Showing papers in "Journal of Molecular Catalysis A-chemical in 1998"
TL;DR: In this article, the decomposition mechanisms of oxygenates on transition metal surfaces focusing primarily on metals of Groups VIII and IB are reviewed and deviations from these serve to illustrate the patterns of oxygenate reactions, and major divisions in the preferred pathways can be rationalized in terms of the affinities of metals for making metaloxygen and metal-hydrogen bonds.
Abstract: The importance of various oxygenates as fuels and as chemical intermediates and products continues to grow. Alcohols and aldehydes have also been the subjects of numerous surface reactivity studies. We review here the decomposition mechanisms of oxygenates on transition metal surfaces focusing primarily on metals of Groups VIII and IB. Common pathways as well as deviations from these serve to illustrate the patterns of oxygenate reactions. Several major divisions in the preferred pathways can be rationalized in terms of the affinities of metals for making metal–oxygen and metal–hydrogen bonds. Other important factors determining oxygenate reactivities include surface crystallographic structure and the detailed molecular structure of the oxygenate. Differences in product distribution between metals are frequent, even in cases where many of the reaction steps are common, primarily because of the plethora of elementary reaction steps usually involved in oxygenate decomposition on transition metal surfaces. As a result, differences late in the reaction sequence can obscure important similarities in the overall reaction network. Spectroscopic identification of common surface reaction intermediates including alkoxides, acyls, and oxametallacycles, has become increasingly important in revealing the underlying similarities in seemingly diverse oxygenate reaction pathways on transition metal surfaces.
347 citations
TL;DR: In this paper, a review of surface science studies relating to the silver-catalyzed epoxidation of ethylene is reviewed from the perspective of industrial catalyst development, and the influence of alkali and halide promoter species via electronic and geometric effects is discussed.
Abstract: Key surface science studies relating to the silver-catalyzed epoxidation of ethylene are reviewed from the perspective of industrial catalyst development. Fundamental studies using primarily Ag(111) and Ag(110) single crystals have provided evidence for the key role of atomic rather than molecular oxygen in both the epoxidation and combustion reactions. Model studies employing higher alkenes are also discussed as they reveal the importance of C–H bond reactivity in the combustion pathways. The influence of alkali and halide promoter species via electronic and geometric effects is discussed. Recent STM and in situ Raman studies of oxygen, chlorine and CO2 adsorption on Ag surfaces showing dynamic silver restructuring and the identification of reactive surface species are highlighted as an area that will further the understanding of the epoxidation reaction by providing key structural information. Such information would not only benefit computational modelling efforts, but could also lead to more rational catalyst development processes.
218 citations
TL;DR: In this article, the photocatalytic degradation of rhodamine B (N,N, N′,N′-tetraethylrhodamine, RB) had been studied in a colloidal TiO2 solution under both UV and visible light irradiation.
Abstract: The photocatalytic degradation of rhodamine B (N,N,N′,N′-tetraethylrhodamine, RB) had been studied in a colloidal TiO2 solution under both UV and visible light irradiation. Two different photodegradation processes of RB, photobleaching and N-deethylation, were examined in detail. The N-deethylation of RB was attributed to the oxidation by HOO· or OH· radicals generated from the dye-photosensitized and/or TiO2 catalyzed reactions; while the photobleaching of RB was found to occur via dye cationic radical. The aromatic rings of RB cannot be destroyed in the absence of oxygen under visible light irradiation. Various additives, Fe3+, EDTA, MV2+, different atmosphere (O2 and N2), time-resolved laser flash photolysis and spin trapping electron paramagnetic resonance (ESR) were used to determine the possible mechanisms involved.
216 citations
TL;DR: The van der Waals interactions between the framework of the zeolite and molecules adsorbed in their intracrystalline free space are the origin of confinement effects which govern their behavior as sorbents and catalysts as mentioned in this paper.
Abstract: In addition to their natural appeal as crystalline solids possessing a void intracrystalline volume consisting of channels and cages, sometimes interconnected, zeolites also have remarkable features resulting from the surface curvature of their internal surface. These features involve non-covalent interactions between the framework of the zeolite and molecules adsorbed in their intracrystalline free space. These interactions extend beyond the molecular level and are thus of supramolecular nature. They are at the origin of confinement effects which govern their behaviour as sorbents and catalysts. By contrast to molecular shape selective effects which result from short range repulsions imposing restrictions on sorbates or reactants, reaction transition states, and/or products, the van der Waals interactions responsible for confinement effects are long ranged and attractive. They favour the adsorption of certain molecules vs. others, sometimes at specific locations in the intracrystalline volume, stabilise reaction transition states, and may eventually enhance the probability of bimolecular reactions. Because of such confinement effects, zeolites can be assimilated to solid solvents and several of their characteristics can be described using solvent effects as an analogy. This review analyses recent literature supporting this view with specific attention to the application of zeolites in selective adsorption and separation and their use as catalysts for fine chemicals synthesis. Some other consequences of confinement effects will also be discussed, in particular those relating to the measurement and optimal use of the acidic properties of zeolites.
210 citations
TL;DR: In this article, some mechanistic models for polymerization and stereo-regulation as well as the factors which affect the activity and stereospecificity of the catalysts are discussed, and the effects of substitutions on Cp ligand of half titanocene complexes were examined.
Abstract: Syndiotactic polystyrene, namely XAREC® is being developed by Idemitsu Petrochemical Co., Ltd. as a major new polymer family. XAREC® is a new crystalline engineering thermoplastic with a crystalline melting point of 270 °C. Because of its crystalline nature, XAREC® has a high heat resistance, an excellent chemical resistance and a water/steam resistance. XAREC® also has the dip soldering resistance. Potential applications for XAREC® include surface-mount electronic devices and electrical connectors. In this paper, some mechanistic models for polymerization and stereo-regulation as well as the factors which affect the activity and stereospecificity of the catalysts are discussed. The effects of substitutions on Cp ligand of half titanocene complexes were examined. The bulky substitution groups reduce the activity. Also, borate compounds as activator and effects of hydrogen are discussed.
171 citations
TL;DR: In this paper, the authors reported that the extent of encapsulation of the Mn(III)Salen complexes in zeolite X varies with the nature of the substituent group on the aromatic ring.
Abstract: Manganese(III) complexes of [ N , N ′-ethylenebis(salicylidene-aminato)] (salen), [ N , N ′-ethylenebis(5-chloro-salicylidene-aminato)] (Cl 2 Salen), [ N , N ′-ethylenebis(5-bromo-salicylidene-aminato)] (Br 2 Salen) and [ N , N ′-ethylenebis(5-nitro-salicylidene-aminato)] [(NO 2 ) 2 Salen] have been encapsulated in the supercages of zeolite X by the zeolite synthesis method. The catalysts have been characterized by FTIR, UV–Vis and EPR spectroscopic techniques, XRD, SEM, thermal and elemental analysis, as well as nitrogen adsorption and cyclic voltammetric studies. The extent of encapsulation of the Mn(III)Salen complexes in zeolite X varies with the nature of the substituent group on the aromatic ring. While bromo groups enhance encapsulation, substitution with –NO 2 groups decreases the amounts of Mn(III) complexes encapsulated in the cavities of the zeolites. Cyclic voltammetric data indicate that the zeolite matrix facilitates the reduction of Mn(III) to Mn(II), suggesting that it behaves like an electron-withdrawing substituent. The aerobic oxidation of styrene to benzaldehyde, styrene oxide and phenylacetaldehyde over these catalysts is also reported.
116 citations
TL;DR: A review of ring-closing metathesis reactions can be found in this paper, where a selection of papers published in the past year can be divided into four groups: (i) those in which RCM has been used as a key step in the total synthesis of a natural product: ( R )-(+)-lasiodiplodin, dactylol, (−)-stemoamide, and epothilone A; (ii) those that have been used to make a sub-unit of natural products: the marine toxins brevetoxin B,
Abstract: A review is given of a selection of papers published in the past year in this rapidly expanding field. Such papers may be divided into four groups: (i) those in which ring-closing metathesis (RCM) has been used as a key step in the total synthesis of a natural product: ( R )-(+)-lasiodiplodin, dactylol, (−)-stemoamide, and epothilone A; (ii) those in which RCM has been used to make a sub-unit of a natural product: the marine toxins brevetoxin B, brevetoxin A, and maitotoxin; (iii) those in which RCM has been used to make compounds of known biological activity: fluvirucin B 1 (Sch 38516); (iv) those in which metathesis reactions have been used to make compounds of potential synthetic importance: crown ethers, aza sugars (polyhydroxylated pyrrolidines), β -lactams, chromenes, and aminoacids. The catalysts used have been one of the following four metal carbene complexes: the Schrock catalyst Mo(CHCMe 2 Ph)(NC 6 H 3 -2,6- i -Pr 2 )[OCMe(CF 3 ) 2 ] 2 ; the Grubbs catalysts Ru(CHCHCPh 2 )Cl 2 (PCy 3 ) 2 and Ru(CHPh)Cl 2 (PCy 3 ) 2 , where Cy=cyclohexyl; and the Tebbe reagent which behaves as Ti(CH 2 )Cp 2 , where Cp=cyclopentadienyl. In this paper these are denoted by Mo-1, Ru-1, Ru-2, and Ti-1, respectively. Ti-1 is especially useful for effecting RCM by the carbonyl–olefination reaction, while the RCM of enynes, catalyzed by Ru-2, is another valuable metathesis reaction.
115 citations
TL;DR: In this article, a variety of planar model supported catalysts prepared by the vapor deposition of catalytically interesting metals onto ultrathin oxide films on refractory metal single crystal substrates in ultrahigh vacuum are presented.
Abstract: Characterization and reaction kinetics studies have been performed over a variety of planar model supported catalysts prepared by the vapor deposition of catalytically interesting metals onto ultrathin oxide films on refractory metal single crystal substrates in ultrahigh vacuum. These unique systems feature many of the advantages for fundamental study associated with single crystals, while addressing important issues for supported catalysts, such as the intrinsic effects of particle size and the role of the support. The oxide thin films have been shown to roughly mimic the chemical and physical properties of the bulk analogs, and yet they are electrically conductive via defects and tunnelling to the single crystal substrate. This renders them amenable to the various charged particle spectroscopies that comprise the core of modern surface science; also, because they are flat as well conductive, they are suitable for scanning–tunnelling and atomic force microscopy. Characterization studies carried out over these models focusing on structural, electronic, and chemical properties as a function of particle size have been related to parallel studies of relevant catalytic reactions, providing fundamental insight into these processes at the atomic level. The select group of experiments presented here provides a broad illustration of the versatility and utility of these materials for elucidating the properties of small, supported metal particles and for simulating catalysis over `real world' high surface area supported catalysts.
111 citations
TL;DR: In this paper, the authors showed that the simultaneous entrapment of a lipase such as Pseudomonas cepacia or Candida antarctica and nanostructured superparamagnetic magnetite (Fe3O4) in hydrophobic sol-gel materials derived from CH3Si(OCH3)3 (MTMOS) or other hydrophilic precursors leads to catalytically active, mechanically stable and magnetically separable heterogeneous biocatalysts.
Abstract: The simultaneous entrapment of a lipase such as Pseudomonas cepacia or Candida antarctica and nanostructured superparamagnetic magnetite (Fe3O4) in hydrophobic sol–gel materials derived from CH3Si(OCH3)3 (MTMOS) or other hydrophobic precursors leads to catalytically active, mechanically stable and magnetically separable heterogeneous biocatalysts. The relative enzyme activity in the test reaction involving the esterification of lauric acid by n-octanol in isooctane is typically 200–300%, with respect to the same reaction using a conventional suspension of the non-immobilized enzyme. Separation of the catalyst by applying a simple magnet poses no problems. In the kinetic resolution of 2-pentylamine, enantioselectivity is essentially complete (ee=97–99%).
111 citations
TL;DR: In this article, a semiconductor composite of CdS particles embedded in a TiO2 gel after heat treatment at 250°C in air is revealed to show catalytic H2 evolution from an EDTA aqueous solution with light of λ⩾450 nm.
Abstract: A semiconductor composite of CdS particles embedded in a TiO2 gel after heat treatment at 250°C in air is revealed to show catalytic H2 evolution from an EDTA aqueous solution with light of λ⩾450 nm, in contrast to bare CdS which is stoichiometrically photocorroded. A significant improvement in the activity even with considerable losses of CdS in the gel during the heat treatment suggests that the TiO2 gel crystallized into anatase can prevent photocorrosion of CdS embedded therein, and serve as a semiconducting matrix to facilitate charge transfer from the embedded CdS to Pt at the surfaces of the gel.
107 citations
TL;DR: In this paper, a direct hydroxylation of benzene and its derivatives with N2O was studied using H-[Al]ZSM-5 zeolite as catalyst.
Abstract: The direct hydroxylation of benzene and its derivatives with N2O is studied using H-[Al]ZSM-5 zeolite as catalyst. The activity of the H-[Al]ZSM-5 zeolite is increased by hydrothermal treatment. This treatment dealuminates the zeolitic framework and creates Lewis acidic extra framework alumina (EFA) which controls the catalytic activity. It is shown by 27Al MAS NMR and IR measurements that the degree of dealumination correlates with the amount of EFA species formed and the yield of phenol obtained. Simple calcination of the H-[Al]ZSM-5 zeolite for a longer time (18 h/550°C) increases the selectivity whereas conversion remains unchanged. Furthermore, benzene derivatives, naphthalene and biphenyl were used as substrates in the direct hydroxylation with N2O. The selectivity to the corresponding phenols is lower than in the case of benzene because the functional groups participate in the reaction mechanism. No correlation is found between the activation or deactivation of the aromatic ring by the functional group and the yield of the corresponding hydroxylated product. In case of naphthalene and biphenyl the yield is limited by substrate decomposition.
TL;DR: In this article, the authors summarized recent reports on (i) hydrogenation (including transfer hydrogenation by HCO 2 Na) of olefins or aldehydes in water, and (ii) reduction of aromatic compounds with carbon monoxide and water by homogeneous transition metal catalysts.
Abstract: This paper summarizes recent reports on (i) hydrogenation (including transfer hydrogenation by HCO 2 Na) of olefins or aldehydes in water, and (ii) reduction of aromatic nitro compounds with carbon monoxide and water by homogeneous transition metal catalysts. The discussion will be focused on the hydrogenation of α , β -unsaturated aldehydes by ruthenium–sulfonated phosphine complexes that show remarkable chemoselectivity toward CO bonds. The author also introduces selective reduction of aromatic nitro-groups by ruthenium or rhodium catalysts under CO/H 2 O conditions. These catalytic reactions are very important from both synthetic and industrial viewpoints, not only because the after-treatment of by-products can be simplified from the conventional methods, but also because the reaction also proceeds with high selectivity affording the desired products.
TL;DR: In this paper, a chiral supported non-chiral catalysts were used for epoxidation reactions with iodosylbenzene, reaching turnover numbers similar or higher than those obtained in solution with related catalysts.
Abstract: Clay-supported non-chiral Mn(salen) catalysts have been prepared by two methods, direct exchange of the complex or treatment of the Mn-exchanged clay with the salen ligand. The first method, which leads to more homogeneously distributed catalysts, has been also used with a chiral Mn(salen) complex. Depending on the size and the amount of exchanged complex and the structure and surface area of the clays, the catalysts can show expanded basal spacings. All the solids promote epoxidation reactions with iodosylbenzene, reaching turnover numbers similar or higher than those obtained in solution with related catalysts. However, the supported chiral catalyst leads to a slightly lower enantioselectivity. The recovery of the catalysts leads to a reduction of activity and mainly of enantioselectivity, as in the case of the chiral catalyst, which cannot be related with leaching of complex. Spectroscopic studies indicate that the decomposition of the ligand is the main reason for this behaviour.
TL;DR: In this paper, the influence of ligand substitution on comonomer incorporation, catalyst activity and molar mass distribution in ethene/1-butene copolymerization with various methylaluminoxane-activated metallocene catalysts was investigated.
Abstract: Ethene/1-octene and ethene/1-butene copolymerization using various methylaluminoxane-activated metallocene catalysts, e.g. silylene-bridged substituted bisindenyl zirconocene systems and halfsandwich titanocene, was performed at 40°C in toluene. The influence of the ligand substitution on comonomer incorporation, catalyst activity, molar mass, molar mass distribution, degree of polymerization and copolymerization parameters was investigated in ethene/1-octene copolymerization at constant comonomer ratio and in ethene/1-butene copolymerization as a function of varying ethene/1-butene feed mass ratios. In ethene/1-octene copolymerization the highest comonomer incorporation was achieved with MAO-activated Me 2 Si(Me 4 Cp)( N - tert -butyl)TiCl 2 catalyst. Best performance in terms of comonomer incorporation combined with high catalyst activity and molar mass was found for silylene-bridged bisindenylzirconocenes, where 2-methyl substitution promoted high degree of polymerization and benzannelation accounted for improved catalyst activity, comonomer incorporation and randomness of comonomer incorporation. In ethene/1-butene copolymerization at high 1-butene feed content silylene-bridged substituted bisindenyl systems showed polymerization characteristics similar to that of ethene/1-octene copolymerization. The influence of 2-methyl substitution on activity and comonomer incorporation was significant only at low 1-butene feed content. Storage moduli and glass transition temperature of the poly(ethene-co-1-butene) copolymers decreased with increasing 1-butene content.
TL;DR: A series of metallocene analogues containing bulky benzamidinate ligands has been prepared and fully characterized in this article, and the polymerization activity is strongly dependent on catalyst and cocatalyst concentrations and on temperature.
Abstract: A series of Ti and Zr metallocene analogues containing bulky benzamidinate ligands has been prepared and fully characterized. Treatment of TiCl 4 (THF) 2 or ZrCl 4 (THF) 2 with two equivalents of the appropriate benzamidinate anions affords the bis(benzamidinato) complexes [C 6 H 5 C(NC 3 H 7 ) 2 ] 2 MCl 2 (M=Ti ( 1 ), Zr ( 2 )) and [C 6 H 5 C(NC 6 H 11 ) 2 ] 2 MCl 2 (M=Ti ( 3 ), Zr ( 4 )). The zirconium complex 2 was structurally characterized by X-ray diffraction. In a similar manner the nonafluoromesityl derivative [(CF 3 ) 3 C 6 H 2 C(NC 6 H 11 ) 2 ] 2 ZrCl 2 ( 5 ) was synthesized from ZrCl 4 (THF) 2 and Li[(CF 3 ) 3 C 6 H 2 C(NC 6 H 11 ) 2 ]. Methylation of 4 with methyllithium yields the dimethyl complex [C 6 H 5 C(NC 6 H 11 ) 2 ] 2 ZrMe 2 ( 6 ). The mixed-ligand metallocene analogues [C 6 H 5 C(NC 3 H 7 ) 2 ](C 5 Me 5 )MCl 2 (M=Ti ( 7 ), Zr ( 8 )) and [C 6 H 5 C(NC 6 H 11 ) 2 ](C 5 Me 5 )TiCl 2 ( 9 ) have been prepared by reacting (C 5 Me 5 )TiCl 3 or (C 5 Me 5 )ZrCl 3 with one equivalent of a lithium N , N ″-dialkylbenzamidinate. The polymerization of ethylene and propylene has been studied by the catalytic precatalyst complexes 1 and 2 upon reaction of an excess of methylalumoxane to obtain the active cationic complexes. The polymerization activity of the complexes is comparable to other benzamidinate ancillary containing ligands although toward shorter amounts of time due to a competitive inhibition presumably obtained by a β -hydrogen elimination from the ligand. Polymerization activity is strongly dependent on catalyst and cocatalyst concentrations and on temperature.
TL;DR: In this article, a water-soluble diphosphine based on a xanthene-type backbone was used for two-phase alkene hydroformylation, which led to the selective formation of linear aldehydes, hex-1-ene and 4-styrenesulfonate.
Abstract: The concept of a large `natural' bite angle in chelating diphosphines has been extended to two-phase alkene hydroformylation by the use of a water-soluble diphosphine based on a xanthene-type backbone. The recently developed diphosphine Xantphos was modified with water-soluble groups by controlled sulfonation in fuming sulfuric acid to form 2,7-bis(SO 3 Na)-4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (2,7-bis(SO 3 Na)-Xantphos) exclusively. The application of this ligand in the two-phase rhodium-catalyzed hydroformylation of alkenes led to the selective formation of linear aldehydes. Some data are reported on the hydroformylation of propene, hex-1-ene and 4-styrenesulfonate. Recycling experiments showed the catalyst to be active up to five cycles. For comparison, the two-phase catalytic results are also given with TPPTS as the ligand. Furthermore, the coordination behaviour of the in situ formed catalytic species HRh(2,7-bis(SO 3 Na)-Xantphos)(CO) 2 was studied by high-pressure NMR spectroscopy which indeed showed the desired bis-equatorial coordination of the ligand to the rhodium center.
TL;DR: The use of olefin metathesis in synthesis has significantly expanded with the development of tunable, functional group tolerant OLEF catalysts as discussed by the authors, which have found extensive use in organic synthesis through RRCM methods.
Abstract: The use of olefin metathesis in synthesis has significantly expanded with the development of tunable, functional group tolerant olefin metathesis catalysts. These structurally defined catalysts have found extensive use in organic synthesis through ring-closing metathesis (RCM) methods. More recently, numerous examples of selective cross-metatheses, including ring-opening metatheses (ROM) and tandem ring-opening/ring-closing metatheses have been reported.
TL;DR: In this paper, a quasi-relativistic density-functional method was used to determine the reaction enthalpies for the steps involved in the dissociation of CH 4 to give surface C and H (CH 4,s →C s +4H s ).
Abstract: Dissociation of methane on different transition metals M (M=Ru, Ir, Rh, Ni, Pd, Pt, Cu, Ag, Au) has been investigated using a quasi-relativistic density-functional method. Reaction enthalpies for the steps involved are determined. The activation energies have been estimated using the analytic BOC-MP formula. The transition metals, Ru, Rh, …, Pt are shown to exhibit high activity in the dissociation of methane, whereas the coinage metals (Cu, Ag, Au) are very inactive. The conclusion is in agreement with experimental observations. The total dissociation enthalpy ΔH for the complete dissociation of CH 4 to give surface C and H (CH 4,s →C s +4H s ) can be regarded as a measure for the activity of the metal in methane dissociation. The order of the calculated ΔH's is consistent with the order of methane conversions over the metals. The dissociation of methane is also examined in the presence of adsorbed oxygen. Oxygen at on-top site promotes methane dehydrogenation. Oxygen at hollow site promotes methane dehydrogenation on Pt and the coinage metals, but is not beneficial to that on the other transition metals.
TL;DR: The gas-phase heterogeneous photooxidation of trichloroethylene (TCE) on ZnO has been investigated in this article, where surface bound and gas phase products and intermediates formed during the photocatalytic oxidation of TCE were characterized with infrared spectroscopy.
Abstract: The gas-phase heterogeneous photooxidation of trichloroethylene (TCE) on ZnO has been investigated. In the presence of ZnO and molecular oxygen, trichloroethylene photooxidizes upon irradiation with wavelengths of light between 300 and 390 nm. Surface-bound and gas-phase products and intermediates formed during the photocatalytic oxidation of TCE were characterized with infrared spectroscopy. The gas-phase product distribution was found to be dependent on TCE pressure. At low TCE pressures, CO2 and CO were produced as the predominant carbon-containing, gas-phase products. At high TCE pressures, additional carbon-containing products were detected. These products include phosgene, and dichloroacetylchloride. It is postulated that phosgene and dichloroacetylchloride form only after the ZnO surface becomes saturated with adsorbed products. These adsorbed photoproducts have been identified as water, bidentate formate and dichloroacetate. These adsorbed photoproducts block sites so that partial oxidation products cannot undergo further oxidation on the ZnO surface.
TL;DR: In this paper, the authors report two group 4 monocyclopentadienyl catalyst systems that yield semicrystalline ethylene/norbornene copolymers with glass transition temperatures over 110°C and melt temperatures over 240°C.
Abstract: Cyclic olefins copolymerize efficiently with ethylene using metallocene catalysts to make rigid engineering polyolefins with high chemical homogeneity. Materials with a high chemical homogeneity have value because they show reduced light scattering resulting in excellent clarity. The ultimate chemical homogeneity in a two component copolymer occurs when an alternating sequence distribution is achieved. Low symmetry bis-metallocenes have been shown to make alternating, stereoregular cyclic olefin copolymers under the right process conditions. These copolymers are semicrystalline and have high melt temperatures. Herein we report two group 4 monocyclopentadienyl catalyst systems that yield semicrystalline ethylene/norbornene copolymers over a range of reactor conditions. The clear copolymers have glass transition temperatures over 110°C and melt temperatures over 240°C. Further characterization data is presented and a short discussion of a plausible polymerization mechanism is given.
TL;DR: In this paper, mesoporous silicates (MCM-41 and Al-containing MCM41) were applied to catalyst supports for liquid-phase benzene oxidation paying attention to their remarkable features, such as large surface area, ordered mesopores and high thermal stability.
Abstract: Mesoporous silicates (MCM-41 and Al-containing MCM-41) were applied to catalyst supports for liquid-phase benzene oxidation paying attention to their remarkable features, such as large surface area, ordered mesopores and high thermal stability. The MCM-41-supported Cu catalysts were prepared by the method of impregnation (Cu/MCM-41) and ion-exchange treatment (Cu–Na·MCM-41, Cu–H·MCM-41). The liquid-phase oxygenation of benzene to phenol over the MCM-41-supported Cu catalysts was studied using molecular oxygen as an oxidant and ascorbic acid as a reducing reagent for the Cu species. The MCM-41-supported Cu catalysts, particularly the Cu ion-exchanged H·MCM-41 (Cu–H·MCM-41), were more active than the corresponding Cu catalysts supported on SiO2, TiO2, MgO, NaZSM-5, NaY, or KL zeolites. The accumulation of hydrogen peroxide (H2O2) was confirmed during the liquid-phase oxidation of benzene catalyzed by the Cu-supported MCM-41.
TL;DR: In this paper, the reaction of hydroxylated silica and alumina supports with methyl aluminoxane in toluene suspension provides chemically modified supports suitable for use in slurry and gas-phase polymerizations of ethylene or propylene on treatment with a variety of metallocene dichloride complexes.
Abstract: Reaction of hydroxylated silica and alumina supports with methyl aluminoxane in toluene suspension provides chemically modified supports suitable for use in slurry and gas-phase polymerizations of ethylene or propylene on treatment with a variety of metallocene dichloride complexes. In particular, aluminas derived from calcination of sol–gel precursors feature high degrees of surface hydroxylation in comparison with commercially available silica (or even alumina) of similar surface area and total porosity. This feature provides a mechanism for increasing the amount of aluminoxane on the former supports, such that commercially acceptable productivities (>10 kg PE/g support×h) are observed at relatively low, total levels of aluminoxane or other alkylaluminum compounds in slurry or gas-phase polymerizations, respectively. A variety of evidence indicates that leaching of active catalyst from these alumina supports occurs to a minor extent under slurry conditions, particularly at higher temperatures in the presence of additional aluminoxane. At lower temperatures, this does not occur to an appreciable extent but the morphology and bulk density of the polymer formed is unsuitable for use in a gas-phase process. This can be attributed to the method for synthesis of the sol–gel alumina precursor which results in irregular particles with a broad particle size distribution. Copolymerization of ethylene with 1-octene or 1-hexene results in formation of linear, low density, PE with a narrow composition distribution as revealed by temperature rising elution fractionation. These studies indicate that less comonomer is incorporated using these supported metallocene catalysts than their soluble analogues under otherwise identical conditions. Finally, some of the resins prepared under slurry conditions (and to a lesser extent in a gas-phase process), exhibit properties consistent with the presence of low levels of long-chain branching; this feature appears to be reasonably general for a variety of simple metallocene complexes.
TL;DR: In this paper, a series of zeolite, amorphous silica-alumina (ASA) and ordered mesoporous aluminium-containing micelle templated silica (MTS) catalysts were investigated for butene oligomerization.
Abstract: The oligomerization of butene at 423 K and 1.5–2 MPa has been investigated over a series of zeolite, amorphous silica–alumina (ASA) and ordered mesoporous aluminium-containing micelle templated silica (MTS) catalysts. While olefin oligomerization into strongly adsorbed residue and fast deactivation prevailed on microporous catalysts and ASA, mesoporous aluminosilicates with uniform pore openings near 3 nm in size exhibited high selectivity and good stability with time for the production of branched dimers. The characterization of the surface properties of the solids, the nature of the adsorbed residue on the spent catalysts, and the identification of reaction intermediates by in situ infrared spectroscopy of co-adsorbed acetonitrile and butene, suggest that the unique catalytic behaviour of MTS-type catalysts is related to the moderate strength and the high dispersion of the acid sites in the mesoporous structure.
TL;DR: In this article, the mechanism of the cross-linking reaction which occurs during the polymerization of dicyclopentadiene with the classical catalyst, WCl6/Et2AlCl (1), and the well-defined preformed alkylidene, Mo(N-2,6-C6H3-i-Pr)(CHC(CH3),2Ph)(OCCH3(CF3)2)2 )2 (2).
Abstract: This contribution presents further information about the mechanism of the cross-linking reaction which occurs during the polymerization of dicyclopentadiene with the classical catalyst, WCl6/Et2AlCl (1), and the well-defined preformed alkylidene, Mo(N-2,6-C6H3-i-Pr)(CHC(CH3)2Ph)(OCCH3(CF3)2)2 (2). When the classical system 1 was used as a catalyst, insoluble polymer was formed in all cases. However, when molybdenum catalyst 2 is employed, solution concentration determines whether soluble or insoluble polymer will form. The formation of insoluble material is attributed to an olefin addition process catalyzed by the heat released upon ring-opening metathesis polymerization of the norbornene subunit of the monomer. If the heat is removed from the polymerization system through dilution or by cooling the solution, soluble linear polymer is formed. These results suggest that an olefin addition process is at least partly responsible for the cross-linking reaction that occurs during the polymerization of dicyclopentadiene. All attempts to cross-link oligomers of linear polydicyclopentadiene with the well-defined molybdenum alkylidene 2 resulted in only the recovery of soluble polymer. With the classical catalyst system 1, insoluble material was obtained, which was assumed to be cross-linked through olefin addition. These results disprove the idea that metathesis cross-linking can be induced by a critical chain length or concentration of polydicyclopentadiene. On the contrary, no indication of metathesis cross-linking was observed whatsoever for these polymerization systems.
TL;DR: The recent development of infrared-visible sum frequency generation (SFG), a surface-specific vibrational spectroscopy, has helped bridge the pressure gap between studies of heterogeneous catalysis under high vacuum and atmospheric pressure as discussed by the authors.
Abstract: The recent development of infrared-visible sum frequency generation (SFG), a surface-specific vibrational spectroscopy, has helped bridge the pressure gap between studies of heterogeneous catalysis under high vacuum and atmospheric pressure. This is achieved by in situ monitoring of surface species at high pressure via their SFG vibrational spectra and correlating the results with the simultaneously measured reaction rate using gas chromatography. Examples of systems studied include olefin hydrogenation and carbon monoxide oxidation over the (111) crystalline face of platinum. In these examples, the studies succeed in revealing the molecular details of the surface reactions. Identification of key intermediates and their concentrations has made it possible for the first time to calculate turn over rates per active surface species rather than just per exposed surface metal atom. In all cases, the key intermediate of the reaction is not detectable on the surface in UHV under similar temperatures.
TL;DR: In this paper, the vanadate anion as tetrabutylammonium salt is used to give methanol, carbon monoxide, formaldehyde, formic acid and carbon dioxide.
Abstract: The oxidation of methane by a combination of air and hydrogen peroxide is effectively catalyzed in solution by a system composed of vanadate and pyrazine-2-carboxylic acid (PCA). In acetonitrile solution, containing the vanadate anion as tetrabutylammonium salt, the reaction gives, over a temperature range of 25 to 50°C, methanol, carbon monoxide, formaldehyde, formic acid and carbon dioxide, the latter three compounds, however, being partially due to the oxidation of the acetonitrile used as the solvent, especially at higher temperatures. In aqueous solution, containing the vanadate anion in the form of the sodium salt, the reaction affords, over a temperature range of 40 to 70°C, selectively methyl hydroperoxide within 4 h. The yield of CH3OOH attains 24%, based on H2O2, after 24 h at 50°C, the catalytic turnover number being 480. The process seems to involve hydroxyl radicals, generated by the catalyst from H2O2 even at low temperatures. At 120°C, methane is oxidized by O2 and H2O2 to give formaldehyde and formic acid, even in the absence of the catalyst, presumably due to the formation of HO· radicals from H2O2 in the presence of very low concentrations of metal ions from the autoclave under high temperature conditions.
TL;DR: In the presence of cyclohexane soluble iron and copper catalysts, tert-butyl hydroperoxide selectively oxidises cycloen-hexane to cycloenanol and cycloenhexanone as mentioned in this paper.
Abstract: In the presence of cyclohexane soluble iron and copper catalysts, tert-butyl hydroperoxide selectively oxidises cyclohexane to cyclohexanol and cyclohexanone (with cyclohexene for the copper catalysts). Under reflux for 24 h, the conversions are 4 to 5% (turnover numbers of 70 to 90) and the selectivities above 90%. Under 25 bar of oxygen at 70°C for 24 h, conversions with the iron catalysts are 9% (turnover numbers of up to 166) but the selectivities are below 80%, as large amounts of adipic acid are also formed. The copper catalysts are more selective under these conditions. Using Cu(tma)2 the conversion is 11% (turnover number of 192) and the selectivity is 91%. Reactions in the presence of cyclohexanone show that the iron catalysts deactivate by complexation with the adipic acid formed by its over-oxidation. The copper catalysts rapidly produce cyclohexene at the beginning of the reactions; this is further oxidised to cyclohexen-3-one and cyclohexen-3-ol, thus reducing the catalyst activity for cyclohexane oxidation.
TL;DR: In this article, the authors performed FT-IR spectroscopy of the CO2/H2 reaction over ZnAl2O4 and Cu-ZnAl 2O4 catalysts at 250°C up to 0.3 MPa and showed that both methanol and carbon monoxide formation were enhanced in presence of copper.
Abstract: The kinetics of the CO2/H2 reaction over ZnAl2O4 and Cu–ZnAl2O4 catalysts at 250°C up to 0.3 MPa have been followed by in situ FT-IR spectroscopy. Both methanol and carbon monoxide formation were enhanced in presence of copper. They were also produced by independent routes through different adsorbed species. Formate (type I, I′ and II), methoxy and carbonate species were identified on the support and, in addition, copper formate and copper carbonyl species when copper was present. The hydrogenation of carbonate species to copper formate species was found rate determining in methanol synthesis over the Cu–ZnAl2O4 catalyst whereas type I formate species were shown to be the active intermediate for this reaction over the ZnAl2O4 support. Carbon monoxide resulted from the water gas shift reaction probably through the same species as methanol formed over the Cu–ZnAl2O4 catalyst whereas it seemed to stem from formate species of type II in the case of the ZnAl2O4 support. Type II formate species were shown inactive in presence of copper whereas the methoxy species adsorbed on the support were found inactive in presence and in absence of copper. The comparison of these results with those previously obtained with the CO/H2 mixture showed that the nature and the role of the detected species strongly depended on the reactive atmosphere and on the presence or not of copper in the catalyst composition.
TL;DR: The acylation of anisole, phenetole or diphenyl ether with carboxylic anhydrides at 100°C in the presence of catalytic quantities of zeolite H-β gives the corresponding para-acylated products in high yield as discussed by the authors.
Abstract: The acylation of anisole, phenetole or diphenyl ether with carboxylic anhydrides at 100°C in the presence of catalytic quantities of zeolite H-β gives the corresponding para-acylated products in high yield; the zeolite can be recovered, regenerated and reused to give almost the same yield as that given by fresh zeolite
TL;DR: In this article, the deactivation of unpromoted MgO and alkali-promoted mgO catalysts in the vapor phase self-condensation of acetone was studied.
Abstract: The deactivation of unpromoted MgO and alkali-promoted MgO catalysts in the vapor-phase self-condensation of acetone was studied. The reaction was catalyzed by basic sites and major products were mesityl oxide, isomesityl oxide and isophorone. Catalysts deactivated because of coke formation. Both the initial catalyst deactivation (d0, h−1) and the product distribution depended on contact time (W/F0): d0, and the selectivity to mesityl oxide increased when W/F0 was increased. It is proposed that non-cyclic trimers, such as phorone, which are produced by aldol condensation of mesityl oxide with acetone, are the key intermediate species for coke formation. These non-cyclic trimers are highly unsaturated compounds that remain strongly bound to the catalyst surface yielding higher non-volatile oligomeric compounds which block basic active sites. Promotion of MgO with alkaline metal ions increased the d0 value measured on unpromoted MgO following the sequence Li