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.     

氮气在MCM－41分子筛中的吸附：实验和分子模拟

用美国Micromeritics公司生产的ASAP2010物理吸附仪测定了低温（77 K） N_2在MCM-41分子筛中的吸附等温线，获得了表征MCM-41特征的BET比表面、BJH孔 容和平均孔径。同时用巨正则Monte Carlo（GCMC）模拟方法考究了N_2在MCM-41中 的吸附，得到了N_2在MCM-41中的模拟吸附等温线，分析了流体在MCM-41分子筛中 的微观结构。GCMC模拟中MCM-41介孔材料模型化为圆柱孔，N_2模型化为Lennard- Jones（LJ）球。N_2和MCM-41介孔墙壁间的相互作用采用Tjatjopoulos-Feke- Mann（TFM）势能模型进行表征。通过使模拟和实验结果有一个好的吻合，确定了 一组有效的MCM-41分子筛的势能参数（σ_(ww) = 0.265 nm，∈_(ww)/k = 190 K ）。这为以后其他吸附质在MCM-41中吸附的预测奠定了基础、提供了依据。     

Adsorption of nitrobenzene from aqueous solution by MCM-41

Adsorption of nitrobenzene onto mesoporous molecular sieves (MCM-41) from aqueous solution has been investigated systematically using batch experiments in this study. Results indicate that nitrobenzene adsorption is initially rapid and the adsorption process reaches a steady state after 1 min. The adsorption isotherms are well described by the Langmuir and the Freundlich models, the former being found to provide the better fit with the experimental data. The effects of temperature, pH, ionic strength, humic acid, and the presence of solvent on adsorption processes are also examined. According to the experimental results, the amount of nitrobenzene adsorbed decreases with an increase of temperature from 278 to 308 K, pH from 1.0 to 11.0, and ionic strength from 0.001 to 0.1 mol/L. However, the amount of nitrobenzene adsorbed onto MCM-41 does not show notable difference in the presence of humic acid. The presence of organic solvent results in a decrease in nitrobenzene adsorption. The desorption process shows a reversibility of nitrobenzene adsorption onto MCM-41. Thermodynamic parameters such as Gibbs free energy are calculated from the experimental data at different temperatures. Based on the results, it suggests that the adsorption is primarily brought about by hydrophobic interaction between nitrobenzene and MCM-41 surface.     

甲烷在中孔分子筛MCM-41中吸附的计算机模拟

采用巨正则系综Monte Carlo方法研究了甲烷在两个不同孔径的MCM-41中不同温度下的吸附等温线和其在孔中的相行为和排列方式.模拟结果显示,在较小孔径的MCM-41中,流体分子达到毛细凝聚所需的化学位较小,并且观察到两个孔径下计算机模拟得到的亚稳态区域都非常宽,使得层状转变（如果有的话）被包含在这个区域.通过比较两种孔径下达到毛细凝聚后的构型,可以看出,在3.5 nm的孔中流体的分子结构出现非常有序的排列,而在5.0 nm的孔中则没有.在常温300 K时甲烷的吸附的计算机模拟表明,孔壁对流体分子的作用仅仅影响较靠近壁面附近的流体分子的排列,而对孔中间的分子几乎没有影响.     

介孔材料MCM-41上汽油吸附深度脱硫

 研究了不同硅铝比的MCM-41介孔材料作为吸附剂对模型汽油以及真实FCC汽油的脱硫性能. 结果表明,在室温和常压下,MCM-41介孔材料对模型溶液中噻吩的吸附随着吸附剂中铝含量的增加而显著提高. 吡啶吸附的红外光谱显示,噻吩吸附容量的提高与吸附材料酸性的明显增大有直接关系. 但在对FCC汽油的吸附脱硫实验中,随着MCM-41中铝含量的提高,脱硫率并未增大. 这主要是由于在FCC汽油中存在大量性质与噻吩类含硫化合物十分相近的芳烃和烯烃,竞争吸附导致MCM-41对有机硫化物的吸附能力显著降低.     

Adsorption of diazinon and fenitothion on MCM-41 and MCM-48 mesoporous silicas from non-polar solvent

The adsorption of two common organophosphorus pesticides, diethoxy-[(2-isopropyl-6-methyl-4-pyrimidinyl)oxy]-thioxophosphorane (diazinon) and dimethoxy-(3-methyl-4-nitrophenoxy)-thioxophosphorane (fenitrothion), by MCM-41 and MCM-48 mesoporous silicas at room temperature was investigated. UVvis and IR spectroscopy, small-angle X-ray diffraction, and the specific surface area analysis (S _BET) were used to study the adsorption behavior of diazinon and fenitrothion. The results show that the MCM-41 and MCM-48 mesoporous silicas adsorb diazinon more efficiently than fenitrothion. The extraction of adsorbed materials from the adsorbents with polar solvents and subsequent analysis by ³¹P NMR showed that the adsorption of diazinon and fenitrothion on mesoporous silicas is destructive and non-destructive, respectively. Nitrogen adsorption measurements showed that the specific surface area of both silicas decreases after the adsorption of pesticides, and the larger effect is observed for diazinon. The article is published in the original.     

Adsorption of acid blue on synthesized polymeric nanocomposites,PPy/MCM-41 and PAni/MCM-41: Isotherm,thermodynamic and kinetic studies

In the this research, removal of acid blue 62 from aqueous solution using mesoporous crystalline material-41 (MCM-41) loaded by polypyrrole (PPy) and polyaniline (PAni) was investigated. Synthesized composites were characterized by SEM, TEM, FTIR, XRD and BET analysis. Langmuir adsorption isotherm showed the best compatibility with the experimental data in comparison with other isotherm models (q_m = 55.55 mg/L). Kinetic studies proved that the adsorption process is compatible with the pseudo-second-order kinetic model. Thermodynamic parameters such as Gibbs free energy changes (ΔG^o), Enthalpy changes (ΔH^o) and Entropy changes (ΔS^o) were calculated. Negative value of ΔG^o and positive value of ΔH^o show that adsorption of Acid blue 62 on PPy/MCM-41 nanocomposite is a spontaneous process also endothermic.     

Adsorption of simple gases in MCM-41 materials: the role of surface roughness

This paper reports the development and testing of atomistic models of silica MCM-41 pores. Model A is a regular cylindrical pore having a constant section. Model B has a surface disorder that reproduces the morphological features of a pore obtained from an on-lattice simulation that mimics the synthesis process of MCM-41 materials. Both models are generated using a similar procedure, which consists of carving the pore out of an atomistic silica block. The differences between the two models are analyzed in terms of small angle neutron scattering spectra as well as adsorption isotherms and isosteric heat curves for Ar at 87 K and Xe at 195 K. As expected for capillary condensation in regular nanopores, the Ar and Xe adsorption/desorption cycles for model A exhibit a large hysteresis loop having a symmetrical shape, i.e., with parallel adsorption and desorption branches. The features of the adsorption isotherms for model B strongly depart from those observed for model A. Both the Ar and Xe adsorption branches for model B correspond to a quasicontinuous pore filling that involves coexistence within the pore of liquid bridges and gas nanobubbles. As in the case of model A, the Ar adsorption isotherm for model B exhibits a significant hysteresis loop; however, the shape of the loop is asymmetrical with a desorption branch much steeper than the adsorption branch. In contrast, the adsorption/desorption cycle for Xe in model B is quasicontinuous and quasireversible. Comparison with adsorption and neutron scattering experiments suggests that model B is too rough at the molecular scale but reproduces reasonably the surface disorder of real MCM-41 at larger length scales. In contrast, model A is smooth at small length scales in agreement with experiments but seems to be too ordered at larger length scales.     

Modified Mesoporous Silicate MCM-41 for Zinc Ion Adsorption: Synthesis, Characterization and Its Adsorption Behavior

Modified MCM‐41 has been prepared by bi‐functionalization of thiol and amino functional groups onto mesoporous silicate MCM‐41. Elemental analysis (EA), thermogravimetry analysis (TGA) and FTIR techniques were used to quantify the attachment of the thiol and amino functional groups to the mesoporous silicate pore wall. Powder X‐ray diffraction (PXRD) and nitrogen adsorption techniques were used to establish the pore diameter, packing of the pores and specific surface area of the modified mesoporous silicate MCM‐41. Adsorption behavior for 18 metal ions on this sorbent has been studied and discussed. This sorbent has high affinity for zinc ions against amino‐ or thiol‐functionalized MCM‐41 sorbents.     

用实验吸附等温线和计算机分子模拟表征中孔分子筛MCM-41

首先比较了表征MCM-41的两个势模型对吸附等温线的影响。发现在一维势模型 中,低压部分的吸附应与选用的势模型的势阱深度有关,而毛细凝聚发生的位置与 孔壁在离壁面较远处与流体分子的相互作用的强弱有关。然后作者使用了一个“混 合”的势模型,即采用作者提出的势函数表征孔壁中氧原子对MCM-41中流体分子的 作用,而采用Tjatjopoulos等提出的势函数近似地表征MCM-41表面硅醇基团以及一 些未知因子对流体分子的作用。虽然这种势模型仍然是一维的,但这种势模型将孔 壁内氧原子的作用和表面上非均匀性分开考虑,具有较明确的物理意义。通过计算 机模拟与实验数据的比较发现,这种势模型可以较好地拟合氮气在MCM-41中的吸附 等温线。     

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.     

Adsorption Behavior for CO on Modified MCM—41 with Zn and CuCl

The Zn-Cu(I)/HMCM-41 catalysts were successfully prepared in laboratory scale by solid-state ion exchange with CuCl salt,together with introduction of Zn to improve the dispersion degree of the active component Cu(I) on the surface. With increase of the amounts of Zn and CuCl from 0% to 9.0% and 25.0% respectively, CO adsorption the amounts of increased from 10.6μmol/g to 183.0μmol/g correspondingly. The in situ FT-IR spectra for CO adsorption demonstrated that there existed two dynamic equilibriums between surface carbonyl complexes:Cu(CO)3^ ←→Cu(CO)2^ CO and Cu(CO)2^ ←→Cu(CO)^ CO.The equilibriums can be shifted reversibly by changing the temperature and pressure.Due to the modified MCM-41 mesoporous materials possess both of the acidic and metallic carbonyl centers, this kind of material can be developed into mesoporous bifunctional catalysts for carbonylation reactions, in which larger molecules are involved.     

Adsorption of 2-methylbenzoic acid onto MCM-41 mesoporous material: kinetics and equilibrium studies

Ordered mesoporous molecular sieves are widely studied as alternative materials in areas where sorptive and catalytic applications are required. MCM-41 type mesoporous material was tested as sorbent of 2-methylbenzoic acid (MBA), an aromatic carboxylic acid selected as model molecule for adsorption studies on mesoporous silicas. Adsorption kinetic studies of MBA on MCM-41 type materials were carried out using ethanol solutions at different MBA concentrations. Experimental results followed Langmuir isotherm model showing large adsorption capacity (3.5?g/g). Two kinetic models, the pseudo first- and second-order, were selected to describe the adsorption process and to determine the best model fitting with the experimental data. Kinetic parameters for each kinetic equation were calculated and discussed. It was shown that the MBA adsorption process onto MCM-41 material could be described by the pseudo-second-order equation and that the MCM-41 performs as a suitable adsorbent material.     

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.     

Adsorption studies and release of selected dyes from functionalized mesoporous MCM-41 silica

Functionalized mesoporous MCM-41 silica was subjected to adsorption and release studies of encapsulated guest molecules of three chosen dyes. These mesoporous systems were composed of three different capping reagents introduced by grafting method on the silica surface to control the release of dye molecules at two different pH values. The amounts of dyes adsorbed on the silica surface were measured using ultraviolet-visible (UV-VIS) spectrophotometry. The efficiency of grafting was calculated on the basis of differential thermal analysis (TG) results and elemental analysis. The release profiles were determined for all obtained systems using USP Dissolution Apparatus 2. Adsorption of the two azo dyes used was the most efficient after the positively charged functionalization and lower after functionalization with neutral and negatively charged capping reagents, while the phthalocyanine dye adsorption was almost functionalization-independent. Grafting efficiency was the highest for neutral capping reagent and much lower for electrically charged molecules of other reagents. Release studies showed clearly that desorption was pH-dependent for azo dyes and pH independent for Alcian Blue. The adsorption and release seem to be connected with the electrical charge of all constituents of these systems. Results obtained can be used for further analysis of different electrically charged molecules.      

Benzene Adsorption Isotherms on MCM-41 and their Use for Pore Size Analysis

This work is focused on the elaboration of methodology for adsorption characterization of porous silicas by using benzene adsorption isotherms measured on good quality MCM-41 materials. Three MCM-41 samples were synthesized by using tetraethyl orthosilicate (TEOS) as silica source and surfactants, octyltrimethylammonium (C8), decyltrimethylammonium (C10) and cetyltrimethylammonium (C16) bromides as templates. A characteristic feature of this synthesis was relatively long hydrothermal treatment (5 days) at 373 K, which gave well ordered samples as evidenced by powder XRD analysis. Benzene adsorption isotherms measured on these MCM-41 samples were used to evaluate such standard quantities as the BET specific surface area, total pore volume, external surface area and the volume of ordered mesopores, and to obtain the statistical film thickness (t-curve) as well as the Kelvin-type relation, which describes the dependence between pore width and condensation pressure for benzene on silica at 298 K. The latter relations were incorporated into the Barrett-Joyner-Halenda algorithm to extend its applicability to calculate the pore size distributions from benzene adsorption data.     

Nitrogen Adsorption Study of MCM-41 Molecular Sieves Synthesized Using Hydrothermal Restructuring

Nitrogen desorption scanning hysteresis loops (DSHLs) for large-pore MCM-41 silicas (pore diameter from 4.0 to 6.5 nm) are reported for the first time. DSHLs for MCM-41 were compared with those of conventional mesoporous silicas and no appreciable differences were found, although hysteresis loops and DSHLs for the latter were usually broader. Since desorption behavior of conventional porous silicas is appreciably influenced by pore connectivity, the observed similarity in hysteresis behavior suggests single-pore blocking effects for MCM-41 due to variation of pore diameter along its nonintersecting channels. It was also shown that the steepness of nitrogen desorption branches at relative pressures close to 0.4 often results from proximity of the lower pressure limit of adsorption-desorption irreversibility and consequently it is not justified to consider it as an indication of narrow pore size distribution. Thus, application of desorption data in calculations of pore size distributions may be grossly misleading.     

Aromaticity of Rees-type hydrocarbons—a DFT computational study

The structure and aromatic properties of Rees hydrocarbons 7bH-cyclopent[cd]indene and its benzo-annelated derivative 1a and 2a, respectively, are examined by the B3LYP/6−31+G(d) calculations employing HOMA criterion of Krygowski and coworkers. It is shown that 1a possesses strong π-electron delocalization over the perimeter of the CC bonds, thus forming a quasi-[10]annulene pattern. Its aromatic character is determined to be 83%. In contrast, 2a is less convenient model system for [14]annulene. The reason behind is that the perimeter network of the potentially aromatic 14π-electrons is supplemented by two additional more local aromatic patterns involving 10π and 6π electrons. Consequently, the π-electron delocalization over the molecular rim is incomplete being thus diminished. The aromatic character of the peripheral bonds in both 1a ⁻ and 2a ⁻ anions formed upon deprotonation of the central C–H bond is decreased, since the role of the smaller rings in forming aromatic subsystems is increasing. Finally, polycyano substitution of 1a and 2a decreases aromaticity due to the price paid for the resonance effect taking place between the carbocyclic π-network and the double bonds of the CN groups. The resonance effect is particularly strong in anions derived by heterolytic cleavage of the C–H bond emanating from the central sp ³ carbon atom. Dedicated to Professor T. M. Krygowski for his outstanding scientific achievements on the occasion of his 70th birthday.     

亚甲基蓝在介孔二氧化硅MCM-41表面的吸附性能研究——介绍一个物理化学综合实验

介绍一个物理化学综合实验。本实验以制备的有序介孔二氧化硅MCM-41为吸附剂,研究了水溶液中亚甲基蓝在其表面的吸附性能。通过利用Langmuir和Freundlich吸附等温式对吸附数据进行分析、处理和拟合。本综合实验教学不仅促进学生对材料结构与性能的认识,而且加强了学生对固体表面吸附过程的理解。