Preparation, thermal decomposition and lifetime of europium tris(dibenzoylmethanato)phenanthroline complex doped xerogel

The Eu tris(dibenzoylmethanato)phenanthroline complex doped xerogel has been synthesized by a catalyst-free sol-gel roure. The non-isothermal kinetic analysis is calculated by Friedman isoconversional method and multivariate non-linear regression method. The overall decomposition process below 600°C is fitted by an Fn model (n order reaction), corresponding to the dehydration of the matrix, and a two-step consecutive reaction of Cn model (n order autocatalytic reaction), corresponding to the decomposition of organic complex. Correlation coefficient is 0.99986. The lifetime values of xerogel, defined as the 5% decomposition of europium organic complex, indicate that the xerogel can find application at near room temperature.     

Influence of Hydrolysis Water Content on the Microstructure of BaTiO3 Xerogels Prepared from High Concentration Metal Alkoxide Solutions

Optically clear and partially crystallized barium titanate (BaTiO₃) monolithic xerogels were prepared by hydrolyzing the high concentration precursor solutions (1.1 mol/L) of metal alkoxides with water vapor to have different hydrolysis water contents (H₂O/Ba-alkoxide mole ratio (r _w) = 2.5–7.0). The microstructural changes of the xerogels have been studied by TEM, XRD and nitrogen adsorption/desorption analysis. The crystallite size of the xerogels was found to increase consecutively with increasing r _w, while the average pore size showed no substantial increase until r _w = 5.0 and it grew rapidly for r _w > 5.0. The influence of r _w on the microstructural development of the xerogels with heat-treatment was also discussed.     

干凝胶无机整体柱的制备及评价

以硅酸钾为硅源,甲酰胺为催化剂在毛细管内原位聚合形成干凝胶柱,制备了一系列致密度不同的整体柱。该法在柱制备及高温干燥过程中不会发生柱床断裂和塌陷现象,此特点明显优于以烷氧基硅烷为前驱体制备无机整体柱的方法。考察了不同模数硅酸钾对整体柱柱床结构的影响,用扫描电镜(SEM)和氮吸附法对整体柱结构进行了表征,考察了整体柱柱压与流速的关系;对整体柱进行十八烷基修饰后,测定了反相整体柱(C18整体柱)对蒽的柱效,通过考察甲苯在该柱上的突破曲线,获得了其对甲苯的柱容量。结果显示该整体柱柱床刚性好,在高温、高柱压、高流速时柱床能保持其物理结构的稳定性,对蒽的柱效达到41400理论塔板/m,对甲苯的柱容量为61 ng。     

New polymorphs of alumina: Part II μ and λ alumina

Abstract

We had reported observation of polymorphs of alumina having rare earth sesquioxide type structures in xerogel γ alumina (XGA) at high pressures and temperatures (High Pressure Research, 1998). In this paper we report observation of two additional phases at somewhat different pressures and temperatures. The XGA quenched from 5.2 GPa and 1450°C showed, besides α Al₂O₃, a new hexagonal metastable polymorph μAl₂O₃. Over a period of 10 to 12 weeks μ A1₂O₃ transforms to a stable hexagonal phase λ Al₂O₃. The cell parameters of λ Al₂O₃ are comparable with those of k Al₂O₃ found by Saalfield on dehydration of gibbsite A1₂O₃3H₂O in air at 800°C. The XGA containing 1 wt% of Cr₂O₃ yielded similar results at 4.56GPa and 1100°C. Further the XGA containing Cr showed complete transformation to n Al₂O₃ at 5.2 GPa and 1520°C.     

Optical Properties and Sensitivity of Xerogel Detection Layers for Fiber-Optic Hydrocarbon Sensors

Xerogel layers were prepared on fiber-optic substrates and glass slides by the dip-coating method from sols composed of tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and phenyltriethoxysilane (PTES), ethanol or isopropan, HCl and water, and by drying the gels at 80°C.Layers with a thickness ranging from 200 to 1500 nm were prepared. Optical properties of the layers applied on the fibers and their sensitivity to toluene and hexane were determined from angular and temporal changes of the output power of the fibers excited by an inclined collimated light beam. These changes were measured at a wavelength of 670 nm and in a range of 1600–1800 nm.The results obtained at 670 nm show that the layers refractive index can be increased from 1.39 to 1.5 by using the PTES sols instead of the MTES or TEOS ones. High optical losses of the layers of the order of 10³ dB/cm can be explained by their porosity. The best detection limits of 0.01 vol.% of toluene and 0.2 vol.% of hexane were obtained for layers containing methyl groups. The presence of toluene in the layers was deduced from spectral bands of toluene in a range of 1600–1800 nm.     

Relationships Between Fluorescence Properties of Benzoquinolines and Physicochemical Changes in the Sol–Gel–Xerogel Transitions of Silicon Alkoxide Systems

Fluorescence and excitation spectra of 3,4-, 5,6- and 7,8-benzoquinolines (BQs) dispersed in the individual sol–gel–xerogel transitions systems of silicon alkoxide have been observed as a function of the reaction time. The fluorescence spectra of excited-state species (neutral, ion-pair and protonated species) of each BQ have been obtained clearly. In the starting sol–gel systems, the fluorescence mainly originated from the neutral species of each BQ. As hydrolysis of the silicon alkoxide proceeded, an interaction between the resulting silanol group and the neutral species led to a formation of the ion-pair species in their ground state. Upon excitation, a part of the ion pair relaxed to the protonated species during the fluorescence lifetime, and the resulting excited species emitted a corresponding fluorescence. As the sol–gel reaction proceeded further, the geometrical relaxation was gradually prevented with an increase in rigidity around the BQ molecules, so that a fluorescence tended to be observed from an unrelaxed conformer of the ion pair. During the gel to xerogel transition, a part of the BQ molecules exists in relatively large spaces of pores, where water plays an important role, and slowly came to exhibit the fluorescence from the protonated species because of concentrated water in the spaces. Contributions of the three excited-state species to the total fluorescence spectra have been estimated by spectral curve fitting. Consequently, the sol–gel reaction has been resolved into four physicochemical reaction stages, which govern the fluorescence behaviors of BQs. Differences in the fluorescence behaviors among the BQ isomers reflect the acid-dissociation constants in the excited states.     

A Chemical Recognition Element for Lithium Ions Based on Optical Electronic Absorption

Xerogel samples with entrapped series of four optical absorption chemosensors were prepared by sol-gel process. These materials are proposed as chemical recognition elements of an optical chemical sensor. The roles of the chemosensors play proton-dissociable chromogenic azocrown ethers bearing phenol and two azo groups as parts of macrocycles. Occurrence of the alkali ion—receptor interaction is signalled by the chemosensors changing their electronic absorption spectra. By this way chemosensor 4O-CH₃ is able to distinguish Li⁺ ions from other alkali metal ions present in aqueous solution, if the chemosensor is entrapped in Glymo-silica (1:1) xerogel matrix. The proposed recognition element for Li⁺ has been exposed to the cycles chemisorption—desorption many times. Besides DRIFT spectra of the used xerogel matrices were analyzed.     

Vanadia-Silica Catalysts Prepared by Sol–Gel Method: Application for Epoxidation Reaction

Vanadia-silica xerogel and aerogel catalysts, with various V/Si molar ratios (0.05, 0.1, and 0.2), were prepared by sol–gel processing. Gels were obtained under acid catalysed conditions (HNO₃) from vanadium (III) acetylacetonate (V(acac)₃) and tetraethoxysilane (TEOS). Under these conditions, it was found that the gelation time increased with V/Si molar ratio. The structure and catalytic properties of the mixed oxides were found to be mainly influenced by the V/Si molar ratio and drying mode. The catalytic behaviour of the xerogels and aerogels was tested in the epoxidation reaction of trans-2-hexen-1-ol. The results showed that the V/Si molar ratio did not significantly influence the catalytic activity, although the effect of drying (xerogel vs. aerogel) was remarkable. Using a molar ratio TBHP/alcohol = 4/7, the epoxide yield was about 50% in the presence of xerogels, with ca. 8 h required to achieve stationary-conversion conditions in this heterogeneous-catalysis system. In contrast, the aerogel catalysts required only 2 h to reach stationary conversion conditions, due to partial leaching of the vanadium species from the solid and the associated contribution of a relatively rapid homogeneous catalysis mechanism to the overall conversion. These differences in catalytic behaviour were related to the vanadium speciation and the texture of the catalysts. To explore the origin of these differences, the catalysts were characterised by nitrogen physisorption, XPS, Raman spectroscopy, TGA/DTA and XRD.     

气体吸附法测定二氧化硅干凝胶的分形维数

提出了一种方便、科学有效的利用气体吸附法测定二氧化硅干凝胶等多孔材料分形维数（表面分形维数和孔分布分形维数）的方法，不需要进行一系列的吸附/脱附实验，只需要利用单一气体的一次吸附/脱附实验得出的样品孔分布、比表面数据，与不同的标尺进行关联，即可同时获得表面分形维数和孔分布分形维数.通过误差分析和校正，保证了结果的可靠性.用上述方法测定了二氧化硅干凝胶的分形维数，以FHH法和SAXS法对所得结果进行了比较和验证，并对吸附/脱附过程所得结果的差异进行了初步分析. 关键词： 分形维数 气体吸附 二氧化硅 干凝胶