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Applied Catalysis A, General (v.287, #2)

Inside front cover - Editorial Board (pp. co2).

A stable, novel catalyst improves hydrogen production in a membrane reactor by S. Irusta; J. M�nera; C. Carrara; E.A. Lombardo; L.M. Cornaglia (pp. 147-158).

The dry reforming of methane as a source of H₂ was performed using a well-known catalyst, Rh/La₂O₃, together with a novel one, Rh/La₂O₃-SiO₂, in a hydrogen-permeable membrane reactor. The catalysts were characterized by XRD, TPR, FTIR, H₂ and CO chemisorption. In all lanthanum-based catalysts, the activity remained constant after 100h on stream at 823K. The basis of their high stability could be traced back to the strong metal-support interaction (TPR) in Rh/La₂O₃ catalysts. The La₂O₃-SiO₂ solids are also stable even though a weaker rhodium–lanthanum interaction (TPR) can be observed. The incorporation of the promoter (La₂O₃) to the silica support induces a parallel increase in the metal dispersion (CO adsorption). The effect of the operation variables upon the performance of the membrane reactor was also studied. The novel Rh (0.6%)/La₂O₃ (27%)-SiO₂ catalyst proved to be the best formulation. Operating the membrane reactor at 823K, both methane and CO₂ conversions were 40% higher than the equilibrium values, producing 0.5mol H₂/mol CH₄. This catalyst, tested at W/ F three times lower than Rh (0.6%)/La₂O₃, showed a similar performance. Both the increase of the sweep gas flow rate and the decrease of the permeation area significantly affected methane conversion and H₂ production. The presence of tiny amounts of graphite only detectable through LRS did not endanger membrane stability. The better performance of Rh (0.6%)/La₂O₃ (27%)-SiO₂ is related to the high dispersion.

Keywords: Membrane reactor; Hydrogen production; Rh catalysts; La; ₂; O; ₃; -SiO; ₂; CO; ₂; reforming

Synthesis, characterization and performance evaluation of Ni/Al₂O₃ catalysts for reforming of crude ethanol for hydrogen production by Abayomi J. Akande; Raphael O. Idem; Ajay K. Dalai (pp. 159-175).

The effects of catalyst synthesis method (i.e. precipitation (PT), coprecipitation (CP) and impregnation (IM)), Ni loading and reduction temperature on the characteristics and performance of Ni/Al₂O₃ catalysts were evaluated for the reforming of crude ethanol for H₂ production. The results showed that in the calcined PT catalysts, no NiAl₂O₄ species were observed whereas this was a major species in CP and IM catalysts. As a result, PT catalysts were more reducible than CP and IM catalysts. PT catalysts exhibited slightly lower crystallite sizes of NiO species than the corresponding CP catalysts. On the other hand, IM catalysts had extremely large crystallite sizes except IM10 (IM catalyst with 10% Ni loading) which had the smallest crystallite size. A combination of small crystallite size and high reducibility for PT catalysts resulted in higher crude ethanol conversions for the PT catalysts. In contrast, the IM catalysts with larger crystallite sizes and lower reducibility yielded the lowest crude ethanol conversions. Catalysts with 15% Ni loading gave the best crude ethanol conversions for each method of synthesis with PT15 giving the best overall crude ethanol conversion of 85mol% again because of its smaller crystallite size and higher reducibility. In terms of H₂ yield, CP15 was the optimum catalyst because of its higher H₂ selectivity as compared to PT15 and IM15 catalysts. Coking was observed at the onset of the reaction but stabilized after 180min TOS.

Keywords: Ni–Al; ₂; O; ₃; catalysts; Synthesis method; Reducibility; Crude ethanol reforming; Crystallite size; Conversion

Poisoning effect of thiophene on the catalytic activity of molybdenum carbide during tri-methyl pentane reforming for hydrogen generation by Praveen K. Cheekatamarla; William J. Thomson (pp. 176-182).

An investigation of the effect of sulfur on the activity of a bulk molybdenum carbide catalyst for the steam and oxidative-steam reforming of tri-methyl pentane, has been undertaken. Using thiophene as a model sulfur compound at concentrations as high as 1000ppmw, the degree of deactivation was found to be dependent on the sulfur concentration but was minimal at concentrations below 100ppmw. While deactivation was completely reversible in the case of steam reforming, spent catalysts from oxidative-steam reforming could only be partially reactivated. Characterization of spent catalysts by XRD and XPS leads to the conclusion that deactivation during steam reforming is due to surface oxidation of the carbide, apparently as a result of sulfur inhibition of carburizing kinetics. On the other hand, deactivation during oxidative steam reforming appears to be due to coking, attributed to increased surface acidity resulting from SO_x species.

Keywords: Hydrogen; Molybdenum carbide; Catalyst; Fuel processor; Gasoline; Thiophene; Sulfur poisoning

Structure-reactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis by David G. Cantrell; Lisa J. Gillie; Adam F. Lee; Karen Wilson (pp. 183-190).

A series of [Mg(1− x₎Al_x(OH)₂] x⁺(CO₃) x/_n²⁻ hydrotalcite materials with compositions over the range x=0.25–0.55 have been synthesised using an alkali-free coprecipitation route. All materials exhibit XRD patterns characteristic of the hydrotalcite phase with a steady lattice expansion observed with increasing Mg content. XPS measurements reveal a decrease in both the Al and Mg photoelectron binding energies with Mg incorporation which correlates with the increased intra-layer electron density. All materials are effective catalysts for the liquid phase transesterification of glyceryl tributyrate with methanol for biodiesel production. The rate increases steadily with Mg content, with the Mg rich Mg_2.93Al catalyst an order of magnitude more active than MgO, with pure Al₂O₃ being completely inert. The rate of reaction also correlates with intralayer electron density which can be associated with increased basicity.

Keywords: Green chemistry; Biodiesel; Renewable fuel; Solid base; Hydrotalcite; Heterogeneous catalysis; Transesterification

Targeting quantitative synthesis for the one-step amination of fatty alcohols and dimethylamine by Hiroshi Kimura; Hideki Taniguchi (pp. 191-196).

One-step amination of dodecyl alcohol (ROH, R=dodecyl group) and dimethylamine (HNMe₂) to N, N-dimethyl-dodecylamine (RNMe₂) using the Cu/Ni/Ca/Ba colloidal catalyst (Cu, 200ppm) proceeded at an initial reaction rate of 974moleh⁻¹mole-Cu⁻¹, with a selectivity of 98.7% and a conversion of 98.8% at 210°C for 4h. The contents of by-products: aldol, N-methyl-dodecylamine (RNHMe) and didodecylmethylamine (R₂NMe) were 0%, 0.1%> and 1.3%, respectively. The possibility for the quantitative synthesis by an advanced catalytic system was discussed.

Keywords: Synergism; Cu/Ni/Ca/Ba; Cu/Ni/Ba; Amination; Metal colloid; Nanoparticles

Highly efficient conversion of propane to maleic anhydride and acetic acid by partial oxidation by Jie Tang; Chuan X. Chen; Wen S. Li; Xiao P. Zhou (pp. 197-202).

Propane-selective oxidation was investigated over VMoO alone or over VMoO catalysts promoted by metal ions. Through this reaction, propane was efficiently converted to maleic anhydride, and acetic acid over VMoO itself and over metal-ion-doped VMoO catalysts. Over 70%Mo20%V10%AgO catalyst, 37.4% of propane single pass conversion was obtained at 300°C with maleic anhydride selectivity of 53.6% and acetic acid selectivity of 38.9%. The studies on propylene oxidation indicated that propylene might not be a reaction intermediate for maleic anhydride formation in propane oxidation. A typical carbon–carbon bond cracking and reformation reaction occurred in propane oxidation by using molecular oxygen as oxidant.

Keywords: Propane oxidation; Catalyst; Maleic anhydride; Acetic acid

Cracking of pentenes to C₂–C₄ light olefins over zeolites and zeotypes by Oleg Bortnovsky; Petr Sazama; Blanka Wichterlova (pp. 203-213).

The cracking of 2-methyl-2-butene was carried out as a model reaction of C₄⁺ olefins cracking by using of various zeolite and zeotype catalysts to obtain light olefins, such as ethylene and propylene. Only ZSM-5 and ZSM-11 zeolites with 10-MRP with intersecting channels and acid sites concentration ranging between 0.03 and 0.1mmol/g exhibit high and stable selectivity to C₂–C₄ olefins at high conversion level. Active and selective catalysts were prepared by three different methods: direct synthesis of H-ZSM-5 with high Si/Al ratio, steaming of H-ZSM-5 and “partial�? ammonium ion exchange of Na-ZSM-5. Neither H-(Al)ZSM-5 with high concentration of acid sites nor H-(Fe)ZSM-5 are suitable catalysts as they catalyze hydrogen-transfer reactions yielding paraffins and aromatics, showing the importance of low concentration and high strength of acid sites for cracking activity-selectivity. High yield of butenes in the reaction products indicates that pentenes cracking in 10-MRP proceeds via formation of voluminous carbenium ion intermediates demanding sufficient space inside of zeolites channel. Therefore, the presence of intersection of 10-MRP is very important. While in ZSM-5 and ZSM-11 zeolites the low amount of active sites mainly situated in the pores intersection is sufficient to achieve high activity-selectivity in the case of mono-dimensional 12-MRP zeolites such as mordenite and ZSM-12 and also with 10-MRP ferrierite much higher total acid concentration is necessary, as vast of the acid sites, situated out of the main channel do not take part in the reaction. AlPO-11 materials with narrow mono-dimensional channel exhibit low conversion with majority of β-scission cracking mechanism independently on the acid sites concentration.

Keywords: Pentene cracking; Zeolite topology; Acid site concentration; Acid site strength; ZSM-5; ZSM-11; AlPO-11; Ferrierite; Mordenite; ZSM-12

A study of effect of particle size on the oxidation of rhenium in the Re/γ-Al₂O₃ catalysts by Janina Okal (pp. 214-220).

The oxidation of the reduced Re/γ-Al₂O₃ catalysts with dispersion of Re from 0.25 to 0.74 and metal particle sizes of 1–9nm, was examined over a temperature range 20–500°C. The structure of the catalysts was characterised using H₂ chemisorption, O₂ uptake, BET and XPS and Raman spectroscopy. The O₂ uptake and XPS results showed that very small Re particles were in direct and strong contact with the oxygen atoms of the support. As a result, the O₂ uptakes were low at room temperature and even at 500°C were lower than expected for oxidation of Re⁰ to R₂O₇ oxide. On the other hand, XPS data indicated that at room temperature clusters and small Re particles were oxidised to Re⁴⁺, Re⁶⁺ and Re⁷⁺ species. The O/Re ratios at temperatures up to 200°C increased with the Re dispersion what implies additionally, high affinity of the highly dispersed Re to oxygen. Fraction of rhenium strongly interacting with the support amounts to 6 or 19% of total Re in the catalysts with average particle sizes of about 5 or 1.5nm, respectively. At higher temperatures the process of oxidation of rhenium accelerates, with instantaneous sublimation of Re₂O₇ oxide and in accord to Raman data, its simultaneous adsorption as the ReO₄ species on γ-alumina. The O₂ uptake at 500°C by the supported Re phase depends on the size of rhenium particles. Small Re particles could be oxidised to an oxidation level lower than heptvalent, while large particles to Re₂O₇. XPS and Raman data showed, however, that after treatment at 500°C, independent on the particle size of the Re and its interaction with the support, whole rhenium was oxidised to Re⁷⁺ species. This species forms some kind of surface complex with an AlOReO₃ or Al(OReO₃)₃ structure, which inhibits the surface migration of rhenium and its loss from the oxidised catalysts.

Keywords: Re/γ-Al; ₂; O; ₃; catalyst; Oxidation; O; ₂; uptake; XPS; Raman spectroscopy

A highly efficient catalyst system of ZnBr₂/ n-Bu₄NI for the synthesis of styrene carbonate from styrene oxide and supercritical carbon dioxide by Jianmin Sun; Shin-Ichiro Fujita; Fengyu Zhao; Masahiko Arai (pp. 221-226).

A catalyst system of ZnBr₂/ n-Bu₄NI is reported for the first time as a highly efficient binary catalyst for the synthesis of styrene carbonate (SC) from styrene oxide and carbon dioxide under supercritical conditions. In a very short reaction time (30min) and even at a mild reaction temperature (80�C), 100% selectivity and nearly 100% yield of SC have been obtained; the TOF value obtained is 46 times larger than those so far reported under similar conditions. The effects of the ZnBr₂/ n-Bu₄NI ratio, types of cation and anion, and other reaction conditions on the reaction were investigated. On the basis of the results, a reaction mechanism has been proposed.

Keywords: Styrene carbonate; Styrene oxide; Carbon dioxide; Binary catalyst; Cycloaddition

“Clean�? limonene epoxidation using Ti-MCM-41 catalyst by M.V. Cagnoli; S.G. Casuscelli; A.M. Alvarez; J.F. Bengoa; N.G. Gallegos; N.M. Samaniego; M.E. Crivello; G.E. Ghione; C.F. P�rez; E.R. Herrero; S.G. Marchetti (pp. 227-235).

The limonene oxidation with H₂O₂ (“clean�? oxidation) using a Ti-MCM-41 catalyst was studied. This catalyst was synthesized by a sol–gel method and characterized by XRD, DRS, ICP and BET. The operative reaction conditions, under which no Ti leaching was detected, were determined. In these conditions the catalyst has shown a high activity, and very good selectivity towards epoxides. The kinetic parameter measurements were carried assuming a heterogeneous reaction and empirical power rate law. Using the initial rate method, a first order with respect to the concentrations of catalyst, limonene and hydrogen peroxide were determined. An apparent activation energy value of 16.4kJ/mol and a pre-exponential Arrhenius�?s factor of 7.1l²/gmolh were obtained.

Keywords: Catalytic limonene epoxidation; Ti-MCM-41 catalysts; Ti leaching; Limonene epoxidation kinetic

Structural and catalytic characteristics of MoO₃/CeO₂ catalysts: CO oxidation activity by Mohamed Mokhtar Mohamed; Saud M.A. Katib (pp. 236-243).

The 8wt.% MoO₃-modified CeO₂ materials were prepared by three different methods namely, impregnation, wet mechanical mixing and co-precipitation and characterized by thermal analyses (TG and DTA), X-ray diffractometry, infrared spectroscopy, UV–vis diffuse reflectance spectroscopy as well as nitrogen sorptometry. Their catalytic activities in the oxidation of CO to CO₂ were also examined. The results showed that the material prepared by co-precipitation provided the highest activity, comparatively. This was due to the concomitant strong metal oxide-support interaction revealed for this material that validated by XRD and FTIR results those showed diminishing in particles size of the two ions and formation of MoOCe linkages (760cm⁻¹), respectively. The UV–vis spectrum of this material showed the predominant presence of tetrahedral Mo⁶⁺ reflecting the highest dispersion among all samples. This material exhibited as well the highest specific surface area (96m²/g) and rather presented mesopores of regular distribution. Important correlations between the latter material and those prepared by impregnation and wet mechanical mixing in view of proposed interaction between ions, morphology, surface texturing and their effects on the activity of CO oxidation were evaluated and discussed. It is inferred that surface molybdate that permit the formation of Mo⁶⁺ in highly dispersed state beside the Ce³⁺/Ce⁴⁺ redox couples were responsible for the revealed catalytic activity.

Keywords: Mo/CeO; ₂; Preparation methods; Structure; CO oxidation

FT-IR and mass spectrometric studies on the interaction of acetaldehyde with TiO₂-supported noble metal catalysts by J. Rask�; T. Kecsk�s; J. Kiss (pp. 244-251).

Surface species formed during the adsorption of acetaldehyde at 300–673K on TiO₂-supported Pt, Rh and Au catalysts were investigated by Fourier transform infrared spectroscopy. Two forms of molecularly adsorbed acetaldehyde–H-bridge bonded on surface OH groups and adsorbed on Lewis sites through one of the oxygen lone pairs–were identified. β-aldolization of acetaldehyde led to the formation of crotonaldehyde, which adsorbs on Lewis sites of TiO₂ through one of the oxygen lone pairs and on metallic sites via the C atom of the aldehyde group. Adsorbed acetaldehyde can be oxidized into surface acetate and it can be reduced resulting in adsorbed ethoxy. Mass spectroscopic analysis of the gas phase composition revealed that the formation of gas phase products (crotonaldehyde, water, benzene, hydrogen, ethylene, acetylene and methane) depends on the nature of the metals and the reaction temperature. An attempt was made to find a possible link between the surface species and the formation of the primary gas phase products.

Keywords: Acetaldehyde adsorption; TiO; ₂; -supported Pt; Rh; Au catalysts; Surface species; Gas phase products; FT-IR; MS

Adsorption and surface reactions of acetaldehyde on TiO₂, CeO₂ and Al₂O₃ by J. Rask�; J. Kiss (pp. 252-260).

The adsorption and surface reactions of acetaldehyde at 300–673K on TiO₂, CeO₂ and Al₂O₃ were investigated by Fourier transform infrared spectroscopy and mass spectroscopy. Acetaldehyde adsorbs molecularly in two forms on the surfaces: (i) in a less stable H-bridge bonded form and (ii) in a more stable form adsorbed on Lewis sites through one of the oxygen lone pairs. Both forms of molecularly adsorbed acetaldehyde transform into crotonaldehyde (CH₃CHCHCHO) by β-aldolization on the surfaces. The reaction of adsorbed acetaldehyde and crotonaldehyde resulted in the formation of benzene at higher temperature. The formation of crotonaldehyde and benzene depended on the nature and the pre-treatments of the oxides: the amount of crotonaldehyde was higher on H₂-pre-treated, while the amount of benzene was higher on O₂-pre-treated surfaces. Primarily the more strongly held acetaldehyde underwent dehydrogenation resulting in H₂ and acetylene. The formation of ethane was interpreted by hydrogenation of the transitorily formed ethylene and/or by catalytic decomposition of ethanol, which formed from adsorbed ethoxy produced by the surface reduction of acetaldehyde. Acetaldehyde could be oxidized into acetate, the decomposition of which resulted in gas phase methane. No CO and CO₂ was detected up to 673K.

Keywords: Acetaldehyde adsorption on oxides; FT-IR and MS; Surface species and gas-phase products; Crotonaldehyde and benzene formation

The kinetics of citral hydrogenation over Pd supported on clinoptilolite rich natural zeolite by Selahattin Yilmaz; Sule Ucar; Levent Artok; Hilal Gulec (pp. 261-266).

The liquid phase hydrogenation of citral has been investigated over Pd (2.42%) supported on clinoptilolite rich natural zeolite catalyst for different reaction conditions. The zeolite support did not affect the active metal properties. This was attributed to the large size of Pd particles on the support. High selectivity (90%) to citronellal was obtained at complete conversion of citral. The catalytic activity increased with reaction temperature (80, 100, 120�C) following an Arrhenius behaviour, while selectivity remained constant for a given conversion. Selectivity to citronellal increased from 78 to 90% with increase in the amount of catalyst in the reaction solution (Citral/Pd mole ratio 293, 176 and 105). The spent catalyst regained its fresh activity and selectivity upon regeneration.

Keywords: Citral; Hydrogenation; Clinoptilolite; Natural zeolite; Pd catalyst

Novelties of reaction in the middle liquid phase in tri-liquid phase transfer catalysis: Kinetics of selective O-alkylation of vanillin with benzyl chloride by Ganapati D. Yadav; Sharad V. Lande (pp. 267-275).

Reactions in three immiscible liquid phases (L–L–L) are attractive, and one of the phases can be the locale of reaction which will have a dramatic effect on product distribution in complex reactions. Thus, converting a bi-liquid system into a tri-liquid phase is of considerable scientific and commercial interest. 4-Benzyloxy-vanillin is used as a perfume and also as a starting material for synthesis of thalifoline and ephedradine as alkaloids and in synthesis of flavonoid compounds. Etherification of vanillin with benzyl chloride under biphasic phase transfer catalysis leads to formation of 4-benzyloxy-vanillin, but selectively suffers due to side reactions. Waste minimization is a major theme of green chemistry. In the traditional liquid–liquid phase transfer catalysis, the catalyst is not recovered but disposed off causing load on environment. However, the transformation of two-liquid phases into three-liquid phases L–L–L, PTC leads to 100% conversion of the limiting reactant benzyl chloride with 100% selectivity to 4-benzyloxy-vanillin, using TBAB as a catalyst. The rates of reactions are very high under L–L–L PTC, and reaction can be completed within 1h as against 8h required in L–L PTC. The catalyst-rich middle phase is recycled many times, thereby leading to profitability. The current work deals with effect of different kinetics and process parameters leading to enhancement in rates and selectivities and greener aspects of phase transfer catalysis.

Keywords: Vanillin; 4-Benzyloxy-vanillin; Benzyl chloride; Phase transfer catalysis (PTC); Liquid–liquid PTC; Liquid–liquid–liquid PTC; TBAB; Kinetics; Selectivity; Reactions in middle phase; Waste minimization

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Applied Catalysis A, General (v.287, #2)