Applied Catalysis A, General (v.232, #1-2)
CONTENTS CONTINUED (N1-N2).
Catalytic activity of palladium-doped iron phosphate in the oxidative dehydrogenation of lactic acid to pyruvic acid by Mamoru Ai (1-6).
The catalytic activity of iron phosphate in the oxidative dehydrogenation of lactic acid increases markedly by doping of a small amount of palladium (Pd) on the surface. At the beginning of the reaction, the selectivity of the Pd-doped catalysts to form pyruvic acid is very low, i.e. the main products are carbon oxides. However, as the time-on-stream increases, the selectivity increases gradually. After about 8 h on stream, the selectivity reaches the level of neat iron phosphate (80 mol%). On the other hand, the activity remains almost unchanged with a large variation in the time-on-stream. For example, the iron phosphate doped with 0.8 wt.% Pd is 10 times more active than the neat iron phosphate.
Keywords: Palladium; Iron phosphate; Lactic acid; Pyruvic acid; Oxidative dehydrogenation;
Formation and evaluation of novel silica supported secondary amines via an effective ‘in-situ’ reduction of silica supported imines by Katherine A. Utting; Duncan J. Macquarrie (7-12).
An effective method for the reduction of silica supported imines has been developed. This represents a useful approach to the selective preparation of silica supported secondary amines. The resultant modifications have produced effective catalysts, which are more hydrolytically stable than the corresponding imines. There is a large spread of different activities in the resulting catalysts, some of which are considerably more active than the simple primary amine, aminopropylsilica (AMPS). The nature of the alkyl group introduced in this technique thus plays a critical role in the catalyst activity. The nitroxyl radical, tetramethylpiperidinyloxy (TEMPO) has also been attached using this methodology, indicating that this approach may be of use in selectively functionalising AMPS leading to a wide range of supported species, bound via a hydrolytically stable linkage.
Keywords: Aminopropylsilica; Silica supported; TEMPO; Borohydride; Base-catalysis;
Ethanol amination catalysis over early transition metal nitrides by M.K. Neylon; S.K. Bej; C.A. Bennett; L.T. Thompson (13-21).
Several high surface area early transition metal nitrides were evaluated for their ability to catalyze ethanol amination. These materials were very active with turnover frequencies comparable to those of supported Pt and Ni catalysts. Their activities ranked in the following order: VN>Mo2N>W2N>TiN>NbN. Like the Pt and Ni catalysts, the nitrides were very selective to the production of ethylamine. In addition, the nitrides demonstrated better selectivities to diethylamine and triethylamine at high conversion. The mechanistic pathways for ethanol amination over the nitride catalysts have been investigated. Ethanol appears to undergo reductive amination to produce ethylamine through an acetaldehyde intermediate. Ethene was produced as a byproduct via ethanol dehydration.
Keywords: Amination; Ethylamine; Nitrides; Transition metals; Deamination;
Low-temperature methane oxidation over oxide-supported Pd catalysts: inhibitory effect of water vapor by Ryuji Kikuchi; Shingo Maeda; Kazunari Sasaki; Stefan Wennerström; Koichi Eguchi (23-28).
The influence of water vapor on the activity for low-temperature methane oxidation over oxide-supported catalysts such as Pd/Al2O3, Pd/SnO2, and Pd/Al2O3-36NiO was studied. It was found that Pd/Al2O3 was deactivated most significantly due to water vapor, and that Pd/Al2O3-36NiO was most insensitive to water vapor. The catalytic activity of Pd/Al2O3 decreased monotonically as water vapor concentration increased, whereas Pd/SnO2 and Pd/Al2O3-36NiO showed almost constant activity under higher water vapor concentrations. The catalytic activity at high steam concentration was in the following order: Pd/SnO2>Pd/Al2O3-36NiO>Pd/Al2O3. Kinetic analysis with methane adsorption as the rate-limiting step was applied to evaluate the water inhibiting effect. Pd/Al2O3 displayed the most negative value of the enthalpy of water adsorption, while Pd/SnO2 and Pd/Al2O3-36NiO exhibited similar water adsorption enthalpy.Deactivation and regeneration of Pd/SnO2 and Pd/Al2O3 catalysts were investigated by cyclic feed of water vapor. Both the catalysts were deactivated rapidly upon switching on water feed, and then they regenerated gradually to the initial activity after the water feed was switched off.
Keywords: Low-temperature oxidation; Pd/Al2O3; Pd/SnO2; Pd/Al2O3-36NiO; Methane combustion; Water inhibition;
In situ magnetic resonance visualisation of the spatial variation of catalytic conversion within a fixed-bed reactor by E.H.L. Yuen; A.J. Sederman; L.F. Gladden (29-38).
We report the first application of magnetic resonance techniques to explore the spatial variation in chemical conversion of a catalysed reaction occurring within a fixed-bed reactor. The reaction studied is the liquid-phase esterification of methanol and acetic acid, catalysed by a proton exchange (Amberlyst 15 ion exchange resin) catalyst. This esterification is considered as a generic liquid-phase reaction, for which the extent of reaction is measured non-invasively, in situ, by monitoring the 1 H chemical shift of the hydroxyl resonance associated with fluid in the inter-particle space of the bed. The paper highlights the application of one-dimensional chemical shift imaging and volume selective magnetic resonance spectroscopy techniques to measure directly and quantitatively the spatial distribution of chemical conversion within a fixed-bed of catalyst particles. In the specific case study considered here it is shown that while conversion within the bed is homogeneous under batch reaction conditions, as expected, when reaction occurs under flowing conditions fractional variations in steady-state conversion of up to ∼20% exist within transverse sections through the bed, perpendicular to the direction of superficial flow. We suggest that magnetic resonance visualisation techniques are now able to provide a quantitative tool to aid in the integrated design of catalyst and reactor.
Keywords: Catalysis; Magnetic resonance imaging; In situ; Chemical reactor;
Steady-state isotopic transient kinetic analysis of the H2/D2 exchange reaction as a tool for characterising the metal phase in supported platinum catalysts by Manuel G. Basallote; Serafı́n Bernal; José M. Gatica; Manuel Pozo (39-50).
The steady-state isotopic transient kinetic analysis (SSITKA) technique has been applied to the characterisation of the metal phase in Pt/CeO2 and Pt/SiO2 catalysts by studying the H2/D2 exchange process. The time scale of the exchange process is close to the delay in the transport of the gases through the reactor. The behaviour of the reactor has been studied by analysing the curves corresponding to blank experiments and it can be satisfactorily modelled with the mathematical equation corresponding to two continuous stirred-tank reactors (CSTRs) in series. The curves in the presence of catalyst can be modelled with a simple mathematical equation that yields the values of the total number of sites active in the isotopic exchange and the apparent first order rate constant for the H2/D2 exchange process. The results so derived are in good agreement with those obtained from the analysis of the TPD experiments following the ITK runs. Although the results suggest a normal equilibrium isotope effect and an inverse kinetic isotope effect, the measured isotope effects are moderate in both cases and include possible contributions from the uncertainties in the calibration of the HD signal.
Keywords: SSITKA; H2/D2 exchange; Pt/CeO2; Pt/SiO2;
Acid soluble oil, by-product formed in isobutane alkylation with alkene in the presence of trifluoro methane sulfonic acid by A.S. Berenblyum; L.V. Ovsyannikova; E.A. Katsman; J. Zavilla; S.I. Hommeltoft; Yu.Z. Karasev (51-58).
The composition of acid soluble oil (ASO) formed during isobutane alkylation with alkene in the presence of trifluoro methane sulfonic acid (TfOH) was studied. Based on the data received, ASO was assumed to be a complicated mixture of unsaturated hydrocarbons with conjugated double bonds. Its main fragment is the substituted five-member cycle of the formula C20H36. ASOs formed (1) by interaction of 2-butene with TfOH; (2) during alkylation in the presence of liquid TfOH; or (3) by degradation of 2,2,4-trimethyl pentane, are practically identical and independent of preparation method.NMR and UV spectroscopy were used to study the interaction of ASO with TfOH. It was demonstrated that in the acid and hydrocarbon phases, a rather strong complex is formed with the composition ASO×2 TfOH. Such an interaction of ASO with TfOH explains the reason for the observed poisoning effect of ASO in the alkylation reaction.
Keywords: Acid soluble oil (ASO); Trifluoro methane sulfonic acid (TfOH); Isobutane alkylation with alkenes; ASO poisoning effect;
On the diffusion of water in silicalite-1: MD simulations using ab initio fitted potential and PFG NMR measurements by C. Bussai; S. Vasenkov; H. Liu; W. Böhlmann; S. Fritzsche; S. Hannongbua; R. Haberlandt; J. Kärger (59-66).
Molecular dynamics simulations of water diffusion in silicalite-1 are reported. The simulations are carried out using an ab initio fitted silicalite-1–water potential based on quantum chemical calculations. In addition, preliminary results of pulsed field gradient (PFG) NMR diffusion measurements of water and small alkane molecules in silicalite-1 samples are presented. Pre-adsorption of water in silicalite-1 samples was found to change the intra-crystalline diffusivities of small alkane molecules in silicalite-1. This is interpreted as an indirect evidence that under our experimental conditions water molecules occupy a significant part of the silicalite-1 channel system. The preliminary results of the PFG NMR diffusion measurements of water in silicalite-1 samples are discussed in terms of the contributions of extra- and intra-crystalline water to the measured signals. An-order-of magnitude agreement between the measured and the simulated intra-crystalline diffusivities of water in silicalite-1 is obtained.
Keywords: Diffusion coefficient; Silicalite-1; Water; Ab initio fitted potential; Molecular dynamics; PFG NMR;
Flexibility of the MCM-41 structure: pore expansion and wall-thickening in MCM-41 derivatives by D. Méhn; Z. Kónya; J. Halász; J.B. Nagy; B. Rác; A. Molnár; I. Kiricsi (67-76).
Utilizing the flexibility of the structure of MCM-41 type mesoporous metal silicates pore size modification was successfully performed for some MCM-41 samples including materials which contain Al or Ti heteroatoms in the wall or organic modifiers covalently bounded to the wall. Changing, i.e. expanding or narrowing the pore diameter was made by post-synthesis treatments. The products of these procedures were characterized by XRD, BET, IR and 29Si MAS NMR techniques. For the expanded samples, a well characteristic thinning while for narrowed ones a definite thickening of pore walls, were established which were also accompanied by changes in the BET areas. The modified MCM-41 specimens retained their semi-crystalline character and the modification did not result in an increase of the concentration of structural imperfections.
Keywords: MCM-41; Pore expansion; Semi-crystalline;
Characterization of tellurium in MoVTeNbO catalysts for propane oxidation or ammoxidation by J.M.M. Millet; H. Roussel; A. Pigamo; J.L. Dubois; J.C. Jumas (77-92).
The oxidation state and local geometry of tellurium in MoVTeNbO catalysts used in the ammoxidation or oxidation of propane were characterized by X-ray absorption, Mössbauer, and X-ray photoelectron (XPS) spectroscopies. The results obtained by Mössbauer, and XPS spectroscopies showed that the catalysts contained Te(IV) in the bulk and mainly Te(VI) at the surface. X-ray absorption fine structure (EXAFS) measurements allowed to determine that the tellurite entities corresponded to TeO4E trigonal bipyramid in the hexagonal phase and to TeO3E somewhat distorted trigonal pyramid in the orthorombic one. The completely determined environment appeared to correspond in both phases to crystallographic sites in hexagonal channels. These results allowed to determine the stoichiometries of the two phases which are TeM3O10 for the hexagonal phase and Te2M20O57 for the orthorombic phase (M=Mo, V, Nb). 125Te Mössbauer isomer shift and pre-peak surface of the X-ray absorption spectra of the Te LIII-edge have been correlated and contributions of the Te(5s) and Te(2p3/2) to the structure have been analyzed.
Keywords: MoVTeNbO catalysts; Propane oxidation; Propane ammoxidation;
Studies on the preparation of supported metal oxide catalysts using JRC-reference catalysts by Atsushi Satsuma; Sakae Takenaka; Tsunehiro Tanaka; Shigeru Nojima; Yoshiya Kera; Hisashi Miyata (93-106).
The effect of preparation conditions on the structure of vanadia–titania catalysts was investigated using two series of 5 wt.%V2O5/TiO2 catalysts prepared by impregnation of ammonium metavanadate using JRC-TIO-3 (rutile) and JRC-TIO-4 (mainly anatase) as supports. The structure and chemical properties of supported vanadia were investigated by FT-IR, Raman, XPS, UV–VIS, NARP, and TPR. The dispersion of surface vanadia was affected by preparation conditions and by the phase of titania, i.e. the catalysts impregnated from oxalic acid solution exhibited higher dispersion of surface vanadia species. The effect of the starting solution was more drastic on the selective oxidation of benzene into maleic anhydride. The production of maleic anhydride was observed in the catalysts prepared from oxalic acid solution of ammonium metavanadate. On the contrary, V2O5/TiO2(rutile) prepared without oxalic acid solution only formed carbon oxides. The correlation between preparation method and catalytic performance in the oxidation reaction was discussed on the basis of the dispersion, surface species, and chemical properties of surface vanadia species.
Keywords: Vanadia–titania; Reference catalysts; Preparation; Support effect; Benzene oxidation;
Selective CO oxidation in rich hydrogen over CuO/samaria-doped ceria by Jenshi B. Wang; Sheng-Chin Lin; Ta-Jen Huang (107-120).
The selective oxidation of CO in a H2-rich environment was studied over copper oxide supported on samaria-doped ceria (SDC). Activity tests, as well as H2- and CO-temperature programmed reduction (CO-TPR), were carried out to seek insight into the effects of metal/support interaction occurring in the reaction. The interfacial active centers at the metal/support interfaces are inferred to be the major active sites of the reaction. By comparing the selective CO oxidation in Ar and in an atmosphere rich in H2, we found that the conversion of CO was inhibited by H2 at temperatures higher than 90 °C. In the H2-rich atmosphere, the oxidation of CO and H2 picked up considerably at 80 and 130 °C, respectively, due to differences in the reactivity of CO and hydrogen toward interfacial oxygen ions. Once bulk copper oxide is reduced by CO or H2, the metastable states formed on the surface of the bulk CuO may serve as active sites, in addition to interfacial active centers, for the selective CO oxidation in excess H2. CO has higher reactivity toward interfacial oxygen ions. Besides the interfacial active centers, the metastable copper clusters may also contribute to the occurrence of hysteresis in H2 oxidation. Selectivity of the CuO/SDC catalyst results mainly from differences in the reactivity of CO and H2 with these oxygen ions. By preparing SDC supports and CuO/SDC catalysts with different surface areas and dispersions, it was found that, although the better-dispersed catalysts exhibited higher CO conversions, more pronounced hydrogen oxidation was observed with these catalysts, thereby resulting in severer decline in selectivity.
Keywords: CO oxidation; Selectivity; Hydrogen-rich environment; CuO catalyst; Samaria-doped ceria;
Influence of lanthanide oxides on the catalytic activity of nickel by Andreea Gluhoi; Petru Mărginean; Dan Lupu; Emil Indrea; Alexandru Radu Biriş (121-128).
The catalytic activity of supported Ni/Ln2O3 (Ln=La, Ce, Gd, Ho, Yb) catalysts for the hydrogen/water isotopic exchange reaction has been studied. The metallic and total surface areas were measured by hydrogen and krypton adsorption, respectively. The Ni surface area evaluated by hydrogen chemisorption is higher than the total BET surface area for Ni/La2O3. The Ni surface area does not change significantly while the total surface area increases with the decreasing Ln3+ ionic radius. The thermodesorption spectra for the adsorbed hydrogen with and without preadsorbed H2O are reported. The mobility of the adsorbed hydrogen decreases from La2O3 to Yb2O3 as support and in the same direction decreases the water uptake normalised to the total surface area. The catalytic activity normalised to the Ni surface area decreases exponentially with the decreasing Ln3+ ionic radii from La2O3 to Yb2O3 as support, in close relationship with the basicity and hydration degree of the support oxide.
Keywords: Hydrogen–water deuterium exchange; Intrinsic activity; Metal-support interaction; Nickel/lanthanide oxides;
Alumina-doped Pt/WO x /ZrO2 catalysts for n-heptane isomerization by Weiming Hua; Jean Sommer (129-135).
n-Heptane isomerization over Al2O3-doped Pt/WO x /ZrO2 (Pt/WZA) in comparison with Pt/WO x /ZrO2 (Pt/WZ) was studied in the presence of H2 at 200 °C. Higher isomerization selectivity was observed for these catalysts. The catalytic activity depends strongly on surface WO x loading and calcination temperature of WO x /Al2O3–ZrO2 (WZA) and WO x /ZrO2 (WZ) supports. The maximum activity for both Pt/WZA and Pt/WZ catalysts occurs at a WO x concentration of 10 wt.% W which is slightly higher than the theoretical monolayer capacities of WZA and WZ supports calcined at 800 °C. Pt/WZA catalyst exhibits the higher maximum activity than Pt/WZ. The loss in activity observed at high H2 pretreatment temperatures appears to be due to the strong interaction between Pt and reduced WO x species.
Keywords: Isomerization of n-heptane; Alumina-doped Pt/WZ; Strong acids; H/D exchange;
Chemical model for dioxin destruction in aqueous medium by G.A. Bogdanovsky; G.L. Vidovich; D.Yu. Kultin; O.K. Lebedeva; A.N. Zakharov (137-145).
Chemical model for destruction of dioxin in aqueous phase was suggested. Eosin being similar to dioxins in structure was chosen as a model substance. Eosin seemed to undergo destruction in electrocatalytic systems. For the first time, the adsorption of dioxin-like pollutant, eosin, was detected on platinum electrode at higher potentials than those of oxygen evolution. The kinetics of electrocatalytic oxidation was studied. A total oxidation of substrate to mineral products (carbonates, bromates) occurred over platinized-platinum catalyst. The scheme for electrocatalytic destruction of eosin was discussed. The similar behavior of dioxin in electrocatalytic systems was considered on the basis of the structural similarity and kinetic relationship in eosin and dioxin oxidation.
Keywords: Electrocatalytic oxidation; Chemical simulation; Destruction; Dioxin-like pollutant; Eosin;
Co/SiO2 catalysts for selective hydrogenation of crotonaldehyde II: influence of the Co surface structure on selectivity by E.L. Rodrigues; J.M.C. Bueno (147-158).
The effect of interaction of silica-supported CoO x on catalytic properties of Co/SiO2 catalysts was investigated. The CoO x precursors supported on silica were obtained by impregnation of the support with cobalt nitrate solution. Cobalt precursors, with different interactions with support, were obtained by changes of impregnation solvent, drying time and temperature; and calcination temperature. The Co surface structure of the catalysts was characterized by diffuse reflectance FTIR spectroscopy (DRIFTS) of adsorbed CO and temperature-programmed desorption of hydrogen (TPD-H2). The TPD-H2 and DRIFT of CO spectra indicate the presence of at least four different Co surface sites, labeled α, β, γ, and σ. The relative amount of these species varied depending on the degree of CoO x interaction with SiO2 in precursors. For crotonaldehyde (CROALD) hydrogenation in gas phase, selectivity to crotyl alcohol and butanol depended strongly on the β/(γ+σ) ratio. Interestingly, selectivity to butyraldehyde does not depend on β, γ, or σ sites present.
Keywords: Co catalysts supported; Crotonaldehyde hydrogenation on supported Co catalysts; Desorption of hydrogen adsorbed on Co catalysts;
Nature of the active site for CO oxidation on highly active Au/γ-Al2O3 by C.K. Costello; M.C. Kung; H.-S. Oh; Y. Wang; H.H. Kung (159-168).
The deactivation and regeneration phenomenon during room temperature CO oxidation was studied over a Au/γ-Al2O3 catalyst, which was as active as the most active supported Au catalysts reported in the literature. The initial rapid loss of activity could be prevented if either hydrogen or water vapor was present in the reaction mixture. Otherwise, it could be recovered by exposure of the deactivated catalyst to either hydrogen or water vapor at room temperature. Thermal treatment above 100 °C in a dry atmosphere also deactivated the catalyst. These results suggested that hydroxyl group, most likely associated with a Au(I) cation, is associated with the active site and support the proposal that the active site is an ensemble of Au+OH− together with Au(0) atoms. The CO oxidation reaction was proposed to proceed via the insertion of CO into the Au+OH− bond to form a hydroxycarbonyl, which is oxidized to a bicarbonate. Decarboxylation of the bicarbonate completes the reaction cycle.
Keywords: Carbon monoxide; Oxidation gold; Active site;
Pt/SO4 2−-ZrO2 catalysts prepared from Pt organometallic compounds by Carlos R. Vera; Carlos L. Pieck; Kiyoyuki Shimizu; Juan C. Yori; José M. Parera (169-180).
New Pt/SO4 2−-ZrO2 catalysts were prepared using precalcined SO4 2−-ZrO2 and organometallic Pt precursors. The objective was to obtain a bifunctional catalyst with improved metal properties, which are mostly suppressed in Pt/SO4 2−-ZrO2 prepared in a standard fashion. The synthesis route used low temperatures to form the Pt particles in order to avoid sulfate decomposition and Pt poisoning. The use of precalcined zirconia decreased ionic diffusion, related to crystallization of the gel and encapsulation of Pt particles. As detected by TPR and cyclohexane dehydrogenation tests, after reducing at 270 °C, Pt on the new catalyst had metallic properties. Its dehydrogenation activity was higher than that of a standard Pt/SO4 2−-ZrO2 catalyst (prepared from H2PtCl6). After activating in air at 300 °C, its activity for n-butane isomerization (300 °C, 1 atm., H2:n-C4=6, WHSV=1 h−1) was low, very likely because of incomplete elimination of adsorbed water. After activation in air at 600 °C, the isomerization activity was almost similar to that of standard Pt/SO4 2−-ZrO2 but the dehydrogenation activity decreased to negligible values. The deleterious interaction between Pt and SO4 2−-ZrO2 that occurred during calcination was possibly related to oxidation of the metal. On the other side, high reduction temperatures did not enhance the metal activity beyond the level obtained after reducing at 270 °C. On the contrary, the higher the reduction temperature, the lower the activity for cyclohexane dehydrogenation that was obtained. This effect was seemingly related to poisoning by sulfur compounds.The new materials could be potentially useful for reacting systems needing a bifunctional catalyst and an acid function like SO4 2−-ZrO2 but their operation is limited if high temperatures in air or H2 are necessary for activation.
Keywords: Organometal; Pt/SO4 2−-ZrO2 catalytic activity; Activation; Sulfur poisoning;
Relations between morphology and catalytic activity of ion exchanger catalysts for synthesis of bisphenol A by Karel Jeřábek; Libuše Hanková; Zdeněk Prokop; Eric G. Lundquist (181-188).
Swollen-state morphology of a series of ion exchanger catalysts was analyzed by inverse steric exclusion chromatography. Correlation of the obtained information with data on activities of the catalysts for synthesis of bisphenol A (BPA) served for quantitative assessment of usefulness of various fractions of polymer mass of the examined ion exchangers for bisphenol A synthesis.
Keywords: Ion exchangers; Catalyst; Morphology; Resin; Bisphenol A;
Oxidative dehydrogenation of isobutane on MCM-41 mesoporous molecular sieves by B. Sulikowski; Z. Olejniczak; E. Włoch; J. Rakoczy; R.X. Valenzuela; V. Cortés Corberán (189-202).
MCM-41 mesoporous molecular sieves containing silicon and vanadium have been prepared by a direct hydrothermal route. Aluminium-containing [Si,Al]-MCM-41 and purely siliceous [Si]-MCM-41 solids were synthesised for comparison purposes. 51V static and magic-angle-spinning NMR studies of [Si,V]-MCM-41 showed one type of VO4 in the as-prepared samples (site I) and two types of monodispersed and distorted to various extent VO4 tetrahedra, chemically bound to the walls of MCM-41 (sites IVa and IVb), in the calcined and rehydrated samples. NMR parameters of vanadium units are given. The catalytic performance of MCM-41 materials was probed in the oxidative dehydrogenation (ODH) of isobutane. The highest selectivities to isobutene (48–59%) were observed for the [Si,V]-MCM-41 materials with Si/V=85 and 30, respectively. On the contrary, a sample containing V2O5 bulk-like species exhibited much lower selectivity to isobutene, especially at the isobutane conversion higher than 5%. The presence of isolated vanadium species, as found by 51V NMR, was responsible for the high olefin selectivity in the ODH of isobutane.
Keywords: Oxidative dehydrogenation; Isobutane; MCM-41; 51V NMR; Vanadium species;
Single step sol–gel made gold on alumina catalyst for selective reduction of NO x under oxidizing conditions: effect of gold precursor and reaction conditions by Erol Seker; Erdogan Gulari (203-217).
We report the effect of gold precursor and reaction conditions on the catalytic activity of gold-alumina catalysts prepared by the sol–gel process. Gold catalysts prepared by the sol–gel method were found to be the most active catalysts ever reported in the literature. We found out that the final activity of Au on alumina was a strong function of the gold loading, the gold precursor and an activation procedure. The Au on alumina catalyst, prepared with 0.8 wt.% gold loading and gold acetate, showed the highest activity. Increasing the feed oxygen concentration had many beneficial effects on the activity and also resulted in the broadening of the temperature window of activity for the 0.8 wt.% Au(acetate) on alumina catalyst. Interestingly, we found out that N2 selectivity of the catalyst was a strong function of water in the feed. The 100% N2 selectivity at peak conversion, obtained under the dry reaction conditions, decreased to ∼58% when there was ∼2% water in the feed.
Keywords: Selective NO reduction; Gold on alumina; Sol–gel preparation; Propene; N2 selectivity; Gold acetate; Hydrogen tetranitratoaurate; Hydrogen tetrachloroaurate;
An EXAFS study of the co-ordination chemistry of hydrogen hexachloroplatinate(IV) by W.A. Spieker; J. Liu; J.T. Miller; A.J. Kropf; J.R. Regalbuto (219-235).
Hydrogen hexachloroplatinate(IV), also called chloroplatinic acid (CPA), is a strong acid that undergoes rapid and extensive hydrolysis. Extended X-ray absorption fine structure (EXAFS) characterization was performed at the Advanced photon source (APS) at Argonne National Laboratory to determine the PtCl and PtO co-ordination chemistry of 200–2000 ppm CPA at pHs of 1.5–12 with different chloride concentrations, light conditions, and time frames. The EXFAS analysis was combined with potentiometric data to postulate the following speciation behavior of the dilute CPA. The initial hydrolysis reaction, aquo ligand exchange of chloride ions, is rapid and reversible, while the latter two reactions, hydroxide ion ligand exchange of chloride and aquo ligands, are relatively slow in acidic solutions but accelerated in the presence of light. Many of the stable Pt complexes in solution are zero valent. High chloride co-ordination is favored at low pH and high chloride concentration. As a result, the [PtCl6]2− species is present only in acidic solutions with a moderate excess of chloride ion or in the neutral solutions in a large excess of chloride ion. Hydroxide ligand formation is favored at low pH and suppressed by chloride ion concentration. As a result, full hydrolysis of CPA by hydroxide ions with precipitation of H2Pt(OH)6 (or Na2Pt(OH)6) is favored only at very low CPA concentrations (ca. 30 ppm).
Keywords: Hydrogen hexachloroplatinate(IV); Chloroplatinic acid; Pt EXAFS; H2PtCl6 hydrolysis; H2PtCl6 photochemical reactions;
Preparation of Pt catalysts supported on activated carbon felts (ACF) by S.R de Miguel; J.I Vilella; E.L Jablonski; O.A Scelza; C Salinas-Martinez de Lecea; A Linares-Solano (237-246).
Activated carbon felts (ACFs) have been used as supports for Pt catalysts. The preparation was carried out by the impregnation method using chloroplatinic acid as metal precursor. The effect of impregnation time and surface chemistry of the support on the catalytic properties and the characteristics of the metallic phase have been investigated. Nitrobenzene (Nbz) hydrogenation in liquid phase at 25 °C and cyclohexane (CH) dehydrogenation in gas phase at 300 °C were used as catalytic tests. The state of platinum in reduced catalysts (at 100 and 350 °C) was studied by TPR and XPS. Oxygen surface groups only produce a slight effect on the catalytic properties. The use of low impregnation times (30 min) during the preparation of Pt/ACF leads to catalysts with Pt mainly deposited in the outer shell of the fibers, while at higher impregnation times, the metallic atoms seem to be deposited inside the pores. Pt(0) species appear in catalysts reduced at 100 °C by effect of the reducing properties of the carbon fiber exhibiting a considerable catalytic activity for Nbz hydrogenation.
Keywords: Activated carbon felts; Metallic supported catalysts; Nitrobenzene hydrogenation; Catalyst characterization;
A new continuous laboratory reactor for the study of catalytic cracking by A. Corma; C. Martı́nez; F.V. Melo; L. Sauvanaud; J.Y. Carriat (247-263).
This article describes a laboratory micro downer (MD) unit, that can be used for fluid cracking (FCC) unit as well as for steam catalytic cracking or selective oxidation. Its performance is compared with that of a fixed bed micro activity test (MAT) and a Davison circulating riser (DCR) riser pilot plant using a commercial FCC catalyst and a standard vacuum gas oil. MD unit simulates a steady state operation similar to FCC riser, catalyst and feedstock being continuously fed and removed during test. This new unit allows a large operation flexibility, being able to work at very low contact times (300 ms) in a wide range of reaction temperatures and catalyst to oil ratios. Important differences are found in product distribution comparing MAT and MD at same reaction temperature and range of conversion. Differences can be explained considering the lower catalyst residence time in the MD unit, leading to lower yields of H2 and dry gas, higher olefinicity in the LPG fraction, and lower amount of coke on the catalyst. In contrast, MD data fit better the yields obtained with DCR pilot plant, especially yields of high interest as isobutene and coke.
Keywords: Catalytic cracking; Micro downer (MD); Micro activity test (MAT); Davison circulating riser (DCR);
The reaction of naphthalene with benzyl chloride over an MCM-41 solid acid catalyst: a kinetic study by Paolo Beltrame; Francesco Demartin; Giovanni Zuretti (265-276).
The reaction of naphthalene with benzyl chloride in cyclohexane at 80 °C was carried out in a slurry batch reactor, over an MCM-41 aluminosilicate catalyst. The concentration of benzyl chloride and of the products, two benzylnaphthalene isomers (M) and a mixture of several dibenzyl derivatives (D), was determined as a function of time, in runs where the initial concentrations of both reactants were varied (C BzCl 0 from 0.03 to 0.05 mol/l; C NA 0 from 0.20 to 0.30 mol/l). Kinetic models were applied in order to interpret the measurements.
Keywords: Benzylnaphthalene; Friedel–Crafts reactions; MCM-41; Reaction kinetics; Solid acid catalysts;
Kinetics of the propane oxidative dehydrogenation on vanadia/titania catalysts from steady-state and transient experiments by R. Grabowski; S. Pietrzyk; J. Słoczyński; F. Genser; K. Wcisło; B. Grzybowska-Świerkosz (277-288).
Oxidative dehydrogenation of propane over V2O5/TiO2 (anatase) catalysts has been studied by steady-state and transient kinetic methods. Steady-state experiments were carried out by measuring conversions and selectivities for various feed compositions including C3H8, O2 and He, and for various contact times, at 225, 250 and 280 °C. Transient tests consisted in submitting a fixed bed of catalyst to a constant flow rate of feed with alternating concentration steps of O2 in He and C3H8 in He, separated with steps of pure He. The influence of reaction temperature and partial pressure of C3H8 on the concentrations of the reaction products (C3H6, CO, CO2, H2O) has been studied in this way in the temperature range 166–446 °C. The results obtained by both methods could be interpreted on the basis of a mechanism in which the C3H6 formation proceeds through a Eley–Rideal sequence of steps, i.e. without participation of the adsorbed propane species. On the basis of the above results, two reaction models have been compared, considering the lattice oxygen as the only oxygen species which participate in the reaction. In the first model, different reaction orders with respect to concentration of the surface oxide species have been considered, for the selective ODH reaction (order=1) and for total combustion reactions (order=2). In the second model, two different types of lattice oxygen were assumed, one of them giving rise to selective oxidation, the other to total combustion products. Both models provided equally good representation of the experimental results, but the first one seems more plausible on the basis of additional arguments. Kinetic constants for both models (activation energies, pre-exponential factors) obtained on the basis of steady-state results are given.
Keywords: Propane oxidative dehydrogenation; Kinetics models; Vanadia-titania catalyst;
Shiego Nishimura: Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis by István Pálinkó (289-290).
AUTHOR INDEX (293-295).
SUBJECT INDEX (297-301).