Channels Of MCM-41 Research Articles

Synthesis and characterization of Cu(II)-Adenine-MCM-41 as stable and efficient mesoporous catalyst for the synthesis of 5-substituted 1H-tetrazoles and 1H-indazolo [1,2-b]phthalazine-triones

A simple and efficient protocol has been developed for the synthesis and characterization of new Cu(II) complex supported into MCM-41 channels modified with adenine (Cu(II)-Adenine-MCM-41) as a reusable and heterogeneous catalyst for the synthesis of 5-substituted 1H-tetrazoles and 1H-indazolo [1,2-b]phthalazine-triones. The key advantages of this method are easy work-up, short reaction times, pure product formation with good to excellent yields and reusability of the catalyst. This catalyst was characterized by TEM, SEM, XRD, TGA, EDS, AAS, BET method and FT-IR spectroscopy.

Creating an Optimal Microenvironment within Mesoporous Silica MCM-41 for Capture of Tobacco-Specific Nitrosamines in Solution.

To meet the requirement of capturing tobacco-specific nitrosamines (TSNA) for environment protection, a unique microenvironment was carefully created inside the channels of mesoporous silica MCM-41. In situ carbonization of template micelles at 923 K, combined with the excess aluminum used in one-pot synthesis of MCM-41, is adopted to tailor the tortuosity of mecsoporous channels, while loaded metal oxides (5 wt %) and the Al component in the framework are employed to exert the necessary electrostatic interaction toward the target carcinogens TSNA in solution. The elaborated microenvironment created in mesoporous sorbents was characterized with XRD, N2 adsorption-desorption, TEM, XPS, and TG-DSC methods. Various solutions of Burley- and Virginia-type tobaccos were used to assess the adsorption performance of new mesoporous sorbents, and the influence of the solid-to-liquid ratio, adsorption time, and loading amount of CuO on the adsorption was carefully examined. The representative sample 5%Cu/AM-10c could capture 27.2% of TSNA in Burley tobacco solution, and its capacity reached 0.3 mg g-1 in Snus tobacco extract solution, offering a promising candidate for the protection of the environment and public health.

Two-dimensional porous SiO2 nanomesh supported high dispersed Ni nanoparticles for CO methanation

Two-dimensional (2D) porous SiO2 nanomesh obtained from mixed acid etching of vermiculite (VMT) was successfully used as a catalyst support for CO methanation. Compared with three-dimensional (3D) MCM-41, 2D VMT-SiO2 provided a superior position for implantation of NiO species. Although NiO/VMT-SiO2 has a total loading of 10wt.% NiO as well as the NiO/MCM-41, the NiO particles on VMT-SiO2 were observed to show a better dispersion as the of 3D MCM-41 channels were easily blocked by large NiO particles. Moreover, on the basis of the H2 temperature-programming reduction results, NiO particles on VMT-SiO2 were more easily reduced than those on MCM-41. These characteristics indicated that NiO/VMT-SiO2 was significantly superior to NiO/MCM-41. The as-obtained NiO/VMT-SiO2 exhibited a CO conversion of 85.9%, CH4 selectivity of 78% at 450°C, 0.1151s−1 turn over frequency at 320°C and a gas hourly space velocity of 20745ml/g/h. All of which were much better than those measured for NiO/MCM-41. We believed that 2D VMT-SiO2 as a new catalyst support open a prospect application of catalyst.

Mean stop paths and diffusion regimes of molecules in one-dimensional zeolite channels

Molecular dynamics methods were used to simulate the diffusion of noble gas molecules, Ne, Ar, and Kr, in the one-dimensional zeolite channels of VET, AFI, VFI, and MCM-41. The gaseous, Knudsen, and configurational diffusion regimes were concisely described by a diffusion regime map. The boundary between the gaseous and the Knudsen diffusion regimes was determined by the probability, Pmm, of bimolecular collision, which was a function of both diameter ratio (dc/dm) and volume density ρV. For a molecule in a channel surrounded by other molecules, the concept of mean stop path (MSP) was proposed as the average distance between collisions, either with another molecule or with the channel wall. The MSP depends on both the mean free path and the difference in channel and molecular diameters (dc-dm). A theoretical model for calculating MSP of noble gas molecules in one-dimensional zeolite channels was derived based on the additivity of diffusion resistances. The mean stop path can describe the characteristics of molecular motion in a zeolite channel in the same way that mean free path does in an infinite space.

Catalytic routes to fuels from C1 and oxygenate molecules.

This account illustrates concepts in chemical kinetics underpinned by the formalism of transition state theory using catalytic processes that enable the synthesis of molecules suitable as fuels from C1 and oxygenate reactants. Such feedstocks provide an essential bridge towards a carbon-free energy future, but their volatility and low energy density require the formation of new C-C bonds and the removal of oxygen. These transformations are described here through recent advances in our understanding of the mechanisms and site requirements in catalysis by surfaces, with emphasis on enabling concepts that tackle ubiquitous reactivity and selectivity challenges. The hurdles in forming the first C-C bond from C1 molecules are illustrated by the oxidative coupling of methane, in which surface O-atoms form OH radicals from O2 and H2O molecules. These gaseous OH species act as strong H-abstractors and activate C-H bonds with earlier transition states than oxide surfaces, thus rendering activation rates less sensitive to the weaker C-H bonds in larger alkane products than in CH4 reactants. Anhydrous carbonylation of dimethyl ether forms a single C-C bond on protons residing within inorganic voids that preferentially stabilize the kinetically-relevant transition state through van der Waals interactions that compensate for the weak CO nucleophile. Similar solvation effects, but by intrapore liquids instead of inorganic hosts, also become evident as alkenes condense within MCM-41 channels containing isolated Ni2+ active sites during dimerization reactions. Intrapore liquids preferentially stabilize transition states for C-C bond formation and product desorption, leading to unprecedented reactivity and site stability at sub-ambient temperatures and to 1-alkene dimer selectivities previously achieved only on organometallic systems with co-catalysts or activators. C1 homologation selectively forms C4 and C7 chains with a specific backbone (isobutane, triptane) on solid acids, because of methylative growth and hydride transfer rates that reflect the stability of their carbenium ion transition states and are unperturbed by side reactions at low temperatures. Aldol condensation of carbonyl compounds and ketonization of carboxylic acids form new C-C bonds concurrently with O-removal. These reactions involve analogous elementary steps and occur on acid-base site pairs on TiO2 and ZrO2 catalysts. Condensations are limited by α-H abstraction to form enolates via concerted interactions with predominantly unoccupied acid-base pairs. Ketonization is mediated instead by C-C bond formation between hydroxy-enolates and monodentate carboxylates on site pairs nearly saturated by carboxylates. Both reactions are rendered practical through bifunctional strategies, in which H2 and a Cu catalyst function scavenge unreactive intermediates, prevent sequential reactions and concomitant deactivation, and remove thermodynamic bottlenecks. Alkanal-alkene Prins condensations on solid acids occur concurrently with alkene dimerization and form molecules with new C-C bonds as skeletal isomers unattainable by other routes. Their respective transition states are of similar size, leading to selectivities that cannot sense the presence of a confining host. Prins condensation reactions benefit from weaker acid sites because their transition states are less charged than those for oligomerization and consequently less sensitive to conjugate anions that become less stable as acids weaken.

Quantum effects in the sorption of hydrogen by mesoporous materials

The sorption and desorption of hydrogen by mesoporous MCM-41 silicate material is studied at temperatures ranging from 6.8 to 290 K. It is found that a thermally activated mechanism with an estimated activation energy Ea ≈ 466 K predominates in the H2 sorption kinetics of an MCM-41 sample for temperatures of 60–290 K. For temperatures of 17–60 K the diffusion coefficient of H2 molecules in MCM-41 is almost entirely temperature independent, which is typical when a tunneling diffusion mechanism predominates over the thermally activated mechanism. Within the 8–17 K range, a change in the mobility of H2 molecules in the channels of MCM-41 is observed that appears to correspond to the formation of a monolayer (or its destruction during heating) and subsequent layers of hydrogen which have condensed on the inner surfaces of the channels. This process has an activation energy Em ≈ 21.2 K. At temperatures below 8 K the diffusion coefficients of H2 depend weakly on temperature. This presumably corresponds to a change in the mechanism for filling of the channels of MCM-41 from the layer-by-layer growth of film on the inner surfaces of the channels to capillary condensation of H2 molecules. These results are compared with previously obtained data on low-temperature sorption of hydrogen by bundles of carbon nanotubes.

Hybrid materials based on polyethylene and MCM-41 microparticles functionalized with silanes: Catalytic aspects of in situ polymerization, crystalline features and mechanical properties

New nanocomposites based on polyethylene have been prepared by in situ polymerization of ethylene in presence of mesoporous MCM-41. The polymerization reactions were performed using a zirconocene catalyst either under homogenous conditions or supported onto mesoporous MCM-41 particles, which are synthesized and decorated post-synthesis with two silanes before polymerization in order to promote an enhanced interfacial adhesion. The existence of polyethylene chains able to crystallize within the mesoporous channels in the resulting nanocomposites is figured out from the small endothermic process, located at around 80 °C, on heating calorimetric experiments, in addition to the main melting endotherm. These results indicate that polyethylene macrochains can grow up during polymerization either outside or inside the MCM-41 channels, these keeping their regular hexagonal arrangements. Mechanical response is observed to be dependent on the content in mesoporous MCM-41 and on the crystalline features of polyethylene. Accordingly, stiffness increases and deformability decreases in the nanocomposites as much as MCM-41 content is enlarged and polyethylene amount within channels is raised. Ultimate mechanical performance improves with MCM-41 incorporation without varying the final processing temperature.

Quantum effects in the sorption kinetics of4He by mesoporous materials

Sorption and desorption of 4He by a mesoporous silicate material MCM-41 was studied in the temperature range of 1.5–290 K. It was shown that for T = 25–290 K the thermal activation mechanism is dominant in the sorption kinetics of 4He atoms by an MCM-41 sample. Its activation energy was estimated as Ea ≈ 164.8 K. For T = 12–23 K, the diffusion of 4He atoms in the MCM-41 was practically independent of temperature, which typically occurs when the tunnelling mechanism of diffusion dominates over the thermally activated one. A change in the mobility of 4He atoms in MCM-41 channels was observed at T = 6–12 K, which may be indicative of the formation upon cooling (or decay upon heating) of a 4He monolayer and subsequent multilayers on the inner surfaces of the channels. Below 6 K, the diffusion coefficients of 4He are only weakly temperature dependent, which may be attributed to the behavior of quantum 4He liquid in the MCM-41 channels covered with several layers of 4He atoms.

Hybrid silica with bimodal mesopore system: Synthesis and catalytic evaluation

As-synthesized CTA–MCM-41 silica possesses basic siloxy (SiO-) catalytic sites, enabling it to be used in reactions such as transesterification and Knoevenagel condensation. Its application in basic catalysis has shown promising results, but attempts to reuse it in more than one cycle has revealed rapid loss of activity due to the removal of cetyltrimethylammonium (CTA) cations from the interior of the channels. The present work describes the encapsulation of nanoparticulate methacrylic polymers during the conventional synthesis of MCM-41, resulting in hybrid silicas containing polymers in the interior of the inorganic and organic matrix. Aqueous dispersions containing surfactant and monomers were characterized by small-angle X-ray scattering (SAXS), which confirmed the expansion of the micelles due to the presence of the encapsulated monomers. Transmission electron microscopy (TEM) was used to observe the formation of polymer nanoparticles stabilized by the surfactant (cetyltrimethy lammonium bromide). Infrared absorption spectroscopy and chemical analysis showed that these particles were present in the material after the synthesis. Analysis of the silicas using SAXS and nitrogen physisorption showed the expansion of the channels of the CTA–MCM-41 and the formation of two mesopore systems in the silica. Catalytic evaluation demonstrated that the silicas synthesized in the presence of the emulsified polymers possessed better catalytic stability when low monomer contents and monomers with larger alkyl chains were used. The encapsulation of nanoparticles of methacrylic polymer in the interior of the CTA–MCM-41 channels helped to avoid leaching of the CTA cations during the reaction.

Preparation and stability of β-carotene loaded using mesoporous silica nanoparticles as carriers system

1,3,5-Trimethylbenzene (1,3,5-TMB) was used as the pore-enlarging modifier to expand the pore size of MCM-41 (mobil company of matter) mesoporous silica nanoparticles. The solvent impregnation method was adopted to assemble non-water-soluble β-carotene into the pore channel of MCM-41. The MCM-41 and drug assemblies were characterized by TEM, FT-IR, elemental analysis and N2 adsorption-desorption. The results showed that MCM-41 has good sphericity and regular pore structure. The research also investigated the optimal loading time, the drug loading and the vitro stability of the β-carotene. As a drug carrier, the modified MCM-41 showing a shorter drug loading time, the drug loading as high as 85.58% and the stability of β-carotene in drug assemblies has improved. The study of this new formulation provides a new way for β-carotene application.

In situ encapsulation of Pd inside the MCM-41 channel.

Pd nanoparticles were successfully introduced into the channels of mesoporous silica MCM-41 with their dispersion well-tuned. We identified the dual role played by CTAB, which was critical for both the micelle template and Pd grafting, leading to the formation of a highly active Pd-MCM-41 nanocomposite for catalysing the Suzuki reaction.

Direct templating assembly route for the preparation of highly-dispersed vanadia species encapsulated in mesoporous MCM-41 channel

Understanding the nature of active sites, including the number and dispersion on the surface of a support, is essential to improve the catalytic activity.

Kinetic study of CTMA+ removal from the pores of the Ti-MCM-41 molecular sieve

Thermogravimetry analysis was used to study the thermal decomposition of Ti-MCM-41 to determine the best calcination conditions. Ti-MCM-41 molecular sieve was synthesized using a hydrothermal route in which Ti ions were incorporated into the pore channels of MCM-41 starting from a hydrogel method according to the following molar composition: 1.00 CTMABr:4.00 SiO2:X TiO2:1.00 Na2O:200.00 H2O. Physicochemical properties of samples were investigated using XRD, FTIR, and N2 adsorption–desorption to confirm the incorporation of TiO2 nanoparticles inside of MCM-41 mesoporous without destroying its structure. The high temperatures facilitate the rapid removal of the surfactant; however, it caused destruction of the MCM-41 structure by breaking the bonds of the silica tetrahedral structure. The procedures to obtain the apparent activation energies of CTMABr decomposition were based on the kinetic model proposed by Flynn-Wall using thermal analysis of data performed with heating rates 5, 10, and 20 °C min−1.

Preparation and characterization of PEI-loaded MCM-41 for CO2 capture

Abstract MCM-41, a molecular sieve, was prepared using tetraethoxysilane and cetyltrimethylammonium bromide and used as an effective adsorbent for CO2. By using the wet impregnation method, various weights of poly(ethyleneimine) (PEI) were modified on mesoporous silicate MCM-41 for increasing the CO2-adsorption capacity. The samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and nitrogen adsorption/desorption isotherms. Thermal gravimetric analysis (TGA) was conducted to investigate the CO2 capture behaviors. PEI was successfully dispersed into the channels of MCM-41, shifting the (110) diffraction peaks in the XRD patterns. The specific surface area and total pore volume of the PEI-loaded MCM-41 adsorbent decreased when the weight of PEI loading increased. From the results, it was concluded that PEI loading influences the CO2 capture performance, resulting from the enhancement of the basic functional groups on MCM-41.

Advantageous delivery of nifedipine from inorganic materials showing increased solubility and biocompatibility

Nifedipine, a potent calcium-channel blocking agent, widely used for treating hypertension has been immobilized inside inorganic matrix materials. The drug was intercalated in Montmorillonite K10 clay by cation exchange, and loaded by an adsorption method inside the channels of mesoporous MCM-41. Spectroscopic evidence (FTIR, powder X-ray diffraction (XRD)) as well as elemental analysis thermal analysis and electron microscopy revealed the preparation of the drug-loaded materials. Release profiling in a simulated body fluid from MCM-41 is fast, indicating that a concentration peak is reached in a short period of time, while the release profile from K10 is much slower. The release profile from MCM-41 leads to higher drug solubility compared even with neat drug solubility, which is advantageous in a drug known for its low solubility (ca. 20μgmL–1). The same is true for K10 clay although its slow release kinetics leave it behind. Spectroscopic data supports this fact by showing that intermolecular host–guest interactions occur thus leading to the observed kinetic effects in the release experiments. In addition it is found that nifedipine inside the host materials is more resistant to photodegradation than in its pure form thus showing another important feature of the hybrid organic/inorganic formulation, as evidenced by GC data.

Dynamics of pH-sensitive nitroxide radicals in water adsorbed in ordered mesoporous molecular sieves by EPR Spectroscopy

A spin pH probe technique was used to study the influence of the channel diameter on the EPR spectra of pH-sensitive nitroxide radicals (NR) located in the channels of the mesoporous molecular sieves MCM-41 and SBA-15 with diameters ranging from 2.3 to 8.1nm. From EPR spectra analysis and the results of the NR retention by the mesoporous molecular sieves upon washing with an aqueous KCl solution, the regularities of NR molecular location inside the channels were studied. The obtained dependence of the fraction of the radical molecules in the fast motional regime (with the rotational correlation times, τc=2×10−11s–9×10−11s) in the channels of the mesoporous molecular sieves as a function of pH indicates that both NR in the fast and slow motional regime (with τc=8×10−9s–7×10−10s) may be used for estimation of the solution acidity inside the channels and of the near-surface electrical potential.

Back Cover: Synthesis of Poly(silyl ether)s by Rhodium(I)–NHC Catalyzed Hydrosilylation: Homogeneous versus Heterogeneous Catalysis (ChemCatChem 5/2013)

Rhodium-catalyzed polymerization goes heavyweightThe cover picture shows a neutral rhodium(I) N-heterocyclic carbene hydrosilylation catalyst immobilized on the surface of MCM-41. In their Full Paper on p. 1133 ff., F. J. Fernández-Alvarez, L. A. Oro et al. describe how these rhodium-containing materials catalyze the preparation of poly(silyl ether)s by hydrosilylation. These results show that the immobilized catalysts provide polymers with higher average molecular weight than those obtained by using the corresponding homogenous systems, which could be attributed to a confinement effect exerted by the channels of MCM-41.

A one-pot synthesis of a ternary nanocomposite based on mesoporous silica, polyaniline and silver

The research on hybrid materials composed of inorganic and organic species in the nanometer range is motivated by the synergic combination of their native properties in a unique material. In this paper, we report a new one-pot synthesis of a ternary nanocomposite based on a conducting polymer (polyaniline – PANI) and silver, using mesoporous silica (MCM-41) as a hard template to promote the controlled growth of both polymer chains and silver nanoparticles. The presence of these reaction products was proved by the appearance of the characteristic diffraction peaks of face-centered cubic metallic silver phase in the X-ray diffractogram and by the presence of typical bands of polyaniline as emeraldine salt in the FTIR and UV-Vis spectra. The preservation of the ordered MCM-41 mesostructure after the polymerization reaction was also attested by XRD and TEM analysis and it was accompanied by a decreasing of the particle size as observed in the SEM images. N2 adsorption–desorption isotherms support the statement that PANI chains and AgNP have been incorporated mainly into the channel of MCM-41, a fact also attested by TEM images. The electrical conductivity of this nanocomposite is on the order of 10−3 S cm−1, which is one million times higher than the value obtained for a MCM-41/polyaniline composite. Although some reports about the isolated incorporation of polyaniline or silver nanoparticles into the MCM-41 pores are found in the literature, to our knowledge this is the first time that such nanocomposite preparation is reported.

Preparation of Organic-Inorganic Hybrid Materials Based on MCM-41 and Its Applications

This work reports the covalent attachment of three different calix[4]arenes (calix[4]arene (C4), p-sulfonatocalix[4]arene (C4S), and p-tert-butyl-calix[4]arene (PC4)) to MCM-41, using a three-step modification process. 3-Chloropropyltrimethoxysilane (ClPTS) was first attached to the mesoporous silica surface and subsequently converted to amides via the reaction with toluene diisocyanate (TDI). Finally, calix[4]arene derivatives attached to the isocyanate ending remained available on toluene di-iso-cyanate. Changes in the surface properties of the mesoporous silica caused by the chemical modification were monitored using the Fourier transform infrared spectroscopy (FTIR), thermal analysis (TGA), and elemental analysis. The FTIR spectra and TGA analysis verify that the calix[4]arene derivatives are covalently attached to the mesoporous silica. The preservation of the MCM-41 channel system was checked by the X-ray diffraction and nitrogen adsorption analysis. These materials were then used to evaluate the sorption properties of some organotins compounds (Tributyltin (TBT), Triphenyltin (TPT), and Dibutyltin (DBT)).

The composite material of perylene bisimide dye in MCM-41 and its photophysical and photochemical properties

The mesoporous material MCM-41 was synthesized by a hydrothermal method, and perylene bisimide dye was incorporated into its channels by impregnation. The absorption, FTIR, fluorescence emission, and decay spectra of perylene bisimide dye in CHCl3 and in MCM-41 were studied to investigate the effect of the one-dimensional channel of MCM-41 on the photophysical and photochemical properties of the dye. The results indicated that the nanochannels of MCM-41 shifted the absorption and emission maxima to red and broadened the spectra, with loss of vibrational structure. The fluorescence decay curves fitted a double-exponential function and the lifetime of perylene bisimide dye in MCM-41 was prolonged. The huge surface area of the mesoporous molecular sieve MCM-41 prevented aggregation of dye molecules, which can thus be used at high concentration.