesized using C22-6-3·Br2 (domain size in b-direction ∼3 nm) and containing 0.24 wt % Fe exhibited the highest catalytic performance. During the first 24 h on stream, this catalyst produced 185 mmolphenol g–1. Calcination to remove the coke deposits completely restored the initial activity.

Hydrogenation of pinene on spent fluid cracking catalyst supported nickel: Langmuir–Hinshelwood kinetic modelling

The kinetics for the pinene hydrogenation to pinane, the transformation of renewable biomass to useful chemicals, using spent fluid catalytic cracking catalyst (SFCC) supported nickel (Ni/SFCC), has been extensively investigated without any organic solvent, in the temperature range 373–393 K and the hydrogen pressure of 2–6 MPa in the absence of diffusional limitation. The non-noble metal catalyst was synthesized by incipient wet impregnation method and characterized by SEM, XPS and ICP. The catalytic effects of various parameters (catalyst weight, temperature and hydrogen pressure) on the reaction were reported, and the novel catalyst exhibited good activity. 17 kinds of Langmuir–Hinshelwood surface reaction mechanisms for the hydrogenation of pinene were proposed and derived. The best model described the experimental data with physically meaningful parameters was obtained by parameter diagnostics and a non-linear regression analysis, in which the surface reaction of adsorbed hydrogen atoms and pinene molecules over nickel catalyst was the rate-controlling step during the entire hydrogenation process. The activation energy of pinene surface hydrogenation to pinane was 118 ± 2.61 kJ mol−1 calculated from the Arrhenius plot.

Development of a membrane adsorber based capture step for the purification of yellow fever virus

Yellow fever (YF) is an endemic disease in some tropical areas of South America and Africa that presents lethality rate between 20 and 50%. There is no specific treatment and to control this disease a highly effective live-attenuated egg based vaccine is widely used for travelers and residents of areas where YF is endemic. However, recent reports of rare, sometimes fatal, adverse events post-vaccination have raised concerns. In order to increase safety records, alternative strategies should be considered, such as developing a new inactivated vaccine using a cell culture based technology, capable of meeting the demands in cases of epidemic. With this goal, the production of YF virus in Vero cells grown on microcarriers and its subsequent purification by chromatographic techniques was studied. In this work we investigate the capture step of the purification process of the YF virus. At first, virus stability was studied over a wide pH range, showing best results for the alkaline region. Considering this result and the pI of the envelope protein previously determined in silico, a strong anion exchanger was considered most suitable. Due to the easy scalability, simplicity to handle, absence of diffusional limitations and suitability for virus handling of membrane adsorbers, a Q membrane was evaluated. The amount of antigen adsorbed onto the membrane was investigated within the pH range for virus stability, and the best pH for virus adsorption was considered to be 8.5. Finally, studies on gradient and step elution allowed to determine the most adequate salt concentration for washing (0.15M) and virus elution (0.30M). Under these operating conditions, it was shown that this capture step is quite efficient, showing high product recovery (93.2±30.3%) and efficient DNA clearance (0.9±0.3ng/dose).

Immobilization of Escherichia coli containing ω‐transaminase activity in LentiKats®

Whole Escherichia coli cells overexpressing ω-transaminase (ω-TA) and immobilized cells entrapped in LentiKats® were used as biocatalysts in the asymmetric synthesis of the aromatic chiral amines 1-phenylethylamine (PEA) and 3-amino-1-phenylbutane (APB). Whole cells were permeabilized with different concentrations of cetrimonium bromide (CTAB) and ethanol; the best results were obtained with CTAB 0.1% which resulted in an increase in reaction rate by 40% compared to the whole cells. The synthesis of PEA was carried out using isopropyl amine (IPA) and L-alanine (Ala) as amino donors. Using whole cell biocatalysis, the reaction with IPA was one order of magnitude faster than with Ala. No reaction was detected when permeabilized E. coli cells containing ω-TA were employed using Ala as the amino donor. Additionally, the synthesis of APB from 4-phenyl-2-butanone and IPA was studied. Whole and permeabilized cells containing ω-TA and their immobilized LentiKats® counterparts showed similar initial reactions rates and yields in the reaction systems, indicating 100% of immobilization efficiency (observed activity/activity immobilized) and absence of diffusional limitations (due to the immobilization). Immobilization of whole and permeabilized cells containing ω-TA in LentiKats® allowed improved stability as the biocatalyst was shown to be efficiently reused for five reaction cycles, retaining around 80% of original activity.

Galacto-oligosaccharides production during lactose hydrolysis by free Aspergillus oryzae β-galactosidase and immobilized on magnetic polysiloxane-polyvinyl alcohol

The synthesis of galacto-oligosaccharides (GOS) by the action of Aspergillus oryzae β-galactosidase free and immobilized on magnetic polysiloxane-polyvinyl alcohol (mPOS-PVA) was studied. A maximum GOS concentration of 26% (w/v) of total sugars was achieved at near 55% lactose conversion from 50%, w/v lactose solution at pH 4.5 and 40 °C. Trisaccharides accounted for more than 81% of the total GOS produced. GOS formation was not considerably affected by pH and temperature. The concentrations of glucose and galactose encountered near maximum GOS concentration greatly inhibited the reactions and reduced GOS yield. GOS formation was not affected by enzyme immobilization in the mPOS-PVA matrix, indicating the absence of diffusional limitations in the enzyme carrier. Furthermore, this water insoluble magnetic derivative was reutilized 10-times and retained about 84% of the initial activity. In addition, the kinetic parameters for various initial lactose concentrations were determined and compared for the free and immobilized enzyme.

Kinetics study of the fluorination of uranium tetrafluoride in a fluidized bed reactor

The kinetics of reaction of the uranium tetrafluoride conversion to the uranium hexafluoride with fluorine gas taking place in a fluidized bed reactor operating in industrial conditions have been studied. The external and internal diffusion effects are investigated by Mears and Weisz–Prater criterions. The kinetic equation for the fluorination of uranium tetrafluoride is developed in the absence of diffusional limitation using an integral method by assuming that the gas flow is of plug or perfectly mixed type. A good agreement is observed between the experimental data and a first-order model with respect to fluorine in the CSTR system. The activation energy of the reaction and the pre-exponential factor are obtained using analytical results from a better model.

Kinetics of photocatalytic VOCs abatement in a standardized reactor

A standardized micro-pilot scale continuous flow apparatus has been built up, using two types of differential photoreactors, and equipped with a refined control of the working conditions in order to study the photocatalytic degradation as a way to abate VOCs in air. Kinetic measurements have been carried out on trichloroethylene (TCE), methanol and benzene as model pollutants. The absence of diffusional limitation and of reaction product effect on the kinetics have both been demonstrated under our working conditions. The influences of concentration, light flux and temperature on the initial degradation rate have been studied. A simple Langmuir–Hinshelwood kinetic model has been verified for TCE and methanol, whereas benzene degradation was more complex and did not follow this simple mechanism. The determined experimental data aim at being useful in the scaling-up of photocatalytic reactors.

Monolithic Bioreactor Immobilizing Trypsin for High-Throughput Analysis

A miniaturized trypsin reactor was prepared by coating a trypsin-containing gel on a porous silica monolith. The trypsin-encapsulated gel was prepared by the sol-gel method. The sol-gel reaction was optimized so that the sol solution containing trypsin forms a thin film on the sol-gel monolith. The trypsin was encapsulated into the gel matrix without losing its activity. The silica monolith was fabricated to fit into a 96-well microtiter plate well and could then be easily removed. The trypsin-immobilized monolith was reacted in the 96-well microtiter plate. After the reaction, the monolith was removed, and the enzymatic activity was measured. The large surface area of the monolith enabled the immobilized trypsin to achieve a high catalytic turnover rate. Furthermore, the kinetic parameter of the immobilized trypsin indicates the absence of diffusional limitations. The durability and repeatability of the fabricated trypsin-coated monolith was tested and found to be satisfactory. The encapsulated trypsin exhibits an increased stability even after continuous use compared with that in free solution. Furthermore, this on-plate bioreactor was applicable to the digestion of protein with multiple cleavage sites.

Scaling behavior of diffusion and reaction processes in percolating porous media

We investigate the diffusion-reaction behavior of two-dimensional pore networks at the critical percolation point. Our results indicate the existence of three distinct regimes of reactivity, determined by parameter xi[triple bond]D/(Kl2), where D is the molecular diffusivity of the reagent, K is its chemical reaction coefficient, and l is the length scale of the pore. First, when the diffusion transport is strongly limited by chemical reaction (i.e., D<<K), we recover the classical scaling behavior Phi approximately Lxi(1/2), where Phi is the mass flux of reagent penetrating the pore space and L is the system size. Second, at an intermediate range of xi values, when the process is influenced by the fractal morphology of the percolation cluster, we observe an anomalous diffusion scaling, Phi approximately L(alpha/2)xiB, with an exponent beta approximately 0.34. Third, in the absence of diffusional limitation (D>>K), the flux of reagent reaches a saturation limit Phi(sat) that scales with the system size as Phi(sat) approximately L(alpha), with an exponent alpha approximately 1.89, corresponding to the fractal dimension of the sample-spanning cluster. We then show that the variation of flux Phi calculated for different network sizes at the second and third regimes can be adequately described in terms of the scaling relation, Phi approximately L(alpha)f(xi/L(z)), where the crossover exponent z approximately 2.69 is consistent with the predicted scaling law alpha=2betaz.

Alkylation of Isobutane with 1-Butene on Zeolite Beta

Alkylation of isobutane with 1-butene was carried out in the liquid phase at 353 K on beta zeolite catalyst samples with varying framework Si/Al ratios and particle sizes. The specific external surface area determined with nitrogen physisorption and t-plot analysis is the key parameter to rationalize alkylation activity and stability. The catalytic activity of zeolite beta samples with specific external surface areas smaller than ca 280 m2g−1suffers from intracrystalline diffusional limitations. The critical particle size to avoid diffusional limitations is estimated at 14 nm. In the absence of diffusional limitations, the alkylate yield obtained during the catalyst lifetime is proportional to the number of Brönsted acid sites in the zeolite and amounts to ca 3 kg mole−1, irrespective of the acid site density. The deactivation of the samples is explained by a site coverage model.

Determination of the acid strength of solid catalysts in water by means of a kinetic tracer

The kinetic analysis of the hydrolysis of acetals in the presence of various solid acid catalysts, dealuminated H-form mordenites, H-montmorillonite and strong ion exchange resins has been carried out. Experiments performed in water as the solvent at different agitation speeds, catalyst weights and acetal concentrations show that the reaction is not controlled by external diffusion, nor by internal diffusion as resulting from the calculation of the Thiele modulus. From the standard plots of the logarithms of the rate constants against the Hammett acidity function H 0 established in homogeneous catalyzed reactions, it is possible to calculate such an acidity function for solid catalysts from the measurement of the reaction rates and extrapolation on the previously established standardization plot. Kinetic analysis of the experimental results allows the determination of the maximum reaction rates in the absence of diffusional limitations and, as a consequence, the determination of the maximum acidity of solids under working reaction conditions. The influence of the dealumination of mordenites is as expected, a maximum of acidity being found for Si Al ratios around 10–15. In a similar way, the perfluorinated sulfonic resin referred to as Nafion is by far the most acidic catalyst. Nevertheless, a leveling effect of the acidity of solids is observed in water as the solvent, the strongest acid being the hydroxonium ion H 3O + with a H 0 limit value of −1.74. For dealuminated mordenites, the relationship found between the measured maximum acidity function and the protonic acidity calculated from the number of protons in the lattice would mean that only 20% of the protons present on the solid are efficient for the catalytic reaction. As in homogeneous catalysis, it is then possible to establish compared acidity scales of solid acids in solvents, water in this work.

Oxygen transfer and culture characteristics of self‐immobilized Solanum aviculare aggregates

Oxygen transfer characteristics of self-immobilized Solanum aviculare cells were measured using aggregates 3.0 to 12.5 mm in diameter. Apparent specific oxygen uptake rates in the absence of external boundary layers varied from 5.9 x 10(-11) to 8.5 x 10(-7) kg kg(-1) s(-1) dry weight, but did not decline continuously with increasing particle size. The effective diffusivity of oxygen in deactivated aggregates increased with particle diameter, varying from 5.0 x 10(-11) to 1.0 x 10(-9) m(2) s(-1) or between 2% and 40% of the molecular diffusivity in water at the same temperature. Gas spaces detected in the larger aggregates were confined to the central core and were not distributed throughout the tissue to facilitate oxygen transfer. Oxygen consumption rates in the absence of diffusional limitations were estimated using the relationship between the observable Thiele modulus and effectiveness factor for zero-order reaction. The calculated results indicated severe oxygen limitations in the aggregates, but were inconsistent with the observation that relatively large S. aviculare aggregates contained a high fraction of viable cells and were capable ofactive growth and steroidal alkaloid synthesis. This work suggests that oxygen delivery is facilitated in living plant cell aggregates by mechanisms which depend on metabolic activity and which do not function in deactivated cells. (c) 1995 John Wiley & Sons Inc.

Spectral analysis of interactions between proteins and dye ligands

Interactions between the triazine dye Cibacron Blue F3GA and the protein lysozyme have been investigated by monitoring the characteristic spectral shift which accompanies the binding phenomenon. This technique, which is both quick and convenient, allows determination of apparent dissociation constants, and in the case of supported dyes, the fraction of total dye available for protein binding. In this paper, this approach has been used to compare experimental results obtained with both free and dextran coupled dye allowing the effects of dye conjugation to be quantified in the absence of diffusional limitations seen with insoluble supports. In addition to equilibrium studies, changes in absorbance have also be used to follow binding kinetics using a “stopped flow” system. Results obtained indicate that dextran-supported dye molecules over a range of dye/dextran ratios show a constant ration of dye availability, but show a decrease in dissociation constant with increased loading. Kinetic studies show the feasibility of applying a standard saturation model to the dye-protein interaction where free dye is used. However, in the case of dextran-supported dyes a model which takes account of dye stacking is required to describe adequately the results obtained.

Studies on the deactivation of immobilized urease

The results of experiments in a fixed-bed reactor and a CSTR containing urease immobilized on a nonporous support and conducted in the absence of diffusional limitations are reported. Kinetic parameters were established by separate batch experiments. The key observation was that the product ammonia attacked the free form of the enzyme and thereby illustrates the importance of mechanism in determining deactivation kinetics.

On the mechanism of immobilized glucose oxidase deactivation by hydrogen peroxide

The result of experiments in a fixed-bed reactor containing glucose oxidase immobilized on a nonporous support and conducted in the absence of diffusional limitations are reported. Kinetic parameters were established by separate batch experiments. The key observation was that, in every case, poisoning by product hydrogen peroxide resulted in a minimum in enzyme activity in the interior of the bed, well away from the ends. The deactivation data were interpreted by fitting the rate constant for poisoning, the only free parameter, to a previously reported theory. The theory postulates several deactivation mechanisms each of which leads to self-consistent kinetics, but the only mechanism which fitted the data assumes that peroxide attack the enzyme when it is the from complexed with glucose. Theory and experiment agreed to within the accuracy (+/- 2%) of the activity measurements.

Determination of intrinsic properties of immobilized enzymes

Staphylococcal nuclease has been insolubilized, directly through its amino groups, on CNBr-activated Sepharose 2B. For kinetic studies, a small substrate (thymidine 5'-(p-nitrophenyl phosphate) 3'-phosphate) has been used to measure the hydrolytic activity. With this system the absence of diffusional limitation has been proven. Eadie-Hofstee analysis of the data has been employed to determine the intrinsic kinetic constants of the insolubilized enzyme. Thek cat-pH andK M-pH profiles and the activation energies are similar for the soluble and for the insolubilized nuclease. At the same time conditions are established in which a stirred batch reactor containing particles of insolubilized nuclease behaves as an open system.

The measurement of effective substrate diffusities within whole cell suspensions using a diffusion‐limited hollow fibre reactor

AbstractAn immobilised whole cell membrane reactor system is described which permits estimation of effective substrate diffusivities within whole cell suspensions. The reactor unit uses a membrane to separate the cell slurry from the nutrient solution (e.g. an artificial kidney hollow fibre dialyser). Whole cells are pumped into the shell side of the unit and clamped off. Substrate solution makes one pass through the dialyser tube side at a flow rate such that its conversion over the complete reactor does not exceed 10%. Under such conditions the reactor operates differentially; previously published mathematical expressions being used in evaluating the effective substrate diffusivity, once η (the effectiveness factor) is determined. The value of η can be estimated readily from the true reaction rate (in the absence of diffusional limitations) and the experimentally observed reaction rate. A hypothetical example is presented. The advantages and disadvantages of the method are discussed.

Hepatic clearance of drugs. I. Theoretical considerations of a "well-stirred" model and a "parallel tube" model. Influence of hepatic blood flow, plasma and blood cell binding, and the hepatocellular enzymatic activity on hepatic drug clearance.

Two commonly used models of hepatic drug clearance are examined. The “well-stirred” model (model I) views the liver as a well-stirred compartment with concentration of drug in the liver in equilibrium with that in the emergent blood. The “parallel tube” model (model II) regards the liver as a series of parallel tubes with enzymes distributed evenly around the tubes and the concentration of drug declines along the length of the tube. Both models are examined under steady-state considerations in the absence of diffusional limitations (cell membranes do not limit the movement of drug molecules). Equations involving the determinants of hepatic drug clearance (hepatic blood flow, fraction of drug in blood unbound, and the hepatocellular enzymatic activity) and various pharmacokinetic parameters are derived. Similarities and differences between the models are explored. Although both models predict similar hepatic drug clearances under a variety of conditions, marked differences between them become apparent in their predictions of the influence of changes in the determinants of drug clearance on various pharmacokinetic parameters.