The catalytic performance of Cs-doped V/Ti/O catalysts in the oxidation of o-xylene to phthalic anhydride: a TPR/TPO and reactivity study

Titania-supported vanadia (V/Ti/O) systems were modified by addition of cesium oxide for application as catalysts in the selective oxidation of o-xylene to phthalic anhydride (PA). Catalytic tests demonstrated that cesium is a strong promoter of the activity and selectivity to PA, but this effect is evident only under well-defined reaction conditions. Samples with a Cs content lower than 0.35 wt.% Cs₂O exhibited a considerable increase in conversion as compared with the undoped V/Ti/O system. Catalytic tests made with varying o-xylene and oxygen concentrations in the feed demonstrated that in Cs-doped V/Ti/O catalysts the rate-determining step is the re-oxidation of vanadium by molecular oxygen. Thermal-programmed reduction (TPR) and thermal-programmed re-oxidation (TPO) tests evidenced that the addition of Cs decreases the vanadium reducibility and increases the re-oxidizability of the reduced vanadium sites. The positive effect of Cs on selectivity to PA was evident only for o-xylene concentrations in feed lower than 1.5 mol%.     

The characterization of the V species and the identification of the promoting effect of dopants in V/Ti/O catalysts for o-xylene oxidation

The present work deals with the study of the role of promoters in TiO₂-supported vanadium oxide, catalyst for the oxidation of o-xylene to phthalic anhydride. Two different series of catalysts were prepared, the first one consisting of undoped samples having different vanadium oxide content, and the second one of samples having 7 wt.% V₂O₅ and variable amounts of Sb and Cs as promoters. All the samples were characterized by means of Raman spectroscopy, X-ray diffraction and thermal-programmed reduction and oxidation, in order to define a method for the quantification of the different V species (i.e., isolated vanadium, dispersed polyvanadate and bulk vanadium oxide) that develop on TiO₂ support in the presence of promoters. It was found that polyvanadate and bulk vanadium oxide spontaneously release molecular oxygen at 600–650 °C, whereas the isolated V is not susceptible of self-reduction. The latter species is predominant in samples having low vanadium oxide loading (≤2 wt.% V₂O₅, with TiO₂ surface area 22.5 m²/g), and possesses the highest intrinsic activity in o-xylene conversion. The presence of Sb, a promoter of activity, increases the dispersion of the most active species and also hinders its segregation in the reaction environment. These promoting effects are more pronounced when both Cs and Sb are present.     

Catalytic oxidation of volatile organic compounds (VOCs) mixture (isopropanol/o-xylene) on zeolite catalysts

The catalytic oxidation of isopropanol and o-xylene alone and in mixture was investigated over basic zeolites (CsX and NaX) and an acidic zeolite (HY). For a given temperature, the conversion of VOCs mixture into CO₂ increases with the basicity of the zeolite. Results show that VOCs are oxidized through a basic mechanism over NaX, and through an acidic mechanism over HY. Over HY, the presence of isopropanol increases the oxidation of o-xylene, probably because of the formation of isopropyldimethylbenzenes obtained via an acidic mechanism. The addition of platinum over zeolites increases the rate of the VOCs oxidation, this behavior being more pronounced over PtHY maybe because of a higher Pt dispersion.     

蓄热催化氧化技术在处理丁苯橡胶尾气中的应用

中国石油吉林石化公司有机合成厂是专业生产丁苯橡胶的企业,因装置建设年代较早,后处理单元的废气一直没有处理设施,而是通过排气管直接排往大气。为降低丁苯橡胶装置对大气的污染程度,完成公司节能减排目标,装置采用北京中电联环保工程有限公司提供的成套专有技术:蓄热催化氧化技术处理橡胶废气,使废气中的有机物在氧化室氧化分解成CO2和H2O,反应后废气通过烟囱达标排放,利用燃料气燃烧提供热量的方式进行开工预热和系统补热。该技术减少燃料消耗,采用新型催化剂,避免催化剂失活现象发生,反应器在点火、运行、停车过程中安全可靠,环保效果明显,苯乙烯排放量下降97%以上。     

Surface characteristics and activity in selective oxidation of o-xylene of supported V2O5 catalysts prepared by standard impregnation and atomic layer deposition

Alumina-, silica-, and titania-supported vanadium oxide systems with V₂O₅ loadings ranging from 3 to 12 wt.%, corresponding to 0.02–0.09 V/(Al,Si,Ti) atomic ratios, were prepared by atomic layer deposition (ALD) and compared with the corresponding impregnated catalysts. The surface acidic properties of the supports and catalysts were investigated using ammonia adsorption microcalorimetry to determine the number and strength of the surface acid sites. Deposition of V₂O₅ on alumina and titania supports gave rise to catalysts with lower amounts of acid sites than the respective supports, while for the samples prepared on silica, an increase of the number of acid sites was observed after V₂O₅ deposition. As a common trend, the surface acid strength was greater for the ALD catalysts than for the impregnated ones, suggesting a stronger interaction of the VO species with the support centers, which act as electron attractor centers creating Lewis-like vanadium species. Redox cycles were performed, involving temperature programmed reduction (TPR) analyses separated by an oxidation treatment (TPO). The results evidenced the good reversibility of the redox behavior of the vanadium centers in every case, while significant differences were observed when comparing the temperatures of reduction (T_max). Lower T_max values were observed for the better dispersed vanadia catalysts. After reduction, the V centers had a final formal average oxidation state corresponding to +3 or less (+2.5 to +2). The reactivity of the vanadia systems was examined by measuring their performance for the oxidation of o-xylene to phthalic anhydride. Activity tests indicated the superior selectivity of the V₂O₅ systems based on the more acidic supports (Al₂O₃ and TiO₂). The nature of the support governed the activity, and the more concentrated catalysts gave rise to improved selectivity to phthalic anhydride.     

复合氧化物的制备及其在碳黑颗粒物氧化中的应用

采用柠檬酸络合燃烧法合成钾铈和钾铈镧复合氧化物催化剂。利用XRD对复合氧化物进行了表征。利用程序升温反应（TPR）方法研究了其对碳黑颗粒物的催化氧化性能,考查了焙烧温度、焙烧时间、前驱体中K、Ce的物质的量比等制备条件对催化剂活性的影响及稳定性。结果表明：钾铈镧复合氧化物对碳黑颗粒物具有较高的催化活性和稳定性。     

Rôles of catalytic oxidation in control of vehicle exhaust emissions

Catalytic oxidation was initially associated with the early development of catalysis and it subsequently became a part of many industrial processes, so it is not surprising it was used to remove hydrocarbons and CO when it became necessary to control these emissions from cars. Later NOx was reduced in a process involving reduction over a Pt/Rh catalyst followed by air injection in front of a Pt-based oxidation catalyst. If over-reduction of NO to NH₃ took place, or if H₂S was produced, it was important these undesirable species were converted to NOx and SOx in the catalytic oxidation stage. When exhaust gas composition could be kept stoichiometric hydrocarbons, CO and NOx were simultaneously converted over a single Pt/Rh three-way catalyst (TWC). With modern TWCs car tailpipe emissions can be exceptionally low. NO is not catalytically dissociated to O₂ and N₂ in the presence of O₂, it can only be reduced to N₂. Its control from lean-burn gasoline engines involves catalytic oxidation to NO₂ and thence nitrate that is stored and periodically reduced to N₂ by exhaust gas enrichment. This method is being modified for diesel engines. These engines produce soot, and filtration is being introduced to remove it. The exhaust temperature of heavy-duty diesels is sufficient (250–400 °C) for NO to be catalytically oxidised to NO₂ over an upstream platinum catalyst that smoothly oxidises soot in the filter. The exhaust gas temperature of passenger car diesels is too low for this to take place all of the time, so trapped soot is periodically burnt in O₂ above 550 °C. Catalytic oxidation of higher than normal amounts of hydrocarbon and CO over an upstream catalyst is used to give sufficient temperature for soot combustion with O₂ to take place.     

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.     

过氧磷钨酸催化氧化柴油深度脱硫实验研究

采用十六烷基三甲基溴化铵和磷钨酸反应制得过氧磷钨酸Q3+{PO4[W(O)O4]3[W(O)O4]}3-(Q3+为RH4+),以其为催化剂,H2O2为氧化剂,考察了该体系对柴油中含硫化合物的氧化脱除效果。结果表明,过氧磷钨酸具有较高的催化活性,在反应时间为60min,反应温度为60℃,V(H2O)∶V(柴油)=0.2,w(催化剂)=1.16%的条件下,柴油可脱硫率可达92.23%,动力学研究结果表明,催化氧化反应为表观一级反应。     

On the role of acidity in catalytic oxidation

The role of the catalyst surface acid-base properties on the heterogeneously-catalyzed oxidation reaction mechanisms is discussed. Acid-base properties depend on the covalent/ionic character of the metal-oxygen bonds and are involved in some steps of the oxidation reactions, such as the activation of the C---H hydrocarbon bonds, the step associated with the evolution of alkoxide species and the desorption/overoxidation of the partial oxidation products. Thus they participate with the cation redox properties in determining the selective/unselective catalyst behavior.     

Bis(o-phenanthroline)copper-catalysed oxidation of lignin model compounds for oxygen bleaching of pulp

The selectivity of the water-soluble and highly active catalyst bis(o-phenanthroline)Cu in the oxygen bleaching of pulp was explored in model compound studies. The experiments were carried out under reaction conditions typical for industrial oxygen bleaching (90 °C, pH 12, p(O₂) 8 bar). The generation of hydrogen peroxide was an essential step in the oxidation of the lignin model compounds, veratryl alcohol and 2,2′-biphenol. Unfortunately, bis(o-phenanthroline)Cu also catalysed the depolymerisation of the carbohydrate model compound, dextran, both with oxygen and with hydrogen peroxide. The results explain well the inadequate selectivity of bis(o-phenanthroline)Cu in the oxygen bleaching of pulp.     

Synthesis, characterization and catalytic application for wet oxidation of phenol of iron-containing clays

High-surface-area pillared clays (PILC) were prepared from naturally occurring montmorillonites by exchanging interlayer ions to polyoxocations containing (i) aluminum (Al₁₃-PILC), (ii) iron adsorpted onto Al₁₃-PILC, and (iii) iron and aluminum located within the same complex (Fe_0.8Al_12.2-PILC). The obtained Fe_0.8Al_12.2-PILCs were characterized by DR–UV–vis and IR spectroscopy, XRD, ESR, scanning electron microscopy and low temperature N₂ adsorption measurements. Important factors affecting catalyst activity and phenol removal efficiencies have been studied, i.e. the effect of pH, temperature, catalyst concentration and stability of the catalyst.     

Comparison among structured and packed-bed reactors for the catalytic partial oxidation of CH4 at short contact times

Three types of catalyst support (foams, honeycomb monoliths with square channels and spheres with approximately equal values of specific geometric surface a_v) were examined and compared by simulation with a 1D, dynamic heterogeneous mathematical model for application to the autothermal partial oxidation of methane. Both cold start-up and steady-state behaviours were investigated.

It was found that mass and, particularly, heat transfer properties markedly affect the reactor behaviour, both at start up and at steady state. Thus, the choice of the catalyst support can lead to greatly different reactor performances. Concerning the reactor start-up, simulations revealed that better interphase heat transport properties and lower bed heat capacity are useful to minimize the total start-up time; on the other hand, more favourable transport properties reduce the maximum flow rate which allows to achieve and maintain an ignited steady state. At steady state, oxygen conversion is strictly governed by interphase mass transfer, while methane conversion depends on a more complex, mixed chemical-diffusional regime.     

相转移催化氧化体系在燃料油脱硫中的应用

介绍了以过氧化氢为氧化荆的相转移催化氧化脱硫研究现状.对过氧化氢/有机酸/季铵盐体系、过氧化氢/杂多酸/季铵盐体系和过氧化氢/合成相转移催化剂体系在脱除燃料油中噻吩、苯并噻吩、二苯并噻吩等方面的应用作了综述.指出了过氧化氢/合成相转移催化剂体系是燃料油深度脱硫的一个重要研究方向.     

水解酸化+生物接触氧化法处理小区生活污水的实验研究

考察了水解酸化配合生物接触氧化法系统处理综合型小区生活污水的工艺条件和效果.试验结果表明:在生化部分的停留时间(HRT)为6 h,水力负荷(VLR)为2.8～3.4 kgCOD/m-2·d,污泥浓度(MLSS)为6 500 rng/L.气水比为16:1时,系统对COD、NH3-N、TP、SS的去除率分别可达到91.9%、81.4%、80%、82.5%,极大降低了环境污袭风险和城市污水处理负担,整个工艺具有流程简单、方便管理、运行稳定、抗冲击负荷能力强,一体化程度高等特点.     

Fuel processor integrated H2S catalytic partial oxidation technology for sulfur removal in fuel cell power plants

H₂S catalytic partial oxidation technology with an activated carbon catalyst was found to be a promising method for the removal of hydrogen sulfide from fuel cell hydrocarbon feedstocks. Three different fuel cell feedstocks were considered for analysis: sour natural gas, sour effluent from a liquid middle distillate fuel processor and a Texaco O₂-blown coal-derived synthesis gas. The H₂S catalytic partial oxidation reaction, its integratability into fuel cell power plants with different hydrocarbon feedstocks and its salient features are discussed. Experimental results indicate that H₂S concentration can be removed down to the part-per-million level in these plants. Additionally, a power law rate expression was developed and reaction kinetics compared to prior literature. The activation energy for this reaction was determined to be 34.4 kJ/g mol with the reaction being first order in H₂S and 0.3 order in O₂.     

有机物对厌氧氨氧化生物脱氮影响研究

以经过处理和未经处理的生活污水中添加碳酸氢铵、亚硝酸钠为试验用水,进行了两阶段对比研究,以期考察有机物对厌氧氨氧化生物脱氮效果的影响。研究结果表明,在平均pH值为8.07,ALK为855～1468mg/L、平均进水NH3-N、NO2--N、COD的质量浓度分别为325.57、301.63、139.35mg/L的条件下,二级串联反应器两阶段的TN去除率分别为78.63%、76.57%,COD去除率分别为64.54%、66.87%。在高氨氮、低碳氮比水质条件下,难降解有机物对厌氧氨氧化细菌活性没有太大影响。同时,扫描电镜观察结果证实,污泥中形成了以厌氧氨氧化球状菌为主,其它杆状菌、丝状菌共存的微生物混培体。此外,厌氧氨氧化菌表面附着的颜色较亮的白色小球,可能是反硝化菌。     

A revision of the mechanism of o-xylene oxidation to phthalic anhydride with V/Ti/O catalysts, and the role of the promoter Cs

This paper describes the effect of Cs oxide, a promoter of an anatase-supported 7 wt.% V₂O₅ catalyst for the oxidation of o-xylene to phthalic anhydride. The effect of Cs was investigated in relation to the reaction network at 320 °C, i.e., under conditions of incomplete o-xylene conversion. It was found that at this temperature the catalytic performance was affected by a surface-saturation effect. However, Cs had the role of favoring the desorption of intermediates and keeping a cleaner and more oxidizing catalyst surface, so accelerating the conversion of the reactant and the consecutive oxidations occurring upon the reaction intermediates finally leading to phthalic anhydride. This de-saturation derived from the enhancement of the V re-oxidation rate, which is one main effect of Cs in V/Ti/O catalysts. Besides the route of o-xylene oxidation to the main reaction intermediate o-tolualdehyde, a parallel reaction pathway included the direct oxidation of o-xylene to phthalaldehyde; the latter was one intermediate in the formation of phthalide, via intramolecular condensation, and of phthalic acid.