Synthesis and surface modification of mesoporous mcm-41 silica materials

Surface modification offers a great opportunity to adjust both the pore diameter and surface properties of MCM-41 type organic–inorganic hybrid materials which result in materials of improved hydrothermal and mechanical stability. Therefore, MCM-41 silica, surface modified with organic ligands, are promising systems with engineered properties and attractive for advanced applications. In the present study, after optimization of the reaction conditions highly ordered MCM-41 silica spheres with uniform mesopores were prepared by the pseudomorphic transformation route. The effect of functionality and alkyl chain length of the alkyl ligands during surface modification was probed by using butyl and octylsilanes with two different functionalities. Due to steric hindrance, the longer chains are assumed to bind only on the outer silica surface and near the entrance of the pores, while the shorter chains are also able to bind to the interior mesopore walls. The resulting materials were comprehensively characterized before and after surface modification using nitrogen sorption techniques, XRD, SEM, solid-state NMR spectroscopy and FTIR spectroscopy. From chromatographic test measurements it was found that the separation power primarily depends on surface coverage and alkyl chain length. On the basis of the present data, surface modified mesoporous silica of MCM-41 type are very promising candidates for future chromatographic applications.     

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。     

Gas adsorption in mesoporous micelle-templated silicas: MCM-41, MCM-48, and SBA-15

This paper reports a molecular simulation and experimental study on the adsorption and condensation of simple fluids in mesoporous micelle-templated silicas MCM-41, MCM-48, and SBA-15. MCM-41 is described as a regular cylindrical silica nanopore, while SBA-15 is assumed to be made up of cylindrical nanopores that are connected through lateral channels. The 3D-connected topology of MCM-48 is described using a gyroid periodic minimal surface. Argon adsorption at 77 K is calculated for the three materials using Grand Canonical Monte Carlo simulations. Qualitative comparison with experiments for nitrogen adsorption in mesoporous micelle-templated silicas is made. The adsorption isotherm for SBA-15 resembles that for MCM-41. In particular, capillary condensation and evaporation are not affected by the presence of the connecting lateral channels. In contrast, the argon adsorption isotherm for MCM-48 departs from that for MCM-41 having the same pore size. While condensation in MCM-41 is a one-step process, filling of MCM-48 involves two successive jumps in the adsorbed amounts which correspond to condensation in different domains of the porosity. The condensation pressure for MCM-48 is larger than that for MCM-41. We attribute this result to the morphology of the MCM-48 surface (made up of both concave and convex regions) that differs from that for MCM-41 (concave only). Our results suggest that the pore connectivity affects pore filling when the size of the connections is comparable to that of the nanopores.     

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。     

Physico-chemical characterization of MCM-41 silica spheres made by the pseudomorphic route and grafted with octadecyl chains

This work reports on the first comprehensive characterization of octadecyl (C(18)) modified MCM-41 silica spheres, prepared via the pseudomorphic route, followed by grafting with mono- or trifunctional octadecyl (C(18)) alkyl chains and endcapping with hexamethyldisilazane. Small angle X-ray scattering (SAXS), nitrogen adsorption-desorption and scanning electron microscopy (SEM) measurements were performed to obtain information about the MCM-41 pore structure, surface properties and morphological features. The degree of grafting and cross-linking of the silanes were determined by (29)Si magic angle spinning NMR spectroscopy, while FTIR and (13)C NMR were employed to study the conformational behavior of the surface-immobilized alkyl chains. The SAXS pattern proved the existence of a hexagonal mesopore arrangement for both the ungrafted and the grafted MCM-41 silica spheres. In addition, there is evidence of some long-range distortion in the pore structure. SEM measurements revealed the same morphological features for the parent silica and the MCM-41 silica spheres before and after C(18) grafting. The achieved surface loading for the MCM-41 material is rather low. It was also shown that a substantial amount of the accessible surface silanol groups is endcapped by trimethylsilane which in turn results in a very low surface coverage due to the octadecyl chains. The nitrogen sorption studies provided values for the surface area, total pore volume and pore diameter which are very typical for mesoporous materials. The reduction in surface area and total pore volume upon surface grafting is related to the binding of trimethylsilane in the interior of the pores, while due to the spatial restrictions octadecyl chains are primarily attached near the pore entrance. The experimental FTIR and (13)C NMR data point to a very low conformational order of the C(18) chains which is in accordance with the observed low surface coverage and the resulting spatial freedom for these surface-immobilized alkyl chains.     

MCM-41分子筛的改性、表面结构与吸附性质的研究

用三甲基氯硅烷、二甲基二氯硅烷、甲基三氯硅烷、四氯化硅与甲基碘改性高硅MCM-41中孔分子筛。XRD,^2^9SiMASNMR,^1^3CMASNMR,以及N~2、水与环己烷吸附的表征,显示改性不同程度地改变了分子筛的表面组成与结构,减小孔径,增加孔壁厚度,因而影响吸附行为,减小吸附容量。硅烷化减少了MCM-41的表面硅羟基含量,增加其疏水性。用CH~3I改性使孔径减小1.4nm,而硅羟基含量并未显著减少。硅烷化以及用CH~3I改性可提高MCM-41分子筛的热稳定性。SiCl~4的改性作用相对不明显。样品的水及环己烷吸附容量与其表面硅羟基含量呈现不同的线性关系,揭示高硅MCM-41分子筛表面吸附中心的本性。     

高骨架铝含量Al-MCM-41的合成

制备了不同Al含量的Al-MCM-41试样，其中Si/Al比值最小为3，即最高含铝量x~A~l=0.303。X射线粉末衍射(XRD)分析表明样品具有MCM-41的特征结构，氮气吸附研究表明，样品呈现Ⅳ型吸附等温线，具有孔径分布均一的中孔结构。文中还利用^2^7AlMASNMR研究了试样中Al的化学环境，结果表明，即使在高铝含量的情况下，样品中的铝原子仍以四配位结合在MCM-41的硅骨架上，未能检测出骨架外六配位铝的存在。文中还就Al含量对孔结构的影响以及Al-MCM-41形成机理作了讨论。     

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.     

Surface modification of MCM-41 and its application in DNA adsorption

Three types of MCM-41 absorbents, namely, Al~(3+)–MCM-41, La~(3+)–MCM-41, and Zn~(2+)–MCM-41, were prepared through amine grafting, phosphonate modification, and metal ion chelation and characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, and N2adsorption–desorption analysis. Results revealed that functionalized MCM-41 maintains the original structure of the molecular sieve and that the pore diameter and surface area are reduced compared with those of pure MCM-41. The adsorption behavior of DNA molecules on the surface of the modified molecular sieves was discussed according to the hard-soft acid–base(HSAB) principle. Experimental results showed that DNA purification could be effectively carried out on functionalized MCM-41 and that DNA is easily released by3–4 molL~(-1)NaCl solution. This study could be used as a general platform for future work on DNA adsorption and enrichment.     

以CTMABr和CTMAOH为共模板剂合成MCM-41

采用共模板剂水热合成了MCM-41.分别用X射线粉末衍射(XRD)、固体核磁共振(²⁷AlMASNMR)和N₂吸附等温线技术考察了用该方法和传统方法所制备的Si-MCM-41和Al-MCM-41样品的晶相结构、孔结构以及Al在分子筛中的化学环境.结果表明,用共模板剂方法合成的MCM-41样品,其纯度和孔径均一性显著提高,特别是当样品中Al含量较高时,仍可保证Al原子以四配位结合在MCM-41的硅骨架上.还就采用共模板剂的理论依据进行了讨论.     

Porous Texture and Surface Character of Dehydroxylated and Rehydroxylated MCM-41 Mesoporous Silicas-Analysis of Adsorption Isotherms of Nitrogen Gas and Water Vapor

Four samples of MCM-41 mesoporous silicas whose average pore diameters are 2.4, 2.8, 3.2, and 3.6 nm were prepared using sodium orthosilicate and cationic surfactants of [CH(3)(CH(2))(n)N(CH(3))(3)]X (n=11, 13, 15, 17). These four samples were calcined at 1123 K in vacuo to obtain the dehydroxylated samples, which were further rehydroxylated at 298 K to obtain the rehydroxylated samples. The adsorption isotherms of nitrogen gas (77 K) for the 12 MCM-41 mesoporous silicas are of Type IVc, giving no adsorption hysteresis. On the other hand, the first adsorption isotherms of water vapor (298 K) for the dehydroxylated MCM-41 samples are quite different from those of nitrogen gas, giving the remarkable adsorption hysteresis. The second water isotherms for the rehydroxylated MCM-41 samples are of Type IV, showing slight hysteresis. Using the nitrogen isotherms, the relation between the pore size and carbon chain length of the surfactant has been determined, and the effect of dehydroxylation and rehydroxylation on the porous texture has been examined. Using the first and second water isotherms, the adsorption model of physisorbed waters adsorbed on the surface silanol groups has been proposed. From the pore size distribution curves of nitrogen and water, the presence of constrictions in the cylindrical pores has been predicted. Copyright 2000 Academic Press.     

Investigation of confined ionic liquid in nanostructured materials by a combination of SANS, contrast-matching SANS, and nitrogen adsorption

Small-angle neutron scattering (SANS), contrast-matching SANS, and nitrogen adsorption have been utilized to investigate the confined ionic liquid (IL) [bmim][PF(6)] phase in ordered mesoporous silica MCM-41 and SBA-15. The results suggest that the pores of SBA-15 are completely filled with IL whereas a small fraction of the pore volume, the pore "core", of MCM-41 is empty. The contrast-matching SANS measurements confirm the enhanced solubility of water in IL. In addition, they provide strong evidence that water does not enter the empty pore core of MCM-41, possibly because of the preferred orientation of the IL molecules in the adsorbed layer.     

A density functional theory for Lennard-Jones fluids in cylindrical pores and its applications to adsorption of nitrogen on MCM-41 materials

A density functional theory (DFT) constructed from the modified fundamental-measure theory and the modified Benedict-Webb-Rubin equation of state is presented. The Helmholtz free energy functional due to attractive interaction is expressed as a functional of attractive weighted-density in which the weight function is a mean-field-like type. An obvious advantage of the present theory is that it reproduces accurate bulk properties such as chemical potential, bulk pressure, vapor-liquid interfacial tension, and so forth when compared with molecular simulations and experiments with the same set of molecular parameters. Capabilities of the present DFT are demonstrated by its applicability to adsorption of argon and nitrogen on, respectively, a model cylindrical pore and mesoporous MCM-41 materials. Comparison of the theoretical results of argon in the model cylindrical pore with those from the newly published molecular simulations indicates that the present DFT predicts accurate average densities in the pore, slightly overestimates the pore pressure, and correctly describes the effect of the fluid-pore wall interaction on average densities and pressures in the pore. Application to adsorption of nitrogen on MCM-41 at 77.4 K shows that the present DFT predicts density profiles and adsorption isotherms in good agreement with those from molecular simulations and experiments. In contrast, the hysteresis loop of adsorption calculated from the mean-field theory shifts toward the low pressure region because a low bulk saturated pressure is produced from the mean-field equation of state. The present DFT offers a good way to describe the adsorption isotherms of porous materials as a function of temperature and pressure.     

Adsorption of crystal violet dye from aqueous solution using mesoporous materials synthesized at room temperature

In this work, batch adsorption experiments are carried out for crystal violet dye using mesoporous MCM-41 synthesized at room temperature and sulfate modified MCM-41 prepared by impregnation method using H₂SO₄ as sulfatising agent. The surface characteristics, pore structure, bonding behavior and thermal degradation of both the MCM-41 samples are characterized by nitrogen adsorption/desorption isotherms, X-ray diffraction (XRD) patterns, Fourier transform infrared (FT-IR) spectroscopy and thermo gravimetric analysis (TGA). The adsorption isotherm, kinetics and thermodynamic parameters are investigated for crystal violet (CV) dye using the calcined and sulfated MCM-41. Results are analysed using Langmuir, Freundlich and Redlich-Peterson isotherm models. It is found that the Freundlich model is an appropriate model to explain the adsorption isotherm. The highest adsorption capacity achieved is found to be 3.4×10⁻⁴ mol g⁻¹ for the sulfated MCM-41. The percentage removal of crystal violet dye increases with increase in the pH for both the MCM-41 adsorbents. Kinetics of adsorption is found to follow the second-order rate equation. From the thermodynamic investigation, it is evident that the adsorption is exothermic in nature.     

MCM-41-PAA杂化介孔材料的制备及其对Hg(Ⅱ)离子吸附研究

本文基于介孔材料具有均一可调的孔径结构、高比表面积以及稳定的骨架,利用硅藻土作为硅源,CTAB(十六烷基三甲基溴化铵)为模板剂,掺杂聚丙烯酸,制备了杂化的功能介孔材料MCM-41-PAA,采用FT-IR、XRD、氮气吸脱附、SEM等手段对该介孔材料的组成、结构与形貌进行了表征,并利用所制备的功能吸附剂MCM-41-PAA对Hg(Ⅱ)进行了吸附研究,探讨了吸附剂的用量、Hg(Ⅱ)初始浓度、pH值和吸附时间等因素对Hg(Ⅱ)离子吸附效果的影响。研究表明:当吸附剂的用量、Hg(Ⅱ)浓度、pH和吸附时间分别为0.2g、300mg/L、4和180min时,对应的吸附量达到132mg/g。     

杂原子介孔Co-MCM-41分子筛的制备及其吸附脱氮性能

采用水热法合成了介孔MCM-41和Co-MCM-41分子筛,并利用XRD、FT-IR、低温N₂吸附-脱附和NH₃-TPD等方法对合成的分子筛进行了表征。考察了晶化时间、晶化温度、陈化时间对合成介孔Co-MCM-41分子筛的影响,确定较适宜的合成条件为陈化时间1 h,晶化温度110 ℃,晶化时间2 d。XRD 和FT-IR表征结果说明,Co原子已经进入MCM-41的骨架。MCM-41和Co-MCM-41的平均孔径均为2.82 nm,BET比表面积分别为986.42和 637.69 m²/g,孔容分别为0.762 1和0.537 2 m³/g。NH₃-TPD的表征结果表明,MCM-41和Co-MCM-41的酸性都较弱,但Co-MCM-41的酸性明显强于MCM-41。在此基础上,利用合成的MCM-41和Co-MCM-41吸附脱除氮含量为1 737.35 μg/g的模拟燃料中的喹啉。喹啉分子尺寸的模拟结果为0.711 6 nm × 0.500 2 nm,说明其可以很容易地进入MCM-41和Co-MCM-41的介孔孔道中。Co-MCM-41分子筛的氮脱除率明显高于MCM-41,这是由于其较强的酸性及与喹啉之间的化学吸附,而且,Co-MCM-41吸附脱氮具有较好的再生性能。     

杂原子介孔Ce-MCM-41分子筛的制备及其吸附脱除甲硫醚性能

以十六烷基三甲基溴化铵为模板剂,正硅酸乙酯(TEOS)为硅源,硝酸铈为铈源,采用水热法合成了杂原子介孔分子筛Ce-MCM-41。XRD和FT-IR表征结果表明,当加入的Ce/Si物质的量比小于0.04时合成了规整有序的介孔结构,并将Ce原子引入到MCM-41骨架中。N₂吸附-脱附测试获得MCM-41和Ce-MCM-41(Ce/Si物质的量比为0.04)的平均孔径分别为2.82和2.46 nm,孔容分别为0.762 1和 0.689 4 m³/g,BET比表面积分别为986.42和756.8 m²/g。NH₃-TPD表征结果表明,Ce-MCM-41的酸性要明显强于MCM-41,但两种分子筛的酸性均较弱。利用合成的MCM-41和Ce-MCM-41吸附脱除甲硫醚浓度为58 μg(甲硫醚)/g的甲硫醚/氮气混合气中的甲硫醚。甲硫醚分子尺寸的模拟结果为0.464 8 nm,可以很容易地进入分子筛的介孔孔道中。由于Ce-MCM-41分子筛具有较多的酸量,其硫吸附容量7.52 mg(S)/g明显高于MCM-41的4.57 mg(S)/g。MCM-41和Ce-MCM-41都具有较好的再生性能,再生3次后硫吸附容量仍可恢复到初始容量的80%,分别为3.52和 5.86 mg(S)/g。     

金属有机化学气相沉积法制备SnO2/MCM-41半导体传感器及其性能研究

采用金属有机化学气相沉积(MOCVD)法制备了SnO2/MCM-41半导体传感器, 考察了沉积时间和沉积温度对SnO2/MCM-41半导体传感器的SnO2沉积量、比表面积和孔径的影响; 研究发现, 随着SnO2沉积量的增加, 孔径有规律地下降, 说明SnO2较均匀地沉积在介孔分子筛MCM-41的孔道之中. SnO2/MCM-41半导体传感器对CO和H2具有较高的传感性能, 其传感性能的大小与CO和H2的浓度成正比.     

MCM-41分子筛的合成及129Xe核磁共振的研究

Purely siliceous MCM-41 with a narrow pore-size distribution, different pore size, high surface area was synthesized . As prepared, calcined and catalytically tested MCM-41 materials have been comprehensively characterized by N₂ adsorption/desorption at 77K and ¹²⁹Xe NMR. By adding mesitylene during the synthesis, the pore size of MCM-41 was enlarged to 5.2nm. The chemical shift in ¹²⁹Xe NMR spectroscopy of adsorbed xenon indicates that the MCM-41 is one dimensional pore channels .     

Nitrogen adsorption on silica surfaces of nonporous and mesoporous materials

We present an accurate comparative analysis of N 2 adsorption at 77 K on nonporous silica and the pore wall surface of MCM-41 materials. The analysis shows that in the low-pressure region of N 2 adsorption obeys a peculiar mechanism governed by short-ranged forces, which makes the surface curvature effect on the N 2 adsorption in mesopores nearly negligible. We used this observation to define more exactly compared to the BET technique the specific surface area of the reference adsorption isotherm on nonporous silica basing on XRD data and linear sections of t-plots. Calculation of the capillary evaporation and condensation pressures seems to confirm our previous finding that the capillary condensation pressure corresponds to the equilibrium transition rather than spinodal condensation at least for pore sizes less than 7 nm. It allowed us to provide more reliable pore size distribution (PSD) analysis of mesoporous silica materials. For example, the PSDs of MCM-41 samples do not show artificial peaks in the micropore range that we obtained in our earlier publications.