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.     

Hydrogen bonding of water confined in mesoporous silica MCM-41 and SBA-15 studied by 1H solid-state NMR

The adsorption of water in two mesoporous silica materials with cylindrical pores of uniform diameter, MCM-41 and SBA-15, was studied by 1H MAS (MAS=magic angle spinning) and static solid-state NMR spectroscopy. All observed hydrogen atoms are either surface -SiOH groups or hydrogen-bonded water molecules. Unlike MCM-41, some strongly bound water molecules exist at the inner surfaces of SBA-15 that are assigned to surface defects. At higher filling levels, a further difference between MCM-41 and SBA-15 is observed. Water molecules in MCM-41 exhibit a bimodal line distribution of chemical shifts, with one peak at the position of inner-bulk water, and the second peak at the position of water molecules in fast exchange with surface -SiOH groups. In SBA-15, a single line is observed that shifts continuously as the pore filling is increased. This result is attributed to a different pore-filling mechanism for the two silica materials. In MCM-41, due to its small pore diameter (3.3 nm), pore filling by pore condensation (axial-pore-filling mode) occurs at a low relative pressure, corresponding roughly to a single adsorbed monolayer. For SBA-15, owing to its larger pore diameter (8 nm), a gradual increase in the thickness of the adsorbed layer (radial-pore-filling mode) prevails until pore condensation takes place at a higher level of pore filling.     

Removal of complexed mercury by dithiocarbamate grafted on mesoporous silica

Mesoporous silica (MCM-41) with d ₍₁₀₀₎ interplanar distance of 38 Å was prepared by a room temperature process through low surfactant templation technique. The surface of MCM-41 was functionalized with dithiocarbamate (dtc) ligand, named as MCM-41-dtc and this was characterized by X-ray diffraction, BET surface area, particle size analysis, ²⁹Si MAS NMR spectra and sulphur analysis. The sorption of mercury from 0.1M HCl solution by MCM-41-dtc was studied as a function of pH, [Hg²⁺], time and temperature. The sorption data obtained at various initial concentrations of mercury were fitted into Langmuir adsorption model. Mercury speciation in solution and the sorption capacity measurements indicated possible formation of a 1 : 1 square planar complex in the solid phase. A very rapid sorption of mercury was observed in the initial stages of equilibration, which can be attributed to the large surface area, wide porosity and fine particle size of MCM-41-dtc, facilitating facile accessibility of mercury into the inner pores of the sorbent. The enthalpy change accompanied by the sorption of mercury was found to decrease from 83.7 to 6.2 kJ/mol, when the initial concentration of mercury was increased from 5^.10^-4M to 1.5^.10^-3M.     

Effect of Phosphorylation of Si-MCM-41 and Ti-Si-MCM-41 Mesoporous Molecular Sieves on Their Structure-Adsorption and Acid Properties

Transmission electron microscopy, X-ray diffraction, nitrogen and argon adsorption, thermal analysis, thermoprogrammed ammonia desorption, and ¹H MAS NMR spectroscopy were used to show that phosphorylation by POCl₃ yields MCM-41 silica gel and Ti-MCM-41 titanium-silica gel mesoporous molecular sieves with about 1.1 mmol/g acid sites consisting largely of hydroxyl group protons of supported phosphoric acid. These materials display catalytic activity in the esterification of acetic acid by ethanol.     

New Routes to the Synthesis of Mesoporous Materials and Their Properties

The factors that influence the hydrothermal synthesis of MCM-41 were investigated, and it was found that compared with those from high H₂O/SiO₂ systems(designated MCM-41-A), the products from low H₂O/SiO₂ systems(designated MCM-41-B) exhibited a less-defined X-ray powder diffraction (XRD) pattern with a broader main reflection peak at a lower 2θ diffraction angle. MCM-41-B possesses a smaller surface area but a larger pore size than MCM-41-A. New routes including direct thermal treatment, room-temperature crystallization and microwave heating were developed for the formation of MCM-41, and the properties of the products prepared from these new routes were compared with those of the MCM-41 hydrothermally synthesized. The pore sizes of MCM-41 materials are uniformly distributed with an effective pore diameter that falls into the range of 2-4 nm, where as the products from wet-gel thermal treatment possess two kinds of mesopores:the well-defined smaller pores distributed at 3 nm and the larger one within 8-20 nm. The MAS NMR spectroscopy revealed that after calcination to remove the organic template in Al-containing MCM-41, a small part of the tetrahedrally-coordinated framework aluminum atoms became octahedrally-coordinated and a considerable amount of Si-OH species were generated.     

Melting and freezing of water in cylindrical silica nanopores

Freezing and melting of H(2)O and D(2)O in the cylindrical pores of well-characterized MCM-41 silica materials (pore diameters from 2.5 to 4.4 nm) was studied by differential scanning calorimetry (DSC) and (1)H NMR cryoporometry. Well-resolved DSC melting and freezing peaks were obtained for pore diameters down to 3.0 nm, but not in 2.5 nm pores. The pore size dependence of the melting point depression DeltaT(m) can be represented by the Gibbs-Thomson equation when the existence of a layer of nonfreezing water at the pore walls is taken into account. The DSC measurements also show that the hysteresis connected with the phase transition, and the melting enthalpy of water in the pores, both vanish near a pore diameter D* approximately equal to 2.8 nm. It is concluded that D* represents a lower limit for first-order melting/freezing in the pores. The NMR spin echo measurements show that a transition from low to high mobility of water molecules takes place in all MCM-41 materials, including the one with 2.5 nm pores, but the transition revealed by NMR occurs at a higher temperature than indicated by the DSC melting peaks. The disagreement between the NMR and DSC transition temperatures becomes more pronounced as the pore size decreases. This is attributed to the fact that with decreasing pore size an increasing fraction of the water molecules is situated in the first and second molecular layers next to the pore wall, and these molecules have slower dynamics than the molecules in the core of the pore.     

Preparation, characterization, and condensation of copper tellurolate clusters in the pores of periodic mesoporous silica MCM-41

The copper-tellurolate cluster [(Cu(6)(TePh)(6)(PPh(2)Et)(5)] has been loaded into the pores of MCM-41 by solid-state impregnation techniques. It was found that the best loading conditions are 110 degrees C and 10(-)(3) Torr static vacuum. The resulting material was analyzed by powder X-ray diffraction (PXRD), nitrogen adsorption isotherms, thermogravimetric analysis (TGA), (31)P CP MAS NMR spectroscopy, and TEM. It was observed that loading is accompanied by loss of the phosphine shell, with retention of the copper-tellurium core. Condensation of the impregnated material may proceed thermally or photochemically. Thermal condensation results in the formation of Cu(2)Te nanoparticles as demonstrated by PXRD, and TEM data suggests that the process has taken place inside the pores of MCM-41. Photochemical condensation yields larger metal-chalcogen clusters in the pores as suggested by the result of UV-vis diffuse reflectance spectroscopy and TEM measurements.     

Solid-state NMR study of MCM-41-type mesoporous silica nanoparticles

A systematic study of the surface of MCM-41-type mesoporous silica nanoparticles prepared under low surfactant concentration was carried out using high-resolution solid-state nuclear magnetic resonance spectroscopy. The structures and concentrations of various species present during dehydration and rehydration of mesoporous silicas between -25 and 500 degrees C were detailed by employing one-dimensional and two-dimensional (1)H, (13)C, and (29)Si NMR, including (1)H signal intensity measurements, (1)H-(1)H homonuclear correlation experiments (double quantum, exchange, and RFDR), and (1)H-(29)Si heteronuclear correlation NMR. These experiments employed high MAS rates of up to 45 kHz. The study shows that the surfactant (CTAB) was almost completely removed by acid extraction. The residual molecules assumed prone positions along the pores, with the tailgroup being most mobile. The weakly adsorbed water was hydrogen bonded to the silanol groups, all of which were involved in such bonds under ambient humidity. Specific structures involving water and silanol groups were proposed for various stages of thermal treatment, which included dehydration, dehydroxylation, and subsequent rehydration.     

MCM-48介孔分子筛的高压合成

采用正硅酸乙酯（TEOS）作硅源,十六烷基三甲基溴化铵（CTAB）为模板剂,在高压 （约7 MPa）和373 K下合成了MCM-48介孔分子筛.用XRD、氮气吸附及29Si MAS NMR对样品 进行了表征.与常压合成的相比,高压下合成的MCM-48具有更高的热稳定性和水热稳定性.2 9Si MAS NMR结果表明,高压有利于分子筛孔壁的聚合,导致分子筛结构更加完善,从而使 其具有更高的稳定性.     

A study of freezing-melting hysteresis of water in different porous materials. Part II: surfactant-templated silicas

The freezing-melting hysteresis of water in mesoporous silicas MCM-48, MCM-41 and SBA-16 has been studied by NMR cryoporometry. The hysteresis in MCM-48 was found to exhibit nearly parallel branches, matching type H1 hysteresis that had been observed earlier in controlled pore glass. The same type of hysteresis is observed in two of three different-sized MCM-41 under study (a pore diameter of 3.6 and 3 nm), superimposed with a secondary, extremely broad, type H3 hysteresis. No hysteresis was found in the smallest MCM-41 with a pore diameter < 3 nm. Finally, water in SBA-16 exhibits type H2 hysteresis with the freezing branch being essentially steeper than the melting one, which is attributed to a pore blockage upon freezing, similar to what we observed earlier in Vycor porous glass. The data were analyzed using the model of curvature-dependent metastability of a solid phase upon melting; the validity of this model has been discussed.     

Molecular organization of hydrophobic molecules and co-adsorbed water in SBA-15 ordered mesoporous silica material

The purpose of this study was to improve our understanding of the molecular organization of hydrophobic guest molecules in the presence of co-adsorbed water inside SBA-15 ordered mesoporous silica material. Understanding this adsorption competition is essential in the development of applications of controlled adsorption and desorption. The poorly water soluble drug compound itraconazole and the fluorescent probe Nile red were selected for the study. The interaction between itraconazole and SBA-15 was investigated using FT-IR, (1)H MAS NMR and (29)Si MAS NMR spectroscopy, by determination of adsorption isotherms and release kinetics in simulated gastric fluid. The distribution and migration of the hydrophobic fluorescent probe Nile red was visualized in situ using confocal fluorescence microscopy. For both molecules, there was a pronounced influence of the co-adsorbed water on adsorption, hydrophobic aggregation and migration in SBA-15 pores. These insights contribute to the development of practical methods for loading ordered mesoporous silica materials with hydrophobic molecules.     

Amine functionalized MCM-41: An active and reusable catalyst for Knoevenagel condensation reaction

This paper reports preparation, characterization of amine modified mesoporous crystalline MCM-41 and its application in Knoevenagel condensation reaction. Amine modified MCM-41 was prepared by co-condensation and post-synthesis methods. The samples were characterized by X-ray powder diffraction, Fourier-transfer infrared spectroscopy (FTIR), thermo gravimetric analysis (TGA), differential thermal analysis (DTA), scanning electron micrograph (SEM), ²⁹Si magic-angle spinning (MAS), nuclear magnetic resonance (NMR), diffuse reflectance spectra (DRS), nitrogen adsorption–desorption and CHN analysis. X-ray diffraction patterns indicate that the modified materials retain the standard MCM-41 structure. SEM study exhibits that the arrangement of particles for 12.8% amine modified MCM-41 is well ordered and spherical in nature. CHN analysis supports that complete hydrolysis of ethoxy groups take place in 12.8% amine modified sample. From the NMR study it is confirmed that the surface coverage is 40% in 12.8% amine modified sample. The base catalytic activity of hybrid MCM-41 materials such as amine (post-synthesis and co-condensation methods) and surfactant functionalized materials for condensation reaction between benzaldehyde and diethyl malonate in solvent free, room temperature synthesis of cinnamic acid was evaluated and correlated with the surface and textural properties. Sample containing 12.8 wt% amine loaded by co-condensation method showed highest malonic ester conversion (92%) and selectivity (98%) for cinnamic acid.     

Unexpected Factors Affecting the Kinetics of Guest Molecule Release from Investigation of Binary Chemical Systems Trapped in a Single Void of Mesoporous Silica Particles

In this work, we present results for loading of well-defined binary systems (cocrystal, solid solution) and untreated materials (physical mixtures) into the voids of MCM-41 mesoporous silica particles employing three different filling methods. The applied techniques belong to the group of “wet methods” (diffusion supported loading – DiSupLo ) and “solvent-free methods” (mechanical ball-mill loading – MeLo , thermal solvent free – TSF ). As probes for testing the guest1-guest2 interactions inside the MCM-41 pores we employed the benzoic acid ( BA ), perfluorobenzoic acid ( PFBA ), and 4-fluorobenzoic acid ( 4-FBA ). The guests intermolecular contacts and phase changes were monitored employing magic angle spinning (MAS) NMR Spectroscopy techniques and powder X-ray diffraction (PXRD). Since mesoporous silica materials are commonly used in drug delivery system research, special attention has been paid to factors affecting guest release kinetics. It has been proven that not only the content and composition of binary systems, but also the loading technique have a strong impact on the rate of guests release. Innovative methods of visualizing differences in release kinetics are presented.     

Quantitative Distinction between Noble Metals Located in Mesopores from Those on the External Surface

We compare three methods for quantitatively distinguishing the location of noble metal (NM) particles in mesopores from those found on the external support surface. MCM-41 and SBA-15 with NM located in mesopores or on the external surface were prepared and characterized by TEM. ³¹P MAS NMR spectroscopy was used to quantify arylphosphines in complexes with NM. Phosphine/NM ratios drop from 2.0 to 0.2 when increasing the probe diameter from 1.08 to 1.54 nm. The reaction between NM and triphenylphosphine (TPP) within 3.0 nm MCM-41 pores takes due to confinement effects multiple weeks. In contrast, external NM react with TPP instantly. A promising method is filling the pores by using the pore volume impregnation technique with tetraethylorthosilicate (TEOS). TPP loading revealed that 66 % of NMs are located on the external surface of MCM-41. The pore filling method can be used in association with any probe molecule, also for the quantification of acid sites.     

Solid State NMR Characterisation of Encapsulated Molecules in Mesoporous Silica

Ibuprofen molecules have been encapsulated in mesoporous MCM-41 type-silica functionalised or not by amino groups. They have been characterised by ¹³C and ¹H solid state NMR spectroscopy. The ¹³C MAS single pulse or cross polarization NMR spectra, as well as the ¹H MAS NMR spectra demonstrate an extremely high mobility of the ibuprofen molecules when the matrix is not functionalised. On the contrary, when the silica matrix is functionalized by amino groups, the ¹³C NMR response shows less mobility suggesting the existence of interactions between the amino groups and the carboxylic groups. Benzoic acid as well as benzamide have also been encapsulated and their NMR responses compared to that of ibuprofen.     

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 .     

Synthesis, structure, and acidic properties of MCM-41 functionalized with phosphate and titanium phosphate groups

Mesoporous molecular sieves Si-MCM-41 (purely siliceous) and Ti-MCM-41 (partly covered with a surface layer of TiO2) were functionalized with phosphate groups by treatment with POCl3 (denoted -MCM-41(P)and Ti-MCM-41(P), respectively). With the use of TEM, X-ray diffraction, and N2 adsorption, it was shown that the initial hexagonal structure, the high specific surface area, and porosity are retained in the functionalized materials but are not as good as in the starting materials. 1H MAS NMR and 31P MAS NMR revealed that the surface of Si-MCM-41(P) consists of silicon phosphate and pyrophosphate species. That of Ti-MCM-41(P) additionally contains titanium dihydro-, hydro-, and pyrophosphate species, the latter being predominant. TPD of adsorbed ammonia for Si-MCM-41(P) and Ti-MCM-41(P) showed that functionalization leads to the creation of moderate and strong acid sites. A combination of mesoporous structure with acidic properties makes the MCM-41 functionalized with phosphate groups promising for use as solid acid catalysts.     

The Synthesis and Characteristics of Vanadoaluminosilicate MCM-41 Mesoporous Molecular Sieves

A series of vanadoaluminosilicate MCM-41 mesoporous molecular sieves with various compositions have been hydrothermally synthesized. Hexadecyltrimethylammonium bromide was used as a surfactant in the synthesis. The samples were characterized with nitrogen sorption, X-ray diffraction, differential thermal analysis, thermogravimetric analysis, Fourier transform-Infrared spectroscopy, UV-visible spectroscopy, scanning electron microscopy, transmission electron microscopy, and solid state NMR. The solid products had the MCM-41 structure and contained only atomically dispersed vanadium and aluminum consistent with framework vanadium and aluminum. The samples were hydrophobic and contained large amount of surfactant in the as-synthesized samples. The surfactant could be removed upon calcination at 450°C. N₂ sorption measurements and TEM demonstrate the high mesoporosity of [V, Al]-MCM-41. The incorporation of vanadium and aluminum into MCM-41 decreased the surface area to some extent. The morphologies of all the samples were the agglomerate of plates. ²⁹Si MAS NMR shows that the pore wall is amorphous. ²⁷Al MAS NMR shows that all of aluminum species were tetrahedrally coordinated even after calcination at 550°C.     

基于MCM-41模板制备β-MnO_2纳米纤维及其催化降解亚甲基蓝染料

以介孔氧化硅材料MCM-41为模板,硝酸锰为锰源,通过浸渍、450℃焙烧4 h得到Mn-MCM-41,用NaOH溶液溶解除去氧化硅模板得到锰氧化物,采用XRD,HRTEM和N2吸附-脱附等测试技术对产物进行了表征.结果表明,所得产物是纯相的β-MnO2纳米纤维,直径小于3 nm.纳米纤维之间有序排列组成类似MCM-41模板的介孔结构,其比表面积达到136.5 m2/g.将所制备的β-MnO2纳米纤维用于催化过氧化氢氧化分解质量浓度为60 mg/L的亚甲基蓝(MB)模拟染料废水,经100 min反应后,亚甲基蓝水溶液脱色率达到97.59%.所制备的催化剂对降解处理高浓度亚甲基蓝溶液,具有降解脱色率高和反应速度快等优点.     

Multinuclear NMR investigations of the oxygen, water, and hydroxyl environments in sodium hexaniobate

Solid-state (1)H, (17)O MAS NMR, (1)H-(93)Nb TRAPDOR NMR, and (1)H double quantum 2D MAS NMR experiments were used to characterize the oxygen, water, and hydroxyl environments in the monoprotonated hexaniobate material, Na(7)[HNb(6)O(19)].15H(2)O. These solid-state NMR experiments demonstrate that the proton is located on the bridging oxygen of the [Nb(6)O(19)](8-) cluster. The solid-state NMR results also show that the NbOH protons are spatially isolated from similar protons, but undergo proton exchange with the water species located in the crystal lattice. On the basis of double quantum (1)H MAS NMR measurements, it was determined that the water species in the crystal lattice have restricted motional dynamics. Two-dimensional (1)H-(17)O MAS NMR correlation experiments show that these restricted waters are preferentially associated with the bridging oxygen. Solution (17)O NMR experiments show that the hydroxyl proton is also attached to the bridging oxygen for the compound in solution. In addition, solution (17)O NMR kinetic studies for the hexaniobate allowed the measurement of relative oxygen exchange rates between the bridging, terminal, and hydroxyl oxygen and the oxygen of the solvent as a function of pH and temperature. These NMR experiments are some of the first investigations into the proton location, oxygen and proton exchange processes, and water dynamics for a base stable polyoxoniobate material, and they provide insight into the chemistry and reactivity of these materials.