Chromium Catalysts Research Articles

NO reduction with CO on Cu–Cr and Co–Cr oxide catalysts supported on Al2O3 and Al2O3 + SiO2

Cu–Cr and Co–Cr oxide systems supported on Al2O3 and Al2O3 + SiO2 were studied in order to find the appropriate combination as a catalyst for the reduction of NO with CO with respect to the active phase and the nature of the support. The catalysts were prepared by successive impregnation of the support with solution mixtures of tartaric acid and nitrate. The catalysts were characterized by atomic absorption spectroscopy (AAS), Brunauer, Emmett, Teller (BET), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) methods. The copper–chromium catalysts are more active than the cobalt–chromium ones. The catalysts supported on Al2O3 + SiO2 are more active than the supported on pure alumina. The probable reasons are the higher content of copper and cobalt on the surface of Al2O3 + SiO2 and a formation of less active in the studied reaction copper and cobalt aluminates on the alumina supported catalysts.

Catalytic Fluorination of Various Chlorinated Hydrocarbons by HF and a Chromium Based Catalyst: Effect of the Presence of Zinc

The Cl/F exchange of chlorinated hydrocarbons with HF was carried out in the gas phase (P = 1 atm. and T = 380 °C) on chromium catalysts doped with zinc. A promoting effect of zinc has been found for various chlorinated hydrocarbons. This positive effect was more significant for CF3CH2Cl than for CF3CHCl2 or CH2Cl2. CF3CH2Cl was less reactive than the two other reactants. The promoting effect observed for an atomic ratio Zn/(Zn + Cr) = 0.15 was higher with a low (Zn + Cr) content, which corresponds to a better dispersion of the different elements on the surface. The presence of zinc is supposed to promote the dispersion of chromium and to favour its fluorination to form the centers which are active in Cl/F exchange and were characterized in XPS by energies corresponding to the presence of “CrOF” and “Cr–F” species. Transformation of CF3CH2Cl (T = 380 °C, CF3CH2Cl/HF = 1/4) over Zn-promoted Cr/AlF3 catalysts. Effect of the Zn/(Zn + Cr) atomic ratio and of (Zn + Cr) total amount (figures on the curves) on the activity.

Experimental and theoretical studies on ethylene polymerization using SiO 2-supported silyl chromate type catalysts prepared by a green method

SiO 2-supported silyl chromate catalyst (UCC S-2 catalyst) is an important catalyst for the commercial production of polyethylene. Cann first reported a possible green synthesis route for the preparation of S-2 catalyst through transformation from Phillips catalyst by addition of triphenylsilanol (TPS) to avoid the use of toxic bis(triphenylsilyl) chromate (BC). In this work, this green synthesis route was further investigated by the combination of experimental and theoretical methods. The obtained catalyst was named as PS catalyst based on the incomplete conversion of Phillips catalyst to S-2 catalyst. Polymerization activity of PS1.5 catalyst was higher than that of S-2 catalyst with either TEA or MAO as cocatalyst. The existence of the simultaneously formed hydroxyl group and its coordination to the reduced Cr site was favorable to the PS catalyst, and this might attribute to the higher polymerization activity of PS catalyst than that of S-2 catalyst. A further modification of PS catalyst by a series of alkyl chlorosilane compounds showed that the role of the electron-withdrawing group only limited at a certain distance away from Cr active site. This preliminary study provided a solid basis for a further innovation of PS catalysts and final substitution of S-2 catalyst in commercial processes.

Effect of support structure on the activity of Cr/nanosilica catalysts and the morphology of prepared polyethylene

AbstractPhillips‐type catalysts are responsible for the commercial production of more than one‐third of all polyethylene sold worldwide. Many types of chromium‐based catalysts are used in the Phillips polymerization process. Ordered mesoporous silica structures were synthesized using various surfactant species. Chromium nitrate nonahydrate (Cr(NO3)3·9H2O) complex was grafted onto the surface of pure silica and was used for ethylene polymerization. The materials were characterized using X‐ray diffraction, nitrogen adsorption‐desorption, inductively coupled plasma optical emission spectroscopy, thermogravimetric analysis and Fourier transform infrared spectroscopy. In the as‐synthesized materials, Cr3+ is present as a surface species in pseudo‐octahedral coordination. After calcination, Cr3+ is almost completely oxidized to Cr6+, which is anchored onto the surface in various oxidative states. The catalyst polymerization activity is dependent on the chromium loading, the pre‐calcination temperature and the support properties. In particular, the chromium catalyst prepared using spherical SBA‐15 is more active than the other catalysts investigated. Porous and nano‐fibrous polyethylene samples were prepared using various silica‐supported chromium catalytic systems. Differential scanning calorimetry results show that the melting point of samples produced with the SBA‐15‐supported catalyst is higher than that of samples produced with Cr/SiO2 under the same conditions, which could be related to the existence of an extended‐chain structure. Copyright © 2010 Society of Chemical Industry

NO Reduction on Carbon-Supported Chromium Catalysts

Carbon-supported chromium catalysts were obtained from activated carbons prepared by chemical activation of orange skin with phosphoric acid. These catalysts present a high development of the porous structure, with a relatively large contribution of micro- and mesopores. The presence of chromium produces a considerable improvement of the NO reduction compared to that obtained for the corresponding support. The removal of the carbon−oxygen complexes by thermal treatment is unfavorable for the NO reduction in both activated carbons and chromium catalysts. Oxygen surface groups improve the chromium dispersion and seem to play an important role in the NO reduction mechanism. A linear relationship between the apparent reaction rate of NO reduction and the amount of acidic surface groups was found. The presence of C3H6 and CO considerably increases the NO reduction on the chromium catalyst. This catalyst exhibited a high resistance toward SO2 poisoning at different experimental conditions. The presence of oxyge...

A Selective Chromium Catalyst System for the Trimerization of Ethene and Its Coordination Chemistry

AbstractIn this paper a novel ligand of the type [PNPNH] is presented for the application in a new homogeneous highly selective ethene trimerization system for the formation of 1‐hexene, which consists of the chromium source CrCl3(thf)3, the ligand Ph2PN(iPr)P(Ph)N(iPr)H (1), and Et3Al as an activator in toluene. The excellent characteristics of this new system, e.g. very high selectivity to C6 with highest purity of the C6 fraction (>99 % 1‐hexene), activity on a constant level on a long timescale, use of small amounts of Et3Al as a cheap activator, and only very low production of PE, make it to a hot candidate for industrial application. Its organometallic background gives an indication of the nature of the active catalyst species.

Effect of preparation method on structure and catalytic activity of Cr-promoted Cu catalyst in glycerol hydrogenolysis

Relationships between surface structure and catalytic properties were investigated for a series of copper chromium catalysts. The catalysts were prepared using methods involving impregnation and precipitation, and their catalytic activities were evaluated for the hydrogenolysis of glycerol. Catalyst (10I and 50I) prepared by the impregnation method contained a mixed phase of both individual copper and chromium oxide structures, while the catalyst (50P) prepared by precipitation showed a single phase, with a copper chromite spinel structure (CuCr2O4). XPS data indicated that, after the reduction step, the copper species in the impregnated catalyst was reduced to Cu0, but the catalyst prepared by the precipitation method retained a spinel structure evidenced by the large amount of Cu2+ species. In hydrogenolysis reactions, the precipitated catalyst showed a higher catalytic activity than the impregnated catalyst. Thus, the reduced copper chromite spinel structure, which constitutes a single phase, appears to be responsible for the high catalytic activity in the hydrogenolysis of glycerol to propylene glycol.

Morphological modifications of Cr/SBA-15 and Cr/Al-SBA-15 ethylene polymerization catalysts: Influence on catalytic behaviour and polymer properties

Mesostructured Phillips catalysts (Cr/SBA-15 and Cr/Al-SBA-15) with different morphologies were synthesized and tested in ethylene polymerization. Morphological properties of mesoporous supports were modified by changing some synthesis variables like stirring rate, concentration of silica source, mixture acidity and the type of cosolvent molecule. Obtained solids were calcined and characterized by N 2 adsorption–desorption, XRD, 27Al NMR and SEM. By decreasing the stirring rate during the synthesis, the size and shape of SBA-15 particles changes from small fibre-like to larger irregular agglomerates. As well, a higher amount of sphere-like particles were obtained by increasing TEOS/P123M ratio. On the contrary, for Al-SBA-15 material, agglomerated particles were obtained even at high stirring rate, but no sphere-like particles were observed by increasing the TEOS/P123M ratio. The increase of HCl/P123M ratio clearly modifies the SBA-15 particle morphology but the structure becomes disordered. Cosolvent incorporation in the SBA-15 synthesis produces a swelling effect in the order trimethylbencene > n-decane > toluene. Only the addition of toluene affects SBA-15 morphology from sphere-like to fibre-like particles. Chromium catalysts prepared by impregnation of SBA-15 supports synthesized at lower stirring rates (50 rpm) present higher activity in ethylene polymerization and produce polymers with larger particle size. On the other hand, Cr/SBA-15 catalysts lead to polyethylenes with slightly lower molecular weight and melting point than traditional Phillips catalyst like Cr/SiO 2.

Synthesis, structures and ethylene polymerization behavior of half-metallocene chromium(iii) catalysts bearing salicylaldiminato ligands

A series of half-metallocene chromium(III) complexes bearing a salicylaldiminato ligand, Cp′[2-R1-4-R2-6-(CH = NR3)C6H2O]CrCl [Cp′ = Cp (1, 5), Cp* (2, 3, 4, 6, 7, 8); R1 = Ph (1, 2, 3, 5, 6, 8), iPr (4), tBu (7); R2 = H (1, 2, 3), Br (4), NO2 (5, 6, 7, 8); R3 = iPr (1, 2, 5, 6, 7), tBu (3, 4, 8)], were synthesized. The structures of complexes 1, 3–5, 7–8 were determined by single crystal X-ray diffraction analysis. The X-ray crystallographic analysis indicates that these complexes adopt a pseudo-octahedral coordination environment with a three-legged piano stool geometry. Upon activation with a small amount of AlEt3, complexes 1–8 exhibit good to high catalytic activity for ethylene polymerization and produce ultra-high molecular weight polyethylene (PE) under mild conditions. The productivity of these complexes is relatively low when activated with AlMe3 and AliBu3. Both the productivity of these catalyst systems and the molecular weight of the produced PE can be tuned in a broad range by changing the ligands and the AlR3 co-catalyst.

Direct conversion of glucose to 5-(hydroxymethyl)furfural in ionic liquids with lanthanide catalysts

The direct conversion of glucose to 5-(hydroxymethyl)furfural (HMF) in ionic liquids with lanthanide catalysts was examined in search of a possibly more environmentally feasible process not involving chromium. The highest HMF yield was obtained with ytterbium chloride or triflate together with alkylimidazolium chlorides. Notably, a higher reactivity was observed when the hydrophobicity of the imidazolium cation was increased, in contrast to analogous chromium catalyst systems. This indicates a different reaction mechanism for the lanthanides than for the chromium catalyst systems.

Ethylene polymerization by the chromium catalysts based on bidentate [O, PO] or [S, P] ligands

AbstractNovel chromium catalysts based on bidentate phenoxy‐phosphinoyl (HO‐2R1‐4R2‐6(Ph2PO)C6H2: R1 = R2 = H, 3a; R1 = tBu, R2 = H, 3b; R1 = R2 = tBu, 3c; R1 = R2 = cumyl, 3d; R1 = anthracenyl, R2 = H, 3e) and thiophenol‐phosphine (HS‐2R1‐4R2‐6(Ph2P)C6H2: R1 = R2 = H, 4a; R1 = SiMe3, R2 = H, 4b) were prepared and characterized. Treatment with modified methyaluminoxane, these catalysts displayed moderate to high‐catalytic activities for ethylene polymerization. The activities of them were higher than those of the corresponding catalysts based on bidentate phenoxy‐phosphine ligands. Both the coordinated donors and the ortho‐substituent of the ligands played an important role in improving catalytic activity. The effects of reaction parameters, such as cocatalyst and Al/Cr molar ratio as well as reaction temperature, on ethylene polymerization behaviors were investigated in detail for two favorable catalytic systems, 3b/CrCl3(thf)3 and 4b/CrCl3(thf)3. Catalyst 4b/CrCl3(thf)3 displayed higher catalytic activity and better temperature tolerance for ethylene polymerization than 3b/CrCl3(thf)3. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 311–319, 2010

Distinguishing Chain Growth Mechanisms in Metal-catalyzed Olefin Oligomerization and Polymerization Systems: C2H4/C2D4 Co-oligomerization/Polymerization Experiments Using Chromium, Iron, and Cobalt Catalysts

A series of co-oligomerization and copolymerization reactions of C2H4/C2D4 (1:1) mixtures have been carried out using various transition metal catalysts based on Cr, Co, and Fe in combination with MAO. The oligomeric α-olefin products have been analyzed by GC and GC/MS, and the experimental results have been compared with the theoretical mass spectra derived from mathematical models. Solid polymer samples have been analyzed by 13C{1H} and 13C DEPT-135 NMR spectroscopy. C2H4/C2D4 co-oligomerization can be used as a method to differentiate between a metallacyclic or a Cossee-type chain growth mechanism in oligomerization systems. In the case of a metallacyclic mechanism, no H/D scrambling is observed, whereas for a Cossee-type mechanism, similar rates of chain propagation and chain termination (β-H elimination) result in rapid H/D scrambling of the C2H4/C2D4 feed. This method is therefore limited to oligomerization systems and cannot be applied in polymerization systems, where the rate of chain propagation ...

Characteristics of Bimodal Polyethylene Prepared via Co‐Immobilization of Chromium and Iron Catalysts on an MgCl2‐Based Support

AbstractBimodal polyethylenes comprising varying proportions of high‐ and low‐molecular‐weight fractions are synthesized in a single polymerization stage, via the co‐immobilization of a chromium and an iron catalyst on an MgCl2/AlEtn(OEt)3−n support. Changes observed in the viscoelastic response of the polymer melt with increasing content of the high‐molecular‐weight fraction indicate effective mixing in the bimodal blend. In flow, chains in the high‐molecular weight fraction tend to orient and stretch under shear. Due to the longer relaxation time of the high‐molecular‐weight component, X‐ray diffraction and scattering reveal that shear‐induced crystallization takes place at temperatures close to the equilibrium melting point of linear polyethylene. The so‐crystallized high‐molecular‐weight component suppresses the nucleation barrier for further crystallization, leading to the formation of a “shish‐kebab” polymer morphology.magnified image

Toolbox Approach to the Search for Effective Ligands for Catalytic Asymmetric Cr-Mediated Coupling Reactions

Chromium catalysts derived from chiral sulfonamides represented by A effect the couplings of aldehydes with vinyl, allyl, or alkyl halides. With three distinct sites for structural modification, A affords access to a structurally diverse pool of chiral sulfonamides. The Cr catalysts derived from these sulfonamides exhibit a broad range of catalyst-substrate matching profiles. A strategy is presented to search for a satisfactory chiral sulfonamide for a given substrate. In order to demonstrate the generality and effectiveness of this approach, five diverse C-C bond-forming cases have been selected from the halichondrin synthesis. For each of the cases, two ligands have been deliberately searched for, to induce the formation of (R)- and (S)-alcohols, respectively, at the arbitrarily chosen efficiency level of ">or=80% yield with >or=20:1 stereoselectivity in the presence of <or=20 mol % of a Cr catalyst". For 9 out of the 10 cases studied, a satisfactory catalyst has been found within this pool of sulfonamides. Even for the remaining case, a Cr catalyst inducing stereoselectivity up to 8:1 has been identified.

Addition of CCl4 to olefins catalyzed by chromium and ruthenium complexes: The influence of water as a nucleophilic additive

The feasibility of the addition of CCl4 to linear and cyclic olefins and dienes in the presence of chromium and ruthenium complexes was established. The influence of water as a nucleophilic additive and the influence of the olefin nature, the catalyst central atom, and its ligand environment on the reaction selectivity and product yields were investigated. It was shown that the addition of water to chromium and ruthenium catalysts leads to the hydrolysis of the chloromethyl group in the resulting 1,1,1,3-tetrachloro derivatives, yielding the corresponding hydroxy derivatives and increasing the selectivity for the formation of (1 + 1) adducts, which is caused by the formation of hypochlorous acid playing the role of a coactivator of the addition of CCl4 to alkenes.

Ethene trimerization at CrI/CrIII — A density functional theory (DFT) study

Catalytic ethene trimerization at a chromium(I) indolate-AlR2Cl centre has been studied by density functional theory (DFT) methods. The reaction is found to follow the standard metallacycle mechanism. At most stages of the reaction, coordination of Cr to the pyrrole ring of the indolate is preferred. In all 13-e intermediates, coordination of the Al-bound chloride to Cr provides additional stabilization: the chloride behaves as a hemilabile ligand. Benzene is found to compete with ethene for coordination to CrI. The final hexene-forming step involves direct Cβ → Cα′ hydrogen transfer; reductive elimination from a possible (hydride)(hexenyl) intermediate is more difficult. The kinetic isotope effect calculated for the direct hydrogen transfer (4.1) agrees well with the experimental value for a bis(phosphino)amide chromium catalyst. Side products obtained in such systems (methylenecyclopentane, methylcyclopentane) can plausibly be explained through routes not involving any (hydride)(alkyl) reductive elimination. Our results indicate that a CrI/CrIII trimerization cycle is possible for some chromium trimerization catalysts, and also suggest that direct hydrogen transfer is most likely a general feature of trimerization at Cr centres.

Half-Sandwich Chromium(III) Catalysts Bearing Hydroxyindanimine Ligands for Ethylene Polymerization

A series of novel half-sandwich chromium(III) complexes bearing hydroxyindanimine ligands, [Cp*Cr[ArN═CC2H3(CH3)C6HR1R2O]Cl (Ar = Ph, R1 = H, R2 = Me (2a), R2 = Cl (2c); Ar = 2,6-iPr2C6H3, R1 = H, R2 = Me (2b), R2 = Cl (2d); Ar = Ph, R1 = tBu, R2 = Me (2e))], were synthesized by the reaction of Cp*CrCl2(THF) and the sodium salts of the hydroxylindanone-imine ligands in THF at −78 °C. Complex 2c has been confirmed by single-crystal X-ray analysis. The complex adopts a three-legged piano stool geometry with a pseudo-octahedral coordination environment. After activation with only a small amount of AlEt3, these catalytic precursors exhibit high activities (up to 1.68 × 105 g PE (mol Cr)−1 h−1) for ethylene polymerization to afford high molecular weight PE ((5.45−9.23) × 105 g mol−1). The catalytic activities of the chromium complexes increase with decreasing steric hindrance on the nitrogen atom of the hydroxyl-indanone-imine ligand. The introduction of the bulky ortho-tBu group and the para-chlorine atom on ...

On the way to biodegradable poly(hydroxy butyrate) from propylene oxide and carbon monoxide via β-butyrolactone: Multisite catalysis with newly designed chiral indole-imino chromium(III) complexes

Abstract Enantioenriched poly(hydroxy butyrate) (PHB) is a biodegradable polyester of significant commercial interest as an environmentally benign substitute of commodity polyolefines. We report on the design and development of new chiral indole-based ligand families and on their chromium(III) complexes as enantioselective catalysts for the conversion of propylene oxide and carbon monoxide to enantioenriched β-butyrolactone, the key monomer for the production of PHB by ring-opening polymerization. The enantioselective carbonylation catalysts are based on new chiral tri- and tetradentate [N2O] and [N4] chromium(III) complexes containing chiral indolaldimine ligand scaffolds. The conceptual design of these ligands is inspired by Jacobsen’s salicylaldimine lead structure; the key difference is an exchange of the salicyl-O-donor against an indole-N-donor, allowing additional structural diversity and stereoelectronic tuning by the indole substitution pattern. Synthetically, chiral indolealdimines are easily accessible from 7-formylindoles by standard Schiff base condensation with chiral amine building blocks; the 7-formylindoles in turn are synthesized from the corresponding 7-bromoindoles by the Rapoport synthesis, and the starting 7-bromoindoles are accessible from 2-bromoaniline by the classical Fischer indole synthesis. Three generations of chiral [N2O] and [N4] chromium(III) catalysts have been developed and evaluated in the enantioselective carbonylation of racemic propylene oxide with carbon monoxide using tetracarbonylcobaltate as the nucleophilic reagent for the insertion of carbon monoxide into the activated propylene oxide/chiral Lewis acid complex. The best catalyst out of 10 candidates showed at a temperature of 80 °C an activity of 37% conversion, 100% chemoselectivity, and 19% stereoselectivity.

ChemInform Abstract: Addition of Water and Carbon Tetrachloride to Cyclododecene in the Presence of Chromium Catalysts.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Improvement of the Reactivity and Selectivity of the oxo‐Diels—Alder Reaction by Steric Modification of the Salen—Chromium Catalyst.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.