Über Oberflächenverbindungen von Übergangsmetallen. VIII. Die Komplexbildung von koordinativ ungesättigtem Oberflächen-Chrom (II) mit Oxiden des Stickstoffs

On Surface Compounds of Transition Metals. VIII. Complex Formation of a Coordinatively Unsaturated Cr^II Surface Compound with Nitrogen Oxides N₂O forms with surface-Cr(II) a relatively unstable light blue compound of the stoichiometry 1:1, while addition of NO results in formation of a very stable dark brown, diamagnetic surface complex . By reaction with O₂ this complex undergoes — depending on reaction temperature — either replacement of NO unter reoxidation of the metal (→Cr(VI)) or/and reaction of the ligand (→NO₂). Direct reaction of NO₂ with results in the same products as stepwise addition of NO and 1/2 O₂. reacts with HCl/ROH under formation of the soluble, paramagnetic kation [Cr(NO)(ROH)_n]²⁺, which is formulated as [Cr(II)(NO)]²⁺ ? [Cr(I)(NO⁺)]²⁺ accordingly to the e.s.r. spectra.     

Oberflächenverbindungen von Übergangsmetallen. XXIX. Die Umsetzung von Oberflächen-Chrom(VI)/Silicagel mit Aluminiumalkylen: „Formierte”︁ Phillips-Katalysatoren

Surface Compounds of Transition Metals. XXIX. Reaction of Surface Chromium (VI)/Silica Gel with Aluminium Alkyles: “Formation” of Phillips Catalysts By the reaction of surface chromium (VI)/silicagel with aluminium alkyles - leading to the so called “Phillips catalysts fo the second generation” - the metal is reduced in two 2e steps via a chromium (IV) intermediate to a final Cr product. The extent of the reduction depends on the ratio Al/Cr, the influence fo light, the presence of higher olefins and the temperature.     

Oberflächenverbindungen von Übergangsmetallen. XVIII. Der Einfluß der Cr-Konzentration auf Oxydationszahl und „Qualität”︁ von Cr-Oberflächenverbindungen auf Silicagel

Surface Compounds of Transition Metals. XVIII. The Influence of Cr Concentration on Oxidation Number and “Quality” of Cr Surface Compounds on Silicagel Mean oxidation number (OZ ) and mean unsaturated character (q ) of reduced Cr surface compounds on silicagel depend on the concentration of Cr in the product. The first effect is caused by the decrease of thermally stable surface Cr^VI with increasing Cr concentration in the activated product, since surface Cr^VI is the only precursor of surface Cr^II (constant yield of reduction～90%); consequently OZ _red shifts to higher values with increasing concentration. — The measured decrease of q with increasing Cr content cannot be correlated with a possible coordination of neighbouring SiOH groups, as it was suggested for the influence of T_act and T_red; other explanations are proposed. For the analyzed product, the results support the postulated model of an ensemble of topologically different individual surface species of typical reactivity.     

Über Trichlorphosphazo-Verbindungen aus Nitrilen. I

The compound [HCCl=C(NPCl₃)(PCl₃)[PCl₆], VI, is formed in the reaction between acetonitrile and PCl₅. Reaction of H₂S with VI results in the formation of the chloride and by further reactions of this chloride with BCl₃ and SbCl₅ the tetrachloroborate and hexachloroantimonate salts respectively, are obtained. Chloroacetonitrile can be made to react with PCl₅ to give HCCl?C(NPCl₃)Cl, II, by use of suitable conditions. There is no indication that mixtures of cis/trans isomers of VI or II exist as has been previously postulated. The ¹H and ³¹P NMR spectra of the above-mentioned compounds and numerous further products from the reactions of CH₃CN, ClCH₂CN and Cl₂CHCN with PCl₅ are discussed in detail. II occurs in a cisoid and a transoid conformation.     

Oberflächenverbindungen von Übergangsmetallen. XXX. Fischer-Tropsch-analoge Umsetzung von CO/H2 bzw. CO2/H2 an Oberflächen-Chrom(II)

Surface Compounds of Transition Metals. XXX. Fischer-Tropsch Analogous Reactions of CO/H₂ and CO₂/H₂ on Surface Chromium(II) Surface chromium(II)/silicagel catalyzes the reduction of CO and CO₂ by hydrogen to CH₄, C₂H₆ and C₃H₈. The catalyst is partially desactivated by this reaction, but a permanent activity of ca. 30% remains. – The reaction can be formulated via the sequence formaldehyde complex → <CH₂>-complex → alkane. If cycloalkenes or chlorobenzene are added simultaneously, these scavenger molecules are methylated by <CH₂>.     

Vinyl ether end‐groups in poly(ethylene glycol)s from the Na2CO3‐promoted degradation of 1,3‐dioxolan‐2‐one polymers

The Na₂CO₃‐promoted polymerization of 1,3‐dioxolan‐2‐one (I) to afford poly(ethylene glycol) III was reinvestigated. The reaction appeared to involve a nucleophilic attack against the carbonyl and methylene groups of I to afford poly(carbonate) II with poly(ethylene glycol) linkages and ethylene oxide IV as a side product (10–22%). As the reaction progressed, poly(carbonate) II decreased and poly(ethylene glycol) III increased. Under some conditions, poly(ethylene glycol)s V and VI with vinyl ether terminal groups were formed unexpectedly. The formation of unsaturated products during the polymerization of I/EO (ethylene oxide) has not been reported in the literature. We believe that vinyl ethers were formed from the degradation of poly(carbonate)s and were accompanied by a reduction in molecular weight. The structures of vinyl ethers V and VI were confirmed by hydrogenation of the double bond into the ethyl ether group in VII and VIII, respectively. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 152–160, 2000     

Über Oberflächenverbindungen von Übergangsmetallen. X [1]. Zum Stöchiometrie-Problem der Bindung von Oberflächen-Chrom(II) an Silicagel

On Surface Compounds of Transition Metals. X. The stoichiometry Problem of Fixation of Surface Chromium (II) on Silicagel. Thermogravimetric analysis of a reduced Philips catalyst (support: silicagel “Merck 7733”) shows that the metal is bound prediminantly (if not only) in chromate-like units up to a concentration of 2,8% Cr^II. At higher Cr^II contents. up to the maximum of 3.5%, the additionally fixed chromium does not consume further OH^s, within the scope of the thermogravimetric method formation of polychromatelike structures and/or use of not-neighbouring OH groups. The analysis further demonstrates that not all the Si? Oh groups comprised by the method are able to accommodate surface chromium. The fixation of the metal seems to proceed most likely by reaction with the vicinal pairs of Si? OH, as a correlation of our data with silica models shows.     

Polymerization of acrylonitrile initiated by thiourea–chromium(VI) redox system

The kinetics of polymerization of acrylonitrile initiated by Cr(VI)–thiourea and Cr(VI)–ethylene thiourea have been studied at 35, 40, and 45°C in nitrogen. The rates of polymerization and of disappearance of Cr(VI) were measured. Chromic acid alone did not initiate the polymerization under deaerated and undeaerated conditions. On the basis of the experimental observation of the dependence of the rate of polymerization R_p, the rate of Cr(VI) disappearance, –R_m, etc., on various variables, a suitable kinetic scheme was proposed and various rate and energy parameters were evaluated.     

Vinyl polymerization initiated by chromic acid–reducing agent systems

A kinetic study of the thermal polymerization of acrylonitrile initiated by chromic acid–reducing agent (n-butanol, ethylene glycol, cyclohexanone, and acetaldehyde) systems was made. Chromic acid alone did not initiate polymerization under deaerated or undeaerated conditions. On the basis of the experimental determination of the dependencies of various variables on the rate of polymerization R_p, the rate of chromium (VI) disappearance ?R_M, the degree of polymerization DP, etc., a reasonable kinetic scheme was arrived at. The mechanism with the reducing agents, n-butanol, cyclohexanone, and ethylene glycol, was found to be similar but different from that with acetaldehyde. Evidence has been presented to prove the formation of radical intermediates formed by the oxidation of the reducing agent by Cr(IV). Rate parameters for oxidation of the reducing agent and polymerization of the monomer were evaluated.     

The photo-induced degradations of tetraneopentylchromium and trimesitylchromium tetrahydrofuranate: some applications to catalysis

Photo-induced reactions of tetraneopentylchromium (I) and trimesitylchromium tetrahydrofuranate (II) have been investigated. Both I and II upon photolysis in solution function as active catalysts for the polymerization of vinyl monomers such as styrene, methyl acrylate, methyl methacrylate, and vinyl acetate, and also for the polymerization of ethylene under very mild conditions. Irradiation of I in tetrahydrofuran in the presence of carbon monoxide leads to the production of Cr(CO)₆ and Cr(CO)₅(THF), demonstrating that low valent chromium species are formed under these conditions.     

Determination of chromium(III) and total chromium in natural waters using a surface ion-imprinted silica gel as selective adsorbent

A sensitive and selective method has been developed to determine Cr(III) and total Cr in natural water samples by ICP-AES with a Cr(III)-imprinted aminopropyl-functionalised silica gel adsorbent. The Cr(III)-imprinted and non-imprinted adsorbent were prepared by an easy one-step reaction with a surface imprinting technique. Their maximum static adsorption capacities for Cr(III) were 11.12 mg g^?1 and 3.81 mg g^?1, respectively. The relative selectivity factors (α _r) for Cr(III)/Co(II), Cr(III)/Au(III), Cr(III)/Ni(II), Cr(III)/Cu(II), Cr(III)/Zn(II), and Cr(III)/Cr(VI), were 377, 21.4, 15.4, 27.7, 26.4, and 31.9, respectively. Under the optimal conditions, Cr(III) can be absorbed quantitatively, but Cr(VI) was not retained. Total chromium was obtained after reducing Cr(VI) to Cr(III) with hydroxyammonium chloride. The detection limit (3σ) for Cr(III) was 0.11 ng mL^?1. The relative standard deviation was 1.2%. The proposed method has been validated by analysing two certified reference materials and successfully applied to the determination and speciation of chromium in natural water samples with satisfactory results.     

Optimization of species stability and interconversion during the complexing reaction for chromium speciation by high‐performance liquid chromatography with inductively coupled plasma mass spectrometry

High‐performance liquid chromatography coupled with inductively coupled plasma mass spectrometry was employed for the determination of chromium species. For simultaneous separation of both chromium species by an anion‐exchange column, ethylenediaminetetraacetic acid was induced to form negatively charged complex with Cr(III) normally. Cr(III) chelating reactions are known to be slow, so a high temperature and long reaction time are needed to ensure the completion of the complexing reaction. However, the stability and interconversion of chromium species during the complexing reaction have not been studied earlier. The main aim of this work was to optimize and investigate complexing reaction conditions between ethylenediaminetetraacetic acid and Cr(III). Through optimizing conditions, the reaction will be finished completely in 15 min at pH 7 and 70°C without any obvious interconversion between Cr(VI) and Cr(III). By compromising analysis time, chromatographic resolution, and sensitivity, 60 mM NH₄NO₃ as competing ion concentration and 1.2 mL/min as flow rate have been selected for real‐sample application. Detection limits for Cr(VI) and Cr(III) were 0.051 and 0.078 μg/L, respectively. The proposed method was used for the determination of chromium species in tap and surface water samples with an acceptable range of spiked recoveries of 95–109%.     

The CrOx/SiO2/Si(100) catalyst – a surface science approach to supported olefin polymerization catalysis

Depositing catalytically active particles onto flat, thin and oxidic support forms an attractive way to make supported catalyst suitable for surface science characterization. Here we show how this approach has been applied to the Phillips (CrO_x/SiO₂) ethylene polymerization catalyst. The model catalyst shows a respectable polymerization activity after thermal activation in dry air (calcination). Combining the molecular information from X‐ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) we can draw a molecular level of the activated catalyst that features exclusively monochromate species, which are anchored to the silica support via ester bonds with the surface silanol groups. These surface chromates form the active polymerization site upon contact with ethylene. Upon increasing calcination temperature we observe a decrease in chromium coverage as some of the surface chromate desorbs from the silica surface. Nevertheless, we also find an increasing polymerization activity of the model catalyst. We attribute this increase in catalytic activity to the isolation of the supported chromium, which prevents dimerization of the coordinatively unsaturated active site. Diluting the amount of chromium to 200 Cr‐atoms/nm² of silica surface enables the visualisation of polyethylene produced by a single active site.     

Ethylene Adsorption and Oxidation Kinetics in the Reactions with Pd(II)/SiO2 and Cr(VI)/SiO2: Consideration of Substrate Distribution between the Gas Phase,Silica Gel,and Reaction Centers

The kinetics of ethylene oxidation by PdCl₂ and CrO₃ complexes supported on silica gel (300 K, closed batch reactor) and the adsorption of C₂H₄ by silica gel and metal complex reaction centers (M ⁿ ) were studied. A new version of the kinetic distribution method was applied to determine the rate constants of ethylene reactions with metal complexes with consideration for the equilibrium distribution of C₂H₄ among the reactor gas phase, silica gel, and M ⁿ . The rate constant of a first-order reaction with respect to Cr(VI) (k _e) remained constant as [M ⁿ ] was increased up to 0.15 mol % with the absence of detectable ethylene adsorption by chromium(VI). In the case of Pd(II)/SiO₂, strong ethylene adsorption by palladium(II) was found, and k _e was an exponential function of [M ⁿ ]. This exponential function is indicative of an increase in the specific activity of Pd(II) with palladium concentration on SiO₂. Taking into account the adsorption of ethylene (physisorption on SiO₂ and chemisorption on Pd(II)), we found an analogy between the kinetic behaviors of Pd(II) in reactions with ethylene on silica gel and with ethylene and other hydrocarbons in solutions.     

Variations of structure and active species in mesoporous Cr-MSU-x catalyst during the dehydrogenation of ethane with CO<Subscript>2</Subscript>

The dehydrogenation of ethane to ethylene under CO₂ over mesoporous Cr-MSU catalyst was investigated with respect to the time on-stream behavior. When ethane was allowed to react for about 240 min, the meso-structure of catalyst remained nearly unchanged in spite of some decrease of surface area. The Cr(VI) species in tetrahedral coordination in fresh Cr-MSU were reduced to Cr(III) species in octahedral coordination, that was expected to cause the activity decrease of catalyst, together with the structure change. Cr(VI) is more active than Cr(III) for ethane dehydrogenation with CO₂, but Cr(III) represent fairly stable active centers for the reaction. The article is published in the original.     

Design of BiOBr0.25I0.75for synergy photoreduction Cr（Ⅵ） and capture Cr（Ⅲ） over wide pH range

Conversion of hexavalent chromium (Cr（Ⅵ）) to trivalent chromium (Cr（Ⅲ）) is an effective way to reduce its environmental risk,especially via photoreduction process.However,over a wide range of p H values,it is still a great challenge to achieve a high removal rate,and the disposal of produced Cr（Ⅲ） should be concerned.In this work,we implemented a high removal rate at 98%for Cr（Ⅵ） and total chromium(Cr（T）) over a wide p H range （4–10） through the synergistic effect of adsorption,photoreduction an...     

Ethylene polymerization over organochromium catalysts: A comparison between closed and open pentadienyl ligands

Divalent organochromium compounds, Cr(Lig)₂, often become active catalysts for the polymerization of ethylene when deposited onto an oxide carrier such as silica or aluminophosphate. Hydroxyl groups are thought to react, releasing one ligand and binding the chromium to the surface. The behavior of the catalyst is then governed by the remaining ligand and the type of carrier. In this study two types of ligands were investigated: cyclopentadienyl and its open ring analog dimethylpentadienyl. This small difference in the type of ligand produces a fundamental difference in the polymerization mechanism. For comparison the mixed ligand chromocene, with one open and one closed ligand, was also synthesized and tested for polymerization.     

Polymerizations with homogeneous chromium catalysts

Homogeneous catalysts derived from chromium complexes and organo-aluminum compounds have been discovered which effect the polymerization of ethylene and the oligomerization of simple olefins. The applicable chromium derivatives are halide complexes of the type CrX₂L₂, CrX₂L₂(NO)₂, CrX₃L₃, and [CrX₃L₂]₂, wherein the ligands L are pyridine, tri-n-butylphosphine, and the like. The nature of the systems with regard to polymerization or oligomerization has been demonstrated to be primarily a function of the organoaluminum halide cocatalyst and of the reaction conditions. In general, systems containing ethylaluminum dichloride cocatalyst afford simple oligomerization products at reaction temperatures above 50°C, while at lower temperatures those containing organoaluminums of the type R₃Al₂X₃ or R₂AlX effect the polymerization of ethylene as the principal reaction.     

Carbonyl-Monoolefin-Derivate des Chroms,Molybdäns und Wolframs. I. Tetracarbonyl-Phosphin-Olefin-Komplexe

Carbonyl Monoolefin Derivatives of the Group VI Transition Metals. I. Tetracarbonyl Phosphine Olefin Complexes Monoolefin complexes cis-M(CO)₄(PR₃)(olefin) (M ? Cr, Mo, W; R ? Et, Bu, Prⁱ, Ph; olefin ? maleic anhydride, dimethyl maleate, dimethyl fumarate, bis(trimethylsilyl) fumarate, ethylene) are obtained from the ionic compounds Et₄N[R₃PM(CO)₄Cl] either via ethanol or acetonitrile derivatives M(CO)₄(PR₃)L, or directly in a two phase system. The olefins are displaced by Lewis-bases such as amines or phosphines under mild conditions.     

The determination of chromium(VI) in natural waters by differential pulse polarography.

A method utilizing differential pulse polarography for the determination of chromium(VI) in natural water is described. Additions of 0.62 μg Cu(II) ml^-1 and 0.55 μg Fe(III) ml^-1 did not interfere with the determination of 0.050 μg Cr(VI) ml^-1. The natural water samples containing chromium(VI) were buffered to approximately pH 7 with 0.1 M ammonium acetate and 0.005 M ethylene diamine and analyzed. Natural water samples of chromium content from 0.035 μg ml^-1 to 2.0 μg ml^-1 may be analyzed directly without further preparation. The detection limit is 0.010 μg ml^-1.