Applied Catalysis A, General (v.274, #1-2)



Carbon nanofibres composites with macroscopic shaping were successfully synthesised by chemical vapour decomposition (CVD) using a low loading in nickel as catalyst (≤1 wt.%). A high carbon nanofibres yield, i.e. 100 wt.% weight gain per hour of synthesis with respect to the initial catalyst weight was obtained, which significantly lowered the synthesis cost while the direct macroscopic shaping rendered more reliable the applications of these materials in conventional catalytic processes.
Keywords: Chemical vapour decomposition; Macroscopic morphology; Nickel catalyst;

The replacement of mineral acids by sulfonic resins in the synthesis of rac-5-(4-hydroxyphenyl)hydantoin from p-hydroxymandelic acid and urea by Carlos Cativiela; José M. Fraile; José I. Garcı́a; Beatriz Lázaro; José A. Mayoral; Antonio Pallarés (9-14).
The sulfonic acid resin Dowex® 50W × 8 can be used to replace mineral acids in the reaction between p-hydroxymandelic acid and urea in water or NaCl solutions. The synthesis of p-hydroxymandelic acid is efficiently carried out using NaOH to promote the reaction between phenol and glyoxylic acid. Alternatively, KF/alumina can be used to obtain methyl p-hydroxymandelate by reaction of phenol with methyl 2-hydroxy-2-methoxyacetate, although, this process is carried out in an organic solvent and gives a lower yield. A comparison between the three alternative methods for the synthesis of 5-(4-hydroxyphenyl)hydantoin shows that the direct route from glyoxylic acid, urea and phenol is the most efficient method, although the p-hydroxymandelic acid route would be an interesting alternative if its heterogeneous synthesis were optimized.
Keywords: Sulfonic acid resins; p-Hydroxymandelic acid; Hydantoins;

Synthesis of composite material MCM-41/Beta and its catalytic performance in waste used palm oil cracking by Yean-Sang Ooi; Ridzuan Zakaria; Abdul Rahman Mohamed; Subhash Bhatia (15-23).
Composite materials composed of MCM-41/Beta were synthesized using two methods: (1) seeding method, and (2) two-step crystallization process. Powder XRD showed the existence of well-structured microphase zeolite Beta and mesophase MCM-41 in the composite materials. The performance of the composite material as a catalyst was investigated in the cracking of waste used palm oil for the production of liquid hydrocarbons and its activity was compared with the catalytic activity of a physical mixture of zeolite Beta and MCM-41. The composite material synthesized via seeding method gave a better performance in terms of conversion and yield of liquid fuel gasoline fraction compared to composite material obtained from two-step crystallization process and physical mixing. The composite material was found to be more selective for liquid fuel gasoline fraction enriched with more olefins compared to zeolite Beta or MCM-41 catalyst alone.
Keywords: Composite MCM-41/Beta; Catalytic cracking; Waste used palm oil; Biofuel;

Activity and characterization of the Co-promoted CuO–CeO2/γ-Al2O3 catalyst for the selective oxidation of CO in excess hydrogen by Jong-Won Park; Jin-Hyeok Jeong; Wang-Lai Yoon; Heon Jung; Ho-Tae Lee; Deuk-Ki Lee; Yong-Ki Park; Young-Woo Rhee (25-32).
For the selective oxidation of CO in excess hydrogen, γ-Al2O3-supported CuO–CeO2-based catalysts were investigated. Effects of the composition of Cu and Ce metals and the addition of Co as a promoter to the catalyst on the catalytic activity and the selectivity for the CO oxidation against H2 were determined as a function of temperature. The effects of the feeding ratio of O2 to CO and the presence of H2O and/or CO2 in the feeding reactant were also examined. Among the catalysts tested, CuO–CeO2/γ-Al2O3 incorporating Co by 0.2 wt.% was the most active to show the CO conversion >99% in the wide temperature window of 150–220 °C with the CO oxidation selectivity of 94–50%. It was found from the catalyst characterization using temperature-programmed reduction/oxidation that the Co-promoted CuO–CeO2/γ-Al2O3, as compared with the catalyst without Co addition, had an increased number of CO oxidation-active sites capable of being easily reduced or oxidized at relatively low temperatures. However, the presence of CO2 and H2O in the feed decreased the low-temperature activity of the catalyst, and shifted the temperature window of CO conversion >99% to higher temperature region, i.e. 205–230 °C. Irrespective of the presence of both CO2 and H2O in the feed, the Co-incorporated catalyst was found to have the temperature window of CO conversion >99.9% within 210–224 °C, whereas no such temperature window existed for the CuO–CeO2/γ-Al2O3 catalyst. Temperature-programmed desorption of CO, CO2, or H2O was also conducted for understanding the catalytic reaction behavior in the presence of CO2 and/or H2O in the reactants.
Keywords: Selective CO oxidation in excess H2; Cu–Ce-based catalyst; Co promoter; Effect of CO2 and H2O; Temperature-programmed reduction/oxidation/desorption;

Synthesis of Rh nano-particles by the microemulsion technology by Manuel Ojeda; Sergio Rojas; Magali Boutonnet; Francisco J. Pérez-Alonso; F. Javier Garcı́a-Garcı́a; José Luis G. Fierro (33-41).
Several alumina-supported rhodium-based catalysts have been prepared using the microemulsion technology. The change of microemulsion properties led to catalysts with different Rh particle size and narrow size distribution while keeping the same metal loading. The metal particle size was determined by XRD and TEM. These catalysts were tested in the CO hydrogenation reaction in order to investigate the influence of the Rh particle size. The catalytic results indicated that turnover frequency (TOF) increased about four times when the Rh particle size increased approximately from <5 to 30 nm. Product selectivity also depended on particle size. As Rh particle size decreased, methane and oxygenated compounds formation was favored at expenses of the higher hydrocarbons (C2+). XPS shows that the electronic metal–support interaction increases as the particle size decreases, leading to partially oxidized Rh atoms. These species are responsible of oxygenate formation, and as a result, its selectivity increases as rhodium particle size decreases.
Keywords: Rh; Microemulsion; Alumina; Particle size; CO hydrogenation;

Thiophene hydrodesulfurization over noble metal modified Co-clay catalysts by M.M. Hossain; M.A. Al-Saleh; M.A. Shalabi; T. Kimura; T. Inui (43-48).
This paper presents the effects of noble metal modification of Co-HPS (high porous saponite) on thiophene hydrodesulfurization. The modification was done by two different ways: (I) single noble metal and (II) bi-noble metal. The noble metal loading was varied from 0.1 to 2 wt.% of the catalyst. In catalyst preparation direct impregnation technique was used for the loading of noble metal and an ion-exchange was followed for Co loading on HPS. The catalysts were characterized by elemental analysis and temperature programmed reduction (TPR). Elemental analysis shows that the Co loading was slightly affected by noble metal modification. TPR study displayed that the reducibility of the catalysts was enhanced in presence of noble metal(s). Thiophene hydrodesulfurization activities were measured in a pulse micro reactor. The noble metal modified catalysts exhibited significantly higher activity as compared to the Co-HPS. Rh shows the highest activity among the three noble metals used for Co-HPS modification. The thiophene hydrodesulfurization activity was further enhanced by a bi-noble metal modification. The Pt-Rh combination shows better performance as compared to the Pd-Rh combination.
Keywords: Clay; Noble metal; Thiophene; Hydrodesulfurization; Pulse reactor;

The physico-chemical properties of primary alcohol conversion catalysts SnO2, CeO2 and tin dioxide modified with rhodium, cerium or both, are presented. The synthesis of pure SnO2 was performed with the Okazaky method. Admixturing with rhodium was made by the impregnation method. Addition of cerium was achieved by co-precipitation. The obtained materials based on tin dioxide were monophase materials of tetragonal structure, characteristic for SnO2. As a result of introducing cerium as a dopant of tin oxide(IV), the character of active centers was changed. The obtained monophase materials of compositions Sn0.93Ce0.07O and Sn0.925Ce0.07Rh0.005O2 revealed a large number of strong Lewis acidic centers on the surface. As a result of the addition of CeO2 to basic tin dioxide there was a clear modification of catalytic properties appearing in these materials.The authors try to explain a mechanism of condensation of alcohol, aldehyde or acid to ketone on their surface with the participation of Lewis acidic centers.
Keywords: Tin dioxide; Cerium dioxide; Rhodium; Ketonization; Lewis acidic centers; Oxygen vacancy;

Two different methods to stabilise alumina-supported nickel particles against sintering have been studied. The first investigated method was co-impregnation using precursors of nickel and different promoters (i.e. oxides of Li, Na, K, Mg, Ca, La and Ce) and the second was formation of a protecting nickel aluminate layer between the nickel particles and the γ-alumina support. The samples were heat-treated in ammonia + hydrogen at 523 K and 250 bar. The sintering process was mainly followed by hydrogen chemisorption. The samples were also characterised by specific surface area measurements, X-ray diffraction and transmission electron microscopy with energy dispersive X-ray mapping.Co-impregnation with oxides of alkali metals, alkaline earths, or lanthanides markedly suppressed the sintering of nickel. The most pronounced effect was achieved for the samples promoted with alkaline earths for which up to 50% of the active surface area remained after the heat-treatment in ammonia + hydrogen compared with the Ni/Al2O3 reference sample. The formation of nickel aluminate by sequential impregnation of the nickel precursors, with calcination at 923 K between the impregnations, enhanced the sintering stability with 35% compared to the reference sample.
Keywords: Sintering; Nickel; Alumina; Catalyst deactivation; Ammonia; Hydrogen; Chemisorption; Promoters; Co-impregnation; Nickel aluminate;

Carbon microfibers were synthesized from hydrogen-diluted methane over an iron catalyst and characterized by scanning electron microscopy, thermogravimetry (TG), X-ray diffraction (XRD), and Raman spectroscopy. The conversion of methane and the overall selectivity of carbon fiber formation were derived reproducibly as a function of the growth conditions. Blank experiments and experiments with different iron loadings helped to identify the catalytic methane decomposition at about 1270 K, whereas the pyrolysis of methane dominated at higher temperatures leading to a core-and-shell structure of the fibers. Prolonged growth using a CH4/H2 = 30/70 mixture at 1423 K for 8 h resulted in a diameter of more than 20 μm and a length approaching a limit of about 40 mm. The growth mode was found to depend strongly on the gas-phase composition and the temperature. By lowering the methane content to 10%, or by lowering the temperature to 1348 K, nanofibers were obtained. When diluting methane with helium instead of hydrogen, significantly shorter fibers were formed. The degree of structural order detected by Raman spectroscopy was hardly influenced by prolonged growth at 1423 K, whereas the interlayer distance was found to decrease slightly to 345 pm after 8 h correlated with an increasing thermal stability in air up to 1024 K (50% weight loss).
Keywords: Carbon microfiber; Raman spectroscopy; Pyrolysis;

Influence of palladium incorporation technique on n-butane hydroisomerization over HZSM-5/bentonite catalysts by Fernando Dorado; Rubı́ Romero; Pablo Cañizares; Amaya Romero (79-85).
The influence of palladium incorporation technique on hydroisomerization of n-butane over bifunctional Pd/HZSM-5 catalysts agglomerated with bentonite was studied to analyse the effect that it has on the catalytic activity. Temperature-programmed desorption of ammonia (TDPA), temperature-programmed reduction (TPR), atomic absorption (AA) spectroscopy, and surface area measurements were used to characterize the catalysts. Pd/HZSM-5/bentonite catalysts were prepared using the impregnation, ion exchange and solid-state ion exchange methods. The impregnated catalyst shows the highest activity. However, the catalysts prepared by solid-state ion exchange had the lowest catalytic activity. Probably, there is not a good mixture between the metal and zeolite for these samples, so that the conversion decreases.
Keywords: Zeolites; Ion exchange; Selectivity; Homogenization; Agglomeration; Metals;

Cobalt and manganese oxide samples supported on cordierite (commercial grade) were prepared by wet impregnation method using finely powdered support material and cobalt or manganese nitrate. The extent of loading was varied between 5 and 20 wt.% Co3O4 or Mn2O3. The physicochemical, surface and catalytic properties of the thermally treated solids (350 and 700 °C) were investigated using XRD, EDX, nitrogen adsorption at −196 °C, CO-oxidation by O2 at 225–275 °C and H2O2-decomposition in a aqueous solution at 30–50 °C.The results obtained revealed that the employed cordierite preheated at 350–700 °C was orthorhombic well crystallized magnesium aluminum silicate (Mg2Al4Si5O18). The manganese oxide-containing cordierite samples consisted of manganese oxide–cordierite solid solution. While all diffraction peaks of Co3O4 were observed in the diffractograms of cobalt-containing cordierite preheated at the same temperatures. The specific surface area of cordierite support material increases progressively by increasing the amount of manganese or cobalt oxide present.The catalytic activity of cordierite was found to increase almost linearly by increasing the amount of cobalt or manganese oxide added. Cobalt oxide-containing catalysts calcined at 350–700 °C exhibited catalytic activity in CO-oxidation by O2 higher than those measured for manganese oxide-containing catalysts calcined at the same temperatures. Opposite results have been observed in the case of H2O2-decomposition. These findings indicate that the active sites taking part in CO-oxidation by O2 are different from those involved in H2O2-decomposition.
Keywords: Cordierite; Cobalt oxide; Manganese oxide; CO-oxidation; H2O2-decomposition;

Mn–Al and Mg–Mn–Al hydrotalcite-like compounds (HTlcs) were synthesized by a coprecipitation method. The activity of the calcined HTlcs for hydrogenation of methyl benzoate to benzaldehyde was tested. The effects of Mn/Al and Mg/Mn ratio, preparation and reaction conditions, and potassium promoter on the catalytic performance of the catalysts were investigated. The prepared catalysts give better performance than conventional 10% Mn/γ-Al2O3 catalyst. The conversion of methyl benzoate and selectivity to benzaldehyde on 3% K/Mg0.2Mn1.8Al1 catalyst at 390 °C are 90.0 and 88.0%, respectively. XRD, TPR and CO2-TPD studies show that the redox and basic properties of the catalysts play important roles in the reaction. A suitable degree of reduction and a small number of basic sites on the catalyst lead to a high activity and selectivity to benzaldehyde.
Keywords: Hydrotalcite-like compounds; Methyl benzoate; Hydrogenation; Benzaldehyde; Manganese oxide catalyst;

Microwave-assisted Suzuki reactions in a continuous flow capillary reactor by Ping He; Stephen J. Haswell; Paul D.I. Fletcher (111-114).
Suzuki cross-coupling reaction of aryl halides with phenylboronic acid to form biaryls has been used to illustrate the development of a microwave-based technique capable of delivering heat locally to a solid Pd-supported catalyst located within a continuous flow capillary reactor. The strong inherent absorption of a thin layer of gold metal on the outside surface of the capillary enabled effective heating to be carried out in the region of the catalyst, enhancing the rate of reaction to give product yields greater than 70% with catalyst/reactant contact times of less than 60 s using a hydrodynamically pumped system.
Keywords: Microwave; Suzuki reaction; Heterogeneous catalysis; Continuous flow capillary reactor;

Effect of reaction conditions on limonene epoxidation with H2O2 catalyzed by supported Keggin heteropolycompounds by S.G. Casuscelli; M.E. Crivello; C.F. Perez; G. Ghione; E.R. Herrero; L.R. Pizzio; P.G. Vázquez; C.V. Cáceres; M.N. Blanco (115-122).
Limonene epoxidation by hydrogen peroxide in the presence of Keggin-type heteropolycompound supported catalysts was studied. Catalysts containing molybdenum or tungsten heteropolycompounds were prepared by pore filling impregnation of carbon or alumina, and were characterized by diffuse reflectance spectroscopy (DRS), Fourier transform infrared (FT-IR) and potentiometric titration. During the reaction, limonene oxide, carvone, carveol, diols and limonene hydroperoxide were produced. Different reaction conditions have been studied, such as nitrogen atmosphere, absence of light, and radical scavenger addition. A catalytic process of oxygen transference could be assumed to occur when a lacunar tungstophosphate supported on carbon is used as catalyst, forming a peroxometalate intermediate. A free radical competitive mechanism was found in the presence of alumina-supported heteropolyacids.
Keywords: Epoxidation; Limonene; Hydrogen peroxide; Heteropolycompounds;

Bulk mixed Mo–V–Te–O catalysts for propane oxidation to acrylic acid by Vadim V. Guliants; Rishabh Bhandari; Jamal N. Al-Saeedi; Vijay K. Vasudevan; Rajiv S. Soman; Olga Guerrero-Pérez; Miguel A. Bañares (123-132).
The model Mo–V–Te–O catalysts containing the orthorhombic (M1) and hexagonal (M2) phases with well-defined crystal morphologies and promising for the selective oxidation of propane to oxygenates were obtained hydrothermally over a wide range of synthesis compositions (Mo0.70–0.30V0.60–0.20Te0.15–0.05). The bulk (ICP), local (EDS) elemental and structural (XRD) analyses of this compositionally simple model system indicated that the M1 phase with the bulk Mo/V ratio of ∼2 was dominant in the Mo–V–Te–O catalysts. This phase has been proposed as active and selective for propane oxidation to oxygenates and ammoxidation to acrylonitrile. The model catalysts displayed high selectivity to acrylic acid in the presence of water vapor in the feed, which is believed to enhance the apparent rates of formation of oxygenates as well as maintain the catalytic surface in a partially reduced oxidation state. The selectivity to acrylic acid over these model catalysts correlated with the extent of exposure of the surface ab planes of the M1 phase proposed to contain the active and selective surface sites. Therefore, the Mo–V–Te–O catalysts represent a well-defined and highly promising model system for elucidating the surface molecular structure–activity/selectivity relationships in propane oxidation to acrylic acid over multicomponent Mo–V–Te–Nb–O catalysts.
Keywords: Mixed Mo–V–Te oxides; Hydrothermal synthesis; Phase diagram; Propane oxidation;

Aqueous phase catalytic oxidation of cyanides over iron-modified cobalt oxide system by M. Stoyanova; St. Christoskova; M. Georgieva (133-138).
The catalytic oxidation of cyanide ions was carried out in an aqueous aerated suspension containing modified with iron Co-oxide system. The dependence of both cyanide oxidation rate and selectivity of the process on the initial pH and the catalyst amount was investigated. The results obtained show that the cyanide degradation proceeds most effectively and with high selectivity to the environmentally harmless carbon dioxide at pH≤9.5. The increase of the alkalinity (with other constant parameters) causes both a decrease in the cyanide conversion degree and lowering the selectivity in respect to complete oxidation products. The activity of Co–Fe-oxide system was compared to that of an individual Co-oxide. The observed high activity of the doped sample is explained by the influence of Fe as a modifying additive, which leads to the appearance of new active sites in the fresh samples—Fe(III)-ions in octahedral coordination and to increasing the mobility of the reactive oxygen.
Keywords: Cyanide oxidation; Iron-modified cobalt oxide system; Catalyst; Kinetics study;

CO2 reforming of methane over coprecipitated Ni–Al catalysts modified with lanthanum by R. Martı́nez; E. Romero; C. Guimon; R. Bilbao (139-149).
The influence of lanthanum oxide added to a Ni/Al2O3 catalyst has been studied in the carbon dioxide reforming of methane. The samples were prepared by variable pH coprecipitation, with different La2O3 contents: 0, 4, 8, and 12 wt.%. Although the NiO reducibility was not altered, the presence of La increased the metallic dispersion (XPS, XRD, TEM), slightly increased conversion levels, and also enhanced the catalyst stability due to a substantial decrease in coke formation during reaction. An increase in Ni dispersion in samples containing La leads to more metallic particles with diameters less than 10 nm that contribute to the catalytic conversion without producing large amounts of coke in filaments. However, there is a limit to the amount of lanthanum oxide (between 8 and 12 wt.%) above which metallic dispersion is not favored, and therefore coke filament formation and quick catalyst deactivation cannot be controlled. This is probably due to the poor La2O3 dispersion in these materials (XPS), leading to a loss in the improvement of Ni–Al–La catalyst properties. La atoms are included in the metallic nickel lattice after 4 h of reaction. After a long reaction time La is segregated and the Ni particle size increases, leading to greater coke production (in filaments).
Keywords: Nickel–alumina catalyst; Lanthanum; Coprecipitation; Methane reforming with CO2; Catalyst deactivation; Coking;

Dehydroisomerization of n-butane over H-Y zeolite supported Pt and Pt,Sn catalysts by S. Scirè; G. Burgio; C. Crisafulli; S. Minicò (151-157).
The direct one-step conversion of n-butane to isobutene was investigated over H-Y zeolite supported platinum and platinum-tin catalysts. Catalytic results over monometallic Pt catalysts showed that dispersion of Pt mainly affects the selectivity towards reaction products, larger Pt ensembles resulting in a higher degree of cracking/hydrogenolysis reactions and consequently to a lower n-butenes and isobutene formation. Addition of tin has been found to significantly improve both the selectivity to isobutene and the resistance to deactivation of the catalytic system. On the basis of characterization results (H2 chemisorption, NH3-TPD, FT-IR of adsorbed CO) the higher isobutene selectivity of Pt,Sn catalysts have been accounted for a dilution effect of Pt ensembles induced by the second metal which inhibits hydrogenolysis reactions, requiring several contiguous Pt sites, and enhances the selectivity for the dehydrogenation reaction that can instead proceed over smaller metal ensembles. The improved stability of Pt,Sn catalysts has been related to the lower surface acidity of the system in the presence of Sn, which results in a reduced formation of carbonaceous deposits responsible for catalysts deactivation.
Keywords: Dehydroisomerization; n-Butane; Isobutene; Zeolite; Platinum; Tin; Bimetallic;

Thermally stable tetragonal zirconia was prepared with the aid of interactions between zirconium atrane complex and surfactant cetyltrimethylammonium bromide (CTAB). The results of N2 adsorption–desorption analysis show that this material contained micropores centered at a pore diameter of about 5 Å. When the prepared zirconia after sulfation was applied to skeletal isomerization of 1-butene, high selectivity to isobutene was observed. This feature may be related to the size of micropores which is the same as the pore diameter of FER zeolite. Two major by-products, propene and pentenes, were formed by the dimerization(oligomerization)-cracking reactions. However, hydrocarbons higher than C6 were not observed in this work. The n-butenes conversion and product selectivities depend on the operating conditions such as the time-on-stream, the space velocity of 1-butene, the partial pressure of 1-butene and the reaction temperature. Especially, the yields of primary products: propene, isobutene and pentenes, were examined under the conditions investigated in this study. The effect of the concentration of sulfuric acid solution utilized in the preparation of the present catalyst was also investigated. On the basis of these results, the mechanism for the skeletal isomerization of 1-butene on the present catalyst was discussed.
Keywords: Sulfated zirconia; Micropore; Mesopore; Skeletal isomerization; 1-Butene; Mechanism;

Synthesis, characterization and catalytic C4 alkene cracking properties of zeolite ZSM-23 by Bin Wang; Qiang Gao; Jiandong Gao; Dong Ji; Xiaolai Wang; Jishuan Suo (167-172).
There has been an increasing interest in recent years in catalytic cracking of mixed C4 alkene to produce ethylene and propylene over zeolite catalysts. Among numerous catalysts which are used in such a process, the zeolite ZSM-23 showed distinctive potential. In this report, pure and well-crystallized ZSM-23 (MTT) catalyst was synthesized under static hydrothermal conditions using pyrrolidine as template. The product obtained was characterized by powder X-ray diffraction (XRD), ICP atom emission, scanning electron microscopy (SEM), thermal analysis (TGA-DTA) and FTIR of adsorbed pyridine. The ZSM-23 was used as catalyst in mixed C4 alkene cracking and showed very high catalytic activity and yields of ethylene and propylene. The effects of reaction temperature, WHSV crystallite size, pore size and Si/Al ratio on the catalytic activity were also studied in detail.
Keywords: Zeolite ZSM-23; Hydrothermal synthesis; Mixed C4 alkene; Catalytic properties;

The effect of mass transfer conditions on the stability of a low surface area Mo2C catalyst for dry (CO2) methane reforming has been studied in a packed bed reactor at temperatures up to 1000 °C and pressures up to 8.3 bar using stoichiometric feeds (CO2:CH4 = 1). Because stoichiometric feeds can be net oxidizing at typical reforming temperatures, the carbide is oxidized to MoO2, forming a moving oxidation front, which will eventually deactivate the entire bed. Based on experiments which isolated the effects of space velocity and molar feed velocity, it is concluded that these catalysts are stable under conditions where gas–solid mass transfer coefficients are low; that is, at high pressures and low mass flow rates. In these situations CO and H2 product gases remain near the catalyst surface, preventing oxidation. Experimental results are also presented which show the temperature above which the carburizing rates are greater than oxidation rates, and this temperature is dependent on molar feed velocity and pressure. These results explain the observations made by previous researchers who observed that stability was greater at higher pressures and that the catalyst oxidized when space velocities were raised. Finally, it is concluded that Mo2C catalysts can be stabilized with respect to oxidation by either operating at low mass velocities, higher temperatures, or by recycling product gases.
Keywords: Molybdenum carbide; Dry methane reforming; Mass transfer; Oxidation; Carburization; Stability;

Ag/alumina catalysts for the selective catalytic reduction of NO x using various reductants by E.F. Iliopoulou; A.P. Evdou; A.A. Lemonidou; I.A. Vasalos (179-189).
A series of Ag-based catalysts supported on unpromoted or Ce-promoted γ-alumina were prepared and studied for the selective catalytic reduction (SCR) of NO x with various reductants. The activity for NO reduction increased as the oxygen-content in the reaction mixture increased from 2% to 10%. The influence of Ag loading, the catalytic support used and the type of the reductant on NO reduction was examined. Moderate silver loadings (3%) exhibit the most efficient deNO x activity, while increasing the metal loading or using a Ce-promoted alumina support inhibits the catalytic activity. Among C3H6, CH4 and CO used as reducing agents, C3H6 has the higher reducing activity. C2H4, C2H6, C3H8, C4H10, 1-C4H8 and 1,3-C4H6 were also tested as reducing agents, suggesting that use of higher and less saturated hydrocarbons results in an enhanced deNO x performance. Higher concentrations of the optimum reductant (1-C4H8) significantly improved the deNO x performance, achieving over 80% NO x reduction and an interestingly broad active temperature window (300–550 °C). NO x is reduced by reacting with intermediates generated from partial oxidation of hydrocarbons used as reductants. Increased availability of these species either by more C atoms in higher hydrocarbons, their easier formation from less saturated hydrocarbons or higher reductant concentration explains the improvement of NO x reduction. The performance of Ag catalysts was linked with the formation of different crystal phases, which are stabilized through a strong interaction with the alumina support. Existence of Ag in oxidation state +1 seems to be the active phase that favors most the NO reduction.
Keywords: Ag; Ce; Alumina; NO reduction; Reductant type;

Chiral diphosphine ligands based on an arene chromium tricarbonyl scaffold: a modular approach to asymmetric hydrogenation by Wolfgang Braun; Albrecht Salzer; Felix Spindler; Elisabetta Alberico (191-203).
A planar-chiral diphosphine ligand-class based on an arene chromium tricarbonyl backbone has been devised featuring a modular architecture that allows for the synthesis of a broad ligand library. The substituents on the donor phosphorus atoms contain aliphatic, alicyclic and aromatic groups of different kinds. A (R,R)-2,5-dimethylphospholane moiety is also included. That library has been subdued to a ligand profiling by employment in a number of homogeneous enantioselective hydrogenations including substrates with C=C, C=N and C=O double bonds. These reactions are described and the results are evaluated and discussed.Based on a modular synthetic approach an extensive diphosphine-ligand library with an arene chromium tricarbonyl backbone has been employed in a screening of enantioselective catalytic hydrogenation of unsaturated substrates with C=C, C=N and C=O double bonds.▪.
Keywords: Asymmetric hydrogenation; Rhodium; Diphosphines; Modular ligands;

Immobilized Rh, Ru, Pd and Ni complexes as catalysts in the hydrogenation of cyclohexene by E.A. Cagnola; M.E. Quiroga; D.A. Liprandi; P.C. L’Argentière (205-212).
Complexes of Rh, Ru, Pd and Ni with tridecylamine and chloride as ligands were prepared and tested as heterogeneous catalysts for the hydrogenation of cyclohexene. The Wilkinson’s complex was used as a reference catalyst. Supported complexes turned out to be more active and more resistant to a sulphur poison than the homogeneous systems also evaluated for comparison. The rhodium-tridecylamine complex was the most active, slightly less than the Wilkinson’s complex but more sulphur resistant than the latter. XPS and FTIR studies revealed that the complexes kept their chemical identity and remained attached to the support even after the reaction. This suggests that the coordination compounds studied are the catalytically active species or that they are converted to the actual active species during the catalytic process.
Keywords: Transition metal complexes; Catalytic hydrogenation; Heterogeneous catalysts;

The recyclability of the 2 wt.% Re2O7/7.5 wt.% B2O3/SiO2–Al2O3 catalytic system upon activation with SnBu4 was evaluated in the metathesis of methyl oleate. The activity and selectivity of this system were kept almost constant for 25 runs, but after 31 runs the catalyst was completely deactivated. XPS analyses showed that tin oxide covers selectively rhenium and boron sites. When all rhenium atoms are covered, the catalytic activity is extinguished.
Keywords: Rhenium oxide; Olefin metathesis; Catalyst recycling;

The catalytic destruction of NH3 and SO2 has been examined over a series of alumina supported transition metal oxide catalysts in the laboratory. It was found that alumina supported vanadium oxide is the best candidate for this reaction. These studies show that employing a feed containing 4% NH3 + 3% SO2 and a residence time of 0.75 s at 700 °C, conversions of 100 and 87.7% for NH3 and SO2, respectively, can be achieved over fresh alumina supported vanadium oxide with the main products being only N2, water and sulfur. The effect of pretreatment of the catalyst by sulfidation was also examined in this study. Based on these results it was shown that feeding a simulated industrial sour water stripper gas (SWSG) plus oxygen over alumina supported vanadium oxide at 750 °C with a residence time of 4 sec, resulted in an NH3 conversion of 100%, while no SO2 was detected in the products.
Keywords: NH3; SO2; Sour water stripper gas; Transition metal oxides; Vanadium oxide;

The effect of the calcination temperature on manganese-based catalysts used in the complete oxidation of acetone has been studied. The catalysts, containing manganese and samarium and being in bulk oxide form, were prepared in the laboratory and calcined at 673, 823, 1073 and 1273 K. The calcination temperature controls and modifies the type of single manganese oxide present, the development of the perovskite structure of samarium-manganese oxides, and the specific surface area of these materials. A positive effect on the catalyst performance has been observed for the samarium-manganese oxides with respect to the single manganese ones.
Keywords: Manganese oxide; Samarium-manganese oxide; Samarium-manganese perovskite; Acetone catalytic combustion; VOCs;

The electrochemical reduction of CO2 has been investigated on Pb granule electrodes at high-pressure conditions and high temperatures (electrode potentials between −1.6 and −2.1 V) by using aqueous 0.2 M K2CO3 as electrolyte in a fixed-bed reactor. Only formic acid was found as a reaction product. The maximum Faradaic efficiency of the product was found to be 94% at −1.8 V (SCE). The space–time yield of the reaction was found to be 1.3 × 10−3  kg L−1  h−1 with an A v value of 1.1 cm−1. Transfer coefficient, β, of the reaction was 0.11, and the exchange current density, j o , was found to be 4.1 × 10−4  A cm−2.
Keywords: Electrochemical reduction; Carbondioxide; Fixed-bed reactor; Pb granule electrodes; HCOOH formation;

Sol–gel synthesis, characterization and catalytic properties of Fe–Ti mixed oxides by G. Neri; G. Rizzo; S. Galvagno; G. Loiacono; A. Donato; M.G. Musolino; R. Pietropaolo; E. Rombi (243-251).
Fe2O3-TiO2 solid acid catalysts have been prepared by a sol–gel route, characterized and tested in the isomerization of α-pinene oxide. The textural, morphological and microstructural characterization of the catalysts was carried out by BET surface area measurements, X-ray diffraction (XRD) and scanning electron microscopy with elemental X-ray analysis (SEM-EDX). Characterization data have shown that, on the pure TiO2 catalyst, titania is mainly in the anatase phase. On the contrary the rutile phase and/or Fe2TiO5, amorphous iron–titanium mixed oxides and hematite were found on the Fe-promoted TiO2 catalysts, depending on the Fe content. Temperature programmed desorption (TPD), microcalorimetry and FT-IR spectroscopy of adsorbed probe basic molecules (ammonia or pyridine) have shown that addition of iron to titania favours formation of new acid sites having a stronger Lewis character. Fe2O3-TiO2 catalysts show a higher activity compared to pure titania in the isomerization of α-pinene oxide to campholenic aldehyde. The products distribution was also influenced by the composition of the catalysts. A correlation between the selectivity to campholenic aldehyde and the amount of Lewis acid sites on the catalysts was found. Based on the characterization and catalytic results, Fe–Ti–O mixed structures were proposed to be the active sites for the selective isomerization of α-pinene oxide to campholenic aldehyde.
Keywords: Ti–Fe metal oxides; α-Pinene oxide; Sol–gel.;

Epoxidation of allyl acetate with tert-butyl hydroperoxide over MoO3/α-Al2O3 promoted by pyridine derivatives by Kenta Shimura; Keisuke Fujita; Hiroyoshi Kanai; Kazunori Utani; Seiichiro Imamura (253-257).
Epoxidation of allyl acetate was carried out over MoO3/α-Al2O3 using tert-butyl hydroperoxide (TBHP) as an oxidizing reagent. Only a small amount of glycidyl acetate was formed. The low yield was due to the cleavage of an epoxy-ring of glycidyl acetate with the acid sites of MoO3/α-Al2O3. The acid sites were neutralized with amines to keep the glycidyl acetate intact during the reaction. Pyridine derivatives greatly improved epoxide selectivity, but addition of aliphatic amines and 2,2′-bipyridine only resulted in retardation of the reaction. The volcano-type plot of Hammett’s p-substituent constant versus epoxide selectivity indicated that pyridine derivative also exerts a ligand effect, and non-substituted pyridine was found to the most suitable additive. The optimum amount of loading of MoO3 was 2 wt.%; at this value, MoO3 was highly dispersed on α-Al2O3. The optimum molar ratio of pyridine to MoO3 for epoxide production was 2/3.
Keywords: Epoxidation; Allyl acetate; TBHP; MoO3/Al2O3; Pyridine derivatives;

Oxidation–reduction of Ni/Al2O3 steam reforming catalysts promoted with Mo by Tadeusz Borowiecki; Andrzej Denis; Wojciech Gac; Roman Dziembaj; Zofia Piwowarska; Marek Drozdek (259-267).
Model Ni/Al2O3 catalysts promoted with different amounts of molybdenum were examined in order to explain a strong increase of the resistance to coking in the steam reforming of hydrocarbons. The properties of catalysts were investigated after various treatment conditions in the water/hydrogen mixtures by means of XRD, XPS, temperature-programmed and gravimetric methods. The studies evidenced the enhancement of the surface or bulk oxidation processes in the presence of Mo promoter. The obtained results indicated that on the surface of Ni–Mo catalysts in the steam reforming reaction the presence of larger number of oxygen adatoms may facilitate transformation of hydrocarbon species, restrain formation of inactive carbon deposit.
Keywords: Ni–Mo catalysts; Steam reforming; TPR; TPO; XPS;

A tapered element oscillating microbalance (TEOM) was used to measure the rates of adsorption and desorption of p-xylene from FCC catalyst at 473 K. The original aim of the work was to investigate whether coke in the catalyst, formed by an earlier reaction with isopropanol at 773 K, influenced the adsorption kinetics. It was actually found that p-xylene reacted in the coked catalyst, even though no reaction occurred in the fresh catalyst at this temperature. This is a rare direct observation of coke causing chemical reaction in a system in which no reaction occurs in the absence of coke.
Keywords: Adsorption; Coke; FCC catalyst; Microbalance; TEOM;

The kinetic rate parameters of a reaction rate model are usually estimated using reaction rate measurements obtained at intrinsic kinetic conditions, viz., without limitation of the reaction rate by mass transport, irrespective of the order of the reaction kinetics. For positive order kinetics, this is good practice and the kinetic rate parameters can be safely applied both at intrinsic and mass transport limited conditions. However, for negative order kinetics, erroneous results are obtained because the kinetic rate parameters estimated at intrinsic kinetic conditions fail to predict the reaction rate when mass transport limitation also plays a role. This is demonstrated for the simple case of Langmuir–Hinshelwood (LH) negative-order kinetics and for the, more complex, kinetics of the oxidation of methyl α -D-glucopyranoside on noble metal catalysts. It is concluded that for a negative-order reaction rate model, which is valid for a wide range of reaction conditions, the kinetic rate parameters need to be estimated both at intrinsic and mass transport limited conditions.
Keywords: Kinetic parameter estimation; Intrinsic kinetics; Negative order; Mass transport limitation; Kinetic model; Langmuir–Hinshelwood kinetics; Reactor modelling;

PEM fuel cells utilizing hydrogen and oxygen have shown great promise as future sources of clean, efficient power. Due to difficulties encountered in hydrogen storage and transport, investigation of on-board generation of hydrogen via fuel processing of liquid hydrocarbons has gained prominence. To achieve these goals, the need for compact reactor systems with effective component and heat integration as well as quick start-up times has been observed.This paper underlines the suitability of microreaction technology for the development of compact fuel processing systems as compared to packed-bed reactor technology which does not scale down in a feasible manner. A microchannel reactor was fabricated in silicon using standard microfabrication tools. A 2% Pt/Al2O3 catalyst was washcoated on the channel walls. Preferential CO oxidation in H2 was chosen as a model reaction and the results were compared with those obtained from a conventional packed-bed microreactor. The silicon microreactor performs very well, while offering distinct advantages such as flexibility of reactor design, integration of structural and functional features and low pressure drop. Use of a wall-coated catalyst as compared to a packed-bed catalyst does not introduce any loss of performance efficiency due to external diffusive limitations. Issues of scale-up and efficient heat integration have also been addressed.
Keywords: PEM fuel cells; Hydrogen; Preferential CO oxidation; Silicon; Microreactor; Heterogeneous catalysis; Washcoat;

Characterization and reactivity of Ru/single oxides catalysts for the syngas reaction by Pérez-Zurita M. Josefina; Dufour Muriel; Halluin Yann; Griboval Anne; Leclercq Lucien; Leclercq Ginette; Goldwasser Mireya; Cubeiro M. Luisa; Bond Geoffrey (295-301).
A series of ruthenium-supported catalysts were prepared and characterised to determine the effect of support interaction in the production of higher alcohol from syngas. Single oxides with various degrees of acidity/reducibility such as MoO3, ZrO2, WO3 and TiO2 were used as supports. Al2O3 was used as reference.XPS and TGA results indicated that, on the studied supports, partial reduction to various extents was attained. The reduction of these supports was very different: while no reduction was observed for Al2O3, TiO2 and ZrO2, WO3 was little reduced. MoO3 being the most reduced support.Catalytic tests indicate that the selectivity towards oxygenates seems to be linked to the reducibility of the support, since with non-easily reducible oxides (Al2O3, ZrO2 and TiO2) the alcohol production was very low, while higher selectivity was obtained on WO3 and especially on MoO3, the reducible oxides. A SMSI effect is invoked.
Keywords: Ruthenium; CO hydrogenation; Syngas; XPS;

Migration of surface species on supports: a proof of their role on the synergism between CoS x or NiS x and MoS2 in HDS by P. Baeza; M.S. Ureta-Zañartu; N. Escalona; J. Ojeda; F.J. Gil-Llambías; B. Delmon (303-309).
Synergetic effects in hydrodesulfurization (HDS) have been investigated in operating conditions different from all those mentioned in literature yet. A synergism is observed between Co(x)/γ-Al2O3 or Ni(x)/γ-Al2O3, and Mo(y)/γ-Al2O3 beds separated by a 5 mm of γ-Al2O3 or SiO2 bed. The results were obtained in conditions very close to those of industrial processes, namely using a high-pressure continuous-flow-reactor and gas-oil as feed, in contrast to model feeds used in most similar experiments. Compared to the activity of the individual beds, a significant increase of HDS activity is observed at 325, 350 and 375 °C, in the case of Co and Ni. As the Co (Ni) containing beds are physically separated from each other, no mutual contamination susceptible to lead to the formation of Co–Mo–S or Ni–Mo–S is possible. Several other alternative explanations have been examined, but it seems that the logical conclusion is that spillover of hydrogen could explain the phenomenon. No synergetic effect was detected using SiC as separator. The effects of changes in the cobalt or nickel (x), and molybdenum (y) contents and of the reaction temperature over the synergetic effect are also described and discussed. The effect of a diffusion of spillover species (probably hydrogen Hso) over a 5 mm distance is so conspicuous that it seems difficult to rule out an important flow of Hso and thus a much stronger effect in real CoMo and NiMo catalysts where the sulfided elements are at nanometer distances from each other. This makes even reasonable the hypothesis that hydrogen spillover could actually play an essential role in the HDS reaction over molybdenum-containing catalysts.
Keywords: Hydrodesulfurization; Spillover hydrogen; Operating conditions; Remote control; Composite bed;