Showing papers in "Journal of Molecular Catalysis A-chemical in 2002"

Ionic liquids: perspectives for organic and catalytic reactions

Abstract: Ionic liquids are attracting a great deal of attention as possible replacement for conventional molecular solvents for catalytic and organic reactions. They complete the use of environmentally friendly water, supercritical fluids or perfluorinated solvents. Features that make ionic liquids attractive include their lack of vapor pressure and the great versatility of their chemical and physical properties. By a judicious combination of cations and anions, it is possible to adjust the solvent properties to the requirement of the reactions, thus creating an almost indefinitely set of “designer solvents”. Besides the possibility of recycling the catalytic system, one main potential interest in using ionic liquids results in the unique interactions of these media with the active species and in the possibility to modify the reaction activity and selectivity. Their successful use as solvents has been demonstrated for a wide range of organic reactions including acid catalyzed reactions and transition metal catalyzed transformations.

Metal-catalyzed hydrocarbon oxygenations in solutions: the dramatic role of additives: a review

Abstract: This review describes examples of remarkable acceleration of metal-catalyzed oxidation reactions by certain additives. In some cases, reactions proceed 2 or 10 times more rapidly in comparison with the process in the additive’s absence, in other cases, reactions become possible only in the presence of the additive. Varying ligands at the metal center or additives, one can not only dramatically improve yields of oxygenates but also control the selectivity of the reaction. Understanding mechanisms of the additive’s action is very important for search of new efficient catalysts and catalytic systems. Additives considered in the review can play roles of the ligands at metal ion or proton or electron transfer reagents and they mimic certain enzymes (the active center or its environment). Often the mechanism of the effect of additives on the reaction rate and the product yield is unknown, and the main aim of the review is to attract investigator’s attention in creating new efficient catalytic systems, which contain not only a metal ion but also a necessary “additive”.

Highly active and stable Ni/Ce-ZrO2 catalyst for H2 production from methane

Abstract: Steam reforming of methane has been carried out aimed at the development of new and highly active catalysts for H2 production The catalytic properties of Ni catalysts supported on various supports have been investigated in connection with characterization results obtained from XRD, H2 chemisorption, TPR and CO2-TPD Among the catalysts examined, Ni/Ce–ZrO2 exhibited the best activity and stability The remarkable catalytic performance is interpreted as a combined result of high oxygen storage capacity of ceria in Ce–ZrO2, strong interaction between Ni and Ce–ZrO2, basic property of the catalyst and rather high capability of H2 uptake

Esterification of different acids over heterogeneous and homogeneous catalysts and correlation with the Taft equation

Abstract: Esterification of acetic, propanoic and pentanoic acid with methanol, ethanol, 1-propanol, 2-propanol, butanol and 2-butanol was studied in the presence of a fibrous polymer-supported sulphonic acid catalyst, Smopex-101. The reaction temperature in the experiments was 60 °C. Comparative experiments were carried out on an ion-exchange resin, Amberlyst 15 and with a homogeneous catalyst, liquid HCl. The effect of different molar ratios between the reactants (acid and alcohol) on the reaction rate was investigated. The rate constants were related to the substituent effects of the reacting molecules according to the Taft equation. The substituent effects of alcohols were found to follow the Taft relationship, which was not the case for the acids. The experimental results were modelled according to a simple second-order model and a more advanced adsorption-based model. The adsorption-based model, which includes the adsorption of carboxylic acid and water and is consistent with structure–activity relationship, was superior to the second-order model.

Occurrence and prevention of photodissolution at the phase junction of magnetite and titanium dioxide

Abstract: A stable magnetic photocatalyst was prepared by coating a magnetic core with a layer of photoactive titanium dioxide. A direct deposition of titanium dioxide onto the surface of magnetic iron oxide particles proved ineffective in producing a stable magnetic photocatalyst, with high levels of photodissolution being observed with these samples. This observed photodissolution is believed to be due to the dissolution of the iron oxide phase, induced by the photoactive the titanium dioxide layer due to electronic interactions at the phase junction in these magnetic photocatalysts. The introduction of an intermediate passive SiO2 layer between the titanium dioxide phase and the iron oxide phase inhibited the direct electrical contact and hence prevented the photodissolution of the iron oxide phase. The magnetic photocatalyst is for use in slurry-type reactors from which the catalyst can be easily recovered by the application of an external magnetic field.

The leaching and re-deposition of metal species from and onto conventional supported palladium catalysts in the Heck reaction of iodobenzene and methyl acrylate in N-methylpyrrolidone

Abstract: When supported palladium catalysts are used for Heck vinylation of iodobenzene with methyl acrylate in N-methylpyrrolidone (NMP) in the presence of triethylamine and sodium carbonate bases, the reaction proceeds homogeneously with dissolved active palladium species that are formed through coordination of NMP and triethylamine with palladium. These active species easily react with iodobenzene (oxidative addition), beginning the catalytic cycle of Heck coupling. The last step of catalyst regeneration takes place with the action of sodium carbonate. The active palladium species are not stable and deposit the metal to support when they cannot find iodobenzene to react in the reaction mixture after this substrate is completely consumed. The re-deposition of palladium occurs on the surfaces of bare support and/or palladium particles remaining on it, depending on the nature of support surface and the number and size of residual metal particles. The growth of palladium particles has been observed after the reuse of catalyst in some case. However, the supported catalysts are recyclable without loss of activity.

Control of Pd leaching in Heck reactions of bromoarenes catalyzed by Pd supported on activated carbon

Abstract: Parameters that influence the palladium leaching during and after Heck reactions of aryl bromides with olefins catalyzed by heterogeneous Pd on activated carbon systems are reported. Pd leaching correlates significantly with the progress of the reaction, the nature of starting materials and products, solvent, base and atmosphere. The Pd concentration in solution is highest at the beginning of the reaction and is reduced when the reaction is finished. Reaction procedures were developed that allow easy and practicable control and minimization of the Pd leaching at the end of the reaction: Pd re-deposition by: (i) increased temperature, (ii) addition of reducing agents and (iii) working under inert conditions. The Pd concentration in solution after the reaction is reduced to 0.05 ppm Pd (μg Pd/g solution). The catalyst meets all demands for practical application: high activity and selectivity under ambient conditions, complete separation (filtration) of Pd from the product mixture, easy recovery of Pd and commercial availability.

Oxidation of cyclohexene with tert-butylhydroperoxide and hydrogen peroxide catalyzed by alumina-supported manganese(II) complexes

Abstract: Alumina-supported Mn(II) complexes with ligands of acetylacetonato (acac), N , N ′-ethylenebis(salicylideneiminato) (salen), ethylenediamine (en) and 2,2′-bipyridine (bpy) catalyze the oxidation of cyclohexene with tert -butylhydroperoxide (TBHP) and hydrogen peroxide. Oxidation of cyclohexene with TBHP gave 2-cyclohexene-1-ol, 2-cyclohexene-1-one and 1-( tert -butylperoxy)-2-cyclohexene whereas, oxidation with H 2 O 2 resulted in the formation of cyclohexeneoxide and cyclohexene-1,2-diol. Mn(II) catalysts with the nitrogen donor ligands show significantly higher catalytic activity than oxygen donor ligands at the same surface coverage. The mechanism of oxidation has also been discussed.

Alumina and silica supported metal catalysts for the production of carbon nanotubes

Abstract: Catalytic activity of iron, cobalt and a mixture of iron and cobalt supported on Al 2 O 3 and SiO 2 has been investigated in the production of carbon nanotubes (NTs). The supports are obtained from different methods. Acetylene was used as the source of carbon and nitrogen as the carrier gas. The pyrolysis of hydrocarbon was carried out at 500, 600 and 700 °C. The effect of other reaction parameters such as rate of flow of the hydrocarbon, carrier gas and the reaction time has also been investigated. Transmission electron microscope (TEM) was used to follow the quality and nature of NTs formed. It was observed that a good quality and quantity of multiwall carbon nanotubes (MWNTs) were produced when alumina prepared from aluminum isopropoxide and deposited with a mixture of iron and cobalt catalysts was used.

Manganese-containing MCM-41 for epoxidation of styrene and stilbene

Abstract: Mn-MCM-41 is found to be the most effective heterogeneous catalyst for the epoxidation of styrene with tert-butyl hydroperoxide (TBHP) among several metal ion-containing mesoporous molecular sieves including Mn-, V-, Cr-, Fe-, and Mo-MCM-41. ESR, XANES, diffuse reflectance UV–VIS, UV–Raman and XPS are used to characterize the Mn-MCM-41 synthesized by both direct hydrothermal (DHT) and template ion exchange (TIE) methods. The results suggest that Mn2+ and Mn3+ coexist in the Mn-MCM-41 samples synthesized by both methods and a large part of manganese atoms could be incorporated into the framework of MCM-41 obtained by the DHT method. The oxidation of either styrene or stilbene with TBHP as the oxidant over the Mn-MCM-41 produces corresponding epoxide as the main product; the reaction probably proceeds through a radical intermediate. The TIE catalyst shows higher activity, while the DHT catalyst gives higher TBHP efficiency for the epoxidation reactions.

Cleavage of the carbon–carbon bond in biphenylene using transition metals

Abstract: Biphenylene has proven to have a rich chemistry with transition metals. In many cases, the aryl–aryl CC bond is cleaved to give a metallocycle complex, which can go on to give a variety of reaction products. Insertion reactions with small molecules lead to the formation of new polycyclic aromatic derivatives with the incorporation of functional groups. In several cases, these reactions are selective and catalytic.

Designing the adequate base solid catalyst with Lewis or Bronsted basic sites or with acid–base pairs

Abstract: A series of heterogeneous catalysts with Lewis and Bronsted basic sites, and acid–base bifunctional pairs has been used in order to perform organic reactions. By changing the chemical composition and activation conditions it is possible to have predominantly Lewis or Bronsted base catalysts within a large range of well defined basicities. This allows to select the most appropriate catalyst for a specific reaction. Thus, MgO, calcined hydrotalcites, rehydrated hydrotalcites and grafted quaternary organic ammonium hydroxides on MCM-41, have been used as catalysts in Knoevenagel condensation, aldolization and Michael additions. Catalysts containing mild acid–base pairs such as those existing in amorphous aluminophosphates (ALPOs) allow to achieve high selectivities with still very reasonable activities.

Selective oxidation of aromatic compounds with dioxygen and peroxides catalyzed by phthalocyanine supported catalysts

Abstract: This article summarizes our research in catalytic oxidation on the design and study of supported metallophthalocyanine catalysts. The catalytic properties of these materials were studied in the oxidation of 2-methylnaphthalene (2MN) to 2-methyl-1,4-naphthoquinone (Vitamin K3, VK3), 2,3,6-trimethylphenol (TMP) to trimethyl-1,4-benzoquinone (precursor of Vitamin E) and in the epoxidation of olefins. Iron tetrasulfophthalocyanine (FePcS) covalently grafted in the dimeric form yielded catalyst more active and selective that those containing monomeric species but suffered from a lack of stability transforming into less selective monomer complexes during catalysis. The stabilization of supported dimer form by covalent link of two adjacent phthalocyanine molecule through appropriate diamine spacer provided more selective and stable catalysts. Trimethyl-1,4-benzoquinone was obtained with 87% yield at 97% conversion of TMP. More demanding oxidation of 2MN afforded 45% yield of VK3. Particular emphasis is placed on the mechanistic aspects of these oxidations using two mechanistic probes, 2-methyl-1-phenylpropan-2-yl hydroperoxide (MPPH) to distinguish between homolytic versus heterolytic cleavage of O–O bond during the formation of active species and thianthrene 5-oxide (SSO) to evaluate nucleophilic versus electrophilic character of formed active species. To illustrate a versatility of the phthalocyanine-based supported catalysts we prepared a novel phthalocyanine complex with eight triethoxysylil substituents which can be directly anchored to the silica without any modification of the silica support. This new catalyst shows good catalytic activity in epoxidation of olefins by dioxygen in the presence of isobutyraldehyde. The same catalytic system was also active in the oxidation of phenols to biphenols with 86% yields. This catalytic system is complementary to previous one that selectively oxidizes phenols to quinones. An appropriate choice of the reaction conditions allows selective oxidation either to quinones or to biaryl compounds.

The role of bismuth as promoter in Pd-Bi catalysts for the selective oxidation of glucose to gluconate

Abstract: Bismuth is a well-established promoter of noble metal-based catalysts for the selective liquid phase oxidation of alcohols, aldehydes and carbohydrates with molecular oxygen. Experiments were carried out to improve the understanding of the promoting role of bismuth in bimetallic Pd-Bi catalysts used for the selective oxidation of glucose to gluconate. In relationship with the fact that these catalysts undergo substantial bismuth teaching under the reaction conditions, particular attention was paid to the possible role played by the soluble fraction of bismuth in. the oxidative process. Carbon-supported Pd-Bi/C catalysts characterized by various Bi-Pd compositions (0.33 less than or equal to Bi/Pd less than or equal to 3.0, 10wt.%Pd + Bi) were prepared from acetate-type precursors, tested under various experimental conditions and analyzed by X-ray diffractometry and X-ray photoelectron spectroscopy (XPS). Whatever the initial catalyst composition, the surface intensity ratio measured by XPS in used catalysts lies in the range 0.4-0.6, suggesting that the dynamic state of the catalyst involves the association of one Bi and two to three I'd atoms. The leaching process and the promoting effect itself are discussed in line with the formation of Bi-glucose and Bi-gluconate complexes present in solution but also as adsorbed species at the catalyst surface. The performances of a monometallic Pd/C catalyst are significantly improved in the presence of adequate amounts of soluble Bi. The involvement of the soluble fraction of bismuth in the overall mechanistic scheme of glucose oxidative dehydrogenation is suggested. The detrimental effect of large amounts of soluble bismuth is attributed to a too extensive adsorption of Bi-glucose complexes on the surface I'd atoms. (C) 2002 Elsevier Science B.V. All rights reserved.

Dehydrogenation of secondary amines to imines catalyzed by an iridium PCP pincer complex: initial aliphatic or direct amino dehydrogenation?

Abstract: The PCP pincer complex, IrH2(C6H3-2,6-(CH2PBut2)2) catalyzes the transfer dehydrogenation of secondary amines to imines. The catalytic system is highly sensitive to both steric and electronic factors and greater than 99% regioselectivity are observed in the dehydrogenation of the asymmetric substrates, cyclohexyethylamine and benzylpropyl amine. Good to excellent yields are obtained when the reactions are carried out in toluene solution. The dehydrogenation of 2,2,2′,2′-tetramethyldibutylamine leads exclusively to the production of the corresponding imine indicating that the catalytic reaction pathway involves direct amino rather than initial aliphatic dehydrogenation.

Catalytic investigation of rhodium nanoparticles in hydrogenation of benzene and phenylacetylene

Abstract: Nanoparticles of rhodium embedded in polyvinylpyrrolidone (PVP), as catalyst, were investigated in the hydrogenation of different substrates (benzene, phenylacetylene, norbornene, quinoline, adiponitrile). The solid was used as a heterogeneous catalyst or as a soluble heterogeneous catalyst in biphasic conditions (liquid/liquid) when the catalyst was dissolved in water. In both cases, the kinetics of the catalytic reaction were found to be zero-order in respect to the substrate and first-order with respect to hydrogen and catalyst. The higher hydrogenation reaction rate was found for benzene by using biphasic conditions. The [Rh-PVP] catalyst has shown an efficient activity for the catalytic hydrogenation of norbornene, quinoline and adiponitrile into norbornane, tetrahydroquinoline and 6-aminocapronitrile.

A new improved catalyst for the palladium-catalyzed amination of aryl chlorides

Abstract: The catalytic amination of aryl chlorides is an interesting subject for the fine chemical industry due to the importance of anilines as building blocks for pharmaceuticals, agrochemicals and new materials. Here we report the application of our recently developed di(1-adamantyl)alkylphosphines for the palladium-catalyzed amination of non-activated aryl chlorides. In general, excellent yields of amination products are obtained using 0.5 mol% of palladium(II)acetate and 1 mol% of di(1-adamantyl)-n-butylphosphine (12) in toluene at 120 °C. Remarkable results are obtained for the coupling of hindered amines with sterically congested aryl chlorides. Comparing the new catalyst system with previously reported best palladium catalysts we found in all cases improved yields of amination products in the presence of our ligands.

Carbon dioxide as building block for the synthesis of organic carbonates: Behavior of homogeneous and heterogeneous catalysts in the oxidative carboxylation of olefins

Abstract: The increasing world market of organic carbonates demands for the development of new synthetic methodologies, alternative to phosgene which represents the starting material for their conventional synthesis. Innovative technologies based on CO 2 are attracting much attention as they are environmentally acceptable and may respond to the atom-economy principle. The oxidative carboxylation of olefins catalyzed by homogeneous or heterogeneous catalysts has a good potential as it uses simple starting materials to produce valuable chemicals. In this paper the conversion of styrene into styrene carbonate is discussed with emphasis on the reaction mechanism and product distribution considering either homogeneous or heterogeneous catalysts.

Synthesis, characterization and study of vanadyl tetradentate Schiff base complexes as catalyst in aerobic selective oxidation of olefins

Abstract: Novel vanadyl Schiff base complexes with the general formula VOL x (x = 7–18) were synthesized and characterized by physicochemical methods. Only an orange polymeric form was obtained for VOL 13 , both brown polymeric and green monomeric forms were obtained for VOL x , x = 16, 17. These observations suggest that in these complexes oxovanadium(IV) exhibit a coordination number of five. Systematic substitution on the ancillary ligand have allowed V(V)/V(IV) reduction potentials to be tuned over a range of approximately 130 mV. The complexes are catalysts for the aerobic oxidation of cyclohexene. Catalytic activity increases with increasing V(V)/V(IV) reduction potential and the catalytic selectivity varied by changes in the ligands. The catalytic system described here is an efficient and inexpensive method for oxidation of olefins, with advantages of high activity, selectivity, re-usability and short reaction times. © 2002 Elsevier Science B.V. All rights reserved.

"Coke" molecules trapped in the micropores of zeolites as active species in hydrocarbon transformations

Abstract: The formation in the channels or cages of zeolites of heavy side products (generally called “coke”) is often responsible for their deactivation owing to poisoning of active sites and/or to pore blockage. However, these coke molecules trapped in the zeolite micropores being relatively simple, are not generally inert with respect to the reactants or intermediates of the desired reactions and, hence can significantly affect the activity and selectivity. This participation in catalytic reactions of the coke molecules trapped in the zeolite micropores is shown here in several examples carried out in liquid or in gas phase, with large, medium or small pore molecular sieves: (i) isopropylation of naphthalene and alkylation of toluene with long chain n-alkenes over HFAU and HBEA zeolites; (ii) selective skeletal isomerization of n-butenes over HFER; (iii) selective hydroisomerization of long chain n-alkanes over PtHTON; (iv) selective methanol conversion into light alkenes over SAPO 34. © 2002 Elsevier Science B.V. All rights reserved.

Synthesis of ethylene carbonate from supercritical carbon dioxide/ethylene oxide mixture in the presence of bifunctional catalyst

Abstract: Ethylene carbonate was rapidly synthesized from supercritical carbon dioxide/ethylene oxide mixture by using as catalyst the system of tetradentate schiff-base aluminum complexes (designated as SalenAlX) coupling with a quaternary ammonium or phosphonium salt. The high rate of reaction was attributed to rapid diffusion and high miscibility of ethylene oxide in supercritical carbon dioxide under employed conditions. Various reaction periods present different formation rate of ethylene carbonate, mainly due to the existence of phase change during the reaction. The synergistic effect of the binary catalyst for ring-opening of ethylene oxide results from nucleophilicity of highly reactive anions of quaternary salts and the electrophilic interaction of SalenAlX with ethylene oxide. The activation of CO 2 was generally initiated by nucleophilic attack of the alcoholate(OCH 2 CH 2 BrNBu 4 ) at the carbon atom of CO 2 , and weak interaction between the central metal ion of SalenAlX and the lone pairs of one oxygen atom of CO 2 . It resulted in the insertion of CO 2 to AlO bond of Salen(X)AlOCH 2 CH 2 BrN( n -Bu) 4 or SalenAlOCH 2 CH 2 X to form linear carbonate which was transformed into ethylene carbonate by intramolecular substitution of halides. The experimental results demonstrate that supercritical carbon dioxide could be used as not only an environmentally benign solvent but also a carbon precursor in synthesis.

Aluminum phthalocyanine complex covalently bonded to MCM-41 silica as heterogeneous catalyst for the synthesis of cyclic carbonates

Abstract: Aluminum phthalocyanine complex was covalently bonded to the silica surface of mesoporous MCM-41 molecular sieve via two different routes. The resulting anchored complex was characterized by X-ray powder diffraction (XRD), nitrogen adsorption and UV–VIS spectroscopy, and used as catalyst for the cycloaddition of CO 2 and epoxides to produce cyclic carbonates. Its catalytic activity can be markedly enhanced in the presence of a quaternary ammonium salt, such as n -Bu 4 NBr. The catalyst exhibits good stability as well as high catalytic activity in reaction conditions, and was subjected to utilization for 10 recycles without obvious loss of activity. The catalytic mechanism of the anchored aluminum phthalocyanine complex, combined with n -Bu 4 NBr as co-catalyst, for ring-opening of epoxides and activation of CO 2 are also discussed in detail.

Catalytic hydroxylation of phenol over ternary hydrotalcites containing Cu, Ni and Al

Abstract: Liquid phase catalytic hydroxylation of phenol was carried out over ternary hydrotalcites containing copper, nickel and aluminum using hydrogen peroxide as oxidant. The influence of various reaction parameters, namely, substrate:catalyst ratio, substrate:oxidant ratio, nature of oxidant, solvent, pH, time-on-stream, reaction temperature and calcination temperature on the activity and selectivity for the “sought for” reaction, were studied. The catalysts were synthesized by the coprecipitation technique using metal nitrates and a NaOH/Na2CO3 mixture. Hydroxylation of phenol over these catalysts resulted mainly in the formation of catechol and hydroquinone. Among the catalysts studied, CuNiAl3-5 (( Cu + Ni)/Al =3.0; Cu/Ni =5.0) and CuNiAl2-1 (( Cu + Ni)/Al =2.0; Cu/Ni =1.0) showed maximum activity with a catechol:hydroquinone ratio close to 1.6. An increase in the substrate:catalyst ratio enhanced the conversion of phenol over these catalysts. With respect to the influence of reaction temperature, the conversion increased up to 65 °C and decreased when the temperature was further increased. Oxidants other than H2O2 and solvents other than water have not showed measurable conversion of phenol. Time-on-stream studies indicated that around 90% of conversion of phenol was achieved in 10 min and longer reaction time did not significantly enhance the conversion. Among the calcined samples studied, that calcined at 800 °C showed a maximum activity for phenol hydroxylation; however, the activities of calcined catalysts were lower than those of fresh hydrotalcites. The observed variation in the activity may be attributed to the copper concentration, especially those present on the surface. A reaction pathway involving hydroxy radical is proposed for the formation of dihydroxybenzenes.

Regioselective hydrophenylation of olefins catalyzed by an Ir(III) complex

Abstract: The novel, anti-Markovnikov, arylation of olefins with benzene to produce straight-chain alkylbenzenes with higher selectivity than branched alkylbenzenes is catalyzed by [Ir(μ-acac-O,O′,C3)(acac-O,O′)(acac-C3)]2 (acac=acetylacetonato), 1 [J. Am. Chem. Soc. 122 (2000) 7414]. The reaction of benzene with propylene gave n-propylbenzene and cumene in 61 and 39% selectivities, respectively. The reaction of benzene and styrene afforded 1,2-diphenylethane in 98% selectivity. Considering the anti-Markovnikov regioselectivity and lack of inhibition by water, we propose that the reaction does not proceed via a Friedel–Crafts, carbocation, mechanism. Complex 1, amongst the various transition metal complexes examined, is the most efficient for catalyzing the anti-Markovnikov olefin arylation. The crystal structure of complex 1 was solved and is consistent with a binuclear Ir(III) structure with three different types of coordinated acac ligands as reported by earlier solution IR and NMR analyses. [Ir(μ-acac-O,O′,C3)(acac-O,O′)Cl]2, 2, was prepared by the reaction of complex 1 with benzoyl chloride, and the crystal structure was also reported.

Decomposition of toluene using an atmospheric pressure plasma/TiO2 catalytic system

Abstract: Application of atmospheric pressure plasma as an alternative technology for the destruction of toluene is demonstrated in this study. Used TiO 2 colloidal solution was obtained by an improved sol–gel method, and coated on glass beads to prepare decomposition of toluene. The physical property of synthesized TiO 2 catalyst film was analyzed by XRD and SEM spectroscopy. From these results, it was identified that the catalyst film exhibited anatase structure with particle size of about 50–100 nm after calcination at 500 °C for 1 h. The decomposition of toluene in TiO 2 /O 2 plasma system was investigated. Amounts of the catalyst and toluene concentration were fixed as 3 wt.% and 1000 ppm, respectively. The analyses for performance of toluene decomposition and intermediates in reaction were done by the in situ method using the mass spectroscopy and gas chromatography. The toluene of 40% was decomposed at pulse voltage of 13 kV in the only O 2 plasma condition without TiO 2 catalyst. Furthermore, the conversion enhanced remarkably in the TiO 2 /O 2 plasma system, and it reached 70% at pulse voltage of 13 kV after 120 min. This result was very notable compared with that in photocatalytic system, with below 40% after 120 min reaction.

Iron perchlorophthalocyanine and tetrasulfophthalocyanine catalyzed oxidation of cyclohexane using hydrogen peroxide, chloroperoxybenzoic acid and tert-butylhydroperoxide as oxidants

Abstract: Polychlorophthalocyanine (Cl 16 PcFe II ) and tetrasulfophthalocyanine ([Fe II TSPc] 4− ) complexes of iron are employed as catalysts for the oxidation of cyclohexane using tert -butyl hydroperoxide (TBHP), m -chloroperoxybenzoic acid ( m -CPBA) and hydrogen peroxide as oxidants. Catalysis using the Cl 16 PcFe II was performed in a dimethylformamide:dichloromethane (3:7) solvent mixture. For the [Fe II TSPc] 4− catalyst, a water:methanol (1:9) mixture was employed. The products of the catalysis are cyclohexanone, cyclohexanol and cyclohexanediol. The relative yields of the products depended on oxidant and the catalyst. TBHP was found to be the best oxidant since minimal destruction of the catalyst and higher selectivity in the products were observed when this oxidant was employed. The mechanism of the oxidation of cyclohexane in the presence of the Cl 16 PcFe II and [Fe II TSPc] 4− involves the oxidation of these catalysts, forming an Fe(III) phthalocyanine species as an intermediate. Higher yields were observed when [FeTSPc] 4− was employed as a catalyst, which is more soluble than the perchlorinated iron phthalocyanine catalyst.

Aromatization of methane over zeolite supported molybdenum: active sites and reaction mechanism

Abstract: The conversion of methane to aromatics, mainly benzene, has been studied. Zeolite type catalysts, ZSM-5, MCM-22, containing molybdenum were used. The reaction was carried out at 973–1073 K and atmospheric pressure. Aromatics were formed after an induction period during which molybdenum carbide was formed. Induction period was followed by analyzing infrared (IR) band of the acidic OH. Zeolite plays an important role for increasing Mo dispersion. Comparison between ZSM-5 and MCM-22 showed that Mo/MCM-22 was twice more active than Mo/ZSM-5. This was attributed to more open topoly of MCM-22 which facilitated the migration of Mo ions towards exchangeable positions thus favoring Mo dispersion, and also reduced diffusion limitation of the aromatic products. Analysis of the products at high space velocity showed that acetylene was formed as primary product along with possibly ethylene. Acetylene was converted into aromatics (benzene, naphthalene) as the space velocity decreased. IR analysis in the OH stretching vibrations showed that H + ions, in H-ZSM-5, are progressively exchanged during the calcination of Mo/HZSM-5, and almost completely removed after the induction period following CH 4 reaction at 823 K. At high temperature in O 2 molybdenum species, possibly MoO 2 + (Mo 2 O 5 ) 2+ , migrate in the zeolite framework and replace H + . These species were further converted into Mo 2 C by reaction with methane, residual carbon species (coke) poisoning the residual zeolite protons. The reaction of CH 4 , C 2 H 2 and C 2 H 4 over acidic H-ZSM-5, H + poisoned Mo 2 C/H-ZSM-5 and non-acidic Mo 2 C/SiO 2 was studied. It resulted that H + sites are not prerequisite for aromatization of C 2 H 2 or C 2 H 4 . It was concluded that the principal route for the aromatization of CH 4 is the formation of C 2 H 2 over Mo 2 C which again over the molybdenum carbide forms benzene. The bifunctional mechanism apparently was less important.

Direct oxidation of hydrogen to hydrogen peroxide over Pd-containing fluorinated or sulfated Al2O3, ZrO2, CeO2, ThO2, Y2O3 and Ga2O3 catalysts in stirred slurry reactor at ambient conditions

Abstract: Direct oxidation of H 2 to H 2 O 2 over powdered PdO-containing fluorinated or sulfated Al 2 O 3 , ZrO 2 , CeO 2 , ThO 2 , Y 2 O 3 , Ga 2 O 3 catalysts with or without pre-reduction by hydrazine, using a pure water or 0.02 M H 2 SO 4 as reaction medium and a gaseous feed consisting of 1.7 vol.% H 2 in O 2 , in a magnetically stirred glass reactor at 22 °C and atmospheric pressure has been investigated. The catalysts have been characterized for their surface area and also by CO chemisorption. The performance of the catalysts in the decomposition of H 2 O 2 at 22 °C has also been studied. The results showed a strong influence of catalyst reduction, fluorination (or chlorination) or sulfatation of the catalyst support and the acidity of reaction medium on both the conversion and H 2 O 2 selectivity in the direct oxidation of H 2 . An acidic medium is essential for achieving both the high conversion and high selectivity in the H 2 oxidation. The reduced catalysts showed much lower H 2 O 2 selectivity in the H 2 oxidation process and much higher H 2 O 2 decomposition activity. The water in the H 2 to H 2 O 2 oxidation over the above catalysts is formed in parallel and/or consecutive reactions, depending upon the catalyst used.

Highly active ethylene polymerization catalysts based on titanium complexes having two phenoxy-imine chelate ligands

Abstract: Five titanium complexes having F(s) or CF 3 (s) containing phenoxy-imine chelate ligands were prepared and investigated for their potential as ethylene polymerization catalysts using methylalumoxane (MAO) as a co-catalyst at 25 °C under atmospheric pressure. Polymerization results showed that the electrophilicity of the titanium center plays a dominant role in determining activity. These results clearly demonstrate, for the first time, the electron-withdrawing substituent effects on polymerization activity. The complexes possessing plural F(s) or CF 3 (s) in the phenoxy-imine ligand displayed very high activities exceeding those for Cp 2 TiCl 2 and Cp 2 ZrCl 2 . The activity, 43,300 kg polyethylene (PE) mol-cat −1 h −1 , exhibited by the complex having three fluorines in the ligand, represents the highest value reported to date for ethylene polymerization catalysts based on titanium complexes under atmospheric pressure at ambient temperatures.

H2 reduction behaviors and catalytic performance of bimetallic tin-modified platinum catalysts for propane dehydrogenation

Abstract: Bimetallic Pt–Sn/MgAl2O4 catalysts with different metal concentrations, used for propane dehydrogenation, were prepared by the impregnation, coprecipitation–impregnation and sol–gel methods. Pore size distribution and surface acidity of the catalysts were studied by N2 physisorption and temperature programmed desorption of ammonia (TPD-NH3), respectively. Reduction behaviors of the catalysts were characterized by temperature programmed reduction (TPR) technique. In the catalysts containing 0.6 wt.% Sn, some platinum-modified by tin crystals were produced by association with acid sites of the support, where hydrogen reduction took place above 500 °C. These tin-modified platinum species were found to be favorable for propane dehydrogenation reaction and also to be responsible for the improvement of the activity as tin content increases from 0.3 to 0.6 wt.%. Metal dispersion, pore size distribution and acidity of the support strongly impact the selectivity and stability of the catalysts. The sol–gel catalyst showed better selectivity but lower stability compared to other catalysts that can be explained by its very narrow pore size distribution and relatively stronger acidity as well as more homogeneous metal distribution on the support. When the catalysts were pretreated with oxygen and then hydrogen, their catalytic activities were significantly enhanced largely due to a better metal distribution on the support.