Characterization of cobalt-impregnated VPO catalysts

Cobalt-impregnated VPO catalysts containing 1–6% of the metal by weight were prepared. The catalytic tests showed that cobalt impregnation significantly increased the overall activity while slightly decreasing the maleic anhydride selectivity. To investigate the origin of the cobalt effect, the solids were characterized using XRD, Raman spectroscopy, FT-IR, TPR and XPS. No structural effects were detected through XRD. TPR showed that the non-equilibrated catalysts contain cobalt oxides which react with the VPO solids to yield a hard to reduce system. Non-equilibrated umpromoted and promoted (low cobalt loadings) catalysts contain small proportions of V(V) phases. After several 100 h on stream, the only phase detected in all cases was V(IV) vanadyl pyrophosphate. The surface oxidation state of vanadium was V(IV). The maleic anhydride yield correlates with the increased exposure of cobalt on the catalysts surface.     

Controlling factors in the selective conversion of n-butane over promoted VPO catalysts at low temperature

Unpromoted and 1.2, 2.3 and 4.3 molar % Co:V cobalt-promoted vanadium-phosphorous-oxide (VPO) catalysts were synthesized via an organic route. The catalyst precursors were calcined and then conditioned in a reactor, forming the active vanadyl pyrophosphate (VO)₂P₂O₇ phase, which was confirmed via X-ray diffraction studies (XRD). The effect of co-promotion on the yield of maleic anhydride (MA) from n-butane oxidation was examined at different temperatures and gas hourly space velocities (GHSV). 2.3% Co:V was the optimum promoter loading for a high yield towards MA. Higher GHSV's proved to enhance the selectivity towards MA.

The catalysts were tested over a 200 h period, generally taking some 24 h to reach steady-state performance. The best performing catalyst yielded 45% MA at 275 °C and a GHSV of 7200 ml g⁻¹ h⁻¹, with an n-butane conversion of 73%, whilst all previously reported VPO catalysts produced far lower MA yields at this temperature.     

The reaction network of the selective oxidation of n-butane on (VO)2P2O7 catalysts: Nature of oxygen containing intermediates

The reaction network of the partial oxidation of n-butane to maleic anhydride on (VO)₂P₂O₇ has been investigated using steady-state and transient experiments in a Temporal-Analysis-of-Products (TAP) reactor under vacuum conditions to identify by mass spectrometry possible intermediate products and in a tubular fixed bed reactor at atmospheric pressure to derive information on the role of the detected and other potential intermediates in the reaction network. The oxidation of butane, butadiene, tetrahydrofuran, dihydrofuran, and furan has been studied in the TAP reactor and, additionally to these compounds, crotonaldehyde, crotonlactone, and malealdehydic acid were oxidized in the tubular flow reactor. From the results obtained it can be concluded that the main reaction pathway from butane to maleic anhydride proceeds via the intermediate products n-butenes, butadiene, crotonaldehyde, dihydrofuran, furan, and crotonlactone.     

Effect of different calcination environments on the vanadium phosphate catalysts for selective oxidation of propane and n-butane

Vanadium phosphate catalysts were synthesized via VOPO₄·2H₂O and were calcined in two different hydrocarbon reaction environments, i.e. n-butane/air and propane/air. Both catalysts are denoted VPDB and VPDP, respectively. Both catalysts exhibited a good crystalline with characteristic peaks of pyrophosphate phase. However, the peaks for VPDP are shown to be more prominent than those of VPDB. BET surface area showed that VPDB gave higher surface area (23 m² g⁻¹) compared to VPDP (18 m² g⁻¹). The average V valence state for VPDP is 4.08 and the higher V valence state for VPDB is 4.26 due to higher amount of V^V for VPDB. Furthermore 14.2% of V^III was found for VPDP but none for VPDB. SEM micrographs clearly revealed that the morphologies of both catalysts composed of plate-like crystallite that was arranged into the characteristic of rosette cluster. However, the catalyst calcined in n-butane/air environment (VPDB) resulted in an increment of the amount of plate-like crystal formed in the rosette rosebud agglomerates. TPR in H₂ profiles of both catalysts gave two reduction peaks corresponding to two kinetically different oxygen species which were associated with V^V and V^IV phases, respectively. VPDB removed larger amount of active oxygen species linked to V^IV phase which eventually caused a higher conversion rate in the selective oxidation of n-butane and propane to maleic anhydride and acrylic acid, respectively.     

VPO transient lattice oxygen contribution

n-Butane partial oxidation to maleic anhydride is practiced commercially in several reactor types with vanadium phosphorous oxide (VPO) catalyst. DuPont operated a Circulating Fluidized Bed facility in which catalyst was shuttled between a net reducing zone (a transport bed) to a net oxidizing zone (air regenerator). Several advantages have been cited for this technology but the role of oxygen is hotly debated and different models have been proposed to characterize the complicated 14e⁻ exchange. To examine the role of surface lattice oxygen, we carried out transient experiments in which catalyst was subjected to high concentrations of butane followed by an extensive re-oxidation treatment. Carbon accumulates on the surface lattice surface thereby contributing to a reduction in reaction rates and the quantity increases with the butane/oxygen ratio. This carbon reacts in the presence of molecular O₂ during the regeneration step. The surface lattice is capable of storing significant amounts of oxygen and when the catalyst is highly oxidized, both selectivity and activity are higher.     

丁烷氧化制顺酐VPO催化剂的制备研究

重点介绍了丁烷氧化制顺酐流化床催化剂的制备工艺：以及催化剂的流化床活性评价装置,还考察了对催化剂的催化性能有重要影响的酸浓度对催化剂催化活性的影响。     

顺酐生产用VPO催化剂的研究进展

综述了正丁烷选择性氧化制顺酐VPO催化剂的研究进展,介绍了催化剂制备过程中添加载体及助剂对催化剂性能的影响,讨论了正丁烷选择氧化制顺酐的反应机理,并结合国内外研究现状对VPO催化剂未来的发展方向作了展望.     

A Model for the Phosphorus Dynamics of VPO Catalysts during the Selective Oxidation of n‐Butane to Maleic Anhydride in a Tubular Reactor

A model for the phosphorus dynamics in vanadium‐phosphorus oxide (VPO) catalysts for the oxidation of n‐butane to maleic anhydride was developed. According to the model, reversible sorption processes determine the phosphorus content of the catalyst. Simulations reveal that several phenomena can be successfully described. If no phosphorus is added to the reactant feed, the catalytic activity increases until runaway occurs. With addition of a proper amount of phosphorus, the loss can be compensated while excessive phosphorus addition results in complete catalyst deactivation. Adjusting the model parameters to experimental data may result in a model that can be used to optimize the performance of maleic anhydride reactors.     

A Study on VPO Specimen Supported on Aluminum-Containing MCM-41 for Partial Oxidation of n-Butane to MA

Dispersed vanadium–phosphorus oxide species supported on Al-MCM-41 with different vanadium loadings have been synthesized for the first time for partial oxidation of butane to MA. It was found that the VPO species was dispersed over the Al-MCM-41 support material, both in the internal channel and on the external surface. With increasing vanadium loading, n-butane conversion increased but MA selectivity decreased considerably under the same reaction conditions. At lower conversions (<30%), rather high MA selectivity (ca. 70%) can be achieved on the low loading sample. Compared with the amorphous structure of large pore SiO₂ support, the unique structure of the MCM-41 and the incorporated Al³⁺ in the framework do have an impact on the reaction behavior of the supported VPO specimen. The chemical nature of the supported VPO species and the interaction between the applied VPO species and the support was found to vary notably with the content of vanadium in the sample and likewise affected the related physico-chemical characteristics and their reaction behaviors.     

Ti/Zr助剂掺杂VPO催化正丁烷选择性氧化制顺酐

采用有机相法制备钒磷氧(VPO)催化剂,并在合成过程中加入Ti/Zr助剂,使用微型固定床反应器评价其催化正丁烷选择性氧化制顺酐的性能,考察了不同助剂元素及其化合物形式和添加量对催化剂性能的影响,分析了添加助剂对催化剂晶相及微观形貌的影响. 结果表明,加入助剂未改变催化剂的晶相结构,但促进了活性相(VO)2P2O7生成,且使反应后催化剂微观形貌发生改变,片层更破碎,活性相暴露,同时生成了利于反应的微量V5+;助剂金属Ti和Zr与V元素间的相互作用对催化剂表面P浓度及V价态都有一定影响;与不加助剂的VPO催化剂相比,相同反应条件下,Ti/Zr助剂修饰的VPO催化性能显著提高,摩尔比Zr/V为1.5%的Zr(NO3)4为助剂的催化剂的催化性能最佳,稳定状态下正丁烷的转化率高达99.1%,顺酐收率为54.4%.     

Water modification of PEG-derived VPO for the partial oxidation of propane

In this study, the PEG-derived VPO precursors were subject to water refluxing (90 °C, 8 h) and in situ activation in a steam-containing environment. For comparison, the VPO precursors without water refluxing were also activated in a similar manner. The IR measurement indicated that the majority of PEG in the precursor has been removed during water refluxing, and the VPO is essentially unenwrapped by PEG. The consequence is an increase of particle size and crystallinity of VPO as well as decreases in surface acidity and site density. The activation of catalysts in a stream-containing environment has an influence on the content of V⁵⁺ species and on the reduction behavior of the VPO catalysts. The VPO catalyst activated with 20% water vapor (by volume) in the feed shows the highest crystallinity. Compared with the non-PEG-derived VPO precursor, the PEG-derived one undergoes structure changes of higher severity during the water/steam treatments. The VPO catalyst generated from the PEG-derived precursor with water refluxing and activated with steam (20%) exhibited a propane conversion of 25% and a (AA + HAc) selectivity of 70%. The superior catalyst behavior can be interpreted in terms of the higher crystallinity of the (VO)₂P₂O₇ phase, the lower content of V⁵⁺ species, and the milder surface acidity as caused by the water/steam treatments.     

A two-phase compartments model for the selective oxidation of n-butane in a circulating fluidized bed reactor

A phenomenological model for the partial oxidation of n-butane to maleic anhydride (MAN) in a gas–solid riser reactor is proposed. The model captures the main transport–kinetic interactions at both the particle and reactor scale. Two different kinetic models from the literature that either include or neglect the role of solid-state diffusion of oxygen in the catalyst bulk are compared in terms of their impact on overall reactor performance. The use of computational fluid dynamics (CFD) as a means of defining a macroscopic cell-type model for describing the flow patterns in an industrial-scale gas–solids riser is also described. Predictions for the conversion of n-butane and yield of MAN are presented as a function of selected operating variables, and the impact of the variable model parameters on the predictions is briefly assessed. It is shown that the predictions are particularly sensitive to the kinetic model. The relative importance of the interactions between the chemical kinetics and the hydrodynamics is also highlighted.     

工业级原料制备正丁烷氧化制顺酐VPO催化剂及其催化性能

采用工业级原料制备正丁烷氧化制顺酐VPO催化剂,在固定床反应器中测试催化剂性能。采用粉末X射线衍射表征合成的催化剂样品。结果表明,在反应温度372℃、空速1 500 h~(-1)和反应压力0.11 MPa下,催化剂活性较好,正丁烷转化率达到86.27%,顺酐选择性73.27%,顺酐收率63.11%。长周期试验结果表明催化剂稳定性良好。     

Influence of Bi–Fe additive on properties of vanadium phosphate catalysts for n-butane oxidation to maleic anhydride

The physico-chemical and catalytic properties of three ways of modified catalysts were studied, i.e. (i) the addition of both Bi and Fe (nitrate form) during the refluxing VOPO₄·2H₂O with isobutanol (Catalyst A), (ii) the simultaneous addition of BiFe oxide powder in the course of the synthesis of precursor VOHPO₄·0.5H₂O (Catalyst B) and (iii) the mechanochemical treatment of precursor VOHPO₄·0.5H₂O and BiFe oxide in ethanol (Catalyst C). It was found that surface area of the modified catalysts has increased except Catalyst B. The reactivity of the oxygen species linked to V⁵⁺ and V⁴⁺ was studied by using H₂-TPR, which also affected the catalytic performance of the catalyst. The conversion of n-butane decreases with an increment of oxygen species associated with V⁵⁺.     

Membrane pilot reactor applied to selective oxidation reactions

The partial oxidation of butane to maleic anhydride was studied in a conventional fixed bed as well as a novel reactor configuration consisting of a porous metallic membrane immersed in a gas–solid fluid bed. The diameter of both reactors was at a commercial scale greater than 30 mm. A range of gas flow rates, temperatures and butane concentrations were tested. Maleic anhydride yield was generally higher in the membrane reactor due to higher butane conversion. Maleic productivity in the fixed bed equalled that observed in the membrane reactor when the gas–solid fluid bed was maintained at a higher temperature of as much as 30 °C. The butane feed rate to the membrane reactor was limited by hot spots. These hot spots were unanticipated and underscore the importance of increasing heat transfer in order to commercialize this technology.     

助剂对VPO 催化剂物性的影响

以V₂O₅ 和浓硫酸为原料, 浓盐酸和柠檬酸作还原剂, 分别加入一定含量的不同金属离子作助剂, 采用超临界流体干燥法制备了VPO 催化剂, 并在不同气氛下进行活化。应用BET、XRD 和XPS 等表征手段, 考察了各种VPO 催化剂粒子的比表面积、晶相结构及表面V 离子价态等物性。实验表明, 采用超临界流体干燥法可使催化剂比表面显著增大。助剂的种类和添加量不同, 活化气氛不同, 对催化剂比表面积的影响也不同; 助剂的种类对催化剂的晶相结构亦有一定的影响, 但对催化剂表面V 离子价态影响不大。     

甲苯氨氧化合成苯甲腈VPO催化剂研究

用浸渍法制备了不同磷钒比（P／V）的VPO和VPO／硅胶催化剂，并用XRD、FTIR、TPR进行了表征，在思苯氨氧化制苯甲腈反应中考察了其催化性能，发现P的引入破坏了V2O5的晶态结构，有效阻止活性晶粒生长。V＝O与V－O－V键在P作用下向催化剂内层移动，并被高层扭曲，催化剂表面生成磷酸盐物质，晶格氧数量明显减少，钒平均氧化态下降，催化剂供氧能力减弱。活性测试表明，催化剂P／V越高，它对苯甲腈选择性也越好，其中V^3 离子和O^2-空位扮演了重要的角色。认为VPO催化剂中内层V＝O氧化－还原中心与P－OH的B酸中心共同形成了一个复合活性中心，P／V比在1到2之间的VPO／硅胶催化剂都具有较好催化性能。在440℃时甲苯转化率达到97％，对苯甲腈的选择性达到94％，故在此温度时苯甲腈产率约为91．2％。     

Millistructured Reactor as Tool for Investigating the Kinetics of Maleic Anhydride Synthesis

A nearly isothermal millistructured fixed-bed reactor was used to investigate the kinetics of the selective oxidation of n-butane to maleic anhydride. In addition to the investigation of a broad range of industrially relevant conditions, the focus was on the influence of the reaction products on the kinetics. For this purpose, a saturator for the accurate dosing of the target product MA was developed. The experimental data were used to derive a reaction network comprising the by-products acetic and acrylic acid. Additionally, the inhibition of the associated reactions by reactants and products was investigated.     

合成温度对VPO催化剂比表面的影响

在不同的温度下制备VPO催化剂，采用BET、XRD和SEM等测试手段考察制备温度对催化剂的比表面及晶相结构的影响。实验结果表明，温度对比表面和晶相结构的影响存在一个最佳值。选择适宜的制备温度可以使催化剂晶粒变小，比表面增大，活性相含量增加。