Dynamics of different molecules adsorbed in porous media

We present in this paper a comparative study on the dynamics of benzene, cyclohexane, and methanol molecules, confined in the pores of MCM-41 molecular sieve and HZSM-5 zeolite. The quasi-elastic neutron scattering (QENS) measurements revealed that the physical state of these adsorbed molecules depended not only on the structural characteristics of the host matrix but also on the chemical properties, such as dipole moment, of the guest molecules. Thus, while no motion was observed in the time-scale of 10⁻¹⁰−10⁻¹² s in the case of methanol, the larger size benzene and cyclohexane molecules are found to perform six-fold and three-fold jump rotation, respectively, when adsorbed inside the cages of HZSM-5 at room temperature. At the same time, all the three molecules are found to undergo a translational motion inside the pores of MCM-41 molecular sieves, the value of diffusion constant being the lowest in case of methanol because of its higher polarity. Translationl motion of the guest molecules inside the pores of MCM-41 can be satisfactorily described by Chudley-Eliott fixed jump length diffusion and accordingly the residence time, jump length and diffusion constant are estimated.     

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.     

Adsorptive removal and kinetics of methylene blue from aqueous solution using NiO/MCM-41 composite

Highly ordered mesoporous material MCM-41 was synthesized from tetraethylorthosilicate (TEOS) as Si source and cetyltrimethylammonium bromide (CTAB) as template. Well-dispersed NiO nanoparticles were introduced into the highly ordered mesoporous MCM-41 by chemical precipitation method to prepare the highly ordered mesoporous NiO/MCM-41 composite. X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), and nitrogen adsorption–desorption measurement were used to examine the morphology and the microstructure of the obtained composite. The morphological study clearly revealed that the synthesized NiO/MCM-41 composite has a highly ordered mesoporous structure with a specific surface area of 435.9 m² g⁻¹. A possible formation mechanism is preliminary proposed for the formation of the nanostructure. The adsorption performance of NiO/MCM-41 composite as an adsorbent was further demonstrated in the removal azo dyes of methyl orange (MO), Congo red (CR), methylene blue (MB) and rhodaming B (RB) under visible light irradiation and dark, respectively. The kinetics and mechanism of removal methylene blue were studied. The results show that NiO/MCM-41 composite has a good removal capacity for organic pollutant MB from the wastewater under the room temperature. Compared with MCM-41 and NiO nanoparticles, 54.2% and 100% higher removal rate were obtained by the NiO/MCM-41 composite.     

Optical properties of (nanometer MCM-41)-(malachite green) composite materials

Nanosized materials loaded with organic dyes are of interest with respect to novel optical applications. The optical properties of malachite green (MG) in MCM-41 are considerably influenced by the limited nanoporous channels of nanometer MCM-41. Nanometer MCM-41 was synthesized by tetraethyl orthosilicate (TEOS) as the source of silica and cetyltrimethylammonium bromide (CTMAB) as the template. The liquid-phase grafting method has been employed for incorporation of the malachite green molecules into the channels of nanometer MCM-41. A comparative study has been carried out on the adsorption of the malachite green into modified MCM-41 and unmodified MCM-41. The modified MCM-41 was synthesized using a silylation reagent, trimethychlorosilane (TMSCl), which functionalized the surface of nanometer MCM-41 for proper host-guest interaction. The prepared (nanometer MCM-41)-MG samples have been studied by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, low-temperature nitrogen adsorption-desorption technique at 77 K, Raman spectra and luminescence studies. In the prepared (nanometer MCM-41)-MG composite materials, the frameworks of the host molecular sieve were kept intact and the MG located inside the pores of MCM-41. Compared with the MG, it is found that the prepared composite materials perform a considerable luminescence. The excitation and emission spectra of MG in both modified MCM-41 and unmodified MCM-41 were examined to explore the structural effects on the optical properties of MG. The results of luminescence spectra indicated that the MG molecules existed in monomer form within MCM-41. However, the luminescent intensity of MG incorporated in the modified MCM-41 are higher than that of MG encapsulated in unmodified MCM-41, which may be due to the anchored methyl groups on the channels of the nanometer MCM-41 and the strong host-guest interactions. The steric effect from the pore size of the host materials is significant. Raman spectra firmly demonstrated the stable form obtained after the MG incorporation into the nanometer MCM-41. Therefore, nanometer MCM-41 appears to have a good potential for its use as a support for dyes and the (nanometer MCM-41)-MG composite materials may give a wide optical application.     

Microwave synthesis and textural property of europium substituted mesoporous molecular sieves

Using cetyltrimethyl ammonium bromide (CTAB) as the template and sodium silicate as the silicon source, the MCM-41 mesoporous molecular sieves with Eu incorporated in the framework were synthesized under microwave irradiation condition and the influence of the Si/Eu molar ratio on the crystalline structure, textural properties and the long-range ordering of the resulting sample was investigated by various physicochemical techniques such as X-ray diffraction (XRD), transmission electron microscope (TEM), diffuse reflectance ultraviolet-visible spectroscopy (UV-vis), thermal gravimetric-differential scanning calorimeter (TG-DSC) and N₂ physical adsorption. The results of N₂ adsorption and XRD reveal that the synthesized sample has the ordered hexagonal mesoporous structure. UV-vis spectra provide the strong evidences that most of europium ions were incorporated into the framework of the MCM-41 sample. The crystalline structure, textural properties and mesoporous ordering of the resultant mesoporous materials are related to the amount of europium incorporation. Small amount europium incorporated into the silica-based MCM-41 does not strongly modify the structure of mesoporous molecular sieve. An increase of the Eu content in sample led to reduction of the specific surface area and the deterioration of the long-range ordering.     

荧光光谱法研究左氧氟沙星与MCM-41的相互作用

采用微波辅助水热法制备介孔分子筛MCM-41,并用浸渍法将左氧氟沙星(LVFX)组装在MCM-41均一的六方形孔道中,制备出新型载药复合物LVFX/MCM-41。用粉末X射线衍射(XRD)、低温氮吸附、傅里叶变换红外光谱(FTIR)及差热-热重(TGA-DTA)分析对MCM-41以及LVFX/ MCM-41复合物进行表征,合成的介孔分子筛MCM-41的孔径为2.382 nm,比表面积为1 015 m2·g-1。对MCM-41、LVFX/MCM-41、LVFX(固态)及LVFX(溶液)的荧光光谱研究结果显示,LVFX/MCM-41的荧光光谱比组装前发生明显红移,表明MCM-41孔道内表面的羟基和LVFX形成氢键,羟基上的电子云向LVFX分子上的吸电子基团转移;同时MCM-41和LVFX之间形成新环,使电子云能在更大的环上移动,药物分子的共轭体系扩大,荧光光谱峰红移。MCM-41与左氧氟沙星之间强的相互作用为研发以MCM-41为载体的新型释药系统提供了理论依据。     

A simple and fast sonication procedure to remove surfactant templates from mesoporous MCM-41

We demonstrate a sonication procedure for the removal of structure-directing micellar templates from mesoporous MCM-41. The method uses a 28 KHz ultrasound in an alcoholic solvent for disrupting micellar aggregation of the surfactant molecules, cetyltrimethylammonium bromide, which have filled the pores of the as-synthesized MCM-41. The majority (93%) of the surfactant molecules are removed out from the powder MCM-41 within a 15 min one-step sonication at a moderate temperature of 40°C. The structural and textural characterization techniques reveal that the resulted surfactant-free MCM-41 exhibits higher features compared to that of those obtained using the conventional calcinations approaches. The surfactant molecules are released into alcohol and can be recovered for reuse. This study provides an easy, cost-effective, mild and useful method for template removal from mesoporous materials at conventional conditions.     

Quantitative characterization of nanoparticle agglomeration within biological media

The electronics industry is one of the world??s fastest growing manufacturing industries. However, e-waste has become a serious pollution problem. This study reports the recovery of e-waste for preparing valuable MCM-48 and ordered mesoporous carbon for the first time. Specifically, this study adopts an alkali-extracted method to obtain sodium silicate precursors from electronic packaging resin ash. The influence of synthesis variables such as gelation pH, neutral/cationic surfactant ratio, hydrothermal treatment temperature, and calcination temperature on the mesophase of MCM-48 materials is investigated. Experimental results confirm that well-ordered cubic MCM-48 materials were synthesized in strongly acidic and strongly basic media. The resulting mesoporous silica had a high surface area of 1,317?m²/g, mean pore size of about 3.0?nm, and a high purity of 99.87?wt%. Ordered mesoporous carbon with high surface area (1,715?m²/g) and uniform pore size of CMK-1 type was successfully prepared by impregnating MCM-48 template using the resin waste. The carbon structure was sensitive to the sulfuric acid concentration and carbonization temperature. Converting e-waste into MCM-48 materials not only eliminates the disposal problem of e-waste, but also transforms industrial waste into a useful nanomaterial.     

Synthesis of multi-wall carbon nanotubes by the pyrolysis of ethanol on Fe/MCM-41 mesoporous molecular sieves

Ordered hexagonal arrangement MCM-41 mesoporous molecular sieves were synthesized by the traditional hydrothermal method, and Fe-loaded MCM-41 mesoporous molecular sieves (Fe/MCM-41) were prepared by the wet impregnation method. Their mesoporous structures were testified by X-ray diffraction (XRD) and the N₂ physical adsorption technique. Carbon nanotubes (CNTs) were synthesized by the chemical vapor deposition (CVD) method via the pyrolysis of ethanol at atmospheric pressure using Fe/MCM-41 as a catalytic template. The effect of different reaction temperatures ranging from 600 to 800 ^°C on the formation of CNTs was investigated. The resulting carbon materials were characterized by various physicochemical techniques such as transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. The results show that multi-wall carbon nanotubes (MWCNTs) with an internal diameter of ca. 7.7 nm and an external diameter of ca. 16.9 nm were successfully obtained by the pyrolysis of ethanol at 800 ^°C utilizing Fe/MCM-41 as a catalytic template.     

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.     

Phase transitions in KNO3 embedded in MCM-41 films with regular nanopores

This paper reports on a comparative study of phase transitions in nanocomposites made up of KNO₃ embedded in 10-μm-thick MCM-41 films with unidirectional pores 4.0 nm in size on an aluminum substrate and of nanocomposites prepared in the form of potassium-nitrate-filled pressed MCM-41 powders with 3.7-nm pores. The temperature dependences of linear permittivity and the amplitude of third harmonic generation have been measured under heating and cooling. The structural transition from phase II to phase I shifts under heating relative to that occurring in bulk KNO₃ toward lower temperatures for potassium nitrate in the film and toward higher temperatures for the pressed MCM-41-based nanocomposite. A significant difference has been observed also within the region of existence of ferroelectric phase III. The data obtained suggest that the shifts of phase transition temperatures observed in the conditions of nanoconfinement are influenced markedly not only by pore size and geometry but also by other factors.     

Does confined water exhibit a fragile-to-strong transition?

The dynamics of supercooled confined water has recently been shown to have a pronounced, apparent fragile-to-strong transition (FST). Here we use broadband dielectric spectroscopy (10^-2–10⁹ Hz) to study the dynamics of water confined in silica matrices MCM-41 C10 and C18, with pore diameter of 21.4 and 36.1 ?, respectively. The local dynamics of water molecules and the dynamics of the hydroxyl groups on the inner wall of the pores are followed up to over 240 K. We argue that the reported FST for confined water is due to the vanishing of the cooperative α relaxation, which implies that it should not be interpreted as a true FST.     

Template synthesis and characterisation of MCM-41 mesoporous molecular sieves containing various transition metal elements—TME (Cu,Fe, Nb,V, Mo)

A series of metallosilicates (transition metal elements—TME) MCM-41 (TME=Fe, Cu, Nb, V, Mo) mesoporous molecular sieves with variable Si/TME ratios have been synthesized and characterised by low temperature N₂ adsorption/desorption, XRD, XPS, H₂-TPR, FTIR combined with NO+NO₂ adsorption, diffuse reflectance UV–Vis spectroscopy, and ESR study. All the materials exhibit hexagonal arrangement of uniform mesopores (with exception of CuMCM-41). Defect holes amid the nanochanels besides well-ordered mesopores characterise mainly Fe-containing materials, in which the highest TME loading was reached. Similar but smaller defects take place in NbMCM-41. The amount of TME included into MCM-41 structure under the preparation conditions used in this work changes in the order: Fe>Nb>Cu≫V≫Mo. This sequence is not related to the oxidation state of metals which was estimated in calcined materials as Fe³⁺, Nb⁵⁺, Cu²⁺, V⁵⁺, Mo⁶⁺. It does not also correlate with cation sizes in a simple way. The possibility of forming tetrahedral coordination seems to limit the TME incorporation into the MCM-41 skeleton if free metal cations are used in the gel (Cu²⁺, Fe³⁺, Nb⁵⁺). Al in the gel makes the isomorphously substitution of silicon by copper easier, but part of Cu occupies extra framework cationic positions in the final material.     

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.     

Synthesis and characterization of ionic liquid-functionalized alumino-silicate MCM-41 hybrid mesoporous materials

Ionic liquid-functionalized alumino-silicate MCM-41 hybrid mesoporous materials have been synthesized with two-step approach, by means of in situ skeleton doping with aluminium and post surface grafting with N-methylimidazole ionic liquid groups. The samples were characterized by X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), N₂ adsorption-desorption, Fourier transform infrared (FTIR) spectra, ²⁷Al and ¹³C MAS NMR spectra and temperature-programmed desorption (TPD) of NH₃. The results indicated that the bifunctionalized MCM-41 possessed ordered mesostructure. Aluminium was efficiently introduced into the framework of the mesostructure, generating Lewis and Brönsted acid sites. N-methylimidazole ionic liquid groups were covalently grafted onto the surface of mesoporous materials. The as-synthesized bifunctional MCM-41 showed good catalytic performance in the coupling reaction of CO₂ and propylene oxide.     

CP MAS Kinetics Study of Ionic Liquids Confined in Mesoporous Silica: Convergence of Non-Classical and Classical Spin Coupling Models

The high data point density measurements of ¹H→¹¹B cross-polarization (CP) kinetics upon magic-angle spinning (MAS) of [bmim][BF₄] confined in mesoporous SBA-15 and MCM-41 were carried out. The complex shaped ¹¹B CP MAS signals were observed in both silica and decomposed into two Lorentz components. This points towards the possibility of bimodal distribution of [bmim][BF₄] in the studied confinements. The convergence of classical and non-classical spin coupling models was deduced processing CP kinetic curves. A good fit of the theoretical curves to the experimental data was achieved using both models without any non-random deviations between theory and experiment to appear. The convergence of spin coupling models was discussed in terms of relatively high mobility of BF₄ ^? anion respect to the cation and the dynamics of anions in pores. These factors delete the borders between spin clusters. The spin diffusion along the pore surfaces in MCM-41 is more than twice faster than in SBA-15.     

Temperature and hydration dependence of proton MAS NMR spectra in MCM-41: Model based on motion induced chemical shift averaging

The proton MAS NMR spectra in MCM-41 at low hydration levels (less than hydration amounting to one water molecule per surface hydroxyl group) show complex proton resonance peak structures, with hydroxyl proton resonances seen in dry MCM-41 disappearing as water is introduced into the pores and new peaks appearing, representing water and hydrated silanol groups. Surface hydroxyl group–water molecule chemical exchange and chemical shift averaging brought about by a water molecule visiting different surface hydrogen bonding sites have been proposed as possible causes for the observed spectral changes. In this report a simple model based on chemical shift averaging, due to the making and breaking of hydrogen bonds as water molecules move on the MCM-41 surface, is shown to fully reproduce the NMR spectra, both as a function of hydration and temperature. Surface proton–water proton chemical exchange is not required in this model at low hydration levels.     

Impact of nanoconfinement on the diisopropylammonium chloride (C6H16ClN) organic ferroelectric

The dielectric studies of diisopropylammonium chloride (DIPAC) nanoparticles embedded into opal and MCM-41 silica matrices are presented. It is shown that the ferroelectric phase transition shifts to low temperatures and broadens for DIPAC within the opal pores compared to bulk. The thermal hysteresis of the transition increases under opal nanoconfinement. No anomalies of the permittivity relevant to the ferroelectric transition are observed for DIPAC within the MCM-41 molecular sieves likely due to formation of the amorphous phase.     

Free volumes evolution during desorption of n-heptane from silica with regular pore geometry. Positron annihilation study

The mechanism of n-heptane evaporation from mesoporous MCM-41 silica was investigated by positron annihilation lifetime spectroscopy (PALS). Experiments were performed in situ during desorption of hydrocarbon under stepwise lowered pressure. Desorption is a quasicontinuous process that corresponds to emptying various types of free volumes in the silica/n-heptane system, including mesopores. Insight into the kinetics of desorption was derived from the changes of PALS spectra associated with succeeding stages of pore emptying. For the investigated system emptying of pores and formation of bubbles in the n-heptane condensate present in regular pores were found to take place at various bulk saturating vapour pressure of the fluid.     

Characterization of mesoporous VOx/MCM-41 composite materials obtained via post-synthesis impregnation

Spherical-particle MCM-41 was synthesized at room temperature, and, then, impregnated with aqueous solutions of NH₄VO₃ to produce variously loaded VO_x/MCM-41 composite materials. Bulk and surface properties of the materials thus produced were characterized by means of X-ray powder diffractometry (XRD), infrared spectroscopy (FTIR), N₂ sorptiometry and X-ray photoelectron spectroscopy (XPS). Results obtained indicated that subsequent calcination at 550 °C (for 2 h) of the blank and impregnated MCM-41 particles, results in materials assuming the same bulk structure of MCM-41, and exposing uniformly mesporous, high area surfaces (P_w = 2.0-2.3 nm; 974-829 m²/g), except for the material obtained at 20 wt%-V₂O₅ that was shown to suffer a considerable loss on surface area (down to 503 m²/g). XPS results implied that the immobilization of the VO_x species occurs via interaction with surface OH/H₂O groups of MCM-41, leading to the formation of vanadate (VO₃⁻) surface species, as well as minor V-O-Si and V₂O₅-like species. However, in all cases, the vanadium sites remained pentavalent and exposed on the surface.