Surface Structure and Catalytic Performance of CuCl/SiO2-Al2O3 Catalysts for Methanol Oxidative Carbonylation

Aluminum was doped into amorphous silica gel to modify its surface structure. The obtained SiO₂-Al₂O₃ support was used to prepare the CuCl/SiO₂-Al₂O₃ catalyst by solid-state ion exchange, and the catalyst activity for liquid-phase oxidative carbonylation of methanol to dimethyl carbonate was investigated. The results showed that the prepared SiO₂-Al₂O₃ support kept the amorphous structure of the silica gel. The BET specific surface area of the silica gel was decreased to 200 m²/g, and the surface acid sites (including Brønsted acid sites) were increased. In the CuCl/SiO₂-Al₂O₃ catalyst, CuCl was not only dispersed on surface but also was ion exchanged with surface Brønsted acid sites of the SiO₂-Al₂O₃ support to form Cu⁺ species, which resulted in a decrease in BET specific surface area to 148 m²/g. These two kinds of Cu⁺ species on the catalyst surface were both active centers for the oxidative carbonylation of methanol to dimethyl carbonate. When the catalyst was prepared with Si/Al molar ratio of 5 and was calcined at 500 °C, the selectivity and space-time yield of dimethyl carbonate reached 74% and 1.27 g/(g·h), respectively.     

Effect of Solvent Diols and Ligands on the Properties of Sol-Gel Alumina-Silicas

Amorphous alumina-silicas were prepared from a tetra-alkoxysilane and anhydrous aluminum trichloride or an aluminum alkoxide by a sol-gel process using 2-methyl-2,4-pentanediol, pinacol, 1,2-propanediol, 2,3-butanediol or ethylene glycol as the solvent or complexing agent, and the effect of diols and alkoxy groups on the physical and chemical properties of the alumina-silicas was examined. When the diol or the alkoxy group was bulky, the alumina-silicas had relatively larger micropores, a larger pore volume and higher surface areas. In the conversion of methanol catalyzed by the alumina-silicas, the bulkier diols and alkoxides gave catalysts that produced dimethyl ether in higher yield and hydrocarbons in lower yield. Thus, when ethylene glycol was used as the diol, the best catalyst for the production of hydrocarbons, especially the production of olefins such as ethylene, propylene and butene, was obtained. Furthermore, in comparison with alumina-silica prepared by a traditional kneading process, it was found that the sol-gel alumina-silica could efficiently convert methanol to dimethyl ether and hydrocarbons, but the material prepared by kneading had a very low conversion of methanol to other compounds.     

Hydrogen-Water Deuterium Exchange Reaction Over Mixed Oxide Supported Nickel Catalysts

Three series of catalysts, Ni/Al₂O₃-SiO₂, Ni/Al₂O₃-Cr₂O₃ and Ni/SiO₂-Cr₂O₃, were prepared by co-precipitation. In all samples the nickel content was kept constant at 70 at.% Ni, while the support composition was varied. The nickel surface areas, which are required to measure the specific catalytic activities, were determined by hydrogen chemisorption. In the case of the single oxide supported nickel catalysts, the order of the specific catalytic activity values was: Ni/Cr₂O₃<Ni/Al₂O₃<Ni/SiO₂. The specific catalytic activity of the Ni/Al₂O₃-SiO₂ samples, as a function of the support composition, follows approximately the weighted sum of the specific activities of the single oxide supported nickel catalysts. The specific catalytic activity value of the Ni/Al₂O₃-Cr₂O₃ and Ni/SiO₂-Cr₂O₃ samples more closely resembled that of Ni/Cr₂O₃ catalyst. The presence on chromia surface of the chromic anhydride and its tendency to spread onto are supposed to be the cause of this behaviour. Due to their enhanced activity, the Ni/Al₂O₃-Cr₂O₃ catalysts can be used for the production and detritiation of heavy water.     

Sol-Gel Synthesis of Multi-Walled Carbon Nanotubes Reinforced Alumina-Silica Fibers

Multi-walled carbon nanotubes (MWCNTs) reinforced alumina-silica fibers were prepared by the sol-gel dry-spinning method. Aluminum isopropoxide (AIP), aluminum nitrite (AN), and tetraethyl orthosilicate (TEOS) were used as starting materials to prepare spinnable aluminosilicate sols in aqueous media. Polyvinylpyrrolidone (PVP) was used as dispersant agent to homogeneous disperses MWCNTs in the spinnable aluminosilicate sol through an in-situ process. The structure and mechanical properties of the MWCNTs reinforced alumina-silica fibers were studied. The results revealed that homogeneous dispersion of MWCNTs in alumina-silica fibers improved their mechanical properties considerably. The addition of only 0.5 wt% acid-treated MWCNTs into alumina-silica fibers result in a 60% increase in tensile strength to 398 ± 45 MPa.     

Mullite Based Compositions Prepared by Sol-Gel Techniques

Ceramic powders and gels in the multi-alumina system with different compositions were prepared by sol-gel method. The preparation of gels was done in strong acidic conditions and also with the addition of ammonia. The powders were prepared by dropping the sols on a hot plate. The effect of water/alkoxide and ethanol/alkoxide ratios was studied together with the type of catalyst used (hydrochloric acid or ammonia). The thermal evolution of materials was followed by thermal analysis, X-ray diffraction, specific surface area measurements and electron microscopy. The morphology of powders was also studied. The characteristic exothermic peak attributed to a good mixing degree of the Al and Si species was observed at ∼980°C. The intensity of this exothermic peak was more pronounced in gels than in sol-gel derived powders. The crystalline phases formed are related with the preparation conditions and with the thermal treatment. The crystallisation of mullite and transient phases of alumina were observed in powder samples heat treated at 1000°C while in gel samples mullite was the only crystalline phase detected.     

Reduction of Eu3 + to Eu2 + in Al-Codoped Silica Glasses Fabricated by the Sol-Gel Technique and CO2-Laser Processing

The spectroscopic properties of europium in aluminium codoped silica glasses produced by the sol-gel technique have been studied with respect to the dopant concentrations and the thermal processing applied to the samples. After thermal annealing at temperatures up to 950_°C the bright red fluorescence around 613 nm characteristic for the trivalent europium ions (Eu^{3 +}) has been observed. The lifetime was measured to be 0.1–2.4 ms depending on dopant concentrations and thermal treatment. Subsequent CO₂-laser processing in air (short time remelting) gave rise to a bright blue fluorescence consisting of two broad bands, lying around 450 and 490 nm, with their peak position depending on the ratio between the aluminium and europium concentrations. The fluorescence lifetimes were found to be shorter than 1 s. This blue fluorescence is attributed to the divalent europium ion (Eu^{2 +}), leading to the conclusion that the CO₂-laser processing of europium doped alumina-silica glasses resulted in the reduction of the trivalent to the divalent europium ion. Laser processing could therefore be a valid alternative to conventional thermal annealing for the generation of Eu^{2 +} in alumina-silica glasses.     

Al2O3-SiO2系纳米粉的微观结构与红外发射特性

Al2O3-SiO2系纳米粉的微观结构与红外发射特性     

CuCl／SiO2-Al2O3催化剂的表面结构及甲醇氧化羰基化催化性能

通过掺杂Al对无定形层析硅胶进行表面改性,采用固体离子交换法制备了CuCl/SiO2-Al2O3催化剂,并考察了它在甲醇液相氧化羰基化合成碳酸二甲酯反应中的催化性能.结果表明,掺杂Al制备的SiO2-Al2O3载体仍保持了硅胶的无定形结构,比表面积降为200m2/g,但表面酸性明显增强,具有B酸中心.CuCl不仅分散于SiO2-Al2O3载体表面,而且与载体表面的B酸发生离子交换作用形成了表面Cu 物种,使催化剂比表面积降为148m2/g,且两种Cu 物种共同构成了催化活性中心.当CuCl/SiO2-Al2O3催化剂的Si/Al比为5,在500℃焙烧时,催化剂上甲醇氧化羰基化反应的碳酸二甲酯的选择性和时空收率分别达到74%和1.27g/(g.h).