MoVTeNbO_x催化剂焙烧条件对丙烷氧化制丙烯酸的影响

制备了Mo1V0.25Te0.13Nb0.12Ox催化剂,研究了焙烧条件（焙烧方式、焙烧气氛和焙烧温度）对该催化剂在丙烷选择氧化制丙烯酸反应中的影响。结果表明,两段间歇式焙烧要优于一段式连续焙烧所得催化剂的催化效果,适宜的预焙烧温度既有利于脱除挥发性物质和化学结合水,又能避免活性组分氧化生成不利的晶相;在惰性气氛中焙烧所得催化剂的催化效果要优于在弱氧化性气氛中焙烧,丙烷转化率和丙烯酸收率随着焙烧温度的升高先增加后减小,在600 ℃时达到最大值。     

Synthesis of acrolein from partial oxidation of propane on Mo–V–Te–P–O catalysts prepared by different methods

The Mo–V–Te–P–O catalysts were prepared by different methods and used for selective oxidation of propane to acrolein. It was found that different preparation methods of the catalysts could greatly affect the elemental valence of the active component and substantially affect the catalytic activity. These catalysts were characterized by X-ray powder diffraction, H_2⁻temperature programmed reduction and O_2⁻derivative thermogravimetric analysis studies, which revealed that the existence of (V_0.07Mo_{0. 93})₅O₁₄ phase in the catalysts could be very important for the selective oxidation of propane to acrolein.     

Effect of calcination atmosphere on the catalytic properties of PtSnNaMg/ZSM-5 for propane dehydrogenation

The influence of calcination atmosphere (flowing air, static air and flowing N₂) on the catalytic properties of PtSnNaMg/ZSM-5 catalysts for propane dehydrogenation was investigated. The catalysts were characterized by XRD, BET, TEM, and TPR. Results showed that the calcination atmosphere affected greatly the catalysts in many ways including surface area, the structure of the metallic phase and the interaction between platinum and tin, which are related to their catalytic behaviors. Among the catalysts studied, the PtSnNaMg/ZSM-5 catalyst calcined in flowing air exhibited the best catalytic performance.     

Effect of calcination atmosphere on the catalytic properties of PtSnNaMg/ZSM-5 for propane dehydrogenation

The influence of calcination atmosphere (flowing air, static air and flowing N₂) on the catalytic properties of PtSnNaMg/ZSM-5 catalysts for propane dehydrogenation was investigated. The catalysts were characterized by XRD, BET, TEM, and TPR. Results showed that the calcination atmosphere affected greatly the catalysts in many ways including surface area, the structure of the metallic phase and the interaction between platinum and tin, which are related to their catalytic behaviors. Among the catalysts studied, the PtSnNaMg/ZSM-5 catalyst calcined in flowing air exhibited the best catalytic performance.     

Influence of gel composition in the synthesis of MoVTeNb catalysts over their catalytic performance in partial propane and propylene oxidation

MoVTeNb mixed oxides catalysts have been prepared by a slurry method with different molar compositions (Mo/Te ratio from 2 to 6 and Nb/(V + Nb) ratio from 0 to 0.7) in the synthesis gel leading to different crystalline phases distribution and catalytic behaviour in the partial oxidation of both propane and propylene to acrylic acid. Chemical analysis indicates that the composition of samples before and after the heat-treatment changes, especially the Te-content, since a significant amount of Te is lost during the heat-treatment step when the amount of oxalate (from niobium oxalate) increases in the synthesis gel. Thus, the nature of the crystalline phases and the catalytic performance of heat-treated materials will be related to the final chemical composition. On the other hand, only the catalysts presenting Te₂M₂₀O₅₇ (M = Mo, V, Nb) crystalline structure, the so-called M1 phase, were active and selective in the partial oxidation of propane to acrylic acid. Moreover, all catalysts were active and relatively selective to the formation of O-containing products, i.e. acrolein and/or acrylic acid, during the partial propylene oxidation although the more active ones were those presenting M1 phase.     

Preparation, Characterisation and Catalytic Behaviour of a New TeVMoO Crystalline Phase

TeM_xMo_1.7O mixed oxides (M = V and/or Nb; x = 0-1.7) have been prepared by calcination of the corresponding salts at 600 °C in an atmosphere of N₂. A new crystalline phase, with a Te/V/Mo atomic ratio of 1/0.2-1.5/1.7, has been isolated and characterised by XRD and IR spectroscopy. This phase is observed in the TeVMo or TeVNbMo mixed oxide but not in the TeNbMo mixed oxide. The new crystalline phase shows an XRD pattern similar to Sb₄Mo₁₀O₃₁ and probably corresponds to the M1 phase recently proposed by Aouine et al. (Chem. Commun. 1180, 2001) to be present in the active and selective MoVTeNbO catalysts. Although these catalysts present a very low activity in the propane oxidation, they are active and selective in the oxidation of propene to acrolein and/or acrylic acid. However, the product distribution depends on the catalyst composition. Acrolein or acrylic acid can be selectively obtained from propene on Nb-free or Nb-containing TeVMo catalysts, respectively. The presence of both V and Nb, in addition to Mo and Te, appears to be important in the formation of acrylic acid from propene.     

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.     

Selective oxidation of propane to oxygenates over mesoporous SBA-15-supported potassium catalysts

As a novel catalyst system for the selective oxidation of low alkanes, mesoporous SBA-15-supported potassium catalysts were firstly employed for the selective oxidation of propane to oxygenates by using molecular oxygen as oxidant. It was found, compared with bare mesoporous SBA-15, that the selectivities to the oxygenates including formaldehyde, acetaldehyde, acrolein and acetone were remarkably enhanced over K _x /SBA-15(K:Si = x:100, mol) catalysts, and the main products were acrolein and acetone. At 500 °C, the yield of the oxygenates can reach 464% over K_3.0/SBA-15, which is the highest value over SBA-15–supported potassium catalysts. The catalysts were characterized by XRD and BET techniques. The results demonstrated that the catalytic performance was strongly dependent on the potassium content of the catalysts. Furthermore, the highly dispersed potassium on the catalyst surface was shown to be important to orientate the reaction toward the production of oxygenates. The obtained results showed that mesoporous structure, uniform pore sizes and appropriate pore surface area were favorable for the selective oxidation of propane. The samples with moderate amount of potassium promoted the selectivity to the oxygenates.     

Effect of preparation conditions on the phase composition of the MoVTe(Nb) oxide catalyst for the oxidative conversions of propane

The replacement of expensive propylene by propane, which requires the development of catalysts for the direct oxidation of propane into acrylonitrile, is an important and insufficiently studied problem. Multicomponent Mo _m V _n Te _k Nb _x oxide systems are promising in one-stage ammoxidation of propane to acrylonitrile. Despite considerable attention of various authors to the preparation methods for these catalysts, the reproducibility of their physicochemical and catalytic properties is low. To optimize the technology of catalyst synthesis, we studied the effect of drying method (evaporation or spray drying) for the aqueous suspension of the initial compounds on the formation of the Mo1V_0.3Te_0.23(Nb_0.12) oxide catalyst. It is shown that the method of drying determines the chemical and phase composition of solid catalyst precursors and the phase composition of the final catalyst in high-temperature treatment. The use of spray drying provides the required physicochemical characteristics of the catalyst (the specific surface area and the phase composition) that determine the high activity and selectivity in the selective conversion of propane. These catalysts contain two crystalline phases: orthorhombic M1 and hexagonal M2 in an optimal ratio of 3: 1.     

Complex metal oxide catalysts for selective oxidation of propane and derivatives: II. The relationship among catalyst preparation, structure and catalytic properties

The effects of the preparation methods on the structure and catalytic performance of the MVTeNb (M=Mo or W) complex metal oxides (CMO) in selective oxidation of propane to acrylic acid (AA) were investigated. Preparation methods can play as significant roles as the chemical composition in determining the structures and catalytic properties of a CMO catalyst. Undesirable preparation methods or conditions can lead to the formation of ineffective crystal phases in the resulting catalysts, leading to poor catalytic activity or selectivity. An effective MoVTeNb oxide catalyst for propane selective oxidation to acrylic acid could be obtained with a combination of a proper metal ratio and proper preparation procedures. Certain drying methods, such as freeze dry and heat evaporation, are undesirable because phase segregation tends to occur during such processes, which negatively affect the catalytic performance. The preferred drying method is the rotavap method, which favors the formation of an effective crystal phase and suppresses the formation of impurity phases. The preferred calcination atmosphere is an inert atmosphere while a calcination atmosphere containing oxygen leads to the formation of significant amounts of MO₃ in MVTeNb oxides (M=Mo or W), which is known to be inactive in propane oxidation. It is proposed that an effective MoVTeNb oxide catalyst for propane selective oxidation to acrylic acid should contain at least two major crystal Phases A and B. Based on studies of oxides of pure Phase A and those exclusively enriched with Phase B, X-ray diffraction (XRD) characteristics of the two major crystal phases (A and B) have been identified and their catalytic functions studied. It is proposed that Phase A is active in propane activation but relatively unselective for the acrylic acid formation, while Phase B is reasonably active for propane activation and fairly selective for acrylic acid formation. It has been shown that using MoVTeNb oxide catalyst with a proper ratio of Phases A and B and synergy between them, very high propane conversion (73%) and acrylic acid selectivity (58%) can be achieved at the same time.     

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.     

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.     

Selective Oxidation of Propane to Acrolein over Ce-Doped Ag–Mo–P–O Catalysts: Influence of Ce Promoter

The selective oxidation of propane to acrolein was performed on Ce-doped Ag_0.3Mo_0.5P_0.3O _x catalysts. The maximal acrolein yield of 4.4% with 28.7% acrolein selectivity was obtained on Ce_0.1Ag_0.3Mo_0.5P_0.3O _y catalyst. The apparent activation energy of Ag_0.3Mo_0.5P_0.3O _x catalysts decreased with the addition of Ce. The addition of Ce facilitated the C-H activation of propane and enhanced conversion of intermediate propene to acrolein. The reducibility and the concentration of Mo⁵⁺ improved as the Ce content increased and was closely related to acrolein selectivity and propane conversion. The role of Ce in these catalysts was proposed: there was formation of the redox cycle Ce³⁺ + Mo⁶⁺ Ce⁴⁺ + Mo⁵⁺ in Ce-doped Ag_0.3Mo_0.5P_0.3O _x catalysts, leading to the modification of properties and catalytic performance of these catalysts.     

Activation of VOHPO4 · 0.5H2O in Propane/Air Mixture: Effect on Structural, Morphological, Oxidant’s Behaviour and Catalytic Property of (VO)2P2O7 Catalysts for Propane Oxidation

VOHPO₄ · 0.5H₂O synthesized by VOPO₄ · 2H₂O and isobutanol was activated in a flow of propane/air mixture (1% propane in air) at 673 K for 36, 75 and 132 h. Three vanadyl pyrophosphate catalysts obtained were denoted as VPD36P, VPD75P and VPD132P. The crystallinity of all propane/air pretreated catalysts as shown in XRD increased with the duration of calcination. SEM micrographs showed the formation of more isolated platelets and more prominent rosebud-shape agglomerate as the pre-treatment was longer. Four reduction peaks maxima at 752, 920, 1026 and 1140 were observed in the rate of hydrogen consumption for VPD36P. As the calcination duration increased to 75 h, the H₂ reduction peaks were shifted to lower temperatures at 750, 882, 1004 and 1140 K. When the calcination duration was further increased to 132 h, only three reduction peaks were observed at 752, 952 and 1142 K. Despite the progressively shifted of the major reduction peak maximum as the duration of calcination increased from 36 to 132 h, the lattice oxygen from VPD36P was found to be the most reactive. The catalytic performance for propane oxidation to acrylic acid (AA) showed that VPD36P gave the highest activity (9.6%) with 83.0% of selectivity to AA.     

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.     

Effect of calcination temperature on characteristics of sulfated zirconia and its application as catalyst for isosynthesis

The effect of catalyst calcination temperature (450 °C, 600 °C, and 750 °C) on catalytic performance of synthesized and commercial grade sulfated zirconia catalysts towards isosynthesis was studied. The characteristics of these catalysts were determined by using various techniques including BET surface area, XRD, NH₃- and CO₂-TPD, ESR, and XPS in order to relate the catalytic reactivity with their physical, chemical, and surface properties. It was found that, for both synthesized and commercial sulfated zirconia catalysts, the increase of calcination temperature resulted in the increase of monoclinic phase in sulfated zirconia, and the decrease of acid sites. According to the catalytic reactivity, at high calcination temperature, lower CO conversion, but higher isobutene production selectivity was observed from commercial sulfated zirconia. As for synthesized sulfated zirconia, the isobutene production selectivity slightly decreased with increasing calcination temperature, whereas the CO conversion was maximized at the calcination temperature of 600 °C. We concluded from the study that the difference in the calcination temperatures influenced the catalytic performance, sulfur content, specific surface area, phase composition, the relative intensity of Zr³⁺, and acid-base properties of the catalysts.     

Catalytic (amm)oxidation of propane with molecular oxygen over complex metal oxides: Involvement of homogeneous reaction in gas phase

Partial oxidation of propane to acrolein and ammoxidafion to acrylonitrile with molecular oxygen proceed over complex metal oxide catalysts under the restricted conditions of the high partial pressures of both propane and oxygen. The selective formations of acrolein and acrylonitrile also required high reaction temperature around 500°C. Effective catalysts for the selective (amm)oxidation were mostly made up of bismuth oxide and molybdenum oxide and were further modified with another metal oxide. From the studies of the volume effect of pre-catalyst zone on the conversion and the selectivities and of the reactions in the absence of the catalyst, it is suggested that the reactions involve homogeneous reactions in the gas phase where thermally activated propane converts into propene, followed by catalytic oxidation of propene over the metal oxide surface.     

Partial Oxidation of Methane to Syngas over Calcined Ni–Mg/Al Layered Double Hydroxides

The catalytic partial oxidation of CH₄ to syngas was carried out over an Ni–Mg/Al mixed-oxide catalyst prepared from layered double hydroxide-type precursors. The catalysts were characterized by XRD, TPR, UV-DRS, XRF, BET and CHNS analysis. The effects of the catalyst composition and the calcination temperature on the catalytic performance and the extent of catalyst deactivation were investigated. Ni–Mg/Al oxide catalysts converted CH₄ into syngas efficiently with high selectivity. The catalyst performance was strongly related to the Ni particle size and the calcination temperature. The catalysts that were calcined at higher temperature exhibited a better catalytic performance. In conclusion, the NiAl₂O₄ spinel phase had a positive effect on the stability of the catalyst.     

Low Temperature Activity of Euro4 Diesel Oxidation Catalysts: Comprehensive Material Analyses and Experimental Evaluation of a Representative Panel

Diesel oxidation catalysts of 16 Euro 4 passenger cars were analyzed and tested on an engine bench to attempt to correlate composition and structure to real-life performances. Particular low temperature behavior was evidenced, which was further studied using simplified gas mixtures on a synthetic gas bench. The long chain hydrocarbons are efficiently trapped, while NO₂ plays a major role in CO abatement before the activation of the catalytic oxidation by O₂.     

Oxidative dehydrogenation of propane over a series of low‐temperature rare earth orthovanadate catalysts prepared by the nitrate method

A series of high‐purity rare earth orthovanadates were prepared by the nitrate method and found to be effective low‐temperature catalysts for the oxydehydrogenation of propane at 320°C, at which no reactions occurred over the catalysts reported in the literature, and, thus, may be of practical significance. The catalytic performances of LnVO₄ (Ln = Y, Ce–Yb) at 500°C were much better than those of rare earth orthovanadate catalysts and also slightly exceeded that of magnesium orthovanadate Mg₃(VO₄)₂ reported in the literature. LnVO₄ (Ln = Y, Ce–Yb) materials were tetragonal active phases which could stabilize the existence of active sites for the oxydehydrogenation of propane. Some catalysts with a certain amount of LnVO₃ reduced from LnVO₄ (Ln = Ho–Yb) under reaction atmosphere exhibited better redox properties and catalytic performances possibly due to the existence of biphasic catalytic synergy. LaVO₄ was a monoclinic unstable active phase, although its bulk structure did not change after reaction. The remarkable deactivation of the LaVO₄ catalyst was probably due to that LaVO₄ could not stabilize the existence of surface active sites. This revised version was published online in July 2006 with corrections to the Cover Date.