Showing papers in "Journal of Molecular Catalysis A-chemical in 1996"

Study of catalysts comprising heteropoly acid H3PW12O40 supported on MCM-41 molecular sieve and amorphous silica

Abstract: With the aim of obtaining pure Keggin-type heteropoly acid (HPA) species on siliceous surfaces, the preparation of HPA catalysts, comprising of 10 to 50 wt.% H3PW12O40 (PW) supported on an amorphous silica or a mesoporous all-silica molecular sieve MCM-41, was studied. The state of HPA on the siliceous surface was characterized by 31P NMR, XRD and TEM. Impregnating MCM-41 or amorphous SiO2 with an aqueous solution of PW gave catalysts with, in general, two HPA species: one with intact Keggin structure (A) and the other with a different structure (B), supposedly, H6P2W18O62 (P2W18) and/or H6P2W21O71 (P2W21). The relative amount of species A and B depends on HPA loading, with A dominating. At higher loadings, 30–50 wt.%, A is practically the only one present on the surface; at lower loadings, both species exist, the amount of B increasing as the HPA loading decreases. In contrast, catalysts prepared by impregnation with a methanol solution of HPA contained exclusively Keggin-type A over the whole range of PW loading. In the PWMCM-41 catalysts, as shown by TEM, the PW species are mainly located inside the MCM-41 pores. The B species was about 8 times as active as A in the liquid-phase trans-de-t-butylation of 2,6-di-t-butyl-4-methylphenol.

Mn-salen catalyst, competitor of enzymes, for asymmetric epoxidation

Abstract: Asymmetric epoxidation of simple olefins using (salen)manganese(III) complexes as catalysts has made a great advances in the last half decade and now finds wide application in organic synthesis. In this article, we describe the scope of the reaction, and the principal achievements to date are presented in Tables. The mechanistic aspect of the reaction is also discussed briefly.

Selective catalytic synthesis of fine chemicals: opportunities and trends

Abstract: Recent, environmentally-driven trends in the manufacture of fine chemicals are reviewed. Comparison of alternative processes on the basis of their atom utilization and E factor (kg byproducts/kg product) i s discussed. The use of chromium-substituted molecular sieves (CrAPO-5 and 11 and Cr silicalite) as recyclable solid catalysts for benzylic and allylic oxidations, the oxidation of secondary alcohols to ketones and the selective decomposition of alkyl hydroperoxides, is described. New developments in the use of zeolite-encapsulated metal complexes as solid catalysts for oxidations and reductions are reviewed. The use of palladium(0) trisulfonated triphenylphosphine complex as a catalyst for carbonylation in water is reported. Finally, the use of supported aqueous phase catalysts in various processes, including enantioselective hydrogenation, is briefly reviewed.

Pd(II) catalyzed acetoxylation of arenes with iodosyl acetate

Abstract: PhI(OAc)2 is an effective oxidant in the acetoxylation of arenes with Pd(OAc)2 as catalyst. The data are most easily interpreted in terms of palladation being the step that determines both the rate and the om p selectivity, and reductive elimination from a PhPd(IV)(OAc) species being the product forming step.

Platinum-oxide species formed by oxidation of platinum crystallites supported on alumina

Abstract: Pt/Al2O3 samples of different platinum loadings are prepared by impregnating γ-Al2O3 with PtCl4 solution. Reduced Pt/Al2O3 samples are oxidized by dioxygen to examine the variation of PtOx (platinum-oxide) species with the oxidation temperature. Four different PtOx species are characterized according to their reduction temperature (Tr) found in the temperature-programmed reduction (TPR) experiments. They are assigned as surface platinum oxide (Tr = −25°C), PtO (Tr = 50°C), PtO2 (Tr = 100°C) and PtAl2O4 (Tr = 220°C), respectively, mainly according to their No/NPt stoichiometry found in the TPR and their desorption kinetics found in the technique of temperature-programmed desorption (TPD). These four species alternate as the dominant PtOx products on Pt/Al2O3 samples when the oxidation temperature is raised to 25°C, 300°C, 500°C and 600°C, respectively.

Synthesis, characterization and photocatalytic properties of iron-doped titania semiconductors prepared from TiO2 and iron(III) acetylacetonate

Abstract: Specimens of iron-doped titania containing different amounts of Fe (0.5–5%) were prepared from TiO2 (Degussa P-25) and Fe(III) acetylacetonate by the wet impregnation method. Samples were characterized by X-ray diffraction analysis, specific surface area (BET) measurements, SEM-EDX, atomic absorption and IR and diffuse reflectance spectra. From the structural point of view, the samples were similar to those obtained with Fe(NO3)3 · 9H2O as the precursor, but with a more homogeneous distribution of iron for each mixed oxide sample on the particle surfaces but not between particles. The photocatalytic activity of these samples under near-UV irradiation was better for oxalic acid degradation than for EDTA, and similar for both types of mixed oxide samples. Mixed oxides showed however lower activity than TiO2. Some photodegradation under visible irradiation, not occurring with TiO2, could be observed for oxalic acid when using 5% Fe-containing samples.

Characterization of nickel species on several γ-alumina supported nickel samples

Abstract: Studies of the chemical preparation, surface areas, pore distributions, powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectra (XPS) of several γ-alumina supported nickel samples active for the catalytic hydrogenation of hexanedinitrile have been carried out. NiO crystallites showed to be larger than the pore mouths of the support. Pore distributions lie in the mesopore region with diameters between 20 and 100 A. Powder XRD detects the oxidized and reduced-nickel phases present and does not detect the nickel aluminate phase, due to the lack of crystallinity of the latter. SEM micrographs detect the presence of octahedral NiO crystallites and amorphous shelling nickel aluminate. XPS results show the presence of surface Ni 2+ (in the form of stoichiometric and non-stoichiometric NiO, and stoichiometric and non-stoichiometric nickel aluminates) and surface reduced nickel. A deconvolution of the experimental curves was carried out in order to obtain a better assignment of the surface nickel species present. Nickel aluminate is detected at calcination temperatures > 623 K and covers all the surface of the support with layers between zero and several atoms thick, depending on calcination temperature and nickel concentration. Catalytically active reduced nickel (for nitrile hydrogenations) either as naked crystallites or as encapsulated nickel inside voided non-stoichiometric aluminate shells lie on top of the underlying catalytically inactive nickel aluminate when precursor calcination temperatures are higher than 623 K and reduction temperatures of 673 K. NiO transformation into nickel aluminate collapses the cubic NiO surface morphology of the small NiO crystallites giving rise to voided shells of nickel aluminate which may hide encapsulated reducible NiO.

Transesterification of soybean oil catalyzed by alkylguanidines heterogenized on different substituted polystyrenes

Abstract: 1,5,7-Triazabicyclo[4.4.0]dec-5-ene and 1,1,3,3-tetramethylguanidine were heterogenized on different chloromethylated poly(styrene/divinylbenzene), as well as on linear polystyrene after introduction of a ‘spacer-arm’. Furthermore, polymeric 2,3-dicyclohexylguanidines were synthesized by the reaction of 1,3-dicyclohexylcarbodiimide with p-(6-aminohexyl)polystyrenes. The guanidine-containing polymers were used in the transesterification of soybean oil with methanol in several consecutive catalytic cycles. The guanidines heterogenized on gel-type poly(styrene/divinylbenzene) with 1 meq Cl/g showed a slightly lower activity than their homogeneous analogous but reached the same high conversions after prolonged reaction time. On the other hand, they slowly leached from the polymers, allowing only nine catalytic cycles. The guanidines heterogenized on linear polystyrene with the use of a ‘spacer-arm’ were less active. Furthermore, they suffered substitution reactions during the recycling experiments to form inactive hexasubstituted guanidinium compounds.

Surface structures of supported tungsten oxide catalysts under dehydrated conditions

Abstract: The molecular structures of the WO3/support (Al2O3, TiO2, Nb2O5, ZrO2, SiO2, and MgO) catalysts under in situ dehydrated conditions have been investigated by Raman spectroscopy. The series of catalysts was synthesized by the aqueous incipient wetness method. The WO3/support catalysts, with the exception of the WO3SiO2 and WO3MgO catalysts, possess a highly distorted, octahedrally coordinated surface tungsten oxide species with one short WO bond (mono-oxo tungsten oxide species) at high surface coverages. The WO3SiO2 catalysts exhibit strong Raman features of crystalline WO3 particles due to the relative low density and reactivity of the surface hydroxyl groups. The WO3MgO catalysts possess non-stoichiometric compounds, Mgx(WO4)y and Cax(WO4)y, at low tungsten oxide contents and crystalline MgWO4 and CaWO4 at high tungsten oxide contents. This result is attributed to the high aqueous solubility of MgO as well as the CaO impurity and the strong acid-base interaction between WO2−4 with Mg(OH)2 and Ca(OH)2. The current findings for supported tungsten oxide catalysts parallel the previous findings for supported molybdenum oxide catalysts and reflect the similar surface structural chemistry of these two oxides.

A perspective on nanocluster catalysis: polyoxoanion and (n-C4H9)4N+ stabilized Ir(0)∼300 nanocluster ‘soluble heterogeneous catalysts’

Abstract: Following a review of the background terms, definitions and key literature necessary to understand the rapidly developing and promising area of nanocluster catalysis, the recent report of polyoxoanion-stabilized Ir(0)∼190–450 (hereafter Ir(0)∼300) nanocluster catalysts is reviewed. Specifically, the experimental observations relating to three principle questions concerning these soluble-oxide-supported, Ir(0)∼300 nanocluster catalysts are reviewed: (i) their catalytic activity (in a prototypical structure-insensitive reaction, cyclohexene hydrogenation); (ii) their lifetime; and (iii) their thermal stability, each in direct comparison to three established heterogeneous Ir(0) catalysts [7.9% dispersed 1% Ir(0) γ- Al 2 O 3 , Exxon's 80% dispersed 1% Ir η- Al 2 O 3 , and a low surface area Ir(0) precipitate]. In addition, observations of WVI to WV· reduction of the polyoxoanion by Ir(0)∼300 and H2 are summarized, results which provide the first soluble-oxide version of the well known spillover of hydrogen in heterogeneous catalysis.

Effect of water vapor on the molecular structures of supported vanadium oxide catalysts at elevated temperatures

Abstract: The effect of water vapor on the molecular structures of V2O5-supported catalysts (SiO2, Al2O3, TiO2, and CeO2) was investigated by in situ Raman spectroscopy as a function of temperature (from 500°;C to 120°;C). Under dry conditions, only isolated surface VO4 species are present on the dehydrated SiO2 surface, and multiple surface vanadium oxide species (isolated VO4 species and polymeric vanadate species) are present on the dehydrated Al2O3, TiO2, and CeO2 surfaces. The Raman features of the surface vanadium oxide species on the SiO2 support are not affected by the introduction of water vapor due to the hydrophobic nature of the SiO2 support employed in this investigation. However, the presence of water has a pronounced effect on the molecular structures of the surface vanadium oxide species on the Al2O3, TiO2, and CeO2 supports. The Raman band of the terminal V = O bond of the surface vanadia species on these oxide supports shifts to lower wavenumbers by 5–30 cm−1 and becomes broad upon exposure to moisture. Above 230°C, the small Raman shift of the surface vanadium oxide species in the presence of water suggests that the dehydrated surface VOx species form a hydrogen bond with some of the adsorbed moisture. Upon further decreasing the temperature below 230°C, the hydrogen-bonded surface VOx species are extensively solvated by water molecules and form a hydrated surface vanadate structure (e.g. decavanadate). The broad Raman band at ≈ 900 cm−1, which is characteristic of the polymeric VOV functionality, appears to be minimally influenced by the presence of water vapor and is a consequence of the broadness of this band. Oxygen-18 isotopic labeling studies revealed that both the terminal V=O and bridging VOV bonds readily undergo oxygen exchange with water vapor. The current observations account for the inhibiting effect of moisture upon oxidation reactions over supported metal oxide catalysts and are critical for interpreting in situ Raman data during hydrocarbon oxidation reactions where H2O is a reaction product.

A novel method for conversion of benzyl alcohols to benzaldehydes by laccase-catalysed oxidation

Abstract: A new catalytic method for the oxidation of substituted benzyl alcohols to the corresponding benzaldehydes by molecular oxygen using the enzyme/cofactor system laccase/2,2′-azino-bis(3-ethyl-benzothiazoline-6-sulfonic acid) is presented. The reaction proceeds under very mild conditions giving the product in quantitative yields. The enzyme requires at least one free ortho -position in the substrate molecule for the reaction to proceed.

Thioether oxidation by hydrogen peroxide using titanium-containing zeolites as catalysts

Abstract: Titanium substituted ZSM-5 (TS-1) and BEA (Ti-beta) are investigated as catalysts for the selective oxidation of thioethers with diluted H2O2, in a protic solvent, at 303 K. The performances of TS-1 and Ti-beta samples are analyzed in relation with structure of thioethers, nature of solvent and shape selectivity effect of catalysts. A mechanism for organic sulfides oxidation on Ti-containing zeolites is proposed.

Shape selective epoxidation of alkenes by metalloporphyrin-dendrimers

Abstract: A series of oxidatively robust manganese-porphyrin-dendrimers were synthesized in good yields for use as shape-selective oxidation catalysts. The poly(phenylester) dendrimers with the bulky tert -butyl terminal groups were synthesized with a convergent approach. These cascade dendrimers were linked at the meta -phenyl positions of the 5,10,15,20-tetrakis(3′,5′-dihydroxyphenyl)porphinato manganese(III) chloride to produce a sterically hindered metal center. The regioselectivity of these catalysts was determined for epoxidation of non-conjugated dienes and 1:1 intermolecular mixture of linear and cyclic alkenes using iodosylbenzene as the oxygen donor. Metalloporphyrin dendrimers exhibit significantly higher substrate selectivity compared to unsubstituted Mn(TPP)(CI), but only moderate selectivity relative to the extremely hindered bis-pocket porphyrin, 5,10,15,20-tetrakis(2′,4′,6′-triphenylphenyl)porphinato manganese(III) acetate. Molecular modeling was performed on the porphyrin dendrimers to elucidate the extent of steric crowding around the porphyrin. There is a relatively large cavity for substrate entrance in the manganese-porphyrin-dendrimers relative to extremely hindered bis-pocket porphyrin.

MCM-41 type molecular sieves as catalysts for the Friedel-Crafts acylation of 2-methoxynaphthalene

Abstract: Molecular sieves of the MCM-41 type were studied as catalysts for the Friedel-Crafts acylation of 2-methoxynaphthalene in the liquid phase. When MCM-41 in the H + form is used as Bronsted-acid catalyst, acylation with acetic anhydride gives substitution predominantly at the 1-position. Increased reaction temperatures lead to a slight decrease of selectivity to acylation at the 1-position. The regenerability of H-MCM-41 is found to be excellent. For the acylation of 2-methoxynaphthalene with acetyl chloride Zn-MCM-41 was studied as a Lewis-acid catalyst, but both selectivity and regenerability are less favourable than in the case of the H-MCM-41 catalysed acylation of with acid anhydride.

Manganese porphyrins covalently bound to silica and montmorillonite K10 as efficient catalysts for alkene and alkane oxidation by hydrogen peroxide

Abstract: New supported Mn III -porphyrin-based catalysts were prepared by covalent binding of meso -tetrakis-(2,6-dichlorophenyl) porphyrin derivatives bearing NH 2 or SO 3 H functions on their meso -aryl rings to either silica or montmorillonite K10. All these supported metalloporphyrins efficiently catalysed the epoxidation of alkenes with PhIO. The Mn III -porphyrin covalently bound to montmorillonite K10 gave remarkably good results for the hydroxylation of linear alkanes such as heptane with PhIO, the obtained yields (54%) being much better than those observed with corresponding homogeneous catalysts. The Mn III -porphyrin covalently bound to silica was an efficient catalyst for epoxidation of alkenes with H 2 O 2 (yields between 65 and 95% under conversion conditions). In the presence of an excess of substrate, this supported catalyst was found to be able to catalyse alkene epoxidation with yields based on H 2 O 2 between 67% and 88%, as well as alkane hydroxylation with yields between 21% (for heptane) and 90% (for cyclooctane).

Binuclear zirconocene cations with μ-CH3-bridges in homogeneous Ziegler-Natta catalyst systems

Abstract: Binuclear zirconocene cations are observed by I H-NMR in C6D6 solutions containing B(C6F5)3 and an excess of a zirconocene dimethyl complex. The CH3-bridged cation [«C5H5)2ZrCH3)ilL-CH3)]+' solvent-separated from the anion H 3 C-B(C 6 F 5 )3' is present in equilibrium with (C5H5)2Zr(CH3)2 and the mononuclear ion pair [(C5H5)2ZrCHj ... H 3 C-B(C 6 F 5 )3]; in more concentrated solutions, a binuclear ion pair [«C5H5)2ZrCH3)ilL-CH3)+ ... H3C­ B(C6F5)3] is the dominant species. Similar equilibria are observed in C6D6 solutions containing B(C 6F5)3 and (CH3)4CiC5H4)2Zr(CH3)2' (CH3)2Si(C5H4)2Zr(CH3)2 or rac-(CH 3)2 Si(indenyJ)2 Zr(CH 3)2' Complexes with sterically more demanding ligands, such as (C5(CH3)5)2Zr(CH3)2 or rac-(CH3)2Si(2-methyl-benz[e]indenyJ)2Zr(CH3)2 do not form any binuclear species under these conditions. In the catalyst system rac-(CH3)2Si(indenyJ)2Zr(CH3)2/Bu3NH+B(C6F5);' activities for the polymerization of propene increase with excess of the dimethyl zirconocene complex. This effect is due in part to a sacrifice of some dimethyl zirconocene for the removal of impurities from the catalyst system and in part to a stabilization of the catalyst in the form of the binuclear cation [«CH3)2Si(indenyJ)2ZrCH3MIL-CH3)]+' The latter appears to act, in the presence of propene, as a source of the mononuclear cation [(CH3)2Si(indenyJ)2ZrCH3(C3H6)]+' rather than as a polymerization catalyst by itself.

State of the art in the development of biomimetic oxidation catalysts

Abstract: Work during the last decade laid down the basis for biomimetic oxidation catalysis, which has become an established technique. Preliminary work relied on the use of metal complexes of 5,10,15,20-tetrakisphenylporphyrin (TPP), but these proved to be inadequate to perform the catalytic function. Further, derivatives of TPP with suitable and adequately positioned substituents, allowed the preparation of high performance catalysts, in terms of catalytic efficiency and stability, for some selected oxidations under specific reaction conditions and for a diversity of oxygen donors. The first oxidations were achieved using the ‘special’ oxygen donor iodosylbenzene, but cheaper and generally available oxidants such as sodium hypochlorite or hydrogen peroxide are presently used as the oxidants. Catalysts capable of promoting stereoselective catalysis are reported. Conditions for the efficient use of the catalysts so far reported require particular specificities and the catalytic activity is only shown, in the majority of the reported works, in the presence of selected species to act as axial ligands to the metal ion of the catalyst. Specific porphyrin structures were reported where the addition of a specific axial ligand can be avoided.

Polyesters modified with 1,4-cyclohexanedimethanol having high clarity prepared utilizing an antimony containing catalyst/stabilizer system

Abstract: This invention relates to a polyester resin prepared by adding one or more dicarboxylic acid components to one or more glycol components containing 1,4-cyclohexanedimethanol equalling 100 mole %, the polyester resin having been prepared in the presence of a catalyst/stabilizer system consisting essentially of antimony compounds and phosphorous compounds and compounds selected from the group consisting essentially of zinc compounds, gallium compounds, and silicon compounds.

Optimization of catalytic production and purification of buckytubes

Abstract: Carbon nanotubes were produced in large amounts by catalytic decomposition of acetylene in the presence of supported Co and Fe catalysts. The influence of various parameters such as the way of catalyst preparation, the nature of the support, the size of active metal particles, and the reaction conditions on the buckytube formation was studied. The process was optimized towards the large-scale production of buckytubes having the same diameters as the fullerene nanotubes obtained from the arc-discharge method. The separation of the buckytubes from the catalyst was also achieved.

Hydrogen spillover in bifunctional catalysis

Abstract: A model is proposed to describe the mechanism of hydroconversions on bifunctional catalysts. This mechanism allows a better interpretation of experimental results. It incorporates the concept of hydrogen spillover, i.e. the surface diffusion of activated hydrogen species. The main consequence of the model is that all reaction steps can occur on one reaction site whereas the classical model requires three reaction steps on different reaction sites, Additionally, the catalytic properties of layered bifunctional catalysts (consisting of a macroscopical metal-containing and acidic part) depending on the geometry of such systems can be easily explained. The new model is based on the assumption that a dynamic equilibrium of two coexisting activated (spilt-over) hydrogen species is formed on the surface; one of them bring neutral (H atom) and the other electrically charged (H+ ion). The influence of the spilt-over hydrogen is illustrated and discussed in the case of the conversion of cyclohexane on different metal-containing and acidic catalysts.

Hydroxylation of phenol over Sn-silicalite-1 molecular sieve: solvent effects

Abstract: Tin-silicalite-1 (MFI) prepared by hydrothermal synthesis has been used as catalyst in the hydroxylation of phenol with aqueous H2O2. Isolated Sn4+ ions which are probably attached to the defect silanols are active in this reaction. At optimum conditions, a H2O2 efficiency of 70% and a ortho to para product ratio of 1.6 have been achieved. The solvent used has a strong influence on the activity. The UV-Vis spectral studies indicate that acetone and acetonitrile probably coordinate strongly with Sn4+ centers preventing the solvolysis of the SiOSn units and the formation of peroxo intermediate. Methanol probably causes the cleavage of SiOSn bond to form SiOH and SnOMe species. Water was found to be an efficient solvent. The formation of EPR active radical ion in presence of H2O2 and H2O and its attenuation in presence of acetone, acetonitrile and methanol support the above conclusion.

Silica-supported 12-molybdophosphoric acid catalysts: Influence of the thermal treatments and of the Mo contents on their behavior, from IR, Raman, X-ray diffraction studies, and catalytic reactivity in the methanol oxidation

Abstract: The thermal behavior of silica-supported 12-molybdophosphoric acid catalysts (PMoH- x series, x = Mo wt%, varying from 16.2 to 0.9) was investigated by using different techniques (IR and Raman spectroscopies, X-ray diffraction, and catalytic reactivity in methanol oxidation). The silica support used in the study (surface area 376 m 2 g −1 ) induces destabilization of 12-molybdophosphoric acid. The transformation into β-MoO 3 begins from 250°C, and is achieved at 350°C. β-MoO 3 is the main species on silica support in a wide range of temperatures, and seems to be stabilized by the support. The role of this molybdenum oxide is discussed in terms of active species in the catalytic reaction, and the capability to rebuild the Keggin unit under exposure to water vapor.

Contribution to water purification using polyoxometalates. Aromatic derivatives, chloroacetic acids

Abstract: Polyoxometalates appear to be effective photocatalysts for a variety of organic pollutants, leading to their decomposition to CO2 and H2O and Cl− in case of chlorinated hydrocarbons. Various aromatic derivatives and chlorinated acetic acids, undergo mineralization upon photolysis with near visible and UV light, in presence of W10O4−32, PW12O3−40, and SiW12O4−40. These catalysts are, at least, as effective as TiO2. The main oxidant appears to be OH radicals formed by the reaction of the excited polyoxometalates with H2O. At the initial stages of photoreaction, dioxygen has a profound effect (up to eighteen times faster) on the photodegradation of chloroacetic acid, whereas, it has minor effect on chlorophenols. On the contrary, at the final stages of photodegradation of chlorophenols, the rates were an order of magnitude faster in presence than in absence of dioxygen. Substitution of WO moiety of SiW12O4−40 for transition metals, namely, [H2OMnIISiW11O39]6− and [H2OCuIISiW11O39]6− reduced, by about an order of magnitude, the effectiveness of the catalyst.

Aerobic oxidation of hydrocarbons catalyzed by electronegative iron salen complexes

Abstract: A number of salen derivatives bearing electronegative substitutents and their corresponding iron(III) chloro complexes have been prepared. Several of the complexes catalyze aerobic oxidation of cyclohexene, primarily to allylic oxidation products. Evidence supports a radical chain autoxidation mechanism, with the complex functioning to decompose intermediate alkyl hydroperoxides. Activity is observed only for complexes with relatively high Fe ( III II ) reduction potentials, but the incomplete correlation of activity with potential indicates that more subtle structural and electronic effects also play an important role in determining the rates of the catalytic reactions.

Modified zeolites as catalysts in the Friedel-Crafts acylation

Abstract: Modified zeolites were found to be active catalysts in the Friedel-Crafts acylation of anisole by acetyl chloride and acetic anhydride. The effect of two different modifications of the zeolites were tested; lanthanum-exchange and varying the Si Al ratio. For the rare-earth modified zeolites, the activity was found to be dependent on the lanthanum content, and the yield increased with the level of lanthanum even up to 93% exchange. Dealuminated Y-zeolites were also found to be very active, and an almost linear increase in the yield with decreasing aluminium fraction was found. This has been attributed to the increased hydrophobicity of dealuminated zeolites. Zeolite Beta was also found to be very active in the reaction.

Photocatalytic reduction of chromium(VI) with titania powders in a flow system. Kinetics and catalyst activity

Abstract: The photocatalytic reduction of chromium(VI) with titania powders has been studied using a continuous flow system The kinetics and the influence of catalyst concentration and pH on the reaction rate have been analyzed Kinetic results show a half-order reaction at pH lower than 4, and a first-order reaction for pH above 4 Deactivation of titania was found when reduction of chromium(VI) was carried out at pH higher than 4, due to the fouling of titania caused by chromium hydroxides A model for catalyst deactivation has been developed It describes the performance of the catalyst at the different experimental conditions tested Furthermore, the results obtained in the kinetic studies are supported by data from electronic microscopy and adsorption techniques

Immobilized catalysts for enantioselective hydrogenation : The effect of site-isolation

Abstract: Functionalized PPM (PPM: (2 S ,4 S )-4-dihpenylphosphino-2-(diphenylphosphino-methyl) pyrrolidine), a chiral diphosphine ligand, was bound to silica gel with different loadings [mmol ligand/g support] and used as model to detect possible effects of site isolation in the enantioselective hydrogenation of methyl-acetamidecinnamate (MAC) with cationic and with neutral Rh-catalysts and of N -(2-methyl-6-ethylphen-1-yl) methoxymethylmethylketimin (MEI) with an Ir-catalyst. To investigate the influence of high local catalyst concentration in absence of a support, two new bis-PPM ligands with spacers of different length were prepared and tested. The results indicate that site isolation effects occur with two of the catalytic systems used: Whereas immobilized cationic Rh-catalysts are practically not affected, the neutral Rh-catalysts lose most of their activity with increasing catalyst loading. Probably, this is due to the formation of chloride-bridged Rh-dimers which is favored by high catalyst concentration as indicated by experiments with the bis-PPM-ligands. Immobilization has a spectacular effect with the Ir-system. It is known that Ir-complexes in presence of hydrogen can form stable hydrogen-bridged dimers that are catalytically inactive. The fact that the activity (and also productivity) of the immobilized catalysts increases with decreasing catalyst loading indicates, that the formation of these dimers can be prevented by immobilization.

Hydrogenation of α,β-unsaturated aldehydes over Ru/Al2O3 catalysts

Abstract: The hydrogenation of citral and cinnamaldehyde has been investigated over Ru/Al 2 O 3 catalysts. The effect of metal particle size on the catalytic activity and selectivity has been studied by using catalysts having a metal dispersion ranging from 0.05 to 0.88. It has been observed that the overall rate of hydrogenation of both the unsaturated aldehydes is not influenced by the Ru particle size. In the hydrogenation of cinnamaldehyde a higher selectivity to cinnamyl alcohol has been observed on catalysts with larger metal particle size. No variations have been found in the hydrogenation of citral. It is suggested that steric effects do not influence significantly the product selectivity. A repulsive interaction between the aromatic ring and the catalyst surface would explain the higher selectivity to cinnamyl alcohol. On all the investigated catalysts the isolated C C double bond of citral shows a low reactivity. It is suggested that the unsaturated aldehyde is adsorbed through the carbonyl group. This strong adsorption prevents the hydrogenation of the isolated olefinic bond which is located far away from the adsorption centers.

Membrane occluded catalysts: a higher order mimic with improved performance

Abstract: A general method to immobilise homogeneous catalysts and to improve the performance of heterogeneous catalysts is discussed. The method consists in embedding the catalysts in hydrophobic PDMS (polydimethylsiloxane)-membranes. Inspired on a complete structural mimic of cytochrome P-450 up to the level of the membrane, this technique gives superior properties to the membrane resident catalyst. The scope and limitations of this method are discussed by two examples of heterogeneous catalysts, i.e., FePc-Y (iron phthalocyanine zeolite Y) and [Mn(bpy) 2 ] 2+ -Y (manganese bis(bipyridyl) zeolite Y), and three examples of homogeneous complexes, i.e., FePc, Ru-binap ([2,2′-bis(diphenylphosphino-1,1′-binaphtyl]chloro( p -cymene)-ruthenium chloride) and the Jacobsen catalyst ( N,N′ -bis(3,5-di- tert -butylsalicylidene)-1,2-cyclohexane-diamine manganese chloride). Due to changed sorption in the zeolites, catalyst activity is enhanced and deactivation is suppressed. Furthermore, the membrane incorporation makes the use of a solvent redundant. For homogenous complexes, this procedure represents a general method for heterogenisation. Moreover, the technique opens new ways in the field of oxidation chemistry, where solvents are necessary to homogenise reagents which usually differ in polarity.