Effect of calcination temperature on isobutane dehydrogenation over Mo/MgAl2O4 catalysts

The catalytic activity of a novel catalyst Mo/MgAl₂O₄ was studied in isobutane dehydrogenation, and the effect of calcination temperature has been comprehensively investigated. For catalysts with MoO₃ loading below monolayer coverage (5Mo/MgAl₂O₄), isobutane conversion increases with calcination temperature, while the selectivity to isobutene decreases. An opposite trend is observed for catalysts with MoO₃ loading above monolayer coverage (30Mo/MgAl₂O₄). In combination with characterization results, it can be concluded that calcination temperature affects catalyst structure and properties in many aspects including surface area, acidity and reducibility, as well as the interaction between Mo species and support, and thus influences the catalytic behaviors.     

Effect of preparation method on sinterability and properties of nanocrystalline MgAl2O4 and ZrO2-MgAl2O4 materials

Abstract

Nanocrystalline MgAl₂O₄ and ZrO₂-MgAl₂O₄ powders were synthesised by combustion and conventional solid state reaction routes. The synthesised powders were processed, dry pressed, and sintered for 3 h at temperatures ranging from 1550 to 1625°C. The sintered pellets were then characterised in terms of phase (XRD), microstructure (SEM), relative density, apparent porosity, water absorption, hardness, three point bend strength, and fracture toughness. The XRD studies revealed that ZrO₂ was present in tetragonal form in the case of combustion synthesised powders (CSP), whereas in powders obtained by solid state reaction (SSP) it was present in the monoclinic form. This study also revealed that the addition of ZrO₂ improved the mechanical properties of sintered MgAl₂O₄ samples: 20 wt-%ZrO₂-MgAl₂O₄ composites prepared from CSPs and conventional SSPs and sintered at 1625°C for 3 h had fracture toughness of 5·96 and 4·33 MPa m^1/2 and three point bend strength of 269 and 98 MPa respectively. Higher sintered density, the presence of tetragonal zirconia as a major phase, and the finer microstructure are probably responsible for the superior mechanical properties exhibited by sintered CSP materials as compared with the sintered SSPs.     

Synthesis andcharacterisation of nanocrystalline MgAl2O4 ceramic powders by use of molten salts

Abstract

Nanocrystalline MgAl₂O₄ powders were prepared by a thermal decomposition method, i.e. by use of molten salts. This method involves co-melting stoichiometric amounts of magnesium nitrate hydrate Mg(NO₃)₂.6H ₂O and aluminium nitrate hydrate Al(N O₃) ₃.9H₂O at 500°C. The spinel content of the co-melted and calcined powders at different firing temperatures up to 1000°C was determined by chemical analysis and the powders were characterised with respect to spinel formation, crystallite and particle sizes by X RD, T EM , and IR spectroscopy. The results obtained revealed that the co-melted materials were amorphous. After heat treatment of the amorphous materials at up to 1000°C, pure spinel powder was obtained, reaching over 98% spinel content. During calcination at different firing temperatures up to 1000°C the amorphous material progressively crystallised, forming nanocrystalline spinel with a maximum crystallite size of about 10 nm and particle size of around 300 nm. Bands in IR spectra were observed corresponding to the ex istence of AlO₆ groups prior to magnesium spinel formation, which was the only crystalline phase at 1000°C.     

No x abatement over VPO catalysts

Vanadium phosphorus oxide (P/V = 1.1) was found to be active for NO reduction by butane to N₂ at 766–816 K. The conversion increased with increasing butane concentration, increasing temperature, and decreasing NO concentration. The activity per surface vanadium ion compared favorably with other active catalytic systems, but was low in the absence of butane. Propane was also effective in reducing NO over this catalyst.     

Interfacial evolution mechanism of MgAl2O4/MgAl2O4 joints bonded with lanthanum glass

The La₂O₃-SiO₂-B₂O₃ (LSB) glass filler with high softening temperature was first used to join MgAl₂O₄ ceramic. An interfacial layer composed of Al₂O₃ was formed due to the solubility difference of MgO and Al₂O₃ in the LSB glass filler. As a result, the addition of Al₂O₃ into the LSB glass filler caused the increase of interfacial layer thickness. On the contrary, the addition of MgO into the LSB glass filler led to the decrease of interfacial layer thickness. When the adding content of MgO was 6 wt%, the interfacial layer disappeared and completely amorphous brazing seam was obtained. The in-line transmittance of joints decreased with the increase of the thickness of interfacial layer. The optimal in-line transmittance of joint bonded with La₂O₃-SiO₂-B₂O₃-MgO (LSB6M) glass filler reached 82.9% at 1000 nm. Meanwhile, the average flexural strength of joints was about 196.2 MPa, which was equal to the strength of MgAl₂O₄ substrate.     

Pd/Co3O4 catalyst for CH4 emissions abatement: study of SO2 poisoning effect

A catalyst with 0.7 wt% Pd load supported over Co₃O₄ oxide was investigated in the methane oxidation by operating under CH₄/O₂ stoichiometric conditions. The effect of the noble metal addition on the activity of bare Co₃O₄ was evaluated. Samples were characterized by BET, XRD, TPR and XPS analyses. The SO₂ poisoning of Pd catalyst and Co₃O₄ was studied by performing CH₄ oxidation tests under stoichiometric conditions in SO₂ (1 ppm or 10 ppm)_. Experiments evidenced that in our conditions the low amount of SO₂ doesn’t influence the Pd behaviour, whereas in presence of 10 ppm of SO₂ some deactivation occurs that becomes more evident above 450 °C at which the catalyst doesn’t reach 100% of methane conversion. Catalytic tests performed over Co₃O₄ and the Pd supported catalyst, after a treatment at 350 °C for 15 h in 10 ppm SO₂/He, suggest that Co₃O₄ is a sulphating support, as confirmed by XPS analysis. Therefore, an important role in lowering the sulphur poisoning of Pd may be played by Co₃O₄.     

Microstructure,mechanical and optical properties of MgAl2O4/MgAl2O4 joints bonded using CaO-Al2O3-SiO2 glass filler

Transparent MgAl₂O₄ ceramics were bonded by using CaO-Al₂O₃-SiO₂ (CAS) glass filler. The CAS glass filler exhibited the same thermal expansion behavior as MgAl₂O₄ ceramic and excellent wetting ability on the surface of MgAl₂O₄ ceramic. When the cooling rate of 15 °C/min was used, no interfacial reaction was observed and the amorphous brazing seam could be obtained. However, low joining temperature (1250 °C) led to the formation of pores and high joining temperature (1400 °C) resulted in the formation of cracks. Furthermore, the slow cooling rate of 5–10 °C/min induced the crystallization of CaAl₂Si₂O₈ and Mg₂Al₄Si₅O₁₈ due to the dissolution of MgAl₂O₄ substrate. The optimal flexural strength of 181–189 MPa was obtained when the joining temperature and cooling rate were 1300–1350 °C and 15 °C/min respectively. Moreover, the in-line transmittance of the joint at 1000 nm was 82.1%, which was slightly lower than that of MgAl₂O₄ ceramic (85.6%).     

Reactions between MgAl2O4 and MgCr2O4 spinels and phosphate bonds

Conclusions A study was made of the reaction between spinels MgAl₂O₄ and MgCr₂O₄ with orthophosphoric acid and sodium chromophosphate bond. The setting of the material is related to the formation of various hydrated phosphates and hydrates. During heating complex phase inversions take place, with the formation of intermediate compounds. At elevated temperatures the stable phases are spinels.Translated from Ogneupory, No. 2, pp. 53–55, February, 1973.     

Interaction of MgAl2O4 and MgCr2O4 spinels with calcoferruginous melts

Conclusions The interaction was studied between MgAl₂O₄ and MgCr₂O₄ spinels (and their solid solutions and compositions with periclase) with calcoferruginous melts, which are the essential constituents of various slag reagents.On exposure to a highly basic calcoferruginous melt, the magnesium-aluminum spinel forms magnesio-ferrite, brownmillerite, and some calcium aluminate.On interaction of magnesiochromite with calcium ferrite, chiefly high-temperature compounds form.With the action of highly ferruginous melts on magnesiochromite, the resulting solid solution increases in volume, and a distortion is observed at the boundary of the reaction and little-change zones of the samples.The addition of granular MgO to spinels or their solid solutions improves the chemical stability of the compositions against calcoferruginous melts [4].During use in contact with highly basic calcoferruginous melts, the addition of periclase to chromium spinellide additives is recommended; for use in contact with highly ferruginous melts, periclase and aluminum spinellide compositions are recommended.Translated from Ogneupory, No. 7, pp. 44–47, July, 1969.     

含双金属的超稳Y沸石负载SO2-4/ZrO2强酸性催化剂上正庚烷临氢异构化

用超稳Y沸石(USY)负载SO2-4/ZrO2固体超强酸,并以此为载体制备含Pt双金属催化剂,用XRD和H2-TPR表征了催化剂的物化性质,并在常压固定床反应器上考察催化剂的正庚烷临氢异构化反应性能.结果表明, USY负载了SO2-4/ZrO2和双金属以后仍能保持沸石原有结构;贵金属Pt、金属助剂以及ZrO2等在USY载体上能够高度分散.在含Pt的催化剂中掺杂了Cr或Al金属助剂以后,正庚烷异构化产物选择性有了明显的提高,且具有更好的稳定性和低温活性;在USY负载SO2-4/ZrO2和0.4%Pt的催化剂上,正庚烷的转化率为42.1%时,异构化产物的选择性只有69.6%,而在掺杂了与Pt摩尔比为5:1的Cr或Al后,正庚烷的转化率分别为44.3%和42.1%时,异构化产物的选择性分别可提高到88.9%和89.5%.     

透明MgAl2O4晶须的制备与表征

本文采用MgO粉和金属Al粉为原料,在氮气气氛下制备MgAl2O4晶须,研究了TiO2、SiO2或Cr2O3等供氧添加剂和制备条件对晶须合成率的影响并分析了晶须的物相组成和结构.结果表明:最佳MgO/Al=2/3;添加SiO2或Cr2O3不利于晶须合成;添加TiO2对合成晶须有促进作用,最佳TiO2加入量为40%;采用氧化铝坩埚比采用釉面陶瓷坩埚好,最佳保温时间为6 h.合成的晶须为MgAl2O4,空间群Fd3M,晶格常数a=b=c=0.8083 nm,MgO 28.3%,Al2O3 71.7%;晶须呈透明柱状,最大长度为20～30 mm,长径比20～400.     

微波辅助溶液燃烧法制备MgAl_2O_4粉体

将硝酸镁、硝酸铝、尿素按物质的量比为1 2 6.66制得透明混合前驱液,用低温燃烧技术与微波加热技术相结合的方法制备了高纯度、低团聚的镁铝尖晶石(MgAl2O4)粉体。研究了燃烧反应过程中,微波输出功率(200、400、600、700 W)对Mg Al2O4粉体晶体结构、形貌及比表面积的影响。结果表明:微波高效加热方式导致燃烧反应瞬间产生大量气体,促进了Mg Al2O4超细颗粒的形成。同时,随着微波输出功率的增加,尿素氧化加速,利于MgAl2O4晶粒的生长发育。在微波功率700W,微波时间2 min的条件下,可制备结晶完整,粒度分布均匀(平均晶粒尺寸为56.03 nm)的Mg Al2O4粉体。     

MgAl2O4透明陶瓷粉体的研究进展

透明镁铝尖晶石陶瓷的制备,对粉体有特殊的要求.高纯、超细、分散性好、高活性的粉体是制备镁铝尖晶石透明陶瓷的首要条件.本文以制备镁铝尖晶石透明陶瓷为目标,从粉体的纯度和粉体颗粒特征两方面分析了粉体的性能对制备透明镁铝尖晶石陶瓷的影响,介绍了几种可用于制备镁铝尖晶石透明陶瓷粉体的方法,分析比较了每种方法的优缺点.     

Self-segregation and solubility in nonstoichiometric MgAl2O4 nanoparticles

Magnesium aluminate spinel (MAS) has a wide range of technological applications owing to its exceptional mechanical properties and good chemical stability. MAS phase diagrams indicate that only stoichiometric MgAl₂O₄ is stable at ∼25°C and Mg²⁺ and Al³⁺ exhibit solubility in nonstoichiometric MAS only at temperatures higher than ∼1200°C. In this study, the synthesis of nonstoichiometric single-phase spinel nanopowders at low temperatures is reported, and the role of the chemical distribution of Al³⁺ and Mg²⁺ excess on the stability of these nanopowders is examined. We performed selective lixiviation to examine the surface segregation of Mg²⁺ and Al³⁺ in nonstoichiometric MAS to investigate its effect on interfacial solubility and consequently the stability of nonstoichiometric MAS. Furthermore, we plotted an experimental phase diagram of nano MAS that predicts the crystallite size limits for the nonstoichiometric single-phase MAS.     

Continuous flow toluene methylation over AlPO4 and AlPO4-Al2O3 catalysts

Toluene methylation with methanol over AlPO₄ (AP) and AlPO₄-Al₂O₃ (APAl) catalysts, obtained through different methods, was carried out in a continuous down-flow fixed bed reactor. The main products were xylenes (XYL), although trimethylbenzenes (TMB) were also found over APAl catalysts. The benzene and ethylbenzene selectivities increased slightly with time on stream at the expense of XYL and TMB selectivities. Isomer distribution was approximately 50, 24 and 26 mol% foro-, m- andp-XYL, and 72, 27 and 0 mol% for 1,2,3-, 1,2,4- and 1,3,5-TMB. The initial reaction rate constants were higher on APA1 catalysts and, furthermore, APAl catalysts exhibited similar catalytic activities, although those obtained in ethylene or propylene oxide are the most active ones. The same occurs on AP catalysts. Moreover, the changes in catalytic activity are similar to the changes in the acidic characteristics measured, in gas phase, versus pyridine. Furthermore, the activity decreased with time on stream due to coke deposition according to the expressionk = k₀ exp(-t). The rate of deactivation, evaluated from the deactivation coefficients (), was greater for APAl than for AP catalysts.     

球磨法制备NiO/MgAl2O4催化剂用于低浓度甲烷催化燃烧

用球磨法制备了MgAl_2O_4尖晶石负载的NiO催化剂NiO/MgAl_2O_4,考察了镍源、NiO负载量、球料比、球磨转速、焙烧温度对低浓度甲烷催化燃烧的影响,并用H_2-TPR、BET、XRD、SEM等手段对催化剂进行了表征。结果表明,与沉淀-沉积法或浸渍法相比,球磨法制备的催化剂前驱体与载体的相互作用较弱,焙烧后生成了高活性NiO物种。球磨法制备时,镍盐种类及NiO负载量、球料比、转速以及焙烧温度均会改变载体表面NiO晶粒的大小和结合状态,从而影响其活性。以乙酸镍为镍源、NiO质量分数10%、球料比2∶1、球磨转速400 r/min、经700℃焙烧,制得的NiO/MgAl_2O_4催化剂稳定性和活性较好;600℃下,700℃焙烧制得催化剂的拟一级反应速率常数(k600)为0.489 L/(g·s),高于MgAl_2O_4尖晶石负载的La_(0.8)Ca_(0.2)FeO_3或La_(0.8)Sr_(0.2)MnO_3钙钛矿催化剂。     

TiO2含量对镁铝尖晶石烧结性能的影响

以分析纯氧化镁、氧化铝、氧化钛为原料,按氧化镁与氧化铝质量比28.33∶71.67配料,在合成体系中分别引入质量分数为0、0.5％、1％、2％、4％、6％、8％和10％的氧化钛,在钼丝炉中1600℃保温2h,烧结法合成镁铝尖晶石.用X射线衍射、扫描电子显微镜和能谱对烧后试样进行分析.结果表明:引入适量的TiO2可显著提高镁铝尖晶石的烧结性能;当TiO2含量低于4％时,随着TiO2的含量增加,试样的线变化率减小,显气孔率下降,体积密度增大,常温耐压强度增大.TiO2的引入提高了镁铝尖晶石晶体空位浓度,活化了晶格,促进镁铝尖晶石的烧结.当TiO2含量高于4％时,试样的线变化率、显气孔率和体积密度没有显著变化,常温耐压强度有所下降;尖晶石晶粒尺寸没有明显变化,且生成了一定量的Mg0.6Al0.8Ti1.6O5,阻碍了镁铝尖晶石之间的接触,影响镁铝尖晶石烧结性能的提高.     

MgAl2O4 spinel prepared by mechanochemical synthesis used as a support of multimetallic catalysts for paraffin dehydrogenation

The catalytic performance in n-butane dehydrogenation of bimetallic PtSn, PtGa and PtIn, and trimetallic PtSnIn and PtSnGa catalysts (with low metal contents) supported on a MgAl₂O₄ prepared by a novel mechanochemical synthesis was evaluated both in flow and pulse equipment. The influence of the addition of different promoters (Sn, Ga and In) to Pt on the activity, selectivity and deactivation in the n-butane dehydrogenation reaction was studied. Stability experiments through successive reaction-regeneration cycles were carried out for selected catalysts. In order to correlate the properties of the metallic phase of the catalysts with the catalytic behavior, several characterization techniques were used, such as test reactions of the metallic phase (cyclohexane dehydrogenation and cyclopentane hydrogenolysis), TPR, XPS, H₂ chemisorption and TEM. Bimetallic PtSn catalyst has a better catalytic behavior than PtIn and PtGa ones. For PtSnM (M: In or Ga) catalysts, whereas Ga addition to the bimetallic catalyst does not practically modify the dehydrogenation performance, the addition of In produces an increase of the activity and the selectivity to butenes. Characterization results indicate the presence of geometric effects for the PtSn catalyst, and geometric and electronic effects for PtIn and PtGa ones. For trimetallic catalysts, the presence of a close contact between Pt, Sn and In or Ga in both trimetallic catalysts was found, mainly due to geometric effects like blocking and dilution of the active sites by the promoters. In stability experiments, the trimetallic PtSnIn/MgAl₂O₄ catalyst clearly displays the best catalytic performance along reaction-regeneration cycles, though PtSnGa and PtSn catalysts also showed a very good behavior through the successive cycles. The characterization of these catalysts after cycles shows that their metallic phases are slightly modified along the cycles.     

γ-Al2O3在CeO2-La2O3催化还原SO2中的作用

考察了反应温度对γ-Al₂O₃、CeO₂-La₂O₃和CeO₂-La₂O₃/γ-Al₂O₃催化CO还原SO₂到单质硫活性的影响。采用XRD表征了催化剂反应前后的物相变化。结果表明，温度过低或过高均不利于γ-Al₂O₃催化CO还原SO₂的反应。γ-Al₂O₃的介入提高了CeO₂和La₂O₃的分散性，使Redox 和COS两种反应机理在同一催化剂上的协同作用增强，使CeO₂-La₂O₃/γ-Al₂O₃在催化还原SO₂的反应中具有更低的反应温度和更高的活性。