Polymerization of olefins through heterogeneous catalysis. XVIII. A kinetic explanation for unusual effects

The development of a detailed kinetic model describing some of the unusual effects observed in catalyzed olefin polymerization is presented. Based on the method of moments, the model describes the rate effects of hydrogen and comonomers, as well as the ability of certain systems to incorporate long chain branches via internal and/or terminal double bond polymerization. Examples are provided demonstrating the model's ability to predict rates and degrees of polymerization with ethylene, propylene, and 1-hexene monomers. In the case of propylene, multiple insertion mechanisms are modeled and compared with experimental sequence length and end group data. In other examples the model is used to simulate an oscillating metallocene catalyst and a metallocene catalyst capable of branch addition via terminal double bond polymerization. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1053–1080, 1997     

Free radical copolymerization and kinetic treatment of styrene with N-phenylmaleimide

The copolymerization of styrene (M₁) with N-phenylmaleimide (M₂) in chloroform with 2,2′-azobis(isobutyronitrile) as an initiator was investigated. The kinetic parameters, such as reactivity ratios, overall activity energy, and the effect of molar fraction of monomers on the initial copolymerization rate, were determined. The bimolecular termination of the copolymerization was proved. The treatment method proposed by Yoshimura and colleagues was used to estimate quantitatively the contribution of the charge-transfer complex (CTC) and the free monomers in the copolymerization process. The propagation reactivity ratios of CTC and free monomers were calculated by a new method. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1535–1542, 1997     

Polymerization of olefins through heterogeneous catalysis. XVI. Design and control of a laboratory stirred bed copolymerization reactor

A novel lab-scale stirred bed gas phase reactor system has been designed and constructed for investigating the kinetic behaviour of olefin copolymerization reactions using heterogeneous catalysts. The system, equipped for basic temperature and pressure control, also includes an on-line composition control system which maintains close control of reactor composition for gaseous and liquid monomers as well as for hydrogen. With this apparatus, it is now possible to investigate the fundamental kinetic features of the catalyst system with high precision. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 373–382, 1997     

Kinetics and mechanism of homogeneous catalytic hydroxylation of maleic acid by hydrogen peroxide: III. Kinetics of the hydrolysis of cis‐epoxysuccinic acid

The objective of this work was to study the hydrolysis kinetics and also the character of the involvement of the epoxidation catalyst (Na₂WO₄ – sodium tungstate) on the hydrolysis of cis‐epoxysuccinic acid (the initial product in the hydroxylation reaction of maleic acid by hydrogen peroxide). The results obtained at 65 °C clearly revealed that the hydrolysis reaction exhibits a considerably low rate in the absence of a catalyst whilst the rate is significantly enhanced by the introduction of catalytic quantities of Na₂WO₄. The phenomenon of end‐product inhibition was observed in this study and the results obtained permitted the development of a kinetic model consistent with experimental observations. Analysis of the kinetic model shows that the reaction is first order with respect to the concentrations of the catalyst and the epoxide. However, tartaric acid has a strong inhibitive influence on the overall reaction rate. © 1999 Society of Chemical Industry     

Oxidation of Aqueous Sulphur Dioxide Catalysed by Poly-4-vinylpyridine–Cu(II) Complex. Part 2: Combined Homogeneous and Heterogeneous Phase Oxidation

Aqueous phase oxidation of sulphur dioxide at low concentrations catalysed by a PVP–Cu complex in the solid phase and dissolved Cu(II) in the liquid phase is studied in a rotating catalyst basket reactor (RCBR). The equilibrium adsorption of Cu(II) and S(VI) on PVP particles is found to be of the Langmuir-type. The diffusional effects of S(IV) species in PVP–Cu resin are found to be insignificant whereas that of product S(VI) are found to be significant. The intraparticle diffusivity of S(VI) is obtained from independent tracer experiments. In the oxidation reaction HSO₃⁻ is the reactive species. Both the S(IV) species in the solution, namely SO₂(aq) and HSO₃⁻, get adsorbed onto the active PVP–Cu sites of the catalyst, but only HSO₃⁻ undergoes oxidation. A kinetic mechanism is proposed based on this feature which shows that SO₂(aq) has a deactivating effect on the catalyst. A rate model is developed for the three-phase reaction system incorporating these factors along with the effect of concentration of H₂SO₄ on the solubility of SO₂ in the dilute aqueous solutions of Cu(II). Transient oxidation experiments are conducted at different conditions of concentration of SO₂ and O₂ in the gas phase and catalyst concentration, and the rate parameters are estimated from the data. The observed and calculated profiles are in very good agreement. This confirms the deactivating effect of non-reactive SO₂(aq) on the heterogeneous catalysis. © 1997 SCI.     

Kinetic study on the poly(methyl methacrylate) seeded soapless emulsion polymerization of styrene. III. Seeds with crosslinking

In this work, a seeded soapless emulsion polymerization was carried out with crosslinking (XL) poly(methyl methacrylate) (PMMA) as seeds, styrene as monomer, and potassium persulfate (K₂S₂O₈) as initiator to synthesize the PMMA XL–PS composite latex, which we knew as the latex interpenetrating polymer network (IPN). The morphology of the latex IPN was observed by transmission electron microscopy (TEM). It showed a core–shell structure. The kinetic data from the early stages of the reaction of seeded soapless emulsion polymerization showed that the square root of polymer yield (W_p)^1/2 was proportional to the reaction time. The reaction rate decreased with the increase of crosslinking density of PMMA seeds. The core–shell model proposed in our previous work^1–2 was modified to predict the conversion of polymerization over the entire course of the synthesis of PMMA (XL)–PS composite latex. Our modified core–shell kinetic model fitted well with the experimental data. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:425–438, 1997     

Kinetic study of olefin polymerization with a supported metallocene catalyst. I. Ethylene/propylene copolymerization in gas phase

A kinetic study of ethylene homopolymerization and copolymerization is conducted with a supported metallocene catalyst in a gas‐phase reactor. An experimental procedure is developed that minimizes the effect of impurities in the reactor and simultaneously yields consistent and reproducible reaction‐rate data. The effects of operational parameters such as reaction temperature, pressure, and comonomer concentration on the kinetics of both homopolymerization and copolymerization are investigated. Online perturbation techniques are implemented to determine key kinetic parameters such as the activation energies for ethylene propagation and catalyst deactivation. A reaction‐rate order close to 2 is obtained for ethylene homopolymerization from pressure perturbations, while near to first‐order dependency is observed in the presence of propylene. To quantify the effects of the operational parameters, a one‐site kinetic model for homopolymerization and a two‐site kinetic model for copolymerization are proposed. The necessary kinetic parameters in the model are estimated using the POLYRED™ package. The resulting kinetic model represents the kinetic data over a wide range of conditions for this supported metallocene catalyst. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 81–114, 2001     

Exact solution of catalyst inhibition problems: Application to hydrodesulfurization for clean fuel production

In catalyst development, a targeted reaction often is inhibited by a strongly adsorbed species. To help develop mitigation means, it is important to quantitatively relate the inhibition dynamics to catalyst properties. The present study develops a combined modeling and experimental approach to address this problem. A general mathematical model consisting of three nonlinear partial differential equations is reduced to quadratures or two first-order ordinary differential equations. The result is a simple parameter estimation method, which is used for kinetic characterization of an unsupported CoMo sulfide catalyst for desulfurizing 4,6-diethyldibenzothiophene with 3-ethylcarbazole as the inhibitor. The active site densities and adsorption-reaction rate constants are determined from modeling of transient response experiments. The unsupported CoMo catalyst has a higher hydrodesulfurization turnover frequency than a commercial sulfided Co_zMo/Al₂O₃–SiO₂ catalyst in the absence of 3-ethylcarbazole. However, the unsupported CoMo sulfide is about three times less resilient to 3-ethylcarbazole inhibition than the Co_zMo/Al₂O₃–SiO₂ catalyst.     

Kinetics and mechanism of maleic acid hydroxylation by hydrogen peroxide. Part I: Formal kinetic studies by immobilized catalysis in a pseudo-homogeneous system

Formal kinetic studies of the epoxidation and hydroxylation of maleic acid by hydrogen peroxide (H₂O₂) have been carried out in the presence of a molybdenum salt, as catalyst, immobilized on Amberlite IRA-400 polymer resin in the chloride form. Immobilization of the catalyst on the resin was by a sorption process. Hydroxylation by immobilized catalysis afforded higher H₂O₂ selectivity and increased product yield in excess of 85%. Analysis of kinetic results shows that the reaction in the pseudo-homogeneous system follows the typical pseudo-zero order dependence on H₂O₂ concentration and is first order with respect to maleic acid concentration. However, the rate model developed for the immobilized catalyst fits the experimental data smoothly and parameter estimation using the Lineweaver–Burk plot fulfils the kinetic consistency tests without additional mathematical manipulation. The problem of activity and stability of the present catalyst was also investigated.     

Kinetics and modeling of 1-phenyl-1,2-propanedione hydrogenation

Kinetics and modeling of 1-phenyl-1,2-propanedione hydrogenation over cinchonidine-modified Pt/Al₂O₃ catalyst is reported. Hydrogenation experiments carried out in a pressurized autoclave (288 K, 1.2–6.5 bar hydrogen) revealed interesting kinetic effects which inspired the model development. The enantioselectivity towards the (R)-configuration, as well as the reaction rate and regioselectivity, depended on the modifier concentration having a maximum. The enantio- and regioselective effects were explained by the kinetic model, which assumes different number of sites for adsorption of the carbonyl groups of the 1-phenyl-1,2-propanedione as well as for the cinchonidine adsorbed in flat and tilted modes. The number of adsorption sites needed for the different species were obtained from molecular considerations and the hydrogenation rate constants were determined along with the adsorption parameters by non-linear regression analysis. A comparison of model predictions with experimental data revealed that the model accounts for the kinetic regularities.     

Kinetics of iodine poisoning of hydrophobic Pt/SDBC catalyst for hydrogen isotopic exchange reaction

Changes with time in the isotopic exchange reaction rate between H₂ and HDO(v) over a hydrophobic Pt/SDBC (styrene-divinylbenzene copolymer) catalyst induced by iodine have been studied experimentally. The addition of a trace amount of I₂ to the reactor caused a remarkable decrease in the activity of the catalyst. The measurement results by means of the gas chromatographic pulse technique show that the H₂ adsorption onto the catalyst surface was strongly inhibited by iodine. A kinetic model for the poisoning was developed in terms of the competitive adsorption of I₂ with H₂ onto the catalytic active site. The experimental results were explained reasonably by the proposed kinetic model based on the site balance by assuming that I₂ adsorbs non-dissociatively on the active site. The poisoning could be prevented by the conversion of I₂ to I^? or IO₃^?through redox reactions.     

Kinetic study of the catalytic reforming of methane with carbon dioxide to synthesis gas over Ni/La2O3 catalyst

The kinetic behavior of the Ni/La₂O₃ catalyst in the reforming reaction of methane with carbon dioxide was investigated as a function of temperature and partial pressures of CH₄ and CO₂. The apparent activation energy of the reforming reaction was estimated to be 13.2 kcal/mol. It was also found that increase of the H₂ partial pressure leads to a continuous enhancement of the rate of CO formation, due to the simultaneous occurrence of the water-gas shift reaction. The mechanism of the CH₄/CO₂ reaction has been investigated using steady-state isotopic tracing and transient experiments, as well as FTIR, XRD, XPS and HR-TEM techniques. Based on the mechanistic results, a kinetic model was developed, which was found to predict satisfactorily the kinetic measurements. Methane cracking and the surface reaction between C and oxycarbonate species, are suggested to be the rate determining steps of the CH₄/CO₂ reaction over the Ni/La₂O₃ catalyst.     

Copolymerization of ethylene and styrene by homogeneous metallocene catalysts. 1. Theoretical studies with rac-ethylenebis-(tetrahydroindenyl)MCl2 [M=Ti, Zr] systems

A computational study of the ethylene-styrene copolymerization with rac-ethylenebis(tetrahydroindenyl)MCl₂ [M=Ti, Zr] systems using DFT methods is presented. The complexation, coordination and insertion energies for ethylene and styrene monomers as well as for the styrene-ethylene copolymerization steps into the catalytic active site models [Et₂(IndH₄)₂]MCH₃⁺ [M=Ti, Zr] were calculated. The goal of this study is to examine the influence of the metal atom [Ti, Zr] on the copolymerization activity. It could be concluded that zirconocene catalyst is much more active than the titanocene based catalyst. This could be explained by the higher steric congestion around the Ti as compared to the Zr complex. Furthermore, it was found that the primary styrene insertion gives rise to complexes in which the active sites are blocked by the phenyl ring in both metal atoms, so that only the secondary insertion of the styrene is possible. These facts might help to clarify the already published experimental results.     

Indirect Synthesis of Bis(2‐PhInd)ZrCl2 Metallocene Catalyst,Kinetic Study and Modeling of Ethylene Polymerization

Bis(2‐phenylindenyl)zirconium dichloride (bis(2‐PhInd)ZrCl₂) catalyst was synthesized via the preparation of bis(2‐phenylindenyl)zirconium dimethyl (bis(2‐PhInd)ZrMe₂) followed by chlorination to obtain the catalyst. Performance of the catalyst for ethylene polymerization and its kinetic behavior were investigated. Activity of the catalyst increased as the [Al]:[Zr] molar ratio increased to 2333:1, followed by reduction at higher ratios. The maximum activity of the catalyst was obtained at a polymerization temperature of 60 °C. The rate‐time profile of the reaction was of a decay type under all conditions. A general kinetic scheme was modified by considering a reversible reaction of latent site formation, and used to predict dynamic polymerization rate and viscosity average molecular weight of the resulting polymer. Kinetic constants were estimated by the Nelder‐Mead numerical optimization algorithm. It was shown that any deviation from the general kinetic behavior can be captured by the addition of the reversible reaction of latent site formation. Simulation results were in satisfactory agreement with experimental data.     

Kinetic Model of Two‐Phase Epoxidation with Ionic Liquids as Micellar Catalysts

The biphasic catalytic epoxidation of cyclooctene using the ionic liquid (IL) 1,2‐dimethyl‐3‐octyl‐imidazolium perrhenate ([OMMIM]ReO₄) as micellar catalyst and H₂O₂ as oxidant was investigated. Kinetic experiments were carried out in the intrinsic kinetic regime as proved by variation of stirring rate and temperature. Variation of catalyst concentration allowed for determination of the critical micellar concentration (CMC) of the catalytic IL. The effect of substrate concentrations on the reaction rate was also assessed. Based on the experiments, a kinetic model adapted from enzyme catalysis was proposed to account for the micellar reaction environment. The model takes into account the onset of micelle formation at the CMC. The application of the kinetic model illustrated the good agreement with the experimental data. The model will be applied to other micellar epoxidation reactions and for the design of an appropriate reaction setup in the future.     

Kinetics and Modeling of Petroleum Residues Hydroprocessing

Several factors must be taken into consideration while interpreting data on kinetics of reactions occurring during hydroprocessing of petroleum residues, i.e., the origin of feed, properties of catalysts and related diffusion phenomena, catalyst deactivation, form of kinetic model, and experimental system employed. Among the operating parameters, temperature, H₂ pressure, H₂S/H₂ ratio, H₂/feed ratio, and contact time have a pronounced effect on kinetic data as well. Moreover, although of the same distillation cut point, residues may vary widely in composition. For example, the content of metals (vanadium+ nickel) may range from few ppm for residues derived from a sweet crude to more than 1000 ppm for one derived from a heavy crude. The content of asphaltenes and CCR may exhibit a similar variability. For the most part, significant discrepancies among reported values of kinetic parameters can be reconciled by closely examining the conditions of experiment used for determination of kinetic data. Kinetic data are incorporat ed in mathematical models used for predicting the life of catalyst during hydroprocessing operation. Good predictions can be made if reliable kinetic data are used for modeling. In this regard, the selection of experimental conditions for determining kinetic results is crucial to ensure their reliability. Thus, properly designed tests for accelerated aging of catalyst may be the source of a valuable information. Otherwise, erroneous conclusions could be reached.     

Kinetic modelling of the catalytic conversion of synthesis gas

A kinetic study for the one-step conversion of synthesis gas to gasoline on a ZnO–Cr₂O₃–ZSM-5 catalyst is described. On this catalyst, three reactions are involved in the overall transformation of synthesis gas: the methanol synthesis, the conversion of methanol to hydrocarbons and the water–gas shift reaction. Under the operating conditions selected for the study, it was found that the water–gas shift was at equilibrium and the methanol was completely converted to hydrocarbons. Consequently, it was postulated that the kinetics of the limiting reaction step, the methanol synthesis on the ZnO–Cr₂O₃ component, was the one that controls the overall reaction rate. Three kinetic model equations describing the rate of synthesis gas conversion on the bifunctional catalyst, were considered to fit the data of the experimental runs performed in a Berty well-mixed reactor. Those equations were derived under very special conditions where the methanol decomposition term could be neglected. It was also observed that in the kinetic equations a term involving the fugacity of CO₂ was required to predict the rate properly. The catalyst deactivation was also taken into account in the analysis.     

Kinetics of Liquid‐Phase Hydrogenation of Iso‐valeraldehyde to Iso‐amyl Alcohol Over A Ru/Al2O3 Catalyst

The liquid‐phase catalytic hydrogenation of iso‐valeraldehyde to iso‐amyl alcohol was studied in a slurry reactor. The kinetics of liquid‐phase hydrogenation of iso‐valeraldehyde over a 5% Ru/Al₂O₃ catalyst was studied in the range of temperature 373‐393 K and H₂ pressure 0.68‐2.72 MPa using 2‐propanol as the solvent. The selectivity to iso‐amyl alcohol was 100%. The kinetic data were analyzed using a simple power law model. A single site Langmuir‐Hinshelwood type model suggesting dissociative adsorption of hydrogen and surface reaction as the rate‐controlling step provided the best fit of the experimental data. The catalyst could be reused thrice without any loss in activity.     

Microkinetic analysis of the oxidative conversion of methane. Dependence of rate constants on the electrical properties of (CaO) x (CeO2)1−x catalysts

Elementary reaction steps of the oxidative conversion of methane to CO _x and ethane were derived from kinetic data for various (CaO) _x (CeO₂)_1–x catalysts. The rate constants depend on electron and O^2– conductivity as well as on the reducibility of the oxides. It is shown hereby that reactions resulting in increased or in decreased ethane selectivity are interrelated via the same catalyst properties.     

Selective kinetic deactivation model for methanol synthesis from simultaneous reaction of CO2 and CO with H2 on a commercial copper/zinc oxide catalyst

A kinetic model for the deactivation of copper/zinc oxide catalyst during the methanol synthesis has been developed. This model is of the Langmuir-Hinshelwood-Hougen-Watson type and considers two types of active sites for the deactivation of catalyst. One of the site types on copper is allocated for the deactivation of the catalyst due to carbon dioxide while another type is assigned for the deactivation of the catalyst due to carbon monoxide. The parameters of the deactivation rate equations based on the above concept have been determined using the experimental data of Hoffmann (1993). The validity of the deactivation model has been checked by comparing the results predicted by the model with experimental data different than of those used to evaluate the parameters of the model. The good agreement that noticed in this comparison confirmed the idea that CO and CO₂ are responsible at different extent for the deactivation of Cu/ZnO catalyst during methanol synthesis.