Characterization and Reactivity of TiO2/SiO2 Supported Vanadium Oxide Catalysts

Several TiO₂/SiO₂ supports and supported vanadium oxide (vanadia) catalysts of varying titania and vanadia content are synthesized and characterized. Titania rutile phase is not detected for the TiO₂/SiO₂ supports even up to a calcinations temperature of 1073 K. The supported vanadia catalysts were also studied for the ethane and propane ODH reaction and compared with TiO₂ (Degussa P25) supported catalysts. Surface vanadia species are present on the titania or silica part of the TiO₂/SiO₂ support depending on the titania and vanadia loading of the catalyst. Depending on the vanadia loading in the V₂O₅/TiO₂/SiO₂ catalyst the anatase to rutile phase transformation may occur below 1073 K. Furthermore, the ODH activity is retained or decreases less rapidly compared to the TiO₂ supported catalyst as the calcination temperature is increased. Consequently, the TiO₂/SiO₂ supported vanadia catalysts do provide certain advantages compared to the TiO₂ (Degussa P25) supported system.     

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.     

Effect of alkali metals additives to V2O5/TiO2 catalyst on physicochemical properties and catalytic performance in oxidative dehydrogenation of propane

The effect of alkali metal additives Li, K, and Rb to V₂O₅/TiO₂ catalyst on the rate of catalyst reduction with propane and reoxidation with oxygen, sorption of propene, and the electron work function has been examined. The results have been correlated with the catalytic performance in oxidative dehydrogenation, ODH, of propane. It has been found that the rates of reduction, reoxidation and the ODH of propane decrease in the order: VTi>LiVTi>KVTi>RbVTi. The activation energies of the reduction and reoxidation are not, however, affected by the presence of the alkali metals. The same sequence has been observed for the work function values of the catalysts. It is argued that alkali metal poisons the centres of the hydrocarbon activation. The yield and selectivity to propene in the ODH of propane increase, however, for the promoted catalysts, following the above sequence. This effect is ascribed to the decrease in the heat of the propene adsorption, which is due to the increase in the basicity and decrease in acidity on the promoted catalysts.     

The Oxidative Dehydrogenation of Propane Using Vanadium Oxide Supported on Nanocrystalline Ceria

Nanocrystalline ceria was prepared as a support for vanadium oxide catalysts and tested for the oxidative dehydrogenation of propane. Nanocrystalline ceria is very active for the total oxidation of propane under conditions used for oxidative dehydrogenation. The addition of vanadium results in a switch of activity to produce propene with appreciable selectivity. The catalyst performance depends on the vanadium loading. Lower vanadium loadings resulted in catalysts with highly dispersed vanadia species, which were selective towards propene production. Higher vanadium loadings resulted in the formation of a mixed cerium–vanadium phase, which was also active for propane selective oxidation. A catalyst with an intermediate loading was far less selective. Catalysts were characterised by a range of techniques (including XRD, laser Raman, TPR, SEM/EDX and XPS), and the activity of the catalysts can be related to their structure and chemistry.     

Effect of Au in V2O5/SiO2 and MoO3/SiO2 catalysts on physicochemical and catalytic properties in oxidation of C3 hydrocarbons and of CO

Silica supported vanadia and molybdena catalysts with, and without Au, were prepared, characterized with XRD, TEM, XPS, H₂-TPR and probe reaction of isopropanol decomposition, and tested in the oxidation of propene, propane and CO. The presence of Au: (a) does not affect markedly structural and textural properties, such as specific surface area, size of V₂O₅ or MoO₃ crystallites, or the electronic state of V and Mo ions, (b) increases the reducibility of vanadia and molybdena phase, (c) enhances the dehydrogenation properties in isopropanol decomposition, and (d) modifies catalytic activity in oxidation reactions. The Au particles increase the total activity in CO oxidation. For propane oxidation at high temperatures the increase in total activity is observed, with the decrease in the selectivity to oxidative dehydrogenation product (propene) and increase in the selectivity to CO₂. The catalytic performance in propene oxidation at 200–300 °C depends on the Au presence and the composition of the reaction mixture. The gold-containing catalysts favour allylic oxidation of propene to acrolein and oxyhydration to acetone, and suppress the C₂ products (ethanal, acetic acid) of partial degradation of a propene molecule. In the presence of hydrogen in the reaction mixture, higher selectivities of acetone (product of oxyhydration) were observed for all the catalysts.     

Optimization of V2O5–MgO/TiO2 catalyst for the oxidative dehydrogenation of propane effect of magnesia loading and preparation procedure

The effect of magnesia loading and preparation procedure of vanadia on titania catalysts on the physicochemical characteristics and the performance in propane oxidative dehydrogenation were investigated. A series of magnesia promoted vanadia catalyst (5 wt% V₂O₅) with varying amounts of MgO (1.9--10 wt%) were synthesized by synchronous and sequential deposition on titania support. The catalysts were characterized using several techniques (BET, XRD, H₂-TPR and NH₃)-TPD). Both MgO loading and preparation procedure affect the catalyst surface properties and the behavior in the oxidative dehydrogenation of propane. Magnesia addition results in drastic increase in propene selectivity, while the effect on activity is negative. The activity is inversely related with the magnesia loading. Deposition of V₂O₅ on previously prepared MgO/TiO₂ presents a beneficial effect in the activity of the sample. The role of acidity and reducibility is explored. There is no correlation between reducibility and activity of the catalysts, whereas the acidity seems to influence the catalytic performance. Catalyst containing 5 wt% V₂O₅ and 1.9 wt% MgO prepared by sequential deposition of V₂O₅ on already doped with MgO titania exhibits the most interesting results.     

Characterisation and catalytic testing of VOx/Al2O3 catalysts for microstructured reactors

With regard to the application in a microstructured reactor, a special VO_x/Al₂O₃ catalyst powder (3 μm) was prepared and characterised by BET, XRD, UV/Vis DRS, and Raman spectroscopy. The higher the vanadium content, the higher the degree of polymerisation of the vanadium species on the support surface and the lower the BET surface area. Pelletised powders were tested in conventional tubular reactors for their catalytic performance in the oxidative dehydrogenation of propane. Their activity increases with vanadia loading, whereas selectivity towards propene decreases at iso-conversion. Catalytic benchmarking was performed to choose a reasonable catalyst for further investigations in a microreactor.     

A General Method for Preparation of PVP-Stabilized Noble Metal Nanoparticles in Room Temperature Ionic Liquids

Catalysts based on a physical mixture of Ga₂O₃ and MoO₃ have been prepared and evaluated for propane partial oxidation to propene. The Ga₂O₃/MoO₃ catalysts demonstrated propene yields greater than a 6 wt% V₂O₅/TiO₂ catalyst, which is known to be active for the reaction. The higher yield of propene was achieved by the alkane activation properties of Ga₂O₃ and the selective oxidation function of MoO₃ combining in a synergistic manner.     

Kinetic modeling of oxidative dehydrogenation of propane with CO2 over MoOx/La2O3–Al2O3 in a fluidized bed

A 4-step kinetic model of CO₂-assisted oxidative dehydrogenation (ODH) of propane to C₂/C₃ olefins over a novel MoO_x/La₂O₃–γAl₂O₃ catalyst was developed. Kinetic experiments were conducted in a CREC Riser Simulator at various reaction temperatures (525–600 °C) and times (15–30 s). The catalyst was highly selective towards propylene at all combinations of the reaction conditions. Langmuir-Hinshelwood type kinetics were formulated considering propane ODH, uni- and bimolecular cracking of propane to produce a C₁-C₂ species. It was found that the one site type model adequately fitted the experimental data. The activation energy for the formation of propylene (67.8 kJ/mol) is much lower than that of bimolecular conversion of propane to ethane and ethylene (303 kJ/mol) as well as the direct cracking of propane to methane and ethylene (106.7 kJ/mol). The kinetic modeling revealed the positive effects of CO₂ towards enhancing the propylene selectivity over the catalyst.     

丙烷氧化脱氢制丙烯的钒基介孔催化材料研究进展

介绍了近年来丙烷氧化脱氢催化技术的最新研究进展,针对其中应用最广泛的钒基介孔催化剂,对其反应机理、载体性能和制备方法进行了综述,并比较了不同催化体系的丙烷氧化脱氢性能,提出了改善钒基介孔催化剂催化活性与选择性的途径。通过比较已经报道的同类研究结果,着重阐述了载体的孔结构对于氧化脱氢过程的影响,比表面积和孔径的优化对于提高催化剂的活性组分分散度以及活性位的数量效果明显。今后研究应着重提高催化剂孔道结构在高温下的稳定性以及使用寿命。     

Vanadium supported on hexagonal mesoporous silica: active and stable catalysts in the oxidative dehydrogenation of alkanes

Hexagonal mesoporous silica (HMS) catalysts post-synthetically doped with vanadia oxo-species were characterized by means of XRD, UV-Vis spectroscopy, H₂-TPR and studied in oxidative dehydrogenation of propane (ODH). The relationship between catalytic activity in ODH and the presence of different vanadia-oxo species (monomeric, oligomeric and oxide-like species) was suggested. Monomeric VOx species are responsible for high catalytic activity and selectivity, oligomeric species containing V-O-V bond are active but non-selective to propene and oxide-like VOx particles are significantly less active and selective.     

Effect of calcium and potassium on V2O5/ZrO2 catalyst for oxidative dehydrogenation of propane: a comparative study

The effect of calcium and potassium on the physiochemical properties and performance of V₂O₅/ZrO₂ catalyst for oxidative dehydrogenation of propane was studied in the temperature range of 385–400 °C. The vanadia loading was kept constant at 5 VO_x/nm² and the atomic ratio A/V (A=Ca, K) was varied from 0.05 to 0.75. The vanadia surface structure was investigated using X-ray diffraction analysis (XRD), electron paramagnetic resonance (EPR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The redox property of the catalysts was studied by temperature programmed reduction (TPR) and temperature programmed oxidation (TPO) whereas surface acidity was measured by temperature programmed desorption (TPD) of ammonia. Calcium and potassium both interact with the surface V=O and stabilize the +5 oxidation state of vanadium. Interaction between calcium and vanadium was more intense though surface concentration of calcium was lower than that of potassium. For doped catalysts, the activity was lower due to an increase in reduction temperature as well as a lower extent of reduction and increased resistance to undergo redox cycles. On the other hand, removal of surface acidic sites by the dopants increased the propene selectivity. Potassium was more effective in decreasing the activity and increasing the propene selectivity.     

Anatase-supported vanadium oxide catalysts for o-xylene oxidation: From consolidated certainties to unexpected effects

In this paper some aspects of the reaction of o-xylene oxidation, catalysed by anatase-supported vanadium oxide, are re-examined and compared with the scientific and patent literature. Specifically, the effect of vanadia loading on turnover frequency and on the distribution of products has been investigated, using catalysts having sub-monolayer and above-monolayer vanadium oxide deposited on a high-purity anatase support. It was found that even catalysts having less-than-monolayer vanadia loading, containing isolated or oligomeric VO₄ species, may give good selectivity to phthalic anhydride, provided the support does not catalyze the formation of heavy compounds. Moreover, the effect of water on the reactivity and chemical–physical characteristics of catalysts was studied by means of in situ Raman spectroscopy. Steam promoted dynamic reversible phenomena occurring at the catalyst surface, by facilitating the dispersion of bulk vanadium oxide and therefore increasing the catalyst activity.     

Catalytic oxidation of propane over molybdenum-based mixed oxides

Catalytic oxidation of propane to produce propene was investigated over molybdenum-based mixed oxide catalysts. Cobalt or magnesium oxide combined with molybdenum oxide exhibits the best catalytic performance for the oxidative dehydrogenation of propane. Catalytic activities of both Co-Mo-O and Mg-Mo-O vary drastically on the catalyst composition and Co(Mg)_0.95Mo_1.0O_x having small amounts of free MoO₃ on the Co(Mg)MoO₄ surface shows the highest catalytic activity keeping a considerably high selectivity to propene. The catalytic activity also depends strongly on the acidic properties of catalysts and MoO₃ clusters formed on the surface of Co(Mg)MoO₄ are responsible for the activities for the oxidative dehydrogenation of propane.     

Propane oxidative dehydrogenation over vanadia catalysts supported on mesoporous silicas with varying pore structure and size

The structural characteristics and the performance of vanadia catalysts (0.7–8 wt.% V) supported on mesoporous (MCM-41, HMS, MCF, SBA-15), microporous (silicalite) and non-porous (SiO₂) silicas in oxidative dehydrogenation of propane were investigated. The structure of vanadia species, the redox and the acidic properties of the catalysts were studied using in situ Raman spectroscopy, TPD- NH₃ and H₂-TPR. The only vanadia species detected on the surface of HMS and MCM-41 for V loadings up to 8 wt.% were isolated monovanadates indicating high vanadia dispersion. Additional bands ascribed to V₂O₅ nanoparticles were evidenced in the case of SBA-15 and MCF supported catalysts while these bands were the only ones identified on the surface of the catalysts supported on silicalite and non-porous silica. The catalysts supported on mesoporous HMS and MCM-41 materials showed the best performance achieving high propane conversions (35–40%) with relatively high propene selectivities (35–47%). Lower activity due to the lower degree of vanadia dispersion, caused by the partial destruction of the pore structure was observed for the SBA-15 and MCF supported catalysts. The degree of dispersion of the V species on the catalyst surface and not the pore size and structure of the mesoporous support or the acidity/reducibility characteristics mainly determine the catalytic activity towards propene production. In addition, it was shown that the pore structure and size of the mesoporous supports did not have any significant effect in the turnover rates (TOF values) of propane conversion (and propene formation at low propane conversion, below ca. 10%). However, the highest propene yield (up to 19%) and stable catalytic behavior was attained for catalysts supported on HMS mesoporous silica, and especially for those combining framework mesoporosity and textural porosity (voids between primary nanoparticles).     

Role of potassium on the structure and activity of alumina-supported vanadium oxide catalysts for propane oxidative dehydrogenation

The influence of potassium on the structure and properties of alumina-supported vanadium oxide catalysts has been studied by in situ Raman spectroscopy, temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), a probe reaction of acid/base–redox sites (methanol chemisorption) and tested in oxidative dehydrogenation (ODH) of propane. Potassium coordinates to surface vanadium oxide species altering its structure but does not form bulk compounds, possibly because the total V+K coverage does not reach the monolayer coverage on alumina. The interaction of K with V weakens the terminal V=O bond. K-doped alumina (KAl)-supported vanadia catalysts show lower acidity, a decrease of reducibility and a decrease of propane conversion values. These trends do not correspond with the changes in the terminal V=O bond energy. Thus, it appears that the terminal V=O bond of surface vanadium oxide species is not the active site for propane ODH, oxidation of methanol to formaldehyde and for the reduction of surface vanadium oxide species by hydrogen. Potassium also changes the acid–base characteristic of the system and decreases the acidic character of surface vanadia. This shift in the acid–base character to a more basic system must also account for the better selectivity in propane ODH due to a variation in the interaction between the intermediates and the surface.     

Novel approaches for the improvement of selectivity in the oxidative activation of light alkanes

Selectivity is the key parameter for the practical application of oxidative activation of light alkanes, improving energy and raw materials utilisation efficiency and reducing CO₂ formation and emission. The intrinsic process complexity and the catalyst multifunctionality imply the need of a close control of many parameters (active centres nature, reactant composition, reaction mechanism, etc.) to improve the selectivity. Novel approaches to get this goal along three complementary directions, say: oxide nanocatalysts preparation by non-conventional routes (to tune the nature of the active centre), oxidant selection (to avoid overoxidation), and catalyst arrangement (to take advantage of the reaction mechanistic features), are presented and discussed by means of representative examples of their application. These include ODH of propane on nanosized molybdates, ODH of ethane with CO₂ on ceria- and MCM-41-based catalysts, and selective oxidation of light alkanes to unsaturated oxygenates over transition metal substituted MCM-41 catalysts.     

Oxidative Dehydrogenation of Propane on V2O5/ZrO2 Catalyst

The performance of zirconia supported vanadia catalyst has been investigated for the oxidative dehydrogenation of propane. Vanadia loading was varied from 1.6 to 22.7 wt%, both below and well above monolayer coverage. The turnover frequency was highest for the catalyst with a vanadia surface density of 5 VO _x /nm². The effect of vanadia loading on the redox behavior of the catalysts was investigated by temperature-programmed reduction and temperature-programmed oxidation. The specific activity increased with ease of reducibility of the catalyst. The extent of reduction and ease of reoxidizability of the catalysts were also found to depend on the surface vanadia structure and to influence the catalytic activity. The effect of vanadia loading on the basicity of the catalysts was also investigated.     

Electrochemical catalysts for hydrocarbon combustion

Two series of electrochemical catalysts were prepared from sputtered Pt thin films onto two kinds of electrolyte membranes, 8 mol% Y₂O₃-stabilized ZrO₂ (YSZ), an O²⁻ conducting oxide and Na₃Zr₂Si₂PO₁₂ (NASICON), a Na⁺ one; respectively. The thickness of the Pt films varied from 8 to 120 nm. Therefore, the Pt loading was extremely low. The catalytic activity of Pt/YSZ and Pt/NASICON systems has been investigated between 200 and 500 °C for propane and propene, respectively. In spite of the low Pt loading, the Pt/YSZ electrochemical catalysts exhibited high activity for propane combustion. Furthermore, the catalytic activity can be in-situ controlled by applying electrical polarisation with high Faradaic efficiency (10³). The catalytic rate of propene deep oxidation on Pt/NASICON electrochemical catalyst was found to be limited by the number of active sites, which is low on very thin Pt films. Moreover, initial anodic polarisation indicate that Na⁺ ions are already present on the top surface of Pt, probably proceeding from the preliminary stabilisation treatment of Pt in the reactive mixture. Nevertheless, polarisation allows the tuning of the catalytic activity of the electrochemical catalysts for propene oxidation. Finally, for both kinds of electrochemical catalysts, our results have evidenced that the measurement of the open-circuit voltage during catalytic process can be an indicator of the hydrocarbon conversion.     

Dynamic states of V‐oxide species: reducibility and performance for methane oxidation on V2O5/SiO2 catalysts as a function of coverage

Temperature‐programmed in situ Raman spectroscopy is used to understand the effect of surface vanadia coverage on the structure, reducibility and performance for the oxidation of methane on V₂O₅/SiO₂ catalysts. The vanadia coverage on silica has no effect on its structure below its dispersion‐limit loading (“monolayer” coverage); however, the interactions among surface vanadia species under reducing conditions become increasingly important. This interaction appears to operate through the sharing of oxygen sites facilitating the reduction, but it does not alter the total reducibility. The probability for this interaction to take place increases with vanadium oxide surface coverage. It is therefore expected that under reaction conditions the catalyst with higher vanadia coverage would have a greater capacity to release oxygen. This would increase the activity per vanadium site. This revised version was published online in July 2006 with corrections to the Cover Date.