Mechanistic Insight in the Ethane Dehydrogenation Reaction over Cr/Al2O3 Catalysts

A Cr/Al₂O₃ alkane dehydrogenation catalyst exhibits a maximum in ethylene yield during an ethane dehydrogenation cycle. Isotopic labelling experiments with monolabelled ¹³C-ethane and deuterium were used to elucidate whether the initial activity increase could be due to formation of an active, larger hydrocarbon intermediate on the surface. The results strongly indicate that this is not the case, and instead point to a traditional reaction cycle involving adsorption of ethane to form an ethyl species, followed by desorption of ethene and hydrogen. Transient kinetic data suggest that ethane adsorption is the rate-determining step of reaction.     

Chromium Oxide Supported on Mesoporous SBA-15 as Propane Dehydrogenation and Oxidative Dehydrogenation Catalysts

The catalytic activity and selectivity of Cr₂O₃ supported on mesoporous SBA-15 for non-oxidative and oxidative dehydrogenation of propane by O₂ and CO₂ have been studied and compared with those of Cr₂O₃/ZrO₂ and Cr₂O₃/-Al₂O₃ catalysts. Cr₂O₃/SBA-15 and Cr₂O₃/ZrO₂/SBA-15 are more selective to propene and more resistant to coking in comparison with Cr₂O₃/ZrO₂ and Cr₂O₃/-Al₂O₃ for non-oxidative dehydrogenation of propane. In oxidative dehydrogenation of propane by O₂ and CO₂, Cr₂O₃/SBA-15 also displays better activity, selectivity and stability than the other two supported catalysts. The propane conversion and propene yield on Cr₂O₃/SBA-15 catalyst for oxidative dehydrogenation of propane by CO₂ at 823 K reach 24.2 and 20.3%, respectively. XPS and TG/DTA have been used to characterize the catalysts before and after reaction. The differences in catalytic behavior of various supported Cr₂O₃ catalysts in the reactions have been discussed on the basis of the characterization results.     

Dehydrogenation of ethylbenzene to styrene over Fe2O3/Al2O3 catalysts in the presence of carbon dioxide

An Fe₂O₃ (10 wt%)/Al₂O₃ (90 wt%) catalyst prepared by a coprecipitation method was found to be effective for dehydrogenation of ethylbenzene to produce styrene in the presence of CO₂ instead of steam used in commercial processes. The dehydrogenation of ethylbenzene over the catalyst in the presence of CO₂ was considered to proceed both via a one-step pathway and via a two-step pathway. CO₂ was found to suppress the deactivation of the catalyst during the dehydrogenation of ethylbenzene. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic Properties of Cr2O3 Doped with MgO Supported on MgF2 and Al2O3

Catalytic properties of Cr₂O₃ supported on MgF₂ or Al₂O₃ have been modified by magnesium oxide. The catalysts have been obtained by the co-impregnation method and characterised by: BET, XRD and TPR. As follows from the results, the oxides supported on magnesium fluorine react with each other already at 400 °C, leading to formation of an amorphous spinel-like phase. On the Al₂O₃ support such an MgCr₂O₄ spinel has appeared at much higher temperatures. The addition of magnesium oxide has a significant effect on the activity and selectivity of the catalysts studied in the CO oxidation reaction at room temperature and in the reaction of cyclohexane dehydrogenation. The magnesium–chromium catalysts supported on MgF₂ have been found to show much higher activity and selectivity than the analogous systems supported on Al₂O₃.     

The Effects of Al2O3 Addition and WO3 Modification on Catalytic Activities of NiO/TiO2 for Acid Catalysis

NiO/Al₂O₃–TiO₂/WO₃ catalysts for acid catalysis were prepared by the addition of Al₂O₃ and the modification with WO₃. The strong acid sites were formed through the bonding between dispersed WO₃ and TiO₂. The larger the dispersed WO₃ amount, the higher both the acidity and catalytic activity. The addition of Al₂O₃ up to 5 mol% enhanced acidity and catalytic activity of NiO/Al₂O₃–TiO₂/WO₃ gradually due to the interaction between Al₂O₃ and TiO₂ and consequent formation of Al–O–Ti bond. The presence of NiO may attract reactants and enhance the local concentration of reactants near acid sites and consequently increase catalytic activity.     

Catalytic Activities of Al2O3-promoted NiSO4/TiO2 for Acid Catalysis

A series of catalysts, NiSO₄/Al₂O₃–TiO₂, for acid catalysis was prepared by the impregnation method, where support, Al₂O₃–TiO₂ was prepared by the coprecipitation method using a mixed aqueous solution of titanium tetrachloride and aluminum nitrate solution followed by adding an aqueous ammonia solution. The addition of nickel sulfate (or Al₂O₃) to TiO₂ shifted the phase transition of TiO₂ from amorphous to anatase to higher temperature because of the interaction between nickel sulfate (or Al₂O₃) and TiO₂. 15-NiSO₄/5-Al₂O₃–TiO₂ containing 15 wt% NiSO₄ and 5 mol% Al₂O₃, and calcined at 400°C exhibited maximum catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation. The catalytic activities for both reactions were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The charge transfer from Ti atoms to the neighboring Al atoms strengthens the Al–O bond between Al and the surface sulfate species. The addition of Al₂O₃ up to 5 mol% enhanced the acidity, thermal property, and catalytic activities of NiSO₄/Al₂O₃–TiO₂ gradually due to the interaction between Al₂O₃ and TiO₂ and consequent formation of Al–O–Ti bond.     

Surface Functionalization of Al2O3 Nanoparticles with Biocompatible Modifiers,Preparation and Characterization of Poly(Vinyl Pyrrolidone)/Modified Al2O3 Nanocomposites

In this present investigation, for increasing compatibility between nanoparticles and polymer matrix as well as preventing of aggregation, surface modification of aluminum oxide nanoparticles was performed with citric acid (CA) and ascorbic acid as biosafe and environmentally friendly modifier agents. For fabrication of nanocomposites, different contents (3, 5, 7?wt%) of nanoparticles were incorporated into the poly(vinyl pyrrolidone) matrix by ultrasonication technique. Transmission electron microscopy images of nanocomposites show a good dispersion of nanoparticles into the polymer matrix. Thermogravimetric analysis results show that the thermal stability of the nanocomposites shifted to higher temperature in comparison with the neat polymer matrix.     

Preparation of MoO3/Al2O3 Catalysts with Sharp Eggshell Mo Distribution by Slurry Impregnation

MoO₃/Al₂O₃ catalysts with eggshell Mo concentration profiles were prepared by reaction of Al₂O₃ extrudates or balls with slurry of MoO₃ in water. The Mo concentration wave penetrating Al₂O₃ particles during this slurry impregnation was almost rectangular. Its height was close to the filled monolayer loading. The thickness of the shell was regulated either by impregnation time or by the MoO₃ amount in the slurry. The hydrodesulfurization activity of Mo species deposited by slurry impregnation was about the same or better (depending on the Al₂O₃ used) as in industrial MoO₃/Al₂O₃ catalyst.     

Characterization and Catalytic Behavior of VO x -CeO2 Catalysts for the Oxidative Dehydrogenation of Propane

A series of ceria-supported vanadium catalysts was prepared via impregnation of the support with an ammonium metavanadate solution. The 723 K calcined samples were tested for propane oxydehydrogenation (ODH) activity and selectivity. The sample exhibiting the highest propane conversion was found to be the ceria support material itself, although this showed essentially no selectivity towards propene. An optimum propene yield of 4.2% was obtained at 673 K for the 6 wt% V₂O₅-CeO₂ sample. Conversion decreased with increasing V loading which was attributed to the formation of cerium vanadate (CeVO₄). This phase was found in all samples after calcination, its abundance rising in proportion to the V loading. In the 6 wt% V₂O₅ catalyst hydrated surface VO_x species were present, although they underwent conversion to CeVO₄ at temperatures above 573 K. The low reducibility of these surface vanadates was linked to the oxidation activity. It is inferred that surface polyvanadate species are responsible for the selective ODH of propane with V-O-V and/or V-O-Ce being the active oxygen species.     

Oxidative Conversion of Propane over Al2O3-Supported Molybdenum and Chromium Oxides

Oxidative dehydrogenation of propane has been studied on Mo/-Al₂O₃ catalysts with 13 wt% of MoO₃ and promoted with Cr. The catalysts were characterized by BET, X-ray diffraction, XPS, TPR, TPO and isopropanol decomposition. The ODH results indicated an important increase in propane conversion with Cr loading increase from 0 to 5 wt%. At 773 K the conversion increased 1.5 times whereas the selectivity to propene was not significantly modified. The higher activities obtained on Cr-doped catalysts provide for the technologically important possibility of carrying out the reaction at lower temperatures.     

Propane Dehydrogenation over a Commercial Pt-Sn/Al2O3 Catalyst for Isobutane Dehydrogenation: Optimization of Reaction Conditions

The applicability of a commercial Pt-Sn/Al₂O₃ isobutane dehydrogenation catalyst in dehydrogenation of propane was studied. Catalyst performance tests were carried out in a fixed-bed quartz reactor under different operating conditions. Generally, as the factors improving propane conversion decrease the propylene selectivity, the optimal operating condition to maximize propylene yield is expected. The optimal condition was obtained by the experimental design method. The investigated parameters were temperature, hydrogen/hydrocarbon (H₂/HC) ratio and space velocity, being changed in three levels. Constrains such as the susceptibility of the catalyst components to sintering or phase transformation were also taken into account. Activity, selectivity and stability of the catalyst were considered as the measured response factors, while the space-time-yield (STY) was considered as the variable to be optimized due to its commercial interest. A STY of 16 mol·kg^-1·h^-1 was achieved under the optimal conditions of T 620℃, H₂/HC 0.6 and, weight hourly space velocity (WHSV) 2.2 h^-1. Single carbon-carbon bond rupture was found to be the main route for the formation of lower hydrocarbon byproducts.     

Surface Properties and Reactivity of Al2O3-Supported MoO3 Catalysts in Ethane Oxidative Dehydrogenation

The effect of MoO₃ loading on the properties and the catalytic performance of a series of alumina-supported molybdena catalysts (0–30 wt% MoO₃) was investigated in the oxidative dehydrogenation of ethane. The molybdena species on alumina were found to be amorphous at submonolayer coverages. At higher loadings, the formation of Al₂(MoO₄)₃ crystallites was detected by XRD. XPS revealed the existence of both Mo(VI) and Mo(V) sites on the catalyst surface, the concentration of which depends on the MoO₃ loading. In terms of catalytic performance, the activity increases with increasing loading in the submonolayer regime, decreasing for higher loadings. High selectivity to ethene is obtained even at relatively high conversion levels for catalysts exceeding monolayer coverage.     

Dehydrogenation of ethylbenzene over TiO2-Fe2O3 and ZrO2-Fe2O3 mixed oxide catalysts

The mixed metal oxides TiO₂-Fe₂O₃ and ZrO₂-Fe₂O₃ were examined as potential catalysts for the dehydrogenation reaction of ethylbenzene. The acidic and basic properties and surface area, pore volume and pore size distribution of these catalysts were measured. The catalytic activities can be correlated very well with the surface area and the acidity and basicity of ZrO₂-Fe₂O₃ catalysts. However, for TiO₂-Fe₂O₃ catalysts, the surface area, the amount of acidic and basic sites and TiFe₂O₅ crystallinity are all important factors affecting the catalytic activities for ethylbenzene dehydrogenation. A synergistic effect was found for the TiO₂-Fe₂O₃ and ZrO₂-Fe₂O₃ catalyst system and also for the TiO₂-Fe₂O₃-ZrO₂ system, i.e. the activities of these catalysts can be ranked in the following order: TiO₂-Fe₂O₃-ZrO₂>TiO₂-Fe₂O₃ >ZrO₂>Fe₂O₃>TiO₂. Meanwhile, all of these catalysts showed higher activities than the conventional potassium-promoted iron catalysts.     

The reduction behavior of Mo/TiO2-Al2O3 catalyst

The effect of TiO₂ modified Al₂O₃ surface on the reducibility of MoO₃ has been studied by TPR and XPS. The results show that Mo⁶⁺ in Mo/TiO₂-Al₂O₃ can be reduced to much lower valency, especially at low Mo loading. The influence of the calcination temperature on the reduction of Mo⁶⁺ on Al₂O₃ and TiO₂-Al₂O₃ carriers is different. The data reveals that the reducibility of Mo⁶⁺ on Al₂O₃ slightly decreased, while that on TiO₂-Al₂O₃ increased when the calcination temperature was raised. It is suggested that the stronger tetrahedral site of the Al₂O₃ surface was first occupied by TiO₂ and main octahedral Mo⁶⁺ in polymeric species-; and a small crystalline MoO₃ formed on TiO₂-Al₂O₃, whereas the formation of tetrahedral Mo⁶⁺ species and Al₂(MoO₄)₃ phase was inhibited.     

Investigation of the sulfidation of Mo/TiO2-Al2O3 catalysts by TPS and LRS

A series of Mo/Al₂O₃ and Mo/TiO₂-Al₂O₃ catalysts were investigated by temperature programmed sulfiding (TPS) and laser Raman spectroscopy (LRS). The effect of TiO₂ on the sulfidability of molybdena was studied in detail. It is found that Mo/Al₂O₃ catalysts can be partially sulfided by O-S exchange at low temperature, forming molybdenum oxysulfide. The Mo-S bond subsequently ruptures in the presence of H₂ to produce H₂S. At 530–550 K deep sulfiding of molybdenum oxysulfide occurs forming crystalline MoS₂. When the surface of Al₂O₃ was covered by a monolayer of TiO₂, the sulfiding rate of molybdena at low temperature was not only greatly increased, but H₂S produced in the reduction of Mo-S species caused deep sulfiding of the catalyst which resulted in a decrease of the TPS peak temperature by 80–100 K. The results indicate that this promotion of the sulfiding of molybdena is enhanced with TiO₂ loading. The function of TiO₂ is explained by the weakened interaction between MoO₃ and Al₂O₃ due to the coverage of the Al₂O₃ surface by TiO₂.     

Ru-Promoted CrO x /Al2O3 Catalyst for the Low-Temperature Oxidative Decomposition of Trichloroethylene in Air

A series of Ru-promoted CrO _x /Al₂O₃ as catalysts for the low-temperature oxidative decomposition of trichloroethylene (TCE) were characterized and evaluated in comparison with an unpromoted CrO _x /Al₂O₃ catalyst. Catalyst characterization was conducted by surface area measurement, X-ray diffraction and X-ray photoelectron spectroscopy. Catalyst performance in the TCE decomposition reaction was evaluated with respect to the initial catalytic activity, the rate of catalyst deactivation, and the product concentrations of CO and Cl₂ under dry or wet air conditions. The presence of a small amount of Ru, as much as 0.4 wt% in a CrO _x /Al₂O₃ catalyst, brought about several beneficial effects on the catalytic reaction performance. As compared with the unpromoted CrO _x /Al₂O₃, this Ru-promoted CrO _x /Al₂O₃ catalyst showed enhanced catalytic activity (249 versus 264 °C in terms of temperature at which 50% of TCE conversion occurred), a reduced concentration of CO (180 versus 325 ppm) in the product, and a decreased propensity to deactivation. Performance improvements of the Ru-promoted CrO _x /Al₂O₃ catalyst were thought to originate from its enhanced oxidation activity due to the coexisting highly-dispersed Ru oxides rendering less active Cr(III) to more active Cr(VI), and facilitating the process of supplying activated oxygen for the reaction system.     

Dynamical Changes in Cu/ZnO/Al2O3 Catalysts

A combination of various transient and steady-state kinetic experiments was used to provide evidence for dynamical changes in a Cu/ZnO/Al₂O₃ catalyst of industrial interest. From these it can be deduced that the reversible structural alterations strongly depend on the reaction conditions as well as on the pretreatment. The pretreatment was found to induce changes in the morphology of the metallic Cu particles to some extent, and surface alloying under more severe reducing conditions.     

Semiconductive and Redox Properties of Ti and Zr Pyrophosphate Catalysts (TiP2O7 and ZrP2O7). Consequences for the Oxidative Dehydrogenation of n-Butane

The electrical conductivity of titanium and zirconium pyrophosphates used as catalysts in n-butane oxidative dehydrogenation has been measured under oxygen and n-butane at 400 and 500 °C and under subsequent exposures to both gases at the catalytic reaction temperature. The two compounds appeared to be p-type semiconductors under air with positive holes as the main charge carriers but became n-type when contacted with n-butane. If their conductivities are comparable as p-type semiconductors (within one order of magnitude), by contrast, they differ by 3 orders of magnitude when being n-type semiconductors. These results explain the difference in catalytic reaction mechanism encountered on the two solids. The alkane activation was proposed to be related in both cases to the p-type semiconducting properties of the solids, likely through hydrogen abstraction by a surface O^- species, forming a C₄H₉ radical which will similarly undergo a second hydrogen abstraction to form butenes. The changes in activation energy and in selectivity on TiP₂O₇ at higher temperatures (>450 C) are indicative of a change in mechanism, possibly with the transient formation of an alkoxide species.     

Reactivity of V2O5/MgF2 Catalysts for the Selective Ammoxidation of 3-Picoline

V₂O₅/MgF₂ catalysts with V₂O₅ contents ranging from 2.1 to 15.7 wt% were prepared, and the influence of the V₂O₅ content of the V₂O₅/MgF₂ catalyst on the structure and activity for the ammoxidation of 3-picoline was investigated. XRD data indicate that V₂O₅ is in a highly dispersed state though segregation of V₂O₅ into tiny crystallites occurs at and above 8 wt% V₂O₅. The 3-picoline ammoxidation activity increased with an increase in V₂O₅ content due not only to the species arising out of interaction of V₂O₅ and MgF₂, but also to the presence of V₂O₅ microcrystals in the catalysts.     

Dehydrogenation of Propane in the Presence and Absence of CO2 Over β-Ga2O3 Supported Chromium Oxide Catalysts

The dehydrogenation of propane to propene in the presence and absence of CO₂ over β-Ga₂O₃ supported chromium oxide catalysts has been studied. The effect of chromium content and feed composition on the dehydrogenation of propane has been investigated. It has been found that deposition of chromium oxide enhances the dehydrogenation activity and resistance to coke deposition. Moreover, it has been shown that CO₂ promotes the dehydrogenation of propane above 848 K. At that temperature in the presence of CO₂ both the yield of propene and conversion of propane were higher than in the inert gas atmosphere.