Iron Uniform-Size Nanoparticles Dispersed on MCM-41 Used as Hydrocarbon Synthesis Catalyst

We have synthesized mesoporous MCM-41 and have used it as a support for iron particles to be employed as a catalyst in the Fischer–Tropsch reaction. The solids were characterized by Mössbauer spectroscopy, X-ray diffraction, BET, TGA, CO chemisorption and volumetric oxidation. Although the catalyst showed a high CO conversion when it was used in the hydrocarbon synthesis from CO and H₂ (14.3% at 1 h of reaction time) mainly methane was formed. The high methane production is likely related to the very small size of the metal particles obtained. We suggest some ways to improve the selectivity.     

Magnetism of iron-containing MCM-41 spheres

Iron species were loaded into the mesopores of MCM-41 spheres by incipient wetness impregnation procedure with Fe(ACAC)₃ as the precursor. The magnetism of the samples was studied by vibrating sample magnetometer (VSM), diffuse reflectance ultraviolet–visible (DRUV-vis) spectra and Mossbauer spectra. The results show that the magnetic behavior of the iron-loaded MCM-41 spheres depends on the content of iron loading, valence, and coordination state of iron ions, testing temperature and the atmosphere for the pyrolysis of the iron precursor.     

Oxidation of ferrocene molecules adsorbed in MCM-41

A mesoporous silica material MCM-41 was synthesized under two different conditions, and ferrocene molecules were adsorbed into one-dimensional pores formed in MCM-41. The pore sizes were determined to be 0.9 and 2.8 nm for the MCM-41 synthesized under an acidic and a basic condition, respectively. The formation of paramagnetic ferricinium ions was observed after the adsorption into pores by ESR, magnetic susceptibility and ⁵⁷Fe Mössbauer measurements. Spin densities attributable to ferricinium ions in the pores are estimated to be 0.74 and 0.065 in MCM-41 synthesized under an acidic and a basic condition, respectively. It was shown that the oxidation of ferrocene molecules to ferricinium ions in MCM-41 prepared in an acid solution is faster than that in base at ambient temperature and pressure.     

介孔材料MCM-41的导热研究

文章根据二氧化硅介孔材料MCM-41纳米孔结构特点,首先建立和验证了纳米结构单元模型,然后使用平衡分子动力学方法模拟了孔壁热导率;接着耦合孔隙内气体导热,开展了一维传热分析,最终提炼出MCM-41的有效热导率表达式;并对壁厚、孔径和孔隙率对热导率的影响进行了分析.研究结果表明,MCM-41具有良好的绝热性能,其有效热导率随孔隙率增大近似呈线性减小,且表现出各向异性;导热性能沿孔道长度方向表现出准一维特性. 关键词： 有效热导率 介孔材料 MCM-41 平衡分子动力学     

Synthesis, characterization, and oxygen sensing properties of Ru(II) complex covalently grafted to mesoporous MCM-41

Novel oxygen sensing materials consisting of [Ru(Bphen)₂bpy]²⁺ (Bphen=4,7-diphenyl-1,10-phenanthroline, bpy=2,2′-bipyridyl) portion covalently grafted to the backbones of the ordered functionalized mesoporous MCM-41 are synthesized by co-condensation of tetraethoxysilane (TEOS) and the functionalized Ru(II) complex [Ru(Bphen)₂Bpy-Si]²⁺ using surfactant cetyltrimethylammoniumbromide (CTAB) as template. The Bpy-Si was used as not only one of the precursors of the sol-gel process but also the second ligand of Ru(Bphen)₂Cl₂·2H₂O complex to prepare the functionalized mesoporous materials for oxygen sensors. Dye leaching shortcoming is overcome due to the Si-C bonds. The derivative mesoporous oxygen sensing materials are characterized by Fourier transform infrared (FT-IR), small angle X-ray diffraction (SAXRD), luminescence intensity quenching Stern-Volmer plots, and excited-state decay analysis. The mesoporous materials show higher sensitivity to the O₂ concentration in N₂ (I₀/I₁₀₀=23.2) and shorter response time (1.2 s) in comparison with those based on sol-gel method. When the concentration of oxygen is 10%, the luminescence intensity of the oxygen sensor can be quenched by 89.9%, suggesting that it is highly sensing at low concentration of oxygen.     

分子筛组装单质半导体碲的拉曼光谱

无机介孔材料MCM41是高有序,直孔道介孔分子筛,具有孔径约4nm的一维均匀孔道,孔壁厚约1nm,介孔体积可达40%,是一种很理想的组装材料主体。半导体碲位于第Ⅵ族,其六方相的晶格排列呈螺旋链状结构。本文采用固相合成反应方法,将单质半导体碲成功地组装在MCM41介孔分子筛中。在组装体中,单质半导体碲保持着六方相的晶体结构,其拉曼晶体振动表现出纳米晶体的结构特征。真空热处理实验表明,组装体具有良好的热稳定性。Te的六方相和所具有的独特的螺旋链状结构使Te分子很容易进入MCM41的直孔道内,同时MCM41均匀而规则的直孔道限制了Te螺旋链的随机排列,因而被组装在直孔道内的Te能螺旋链式生长,形成一维半导体纳米晶体,排列均匀,尺寸单一,具有稳定的空间构型     

Melting of Naphthalene Confined in Mesoporous Silica MCM-41

The ²H nuclear magnetic resonance (NMR) solid-echo spectra of naphthalene molecules as guests in the mesopores of neat MCM-41 with a pore width of 3.3 nm were measured in the temperature regime from 180 to 250 K. A strong reduction of the melting point of the naphthalene molecules by 152 K is observed. The line shape changes in the melting region were simulated with two different models, namely, the model of a narrow distribution of activation energies, which is typical for a crystal-like phase, and a two-phase model. Both models indicate a relatively narrow distribution of melting points of the naphthalene molecules inside the pores, indicative of a rather well-defined structure of the naphthalene molecules inside the pores. This finding supports the proposal of a plastic crystalline phase previously proposed by other groups.     

Theoretical studies of argon adsorption in MCM-41 mesoporous systems

Adsorption isotherms are predicted for spherical adsorbates in cylindrical channels of MCM-41 mesoporous materials over a wide range of temperatures by using the “fragment method”. This prediction shows that an equilibrium capillary condensation is impossible for pores with diameters smaller than 2.5 nm. The adsorbate distribution in relatively large pore channels was described by the quasi-chemical approximation (QCA) that takes into account direct pair correlations between interacting molecules. In order to improve the lattice-gas model in the vicinity of the critical point, a calibration function that takes into account information from the fragment method, was introduced into the QCA equations. The influence of the size factor of pores on argon adsorption isotherms was demonstrated.     

Strong visible photoluminescence of TEEDPS-modified silica MCM-41 materials

The photoluminescence (PL) of mesoporous silica MCM-41 modified with triethoxy-3-ethylenediaminepropyl-silane (TEEDPS) has been investigated using UV-Vis fluorescence and absorption spectroscopy methods and thermogravimetry. Results suggest that the TEEDPS-modified MCM-41 has a greatly enhanced (by 20 times) PL intensity at 420 nm relative to that of TEEDPS, and this enhanced PL still remains even after post heat treatment at 180 °C under vacuum (yielding a sample MTV). Meanwhile, a strong PL band at 450 nm appeared in the sample MTV when excited at 380 nm. The TEEDPS-modified material could be partly oxidized at 180 °C in air (yielding a sample MTA); therefore obvious absorption was found up to about 600 nm. The interaction between this absorption and the vacancy-related emission resulted in the maximum of the emission bands of the sample MTA at around 515 nm when excited at 440 nm. PACS 78.55.Mb; 78.55.Hx; 79.60.Jv; 78.40.Ha     

Structure and dynamic properties of liquids confined in MCM-41 mesopores

The microscopic structure and dynamic properties of water, methanol, and acetonitrile confined in mesoporous MCM-41 materials have been investigated under monolayer and capillary-condensation conditions as a function of pore size and temperature by in situ FTIR and X-ray diffraction and quasi-elastic neutron scattering techniques. Both interfacial and confinement effects on the structure and dynamics of the liquids in hydrophilic pores are discussed at the molecular level.     

Water confined in MCM-41: a mode coupling theory analysis

In this paper we analyze molecular dynamics simulation results on supercooled water in a MCM-41 pore in order to test the mode coupling theory. A layer analysis must be performed for water in the pore in order to exclude the contribution of water bound to the strongly hydrophilic surface. Upon supercooling a range of temperatures is reached where the liquid follows the mode coupling theory. From the power law behavior of the relaxation times extracted from the Kohlrausch-William-Watts fit to the self-intermediate scattering function, we obtain the crossover temperature T(C) and the γ exponent of the theory. The time-temperature superposition principle is also satisfied. A fit to the von Schweidler law yields a coefficient b from which all the other parameters of the theory have been calculated. In particular, we obtained the same value of γ as extracted from the power law fit to the relaxation times, in agreement with the requirements of the theory. For very low temperatures, the mode coupling theory no longer holds as hopping processes intervene and water turns its behavior to that of a strong liquid.     

乙烷在中孔分子筛MCM-41中吸附的计算机分子模拟

采用计算机模拟方法研究不同温度下乙烷在不同孔径的MCM-41中的吸附.其中乙烷分子采用两个LJ中心的势模型表征，乙烷分子与MCM-41孔壁的相互作用采用一个连续的势模型表示.除考察了温度、孔径对吸附量的影响外，还研究这些量对乙烷分子在孔中的甲基和质心的分布，以及它们对乙烷分子在孔中的排列方向的影响.GCMC模拟结果发现，在180K和300K时壁面处都有较多的乙烷分子倾向于沿着壁面排列，同时在180K时其余的流体分子倾向于垂直于壁面排列，而在300K时其余的分子并不像在180K时一样倾向于垂直于壁面排列.模拟结果还表明，除壁面附近外，即使是在180K的较低温度下，乙烷分子的排列也是混乱和无序的.     

MCM-41负载磷钨酸的制备与表征

以十六烷基三甲基溴化铵(CTAB)为表面活性剂,以正硅酸乙酯(TEOS)为硅源,采用水热合成法制备了介孔分子筛MCM-41,并利用浸渍法将磷钨酸(HPW)负载在MCM-41分子筛上,得到HPW/MCM-41催化剂f采用X射线衍射、傅里叶变换红外光谱、透射电镜等手段对负载型催化剂进行了表征.结果表明,磷钨酸负载到MCM-...     

Density profile of water confined in cylindrical pores in MCM-41 silica

Recently, water absorbed in the porous silica material MCM-41-S15 has been used to demonstrate an apparent fragile to strong dynamical crossover on cooling below ～220 K, and also to claim that the density of confined water reaches a minimum at a temperature around 200 K. Both of these behaviours are purported to arise from the crossing of a Widom line above a conjectured liquid-liquid critical point in bulk water. Here it is shown that traditional estimates of the pore diameter in this porous silica material (of order 15 ?) are too small to allow the amount of water that is observed to be absorbed by these materials (around 0.5 g H(2)O/g substrate) to be absorbed only inside the pore. Either the additional water is absorbed on the surface of the silica particles and outside the pores, or else the pores are larger than the traditional estimates. In addition the low Q Bragg intensities from a sample of MCM-41-S15 porous silica under different dry and wet conditions and with different hydrogen isotopes are simulated using a simple model of the water and silica density profile across the pore. It is found the best agreement of these intensities with experimental data is shown by assuming the much larger pore diameter of 25 ? (radius 12.5 ?). Qualitative agreement is found between these simulated density profiles and those found in recent empirical potential structure refinement simulations of the same data, even though the latter data did not specifically include the Bragg peaks in the structure refinement. It is shown that the change in the (100) peak intensity on cooling from 300 to 210 K, which previously has been ascribed to a change in density of the confined water on cooling, can equally be ascribed to a change in density profile at constant average density. It is further pointed out that, independent of whether the pore diameter really is as large as 25 ? or whether a significant amount of water is absorbed outside the pore, the earlier reports of a dynamic crossover in supercooled confined water could in fact be a crystallization transition in the larger pore or surface water.     

ZnO/MCM-41复合材料的制备及其光谱性质

以硅酸钠(NaSiO3·9H2O)为硅源,三甲基十六烷基溴化铵(CTAB)为模板剂在碱性条件下水热合成介孔分子筛MCM-41,然后以其为主体,采用锌盐浸渍-灼烧的方法.在介孔分子筛MCM-41上负载了氧化锌(ZnO)纳米微粒,并通过扫描电子显微镜,红外光谱(1R).紫外可见吸收光谱(UV-Vis),N2-吸附脱附和荧光光谱等手段对产品进行了系列表征.结果表明,分子筛平均直径约为1.7μm,对不同温度下灼烧后得到的ZnO/MCM-41组装体的光谱性质进行研究表明介孔分子筛MCM-41的尺寸所限,制备出的ZnO粒子粒径小于2 nm.量子尺寸效应使得ZnO/MCM-41组装体中的ZnO纳米粒子的紫外可见吸收光谱及荧光光谱均发生蓝移.通过红外光谱分析得知ZnO负载到MCM-41上并没有改变介孔分子筛原有的骨架结构.     

Synthesis and characterization of MCM-41 decorated with CuO particles

CuO particles have decorated on the external surface of MCM-41 by in situ introducing cupric nitrate during the hydrothermal synthesis followed by the calcination. The textural and structural properties of CuO/MCM-41 are compared with those of pure MCM-41. The results show that CuO particles are about 40 nm in size and are not agglomerated. The addition of cupric nitrate to the synthesis gel leads to materials with somewhat reduced quality as evidenced from X-ray diffraction patterns and nitrogen adsorption measurements. CuO/MCM-41 is less ordered relative to pure MCM-41 and there are inter-aggregate pores resulting in a higher average pore diameter in the material. The formation of CuO particles on the external surface of MCM-41 and the possible reason for the less ordered structure of CuO/MCM-41 are also discussed in the present paper.     

超声波酸性体系下MCM-41的合成及表征

以十六烷基三甲基溴化铵(CTAB)为模板剂,正硅酸乙酯(TEOS)为硅源,以超声波为辅助手段于酸性介质中制备出了MCM-41介孔分子筛,采用XRD、TEM、FTIR等手段对样品进行了分析表征.结果表明,所合成样品具备MCM-41所特有的六方排列的一维孔道结构和较高的有序度,所合成的MCM-41平均粒径为11nm,孔径约为2.7nm.     

罗丹明6G/MCM-41纳米复合物的发光蓝移

介孔分子筛MCM-41具有规则排列的纳米量级的孔道，可以作为宿主封装其他材料。利用这一性质，将激光染料罗丹明6G分子封装在介孔分子筛MCM-41中形成了纳米复合物。用透射电镜、小角度X射线衍射和荧光光谱等方法对纳米复合物的性质进行了分析。用波长为480nm的光对纳米复合物进行激发，得到的发光峰为531nm。与无限稀罗丹明6G乙醇溶液的发光峰相比，这一发光峰有15nm的蓝移和明显的宽化。我们认为当罗丹明6G分子封装在MCM-41的介孔中时，罗丹明6G分子与介孔分子筛孔道表面分子之间存在较强的相互作用(包括氢键、静电吸引等)，导致电子云被局域在具有较强吸引力的分子筛表面原子周围。引起激发态能量升高，发光峰蓝移。     

Comparative studies of Zr-based MCM-41 and MCM-48 mesoporous molecular sieves: Synthesis and physicochemical properties

Two surfactant-templated synthetic routes are developed for the preparation of new types of mesoporous molecular sieves, Zr-MCM-41 and Zr-MCM-48, using different Si sources but keeping the same zirconium precursor (zirconium-n-propoxide). When fumed silica was used as Si precursor, a Zr-MCM-48 material of cubic structure was formed with a surface area of 654.8 m²/g and an unimodal pore diameter distribution. It shows low stability: after calcination at 600 °C, the ordered structure was transformed into a relatively disordered worm-like mesostructure with many defects and silanol groups. The use of tetraethyl orthosilicate as Si source led to the formation of a Zr-MCM-41 mesoporous solid, which had good thermal stability and a highly ordered hexagonal arrangement, with a surface area 677.9 m²/g and an uniform pore diameter distribution. Fourier transform infrared (FT-IR) characterization and ²⁹Si NMR analysis confirm that zirconium ions indeed incorporated into the framework of the solid. The in situ FT-IR spectroscopy of pyridine adsorption reveals that both, Lewis and Brönsted acid sites, were formed on the surface of these mesoporous materials. The strength and number of the Brönsted acid sites of the Zr-MCM-48 solid were greater than those of the Zr-MCM-41, due to a lower degree of condensation reaction during the synthesis that led to more structural defects in the framework and more silanol groups stretching from the solid surface.     

Water-filled MCM-41 characterized by double-quantum-filtered2H NMR spectral analysis

The dynamics of water molecules confined in adsorbed layers of siliceous MCM-41 with a pore diameter of 2.8 nm is investigated at 230 K by deuteron nuclear magnetic resonance (NMR) relaxation studies including line shapes of theT ₁ process and double quantum filtered (DQF) spectral analyses.²H DQF NMR is a particularly sensitive tool for the determination of the adsorbate dynamics resulting from residual quadrupolar interaction due to the local order. The amount of monolayer water is determined. The monolayer water is composed of two different water components characterized by water, with isotropic reorientational motions, exchanging with water displaying a solid-like spectrum with 4 kHz edge splitting. One may expect that the latter water is situated on surface sites in MCM-41. The restricted wobbling motion of the D-O bond is used to describe its dynamics which is one order of magnitude slower than the isotropic reorientational motion. The order parameter, the motional correlation time, and the exchange rate thus determined provide useful information on the structure and the adsorptive properties of the mesoporous system.