Surface science approach to the preparation and characterization of model Ziegler–Natta heterogeneous polymerization catalysts

Ziegler–Natta heterogeneous catalytic systems are extensively used to polymerize ethylene and propylene. Some industrial catalysts consist of TiCl₄ chemisorbed on activated MgCl₂ and subsequently reduced and alkylated by reaction with an aluminum alkyl (generally AlEt₃). Lewis bases are added to the catalytic systems to control the enantio-selectivity for the production of isotactic polypropylene. Our aim is to clarify the chemical composition of the active centers by modern surface science methods. Model catalysts are prepared in the form of ultra-thin films by gas-phase deposition on a gold foil in ultrahigh vacuum. Under these conditions, MgCl₂ films grow to controlled thickness via a layer-by-layer mechanism, as revealed by AES and XPS. TiCl₄ can be deposited on these films near room temperature by both electron irradiation-induced and metallic magnesium-induced chemical vapor deposition. Angle-resolved XPS studies indicate that these films consist of a few layers of TiCl₂ with one monolayer of TiCl₄ chemisorbed on its surface. The exposure of these titanium chloride films to the co-catalyst AlEt₃ produces an active model Ziegler–Natta catalyst. XPS analysis reveals the presence of TiCl₂Et on the catalyst surface: this is believed to be the active site. Prolonged reaction with the co-catalyst reduces the titanium sites to TiClEt_n (n = 1 and/or 2). High molecular weight polyethylene and polypropylene are synthesized on these catalysts, as shown by Raman spectroscopy. Highly isotactic polypropylene is produced without need for stereo-regulating Lewis bases. This revised version was published online in June 2006 with corrections to the Cover Date.     

Estimation of the chain propagation rate constants of propylene polymerization and ethylene-1-hexene copolymerization catalyzed with MgCl2-supported Ziegler–Natta catalysts

In olefin polymerization with MgCl₂-supported Ziegler–Natta (Z–N) catalysts, the apparent propagation rate constant (k_p)_a calculated by R_p = (k_p)_a [C*] C_Me (C_Me is equilibrium monomer concentration in the reaction system) declines with reaction time for gradually developed monomer diffusion limitation in the polymer/catalyst particles. In this work, a simplified multi-grain particle model was proposed to build correlation between (k_p)_a and other kinetic parameters that can be determined experimentally. Rate profiles of propylene polymerization and ethylene-1-hexene copolymerization by three MgCl₂-supported Z–N catalysts were determined, and the (k_p)_a data was calculated using [C*] determined by quench-labelling the propagation chains with acyl chloride. Decline of (k_p)_a in each polymerization process was precisely fitted by the linear correlation between lg(k_p)_a and [(ρ_catm_p)/(ρ_pm_cat) + 1]^1/3 developed on the particle model. Real propagation rate constant (k_p) was estimated by extrapolating the fitting line to the starting point of polymerization, where no concentration gradient exists. According to the particle model, the slope of the lg(k_p)_a versus [(ρ_catm_p)/(ρ_pm_cat) + 1]^1/3 line (lgd) represents the degree of monomer diffusion limitation. Variations of parameter d found in the studied reaction systems can be reasonably explained based on the knowledge of olefin diffusion in the polymer phase.     

A density functional study of Ti/MgCl 2 -supported Ziegler–Natta catalysts

Total energy pseudopotential calculations have been used to examine the adsorption of TiCl₄ at both the 110 and 100 surfaces of magnesium chloride. Titanium(IV) chloride is found to bind most strongly on the 100 surface resulting in the formation of a complex with approximately trigonal bipyramidal coordination of titanium, which will dissociate to form TiCl ₃ ⁺ and Cl⁻ with an energy of 127.7 kJ mol⁻¹. Cluster calculations indicate that this site only weakly binds ethene, but does catalyse the formation of C–C bonds with an activation energy consistent with experimental estimates. This revised version was published online in June 2006 with corrections to the Cover Date.     

Heterogeneous Ziegler–Natta catalysts with various sizes of MgCl<Subscript>2</Subscript> crystallites: synthesis and characterization

A MgCl₂-based Ziegler–Natta catalyst was characterized using X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and IR spectra. We focused on the XRD reflection at 2θ = 50° to determine the thickness of MgCl₂ crystals, and validated these results with TEM pictures. SEM pictures were taken in order to measure the size of the nanoparticles formed by the MgCl₂ crystals. Several compounds were synthesized for comparison and to aid interpretation of the infrared (IR) spectra. The catalysts were prepared by precipitating MgCl₂, which was used as support material and subsequently treated with TiCl₄. The thickness of the catalyst crystals was calculated from the XRD reflection at 2θ = 50°. Changing the precipitation temperature within a range from 40 to 90 °C altered the thickness of the MgCl₂ crystal plates. The maximum thickness of 7 nm was achieved at a precipitation temperature of 60 °C. The SEM pictures showed that the nanoparticles had a diameter of ~200 nm. A crystal base unit had a volume that corresponded to that of a sphere of 3.5 nm radius. Thus, we estimated that a typical catalyst particle with a diameter of 20 μm contained about one million nanoparticles, each of which consisted of about 25,000 MgCl₂ crystal units.     

Melt fracture of HDPEs: Metallocene versus Ziegler–Natta and broad MWD effects

The melt fracture of high-density polyethylenes (HDPEs) is studied primarily as a function of molecular weight and its distribution for broad molecular weight distribution metallocene and Ziegler–Natta catalyst resins. It is found that sharkskin and other melt fracture phenomena are very different for these two classes of polymers, although their rheological behaviors are nearly the same for many of these. Moreover, the metallocene HDPE shows significant slip at the die wall without exhibiting stick-slip transition. Important correlations are derived between the critical conditions for the onset of melt fracture and molecular characteristics.     

Kinetics of methane combustion over Pt and Pt–Pd catalysts

A kinetic study was performed over thermally aged and steam-aged Pt and Pt–Pd catalysts to investigate the effect of temperature, and methane and water concentrations on the performance of catalysts in the range of interest for environmental applications. It was found that both catalysts permanently lose a large portion of their initial activity as result of exposure to 5 vol.% water in the reactor feed. Empirical power-law and LHHW type of rate equations were proposed for methane combustion over Pt and Pt–Pd catalysts respectively. Optimization was used to determine the parameters of the proposed rate equations using the experimental results. The overall reaction orders of one and zero in methane and water concentration was found for stabilized steam-aged Pt catalyst in the presence and absence of water. The apparent self-inhibition effect caused by methane over Pt–Pd catalyst in the absence of water was associated with the inhibiting effect of water produced during the combustion of methane. A significant reversible inhibition effect was also observed over steam-aged Pt–Pd catalyst when 5 vol.% water vapor was added to the reactor feed stream. A significant reduction in both activity and activation energy was observed above temperatures of approximately 550 °C for steam-aged Pt–Pd catalyst in the presence of water (the activation energy dropped from a value of 72.6 kJ/mol to 35.7 kJ/mol when temperature exceeded 550 °C).     

Effects of Ti oxidation state on ethylene, 1-hexene comonomer polymerization by MgCl<Subscript>2</Subscript>-supported Ziegler–Natta catalysts

In this study, the influences of the Ti oxidation state on the catalytic properties of MgCl₂-supported Ziegler–Natta catalysts in ethylene homo- and co-polymerization with 1-hexene were investigated. Three catalysts having different Ti oxidation states were synthesized by milling TiCl₄, TiCl₃, or TiCl₂ together with MgCl₂. With these catalysts having different Ti oxidation states, the polymerization conditions such as the Al concentration, temperature, and 1-hexene concentration were varied to figure out their catalytic abilities in ethylene homo- and co-polymerization. The Ti oxidation state affected the catalyst activity largely, having unique dependences on the polymerization conditions. A higher oxidation state led to a higher activity, slightly larger comonomer incorporation, and lower molecular weight as well as its narrower distribution. However, rough characteristics of copolymers were similar among the different Ti oxidation states.     

Dimerization of 1‐butene via zirconium‐based Ziegler–Natta catalyst

A zirconium‐based Ziegler–Natta catalytic system has been tested in the dimerization of 1‐butene. It was found that the concentration of Et₂AlCl, Ph₃P and PhONa as well as the reaction temperature had great influences on the activity and selectivity of the catalyst. Under the optimum reaction conditions, the conversion of 1‐butene is 91.9%, and the selectivity of dimers is 76.7%. Basic ligands such as Ph₃P and PhONa can inhibit isomerization of 1‐butene to 2‐butene effectively. In addition, the metal hydride mechanism was also suggested and some indirect evidence was obtained in favor of this mechanism. This revised version was published online in July 2006 with corrections to the Cover Date.     

Polymerization of propylene with Ziegler–Natta catalyst: optimization of operating conditions by response surface methodology (RSM)

Optimization of operational conditions for the polymerization of propylene with Ziegler–Natta catalyst was carried out via RSM. Response surface methodology (RSM) based on a three-level, four-variable Box–Behnken design was used to evaluate the interactive effects of reaction conditions such as reaction temperature (60–80 °C), monomer pressure (5–8 bar), hydrogen volume (130–170 mL), and cocatalyst to catalyst ratio (Al/Ti, 340–500) on the catalyst activity and melt flow rate (MFR). The optimum reaction conditions derived via RSM were: temperature 70 °C, pressure 8 bar, hydrogen volume 151 mL, and cocatalyst to catalyst ratio 390. The experimental catalyst activity and MFR were 8 g polypropylene/mg catalyst and 10.9 g/10 min, respectively, under optimum conditions. Optimum values were determined from process cost point of view and offered better operational conditions.     

Kinetics and selectivity of methyl-ethyl-ketone combustion in air over alumina-supported PdOx–MnOx catalysts

A study is presented of the kinetics and oxidation selectivity of methyl-ethyl-ketone (MEK) in air over bimetallic PdO_x(0–1 wt% Pd)–MnO_x(18 wt% Mn)/Al₂O₃ and monometallic PdO_x(1 wt% Pd)/Al₂O₃ and MnO_x(18 wt% Mn)/Al₂O₃ catalysts. Reaction rate data were obtained at temperatures in the 443–523 K range and for MEK partial pressures in the reactor feed of between 6.5 and 126.6 Pa. Products of both MEK combustion and partial oxidation reactions were found. Monometallic Pd/Al₂O₃ was the most selective catalyst for complete oxidation whereas the partial oxidation of MEK in the presence of manganese oxides was significant. The maximum yield for the partial oxidation products (acetaldehyde, methyl-vinyl-ketone, and diacetyl) was always below 10%. Kinetic studies showed that the rates of CO₂ formation over PdO_x/Al₂O₃ were well-fitted by the surface redox Mars–van Krevelen (MvK) kinetic expression and also by a Langmuir–Hinshelwood (LH) model derived after considering the surface reaction between adsorbed MEK and oxygen as the rate-determining step. In the case of the Mn-containing catalysts the MvK model provides the best fit. Irrespective of the model, the kinetic parameters for the bimetallic Pd–Mn catalysts were between the values obtained for the monometallic samples, suggesting an additive rather than a cooperative effect between palladium and manganese species for MEK combustion.     

Effects of chemical structure of phenolic antioxidants on Ziegler–Natta catalyst performance during propylene polymerization

Present work studied the synthesis of in-reactor stabilization of polypropylene via introducing antioxidant into polymerization media. Special attention was dedicated to assess the efficiency of antioxidant in catalyst deactivation. Three different types of antioxidants (Irganox 1076, Irganox 1010 and Irganox 1330) which contain ester and/or phenolic OH functional groups were chosen to investigate their impact on Ziegler–Natta catalyst performance during slurry propylene polymerization. Our Results indicated that not only phenolic OH groups but also esteric bond of antioxidants are capable of interacting with active center of catalyst and consequently decreasing the catalyst activity. Our propylene polymerization results showed that determining factors such as antioxidant chemical structures and its steric hindrance effect and the number of functional groups (phenolic and esteric groups) affected on the Ziegler–Natta catalyst performance. Therefore, effects of these three types of antioxidants on polymer characteristics such as particle size distribution, morphology, T _m , T _c , X _c , and isotacticity were evaluated. Morphological analysis using scanning electron microscopy (SEM) showed that introducing antioxidant during propylene polymerization did not destroy the spherical morphology of the polypropylene particles. Conclusively, due to the negative effect of esteric bond of antioxidant on Ziegler–Natta catalyst performance, the use of antioxidant without ester groups (Irganox 1330) is more recommended during propylene polymerization.     

Kinetics and mechanism of the preparation of Raney® copper

Raney® copper is an active hydrogenation catalyst formed by the selective dissolution of aluminium from a Cu–Al alloy. The structure of Raney® copper is presented in a series of images taken using a focussed ion beam miller (FIB). The images show a structure consisting of a uniform three-dimensional network of fine copper ligaments. A rotating disc electrode, used to control the diffusion layer, enabled a study of the kinetics of the leaching reaction at 269–303K in 2–8m NaOH. Under these conditions, the reaction rate was constant and independent of hydroxide concentration. The activation energy for leaching was determined as 69±7kJmol^–1. The mixed corrosion potential of the dissolving alloy has been related to the exposed copper surface area, which in turn is dependent on the leaching rate and the mechanism of rearrangement. The overall mechanism of formation/rearrangement of the Raney® copper structure was found to be mainly dissolution/redeposition of copper atoms, with surface or volume diffusion, or possibly both, playing a minor role.     

Ziegler–Natta catalyst activation. Thermodynamic and kinetic aspects of metallocenium ion-pair formation,dissociation, and structural reorganization

This contribution reviews thermochemical and dynamic NMR studies of metallocenium olefin polymerization catalyst formation from, and dissociation to, the neutral metallocene dimethyl precursors and either fluoroarylboranes or methylalumoxane (MAO), as well as the kinetics of ion pair separation/recombination/reorganization. Significant correlations are found between borane Lewis acidity, the enthalpy of catalyst formation, catalyst NMR spectral parameters, the free energy of activation of catalyst ion pair separation/recombination/reorganization, and ethylene polymerization activity. This revised version was published online in June 2006 with corrections to the Cover Date.     

Numerical study of homogeneous–heterogeneous reactions on stagnation point flow of ferrofluid with non-linear slip condition

This study deals with the stagnation point flow of ferrofluid over a flat plate with non-linear slip boundary condition in the presence of homogeneous-heterogeneous reactions.Three kinds of ferroparticles,namely,magnetite(Fe_3O_4),cobalt ferrite(CoFe_2O_4) and manganese zinc ferrite(Mn-ZnFe_2O_4) are taken into account with water and kerosene as conventional base fluids.The developed model of homogeneous-heterogeneous reactions in boundary layer flow with equal and unequal diffusivities for reactant and autocatalysis is considered.The governing partial differential equations are converted into system of non-linear ordinary differential equations by mean of similarity transformations.These ordinary differential equations are integrated numerically using shooting method.The effects of pertinent parameters on velocity and concentration profiles are presented graphically and discussed.We found that in the presence of Fe_3O_4-kerosene and CoFe_2O_4-kerosene,velocity profiles increase for large values of α and β whereas there is a decrement in concentration profiles with increasing values of if and K_s.Furthermore,the comparison between non-magnetic(A1_2O_3) and magnetic Fe_3O_4 nanoparticles is given in tabular form.     

Synthesis of cardanol-based phthalonitrile monomer and its copolymerization with phenol–aniline-based benzoxazine

The cardanol-based phthalonitrile (PN) monomer was successfully produced via the nucleophilic substitution reaction of cardanol with 4-nitrophthalonitrile in potassium carbonate media. The conventional methods were employed to predict the chemical structure. The influence of long alkyl chains of cardanol was observed on the thermomechanical properties, recorded values were much below than the poly(Baph) standards. However, the thermal stabilities were recorded in good agreement to PN resin values. Furthermore, the 100 kGy dose of Co⁶⁰ irradiation does not show any remarkable changes in the studied properties. The copolymers from P-a benzoxazine and cardanol-based PN (CPN) on the different wt % blending were prepared. The curing behavior and mechanism of the monomer blends were analyzed. The curing of CPN was improved in the presence of active hydrogen produced from the P-a polymerization. The T _g and thermal properties of the copolymer were much better than the neat poly(P-a). © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47505.     

Control of molecular weight distribution for polypropylene obtained by commercial Ziegler–Natta catalyst: effect of temperature

Polymerization of propylene was carried out by using MgCl₂-supported TiCl₄ catalyst in conjunction with triethylaluminium (TEA) as cocatalyst. The effect of polymerization temperature on polymerization of propylene was investigated. The catalyst activity was influenced by the polymerization temperature significantly and the maximum activity of the catalyst was obtained at 40 °C. With increasing the polymerization temperature, the molecular weight of polypropylene (PP) drastically decreased, while the polydispersity index (PDI) increased. The effect of the two-stepwise polymerization procedure on the molecular weight and molecular weight distribution of PP was studied and the broad PDI of PP was obtained. It was also found that the PDI of PP could be controlled for propylene polymerization through regulation of polymerization temperature. Among the whole experimental cases, the M _w of PP was controlled from 14.5 × 10⁴ to 75.2 × 10⁴ g/mol and the PDI could be controlled from 4.7 to 10.2.     

Macrokinetics of Polybutadiene Production on a Titanium Ziegler–Natta Catalyst System Prepared in Turbulent Flows

The research was directed at establishing the dependence of the disperse composition of particles of TiCl₄–Al(i-C₄H₉)₃ catalytically active precipitate, and their activity and molecular weight characteristics of polybutadiene formed in their presence on the length of sections and the confuser diameter of the tubular turbulent apparatus of diffuser–confuser design used to prepare a catalytic system in turbulent flows. An approach that combines methods of computational fluid dynamics (ANSYS Workbench 17.1 platform) and formal kinetics, and solves inverse problems of kinetics has been used to solve the stated problem. The following required dependences were established based on numerical experiments on the models developed using this approach: (1) the hydrodynamics of the process of dispersing particles of TiCl₄–Al(i-C₄H₉)₃ catalytically active precipitate in a tubular turbulent apparatus with diffuser–confuser design, and (2) macrokinetics of butadiene polymerization on the particles of TiCl₄–Al(i-C₄H₉)₃ catalytically active precipitate dispersed in turbulent flows (batch reactor, the solvent was toluene, and the temperature of the process was 25°С).     

Random copolymerization of ε-caprolactone and l-lactide with molybdenum complexes

Dioxomolybdenum complexes were examined as catalysts for the copolymerization of ε-caprolactone (ε-CL) and l-lactide (l-LA). The bis-[(5-OMe)salicylaldehydato]dioxomolybdenum complex completed the copolymerization after 20 h at 110 °C with 0.05 mol% of the catalyst to produce a copolymer in high yield. The microstructure of the copolymer was analyzed using ¹H and ¹³C NMR spectroscopy and was determined to have a random structure. The r values calculated from the heterodiad analysis of the ¹H NMR data were r _LA = 0.91 and r _CL = 0.93, and the L values calculated from the triad analysis of the ¹³C NMR data were L _LA = 1.58 and L _CL = 1.81. Other dioxomolybdenum complexes, such as cis-α MoO₂[(3-MeO)DiMeSaltn], MoO₂(acac)₂ and (NH₄)₈[Mo₁₀O₃₄] exhibited comparable or slightly lower reactivity for the copolymerization. Consecutive polymerization of ε-CL followed by l-LA afforded a block copolymer without trans-esterification.     

Self-propagating high-temperature synthesis of advanced ceramics in the Mo–Si–B system: Kinetics and mechanism of combustion and structure formation

The goal of this work is to investigate the combustion mechanisms of reactions in the Mo–Si–B system and to obtain ceramic materials using the SHS method. It is concluded that the following processes are defined by the SHS for Si-rich Mo–Si–B compositions: silicon melting, its spreading over the surfaces of the solid Mo and B particles, followed by B dissolution in the melt, and formation of intermediate Mo₃Si-phase film. The subsequent diffusion of silicon into molybdenum results in the formation of MoSi₂ grains and molybdenum boride phase forms due to the diffusion of molybdenum into B-rich melt. The formation of MoB phase for B-rich compositions may occur via gas-phase mass transfer of MoO₃ gaseous species to boron particles. The stages of chemical interaction in the combustion wave are also investigated. The obtained results indicate the possibility of both parallel and consecutive reactions to form molybdenum silicide and molybdenum boride phases. Thus the progression of combustion process may occur through the merging reaction fronts regime and splitting reaction fronts regime. Molybdenum silicide formation leads the combustion wave propagation during the splitting regime, while the molybdenum boride phase appears later. Finally, targets for magnetron sputtering of promising multi-phase Mo–Si–B coatings are synthesised by forced SHS compaction method.