Thermal degradation of CTAB in as-synthesized MCM-41

Thermal evacuation of a surfactant template from pure siliceous MCM-41 and MCM-41 containing aluminium in hydrogen flow was investigated. Micelle templated MCM-41 were prepared using hexadecyltrimethylammonium bromide (CTAB). The products of thermal surfactant degradation outside and inside pores were identified at various temperatures using ¹³C solid-state nuclear magnetic resonance (NMR) spectroscopy, gas chromatography coupled with mass spectrometer (GC-MS) and temperature programmed desorption coupled with mass spectrometer (TPD-MS). The GC-MS and ¹³C MAS NMR results obtained from this study provide an insight into the mechanism of surfactant transformation during MCM-41 synthesis on molecular level.     

Photoluminescence study of mesoporous MCM-41 and Ti-grafted MCM-41

Photoluminescence combined with diffuse reflectance UV—Vis spectroscopy was used to characterise both the pure siliceous MCM-41 and Ti-MCM-41 containing tetrahedral Ti(IV) species anchored onto the inner walls of the siliceous MCM-41. Both Ti-MCM-41 and MCM-41 showed complex photoemission signals and this study allows, for the first time, the distinction of the emission signals of tetrahedral Ti9IV) ions from those due to silica surface centres.     

Thermal oxidation of oxynitride perovskites in different atmospheres

Oxynitride perovskites with bright and variable colour have the potential to be employed as non-toxic pigments, and to substitute colourants that contain harmful heavy metals. For this application it is extremely important to have a precise knowledge about the materials stability. The thermal stability of oxynitride perovskites in different atmospheres was measured by thermogravimetry in combination with mass spectroscopy (MS). The studied compounds, namely LaTiO₂N, SrNbO₂N and SrTaO₂N, were heated up to 1200 °C in argon–oxygen mixtures with varying oxygen contents. It was found that the thermal behaviour of the studied oxynitrides changes drastically with the oxygen concentration. When heated up in pure argon the oxynitrides transform to oxides containing transition metals of lower oxidation state and/or binary nitrides. For oxygen contents between 0.5% and 20% the samples were completely oxidised. The oxidation rate increases with oxygen content. MS analysis of the gaseous products (including N₂, NO and NO₂) reveals a complex reaction mechanism, which is strongly modified by the composition of the atmosphere.     

不同有机胺修饰MCM-41的CO2吸附性能和热稳定性

将乙二胺(EDA,60 g/mol)、四乙烯五胺(TEPA,189 g/mol)和两种聚乙烯亚胺600(PEI600g/mol; PEI1800g/mol)分别负载在MCM-41上制备氨基功能化介孔材料,研究其对CO₂的吸附性能和热稳定性.结果表明,除了EDA-MCM41,其他三种材料随着胺分子量增大CO₂吸附性能下降,但是热稳定性却有所提高,其中,TEPA-MCM41的吸附容量最大,达到2.7 mmol/g.同时发现,乙二胺在制备过程中随溶剂挥发而难以完全负载在MCM-41上.在纯N₂气氛和再生温度100 ℃条件下,经10次循环实验后,TEPA-MCM41的吸附能力下降了7.4%,而PEI600-MCM41和PEI1800-MCM41吸附能力保持不变,且质量变化在1%以内,表现出良好的再生稳定性.采用80%CO₂/20%N₂对吸附饱和的材料进行再生,四种材料的再生温度将提高到160 ℃以上,高分子量PEI600-MCM41和PEI1800-MCM41相比于TEPA-MCM41具有更好的热稳定性.     

Formation Process of MCM-41 Precursor and Porous Texture of MCM-41

The formation process of the MCM-41 precursors (silicate/surfactant complex) was investigated on the basis of the pH titration curves of Na(4)SiO(4) in the presence of [C(16)H(33)N(CH(3))(3)]Cl. Measurements of the pH titration curves were carried out using the computer-controlled gravimetric titrator constructed in our laboratory. The white precipitate (MCM-41 precursor) was abruptly formed at pH 11.1 (298 K) and at pH 9.9 (343 K). Formation of the MCM-41 precursor can be explained by coagulation of the rod-like micelle colloids whose surface is covered by the condensed silicate anions of (HSiO(3))(n)(n-). The porous texture of the MCM-41 samples whose precursors were synthesized under different conditions was analyzed on the basis of the adsorption isotherms of nitrogen at 77 K. It has been shown that the MCM-41 sample whose precursor was prepared at pH 9.9 and 343 K shows one sharp peak (r(p) = 1.65 nm) in the pore size distribution curve, but the MCM-41 samples whose precursors were prepared at pH 6.5-5.0 and 343 K give two peaks (r(p) = 1.66 nm and r(p) = 2.12-2.36 nm). The appearance of the second peak (r(p) = 2.12-2.36 nm) has been considered to be in connection with the destruction of the MCM-41 precursor into small fragments in acidic medium. Copyright 2000 Academic Press.     

Metastability of MCM-41 and Al-MCM-41

The apparent stability of MCM-41 and Al-MCM-41 in water was appraised in a series of solubility experiments. MCM-41 is a siliceous, mesoporous material of hexagonal symmetry and exceptionally high surface area first synthesized in 1992. The dissolution experiments were carried out at several solid/water ratios: 1/200, 1/100, and 1/75. Results indicated that MCM-41 and Al-MCM-41 are more soluble than amorphous silica at ambient temperatures. Using standard thermodynamic data, a minimum Gibbs free energy of formation of -847.9 kJ/mol for MCM-41 was calculated compared to -848.85 kJ/mol for amorphous silica and -856.3 kJ/mol for quartz. X-ray diffraction (XRD) analyses of recovered solids indicated a progressive loss of crystallinity in MCM-41 and Al-MCM-41 over the 79 day dissolution experiment. BET nitrogen surface area analyses of recovered solids revealed no appreciable change in the surface area of either material after 79 days of reaction in water. Field emission scanning electron microscope (SEM) images taken of the 79 day MCM-41 sample showed some degradation of the initial structure-fine, worm-like particles.     

CO_2 adsorption performance of different amine-based siliceous MCM-41 materials

A series of amine-based adsorbents were synthesized using siliceous MCM-41 individually impregnated with four different amines(ethylenediamine(EDA),diethylenetriamine(DETA),tetraethylenepentamine(TEPA) and pentaethylenehexamine(PEHA)) to study the effect of amine chain length and loading weight on their CO2 adsorption performances in detail.The adsorbents were characterized by FT-IR,elemental analysis,and thermo-gravimetric analysis to confirm their structure properties.Thermo-gravimetric analysis was also used to evaluate the CO2 adsorption performance of adsorbents.Longer chain amine-based materials can achieve higher amine loadings and show better thermal stability.The CO2 adsorption capacities at different temperatures indicate that the CO2 adsorption is thermodynamically controlled over EDAMCM41 and DETA-MCM41,while the adsorption over TEPA-MCM41 and PEHA-MCM41 is under kinetic control at low temperature.The chain length of amines affects the CO2 adsorption performance and the adsorption mechanism significantly.The results also indicate that CO2 adsorption capacity can be enhanced despite of high operation temperatures,if appropriate amines(TEPA and PEHA) are applied.However,adsorbents with short chain amine exhibit higher adsorption and desorption rates due to the collaborative effect of rapid reaction mechanisms of primary amines and less diffusion resistance of shorter chain length amines.     

高硅分子筛MCM-41热稳定性与水热稳定性的初步研究

高硅分子筛ＭＣＭ－４１热稳定性与水热稳定性的初步研究董维阳,龙英才,郜晓琴,许太明,谢关根,高滋（复旦大学化学系,上海,２００４３３）关键词高硅分子筛ＭＣＭ－４１,热稳定性,水热稳定性,环己烷吸附等温线ＭＣＭ－４１是最近几年刚刚发现的中孔分子筛（Ｍ４...     

Sorption on as-synthesized MCM-41

Mesoporous materials MCM-41 with the hexagonal arrangement of pores were obtained using dodecyltrimethylammonium bromide and octadecyltrimethylammonium bromide as templating surfactants. Adsorption of toluene and propan-1-ol on the as-synthesized MCM-41 silica samples was investigated using the TG-DTG, DTA and DSC techniques. The sorption mechanism of used adsorptives is discussed in terms of hydrophobicity of pore interior filled with template as well as pore dimensions.     

Template transformations in preparation of MCM-41 silica

Template transformation in MCM-41 material during thermal treatment under different conditions was investigated on the basis of thermogravimetry (TG-DTA), X-ray diffraction (XRD) and positron annihilation lifetime spectroscopy (PALS). Micelle templated silica was prepared using C18 trimethylammonium bromide. The pore structure of MCM-41 samples obtained after removal of the surfactant in air, argon flow and vacuum was analyzed on the basis of the adsorption isotherms of nitrogen at 77 K and XRD experiments. The TG-DTA experiments confirm the mechanism of the template removal known from literature. However, the sequence of the processes during thermal treatment of as-synthesized sample and temperature of transformations depended strongly on the presence of oxygen and the heating rate. The main template degradation took place below 573 K and was independent of the kind of atmosphere above the sample. Residual carbonaceous species are removed from pores and the external surface of MCM-41 silica upon heating to 823 K by combustion or evaporation. The latter process as well as translocation of liquid-like products of template degradation from the pore interior to external surface was confirmed by PALS experiment in vacuum.     

高表面积MCM-41的合成与性能

通过易于控制的手段，合成出了超高表面积的MCM 41分子筛，并采用XRD、N2吸附脱附、热重 差热分析、SEM等测试手段对合成样品进行分析表征，主要考察了晶化时间对分子筛表面积的影响.其XRD结果表明,合成的固体产物具有六方排列的孔道结构，结晶度高.氮吸附脱附实验表明，产物具有单一的孔径分布，其BET表面积可达1300 m2•g－1左右，合成样品的平均孔径约3.2 nm，适宜的晶化时间是50～100 h.热重分析表明，样品热稳定性好，吡啶升温脱附曲线表明合成的样品具有弱酸性.     

功能化MCM-41在有机合成中的应用

介孔分子筛MCM-41具有较高的比表面和规整的结构以及表面带有羟基,因此可对MCM-41进行官能团化,从而实现均相催化剂在MCM-41中的组装,达到均相催化剂多相化的目的。本文介绍了最近几年以MCM-41为主体,组装均相配合物得到固相化催化剂在氧化、还原、烷基化、氢甲酰化、Heck、环丙烷化等反应中的应用。     

Diffusion of guest molecules in MCM-41 agglomerates

The pulsed field gradient nuclear magnetic resonance method has been used to study self-diffusion of cyclohexane in a commercial MCM-41 material at different external gas pressures from zero to saturated vapor pressure. It is found that the effective diffusivities exhibit three different regions with increasing pressure: decrease at low pressures, a sudden drop at intermediate pressures, and increase at higher pressures. In addition, in the region of irreversible adsorption (hysteresis loop) the diffusivities are also found to differ on the adsorption and the desorption branches. A simple analytical model taking account of different molecular ensembles with different transport properties due to the complex architecture of the porous structure is developed which provides a quantitative prediction of the experimental data. The analysis reveals that the effective diffusivity is predominantly controlled by the adsorption properties of the individual mesoporous MCM-41 crystallites which, in combination with high transport rates, provide a simple instrument for fine tuning of the transport properties by a subtle variation of the external conditions.     

Thermal analysis in reactive atmospheres

By using reactive atmospheres, the area of application of thermal analysis is expanded considerably to cover many aspects of high temperature research into fuels, extractive metallurgy, materials and catalysts. This article reviews the design of apparatus and its application in kinetic and thermodynamic studies involving atmospheres such as H₂, CO, N₂, NH₃, CO₂, H₂O, SO_2or3, H₂S, S₂, Cl₂, HO, F₂ and HF at low or high pressures and as low pressure plasmas. Apart from gas-solid reactions, the important influence of a controlled product gas atmosphere on decomposition reactions is discussed also. Gas-solid adsorption and solubility studies are not included.

---

Zusammenfassung Das Anwendungsgebiet der thermischen Analyse wird durch Arbeiten in reaktiver Atmosphäre beträchtlich erweitert, so dass Hochtemperaturuntersuchungen auf solchen Gebieten wie Brennstoffe, extraktive Metallurgie, Material- und Katalyseforschung möglich werden. Dieser Übersichtsartikel behandelt die Konstruktion der Geräte und ihre Anwendungen bei kinetischen und thermo-dynamischen Untersuchungen in solchen Gasphasen wie H₂, CO, N₂, NH₃, CO₂, SO_{2o der 3}, H₂S, S₂, Cl₂, HCl, F₂ und HF sowie im Niederdruckplasma. Ausser Gas-Festkörperumsetzungen wird auch der Einfluss einer geregelten Produktgas-Atmosphäre auf Zersetzungsreaktionen diskutiert. Auf die Adsorption von Gasen an Festkörpern und Löslichkeitsuntersuchungen wird nicht eingegangen.

---

, , , . , , , , , , , - , , , , . — , . - .

     

Synthesis and Characterization of Ce-Containing MCM-41

SUMMARY. The cerium-containing MCM-41 (Ce-MCM-41) has been synthesized with size in the micrometer range by direct hydrothermal method. Transmission electron microscopy shows the regular hexagonal array of uniform channel characteristics of MCM-41. Five peaks were detected in the lov-angle XRD patterns,an interplanar spacing d100 = 40.6 A was obtained that can be indexed on a hexagonal unit cell with a0 = 46.9 A. Nitrogen adsorption isotherm at 77 K revealed a surface area of 920 m2/g, pore size of 26.2 A and wall thickness of 18.1 A. A cell contraction of 2.6 A upon calcination was observed. The spectroscopic studies indicate that the synthesized sample is with MCM-41 structure and Ce is in the framework position. A weak Lewis acidity was indicated by infrared spectra of pyridine adsorption. The synthesized Ce-MCM-41 exhibits fairly catalytic activity for the NO reduction by CO.     

Characterization of methylated nanoscale MCM-41 material

In this study, nanoscale MCM-41 molecular sieve was prepared under a basic condition by a hydrothermal method using cetyltrimethylammonium bromide as a template and tetraethyl orthosilicate as a silica source. Methylated nanoscale MCM-41 molecular sieve was prepared from the nanoscale MCM-41 by post-synthesis method using trimethylchlorosilane (TMCS) as coupling agent. The product was characterized by means of element analysis, powder X-ray diffraction, Fourier transform infrared (FT-IR) spectroscopy, low-temperature nitrogen adsorption-desorption technique at 77 K, scanning electron microscopic (SEM), thermogravimetry-differential thermal analysis (TG-DTA). Powder XRD showed that the framework of the molecular sieve was well retained and the degree of ordering of the methylated MCM-41 decreases. IR spectra and the low-temperature nitrogen adsorption-desorption technique suggested that methyl was successfully grafted to the inner surface of the methylated MCM-41 and the mesoporous channels of the methylated MCM-41 were still maintained. Scanning electron microscopic results showed that the average size of the methylated MCM-41 prepared was 112 nm. Differential thermal analysis showed that the prepared material has preferable thermal stability and the methylated MCM-41 can be stable at 903 °C.     

Synthesis of nopol over MCM-41 catalysts

MCM-41 was found to be an active heterogeneous catalyst for the synthesis of nopol by the Prins condensation of beta-pinene and paraformaldehyde, but Sn-MCM-41 in which Sn has been grafted on MCM-41 by chemical vapor deposition is far more active and combines high efficiency and recyclability.