Intraparticle Diffusion Limitations Research Articles

High‐Resolution Single‐Turnover Mapping Reveals Intraparticle Diffusion Limitation in Ti‐MCM‐41‐Catalyzed Epoxidation

High‐Resolution Single‐Turnover Mapping Reveals Intraparticle Diffusion Limitation in Ti‐MCM‐41‐Catalyzed Epoxidation

High‐Resolution Single‐Turnover Mapping Reveals Intraparticle Diffusion Limitation in Ti‐MCM‐41‐Catalyzed Epoxidation

Shedding light on catalysis: For the first time, transport phenomena and their influence on catalysis are directly observed for individual catalytic particles by single-turnover mapping using high-resolution fluorescence microscopy. The Thiele modulus could be measured with just one experiment, proving the existence of diffusional limitations for the Ti-MCM-41 catalyzed epoxidation of a fluorescent reporter molecule (see picture; TBHP=tert-butylhydroperoxide).

Synthesis of carvacrol by Friedel–Crafts alkylation of o‐cresol with isopropanol using superacidic catalyst UDCaT‐5

AbstractBACKGROUND: Alkylation of o‐cresol with propylene or isopropyl alcohol (IPA) over solid acid catalysts results in the formation of carvacrol, which finds potential applications in the synthesis of fine chemicals, intermediates, specialty chemicals, flavours and fragrances.RESULTS: The present work covers evaluation of novel mesoporous superacidic catalysts namely UDCaT‐4, UDCaT‐5 and UDCaT‐6 in the greener synthesis of carvacrol. The catalysts are modified versions of zirconia showing high catalytic activity, stability and reusability. The catalytic activity increases in the following order: UDCaT‐5 > UDCaT‐4 > UDCaT‐6 > sulfated zirconia. The process was optimized and a mathematical model developed to describe the reaction pathway in liquid phase. Carvacrol could be efficiently obtained with a selectivity up to 82% at an isopropanol conversion of 98% after 2 h over UDCaT‐5 at 180 °C.CONCLUSION: The reaction is free from any external mass transfer as well as intraparticle diffusion limitations and is intrinsically kinetically controlled. An overall second‐order kinetic equation was used to fit the experimental data. The activation energy was found to be 19.2 kcal/mol. The reaction was carried out without any solvent in order to make the process cleaner and greener. Copyright © 2009 Society of Chemical Industry

Selectivity engineering in isopropylation of mesitylene with isopropyl alcohol over cesium substituted heteropolyacid supported on K-10 clay

Alkylation of aromatics catalyzed by solid acids constitutes a class of reactions of both academic and industrial importance. Among alkylation reactions, isopropylation of aromatic compounds has attracted considerable attention. Use of propylene as alkylating agent at very high temperatures leads to coke formation which results in deactivation of the catalyst. The use of isopropanol (IPA) as an alkylating agent is attractive when propylene is not readily available. In situ dehydration of IPA leads to prolonged activity since water of reaction suppresses coke formation. Further, IPA dehydration also generates diisopropyl ether which itself is an excellent alkylating agent. Alkylation of mesitylene with propylene or IPA results in the formation of 2-isopropyl-mesitylene (2-IPMT), which is almost extensively used as a precursor in a number of industrial chemicals. This work covers the evaluation of clay-supported heteropolyacids and sulfated zirconia. A variety of solid acid catalysts such as K-10 clay, sulfated zirconia, Filtrol-24, 20% w/w dodecatungstophosphoric acid (H3PW12O40, DTP) supported on K-10 montmorillonite clay and 20% w/w cesium substituted dodecatungstophosphoric acid (Cs2.5H0.5PW12O40, Cs-DTP) supported on K-10 montmorillonite clay were investigated for the liquid phase isopropylation of mesitylene to 2-IPMT using IPA at much milder conditions vis-a-vis other catalysts reported so far. 20% w/w Cs-DTP/K-10 clay was found to be the best catalyst which gives 98% conversion of limiting component, IPA and 98% selectivity towards the desired product, 2-IPMT after 2 h of total reaction time. This catalyst could be reused without any further chemical treatment, eliminating the effluent disposal problems. The reaction was carried out without using any solvent and the process subscribes to the principles of green chemistry. The catalytic activity is in the following order: 20% w/w Cs-DTP/K-10 clay (most active) > 20% w/w DTP/K-10 clay > Filtrol-24 > Sulfated zirconia > K-10 clay (least active). The effect of various operating parameters and catalyst reusability were also systematically investigated. A mathematical model was proposed to probe into the intricate reaction kinetics and mechanism consistent with the experimental results. The reaction is free from any external mass transfer as well as intraparticle diffusion limitations and is intrinsically kinetically controlled. An overall second order kinetic equation was used to fit the experimental data, under the assumption that all the species are weakly adsorbed on the catalytic sites.

Influence of mass transfer limitations on the enzymatic synthesis of β-lactam antibiotics catalyzed by penicillin G acylase immobilized on glioxil-agarose

Mass transfer effects were investigated for the synthesis of ampicillin and amoxicillin, at pH 6.5 and 25 degrees C, catalyzed by penicillin G acylase immobilized on agarose. The influence of external mass transfer was analysed using different stirring rates, ranging form 200 to 800 rpm. Above 400 rpm, the film resistance may be neglected. Intra-particle diffusion limitation was investigated using biocatalysts prepared with different enzyme loads and agarose with different mean pore diameters. When agarose with 6, 8 and 10% of crosslinking were used, for the same enzyme load, substrates and products concentration profiles presented no expressive differences, suggesting pore diameter is not important parameter. An increase on enzyme load showed that when more than 90 IU of enzyme activity were used per mL of support, the system was influenced by intra-particle mass transfer. A reactive-diffusive model was used to estimate effective diffusivities of substrates and products.

Effect of Mass-Transport Limitations on the Performance of a Packed Bed Membrane Reactor for Partial Oxidations. Intraparticle Mass Transport

For partial oxidation systems, where the reaction order in oxygen of the formation rate of the target product is smaller than the reaction order in oxygen of the consecutive reaction rate toward the waste product, a packed bed membrane reactor can be applied to distributively dose oxygen along the reactor in order to improve the selectivity toward the desired intermediate product. In this paper, how the performance of a PBMR for partial oxidations is influenced by the extent of intraparticle diffusion limitations has been investigated. First, the intrinsic effects for a single catalyst particle are discussed, and subsequently the integral effects over the entire reactor on the conversion and selectivity are studied with numerical models (considering different sets of reaction orders, different ratios of reaction rates of the primary and secondary reaction, and different stoichiometric coefficients). In Part 2, the effects of radial oxygen concentration gradients from the membrane wall to the center of the packed bed are investigated. The main objective of this work is to provide guidelines to assess whether mass-transfer limitations inside the particles (Part 1) and/or from the membrane wall to the center of the catalyst bed (Part 2) affect the reactor performance and to quantify these effects.

Effect of Mass-Transport Limitations on the Performance of a Packed Bed Membrane Reactor for Partial Oxidations. Transport from the Membrane to the Packed Bed

With a packed bed membrane reactor, the product yield can be significantly enhanced for partial oxidation systems, via distributive addition of oxygen to the reaction mixture along the axial coordinate of the reactor, provided that the reaction order in oxygen of the formation rate of the target product is smaller than the reaction order in oxygen of the consecutive reaction toward the waste product. In Part 1, the intrinsic and integral effects of intraparticle diffusion limitations on the performance of the packed bed membrane reactor have been investigated. In this paper, the study is extended by studying the effects of mass-transfer limitations from the membrane wall to the center of the packed bed. Again, first the intrinsic effects are examined, followed by a study of the integral effects over the entire reactor using a two-dimensional reactor model. A modified Thiele modulus Φ‘‘ was defined and it was found that only for Φ‘‘> 1 radial concentration profiles inside the packed bed does the integral reactor performance deteriorate and that for values <1, a one-dimensional reactor model suffices. Asymptotic solutions for high values of the modified Thiele modulus for the relative conversion rate of the main reaction and the relative product selectivity have been derived, which allows a quick (conservative) estimate of the influence of the mass-transfer limitations from the membrane wall to the center of the packed bed.

Study of intra-particle diffusion effect on hydrodesulphurization of dibenzothiophenic compounds

In this paper, the intra-particle diffusion effect on hydrodesulphurization of dibenzothiophenic compounds in light cycle oil was experimentally studied. The catalyst effectiveness factors were determined with a full-size commercial NiMo/Al 2O 3 hydrotreating catalyst and its crushed catalyst particles under similar operating conditions. In the full-size commercial catalyst, significant intra-particle diffusion resistance was present, resulting in reduced sulphur removal reaction rates for all the dibenzothiophenic compounds investigated. The effectiveness factors ranged from only 0.300 to 0.822 in the temperature range of commercial importance. In the crushed catalyst, the intra-particle diffusion limitation was mostly eliminated with the corresponding effectiveness factors between 0.813 and 0.985. The DBTs with little or no steric hindrance effect, and thus with high intrinsic reactivities, had lower effectiveness factors, indicating a stronger intra-particle diffusion resistance. Those with strong steric hindrance effect, and thus with low intrinsic reactivities, had higher effectiveness factors, showing a weaker intra-particle diffusion limitation.

Kinetics of hydrolysis of tetrahydrofurfuryl butyrate in a three phase system containing immobilized lipase from Candida antarctica

Hydrolysis of tetrahydrofurfuryl butyrate (THFB) in heptane was carried out at 30 °C in a triphasic system by using stoichiometric amounts of water and immobilized Candida antarctica lipase (Novozym 435) as catalyst. The three phase system has several advantages in hydrolysis reaction due to the very high solubility of the hydrophobic substrate in the organic medium and suppression of enzyme inhibition by the products alcohol and acid, which are partitioned into the bulk organic phase. The effects of various parameters such as speed of agitation, concentration of the ester, quantity of water, temperature, catalyst loading, and reusability of the enzyme were systematically studied in detail. The catalyst reusability studies showed that the deactivation was reversible. The reaction was free from external mass transfer and intra-particle diffusion limitations. Based on the initial rates and concentration profiles, the hydrolysis reaction was found to obey the ping-pong bi-bi mechanism with inhibition by the reactant water. This also demonstrates the significance of the three phase hydrolysis by enzyme catalysis. It indicates that by taking stoichiometric amounts of reagents, inhibition due to reactants can be reduced in a three phase system and the reusability of the catalyst also can be improved.

Selective Hydrogenation of 1‐Butene Rich Cuts: the Impact of the Intraparticle Diffusion Limitations on the Selectivity

AbstractThe impact of intraparticle diffusion limitations on the selectivity of an industrial reactor for selective hydrogenation of 1‐butyne and 1,3‐butadiene contained in 1‐butene rich cuts was evaluated. To this end, a simple model of a trickle‐bed reactor was employed and actual process operating conditions were chosen. A kinetic model was chosen whose parameters correspond to a commercial catalyst. These parameters were calculated from experiments conducted under industrial operating conditions. The complex diffusion and reaction phenomena occurring inside catalyst pellets placed at different depths of the reactor are comprehensively described. 1‐Butene losses in the range 20–30 %, which are usual in commercial plants, were predicted. It was concluded that the operating pressure is crucial for enhancing process selectivity.

Adsorption-enhanced steam-methane reforming with intraparticle-diffusion limitations

The adsorption-enhanced reaction (AER) process is theoretically analyzed for hydrogen production by steam-methane reforming (SMR). It uses a fixed-bed packed column of an admixture of a SMR catalyst and an adsorbent for selective removal of CO 2 from the reaction zone. A mathematical model taking into account multicomponent (six species) mass balances, overall mass balance, Ergun relation for pressure drop, energy balance for bed-volume element including the heat-transfer to the column wall, and nonlinear adsorption equilibrium isotherm coupled with three main reactions was derived to describe AER process with the intraparticle-diffusion limitations. The numerical solution of the model equations for this process was obtained by using the method of orthogonal collocation. The validity of the model prediction was checked by comparing the simulated results with experimental data from literature. The mechanism of the adsorption-enhanced SMR is studied by analysis of the profiles of the bed concentrations, temperature, velocity, pressure, and reaction and adsorption rates. The intraparticle-diffusion limitations on the adsorption-enhanced SMR are evaluated by the effectiveness factors. The effect of the operating conditions (reaction temperature, pressure and length of adsorptive reactor) on the hydrogen purity, hydrogen productivity and methane conversion is studied by numerical simulation; a high purity of hydrogen product gas (90–98%) with methane as the prime impurity and traces of CO 2 (below 400 ppm) and CO (below 30 ppm) can be produced directly from the adsorptive reactor under conditions of 450–490 °C and 222.9–891.4 kPa.

Cesium-substituted dodecatungstophosphoric acid on K-10 clay for benzoylation of anisole with benzoyl chloride

Synthesis of cesium-substituted dodecatungstophosphoric acid (Cs 2.5H 0.5PW 12O 40) supported on K-10 clay is reported. The catalyst is fully characterized. It serves as a promising catalyst for the acylation of aromatic compounds. The acylation of anisole with benzoyl chloride was studied systematically by using 20% w/w Cs 2.5H 0.5PW 12O 40/K-10 and this catalyst can be reused without any further chemical treatment, eliminating the effluent disposal problem. This acylation reaction is 100% selective toward 4-methoxybenzophenone, which is the desired perfumery intermediate. The effect of various important kinetic parameters was evaluated systematically to establish that the reaction obeys the Eley–Rideal type of mechanism with a very weak adsorption of the reactants. The reaction is free from any external mass transfer as well as intraparticle diffusion limitations and is intrinsically kinetically controlled.

Influence of different silica derivatives in the immobilization and stabilization of a Bacillus licheniformis protease (Subtilisin Carlsberg)

Alcalase 2T, a commercial preparation of Subtilisin Carlsberg, was covalent immobilized onto physiochemically characterized silica supports. The effect of mean pore diameter and surface chemistry on enzyme activity in the hydrolysis of casein has been examined. Two sets of chemically distinct silica supports were used presenting terminal amino (S APTES) or hydroxyl groups (S TESPM-pHEMA). The percentage of immobilized protein was smaller in S APTES (31–39%) than in S TESPM-pHEMA (62–71%), but presented higher total and specific activity. Silicas with large pores (S 1000, 130/1200 Å) presented higher specific activities relative to those with smaller pore sizes (S 300, 130/550 Å). The influence of glutaraldehyde concentration and the time of enzyme coupling to the S 1000S APTES supports was examined. The apparent K m value for the S 1000S APTES immobilized enzyme is lower than the soluble one which may be explained by the partitioning effects of the substrate. No intraparticle diffusion limitations were observed for the immobilized enzyme and therefore the substrate diffusion does not influence the observable kinetics. Finally, the optimum pH, optimum temperature, thermal stability, operational stability, and storage stability of the immobilized and freely soluble enzymes were compared.

Mathematical modelling of Claus reactors undergoing sulfur condensation and evaporation

The paper is dedicated to the mathematical modelling of Claus reaction performance in the packed catalyst bed under conditions of sulfur condensation and evaporation. The proposed mathematical model accounts for heat and mass transfer between reaction gas and solid catalyst, condensation and evaporation of sulfur, reversible catalyst deactivation by liquid sulfur, Claus reaction reversibility, intraparticle diffusion limitations, heat conductivity of the catalyst bed frame. Reverse-flow operation of the catalyst bed have been investigated. The complex process flow-sheets, comprising two and three catalyst beds with intermediate sulfur condensers, have been simulated. It has been shown, that application of the reverse-flow technique provides increase of process efficiency.

Selective hydrogenation of sunflower seed oil in a three‐phase catalytic membrane reactor

AbstractContinuous hydrogenation of sunflower seed oil has been carried out in a novel three‐phase catalytic membrane hydrogenation reactor. The membrane reactor consisted of a membrane impregnated with Pd as the active catalyst, which provided a catalytic interface between the gas phase (H2) and the oil. Hydrogenations were carried out at different pressures, temperatures, and selectivities, and the formation of trans isomers was monitored during the hydrogenation runs. For the three‐phase catalytic membrane reactor, interfacial transport resistances and intraparticle diffusion limitations did not influence the hydrogenation reaction. Hydrogenation runs under kinetically controlled conditions showed that oleic and elaidic acid were not hydrogenated in the presence of linoleic acid. Initial formation of stearic acid was caused by direct conversion of linoleic acid into stearic acid by a shunt reaction. Furthermore, high selectivities led to high trans levels, which is in accordance with the many published data on hydrogenation of vegetable oils in slurry reactors. Finally, the catalytic membrane showed severe catalyst deactivation. Only partial recovery of the catalyst activity was possible.

Mass transfer kinetics in pressure swing adsorption

Many mass transfer kinetic models are used to study varying pressure steps of a pressure swing adsorption (PSA) cycle, namely pressurization and blowdown steps. It is shown that the choice of an appropriate model to account for intra-particle diffusional limitations is essential to simulate accurately PSA processes. It has been demonstrated that, besides mathematical approximations (parabolic profile within the particle), a very important factor that can affect simulation results remarkably is the correctness of the mass transfer kinetic model when regarded as being a mass balance for the adsorbent particle. In fact, neglecting intra-particle gas phase leads to erroneous simulations. Some models widely used in the literature, such as the classic linear driving force (LDF) and the solid diffusion models prove inadequate. It is recommended to use pore diffusion model as well as a new version of the solid diffusion model to get reliable predictions.

Synthesizing Alcohols and Ketones by Photoinduced Catalytic Partial Oxidation of Hydrocarbons in TiO2 Film Reactors Prepared by Three Different Methods

The partial oxidation of cyclohexane to cyclohexanol and cyclohexanone on UV-irradiated titanium dioxide films in the presence of molecular oxygen at low temperatures and atmospheric pressure was studied. Three different coating methodologies (dip coating using titanium isopropoxide and commercially available titanium dioxide particles, sol−gel process, and flame aerosol process) were used to deposit the titanium dioxide films, and their effectiveness in partial oxidation of cyclohexane was compared. Conversions of the cyclohexane in the gas-phase reactor averaged between 1.1 and 8.7% per pass (8-s contact time) for the different film reactors. The yield (formation of ketone and alcohol) exceeded 96% for all the film reactors. No detectable amount of carbon dioxide was generated. The selectivity for ketone formation ranged from 59 to 91%. The films produced by the flame aerosol method resulted in the highest yield per mass of catalyst used and showed no coking and deactivation for a total run time of approximately 10 h (2 cycles). The films were characterized by XRD, SEM, and TEM to establish the phase compositions, morphologies, and primary particle sizes, respectively. The flame aerosol coating resulted in the formation of high surface area aggregates consisting of nanometer-sized primary particles with high density (minimal internal porosity), whereas dip coating resulted in the formation of bulk crystallites that were more susceptible to coking and deactivation. The flame-aerosol-deposited titania particles had more surface sites per unit mass for photooxidation and minimal intraparticle diffusion limitations.

Mathematical modeling of unsteady-state operation taking into account adsorption and chemisorption processes on the catalyst pellet

In this paper the dynamics of hydrocarbons deep oxidation in the catalyst pellet with account of intraparticle diffusion limitations under alternating adsorption stage and oxidation one is studied by means of mathematical modeling. As an example the isopropyl benzene oxidation by oxygen on Cu/Cr/Al oxide catalyst is considered. It is shown that intraparticle diffusion limitation significantly influence both adsorption and oxidation performance. The modeling results demonstrate the existence of the optimal catalyst pellet size, providing the maximum purification efficiency in the adsorption-catalytic process of VOC abatement.

Intraparticle Diffusion Limitations in the Hydrogenation of Monounsaturated Edible Oils and Their Fatty Acid Methyl Esters

Intraparticle diffusion limitation in the hydrogenation and isomerization of fatty acid methyl esters (FAMEs) and edible oils (triacylglycerol, TAG) in porous nickel catalyst was investigated both under reactive and under inert conditions. Under reactive conditions, the diffusion coefficients were determined from the best fits of the model simulations applying the intrinsic reacting kinetics of monounsaturated FAME hydrogenation to experiments under diffusion limited conditions. For 0.02 < PH2 < 0.50 MPa, an effective hydrogen diffusion coefficient of De,H2 = (1.6 ± 0.7) × 10-10 m2/s (T = 443 K) was obtained in Pricat 9910 (sample 1992). TAG hydrogenations, at 373 < T < 443 K and 0.30 < PH2 < 0.50 MPa, appear to be limited by diffusion limitation of TAG, yielding De,TAG = (4.5 ± 2.2) × 10-13 (T = 373 K), (2.0 ± 0.9) × 10-12 (T = 403 K), and De,TAG = (3.3 ± 1.1) × 10-12 m2/s (T = 443 K). The temperature dependency of De follows the relation Deη/T = 2 × 10-7 (m2 Pa)/K (η is the fluid viscosity, Pa s) for 37...

Kinetics and Intraparticle Diffusion Modelling of a Complex Multistep Reaction: Hydrogenation of Acetophenone over a Rhodium Catalyst

In a first step the intrinsic kinetics of acetophenone hydrogenation on a Rh/C catalyst was studied in a semi-batch reactor. Tiny catalyst grains of 17 μm in average diameter were used in order to avoid any intraparticle diffusion limitation. Experiments were performed over a wide range of operating conditions and various Langmuir–Hinshelwood kinetic equations were discriminated over the complete conversion range. The model based on nondissociative adsorption of hydrogen and noncompetitive adsorption of the organic species with the gas molecules was found to fit better all the experimental data. The relevance of its optimized parameters was then discussed. In a second step the intraparticle diffusion limitations were also studied in a semi-batch reactor by varying the particle size. Two models based on the complex kinetics previously established were developed, taking into account the catalyst grain size and shape. After an additional adjustment of one of the adsorption constants ratios, they were found to provide a good representation of the data in terms of activity and selectivity.