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Applied Catalysis A, General (v.305, #2)

Editorial Board (pp. co2).

Effect of Al-SBA-15 support on catalytic functionalities of hydrotreating catalysts by G. Muthu Kumaran; Shelu Garg; Kapil Soni; Manoj Kumar; L.D. Sharma; G. Murali Dhar; K.S. Rama Rao (pp. 123-129).

SBA-15 and Al-SBA-15 of varying Si/Al ratios (10–40) were synthesized by well-known procedures. Characterization using XRD, pore size distribution,²⁷Al NMR confirmed that hexagonal mesoporous structure is obtained and majority of Al is in tetrahedral positions. Mo, CoMo and NiMo catalysts prepared using these supports were examined by XRD, oxygen uptakes in sulfided state and temperature programmed reduction (TPR). The catalysts were tested for hydrodesulfurization (HDS) of thiophene and hydrogenation of cyclohexene. Systematic variation of oxygen chemisorption and catalytic activities were observed as a function of Si/Al ratio. The systematic variation of activities, oxygen chemisorption and TPR hydrogen consumption of Mo indicated that the molybdenum dispersion and anion vacancies, and catalytic activities are significantly influenced by support Al content. Compared to SBA-15, Al-SBA-15 supported Mo, CoMo and NiMo catalysts show outstanding activities for hydrogenation.

Keywords: SBA-15; Al-SBA-15; HDS; Hydrogenation; Mo, CoMo and NiMo hydrotreating catalysts

Physico-chemical properties of mixed molybdenum and cerium oxides supported on silica–alumina and their use as catalysts in the thermal-catalytic cracking (TCC) of n-hexane by Nabil Al-Yassir; Raymond Le Van Mao (pp. 130-139).

Mixed oxides, MoO₃–CeO₂, were being used as catalysts for the cracking (TCC) of liquid hydrocarbon feedstocks. The dispersion and interactions of MoO₃, CeO₂ and mixtures thereof impregnated into the silica–alumina surface were investigated using several techniques, which included X-ray diffraction (XRD) and laser Raman spectroscopy (LRS). The loadings and the chemical states of metal oxides incorporated separately had significant effects on the catalytic activities of the resulting monocomponent catalysts. Addition of cerium to molybdenum had a favorable effect on the production of light olefins in the TCC of n-hexane up to a certain level of cerium loading. In fact, high loadings of molybdenum and/or cerium favored the formation of aromatics, instead. The catalytic performance of the bicomponent catalysts also depended significantly on the incorporation methods. It was found that the co-impregnation of MoO₃ and CeO₂, which led to the highest production of light olefins, corresponded to the formation of (surface) cerium molybdate to the highest extent. On the other hand, the catalysts prepared by the two-step impregnation methods (sequential and reverse sequential impregnation) showed much lower catalytic performance due to low Mo–Ce interactions as suggested by an important segregation of the active phases, mostly MoO₃. The sequence of catalytic performance (to the desired products, i.e. light olefins) fully coincided with that of the dispersion of molybdate species on the support surface.

Keywords: Thermal-catalytic cracking (TCC); Supported MoO; ₃; –CeO; ₂; X-ray diffraction (XRD); Laser Raman spectroscopy (LRS)

Cyclohexane oxidation catalyzed by mononuclear iron(III) complexes by Nakédia M.F. Carvalho; Adolfo Horn Jr.; O.A.C. Antunes (pp. 140-145).

In this work, we present the oxidation of cyclohexane catalyzed by a family of mononuclear iron(III) complexes: [Fe(BMPA)Cl₃]1, [Fe(MPBMPA)Cl₃]2, [Fe(PBMPA)Cl₂]3 and [Fe(PABMPA)Cl₂](ClO₄)4 using hydrogen peroxide or tert-butyl hydroperoxide as oxidant, in acetonitrile solution. These complexes were able to oxidize the cyclohexane into cyclohexanol and cyclohexanone with good yields. It was also possible to characterize by gas chromatography and mass spectrometry the by-products, cyclohexyl hydroperoxide and tert-butyl cyclohexyl peroxide. Adipic acid (AA) was also formed in the reaction and it was determined by titration. The reactions with hydrogen peroxide exhibited much greater yields (about 30% for all the complexes) than when tert-butyl hydroperoxide was employed (about 17% of yield with complex1). The alcohol/ketone ratio in the reactions with hydrogen peroxide after 24h was around 1.5, indicating cyclohexanol selectivity, while with tert-butyl hydroperoxide the ratio was around 0.7–1.0. In conclusion, the studied complexes can be considered good catalysts to oxidize the cyclohexane in mild conditions.

Keywords: Selective cyclohexane oxidation; Cyclohexanol; Cyclohexanone; Cyclohexyl hydroperoxide; tert; -Butyl cyclohexyl peroxide; Adipic acid; Iron(III) mononuclear complexes; Peroxides; Methane monooxygenase models

Partial oxidation of n-hexadecane at short contact times: Catalyst and washcoat loading and catalyst morphology by N.J. Degenstein; R. Subramanian; L.D. Schmidt (pp. 146-159).

Rhodium supported on alumina foam is an exceptionally active catalytic partial oxidation catalyst, giving high selectivities to syngas or olefins by changing reaction stoichiometry or catalyst properties such as rhodium loading and washcoat loading. Rhodium catalyst loading and γ-alumina washcoat loading were varied systematically on α-alumina foams to investigate resulting microstructure and products of the partial oxidation of n-hexadecane.At conditions favorable for producing syngas, varying rhodium loading between 0.05 and 10.0wt.% (a factor of 200) had little affect on H₂ and CO selectivities or fuel conversions. However, non-washcoated catalysts gave low selectivities to syngas. Increasing washcoat loading results in high selectivities of olefins, while varying rhodium loading has little affect on production of olefins. Selectivitiy data from multi-metallic catalysts, Rh–Ce and Rh–Pt, was also investigated.The morphology of catalysts was examined using scanning electron microscopy. The micrographs indicate that increasing rhodium loading above a few percent led to the formation of a rhodium film on the catalyst support. The presence of washcoat seems to keep rhodium spread on the surface at high reaction temperatures enhancing heterogeneous products. Very high washcoat loadings (10wt.%) decrease pore diameters, increasing the likelihood of reactant contact with the rhodium surface, leading to increased H₂ and decreased olefin selectivities.

Keywords: Partial oxidation; Rhodium; Hexadecane; Millisecond reactor; Monolith

Open flow hot isostatic pressing assisted synthesis of unsupported MoS₂ catalysts by Mohammad H. Siadati; Gabriel Alonso; Brenda Torres; Russell R. Chianelli (pp. 160-168).

We report the development and application of a new technique for synthesizing highly porous unsupported catalytic materials. The new technique is called open-flow hot isostatic pressing (OFHIP) technique. In this technique, an isostatic pressure is first applied to the catalyst precursor in an open flow fashion, and then heat is applied. Under this condition, as the organic components gradually decompose and leave the material, the voids left behind are immediately filled by the gas (pressure medium) in flow. This substitution warrants the preservation as well as the uniformity of the voids/pores. The result is a very porous material with uniform pore size distribution.The general goal was to determine the optimal conditions of temperature and pressure for best catalytic activity results. Results indicated that besides temperature and pressure, the catalyst precursor had significant effects, as well.The unpromoted MoS₂ samples synthesized at 300°C indicate that as the synthesis pressure increased, both surface area and catalytic activity of the materials produced increased. The catalytic activity k value increased by a factor of 2 from 3 to 6×10⁻⁷mol/gs that corresponds to increase in pressure from 100 to 800psi (6.9 to 55.2bar), respectively. The N₂ gas used as pressure medium resulted in highly porous materials but low activity. H₂ appeared to be the ideal gas for both pressure medium and reducing agent. Co-promoted MoS₂ catalysts synthesized at 1400psi (96.5bar) and 300°C showed catalytic activity as high as 37mol/gs.

Keywords: Unsupported catalyst synthesis; HDS; Porous materials; Open flow method; Hot isostatic pressure

Enhancement of gas–liquid mass transfer during the unsteady-state catalytic decomposition of ozone in water by R. Rosal; A. Rodríguez; M. Zerhouni (pp. 169-175).

Unsteady mass transfer and kinetic experiments were conducted in a gas–liquid reactor to study the catalytic and non-catalytic decomposition of ozone in water. In the absence of catalyst, first-order decomposition constants and volumetric mass transfer coefficients were determined at temperatures between 20°C and 30°C. Catalytic runs were performed on fumed colloidal TiO₂, which primary particles had an average diameter of 20nm. The evolution of the concentration of ozone in water after introducing a given amount of catalyst was fitted to a kinetic model that assumed two-stage adsorption–decomposition on the catalytic surface. Runs were performed under slightly acidic conditions allowing the adsorption of ozone molecules on Lewis acid sites. A significant enhancement of mass transfer has been linked to the presence of particles. This effect could be attributed to a physical shuttle mechanism involving the adsorption of ozone on surface adhering particles. For bulk concentrations of catalyst up to 0.65kgm⁻³, the fraction of interface coverage did not reach saturation. The observed behaviour is consistent with the hydrophilic character of titanium oxide. Reaction constants and activation energies were determined for the two chemical steps describing catalytic ozone decomposition.

Keywords: Adsorption; Catalysis; Ozonation; Enhancement; Titanium oxide

Liquid phase oligomerization of 1-hexene over different mesoporous aluminosilicates (Al-MTS, Al-MCM-41 and Al-SBA-15) and micrometer/nanometer HZSM-5 zeolites by R. Van Grieken; J.M. Escola; J. Moreno; R. Rodríguez (pp. 176-188).

The liquid phase oligomerization of 1-hexene at 200°C and 5MPa using n-octane as solvent towards hydrocarbon mixtures useful as fuels (gasoline and diesel) was tested over several acid catalysts: micrometer (μ-) and nanocrystalline (n-) HZSM-5 zeolites, mesoporous hydrothermal Al-MCM-41, and sol–gel Al-MTS and Al-SBA-15 catalysts. The conversion was always above 75% except for μ-HZSM-5 (just 8.4%) due to its low external surface area (5m²g⁻¹) and the fast deactivation in the reaction conditions used in this work. The total selectivity towards oligomers was around 95% and the highest share of C₉–C₁₂ dimers (47%), C₁₃–C₁₈ trimers (33%) and heavy C₁₉–C₃₀ compounds (33%) were yielded over Al-SBA-15, n-HZSM-5 and sol–gel Al-MTS, respectively. The remarkable oligomerization performance of n-HZSM-5 was ascribed to its high external surface area (102m²g⁻¹) and for the mesoporous catalysts, to their large BET surface area. In particular, Al-MTS showed the best behaviour due to its higher BET surface area and slightly weaker acidity. All the catalysts exhibited steady-state performance with time on stream (TOS) without drastic changes in activity up to 180min. Simulated distillation analyses proved that the lighter fuel (gasoline+diesel) was obtained over Al-MTS (final distillation temperature=463°C; C₂₆–C₃₂=8.4%) while the heaviest was obtained over n-HZSM-5 zeolite (final distillation temperature=524°C; C₂₈–C₄₀=11.7%), probably related to its stronger acidity and microporous nature. The similar nature of hydrocarbons compounds retained over the catalysts after reaction proved by FTIR spectroscopy together with the thermogravimetric analyses results, showed the stronger adsorption of the reaction products promoted by the microporous nature of zeolites.

Keywords: Oligomerization; 1-Hexene; Nanocrystalline HZSM-5; Al-MTS; Al-SBA-15; Fuels

Copper ion-exchanged and impregnated Fe-pillared clays by Fernando Dorado; Antonio de Lucas; Prado B. García; Amaya Romero; José L. Valverde (pp. 189-196).

Copper ion-exchanged and impregnated iron-pillared interlayer clays (Fe-PILCs), prepared by using copper solutions with different pH values and copper precursors, were tested in the selective catalytic reduction (SCR) of NO_x by propene. Low pH values led to dehydroxylation of the iron pillars and damages in the clay structure. Samples prepared without pH control presented isolated Cu²⁺ ions; those prepared by using alkaline pH solutions presented Cu²⁺ ions and CuO clusters. It was observed that the presence of Cu²⁺ ions accompanied by CuO clusters led to an improvement in the NO_x yield to N₂, which was in agreement with the best activity for C₃H₆ combustion showed by these catalysts. The best catalytic results were obtained with samples prepared by ion exchange with copper II acetate under alkaline pH, where Cu²⁺ ions and CuO clusters were simultaneously formed in an adequate proportion.

Keywords: Cu; NO-SCR; Iron-pillared clays; Impregnation; Ion exchange

Preparation and characterization of VO_x/TiO₂ catalytic coatings on stainless steel plates for structured catalytic reactors by Thierry Giornelli; Axel Löfberg; Elisabeth Bordes-Richard (pp. 197-203).

The parameters to be controlled to coat metallic walls by VO_x/TiO₂ catalysts which are used in the mild oxidation of hydrocarbons and NO_x abatement are studied. Stainless steel (316L) was chosen because of its large application in industrial catalytic reactors. TiO₂ films on stainless steel were obtained by dip-coating in two steps. Superficially oxidized plates were first dipped in Ti-alkoxide sol–gel to be coated by a very thin layer of TiO₂. On this anchoring layer was then deposited a porous film of titania by dipping plates in an aqueous suspension of TiO₂ particles. After calcination, VO_x species were grafted to TiO₂/SS plates and their loading was determined by means of X-ray photoelectron spectroscopy. The chemical and mechanical resistances of films were controlled by several tests. Laser Raman spectroscopy, X-ray diffraction and scanning electron microscopy were used to characterize the samples after each step of preparation. The porous texture was determined using a thermobalance. The dispersion and the nature of VO_x species and the value of theoretical monolayer of VO_x on TiO₂/stainless steel are shown to depend on the surface V/Ti ratio, in the same manner as for VO_x/TiO₂ coating anodised aluminum plates and as for VO_x/TiO₂ powders. Therefore, we have demonstrated that the shaping of TiO₂ has no influence on the characteristics of the active phase, which is of prime importance for catalytic applications in structured reactors.

Keywords: Structured reactors; Catalytic wall reactors; Stainless steel; Dip-coating; V; ₂; O; ₅; /TiO; ₂; catalyst; XPS

Role of redox couples of Rh⁰/Rh δ⁺ and Ce⁴⁺/Ce³⁺ in CH₄/CO₂ reforming over Rh–CeO₂/Al₂O₃ catalyst by Rui Wang; Hengyong Xu; Xuebin Liu; Qingjie Ge; Wenzhao Li (pp. 204-210).

The interaction between Rh and CeO₂ over the Rh–CeO₂/Al₂O₃ catalyst was investigated for the reaction of methane reforming with CO₂. It was shown that the activity and coke resistance of Rh/Al₂O₃ were enhanced by the addition of CeO₂, which greatly depend on the interaction between rhodium and ceria under the reaction atmosphere. In situ electrical conductivity results showed that the reducing agents (CH₄, H₂) in the reaction system could be activated and dissociated on Rh⁰, releasing electrons to CeO₂ in close contact with Rh⁰ and generating the Ce⁴⁺/Ce³⁺ redox couple. Meanwhile, it was found from XPS measurements that the electron transfer could also happen from Rh⁰ to CeO₂, creating the Rh⁰/Rh δ⁺ couple. Thus, the very coexistence of Ce⁴⁺/Ce³⁺ and Rh⁰/Rh δ⁺ redox couples facilitated the activation of CH₄ and CO₂ and further enhanced the catalytic activity and coke resistance of Rh/Al₂O₃. For CH₄ activation, the electron-deficient state of Rh δ⁺ had higher ability to accept σ electrons of CH₄ to promote CH₄ adsorption and C–H bond cleavage, while CO₂ activation was mainly facilitated by accepting free electrons of Ce³⁺ species, which resulted in the enhancement of carbon elimination to yield CO. Finally, a cycle mechanism of redox couples over Rh–CeO₂/Al₂O₃ in CH₄/CO₂ reforming was proposed.

Keywords: CH; ₄; /CO; ₂; reforming; Rh–CeO; ₂; catalyst; Redox couples

Acid–base reactions on alumina-supported niobia by Mona A. Abdel-Rehim; Ana Carlota B. dos Santos; Vera Lúcia L. Camorim; Arnaldo da Costa Faro Jr. (pp. 211-218).

Catalysts containing from 8 to 28%wt. of niobia supported on γ-alumina by impregnation with niobium pentaethoxide solutions were characterized by infrared spectroscopy of adsorbed pyridine as a probe for acidic sites and CO₂ adsorption as a probe for basic sites. The catalysts had their activity measured in isopropanol dehydration at 453K, 1-butene isomerization at 348K and in cumene dealkylation at 728K. The density and strength of the Lewis acid sites and of the basic sites decreased with niobium content. On the other hand, the density of Brønsted acid sites increased in the same direction. Different reactions responded differently to niobium addition. In isopropanol dehydration, the main factor responsible for the observed decrease in catalytic activity was the decrease in concentration of basic sites. Activity for 1-butene isomerization also decreased with niobium addition, but the main factor seemed to be the decrease in concentration of alumina-associated Lewis acidic sites. Evidence from cis-/ trans-2-butene ratio indicated that niobium addition modifies the properties of neighboring aluminum sites. Brønsted acidic sites created by niobium addition are responsible for the development of activity in the cumene dealkylation reaction, but the sites associated with tridimensional niobia species seem to be more effective in this reaction.

Keywords: Niobium oxide; Alumina; Isopropanol dehydration; 1-Butene isomerization; Cumene cracking

Improvement of the thermal stability of hydrous zirconia by post-synthesis treatment with NaOH and NH₄OH solutions by G. Aguila; S. Guerrero; F. Gracia; P. Araya (pp. 219-232).

The structural stability of a commercial hydrous zirconia and a hydrous zirconia synthesized in our laboratory by the sol–gel method has been improved by a post-synthesis reflux treatment with aqueous NH₄OH and NaOH solutions (100°C for 9h). Direct calcination of both materials at 700°C for 3h, rendered zirconium oxides with a mixture of monoclinic and tetragonal structures and specific areas of 36 and 27m²/g, respectively. In contrast, if the materials were refluxed before calcination with NaOH or NH₄OH solutions, zirconium oxides with tetragonal structure and specific areas between 90 and 200m²/g were obtained after calcination at 700°C. In the treatments with NaOH solution, addition of Si coming from dissolution of the flask glass walls or from an external Si source in a plastic flask is responsible for the stability of the final zirconium oxides. Nevertheless, the porous structure of these materials is determined by the treatment conditions in basic solution, thus allowing to control the physical properties of the final oxide. Short treatment times are sufficient to stabilize the hydrous zirconia. On the contrary, if the reflux is made with a NH₄OH solution, the stabilization mechanism appears to be different since there is no evidence of Si addition in the oxides. In this case, the pore size distribution strongly depends on the reflux time, and the highest specific area is obtained for the largest treatment time (9h).

Keywords: Zirconium oxide; High surface area; NaOH treatment; NH; ₄; OH treatment

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Applied Catalysis A, General (v.305, #2)