Selective ammoxidation of propane over multicomponent oxide catalysts

The ammoxidation of propane to acrylonitrile was investigated over various multicomponent mixed oxidecatalysts. Bi₃GaMo₂ oxide, Bi_0.85V_0.55Mo_0.45 oxide and Bi₂TiMo oxide showed a good selectivity to acrylonitrile. The catalytic performance was strongly dependent on the components constituting the oxides due to the concentration of defects, the nature of the component and the mixed oxide phases. The effect of support type on the catalytic performance of Bi_0.85V_0.55Mo_0.45 oxide showed that selectivity to acrylonitrile decreased in the order silica > alumina > magnesia. The catalytic performance of Bio_0.85V_0.55Mo_0.45 oxide was improved by the addition of a small amount of a component such as magnesium, calcium and tellurium. The catalytic performance of Bi₃GaMo₂, oxide catalyst, however, decreased significantly with the addition of a smallamount of alkali metal (Li, Na, K, Cs).     

Transient behaviour of dense catalytic membranes based on Cu- and Co-doped Bi4V2O11 (BIMEVOX) in the oxidation of propene and propane

ME-doped γ-Bi₄V₂O₁₁ (BIMEVOX) oxides are highly oxide ion conducting materials and this property may be profitably used in selective oxidation of hydrocarbons. The catalytic properties of BICUVOX and BICOVOX when shaped as dense membranes displayed in catalytic dense membrane reactor are examined in the oxidation of propene and of propane. Mirror-polished BICUVOX and BICOVOX membranes studied previously were poorly active for propene oxidation because of a small number of active sites but showed an excellent stability and reproducibility (lasting more than 1 month) during which products of mild oxidation (acrolein, hexadiene) and CO were formed. Membranes with depolished surfaces exhibit high conversions of propene (up to 60 mol%), and also of propane (up to 20 mol%) but – contrary to mirror-polished membranes – a complex transient behaviour is observed during which syngas production occurs. The membrane polarisation followed by in situ solid electrolyte potentiometry shows that the oxygen reservoir is far higher than expected on the reaction side which is separated (by the membrane) from the oxidising side where (diluted) oxygen is reduced to O²⁻ specie. The influence of oxygen partial pressure on the catalytic performance suggests that the electronic conductivity of the material is limiting the oxygen flux through the membrane, and thus is determining the catalytic properties and transient behaviours of depolished membranes.     

Heteropolyacid salts catalysts supported on silylated attapulgite for cyclohexane oxidation

以硅烷化凹凸棒黏土为载体,满孔浸渍法制备了负载型杂多酸盐催化剂,采用X射线衍射、比表面积测定、傅里叶变换红外光谱等手段对催化剂进行了表征。分别考察了双氧水用量、杂多酸盐负载量及循环使用次数等因素对环己烷氧化反应的影响。结果表明,负载型杂多酸盐CoH₇P₂Mo₁₅V₃O₆₂能显著提高环己烷氧化能力;催化剂回收烘干后可直接重复利用,催化活性没有明显降低。在优化条件下,环己烷转化率达到35.28%,目标产物环己醇和环己酮的总收率达到12.48%。实验表明,负载型杂多酸盐CoH₇P₂Mo₁₅V₃O₆₂是一种具有应用前景的催化剂,便于与产物分离,且对设备和环境污染小。     

High-throughput preparation and testing of MoNbSbV mixed oxide catalysts for direct oxidation of propane to acrylic and acetic acids

An automated robotic method using a solution at pH=2 containing four precursor salts dissolved has been developed and validated for high-throughput preparation of Mo, Nb, Sb and V mixed metal oxide solids, which are known to be selective for propane oxidation to acrylic (AA) and acetic (AcA) acids. Spherical shaped silica beads of exceptionally narrow size distribution were synthesised using oil drop technique from a Brace™ instrument. Automated impregnation of the beads by the previous solution has been developed and validated. Catalytic studies were performed using a conventional micro-reactor system with an Ultra-Fast™ GC analysis (<2 min against >30 min). After calcination of the samples under either N₂ or air at 873 K, a mixture of phases was obtained, such as VSbO₄, Mo_xM_1−xO_2.8 (M=V and/or Nb), Sb₄₍₂₎Mo₁₀O₃₁, and other minor phases, such as MoO₃ if activated in air. Mixed oxide samples calcined under N₂ gave better catalytic activity and selectivity to AA/AcA compared to those calcined under air. Measures of catalytic performance of 16 supposedly identical materials fell within a ±5% range of the median values, showing that our experimental set-up is relevant to combinatorial studies. By preparing 15 samples of different chemical composition, optimum catalytic performance was found to correspond to Mo_0.55 Nb_0.09 Sb_0.18V_0.18 mixed oxide calcined at 773 K under N₂, containing a mixture of phases, in particular Mo_xM_1−xO_2.8 and Sb₂Mo₁₀O₃₁, similarly to the M1 and M2 phases observed for MoNbTeV mixed oxide catalysts.     

Effects of calcium cations incorporated into magnesium vanadates on the redox behaviors and the catalytic activities for the oxidative dehydrogenation of propane

The influence of the incorporation of Ca²⁺ into MgV₂O₆, Mg₂V₂O₇ and Mg₃V₂O₈ on the catalytic activities for the oxidative dehydrogenation of propane and the mobility of lattice oxygen in those catalysts has been investigated. On Mg₂V₂O₇ and Mg₃V₂O₈ incorporated with Ca²⁺, the selectivity to propylene evidently increased with increasing the content of Ca²⁺ in those catalysts. A decrease of the conversion of propane was observed on Ca²⁺-added Mg₂V₂O₇ with increasing Ca²⁺-content; while on Ca²⁺-added Mg₃V₂O₈, no dependence on the Ca²⁺-content was observed. The incorporation of Ca²⁺ was rather insensitive to both the selectivity and the conversion on Ca²⁺-added MgV₂O₆. In order to examine the mobility of lattice oxygen in those catalysts, those catalysts were employed for the oxidative dehydrogenation in the absence of oxygen for 2.25 h, followed by the addition of gaseous oxygen into the feedstream. After the addition of gaseous oxygen under the present conditions, oxygen in the effluent was detected at 9, 3.5 and 2 min with Mg₂V₂O₇ incorporated with Ca²⁺ at these Ca/(Mg + Ca) ratios: 0, 0.05 and 0.1. The times were 1.4, 2 and 4 min with Mg₃V₂O₈ incorporated with Ca²⁺ as Mg₂V₂O₇. However, no oxygen was detected with MgV₂O₆ incorporated with Ca²⁺ even 15 min after the addition of the gaseous oxygen. The present results reveal that the mobility of the lattice oxygen in those catalysts is strongly influenced by the amount of Ca²⁺ incorporated, resulting in significant effects on the activities for the oxidative dehydrogenation of propane. ⁵¹V MAS NMR was also employed for the observation of redox behaviors of vanadium species in those catalysts during the reaction.     

Partial oxidation of propane to acrolein in a membrane reactor – Experimental data and computer simulation

Acrolein is synthesized in a tubular membrane reactor with a porous membrane acting as oxygen distributor at 450–550 °C with the catalyst Ag_0.01Bi_0.85V_0.54Mo_0.45O₄ by direct partial oxidation of propane. The reaction in the membrane reactor is compared with the reaction in the classical co-feed reactor. If the oxygen is dosed through the porous reactor wall to the tube side of the reactor, higher acrolein yields and selectivities are obtained. The catalyst is located inside the tube, where the propane streams through. For the simulation – based on a kinetic model – the membrane reactor was partitioned into 14 separate sections through which the oxygen supply takes place. In accordance with the experiment the simulations show that the acrolein selectivities will increase if the oxygen is dosed through the reactor wall acting as membrane oxygen distributor.     

Comparison of different catalysts in the membrane-supported dehydrogenation of propane

Dehydrogenation of propane is studied in a high temperature packed bed catalytic membrane reactor with a hydrogen-selective silica membrane. The silica membrane is prepared by a two-step sol–gel process. The removal of hydrogen in the membrane reactor results in higher propane conversion and higher propene yields in comparison to an equivalent fixed-bed reactor. Unfortunately, as a result of the H₂ removal coking is favoured in the membrane reactor. Therefore, the higher propene yields are found only for the first 100–120 min time on stream. However, the lower selectivity of the membrane reactor due to coking is compensated to some extend by a reduced hydrogenolysis. Two commercial dehydrogenation catalysts of different activity were tested in the membrane reactor: Cr₂O₃/Al₂O₃ and Pt–Sn/Al₂O₃. The two catalysts show a different activity, coking, and regeneration behaviour in the membrane-supported propane dehydrogenation.     

Selective oxidation of propylene to acrolein over supported V2O5/Nb2O5 catalysts: An in situ Raman, IR, TPSR and kinetic study

The vapor-phase selective oxidation of propylene (H₂CCHCH₃) to acrolein (H₂CCHCHO) was investigated over supported V₂O₅/Nb₂O₅ catalysts. The catalysts were synthesized by incipient wetness impregnation of V-isopropoxide/isopropanol solutions and calcination at 450 °C. The catalytic active vanadia component was shown by in situ Raman spectroscopy to be 100% dispersed as surface VO_x species on the Nb₂O₅ support in the sub-monolayer region (<8.4 V/nm²). Surface allyl species (H₂CCHCH_2*) were observed with in situ FT-IR to be the most abundant reaction intermediates. The acrolein formation kinetics and selectivity were strongly dependent on the surface VO_x coverage. Two surface VO_x sites were found to participate in the selective oxidation of propylene to acrolein. The reaction kinetics followed a Langmuir–Hinshelwood mechanism with first-order in propylene and half-order in O₂ partial pressures. C₃H₆-TPSR spectroscopy studies also revealed that the lattice oxygen from the catalyst was not capable of selectively oxidizing propylene to acrolein and that the presence of gas phase molecular O₂ was critical for maintaining the surface VO_x species in the fully oxidized state. The catalytic active site for this selective oxidation reaction involves the bridging VONb support bond.     

Evaluation of the role played by bismuth molybdates in Bi2Sn2O7–MoO3 catalysts used for partial oxidation of isobutene to methacrolein

The catalytic behaviour of multiphasic catalysts based on -bismuth pyrostannate, Bi₂Sn₂O₇, was investigated in the selective oxidation of isobutene into methacrolein. When -Bi₂Sn₂O₇ is mixed with MoO₃, strong cooperation effects on the yield and selectivity in methacrolein occur. However, XRD analyses performed on samples after test revealed the formation of a low quantity of -bismuth molybdate, -Bi₂Mo₃O₁₂, when the reaction temperature exceeded 673 K. Additional experiments were therefore carried out on the “Bi–Sn–Mo–O” catalysts in order to shed light on the role of Bi₂Mo₃O₁₂ in the synergetic effects observed in the Bi₂Sn₂O₇–MoO₃ system. The experimental results are discussed in terms of several hypotheses. First, the intrinsic activity of Bi₂Mo₃ O₁₂ is probably the simplest explanation for the synergetic effects, although experiments have shown that this phase present in a low quantity is only poorly active. Second, catalytic tests made on Bi₂Sn₂O₇–Bi₂Mo₃O₁₂ mechanical mixtures have evidenced a cooperation between these two ternary oxides, particularly when Bi₂Sn₂O₇ was the major component of the mixture. Consequently, it is likely that a synergy between Bi₂Sn₂O₇ and the in situ generated Bi₂Mo₃O₁₂ might play a role in the synergy observed in the Bi₂Sn₂O₇–MoO₃ association. Third, as bismuth pyrostannate was previously shown to behave as an oxygen donor phase with respect to WO₃, a remote control mechanism could therefore occur between Bi₂Sn₂O₇ and MoO₃, independently from the formation of -Bi₂Mo₃O₁₂.     

柱撑型钒基催化剂的制备及其丙烷氧化脱氢催化性能

以人工合成的3种不同层板组成的层状双羟基金属氢氧化物MgAl-CO₃-LDHs、CoAl-CO₃-LDHs和NiCr-CO₃-LDHs为载体，以偏钒酸钠NaVO₃为柱撑剂和钒前体，采用离子交换法制备了钒柱撑的催化剂样品（MgAlVO、CoAlVO和NiCrVO），通过XRD、FTIR、XPS和Raman等手段表征了样品的物相、钒物种的存在形态以及钒的价态，以丙烷氧化脱氢制丙烯为模型反应，表征了所制备的催化剂样品的催化性能，着重探讨了LDHs载体的层板组成以及钒的含量对催化剂样品中的钒氧物种存在形态及其丙烷氧化脱氢催化性能的影响。结果表明：以20%理论含量的钒柱撑MgAl-CO₃-LDHs所制备的催化剂样品20%MgAlVO对丙烷氧化脱氢反应的催化性能较好，当反应温度为560℃时，丙烯收率可以达到11.3%，Raman光谱显示该催化剂样品中的钒以Mg₃V₂O₈和α-Mg₂V₂O₇两种形式共存，且晶格氧O^2-和吸附氧O^-所占的比例较为均衡，有利于获得较好的丙烷氧化脱氢催化性能，而在同样条件下制备的催化剂样品20%CoAlVO和20%NiCrVO中的钒物种只观察到分别以Co₃V₂O₈和Ni₃V₂O₈存在的正钒酸盐，前者对丙烯的收率不到8%，后者甚至完全得不到丙烯。     

Selective oxidation of ammonia over copper-silver-based catalysts

A novel catalyst based on copper-silver was developed to solve the contradiction between the high conversion temperature of Cu-based catalyst and low N₂ selectivity of Ag-based catalyst during selective oxidation of ammonium gas. The Cu-Ag-based catalyst (Cu 5 wt.%-Ag 5 wt.%/Al₂O₃) displayed a relatively low complete conversion temperature (<320 °C) with a high N₂ selectivity (>95%). Increasing loading of Cu and Ag decreases N₂ selectivity. The low N₂ selectivity of Ag-based catalyst is possibly related to the formation of Ag₂O crystals. Improvement of N₂ selectivity of Ag-based catalyst was obtained by doping Cu to decrease crystallized Ag₂O phase. The temperature programmed reaction (TPR) data show that N₂O is the main byproduct of oxidation of ammonia at temperature lower than 200 °C. Two bands of nitrate species at 1541 and 1302 cm⁻¹ were observed on Ag 10 wt.%/Al₂O₃ at the temperature higher than 250 °C, which indicates the formation of NO_x during the selective catalytic oxidation of ammonia. No nitrate species was observed on Cu 10 wt.%/Al₂O₃ and Cu 5 wt.%-Ag 5 wt.%/Al₂O₃, while only one nitrate species (1543 cm⁻¹) existed on Cu 10 wt.%-Ag 10 wt.%/Al₂O₃. We proposed that mixing Ag with Cu inhibited the formation of NO_x during the selective catalytic oxidation of ammonia over Cu-Ag/Al₂O₃.     

Solid state chemistry of bulk mixed metal oxide catalysts for the selective oxidation of propane to acrylic acid

Syntheses of Mo–V–Sb–Nb–O bulk materials, which are candidate catalyst systems for the selective oxidation of propane to acrolein and acrylic acid, were made using soluble precursor materials. The products were characterized by X-ray powder diffraction and Raman spectroscopic studies. The objectives of this work were to explore the utility of liquid phase automated synthesis for the preparation of bulk mixed metal oxides, and the identification of the oxide phases present in the system. This is the first published study of the phase composition for these materials. After calcination of these bulk oxides under flowing nitrogen at 600°C, and using stoichiometric ratios of Mo–V–Sb–Nb (1:1:0.4:0.4) and Mo–V–Sb–Nb (3.3:1:0.4:0.4) it was demonstrated that a mixture of phases were obtained for the syntheses. X-ray powder diffraction studies distinguished SbVO₄, Mo₆V₉O₄₀, MoO₃, and a niobium-stabilized defect phase of a vanadium-rich molybdate, Mo_0.61–0.77V_0.31–0.19Nb_0.08–0.04O_x, as the major phases present. Complementary data were provided by the Raman spectroscopic studies, which illustrated the heterogeneity of the phases present in the mixture. Raman also indicated bands attributable to the presence of phases containing terminal M=O bonds as well as M–O–M polycrystalline phases. Previous studies on this system have identified SbVO₄ and niobium-stabilized vanadium molybdate species as the active phases necessary for the selective oxidation of alkanes.     

K改性Mo-SBA-16催化剂催化乙烷选择氧化制乙烯和醛类

采用超声辅助等体积浸渍法制备了K改性Mo掺杂SBA-16催化剂x K/Mo-SBA-16,评价其对乙烷选择氧化反应的催化性能,并利用XRD、TEM、N_2吸附-脱附、UV-Vis、UV-Raman和FT-IR等对催化剂物化性质进行表征,同时考察反应温度和K含量等因素对催化剂性能的影响。结果表明,K的添加提高了产物中醛类选择性。在V(C_2H_6)∶V(O_2)=3∶1和反应温度550℃条件下,0. 1K/5. 0Mo-SBA-16催化剂上乙醛选择性为17. 1%,总醛(包括乙醛和丙烯醛)产率7. 4%。碱金属K的添加改变了催化剂中活性组分的结构和分散度,孤立和高分散的MoO_4四面体逐渐转变为低聚扭曲的MoO_x四面体,可能形成了K_2Mo_2O_7新相。     

Preparation of H5PMo10V2O40 catalyst immobilized on nitrogen-containing mesostructured cellular foam carbon (N-MCF-C) and its application to the vapor-phase oxidation of benzyl alcohol

Nitrogen-containing mesostructured cellular foam carbon (N-MCF-C) was synthesized by a templating method using mesostructured cellular foam silica (MCF-S) and polypyrrole as a templating agent and a carbon precursor, respectively. The N-MCF-C was then modified to have a positive charge, and thus, to provide a site for the immobilization of [PMo₁₀V₂O₄₀]⁵⁻. By taking advantage of the overall negative charge of [PMo₁₀V₂O₄₀]⁵⁻, H₅PMo₁₀V₂O₄₀ (PMo₁₀V₂) catalyst was chemically immobilized on the N-MCF-C support as a charge-matching component. Characterization results showed that the PMo₁₀V₂ catalyst was finely dispersed on the N-MCF-C support via strong chemical interaction, and that the pore structure of N-MCF-C was still maintained even after the immobilization of PMo₁₀V₂. In the vapor-phase oxidation of benzyl alcohol, the PMo₁₀V₂/N-MCF-C catalyst showed a higher conversion and a higher oxidation activity (formation of benzaldehyde) than the unsupported PMo₁₀V₂ and PMo₁₀V₂/MCF-S catalysts.     

无人机用锂离子电池正极材料Li1.20Mn0.54Ni0.13Co0.13O2的Mo6+掺杂改性研究

采用碳酸盐共沉淀法和高温烧结工艺将一定量的Mo⁶⁺掺杂到Li_1.20Mn_0.54Ni_0.13Co_0.13O₂正极材料中。利用XRD、SEM、EDS和恒流测试仪研究Mo⁶⁺掺杂对Li_1.20Mn_0.54Ni_0.13Co_0.13O₂正极材料的晶体结构、微观形貌和电化学性能的影响。结果显示,Li_1.20Mn_0.52Ni_0.13Co_0.13Mo_0.02O₂表现出更低的阳离子混排和优异的电化学性能。经过Mo⁶⁺掺杂后的正极,由于Li⁺高速的迁移速率,使得首次不可逆容量损失降低,并展现出更好的高倍率性能和优异的循环稳定性。在0.5C倍率下循环100周后,Li_1.20Mn_0.52Ni_0.13Co_0.13Mo_0.02O₂的容量保持率达到92.2%,远远大于Li_1.20Mn_0.54Ni_0.13Co_0.13O₂的87.5%。另外,当放电倍率增大到5C时,Li_1.20Mn_0.54Ni_0.13Co_0.13O₂的放电比容量要比Li_1.20Mn_0.52Ni_0.13Co_0.13Mo_0.02O₂低21.0 mA·h/g。因此,采用Mo⁶⁺掺杂改性Li_1.20Mn_0.54Ni_0.13Co_0.13O₂正极材料,可以有效提高锂电池的循环保持率和高倍率放电性能。     

催化剂上丙烷氧化脱氢制丙烯

以浸渍法制备VMo/γ-Al₂O₃和VMoMg/γ-Al₂O₃催化剂,考察其催化丙烷氧化脱氢制丙烯的反应活性,采用XRD、UV-Vis DRS和In suit IR对催化剂进行表征。结果表明,V负载质量分数为3%、Mo负载质量分数为7%时的3V7Mo/γ-Al₂O₃催化剂表现出较好的催化性能;添加Mg后催化剂的催化性能有所改善,反应温度500 ℃时,丙烷转化率为18.19%,丙烯选择性74.76%。丙烷和丙烯在3V7Mo/γ-Al₂O₃和3V₇Mo₄Mg/γ-Al₂O₃催化剂上吸附后,C—H键的H与催化剂活性中心的晶格氧发生作用形成H—O键,且3V₇Mo₄Mg/γ-Al₂O₃催化剂上出现C—O键的温度比3V₇Mo/γ-Al₂O₃催化剂高,表明加入Mg有利于提高丙烯选择性。     

SrFe0.6Cu0.3Ti0.1O3-δ透氧膜反应器中甲烷部分氧化反应工艺及膜稳定性考察

采用SrFe_0.6Cu_0.3Ti_0.1O_3-δ混合导体透氧膜组装成膜催化反应器,进行甲烷部分氧化制合成气反应,考察了反应温度、空速、催化剂粒径等条件的影响,并分析了反应气氛引起的透氧膜结构变化情况。结果表明,在膜反应器内,催化反应与透氧过程存在相互制约和相互促进的关系。在膜反应器内进行甲烷部分氧化反应后,透氧膜的两侧表面均发生蚀刻现象,结晶度显著降低,反应侧蚀刻现象较为严重,膜表面形成了疏松的多孔层,反应气氛使膜表面晶体结构发生了较大改变,Sr容易从钙钛矿结构中析出并与CO₂结合形成SrCO₃,Sr的析出导致组成不平衡,促进了钙钛矿结构分解及其他物相的产生。     

Oxidation of isobutane on a heteropolycompound hydrogen reservoir

The selective oxidation of isobutane to methacrylic acid (MAA) has been studied on Cs_1.6H_2.4P_1.7Mo₁₁V_1.1O₄₀ which is a heteropolycompound (HPC). The oxidation mechanism isobutane → isobutene → MAL → MAA has already been proposed previously with a rate determining step corresponding to the first step of the reaction, i.e., to dehydrogenation of isobutane leading to isobutene. Here we show that the HPC which is active in the oxidation of isobutane possesses, in the reduced state, anionic vacancies able to store reactive hydrogen species H ^* presenting the ability to diffuse through the solid. Such a result is of great importance since dehydrogenation requires the abstraction of hydrogen from the molecule which could be performed by a lacunar phase. Therefore, a mechanism of the dehydrogenation step is proposed, involving a heterolytic abstraction of a H⁻ species from the alkane.     

Oxidation of unsymmetrical dimethylhydrazine over heterogeneous catalysts Solution of environmental problems of production, storage and disposal of highly toxic rocket fuels

The catalytic oxidation of unsymmetrical dimethylhydrazine (UDMH) by air has been studied in a vibro-fluidized catalyst bed laboratory kinetic setup over catalysts Cu_xMg_1−xCr₂O₄/γ-Al₂O₃, 32.9%Ir/γ-Al₂O₃ and β-Si₃N₄ in a temperature range 150–400 °C. The catalyst Cu_xMg_1−xCr₂O₄/γ-Al₂O₃ was found to be optimal regarding high yields of CO₂ and low yields of NO_x. A probable mechanism of UDMH heterogeneous catalytic oxidation is proposed. Catalyst Cu_xMg_1−xCr₂O₄/γ-Al₂O₃ has been further used in the pilot plant specially designed for the destruction of UDMH. Results of testing the main fluidized bed catalytic reactor for UDMH oxidation and the reactor for selective catalytic reduction of NO_x with NH₃ are presented. These results prove that the developed UDMH destruction technology is highly efficient and environmentally safe.     

Concentration and residence time effects in packed bed membrane reactors

A fixed bed reactor (FBR) and a packed bed membrane reactor (PBMR) were compared with respect to their performance in the oxidative dehydrogenation of ethane over VO_x/γ-Al₂O₃ catalyst. The experiments were carried out at high space velocities and under oxygen excess conditions. In the PBMR, the oxidant air was distributed from the shell side of the membrane.

At similar overall feed configurations, the conversion of ethane was found to be higher in the PBMR. This effect was most pronounced at the highest space velocity. Mostly ethylene yield was higher in the PBMR than in the FBR. However, the yield of carbon oxides increased more. Thus, an improvement of olefin selectivity was not observed. There were even sets of experimental conditions, where the ethylene yield in the PBMR fell below the corresponding value for the FBR. In the PBMR under oxygen excess conditions, the consecutive oxidation of ethylene is more favoured than in the FBR.

Two essential reasons for the observed differences in the reactor performances are discussed. At first, there are different local reactant concentrations. Secondly, there are essential differences in the residence time behaviour of the reactants in the FBR and PBMR. In order to exemplify the latter aspect additional experiments have been carried out using a cascade of three identical PBMRs. Varying the specific oxygen flow rates over the individual membrane segment walls different dosing profiles were implemented. The results obtained in this study emphasise the general potential, but also the limits of membrane reactors compared to the FBR.