Untersuchungen an Nickeloxid-Mischkatalysatoren. XI. Oberflächenchemische Eigenschaften von NiO/SiO2-Katalysatoren

Studies on Nickel Oxide Mixed Catalysts. XI. Surface Chemical Properties of NiO/SiO₂ Catalysts NiO/SiO₂ catalysts with different composition prepared by precipitation have been investigated which are characterized by a nickel layersilicate-like structure. The determination of the surface chemical properties was carried out by infrared spectroscopy (before and after pyridine adsorption), ¹H-NMR-measurements, chemisorption of ammonia, and titration with n-butylamine. It has been found three kinds of hydroxid groups which are assigned to Ni? OH and Si? OH groups with respect to investigations on definite nickel layersilicates. Furthermore, coordinatively unsaturated Ni^II surface sites were indicated. The number of the OH groups as well as the centers determined by chemisorption of bases increase with increasing NiO content. The obtained results allow the conclusion that the OH groups and the coordinatively unsaturated Ni^II surface sites are weakly acid.     

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 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.     

Polymerization of Ethylene by Silica‐Supported Dinuclear CrIII Sites through an Initiation Step Involving CH Bond Activation

The insertion of an olefin into a preformed metal–carbon bond is a common mechanism for transition‐metal‐catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal–carbon bond is not present in the precatalyst. The Phillips catalyst, CrO₃ dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first Cr? C bond is formed remain unknown. We synthesized well‐defined dinuclear Cr^II and Cr^III sites on silica. Whereas the Cr^II material was a poor polymerization catalyst, the Cr^III material was active. Poisoning studies showed that about 65 % of the Cr^III sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si–(μ‐OH)–Cr^III species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr^III? O bonds.     

Untersuchungen an katalytisch aktiven Oberflächenverbindungen. VII. Katalytische Eigenschaften von Vanadiumoxid-Chromoxid-Aufschicht-Katalysatoren

Studies on Catalytically Active Surface Compounds. VII. Catalytic Properties of Oxidic Vanadium-Chromium Catalysts The catalytic properties of silica supported vanadium-chromium catalysts for the oxidation of ethanol, butane, propene, and butene are described and completed by ESR measurements. Differences in catalytic activity of chromium-free vanadium catalysts which are caused by different values of the silanol group concentration of the support are shown to become levelled by the addition of chromium. V? O? Cr species of medium catalytic activity are considered to be the catalytic active center. ESR spectra taken after the reaction show the existence of Cr³⁺ and VO⁺² species. A correlation between the intensity of the VO²⁺ signal and the catalytic activity of the samples is observed.     

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.     

Über die Natur von Schwermetallkomplexen aus 2-(2-Hydroxyphenyl)-4-carboxy-2-H-napht[1.2-d]triazolen

2-(2-hydroxyphenyl)-4-carboxy-naphtho [1.2-d] triazoles are terdentate ligands which can form 1:1 complexes with Cu^II and Ni^II ions, and 1:1 as well as 1:2 complexes with Cr^III-ions. All these complexes are characterized by ortho-condensed 6/6-membered rings, the 1:2 chromium complexes having the DREW -PFITZNER arrangement. This behaviour, which differs from that of comparable o-hydroxy-o′-carboxy-diaryl azo-dyestuffs, is due to the different dimensions of the compared ligands.     

Polymerization of Ethylene by Silica‐Supported Dinuclear CrIII Sites through an Initiation Step Involving C?H Bond Activation

The insertion of an olefin into a preformed metal–carbon bond is a common mechanism for transition‐metal‐catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal–carbon bond is not present in the precatalyst. The Phillips catalyst, CrO₃ dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first Cr C bond is formed remain unknown. We synthesized well‐defined dinuclear Cr^II and Cr^III sites on silica. Whereas the Cr^II material was a poor polymerization catalyst, the Cr^III material was active. Poisoning studies showed that about 65 % of the Cr^III sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si–(μ‐OH)–Cr^III species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr^III O bonds.     

Über die Chemie der Oberfläche des Titandioxids. IV. Austausch von Hydroxidionen gegen Fluoridionen

On reaction of anatase of high surface area with NaF solutions, OH groups of the TiO₂ surface were replaced by F^?. Only half the surface OH groups were exchanged in unbuffered solutions or at a pH at or slightly above the zero point of charge (pH 6.6). These are the OH groups exhibiting basic character. The remaining OH groups of acidic character exchange H⁺ for Na⁺ concurrently. They too, are replaced by fluorine in more acidic solutions (pH 4.6).     

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>.     

Über Chrom(II)-carboxylate und Halogenochromate(II)

Highly pure chromium(II) acetate, was obtained from chromium powder and anhydrous acetic acid in the presence of a small amount of acetylhalide. Cr^II acetate reacts with acetyl halide in anhydrous acetic acid to sesquisolvates like CrCl₂ · 3/2 CH₃COOH. If one equivalent of alkali acetate or an organic nitrogen base is added to solutions of Cr^II acetate in acetyl halide/acetic acid mixtures, trihalochromates(II) are precipitated which are hexagonal in structure, except the ammonium and the potassium salt. With two equivalents of alkali or ammonium acetate tetrachlorochromates(II) of cesium, rubidium and ammonium are precipitated. They are tetragonal in structure (K₂NiF₄ type). Using pyridinium acetate with various mixtures of acetyl bromide and acetic acid, only the solvates (PyH)₃(CrBr₅) · 2CH₃COOH is formed.     

Kationenverteilung und Überstrukturordnung in ternären und quaternären Sulfidspinellen MIIMS4 – Einkristallstrukturuntersuchungen

Cation Distribution and Superstructure Ordering in Ternary and Quaternary Sulfide Spinels M^IIM₂^III S₄ – Single Crystal Structure Determinations The crystal structures of spinel type MIn₂S₄ (M ? Mn, Co, Ni), MCr_2?2xIn_2xS₄ (M ? Mn, Ni), and Cd_0.52Co_0,48Cr₂S₄ were reinvestigated by X-ray methods using single crystals grown by vapour phase transport technique. The indium sulfides possess a partially inverse distribution of the metal ions on the tetrahedral (8a) and octahedral sites (16d) of the structure. The degrees of inversion λ are 0.34 (MnIn₂S₄, a = 1072.0(1) pm, structural parameter u = 0.25726(2)), 0.84 (CoIn₂S₄, a = 1058.1(1) pm, u = 0.26921(5)) und 0.93 (NiIn₂S₄ a = 1050.5(1), u = 0.26040(3)). In the case of the chromium indium sulfide solid solutions, the degrees of inversion (and the structural parameters) increase (and decrease) linearly with increase in indium content x. ψ-scans of reflections not allowed in the space group Fd3 m do not prove simultaneous diffraction. Refinement of the structure of MnIn₂S₄ in space group F4 3m results in a partial superstructure ordering of Mn and In on the tetrahedral sites, 4a Mn_0.83In_0.17, 4c Mn_0.49In_0.51. In the case of Cd_0.52Co_0.48Cr₂S₄, superstructure ordering is like Cd_0.41Co_0.59 and Cd_0.62Co_0.38, respectively.     

Jahn-Teller-Verzerrungen von Übergangsmetallionen in tetraedrischer Koordination – die Strukturen von Cat[MII(NCS)4] (MII: Co,Ni, Cu) und von Spinellmischkristallen MIICr2O4(MII: Zn?Ni,Zn?Cu,Cu?Ni)

Jahn-Teller Distortions of Transition Metal Ions in Tetrahedral Coordination — The Structures of Cat[M^II(NCS)₄]^II (M^II: Co, Ni, Cu) and of Mixed Crystals M^IICr₂O₄(M^II: Zn? Ni, Zn? Cu, Cu? Ni) of the Spinel Type The structure determination of compounds Cat[M^II(NCS)₄] with Cat = p-xylylenebis(triphenylphosphonium)²⁺ and M^II = Co, Ni, Cu [space group P2₁/n, Z = 4] yielded pseudotetrahedral M^IIN₄-polyhedra, which are distorted by packing forces and vibronic coupling effects of the Jahn-Teller type. Spinel mixed crystals with M^II = Zn? Ni, Zn? Cu, Ni? Cu in the tetrahedral sites exhibit phase transition to tetragonal and o-rhombic structures, induced by cooperative Jahn-Teller interactions. The distortion symmetries of the M^IIN₄ and M^IIO₄ tetrahedra are analysed on the basis of the respective electronic groundstate and the possible Jahn-Teller active vibrational modes.     

Neue Redoxserien auf der Basis von Übergangsmetallkomplexen heterocyclischer Arensulfonylhydrazone

New Redox Series Based on Transition Metal Complexes of Heterocyclic Arenesulfonylhydrazones Heterocyclic bidentate arenesulfonylhydrazones (2-acetylpyridine-p-toluenesulfonylhydrazone ? APSH? H, 2-acetylquinoline-p-toluenesulfonylhydrazone ? ACSH? H) with transition metal ions afford tetrahedral 1,2-complexes M^II(APSH)₂ and M^II(ACSH)₂. In most cases the E-isomers of APSH^? and ACSH^? are coordinated, five-membered chelate rings are formed with the pyridine and hydrazone nitrogen atoms as donor atoms. In the complexes Zn(APSH)₂ and Cu(APSH)₂ probably the Z-isomer of APSH^? is present with the pyridine and the sulfonylamide nitrogen atoms as a donor set, yielding a six-membered chelate ring. These proposals are based on the magnetic moments, the ligand field spectra, and the binding energies (ESCA). Normally the complexes M^II(APSH)₂ and M^II(ACSH)₂ are reduced in two reversible steps. For Fe(APSH)₂ and Fe(ACSH)₂ a third anodic wave, but for Cu(APSH)₂ only one wave is observed. Relations between redox properties and structure of the new complexes are discussed, and the redox series are compared with that of the M(dipy)₃ⁿ⁺-type complexes.     

Studies regarding the formation from metal nitrates and diol of NiM 2 III O4 spinels, inside a silica matrix

The present study deals with preparation and characterization of spinel mixed oxide systems NiM ₂ ^III O₄, where M^III?=?Fe^III, Cr^III. In order to obtain 50% NiFe₂O₄/50% SiO₂ and 50% NiCr₂O₄/50% SiO₂ nanocomposite, we have used a versatile route based on the thermal decomposition inside the SiO₂ matrix, of some particular precursors, coordination compounds of the involved M^II and M^III cations with dicarboxylate ligands. The ligands form in the redox reaction between metal nitrates mixture and 1,3-propanediol at the heating around 140?°C of the gels (tetraethylorthosilicate?Cmetal nitrates?C1,3-propanediol?Cwater). The as-obtained precursors, embedded in silica gels, have been characterized by FT-IR spectrometry and thermal analysis. Both precursors thermally decompose up to 350?°C leading to the formation of the corresponding metal oxides inside the silica matrix. X-ray diffraction of the annealed powders have evidenced the formation of NiFe₂O₄ starting with 600?°C, and NiCr₂O₄ starting with 400?°C. This behavior can be explained by the fact that, by thermal decomposition of the Fe(III) carboxylate at 300?°C, the spinelic phase ??-Fe₂O₃ is formed, which interacts with the NiO, forming the ferrite nuclei. By thermal decomposition of chromium carboxylate, a nonstoichiometric chromium oxide (Cr₂O_3+x ) is formed. In the range 380?C400?°C, Cr₂O_3+x turns into Cr₂O₃ which immediately interacts with NiO leading to the formation of nickel chromites nuclei inside the pores of silica matrix. Both spinels have been obtained as nanocrystalites homogenously dispersed as resulted from XRD and TEM data.     

Untersuchungen an oxidischen Katalysatoren. XV. Der Einfluß von Oberflächenhydratation und -acidität der Übergangsformen des Aluminiumoxids auf die katalytische Aktivität bei der Dehydratisierung von Isopropanol

Studies on Oxide Catalysts. XV. The Influence of Surface Hydration and Acidity of Transient Forms of Alumina on the Catalytic Activity in the Dehydration of Isopropanol Surface OH groups were determined quantitatively on a aeries fo different samples of transient aluminas by means of D₂O exchange as well as by interaction with thionyl chloride and potassium vapour, and the acid centers were determined by titration with potassium methoxide and high temperature chemisorption of ammonia. Only the deuterium exchange and chemisorption of ammonia proved to be suitable for a quantitative characterization of the surface chemical properties mentioned in the paper. The catalytic activity is dependent directly on the number of acid centers. Between the number of surface OH groups and the results of the catalytic dehydration of isopropanol, however, no correlation was found.     

Molecular aspects of glucose dehydration by chromium chlorides in ionic liquids

A combined experimental and computational study of the ionic‐liquid‐mediated dehydration of glucose and fructose by Cr^II and Cr^III chlorides has been performed. The ability of chromium to selectively dehydrate glucose to 5‐hydroxymethylfurfural (HMF) in the ionic liquid 1‐ethyl‐3‐methyl imidazolium chloride does not depend on the oxidation state of chromium. Nevertheless, Cr^III exhibits higher activity and selectivity to HMF than Cr^II. Anhydrous CrCl₂ and CrCl₃?6 H₂O readily catalyze glucose dehydration with HMF yields of 60 and 72 %, respectively, after 3 h. Anhydrous CrCl₃ has a lower activity, because it only slowly dissolves in the reaction mixture. The transformation of glucose to HMF involves the formation of fructose as an intermediate. The exceptional catalytic performance of the chromium catalysts is explained by their unique ability to catalyze glucose to fructose isomerization and fructose to HMF dehydration with high selectivity. Side reactions leading to humins by means of condensation reactions take predominantly place during fructose dehydration. The higher HMF selectivity for Cr^III is tentatively explained by the higher activity in fructose dehydration compared to Cr^II. This limits the concentration of intermediates that are involved in bimolecular condensation reactions. Model DFT calculations indicate a substantially lower activation barrier for glucose isomerization by Cr^III compared to Cr^II. Qualitatively, glucose isomerization follows a similar mechanism for Cr^II and Cr^III. The mechanism involves ring opening of D ‐glucopyranose coordinated to a single Cr ion, followed by a transient self‐organization of catalytic chromium complexes that promotes the rate‐determining hydrogen‐shift step.     

Zur Addition von Übergangsmetallhalogeniden an Acetylacetonate zweiwertiger Metallionen

Addition of Transition Metal Dihalides to Acetylacetonates of Divalent Metal Ions Transition metal dihalides ^aM^IIX₂ (FeCl₂, CoCl₂ NiBr₂ etc.) are added by the chelates M^II(acac)₂ under formation of binuclear complexes (THF)₂M^II(acac)₂(^aM^IIX₂). The octahedral and the tetrahedral centre of these compounds are connected by tridentate oxygen atoms of the two acetylacetonato ligands which are simultaneously included in four-membered rings (M^IIOM^II). The addition is combined with a deformation of the octahedral centre, as a prerequisite of a closest package of the atoms within the M^IIOM^II-ring. In the trinuclear complex (THF)₂Ni(acac)₂(HgCl₂)₂ III the interaction between the three coordination centres is weak. No structural change of the octahedral centre (THF)₂Ni(acac)₂ is found, but the HgCl₂-groups diverge slightly from linearity (Cl? Hg? Cl 171.1°). No binuclear complexes with a central ion of the oxidation state III in the octahedral centre were obtained. One reason is the lowered donor strength of the bidentate Lewis base function of the octahedral centre [(THF)₂Mⁿ⁺(acac)₂]^n-2 with M⁺³ as a centralatom. Reacting systems with di- and trivalent ions prefer ionic complexes, as it is shown by the formation of [(THF)₂V(acac)₂][(THF)CoCl₃] IV from VCl₃ and Co(acac)₂. The crystal structures of (THF)₂Co(acac)₂CoCl₂ II and [(THF)₂V(acac)₂][(THF)CoCl₃] IV were determined by x-ray diffraction. II : orthorhombic-primitive; space group P2₁2₁2₁, Z = 4; a = 967.4(2), b = 1453.4(3), c = 1715.9(4) pm; R = 0.049 for 3084 observed reflections. IV : triclinic; space group P1, Nr. 2; Z = 2; a = 871,5(2), b = 930,6(3), c = 1865,6(6) pm; α = 101,70(2), b? = 92,45(2), γ = 91,06(2)°; R = 0,060 für 4221 observed reflections.     

Über die Chemie der Oberfläche des Titandioxids. III. Reaktionen der basischen Hydroxylgruppen auf der Oberfläche

Adsorption of various acids on anatase of high surface area was studied. Phosphoric, arsenic, sulphuric and acetic acid are specifically adsorbed; hydrochloric and perchloric acid are not adsorbed. Phosphate ions are bound on the TiO₂ surface also from NaH₂PO₄ and Na₂HPO₄ solutions; sodium ions are adsorbed at the same time. OH^? ions on the surface are replaced by anions such as H₂PO in these reactions. The bonding of adsorbed phosphate ions is not purely ionic. Infrared spectra show that adsorbed acetic acid is bound as acetate. NO₂ reacts with the basic OH^? ions undergoing disproportionation; OH^? ions are replaced by NO ions. Phophoric acid adsorption corresponded always to half the total OH population on five different TiO₂ samples. The TiO₂ surface is not completely covered by OH groups. The maximum coverage is ca. 7.5 μMol OH/m².