Layered double hydroxide-derived vanadium catalysts for oxidative dehydrogenation of propane: Influence of interlayer-doping versus layer-doping

Mixed-oxide vanadium catalysts for oxidative dehydrogenation (ODH) of propane have been prepared by thermal decomposition of Mg, Al-layered double hydroxides (LDHs) containing vanadium either in the brucite layer or in the interlayer. The materials have been characterised by XRD, ICP-AES chemical analysis, XPS, BET and ESR. The catalytic performance of the samples depended on the manner of incorporation of the vanadium into the LDH structure. The sample obtained from interlayer-doped precursor was more active and more selective than mixed oxides obtained from layer-doped LDHs. The difference in the catalytic properties was attributed to the different magnesium vanadates nucleating in the calcined samples, the pyrovanadate formed from the interlayer-doped LDH giving better performance than ortho-vanadate crystallising from the layer-doped precursor. It has been suggested that one of the factors contributing to the difference in the behaviour of both types of catalysts might be the difference in the covalency of V---O in-plane bonds around the reduced V centres.     

Li_(1.02)Mn_(1.92)Al_(0.02)Cr_(0.02)Mg_(0.02)O_(4-x)F_x尖晶石正极材料的研究

采用溶胶-凝胶法合成了复合离子掺杂的尖晶石型锰酸锂Li1.02Mn1.92Al0.02Cr0.02Mg0.02O4-xFx(x=0,0.06)正极材料,并用XRD、CV、EIS和充放电测试等研究了其结构和电化学性能。结果表明,F与金属离子(Li、Al、Cr、Mg)的复合掺杂不仅提高了材料的比容量,还增加了尖晶石结构的稳定性,改善了材料的循环性能和可逆性能;充放电测试结果表明,Li1.02Mn1.92Al0.02Cr0.02Mg0.02O3.94F0.06具有优越的循环性能,常温下,以1/3C充放电的首次放电容量及50个循环后的容量保持率分别为117.9 mAh/g,96.9%。     

High-temperature catalysts with a synergetic effect of Pd and manganese oxides

A synergetic effect in the catalytic activity has been found after palladium introduction in Mn–Al–O systems. The magnitude of the synergetic effect depends on the types of the oxidic manganese species: oxide Mn₃O₄, spinel (Mn, Mg)[Mn, Al]₂O₄ or hexaaluminate (Mn, Mg)LaAl₁₁O₁₉. The synergetic effect of Pd and manganese-containing compounds is observed only if palladium is introduced to the low-temperature precursor of the manganese alumina spinel or manganese hexaaluminate. The synergetic effect is not observed when high-temperature samples with formed spinel or hexaaluminate phases are modified with Pd.     

Catalytic combustion of toluene over mixed Cu–Mn oxides

The influence of the synthesis pH and of the Zn and/or Al additives on the Cu–Mn precursors, obtained by co-precipitation at a constant pH from aqueous solutions of appropriate nitrates at (Cu + Zn)/(Mn + Al) ratio equal 2, was investigated. The relative content of Mn increased with the pH of precipitation. Depending on the sample composition the identified crystalline phases included layered hydroxy double salt, hydrotalcite-like structure, CuO and ZnO. Some precursors had strongly amorphous character. Calcination of the precursors at 673 K resulted in mixed oxides, in which CuO of various degrees of crystallinity could be identified. The Mn-containing phases remained amorphous. All calcined materials proved extremely active in catalytic combustion of toluene. Some catalysts reached 100% conversion already at 403 K. High conversions observed in the low temperature regime were partially due to the strong sorption of toluene. In the catalysts containing Al additive this effect was suppressed.     

Layered double hydroxides as catalysts for the efficient growth of high quality single-walled carbon nanotubes in a fluidized bed reactor

A family of layered double hydroxides (LDHs), such as Fe/Mg/Al, Co/Mg/Al, and Ni/Mg/Al LDHs, were used as catalysts for the efficient growth of single-walled carbon nanotubes (SWCNTs) in a fluidized bed reactor. The LDH flakes were agglomerated into clusters with sizes ranging from 50 to 200 μm, and they can be easily fluidized with a gas velocity ranging from 2.3 to 24 cm/s. After calcination and reduction, small metal catalyst particles formed and distributed uniformly on the flakes. At the reaction temperature, the introduction of methane realized the growth of SWCNTs with the diameter of 1-4 nm. The loose structure of LDH agglomerates afforded a yield as high as 0.95 g_CNT/(g_cat h) of SWCNTs with a surface area of 930 m²/g. Compared with Fe/Mg/Al LDH, Ni/Mg/Al and Co/Mg/Al LDHs showed a better selectivity to SWCNTs. The highest selectivity for metallic SWCNTs was obtained using Co/Mg/AI LDHs as the catalyst.     

Co-doping effects of (Al,Ti, Mg) on the microstructure and electrical behavior of ZnO-based ceramics

Co-doped ZnO-based ceramics using Al, Ti, and Mg ions in different ratios were synthesized with the objective to investigate the doping effects on the crystalline features, microstructure and the electrical behavior. For Al and Ti doping, a coexistence of crystalline phases was shown with a major wurtzite ZnO structure and secondary spinel phases (ZnAl₂O₄, Zn₂TiO₄, or Zn_aTi_bAl_cO_d), while Mg doping did not alter significantly the structural features of the wurtzite ZnO phase. The electrical behavior induced by Al, Ti, and Mg co-doping in different ratios was investigated using Raman, electron paramagnetic resonance (EPR) and ²⁷Al and ⁶⁷Zn solid-state nuclear magnetic resonance (NMR). Al doping induces a high electrical conductivity compared to other doping elements. In particular, shallow donors from Zn_i-Al_Zn defect structures are inferred from the characteristic NMR signal at about 185 ppm; that is, quite far from the usual oxygen coordinated Al. The Knight shift effect emanating from a highly conducting Al-doped ZnO ceramics was considered as the origin of this observation. Oppositely, as Ti doping leads to the formation of secondary spinel phases, EPR analysis shows a high concentration of Ti³⁺ ions which limit the electrical conductivity. The correlation between the structural features at the local order, the involved defects and the electrical behavior as function of the doping process are discussed.     

Mesoporous Cu–Mn Hopcalite catalyst and its performance in low temperature ethylene combustion in a carbon dioxide stream

Complete combustion of trace amounts of ethylene in food grade CO₂ over a Cu–Mn Hopcalite catalyst has been investigated. A mesoporous structure is identified in the catalyst. Low temperature calcined samples are found to be more active than the high temperature calcined ones. The presence of Cu²⁺ and Mn³⁺ is essential for the high activity of the catalyst. The Cu–Mn catalyst without a third component deactivates quickly in the reaction stream. However, doping with Al or Mg individually and with Ni–Al or Mg–Al simultaneously increases the lifetime. In situ DRIFTS measurements provide evidence that hydroxyl groups form and adsorb on Mn species. With the doping of Al, Mg and Ni ions, the amount of hydroxyl groups adsorbed reduces and the stability improves. Doping with Al and Mg simultaneously gives the best stability. A synergetic effect between CuO and amorphous Cu–Mn oxide phases is also confirmed.     

The Effect of Thermal Pre-Treatment on Structure, Composition, Basicity and Catalytic Activity of Mg/Al Mixed Oxides

The goal of this work is to study the effect of thermal pre-treatment of Mg/Al mixed oxides (450–1,050 °C) on their structure, basicity and catalytic activity in transesterification of rapeseed oil. The catalytic activity of Mg/Al catalysts was shown to depend not only on the amount of basic sites, but also on crystallite size of MgO, specific surface area and population of medium/strong basic sites. Moreover, high stability of Mg/Al-550 was established by re-using the catalyst four times. It was associated with negligible magnesium leaching from the solid catalyst to liquid phases.     

Mg/Al复合氧化物催化丙酮和碳酸二甲酯合成乙酰乙酸甲酯

利用先共沉淀后水热处理的方法制备出水滑石前体,将其焙烧后,得到具有不同Mg/Al摩尔比的复合氧化物,并借助电感耦合等离子原子发射光谱、XRD、CO2-TPD和氮气吸脱附等手段对其进行表征。结果表明,Mg/Al摩尔比对催化剂的结构和碱性质有较大的影响,并且在催化丙酮和碳酸二甲酯合成乙酰乙酸甲酯的反应中,其比单纯的氧化镁和氧化铝表现出了更高的催化活性。进一步的研究表明,在该反应中是复合氧化物的中强碱性位起催化活性的作用,当Mg/Al摩尔比为2时,催化剂具有最高的活性。此外,重复性实验表明,这些催化剂具有较好的稳定性和重复使用性。     

Environmentally friendly, heterogeneous acid and base catalysis for the methylation of catechol: Chances for the control of chemo-selectivity

The properties of catalysts with (i) Brønsted-type acidity (H-mordenite and Al/P mixed oxide), (ii) Lewis-type acidity (Al trifluoride) or (iii) basic characteristics (Mg/Fe mixed oxide) were investigated in the gas-phase methylation of catechol. When methanol was used as the methylating agent, H-mordenite and AlF₃ gave high selectivities to guaiacol (the product of O-methylation) under mild reaction conditions, that is at very low catechol conversions. An increase in temperature led to the transformation of guaiacol to phenol and cresols, and to considerable catalyst deactivation. The basic catalyst Mg/Fe/O also favored an extensive degradation of guaiacol to phenol. On the mildly acidic catalyst Al/P mixed oxide a stable catalytic performance and a high selectivity to guaiacol at 40% catechol conversion were obtained. When methylformate, a more reactive methylating agent, was used with AlF₃ and Mg/Fe mixed oxide as catalysts, higher catechol conversions and slower deactivation rates could be achieved under mild reaction conditions, with a low extent of guaiacol degradation. However, methylformate rapidly decomposed when temperatures above 350 °C were used. Finally, tests were made by reacting catechol and diethoxymethane with acid catalysts, with the aim of synthesizing methylenedioxybenzene. The latter product was obtained with high selectivity, but with very low yield, due to both catalyst deactivation and decomposition of diethoxymethane.     

Total oxidation of selected mono-carbon VOCs over hydrotalcite originated metal oxide catalysts

Hydrotalcite originated Cu–Mg–Al, Co–Mg–Al and Cu–Co–Mg–Al oxide systems were tested as catalysts for the total oxidation of mono-carbon VOCs (methane, methanol, and formic acid). Both calcination temperature of the hydrotalcite precursors as well as doping of the catalysts with potassium promoter influenced their catalytic activity. Increased calcination temperature, which resulted in a decrease of the surface area of the samples and formation of the spinel phases, activated the Co–Mg–Al catalyst, while the opposite effect was observed for the Cu–Mg–Al and Cu–Co–Mg–Al catalysts. On the other hand doping of the catalysts with potassium promoter significantly activated the Cu–Mg–Al and Cu–Co–Mg–Al catalysts in the processes of methanol and formic acid conversion, while only slightly influenced the catalytic performance of the Co–Mg–Al sample.     

焙烧温度对双金属催化剂甲苯加氢重整反应催化性能的影响

以类层柱Ni-Co/Mg(Al)O水滑石为前驱体,经不同温度焙烧制备了系列焦炉煤气中焦油模型化合物甲苯加氢重整制合成气催化剂. 在常压、反应温度800℃、水/碳摩尔比0.7和体积空速12000 h-1的条件下,在35 h评价时间内850℃焙烧的催化剂可完全转化甲苯,CH4和CO的平均产率分别为34%和66%,而低于750和高于950℃焙烧的催化剂活性较差. 850℃焙烧时催化剂比表面积较大,形成了尖晶石和固溶体,活性金属与载体间的相互作用较强,还原后活性金属颗粒小且均匀分布. 催化剂上有少量须状碳生成,绝大部分积碳可被H2消除,积碳是可逆过程.     

复原镁铝水滑石固体碱在Aldol反应中的应用

以镁铝水滑石为前体焙烧制得镁铝复合氧化物,在含水体系中水合催化取代苯甲醛和丙酮的aldol反应。结果显示,水滑石前体经过复原处理后活性高于普通Mg/Al水滑石;Mg/Al比为4∶1,NaOH/Na2CO3为沉淀剂,镁铝的硝酸盐作为金属盐来源制备的催化剂对对硝基苯甲醛和丙酮之间的aldol反应具有最好的催化效果。     

Enhancement of thermal properties of polyacrylonitrile by reinforcement of Mg‐Al layered double hydroxide

The in situ polymerization technique was used for synthesis of polyacrylonitrile nanocomposites (PAN/LDH) by reinforcement of different concentrations of Mg‐Al layered double hydroxide (LDH). Molecular interaction between Mg/Al LDH and PAN matrix was studied by Fourier transform infrared spectroscopy. The morphology of PAN/LDH nanocomposite was investigated by field emission scanning electron microscopy. Energy dispersive spectroscopy of nanocomposites were noticed to establish the elemental composition of the composite. High resolution transmission electron microscopy was used to study the nanostructural distribution of LDH with PAN matrix, whereas crystalline patterns were explained by selected area electron diffraction studies. The thermal properties of nanocomposites were studied by thermo‐gravimetric analysis. It was found that, the thermal property of PAN was substantially enhanced due to strong interfacial adhesion of Mg‐Al‐LDH with PAN matrix. Further, it was noticed that the thermal stability of PAN/LDH nanocomposites were increased with increasing concentration of LDH. POLYM. COMPOS., 36:2140–2144, 2015. © 2014 Society of Plastics Engineer     

混合溶剂制备Mg/Al水滑石

在水与有机溶剂组成的混合溶剂中,采用均相沉淀水热法制备了Mg/Al水滑石.研究了有机溶剂种类和添加量对水滑石性能的影响,通过X射线粉末衍射、热重分析、扫描电子显微镜和氮吸附仪表征了水滑石的结晶度、热稳定性、形貌以及比表面积.结果表明:添加少量有机溶剂后可得到形貌规整,结晶度和热稳定性能没有明显变化的水滑石粉体,但少量有机溶剂的添加对水滑石粒径尺寸影响较大,粒径尺寸从2μm减小到1 μm左右.随着有机溶剂添加量的增加,粉体粒径明显减小,比表面积则随之增大.     

Memory effect-enhanced catalytic ozonation of aqueous phenol and oxalic acid over supported Cu catalysts derived from hydrotalcite

Mg/Al supported metal (Fe, Co, Ni and Cu) oxide catalysts were prepared by co-precipitation of hydrotalcite-like clay materials as precursors, calcined, and used for the ozonation reaction of phenol and oxalic acid. The reaction was carried out using the catalyst and aqueous solution of phenol or oxalic acid in an O₃/O₂ mixed gas-flow at 20 °C. In the ozonation of phenol, the combination of ozone and supported metal oxide catalysts was effective for the removal of total organic carbon (TOC). Also in the ozonation of oxalic acid as the main TOC component, Cu/Mg/Al catalysts showed the highest activity, followed by Ni/Mg/Al catalyst, while both Fe/Mg/Al and Co/Mg/Al catalysts were not active. Leaching of Cu and Ni, probably due to the chelation of metals by oxalic acid, was significantly observed at the beginning of the reaction. However the metal leaching disappeared at the end of the reaction possibly due to the entire consumption of oxalic acid during the reaction. The best result of oxalic acid mineralization was observed over Cu/Mg/Al catalyst calcined at 600 °C, on which least leaching of the metal was detected. Moreover, a “memory effect” of hydrotalcite accelerated the mineralization of oxalic acid over the Cu/Mg/Al catalyst; oxalate anions were captured and decomposed in the reconstituted hydrotalcite interlayer space on the surface of the Cu/Mg/Al catalyst, resulting in a remarkable enhancement in the catalytic activity of the ozonation.     

Ni catalysts from NiAl2O4 spinel for CO2 reforming of methane

A series of mixed oxides close to NiAl₂O₄ was obtained by a sol–gel like method (propionic acid). The characterization of the different structures was made by X-ray diffraction (XRD), scanning electron microscopy (SEM) or transmission electron microscopy (TEM). For the stoichiometric ratio of Ni to Al exactly equal to 0.5, homogeneous crystalline spinel phase was formed for a temperature of calcination equal or higher than 725 °C. A solid solution was obtained for a Ni/Al ratio lower than 0.5. The spinel structure is non-tolerant concerning a change of nickel to aluminum ratio higher than 0.5: an excess of nickel gives large particles of NiO on spinel phase. Comparative reduction and dry reforming of these oxides was made to control the formation of Ni and its sintering for applications in methane dry reforming. Preliminary reactivity results in dry reforming of methane are given.     

Structural Analysis of the Transition Phases in the Kaolinite-Mullite Thermal Sequence

Dehydroxylation phases of a kaolinite from Zettlitz, Czechoslovakia, were investigated by radial electron density distribution (RED) and X-ray spectroscopy. Special emphasis was placed on the 2 amorphous or poorly crystallized transition phases, i.e. metakaolinite (600° to 800°C) and the spinel structure (900°C). The RED diffraction method, which is particularly suited to the study of such materials, indicated (1) a particularly expanded tetrahedral configuration for the Al atoms in metakaolinite and (2) a pure Al₂O₃ spinel structure. The first result confirms earlier observations; the second eliminates, for the experimental conditions and kaolinite studied, the possibility of the formation of an Al-Si spinel structure with the AI and Si occupying the octahedral and tetrahedral interstices, respectively. Complementary measurements on the AlKα line shifts for the same samples support the present interpretation. Determination of the apparent Si coordination number by the same spectroscopy procedure revealed a regular 4 coordination for all phases except the end-member mullite.     

Characterization and activity of novel copper-containing catalysts for selective catalytic reduction of NO with NH3

Cu/Mg/Al mixed oxides (CuO = 4.0–12.5 wt%), obtained by calcination of hydrotalcite-type (HT) anionic clays, were investigated in the selective catalytic reduction (SCR) of NO by NH₃, either in the absence or presence of oxygen, and their behaviours were compared with that of a CuO-supported catalyst (CuO = 10.0 wt%), prepared by incipient wetness impregnation of a Mg/Al mixed oxide also obtained by calcination of an HT precursor. XRD analysis, UV-visible-NIR diffuse reflectance spectra and temperature-programmed reduction analyses showed the formation, in the mixed oxide catalysts obtained from HT precursors, mainly of octahedrally coordinated Cu²⁺ ions, more strongly stabilized than Cu-containing species in the supported catalyst, although the latter showed a lower percentage of reduction. The presence of well dispersed Cu²⁺ ions improved the catalytic performances, although similar behaviours were observed for all catalysts in the absence of oxygen. On the contrary, when the mixture with excess oxygen was fed, very interesting catalytic performances were obtained for the catalyst richest in copper (CuO = 12.5 wt%). This catalyst exhibited a behaviour comparable to that of a commercial V₂O₅–WO₃TiO₂ catalyst, without any deactivation phenomena after four consecutive cycles and following 8 h of time-on-stream at 653 K. Decreasing the copper content or increasing the calcination time and temperature led to considerably poorer performances and catalytic behaviours similar to that of the CuO-supported catalyst, due to the side-reaction of NH₃ combustion on the free Mg/Al mixed oxide surface.     

MgAl2O4 spinel synthesis by combustion and detonation reactions: A thermochemical evaluation

The production of MgAl₂O₄ spinel is evaluated using a thermochemical program, THOR, which calculates the equilibrium products resulting from the adiabatic combustion (isobaric and isochoric) or detonation of energetic materials. A classic metalized emulsion was used (ammonium nitrate, fuel oil, aluminium, water and glass microballoons), to which MgO was added. The Al/MgO proportion and the reaction regime were varied, the maximum spinel yield being achieved for the detonation of a fuel-rich emulsion with 5Al:3MgO (mol). In parallel, MgAl₂O₄ was experimentally synthesized by solution combustion of Al and Mg nitrates with various urea contents. The same reactions were simulated with THOR and the results obtained for products type and amounts were found to be in good agreement. Additionally, THOR simulations provided clear explanations for the experimental observations, which can be of invaluable help in the selection of fuel type and content in solution combustion synthesis of any given mixed oxide.