Showing papers in "Journal of Molecular Catalysis A-chemical in 2001"

Hydrogenation of carbon–carbon multiple bonds: chemo-, regio- and stereo-selectivity

Abstract: Results of the last decade with respect to the selective hydrogenation of hydrocarbons with multiple unsaturation (dienes and alkynes) over heterogeneous palladium catalysts are reviewed. Factors such as metal dispersion, carbon deposits, and the use of promoters and additives controlling catalytic activities and chemo-, regio- and stereo-selectivity are discussed. A detailed treatment of the status of the selective removal of hydrocarbon impurities with multiple unsaturation from industrial feedstocks is also given.

Membrane reactors for hydrogenation and dehydrogenation processes based on supported palladium

Abstract: Membrane reactors applied to catalytic reactions are currently being studied in many places world-wide. Significant developments in membrane science and the vision of process intensification by multifunctional reactors have stimulated a lot of academic and industrial research, which is impressively demonstrated by more than 100 scientific papers on catalytic membrane reactors being published per year. Palladium as a noble metal with exceptional hydrogen permeation properties and, at the same time, broad applicability as a catalyst, first of all for hydrogenation, is part of many of these developments. This paper discusses two different membrane reactor concepts which both rely on supported palladium, on the one hand as a permselective membrane material, and on the other hand as base component of a membrane-type hydrogenation catalyst. Dense palladium composite membranes can be used for hydrogen separation from packed-bed catalysts in gas-phase hydrocarbon dehydrogenation reactions. Mesoporous membranes containing dispersed bimetallic Pd/X-clusters can be employed as so-called catalytic diffusers for liquid-phase hydrogenation, e.g. of nitrate and nitrite in water. The principles of both concepts are introduced, recently obtained experimental data are evaluated in connection with literature results, and the perspectives for further development are highlighted.

Palladium metal catalysts in Heck CC coupling reactions

Abstract: This review is devoted to the application of palladium metal catalysts to the Heck reaction (arylation or vinylation of alkenes with aryl or vinyl halides and pseudohalides). The number of relevant articles published in recent years shows steadily increasing interest in this field. A brief outline of the historical development of heterogeneous catalysis as applied to the Heck reaction is given (Section 2). Both supported metal catalysts (Section 3) and stabilized colloidal palladium catalysts (Section 4) are included. Heterogeneous catalysts supported over different kinds of supports (carbon, inorganic oxides, molecular sieves, polymeric materials, etc.) are reviewed under separate headings. Particular attention is paid to the metal leaching and the nature of catalysis, two tightly connected problems which appear to be still controversial (Section 5). Some perspectives are shortly discussed in the final section (Section 6).

Bimetallic palladium catalysts: influence of the co-metal on the catalyst performance

Abstract: An overview about the effect of co-metal on the performance of Pd in bimetallic catalysts is presented. In the first part, the promotion of monometallic catalysts by addition of a second metal is described from a general point of view in terms of: (i) electronic effects; (ii) geometric effects; (iii) the occurrence of mixed sites; and (iv) the disappearance of the β-PdH phase. The second part presents the main methods to prepare bimetallic catalysts through the unselective and selective depositions of the co-metal precursor and the use of heterobinuclear complexes. The influence of preparation methods on the chemical state and the spatial distribution of both components are highlighted. In the last part, a short overview is given about the influence of co-metal addition on some important reactions in which Pd is the reference active phase: (i) selective hydrogenation of highly unsaturated aliphatic hydrocarbons; (ii) hydrogenation of aromatics or hydrodearomatisation; (iii) hydrogenation of nitrogen-containing compounds; (iv) reactions involving CO; and (v) hydrodechlorination.

Synthesis of supported palladium catalysts

Abstract: The synthesis of supported palladium catalysts is reviewed with emphasis on a summary of the chemistry of catalyst synthesis, oxide and carbon support properties and case studies from the period 1990–2000. With oxide supports aqueous precursor solutions are often used. The charge of the complex in combination with the iso-electric point (IEP) of the support is an important property to consider during synthesis affecting both dispersion and distribution over large support bodies. With carbon supports direct reduction of palladium precursor may occur that leads to large metallic Pd particles. With the latter support lyophilicity can largely affect Pd emplacement. With oxide and carbon supports sol–gel, deposition–precipitation, deposition–reduction, ion-exchange and impregnation methods have been applied. In general it turns out that the thermal treatment often dominates the primary application of the Pd-precursor in establishing the ultimate metal dispersion. Thermal treatment in an inert atmosphere at temperatures not exceeding 773 K prior to reduction is beneficial for the Pd dispersion. Gas-phase reduction leads to smaller Pd particles than does liquid-phase reduction. In our opinion large progress can be made to develop highly loaded thermostable Pd catalysts by utilizing deposition–precipitation techniques in combination with anchoring sites on the support.

Hydrogenolysis of organohalogen compounds over palladium supported catalysts

Abstract: Catalytic hydrodehalogenation is one of the most promising methods for detoxification of organohalogen waste. A variety of catalytic systems has been used, although palladium catalysts are the best choice because of their optimal catalytic properties. Catalytic hydrodehalogenation on palladium catalysts is carried out in both the liquid and gas-phases. The experimental procedures for the catalytic hydrodehalogenation are reviewed; the hydrogen source can be either molecular hydrogen or a hydrogen donor in the so called hydrogen-transfer hydrogenolysis. A wide variety of hydrogen donors has been used, including inorganic salts (phosphinates) organic salts (formates), alcohols, proaromatic compounds, etc. Kinetics and mechanisms proposed for hydrodehalogenation are presented, and both agreements and disagreements on the mechanistic aspects are discussed. Catalysts modification (base addition, support modification, second metal addition, etc.) as well as its transformation during the hydrogenolytic process (i.e. deactivation) are discussed.

Supported bimetallic palladium catalysts for water-phase nitrate reduction

Abstract: Technologies for the removal of nitrate from drinking water and waste water will be required in the near future, and the catalytic nitrate reduction is one of the most promising methods. So far, the understanding of nitrate reducing catalysts has been very poor. Experimental trends in nitrate removal activity and selectivity for various pH values, and for different bimetallic catalysts, as well as results described in literature were used to set up a mechanistic model for the reaction. The nitrate reduction activity is determined by bimetallic ensembles, at which nitrate is adsorbed and reduced to nitrite by hydrogen, which is supposed to spillover from palladium sites to the bimetallic sites. Formic acid, on the other hand, reacts with nitrate by a transfer hydrogenation mechanism from neighboring palladium sites. The extent of positive charging of the bimetallic sites is supposed to influence the activity at different pH values. At a high pH, strongly adsorbing oxygenated species block bimetallic nitrate adsorption sites as well as palladium sites. The selectivity is determined by the ratio of nitrogen species to reductant species at monometallic palladium sites. At these sites, the reduction of nitrite and other intermediates take place, finally leading to the end products. If this ratio of nitrogen to reductant species changes, the selectivity changes as well, e.g. at different ratios of the two metals. The trends in the experimental data are well described by this model.

Poisoning and deactivation of palladium catalysts

Abstract: Some of the major causes for deactivation and premature degradation of palladium catalysts are briefly summarised. These include particle growth for various reasons, coke deposition and coke transformation, the influence of the support material on long term stability and modifications at the palladium surface itself such as valency changes or the formation of simple but stable molecular surface species. In addition, variations of physical properties as well as chemical poisoning, corrosion and leaching are discussed. In spite of the large body of literature concerning the common phenomenon catalyst deactivation specific information accessible for the various kinds of palladium catalysts being used worldwide is still quite limited. There is a serious need for future studies on properties of palladium-based catalytic systems regarding, for example, the interactions between carbon, hydrogen and supported palladium as a ternary system. Observations on different deactivation processes on Pd/SiO2 catalysts occuring under industrial conditions are compiled. It was tried to roughly differentiate between different degrees of coking and coke transformation in mainly thermally or purely catalytically driven catalyst coking on the one hand and of moderate or enhanced corrosion phenomena or changes of the properties of the palladium itself on the other. © 2001 Elsevier Science B.V. All rights reserved.

Supported palladium catalysts for fine chemicals synthesis

Abstract: The contribution reviews the application of heterogeneous Pd catalysts for the manufacture of fine chemicals with special emphasis of their organic synthetic potential. In a first part, some background is given on the scope and limitations of homogeneous and heterogeneous catalysis and on significant parameters and the various types of supported Pd catalysts. Then, the application of supported Pd catalysts for important classes of transformations are reviewed organized according to reaction type. The general statements are illustrated with relevant examples from the literature and from our own laboratories. Hydrogenation, hydrogenolysis, and dehydrogenation reactions are considered to be mature technologies with an extremely broad scope both for small-scale laboratory applications and large to very large-scale manufacturing processes. Oxidation reactions and CC coupling reactions usually have a rather narrow scope and only relatively few have been developed to the technical stage. Especially for CC coupling reactions, the nature of the active Pd species is under debate because it is not clear whether the reaction takes place on the metallic surface or whether leached soluble Pd complexes are the active catalysts.

Catalysis by metal nanoparticles supported on functional organic polymers

Abstract: The preparation and catalytic applications of dispersed metal catalysts supported on organic functional polymers are presented. The advantages of these catalysts, such as the easy tailoring with respect to the nature of the used support, the “nanoscale” size control of metal crystallites by the polymer framework, the high accessibility and consequent catalytic activity in a proper liquid or liquid–vapor reaction systems are stressed. Various proposed catalytic processes making use of these materials are presented and evaluated, including multifunctional catalysis, e.g. redox-acid. Interesting peculiar aspects such as the enhancement of the hydrogenation rate by nitrogen containing moieties anchored to the polymer backbone are emphasised. When suitable, a comparison with catalysts based on inorganic supports is given.

Metal catalysts for the vinyl polymerization of norbornene

Abstract: A full literature and patent account (about 100 references) is given on work describing the vinyl polymerization to homo-polynorbornene. The interest in vinyl homo-polynorbornene is driven by its dielectric and mechanical properties for the technical application as an interlevel dielectric in microelectronics applications. The norbornene/olefin copolymerization is covered to some extent for comparison. The metal catalysts are presented and important polymer product properties are emphasized.

A transient FTIR study of species formed during NOx storage in the Pt/BaO/Al2O3 system

Abstract: FTIR spectroscopy has been used to study samples of Al 2 O 3 , Pt/Al 2 O 3 , BaO/Al 2 O 3 and Pt/BaO/Al 2 O 3 under exposure to either NO 2 and O 2 or NO and O 2 , between 100 and 400°C. The resulting spectra are fitted to various species, mainly nitrites and nitrates, formed on Al 2 O 3 and BaO. The exposure of NO 2 and O 2 on the Al 2 O 3 sample results in the formation of monodentate, bridged and bidentate nitrates. The relative abundance of these nitrate species change with temperature. The obtained enthalpy changes for transition from monodentate to bridged nitrate and from monodentate to bidentate nitrate are −7.7 and 7.4 kJ/mol, respectively. The corresponding entropy changes are −7.1 and 30.2 J/mol K, respectively. For NO and O 2 , lower concentrations of nitrates are detected as well as both linear and bridged nitrites. When Pt is present in the catalyst, NO is oxidised to NO 2 at temperatures above 150°C resulting in the formation of large amounts of nitrates. It seems as if the formation of nitrites proceed the formation of nitrates. For the samples with both BaO and Al 2 O 3 , similar species as outlined above are observed on Al 2 O 3 while mainly nitrites and hyponitrites are observed on Ba when exposed to NO+O 2 below 200°C. When exposed to NO 2 +O 2 , nitrates are formed on BaO as well as two features attributed to bidentate and bridged nitrate on alumina at the alumina–barium oxide interface. Formation of bulk Ba(NO 3 ) 2 was not observed. It is concluded that Al 2 O 3 plays an important role as a storage site at 100°C≤ T ≤300°C and by providing a large surface area for BaO. There are also indications that nitrates formed on Al 2 O 3 may spill over to BaO.

Metal-support interactions in cobalt-aluminum co-precipitated catalysts: XPS and CO adsorption studies

Abstract: Cobalt-aluminum catalysts were prepared using either the precipitation of Co2+ in the presence of freshly prepared Zn-Al hydrotalcite (the promoted sample) or the co-precipitation of Co2+ and Al3+ (the unpromoted samples). The evolution of the initial hydrotalcite-like structure was monitored during its calcination and the reductive treatment by means of XPS. It was shown that at 480°C the reduction of the calcined samples results in the formation of Co0 species, the further reduction at 650°C results in an increase of the amount of the Co0 species. The samples reduced at 650°C chemisorb readily carbon monoxide at 77 K, while the sample reduced at 480°C does not chemisorb CO at 77 K. At elevated temperatures, all reduced samples are found to be able to chemisorb CO. Terminal CO moieties as well as monodentate carbonates, formates and carboxyl species were detected at the surface of the reduced samples at their exposure to the CO medium at the elevated temperature. The intensity of the IR absorption bands of chemisorbed CO are found proportional to the surface fraction of the Co0 species, measured by XPS. The apparent red shift of the IR absorption bands is observed for CO adsorbed on the samples reduced at 480°C. The obtained data correlate with the catalytic properties of the Co-Al samples in hydrogenation reactions. The conclusion on the existence of a strong metal–support interaction in the samples under the study is made.

Carbon nanofiber supported palladium catalyst for liquid-phase reactions: An active and selective catalyst for hydrogenation of cinnamaldehyde into hydrocinnamaldehyde

Abstract: Carbon nanofibers (CNFs) prepared by decomposition of ethane over a Ni/alumina catalyst, are used as support for palladium clusters. The carbon support displays a mean diameter of 40–50 nm, lengths up to several tens of micrometers, as highlighted by transmission electron microscopy (TEM) observations and a specific surface area of about 50 m2/g. The spheroidal palladium particles have a relatively homogeneous and sharp size distribution, centered at around 4 nm. This novel Pd/carbon nanofiber catalyst displays unusual catalytic properties and is successfully used in the selective hydrogenation of the C C bond in cinnamaldehyde at a reaction temperature of around 80°C, under continuous hydrogen flowing at atmospheric pressure. The high performances of this novel catalyst in terms of efficiency and selectivity are, respectively, related to the inhibition of the mass-transfer processes over this non-porous material and to peculiar palladium–carbon interactions. It is concluded that the absence of microporosity in the carbon nanofibers favours both the high activity and selectivity which is confirmed by comparison with the commercially available high surface area charcoal supported palladium catalyst.

Oxidations by the system “hydrogen peroxide–manganese(IV) complex–carboxylic acid”: Part 3. Oxygenation of ethane, higher alkanes, alcohols, olefins and sulfides

Abstract: The manganese(IV) complex salt [L2Mn2O3](PF6)2 (L = 1,4,7-trimethyl-1,4,7-triazacyclononane) (compound 1, see Scheme 1) very efficiently catalyzes the hydroperoxidation of saturated hydrocarbons, including ethane by H2O2 in acetontitrile or nitromethane solution at low (room or lower) temperature, provided a carboxylic (typically acetic) acid is present. The hydroperoxidation of tertiary positions in disubstituted cyclohexanes proceeds with partial retention of configuration in nitromethane or acetonitrile solution, while the stereoselectivity of the reaction is only negligible in acetone solution. The system “H2O2–compound 1–MeCO2H” also transforms secondary alcohols into the corresponding ketones with quantitative yields at room temperature within a few minutes; the yields of aldehydes and carboxylic acids in the oxidation of primary alcohols are lower. Terminal aliphatic olefins such as hexene-1 are quantitatively epoxidized by the same system in acetonitrile at room temperature within 20 min, while the epoxide yield in the analogous reaction with styrene attains only 60% under the same conditions. Finally, dimethylsulfide can be quantitatively and selectively converted into dimethylsulfoxide within 3 h at room temperature. The system “tert-BuOOH–compound 1” also oxidizes alkanes, addition of acetic acids has less pronounced effect on the direction and efficiency of the reaction. Two other checked derivative of Mn(IV) (compounds 2 and 3) as well a porphyrin complex of Mn(III) (compound 4) exhibited lower activity in catalysis of alkane oxidation with tert-BuOOH. © 2001 Elsevier Science B.V. All rights reserved.

Supported palladium catalysts in environmental catalytic technologies for gaseous emissions

Abstract: Palladium is the active component in several catalytic formulations for environmental technologies, due to its superior performances in the conversion of some hydrocarbons (for example, methane) and halocarbons, and the thermal stability and low volatility of Pd species. The properties and reactivity of Pd-based catalysts in the conversion of methane catalytic combustion for gas turbine applications, reduction of greenhouse gas (methane, N2O) emissions, hydrodehalogenation and oxidative destruction of halocarbons and their applications in the elimination of other pollutants from gaseous emissions are reviewed, with emphasis on the structure-activity relationships, reaction mechanism and sensitivity to poisoning.

Radical and non-radical chemistry of the Fenton-like systems in the presence of organic substrates

Abstract: The review deals with the chemistry concerning the processes of mild and selective oxidation of alkanes and cycloalkanes by using synthetic iron complexes as catalytic systems. These cover a number of coordinated species of iron ions including the so-called Gif systems up to the polydentated Que’s models, aimed to mimic or emulate the non-haem enzymatic oxidation of alkanes. Special attention is paid to the mechanisms proposed by the various Authors and to the arguments set out for and against the free-radical interpretation to account for the activation of the CH bonds. The schemes of the free-radical chemistry of Fenton reagents in the presence of organic substrates are quoted as a reference basis for the discussion.

Hydrogenation and hydrogenolysis of nitro-, nitroso-, azo-, azoxy- and other nitrogen-containing compounds on palladium

Abstract: The formation of nitroso-, azo- and azoxy-aromatics depends of a relatively complex scheme involving consecutive reactions. However, all different products can be produced in good yields with palladium catalysts, using optimised kinetics, selective poisoning or alloying. These reactions show different dependencies of the reaction rate on Pd particle size and alloying. Solvents have different roles, in addition to the usual one consisting in the solubilisation of the reactants: they can change the solubility of hydrogen, compete with the reactants for adsorption at the metal surface or catalyse side reactions.

Ionic liquid as a green catalytic reaction medium for esterifications

Abstract: Esterifications of alcohols with carboxylic acids in the room temperature ionic liquid 1-butylpyridinium chloride–aluminium(III) chloride as green reaction medium in catalytic quantities have been investigated. Excellent conversion and selectivities were achieved, and the most of resultant esters could be easily recovered due to immiscibility with the ionic liquid.

Catalysis and polymer networks — the role of morphology and molecular accessibility

Abstract: Catalysis and polymer networks — the role of morphology and molecular accessibility B. Corain a, M. Zecca a, K. Jeřabek b,∗ a Universita’ di Padova, Dipartimento di Chimica Inorganica, Metallorganica e Analitica and Centro di Studio sulla Stabilita’ e Reattivita’ dei Composti di Coordinazione, C.N.R., via Marzolo 1, 35131 Padova, Italy b Institute of Chemical Processes Fundamentals, Academy of Sciences of the Czech Republic, Rozvojova 135, 165 02 Praha 6 Suchdol, Czech Republic

Alkylations of benzene in room temperature ionic liquids modified with HCl

Abstract: Alkylations of benzene with dodecene and/or chloromethanes in the chloroaluminate room temperature ionic liquid modified with HCl as catalysts have been investigated. Different product distributions and enhanced activity, were respectively observed for these two alkylation reactions. This may be attributed to the superacidity of the ionic liquid induced by the presence of HCl.

A study of the surface acidity of acid-treated montmorillonite clay catalysts

Abstract: The surface acidity of a series of commercial Sud Chemie acid-treated montmorillonite clays (K-catalysts) has been evaluated by a wide range of complementary experimental techniques. The different methods applied allow a rather complete characterisation of the surface acidity providing a complete picture of the Lewis/Bronsted acid strength/density of the surface sites. IR data show that the Bronsted sites on these catalysts are relatively weak and provide evidence for a slight increase of the strength and the density of Bronsted sites in the order K5 < K10 K20 < K30 in full agreement with the trend in iso-butene conversion, which is a measure of the strength and/or the abundancy of Bronsted sites. The apparent contradiction of these data with those obtained from the ammonia adsorption and iso-propanol conversion experiments can be explained by the structural and chemical modification of the clays upon acid treatment. © 2001 Elsevier Science B.V. All rights reserved.

Methane combustion over supported cobalt catalysts

Abstract: A series of cobalt-based catalysts with different supports have been prepared using impregnation method, and characterised by X-ray diffraction (XRD), laser Raman (LR), and infrared spectroscopy (IR). The catalytic activities for methane combustion were assessed in a micro-reactor. The ZrO2 and Al2O3 supports are themselves active in methane combustion, and ZrO2 supported cobalt catalyst was found to have the highest activity amongst the TiO2, Al2O3, MgO supported catalysts and bulk Co3O4. The Co content has a marked effect on the activity of the ZrO2 supported catalyst with 1.0 and 15 wt.% of Co having the lowest light-off temperature in methane combustion. In the MgO supported catalyst, Co oxide was highly dispersed over the MgO support surface or enters the lattice of MgO to form a solid solution, whose activity for methane combustion is reasonable. The zirconia supported cobalt catalysts are very active and stable when calcination or reaction temperature is no more than 900°C. Calcining the catalysts at temperatures above 900°C for more than 1 h decreases the catalyst activity. The deactivation of the catalyst probably results from the decrease of the surface area.

Chiral single crystal surface chemistry

Abstract: Several experiments have been performed to probe the enantiospecific properties of chiral single crystal surfaces. The surfaces chosen have been the (643) planes of Ag and Cu, both face centered cubic structures. The chirality of these surfaces arises from the handedness of their kinked step structures. These structures are such that the (643) and the .643/ surfaces are related by mirror symmetry but are non-superimposable. We denote them as (643) R and (643) S . As a consequence of this handedness it is expected that the interactions of these surfaces with the left- and right-handed enantiomers of a chiral molecule should be different. In other words the chemistry of chiral molecules on these surfaces should be enantiospecific. We have observed that the desorption energies of R-3-methyl-cyclohexanone differ by 0:22 0:05 kcal/mole on the Cu(643) R and the Cu(643) S surfaces. Similarly, on the Ag(643) R surface we have observed that the orientations of R- and S-2-butanoxy groups differ. This enantiospecific orientation is revealed by the intensities of the absorption bands in an infrared absorption spectra of these species on the Ag(643) R surface. These two results expand the small but growing set of observations of the enantiospecific properties of chiral single crystal surfaces. © 2001 Elsevier Science B.V. All rights reserved.

Ethylene polymerization activity under practical conditions displayed by zirconium complexes having two phenoxy-imine chelate ligands

Abstract: Ethylene polymerization using zirconium complexes having two phenoxy-imine chelate ligands was investigated at 50 and 75°C at 0.9 MPa ethylene pressure. The activity of the proto-catalyst, N-[(3-t-butylsalicylidene)anilinato]zirconium(IV)dichloride (1), decreased by increasing the polymerization temperature (50°C: 1192 kg-PE/mmol-Zr·h, 75°C: 209 kg-PE/mmol-Zr·h). The introduction of an electron donating group, such as an n-hexyl and a methoxy group, on the imine nitrogen or at the para-position of phenoxy oxygen in a benzene ring suppressed the activity decrease, however, the activity at 75°C was still lower than that at 50°C. The combination of an n-hexyl or a cyclohexyl group on the imine nitrogen and a methoxy group at the para-position of the phenoxy oxygen enhanced activity at 75°C. Thus, N-[(3-t-butyl-5-methoxysalicylidene)-n-hexylaminato]zirconium(IV)dichloride (5) and N-[(3-t-butyl-5-methoxysalicylidene)cyclohexylaminato]zirconium(IV)dichloride (6) displayed activities of greater than 1000 kg-PE/mmol-Zr h. Moreover, the introduction of an adamantyl or a cumyl group at the position adjacent to the phenoxy oxygen further increased the activity. N-[(3-Adamantyl-5-methoxysalicylidene)cyclohexylaminato]zirconium(IV)dichloride (7), N-[(3-cumyl-5-methoxysalicylidene)-n-hexylaminato]zirconium(IV)dichloride (8), and N-[(3-cumyl-5-methoxysalicylidene)cyclohexylaminato]zirconium(IV)dichloride (9) displayed exceptionally high activity (7: 3052 kg-PE/mmol-Zr·h, 8: 7078 kg-PE/mmol-Zr·h, 9: 7224 kg-PE/mmol-Zr·h).

Comparative photosensitised transformation of polychlorophenols with different sulphonated metallophthalocyanine complexes in aqueous medium

Abstract: The relative efficiencies for the oxidation of trichlorophenol (TCP) and pentachlorophenol (PCP), sensitised by sulphonated phthalocyanine complexes containing Zn, Al, Sn and Si as central metals, were studied in aqueous solutions. For the first time, sulphonated silicon and tin phthalocyanines were synthesised and their activity towards photosensitisation of singlet oxygen and photo-oxidation of polychlorophenols was examined. The efficiency of the sensitised photo-oxidative degradation of polychlorophenols depends not only on singlet oxygen quantum yield of sensitiser, but also on its stability. The influence of substrate structure and the pH of the solution on the photo-oxidation efficiency, as well as on the sensitiser photodegradation were studied. It was found that the contribution of the Type II (singlet oxygen-mediated) mechanism to the relative efficiency of the photosensitised photo-oxidation of the phenol, decreased from phenol to p -chlorophenol, TCP and PCP, whereas Type I radical pathway increased. The results obtained for PCP give evidence that electron transfer from the excited sensitiser to the substrate in its unionised form is responsible for enhanced sensitiser photodegradation.

Ethylene/α-olefin copolymerization by various nonbridged (cyclopentadienyl)(aryloxy)titanium(IV) complexes — MAO catalyst system

Abstract: Effect of ligand in copolymerization of ethylene with 1-hexene by (cyclopentadienyl)(aryloxy)titanium(IV) complexes of the type, Cp′TiCl2(OAr) [OAr=O-2,6-iPr2C6H3 and Cp′=C5Me5 (1), 1,3-tBu2C5H3 (2), tBu2C5H4 (3), 1,3-Me2C5H3 (5), Cp (6); OAr=O-2,6-Me2C6H3 and Cp′=C5Me5 (7), 1,3-tBu2C5H3 (8)], has been explored in the presence of MAO, and [Me2Si(C5Me4)(NtBu)]TiCl2 (4) was chosen as the reference. It was revealed that 1 exhibited the highest catalytic activity, and the resultant poly(ethylene-co-1-hexene)s possessed relatively high molecular weights with narrow molecular weight distributions ( M w =29.5×10 4 to 34.5×10 4 ,M w /M n =1.74 –1.88), and contained 1-hexene in relatively high extents (36.6–43.5 mol%). Effect of both cyclopentadienyl and aryloxy groups plays an essential key role in the copolymerization not only to exhibit high catalytic activity but also to afford relatively high molecular weight copolymer with narrow polydispersity. It also turned out that α-olefin incorporation into the copolymer is highly affected by the substituents on the cyclopentadienyl group. The monomer reactivity ratios were not affected by both the polymerization temperature and Al/Ti molar ratios, but the monomer sequence distributions and the monomer reactivity ratios depended upon cyclopentadienyl fragment used. In spite of the rather wide bond angle of Cp–Ti–O (ca. 120.5°) in 1–3 than that of Cp–Ti–N in 4 (107.6°), a high level of 1-hexene incorporation and smaller rErH value by 1–3 than those by 4 were attained, which would be due to the flexible internal rotation of both cyclopentadienyl and aryloxy groups.

Liquid-phase oxidation of benzene to phenol by CuO–Al2O3 catalysts prepared by co-precipitation method

Abstract: The liquid-phase catalytic oxidation of benzene to directly produce phenol was attempted under mild reaction conditions using supported Cu catalysts in aqueous acetic acid solvent. Gaseous oxygen and ascorbic acid were used as an oxidant and a reductant, respectively. Among the supported Cu catalysts studied, the Cu catalysts prepared by the impregnation method, irrespective of the oxide supports, the Cu species were considerably leached during the benzene oxidation. A supported Cu catalyst (CuO–Al 2 O 3 ) prepared by co-precipitation of Cu(NO 3 ) 2 ·3H 2 O and Al(NO 3 ) 3 ·9H 2 O inhibited the leaching of Cu species in comparison with the Cu catalysts supported by the impregnation method on Al 2 O 3 , SiO 2 , MCM-41, etc. The influences of the amount of supported Cu, the partial pressure of O 2 , the amount of ascorbic acid, the concentration of acetic acid in the solvent, and the reaction temperature on the phenol yield were investigated using the CuO–Al 2 O 3 catalyst. The aqueous solvent including high concentration of acetic acid (around 80 vol.%) dramatically inhibited the leaching of Cu species in the CuO–Al 2 O 3 catalyst. H 2 O 2 , which is considered to play an important role in phenol formation, was detected during the benzene oxidation catalyzed by CuO–Al 2 O 3 .

Mixed molybdenum oxide based partial oxidation catalyst: 2. Combined X-ray diffraction, electron microscopy and Raman investigation of the phase stability of (MoVW)5O14-type oxides

Abstract: Thermal activation of a nanocrystalline Mo5O14-type Mo0.64V0.25W0.09Ox catalyst leads to enhanced catalytic performance in the partial oxidation of methanol, propylene and acrolein. This thermal activation process was invest igated by X-ray diffraction, transmission electron microscopy and Raman microspectroscopy. Ther mal activation of the nanocrystalline Mo0.64V0.25W0.09Ox precursor oxide in inert atmospheres induces partial crystallization of a Mo5O14-type oxide only in a narrow temperature range ending at 818 K. The Raman spectrum of the crystalline Mo5O14 oxide was identified by statistical analysis and by comparison with XRD and TEM results. The observed Raman bands in the M=O stretching mode regime were attributed to the different Mo=O bond distances in Mo 5O15. A fraction of the precursor oxide remains nanocrystalline after activation as shown by Raman spectroscopy. HRTEM identified amorphous surface layers on top crystalline cores. Above 818 K, the Mo5O14-type structure disproportionates into the stable phases MoO 2 and MoO3. This disproportionation occurs via an intermediate state which is formed by bundles of molybdenum oxide chains exhibiting structural order in only one dimension as shown by HRTEM. These results from the combined structural analysis suggest that the improvement of the catalytic performance of the MoVW oxide catalyst in the partial oxidation of methanol is related to the formation of the Mo5O14 type mixed oxide.

High resolution TEM studies on palladium nanoparticles

Abstract: In the present work, we discuss the analysis of Pd nanoparticles in the range of 1–5 nm using high resolution electron microscopy (HREM). Particles were grown using colloidal methods. The principles of the observation of HREM images and their study using the corresponding FFT of the images are discussed. The results show that particles have the following shapes: FCC cubo-octahedrons, icosahedrons, truncated decahedrons and single-twinned FCC configuration. Each one of those structures is discussed and images are shown. We also found that a fraction of the particles corresponds to a short range ordered amorphous phase. The future of HREM observations of nanoparticles is also discussed.