Polyamide-bound metal complex catalysts: Synthesis,characterization and catalytic properties

A series of polyamides having different numbers of methylene groups in their repeating units have been synthesized by interfacial polycondensation of terephthaloyl chloride with piperazine and aliphatic diamines H₂N(CH₂)_nNH₂ (n = 2, 6, 10). These materials, which have high thermal stability, were used for immobilization of rhodium and platinum complexes. Chloroplatinic acid and the compounds PtCl₂(CH₃CN)₂ and [RhCl(CO)₂]₂ were used as precursors of the supported catalysts. Low molecular weight analogues of the polyamides were prepared for a study of the coordination mode between the metal ion and the polymer by IR spectroscopy. The results suggest that the carbonyl oxygen of the polyamide is the site of coordination to both rhodium and platinum. The bound catalysts exhibited high activity in hydrosilylation of hexene-1. The activities of the rhodium complexes were found to be dependent on the structure of the polyamide support, decreasing with increasing distance between the amide groups, and closely paralled the changes in the degree of crystallinity of the polymers. Repeated use of the polymers bearing rhodium complexes showed that the bond between the metal and polyamide is fairly stable.     

Methane conversion by various metal, metal oxide and metal carbide catalysts

The progress in the field of methane conversion into higher hydrocarbons including aromatics and oxygenated compounds in the recent five years will be reviewed shortly, together with a new type of the methane conversion reaction with carbon monoxide at lower temperatures (600–700 K) by supported group VIII metal catalysts. Benzene was formed selectively among hydrocarbons in the CH₄–CO reaction over silica-supported Rh, Ru, Pd and Os catalysts under atmospheric pressure. Both CH₄ and CO were required for benzene formation, and only ethane and ethylene were formed besides benzene. The amount of C₃–C₅ hydrocarbons was negligible, which suggests that a completely different mechanism from the CO–H₂ reaction may be operating over these catalysts despite of the similarity in the reaction conditions with the CO–H₂ reaction. The mechanism of benzene formation was studied deeply by means of kinetical investigation as well as infrared spectroscopy and isotopic tracer method in connection with that of CO hydrogenation.     

Effect of ultra-fine gold particle addition to metal oxides in ethylene oxidation

Oxidation of ethylene was carried out over alumina-supported metal oxide catalysts and highly dispersed gold catalysts, respectively, under atmospheric pressure. The ethylene was completely oxidized to produce carbon dioxide and water with both metal oxide and gold catalysts. The activity of gold catalyst prepared by deposition method was much higher than that of supported metal oxide catalysts. Ultra-fine gold particles on Co₃O₄ were more active than on Al₂O₃. Fe₂O₃/Al₂O₃ and MnO₂/Al₂O₃ catalysts were more active than MoO₃/Al₂O₃ catalyst. The activity of the supported metal oxide catalysts was greatly enhanced by addition of gold particles. It was therefore considered that gold particles promote dissociative adsorption of oxygen and the adsorbed oxygen reacts with adsorbed ethylene on support adjacent to the active site.     

Synthesis of dual-doped non-precious metal electrocatalysts and their electrocatalytic activity for oxygen reduction reaction

The pyrolyzed carbon supported ferrum polypyrrole(Fe-N/C) catalysts are synthesized with or without selected dopants, p-toluenesulfonic acid(TsOH), by a facile thermal annealing approach at desired temperature for optimizing their activity for the oxygen reduction reaction(ORR) in O2-saturated 0.1 mol/L KOH solution. The electrochemical techniques such as cyclic voltammetry(CV) and rotating disk electrode(RDE) are employed with the Koutecky-Levich theory to quantitatively obtain the ORR kinetic constants and the reaction mechanisms. It is found that catalysts doped with TsOH show significantly improved ORR activity relative to the TsOH-free one. The average electron transfer numbers for the catalyzed ORR are determined to be 3.899 and 3.098, respectively, for the catalysts with and without TsOH-doping. The heat-treatment is found to be a necessary step for catalyst activity improvement, and the catalyst pyrolyzed at 600℃ gives the best ORR activity. An onset potential and the potential at the current density of-1.5 mA/cm2 for TsOH-doped catalyst after pyrolysis are 30 mV and 170 mV, which are more positive than those without pyrolized. Furthermore, the catalyst doped with TsOH shows higher tolerance to methanol compared with commercial Pt/C catalyst in 0.1 mol/L KOH. To understand this TsOH doping and pyrolyzed effect, X-ray diffraction(XRD), scanning electron microscope(SEM) and X-ray photoelectron spectroscopy(XPS) are used to characterize these catalysts in terms of their structure and composition. XPS results indicate that the pyrrolic-N groups are the most active sites, a finding that is supported by the correspondence between changes in pyridinic-N content and ORR activity that occur with changing temperature. Sulfur species are also structurally bound to carbon in the forms of C–Sn–C, an additional beneficial factor for the ORR.     

An evaluation of silica-supported catalysts for the reaction between ethanol and polymethylhydrogensiloxane

Catalysis of the reaction between Me₃SiO[Si(H)Me)O]_nSiMe₃ (n ≈ 50) and ethanol by silica-supported rhodium and iridium catalysts has been investigated. Donor groups in the anchored ligands were PPh₂, S^?, or C₅H₄^?. The rhodium-PPh₂ system showed marked inhibition by dihydrogen. The supported iridium catalysts all showed high activity which declined rapidly during successive cycles of re-use, but the iridium-PPh₂ catalyst was the least affected. In every case, the separated liquid products showed activity as homogeneous catalysts, indicating that leaching of the metal from the support was occurring. That ligand was also being leached was shown by labelling with tritium. The results demonstrate the necessity to test supported catalysts through more than one cycle.     

Synthesis and properties of PdSn/Al2O3 and PdSn/SiO2 prepared by solvated metal atom dispersed method

A series of solvated metal atom dispersion (SMAD) catalysts: Pd/SiO₂, Pd/Al₂O₃, Sn/SiO₂, Sn/Al₂O₃, Pd_xSn_y/SiO₂ and Pd_xSn_y/Al₂O₃. It was prepared by simultaneous evaporation of Pd and Sn. The metals were co-deposited at 77 K using acetone, 2-propanol and THF to produce colloids “in situ” all the supported catalyst were characterized by chemisorption, transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and TPR. This series of catalyst were tested for crotonaldehyde hydrogenation in gas phase to obtain crotyl alcohol.     

Non-metallocene rare earth metal catalysts for the diastereoselective and enantioselective hydroamination of aminoalkenes

In this short review we summarize our work on new cyclopentadienyl-free rare earth metal catalysts for the diastereoselective and enantioselective hydroamination of aminoalkenes. Non-metallocene rare earth metal catalysts based on diamidoamine, biphenolate and binaphtholate ligands are readily available through alkane and amine elimination procedures. Diamidoamine yttrium complexes are efficient catalysts for the highly diastereoselective cyclization of 1-methyl-pent-4-enylamine to yield trans-2,5-dimethyl-pyrroldine with trans to cis ratios of up to 23:1. The X-ray crystal structural analysis of [((2,6-Et₂C₆H₃NCH₂CH₂)₂NMe)Y{N(SiMe₃)₂}] is reported in which yttrium is coordinated in a highly distorted tetrahedral fashion. 3,3′-Di-tert-butyl substituted biphenolate complexes tend to form phenolate-bridged hetero- and homochiral dimers. The low steric demand of the tert-butyl substituents resulted also in low enantioselectivities in hydroamination/cyclization reactions. Binaphtholate complexes with sterically more demanding tris(aryl)silyl substituents were more efficient catalysts; giving enantioselectivities of up to 83% ee. These catalysts could also be applied in kinetic resolution of chiral aminoalkenes giving k_rel values as high as 16. Catalyst activities strongly depend on the reactivity of the leaving group which is protolytically exchanged for the substrate during the initiation step. Complexes with bis(dimethylsilyl)amido ligand initiate rather sluggishly because of the low basicity of this amido ligand and appreciable catalytic activity is only observed at elevated temperatures. Aryl and Alkyl complexes showed significant better rates comparable in magnitude to lanthanocene catalysts.     

Models of active sites in supported Cu metal catalysts in 1,2-dichloroethane dechlorination. DFT analysis

It is suggested that a set of discrete Cu nanoclusters satisfying the conditions of structural and electron stability should be used as models of active sites on supported metal catalysts. The close-packed Cu₂₀ tetrahedral nanocluster that satisfies these two conditions was taken as a base model of active sites on supported copper catalysts. Theoretical analysis of two possible mechanisms of C-Cl bond dissociation of 1,2-dichloroethane on copper catalysts was performed by the density functional theory method. The first mechanism involves sequential splitting of C-Cl bonds in the molecule in three stages with further stabilization of chloroalkyl intermediates (stepwise mechanism). All these stages are activated. The limiting stage is the one that corresponds to dissociation of the first C-Cl bond with an activation energy of E# = 34.3 kcal/mol. The second mechanism corresponds to the simultaneous elimination of two chlorine atoms from 1,2-dichloroethane with liberation of ethylene in the gas phase; this is a one-stage process with an activation energy of E# = 26.1 kcal/mol (direct mechanism). A comparison of the two reaction routes shows that the direct mechanism is most probable on copper catalysts.     

Supported oxorhenate catalysts prepared by thermal spreading of metal Re for methanol conversion to methylal

TiO₂-anatase and SiO₂ supported oxorhenate catalysts were prepared by an original and simple technique based on the oxidative dispersion of metallic rhenium under dry conditions. The dispersion process of the supported oxorhenate phase as a function of the rhenium coverage and the support properties are discussed on the base of in situ characterization. The structures of the as prepared catalysts were found to be comparable to those of materials prepared using the incipient wetness impregnation technique. The absence of water in the preparation technique has made it possible to highlight the role of the hydration level on the rhenium oxide volatilization. The as-prepared Re/TiO₂ catalysts were found to be effective for the direct conversion of methanol to methylal.     

Catalysts based on cordierite modified with transition metal oxides

Catalysts active in ammonia oxidation have been obtained by the substitution of transition metal (Mn, Fe, Co, Ni, and Cu) ions for Mg ions in the cordierite structure 2MgO · 2Al₂O₃ · 5SiO₂ at 1100°C. Their phase composition, texture, and activity depend on the type and amount of introduced transition metal oxide. The Mn- and Cu-containing catalysts, which consist of substituted cordierites 2(Mg_{1 ? x} M _x )O · 2Al₂O₃ · 5SiO₂ and Mn₂O₃ or CuO crystallites located on their surface, are most active in ammonia oxidation. The catalysts are characterized by a small specific surface area and have large pores, whose total volume is small. The Fe-containing catalysts consist of the Fe-substituted cordierite phase and particles of an iron oxide phase. These particles are mostly located in internal pores of the catalysts and are, therefore, hardly accessible to ammonia molecules. The introduction of Co or Ni oxide leads to the formation of a low-active spinel phase rather than the cordierite phase.     

Highly active double metal cyanide complexes： Effect of central metal and ligand on reaction of epoxide/CO2

Various novel double metal cyanide (DMC) catalysts were successfully prepared by modifying the central metal (M) and one of cyanide ion (CN-) in Zna[M(CN)b]c complex. Such modifications have significant impact on the catalytic efficiency as well as the polymer selectivity for the reaction of PO/CO2. Zn–Ni(Ⅱ) DMC is a potential catalyst for alternating copolymerization of PO/CO2, and DMC catalysts based on Zn3[Co(CN)5X]2 (X = Br-and N3-) exhibit moderate efficiency for the production of polycarbonates. This research presents the preliminary exploration of novel DMC complex via chemical modification of its central metal and ligand.     

Time-on stream behavior of Pd-, Co-, and Mn-zeolite catalysts supported on metal blocks in high-temperature methane oxidation

Stability of the Pd-, Co-, and Mn-zeolite catalysts supported on metal blocks was studied in high-temperature methane oxidation. The temperature regions were found in which the starting catalysts exhibit stable performance. The temperature was determined at which a partial deactivation is followed by stabilization of catalysts in reaction environment. In terms of specific activity, the partially deactivated Pd-zeolite catalyst is several times more active than conventional oxidation catalysts Pd/Al₂O₃, Pt/Al₂O₃, and the most active oxide CeO·6Al₂O₃.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2075–2078, October, 2004.     

Cyclohexane transformations over metal oxide catalysts. 1. Effect of the nature of metal and support on the catalytic activity in cyclohexane ring opening

The activities of monometallic Pt-, Ru-, and Rh-containing catalysts supported on Al₂O₃, Al₂O₃—F, SiO₂, WO₃/ZrO₂, and La₂Î₃/ZrO₂, in cyclohexane ring opening to form n-hexane were studied. The most active catalyst is Rh/Al₂O₃. Cyclohexane hydrogenolysis to n-hexane also occurs over the Pt/Al_;>2O₃ and Pt/La₂Î₃/ZrO₂ catalysts. Ring opening over the Ru catalysts proceeds at significantly lower temperatures (210—230 °C) than over the Pt and Rh catalysts (350—400 °C), but the ruthenium systems are less selective for n-hexane formation than Rh/Al₂O₃ catalysts. The effects of acid-basic properties of the support and the reaction conditions on the activities of the catalytic systems in cyclohexane ring opening was studied.     

Molybdenum hexacarbonyl supported on functionalized multi-wall carbon nanotubes: Efficient and highly reusable catalysts for epoxidation of alkenes with tert-butyl hydroperoxide

In the present work, highly efficient epoxidation of alkenes catalyzed by Mo(CO)₆ supported on amines modified multi-wall carbon nanotubes, MWCNTs, is reported. The prepared catalysts were characterized by elemental analysis, scanning electron microscopy, FT-IR and diffuse reflectance UV-Vis spectroscopic methods. These new heterogenized catalysts, [Mo(CO)₆@amines-MWCNT], were used as highly efficient catalysts for epoxidation of alkenes with tert-BuOOH. These robust catalysts could be reused several times without loss of their catalytic activities.     

Tailoring the surface structure of iron compounds to optimize the selectivity of 3-nitrostyrene hydrogenation reaction over Pt catalyst

Selective hydrogenation of substituted nitroarenes is an important reaction to obtain amines.Supported metal catalysts are wildly used in this reaction because the surface structure of supports can tune the properties of the supported metal nanoparticles （NPs） and promote the selectivity to amines.Herein,Pt NPs were immobilized on Fe OOH,Fe₃O₄andα-Fe₂O₃nanorods to synthesize a series of iron compounds supported Pt catalysts by liquid phase reduction me...     

A study of the dispersity of iron oxide and iron oxide-noble metal (Me = Pd,Pt) supported systems

Samples of one-(Fe) and two-component (Fe-Pd and Fe-Pt) catalysts were prepared by incipient wetness impregnation of four different supports: TiO₂ (anatase), γ-Al₂O₃, activated carbon, and diatomite. The chosen synthesis conditions resulted in the formation of nanosized supported phases—iron oxide (in the one-component samples), or iron oxide-noble metal (in the two-component ones). Different agglomeration degrees of these phases were obtained as a result of thermal treatment. Ultradisperse size of the supported phase was maintained in some samples, while a process of partial agglomeration occurred in others, giving rise to nearly bidisperse (ultra-and highdisperse) supported particles. The different texture of the used supports and their chemical composition are the reasons for the different stability of the nanosized supported phases. The samples were tested as heterogeneous catalysts in total benzene oxidation reaction. The text was submitted by the authors in English.     

Ether-directed diastereoselectivity in catalysed Overman rearrangement: comparative studies of metal catalysts

Ether-directed diastereoselectivity in Overman rearrangement of δ-methoxy and δ-TBDMSO substituted allylic trichloroacetimidates has been explored using PtCl₂, PtCl₄, AuCl and AuCl₃ catalysts in comparison with commonly used Pd(II) catalysts. For both substrates the use of PtCl₂ catalyst gave notably improved anti/syn-ratio of 1,2-aminoalcohol derivatives (anti/syn=11:1 for δ-methoxy; 6:1 for δ-TBDMSO) compared to all metal catalysts known to promote Overman rearrangement. Formation of 2-trichloromethyloxazoline was observed as a dominant side reaction in the metal catalysed rearrangement of δ-methoxy substituted allylic trichloroacetimidates considerably reducing the yield of the desired product. This side reaction was suppressed when δ-TBDMS-ether was used as a directing group.     

Preparation,characterization, and application of fine metal particles and metal colloids using hydrotriorganoborates

The reduction of transition metal salts and oxides using hydrotriorganoborates in organic media allows the production of X-ray amorphous nanopowders of metals and alloys under mild conditions. For example, the reduction of needle-shaped iron oxides at 80°C in organic solvents leads to acicular iron-magnet pigments suitable for recording magnetic signals. The reduction of TiCl₄ with K[BEt₃H] gives an ether-soluble [Ti(0)·0.5THF]_x which serves as a catalyst for the hydrogenation of titanium or zirconium sponges and related systems and as a powerful activator for heterogeneous hydrogenation catalysts. The use of tetraalkylammonium hydrotriorganoborates as reducing agents leads to colloidal transition metals in organic phases. These colloids may also be obtained using conventional reducing agents after first reacting the metal salts with the stabilizing tetraalkylammonium halide. Colloidal metals prepared in this way serve as sources for heterogeneous metal catalysts.     

Oxide supported platinum electrocatalysts for hydrogen and alcohol fuel cells

Efficient oxide supported electrocatalysts for hydrogen and alcohol fuel cells are developed. They are characterized by a low content of platinum, exhibit high activity in the oxidation of low-molecular alcohols and tolerance to the CO poisoning. It is shown that the application of catalysts developed (Pt/SnO₂-SbO _x ) enables one to raise the power of fuel cells operating on ethanol approximately by two times as compared with similar fuel cells with commercial PtRu/C catalysts.     

Transition metal loaded TiO2 for phenol photo-degradation

Photocatalytic degradation of phenol under both UV radiation and visible light, using TiO₂ (Degussa P-25) and TiO₂ loaded with some transition metal ions (Co, Cu, Fe and Mo) was examined. From the series of metal loaded catalysts, Mo/TiO₂ was the most efficient one. In the presence of Mo, neither TiO₂ anatase/rutile fraction nor its pore size diameter has been affected. However, Mo made its surface more acidic. The percentage of phenol degradation reached under visible light was significantly lower than that under UV radiation due to the lower degree of light absorption by the catalyst surface. From the series of studied catalysts, 2 wt% Mo/TiO₂ was the most efficient one. The synergetic effect between S_BET, mean pore size diameter, catalyst agglomerate size, band gap, ZPC and the type of Mo_xO_y species on TiO₂ surface, depending on Mo loading, created its photocatalytic performance.