Kinetic parameter estimation for supported vanadium oxide catalysts for propane ODH reaction: Effect of loading and support

Kinetic parameters are estimated for a sequential Mars van Krevelan (MVK) reaction model occurring over several supported vanadium oxide (vanadia) catalysts involved in the propane oxidative dehydrogenation (ODH) reaction. The estimated kinetic parameters, pre-exponential factors and activation energies, are used to understand the effect of vanadia loading and oxide support. The pre-exponential factors and vanadia normalized pre-exponential factors vary with vanadia loading and oxide support. The monotonic increase in normalized pre-exponential factors with vanadia loading and the variation of pre-exponential factors with oxide support appears to be related to the change in acidity/basicity of the catalyst and the redox nature of the catalyst, respectively. The activation energy for propene degradation does not significantly change with catalyst; however, the activation energy for propane oxidation is different for the V₂O₅/Al₂O₃ catalyst. It appears that two important considerations are required for the development of an efficient propane ODH catalyst: a high rate constant associated with the propane oxidation reaction, and a high ratio of the rate constant for propene formation to degradation reaction. Based on the observations in the present study it is proposed that a higher TiO₂ support surface area will assist in increasing the propane oxidation activity and propene yield.     

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.     

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.     

Selective conversion of propane to propene by the catalytic oxidative dehydrogenation over cobalt and magnesium molybdates

Catalytic performances of various metal molybdates were tested in the oxidative dehydrogenation of propane to propene with molecular oxygen under an atmospheric pressure. Most of the molybdates tested promoted the selective oxidative conversion of propane to propene and among them cobalt and magnesium molybdates were found highest in the activity and selectivity. It was also found that their catalytic activities were highly sensitive to the catalyst composition, and it turned out that Co_0.95MoO _x and Mg_0.95MoO _x catalysts which have slightly excess molybdenum showed the highest activity in the oxidative dehydrogenation of propane. Under the optimized reaction conditions, higher reaction temperatures and lower partial pressures of oxygen, these catalysts gave 60% selectivity to propene at 20% conversion of propane. Since the molybdates having the surface enriched with molybdenum oxide tended to show high activity for the propane oxidation, surface molybdenum oxide clusters supported on metal molybdate matrix seem to be the active sites for the selective oxidative dehydrogenation of propane.     

Chemisorption of C3 hydrocarbons on cobalt silica supported Fischer–Tropsch catalysts

Chemisorption of propene and propane was studied in a pulse reactor over a series of cobalt silica-supported Fischer–Tropsch catalysts. It was shown that interaction of propene with cobalt metal particles resulted in its rapid autohydrogenation. The reaction consists in a part of the propene being dehydrogenated to surface carbon and CH_x chemisorbed species; hydrogen atoms released in the course of propene dehydrogenation are then involved in hydrogenation of remaining propene molecules to propane at 323–423 K or in propene hydrogenolysis to methane and ethane at temperatures higher than 423 K. The catalyst characterization suggests that propene chemisorption over cobalt catalysts is primarily a function of the density of cobalt surface metal sites. A correlation between propene chemisorption and Fischer–Tropsch reaction rate was observed over a series of cobalt silica-supported catalysts. No propane chemisorption was observed at 323–373 K over cobalt silica-supported catalysts. Propane autohydrogenolysis was found to proceed at higher temperatures, with methane being the major product of this reaction over cobalt catalysts. Hydrogen for propane autohydrogenolysis is probably provided by adsorbed CH_x species formed via propane dehydrogenation. Propene and propane chemisorption is dramatically reduced upon the catalyst exposure to synthesis gas (H₂/CO = 2) at 323–473 K. Our results suggest that cobalt metal particles are probably completely covered by carbon monoxide molecules under the conditions similar to Fischer–Tropsch synthesis and thus, most of cobalt surface sites are not available for propene and propane chemisorption.     

Effect of Modifiers on the Reactivity of Cr2O3/Al2O3 and Cr2O3/TiO2 Catalysts for the Oxidative Dehydrogenation of Propane

Chromium oxide supported on alumina and titania supports was modified with oxides of sodium, vanadium and molybdenum. The modified and unmodified chromium oxide catalysts were characterized by several techniques. The presence of surface chromium oxide and surface molybdenum and vanadium oxide species was detected in the unmodified and molybdenum and vanadium oxide modified supported chromium oxide catalysts. The reducibility (T_max and H/Cr ratio) of the surface chromium species was not affected for the vanadium and molybdenum oxide modified catalysts; however, the reducibility changed noticeably for sodium modified supported chromium oxide catalysts. Studies of the reactivity of the ODH of propane revealed the effect of modifiers on the reactivity properties of the surface chromium oxide species. The activity and propene selectivity decreased for sodium modified supported chromium oxide catalysts. However, the activity increased for vanadium oxide modified catalysts and was similar for molybdenum oxide modified catalysts irrespective of the support. The propene selectivity was higher for molybdenum oxide modified chromium oxide catalysts. However, the propene selectivity for vanadium oxide modified catalysts depends on the support since it appears that the inherent selectivity of the surface vanadium oxide species is reflected.     

Activities of γ-Al2O3-Supported Metal Oxide Catalysts in Propane Oxidative Dehydrogenation

The activities of metal oxide catalysts in propane oxidative dehydrogenation to propene have been studied. The catalysts are M/-Al₂O₃ (where M is an oxide of Cr, Mn, Zr, Ni, Ba, Y, Dy, Tb, Yb, Ce, Tm, Ho or Pr). Both transition metal oxides (TMO) and rare-earth metal oxides (REO) are found to catalyze the reaction at 350-450 °C, 1 atm and a feed rate of 75 cm³/min of a mixture of C₃H₈, O₂ and He in a molar ratio of 4:1:10. Among the catalysts, Cr-Al-O is found to exhibit the best performance. The selectivity to propene is 41.1% at 350 °C while it is 54.1% at 450 °C. Dy-Al-O has the highest C₃H₆ selectivity among the REO. At 450 °C, the other catalysts show C₃H₆ selectivity ranging from 16.2 to 37.7%. In general TMO show higher C₃H₆ selectivity than REO, which, however, show higher C₂H₄ selectivity. An attempt is made to correlate propane conversion and selectivity to C₃H₆ with metal-oxygen bond strength in the catalysts. For the TMO a linear correlation is found between the standard aqueous reduction potential of the metal cation of the respective catalyst and its selectivity to propane at 11% conversion. No such correlation has been found in the case of REO. Analyses of the product distributions suggest that for TMO propane activation the redox mechanism seems to prevail while the REO activate it by adsorbed oxygen.     

Oxidative Dehydrogenation of Light Alkanes on Supported Molten Alkali Metal Chloride Catalysts

Potential and limitations of molten alkali metal (Li, Na, and K) chlorides supported on Dy₂O₃/MgO were explored for the oxidative dehydrogenation of lower alkanes, such as ethane and propane. The catalysts have high activity and selectivity to olefins compared to conventional catalysts. Optimum performance is obtained with catalysts on which the alkali metal chloride phase is molten under reaction conditions. Lower chloride melting point correlates with higher selectivity. The high selectivity to ethene or propene is attributed to the high mobility of cations and anions, which facilitates desorption of alkene (limiting further oxidation) and the generation of spatially isolated hypochloride anions acting as the active sites for the primary C–H bond activation.     

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。     

NO reduction by hydrocarbons in an oxidizing atmosphere over transition metal-zirconium mixed oxides

Cu-Zr-O catalyst is active and selective in the NO reduction by propene in an oxidizing atmosphere. This is due to the inertness of ZrO₂ in hydrocarbon oxidation and its ability to disperse Cu. Other transition metal-zirconium mixed oxides were also tested for NO reduction by hydrocarbons in an oxidizing atmosphere. When propene is used as a reductant, Cu, Ni and Co supported on zirconia are active and selective catalysts. When propane is used as a reductant NO conversion decreases for all of the catalysts. The decrease is mild for Ni and Co but is very severe for Cu.     

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.     

Innovative Catalysts for Oxidative Dehydrogenation in the Gas Phase – Metallic Short Fibers and Coated Glass Fabrics

The catalytic activity of metallic short fibers with chosen alloy components and textures was investigated in the oxidative dehydrogenation (ODH) of propane to yield propene, and of isopropanol to yield acetone. The short fibers were synthesized using a melt extraction process and the properties of the fibers were intensely characterized. A correlation between the structure and the catalytic activity of the material was established. Optical microscopic, DSC, XRD, REM and EDX methods were used to characterize the fibers. Selective results of the dependency of the temperature on the propane conversion are presented in this work. A yield of more than 35 % propene is obtained at a propane conversion of 50 %. The ODH of isopropanol to acetone occurred with attractive yields of over 80 %. The results demonstrate the high innovative potential of the metal fiber materials. The use of coated glass fabrics as catalysts for the ODH and total oxidation of propane were also part of this investigation.     

On the Role of the Vanadium Distribution in MoVTeNbO x Mixed Oxides for the Selective Catalytic Oxidation of Propane

The selective oxidation of propane to acrylic acid is studied over a series of nearly pure M1-phase MoVTeNbO _x catalysts. Quantitative analysis of the reaction network shows that the ratio of the rate constants for propane oxidative dehydrogenation to propene and for the further oxidation of propene is constant. The rates towards acrolein and acetone, however, vary subtly with the concentration of vanadium and the location of its substitution. The reaction of acrolein to acetic acid and carbon oxides, associated with accessible metal cations, contributes two-thirds towards the non-selective pathway. The other third is associated with acetone formation. Vanadium is first substituted selectively at sites that are inactive for propane activation. Depending on the selectivity of this substitution two groups of materials have been identified, which show a distinctly different dependence on the concentration of vanadium. Statistic distribution of vanadium in the M1 phase appears to be the most promising strategy to improve the performance of MoVTeNbO _x catalysts for a given vanadium concentration.     

Activity of the Ni?CAl Mixed Oxides Prepared from Hydrotalcite-Like Precursors in the Oxidative Dehydrogenation of Ethane and Propane

The activity of Ni?CAl mixed oxides obtained by the thermal pre-treatment of Ni?CAl hydrotalcite-like precursors was studied in the ODH of ethane and propane. The activity of the Ni?CAl mixed oxide catalysts was studied with respect to (i) the role of Ni content and (ii) the role of temperature during Ni?CAl HTs thermal pre-treatment. The structure analysis and the activity of Ni?CAl mixed oxides were discussed in three groups; (A) the catalysts pre-treated at 500???C, (B) the catalysts pre-treated at 600???C and (C) the Ni2?CAl catalyst with constant Ni content pre-treated at 500?C900???C. Ni?CAl mixed oxides were active and selective catalysts in the ODH of ethane even at 450???C. On the other hand, the catalysts posses low selectivity to propene. It is supposed that the interaction of NiO with alumina phase plays the critical role in the active and selective catalysts. The Ni?CAl mixed oxides were characterized by XRD, H₂-TPR and diffuse reflectance spectroscopy.     

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.     

Propane oxydehydrogenation reaction on a VPO/TiO2 catalyst. Role of the nature of acid sites determined by dynamic in-situ IR studies

A VPO/TiO₂ catalyst tested in the oxydehydrogenation reaction (ODH) of propane between 300 and 400°C shows satisfactory performances (up to 80% of propene selectivity at 2% of propane conversion at 300°C or 56% of propene selectivity at 9% of propane conversion at 400°C). Addition of water or pyridine in the feed gas tends to decrease the propane conversion and enhances the propene selectivity. It is shown that water increases the number of Brönsted surface acid sites by dissociative adsorption which, in turn, enhances propene selectivity at the expense of the CO_x selectivity. These results are in good agreement with spectroscopic IR observations performed under catalytic conditions showing that the Lewis acid sites are linked to CO_xformation, whereas it seems that Brönsted sites would rather be linked to propene formation.     

The selective oxidative dehydrogenation of propane on vanadium aluminophosphate catalysts

VAPO-5 and V/ ALPO-5 catalysts have been tested for the oxidative dehydrogenation of propane. Depending on the vanadium contents and the preparation procedure, different vanadium species and different catalytic behavior are observed. The catalyst containing V⁵⁺ species with a tetrahedral coordination presents the higher yield of propene in the oxidative dehydrogenation of propane. The same yields of CO₂ are observed on all vanadium aluminophosphate catalysts, while the higher the yield of propene the lower the yield of CO is.     

Influence of the different dechlorination time on catalytic performances of PtSnNa/ZSM-5 catalyst for propane dehydrogenation

Catalytic dehydrogenation of propane has recently received considerable attention because of the increasing demand for propene. Among several catalysts, PtSnNa/ZSM-5 catalyst is one of the most suitable ones. In this study, PtSnNa/ZSM-5 catalysts with different content of chlorine were prepared by changing the time of catalyst dechlorination. The obtained catalysts were characterized by X-ray fluorescence (XRF), XRD, nitrogen adsorption, ²⁷Al MAS NMR, NH₃-TPD, H₂ chemisorption and TPR. It was found that with the increase of treatment time, more framework aluminum atoms were removed from tetrahedral positions, leading to the loss of Sn species and the decrease of catalyst acidity. Meantime, the porous properties and the interactions between Pt and Sn of the catalysts changed remarkably, which was disadvantageous to the reaction. Compared with the dechlorinated catalysts, the fresh sample with suitable content of chlorine exhibited the best reaction activity and stability. The average yield of propene was about 30.4% over 45 h for the reaction of propane dehydrogenation at 590 °C. Finally, a model was proposed for the influence of dechlorinated treatment on catalytic properties of PtSnNa/ZSM-5 catalyst for propane dehydrogenation.     

Effect of doping of TiO2 support with altervalent ions on physicochemical and catalytic properties in oxidative dehydrogenation of propane of vanadia–titania catalysts

Vanadia phase (one monolayer) was deposited on TiO₂ anatase doped with Ca²⁺, Al³⁺, Fe³⁺ and W⁶⁺ ions and the catalysts thus obtained (VMeTi) were characterized by XPS, work function technique, decomposition of isopropanol (a probe reaction for acido–basic properties) and tested in oxidative dehydrogenation of propane. The doping of the TiO₂ support modifies physicochemical and catalytic properties of the active vanadia phase with respect to the undoped TiO₂. The specific activity in the propane oxydehydrogenation decreases in the order: VFeTi>VWTi>VTi>VAlTi>VCaTi (3), whereas the selectivity to propene follows the sequence: VWTiVTi>VFeTi>VAlTi>VCaTi. This implies that the lower is the surface energy barrier for transfer of electrons from the catalyst to the reacting molecules the higher is the selectivity to the partial oxidation product. It is argued that owing to the decrease in this energy barrier the reoxidation step in the catalytic reaction, involving such a transfer: O₂+4e→2O²⁻ is fast, thus, preventing the presence of intermediate non-selective electrophilic oxygen species on the surface.     

Ga2O3/HZSM-48 for dehydrogenation of propane: Effect of acidity and pore geometry of support

The catalytic performance of propane dehydrogenation over HZSM-48 supported Ga₂O₃ catalysts in the presence of CO₂ was investigated, and compared with that of HZSM-5 supported ones. The activity decreases with the increase of Si/Al ratio of catalyst support while the selectivity to propene shows a contrary trend. Ga₂O₃/HZSM-48 with Si/Al ratio of 130 has the best propene yield of 22%. HZSM-48 supported catalysts exhibit higher selectivities to propene than the HZSM-5 supported ones at similar propane conversion, due to their weak acid strength. However, their stabilities are not so good as those of the latters, owing to their more weak acid sites and unidimensional pore structures.