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.     

Adsorption in cylindrical pores: Mixed lattice-site/off-site Monte Carlo simulations in pores with heterogeneous wall structure

We present results of grand canonical Monte Carlo simulations of adsorption in cylindrical pores with rough surface modeled by lattice-site approach. Each site is characterized by two parameters: structural and energetic, which locally modify the structure and energy properties of the surface. There are three types of sites, randomly distributed over the wall: attractive, neutral and repulsive with respect to the smooth pore model. The results presented here show how this model affects the mechanism of adsorption and how it changes the forms of adsorption isotherm. We compare our numerical results with the experimental data of adsorption of a simple fluid (CH₄, T = 77 K) in cylindrical silica pore of diameter d = 4 nm (MCM-41 material).     

Pore size distribution calculation from H NMR signal and N2 adsorption–desorption techniques

The pore size distribution (PSD) of nano-material MCM-41 is determined using two different approaches: N₂ adsorption–desorption and ¹H NMR signal of water confined in silica nano-pores of MCM-41. The first approach is based on the recently modified Kelvin equation [J.V. Rocha, D. Barrera, K. Sapag, Top. Catal. 54(2011) 121–134] which deals with the known underestimation in pore size distribution for the mesoporous materials such as MCM-41 by introducing a correction factor to the classical Kelvin equation. The second method employs the Gibbs–Thompson equation, using NMR, for melting point depression of liquid in confined geometries. The result shows that both approaches give similar pore size distribution to some extent, and also the NMR technique can be considered as an alternative direct method to obtain quantitative results especially for mesoporous materials. The pore diameter estimated for the nano-material used in this study was about 35 and 38 Å for the modified Kelvin and NMR methods respectively. A comparison between these methods and the classical Kelvin equation is also presented.     

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.     

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.     

Water-filled MCM-41 characterized by double-quantum-filtered2H NMR spectral analysis

The dynamics of water molecules confined in adsorbed layers of siliceous MCM-41 with a pore diameter of 2.8 nm is investigated at 230 K by deuteron nuclear magnetic resonance (NMR) relaxation studies including line shapes of theT ₁ process and double quantum filtered (DQF) spectral analyses.²H DQF NMR is a particularly sensitive tool for the determination of the adsorbate dynamics resulting from residual quadrupolar interaction due to the local order. The amount of monolayer water is determined. The monolayer water is composed of two different water components characterized by water, with isotropic reorientational motions, exchanging with water displaying a solid-like spectrum with 4 kHz edge splitting. One may expect that the latter water is situated on surface sites in MCM-41. The restricted wobbling motion of the D-O bond is used to describe its dynamics which is one order of magnitude slower than the isotropic reorientational motion. The order parameter, the motional correlation time, and the exchange rate thus determined provide useful information on the structure and the adsorptive properties of the mesoporous system.     

Preparation, characterization, and luminescence properties of novel hybrid material containing europium(III) complex covalently bonded to MCM-41

In this paper, a novel luminescent organic-inorganic hybrid material containing covalently bonded ternary europium complex in mesoporous silica MCM-41 has been successfully prepared by co-condensation of tetrethoxysilane (TEOS) and the modified ligand 2-phenyl-1H-imidazo[4,5-f][1,10]phen-3-(triethoxysilyl)propylcarbamate (PIP-Si) in the presence of cetyltrimethylammonium bromide (CTAB) surfactant as template. PIP-Si containing 1,10-phenanthroline covalently grafted to 3-(triethoxysilyl)propyl isocyanate is used not only as a precursor but also as the second ligand for Eu(TTA)₃·2H₂O (TTA: 2-thenoyltrifluoroacetate) complex to prepare a novel functionalized mesoporous material. The resulted mesoporous composite materials, which demonstrate strong characteristic emission lines of Eu³⁺⁵D₀-⁷F_J (J=0, 1, 2, 3, 4), were characterized by Fourier transform infrared (FT-IR), small-angle X-ray diffraction, excited-state decay analysis. Emission intensity of the Eu(III) complex covalently linked to MCM-41 (Eu-MCM-41) increases with the increasing irradiation time, demonstrating better photostability compared with both pure Eu(III) complex and physically incorporated sample.     

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.     

Effect of the thermal and hydrothermal treatment on textural properties of Zr-MCM-41 mesoporous molecular sieve

Zr-containing mesoporous molecular sieves were synthesized by hydrothermal method using cetyltrimethyl ammonium bromide as a template and sodium silicate and zirconium sulfate as raw materials. The structure and morphology of the synthesized samples were characterized via various physicochemical methods, including X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, solid state nuclear magnetic resonance (²⁹Si MAS-NMR) techniques, thermal gravimetric-differential scanning calorimeter (TG-DSC) and N₂ physical adsorption, respectively. The effect of the different initial ZrO₂:SiO₂ molar ratio, the different thermal treatment temperature and the different hydrothermal treatment time on textural property was investigated. The experimental results reveal that the as synthesized samples possess a typical mesoporous structure of MCM-41. On the other hand, the specific surface area and pore volume of the synthesized Zr-MCM-41 mesoporous molecular sieve decrease with the increase of the amount of zirconium incorporated in the starting material, the rise of thermal treatment temperature and the prolonging of hydrothermal treatment time, the mesoporous ordering becomes poor. Also, when the molar ratio of ZrO₂:SiO₂ in the starting material is 0.1, the mesoporous structure of the Zr-MCM-41 mesoporous molecular sieve still retains after calcination at 750 °C for 3 h or hydrothermal treatment at 100 °C for 6 d, and have specific surface areas of 423.9 and 563.9 m²/g, respectively.     

The nature of the electronic factor governing diastereofacial selectivity in remotely substituted (X) 2‐adamantyl cations: 5‐X versus 4‐X substitution

A limited series of 4^eq‐substituted (X) 2‐methyleneadamantanes ( 6 , Y?CH₂, X?F, Cl, Br, I, and SnMe₃) has been synthesized and diastereoselectivities for their hydrochlorination (HCl/CH₂Cl₂) have been determined. Diastereoselectivities for the fluorination (DAST/CH₂Cl₂) of secondary alcohol mixtures, obtained from the hydride reduction of the precursor ketones ( 6 ,Y?O) to the alkenes, have also been measured. A comparison of this selectivity data for nucleophilic trapping of 4^eq‐substituted (X) 2‐adamantyl cations ( 4 , R?H and Me) with the corresponding information for 5‐substituted (X) 2‐adamantyl cations ( 1 , R?H and Me) has revealed important distinctions between the two series. In particular, whereas extended hyperconjugative effects appear to be the predominant electronic effect governing facial selectivity in the 5,2‐series, electrostatic influences prevail in the 4,2‐disposition. Copyright © 2007 John Wiley & Sons, Ltd.     

Computational studies on the thermal decomposition of the CH2FOCHFO radical

Theoretical studies have been carried out on the kinetics and thermochemistry of the thermal decomposition of the CH₂FOCHFO radical formed during the photo-oxidation of CH₂FOCH₂F (HFE-152E) using the dual-level method of obtaining the optimised structure at DFT(M06-2X)/6-311++G(d,p) followed by a single-point energy calculation at the G3 level of theory. The rate constant for different reaction channels involved during the decomposition processes of CH₂FOCHFO is evaluated at 298 K and 1 atm using canonical transition-state theory. The results point out that the C–H bond scission is the dominant path involving an energy barrier of 9.5 kcal mol^?1 determined at the G3 level of theory. A potential energy diagram is constructed and the results are compared with the data available from the literature for a structurally similar molecule.     

Oxidation of ferrocene molecules adsorbed in MCM-41

A mesoporous silica material MCM-41 was synthesized under two different conditions, and ferrocene molecules were adsorbed into one-dimensional pores formed in MCM-41. The pore sizes were determined to be 0.9 and 2.8 nm for the MCM-41 synthesized under an acidic and a basic condition, respectively. The formation of paramagnetic ferricinium ions was observed after the adsorption into pores by ESR, magnetic susceptibility and ⁵⁷Fe Mössbauer measurements. Spin densities attributable to ferricinium ions in the pores are estimated to be 0.74 and 0.065 in MCM-41 synthesized under an acidic and a basic condition, respectively. It was shown that the oxidation of ferrocene molecules to ferricinium ions in MCM-41 prepared in an acid solution is faster than that in base at ambient temperature and pressure.     

On the synthesis of AlPO4-21 molecular sieve by vapor phase transport method and its phase transformation to AlPO4-15 molecular sieve

An experimental design was applied to the synthesis of AlPO₄-21 molecular sieve (AWO structure) by vapor phase transport (VPT) method, using tetramethylguanidine (TMG) as the template. In this study, the effects of crystallization time, crystallization temperature, phosphor content, template content and water content in the synthesis gel were investigated. The materials obtained were characterized by X-ray diffraction, scanning electron microscopy and fourier transform infrared spectroscopy (FT-IR). Microstructural analysis of the crystal growth in vapor synthetic conditions revealed a revised crystal growth route from zeolite AlPO₄-21 to AlPO₄-15 in the presence of the TMG. Homogenous hexagonal prism AlPO₄-21 crystals with size of 7 × 3 μm were synthesized at a lower temperature (120 °C), which were completely different from the typical tabular parallelogram crystallization microstructure of AlPO₄-21 phase. The crystals were transformed into AlPO₄-21 phase with higher crystallization temperature, longer crystallization time, higher P₂O₅/Al₂O₃ ratio and higher TMG/Al₂O₃ ratio.     

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.     

Proton conduction in CsHSO4–mesoporous silica composite electrolytes

Novel proton-conducting composites were prepared by incorporating molten CsHSO₄ (CHS) into two types of mesoporous silica, MCM-41 with a one-dimensional (1-D) hexagonal structure and MCM-48 with a three-dimensional (3-D) cubic structure. Their proton conductivities (σ) were measured to examine effects of the incorporation and the dimensionality of the mesopores on their conductivity. Incorporation of proper amounts of MCMs maintained high proton conductivities as high as ~ 10^? 3 S cm^? 1 at temperatures above the superprotonic phase-transition temperature (T_s: 414 K) of CHS and improved the conductivity by 2 to 3 orders of magnitude at temperatures below T_s. In the case of MCM-41, more than 40 mol% mixing, however, caused steep drops in σ in both temperature ranges. On the other hand, the CHS/MCM-48 composite showed a linear increase in σ below T_s and a gradual decrease in σ above T_s with an increase in the MCM-48 content at least up to 60 mol%. X-ray diffraction (XRD) analysis revealed that CHS filled in the MCM mesopores became X-ray crystallographically amorphous and the amount of the amorphous phase increased with an increase in the MCM contents. In the case of the CHS/MCM-48 composites, the activation energy (E_a) for proton conduction below T_s drastically came close to that above T_s by mixing with MCM-48 up to 30 mol%. This indicates that the proton can transport similarly to the conduction mechanism in the superprotonic phase even below T_s. These results suggest that CHS adopts a highly proton-conducting amorphous phase in the mesopores at temperatures below T_s, and that MCM-48 with the 3-D cubic structure is more suitable for formation of high proton-conducting percolation paths.     

Post-treatment of mesoporous material with high temperature for synthesis super-microporous materials with enhanced hydrothermal stability

Super-microporous silicon material with high hydrothermal stability denoted as MCM-48-T has been prepared from mesoporous MCM-48 by high temperature treatment. The structural and chemical property of MCM-48-T has been characterized by variety of techniques such as small-angle X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ adsorption-desorption, infrared spectroscopy (IR) and ²⁹Si MAS NMR, etc. The results showed that Si-OH groups are forced to condense by the treatment of high temperature and the pore size of MCM-48-T is around 1.03 nm in the super-microporous range. Besides, the ratio of Q⁴/Q³ increases considerably. Compared with the original material (MCM-48), the hydrothermal stability of MCM-48-T has significantly enhanced.     

Thermal stability and surface acidity of mesoporous silica doubly doped by incorporation of sulfate and zirconium ions

A sulfated Si-Zr-MCM-41 solid with highly ordered mesostructure was synthesized through a templated synthesis route where the CTAB surfactant was used as template. During the synthesis procedure, various amounts of (NH₄)₂SO₄ were added into the mixed solution of Zr and Si precursors to in situ sulfate the MCM-41 solids, aiming to enhance the acidity and thermal stability. The resultant materials showed a long-range ordered hexagonal arrangement with high surface area larger than 797 m²/g and an average pore size distributed at approximate 2.5-2.8 nm. The high-resolution TEM observations confirmed that the order of the mesostructure gained when the molar ratio of SO₄²⁻/(ZrO₂ + SiO₂) increased from 0.1 to 0.3 but decreased as it reached 0.5, which is consistent with the results of ²⁹Si MAS-NMR and XRD analysis. Compared to Si-MCM-41, the (Q² + Q³)/Q⁴ ratio derived from the NMR spectra of the Zr-doped sample was higher, indicating that zirconium atoms were incorporated into the silica framework. Unexpectedly, in situ sulfation does not enhance the surface Brönsted acidity, most likely due to the sulfur retained within the bulk of the materials; however, it indeed improved the thermal stability of the solid and long-range order of the structure.     

Incorporation of iodine into the channels of AlPO4-5 crystals

Iodine molecules have been incorporated into the channels of calcined AlPO₄-5 crystals by using vapor-phase diffusion method. XRD results indicate that the iodine-loaded AlPO₄-5 crystals are of similar structure as that of as-synthesized AlPO₄-5 crystals. TG and XRF analyses reveal that the iodine molecules have not compactly filled the channels of AlPO₄-5 crystals, and the desorbing process of adsorbed iodine is quite different from that of adsorbed water. Polarized Raman spectra imply that the iodine exists in the channels of AlPO₄-5 crystals as vapor-like I₂ molecules, and the I₂ molecules orientate randomly.     

Isotopically selective infrared multiphoton dissociation of 2-chloro-1,1,1-trifluoroethane: 13C selectivity and mechanism

13 C-selective infrared multiphoton dissociation of CF₃CH₂Cl has been studied by analyzing the distribution of ¹³C concentrations of the main products CF₂=CHCl, CF₂=CH₂, CF₂=CHF, C₂F₆, and the trace products CF₃CH₂CF₃ and CF₃CH=CHF₃. The mechanism mainly concerns the dissociation of energized CF₃CH₂Cl, the collisional stabilization of excited CF₃CH and CF₃CH₂ and the recombination of the nascent radicals. No significant radical–molecule reactions degrade the intrinsic ¹³C dissociation selectivity. High ¹³C production yield and ¹³C concentration can be attained at a laser fluence of 1.6 J/cm². Such low fluence can be used to improve focus condition and enhance photon utilization efficiency for practicable ¹³C separation. Received: 10 March 1998/Revised version: 17 September 1998     

Ordered mesoporous silica materials (SBA-15) with good heat-resistant magnetism

A series of highly ordered mesoporous materials (CF-SBA-15) with heat-resistant magnetism have been successfully prepared from impregnation of cobalt salt, iron salt, and citric acid with as-synthesized SBA-15. XRD and N₂ isotherms indicate that these materials have highly ordered hexagonal mesoporous symmetry and open pore systems. The measurement of magnetic property shows that these materials are ferromagnetic even if calcined at 550 °C for 10 h in air, indicating their good heat-resistant magnetism. These results would be very important for recycle and regeneration of adsorbents and catalysts in practical applications. Moreover, this method may be useful for other mesoporous materials with thermally stable magnetism from a combination of other mesoporous materials such as MCM-41 with magnetic nanoparticles of MnFe₂O₄ and NiFe₂O₄.