Chromium Catalysts Research Articles

A new chromium(III)/ p-tert-butylcalix[4]arene/alkylaluminum catalyst system for the preparation of low molecular weight polyethylene

A new chromium(III)/ p-tert-butylcalix[4]arene/alkylaluminum catalyst system was applied to assist the ethylene polymerization. The catalyst system was found to be very active for the ethylene polymerization, and gave strictly linear polyethylene, with high crystallinity and low number average molecular weight ( M n) ranging from 910 to 9740, without using molecular hydrogen as a molecular weight reducer. The effects of polymerization parameters such as temperature, ethylene pressure, chromium catalyst concentration, type of alkylaluminum compounds, and Al/Cr molar ratio on the catalytic activity and properties of the polyethylene were investigated. The formation of low molecular weight polyethylene using the chromium(III)/ p-tert-butylcalix[4]arene/alkylaluminum catalyst system was explained by the coordination effect of structurally hindered and less electron-donating p-tert-butylcalix[4]arene ( LH 4) anion ligand.

Distribution of chromium species in catalysts supported on ZrO 2/Al 2O 3 and performance in dehydrogenation

Chromium catalysts (2–3 wt% Cr) supported on Al 2O 3 and ZrO 2/Al 2O 3, prepared with Cr(NO 3) 3·9H 2O or K 2CrO 4, were characterized and studied in the isobutane dehydrogenation. Depending on the chromium salt and the support, CrO 3, Cr 2O 3, and K 2CrO 4 species were observed in the calcined samples, and amorphous Cr 2O 3 remained after reduction with H 2. The preparation of ZrO 2/Al 2O 3 support yields zirconia dispersed on the surface of alumina. In the Cr/ZrO 2/Al 2O 3 calcined catalysts, the chromium sites were distributed on the ZrO 2 and Al 2O 3 surfaces. The O 2 chemisorption and the activity and selectivity to isobutene were also affected by the chromium species distribution. The addition of potassium decreased the O 2 chemisorption. In the isobutane dehydrogenation, the turnover number and selectivity to isobutene increased for the catalysts prepared with K 2CrO 4.

The Effect of Aromatic Ethers on the Trimerisation of Ethylene using a Chromium Catalyst and Aryloxy Ligands

AbstractA catalyst system consisting of a chromium source, a 2,6‐disubstituted phenol and an aromatic ether solvent, on activation with triethylaluminium, is active and selective for the trimerisation of ethylene to form 1‐hexene. The aromatic ether appears to play a role as both solvent and reagent and is pivotal in ensuring both the good activity of the system as well as selectivity to the desired product, 1‐hexene.

Characterization of chromium species in catalysts for dehydrogenation and polymerization

Abstract Chromium catalysts, prepared with different chromium contents, supports and precursor compounds, were characterized by O2 and CO chemisorption, in order to quantify the Cr3+ and Cr2+ sites, active species to dehydrogenation and ethylene polymerization reactions. Both probe molecules, O2 and CO, showed to be selective to the chromium active species, Cr3+ and Cr2+, respectively, and a valid method to quantify these sites. Distinct chromium species, Cr6+, amorphous Cr3+, crystalline Cr3+ and Cr2+, were characterized by XRD, DRS, TPR, FT-IR and DRIFTS in the calcined and reduced catalysts. The distribution of chromium species, which depends on the content, precursor compound and support, was related to the catalytic activity in the dehydrogenation and polymerization reactions. The catalytic activity to cyclohexane dehydrogenation depends on the dispersion of amorphous Cr3+ species. The activity in the ethylene polymerization is related to the amount of CrA2+ and CrB2+ species. Both sites showed similar activities.

Selection of optimum chromium oxide-based catalysts for propane oxidehydrogenation

Propane oxidative dehydrogenation has been studied at 350–500 °C, 1.0 bar and feed flow rate of 75 cm 3/min over several supported chromium oxide-based catalysts. Effects of various chromium loadings, different supports, catalyst precursors and reaction conditions were investigated in an attempt to select an optimum catalyst for the reaction. Chromium oxides of different loadings ranging from 0.1 to 20 wt.% on γ-Al 2O 3 were tested. At 450 °C, 10 wt.% loading exhibited propane conversion of 19%. γ-Al 2O 3 gave the best performance when MgO, TiO 2, SiO 2 and γ-Al 2O 3 were tested as supports. As a precursor, Cr(NO 3) 3·9H 2O exhibited the best results compared with K 2Cr 2O 7, CaCr 2O 7, Na 2Cr 2O 7, Cr 2SO 4·12H 2O and CrO 3. Effects of reaction temperatures and feed compositions were also evaluated on a 10 wt.% Cr-Al-O catalyst. The maximum selectivity to propene obtained was 61% while the yield was 18%. The 10 wt.% Cr-Al-O catalyst was characterized by X-ray diffraction patterns (XRD), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) which confirmed the presence of both Cr 3+ and Cr 6+ in the calcined catalyst and, also, the predominance of Cr 3+ in the spent catalyst.

Long Chain Branching in Polyethylene from the Phillips Chromium Catalyst

Supported chromium oxide polymerization catalysts can impart significant levels of long chain branching (LCB) to polyethylene made in a slurry process. LCB often dominates the rheological behavior of the polymer, and is thus responsible for the performance of many HDPE grades during commercial molding operations. LCB is difficult to accurately measure but its presence can be inferred from the visco‐elastic character of the polymer, relative to its molecular weight (MW) and MW distribution. Depending on catalyst choice, LCB levels from chromium oxide catalysts can range from near zero, to very high, rivaling the tightly bridged metallocenes. The most common catalyst and reactor variables were investigated with respect to their effect on LCB formation. The simple “macromonomer insertion” view does not in itself account for many of these LCB responses. In fact, no overall correlation was found between LCB and the catalyst's ability to incorporate large comonomers. Instead, variables that influenced LCB most also govern the active site density, including activation temperature, chromium loading, and the presence of cocatalysts or poisons in the reactor. The physical structure of the catalyst support also had a strong influence on the elastic behavior of the polymer, independent of any effect on MW or MW distribution. These and other variables are discussed.

Zeolite MCM-22 supported heterogeneous chromium catalyst for ethylene polymerization

The chromium alkyl complex Cp*Cr(py)Me2 (Cp* = η5-pentamethylcyclopentadienyl, py = pyridine, Me = methyl) has been affixed to the surface of zeolite MCM-22 via methane elimination. The resulting composite material has been characterized by a variety of physical methods. The results are consistent with the presence of isolated organometallic fragments linked to the solid support by a covalent chromium–oxygen bond. Activation of this material with methylaluminoxane (MAO) yielded a highly active catalyst for the polymerization of ethylene. The polyethylene thus formed featured high molecular weights (Mw ∼ 4 × 106) and narrow molecular weight distributions (Mw/Mn ∼ 3.5).

Kinetics of an Intratracheally Administered Chromium Catalyst in Rats

Chromium-based catalysts are used for the synthesis of polyethylene, but little is known about the hazard and biomonitoring possibilities of this type of chromium for workers who may be occupationally exposed to such compounds. Therefore, the bioavailability and toxicokinetics of chromium were studied in male Wistar rats after a single intratracheal instillation (2 ml/kg body weight) of various doses (1, 5, or 25 mg/kg body weight) of the catalyst (approximately 1% chromium bound to an amorphous silica matrix), either before (CAT-Cr[III]) or after (CAT-Cr[ VI]) heat treatment. The results were compared with those of equivalent amounts of two chromium salts (CrCl 3 and K 2 Cr 2 O 7 ). Each dose group was composed of three rats. The concentration of chromium was determined by atomic absorption spectrometry in urine (collected daily for 7 d) and in plasma, erythrocytes, lung, and liver tissue obtained 2 d (only highest concentration) and 7 d after dosing. On d 2, a significant increase in lung weight was found in the animals treated with the highest dose of the hexavalent Cr products. On d 7, on the basis of body weights, lung weights, and lung histology, there was no overt toxicity, except after the highest dose of CAT-Cr(VI). The elimination of all forms of chromium was apparently monoexponential, with calculated half-life elimination times in urine of 4-11 h for Cr(III) (CAT-Cr[III] and CrCl 3 ) and 8-21 h for Cr(VI) (CAT-Cr[VI] and K 2 Cr 2 O 7 ). On d 2, the erythrocytes Cr concentrations were significantly higher for the hexavalent Cr products than for the trivalent Cr products. After 7 d, the erythrocytes Cr concentrations were significantly increased above control values (3 w g/L) only in rats treated with the 2 highest doses of Cr( VI) compounds (12 and 64 w g/L for K2 Cr2 O7 , and 14 and 79 w g/L for CAT-Cr[VI]). The present study shows that intratracheally instilled Cr(VI) and Cr(III) have different toxicokinetic profiles and that the Cr(VI) catalyst has the same bioavailability and excretion kinetics as a water-soluble Cr(VI) salt. Exposure to chromium compounds could be monitored by measuring Cr concentrations in urine (shortly after exposure) and in erythrocytes (also at later time points after high Cr[VI] exposure).

Chromium(III) supported on aluminum-nitride-surfaced alumina: characteristics and dehydrogenation activity

Chromium catalysts were prepared by the saturating chemisorption of chromium(III) acetylacetonate (Cr(acac) 3) at 200 °C on alumina and on alumina that had been surfaced with aluminum nitride. According to elemental analysis, infrared spectroscopy, and electron spin resonance spectroscopy, chemisorption occurred through ligand exchange reaction of Cr(acac) 3 with surface OH and NH x groups and dissociation of Cr(acac) 3 or Hacac on coordinatively unsaturated sites on the surface. Steric hindrance imposed by the acac ligands defined saturation of the surface with adsorbed species; the same chromium loading (0.64±0.06 atoms per nm 2) was obtained on all supports. After removal of the ligands with oxygen, water, or ammonia, the samples were active in isobutane dehydrogenation. For chromium sites with high activity, nearby oxygen ions were needed. The activity of a Cr 3+ ion was mainly determined by its environment and not by whether it had been formed through reduction.

Development of a polyfunctional catalyst for benzene production from pyrolysis gasoline

The concurrence of hydrodealkylation, hydrocracking, and hydrodesulfurization reactions necessitates a polyfunctional catalyst in the production of pure benzene from pyrolysis gasoline or other industrial mixtures containing various alkylaromatic and nonaromatic hydrocarbons. A chromium catalyst was developed in this study with comparable performance to the commercial catalyst Pyrotol H-9430 (Houdry, USA). The new catalyst contained 15 wt.% Cr and was doped with KF. When tested utilizing a hydrocleaned BTX fraction of pyrolysis gasoline, it preserved its benzene selectivity and hydrocracking activity for 3000 h, but lost about 10% of its hydrodealkylation activity. The deactivated catalyst completely recovered its original surface properties and hence activity when subjected to oxidative regeneration. The developed catalyst maintained its Cr and KF content in regeneration as it did in drying, calcination, and reaction. The characteristics of the catalyst were examined by BET, XRD, DTA, TGA and electron microscope.

Ethylene Polymerization with Silica-Supported Cr[N(SiMe3)2]3/Alumoxane Catalyst

A new silica-supported non-cyclopentadienyl chromium catalyst having high activity for ethylene polymerization is presented. The catalyst was prepared by supporting tris[bis(trimethylsilyl)amido]chromium(III) Cr[N(SiMe3)2]3 and alumoxane on silica. The best activity was attained by supporting Cr[N(SiMe3)2]3 (0.2 wt%-Cr for silica) on silica calcined at 350°C and next supporting a modified-methylalumoxane (Al/Cr molar ratio = 20). Activity during the slurry-phase polymerization was 75100 g-PE/mmol-Cr·hr at 103°C and 1.4 MPa ethylene partial pressure in isobutane. The obtained polyethylene had a relatively high molecular weight and broad molecular weight distribution (HLMFR = 15.6 g/10 min, Mw = 330000, Mw/Mn = 33.0). The calcination temperature of silica had significant influence on the catalyst performance. As calcination temperature increased, short branches, mainly n-butyl and some ethyl branches, were introduced into the resulting polyethylene. Therefore, poly[ethylene-co-(1-hexene)] was obtained from the ethylene monomer alone. This shows two Cr active sites, a polymerization and trimerization site. The relative amount of each site is considered to be determined by the type of silanol groups obtained after calcination.

Discovery of a new family of chromium ethylene polymerisation catalysts using high throughput screening methodology.

Following a discovery that salicylaldimines bearing bulky ortho-phenoxy substituents and small imine substituents give very active chromium catalysts for ethylene polymerisation, High Throughput Screening (HTS) methodology has been employed to facilitate a further discovery of exceptionally active catalysts based on tridentate salicylaldimine ligands with bulky triptycenyl groups.

Allylic CH Oxidation versus Epoxidation of 2-Cyclohexenols, Catalyzed by Chromium- and Manganese-Substituted Polyoxometalates and Salen Complexes

2-Cyclohexenol (1) is oxidized chemoselectively to 2-cyclohexenone (2a) by the α-Keggin chromium-substituted polyoxometalate (POM) Ia as the catalyst and iodosobenzene as the oxygen source. For the chromium(salen) catalyst IIa the same chemoselectivity in favor of allylic CH oxidation is observed. The manganese-substituted POM Ib and the manganese(salen) complex IIb, however, afford appreciable amounts of the epoxy alcohol 2b. For the stereolabeled 5-tert-butyl-2-cyclohexenols 5, the diastereoselectivity of the epoxidation was appreciable (syn:anti 82:18) in the case of the manganese(salen) complex IIb with the cis isomer, while the manganese-substituted POM Ib exhibited no syn versus anti π-facial differentiation for the cis or trans diastereomer of the cyclohexenol 5. The observed syn hydroxy directivity for the manganese(salen) complex IIb is rationalized in terms of optimal hydrogen bonding between the MnV oxo complex IIb and the trans diastereomer of the allylic alcohol substrate 5. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Chromium and Lanthanum on Transition Alumina Surfaces: The Role of Bulk Point-Defect Distributions on Catalytic Activity

ABSTRACTWe employ a combination of Z-contrast scanning transmission electron microscopy (Z-STEM) and first-principles density-functional calculations to investigate the interaction between metal atoms and cubic alumina catalytic supports. We show that there are two observed La sites at the porous γ- alumina surface, and that single La atoms do not exhibit a tendency to cluster. Cr behaves very differently from La - it has a tendency to cover the alumina surface in ordered, periodic raft-like patches. The degradation of the chromia/alumina catalyst is related to the possibility for the chromium to move away from the surface into the bulk, and the activation barrier for such a process is higher in η- alumina than in γ- alumina, i.e., η- alumina is a more durable support for chromium catalyst.

Estimating the Parameters of an Adiabatic Steam Methane Conversion Reactor with a Regular Catalytic Block

A mathematical model and a computer program are developed for the design of a catalytic block (plane-parallel heat-conducting substrate coated with a thin catalyst layer) for adiabatic steam methane conversion. An experimental rate function for steam methane conversion on a nickel–chromium catalyst is used to estimate the dimensions and flow resistance of the adiabatic reactor at a large stoichiometric excess of steam.

Synthesis of Synthetic Lubricants by Trimerization of 1-Decene and 1-Dodecene with Homogeneous Chromium Catalysts

The Cr-catalyzed trimerization of 1-decene and 1-dodecene is a highly selective method to synthesize C30 and C36 olefins. After hydrogenation, these products display very attractive viscosity indices (VI's). Due to the high trimer selectivity of the reaction no additional work-up is needed to separate C20, C24 or C40+ oligomers from the product prior to a potential use of these products in synthetic lubricants for automotive applications.

Chemoselectivity and Stereospecificity of Chromium(II) Catalysts for 1,3-Diene Polymerization

1,3-Butadiene, isoprene, (E)- and (Z)-1,3-pentadiene were polymerized with catalyst systems based on CrCl2(dmpe)2 and Cr(CH3)2(dmpe)2 (dmpe = 1,2-bis(dimethylphosphino)ethane) and Al(C2H5)3 or methylaluminoxane (MAO). The chemo- and stereoselectivity of these systems are strongly influenced by monomer structure. They give 1,2-syndiotactic polybutadiene, 3,4-amorphous polyisoprene, cis-1,4-isotactic poly[(E)-pentadiene] and cis-1,4-atactic poly[(Z)-pentadiene]. The factors influencing chemo- and stereoselectivity are discussed.

An electrochemical coupling of organic halide with aldehydes, catalytic in chromium and nickel salts. the Nozaki-Hiyama-Kishi reaction.

[reaction: see text] Electrochemical arylation of arenecarboxaldehydes using an iron sacrificial anode in the presence of chromium and nickel catalysts afforded the corresponding arylated secondary alcohols in moderate to good yields. The chromium and nickel salts as catalysts are obtained by oxidation of a stainless steel rod during a preelectrolysis in 7% and 3%, respectively. The process was also applied to the addition of vinyl halide, allyl acetate, or alpha-chloroester to aromatic aldehydes.

Vinylic Polymerization and Copolymerization of Norbornene and Ethene by Homogeneous Chromium(III) Catalysts

Homogeneous chromium(III) catalysts of the type [Cp′CrMeCl]2/MAO (Cp′ = cyclopentadienyl, pentamethylcyclopentadienyl, indenyl, fluorenyl) have been synthesized to investigate the influence of the electronic nature and the sterical demand of different Cp′-ligands on the polymerization and copolymerization of norbornene and ethene. In the case of norbornene polymerization the catalyst activity is increased by intensifying the electron donating character of the Cp′-ligand whereas the sterical demand affects the crystallinity of polynorbornene. The use of the [Cp*CrMeCl]2 /MAO and the [FluCrMeCl]2 /MAO catalyst for norbornene-ethene copolymerization under various polymerization conditions led to the formation of norbornene-ethene copolymers with different microstructures ranging from statistical norbornene-ethene copolymers over vinyl-terminated norbornene macromonomers to norbornene-ethene block copolymers. Furthermore highly linear, ultra high molecular polyethylene can be obtained using the [FluCrMeCl]2/MAO catalyst.

Silica Supported Chromium Catalyst. Reactivity Studies with Alcohols

Fil: Parentis, Monica Liliana. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Salta. Instituto de Investigaciones para la Industria Quimica. Universidad Nacional de Salta. Facultad de Ingenieria. Instituto de Investigaciones para la Industria Quimica; Argentina