Catalyst Fragmentation Research Articles

Catalyst fragmentation during propylene polymerization. III: Bulk polymerization process simulation

AbstractFragmentation of support/catalyst particles during propylene bulk polymerization is analyzed by means of a mathematical model including energy and mass balances, with chemical reaction. The rupture phenomenon is specifically considered by the model and analyzed as it proceeds along time. Model predictions concerning the effects of fragmentation on polymerization are discussed. The influence of mass‐transfer resistances at the macroparticle and microparticle level, as well as the microparticle nucleus‐size effects over the polymerization process, are analyzed. Macroparticle mass‐transfer resistance affects both the rate of fragmentation and temperature excursions. Microparticle nucleus‐size exerts a strong influence over the whole polymerization process. A small micronucleus‐size produces both a delay in the fragmentation process and a greater value of she final catalyst yield. The effects of major critical parameters are evaluated via model simulation, and the results are discussed. The analysis shows that fragmentation depends on the combined effect of the parameters studied. Modeling of the process considering all parameters simultaneously is the proper way of predicting the fragmentation sequence for a given support/catalyst particle. Crystallinity of the produced polymer affects the rate of fragmentation, either increasing or decreasing the rupture rate depending on macroparticle porosity and compactness. Heat transfer conditions in the liquid‐phase system make the temperature runaway problem easy to predict and control, in spite of high polymer yields. The design of “tailor‐made” support/catalyst macroparticles in accordance with catalytic activity is necessary in order to obtain high yields and controlled process temperatures.

Effects of catalyst fragmentation during propylene polymerization. IV: Comparison between gas phase and bulk polymerization processes

AbstractA comparative study of the effects of catalyst fragmentation in the gas phase and bulk polymerization processes in performed. Typical operating conditions for each process are used for the comparison. The monomer concentration in the bulk process is nearly one order of magnitude larger than that in the gas phase process. The energy transfer conditions between the particle and the fluid phase are better in the liquid phase. The rate of mass transfer within the macroparticle at the initial steps of the polymerization is found to be slower in the bulk process than in the gas phase process. In the liquid phase process, fragmentation takes place more slowly. Temperature excursion values are smaller even though the dimensionless monomer concentrations are, in fact, greater because of the higher monomer concentration available in the liquid phase. The final steady rate of reaction and the ultimate catalyst yield reflect this phenomenon. Although the microparticle nucleus size affects both processes, the diffusion control that occurs when the fragments are large affects the gas phase process more intensely. In the bulk process, even at the lowest value reached by the rate of reaction, this is still sufficiently high so as to produce high yields.

Phillips-type polymerization catalysts: kinetic behaviour and active centre determination

The kinetic behaviour of a Phillips-type catalyst in the presence of aluminium alkyls has been studied as a function of ethylene pressure in the range 1–11 atm. The results highlight the importance of catalyst fragmentation, which is further substantiated by morphological electron microscopic studies. Active centre concentration studies have been carried out using a 14CO radio-labelling technique which enabled values of active propagating centres and rate coefficients to be obtained, both of which were found to depend on the ethylene pressure. Only a small percentage of the chromium (3.2–7.3%) is found to be involved in active centres. These results provide valuable information concerning the factors controlling ethylene polymerization by this type of catalyst.

Catalyst fragmentation during propylene polymerization: Part II. Microparticle diffusion and reaction effects

AbstractA mathematical model is used to predict the behavior of the support/catalyst/polymer macroparticle during the gas phase polymerization of propylene. The problem is focused on the micrograin level of the macroparticle, and the emphasis placed on the combined reaction and diffusion processes taking place in the micropartiele. Particular attention is given to the effect of the main geometry parameter, the size of the catalytic nucleus, on polymerization variables. Fragmentation rate, monomer concentration, and temperature are studied in terms of their dependence on the geometry‐related dimensionless numbers typical of the process. A strong influence of the micrograin size on the commanding process variables is predicted by the model. Fragmentation proves relevant to the overall process and critical to define the time scale of temperature excursions. Micrograin nucleus size are found to be important when computing both maximum temperatures and minimum yields. The model suggests criteria for predicting “a priori” the combination of reaction parameters that will produce a diffusion‐limited reaction regime in the microparticle.

Catalyst fragmentation during propylene polymerization: Part I. The effects of grain size and structure

AbstractFragmentation of support/catalyst particles during propylene polymerization in the gas phase is analyzed via a mathematical model including energy and mass transfer with chemical reaction processes. The rupture phenomenon is considered specifically by the model, and evaluated as it proceeds in time, Two different regions are recognized in the polymerizing particle at fragmentation time: an inner core resembling the original solid support/catalyst structure, and an external set of layers where most of the polymerization occurs. Model predictions concerning the effects of fragmentation on polymerization are discussed. The influence of different degrees of fragmentation on thermal runaways and monomer availability at active sites located inside the support/catalyst/polymer complex is shown. Monomer concentration profiles inside the growing particles are explained in terms of the combined fragmentation‐polymerization interaction. Results show a strong influence of catalyst structure on critical phenomena during early polymerization stages, and suggest the possibility of controlling critical parameters via the definition of fragment structure at catalyst preparation time.

Highly Asymmetric-selective and Stereoselective Polymerization of (RS)-α-Methylbenzyl Methacrylate with Cyclohexyl-magnesium Bromide-Axially Dissymmetric 2,2′-Diamino-6,6′-dimethylbiphenyl System

Enantiomer-selective polymerization of (RS)-α-methylbenzyl methacrylate [(RS)-MBMA] was investigated in toluene at −30°C. Reaction products between cyclohexylmagnesium bromide (cHexMgBr) and axially dissymmetric 2,2′-diamino-6,6′-dimethylbiphenyl (AMB) in the mole ratio of 1.5:1 were used as a chiral initiating system. The polymer produced contained a biphenyl group from the catalyst fragment. The polymerization proceeded in an anionic coordination mechanism and the racemic monomer was kinetically resolved during the course of the reaction. The enantiomer selectivity ratio when using (R)-AMB was estimated to be r(S)=18.0 according to the integrated composition equation for an “ideal” copolymerization of the enantiomeric monomers. Most parts of the polymer were found to be of full isotacticity from inspection of the 13C NMR spectrum. Some physical properties of the polymer strongly suggested that it is a mixture of (R)- and (S)-homopolymers. This may be an example of an asymmetric-selective (stereoelective) and stereoselective polymerization of vinyl monomer. In the copolymerizations between (RS)-MBMA and achiral methacrylates, the catalyst also showed high selectivity toward (RS)-MBMA. Each of the copolymers from methacrylates of methyl, benzyl, and diphenylmethyl alcohols was coisotactic, and the enantiomer selections were consistent with that observed for the homopolymerization. On the other hand, both the isotacticity of copolymer and the selection greatly decreased in the copolymerization with α,α-dimethylbenzyl methacrylate which has no hydrogen at the α-carbon of the ester group.

Disproportionation of CO on iron-cobalt alloys—III: Kinetic laws of the carbon growth and catalyst fragmentation

The rate of CO disproportionate catalysed by filings of an iron-cobalt alloy (49wt% Co, 49wt% Fe, 2wt% V) has been studied between 400°C and 650°C using C0 C0 2 mixtures ranging from 95% CO, 5% CO 2 to 35% CO, 65% CO 2. Consistent with a previous thermodynamic study, poisoning of the reaction is observed when the temperature and the gas composition are such that carburizing or oxidizing of the catalyst may occur. When the metal is thermodynamically stable, the only processes to be taken into account are the fragmentation of the catalyst and the carbon growth on the metal fragments. The dependence of the rate of these two processes on the thermodynamic activity of the carbon in the gas phase has been determined. The influence of this thermodynamic activity and of the temperature on the size of the metal fragment has been studied. Our results support a mechanism of carbon deposition controlled by a bulk diffusion of carbon atoms through the metal fragment driven by a concentration gradient.

Disproportionation of CO on iron-cobalt alloys—II: Kinetic study on iron-cobalt alloys of different compositions

The catalytic activities in CO disproportionate of iron-cobalt alloys of various composition have been compared to the activities of the pure metals at 600°C using a C0 C0 2 gas mixture (75% CO). The catalytic activities of the alloys are all greater than those of the pure metals. The synergetic effect is maximum for cobalt content between 37.5 and 50 wt %. These findings are consistent with a previous thermodynamic study which predicts that such compositions are the least liable to be poisoned by side reactions of carburization and oxidization. Furthermore X-ray diffraction studies show that contrary to pure metals the structures of the alloys are not modified by carbon deposition. The filamentous carbon deposit induces an important fragmentation of the metal catalyst originally under the form of coarse filings. The smallest particle sizes are obtained from the most active alloy composition.

Stability of polyvinyl chloride latex: III. Effects of simple electrolytes

The stability and electrophoretic mobility of a polyvinyl chloride sol at various concentrations of sodium and magnesium salts was studied as a function of the surface concentration of an anionic surfactant, the dodecyl sulfate. The adsorption isotherm of the latter was determined spectrophotometrically as a function of the sol concentration and the electrolyte concentration in the medium. Stability results were found to compare favorably with DLVO theory predictions. The mobility study showed electrophoretically derived surface concentrations to be poor indicators of sol stability. In addition, the presence of a permanent particle surface charge, attributed to catalyst fragments, was established.

Thermal degradation of poly(α‐methyl styrene) in a closed system

AbstractPoly(α‐methyl styrene), produced in various ways, was degraded in a closed system over a range of temperatures from 260 to 320°C. in the absence of air. Except for the initial parts of the degradation reaction (up to about 20% conversion to monomer), all samples had a random initiation reaction, followed by a depropagation step with a number‐average kinetic chain length larger than the number‐average polymer chain length. The energies of activation, after the initial part of the degradation, were all of similar magnitude (ca. 63 kcal./mole) and agreed with values found by previous workers. A few samples showed distinct maxima during the first 20% conversion in their curves of rate of monomer versus conversion, whereas others showed only constant rates or slowly decreasing rates during the initial periods of the reaction. The latter rates can be accounted for by chain‐end initiation (kinetic chain length ≫ polymer chains) or by a random initiation process followed by depropagation and disproportionation as termination reaction. The initial maxima found in some samples could be accounted for by a theory of random initiation in conjunction with a number‐average kinetic chain length larger than the polymer chains, which is inhibited by a reaction of polymer radicals with catalyst fragments. The inhibitor is consumed during the reaction.

Chain Transfer to Telomer. II. In the Case of Catalyzed β-Polymerization

Abstract Catalyzed styrene polymerization in the presence of azobisisobutyronitrile and telomer has been carried out to investigate the effects of the conversion of monomer and of the addition of catalyst on the degree of polymerization of polymer under the same conditions as in the previous report. The apparent degree of polymerization of polymer was obtained as a rather larger value than expected. This situation is qualitatively explained by the reactions of a catalyst fragment radical with telomer and with polymer freshly produced.

The incorporation of aroyl peroxide fragments in polystyrene

AbstractInitiation of the solution polymerization of styrene by catalytic quantities of p‐chloro‐,p‐bromo‐, and p‐nitrobenzoyl peroxides leads to incorporation of catalyst fragments which are primarily (88–95%) aroyloxy groups. Use of p‐iodobenzoyl peroxide gives polymers of somewhat higher molecular weight and lower (79%) aroyloxy fragment content. This anomalous behavior of the iodo compound may be due to its rearrangement to an iodosobenzoate‐type compound. Treatment of polystyrene in benzene solution with these same peroxides leads to incorporation of catalyst fragments 96% chlorobenzoyloxy and 48% bromobenzoyloxy groups. p‐Nitrobenzoyl peroxide is not incorporated under these conditions. This is due to a scavenging action of the benzene solvent. p‐Iodobenzoyl peroxide again did not fit a general pattern and introduced 68% aroyloxy groups. In all of these reactions and increase in molecular weight of the polymer occurred. Treatment of bulk polystyrene with the peroxides led to incorporation of 70% chlorobenzoyloxy, 55% bromobenzoyloxy, and 3% p‐nitrobenzoyloxy groups. In these reactions the molecular weights of the polystyrene samples always decreased.

Detection of End Groups in Polymer Molecules

THE radical theory of vinyl polymerization is confirmed by the identification of so-called catalyst fragments in the polymer molecule. The usual method is to purify the polymer by repeated reprecipitation and to use a catalyst containing an easily identifiable element. Thus when tribromobenzoyl peroxide has been used as the catalyst and the molecular weight of the product is not too high, it is possible to identify bromine in the polymer after repeated purification1.

Determination of catalyst fragments in polymers by absorption in the ultraviolet

Determination of catalyst fragments in polymers by absorption in the ultraviolet