Low temperature properties of acetonitrile confined in MCM-41

The effect of confinement on the phase changes and dynamics of acetonitrile in mesoporous MCM-41 was studied by use of adsorption, FT-IR, DSC, and quasi-elastic neutron scattering (QENS) measurements. Acetonitrile molecules in a monolayer interact strongly with surface hydroxyls to be registered and perturb the triple bond in the C[triple bond]N group. Adsorbed molecules above the monolayer through to the central part of the cylindrical pores are capillary condensed molecules (cc-acetonitrile), but they do not show the hysteresis loop in adsorption-desorption isotherms, i.e., second order capillary condensation. FT-IR measurements indicated that the condensed phase is very similar to the bulk liquid. The cc-acetonitrile freezes at temperatures that depend on the pore size of the MCM-41 down to 29.1 A (C14), below which it is not frozen. In addition, phase changes between alpha-type and beta-type acetonitriles were observed below the melting points. Application of the Gibbs-Thomson equation, assuming the unfrozen layer thickness to be 0.7 nm, gave the interface free energy differences between the interfaces, i.e., Deltagamma(l/alpha) = 22.4 mJ m(-2) for the liquid/pore surface (ps) and alpha-type/ps, and Deltagamma(alpha/beta) = 3.17 mJ m(-2) for alpha-type/ps and beta-type/ps, respectively. QENS experiments substantiate the differing behaviors of monolayer acetonitrile and cc-acetonitrile. The monolayer acetonitrile molecules are anchored so as not to translate. The two Lorentzian analysis of QENS spectra for cc-acetonitriles showed translational motion but markedly slowed. However, the activation energy for cc-acetonitrile in MCM-41 (C18) is 7.0 kJ mol(-1) compared to the bulk value of 12.7 kJ mol(-1). The relaxation times for tumbling rotational diffusion of cc-acetonitrile are similar to bulk values.     

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.     

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.     

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.     

NSAID naproxen in mesoporous matrix MCM-41: drug uptake and release properties

Hexagonally ordered mesoporous silica material MCM-41 (S_BET?=?1090?m²/g, pore size?=?31.2 ?) was synthesized and modified by 3-aminopropyl ligands. The differences in an uptake and subsequent release of anti-inflammatory drug naproxen from unmodified and amino modified MCM-41 samples were studied. The prepared materials were characterized by high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM), nitrogen adsorption/desorption, Fourier-Transform Infrared Spectroscopy (FT-IR), Small-angle X-ray scattering (SAXS), thermoanalytical methods (TG/DTA) and elemental analysis. The amount of the drug released was monitored with thin layer chromatography (TLC) with densitometric detection in defined time intervals. The amounts of the released naproxen from mesoporous silica MCM-41/napro and amine-modified silica sample A-MCM-41/napro were 95 and 90% of naproxen after 72?h. In this study we compare the differences of release profiles from mesoporous silica MCM-41 and mesoporous silica SBA-15.     

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.     

The synthesis of MCM-41 nanomaterial from Algerian Bentonite: The effect of the mineral phase contents of clay on the structure properties of MCM-41

This study focuses on the MCM-41 material (Mobil Composition of Matter). The MCM-41 nanomaterial presents higher physical properties such as pore sizes, surface areas and pore volumes. This material is usually synthesized by using laboratory reagents as silicate sources and aluminium source. These laboratory reagents are still expensive and toxic for large scale production. The main aim of this work is to resolve this problem and to replace these expensive laboratory reagents by more cost effective ones. The volclay and Algerian bentonite low-cost mass clay materials are used as silicate and acuminate sources separately by adopting an alkaline fusion process to extract both silicon and aluminium (1 kg of silicium and aluminium from volclay and Algerian bentonite cost around 0.03 and 0.01 € whereas the same amount of silicon from ludox and aluminium from sodium aluminates cost around 350 €). The synthesis of MCM-41 from bentonite was carried out by the hydrothermal method using the supernatants of bentonite (in the form of sodium silicate and sodium aluminate). On the basis of the data obtained from powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and N₂ adsorption and desorption, the results revealed that the properties of MCM-41 synthesized from Algerian bentonite and volclay separately depend on both elemental composition and mineral phase contents of the used bentonite. Pure and highly ordered hexagonal mesoporous MCM-41 with uniform pore sizes and a high specific surface area have been successfully synthesized without any phases which exist in natural bentonite. The Algerian bentonite was chosen because of its low cost compared to volclay, another commercial clay source.     

TiO~2修饰的介孔分子筛MCM-41的合成、表征及光 催化性研究

首次以三种不同的含钛有机物为氧化钛的前驱体,合成了三种TiO~2呈单层分散状态的TiO~2修饰的介孔分子筛MCM-41,并以XRD,FT-IR,液氮温度下N~2吸附-脱附曲线,固体UV-vis漫反射等表征手段对其结构特征和氧化钛分散状态进行了研究。TiO~2在介孔分子筛MCM-41孔道中分散,MCM-41仍能保持骨架结构,没有晶相TiO~2生成;TiO~2与MCM-41孔道表面的≡Si－OH以化学键连接,生成Si－O－Ti键;TiO~2在MCM-41内孔壁呈均匀单层分散状态;TiO~2粒子的减小使其对紫外光的吸收发生明显的蓝移现象。并以苯酚降解反应为模型检验合成的具有光催化性的TiO~2修饰的MCM-41的光催化活性。     

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。     

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.     

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.     

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.     

Effect of titanium on the structure and properties of mesoporous MCM-41 molecular sieves

Mesoporous MSM-41 titanium silicate molecular sieves with high titanium content were synthesized. The hexagonal structure along with high specific surface and pore volume are retained up to Si/Ti≥5. L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prospekt Nauki, Kiev 252039, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 3, pp. 177–182, May–June, 1999.     

萘普生在介孔分子筛MCM-41中的负载及其溶出性能

用含氨基的偶联剂修饰介孔分子筛MCM-41的表面,将修饰前和修饰后的介孔分子筛分别负载难溶性药物萘普生（NAP）,利用X射线衍射、傅里叶变换红外光谱、差示扫描量热分析、扫描电子显微镜、透射电子显微镜和N2吸附 脱附分析等技术分别对其进行了结构表征和性能测试。 结果表明,药物分子存在于分子筛的孔道中。 负载在MCM-41中的萘普生溶出速率远优于原料药,60 min时大约溶出70%。 氨基修饰后的药物释放速率比修饰前有所减慢,表明可通过氨基修饰调节释放速率。     

介孔分子筛MCM-41的硅烷化改性及吸附性能研究

高硅分子筛MCM-41具有均匀的中孔孔径分布、较大的比表面积(>900m2/g)[1-2],应用前景广阔.已在硅基分子筛的基础上合成了各种含有杂原子分子筛、金属氧化物分子筛、有机官能团分子筛[3-8].改变介孔MCM-41分子筛的表面组成可以较好地调节其吸附性能[9].本文以直接反应的方法用三甲基氯硅烷来修饰MCM-41.通过XRD、红外吸收光谱和比表面和孔隙率测定对样品进行了表征,并分别对改性前后样品进行了水与苯的吸附性能研究.