Applied Catalysis A, General (v.235, #1-2)
New evidences of redox mechanism in n-butane oxidative dehydrogenation over undoped and Cs-doped nickel molybdates by L.M. Madeira; J.M. Herrmann; J. Disdier; M.F. Portela; F.G. Freire (1-10).
Redox-type mechanisms of the n-butane oxidative dehydrogenation (ODH), over unpromoted and 3% cesium-promoted nickel molybdate, are clearly put in evidence by a study of the catalyst’s electrical conductivity. The in situ data, recorded submitting the catalysts to a sequence of gaseous atmospheres (oxygen–butane–oxygen–reaction mixture), also evidences the n-type semiconductor behavior of the nickel molybdate.The above-mentioned mechanism is also confirmed by temperature-programmed reduction (TPR) and catalytic data. The experiments show that cesium inhibits strongly NiMoO4 reduction under the reaction conditions, keeping the catalyst in a higher oxidation state, which can be useful for practical applications.
Keywords: Electrical conductivity; Redox mechanism; n-Butane; Oxidative dehydrogenation; Nickel molybdate; Cesium-doping;
Photocatalytic degradation of aqueous formic acid over the silica composite films based on lacunary Keggin-type polyoxometalates by Danfeng Li; Yihang Guo; Changwen Hu; Li Mao; Enbo Wang (11-20).
The composite films [X n+W11O39](12−n)−/SiO2 (X=Si, Ge, P) (abbreviated XW11/SiO2) were prepared by tetraethoxysilane (TEOS) hydrolysis sol–gel method via a spin-coating technique. Mono-vacant Keggin-type polyoxometalates (POMs) [X n+W11O39](12−n)− were the inorganic precursors used in this method. Formation of the composite films is due to chemical grafting of organic silanol groups to the surface oxygen atoms at the vacant sites of [X n+W11O39](12−n)−, resulting in the saturation of the surface of the lacunary POM. Therefore, a coordination structural model of the films was proposed. As for the films, retention of the primary Keggin structure was confirmed by UV–VIS, FT-IR, and MAS NMR spectra. The surface morphology of the films was characterized by scanning electron microscopy (SEM): the film surface is highly uniform, and the layer thickness is in the range of 250–350 nm. Aqueous formic acid (FA) (0–20 mmol/l) was degraded and mineralized into CO2 and H2O by irradiating the films in the near-UV area. The disappearance of FA follows Langmuir–Hinshelwood first-order kinetics.
Keywords: Polyoxometalates; Sol–gel; Spin-coating; Composition film; Photocatalysis; Formic acid;
Selective hydrogenation of phenol to cyclohexanone over supported Pd and Pd-Ca catalysts: an investigation on the influence of different supports and Pd precursors by Salvatore Scirè; Simona Minicò; Carmelo Crisafulli (21-31).
The gas-phase selective hydrogenation of phenol to cyclohexanone has been investigated over supported Pd catalysts in order to clarify the role of the support, the Pd precursor and the addition of Ca in controlling activity, selectivity and stability of the catalytic system. The catalytic results showed that over monometallic Pd catalysts prepared by PdCl2 as precursor the order of activity and selectivity to cyclohexanone was the following: Pd/La2O3>Pd/CeO2>Pd/Al2O3. The same order of activity was observed over ex-Pd(CH3COO)2 catalysts. However, over these latter samples the selectivity to cyclohexanone was very high regardless of the support used. Addition of calcium strongly improved the catalytic performance of both ex-chloride and ex-acetate Pd catalysts supported on alumina, whereas no significant improvement was observed on Pd catalysts supported on CeO2 and La2O3.On the basis of characterization data (CO chemisorption, TPD and FT-IR) reported in this paper it has been suggested that the acid–base properties of supported Pd catalysts strongly influence the adsorption–desorption equilibrium of the reactant and products, being responsible for directing the selectivity to the reaction products. The catalytic activity of the system appears to be, instead, mainly controlled by the palladium sites on which hydrogen is activated.
Keywords: Phenol hydrogenation; Cyclohexanone; Palladium; Alumina; Ceria; Lanthana; Precursor; Acidity; FT-IR;
A new chromium-based catalyst coated with paraffin for ethylene oligomerization and the effect of chromium state on oligomerization selectivity by Yiqun Fang; Yuanxia Liu; Yucai Ke; Cunyue Guo; Ning Zhu; Xia Mi; Zhi Ma; Youliang Hu (33-38).
A new method is proposed in which a chromium-based catalyst system containing chromium(III) tris (2-ethylhexanoate) (Cr(EH)3), 2,5-dimethylpyrrole (2,5-DMP), GeCl4 and triethylaluminum (TEA) coated with solid paraffin is used for oligomerization of ethylene. GC-MS and 1 H -NMR analyses show that the coated catalyst system prepared via this approach is air-insensitive so it can be stored under ambient conditions. The selectivity for 1-hexene is up to 97.91 wt.%, and only slightly decreases when the catalyst system is exposed in air for 24 h; in contrast, the selectivity is only 35.6% for its non-coated counterpart. XPS reveals that an attachment of atmospheric oxygen to catalyst makes the chemical state of chromium change from Cr3+ to Cr6+. Such a change, in turn, reduces remarkably its selectivity for 1-hexene.
Keywords: Chromium-based catalysts; Paraffin (alkanes); Ethylene oligomerization; Trimerization; Hexene;
The partial oxidation of methane to syngas over the nickel-modified hexaaluminate catalysts BaNi y Al12−y O19−δ by Wenling Chu; Weishen Yang; Liwu Lin (39-45).
A series of Ni-modified hexaaluminates BaNi y Al12−y O19−δ (y=0.3, 0.6, 0.9, and 1.0) were prepared by decomposition of nitrates and calcination at high temperature. The crystalline structure and catalytic properties for partial oxidation of methane to synthesis gas were investigated by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and TG techniques. Over all compositions of the systems, the reduced Ni-modified hexaaluminates BaNi y Al12−y O19−δ exhibit significant catalytic activity and stability for the partial oxidation of methane to syngas, with 92% of CH4 conversion and 95% CO selectivity at 850 °C. After 100 h of time on stream, no loss of nickel was detected and carbon deposition is nearly negligible. Both advantageous results were attributed to the strong interaction between nickel and its neighboring atoms in hexaaluminates.
Keywords: Hexaaluminate BaNi y Al12−y O19−δ ; Catalyst; Partial oxidation of methane; Synthesis gas;
Experimental evidence for hydrogen spillover during hydrocracking in a membrane reactor by Negar Izadi Najafabadi; Goutam Chattopadhyaya; Kevin J Smith (47-60).
The significance of hydrogen transport by spillover during diphenylmethane (DPM) hydrocracking in a ceramic membrane reactor has been investigated. In the first part of the study, hydrogen spillover across Al2O3 particles of a ceramic membrane was confirmed by the temperature-programmed hydrogenation (TPH) of coke deposited on SiO2–Al2O3, and placed at the bottom of a layered bed of Co-SiO2 catalyst and Al2O3 membrane particles. CH4 production in the temperature range 430–450 °C was shown to be due to hydrogen activation on the catalyst, spillover across the Al2O3 membrane particles and subsequent reaction with the coked SiO2–Al2O3.The second part of the study focused on the hydrocracking of DPM in a membrane reactor configured so that the catalyst and DPM were separated by a ceramic membrane. Conversion of DPM at temperatures <430 °C was indicative of the reaction between DPM and hydrogen, the latter activated on the catalyst and transferred across the membrane to react with the DPM. Experiments showed that despite the separation of reactant and catalyst, the presence of catalyst enhanced the yield of benzene and toluene compared to thermal cracking at the same reaction conditions. Although contact between liquid and catalyst could not be completely eliminated by the porous ceramic membrane, the yield of benzene and toluene could only be accounted for by the spillover of hydrogen from the catalyst to the membrane, followed by reaction with DPM on the shell-side of the reactor.
Keywords: Hydrocracking; Membrane reactor; Hydrogen spillover;
Studies on sulfate formation during the conversion of H2S and SO2 to sulfur over activated alumina by P.D Clark; N.I Dowling; M Huang; O Okemona; G.D Butlin; R Hou; W.S Kijlstra (61-69).
A mechanism for the conversion of H2S and SO2 over alumina is proposed in which surface thiosulfate is formed and reacts with SO2 and H2S via a series of oxy-anions to form sulfur and H2O. In addition to thiosulfate being a key species of the catalytic cycle, it is proposed that sulfate is also formed as part of the catalytic cycle but that its surface concentration is limited to some steady-state concentration by virtue of its reduction by H2S. The initial formation of thiosulfate and sulfate on alumina probably occurs by two mechanisms. One pathway involves interaction of either H2S or SO2 with oxide sites on the alumina surface. Such oxide sites have been suggested by Sohlberg et al. to arise as a consequence of the interaction of vacant sites in the defect spinel structure of alumina with H2O [J. Phys. Chem. 100 (1996) 7550]. The other pathway, the first step of which has been referred to numerous times in the literature, likely involves the adsorption of SO2 at Lewis acid–base sites, and reaction with H2S to form sulfur oxy-anions and eventually, sulfur and water.
Keywords: Claus; H2S/SO2 conversion; Sulfate formation;
Sol–gel synthesis of titania–alumina catalyst supports by E.Y Kaneko; S.H Pulcinelli; V Teixeira da Silva; C.V Santilli (71-78).
Titanium oxide is a good candidate as new support for hydrotreating (HDT) catalysts, but has the inconvenience of presenting small surface area and poor thermal stability. To overcome these handicaps TiO2–Al2O3 mixed oxides were proposed as catalyst support. Here, the results concerning the preparation, characterization and testing of molybdenum catalyst supported on titania–alumina are presented. The support was prepared by sol–gel route using titanium and aluminum isopropoxides, chelated with acetylacetone (acac) to promote similar hydrolysis ratio for both the alcoxides. The effect of nominal complexing ratios [acac]/[Ti] and of sol aging temperature on the structural features of nanometric particles was analyzed by quasi-elastic light scattering (QELS) and N2 adsorption isotherm measurements. These characterizations have shown that the addition of acac and the increase of aging temperature favor the full dispersion of primary nanoparticles in mother acid solution. The dried powder presents a monomodal distribution of slit-shaped micropores, formed by irregular packing of platelet primary particles, surface area superior to 200 m2 g−1 and mean pore size of about 1 nm. These characteristics of porous texture are preserved after firing at 673 K. The diffraction patterns of sample fired above 973 K show only the presence of anatase crystalline phase. The crystalline structure of the support remained unaltered after molybdenum adsorption, but the surface area and the micropore volume were drastically reduced.
Keywords: Sol–gel process; Molybdenum catalysts; Thiophene hydrodesulfurization;
Surface properties and catalytic performance in CO oxidation of cerium substituted lanthanum–manganese oxides by Y Zhang-Steenwinkel; J Beckers; A Bliek (79-92).
Perovskite type oxides may potentially replace noble metal catalysts as full oxidation catalysts of hydrocarbons, as present in combustion exhaust streams, due to their high activity and fair hydrothermal stability. Presently we have investigated the catalytic activities of a series of Ce substituted La–Mn perovskites, prepared by coprecipitation, in the oxidation of carbon monoxide. The composition, bulk structure and the surface properties were established using elemental analysis, XRD and XPS. The properties tested include the reducibility and reoxidation behavior, the thermal stability of these perovskites and the oxidation activity. It was observed that Ce is not totally incorporated in the perovskite lattice. For a high degree of substitution, an excess Ce forms a separate CeO2 phase. Simultaneously, an increase in the atomic Mn4+/Mn3+ ratio and a decreasing surface oxygen concentration are observed. This suggests that cation vacancies are created at A (La) sites, resulting in the formation of unsaturated Mn (B) site ions on the surface. The catalytic activity of the La–Mn perovskites systematically changes as a function of the degree of Ce substitution. A plausible explanation for this behavior is the nonstoichiometry induced by Ce substitution, which results in the formation of cation/anion vacancies near the surface. The specific surface area and the atomic surface composition play a less pronounced role in the catalytic activity.
Keywords: Perovskites; La–Ce–Mn; Surface properties; Oxidation catalysis; CO oxidation;
Evolution of coke composition during ageing under nitrogen by P Magnoux; H.S Cerqueira; M Guisnet (93-99).
The ageing under nitrogen flow at 250–450 °C of coke formed during different model molecules (m-xylene, n-heptane, propene) over a USHY zeolite is discussed. This ageing treatment causes a decrease in the zeolite coke content (essentially for low reaction temperature) and an increase in the size and aromaticity of coke molecules. Pyridine adsorption followed by IR spectroscopy, carried out on the coked samples before and after ageing, shows that ageing causes a significant decrease in the concentrations of protonic and Lewis acid sites able to retain pyridine adsorbed at 150 °C. The deactivating effect of coke is also more pronounced after than before ageing. A change in the mode of deactivation from site poisoning to pore blockage is proposed to explain these observations. The same trends were observed for ageing a commercial coked catalyst, sampled from the fluid catalytic cracking (FCC) unit.
Keywords: Deactivation; Coking; Ageing under nitrogen; Fluid catalytic cracking unit; USHY zeolite;
Isothermal kinetic measurements for hydrogen production from hydrocarbon fuels using a novel kinetic reactor concept by S. Springmann; G. Friedrich; M. Himmen; M. Sommer; G. Eigenberger (101-111).
The development of efficient reformers for the on-board-production of hydrogen from liquid hydrocarbon fuels like gasoline is one of the challenges for the implementation of fuel cell propulsion systems for automobiles. It requires detailed kinetic information about the reforming reactions of the main gasoline components. Such information is presently only scarcely available, mainly because of the unusual high temperature and short contact time reaction conditions. Therefore, a novel isothermal reactor for kinetic measurement of heterogeneous catalytic reforming reactions under actual operation conditions (T=800 °C, p=5 bar) has been developed. Different hydrocarbon fuels (iso-octane, hexene, toluene, gasoline) were studied under steam reforming (SR) and autothermal reforming (ATR) conditions using a proprietary noble metal catalyst supported on metallic monolith structures. On the basis of these results different reaction pathways for SR and ATR are discussed. The results obtained will be used for design and optimization of mobile autothermal reformers.
Keywords: Hydrogen; Gasoline; Reforming; Kinetic reactor;
Effect of synthesis time and mode of stirring on physico-chemical and catalytic properties of ZSM-5 zeolite catalysts by N Kumar; V Nieminen; K Demirkan; T Salmi; D Yu. Murzin; E Laine (113-123).
The H-ZSM-5 zeolite catalysts were synthesized by varying the synthesis time and mode of stirring. The catalysts were characterized using X-ray powder diffraction, scanning electron microscope, X-ray fluorescency, nitrogen adsorption and FTIR of adsorbed pyridine. It was found that the synthesis time and mode of stirring influenced the morphology, phase purity, surface area, acidic and catalytic properties of H-ZSM-5. The XRD patterns of Na-ZSM-5 zeolite synthesized in 3 and 6 h exhibited the presence of amorphous phase. The crystal size of ZSM-5 increased with increase in synthesis time. The FTIR spectra of adsorbed pyridine on H-ZSM-5 catalysts synthesized in 72, 24 and 12 h showed the presence of Brønsted and Lewis acid sites. The H-ZSM-5 catalyst synthesized in 72 h exhibited highest number of Brønsted acid sites. The reaction of n-butane to iso-butane was used as a test reaction to investigate the catalytic property of the synthesized H-ZSM-5 catalysts. The effect of time on stream, temperature and weight hour space velocity (WHSV) on the conversion of n-butane and yield of iso-butane over the H-ZSM-5 catalysts were studied. The H-ZSM-5 catalyst synthesized in 72 h in the static mode showed the highest conversion of n-butane and yield of iso-butane.
Keywords: ZSM-5; Synthesis time; Mode of stirring; Physico-chemical; Catalytic properties; iso-Butane;
Continuous hydrogenation of 1-phenyl-1,2-propanedione under transient and steady-state conditions: regioselectivity, enantioselectivity and catalyst deactivation by E. Toukoniitty; P. Mäki-Arvela; A. Kalantar Neyestanaki; T. Salmi; D.Yu. Murzin (125-138).
Continuous enantioselective hydrogenation of 1-phenyl-1,2-propanedione was investigated at 25 °C and 5 bar H2 over a knitted Pt silica fiber catalyst modified with cinchonidine. The main reaction parameters were the reactant and modifier concentrations, the liquid space time and the pre-treatment of the catalyst. The initial hydrogen uptake rate decreased about 25–45% within 20 min time-on-stream. The catalyst deactivation was quantitatively described with a mechanistic model, which assumed that the deactivation is of first-order in reactant concentration. The amount of modifier did not affect the reaction rate. The steady-state regioselectivity of 1-hydroxy-1-phenylpropanone and enantioselectivity of the main product, (R)-1-hydroxy-1-phenylpropanone, were independent of reactant inlet concentration and space time. The enantioselectivity of the main product, (R)-1-hydroxy-1-phenylpropanone, increased from zero to the highest enantiomeric excess (ee=60%) with increasing time-on-stream, after which it attained a steady-state value, dependent on the modifier concentration. Continuous feeding of the modifier was necessary to maintain a constant ee.
Keywords: Continuous liquid-phase hydrogenation; Dione; Fibre catalyst; Deactivation;
Pt-Cr/ZSM-5 catalysts for propane and cyclohexane conversions by Lucia R. Raddi de Araujo; Martin Schmal (139-147).
The effect of chromium on the structural and catalytic properties was investigated for Pt-Cr/ZSM-5 system. From TPR measurements it is seen that the reduction profiles were modified by the addition of Cr to the Pt/ZSM-5. Chemissorption data showed that Cr promoted the Pt dispersion. The propane and cyclohexane reactions showed higher activity and aromatic selectivity for the bimetallic samples when compared to the single Pt/ZSM-5 system. Unlike the Pt/ZSM-5 catalyst at higher cyclohexane conversions the hydrogenolysis did not occur on the bimetallic system, and this behavior increases the efficiency by eliminating undesirable parallel reactions.The Pt–Cr interaction seems to be favored and not only prevents the migration of Pt particles to the external zeolite surface but also weakens the Pt interaction with reactant intermediates, inhibiting hydrogenolysis
Keywords: Propane conversion; Pt-Cr catalyst;
Electrooxidation of methanol over a membrane-based electrode and effect of tungsten and molybdenum on the activity by Shijun Liao; Vladimir Linkov; Leslie Petrik (149-155).
A new type of electrode, in which porous ceramic membranes are used as substrate, has been prepared by electrodepositing the active components on a carbonized or Au sputtered membrane. The electrocatalytic oxidation of methanol on the electrodes was investigated. The membrane-based electrode showed very good activity, which was much better than a platinum-based electrode. Tungsten and molybdenum were very effective promoters; the activity of a tungsten-containing electrode was three times higher than that of an electrode without tungsten. However, ruthenium showed a negative effect on the activity of electrodes whilst tungsten was used as a promoter.
Keywords: Electrooxidation; Methanol; Membrane-based electrode; Tungsten; Molybdenum;
Modification of surface activity of Cu-based amorphous alloys by chemical processes of metal degradation by M Janik-Czachor; A Szummer; J Bukowska; A Molnar; P Mack; S.M Filipek; P Kedzierzawski; A Kudelski; M Pisarek; M Dolata; M Varga (157-170).
Changes in morphology, local chemical composition, catalytic activity and local surface-enhanced Raman scattering (SERS) activity due to various methods of modification of surfaces of Cu–Zr, Cu–Hf and Cu–Ti amorphous alloys were investigated. Our recent investigations have shown that otherwise detrimental processes of material degradation may help to segregate clusters of Cu and to develop a large specific surface area of Cu on ZrO x or HfO x supports. This transforms the original Cu-based amorphous ribbons of low surface area into efficient and stable catalysts. The following methods of activation based on the degradation processes, and their combinations are presented: ageing/oxidation in air, anodic activation, and pre-treatment with hydrogen (hydrogenation under high hydrogen pressure, or cathodic hydrogen charging). Structural changes in the bulk of the amorphous alloys introduced by the above pre-treatments were examined by XRD. Morphological and chemical changes on the surface were followed microscopically (optical microscopy and high resolution SEM) and by using a high resolution scanning Auger microprobe (SAM) and an X-ray electron microprobe. The amount of segregated Cu on the surface was estimated by an electrochemical method. A test reaction of dehydrogenation of 2-propanol was used to compare the catalytic activity of the materials after different chemical pre-treatments. Proposed mechanisms of changes in the local adsorption properties, local enhancement of Raman spectra and the catalytic activities due to the above combined pre-treatments are discussed.
Keywords: Surface-enhanced Raman scattering; Cu-based catalysts; Dehydrogenation of alcohols; Devitrification; Surface analysis; SEM/SAM investigations;
Structural characterisation of silica supported CoMo catalysts by UV Raman spectroscopy, XPS and X-ray diffraction techniques by V La Parola; G Deganello; C.R Tewell; A.M Venezia (171-180).
The structure of Co-Mo catalysts supported on commercial silica, doped with various amounts of sodium ions, was investigated by means of X-ray diffraction (XRD), UV-Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Two series of samples were considered. One series was prepared by classic incipient wet impregnation (WI) and the other series by co-impregnation in the presence of nitrilotriacetic acid (NTA). The effect of sodium on the catalyst structure depended on the preparation procedure: in the case of the WI catalysts, sodium promoted the transformation of the polymolybdate species into monomolybdate Na2MoO4; in the case of the NTA prepared samples, sodium induced transformation from heptamolybdates, Mo2O7 2− and mixed CoMo oxides into MoO4 2− units with distorted tetrahedral symmetry without forming Na2MoO4 compound. In addition to purely structural modifications, sodium induced an electronic effect, evidenced by Mo 3d and Co 2p XPS binding energy shifts. The catalytic behaviour of the samples, previously tested in the HDS of thiophene, was explained in terms of structural and electronic changes.
Keywords: CoMo/silica; HDS; Sodium; Raman; XPS;
Oxidative dehydrogenation and ammoxidation of ethane and propane over pentasil ring Co-zeolites by Roman Bulánek; Kateřina Novoveská; Blanka Wichterlová (181-191).
The activity of Co-zeolites with MFI, BEA, FER, MOR topologies was studied in ethane and propane oxidative dehydrogenation and ammoxidation in a through-flow arrangement and temperature range 350–500 °C. The products of oxidative dehydrogenation of paraffins were corresponding olefins, CO and CO2, in the propane reaction only a small part was cracked to ethylene and methane. Besides CO and CO2, the ammoxidation of ethane yielded ethylene and acetonitrile, and of propane brought propylene and acetonitrile too. Presence of ammonia increased substantially the paraffin conversion as well as selectivity and yield of the sum of olefin and nitrile for CoH-BEA and CoH-MFI zeolites, while with CoNa-zeolites the paraffin conversion and selectivity were similar. The sequence in activity in both the oxidative dehydrogenation and ammoxidation (in TOF values per Co ion in conversion of paraffins and yields of olefin and olefin+acetonitrile) of Co-zeolites (more pronounced for ammoxidation) Co-BEA>CoMFI⪢Co-MOR>Co-FER is ascribed to the location of prevailing amount of Co ions in β cationic position of pentasil ring zeolites, i.e. at the exposed site at the intersection of straight and sinusoidal channel in ZSM-5, and in open channels of beta-zeolite, in contrast to Co-β ions in MOR and FER structures occurring in the narrow interconnected channels and in eight-member ring channel, respectively. Formation of acetonitrile at propane ammoxidation can be attributed to interaction of ammonia with double bond of propylene at β position, as suggested in literature, as well as to the observed easy decomposition of acrylonitrile to acetonitrile in the presence of ammonia.
Keywords: Ethane; Propane; Ammoxidation; Acetonitrile; Co-zeolites; Cobalt; Co-β; Co-ZSM-5;
Preparation, characterization and application of a catalytic NaA membrane for CH4/CO2 reforming to syngas by B.S Liu; L.Z Gao; C.T Au (193-206).
The NaA and La2NiO4/NaA composite membranes were prepared by means of in situ hydrothermal synthesis. The structure, morphology, separation performance, as well as chemical state and surface composition of a La2NiO4/NaA composite membrane were investigated by means of techniques, such as XRD, SEM-EDX, GC, TPD, XPS, and XAES. When the membrane was applied in a membrane reactor for CH4/CO2 reforming, CH4 and CO2 conversions, and CO and H2 selectivities were significantly higher than the values observed over a traditional fixed-bed reactor. We studied the effects of the flow rate of sweeping N2, gas hour space velocity (GHSV), and reaction temperature on the conversions of CH4 and CO2. The carbon deposited on the surface of membrane was studied by means of the TPO–MS technique.
Keywords: La2NiO4/NaA composite membrane; Membrane reactor; CH4/CO2 reforming; NaA synthesis;
Benzylation and benzoylation of substituted benzenes over solid catalysts containing Ga- and Mg-oxides and/or chlorides derived from Ga–Mg-hydrotalcite by its HCl pre-treatment or calcination by Vasant R Choudhary; Suman K Jana; Vijay S Narkhede (207-215).
Influence of HCl pre-treatment conditions (temperature: 26 or 110 °C and period: 0.1, 0.5 or 1.0 h) and calcination temperature (300, 600 and 800 °C) of Ga–Mg-hydrotalcite (Mg/Ga=3.0) anionic clay on its catalytic activity in the benzylation (by benzyl chloride) or benzoylation (by benzoyl chloride) of toluene, p-xylene, mesitylene and anisole at 110 °C has been investigated. The hydrotalcite before and after its HCl pre-treatment or calcination is characterized by measuring its surface area and crystalline phases. Its catalytic activity in the benzylation reactions is increased very markedly by the HCl pre-treatment, depending upon the severity of the pre-treatment. It showed no activity in the benzoylation of substituted benzenes (for 3.0 h) but, after its pre-treatment by HCl gas (at 110 °C for 1.0 h), it showed high activity for the benzoylation reactions. Its catalytic activity in the benzylation of toluene is also increased very markedly after its calcination (at 300–800 °C): the higher the calcination temperature, the higher is the catalytic activity. The induction period for the benzylation reactions is drastically reduced after the HCl pre-treatment or calcination of the hydrotalcite. The catalyst activation by the HCl pre-treatment is attributed to the formation of catalytically active mixed GaCl3 and MgCl2 species dispersed on MgO and Ga2O3. The catalyst activation by the calcination at the lower temperature (300 °C) is attributed mostly to the dehydration of the hydrotalcite, causing an increase in the surface area. However, that at the higher temperatures is attributed to both the dehydration and the decomposition of the hydrotalcite, forming metal oxides (at 600 °C) or a mixture of MgO and MgGa2O4 (at 800 °C), with a large increase in the surface area of the catalyst.
Keywords: Ga–Mg-hydrotalcite; Activation of Ga–Mg-hydrotalcite by HCl pre-treatment; Activation of Ga–Mg-hydrotalcite by calcination; Benzylation of substituted benzenes; Benzoylation of substituted benzenes;
Synthesis of highly dispersed zirconia-supported iron-based catalysts for Fischer–Tropsch synthesis by F.R van den Berg; M.W.J Crajé; P.J Kooyman; A.M van der Kraan; J.W Geus (217-224).
Zirconia-supported iron-based Fischer–Tropsch catalysts were prepared using incipient wetness impregnation. The choice of the precursor, in this case the chelating ammonium iron(III) citrate, and the applied calcination temperature determine the final distribution of the precursor on the zirconia support. Several techniques reveal that both an unpromoted and a potassium-promoted catalyst can be prepared, of which the iron(III) oxide exhibits a high dispersion and is, moreover, monodisperse.
Keywords: Fischer–Tropsch catalysts; Iron based; Potassium-promoted; Zirconia-supported; Calcination temperature;
Effect of catalyst and substituents on the hydrogenation of chloronitrobenzenes by V. Kratky; M. Kralik; M. Mecarova; M. Stolcova; L. Zalibera; M. Hronec (225-231).
The liquid phase hydrogenation of chloronitrobenzene isomers (x-CNB, x=2, 3, 4) to corresponding chloroanilines (x-CAN) at 0.5 MPa and 25 °C was studied. Palladium catalysts containing 1 wt.% of Pd on charcoal and sulphonated poly(styrene-co-divinylbenzene) Pd/D were used. At stoichiometric consumption of hydrogen (3 mol of H2/mol of x-CNB) 85 and 90% selectivities were obtained with Pd/C and Pd/D catalysts, respectively. Mathematical treatment of kinetic data revealed the dependence of reaction mechanism on the type of catalyst. A simultaneous hydrogenation of the nitro group and hydrogenolysis of chlorine in x-CNB was observed for the reaction over the Pd/C catalyst. However, over the Pd/D catalyst hydrogenolysis of chlorine is a subsequent reaction. The influence of substituents and sterical hindrance on the substrate reactivity is also discussed.
Keywords: Hydrogenation; Liquid phase; Chloronitrobenzenes; Dechlorination; Palladium catalysts; Substrate reactivity;
Mo and W heteropolyacid based catalysts applied to the preparation of flavones and substituted chromones by cyclocondensation of o-hydroxyphenyl aryl 1,3-propanediones by P Vázquez; L Pizzio; G Romanelli; J Autino; C Cáceres; M Blanco (233-240).
Cyclization reaction of substituted 1-(2-hydroxyphenyl)-3-aryl-1,3-propanediones to obtain flavones and 2-arylchromones was studied at 90 °C. Tungstophosphoric and molybdophosphoric acids were used as catalysts, both bulk or supported on silica by means of pore filling impregnation. These catalysts were washed with glacial acetic acid before being used. The characterization of the catalysts by FT-IR, DRS and XRD showed that the heteropolyacids present undegraded Keggin structure. Catalyst acidity was measured by means of potentiometric titration with n-butylamine. The washed catalysts present a higher number of acidic sites and a higher acidic strength than that of the support. The acidity of the supported catalysts based on molybdophosphoric acid was lower than that of those based on tungstophosphoric acid. Conversion to flavones and substituted chromones at different reaction times and molar ratios of heteropolyacid to initial ketone was evaluated. The desired products were obtained in all cases with high conversion and high selectivity, the conversion being higher for the bulk catalysts.
Keywords: Keggin heteropolyacid; Catalysts; Flavones; Chromones; Cyclocondensation;
Kinetics of selective catalytic reduction of NO with NH3 on Fe-ZSM-5 catalyst by H.Y. Huang; R.Q. Long; R.T. Yang (241-251).
Fe-exchanged ZSM-5 has been found previously to be much more active than commercial vanadia-based catalysts for selective catalytic reduction (SCR) of NO with NH3. The kinetics of the SCR reaction is studied in this work. The results show that the rate of NO conversion on Fe-ZSM-5 is first-order with respect to NO, zeroth-order w.r.t. NH3 and nearly half-order w.r.t. O2, at 260–300 °C. This is in good agreement with our previous FTIR result that the catalyst surface is nearly completely covered by ammonia adsorbed species (i.e. NH4 + ions) under reaction conditions. The present results further support that the SCR mechanism on Fe-ZSM-5 involves NO2 as an intermediate and the formation of NO2 from NO oxidation (NO+(1/2)O2→NO2) is probably the rate-determining step. The apparent activation energy for the reaction is found to be 54 kJ/mol.
Keywords: Fe-exchanged ZSM-5; Selective catalytic reduction; SCR of NO with ammonia; Kinetics; Activation energy;
On the role of ruthenium associated with Pd and/or Bi in carbon-supported catalysts for the partial oxidation of glucose by S Hermans; M Devillers (253-264).
The role of ruthenium when associated with Pd and/or Bi in carbon-supported catalysts for the selective oxidation of d-glucose in aqueous phase was investigated. It was found that ruthenium does not have an activity by itself, but acts as a “promoter” for the catalytic activity of monometallic Pd/C catalysts. Catalysts containing bismuth in association with ruthenium displayed a catalytic activity only when the temperature was raised above 60 °C, but to produce essentially glucuronic and oxalic acid. When incorporated into bimetallic Pd-Bi/C catalysts, Ru was shown to exert a slightly inhibitory effect on the production of gluconic acid. In contrast with Bi, which was found to leach as in previous studies, no significant Ru leaching was observed under the reaction conditions.
Keywords: Glucose oxidation; Ruthenium; Palladium; Bismuth; Bimetallic catalysts;
Light alkanes aromatization to BTX over Zn-ZSM-5 catalysts by Louis M. Lubango; Mike S. Scurrell (265-272).
The selectivity obtained in the aromatization of propane over Zn-ZSM-5 based catalysts (Si/Al nominal ratio=35) increased from about 75–80% (expressed on a carbon basis) to over 91% in the case of catalysts where iron (or other second transition metal) ions have been separately introduced by means of solid state ion exchange (SSIE) procedures. The observed improvements are not seen for all catalyst compositions. The ideal zinc and iron contents are 0.5 and 1.0 mass%, respectively, corresponding closely in total to the expected ion-exchange capacity of the parent ZSM-5. The introduction of iron cations is associated with the reappearance of substantial Bronsted acidity (present in the parent H-ZSM-5 but lost as a result of incorporation of Zn by SSIE as evidenced by the restoration of high temperature peak for ammonia as observed during temperature programmed desorption. Attempts are made to understand the origin of the improved BTX selectivities seen in the presence of ions of a second transition metal such as iron, chromium or manganese.
Keywords: Zn-ZSM-5 catalysts; Solid state ion exchange; BTX selectivities; Aromatization;
Modified hydroxyapatite with sodium nitrate: an efficient new solid catalyst for the Claisen–Schmidt condensation by Saı̈d Sebti; Abderrahim Solhy; Rachid Tahir; Abdellatif Smahi (273-281).
A simple and convenient synthesis of chalcones has been accomplished by heterogeneous catalysis of the Claisen–Schmidt condensation using a small amount of sodium nitrate/hydroxyapatite. The reaction was carried out under very mild conditions at room temperature. The addition of a quaternary ammonium salt enhanced the rate and the yield of the reaction. NaNO3/hydroxyapatite can be easily regenerated and reused. Several chalcones have been synthesised with very high yields.
Keywords: Hydroxyapatite; Sodium nitrate; Heterogeneous catalysis; Claisen–Schmidt condensation; Chalcone;
AUTHOR INDEX (285-287).
SUBJECT INDEX (289-293).
INSTRUCTIONS TO AUTHORS (295-299).
CONTENTS CONTINUED (300-301).